DE10325487A1 - Thermal treatment of precursor material of catalytically active material, for partial gas-phase oxidation of acrolein to acrylic acid, involves circulating gas stream flowing through rotary tube furnace - Google Patents

Abstract

Thermal treatment of the precursor material of a catalytically active material in a rotary tube furnace involves circulating at least a portion of a gas stream flowing through the rotary tube furnace. Independent claims are also included for: (1) A rotary tube furnace apparatus comprising recycle gas compressor(s); exit gas compressor(s); pressure reducer(s); fresh gas feed(s); heatable rotary tube(s); and recycle gas pipe(s); and (2) A tube-bundle reactor comprising 5000-40000 catalyst tubes filled with a load whose catalysts contain a multi-element oxide material as active material. The load of the catalyst tube for a partial oxidation of acrolein to acrylic acid is such that when a random sample of 12 catalyst tubes is taken, the difference between the arithmetic mean activity and the highest or lowest activity is not >8[deg]C.

Description

This Invention relates to a method for the thermal treatment of the precursor material a catalytic active mass in a rotary kiln through which a gas stream flows.

Under the term “catalytic Active mass " in this document solids are understood that are in chemical Reactions, e.g. in chemical gas phase reactions, in addition to the reactants are used to either carry out the chemical reaction to reduce required temperature and / or the selectivity to increase the target product formation. It runs the chemical reaction usually at the surface (interface) of the catalytic active mass.

Examples for such heterogeneously catalyzed reactions, which are in principle both in the liquid phase can also be carried out in the gas phase heterogeneously catalyzed hydrogenations, dehydrogenations and oxide hydrogenations, but also partial oxidations and partial ammoxidations. there can the active mass in powder form as well as geometric moldings shaped (the latter can be used, for example, by inserting (absorbing) into the inner surface a preformed support body (man then speaks of a supported catalyst), by applying it to the outer surface of a preformed carrier body (one speaks then by shell catalyst) or by pressing (one speaks then by full catalysts). The shape can be already the precursor mass or are first impressed on the catalytic active composition.

A complete oxidation of an organic compound (for example a saturated or unsaturated hydrocarbon, an alcohol or an aldehyde) is understood in this document to mean that the total carbon contained in the organic compound is converted into oxides of carbon (CO, CO ₂ ).

All other reactions of organic compounds with oxygen (including the Oxydehydrogenations) are described in this document as partial oxidations considered. The partial ammoxidation differs from that partial oxidation due to the additional presence of ammonia.

It is now generally known that catalytic active materials already shaped or unshaped, usually available by placing one in usually not catalytically active or at most reduced active precursor mass generated, which may already be shaped or still unshaped, and this with increased Exposes temperature to a specific gas atmosphere.

For example, the DE-A 10211275 in their exemplary embodiments, the activation of a precursor mass of a dehydrogenation catalyst at elevated temperature (500 ° C.) in changing gas streams (hydrogen, air, nitrogen) and their use in catalysts for the heterogeneously catalyzed dehydrogenation of hydrocarbons in the gas phase.

The scriptures describe in a similar way EP-A 529853 . EP-A 318295 . EP-A 608838 , WO 01/96270, EP-A 731077 . EP-A 1260495 . EP-A 1254709 . EP-A 1192987 . EP-A 962253 . EP-A 1262235 . EP-A 1090684 . DE-A 19835247 . EP-A 895809 . DE-A 10261186 . EP-A 774297 , WO 02/24620, EP-A 668104 . DE-A 2161450 . EP-A 724481 . EP-A 714700 . DE-A 10046928 and DE-A 19815281 the thermal treatment of precursor masses of multielement oxide active materials in a wide variety of gas atmospheres. The resulting multielement oxide active materials are recommended and used in these documents as catalytic active materials in catalysts for a wide variety of heterogeneously catalyzed partial gas-phase oxidations and -ammoxidations of organic compounds.

in principle can such thermal treatments in a wide variety of furnace types such as e.g. tray ovens, rotary kilns, Belt calciners, fluid bed furnaces or shaft furnaces carried out become.

Of It is of increasing importance (cf. WO 02/24620) that the total precursor mass to be treated thermally under possible uniform conditions is dealt with in order to get the most to obtain a uniformly procured total amount of active mass.

For example are as suitable as possible uniformly procured amounts of active mass better for heterogeneous Gas phase catalysts with high reactant loading of the active mass, since they have particularly uniform thermal reaction ratios over a Enable reactor cross section.

In front against this background WO 02/24620 recommends thermal treatment such precursors using a special belt calciner. adversely What is associated with a band calcination, however, is that it is at rest the precursor mass he follows. However, normally form within such a stationary bed Temperature gradients from that a uniform thermal treatment over the entire precursor mass to be treated thermally counteract.

In contrast, a thermal would be preferred Treatment of the precursor mass in moving bed, as is the case with thermal treatments in rotary kilns. Due to the constant movement of the precursor mass in rotary kilns, it forms a continuously self-homogenizing bed, within which, for example, there is no formation of so-called "hot points" or "cold points" (in most thermal treatments of precursor masses, exothermic or endothermic processes take place , which lead to the formation of so-called "hot points" (locations of maximum temperature within the thermally treated precursor mass or to the formation of so-called "cold points" (locations of minimum temperature within the thermally treated precursor mass). Both too high and too low However, temperatures affect the catalytic properties of the active composition.

On another point of view is that with the vast number of thermal Treatments of precursor masses thermal decomposition processes of chemical substances contained in the precursor mass Components go hand in hand, whereby gaseous decomposition products are formed, which is advantageous to the resulting active mass quality or can have an adverse effect. In both cases one would be Self-homogenization in moving bed advantageous.

A thermal treatment of active mass precursor mass in the rotary kiln is described in the documents DE-A 19815281 (e.g. example 1), DE-A 10046928 (eg production example 1) and EP-A 714700 (Examples) stimulated.

Usually such a thermal treatment in a rotary kiln is on an industrial scale done so that the angle of inclination of the rotary tube to the horizontal on one value other than zero is set. The highest one The place of the rotary tube is the entry point of the precursor mass and the lowest point is the location of the material discharge. The rotary tube is operated continuously. That is, the thermal precursor mass to be treated is continuously fed to the rotary tube on one side, in Rotary tube continuously from the highest to the lowest Location transported and carried out there continuously. On the Learns way through the rotary tube the precursor mass usually thermal treatment.

adversely is that such a continuous driving style is only comparative short dwell times of the material to be thermally treated in the rotary tube allows.

to Setting the desired one gas atmosphere usually becomes countercurrent to the funded precursor mass a corresponding gas flow is passed through the rotary tube. in the In the simplest case, this can be made of air, but in other cases et al also from valuable gases (e.g. reducing gases such as hydrogen or ammonia or inert gases such as e.g. Nitrogen).

A disadvantage of the procedure described is the comparatively high demand for gases which are not used after leaving the rotary tube. Another disadvantage is that in the context of can not continue to exert their beneficial effect formed in the material stock by thermal decomposition, low applied, gases are discharged with the gas stream, and (for example, of NH ₄ ⁺ formed NH _3, or NO ₃ ^- formed NO ₂ , or formed from CO ₃ ^2- CO ₂ or CO). Such an advantageous effect can consist, for example, of a reducing effect.

The The desired temperature in the material in the rotary tube normally becomes generated indirectly by turning the rotary tube on from the outside certain temperature is brought.

Around pronounced in the rotary tube to avoid radial and axial temperature gradients, it would be desirable the one passed through the rotary tube Gas flow to the for the material in the rotary tube desired Preheated temperature into the rotary tube.

Usually would this Heat content of Gas flow when leaving the rotary tube in a disadvantageous way continue to be used.

The In view of this state, the object of the present invention was the technology therein, a process for the thermal treatment of the precursor material a catalytic active mass in a rotary tube furnace through which a gas stream flows disposal to face the disadvantages of the prior art methods largely remedies.

Accordingly was a process for the thermal treatment of the precursor mass a catalytic active mass in a rotary kiln through which a gas stream flows found, which is characterized in that at least a subset of the flowing through the rotary kiln Gas flow in a circle becomes.

The gas stream flowing through the rotary kiln can in the simplest case consist of air, but also contain valuable gases such as CO, CO ₂ , NH ₃ , N ₂ , NO _x , SO ₂ or acetonitrile or consist of these gases. However, it can also contain a mixture of these gases or consist of such a mixture.

The gas flow flowing through the rotary kiln can be> 0 to 500 Nm ³ / h (or up to 300 Nm ³ / h in the case of pulverulent precursors) in the process according to the invention. Often the gas flow through the rotary kiln will be ≥ 50 to 500 (or 300) Nm ³ / h and usually 100 to 300 Nm ³ / h.

to exercise of the method according to the invention a suitable rotary kiln device is required. Hereinafter the design of such should be shown without the inventive method or the rotary kiln devices used therefor in any one Restrict shape.

A schematic representation of this rotary kiln device, which is suitable as an example for the method according to the invention (and used in the exemplary embodiment) is shown in FIG 1 , to which the reference numbers used below also refer. The regulations are preferably computer-controlled.

The central element of such a rotary tube furnace device suitable for carrying out the method according to the invention is the rotary tube ( 1 ).

It can e.g. 4000 mm long and an inner diameter of 700 mm exhibit. It can be made of stainless steel 1.4893 and have a wall thickness of 10 mm (of course, come for the inventive method also tube lengths up to 15 m, often up to 12 m; As a rule, the length of the rotary tube is inventive method ≥ 1 m, in the Rule ≥ 4 m are; the inside diameter is usually 20 cm to 2 m, often at 20 cm to 1 m).

