MXPA01008963A - Method for the catalytic gas phase oxidation of propene into acrylic acid - Google Patents

Abstract

The invention relates to a method for the catalytic gas phase oxidation of propene into acrylic acid in which the initial mixture of reaction gas, with an increased propene charge, is oxidized on a first fixed-bed catalyst during a first reaction step, and the mixture of product gas which contains acrolein and which is of the first reaction step is, under an increased acrolein charge, subsequently oxidized on a second fixed-bed catalyst during a second reaction step, whereby the catalyst shaped bodies comprise an annular geometry in both reaction steps.

Description

METHOD OF CATALYTIC OXIDATION IN GASEOUS PHASE OF PROPENO IN ACRYLIC ACID The present invention relates to a process for the catalytic oxidation, in gas phase, of propene to acrylic acid, in which an initial mixture of the gas is reaction 1 consisting of propene, molecular oxygen and at least one inert gas, and contains molecular oxygen and propene in a molar ratio 02: C3H6 of > 1 first passes, in a first stage reaction at elevated temperatures, on a first fixed bed catalyst whose active material is at least one multimetal oxide containing molybdenum and / or tungsten and bismuth, tellurium, antimony, tin and / or copper, in such a way that the conversion of propene in a single step is > 90% molar, and the associated selectivity of acrolein formation and the formation of the acrylic acid byproduct together is > 90% molar, the temperature of the gas mixture product leaving the first reaction stage, if required, is reduced by direct and / or indirect cooling and, if required, molecular oxygen and / or inert gas is added to the gaseous product mixture, and the product gas mixture, as the initial mixture of the reaction gases 2 consisting of acrolein, molecular oxygen and at least one inert gas, and containing the molecular oxygen and acrolein in a molar ratio 02: C3H40 of > 0.5, then passes, in a second stage of reaction at elevated temperatures, on a second fixed-bed catalyst, whose active material is at least one multimetal oxygen consisting of molybdenum and vanadium, in such a way that the conversion of acrolein into a single step is > 90% molar and the selectivity of the balanced acrylic acid formation over the reaction steps is > 80% molar, based on converted propene.

The aforementioned process for catalytic gas phase oxidation of propene to acrylic acid is usually known (see for example DE-A 3002829). In particular, the two reaction steps are known per se (see, for example, EP-A 714700, EP-A 700893, EP-A 15565, DE-C 2830765, DE-C 3338380, JP-A 91/294239, EP-A 807465, WO 98/24746, EP-B 279374, DE-C 2513405, DE-A 3300044, EP-A 575897 and DE-A 19855913).

Acrylic acid is an important monomer that is used as such or with alkyl esters to produce, for example, suitable polymers as adhesives.

The objective of each of the two stages of oxidation in the gas phase, in fixed bed from propene to acrylic acid, is, in principle, to obtain a very high space-time yield of acrylic acid (STYAA) (in a continuous process, ie , the total amount of acrylic acid, in liters, produced per hour and the total volume of the catalyst bed used).

Therefore, there is a general interest to carry out gas phase oxidation, in a fixed bed, in two stages, from propene to acrylic acid on the one hand with a very high load of the first fixed-bed catalyst with propene (which must be understand as the amount of propene in liters (-PTN) (= 1 (PTN), the volume in liters that would occupy the corresponding amount of propene under normal conditions of temperature and pressure, that is, at 25 ° C and 1 baria) it passes as a component of the initial mixture of the reaction gases 1 for one hour through a liter of the catalyst bed 1) and, on the other hand, with a very high load of the second fixed catalyst bed with acrolein (this is understood as the amount of acrolein in liters (PTN) (= 1 (PTN), the volume in liters that would occupy the corresponding amount of acrolein under normal conditions of temperature and pressure, that is, at 25 ° C and 1 baria) that passes as a component of the reaction mixture 2 for one hour through a liter of the catalyst bed 2) without significantly deteriorating the conversion of propene and acrolein which takes place during a single step of the two initial mixtures of reaction gases 1, 2 through the fixed catalyst beds and the selectivity, balanced in the reaction stages, of the formation of the associated acrylic acid (based on the converted propene).

The execution of the aforementioned is adversely affected by the fact that the oxidation in gas phase, in fixed bed from propene to acrolein and oxidation in gas phase, in fixed bed of acrolein to acrylic acid are highly exothermic on the one hand and, on the other hand, they are accompanied by a variety of possible simultaneous and subsequent reactions.

In principle, the multimetal oxides suitable as active catalytic materials can be used in the form of a powder in the gas phase oxidation, in a fixed bed from propene to acrylic acid described at the beginning. Usually, however, these are formed in specific catalyst geometries before being used, since the powder of the active material is unsuitable for industrial use due to the bad gas permeation.

An object of the present invention is to select the geometry of the molded parts of the catalyst to be used in the two fixed catalytic beds of the gas phase oxidation, in a fixed bed, catalytic described at the beginning so that, with high loads of the fixed catalyst beds with the initial materials and a certain conversion of the raw materials, a very high selectivity of the desired composite acrylic acid of origin in a single step through the fixed catalyst beds.

As a basis it is possible to use the following prior art.

The traditional processes for catalytic oxidation in the gas phase, in a fixed bed from propene to acrolein or from acrolein to acrylic acid, where nitrogen is used as a main component of the inert diluent gas, and a fixed-bed catalyst present in the reaction zone and homogeneous throughout this reaction zone, ie, having a uniform chemical composition on the fixed catalyst bed, and the temperature of the reaction zone is maintained at a uniform value over the reaction zone (the The temperature of a reaction zone is understood here as the temperature of the fixed catalyst bed present 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 the reaction zone in this case means the average number of the temperature of the catalyst bed along the reaction zone. ón), limits the applicable charge of the propene or acrolein of the fixed catalyst bed to comparatively low values.

Thus, the propene load used in the fixed catalyst bed is usually < 155 liters (P.T.N.) of propene / liter of the catalyst bed per hour (see, for example, EP-A 15565 (maximum propene load = 120 liters) (propene P.T.N.) / liter per hour), DE-C 2830765 (maximum propene load = 94.5 liters (P.T.N.) of propene / liter per hour), EP-A 804465 (maximum propene load = 128 liters (P.T.N.) of propene / liter per hour), EP-A 279374 (maximum load of propene = 112 liters (PTN) of propene / liter per hour), DE-C 2513405 (maximum propene load = 110 liters (PTN) of propene / liter per hour), DE-A 3300044 (maximum propene load = 112 liters (PTN) of propene / liter per hour), and EP-A 575897 (maximum propene load = 120 liters (PTN) of propene / liter per hour).

Furthermore, in almost all examples of DE-C 3338380, the maximum propene load is 126 liters (PTN) of propene / liter per hour: only in Example 3 of this publication is a propene load of 162 liters (PTN) / liter per hour performed, the molded parts of catalyst used being exclusively solid cylinders consisting of active material and with a length of 7 mm and a diameter of 5 mm.

EP-B 450596 discloses a propene loading of the catalyst bed of 202.5 liters (P.T.N.) of propene / liter per hour, with the use of a structured catalyst bed in an otherwise traditional process. The molded catalyst parts used are coated, spherical catalysts.

EP-A 293224 likewise describes propene loads above 160 liters (P.T.N.) of propene / liter per hour, using an inert diluent gas completely free of molecular nitrogen. The geometry of the catalyst used is not mentioned.

EP-A 253409 and the associated equivalent, EP-A 257565, disclose that, when an inert diluent gas having a molar heat capacity greater than molecular nitrogen is used, the proportion of propene in the initial mixture of reaction gases may increase. However, in the two aforementioned publications also the maximum propene load made from the catalyst bed is 140 liters (P.T.N.) of propene / liter per hour. None of the publications offers any information related to the geometry of the catalyst used.

In a form corresponding to the traditional processes for catalytic oxidation in the gas phase, in a fixed bed from propene to acrolein, the traditional processes for catalytic oxidation in the gas phase, in a fixed bed from acrolein to acrylic acid, also usually limit the load of acrolein from the fixed catalyst bed to < 150 liters (P.T.N.) of acrolein / liter of catalyst per hour (see, for example, EP-B 700893, the molded parts of catalyst used are coated, spherical catalysts).

Oxidations in the gas phase, in two stages from propene to acrylic acid, in which the two oxidation phases are operated with a high charge of propene and a high acrolein charge of the respective fixed-bed catalyst, are practically unknown in the art previous.

Among the exceptions are EP-A 253409 and the associated equivalent, EP-A 257565, already cited. Another exception is EP-A 293224 already mentioned, the coated, spherical catalysts being used in the oxidation step of acrolein.

The fundamental possibility of using annular geometry for the catalyst molded parts to be used in the two pertinent oxidation steps is known from the prior art.

For example, the ring geometry in DE-A 3113179 is suggested very generally for exothermic oxidation in gas phase, in fixed bed, from the point of view of "very small pressure drop". However, this publication points out that the influence of the geometries on the selectivity of the product formation can differ from one reaction to another.

Otherwise, these catalyst molded parts having ring geometry for the gas phase catalytic oxidation of propene and / or acrolein are described, for example, in EP-A 575897, DE-A 19855913 and EP-A 700893. In all However, the use of catalytic molded parts having ring geometry was limited to low loads of the raw material.

The present invention, therefore, relates to a process for the catalytic gas-phase oxidation of propene to acrylic acid, in which an initial mixture of reaction gases 1 comprising propene, molecular oxygen and at least one inert gas, and contains molecular oxygen and propene in a molar ratio 02: C3H6 of > 1 first passes, in a first stage of reaction at elevated temperatures, on a first fixed-bed catalyst whose active material is at least one multimetal oxide containing molybdenum and / or tungsten and bismuth, telum, antimony, tin and / or copper, in such that the conversion of propene in a single step is > 90% molar, and the associated selectivity of acrolein formation and the formation of the acrylic acid byproduct together is > 90% molar, the temperature of the gas mixture product leaving the first reaction stage, if required, is reduced by direct and / or indirect cooling and, if required, molecular oxygen and / or inert gas is added to the gaseous product mixture, and the product gas mixture, as the initial mixture of reaction gases 2 containing acrolein, molecular oxygen and at least one inert gas and contains the molecular oxygen and acrolein in a molar ratio 02: C3H4? of >0.5, then passes, in a second stage of reaction at elevated temperatures, on a second fixed-bed catalyst whose active material is at least one multimetal oxide containing molybdenum and vanadium, in such a way that the conversion of acrolein in a single step is > 90% molar, and the selectivity of the balanced acrylic acid formation over the reaction steps is > 80% molar, based on the converted propene, where: a) the loading of the first fixed-bed catalyst with the propene contained in the initial mixture of reaction gases 1 is > 160 liters (S. T. P.) of propene / liter of the catalyst bed per hour b) the loading of the second fixed-bed catalyst with the acrolein contained in the initial mixture of reaction gases 2 is > 140 liters (STP) of acrolein / liter of catalyst bed per hour, and c) the geometry of the catalytic molded parts of the first fixed-bed catalyst and the geometry of the catalytic molded parts of the second fixed-bed catalyst are ring-shaped, with the condition of what: - the external diameter of the ring is from 2 to 11 mm - the length of the ring is from 2 to 11 mm, and - the thickness of the wall of the ring is from 1 to 1 mm.

Suitable annular geometries according to the invention for the catalyst molded parts of the two fixed catalyst beds include those described in EP-A 184790, DE-A 3113179, DE-A 3300044 and EP-A 714700.

In addition, the annular catalyst molded parts, according to the invention, of both reaction stages can be unsupported catalysts (consisting exclusively of the catalytically active multimetal oxide material), coated catalysts (an annular support has a layer of catalytic multimetal oxide material). active adsorbed on its outer surface) or supported catalysts (an annular support contains catalytically active multimetal oxide material adsorbed).

