EP1261597A1 - Method for producing maleic acid anhydride - Google Patents

Abstract

A method for producing maleic acid anhydride by heterogeneous catalytic gas phase oxidation of hydrocarbons comprising at least four carbon atoms with gases containing oxygen in the presence of a volatile phosphorus bond on a catalyst containing vanadium, phosphorus and oxygen in a multitube fixed-bed reactor unit consisting of at least two successive cooled reaction areas, wherein the temperature of the first reactor area is 350 - 450 °C and the temperature of the second and other reaction area is 350 - 480 °C, the difference in temperature between the hottest and coldest reaction area being at least 2 °C.

Description

Process for the preparation of maleic anhydride

description

The present invention relates to a process for the preparation of maleic anhydride by heterogeneous catalytic gas phase oxidation of hydrocarbons having at least four carbon atoms with gases containing oxygen in the presence of a volatile phosphorus compound on a vanadium, phosphorus and oxygen-containing catalyst in at least two successive and cooled Tube bundle having reaction zones - reactor unit.

Maleic anhydride is an important intermediate in the synthesis of γ-butyrolactone, tetrahydrofuran and 1,4-butanediol, which in turn are used as solvents or are further processed, for example, to give polymers such as polytetrahydrofuran or polyvinylpyrrolidone.

The production of maleic anhydride by heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms with oxygen in a tube bundle reactor over a vanadium-, phosphorus- and oxygen-containing catalyst-is generally known and, for example, in Ullmann's Encyclopedia of Industrial Chemistry, - edition 6 ^th , 1999 Electronic Release, Chapter "MALEIC AND FUMARIC ACID - Maleic Anhydride". In general, benzene or C-hydrocarbons, such as 1,3-butadiene, n-butenes or n-butane, are used. The vanadium, phosphorus and oxygen-containing catalysts, which are referred to hereinafter as "VPO catalysts", are unpromoted (see US 4,429,137, US 4,562,268, US 5,641,722 and US 5,773,382) or promoted (see US 5,011,945, US 5,158,923 and US 5,296,436) used. According to US 4,562,268, Example 18, a maximum space / time yield of 76 g / lh, ie 76 g maleic anhydride per 1 catalyst per hour was achieved. The space / time yield means the amount of product of value, ie maleic anhydride, in grams per hour and volume of the catalyst bed used, in liters.

The most important objective of the heterogeneous catalytic gas phase oxidations of hydrocarbons to maleic anhydride is basically to achieve the highest possible space / time yield over a long period of several months. It has already been recognized in US Pat. No. 3,296,282 that the deactivation of the VPO catalyst can be suppressed by adding an organic phosphorus compound. The best results were obtained when the phosphorus compound was fed after the oxidation reaction was interrupted. A maximum space / time yield of 77 g / lh could be achieved in the oxidation of 2-butene (see example 1 in the citation).

The published patent application EP-A-0 123 467 teaches that the phosphorus compound can also be added continuously during the oxidation reaction with the addition of steam. A space / time yield of 68 g / lh was achieved in the oxidation of n-butane (see example 1 in the citation).

It was recognized in US Pat. No. 4,515,899 that the activity of the VPO catalyst is damped by adding a phosphorus compound. Thus, a higher hydrocarbon load with improved selectivity is possible, which leads to an increased space / time yield. A space / time yield of 90 g / lh was achieved in the oxidation of n-butane (see example 3 in the citation).

US Pat. No. 5,185,455 discloses an optimization of the process parameters in the oxidation of n-butane to maleic anhydride over a VPO catalyst in the constant presence of trimethyl phosphate and water vapor. The maximum space / time yield achieved was 104 g / lh (see example 2 in the citation).

The conversion of the hydrocarbons to maleic anhydride is highly exothermic and is accompanied by a large number of possible parallel and subsequent reactions. With increasing hydrocarbon pollution, which is to be striven for in order to achieve a high space / time yield, the selectivity of the product of value decreases with constant hydrocarbon conversion due to the increased heat production.

In the published patent application EP-A-0 099 431 it was proposed to counter the mentioned effect of the reduced selectivity with increased heat production by using a structured catalyst bed, ie a catalyst bed which is adapted in terms of activity. The lowest catalyst activity is at the reactor inlet, the highest at the reactor outlet. In between, it can vary continuously or in stages. For the targeted adjustment of the catalyst activities, the published patent essentially teaches the dilution of the active catalyst particles with inert material and the use of differently active catalysts and, if appropriate, mixtures thereof. US Pat. No. 5,011,945 describes the use of a structured catalyst bed in the oxidation of n-butane over a VPO catalyst in the presence of a volatile phosphorus compound, the unreacted n-butane being recirculated after separation of maleic anhydride and by-products. The lowest catalyst activity is at the reactor inlet, the highest at the reactor outlet. A maximum space / time yield of 95 g / lh was described.

WO 93/01155 discloses a process for the preparation of maleic anhydride from n-butane over a VPO catalyst in the presence of a volatile phosphorus compound, in which the catalyst activity varies with the temperature and the n-butane concentration in the direction of flow of the gas such that the reaction rate is promoted by high activity in a range of low temperature and low n-butane concentration within the bed and by low activity in a critical area within the bed where the combination of temperature and n-butane concentration increases to an excessive level Sales and reaction temperature would result is limited. The maximum space / time yield achieved was 129 g / lh (see example 6 in the citation).

Wellauer et al. , Chem. Eng. Be. Vol. 41, No. 4 (1986), pages 765 to 772, describes a simulation model for the oxidation of n-butane to maleic anhydride based on experimental data from a known catalyst. The hotspot maximum, i.e. the maximum temperature in the catalyst bed caused by the exothermic reaction was identified as a critical parameter in relation to the yield. The publication teaches two measures to optimize the process: (a) the use of two catalyst beds with a low activity at the reactor inlet and a high activity at the reactor outlet and (b) the setting of two different salt bath temperatures. Even by means of computational optimization, only an increase in the experimentally determined space / time yield from 60 g / lh to 62 g / lh for (a) and 63 g / lh for (b) was obtained compared to the comparison process with a catalyst bed at a constant salt bath temperature (see table 2 in the quote).

According to the invention it was recognized that among the in Wellauer et al. disclosed conditions, the n-butane load on the catalyst can no longer be increased significantly, since the temperature of the hot spot maxima would rise significantly. This in turn would have one significant reduction in selectivity and thus a significant decrease in yield and space / time yield.

