Two-stage production process of α,β-ethylenically unsaturated carboxylic acids and plant therefor

Description

[0001] The present invention relates to a process for the production of α,β-ethylene 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 an apparatus therefor.

[0002] Such a two-step process for the production of acrylic acid is known per se and is described, for example, in WO 02 / 081422 A1 and the publications cited therein. Furthermore, US 6,069,271 describes a process for the production of acrylic acid.

[0003] First, propylene and molecular oxygen are fed into a first reactor. The oxygen can be introduced, for example, in the form of air. An inert gas such as nitrogen can also be added. In the first reactor, propylene is converted to acrolein via catalytic oxidation. The exhaust gas from the first reactor thus contains, among other things, acrolein and unreacted oxygen. This exhaust gas is introduced into the second reactor, where the acrolein and oxygen are converted to acrylic acid.

[0004] Suitable catalysts for the gas-phase oxidation of propylene to acrolein or of acrolein to acrylic acid are used in the first and second reactors. A large number of catalysts suitable for these reactions have been proposed. Multimetal oxides of molybdenum are commonly used in industry.

[0005] To ensure that the acrolein introduced into the second reactor is converted to acrylic acid as completely as possible, sufficient oxygen must also be available in the second reactor. This can be achieved by introducing enough oxygen into the first reactor so that, after the conversion of propylene to acrolein, sufficient oxygen remains for the reaction in the second reactor. Alternatively, oxygen can be added between the first and second reactors. Since increasing the oxygen content in a mixture of propylene and oxygen increases the risk of explosion, it is advantageous to introduce only as much oxygen into the first reactor as is required for the conversion of propylene to acrolein. Subsequently, as much oxygen is added between the first and second reactors as is required for the conversion of the acrolein to acrylic acid.

[0006] It was observed that in a plant operating this process, a pressure drop occurs in the second reactor when the reaction gas passes through. Operating the synthesis at elevated pressure leads to a reduction in acrylic acid selectivity. When the pressure drop reaches a certain level, the reactor must be shut down and serviced. This process is also known as skimming. This maintenance is both time-consuming and resource-intensive, as the plant's downtime is extended by cooling and reheating.

[0007] US Patent 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. A first catalyst layer is placed in the lower part of each reaction tube, and a second catalyst layer in the upper part. An inert layer with a void ratio of 40 to 99.5% is placed between them. The process is designed to prevent the problem of reaction tube clogging caused by sublimated catalyst components from the first catalyst being carried downstream by the reaction gas stream.

[0008] The invention is based on the objective of further developing the aforementioned method and the aforementioned system in such a way that the maintenance effort is reduced.

[0009] According to the invention, the problem is solved by a method according to claim 1 and a system according to claim 6. Advantageous embodiments and further developments are described in the dependent claims.

[0010] The inventive process for the production of α,β-ethylene-unsaturated carboxylic acids by two-stage catalytic gas-phase oxidation of alkenes is a process in which one a) a gas stream containing at least one alkene is subjected in a first reactor in the presence of oxygen to a first catalytic oxidation reaction on a first catalyst in the form of a multimetal oxide of molybdenum, yielding a gas stream containing at least one α,β-ethylene-unsaturated aldehyde, b) the gas stream containing at least one α,β-ethylene-unsaturated aldehyde is directed through a connecting line to a second reactor, and c) the gas stream containing at least one α,β-ethylene-unsaturated aldehyde is subjected in the second reactor in the presence of oxygen to a second catalytic oxidation reaction on a second catalyst, yielding a gas stream containing at least one α,β-ethylene-unsaturated carboxylic acid.

[0011] The process is characterized in that the gas stream containing at least one α,β-ethylene unsaturated aldehyde is passed through an exchangeable structure with a large specific surface area arranged in the connecting line.

[0012] The gas stream containing at least one alkene contains one or more alkenes. Suitable alkenes comprise, for example, 3 to 5 carbon atoms per molecule and are selected, for example, from propylene, 2-methylpropylene, 1-butene, 2-butene, or 1-pentene. Preferably, the alkene is selected from propylene and 2-methylpropylene. Propylene is particularly preferred.

[0013] The 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 supplied. Suitable external oxygen sources include, for example, oxygen, synthetic air, and air. Air is particularly preferred.

