DE102005009885A1 - Preparation of acrolein or acrylic acid involves conducting starting reaction gas mixture having propylene and molecular oxygen reactants and inert molecular nitrogen and propane diluent gasses through fixed catalyst bed - Google Patents

Abstract

Acrolein or acrylic acid is prepared by catalyzing partial gas phase oxidation of propene, where a starting reaction gas mixture having propylene and molecular oxygen reactants and inert molecular nitrogen and propane diluent gasses is conducted at elevated temperature through a fixed catalyst bed whose active composition is a multimetal oxide having molybdenum, iron and bismuth. Preparation of acrolein or acrylic acid involves catalyzing partial gas phase oxidation of propene, where a starting reaction gas mixture having propylene and molecular oxygen reactants and the inert molecular nitrogen and propane diluent gasses and having the molecular oxygen and the propylene in a molar oxygen;C 3H 6 ratio of >=1 is conducted at elevated temperature through a fixed catalyst bed whose active composition is a multimetal oxide having molybdenum, iron and bismuth based on its total volume of 6-9 vol.% propylene, 8-18 vol.% molecular oxygen. 6-30 vol.% propane and 32-72 vol.% molecular nitrogen. The molar ratio V 1 of propane in starting reaction gas mixture to propylene in starting reaction gas mixture is 1-4. The molar V 2 of molecular nitrogen present in starting reaction gas mixture to molecular oxygen in starting reaction the mixture is 2-6, and the molar ratio of the molecular oxygen in starting reaction gas mixture to propylene in starting reaction mixture is 1.3-2.4.

Description

The present invention relates to a process for the preparation of acrolein or acrylic acid or a mixture thereof by heterogeneously catalyzed partial gas phase oxidation of propene, comprising reacting the reactants propylene and molecular oxygen and the inert diluent gases molecular nitrogen and propane containing reaction gas starting mixture containing the molecular oxygen and propylene in a molar ratio O ₂ : C ₃ H ₆ ≥ 1, at elevated temperature leads through a fixed catalyst bed, the active material is at least one of the elements Mo, Fe and Bi containing multimetal.

acrolein forms a reactive monomer, especially as an intermediate, z. B. in the production of acrylic acid by two-stage heterogeneous catalyzed partial gas phase oxidation of propene of importance is. acrylic acid is as such or in the form of their alkyl esters z. B. to manufacture of polymers, i.a. as adhesives or water absorbent Materials can be used, suitable.

The preparation of acrolein by the process of heterogeneously catalyzed partial gas phase oxidation described in the preamble of this document is known (cf., for example, EP-A 11 06 598, WO 97/36849, EP-A 293 224, WO 01/96271, US Pat. DE-A 198 37 517, EP-A 274 681, DE-A 198 37 519, DE-A 198 37 520, EP-A 117 146, WO 03/11804, US 3,161,670 , WO 01/96270, DE-A 195 08 558, DE-A 33 13 573, DE-A 102 45 585, WO 03/076370, DE-A 103 16 039, WO 04/031106, and the German application DE- A 10 2004 032 129 and the prior art cited in these documents). It usually forms the first stage of a two-stage heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid. In the first reaction stage, the propene is substantially partially oxidized to acrolein, and in the second reaction stage, the acrolein formed in the first reaction stage is substantially partially oxidized to acrylic acid. Of importance is that the technical embodiment is normally designed so that the acrolein formed in the first reaction stage is not separated, but as part of the first reaction stage leaving product gas mixture, optionally supplemented by molecular oxygen and inert gas, and optionally by direct and / or cooled indirect cooling, is led to the second reaction stage.

target product a heterogeneously catalyzed gas phase partial oxidation of propene to acrolein is acrolein.

problem point all heterogeneously catalyzed gas phase partial oxidations in the fixed catalyst bed is that the reaction gas mixture when flowing through the cata- torfestbett a maximum, the so-called hot spot value passes. This maximum value is composed of the external temperature of the fixed catalyst bed and the heat of reaction.

Out establish the expediency Therefore, the temperature of the fixed catalyst bed and the effective Temperature of the fixed catalyst bed differ from each other. there becomes the temperature below the temperature of the fixed catalyst bed the fixed catalyst bed during exercise of the partial oxidation process, but in notional absence a chemical reaction (i.e., without the influence of the heat of reaction) understood. By contrast, the effective temperature of the fixed catalyst bed becomes the actual Temperature of the catalyst fixed bed with the inclusion of the heat of reaction Partial oxidation understood. Is the temperature of the fixed catalyst bed along the Fixed catalyst bed not constant (eg in the case of several Temperature zones), the term temperature of the fixed catalyst bed means the (numbers) average of the temperature along the fixed catalyst bed. Of course For example, the temperature of the fixed catalyst bed may be configured along its length. that the temperature over a certain length of section is constant, then changes abruptly and over another length of section u.s.w .. persists in this new value. One then speaks of one more than one temperature zone (or reaction zone) having or in more than one temperature zone (or reaction zone) fixed catalyst bed (fixed catalyst bed). Realizing such temperature zones (or reaction zones), Catalyst-fed reactors are used in a similar manner referred to as "single" or "multi-zone reactors" (cf. WO 04/085369). With the temperature of the reaction gas mixture passes through the effective temperature of the fixed catalyst bed in the flow direction the reaction gas mixture also the hot spot value.

One goal of the heterogeneously catalyzed partial gas phase oxidations of propylene to acrolein is to make the hotspot temperature and with it its sensitivity to an increase in the temperature of the fixed catalyst bed as low as possible. Too high hotspot temperatures usually reduce both the service life of the fixed catalyst bed and the selectivity of target product formation (here: Acrolein). Lower hot spot temperatures are usually accompanied by a reduction in their sensitivity to an increase in the temperature of the fixed catalyst bed. This proves z. B. in carrying out the partial oxidation in tube bundle reactors as low, if the individual catalyst tubes due to temperature differences over the reactor cross-section different temperatures of the catalyst fixed bed located in the respective catalyst tube. The presence of propane in the reaction gas mixture favors lower hot-spot temperatures as a result of the comparatively increased specific heat of the propane (cf., for example, EP-A 293 224).

Farther favored a presence of propane due to its flammability the explosive behavior of the Reaction gas mixture (eg DE-A 195 08 558).

On the other hand, it is known that a presence of propane in the reaction gas mixture the unwanted Formation of propionaldehyde and / or propionic acid in undesired Way favors (See, for example, WO 01/96270). WO 01/96270 therefore recommends the reaction gas mixture to dilute with molecular nitrogen and as an oxygen source for example, to use air. An advantage of this procedure would be simultaneous the use of a comparatively economical source of oxygen. Another object of the prior art methods is therein, as a propene source, a heterogeneously catalyzed oxydehydrogenation and / or To be able to use dehydrogenation of propane to propylene and thereby on a subsequent separation of the formed propylene from unreacted propane, so as to have a commercial propylene source to dispose of (See, for example, WO 03/011804, DE-A 198 37 517, DE-A 198 37 519, DE-A 198 37 520, WO 01/96370, DE-A 102 45 585 and WO 03/76370).

In WO 01/96270 is in this regard a heterogeneously catalyzed propane dehydrogenation with comparatively low propane conversions recommended.

On the other side demand z. For example, both EP-A 990 636 and EP-A 10 70 700 as high as possible Propene contents in the reaction gas starting mixture of a propene partial oxidation to acrolein, if possible to be able to achieve high space-time yields on the target product.

Although high propane conversions now promote the possibility of adjusting increased propylene contents in the starting reaction gas mixture of propylene partial oxidation. However, they are disadvantageous in that on the one hand the remaining propane content is reduced, which has an unfavorable effect on the hot spot formation and the explosion behavior and at the same time increases with high propane conversions associated with the dehydrogenation or oxydehydrogenation secondary component formation (eg H ₂ or H ₂ O). If the resulting dehydrogenation and / or Oxydehydriergemisch used in this case as such for the production of the reaction gas starting mixture of the subsequent propylene partial oxidation, as z. For example, EP-A 117 146, DE-A 33 13 573 and US-A 3,161,670 recommend results in a comparatively voluminous reaction gas starting mixture. However, small volumes are advantageous from the point of view of the most economical possible promotion of the reaction gas streams. In particular, when high loads of the catalyst charge according to the in the documents WO 04/85365, WO 04/85367, WO 04/85369, WO 04/85370, WO 04/85363, WO 00/53559, WO 04/85362, WO 00 / 53557 and DE-A 199 48 248 are to be implemented.

on the other hand are high dehydrogenation or Oxidehydrierumsätze in the sense of a possible economic propylene production favorable. Conversely requires an elevated one Propene content in the reaction gas starting mixture an increased molar relationship from molecular oxygen to propylene in same to the activity of the catalyst feed to develop optimally.

simultaneously proves too high residual oxygen content in the product gas mixture the partial oxidation insofar as disadvantageous, as he in an advantageous Return of the remaining after separation of the target product from this product gas mixture, unreacted propane containing, residual gas in the propylene for production applied heterogeneously catalyzed propane dehydrogenation to be dehydrogenated Propane in unwanted Attack mode and reduces the selectivity of propene formation. In general, a heterogeneously catalyzed propane dehydrogenation over a heterogeneously catalyzed Propanoxidehydrierung preferred.

In contrast to the exothermic heterogeneously catalyzed oxydehydrogenation, which is enforced by oxygen present and in the intermediately no free hydrogen is formed (the torn from the dehydrogenated propane entrissene hydrogen is directly as water (H ₂ O) snatched) is detectable under one heterogeneously catalyzed dehydrogenation can be understood as a ("conventional") dehydrogenation, the catalytic effect of which is endothermic, in contrast to the oxydehydrogenation (as a subsequent step, an exo thermic hydrogen combustion may be involved in the heterogeneously catalyzed dehydrogenation) and in which at least intermediate free molecular hydrogen is formed. This usually requires different reaction conditions and different catalysts than for oxydehydrogenation.

Under Fresh propane is understood in this writing propane, that still did not participate in any chemical reaction. Usually will it crude propane (which preferably the specification according to DE-A 102 46 119 and DE-A 102 45 585 met) which contains small amounts of propane also different components.

The Reaction gas starting mixture for the propene partial oxidation to acrolein satisfies in this document in an expedient manner likewise the specifications recommended in DE-A 102 46 119 and DE-A 102 45 585.

Under the loading of a catalyst bed catalyzing a reaction step with reaction gas (exit) mixture is in this document the amount on reaction gas (outlet) mixture in standard liters (= Nl, the volume in liters, the corresponding reaction gas (outlet) mixed amount at normal conditions (0 ° C, 1 bar) would take) understood that per hour by a liter of fixed catalyst bed guided becomes.

The Strain can also be mixed only on one component of the reaction gas (outlet) mixture be related. Then it is the amount of this ingredient in Nl / l · h that per hour through one liter of fixed catalyst bed (pure inert material beds are not counted to the fixed catalyst bed).

When an inert gas in this document is a reaction gas component be understood, under the conditions of the corresponding Reaction essentially as inert and -any inert reactant gas component for themselves considered - too more than 95 mol%, preferably more than 99 mol% chemically unchanged remains.

adversely to the above compiled recommendations of the prior Technology for a heterogeneously catalyzed partial oxidation of propene to acrolein is that they only illuminate individual aspects. The task The present invention was therefore an improved Process of heterogeneously catalyzed partial oxidation of propene to acrolein available to provide the various individual aspects cited in the prior art in its entirety in an optimizing way.

Accordingly, a process for the production of acrolein or acrylic acid or a mixture thereof by heterogeneously catalyzed partial gas phase oxidation of propene, in which one comprising the reactants propylene and molecular oxygen and the inert diluent gases molecular nitrogen and propane starting reaction gas mixture 2 containing the molecular oxygen and the propylene in a molar ratio O ₂ : C ₃ H ₆ ≥ 1 at elevated temperature through a fixed catalyst bed (through a fixed catalyst bed) whose active material is at least one multimetal oxide containing the elements Mo, Fe and Bi , which is characterized in that the reaction gas starting mixture 2 , based on its total volume, the following contents 6 to 9 vol.% propylene, 8 to 18% by volume molecular oxygen, 6 to 30 vol.% Propane and 32 to 72 vol.% molecular nitrogen with the proviso that the molar ratio V, of the reaction gas starting mixture 2 Propane contained in the starting reaction gas mixture 2 contained propylene 1 to 4, the molar ratio of V ₂ in the reaction gas starting mixture 2 contained molecular nitrogen to in the starting reaction gas mixture 2 contained molecular oxygen 2 to 6 and the molar ratio of V ₃ in the reaction gas starting mixture 2 contained molecular oxygen to in the reaction gas starting mixture 2 contained propylene 1.3 to 2.4.

The method according to the invention differs from the methods of the prior art primarily by the requirement V ₁ = 1 to 4. In all exemplary embodiments of the prior art, V ₁ ≥ 4.5. The latter has a detrimental effect on the selectivity of byproduct formation on propionaldehyde and propyne and limits the possible conversion of a heterogeneously catalyzed propane dehydrogenation as a propylene source, without being indispensable in view of the explosion behavior.

According to the invention, preference is given to the contents of the reaction gas starting mixture 2 7 to 9% by volume of propylene, propylene, 9.8 to 16% by volume of molecular oxygen, molecular oxygen, 9 to 25% by volume of propane and Propane and From 35 to 65% by volume of molecular nitrogen, molecular nitrogen, with the proviso that
V ₁ = 1 to 3.5
V ₂ = 3 to 4.5 and
V ₃ = 1.4 to 2.2
is.

According to the invention, the contents of the reaction gas starting mixture are particularly preferred for the contents 2 7 to 9 vol.% propylene, 9.8 to 15 vol.% molecular oxygen, 10.5 to 20 vol.% Propane and 40 to 60 vol.% molecular nitrogen, with the proviso that
V ₁ = 1.5 to 2.5
V ₂ = 3.5 to 4.5 (preferably 3.5 to 4)
V ₃ = 1.4 to 2.14 (preferably 1.5 to 2.0)
is.

Very particular preference according to the invention applies to the contents of the reaction gas starting mixture 2 7 to 9 vol.% propylene, 9.8 to 15.5% by volume molecular oxygen, 10.5 to 15.5% by volume Propane and 40 to 60 vol.% molecular nitrogen with the proviso that
V ₁ = 1.5 to 2.2
V ₂ = 3.5 to 4.5 (preferably 3.5 to 4)
V ₃ = 1.4 to 2.14 (preferably 1.5 to 2.0)
is.

In the invention most preferred embodiment of the method according to the invention applies to the contents of the starting reaction gas mixture 2 7 to 8 vol.% propylene, 11.9 to 15.5% by volume molecular oxygen, 11.9 to 15.5% by volume Propane and 50 to 60 vol.% molecular nitrogen, with the proviso that
V ₁ = 1.7 to 2.1,
V ₂ = 3.5 to 4.5 (preferably 3.5 to 4)
V ₃ = 1.7 to 2.1 (preferably 1.8 to 2.0)
is.

In an alternative embodiment of the method according to the invention, the contents of the reaction gas starting mixture apply 2 7 to 9 vol.% propylene, 9.8 to 15 vol.% molecular oxygen, 21 to 28% by volume Propane and 40 to 60 vol.% molecular nitrogen, with the proviso that
V ₁ = 3 to 4,
V ₂ = 3.5 to 4.5 (preferably 3.5 to 4)
V ₃ = 1.4 to 2.14 (preferably 1.5 to 2.0)
is.

Quite generally, it is the case for all starting reaction gas starting mixtures according to the invention 2 favorable if their total content of components other than propylene, molecular oxygen, propane and molecular nitrogen is ≦ 10% by volume. Of these up to 10% by volume of other constituents, up to 8% by volume of ethane and / or methane may be present (but otherwise this content of ethane and methane may also be present). As a rule, the total content of starting reaction gas mixtures according to the invention is 2 but ethane and / or methane ≤ 5 vol .-%, usually ≤ 3 vol .-%, and often ≤ 2 vol .-%. However, such contents of ≥ 0.5% by volume are entirely possible according to the invention as a result of their largely inert behavior and are advantageous according to the invention due to the favorable thermal conductivity of the methane and ethane. Advantageously according to the invention, all the reaction gas starting mixtures according to the invention comprise 2 ≤ 5% by volume of water and ≤ 5% by volume of carbon oxides (CO ₂ and / or CO). Particularly advantageously, all the reaction gas starting mixtures according to the invention comprise 2 ≤ 3% by volume of water and ≤ 3% by volume of carbon oxides. The same content limits are preferably for molecular hydrogen, the content of which is particularly advantageous vanishing. Very particularly advantageous all mentioned reaction gas starting mixtures according to the invention comprise 2 ≤ 2% by volume of water and ≤ 2% by volume of carbon oxides. Water contents of ≥ 0.5% by volume in the starting reaction gas mixture are generally favorable 2 ,

Furthermore, it is favorable according to the invention if the total content of all the abovementioned starting reaction gas mixtures according to the invention 2 is different from propylene, molecular oxygen, propane and molecular nitrogen components ≤ 5 vol .-%, particularly advantageously ≤ 3 vol .-%. According to the invention, V _{2 is} substantially 3.73 for all of the abovementioned reaction gas starting mixtures according to the invention. The preferred ranges V ₂ and V ₃ mentioned in the listed reaction gas starting mixtures according to the invention apply independently of one another.

Advantageous for the inventive method is that as propene source that in process of continuous heterogeneously catalyzed partial dehydrogenation and / or oxydehydrogenation propylene formed in the gas phase by propane is contemplated, without that of this propylene previously catalyzed in the heterogeneous partial dehydrogenation and / or oxydehydrogenation unreacted Propane must be separated.

In principle, all known heterogeneously catalyzed partial dehydrogenation of propane come into consideration, as described, for. From the documents WO 03/076370, WO 01/96271, EP-A 117 146, WO 03/011804, US 3,161,670 , WO 01/96270, DE-A 33 13 573, DE-A 102 45 585, DE-A 103 16 039 and from the German application DE-A 10 2004 032 129 are previously known.

As well All known in the art known dehydrogenation catalysts into consideration. It is advantageous to dehydrate in the fixed catalyst bed.

The Dehydrogenation catalysts can be roughly divided into two groups. Namely into those which are oxidic in nature (for example chromium oxide and / or Alumina) and those made up of at least one on one, usually oxidic, carrier deposited, usually comparatively noble, metal (for example Platinum). Among other things, so that all dehydrogenation catalysts used in DE-A 102 19 879, WO 01/96270, the EP-A 731077, DE-A 10211275, DE-A 10131297, WO 99/46039, US-A 4,788,371, EP-A-0 705 136, WO 99/29420, US-A 4 220 091, US-A No. 5,430,220, US Pat. No. 5,877,369, EP-A-0 117 146, DE-A 199 37 196, DE-A 199 37 105 and DE-A 199 37 107 recommended be as well as the catalyst according to Example 4 of DE-A 102 19 879. In particular, you can both the catalyst according to Example 1, Example 2, Example 3, and Example 4 of DE-A 199 37 107 as also the catalyst according to Example 4 of DE-A 102 19 879 and the catalysts of WO 02/51547, WO 02/51540 and DE-A 102005002127 are used.

there are dehydrogenation catalysts containing 10 to 99.9% by weight Zirconia, 0 to 60 weight percent alumina, silica and / or Titanium dioxide and 0.1 to 10 wt .-% of at least one element of first or second main group, an element of the third subgroup, an element of the eighth subgroup of the Periodic Table of the Elements, Lanthanum and / or tin contain, with the proviso that the sum of the weight percentages 100 wt .-% results.

Especially also suitable in the examples and comparative examples Dehydrogenation catalyst used in this document.

As a general rule For example, the dehydrogenation catalysts may be catalyst strands (diameter typically 1 to 10 mm, preferably 1.5 to 5 mm; Length typically 1 to 20 mm, preferably 3 to 10 mm), tablets (preferably the same dimensions as in the strands) and / or catalyst rings (outer diameter and length each typically 2 to 30 mm or to 10 mm, wall thickness expediently 1 to 10 mm, or to 5 mm, or to 3 mm).

In As a rule, the dehydrogenation catalysts (especially those in this Example font used and those in DE-A 19937107 recommended (in particular the exemplary catalysts of this DE-A) so that they both the dehydration of propane and catalyze the combustion of molecular hydrogen assets. The Hydrogen combustion proceeds in comparison with the dehydration of propane in the case of a competitive situation at the catalysts much faster.

to execution the heterogeneously catalyzed propane dehydrogenation come in principle all known in the art reactor types and process variants into consideration. Descriptions of such process variants included for example, all concerning the dehydrogenation and dehydrogenation catalysts quoted writings of the prior art.

Characteristic of the partial heterogeneously catalyzed dehydrogenation of propane is that it proceeds endothermically. That is, for the adjustment of the required reaction temperature and the heat required for the reaction (energy) must either the starting reaction gas mixture (the reaction gas (exit) mixture for the heterogeneously catalyzed propane dehydrogenation in this document as reaction gas (exit) mixture 1 be referred to) be supplied in advance and / or in the course of the heterogeneously catalyzed dehydrogenation. Optionally, the reaction gas mixture must escape the required heat of reaction itself.

Further is it for heterogeneously catalyzed dehydrogenation of propane due to the high required reaction temperatures typical that in small quantities high-boiling high molecular weight organic Compounds, down to the carbon, are formed on the catalyst surface Separate and thereby deactivate selbige. To this detrimental This can be catalyzed by heterogeneously catalyzed side effects Dehydration at elevated Temperature over the catalyst surface to be passed, propane-containing reaction gas mixture with steam dilute become. Depositing carbon is among those given Partially or completely eliminated conditions according to the principle of coal gasification.

A different possibility, eliminating deposited carbon compounds is to the dehydrogenation catalyst from time to time (daily if necessary) increased Temperature with an oxygen-containing gas (useful in the absence of hydrocarbons) and thus the separated Burning off carbon, so to speak. An extensive suppression of Formation of carbon deposits is also possible because of the heterogeneously catalyzed propane to be dehydrogenated before it at elevated Temperature over led the dehydrogenation catalyst is added, molecular hydrogen.

Of course, there is also the possibility the heterogeneously catalysed to be dehydrogenated propane a mixture of Water vapor and molecular hydrogen to add. An addition of molecular hydrogen for heterogeneously catalyzed dehydrogenation tion of propane also reduces the unwanted formation of allenes (propadiene), Propyne and acetylene as by-products. Partial oxidation of such added Hydrogen can also provide the required heat of reaction.

It is essential according to the invention that heterogeneously catalyzed partial propane dehydrogenations in which the propane conversion is from 20 to 30 mol% (based on the one-time passage of fresh propane through the dehydrogenation) are also suitable as the propene source. However, heterogeneously catalyzed partial propane dehydrogenations are particularly advantageous for the process according to the invention as a propene source NEN the aforementioned propane conversion is 30 to 60 mol .-%, preferably 35 to 55 mol .-% and particularly preferably 35 to 45 mol .-%.

For the realization of the abovementioned propane conversions, it is favorable to carry out the heterogeneously catalyzed propane dehydrogenation at a working pressure of 0.3 to 10 bar, or advantageously up to 3 bar. Furthermore, it is favorable to dilute the heterogeneously catalytically dehydrogenated propane with steam. On the one hand, the heat capacity of the water makes it possible to compensate for part of the effect of dehydrogenation and on the other hand dilution with water vapor reduces the educt and product partial pressure, which has a favorable effect on the equilibrium dehydrogenation. Furthermore, the water vapor co-use, as already mentioned, has an advantageous effect on the service life of noble metal-containing dehydrogenation catalysts, in particular in the case of desired high propane conversions. If required, molecular hydrogen can also be added as a further constituent. The molar ratio of molecular hydrogen to propane is in the reaction gas starting mixture 1 usually ≤ 5. The molar ratio of water vapor to propane is in the reaction gas starting mixture 1 expediently ≥ 0.05 to 2 or to 1.

In general, the smallest possible water vapor contents in the reaction gas starting mixture 1 preferred and desired. In the case of fresh propane-based propane conversions in the range from 20 to 30 mol .-%, the reaction mixture is usually in the starting gas mixture 1 recycled amount of steam contained sufficient amount of steam as a steam supply for the heterogeneously catalyzed dehydrogenation. For higher propane conversions based on fresh propane, water vapor is normally added beyond this, in which, for example, water vapor is added. B. can act to separate process water. As oxidation cycle gas (cf.

EP-A 11 80 508 and the German application DE-A 10 2004 032 129) is customary the residual gas referred to after a single-stage or multi-stage heterogeneously catalyzed gas phase partial oxidation of at least one organic compound (here: propylene and / or acrolein) then remains, if one from the product gas mixture of the partial oxidation of the target product more or less selective (eg by absorption into a suitable solvent or by fractional condensation). As a general rule it exists predominantly from the for the partial oxidation used inert diluent gases and from in the Partial oxidation usually by-produced (or added as a diluent gas, which is less according to the invention preferred) is water vapor and by unwanted complete oxidation formed carbon oxides.

Besides, it usually contains still residual amounts of oxygen not consumed in the partial oxidation and unreacted organic starting compounds (here: Propylene, acrolein) and lowest target product levels.

at the partial oxidation according to the invention contains the oxidant gas remaining propane and is therefore advantageous in the expediently serving as propylene source heterogeneously catalyzed partial propane dehydrogenation recycled.

Basically a heterogeneously catalyzed partial, acting as propylene source Propane dehydrogenation (quasi) adiabatic and thereby be carried out endothermically. The reaction gas starting mixture is usually initially on a temperature of 500 to 700 ° C (or from 550 to 650 ° C) heated (For example, by direct firing of the surrounding wall). Adiabatic passage through the at least one catalyst bed then the reaction gas mixture is then depending on the conversion and dilution about 30 ° C up to 200 ° C cooling down. A presence of water vapor as a heat carrier is also under the Viewpoint of adiabatic driving advantageously noticeable. lower Allow reaction temperatures longer Service life of the catalyst bed used. Higher reaction temperatures promote increased sales.

application point becomes appropriate a heterogeneously catalyzed propane dehydrogenation as propylene source for the inventive method as a horde reactor realize.

This contains spatial successively more useful as a dehydrogenation catalyzing catalyst bed. The number of catalyst beds can 1 to 20, expedient 2 to 8, but also 3 to 6 amount. Let with increasing Hordenzahl Increased increasingly easier propane conversions to reach. The catalyst beds are preferably radial or axial arranged one behind the other. Application technology is appropriate applied in such a tray reactor, the fixed catalyst bed type.

in the In the simplest case, the fixed catalyst beds are in a shaft furnace reactor axially or in the annular gaps of centric nestled arranged cylindrical gratings. However, it is also possible that Ring gaps in segments one above the other to arrange and the gas after radial passage in a segment in the next about that or underlying segment.

Conveniently, the reaction gas mixture 1 on its way from one catalyst bed to the next catalyst bed, for example, by passing it over hot-gas heated heat exchanger surfaces (eg ribs) or by passing it through hot-gas heated tubes, in the hopper reactor.

If, in addition, the tray reactor is operated adiabatically, it is sufficient for fresh propane-based propane conversions ≦ 30 mol%, especially when using the catalysts described in DE-A 199 37 107, in particular the exemplary embodiments, the reaction gas mixture 1 preheated to a temperature of 450 to 550 ° C in the dehydrogenation reactor and keep within the Horde reactor in this temperature range. That is, the entire propane dehydrogenation is to be realized at extremely low temperatures, which proves to be particularly favorable for the lifetime of the fixed catalyst beds between two regenerations. For higher propane conversions, the reaction gas mixture 1 expediently preheated to higher temperatures in the dehydrogenation reactor (this can be up to 700 ° C) and kept within the Horde reactor in this elevated temperature range.

