DE102005056377A1 - Preparation of acrolein and/or acrylic acid useful for e.g. preparing polymers involves dehydrogenating propane to hydrogen and product gas comprising propylene; and oxidizing molecular hydrogen to steam and product gas to target products - Google Patents

Abstract

Preparation of acrolein and/or acrylic acid (target product) involves: in reaction zone (A): partially dehydrogenating propane catalytically from a mixture of fresh propane and recycled mixture forming molecular hydrogen and propylene; oxidizing molecular hydrogen to steam and forming product gas (A); in reaction zone (B): partially oxidizing product gas (A) to product gas (B) comprising target product; in separation zones: removing target product from product gas (B), scrubbing of carbon dioxide present in the residual gas (I); and recycling residual gas (I) into reaction zone (A). Preparing acrolein and/or acrylic acid (target product) from propane involves: (i) in reaction zone (A): feeding gaseous streams of fresh propane, and recycled mixture to a first reaction zone (A) to form a reaction gas (A), passing the reaction gas A through at least one catalyst bed in which partial heterogeneously catalyzed dehydrogenation of the propane forms molecular hydrogen and propylene; oxidizing molecular hydrogen by molecular oxygen to steam; and feeding the resulting product gas (A) comprising molecular hydrogen, steam, propylene and propane is withdrawn from reaction zone (A) to reaction zone (B); (ii) in reaction zone (B), the product gas A from reaction zone A, with feeding of molecular oxygen, forming a reaction gas (B) comprising molecular hydrogen, steam, propane, propylene and molecular oxygen, and the propylene is subjected to a heterogeneously catalyzed partial gas phase oxidation to give a product gas (B) comprising target product, unconverted propane, molecular hydrogen, steam, carbon dioxide as a by-product, and also other secondary components having lower and higher boiling points than water; (iii) in a first separation zone: removing target product, water and secondary components having a higher boiling point than water present from the product gas (B) to leave a residual gas (I) comprising unconverted propane, carbon dioxide, molecular hydrogen, the secondary components and any unconverted propylene and any unconverted molecular oxygen; (iv) in second separation zone: as an aftertreatment measure (1), scrubbing out carbon dioxide present in residual gas (I) and condensing out water present if any; as an aftertreatment measure (2), discharging a remaining portion of residual gas (I); as aftertreatment treatment measure (3), removing molecular hydrogen by a separating membrane in a third separation zone (III); and as an aftertreatment measure (4), chemically reducing any molecular oxygen present in residual gas (I), where the sequence of use of aftertreatment measures 1 - 4 is as desired; and (v): recycling aftertreated residual gas (I) which comprises unconverted propane into reaction zone (A) as at least one of the two feed streams comprising propane. The amount of molecular hydrogen (M), which is at least 5 - 95 mol % of the total amount of molecular hydrogen produced in reaction zone (A) and that fed to reaction zone (A), is oxidized to steam in reaction zone (A).

Description

• A) – einer ersten Reaktionszone A unter Ausbildung eines Reaktionsgases A wenigstens zwei gasförmige, Propan enthaltende Zufuhrströme zuführt, von denen wenigstens einer Frischpropan enthält, – in der Reaktionszone A das Reaktionsgas A durch wenigstens ein Katalysatorbett führt, in welchem durch partielle heterogen katalysierte Dehydrierung des Propans molekularer Wasserstoff und Propylen gebildet werden, – der Reaktionszone A molekularen Sauerstoff zuführt, der in der Reaktionszone A im Reaktionsgas A enthaltenen molekularen Wasserstoff zu Wasserdampf oxidiert, und – der Reaktionszone A Produktgas A entnimmt, das molekularen Wasserstoff, Wasserdampf, Propylen und Propan enthält,
• B) in einer Reaktionszone B das der Reaktionszone A entnommene Produktgas A unter Zufuhr von molekularem Sauerstoff zur Beschickung wenigstens eines Oxidationsreaktors mit einem molekularen Wasserstoff, Wasserdampf, Propan, Propylen und molekularen Sauerstoff enthaltenden Reaktionsgas B verwendet und das in selbigem enthaltene Propylen einer heterogen katalysierten partiellen Gasphasenoxidation zu einem Acrolein, oder Acrylsäure, oder deren Gemisch als Zielprodukt, nicht umgesetztes Propan, molekularen Wasserstoff, Wasserdampf, Kohlendioxid als Nebenprodukt sowie andere leichter und schwerer als Wasser siedende Nebenkomponenten enthaltenden Produktgas B unterwirft,
• C) aus der Reaktionszone B Produktgas B herausführt und aus selbigem in einer ersten Trennzone I darin enthaltenes Zielprodukt, Wasser und schwerer als Wasser siedende Nebenkomponenten unter Verbleib eines Restgases I abtrennt, das nicht umgesetztes Propan, Kohlendioxid, molekularen Wasserstoff, leichter als Wasser siedende Nebenkomponenten sowie in der Reaktionszone B gegebenenfalls nicht umgesetztes Propylen und gegebenenfalls nicht umgesetzten molekularen Sauerstoff enthält,
• D) – als Nachbehandlungsmaßnahme 1 in einer zweiten Trennzone II in Restgas I enthaltenes Kohlendioxid auswäscht und gegebenenfalls in Restgas I enthaltenes Wasser auskondensiert, – als Nachbehandlungsmaßnahme 2 eine Teilmenge an Restgas I auslässt, – gegebenenfalls als Nachbehandlungsmaßnahme 3 in einer dritten Trennzone III über eine Trennmembran in Restgas I enthaltenen molekularen Wasserstoff abtrennt und – gegebenenfalls als Nachbehandlungsmaßnahme 4 in Restgas I gegebenenfalls enthaltenen molekularen Sauerstoff chemisch reduziert, wobei die Abfolge der Anwendung der Nachbehandlungsmaßnahmen 1 bis 4 beliebig ist, und
• E) nach Anwendung der Nachbehandlungsmaßnahmen 1 und 2 sowie gegebenenfalls 3 und/oder 4 verbleibendes, nicht umgesetztes Propan enthaltendes Restgas I (in dieser Schrift auch Kreisgas I genannt) als wenigstens einen der wenigstens zwei Propan enthaltenden Zufuhrströme in die Reaktionszone A zurückführt.

The present invention relates to a process for the preparation of acrolein, or acrylic acid, or their mixture of propane, wherein

• A) - a first reaction zone A with the formation of a reaction gas A at least two gaseous feed streams containing propane, of which contains at least one fresh propane, - in the reaction zone A, the reaction gas A through at least one catalyst bed leads, in which by partial heterogeneously catalyzed dehydrogenation of the Propane molecular hydrogen and propylene are formed, - the reaction zone A molecular oxygen supplies, which oxidized in the reaction zone A in the reaction gas A molecular hydrogen to water vapor, and - the reaction zone A product gas A takes, containing molecular hydrogen, water vapor, propylene and propane .
• B) in a reaction zone B, the product gas A taken from the reaction zone A with supply of molecular oxygen for charging at least one oxidation reactor with a molecular hydrogen, water vapor, propane, propylene and molecular oxygen-containing reaction gas B and the propylene contained in the same heterogeneously catalyzed partial Subjecting gas phase oxidation to an acrolein, or acrylic acid, or their mixture as target product, unreacted propane, molecular hydrogen, water vapor, by-product carbon dioxide and other product gas B containing lighter and heavier secondary boiling water components,
• C) from the reaction zone B product gas B leads and from the same in a first separation zone I contained therein target product, water and heavier water than boiling secondary components leaving a residual gas I separates the unreacted propane, carbon dioxide, molecular hydrogen, lighter than boiling water secondary components as well as in the reaction zone B optionally unreacted propylene and optionally unreacted molecular oxygen,
• D) - scrubbing carbon dioxide contained in residual gas I as after-treatment measure 1 in a second separation zone II and optionally condensing water contained in residual gas I, - omitting a subset of residual gas I as after-treatment measure 2, optionally as after-treatment measure 3 in a third separation zone III via a separation membrane in the residual gas I contained molecular hydrogen and optionally chemically reduced as a post-treatment measure 4 in residual gas I molecular oxygen contained, the sequence of application of the aftertreatment measures 1 to 4 is arbitrary, and
• E) after application of the aftertreatment measures 1 and 2 and optionally 3 and / or 4 remaining, unreacted propane containing residual gas I (also called recycle gas I in this document) as at least one of the at least two propane-containing feed streams back into the reaction zone A.

Acrylic acid is an important basic chemical that is used inter alia as a monomer for the production of polymers is used, for example in disperse distribution in aqueous Medium be used as a binder. Another one Field of application of acrylic acid polymers form superabsorbents in the hygiene sector and other application areas.

acrolein is a significant intermediate, for example for manufacturing of glutaric dialdehyde, methionine, 1,3-propanediol, 3-picoline, folic acid and Acrylic acid.

The in the preamble this method described for the production of acrolein, or acrylic acid, or their mixture of propane is known in a similar manner (cf. e.g. DE-A 33 13 573 and EP-A 117 146).

It differs from the documents DE-A 10 2004 032 129, EP-A 731 077, DE-A 10 2005 049 699, DE-A 10 2005 052 923, WO 01/96271, WO 03/011804, WO 03 / 076370, WO 01/96270, DE-A 10 2005 009 891, DE-A 10 2005 013 039, DE-A 10 2005 022 798, DE-A 10 2005 009 885, DE-A 10 2005 010 111, DE-A 102 45 585, DE-A 103 16 039, WO 03/011804 described in particular in that at the reaction zone A withdrawn product gas A no material separation is made before it is used to feed the reaction zone B. This is advantageous in that with such material separations (especially in the case of thermal separation process) normally associated value product and energy losses as well as the required expenditure on equipment are avoided at this point. However, the method according to the preamble of this document can not completely avoid such separations. Rather, like all cycle gas processes, it not only requires at least one target product son Also, at least one minor component outlet. However, the method described at the beginning of this document has charm in that it brings the secondary component separation close to the destination product separation and thus to the point where material and thermal gradients must necessarily be forced under energy and apparatus expenditure anyway to reach the goal reach. A coupling of the secondary component separation to such expenses reduces the total expenditure required.

A individually outstanding secondary component in the relevant cycle gas process forms the molecular hydrogen formed in the reaction zone A.

In contrast to the exothermic heterogeneously catalyzed oxydehydrogenation, which is enforced by oxygen present and in the intermediately no free hydrogen is formed (the hydrogen torn from the hydrocarbon to be dehydrogenated is immediately snatched as water (H ₂ O)) or is detectable under the in the reaction zone A to be carried out heterogeneously catalyzed dehydrogenation a ("conventional") dehydrogenation are understood, the heat of reaction is different from the oxydehydrogenation endothermic (as a subsequent step, an exothermic hydrogen combustion in the reaction zone A be included) and is formed at the at least intermediate free molecular hydrogen This usually requires different reaction conditions and different catalysts than for oxydehydrogenation.

That is, the relevant process proceeds in the reaction zone A necessarily under H ₂ evolution. Since the molecular hydrogen thus developed in the reaction zone A is not a reactant of the reaction which takes place in the reaction zone B in the true sense of the word, it does not naturally absorb itself in the relevant cycle gas process. It is therefore the operator's decision to determine where and in what form the molecular hydrogen formed in the reaction zone A (optionally added to the reaction zone A) should be discharged again in the cycle gas process described (ideally, in the reaction zone A the propylene molecule formed will be formed) a hydrogen molecule is formed).

One in the writings of the closest prior art (see, eg US 3,161,670 , EP-A 117 146, DE-A 33 13 573) made proposal is to operate the reaction zone A with the exclusion of molecular oxygen and carry the molecular hydrogen formed in its entirety as an inert gas to behind the separation zone I and then with him in the reaction zone B unreacted molecular oxygen heterogeneously catalyzed to completely burn to water and recirculate the water formed in its entirety in the reaction zone A and thus formed in the reaction zone A molecular hydrogen only on the second pass through the loop and only in the separation zone I in its oxidized form, namely as water, separate. Such a procedure is disadvantageous in several ways. On the one hand, uncoupled molecular hydrogen pollutes the reaction gas B with an explosive gas-capable, both a special diffusion behavior (certain construction materials are permeable to molecular hydrogen) and a pronounced reduction potential (own investigations have shown that a presence of increased amounts of molecular hydrogen in the reaction gas B on the other hand, in the separation zone I, the target product must be separated from a significant amount of water, which is associated with a significant energy requirement, since the target products have a high affinity to Have water (increased amounts of water in the reaction gas B usually also reduce the life of the catalysts used for the partial oxidation, usually Mo in oxide form, since H ₂ O the Promotes sublimation of Mo oxides). In addition, in the described hydrogen combustion behind the separation zone I selectively an extremely high heat of combustion is released, which is locally difficult to exploit advantageous in their extent. Inextricably linked to the problems described above, the use of an excess of molecular oxygen relative to the target reaction stoichiometry for increased catalyst lifetime in the partial oxidation of reaction zone B naturally results in increased explosion risks in reaction zone B (acrylic acid being the target product and becoming the target) Reaction gas B, the previously required amount of molecular oxygen is added in advance, a molar ratio of molecular oxygen contained in the reaction gas B to propylene contained therein of at least> 1.5 is required (while a slight propylene full combustion is already taken into account)).

As an alternative, the prior art also proposes a process variant in which the total amount of molecular hydrogen formed in the reaction zone A is also carried as such to the rear of the separation zone I. It is then recommended to separate the total propane and propylene contained in the residual gas I from all other constituents of the residual gas I, including the molecular hydrogen, and to recycle only the thus separated stream of C ₃ hydrocarbons into the reaction zone A. A disadvantage of this process variant, however, is that it goes along with the necessity is going to convert the total amount of C ₃ hydrocarbons in the full extent in the condensed phase and then return from selbiger back into the gas phase.

When another proposal, DE-A 33 13 573 includes the possibility the molecular hydrogen formed in the reaction zone A still completely in the reaction zone A by means of added molecular oxygen To oxidize water, and either at least a portion of the formed Condensate water even before the reaction zone B and on this Separate way, or the same in the total amount formed entrain into the reaction zone B. Both are disadvantageous. For one thing requires a condensation a cooling off of water of the product gas A to temperatures well below that in the reaction zone B required temperatures are.

To the other, this temperature reduction must be from a very high temperature level starting, as a combustion of molecular hydrogen about twice that amount of heat, which is consumed to its formation in the context of dehydration.

Farther results in leaving the total amount of water generated in the reaction gas in the separation zone I as described above increased separation effort, and in the reaction zone B usually a shortened catalyst life.

in view of the described prior art, the object of the present invention Invention in it, one of the preamble according to the present document according to the method to disposal to put that the disadvantages described at best even in diminished Form has.

The comparatively simple solution The task is to both the outlet of the in the reaction zone A formed molecular hydrogen as well the generation of the outlet over a larger process route to smear. Adhering to the specification, the outlet exclusively behind the Reaction zone B and advantageously (only) a partial outlet in the separation zone I in the form of advance in the reaction zone A by Hydrogen combustion produced water is realized in the separation zone I in the reaction zone B indispensably formed Partialoxidationswasser in the known methods for target product separation anyway with separated, so that the separation zone I corresponding separating effective equipment in natural Way already included.

