Two-stage preparation process for α,β-ethylenically unsaturated carboxylic acids and plant for the purpose

Abstract

The present invention relates to a process for producing α,β-ethylenically unsaturated carboxylic acids by two-stage catalytic gas-phase oxidation of alkenes, in which a gas stream (1) comprising at least one alkene is subjected to a first catalytic oxidation reaction in the presence of oxygen in a first reactor (A) on a first catalyst (K1) in the form of a polymetallic oxide of molybdenum, in order to obtain a gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde, and said gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde is conducted through a connecting line (V) to a second reactor (B), c) a process in which the gas stream (2) containing at least one α,β-ethylenically unsaturated aldehyde is subjected to a second catalytic oxidation reaction in the presence of oxygen in the second reactor (B) with a second catalyst (K2) to obtain a gas stream (3) containing at least one α,β-ethylenically unsaturated carboxylic acid. The method of the present invention comprises a step of conducting the gas stream (2) containing at least one α,β-ethylenically unsaturated aldehyde through an exchangeable structure (S) with a high specific surface area, which is arranged in the connecting conduit (V).

Description

[Technical Field]

[0001] The present invention relates to a process for producing α,β-ethylenically unsaturated carboxylic acids by two-stage catalytic gas-phase oxidation of alkenes, in particular a process for the two-stage oxidation of propylene to acrylic acid, and to a plant for that purpose. [Background technology]

[0002] Such two-stage processes for producing acrylic acid are known per se and are described, for example, in WO 02 / 081422 and the references cited therein.

[0003] First, propylene and molecular oxygen are fed into the first reactor. The oxygen may be fed, for example, in the form of air. Additionally, an inert gas, such as nitrogen, may be fed. In the first reactor, propylene is converted to acrolein by catalytic oxidation. The outlet gas from the first reactor therefore contains, among other things, acrolein and unconverted oxygen. This outlet gas is then fed into the second reactor, where the acrolein and oxygen are converted to acrylic acid.

[0004] The catalysts used in the first and second reactors are catalysts suitable for the gas-phase oxidation of propylene to acrolein or acrolein to acrylic acid. Numerous catalysts suitable for the reactions mentioned have been suggested. Typically, multimetal oxides of molybdenum are used commercially.

[0005] Sufficient oxygen must also be available in the second reactor to convert the acrolein introduced therein to a sufficient amount of acrylic acid. For this purpose, the first reactor may already be charged with a large amount of oxygen, so that there is still sufficient oxygen remaining in the second reactor after propylene has been converted to acrolein. Alternatively, oxygen can be added between the first and second reactors. Because increasing the oxygen content in the propylene-oxygen mixture increases the risk of explosion, it is advantageous to introduce only the amount of oxygen necessary to convert propylene to acrolein into the first reactor. The amount of oxygen necessary to convert acrolein to acrylic acid is then added between the first and second reactors.

[0006] In plant operation, it was found that a pressure drop occurred in the second reactor as the reaction gas passed through. Operating the synthesis at high pressure resulted in a loss of acrylic acid selectivity. When the pressure drop reached a certain level, the reactor had to be shut down for maintenance, a process also known as skimming. This maintenance process consumed both time and resources, as the plant required extended downtime due to cooling and heating.

[0007] U.S. Patent No. 6,069,271 describes a process for producing acrylic acid from propylene by two-stage catalytic oxidation using a single fixed-bed shell-and-tube heat exchanger reactor. Each reactor tube contains a first catalyst layer at the bottom and a second catalyst layer at the top. Between these layers is an inert layer with a porosity of 40% to 99.5%. This process is intended to prevent the problem of reactor tube blockage caused by sublimated catalyst components from the first catalyst being carried downstream by the reaction gas flow. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 02 / 081422 Brochure [Patent Document 2] National Patent No. 6,069,271 Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to develop originally specified processes and originally specified plants in such a way that they require less maintenance.

[0010] According to the invention, the object is achieved by a process according to claim 1 and a plant according to claim 6. Advantageous embodiments and further developments are revealed by the dependent claims. [Means for solving the problem]

[0011] The process of the present invention for producing an α,β-ethylenically unsaturated carboxylic acid by two-step catalytic vapor phase oxidation of an alkene comprises: a) a gas stream comprising at least one alkene is subjected to a first catalytic oxidation reaction in a first reactor in the presence of oxygen with a first catalyst in the form of a multimetallic oxide of molybdenum in order to obtain a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde, b) a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde is conducted through a connecting conduit to a second reactor, c) The gas stream comprising at least one α,β-ethylenically unsaturated aldehyde is subjected to a second catalytic oxidation reaction with a second catalyst in the presence of oxygen in a second reactor to obtain a gas stream comprising at least one α,β-ethylenically unsaturated carboxylic acid.

[0012] The process comprises directing a gas stream containing at least one α,β-ethylenically unsaturated aldehyde through an exchangeable structure having a high specific surface area disposed within a connecting conduit.

[0013] The gas stream containing at least one alkene contains one or more alkenes. Suitable alkenes, for example, contain 3 to 5 carbon atoms per molecule and are selected from, for example, propylene, 2-methylpropylene, 1-butene, 2-butene, or 1-pentene. The alkene is preferably selected from propylene and 2-methylpropylene. Propylene is particularly preferred.

[0014] A gas stream containing at least one alkene is subjected to a first catalytic oxidation reaction in the presence of oxygen. An external oxygen source can be provided. Suitable external oxygen sources include, for example, oxygen, synthetic air, and air. Air is particularly preferred.