In expedient way can so-called lifting lances attached to the inner wall of the rotary kiln his. They serve primarily the purpose, the precursor mass to be thermally treated in the rotary kiln (the material to be thermally treated) and so the To promote self-homogenization in moving fill. Relative to the others as an example mentioned dimensions of the example described suitable according to the invention Rotary tube device (which all relate to each other and in the embodiment the height can be applied the lances are 5 cm. In principle, a single Full length lance extend the rotary tube. Conveniently, one will However, only extend the lance to a part of the length of the rotary tube. In the exemplary design variant can they e.g. a length of 23.5 cm. In these cases, it is advantageous according to the invention if at the same height of the rotary kiln equidistant around the circumference (e.g. 90 ° each) several (one multiple) lifting lances (e.g. four = one quadruple) attached are. Along The rotary tube is advantageous in terms of application technology such multiple. There are in the exemplary embodiment variant along the Rotary tube eight such quadruples (each at a distance of 23.5 cm). The stroke lances of two neighboring multiples (quadruples) sit on The circumference of the rotary tube is preferably offset from one another. At the beginning and at the end of the rotary kiln (first and last 23.5 cm in the exemplary Configuration) there are advantageously no lifting lances.

The speed of rotation of the rotary tube can advantageously be set variably. Typical speeds are> 0 to 5 or 3 revolutions per minute. The rotary tube is expediently both left and right rotatable. For example, when turning to the right, the material remains in the rotating tube, e.g. when turning to the left, the precursor mass to be treated thermally is determined by the inclination of the rotating tube from the entry ( 3 ) to discharge ( 4 ) conveyed and removed from the rotary tube using a discharge aid (eg a shovel).

The Angle of inclination of the rotary tube to the horizontal can be advantageous can be set variably. Typical range values are 0 ° to 4 ° or 2 °. In discontinuous In operation, it is actually 0 °. The deepest point is in continuous operation the rotary tube during material discharge.

The Material entry is advisable via a Cell rotary valve volume-controlled or mass-controlled using a scale carried out. The As already mentioned, material discharge is carried out via the direction of rotation of the rotary tube controlled.

at discontinuous operation of the rotary tube dimensioned as above (that for exercise of the method according to the invention, As already mentioned, also bigger or could be smaller) can thermally treat a material quantity of 200 to 600 kg become. It is usually only in the heated part of the rotary tube.

Around, as required according to the invention, at least a subset of the gas stream flowing through the rotary kiln lead in a circle to be able needed the rotary kiln device to be used according to the invention a circular line required for this (in the simplest case a pipe routing system). During this is set up statically, the rotary tube is necessary attached movably (rotation).

To at the rotary tube inlet and at the rotary tube out If the stationary and rotating elements are connected to each other, either ball bearings or gaps can be used, which are sealed by graphite or Teflon rings (graphite or Teflon press seals). Because of their high temperature resistance, graphite press seals are preferred.

Conveniently, rejuvenated the rotary tube protrudes at its beginning and end into the feeding or leading away Pipe of the circular line into it.

These compounds are advantageously flushed with sealing gas. The two flushing streams ( 11 ) supplement the gas flow led through the rotary tube at the entry of the gas stream into the rotary tube and at the outlet of the gas stream from the rotary tube. The chemical composition of the flushing streams can be adapted to the temperature desired in the rotary tube for the thermal treatment. For example, air can be used as the sealing gas. In particular in the case of thermal treatments carried out at higher temperatures, however, inert gas (for example nitrogen) is preferably used as the sealing gas, since in addition to the blocking effect, this also produces an oxidation protection. The amount of sealing gas is preferably kept low. A single flushing stream of this type is therefore expediently> 0 to 50 Nm ³ / h, preferably 1 to 50 Nm ³ / h.

The blocking effect is important, among other things, to prevent the gas atmosphere prevailing in the rotary tube itself, which often contains components that are not completely toxic (e.g. CO ₂ , NO _x , NH ₃ , CO, SO ₂ , acetonitrile) Static / rotating connection point does not get into the surrounding atmosphere.

The circulation gas flow (the gas circulation) is promoted by means of a circulation gas compressor ( 13 ) (Fan, for example of the type KXE 160-004030-00, from Konrad Reitz GmbH), which draws in in the direction of the gas stream flowing out of the rotary tube and presses in the other direction. Immediately behind the cycle gas compressor, the gas pressure is usually above atmospheric pressure (ie above one atmosphere). There is a circulating gas outlet behind the circulating gas compressor (via a control valve ( 14 ) Circulating gas can be discharged, which is discharged via the exhaust gas compressor ( 17 ) (Fan; e.g. type T100 / 315-R3 / 500, from Meidinger) is sucked in). According to the invention, a pressure reducer is expediently located behind the outlet point for the cycle gas ( 15 ), for example an aperture (cross-sectional taper; e.g. by a factor of 3). This opens up the possibility in a simple manner to draw in more gas on the suction side of the cycle gas compressor than to pass on the pressure side of the cycle gas compressor.

On this way, a slight negative pressure can be set in the rotary tube become. That is, the pressure of the gas stream flowing through the rotary tube can be below the ambient pressure when leaving the rotary tube of the rotary tube. This measure also contributes to this at that from the gas atmosphere nothing comes out in the rotary tube. A slight overpressure can also be present in the rotary tube relative to the ambient pressure can be set. Frequently one gets the pressure immediately behind the exit of the gas stream from the rotary tube to a value in the range of +1.0 mbar above up to –1.2 mbar below the external pressure to adjust. That is, the pressure of the gas stream flowing through the rotary tube can be below the ambient pressure when leaving the rotary tube of the rotary tube. This is preferred according to the invention.

The pressure in the rotary tube is expediently set on the basis of pressure measurements by means of a pressure sensor. Suitable pressure sensors are, for example, those of type AMD 220, from Hartmann & Braun, which operate on the pressure transmitter (diaphragm measuring principle) principle. The pressure sensor ( 16 ) positioned immediately behind the outlet of the gas flow from the rotary tube. Another pressure sensor can be located before the gas flow enters the rotary tube. This can also be the only pressure sensor. In the interaction of pressure sensor ( 16 ), Exhaust gas compressor ( 17 ) (Fan that draws in towards the control valve), cycle gas compressor and fresh gas supply can then be used to adjust the pressure. If the two compressors and the fresh gas supply are operated stationary (or non-controllable compressors are used), the control valve ( 14 ) the sole pressure adjusting screw.

The The speed of the gas flow through the rotary tube is more typical Way with values ≥ 0.1 m / s and ≤ 2 m / s.

The Heating the material in the rotary tube is advantageously carried out according to the invention by Heating the rotary tube from the outside. In principle, this heating can be done by direct firing. That is, there are below the rotary tube, for example attached burners, their hot ones Fuel gases, for example with the help of fans, from an envelope surrounded rotating tube become.

However, the rotary tube is preferably surrounded by an envelope, for example a cuboid, which has electrically heated (resistance heating) elements on its inside. For reasons of temperature uniformity, the envelope should be such on at least two opposite sides Have heating elements. However, the heating elements preferably surround the rotary tube on all sides. Their distance from the outer surface of the rotary tube is typically 10 to 20 cm. In the embodiment described here as an example ( 1 ) the rotating tube rotates freely in a cuboid ( 2 ), which has four successive, equally long, electrically heated (resistance heating) heating zones in the length of the rotary tube, each of which encloses the circumference of the rotary tube furnace. Each of the heating zones can heat the corresponding rotary tube section to temperatures between room temperature and 850 ° C. The maximum heating output of each heating zone is 30 kW. The distance between the electrical heating zone and the outer surface of the rotary tube is approximately 10 cm. At the beginning and at the end, the rotary tube protrudes approx. 30 cm from the cuboid.

In addition, a heater ( 10 ) the possibility of heating the gas flow into the rotary tube to the desired temperature in advance of its entry into the rotary tube (for example to the temperature desired for the material in the rotary tube), in order to support the temperature control of the material in the rotary tube furnace. If the thermal treatment of the precursor mass is accompanied by an exothermic chemical conversion in the precursor mass, the gas stream led into the rotary tube will advantageously be led into the rotary tube according to the invention at a temperature which is below the temperature (for example up to 50 ° C., often up to 20 °) C), which is provided for the material in the rotary kiln. If, on the other hand, the thermal treatment of the precursor mass is accompanied by an endothermic chemical conversion in the precursor mass, the gas stream led into the rotary tube will advantageously be led into the rotary tube according to the invention at a temperature which is above the temperature (for example up to 50 ° C., often up to 20 ° C), which is intended for the material in the rotary kiln.

In principle, the heater ( 10 ) be a heater of any kind. For example, it can be an indirect heat exchanger. In principle, such a heater can also be used as a cooler (by adjusting the temperature of the heat transfer medium, eg using a cooling sole). In principle, the heater can also be heated by means of direct or indirect firing. Direct or indirect steam heating can also be considered. In the case of direct steam heating, the circulating gas line will expediently be heated at least partially in order to prevent undesired condensation of the water vapor contained in the gas stream or to condensate the water vapor in a targeted manner if necessary. Often it will be an electric heater, in which the gas flow is conducted over electrically heated (resistance heating) metal wires. In the embodiment described by way of example, a CSN instantaneous water heater, type 97 D 80 from Schniewindt KG, 58805 Neuerade-DE, is used, the maximum output of which is 1 × 50 kW + 1 × 30 kW.