It is even more clever to carry out the above described intermediate heating directly (autothermal mode). This is the reaction gas mixture 1 either already before flowing through the first catalyst bed (then the starting reaction gas mixture 1 molecular hydrogen added) and / or added between the subsequent catalyst beds to a limited extent molecular oxygen. Thus, (usually catalyzed by the dehydrogenation catalysts themselves) a limited combustion of the reaction gas mixture 1 contained, formed in the course of heterogeneously catalyzed propane dehydrogenation and / or the reaction gas mixture 1 added molecular hydrogen (optionally accompanied by a small amount of propane combustion) (it may also be appropriate in terms of application technology to insert catalyst beds in the tray reactor, which are charged with catalyst, which specifically (selectively) catalyzes the combustion of hydrogen (as such catalysts come, for example, those of the scriptures US 4,788,371 . US 4,886,928 . US 5,430,209 . US 5,530,171 . US 5,527,979 and US 5,563,314 into consideration; for example, such catalyst beds may be accommodated in an alternating manner with the beds containing dehydrogenation catalyst in the tray reactor). The released heat of reaction thus enables quasi-autothermal (the gross carbonization is essentially zero) a nearly isothermal mode of operation of heterogeneously catalyzed propane dehydrogenation. With increasing selected residence time of the reaction gas in the catalyst bed as a propane dehydrogenation at decreasing or substantially constant temperature is possible, which allows particularly long service life between tween two regenerations (in extreme cases can be burned to ensure the autothermal power only propane).

In general, according to the invention, an oxygen feed as described above should be carried out so that the oxygen content of the reaction gas mixture 1 , based on the amount of molecular hydrogen contained therein, 0.5 to 50 or 30, preferably 10 to 25 vol .-%. As oxygen source, both pure molecular oxygen or inert gas, for example CO, CO ₂ , N ₂ and / or noble gases, dilute oxygen, but in particular also air (apart from oxidation gas preferably only air is used as source of oxygen) come into consideration. The resulting combustion gases generally additionally dilute and thus promote heterogeneously catalyzed propane dehydrogenation. This applies in particular to the water vapor formed during combustion.

The Isothermia of for the inventive method in an appropriate manner acting as a propylene heterogeneously catalyzed propane dehydrogenation let yourself thereby further improve that one in the Horden reactor in the spaces between the catalyst beds closed, but before its filling conveniently but not necessarily evacuated, installations (for example, tubular) attaches. These fittings Contain suitable solids or liquids above one evaporate or melt at a certain temperature while consuming heat and condense again where this temperature falls below and heat release.

Needless to say, let yourself the heterogeneously catalyzed propane dehydrogenation also as in DE-A 102 11 275 described (as "loop variant") realized that forms an integral part of this patent application.

• – einer Dehydrierzone ein das zu dehydrierende Propan enthaltende Reaktionsgasausgangsgemisch 1 kontinuierlich zugeführt wird,
• – das Reaktionsgasausgangsgemisch 1 in der Dehydrierzone durch wenigstens ein Katalysatorfestbett geführt wird, an welchem durch katalytische Dehydrierung molekularer Wasserstoff und (partiell) Propylen gebildet werden,
• – dem Reaktionsgasausgangsgemisch 1 vor und/oder nach Eintritt in die Dehydrierzone wenigstens ein molekularen Sauerstoff enthaltendes Gas zugesetzt wird,
• – der molekulare Sauerstoff in der Dehydrierzone im Reaktionsgasgemisch 1 enthaltenen molekularen Wasserstoff teilweise zu Wasserdampf oxidiert und
• – der Dehydrierzone ein Produktgas entnommen wird, das molekularen Wasserstoff, Wasserdampf, Propylen und nicht umgesetztes Propan enthält und das dadurch gekennzeichnet ist, dass das der Dehydrierzone entnommene Produktgas in zwei Teilmengen identischer Zusammensetzung aufgeteilt und eine der beiden Teilmengen als Dehydrierkreisgas (vorzugsweise als Bestandteil des Reaktionsgasausgangsgemischs 1) in die Dehydrierzone zurückgeführt wird, wie es auch die WO 03/076370 empfiehlt.

In other words, it is advantageous for the process according to the invention that the propene source can be a process of continuous heterogeneously catalyzed partial dehydrogenation of propane in the gas phase, in which

• A dehydrogenation zone containing a reaction gas starting mixture containing the propane to be dehydrogenated 1 is fed continuously,
• - The starting reaction gas mixture 1 is passed through at least one fixed catalyst bed in the dehydrogenation zone, at which molecular hydrogen and (partially) propylene are formed by catalytic dehydrogenation,
• - The starting reaction gas mixture 1 before and / or after entry into the dehydrogenation zone at least one molecular oxygen-containing gas is added,
• - The molecular oxygen in the dehydrogenation in the reaction gas mixture 1 contained molecular hydrogen partially oxidized to water vapor and
• A product gas containing molecular hydrogen, water vapor, propylene and unconverted propane is withdrawn from the dehydrogenation zone and characterized in that the product gas withdrawn from the dehydrogenation zone is divided into two subsets of identical composition and one of the two subsets is used as dehydrogenating gas (preferably as part of the starting reaction gas mixture 1 ) is recycled to the dehydrogenation, as well as WO 03/076370 recommends.

In this case, the oxidation cycle gas component of the starting reaction gas mixture 1 be and / or according to the teaching of the German application DE-A 10 2004 032 129 only after already partially completed dehydrogenation the reaction gas mixture 1 be supplied.

Is the oxidation gas gas part of the starting reaction gas mixture 1 , This expediently contains only the oxidation gas derived molecular oxygen.

For the inventive method it is favorable in the context of the described loop method, when the amount of Dehydrierkreisgases, based on that in the dehydrogenation zone product gas formed, 30 to 70% by volume, advantageously 40 to 60% by volume, preferably 50% by volume.

With regard to the partial oxidation according to the invention, the reaction gas starting mixture contains 1 in the case of a z. B. loop reactor carried out in the tray reactor (Horden loop reactor then = dehydrogenation) in the steady state in an appropriate manner: 15 to 25 vol.% Propane, 2 to 6% by volume propylene, 5 to 20 vol.% Steam, 2 to 10 vol.% molecular hydrogen, 40 to 75 vol.% molecular nitrogen, and > 0 to 3 vol.% molecular oxygen.

The conversion of propane (based on the single pass of the aforementioned reaction gas starting mixture 1 by the loop reactor operated in loop mode) and the loop circle gas ratio (amount of Dehydrierkreisgases based on the total amount of product gas in the dehydrogenation zone) are selected with regard to the heterogeneously catalyzed gas phase partial oxidation of propylene according to the invention with advantage (eg in Horden loop reactor as dehydrogenation), the product gas formed in the dehydrogenation zone contains unreacted propane and desired propylene in the propene to propane molar ratio of 0.25 and 0.3 to 0.5 (optionally to 0.66). For a loop nitrogen gas ratio of 0.5, this corresponds to the single pass of the starting reaction gas mixture 1 by the dehydrogenation zone related conversion of propane contained therein of 15 to 25 mol%.

Typical loads of the dehydrogenation catalyst beds with reaction gas mixture 1 are from 250 to 5000 h ^-1 (in heavy load mode also up to 4000 h ^-1 ), preferably from 10,000 to 25,000 Nl / l · h, particularly preferably from 15,000 to 20,000 Nl / l · h. The corresponding charges with propane are typically 50 to 1000 h ^-1 (in high load mode also up to 40 000 h ^-1 ), preferably at 2000 to 5000 Nl / l · h, particularly preferably 3000 to 4000 Nl / l · h.

The dehydrogenation product gas taken from the dehydrogenation zone (the dehydrogenation reactor) as propylene source is in accordance with the reaction conditions chosen for the heterogeneously catalyzed propane dehydrogenation at a pressure of 0.3 to 10 bar, preferably 1 to 3 bar, and often has a temperature of 450 to 650 ° C, often a temperature of 500 to 600 ° C. , In general, it contains propane, propene, H ₂ , N ₂ , H ₂ O, methane, ethane (the latter two usually result from thermal decomposition of a small amount of propane), ethylene, butene-1, other butenes such as isobutene, others C ₄ hydrocarbons such as n-butane, isobutane, butadiene etc., CO and CO ₂ , but usually also oxygenates such as alcohols, aldehydes and carboxylic acids (usually with ≤ 9 C-atoms). Furthermore, it may be contained in small amounts of the oxidation gas derived components.

While the EP-A 117 146, DE-A 33 13 573 and US-A 3,161,670 recommend the product gas formed in the propane dehydrogenation as such Charging the partial oxidation according to the invention Use it for it a partial oxidation according to the invention advantageous, from the propylene-containing product gas of the propane dehydrogenation before its use as a propene source for the propene partial oxidation according to the invention at least a subset of the contained therein, of propane and Propylene different, components separate. It should the requirements of DE-A 102 11 275 are observed.

According to the invention advantageous it will be at least 50 vol .-%, preferably at least 75 vol .-%, more preferably at least 90% by volume, and most preferably at least 95% by volume of the product gas contained in the propane dehydrogenation, Separate components from propane and propylene before the same as Propenquelle for the partial oxidation according to the invention is used.

A for the needs of the invention appropriate possibility there is e.g. therein, preferably cooled (preferably to temperatures from 10 to 100 or 70 ° C) Product gas mixture of propane dehydrogenation, z. B. at a pressure from 0.1 to 50 bar, preferably 5 to 15 bar and a temperature of z. From 0 to 100 ° C, preferably 20 to 40 ° C, with a (preferably high boiling) organic solvent (preferably a hydrophobic) in which propane and propylene (opposite to the other components of the product gas mixture of the propane dehydrogenation expediently preferred) be brought into contact (eg by simple Passing). By subsequent desorption, rectification and / or stripping with a respect the partial oxidation according to the invention behaving inertly and / or in this partial oxidation as Reactant needed Gas (eg, air or another mixture of molecular oxygen and inert gas) recovered the propane and propylene in a mixture in purified form and this mixture for the partial oxidation can be used as propylene source (preferably becomes as in Comparative Example 1 of the German application DE-A 10th 2004 032 129 described proceeded) The possibly molecular Hydrogen-containing offgas of such absorption can be e.g. again a pressure swing adsorption and / or membrane separation (for example according to DE-A 10235419) subject and then, if necessary, use the separated hydrogen.

Indeed is the C3 hydrocarbons / C4 hydrocarbons separation factor in the above separation method comparatively limited and for in DE-A 10245585 described needs often not sufficient.

When alternative for the separation step described above via absorption is for the purposes according to the invention therefore often a pressure swing adsorption or a pressure rectification preferred.

In principle, all absorbents which are capable of absorbing propane and propylene are suitable as absorbents for the absorptive separation described above. The absorbent is preferably an organic solvent which is preferably hydrophobic and / or high boiling. Advantageously, this solvent has a boiling point (at a normal pressure of 1 atm) of at least 120 ° C, preferably of at least 180 ° C, preferably from 200 to 350 ° C, especially from 250 to 300 ° C, more preferably from 260 to 290 ° C. Conveniently, the flash point (at a normal pressure of 1 bar) is above 110 ° C. Generally suitable as absorbent relatively non-polar organic solvents, such as aliphatic hydrocarbons, which preferably contain no outwardly polar group, but also aromatic hydrocarbons. In general, it is desirable that the absorbent has the highest possible boiling point with the highest possible solubility for propane and propylene. Examples which may be mentioned as absorbents are aliphatic hydrocarbons, for example C ₈ -C ₂₀ alkanes or alkenes, or aromatic hydrocarbons, for example, middle oil fractions from the Paraffindestillation or ethers with bulky (sterically demanding) groups on the O atom, or mixtures thereof, wherein this may be added a polar solvent, such as, for example, the 1,2-dimethyl phthalate disclosed in DE-A 43 08 087. Also suitable are esters of benzoic acid and phthalic acid with straight-chain alkanols containing 1 to 8 carbon atoms, such as n-butyl benzoate, benzoin acid methyl ester, ethyl benzoate, dimethyl phthalate, diethyl phthalate, and so-called heat transfer oils, such as diphenyl, diphenyl ether and mixtures of diphenyl and diphenyl ether or their chlorinated derivatives and triaryl alkenes, for example 4-methyl-4'-benzyl-diphenylmethane and its isomers 2-methyl-2'- benzyl-diphenylmethane, 2-methyl-4'-benzyldiphenylmethane and 4-methyl-2'-benzyl-diphenylmethane, and mixtures of such isomers. A suitable absorbent is a solvent mixture of diphenyl and diphenyl ether, preferably in the azeotropic composition, in particular from about 25 wt .-% diphenyl (biphenyl) and about 75 wt .-% of diphenyl ether, for example the commercially available Diphyl ^® (z. B. obtained from Bayer Aktiengesellschaft). Frequently, this solvent mixture contains a solvent such as dimethyl phthalate in an amount of 0.1 to 25 wt .-%, based on the total solvent mixture added. Particularly suitable absorbents are also octanes, nonanes, decanes, undecanes, dodecanes, tridecanes, tetradecanes, pentadecanes, hexadecanes, heptadecans and octadecanes, with tetradecanes in particular having proven to be particularly suitable. It is favorable if the absorbent used on the one hand fulfills the above-mentioned boiling point, but on the other hand has a molecular weight which is not too high. Advantageously, the molecular weight of the absorbent is ≦ 300 g / mol. Also suitable are the paraffin oils having 8 to 16 carbon atoms described in DE-A 33 13 573. Examples of suitable commercial products are products sold by Haltermann, such as Halpasols i, such as Halpasol 250/340 i and Halpasol 250/275 i, as well as printing ink oils under the names PKWF and Printosol. Preference is given to aromatics-free commercial products, eg. B. those of the type PKWFaf. If they contain a low content of residual aromatics, this can advantageously be reduced by rectification and / or adsorption in advance of the described use and be pressed to values significantly below 1000 ppm by weight.

The performance of absorption is not particularly limited. All methods and conditions customary to the person skilled in the art can be used. Preferably, the gas mixture at a pressure of 1 to 50 bar, preferably 2 to 20 bar, more preferably 5 to 15 bar, and a temperature of 0 to 100 ° C, in particular from 20 to 50 or 40 ° C, with the absorbent brought into contact. The absorption can be made both in columns and in quench apparatus who the. It can be done in cocurrent or in countercurrent (is preferred). Suitable absorption columns are, for example tray columns (having bubble and / or sieve trays), columns with structured packings (for example, sheet metal packings with a specific surface area of 100 to 1000, or up to 750 m ² / m ^3, for example Mellapak ^® 250 Y) and Packed columns (filled, for example, with Raschig fillers). However, it is also possible to use trickle and spray towers, graphite block absorbers, surface absorbers such as thick-film and thin-layer absorbers, dishwashers, cross-flow scrubbers and rotary scrubbers. In addition, it may be beneficial to let the absorption take place in a bubble column with and without internals.

The Separation of the propane and propylene from the absorbent can by stripping, flash evaporation and / or distillation respectively.

The Separation of the propane and propylene is carried out by the absorbent preferably by stripping and / or desorption. The desorption can in usual Way over a pressure and / or temperature change carried out be, preferably at a pressure of 0.1 to 10 bar, in particular 1 to 5 bar, stronger preferably 1 to 3 bar, and a temperature of 0 to 200 ° C, in particular 20 to 100 ° C, stronger preferably 30 to 70 ° C, more preferably 30 to 50 ° C. One for the stripping suitable gas is for example steam, preferably however, are in particular oxygen / nitrogen mixtures, for example Air. When using air or oxygen / nitrogen mixtures, where the oxygen content is over 10 vol.%, It may be useful before and / or during the Stripping process to add a gas that reduces the explosion area. Especially suitable for it are gases with a specific heat capacity ≥ 29 J / mol · K at 20 ° C, such as methane, Ethane, propane (preferred), propene, benzene, methanol, ethanol, as well as Ammonia, carbon dioxide, and water. C4 hydrocarbons are preferred according to the invention as such additives however to avoid. Bubble columns are also particularly suitable for stripping and without fittings.

The Separation of the propane and propylene from the absorbent can also over a distillation or rectification are carried out, with those skilled in the art common Columns with packs, packing or corresponding internals can be used. Preferred conditions in the distillation or rectification are a pressure of 0.01 up to 5 bar, in particular 0.1 to 4 bar, more preferably 1 to 3 bar, and a temperature (in the bottom) of 50 to 300 ° C, especially 150 to 250 ° C.

A propylene source which is principally suitable for the process according to the invention and is obtained by stripping from the absorbent can, before it is used for charging the partial oxidation, be fed to a further process stage in order to obtain, for example, B. the losses of stripped absorbent to re Ducate (for example, separation in demisters and / or depth filters) and the inventive partial oxidation at the same time to protect against absorbent or to further improve the separation effect between C3 / C4 hydrocarbons. Such separation of the absorbent can take place according to all the process steps known to the person skilled in the art. A preferred embodiment of such a separation in the context of the method according to the invention is z. As the quenching of the output stream from the stripping with water. In this case, the absorbent from this loaded output stream is washed out with water and the output stream simultaneously charged with water (small amounts of water have a beneficial effect on the activity of the catalysts for the partial oxidation of the invention). This laundry or quenching can z. B. at the top of a desorption column on a liquid-catching ground by spraying water or in a separate apparatus.

to support of the separation effect can in the quench space, the quench surface enlarging internals be installed, as the expert of rectifications, absorptions and Desorptionsen ago known.

water is in this respect a preferred detergent, as it is in the following normally does not bother at least one partial zone. After this the water is the absorbent from the propane and propylene has washed out loaded feed stream, the water / absorbent mixture fed to a phase separation and the treated, low-volume, output stream, directly the partial oxidation of the invention supplied become.

In an advantageous manner for the process according to the invention, in particular when the propylene / propane mixture is stripped from the absorbate by means of air, generally starting reaction gas mixtures which are directly required can be obtained 2 win. In the event that their propane content should not satisfy the present invention, fresh propane may be added to them prior to their use for the inventive partial oxidation of the propylene present. The same is then expediently used in the heterogeneously catalyzed dehydrogenation (as a constituent of the starting reaction gas mixture) via the oxidizing gas 1 ) are returned. The supply of fresh propane into the reaction gas starting mixture can then be added to the corresponding amount of propane 1 be mitigated. In an extreme case, the supply of fresh propane required in the heterogeneously catalyzed propane dehydrogenation can then be completely eliminated if this supply of fresh propane before the partial oxidation of propylene according to the invention is carried out completely into the reaction gas starting mixture 2 takes place, from where it then as a remaining constituent in the oxidation gas only after passing through the partial oxidation of the reaction mixture gas mixture according to the invention 1 for the heterogeneously catalyzed propane dehydrogenation is supplied. If appropriate, a fresh propane feed can also be carried out in a C ₃ separation which may be present between heterogeneously catalyzed dehydrogenation and propylene partial oxidation (for example as stripping gas).

If it is a two-stage partial oxidation of propylene to acrylic acid, a part or the complete supply of fresh propane can also in the starting reaction gas mixture 3 take place (however, the starting reaction gas mixture 3 sometimes already not explosive, if this qualification already on the reaction gas starting mixture 2 experienced). This is advantageous above all because the undesired side reaction of propane to propionaldehyde and / or propionic acid proceeds primarily from the first reaction stage under its conditions. It is also advantageous to distribute a Frischpropanzufuhr to the first and second oxidation stage substantially uniformly.

By this possibility of the supply of fresh propane into the reaction gas starting mixture 2 and or 3 into, the composition of the reaction gas starting mixtures can be 2 and 3 unerringly designed not explosive. Decisive for answering the question whether the reaction gas starting mixture 2 respectively. 3 is explosive or not, is whether in the starting material under certain conditions (pressure, temperature) reaction gas starting mixture 2 respectively. 3 a combustion (ignition, explosion) initiated by a local ignition source (eg glowing platinum wire) or not (see DIN51649 and the examination description in WO 04/007405). If it spreads, the mixture should be called explosive here. If not spread, the mixture in this document is classified as non-explosive. Is the starting reaction gas mixture 2 respectively. 3 not explosive, this also applies to the reaction gas mixtures formed in the course of the partial oxidation of propylene according to the invention 2 respectively. 3 (see WO 04/007405).

• – in einer vorgeschalteten ersten Stufe Propan als Bestandteil eines Reaktionsgasausgangsgemisch 1 einer partiellen heterogen katalysierten Dehydrierung in der Gasphase unter Bildung eines Produktgasgemischs 1, das Propylen und nicht umgesetztes Propan enthält, unterwirft;
• – aus dem Propylen und nicht umgesetztes Propan enthaltenden Produktgasgemisch 1 der vorgeschalteten Stufe von den darin enthaltenen, von Propan und Propylen verschiedenen, Bestandteilen gegebenenfalls eine Teilmenge (vorteilhaft wenigstens 50 Vol.-%, mit Vorteil wenigstens 75 Vol.-%, besonders bevorzugt wenigstens 90 Vol.-% und ganz besonders bevorzugt wenigstens 95 Vol.-% und am besten 100 Vol.-%) abtrennt und es dann als Bestandteil des Reakti onsgasausgangsgemisch 2 für die Herstellung von Acrolein oder Acrylsäure oder deren Gemisch als Zielprodukt durch die heterogen katalysierte partielle Gasphasenoxidation (einstufige oder zweistufige) des im Reaktionsgasausgangsgemisch 2 enthaltenen Propylens verwendet;
• – aus dem im Rahmen der partiellen Oxidation des Propylens anfallenden Produktgasgemisch 2 bzw. 3 in einer Abtrennstufe Zielprodukt (Acrolein, oder Acrylsäure, oder deren Gemisch) abtrennt und wenigstens dabei verbleibendes nicht umgesetztes Propan in die vorgeschaltete erste Stufe der partiellen heterogen katalysierten Propandehydrierung zurückführt (bevorzugt wird die gesamte bei der Zielproduktabtrennung verbleibende Restgasmenge als Oxidationskreisgas in die vorgeschaltete erste Stufe rückgeführt).

That is, the inventive method comprises in particular a process according to the invention for the production of acrolein or acrylic acid or a mixture thereof by heterogeneously catalyzed partial gas phase oxidation of propylene, which is characterized in that

• - In an upstream first stage propane as part of a reaction gas starting mixture 1 a partial heterogeneously catalyzed dehydrogenation in the gas phase to form a product gas mixture 1 submitting propylene and unreacted propane subject;
• - From the propylene and unreacted propane containing product gas mixture 1 the upstream stage of the components contained therein, other than propane and propylene optionally a subset (advantageously at least 50 vol .-%, advantageously at least 75 vol .-%, more preferably at least 90 vol .-% and most preferably at least 95th Vol .-% and most preferably 100 vol .-%) and then separates it as a constituent of Reacti onsgasausgangsgemisch 2 for the production of acrolein or acrylic acid or their mixture as the target product by the heterogeneously catalyzed partial gas phase oxidation (single-stage or two-stage) of the starting reaction gas mixture 2 contained propylene used;
• - From the resulting in the context of partial oxidation of propylene product gas mixture 2 respectively. 3 in a separation step target product (acrolein, or acrylic acid, or mixture thereof) separates and at least while remaining unreacted propane in the upstream first stage of the partial heterogeneously catalyzed propane dehydrogenation leads back (preferably the entire remaining in the Zielproduktabtrennung residual gas as oxidation gas in the upstream first stage recycled).

In particular, the present invention relates to an advantageous embodiment of the above method, wherein the required for the process propane (fresh propane) completely the reaction gas starting mixture 1 supplies.

Furthermore, the present invention relates to an advantageous embodiment of the above method, wherein the propane (fresh propane) required for the process at most partially (eg only 75%, or only 50%, or only 25%) of the reaction gas starting mixture 1 and at least partially (usually the residual amount, optionally the total amount) of the reaction gas starting mixture 2 (and / or the starting reaction gas mixture 3 ) feeds. Otherwise, as described in WO 01/96170, which forms an integral part of this application.

In in a known manner runs the heterogeneously catalyzed gas phase partial oxidation of propylene to acrylic acid with molecular oxygen basically in two along the Reaction coordinate of successive steps, of which the first leads to acrolein and the second from acrolein to acrylic acid.

This Reaction process in two successive steps open in a known manner the possibility the inventive method at the stage of acrolein (the stage of predominant acrolein formation) abort and perform the target product separation at this stage, or the method according to the invention to the majority Acrylic acid formation continue and then carry out the target product separation.

Leading the inventive method to the majority Acrylic acid formation, it is advantageous according to the invention the process is two-stage, d. H. arranged in two consecutive To carry out oxidation steps, where expediently in each of the two oxidation states, the fixed catalyst bed to be used and preferably also the other reaction conditions, such. As the temperature of the fixed catalyst bed, in an optimizing manner be adjusted.

Though capital the for the catalysts of the first oxidation state (propylene → acrolein) particularly suitable as active compositions containing the elements Mo, Fe, Bi Multimetal oxides, to some extent also the second oxidation state (Acrolein → acrylic acid) catalyze, but be for the second oxidation step normally prefers catalysts, whose active material contains at least one element Mo and V Multimetal is.

In order to the method according to the invention is suitable the heterogeneously catalyzed partial oxidation of propylene over fixed catalyst beds, their catalysts as active mass at least one of the elements Mo, Fe and Bi containing multimetal have, in particular as one-step process for the preparation of acrolein (and optionally Acrylic acid) or as the first reaction stage for the two-stage preparation of acrylic acid.

The realization of the single-stage heterogeneously catalyzed partial oxidation of propylene to acrolein and optionally acrylic acid or the two-stage heterogeneously catalyzed partial oxidation of propylene to acrylic acid using a reaction gas starting mixture according to the invention 2 can be described in detail as in EP-A 70 07 14 (first reaction stage, as described there, but also in ent EP-A 70 08 93 (second reaction stage, as described there, but also in a corresponding countercurrent procedure), WO 04/085369 (in particular this document is considered an integral part of this document) (as two-stage process), WO 04/85363, DE-A 103 13 212 (first reaction stage), EP-A 11 59 248 (as a two-stage process), EP-A 11 59 246 (second reaction stage), EP-A 11 59 247 ( as a two-stage process), DE-A 199 48 248 (as a two-stage process), DE-A 101 01 695 (one-stage or two-stage), WO 04/085368 (as a two-stage process), DE 10 2004 021 764 (two-stage), WO 04 No. 085362 (first reaction stage), WO 04/085370 (second reaction stage), WO 04/085365 (second reaction stage), WO 04/085367 (two-stage), EP-A 99 06 36, EP-A 10 07 007 and EP-A 11 06 598 described.

This applies in particular to all embodiments contained in these documents. They can be carried out as described in these documents, but with the difference that as reaction gas starting mixture for the first reaction stage (propylene to acrolein), a reaction gas starting mixture according to the invention 2 is used. As far as the other parameters are concerned, the procedure is as in the exemplary embodiments of the cited documents (in particular concerning the fixed catalyst beds and reactant loading of the fixed catalyst beds). If two stages are used in the abovementioned exemplary embodiments of the prior art and a secondary oxygen (secondary air) feed takes place between the two reaction stages, this is carried out in a corresponding manner, but adjusted in amount such that the molar ratio of molecular oxygen to acrolein in the feed mixture the second reaction stage corresponds to that in the embodiments of said writings.

For the respective Reaction stage particularly suitable Multimetalloxidkatalysatoren are widely described above and well known to those skilled in the art. For example EP-A 25 34 09 on page 5 refers to corresponding US patents.

