DE10313211A1 - Partial oxidation of acrolein to acrylic acid by conducting starting reaction gas mixture comprising acrolein, molecular oxygen, and inert gas(es) in one reaction stage over fixed catalyst bed comprising multimetal oxide(s) - Google Patents

Abstract

Partial oxidation of acrolein to acrylic acid comprises conducting a starting reaction gas mixture comprising acrolein, molecular oxygen, and an inert gas(es), containing molecular oxygen and the acrolein in a molar ratio of >=0.5 in one reaction stage over a fixed catalyst bed whose active composition is a multimetal oxide(s) comprising the elements molybdenum and vanadium. Partial oxidation of acrolein to acrylic acid in the gas phase under heterogeneous catalysis comprises conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and an inert gas(es) containing >=20 vol.% of molecular nitrogen and contains molecular oxygen and the acrolein in a molar 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 230-320[deg]C, and the temperature of reaction zone B likewise being 230-320[deg]C, whose active composition is at least one multimetal oxide comprising the elements molybdenum and vanadium, such that reaction zone A extends to an acrolein conversion of 45-85 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, where the hourly space velocity of the acrolein contained in the starting reaction gas mixture on over the fixed catalyst bed is =145 L (STP) acrolein/L of fixed catalyst bed-hour and >=70 L (STP) acrolein/L fixed catalyst bed-h, 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 the difference T(maxA)-T(maxB), determined from the highest temperature T(maxA) which the reaction gas mixture has within the reaction zone A and the highest temperature T(maxB) which the reaction gas mixture has within reaction zone B, is 0[deg]C.

Description

The present invention relates to a process for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid, in which reaction gas starting mixture containing the molecular oxygen and the acrolein contains an acrolein, molecular oxygen and at least one inert gas which consists of at least 20% of its volume of molecular nitrogen contains in a molar ratio O ₂ : C ₃ H ₄ O ≥ 0.5, in one reaction stage via a fixed bed 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 of 230 to 320 ° C and the temperature of reaction zone B is also a temperature in the range of 230 to 320 ° C, and the active composition of which is at least one multimetal oxide containing the elements Mo and V, means that reaction zone A leads to a conversion of the Acroleins extends from 45 to 85 mol% and with one pass of the R the starting gas mixture through the entire fixed bed catalyst bed the acrolein conversion ≥ 90 mol% and the selectivity of acrylic acid formation, based on converted acrolein, is ≥ 90 mol%, the chronological sequence in which the reaction gas starting mixture flows through the reaction zones corresponds to the alphabetical sequence of the reaction zones.

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

In addition to molecular oxygen and contains the reactants the reaction gas starting mixture i.a. therefore inert gas to the reaction gas outside of the explosion area.

One objective of such a heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid is to achieve the highest possible yield A ^AA of acrylic acid (that is the mole number of acrolein converted to acrylic acid, based on, with one pass of the reaction gas mixture through the reaction stage under otherwise specified boundary conditions) the number of moles of acrolein used).

Another objective of such a heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid is to achieve the highest possible space-time yield (RZA ^AA ) of acrylic acid (that is, in a continuous procedure, the volume generated per hour and volume of the fixed bed catalyst bed used in liters Total amount of acrylic acid).

With the given yield A ^AA remaining the same, the space-time yield is greater, the greater the load on the fixed bed catalyst bed of the reaction stage with acrolein (below this is the amount of acrolein in standard liters (= Nl; the volume in liters that corresponds to the corresponding Amount of acrolein under normal conditions, ie at 25 ° C and 1 bar, would understand), which is passed as part of the reaction gas starting mixture per hour through a liter of fixed bed catalyst bed).

The teachings of the writings WO 01/36364, DE-A 19927624 . DE-A 19948248 . DE-A 19948523 . DE-A 19948241 and DE-A 19910506 are therefore aimed at significantly increasing the acrolein loading of the fixed bed catalyst bed of the reaction stage with acrolein while the A ^AA remains essentially the same. This is essentially achieved by arranging the fixed bed catalyst bed in the reaction stage in two spatially successive temperature zones (reaction zones). The acrolein load on the fixed bed catalyst bed is chosen to be ≥ 150 Nl / l fixed bed catalyst bed h and the temperature of the second (in the direction of flow of the reaction gas mixture) temperature zone must be at least 10 ° C. above the temperature of the first temperature zone.

The teaches in a similar way EP-A 1106598 a method of high-load operation for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid, at which the fixed bed catalyst bed the reaction stage is arranged in several temperature zones. there can be the temperature difference in the flow direction of the reaction gas mixture subsequent temperature zone according to the teaching of EP-A 1106598 both more and less than 5 ° C above the temperature of the previous temperature zone, which is EP-A 1106598 completely leaves open under what conditions a larger and under what conditions a smaller temperature difference is applied should be.

EP-A 1106598 also leaves completely open, what is meant by the temperature of a reaction or temperature zone.

In contrast to this, in the other documents of the cited prior art, the temperature of a reaction zone is understood to mean the temperature of the fixed bed catalyst bed in the reaction zone when the process is carried out in the absence of a chemical reaction. If this temperature is not constant within the reaction zone, the term temperature of a reaction zone here means the (number) average of the temperature of the fixed bed catalyst bed along the reaction zone. It is essential that the temperature of the individual reaction zones is essentially independent of one another, which is why a temperature zone always corresponds to one reaction zone. The above definition of the temperature of a The reaction zone should also apply in this document.

Because the heterogeneously catalyzed partial Gas phase oxidation of acrolein to acrylic acid is a distinctly exothermic Reaction is, the temperature of the reaction gas mixture is at reactive passage through the fixed bed catalyst bed in usually different from the temperature of a reaction zone. she is usually above the temperature of the reaction zone and goes through usually a maximum within a reaction zone (hotspot maximum) or falls starting from a maximum value.

A disadvantage of the teachings of the prior art, however, is that they are aimed exclusively at operating a multi-zone arrangement under high acrolein load. This is disadvantageous insofar as a high RZA ^{AA is inevitably} associated with such a procedure. However, this requires a corresponding market need for acrylic acid. If the latter is not the case (e.g. temporarily), the multi-zone arrangement must necessarily be operated with lower acrolein loads, with the highest possible selectivity of acrylic acid formation, based on converted acrolein, being the main target (S ^AA ). This is the molar amount of acrylic acid formed in a single pass through the multi-zone arrangement, based on the number of moles of acrolein reacted.

The object of the present invention therefore consisted of a process of heterogeneously catalyzed partial Gas phase oxidation of acrolein to acrylic acid in a multi-zone arrangement to disposal to be set at which the acrylic acid formation at acrolein loads of <150 Nl / l · h with if possible high selectivity he follows.

• a) die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch enthaltenen Acrolein ≤ 145 Nl Acrolein/l Festbettkatalysatorschüttung·h und ≥ 70 Nl Acrolein/l Festbettkatalysatorschüttung·h beträgt,
• b) die volumenspezifische Aktivität der Festbettkatalysatorschüttung in Strömungsrichtung des Reaktionsgasgemisches über die Festbettkatalysatorschüttung entweder konstant ist, oder wenigstens einmal zunimmt, und
• c) die Differenz T^maxA – T^maxB, gebildet aus der höchsten Temperatur T^maxA, die das Reaktionsgasgemisch innerhalb der Reaktionszone A aufweist, und der höchsten Temperatur T^maxB, die das Reaktionsgasgemisch innerhalb der Reaktionszone B aufweist, ≥ 0°C beträgt.