Both the heating of the rotary kiln and of the gas flow led into the rotary tube are controlled in an expedient manner via the temperature of the material in the rotary tube. This is usually determined using thermocouples protruding into the material. In the embodiment described by way of example, a lance is located on the central axis of the rotary tube ( 8th ), of which a total of three thermocouples at intervals of 800 mm ( 9 ) vertically into the material. The arithmetic mean of the three thermocouple temperatures is then understood as the temperature of the material. Within the material contained in the rotary tube, the maximum deviation of two measured temperatures is expediently less than 30 ° C., preferably less than 20 ° C., particularly preferably less than 10 ° C. and very particularly preferably less than 5 or 3 ° C.

Frequently one in the inventive method thermal treatment at a temperature of 100 to 1100 ° C, or 200 Carry out up to 800 ° C applies in particular to the case that the catalytic active composition is a multi-element oxide composition, e.g. is a multimetal oxide mass.

Especially the method according to the invention is preferred in the case of a multielement oxide mass as a catalytic active mass either at 600 to 800 ° C or at 300 to 600 ° C carried out (The aforementioned temperatures always mean the material temperature).

All is generally the result of the method according to the invention receive a catalytic active composition.

The Temperature rate with which the precursor mass in the rotating tube is heated for the purpose of their thermal treatment, is included method according to the invention normally at values ≤ 10 ° C / min. The temperature rate is often ≤ 8 ° C / min, often ≤ 5 ° C / min or ≤ 3 ° C / min and it is often at values of ≤ 2 ° C / min or ≤ 1 ° C / min. In the However, this temperature rate usually becomes ≥ 0.1 ° C / min, usually ≥ 0.2 ° C / min and often ≥ 0.3 ° C / min or ≥ 0.4 ° C / min.

How already mentioned, goes with the thermal treatment of the precursor mass of a catalytic Active mass often an exothermic chemical reaction, in which components of the through the rotary tube Gas flow are involved.

For the method according to the invention it is essential that this chemical reaction is not uncontrolled runs, the heat released by the chemical reaction is rapidly dissipated, as an uncontrolled one Course of excessive heat development and uncontrolled temperature increase of the thermally treated Lead material goods can, which ultimately leads to a reduction in the active mass properties leads.

In order to be able to ward off such an uncontrolled course of the thermal treatment of the material in good time, a rotary tube device suitable for the method according to the invention advantageously has rapid cooling. This can be done, for example, in a particularly efficient manner in that the envelope surrounding the rotary tube (the cuboid in the exemplary embodiment) has openings (longitudinal extension typically 60 cm), holes, on one side (for example in the lower part), through which means a fan ( 5 ) (e.g. one of the type E 315 / 40-63, from Ventapp GmbH) Ambient air or previously cooled air is sucked in and through flaps on the other (opposite) side (e.g. in the upper part) of the envelope ( 7 ) can be brought out with a variably adjustable opening. The rotary tube heating is often switched off at the same time. Such rapid cooling also makes it possible to terminate the thermal treatment precisely and thus to prevent thermal treatment that is too extensive. As an additional measure, the gas stream can be cooled (in the "heater ( 10 ) ") into the rotary tube.

The Quick cooling allows e.g. at the end of the thermal treatment of the precursor mass, the temperature of the material inside the rotary kiln a period of ≤ 5 h, mostly ≤ 4 h, often ≤ 2 h, at least To lower 300 ° C. As a rule, however, this cooling period not less than 0.5 h.

Between the pressure reducer ( 15 ) and the heater ( 10 ) you become the actually recirculated gas fraction ( 19 ) Appropriately perform the fresh gas metering in terms of application technology. As in the in 1 illustrated embodiment is often a fresh gas base load (e.g. nitrogen ( 20 )) added. The at least one splitter can then be used to react to the measured values of the various constituent sensors used and to regulate the constituent contents of the gas stream supplied in the rotary tube in detail. In the exemplary embodiment, this is a nitrogen / air splitter ( 21 ), which reacts to the measured value of the oxygen sensor.

The component sensors are conveniently located in front of the cycle gas compressor ( 18 ). However, they could also be located elsewhere.

These sensors often determine reducing gas components such as NH ₃ , CO, NO, N ₂ O, acetic acid, aerosols (eg ammonium acetate) and oxidizing components such as NO ₂ and O ₂ , the content of which often has to be within particularly narrow limits.

In the exemplary embodiment, two sensors ( 18 ) attached, namely one to determine the ammonia content and one to determine the oxygen content. The ammonia sensor generally works preferably according to an optical measuring principle (the absorption of light of a certain wavelength correlates in proportion to the ammonia content of the gas). In the exemplary embodiment, it is a device from Perkin & Elmer of the MCS 100 type. Oxygen sensors, on the other hand, are generally based on the paramagnetic properties of the oxygen. In the exemplary embodiment, an oximat from Siemens of the type Oxymat MAT SF 7MB 1010-2CA01-1AA1-Z is expediently used as the oxygen sensor.

in the The case of catalytic multimetal oxide active materials takes place at the transition from the precursor mass to the catalytic active composition in the context of the thermal according to the invention Treatment often a Oxygen absorption from the gas atmosphere in the rotary tube. through in front of the gas flow inlet into the rotary tube and behind the gas flow outlet Oxygen sensors attached to the rotary tube can absorb this oxygen quantitatively detected online and the oxygen supply in the rotary tube flowing through Gas flow can be adjusted accordingly (such an adjustment takes place not, usually the oxygen supply in the rotary tube supplied Gas flow kept essentially constant). Over- or underoxidation the active mass can be avoided and thus the catalyst performance improve. Completely the same can also be done with reducing gas components become.

The Circular gas operation according to the invention proves proves to be particularly advantageous in this context, if in the course of the thermal treatment the character of the im Rotary furnace prevailing gas atmosphere changed from "reducing" to "oxidizing" or vice versa becomes. Let them allow this change in a simple way perform, that the gas atmosphere always outside in the rotary tube of the explosion area.

Tests of the method according to the invention have frequently shown that the gas component sensors have displayed incorrect values.

accurate Studies of this state of affairs have shown that this is due is due that the gas stream flowing through the rotary tube usually from the thermally treated precursor mass stemming Very fine dust with which is on the sensor surface (The sensors usually work in such a way that they come from the gas flow Suck in partial quantities) and falsify the measurements. The sole application of filters installed in front of the sensors could solve this problem not help as they are dependent clogged by time and thus also falsify the measurement results.

A cyclone attached behind the outlet (but in front of the sensors) of the gas flow led through the rotary tube ( 12 ), for the separation of solid particles entrained in the gas flow (the centrifugal separator separates solid particles suspended in the gas phase by the interaction of centrifugal and gravity; the centrifugal force of the gas stream rotating as a spiral vortex accelerates the sedimentation of the suspended particles).

The use of such a cyclone also prevents the fine dust from depositing on the walls of the circulating gas line (such fine dust precipitation would be disadvantageous, for example, in the event of a product change; i.e. if the precursor mass to be treated thermally is changed, the subsequent mass is recycled as part of a recycle gas flow If necessary, contaminated with the dust of the precursor mass previously treated deposited on the walls; the fine dust can also contaminate the heater ( 10 ) damage (change in heat transfer) or clog).

The Solid matter separated in the cyclone is disposed of.

The Pressure loss caused by the cyclone is by the cycle gas compressor balanced.

Instead of In principle, a cyclone can also use any other device be, which is suitable for fine-particle solids from a gaseous To separate the dispersion phase.

The discharged cycle gas component ( 22 ) (Exhaust gas) often contains not completely harmless gases such as NO _x , CO, acetic acid, NH ₃ , etc., which is why they are normally separated in an exhaust gas cleaning device ( 23 ).

To the exhaust gas is usually initially via a Wash column performed (This is essentially a column free of internals, the front contains a separating pack at the outlet; the exhaust gas and watery spray are guided in cocurrent and in countercurrent (2 spray nozzles with opposite spray direction).

Out Coming to the scrubbing column, the exhaust gas is expediently led into a device that has a Contains fine dust filters (usually a bundle of bag filters) the inside of which the penetrated exhaust gas is carried away. Then it will be beneficial in the end burned in a muffle.

to Regulation of the amount of the rotary tube other than the sealing gas total fed Gas flow, is particularly suitable in the method according to the invention Principle of mass flow measurement based on Coriolis forces Principle of orifice measurement based on pressure differences and the principle of thermal convection.

In the exemplary embodiment, the amount of the gas flow which is different from the sealing gas and which is fed to the rotary tube is measured by means of a sensor ( 28 ) from KURZ Instruments, Inc., Montery (USA), model 455 Jr (measuring principle: thermal convective mass flow measurement using a temperature anemometer).

The rotary kiln device described can be operated both continuously and discontinuously. According to the invention, it is preferably operated batchwise. In continuous operation, material and gas phase are preferably passed through the rotary kiln in counterflow. For example, the multimetal oxide masses DE-A 19855913 or WO 02/24620 and the pre-formed phases required for their production (for example the bismuth tungstates and the bismuth molybdates) can be obtained in an outstanding manner by continuous thermal treatment of the precursor mass in a rotary kiln.