Cheap catalysts for the respective oxidation state is also disclosed in DE-A 4 431 957, DE-A 10 2004 025 445 and DE-A 4 431 949. This applies in particular for those the general formula I in the two aforementioned publications. Especially advantageous catalysts for the respective oxidation state is disclosed by the publications DE-A 103 25 488, DE-A 103 25 487, DE-A 103 53 954, DE-A 103 44 149, DE-A 103 51 269, DE-A 103 50 812 and DE-A 103 50 822.

For the reaction stage according to the invention the heterogeneously catalyzed gas phase partial oxidation of propylene to acrolein or acrylic acid or their mixture, are in principle all Mo, Bi and Fe containing Multimetal oxide compositions as active mass into consideration.

This In particular, the multimetal oxide active materials are the general ones Formula I of DE-A 199 55 176, the Multimetalloxidaktivmassen the general formula I of DE-A 199 48 523, the Multimetalloxidaktivmassen of the general formula I of DE-A 199 48 523, the multimetal oxide active compositions of the general formulas I, II and III of DE-A 101 01 695, the multimetal oxide active compositions the general formulas I, II and III of DE-A 199 48 248 and Multimetal oxide active compounds of the general formulas I, II and III DE-A 199 55 168 and mentioned in EP-A 70 07 14 Multimetalloxidaktivmassen.

Furthermore, the Mo, Bi and Fe-containing multimetal oxide catalysts suitable for this reaction stage are those described in DE-A 100 46 957, DE-A 100 63 162, DE-C 3 338 380, DE-A 199 02 562, EP-A 15 565, DE-C 2 380 765, EP-A 8 074 65, EP-A 27 93 74, DE-A 330 00 44, EP-A 575897, US-A 4438217, DE-A 19855913, WO 98/24746, DE-A 197 46 210 (those of the general formula II), JP-A 91/294239, EP-A 293 224 and EP-A 700 714 are disclosed. This applies in particular to the exemplary embodiments in these documents, among which those of EP-A 15565, EP-A 575897, DE-A 197 46 210 and DE-A 198 55 913 are particularly preferred. Of particular note in this context are a catalyst according to Example 1c from EP-A 15 565 and a catalyst to be prepared in a corresponding manner, the active composition but the composition Mo ₁₂ Ni _6.5 Zn ₂ Fe ₂ Bi ₁ P _0.0065 K _{0, 06} O _x · 10SiO ₂ . Further mention should be made of the example with the consecutive no. 3 from DE-A 198 55 913 (stoichiometry: Mo ₁₂ Co ₇ Fe ₃ Bi _0.6 K _0.08 Si _1.6 O _x ) as hollow cylinder full catalyst of the geometry 5 mm × 3 mm × 2 mm (outer diameter × height × inner diameter) and the multimetal II - full catalyst according to Example 1 of DE-A 197 46 210. Further, the multimetal oxide catalysts of US-A 4,438,217 should be mentioned. The latter applies in particular when these hollow cylinders have a geometry of 5.5 mm × 3 mm × 3.5 mm, or 5 mm × 2 mm × 2 mm, or 5 mm × 3 mm × 2 mm, or 6 mm × 3 mm × 3mm, or 7mm x 3mm x 4mm (each outside diameter x height x inside diameter). Other possible catalyst geometries in this context are strands (eg 7.7 mm in length and 7 mm in diameter, or 6.4 mm in length and 5.7 mm in diameter).

A multiplicity of the multimetal oxide active compositions which are suitable for the step of propylene to acrolein and optionally acrylic acid can be obtained under the general formula IV Mo ₁₂ Bi _a Fe _b X ¹ _c X ² _d X ³ _e X ⁴ _f O _n (IV) in which the variables have the following meaning:
X ¹ = nickel and / or cobalt,
X ² = thallium, an alkali metal and / or an alkaline earth metal,
X ³ = zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and / or tungsten,
X ⁴ = silicon, aluminum, titanium and / or zirconium,
a = 0.5 to 5,
b = 0.01 to 5, preferably 2 to 4,
c = 0 to 10, preferably 3 to 10,
d = 0 to 2, preferably 0.02 to 2,
e = 0 to 8, preferably 0 to 5,
f = 0 to 10 and
n = a number determined by the valence and frequency of the elements other than oxygen in IV,
subsume.

she are available in a manner known per se (see, for example, DE-A 4 023 239) and are customary in substance to spheres, rings or cylinders shaped or in Shape of shell catalysts, d. h., coated with the active composition preformed, inert support bodies used. Of course you can but also be used in powder form as catalysts.

In principle, active compounds of the general formula IV can be prepared in a simple manner by producing an intimate, preferably finely divided, stoichiometrically composed, dry mixture of suitable sources of their elemental constituents and calcining this at temperatures of 350 to 650 ° C. The calcination can be carried out both under inert gas and under an oxidative atmosphere such. As air (mixture of inert gas and oxygen) and also under a reducing atmosphere (eg., Mixture of inert gas, NH ₃ , CO and / or H ₂ ) take place. The calcination time can be several minutes to several hours and usually decreases with temperature. Suitable sources of the elemental constituents of the multimetal oxide active compositions IV are those compounds which are already oxides and / or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.

In addition to the oxides, suitable starting compounds in particular are halides, nitrates, formates, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and / or hydroxides (compounds such as NH ₄ OH, (NH ₄ ) ₂ CO ₃ , NH ₄ NO ₃ , NH ₄ CHO ₂ , CH ₃ COOH, NH ₄ CH ₃ CO ₂ and / or ammonium oxalate, which can disintegrate and / or decompose into gaseous compounds at the latest during later calcination, can be additionally incorporated into the intimate dry mixture) ,

The intimate mixing of the starting compounds for the preparation of multimetal oxide active materials IV can be done in dry or wet form. It happens in dry Form, the starting compounds are expediently as finely divided Powder used and after mixing and optionally compacting subjected to calcination. Preferably, the intimate mixing takes place but in wet form. Usually be the starting compounds in the form of an aqueous solution and / or suspension mixed together. Especially intimate dry mixtures are then obtained in the described mixing method, if only from in dissolved Form present sources of elementary constituents becomes. As a solvent Water is preferably used. Subsequently, the resulting aqueous mass dried, the drying process preferably by spray drying the aqueous Mixture with outlet temperatures of 100 to 150 ° C takes place.

The multimetal oxide of the general formula IV can be used for the step "propylene → acrolein (and optionally acrylic acid)" formed both in powder form and to certain catalyst geometries, wherein the shaping can be carried out before or after the final calcination. For example, solid catalysts can be prepared from the powder form of the active composition or its uncalcined and / or partially calcined precursor composition by compacting to the desired catalyst geometry (for example by tableting, extruding or extrusion molding). As graphite or stearic acid as a lubricant and / or molding aids and reinforcing agents such as microfibers Glass, asbestos, silicon carbide or potassium titanate can be added. Suitable Vollkatalysatorgeometrien are z. B. solid cylinder or hollow cylinder with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinder, a wall thickness of 1 to 3 mm is appropriate. Of course, the full catalyst may also have spherical geometry, wherein the ball diameter may be 2 to 10 mm.

A especially cheap Hollow cylinder geometry is 5 mm × 3 mm × 2 mm (outer diameter × length × inner diameter), especially in the case of full catalysts.

Of course you can the shape of the powdered Active mass or its powdery, yet non-calcined and / or partly calcined, precursor material also by application carried out on preformed inert catalyst support. The coating the carrier body to Preparation of the coated catalysts is usually in one suitable rotatable container executed as it is z. From DE-A 2 909 671, EP-A 293 859 or from EP-A 714 700 is known. Conveniently, is applied to the coating of the carrier body to be applied Moistened powder mass and after application, for. B. by means of hot air, dried again. The layer thickness of the applied to the carrier body Powder mass is expediently in the range 10 to 1000 μm, preferably in the range 50 to 500 microns and more preferably in the range 150 to 250 microns, selected.

When support materials can usual porous or nonporous Aluminas, silica, thoria, zirconia, silicon carbide or silicates such as magnesium or aluminum silicate are used. They are behaving the method of the invention underlying target reaction is generally substantially inert. The carrier bodies can be regular or irregular shaped being regularly shaped Carrier body with clearly formed surface roughness, e.g. Spheres or hollow cylinders are preferred. Suitable is the Use of substantially non-porous, surface rough, spherical carriers Steatite whose diameter is 1 to 10 mm or 8 mm, preferably 4 to 5 mm. But is also suitable for the use of cylinders as a carrier body whose Length 2 up to 10 mm and their outer diameter 4 to 10 mm. In the case of suitable according to the invention Rings as a carrier body lies the wall thickness over it usually at 1 to 4 mm. According to the invention preferred to be used annular Own carrier body a length from 2 to 6 mm, an outer diameter from 4 to 8 mm and a wall thickness of 1 to 2 mm. Suitable according to the invention are above all rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) as a carrier body. The Fineness of the surface the Trägerkör pers applied catalytically active oxide masses will of course be of the desired shell thickness adapted (see EP-A 714 700).

For the step from propylene to acrolein (and optionally acrylic acid) to be used Multimetalloxidaktivmassen are also compounds of general formula V, [Y ¹ _{a '} Y ² _b' O _{x '} ] _p [Y ³ _c' Y ⁴ _{d '} Y ⁵ _e' Y ⁶ _{f '} Y ⁷ _g' Y ² _{h '} O _y' ] _q (V) in which the variables have the following meaning:
Y ¹ = only bismuth or bismuth and at least one of the elements tellurium, antimony, tin and copper,
Y ² = molybdenum or molybdenum and tungsten,
Y ³ = an alkali metal, thallium and / or samarium,
Y ⁴ = an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and / or mercury,
Y ⁵ = iron or iron and at least one of the elements chromium and cerium,
Y ⁶ = phosphorus, arsenic, boron and / or antimony,
Y ⁷ = a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and / or uranium,
a '= 0.01 to 8,
b '= 0.1 to 30,
c '= 0 to 4,
d '= 0 to 20,
e '> 0 to 20,
f '= 0 to 6,
g '= 0 to 15,
h '= 8 to 16,
x ', y' = numbers which are determined by the valence and frequency of the elements other than oxygen in V, and
p, q = numbers whose ratio p / q is 0.1 to 10,
containing three-dimensionally extended, from their local environment due to their local environment A range of chemical composition Y ¹ _{a '} Y ² _b' O _{x '} whose maximum diameter (longest passing through the center of gravity of the region of two points located on the surface (interface) of the area) 1 nm to 100 microns , huh 10 nm to 500 nm or 1 micron to 50 or 25 microns, is.

Particularly advantageous multimetal oxide compositions V are those in which Y ¹ is only bismuth.

Among these, those which are of the general formula VI, [Bi _{a "} Z ² _b" O _{x "} ] [Z ² ₁₂ Z ³ _c" Z ⁴ _{d "} Fe _e" Z ⁵ _{f "} Z ⁶ _g" Z ⁷ _{h "} O _{y ''} ] _{q ''} (VI), in which the variants have the following meaning:
Z ² = molybdenum or molybdenum and tungsten,
Z ³ = nickel and / or cobalt,
Z ⁴ = thallium, an alkali metal and / or an alkaline earth metal,
Z ⁵ = phosphorus, arsenic, boron, antimony, tin, cerium and / or lead,
Z ⁶ = silicon, aluminum, titanium and / or zirconium,
Z ⁷ = copper, silver and / or gold,
a '' = 0.1 to 1,
b '' = 0.2 to 2,
c '' = 3 to 10,
d "= 0.02 to 2,
e "= 0.01 to 5, preferably 0.1 to 3,
f '' = 0 to 5,
g '' = 0 to 10,
h '' = 0 to 1,
x '', y '' = numbers determined by the valency and frequency of the element other than oxygen in VI,
p '', q '' = numbers whose ratio p '' / q '' is from 0.1 to 5, preferably 0.5 to 2, with very particular preference being given to those masses VI in which Z ² _{b ''} = (Tungsten) _b'' and Z ² ₁₂ = (molybdenum) ₁₂ .

Furthermore, it is advantageous if at least 25 mol% (preferably at least 50 mol% and particularly preferably at least 100 mol%) of the total fraction [Y ¹ _{a '} Y ² _b' O _{x '} ] _p ([Bi _a'' Z ² _b'' O _x'' ] _p'' ) of the invention suitable Multimetalloxidmassen V (Multimetalloxidmassen VI) in the present invention suitable Multimetalloxidmassen V (Multimetalloxidmassen VI) in the form of three-dimensionally extended, different from their local environment because of their local environment Chemical composition demarcated, areas of chemical composition Y ¹ _{a '} Y ² _b' O _{x '} [Bi _a'' Z ² _b'' O _x'' ] are present, the largest diameter in the range 1 nm to 100 microns.

Regarding the shaping applies with respect Multimetal oxide V-catalysts in the multimetal oxide materials IV catalysts said.

The Preparation of Multimetal Oxide Compounds V-Active Compounds is, for. In EP-A 575 897 and in DE-A 198 55 913 described.

The The above-recommended inert support materials include i.a. also as inert materials for dilution and / or delimitation of corresponding fixed catalyst beds, or as their protective and / or the gas mixture heating pre-feed into consideration.

At this point should be mentioned that all catalysts and multimetal oxide materials used for the step from propylene to acrolein were also recommended in principle for the Partialammoxidation of propylene to acrylonitrile are suitable.

For the second Step (the second reaction step), the heterogeneously catalyzed Gas-phase partial oxidation of acrolein to acrylic acid, come, As already stated, in principle all Mo and V-containing multimetal oxide materials as active masses for The necessities Catalysts into consideration, for. B. those of DE-A 100 46 928.

A variety of the same, for. As those of DE-A 198 15 281, can be under the general formula VII, Mo ₁₂ V _a X ¹ _b X ² _c X ³ _d X ⁴ _e X ⁵ _f X ⁶ _g O _n (VII) in which the variables have the following meaning:
X ¹ = W, Nb, Ta, Cr and / or Ce,
X ² = Cu, Ni, Co, Fe, Mn and / or Zn,
X ³ = Sb and / or Bi,
X ⁴ = one or more alkali metals,
X ⁵ = one or more alkaline earth metals,
X ⁶ = Si, Al, Ti and / or Zr,
a = 1 to 6,
b = 0.2 to 4,
c = 0.5 to 18,
d = 0 to 40,
e = 0 to 2,
f = 0 to 4,
g = 0 to 40 and
n = a number which is determined by the valence and frequency of the elements other than oxygen in VII,
subsume.

Preferred embodiments within the active multimetal oxides VII according to the invention are those which are covered by the following meanings of the variables of general formula VII:
X ¹ = W, Nb, and / or Cr,
X ² = Cu, Ni, Co, and / or Fe,
X ³ = Sb,
X ⁴ = Na and / or K,
X ⁵ = Ca, Sr and / or Ba,
X ⁶ = Si, Al, and / or Ti,
a = 1.5 to 5,
b = 0.5 to 2,
c = 0.5 to 3,
d = 0 to 2,
e = 0 to 0.2,
f = 0 to 1 and
n = a number determined by the valence and frequency of the elements other than oxygen in VII.

However, very particularly preferred multimetal oxides VII according to the invention are those of the general formula VIII Mo ₁₂ V _{a '} Y ¹ _b' Y ² _{c '} Y ⁵ _f' Y ⁶ _g O _{n '} (VIII), With
Y ¹ = W and / or Nb,
Y ² = Cu and / or Ni,
Y ⁵ = Ca and / or Sr,
Y ⁶ = Si and / or Al,
a '= 2 to 4,
b '= 1 to 1.5,
c '= 1 to 3,
f '= 0 to 0.5
g '= 0 to 8 and
n '= a number which is determined by the valence and frequency of the elements other than oxygen in VIII.

The suitable according to the invention Multimetal oxide active materials (VII) are known per se, for. B. in DE-A 43 35 973 or in EP-A 714 700 disclosed manner available.

In principle, suitable for the step "acrolein → acrylic acid" suitable Multimetalloxidaktivmassen, especially those of the general formula VII, in a simple manner be prepared by suitable sources of their elemental constituents an intimate, preferably finely divided, their stoichiometry correspondingly composed, dry mixture and this calcined at temperatures of 350 to 600 ° C. The calcination can be carried out both under inert gas and under an oxidative atmosphere such. As air (mixture of inert gas and oxygen) as well as under a reducing atmosphere (eg., Mixtures of inert gas and reducing gases such as H ₂ , NH ₃ , CO, methane and / or acrolein or said reducing acting gases per se) performed become. The calcination time can be several minutes to several hours and usually decreases with temperature. Suitable sources of the elemental constituents of the multimetal oxide active compounds VII are those compounds which are already oxides and / or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.

The intimate mixing of the starting compounds for the preparation of multimetal oxide materials VII can be done in dry or wet form. Is it done in dry form, so the starting compounds are suitably used as finely divided powder and after mixing and optionally Compacting subjected to calcination. Preferably, this is done intimate mixing but in wet form.

Usually be the starting compounds in the form of an aqueous solution and / or suspension mixed together. Especially intimate dry mixtures are then obtained in the described mixing method, if only from in dissolved Form present sources of elementary constituents becomes. As a solvent Water is preferably used. Subsequently, the resulting aqueous mass dried, the drying process preferably by spray drying the aqueous Mixture with outlet temperatures of 100 to 150 ° C takes place.

The resulting Multimetalloxidmassen, in particular those of the general Formula VII, can for the Acrolein oxidation both in powder form and to certain catalyst geometries be used, wherein the shaping before or after the final Calcination can be done. For example, from the powder form of Active mass or its uncalcined precursor mass by compaction to the desired Catalyst geometry (eg by tabletting, extruding or Extrusion) Vollkatalysatoren be prepared, where appropriate Aids such. As graphite or stearic acid as a lubricant and / or Molding aids and reinforcing agents such as glass microfibers, asbestos, silicon carbide or potassium titanate can be added. Suitable Vollkatalysatorgeometrien are z. B. solid cylinder or Hollow cylinder with an outer diameter and a length from 2 to 10 mm. In the case of the hollow cylinder is a wall thickness of 1 to 3 mm appropriate. Of course you can the full catalyst also have spherical geometry, wherein the ball diameter 2 to 10 mm (eg 8.2 mm or 5.1 mm).

Of course you can the shape of the powdered Active mass or its powdery, yet uncalcined, precursor material also be carried out by applying to preformed inert catalyst support. The coating of the carrier body for Preparation of the coated catalysts is usually in a suitable rotatable container executed as it is z. B. from DE-A 2 909 671, EP-A 293 859 or from EP-A 714 700 is known.

Conveniently, is used to coat the carrier body applied powder compound moistened and after application, z. B. by means of hotter Air, dried again. The layer thickness of the applied to the carrier body Powder mass is expediently in the range 10 to 1000 μm, preferably in the range 50 to 500 microns and more preferably in the range 150 to 250 microns, selected.

As carrier materials, it is possible to use customary porous or non-porous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates, such as magnesium silicate or aluminum silicate. The carrier bodies may be regularly or irregularly shaped, with regularly shaped carrier bodies having a distinct surface roughness, e.g. As balls or hollow cylinder with chippings, are preferred. Suitable is the use of substantially nonporous, surface rough, spherical steatite supports whose diameter is 1 to 10 mm or 8 mm, preferably 4 to 5 mm. That is, suitable ball geometries may have diameters of 8.2 mm or 5.1 mm. However, it is also suitable to use cylinders as support bodies whose length is 2 to 10 mm and whose outer diameter is 4 to 10 mm. In addition, in the case of rings as a carrier body, the wall thickness is usually 1 to 4 mm. Preferably to be used annular support body have a length of 2 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness of 1 to 2 mm. Particularly suitable are rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) as Trä gerkörper. The fineness of the catalytically active oxide masses to be applied to the surface of the carrier body is, of course, adapted to the desired shell thickness (cf EP-A 714 700).

Favorable for the step "acrolein → acrylic acid" to be used Multimetalloxidaktivmassen are also compounds of general formula IX, [D] _p [E] _q (IX), in which the variables have the following meaning:
D = Mo ₁₂ V _{a "} Z ¹ _b" Z ² _{b "} Z ³ _d" Z ⁴ _{e "} Z ⁵ _f" Z ⁶ _{g "} O _x"
E = Z ⁷ ₁₂ Cu _{h "} H _i" O _{y "} ,
Z ¹ = W, Nb, Ta, Cr and / or Ce,
Z ² = Cu, Ni, Co, Fe, Mn and / or Zn,
Z ³ = Sb and / or Bi,
Z ⁴ = Li, Na, K, Rb, Cs and / or H
Z ⁵ = Mg, Ca, Sr and / or Ba,
Z ⁶ = Si, Al, Ti and / or Zr,
Z ⁷ = Mo, W, V, Nb and / or Ta, preferably Mo and / or W.
a '' = 1 to 8,
b '' = 0.2 to 5,
c '' = 0 to 23,
d '' = 0 to 50,
e '' = 0 to 2,
f '' = 0 to 5,
g '' = 0 to 50,
h '' = 4 to 30,
i '' = 0 to 20 and
x '', y '' = numbers which are determined by the valence and frequency of the element different from oxygen in IX and
p, q = non-zero numbers whose ratio p / q is 160: 1 to 1: 1 and which are obtainable by using a multimetal oxide composition E Z ⁷ ₁₂ Cu _{h "} H _i" O _{y "} (E), in finely divided form (initial mass 1 ) and then the preformed solid starting material 1 in an aqueous solution, an aqueous suspension or in a finely divided dry mixture of sources of elements Mo, V, Z ¹ , Z ² , Z ³ , Z ⁴ , Z ⁵ , Z ⁶ , the aforementioned elements in the stoichiometry D Mo ₁₂ V _{a "} Z ¹ _b" Z ² _{c "} Z ³ _d" Z ⁴ _{e "} Z ⁵ _f" Z ⁶ _{g "} (D) contains (initial mass 2 ), in the desired ratio p: q incorporated, which thereby optionally drying resulting aqueous mixture, and calcining the dry precursor material given before or after drying to the desired catalyst geometry at temperatures of 250 to 600 ° C.

Preference is given to the multimetal oxide compositions IX in which the incorporation of the preformed solid starting material 1 in an aqueous starting material 2 at a temperature <70 ° C takes place. A detailed description of the preparation of multimetal oxide materials VI catalysts include, for. For example, EP-A 668 104, DE-A 197 36 105, DE-A 100 46 928, DE-A 197 40 493 and DE-A 195 28 646.

Regarding the shaping applies with respect Multimetal oxide materials IX catalysts that in the case of the multimetal oxide materials VII catalysts said.

For the step "acrolein → acrylic acid" in excellent Suitable multimetal oxide catalysts are also those of DE-A 198 15 281, in particular with Multimetalloxidaktivmassen the general formula I of this document.

Advantageous be for the step from propylene to acrolein full-catalyst rings and for the step from Acrolein to acrylic acid Shell catalyst rings used.

The implementation of the method according to the invention, from propylene to acrolein (as well as give Acrylic acid, if appropriate), z with the catalysts described. B. in a one-zone multi-contact fixed bed reactor, as described in DE-A 4 431 957. In this case, reaction gas mixture 2 and heat transfer medium (heat exchange medium) are viewed over the reactor in cocurrent or in countercurrent.

The reaction pressure is usually in the range of 1 to 3 bar and the total space load of the fixed catalyst bed with reaction gas (exit) mixture 2 is preferably 1500 to 4000 or 6000 Nl / l · h or more. The propylene load (the propylene load of the fixed catalyst bed) is typically 90 to 200 Nl / l · hr or 300 Nl / l · hr or more. Propylene loads above 135 Nl / l · h or ≥ 140 Nl / l · h, or ≥ 150 Nl / l · h, or ≥ 160 Nl / l · h are particularly preferred according to the invention, since the reaction gas starting mixture according to the invention 2 a favorable hot spot behavior conditioned (all the above also applies regardless of the specific choice of the fixed bed reactor).

Preferably, the single-zone multiple contact tube fixed bed reactor is supplied with the feed gas mixture from above. As a heat exchange agent is advantageously a molten salt, preferably consisting of 60 wt .-% potassium nitrate (KNO ₃ ) and 40 wt% sodium nitrite (NaNO ₂ ), or from 53 wt .-% potassium nitrate (KNO ₃ ), 40 wt .-% Sodium nitrite (NaNO ₂ ) and 7 wt .-% sodium nitrate (NaNO ₃ ) used.

Considered above the reactor, as already stated, molten salt and reaction gas mixture 2 be performed both in DC and in countercurrent. The molten salt itself is preferably guided meander-shaped around the catalyst tubes.

• – zunächst auf einer Länge von 40 bis 80 bzw. bis 60 % der Kontaktrohrlänge entweder nur Katalysator, oder ein Gemisch aus Katalysator und Inertmaterial, wobei letzteres, bezogen auf die Mischung, einen Gewichtsanteil von bis zu 20 Gew.-% ausmacht (Abschnitt C);
• – daran anschließend auf einer Länge von 20 bis 50 bzw. bis 40 % der Gesamtrohrlänge entweder nur Katalysator, oder ein Gemisch aus Katalysator und Inertmaterial, wobei letzteres, bezogen auf die Mischung, einen Gewichtsanteil von bis zu 40 Gew.-% ausmacht (Abschnitt B); und
• – abschließend auf einer Länge von 10 bis 20 % der Gesamtrohrlänge eine Schüttung aus Inertmaterial (Abschnitt A), die vorzugsweise so gewählt wird, dass sie einen möglichst geringen Druckverlust bedingt.

If the contact tubes are flown from top to bottom, it is advisable to charge the catalyst tubes from the bottom to the top as follows (for the flow from bottom to top, the feed sequence is suitably reversed):

• - First, on a length of 40 to 80 or 60% of the catalyst tube length either catalyst, or a mixture of catalyst and inert material, the latter, based on the mixture, a weight fraction of up to 20 wt .-% makes (Section C );
• - Thereafter, over a length of 20 to 50 or 40% of the total tube length, either catalyst, or a mixture of catalyst and inert material, the latter, based on the mixture, a weight fraction of up to 40 wt .-% makes (Section B); and
• - Finally, over a length of 10 to 20% of the total tube length, a bed of inert material (Section A), which is preferably chosen so that it causes the lowest possible pressure loss.

Prefers section C is undiluted.

The aforementioned Loading variant is particularly useful if as catalysts such example 1 of DE-A 100 46 957 or according to Example 3 of DE-A 100 46 957 and as an inert material rings of steatite with the geometry 7 mm × 7 mm × 4 mm (outer diameter × height × inner diameter) used become. With regard to the salt bath temperature, the same applies in DE-A 4 431 957.

The implementation of the partial oxidation according to the invention, from propylene to acrolein (and, where appropriate, acrylic acid), can also be carried out with the described catalysts, but also z. B. be carried out in a two-zone multi-contact fixed bed reactor, as described in DE-A 199 10 506. In both cases described above (and more generally in the process according to the invention), the propene conversion achieved in a single pass is normally ≥90 mol% or ≥95 mol% and the selectivity of acrolein formation is ≥90 mol%. According to the invention, the partial oxidation of propene according to the invention to acrolein, or acrylic acid or a mixture thereof is carried out as described in EP-A 1159244 and very particularly preferably as in WO 04/085363 and in WO 04/085362, but with the difference that as the starting reaction gas mixture, an inventive reaction gas starting mixture 2 is used. In particular, all embodiments of the aforementioned publications can be carried out as described in these documents, but using a reaction gas starting mixture 2 as a feed gas mixture. In particular with the reaction gas starting mixtures which are particularly preferred in this document and which are exemplary 2 ,

The Documents EP-A 1159244, WO 04/085363 and WO 04/085362 are known as integral part of this document.

In other words, the partial oxidation of propylene according to the invention can be carried out particularly advantageously on a fixed catalyst bed with increased propylene loading, which comprises at least two temperature zones has.