Accordingly, a process of heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid has been achieved in which a reaction gas starting mixture containing the molecular oxygen and the acrolein in one contains an acrolein, molecular oxygen and at least one inert gas which consists of at least 20% of its volume of molecular nitrogen contains molar ratio O ₂ : C ₃ H ₄ O ≥ 0.5, in one reaction stage via a fixed bed 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 is also a temperature in the range of 230 to 320 ° C, and the active composition of which is at least one multimetal oxide containing the elements Mo and V, means that reaction zone A leads to a conversion of acrolein of 45 to 85 mol% and with a single pass of the reaction gas Ausg mixed with the total fixed bed catalyst bed, the acrolein conversion ≥ 90 mol% and the selectivity of acrylic acid formation, based on converted acrolein, is ≥ 90 mol%, the chronological sequence in which the reaction gas starting mixture flows through the reaction zones corresponds to the alphabetical sequence of the reaction zones, found, which is characterized in that

• a) the loading of the fixed bed catalyst bed with the acrolein contained in the reaction gas starting mixture is ≤ 145 Nl acrolein / l fixed bed catalyst bed · h and ≥ 70 Nl acrolein / l fixed bed catalyst bed · h,
• b) the volume-specific activity of the fixed bed catalyst bed in the flow direction of the reaction gas mixture over the fixed bed catalyst bed is either constant, or increases at least once, and
• c) the difference T ^maxA - T ^maxB , formed from the highest temperature T ^maxA that the reaction ^gas mixture has in reaction ^zone A and the highest temperature T ^maxB that the reaction ^gas mixture has in reaction ^zone B is ≥ 0 ° C.

The volume-specific takes advantage activity the fixed bed catalyst bed in the direction of flow at least once.

As a rule, the difference T ^maxA -T ^{maxB in the method according to the invention will} not be more than 75 ° C. According to the invention, T ^maxA -T ^{maxB is preferably} ≥ 3 ° C and ≤ 60 ° C. In the method ^{according to} the invention, T ^maxA -T ^maxB is very particularly preferably 5 5 ° C and 40 40 ° C.

The method according to the invention proves to be e.g. as advantageous if the loading of the fixed bed catalyst bed with the acrolein contained in the reaction gas starting mixture ≥ 70 Nl acrolein / l · h and ≤ 140 Nl acrolein / l · h, or ≥ 70 Nl acrolein / l · h and ≤ 135 Nl acrolein / l · h, or ≥ 70 Nl acrolein / l · h h and ≤ 140 Nl acrolein / l · h, or ≥ 80 Nl acrolein / l · h and <130 Nl acrolein / l · h, or ≥ 90 Nl acrolein / l · h and <125 Nl acrolein / l · h, or ≥ 100 Nl acrolein / l · h and ≤ 120 Nl acrolein / l · h, or ≥ 105 Nl acrolein / l · h and ≤ 115 Nl acrolein / l · h.

Of course, the method according to the invention can can also be used when the load on the fixed bed catalyst bed the acrolein contained in the reaction gas mixture is <70 Nl acrolein / l · h and / or the volume-specific activity the fixed bed catalyst bed is constant. The operation of a multi-zone arrangement at such a low level Reactant loads would be however hardly economical anymore.

The differences T ^maxA -T ^maxB required ^{according to} the invention are usually set when the process ^according to the invention is carried out if, on the one hand, both the temperature of the reaction temperature ne A and the temperature of reaction zone B is in the range from 230 to 320 ° C. and on the other hand the difference between the temperature of reaction zone B (T _B ) and the temperature of reaction zone A (T _A ), ie, T _B - T _A , ≤ 0 ° C and ≥ -20 ° C or ≥ -10 ° C, or ≤ 0 ° C and ≥ -5 ° C, or often ≤ 0 ° C and ≥ -3 ° C.

That is, contrary to the teaching of State of the art for high loads, the temperature in the method according to the invention downstream zone is usually less than temperature the previous reaction zone.

The above statement regarding the temperature differences T _B - T _{A also} applies if the temperature of the reaction zone A is in the preferred range from 250 to 300 ° C. or in the particularly preferred range from 260 to 280 ° C.

The working pressure in the method according to the invention both below normal pressure (e.g. up to 0.5 bar) and are above normal pressure. Typically the working pressure at values from 1 to 5 bar, often 1 to 3 bar. Usually the reaction pressure is 100 bar do not exceed.

According to the invention preferably extends reaction zone A up to a conversion of acrolein of 50 to 85 mol% or 60 to 85 mol%.

As a rule, the simple Pass-related acrolein conversion in the method according to the invention ≥ 92 mol%, or ≥ 94 mol%, or ≥ 96 mol%, or ≥ 98 mol% and often even ≥ 99 mol%. The selectivity of acrylic acid formation is regularly ≥ 92 mol%, or ≥ 94 mol%, often ≥ 95 mol% or ≥ 96 mol% or ≥ 97 mol%.

According to the invention, the molar ratio of O ₂ : acrolein in the reaction gas starting mixture must be 0,5 0.5. It is often values ≥ 1. Usually this ratio will be values ≤ 3. According to the invention, the molar ratio of O ₂ : acrolein in the reaction gas starting mixture is frequently 1 to 2 or 1 to 1.5.

Suitable catalysts for the fixed bed catalyst bed of the gas phase catalytic acrolein oxidation according to the invention are all those whose active composition is at least one Mo and V containing multimetal oxide. Such suitable multi-metal oxide catalysts can be found, for example, in US Pat. No. 3,775,474, US Pat. No. 3,954,555, US Pat. No. 3,89,3951 and US Pat. No. 4,339,355. The multimetal oxide compositions of EP-A 427508, DE-A 2909671, DE-C 3151805, DE-AS 2626887, DE-A 4302991, EP-A 700893, EP-A 714700 and EP are also particularly suitable the DE-A 19736105 and DE-A 10046928.

The exemplary embodiments of EP-A 714700 and US Pat DE-A 19736105 ,

A large number of the multimetal oxide active compositions suitable for the fixed bed catalyst bed, for example those of DE-A 19815281, can be obtained using the general formula IV Mo ₁₂ V _a X ¹ _b X ² _c X ³ _d X ⁴ _e X ⁵ _f X ⁶ _g O _n (IV) in which the variables have the following meaning:
X1 = W, Nb, Ta, Cr and / or Ce,
X ² = Cu, Ni, Co, Fe, Mn and / or Zn,
X ³ = Sb and / or Bi,
X ⁴ = one or more alkali metals,
X ⁵ = one or more alkaline earth metals,
X ⁶ = Si, Al, Ti and / or Zr,
a = 1 to 6,
b = 0.2 to 4,
c = 0.5 to 18,
d = 0 to 40,
e = 0 to 2,
f = 0 to 4,
g = 0 to 40 and
n = a number which is determined by the valency and frequency of the elements in IV different from oxygen,
subsumed.

Preferred embodiments according to the invention within the active multimetal oxides IV are those which are covered by the following meanings of the variables of the general formula IV:
X1 = W, Nb, and / or Cr,
X ² = Cu, Ni, Co, and / or Fe,
X ³ = Sb,
X ⁴ = Na and / or K,
X ⁵ = Ca, Sr and / or Ba,
X ⁶ = Si, Al, and / or Ti,
a = 1.5 to 5,
b = 0.5 to 2,
c = 0.5 to 3,
d = 0 to 2,
e = 0 to 0.2,
f = 0 to 1 and
n = a number determined by the valency and frequency of elements other than oxygen in IV.

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

The multimetal oxide active compositions suitable according to the invention (IV) are known per se, e.g. in DE-A 4335973 or in disclosed in EP-A 714700.

