ES2545257T3 - Partial oxidation process in gas phase heterogeneously catalyzed acrolein to give acrylic acid - Google Patents

Abstract

A heterogeneously catalyzed gas-phase partial oxidation process of acrolein to acrylic acid, in which a reaction gas starting mixture containing acrolein, molecular oxygen and at least one inert gas is conducted, which is made up of at least least 20 % of its volume by molecular nitrogen, which contains molecular oxygen and acrolein in a molar ratio of O2 : C3H4O >= 0.5, to a reaction stage through a fixed-bed catalyst packing, which is arranged in two reaction zones A, B arranged spatially consecutively, the temperature of reaction zone A being a temperature in the range of 230 to 320 °C and the temperature in reaction zone B also a temperature in the range of 230 at 320 ºC, and whose active mass is at least one multimetallic oxide containing the elements Mo and V, so that the reaction zone A extends up to a reaction of acrolein from 45 to 85 mol% and in a single step d e the reaction gas starting mixture through the entire fixed-bed catalyst packing the acrolein reaction is >= 90 mol % and the selectivity of acrylic acid formation, with respect to the reacted acrolein, is >= 90 mol %, the time sequence in which the reaction gas starting mixture passes through the reaction zones corresponds to the alphabetical sequence of the reaction zones, which is characterized in that a) the loading of the catalyst packing in fixed bed with acrolein contained in the reaction gas starting mixture is <= 125 Nl acrolein/l fixed bed catalyst packing h and >= 70 Nl acrolein/l fixed bed catalyst packing h, b) the specific activity of the volume of the fixed-bed catalyst packing in the direction of the flow of the reaction gas mixture through the fixed-bed catalyst packing is constant or increases at least once, and c) the difference TmaxA - tm axB, constituted by the highest temperature, TmaxA, that the gaseous reaction mixture presents within the reaction zone A and the highest temperature, TmaxB, that the gaseous reaction mixture presents within the reaction zone B is >= 0ºC.

Description

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DESCRIPTION

Partial oxidation process in gas phase heterogeneously catalyzed acrolein to give acrylic acid

The present invention relates to a heterogeneously catalyzed partial phase oxidation process of acrolein to give acrylic acid, in which a starting mixture of reaction gas containing acrolein, molecular oxygen and at least one inert gas is conducted , which is constituted in at least 20% of its volume by molecular nitrogen, which contains molecular oxygen and acrolein in a molar ratio of O2: C3H4O ≥ 0.5, at a reaction stage through a catalyst filler in a fixed bed, which is arranged in two reaction zones A, B arranged spatially consecutively, the temperature of the reaction zone A being a temperature in the range of 230 to 320 ° C and the temperature in the reaction zone B also a temperature in the range of 230 to 320 ° C, and whose active mass is at least one multimetallic oxide containing the elements Mo and V, so that the reaction zone A extends to an area ction of acrolein from 45 to 85 mol% and in a single step of the starting mixture of reaction gas through the entire catalyst bed filler the reaction of acrolein is ≥ 90 mol% and the selectivity of the formation of acrylic acid, with respect to the reacted acrolein, is ≥ 90 mol%, corresponding to the temporal succession in which the starting mixture of reaction gas crosses the reaction zones to the alphabetic succession of the zones of reaction.

The present acrolein catalytic gas phase oxidation process for acrylic acid is generally known (see, for example, DE-A 19910508) and is especially important as a second oxidation stage in acid production. acrylic by oxidation in catalytic gas phase in two stages starting from propene. Acrylic acid is an important monomer that is used as such or in the form of an alkyl ester for the production of suitable polymers, for example, as adhesives.

In addition to molecular oxygen and reactants, the starting mixture of reaction gas contains, among other materials, therefore, inert gas, to keep the reaction gas out of the explosion region.

An objective of a heterogeneously catalyzed partial gas phase oxidation of this type of acrolein to give acrylic acid is to achieve a single step of the reaction mixture through the reaction stage under the otherwise pre-established boundary conditions. highest AAA yield of acrylic acid (this is the number of moles of acrolein reacted to give acrylic acid, with respect to the number of moles of acrolein introduced).

Another objective of a partial gas phase oxidation catalyzed in a heterogeneous manner of acrolein to give acrylic acid is to achieve the highest possible space-time yield (RZAAA) of acrylic acid (that is in a continuous process mode the amounts of Acrylic acid produced per hour and volume of catalyst filler in fixed bed used in liters).

In the case of a constant AAA given performance, the space-time efficiency is higher when the charge of the fixed-bed catalyst filler of the acrolein reaction stage is higher (hence the amount of acrolein in standard liters is understood (= Nl; the volume in liters that would take the amount of acrolein under normal conditions, that is, at 25 ° C and 100 kPa) that is conducted as a component of the reaction mixture starting gas per hour through a liter of filler fixed bed catalyst.

The teachings of documents WO 01/36364, DE-A 19927624, DE-A 19948248, DE-A 19948523, DE-A 19948241 and DE-A 19910506 are aimed at increasing, therefore, to an essentially constant AAA, the Acrolein loading of the catalyst bed in the fixed bed of the reaction step with acrolein significantly. This is essentially achieved so that the fixed-bed catalyst filling in the reaction stage is arranged in each case in two spatially consecutive temperature zones (reaction zones). The acrolein charge of the fixed bed catalyst filler is chosen, in this respect, to be ≥ 150 NI / I of fixed bed catalyst filler and the temperature of the corresponding second temperature zone (in the direction of the flow of the reaction gas mixture) must be at least 10 ° C above the temperature of the first temperature zone.

Similarly, EP-A 1106598 also teaches a method of the high-load mode of operation for the partial oxidation in gas phase heterogeneously catalyzed acrolein to give acrylic acid, in which the fixed-bed catalyst filler of the reaction stage is arranged in several temperature zones. In this regard, the temperature difference of a subsequent temperature zone in the direction of flow of the reaction gas mixture according to the teachings of EP-A 1106598 is both greater than and also less than 5 ° C above the temperature of the above temperature zone, leaving EPA 1106598 fully open under what conditions a higher temperature difference should be used and under what conditions a lower temperature difference should be used.

Also EP-A 1106598 leaves what is understood by the temperature of a reaction or temperature zone fully open.

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Unlike this in the remaining documents of the cited prior art, by temperature of a reaction zone, the temperature of the catalyst bed filling in the fixed bed is understood as being in the reaction zone when the procedure is implemented in the absence of a chemical reaction This temperature within the reaction zone is not constant, so that the concept temperature of a reaction zone here means the average value (in number) of the catalyst bed temperature in a fixed bed along The reaction zone. It is essential, in this regard, that the heating of the individual reaction zones be carried out essentially independently of each other, whereby a reaction zone always corresponds to a temperature zone. The above definition of the temperature of a reaction zone must also be valid here.

Since partial gas phase oxidation catalyzed in a heterogeneous manner of acrolein to give acrylic acid is a markedly exothermic reaction, the temperature of the reaction gas mixture in a reactive passage through the fixed bed catalyst filler is generally different from the temperature of a reaction zone. It is normally higher than the temperature of the reaction zone and generally crosses a maximum (maximum hot spot) within a reaction zone or decreases from a maximum value.