The object of the present invention was to develop a process for the preparation of maleic anhydride by heterogeneous gas-phase oxidation of a hydrocarbon having at least four carbon atoms with oxygen, which process has a high conversion, a high selectivity and a high rate when the catalyst is exposed to high hydrocarbons Yield of product of value and therefore a significantly higher space / time yield than is possible according to the prior art. Another object of the present invention was to find a flexible reaction procedure which, even with fluctuations in the amount, quality or purity of the starting materials or with progressive deactivation of the catalyst, enables a high space / time yield over a long period of time.

Accordingly, a process for the production of maleic anhydride by heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms with oxygen-containing gases in the presence of a volatile phosphorus compound on a vanadium, phosphorus and oxygen-containing catalyst was found in a tube bundle reactor unit having at least two successive and cooled reaction zones , characterized in that the temperature of the first reaction zone is 350 to 450 ° C and the temperature of the second and further reaction zone is 350 to 480 ° C, the temperature difference between the hottest and the coldest reaction zone being at least 2 ° C.

The term tube bundle reactor unit is understood to mean a unit comprising at least one tube bundle reactor. A tube bundle reactor in turn consists of at least one reactor tube which is surrounded by a heat transfer medium for heating and / or cooling. In general, the tube bundle reactors used industrially contain a few hundred to several tens of thousands of reactor tubes connected in parallel. If several individual tube bundle reactors (in the sense of tube bundle reactor apparatus) are connected in parallel, these are to be understood as the equivalent of a tube bundle reactor and are included in the term tube bundle reactor below.

If the tube bundle reactor unit consists of several tube bundle reactors, for example two, three, four or more, these are connected in series. In general, the tube bundle reactors are then connected in direct succession, ie the output stream of the tube bundle reactor becomes direct headed into the entrance of the following. However, it is also possible to remove and / or supply mass and / or energy between the two tube bundle reactors. For example, a part of the gas stream or a component thereof can be removed or a further gas stream can be supplied or the existing gas stream can be passed through a heat exchanger.

It is essential in the method according to the invention to use a tube bundle reactor unit which has at least two successive reaction zones and is operated at the aforementioned temperatures while maintaining the temperature difference mentioned. The term reaction zone is to be understood as an area within a tube bundle reactor which contains a catalyst and in which the temperature is kept at a uniform value. The temperature of a reaction zone is understood to mean the temperature of the catalyst bed located in this reaction zone, which would be present in the absence of a chemical reaction if the process were carried out. If this temperature is not exactly the same at all points, the term means the number average of the temperatures along the reaction zone. The first, second or further reaction zone is to be understood in each case as the first, second or further reaction zone lying in the direction of passage of the gas.

Depending on the embodiment, a tube bundle reactor can contain one, two, three or even further successive reaction zones. If a tube bundle reactor contains more than one reaction zone, generally spatially separated temperature control media are used.

The tube bundle reactor unit which can be used in the process according to the invention can be used, for example, for

two successive reaction zones are realized by

* a two-zone tube bundle reactor

* two single-zone tube bundle reactors connected in series

or with three successive reaction zones

* a three-zone tube bundle reactor

* a two-zone tube bundle reactor with a single-zone tube bundle reactor connected in series

* a single-zone tube bundle reactor with a two-zone tube bundle reactor connected in series * three single-zone tube bundle reactors connected in series.

The tube bundle reactor unit can furthermore also contain one or more preheating zones which heat the incoming gas mixture. A preheating zone integrated in a tube bundle reactor can be realized, for example, by reactor tubes filled with inert material, which are also surrounded by a heat transfer medium. In principle, all moldings which are chemically inert, ie do not induce or catalyze a heterogeneous catalytic reaction, and which have a maximum pressure drop below the respective, maximum tolerable, plant-specific value are suitable as inert material. Suitable are, for example, oxidic materials, such as A1 ₂ 0 ₃ , or metallic materials, such as stainless steel. Shaped bodies are, for example, spheres, tablets, hollow cylinders, rings, trilobes, tristars, wagon wheels, extrudates, randomly broken shaped bodies.

The temperature of the first reaction zone in the direction of passage is 350 to 450 ° C., preferably 380 to 440 ° C. and particularly preferably 380 to 430 ° C. The temperatures of the second and further reaction zones in the direction of passage are 350 to 480 ° C, preferably 380 to 460 ° C and particularly preferably 400 to 450 ° C. The temperature difference between the hottest and the coldest reaction zone is at least 2 ° C. In general, the hottest reaction zone is downstream of the coldest reaction zone, with other reaction zones in between. For the embodiment with two successive reaction zones, this means that the temperature of the second reaction zone is at least 2 ° C. above the temperature of the first reaction zone.

In principle, there are several possibilities according to the invention for the embodiments with three successive reaction zones. To simplify matters, the zones are numbered from 1 to 3 in the transmission direction and the temperature is abbreviated to T in the following.

a) "T (Zone 2) - T (Zone 1)" meets the minimum temperature difference according to the invention, i.e. T (Zone 2) is the hottest temperature and T (Zone 1) is the coldest temperature before the hottest zones. Thus T (Zone 3) is by definition lower than T (Zone 2).

b) "T (Zone 3) - T (Zone 1)" fulfills the minimum temperature difference according to the invention, ie T (Zone 3) is the hottest temperature and T (Zone 1) the coldest, before the hottest zones sensitive temperature. Thus T (Zone 2) is by definition between T (Zone 1) and T (Zone 3).

c) "T (Zone 3) - T (Zone 2)" meets the minimum temperature difference according to the invention, i.e. T (Zone 3) is the hottest temperature and T (Zone 2) is the coldest temperature before the hottest zones. Thus T (Zone 1) is by definition between T (Zone 2) and T (Zone 3).

Case (b) is preferred in which the temperature increases in the direction of passage from zone to zone. This also applies to the existence of more than three successive reaction zones.

The temperature difference between the hottest and the coldest reaction zone is preferably at least 5 ° C., particularly preferably at least 8 ° C., very particularly preferably at least 10 ° C. and in particular at least 12 ° C.

It was also surprisingly found that the yield of product of value, i.e. on maleic anhydride, significantly depends on the temperature difference of the emerging hot spot maxima. A hotspot maximum is the maximum of the temperature measured in the catalyst bed during the chemical reaction within a reaction zone. In general, the hotspot maximum is neither directly at the beginning nor directly at the end of a reaction zone, so that the temperature before and after the hotspot maximum is lower. If there is no maximum temperature within a catalyst bed of a reaction zone, i.e. Every point is at the same temperature, so this temperature is to be used as the maximum hot spot. With increasing temperature difference between the hot spot maximum of the second or subsequent reaction zone and the hot spot maximum of a reaction zone lying in front of it, the yield of product of value increases. The hot point maximum of the first, second or subsequent reaction zone is to be understood in each case as the hot point maximum of the first, second or subsequent reaction zone lying in the direction of passage of the gas.