In principle, multimetal oxide active compositions suitable for the catalysts of the fixed bed catalyst bed, in particular those of the general formula IV, can be prepared in a simple manner by generating an intimate, preferably finely divided, dry mixture of suitable stoichiometry from suitable sources of their elemental constituents, and this at temperatures calcined from 350 to 600 ° C. The calcination can take place both under inert gas and under an oxidative atmosphere such as air (mixture of inert gas and oxygen) and also under a reducing atmosphere (eg mixtures of inert gas and reducing gases such as H ₂ , NH ₃ , CO, methane and / or acrolein or the reducing gases mentioned are carried out by themselves). The calcination time can range from a few minutes to a few hours and usually decreases with temperature. Suitable sources for the elementary constituents of the multimetal oxide active compositions IV are those compounds which are already oxides and / or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.

The intimate mixing of the starting compounds for the production of multimetal oxide masses IV can be in dry or done in wet form. If it is done in a dry form, it will be the starting compounds expediently used as a fine powder and after mixing and optionally Compacting subjected to calcination. This is preferably done intimate mixing, however, in wet form.

Usually the starting compounds are in the form of an aqueous solution and / or suspension mixed together. Particularly intimate dry mixes are obtained in the described mixing process if only from in dissolved Form of available sources of the elementary constituents becomes. As a solvent water is preferred. Then the aqueous mass obtained dried, the drying process preferably by spray drying the watery Mixing with outlet temperatures of 100 to 150 ° C takes place.

The resulting multimetal oxide compositions, in particular those of the general formula IV, are generally used in the fixed bed catalyst bed, not in powder form, but shaped into specific catalyst geometries, the shaping being able to take place before or after the final calcination. For example, solid catalysts can be produced from the powder form of the active composition or its uncalcined precursor composition by compression to the desired catalyst geometry (for example by tableting, extrusion or extrusion), where appropriate auxiliaries such as graphite or stearic acid as lubricants and / or shaping auxiliaries and reinforcing agents such as glass microfibers, Asbestos, silicon carbide or potassium titanate can be added. Suitable full catalyst geometries are, for example, full cylinders or hollow cylinders with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinder, a wall thickness of 1 to 3 mm is appropriate. Of course, the full catalytic converter can also have a spherical geometry have, the ball diameter can be 2 to 10 mm.

Of course, the shape the powdery Active mass or its powdery, still not calcined, precursor mass also by applying to preformed inert catalyst supports. The coating of the carrier body for Manufacture of coated catalysts is usually carried out in a suitable rotatable container executed as it e.g. from DE-A 2909671, EP-A 293859 or from EP-A 714700 is known.

It is useful for coating the carrier body the powder mass to be applied is moistened and after application, e.g. by means of hot Air, dried again. The layer thickness of the applied to the carrier body Powder mass is convenient in the range 10 to 1000 μm, preferably in the range from 50 to 500 μm and particularly preferably in the range from 150 to 250 μm, chosen as support materials can usual porous or non-porous Aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium or aluminum silicate can be used. The carrier body can be regular or irregularly shaped be, being regularly shaped Carrier body with clearly developed surface roughness, e.g. Balls or hollow cylinders with a split layer are preferred. It is suitable to use essentially non-porous, rough spherical carriers made of steatite (e.g. steatite C220 from CeramTec), their diameter 1 to 8 mm, preferably 4 to 5 mm. But it is also suitable the use of cylinders as support bodies, the length of which is 2 to 10 mm and their outer diameter 4 to 10 mm. In the case of rings as a carrier body the wall thickness above usually at 1 to 4 mm. Ring-shaped carrier bodies to be used preferably have a length of 2 to 6 mm, an outer diameter from 4 to 8 mm and a wall thickness from 1 to 2 mm. Are suitable before especially rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) as Carrier body. The Fineness of the surface of the carrier body to be applied catalytically active oxide masses is of course the desired shell thickness adapted (see EP-A 714 700).

Favorable multimetal oxide active compositions to be used for the catalysts of the fixed bed catalyst bed are also compositions of the general formula VI, [D] _P [E] _q (VI), in which the variables have the following meaning:
D = Mo ₁₂ V _{a ''} Z ¹ _{b ''} Z ² _{c ''} Z ³ _{d ''} Z ⁴ _{e ''} Z ⁵ _{f ''} Z ⁶ _{g ''} O _{x ''} ,
E = Z ⁷ ₁₂ Cu _{h ''} H _{i ''} O _{y ''} ,
Z ¹ = W, Nb, Ta, Cr and / or Ce,
Z ² = Cu, Ni, Co, Fe, Mn and / or Zn,
Z ³ = Sb and / or Bi,
Z ⁴ = Li, Na, K, Rb, Cs and / or N
Z ⁵ = Mg, Ca, Sr and / or Ba,
Z ⁶ = Si, Al, Ti and / or Zr,
Z ⁷ = Mo, W, V, Nb and / or Ta, preferably Mo and / or W.
a '' = 1 to 8,
b '' = 0.2 to 5,
c '' = 0 to 23,
d '' = 0 to 50,
e '' = 0 to 2,
f '= 0 to 5,
g '' = 0 to 50,
h '' = 4 to 30,
i '' = 0 to 20 and
x '', y '' = numbers determined by the valency and frequency of the element other than oxygen in VI and
p, q = non-zero numbers, whose ratio p / q is 160: 1 to 1: 1, and which can be obtained by using a multimetal oxide mass E Z ⁷ ₁₂ Cu _{h ''} H _{i ''} O _{y ''} (E),
separately in finely divided form (starting mass 1) and then the preformed solid starting mass 1 in an aqueous solution, an aqueous suspension or in a finely divided dry mixture of sources of the elements Mo, V, Z ¹ , Z ² , Z ³ , Z ⁴ , Z ⁵ , Z ⁶ , which the aforementioned elements in the stoichiometry D Mo ¹² V _{a ''} Z ¹ _{b ''} Ze ² _{c ''} Z ³ _{d ''} Z ⁴ _{e ''} Z ⁵ _{f ''} Z ⁶ _{g ''} (D), contains (starting mass 2), incorporated in the desired ratio p: q, the resulting aqueous mixture dries, and the resulting dry precursor is calcined before or after drying to the desired catalyst geometry at temperatures of 250 to 600 ° C.

The multimetal oxide compositions VI are preferred, in which the preformed solid starting composition 1 is incorporated into an aqueous starting composition 2 at a temperature <70 ° C. A detailed description of the production of multimetal oxide VI catalysts contains, for example, EP-A 668104, which DE-A 19736105 , DE-A 10046928, DE-A 19740493 and DE-A 19528646.

Regarding the shape applies in terms of Multimetal oxide masses VI catalysts that in the multimetal oxide masses IV catalysts said.

They are also suitable as for the Suitable multimetal oxide masses of catalysts of the fixed bed catalyst bed that of DE-A 19815281, in particular all exemplary embodiments from this writing. This applies to the shape above Said.

Catalysts S1 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _{2, 4} O _n ) and S7 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _1.6 Ni ₀ ) are particularly suitable catalysts for the fixed bed catalyst bed of the process according to the invention _{, 8} O _n ) from DE-A 4442346 with an active mass fraction of 27% by weight and a shell thickness of 230 μm, the shell catalyst from preparation example 5 of DE-A 10046928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _{2 , 4} O _n ) with an active mass fraction of 20% by weight, the shell catalysts according to Examples 1 to 5 from DE-A 19815281, but also like the shell catalysts mentioned above for the second reaction stage on support rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) with an active mass fraction of 20% by weight (based on the total mass of the shell catalyst), and a shell catalyst with two-phase active mass of stoichiometry (Mo ₁₂ V ₃ W _1.2 Cu _{2, 4} O _x ) (Cu Mo _{0 , 5} W _{0, 5} O ₄ ) _{1, 6} , and manufactured according to the DE-A 19736105 and an active mass fraction of 20% by weight applied to the aforementioned 7 mm × 3 mm × 4 mm carrier.