In the novel process, the ringless supported catalysts are preferably used in the first reaction stage and the ring catalysts coated in the second reaction stage. However, of course it is also possible to use the combinations "coated catalyst / catalyst without support" or "catalyst without support / catalyst without support" or "coated catalyst / coated catalyst" in the two successive reaction stages.

According to the invention, the preferred coated catalysts are those whose support rings have a length from 2 to 10 mm (or from 3 to 6 mm), an external diameter from 2 to 10 mm (or from 4 to 8 mm) and a wall thickness from 1 to 4 mm (or from 1 to 2 mm). Very particularly preferably, the support rings measure 7 mm x 3 mm x 4 mm (external diameter x length x internal diameter). The thickness of the catalytically active oxide material applied as a layer to the annular support is, in general, from 10 to 1000 μ, very preferably from 50 to 500 μ, particularly preferably from 100 to 500 μ, very particularly preferably from 150 to 500 μm. 250 μ.

The claims made in the above with respect to the preferred geometry of the supporting rings of the convenient coated catalysts according to the invention are applied in the same way to the suitable supported catalysts according to the invention.

In the case of suitable unsupported catalyst rings according to the invention, those of which the internal diameter is from 0.1 to 0.7 times in external diameter and the length is from 0.5 to 2 times the outer diameter are particularly convenient.

Suitable unsupported catalyst rings that can be used according to the invention have an external diameter of from 2 to 10 mm (or from 3 to 7 mm), an internal diameter of at least 1.0 mm, a wall thickness from 1 to 2 mm (or not more than 1.5 mm) and a length from 2 to 10 mm (or from 3 to 6 mm). In the cases of the suitable unsupported catalyst rings according to the invention, the external diameter is often from 4 to 5 mm, the internal diameter is from 1.5 to 2.5 mm, the wall thickness from 1.0 to 1.5 mm and the length from 2 to 6 mm.

This means that the catalysts without support in the shape of a hollow cylinder measures (in each case the external diameter per height per internal diameter) 5 mm x 3 mm x 2 mm, 5 mm x 2 mm x 2 mm, 5 mm x 3 mm x 3 mm, 6 mm x 3 mm x 3 mm or 7 mm x 3 mm x 4 mm.

The fixed-bed catalysts 1, suitable for the novel process, are all those whose active material is at least one multimetal oxide containing Mo, Bi and Fe.

This means that, in principle, all those multimetal oxides which are described in 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 formula II), JP-A 91/294239, EP-A 293224 and EP-A 700714 can be used as active materials for fixed-bed catalysts 1. This applies, in particular, to exemplary embodiments in these publications, among which EP-A 15565, EP-A 575897, DE-A 19746210 and DE are particularly preferred. -A 19855913. Particularly notable in this context are the multimetal oxide active material according to Example lc of EP-A 15565 and an active material which has to be prepared in a corresponding form but having the composition o12Ni6.5Zn2Fe2Bi1P0.0065K0. 0eOx • 10SiO2.

Suitable fixed-bed catalysts according to the invention which can be chosen are the example with serial number 3 of DE-A 19855913 (stoichiometry:? 2Co7Fe3Bio.6 o.o8Si? 6? X) as a catalyst without support in the form of hollow cylinders (rings) and measuring 5 mm? 3 mm x 2 mm (external diameter x length x internal diameter) and the catalyst without support consisting of multimetal oxide II according to Example 1 of DE-A 19746210. Other examples would be the unsupported multimetal oxide catalysts, annular of US- A 4438217. The latter applies in particular when these hollow cylinders measure 5 mm x 2 mm x 2 mm or 5 mm x 3 mm x 2 mm or 6 mm x 3 mm x 3 mm or 7 mm x 3 mm x 4 mm ( in each case external diameter x length x internal diameter).

A large number of suitable multimetal oxide active materials for the fixed bed catalysts 1 can be represented by the formula I: where X1 is nickel or cobalt, or both X2 is thallium, an alkali metal or an alkaline earth metal, - or both X3 is zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and / or tungsten X4 is silicon, aluminum, titanium and / or zirconium a is from 0.5 to 5 b is from 0.01 to 5, preferably from 2 to 4 c is from 0 to 10, preferably from 3 to 10, d is from 0 to 2, preferably from 0.02 to 2, e is from 0 to 8, preferably from 0 to 5 f is from 0 to 10, and n is a number that is determined by the valence and frequency and two different elements of oxygen in I.

These can be obtained in a manner known per se (see, for example DE-A 4023239) and according to the invention are, for example, any form as it may be in rings or used in the form of coated, ring catalysts, ie , inert supports preformed in rings and coated with the active material.

In principle, the active materials suitable for the fixed-bed catalysts 1, in particular active materials of the formula I, can be prepared in a simple form by producing, from suitable sources of their elementary constituents, an anhydrous, very intimate mixture of finely divided preference with a composition corresponding to its stoichiometry and calcining the anhydrous mixture at a temperature from 350 to 650 ° C. The calcination can be carried out in an inert gas or in an oxidizing atmosphere, for example air (mixture of inert gas and oxygen) or in a reducing atmosphere (for example, a mixture of inert gas, NH3, CO and / or H2). The duration of the calcination can be from a few minutes to a few hours and usually decreases with temperature. Suitable sources of 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, suitable starting compounds of this type are in particular halides, nitrates, formations, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and / or hydroxides (compounds such as NHOH, (NH4) C03, NH4N03 , NH4CH02, CH3COOH, NHCH3C02 and / or ammonium oxalate, which decomposes and / or can be decomposed at least during the subsequent calcination to obtain compounds that escape completely in gaseous form, can also be incorporated into the intimate anhydrous mixture).

The intimate mixing of the initial compounds for the preparation of multimetal oxide materials I can be carried out in anhydrous or in wet form. If it is carried out in anhydrous form, the initial compounds are conveniently used in the form of finely divided powders and, after mixing and any compaction, they are subjected to calcination. However, the intimate mixing preferably takes place in a moist form. Usually, the initial compounds are mixed together in the form of a solution and / or aqueous suspension. Particularly intimate anhydrous mixtures are obtained in the mixing method described when only dissolved sources of the constituents-elementals are used as raw materials. A preferred solvent used is water. The aqueous material that is obtained is then dried, the drying process preferably being effected by spray drying the aqueous mixture at an exit temperature of from 100 to 150 ° C.

Suitable multimetal oxide materials for the novel fixed-bed catalysts 1, in particular those multimetal oxide materials of the formula I, are formed in an annular catalyst geometry before being used for the novel process, it being possible to carry out the formation before or after the final calcination. For example, the unsupported, ring-shaped catalysts can be prepared from the powder form of the active material or its uncalcined and / or partially calcined precursor material by compaction to obtain the desired catalyst geometry (for example by extrusion), possible, if required, add auxiliaries, for example graphite or stearic acid as lubricants and / or auxiliaries for molding and reinforcing agents, such as glass microfibers, asbestos, silicon carbide or potassium titanate.

Of course, the shaping of the pulverulent active material or its pulverulent, uncalcined and / or partially calcined precursor material can also be effected by applying it to inert catalyst supports which are preformed into rings. The coating of the annular supports for the preparation of the coated catalysts is, as a rule, carried out in a suitable rotary vessel, as described for example in DE-A 2909671, EP-A 293859 or EP-A 714700. To coat the annular supports the powder material to be applied or the support is wetted and, after application, dried again, for example by means of hot air. The thickness of the cover of the powder material applied to the annular support is suitably chosen to be from 10 to 1000 μ, preferably from 50 to 500 μ, particularly preferably from 150 to 150 μ.

Suitable support materials are aluminas, silicas, thorium dioxide, zirconium dioxide, silicon carbide or silicates, such as porous or non-porous traditional magnesium silicate or aluminum silicate. Brackets that have substantial rough surface are preferred. These ring-shaped, virtually non-porous steatite supports having a rough surface are convenient. It is convenient to use annular cylinder supports whose length is from 2 to 10 mm and whose external diameter is from 4 to 10 mm. The wall thickness is usually from 1 to 4 mm. The annular supports to be used according to the invention preferably have a length from 3 to 6 mm, an external diameter from 4 to 8 mm and a wall thickness from 1 to 2 mm. Other supports particularly suitable according to the invention are rings measuring 7 mm x 3 mm x 4 mm (external diameter x length x internal diameter). The fineness of the catalytically active oxide material to be applied to the surface of the support, of course, is adapted to the desired thickness of the coating (see EP-A 714 700).

Otherwise, for the purpose of forming, the annular support can also be impregnated with a solution and / or suspension containing the initial compounds of the elementary constituents of the relevant multimetal oxide material, dried and finally calcined, as described, to give the catalysts with support.

The advantageous multimetal oxide active material to be used according to the invention for the fixed bed catalysts 1 are also materials of the formula II X a'Y b'Ox '] p [Y c'Y d'Y e'Y f'Y g? h'Oy '] < (II) where Y1 is bismuth, tellurium, antimony, tin and / or copper And it's molybdenum and / or tungsten Y3 is an alkali metal, thallium and / or samarium Y4 is an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and / or mercury Y5 is iron, chromium, cerium and / or vanadium Y6 is phosphorus, arsenic, boron and / or antimony Y7 is a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and / or uranium a 'is from 0.1 to 8 b 'is from 0.1 to 30 c 'is from 0 to 4 d 'is from 0 to 20 e 'is from 0 to 20 f is from 0 to 6 g 'is from 0 to 15 h' is from 9 to 16 x 'and x' are numbers that are determined by the valence and frequency of the elements other than oxygen in II and p and q are numbers whose ratio p / q is from 0.1 to 10 which contain three-dimensional regions that are delimited from their local environment due to their composition different from the local environment and have the chemical composition Y1aX2i > < Ox > and whose maximum diameter (largest distance passing through the center of gravity of the region and connecting two points present on the surface (interface) of the region) is from 1 nm to 100 μ, often from 10 nm to 500 nm or from 1 μ to 50 or 25 μ.

The novel multimetal oxide materials II, particularly advantageous, are those in which Y is bismuth.

Preferred among these, in turn, are those of the formula III [Bi • z2b.o, [Z 12Z c 'z4d. Fee 'Z f "Z g" Z h "Oy"] (or: where Z2 is molybdenum and / or tungsten Z3 is nickel and / or cobalt Z4 is thallium, an alkali metal and / or an alkaline earth metal Z5 is phosphorus, arsenic, boron, antimony, tin, cerium and / or lead Z6 is silicon, aluminum, titanium and / or zirconium Z7 is copper, silver and / or gold a "is from 0.1 to 1 b "is from 0.2 to 2 c "is from 3 to 10 d "is from 0.2 to 2 e" is from 0.1 to 5, preferably from 0.1 to 3 f "is from 0 to 5 g "is from 0 to 10 h "is from 0 to 1 x "y and" are numbers that are determined by the valence and frequency of the different elements of oxygen in III, and p "and q" are numbers whose relation p "/ q" is from 0.1 to 5, preferably from 0.5 to 2.

The materials III are very particularly preferred being those in which Z2b- is (tungsten) b "and Z2i2 is (molybdenum)? 2.

It is also advantageous if at least 25 mol% (preferably at least 50 mol%, most preferably at least 100 mol%) of the total amount [Y1a'Y2bOx-] p ([Bia "Z2b" O? "] P») of multimetal oxide materials II (multimetal oxide materials III) suitable according to the invention as fixed-bed catalysts I are present in the multimetal oxide materials II (multimetal oxide materials III) suitable according to the invention in the form of three-dimensional regions which are delimited from their local environment due to their chemical composition different from their local environment and have the chemical composition Y1a'Y2OX '([Bia "Z2b "0X"] and whose maximum diameter is from 1 nm to 100 microns.

With respect to shaping, the statements made in relation to the catalysts comprising metal oxide materials I are applicable with respect to the catalysts comprising multimetal oxide materials -II.