• a) wenigstens einen Kreisgasverdichter (13);
• b) wenigstens einen Abgasverdichter (17);
• c) wenigstens einen Druckminderer (15);
• d) wenigstens eine Frischgaszufuhr ((20), (21));
• e) wenigstens ein beheizbares Drehrohr (1); und
• f) wenigstens eine Kreisgasleitung.

Rotary kiln devices, which include:

• a) at least one cycle gas compressor ( 13 );
• b) at least one exhaust gas compressor ( 17 );
• c) at least one pressure reducer ( 15 );
• d) at least one fresh gas supply (( 20 ), ( 21 ));
• e) at least one heatable rotary tube ( 1 ); and
• f) at least one cycle gas line.

Become a compressor ( 13 ) and ( 17 ) that are not adjustable, the rotary kiln device is still at least one control valve ( 14 ) include.

Embodiments which are advantageous according to the invention also comprise at least one heater ( 10 ).

All particularly preferred embodiments included in addition integrated at least one cyclone.

Furthermore it is advantageous if the rotary kiln device is a device for rapid cooling having.

If it is intended to use the rotary kiln device also for thermal treatment of material, in which the gas flow through the rotary tube is not at least partially circulated, it is expedient if the connection between the cyclone ( 12 ) and cycle gas compressor ( 13 ) according to the three-way valve principle ( 26 ) closed and the gas flow directly into the exhaust gas purification device ( 23 ) can be performed. The connection to the exhaust gas cleaning device located behind the cycle gas compressor is also closed in this case according to the three-way valve principle. If the gas flow consists essentially of air, this is in this case via the cycle gas compressor ( 13 ) sucked in ( 27 ). The connection to the cyclone is also closed using the three-way valve principle. In this case, the gas stream is preferably drawn through the rotary tube, so that the internal pressure of the rotary tube is less than the ambient pressure.

Prefers the above treatment is carried out continuously and in countercurrent of material and gas flow. That is, the material to be thermally treated becomes at the opposite end like the gas flow (at the highest Point of the rotary tube) in the rotary tube furnace, and at the opposite End like the gas stream discharged from the rotary kiln.

at the exemplary embodiment the rotary tube device, the pressure behind the rotary tube outlet (of the gas flow) in continuous operation advantageously –0.8 mbar below the external pressure set horizontally. The same is the case for discontinuous operation Pressure setting –0.2 mbar.

How already mentioned, the method according to the invention is suitable for the thermal treatment of precursor masses of catalytic Active compounds of all kinds. The precursor compound can be in powder form as well as special (catalyst) shaped bodies of the thermal according to the invention Treatment.

• – zu jedem Zeitpunkt der thermischen Behandlung 0,5 bis 4 Vol.-% O₂,
• – über die Gesamtdauer der reduktiven thermischen Behandlung gemittelt 1 bis 8 Vol.-% NH₃, sowie
• – Wasserdampf und/oder Inertgas als Restmengen enthält,

wobei der NH₃-Gehalt der Atmosphäre während der thermischen Behandlung ein Maximum durchläuft, das unterhalb von 20 Vol.-% liegt.For example, it is suitable for the production of catalytically active multielement oxide materials which contain at least one of the elements Nb and W and the elements Mo and V, the molar proportion of the Mo element in the total amount of all elements of the catalytically active multielement oxide material other than oxygen being 20 mol%. % to 80 mol%, the molar ratio of Mo contained in the catalytically active multielement oxide composition to V, Mo / V contained in the catalytically active multielement oxide composition is 15: 1 to 1: 1, and the corresponding molar ratio Mo / ( Total amount from W and Nb) is 80: 1 to 1: 4. Such multielement oxide materials often still contain Cu in the corresponding molar Mo / Cu ratio of 30: 1 to 1: 3. The temperature range of the thermal treatment of the precursor mass typically extends to 300 to 600 ° C. In the following, the various advantages of the method according to the invention are shown again using examples of such multi-element oxide materials. They can be obtained, for example, by producing an intimate, also containing ammonium ions, dry mixture (the precursor mass) from starting compounds that contain the elemental constituents of the multi-element oxide mass other than oxygen, and this at temperatures in the range from 300 to 450 ° C ( Material temperature) thermally treated in a gas atmosphere containing O ₂ and NH ₃ . The gas atmosphere in which the thermal treatment should take place is according to the teaching of EP-B 72448 eg one that

• 0.5 to 4% by volume of O ₂ at any time during the thermal treatment,
• - Averaged over the total duration of the reductive thermal treatment 1 to 8 vol .-% NH ₃ , and
• Contains water vapor and / or inert gas as residual amounts,

the NH ₃ content of the atmosphere passes through a maximum during the thermal treatment, which is below 20% by volume.

The aforementioned Multielementoxidmassen can in addition to the elements Nb and / or W, and Mo, V and, if appropriate Cu additionally e.g. the elements Ta, Cr, Ce, Ni, Co, Fe, Mn, Zn, Sb, Bi, Alkali (Li, Na, K, Rb, Cs), H, alkaline earth (Mg, Ca, Sr, Ba), Si, Al, Ti and Zr contain. Naturally the multielement oxide active material can also consist only of the elements Nb and / or W as well as Mo, V and optionally Cu exist. You are suitable are particularly heterogeneous as active compositions for catalysts catalyzed partial gas phase oxidation of acrolein to acrylic acid.

Particularly suitable catalytically active multielement oxide compositions are sufficient as the active composition for catalysts for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid the following general stoichiometry I Mo ₁₂ V _a X ¹ _b X ² _c X ³ _d X ⁴ _e X ⁵ _f X ⁶ _g O _n (I), in which the variables have the following meaning:
X ¹ = W, Nb, Ta, Cr and / or Ce,
X ² = Cu, Ni, Co, Fe, Mn and / or Zn,
X ³ = Sb and / or Bi,
X ⁴ = one or more alkali metals (Li, Na, K, Rb, Cs) and / or H,
X ⁵ = one or more alkaline earth metals (Mg, Ca, Sr, Ba),
X ⁶ = Si, Al, Ti and / or Zr,
a = 1 to 6,
b = 0.2 to 4,
c = 0 to 18, preferably 0.5 to 18,
d = 0 to 40,
e = 0 to 2,
f = 0 to 4,
g = 0 to 40 and
n = a number determined by the valency and frequency of the elements in I other than oxygen, and
the variables within the specified ranges are to be selected with the proviso that the molar proportion of the element Mo in the total amount of all elements of the multi-element oxide mass (I) other than oxygen is 20 mol% to 80 mol%, the molar ratio of in Mo containing catalytically active multielement oxide composition (I) to V, Mo / V contained in the catalytically active multielement oxide composition (I) is 15: 1 to 1: 1, and the corresponding molar ratio Mo / (total amount of W and Nb) is 80: 1 is up to 1: 4 (and the corresponding molar Mo / Cu ratio is 30: 1 to 1: 3 if the multielement oxide mass contains Cu).

Among the active multielement oxide materials (I), those in which the variables lie in the following ranges are preferred:
X ¹ = W, Nb and / or Cr,
X ² = Cu, Ni, Co and / or Fe,
X ³ = Sb,
X ⁴ = Na and / or K,
X ⁵ = Ca, Sr and / or Ba,
X ⁶ = Si, Al and / or Ti,
a = 2.5 to 5,
b = 0.5 to 2,
c = 0.5 to 3,
d = 0 to 2,
e = 0 to 0.2,
f = 0 to 1,
g = 0 to 15 and
n = a number which is determined by the valency and frequency of the elements in I other than oxygen.

However, the following multielement oxide active materials II are very particularly preferred process products of the process according to the invention: Mo ₁₂ V _a X ¹ _b X ² _c X ⁵ _f x ⁶ _g O _n (II), in which the variables have the following meaning:
X ¹ = W and / or Nb,
X ² = Cu and / or Ni,
X ⁵ = Co and / or Sr,
X ⁶ = Si and / or Al,
a = 3 to 4.5,
b = 1 to 1.5,
c = 0.75 to 2.5,
f = 0 to 0.5,
g = 0 to 8 and
n = a number determined by the valency and frequency of the elements other than oxygen in II, and
the variables within the specified ranges are to be selected with the proviso that the molar proportion of the element Mo in the total amount of all elements of the multielement oxide active composition (II) other than oxygen is 20 mol% to 80 mol%, the molar ratio of in Mo containing catalytically active multielement oxide composition (II) to V, Mo / V, contained in the catalytically active multielement oxide composition (II) is 15: 1 to 1: 1, the corresponding molar ratio Mo / Cu 30: 1 to 1: 3 and the corresponding molar ratio Mo / (total amount of W and Nb) is 80: 1 to 1: 4.

to The production of such multi-element oxide materials is carried out as before said from sources suitable in a manner known per se (starting compounds) the elemental constituents of the desired multielement oxide active composition other than oxygen in the respective stoichiometric target aimed at in the multielement oxide active composition relationship and creates the most intimate, preferably finely divided, dry mixture, which then undergoes thermal treatment is subjected to the thermal treatment before or after the formation into (catalyst) shaped bodies of certain geometry can be done. According to the invention advantageous it occurs before that. The sources can either already be Act oxides, or such compounds that are caused by heating, at least in the presence of oxygen, can be converted into oxides. In addition to the oxides, the main starting compounds are therefore Halides, nitrates, formates, oxalates, acetates, carbonates or Hydroxides into consideration.

suitable The starting compounds of Mo, V, W and Nb are also their oxo compounds (Molybdate, Vanadate, Wolframate and Niobate) or those of these derived acids. Sources containing oxygen are also cheap.