[0014] The first catalytic oxidation reaction is carried out in the first reactor. The first reactor is any gas-phase oxidation reactor. 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 enables uniform heat removal and good heat exchange.

[0015] Typically, such fixed-bed shell-and-tube heat exchange reactors consist of a vessel, usually cylindrical, containing a multitude of contact tubes (a tube bundle), usually arranged vertically. Each of these contact tubes contains a fixed-bed array of the catalytically active multimetal oxide (e.g., the first catalyst in the first reactor). The ends of the contact tubes are sealed to tube sheets and terminate in a hood connected to the vessel at the top and bottom, respectively. The gas flow through the contact tubes is fed in and discharged via these hoods, so that each contact tube corresponds to an elongated reaction unit zone. The contact tubes typically have a wall thickness of 1 to 3 mm, an inner diameter of 20 to 30 mm, and a tube length of 2 to 4 m. For practical purposes, the number of contact tubes housed in the vessel is at least 5.000, preferably at least 10,000. The number of contact tubes housed in the reaction vessel is often between 15,000 and 30,000. Within the vessel, the contact tubes are normally arranged homogeneously, with the distribution being expediently chosen such that the distance between the central inner axes of the nearest contact tubes is 35 to 45 mm.

[0016] Furthermore, a heat exchange medium is circulated through the space surrounding the contact tubes to dissipate the process heat. After exiting the vessel, the heat exchange medium is brought back to its original temperature, for example in external heat exchangers, before re-entering the reaction vessel. Fluid temperature control media are particularly suitable as heat exchange media. The use of melts of salts ("salt bath") such as potassium nitrate, potassium nitrite, sodium nitrite, and / or sodium nitrate, or of low-melting-point metals such as sodium, mercury, and alloys of various metals, is especially advantageous.

[0017] The first catalytic oxidation reaction is carried out in the presence of the first catalyst. The first catalyst is in the form of a multimetal oxide of molybdenum. Such catalysts are known per se. In one embodiment, the multimetal oxide has a stoichiometry of the general formula (I) Mo 12 Bi a Fe b X 1< c X 2< d X 3< e X 4< f O n (I), wherein X 1< = Nickel and / or Cobalt, X 2< = Thallium, Samarium, an alkali metal and / or an alkaline earth metal, X 3< = Zinc, Phosphorus, Arsenic, Boron, Antimony, Tin, Cerium, Lead, Vanadium, Chromium, Niobium and / or Tungsten, X 4< = Silicon, Aluminium, 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, and n is a number determined by the valence and abundance of the elements in (I) except for oxygen.

[0018] In preferred embodiments, 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, and f = 0.5 or 1 to 10.

[0019] X1< is preferably cobalt, X2< is preferably K, Cs and / or Sr, more preferably K, X3< is preferably tungsten, zinc and / or phosphorus, and X4< is preferably Si. It is particularly preferred that the variables X1< to X4< simultaneously have the above-mentioned definitions. It is even more preferred that all stoichiometric coefficients a to f and all variables X1< to X4< simultaneously have the above-mentioned advantageous definitions.

[0020] In one embodiment, the catalytically active multimetal oxide has a stoichiometry of the general formula (II) [Y 1< a' Y 2< b' O x' ]p[Y 3< c' Y 4< d' Y 5< e' Y 6< f' Y 7< g' Y 8< h' O y ]q (II) wherein Y 1< = only bismuth or bismuth and at least one of the elements tellurium, antimony, tin and copper, Y 2< = molybdenum and / or tungsten, Y 3< = an alkali metal, thallium and / or samarium, Y 4< = an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and / or mercury, Y 5< = iron or iron and at least one of the elements vanadium, chromium and cerium, Y 6< = phosphorus, arsenic, boron and / or antimony, Y 7< = a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminium, gallium, indium, silicon, germanium, lead, thorium and / or uranium, Y 8< = molybdenum and / 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' are numbers determined by the valence and abundance of the elements in (II) except oxygen, and p, q are numbers whose p / q ratio is between 0.1 and 10.

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

[0022] Catalytically active multimetal oxides of stoichiometry (II) comprise three-dimensional regions of chemical composition Y 1< a' Y 2< b' O x' , dispersed in a matrix phase of chemical composition Y 3< c' Y 4< d' Y 5< e' Y 6< f' Y 7< g Y 8< h' O y'.

