ZA200508593B

ZA200508593B - Method for the heterogeneously catalyzed partial gas phase oxidation of acrolein into acrylic acid

Info

Publication number: ZA200508593B
Application number: ZA200508593A
Authority: ZA
Inventors: Dieterle Martin; Petzoldt Jochen; Mueller-Engel Klaus Joachim; Arnold Heiko
Original assignee: Basf Ag
Priority date: 2003-03-25
Filing date: 2005-10-24
Publication date: 2007-02-28
Also published as: TW200420535A; ES2545257T3; TWI324994B; CN100378058C; DE10313211A1; MY136503A; CN1764631A

Description

PF 54307

Heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid

The present invention relates to a process for partially oxidizing acrolein to acrylic acid in the gas phase under heterogeneous catalysis by conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and at least one inert gas contain- ing at least 20% by volume of molecular nitrogen and contains the molecular oxygen and the acrolein in a molar O,:C3H,O ratio of > 0.5 in one reaction stage over a fixed catalyst bed which is arranged in two spatially successive reaction zones A,B, the tem- perature of reaction zone A being a temperature in the range from 230 to 320°C and the temperature of reaction zone B likewise being a temperature in the range from 230 to 320°C, whose active composition is at least one multimetal oxide comprising the elements Mo and V, in such a way that reaction zone A extends to an acrolein conver- sion of from 45 to 85 mol% and, on single pass of the starting reaction gas mixture through the overall fixed catalyst bed, the acrolein conversion is > 90 mol% and the i5 selectivity of acrylic acid formation, based on acrolein converted, is > 90 moi%, the chronological sequence in which the starting reaction gas mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones.

The abovementioned process for catalytically oxidizing acrolein to acrylic acid in the gas phase is generally known (cf, for example, DE-A 19910508) and is especially sig- nificant as the second oxidation stage in the preparation of acrylic acid by two-stage catalytic gas phase oxidation starting from propene. Acrylic acid is an important mono- mer which finds use as such or in the form of its alkyl ester for obtaining polymers suit- able, for example, as adhesives.

In addition to molecular oxygen and the reactants, the starting reaction gas mixture contains inert gas in order to keep the reaction gas outside the explosion range, among other reasons.

One need of such a heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid is to achieve a very high yield Y** of acrylic acid (this is the number of moles of acrolein converted to acrylic acid, based on the number of moles of acrolein used) on single pass of the reaction gas mixture through the reaction stage under oth- erwise predefined boundary conditions.

A further need of such a heterogeneously catalyzed partial gas phase oxidation of ac- rolein to acrylic acid is to achieve a very high space-time yield (STY*) of acrylic acid (in a continuous procedure, this is the total amount of acrylic acid obtained per hour and per liter of volume of the fixed catalyst bed used).

PF 54307

At a constant given yield Y**, the greater the hourly space velocity of acrolein from the fixed catalyst bed of the reaction stage (this refers to the amount of acrolein in liters (STP) = 1 (STP); the volume in liters which would be taken up by the appropriate amount of acrolein under standard conditions, i.e. at 25°C and 1 bar) which is conducted as a constituent of the starting reaction gas mixture per hour through one liter of fixed catalyst bed).

The teachings of the documents WO 01/36364, DE-A 19927624, DE-A 19948248, DE-

A 19948523, DE-A 19948241 and DE-A 19910506 are therefore directed toward significantly increasing the hourly space velocity of acrolein on the fixed catalyst bed of the reaction stage at substantially constant YA*. This is achieved substantially by arranging the fixed catalyst bed in the reaction stage in two spatially successive temperature zones (reaction zones). The hourly space velocity of acrolein on the fixed catalyst bed is selected at > 150 | (STP)/I of fixed catalyst bed-h and the temperature of the second (in the flow direction of the reaction gas mixture) temperature zone has to be at least 10°C above the temperature of the first temperature zone.

In a similar manner, EP-A 1106598 also teaches a process of the high loading method for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid, in which the fixed catalyst bed of the reaction stage is arranged in a plurality of temperature zones. According to a teaching of EP-A 1106598, the temperature difference of a subsequent temperature zone in the flow direction of the reaction gas mixture can be either more or less than 5°C above the temperature of the preceding temperature zone, and EP-A 1106598 leaves completely open the question of under which conditions a larger and under which conditions a smaller temperature difference should be used.

EP-A 1106598 also leaves completely open the definition of the temperature of a reaction zone or a temperature zone.