In a preferred embodiment, at least one hot spot maximum in the second or subsequent reaction zone is higher than all hot spot maxima in the reaction zones lying in front of it. For the simple embodiment with two successive reaction zones, this means that the hot spot maximum of the second reaction zone is higher than that of the first. For the embodiment with three successive reaction zones, this means that either the hot spot maximum of the third reaction zone is higher than the hot spot maxima of the first and second, or that at least the hot spot maximum of the second reaction zone is higher than the hot spot maximum of the first.

In the process according to the invention, the hydrocarbons used are aliphatic and aromatic, saturated and unsaturated hydrocarbons with at least four carbon atoms, for example 1,3-butadiene, 1-butene, 2-cis-butene, 2-trans-butene, n-butane, C - Mixture, 1, 3-pentadiene, 1, 4-pentadiene, 1-pentene, 2-cis-pentene, 2-trans-pentene, n-pentane, cyclopentadiene, dicyclopentadiene, cyclopentene, cyclopentane, Cs mixture, Hexenes, hexanes, cyclohexane and benzene. 1-Butene, 2-cis-butene, 2-trans-butene, n-butane, benzene or mixtures thereof are preferably used. The use of n-butane is particularly preferred, for example as pure n-butane or as a component in n-butane-containing gases and liquids. The n-butane used can originate, for example, from natural gas, steam crackers or FCC crackers.

The hydrocarbon is generally added in a quantity-controlled manner, i.e. with constant specification of a defined quantity per unit of time. The hydrocarbon can be metered in liquid or gaseous form. Dosing in liquid form with subsequent evaporation before entry into the tube bundle reactor unit is preferred.

Oxygen-containing gases, such as air, synthetic air, an oxygen-enriched gas or so-called "pure", i.e. e.g. Oxygen from air separation is used. The oxygen-containing gas is also added in a quantity-controlled manner.

The gas to be passed through the tube bundle reactor unit generally contains inert gas. The proportion of inert gas is usually 30 to 90% by volume at the start. Inert gases are all gases that do not directly contribute to the formation of maleic anhydride, such as nitrogen, noble gases, carbon monoxide, carbon dioxide, water vapor, oxygenated and non-oxygenated hydrocarbons with less than four carbon atoms (e.g. methane, ethane, propane, methanol, formaldehyde, Formic acid, ethanol, acetaldehyde, acetic acid, propanol, propionaldehyde, propionic acid, acrolein, crotonaldehyde, acrylic acid) and mixtures thereof. Generally, the inert gas is introduced into the system via the oxygen-containing gas. However, it is also possible to add further inert gases separately. Enrichment with other inert gases, which result, for example, from the partial oxidation of the coal Hydrogen can originate is possible via a partial recycling of the possibly prepared reaction discharge.

In order to ensure a long catalyst life and further increase in conversion, selectivity, yield, catalyst load and space / time yield, a volatile phosphorus compound is added to the gas in the process according to the invention. At the start, ie at the reactor inlet, their concentration is at least 0.1 ppm by volume, ie 0.1-10 ^{-6 parts by} volume of the volatile phosphorus compounds, based on the total volume of the gas at the reactor inlet. A content of 0.2 to 20 ppm by volume is preferred, particularly preferably 0.5 to 5 ppm by volume. Volatile phosphorus compounds are to be understood as all those phosphorus-containing compounds which are gaseous in the desired concentration under the conditions of use. For example, the general formulas (I) and (II) may be mentioned

0 II

where X ¹ , X ² and X ³ independently of one another are hydrogen, halogen, Ci to C ₆ alkyl, C _{3 to} C ₆ cycloalkyl, C ₆ to Cio aryl, Ci to C ₆ alkoxy, C ₃ -bis C _δ cycloalkoxy and C ₆ - to Cιo ~ Aroxy mean. Compounds of the formula (III) are preferred

RO — P — OR ₃ OR, (in:

where R ¹ , R ² and R ³ independently of one another are hydrogen, C ₁ -C ₆ -alkyl, C ₃ -C ₆ -cycloalkyl and C ₆ -C 10 -aryl. The compounds of the formula (III) in which R ¹ , R ² and R ³ independently of one another are C 1 -C ₄ -alkyl, for example methyl, ethyl, propyl, 1-methylethyl, butyl, 1-methylpropyl, 2 are particularly preferred -Methylpropyl and 1, 1-dimethylethyl, preferably tri- (Cι ~ to C ₄ alkyl) phosphate. Trimethyl phosphate, triethyl phosphate and tripropyl phosphate, in particular triethyl phosphate, are very particularly preferred.

The process according to the invention can be carried out at a pressure below normal pressure (for example up to 0.05 MPa abs) as well as above normal pressure (for example up to 10 MPa abs). Below that is the in to understand the pressure in the tube bundle reactor unit. A pressure of 0.1 to 1 MPa abs is preferred, particularly preferably 0.1 to 0.5 MPa abs.

Suitable catalysts for the process according to the invention are all those whose active composition comprises vanadium, phosphorus and oxygen. For example, catalysts which do not contain promoters can be used, as described, for example, in US Pat. No. 5,275,996, US Pat. No. 5,641,722, US Pat. No. 5,137,860, US Pat. No. 5,095,125, US Pat. No. 4,933,312 or EP-A-0 056 901.

Furthermore, catalysts can be used which contain other components in addition to vanadium, phosphorus and oxygen. In principle, components containing elements of the 1st to 15th group of the periodic table can be added. The content of additives in the finished catalyst is generally not more than about 5% by weight, calculated as oxide. Examples can be found in the documents WO 97/12674, WO 95/26817, US 5,137,860, US 5,296,436, US 5,158,923 or US 4,795,818. Preferred additives are compounds of the elements cobalt, molybdenum, iron, zinc, hafnium, zircon, lithium, titanium, chromium, manganese, nikkei, copper, boron, silicon, antimony, tin, niobium and bismuth.

Suitable catalysts can be produced, for example, as follows:

a) reaction of a pentavalent vanadium compound (for example V ₂ 0 ₅ ) with an organic reducing solvent (for example alcohol, such as isobutanol) in the presence of a pentavalent phosphorus compound (for example ortho- and / or pyrophosphoric acid) with heating.

b) Isolation of the VPO catalyst precursor formed (e.g. by filtration or evaporation).

c) Drying of the VPO catalyst precursor, optionally starting preforming by additional elimination of water from the VPO mixture.

d) shaping (e.g. tableting, optionally adding a so-called lubricant such as graphite and optionally adding a so-called pore former such as stearic acid).

e) preforming the VPO catalyst precursor by heating in an atmosphere containing oxygen, nitrogen, noble gases, carbon dioxide, carbon monoxide and / or water vapor. By A suitable combination of temperatures, treatment times and gas atmospheres adapted to the respective catalyst system can influence the catalyst performance.