[0015] The first catalytic oxidation reaction is carried out in a first reactor. The first reactor can be any reactor for gas-phase oxidation. In a preferred embodiment, the first reactor is a fixed-bed shell-and-tube heat exchanger reactor. Carrying out the reaction in a fixed-bed shell-and-tube heat exchanger reactor allows for uniform heat removal and good heat exchange.

[0016] Such fixed-bed shell-and-tube heat exchange reactors typically consist of a generally cylindrical vessel containing a number of catalyst tubes (tube bundles), typically arranged vertically. Each of these catalyst tubes contains a fixed-bed arrangement of catalytically active multimetal oxides (e.g., the first catalyst in the first reactor). The ends of the catalyst tubes are sealed and fixed in tube sheets, and each of them opens into hoods connected to the vessel at its upper and lower ends. Gas streams flowing through the catalyst tubes are supplied and removed through these hoods, so that each catalyst tube corresponds to an elongated reaction unit zone. Typically, the catalyst tubes have a wall thickness of 1 to 3 mm, an internal diameter of 20 to 30 mm, and a tube length of 2 to 4 m. In applications where the number of catalyst tubes contained within the vessel is appropriate, the number of catalyst tubes contained within the vessel may reach at least 5,000, preferably at least 10,000. Often, the number of catalyst tubes contained within a reactor vessel is between 15,000 and 30,000. Within the vessel, the catalyst tubes are typically arranged in a uniform distribution, the distribution being suitably selected so that the separation of the central inner axes of adjacent catalyst tubes is 35 to 45 mm.

[0017] Furthermore, a heat exchange medium is passed through the space surrounding the catalyst tubes to remove process heat. After leaving the vessel, the heat exchange medium is returned to its original temperature, for example in an external heat exchanger, before re-entering the reactor vessel. Suitable heat exchange media are, in particular, fluid temperature control media. It is particularly preferred to use a melt of salts ("salt bath") such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or a melt of low-melting metals such as sodium, mercury and alloys of different metals.

[0018] The first catalytic oxidation reaction is carried out in the presence of a first catalyst. The first catalyst takes the form of a multimetal oxide of molybdenum. Such catalysts are known per se. In one embodiment, the multimetal oxide has the general formula (I): Mo 12 Bi a Fe b X 1 c X 2 d X 3 e X4 f O n (I) and having a stoichiometry of During the ceremony, X 1 = nickel and / or cobalt, X 2 = thallium, samarium, alkali metals and / or alkaline earth metals, X 3 = zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead, vanadium, chromium, niobium and / or tungsten X 4 = silicon, aluminum, titanium and / or zirconium, a=0.2 to 5, b=0.01 to 5, c=0 to 10, d=0 to 2, e=0 to 8, f=0 to 10, n is a number determined by the valence and frequency of the elements (I) excluding oxygen. In a preferred embodiment, the stoichiometric coefficients are as follows: a=0.4 to 2, b=2 to 4, c=3 to 10, d=0.02 to 2, e=0 to 5, f=0.5 or 1 to 10.

[0019] X 1 is preferably cobalt, and X 2 is preferably K, Cs and / or Sr, more preferably K, and X 3 is preferably tungsten, zinc and / or phosphorus, and X 4 is preferably Si. More preferably, the variable X 1 From X 4 simultaneously have the above definitions. All stoichiometric coefficients a through f and all variables X 1 From X 4 It is further preferred that simultaneously have the above advantageous definitions.

[0020] In one embodiment, the catalytically active multimetal oxide has the general formula (II): [Y 1 a’ Y 2 b’ O x’ ]p[Y 3 c’ Y 4 d’ Y 5 e’ Y 6 f’ Y 7 g’ Y 8 h’ O y’ ]q (II) and During the ceremony, Y 1 = bismuth alone or bismuth with at least one of the elements tellurium, antimony, tin and copper; Y 2 = molybdenum or tungsten, Y 3 = alkali metals, thallium and / or samarium, Y 4 = alkaline earth metals, nickel, cobalt, copper, manganese, zinc, tin, cadmium and / or mercury, Y 5 = iron or iron and at least one of the elements vanadium, chromium and cerium, Y 6 = phosphorus, arsenic, boron and / or antimony, Y 7 = rare earth metals, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and / or uranium, Y 8 = molybdenum or tungsten, a'=0.01 to 8, b'=0.1 to 30, c'=0 to 4, d'=0 to 20, e'>0 to 20, f'=0 to 6, g'=0 to 15, h'=8 to 16, x', y' = numbers determined by the valence and frequency of elements in (II) other than oxygen, p, q are numbers such that the p / q ratio is between 0.1 and 10.

[0021] Particularly advantageous catalytically active multimetal oxides of stoichiometry (II) are Y 1 is bismuth only.

[0022] Stoichiometric (II) catalytically active multimetal oxides have the chemical composition Y 3 c’ Y 4 d’ Y 5 e’ Y 6 f’ Y 7 g’ Y 8 h’ O y’ Chemical composition Y dispersed in the matrix phase 1 a’ Y 2 b’ O x’ It includes a three-dimensional area of

[0023] The preparation of such catalysts is described in detail in documents DE 4407020A1, EP 575897A1, DE 3338380A1 and EP 2114562A1.