The preparation of the active material comprising multimetal oxide materials II is described, for example, in EP-A 575897 and DE-A 19855913.

Usually, the first reaction step of the novel process is carried out in a tubular bundle reactor loaded with the ring catalysts, as described for example in EP-A 700714.

In the simplest form, this means that the fixed bed catalyst 1 to be used according to the invention is present in the metal tubes of a tubular bundle reactor, and a thermostatic medium (operation in a zone), as a general rule a saline melt, is passed around the metal tubes. The salt melt and the reaction gas mixture can be fed co-current or counter-current in a simple manner. The salt melt (the thermostatic medium) can, however, be fed in a meandering manner around the tubular bundles, considered on the reactor, so that only when considering the reactor above all there is a co-current or countercurrent with with respect to the flow direction of the reaction gas mixture. The flow velocity of the thermostatic medium (heat exchange medium) is usually established such that the temperature increase (due to the exothermic nature of the reaction) of the heat exchanger medium from the point of entry to the reactor to the exit point of the reactor is from > 0 to 10 ° C, frequently from > 2 to 8 ° C, or from > 3 to 6 ° C. The temperature of the heat exchanging medium at the inlet of the tubular bundle reactor is, as a rule, from 310 to 360 ° C, often from 320 to 340 ° C.

The heat exchange medium, particularly convenient, are fluid thermostatic means. The use of fluids of salts, such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or metals that have a low melting point, such as sodium, mercury and alloys of different metals, is particularly advantageous .

The initial mixture of reaction gases 1 of the catalyst bed 1 is conveniently preheated to the temperature of the reaction before being fed. In the novel variant of the first reaction stage described above, this is often from 310 to 360 ° C, usually from 320 to 340 ° C.

A convenient form with respect to the technology of the application, the first reaction stage of the novel process is carried out in a tubular bundle reactor, in two zones. A preferred variant of a tubular bundle reactor in two zones which can be used according to the invention is described in DE-C 2830765. However, the tubular bundle reactors in two zones described in DE-C 2513405, US-A 3147084 , DE-A 2201528, EP-A 383224 and DE-A 2903218 are also suitable for carrying out the first reaction step of the novel process.

In the simplest form, this means that the fixed bed catalyst 1 to be used according to the invention is present in the metallic tubes of a tubular bundle reaction, and two thermostatic means in spaces practically separated from each other, as a general rule , saline fused, pass around the metal tubes. According to the invention, the tubular section on which the respective salt bath extends represents a reaction zone. In a simpler way, this means that a saline bath A flows around this section of the tubes (the reaction zone A) in the Gual where the oxidative reaction of the propene takes place (in the single step) until a conversion is obtained from 40 to 80 mole% and a salt bath B flows around this section of the tubes (the reaction zones B) in which the subsequent oxidative reaction of the propene takes place (in a single step) until a conversion of at least 90 mole% (if required, the reaction zones A, B to be used according to the invention can be followed by other reaction zones which are maintained at individual temperatures).

In terms of the application technology, the first reaction step of the novel process conveniently comprises no further reaction zones, that is, the salt bath B flows conveniently around this section of the tubes in which the subsequent oxidative reaction of the propene (in one step) until a conversion of > 92% molar or > 94% molar or more.

Typically, the start of reaction zone B is behind the maximum hot spot of reaction zone A. The maximum hot spot of reaction zone B is usually below the maximum hot point temperature of the reaction zone. the reaction zone A.

According to the invention, saline baths A, B can pass co-current or countercurrent through the space surrounding the reaction tubes, relative to the flow direction of the mixture of the reaction gases flowing through of the reaction tubes. According to the invention, the co-current flow can of course also be used in the reaction zone A, and the countercurrent flow in the reaction zone B (or vice versa).

In all the aforementioned configurations, within the respective reaction zone, a transverse flow can also be superimposed on the flow of the salt melt parallel to the reaction tubes, so that the individual reaction zone corresponds to a tubular bundle reactor like it is described in EP-A 700714 or in EP-A 7008943 and, throughout the longitudinal section, a serpentine flow of the exchanging medium through the tubular bundles of the catalyst [sic] results.

The initial mixture of reaction gases 1 of the catalyst bed 1 is conveniently preheated to the reaction temperature before being fed.

The reactors in tubular bundles mentioned above (this also applies to the reactors of tubular bundles in a zone), the tubes of the catalyst are usually made of ferritic steel and usually have a wall thickness from 1 to 3 mm. The internal diameter is, as a rule, from 20 to 30 mm, often from 21 to 26 mm. In terms of the application technology, the number of catalyst tubes housed in the tubular bundle container is conveniently at least 5000, preferably at least 10,000. It is frequent that the number of catalyst tubes housed in the reaction vessel is from 15,000 to 30,000. The reactors in tubular bundles having more than 40,000 catalytic tubes tend to be the exception. Within the container, the catalyst tubes are usually homogeneously distributed, the distribution being conveniently chosen so that the distance between the central internal shafts or the adjacent catalyst tubes (i.e., the separation of the catalyst tubes) is 35 at 45 mm (see, for example, EP-B 468290).

The particularly convenient heat exchange media are thermostatic fluid media. The use of salt melts, such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or metals that have a low melting point, such as sodium, mercury and some metal alloys, is particularly convenient.

As a rule, in all the aforementioned configurations of the flow in the reactors in tubular bundles with two zones, the flow velocity within the two required circulations of the heat exchange medium is chosen so that the temperature of the exchange medium of the heat increases from the point of entry to the reaction zone to the point of exit of the reaction zone (due to the exothermic nature of the reaction) from 0 to 15 ° C, ie, according to the invention,? T mentioned above can be from 1 to 10 ° C or from 2 to 8 ° C or from 3 to 6 ° C.

According to the invention, the temperature of the ion exchange medium at the inlet of the reaction zone B is usually, on the one hand, from 315 to 380 ° C and, on the other hand, at the same time at least 5 ° C above the temperature of the heat exchange medium at the inlet - of the reaction zone A.

Preferably, the temperature of the heat exchange medium at the inlet of the reaction zone B is at least 10 ° C above the temperature of the heat exchange medium at the inlet of the reaction zone A. The difference between the temperatures at the entrance of the reaction zones A and B can be like this, according to the invention, up to 20 ° C, up to 25 ° C, up to 30 ° C, up to 40 ° C, up to 45 ° C or up to 50 ° C. In general, the aforementioned temperature difference, however, will not be greater than 50 ° C. The greater the charge chosen from the propene of the catalyst bed 1 in the novel process, the greater must be the difference between the temperature of the heat exchange medium at the entrance of the reaction zone A and the temperature of the heat exchange medium at the inlet from reaction zone B.

According to the invention, the temperature of the heat exchange medium at the inlet of the reaction zone B is conveniently from 330 to 370 ° C and, particularly convenient from 340 to 370 ° C.

In the novel process, the two reaction zones A, B can, of course, also be carried out in the tubular beam reactors spatially separated from each other. If required, a heat exchanger can also be mounted between the two reaction zones, A, B. Of course, the two reaction zones A, B can also be designed as a fluidized bed.

In addition, in the novel process (both in an area and in two zones as a variant), it is also possible to use catalyst beds 1 whose specific volume activity increases continuously, suddenly or gradually in the flow direction of the Initial mixture of reaction gases 1 (this can be done, for example, as described in WO 98/24746 or as described in JP-A 91/294239 or by using an inert material as a diluent). In addition to nitrogen, steam and / or carbon oxides, it is also possible to use the inert diluent gases recommended in EP-A 293224 and in EP-B 247565 (for example, only propane or only methane, etc.). The latter, if required, can also be used in combination with a specific activity of the volume of the catalyst bed which increases in the flow direction of the reaction gas mixture.

Also, once again it should be noted in this case that in particular the type of tubular bundle reactor, of two zones described in the published German Application DE-B 2,201,528 can also be used to perform the reaction step 1 of the novel process, the type of reactor providing the possibility of transferring a portion of the relatively hot heat transfer medium from the reaction zone B to the reaction zone A for , if required, heat an initial mixture of cold reaction gases or a cold recycle gas.

In addition, the characteristic tubular bundle within an individual reaction zone can be designed as described in EP-A 382098.

According to the invention, it has been found convenient to cool the product gas mixture leaving the first reaction zone before entering the second reaction stage, for this means to suppress the subsequent complete combustion of parts of the acrolein formed in the reaction zone. The first stage of reaction. For this purpose, a subsequent cooler is usually connected between the two reaction stages. In the simplest case, this may be an indirect tubular bundle heat exchanger. The product gas mixture, as a rule, is passed through the tubes and the heat exchange medium whose type may correspond to the heat exchange medium recommended for the tubular beam reactors which is fed around the tubes. For convenience, the interior of the tubes is filled with inert supports (for example, stainless steel spirals, steatite rings, soapstone spheres, etc.). These improve the heat exchange and trap any molybdenum trioxide that is sublimed from the fixed catalyst bed of the first reaction stage, before the entrance of the molybdenum trioxide into the second reaction zone. It is convenient if the rear cooler is made of stainless steel coated with zinc silicate coating material.

As a rule, the conversion of propene into the first reaction stage in the novel process is > 92% molar or > 94% molar, based on a single step. The selectivity of the acrolein formation and the formation of the acrylic acid by-product which originates in a single step in the first reaction step is, as a rule, > 92% molar or > 94% molar or frequently > 95% molar or > 96% molar or > 97% molar.

The novel process is suitable for propene loads of the catalyst bed of > 165 liters (P.T.N.) / L x h o > 170 L (P.T.N.) / L x h or > 175 L (P.T.N.) / L x h or > 180 L (P.T.N.) 1 h, but also for propene loads of the catalyst bed 1 of > 185 L (P.T.N.) / L x h or 190 L (P.T.N.) / L x h or > 200 L (P.T.N.) / L x h or > 210 L (P.T.N.) / L x h y for loads of > 220 L (P.T.N.) / L x h or > 230 L (P.T.N.) / L x h or > 240 L (P.T.N.) / L x h or > 250 L (P.T.N.) / L x h.

The inert gas to be used according to the invention for the initial mixture of reaction gases 1 can comprises > 20 or > 30 or > 40 or > 50 'or > 60 or > 70 or > 80 or > 90 or > 95% by volume of molecular nitrogen.

In the case of propene loads of the catalyst bed 1 above 250 L (PTN) / L xh, the presence of inert diluent gases (in this case, inert diluent gases generally means those that suffer less than 5%, preference less than 2% conversion during the single step through the respective reaction stage) such as propane, ethane, methane, pentane, butane, CO2, CO, steam and / or noble gases, is recommended for the initial gas mixture of reaction 1 for the novel process. However, these gases and mixtures thereof may also be present at lower novel propene loads of the catalyst bed 1 in the initial mixture of reaction gases 1 or can be used as single diluent gases. It is surprising that the novel process can also generally be performed with good selectivities in the case of the homogeneous, ie chemically uniform, catalyst bed 1.

With increasing propene loads, the process in the two zones described is preferred in comparison with the process in an area described in the first reaction stage.

In the novel process according to the invention, the propene loading of the first fixed-bed catalyst will usually not exceed 600 L (P.T.N.) / L x h. Typically, the propene charges of the first fixed bed catalyst in the novel process are < 300 L (P.T.N.) / L x h, frequently < 200 L (P.T.N.) / L x h.

In the novel process, the operating pressure of the first reaction stage may be below atmospheric pressure (usually up to 0.5 bar) or above atmospheric pressure. In general, the operating pressure in the first reaction stage is from 1 to 5, often from 1.5 to 3.5 bar. Usually, the reaction pressure in the first reaction stage will not exceed 100 bar.

The molar ratio 02: C3H6 in the initial mixture of reaction gases 1 should be, according to the invention, > 1. Usually, this relationship is < 3. Frequently, the molar ratio 02: C3H6 and [sic] the initial mixture of reaction gases 1 is, according to the invention, > 1.5 and < 2.0.