In contrast, the remaining prior art documents define the temperature of a reaction zone as the temperature of the fixed catalyst bed disposed in the reaction zone when performing the process in the absence of a chemical reaction. When this temperature is not constant within the reaction zone, the term temperature of a reaction zone refers to the (numerical) mean of the temperature of the fixed catalyst bed along the reaction zone. It is essential that the individual reaction zones are heated substantially independently of one another, so that one reaction zone always corresponds to one temperature zone. The above definition of the temperature of a reaction zone also : applies in this document.

PF 54307

Since the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid is a markedly exothermic reaction, the temperature of the reaction gas mixture on reactive pass through the fixed catalyst bed is generally different to the temperature of a reaction zone. it is normally above the temperature of the reaction zone and gener- ally proceeds within a reaction zone through a maximum (heating point maximum) or falls starting from a maximum value.

However, a disadvantage of the teachings of the prior art is that they are directed ex- clusively toward operating a multizone arrangement under a higher acrolein loading.

This is disadvantageous in that such a procedure is inevitably accompanied by a high

STY*. However, this requires an appropriate market demand for acrylic acid. When the latter is absent (for example temporarily), the multizone arrangement necessarily has to be operated at lower acrolein loadings, and a target parameter to be pursued which then comes to the forefront is a very high selectivity of acrylic acid formation, based on acrolein converted (S**). This is the molar amount of acrylic acid formed in single pass through the muttizone arrangement, based on the number of moles of ac- rolein converted.

A need exists to provide a process for heterogeneously catalyzed partial gas phase 50 oxidation of acrolein to acrylic acid in a multizone arrangement, in which acrylic acid is formed with very high selectivity at acrolein loadings of < 1501 (STP)/I-h.

We have found that this need is fulfilled by a process for partially oxidizing acrolein to acrylic acid in the gas phase under heterogeneous catalysis by conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and at least one inert gas containing at least 20% by volume of molecular nitrogen and contains the molecu- lar oxygen and the acrolein in a molar 0,:C;H.0 ratio of > 0.5 in one reaction stage over a fixed catalyst bed which is arranged in two spatially successive reaction zones

A,B, the temperature of reaction zone A being a temperature in the range from 230 to 320°C and the temperature of reaction zone B likewise being a temperature in the range from 230 to 320°C, whose active composition is at least one multimetal oxide comprising the elements Mo and V, in such a way that reaction zone A extends to an acrolein conversion of from 45 to 85 mol% and, on single pass of the starting reaction gas mixture through the overall fixed catalyst bed, the acrolein conversion is > 90 mol% and the selectivity of acrylic acid formation, based on acrolein converted, is > 90 mol%, the chronological sequence in which the starting reaction gas mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones, whe- rein

PF 54307 a) the hourly space velocity of the acrolein contained in the starting reaction gas mixture on the fixed catalyst bed is < 145 | (STP) of acrolein/l of fixed catalyst bed-h and > 70 | (STP) of acrolein/l of fixed catalyst bed-h, b) the volume-specific activity of the fixed catalyst bed is either constant or increases at least once in the flow direction of the reaction gas mixture over the fixed catalyst bed, and c) the difference T™* — T™® formed from the highest temperature T™®* which the reaction gas mixture has within the reaction zone A and the highest temperature

T™® which the reaction gas mixture has within reaction zone B, is > 0°C.

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

In general, the difference T™* — T"®in the process according to the invention will not be more than 75°C. According to the invention, T"* — T"@8 js preferably > 3°C and < 60°C. Very particular preference is given to T"* — T™® in the process according to the invention being > 5°C and < 40°C.

The process according to the invention proves advantageous, for example, when the hourly space velocity of the acrolein contained in the starting reaction gas mixture on the fixed catalyst bed is > 70 | (STP) of acrolein/l-h and < 140 | (STP) of acrolein/I-h, or > 701 (STP) of acrolein/I-h and < 135 | (STP) of acrolein/l-h or > 70 | (STP) of acrolein/l-h and < 1401 (STP) of acrolein/I-h, or > 80 | (STP) of acrolein/I‘h and < 130 (STP) of acrolein/I-h, or > 90 | (STP) of acrolein/I-h, and < 125 | (STP) of acrolein/l-h, or > 1001 (STP) of acrolein/I-h and < 120 | (STP) of acrolein/I-h, or > 105 | (STP) of acrolein/l-h and < 1151 (STP) of acrolein/I-h.

