DE10313210A1 - Partial oxidation of propene for manufacture of acrolein, involves passing reaction mixture over catalyst bed having preset temperature zones whose transition is different from transition between preset fixed catalyst bed zones - Google Patents

Abstract

A mixture of propene, molecular oxygen and inert gas, is passed over a fixed catalyst bed having temperature zones (A,B) maintained at 290-380 [deg] C. The fixed catalyst bed has two fixed catalyst bed zones having preset volume-specific activity. The transition from zone A to zone B in fixed catalyst bed do not coincide with transition from one fixed catalyst bed zone to another. The partial oxidation of propene in gaseous phase is performed in presence of heterogeneous catalyst. A reaction gas mixture comprising propene, molecular oxygen and at least one inert gas is passed over a fixed catalyst bed containing at least one multimetal oxide comprising molybdenum, iron and bismuth. The molar ratio of molecular oxygen and propene is = 1. The fixed catalyst bed is arranged in two spatially successive temperature zones (A,B), both maintained at 290-380 [deg] C. The fixed catalyst bed consists of at least two spatially successive fixed catalyst bed zones. The volume-specific activity within one fixed catalyst bed zone is substantially constant and increases sharply in the flow direction of the reaction gas mixture at the transition from one fixed catalyst bed zone to another fixed catalyst bed zone. The temperature zone (A) extends up to a conversion of propene of 40-80 mol%. On single pass of the starting reaction gas mixture through the entire fixed catalyst bed, the propene conversion is >=90 mol% and the selectivity of acrolein formation based on converted propene is >=90 mol %. The sequence in time in which the reaction gas mixture flows through the temperature zones (A,B) corresponds to the alphabetic sequence of the temperature zones. The hourly space velocity of the propene contained in the reaction gas mixture on the fixed catalyst bed is >= 90 I(STP) of propene/I of fixed bed catalyst.h. The difference (TmaxA-TmaxB) between the highest temperature (TmaxA) of reaction gas mixture in zone A, and the highest temperature (TmaxB) of reaction gas mixture in zone B, is >=0 [deg] C. The transition from temperature zone A to temperature zone B in the fixed catalyst bed do not coincide with a transition from one fixed catalyst bed zone to another fixed catalyst bed zone.

Description

• – die Festbettkatalysatorschüttung in zwei räumlich aufeinander folgenden Temperaturzonen A, B angeordnet ist,
• – sowohl die Temperatur der Temperaturzone A als auch die Temperatur der Temperaturzone B eine Temperatur im Bereich von 290 bis 380°C ist,
• – die Festbettkatalysatorschüttung aus wenigstens zwei räumlich aufeinanderfolgenden Festbettkatalysatorschüttungszonen besteht, wobei die volumenspezifische Aktivität innerhalb einer Festbettkatalysatorschüttungszone im wesentlichen konstant ist und in Strömungsrichtung des Reaktionsgasgemisches beim Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone sprunghaft zunimmt,
• – sich die Temperaturzone A bis zu einem Umsatz des Propens von 40 bis 80 mol.-% erstreckt,
• – bei einmaligem Durchgang des Reaktionsgasausgangsgemisches durch die gesamte Festbettkatalysatorschüttung der Propenumsatz ≥90 mol.-% und die Selektivität der Acroleinbildung, bezogen auf umgesetztes Propen, ≥90 mol.-% beträgt,
• – die zeitliche Abfolge, in der das Reaktionsgasgemisch die Temperaturzonen A, B durchströmt der alphabetischen Abfolge der Temperaturzonen entspricht,
• – die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch enthaltenen Propen ≥90 Nl Propen/l Festbettkatalysatorschüttung μ h beträgt, und
• – die Differenz T^maxA – T^maxB, gebildet aus der höchsten Temperatur T^maxA, die das Reaktionsgasgemisch innerhalb der Temperaturzone A aufweist, und der höchsten Temperatur T^maxB, die das Reaktionsgasgemisch innerhalb der Temperaturzone B aufweist, ≥0°C beträgt.

The present invention relates to a process of heterogeneously catalyzed partial gas phase oxidation of propene to acrolein, in which 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 contains, in a reaction stage with the proviso via a fixed bed catalyst bed, the active mass of which is at least one multimetal oxide containing the elements Mo, Fe and Bi, that

• The fixed bed catalyst bed is arranged in two spatially successive temperature zones A, B,
• Both the temperature of temperature zone A and the temperature of temperature zone B are a temperature in the range from 290 to 380 ° C.,
• The fixed bed catalyst bed consists of at least two spatially successive fixed bed catalyst bed zones, the volume-specific activity within a fixed bed catalyst bed zone being essentially constant and increasing suddenly in the direction of flow of the reaction gas mixture during the transition from a fixed bed catalyst bed zone into another fixed bed catalyst bed zone,
• Temperature zone A extends to a propene conversion of 40 to 80 mol%,
• The propene conversion is ≥90 mol% and the selectivity of acrolein formation, based on converted propene, is ≥90 mol% when the reaction gas starting mixture passes through the entire fixed bed catalyst bed once,
• The chronological sequence in which the reaction gas mixture flows through the temperature zones A, B corresponds to the alphabetical sequence of the temperature zones,
• - The loading of the fixed bed catalyst bed with the propene contained in the reaction gas starting mixture is ≥90 Nl propene / l fixed bed catalyst bed μ h, and
• - The difference T ^maxA - T ^maxB , formed from the highest temperature T ^maxA , which the reaction ^gas mixture has within the temperature ^zone A, and the highest temperature T ^maxB , which the reaction ^gas mixture has within the temperature zone B, is ≥0 ° C.

The aforementioned process of heterogeneous catalyzed partial gas phase oxidation of propene to acrolein is e.g. known in principle from WO 00/53556 and in particular as the first oxidation stage in the production of acrylic acid two-stage catalytic gas phase oxidation based on propene significant. acrylic acid is a significant monomer, as such or in the form of its Alkyl esters to produce e.g. Polymerization suitable as adhesives Is used. Acrolein is an important intermediate.

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 catalyzed partial gas phase oxidation of propene to acrolein is to achieve the highest possible selectivity S ^AC for acrolein (that is the mole number of propene converted to acrolein, based on that) when the reaction gas mixture passes through the reaction stage once under otherwise specified boundary conditions Number of moles of propene reacted).

Another objective is to achieve the highest possible conversion U ^P of propene when the reaction gas mixture passes through the reaction stage under otherwise specified boundary conditions (this is the mole number of propene converted, based on the mole number of propene used).

If the S ^{AC is} high at a high U ^P , this corresponds to a high yield A ^AC of acrolein (A ^AC is the product U ^P · S ^AC ; that is, the mole number of propene converted to acrolein, based on the mole number of propene used).

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

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

The DE-A 19948241 , the DE-A 19910506 and WO 0053556 teach that the above-mentioned goals can in principle be achieved by means of the heterogeneously catalyzed partial gas phase oxidation of propene to acrolein described above.