However, the disadvantage of the teachings of the state of the art is that they are exclusively aimed at operating a multi-zone arrangement with a high acrolein load. This is disadvantageous in the sense that such a mode of procedure necessarily entails a high RZAAA. This implies, however, a corresponding market need for acrylic acid. If the latter does not occur (for example, temporarily), the arrangement of several zones must necessarily be operated at a lower acrolein load, then having as high a selectivity as possible of the formation of acrylic acid with respect to the reacted acrolein than the intended target amount (SAA). This is the molar amount formed in a single step through the arrangement of several areas of acrylic acid with respect to the molar number of the acrolein, in this respect, reacted.

The objective of the present invention is, therefore, to provide a process of partial oxidation in gas phase heterogeneously catalyzed acrolein to give acrylic acid in a multi-zone arrangement, in which for acrolein charges <150 Nl / 1 · Acrylic acid formation is carried out with the highest possible selectivity.

In addition, a heterogeneously catalyzed partial gas phase oxidation process of acrolein to acrylic acid has been found, in which a starting mixture of reaction gas containing acrolein, molecular oxygen and at least one inert gas is conducted, which is constituted in at least 20% of its volume by molecular nitrogen, which contains molecular oxygen and acrolein in a molar ratio of O2: C3H4O ≥ 0.5, in a reaction stage through a catalyst filler in fixed bed, which is arranged in two reaction zones A, B arranged spatially consecutively, the temperature of the reaction zone A being a temperature in the range of 230 to 320 ° C and the temperature in the reaction zone B also a temperature in the range of 230 to 320 ° C, and whose active mass is at least one metal oxide containing the elements Mo and V, so that the reaction zone A extends to a reaction of the acrole from 45 to 85 mol% and in a single step of the starting mixture of reaction gas through the entire catalyst bed filling the acrolein reaction is ≥ 90 mol% and the selectivity of the Acrylic acid formation, with respect to the reacted acrolein, is ≥ 90 mol%, corresponding to the temporal sequence in which the reaction mixture of the starting gas crosses the reaction zones to the alphabetic sequence of the reaction zones, which is characterized because

a) the charge of the fixed-bed catalyst filler with the acrolein contained in the reaction mixture starting mixture is ≤ 125 Nl of acrolein / l of fixed-bed catalyst load h and ≥ 70 Nl of acrolein / l of loading fixed bed catalyst h,

b) the specific volume activity of the fixed bed catalyst filler in the direction of the flow of the reaction gas mixture through the fixed bed catalyst filler is constant or at least ever increases, and

c) the difference TmaxA - TmaxB, formed by the highest temperature, TmaxA, which presents the gaseous reaction mixture within the reaction zone A and the highest temperature, TmaxB, which presents the gaseous reaction mixture within the zone Reaction B is ≥ 0 ° C.

Advantageously, the specific volume activity of the catalyst bed in a fixed bed in the direction of the current increases at least once.

Generally, in the process according to the invention, the difference TmaxA-TmaxB is not greater than 75 ° C. According to the invention, preferably, TmaxA-TmaxB is ≥ 3 ° C and ≤ 60 ° C. Very particularly preferably TmaxA-TmaxB in the process according to the invention is ≥ 5 ° C and ≤ 40 ° C.

It has been shown that the process according to the invention is advantageous, for example, when the filler load of the catalyst in a fixed bed with the acrolein contained in the starting reaction gas mixture is ≥ 80 NI of

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acrolein / l · h and ≤ 125 NI of acrolein / l · h, or ≥ 90 NI of acrolein / l · h and ≤ 125 NI of Acrolein / l · h, or ≥ 100 Nl of acrolein / l · h and ≤ 120 Nl of acrolein / l · h, or ≥ 105 Nl of acrolein / l · h and ≤ 115 Nl of acrolein / l · h.

Of course, the process according to the invention can also be used when the charge of the catalyst bed in the fixed bed with the acrolein contained in the reaction gas mixture is <70 Nl of acrolein / l · h and / or the specific volume activity of the filler Fixed bed catalyst is constant. The operation of a multi-zone arrangement for reduced reactant loads in this way would, however, still be inexpensive.

The differences required according to the invention TmaxA-TmaxB are adjusted in the implementation of the process according to the invention if, on the one hand, both the temperature of the reaction zone A and also the temperature of the reaction zone B are in the range of 230 to 320 ° C and, on the other hand, the difference between the temperature of the reaction zone B (TB) and the temperature of the reaction zone A (TA), that is, TB-TA, is ≤ 0 ° C and ≥ -20 ° C or ≥ -10 ° C, or ≤ 0 ° C and ≥ -5 ° C, or often ≤ 0 ° C and ≥ -3 ° C.

That is, contrary to the teachings of the state of the art for high loads, in the process according to the invention the temperature of the posterior zone will normally be lower than the temperature of the preceding reaction zone.

The above statement regarding the TB-TA temperature difference is also valid when the temperature of the reaction zone A is in the preferred range of 250 to 300 ° C or in the particularly preferred range of 260 to 280 ° C.

In the process according to the invention, the operating pressure may be lower than normal pressure (for example up to 50 kPa) or higher than normal pressure. Normally the working pressure is in values of 100 to 500 kPa, often 100 to 300 kPa. Normally the reaction pressure of 10,000 kPa is not exceeded.

According to the invention, the reaction zone A preferably extends to an acrolein reaction of 50 to 85 mol% or 60 to 85 mol%.

Generally the acrolein reaction with respect to a single step in the process according to the invention is ≥ 92% in moles, or ≥ 94% in moles, or ≥ 96% in moles, or ≥ 98% in moles and even often ≥ 99 mol% In this regard, the selectivity of the acrylic acid formation is normally ≥ 92% in moles, or ≥ 94% in moles, often ≥ 95% in moles or ≥ 96% in moles or ≥ 97% in moles.

The molar ratio O2: acrolein in the gaseous mixture of the initial reaction must be according to the invention ≥ 0.5. It is often found in values ≥ 1. Usually this ratio is found in values ≤ 3. Often the molar ratio of O2: acrolein in the gaseous starting reaction mixture is according to the invention from 1 to 2 or from 1 to 1, 5.

As catalysts for the filler of a fixed-bed catalyst of the gas phase catalyzed acrolein oxidation according to the invention are those whose active mass is a multimetallic oxide containing Mo and V. Suitable multimetallic oxide catalysts of this type can be extracted, for example, from US-A 3 775 474, from US 3954855, from US-A 3893951 and from US-A 4339355. In addition, the multimetallic oxide masses of EP-A 427508 are particularly suitable, of document DE-A 2909671, document DE-C 3151805, document DE-AS 2626887, document DE-A 4302991, document EP-A 700893, document EP-A 714700 and document DE-A 19736105, as well as of DE-A 10046928.

Exemplary embodiments of EP-A 714700, as well as DE-A 19736105, are also suitable in this context.

A plurality of the multimetallic oxide active masses suitable for the fixed bed catalyst filler, for example those of DE-A 19815281, are summarized with the general formula IV

image 1

in which the variables have the following meanings: X1 = W, Nb, Ta, Cr and / or Ce, X2 = Cu, Ni, Co, Fe, Mn and / or Zn, X3 = Sb and / or Bi, X4 = one or more alkali metals, X5 = one or more alkaline earth metals,

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X6 = 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 a2, f = 0 to 4 , g = 0 to 40 and n = a number that is determined by the assessment and frequency of non-oxygen elements

of IV.