[0024] Among the stoichiometries of general formula (II), general formula (IIa) [Bi a” Z 2 b”O x” ]p”[Z 8 12 Z 3 c” Z 4 d” Fe e” Z 5 f” Z 6g” Z 7 h” O y” ]q” (IIa) Preferably, it conforms to During the ceremony Z 2 = molybdenum and / or tungsten, Z 3 = nickel and / or cobalt, Z 4 = thallium, alkali metals and / or alkaline earth metals, preferably K, Cs and / or Sr, Z 5 = phosphorus, arsenic, boron, antimony, tin, cerium, vanadium, chromium and / or Bi, Z 6 = silicon, aluminum, titanium and / or zirconium, preferably Si Z 7 = copper, silver and / or gold, Z 8 = molybdenum and / or tungsten, a”=0.1 to 1, b”=0.2 to 2, c”=3 to 10, d”=0.02 to 2, e"=0.01 to 5, preferably 0.1 to 3, f=0 to 5, g"=0 to 10, preferably >0 to 10, more preferably 0.2 to 10, most preferably 0.4 to 3; h”=0 to 1, x" and y" are numbers determined by the valence and frequency of elements in (IIa) other than oxygen, p, q are numbers such that the p / q ratio is between 0.1 and 5, preferably between 0.5 and 2.

[0025] Among catalytically active multimetal oxides of stoichiometry (IIa), Z 2 b” = (tungsten) b” and Z 8 12 = (molybdenum) 12It is preferable that:

[0026] The first catalytic oxidation reaction in the first reactor is carried out at a temperature in the range of 200 to 420° C., an operating pressure in the range of 2 to 3 bar absolute, and a residence time in the range of 1.5 to 2.5 seconds.

[0027] The product stream from the first catalytic oxidation reaction contains at least one α,β-ethylenically unsaturated aldehyde formed by the partial oxidation of the alkene, in addition to any unconverted portion of the alkene. Thus, the partial oxidation of propylene results in acrolein, and the partial oxidation of 2-methylpropylene results in methacrolein. The α,β-ethylenically unsaturated aldehyde is preferably selected from acrolein.

[0028] A gas stream comprising at least one α,β-ethylenically unsaturated aldehyde is removed at the outlet from the first reactor via a connecting conduit and introduced into the second reactor.

[0029] The gas stream containing at least one α,β-ethylenically unsaturated aldehyde is subjected to a second catalytic oxidation reaction in the presence of oxygen. An external oxygen source can be provided. Suitable external oxygen sources include, for example, oxygen, synthetic air, and air. Air is particularly preferred.

[0030] The second catalytic oxidation reaction is carried out in a second reactor. The second reactor can be any reactor for gas-phase oxidation. In a preferred embodiment, the second reactor is a fixed-bed shell-and-tube heat exchanger reactor. Carrying out the reaction in a fixed-bed shell-and-tube heat exchanger reactor allows for uniform heat removal and good heat exchange.

[0031] The second catalytic oxidation reaction is carried out in the presence of a second catalyst. The second catalyst takes the form of a multimetal oxide of molybdenum. Such catalysts are known per se. In one embodiment, the multimetal oxide has the general formula (III): Mo 12 V a X 1b X 2 c X 3 d X 4 e X 5 f X 6 g O n (III) stoichiometry, During the ceremony, 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, n is a number determined by the valence and frequency of the elements in (III) except for oxygen.

[0032] Preferably, the variables should be selected within the ranges specified by the proviso that the molar ratio of element Mo is 20 to 80 mol %, based on the total amount of all elements except oxygen in the multimetal oxide material (III), the molar ratio of Mo present in the catalytically active multimetal oxide material (III) to V present in the catalytically active multimetal oxide material (III), Mo / V, is 15:1 to 1:1, and the corresponding molar ratio Mo / (total amount of W and Nb) is 80:1 to 1:4 (if the multimetal oxide material contains Cu, the corresponding molar Mo / Cu ratio is 30:1 to 1:3).

[0033] Preferred multimetal oxide catalysts (III) are 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, n is a number determined by the valence and frequency of the elements in (III) except for oxygen.

[0034] Preferred multimetal oxide catalysts have the following general stoichiometry (IIIa): Mo 12 V a X 1 b X 2 c X 5 f X 6 g O n According to (IIIa), During the ceremony, 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, n is a number determined by the valence and frequency of the elements in (IIIa).

[0035] Mo / Cu is 30:1 to 1:3, and the corresponding molar ratio Mo / (total amount of W and Nb) is 80:1 to 1:4.

[0036] Preferably, the variables should be selected within the ranges specified with the proviso that the molar ratio of element Mo is 20 to 80 mol %, based on the total amount of all elements except oxygen in the catalytically active multimetal oxide material (IIIa), the molar ratio of Mo present in the catalytically active multimetal oxide material (IIIa) to V present in the catalytically active multimetal oxide material (IIIa), Mo / V, is 15:1 to 1:1, the corresponding molar ratio Mo / Cu is 30:1 to 1:3, and the corresponding molar ratio Mo / (total amount of W and Nb) is 80:1 to 1:4.

[0037] The first and second catalysts can be in the form of pellets, beads, or rings with holes, for example, produced by a tablet press or extruder. Alternatively, they can be used in the form of catalyst components deposited on a refractory support, with similar activity.

[0038] The first and second catalysts and their preparation are described in detail, for example, in US Pat. No. 8,232,425.

[0039] The second catalytic oxidation reaction in the second reactor is carried out at a temperature in the range of 240 to 320° C., an operating pressure in the range of 1.5 to 2.5 bar absolute, and a residence time in the range of 1 to 1.7 seconds.

[0040] The product stream of the second catalytic oxidation reaction, as well as any unconverted portion of the alkene and / or any unconverted portion of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde, contains at least one α,β-ethylenically unsaturated carboxylic acid formed by partial oxidation of the aldehyde. Thus, partial oxidation of acrolein results in acrylic acid, and partial oxidation of methacrolein results in methacrylic acid. The α,β-ethylenically unsaturated carboxylic acid is preferably selected from acrylic acid.

[0041] The gas stream produced from the second reactor, which contains, in addition to propylene, acrolein, and oxygen, at least one α,β-ethylenically unsaturated carboxylic acid, preferably acrylic acid, essentially comprises acrylic acid. The carboxylic acid, preferably acrylic acid, is typically obtained by absorption or fractional distillation.