Suitable molecular oxygen sources necessary in the first reaction step are air and spent air of molecular nitrogen (eg,> 90% by volume of 02, <10% by volume of N2).

According to the invention, the propene fraction of the initial mixture of reaction gases 1 can be, for example, from 4 to 15, often from 5 to 12% by volume or from 5 to 8% by volume (based on in each case in the total volume).

The novel process is often carried out at a volume ratio of propene to oxygen to inert gases (including steam) in the initial mixture of reaction gases 1 from 1: 1.0 to 3.0): (from 5 to 25) , preferably 1: (1.5 to 2.3): (10 to 15).

In addition to the components, as a general rule, the initial mixture of reaction gases 1 contains practically no more components.

In terms of the application technology, the gas mixture resulting from the first reaction stage is conveniently cooled, in the aforementioned back-up cooler, to a temperature from 210 to 290 ° C, often from 220 to 260 ° C or from 225 to 245 ° C. It is completely possible to cool the gas mixture produced by the first reaction stage at temperatures that are below the temperature of the second reaction stage. However, the subsequent cooling described is by no means compulsive and can be omitted, as a rule, in particular if the path of the gas mixture resulting from the first reaction stage to the second reaction stage is kept short. Usually, the novel process is further carried out in such a way that the oxygen demand in the second reaction stage is not covered by a correspondingly high oxygen content of the initial mixture of reaction gases., but the required oxygen is added in the region between the first and second reaction stages. This can be done before, during and / or after the subsequent cooling. Suitable molecular oxygen sources, necessary in the second stage of reaction of pure oxygen and mixtures of oxygen and inert gas, for example, air or air depleted of molecular nitrogen (for example >; 90% by volume of 02, > 10% by volume of N2). The oxygen source is usually added in compressed form to the reaction pressure.

According to the invention, the acrolein fraction of the initial mixture of reaction gases 2 thus produced can be, for example, from 3 to 15, often from 4 to 10% by volume or from 5 to 8% by volume ( based in each case on the total volume).

According to the invention, the molar ratio 02: acrolein in the initial mixture of reaction gases 2 should be > 0.5 or 1. Usually, this relationship is < 3. Frequently, the molar ratio 02: acrolein and the initial mixture of reaction gases 2 is, according to the invention, from 1 to 2 or from 1 to 1.5. Frequently, the novel process is performed with a volume ratio (1 (PTN)) of acrolein to oxygen to steam to inert gas in the initial mixture of reaction gases 2 of 1: (0.5 or 1 to 3): (0 to 20): (3 to 30), preferably 1: (1 to 3): (0.5 to 10): (7 to 10).

The operating pressure in the second reaction stage can be below atmospheric pressure (for example, up to 0.5 bar) above atmospheric pressure. In general, the operating pressure in the second reaction zone is, according to the invention, from 1 to 5, often from 1 to 3 bar. In general, the reaction pressure in the second reaction zone does not exceed 100 bar.

Like the first reaction stage, the second reaction step of the novel process can be carried out in a simple manner in a tubular beam reactor loaded with ring catalysts and as described, for example, in EP-A 700893.

In the simplest form, this means that the fixed bed catalyst 2 to be used according to the invention is present in the metal tubes of a tubular beam reactor, and a thermostatic medium (one zone procedure, as a rule a melt The salt melt and the mixture of reaction gases can be fed co-current or countercurrent in a simple way.The thermostatic medium, however, can pass in a serpentine form around the tubular beam, considered above the reactor, so that, considered only on the total reactor, there is flow in co-current or countercurrent with respect to the direction of flow of the mixture of reaction gases. thermostatic medium (heat exchanger medium) is usually such that the temperature increase (due to the exothermic nature of the reaction) of the heat exchanger medium from the point or input to the reactor to the exit point of the reactor is from > 0 to 10 ° C, frequently from > 2 to 8 ° C or from > 3 to 6 ° C. The temperature of the heat exchanger medium at the inlet in the tubular beam reactor is generally from 230 to 300 ° C, frequently from 245 to 285 ° C or from 245 to 265 ° C. Convenient heat exchange media are the fluid thermostatic media that have been described for the first reaction stage.

Conveniently, the initial mixture of reaction gases 2 from the catalyst bed 2 is preheated to the reaction temperature before being fed. In the novel variant of the second reaction stage described in the foregoing, this is often from 230 to 300 ° C, often from 245 to 285 ° C or from 245 to 265 ° C.

By general gift, the process in a first reaction zone is combined with the process in an area of the second reaction stage, the relative flow of the reaction gas mixture and the thermostatic medium being chosen to be identical in the two stages.

However, the second reaction step of the novel process can, of course, also be carried out in a manner corresponding to the first reaction stage, such as two reaction zones in successive spaces C, D, the temperature of the reaction zone C suitably being from 230 to 270 ° C and the temperature of the reaction zone D from 250 to 300 ° C, and at the same time at least 10 ° C above the temperature of the reaction zone C.r.

The reaction zone C preferably extends to an acrolein conversion from 65 to 80 mol%. In addition, the temperature of the reaction zone C is conveniently from 245 to 260 ° C. The temperature of the reaction zone D is preferably at least 20 ° C above the temperature of the reaction zone C and conveniently is from 265 to 285 ° C.

The higher the chosen charge of acrolein in the catalyst bed 2 in the novel process, the greater should be the difference chosen between the temperature of the reaction zone C and the temperature of the reaction zone D. However, usually the difference in temperature mentioned above in the novel process will not be more than 40 ° C, that is, the difference between the temperature of the reaction zone C and that of the reaction zone D can be, according to the invention, up to 15 ° C, up to 25 ° C, up to 30 ° C, up to 35 ° C or up to 40 ° C.

In the novel process the conversion of acrolein into the novel process may be, in general, > 92 or 94 or > 96 or > 98 and often even > 99% molar, based on a single step of the second reaction stage. The selectivity of the acrylic acid formation can be, as a rule, > 92 or > 94% molar, frequently > 95 or > 96 or > 97% molar, based on the acrolein converted.

The novel process is suitable for acrolein loads of the catalyst bed 2 of > 140 L (P.T.N.) / L x h or > 150 L (P.T.N.) / L x h or > 160 L (P.T.N.) / L x h or > 170 L (P.T.N.) 1 h or > 175 L (P.T.N.) / L x h or > 180 L (P.T.N.) / L x h but also for acrolein loads of the catalyst bed 2 of > 185 L (P.T.N.) / L x h or > 190 L (P.T.N.) / L x h or > 200 L (P.T.N.) / L x h or > 210 L (P.T.N.) / L x h y for loads of > 220 L (P.T.N.) / L x h or > 230 L (P.T.N.) / L x h or > 240 L (P.T.N.) / L x h or > 250 L (P.T.N.) / L x h.

The inert gas that can be used concomitantly according to the invention in the second reaction step can comprise > 30 or > 40 or > 50 or > 60 or > 70 or > 80 or > 90 or > 95% by volume of molecular nitrogen Conveniently, the inert diluent gas in the second reaction step in the novel process comprises from 5 to 20% by weight of H20 (formed in the first reaction step) and from 70 to 90% by weight of N2.

In addition to the components mentioned in this publication, the initial mixture of reaction gases 2 usually contains -practically no more components.

In the case of acrolein loads of the second fixed bed catalyst above 250 L (PTN) / L xh, the presence of inert diluent gases, such as propane, ethane, methane, butane, pentane, CO2, CO, vapor and / or inert gases, it is recommended for the initial mixture of reaction gases 2. However, these gases can of course also be present with lower charges of acrolein. The novel process can generally be carried out with good selectivity using a homogeneous, ie chemically uniform, catalyst bed 2.

In the novel process, the acrolein loading of the second fixed bed catalyst usually does not exceed 600 L (P.T.N.) / L x h. Typically, the acrolein loading of the catalyst bed 2 in the novel process is < 300 L (P.T.N.) / L x h, frequently < 250 L (P.T.N.) / L x h, without a significant decline in conversion and selectivity.

As a rule, the acrolein loading of the second catalyst bed in the novel process is approximately 10 L (PTN) / L xh, often approximately 20 or 25 L (PTN) / L xh, below the propene load of the first catalyst bed. This is mainly due to the fact that, in the first reaction stage, the conversion and selectivity with respect to acrolein, as a rule, does not reach 100%. In addition, the oxygen demand of the second reaction stage is usually covered with air. With increasing charge of acrolein, the two-zone process described in comparison with the process in an area carried out in the second reaction stage is preferred.

It is worth noting that, in the novel process, the selectivity of acrylic acid formation, balanced on both reaction steps and based on the converted propene, may be as a general rule > 83% molar, frequently > 85% molar or > 88% molar, frequently > 90% molar or 93% molar, even in the highest loads of propene and acrolein.

The annular fixed-bed catalysts 2 to be used according to the invention, which are suitable for the catalytic oxidation in the gas phase of acrolein in the second reaction stage, are all those whose active material is at least one multimetal oxide containing Mo and V.

Such suitable multimetal oxide active materials are described in, for example, US-A 3 775 474, US-A 3 954 855, US-A 3 893 951 and US-A 4 339 355. Also particularly convenient are the active materials of multimetal oxides of EP-A 427 508, DE-A 2 909 671, DE-C 31 51 805, German Published Application DE-B 2,626,887, DE-A 43 02 991, EP-A 700 893, EP-A 714 700 and DE-A 19 73 6105. Particularly preferred in this context are the exemplary embodiments of EP-A 714 700 and DE-A 19 73 6105.

A large number of suitable multimetal oxide active materials for the fixed-bed catalysts 2 can be summarized in formula IV: ??? 2VaX1bX2cX3clX4eX5fX6qOn ilV) where: X1 is W, Nb, Ta, Cr and / or Ce X¿ is Cu, Ni , Co, Fe, Mn and / or Zn X4 is one or more alkali metals X5 is one or more alkaline earth metals X6 is Si, Al, Ti and / or Zr a is from 1 to 6 b is from 0.2 to 4 c is from 0.5 to l is from 0 to 40 e is from 0 to 2 f is from 0 to 4 g is from 0 to 40, and n is a number that is determined by the valence and frequency of the different oxygen elements in IV.

Preferred embodiments of the active multimetal oxides IV are those of the formula IV, where: X1 is W, Nb, and / or Cr X2 is Cu, Ni, Co and / or Fe XJ is Sb X4 is Na and / or K X5 is Ca, Sr and / or Ba X6 is Yes, Al and / or Ti a is from 1.5 to 5 b is from 0.5 to 2 c is from 0.5 to 3 d is from 0 to 2 e is from 0 to 0.2 f is from 0 to 4 g is from 0 to 1, and n is a number that is determined by the valence and frequency of the different elements of oxygen in IV.

The multimetallic oxides IV very particularly preferred are, however, those of the formula V: 2Va- YV Y2c 'Y5f? 6g' ° n. (V) where : Y1 is W and / or Nb Y2 is Cu and / or Ni Y5 is Ca and / or Sr Y6 is Yes and / or Al a 'is from 2 to 4 b 'is from 1 to 1.5 c 'is from 1 to 3 f is from 0 to 0.5 g 'is from 0 to 8, and n 'is a number that is determined by the valence and frequency of the different elements of oxygen in V.

Suitable multimetal oxide (IV) active materials according to the invention can be obtained in a manner known per se, for example, described in DE-A 4335973 or in EP-A 714700.