• A) – einer ersten Reaktionszone A unter Ausbildung eines Reaktionsgases A wenigstens zwei gasförmige, Propan enthaltende Zufuhrströme zuführt, von denen wenigstens einer Frischpropan enthält, – in der Reaktionszone A das Reaktionsgas A durch wenigstens ein Katalysatorbett führt, an welchem durch partielle heterogen katalysierte Dehydrierung des Propans molekularer Wasserstoff und Propylen gebildet werden, – der Reaktionszone A molekularen Sauerstoff zuführt, der in der Reaktionszone A im Reaktionsgas A enthaltenen molekularen Wasserstoff zu Wasserdampf oxidiert, und – der Reaktionszone A Produktgas A entnimmt, das molekularen Wasserstoff, Wasserdampf, Propylen und Propan enthält,
• B) in einer Reaktionszone B das der Reaktionszone A entnommene Produktgas A unter Zufuhr von molekularem Sauerstoff zur Beschickung wenigstens eines Oxidationsreaktors mit einem molekularen Wasserstoff, Wasserdampf, Propan, Propylen und molekularen Sauerstoff enthaltenden Reaktionsgas B verwendet und das in selbigem enthaltene Propylen einer heterogen katalysierten partiellen Gasphasenoxidation zu einem Acrolein, oder Acrylsäure, oder deren Gemisch als Zielprodukt, nicht umgesetztes Propan, molekularen Wasserstoff, Wasserdampf, Kohlendioxid als Nebenprodukt sowie andere leichter und schwerer als Wasser siedende Nebenkomponenten enthaltenden Produktgas B unterwirft,
• C) aus der Reaktionszone B Produktgas B herausführt und aus selbigem in einer ersten Trennzone I darin enthaltenes Zielprodukt, Wasser und schwerer als Wasser siedende Nebenkomponenten unter Verbleib eines Restgases I abtrennt, das nicht umgesetztes Propan, Kohlendioxid, molekularen Wasserstoff, leichter als Wasser siedende Nebenkomponenten sowie in der Reaktionszone B gegebenenfalls nicht umgesetztes Propylen und gegebenenfalls nicht umgesetzten molekularen Sauerstoff enthält,
• D) – als Nachbehandlungsmaßnahme 1 in einer zweiten Trennzone II in Restgas I enthaltenes Kohlendioxid auswäscht und gegebenenfalls in Restgas I enthaltenes Wasser auskondensiert, – als Nachbehandlungsmaßnahme 2 eine Teilmenge an Restgas I auslässt, – gegebenenfalls als Nachbehandlungsmaßnahme 3 in einer dritten Trennzone III über eine Trennmembran in Restgas I enthaltenen molekularen Wasserstoff abtrennt und – gegebenenfalls als Nachbehandlungsmaßnahme 4 in Restgas I gegebenenfalls enthaltenen molekularen Sauerstoff chemisch reduziert, wobei die Abfolge der Anwendung der Nachbehandlungsmaßnahmen 1 bis 4 beliebig ist, und
• E) nach Anwendung der Nachbehandlungsmaßnahmen 1 und 2 sowie gegebenenfalls 3 und/oder 4 verbleibendes, nicht umgesetztes Propan enthaltendes nachbehandelte Restgas I (in dieser Schrift auch Kreisgas I genannt) als wenigstens einen der wenigstens zwei Propan enthaltenden Zufuhrströme in die Reaktionszone A zurückführt, zur Verfügung gestellt, das dadurch gekennzeichnet ist, dass man in der Reaktionszone A eine Menge M an molekularem Wasserstoff zu Wasserdampf oxidiert, die wenigstens 5 mol-% jedoch nicht mehr als 95 mol.-% der Gesamtmenge aus in der Reaktionszone A produziertem und der Reaktionszone A gegebenenfalls zugeführtem molekularem Wasserstoff ist.

Accordingly, as a solution to the object of the invention, a process for the preparation of acrolein, or acrylic acid, or their mixture of propane, wherein

• A) - a first reaction zone A with the formation of a reaction gas A at least two gaseous feed streams containing propane, of which contains at least one fresh propane, - in the reaction zone A, the reaction gas A through at least one catalyst bed leads to which by partial heterogeneously catalyzed dehydrogenation of Propane molecular hydrogen and propylene are formed, - the reaction zone A molecular oxygen supplies, which oxidized in the reaction zone A in the reaction gas A molecular hydrogen to water vapor, and - the reaction zone A product gas A takes, containing molecular hydrogen, water vapor, propylene and propane .
• B) in a reaction zone B, the product gas A taken from the reaction zone A with supply of molecular oxygen for charging at least one oxidation reactor with a molecular hydrogen, water vapor, propane, propylene and molecular oxygen-containing reaction gas B and the propylene contained in the same heterogeneously catalyzed partial Subjecting gas phase oxidation to an acrolein, or acrylic acid, or their mixture as target product, unreacted propane, molecular hydrogen, water vapor, by-product carbon dioxide and other product gas B containing lighter and heavier secondary boiling water components,
• C) from the reaction zone B product gas B leads and from the same in a first separation zone I contained therein target product, water and heavier water than boiling secondary components leaving a residual gas I separates the unreacted propane, carbon dioxide, molecular hydrogen, lighter than boiling water secondary components as well as in the reaction zone B optionally unreacted propylene and optionally unreacted molecular oxygen,
• D) - scrubbing carbon dioxide contained in residual gas I as after-treatment measure 1 in a second separation zone II and optionally condensing water contained in residual gas I, - omitting a subset of residual gas I as after-treatment measure 2, optionally as after-treatment measure 3 in a third separation zone III via a separation membrane separated in residual gas I molecular hydrogen and separated Optionally chemically reduced as a post-treatment measure 4 in residual gas I molecular oxygen contained, the sequence of application of the aftertreatment measures 1 to 4 is arbitrary, and
• E) after application of the aftertreatment measures 1 and 2 and optionally 3 and / or 4 remaining, unreacted propane containing post-treated residual gas I (also called recycle gas I in this document) as at least one of the at least two propane-containing feed streams in the reaction zone A, to , Characterized in that in the reaction zone A, an amount M of molecular hydrogen is oxidized to water vapor, the at least 5 mol% but not more than 95 mol .-% of the total amount of produced in the reaction zone A and the reaction zone A is optionally supplied molecular hydrogen.

According to the invention preferred is the amount M (always in a corresponding manner) at least 10 mol%, but not more than 90 mol%. Particularly preferred is the amount M is at least 15 mol% but not more than 85 mol%. Even better the amount M is at least 20 mol%, but not more than 80 mol%. Most preferably, the amount M is at least 25 mol%, however, not more than 75 mol%. Even better, the amount M is at least 30 mol% and at most 70 mol%. Furthermore, a quantity M of at least 35 is advantageous mol% and at most 65 mol%. Even better, the amount M is at least 40 mol%, but not more than 60 mol%. At best, the amount M is at least 45 mol% and not more than 55 mol%. M = 50 mol% is very particular according to the invention advantageous.

One significant advantage of the procedure according to the invention consists in that the propane is predominant throughout its entire district leadership in gaseous form State of aggregation remains, i.e. not converted to the condensed liquid phase got to. This is advantageous not least because propane a comparatively nonpolar molecule is, the condensation is relatively expensive.

One Another advantageous feature of the method according to the invention is that the reaction gas B necessary molecular Hydrogen limited according to the invention Contains quantity. Because this has, in addition to the aforementioned adverse properties according to the teaching DE-A 33 13 573 and EP-A 117 146 also the advantageous Characteristic that it is chemically inert in the reaction zone B. behaves. That is, at least 95 mol%, usually even at least 97 mol% or at least 99 mol% of the reaction zone B supplied molecular Hydrogen remains chemical when passing through the reaction zone B. unchanged receive.

He but at the same time has the advantage of highest thermal conductivity within the gases. According to Walter J. Moore, Physical Chemistry, WDEG Verlag, Berlin (1973), page 171, this is z under normal conditions. B. more than ten times as large, like the thermal conductivity of carbon dioxide and about eight times that of molecular Nitrogen or molecular oxygen.

It is this increased thermal conductivity which, according to Table I of DE-A 33 13 573, is responsible for the fact that, in reaction zone B, the selectivity of CO _x formation in the presence of molecular hydrogen is significantly lower than in the absence of molecular hydrogen. It causes lower catalyst surface temperatures due to faster heat removal from the reaction site in reaction zone B, and consequently a lower proportion of full propylene combustion. This applies in particular if, as in the process according to the invention, highly thermally conductive molecular hydrogen and highly thermally absorbable propane are synergistically involved in heat removal.

Under Fresh propane is understood in this writing propane, that still not participated in dehydrogenation in the reaction zone A. Has. It is usually used as a component of crude propane (which is preferably the specification according to DE-A 102 46 119 and DE-A 102 45 585 met) supplied in small quantities also of propane different components contains. Such crude propane is, for example, in DE-A 10 2005 022 798 described method available. In general, the reaction zone A next to fresh propane as another Propane-containing feed stream only according to the invention aftertreated residual gas I fed.

In the process according to the invention, a feed of fresh propane exclusively into the reaction zone A preferably occurs as a constituent of the feed gas mixture for the reaction zone A. In principle, partial quantities of the fresh propane can also be introduced into the feed gas mixtures of the first and / or second oxidation stage of the reaction zone B for reasons of explosion safety fed who the.

The Reaction gas B, with which the reaction zone B is charged, meets in this Font in a convenient way also recommended in DE-A 102 46 119 and DE-A 102 45 585 Specification. Further, between reaction zone A and reaction zone B according to the invention expedient one mechanical separation operation according to DE-A 103 16 039 switched.

Under the loading of a catalyst bed catalyzing a reaction step with reaction gas, the amount of reaction gas in this document in norm liters (= NI, the volume in liters, which is the corresponding Reaction gas amount under normal conditions (0 ° C, 1 bar) would take) understood, per hour through one liter of catalyst bed (eg fixed catalyst bed) guided becomes.

The Strain can only affect one component of the reaction gas be related. Then it is the amount of this ingredient in Nl / l · h that per hour through a liter of catalyst bed is performed (pure inert material beds are not counted to the fixed catalyst bed). Is the catalyst bed from a mixture of catalyst and inert diluent bodies the load, as far as this is mentioned, only on the Volume unit be based on catalyst contained.

When An inert gas should generally be a reaction gas constituent in this document be understood, under the conditions of the corresponding Reaction essentially as chemically inert and behaves - each inert reaction gas component for themselves considered - too more than 95 mol%, preferably more than 97 mol% or more as chemically unchanged as 99 mol% preserved.

Typical reaction gases B, with which the reaction zone B can be charged in the process according to the invention, have the following contents: 4 to 25 vol.% propylene, 6 to 70 vol.% Propane, 5 to 60 vol.% H ₂ O, 5 to 65 vol.% O ₂ and 0.3 to 20 vol.% H ₂ .

Reaction gases B preferred according to the invention have the following contents: 6 to 15 vol.% propylene, 6 to 60 vol.% Propane, 5 to 30% by volume H ₂ O, 8 to 35 vol.% O ₂ and 2 to 18% by volume H ₂ .

Very particularly preferred reaction gases B according to the invention for the aforementioned feed have the following contents: 8 to 14 vol.% propylene, 20 to 55% by volume, preferably 20 to 45% by volume Propane, 10 to 25 vol.% H ₂ O, 10 to 25% by volume, preferably 15 to 20% by volume O ₂ and 6 to 15 vol.% H ₂ .

The Advantageous average propane content in the reaction gas B results z. B. from DE-A 102 45 585.

Within the aforementioned composition grid, it is favorable if the molar ratio V ₁ of propane contained in reaction gas B to propylene contained in reaction gas B is 1 to 9 (ie, according to the invention, a propane / propene separation according to WO 04/094041 can advantageously be omitted). , Furthermore, it is advantageous within the aforementioned composition grid if the ratio V ₂ of molecular oxygen contained in the reaction gas B to propylene contained in the reaction gas B is 1 to 2.5. Furthermore, within the meaning of the present invention, it is advantageous within the aforementioned composition grid if the ratio V ₃ of propylene contained in the reaction gas B to the molecular hydrogen contained in the reaction gas B is 0.5 to 20. It is also within the above composition grid advantageous if the molar ratio of V ₄ contained in the reaction gas B water vapor to molar total amount of propane contained in the reaction gas B and propylene is 0.005 to 10.

With particular advantage, V ₁ in the reaction gas B used for charging the reaction zone B (in the reaction gas B starting mixture) is 1 to 7 or up to 4 or 2 to 6 and particularly favorably 2 to 5 or 3.5 to 4.5 , Furthermore, it is preferred for the reaction gas B starting mixture when V _{2 is} 1.2 to 2.0, or 1.4 to 1.8. Furthermore, it is advantageous for the reaction gas B starting mixture if V _{3 is from} 0.5 to 15, or from 0.5 to 10, or from 0.5 to 1.5. In a corresponding manner, V ₄ in the reaction gas B starting mixture is preferably 0.01 to 5, more preferably 0.05 to 3, advantageously 0.1 to 1 and particularly advantageously 0.1 to 0.5 or even 0.3.

Not explosive reaction gas B starting mixtures are preferred according to the invention.

critical for the Answering the question whether the reaction gas B starting mixture explosive is or is not, whether under the condition under certain conditions (Pressure, temperature) mixture one by a local ignition source (eg glowing Platinum wire) initiated combustion (ignition, explosion) propagates or not (see DIN 51649 and the examination description in WO 04/007405). If there is a spread, the mixture should be here be described as explosive. If it does not spread, it will Mixture in this document classified as non-explosive. Is this Reaction gas starting mixture of a partial oxidation according to the invention not explosive, this also applies to the reaction gas mixtures formed from the same in the course of the partial oxidation (see WO 04/007405).

When Source for needed in reaction zone B, not contained in product gas A, molecular oxygen can be molecular Oxygen as such, or a mixture of molecular oxygen and one (or a mixture of such inert gases) in the Reaction zone B chemically inert gas (e.g., noble gases such as Argon, molecular nitrogen, water vapor, carbon dioxide, etc.) be (e.g., air). According to the invention, the molecular weight is preferred Oxygen supplied as gas, not more than 30% by volume, preferably not more than 25% by volume, advantageously not more than 20 vol .-%, particularly advantageous not more as 15% by volume, more preferably not more than 10% by volume, and more preferably not more than 5% by volume of other (other than molecular oxygen) Contains gases. Very cheap it is when the reaction zone B of the molecular oxygen to be supplied supplied in its pure form becomes.

The The aforementioned applies in principle in the same way for the method according to the invention the reaction zone A but the post-treated in the reaction zone A recycled residual gas I optionally contained molecular oxygen going supplied molecular oxygen. Since the oxygen demand in the reaction zone However, A is a comparatively lesser, but is also one Use of air as an oxygen source for oxygen demand in the Reaction zone A in the inventive method in particular for reasons the economy low.

Although in the process according to the invention the aftertreated residual gas I recycled to the reaction zone A will still contain molecular oxygen still remaining in the reaction zone B and in principle this oxygen content of the aftertreated residual gas I recycled to the reaction zone A can also be such that it exceeds that no further supply of molecular oxygen into the reaction zone A is required in the process according to the invention. In principle, however, the post-treated residual gas I recycled into the reaction zone A does not necessarily have to contain oxygen in accordance with the invention, and in many cases according to the invention, an additional oxygen supply is required in the reaction zone A. This can then be carried out as pure oxygen or as a mixture of molecular oxygen and one or more gas (eg N ₂ , H ₂ O, noble gases and / or CO ₂ ) that is chemically inert in the reaction zones A, B (eg. As air). According to the invention, it is preferably carried out as a gas which is not more than 30% by volume, preferably not more than 25% by volume, advantageously not more than 20% by volume, more preferably not more than 15% by volume, better not contains more than 10% by volume and more preferably not more than 5% by volume or not more than 2% by volume of other gases other than molecular oxygen. Also particularly advantageous at this point is the supply of pure oxygen.

above described, preferred according to the invention, Supply of pure oxygen in both the reaction zone A and in the reaction zone B is particularly advantageous because they the circle process according to the invention no over the indispensable burdened with inert gas in the further Course of the circular procedure must be left out again.

Quite generally, it is typical for the process according to the invention that the total content of the reaction gas B starting mixture of propylene, molecular hydrogen, water vapor, propane and mole Mostly ≦ 40% by volume, or ≦ 35% by volume, or ≦ 30% by volume, or ≦ 25% by volume, or ≦ 20% by volume, often ≦ 15% by volume, often ≦ 10% by volume. As a result, total contents amounting to ≦ 5% by volume are difficult to realize within the scope of the procedure according to the invention. Of these other constituents of the reaction gas B starting mixture, up to 80% by volume of ethane and / or methane may be present. Incidentally, such contents will be primarily carbon oxides (CO ₂ , CO) and noble gas, but also minor component oxygenates such as formaldehyde, benzaldehyde, methacrolein, acetic acid, propionic acid, methacrylic acid and the like. Of course, ethylene, isobutene, n-butane, n-butenes and molecular nitrogen also belong to these possible other constituents of the reaction gas B starting mixture. However, an advantage (see EP-A 293 224) of the procedure according to the invention is in principle that the reaction gas B starting mixture contains 0.1 to 30% by volume, or 1 to 25 or 20% by volume, often 5% may contain up to 15% by volume of CO ₂ . The CO content will normally be ≦ 5 vol.%, Or ≦ 4 vol.%, Or ≦ 3 vol.%, Or ≦ 2 vol.%, Usually ≦ 1 vol.%. Usually, however, it contains ≦ 20% by volume, preferably ≦ 15% by volume, more preferably ≦ 10% by volume, and most preferably ≦ 5% by volume of N ₂ .

In principle, all known heterogeneously catalyzed partial dehydrogenations of propane are suitable in reaction zone A, as described, for example, in WO 03/076370, WO 01/96271, EP-A 117 146, WO 03/011804, EP-A 731 077, US Pat. US 3,161,670 DE-A 10 2005 009 891, DE-A 10 2005 013039, DE-A 10 2005 022 798, DE-A 10 2005 009 885, DE-A 10 2005 010 111, DE-A 10 2005 049 699 and from the German application DE-A 10 2004 032 129 are previously known.

D. h., Based on the single passage of the reaction zone A. supplied Propane through the reaction zone A, the reaction zone A can through targeted heat exchange with outside the reaction zone A guided fluid (i.e., liquid or gaseous) Heat transfer isothermal be designed. It can also be adiabatically that is, substantially without such targeted heat exchange with outside the reaction zone A guided Heat carriers are running. In the latter case, the gross monetary value, based on the one-off Passage of the reaction zone A supplied propane through the reaction zone A, by adopting in the preceding writings recommended and measures to be described below both endothermic (negative), or authoterm (essentially zero) or exothermic (positive) become. Likewise the catalysts recommended in the aforementioned publications inventive method be applied. In principle, the heterogeneously catalyzed propane dehydrogenation independently whether adiabatically or isothermally operated, both in a fixed bed reactor as well as in a moving bed, fluidized bed or fluidized bed reactor (The latter is particularly suitable because of its backmixing for one Heating the reaction gas A starting mixture to the reaction temperature in the reaction zone A by hydrogen combustion in the reaction gas A, when the recycle gas I contains molecular oxygen) feasible.

