EP1144352A2

EP1144352A2 - Method for producing acrolein by heterogeneous catalytic gas-phase partial oxidation of propene

Info

Publication number: EP1144352A2
Application number: EP00912429A
Authority: EP
Inventors: Peter Zehner; Otto Machhammer; Heiko Arnold; Klaus Joachim MÜLLER-ENGEL
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1999-01-22
Filing date: 2000-01-15
Publication date: 2001-10-17
Also published as: WO2000043341A3; DE19902562A1; CN1336908A; WO2000043341A2; US6410785B1; BR0007601A

Abstract

The invention relates to a method for producing acrolein by heterogeneous catalytic gas-phase partial oxidation. A reaction formulation containing propene and molecular oxygen in a molar ratio of C>3<H>6< : O>2< > 1 is converted in successive reaction zones in a catalyst which is in a solid state at an increased temperature. Additional molecular oxygen is added to the reaction gas mixture during the partial oxidation.

Description

Process for the production of acrolein by heterogeneously catalyzed gas phase partial oxidation of propene

description

The present invention relates to a process for the preparation of acrolein by heterogeneously catalyzed gas phase partial oxidation of propene with molecular oxygen over catalysts in the solid state.

Acrolein is an important intermediate, for example for the production of glutardialdehyde, methionine, polyacid and acrylic acid.

It is generally known to produce acrolein by heterogeneously catalyzed gas phase oxidation of propene with molecular oxygen over catalysts in the solid state of aggregation (cf. for example DE-A 1 962 431, DE-A 2 943 707, DE-PS 1 205 502, EP-A 257 565, EP-A 253 409, DE-AS 2 251 364, EP-A 117 146, GB-PS 1 450 986 and EP-A 293 224). The catalysts to be used are usually oxide compositions which usually ensure a high selectivity of acrolein formation. As part of the by-product spectrum, acrolein formation is often accompanied by a certain formation of acrylic acid. In addition to oxygen, the catalytically active oxide composition can contain only one other element or more than one other element (multielement oxide compositions). Particularly frequently used as catalytically active oxide compositions are those which comprise more than one metallic, in particular transition metallic, element. In this case one speaks of multimetal oxide masses. The multimetal oxide materials are usually not simple physical mixtures of oxides of the elemental constituents, but rather heterogeneous mixtures of complex poly compounds of these elements. Such multimetal oxide compositions generally contain the elements Mo, Bi and Fe. As a rule, the heterogeneously catalyzed gas phase oxidation of propene to acrolein takes place at elevated temperature (usually a few hundred ° C., typically 200 to 450 ° C.).

Since the heterogeneously catalyzed gas phase oxidation of propene to acrolein is highly exothermic, it is best carried out in a fluidized bed or in a multi-contact tube fixed-bed reactor, through the space surrounding the contact tubes of which a heat exchange medium is passed. The latter procedure is the preferred one (see, for example, DE-A 4 431 957 and DE-A 4 431 949). The working pressure (absolute pressure) is normally 1 to 10 bar. The target implementation takes place during the dwell time of the reaction gas mixture in the catalyst feed through which it is passed.

As a result of the markedly exothermic nature of the partial oxidation of propene, the oxidation reactors are usually charged with a mixture which contains the reactants molecular oxygen and propylene diluted with a gas which is essentially inert under the conditions of gas-phase catalytic partial oxidation. Here, dilution gases are understood, the constituents of which, under the conditions of the heterogeneously catalyzed gas phase partial oxidation, each constituent per se, remain unchanged at more than 95 mol%, preferably at more than 98 mol%. The inert diluent gas usually combines the largest volume fraction of the three components of the feed gas. One task of the inert diluent gas is to absorb and dissipate the heat released during the partial oxidation. A second task of the inert diluent gas is to reduce the tendency of the reaction mixture to explode. According to DIN 51 649 a molecular oxygen and a combustible gas such as propylene containing gas mixture is outside the explosion range under specified boundary conditions (pressure, temperature) if there is a combustion (ignition, ignition, etc.) initiated by a local ignition source (e.g. glowing platinum wire). Explosion) can no longer spread from the ignition source in the gas mixture.

The classic processes of heterogeneously catalyzed gas phase oxidation of propene to acrolein recommend water vapor and / or nitrogen as an inert diluent gas (cf., for example, US Pat. No. 4,147,885, DE-A 2 056 614, DE-A 52 009 172, DE-A 2 202 734, DE-A 3 006 894 and DE-A 2 436 818). The advantage of using nitrogen as an inert diluent gas is the fact that this opens up the possibility of using air as a source for the molecular oxygen required for the partial oxidation of the propene.