Preferred embodiments according to the invention within the active multimetallic oxides IV are those

which consider the following meanings of the variables of the general formula IV:

X1 = W, Nb and / or Cr,

X2 = Cu, Ni, Co and / or Fe,

X3 = Sb,

X4 = Na and / or K,

X5 = Ca, Sr and / or Ba,

X6 = Yes, 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 that is determined by the assessment and frequency of non-oxygen elements

of IV. According to the invention, metal oxides IV are very particularly preferred those of the general formula V

image2

with

Y1

= W and / or Nb,

Y2

= Cu and / or Ni,

Y5

= Ca and / or Sr,

Y6

= Yes and / or Al,

to'

= 2 to 4,

b ’

= 1 to 1.5,

C'

= 1 to 3,

F'

= 0 to 0.5

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g ’= 0 to 8 and

n ’= a number determined by the assessment and frequency of non-oxygen elements of V.

The suitable multimetallic oxide (IV) masses according to the invention can be obtained in known ways per se, for example those disclosed in DE-A 4335973 or in EP-A 714700.

Mainly, suitable multimetallic oxide active masses can be prepared for catalysts of the fixed-bed catalyst filler, especially those of the general formula IV, in a simple way so that the most intimate dry mixture is produced from suitable sources of its elementary constituents possible, preferably finely divided, composed corresponding to its stoichiometry and this is calcined at temperatures of 350 to 600 ° C. The calcination can be carried out both in an inert gas atmosphere and in an oxidative atmosphere such as, for example, air (mixture of inert gas and oxygen), as well as in a reducing atmosphere (for example, mixtures of inert gas and reducing gases such as H2, NH3, CO, methane and / or acrolein or the gases with reducing activity mentioned in themselves). The duration of calcination can be from a few minutes to a few hours and usually decreases with temperature. As sources for the elemental constituents of the active masses of multimetallic oxide are considered compounds that are already oxides and / or compounds that can be transformed into oxides by heating, at least in the presence of oxygen.

The intimate mixing of the starting compounds for the mass production of multimetallic oxide IV can be carried out in dry form or in wet form. If performed in dry form, the compounds are conveniently used as finely divided powder and after mixing and if necessary a compaction, they are subjected to calcination. Preferably, intimate mixing is carried out, however, in wet form.

In this regard, usually, the starting compounds are mixed in the form of an aqueous solution and / or suspension with each other. Particularly intimate dry blends are then obtained in the described mixing process, if it is based exclusively on sources present in dissolved form of the elemental constituents. Water is preferably used as solvent. The aqueous mass obtained is then dried, the drying process being preferably carried out by spray drying the aqueous mixture with outlet temperatures of 100 to 150 ° C.

The masses of multimetallic oxide that are obtained, especially those of the general formula IV, are not generally used in powder form, but are formed in certain catalyst geometries in the catalyst bed in a fixed bed, the conformation being possible before or after the final calcination. For example, complete catalysts can be produced from the powdered form of the active mass or its uncalcined precursor mass by compaction to give the desired catalyst geometry (for example, by compressing into tablets, extrusion or pressing extrusion) , adjuvants such as, for example, graphite or stearic acid may be added as lubricants and / or molding aids and reinforcing agents such as glass microfibers, asbestos, silicon carbide or potassium titanate. Suitable complete catalyst geometries are, for example, full cylinders or hollow cylinders with an outside diameter and a length of 2 to 10 mm. In the case of hollow cylinders a wall thickness of 1 to 3 mm is suitable. Naturally, the complete catalyst can also have a spherical geometry, the diameter of the sphere being 2 to 10 mm.

Of course, the formation of the active mass in the form of a powder or its precursor mass in the form of a powder that is not yet calcined can also be carried out, also by application on inert catalyst supports. The coating of the support bodies for the production of the housing catalysts is generally carried out in a suitable rotating container, as is known, for example, from DE-A 2909671, EP-A 293859 or from the document EP-A 714700.

Conveniently, the dough to be applied for the coating of the molded body is wetted and after application dried, for example by means of hot air. The layer thickness of the mass applied on the support body is conveniently chosen in the range of 10 to 1000 mm, preferably in the range of 50 to 500 mm and is particularly preferably in the range of 150 to 250 mm. .

As support materials, porous and non-porous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium or aluminum silicate can be used in this regard. The support bodies must be shaped regularly or irregularly, with support bodies generally formed with a clearly formed surface roughness being preferred, for example, hollow spheres or cylinders with a gravel layer. The use of spherical supports with essentially non-porous surface roughness of soapstone (for example, C220 soapstone from CeramTec), whose diameter is 1 to 8 mm, preferably 4 to 5 mm, is suitable. However, the use of cylinders as support bodies, whose length is from 2 to 10 mm and its outer diameter from 4 to 10 mm, is also suitable. In the case of rings as support bodies the wall thickness is also usually 1 to 4 mm. The annular support bodies to be used preferably have a length of 2 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness of 1 to 2 mm. Especially suitable are also rings with a geometry of 7 mm x 3 mm x 4 mm (outer diameter x

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length x inside diameter) as support bodies. The fineness of the catalytically active oxide masses to be applied on the surface of the support body naturally adapts to the desired shell thickness (see EP-A 714 700).

The active masses of multimetallic oxide which are to be used suitably for the catalyst of the fixed bed catalyst filler are also masses of the general formula VI,

image3

in which the variables have the following meanings:

image4

Z1 = W, Nb, Ta, Cr and / or Ce, Z2 = Cu, Ni, Co, Fe, Mn and / or Zn, Z3 = Sb and / or Bi, Z4 = Li, Na, K, Rb, Cs and / or H Z5 = Mg, Ca, Sr and / or Ba, Z6 = Si, Al, Ti and / or Zr, Z7 = 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", and "= numbers determined by the assessment and frequency of non-oxygen elements

of VI and p, q = non-zero numbers whose ratio p / q is 160: 1 to 1: 1, and which can be obtained by previously producing a mass of multimetallic oxide E

image5

in a finely divided form separately (starting mass 1) and then the solid starting mass 1 previously produced is incorporated into an aqueous solution, an aqueous suspension or a finely divided dry mixture of sources of the elements Mo, V, Z1, Z2 , Z3, Z4, Z5, Z6, which contains the elements mentioned above in stoichiometry D

image6

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(starting mass 2), in the desired quantity ratio p: q, the aqueous mixture, if necessary, in this respect is dried and the dried precursor mass obtained in this way is calcined at temperatures of 250 to 600 ° C to give the Desired catalyst geometry before or after drying.

The masses of multimetallic oxide VI are preferred, in which the incorporation of the previously formed solid starting mass 1 is carried out at an aqueous starting mass 2 at a temperature <70 ° C. A detailed description of the preparation of multimetallic oxide mass catalysts VI is presented, for example, in EP-A 668104, DE-A 19736105, DE-A 10046928, DE-A 19740493 and DE-A 19528646.

With respect to the conformation, what is said in the multimetallic oxide mass catalysts IV is valid with respect to multimetallic oxide mass catalysts VI.

In addition, those of DE-A 19815281, especially all exemplary embodiments of this document, are suitable as masses of multimetallic oxide suitable for catalysts in the fixed bed catalyst filler. With regard to the conformation, the above is valid.