It will be appreciated that the process according to the invention can also be applied when the hourly space velocity of acrolein contained in the reaction gas mixture on the fixed catalyst bed is < 70 | (STP) of acrolein/I-h and/or the volume-specific activity of the fixed catalyst bed is constant. However, the operation of a multizone arrangement at such low reactant loadings would hardly be economic.

When performing the process according to the invention, the differences T™# — Tm8 required in accordance with the invention are normally attained when, on the one hand, both the temperature of reaction zone A and the temperature of reaction zone B are in the range from 230 to 320°C and, on the other hand, the difference between the

PF 54307 . oo. { <905/08593 temperature of reaction zone B (Tg) and the temperature of reaction zone A (Tp), i.€.

Te-Ta, is <0°C and > -10°C, or < 0°C and > -5°C, or frequently < 0°C and > -3°C.

In other words, in contrast to the teaching of the prior art for high loadings, the 5 temperature of the subsequent zone in the process according to the invention will normally be lower than the temperature of the preceding reaction zone.

The above statement relating to the temperature differences Tg-Ta also applies when the temperature of reaction zone A is in the preferred range of from 250 to 300°C or in the preferred range of from 260 to 280°C.

The working pressure in the process according to the invention can either be below atmospheric pressure (for example down to 0.5 bar) or above atmospheric pressure.

Typically, the working pressure will be at values of from 1 to 5 bar, frequently from 1 to 3 bar. Normally, the reaction pressure will not exceed 100 bar.

According to the invention, preference is given to reaction zone A extending to an acrolein conversion of from 50 to 85 mol% or from 60 to 85 mol%.

In general, the acrolein conversion based on single pass in the process according to the invention can be > 92 mol%, or > 94 mol%, or > 96 mol%, or > 98 moi% and frequently even > 99 mol%. The selectivity of acrylic acid formation will generally be > 92 mol%, or > 94 mol%, frequently > 95 mol% or > 96 mol% or > 97 mol%.

According to the invention, the molar O:acrolein ratio in the starting reaction gas mixture has to be > 0.5. It is frequently at values of > 1. Typically, this ratio will be at values of < 3. According to the invention, the molar O,:acrolein ratio in the starting reaction gas mixture will be from 1 to 2 or from 1 to 1.5.

Useful catalysts for the fixed catalyst bed of the catalytic gas phase oxidation of acrolein according to the invention are all of those whose active composition is at least one multimetal oxide comprising Mo and V. Such suitable multimetal oxide catalysts can be taken, for example, from US—A 3 775 474, US-A 3954855, US-A 3893951 and

US-A 4339355. Also particularly suitable are the multimetal oxide compositions of EP-

A 427508, DE-A 2909671, DE-C 3151805, DE-B 2626887, DE-A 4302991, EP-

A 700893, EP-A 714700 and DE-A 19736105 and also DE-A 10046928.

Also suitable in this context are the exemplary embodiments of EP-A 714700 and also of DE-A 19736105.

PF 54307

A multiplicity of the multimetal oxide active compositions suitable for the fixed catalyst bed, for example those of DE-A 19815281, can be encompassed by the general formula IV

MO1,Vo Xp X2 XXX X50, (IV) where the variables are defined as follows:

X'= W, Nb, Ta, Cr and/or Ce,

X?= Cu, Ni, Co, Fe, Mn and/or Zn,

X* = Sb and/or Bi,

X* = one or more alkali metals,

X® = one or more alkaline earth metals,

X® = Si, Al, Ti and/or Zr, a= from1to6, b= from0.2to4, c= from0.5to0 18, d= from O to 40, e= fromOto2, f= from Oto4, g= from Oto 40 and n= a number which is deterrnined by the valency and frequency of the elements in IV other than oxygen.

Embodiments among the active muitimetal oxides IV which are preferred according to the invention are those which are encompassed by the following definitions of the variables of the general formula IV:

X'= W, Nb, and/or Cr,

X?= Cu, Ni, Co, and/or Fe,

X= 8b,

X*= Na and/or K,

X® = Ca, Srand/or Ba,

X°®= Si, Al, and/or Ti, a= from1.5to05, b= from0.5t0 2, c= from0.5to 3, 40 d= fromOto 2,

PF 54307 e= from0to0.2 f= from0Oto1and n= a number which is determined by the valency and frequency of the elements in V other than oxygen.

However, multimetal oxides IV which are very particularly preferred according to the invention are those of the general formula V

Mo12Va Yu Y2. Yor Yo5 On Vv) where

Y'= W and/or Nb,

Y2= Cu and/or Ni,

Y®= Ca and/or Sr,

Y® = Siand/or Al, a'= from 2to4, b'= from 1to 1.5, c= from1to3, f= from0to0.5 g = fromOto 8 and n' = a number which is determined by the valency and frequency of the elements in V other than oxygen.