EP-A 293 224 recommends using a saturated hydrocarbon gas mixture consisting of carbon dioxide, water vapor and 1 to 5 carbon atoms as the inert diluent gas.

EP-A 253 409 teaches that inert diluent gases which have an increased molar specific heat are advantageous. DE-A 19 508 531 teaches that inert diluent gases, which in addition to an increased molar specific heat also have the property of flammability, are particularly suitable inert diluent gases. In all of the above cases, the State of the art furthermore to choose the ratio of molecular oxygen to propene in the reaction gas starting mixture for the catalytic partial oxidation of the propene> 1 and to add the total amount of molecular oxygen required for the catalytic partial oxidation of the propene to the reaction gas starting mixture in full.

A disadvantage of the procedure of the prior art is that the inert diluent gases to be used are invariably valuable substances which, for cost reasons, are normally separated from the target product as components of the product gas mixture and are recycled (see, for example, EP-A 253 409 ) can be reused as an inert diluent gas (this also applies to the sole use of nitrogen as an inert diluent gas, since the amount of nitrogen introduced when using air as an oxygen source as an oxygen companion would not be sufficient in its quantity as the sole diluent gas for safe process control ; ie, for safety reasons, an additional nitrogen source (usually circular nitrogen) is always required). However, the aforementioned circuitry is complex (in the event that the product gas mixture of the partial oxidation of propene to acrolein is used directly for a subsequent partial oxidation of the acrolein contained therein to acrylic acid, the cycle gas is usually separated off only after the acrylic acid stage with recycling to the acrolein stage) . The object of the present invention was therefore to provide a process for the preparation of acrolein by heterogeneously catalyzed gas-phase partial oxidation of propene with molecular oxygen on catalysts in the solid state of aggregation, which either only describes the disadvantages of the processes of the prior art reduced form or no longer at all.

Accordingly, a process for the production of acrolein by heterogeneously catalyzed gas phase partial oxidation of propene with molecular oxygen over catalysts in the solid state was found, which is characterized in that a reaction gas starting mixture, the propene and molecular oxygen in a ratio CH ₆ : 0 ₂ > 1 contains, at elevated temperature, first passes through a first reaction zone I equipped with a first catalyst charge I in the solid state of aggregation and thereby oxidizes part of the propene contained in the reaction gas starting mixture to acrolein and then to complete the partial oxidation of the propene to acrolein the product gas mixture I emerging from the reaction zone I at elevated temperature by at least one conducts a further reaction zone having a fixed catalyst charge and thereby increases the molar ratio of molecular oxygen to propene present in the reaction gas mixture in at least one of the further reaction zones by metering in molecular oxygen and / or a gas containing molecular oxygen, with the proviso that that in the product gas mixture emerging from the last reaction zone, based on the propene fed to reaction zone I, at least 90 mol% of the propene are converted with a selectivity of acrolein formation of> 80 mol%.

The advantage of the process according to the invention over the processes of the prior art lies in the fact that the reaction gas mixture in each reaction zone, based on the molar amount of molar oxygen contained, has an increased molar

Contains the sum of propene and acrolein, which in the same way as the inert diluent gas reduces the tendency of the reaction gas mixture to explode and which can absorb and dissipate heat released during the partial oxidation. Since both propene and acrolein also have an increased molar specific heat and the property of flammability, their abovementioned effect is particularly pronounced, which enables a significant reduction in the amount of inert diluent gas to be added without impairing safety. On the other hand, the propene is largely converted into acrolein as it passes through the reaction zones and is separated as such from the product gas mixture leaving the last reaction zone as the target product and not recycled as cycle gas, which is why the advantage of the procedure according to the invention is primarily that Reduced amount of inert gas to be circulated. at

The use of nitrogen as the sole inert diluent gas in the procedure according to the invention can even be completely avoided under favorable circumstances, since the amount of nitrogen in the oxygen source air is sufficient as an inert diluent gas. This also applies if the product gas mixture leaving the last reaction zone of the process according to the invention is directly fed to a heterogeneously catalyzed acrolein partial oxidation to acrylic acid and the target product is only separated off after the acrylic acid stage.

The advantage of the process according to the invention outlined above is, of course, the more pronounced the greater the number of reaction zones used into which molecular oxygen or a gas containing molecular oxygen is metered, ie the lower the proportion of molecular oxygen in the reaction gas mixture chosen within a reaction zone becomes. Out For the sake of expediency, the number of reaction zones into which molecular oxygen or a gas containing molecular oxygen is metered in is generally not more than three in the process according to the invention; the process according to the invention, including the first reaction zone, preferably comprises two reaction zones. Modern method no further reaction zone in both no molecular oxygen and no molecular oxygen ^corresponds' supporting gas is added - with the advantage fiction, includes. If the molecular oxygen is metered in as a constituent of a gas mixture, for example in the form of air, the other constituents of the gas mixture normally form inert gases with respect to the process according to the invention.