If additives are used, they can in principle be added at any stage of the catalyst preparation, be it as a solution, solid or gaseous component. Advantageously, the additives are already added in step (a) of the general description above in the preparation of the VPO catalyst precursor.

In general, the catalysts used in the process according to the invention are characterized by a phosphorus / vanadium atom ratio of 0.9 to 1.5, preferably 0.9 to 1.2 and particularly preferably 1.0 to 1.1. The average oxidation state of the vanadium is generally +3.9 to +4.4 and preferably 4.0 to 4.3. The catalysts used generally have a BET surface area of 10 to 50 m ² / g and preferably 15 to 30 m ² / g. They generally have a pore volume of 0.1 to 0.5 ml / g and preferably 0.1 to 0.3 ml / g. The bulk density of the catalysts used is generally 0.5 to 1.5 kg / 1 and preferably 0.5 to 1.0 kg / 1.

The catalysts are generally used as shaped articles with an average size of more than 2 mm. Due to the pressure loss to be observed during the practice of the process, smaller molded articles are generally unsuitable. Examples of suitable moldings are tablets, cylinders, hollow cylinders, rings, balls, strands, wagon wheels or extrudates. Special shapes, such as "Trilobes" and "Tristars" (see EP-A-0 593 646) or moldings with at least one notch on the outside (see US Pat. No. 5,168,090) are also possible.

So-called full catalysts are generally used, i.e. the entire shaped catalyst body consists of the VPO-containing active composition, including any auxiliaries, such as graphite or pore formers, and other components. Furthermore, it is possible to apply the active composition to a carrier, for example an inorganic, oxidic shaped body. Such catalysts are usually referred to as shell catalysts.

Various variants are possible with regard to the use of the catalyst in the method according to the invention. In the simplest case, all reaction zones of the tube bundle reactor unit are filled with the same catalyst bed. Under catalyst bed is to be understood catalyst material, which per unit volume

Medium media are preferably used in molten salt. The individual reaction zones can be divided at equal intervals, but they can also deviate significantly from them.

The process according to the invention is carried out in a manner which is expedient in terms of application technology, using a tube bundle reactor unit having two, three or four successive reaction zones. The use of a two-zone tube bundle reactor, a three-zone tube bundle reactor or a four-zone tube bundle reactor is preferred. Carrying out with two successive reaction zones is particularly preferred, with the use of a two-zone tube bundle reactor being particularly preferred in this embodiment. Suitable technical embodiments are described, for example, in the documents DE-A 22 01 528, DE-A 25 13 405, DE-A 28 30 765, DE-A 29 03 582, US 3,147,084 and EP-A 0 383 224. At this point, reference is made to the special two-zone reactor disclosed in the published patent application DE-A 22 01 528, which includes the possibility of removing a partial amount of the hotter heat transfer medium of the "rear" reaction zone to the colder heat transfer medium of the "front" reaction zone. to allow heating if necessary.

It is also possible to superimpose a cross-flow in all of the aforementioned case constellations within the respective reaction zone of the parallel flow of the heat transfer medium relative to the reaction tubes, so that the individual reaction zones, as described in published patent application EP-A 0 700 714 or EP-A 0 700 893 described tube bundle reactor and, overall, a longitudinal section through the contact tube bundle results in a meandering flow profile of the heat transfer medium.

The reaction mixture is expediently preheated to the desired reaction temperature before contact with the catalyst, for example in a so-called preheating zone.

In the aforementioned tube bundle reactors, the reactor tubes are usually made of ferritic steel and typically have a wall thickness of 1 to 3 mm. Their inner diameter is usually 20 to 30 mm. The number of reactor tubes per tube bundle reactor is usually in the range between 5000 and 35000, although a number over 35000 can also be realized in particularly large plants. The reactor tubes are normally arranged homogeneously distributed within the reactor body. Fluid temperature control media are particularly suitable as heat transfer media. The use of molten salts, such as potassium nitrate, potassium nitrite, sodium nitrate and / or sodium nitrite, or of low-melting metals such as sodium and alloys of various metals is particularly favorable.

According to the invention, the inlet temperatures of the heat transfer medium are 350 to 450 ° C. for the first reaction zone and 350 to 480 ° C. for the second and each further reaction zone, it being important to note that, according to the invention, the temperature difference of the hottest reaction zone minus the temperature of the is at least 2 ° C in the direction of passage before the hottest reaction zone, coldest reaction zone.

The process according to the invention can be carried out in two preferred process variants, the variant with "straight passage" and the variant with "return".

a) "straight passage"

The process variant of the "straight pass" is characterized in that the conversion of hydrocarbons per reactor pass is 75 to 95% and maleic anhydride and, if appropriate, oxygenated hydrocarbon by-products are removed from the reactor discharge. The reaction gas passed through the reactor, in particular the unreacted hydrocarbons, is / are not fed to a direct recycle during the "straight pass". The total conversion of hydrocarbons per reactor pass is preferably 80 to 90%. The remaining residual stream, which contains inert gases, unreacted hydrocarbons and, if appropriate, further, non-separated components, is generally discharged from the plant.

The concentration of hydrocarbons at the beginning, ie at the reactor inlet, is preferably 1.0 to 4.0% by volume, particularly preferably 1.5 to 3.0% by volume. The concentration of oxygen is preferably 5 to 50 vol .-% at the beginning, particularly preferably 15 to 30 vol .-%. The origin of the oxygen used is in principle insignificant for the process according to the invention, provided that no harmful impurities are present. Air is preferred as the oxygen source for simple technical considerations. In the simplest case, this can be used directly or preferably after particle cleaning. An enrichment of oxygen, for example through air liquefaction and subsequent de- Stillation or pressure swing adsorption is possible in principle.

The loading of the catalyst with hydrocarbons is generally at least 20 Nl / l-h, preferably at least 30 Nl / l-h, particularly preferably at least 35 Nl / l-h.