[0023] The production of such catalysts is described in detail in documents DE4407020 A1, EP575897 A1, DE3338380 A1 and EP2114562 A1.

[0024] Within the stoichiometries of the general formula (II), those are preferred which correspond to the 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 6< g" Z 7< h" O y" ]q" (IIa) wherein Z 2< = molybdenum and / or tungsten, Z 3< = nickel and / or cobalt, Z 4< = thallium, an alkali metal and / or an alkaline earth metal, 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 and most preferably 0.4 to 3, h" = 0 to 1, x", y" numbers are determined by the valency and abundance of the elements in (IIa) except oxygen, and p", q" numbers whose p" / q" ratio is between 0.1 and 5, preferably between 0.5 and 2.

[0025] Within the catalytically active multimetal oxides of stoichiometry (IIa), those are preferred in which Z 2< b" = (tungsten) b" and Z 8< 12 = (molybdenum) 12 .

[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 of the first catalytic oxidation reaction comprises, optionally in addition to an unreacted portion of the alkene, at least one α,β-ethylene-unsaturated aldehyde formed by partial oxidation of the alkene. For example, the partial oxidation of propylene leads to acrolein, and the partial oxidation of 2-methylpropylene leads to methacrolein. Preferably, the α,β-ethylene-unsaturated aldehyde is selected from acrolein.

[0028] The gas stream containing at least one α,β-ethylene unsaturated aldehyde is discharged at the outlet of the first reactor via the connecting line and introduced into the second reactor.

[0029] The gas stream containing at least one α,β-ethylene unsaturated aldehyde is subjected to a second catalytic oxidation reaction in the presence of oxygen. An external oxygen source can be supplied. 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 the second reactor. The second reactor is any gas-phase oxidation reactor. In a preferred embodiment, the second reactor is a fixed-bed shell-and-tube heat exchanger reactor. Conducting the reaction in a fixed-bed shell-and-tube heat exchanger reactor allows for uniform heat removal and efficient heat exchange.

[0031] The second catalytic oxidation reaction is carried out in the presence of a second catalyst. The second catalyst is in the form of a multimetal oxide of molybdenum. Such catalysts are known per se. In one embodiment, the multimetal oxide has a stoichiometry of the general formula (III) Mo 12 V a X 1< b X 2< c X 3< d X 4< e X 5< f X 6< g O n (III), wherein X 1< = W, Nb, Ta, Cr and / or Ce, X 2< = Cu, Ni, Co, Fe, Mn and / or Zn, X 3< = Sb and / or Bi, X 4< = one or more alkali metals (Li, Na, K, Rb, Cs) and / or H, X 5< = one or more alkaline earth metals (Mg, Ca, Sr, Ba), X 6< = Si, Al, Ti and / or Zr, a = 1 to 6, b = 0.2 to 4, c = 0 to 18, preferably 0.5 to 18, d = 0 to 40, e = 0 to 2, f = 0 to 4, g = 0 to 40, and n is a number determined by the valency and abundance of the elements in (III) except for oxygen.

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

[0033] Preferred multimetal oxide catalysts (III) are those in which X 1< = W, Nb and / or Cr, X 2< = Cu, Ni, Co and / or Fe, X 3< = Sb, X 4< = Na and / or K, X 5< = Ca, Sr and / or Ba, X 6< = Si, Al and / or Ti, a = 2.5 to 5, b = 0.5 to 2, c = 0.5 to 3, d = 0 to 2, e = 0 to 0.2, f = 0 to 1, g = 0 to 15, and n is a number determined by the valence and abundance of the elements in (III) except for oxygen.

[0034] Preferred multimetal oxide catalysts conform to the following general stoichiometry (IIIa) Mo 12 V a X 1< b X 2< c X 5< f X 6< g O n (IIIa), wherein X 1< = W and / or Nb, X 2< = Cu and / or Ni, X 5< = Co and / or Sr, X 6< = Si and / or Al, a = 3 to 4.5, b = 1 to 1.5, c = 0.75 to 2.5, f = 0 to 0.5, g = 0 to 8, and n is a number determined by the valence and abundance of the elements in (Illa) except for oxygen.