Typically, the heterogeneously catalyzed partial dehydrogenation of propane to propylene requires comparatively high reaction temperatures. The achievable conversion is usually limited by the thermodynamic equilibrium. Typical reaction temperatures are 300 to 800 ° C and 400 to 700 ° C, respectively. One molecule of hydrogen is generated per molecule of propane dehydrogenated to propylene. The working pressure in the reaction zone A is typically 0.3 to 5 or 3 bar. According to the invention, the working pressure in the reaction zone A is preferably 2 to 5 or 4 bar. But it can also be up to 20 bar. If the reaction zone A is operated at very high pressures (eg> 5 or 10 to 20 bar) and burnt in it at least one amount of hydrogen corresponding to at least 50 mol% of the total amount of produced in the reaction zone A molecular hydrogen is It is advantageous to relax the product gas A by expansion in an expansion turbine, and with the work done to co-drive the compressor for the remainder I in advance of its CO ₂ scrubbing. At the same time, the product gas A cools down to the temperature required for further use in the reaction zone 8. High temperatures and removal of the reaction product H ₂ favor the equilibrium position in reaction zone A in the sense of the target product.

Since the heterogeneously catalyzed dehydrogenation reaction proceeds under volume increase, the conversion can be increased by lowering the partial pressure of the dehydrogenation products. This can be achieved in a simple manner, for example, by dehydration under reduced pressure (although carrying out at elevated pressure is generally advantageous for the catalyst service life) and / or by admixing substantially inert diluent gases, such as, for example, steam Dehydrogenation reaction is normally an inert gas. A dilution with water vapor as a further advantage, as a rule, a reduced coking of the catalyst used, since the water vapor reacts with coke formed according to the principle of coal gasification. The heat capacity of the water vapor can also be a part of En To compensate for dehydrogenation dehydrogenation.

While Water vapor in limited quantities in the subsequent reaction zone B for the activity the partial oxidation catalysts usually as beneficial proves one is about it exceeding amount for the reasons already mentioned in the reaction zone B. disadvantageous. In addition, according to the invention in the reaction zone A, a hydrogen partial amount to water vapor be oxidized. It is therefore advantageous according to the invention, if the Reaction zone A supplied Amount of water vapor, based on the catalyst charge the reaction zone A supplied Reaction gas A ≤ 20 Vol .-%, preferably ≤ 15 Vol .-% and particularly preferably ≦ 10 Vol .-% is. In general, the amount of water vapor fed to the reaction zone A, in the same way, but normally ≥ 1 vol.%, often ≥ 2 vol.%, or ≥ 3 vol.% and frequently ≥ 5 vol.% be.

Other suitable for the heterogeneously catalyzed propane dehydrogenation diluents are, for. As nitrogen, noble gases such as He, Ne and Ar, but also compounds such as CO, CO ₂ , methane and ethane. All diluents mentioned can be used either alone or in the form of different mixtures. As far as the aforementioned diluent gases are formed as by-product in the cycle gas process according to the invention, or supplied as fresh gas (or fresh gas constituent), it requires in the inventive method an outlet in an appropriate amount, which is why a corresponding fresh gas supply according to the invention is less preferred. In principle, however, a substantially constant amount of a diluent gas can be circulated in the process according to the invention and any losses associated with it can be freshly supplemented.

in principle come for the heterogeneously catalyzed propane dehydrogenation all in the state of Technically known dehydrogenation catalysts into consideration. They leave roughly divide into two groups. Namely in those that are more oxidic Nature are (for example, chromium oxide and / or alumina) and in those that consist of at least one on one, usually oxidic, carrier isolated, usually comparatively noble, metal (for Example platinum) exist. Among other things, so all dehydrogenation catalysts used in WO 01/96270, EP-A 731077, the 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, the US Pat. 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 become. In particular, you can both the catalyst according to Example 1, Example 2, Example 3, and Example 4 of DE-A 199 37 107 used become.

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 is also suitable in the embodiment 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). To carry out the heterogeneously catalyzed Dehydration in a fluidized bed (or fluid or moving bed) is one use corresponding finely divided catalyst. According to the invention preferred is for the Reaction zone A, the fixed catalyst bed.

In As a rule, the dehydrogenation catalysts (especially those in this Font used by way of example and in DE-A 199 37 107 recommended (in particular the exemplary catalysts of this DE-A) so that they can 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 catalysts quoted prior art writings and the prior art cited at the beginning of this specification.

A comparatively comprehensive description of dehydrogenation processes suitable according to the invention also includes Catalytica® ^® Studies Division, Oxidative Dehydrogenation and Alternative Dehydrogenation Processes, Study Number 4192 OD, 1993, 430 Ferguson Drive, Mountain View, California, 94043-5272 USA.

Characteristic for the partial heterogeneously catalyzed dehydrogenation of propane is how already said that it is endothermic. That is, the for the Adjustment of the required reaction temperature as well as for the reaction needed Thermal energy) must either the reaction gas A in advance and / or in the course of heterogeneous catalyzed dehydration supplied become. Optionally, the reaction gas A has the required heat of reaction itself withdraw yourself.

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 A as already discussed diluted with water vapor 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 at elevated temperature with an oxygen-containing gas (expediently in the absence of hydrocarbons) to flow through and burn off the deposited carbon, so to speak. A certain oppression The 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 of propane also reduces the unwanted formation of allenes (propadiene), Propyne and acetylene as by-products.

In order to can the reaction zone A to form a through the at least one Catalyst bed leading to Reaction gas A (in this document also feed gas mixture the reaction zone A or reaction gas A-starting mixture called) in the simplest case, only fresh propane and recycle gas I are supplied. The latter can already be the amount of molecular oxygen included that needed is required to in the reaction zone A according to the invention required To cause hydrogen combustion. This is due to the fact that in the reaction zone B relative to the reaction stoichiometry molecular oxygen is preferably used in excess, the method of the invention normally largely remains in the recycle gas I.

The Circulating gas I is in the above case also regularly straight so much water vapor that this, together with the im Hydrogen combustion in the reaction zone A formed water vapor, develop its advantageous properties for the overall process can. The supply of additional gaseous streams in the reaction zone is not required in the above case. The in the reaction zone A desired Implementation takes place in the simple passage of the reaction gas A through the same.

Of course, in order to form the reaction gas A to be led through the at least one catalyst bed, in addition to fresh propane and recycle gas I, steam and / or molecular hydrogen may also be fed in order to develop their advantageous effect described in this document. The molar ratio of molecular hydrogen to propane in the feed gas mixture of the reaction zone A is usually ≦ 5. The molar ratio of water vapor to propane can in the feed gas mixture of the reaction zone A z. B. ≥ 0 to 30, suitably 0.1 to 2 and conveniently 0.5 to 1. In addition, the feed gas mixture for the reaction zone A can be supplied with extra molecular oxygen (in pure form and / or as a mixture with inert gas) and / or extra inert gas as required. The desired reaction in the reaction zone A can then again in the simple passage of the reaction gas A (of the feed gas of the reaction zone A) by the same take place without along the reaction path, a supply of other gaseous streams. Under the reaction path in the reaction zone A in this document is the flow path of that propane through the reaction zone A depending on the dehydrogenative conversion (the conversion in the heterogeneously catalyzed dehydrogenation) understood this propane that is fed to the reaction gas A prior to its first passage through at least one catalyst bed of the reaction zone A.

A suitable reactor form for such a single pass, heterogeneously catalyzed propane dehydrogenation the feed gas mixture through the reaction zone A and without intermediate gas feed is z. B. the fixed bed tube or tube bundle reactor. It is located the dehydrogenation catalyst in one or in a bundle of Reaction tubes as a fixed bed. Is the invention required Hydrogen combustion in the reaction zone A such that the reaction taking place in reaction zone A is endothermic Heating the reaction tubes according to the invention expedient from the outside (of course can if necessary, they are also cooled become). This can be z. B. be done by that in the reaction tubes surrounding space a gas, z. A hydrocarbon such as methane, is burned. Cheap It is this direct form of contact tube heating only at first 20 to 30% of the fixed bed apply and the remaining bed length by in the context of Combustion released radiant heat to the required reaction temperature warm. In this way is an approximate isothermal reaction regime reachable. Suitable reaction tube internal diameters are approximately 10 to 15 cm. A typical Dehydrierrohrbündelreaktor comprises 300 to 1000 or more reaction tubes. The temperature in the reaction tube inside ranges from 300 to 700 ° C, preferably in the range of 400 to 700 ° C. With Advantage is the reaction gas A starting mixture the tubular reactor supplied preheated to the reaction temperature. It is possible that the product gas (mixture) A, the reaction tube with a 50 to 100 ° C lower located Temperature leaves. This starting temperature can also be higher or at the same level lie. In the context of the aforementioned procedure is the use oxidic dehydrogenation catalysts based on chromium and / or alumina appropriate. Of the Dehydrogenation catalyst is usually used undiluted. Industrially you can have several such tube bundle reactors operate in parallel and mix their product gases A for charging of the reaction zone B use. Optionally, only two of these can Reactors are in dehydrogenation, while in the third reactor the Catalyst feed is regenerated.

One easy passage of the feed gas through the reaction zone A can also be used in a moving bed or fluid bed or fluidized bed reactor done as it is z. For example, DE-A 102 45 585 and in this document in this regard cited literature describes.

Basically the reaction zone A of the inventive method also of two Sections exist. Such a structure of the reaction zone A is especially recommended if the feed gas for the reaction zone A does not comprise molecular oxygen (this can then, for example, the Be the case when the recycle gas I contains no molecular oxygen).

In In this case, in the first section, the actual heterogeneously catalyzed Dehydration takes place and in the second section, after an intermediate feed of molecular oxygen and / or a mixture of molecular Oxygen and inert gas, the heterogeneously catalyzed required according to the invention Hydrogen combustion.

All In general, the reaction zone will be used in the process according to the invention A operate appropriately, that, based on a single pass through the reaction zone A, ≥ 5 mol% to ≤ 60 mol%, preferably ≥ 10 mol% to ≤ 50 mol%, more preferably ≥ 15 mol% to ≤ 40 mol%, and most preferably ≥ 20 mol% to ≤ 35 mol% the total of the reaction zone A supplied propane in the reaction zone A dehydrating be implemented. Such a limited turnover in the reaction zone A is normally according to the invention therefore sufficient because the remaining amount of unreacted propane in the subsequent reaction zone B substantially as a diluent gas acts and in the further course of the procedure according to the invention can be recycled largely lossless in the reaction zone A. The advantage a procedure with low propane conversion is that at one time Passage of the reaction gas A through the reaction zone A for the endothermic Dehydration needed heat is comparatively low and comparatively for sales low reaction temperatures are sufficient.

According to the invention, it can, as already mentioned, be expedient to carry out (quasi) adiabatically the propane dehydrogenation in the reaction zone A (eg with comparatively low propane conversion). That is, the charge gas mixture for the reaction zone A is usually first heated to a temperature of 500 to 700 ° C (or from 550 to 650 ° C) (for example, by direct firing of the surrounding wall). Normally, a single adiabatic passage through a catalyst bed will then be sufficient to achieve both the desired dehydrogenative conversion and the desired according to the required hydrogen combustion to achieve, wherein the reaction gas, depending on the quantitative ratio of endothermic dehydrogenation and exothermic hydrogen combustion in the gross heat, cool or thermally neutral behavior. According to the invention, preference is given to an adiabatic mode of operation in which the reaction gas cools off in the simple passage in about 30 to 200 ° C. If required, hydrogen formed in the course of the dehydrogenation in a second section of the reaction zone A can be post-combusted with heterogeneously catalyzed intermolecular oxygen. This combustion can likewise be carried out adiabatically.

Remarkably, is in particular in adiabatic operation a single shaft furnace reactor as Fixed bed reactor sufficient, the reaction gas A axially and / or flows through radially becomes.

in the the simplest case is a single closed one Reaction volume, for example, a container whose inner diameter 0.1 to 10 m, possibly also 0.5 to 5 m and in which the fixed catalyst bed on a carrier device (For example, a grid) is applied. That with catalyst charged reaction volume, which in adiabatic operation substantially thermally insulated is, is going to mean, Propane containing, reaction gas A flows through axially. The Catalyst geometry can be both spherical and annular or strand-like be. Since in this case the reaction volume by a very cost-effective Apparatus is to be realized, are all catalyst geometries, the have a particularly low pressure loss, to be preferred. These are mainly catalyst geometries leading to a large void volume to lead or structured, such as monoliths or Honeycombs. To realize a radial flow of the propane-containing Reaction gas A, the reactor can, for example, two in one liner shell located, concentric nested, cylindrical Gratings exist and arranged the catalyst bed in the annular gap be. In the adiabatic case would be the metal shell optionally in turn thermally isolated.

When Inventive catalyst charge for one heterogeneously catalyzed propane dehydrogenation are particularly suitable also those disclosed in DE-A 199 37 107, above all all by way of example disclosed catalysts and their mixtures with respect to the heterogeneously catalyzed dehydrogenation inert geometric shaped bodies.

After a prolonged period of use, the aforementioned catalysts can be regenerated in a simple manner, for example, by dilute air at first at initial temperatures of from 300 to 600 ° C., often at 400 to 550 ° C., first in regeneration stages with nitrogen and / or water vapor the catalyst bed passes. The catalyst (bed) load with regeneration gas (eg air) can be, for example, 50 to 10,000 h ^-1 and the oxygen content of the regeneration gas 0.1 or 0.5 to 20% by volume.

In subsequent further regeneration stages, air can be used as regeneration gas under otherwise identical regeneration conditions. In terms of application, it is advisable to purge the catalyst with inert gas (for example N ₂ ) before it is regenerated.

Then it is It is usually recommended, even with pure molecular hydrogen or with inert gas (preferably water vapor and / or nitrogen) diluted molecular hydrogen (the hydrogen content should be ≥ 1 Vol .-% be) in the otherwise same condition grid to regenerate.

The heterogeneously catalyzed propane dehydrogenation in reaction zone A of the process according to the invention can be operated with comparatively low propane conversion (≤30 mol%) in all cases at the same catalyst bed loading (both the reaction gas as a whole and the propane contained in same) become like the variants with high propane conversion (> 30 mol%). This loading with reaction gas A can be, for example, 100 to 40,000 or 10000 h ^-1 , often 300 to 7000 h ^-1 , that is often about 500 to 4000 h ^-1 .

In particularly elegant way the heterogeneously catalyzed propane dehydrogenation in the reaction zone A (especially in single pass, propane conversions of 15 to 35 mol%) in a tray reactor.

This contains spatial successively more than one dehydrogenation catalyzing Catalyst bed. The catalyst bed number can be 1 to 20, suitably 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.

In expedient manner the reaction gas A is on its way from a catalyst bed to the next Catalyst bed, for example, by passing over heated with hot gases heat exchanger surfaces (e.g. Ribs) or by passing through pipes heated with hot fuel gases, subjected in the Horden reactor of a Zwischenerhitzung (if necessary can be done in a corresponding manner, an intermediate cooling).

Becomes the Horden reactor in the rest operated adiabatically, it is especially for propane conversions ≤ 30 mol%, especially when used the catalysts described in DE-A 199 37 107, in particular the exemplary embodiments, sufficiently, the reaction gas mixture to a temperature of 450 up to 550 ° C (preferably 450 to 500 ° C) preheated to pass into the dehydrogenation reactor and within the tray reactor to keep in this temperature range. That is, the entire propane dehydrogenation is so at extremely low Temperatures to realize what the service life of the fixed catalyst beds between two regenerations proves to be particularly favorable. For higher propane conversions will the reaction gas mixture expediently higher Temperatures preheated in the dehydrogenation reactor led (this can up to 700 ° C) and kept within this elevated temperature range within the tray reactor.

Yet more skillful is the above-described intermediate heating to carry out in a direct way (allows autothermal driving). For this purpose, the reaction gas A is either already before (eg as part of circulating gas I) flow through the first catalyst bed (then the reaction gas A-starting mixture advantageously contain molecular hydrogen added) and / or between subsequent catalyst beds to a limited extent added molecular oxygen. So can (usually by the Dehydrogenation catalysts themselves catalyzed) the required according to the invention limited combustion of contained in the reaction gas A, in the course the heterogeneously catalyzed propane dehydrogenation formed and / or the reaction gas A added molecular hydrogen to particularly purposeful and controlled manner (it can also be appropriate in terms of application technology, to insert catalyst beds in the tray reactor which are fed with catalyst are the most specific (selective) the combustion of hydrogen catalyzes (as such catalysts, for example, those of Fonts US-A 4,788,371, US-A 4,886,928, US-A 5,430,209, US-A 5 530,171, US-A 5,527,979 and US-A 5,563,314; for example can such catalyst beds in an alternating manner to the dehydrogenation catalyst containing beds in Horden reactor be housed; these catalysts are also suitable for the already described hydrogen combustion in a second Section of the reaction zone A). The released heat of reaction thus allows dependent on a total of the amount of molecular hydrogen burned exothermic, or a total autothermal (which is gross thermal essentially zero), or an overall endothermic mode of operation the heterogeneously catalyzed propane dehydrogenation. With increasing selected Residence time of the reaction gas in the catalyst bed is thus a propane dehydrogenation at decreasing or substantially constant temperature possible, which particularly long service life between two regenerations possible and preferred according to the invention is.