[0042] Acrylic acid is absorbed in an absorption liquid suitable for absorbing acrylic acid, such as diphenyl ether, diphenyl ether, dimethyl phthalate, ethylhexanoic acid, N-methylpyrrolidone, paraffin fractions, or mixtures thereof, such as oligomeric acrylic acid, e.g., a mixture containing diacrylic acid, triacrylic acid, and tetraacrylic acid, or water. The absorption liquid is cooled, e.g., at a temperature of 100 to 180 °C, using a heat exchanger, and then countercurrently contacted with a gas stream containing at least one α,β-ethylenically unsaturated carboxylic acid in an absorption column. Suitable absorption columns include, for example, columns with random packing, columns with structured packing, valve tray columns, or bubble-cap tray columns. The acrylic acid-containing absorption liquid generally contains volatile impurities, such as water, acrolein, formaldehyde, formic acid, and / or acetic acid. These can be at least partially removed by stripping with a stripping gas, e.g., nitrogen or air, in a desorption column in countercurrent flow. The crude acrylic acid is typically obtained by rectification removal under reduced pressure, e.g., 0.04 to 0.1 bar, e.g., in a random packing column or a tray column. The crude acrylic acid is removed as a surface product in the upper region of the rectification column or via a side draw, and the absorption liquid is appropriately recycled and reused for absorption. When water is used as the absorption liquid, the crude acrylic acid is isolated from the aqueous acrylic acid solution by countercurrent extraction with an extractant, e.g., ethyl acetate, butyl acetate, ethyl acrylate, 2-butanone, or a mixture thereof, in an extraction column, followed by distillation of the extract.

[0043] For fractional condensation, a gas stream containing at least one α,β-ethylenically unsaturated carboxylic acid is cooled to 100 to 180°C and appropriately introduced into the lower region of a separating column. As the gas flows up through the column, a medium boiler fraction can be removed as a crude acrylic acid fraction via a properly installed collection tray. This crude acrylic acid fraction can be further purified by crystallization. The crystallization method is not subject to any restrictions.

[0044] In all post-treatment steps, stabilizers and / or polymerization inhibitors for acrylic acid can be added in a manner known per se. An example of a suitable stabilizer is phenothiazine. Examples of suitable polymerization inhibitors are hydroquinone, hydroquinone monomethyl ether, p-nitrosophenol, tert-butylphenol, or mixtures thereof.

[0045] All plant components that come into contact with the reaction gases in the described process are manufactured from materials that are acetic acid-, acrolein- or acrylic acid-resistant under the prevailing reaction conditions.

[0046] It was found that the molybdenum impurities used in the first catalyst tend to sublime under the reaction conditions present in the first reactor. In conventional plants with two reactors, the discharged catalyst components enter the second reactor via a connecting conduit along with the reaction gas from the first reactor. They are deposited, for example, on the inert random packing in the preheating zone and / or on the second catalyst. This narrows the free cross section in the second reactor (fouling) and increases the pressure drop during the passage of the reaction gas through the second reactor. Therefore, operating the synthesis at high pressure results in a loss of acrylic acid selectivity. When the pressure drop reaches a certain level, the reactor must be shut down and the bed or the top layer of the inert random packing must be replaced (skimming).

[0047] According to the present invention, the need for maintenance is reduced in that after leaving the first reactor, a gas stream containing at least one α,β-ethylenically unsaturated aldehyde, which incorporates a partial level of the catalytic components discharged from the first catalyst, passes through a replaceable structure with a high specific surface area arranged in a connecting conduit before entering the second reactor. Advantageously, as a result, the catalytic components of the first catalyst discharged from the first reactor together with the reaction gas do not proceed unhindered into the second reactor, but are partially or completely desublimated at the high specific surface area of ​​the replaceable structure.

[0048] The "specific surface area" of an exchangeable structure is understood to mean the surface area provided by the exchangeable structure based on the volume of the exchangeable structure. The "surface area provided by the exchangeable structure" refers to the corresponding surface area in macroscopic terms (geometric surface area), without taking into account any possible roughness of this surface. The "high specific surface area" of an exchangeable structure refers in particular to a surface area that is greater than the surface area of ​​the connecting conduit in the absence of the exchangeable structure.

[0049] In a further configuration, the microscopic surface area of ​​the exchangeable structure is increased in that the roughness of this surface area is increased. a is, for example, greater than 0.35 μm, preferably greater than 1 μm, and more preferably 2 μm or more. a is, for example, in the range of 2 μm to 3 μm, in particular in the range of 2 μm to 2.5 μm.

[0050] In one embodiment, the flow rate of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde is reduced in the region of the exchangeable structure. The reduction in flow rate is achieved, for example, by increasing the cross-sectional area of ​​the connecting conduit. The reduction in flow rate increases the residence time of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde in the exchangeable structure. The increased residence time promotes desublimation of the discharged catalyst components.

[0051] In one embodiment, the gas stream containing at least one α,β-ethylenically unsaturated aldehyde is cooled to 80 to 160°C, preferably 110 to 145°C, after leaving the first reactor and before passing through the exchangeable structure. The cooling is performed, for example, by an external cooler disposed in the connecting conduit. External cooling of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde, which may contain the catalytic component of the first catalyst discharged from the first reactor together with the reaction gas, promotes desublimation on the high specific surface area of ​​the exchangeable structure.