The required content of ammonium ions in the intimate dry mixture can be achieved in a simple manner by incorporating a corresponding amount of ammonium ions into the intimate dry mixture. The ammonium ions can expediently be introduced into the intimate dry mixture, for example, by using the corresponding ammonium oxometalates as sources of the elements Mo, V, W or Nb. Examples include ammonium metaniobate, ammonium metavanadate, ammonium heptamolybdate tetrahydrate and ammonium paratungstate heptahydrate. Of course, ammonium suppliers such as NH ₄ NO ₃ , or NH ₄ Cl, or ammonium acetate, or ammonium carbonate, or ammonium hydrogen carbonate, or NH ₄ OH, or NH ₄ CHO, can also be added to the intimate dry mixture to be thermally treated, regardless of the starting compounds required as sources of the multielement oxide active composition ₂ , or ammonium oxalate can be incorporated.

The intimate mixing of the starting compounds can in principle be carried out in a dry state or done in wet form. If it is done in dry form, the Starting compounds expediently used as fine powder and after mixing e.g. to (catalyst) moldings more desired Compressed geometry (e.g. tableted), which is then the thermal according to the invention Treatment.

Preferably however, the intimate mixing takes place in wet form. Usually the starting compounds are in the form of an aqueous solution and / or Suspension mixed together. Particularly intimate dry mixes are obtained in the described mixing process if only from in dissolved Form from existing sources and starting compounds is assumed. As a solvent water is preferred. Then the aqueous mass (Solution or suspension) and the intimate dry mixture thus obtained if necessary treated directly thermally. Preferably done the drying process by spray drying (the outlet temperatures are usually 100 to 150 ° C) and immediately following the completion of the aqueous solution or suspension. That included Powder can be formed immediately by pressing. Frequently however, it turns out to be one immediate processing as too fine, which is why it is with the addition of e.g. Water is appropriately kneaded. frequently When kneading, an addition of a lower organic carboxylic acid (e.g. Acetic acid) as advantageous (typical additional amounts are 5 to 10% by weight on powder mass used).

The modeling clay is then either the desired Geometry molded, dried and then subjected to thermal treatment (leads to so-called full catalysts) or only to strands molded, thermally treated and then ground to a powder (usually <80 μm) usually with the addition of a small amount of water and optionally more common Binder is applied as a moist mass to inert carrier bodies. To When the coating is finished, it is dried again and thus an operational shell catalyst receive. The intimate mixing of the starting compounds takes place in the form of e.g. aqueous solution, so can impregnated with the same, also inert porous carrier body, dried and then to supported catalysts thermally treated according to the invention become. Coating can be used in the production of coated catalysts the carrier body too prior to the thermal treatment, i.e. e.g. with the humidified spray powder, respectively.

For shell catalysts suitable carrier materials are e.g. porous or non-porous Aluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such as magnesium or aluminum silicate (e.g. steatite of the type C 220 from CeramTec).

The Carrier bodies can be shaped regularly or irregularly be, being regularly shaped Carrier body with clearly developed surface roughness, e.g. Balls or hollow cylinders with a split layer are preferred.

Suitable is the use of essentially non-porous, rough-surface, spherical supports Steatite (e.g. type C 220 steatite from CeramTec), their diameter 1 to 8 mm, preferably 4 to 5 mm. But that is also suitable Use of cylinders as support bodies, the Length 2 up to 10 mm and their outer diameter 4 to 10 mm. In the case of rings as a carrier body the wall thickness above usually at 1 to 4 mm. Ring-shaped carrier bodies to be used preferably have a length of 2 up to 6 mm, an outer diameter from 4 to 8 mm and a wall thickness from 1 to 2 mm. Are suitable especially rings of geometry 7 mm × 3 mm × 4 mm (outer diameter × length × inner diameter) as a carrier body.

The coating of the carrier body with finely divided multielement oxide active composition obtainable by thermal treatment or its finely divided leading mass to be thermally treated (intimate dry mixture) is generally carried out in a rotatable container, as is known, for example, from DE-A 2909671 , the EP-A 293859 or from the EP-A 714700 is known. The procedure of EP-A 714700 is preferred.

Conveniently, is used to coat the carrier body the powder mass to be applied moistens the carrier body. After applying is usually by means of hot Air dried. The layer thickness of the powder mass applied to the carrier will be convenient in the range 10 to 1000 μm, preferably in the range from 50 to 500 μm and particularly preferably in the range from 150 to 250 μm.

in the In the case of full catalysts, the shaping can, as already mentioned, also before or after performing the thermal treatment.

For example can from the powder form of the multielement oxide active composition obtainable according to the invention or their not yet thermally treated precursor mass (the intimate dry mixture) by compression to the desired one Catalyst geometry (e.g. by tableting, extruding or Extrusion) full catalysts are prepared, where appropriate Tools such as Graphite or stearic acid as a lubricant and / or molding aid and reinforcing agents such as microfibers made of glass, asbestos, silicon carbide or potassium titanate can be added. Suitable unsupported catalyst geometries are e.g. Solid cylinder or Hollow cylinder with an outer diameter and a length of 2 to 10 mm. In the case of the hollow cylinder, the wall thickness is 1 to 3 mm appropriate. Of course you can the full catalyst also have spherical geometry, the spherical diameter Can be 2 to 10 mm.

Of course they can relevant multielement oxide active materials also in powder form, i.e. not shaped into certain catalyst geometries, as catalysts for the heterogeneously catalyzed partial oxidation of acrolein to acrylic acid used (e.g. also in a fluidized bed).

The partial gas phase oxidation of acrolein to acrylic acid itself can be carried out using the multielement oxide active compositions described, for example as in US Pat EP-A 724481 , or as in the DE-A 19910508 described.

With regard to the thermal treatment of the precursor compositions of the multi-element oxide compositions described above, the EP-B 724481 now fixed (p.5, lines 25 ff):
“The calcination atmosphere required according to the invention can be achieved in a simple manner, for example, by calcining in an oven through which a gas mixture is passed which has the appropriate composition with regard to O ₂ , NH ₃ , and inert gases / water vapor. In a less preferred embodiment, the required average ammonia content of the calcining atmosphere can also be achieved by incorporating a corresponding amount of ammonium ions into the dry mass to be calcined, which ammonium ions decompose during the calcination while releasing NH _3. "

The calcination was carried out in the EP-B 724481 in convection ovens, where a gas mixture containing a certain volume fraction of NH ₃ (vol.%) was supplied to them.

With the invention underlying this application, the less preferred embodiment of the EP-B 724481 a preferred embodiment. This can be attributed, among other things, to the fact that the process according to the invention no longer requires an external addition of NH ₃ into the calcination atmosphere, but rather, as a source of ammonia, requires the ammonium ions contained in the precursor mass to be thermally treated by the recycle gas. As a further advantage, the process of the present patent application reduces the expenditure of energy for the thermal treatment by virtue of the recycle gas guidance, since it uses the energy content contained in the recycle gas. In addition, the process according to the invention allows the production of large amounts of catalytic active compositions (in particular catalytically active multielement oxide compositions) with a particularly narrow activity distribution within the batch produced, owing to the bed which is continuously self-homogenizing in the rotary tube. This is particularly important in the case of the catalytically active multielement oxide compositions which contain at least one of the elements Nb and W and the elements Mo, V and Cu and which are to be used for the production of catalysts for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid , Especially if this gas phase partial oxidation is carried out at high acrolein loads, such as that DE-A 10307983 , the DE-A 19948523 and the DE-A 19910508 describe.

As a rule, this gas phase partial oxidation of acrolein is carried out in a tube bundle reactor having one or more temperature zones, such as, for example, that of EP-A 700714 , the EP-A 700 893 , the DE-A 19910508 , the DE-A 19948523 , the DE-A 19910506 , the DE-A 19948241 , the DE-C 2830765 , the DE-C 2513405 , the US-A 3147084 , the DE-A 2201528 , the EP-A 383224 and the DE-A 2903218 describe.

The solid catalyst bed is located in the metal tubes (catalysts) of the tube bundle reactor and the temperature control media or media are conducted around the metal tubes (if there is more than one temperature zone, a corresponding number of spatially separated temperature control media is conducted around the metal tubes). The temperature control medium is usually a molten salt. The reaction mixture is passed through the catalyst tubes.

The Contact tubes are common made of ferritic steel and typically have a Wall thickness from 1 to 3 mm. Their inside diameter is in Rule 20 to 30 mm, often 21 to 26 mm. Your length is expedient 2 to 4 m.

Appropriately from an application point of view, the number of contact tubes accommodated in the tube bundle container is at least 5000, preferably at least 10,000. The number of contact tubes accommodated in the reactor vessel is frequently 15,000 to 30,000. Tube bundle reactors with a number of contact tubes above 40,000 are more likely to form that Exception. The contact tubes are normally arranged homogeneously distributed within the container (preferably 6 equidistant neighboring tubes per contact tube), the distribution being expediently chosen such that the distance between the central inner axes from the closest contact tubes (the so-called contact tube division) is 35 to 45 mm (cf. e.g. EP-B 468290 ).

As Heat exchange means very cheap is the use of melting salts such as potassium nitrate, potassium nitrite, Sodium nitrite and / or sodium nitrate, or from low melting Metals such as sodium, mercury and alloys of various Metals.