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

[0036] Preferably, the variables are to be selected within the specified ranges, provided that the molar fraction of the element Mo, based on the total amount of all elements except oxygen in the catalytically active multimetal oxide material (Illa), is 20 to 80 mol%, the molar ratio of Mo contained in the catalytically active multimetal oxide material (Illa) to V, Mo / V contained in the catalytically active multimetal oxide material (Illa), 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 a through-hole, produced by a tableting or extrusion machine. Alternatively, they can be used in a form with catalytic components deposited on a refractory support, achieving similar effectiveness.

[0038] The first catalyst and second catalyst, as well as their manufacture, are described in detail, for example, in US 8,232,425 B2.

[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 comprises, optionally in addition to an unreacted portion of the alkene and / or an unreacted portion of the gas stream containing at least one α,β-ethylene-unsaturated aldehyde, at least one α,β-ethylene-unsaturated carboxylic acid formed by partial oxidation of the aldehyde. For example, the partial oxidation of acrolein leads to acrylic acid, and the partial oxidation of methacrolein to methacrylic acid. Preferably, the α,β-ethylene-unsaturated carboxylic acid is selected from acrylic acid.

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

[0042] Acrylic acid is absorbed in an absorption liquid suitable for acrylic acid absorption, such as diphenyl, diphenyl ether, dimethyl phthalate, ethylhexanoic acid, N-methylpyrrolidone, paraffin fractions, or mixtures thereof; oligomeric acrylic acids, such as mixtures containing di-, tri-, and tetraacrylic acid; or water. The absorption liquid is brought into contact with the gas stream containing at least one α,β-ethylene unsaturated carboxylic acid in a countercurrent absorption column, e.g., after cooling by means of a heat exchanger at a temperature of 100 to 180 °C. Suitable absorption columns include, for example, packed, valve tray, or bubble-cap columns. The absorption liquid loaded with acrylic acid typically contains volatile impurities such as water, acrolein, formaldehyde, formic acid, and / or acetic acid. These can be removed by stripping with a stripping gas, e.g.,Nitrogen or air are at least partially removed in a countercurrent desorption column. The crude acrylic acid is typically recovered by rectification at reduced pressure, e.g., 0.04 to 0.1 bar, for example, in a packed or tray column. The crude acrylic acid is collected as the overhead product or via a side draw-off at the top of the rectification column, with the absorption liquid being advantageously 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 extraction in a countercurrent extraction column with an extraction solvent, such as ethyl acetate, butyl acetate, ethyl acrylate, 2-butanone, or mixtures thereof, followed by distillation of the extract.

[0043] For fractional condensation, the gas stream containing at least one α,β-ethylene unsaturated carboxylic acid is cooled to 100 to 180 °C and expediently introduced into the lower section of a column with separating internals. As it rises within the column, a middle-boiling fraction, known as the crude acrylic acid fraction, can be collected via a suitably positioned tray. This crude acrylic acid fraction can be further purified by crystallization. The crystallization process is not subject to any limitations.

[0044] Stabilizers and / or polymerization inhibitors for acrylic acid can be added during all processing steps in a manner known per se. A suitable stabilizer is, for example, phenothiazine. Suitable polymerization inhibitors include, for example, hydroquinone, hydroquinone monomethyl ether, p-nitrosophenol, tert-butylphenols, or mixtures thereof.

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

[0046] It was found that the molybdenum compounds used in the first catalyst tend to sublimate under the reaction conditions prevailing in the first reactor. In conventional two-reactor plants, the released catalyst components are carried by the reaction gas from the first reactor to the second reactor via the connecting line. There, they deposit, for example, on inert packing material 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 when the reaction gas passes through the second reactor. Operating the synthesis at this elevated pressure therefore leads to a loss in acrylic acid selectivity. When the pressure drop reaches a certain level, the reactor must be shut down and the top layer of the bed or the inert packing material replaced (skimming).

[0047] According to the invention, this maintenance effort is reduced by passing the gas stream, containing at least one α,β-ethylene unsaturated aldehyde and carrying partially discharged catalyst components from the first catalyst, through a replaceable structure with a large specific surface area arranged in the connecting line after it leaves the first reactor and before entering the second reactor. Advantageously, this prevents the catalyst components of the first catalyst, which are carried out of the first reactor with the reaction gas, from entering the second reactor unhindered, but are partially or completely desublimated on the large specific surface area of ​​the replaceable structure.