The above for the reaction step according to the invention recommended catalysts are for the reaction stage according to the invention but also suitable if you keep everything and only the carrier geometry to 5 mm × 3 mm × 1.5 mm changed (Outer diameter × length × inner diameter). Can also the multimetal oxides mentioned but also in the form of the corresponding Fully catalyst rings used in the reaction stage according to the invention become.

To prepare the fixed bed catalyst bed at method according to the invention only the corresponding multimetal oxide active catalyst body or also largely homogeneous mixtures of multimetal oxide active material having shaped catalyst bodies and not having a multimetal oxide active composition, regarding the heterogeneously catalyzed partial gas phase oxidation essentially inert, shaped bodies (Dilution molded body) used become. As materials for such inert shaped body In principle, all those who also consider themselves as support material for suitable according to the invention Shell catalysts are suitable. As such materials come e.g. porous or non-porous Aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide, Silicates such as magnesium or aluminum silicate or the steatite already mentioned (e.g. Steatite C220 from CeramTec).

The geometry of such inert shaped dilution bodies can be arbitrary in principle. That means, for example, balls, Polygons, solid cylinders or rings. Preferred according to the invention is such an inert diluent choose, whose geometry corresponds to that of the shaped catalyst bodies to be diluted with them.

According to the invention it is favorable if the chemical composition of the active mass used over the Fixed catalyst bed not changed. That is, the for a single shaped catalyst body Active mass used can be a mixture of different Multimetal oxides containing elements Mo and V, for all shaped catalyst bodies Fixed catalyst bed however, the same mixture must then be used.

The volume-specific (i.e., the activity standardized to the unit of volume can in this case e.g. can be easily reduced by having a basic set of homogeneously produced shaped catalyst bodies with shaped dilution bodies homogeneous diluted. The higher the proportion of the shaped diluent is selected, the lower the active mass contained in a certain volume of the bed or catalyst activity.

A volume-specific activity which increases at least once in the direction of flow of the reaction gas mixture via the fixed bed catalyst bed can thus be set in a simple manner for the process according to the invention, for example, by starting the bed with a high proportion of inert diluent shaped bodies, based on a type of shaped catalyst bodies, and then this proportion on shaped dilution bodies in the flow direction either continuously or at least once or several times abruptly (for example step-wise). However, an increase in the volume-specific activity is also possible, for example, by increasing the thickness of the active mass layer applied to the support while maintaining the same geometry and type of active mass of a shell catalyst or in a mixture of shell catalysts with the same geometry but with a different proportion by weight of the active mass increases the proportion of shaped catalyst bodies with a higher proportion of active mass. Alternatively, one can also dilute the active materials themselves, for example, by incorporating inert, diluting materials, such as highly-fired silicon dioxide, into the dry mixture of starting compounds to be calcined in the active material preparation. Different amounts of diluting material automatically lead to different activities. The more diluting material is added, the lower the resulting activity will be. An analogous effect can also be achieved, for example, by changing the mixing ratio in a corresponding manner in mixtures of unsupported catalysts and shell catalysts (with identical active mass). Of course, the variants described can also be used in combination.

Of course, for the fixed bed catalyst bed also mixtures of catalysts with chemically different ones Active composition and as a result of this different composition different activity be used. These mixtures can in turn be diluted with inert diluents.

Normally, the method according to the invention within the fixed bed catalyst bed in the direction of flow of the reaction gas mixture does not have volume-specific activity lose weight once.

Before and / or after the Fixed catalyst bed can yourself consisting of inert material (e.g. only dilution tablets) packings (they are not conceptually assigned to the fixed bed catalyst bed in this document, since they are not molded articles contain the multimetal oxide active material). The for the inert bed used dilution moldings same geometry as that used in the fixed bed catalyst bed Catalyst bodies exhibit. The geometry of the for the inertia used dilution tablets but also different from the aforementioned geometry of the shaped catalyst bodies be (e.g. spherical instead of circular).

Frequently assign the for such inertia Shaped body used the ring-shaped Geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) or the spherical Geometry with the diameter d = 4 - 5 mm.

According to the invention, preference is given to the method according to the invention the fixed bed catalyst bed in flow direction of the reaction gas mixture structured as follows.

First of all on a length of 10 to 60%, preferably 10 to 50%, particularly preferably 20 to 40 %, and most preferably 25 to 35% (i.e., e.g. on one length of 0.70 to 1.50 m, preferably 0.90 to 1.20 m), in each case the total length of the Fixed catalyst bed, a homogeneous mixture or two (with decreasing dilution) on top of each other the following homogeneous mixtures of shaped catalyst bodies and shaped dilution bodies (where both preferably have essentially the same geometry), where the weight fraction of the dilution tablets (the mass densities of Catalyst bodies and differentiate from dilution tablets usually only slightly) normally 10 to 50% by weight, preferably 20 to 45% by weight and is particularly preferably 25 to 35% by weight. Following this first zone the fixed bed catalyst bed is then advantageous according to the invention to the end of the length the fixed bed catalyst bed (i.e. over a length from 2.00 to 3.00 m, preferably 2.50 to 3.00 m) either one only to a lesser extent (than in the first zone) diluted bed of the Catalyst bodies, or, very particularly preferably, a sole pouring of the same Catalyst bodies, which were also used in the first zone.

The above is particularly true then when shell catalyst rings are used as shaped catalyst bodies in the fixed bed catalyst bed or shell catalyst balls are used (especially those that be cited as preferred in this document). Show with advantage within the framework of the aforementioned structuring, both the shaped catalyst bodies or their carrier rings as well as the dilution tablets in the process according to the invention essentially the ring geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) on.

The above also applies to if shell catalyst moldings are used instead of inert diluents whose active mass fraction is 2 to 15% by weight - points lower is, than the active mass fraction of the molded shell catalyst body on End of the fixed bed catalyst bed.

In an expedient manner in terms of application technology, 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 FIGS DE-A 19910508 , 19948523, 19910506 and 19948241. DE-C 2830765 discloses a preferred variant of a two-zone tube bundle reactor which can be used according to the invention. 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.

That is, located in the simplest way the one to be used according to the invention Fixed catalyst bed (possibly with upstream and / or downstream inert fillings) in the metal tubes of a tube bundle reactor and around the metal tubes are two essentially spatially separated from each other Temperature control media, usually molten salts. The pipe section over which extends the respective salt bath, represents a reaction zone according to the invention.

That is, flows around in a simple manner, e.g. a salt bath A those sections of the tubes (the reaction zone A), in which the oxidative conversion of acrolein (at simple pass) until a sales value is reached in the area from 45 to 85 mol% (preferably 50 to 85 mol%, particularly preferably 60 up to 85 mol%) and a salt bath B flows around the section of the tubes (the reaction zone B), in which the oxidative connection reaction takes place of acrolein (in a single pass) until reaching one Sales value of at least 90 mol% takes place (if necessary to those to be used according to the invention Reaction zones A, B connect further reaction zones that are kept at individual temperatures).

Appropriately applied in terms of application technology the reaction stage of the process according to the invention is no further Reaction zones. That is, the salt bath B expediently flows around the section of the pipes, in which the oxidative subsequent conversion of acrolein (at single pass) up to a conversion value of ≥ 92 mol%, or ≥ 94 mol% or ≥ 96 mol% or ≥ 98 mol% and often even ≥ 99 mol% or more.

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

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

Of course, according to the invention also in the reaction zone A is a co-current and in the reaction zone B a counter flow (or vice versa).