All those which are known from the prior art can be used as inert or dilution gases for the process according to the invention. These are, for example, N ₂ , CO, C0, H ₂ 0, saturated hydrocarbons (in particular Ci to C ₅ alkanes) and / or noble gases.

Within the scope of the process according to the invention, the different reaction zones can be charged with one and the same but also with different catalysts. It is only essential according to the invention that the catalyst feed ensures sufficient selectivity of acrolein formation. This is the case with numerous prior art catalysts. Such catalysts are e.g. those of DE-A 2 909 592, especially those from Example 1 of said document. Alternatively, however, the multimetal oxide catalysts II or II 'of DE-A 19 753 817 can also be used. This applies in particular to the exemplary embodiments listed in these documents. Especially when they are designed as hollow cylinder full catalysts as described in EP-A 575 897. The ACF-2 multimetal oxide catalyst from Nippon Shokubai, which contains Bi, Mo and Fe, can of course also be used. The catalyst feed to a single reaction zone can consist of a single catalyst, a mixture of catalysts or a sequential arrangement of different catalysts. The reaction temperature in the reaction zones of the process according to the invention is expediently chosen to be from 300 ° C. to 450 ° C., preferably from 320 to 390 ° C.

Of course, the reaction temperature can be made uniform or different in all reaction zones. As a rule, it is advantageous if the reaction temperatures in Direction of increasing propene conversion within the reaction zone increases.

The at least two reaction zones required according to the invention can be designed as a fluidized bed and / or as a fixed bed. Furthermore, according to the invention, they can be implemented in a single reactor or else in separate reactors connected in series. The process according to the invention is preferably carried out in multi-contact tube fixed bed reactors. The two-zone multi-contact tube fixed-bed reactor described in US Pat. No. 4,203,906 is suitable, for example, for carrying out the process according to the invention in a single rector if, when the reaction gas passes into the second reaction zone, the possibility of metering in molecular oxygen or a molecular oxygen-containing one Gases is created. In both reaction zones, the reaction gas mixture and the heat exchange medium, viewed in the individual reaction zone, can be conducted in cocurrent and / or in countercurrent. The flow of the heat exchange medium can be designed as a pure longitudinal flow, as a longitudinal flow with a superimposed transverse flow or as a radial flow, as described in DE-A 2 201 528.

As a rule, however, a single reaction zone will be designed as a separate multi-contact tube fixed bed reactor within the process according to the invention. The latter can be designed and operated, for example, like those described in EP-A 700 714.

Of course, the counter-current mode of operation described in EP-A 700 714 is also possible. It is essential according to the invention that when the individual reaction zone is implemented as a multi-contact tube fixed bed reactor, in contrast to that described in "Encyclopedia of Chemical Processing and Design, Marcel Dekker, Inc., Vol. 1, Abrasives to Acrylonitrile, 1976, pp 410 to 412 "Reactor parallel connection suggests a series (series) connection of reactors, which underlines the difference of the procedure according to the invention from the procedure of the prior art.

If the process according to the invention is implemented as a two-zone process, it is expedient to connect two multi-contact tube fixed bed reactors in series in this regard and to operate both in the meandering uniform mode of operation described in EP-A 700 714. For example, the reaction gas starting mixture fed to the first reaction zone can contain a propene: oxygen: essentially indifferent gases volume (NL) ratio of (> 1.0 to 3.0): 1: (10 to 1.5), preferably of (1, 1 to 2.0): 1: (10 to 1.5), particularly preferably from (1.2 to 1.5): 1: (10 to 1.5). This is particularly the case if the method according to the invention is designed as a two-zone method. The reaction pressure is usually 0.5 to 5 bar, preferably 1 to 3 bar. In the case of a multi-contact tube fixed bed design of the method according to the invention, the total space load is frequently 1500 to 2500 Nl / l / h.

While in the process according to the invention the molar ratio of propene: molecular oxygen in the reaction gas starting mixture fed to the first reaction zone is inevitably> 1 (as a rule it will be <3), it is preferred according to the invention to add molecular oxygen to the subsequent reaction zones in this way ensure that there is a molar ratio of molecular oxygen: propene> 1 at least in the reaction gas mixture fed to the last reaction zone. According to the invention, it is also advantageous if the molar ratio of (propene and acrolein): molecular oxygen in the reaction gas mixture of each reaction zone is> 1.

In the process according to the invention, in the product gas mixture emerging from the last reaction zone, based on the propene fed to reaction zone I, at least 95 mol% of the propene are preferably reacted with a selectivity of acrolein formation of> 85 mol%. Furthermore, the required molecular oxygen, preferably in the form of air, will be added to both the reaction gas starting mixture and the further reaction zones.