[0048] The "specific surface area" of the interchangeable structure refers to the surface area provided by the interchangeable structure relative to its volume. The "surface area provided by the interchangeable structure" denotes a corresponding, macroscopically considered surface (geometric surface), without taking any potential roughness of this surface into account. The "large specific surface area" of the interchangeable structure specifically refers to a surface area that is larger than the surface area of ​​the connecting pipe if the interchangeable structure were not present.

[0049] According to a further embodiment, the microscopic surface area of ​​the exchangeable structure is increased by increasing the roughness of this surface. The mean roughness Ra is, for example, greater than 0.35 µm, preferably greater than 1 µm, and particularly preferably greater than or equal to 2 µm. The mean roughness Ra is, for example, in the range of 2 µm to 3 µm, and particularly in the range of 2 µm to 2.5 µm.

[0050] In one embodiment, the flow velocity of the gas stream containing at least one α,β-ethylene-unsaturated aldehyde is slowed down in the region of the exchangeable structure. This reduction in flow velocity is achieved, for example, by increasing the cross-sectional area of ​​the connecting line. The slower flow velocity results in a longer residence time of the gas stream containing at least one α,β-ethylene-unsaturated aldehyde in the exchangeable structure. A longer residence time promotes the desublimation of the discharged catalyst components.

[0051] In one embodiment, the gas stream containing at least one α,β-ethylene unsaturated aldehyde is cooled by 80 to 160 °C, preferably 110 to 145 °C, after leaving the first reactor and before passing through the exchangeable structure. Cooling is achieved, for example, by an external cooler arranged in the connecting line. External cooling of the gas stream containing at least one α,β-ethylene unsaturated aldehyde, which may contain catalyst components of the first catalyst carried out of the first reactor along with the reaction gas, facilitates desublimation at the large specific surface area of ​​the exchangeable structure.

[0052] In the process according to the invention, desublimation can be promoted from a thermodynamic point of view by low temperatures of the gas stream containing at least one α,β-ethylene unsaturated aldehyde or by low water concentrations in said gas stream. Since water is formed as a byproduct in the present oxidation, the water concentration can only be controlled to a limited extent. Promoting desublimation is therefore advantageously achieved by cooling, i.e., at low temperatures, the gas stream after it leaves the first reactor. However, at excessively low temperatures, there is a risk that higher-boiling byproducts will condense, which can lead to fouling of plant components.In general, therefore, the catalyst components of the first catalyst, which are carried out with the reaction gas from the first reactor, are partially desublimated on the large specific surface of the exchangeable structure, while another part can also desublimate, for example, on the inner wall of the connecting pipe between the two reactors.

[0053] The replaceable structure is interchangeable. In one embodiment, the replaceable structure is exchanged once or periodically while the first reactor (A) and / or second reactor (B) are not cooled below 150 °C. Because the first reactor and / or second reactor are not completely cooled, they can be kept thermally close to the reaction conditions, for example, at 150 °C and 1 bar absolute. This temperature is higher than the melting point of the temperature control medium surrounding the reaction tubes (e.g., the salt bath). Since the salt bath is kept in a molten state, the reaction conditions are quickly restored after restarting the synthesis. Thus, both the time and maintenance effort can be significantly reduced compared to conventional maintenance or cleaning processes (skimming) (a few hours instead of approximately two weeks), since, for example,No cooling of the molten salt to room temperature and reheating to at least 150 °C is necessary.

[0054] A preferred embodiment comprises the production of acrylic acid by two-stage catalytic gas-phase oxidation of propylene, wherein propylene is oxidized to acrolein in the first catalytic oxidation reaction, which is further oxidized to acrylic acid in the second catalytic oxidation reaction. This process enables the efficient production of the valuable product acrylic acid, which, for example, finds large-scale application in polymerization.