[0052] In the process of the present invention, from a thermodynamic point of view, desublimation can be promoted by a low temperature of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde or by a low water concentration in the gas stream. Since the current oxidation produces water as a by-product, the water concentration can only be controlled to a limited extent. Therefore, promotion of desublimation is advantageously achieved by cooling the gas stream after it leaves the first reactor, i.e., by cooling at low temperatures. However, excessively low temperatures can lead to condensation of secondary components with higher boiling points, which can lead to contamination of plant components. Therefore, the catalytic components of the first catalyst discharged from the first reactor together with the reaction gas are generally partially desublimated at the high specific surface area of ​​the exchangeable structure, while a further portion can also be desublimated, for example, on the inner tube wall of the connecting conduit between the two reactors.

[0053] The replaceable structure is replaceable. In one embodiment, the replaceable structure is replaced once or periodically while the first reactor (A) and / or the second reactor (B) are not cooled below 150°C. Because the first reactor and / or the second reactor are not completely cooled, they may be maintained at thermally close to reaction conditions, for example, at 150°C and 1 bar absolute. This temperature is higher than the melting temperature of the temperature-controlling medium surrounding the reaction tube (e.g., a salt bath). By keeping the salt bath molten, reaction conditions are achieved again very quickly after synthesis is resumed. Therefore, for example, since there is no need to cool the salt melt to room temperature and then reheat it to at least 150°C, both the time required and the need for maintenance can be significantly reduced (to a few hours instead of about two weeks) compared to traditional maintenance or cleaning processes (skimming).

[0054] A preferred embodiment involves the production of acrylic acid by a two-step catalytic vapor-phase oxidation of propylene, where propylene is oxidized to acrolein in a first catalytic oxidation reaction, and acrolein is further oxidized to acrylic acid in a second catalytic oxidation reaction. This process allows for the efficient production of valuable acrylic acid products, which find use on an industrial scale, for example, in polymerization.

[0055] The present invention also provides a plant for producing an α,β-ethylenically unsaturated carboxylic acid by two-stage catalytic vapor phase oxidation of an alkene, comprising: a) a first reactor designed to carry out a first catalytic oxidation reaction of a gas stream comprising at least one alkene in the presence of oxygen on a first catalyst in the form of a multimetal oxide of molybdenum, in order to obtain a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde; b) a second reactor designed to carry out a second catalytic oxidation reaction of a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde in the presence of oxygen over a second catalyst to obtain a gas stream comprising at least one α,β-ethylenically unsaturated carboxylic acid; c) a connecting conduit arranged between the first reactor and the second reactor for conducting a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde into the second reactor; d) an exchangeable structure with a high specific surface area, which is arranged in the connecting conduit and through which a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde can pass.

[0056] The "specific surface area" of the exchangeable structure is understood to mean the surface area provided by the exchangeable structure based on the volume of the exchangeable structure. The "high specific surface area" of the exchangeable structure refers in particular to a surface area that is greater than the surface area of ​​the connecting conduit in the absence of the exchangeable structure.

[0057] In one embodiment, the replaceable structure has a length of at least 400 m 2 / m 3 , preferably at least 600 m 2 / m 3 It has a specific surface area of ​​at least 400 m 2 / m 3 The installation of an exchangeable structure having a specific surface area of ​​0.15 MPa promotes the desublimation of the discharged catalyst components and thus reduces the need for maintenance of the plant.

[0058] Furthermore, the structure is selected to create a minimum pressure drop through the structure. This can be achieved through the installed absolute surface area of ​​the structure (S). This can be, for example, at least 350 m 2 is.

[0059] In one embodiment, the replaceable structure has a length of at least 600 m 2 / m 3 , preferably at least 800 m 2 / m 3 The "specific surface area per unit gas volume" of an exchangeable structure is understood to mean the surface area provided by the exchangeable structure through which a gas stream, such as an acrolein-containing gas stream, flows directly and comes into contact with the surface of the structure, based on the void volume of the structure through which the gas stream flows.

[0060] At least 600 m 2 / m 3 The provision of an exchangeable structure having a specific surface area per unit gas volume of facilitates the desublimation of the discharged catalyst components and thus reduces the need for maintenance of the plant.

[0061] In one embodiment, the replaceable structure is a structured packing. The term "structured packing" refers to, for example, metal plates (trays), metal meshes, metal grids, etc. Which It is understood to mean that the trays may be, for example, smooth, corrugated, perforated, or embossed. The channels of the metal grid may have, for example, a straight or inclined configuration. In general, structured packings offer a high (contact) surface area but low resistance to gas flow.

[0062] The prior art discloses the production of acrylic acid from propylene, in which the catalyst components discharged from the reactor are removed in suitable equipment, for example, on random packings arranged in the reactor tubes. Random packings are considered unstructured packings, not structured packings. Desublimation of the discharged catalyst components on the body of such random packings (unstructured packings) results in a higher pressure drop than desublimation on structured packings with the same absolute surface area.

[0063] In one embodiment, the replaceable structure is made of ceramic. The ceramic is preferably a monolithic ceramic. Monolithic ceramics can withstand high temperatures and have high erosion resistance, for example, compared to composite materials with a ceramic matrix. Furthermore, monolithic ceramics use inexpensive raw materials and can be formed into specific shapes. Monolithic ceramics may be made of, for example, silicon dioxide (SiO), silicon nitride (SiN), silicon aluminum oxynitride (SiAlON), silicon carbide (SiC), silicon oxynitride (SiNO), aluminum nitride (AlN), aluminum oxide (AlO), hafnium dioxide (HfO), zirconium dioxide (ZrO), silicon silicide carbide (Si-SiC), or other oxides, carbides, or nitrides, or combinations thereof, preferably silicon dioxide (SiO) and / or aluminum oxide (AlO).