If the process according to the invention is implemented as a series connection of two multi-contact fixed bed reactors, the molar ratio of propene: oxygen in the reaction gas starting mixture can be, for example (> 1.0 to 3.0): 1, often (1.1 to 2.0) : 1 and in the reaction gas mixture fed to the second multi-contact tube fixed bed reactor after the addition of molecular oxygen, for example 1: (> 1.0 to 3.0), often 1: (1.5 to 2.0). Furthermore, the propene conversion in the first multi-contact tube fixed bed reactor will advantageously be 20 to 60 mol%, often 40 to 60 mol%, based on the propene supplied. The above applies in general in the case of a two-zone implementation. In particular in the two-zone implementation of the method according to the invention, it makes sense to determine the total molecular oxygen required in the form of air and the add nitrogen as the sole diluent gas. In this case, recycle gas routing can normally be completely dispensed with or restricted to recycling unreacted propene.

Of course, in the context of the process according to the invention, no pure acrolein is obtained, but a gas mixture from which the acrolein can be separated in a manner known per se (e.g. by absorption in an aqueous medium with subsequent rectificative separation). The acrolein so separated can be used as an intermediate for the synthesis of various end products. However, it can also be used in a heterogeneously catalyzed gas-phase partial oxidation of acrolein with molecular oxygen on catalysts in the solid state to produce acrylic acid. If the acrolein is used in this way for the production of acrylic acid in at least one further gas-phase-catalytic oxidation zone, the reaction gases containing the acrolein of the last propenoxidation zone are normally transferred to this at least one further oxidation zone without removal of secondary components. If necessary, they undergo intermediate cooling beforehand.

In principle, this further heterogeneously catalyzed gas-phase partial oxidation of acrolein to acrylic acid can be carried out in several reaction zones connected in series, in a completely analogous manner to the propene partial oxidation according to the invention. However, it can also be carried out in a manner known per se in a single reaction zone or in a plurality of reaction zones connected in parallel with one another.

The reaction zones are also advantageously implemented as separate multi-contact tube fixed-bed reactors, as is e.g. is described in EP-A 700 893 and the prior art cited therein or in DE-A 4 431 949, DE-A 4 442 346, DE-A 19 736 105 or EP-A 731 082. Multi-metal oxides suitable as catalysts in this regard are e.g. those that contain the elements Mo and V. The reaction temperature in the reaction zones is expediently chosen to be 200 to 300 ° C., preferably 220 to 290 ° C.

The reaction pressure in the reaction zones is usually 0.5 to 5 bar, preferably 1 to 3 bar. The total space load of the multi-contact tube fixed bed reactors is usually 1000 to 2500 Nl / l / h. Suitable catalysts for the acrolein partial oxidation to acrylic acid are, for example, those of the general formula I or I 'from DE-A 4 442 346. Alternatively, however, the multimetal oxide catalysts of DE-A 19736105, in particular the exemplary embodiments mentioned in the abovementioned document, can also be used. The ACS-4 multimetal oxide catalyst from Nippon Shokubai, which includes Bi, Mo and Fe, can of course also be used in the acrolein oxidation stage. In the case of a series connection of the reaction zones for the partial oxidation of acrolein, the statements regarding the propene partial oxidation according to the invention apply in a corresponding manner.

In the case of parallel connection of the reaction zones or in the case of a restriction to a single reaction zone, with an acrolein: oxygen: water vapor: other essentially indifferent gases, volume (Nl) ratio of 1: (0.90 to 3): ( > 0 to 20): (3 to 30), preferably from 1: (0.90 to 3): (0.5 to 10): (7 to 18) worked in the reaction gas inlet mixture. To achieve the desired ratios, it is usually necessary to add additional molecular oxygen to the product gas mixture containing acrolein from the last propene oxidation zone before it is introduced into the at least one acrolein oxidation zone. This can be in the form of air, in the form of nitrogen-depleted air or in the form of pure oxygen. Of course, additional dilution gases, known essentially as indifferent, can be added at this point as desired.

The gas mixture leaving the last acrolein oxidation zone naturally does not consist of pure acrylic acid, but of a gas mixture containing the latter, from which acrylic acid can be separated in a manner known per se.

The various known variants of acrylic acid separation are summarized, for example, in DE-A 19 600 955. Correspondingly, the acrolein could also be separated from the reaction gas mixture leaving the last propene oxidation zone. A common feature of the separation process is that the desired product is separated from the product gas mixture either by absorption with a solvent (cf. also DE-4308087) or by absorption with water and / or by partial condensation. The resulting absorbate or condensate is then worked up by distillation (optionally with the addition of an azeotropic entrainer) and / or crystallization ^', and essentially pure acrylic acid or pure acrolein is thus obtained. The dividing line is drawn in all cases so that a residual gas stream essentially free of acrylic acid and / or acrolein is formed, the main component of which is the indifferent diluent gases and the partial or can be completely reused as inert diluent via recycle gas.