This is advantageous in that low volume-specific activity within a fixed bed catalyst bed or Fixed catalyst bed zone e.g. by doing so that one with the actual active mass-carrying shaped catalyst body inert diluent shaped bodies free of active composition, as a result of which the fixed bed catalyst bed as a whole cost-effective becomes.

A disadvantage of the teachings of DE-A 19948241 , the DE-A 19910506 and WO 0053556, however, is that they do not have a corresponding exemplary embodiment and thus leave the concrete design of such a procedure open.

Similarly, the teaches EP-A 1106598 a process of heterogeneously catalyzed partial gas phase oxidation of propene to acrolein, in which the fixed bed catalyst bed of the reaction stage consists of several spatially successive fixed bed catalyst bed zones, the volume-specific activity of which is essentially constant within a fixed bed catalyst bed zone and in the direction of flow of the reaction gas mixture when changing from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone increases and can be arranged in several temperature zones.

According to the teaching of EP-A 1106598 it is expedient if fixed bed catalyst bed zones and temperature zones coincide spatially.

In-depth investigations on the part of the applicant has shown, however, that a spatial coincidence of Fixed catalyst bed zones and temperature zones for an optimal achievement of the different objectives mentioned usually not beneficial is.

This includes due to the fact that the transition from temperature zone A to temperature zone B and the transition from a fixed bed catalyst bed zone in a different fixed bed catalyst bed zone than either in essentially equivalent or as essentially opposite effective measures be taken.

If they are taken as essentially equivalent measures (e.g. transition from a colder temperature zone A to a hotter temperature zone B and transition from a fixed-bed catalyst bed zone with low volume-specific activity to a fixed-bed catalyst bed zone with higher volume-specific activity; both measures have the purpose of promoting the reaction), the overall effect achieved in the transition area often goes beyond the desired goal and, for example, there is a reduction in the S ^AC .

If they are taken as essentially counteracting measures (e.g. transition from a hotter temperature zone A to a colder temperature zone B and transition from a fixed-bed catalyst bed zone with lower volume-specific activity to a fixed-bed catalyst bed zone with higher volume-specific activity; the first measure has the purpose of reducing the reaction , the second measure has the purpose of promoting the reaction), the effects in the transition area are often neutralized and the result is a reduced U ^P.

The object of the present invention therefore consisted of a process of heterogeneously catalyzed partial Gas phase oxidation of propene to acrolein in a multi-zone arrangement to disposal to face the disadvantages of the prior art methods does not have.

• – die Festbettkatalysatorschüttung in zwei räumlich aufeinanderfolgenden Temperaturzonen A, B angeordnet ist,
• – sowohl die Temperatur der Temperaturzone A als auch die Temperatur der Temperaturzone B eine Temperatur im Bereich von 290 bis 380°C ist,
• – die Festbettkatalysatorschüttung aus wenigstens zwei räumlich aufeinander folgenden Festbettkatalysatorschüttungszonen besteht, wobei die volumenspezifische Aktivität innerhalb einer Festbettkatalysatorschüttungszone im wesentlichen konstant ist und in Strömungsrichtung des Reaktionsgasgemisches beim Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone sprunghaft zunimmt,
• – sich die Temperaturzone A bis zu einem Umsatz des Propens von 40 bis 80 mol.-% erstreckt,
• – bei einmaligem Durchgang des Reaktionsgasausgangsgemisches durch die gesamte Festbettkatalysatorschüttung der Propenumsatz ≥90 mol.-% und die Selektivität der Acroleinbildung, bezogen auf umgesetztes Propen, ≥90 mol.-% beträgt,
• – die zeitliche Abfolge, in der das Reaktionsgasgemisch die Temperaturzonen A, B durchströmt, der alphabetischen Abfolge der Temperaturzonen entspricht,
• – die Belastung der Festbettkatalysatorschüttung mit dem im Reaktionsgasausgangsgemisch enthaltenen Propen ≥90 Nl Propen/l Festbettkatalysatorschüttung · h beträgt, und
• – die Differenz T^maxA – T^maxB gebildet aus der höchsten Temperatur T^maxA, die das Reaktionsgasgemisch innerhalb der Temperaturzone A aufweist, und der höchsten Temperatur T^maxB, die das Reaktionsgasgemisch innerhalb der Temperaturzone B aufweist, ≥0°C beträgt,

gefunden, das dadurch gekennzeichnet ist, dass der Übergang von der Temperaturzone A in die Temperaturzone B in der Festbettkatalysatorschüttung (räumlich) nicht mit einem Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone zusammenfällt.Accordingly, a process of heterogeneously catalyzed partial gas phase oxidation of propene to acrolein was carried out in which a starting gas mixture containing propene, molecular oxygen and at least one inert gas, which contained the molecular oxygen and the propene in a molar ratio of O ₂ : C ₃ H ₆ ≥1 , in a reaction stage with the proviso via a fixed bed catalyst bed, the active mass of which is at least one multimetal oxide containing the elements Mo, Fe and Bi, that

• The fixed bed catalyst bed is arranged in two spatially successive temperature zones A, B,
• Both the temperature of temperature zone A and the temperature of temperature zone B are a temperature in the range from 290 to 380 ° C.,
• The fixed bed catalyst bed consists of at least two spatially consecutive fixed bed catalyst bed zones, the volume-specific activity within a fixed bed catalyst bed zone being essentially constant and increasing suddenly in the direction of flow of the reaction gas mixture upon transition from a fixed bed catalyst bed zone into another fixed bed catalyst bed zone,
• Temperature zone A extends to a propene conversion of 40 to 80 mol%,
• The propene conversion is ≥90 mol% and the selectivity of acrolein formation, based on converted propene, is ≥90 mol% when the reaction gas starting mixture passes through the entire fixed bed catalyst bed once,
• The chronological sequence in which the reaction gas mixture flows through the temperature zones A, B corresponds to the alphabetical sequence of the temperature zones,
• - The loading of the fixed bed catalyst bed with the propene contained in the reaction gas starting mixture is ≥90 Nl propene / l fixed bed catalyst bed · h, and
• The difference T ^maxA - T ^maxB formed from the highest temperature T ^maxA that the reaction ^gas mixture has within the temperature ^zone A and the highest temperature T ^maxB that the reaction ^gas mixture has within the temperature zone B is ≥0 ° C.

found, which is characterized in that the transition from temperature zone A to temperature zone B in the fixed bed catalyst bed (spatially) does not coincide with a transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone.

Under the temperature of a temperature zone in this document the temperature of the in the temperature zone located part of the fixed bed catalyst bed when exercising method according to the invention, however understood in the absence of a chemical reaction. is this temperature is not constant within the temperature zone, so The term temperature of a temperature zone here means the (number) mean of Fixed bed catalyst bed temperature along the temperature zone.

It is essential that the temperature control of the individual temperature zones essentially independent of each other he follows.

Because the heterogeneously catalyzed partial Gas phase oxidation of propene to acrolein 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 temperature zone. she is usually above the temperature of the temperature zone and goes through usually a maximum within a temperature zone (hotspot maximum) or falls starting from a maximum value.