Maleic anhydride can be separated off, for example, by absorption in a suitable absorbent. Suitable absorbents are, for example, water or organic solvents. When absorbed in water, maleic anhydride is hydrated to maleic acid. Absorption in an organic solvent is preferred. Suitable organic solvents are, for example, the high-boiling solvents mentioned in WO 97/43242, such as tri-cresyl phosphate, dibutyl maleate, high molecular weight wax, aromatic hydrocarbons with a boiling point above 140 ° C. or di-CC ₈ alkyl phthalates, such as dibutyl phthalate. Oxygenated hydrocarbon by-products are generally also absorbed in the solvents mentioned. The absorption can be carried out, for example, at a temperature of 60 to 160 ° C. and a pressure of 0.1 to 0.5 MPa abs or above. Suitable procedures are, for example, passing the gaseous, optionally cooled reactor discharge through a container filled with absorption liquid or spraying the absorption liquid in the gas stream. Appropriate methods for washing out gas streams are known to the person skilled in the art.

"Return"

The process variant of the "recycle" is characterized in that the conversion of hydrocarbons per reactor pass is 30 to 60%, maleic anhydride and optionally oxygenated hydrocarbon by-products are removed from the reactor discharge and at least part of the remaining stream or at least part of the not - Recycled, optionally separated hydrocarbons returns to the reaction zones. The total conversion of hydrocarbons per reactor pass is preferably 40 to 50%.

The concentration of hydrocarbons at the beginning, ie at the reactor inlet, is preferably at least 2.0% by volume, particularly preferably at least 2.5% by volume. The concentration of oxygen is preferably 5 to 60 vol .-% at the beginning, particularly preferably 15 to 50 vol .-%. The origin of the Set oxygen is in principle insignificant for the process according to the invention, provided that no harmful impurities are present. For simple technical considerations, the oxygen used generally comes from the air, with the oxygen usually being enriched. It can be done, for example, by air liquefaction and subsequent distillation or pressure swing adsorption. An oxygen-containing gas with a concentration of oxygen of 20 to 100 vol .-% is preferably used.

The loading of the catalyst with hydrocarbons is generally at least 20 Nl / l-h, preferably at least 30 Nl / l-h, particularly preferably at least 35 Nl / l-h.

The integral total sales of hydrocarbons, i.e. the conversion based on the entire plant is 80 to 100%, preferably 90 to 100%, in the process according to the invention in the variant with "recycling".

Maleic anhydride can be separated off, for example, as described under (a).

The gas stream remaining after the separation of maleic anhydride or at least the unconverted hydrocarbons contained therein are at least partially returned to the reaction zones in the process variant with “recycle”.

(i) When the gas stream is returned without enrichment of the hydrocarbons, it is advantageous to discharge part of the gas stream from the system (so-called purge stream) in order to counteract an enrichment of impurities. The remaining gas stream can generally be returned to the reaction zones. The corresponding

The amount of hydrocarbon and oxygen consumed is added as usual.

(ii) For example, in order to reduce the amount of inert gas to be recycled, it may be advantageous to enrich the hydrocarbons present. Depending on the type of hydrocarbons used, different methods can be considered. Examples include condensation or adsorption on suitable adsorbents (for example in the form of pressure swing or temperature swing adsorption). For example, for the enrichment of n-butane, adsorption on activated carbon or zeolites with subsequent de- Sorption possible at reduced pressure and / or elevated temperature.

The two described process variants "straight passage" and "recirculation" represent two preferred special cases of the process according to the invention. They have no limiting effect on other possible variants or on the process parameters mentioned as preferred.

The independent temperature control of the different reaction zones makes it possible to react to fluctuations in the quantity, quality or purity of the starting materials or a progressive deactivation of the catalyst and to maintain a stable and high yield of maleic anhydride. For example, it is possible to a catalyst deactivation in the first reaction zone (for example by catalyst poisons which deactivate the upper layers of the VPO catalyst or by temperature-related deactivation in the region of the hotspot maximum of the first reaction zone) by increasing the temperature of this first reaction zone again to increase sales. Furthermore, the targeted adjustment of the temperatures of the individual reaction zones enables a targeted adjustment of the conversions in these reaction zones, which has an influence on the selectivity and thus on the yield.

The maleic anhydride obtained can, for example, be processed further to give γ-butyrolactone, tetrahydrofuran, 1, 4-butanediol or mixtures thereof. Suitable methods are known to the person skilled in the art. For the sake of completeness, reference is made to the two publications WO 97/43234 (direct hydrogenation of maleic anhydride in the gas phase) and WO 97/43242 (hydrogenation of a maleic acid diester in the gas phase).

The invention furthermore relates to a circuit for the production of maleic anhydride according to the process of the invention, comprising heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms with gases containing oxygen in the presence of a volatile phosphorus compound

a) a quantity-controlled feed unit for the hydrocarbon, for the oxygen-containing gas and, if appropriate, for the phosphorus compound, b) a tube bundle reactor unit which has at least two successive and cooled reaction zones and enables different setting of the temperatures of the individual reaction zones and

c) a unit for separating the maleic anhydride formed and optionally oxygenated hydrocarbon by-products.

What is essential in the connection according to the invention is the existence of the units (a), (b) and (c) which follow one another in the direction of transmission. The units mentioned can follow one another directly as well as indirectly by interposing further units or apparatus. This also includes apparatuses and connections which allow mass and / or energy to be supplied and / or removed. The three units are described in more detail below.

a) Feed unit

The supply unit ensures the quantity-controlled supply of the hydrocarbon, the oxygen-containing gas, optionally the inert gas and optionally the phosphorus compound.

The hydrocarbons can be metered in both liquid and gaseous form. Dosage in the liquid phase is preferred.

The oxygen-containing gas and optionally the inert gas are added in gaseous form. It is preferred to add only one gas which has the desired composition. For the process variant of the "straight passage" this is preferably air, for the process variant of the "recycle" this is preferably a gas with a higher oxygen content than air.

The volatile phosphorus compound is generally added separately in liquid form. However, it is also possible to add the desired amount of the volatile phosphorus compound to the hydrocarbon before the plant. In this case, the corresponding addition unit on the circuit is not required.

The order of additions is not important for the successful implementation of the procedure. So all streams can be in any order one after the other or for example can also be metered together in one place in the system. It is important to mix thoroughly before entering the first reaction zone. The mixing can take place, for example, in a static mixer which is located between the feed unit and the tube bundle reactor unit. Appropriate installations in the tube bundle reactor unit also enable the required mixing to be carried out there. Under these circumstances, the feed unit can also be located directly in the entrance area of the tube bundle reactor unit.