Particularly suitable catalysts for the fixed bed catalyst filling of the process of the invention are the S1 (stoichiometry: MO12V3W1,2Cu2,4On) and S7 (stoichiometry: Mo12V3W1,2Cu1,6Ni0,8On) housing catalysts of DE-A 4442346 with an active mass ratio of 27% by weight and a shell thickness of 230 mm, the housing catalyst of preparation example 5 of DE-A 10046928 (stoichiometry: Mo12V3W1,2Cu2,4On) with an active mass ratio of 20% by weight, the housing catalysts according to examples 1 to 5 of DE-A 19815281, however also that the housing catalysts mentioned above for the second reaction stage are applied on support rings of a geometry of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) with an active mass ratio of 20% by weight (with respect to the total mass of the housing catalyst), as well as a car catalyst House with two-phase active mass of stoichiometry (Mo10.4V3W1.2Ox) (CuMo0.5W0.504) 1.6, and prepared according to DE-A 19736105 and a proportion of active mass of 20% by weight applied on the 7 mm x 3 mm x 4 mm support mentioned above.

The catalysts recommended above for the reaction stage according to the invention are also suitable, however, for the reaction stage according to the invention if everything is preserved and only the geometry of the support is modified to 5 mm x 3 mm x 1.5 mm (outer diameter x length x inner diameter). Furthermore, the multimetallic oxides mentioned in the form of corresponding complete catalyst rings can also be used in the reaction step according to the invention.

For the preparation of the catalyst bed in a fixed bed, only the catalyst molded bodies having active masses of multimetallic oxide or also broadly homogeneous mixtures of catalyst molded bodies having active masses of multimetallic oxide and bodies can be used in the process according to the invention. molded (dilution molded bodies) that do not have any active mass of multimetallic oxide, which behave with respect to partial oxidation in a gas phase heterogeneously catalyzed in an essentially inert manner. As materials for said inert molded bodies, all those which are also suitable as a support material for suitable housing catalysts according to the invention are considered. As such materials are considered, for example, porous and non-porous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide, silicates such as magnesium or aluminum silicate or the soapstone already mentioned (for example , soapstone C-220 from CeramTec).

The geometry of said inert dilution molded bodies may, in principle, be discretionary. That is, they can be, for example, spheres, polygons, full cylinders or also rings. According to the invention, preferably, inert dilution bodies are chosen whose geometry corresponds to any of the catalyst molded bodies that are diluted therewith.

It is suitable according to the invention that the chemical composition of the active mass used is not altered through the catalyst bed in a fixed bed. That is, the active mass used for an individual catalyst molded body may, in fact, be a mixture of different multimetallic oxides containing the Mo and V elements, for all catalyst molded bodies of the fixed bed catalyst filler must be used. , however, the same mixture.

The specific volume activity (that is, the one normalized to one unit of volume) can be reduced in this case, for example, in a simple way, homogeneously diluting a basic amount of catalyst molded bodies produced uniformly with molded bodies. dilution The higher the proportion of the dilution molded bodies, the lower the active mass present in a given volume of filler or the activity of the catalyst.

A specific activity of increasing volume at least once in the direction of the flow of the reaction gas mixture through the catalyst bed in a fixed bed can be adjusted for the process according to the invention, therefore, in a simple way, by example so that the filling begins with a high proportion of inert dilution molded bodies with respect to the class of catalytic molded bodies and then this

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proportion of dilution molded bodies is reduced in the direction of the current either continuously or at least once or several times abruptly (eg, in stages). An increase in the specific volume activity is also, however, possible, for example, by increasing the thickness of the active mass layer applied to the support for a conserved geometry and an active mass type of a shell catalyst body. in a mixture of housing catalysts with the same geometry but with different weight ratio of the active mass, the proportion of catalyst castings with a higher weight ratio of active mass. Alternatively, the active mass itself can be diluted by incorporating in the production of the active mass, for example in the dry mixture to be calcined from starting compounds, inert diluent materials such as highly calcined silicon dioxide. With different compositions of diluent material, different activities are automatically obtained. The more diluent material added, the lower the resulting activity. An analogous effect can also be achieved, for example, by changing the mixture ratio in mixtures of complete catalysts and of housing catalysts (in the case of an identical active mass). Naturally, the variants described can also be used in combination.

Of course, mixtures of catalysts with a chemically different active dough composition and as a consequence a different composition with a different activity can also be used for the fixed bed catalyst filling. These mixtures can be diluted, in turn, with inert dilution bodies.

In the normal case, in the process of the invention, the specific activity of volume is never reduced within the fixed bed catalyst filling in the direction of the flow of the reaction gas mixture.

Before and / or subsequent to the fixed-bed catalyst filler, fillers consisting exclusively of inert material (for example, only dilution molded bodies) can be found (these are not conceptually included in the fixed-bed catalyst filler, since they do not contain molded bodies that have an active mass of multimetallic oxide). In this regard, the dilution molded bodies used for the inert filler have the same geometry as the catalyst molded bodies used in the fixed bed catalyst filler. The geometry of the dilution molded bodies used for the inert filler may, however, also be different from the aforementioned geometry of the catalyst molded bodies (for example spherical rather than annular).

Often, the molded bodies used for such inert fillers have an annular geometry of 7 mm x 7 mm x 4 mm (outer diameter x length x inner diameter) or a spherical geometry with a diameter d = 4-5 mm.

According to the invention, it is preferred in the process according to the invention to structure the catalyst filling in a fixed bed in the flow direction of the reaction gas mixture as follows.

In the first place at a length of 10 to 60%, preferably from 10 to 50%, particularly preferably from 20 to 40% and very particularly preferably from 25 to 35% (ie, for example at a length of 0 , 70 to 1.50 m, preferably 0.90 to 1.20 m), in each case of the total length of the catalyst bed in a fixed bed, a homogeneous mixture or two consecutive homogeneous mixtures (with decreasing dilution) are arranged of catalyst molded bodies and dilution molded bodies (both preferably having essentially the same geometry), the proportion being by weight of dilution molded bodies (the mass densities of catalyst molded bodies and dilution molded bodies are generally differentiated only minimally) normally 10 to 50% by weight, preferably 20 to 45% by weight and particularly preferably 25 to 35% by weight. Following this first zone of the catalyst bed filling, it is then advantageously according to the invention to the end of the length of the catalyst bed filling (ie, for example, at a length of 2.00 to 3.00 m, preferably from 2.50 to 3.00 m) either a filler diluted only to a small extent (as in the first zone) of the catalyst molded bodies, or, very particularly preferably, a filler unique from the same catalyst molded body that has also been used in the first zone.

The above applies especially when in the fixed-bed catalyst filling as a catalyst molded body, shell catalyst rings or shell catalyst spheres are used (especially those indicated as preferred herein). Advantageously, within the framework of the aforementioned structuring, both the catalyst molded bodies or their support rings and the dilution molded bodies in the process according to the invention have essentially an annular geometry of 7 mm x 3 mm x 4 mm ( outer diameter x length x inner diameter).

The aforementioned also applies when instead of dilution bodies, molded bodies of shell catalyst are used, whose proportion of active mass is reduced by 2 to 15% by weight with respect to the proportion of active mass of the molded body of housing catalyst at the end of the fixed bed catalyst filling.

In a suitable technical use mode, the completion of the reaction step of the process according to the invention is carried out in a two-zone tube bundle reactor, as described, for example, in DE documents.

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A 19910508, 19948523, 19910506 and 19948241. A preferred variant of a two-zone tube beam reactor that can be used according to the invention is disclosed in DE-C 2830765. But also the disclosed two-zone tube beam reactors. in DE-C 2513405, US-A 3147084, DEA-2201528, EP-A 383224 and DE-A 2903218 are suitable for an embodiment of the reaction step of the process according to the invention.