In principle, the suitable multimetal oxide active materials according to the invention for the fixed bed catalysts 2, in particular the active materials of the formula IV, can be prepared in a simple manner by producing, from the convenient sources of the elementary constituents of these, a very intimate, preferably finely divided, anhydrous mixture having a composition corresponding to its stoichiometry, and calcining the anhydrous mixture at a temperature from 350 to 600 ° C. The calcination may be carried out in an inert gas or in an oxidizing atmosphere, for example air (mixture of inert gas and oxygen) or in a reducing atmosphere (for example, mixture of inert gas and reducing gases, such as H2, NH3, CO, methane and / or acrolein or the reducing gases themselves). The duration of the calcination can be from a few minutes to a few hours and by regular decreases with the increase in temperature. The sources of the suitable elementary constituents of the multimetal oxides active materials 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 initial compounds for the preparation of the multimetal oxide materials IV can be carried out in anhydrous or wet form. If it is carried out in the anhydrous form, the initial compounds are conveniently used in the form of finely divided powder and, after mixing and any compaction, they are subjected to calcination. The intimate mixing, however, preferably takes place in a moist form.

Usually, the initial compounds are mixed together in the form of a solution and / or aqueous suspension. Particular intimate anhydrous mixtures are obtained in the mixing process described when only dissolved sources of the elemental constituents are used as raw materials. A preferred solvent used is water. The aqueous material obtained is then dried, the drying process preferably being carried out by spray drying the aqueous mixture at discharge temperatures from 100 to 150 ° C.

Suitable multimetal oxide materials for the fixed-bed catalysts 2, in particular the multimetal oxide materials of the formula IV, are used for the novel process after shaping to obtain annular catalyst geometries, it being possible to carry out the molding before or after the final calcination, in a form particularly corresponding to that of the fixed-bed catalysts 1. For example, the unsupported, ring-shaped catalysts can be prepared in a virtually analogous manner from the powder form of the active material or its precursor material not calcined by compaction to obtain the desired catalyst geometry (for example by extrusion), it being possible, if required, to add auxiliaries, for example graphite or stearic acid as lubricants and / or modeling aids and reinforcing agents, such as glass microfibers , asbestos, silicon carbide or potassium titanate. The geometries of suitable unsupported catalysts, as stated in the foregoing, are hollow cylinders with an external diameter and a length from 2 to 10 mm. A suitable wall thickness is from 1 to 3 mm.

Of course, the modeling of pulverulent active material or its pulverulent uncalcined precursor material can also be carried out by applying it to catalyst supports, inert, preformed into rings. The coating of the supports for the preparation of coated catalysts is carried out as a rule in a suitable rotary vessel as described, for example in DE-A 2909671, EP-A 293859 or EP-A 714700.

To coat the supports, the powder material to be applied is conveniently moistened and, after application, dried again, for example by means of hot air. The thickness of the layer of the powder material applied to the support is conveniently chosen to be from 10 to 1000 microns, preferably from 50 to 500 microns, particularly preferably from 150 to 250 microns.

Suitable support materials are aluminas, silicas, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as porous or non-porous traditional magnesium silicate or aluminum silicate. Supports having substantial surface roughness are preferred. It is convenient to use hollow cylinders whose length is from 2 to 10 mm and whose external diameter is from 4 to 10 mm. In addition, the wall thickness is usually from 1 to 4 mm. The annular supports preferably for use according to the invention have a length from 3 to 6 mm, an external diameter from 4 to 8 mm and a wall thickness from 1 to 2 mm. According to the invention, rings measuring 7 mm x 13 mm x 4 mm (external diameter x length x internal diameter) are also particularly suitable as supports. The fineness of the catalytically active oxidic materials to be applied to the surface of the support is of course designed for the desired thickness of the coating (see EP-A 714 700).

Of course, the multi-metal oxide active material of the fixed-bed catalysts 2 can also be molded into ring-supported catalysts.

The active materials of advantageous multimetal oxides to be used according to the invention as fixed bed catalysts 2 are also materials of the formula VI: [D] p [E] q (VI), where: D is M? 2Va »Z1b» Z2c »Z3d« Z4e «Z5f» Z6g «03i E is Z7? 2Cuh« Hi »Oy» Z1 is W, Nb, Ta, Cr, and / or Ce Z2 is Cu, Ni, Co, Fe, Mn and / or Zn Z3 is Sb and / or Bi Z4 is Li, Na, K, Rb, Cs and / or H Z5 is Mg, Ca, Sr and / or Ba Z6 is Si, Al, Ti and / or Zr Z7 is Mo, W, V, Nb and / or Ta a "is from 0 to 8 b" is from 0.2 to 5 c "is from 0 to 23 d" is from 0 'to 50 e "is from 0 to 2 f" is from 0 to 5 g "is from O to 50 h "is from 4 to 30 i "is from 0 to 20, and x "y and" are numbers that are determined by the valence and frequency of the different oxygen elements in VI, and p and q are nonzero numbers, whose p / q ratio is from 160: 1 to 1: 1, which can be obtained by separately preforming a multimetal oxide material E: Z712Cuh «Hi« O (E), in finely divided form (raw material 1) and then incorporating the solid raw material 1, preformed in an aqueous solution, an aqueous suspension or a finely divided anhydrous mixture of sources of the elements Mo, V, Z1, Z2, Z3, Z4, Z5 and Z6, which contains the aforementioned elements in the D stoichiometry 2Va »z z2c» z z4e "Z5f" Z (D) (raw material 2), in the desired ratio p: q, if required by drying the resulting aqueous mixture and calcining the anhydrous precursor material thus obtained, at a temperature from 250 to 600 ° C, before or after it is molded to obtain the desired catalyst geometry.

The multimetal oxide materials VI are preferred where the preformed solid raw material 1 is incorporated into an aqueous raw material 2 to < 70 ° C. a detailed description of the preparation of the active materials comprising the multimetal oxide VI is contained, for example, in EP-A 668104, DE-A 19736105 and DE-A 19528646.

With respect to molding, the claims made in the case of the active materials comprising the multimetal oxide IV are applicable to the active materials comprising the multimetal oxide VI.

In a convenient form for the application technology, the second reaction stage of the novel process is carried out in a tubular bundle reactor in two zones. A preferred variant of a tubular beam reactor in two zones that can be used according to the invention for the second reaction stage is described in DE-C 2830765. However, the tubular beam reactors in two zones described in DE-C 2513405, US-A 3147084, DE-A 2201528, EP-A 383224 and DE-A 2903582 are also suitable for carrying out the second reaction step of the novel process.

In a simpler form, this means that the fixed-bed catalyst to be used according to the invention is present in the metallic tubes of the tubular bundle reactor, and two thermostatic means in spaces practically separated from each other, as a rule , saline fused, are passed around the metal tubes. The tubular section on which the respective salt bath extends and represents, according to the invention, a reaction zone.

In a simpler form, this means that a saline bath C flows around those sections of the tubes (reaction zone C) in which the oxidative reaction of acrolein (in a single bath) takes place until a conversion from 55 to 85 mole% is achieved, and a salt bath D flows around that section of the tubes (reaction zone D) in which the subsequent oxidative reaction of acrolein takes place (in a single step) until it reaches a conversion of at least 90 mol% (if required, the reaction zones C, D to be used according to the invention can be followed by other reaction zones which are maintained at individual temperatures).

It is convenient in terms of the application technology if the reaction stage 2 of the novel process no longer comprises reaction zones, that is, the salt bath D conveniently flows around that section of the tubes in which the oxidative reaction takes place. Subsequent Acrolein (in a single step) for a conversion of > 92% molar or > 94% molar or > 96% molar or > 98% molar, and often even > 99% molar or more.

Typically, the start of reaction zone D is behind the maximum hot spot of reaction zone C. The temperature of the maximum hot spot of reaction zone D is usually below the maximum point temperature hot from reaction zone C.

The two salt baths C, D can, in accordance with the invention, pass co-current or countercurrent through the space surrounding the reaction tubes, relative to the direction of flow of the reaction gas mixture flowing to through the reaction tubes. According to the invention, the co-current flow can, of course, also be used in the reaction zone C, and the countercurrent flow in the reaction zone D (or vice versa).

In all the aforementioned configurations within the respective reaction zone, a transverse flow can also be superimposed on the flow of the salt melt parallel to the reaction tubes, so that the individual reaction zone corresponds to a tubular beam reaction as described in EP-A 7001714 and in EP-A 700893 and, on the longitudinal section as a whole, a serpentine flow of the resulting heat exchange medium through the tubular bundle of the catalyst.

In the aforementioned tubular beam reactors (as in the tubular bundle reactors of the one zone process), the catalyst tubes used are usually made of ferritic steel and usually have a wall thickness of from 1 to 3 mm. Its internal diameter is, as a rule, from 20 to 30 mm, often from 22 to 26 mm. In terms of the application technology, the number of catalyst tubes housed in the tubular bundle container is conveniently at least 5000, preferably at least 10,000. Frequently, the number of catalyst tubes housed in the reaction vessel is from 15,000 to 30,000. Tube bundle reactors having more than 40,000 catalyst tubes tend to be the exception. Within the container, the catalyst tubes are usually distributed in a homogeneous manner, the distribution being conveniently chosen so that the distance between the central internal shafts of the adjacent catalyst tubes (ie, the separation of the catalyst tubes) is from up to 45 mm (see, EP-B 468290).

The particularly convenient heat exchange means are thermostatic fluid media. The use of salt melt, such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or metals that have a low melting point, such as sodium, mercury and some metal alloys, is particularly advantageous .

As a general rule, in all the above mentioned flow configurations in the tubular beam reactors in two reaction zones, the flow velocity within the two required circulations of heat exchange means is chosen so that the temperature of the medium of heat exchange increases from the point of entry to the reaction zone to the exit point of the reaction zone at a temperature from 0 to 15 ° C, ie, the aforementioned? T can be, according to the invention , from 1 to 10 ° C or from 2 to 8 ° C or from 2 to 6 ° C.

In a novel process in two zones, the temperature of the heat exchange medium at the inlet of the reaction zone C is usually from 230 to 270 ° C in the second reaction stage. The temperature of the heat exchange medium at the inlet of the reaction zone D is, according to the invention, usually on the one hand from 250 ° C to 300 ° C and, on the other hand, at the same time at least 10 ° C above the temperature of the heat exchange medium entering the reaction zone C.

Preferably, the temperature of the heat exchange medium at the inlet of the reaction zone D is at least 20 ° above the temperature of the heat exchange medium entering the reaction zone C. The difference between the temperatures at the entrance to the reaction zones C and D can thus be, according to the invention, up to 15 ° C, up to 25 ° C, up to 30 ° C, up to 35 ° C or up to 40 ° C. Usually, however, the aforementioned temperature is no higher than 50 ° C. The higher the charge of acrolein chosen from the catalyst bed 2 in the novel process, the greater should be the difference between the temperature of the heat exchange medium at the inlet of the reaction zone C and the temperature of the heat exchange medium at the inlet of the reaction zone D. Preferably, the temperature at the inlet of the reaction zone C is from 245 to 260 ° C and the temperature at the inlet of the reaction zone D is from 265 to 285 ° C.

In the novel process, the two reaction zones C, D can, of course, also be carried out in reactors of tubular bundles separated from each other. If required, a heat exchanger can also be mounted between the two reaction zones C, D. The two reaction zones C, D can, of course, also be designed as a fluidized bed.

Furthermore, in the novel process, it is also possible to use, in general, catalyst beds 2 whose specific activity of the volume in the flow direction of the reaction gas mixture increases continuously, in a sudden or gradual manner (this can be manifested, for example, by dilution with inert material or variation of the activity of multimetal oxide).

The inert diluent gases (for example only propane or only methane, etc.) recommended in EP-A 293224 and in EP-B 257565 can also be used for the novel process of the second reaction stage. These diluent gases, if required, can also be combined with a specific activity of the volume of the catalyst bed 2 decreasing in the direction of flow of the reaction gas mixture.

Again, it should be noted here that, in particular, the type of two-zone tubular bundle reactor described in the German published application DE-B 2,201,528 can be used to carry out the second reaction step of the novel process, whose type of reactor provides the possibility of transferring a part of the relatively hot heat exchanger medium from the reaction zone B to the reaction zone C, if it is required to heat an initial mixture of reaction gases 2 that is too cold or a cold recycled gas . In addition, characteristic tubular beams within an individual reaction zone can be designed as described in EP-A 382 098.