The multimetal oxide compositions (IV) which are suitable according to the invention are obtainable in a manner known per se, for example disclosed in DE-A 4335973 or in EP-A 714700.

In principle, multimetal oxide active compositions, in particular those of the general formula IV, suitable for the catalysts of the fixed catalyst bed can be prepared in a simple manner by obtaining a very intimate, preferably finely divided dry mixture having a composition corresponding to their stoichiometry from suitable sources of their elemental constituents and calcining it at temperatures of from 350 to 600°C. The calcination may be carried out either under inert gas or under an oxidative atmosphere, for example air (mixture of inert gas and oxygen), and also under a reducing atmosphere (for example mixtures of inert gas and reducing gases such as H,, NH,

CO, methane and/or acrolein or the reducing gases mentioned themselves). The calcination time can be from a few minutes to a few hours and typically reduces with 40 temperature. Useful sources for the elemental constituents of the multimetal oxide

PF 54307 active compositions 1V include those compounds which are already oxides and/or those compounds which can be converted to oxides by heating, at least in the presence of oxygen.

The starting compounds for preparing multimetal oxide compositions V can be intimately mixed in dry or in wet form. When they are mixed in dry form, the starting compounds are advantageously used as finely divided powder and subjected to calcining after mixing and optional compaction. However, preference is given to effecting the intimate mixing in wet form.

This is typically done by mixing the starting compounds in the form of an aqueous solution and/or suspension. Particularly intimate dry mixtures are obtained in the mixing processes described when the starting materials are exclusively sources of the elemental constituents in dissolved form. The solvent used is preferably water.

Subsequently, the aqueous composition obtained is dried, and the drying process is preferably effected by spray-drying the aqueous mixture at exit temperatures of from 100 to 150°C.

The resulting multimetal oxide compositions, in particular those of the general formula

IV, are generally used in the fixed catalyst bed not in powder form, but rather shaped to certain catalyst geometries, and the shaping may be effected before or after the final calcination. For example, unsupported catalysts can be prepared from the powder form of the active composition or its uncalcined precursor composition by compacting to the desired catalyst geometry (for example by tableting or extruding), optionally with the addition of assistants, for example graphite or stearic acid as lubricants and/or shaping assistants and reinforcing agents such as microfibers of glass, asbestos, silicon carbide or potassium titanate. Examples of suitable unsupported catalyst geometries are solid cylinders or hollow cylinders having an external diameter and a length of from 2 to 10 mm. In the case of the hollow cylinders, a wall thickness of from 1 to 3 mm is advantageous. It will be appreciated that the supported catalyst may also have spherical geometry and the spherical diameter may be from 2 to 10 mm.

It will be appreciated that the pulverulent active composition or its pulverulent precursor composition which is yet to be calcined can also be shaped by applying to preshaped inert catalyst supports. The coating of the support bodies to prepare the coated catalysts is generally performed in a suitable rotatable vessel, as disclosed, for example, by DE-A 2909671, EP-A 293858 or from EP-A 714700.

To coat the support bodies, the powder composition to be applied is advantageously 40 - moistened and is dried again after application, for example by means of hot air. The

Lo : PF 54307 layer thickness of the powder composition applied to the support body is advantageously selected within the range from 10 to 1 000 pm, preferably within the range from 50 to 500 ym and more preferably in the range from 150 to 250 um.

S Useful support materials are customary porous or nonporous aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium silicate or aluminum silicate. The support bodies may have a regular or irregular shape, although preference is given to regularly shaped support bodies having distinct surface roughness, for example spheres or hollow cylinders having a grit layer. Iltis suitable to use substantially nonporous, surface-roughened, spherical supports made of steatite (e.g. Steatite C220 from CeramTec) whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm. However, suitable support bodies also include cylinders whose length is from 2 to 10 mm and whose external diameter is from 4 to 10 mm. In the case of rings as support bodies, the wall thickness is also typically from 1 to 4 mm. Annular support bodies to be used with preference have a length of from 2 to 6 mm, an external diameter of from 4 to 8 mm and a wall thickness of from 1 to 2 mm. Suitable support bodies are also in particular rings of geometry 7 mm x 3 mm x 4 mm (external diameter x length x internal diameter). It will be appreciated that the fineness of the catalytically active oxide compositions to be applied to the surface of the support body is adapted to the desired coating thickness (cf. EP-A 714 700).