If the circuit is equipped with a recirculation, it opens into the feed gas stream before the mixing unit.

b) tube bundle reactor unit

The tube bundle reactor unit is to be designed as described above. In particular, this comprises at least two successive reaction zones, which allow different temperatures to be set. The individual reaction zones can be implemented in a tube bundle reactor as a so-called multi-zone tube bundle reactor as well as in a plurality of tube bundle reactors connected in series, which in turn can contain one or more reaction zones. The tube bundle reactor unit preferably contains a multi-zone tube bundle reactor, particularly preferably a two-zone tube bundle reactor, a three-zone tube bundle reactor or a four-zone tube bundle reactor. The use of a two-zone tube bundle reactor is very particularly preferred.

c) Unit for separating the maleic anhydride formed

The separation of the maleic anhydride formed is to be carried out as described above. In particular, various apparatuses which are suitable for the absorption of gases in liquids can be used for this purpose. Several identical or different apparatuses for separating the maleic anhydride can also be combined into one unit.

A highly simplified block diagram of the interconnection according to the invention for the two special method variants "straight passage" and "return" is contained in FIG. 2. 2a shows the variant of the "straight pass", where (1) feed unit, (2) tube bundle reactor unit and (3) unit for separating the maleic anhydride formed. Streams (A), (B), (C) and (D) correspond to the feed streams hydrocarbon, oxygen-containing gas, optionally inert gas and optionally volatile phosphorus compound. The solution containing maleic anhydride, which is discharged from the system, is referred to as stream (E). Stream (F) corresponds to the remaining gas stream, which is also discharged from the plant.

2b shows the variant of the "recirculation", the units (1), (2) and (3) and the streams (A), (B), (C), (D) and (E) being those mentioned above Have meanings. Current (G) is optional depending on the embodiment and corresponds to the so-called "purge current" to be discharged from the system. Also optional is the unit (4), which symbolizes the unit for the enrichment of the unreacted hydrocarbon and contains an exhaust gas stream (H). The recirculated hydrocarbon-containing gas stream is fed to the feed unit (1).

As mentioned above, the units mentioned can follow one another directly or indirectly by interposing further units or apparatus. Examples of further units or apparatuses contained therein are, without acting as a limitation, heat exchangers (for heating or cooling), pumps (for conveying liquids, such as the liquid feeds or the solution of the separated maleic anhydride) or gas thinners (for conveying) and compression of gases, such as the gaseous inlets or the gas to be recycled in the variant with "recirculation").

In a particularly preferred embodiment for the production of maleic anhydride, n-butane is used as the starting hydrocarbon and the heterogeneous catalytic gas phase oxidation is carried out in a tube bundle reactor unit with two successive reaction zones in a "straight pass". The most important characteristics of a preferred connection and the preferred method are described below.

Air as gas containing oxygen and inert gas is fed into the feed unit in a quantity-controlled manner. The quantity of n-butane is also regulated, but is preferably supplied in liquid form via a pump and evaporated in the gas stream. The ratio between the supplied amounts of n-butane and oxygen is generally adjusted according to the exothermic nature of the reaction and the desired space / time yield and is therefore, for example, from

Another significant advantage of the invention lies in the high flexibility of the reaction procedure. Due to the independently adjustable temperatures of the individual reaction zones, the conversions and selectivities in these reaction zones can be specifically set and optimized. This makes it possible to counteract a wide variety of influences, such as fluctuations in the quantity, quality or purity of the starting materials or a progressive deactivation of the catalyst.

Examples

Unless otherwise stated, the sizes used in this document are defined as follows:

^m maleic anhydride

Space / time yield V, catalyst t

V hydrocarbon f

Load = catalyst "

^ KW, Reakto: rJ3in - ^ K _» Reaktoj-aus

Sales U

^ T reactor, a

τCW plant in ^ KW plant, from integral total turnover U _tot = ^ KW plant

^{• Ω} MSA, reactor, off

Selectivity S - ü ^* ^ ^ _e actuator, an ⁿ w, reactor, out

integral e total selectivity S _tot = ^ KW plant ^ in ^ K plant ^ us

A yield u • s integral total yield A _tot = ^Uges' ^ ges

Selectivity S ^{co + C02} = 1 ^ O + CO 2, reactor, from ^ co + CO 2, reactor, a ^ - ^ K, reactor, a ^ KW -L ^ O + CO 2 plant, from TO + CO 2 plant, on

Selectivity S _total ^{co + c02} ^ ^ TW reactor, a ^ TW reactor

T _p temperature of the pth reaction zone

^T _{p, He} i _ßpunkt temperature of the hot spot maximum in the p-th reaction zone m _M aiei _nsäu re _a nhyd _r id mass of maleic anhydride produced [g] ^v κ _ata iy _sator bulk volume of the catalyst summed over all reaction zones [1] t time unit [h ]

Vκ _oh i _{en hydrogen} to 0 ° C and 0.1013 MPa abs normalized volume of the hydrocarbon in the gas phase [Nl]

(Calculated size. If a hydrocarbon is in the liquid phase under these conditions, the ideal gas law is used to calculate the hypothetical gas volume.) U Conversion of hydrocarbons per reactor pass

U _total integral sales of hydrocarbons

maleic anhydride (ie, on the overall plant based) S selectivity of. per pass through the reactor S _ges integral overall selectivity of. maleic anhydride (ie, on the overall plant based) (ie, based on the overall plant) _is integral total yield of maleic anhydride A yield of maleic anhydride per pass through the reactor Ag

Sco + co ₂ selectivity with regard to CO plus C0 per reactor run Sges, co ₊ co ₂ integral overall selectivity with respect to CO plus C0 ₂

(ie, based on the overall plant) ⁿ κw, reactor, a molar flow rate of hydrocarbons at the reactor inlet [mol / h] ⁿ κ, reactor _au s mass flow of hydrocarbons at the

Age reactor outlet [mol / h] ⁿ κw, with _l, ei _n molar flow rate of hydrocarbons at the entrance of the complex [mol / h] NICW, plant, out mass flow of hydrocarbons at the outlet of the plant [mol / h] ⁿ MSA reactor, from Mass flow of maleic anhydride on

Reactor outlet [mol / h] ⁿ MΞA, plant, from mass flow of maleic anhydride at the outlet of the plant [mol / h] Catalyst 1

11.8 kg of 100% orthophosphoric acid were dissolved in 150 l of isobutanol in a 240 l kettle with stirring, and 9.09 kg of vanadium pentoxide were then added. This suspension was refluxed for 16 hours and then cooled to room temperature. The resulting precipitate was filtered off, washed with isobutanol and dried at 150 ° C. in vacuo at 8 kPa abs (80 mbar abs). The dried powder was then treated in a rotary tube at 250 to 300 ° C. for 2 hours. After cooling to room temperature, 3% by weight of graphite was added and mixed intimately. The powder was tableted into 5x3x2 mm hollow cylinders. The hollow cylinders were first heated in air in a muffle furnace at 7 ° C / min to 250 ° C and then at 2 ° C / min to 385 ° C. The catalyst was left at this temperature for 10 minutes before the atmosphere was switched from air to N / H ₂ 0 (1: 1). The mixture was heated to 425 ° C. under the N ₂ / H ₂ O atmosphere (1: 1) and the system was left at this temperature for 3 hours. Finally, the mixture was cooled to room temperature under nitrogen.