However, the advantage of the process according to the invention is that it minimizes the amount of circulating gas in the inert diluent gas used. It is of particular importance according to the invention that this is possible without reducing the space-time yield. The process according to the invention also makes use of the fact that H ₂ 0 is formed as a by-product of the relevant catalytic gas phase oxidation and acts as an additional inert diluent gas along the reaction path.

In conclusion, it should be noted that the process according to the invention can of course be applied in a completely corresponding manner to the heterogeneously catalyzed gas-phase partial oxidation of isobutyric acid, tert. -Butanol, isobutene, isobutylraldehyde and / or the methyl ether of tert. -Butanols to methacrolein and / or methacrylic acid.

Examples

Production of acrolein and acrylic acid by heterogeneously catalyzed gas phase partial oxidation of propene with molecular oxygen on solid-state catalysts (in the context of the analysis given below, the small proportion of noble gases in air is added to the nitrogen content of the air)

A) Series connection according to the invention

14.0 mol / h of technical propene (that is a mixture of propene and propane with a propane content of 4.3% by volume) and 52.4 mol / h of air become a 66.4 mol / h reaction gas starting mixture of the composition

20.19 vol.% Propene, 60.96 vol.% N ₂ , 16.15 vol.% 0 ₂ , 1.77 vol.% H ₂ 0, 0.03 vol.% CO ₂ and

0.9 vol .-% other components

combined, compressed to a pressure of 1.90 bar and heated to a temperature of 200 ° C. A first reaction tube (V2A steel; length 3.80 m; 2.0 mm wall thickness; 2.6 cm inner diameter) is charged with the aforementioned reaction gas starting mixture as the first propene oxidation zone Temperature of 340 ° C is salt bath cooled. In the direction of flow, the reaction tube is loaded with a pre-fill of steatite balls (diameter: 4-5 mm) over a length of 50 cm. A bed of the 5 multimetal oxide catalyst according to Example 1, 3. / Multimetal Oxide II from DE-A 19753817 follows on a contact tube length of 3.00 m. The product gas mixture leaving the first reaction tube in an amount of 66.5 mol / h is indirectly cooled directly to 200 ° C. to avoid undesired afterburning and has the following composition 10:

10.07% by volume of propene,

9.11% by volume of acrolein,

0.50 vol .-% acrylic acid, 15 60.79 vol .-% N ₂ ,

4.50 vol.% 0 ₂ ,

12.74 vol. H ₂ 0.

0.60 vol.% CO,

0.78 vol.% C0 ₂ and 20 0.91 vol.% Other components.

36.3 mol / h of air are mixed with it and the resulting 102.8 mol / h reaction gas mixture of the composition

25 6.51% by volume of propene,

5.89 vol.% Acrolein,

0.33% by volume of acrylic acid,

66, 61 vol. -% N ₂ ,

10, 14 vol. -% 0 ₂ , 30 9, 03 vol. -% H ₂ 0,

0, 39 vol. -% CO,

0.52 vol. -% C0 ₂ and

0.58% by volume of other components,

35, a second reaction tube (V2A steel, length 3.80 m; 2.0 mm wall thickness; 2.6 cm inner diameter) is charged as the second propene oxidation zone at an inlet pressure of 1.75 bar and an inlet temperature of 200 ° C is cooled along its entire length to a temperature of 350 ° C salt bath. In flow direction

40 tion is the second reaction tube over a length of 50 cm with a pre-fill of steatite balls (diameter: 4-5 mm). A bed of the multimetal oxide catalyst according to Example 1, 3rd / multimetal oxide II from DE-A 19753817 follows on a contact tube length of 3.00 m. The product gas mixture leaving the second reaction tube in an amount of 103.0 mol / h is used to avoid undesired afterburning. indirectly cooled directly to 200 ° C and has the following composition:

0.33 vol.% Propene, 11.47 vol. Acrolein, 0.63 vol.% Acrylic acid, 66.49 vol.% N ₂ , 3.0 vol.% 0 ₂ , 15.75 vol .% H ₂ 0, 0.76 vol.% CO,

0.98 vol.% C0 ₂ and

0.59% by volume of other components.