Advantageous multimetal oxide active compositions to be used for the catalysts of the fixed catalyst bed are also compositions of the general formula VI [DLE (VI) where the variables are defined as follows:

D = MowVaZ'vZ2eZi¢Z'Z%2%0x ,

E = Z;,CuyHO,,

Z' = W,Nb, Ta, Crand/or Ce,

Z> = Cu, Ni, Co, Fe, Mn and/or Zn,

Z® = Sband/orBi 2* = Li Na K Rb, Cs and/or H

Z> = Mg, Ca, Srand/or Ba,

Z° = Si, Al, Tiandlor Zr,

Z’ = Mo, W,V, Nband/or Ta, preferably Mo and/or W 40 a" = from 1 to 8,

os ; PF 54307 b> = from0.21t0 5, c’ = from 0 to 23, d’ = from 0 to 50, e’ = from 0 to 2, f = from 0 to 5, g’ = from O to 50, h’ = from 4 to 30, i” = from 0 to 20 and xy" = numbers which are determined by the valency and frequency of the elements in VI other than oxygen and pq = numbers other than zero whose p/q ratio is from 160:1 to 1:1, and which are obtainable by separately preforming a multimetal oxide composition E

Z",CunH:Op (E) in finely divided form (starting composition 1) and subsequently incorporating the preformed solid starting composition 1 into an aqueous solution, an aqueous suspension or into a finely divided dry mixture of sources of the elements Mo, V, yARYS 2% 2° 2°, Z° which comprises the abovementioned elements in the stoichiometry D

Mo", Z",-2%- 2°42" 2% 2° (D) (starting composition 2) in the desired p:q ratio, drying the aqueous mixture which may result, and calcining the resulting dry precursor composition at temperatures of from 250 to 600°C to give the desired catalyst geometry before or after drying.

Preference is given to the multimetal oxide compositions VI in which the preformed solid starting composition 1 is incorporated into an aqueous starting composition 2 at a temperature of <70°C. A detailed description of the preparation of multimetal oxide VI catalysts is contained, for example, in EP-A 668104, DE-A 19736105,

DE-A 10046928, DE-A 19740493 and DE-A 19528646.

With regard to the shaping, the statements made for the multimetal oxide IV catalysts apply to the multimetal oxide Vi catalysts.

Further suitable multimetal oxide compositions for the catalysts of the fixed catalyst bed are those of DE-A 19815281, in particular all exemplary embodiments from this document. With regard to the shaping, the same applies as was stated above.

ro ; PF 54307 : 11 2005/0593

For the fixed catalyst bed of the process according to the invention, particularly suitable catalysts are the coated catalysts S1 (stoichiometry: Mo,,V;W, ,Cu, 40,) and S7 (stoichiometry: Mo4,VsW; 2Cu; gNigeO,) from DE-A 4442346 having an active composition fraction of 27% by weight and a coating thickness of 230 um, the coated catalyst from preparative example 5 of DE-A 10046928 (stoichiometry:

Mo,,V3W, ,Cu, 40.) having an active composition fraction of 20% by weight, the coated catalysts of examples 1 to 5 from DE-A 19815281, but equally the abovementioned coated catalysts for the second reaction stage applied to support rings of geometry 7 mm x 3 mm x 4 mm (external diameter x length x internal diameter) having an active composition fraction of 20% by weight (based on the overall composition of the coated catalyst), and also a coated catalyst having a biphasic active composition of stoichiometry (Mo4g 4V3W,;20,) (CuMo, sW 50,4)16 and prepared according to DE-

A 19736105 and having an active composition fraction of 20% by weight applied to the abovementioned 7 mm x 3 mm x 4 mm support.

The catalysts recommended above for the reaction stage according to the invention are only suitable for the reaction stage according to the invention when everything is retained except the support geometry which is changed to 5 mm x 3 mm x 1.5 mm (external diameter x length x internal diameter). The multimetal oxides mentioned can also be used in the reaction stage according to the invention in the form of the corresponding unsupported catalyst rings.

To prepare the fixed catalyst bed in the process according to the invention, it is possible to use only the shaped catalyst bodies having the appropriate multimetal oxide active composition or else substantially homogeneous mixtures of shaped catalyst bodies having multimetal oxide active composition and shaped bodies (shaped diluent bodies) behaving substantially inertly with regard to the heterogeneously catalyzed partial gas phase oxidation and having no multimetal oxide active composition. Useful materials for such inert shaped bodies include in principle all of those which are also suitable as support material for coated catalysts suitable according to the invention.