The novel process, of course, can also be performed in a single tubular bundle reactor in two zones as described, for example, in DE-C 2830765, EP-A 911313 and EP-A .383224, so that the first stage of reaction to be implemented in the first reaction zone and the second reaction stage in the second reactor reaction zone of tubular bundles of two zones.

The novel process is particularly convenient for a continuous process. It is surprising that this allows good selectivities in the formation of the desired product in a single step with a high loading of the fixed bed catalysts with raw materials.

The novel process provides not the acrylic acid in the pure form but a mixture of the secondary components of which the acrylic acid can be separated in a manner known per se (for example by rectification and / or crystallization). The unconverted acrolein, and the propene and inert diluent gas that is used and / or formed in the course of the reaction may be recycled to the gas phase oxidation. In the oxidation in gas phase, in two stages, novel starting from propene, the recycling is conveniently carried out in the first reaction stage. If required, the novel process can of course also be used in the case of usual propene loads.

Otherwise, in this publication the conversion, selectivity and dwell time are defined as follows, unless otherwise stated: Number of moles of the raw material converted Conversion of the raw material (%) = x 100 Number of moles of the raw material used Number of moles of the material Selectivity of the converted premium to product product formation = x 100 Number of moles of the converted raw material Empty reactor volume Time spent filling with catalyst (1) (sec) = x 3600 Performance of the initial mixture of reaction gases (1 (P.T.N.) / h Examples and comparative examples a) Preparation of a novel fixed bed catalyst 1 1. Preparation of raw material 1 209. 3 kg of tungstic acid (72.94% by weight of W) were stirred, little by little, at 25 ° C, in 775 kg of an aqueous solution of bismuth nitrate containing nitric acid (11.2% by weight of Bi, from 3 to 5% by weight of free nitric acid, density: from 1.22 to 1.27 g / mol The resulting aqueous mixture was then stirred for more than 2 hours at 25 ° C and then spray-dried.

Spray drying was performed in a spray tower with rotating discs by the co-current method at a gas outlet temperature of 300 ± 10 ° C and a gas outlet temperature [sic] of 100 ± 10 ° C. The spray-dried powder obtained was then calcined at a temperature of 780 to 810 ° C (in a rotary tubular furnace through which air flowed (1.54 3 internal volume, 200 m3 (P.T.N.) of air / h)). What is important with respect to the exact determination of the calcination temperature is that it was designed for the composition of the desired phase of the calcination product. The phases W03 (monoclinic) and BÍ2 2O9 are desired; the presence of? -Bi2W06 (ruselite) is unwanted. Consequently, if the compound? -Bi2W06 is still detectable after calcination, based on a reflection in the powder X-ray diffraction pattern at a reflection angle of 2? = 28.4 ° (CuKa radiation), the preparation must be repeated and the calcination temperature must be increased within the established temperature range until the reflection disappears. The mixed, calcined, preformed, thus obtained oxide was crushed so that the X50 value (see Ullmann's Encyclopedia of Industrial Chemistry, 6th edition (1998) Electronic Relay, section 3.1.4 or DIN 66141) of the resulting particles outside 5 μ . The crushed material was then mixed with 1% by weight (based on the crushed material) of finely divided Si02 (bulk density 150 g / 1; X50 value of the Si02 particles was 10 μ and the BET surface area was 100 m2 / g). 2. Preparation of raw material 2 A solution A was prepared by dissolving 213 kg of ammonium heptamolybdate in 600 1 of water at 60 ° C with stirring, and 0.97 kg of an aqueous solution of potassium hydroxide (46.8% by weight of KOH) was added at 20 ° C to the resulting solution while maintaining at 60 ° C and with agitation.

A solution B was prepared by introducing 116.25 kg of an aqueous solution of iron nitrate (14.2% by weight of Fe) into 262.9 kg of an aqueous solution of cobalt nitrate (12.4% by weight of Co) at 60 ° C. Then solution B was pumped continuously to solution A initially taken for a period of 30 minutes while maintaining it at 60 ° C. It was then stirred for 15 minutes at 60 ° C. Then, 19.16 kg of a silica gel (46.80% by weight of SIO2, density from 1.36 to 1.42 g / ml, pH from 8.5 to 9.5, maximum alkali metal content 0.5% by weight) were added to the resulting aqueous mixture and it was then stirred for another 15 minutes at 60 ° C.

The spray drying was then carried out in a spraying tower with rotating discs by the co-current method (gas inlet temperature: 400 ± 10 ° C, gas outlet temperature: 140 ± 5 ° C. by resulting spray had an ignition loss of approximately 30% by weight (3 h at 600 ° C). 3. Preparation of the multimetal oxide active material The raw material 1 was mixed homogeneously with the raw material 2 in the amount necessary for the active material multimetal oxide with the stoichiometry: [Bi2W29 • 2wO3lo.5C 012C05.5Fe2.9Si1.59Ko.08Ox]! Based on the aforementioned total material, further 1.5% by weight finely divided graph (sieve analysis: minimum 50% by weight <24 μ, maximum 10% by weight> 24 μ and <48 μ, maximum 5 % by weight> 48 μ, BET surface area: from 6 to 13 m2 / g) were mixed homogeneously. The resulting anhydrous mixture was compressed to obtain hollow cylinders with a length of 3 mm, external diameter of 5 mm and a wall thickness of 1.5 mm and then they were subjected to thermal treatment as follows.

In a muffle furnace through which air flowed (60 1 internal volume, 1 1 / h of air per gram of the precursor of the active material), the heating was carried out at a heating rate of 180 ° C / h, initially from the room temperature (25 ° C) up to 190 ° C. This temperature was maintained for one hour then increased to 210 ° C at a heating rate of 60 ° C / h. The 210 ° C, in turn, was maintained for one hour before increasing at a heating rate of 60 ° C / h to 230 ° C. This temperature was similarly maintained for one hour before it was increased to 265 ° C, once again at a heating rate of 60 ° C / h. then, the 265 ° C was maintained in the same way for one hour. Then, the cooling was performed first at room temperature, and the decomposition phase was thus practically complete. Then, the heating was carried out at a heating rate of 180 ° C / h up to 465 ° C and this calcination temperature was maintained for 4 hours. A bed of the catalyst rings without resultant support formed a novel fixed bed catalyst. b) Preparation of a novel fixed bed catalyst 2 1. Preparation of the catalytically active oxide material M? I2V3W? .2Cu2.4On 190 g of copper (II) acetate monohydrate were dissolved in 2700 g of water to obtain a solution I. 860 g of ammonium heptamclibdate tetrahydrate, - 143 g of ammonium metavanadate and 126 g of ammonium paratungstate heptahydrate were dissolved in succession in 5500 g of water at 95 ° C to obtain a solution II. Then, solution I was stirred all at once in solution II, and then an aqueous solution at 25% by weight concentration of NH3 was added in an amount sufficient to form a solution again. This was spray-dried at an exit temperature of 110 ° C. The resulting spray-dried powder was kneaded with 0.25 kg of an aqueous solution of acetic acid at 30% concentration by weight per kg of powder using a ZS1-80 type kneader from Werner and Pfleiderer, and then dried at 110 ° C. for 10 hours in a dryer oven. 700 g of the catalyst precursor thus obtained were calcined in an air / nitrogen mixture [(200 1 N2 / 15 1 air) / h] in a rotary tubular oven (50 cm long, 12 cm internal diameter). During calcination, the kneaded material was first heated continuously from room temperature (approximately 25 ° C) to 325 ° C during the course of one hour. This temperature was then maintained for 4 hours. Then the heating was carried out at 400 ° C in the course of 15 minutes, this temperature was maintained for one hour and then the cooling was carried out up to room temperature.

The catalytically active material, calcined, was crushed to obtain a finely divided powder, 50% of whose particles passed through a sieve of mesh size from 1 to 10 μ and whose fraction of the particles with a maximum dimension greater than 50 μ It was less than 1%.

Preparation of the coated catalyst 28 kg of annular supports (external diameter 7 mm, length 3 mm, internal diameter 4 mm, steatite, having a surface roughness Rz in accordance with EP-B 714700 of 45 μ and with a total volume of pore, based on the volume of supports, from <; 1% by volume, manufacturer: Caramtec DE) were introduced into a coating vessel (90 ° inclination angle: Lódige Hicoater, DE) having an internal volume of 200 1. The container for the coating then rotated at 16 rpm. 2000 g of an aqueous solution consisting of 75% by weight of H20 and 25% by weight of glycerol were sprayed onto the supports by means of a nozzle in the course of 25 minutes. At the same time, 7 kg of the catalytically active oxide powder of a) were introduced continuously through the vibrating channel out of the spray cone of the atomizing nozzle in the same period. During the coating, the feed powder was completely adsorbed on the surface of the supports, and agglomeration of the finely divided oxide active material was not observed. After the completion of the addition of the powder and the aqueous solution, hot air at 110 ° C was introduced into the coating vessel for 20 minutes at a speed of 2 rpm. The drying was then carried out for another 2 hours at 250 ° C in a stationary bed (tray dryer) with air. Coated annular catalysts were obtained whose proportion of the oxide active material was 20% by weight, based on the total material. The thickness of the coating was 230 ± 25 μ, both on the surface of a support and on the surface of different supports. A bed of the resulting coated catalyst rings formed a novel fixed bed catalyst. c) Preparation of - a coated, fixed-bed, comparative, spherical catalyst 1 1. Preparation of raw material 1 209. 3 kg of tungstic acid (72.94% by weight of W) were stirred, little by little, at 25 ° C, in 775 kg of an aqueous solution of bismuth nitrate containing nitric acid (11.2% by weight of Bi, from 3 to 5% by weight of free nitric acid, density: from 1.22 to 1.27 g / mol The resulting aqueous mixture was then stirred for more than 2 hours at 25 ° C and then spray-dried.

Spray drying was performed in a spray tower with rotating discs by the co-current method at a gas outlet temperature of 300 ± 10 ° C and a gas outlet temperature [sic] of 100 ± 10 ° C. The spray-dried powder obtained was then calcined at a temperature of 780 to 810 ° C (in a rotary tubular furnace through which air flowed (1.54 m3 internal volume, 200 m3 (P.T.N.) of air / h)). What is important with respect to the exact determination of the calcination temperature is that it was designed for the composition of the desired phase of the calcination product. The phases W03 (monoclinic) and Bi2W2? 9 are desired; the presence of? -BI2W06 (ruselite) is unwanted. Consequently, if the compound? -Bi2W06 is still detectable after calcination, based on a reflection in the powder X-ray diffraction pattern at a reflection angle of 2? = 28.4 ° (CuKa radiation), the preparation must be repeated and the calcination temperature must be increased within the established temperature range until the reflection disappears. The mixed, calcined, preformed, thus obtained oxide was crushed so that the X50 value (see Ullmann's Encyclopedia of Industrial Chemistry, 6th edition (1998) Electronic Relay, section 3.1.4 or DIN 66141) of the resulting particles outside 5 μ . The crushed material was then mixed with 1% by weight (based on the crushed material) of finely divided SiO (bulk density 150 g / 1; X50 value of the SiO2 particles was 10 μ and the BET surface area was 100 m2 / g). 2. Preparation of raw material 2 A solution A was prepared by dissolving 213 kg of ammonium heptamolybdate in 600 1 of water at 60 ° C with stirring, and 0.97 kg of an aqueous solution of potassium hydroxide (46.8% by weight of KOH) was added at 20 ° C to the resulting solution while maintaining at 60 ° C and with agitation.