That is to say, in a simple way, the fixed-bed catalyst filler to be used according to the invention is found (possibly with inert fillers arranged before and / or subsequently) in the metal tubes of a tube bundle reactor and around From the metal tubes, two heating means essentially spaced apart from each other are conducted, generally salt melts. The section of the tube, through which the corresponding saline bath extends, represents according to the invention a reaction zone.

That is, in a simple way, for example, a salt bath A is circulated around those sections of the tube (the reaction zone A) in which the oxidative reaction of the acrolein is carried out (in a single step ) until a reaction value is achieved in the range of 45 to 85 mol% (preferably 50 to 85 mol%, particularly preferably 60 to 85 mol%) and B is circulated around the tube section (reaction zone B), in which the final oxidative reaction of acrolein (in a single step) is carried out until a reaction value of at least 90 mol% is achieved (if required they can be added to the zone of reaction A, B which is used according to the invention other reaction zones that are maintained at individual temperatures).

With regard to technical use, conveniently, the reaction step of the process according to the invention does not comprise any additional reaction zone. That is, the saline bath B is conveniently circulated around the section of the tubes, in which the final oxidative reaction of the acrolein is carried out (for a single step) to a reaction value> 92 mol%, or> 94% in moles, or> 96% in moles, or> 98% in moles and often even> 99% in moles or higher.

Usually, the beginning of reaction zone B is behind the maximum hot spot of reaction zone A.

Both saline baths A, B can be conducted according to the invention with respect to the current direction of the reaction gas mixture flowing through the reaction tube in the same direction of current or countercurrent through the space surrounding the tube of reaction. Naturally, according to the invention, a current with the same current direction can also be used in the reaction zone A and in the reaction zone B a countercurrent current (or vice versa).

Naturally, in all the case configurations mentioned above, the parallel flow that occurs within the corresponding reaction zone, with respect to the reaction tubes, of the molten saline material a transverse current is superimposed, so that the reaction zone individual corresponds to a tube bundle reactor described as in EP-A 700714 or in EP-A 700893 and, as a whole, in the longitudinal section by means of the beam of contact tubes a result of a path of the meander current of the heat exchange medium.

Usually, in the tube beam reactors mentioned above for the reaction stage according to the invention, the contact tubes are made of ferritic steel and normally have a wall thickness of 1 to 3 mm. Its inner diameter is generally 20 to 30 mm, often 22 to 26 mm. Its length is conveniently from 3 to 4, preferably 3.5 m. In each temperature zone the catalyst bed in fixed bed covers at least 60% or at least 75% or at least 90% of the length of the zone. The remaining length given the remaining case is covered, if necessary, with an inert filler. With respect to technical use, the number of contact tubes arranged in the tube bundle container amounts to 5000, preferably at least

10,000 Often the number of contact tubes arranged in the reaction vessel is 15,000 to 30,000. Tube beam reactors with a number of contact tubes greater than 40,000 are certainly an exception. In the interior of the container the contact tubes are arranged in a normal case in a homogeneously distributed manner (preferably 6 adjacent tubes equidistant by contact tube), the distribution being conveniently chosen so that the distance between the central inner axes of contact tubes more close to each other (the so-called contact tube separation) is 35 to 45 mm (see EP-B 468290).

Especially suitable for heat exchange are fluid heating means. Particularly suitable is the use of salt melts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or low melting metals such as sodium, mercury or alloys of different metals.

Generally, in all the above-mentioned configurations of current conduction in the two-zone tube beam reactors of the reaction stage according to the invention, the flow rate within both necessary heat exchange medium circuits is chosen so that the Temperature of the heat exchange medium of the entry site in the reaction zone increases to the exit site of the reaction zone by 0 to 15 ° C. That is, the mentioned ΔT can be according to the invention from 1 to 103 ° C or from 2 to 8 ° C or from 3 to 6 ° C.

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The inlet temperature of the heat exchange medium in the reaction zone A is according to the invention normally in the range of 230 to 320 ° C, preferably in the range of 250 to 300 ° C and particularly preferably in the range of 260 at 280 ° C. The inlet temperature of the heat exchange medium in the reaction zone B is according to the invention usually also in the range of 230 to 320 ° C or 280 ° C, simultaneously, however, it is usually> 0 ° C to <20 ° C or at <10 ° C, or> 0 ° C and <5 ° C, or, often> 0 ° C and <3 ° C below the inlet temperature of the heat exchange medium entering the reaction zone A.

At this point it should also be noted that for one embodiment of the reaction stage according to the invention of the process according to the invention, the type of two-zone tube bundle reactor described in DE-AS 2201528 can also be used, which it is possible to discharge a partial amount from the hottest heat exchange medium of the reaction zones B to the reaction zone A, in order to produce a heating of a cold-starting reaction gas mixture or a circuit gas cold. In addition, the tube bundle characteristic can be configured within an individual reaction zone as described in EP-A 382 098.

Normally, acrolein is used in the process according to the invention, which is produced by catalytic gas phase oxidation of propene. Generally, the acrolein-containing reaction gases are used in this propene oxidation without intermediate purification, whereby the starting reaction gas mixture according to the invention may also contain reduced amounts of, for example, unreacted propene or by-products of the oxidation of propene. Normally, in this respect, the oxygen necessary for the oxidation of acrolein must be added to the gas mixture of propene oxidation product.

Advantageously, a gas phase catalyzed propene oxidation of this type prior to the process according to the invention to give acrolein is carried out analogously to the process according to the invention, conducting a starting reaction gas mixture containing propene, molecular oxygen and at least one inert gas, which contains the molecular oxygen and the propene in a molar ratio O2: C3H6 ≥ 1, in a reaction stage through a catalyst bed in a fixed bed, which is arranged in two reaction zones A ' B 'spatially arranged consecutively, the temperature of the reaction zone A' being a temperature in the range of 290 to 380 ° C and a temperature of the reaction zone B 'also a temperature in the range of 290 to 380 ° C, and its active mass is a metal oxide containing at least one of the elements Mo, Fe and Bi, so that the reaction zone A 'extends to a reaction of the propene of at 80 mol% and the propene reaction for a single passage through the catalyst bed in a fixed bed 1 is ≥ 90 mol% and the corresponding selectivity of acrolein formation, as well as by-product formation of acrylic acid in total is ≥ 90% in moles, characterized in that

a) the charge of the catalyst bed in a fixed bed with the propene contained in the initial reaction gas mixture is ≤ 160 Nl of propene / l of catalyst filling in a fixed bed 1 h and ≥ 90 Nl of acrolein / l of the filler fixed bed catalyst 1 h,

b) the specific volume activity of the fixed bed catalyst filler in the direction of the flow of the reaction gas mixture through the fixed bed catalyst filler is constant or at least ever increases, and

c) the difference TmaxA '- TmaxB', formed by the highest temperature, TmaxA ', which presents the gaseous reaction mixture within the reaction zone A and the highest temperature, TmaxB', which presents the gaseous reaction mixture within the reaction zone B is ≥ 0 ° C.

Generally, in the process according to the invention, the difference TmaxA '- TmaxB' is not greater than 80 ° C.

Preferably, TmaxA ’- TmaxB’ is ≥ 3 ºC and ≤ 70 ºC. Very particularly, TmaxA ’- TmaxB’ is ≥ 20ºC and ≤ 60ºC.