Of course you can in all of the above Case constellations within the respective reaction zone of the relative to the reaction tubes, parallel flow of the A molten salt layer over a cross flow, so that the individual reaction zone as in EP-A 700714 or corresponds to the tube bundle reactor described in EP-A 700893 and overall in longitudinal section through the contact tube bundle a meandering flow of the heat exchange medium results.

Usually are in the aforementioned two-zone tube bundle reactors for the reaction stage according to the invention the contact tubes are made of ferritic steel and have typically a wall thickness of 1 to 3 mm. Your inside diameter is usually 20 to 30 mm, often 22 to 26 mm. Their length is expediently 3 to 4, preferably 3.5 m. The fixed bed catalyst bed occupies at least in each temperature zone 60%, or at least 75%, or at least 90% of the length of the Zone. The remaining length, if any, is determined by an inertia busy. Appropriately from an application point of view, the one accommodated in the tube bundle container Number of contact tubes to at least 5000, preferably to at least 10000. Often is the number of in the reaction vessel housed contact tubes 15000 to 30000. Tube bundle reactors with a number of contact tubes above 40,000 rather the exception. The contact tubes are inside the container normally distributed homogeneously (preferably 6 equidistant Neighboring tubes per contact tube), the distribution being appropriately selected so that the distance of the central inner axes from the nearest to each other Contact tubes (the so-called contact tube division) 35 to 45 mm is (see EP-B 468290).

Are suitable as heat exchange medium in particular fluid tempering media. The use of is particularly favorable Melting salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or of low melting metals such as Sodium, mercury and alloys of various metals.

As a rule, all of the above mentioned Constellations of the current routing in the two-zone tube bundle reactors the reaction stage according to the invention the flow rate selected within the two required heat exchange medium circuits so that the temperature of the heat exchange medium from the entry point into the reaction zone up to the exit point from the reaction zone around 0 to 15 ° C increases. That is, the aforementioned ΔT can 1 to 10 ° C, or 2 to 8 ° C or 3 to 6 ° C be.

The inlet temperature of the heat exchange medium According to the invention, reaction zone A is normally in the range from 230 to 320 ° C, preferably in the range from 250 to 300 ° C. and particularly preferably in Range from 260 to 280 ° C. The inlet temperature of the heat exchange medium according to the invention, reaction zone B is normally also in Range from 230 to 320 ° C, or up to 280 ° C, at the same time, however, normally ≥ 0 ° C to ≤ 20 ° C or up to ≤ 10 ° C, or ≥ 0 ° C and ≤ 5 ° C, or often ≥ 0 ° C and ≤ 3 ° C below the inlet temperature of that entering reaction zone A. Heat exchange medium.

At this point, too once noted that for carrying out the reaction stage according to the invention of the method according to the invention in particular also the two-zone tubular reactor type described in DE-AS 2201528 can be used, the possibility includes, from the hotter Heat exchange means the reaction zone B a portion to the reaction zone A to if necessary warming up a reaction gas starting mixture which is too cold or a cold circulating gas to effect. Furthermore, the tube bundle characteristic can be within an individual reaction zone as described in EP-A 382 098 be designed.

Acrolein, which is produced by catalytic Gas phase oxidation of propene was generated. As a rule, the reaction gases of this propene oxidation containing acrolein are used without intermediate purification, which is why the reaction gas starting mixture according to the invention can also contain small amounts of, for example, unreacted propene or of by-products of propene oxidation. Normally, the oxygen required for acrolein oxidation must also be added to the product gas mixture from the propene oxidation.

• a) die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch enthaltenen Propen ≤ 160 Nl Propen/l Festbettkatalysatorschüttung 1·h und ≥ 90 Nl Propen/1 Festbettkatalysatorschüttung 1·h beträgt;
• b) die volumenspezifische Aktivität der Festbettkatalysatorschüttung in Strömungsrichtung des Reaktionsgasgemisches über die Festbettkatalysatorschüttung entweder konstant ist oder wenigstens einmal zunimmt, und
• c) die Differenz T^maxA' – T^maxB' gebildet aus der höchsten Temperatur T^maxA', die das Reaktionsgasgemisch innerhalb der Reaktionszone A' aufweist, und der höchsten Temperatur T^maxB', die das Reaktionsgasgemisch innerhalb der Reaktionszone B aufweist, ≥ 0°C beträgt.

Such a gas phase catalytic oxidation of propene to acrolein preceding the process according to the invention is advantageously carried out in analogy to the process according to the invention in such a way that a starting gas mixture containing propene, molecular oxygen and at least one inert gas, the molecular oxygen and the propene in a molar ratio O ₂ : C ₃ H ₆ ≥ 1, in one reaction stage via a fixed bed catalyst bed which is arranged in two spatially successive reaction zones A ', B', the temperature of the reaction zone A 'being in the range from 290 to 380 ° C and the temperature of the reaction zone B 'is also a temperature in the range from 290 to 380 ° C, and the active composition of which is at least one multimetal oxide containing the elements Mo, Fe and Bi, means that the reaction zone A' leads to a conversion of the propene from 40 to 80 mol .-% and the propene conversion in a single pass d Due to the fixed bed catalyst bed 1 90 90 mol% and the associated selectivity of acrolein formation and acrylic acid by-product formation taken together ≥ 90 mol%, which is characterized in that

• a) the loading of the fixed bed catalyst bed with the propene contained in the reaction gas starting mixture is ≤ 160 Nl propene / l fixed bed catalyst bed 1 · h and ≥ 90 Nl propene / 1 fixed bed catalyst bed 1 · h;
• b) the volume-specific activity of the fixed bed catalyst bed in the flow direction of the reaction gas mixture over the fixed bed catalyst bed is either constant or increases at least once, and
• c) the difference T ^{maxA '} - T ^maxB' formed from the highest temperature T ^{maxA '} , which the reaction ^gas mixture has within the reaction ^zone A', and the highest temperature T ^{maxB '} , which the reaction ^gas mixture has within the reaction ^zone B, ≥ 0 ° C.

As a rule, the difference T ^maxA '- T ^{maxB' will} not be more than 80 ° C.

T ^{maxA '} - T ^{maxB' is preferably} ≥ 3 ° C and ≤ 70 ° C. T ^maxA '- T ^maxB' is very particularly preferably ≥ 20 ° C and ≤ 60 ° C.

It proves to be advantageous if the loading of the fixed bed catalyst bed with that in the reaction gas starting mixture contained propene ≥ 90 Nl propene / l h and ≤ 155 Nl propene / l · h, or ≥ 100 Nl Propene / lh and ≤ 150 Nl propene / l · h, or ≥ 110 Nl propene / l · h and ≤ 145 Nl propene / l · h, or ≥ 120 Nl propene / l · h and ≤ 140 Nl propene / l · h, or ≥ 125 Nl propene / l · h and ≤ 135 Nl propene / l · h is.

The differences T ^{maxA '} - T ^maxB' required in this case usually occur when the process is carried out if, on the one hand, both the temperature of reaction ^zone A 'and the temperature of reaction zone B' are in the range from 290 to 380 ° C and, on the other hand the difference between the temperature of reaction zone B '(T _B' ) and the temperature of reaction zone A '(T _A' ), ie, T _{B '} - T _A' , ≤ 0 ° C and ≥-10 ° C, or ≤ 0 ° C and ≥ -5 ° C, or often ≤ 0 ° C and ≥ -3 ° C.

That is, to carry out the method according to the invention can two two-zone tube bundle reactors connected in series or to a four-zone tube bundle reactor be fused, e.g. described in WO 01/36364 is, the propene oxidation in the first two-zone part and in the second Two-zone part of the acrolein oxidation according to the invention carried out becomes.