A solution B was prepared by introducing 116.25 kg of an aqueous solution of iron nitrate (14.2% by weight of Fe) into 262.9 kg of an aqueous solution of cobalt nitrate (12.4% by weight of Co) at 60 ° C. Then solution B was pumped continuously to solution A initially taken for a period of 30 minutes while maintaining it at 60 ° C. It was then stirred for 15 minutes at 60 ° C. Then, 19.16 kg of a silica gel (46.80% by weight of SiO2, density from 1.36 to 1.42 g / ml, pH from 8.5 to 9.5, maximum alkali metal content 0.5% by weight) were added to the resulting aqueous mixture and it was then stirred for another 15 minutes at 60 ° C.

The spray drying was then carried out in a spray tower with rotating discs by the countercurrent method (gas inlet temperature: 400 ± 10 ° C, gas outlet temperature: 140 ± 5 ° C.) The spray-dried powder The resultant had an ignition loss of approximately 30% by weight (ignition for 3 h at 600 ° C). 3. Preparation of the multimetal oxide active material The raw material 1 was mixed homogeneously with the raw material 2 in the amount necessary for the active material multimetal oxide with the stoichiometry: [Bi2W29 • 2W03] or .5 [MO12CO5.5Fe2.94Si1.59K0. 08Ox .i l Based on the aforementioned total material, in addition 1. 5% by weight finely divided graph (sieve analysis: minimum 50% by weight <24 μ, maximum 10% by weight> 24 μ and <48 μ, maximum 5% by weight> 48 μ, area of BET surface: from 6 to 13 m2 / g) were mixed homogeneously. The resulting anhydrous mixture was compressed to obtain hollow cylinders with a length of 3 mm, external diameter of 5 mm and a wall thickness of 1.5 mm and then they were subjected to thermal treatment as follows.

In a muffle furnace through which air flowed (60 1 internal volume, 1 1 / h of air per gram of the precursor of the active material), the heating was carried out at a heating rate of 180 ° C / h, initially from the room temperature (25 ° C) up to, 190 ° C. This temperature was maintained for one hour then increased to 210 ° C at a heating rate of 60 ° C / h. The 210 ° C, in turn, was maintained for one hour before increasing at a heating rate of 60 ° C / h to 230 ° C. This temperature was similarly maintained for one hour before it was increased to 265 ° C, once again at a heating rate of 60 ° C / h. then, the 265 ° C was maintained in the same way for one hour. Then, the cooling was performed first at room temperature, and the decomposition phase was thus practically complete. Then, the heating was carried out at a heating rate of 180 ° C / h up to 465 ° C and this calcination temperature was maintained for 4 hours.

The catalytic active material, calcined, was crushed to a finely divided powder, 50% of whose particles passed through a sieve of mesh size from 1 to 10 μ and whose proportion of particles had a maximum dimension above 50 μ It was less than 1%. 4. Preparation of the coated catalyst kg of spherical supports (diameter 4-5 mm, having a surface roughness Rz in accordance with EP-B 714700 of 45 mm and with a total pore volume, based on the volume of supports, of <1% by volume , manufacturer: Ceramtec DE) were introduced into a coating vessel (90 ° inclination angle: Lodige Hicoater, DE) having an internal volume of 200 1. The container for the coating then rotated at 16 rpm. 2000 g of an aqueous solution consisting of 75% by weight of H20 and 25% by weight of glycerol were sprayed onto the supports by means of a nozzle in the course of 25 minutes. At the same time, 13 kg of the catalytically active oxide powder of a) were introduced continuously through the vibrating channel out of the spray cone of the atomizing nozzle in the same period. During the coating, the feed powder was completely adsorbed on the surface of the supports, and agglomeration of the finely divided oxide active material was not observed. After the completion of the addition of the powder and the aqueous solution, hot air at 110 ° C was introduced into the coating vessel for 20 minutes at a speed of 2 rpm. The drying was then carried out for another 2 hours at 250 ° C in a stationary bed (tray dryer) with air. Coated spherical catalysts were obtained whose proportion of the oxide active material was 30% [sic], based on the total material. The thickness of the coating was 280 ± 25 μ, both on the surface of a support and on the surface of different supports. A bed of the resulting coated catalyst beads formed the spherical, fixed bed, comparative catalyst 1.

Preparation of a coated, fixed-bed, comparative, spherical catalyst 2 Preparation of the catalytically active oxide material M012V3 1.2Cu2.4On 190 g of copper (II) acetate monohydrate were dissolved in 2700 g of water to obtain a solution I. 860 g of ammonium heptamolybdate tetrahydrate, 143 g of ammonium metavanadate and 126 g of ammonium paratungstate heptahydrate were dissolved in succession in 5500 g of water at 95 ° C to obtain a solution II. Then, solution I was stirred all at once in solution II, and then an aqueous solution at 25% by weight concentration of NH3 was added in an amount sufficient to form a solution again. This was spray-dried at an exit temperature of 110 ° C. The resulting spray-dried powder was kneaded with 0.25 kg of an aqueous solution of acetic acid at 30% concentration by weight per kg of powder using a ZS1-80 type kneader from Werner and Pfleiderer, and then dried at 110 ° C. for 10 hours in a dryer oven. 700 g of the catalyst precursor thus obtained were calcined in an air / nitrogen mixture [(200 1 N2 / 15 1 air) / h] in a rotary tubular oven (50 cm long, 12 cm internal diameter). During calcination, the kneaded material was first heated continuously from room temperature (approximately 25 ° C) to 325 ° C during the course of one hour. This temperature was then maintained for 4 hours. After the heating was carried out at 400 ° C in the course of 15 minutes, this temperature was maintained for one hour and then the cooling was carried out up to room temperature.

The catalytically active material, calcined, was crushed to obtain a finely divided powder, 50% of whose particles passed through a sieve of mesh size from 1 to 10 μ and whose fraction of the particles with a maximum dimension greater than 50 μ It was less than 1%.

Preparation of the coated catalyst kg of spherical supports (diameter 4-5 mm, steatite, having a surface roughness Rz in accordance with EP-B 714700 of 45 μ and with a total pore volume, based on the volume of supports, of <1% in volume, manufacturer: Caramtec DE) were introduced in a coating vessel (90 ° inclination angle: Lódige Hicoater, DE) having an internal volume of 200 1. The container for the coating then rotated at 16 rpm. 1600 g of an aqueous solution [sic] were sprayed onto the supports by means of a nozzle in the course of 25 minutes. At the same time, 5.3 kg of the catalytically active oxide powder of a) were introduced continuously through the vibrating channel out of the spray cone of the atomizing nozzle in the same period. During the coating, the feed powder was completely adsorbed on the surface of the supports, and agglomeration of the finely divided oxide active material was not observed. After the completion of the addition of the powder and the aqueous solution, hot air at 110 ° C was introduced into the coating vessel for 20 minutes at a speed of 2 rpm. Coated spherical catalysts were obtained whose proportion of the oxide active material was 15% by weight, based on the total material. The thickness of the coating was 210 ± 5 μ, both on the surface of a support and on the surface of different supports. A bed of the resulting coated catalyst beads formed a coated, fixed bed, comparative, spherical catalyst 2. 1. Preparation of raw material 1 209. 3 kg of tungstic acid (72.94 wt.% Of W) were stirred at 25 ° C, in 775 kg of an aqueous solution of bismuth nitrate containing nitric acid (11.2 wt.% Of Bi, from 3 to 5 wt.% Of free nitric acid, density: from 1.2 to 1.27 g / mol The resulting aqueous mixture was then stirred for more than 2 hours at 25 ° C and then spray-dried.

The spray-drying was carried out in a spraying tower with rotating discs by the co-current method at a gas outlet temperature of 300 ± 10 ° C and a gas outlet temperature [sic] of 100 ± 10 ° C. The spray-dried powder obtained was then calcined at a temperature of 780 to 810 ° C (in a rotary tubular furnace through which air flowed (1.54 m3 internal volume, 200 m3 (P.T.N.) of air / h)). What is important with respect to the exact determination of the calcination temperature is that it was designed for the composition of the desired phase of the calcination product. The phases W03 (monoclinic) and Bi2W209 are desired; the presence of? -Bi2W06 (ruselite) is unwanted. Consequently, if the compound? -Bi2W06 is still detectable after calcination, based on a reflection in the powder X-ray diffraction pattern at a reflection angle of 2? = 28.4 ° (CuKa radiation), the preparation must be repeated and the calcination temperature must be increased within the established temperature range until the reflection disappears. The mixed, calcined, preformed, thus obtained oxide was crushed so that the X50 value (see Ullmann's Encyclopedia of Industrial Chemistry, 6th edition (1998) Electronic Relay, section 3.1.4 or DIN 66141) of the resulting particles outside 5 μ. The crushed material was then mixed with 1% by weight (based on the crushed material) of finely divided Si02 (bulk density 150 g / 1; X5o value of the Si02 particles was 10 μ and the BET surface area was 100 m2 / g). 2. Preparation of raw material 2 A solution A was prepared by dissolving 213 kg of ammonium heptamolybdate in 600 1 of water at 60 ° C with stirring, and 0.97 kg of an aqueous solution of potassium hydroxide (46.8% by weight of KOH) was added at 20 ° C to the resulting solution while maintaining at 60 ° C and with agitation.

A solution B was prepared by introducing 116.25 kg of an aqueous solution of iron nitrate (14.2% by weight of Fe) into 262.9 kg of an aqueous solution of cobalt nitrate (12.4% by weight of Co) at 60 ° C. Then solution B was pumped continuously to solution A initially taken for a period of 30 minutes while maintaining it at 60 ° C. Then it was stirred for 15 minutes at 60 ° C. 19.16 kg of a silica gel (46.80% by weight of Si02, density from 1.36 to 1.42 g / ml, pH from 8.5 to 9.5, maximum alkali metal content 0.5% by weight) were added to the resulting aqueous mixture and then stirred for another 15 minutes at 60 ° C.

The spray drying was then carried out in a spraying tower with rotating discs by the co-current method (gas inlet temperature: 400 ± 10 ° C, gas outlet temperature: 140 ± 5 ° C. by resulting spray had an ignition loss of approximately 30% by weight (3 h at 600 ° C). 3. Preparation of the multimetal oxide active material The raw material 1 was mixed homogeneously with the raw material 2 in the amount necessary for the active material multimetal oxide with the stoichiometry: [Bi2W29 • 2wO3lo.5fM012C05.5 e2.54Si1.5sKo.oeOx]! Based on the aforementioned total material, further 1.5% by weight finely divided graph (sieve analysis: minimum 50% by weight <24 μ, maximum 10% by weight> 24 μ and <48 μ, maximum 5 % by weight> 48 μ, BET surface area: from 6 to 13 pr / g) were mixed homogeneously. The resulting anhydrous mixture was compressed to obtain solid cylinders with a length of 3 mm, outer diameter of 5 mm and then subjected to heat treatment as follows.