• a) die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch 2 enthaltenen Propen ≥ 160 Nl Propen/l Festbettkatalysatorschüttung·h beträgt,
• b) die Festbettkatalysatorschüttung aus einer in zwei räumlich aufeinanderfolgenden Reaktionszonen A*, B* angeordneten Festbettkatalysatorschüttung besteht, wobei die Temperatur der Reaktionszone A* 300 bis 390°C und die Temperatur der Reaktionszone B* 305 bis 420°C beträgt und gleichzeitig wenigstens 5°C oberhalb der Temperatur der Reaktionszone A* liegt,
• c) das Reaktionsgasausgangsgemisch 2 die Reaktionszonen A*, B* in der zeitlichen Abfolge „erst A*'', „dann B*'' durchströmt und
• d) sich die Reaktionszone A* bis zu einem Umsatz des Propens von 40 bis 80 mol-% erstreckt.

That is, an advantageous embodiment of the inventive partial oxidation of propylene to acrolein, or acrylic acid or a mixture thereof is a process according to the invention in which the reaction gas starting mixture 2 with the proviso over (through) a fixed-bed catalyst bed whose active material is at least one Mo, Fe and Bi-containing multimetal, leads to the fact that the propylene conversion in a single pass ≥ 90 mol% and the associated selectivity of acrolein formation and the Acrylsäurenebenproduktbildung taken together ≥ 90 mol% and which is characterized in that

• a) the load of the fixed bed catalyst bed with that in the reaction gas starting mixture 2 Propene ≥ 160 Nl propene / l fixed-bed catalyst bed · h,
• b) the fixed-bed catalyst charge consists of a fixed-bed catalyst charge arranged in two spatially successive reaction zones A *, B *, the temperature of the reaction zone A * being from 300 to 390 ° C. and the temperature of the reaction zone B being from 305 to 420 ° C. and at the same time at least 5 ° C is above the temperature of reaction zone A *,
• c) the reaction gas starting mixture 2 the reaction zones A *, B * in the time sequence "first A *", then "B *" flows through and
• d) the reaction zone A * extends to a conversion of the propene of 40 to 80 mol%.

Otherwise, will to EP-A 1159244.

• – die Festbettkatalysatorschüttung in zwei räumlich aufeinanderfolgenden Temperaturzonen A, B angeordnet ist,
• – sowohl die Temperatur der Temperaturzone A als auch die Temperatur der Temperaturzone B eine Temperatur im Bereich von 290 bis 380°C ist,
• – die Festbettkatalysatorschüttung aus wenigstens zwei räumlich aufeinander folgenden Festbettkatalysatorschüttungszonen besteht, wobei die volumenspezifische Aktivität innerhalb einer Festbettkatalysatorschüttungszone im wesentlichen konstant ist und in Strömungsrichtung des Reaktionsgasgemisches 2 beim Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone sprunghaft zunimmt,
• – sich die Temperaturzone A bis zu einem Umsatz des Propens von 40 bis 80 mol.-% erstreckt,
• – bei einmaligem Durchgang des Reaktionsgasausgangsgemisches 2 durch die gesamte Festbettkatalysatorschüttung der Propenumsatz ≥ 90 mol.-% und die Selektivität der Acroleinbildung, bezogen auf umgesetztes Propen, ≥ 90 mol.-% beträgt,
• – die zeitliche Abfolge, in der das Reaktionsgasgemisch 2 die Temperaturzonen A, B durchströmt, der alphabetischen Abfolge der Temperaturzonen entspricht,
• – die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch 2 enthaltenen Propen ≥ 90 Nl Propen/l Festbettkatalysatorschüttung·h beträgt, und
• – die Differenz T^maxA – T^maxB, gebildet aus der höchsten Temperatur T^maxA, die das Reaktionsgasgemisch 2 innerhalb der Temperaturzone A aufweist, und der höchsten Temperatur T^maxB, die das Reaktionsgasgemisch 2 innerhalb der Temperaturzone B aufweist, ≥ 0°C beträgt,

und das zusätzlich dadurch gekennzeichnet ist, dass der Übergang von der Temperaturzone A in die Temperaturzone B in der Festbettkatalysatorschüttung nicht mit einem Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone zusammenfällt.Ie. but also, a very particularly preferred embodiment of the inventive partial oxidation of propylene to acrolein, or acrylic acid, or their Ge mixture, is an inventive method in which the reaction gas starting mixture 2 with the proviso over a fixed bed catalyst bed, the active material is at least one of the elements Mo, Fe and Bi-containing multimetal, leads that

• The fixed bed catalyst bed is arranged in two spatially consecutive temperature zones A, B,
• Both the temperature of the temperature zone A and the temperature of the temperature zone B is a temperature in the range from 290 to 380 ° C,
• - The fixed bed catalyst bed consists of at least two spatially sequential fixed bed catalyst bed zones, wherein the volume-specific activity is substantially constant within a fixed bed catalyst bed and in the flow direction of the reaction gas mixture 2 increases abruptly during the transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone,
• The temperature zone A extends to a conversion of the propene of 40 to 80 mol%,
• - With a single pass of the starting reaction gas mixture 2 the propene conversion is ≥ 90 mol% and the selectivity of acrolein formation, based on reacted propene, is ≥90 mol%, through the entire fixed-bed catalyst bed,
• - The time sequence in which the reaction gas mixture 2 the temperature zones A, B flows through, corresponds to the alphabetical sequence of the temperature zones,
• - The burden of fixed bed catalyst bed with that in the starting reaction gas mixture 2 Propene contained ≥ 90 Nl propene / l fixed-bed catalyst bed · h, and
• - The difference T ^maxA - T ^maxB , formed from the highest temperature T ^maxA , the reaction ^gas mixture 2 within the temperature ^zone A, and the highest temperature T ^maxB containing the reaction ^gas mixture 2 within the temperature zone B, ≥ 0 ° C,

and additionally characterized in that the transition from the temperature zone A to the temperature zone B in the fixed bed catalyst bed does not coincide with a transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone.

Details to this procedure can be found in WO 04/085362, the integral Part of this document is, as well as in the further course of this Scripture in the description of the most preferred two-stage Partial oxidation of propylene to acrylic acid.

The implementation of the second step in the case of a two-stage partial oxidation of propylene to acrolein, namely the partial oxidation of acrolein to acrylic acid, can be carried out with the described catalysts z. B. in a one-zone Vielkontaktrohr fixed bed reactor, as described in DE-A 44 31 949. It can Reaktionsgsgasgemisch 3 and heat transfer medium are viewed over the reactor in DC. In general, the product gas mixture of the preceding inventive Propylene partial oxidation to acrolein in principle as such (optionally after the intermediate cooling (this can be done indirectly or directly by eg., Secondary air addition) thereof), ie, without Nebenkomponentenabtrennung, in the second reaction stage, ie led to the Acroleinpartialoxidation.

The molecular oxygen required for the second step, the acrolein partial oxidation, can already be present in the reaction gas starting mixture 2 be included for the invention propylene partial oxidation to acrolein. However, it can also be partially or completely added directly to the product gas mixture of the first reaction stage, ie the propylene partial oxidation according to the invention to acrolein (this is preferably done as (secondary) air, but can also be in the form of pure oxygen or mixtures of inert gas or oxygen ). Regardless of what is done, the feed gas mixture (the reaction gas starting mixture 3 ) such, for the same reasons as the inventive partial oxidation of propylene to acrolein, or acrylic acid, or a mixture thereof, inventive partial oxidation of the acrolein to acrolic acid, advantageously following contents on: 4.5 to 8 vol.% acrolein, 2.25 or 4.5 to 9 vol.% molecular oxygen, 6 to 30 vol.% Propane, 32 to 72 vol.% molecular nitrogen and 5 to 15 vol.% Steam.

Preferably, the reaction gas starting mixture 3 following contents: 5.5 to 8 vol.% acrolein, 2.75 or 5.5 to 9 vol.% molecular oxygen, 10 to 25 vol.% Propane, 40 to 70 vol.% molecular nitrogen and 5 to 15 vol.% Steam.

Very particular preference is given to the starting reaction gas mixture 3 following contents: 6 to 8 vol.% Acrolein (preferably 6 to 7% by volume), 3 or 6 to 9 vol.% molecular oxygen, 10 to 20 vol.% Propane (preferably 10 to 16% by volume), 50 to 65 vol.% molecular nitrogen and 7 to 13 vol.% Steam, wherein the preferred ranges apply independently, but with advantage be realized simultaneously.

Quite generally, it is the case for all starting reaction gas starting mixtures according to the invention 3 favorable if their total content of components other than those specified above is ≦ 10% by volume, preferably ≦ 8% by volume, more preferably ≦ 6 or ≦ 5% by volume and most preferably ≦ 4% by volume be. Particularly advantageous is the total content of the reaction gas starting mixture 3 to carbon oxides ≤ 5 vol .-% and most preferably ≤ 3 vol .-% or ≤ 2 vol .-%.

The content of the reaction gas starting mixture 3 of propylene is preferably ≦ 2 vol .-% and particularly advantageously ≦ 1 vol .-%.

Furthermore, it is for the reaction gas starting mixture 3 advantageous if the molar ratio of in the reaction gas starting mixture 3 contained molecular oxygen to in the reaction gas starting mixture 3 acrolein ≥ 0.5 and ≤ 2, with advantage ≥ 1 and ≤ 1.75, more advantageously ≥ 1 and ≤ 1.5, and most preferably ≥ 1 and ≤ 1.25.

Furthermore, it is for the reaction gas starting mixture 3 advantageous when the molar ratio of propane contained in it to acrolein contained in it 1 to 4 , with preference from 1.5 to 3.5, particularly preferably 1.5 to 3 and very particularly preferably 1.5 or 2 to 2.5.

The content of the reaction gas starting mixture 3 As a rule, methane and / or ethane will be ≦ 8% by volume, usually ≦ 5% by volume and usually ≦ 3% by volume or ≦ 2% by volume. Often, however, it will advantageously amount to ≥ 0.5% by volume.

As in the first reaction stage, the reaction pressure is also more common in the second reaction stage in the range of 1 to 3 bar, and the total space load of the fixed catalyst bed with reaction gas (exit) mixture 3 is preferably 1500 to 4000 or 6000 Nl / l · h or more. The acrolein load (the acrolein loading of the fixed catalyst bed) is typically 90 to 190 Nl / lh, or to 290 Nl / lh or more. Acrolein load above 135 Nl / l · h, or ≥ 140 Nl / l · h, or ≥ 150 Nl / l · h, or ≥ 160 Nl / l · h are particularly preferred because the reaction gas starting mixture according to the invention 3 also a favorable hot spot behavior caused.

The acrolein conversion, based on a single pass of the reaction gas mixture 3 through the fixed catalyst bed, it is expedient to be ≥90 mol% and the associated acrylic acid selectivity selectivity ≥90 mol%.

Preferably, the single-zone multiple contact tube fixed bed reactor is also supplied with the feed gas mixture from above. As a heat exchange agent, a salt melt, preferably from 60% by weight of potassium nitrate (KNO ₃ ) and 40% by weight of sodium nitrite (NaNO ₂ ) or of 53% by weight of potassium nitrate (KNO ₃ ) is expediently also used in the second stage. 40 wt .-% sodium nitrite (NaNO ₂ ) and 7 wt .-% sodium nitrate (NaNO ₃ ) consisting used. Considered above the reactor, as already stated, molten salt and reaction gas mixture 3 be performed both in DC and in countercurrent. The molten salt itself is preferably guided meander-shaped around the catalyst tubes.

• – zunächst auf einer Länge von 50 bis 80 bzw. bis 70 % der Kontaktrohrlänge entweder nur Katalysator oder ein Gemisch aus Katalysator und Inertmaterial, wobei letzteres, bezogen auf die Mischung, einen Gewichtsanteil von bis zu 20 Gew.-% ausmacht (Abschnitt C);
• – daran anschließend auf einer Länge von 20 bis 40 % der Gesamtrohrlänge entweder nur Katalysator, oder ein Gemisch aus Katalysator und Inertmaterial, wo bei letzteres, bezogen auf die Mischung, einen Gewichtsanteil von bis zu 50 bzw. bis zu 40 Gew.-% ausmacht (Abschnitt B); und
• – abschließend auf einer Länge von 5 bis 20 % der Gesamtrohrlänge eine Schüttung aus Inertmaterial (Abschnitt A), die vorzugsweise so gewählt wird, dass sie einen möglichst geringen Druckverlust bedingt.

If the contact tubes are flown from top to bottom, it is advisable to feed the contact tubes from bottom to top as follows:

• Initially at a length of 50 to 80 or up to 70% of the catalyst tube length, either catalyst or a mixture of catalyst and inert material, the latter, based on the mixture, constituting a weight fraction of up to 20% by weight (Section C) ;
• - Then thereafter to a length of 20 to 40% of the total tube length either catalyst only, or a mixture of catalyst and inert material, where in the latter, based on the mixture, a weight fraction of up to 50 or up to 40 wt .-% makes (Section B); and
• - Finally, on a length of 5 to 20% of the total tube length, a bed of inert material (Section A), which is preferably chosen so that it causes the lowest possible pressure loss.

Prefers section C is undiluted. As is generally the case with heterogeneously catalyzed gas phase partial oxidation from acrolein to acrylic acid (especially at high acrolein loadings of the catalyst bed and high water vapor contents of the feed gas mixture), can Section B also consists of two consecutive catalyst dilutions (for the purpose of minimizing hot spot temperature and hot spot temperature sensitivity). From bottom to top first with up to 20 wt .-% inert material and subsequently with> 20 wt .-% to 50 or to 40 wt .-% inert material. The section C is then preferred undiluted.

For a flow of the Contact tubes from bottom to top, is the contact tube feed in expediently vice versa.

The aforementioned feed variant is particularly useful if, as catalysts, those according to the preparation example 5 DE-A 100 46 928 or those according to DE-A 198 15 281 and as inert material rings made of steatite with the geometry 7 mm × 7 mm × 4 mm or 7 mm × 7 mm × 3 mm (outer diameter × height × inner diameter) be used. With regard to the salt bath temperature, the statements made in DE-A 443 19 49 apply. It is usually chosen so that the achieved acrolein conversion in a single pass is normally ≥ 90 mol%, or ≥ 95 mol% or ≥ 99 mol%.

The implementation of the partial oxidation of acrolein to acrylic acid, but with the catalysts described but also z. B. in a two-zone multi-contact fixed bed reactor, as described in DE-199 10 508. For acrolein sales the above applies. Also in the case of carrying out acroleinpartialoxidation as described above as the second reaction stage of a two-stage propylene oxidation to acrylic acid in a two-zone Vielkontaktrohr fixed bed reactor to produce the feed gas mixture (reaction gas starting mixture 3 ) expediently directly use the product gas mixture of the partial oxidation directed to the first step (if appropriate after indirect or direct (eg by supplying secondary air) intercooling thereof) (as already described above). The oxygen required for the acrolein partial oxidation is preferably added as air (but optionally also as pure molecular oxygen or as a mixture of molecular oxygen and an inert gas) and z. B. the product gas mixture of the first step of the two-stage partial oxidation (propylene → acrolein) is added directly. But he can also, as already described, be already in the reaction gas starting mixture 2 be included for the first reaction stage.

at a two-stage partial oxidation of propylene to acrylic acid with immediate reuse of the product gas mixture of the first Partial oxidation step for charging the second step of Partial oxidation, one is usually two single-zone multi-contact fixed bed reactors (at high reactant loading of the catalyst bed is how completely generally valid, a Gleichstromfahrweise between reaction gas and salt bath (heat carrier) over the Tube reactor preferably) or two two-zone multi-contact tube fixed bed reactors daisy-chaining.

A mixed in series (one-zone / two-zone or vice versa) is possible, too.

Between The reactors may be an intercooler, if necessary Inert fillings included can, which can exercise a filter function. The salt bath temperature of multi-contact tube reactors for the first step of the two-stage partial oxidation of propylene to acrylic acid is usually 300 to 400 ° C. The salt bath temperature of multi-contact tube reactors for the second Step of the partial oxidation of propylene to acrylic acid, the Partial oxidation of acrolein to acrylic acid, is usually 200 to 350 ° C. In addition, will the heat exchange agents (preferably molten salts) normally in such amounts by the relevant multi-contact tube fixed bed reactors guided, that the difference between their input and their output temperature usually ≤ 5 ° C. As already mentioned, can however, both steps of the partial oxidation of propylene to acrylic acid also as described in DE-A 101 21 592 in a reactor on a Charging be done.

It should also be mentioned again that a part of the reaction gas starting mixture 2 for the first step ("propylene → acrolein") may be from the partial oxidation oxidizing gas (residual gas).

in this connection it is, as already said, a molecular oxygen containing gas which, after the target product separation (acrolein and / or acrylic acid separation) from the product gas mixture of the partial oxidation remains and partially as inert diluent gas in the feed for the first and / or optionally second step of the partial oxidation from propylene to acrolein and / or acrylic acid can be recycled.

Prefers Will such propane, molecular oxygen and possibly not reacted propylene-containing oxidation gas, however, advantageous exclusively in the optionally acting as a propylene heterogeneously catalyzed Propane dehydrogenation recycled.

All in all forms a tube bundle reactor within which the catalyst feed along the individual Change contact tubes with completion of the first reaction step accordingly (such Two-stage propylene partial oxidations in the so-called "single reactor" teach, for example, the EP-A 91 13 13, EP-A 97 98 13, EP-A 99 06 36 and DE-A 28 30 765) the simplest form of realization of two oxidation states for the two Steps of partial oxidation of propylene to acrylic acid. Possibly In this case, the feed of the catalyst tubes with catalyst an inert material bed interrupted.

Preferably However, the two oxidation states are in the form of two consecutively switched tube bundle systems realized. these can are in a reactor, the transition from a tube bundle to other tube bundle from a not in the contact tube accommodated (appropriately walkable) fill is formed from inert material. While the contact tubes in the Usually be lapped by a heat transfer medium, this does not reach an inert material bed attached as described above. Advantageously, therefore, the two catalyst tube bundles in spatially separate reactors accommodated. As a rule, this is between the two Tube reactors an intercooler, an optional acrolein afterburning in the product gas mixture, which leaves the first oxidation zone to reduce. The reaction temperature in the first reaction stage (propylene → acrolein) is usually at 300 to 450 ° C, preferably at 320 to 390 ° C. The reaction temperature in the second reaction stage (acrolein → acrylic acid) is usually at 200 to 370 ° C, often at 220 to 330 ° C. Of the Reaction pressure is in both oxidation zones appropriate 0.5 to 5, advantageously 1 to 3 bar. The load (Nl / l · h) of Oxidation catalysts with reaction gas is in both reaction stages often 1500 to 2500 Nl / l · h or up to 4000 Nl / l · h. The load of propylene can be 100 to 200 or 300 and more Nl / lh lie.

In principle, the two oxidation states in the process according to the invention can be designed as described, for. In DE-A 198 37 517, DE-A 199 10 506, DE-A 199 10 508 and DE-A 198 37 519 is described.

In Both reaction stages have an excess of molecular oxygen relative to the reaction stoichiometric required amount usually beneficial to the kinetics of respective gas phase partial oxidation.

in principle is the heterogeneous invention catalyzed gas phase partial oxidation of propylene to acrylic acid but also in a single single tube bundle reactor as follows feasible. Both reaction steps take place in an oxidation reactor of is charged with one or more catalysts whose active composition is a multimetal oxide containing Mo, Fe and Bi, which is capable of catalyzing the reaction of both reaction steps. Of course, this catalyst feed can also be along the reaction coordinate change continuously or abruptly. Naturally can in one embodiment a two-stage according to the invention Partial oxidation of propylene to acrylic acid in the form of two series-connected Oxidation levels from that leaving the first oxidation stage Product gas mixture contained selbigem, in the first oxidation stage by-product, carbon monoxide and water vapor as needed before passing to the second oxidation stage partially or Completely be separated. Preferably, according to the invention, a procedure choose, which does not provide for such a separation.

As sources for an oxygen intermediate feed carried out between the two oxidation stages, as already mentioned, in addition to air (preferably) both pure molecular oxygen and inert molecular gas diluted with inert gas such as CO ₂ , CO, noble gases, N ₂ and / or saturated hydrocarbons are considered.

By metering z. B. cold air to hot product gas mixture 2 can be effected in the context of the method according to the invention on a direct route, a cooling of the same, before it as a constituent of a reaction gas starting mixture 3 is used.

According to the invention, the partial oxidation of acrolein to acrylic acid is carried out as described in EP-A 11 59 246, and very particularly preferably as described in WO 04/085365 and in WO 04/085370. According to the invention, however, a starting reaction gas mixture containing acrolein is preferred 3 used (this may in particular be the product gas mixture of a first-stage partial oxidation of propylene to acrolein according to the invention, which was optionally supplemented with sufficient secondary air that the ratio of molecular oxygen to acrolein in the resulting reaction gas starting mixture 3 in each case 0.5 to 1.5). In particular, all embodiments of the aforementioned publications can be carried out as described in these documents, but using a reaction gas starting mixture 3 as a feed gas mixture. In particular with the reaction gas starting mixtures which are particularly preferred in this document and which are exemplary 3 , The documents EP-A 1159246, WO 04/08536 and WO 04/085370 are considered an integral part of this document.

that is, the partial oxidation according to the invention from acrolein to acrylic acid let yourself perform advantageous on a fixed catalyst bed with increased Acroleinbelastung, the has at least two temperature zones.

• a) die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch 3 enthaltenen Acrolein ≥ 150 Nl Acrolein/l Festbettkatalysatorschüttung·h beträgt,
• b) die Festbettkatalysatorschüttung aus einer in zwei räumlich aufeinanderfolgenden Reaktionszonen C*, D* angeordneten Festbettkatalysatorschüttung besteht, wobei die Temperatur der Reaktionszone C* 230 bis 270°C und die Temperatur der Reaktionszone D* 250 bis 300°C beträgt und gleichzeitig wenigstens 5°C oberhalb der Temperatur der Reaktionszone C* liegt,
• c) das Reaktionsgasausgangsgemisch 3 die Reaktionszonen C*, D* in der zeitlichen Abfolge „erst C*'', „dann D*'' durchströmt und
• d) sich die Reaktionszone C* bis zu einem Umsatz des Acroleins von 55 bis 85 molerstreckt.

That is, an advantageous embodiment of the inventive partial oxidation of acrolein to acrylic acid is a process in which the reaction gas starting mixture 3 with the proviso over (through) a fixed-bed catalyst bed whose active material is at least one multimetal oxide containing the elements Mo and V, the acrolein conversion in a single pass is ≥ 90 mol% and the associated selectivity of acrylic acid formation is ≥ 90 mol% and which is characterized in that

• a) the load of the fixed bed catalyst bed with that in the reaction gas starting mixture 3 acrolein ≥ 150 Nl acrolein / l fixed-bed catalyst bed · h,
• b) the fixed-bed catalyst charge consists of a fixed-bed catalyst charge arranged in two successive reaction zones C *, D *, wherein the temperature of the reaction zone C * is 230 to 270 ° C and the temperature of the reaction zone D * 250 to 300 ° C and at the same time at least 5 ° C is above the temperature of the reaction zone C *,
• c) the reaction gas starting mixture 3 the reaction zones C *, D * in the time sequence "first C *", then "D *" flows through and
• d) the reaction zone C * extends to a conversion of acrolein of 55 to 85 mol.

Otherwise, will to EP-A 1159246.

• – die Festbettkatalysatorschüttung in zwei räumlich aufeinanderfolgenden Temperaturzonen C, D angeordnet ist,
• – sowohl die Temperatur der Temperaturzone C als auch die Temperatur der Temperaturzone D eine Temperatur im Bereich von 230 bis 320°C ist,
• – die Festbettkatalysatorschüttung aus wenigstens zwei räumlich aufeinanderfolgenden Festbettkatalysatorschüttungszonen besteht, wobei die volumenspezifische Aktivität innerhalb einer Festbettkatalysatorschüttungszone im wesentlichen konstant ist und in Strömungsrichtung des Reaktionsgasgemisches 3 beim Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone sprunghaft zunimmt,
• – sich die Temperaturzone C bis zu einem Umsatz des Acroleins von 45 bis 85 mol-% erstreckt,
• – bei einmaligem Durchgang des Reaktionsgasausgangsgemisches 3 durch die gesamte Festbettkatalysatorschüttung der Acroleinumsatz ≥ 90 mol-% und die Selektivität der Acrylsäurebildung, bezogen auf umgesetztes Acrolein, ≥ 90 mol-% beträgt,
• – die zeitliche Abfolge, in der das Reaktionsgasgemisch 3 die Temperaturzonen C, D durchströmt, der alphabetischen Abfolge der Temperaturzonen entspricht,
• – die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch 3 enthaltenen Acrolein ≥ 70 Nl Acrolein/l Festbettkatalysatorschüttung·h beträgt, und
• – die Differenz T^maxC – T^maxD, gebildet aus der höchsten Temperatur T^maxC, die das Reaktionsgasgemisch 3 innerhalb der Temperaturzone C aufweist, und der höchsten Temperatur T^maxD die das Reaktionsgasgemisch 3 innerhalb der Temperaturzone D aufweist, ≥ 0°C beträgt,

und das zusätzlich dadurch gekennzeichnet ist, dass der Übergang von der Temperaturzone C in die Temperaturzone D in der Festbettkatalysatorschüttung nicht mit einem Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone zusammenfällt.In other words, a very particularly preferred embodiment of the partial oxidation of the acrolein according to the invention to acrylic acid is a process according to the invention in which the starting reaction gas mixture 3 with the proviso of a fixed bed catalyst bed, the active material is at least one of the elements Mo and V-containing multimetal, leads that

• The fixed catalyst bed is arranged in two spatially successive temperature zones C, D,
• Both the temperature of the temperature zone C and the temperature of the temperature zone D is a temperature in the range of 230 to 320 ° C,
• - The fixed bed catalyst bed consists of at least two spatially sequential fixed bed catalyst bed zones, wherein the volume-specific activity is substantially constant within a fixed bed catalyst bed and in the flow direction of the reaction gas mixture 3 increases abruptly during the transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone,
• The temperature zone C extends to a conversion of the acrolein of 45 to 85 mol%,
• - With a single pass of the starting reaction gas mixture 3 the acrolein conversion is ≥90 mol% and the selectivity of the acrylic acid formation, based on reacted acrolein, is ≥90 mol% through the entire fixed-bed catalyst bed,
• - The time sequence in which the reaction gas mixture 3 the temperature zones C, D flows through, which corresponds to the alphabetical sequence of the temperature zones,
• - The burden of fixed bed catalyst bed with that in the starting reaction gas mixture 3 acrolein ≥ 70 Nl acrolein / l fixed bed catalyst bed · h, and
• - The difference T ^maxC - T ^maxD , formed from the highest temperature T ^maxC , the reaction ^gas mixture 3 within the temperature ^zone C, and the highest temperature T ^maxD the the reaction ^gas mixture 3 within the temperature zone D, ≥ 0 ° C,

and additionally characterized in that the transition from the temperature zone C to the temperature zone D in the fixed bed catalyst bed does not coincide with a transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone.

Details For this procedure can be found in WO 04/085370, the integral Part of this document is, as well as in the further course of this Scripture in the description of the most preferred two-stage Partial oxidation of propylene to acrylic acid.

Such a preferred two-stage partial oxidation of propylene to acrylic acid can advantageously be carried out as described in EP-A 1159248 and in WO 04/085367, but with the difference that the reaction gas starting mixture for the first oxidation stage (propylene to acrolein) is a reaction gas starting mixture according to the invention 2 is used (especially in the embodiments of EP-A 1159248 and WO 04/085367, both writings form an integral part of this document).