Useful such materials include, for example, porous or nonporous aluminum oxide, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide, silicates such as magnesium silicate or aluminum silicate or the steatite already mentioned above (e.g.

Steatit C-220 from CeramTec).

The geometry of such inert shaped diluent bodies may in principle be as desired. In other words, they may, for example, be spheres, polygons, solid cylinders or else rings.

According to the invention, the inert shaped diluent bodies selected will preferably be those whose geometry corresponds to that of the shaped catalyst bodies to be diluted 40 by them.

2.4 PF 54307 = 12

According to the invention, it is advantageous when the chemical composition of the active composition used does not vary over the fixed catalyst bed. In other words, although the active composition used for a single shaped catalyst body can be a mixture of different multimetal oxides comprising the elements Mo and V, the same mixture then has to be used for all shaped catalyst bodies of the fixed catalyst bed.

In this case, the volume-specific (i.e. normalized to the unit of volume) activity can be reduced in a simple manner, for example, by homogeneously diluting a basic amount of shaped catalyst bodies prepared in a uniform manner with shaped diluent bodies.

The higher the proportion of shaped diluent bodies selected, the less the active composition and catalyst activity contained in a certain volume of the bed.

A volume-specific activity increasing at least once in the flow direction of the reaction gas mixture over the fixed catalyst bed can therefore be attained for the process according to the invention in a simple manner, for example, by beginning the bed with a high proportion of inert shaped diluent bodies based on one type of shaped catalyst bodies, and then either continuously or, at least once or more than once, abruptly (for example stepwise) reducing this proportion of shaped diluent bodies in the flow direction. However, an increase in the volume-specific activity is also possible, for example, by increasing the thickness of the active composition layer applied to the support at a constant geometry and active composition type of a coated shaped catalyst body, or, in a mixture of coated catalysts having the same geometry but having different proportions by weight of the active composition, increasing the proportion of shaped catalyst bodies having higher active composition contents. Alternatively, the active compositions themselves can be diluted in the course of the active composition preparation by, for example, incorporating inert, diluting materials such as hard-fired silica into the dry mixture of the starting compounds to be calcined. Different amounts of diluting material added lead automatically to different activities. The more diluting material is added, the lower the resulting activity will be. A similar effect can also be achieved, for example, in mixtures of unsupported catalysts and of coated catalysts (having identical active composition) by varying the mixing ratio in an appropriate manner. It will be appreciated that the variants described can also be used in combination.

Itis of course also possible to use mixtures of catalysts having chemically different active compositions and, as a consequence of these different compositions, having different activities for the fixed catalyst bed. These mixtures may in turn be diluted in inert diluent bodies.

Bey PF 54307

Normally, the volume-specific activity will decrease not once within the fixed catalyst bed in the flow direction of the reaction gas mixture in the process according to the invention.

Upstream and/or downstream of the fixed catalyst bed may be disposed beds consisting exclusively of inert material (for example only fixed diluent bodies) (in this document, they are not included for terminology purposes in the fixed catalyst bed, since they contain no shaped bodies which have multimetal oxide active composition).

The shaped diluent bodies used for the inert bed can have the same geometry as the shaped catalyst bodies used in the fixed catalyst bed. However, the geometry of the shaped diluent bodies used for the inert bed can also be different to the abovementioned geometry of the shaped catalyst bodies (for example, spherical instead of annular).

Frequently, the shaped bodies used for such inert beds have the annutar geometry 7 mm x 7 mm x 4 mm (external diameter x length x internal diameter) or the spherical geometry having the diameter d = 4-5 mm.

According to the invention, the fixed catalyst bed in the process according to the invention is preferably structured as follows in the flow direction of the reaction gas mixture:

First, to a length of from 10 to 60%, preferably from 10 to 50%, more preferably from 20 to 40% and most preferably from 25 to 35% (i.e., for example, to a length of from 0.70 to 1.50 m, preferably from 0.90 to 1.20 m), each of the total length of the fixed bed catalyst bed, a homogeneous mixture or two (having decreasing dilution) successive homogeneous mixtures of shaped catalyst bodies and shaped diluent bodies (both preferably having substantially the same geometry), in which the proportion by weight of shaped diluent bodies (the mass densities of shaped catalyst bodies and of shaped diluent bodies generally differing only slightly) is normally from 10 to 50% by weight, preferably from 20 to 45% by weight and more preferably from 25 to 35% by weight.