[0064] The replaceable structure is particularly composed of blocks, preferably rectangular blocks. The replaceable structure typically contains 1 to 200 blocks, preferably 50 to 170 blocks, and more preferably 100 to 150 blocks. Typically, every rectangular block has an edge length of 100 to 200 mm (length) and 100 to 200 mm (width). Rectangular blocks with long sides are preferred. The depth of the blocks, extending in the direction of gas flow, is 150 to 350 mm. This corresponds to the pressure drop and the total surface area of ​​the replaceable structure. The blocks are arranged side by side. The blocks preferably fill the cross-sectional area of ​​the replaceable structure. The "cross-sectional area" of the replaceable structure is understood to mean any area of ​​the replaceable structure perpendicular to the flow direction of the gas flow containing at least one α,β-ethylenically unsaturated aldehyde passing through it. Essentially no gaps exist between the blocks. Any gaps can be hermetically closed, for example, by glass fabric. The block more preferably completely fills the cross-sectional area of ​​the replaceable structure.

[0065] In one embodiment, the replaceable structure comprises channels. The channels may be formed, in particular, in the aforementioned blocks. The channels may have different cross sections, for example, circular, honeycomb, rectangular, and square. Square cross sections are preferred. The longitudinal direction of the channels is in each case aligned with the flow direction of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde. The surface of the channels provides a surface area of ​​the replaceable structure on which the discharged catalyst components are desublimated, reducing the need for plant maintenance.

[0066] The cross-sectional area of ​​the connecting conduit is completely filled by the exchangeable structure. The gas stream containing at least one α,β-ethylenically unsaturated aldehyde is directed particularly through the channels of the exchangeable structure, thus over its surface area, and cannot flow past the exchangeable structure. More preferably, the gas stream containing at least one α,β-ethylenically unsaturated aldehyde cannot reach the second reactor without first flowing through the channels of the exchangeable structure. This means that there are no gaps in the cross-sectional area of ​​the connecting conduit. The channels in the blocks of the exchangeable structure are not considered "gaps" here. "Gap" is understood to mean, for example, an opening between the outer end of the exchangeable structure and the housing, or an opening between two blocks when their outer walls are not aligned closely enough to contact.

[0067] In one embodiment, the connecting conduit has an installation housing in which the replaceable structure is arranged. The housing is airtight. The replaceable structure is particularly releasably mounted in the housing so that it can be replaced. For example, the replaceable structure can be removed and replaced with a new replaceable structure.

[0068] Alternatively or additionally, the housing may be split and removed from the remainder of the connecting conduit. For this purpose, the housing has a connection port at each end, through which the housing can be removably connected to the connecting conduit via a screw and secured by a flange. In that case, the housing together with the replaceable structure may be replaced with a new housing together with a new replaceable structure. The replaceable structure may then be permanently attached in the housing. Arranging the replaceable structure in a housing installed in the connecting conduit allows for easy and quick replacement of the replaceable structure arranged in the housing, in that a used replaceable structure arranged in the housing is replaced with a new structure or a new housing including the replaceable structure.

[0069] Typically, the replaceable structure rests on a support element mounted below the replaceable structure within the housing. Typical support elements include, for example, grids and meshes. The openings in the support element have a cross-sectional area greater than the cross-sectional area of ​​the channels in the structured packing. The support element functions to prevent any movement, e.g., by sliding, in the direction of the replaceable structure or connecting conduits within the housing.

[0070] In one embodiment, the cross-sectional area of ​​the housing of the exchangeable structure is larger than the cross-sectional area of ​​the connecting conduit. The "cross-sectional area" of the housing is understood to mean any area of ​​the housing perpendicular to the flow direction of the gas stream containing at least one α,β-ethylenically unsaturated aldehyde flowing therethrough. The cross-sectional area of ​​the housing of the exchangeable structure is preferably at least 25% larger, more preferably at least 150% larger, than the cross-sectional area of ​​the connecting conduit. An embodiment in which the cross-sectional area of ​​the housing of the exchangeable structure is larger than the cross-sectional area of ​​the connecting conduit allows the pressure drop in the process of the present invention to be kept low. An embodiment that can minimize the pressure drop results in higher selectivity for acrylic acid in the oxidation of propylene to acrylic acid and is therefore particularly preferred.

[0071] Furthermore, the surface area available for the sublimation of the catalytic components is increased by the larger cross-sectional area. In particular, since the surfaces are parallel to one another, no pressure drop occurs even if the catalytic components are adsorbed onto the surface. As a result, the cross-section remains sufficient for gas passage over a long period of time. This advantageously extends the maintenance intervals or reduces the need for plant maintenance.

[0072] In one embodiment, the plant includes a cooler for cooling the gas stream containing at least one α,β-ethylenically unsaturated aldehyde after it leaves the first reactor and before it passes through the exchangeable structure. Cooling the gas stream containing at least one α,β-ethylenically unsaturated aldehyde, which may contain a catalytic component of the first catalyst discharged from the first reactor together with the reaction gas, promotes partial or complete desublimation on the high specific surface area of ​​the exchangeable structure.

[0073] Unless otherwise stated, the above embodiments, explanations and preferences apply equally to the process of the invention and the plant of the invention.

[0074] The invention will now be explained in detail by means of the accompanying drawings using the example of the production of acrylic acid from propylene. [Brief explanation of the drawings]

[0075] [Figure 1] 1 is a schematic diagram of a working example of a plant according to the invention for the production of acrylic acid by two-stage catalytic gas-phase oxidation of propylene. [Figure 2] 1 is a schematic perspective view of a cross section of an exchangeable structure having a high specific surface area. DETAILED DESCRIPTION OF THE INVENTION

[0076] An example of the operation of the plant of the present invention will be described with reference to FIGS.