As a rule, the difference T ^maxA -T ^{maxB in the method according to the invention will} not be more than 80 ° C. According to the invention, T ^maxA -T ^{maxB is} preferably ≥3 ° C and ≤70 ° C. In the method according to the invention, T ^maxA -T ^maxB is very particularly preferably 2020 ° C and 6060 ° C.

The differences T ^maxA - T ^{maxB required in accordance} with the invention occur when the process ^according to the invention is carried out in the case of rather low (≥90 Nl / l · h and <160 Nl / l · h) propene loads on the fixed bed catalyst bed normally when both Temperature of temperature zone A and the temperature of temperature zone B is in the range from 290 to 380 ° C. and, on the other hand, the difference between the temperature of temperature zone B (T _B ) and the temperature of temperature 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.

When the process according to the invention is carried out under increased propene loads (1160 Nl / l · h and 300300 Nl / l · h or 600600 Nl / l · h), the differences T ^maxA - T ^maxB required ^according to the invention usually occur when on the one hand, both the temperature of temperature zone A and the temperature of temperature zone B are in the range from 290 to 380 ° C and T _B - T _A > 0 ° C and ≤50 ° C, or ≥5 ° C and ≤45 ° C, or ≥10 ° C and ≤40 ° C, or ≥15 ° C and ≤30 ° C, or ≤35 ° C (e.g. 20 ° C or 25 ° C).

The above statement regarding the temperature differences T _B - T _{A also} applies regularly if the temperature of the temperature zone A is in the preferred range from 305 to 365 ° C or in the particularly preferred range from 310 to 340 ° C.

The propene load on the fixed bed catalyst bed can thus in the method according to the invention e.g. ≥90 Nl / l · h and ≤300 Nl / lh, or ≥110 Nl / l · h and ≤280 Nl / lh, or ≥130 Nl / l · h and ≤260 Nl / l · h, or ≥150 Nl / l · h and ≤240 Nl / l · h, or ≥170 Nl / l · h and ≤220 Nl / l · h, or ≥190 Nl / l · h and ≤ 200 Nl / l · h.

According to the invention preferably extends the temperature zone A up to a propene conversion of 50 to 70 mol% or 60 to 70 mol%.

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, the molar ratio of O ₂ : propene in the reaction gas starting mixture must be ≥1. Often it will be values> 1. This ratio will usually be at values ≤3. According to the invention, the molar ratio of O ₂ : propene in the reaction gas starting mixture is frequently 1 to 2 or 1 to 1.5.

As a rule, the simple Pass-related propene conversion in the process according to the invention ≥92 mol%, or ≥94 mol%, or ≥96 mol%. The selectivity the formation of acrolein is regularly ≥92 mol% or ≥94 mol%, often ≥95 mol%, or ≥96 mol% or ≥97 mol%.

As a source of the required molecular Oxygen comes in both air and molecular nitrogen de-enriched air into consideration.

As catalysts for the fixed bed catalyst bed inventive method are all those whose active mass at least is a multimetal oxide containing Mo, Bi and Fe.

These are in particular the multimetal oxide active compositions of the general formula I DE-A 19955176 , the multimetal oxide active compositions of the general formula I DE-A 19948523 , the multimetal oxide active compositions of the general formula I DE-A 19948523 , the multimetal oxide active compositions of the general formulas I, II and III of DE-A 10101695 , the multimetal oxide active compositions of the general formulas I, II and III of DE-A 19948248 and the multimetal oxide active compositions of the general formulas I, II and III of DE-A 19955168 as well as those in the EP-A 700714 called multimetal oxide active materials.

The Mo, Bi and Fe-containing multimetal oxide catalysts in the documents are also suitable for the fixed bed catalyst bed DE-A 10046957 . DE-A 10063162 . DE-C 3338380 . DE-A 19902562 . EP-A 15565 . DE-C 2380765 . EP-A 807465 . EP-A 279374 . DE-A 3300044 . EP-A 575897 . US-A 4438217 . DE-A 19855913 , WO 98/24746, DE-A 19746210 (those of the general formula II), JP-A 91/294239 . EP-A 293224 and EP-A 700714 be disclosed. This applies in particular to the exemplary embodiments in these documents, among which those of EP-A 15565 , the EP-A 575897 , the DE-A 19746210 and the DE-A 19855913 are particularly preferred. In this context, a catalyst according to Example 1c from the EP-A 15565 and a catalyst to be produced in a corresponding manner, the active composition of which, however, has the composition Mo ₁₂ Ni _6.5 Zn ₂ Fe ₂ Bi ₁ P _0.0065 K _0.06 Ox · 10 SiO ₂ . Also to be emphasized is the example with the current number 3 from the DE-A 19855913 (Stoichiometry: Mo ₁₂ Co ₇ Fe ₃ Bi _0.6 K _0.08 Si _1.6 Ox) as a hollow cylinder full catalyst with a geometry of 5 mm × 3 mm × 2 mm or 5 mm × 2 mm × 2 mm Qeweils outer diameter × height × Inner diameter) and the multimetal oxide II full catalyst according to Example 1 of DE-A 19746210 , Furthermore, the multimetal oxide catalysts would be the US-A 4438217 to call. The latter applies in particular if it has a hollow cylinder geometry of the dimensions 5.5 mm × 3 mm × 3.5 mm, or 5 mm × 2 mm × 2 mm, or 5 mm × 3 mm × 2 mm, or 6 mm × 3 mm × 3 mm, or 7 mm × 3 mm × 4 mm (each outer diameter × height × inner diameter). The multimetal oxide catalysts and geometries are also suitable DE-A 10101695 or WO 02/062737.

Furthermore, Example 1 from the DE-A 10046957 (Stoichiometry: [Bi ₂ W ₂ O ₉ × 2WO _{3] 0, 5} x [MO ₁₂ CO _5.6 Fe _2.94 Si _1.59 K _0.08 O _{x] 1)} a hollow cylinder (ring) unsupported catalyst geometry 5 mm × 3 mm × 2 mm or 5 mm × 2 mm × 2 mm (each outer diameter × length × inner diameter), as well as the shell catalysts 1, 2 and 3 from the DE-A 10063162 (Stoichiometry: Mo ₁₂ Bi _1.0 Fe ₃ Co ₇ Si _1.6 K _0.08 ), however as ring-shaped shell catalysts of appropriate shell thickness and on carrier rings of the geometry 5 mm × 3 mm × 1.5 mm or 7 mm × 3 mm × 1.5 mm (each outer diameter × length × inner diameter) applied, suitable.