According to the invention, this first zone is then advantageously followed to the end of the length of the fixed catalyst bed (i.e., for example, to a length of from 2.00 to 3.00 m, preferably from 2.50 to 3.00 m) by either a bed of the shaped catalyst bodies diluted only to a slighter extent (than in the first zone) or, most preferably, an unaccompanied bed of the same shaped catalyst bodies which have also been used in the first zone. : The aforementioned applies in particular when the shaped catalyst bodies used in the fixed catalyst bed are coated catalyst rings or coated catalyst spheres (in particular 40 - those which are listed in this document as preferred). For the purposes of the

Ce PF 54307 aforementioned structuring, both the shaped catalyst bodies or their support rings and the shaped diluent bodies in the process according to the invention substantially have the ring geometry 7 mm x 3 mm x 4 mm (external diameter x length x internal diameter).

The abovementioned also applies when, instead of inert shaped diluent bodies, shaped coated catalyst bodies are used whose active composition content is from 2 to 15% by weight lower than the active composition content of the shaped coated catalyst bodies at the end of the fixed catalyst bed 2.

In an advantageous manner from an application point of view, the reaction stage of the process according to the invention is carried out in a two-zone tube bundle reactor, as described, for example, in DE-A 19910508, 19948523, 19910506 and 19948241. A preferred variant of a two-zone tube bundle reactor which can be used in accordance with the invention is disclosed by DE-C 2830765. However, the two-zone tube bundle reactors disclosed in DE-C 2513405, US-A 3147084, DE-A 2201528, EP-A 383224 and DE-A 2903218 are also suitable for carrying out the reaction stage of the process according to the invention.

In other words, in the simplest manner, the fixed catalyst bed to be used in accordance with the invention (possibly with downstream and/or upstream inert beds) is disposed in the metal tubes of a tube bundle reactor and two spatially separated heating media, generally salt melts, are conducted around the metal tubes. The tube section over which the particular salt bath extends represents a reaction zone in accordance with the invention.

In other words, in the simplest manner, for example, a salt bath A flows around that section of the tubes (reaction zone A) in which acrolein is oxidatively converted (on single pass) until a conversion value in the range from 45 to 85 mol%, (preferably from 50 to 85 mol%, more preferably from 60 to 85 mol%) is achieved and a salt bath B flows around the section of the tube (reaction zone B) in which the acrolein is subsequently oxidatively converted (on single pass) until a conversion value of at least 90 mol% is achieved (if required, the reaction zones A, B to be used in accordance with the invention can be followed by further reaction zones which are maintained at individual temperatures).

It is advantageous from an application point of view if the reaction stage of the process according to the invention includes no further reaction zones. In other words, the salt bath B advantageously flows around the sections of the tubes in which acrolein is 40 subsequently oxidatively converted (on single pass) up to a conversion value of

Ce PF 54307 > 92 mol%, or > 94 mol% or > 96 mol% or > 98 mol% and frequently even > 99 mol% or more.

Typically, the beginning of the reaction zone B lies beyond the heating point maximum of reaction zone A.

According to the invention, both salt baths A, B can be conducted in cocurrent or in countercurrent through the space surrounding the reaction tubes relative to the flow direction of the reaction gas mixture flowing through the reaction tubes. It will be appreciated that, in accordance with the invention, cocurrent flow may be applied in reaction zone A and countercurrent flow in reaction zone B (or vice versa).

In all of the aforementioned cases, it will be appreciated that a transverse flow can be superimposed on the parallel flow of the salt melts relative to the reaction tubes taking place within the particular reaction zone, so that the individual reaction zone corresponds to a tube bundle reactor as described in EP-A 700714 or in EP-A 700893, which results overall in a meandering flow profile of the heat exchange medium in a longitudinal section through the catalyst tube bundle.

Typically, the catalyst tubes for the reaction stage according to the invention in the aforementioned two-zone tube bundle reactors are manufactured from ferritic steel and typically have a wall thickness of from 1 to 3 mm. Their internal diameter is generally from 20 to 30 mm, frequently from 22 to 26 mm. Their length is advantageously from 3 to 4 m, preferably 3.5 m. In each temperature zone, the fixed catalyst bed occupies at least 60%, or at least 75%, or at least 90%, of the length of the zone. Any remaining length is optionally occupied by an inert bed. it is advantageous from an application point of view for the number of catalyst tubes accommodated in the tube bundle vessel to be at least 5 000, preferably at least 10 000. Frequently, the number of catalyst tubes accommodated in the reaction vessel is from 15 000 to 30 000. Tube bundle reactors having a number of catalyst tubes above 40 000 are usually exceptional.