The catalyst showed a bulk density of 0.9 1 / kg, a total pore volume of 0.2 ml / g, a BET surface area of 17 m ² / g, a phosphorus / vanadium ratio of 1.04 to 1.05 and a mean oxidation level of vanadium of + 4.1.

Attachment 1

The test facility was equipped with a feed unit and a reactor tube. The replacement of a tube bundle reactor by a reactor tube is very possible on a laboratory or pilot plant scale, provided the dimensions of the reactor tube are in the range of a technical reactor tube. The system was operated in a "straight pass".

The hydrocarbon was added in a quantity-controlled manner in liquid form via a pump. Air was added in a quantity-controlled manner as the oxygen-containing gas. Triethyl phosphate (TEP) was also added in liquid form, as a solution of 0.25% by weight of TEP in water, in a quantity-controlled manner.

The tube bundle reactor unit consisted of a two-zone reactor tube. The length of the reactor tube was 6.4 m, the inner diameter 22.3 mm. A multi-thermocouple with 20 temperature measuring points was located in a protective tube inside the reactor tube. The reactor tube was divided into two reaction zones. Both reaction zones were 3.2 m long. They were surrounded by separate heat transfer circuits. The Flow through the tube bundle reactor was from top to bottom. The upper 0.3 m of the reactor tube were filled with inert material and formed the preheating zone. The first and second reaction zones each contained 1.0 l of catalyst 1. A molten salt was used as the heat transfer medium. The pressure at the reactor inlet was 0.1 to 0.4 MPa abs (1 to 4 bar abs) in all tests.

Immediately after the tube bundle reactor unit, gaseous product was removed and fed to the gas chromatographic on-line analysis. The main stream of the gaseous reactor discharge was discharged from the plant.

All examples were carried out with n-butane as the hydrocarbon over catalyst 1 in Appendix 1.

Since the plant was operated in a "straight pass", sales, selectivities and yields per reactor pass are identical to the integral values relating to the entire plant.

example 1

In Example 1A *, the reactor was operated as a single-zone tube bundle reactor (not according to the invention). The temperatures of both reaction zones were 429 ° C. The gas stream initially contained, i.e. at the reactor inlet, an n-butane concentration of 2.0 vol .-%. The loading of the catalyst with n-butane was 40 Nl / l-h. Triethyl phosphate (TEP) was added continuously as a volatile phosphorus compound in an amount of 4 ppm by volume, based on the total amount of gas at the beginning. A hotspot maximum of 455 ° C. was observed in the area of the first reaction zone. With a conversion U of n-butane of 85.3%, a yield A of maleic anhydride of 54.2% was obtained. This results in a space / time yield of 94.9 g / l-h. The experimental data are shown in Table 1.

In Examples 1B to IG, the n-butane concentration or the load on the catalyst were increased, the two reaction zones now being operated according to the invention at different temperatures (two-zone tube bundle reactor). In order to counteract a yield-increasing increase in the internal reactor temperature at higher loads, the temperature of the first reaction zone was reduced compared to Example 1A *. The temperature of the second reaction zone was adjusted in accordance with the requirements to achieve a high space / time yield. In example IG at a concentration of n-butane of 3.0 vol. -% and a loading of the catalyst of 75 Nl / lh achieved a space / time yield of 149 g / lh.

A space / time yield of 149 g / l-h can easily be achieved by the process according to the invention.

Example 2

In Example 2, the reactor was used as a two-zone tube bundle reactor with a constant load of 50 1/1-h, a constant n-butane concentration of 2.0% by volume and a steady supply of triethyl phosphate (TEP) in an amount of 4 volumes -ppm, based on the total amount of gas at the beginning. The conversion U was 85 ± 1% during the test series. The two temperatures of the reaction zones Ti and T ₂ were varied and thus also the difference between the temperatures of the two hot spot maxima T _#He i _ß p _unkt ~

Tl, hotspot

The results are shown in Table 2. In Example 2A according to the invention, a differential temperature between the two reaction zones T - i of 2 ° C. was set. The hot point maximum obtained under these conditions of the second reaction zone T ₂ , hot point was still relatively weak compared to Tι _rHeiß p _Unk t and, at 441 ° C, showed a 21 ° C lower temperature than Ti, _He i _ß p _unkt - Nevertheless, a yield was obtained A obtained from maleic anhydride of 52.4%.

By lowering i and increasing T ₂ , the difference in temperature between the two reaction zones T ₂ - i was increased to values greater than 2 ° C. In example 2C according to the invention, T ₂ - i was, for example, 6 ° C. Due to the changed temperature setting, the conversion in the first reaction zone was reduced and increased in the second reaction zone. This manifests itself in a lowering of the temperature of the hot spot maximum of the first reaction zone Tι _{/ He} i _ßpunkt and an increase in the temperature of the hot spot maximum of the second reaction _zone T _{#He ßpunkt} . The differential _{temperature / He} i _ßpunkt - Tι _fHe i _{ß point} dropped to a value of -12 ° C, which led to an increase in the yield A of maleic anhydride to 53.0%.

By further lowering i and increasing T, a further lowering of the temperature of the hotspot maximum of the first reaction zone Ti, _He i _{ß point} and an increase of the temperature of the hotspot maximum of the second reaction zone T, hotpoint could be achieved. Thus were ^τ 2 for the temperature difference, the hotspot - T I _#H hite _P oint values of> 0 ° C was reached. In Example 2E this was + 9 ° C, ie the hot spot maximum of the second reaction zone T _{2 He} i _ß p _unk t was 9 ° C above the hot spot maximum of the first reaction zone Tι _{/ He} ißpun _kt • The yield A of maleic anhydride rose significantly to 54.2%. Compared to comparative example 2A *, this corresponds to an increase in yield of 3.4% by rel.

3 shows the dependency of the yield A of maleic anhydride determined in Example 2 on the difference in temperature of the two hot spot maxima T ₂ , _He i _ßpunkt - T- ^ _He i _ß p _un ^. The yield increases with increasing temperature difference T _{2, H} eißpunkt - ^{τ ι,} Hei pu _{ß n} kt steadily.

With increasing temperature difference of the two hot spot maxima T, _H pu _ß ei _NKT ~ T I _{/ He} ißpunkt the yield A of maleic anhydride ^¬ increases steadily. The highest yields are at T A _{2 /} hotspot - T I, _{H P} oint hite> 0 ° C was obtained.