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 A, based on the amount of molecular hydrogen contained therein, 0.5 to 50 or 30, preferably 10 to 25 vol .-%. Pure oxygen (preferably according to the invention) or inert gas, for example CO, CO ₂ , N ₂ and / or noble gases, dilute molecular oxygen, but in particular air are also suitable as the source of oxygen. According to the invention, the molecular oxygen is preferably fed in as gas which is not more than 30% by volume, preferably not more than 25% by volume, advantageously not more than 20% by volume, particularly advantageously not more than 15% by volume. more preferably not more than 10% by volume and more preferably not more than 5% by volume of other gases (other than molecular oxygen). Particularly advantageous is the oxygen feed described in pure form.

Since the combustion of 1 mol of molecular hydrogen to give H ₂ O about twice as much (about 240 kJ / mol) of energy, as the dehydrogenation of 1 mol of propane to propylene and H ₂ consumed (about 120 kJ / mol), is a As described autothermal design of the reaction zone A in the adiabatic Horde reactor with regard to envisaged advantage of the procedure according to the invention particularly expedient, but it just requires the combustion of a hydrogen amount of about 50 mol% of the formed in the context of dehydrogenation in the reaction zone A molecular hydrogen.

The Advantageousness of the method according to the invention, however not only when one in the reaction zone A a Hydrogen amount of about 50 mol% of the formed in the reaction zone A. molecular quantity of hydrogen burns. This advantage Rather, it comes into its own even when you are in the reaction zone A is a hydrogen amount of 5 to 95 mol%, preferably 10 to 90 mol%, more preferably 15 to 85 mol%, most preferably 20 to 80 mol%, more preferably 25 to 75 mol%, more favorably 30 to 70 mol%, still more preferably from 35 to 65 mole%, and most preferably from 40 to 60 mol% or 45 to 55 mol% of that formed in the reaction zone A. molecular hydrogen amount to water burns (preferably in the above-described operation of an adiabatic tray reactor).

In As a rule, an oxygen feed as described above should be used be made so that the oxygen content of the reaction gas A, based on the amount of propane and propylene contained therein 0.01 or 0.5 to 3 vol .-% is.

The Isothermia of heterogeneously catalyzed propane dehydrogenation can be 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 internals (eg tubular) attaches. Such installations can be placed in the respective catalyst bed. 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.

One way to heat the charge gas mixture for the heterogeneously catalyzed propane dehydrogenation in the reaction zone A to the required reaction temperature is also to burn a portion of the propane and / or H ₂ contained therein by means of molecular oxygen contained in the feed gas mixture entering the reaction zone A. (For example, on suitable specifically acting combustion catalysts, for example by simply passing and / or passing) and by means of the combustion heat thus released, the heating to the desired temperature for the dehydrogenation reaction (such a procedure is (as already mentioned) in particular in a fluidized bed reactor advantageous).

the As mentioned above, one can use the reaction zone A of the process according to the invention like the documents DE-A 10 2004 032 129 and DE-A 10 2005 013 39, but with the difference that as feed gas mixture the reaction zone A, a mixture of water vapor, fresh propane and Circulating gas I is used. In this case, the reaction zone A is as (preferably adiabater) Horden reactor realizes in which catalyst beds (preferably fixed beds) arranged radially or axially one behind the other are. With advantage amounts the number of catalyst bed grades in such a tray reactor three. Preference is given to the heterogeneously catalyzed partial propane dehydrogenation doing autothermal. For this purpose, the charge gas mixture of the reaction zone A is behind the first catalyst passed through (fixed) bed and between the in the direction of flow bed first catalyst (fixed) bed subsequent catalyst (fixed) to a limited extent molecular oxygen or a containing such Added mixture with inert gas. So, usually, by the Dehydrogenation catalysts themselves catalyzed, a limited combustion of in the course of heterogeneously catalyzed propane dehydrogenation formed Hydrogen (and, if appropriate, to a limited extent of Propane and / or propylene) whose exothermic heat of the Dehydration temperature is substantially maintained.

It is useful the partial heterogeneously catalyzed dehydrogenation of propane thereby operated essentially on the three catalyst hordes so operated, that the conversion of the propane fed into the reactor, based on one-time reactor passage, about 20 mol .-% is (of course he can at inventive method but also 30 mol .-%, or 40 mol .-%, or 50 mol .-%). The achieved selectivity The propylene is regularly at 90 mol .-%. The maximum Sales contribution of a single Horde migrates with increasing operating time in the flow direction from the front to the back. In general, the catalyst feed regenerated before flowing in the direction of flow third horde provides the maximum revenue contribution. With advantage regeneration when the coking of all hordes is identical Extent reached Has.

It is favorable in general for the above-described heterogeneously catalyzed partial dehydrogenation of propane if the total catalyst charge (sum over all beds) with the total amount of propane and propylene is ≥ 500 Nl / l ·h and ≦ 20000 Nl / l ·h ( typical values are 1500 Nl / l · h to 2500 Nl / l · h). The maximum reaction temperature within an individual fixed catalyst bed is advantageously maintained at 500 ° C to 600 ° C (or up to 650 ° C). With particular advantage is the feed gas Mixture for the reaction zone A in the above heterogeneously catalyzed partial propane dehydrogenation in the tray reactor only from fresh propane and recycled from the partial oxidation in the dehydrogenation cycle gas I, which from the partial oxidation resulting usually contains a sufficient amount of water vapor to a satisfactory service life of Dehydrierkatalysatorbetten too require.

adversely the method described above is that almost all catalysts, which catalyze the dehydrogenation of propane, including combustion (full Oxidation of propane and propylene to carbon oxides and water vapor) catalyze propane and propylene with molecular oxygen and, as already stated, usually molecular oxygen in the heterogeneously catalyzed partial dehydrogenation of the propane recycled circulating gas I from the partial oxidation is included.

the can be according to DE-A 102 11 275 by removing that from the dehydrogenation zone Divides product gas into two subsets of identical composition, by only one of the two subsets as product gas A of the partial oxidation supply, while the other subset as part of the reaction gas A in the Dehydration is recycled. The in this from the dehydrogenation itself circulating gas II contained molecular hydrogen, should then be in the feed gas mixture for the Reaction zone A contained propane and optionally propylene protect the same in the same contained molecular oxygen. This Protection is based insist that they are usually heterogeneous by the same catalysts catalyzed combustion of molecular hydrogen to water over the Full combustion of propane and / or propylene is kinetically preferred.

Of the According to the teaching of DE-A 102 11 275, the dehydrogenating gas circulation is expediently carried out according to the Realize jet pump principle (it is also called loop mode designated). Furthermore, in this document the possibility is addressed, the Feed gas mixture for the reaction zone A as additional oxidation protection in addition molecular Add hydrogen. about the requirement of molecular hydrogen in the propellant jet for the Metering jet pump in certain feed hierarchy, gives the DE-A 10 2005 049 699 corresponding information.

According to the teaching DE-A 10 2004 032 129 and DE-A 10 2005 013 039 will be the return of a from the heterogeneously catalyzed partial oxidation, mo molecular oxygen-containing, circulating gas I advantageously not in the feed gas mixture for carry out the heterogeneously catalyzed partial propane dehydrogenation. Rather, this feedback is with Advantage only a certain Dehydrierumsatz in the reaction gas A of the reaction zone A into take place. In this case, in DE-A 10 2004 032 129 also proposed in advance of this recycling the feed gas mixture the reaction zone A preferably additionally external molecular Add hydrogen. Furthermore, also in DE-A 10 2004 032 129 the loop method for the dehydration propagates. Propulsion jet is transferred to the procedure according to the invention exclusively the recycled from the partial oxidation in the dehydrogenation cycle gas I.

The Teaching the embodiment II of DE-A 10 2005 009 885 following is in the reaction zone according to the invention A preferably use a loop procedure in which the feed gas mixture for the same is composed of recycle gas I, from fresh propane, from external molecular hydrogen, from a minimal amount of external water vapor and from the dehydrogenation itself recirculated cycle gas II (on the external water vapor could also be waived). The propulsion jet is a mixture of fresh propane, external molecular hydrogen, cycle gas I from the partial oxidation and external steam used. Regarding the advantageous To be maintained Zudosierungsabfolge in Treibstrahlerzeugung gives DE-A 10 2005 049 699 advantageous recommendation.

According to the invention advantageously includes cycle gas I usually not only molecular oxygen and water vapor, but also molecular hydrogen, carbon monoxide and carbon dioxide. As a rule, it also contains CO. According to the invention, cycle gas I preferably contains from 5 to 15% by volume of CO ₂ .

According to the invention, the product gas A withdrawn from the reaction zone A advantageously has the following contents in the process according to the invention: propane 25 to 60 vol.%, propene 8 to 25 vol.%, H ₂ 0.04 to 25 vol.%, Often up to 15 vol.%, H ₂ O 2 to 25 vol.% And CO ₂ > 0 to 30 vol .-%, often up to 15 vol .-%.

The Temperature of the product gas A is typically 400 to 700 ° C, preferably 450 to 650 ° C.

Of the Pressure of the reaction gas A leaving the product gas A is inventively preferred 2 to 4 bar. But it can also, as already mentioned, be up to 20 bar.

According to the invention will now the product gas A without further secondary component separation to the feed of the at least one oxidation reactor in the reaction zone B with Reaction gas B used.

Advantageously, it is sufficient to add to the product gas A the amount of molecular oxygen required to achieve the objective in the reaction zone B. This additive can in principle be used as pure oxygen or as a mixture (eg air) of molecular oxygen and one or more gases which behave chemically inert in reaction zone B (eg N ₂ , H ₂ O, noble gases, CO ₂ ). respectively. According to the invention, it preferably takes place as a gas which is not more than 30% by volume, preferably not more than 25% by volume, advantageously not more than 20% by volume, particularly advantageously not more than 15% by volume, better not more than 10% by volume and more preferably not more than 5% by volume or not more than 2% by volume of other gases other than molecular oxygen. Especially advantageous at this point is the use of pure oxygen. Normally, the amount of molecular oxygen supplied is such that in the feed gas for the reaction zone B (in the starting reaction gas B mixture), the molar ratio of molecular oxygen to propylene contained is ≥ 1 and ≦ 3. Before the supply of the molecular oxygen to the product gas A, the product gas A is expediently cooled according to the invention to a temperature in the range from 250 to 350 ° C., preferably in the range from 270 to 320 ° C. This cooling is advantageous according to the invention by indirect heat exchange (preferably in countercurrent operation, this statement applies quite generally to indirect heat exchange in this document, unless explicitly stated otherwise). The cooling agent used here is preferably the reaction gas A starting mixture for reaction zone A, which in this way is simultaneously brought to the reaction temperature desired in reaction zone A (in principle, however, cooling can, as already mentioned, also be carried out by expansion in an expansion turbine if the outlet pressure is sufficiently high in this respect).

Sent according to the invention let yourself the supply of molecular oxygen-containing gas for preferably previously cooled as described Product gas A accomplish so that one with the product gas A as Propulsion jet operates a jet pump, which has a motive nozzle, a Includes a mixing section, a diffuser and a suction nozzle, wherein the conveying direction the through the motive nozzle over the Mixing section and the diffuser relaxed jet of fuel into the inlet the at least one oxidation reactor in the reaction zone B as well the suction of the suction nozzle in the direction of the source of the molecular oxygen-containing gas has and thereby the negative pressure generated in the suction that the molecular oxygen Contains containing gas, while mixing with the Propulsion jet transported through the mixing section via the diffuser and the thereby formed reaction gas B starting mixture in the inlet of the second Reaction zone B, or in the inlet of the at least one oxidation reactor the second reaction zone B releases. The above variant is particularly applicable when the product gas A has a pressure of 2 to 5 or more, or up to 4 bar.

Of course you can but the molecular oxygen-containing gas but also on conventional Way with the product gas A to the feed gas mixture for the reaction zone Mix B (reaction gas B starting mixture). According to the invention may be appropriate between reaction zone A and reaction zone B a mechanical separation operation according to DE-A 103 16 039 switched.

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 in the reaction zone B at the stage of acrolein (the stage predominant Acrolein formation) and the target product separation on this Stage to make, or the inventive method to the majority Acrylic acid formation continue and then carry out the target product separation.

If the process according to the invention is carried out up to the predominant formation of acrylic acid, it is advantageous according to the invention to carry out the process in two stages, ie in two consecutively arranged oxidation states, the catalyst to be used being expediently in each of the two oxidation states peat test bed and preferably also the other reaction conditions, such. As the temperature of the fixed catalyst bed can be adjusted in an optimizing manner.

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 invention is in the Reaction zone B to be carried out Process of heterogeneously catalyzed partial oxidation of propylene on fixed catalyst beds, their catalysts as the active material at least have a multimetal oxide containing the elements Mo, Fe and Bi, in particular as a one-step process for the production 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 B produced according to the invention can be described in detail as in EP-A 70 07 14 (first reaction stage; as described there, but also in a corresponding countercurrent procedure of salt bath and reaction gas starting mixture via the tube reactor), 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 an integral Part of this document) (as a two-stage process), WO 04/85363, DE-A 103 13 212 (first reaction stage), EP-A 1 159 248 (as a two-stage process), EP-A 1 159 246 (second reaction stage), EP-A 1 159 247 (as a two-stage process), DE-A 199 48 248 (as a two-stage process), DE-A 101 01 6 95 (single-stage or two-stage), WO 04/085368 (as a two-stage process), DE 10 2004 021 764 (two-stage), WO 04/085362 (first reaction stage), WO 04/085370 (second reaction stage), WO 04/085365 (second reaction stage), WO 04/085367 (two-stage), EP-A 990 636, EP-A 1 007 007 and EP-A 1,106,598.

This especially applies to all embodiments contained in these documents. You can like in described in these publications, but with the Difference that as reaction gas starting mixture for the first Reaction stage (propylene to acrolein) an inventively produced Reaction gas B is used. The other parameters concerning becomes as in the embodiments the said writings method (in particular the fixed catalyst beds and reactant load of fixed catalyst beds). Becomes in the aforementioned embodiments of the prior art two-stage procedure and takes place between the two reaction stages, a secondary oxygen (inventively less preferably secondary air) feed-in, this is done in a similar way, however, adjusted in their quantity so that the molar ratio of molecular oxygen to acrolein in the feed gas mixture of second reaction stage that in the embodiments of said writings equivalent.

According to the invention advantageous the amounts of oxygen in the reaction zone B are measured so that the product gas B contains unreacted molecular oxygen (suitably ≥ 0.5 to 6% by volume, advantageously 1 to 5% by volume, preferably 2 to 4% by volume). In the case of one two-step procedure, the above applies to each of the both oxidation states.

For the respective Oxidation stage particularly suitable Multimetalloxidkatalysatoren are widely described above and well known to those skilled in the art. For example EP-A 253 409 on page 5 refers to corresponding US patents.

Cheap catalysts for the respective oxidation state also disclose DE-A 44 31 957, DE-A 10 2004 025 445 and DE-A 44 31 949. This applies in particular for those the general formula I in the two aforementioned earlier writings. Particularly advantageous catalysts for the respective oxidation state disclose the writings 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 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 the multimetal oxide active compositions mentioned in EP-A 7 00 714.

Also suitable for this reaction stage are the Mo, Bi and Fe-containing multimetal oxide catalysts described in Research Disclosure No. 497012 of 29.08.2005, 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 15 565, EP-A 575 897, 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 × 3 mm, or 7 mm × 3 mm × 4 mm (outer diameter × height × inner 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 (z. B. in fluidized bed reactors).

In principle, active compounds of the general formula IV can be prepared in a simple manner by producing a very intimate, preferably finely divided, stoichiometrically composed, dry mixture of suitable sources of their elemental constituents and this calcine 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 active compounds of the general formula IV can be used for the step "propylene → acrolein (and optionally acrylic acid) "both in powder form used as well as molded to specific catalyst geometries The shaping before or after the final calcination can be done. For example, you can from the powder form of the active composition or its uncalcined and / or partially calcined precursor by compacting 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. Instead of graphite but can also hexagonal boron nitride as an aid be used in the shaping, as described in DE-A 10 2005 037 678 recommends. 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 can be.