• a) die Belastung der Festbettkatalysatorschüttung 1 mit dem im Reaktionsgasausgangsgemisch 2 enthaltenen Propen > 160 Nl Propen/l Festbettkatalysatorschüttung 1·h beträgt,
• b) die erste Festbettkatalysatorschüttung aus einer in zwei räumlich aufeinanderfolgenden Reaktionszonen A*, B* angeordneten Festbettkatalysatorschüttung besteht, wobei die Temperatur der Reaktionszone A* 300 bis 390°C und die Temperatur der Reaktionszone B* 305 bis 420°C beträgt und gleichzeitig wenigstens 5°C oberhalb der Temperatur der Reaktionszone A* liegt,
• c) das Reaktionsgasausgangsgemisch 2 die Reaktionszonen A*, B* in der zeitlichen Abfolge „erst A*'', „dann B*'' durchströmt,
• d) die Reaktionszone A* sich bis zu einem Umsatz des Propens von 40 bis 80 mol-% erstreckt,
• e) die Belastung der Festbettkatalysatorschüttung 2 mit dem im Reaktionsgasausgangsgemisch 3 enthaltenen Acrolein ≥ 140 Nl Acrolein/l Festbettkatalysatorschüttung 2·h beträgt,
• f) die Festbettkatalysatorschüttung 2 aus einer in zwei räumlich aufeinanderfolgenden Reaktionszonen C*, D* angeordneten Festbettkatalysatorschüttung 2 besteht, wobei die Temperatur der Reaktionszone C* 230 bis 270°C und die Temperatur der Reaktionszone D* 250 bis 300°C beträgt und gleichzeitig wenigstens 10°C oberhalb der Reaktionszone C* liegt,
• g) das Reaktionsgasausgangsgemisch 3 die Rektionszonen C*, D* in der zeitlichen Abfolge „erst C*'', „dann D*'' durchströmt und
• h) sich die Reaktionszone C* bis zu einem Umsatz des Acroleins von 55 bis 85 mol-% erstreckt.

That is, it is first an inventive reaction gas starting mixture 2 in a first reaction stage with the proviso over (through) a fixed bed catalyst bed 1 , whose active material is at least one of the elements Mo, Fe and Bi containing multimetal lead, that the propylene conversion in a single pass 90 mol% and the concomitant selectivity of acrolein formation and Acrylsäurenebenproduktbildung taken together be ≥ 90 mol%, the temperature of the first reaction stage leaving product gas mixture 2 optionally reduce by indirect and / or direct cooling and the product gas mixture 2 optionally add molecular oxygen and / or inert gas, and thereafter as acrolein, molecular oxygen and at least one inert gas containing reaction gas starting mixture 3 containing the molecular oxygen and the acrolein in a molar ratio O ₂ : C ₃ H ₄ O ≥ 0.5, in a second reaction stage, with the proviso over (through) a fixed bed catalyst bed 2 , whose active material is at least one multimetal oxide containing molybdenum and vanadium, lead to the acrolein conversion in a single pass ≥ 90 mol% and the selectivity of the averaged over both reaction stages of acrylic acid formation, based on reacted propylene, ≥ 80 mol% and continue to do so proceed that way

• a) the load of the fixed bed catalyst bed 1 with the starting gas in the reaction mixture 2 propene> 160 Nl propene / l fixed-bed catalyst bed 1 × h,
• b) the first fixed-bed catalyst charge consists of a fixed-bed catalyst charge arranged in two spatially successive reaction zones A *, B *, the temperature of the reaction zone A * 300 to 390 ° C and the temperature of the reaction zone B * is 305 to 420 ° C and at the same time at least 5 ° C above the temperature of the reaction zone A *,
• c) the reaction gas starting mixture 2 flows through the reaction zones A *, B * in the time sequence "first A *", then "B *",
• d) the reaction zone A * extends to a conversion of the propene of 40 to 80 mol%,
• e) the load of the fixed bed catalyst bed 2 with the starting gas in the reaction mixture 3 acrolein ≥ 140 Nl acrolein / l fixed-bed catalyst bed 2 × h,
• f) the fixed bed catalyst bed 2 from a fixed bed catalyst bed arranged in two spatially successive reaction zones C *, D * 2 where the temperature of the reaction zone C * is 230 to 270 ° C and the temperature of the reaction zone D * is 250 to 300 ° C and at the same time at least 10 ° C above the reaction zone C *,
• g) the reaction gas starting mixture 3 the reaction zones C *, D * in the time sequence "first C *", then "D *" flows through and
• h) the reaction zone C * extends to a conversion of the acrolein of 55 to 85 mol%.

Otherwise, will to EP-A 1159248.

However, it is particularly preferred to carry them out according to WO 04/085369, which is an integral part of this document, but with the difference that the reaction gas starting mixture for the first oxidation stage (propylene to acrolein) is a reaction gas starting mixture according to the invention 2 is used (especially in the embodiments of WO 04/085369).

• – die Festbettkatalysatorschüttung 1 in zwei räumlich aufeinanderfolgenden Temperaturzonen A, B angeordnet ist,
• – sowohl die Temperatur der Temperaturzone A als auch die Temperatur der Temperaturzone B eine Temperatur im Bereich von 290 bis 380°C ist,
• – die Festbettkatalysatorschüttung 1 aus wenigstens zwei räumlich aufeinander folgenden Festbettkatalysatorschüttungszonen besteht, wobei die volumenspezifische Aktivität innerhalb einer Festbettkatalysatorschüttungszone im wesentlichen konstant ist und in Strömungsrichtung des Reaktionsgasgemisches 2 beim Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone sprunghaft zunimmt,
• – sich die Temperaturzone A bis zu einem Umsatz des Propens von 40 bis 80 mol-% erstreckt,
• – bei einmaligem Durchgang des Reaktionsgasausgangsgemisches 2 durch die gesamte Festbettkatalysatorschüttung 1 der Propenumsatz ≥ 90 mol-% und die Selektivität der Acroleinbildung sowie der Acrylsäurenebenproduktbildung zusammengenommen und bezogen auf umgesetztes Propen ≥ 90 mol-% beträgt,
• – die zeitliche Abfolge, in der das Reaktionsgasgemisch 2 die Temperaturzonen A, B durchströmt, der alphabetischen Abfolge der Temperaturzonen A, B entspricht,
• – die Belastung der Festbettkatalysatorschüttung 1 mit dem im Reaktionsgasausgangsgemisch 2 enthaltenen Propen ≥ 90 Nl Propen/l Festbettkatalysatorschüttung 1·h beträgt, und
• – die Differenz T^maxA – T^maxB, gebildet aus der höchsten Temperatur T^maxA, die das Reaktionsgasgemisch 2 innerhalb der Temperaturzone A aufweist, und der höchsten Temperatur T^maxB, die das Reaktionsgasgemisch 2 innerhalb der Temperaturzone B aufweist, ≥ 0°C beträgt, danach die Temperatur des die erste Reaktionsstufe verlassenden Produktgasgemisches 2 durch Kühlung gegebenenfalls verringern und dem Produktgasgemisch 2 gegebenenfalls molekularen Sauerstoff und/oder Inertgas, vorzugsweise gegebenenfalls Luft, zugeben, und es anschließend als Acrolein, molekularen Sauerstoff und wenigstens ein Inertgas enthaltendes Reaktionsgasausgangsgemisch 3, das den molekularen Sauerstoff und das Acrolein in einem molaren Verhältnis O₂:C₃H₄O ≥ 0,5 enthält, in einer zweiten Reaktionsstufe mit der Maßgabe über eine Festbettkatalysatorschüttung 2, deren Aktivmasse wenigstens ein die Elemente Mo und V enthaltendes Multimetalloxid ist, führen, dass
• – die Festbettkatalysatorschüttung 2 in zwei räumlich aufeinanderfolgenden Temperaturzonen C, D angeordnet ist,
• – sowohl die Temperatur der Temperaturzone C als auch die Temperatur der Temperaturzone D eine Temperatur im Bereich von 230 bis 320°C ist,
• – die Festbettkatalysatorschüttung 2 aus wenigstens zwei räumlich aufeinanderfolgenden Festbettkatalysatorschüttungszonen besteht, wobei die volumenspezifi sche Aktivität innerhalb einer Festbettkatalysatorschüttungszone im wesentlichen konstant ist und in Strömungsrichtung des Reaktionsgasgemisches 3 beim Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone sprunghaft zunimmt,
• – sich die Temperaturzone C bis zu einem Umsatz des Acroleins von 45 bis 85 mol-% erstreckt,
• – bei einmaligem Durchgang des Reaktionsgasausgangsgemisches 3 durch die gesamte Festbettkatalysatorschüttung 2 der Acroleinumsatz ≥ 90 mol-% und die Selektivität der Acrylsäurebildung, bezogen auf über beide Reaktionsstufen umgesetztes Propen, ≥ 80 mol-% beträgt,
• – die zeitliche Abfolge, in der das Reaktionsgasgemisch 3 die Temperaturzonen C, D durchströmt, der alphabetischen Abfolge der Temperaturzonen C, D entspricht,
• – die Belastung der Festbettkatalysatorschüttung 2 mit dem im Reaktionsgasausgangsgemisch 3 enthaltenen Acrolein ≥ 70 Nl Acrolein/l Festbettkatalysatorschüttung 2·h beträgt, und
• – die Differenz T^maxC – T^maxD gebildet aus der höchsten Temperatur T^maxC, die das Reaktionsgasgemisch 3 innerhalb der Temperaturzone C aufweist, und der höchsten Temperatur T^maxD, die das Reaktionsgasgemisch 3 innerhalb der Temperaturzone D aufweist, ≥ 0°C beträgt,

mit der Maßgabe, dass das Verfahren zusätzlich dadurch gekennzeichnet ist, dass weder der Übergang von der Temperaturzone A in die Temperaturzone B in der Festbettkatalysatorschüttung 1, noch der Übergang von der Temperaturzone C in die Temperaturzone D in der Festbettkatalysatorschüttung 2 mit einem Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone zusammenfällt.D. h., It is first an inventive reaction gas starting mixture 2 in a first reaction stage with the proviso over a fixed bed catalyst bed 1 , whose active material is at least one containing the elements Mo, Fe and Bi multimetal, lead that

• - The fixed bed catalyst bed 1 is arranged in two spatially successive temperature zones A, B,
• Both the temperature of the temperature zone A and the temperature of the temperature zone B is a temperature in the range from 290 to 380 ° C,
• - The fixed bed catalyst bed 1 consists of at least two spatially sequential fixed-bed catalyst bed zones, wherein the volume-specific activity within a fixed-bed catalyst bed zone is substantially constant and in the flow direction of the reaction gas mixture 2 increases abruptly during the transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone,
• The temperature zone A extends to a conversion of the propene of 40 to 80 mol%,
• - With a single pass of the starting reaction gas mixture 2 through the entire fixed bed catalyst bed 1 the propene conversion is ≥90 mol% and the selectivity of the formation of acrolein and the by-product of acrylic acid by-product taken together is ≥90 mol%, based on the amount of propene converted,
• - The time sequence in which the reaction gas mixture 2 the temperature zones A, B flows through, corresponds to the alphabetical sequence of the temperature zones A, B,
• - The burden of fixed bed catalyst bed 1 with the starting gas in the reaction mixture 2 Propene ≥ 90 Nl propene / l fixed-bed catalyst bed 1 · H is, and
• - The difference T ^maxA - T ^maxB , formed from the highest temperature T ^maxA , the reaction ^gas mixture 2 within the temperature ^zone A, and the highest temperature T ^maxB containing the reaction ^gas mixture 2 within the temperature zone B, ≥ 0 ° C, then the temperature of the first reaction stage leaving the product gas mixture 2 optionally reduce by cooling and the product gas mixture 2 optionally molecular oxygen and / or inert gas, preferably optionally air, and then adding it as acrolein, molecular oxygen and at least one inert gas containing reaction gas starting mixture 3 containing the molecular oxygen and the acrolein in a molar ratio of O ₂ : C ₃ H ₄ O ≥ 0.5, in a second reaction stage, as determined by a fixed bed catalyst bed 2 , whose active material is at least one multimetal oxide containing the elements Mo and V, lead that
• - The fixed bed catalyst bed 2 is arranged in two spatially successive temperature zones C, D,
• Both the temperature of the temperature zone C and the temperature of the temperature zone D is a temperature in the range of 230 to 320 ° C,
• - The fixed bed catalyst bed 2 consists of at least two spatially sequential fixed bed catalyst bed zones, the volume-specific activity within a fixed bed catalyst bed zone being substantially constant and in the flow direction of the reaction gas mixture 3 in the transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone increases dramatically,
• The temperature zone C extends to a conversion of the acrolein of 45 to 85 mol%,
• - With a single pass of the starting reaction gas mixture 3 through the entire fixed bed catalyst bed 2 the acrolein conversion is ≥90 mol% and the selectivity of the acrylic acid formation, based on propene reacted over both reaction stages, is ≥80 mol%,
• - The time sequence in which the reaction gas mixture 3 the temperature zones C, D flows through, corresponds to the alphabetical sequence of the temperature zones C, D,
• - The burden of fixed bed catalyst bed 2 with the starting gas in the reaction mixture 3 acrolein ≥ 70 Nl acrolein / l fixed-bed catalyst bed 2 × h, and
• - The difference T ^maxC - T ^maxD formed from the highest temperature T ^maxC , which is the reaction ^gas mixture 3 within the temperature ^zone C, and the highest temperature T ^maxD , which is the reaction ^gas mixture 3 within the temperature zone D, ≥ 0 ° C,

with the proviso that the method is additionally characterized in that neither the transition from the temperature zone A in the temperature zone B in the fixed bed catalyst bed 1 , nor the transition from the temperature zone C in the temperature zone D in the fixed-bed catalyst bed 2 coincides with a transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone.

Under The temperature of a temperature zone is the temperature of the in the temperature zone located part of the fixed bed catalyst bed at exercise of the method according to the invention, but understood in the absence of a chemical reaction. is this temperature is not constant within the temperature zone, so The term temperature of a temperature zone here means the (numbers) mean value the temperature of the fixed catalyst bed along the reaction zone. Essential is that the temperature of the individual temperature zones in essentially independent from each other.

Since both the heterogeneously catalyzed partial gas phase oxidation of propene to acrolein and the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid is a pronounced exothermic reaction, both the temperature of the reaction gas mixture 2 as well as the temperature of the reaction gas mixture 3 during the reactive passage through the fixed-bed catalyst bed 1 or Festbettkata lysatorschüttung 2 usually different from the temperature of a temperature zone. It is usually above the temperature of the temperature zone and usually passes through a maximum (hotspot maximum) within a temperature zone or decreases starting from a maximum value.

As a rule, in the method ^{according to} the invention the difference T ^maxA - T ^{maxB will} not be more than 80 ° C. According to the invention, T ^maxA -T ^maxB ≥ 3 ° C. and ^≦ 70 ° C. is preferred. ^With very particular preference, T ^maxA -T ^maxB in the process ^{according to} the invention is ≥ 20 ° C. and ≦ 60 ° C.

The inventively required differences T ^maxA - T ^maxB are in the practice of the method ^according to the invention in the case of rather low (≥ 90 Nl / l · h and ≤ 160 Nl / l · h) Propenbelastungen the fixed bed catalyst bed 1 normally when on the one hand both the temperature of the reaction zone A and the temperature of the reaction zone B is in the range of 290 to 380 ° C and on the other hand the difference between the temperature of the reaction zone B (T _B ) and the temperature of the reaction zone A (T _A ), ie, T _B - T _A , ≤ 0 ° C and ≥ -20 ° C or ≥ -10 ° C or ≤ 0 ° C and ≥ -5 ° C, or frequently ≤ 0 ° C and ≥ -3 ° C is.

When the process according to the invention is carried out under increased propene loadings (≥ 160 Nl / l · h and ≤ 300 Nl / l · h, or ≤ 600 Nl / l · h, respectively), the differences T ^maxA -T ^maxB required ^according to the invention ^arise normally when on the one hand both the temperature of the reaction zone A and the temperature of the reaction zone B in the range of 290 to 380 ° C and T _B - T _A ≥ 0 ° C and ≤ 50 ° C, or ≥ 5 ° C and ≤ 45 ° C, or ≥ 10 ° C and ≤ 40 ° C, or ≥ 15 ° C and ≤ 30 ° C or ≤ 35 ° C (eg 20 ° C or 25 ° C).

The aforementioned statement regarding the temperature differences T _B - T _A applies regularly even if the temperature of the reaction zone A in the preferred range of 305 to 365 ° C or in the most preferred range of 310 to 340 ° C.

The propene loading of the fixed bed catalyst bed 1 Thus, in the described process, for example, 90 Nl / l · h and ≦ 300 Nl / l · h, or ≥ 110 Nl / l · h and ≦ 280 Nl / l · h or ≥ 130 Nl / l · h and ≦ 260 Nl / lh, or ≥ 150 Nl / l · hr and ≤ 240 Nl / l · hr, or ≥ 170 Nl / l · h and ≤ 220 Nl / l · h, or ≥ 190 Nl / l · h and ≤ 200 Nl / l · h.

According to the invention, the temperature zone A preferably extends to a conversion of the propene from 50 to 70 mol% and 60 to 70 mol%, respectively.

As a rule, in the method ^{according to} the invention the difference T ^maxC - T ^{maxD will} not be more than 75 ° C. According to the invention, T ^maxC - T ^maxD ≥ 3 ° C and ≤ 60 ° C. ^With very particular preference T ^maxC - T ^maxD in the process ^{according to} the invention is ≥ 5 ° C and ≤ 40 ° C.

The inventively required differences T ^maxC - T ^maxD are in the practice of the inventive method in the case of rather low (≥70 Nl / l · h and ≤ 150 Nl / l · h) Acroleinbelastungen the fixed bed catalyst bed 2 normally when on the one hand both the temperature of the reaction zone C and the temperature of the reaction zone D is in the range of 230 to 320 ° C and on the other hand, the difference between the temperature of the reaction zone D (T _D ) and the temperature of the reaction zone C (T. _C ), ie, T _D -T _C , ≤ 0 ° C and ≥ -20 ° C or ≥ -10 ° C or ≤ 0 ° C and ≥ -5 ° C, or frequently ≤ 0 ° C and ≥ -3 ° C is.

When the process according to the invention is carried out under increased propene loadings and thus simultaneously increased acrolein loadings (≥ 150 Nl / l ·h and ≦ 300 Nl / l ·h, or ≦ 600 Nl / l ·), the differences T ^maxC -T ^maxD required ^according to the invention ^arise normally when on the one hand both the temperature of the reaction zone C and the temperature of the reaction zone D in the range of 230 to 320 ° C and T _D - T _C ≥ 0 ° C and ≤ 40 ° C, or ≥ 5 ° C and ≤ 35 ° C, or 30 ° C, or ≥ 10 ° C and ≤ 25 ° C, or ≤ 20 ° C, or ≤ 15 ° C.

The aforementioned statement regarding the temperature differences T _C - T _C applies regularly even if the temperature of the reaction zone C in the preferred range of 250 to 300 ° C or in the most preferred range of 260 to 280 ° C.

The acrolein loading of the packed catalyst bed 2 can thus z. ≥ 70 Nl / l · or ≥ 90 Nl / l · h and ≤ 300 Nl / l · h, or ≥ 110 Nl / l · and ≤ 280 Nl / l · or ≥ 130 Nl / l · h and ≤ 260 Nl / l ·, or ≥ 150 Nl / l · h and ≤ 240 Nl / l ·, or ≥ 170 Nl / l · and ≤ 220 Nl / l · h, or ≥ 190 Nl / l · h and ≤ 200 Nl / l · amount.

According to the invention preferred The temperature zone C extends to a conversion of the acrolein of 50 to 85 mol% or 60 to 85 mol%.

Of the Working pressure can be in both reaction stages both below from normal pressure (eg up to 0.5 bar) as well as above atmospheric pressure lie. Typically, the working pressure in the two reaction stages at values of 1 to 5 bar, often 1 to 3 bar.

Usually the reaction pressure in any of the two reaction stages 100 bar.

As a rule, it is based on the simple passage of the fixed-bed catalyst bed 1 relative propene conversion in the procedure described ≥ 92 mol% or 94 mol%. The selectivity of the desired product formation (sum of acrolein formation and Acrylsäurenebenproduktbildung) is in a conventional manner suitable choice (see catalysts recommended in this document) of fixed bed catalyst bed 1 regularly ≥ 92 mol%, or ≥ 94 mol%, often ≥ 95 mol%, or ≥ 96 mol% or ≥ 97 mol%.

In general, in the process described above, the acrolein loading of the fixed bed catalyst bed 2 furthermore, about 10 Nl / l · h, often about 20 or 25 Nl / l · h below the propene loading of the fixed bed catalyst bed 1 lie. This is primarily due to the fact that in the first reaction stage both the propane conversion and the selectivity of the acrolein formation generally do not reach 100%.

As a rule, it is based on the simple passage of the fixed-bed catalyst bed 2 relative acrolein conversion in the process described above ≥ 92 mol%, or ≥ 94 mol%, or ≥ 96 mol%, or ≥ 98 mol% and often even ≥ 99 mol% or more.

In a conventional manner suitable choice of Festbettkatalysatorschüttungen 1 and 2 (see the catalyst recommendations given in this document), the selectivity of acrylic acid formation, based on reacted propene, in the procedure described above over both reaction stages, is ≥83 mol%, frequently ≥85 mol%, or ≥88 mol%. %, often at ≥90 mol%, or ≥93 mol%.

For the procedure described is essential that the fixed-bed catalyst bed 1 consists of at least two spatially sequential fixed-bed catalyst bed zones, wherein the volume-specific activity within a fixed-bed catalyst bed zone is substantially constant and in the flow direction of the reaction gas mixture 1 during the transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone increases rapidly.

The volume specific (i.e., based on the unit of the respective bulk volume normalized) activity a fixed bed catalyst bed zone can now in about the fixed bed catalyst bed zone be adjusted in a substantially constant manner that one starts from a basic amount of uniformly prepared catalyst molding (her fill corresponds to the maximum achievable volume-specific activity) and these in the respective fixed bed catalyst bed zone with respect to the heterogeneously catalyzed partial gas phase oxidation substantially inertly behaving moldings (Dilution molding) homogeneous diluted. The higher the proportion of dilution molded bodies is selected the lower is the volume contained in a certain volume of the bed Active mass or catalyst activity. As materials for such come inert diluent molding in principle, all those considered as carrier material suitable for the invention Own shell catalysts.

When such materials come z. B. porous or nonporous aluminas, Silica, thoria, zirconia, silicon carbide, silicates such as magnesium or aluminum silicate or the already mentioned steatite (eg Steatite C-220 from Ceram Tec).

The Geometry of such inert diluent bodies can in principle be arbitrary. That is, for example, balls, Polygons, solid cylinders or rings. According to the invention preferred Will be as inert diluent such choose, whose geometry corresponds to that of the catalyst molding to be diluted with them.

According to the invention, it is advantageous if the chemical composition of the active composition used over the entire fixed bed catalyst bed 1 not changed. That is to say, the active composition used for a single shaped catalyst body may indeed be a mixture of different multimetal oxides containing the elements Mo, Fe and Bi, for all catalyst bodies of the fixed bed catalyst bed 1 but then the same mixture is to be used.

One in the flow direction of the reaction gas mixture 2 zone-wise increasing volume-specific activity can thus be achieved in a simple manner for the described process by way of the fixed catalyst bed. B. adjust by starting the bed in a first fixed bed catalyst bed zone with a high proportion of inert diluent with respect to a variety of shaped catalyst bodies, and then reduces this proportion of Verdünnungsformkörpern in the flow direction zones.

A However, there is a zonal increase in volume-specific activity also z. B. thereby possible that one at a constant geometry and Aktivmassenart a shell catalyst molding zonewise the thickness of the on the support applied active mass layer increased or in a mixture of coated catalysts with the same geometry but with different weights the active mass zone by piece the proportion of catalyst bodies with higher Active mass fraction increases. Alternatively, you can also the active masses even dilute, in which one in the active mass production z. B. in the calcined Dry mixture of starting compounds inert diluent acting Materials such as burned-in silica incorporated. different Additional amounts of diluting acting material lead automatically to different activities. The more diluting active material is added, the lower the resulting activity be. Analogous effect leaves z. B. also achieve that in mixtures of solid catalysts and of shell catalysts (with identical active material) in a corresponding Way the mixing ratio changed. Furthermore, lets a variation of the volume-specific activity the use of catalyst geometries with different bulk density (e.g., in full catalysts having identical active composition the different geometries). Needless to say, the described Apply variants also in combination.

Of course, for the fixed bed catalyst bed 1 but also mixtures of catalysts with chemically different active composition and as a result of this different composition of different activity can be used. In turn, these mixtures can be varied in their composition in zones and / or diluted with different amounts of inert shaped diluent bodies.

In advance and / or after the fixed catalyst bed 1 may consist exclusively of inert material (for example, only shaped diluent bodies) of found beds (in this document, they do not become the fixed bed catalyst charge in conceptual terms 1 attributed, since they contain no moldings having multimetal oxide active material). The shaped bodies used for the Inertmaterialschüttung the same geometry as in the fixed bed catalyst bed 1 having used catalyst bodies. However, the geometry of the shaped bodies used for the Inertmaterialschüttung may also be different from the aforementioned geometry of the shaped catalyst body (eg, spherical instead of annular).

Often point the for Such Inertmaterialschüttungen used moldings the ring-shaped Geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) or the spherical one Geometry with the diameter d = 4 - 5 mm. The temperature zones A and B can in the method according to the invention also on the Inertmaterialschüttungen extend. According to the invention advantageous capture both the temperature zone A and the temperature zone B each no more than three fixed bed catalyst bed zones (mandatory according to the invention becomes at least one fixed bed catalyst bed zone of both temperature zones detected).

Particularly according to the invention Advantageously, the entire fixed bed catalyst bed does not comprise more than five, not functional more than four or three fixed bed catalyst bed zones.

In the transition from a fixed bed catalyst bed zone into another fixed bed catalyst bed zone (in the flow direction of the reaction gas mixture 2 ) of the fixed bed catalyst bed 1 should (with a uniform active mass over the entire fixed-bed catalyst bed 1 ) the volume-specific active composition (ie, the weight of the multimetal oxide active mass contained in the unit bulk volume) expediently increases according to the invention by at least 5% by weight, preferably by at least 10% by weight (this also applies in particular to uniform shaped catalyst bodies over the entire fixed bed catalyst charge 1 ). As a rule, this increase in the process according to the invention will not be more than 50% by weight, usually not more than 40% by weight. Furthermore, with a uniform active mass over the entire fixed-bed catalyst bed 1 the difference in the volume-specific active mass of that fixed-bed catalyst bed zone having the lowest volume-specific activity and that fixed bed catalyst bed zone having the highest volume-specific activity according to the invention not more than 50 wt .-%, preferably not more than 40 wt .-% and usually not more than 30 wt. -%.

Frequently in the process according to the invention, the fixed bed catalyst bed 1 consist of only two fixed bed catalyst bed zones.

According to the invention, preference is given in the flow direction of the reaction gas mixture 2 last fixed bed catalyst bed zone of the fixed bed catalyst bed 1 undiluted. D. h., It consists preferably exclusively of shaped catalyst bodies. If necessary, it can also consist of a bed of shaped catalyst bodies whose volume-specific activity z. B. lowered by dilution with inert material, for. B. by 10%, there is the fixed bed catalyst bed 1 only from two fixed bed catalyst bed zones, it is generally advantageous according to the invention (as in the process according to the invention) if the fixed bed catalyst bed zone with the highest volume-specific activity does not protrude into the temperature zone A (especially if in the temperature zone A and in the temperature zone B) the temperature is controlled by means of a flowing heat carrier, which flows in countercurrent to the reaction gas mixture). That is, conveniently, the fixed bed catalyst bed zone having the lower volume specific activity will protrude into the temperature zone B and begin and end the fixed bed catalyst bed zone having the higher volume specific activity in the temperature zone B. (ie, start behind the transition from temperature zone A to temperature zone B.