In a muffle furnace through which air flowed (60 1 internal volume, 1 1 / h of air per gram of the precursor of the active material), the heating was carried out at a heating rate of 150 ° C / h, initially from the room temperature (25 ° C) up to 180 ° C. This temperature was maintained for 1.5 hours and then increased to 200 ° C at a heating rate of 60 ° C / h. The 200 ° C, in turn, was maintained for 1.5 hours before increasing at a heating rate of 60 ° C / h to 220 ° C. This temperature was similarly maintained for 1.5 hours before it was increased to 250 ° C, once again at a heating rate of 60 ° C / h. Then, the 250 ° C was maintained in the same way for 1.5 hours. Then, the cooling was performed first at room temperature, and the decomposition phase was thus practically complete. Then, the heating was carried out at a heating rate of 180 ° C / h up to 465 ° C and this calcination temperature was maintained for 4 hours. A catalyst bed without resultant support formed the comparative fixed-bed catalyst 1 in the form of solid cylinders. f) Preparation of the comparative fixed-bed catalyst 2 in the form of solid cylinders 1. Preparation of the catalytically active oxide material M ?? 2V3W? .2CU2.4On 190 g of copper (II) acetate monohydrate were dissolved in 2700 g of water to obtain a solution I. 860 g of ammonium heptamolybdate tetrahydrate, 143 g of ammonium metavanadate and 126 g of ammonium paratungstate heptahydrate were dissolved in succession in 5500 g of water at 95 ° C to obtain a solution II. Then, solution I was stirred all at once in solution II, and then an aqueous solution at 25% by weight concentration of NH3 was added in an amount sufficient to form a solution again. This was spray-dried at an exit temperature of 110 ° C. The resulting spray-dried powder was kneaded with 0.25 kg of an aqueous solution of acetic acid at 30% concentration by weight per kg of powder using a kneader of type ZS1-80 of Werner and Pfleiderer, and then dried at 110 ° C. for 10 hours in a dryer oven. 700 g of the catalyst precursor thus obtained were calcined in an air / nitrogen mixture [(200 1 N2 / 15 1 air) / h] in a rotary tubular oven (50 cm long, 12 cm internal diameter). During calcination, the kneaded material was first heated continuously from room temperature (approximately 25 ° C) to 325 ° C during the course of one hour. This temperature was then maintained for 4 hours.

Then the heating was carried out at 400 ° C in the course of 15 minutes, this temperature was maintained for one hour and then the cooling was carried out up to room temperature.

The catalytically active material, calcined, was crushed to obtain a finely divided powder, 50% of whose particles passed through a sieve of mesh size from 1 to 10 μ and whose fraction of the particles with a maximum dimension greater than 50 μ It was less than 1%.

After mixing 3% by weight (based on the active material) of graphite, the catalytically active material thus obtained was compressed to obtain solid cylinders with a length of 3 mm and an external diameter of 5 mm.

A bed of the resulting unsupported catalysts formed the comparative fixed bed catalyst 2 in the form of solid cylinders.

Catalytic oxidation in gas phase from propene to acrylic acid The first reaction stage A first reaction tube (stainless steel V2A, external diameter 30 mm, wall thickness 2 mm, internal diameter 26 mm, length: 436 cm, with a thermal tube (external diameter 4 mm) centered in the middle of the reaction tube for receiving a thermocouple with which the temperature in the reaction tube can be determined) was loaded from the base to the top on a catalyst supporting slab (44 cm long), first with soapstone beads with a surface rough (from 4 to 5 mm in diameter, inert material for heating the initial mixture of reaction gases 1) over a length of 30 cm and then with the fixed bed catalyst 1 prepared in a) (or in c) or in e)) over a length of 300 cm, before the load was complete over a length of 30 cm with the above-mentioned soapstone beads as a subsequent bed. The remaining 35 cm of the catalyst tube was left empty.

This part of the first reaction tube that had been charged with solid was thermostated by means of 12 aluminum blocks fused in cylindrical form around the tube and each having a length of 30 cm and being heated by electrical heating tapes (comparative experiments with a corresponding reaction tube heated by means of a salt bath through which nitrogen was bubbled showed that the thermostatization by means of aluminum block could simulate the thermostating by means of a salt bath). The first six aluminum blocks in the flow direction defined a reaction zone A and the remaining aluminum blocks defined a reaction zone B. The ends of the reaction tube that were free of solids were maintained at 220 ° C by means of steam at superatmospheric pressure.

The second reaction stage A second reaction tube (stainless steel V2A, external diameter 30 mm, wall thickness 26 mm, internal diameter 26 mm, length: 439 cm, with a thermal tube (external diameter 4 mm) centered in the middle of the reaction tube for receiving a thermocouple with which the temperature in the reaction tube could be determined) was loaded from the bottom to the top on a catalyst support plate (44 cm long), first with steatite beads with a rough surface (from 4 to 5 mm in diameter, inert material for heating the initial mixture of reaction gases 2) for a length of 30 cm and then with the fixed-bed catalyst prepared in B) (or in d) of)) on a length of 300 cm, before the load was complete in a length of 30 cm with the soapstone beads mentioned above as a preliminary bed. The remaining 35 cm of the catalyst tube was left empty.

This part of the second reaction tube that had been charged with solid was thermostated by means of 12 aluminum blocks cast in cylindrical form around the tube and each having a length of 30 cm (in comparative experiments with a corresponding heated reaction tube). by means of a salt bath through which nitrogen was bubbled, it was demonstrated that the thermostatization by means of the aluminum block was able to simulate the thermostating by means of a salt bath). The first 6 aluminum blocks in the flow direction defined a reaction zone C and the remaining 6 aluminum blocks defined a reaction zone D. The ends of the reaction tube that are free of solid were maintained at 220 ° C by of superatmospheric pressure steam.

Oxidation in gas phase The first reaction tube described above was fed continuously with an initial mixture of reaction gases having the following composition, the charge and the thermostatization of the first reaction tube was established as follows: from 6 to 6.5% by volume of propene from 3 to 3.5% by volume of H20 from 0.3 to 0.5% by volume of CO from 0.8 to 1.2% by volume of C02 from 0.025 to 0.04% by volume of acrolein, and from 10.4 to 10.7% by volume of 02 The remaining amount up to 100% consisting of molecular nitrogen.

A small sample of the gas mixture resulting from the first reaction stage was taken at the outlet of the first reaction tube for gas chromatographic analysis. Otherwise, the product gas mixture was fed directly to the oxidation step of the subsequent acrolein (oxidation to acrylic acid) with introduction of air at 25 ° C through a nozzle (reaction step 2). A small sample of the gas mixture produced by the acrolein oxidation step in the same way was taken for gas chromatographic analysis. Otherwise, acrylic acid was separated from the gas mixture resulting from the second reaction step in a manner known per se, and a part of the waste gas was reused to charge the propene oxidation step (ie, as recycled gas). ), which explains the acrolein content of the aforementioned feed gas and the small variants in the composition of the feed.

The pressure at the inlet of the first reaction tube varied, as a function of the chosen propene load, in a range from 3.0 to 0.9 bar. A test point was also present at the end of reaction zones A, C. The pressure at the entrance of the second reaction tube varied, as a function of the acrolein load, in the range of 2 to 0.5 bar.

The results obtained for the various catalyst beds as a function of the chosen loads and the chosen aluminum thermostatization and the air supply implemented (after the first reaction stage) are shown in the following tables (the upper example can, according to the With the invention, it can also be carried out in tubular reactors in an area in a corresponding manner (ie, for example with the same load), the temperature in the first stage should then conveniently be from 320 to 360 ° C and that of the second stage from 245 to 275 ° C, in this context, it is also advisable, in the first reaction stage, to replace the 300 cm bed of the fixed bed catalyst 1 by the following bed conveniently only in a length of 270 cm: the direction of flow, first a mixture of 65% by volume of fixed bed catalyst 1 and 35% by volume of steatite rings (external diameter per internal diameter per length) ud equal 5 mm x 3 mm x 2 mm) in a length of 100 cm and then a mixture of 90% by volume of the fixed bed catalyst 1 and 10% by volume of the above soapstone rings in a length of 170 cm; it was also advisable in this context, in the second reaction stage, to replace the 300 cm bed of the fixed bed catalyst 2 by the following bed of corresponding length: in the flow direction, first a 70% by volume mixture of the catalyst fixed bed 2 and 30% by volume of steatite rings (external diameter x internal diameter x length = 7 mm x 3 mm x 4 mm) over a length of 100 cm and then 200 cm of pure bed 2 fixed catalyst) .

TA, TB, TC and TD are the temperatures of the aluminum blocks in reaction zones A, B, C and D.

CPA is the propene conversion at the end of the reaction zone A.

BpB is the conversion of propene to the end of reaction zone B.

SDP is the selectivity of the acrolein formation and the formation of the acrylic acid by-product together after the first reaction stage and based on the converted propene.

CAc is the conversion of acrolein at the end of the reaction zone C.

CAD is the conversion of acrolein at the end of reaction zone B.

CPD is the propene conversion at the end of the reaction zone D.

SAA is the selectivity of acrylic acid formation after the second reaction step based on the converted propene.

STYAA is the space-time yield of acrylic acid at the outlet of the second reaction tube.

R is the molar ratio of molecular oxygen to acrolein in the initial mixture of reaction gases 2.

M is the amount of air fed through a nozzle after the first reaction stage.

Table 1 Catalyst bed: Novel fixed bed catalyst 1 / novel fixed bed catalyst 2 Table 2 Catalyst bed: Coated catalyst, fixed bed, comparative 1 / coated catalyst, fixed bed, comparative 2 Table 3 Catalyst bed: cylindrical catalyst without support, fixed bed, comparative 1 / cylindrical catalyst, without support, fixed bed, comparative 2

Claims (1)

1. A process for the gas phase catalytic oxidation of propene to acrylic acid, in which an initial mixture of reaction gases 1 comprising propene, molecular oxygen and at least one inert gas, and contains the molecular oxygen and propene in a ratio molar 02: C3Hd de = l first passes, in a first stage reaction at elevated temperatures, on a first fixed-bed catalyst whose active material is at least one multimetal oxide containing molybdenum and / or tungsten and bismuth, telurine [sic], antimony, tin and / or copper, in such a way that the conversion of propene in a single step is > 90% molar, and the associated selectivity of acrolein formation and the formation of the acrylic acid byproduct together is > 90% molar, the temperature of the gas mixture product leaving the first reaction stage, if required, is reduced by direct and / or indirect cooling and, if required, molecular oxygen and / or inert gas is added to the gaseous product mixture, and the product gas mixture, as the initial mixture of reaction gases 2 containing acrolein, molecular oxygen and at least one inert gas and contains the molecular oxygen and acrolein in a molar ratio 02: C3H40 of > 0.5, then passes, in a second stage of reaction at elevated temperatures, on a second fixed-bed catalyst whose active material is at least one multimetal oxide containing molybdenum and vanadium, in such a way that the conversion of acrolein in a single step is > 90% molar, and the selectivity of the balanced acrylic acid formation over the reaction steps is > 80% molar, based on the converted propene, where: a) the loading of the first fixed-bed catalyst with the propene contained in the initial mixture of reaction gases 1 is > 160 liters (S. T. P.) of propene / liter of the catalyst bed per hour b) the loading of the second fixed-bed catalyst with the acrolein contained in the initial mixture of reaction gases 2 is > 140 liters (S. T. P.) of acrolein / liter of catalyst bed per hour, and c) the geometry of the catalytic molded parts of the first fixed-bed catalyst and the geometry of the catalytic molded parts of the second fixed-bed catalyst are ring-shaped, with the proviso that: the external diameter of the ring is from 2 to 11 mm - the length of the ring is from 2 to 11 mm, and - the thickness of the wall of the ring is from 1 to 1 mm. A process for the gas phase catalytic oxidation of propene to acrolein and / or acrylic acid, in which an initial mixture of reaction gases 1 consisting of propene, molecular oxygen and at least one inert gas and contains the molecular oxygen and the propene in a molar ratio 02: C3H6 of = 1 passes, in a first stage reaction at elevated temperatures, on a first fixed-bed catalyst whose active material is at least one multimetal oxide containing molybdenum and / or tungsten and bismuth, tellurium, antimony, tin and / or copper, in such a way that the conversion of propene in a single step is > 90% molar and the associated selectivity of acrolein formation and the formation of the acrylic acid byproduct together is > 90% molar, where: ^ gggge ^ a) the charge of the fixed-bed catalyst with the propene contained in the initial mixture of reaction gases 1 is = 160 L (S.T.P) of propene / 1 of the catalyst bed per h, and b) the geometry of the molded parts of the catalyst of the fixed-bed catalyst is annular, with the proviso that - the external diameter of the ring is from 2 to 11 mm - the length of the ring is from 2 to 11 mm, and - the thickness of the ring wall is from 1 to 5 mm.