[0077] 1 shows a plant according to the invention for the catalytic gas-phase oxidation of propylene to acrylic acid. The plant comprises a first shell-and-tube heat exchanger reactor A, which is salt-bath regulated, and has reactor tubes R1 filled with a first catalyst K1. A connecting conduit V connects the first shell-and-tube heat exchanger reactor A to a second shell-and-tube heat exchanger reactor B, which is salt-bath regulated. The second shell-and-tube heat exchanger reactor B comprises reactor tubes R2 filled with a second catalyst K2.

[0078] The connecting conduit V has a cooler K' between the first shell-and-tube heat exchange reactor A and the second shell-and-tube heat exchange reactor B. The connecting conduit V also has a housing G with a replaceable structure S disposed therein. The housing G is a part of the connecting conduit V. The housing G is inserted into the connecting conduit V between the cooler K' and the second shell-and-tube heat exchange reactor B. The connecting conduit V between the first shell-and-tube heat exchange reactor A and the housing G has a diameter of 1.4 m. The connecting conduit V between the housing G and the second shell-and-tube heat exchange reactor V has a diameter of 1.6 m. The housing with the replaceable structure S has a diameter of 2 m.

[0079] The exchangeable structure S, arranged in the housing G, has blocks U and channels C formed in the flow direction of the acrolein-containing gas stream 2 flowing therethrough. A schematic cross-section of the exchangeable structure S is shown in Figure 2. The channels C are indicated by dotted lines in Figure 2. The surfaces of the channels provide a high specific surface area for the exchangeable structure compared to the surface area provided by the connecting conduits themselves. This high specific surface area is 600 m 2 / m 3 The specific surface area per unit gas volume is 800 m 2 / m 3 The specific surface area is the geometric surface area that ignores the increase in surface area due to roughness. The average roughness R of the structure a is in the range of 2 μm to 2.5 μm.

[0080] The blocks of the replaceable structure S are placed on a grid Y in a housing G. They essentially completely fill the cross section of the housing G and therefore of the connecting conduit V. Any gaps are filled, for example, by a glass fabric. The housing G has one connection port at one end and one at the other end, via which the housing G is detachably connected to the other part of the connecting conduit V via a screw and fixed by a flange. Below follows a description of an example embodiment of the process of the invention, followed by further details of a working example of the plant of the invention.

[0081] A propylene-containing gas stream 1 is introduced into a first shell-and-tube heat exchange reactor A and flows through a reactor tube R1 filled with a first catalyst K1. In the first shell-and-tube heat exchange reactor A, propylene reacts with atmospheric oxygen over the first catalyst K1 in a first catalytic oxidation reaction to produce acrolein. The first catalyst K1 used is a polymetallic oxide of molybdenum. The resulting gas stream 2, essentially containing acrolein and unconverted atmospheric oxygen, exits the first shell-and-tube heat exchange reactor A via a connecting line V, as indicated by the arrow at the bottom end of the first shell-and-tube heat exchange reactor A in Figure 1. Under the prevailing reaction conditions of 300 to 400 °C and 2 to 3 bar absolute in the first shell-and-tube heat exchange reactor A, the components of the first catalyst K1 sublimate to some extent and are discharged into the connecting line V by the acrolein-containing gas stream 2.

[0082] Before reaching the second shell-and-tube heat exchanger reactor B, the acrolein-containing gas stream 2 passes through a cooler K', which cools it from 350°C to 240°C. After leaving the cooler K', the acrolein-containing gas stream 2 is further cooled from 240°C to 150°C due to heat losses in the pipeline before reaching the exchangeable structure S. The acrolein-containing gas stream 2 is guided through the exchangeable structure S, in that it flows through the channel C of the block U of the exchangeable structure S, as indicated by the arrows in FIG. 1. The sublimated components of the first catalyst K1 present in the acrolein-containing gas stream 2 are subsequently partially or completely desublimated on the surface of the channel C of the block U of the exchangeable structure S. The resulting acrolein-containing gas stream 2, from which the sublimated components of the first catalyst K1 have essentially been removed, then exits the exchangeable structure S via a connecting conduit V (not shown in FIG. 2).

[0083] Subsequently, the acrolein-containing gas stream 2 enters and flows through the second shell-and-tube heat exchange reactor B, as shown by the arrow at the reactor inlet of the second shell-and-tube heat exchange reactor B in Figure 1. In the second shell-and-tube heat exchange reactor B, the acrolein undergoes a second oxidation reaction in the presence of atmospheric oxygen over a second catalyst K2 to form acrylic acid. The second catalyst K2 used is a multimetal oxide of molybdenum. The resulting acrylic acid-containing gas stream 3 essentially contains acrylic acid as well as propylene, acrolein, and oxygen, which are removed at the outlet from the second shell-and-tube heat exchange reactor B.

[0084] As the deposition of sublimated components of the first catalyst K1 in the channels C of the exchangeable structure S increases, the cross section of the channels C shrinks due to the accumulation of desublimated components of the first catalyst K1. This can lead to an increase in pressure in the first shell-and-tube heat exchanger reactor A, necessitating the replacement (maintenance) of the exchangeable structures. During the life of the first catalyst 1 and / or the second catalyst 2, the exchangeable structures may be replaced once or periodically. Replacement naturally entails an interruption of synthesis in the first shell-and-tube heat exchanger reactor A and the second shell-and-tube heat exchanger reactor B due to a plant shutdown (standby). For this purpose, the reactors are typically maintained at 1 bar absolute and approximately 150 °C. The stated temperature is higher than the melting temperature of the salt bath. By keeping the salt bath molten, reaction conditions are achieved again very quickly after synthesis is resumed. In comparison, cooling the salt melt to room temperature and reheating it to at least 150° C. is significantly more time consuming and resource intensive.