Is there the fixed bed catalyst bed 1 only from three fixed bed catalyst bed zones, it is inventively also usually advantageous if the fixed bed catalyst bed zone with the higher volume specific activity does not protrude into the temperature zone A but begins and ends in the temperature zone B, ie, behind the transition from the temperature zone A in the temperature zone B has its beginning (especially when in the temperature zone A and in the temperature zone B, the temperature control by means of a flowing heat carrier takes place, each of which flows in countercurrent to the reaction gas mixture). That is, normally, in this case, the fixed bed catalyst bed zone having the second highest volume specific activity will protrude into both the temperature zone A and the temperature zone B.

Is there the fixed bed catalyst bed 1 from four fixed-bed catalyst bed zones, it is erfin According to the invention, it is generally advantageous if the fixed-bed catalyst bed zone with the third highest volume-specific activity protrudes both into the temperature zone A and into the temperature zone B (in particular if the temperature control takes place in the temperature zone A and in the temperature zone B by means of a flowing heat carrier) in countercurrent to the reaction gas mixture 2 flows).

In the case of a DC flow of reaction gas mixture and heat carriers in the temperature zones A and B, it may be advantageous if in the process according to the invention within the fixed-bed catalyst bed 1 the fixed-bed catalyst bed zone with the maximum volume-specific activity protrudes into the temperature zone A.

In general, the volume-specific activity between two fixed-bed catalyst bed zones of a fixed-bed catalyst bed can be determined 1 experimentally differentiate in a simple manner so that under identical boundary conditions (preferably the conditions of the contemplated method) on Festbettkatalysatorschüttungen the same length, but in each case the composition of the respective fixed bed catalyst bed corresponding to the same propene-containing reaction gas mixture is performed. The higher amount of propene reacted has the higher volume specific activity.

bzw. im Bereich

bzw. im Bereich

kein Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone befindet, wobei X der Ort (die Stelle) innerhalb der Festbettkatalysatorschüttung 1 ist, an dem der Übergang von der Temperaturzone A in die Temperaturzone B erfolgt.If the total length of the fixed-bed catalyst charge is 1 L ¹ , it is advantageous according to the invention if it is within the range

or in the area

or in the area

there is no transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone where X is the location within the fixed bed catalyst bed 1 is where the transition from the temperature zone A to the temperature zone B takes place.

In the method described above, preference is given to the fixed bed catalyst bed 1 in the flow direction of the reaction gas mixture 2 structured as follows.

Initially at a length of 10 to 60%, preferably 10 to 50%, particularly preferably 20 to 40% and very particularly preferably 25 to 35% (ie, for example, over a length of 0.70 to 1.50 m, preferably 0, 90 to 1.20 m), in each case the total length of the fixed bed catalyst bed 1 , a homogeneous mixture of shaped catalyst bodies and shaped diluents (both preferably having essentially the same geometry), the proportion by weight of the diluent bodies (the mass densities of shaped catalyst bodies and diluent tablets generally differing only slightly), normally from 5 to 40% by weight, or 10 to 40% by weight, or 20 to 40% by weight, or 25 to 35% by weight. Following this first zone of the fixed-bed catalyst charge, it is then advantageous to either end the length of the fixed bed catalyst charge (ie, for example, over a length of 2.00 to 3.00 m, preferably 2.50 to 3.00 m) a bed of the shaped catalyst bodies diluted only to a lesser extent (than in the first zone), or, with very particular preference, a sole (undiluted) bed of the same shaped catalyst bodies which have also been used in the first zone. The abovementioned is particularly true when in the fixed-bed catalyst bed solid catalyst rings or coated catalyst rings (in particular those which are mentioned as being preferred in this document) are used as shaped catalyst bodies. In the context of the aforementioned structuring, both the shaped catalyst bodies and the shaped diluent bodies in the method according to the invention advantageously have essentially the ring geometry 5 mm.times.3 mm.times.2 mm (outer diameter × length × inner diameter).

The abovementioned also applies if, instead of inert dilution molded bodies, shell catalyst molded bodies are used whose active mass fraction is lower by 2 to 15% by weight than the active mass fraction at the end of the fixed bed catalyst bed 1 optionally used coated catalyst molding.

A pure Inertmaterialschüttung, whose length, based on the length of the fixed bed catalyst bed 1 , Expediently 5 to 20%, passes in the flow direction of the reaction gas mixture 2 usually the fixed bed catalyst bed 1 one. It is usually used as a heating zone for the reaction gas mixture 2 used. Instead of the bed of inert material, a catalyst fixed bed diluted with inert material can also be used as the heating zone.

Advantageously according to the invention now extends to the aforementioned fixed-bed catalyst beds 1 the fixed-bed catalyst bed zone with the lower volume-specific activity still at 5 to 20%, often to 5 to 15% of its length in the temperature zone B.

Suitably, the temperature zone A also extends to a fixed bed catalyst bed 1 optionally used bed of inert material.

For the advantage of the procedure described is also essential that the fixed-bed catalyst bed 2 consists of at least two spatially sequential fixed-bed catalyst bed zones, wherein the volume-specific activity within a fixed-bed catalyst bed zone is substantially constant and in the flow direction of the reaction gas mixture 3 during the transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone increases rapidly.

The volume-specific (that is, the unit of the respective bulk volume normalized) activity a fixed bed catalyst bed zone can now in about the fixed bed catalyst bed zone be adjusted in a substantially constant manner that one starts from a basic amount of uniformly prepared catalyst molding (her fill corresponds to the maximum achievable volume-specific activity) and these in the respective fixed bed catalyst bed zone with respect to the heterogeneously catalyzed partial gas phase oxidation substantially inertly behaving moldings (Dilution molding) homogeneous diluted. The higher the proportion of dilution molded bodies is selected the lower is the volume contained in a certain volume of the bed Active mass or catalyst activity. As materials for such come inert diluent molding in principle, all those considered as carrier material suitable for the invention Shell catalysts are suitable.

When such materials come z. B. porous or nonporous aluminas, Silica, thoria, zirconia, silicon carbide, silicates such as magnesium or aluminum silicate or the already mentioned steatite (eg steatite C-220 from Cerm Tec).

The Geometry of such inert diluent bodies can in principle be arbitrary. That is, for example, balls, Polygons, solid cylinders or rings. According to the invention preferred Will be as inert diluent such choose, whose geometry corresponds to that of the catalyst molding to be diluted with them.

According to the invention, it is advantageous if the chemical composition of the active composition used over the entire fixed bed catalyst bed 2 not changed. That is, the active material used for a single catalyst molding may indeed be a mixture of different, containing the elements Mo and V, Multime talloxiden be, for all catalyst bodies of fixed bed catalyst bed 2 but then the same mixture is to be used.

One in the flow direction of the reaction gas mixture 3 over the fixed bed catalyst bed 2 Zone-wise increasing volume-specific activity can thus be in a simple manner for the described method z. B. set by starting the bed in a first fixed bed catalyst bed zone with a high proportion of inert diluent moldings based on a variety of shaped catalyst bodies, and then reduces this proportion of diluent bodies in the flow direction zones.

A zonal increase in volume-specific activity is also z. Example, by the fact that the thickness of the applied on the support active mass layer increases or increases in a mixture of shell catalysts with the same geometry but with different weight fraction of the active composition zone by zone the proportion of catalyst moldings with higher active mass fraction in a constant geometry and Aktivmassenart a shell catalyst. Alternatively, one can also dilute the active compounds themselves, in which one in the active mass production z. B. incorporated into the calcined dry mixture of starting compounds inert diluting materials such as burned silica. Different additional amounts of diluting material automatically lead to different activities. The more diluting material is added, the lower will be the resulting activity. Analogous effect can be z. B. also achieve that changes in mixtures of solid catalysts and coated catalysts (with identical active material) in a corresponding manner, the mixing ratio. Furthermore, it is possible to achieve a variation of the volume-specific activity by using catalyst geometries with different bulk density (for example in the case of unsupported catalysts with identical active-mass composition of the different geometries). Needless to say, the variants described can also be used in combination.

Of course, for the fixed bed catalyst bed 2 but also mixtures of catalysts with chemically different active composition and as a result of this different composition of different activity can be used. In turn, these mixtures can be varied in their composition in zones and / or diluted with different amounts of inert shaped diluent bodies.

In advance and / or after the fixed catalyst bed 2 may consist exclusively of inert material (for example, only shaped diluent bodies) of found beds (in this document, they do not become the fixed bed catalyst charge in conceptual terms 2 attributed, as they contain no moldings having multimetal oxide active mass). In this case, the diluent tablets used for the inert material bed may have the same geometry as the diluent tablets used in the fixed catalyst bed. The geometry of the shaped bodies used for the Inertmaterialschüttung but can also be different from the aforementioned geometry of the shaped catalyst body (eg, spherical instead of annular).

Often point the for Such Inertmaterialschüttungen used moldings the ring-shaped Geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) or the spherical one Geometry with the diameter d = 4 - 5 mm. The temperature zone C and D can in the method according to the invention also on the Inertmaterialschüttungen extend. According to the invention advantageous capture both the temperature zone C and the temperature zone D no more than three fixed bed catalyst bed zones each (mandatory according to the invention becomes at least one fixed bed catalyst bed zone of both temperature zones detected).

According to the invention, the entire fixed bed catalyst charge is particularly advantageous 2 not more than five, suitably not more than four or three fixed bed catalyst bed zones.

In the transition from a fixed bed catalyst bed zone into another fixed bed catalyst bed zone (in the flow direction of the reaction gas mixture 3 ) should (with a uniform active mass over the entire fixed-bed catalyst bed 2 ) the volume-specific active composition (ie, the weight of the multimetal oxide active mass contained in the unit bulk volume) expediently increases according to the invention by at least 5% by weight, preferably by at least 10% by weight (this also applies in particular to uniform shaped catalyst bodies over the entire fixed bed catalyst charge 2 ). As a rule, this increase in the process according to the invention will not be more than 50% by weight, usually not more than 40% by weight. Furthermore, with a uniform active mass over the entire fixed-bed catalyst bed 2 the difference in the volume-specific active mass of the fixed-bed catalyst bed zone having the lowest volume-specific activity and that fixed bed catalyst bed zone having the highest volume-specific activity according to the invention not more than 50 wt .-%, preferably not more than 40 wt .-% and particularly preferably not more than 30 wt. %.

Frequently in the process according to the invention, the fixed bed catalyst bed 2 consist of only two fixed bed catalyst bed zones.

According to the invention, preference is given in the flow direction of the reaction gas mixture 3 last fixed bed catalyst bed zone of the fixed bed catalyst bed 2 undiluted. D. h., It consists preferably exclusively of shaped catalyst bodies. If necessary, it can also consist of a bed of shaped catalyst bodies whose volume-specific activity z. B. lowered by dilution with inert material, for. B. by 10%, is. Is there the fixed bed catalyst bed 2 only from two fixed bed catalyst bed zones, it is inventively usually advantageous (as in the process of the invention in general), when the fixed bed catalyst bed zone with the highest volume specific activity extends into the temperature zone C (especially if in the temperature zone C and in the temperature zone D the Tempering by means of a flowing heat carrier takes place, each in countercurrent to the reaction gas mixture 3 flows).

Is there the fixed bed catalyst bed 2 only from three fixed-bed catalyst bed zones, it is also generally advantageous according to the invention if the fixed-bed catalyst bed zone with the highest volume-specific activity projects into the temperature zone C (in particular if the temperature is controlled in the temperature zone C and in the temperature zone D by means of a flowing heat carrier, in each case in countercurrent to the reaction gas mixture 3 flows).

Is there the fixed bed catalyst bed 2 From four fixed bed catalyst bed zones, it is inventively generally advantageous if the fixed bed catalyst bed zone protrudes with the second highest volume specific activity in both the temperature zone C and in the temperature zone D (especially if in the temperature zone C and in the temperature zone D, the temperature by means of a flowing heat carrier takes place, each in countercurrent to the reaction gas mixture 3 flows.

In the case of a DC flow of reaction gas mixture 3 and heat carriers in the temperature zones C and D, it may be advantageous according to the invention, if within the fixed bed catalyst bed 2 the fixed bed catalyst bed zone with the highest volume-specific activity does not protrude into the temperature zone C, but only begins after the transition from the temperature zone C into the temperature zone D.

The volume-specific activity between two fixed bed catalyst bed zones within the fixed bed catalyst bed 2 can be differentiated experimentally in a simple manner so that under identical boundary conditions (preferably the conditions of the envisaged method) on Festbettkatalysatorschüttungen the same length, but in each case the composition of the respective fixed bed catalyst bed corresponding zone, the same acrolein containing reaction gas starting mixture is performed. The higher amount of acrolein reacted has the higher volume specific activity.

bzw. im Bereich

bzw. im Bereich

kein Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone befindet, wobei X der Ort innerhalb der Festbettkatalysatorschüttung 2 ist, an dem der Übergang von der Temperaturzone A in die Temperaturzone D erfolgt.Is the total length of the fixed bed catalyst bed 2 L ² , it is inventively advantageous when in the area

or in the area

or in the area

there is no transition from one fixed bed catalyst bed zone to another fixed bed catalyst bed zone, where X is the location within the fixed bed catalyst bed 2 is at which the transition from the temperature zone A to the temperature zone D takes place.

In the method described above, preference is given to the fixed bed catalyst bed 2 in the flow direction of the reaction gas mixture 3 structured as follows.

Initially at a length of 10 to 60%, preferably 10 to 50%, particularly preferably 20 to 40% and very particularly preferably 25 to 35% (ie for a length of 0.70 to 1.50 m, for example, preferably 0.90 to 1.20 m), in each case the total length of the fixed bed catalyst charge 2 , a homogeneous mixture or two (with decreasing dilution) successive homogeneous mixtures of shaped catalyst bodies and Verdünnungsformkörpern (both preferably have substantially the same geometry), wherein the proportion of the shaped diluent is such that the volume-specific active composition, based on a only the catalyst moldings existing bed, by 10 to 50 wt .-%, preferably 20 to 45 wt .-% and particularly preferably 25 to 35 wt .-% is lowered. Following this first or at these two first zones is then inventively advantageous until the end of the length of the fixed bed catalyst bed 2 (ie, for a length of 2.00 to 3.00 m, preferably 2.50 to 3.00 m) either only to a lesser extent (than in the first or in the first two zones) diluted Batch of the catalyst bodies, or, most preferably, a sole bed of the same shaped catalyst bodies, which have also been used in the first zones.

The above is particularly true when in the fixed bed catalyst bed 2 shell catalyst rings or shell catalyst spheres are used as catalyst shaped bodies (in particular those which are mentioned as preferred in this document). In the context of the aforementioned structuring, both the catalyst or the carrier rings and the shaped diluent bodies in the process according to the invention advantageously have essentially the ring geometry of 7 mm.times.3 mm.times.4 mm (external diameter.times.length.times.indical diameter).

The The above also applies if, instead of inert dilution molded bodies, coated catalyst tablets are used whose active mass fraction by 2 to 15 wt .-% points deeper is, as the active material content of the shell catalyst moldings on End of the fixed bed catalyst bed.

A pure Inertmaterialschüttung, whose length, based on the length of the fixed bed catalyst bed 2 , Expediently 5 to 20%, passes in the flow direction of the reaction gas mixture 3 usually the fixed bed catalyst bed 2 one. It normally serves the purpose of tempering the reaction gas mixture 3 , Instead of the bed of inert material, a catalyst fixed bed diluted with inert material can also be used as the heating zone.

Advantageously according to the invention now extends to the aforementioned fixed-bed catalyst beds 2 the temperature zone C (which according to the invention also advantageously extends to the feedstock of inert material) is still 5 to 20%, frequently 5 to 15% of the length of the flow direction of the reaction gas mixture 3 last (volume-specific most active) fixed bed catalyst bed zone of the fixed bed catalyst bed 2 ,

In in terms of application technology appropriate manner the implementation takes place the first reaction stage of the method described above in a two-zone tube bundle reactor, as he z. In DE-A's 199 10,508, 199 48 523, 199 10 506 and 199 48 241. A preferred variant of an inventively employable two-zone tube bundle reactor DE-C 28 30 765. But also in DE-C 25 13 405, the US-A 3,147,084, DE-A 22 01 528, EP-A 38 32 24 and DE-A 29 03 218 disclosed two-zone tube bundle reactors are for a implementation of the first reaction stage of the method described above.

D. h., In the simplest way is to be used fixed bed catalyst bed 1 (possibly with upstream and / or downstream Inertmaterialschüttungen) in the metal tubes of a tube bundle reactor and the metal tubes are two mutually substantially spatially separate tempering, usually molten salt, out. The pipe section, over which the respective salt bath extends, represents according to the invention a temperature zone. D. h., In the simplest way flows around z. B. a salt bath A that portion of the tubes (the temperature zone A), in which the oxidative conversion of the propene (in single pass) to reach a conversion in the range of 40 to 80 mol% and a salt bath B flows around the section the tubes (the reaction zone B), in which the oxidative terminal conversion of the propene (in single pass) takes place until a conversion value of at least 90 mol% is reached (if necessary, further reaction zones can follow the temperature zones A, B to be used according to the invention), which are kept at individual temperatures).

application point expediently includes the first reaction stage of the method described no further Temperature zones. D. h., The salt bath B flows around the appropriate section of the tubes, in which the oxidative Anschlussumset tion of Propens (in single pass) up to a conversion value ≥ 90 mol%, or ≥ 92 mol% or ≥ 94 mol% or more.

Usually the beginning of the temperature zone B is behind the hotspot maximum the temperature zone A.

The two salt baths A, B can relative to the invention to the flow direction of the flowing through the reaction tubes Reaction gas mixture in cocurrent or in countercurrent through the the reaction tubes surrounding space are performed.

Of course you can also in accordance with the invention the temperature zone A is a direct flow and in the temperature zone B is a counterflow (or vice versa).

Of course, in all the above case constellations within the respective temperature zone of the, taking place relative to the reaction tubes, parallel flow of the molten salt one more Superimpose cross flow, so that the individual reaction zone corresponds to a tube bundle reactor as described in EP-A 700 714 or in EP-A 700 893 and a meandering in longitudinal section through the contact tube bundle results in a meandering flow pattern of the heat exchange medium.

Appropriately, in the described method, the starting reaction gas mixture 2 the fixed bed catalyst bed 1 supplied preheated to the reaction temperature.

Usually, in the two-zone tube bundle reactors, the contact tubes are made of ferritic steel and typically have a wall thickness of 1 to 3 mm. Their inner diameter is usually 20 to 30 mm, often 21 to 26 mm. Their length is suitably 2 to 4 m, preferably 2.5 to 3.5 m. In each temperature zone, the fixed-bed catalyst bed occupies 1 at least 60%, or at least 75%, or at least 90% of the length of the zone. The optionally remaining residual length is optionally occupied by an inert material bed. In terms of application, the number of contact tubes accommodated in the tube bundle container amounts to at least 5000, preferably to at least 10000. Frequently, the number of contact tubes accommodated in the reaction container is 15000 to 30000 , Tube bundle reactors with a number of contact tubes above 40000 tend to be the exception. Within the container, the contact tubes are normally distributed homogeneously (preferably 6 equidistant adjacent tubes per contact tube), the distribution is suitably chosen so that the distance between the central inner axes of closest contact tubes (the so-called contact tube pitch) is 35 to 45 mm (see eg EP-B 468 290).

When Heat exchange means are also suitable for the two-zone mode of operation, in particular fluid temperature control media. Especially Cheap is the use of melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or of low-melting metals such as Sodium, mercury and alloys of various metals.

In The rule is in all the above-mentioned constellations of power in the two-zone tube bundle reactors the flow rate within the two required heat exchange fluid circuits so selected that the temperature of the heat exchange medium from the point of entry into the temperature zone to the exit point from the temperature zone (due to the exothermic nature of the reaction) around 0 to 15 ° C increases. That is, the aforementioned ΔT can according to the invention 1 to 10 ° C, or 2 to 8 ° C or 3 to 6 ° C.

The inlet temperature of the heat exchange medium in the temperature zone A according to the invention is normally in the range of 290 to 380 ° C, preferably in the range of 305 to 365 ° C and more preferably in the range of 310 to 340 ° C or at 330 ° C. For propene loads of the fixed bed catalyst bed 1 of ≥ 90 Nl / l · h and ≤ 160 Nl / l · h, the inlet temperature of the heat exchange medium in the temperature zone B according to the invention also in the range of 290 to 380 ° C, but at the same time normally suitably according to the invention ≥ 0 ° C to ≤ 20 ° C. , or ≤ 10 ° C, or ≥ 0 ° C and ≤ 5 ° C, or often ≥ 0 ° C and ≤ 3 ° C below the inlet temperature of the entering into the temperature zone A heat exchange medium. For propene loads of the fixed bed catalyst bed 1 of ≥ 160 Nl / l · h and (usually) ≤ 300 Nl / l · h (or 600 Nl / l · h), the inlet temperature of the heat exchange medium in the temperature zone B according to the invention also in the range of 290 to 380 ° C. but normally according to the invention suitably ≥ 0 ° C to ≤ 50 ° C, or ≥ 5 ° C and ≤ 45 ° C, or ≥ 10 ° C and ≤ 40 ° C, or ≥ 15 ° C and ≤ 30 ° C or ≤ 35 ° C (eg 20 ° C or 25 ° C) are above the inlet temperature of entering the temperature zone A heat exchange medium.

It should be noted at this point again that for carrying out the reaction stage 1 in particular also the two-zone tubular reactor type described in DE-AS 22 01 528 can be used, which includes the possibility of dissipating a portion of the hotter heat exchange medium of the temperature zone B to the temperature zone A, to optionally a warming of a cold reaction gas starting mixture or a cold cycle gas to effect. Furthermore, the tube bundle characteristic can be designed within an individual temperature zone as described in EP-A 382 098.

Incidentally, it has proven expedient to cool the product gas mixture leaving the first reaction stage directly and / or indirectly before it enters the second reaction stage so as to suppress post-combustion of parts of the acrolein formed in the first reaction stage. Usually, an aftercooler is connected between the two reaction stages. In the simplest case, this can be an indirect shell-and-tube heat exchanger. The product gas mixture is usually passed through the pipes and around the pipes a heat exchanger medium is performed, the nature of the for the tube bundle reactors may correspond to recommended heat exchange media. Advantageously, the interior of the tube is filled with inert packing materials (eg spirals made of stainless steel, rings made of steatite, balls made of steatite, etc.). The same improves the heat exchange and, if appropriate, from the fixed bed catalyst bed of the first reaction stage, subliming molybdenum trioxide precedes its entry into the second reaction stage. It is advantageous if the aftercooler is made of zinc silicate coated stainless steel.

In The rule is the propene conversion related to the single pass in the method according to the invention in the first reaction stage ≥ 92 mol% or ≥ 94 mol%. The resulting in the first reaction step in a single pass selectivity acrolein formation as well as acrylic by-product formation according to the invention together regularly ≥ 92 mol% or ≥ 94 mol%, often ≥ 95 mol% or ≥ 96 mol% or ≥ 97 mol%.

In terms of application, the product gas mixture of the first reaction stage in the already mentioned aftercooler is cooled to a temperature of 210 to 290 ° C., frequently 230 to 280 ° C. or 250 to 270 ° C. The cooling of the product gas mixture of the first reaction stage may well be at temperatures below the temperature of the second reaction stage. However, the aftercooling described is by no means compulsory and can be dispensed with, in particular, when the route of the product gas mixture from the first reaction stage to the second reaction stage is kept short. Advantageously, the described two-stage process is further realized in such a way that the oxygen requirement in the second reaction stage is not already reached by a correspondingly high oxygen content of the reaction gas starting mixture 2 but adds the required oxygen in the range between the first and second reaction stage ("secondary oxygen addition") .This can be done before, during, after and / or for aftercooling The source of the molecular oxygen required in the second reaction stage is pure oxygen It is also possible to use mixtures of oxygen and inert gas, for example air (is preferred according to the invention) or air enriched in molecular nitrogen (for example 90% by volume O ₂ , ≦ 10% by volume N ₂ ) Of course, in the process according to the invention, the oxygen demand in the second reaction stage can already be covered by a correspondingly high oxygen demand in the first reaction stage.

As the implementation In the first reaction stage, the implementation of the second reaction stage of the process according to the invention in application technology expedient manner in a two-zone tube bundle reactor, as he for the first reaction stage has already been described. The designs with regard to the two-zone tube bundle reactor for the first reaction stage therefore also apply to the two-zone tube bundle reactor for the second reaction stage.

D. h., In a simple way is the present invention to be used Festbettkatalysatorschüttung 2 (optionally including the Inertmaterialschüttungen) in the metal tubes of a tube bundle reactor and the metal tubes are two mutually substantially spatially separated tempering, usually salt melts, out. The pipe section, over which the respective salt bath extends, represents according to the invention a temperature zone.

D. h., flows around in a simple manner z. B. a salt bath C those sections of the tubes (the temperature zone C), in which the oxidative conversion of acrolein (at easy passage) until reaching a sales value in the range from 45 to 85 mol% (preferably 50 to 85 mol%, particularly preferably 60 to 85 mol%) and a salt bath D flows around the section of the tubes (the temperature zone D), in which the oxidative terminal conversion of acrolein (in single pass) until reaching one Sales value of at least 90 mol% completes (if necessary, can to be applied to the invention Temperature zones C, D connect further temperature zones, the kept at individual temperatures).

Appropriately from an application point of view comprises the reaction stage 2 the process of the invention no further temperature zones. In other words, the salt bath D expediently flows around the section of the tubes in which the oxidative terminal conversion of the acrolein (in a single pass) up to a conversion value of ≥ 92 mol%, or ≥ 94 mol% or ≥ 96 mol%. % or ≥ 98 mol% and often even ≥ 99 mol% or more.

Usually the beginning of the temperature zone D is behind the hotspot maximum the temperature zone C

The two salt baths C, D can relative to the invention to the flow direction of the flowing through the reaction tubes Reaction gas mixture in cocurrent or in countercurrent through the the reaction tubes surrounding space are performed.

Of course you can also in accordance with the invention the temperature zone C a DC flow and in the temperature zone D a counterflow (or vice versa).

Needless to say, you can in all the above case constellations within the respective temperature zone of, taking place relative to the reaction tubes parallel flow the molten salt still overlap a cross-flow, so that the individual Reaction zone one as in EP-A 700 714 or in EP-A 700th 893 described tube bundle reactor corresponds and overall in longitudinal section through the contact tube bundle a meandering flow pattern of the heat exchange medium results.

Usually, in the aforementioned two-zone shell and tube reactors for the second reaction stage, the catalyst tubes are made of ferritic steel and typically have a wall thickness of 1 to 3 mm. Their inner diameter is usually 20 to 30 mm, often 21 to 26 mm. Their length is suitably 3 to 4, preferably 3.5 m. In each temperature zone, the fixed-bed catalyst bed occupies 2 at least 60%, or at least 75%, or at least 90% of the length of the zone. The optionally remaining residual length is optionally occupied by an inert material bed. In terms of application, the number of catalyst tubes accommodated in the tube bundle container amounts to at least 5,000, preferably to at least 10,000. Frequently, the number of catalyst tubes accommodated in the reaction container is 15,000 to 30,000. Tube bundle reactors with a number of contact tubes above 40,000 tend to be the exception. Within the container, the contact tubes are normally distributed homogeneously (preferably 6 equidistant adjacent tubes per contact tube), the distribution is suitably chosen so that the distance between the central inner axes of closest contact tubes (the so-called contact tube pitch) is 35 to 45 mm (see EP-B 468 290).