A large number of the multimetal oxide active compositions suitable for the catalysts of the fixed bed catalyst bed can be found under the general formula I Mo ₁₂ Bi _a Fe _b X ¹ _c X ² _d X ³ _e X ⁴ _f O _n (I), in which the variables have the following meaning:
X ¹ = nickel and / or cobalt,
X ² = thallium, an alkali metal and / or an alkaline earth metal,
X ³ = zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and / or tungsten,
X ⁴ = silicon, aluminum, titanium and / or zirconium,
a = 0.5 to 5,
b = 0.01 to 5, preferably 2 to 4,
c = 0 to 10, preferably 3 to 10,
d = 0 to 2, preferably 0.02 to 2,
e = 0 to 8, preferably 0 to 5,
f = 0 to 10 and
n = a number which is determined by the valency and frequency of the elements in I other than oxygen,
subsumed.

They are in a manner known per se available (see e.g. DE-A 4023239) and are usually added in substance Spheres, rings or cylinders shaped or in the form of shell catalysts, i.e., preformed, inert carrier bodies coated with the active composition. Of course can you but can also be used in powder form as catalysts.

In principle, active compositions of the general formula 1 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 calcining them at temperatures of 350 to 650 ° 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 mixture of inert gas, NH ₃ , CO and / or H ₂ ). 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 materials I are those compounds which are already oxides and / or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.

In addition to the oxides, the main starting compounds are halides, nitrates, formi ate, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and / or hydroxides (compounds such as NH ₄ OH, (NH ₄ ) ₂ CO ₃ , NH ₄ NO ₃ , NH ₄ CHO ₂ , CH ₃ COOH, NH ₄ CH ₃ CO ₂ and / or ammonium oxalate, which can decompose and / or decompose into gaseous compounds at the latest during later calcination, can also be incorporated into the intimate dry mixture).

The intimate mixing of the starting compounds for the production of multimetal oxide active materials I can 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 in the form of an aqueous solution and / or suspension with one another mixed. Particularly intimate dry mixtures are described in the Mixing process obtained when only in dissolved form existing sources of elementary constituents are assumed. As a solvent water is preferred. Then the aqueous mass obtained dried, the drying process preferably by spray drying the aqueous mixture with Outlet temperatures of 100 to 150 ° C takes place.

Usually the multimetal oxide active compositions of the general formula I in the fixed bed catalyst bed not in powder form but with certain catalyst geometries molded used, the shape before or after the final calcination can be done. For example from the powder form of the active composition or its uncalcined and / or partially calcined precursor mass by compression to the desired one Catalyst geometry (e.g. by tableting, extruding or Extrusion) full catalysts are prepared, where appropriate Tools such as Graphite or stearic acid as a lubricant and / or Molding aids and reinforcing agents such as microfibers made of glass, asbestos, silicon carbide or potassium titanate can be added. Suitable unsupported catalyst geometries are e.g. Solid cylinder or Hollow cylinder with an outer diameter and a length from 2 to 10 mm. In the case of the hollow cylinder, the wall thickness is 1 to 3 mm appropriate. Of course you can the full catalyst also have spherical geometry, the spherical diameter Can be 2 to 10 mm.

A particularly favorable hollow cylinder geometry is 5 mm × 3 mm × 2 mm (outer diameter × length × inner diameter), especially in the case of full catalysts.

Of course, the powdery active composition or its powdery, not yet and / or partially calcined, precursor composition can also be shaped by application to preformed inert catalyst supports. The coating of the support bodies for the production of the coated catalysts is generally carried out in a suitable rotatable container, such as that from the DE-A 2909671 , the EP-A 293859 or from the EP-A 714700 is known. To coat the carrier bodies, the powder mass to be applied is expediently moistened and, after application, for example by means of hot air, dried again. The layer thickness of the powder composition applied to the carrier body is expediently selected in the range from 10 to 1000 μm, preferably in the range from 50 to 500 μm and particularly preferably in the range from 150 to 250 μm.

Conventional 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 as carrier materials. As a rule, they are essentially inert with respect to the target reaction underlying the process according to the invention in the first reaction stage. The carrier bodies can have a regular or irregular shape, preference being given to regularly shaped carrier bodies with a clearly formed surface roughness, for example balls or hollow cylinders. It is suitable to use essentially non-porous, rough-surface, spherical supports made of steatite (for example steatite C220 from CeramTec), the diameter of which is 1 to 8 mm, preferably 4 to 5 mm. However, it is also suitable to use cylinders as carrier bodies, the length of which is 2 to 10 mm and the outside diameter is 4 to 10 mm. In the case of rings suitable as support bodies according to the invention, the wall thickness is moreover usually 1 to 4 mm. Annular support bodies to be preferably used according to the invention have a length of 2 to 6 mm, an outer diameter of 4 to 8 mm and a wall thickness of 1 to 2 mm. According to the invention, rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) are particularly suitable as carrier bodies. The fineness of the catalytically active oxide materials to be applied to the surface of the carrier body is of course adapted to the desired shell thickness (cf. EP-A 714 700 ).

Multimetal oxide active compositions suitable for the catalysts of the reaction stage are also compositions of the general formula II [Y ¹ _{a '} Y ² _{b '} O _{x '} ] P (Y ³ _c' Y ⁴ _{d '} Y ⁵ _e' Y ⁶ _{f '} Y ⁷ _g' Y ² _{h '} O _y' ] _q (II), in which the variables have the following meaning:
Y ¹ = only bismuth or bismuth and at least one of the elements tellurium, antimony, tin and copper,
Y ² = molybdenum or molybdenum and tungsten,
Y ³ = an alkali metal, thallium and / or samarium,
Y ⁴ = an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and / or mercury,
Y ⁵ = iron or iron and at least one of the elements chromium and cerium,
Y ⁶ = phosphorus, arsenic, boron and / or antimony,
Y ⁷ = a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and / or uranium,
a '= 0.01 to 8,
b '= 0.1 to 30,
c '= 0 to 4,
d '= 0 to 20,
e '> 0 to 20,
f '= 0 to 6,
g '= 0 to 15,
h '= 8 to 16,
x ', y' = numbers which are determined by the valency and frequency of the elements in II other than oxygen and
p, q = numbers whose ratio p / q is 0.1 to 10,
containing three-dimensionally extended areas of the chemical composition Y ¹ _{a '} Y ² _b' O _{x '} , which are separated from their local surroundings due to their composition which differs from their local surroundings, the largest diameter of which (longest connecting section through the center of gravity of the area of two on the surface ( Interface) of the area of located points) 1 nm to 100 μm, frequently 10 nm to 500 nm or 1 μm to 50 or 25 μm.

Particularly advantageous multimetal oxide compositions II according to the invention are those in which Y ^{1 is} only bismuth.