Example 3

Example 3 shows the long-term stability of the VPO catalyst used under the conditions according to the invention at high

Space / time yield. Even after more than 3300 operating hours at a concentration of 3.0% by volume of n-butane and a loading of the catalyst of 75 Nl / lh, the process according to the invention continued to achieve a constantly high space / time yield of 149 to 150 g / lh reached.

The method according to the invention enables a stable driving style with a high space / time yield of 149 to 150 g / l-h even after more than 3300 operating hours under high load.

Example 4

In Example 4, the reactor was used as a two-zone tube bundle reactor at a high load of 75 Nl / lh, a constant n-butane concentration of 3.0% by volume and an initial supply of triethyl phosphate (TEP) in an amount of 4% by volume , based on the total amount of gas at the beginning. The temperatures of the two reaction zones were kept constant at i = 15 ° C and T ₂ = 422 ° C.

The results are shown in Table 4. The selectivity of the CO and C0 formation is given as a further measure of the activity of the catalyst. Under the set conditions, this was between 40.3 and 40.6%. At time t _D , the supply to TEP was now stopped and the performance data were determined at intervals of approximately one hour. A drop in the maleic anhydride selec- activity can be observed. After about 11 hours, the maleic anhydride selectivity had dropped from originally 57.7% to 54.3%. The selectivity of CO and C0 formation increased significantly from 40.3 to 44.3%. About 12 hours after the TEP metering had been switched off, the temperature of the hot spot maximum of the first reaction zone had risen from originally stable 462 ° C. to over 490 ° C. Controlled execution of the reaction with high selectivity and yield was no longer possible.

Example 4 shows that the addition of a volatile phosphorus compound is essential for achieving a stable reaction, especially in high-load operation with a high load on the catalyst. Without metering the phosphorus component or if the metering is too low, the activity of the VPO catalyst increases, which leads to a significant increase in the temperature of the hot spot maximum of the first reaction zone _Tι # _H ei _ß p _{un t} / a significant increase in the conversion U and a decrease in the selectivity S leads. It is also possible to go through the reaction under these circumstances.

Examples 1 to 4 show that the process according to the invention enables a high conversion, a high selectivity and a high yield of product of value and therefore a high space / time yield when the catalyst is exposed to high hydrocarbons. In the gas phase oxidation of n-butane over VPO catalysts according to the invention, a space / time yield of 150 g / lh can thus be achieved without problems. Thanks to the stable reaction management, this high value is safely maintained even after several thousand hours of operation. A targeted adjustment of the catalyst activities, the catalyst amounts and the temperatures of the individual reaction zones enables an optimization of the yield of maleic anhydride. Thus, the yield rises steadily with increasing differential temperature between the hot spot maximum of the second or subsequent reaction zone and a hot spot maximum of a reaction zone located in front of it. Furthermore, the method according to the invention offers flexible reaction control with regard to the desired high space / time yield, which also offers the possibility of optimization in the case of fluctuations in the quantity, quality or purity of the starting materials or a progressive deactivation of the catalyst. Table 1: Results of Example 1,

* Comparative example

The following parameters were kept constant throughout the test series: concentration of triethyl phosphate (TEP) = 4 volume ppm.

Table 2: Results of Example 2,

The following parameters were kept constant throughout the test series:

Sales U 85 + 1%

Concentration of triethyl phosphate (TEP) = 4 volume ppm

Concentration of n-butane = 2.0% by volume

Load = 50 Nl / l-h.

Table 3: Results of Example 3.

The following parameters were kept constant throughout the test series:

Concentration of triethyl phosphate (TEP) = 4 volume ppm

Concentration of n-butane = 3.0% by volume

Load = 75 Nl / l-h

Tl = 412 to 423 ° C

T ₂ = 424 to 426 ° C

Table 4: Results of Example 4.

nb: not determined

The following parameters were kept constant throughout the test series: Concentration of n-butane = 3.0% by volume load = 75 Nl / lh _X = 415 ° C

Claims

claims

1. Process for the preparation of maleic anhydride by heterogeneous catalytic gas phase oxidation of hydrocarbons having at least four carbon atoms with gases containing oxygen in the presence of a volatile phosphorus compound on a catalyst containing vanadium, phosphorus and oxygen in a tube bundle reactor having at least two successive and cooled reaction zones Unit, characterized in that the temperature of the first reaction zone is 350 to 450 ° C and the temperature of the second and further reaction zone is 350 to 480 ° C, the temperature difference between the hottest and the coldest reaction zone being at least 2 ° C.

2. The method according to claim 1, characterized in that the temperature difference between the hottest and the coldest reaction zone is at least 5 ° C.

3. The method according to claims 1 to 2, characterized in that at least one hot spot maximum of the second or subsequent reaction zone is higher than all hot spot maxima in reaction zones lying in front of it.

4. Process according to claims 1 to 3, characterized in that n-butane is used as the hydrocarbon.

5. The method according to claims 1 to 4, characterized in that as a volatile phosphorus compound tri- (Ci to

C-alkyl) phosphate is used.

6. Process according to claims 1 to 5, characterized in that one carries out the heterogeneous catalytic gas phase oxidation at a pressure of 0.1 to 1 MPa abs.

7. Process according to claims 1 to 6, characterized in that the gas phase oxidation is operated in a single pass, the conversion of hydrocarbons per pass through the reactor is 75 to 90% and maleic anhydride and optionally oxygenated hydrocarbon by-products are removed from the reactor discharge.

8. The method according to claims 1 to 6, characterized in that one operates the gas phase oxidation with recycle, the conversion of hydrocarbons per reactor pass is 30 to 60%, one removes maleic anhydride and optionally oxygenated hydrocarbon by-products from the reactor discharge and at least one Part of the remaining stream or at least a portion of the unreacted, optionally separated hydrocarbons is returned to the reaction zones.

9. Circuit for the production of maleic anhydride according to claims 1 to 8, comprising heterogeneously catalyzed gas phase oxidation of hydrocarbons having at least four carbon atoms with oxygen-containing gases in the presence of a volatile phosphorus compound

(a) a quantity-controlled feed unit for the hydrocarbon, for the oxygen-containing gas and optionally for the phosphorus compound,

(b) a tube bundle reactor unit which has at least two successive and cooled reaction zones and enables different settings of the temperatures of the individual reaction zones and

(c) a unit for separating the maleic anhydride formed and, if appropriate, oxygenated hydrocarbon by-products.

10. Circuit according to claim 9, characterized in that the tube bundle reactor unit contains a multi-zone tube bundle reactor.