When Heat exchange means In particular, fluid temperature control media are suitable. Particularly favorable is the Use of melts of salts such as potassium nitrate, potassium nitrite, Sodium nitrite and / or sodium nitrate, or of low-melting Metals such as sodium, mercury and alloys of various kinds Metals.

In The rule is in all the above-mentioned constellations of power in the two-zone tube bundle reactors the second reaction stage the flow rate within the two required heat exchange medium cycles chosen so that the temperature of the heat exchange medium from the entry point into the temperature zone to the exit point rises from 0 to 15 ° C from the temperature zone. That is, the aforementioned ΔT can according to the invention 1 to 10 ° C, or 2 to 8 ° C or 3 to 6 ° C be.

The inlet temperature of the heat exchange medium into the temperature zone C according to the invention is normally in the range of 230 to 320 ° C, preferably in the range of 250 to 300 ° C and particularly preferably in the range of 260 to 280 ° C. At acrolein loadings of the fixed bed catalyst bed 2 of ≥ 70 Nl / l · h and ≤ 150 Nl / l · h, the inlet temperature of the heat exchange medium into the temperature zone D according to the invention also in the range of 230 to 320 ° C, but at the same time normally suitably according to the invention ≥ 0 ° C to ≤ 20 ° C. or ≤ 10 ° C, or ≥ 0 ° C and ≤ 5 ° C, or frequently ≥ 0 ° C and ≤ 3 ° C below the inlet temperature of the entering into the temperature zone C heat exchange medium. In the case of acrolein loading of the fixed catalyst bed of ≥ 150 Nl / l · h and (usually) ≤ 300 Nl / l · h (resp.

600 Nl / lh) is the inlet temperature of the heat exchange medium in the Temperature zone D according to the invention also in the range of 230 to 320 ° C, at the same time but normally according to the invention suitably ≥ 0 ° C to ≤ 40 ° C, or ≥ 5 ° C and ≤ 35 ° C, or 30 ° C, or ≥ 10 ° C and ≤ 25 ° C, or 20 ° C, or 15 ° C above the inlet temperature of the entering into the temperature zone C heat exchange means are.

It should also be pointed out once again that for carrying out the second reaction stage of the described method in particular also the described in DE-AS 22 01 528 Zweizonenrohrbündelreaktortyp can be used, which includes the possibility of the hotter heat exchange medium of the temperature zone D a Partial amount to the temperature zone C dissipate, if necessary, a warming of a too cold reaction gas starting mixture 3 to effect. Furthermore, the tube bundle characteristic can be designed within an individual reaction zone as described in EP-A 382 098.

Of course, two two-zone tube bundle reactors can be fused to form a four-zone tube bundle reactor for carrying out the method described, as described in WO 01/36364. Normally, in these cases, there is between the fixed bed catalyst bed 1 and the fixed bed catalyst bed 2 an inert material bed. However, such a Zwischeninertschüttung can also be dispensed with. The length of the reaction tubes corresponds to the merger case often the sum of the lengths of the unfused tube bundle reactors.

At this point it should be noted that as active materials both for the fixed-bed catalyst bed 1 as well as for the fixed bed catalyst bed 2 also the multimetal oxide materials of DE-A 102 61 186 are favorable.

Favorable embodiments of a two-zone tube bundle reactor for the first reaction stage may be as follows (the structural detail may be as in the utility model applications 202 19 277.6, 2002 19 278.4 and 202 19 279.2 or in the PCT applications PCT / EP02 / 14187, PCT / EP02 / 14188 or PCT / EP02 / 14189): contact tubes: Material of the contact tubes: ferritic steel; Dimensions of the contact tubes: z. B. 3500 mm in length; z. B. 2 mm wall thickness; z. B. 30 mm outside diameter;

• – von außen nach innen,
• – von innen nach außen,

um die Kontaktrohre;
durch um den Behälterumfang angebrachte Fenster sammelt sich die Salzschmelze an jedem Zonenende in einem um den Reaktormantel angebrachten Ringkanal, um einschließlich Teilstromkühlung im Kreis gepumpt zu werden;
über jede Temperaturzone wird die Salzschmelze von unten nach oben geführt;
Das Reaktionsgasgemisch verlässt den Reaktor der ersten Stufe mit einer Temperatur wenige Grad höher als die Salzbadeintrittstemperatur des ersten Reaktors. Das Reaktionsgasgemisch wird für die Weiterverarbeitung zweckmäßigerweise in einem separaten Nachkühler, der dem Reaktor der 1. Stufe nachgeschaltet ist, auf 220°C bis 280°C, bevorzugt 240°C bis 260°C abgekühlt.Number of contact tubes in the tube bundle: z. 30,000, or 28,000, or 32,000, or 34,000; additionally up to 10 thermotubes (as described in EP-A 873 783 and EP-A 12 70 065), which are charged like the contact tubes (spiraling from the very outward to the inside) z. B. the same length and wall thickness, but with an outside diameter of z. B. 33.4 mm and a centered thermal sleeve of z. B. 10 mm outer diameter and z. B. 1 mm wall thickness;
Reactor (material like the contact tubes):
Cylindrical container with an inside diameter of 6000 - 8000 mm;
Reactor hoods plated with Type 1.4541 stainless steel; Plating thickness: a few mm;
annularly arranged tube bundle, z. B. with a free central space;
Diameter of the central free space: z. 1000 - 2500 mm (eg 1200 mm, or 1400 mm, or 1600 mm, or 1800 mm, or 2000 mm, or 2200 mm, or 2400 mm);
normally homogeneous contact tube distribution in the tube bundle ( 6 equidistant neighboring tubes per contact tube), arrangement in equilateral triangle, contact tube pitch (distance of the centric inner axes of closest contact tubes): 35-45 mm, z. B. 36 mm, or 38 mm, or 40 mm, or 42 mm, or 44 mm;
the contact tubes are sealingly fixed with their ends in contact tube bottoms (upper bottom and lower bottom, for example with a thickness of 100-200 mm) and open at the upper end into a hood connected to the container, which is an admission for the reaction gas starting mixture 2 having; a z. B. on half the contact tube length separating plate of a thickness of 20 - 100 mm, dividing the reactor space symmetrically into two temperature zones A (upper zone) and B (lower zone); each temperature zone is divided by a deflection disk into 2 equidistant longitudinal sections;
the deflection pulley preferably has ring geometry; the contact tubes are sealingly attached to the separating plate with advantage; they are not attached sealingly to the deflecting disks, so that the transverse flow velocity of the molten salt within a zone is as constant as possible;
each zone is supplied with salt melt as a heat carrier by its own salt pump; the supply of molten salt is z. B. below the deflection pulley and the removal is z. B. above the deflection pulley;
from both molten salts z. B. removed a partial flow and z. B. cooled in a common or two separate indirect heat exchangers (steam generation);
in the first case, the cooled molten salt stream is split, combined with the respective residual stream and pressed by the respective pump into the corresponding annular channel, which distributes the molten salt over the container circumference, into the reactor;
through window located in the reactor jacket the molten salt reaches the tube bundle; the inflow occurs z. B. in the radial direction to the tube bundle;
the molten salt flows in each zone of the specification of the baffle following z. In sequence

• - from the outside to the inside,
• - from the inside to the outside,

around the contact tubes;
through windows mounted around the vessel periphery, the molten salt at each zone end collects in a ring channel mounted around the reactor jacket to be circulated, including partial flow cooling;
over each temperature zone, the molten salt is passed from bottom to top;
The reaction gas mixture leaves the first stage reactor at a temperature a few degrees higher than the salt bath inlet temperature of the first reactor. The reaction gas mixture is suitably cooled to 220 ° C to 280 ° C, preferably 240 ° C to 260 ° C for further processing in a separate aftercooler downstream of the reactor of the 1st stage.

Of the aftercooler is usually flanged below the lower tube plate and usually consists of tubes with ferritic steel. In the Pipes of the Nachküh lers are beneficial inside stainless steel sheet metal spirals, which are partially or fully wound could be, to improve the heat transfer introduced.

Molten salt:

As a molten salt, a mixture of 53 wt .-% potassium nitrate, 40 wt .-% sodium nitrite and 7 wt .-% sodium nitrate can be used; both reaction zones and the aftercooler advantageously use a molten salt of the same composition; The amount of salt circulated in the reaction zones can be about 10000 m ³ / h per zone.

Flow Control:

• Abschnitt 1: 50 cm Länge Steatitringe der Geometrie 7 mm × 7 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) als Vorschüttung.
• Abschnitt 2: 140 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 30 Gew.-% an Steatitringen der Geometrie 5 mm × 3 mm × 2 mm (Außendurchmesser × Länge × Innendurchmesser) und 70 Gew.-% Vollkatalysator aus Abschnitt 3.
• Abschnitt 3: 160 cm Länge Katalysatorbeschickung mit ringförmigem (5 mm × 3 mm × 2 mm = Außendurchmesser × Länge × Innendurchmesser) Vollkatalysator gemäß Beispiel 1 der DE-A 10046957 (Stöchiometrie: [Bi₂W₂O₉ × 2 WO₃]_0,5 [Mo₁₂Co_5,5Fe_2,94Si_1,59K_0,08O_x]₁).

The reaction gas starting mixture 2 expediently flows from top to bottom through the first-stage reactor, while the different temperature salt melts of the individual zones are promoted expedient from bottom to top; Contact tube and thermal tube feed (from top to bottom) z. B .:

• Section 1: 50 cm long steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
• Section 2: 140 cm length of catalyst feed with a homogeneous mixture of 30 wt% steatite rings of geometry 5 mm x 3 mm x 2 mm (outside diameter x length x inside diameter) and 70 wt% of full catalyst from section 3.
• Section 3: 160 cm length catalyst charge with annular (5 mm × 3 mm × 2 mm = outer diameter × length × inner diameter) unsupported catalyst according to Example 1 of DE-A 10046957 (stoichiometry: [Bi ₂ W ₂ O ₉ × 2 WO ₃ ] _{0 , 5} [Mo ₁₂ Co _5.5 Fe _2.94 Si _1.59 K _0.08 O _x ] ₁ ).

Favorable embodiments of a two-zone tube bundle reactor for the second reaction stage may be as follows:
Everything like the two-zone tube bundle reactor for the first reaction stage. The thickness of the upper and lower contact tube bottom is often 100-200 mm, z. B. 110 mm, or 130 mm, or 150 mm, or 170 mm, or 190 mm.

Of the aftercooler eliminated; instead of this the contact tubes with their lower openings in one at the bottom End with the container connected hood with outlet for the product gas mixture; the upper temperature zone is zone C and the lower temperature zone is the temperature zone D. Between outlet "aftercooler" and inlet "reactor for the second reaction stage" is appropriate one supply ability for compressed Air.

• Abschnitt 1: 20 cm Länge Steatitringe der Geometrie 7 mm × 7 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) als Vorschüttung.
• Abschnitt 2: 90 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 30 Gew.-% an Steatit-Ringen der Geometrie 7 mm × 3 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) und 70 Gew.-% Schalenkatalysator aus Abschnitt 4.
• Abschnitt 3: 50 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 20 Gew.-% an Steatit-Ringen der Geometrie 7 mm × 3 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) und 80 Gew.-% Schalenkatalysator aus Abschnitt 4.
• Abschnitt 4: 190 cm Länge Katalysatorbeschickung mit ringförmigem (7 mm × 3 mm × 4 mm = Außendurchmesser × Länge × Innendurchmesser) Schalenkatalysator gemäß Herstellungsbeispiel 5 der DE-A 100 46 928 (Stöchiometrie: Mo₁₂V₃W_1,2Cu_2,4O_x).

The contact tube and thermal tube charge (from top to bottom) may, for. B. be as follows :.

• Section 1: 20 cm length steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a feed.
• Section 2: 90 cm length of catalyst feed with a homogeneous mixture of 30 wt% steatite rings of geometry 7 mm x 3 mm x 4 mm (outside diameter x length x inside diameter) and 70 wt% shell catalyst from section 4.
• Section 3: 50 cm in length Catalyst feed with a homogeneous mixture of 20 wt% steatite rings of geometry 7 mm x 3 mm x 4 mm (outside diameter x length x inside diameter) and 80 wt% shell catalyst from section 4.
• Section 4: 190 cm length catalyst charge with annular (7 mm × 3 mm × 4 mm = outer diameter × length × inner diameter) coated catalyst according to Preparation Example 5 of DE-A 100 46 928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _{2, 4} O _x ).

• Abschnitt 1: 20 cm Länge Steatitringe der Geometrie 7 mm × 7 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) als Vorschüttung.
• Abschnitt 2: 140 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 25 Gew.-% an Steatit-Ringen der Geometrie 7 mm × 3 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) und 75 Gew.-% Schalenkatalysator aus Abschnitt 3.
• Abschnitt 3: 190 cm Länge Katalysatorbeschickung mit ringförmigem (7 mm × 3 mm × 4 mm = Außendurchmesser × Länge × Innendurchmesser) Schalenkatalysator gemäß Herstellungsbeispiel 5 der DE-A 100 46 928 (Stöchiometrie: Mo₁₂V₃W_1,2Cu_2,4O_x).

The second-stage contact tube and thermal tube feed can also look like this from top to bottom:

• Section 1: 20 cm length steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a feed.
• Section 2: 140 cm length of catalyst feed with a homogeneous mixture of 25 wt% steatite rings of geometry 7 mm x 3 mm x 4 mm (outside diameter x length x inside diameter) and 75 wt% shell catalyst from section 3.
• Section 3: 190 cm length catalyst charge with annular (7 mm × 3 mm × 4 mm = outer diameter × length × inner diameter) coated catalyst according to Preparation Example 5 of DE-A 100 46 928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _{2, 4} O _x ).

• a) einen Katalysator gemäß Beispiel 1 c aus der EP-A 15 565 oder einen gemäß diesem Beispiel herzustellenden Katalysator, der jedoch die Aktivmasse Mo₁₂Ni_6,5Zn₂Fe₂Bi₁P_0,0065K_0,06O_x·10 SiO₂ aufweist;
• b) Beispiel Nr. 3 aus der DE-A 198 55 913 als Hohlzylindervollkatalysator der Geometrie 5 mm × 3 mm × 2 mm bzw. 5 mm × 2mm × 2mm;
• c) Multimetalloxid II – Vollkatalysator gemäß Beispie 1 der DE-A 197 46 210;
• d) einen der Schalenkatalysatoren 1, 2 und 3 aus der DE-A 100 63 162, jedoch bei gleicher Schalendicke auf Trägerringe der Geometrie 5 mm × 3 mm × 1,5 mm bzw. 7 mm × 3 mm × 1,5 mm aufgebracht;
• e) durch die Katalysatoren, insbesondere die Ausführungsbeispiele, der DE-A 10344149 und der DE-A 10353954.

In the aforementioned first-stage feed, the unsupported catalyst from Example 1 of DE-A 100 46 957 can also be replaced by:

• a) a catalyst according to Example 1c from EP-A 15 565 or a catalyst to be prepared according to this example but containing the active composition Mo ₁₂ Ni _6.5 Zn ₂ Fe ₂ Bi ₁ P _0.0065 K _0.06 O _x · 10 SiO ₂ ;
• b) Example No. 3 from DE-A 198 55 913 as hollow cylinder full catalyst of the geometry 5 mm × 3 mm × 2 mm or 5 mm × 2 mm × 2 mm;
• c) multimetal oxide II - full catalyst according to Example 1 of DE-A 197 46 210;
• d) one of the shell catalysts 1, 2 and 3 from DE-A 100 63 162, but applied with the same shell thickness on carrier rings of geometry 5 mm × 3 mm × 1.5 mm or 7 mm × 3 mm × 1.5 mm ;
• e) by the catalysts, in particular the embodiments, DE-A 10344149 and DE-A 10353954.

• a) Schalenkatalysator S1 oder S7 aus der DE-A 4442346 mit einem Aktivmassenanteil von 27 Gew.-% und einer Schalendicke von 230 μm;
• b) einem Schalenkatalysator gemäß den Beispielen 1 bis 5 aus der DE-A 198 15 281, jedoch auf Trägerringe der Geometrie 7 mm × 3 mm × 4 mm mit einem Aktivmassenanteil von 20 Gew.-% aufgebracht;
• c) Schalenkatalysator mit zweiphasiger Aktivmasse der Stöchiometrie (Mo_10,4V₃W_1,2O_x) (CuMo_0,5W_0,5O₄)_1,6, hergestellt gemäß DE-A 197 36 105 und mit einem Aktivmassenanteil von 20 Gew.-% auf den vorgenannten 7 mm × 3 mm × 4 mm Träger aufgebracht.

In all the aforementioned second-stage feeds, the coated catalyst according to Preparation Example 5 of DE-A 100 46 928 can be replaced by:

• a) coated catalyst S1 or S7 from DE-A 4442346 with an active composition content of 27 wt .-% and a shell thickness of 230 microns;
• b) a coated catalyst according to Examples 1 to 5 from DE-A 198 15 281, but applied to support rings of geometry 7 mm × 3 mm × 4 mm with an active material content of 20 wt .-%;
• c) coated catalyst with two-phase active mass of stoichiometry (Mo _10.4 V ₃ W _1.2 O _x ) (CuMo _0.5 W _0.5 O ₄ ) _1.6 , prepared according to DE-A 197 36 105 and with an active mass fraction of 20 wt .-% applied to the aforementioned 7 mm × 3 mm × 4 mm support.

Incidentally, the fixed bed catalyst bed 1 and the fixed bed catalyst bed 2 According to the invention, it is expedient to choose (for example by dilution with inert material, for example) that the temperature difference between the hotspot maximum of the reaction gas mixture in the individual reaction zones and the particular temperature of the reaction zone does not generally exceed 80.degree. Most of this temperature difference is ≤ 70 ° C, often it is 20 to 70 ° C, preferably this temperature difference is low. In addition, the fixed bed catalyst beds 1 and 2 selected for safety reasons in a manner known per se to the person skilled in the art (for example by dilution with, for example, inert material) that the peak-to-salt temperature sensitivity as defined in EP-A 1106598 ≦ 9 ° C., or ≤ 7 ° C, or ≤ 5 ° C, or ≤ 3 ° C.

Aftercooler and Reactor for the second stage are connected by a connecting tube, the Less length than 25 m.

In the two-zone examples contained in this document and in the above reactor arrangement, in the second reaction stage, the annular shaped diluent bodies and the annular shaped catalyst bodies can also be formed by spherical diluent bodies and spherical catalyses torformkörper (each with a radius of 2 to 5 mm and with an active material content of 10 to 30 wt .-%, often 10 to 20 wt .-%) are replaced.

The product of the present invention partial oxidation (after the first and / or second reaction stage) leaving product gas mixture (here product gas mixture 2 (after the first reaction stage) or 3 (after the second reaction stage)) is in the case of a production of acrolein and / or acrylic acid usually composed essentially of the target product acrolein or acrylic acid or its mixture with acrolein, unreacted molecular oxygen (with Looking at the life of the catalysts used, it is beneficial if the oxygen content in both the product gas mixture 2 as well as in the product gas mixture 3 is still at least 1.5 to 4 vol .-%), propane, unreacted propylene, molecular nitrogen, by-produced and / or used as diluent gas steam, as a by-product and / or diluent gas co-used carbon oxides, and small amounts of other lower aldehydes lower alkanecarboxylic acids (eg acetic acid, formic acid and propionic acid) as well as maleic anhydride, benzaldehyde, aromatic carboxylic acids and aromatic carboxylic acid anhydrides (eg phthalic anhydride and benzoic acid), optionally further hydrocarbons, such as. C4 hydrocarbons (eg butene-1 and possibly other butenes) and other inert diluent gases.

The target product may be from the product gas mixture 3 respectively. 2 in a manner known per se in a separation zone are separated (for example, by partial or complete and optionally fractional condensation of acrylic acid or by absorption of acrylic acid in water or in a high-boiling hydrophobic organic solvent or by absorption of acrolein in water or in aqueous solutions lower carboxylic acids and subsequent workup of the condensates and / or absorbates; according to the invention, the product gas mixture is preferred 3 respectively. 2 be condensed fractionally; see. z. EP-A 13 88 533, EP-A 13 88 532, DE-A 102 35 847, EP-A 79 28 67, WO 98/01415, EP-A 10 15 411, EP-A 10 15 410, WO 99/50219, WO 00/53560, WO 02/09839, DE-A 102 35 847, WO 03/041833, DE-A 102 23 058, DE-A 102 43 625, DE-A 103 36 386, EP-A 85 41 29, US-A 4,317,926, DE-A 198 37 520, DE-A 196 06 877, DE-A 190 50 1325, DE-A 102 47 240, DE-A 197 40 253, EP-A 69 57 36 , EP-A 98 22 87, EP-A 10 41 062, EP-A 11 71 46, DE-A 43 08 087, DE-A 43 35 172, DE-A 44 36 243, DE-A 19 924 532, DE-A 103 32 758 and DE-A 19 924 533). An acrylic acid removal can also as in EP-A 98 22 87, EP-A 98 22 89, DE-A 103 36 386, DE-A 101 15 277, DE-A 196 06 877, DE-A 197 40 252, DE-A 196 27 847, EP-A 92 04 08, EP-A 10 68 174, EP-A 10 66 239, EP-A 10 66 240, WO 00/53560, WO 00/53561, DE-A 100 53 086 and EP-A 98 22 88 are made. Preferably, as in 7 WO / 0196271 or as described in DE-A 10 2004 032 129 and their equivalent patents described separated. Favorable separation methods are also the methods described in WO 2004/063138, WO 2004/035514, DE-A 102 43 625 and DE-A 102 35 847. The further processing of a crude acrylic acid thereby obtained can, for. As described in the publications WO 01/77056, WO 03/041832, WO 02/055469, WO 03/078378 and WO 03/041833.

A common feature of the aforementioned separation process is (as already mentioned), that at the top of the respective separating internals containing separation column in the lower part of the product mixture 3 respectively. 2 , usually after prior direct and / or indirect cooling thereof, normally leaving a residual gas stream, essentially those components of the product gas mixture 2 respectively. 3 contains, whose boiling point at atmospheric pressure (1 bar) ≤ -30 ° C (ie, the hardly condensable or volatile constituents).

In the lower part of the separation column usually fall the less volatile constituents of the product gas mixture 3 respectively. 2 , including the respective target product, in a condensed phase.

The Residual gas components are primarily propane, if necessary in the partial oxidation unreacted propylene, molecular Oxygen as well as often the other in-use diluent gases used in the partial oxidation, such as As nitrogen and carbon dioxide. Water vapor can vary depending on applied separation process in the residual gas only in traces or in Be contained in amounts of up to 20 vol .-% or more.

From This main gas radical is preferably at least one according to the invention (preferably having residual gas composition) propane, molecular Oxygen and optionally unreacted propylene containing Subset (preferably the total amount, but possibly also just half, or two thirds, or three quarters of that total) as one Propane-containing feed stream in the optionally as a propene source serving heterogeneously catalyzed propane dehydrogenation and / or Propanoxidehydrierung recycled (oxidation gas). Residual gas subsets can but also recycled to one or both stages of the partial oxidation and / or burned for the purpose of generating energy.

In the work-up of the condensed phase (for the purpose of separation of the target product), other residual gases may be incurred, as will normally be attempted, the total in the product gas mixture 3 respectively. 2 attributable amount of unreacted propane in the optionally used as a propylene source heterogeneously catalyzed propane dehydrogenation and / or Propanoxidehydrierung and recover within the scope of the target separation. These usually still contain propane and possibly propylene, but often no molecular oxygen more. Usually, they are combined with the main residual gas to form a total residual gas and are recycled to the heterogeneously catalyzed propane dehydrogenation and / or propane oxydehydrogenation which may be used as the propylene source. But it is also a separate utilization of such other residual gases possible.

By the preferably complete Return of the Total residual gas can thus be continuous in continuous operation Conversion of propane to acrylic acid and / or acrolein.

Essential is doing that, by the described repatriation of the optionally heterogeneously catalyzed propane dehydrogenation serving as propylene source and / or Propanoxidehydrierung in the latter a transfer of Propane can be reached to propylene with almost 100% selectivity is.

The advantageousness of such a procedure is given in the case of both lower (≦ 30 mol%) and high (≥ 30 mol%) dehydrogenation conversions (based on the one-time passage of fresh propane through the dehydrogenation). In general, it is favorable in such a recycling of oxidation gas when the hydrogen content in the reaction gas starting mixture 1 in an at least stoichiometric ratio (with respect to an oxygen combustion to water) over the oxidation gas into the reaction gas starting mixture 1 returned oxygen amount is.

The aforementioned Kreisgasfahrweise is applicable in a corresponding manner when the Partial oxidation a partial ammoxidation of propene to acrylonitrile is. It is also applicable accordingly when in dehydration the propane is replaced by iso-butane and the resulting iso-butene in a corresponding manner in a partial oxidation to methacrolein and / or methacrylic acid is partially oxidized.

At this point it should again be noted that the separation of acrylic acid from a product gas mixture obtained according to the invention 3 (in particular by way of example) preferably takes place in such a way that the product gas mixture previously cooled, if appropriate, by direct and / or indirect cooling 3 in a separation-containing internals-containing column with side extraction of a crude acrylic acid (eg., In itself) ascending fractionally condensed and / or absorbed with water or aqueous solution, as WO 2004/035514 and DE-A 102 43 625 exemplifies describe. The withdrawn crude acrylic acid is subsequently preferably subjected to a suspension crystallization, and the acrylic acid suspension crystallizate formed in this way is preferably separated from residual mother liquor by means of a wash column. Advantageously, the melt used in this case as washing liquid is the acrylic acid crystals previously separated in the wash column. Further, the washing column is preferably one having forced transport of the crystal bed. Particularly preferred is a hydraulic or a mechanical wash column. In detail, the description of WO 01/77056, WO 03/041832 and WO 03/041833 can be followed. D. h., Preferably, remaining mother liquor is recycled to the fractionating condensation (see also EP-A 10 15 410). The minor component outlet is normally below the side draw of the crude acrylic acid as a purge stream.

Under Use of only one crystallization stage is so acrylic acid with a purity ≥ 99.8% by weight available, which are outstanding for the production of superabsorbents based on poly-Na-acrylate. Also, it should be noted that an advantage of the procedure according to the invention in principle therein exists that in all places of this writing, including the Embodiments, where diluted with inert material Catalyst feeds are described and / or required, the corresponding catalysts at the same bed length also undiluted can be used (used).

I. Heterogeneous catalyzed partial propane dehydrogenation as propylene source

general Experimental setup and test execution as well as results (described in all cases the stationary operating state).

The heterogeneously catalyzed partial propane dehydrogenations were carried out in a tray loop reactor according to 1 performed, to which the numerical addresses refer in the following.

A vertical tubular reactor ( 11 ) (Diameter: 80 mm) was with a thermal insulation ( 10 ) provided in a support heater ( 9 ). The temperature of the support heater was 500 ° C. In the center of the tubular reactor was a central tube (diameter: 20 mm), which contained a sleeve for a tensile thermocouple and a sleeve for a step thermocouple. In addition, it contained in the tubular reactor leading lines through which reaction gas samples could be taken from the tube reactor, as well as in the tube reactor leading lines through which air could be injected into the tubular reactor.