Usually located in the second Fall between the fixed bed catalyst beds for the two reaction stages an inertia. However, such an intermediate pouring can also be dispensed with.

Of course, the propene oxidation level but also a separate one or with the subsequent acrolein oxidation stage in a correspondingly fused single-zone tube bundle reactor his.

The length of the reaction tubes corresponds in the event of a merger, often the sum of the lengths of the non-fused ones Tube reactors.

The proportion of propene in the reaction gas starting mixture for the the inventive method preceding reaction stage can e.g. at values from 4 to 15% by volume, often 5 to 12% by volume or 5 to 8% by volume (each related on the total volume).

Frequently, the process preceding the process according to the invention will be carried out with a propene: oxygen: indifferent gases (including water vapor) volume ratio in the reaction gas starting mixture of 1: (1.0 to 3.0): (5 to 25), preferably 1: (1.4 to 2.3): Carry out (10 to 15). As a rule, the indifferent (inert) gas will consist of at least 20% of its volume of molecular nitrogen. However, it can also be ≥ 30% by volume, or ≥ 40% by volume, or ≥ 50% by volume, or ≥ 60% by volume, or ≥ 70% by volume, or ≥ 80 vol .-%, or ≥ 90 vol .-%, or ≥ 95 vol .-% consist of molecular nitrogen (in general, in this document, inert diluent gases are supposed to be those that occur during a single pass implement less than 5%, preferably less than 2%, of the respective reaction stage; in addition to molar nitrogen, these include, for example, gases such as propane, ethane, methane, pentane, butane, CO ₂ , CO, water vapor and / or noble gases). Of course, the inert diluent gas in the process preceding the process according to the invention can also consist of up to 50 mol% or up to 75 mol% and more of propane. Circulating gas can also be part of the diluent gas, as it remains after the acrylic acid has been separated off from the product gas mixture of the process according to the invention.

As catalysts for such Gas phase catalytic propene oxidation comes especially from those EP-A 15565, EP-A 575897, DE-A 19746210 and DE-A 19855913 into consideration.

The acrolein content in the reaction gas starting mixture for the inventive method can e.g. at values of 3 to 15 vol.%, often at 4 to 10 vol.% or 5 to 8 vol .-% (each based on the total volume).

Frequently becomes the method of the invention with an acrolein present in the reaction gas starting mixture: Oxygen: water vapor: inert gas - volume ratio (Nl) from 1: (1 to 3): (0 to 20): (3 to 30), preferably from 1 : (1 to 3): (0.5 to 10): (7 to 10) execute.

Of course, you can use the inventive method but also with one present in the reaction gas starting mixture Acrolein: Oxygen: Water vapor Other - volume ratio (Nl) from 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 bed of the method according to the invention the multimetal oxide compositions of DE-A 10261186 are also favorable.

In general, the volume-specific activity experimentally between two fixed bed catalyst bed zones differentiate easily in such a way that under identical boundary conditions (preferably the conditions of the envisaged process) over fixed bed catalyst beds same length, but in each case the composition of the respective fixed bed catalyst bed zone correspondingly, the same acrolein-containing reaction gas starting mixture guided becomes. The higher one converted amount of acrolein shows the higher volume-specific activity.

Favorable designs according to the invention of a two-zone tube bundle reactor for a propene partial oxidation stage preceding the reaction stage according to the invention can be as follows (the constructive detailed design can be as in utility model applications 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: Contact tube material: ferritic steel; Dimensions of the contact tubes: eg 3500 mm length; eg 30 mm outside diameter; eg 2 mm wall thickness;

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

um die Kontaktrohre;
durch um den Behälterumfang angebrachte Fenster sammelt sich die Salzschmelze an jedem Zonenende in einem um den Reaktormantel angebrachten Ringkanal, um einschließlich Teilstromkühlung im Kreis gepumpt zu werden;
über jede Reaktionszone wird die Salzschmelze von unten nach oben geführt.Number of contact tubes in the tube bundle: e.g. 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 loaded like the contact tubes (rotating helically from the very outside to the inside), e.g. of the same length and wall thickness, but with an outside diameter of eg 33.4 mm and a centered thermal sleeve with eg 8 mm outer diameter and eg 1 mm wall thickness;
Reactor (material like the contact tubes):
Cylindrical container with an inner diameter of 6000 - 8000 mm;
Reactor covers clad with 1.4541 stainless steel; Plating thickness: a few mm;
ring-shaped tube bundle, for example with a free central space;
Diameter of the central free space: eg 1000 - 2500 mm (eg 1200 mm, or 1400 mm, or 1600 mm, or 1800 mm, or 2000 mm, or 2200 mm, or 2400 mm);
Normally homogeneous contact tube distribution in the tube bundle (6 equidistant neighboring tubes per contact tube), arrangement in an equilateral triangle, contact tube division (distance of the central inner axes from the closest contact tubes): 35 - 45 mm, e.g. 36 mm, or 38 mm, or 40 mm, or 42 mm , or 44 mm; the contact tubes are sealed with their ends in contact tube plates (upper plate and lower plate, for example with a thickness of 100-200 mm) and open at the upper end into a hood connected to the container, which has an approval for the reaction gas starting mixture; a separating plate, for example on half 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' (lower zone); each reaction zone is divided into two equidistant longitudinal sections by a deflection disk;
the deflection disk preferably has a ring geometry; the contact tubes are advantageously fastened in a sealing manner to the separating plate; they are not attached in a sealing manner to the deflection disks, so that the cross-flow speed of the molten salt is as constant as possible within a zone;
each zone is supplied with molten salt as a heat transfer medium by its own salt pump; the supply of the molten salt is, for example, below the deflection plate and the removal is, for example, above the deflection plate;
For example, a partial flow is taken from both molten salt circuits and cooled, for example, in a common or two separate indirect heat exchangers (steam generation);
in the first case the cooled salt melt stream is divided, combined with the respective residual stream and pressed into the reactor by the respective pump into the corresponding ring channel which distributes the salt melt over the circumference of the container;
the salt melt reaches the tube bundle through windows in the reactor jacket; the inflow takes place, for example, in the radial direction to the tube bundle;
the salt melt flows in each zone according to the specification of the baffle, for example in the sequence

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

around the contact tubes;
through windows arranged around the circumference of the vessel, the molten salt collects at each end of the zone in an annular channel arranged around the reactor jacket in order to be pumped in a circuit, including partial flow cooling;
The molten salt is passed from bottom to top over each reaction zone.

The reaction gas mixture leaves the Reactor of the reaction stage according to the invention upstream reaction stage with a temperature of a few degrees higher than the salt bath inlet temperature of this reactor. The reaction gas mixture is for the further processing expediently in a separate aftercooler, which is downstream of this reactor, cooled to 220 ° C to 280 ° C, preferably 240 ° C to 260 ° C.

The aftercooler is usually below flanged to the lower tube plate and usually consists of Pipes with ferritic steel. Into the aftercooler pipes with advantage inside stainless steel sheet spirals that are partially or fully coiled could be, to improve heat transfer introduced.

Molten salt:
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 as the molten salt; both reaction zones and the aftercooler advantageously use a molten salt of the same composition; the amount of salt pumped around in the reaction zones can be approximately 10000 m ³ / h per zone.