In this regard, it has been shown that it is advantageous that the charge of the catalyst bed in the fixed bed with the propene contained in the starting reaction gas mixture is ≥ 90 Nl of propene / l · h, or ≤ 155 Nl of propene / l · ho ≥ 100 Nl of propene / l · h and ≤ 150 Nl of propene / l · h, or ≥ 110 Nl of propene / l · h and ≤ 145 Nl of propene / l · h, or ≥ 120 Nl of propene / l · H ≤ 140 Nl of propene / l · h, or ≥ 125 Nl of propene / l · h and ≤ 135 Nl of propene / l · h.

The differences required in this respect of TmaxA '- TmaxB' are adjusted in the implementation of the procedure if, on the one hand, both the temperature of the reaction zone A 'and also the temperature of the reaction zone B' are found in the range of 230 to 380 ° C and, on the other hand, the difference between the temperature of the reaction zone B '(TB') and the temperature of the reaction zone A '(TA'), that is, TB ' -TA ', is ≤ 0 ºC and ≥ -10 ºC, or ≤ 0 ºC and ≥ 5 ºC, or often ≤ 0ºC and ≥ -3ºC.

That is, for carrying out the process according to the invention, two two-zone tube bundle reactors can be connected in series or fused in a four-zone tube bundle reactor, as described, for example, in WO 01 / 36364, the oxidation of propene being carried out in the first portion of two zones and in the second portion of two zones the oxidation of acrolein according to the invention.

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Normally, in the second case, an inert filler is located between the fixed bed catalyst fillers for the two reaction stages. However, an intermediate inert filling of this type can also be renounced.

Naturally, the propene oxidation stage may be a tube bundle reactor of a separate zone or fused with the acrolein oxidation stage in a corresponding manner.

The length of the reaction tubes corresponds in the case of fusion often with the sum of the lengths of the unbranched tube beam reactors.

The proportion of propene in the initial reaction gas mixture for the reaction stage preceding the process according to the invention can be, in this respect, for example, of values of 15% by volume, usually 5 to 12% by volume or 5 to 8% by volume (in each case with respect to the total volume).

The procedure according to the invention is often carried out at a volumetric ratio of propene: oxygen: indifferent gases (including water vapor) in the starting gas reaction mixture of 1: (1.0 to 3.0 ): (5 to 25), preferably 1: (1.4 to 2.3): (10 to 15). Normally the indifferent (inert) gas is constituted in at least 20% of its volume by molecular nitrogen. However, it may also consist of ≥ 30% by volume, or ≥ 40% by volume, or ≥ 50% by volume, or ≥ 60% by volume,

or in ≥ 70% by volume, or in ≥ 80% by volume, or by ≥ 90% by volume, or by ≥ 95% by volume by molecular nitrogen (generally inert dilution gases that react in a single pass through the corresponding reaction stage in less than 5%, preferably in less than 2%; these are in addition to molecular nitrogen, for example, gases such as propane, ethane, methane, pentane, butane, CO2, CO, water vapor and / or noble gases). Naturally, the inert dilution gas in the process preceding the process of the invention is also made up to 50 mol%, or up to 75 mol% and more propane. The dilution gas component may also be circuit gas, as preserved after the separation of the acrylic acid from the gas mixture of the product of the process according to the invention.

Catalysts for an oxidation of gas phase catalytic propene of this type are especially those of EP-A 15565, EP-A 575897, DE-A 19746210 and DE-A 19855913.

The proportion of acrolein in the initial reaction gas mixture for the process according to the invention can be, for example, values of 3 to 15% by volume, often 4 to 10% by volume or 5 to 8% in volume (in each case with respect to the total volume). The process according to the invention is often carried out with a volumetric ratio present in the acrolein starting reaction gas mixture: oxygen: water vapor: inert gas (NI) of 1: (1 to 3): (0 to 20) : (3 to 30), preferably 1: (1 to 3): (0.5 to 10): (7 to 10).

Of course, however, the process according to the invention can also be carried out with a volumetric ratio present in the initial gas reaction mixture of acrolein: oxygen: water vapor: other (NI) of 1: (0.9 to 1, 3): (2.5 to 3.5): (10 to 12).

At this point it should be noted that, as active masses for the fixed bed catalyst filler of the process according to the invention, the multimetallic oxide masses of DE-A 10261186 are also suitable.

In general, the specific activity of volume between two fixed-bed catalyst filling zones can be experimentally differentiated in a simple way so that under identical boundary conditions (preferably the process conditions considered) through fixed-bed catalyst fillers of the same length, but in each case corresponding to the composition of the corresponding fixed bed catalyst filling zone, the same starting reaction gas mixture containing acrolein is conducted. The highest reacted amount of acrolein has the highest specific volume activity.

Suitable designs according to the invention of a two-zone tube bundle reactor for a stage of partial oxidation of propene preceding the reaction stage according to the invention can be obtained as follows (the design of the construction details can be performed as in applications for utility models 202 19 277.6, 2002 19 278.4 and 202 19 279.2 or in PCT applications PCT / EP02 / 14187, PCT / EP02 / 14188 or PCT / EP02 / 14189):

Contact Tubes:

Material of the contact tubes: ferritic steel;

Contact tube sizes: for example, 3500 mm in length;

for example, 30 mm outside diameter;

for example, 2 mm wall thickness;

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Number of contact tubes in the tube bundle:

for example 30,000, or 28,000, or 32,000, or 34,000; additionally up to 10 thermotubes (as described in EP-A 873 783 and EP-A 12 70 065), which are fed as the contact tubes (helical type that rotate from the outside to the inside), for example of the same wall length and thickness, but with an outside diameter of, for example, 33.4 mm and a centered thermovaline of, for example, 8 mm outside diameter and, for example, 1 mm wall thickness;

Reactor (material such as contact tubes):

Cylinder container with an inner diameter of 6000-8000 mm;

Reactor cover plated with stainless steel type 1.4541; plating thickness: some mm;

bundle of tubes arranged annularly, for example with a free central space;

diameter of the central clearance: for example 1000 - 2500 mm (for example 1200 mm, or 1400 mm, or 1600 mm, or 1800 mm, or 2000 mm, or 2200 mm, or 2400 mm);

normally homogeneous contact tube distribution in tube bundles (6 adjacent equidistant tubes per contact tube), equilateral triangle arrangement, contact tube distribution (distance from the central axes of contact tubes closest to each other: 35 - 45 mm, for example 36 mm, or 38 mm, or 40 mm, or 42 mm, or 44 mm;

the contact tubes are hermetically fixed with their ends in contact tube floors (upper floor and lower floor, for example with a thickness of 100-200 mm) and end at the upper end in a bell connected to the container that has a input for the gaseous mixture of starting reaction; a separation plate which is, for example, in the middle of the length of the contact tube with a thickness of 20-100 mm divides the reactor space symmetrically into two reaction zones (temperature zones) A '(upper zone ) and B '(inner zone); each reaction zone is divided by a shunt plate into 2 equidistant longitudinal sections;

the shunt plate preferably has a ring geometry; the contact tubes are hermetically fixed to the separation plate; in the shunt plates they are not hermetically fixed, so that the transverse current velocity of the salt melts within an area is as stable as possible;

each zone is supplied by its own salt pump with salt melt as a heat carrier; the salt melt feed is, for example, below the bypass plate and the discharge is, for example, above the bypass plate;

from both circuits of molten salt, a partial current is extracted, for example, and cooled in a separate or two separate indirect heat exchanger (steam production);

in the first case, the cooled saline melt stream is separated, purified with the corresponding residual current and is conducted under pressure by means of the pump corresponding to the corresponding annular channel, which distributes the salt melt through the perimeter of the vessel, of the reactor ;

the salt melt reaches the tube bundle through the window in the reactor jacket; the inflow is carried out, for example, in the radial direction to the tube bundle;

the salt melt flows in each zone of the bypass plate specification following, for example, the sequence

 from outside to inside,

 from inside to outside,

around the contact tubes;

through the window surrounding the perimeter of the vessel, the salt melt is collected at each end of the zone in an annular channel that surrounds the reactor jacket, to even pump partial current cooling into the circuit; Through each reaction zone, the salt melt is conducted from the bottom up.