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 partially 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. B. from DE-A 29 09 671, 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, z. As balls 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 of the carrier body to be 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 areas of the chemical composition Y ¹ _{a '} Y ² _b' O _{x '} , delimited from their local environment due to their different composition from their local environment, whose largest diameter (longest passing through the center of gravity of the area direct connection of two on the surface (Interface) of the area of points) 1 nm to 100 microns, often 10 nm to 500 nm or 1 micron to 50 or 25 microns, is.

Particularly advantageous inventively suitable Muitimetalloxidmassen 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 ''} ] _{p ''} [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 variables 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 valence and frequency of the elements other than oxygen in VI,
p ", q" = numbers whose ratio p "/ q" is 0.1 to 5, preferably 0.5 to 2,
wherein those masses VI are particularly preferred, 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 especially preferably 100 mol%) of the total proportion of [Y ¹ _{a 'Y} ² _b' O _{x '] p} ([Bi _a''Z ² _b' O _{x '] p'')} of the multimetal oxide V (multimetal VI) in the multimetal oxide compositions V (multimetal oxide materials VP) which are suitable according to the invention in the form of three-dimensionally extended regions of the chemical composition Y ¹ _{a '} Y ² _b' O _{x '} from their local environment because of their chemical composition different from their local environment [Bi _{a ''} Z ² _{b ''} O _{x ''} ] are present, whose largest diameter is 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 fixed catalyst corresponding beds, or as their protective and / or the gas mixture heating pre-feed into consideration.

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. B. those of DE-A 198 15 281, can be found 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 which is denoted by the valence and frequency of the elements other than oxygen in VII is true.

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 (eg in a fluidized bed reactor) used as well as molded to specific catalyst geometries The shaping before or after the final calcination can be done. For example, you can from the powder form of the active composition or its uncalcined precursor composition by compacting to the desired Catalyst geometry (eg by tabletting, extrusion or extrusion) Full catalysts are prepared, optionally with auxiliary agents such as As graphite or stearic acid as a lubricant and / or molding aid and reinforcing agent 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.

When support materials can usual porous or nonporous Aluminas, silica, thoria, zirconia, silicon carbide or silicates such as magnesium or aluminum silicate is used. The carrier bodies can be regular or irregular shaped be regularly formed carrier body with clearly formed surface roughness, z. As balls or hollow cylinder with chippings, are preferred. Suitable is the use of essentially nonporous, surface roughening, spherical carriers made of steatite whose diameter is 1 to 10 mm or 8 mm, preferably 4 to 5 mm. D. h., suitable ball geometries can Diameter of 8.2 mm or 5.1 mm. But it is suitable also the use of cylinders as carrier bodies whose length is 2 to 10 mm and its outer diameter 4 to 10 mm. In the case of rings as a carrier body is the wall thickness over it usually at 1 to 4 mm. Preferably to be used annular carrier body have a length of 2 to 6 mm, an outer diameter from 4 to 8 mm and a wall thickness of 1 to 2 mm. Are suitable especially rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) as a carrier body. The Fineness of the surface of the carrier body to be applied catalytically active oxide masses will of course be of the desired shell thickness adapted (see 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 _{c "} 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 = numbers other than zero 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 vorbildet (starting material I) 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 , which 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 (starting material 2), in the desired ratio p: q incorporated, which optionally dried resulting aqueous mixture, and the dry precursor material thus calcined prior to or after drying to the desired catalyst geometry at temperatures of 250 to 600 ° C.

Prefers are the Multimetalloxidmassen IX, in which the incorporation of Preformed solid starting material 1 in an aqueous starting material 2 at a temperature <70 ° C is carried out. A detailed description of the preparation of multimetal oxide compositions VI catalysts contain z. For example, EP-A 668 104, DE-A 197th 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 execution the parital oxidation of the process according to the invention, of propylene to acrolein (and optionally acrylic acid), can with the described Catalysts z. B. in a one-zone Vielkontaktrohr fixed bed reactor carried out be as described in DE-A 44 31 957. In this case, reaction gas mixture and heat transfer medium (heat exchange medium) over the Reactor can be viewed in cocurrent or countercurrent.

Of 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 B is preferably 1500 to 4000 or 6000 Nl / l · h or more. The propylene load (the propylene loading of the fixed catalyst bed) is typical 90 to 200 Nl / l · h or up to 300 Nl / l · h or more. Propylene loads above 135 Nl / lh or ≥ 140 Nl / lh, or ≥ 150 Nl / lh, or ≥ 160 Nl / l · h are particularly preferred according to the invention preferred, since the reaction gas starting mixture according to the invention for the Reaction zone B due to the presence of unreacted propane and molecular hydrogen causes a favorable hot spot behavior (everything The aforementioned also applies independently 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.

On the Reactor can be considered As already stated, molten salt and reaction gas mixture both be conducted in DC and in countercurrent. The molten salt itself is preferably meandering around guided the contact 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 30 bzw. 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 contact tube length either catalyst only, or a mixture of catalyst and inert material, the latter, based on the mixture, a weight fraction of up to 30 or up to 20 wt. % (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 feed variant is particularly useful when sol as catalysts according to Research Disclosure No. 497012 of 29.08.2005, or according to Example 1 of DE-A 100 46 957 or according to Example 3 of DE-A 100 46 957 and as inert material rings of steatite with the geometry 7 mm × 7 mm × 4 mm (outer diameter × height × inner diameter) can be used. With regard to the salt bath temperature, what has been stated in DE-A 4 431 957 applies.

The execution the partial oxidation in the reaction zone B, from propylene to acrolein (and optionally acrylic acid), can with the described catalysts but also z. In one Two-zone multi-contact tube fixed bed reactor can be carried out as described in DE-A 199 10 506, DE-A 10 2005 009 885, DE-A 10 2004 032 129, DE-A 10 2005 013 039 and DE-A 10 2005 009 891 and DE-A 10 2005 010 111 describes. In both above described cases (And more generally in the process of the invention) is the achieved Propene conversion in a single pass usually at throw> 90 mol%, or> 95 mol% and the selectivity of acrolein formation at values> 90 mol%. According to the invention advantageous the partial oxidation according to the invention takes place from propene to acrolein, or acrylic acid or a mixture thereof in EP-A 1 159 244 and most preferably as in WO 04/085363 and in WO 04/085362.

The Documents EP-A 1 159 244, WO 04/085363 and WO 04/085362 are known as integral part of this document.

D. h., A to be carried out in the reaction zone B partial oxidation of Propylene to acrolein (and optionally acrylic acid) can be particularly advantageous on a fixed catalyst bed with increased propylene load perform the has at least two temperature zones.

This will be z. Reference is made, for example, to EP-A 1 159 244 and WO 04/085362.

The execution of the second step in the case of a two-stage partial oxidation from propylene to acrolein, viz the partial oxidation of acrolein to acrylic acid, can with the described Catalysts z. B. be carried out in a one-zone Vielkontaktrohr fixed bed reactor, as described in DE-A 44 31 949. Reaction gas mixture and Heat transfer medium over the Reactor considered to be run in DC. Usually will the product gas mixture of the preceding propylene partial oxidation according to the invention to acrolein in principle as such (optionally after intercooling (this can indirectly or directly by z. B. secondary oxygen (or less according to the invention preferably secondary air -) - Addition the same), that is, without secondary component separation, in the second reaction stage, i. into the acrolein partial oxidation guided.

The molecular oxygen required for the second step, the acrolein partial oxidation, may already be present in the reaction gas B starting mixture for the propylene partial oxidation to the 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 to acrolein (this can be in the form of secondary air, but preferably in the form of pure oxygen or mixtures of inert gas and oxygen (preferably ≦ 50 vol. %, or ≦ 40 vol%, or ≦ 30 vol%, or ≦ 20 vol%, or ≦ 10 vol%, or ≦ 5 vol%, or ≦ 2 vol% respectively)). Regardless of the procedure, the feed gas mixture (the starting reaction gas mixture) of such a partial oxidation of the acrolein to acrylic acid advantageously has the following contents: 3 to 25 vol.% acrolein, 5 to 65 vol.% molecular oxygen, 6 to 70 vol.% Propane, 0.3 to 20 vol.% molecular hydrogen and 8 to 65 vol.% Steam.

The aforementioned reaction gas starting mixture preferably has the following contents: 5 to 14% by volume acrolein, 6 to 25 vol.% molecular oxygen, 6 to 60 vol.% Propane, 2 to 18% by volume molecular hydrogen and 7 to 35 vol.% Steam.

Most preferably, the aforementioned reaction gas starting mixture has the following contents: 6 to 12 vol.% acrolein, 8 to 20 vol.% molecular oxygen, 20 to 55 vol.% Propane, 6 to 15 vol.% molecular hydrogen and 11 to 26 vol.% Steam, wherein the preferred ranges apply independently, but with advantage be realized simultaneously. The nitrogen content in the abovementioned mixtures is generally ≦ 20% by volume, preferably ≦ 15% by volume, more preferably ≦ 10% by volume and very particularly preferably ≦ 5% by volume. The ratio of the molar amounts of O ₂ and acrolein, O ₂ : acrolein present in the feed gas mixture for the second oxidation stage is advantageously in accordance with the invention generally ≥ 0.5 and ≦ 2, frequently ≥ 1 and ≦ 1.5.

The CO ₂ content of the feed gas mixture for the second oxidation stage may be 1 to 40, or 2 to 30, or 4 to 20, or often 6 to 18, volume percent.

As in the first reaction stage (propylene → acrolein) is also in the second reaction stage (acrolein → acrylic acid) the reaction pressure usually in the range of 1 to 3 bar, and the total space load of the fixed catalyst bed with reaction gas (exit) mixture is preferably from 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 / l · h or more. Acrolein load above 135 Nl / lh, or ≥ 140 Nl / lh, or ≥ 150 Nl / l · h, or ≥ 160 Nl / l · h are particularly preferred, since the invention to be used Reaction gas starting mixture due to the presence of propane and molecular hydrogen also causes a favorable hot spot behavior.

Of the Acroleinumsatz, based on a single pass of the reaction gas mixture through the fixed catalyst bed of the second oxidation stage, is expediently normally ≥ 90 mol% and the associated selectivity of acrylic acid formation ≥ 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. Viewed over the reactor, as already stated, molten salt and reaction gas mixture can be conducted both in cocurrent and 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 30 (bzw. 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, wobei 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:

• Either initially catalyst or a mixture of catalyst and inert material over a length of 50 to 80 or up to 70% of the catalyst tube length, the latter, based on the mixture, having a weight fraction of up to 30 (or 20)% by weight (Section C);
• - Thereafter, over a length of 20 to 40% of the total tube length, either catalyst only, or a mixture of catalyst and inert material, the latter, based on the mixture, a weight fraction of up to 50 and 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 hotspot temperature and hotspot temperature sensitivity). From bottom to top first with up to 30 (or 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 an appropriate manner vice versa.

The aforementioned feed variant is particularly useful if, as catalysts, those according to Preparation Example 5 of DE-A 100 46 928 or those according to DE-A 198 15 281 and as In Steatite rings with the geometry 7 mm × 7 mm × 4 mm or 7 mm × 7 mm × 3 mm (outer diameter × height × inner diameter) are 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 execution Partial oxidation of acrolein to acrylic acid, can 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 10 508. For acrolein sales this applies mentioned above. Also in the case of performing a as above Acroleinpartialoxidation described as the second reaction stage a two-stage propylene partial oxidation to acrylic acid in one Two-zone multiple catalyst tube fixed bed reactor Is used to produce the feed gas mixture (reaction gas starting mixture) appropriate immediately the product gas mixture of the first step (propylene → acrolein) directed partial oxidation (if necessary, after indirect or direct (for example by supplying secondary oxygen) intercooling of the same) use (as described above). The one for the acrolein partial oxidation needed Oxygen is preferably used as pure molecular oxygen (optionally but also as air or as a mixture of molecular oxygen and an inert gas with an inert gas content of preferably ≦ 50% by volume, particularly preferably ≦ 40 Vol .-%, or ≤ 30 Vol .-%, or ≤ 20 Vol .-%, even better ≤ 10 Vol .-%, or ≤ 5 Vol .-%, or ≤ 2 Vol .-%) is added and z. B. the product gas mixture of the first step the two-stage partial oxidation (propylene → acrolein) added immediately. But it can also, as already described, already in the starting reaction gas mixture for the be included first reaction step.

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 series connection (one-zone / two-zone or vice versa) is also possible.

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 be mentioned here, too that a part of the reaction gas B starting mixture for the first Step ("propylene → acrolein") from the separation zone I coming residual gas I and or inventively post-treated residual gas I can be.

According to the invention preferred However, the above-mentioned residual gases I but exclusively as recycle gas I in the heterogeneously catalyzed propane dehydrogenation in the reaction zone A returned and this expedient exclusively as Component of the reaction gas A starting mixture. The completeness It should be noted at this point that both the feed gas mixture for the first oxidation stage as well as the feed gas mixture for the second Oxidation level or just one of both additional fresh propane added can be. It is not preferred according to the invention but can if appropriate, an ignitability to exclude the feed gas mixtures.

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 911 313, EP-A 979 813, EP-A 990 636 and DE-A 28 30 765) the simplest realization of two oxidation states for the both steps of the 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 series-connected Tube bundle systems realized. These may be located in a reactor, wherein the transition from one tube bundle to the other tube bundle is formed by a bed of inert material that is not accommodated in the contact tube (expediently passable). While the contact tubes are usually lapped by a heat transfer medium, this does not reach an inert material fill attached as described above. Advantageously, therefore, the two catalyst tube bundles are housed in spatially separate reactors. As a rule, an intercooler is located between the two tube bundle reactors in order to reduce any subsequent acrolein afterburning in the product gas mixture leaving the first oxidation zone. The reaction temperature in the first reaction stage (propylene → acrolein) is generally from 300 to 450 ° C, preferably from 320 to 390 ° C. The reaction temperature in the second reaction stage (acrolein → acrylic acid) is generally 200 to 370 ° C, often 220 to 330 ° C. The reaction pressure is expedient in both oxidation zones 0.5 to 5, advantageously 1 to 3 bar. The load (Nl / lh) of the oxidation catalysts with reaction gas in both reaction stages is frequently 1500 to 2500 Nl / lh or up to 4000 Nl / lh. The loading with propylene or acrolein can be 100 to 200 or 300 and more Nl / lh.

in principle can designed the two oxidation states in the process of the invention be like z. 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 advantageous to the kinetics of the respective Gas phase partial oxidation and on the catalyst life.

in principle is the heterogeneous to be carried out according to the 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 with a or more catalysts is charged, the active mass a the element Mo, Fe and Bi containing multimetal is, the catalyzes the implementation of both reaction steps. Needless to say, Also, this catalyst feed 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 already mentioned, in addition to air (this is less preferred according to the invention), both pure molecular oxygen and molecular oxygen diluted with inert gas such as CO ₂ , CO, noble gases, N ₂ and / or saturated hydrocarbons are used as sources for an intermediate oxygen feed into consideration. According to the invention, the oxygen source preferably contains ≦ 50% by volume, preferably ≦ 40% by volume, more preferably ≦ 30% by volume, very particularly preferably ≦ 20% by volume, better ≦ 10% by volume or ≦ 5% by volume .-%, or ≤ 2% of gases other than molecular oxygen.

By Metering of z. B. cold oxygen source to the product gas mixture the first partial oxidation stage can in the context of the method according to the invention also on direct way a cooling off be effected before it as part of a starting reaction gas mixture for the second partial oxidation stage is used.

According to the invention advantageous the partial oxidation of acrolein to acrylic acid takes place as in EP-A 1 159 246, and most preferably as in WO 04/085365 and described in WO 04/085370. According to the invention preferred is used as acrolein-containing reaction gas starting mixture However, a starting reaction gas mixture is used, which is the product gas mixture a first-stage partial oxidation according to the invention from propylene to acrolein, which was optionally supplemented with so much secondary air that The relationship from molecular oxygen to acrolein in the resulting reaction gas starting mixture in each case 0.5 to 1.5. The documents EP-A 1 159 246, WO 04/08536 and WO 04/085370 are considered as an integral part of this document.

D. h., The inventive partial oxidation from acrolein to acrylic acid let yourself perform advantageous on a fixed catalyst bed with increased Acroleinbelastung, the has at least two temperature zones.

All in all If a two-stage partial oxidation of propylene to acrylic acid is preferred as in EP-A 1 159 248 or in WO 04/085367 or WO 04/085369 described described.

The product gas B leaving the inventive partial oxidation (after the first and / or second oxidation stage) contains essentially acrolein, or acrylic acid, or their mixture as the target product, unreacted propane, molecular hydrogen, (by-produced product and / or diluent gas) Water vapor, optionally unreacted molecular oxygen (in view of the life of the catalysts used, it is usually favorable if the oxygen content in the product gas mixture of the partial oxidation stages, for example, at least 1.5 to 4 vol .-% is), as well as other (in Normal pressure, ie, 1 bar) lighter and heavier than water boiling by-products or components (such as CO, lower aldehydes, lower alkane carboxylic acids (eg., Acetic, formic and propionic), maleic anhydride, benzaldehyde, aromatic carboxylic acids and aromatic carboxylic anhydrides (e.g., phthalic anhydride and Benzoic acid), optionally further hydrocarbons, such as. B. C ₄ hydrocarbons (eg., Butene-1 and possibly other butenes) and optionally further inert diluent gases such. B. N ₂ ).