Even though through suitable flow conditions in the tube bundle reactors In general, attempts are made to achieve that by looking at the reactor section the same salt bath temperature acts on each contact tube, can the existence of temperature gradients across the reactor cross section in in practice not entirely be avoided. If so now to the temperature gradients over the Distinct reactor cross-section Activity gradients across the individual contact tube feeds come, so this can Affect the safety of the operation of the tube bundle reactor, because the heat development due to the exothermic heterogeneously catalyzed fixed bed partial oxidation from acrolein to acrylic acid in the individual contact tubes of a tube bundle reactor in such a case would be significantly different. The latter because an increased activity the contact tube loading means that at the same temperature per Unit of time implemented more and thus more heat is developed.

The inventive method now allows regular loading of tube bundle reactors (with catalysts that act as catalytic active said Multielementoxidmassen, the at least one of the elements Nb and W and the elements Mo, V and optionally contain Cu) with 5,000 to 40,000 contact tubes, which is designed so that at one arbitrary picked sample of 12 contact tubes the difference between the arithmetic mean activity and the highest or lowest activity more than 8 ° C, frequently not more than 6 ° C, often not more than 4 ° C and cheap make is not more than 2 ° C.

Remarkable on the method according to the invention is that the above result is achieved even if the in Tube reactor total active mass contained in less than 100 or less than 75 or less than 50 approaches the thermal treatment of a precursor mass was produced. Frequently is the number of these approaches 5 to 40.

As Measure of the activity of the contact tube loading the temperature that the individual contact tube uses is used flowing around Salt bath (mixture of 53% by weight Potassium nitrate, 40% by weight sodium nitrite and 7% by weight sodium nitrate) must have thus with a single pass of a reaction gas mixture from 4.8 Vol.% Acrolein, 7 vol.% Oxygen, 10 vol.% Water vapor and 78.2% by volume nitrogen (when the catalyst feed is loaded with 85 Nl acrolein / l catalyst feed × h) through the loaded catalyst tube an acrolein conversion of 97 mol% is achieved (under "1 catalyst feed" the Volumes inside the catalyst tube not included where pure pre- or postfill are made of inert material, but only the bulk volumes, the shaped catalyst bodies (if necessary diluted with inert material) contain.

A as described achievable feeding of contact tubes in tube bundle reactors is especially important when the tube bundle reactor is loaded with acrolein in the catalyst feed operated ≥ 135 Nl / lh, or ≥ 150 Nl / l · h, or ≥ 160 Nl / l · h, or ≥ 170 Nl / l · h, or ≥ 180 Nl / l · h, or ≥ 200 Nl / l · h, or ≥ 220 Nl / l · h, or ≥ 240 Nl / l · h. Needless to say, is such a catalyst feed even with smaller acrolein loads advantageous.

In usually the acrolein load on the catalyst coating however ≤ 350 Nl / lh, or ≤ 300 Nl / lh, or ≤ 250 Nl / lh be.

Otherwise, the tube bundle reactor with the partial oxidation of acrolein to acrylic acid can be operated as it is DE-A 10307983 , the DE-A 19948523 and the DE-A 19910508 describe.

Finally, it should be noted that With the method according to the invention, residence times of the thermally treated precursor mass in the rotary tube furnace of ≥ 5 h, or ≥ 10 h, or ≥ 15 h, or ≥ 20 h, or ≥ 25 h are possible. As a rule, this dwell time will be ≤ 50 h.

Further it should be noted that the inventive method especially suitable for the production of multi-element oxide active materials the elements Mo, V, at least one of the two elements Te and Sb, and at least one of the elements from the group comprising Nb, Pb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, B, Ce, Sn, Zn, Si, Na, Li, K, Mg, Ag, Au and In in combination contain.

The thermal treatment is often carried out first at 150 to 350 ° C., preferably 250 to 350 ° C. under an oxidizing (oxygen-containing) atmosphere (for example air) and then at 350 to 1000 ° C., or 400 to 700 ° C. or 400 to 650 ° C with exclusion of oxygen (e.g. under N ₂ ). The thermal treatment in air can also be carried out continuously.

Prefers contains the combination of elements from the last element group Nb, Ta, W and / or Ti and particularly preferably the element Nb.

The relevant multielement oxide active materials preferably contain the aforementioned combination of elements in stoichiometry III Mo ₁ V _b M ¹ _c M ² _d (III), With
M ¹ = Te and / or Sb,
M ² = at least one of the elements from the group include Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, Ce, Sn , Zn, Si, Na, Li, K, Mg, Ag, Au and In,
b = 0.01 to 1,
c => 0 to 1, and
d => 0 to 1.

M ¹ = Te and M ² = Nb, Ta, W and / or Ti are preferred. M ² = Nb is preferred.

The stoichiometric Coefficient b is with Advantage 0.1 to 0.6. The preferred range is correspondingly for the stoichiometric Coefficients c from 0.01 to 1 or from 0.05 to 0.4 and favorable values for d 0.01 to 1 or 0.1 to 0.6.

Especially Cheap it is when the stoichiometric Coefficients b, c and d simultaneously in the aforementioned preferred ranges lie.

The The above applies in particular if the active composition with regard to their elements other than oxygen from a aforementioned combination of elements consists.

These are then in particular the multielement oxide active materials of general stoichiometry IV Mo ₁ V _b M ¹ _c M ² _d O _n (IV), where the variables have the meaning given with respect to stoichiometry III and n = a number which is determined by the valency and frequency of the elements other than oxygen in (IV).

Further the method according to the invention is suitable preferred for the production of such multielement oxide active materials which on the one hand either contain one of the aforementioned combinations of elements, or in terms of the elements other than oxygen, consisting of it and at the same time an X-ray diffractogram which shows diffraction reflexes h and i, their vertices at the diffraction angles (2⊝) 22.2 ± 0.5 ° (h) and 27.3 ± 0.5 ° (i) (all in this document related to an X-ray diffractogram Specifications relate to using Cu-Kα radiation as x-rays generated X-ray diffractogram (Siemens diffractometer theta-theta D-5000, tube voltage: 40 kV, tube current : 40mA, aperture diaphragm V20 (variable), anti-scatter diaphragm V20 (variable), secondary monochromator diaphragm (0.1 mm), detector aperture (0.6 mm), measuring interval (2⊝): 0.02 °, measuring time each Step: 2.4s, detector scintillation counter tube).

The The half-width of these diffraction reflections can be very small or also be very pronounced.

Especially the method according to the invention is suitable for the production of those of the aforementioned multi-element oxide active materials, their X-ray diffractogram additional has a diffraction reflex k for the diffraction reflexes h and i, the apex of which is 28.2 ± 0.5 ° (k).

Among the latter, those in which the diffraction reflex h is the most intense within the X-ray diffractogram and which has a half-value width of at most 0.5 ° are preferred for a production according to the invention, and the method according to the invention is very particularly suitable for those in which the Half-width of the diffraction reflex i and the diffraction reflex k is simultaneously ≤ 1 ° and the intensity P _{k of} the diffraction reflex k and the intensity P _{i of} the diffraction reflex i the relationship 0.2 ≤ R ≤ 0.85, better 0.3 ≤ R ≤ 0 , 85, preferably 0.4 ≤ R ≤ 0.85, particularly preferably 0.65 ≤ R ≤ 0.85, even more preferably 0.67 ≤ R ≤ 0.75 and very particularly preferably R = 0.70 to 0.75 or R = 0, 72, in the R that by the formula R = P i / (P i + P k ) is defined intensity ratio. The aforementioned X-ray diffractograms preferably have no diffraction reflection, the maximum of which is 2⊝ = 50 ± 0.3 °.

In this document, the definition of the intensity of a diffraction reflex in the X-ray diffractogram relates to that in the DE-A 19835247 , the DE-A 10122027 , as well as in the DE-A 10051419 and DE-A 10046672 laid down definition. The same applies to the definition of the full width at half maximum.

In addition to the diffraction reflections h, i and k, the aforementioned X-ray diffractograms of multielement oxide active materials which can be advantageously produced according to the invention also contain further diffraction reflections whose apex points lie at the following diffraction angles (2 ():
9.0 ± 0.4 ° (l)
6.7 ± 0.4 ° (o) and
7.9 ± 0.4 ° (p).

Is cheap it further if the x-ray diffractogram additionally contains a diffraction reflex, whose apex is at the diffraction angle (2⊝) = 45.2 ± 0.4 ° (q).

Often the X-ray diffractogram also contains the reflections 29.2 ± 0.4 ° (m) and 35.4 ± 0.4 ° (n).

It is also advantageous if the element combinations defined in the formulas III and IV are present as pure i-phase. If the catalytically active oxide mass also contains the k phase, its X-ray diffractogram contains, in addition to the above-mentioned, other diffraction reflections, the apex of which lies at the following diffraction angles (2⊝): 36.2 ± 0.4 ° (m) and 50 ± 0, 4 ° (the terms i- and k-phase are used in this document as in the DE-A 10122027 and DE-A 10119933 fixed used).

If the intensity 100 is assigned to the diffraction reflex h, it is advantageous according to the invention if the diffraction reflexes i, l, m, n, o, p, q have the following intensities in the same intensity scale:
i: 5 to 95, often 5 to 80, sometimes 10 to 60;
l: 1 to 30;
m: 1 to 40;
n: 1 to 40;
o: 1 to 30;
p: 1 to 30 and
q: 5 to 60.

Contains the X-ray diffractogram of the aforementioned additional Diffraction reflections, the full width at half maximum is usually ≤ 1 °.