Flow Control:
the reaction gas starting mixture for the propene oxidation expediently flows through the reactor from top to bottom, while the differently tempered salt melts of the individual zones are expediently conveyed from bottom to top; Contact tube and thermotube feeding (from top to bottom) eg:
Section 1: 50 cm in length
Steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
Section 2: 140 cm in length
Catalyst feed with a homogeneous mixture of 30% by weight of steatite rings of geometry 5 mm × 3 mm × 2 mm (outer diameter × length × inner diameter) and 70% by weight of full catalyst from section 3.
Section 3: 160 cm in length
Catalyst feed with annular (5 mm × 3 mm × 2 mm = outer diameter × length × inner diameter) full catalyst according to Example 1 of DE-A 10046957 (stoichiometry: [Bi ₂ W ₂ O ₃ × 2WO ₃ ] _0.5 [Mo ₁₂ Co _{5 , 5} Fe _2.94 Si _1.59 K _0.08 O _x ] ₁ ).

Inexpensive embodiments of a Two-zone tube bundle reactor for the acrolein reaction step according to the invention can be as follows: Everything as in the two-zone tube bundle reactor for the propene reaction stage. However, the thickness of the upper and lower contact tube plate is frequently 100-200 mm, e.g. 110 mm, or 130 mm, or 150 mm, or 170 mm, or 190 mm.

The aftercooler is omitted; instead the Contact tubes with their lower openings into a hood with an outlet connected to the container at the lower end for the Product gas mixture; the upper reaction zone is zone A and the lower reaction zone is reaction zone B. Between outlet "aftercooler" and inlet "reactor for the reaction stage according to the invention" there is expediently one supply ability for compressed Air.

The contact tube and thermal tube loading (from top to bottom) can be as follows, for example:
Section 1: 20 cm long
Steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
Section 2: 90 cm in length
Catalyst feed with a homogeneous mixture of 30% by weight of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) and 70% by weight coated catalyst from section 4.
Section 3: 50 cm long
Catalyst feed with a homogeneous mixture of 20% by weight of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) and 80% by weight coated catalyst from section 4.
Section 4: 190 cm length
Catalyst feed with ring-shaped (7 mm × 3 mm × 4 mm = outer diameter × length × inner diameter) coated catalyst according to production example 5 of DE-A 10046928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _{2 , 4} O _x ).

Alternatively, the propene step contact tube and thermotube loading (from top to bottom) can also look like this:
Section 1: 50 cm in length
Steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
Section 2: 300 cm in length
Catalyst feed with annular (5 mm × 3 mm × 2 mm = outer diameter × length × inner diameter) full catalyst according to Example 1 of DE-A 10046957 (stoichiometry: [Bi ₂ W ₂ O ₉ × 2WO ₃ ] _0.5 [Mo ₁₂ Co _{5 , 6} Fe _2.94 Si _1.59 K _0.08 O _x ] ₁ ).

The acrol-stage contact tube and thermotube feeding can also look like this (from top to bottom):
Section 1: 20 cm long
Steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
Section 2: 140 cm in length
Catalyst feed with a homogeneous mixture of 25% by weight of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) and 75% by weight coated catalyst from section 3.
Section 3: 190 cm length
Catalyst feed with ring-shaped (7 mm × 3 mm × 4 mm = outer diameter × length × inner diameter) coated catalyst according to production example 5 of DE-A 10046928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _2.4 O _x ).

• a) einen Katalysator gemäß Beispiel 1 c aus der EP-A 15565 oder einen gemäß diesem Beispiel herzustellenden Katalysator, der jedoch die Aktivmasse Mo₁₂Ni_6,5Zn₂Fe₂Bi₁P_0,0065K_0,06O_x·10 SiO₂ aufweist;
• b) Beispiel Nr. 3 aus der DE-A 19855913 als Hohlzylindervollkatalysator der Geometrie 5 mm × 3 mm × 2 mm bzw.5 mm × 2 mm × 2 mm;
• c) Multimetalloxid II – Vollkatalysator gemäß Beispiel 1 der DE-A 19746210;
• d) einen der Schalenkatalysatoren 1, 2 und 3 aus der DE-A 10063162, jedoch bei gleicher Schalendicke auf Trägerringe der Geometrie 5 mm × 3 mm × 1,5 mm bzw. 7 mm × 3 mm × 1,5 mm aufgebracht;
• e) einen Schalenkatalysator gemäß den Ausführungsbeispielen der DE-A 19815281 jedoch bei gleicher Schalendicke auf Trägerringe der Geometrie 7 mm × 3 mm × 4 mm, oder 8 mm × 6 mm × 4 mm (stets Außendurchmesser × Länge × Innendurchmesser) aufgebracht.

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

• a) a catalyst according to Example 1 c from EP-A 15565 or a catalyst to be produced according to this example, but which has the active composition Mo ₁₂ Ni _6.5 Zn ₂ Fe ₂ Bi ₁ P _0.0065 K _0.06 O _x · 10 SiO ₂ ;
• b) Example No. 3 from DE-A 19855913 as a hollow cylinder full catalyst of the geometry 5 mm × 3 mm × 2 mm or 5 mm × 2 mm × 2 mm;
• c) multimetal oxide II - full catalyst according to Example 1 of DE-A 19746210;
• d) one of the shell catalysts 1, 2 and 3 from DE-A 10063162, but with the same shell thickness applied to carrier rings of geometry 5 mm × 3 mm × 1.5 mm or 7 mm × 3 mm × 1.5 mm;
• e) a coated catalyst according to the exemplary embodiments of DE-A 19815281 but with the same shell thickness applied to carrier rings of geometry 7 mm × 3 mm × 4 mm, or 8 mm × 6 mm × 4 mm (always outer diameter × length × inner diameter).

In the above listing is called support material preferably used steatite from CeramTec (type: C220).

• a) Schalenkatalysator S1 oder S7 aus der DE-A 4442346 mit einem Aktivmassenanteil von 27 Gew.-% und einer Schalendicke von 230 μm;
• b) einem Schalenkatalysator gemäß den Beispielen 1 bis 5 aus der DE-A 19815281, jedoch auf Trägerringe der Geometrie 7 mm × 3 mm × 4 mm mit einem Aktivmassenanteil von 20 Gew.-% aufgebracht;
• c) Schalenkatalysator mit zweiphasiger Aktivmasse der Stöchiometrie (Mo_10,4V₃W_1,2O_x) (CuMo_0,5W_0,5O₄)_1,6. hergestellt gemäß DE-A 19736105 und mit einem Aktivmassenanteil von 20 Gew.-% auf den vorgenannten 7 mm × 3 mm × 4 mm Träger aufgebracht.

In all of the abovementioned acrolein stage feeds according to the invention, the coated catalyst according to preparation example 5 of DE-A 10046928 can be replaced by:

• a) shell catalyst S1 or S7 from DE-A 4442346 with an active mass fraction of 27% by weight and a shell thickness of 230 μm;
• b) a coated catalyst according to Examples 1 to 5 from DE-A 19815281, but applied to carrier rings of geometry 7 mm × 3 mm × 4 mm with an active mass fraction of 20% by weight;
• c) Shell catalyst with two-phase active mass of stoichiometry (Mo _10.4 V ₃ W _1.2 O _x ) (CuMo _0.5 W _0.5 O ₄ ) _1.6 . manufactured according to DE-A 19736105 and applied with an active mass fraction of 20% by weight to the aforementioned 7 mm × 3 mm × 4 mm support.

Incidentally, the fixed bed catalyst bed for the propene oxidation stage and the fixed bed catalyst bed for the acrolein oxidation stage according to the invention appropriately chosen (e.g. by dilution with e.g. Inert material) that the temperature difference between the hotspot maximum of the reaction gas mixture in the individual reaction zones and respective temperature of the reaction zone usually does not exceed 80 ° C. Most is this temperature difference ≤ 70 ° C, is often at 20 to 70 ° C, this temperature difference is preferably small. Also be these fixed bed catalyst beds for safety reasons chosen in a manner known per se to the person skilled in the art (e.g. by dilution with e.g. Inert material) that the "peak-to-salt-temperature sensitivity "as defined in EP-A 1106598 is ≤ 9 ° C, or ≤ 7 ° C, or ≤ 5 ° C, or ≤ 3 ° C.