When Process for the separation of target product contained in product gas B, Water and heavier than water boiling secondary components come for the Invention according to the method in the first separation zone I in principle all in the prior art in this regard into consideration. They are essentially by it characterized in that the target product z. B. by absorptive and / or condensing measures from the gaseous transferred to the condensed phase and subsequently the condensates or absorbates for the purpose of further target product separation by extractive, distillative, crystallative and / or desorptive activities worked up. Together with the target product and / or on the transfer of the Target product in the condensed phase following these Procedure usually also in the reaction gas B contained water vapor and heavy as water boiling secondary components converted into the condensed phase and thus separated (preferred according to the invention a quantity of water vapor is converted into the condensed phase, the at least 70 mol%, preferably at least 80 mol%, better at least 90 mol% and particularly preferably at least 95 mol% of in the Reaction zone B formed (or more preferably in the product gas B total contained) amount of water vapor.

As an absorption agent come z. As water, aqueous solutions and / or organic solvents into consideration. In the context of this "condensation" (separation) of target product, water and secondary components boiling heavier than water, a residual gas which does not condense into the condensed phase normally remains, which comprises the components of the product gas B which are comparatively difficult to condense (lighter than water) usually especially those components whose boiling point at normal pressure (1 bar) ≤ -30 ° C (their total content of the residual gas B is usually ≥ 60 vol .-%, often ≥ 70 vol .-%, often ≥ 80 vol. These include primarily unreacted propane, molecular hydrogen, carbon dioxide, optionally unreacted propylene, optionally unreacted molecular oxygen, and other secondary components which boil more easily than water, such as CO, for example , Ethane, methane, optionally N ₂ , optionally noble gases (eg He, He, Ar, etc.). To a lesser extent, the residual gas may also be Acry lsäure, acrolein and / or H ₂ O contained. According to the invention, the residual gas preferably contains ≦ 10% by volume, with advantage ≦ 5% by volume and with particular advantage ≦ 3% by volume of water vapor. This aforementioned residual gas forms (based on the amount of propane contained therein) normally at least the main amount (usually at least 80%, or at least 90%, or at least 95% or more) of the residual gas formed in the first separation zone I and is in this document referred to as (main) residual gas I.

Especially when the condensation of the target product by absorption by means of an organic solvent takes place, falls in the separation zone I usually at least a second unreacted propane and optionally unreacted propylene containing residual Gas on (based on propane contained in it is its amount in Compared to the amount of (main) residual gas I normally much lower (usually ≤ 20%, usually ≤ 10%, or ≤ 5%, or ≤ 1%)). This is due to the fact that the forming condensed phase to some extent not converted propane and optionally unreacted propylene receives.

in the further course of the extractive, distillative, crystallizing and / or desorptive separation of the target product from the condensed Phase, this unreacted propane and, where appropriate Propylene usually as part of at least one other Gas phase recovered and referred to in this document as (by-product) residual gas I.

The Sum of (main) residual gas I and (secondary) residual gas I forms in this Case the total amount of residual gas I. If no (minor) residual gas I in the separation zone I, the (main) residual gas I is automatically the total amount residual gas I (also called (total) residual gas I).

Preferably according to the invention, the conversion of the target product from the product gas B into the condensed phase takes place by fractional condensation. This especially if the target product is acrylic acid. Basically, however, for target product separation z. B. all in the documents DE-A 102 13 998, DE-A 22 63 496, US 3,433,840 (Processes described in the abovementioned publications are particularly recommended for the acrolein removal), EP-A 1 388 533, EP-A 1 388 532, DE-A 102 35 847, EP-A 792 867, WO 98/01415, EP-A 1 015 411, EP-A 1 015 410, 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 854 129, 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 695 736, EP-A 982 287, EP-A 1 041 062, EP-A 1 171 46, DE-A 43 08 087, DE-A 43 35 172, DE-A 44 36 243, DE-A 199 24 532, DE-A 103 32 758 and DE-A 199 24 533 described absorptive and / or condensative process for "target product condensation" and further workup of the "condensate" suitable. An acrylic acid removal can also be carried out as described in EP-A 982 287, EP-A 982 289, 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 920 408, EP-A 1 068 174, EP-A 1 066 239, EP-A 1 066 240, WO 00/53560, WO 00 / 53561, DE-A 100 53 086 and EP-A 982 288 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 processes described in the publications WO 04/063138, WO 04/35514, 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.

common A feature of the aforementioned separation method is (as already mentioned) that z. B. at the top of the respective separating internals containing Separating column, in the lower part of the product gas B, usually after prior direct and / or indirect cooling thereof, z. B. is fed, normally a stream of residual gas I remains, mainly those components contains the product gas B, their boiling point at atmospheric pressure (1 bar) lower than that of Water is and is usually ≤ -30 ° C (i.e. the hardly condensable or volatile constituents). residual gas I can but z. B. also contain components such as water vapor and acrylic acid.

in the lower part of the separation column usually fall mainly the less volatile Constituents of the product gas B, including the respective target product, in condensed phase. Condensed aqueous phase is usually over sidestream taken.

Typically, (main) residual gas I contains the following contents: 1 to 20 vol.% H ₂ O, 40 to 90 vol.% Propane, 0 to 10 vol.% O ₂ , 5 to 30% by volume CO ₂ and > 0 to 2 vol.% CO.

Of the Content of acrolein and acrylic acid is usually each <1 Vol .-%.

According to the invention, it is now necessary for the residual gas I obtained in the separation zone I to undergo certain after-treatment measures. It is essential according to the invention that the respective post-treatment measure does not have to be carried out in a necessary manner on the total amount of residual gas I. D. h., It may be appropriate according to the invention, the post-treatment measure only on a subset (eg., Only on (main) residual gas I) of the total amount of the residual gas I perform. In this case, the sum of the untreated subset of the total amount of the residual gas I and the residual gas remaining after the aftertreatment measure has been carried out on the other subset of the total amount of the residual gas I forms the total amount of aftertreated residual gas I (the "new" Residual gas I) which can be subjected in a corresponding manner to further aftertreatment according to the invention (the abovementioned subsets may also have different chemical compositions.) From the unreacted propane-containing total amount of (post-treated) residual gas I remaining after the completion of all after-treatment measures deemed necessary in the respective case , According to the invention, at least one subset as at least one of the at least two propane containing feed streams recycled to the reaction zone A. Another subset (if appropriate also an untreated subset of residual gas I) can optionally be recycled to the first and / or second oxidation stage of reaction zone B in order there to make the explosion behavior of the reaction gas advantageous as a constituent of the respective feed gas.

According to the invention advantageous is the invention as at least one of the at least two propane-containing feed streams into the Reaction zone A recycled amount aftertreated according to the invention However, residual gas I should be such that it is at least 80 mol%, preferably at least 85 mole%, more preferably at least 90 mole%, or at least 92 mol%, or at least 94 mol%, or at least 96 mol%, or at least 98 mol% of the product gas B from the Reaction zone B led out propane contains. This return does not have to necessarily at one and the same point of the reaction zone A done. Rather, this can also be done via several different over the Reaction zone A distributed feed points happen.

Substantially according to the invention is in the context of the aforementioned again that the order of Residual gas I carried out treatment measures in accordance with the invention is.

In According to the invention advantageously However, you will be following sequence of post-treatment measures comply (they refer in particular to the aftertreatment the (main) residual gas I).

First, will you omit a partial amount of residual gas I (the outlet is particular required to deal with the supply of molecular oxygen involved in the overall procedure remove inert components). This omitted amount (in As a rule, the exhausted amount of residual gas I of an exhaust gas combustion fed) can have the same composition as the residual gas I myself. This will usually be the case if the pro Time unit in the discharged residual gas I contained amount of propane, based on the amount fed into the process per unit of time fresh propane ≤30 mol%, better ≤ 25 mol% or ≤ 20 mol%, or ≤ 15 mol%, preferably ≤ 10 mol%, more preferably ≤ 5 mol% and most preferably ≤ 3 mol% or ≤ 1 mol%.

Lying the exuberant Amount of propane higher than the aforementioned values, or even in the case of the aforementioned Values, the outlet can also be done so that the in the amount of residual gas I to be omitted propane and optionally Separate propylene in advance of the outlet from the outlet, where by thus separated propane and optionally propylene component of nachbehandeltem according to the invention Residual gas I remains.

A simple way for the aforementioned separation is z. B. therein, the corresponding amount of residual gas I with a (preferably high-boiling) organic solvent (preferably a hydrophobic, eg., Tetradekan or mixtures of C ₈ -C ₂₀ alkanes), in which propane and propylene (compared to the other constituents the residual gas I is expediently preferred) are brought into contact (eg by simply passing through). By subsequent desorption (under reduced pressure), distillation and / or stripping with a non-interfering in the reaction zone A gas (eg., Water vapor, molecular hydrogen, molecular oxygen and / or other inert gas (eg., Air)) the propane and (optionally) recovered propylene and added to the post-treated residual gas I. In particular, it is possible to proceed from product gas A as in the analogous absorptive separation of propane described in DE-A 10 2004 032 129. If the above-described separation, which is less preferred according to the invention, is carried out in the process according to the invention, it will always extend only to a partial amount of residual gas I and thus only to a subset of the propane contained in the product gas B. According to the invention, the aforementioned amount of propane is preferably ≦ 50 mol%, more preferably ≦ 40 mol%, particularly preferably ≦ 30 mol%, particularly preferably ≦ 20 mol%, very particularly preferably ≦ 10 mol% and most preferably ≦ 5 mol% ,

After a partial amount of residual gas I has been discharged, it will be washed from at least a subset of the residual gas I (in a second separation zone II) contained therein CO ₂ . Contains the residual gas I also CO and O ₂ , you can (with simultaneous chemical reduction of the molecular oxygen) previously contained in the residual gas I CO z. B. by passing over corresponding catalysts selectively to CO ₂ oxidize (if necessary, the residual gas I in this regard required molecular oxygen can also be pre-dosed). With the catalysts recommended in WO 01/60738 (complex oxides of the stoichiometry Cu _x Ce _1-x O _2-y , where x = 0.01 to 0.3 and y ≥ x), this is also possible in the presence of molecular hydrogen , According to the invention, however, such a CO conversion is not preferred. This is due to the fact that any CO remaining in the recycle gas I would anyway be at least partially oxidized to CO ₂ within the subsequent loop in the reaction zones A and / or B, thus having a natural outlet in the aforementioned CO ₂ scrubbing. At the same time, however, a reduction of O ₂ contained in the residual gas I with H ₂ contained in the residual gas I can also be accompanied by CO conversion. This reduction can also be implemented singularly.

As a rule, it is sufficient according to the invention to wash out CO ₂ contained only in a subset of the residual gas I. Advantageously, the CO ₂ - scrubbing at elevated pressure (typically 3 to 50 bar, preferably 5 to 30, preferably 8 to 20, more preferably 10 to 20 or 15 bar) and according to the invention suitably by means of a basic (in Brönsted make sense) liquid.

As such basic liquids come z. As organic amines such as monoethanolamine, aqueous solutions of organic amines such. B. of monoethanolamine, aqueous alkali hydroxide or aqueous alkali carbonate or aqueous solutions of alkali metal carbonates and alkali metal bicarbonates into consideration (see, for example, also WO 05/05347). According to the invention, preference is given to using a K ₂ CO ₃ -containing aqueous solution for CO ₂ scrubbing. As such aqueous solutions come z. B. solutions of K ₂ CO ₃ in water, or of K ₂ CO ₃ and KHCO ₃ in water or of K ₂ CO ₃ , KHCO ₃ and KOH in water considered. Such an aqueous potassium carbonate solution advantageously has a solids content of from 10 to 30 or 40% by weight, particularly preferably from 20 to 25% by weight. By adding small amounts of alkali metal hydroxide (eg KOH), the aqueous alkali carbonate solution may additionally be made alkaline prior to washing in order to neutralize optionally present carboxylic acids (eg acetic acid, formic acid, acrylic acid) in the residual gas I. Cheap aqueous washing solutions are also those which contain dissolved KHCO ₃ and K ₂ CO ₃ in a weight ratio of about 1: 2, wherein their solids content is advantageously 20 to 25 wt .-%. Taking up CO ₂ and water, a molar unit of K ₂ CO _{3 is converted} into two molar units of KHCO ₃ (K ₂ CO ₃ + H ₂ O + CO ₂ → 2 KHCO ₃ ) during the wash.

Instead of carrying out the CO ₂ scrubbing by means of a basic liquid, the CO ₂ scrubbing can z. B. also under CO ₂ clathrate formation according to the teaching of EP-A 900 121 performed.

From an application point of view, the CO ₂ scrubbing is carried out in a scrubbing column and in countercurrent. The residual gas I to be washed flows in the wash column usually from bottom to top and the washing liquid from top to bottom. In known manner, the wash column contains the replacement surface enlarging internals. These can be fillers, mass transfer trays (eg sieve trays) and / or packings.

At the top of the wash column, the washed residual gas I is advantageously omitted and from the bottom of the wash column z. B. advantageously, the aqueous, predominantly potassium hydrogen carbonate-containing solution (or an aqueous CO ₂ clathrate solution) removed. For example, the sump solution KHCO ₃ and K ₂ CO _{3 dissolved} in a weight ratio of 2.5: 1 included.

By introducing hot steam into the aqueous potassium bicarbonate solution, the bicarbonate can thermally (in carbonate and CO ₂ , H ₂ O) zurückgespal th and the released CO _{2 are} expelled. The aqueous potassium carbonate solution thus recovered can, as a rule after evaporation of water vapor condensed in the course of the cleavage, be recycled to the CO ₂ scrubber to be carried out as described. The evaporation of the condensing water vapor can also be operated continuously by evaporators integrated in the column. The above cleavage is also carried out in an expedient manner in a column containing the insert surface enlarging the exchange surface (eg (sieve) tray column, packed column and / or packed column). Also advantageous here is the countercurrent operation. The hot steam is advantageously fed in the lower part of the column and the aqueous potassium carbonate solution is given up with advantage at the top of the column.

While the CO ₂ scrubbing is preferably carried out at low temperatures, the bicarbonate cleavage is preferably carried out at elevated temperatures. In the described composite of CO ₂ scrubbing, cleavage of the wash solution and reuse as a scrubbing liquid, the cleavage is carried out appropriately by means of steam having a temperature of 130 to 160 ° C, preferably from 140 to 150 ° C. The rückzuspaltende aqueous solution is expediently applied at a temperature of 80 to 120 ° C, more preferably from 90 to 110 ° C. Conversely, the residual gas I according to the invention advantageously with a temperature of 60 to 90 ° C, more preferably 70 to 80 ° C in the wash column, and the washing liquid at a temperature of 70 to 90 ° C, preferably 75 to 85 ° C, abandoned ,

In particular, if only a subset of the residual gas I is subjected to CO ₂ scrubbing, it is advantageous to carry out the compression of the residual gas I to the pressure preferred for CO ₂ scrubbing in several stages (eg with the aid of a multistage radial compressor; compressed gas can be taken behind each compression stage).

Typically, the residual gas I leaves the separation zone I in the process according to the invention with a pressure of ≥ 1 bar and ≤ 2.5 bar, preferably ≤ 2.0 bar. The heterogeneously catalyzed dehydrogenation in the reaction zone A is advantageously carried out at a pressure of ≥ 2 to ≦ 4 bar, preferably ≥ 2.5 to ≦ 3.5 bar. The reaction zone B is inventively operated at a pressure of ≥ 1 bar and ≤ 3 bar, preferably ≥ 1.5 and ≤ 2.5 bar. The preferred working pressures for the CO ₂ scrubbing are 3 to 50 bar, or 5 to 30 bar, or 8 to 20 bar, preferably 10 to 20 or 15 bar.

Application technology is useful now in a first compression stage using a multi-stage z. B. turbocompressor (radial compressor, eg of the type MH4B, the company Mannesmann DEMAG, DE) (also referred to here as recycle gas I compressor) a compression of the residual gas I to the working pressure in the reaction zone A carried out (eg , 2 to 3.2 bar). If not the total amount of residual gas I subjected to the CO ₂ scrubbing, divides the compressed as described residual gas I expediently in two subset of identical composition. The proportions can be z. B. amount to 70 to 30% of the total amount. The subset that is not subject to CO ₂ scrubbing is ready for recycling to reaction zone A. Only the other subset is further compressed to the working pressure envisaged for CO ₂ scrubbing. This can be done in just one additional compression stage. According to the invention, however, at least two additional compression stages will be used for this purpose. This is due to the fact that with the compression of a gas at the same time a temperature increase is accompanied by the same. Conversely, the relaxation (expansion) of the CO ₂ -washed residual gas I is accompanied by a cooling of the CO ₂ -washed residual gas I to the working pressure adequate for the return to the reaction zone A. If the relaxation is likewise carried out in several stages, it is possible to carry out an indirect heat exchange between heated, compressed unwashed residual gas I and cooled, expanded, CO ₂ -washed residual gas I before the respective next stage. Such a heat exchange can also be applied before the first expansion. As a result of the indirect heat exchange, condensation of water vapor still contained therein may occur in the residual gas I. The aforementioned expansions are advantageously carried out in (advantageously also multi-stage) expansion turbines (this serves to recover compaction energy). Depending compression or expansion stage, the difference between input and output pressure z. B. 2 to 10 bar.