Among these, those are preferred which have the general formula III [Bi _{a ''} Z ² _{b ''} O _{x ''} ] _{p ''} [Z ² ₁₂ Z ³ _{c ''} Z ⁴ _{d ''} Fe _{e ''} Z ⁵ _{f ''} Z ⁶ _{g ''} Z ⁷ _{h ''} O _y'' ] _{q ''} (III) in which the variants have the following meaning:
Z ² = molybdenum or molybdenum and tungsten,
Z ³ = nickel and / or cobalt,
Z ⁴ = thallium, an alkali metal and / or an alkaline earth metal,
Z ⁵ = phosphorus, arsenic, boron, antimony, tin, cerium and / or lead,
Z ⁶ = silicon, aluminum, titanium and / or zirconium,
Z ⁷ = copper, silver and / or gold,
a '' = 0.1 to 1,
b '' = 0.2 to 2,
c '' = 3 to 10,
d '' = 0.02 to 2,
e '' = 0.01 to 5, preferably 0.1 to 3,
f '' = 0 to 5,
g '' = 0 to 10,
h '' = 0 to 1,
x '', y '' = numbers determined by the valency and frequency of the element other than oxygen in III,
p '', q '' = numbers whose ratio p '' / q '' is 0.1 to 5, preferably 0.5 to 2,
correspond, those masses III being very particularly preferred in which Z ² _{b ''} = (tungsten) _{b ''} and Z ² ₁₂ = (molybdenum) ₁₂ .

It is also advantageous if at least 25 mol% (preferably at least 50 mol% and particularly preferably at least 100 mol%) of the total fraction [Y ¹ _{a '} Y ² _b' O _{x '} ] _p [Bi _a'' Z ² _{b "} O _x" ] _{p "of} the multimetal oxide compositions II (multimetal oxide compositions III) which are suitable according to the invention in the multimetal oxide compositions II (multimetal oxide compositions III) which are suitable according to the invention in the form of three-dimensionally expanded chemical compositions which are different from their local surroundings because of their local surroundings delimited areas of the chemical composition [Y ¹ _{a '} Y ² _b' O _{x '} [Bi _a'' Z ² _b'' O _x'' ), the size of which is in the range 1 nm to 100 µm.

Regarding the shape applies in terms of Multimetal oxide masses II catalysts in the multimetal oxide masses I catalysts said.

The production of multimetal oxide II active materials is for example in the EP-A 575897 as well as in the DE-A 19855913 described.

It is essential according to the invention that the fixed bed catalyst bed consists of at least two spatially successive fixed bed catalyst bed zones, the volume-specific activity within a fixed bed catalyst bed zone being essentially constant and increasing suddenly in the direction of flow of the reaction gas mixture during the transition from a fixed bed catalyst bed zone into another fixed bed catalyst bed zone.

The volume-specific (i.e., the activity standardized to the unit of the respective bulk volume Fixed catalyst bed zone can now in over the fixed bed catalyst bed zone be set essentially constant way that one starts from a basic quantity of uniformly produced shaped catalyst bodies (their fill corresponds to the maximum achievable volume-specific activity) and this in the respective fixed bed catalyst bed zone with respect to the heterogeneously catalyzed partial gas phase oxidation essentially inert shaped bodies (Shaped diluent) homogeneous diluted. The higher the proportion of the shaped diluent is selected, the lower the amount contained in a certain volume of the bed Active mass or catalyst activity. As materials for such inert dilution tablets come in principle all those who are also considered as carrier 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. Steatit C-220 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 one becomes such as an inert shaped 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 total fixed bed catalyst bed not changed. That is, the for a single shaped catalyst body Active mass used can be a mixture of different, the Elements containing Mo, Fe and Bi, be multimetal oxides, for all shaped catalyst bodies Fixed catalyst bed however, the same mixture must then be used.

One in the flow direction of the reaction gas mixture over the Fixed catalyst bed Zone-specific increasing volume-specific activity can be achieved for the method according to the invention thus e.g. by adjusting the fill in a first fixed bed catalyst bed zone with a high Proportion of inert diluent tablets starts on a kind of shaped catalyst bodies, and then this Proportion of dilution tablets in flow direction reduced by zones.

A zone-wise increase in volume-specific activity but is also e.g. thereby possible that one with the same geometry and type of active mass Shaped coated catalyst body zone by zone the thickness of the active mass layer applied to the carrier elevated or in a mixture of shell catalysts with the same geometry but with different proportions by weight of the active composition Proportion of shaped catalyst bodies with higher Active mass weight percentage increases. Alternatively, you can also use the Dilute active materials yourself, by using e.g. active mass production into the one to be calcined Dry mixture of starting compounds inert diluting Incorporated materials such as high-fused silica. different Additional amounts of thinning lead effective material automatically to different activities. The more thinning Material is added, the lower the resulting activity will be. Analog effect leaves e.g. also achieved by using mixtures of full catalysts and from shell catalysts (with identical active mass) in a corresponding Way the mixing ratio changed. Furthermore lets a variation in volume-specific activity the use of catalyst geometries with different bulk densities achieve (e.g. with full catalysts with an identical composition of active materials of different geometries). It goes without saying that the described Use variants 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 in zones their composition varies and / or with different amounts inert dilution tablets are diluted.

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).

The shaped bodies used for such inert beds often have the annular geometry 7 mm × 7 mm × 4 mm (outer diameter × length × inner diameter) or the spherical geometry with the diameter d = 4 - 5 mm. The temperature zones A and B can also extend to the inert beds in the method according to the invention. According to the invention, both the temperature zone A and the temperature zone B advantageously do not record more than three fixed-bed catalyst bed zones (according to the invention, at least one fixed-bed catalyst bed zone is detected by both temperature zones).

Particularly advantageously comprises according to the invention the total fixed bed catalyst bed not more than five, expediently not more than four or three fixed bed catalyst bed zones.

When moving from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone (in the direction of flow of the reaction gas mixture) should (with a uniform active mass over the entire Fixed catalyst bed) the volume-specific active mass (i.e., the weight of the in the bulk volume contained multimetal oxide mass) expediently according to the invention by at least 5% by weight, preferably increase by at least 10% by weight (this applies in particular even with uniform shaped catalyst bodies over the entire fixed bed catalyst bed). As a rule, this increase in the method according to the invention not be more than 50% by weight, usually not more than 40% by weight. Furthermore, with a uniform active composition over the entire fixed bed catalyst bed Difference in the volume-specific active mass of that fixed bed catalyst bed zone with the lowest volume-specific activity and that fixed bed catalyst bed zone with the highest volume-specific activity advantageous according to the invention not more than 50% by weight, preferably not more than 40% by weight and usually not more than 30% by weight.

Frequently is in the inventive method the fixed bed catalyst bed consist of only two fixed bed catalyst bed zones.

According to the invention, this is preferred in the direction of flow of the reaction gas mixture last fixed bed catalyst bed zone undiluted. That is, it preferably consists exclusively of shaped catalyst bodies. in the If necessary, it can also consist of a bed of shaped catalyst bodies, their volume-specific activity e.g. by dilution lowered with inert material, e.g. by 10%.

Is the fixed bed catalyst bed only from two fixed bed catalyst bed zones, it is according to the invention in the Generally advantageous (as is very general in the method according to the invention) if the fixed bed catalyst bed zone with the highest volume-specific activity does not protrude into temperature zone A (especially if if in temperature zone A and in temperature zone B tempering by means of a flowing Heat transfer takes place, which flows in countercurrent to the reaction gas mixture). that is, in cheaper The fixed bed catalyst bed zone with the smaller becomes volume-specific activity protrude into the temperature zone B and the fixed bed catalyst bed zone with the higher volume-specific activity start and end in temperature zone B (i.e., after the transition from temperature zone A to temperature zone B).