The reaction mixture leaves the reactor of the reaction stage preceding the reaction stage according to the invention with a temperature a few degrees higher than the inlet temperature of the salt bath of this reaction. The reaction mixture is conveniently cooled for processing in a separate rear cooler, which is subsequently connected to this reactor, at 220 ° C to 280 ° C, preferably 240 ° C to 260 ° C.

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The rear cooler is generally connected with flanges below the bottom of the lower tubes and is normally made up of tubes with ferritic steel. Stainless steel sheet spirals are advantageously incorporated internally in the tubes of the rear cooler, which can be partially or totally screwed in to improve heat transfer.

5 Saline melt:

As the salt melt, a mixture of 53% by weight of potassium nitrate, 40% by weight of sodium nitrite and 7% by weight of sodium nitrate can be used; both reaction zones and the subsequent refrigerator advantageously use a salt melt of the same composition; The amount of salt pumped in the reaction zones can be according to the area of approximately 10,000 m3 / h.

10 Current conduction:

the initial reaction gas mixture for the oxidation of propene conveniently flows from the top downwards through the reactor, while the salt melts heated differently from the individual zones are conveniently transported from the bottom up; the contact tube and thermotube feed (from top to bottom), for example:

15 Section 1: 50 cm in length

7mm x 7mm x 4mm geometry soapstone ring (outside diameter x length x inside diameter) as pre-filled.

Section 2: 140 cm in length

catalyst feed with a homogeneous mixture of 30% by weight of soapstone rings of

20 geometry of 5 mm x 3 mm x 2 mm (outer diameter x length x inner diameter) and 70% by weight of complete catalyst from section 3.

Section 3: 160 cm in length

ring-shaped catalyst feed (5 mm x 3 mm x 2 mm = outer diameter x length x inner diameter) of complete catalyst according to example 1 of DE-A 10046957 (stoichiometry: [Bi2W2O9 x 2

WO3] 0.5 [Mo12Co5.5Fe2.94 Si1.59K0.08Ox]) 1.

According to the invention, suitable designs of a two-zone tube beam reactor for the acrolein reaction stage according to the invention can be achieved as follows:

All such as in the two-zone tube beam reactor for the propene reaction stage. The floor thickness of upper and lower contact tubes is, however, often 100-200 mm, for example 110

30 mm, or 130 mm, or 150 mm, or 170 mm, or 190 mm.

The rear refrigerator is suppressed; instead, the contact tubes with their lower opening flow into a bell attached to the lower end with the outlet vessel for the gaseous product mixture; the upper reaction zone is zone A and the lower reaction zone is reaction zone B. Between the "rear cooler" outlet and the "reactor for the reaction stage according to the invention" input is present

35 conveniently a power supply for compressed air.

The feeding of the contact tube and the thermotube (from top to bottom) can be, for example, as follows:

Section 1: 20 cm in length

7mm x 7mm x 4mm geometry soapstone ring (outside diameter x length x inside diameter) as pre-filled.

40 Section 2: 90 cm in length

catalyst feed with a homogeneous mixture of 30% by weight of geometry soapstone rings of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) and 70% by weight of housing catalyst from the section Four.

Section 3: 50 cm in length

45 catalyst feed with a homogeneous mixture of 20% by weight of geometry soapstone rings of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) and 80% by weight of shell catalyst from the section 4.

Section 4: 190 cm in length

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ring-shaped catalyst feed (7 mm x 3 mm x 4 mm = outer diameter x length x lower diameter) of housing catalyst according to example 5 of DE-A 10046928 (stoichiometry: Mo12V3W, 2Cu12.4O x).

Alternatively, the supply of the contactor tube and the thermotube for the propene stage (from top to bottom) 5 can also have the form of:

Section 1: 50 cm in length

7mm x 7mm x 4mm geometry soapstone ring (outside diameter x length x inside diameter) as pre-filled.

Section 2: 300 cm in length

10 ring-shaped catalyst feed (5 mm x 3 mm x 2 mm = outer diameter x length x lower diameter) of complete catalyst according to example 1 of DE-A 10046957 (stoichiometry: [Bi2W2O9 x 2WO3] 0.5 • [Mo12Co5,6Fe2,94Si1,59K0,08 Ox] 1).

Alternatively, the supply of the contactor tube and the thermo tube for the acrolein stage (from top to bottom) may also have the form of:

15 Section 1: 20 cm in length

7mm x 7mm x 4mm geometry soapstone ring (outside diameter x length x inside diameter) as pre-filled.

Section 2: 140 cm in length

catalyst feed with a homogeneous mixture of 25% by weight of soapstone rings of 20 geometry of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) and 75% by weight of housing catalyst from the section 3.

Section 3: 190 cm in length

ring-shaped catalyst feed (7 mm x 3 mm x 4 mm = outer diameter x length x lower diameter) of housing catalyst according to example 5 of document DE-A 10046928 25 (stoichiometry: Mo12V3 W1,2 Cu2,4Ox ).

In all of the mentioned propene stage feeds, the complete catalyst of example 1 of DE-A 10046957 can also be replaced by:

a) a catalyst according to example 1 c of EP-A 15565 or a catalyst produced according to this example, which nevertheless has the active mass Mo12Ni6,5Zn2Fe2Bi1P0,0065K0,06Ox • 10 SiO2;

30 b) Example No. 3 of DE-A 19855913 as a complete hollow cylinder catalyst of 5 mm x 3 mm x 2 mm geometry or 5 mm x 2 mm x 2 mm;

c) multimetallic oxide II - complete catalyst according to example 1 of DE-A 19746210;

d) one of the housing catalysts 1, 2 and 3 of DE-A 10063162, but for the same shell thickness applied on 5 mm x 3 mm x 1.5 mm or 7 mm x geometry support rings 3 mm x 1.5 35 mm;

e) a housing catalyst according to the embodiments of DE-A 19815281, but for the same housing thickness applied on geometry support rings of 7 mm x 3 mm x 4 mm, or 8 mm x 6 mm x 4 mm (always outside diameter x length x inside diameter).

In the list indicated above, preferably Steatite from the company 40 CeramTec (type: C220) is used as support material.

In all the acrolein stage feeds according to the invention mentioned above, the housing catalyst according to the example prepared in document DE-A 10046928 can be replaced by:

a) housing catalyst S1 or S7 of DE-A 4442346 with an active mass ratio of 27% by weight and a shell thickness of 230 µm;

45 b) a carcass catalyst according to examples 1 to 5 of DE-A 19815281, but applied on a geometry support ring of 7 mm x 3 mm x 4 mm with an active mass ratio of 20% by weight;

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c) Two-phase stoichiometry active mass housing catalyst (Mo10.4V3W1.2Qx) (CuMo0.5W0.6O4) 1.6, produced according to DE-A 19736105 and with an active mass ratio of 20% in weight applied on the 7 mm x 3 mm x 4 mm support mentioned above.