40.0 mol / h of air are added to it, so that 143.0 mol / h of a reaction gas mixture of the following composition is formed:

0.23 vol.% Propene,

8.27% by volume of acrolein,

0.46% by volume acrylic acid, 69.51% by volume N ₂ ,

7.88 vol.% 0 ₂ ,

11.97% by volume H ₂ 0,

0.55 vol.% CO,

0.71 vol.% C0 ₂ and 0.42 vol.% Other components.

The above-mentioned reaction gas mixture is divided into two partial streams of equal size, which are used to feed two reaction tubes connected in parallel as acrolein oxidation zones (inlet pressure = 1.45 bar, inlet temperature = 200 ° C.). These reaction tubes (V2A steel; length: 3.80 m, 2.0 mm wall thickness; 2.6 cm inner diameter) are each initially in the flow direction over a length of 50 cm with a bed of steatite balls (diameter: 4-5 mm ) loaded. A bed of the multimetal oxide catalyst according to Example b, Sl of DE-A 4442346 follows on a contact tube length of 2.70 m. The entire length of the reaction tubes is kept at 270 ° C. with a salt bath. The outlet pressure of the reaction tubes is 1.35 bar. The product gas mixture leaving the two parallel reaction tubes is combined to form 137.6 mol / h of a total product gas mixture of the following composition:

0.24 vol.% Propene, 0.04 vol.% Acrolein, 8.55 vol.% Acrylic acid, 72.24 vol.% N, 3.15 vol.% 0 ₂ , 12.87 vol.% H ₂ 0, 0.83 vol.% CO, 1.38 vol.% C0 ₂ and 0.7 vol.% Other components.

The leaving the acrolein oxidation hot total product - is the gas mixture in a venturi scrubber (quench apparatus) by di ^¬ rect contact with the region of the narrowest cross-section of the Venturi tube mounted slots einzudüsende quench liquid (140-150 ° C) from 57.4 wt -.% Diphenyl ether, 20.7% by weight diphenyl and 20% by weight o-dimethyl phthalate cooled to a temperature of approx. 160 ° C. Subsequently, the drop-like liquid portion of the quench liquid is separated from the gas phase consisting of reaction gas and vaporized quench liquid in a downstream droplet separator (supply container with gas pipe carried away at the top) and recycled in a circuit I to the venturi scrubber. A partial stream of the recycled quench liquid is subjected to a solvent distillation, the quench liquid being distilled over and high-boiling secondary components which are burned up remaining. The over-distilled quench liquid is fed to the outlet of the absorption column described below.

The gas phase, which has a temperature of approx. 160 ° C., is fed into the lower part of a packed column (3 m high; double jacket made of glass; inner diameter 50 mm; packed zones of lengths (from bottom to top) 90 cm, 90 cm and 50 cm; the packing zones are thermostatted from bottom to top as follows: 90 ° C, 60 ° C, 20 ° C; the penultimate and the last packing zone are separated by a chimney tray; the packing bodies are stainless steel metal coils with a coil diameter of 5 mm and a helix length of 5 mm; the absorbent is fed in directly above the middle packing zone) and the counterflow of 4900 g / h which also consists of 57.4% by weight diphenyl ether, 20.7% by weight diphenyl and 20% by weight o-Dimethylphthalat composite, applied at a temperature of 50 ° C exposed absorbent. Includes such as acrolein and acetic acid absorbed the discharge of the absorption column, which in addition to acrylic acid and low-boiling by-products, is heated in a heat exchanger indirectly to 100 ° C and at the head of a desorption column given that run also as a packed column, a ^■ length of 2 m is (double jacket made of glass; 50 mm inner diameter; filler: stainless steel helixes with a helix diameter of 5 mm and a helix length of 5 mm; a filler zone length 1 m; thermostatted to 120 ° C). In the desorption column, the low boiling point compared to acrylic acid is the components such as acrolein and acetic acid are largely removed from the acrylic acid / absorbent mixture by stripping with residual gas leaving the absorption column (22.8 mol / h residual gas; countercurrent; feed temperature 120 ° C.). The loaded stripping gas leaving the desorption column is recirculated and combined with the hot reaction gas of the acrolein oxidation stage before it enters the venturi quench.

The non-absorbed gas mixture leaving the second packed zone upwards in the absorption column is further cooled in the third packed zone in order to remove the easily condensable part of the secondary components contained therein, e.g. Separate water and acetic acid by condensation. This condensate is called acid water. To increase the separation effect, part of the acid water above the third packing zone of the absorption column is returned to the absorption column at a temperature of 20 ° C. The acid water is extracted below the uppermost packing zone from the chimney floor attached there. The reflux ratio is 200. The amount of acid water to be taken off continuously is 15.6 mol / h. In addition to 90.3% by weight of water, it also contains 2.60% by weight of acrylic acid. If necessary, this can be recovered as described in DE-A 19600955. The residual gas ultimately leaving the absorption column is partly used for stripping and otherwise forms exhaust gas.