Is the fixed bed catalyst bed only from three fixed bed catalyst bed zones, it is according to the invention in the Usually advantageous if the fixed bed catalyst bed zone with the highest volume-specific activity does not protrude into temperature zone A but in the temperature zone B begins and ends, i.e. after the transition from the temperature zone A begins in temperature zone B (especially if in temperature zone A and in temperature zone B tempering by means of a flowing Heat transfer takes place, which flows in countercurrent to the reaction gas mixture). That is, normally in this case the fixed bed catalyst bed zone with the second highest volume-specific activity both in temperature zone A and in temperature zone B protrude.

Is the fixed bed catalyst bed four fixed bed catalyst bed zones, it is according to the invention in the Usually advantageous if the fixed bed catalyst bed zone with the third highest volume-specific activity both in temperature zone A and in temperature zone B protrudes (especially if in temperature zone A and in temperature zone B the temperature is controlled by means of a flowing heat transfer medium, which flows in countercurrent to the reaction gas mixture).

In the case of direct current guidance from Reaction gas mixture and heat transfer media in the temperature zones A and B, it can be advantageous if the method according to the invention the fixed bed catalyst bed zone with the highest volume-specific activity protrudes into temperature zone A.

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 propene-containing reaction gas starting mixture guided becomes. The higher one The amount of propene converted shows the higher volume-specific activity.

bzw. im Bereich

bzw. im Be reich

kein Übergang von einer Festbettkatalysatorschüttungszone in eine andere Festbettkatalysatorschüttungszone befindet, wobei X der Ort (die Stelle) innerhalb der Festbettkatalysatorschüttung ist, an dem der Übergang von der Temperaturzone A in die Temperaturzone B erfolgt.If the total length of the fixed bed catalyst bed L is, it is advantageous according to the invention if there is in the area

or in the area

or in the area

there is no transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone, where X is the location within the fixed bed catalyst bed where the transition from temperature zone A to temperature zone B takes place.

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 1, a homogeneous mixture of shaped catalyst bodies and shaped diluent 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 5 to 40% by weight, or 10 to 40% by weight, or 20 to 40% by weight, or 25 to 35% by weight. Following this first Fixed bed catalyst bed zone is then advantageously 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 single (undiluted) bed of the same shaped catalyst bodies, the have also been used in the first zone.

The above is particularly true then when full catalyst rings are used as shaped catalyst bodies in the fixed bed catalyst bed or coated catalyst rings (especially those described in this document be mentioned as preferred) are used. Show with advantage in the context of the aforementioned structuring both the shaped catalyst bodies and also the dilution moldings at inventive method in essentially the ring geometry 5 mm × 3 mm × 2 mm (outer diameter × length × inner diameter).

The above also applies to if shell catalyst moldings are used instead of inert shaped diluent bodies whose active mass fraction is 2 to 15% by weight points lower lies as the active mass fraction at the end of the fixed bed catalyst bed, if appropriate shell catalyst molded body used.

A pure inert material fill, whose Length, based on the length the fixed bed catalyst bed, expedient 5 to Is 20%, conducts in the direction of flow of the reaction gas mixture usually the fixed bed catalyst bed. It is normally used as a heating zone for the reaction gas mixture.

Advantageously extends according to the invention now the fixed bed catalyst bed zone in the aforementioned fixed bed catalyst beds with the lower volume-specific activity still at 5 to 20%, often on 5 to 15% of their length into temperature zone B.

The temperature zone expediently extends according to the invention A also on a possibly applied advance payment Inert material.

The reaction stage of the process according to the invention is carried out in a manner which is expedient in terms of application technology, in a two-zone tube bundle reactor, as described, for example, in DE-A's 19910508, 19948523, 19910506 and 19948241. A preferred variant of a two-zone tube bundle reactor which can be used according to the invention is disclosed in US Pat DE-C 2830765 , But also those in the DE-C 2513405 , the US-A 3147084 , the DE-A 2201528 , the EP-A 383224 and the DE-A 2903218 The two-zone tube bundle reactors disclosed are suitable for carrying out the first reaction stage of the process according to the invention.

In other words, the fixed bed catalyst bed to be used according to the invention (possibly with upstream and / or downstream inert beds) is in the simplest manner in the metal tubes of a tube bundle reactor and two temperature-regulating media, generally molten salts, which are essentially spatially separated from one another, are guided around the metal tubes. According to the invention, the pipe section over which the respective salt bath extends represents a temperature zone. Ie, flows around in the simplest way Salt bath A is the section of the pipes (temperature zone A) in which the oxidative conversion of the propene (in a single pass) takes place until conversion in the range from 40 to 80 mol% takes place and a salt bath B flows around the section of the pipes ( temperature zone B), in which the oxidative subsequent conversion of the propene (in a single pass) takes place until a conversion value of at least 90 mol% is reached (if necessary, further temperature zones can be connected to the temperature zones A, B to be used according to the invention, which are based on individual Temperatures are maintained).

Appropriately applied in terms of application technology the reaction stage of the process according to the invention is no further Temperature zones. That is, the salt bath B expediently flows around the section of the pipes, in which the oxidative subsequent conversion of the propene (at single pass) up to a sales value ≥90 mol%, or ≥92 mol% or ≥94 mol% or does more.

Usually is the start of temperature zone B behind the hotspot maximum the temperature 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 temperature zone A is a co-current and in the temperature zone B a counter flow (or vice versa).

Of course, in all of the above-mentioned constellations, a transverse flow can still be superimposed on the parallel flow of the molten salt relative to the reaction tubes within the respective temperature zone; so that the single temperature zone like one in the EP-A 700714 or in the EP-A 700893 corresponds to the tube bundle reactor described and overall a longitudinal section through the contact tube bundle results in a meandering flow profile of the heat exchange medium.

The method according to the invention is expedient the reaction gas starting mixture of the fixed bed catalyst bed preheated the reaction temperature fed.

The contact tubes in the two-zone tube bundle reactors are usually made from ferritic steel and typically have a wall thickness of 1 to 3 mm. Their inner diameter is usually 20 to 30 mm, often 21 to 26 mm. Their length is expediently 2 to 4 m, preferably 2.5 to 3.5 m. In each temperature zone, the fixed bed catalyst bed occupies at least 60% or at least 75%, or at least 90% of the length of the zone. The remaining length, if any, may be occupied by an inert bed. Appropriately from an application point of view, the number of contact tubes accommodated in the tube bundle container is at least 5000, preferably at least 10,000. The number of contact tubes accommodated in the reaction vessel is often 15,000 to 30,000. Tube bundle reactors with a number of contact tubes above 40,000 are rather the exception. The contact tubes are normally arranged homogeneously distributed within the container (preferably 6 equidistant neighboring tubes per contact tube), the distribution being expediently chosen such that the distance between the central inner axes from the closest contact tubes (the so-called contact tube division) is 35 to 45 mm (cf. e.g. EP-B 468290 ).