In addition, the fixed bed catalyst filling for the propene oxidation stage and the fixed bed catalyst filling for the acrolein oxidation stage according to the invention are chosen (for example by dilution with, for example, inert material) of so that the temperature difference between the maximum hot spot of the reaction gas mixture in the individual reaction zones and the corresponding temperature of the reaction zone does not generally exceed 80 ° C. Usually, this temperature difference is ≤ 70 ° C, often it is 20 to 70 ° C, preferably this temperature difference is small. Furthermore, these fixed-bed catalyst fillings, for safety reasons, are chosen in a manner known to the expert (for example by dilution with, for example, inert material), so that the "maximum salt temperature sensitivity "As defined in EP-A 1106598 is ≤ 9 ° C, or ≤ 7 ° C, or ≤ 5 ° C, or ≤ 3 ° C.

The rear cooler and the reactor for the acrolein reaction stage are connected by a junction tube whose length is less than 25 m.

The order of the reactors described and all other orders of reactors and feeds described herein with a fixed bed catalyst can also be operated at high propene or acrolein loads as described in WO 01/36364, DE- A 19927624, DE-A 19948248, DE-A 19948523, DE-A 19948241, DE-A 19910506, DE-A 10302715 and EP-A 1106598.

Preferably, the reaction zones A ', B' or A, B can be operated in this respect, which have the temperatures recommended herein, however, so that the corresponding second reaction zone, according to the teachings of the previous documents, present a higher temperature than the corresponding first reaction zone. The hot spot temperature in the second corresponding reaction zone is preferably always lower than that of the corresponding first reaction zone.

In the acrolein fillers according to the invention, however, with the method of the invention according to the invention, an increased selectivity of acrylic acid formation is obtained with respect to a recommended process mode for high acrolein fillers.

In the following examples and comparative examples, as well as in the previous reactor arrangement, the ring-shaped dilution bodies and the ring-shaped catalyst molded bodies can also be replaced by dilution molded bodies in the acrolein reaction stage spherical and molded spherical catalyst bodies (in each case with a radius of 2 to 5 mm and with an active mass ratio of 10 to 30% by weight, often 10 to 20% by weight). What has been said with respect to the operation in case of high acrolein loads according to the previous stages maintains, in this respect, its validity.

Examples and comparative examples

A reaction tube (V2A steel; 30 mm outside diameter, 2 mm wall thickness, 26 mm inside diameter, length: 350 cm, as well as a thermotube centered in the middle of the reaction tube (4 mm outside diameter ) to collect a thermoelement with which the temperature in the reaction tube can be determined along its entire length) is fed from top to bottom as follows:

Section 1: 20 cm in length

7mm x 7mm x 4mm geometry soapstone ring (outside diameter x length x inside diameter) as pre-filled.

Section 2: 90 cm in length

catalyst feed with a homogeneous mixture of 30% by weight of geometry soapstone rings of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) and 70% by weight of housing catalyst from the section Four.

Section 3: 50 cm in length

catalyst feed with a homogeneous mixture of 20% by weight of geometry soapstone rings of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) and 80% by weight of housing catalyst from the section Four.

Section 4: 190 cm in length

ring-shaped catalyst feed (7 mm x 3 mm x 4 mm = outer diameter x length x lower diameter) of housing catalyst according to example 5 of DE-A 10046928 (stoichiometry: Mo12V3W1,2Cu2,4Ox).

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From top to bottom the first 175 cm are thermostatized by means of a saline bath A pumped against the current. The second 175 cm are thermostatized by means of a saline bath B pumped in countercurrent. Gas phase oxidation: The reaction tube described above is continuously fed with a reaction starting mixture of the following composition, the charge and thermostatization of the reaction tube being varied: 5.5% by volume of acrolein, 0, 3% by volume of propene, 6.0% by volume of molecular oxygen, 0.4% by volume of CO, 10 0.8% by volume of CO2, 9.0% by volume of water and 78.0% in volume of nitrogen, The reaction gas mixture is passed through the reaction tube from top to bottom. The pressure at the inlet of the reaction tube varies depending on the acrolein load between 160 and 210 kPa. 15 From the reaction gas mixture a small sample is collected at the outlet of the reaction tube

by gas chromatography. At the end of reaction zone A is also an analysis site. The results depending on the loads and temperatures of the salt bath are shown in the following table (letter E in parentheses means example and letter C in parentheses means comparative example).

TA, TB, represent the temperatures of the saline baths pumped to reaction zones A and B.

20 UAA is the reaction of acrolein at the end of reaction zone A in mol%. UAB is the reaction of acrolein at the end of reaction zone B in mol%. SAA is the selectivity of the formation of acrylic acid in the product gas mixture with respect to

unreacted acrolein in mol%. TmaxA, TmaxB

they represent the highest temperature of the reaction gas mixture within reaction zones A and B in ° C.

Table

Acrolein load (Nl / l • h)

TA TB Tmax Tmaxb UAA UAB SAA

106 (E)

260 260 302 276 80.7 99.3 95.4

108 (E)

262 259 312 275 84.8 99.3 95.8

104 (C)

257 262 285 291 64.0 99.3 94.9

152 (C)

263 269 303 287 78.8 99.3 95.8

147 (C)

257 278 278 310 61.3 99.3 95.0

Claims (1)

one.

A process of partial oxidation in gas phase catalyzed in a heterogeneous form of acrolein to give

Acrylic acid, in which a starting mixture of reaction gas containing acrolein, oxygen is conducted

molecular and at least one inert gas, which is made up of at least 20% of its volume by nitrogen

5

molecular, which contains molecular oxygen and acrolein in a molar ratio of O2: C3H4O ≥ 0.5, at a

reaction step through a catalyst bed in a fixed bed, which is arranged in two areas of

reaction A, B spatially arranged consecutively, the temperature of the reaction zone A being a

temperature in the range of 230 to 320 ° C and the temperature in reaction zone B also a

temperature in the range of 230 to 320 ° C, and whose active mass is at least one multimetallic oxide that

10

it contains the elements Mo and V, so that the reaction zone A extends until a reaction of the

45 to 85 mol% acrolein and in a single step of the reaction gas starting mixture through the

the whole of the catalyst bed in the fixed bed the acrolein reaction is ≥ 90 mol% and the

selectivity of acrylic acid formation, with respect to reacted acrolein, be ≥ 90 mol%,

fifteen

the temporal sequence corresponding to the reaction mixture starting through the reaction zones corresponds to the alphabetic sequence of the reaction zones, which is characterized in that

a) the charge of the catalyst bed in a fixed bed with the acrolein contained in the starting gas mixture of

reaction is ≤ 125 Nl of acrolein / l of catalyst filler in fixed bed h and ≥ 70 Nl of acrolein / l of filler

fixed bed catalyst h,

b) the specific activity of the volume of the catalyst bed in a fixed bed in the direction of the flow of

twenty

the gaseous reaction mixture through the catalyst bed filler is constant or at least

increases once, and

c) the difference TmaxA - TmaxB, consisting of the highest temperature, TmaxA, which presents the reaction gas mixture within the reaction zone A and the highest temperature, TmaxB, which presents the gas mixture

Reaction within the reaction zone B is ≥ 0 ° C.

25

2. Method according to claim 1, characterized in that the difference TmaxA-TmaxB is ≥ 0 ° C and ≤ 75 ° C.

18