The bottom liquid of the desorption column is fed from the bottom to a bottom column containing 57 dual flow trays on the 8th tray (inside diameter: 50 mm; length: 3.8 m; top pressure: 100 mbar; bottom pressure: 280 mbar; bottom temperature: 195 ° C; on the 9th floor there is a pressure loss resistor;) and rectified in the same. From the 48th bottom, 11.7 mol / h of a crude acrylic acid are removed per hour via side draw. The purity of the crude acrylic acid is 99.5% by weight. After partial condensation, a gas stream enriched with low boilers and containing acrylic acid is drawn off at the top of the rectification column and, after its complete condensation, is recirculated to the absorption column in a cooling case (32 g / h) above the lowest packing zone. The absorbent free of low boilers and almost free of acrylic acid is withdrawn from the bottom of the rectification column and recycled into the absorption column above the second packing zone (viewed from below). The reflux at the head of the rectification column ^is' phenothiazine was added as a polymerization inhibitor and in such amounts that the side draw containing 300 ppm of phenothiazine (a schematic representation of the work-up procedure of the reaction gas of the acrolein oxidation stage is shown in DE-A 19600955; moreover, the working-up procedure is also - DE-A 4308087 shown). The composition of the residual gas leaving the absorption column is

0.30 vol.% Propene, 5 0.02 vol.% Acrolein, 0 vol.% Acrylic acid, 90.08 vol.% N ₂ , 3.93 vol.% 0 ₂ , 2.36 vol .% H ₂ 0, 10 1.03 vol.% CO,

1.72 vol.% C0 and

0.557% by volume of other components.

The amount of the residual gas stream is 110.3 mol / h.

15

As the figure attached to this application shows, the gas phase partial oxidation is safely outside the explosion area from the beginning (the figure shows in the nitrogen (N ₂ ) propene (C ₃ H ₆ ) -Oxygen (0 ₂ ) triangle diagram below the boundary condition

20 180 ° C, 1 bar the dividing line between "in the explosion area" [φ] and "outside the explosion area" [Q]). Circulating gas flow can be completely omitted.

B) Classic parallel connection 25

14.0 mol / h technical propene, 107.3 mol / h air and 140.5 mol / h cycle gas of the composition

0.28 vol.% Propene, 30 0.02 vol.% Acrolein,

0 vol .-% acrylic acid,

90, 45 vol. -% N ₂ ,

3, 50 vol. -% 0 ₂ ,

2, 36 vol. -% H ₂ 0, 35 1, 06 vol. -% CO,

1.77 vol. -% C0 ₂ and

0.56% by volume of other components,

become 261.8 mol / h reaction gas starting mixture of the composition

5.26 vol.% Propene, 0.01 vol.% Acrolein, 0 vol.% Acrylic acid, 45 80.21 vol.% N ₂ , 10.27 vol.% 0 ₂ , 2.19 vol .% H ₂ 0, 0.57 vol. -% CO,

0.96 vol. -% C0 ₂ and

0.53% by volume of other components,

combined, compressed to a pressure of 1.90 bar and heated to a temperature of 200 ° C.

The aforementioned reaction gas mixture is divided into two equal-sized partial flows, which are used to feed two reaction tubes (V2A steel; length 3.80 m; 2.0 mm wall thickness; 2.6 cm inside diameter) connected in parallel as propenoxidation zones. These reaction tubes, like the propenoxidation tubes in Example A), are each filled with a pre-fill of steatite spheres (diameter: 4-5 mm) over a length of 50 cm. A bed of the multimetal oxide catalyst according to Example 1, 3rd / multimetal oxide II from DE-A 19753817 follows on a contact tube length of 3.00 m. The length of both reaction tubes is salt bath cooled to a temperature of 350 ° C. The product gas streams leaving the reaction tubes are combined to a total product gas stream of 262.1 mol / h of the following composition:

0.26 vol.% Propene,

4.52% by volume of acrolein, 0.25% by volume of acrylic acid,

80.10% by volume of N ₂ ,

4.5% by volume 0 ₂ ,

7.62 vol.% H0,

0.87 vol.% CO, 1.34 vol.% CO ₂ and

0.54 vol - other components.

18.8 mol / h of air are mixed into the total product gas stream, so that 280.9 mol / h of a reaction gas mixture of the following composition is formed:

0.25 vol.% Propene,

4.22% by volume of acrolein,

0.23% by volume of acrylic acid, 79.91% by volume of N ₂ ,

5.57 vol.% 0.

7.26 vol.% H ₂ 0.