The specific surface area of multielement oxide active compositions of the general formula IV to be produced according to the invention or of multielement oxide active compositions which contain element combinations of the general formula III is in many cases 1 to 30 m ² / g (BET surface area, nitrogen), especially if their X-ray diffractogram is designed as described ,

The preparation of the precursor compositions of the multi-element oxide active compositions obtainable as described can be found in connection with the cited prior art. These include in particular DE-A 10122027 , the DE-A 10119933 . DE-A 10033121 , the EP-A 1192987 , the DE-A 10029338 , JP-A 2000-143244, the EP-A 962253 , the EP-A 895809 , the DE-A 19835247 , WO 00/29105, WO 00/29106, the EP-A 529853 and the EP-A 608838 (Spray drying is to be used as the drying method in all exemplary embodiments of the latter two documents, for example with an inlet temperature of 300 to 350 ° C. and an outlet temperature of 100 to 150 ° C.; countercurrent or cocurrent). The conditions of the thermal treatment of these precursor masses can also be found in these documents.

The multielement oxidative compositions described can be shaped as such (ie in powder form) or into suitable geometries (cf. for example the shell catalysts of the DE-A 10051419 as well as the geometric variants of the DE-A 10122027 ) for the gas phase catalytic partial oxidation and / or ammoxidation of lower hydrocarbons and lower organic compounds such as propylene, propane and acrolein. They are particularly suitable for the production of acrolein and / or acrylic acid and for the production of acrylonitrile from propane and / or propene. The reaction conditions also contain the cited prior art.

The method according to the invention now also allows a regular loading of tube bundle reactors (with catalysts which, as the active composition, said catalytically active multi-element oxide compositions comprising the elements Mo, V, at least one of the two elements Te and Sb and at least one of the elements from the group comprising Nb, Ta etc. included in combination) with 5,000 to 40,000 contact tubes, which is designed so that, in the case of an arbitrarily selected sample of 12 contact tubes, the difference between the arithme table average activity and the highest or lowest activity is not more than 8 ° C, often not more than 6 ° C, often not more than 4 ° C and in favorable cases not more than 2 ° C.

Remarkable on the method according to the invention is that the above result is achieved even if the in the tube bundle reactor as a whole Active mass contained in less than 100 or less than 75 or less than 50 approaches to the thermal treatment of a precursor mass was produced. Frequently is the number of these approaches 5 to 40.

As Measure of the activity of the contact tube loading the temperature is used, which the individual Contact tube washing around Salt bath (mixture of 53% by weight potassium nitrate, 40% by weight sodium nitrite and 7 wt .-% sodium nitrate) must have, so with a single pass a reaction gas mixture of 4.8 vol.% acrolein, 7 vol.% oxygen, 10 vol.% Water vapor and 78.2 vol.% Nitrogen (with one load the catalyst feed with 85 Nl acrolein / l catalyst feed × h) the charged catalyst tube achieved an acrolein conversion of 97 mol% will (under “l Catalyst feed " the volumes inside the catalyst tube are not included, where there are pure prepayments and refills are made of inert material, but only the bulk volumes, the shaped catalyst bodies (if necessary diluted with inert material) contain.

A such feeding of contact tubes is particularly important if the tube bundle reactor operated with an acrolein load on the catalyst feed that is ≥ 135 Nl / lh, or ≥ 150 Nl / l · h, or ≥ 170 Nl / l · h, or ≥ 200 Nl / l · h, or ≥ 240 Nl / l · h. In the Usually it becomes ≤ 350 Nl / lh, or ≤ 300 Nl / lh or ≤ 250 Nl / lh be.

embodiment

A) Production of a precursor mass for the purpose of producing a multielement oxide mass of the stoichiometry Mo ₁₂ V ₃ W _1.2 Cu _2.4 O _x

16.3 kg of copper (II) acetate hydrate (content: 40.0% by weight CuO) were in 274 l water of temperature 25 ° C with stirring solved. It became a clear solution 1 received.

Spatially separated from this, 614 l of water were heated to 40 ° C. and 73 kg of ammonium heptamolybdate tetrahydrate (81.5% by weight of MoO ₃ ) were stirred in while maintaining the 40 ° C. The mixture was then heated to 90 ° C. in the course of 30 minutes and, while maintaining this temperature, 11.3 kg of ammonium metavanadate and 10.7 kg of ammonium paratungstate heptahydrate (88.9% by weight of WO ₃ ) were stirred in successively and in the order mentioned. A clear solution 2 was obtained.

Solution 2 was cooled to 80 ° C. and then solution 1 was stirred into solution 2. The resulting mixture was mixed with 130 l of a 25% by weight aqueous NH ₃ solution which had a temperature of 25 ° C. With stirring, a clear solution was formed which briefly had a temperature of 65 ° C. and a pH of 8.5. This was another 20 l of water at 25 ° C added. The temperature of the resulting solution then rose again to 80 ° C. and this was then spray-dried with a spray dryer from Niro-Atomizer (Copenhagen), type S-50-N / R (gas inlet temperature: 350 ° C., gas outlet temperature: 110 ° C). The spray powder had a particle diameter of 2 to 50 μm. 60 kg of the spray powder obtained in this way were metered into a kneader from the company VMK (Aachener Misch- und Knetmaschinen Fabrik) of type VM 160 (Sigma blades) and with the addition of 5.5 l acetic acid (≈ 100% by weight, glacial acetic acid) and 5.2 l of water kneaded (speed of the screw: 20 rpm). After a kneading time of 4 to 5 minutes, a further 6.5 l of water were added and the kneading process was continued until 30 minutes had passed (kneading temperature approx. 40 to 50 ° C.). The kneaded material was then emptied into an extruder and strands (length: 1-10 cm; diameter) using the extruder (Bonnot Company (Ohio), type: G 103-10 / D7A-572K (6 '' extruder W Packer) The strands were dried on the belt dryer for 1 hour at a temperature of 120 ° C. (material temperature) The dried strands formed the precursor mass to be treated thermally by the process according to the invention.

B) Production of the catalytic active composition by thermal according to the invention Treatment in a rotary kiln device

• – die thermische Behandlung erfolgte diskontinuierlich mit einer Materialgutmenge von 300 kg, die wie wie in A) beschrieben hergestellt worden war;
• – der Neigungswinkel des Drehrohres zur Horizontalen betrug ≈ 0°;
• – das Drehrohr rotierte mit 1,5 Umdrehungen/min rechts herum;
• – während der gesamten thermischen Behandlung wurde durch das Drehrohr ein Gasstrom von 205 Nm³/h geführt, der (nach Verdrängung der ursprünglich enthaltenen Luft) wie folgt zusammengesetzt war und an seinem Ausgang aus dem Drehrohr durch weitere 25 Nm³/h Sperrgasstickstoff ergänzt wurde: 80 Nm³/h zusammengesetzt aus Grundlast-Stickstoff (20) und im Drehrohr freigesetzten Gasen, 25 Nm³/h Sperrgasstickstoff (11), 30 Nm³/h Luft (Splitter (21)); und 70 Nm³/h rezirculiertes Kreisgas (19). Der Sperrgas-Stickstoff wurde mit einer Temperatur von 25°C zugeführt. Das Gemisch der anderen Gasströme wurde aus dem Erhitzer kommend jeweils mit der Temperatur ins Drehrohr geführt, die das Materialgut im Drehrohr jeweils aufwies.
• – innerhalb von 10 h wurde die Materialguttemperatur von 25°C im wesentlichen linear auf 300°C erhitzt; anschließend wurde die Materialguttemperatur innerhalb von 2 h im wesentlichen linear auf 360°C erhitzt; nachfolgend wurde die Materialguttemperatur innerhalb von 7 h im wesentlichen linear auf 350°C gesenkt; dann wurde die Materialguttemperatur innerhalb von 2 h im wesentlichen linear auf 420°C erhöht und diese Materialguttemperatur während 30 min gehalten;
• – dann wurden im durch das Drehrohr geführten Gasstrom die 30 Nm³/h Luft durch eine entsprechende Erhöhung des Grundlast-Stickstoff ersetzt (wodurch der Vorgang der eigentlichen thermischen Behandlung beendet wurde), die Beheizung des Drehrohres abgeschaltet und das Materialgut durch Einschalten der Schnellkühlung des Drehrohres durch Ansaugen von Umgebungsluft innerhalb von 2 h auf eine unterhalb von 100°C liegende Temperatur und schließlich auf Umgebungstemperatur abgekühlt; der Gasstrom wurde dem Drehrohr dabei mit einer Temperatur von 25°C zugeführt;
• – während der gesamten thermischen Behandlung lag der Druck (unmittelbar) hinter dem Drehrohrausgangs des Gasstroms 0,2 mbar unterhalb des Außendrucks.