Within the vessel, the catalyst tubes are normally homogeneously distributed (preferably 6 equidistant adjacent tubes per catalyst tube), and the distribution is advantageously selected in such a way that the distance between the central internal axes of immediately adjacent catalyst tubes (the catalyst tube pitch) is from 35 to 45 mm (cf, for example, EP-B 468290).

Useful heat exchange media are in particular fluid heating media. it is particularly advantageous to use melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and/or sodium nitrate, or of low-melting metals such as sodium, mercury and 40 also alloys of different metals.

Claims

PF 54307 We claim:

1. A process for partially oxidizing acrolein to acrylic acid in the gas phase under heterogeneous catalysis by conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and at least one inert gas containing at least 20% by volume of molecular nitrogen and contains the molecular oxygen and the acrolein in a molar O,:C;H,O ratio of > 0.5 in one reaction stage over a fixed catalyst bed which is arranged in two spatially successive reaction zones A,B, the temperature of reaction zone A being a temperature in the range from 230 to 320°C and the temperature of reaction zone B likewise being a tempera- ture in the range from 230 to 320°C, whose active composition is at least one multimetal oxide comprising the elements Mo and V, in such a way that reaction zone A extends to an acrolein conversion of from 45 to 85 mol% and, on single pass of the starting reaction gas mixture through the overall fixed catalyst bed, the acrolein conversion is > 90 mol% and the selectivity of acrylic acid formation, based on acrolein converted, is > 90 mol%, the chronological sequence in which the starting reaction gas mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones, wherein a) the hourly space velocity of the acrolein contained in the starting reaction gas mixture on the fixed catalyst bed is < 145 | (STP) of acrolein/l of fixed catalyst bed-h and > 70 | (STP) of acrolein/l of fixed catalyst bed-h, b) the volume-specific activity of the fixed catalyst bed is either constant or in- creases at least once in the flow direction of the reaction gas mixture over the fixed catalyst bed, and c) the difference T™ — T™® formed from the highest temperature TmaxA which the reaction gas mixture has within the reaction zone A and the high- est temperature T™® which the reaction gas mixture has within reaction zone B, is > 0°C.

2. A process as claimed in claim 1, wherein the difference T™* — T"*® is 2 0°C and < 75°C.

3. A process as claimed in claim 1, wherein the difference T™* — T"*® is 2 3°C and < 60°C.

4. A process as claimed in claim 1, wherein the difference T™* — T™® is 2 5°C 40 and < 40°C. 127/2003 Mr/Bei 02.27.2003

PF 54307

5. A process as claimed in any one of claims 1 to 4, wherein the hourly space ve- locity of the acrolein contained in the starting reaction gas mixture on the fixed catalyst bed is = 70 | (STP) of acrolein/I-h and s 140 | (STP) of acrolein/I-h.

6. A process as claimed in any one of claims 1 to 4, wherein the hourly space ve- locity of the acrolein contained in the starting reaction gas mixture on the fixed catalyst bed is 2 80 | (STP) of acrolein/I-h and < 130 | (STP) of acrolein/l-h.

7. A process as claimed in any one of claims 1 to 5, wherein the active composition of the fixed catalyst bed is at least one multimetal oxide active composition of the general formula IV Mo, VX XXX XX%00 (IV) where the variables are defined as follows: X' = W, Nb, Ta, Cr and/or Ce, X2 = Cu, Ni, Co, Fe, Mn and/or Zn, x3 = Sb and/or Bi, X* = one or more alkali metals, X® = one or more alkaline earth metals, X8 = Si, Al, Tiand/or Zr, a = from 1to6, b = from 0.2to 4, c = from 0.5to 18, d = from 0 to 40, e = from 0 to 2, f = from Oto 4, g = from 0 to 40, and n = a number which is determined by the valency and frequency of the ele- ments other than oxygen in IV.

8. A process as claimed in any one of claims 1 to 7, wherein the volume-specific activity of the fixed catalyst bed increases at least once in the flow direction of the reaction gas mixture over the fixed catalyst bed.

9. A process according to the invention for partially oxidizing acrolein to acrylic acid, substantially as hereinbefore described or exemplified.

PF 54307

10. A process for partially oxidizing acrolein to acrylic acid including any new and : inventive integer or combination of integers, substantially as herein described.