The tube reactor contained three hordes ( 5 . 6 . 7 ), which consisted of three identical, applied on a stainless steel wire mesh beds of inert material and dehydrogenation catalyst. In the flow direction of the reaction gas through the dehydrogenation reactor, the beds were constructed as follows:
First on a 60 mm bulk length, a bed of steatite spheres (diameter: 4 to 5 mm) made of steatite C-220 from Ceram Tec. Subsequently, on a packed length of 120 mm dehydrogenation catalyst (Pt / Sn alloy containing the elements Cs, K and La were promoted in oxidic form and which were coated on the outer and inner surface of ZrO ₂ .SiO ₂ mixed oxide carrier strands (average length (gauss distributed in the range of 3 mm to 12 mm with a maximum of about 6 mm): 6 mm, Diameter: 2 mm) in elemental stoichiometry (mass ratios including support) Pt _0.3 Sn _0.6 La _3.0 Cs _0.5 K _0.2 (ZrO ₂ ) _88.3 (SiO ₂ ) _{7.1 was} applied ( Catalyst precursor preparation and activation to the active catalyst as in Example 4 of DE-A 10219879).

The product gas mixture emerging from the last horde 1 was divided into two halves of identical composition. One half ( 2 ) was used as part of the reaction gas starting mixture ( 4 ) returned to the dehydrogenation. The other half ( 1 ) was taken out of the dehydrogenation zone.

By absorption in technical Tetradecan the company Haltermann, DE type PKWF 4/7 of as an absorbent and subsequent stripping with air (as described in DE-A 10 2004 032 129), the components other than propylene and propane largely separated and a for the partial oxidation of propylene to acrolein and / or acrylic acid directly suitable feed gas mixture can be obtained. The reaction gas starting mixture ( 4 ) consisted of dehydrogenating gas ( 2 ) as well as steam, oxidizing gas (simulated by a mixture of oxygen and nitrogen for simplicity) and fresh propane (all three summarized here by the address ( 3 ) symbolizes).

The Fresh propane has the following contents:

Vol .-% methane 0.005 Ethan 0.005 ethene 0 propane 99.982 propene (Propylene) 0,003 isobutane 0.005

The loading of the first catalyst bed with propane was in all cases 305 Nl / lh.

The inlet pressure of the starting reaction gas mixture was 2.3 bar. The pressure drop across the dehydrogenation reactor was about 200 mbar. If appropriate, air (500 ° C., reaction pressure) was added to the reaction gas mixture before the second and before the third catalyst bed (in the flow direction). The following Tables 1 to 3 show the in three experiments depending on the different reaction gas starting mixtures ( 4 ) as well as the different Luftzudosierungen results obtained.

there the following symbols apply:

L2

= Air intake before second catalyst bed in Nl / h

L3

= Air intake before third catalyst bed in Nl / h

RA

= Contents of the reaction gas starting mixture ( 4 ) (contained in all cases, based on the amount of propane contained, about 70% by volume of water vapor)

R1

= Contents of the reaction gas, after passing through the first catalyst bed

R2

= Contents of the reaction gas, after passing through the second catalyst bed

P

= Contents of the product gas, this is the reaction gas after passing through the third catalyst bed

TE

= Inlet temperature of the reaction gas starting mixture in the first catalyst bed in ° C

T1

= highest temperature in the first catalyst bed in ° C

T2

= highest temperature in the second catalyst bed in ° C

T3

= highest temperature in the third catalyst bed in ° C

TA

= Exit temperature of the product gas

B

= Contents of the resulting Feed gas mixture for a partial oxidation of the invention (all Feed gas mixtures were not explosive)

U

= Propane conversion, based on fresh propane and once through the dehydrogenation reactor

S

= Selectivity of propylene formation (all contents in% by volume based on the total gas, calculated anhydrous).

Example 1 / Table 1

Example 2 / Table 2

Example 3 / Table 3

II. Inventive heterogeneous catalyzed two-stage partial oxidation of propylene to acrylic acid.

 (described is the stationary Operating condition)

experimental arrangement

First reaction stage:

• Abschnitt 1: 50 cm Länge Steatitringe der Geometrie 7 mm × 7 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) als Vorschüttung.
• Abschnitt 2: 140 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 30 Gew.-% an Steatitringen der Geometrie 5 mm × 3 mm × 2 mm (Außendurchmesser × Länge × Innendurchmesser) und 80 Gew.-% Vollkatalysator aus Abschnitt 3.
• Abschnitt 3: 160 cm Länge Katalysatorbeschickung mit ringförmigem (5 mm × 3 mm × 2 mm = Außendurchmesser × Länge × Innendurchmesser) Vollkatalysator gemäß Beispiel 1 der DE-A 10046957 (Stöchiometrie: (Bi₂W₂O₉ × 2WO₃]_0,5[Mo₁₂Co_5,5Fe_2,94Si_1,59K_0,08O_x]₁).

A reaction tube (V2A steel, 30 mm outer diameter, 2 mm wall thickness, 26 mm inner diameter, length: 350 cm, and a centered in the reaction tube tube thermal tube (4 mm outer diameter) for receiving a thermocouple with the temperature in the reaction tube over its entire length determined can be) is loaded from top to bottom as follows:

• Section 1: 50 cm long steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
• Section 2: 140 cm in length Catalyst feed with a homogeneous mixture of 30 wt% steatite rings of geometry 5 mm x 3 mm x 2 mm (outside diameter x length x inside diameter) and 80 wt% of full catalyst from section 3.
• Section 3: 160 cm length catalyst charge with annular (5 mm × 3 mm × 2 mm = outer diameter × length × inner diameter) unsupported catalyst according to Example 1 of DE-A 10046957 (stoichiometry: (Bi ₂ W ₂ O ₉ × 2WO ₃ ) _{0, 5} [Mo ₁₂ Co _5.5 Fe _2.94 Si _1.59 K _0.08 O _x ] ₁ ).

From top down are the first 175 cm by means of a counter-current pumped salt bath A thermostated. The second will be 175 cm thermostated by means of a countercurrently pumped salt bath B.

Second reaction stage:

• Abschnitt 1: 20 cm Länge Steatitringe der Geometrie 7 mm × 7 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) als Vorschüttung.
• Abschnitt 2: 90 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 30 Gew.-% an Steatit-Ringen der Geometrie 7 mm × 3 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) und 75 Gew.-% Schalenkatalysator aus Abschitt 4.
• Abschnitt 3: 50 cm Länge Katalysatorbeschickung mit einem homogenen Gemisch aus 20 Gew.-% an Steatit-Ringen der Geometrie 7 mm × 3 mm × 4 mm (Außendurchmesser × Länge × Innendurchmesser) und 85 Gew.-% Schalenkatalysator aus Abschnitt 4.
• Abschnitt 4: 190 cm Länge Katalysatorbeschickung mit ringförmigem (7 mm × 3 mm × 4 mm = Außendurchmesser × Länge × Innendurchmesser) Schalenkatalysator gemäß Herstellungsbeispiel 5 der DE-A 10046928 (Stöchiometrie: Mo₁₂V₃W_1,2Cu_2,4O_x).

A reaction tube (V2A steel, 30 mm outer diameter, 2 mm wall thickness, 26 mm inner diameter, length: 350 cm, and a centered in the reaction tube tube thermal tube (4 mm outer diameter) for receiving a thermocouple with the temperature in the reaction tube over its entire length determined can be) is loaded from top to bottom as follows:

• Section 1: 20 cm length steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a feed.
• Section 2: 90 cm length of catalyst feed with a homogeneous mixture of 30 wt% steatite rings of geometry 7 mm x 3 mm x 4 mm (outside diameter x length x inside diameter) and 75 wt% shell catalyst from section 4.
• Section 3: 50 cm length of catalyst feed with a homogeneous mixture of 20 wt% steatite rings of geometry 7 mm x 3 mm x 4 mm (outside diameter x length x inside diameter) and 85 wt% shell catalyst from section 4.
• Section 4: 190 cm length catalyst charge with annular (7 mm × 3 mm × 4 mm = outer diameter × length × inner diameter) coated catalyst according to Preparation Example 5 of DE-A 10046928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _2.4 O _x ).

From top down are the first 175 cm by means of a counter-current pumped salt bath C thermostated. The second will be 175 cm thermostated by means of a countercurrently pumped salt bath D.

Propylene source and two-stage partial oxidation to acrylic acid First, a tray reactor is operated as in I. The composition of the reaction gas starting mixture for the heterogeneously catalyzed dehydrogenation is as follows (% by volume, based on total gas): Vol .-% acrylic acid 0.02 acetic acid 0.03 water 9.23 1-butene 0.01 isobutene 0.02 propane 18.46 propylene 3.98 Ethan 1.16 ethylene 0.22 CO ₂ 2.34 CO 0.26 N ₂ 59.7 O ₂ 1.62 CH ₄ 0.12 H ₂ 2.83

• – 41,9 Vol.-% Oxidationskreisgas, das folgende Gehalte aufweist:

Vol.-% Acrylsäure 0,02 Essigsäure 0,04 H₂O 2,73 iso-Buten 0,01 Acrolein 0,05 Propan 17,30 Propylen 0,32 Ethan 1,20 Ethylen 0,22 CO₂ 2,41 CO 0,61 N₂ 71,21 O₂ 3,87

• – 3,9 Vol.-% Frischpropan, das folgende Gehalte aufweist:

Vol.-% Propan 98,91 iso-Butan 0,05 Propylen 0,1 Ethan 0,92 Ethylen 0,01

• – 1,02 Vol.-% Wasserstoff
• – 2,03 Vol.-% Wasserdampf und
• – 51,15 Vol.-% Dehydrierkreisgas.

It consists of:

• 41.9% by volume of oxidizing gas having the following contents:

Vol .-% acrylic acid 0.02 acetic acid 0.04 H ₂ O 2.73 isobutene 0.01 acrolein 0.05 propane 17.30 propylene 0.32 Ethan 1.20 ethylene 0.22 CO ₂ 2.41 CO 0.61 N ₂ 71.21 O ₂ 3.87

• 3.9% by volume of fresh propane, which has the following contents:

Vol .-% propane 98.91 isobutane 0.05 propylene 0.1 Ethan 0.92 ethylene 0.01

• - 1.02 vol .-% hydrogen
• 2.03% by volume of steam and
• - 51.15 vol .-% Dehydrierkreisgas.

The remaining product gas mixture is composed as follows: Vol .-% acrylic acid 0.02 acetic acid 0.03 H ₂ O 11.84 isobutene 0.01 propane 14.32 propylene 7.52 Ethan 1.21 ethylene 0.26 CO ₂ 2.61 N ₂ 58.41 O ₂ 0.23 H ₂ 3.55

The propane and propylene contained in the product gas mixture of the heterogeneously catalyzed propane dehydrogenation is separated off by adsorption as described in I and freed again by means of air to give the following feed gas for the partial oxidation, which has the following contents: Vol .-% H ₂ O 2.39 tetradecane 0.01 isobutene 0.01 propane 15.15 propylene 7.95 Ethan 1.10 ethylene 0.20 CO ₂ 1.05 N ₂ 56,99 O ₂ 15.16

• U^P _A' der Propenumsatz am Ende der Reaktionszone A, beträgt 64,5 mol.-%.
• U^P _B' der Propenumsatz am Ende der Reaktionszone B, beträgt 94,9 mol.-%.

With this feed gas mixture (it is outside the explosion range), the described first partial oxidation reaction stage is charged. The propylene loading of the fixed bed catalyst feed is chosen to be 185 Nl / lh. The pressure at the entrance of the first reaction stage is 3.1 bar. T _A = 322 ° C; T _B = 328 ° C;
The product gas mixture leaving the first reaction stage has the following contents: Vol .-% acrylic acid 0.46 acetic acid 0.14 H ₂ O 10.65 1-butene 0.01 acrolein 6.99 propane 15.16 propylene 0.17 Ethan 1.10 ethylene 0.20 CO ₂ 1.62 CO 0.23 N ₂ 57,02 O ₂ 6.25

• U ^P _{A ',} the propene conversion at the end of the reaction zone A, is 64.5 mol .-%.
• U ^P _{B 'is} the propene conversion at the end of the reaction zone B, is 94.9 mol .-%.

As much air (25 ° C) is metered into the product gas mixture of the first stage that the ratio of acrolein: O ₂ in the resulting mixture is 6.59 to 7.07.

With This mixture is then fed directly to the second reaction stage (T = 231.7 ° C).

The acrolein loading of the fixed catalyst bed is 152 Nl / lh. T _C = 263 ° C; T _D = 269 ° C. The pressure at the inlet of the second reaction stage is 2.1 bar.

• U^A _C' der Acroleinumsatz am Ende der Reaktionszone C, beträgt 68,1 mol.-%.
• U^A _D' der Acroleinumsatz am Ende der Reaktionszone D, beträgt 98,8 mol.-%.

The product gas mixture leaving the second reaction stage has the following contents: Vol .-% acrylic acid 6.72 acetic acid 0.22 H ₂ O 11.06 formaldehyde 0.14 acrolein 0.05 formic acid 0.03 maleic anhydride 0.06 benzoic acid 0.01 propane 14.62 propylene 0.28 Ethan 1.02 ethylene 0.18 CO ₂ 2.03 CO 0.52 N ₂ 59.86 O ₂ 3.20 propionic 0.0032

• U ^A _{C '} the acrolein conversion at the end of the reaction zone C, is 68.1 mol .-%.
• U ^A _{D '} the acrolein at the end of the reaction zone D, is 98.8 mol .-%.

Of the on the amount of acrylic acid contained related propionic acid content is significantly lower than those of the examples of WO 01/96270.

In flows through both reaction stages the reaction gas mixture, the two catalyst tubes from top to bottom.

The Content analyzes are carried out by means of gas chromatographic analysis.

The acrylic acid is from the product gas mixture as in the exemplary embodiments DE-A 10 2004 032 129 separated and the residual gas as the oxidation gas returned to the heterogeneously catalyzed dehydrogenation.

Claims (53)

A process for the preparation of acrolein or acrylic acid or a mixture thereof by heterogeneously catalyzed partial gas-phase oxidation of propene, comprising reacting the reaction gas starting mixture comprising the propylene and molecular oxygen reactants and the inert diluent gases, molecular nitrogen and propane 2 containing the molecular oxygen and the propylene in a molar ratio O ₂ : C ₃ H ₆ ≥ 1, at elevated temperature through a fixed catalyst bed whose active material is at least one multimetal oxide containing the elements Mo, Fe and Bi, characterized in that the reaction gas starting mixture 2 , based on its total volume, the following contents 6 to 9 vol.% propylene, 8 to 18% by volume molecular oxygen, 6 to 30 vol.% Propane and 32 to 72 vol.% molecular nitrogen, with the proviso that the molar ratio V, of the reaction gas starting mixture 2 Propane contained in the starting reaction gas mixture 2 contained propylene 1 to 4, the molar ratio of V ₂ in the reaction gas starting mixture 2 contained molecular nitrogen to in the starting reaction gas mixture 2 contained molecular oxygen 2 to 6 and the molar ratio of V ₃ in the reaction gas starting mixture 2 contained molecular oxygen to in the reaction gas starting mixture 2 contained propylene 1.3 to 2.4. A method according to claim 1, characterized in that the reaction gas starting mixture 2 the following contents 7 to 9 vol.% propylene, 9.8 to 16% by volume molecular oxygen, 9 to 25 vol.% Propane and 35 to 65 vol.% molecular nitrogen, with the proviso that V ₁ = 1 to 3.5 V ₂ = 3 to 4.5 and V ₃ = 1.4 to 2.2. A method according to claim 1, characterized in that the reaction gas starting mixture 2 the following contents 7 to 9 vol.% propylene, 9.8 to 15 vol.% molecular oxygen, 10.5 to 20 vol.% Propane and 40 to 60 vol.% molecular nitrogen, with the proviso that V ₁ = 1.5 to 2.5 V ₂ = 3.5 to 4.5 and V ₃ = 1.4 to 2.14. A method according to claim 3, characterized in that V _{2 is} 3.5 to 4. A method according to claim 3 or 4, characterized in that V _{3 is} 1.5 to 2.0. A method according to claim 1, characterized in that the reaction gas starting mixture 2 the following contents 7 to 9 vol.% propylene, 9.8 to 15.5% by volume molecular oxygen, 10.5 to 15.5% by volume Propane and 40 to 60 vol.% molecular nitrogen, with the proviso that V ₁ = 1.5 to 2.2 V ₂ = 3.5 to 4.5 and V ₃ = 1, 5 to 2.14. A method according to claim 6, characterized in that V _{2 is} 3.5 to 4. A method according to claim 6 or 7, characterized in that V _{3 is} 1.5 to 2.0. A method according to claim 1, characterized in that the reaction gas starting mixture 2 the following contents 7 to 8 vol.% propylene, 11.9 to 15.5% by volume molecular oxygen, 11.9 to 15.5% by volume Propane and 50 to 60 vol.% molecular nitrogen, with the proviso that V ₁ = 1.7 to 2.1 V ₂ = 3.5 to 4.5 and V ₃ = 1.7 to 2.1. A method according to claim 9, characterized in that V _{2 is} 3.5 to 4. A method according to claim 9 or 10, characterized in that V _{3 is} 1.8 to 2.0. A method according to claim 1, characterized in that the reaction gas starting mixture 2 the following contents 7 to 9 vol.% propylene, 9.8 to 15 vol.% molecular oxygen, 21 to 28% by volume Propane and 40 to 60 vol.% molecular nitrogen, with the proviso that V ₁ = 3 to 4 V ₂ = 3.5 to 4.5 and V ₃ = 1.4 to 2.14. A method according to claim 12, characterized in that V _{2 is} 3.5 to 4. A method according to claim 12 or 13, characterized in that V _{3 is} 1.5 to 2.0. Method according to one of claims 1 to 14, characterized in that the total content of the reaction gas starting mixture 2 at components other than propylene, molecular oxygen, propane and molecular nitrogen is ≦ 10% by volume. Method according to one of claims 1 to 15, characterized in that the reaction gas starting mixture 2 from 0.5 to 8% by volume of at least one of the compounds methane and ethane. Method according to one of claims 1 to 15, characterized in that the reaction gas starting mixture 2 from 0.5 to 5% by volume of at least one of the compounds methane and ethane. Method according to one of claims 1 to 15, characterized in that the reaction gas starting mixture 2 from 0.5 to 3% by volume of at least one of the compounds methane and ethane. Method according to one of claims 1 to 17, characterized in that the reaction gas starting mixture 2 Contains ≦ 5% by volume of water and ≦ 5% by volume of carbon oxides. Method according to one of claims 1 to 17, characterized in that the reaction gas starting mixture 2 Contains ≦ 3% by volume of water and ≦ 3% by volume of carbon oxides. Method according to one of claims 1 to 17, characterized in that the reaction gas starting mixture 2 Contains ≤ 2 vol .-% of water and ≤ 2 vol .-% carbon oxides. Method according to one of claims 1 to 21, characterized in that the reaction gas starting mixture 2 Contains ≥ 0.5 vol .-% of water. Method according to one of claims 1 to 22, characterized in that the total content of the reaction gas starting mixture 2 is different from propylene, molecular oxygen, propane and molecular nitrogen components ≤ 5 vol .-%. Method according to one of claims 1 to 22, characterized in that the total content of the reaction gas starting mixture 2 at components other than propylene, molecular oxygen, propane and molecular nitrogen is ≦ 3 vol%. Method according to one of claims 1 to 24, characterized in that as a source for the reaction gas in the starting mixture 2 contained propylene is used in continuous heterogeneously catalyzed partial dehydrogenation and / or oxydehydrogenation of propane in the gas phase formed propylene, without that of this propylene unreacted in the heterogeneously catalyzed partial dehydrogenation and / or oxydehydrogenation unreacted propane is separated. Method according to claim 25, characterized in that that the method of heterogeneously catalyzed partial dehydrogenation of propane is an autothermal dehydration. A method according to claim 25 or 26, characterized in that the method of heterogeneously catalyzed partial dehydrogenation of propane is one in which - a dehydrogenation zone containing the propane to be dehydrogenated reaction gas starting mixture 1 is fed continuously, - the starting reaction gas mixture 1 in the dehydrogenation zone is passed through at least one fixed catalyst bed, are formed at which by catalytic dehydrogenation of molecular hydrogen and propylene, - the reaction gas starting mixture 1 before and / or after entry into the dehydrogenation zone at least one molecular oxygen-containing gas is added, - the molecular oxygen in the dehydrogenation zone in the reaction gas mixture 1 - Partially oxidized hydrogen contained in the molecular hydrogen contained in the dehydrogenation zone and a product gas containing molecular hydrogen, water vapor, propylene and unreacted propane is taken with the proviso that the product gas removed from the dehydrogenation divided into two subsets of identical composition and one of the two subsets Dehydrierkreisgas is recycled to the dehydrogenation zone. A method according to claim 27, characterized in that the Dehydrierkreisgas in the starting reaction gas mixture 1 is returned. A method according to claim 28, characterized in that the reaction gas starting mixture 1 includes: 15 to 25 vol.% Propane, 2 to 6% by volume propylene, 5 to 20 vol.% Steam, 2 to 10 vol.% molecular hydrogen, 40 to 75 vol.% molecular nitrogen and > 0 to 3 vol.% molecular oxygen. Method according to claim 28 or 29, characterized that the product gas propane and propylene propene in the molecular ratio to propane of 0.3 to 0.66. Method according to one of claims 1 to 24, characterized in that as a source for the reaction gas in the starting mixture 2 Propylene contained, the product gas mixture of a partial propane dehydrogenation is used after at least 50 vol .-% of the contained in the product gas of the propane dehydrogenation, other than propane and propylene, components have been separated. Method according to one of claims 1 to 31, characterized that the loading of the fixed catalyst bed with propylene ≥ 135 Nl / l · h to 300 Nl / lh is. Method according to one of claims 1 to 31, characterized in that the propylene conversion in a single pass ≥ 90 mol .-% and the associated selectivity of acrolein formation and Acrylsäurenebenproduktbildung taken together ≥ 90 mol .-% and a) the load of the catalyst test bed with in the reaction gas starting mixture 2 b) the fixed catalyst bed consists of a fixed catalyst bed arranged in two spatially successive reaction zones A *, B *, the temperature of the reaction zone A * being from 300 to 390 ° C. and the temperature of the reaction zone B * is 305 to 420 ° C and at the same time at least 5 ° C above the temperature of the reaction zone A *, c) the reaction gas starting mixture 2 the reaction zones A *, B * in the time sequence "first A *", then "B *" flows through and d) the reaction zone A * extends to a conversion of the propene of 40 to 80 mol%. A process according to any one of claims 1 to 33, characterized in that the process according to any one of claims 1 to 33 is followed by a process of heterogeneously catalyzed partial gas-phase oxidation of acrolein formed in a process according to any one of claims 1 to 33, in which a reaction gas starting mixture containing the acrolein 3 passing through a fixed catalyst bed, the active material is at least one of the elements Mo and V-containing multimetal. A method according to claim 34, characterized in that the reaction gas starting mixture 3 has the following contents: 4.5 to 8 vol.% acrolein, 2.25 to 9% by volume molecular oxygen, 6 to 30 vol.% Propane, 32 to 72 vol.% molecular nitrogen, 5 to 15 vol.% Steam. A method according to claim 35, characterized in that the reaction gas starting mixture 3 Contains 4.5 to 9 vol .-% molecular oxygen. A method according to claim 34, characterized in that the reaction gas starting mixture 3 has the following contents: 5.5 to 8 vol.% acrolein, 2.75 to 9 vol.% molecular oxygen, 10 to 25 vol.% Propane, 40 to 70 vol.% molecular nitrogen, 5 to 15 vol.% Steam. A method according to claim 37, characterized in that the reaction gas starting mixture 3 Contains 5.5 to 9 vol .-% molecular oxygen. A method according to claim 34, characterized in that the reaction gas starting mixture 3 has the following contents: 6 to 8 vol.% acrolein, 3 to 9 vol.% molecular oxygen, 10 to 20 vol.% Propane, 50 to 65 vol.% molecular nitrogen and 7 to 13 vol.% Steam. A method according to claim 39, characterized in that the reaction gas starting mixture 3 Contains 6 to 9 vol.% Of molecular oxygen. A method according to claim 39 or 40, characterized in that the reaction gas starting mixture 3 Contains 6 to 7% by volume of acrolein. Method according to one of claims 39 to 41, characterized in that the reaction gas starting mixture 3 Contains 10 to 16% by volume of propane. Method according to one of claims 35 to 42, characterized in that the reaction gas starting mixture 3 Contains 0.5 to 8 vol .-% of methane and / or ethane. Method according to one of Claims 34 to 43, characterized the acrolein loading of the fixed catalyst bed is ≥ 135 Nl / l · h to 290 Nl / lh is. Method according to one of claims 34 to 43, characterized in that the acrolein conversion in a single pass ≥ 90 mol .-% and the concomitant selectivity of acrylic acid formation ≥ 90 mol .-%, and a) the load of the fixed catalyst bed with that in the reaction gas starting mixture 3 b) the fixed catalyst bed consists of a fixed catalyst bed arranged in two spatially successive reaction zones C *, D *, the temperature of the reaction zone C * being from 230 to 270 ° C. and the temperature of the reaction zone D * is 205 to 300 ° C and at the same time at least 5 ° C above the temperature of the reaction zone C *, c) the reaction gas starting mixture 3 the reaction zones C *, D * in the time sequence "first C *", then "D *" flows through and d) the reaction zone C * extends to a conversion of the acrolein of 55 to 85 mol%. Method according to one of claims 1 to 24, characterized in that - The process according to any one of claims 1 to 24 optionally followed by a process of heterogeneously catalyzed partial gas phase oxidation of acrolein formed in a process according to any one of claims 1 to 24 in which one of this acrolein as a constituent of a reaction gas starting mixture 3 leading through a fixed catalyst bed whose active composition is at least one multimetal oxide containing the elements Mo and V, and - in a first stage preceding the process according to one of claims 1 to 24, propane as constituent of a reaction gas starting mixture 1 a partial heterogeneously catalyzed dehydrogenation in the gas phase to form a product gas mixture 1 containing propylene and unreacted propane subjects, - from the propylene and unreacted propane-containing product gas mixture 1 the upstream stage of the components contained therein, which are different from propane and propylene optionally a partial amount and then separates it as part of the reaction gas starting mixture 2 used, - Acrolein, acrylic acid or their mixture as Zielpro product separates from the product gas mixture resulting in the partial gas phase oxidation and at least while remaining unreacted propane is recycled to the upstream first stage and - the reaction gas starting mixture 2 and / or the reaction gas starting mixture 3 Added fresh propane. Method according to one of Claims 34 to 46, characterized that followed by a process in which the product gas mixture the acroleinpartialoxidation after direct if necessary and / or indirect cooling rising in a separation-containing internals column Fractionally condensed under side draw of a crude acrylic acid and / or with water and / or aqueous solution absorbed. Method according to claim 47, characterized that is followed by a process in which the crude acrylic acid of a Suspension crystallization to form acrylic acid suspension crystallizate and remaining mother liquor. Method according to claim 48, characterized that is followed by a process in which the acrylic acid suspension crystallizate is separated by means of a wash column of residual mother liquor. Method according to claim 49, characterized that the wash column is one with forced transport of Crystal bed is. Method according to claim 49 or 50, characterized the wash column is a hydraulic wash column. Method according to one of Claims 49 to 51, characterized that as washing liquid the Melt of pre-separated in the wash column acrylic acid crystals is used. Method according to one of Claims 49 to 52, characterized that followed by a process in which the separated Acrylsäureauspensionskristallisat melted and copolymerized in free-radically polymers becomes.