[0085] The housing G with the replaceable structure S is then removed. For this purpose, the screws of the flange connecting the housing G to the connecting conduit V are released. The connecting conduit V is then opened at the connection port attached to the end of the housing G. The housing G with the used replaceable structure S is then lifted with a crane. A new housing G with a new replaceable structure S is then inserted, and the connecting conduit V is again hermetically sealed with the flange and screwed in. In another working example, a bypass or two structures are connected in parallel, in which only one operates at a time and can be switched to the other during operation. Afterwards, synthesis in the first shell-and-tube heat exchanger reactor A and the second shell-and-tube heat exchanger reactor B can be resumed.

[0086] The placement of the replaceable structure in a housing installed in the connecting conduit allows for easy and quick replacement of the replaceable structure placed in the housing. Furthermore, replacement is much simpler than the processes previously described in the prior art. Until now, the discharged catalyst components have accumulated on the surfaces (e.g., the bed) of the second shell-and-tube heat exchanger reactor B. As a result, if an increase in pressure drop occurred, it was necessary to shut down the process to allow the reactor to cool, open the reactor, and perform maintenance in the form of cleaning the individual reactor tubes. The maintenance needs of the plant of the present invention are therefore many times smaller. [Explanation of symbols]

[0087] 1. A gas stream containing at least one alkene 2. A gas stream containing at least one α,β-ethylenically unsaturated aldehyde 3. A gas stream containing at least one α,β-ethylenically unsaturated carboxylic acid A. First shell-and-tube heat exchange reactor B. Second shell-and-tube heat exchange reactor C channel G Housing K' cooler K1 First catalyst K2 Second catalyst R1 Reaction tube (in the first shell-and-tube heat exchanger reactor A) R2 Reaction tube (in the second shell-and-tube heat exchanger reactor B) S Exchangeable Structure U-block V Connecting conduit Y Grid

Claims

1. 1. A process for producing α,β-ethylenically unsaturated carboxylic acids by two-stage catalytic vapor phase oxidation of alkenes, comprising: a) a gas stream (1) comprising at least one alkene is subjected to a first catalytic oxidation reaction in the presence of oxygen in a first reactor (A) with a first catalyst (K1) in the form of a polymetallic oxide of molybdenum, in order to obtain a gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde, b) said gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde is conducted through a connecting line (V) into a second reactor (B), c) the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde is subjected to a second catalytic oxidation reaction with a second catalyst (K2) in the presence of oxygen in the second reactor (B) to obtain a gas stream (3) comprising at least one α,β-ethylenically unsaturated carboxylic acid, A process comprising the step of directing said gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde through an exchangeable structure (S) having a specific surface area of ​​at least 400 m 2 / m 3 arranged in said connecting conduit (V).

2. 2. The process according to claim 1, wherein the flow rate of the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde is slowed in the region of the exchangeable structure (S).

3. 3. The process according to claim 1 or 2, wherein the gas stream (2) comprising the at least one α,β-ethylenically unsaturated aldehyde is cooled by 80 to 160° C. after leaving the first reactor (A) and before passing through the exchangeable structure (S).

4. 4. The process according to any one of claims 1 to 3, wherein the replaceable structure (S) is replaced once or periodically while the first reactor (A) and / or the second reactor (B) are not cooled below 150°C.

5. 5. A process according to any one of claims 1 to 4 for the production of acrylic acid by two-stage catalytic gas phase oxidation of propylene.

6. 1. A plant for producing α,β-ethylenically unsaturated carboxylic acids by two-stage catalytic vapor phase oxidation of alkenes, comprising: a) a first reactor (A) designed to carry out a first catalytic oxidation reaction of a gas stream (1) comprising at least one alkene in the presence of oxygen on a first catalyst (K1) in the form of a multimetal oxide of molybdenum, in order to obtain a gas stream comprising at least one α,β-ethylenically unsaturated aldehyde; b) a second reactor (B) designed to carry out a second catalytic oxidation reaction of said gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde in the presence of oxygen over a second catalyst (K2) to obtain a gas stream (3) comprising at least one α,β-ethylenically unsaturated carboxylic acid; c) a connecting conduit (V) arranged between the first reactor (A) and the second reactor (B) for conducting the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde into the second reactor (B); d) an exchangeable structure (S) arranged in said connecting conduit (V) and having a specific surface area of ​​at least 400 m 2 / m 3 and through which said gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde can pass.

7. The replaceable structure (S) has a length of at least 600 m ２ / m ３ 7. The plant according to claim 6, having a specific surface area of

8. The replaceable structure (S) has a length of at least 800 m ２ / m ３ 7. The plant according to claim 6, wherein the specific surface area per unit gas volume is:

9. 9. A plant according to any one of claims 6 to 8, wherein the replaceable structure (S) is a structured packing.

10. 10. A plant according to any one of claims 6 to 9, wherein the replaceable structure (S) is made of ceramic.

11. 11. A plant according to any one of claims 6 to 10, wherein the replaceable structure (S) comprises a channel (C).

12. 12. A plant according to any one of claims 6 to 11, wherein the connecting conduit (V) has an installed housing (G), and the replaceable structure (S) is arranged in the housing (G).

13. 13. A plant according to claim 12, wherein the cross-sectional area of ​​the housing (G) of the replaceable structure (S) is greater than the cross-sectional area of ​​the connecting conduit (V).

14. 14. The plant according to any one of claims 6 to 13, wherein the first reactor (A) and / or the second reactor (B) are fixed-bed shell-and-tube heat exchange reactors.

15. 15. The plant according to any one of claims 6 to 14, further comprising a cooler (K') for cooling the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde after leaving the first reactor (A) and before passing through the exchangeable structure (S).

Images