In general, at least 50 mol%, usually at least 60 mol%, or at least 80 mol% and often at least or more than 90% by volume of the CO ₂ contained in the radical I are washed out in the separation zone II.

Advantageously, the washed CO ₂ amount corresponding to the amount of CO ₂ products formed in the reaction zones A and B.

As a rule, CO ₂ -washed residual gas still contains 1 to 20, often 5 to 10 vol .-% of CO ₂ .

Before the CO ₂ -washed residual gas I is released, it can still be subjected to a membrane separation in a third separation zone III in order to separate off at least a portion of the molecular hydrogen contained therein before recycling residual gas I into the reaction zone A (this measure will be particularly then seize, if in the reaction zone A only a comparatively small amount of hydrogen is burned). In this regard, suitable separation membranes are z. B. aromatic polyimide membranes, for. Those of UBE Industries Ltd .. Among the latter, in particular the membrane types A, B-H, C and D come into consideration. Particularly preferred for such a separation is the use of a UBE Industries Ltd. Polyimide membrane type B-H. The hydrogen permeation rate for H ₂ for this membrane is 0.7 × 10 ^-3 [STP × cc / cm ² · sec · cm Hg] at 60 ° C. For this purpose, the CO ₂ -washed residual gas I via the, usually to a tube (but it is also possible a plate or a winding module) designed membrane are passed, which is permeable only to the molecular hydrogen. The thus separated molecular hydrogen can be used in the context of other chemical syntheses or z. B. together with discharged in the context of the process according to the invention residual gas I combustion. In this combustion, the released CO ₂ (it can also be easily released into the atmosphere) and the aqueous condensates formed in the separation zones I and II can be included.

Furthermore, the above-mentioned combustion can separate the heavy matter separated in the separation zone I. to be supplied. Usually, the combustion takes place under air supply. The execution of the combustion can, for. B. as described in EP-A 925 272.

The Membrane separation of molecular hydrogen is preferred also under high pressure (eg 5 to 50 bar, typically 10 to 15 bar).

It is therefore appropriate according to the invention, either immediately before the CO ₂ scrubbing or immediately after the CO ₂ scrubbing (preferably) of the residual gas I perform in order to use the once increased pressure level 2 times in an advantageous manner. In addition to plate membranes, wound membranes or capillary membranes, tubular membranes (hollow-fiber membranes) are particularly suitable for hydrogen separation. Your inner diameter can z. B. a few microns to a few mm. Analogous to the tubes of a tube bundle reactor is z. B. cast a bundle of such tubular membranes at the two hose ends in each case a plate. Outside the tube membrane there is preferably a reduced pressure (<1 bar). The residual gas I under increased pressure (> 1 bar) is guided against one of the two ends of the plate and pushed through the inside of the tube to the hose outlet located at the opposite end of the plate. Along the flow path predetermined in the interior of the tube, molecular hydrogen is released to the outside via the H ₂ permeable membrane.

For the rest, can the post-treated as described residual gas I (sum of post-treated Subset and possibly not post-treated subset) as needed as one of the at least two gaseous propane containing feed streams be recycled to the reaction zone A. According to the invention preferred is the Ge total amount of inventively post-treated residual gas I the reaction gas A starting mixture (fed to the feed gas mixture of the reaction zone A).

According to the invention, preference is given to a method according to the invention in which the highest working pressure level is present in the process step of the CO ₂ scrubbing of the residual gas I. This allows, as described, the implementation of the method according to the invention with the concomitant use of only one compressor (preferably a multistage centrifugal compressor), which is to be positioned between the formation of residual gas I and CO ₂ scrubbing of residual gas I. In particular, between reaction zone A and reaction zone B, it is not necessary in the process according to the invention, as shown, to concomitantly use any other compressor. Of course, however, such a further compressor can be integrated into the method according to the invention.

One Another advantage of the method according to the invention is that the realization is possible allows small circulatory flow. Further opened it's the possibility that the reaction zone A supplied reaction gas A starting mixture in naturally Way contains molecular hydrogen, which in this starting mixture contained hydrocarbon before combustion with simultaneously protects in this mixture contained molecular oxygen. Based on converted fresh propane resulting from it comparatively high target product selectivities. Finally It should be noted again that a basis of the method according to the invention This consists of the non-inert minor components that are in the course the method according to the invention be formed or introduced into the same, their outlet in naturally Way in the in the separation zones I and II of the method according to the invention have formed condensates.

In typically contains in the reaction zone A recycled circulating gas I:

50 up to 90 vol.% Propane, > 0 to 5, usually up to ≤ 2 or ≤ 1 Vol .-% CO, 1 to 20 vol.% H ₂ , 1 to 20 vol.% CO ₂ , 1 to 20 vol.% H ₂ O, 0 to 10, often up to 5 vol.% O ₂ and 0 (often ≥ 0.1 to) up to 5 vol.% Propylene.

Finally, be still noted that both in the separation zone I and in the Separation zone II, whenever condensed phases occur, polymerization inhibitors be added. As such, in principle, all known process inhibitors into consideration. Particularly according to the invention are suitable for. As phenothiazine and the methyl ether of hydroquinone. The presence of molecular oxygen increases its effectiveness of the polymerization inhibitors increased.

Examples and Comparative Examples

I. Long-term operation of a heterogeneously catalyzed two-stage partial oxidation of propylene to acrylic acid in the absence and in the presence of molecular hydrogen

• A) General experimental setup of the reaction apparatus

Reactor for the first oxidation state

Of the Reactor consisted of a double-walled cylinder made of stainless steel (cylindrical guide tube, surrounded from a cylindrical outer container). The Wall thicknesses were everywhere 2 to 5 mm. The inner diameter of the outer cylinder was 91 mm. The inner diameter of the guide tube was about 60 mm.

Above and below was the double-walled cylinder through a lid, respectively Floor completed.

The Contact tube (400 cm total length, 26 mm inner diameter, 30 mm outer diameter, 2 mm wall thickness, stainless steel) was in the guide tube of the cylindrical container housed so that it at the top or bottom of the same (sealed) by the lid or bottom each protruding straight. The heat exchange agent (Salt melt, consisting of 53 wt .-% potassium nitrate, 40 wt .-% sodium nitrite and 7% by weight of sodium nitrate) was included in the cylindrical container. To over the whole in the cylindrical container located contact tube length (400 cm) as possible uniform thermal Boundary conditions on the outer wall to ensure the contact tube became the heat exchange agent pumped by a propeller pump.

An applied to the outer jacket electrical heating, the temperature of the heat exchange medium could be controlled to the desired level. Otherwise, there was air cooling. Reactor charge: Viewed over the first stage reactor, the molten salt and the feed gas mixture of the first stage reactor were passed in cocurrent. The feed gas mixture entered the bottom of the first stage reactor. It was fed into the reaction tube at a temperature of 165 ° C. The salt melt entered the bottom with a temperature T ^a in the cylindrical guide tube and a top having a temperature T ^out of the cylindrical guide tube, the + was up to 2 ° C above T. T ^an was set so that there is always a propylene conversion of 97.8 ± 0.1 mol% resulted in single pass at the output of the first oxidation stage. Contact tube charge: (from bottom to top) Section A: 90 cm length Prefill of steatite balls of 4-5 mm diameter. Section B: 100 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 C. Section C: 200 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 100 46 957 (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 ). Section D: 10 cm length of bedding out of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) Intermediate cooling and intermediate oxygen supply (pure O ₂ as secondary gas)

The the first fixed bed reactor leaving product gas mixture was the Purpose of intercooling (indirectly by air) through a connecting tube (40 cm in length, 26 mm inner diameter, 30 mm outer diameter, 2 mm wall thickness, Stainless steel, wrapped with 1 cm of insulating material) led, the on a length of 20 cm centered, with an inert bed out Steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) fed and flanged directly to the first-stage contact tube was.

The product gas mixture always entered with a temperature of> T ⁱⁿ (first stage) in the connecting pipe and left it with a located above 200 ° C and below 270 ° C temperature.

At the end of the connecting tube molecular oxygen present at the pressure level of the product gas mixture was metered into the cooled product gas mixture. The resulting gas mixture (feed gas mixture for the second oxidation stage) was led directly into the second-stage contact tube to which the above-mentioned connection tube with its other end was also flanged. The amount of molecular oxygen metered in was such that the molar ratio of O ₂ in the resulting gas mixture to acrolein in the resulting gas mixture was 1.3.

Reactor for the second oxidation state

A contact-tube fixed-bed reactor was used, which was identical to that for the first oxidation stage. Salt melt and feed gas mixture were conducted in direct current over the reactor. The molten salt entered at the bottom, the feed gas mixture also. The inlet temperature ^T of the molten salt was set so that there is always an acrolein conversion of 99.3 ± 0.1 mol% resulted in single pass at the output of the second oxidation stage. T ^from the molten salt was up to 2 ° C above T ^a .

• B) In Abhängigkeit von der Zusammensetzung des Beschickungsgasgemischs der ersten Oxidationsstufe erzielte Ergebnisse (die Propenbelastung wurde auf 150 Nl/l·h eingestellt, die Selektivität der Acrylsäurebildung betrug stets ≥ 94 mol-%). a) Die Zusammensetzung des Beschickungsgasgemischs für die erste Oxidationsstufe betrug im wesentlichen:

7 Vol.-% Propylen, 12 Vol.-% O₂, 20 Vol.-% H₂, 5 Vol.-% H₂O und 56 Vol.-% N₂. The contact tube charge (from bottom to top) was: Section A: 70 cm length of fill of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter). Section B: 100 cm length of catalyst charge with a homogeneous mixture of 30% by weight of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × internal diameter) and 70% by weight of coated catalyst from section C. Section C: 200 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 ). Section D: 30 cm length of bedding out of steatite balls with a diameter of 4-5 mm.

• B) Depending on the composition of the feed gas mixture of the first oxidation stage results achieved (the propene load was set to 150 Nl / l · h, the selectivity of acrylic acid formation was always ≥ 94 mol%). a) The composition of the feed gas mixture for the first oxidation stage was essentially:

7 vol.% propylene, 12 vol.% O ₂ , 20 vol.% H ₂ , 5 vol.% H ₂ O and 56 vol.% N ₂ .

At the beginning of the fresh catalyst-fed reaction apparatus, the inlet temperatures were: T ^a (first oxidation state): 320 ° C T ^a (second oxidation state): 268 ° C

• b) Die Zusammensetzung des Beschickungsgasgemischs für die erste Oxidationsstufe betrug im wesentlichen:

7 Vol.-% Propylen, 12 Vol.-% O₂, 5 Vol.-% H₂O und 76 Vol.-% N₂. After 3 months of operation, the inlet temperatures were: T ^a (first oxidation state): 330 ° C T ^a (second oxidation state): 285 ° C

• b) The composition of the feed gas mixture for the first oxidation stage was essentially:

7 vol.% propylene, 12 vol.% O ₂ , 5 vol.% H ₂ O and 76% by volume N ₂ .

To Beginning of the fresh catalyst-fed reaction apparatus the inlet temperatures were:

T ^a (first Oxidation state): 320 ° C T ^a (second Oxidation state): 268 ° C

After 3 months of operation, the inlet temperatures were: T ^a (first oxidation state): 324 ° C T ^a (second oxidation state): 276 ° C

II. Process for the preparation of acrylic acid from propane (a stationary operating state is described)

• A) Design of the reaction zone A

The Reaction zone A consists of a designed as a tray reactor and adiabatically designed shaft furnace reactor, the three in the flow direction Has fixed catalyst beds arranged one behind the other. The respective Fixed catalyst bed is a one applied on a stainless steel wire net fill out (in the flow direction arranged in said sequence) inert material (bed height: 26 mm; Steatite balls of diameter 1.5 to 2.5 mm) and a mixture from fresh dehydrogenation catalyst and steatite spheres (1.5 to 2.5 mm diameter) in the bulk volume ratio dehydrogenation catalyst : Steatite balls = 1: 3 (alternatively, at this position also the same amount of catalyst used undiluted).

Before each fixed bed is a static gas mixer. The dehydrogenation catalyst is a Pt / Sn alloy promoted with the elements Cs, K and La in oxidic form and deposited on the outer and inner surfaces of ZrO ₂ .SiO ₂ mixed oxide support strands (average length (Gauss distributed in the range of 3 mm to 6 mm, 6 mm diameter: 2 mm) in elemental stoichiometry (mass ratio including support) Pt _0.3 Sn _0.6 La _3.0 Cs _0.5 K _0.2 (ZrO ₂ ) _88.3 (Si0 ₂ ) _7.1 (catalyst precursor preparation and activation to the active catalyst as in Example 4 of DE-A 102 19 879).

The heterogeneously catalyzed partial propane dehydrogenation is described in Horden reactor performed in a straight pass. The burden of total catalyst amount (calculated without inert material) of all hordes with propane is 1500 Nl / lh.

• – 50,87 Nm³/h Kreisgas I mit folgenden Gehalten:

Acrolein 0,01 Vol.-%, Acrylsäure 0,01 Vol.-%, Propan 63,49 Vol.-%, Propylen 0,86 Vol.-%, H₂ 14,50 Vol.-%, O₂ 4,72 Vol.-%, H₂O 2,05 Vol.-%, CO 1,17 Vol.-%, CO₂ 11,17 Vol.-%;

• – 3,09 Nm³/h Wasserdampf; und
• – 8,5 Nm³/h Roh-Propan, das zu > 98 Vol.-% Propan enthält.

62.46 Nm ³ / h reaction gas A starting mixture (T = 438 ° C, P = 2.92 bar) are fed to the first catalyst bed in the flow direction, which is composed as follows:

• - 50.87 Nm ³ / h cycle gas I with the following contents:

acrolein 0.01 vol.%, acrylic acid 0.01 vol.%, propane 63.49 vol.%, propylene 0.86 vol.%, H ₂ 14.50 vol.%, O ₂ 4.72 vol.%, H ₂ O 2.05 vol.%, CO 1.17 vol.%, CO ₂ 11.17% by volume;

• - 3.09 Nm ³ / h water vapor; and
• - 8.5 Nm ³ / h of crude propane containing> 98% by volume of propane.

The Reaction gas A starting mixture is by indirect heat exchange with product gas A (T = 581 ° C; P = 2.88 bar) of 80 ° C to the 438 ° C heated. The evaporation of the crude propane takes place by means of in the separation zone I accruing aqueous Condensate (sour water (T = 33 ° C), indirect heat exchange). Of the Steam is available at a temperature of 152 ° C and 5 bar.

The bed height of the first catalyst bed through which the reaction gas A starting mixture flows is such that the reaction gas A leaves this catalyst bed at a temperature of 549 ° C. and a pressure of 2.91 bar with the following contents: propane 59.0 vol.%, propylene 5.94 vol.%, H ₂ 10.25 vol.%, H ₂ O 12.98 vol.%, CO ₂ 9.97% by volume, and O ₂ 0% by volume.

The maximum temperature in the first catalyst bed is 592 ° C.

The leaving amount is 63.39 Nm ³ / h. After the first catalyst bed, the reaction gas A is metered with 0.99 Nm ³ / h of molecular oxygen (purity> 99% by volume). The oxygen is preheated to 176 ° C. It is throttled at a pressure of 3.20 bar located, so that the resulting pressure of the resulting reaction gas A was still 2.91 bar.

The bed height of the second catalyst bed is such that the reaction gas A leaves the second catalyst bed at a temperature of 566 ° C. and a pressure of 2.90 bar with the following contents: propane 51.91 vol.%, propylene 9.55 vol.%, H ₂ 10.88 vol.%, H ₂ O 15.38 vol.%, CO ₂ 9.57% by volume and O ₂ 0% by volume.

The maximum temperature in the second catalyst bed is 595 ° C.

The leaving quantity is 66.06 Nm ³ / h. 0.92 Nm ³ / h of molecular oxygen (purity> 99% by volume) are metered into this reaction gas A before the static mixer located in front of the third catalyst bed (T = 176 ° C., throttled at a pressure of 3.20) , The resulting pressure of the resulting reaction gas A is still 2.90 bar.

The bed height of the third catalyst bed is such that the reaction gas A leaves the third catalyst bed as product gas A with a temperature of 581 ° C. and a pressure of 2.88 bar with the following contents: propane 47.19 vol.%, propylene 12.75 vol.%, H ₂ 11.38 vol.%, H ₂ O 17.46% by volume CO ₂ 9.21 vol.% And O ₂ 0% by volume.

The maximum temperature in the third catalyst bed is 612 ° C.

The abandoned amount is 68.52 Nm ³ / h. By indirect heat exchange with reaction gas A starting mixture, the product gas A is cooled to a temperature of 290 ° C.