0.81 vol.% CO,

1.25% by volume of CO ₂ and 0.5% by volume of other components. This reaction gas mixture is divided into two partial streams of equal size, which are fed with an inlet pressure of 1.55 bar and an inlet temperature of 200 ° C to feed two reaction tubes connected in parallel as acrolein oxidation zones (V2A steel; length: 3.80 m; 2, 0 mm wall thickness; 2.6 cm inner diameter) can be used. These reaction tubes, like the acrolein oxidation tubes in Example A), are each supplied with a pre-fill of steatite balls (diameter: 4-5 mm) over a length of 50 cm. A bed of the multimetal oxide catalyst according to Example b, S1 of DE-A 4442346 follows on a contact tube length of 3.00 m. The length of both reaction tubes is salt bath cooled to a temperature of 270 ° C. The product gas streams leaving the reaction tubes are indirectly cooled directly to 200 ° C. in order to avoid undesired afterburning and combined to a total product gas stream of 275.4 mol / h of the following composition:

0.25 vol.% Propene,

0.02% by volume of acrolein, 4.29% by volume of acrylic acid,

81.49 vol.% N ₂ ,

3.15 vol.% 0 ₂ ,

7.62 vol.% H ₂ 0.

0.95 vol.% CO, 1, 59 vol.% C0 ₂ and

0.64% by volume of other components.

The hot total product gas stream leaving the acrolein oxidation stage is worked up in a manner corresponding to that in Example A). There are:

15.8 mol / h of acid water, containing 82.8% by weight of water and 2.60% by weight of acrylic acid and

11.7 mol / h 99.5 wt. -% raw acrylic acid.

140.5 mol / h of residual gas are returned to the propene oxidation, 107.7 mol / h of residual gas are fed to the combustion.

The space-time yield of crude acrylic acid in the classic parallel connection corresponds to that in Example A) according to the invention. In contrast to the procedure according to the invention, the classic parallel connection, however, requires the recirculation of 140.5 mol / h of recycle gas in order to operate the gas phase partial oxidation safely outside the explosion area from the start.

Claims

claims

1. A process for the preparation of acrolein by heterogeneously catalyzed gas phase partial oxidation of propene with molecular oxygen over catalysts in the solid state, characterized in that a reaction gas starting mixture, the propene and molecular oxygen in a molar ratio C ₃ H ₆ : 0> 1 contains, at elevated temperature, first passes through a first reaction zone I equipped with a first catalyst charge I in the solid state of aggregation and thereby oxidizes a portion of the propene contained in the reaction gas starting mixture to acrolein and then to complete the partial oxidation of the propene to acrolein

Product gas mixture I emerging from reaction zone I passes at elevated temperature through at least one further reaction zone having a fixed catalyst charge and in at least one of the at least one further reaction zones the molar ratio of molecular oxygen to propene present in the reaction gas mixture by metering in molecular oxygen and / or a molecular oxygen-containing gas increased, with the proviso that in the product gas mixture emerging from the last reaction zone, based on the propene supplied to reaction zone I, at least 90 mol% of the propene are converted with a selectivity of acrolein formation of 80 mol% .

2. The method according to claim 1, characterized in that in the product gas mixture emerging from the last reaction zone, based on the propene fed to reaction zone I, at least 95 mol% of the propene are reacted with a selectivity of acrolein formation of 85 mol%.

Process according to Claim 1 or 2, characterized in that the reaction gas starting mixture contains the propene and molecular oxygen in a molar ratio of C ₃ H ₆ : 0 ₂ > 1.1 and <3.

Method according to one of claims 1 to 3, characterized in that the total number of reaction zones is two.

Sign.

5. The method according to claim 4, characterized in that the

Propene conversion in the first reaction zone, based on the propene supplied, is 20 to 60 mol%.

6. The method according to any one of claims 1 to 5, characterized in that the total amount of molecular oxygen required in the course of the method is supplied as a component of air.

7. The method according to any one of claims 1 to 6, characterized in that a molar ratio of molecular oxygen: propene> 1 is present at least in the reaction gas mixture fed to the last reaction zone.

8. The method according to any one of claims 1 to 7, characterized in that it is carried out continuously without a component of the reaction gas mixture being circulated.

9. A process for the preparation of acrylic acid from propene, characterized in that it comprises a process according to any one of claims 1 to 8.

10. Device, comprising a series connection of at least two multi-contact tube fixed bed reactors, which with

Catalysts are loaded which are suitable for a gas phase oxidation of propene to acrolein.

11. Device comprising a series connection of two series-connected fixed-contact tube fixed bed reactors, which are charged with catalysts which are suitable for a gas phase oxidation of propene to acrolein, and then two parallel-connected fixed-bed reactors connected in parallel, which are equipped with catalysts are loaded, which are suitable for a gas phase oxidation of acrolein to acrylic acid.