The thermal treatment was carried out in a rotary kiln device according to 1 with the dimensions and auxiliary elements according to the exemplary embodiment in the description of this document and under the following conditions:

• - The thermal treatment was carried out discontinuously with a material quantity of 300 kg, which had been produced as described in A);
• - The angle of inclination of the rotary tube to the horizontal was ≈ 0 °;
• - The rotary tube rotated clockwise at 1.5 revolutions / min;
• - During the entire thermal treatment, a gas flow of 205 Nm ³ / h was passed through the rotary tube, which (after displacement of the air originally contained) was composed as follows and at its outlet from the Rotary tube was supplemented by a further 25 Nm ³ / h barrier gas nitrogen: 80 Nm ³ / h composed of base load nitrogen ( 20 ) and gases released in the rotary tube, 25 Nm ³ / h barrier gas nitrogen ( 11 ), 30 Nm ³ / h air (splitter ( 21 )); and 70 Nm ³ / h recirculated cycle gas ( 19 ). The sealing gas nitrogen was supplied at a temperature of 25 ° C. The mixture of the other gas streams coming from the heater was fed into the rotary tube at the temperature that the material had in the rotary tube.
• - The material temperature was heated from 25 ° C to 300 ° C essentially linearly within 10 h; the material temperature was then heated essentially linearly to 360 ° C. in the course of 2 h; subsequently the material temperature was reduced substantially linearly to 350 ° C within 7 h; the material temperature was then increased substantially linearly to 420 ° C. in the course of 2 h and this material temperature was held for 30 min;
• - Then the 30 Nm ³ / h of air in the gas flow led through the rotary tube were replaced by a corresponding increase in the base load nitrogen (which ended the process of the actual thermal treatment), the heating of the rotary tube was switched off and the material was switched on by switching on the rapid cooling of the The rotary tube is cooled to a temperature below 100 ° C. within 2 h and finally to ambient temperature by sucking in ambient air; the gas stream was fed to the rotary tube at a temperature of 25 ° C;
• - During the entire thermal treatment, the pressure (immediately) behind the rotary tube outlet of the gas stream was 0.2 mbar below the external pressure.

The Oxygen content of the gas atmosphere in the rotary kiln in all phases of the thermal treatment 2.99 vol.%. about the total duration of the reductive thermal treatment arithmetic the ammonia concentration of the gas atmosphere in the rotary kiln was averaged at 4 vol.%.

2 shows the amount of ammonia released as a function of the material temperature in ° C from the precursor mass as a percentage of the total amount of ammonia released from the precursor mass in the overall course of the thermal treatment.

3 shows the ammonia concentration of the atmosphere in vol .-% in which the thermal treatment took place depending on the material temperature in ° C during the thermal treatment.

4 shows, depending on the material temperature, the molar amounts of molecular oxygen and ammonia that were fed into the rotary tube per kg of precursor mass and hour via the thermal treatment with the gas stream.

C) Shape of the multimetal oxide active material

The catalytically obtained under "B)" active material was measured using a biplex cross-flow classifier mill (BQ 500) (from Hosokawa-Alpine Augsburg) into a fine powder ground, of which 50% of the powder particles are a sieve with a mesh size 1 to 10 μm passed and its proportion of particles with a longest dimension above 50 μm was less than 1%.

Using the ground powder, as in S1 EP-B 714700 annular support body (7 mm outer diameter, 3 mm length, 4 mm inner diameter, Steatite C220 from CeramTec with a surface roughness R ₂ of 45 μm) coated. The binder was an aqueous solution of 75% by weight of water and 25% by weight of glycerin.

The Active mass fraction of the resulting coated catalysts was however, in contrast to the aforementioned example S1 to 20 wt .-% (based on the total weight of carrier body and active mass) selected. The quantitative ratio powder and binder were adjusted proportionally.

D) Testing the coated catalysts

The coated catalysts were tested as follows in a model contact tube surrounded by a salt bath (mixture of 53% by weight of potassium nitrate, 40% by weight of sodium nitrite and 7% by weight of sodium nitrate):
Model contact tube: V2A steel, 2 mm wall thickness, 26 mm inside diameter, centered a thermo sleeve (to accommodate a thermocouple) with an outside diameter of 4 mm, 1.56 l of the free model contact tube space were filled with the shell catalyst;

The reaction gas mixture had the following starting composition:
4.8 vol% acrolein, 7 vol% oxygen, 10 vol% water vapor, 78.2 vol% nitrogen.

The Model contact tube was at 2800 Nl / h of reaction gas starting mixture loaded. This corresponds to a load on the catalyst feed of 86 Nl / l · h. The temperature of the salt bath was set so that with simple Passing an acrolein conversion of 97 mol% resulted.

In ten independent from each other 12 model contact tubes were loaded as described compared with each other.

In all cases lay for the required bath temperature in acrolein conversion Interval 257 ± 4 ° C. The selectivity of acrylic acid formation lay in all cases at about 94.8 mol%.

Claims (21)

Process for the thermal treatment of the precursor mass of a catalytic active composition in a rotary tube furnace through which a gas stream flows, characterized in that at least a partial amount of the gas stream flowing through the rotary tube furnace is circulated. A method according to claim 1, characterized in that the active mass is a multi-element oxide mass. A method according to claim 1 or 2, characterized in that that the active composition is a multi-element oxide composition, at least contains one of the elements Nb and W and the elements Mo and V, where the molar proportion of the element Mo in the total amount of all of Oxygen different elements of the catalytically active multi-element oxide mass Is 20 mol% to 80 mol%, the molar ratio of contained in the catalytically active multielement oxide mass Mo to contained in the catalytically active multielement oxide mass V, Mo / V, is 15: 1 to 1: 1, and the corresponding molar ratio Mo / (total amount from W and Nb) is 80: 1 to 1: 4. A method according to claim 1 or 2, characterized in that that the active mass is a multi-element oxide mass that the elements Mo, V, at least one of the two elements Te and Sb, and at least one of the elements from the group comprising Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, B, Ce, Sn, Zn, Si, Contains Na, Li, K, Mg, Ag, Au and In in combination. Method according to one of claims 1 to 4, characterized in that that the pressure in the rotary kiln is below the ambient pressure. Method according to one of claims 1 to 5, characterized in that the gas atmosphere in the rotary tube contains at least one of the components CO ₂ , acetic acid, NO _x , NH ₃ , CO, SO ₂ and acetonitrile. Method according to one of claims 1 to 6, characterized in that that the subset circulated of the gas stream flowing through the rotary kiln via a Cyclone guided becomes. Method according to one of claims 1 to 7, characterized in that that the temperature of the precursor mass at least at the end of the thermal treatment within a period of ≥ 0.5 h and ≤ 5 h Lowered 300 ° C becomes. Method according to one of claims 1 to 8, characterized in that that to regulate the total amount of gas flow supplied to the rotary tube either based on the principle of mass flow measurement of Coriolis forces, the principle of orifice measurement based on pressure differences or the principle of thermal convection is applied. Method according to one of claims 1 to 9, characterized in that that it is carried out in a rotary kiln device which includes: a) at least one cycle gas compressor; b) at least one exhaust gas compressor; c) at least one pressure reducer; d) at least one fresh gas supply; e) at least one heatable Rotary tube; and f) at least one cycle gas line. A method according to claim 10, characterized in that the rotary kiln device additionally at least one Control valve includes. A method according to claim 10 or 11, characterized in that the rotary kiln device additionally at least one Cyclone covers. Method according to one of claims 10 to 12, characterized in that that the rotary kiln device is also a device for rapid cooling has, which from an envelope surrounding the rotary tube consists of openings on one side has, through which ambient air is sucked in by means of a fan and through on the opposite Side of the envelope existing flaps with a variably adjustable opening can be brought out can. Method according to one of claims 1 to 13, characterized in that that the dwell time of the precursor mass in the rotary kiln ≥ 5 h is. Rotary kiln device comprising: a) at least a cycle gas compressor; b) at least one exhaust gas compressor; c) at least one pressure reducer; d) at least one fresh gas supply; e) at least one heatable rotary tube and f) at least one Circulating gas line. Rotary tube furnace device according to claim 15, which additionally comprises at least one control valve. Rotary tube furnace device according to claim 15 or 16, the additional still includes at least one cyclone. Rotary tube furnace device according to one of claims 15 to 17 that in addition still has a device for rapid cooling, which consists of a the envelope surrounding the rotary tube consists of openings on one side has, through which ambient air is sucked in by means of a fan and through on the opposite Side of the envelope Flaps with a variably adjustable opening can be brought out can. Tube reactor with 5000 to 40,000 contact tubes that are filled with one feed, whose catalysts contain a multielement oxide mass as active mass, the at least one of the elements Nb and W and the elements Mo and V contain, the molar proportion of the element Mo on the Total amount of all elements other than oxygen of the catalytic active multielement oxide mass is 20 to 80 mol%, that molar ratio of contained in the catalytically active multielement oxide mass Mo to contained in the catalytically active multielement oxide mass V, Mo / V, is 15: 1 to 1: 1, and the corresponding molar ratio Mo / (total amount from W and Nb) is 80: 1 to 1: 4, and by a method according to any one of claims 1 to 14 available is, the loading of the contact tubes with respect to a partial oxidation from acrolein to acrylic acid is such that in the case of an arbitrarily selected sample of 12 contact tubes, the difference between the arithmetic medium activity and the highest or smallest activity is not more than 8 ° C. Tube reactor with 5000 to 40,000 contact tubes that are filled with one feed, whose catalysts contain a multielement oxide mass as active mass, the elements Mo, V, at least one of the two elements Te and Sb, and comprising at least one of the elements from the group Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, B, Ce, Sn, Zn, Si, Na, Li, K, Mg, Ag, Au and In in combination contains and by a method according to any one of claims 1 to 14 available is, the loading of the contact tubes with respect to a partial oxidation from acrolein to acrylic acid is such that in the case of an arbitrarily selected sample of 12 contact tubes, the difference between the arithmetic mean activity and the highest or smallest activity not more than 8 ° C is. Tube reactor according to one of the claims 19 or 20, with an acrolein loading on the catalyst feed from ≥ 135 Nl / lh is operated.