13.31 Nm ³ / h of molecular oxygen (purity> 99% by volume) are metered into this product gas A (T = 176 ° C., throttled at a pressure of 3.20 bar). The resulting pressure and the resulting temperature of the charge gas mixture (reaction gas B starting mixture) thus obtained for the first partial oxidation stage of a two-stage partial oxidation of the propylene to acrylic acid contained in the product gas B are 283 ° C. and 1.75 bar.

The first oxidation stage is a two-zone multi-tube reactor. The reaction tubes are as follows: V2A steel; 30 mm outer diameter, 2 mm wall thickness, 28 mm inner diameter, length: 350 cm. From top to bottom, the reaction tubes are charged as follows: Part 1: 50 cm length steatite length of the geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill. Section 2: 140 nm catalyst charge with a homogeneous mixture of 20 wt .-% (alternatively 30 wt .-%) of steatite rings of geometry 5 mm × 3 mm × 2 mm (outer diameter × length × inner diameter) and 80 wt .-% (alternatively 70 % By weight) unsupported 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 100 46 957 (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 ). Alternatively, one of the catalysts BVK 1 to BVK 11 from Research Disclosure No. 497012 of 29.08.2005 can also be used here.

From top down are the first 175 cm by means of a counter-current to the reaction gas B pumped salt bath A thermostated. The second 175 cm are by means of a countercurrent to the reaction gas B pumped salt bath B thermostated.

The second oxidation stage is also a two-zone multi-tube reactor. The reaction tubes are charged from top to bottom as follows: Part 1: 20 cm long steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill. Section 2: 90 cm length of catalyst charge with a homogeneous mixture of 25 wt% (alternatively 30 wt%) steatite rings of geometry 7 mm x 3 mm x 4 mm (outside diameter x length x inside diameter) and 75 wt% (alternatively 70% by weight) coated catalyst from Section 4. Section 3: 50 cm length of catalyst charge with a homogeneous mixture of 15% by weight (alternatively 20% by weight) of steatite rings of geometry 7 mm × 3 mm × 4 mm (external diameter × length × internal diameter) and 85% by weight ( alternatively 80% by weight) coated catalyst from section 4. Section 4: 190 cm length of 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 ).

From top down are the first 175 cm by means of a counter-current to the reaction gas pumped salt bath C thermostated. The second 175 cm are pumped by means of a countercurrent to the reaction gas Salt bath D thermostated.

Between the two oxidation states is a shell-and-tube heat exchanger cooled by salt bath, with which the product gas of the first oxidation stage are cooled can. Before entering the second oxidation stage is a valve for supplying molecular oxygen (purity> 99% by volume).

The propylene loading of the catalyst feed of the first oxidation stage is chosen to be 145 Nl / lh. The salt melts (53% by weight of KNO ₃ , 40% by weight of NaNO ₂ , 7% by weight of NaNO ₃ ) have the following entering temperatures: T _A = 324 ° C T _B = 328 ° C T _C = 265 ° C T _D = 269 ° C

As much molecular oxygen (176 ° C., 3.20 bar) is metered into the product gas mixture of the first oxidation stage that the molar ratio of O ₂ : acrolein in the resulting feed gas mixture for the second oxidation stage is 1.25. The acrolein loading of the catalyst charge of the second oxidation stage is 140 Nl / lh. The pressure at the inlet of the second oxidation stage is 1.61 bar. The reaction gas leaves the intercooler at a temperature of 260 ° C and the inlet temperature of the feed gas mixture in the second oxidation stage is 258 ° C. The reaction gas leaves the intercooler at a temperature of 260 ° C and the inlet temperature of the feed gas mixture in the second oxidation stage is 258 ° C. The product gas mixture of the first oxidation state has the following contents: acrolein 9.02% by volume, acrylic acid 0.61 vol.%, propane 39.5 vol.%, propylene 0.53 vol.%, H ₂ 9.49 vol.%, O ₂ 3.90 vol.%, H ₂ O 25.75 vol.% And CO ₂ 8.77% by volume.

The Temperature of the product gas of the first oxidation stage is before entry in the aftercooler 335 ° C.

The product gas mixture of the second oxidation stage (the product gas B) has a temperature of 270 ° C and a pressure of 1.55 bar and the following contents: acrolein 0.049 vol.%, acrylic acid 9.12% by volume, acetic acid 0.26 vol.%, propane 39.43 vol.%, propylene 0.53 vol.%, H ₂ 9.52% by volume, O ₂ 2.94 vol.%, H ₂ O 26.10 vol.%, CO 0.72 vol.% And CO ₂ 9.3% by volume.

The Product gas B is as described in WO 2004/035514 in a Tray column (separation zone I) fractionally condensed.

Of the waste incineration be the first fuel 118 kg / h of high boilers (polyacrylic acids (Michael adducts) etc.).

From the second capture bottom above the supply of the reaction gas B in the tray column 23577 kg / h of a condensed crude acrylic acid are removed, which has a temperature of 15 ° C and 96.99 wt .-% acrylic acid. This is suspension crystallized as described in WO 2004/035514 after addition of a small amount of water, the suspension crystals in hydraulic wash columns of the Mother liquor separated and the mother liquor as described in WO 2004/035514 recirculated to the condensation column. The purity of the washed suspension crystallizate is> 99.87% by weight of acrylic acid and is directly suitable for the preparation of water "superabsorbent" polymers for use in the hygiene sector.

The amount of acid water condensate withdrawn from the third collecting tray above the feed of the reaction gas B into the condensation column and not returned to the condensation column is 18145 kg / h, has a temperature of 33 ° C. and has the following contents: 0.11% by weight acrolein, 1.30% by weight Acrylic acid, 0.95% by weight Acetic acid and 95.6% by weight Water.

At the top of the condensation column leave 53.38 Nm ³ / h of residual gas I, the separation zone I with a temperature of 33 ° C and a pressure of 1.20 bar and the following contents: acrolein 0.03 vol.%, acrylic acid 0.02 vol.%, propane 61.07% by volume, propylene 0.82 vol.%, H ₂ 14.02% by volume, O ₂ 4.55% by volume, H ₂ O 2.00 vol.%, CO 1.13 vol.% And CO ₂ 14.36 vol .-%.

0.084 Vol .-% of the residual gas I are with the composition of the residual gas I left out (as a third fuel).

The Residual amount of the residual gas I (hereinafter linguistically simplified "residual gas I ") in the first compressor stage of a multistage centrifugal compressor from 1.20 bar to 3.20 bar compressed, the temperature of the residual gas I at 92 ° C increases. Then the residual gas I in two halves of identical composition divided up. One half directly forms a subset of the returned to the reaction zone A circulating gas I. The other half of the residual gas I is in a second compressor stage of 3.20 bar compressed to 5.80 bar. It heats up to 127 ° C.

In an indirect heat exchanger, it is cooled to 78 ° C, without causing condensation (coolant is CO ₂ -washed and depleted by molecular membrane subsequent separation of molecular hydrogen and subsequently relaxed in an expansion turbine residual gas I temperature 42 ° C and pressure 4, 25 bar). By means of indirect air cooling, the residual gas I is cooled without condensation from 78 ° C to 54 ° C. In a further compressor stage, the residual gas I is compressed from 5.80 bar to 12.0 bar, wherein it is heated from 54 ° C to 75 ° C.

The thus compressed residual gas I is fed into the lower part of a packed column (separation zone II). At the top of this wash column 18000 kg / h of an aqueous K ₂ CO ₃ solution are introduced, which has a temperature of 82 ° C and contains phenotiazion as a polymerization inhibitor. At the top of the scrubbing column escapes CO ₂ -washed residual gas I, which has the following contents at a pressure of 12.00 bar and a temperature of 85 ° C: propane 65.94 vol.%, propylene 0.89 vol.%, H ₂ 15.13 vol.%, O ₂ 4.91 vol.%, H ₂ O 2.16 vol.%, CO 1.22 vol.% And CO ₂ 7.75% by volume.

The CO ₂ -washed residual gas I (24.53 Nm ³ / h) is applied to a bundle of molded tube membranes (external pressure 0.1 bar; polyimide membrane type B-H of Fa.

UBE Industries Ltd.) and leaves the same (p = 12.00 bar, T = 85 ° C) with the following contents: propane 66.1 vol.%, propylene 0.89 vol.%, H ₂ 15.03% by volume, O ₂ 4.92 vol.%, H ₂ O 2.10 vol.%, CO 1.22 vol.% And CO ₂ 7.73 vol.

In a multistage expansion turbine, the residual gas I (24.46 Nm ³ / h) is now released from 12.00 bar to 4.25 bar and cooled from 85 ° C to 42 ° C. In indirect heat exchange with coming out of the separation zone I, compressed in a first stage residual gas I, the CO ₂ washed and H ₂ depleted residual gas I from 42 ° C to 97 ° C and is heated together with the other half of the fractional condensation of the product gas B remaining (not CO ₂ -washed) residual gas I recycled as recycle gas I into the reaction gas A-starting mixture for the reaction zone A.

The permeate stream (0.0672 Nm ³ / h) has the following contents: propane 0.12 vol.%, propylene 0.01 vol.%, H ₂ 56.2% by volume, H ₂ O 27.2 vol.%, CO 0.15% by volume and CO ₂ 12.39 Vol.

It is fed as the fourth fuel of the common incinerator.

21846 kg / h of aqueous potassium bicarbonate solution are removed from the bottom of the CO ₂ scrubbing column at a temperature of 75 ° C. and heated to 100 ° C. by indirect heat exchange from 75 ° C. (the heat carrier used is back-split potassium bicarbonate solution) to the top of a second Packed column (back-flow column). In countercurrent 26 Nm ³ / h of steam (144 ° C, 4 bar) are fed into the back-column in the lower part. The gas stream escaping from the top of the column is subjected to direct cooling (coolant: previously formed condensate) and the resulting aqueous condensate is added as reflux to the column. The escaping CO ₂ is supplied as a fifth of the common fuel combustion plant. From the outlet of the back-column (32968 kg / h) 14968 kg / h of water are evaporated. The remaining amount is (optionally post-neutralized with KOH) as a washing solution recycled to the top of the CO ₂ scrubbing column.

The fuels 1 to 5 are burned together with the addition of air (17.7 Nm ³ / h) in an incinerator.

Claims (24)

A process for the preparation of acrolein, or acrylic acid, or their mixture of propane, wherein A) - a first reaction zone A with formation of a reaction gas A at least two gaseous feed streams containing propane, at least one of which contains fresh propane, - in the reaction zone A the reaction gas A through at least one catalyst bed leads, in which are formed by partial heterogeneously catalyzed dehydrogenation of propane molecular hydrogen and propylene, - the reaction zone A molecular oxygen feeds, which oxidized in the reaction zone A in the reaction gas A molecular hydrogen to water vapor, and The reaction zone A receives product gas A containing molecular hydrogen, water vapor, propylene and propane; B) in a reaction zone B the product gas A withdrawn from the reaction zone A with supply of molecular oxygen for charging at least one oxidation reactor with a molecular water reaction gas B containing propane, water vapor, propane, propylene and molecular oxygen and the propylene contained in the same heterogeneously catalyzed partial gas phase oxidation to an acrolein, or acrylic acid, or their mixture as a target product, unreacted propane, molecular hydrogen, steam, carbon dioxide as a byproduct and other product gas B which contains lighter and heavier water-boiling secondary components, C) from the reaction zone B product gas B leads and from the same in a first separation zone I contained therein target product, water and heavier water than boiling secondary components leaving a residual gas I separates the unreacted propane, carbon dioxide, molecular hydrogen, lighter than boiling water secondary components and optionally in the reaction zone B unreacted propylene and optionally unreacted molecular oxygen contains, D) - as aftertreatment measure 1 in a second separation zone II in residual gas I contained carbon dioxide washes and optionally condensed water contained in residual gas I, - as aftertreatment measure 2 a subset Residual gas I leaves, - optionally separated as aftertreatment measure 3 in a third separation zone III via a separation membrane contained in residual gas I molecular hydrogen and - optionally contained as after-treatment measure 4 in residual gas I optionally chemically reduced molecular oxygen, the sequence of application of the aftertreatment measures 1 to 4 is arbitrary, and E) after application of the aftertreatment measures 1 and 2 and optionally 3 and / or 4 remaining unreacted propane containing aftertreated residual gas I as at least one of the at least two Propane-containing feed streams in the reaction zone A, characterized in that oxidized in the reaction zone A, an amount M of molecular hydrogen to water vapor, the at least 5 mol% but not more than 95 mol .-% of the total amount in the reaction zone A produced and the reaction zone A optionally supplied molecular hydrogen. Method according to claim 1, characterized in that that in the reaction zone A is a quantity M of molecular hydrogen oxidized to water vapor, but not more than 10 mol% as 90 mol% of the total amount of produced in the reaction zone A. and the reaction zone A optionally supplied molecular hydrogen is. Method according to claim 1, characterized in that that in the reaction zone A is a quantity M of molecular hydrogen oxidized to water vapor, but not more than 20 mol% as 80 mol% of the total amount of produced in the reaction zone A. and the reaction zone A optionally supplied molecular hydrogen is. Method according to claim 1, characterized in that that in the reaction zone A is a quantity M of molecular hydrogen oxidized to water vapor, but not more than 40 mol% as 60 mol% of the total amount of produced in the reaction zone A. and the reaction zone A optionally supplied molecular hydrogen is. Method according to one of claims 1 to 4, characterized in that the reaction gas B, with which the at least one oxidation reactor is charged, the following contents: 4 to 25 vol .-% propylene, 6 to 70 vol .-% propane, 5 bis 60% by volume H ₂ O, 8 to 65% by volume O ₂ and 0.3 to 20% by volume H ₂ . Method according to one of claims 1 to 4, characterized in that the reaction gas B, with which the at least one oxidation reactor is charged, the following contents: 6 to 15 vol .-% propylene, 6 to 60 vol .-% propane, 5 bis 30 vol.% H ₂ O, 8 to 35 vol.% O ₂ and 2 to 18 vol.% H ₂ . A method according to claim 5 or 6, characterized in that the molar ratio V ₁ of propane contained in the reaction gas B to propylene contained in the reaction gas B is 1 to 9. A process according to claim 5 or 6, characterized in that the molar ratio V ₁ of propane contained in reaction gas B to propylene contained in reaction gas B is 1 to 7. Method according to one of claims 1 to 8, characterized in that as a source of required in the reaction zone B molecular oxygen pure molecular oxygen or a mixture of molecular oxygen and inert gas containing not more than 10% by volume of inert gas. Method according to one of claims 1 to 9, characterized that as a source for needed in the reaction zone A. molecular oxygen pure molecular oxygen or a mixture This is not used for molecular oxygen and inert gas contains more than 10 vol .-% inert gas. Method according to one of claims 1 to 8, characterized that as a source for needed in the reaction zone A. molecular oxygen air and as a source of required in the reaction zone B molecular Oxygen is pure molecular oxygen or a mixture of molecular oxygen Oxygen and inert gas is used, which is not more than 10 vol .-% Inert gas contains. Method according to one of claims 1 to 11, characterized in that the reaction gas B, with which the at least one oxidation reactor is charged, contains 0.1 to 30 vol .-% CO ₂ . Method according to one of claims 1 to 11, characterized in that the reaction gas B, with which the at least one oxidation reactor is charged, 1 to 20 vol .-% CO ₂ . Method according to one of claims 1 to 13, characterized that the reaction gas B, with which the at least one oxidation reactor is charged, ≤ 5 Contains vol .-% of molecular nitrogen. Method according to one of claims 1 to 14, characterized the reaction zone A is designed adiabatically. Method according to one of claims 1 to 15, characterized the reaction zone A is carried out as a tray reactor. Method according to one of claims 1 to 16, characterized in that the recycled to the reaction zone A post-treated residual gas I molecular oxygen, water vapor, molecular hydrogen, CO and CO ₂ . Method according to one of claims 1 to 17, characterized in that the recycled to the reaction zone A after-treated residual gas I contains 5 to 15 vol .-% CO ₂ . Method according to one of claims 1 to 18, characterized in that the washing out of CO ₂ from residual gas I by means of a potassium carbonate-containing aqueous solution. Method according to one of claims 1 to 19, characterized in that the washing out of CO ₂ from residual gas I is carried out at a pressure of 3 to 50 bar. Method according to one of claims 1 to 20, characterized that in the separation zone I from the product gas B, an amount of water separated is at least 70 mol .-% of that formed in the reaction zone B. Amount of water is. Method according to one of claims 1 to 20, characterized that in the separation zone I from the product gas B, an amount of water separated which is at least 90 mol% of that formed in the reaction zone B. Amount of water is. Method according to one of claims 1 to 22, characterized that the amount of per unit time contained in the discharged residual gas I. Propane less than 30 mol .-%, based on the pro Time unit supplied Amount of fresh propane is. Method according to one of claims 1 to 22, characterized that the amount of in post-treated residual gas I in the reaction zone A recirculated propane at least 90 mol% of the amount of propane contained in the product gas B is.