Are suitable as heat exchange medium also for the two-zone procedure, in particular fluid temperature control media. Especially Cheap is the use of 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 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 flow rate within the two required heat exchange medium circuits selected that the temperature of the heat exchange medium from the entry point into the temperature zone to the exit point from the temperature zone (due to the exothermic nature of the reaction) 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, temperature zone A is normally in the range from 290 to 380 ° C, preferably in the range from 305 to 365 ° C. and particularly preferably in Range from 310 to 340 ° C or up to 330 ° C. With propene loads of the fixed bed catalyst bed of ≥90 Nl / l · h and <160 Nl / l · h, the entry temperature becomes of the heat exchange medium to temperature zone B according to the invention also in the range of 290 to 380 ° C, at the same time, however, according to the invention normally expediently ≥0 ° C to ≤20 ° C or ≤10 ° C, or ≥0 ° C and ≤5 ° C, or frequently ≥0 ° C and ≤3 ° C below the inlet temperature of the heat exchange medium entering temperature zone A. With propene loads of the fixed bed catalyst bed of ≥160 Nl / l · h and (usually) ≤300 Nl / l · h (or 600 Nl / lh) the inlet temperature of the heat exchange medium in the Temperature zone B also according to the invention in the range from 290 to 380 ° C, but normally according to the invention expediently> 0 ° C and ≤50 ° C, or ≥5 ° C and ≤45 ° C, or ≥10 ° C and ≤40 ° C, or ≥15 ° C and ≤30 ° C or ≤35 ° C (e.g. 20 ° C or 25 ° C) above the inlet temperature of the heat exchange medium entering temperature zone A.

At this point, it should also be pointed out once again that, in particular, that in FIG DE-AS 2201528 described two-zone tube bundle reactor type can be used, which includes the ability to remove a portion of the hotter heat exchange medium of the temperature zone B to the temperature zone A, in order to possibly heat up a cold reaction gas starting mixture or a cold circulating gas. Furthermore, the tube bundle characteristic can be within an individual temperature zone as in FIG EP-A 382098 described.

From the one leaving the reaction stage The product gas mixture can separate the acrolein in a manner known per se and the remaining gas at least in a manner known per se partly as a diluent gas be returned to the propene partial oxidation. In a convenient way however, it becomes heterogeneous for feeding a subsequent one catalyzed partial oxidation of the acrolein contained therein acrylic acid use. Zeckmäßigerweise it becomes one before entering such a subsequent reaction stage Cool down in a direct and / or indirect way, so as to achieve a full afterburn parts of the acrolein formed in the propene oxidation step to suppress.

Usually an aftercooler is connected between the two reaction stages. In the simplest case, this can be an indirect tube bundle heat transfer medium. Subsequently oxygen is usually supplemented in the form of air, the one for such Acrolein oxidation beneficial overstoichiometric Oxygen content available to deliver. Such an acrolein partial oxidation is advantageous in analogy to the method according to the invention carried out.

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

The process according to the invention is often used in the case of a propene: oxygen-inert gas (including water vapor) volume ratio in the reaction gas starting mixture 1 of 1: (1.0 to 3.0): (5 to 25), preferably 1: (1.7 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 vol.%, Or ≥40 vol.%, Or ≥50 vol.%, Or ≥60 vol.%, Or ≥70 vol.%, Or ≥80 vol .-%, or ≥90 vol .-%, or ≥95 vol .-% consist of molecular nitrogen (generally, in this document, inert diluent gases should be those that pass through the reaction step less than 5%, preferably less than 2%; in addition to molecular nitrogen, these include 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 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 acrolein and / or acrylic acid have been separated off (for example in the case of a downstream second oxidation stage) from the product gas mixture.

Reaction gas starting mixtures which are favorable according to the invention are, for example, also those which consist of 6 to 15 (preferably 7 to 11)% by volume propene, 4 to 20 (preferably 6 to 12)% by volume Water, ≥0 to 10 (preferably ≥0 to 5)% by volume components other than propene, water, oxygen and nitrogen, so much molecular oxygen that the molar ratio of molecular oxygen to propene contained is 1.5 to 2.5 (preferably 1.6 to 2.2), and the residual amount is composed of up to 100% by volume total of molecular nitrogen how they the DE-A 10302715 recommends.

At this point it should be noted that the multimetal oxide masses of the DE-A 10261186 are cheap.

Configurations of a two-zone tube bundle reactor for the reaction stage according to the invention which are favorable 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 Temperaturzone wird die Salzschmelze von unten nach oben geführt;
Das Reaktionsgasgemisch verlässt den Reaktor der erfindungsgemäßen Reaktionsstufe mit einer Temperatur wenige Grad höher als die Salzbadeintrittstemperatur. Das Reaktionsgasgemisch wird für die Weiterverarbeitung in einer nachfolgenden Acroleinoxidationsstufe zweckmäßigerweise in einem separaten Nachkühler, der dem Reaktor der 1. Stufe nachgeschaltet ist, auf 220°C bis 280°C, bevorzugt 240°C bis 260°C abgekühlt.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 in EP-A 873 783 and EP-A 12 70 065 described), which are loaded like the contact tubes (rotating like a screw from the very outside to the inside), for example of the same length and wall thickness, but with an outer diameter of, for example, 33.4 mm and a centered thermal sleeve of, for example, 8 mm outer diameter and, for example, 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 inlet 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 temperature zones (temperature zones) A (upper zone) and B (lower zone); each temperature zone is divided into 2 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 velocity 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 led from bottom to top over each temperature zone;
The reaction gas mixture leaves the reactor of the reaction stage according to the invention at a temperature a few degrees higher than the salt bath inlet temperature. For further processing in a subsequent acrolein oxidation stage, the reaction gas mixture is expediently cooled to 220 ° C. to 280 ° C., preferably 240 ° C. to 260 ° C., in a separate aftercooler, which is connected downstream of the first stage reactor.

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

Flow Control:

the reaction gas starting mixture expediently flows from top to bottom through the first stage reactor, 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 the DE-A 10046957 (Stoichiometry: [Bi ₂ W ₂ O ₉ × 2 WO ₃ ] _0.5 [Mo ₁₂ Co _5.5 Fe ₂ , ₉₄ Si _1.59 K _0.08 O _x ) ₁ ).

• 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 Beispie 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.

In the feed mentioned above, the unsupported catalyst from Example 1 can DE-A 10046957 can also be replaced by:

• a) a catalyst according to Example 1 c from the 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 the DE-A 19855913 as a hollow cylinder full catalyst with a geometry of 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 the DE-A 10063162 , but with the same shell thickness applied to carrier rings with a geometry of 5 mm × 3 mm × 1.5 mm or 7 mm × 3 mm × 1.5 mm.