After cooler and reactor for the acrolein reaction stage are connected by a connecting pipe, the length of which is less than 25 m.

The described reactor arrangement and all other reactor arrangements and feeds with fixed bed catalyst described in this document can also be used with high propene or acrolein loads as in the documents WO 01/36364, DE-A 19927624 . DE-A 19948248 . DE-A19948523 , DE-A 19948241, DE-A 19910506, DE-A 10302715 and EP-A 1106598 are described.

The reaction zones A ', B' or A, B can preferably be used have the temperatures recommended in this document, however such that the second reaction zone in each case, according to the teaching of the abovementioned documents, a higher one Has temperature than the first reaction zone. The hot spot temperature in the second reaction zone in each case is preferably always below that of the first reaction zone.

With the acrolein loads according to the invention However, with the procedure according to the invention, a results relatively to the for high acrolein loads recommended procedure, increased selectivity of acrylic acid formation.

In the examples below and Comparative examples and in the above reactor arrangement can in the acrolein reaction stage the annular shaped diluent bodies and the annular shaped catalyst bodies also through spherical Dilution tablets and spherical Catalyst bodies (each with a radius of 2 to 5 mm and with an active mass fraction of 10 to 30% by weight, often 10 to 20 wt .-%) are replaced. The statement regarding the Operation with flea acrolein loads according to the aforementioned writings reserves their 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) for receiving a thermocouple with which the temperature in the reaction tube is determined over its entire length is loaded from top to bottom as follows:
Section 1: 20 cm long
Steatite rings of geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) as a pre-fill.
Section 2: 90 cm in length
Catalyst feed with a homogeneous mixture of 30% by weight of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) and 70% by weight coated catalyst from section 4.
Section 3: 50 cm long
Catalyst feed with a homogeneous mixture of 20% by weight of steatite rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) and 80% by weight coated catalyst from section 4.
Section 4: 190 cm length
Catalyst feed with ring-shaped (7 mm × 3 mm × 4 mm = outer diameter × length × inner diameter) coated catalyst according to production example 5 of DE-A 10046928 (stoichiometry: Mo ₁₂ V ₃ W _1.2 Cu _{2, 4} O _x ).

From top to bottom are the first 175 cm thermostatted by means of a salt bath A pumped in countercurrent. The second 175 cm are pumped in countercurrent Salt bath B thermostatted.

Gas-phase oxidation:
The reaction tube described above is continuously charged with a reaction gas starting mixture of the following composition, the load and the thermostatting of the reaction tube being varied:
5.5% by volume of acrolein,
0.3% by volume of propene,
6.0 vol% molecular oxygen,
0.4 vol.% CO,
0.8 vol% CO ₂ ,
9.0% by volume of water and,
78.0% by volume of nitrogen.

The reaction gas mixture flows through the reaction tube from top to bottom.

The pressure at the entrance to the reaction tube varies depending on of the acrolein load between 1.6 and 2.1 bar.

The product gas mixture is at the reaction tube outlet a little sample for the gas chromatographic analysis taken. At the end of the reaction zone A is also an analysis point.

The results as a function of loads and salt bath temperatures are shown in the table below (the letter B in brackets means example and the letter V in brackets means comparative example).
T _A , T _B stand for the temperatures of the pumped salt baths in reaction zones A and B.
U _AA is the acrolein conversion at the end of reaction zone A in mol%.
U _AB is the acrolein conversion at the end of reaction zone B in mol%.
S ^AA is the selectivity of the acrylic acid formation in the product gas mixture based on converted acrolein in mol%.

T ^maxA , T ^maxB stand for the highest temperature of the reaction ^gas mixture within reaction ^zones A and B in ° C.

table

Claims (8)

A process for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid, in which one contains a reaction gas starting mixture containing the molecular oxygen and the acrolein in one molar, containing acrolein, molecular oxygen and at least one inert gas which consists of at least 20% of its volume of molecular nitrogen Ratio contains O ₂ : C ₃ H ₄ O ≥ 0.5, in one reaction stage via a fixed bed catalyst bed which is arranged in two spatially successive reaction zones A, B, the temperature of reaction zone A being in the range from 230 to 320 ° C and the temperature of reaction zone B is also a temperature in the range of 230 to 320 ° C, and the active composition of which is at least one multimetal oxide containing the elements Mo and V, means that reaction zone A leads to a conversion of acrolein of 45 to 85 mol% and with a single pass of the reaction gas starting mixture d Due to the total fixed bed catalyst bed, the acrolein conversion is ≥ 90 mol% and the selectivity of acrylic acid formation, based on converted acrolein, is ≥ 90 mol%, the chronological sequence in which the reaction gas starting mixture flows through the reaction zones corresponds to the alphabetical sequence of the reaction zones, that characterized in that a) the loading of the fixed bed catalyst bed with the acrolein contained in the reaction gas starting mixture is ≤ 145 Nl acrolein / l fixed bed catalyst bed · h and ≥ 70 Nl acrolein / l fixed bed catalyst bed · h, b) the volume-specific activity of the fixed bed catalyst bed in the direction of flow of the reaction gas mixture the fixed bed catalyst bed is either constant or increases at least once, and c) the difference T ^maxA - T ^maxB formed from the highest temperature T ^maxA that the reaction ^gas mixture has within reaction ^zone A and the highest temperature temperature T ^maxB , which the reaction ^gas mixture has in reaction ^zone B, is ≥ 0 ° C. A method according to claim 1, characterized in that the difference T ^maxA - T ^maxB ≥ 0 ° C and ≤ 75 ° C. A method according to claim 1, characterized in that the difference T ^maxA - T ^maxB ≥ 3 ° C and ≤ 6 0 ° C. A method according to claim 1, characterized in that the difference T ^maxA - T ^maxB ≥ 5 ° C and ≤ 40 ° C. Method according to one of claims 1 to 4, characterized in that that the loading of the fixed bed catalyst bed with that in the reaction gas starting mixture contained acrolein ≥ 70 Nl acrolein / l · h and ≤ 140 Nl acrolein / l · h is. Method according to one of claims 1 to 4, characterized in that that the loading of the fixed bed catalyst bed with that in the reaction gas starting mixture contained acrolein ≥ 80 Nl acrolein / l · h and ≤ 130 Nl acrolein / l · h is. Method according to one of claims 1 to 5, characterized in that the active composition of the fixed bed catalyst bed at least one multimetal oxide active composition of the general formula IV MO ₁₂ V _a X ¹ _b X ² _c X ³ _d X ⁴ _e X ⁵ _f X ⁶ _g X _n (IV), in which the variables have the following meaning: X ¹ = W, Nb, Ta, Cr and / or Ce, X ² = Cu, Ni, Co, Fe, Mn and / or Zn, X ³ = Sb and / or Bi, X ⁴ = one or more alkali metals, X ⁵ = one or more alkaline earth metals, X ⁶ = Si, Al, Ti and / or Zr, a = 1 to 6, b = 0.2 to 4, c = 0.5 to 1 8 , d = 0 to 40, e = 0 to 2, f = 0 to 4, g = 0 to 40 and n = a number which is determined by the valency and frequency of the elements in IV other than oxygen. Method according to one of claims 1 to 7, characterized in that that the volume-specific activity of the fixed bed catalyst bed in flow direction of the reaction gas mixture the fixed bed catalyst bed increases at least once.