In addition, the fixed bed catalyst bed is expediently chosen according to the invention (for example by dilution with, for example, inert material) such that the temperature difference between the hotspot maximum of the reaction gas mixture in the individual temperature zones and the respective temperature in the temperature zone generally does not exceed 80.degree. This temperature difference is usually ≤70 ° C, often it is 20 to 70 ° C, preferably this temperature difference is small. In addition, the fixed bed catalyst bed is chosen for safety reasons in a manner known per se to the person skilled in the art (for example by dilution with, for example, inert material) such that the "peak-to-salt-temperature sensitivity" as defined in the EP-A 1106598 ≤ 9 ° C, or ≤7 ° C, or ≤5 ° C, or ≤3 ° C.

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: 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 with the 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 the DE-A 10046957 (Stoichiometry: [Bi ₂ W ₂ O ₉ × 2WO ₃ ] _0.5 [Mo ₁₂ Co _5.5 Fe _2.94 Si _1.59 K _0.08 O _x ] ₁ ).

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.

The first reaction stage described above is fed continuously with a reaction gas starting mixture 1 of the following composition, wherein
the load and the thermostatization of the first reaction tube can be varied: 6 to 6.5 vol.% propene,
3 to 3.5% by volume H ₂ O,
0.3 to 0.5 vol.% CO,
0.8 to 1.2 vol% CO ₂ ,
0.01 to 0.04% by volume of acrolein,
10.4 to 10.7% by volume of O ₂ and
as a residual amount of 100% molecular 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 from the chosen one Propene load between 2.3 and 3.1 bar.

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

The results depend The table below shows the loads and salt bath temperatures 1 (the letter B in parentheses means example).

T _A , T _B stand for the temperatures of the pumped salt baths in temperature zones A and B.

U P / A is the propene turnover at the end of the Temperature zone A in mol%.

U P / B is the propene turnover at the end of the Temperature zone B in mol%.

S ^AC is the selectivity of acrolein formation in the product gas mixture based on converted propene in mol%.

S ^AA is the selectivity of acrylic acid by-product formation in the product gas mixture based on converted propene in mol%.

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

Table 1

Comparative Examples

Everything is done as in the examples. However, the length of the feed sections of the fixed bed catalyst feed are changed as follows:
Section 2: 125 cm instead of 140 cm.
Section 3: 175 cm instead of 160 cm.

The results are shown in Table 2. (V) means comparative example. Table 2

In comparison with the results in Table 1, lower S ^{AC are} achieved with comparable UP / B values.

Claims (18)

Process of heterogeneously catalyzed partial gas phase oxidation of propene to acrolein, in which a reaction gas starting mixture containing propene, molecular oxygen and at least one inert gas, which contains the molecular oxygen and the propene in a molar ratio O ₂ : C ₃ H ₆ ≥1, in one reaction step with the proviso of a fixed bed catalyst bed, the active mass of which is at least one multimetal oxide containing the elements Mo, Fe and Bi, leads to the fact that the fixed bed catalyst bed is arranged in two spatially successive temperature zones A, B, both the temperature of temperature zone A and the temperature temperature zone B is a temperature in the range from 290 to 380 ° C., - The fixed bed catalyst bed consists of at least two spatially successive fixed bed catalyst bed zones, the volume-specific activity within a fixed bed catalyst bed zone being essentially constant and in the direction of flow of the reaction gas mixture during the transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone, the temperature of the zone A increases rapidly to one Propene extends from 40 to 80 mol%, - with a single passage of the reaction gas starting mixture through the entire fixed bed catalyst bed, the propene conversion ≥90 mol% and the selectivity of acrolein formation, based on converted propene, is ≥90 mol%, - which time sequence in which the reaction gas mixture flows through the temperature zones A, B, corresponds to the alphabetical sequence of the temperature zones, - the loading of the fixed bed catalyst bed with that in the reaction gas starting propylene contained ≥90 Nl propene / l fixed bed catalyst bed · h, and - the difference T ^maxA - T ^maxB , formed from the highest temperature T ^max , which the reaction ^gas mixture has within the temperature ^zone A, and the highest temperature T ^maxB , which Reaction gas mixture within temperature zone B is ≥0 ° C, characterized in that the transition from temperature zone A to temperature zone B in the fixed bed catalyst bed does not coincide with a transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone. A method according to claim 1, characterized in that T ^maxA - T ^{maxB is} ≥3 ° C and ≤70 ° C. A method according to claim 1, characterized in that T ^maxA - T ^{maxB is} ≥20 ° C and ≤60 ° C. A method according to claim 1 to 3, characterized in that the propene loading of the fixed bed catalyst bed is ≥90 Nl / l · h and <160 Nl / l · h. Method according to one of claims 1 to 3, characterized in that that the propene load of the fixed bed catalyst bed ≥160 Nl / l · h and ≤300 Nl / l h is. Method according to one of claims 1 to 5, characterized in that that the temperature of reaction zone A is 305 to 365 ° C. Method according to one of claims 1 to 5, characterized in that that the temperature of reaction zone A is 310 to 340 ° C. Method according to one of claims 1 to 7, characterized in that that the temperature zone A up to a propene conversion of Extends from 50 to 70 mol%. Method according to one of claims 1 to 8, characterized in that that the temperature zone A up to a propene conversion of 60 to 70 mol .-% extends. Method according to one of claims 1 to 9, characterized in that the ratio O ₂ : propene in the reaction gas starting mixture is 1 to 2. Method according to one of claims 1 to 10, characterized in that that the chemical composition of the active mass used is above the total fixed bed catalyst bed unchanged is. Method according to one of claims 1 to 11, characterized in that that the total fixed bed catalyst bed no more than four fixed bed catalyst bed zones includes. Method according to one of claims 1 to 12, characterized in that that with a uniform active mass over the entire fixed bed catalyst bed during the transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone in the direction of flow of the reaction gas mixture the volume-specific active mass by at least 5 wt .-% increases. Method according to one of claims 1 to 12, characterized in that that with a uniform active mass over the entire fixed bed catalyst bed during the transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone in the direction of flow of the reaction gas mixture the volume-specific active mass by at least 10 wt .-% increases. Method according to one of claims 1 to 14, characterized in that that with a uniform active mass over the entire fixed bed catalyst bed Difference in the volume-specific active mass of that fixed bed catalyst bed zone with the lowest volume-specific activity and that fixed bed catalyst bed zone with the highest volume-specific activity is not more than 40% by weight. Method according to one of claims 1 to 15, characterized in that that the flow direction of the reaction gas mixture last fixed bed catalyst bed zone undiluted is and only from shaped catalyst bodies consists. Method according to one of claims 1 to 16, characterized in that that the fixed bed catalyst bed zone with the highest volume-specific activity does not protrude into temperature zone A. Method according to one of claims 1 to 17, characterized in that in the fixed bed catalyst bed in the area

there is no transition from a fixed bed catalyst bed zone to another fixed bed catalyst bed zone, where L is the length of the fixed bed catalyst bed and X is the location within the fixed bed catalyst bed where the transition from temperature zone A to temperature zone B takes place.