[0055] The invention also relates to a plant for the production of α,β-ethyleneistically unsaturated carboxylic acids by two-stage catalytic gas-phase oxidation of alkenes, comprising a) a first reactor configured to carry out a first catalytic oxidation reaction of a gas stream containing at least one alkene in the presence of oxygen over a first catalyst in the form of a multimetal oxide of molybdenum, yielding a gas stream containing at least one α,β-ethylene-unsaturated aldehyde; b) a second reactor configured to carry out a second catalytic oxidation reaction of the gas stream containing at least one α,β-ethylene-unsaturated aldehyde in the presence of oxygen over a second catalyst, yielding a gas stream containing at least one α,β-ethylene-unsaturated carboxylic acid; c) a connecting line arranged between the first reactor and the second reactor to direct the gas stream containing at least one α,β-ethylene-unsaturated aldehyde into the second reactor.and d) an exchangeable structure with a large specific surface area arranged in the connecting line, through which the gas stream containing at least one α,β-ethylene unsaturated aldehyde can pass.

[0056] The "specific surface area" of the interchangeable structure refers to the surface area provided by the interchangeable structure relative to its volume. The "large specific surface area" of the interchangeable structure specifically denotes a surface area that is larger than the surface area of ​​the connecting line if the interchangeable structure were not present.

[0057] According to the invention, the replaceable structure has a specific surface area of ​​at least 400 m² / m³, preferably at least 600 m² / m³. The incorporation of replaceable structures with a specific surface area of ​​at least 400 m² / m³ promotes the desublimation of the carried-out catalyst components and thereby reduces the maintenance requirements of the system.

[0058] Furthermore, the structure is chosen to minimize pressure loss through it. This can be achieved through the structure's absolute surface area (S), which is, for example, at least 350 m².

[0059] In one embodiment, the replaceable structure has a specific surface area per gas volume of at least 600 m² / m³, preferably at least 800 m² / m³. "Specific surface area per gas volume" of the replaceable structure refers to the surface area provided by the replaceable structure over which the flowing gas stream, e.g., the gas stream containing acrolein, flows directly and thereby comes into contact with the surface of the structure, relative to the cavity volume of the structure through which the gas stream flows.

[0060] The installation of replaceable structures with a specific surface area per gas volume of at least 600 m² / m³ promotes the desublimation of the carried-out catalyst components and thereby reduces the maintenance effort of the plant.

[0061] In one embodiment, the interchangeable structure is a structured packing. The term "structured packing" refers to internal components such as metal plates (bases), metal meshes, metal grids, etc. The bases can be, for example, smooth, corrugated, perforated, or embossed. The flow channels of the metal grids can be, for example, straight or inclined. In general, structured packings offer a large (contact) surface area but low resistance to gas flow.

[0062] The production of acrylic acid from propylene is known from the prior art, whereby catalyst components discharged from the reactor are separated, for example, at packed beds arranged in the reaction tubes, in suitable systems. These packed beds are not structured packings but are considered unstructured packings. The pressure drop increases more significantly when the discharged catalyst components are desublimated at the packing material of such unstructured packings than when desublimated at structured packings with the same absolute surface area.

[0063] In one embodiment, the replaceable structure consists of a ceramic. The ceramic is preferably a monolithic ceramic. Monolithic ceramics are able to withstand high temperatures and exhibit high erosion resistance, for example, compared to composite materials with a ceramic matrix. Furthermore, monolithic ceramics utilize cost-effective raw materials and can be molded into specific shapes. The monolithic ceramic can consist of, for example, silicon dioxide (SiO2), silicon nitride (Si3N4), silicon aluminum oxynitride (SiAION), silicon carbide (SiC), silicon oxynitride (Si2N2O), aluminum nitride (AlN), aluminum oxide (Al2O3), hafnium dioxide (HfO2), zirconium dioxide (ZrO2), silicified silicon carbide (Si-SiC) or other oxides, carbides or nitrides or a combination thereof, preferably silicon dioxide (SiO2) and / or aluminum oxide (Al2O3).

[0064] The replaceable structure consists in particular of blocks, preferably rectangular blocks. Typically, the replaceable structure contains 1 to 200, preferably 50 to 170, and most preferably 100 to 150 blocks. Each rectangular block typically has edge lengths of 100 to 200 mm and widths of 100 to 200 mm. Larger rectangular blocks are preferred. The depth of the blocks is 150 to 350 mm in the direction of gas flow. It is adapted to the pressure drop and the total surface area of ​​the replaceable structure. The blocks are arranged side by side. Preferably, the blocks fill the cross-sectional area of ​​the replaceable structure. The "cross-sectional area" of the exchangeable structure is understood to be an area of ​​the exchangeable structure perpendicular to the flow direction of the gas stream passing through it, containing at least one α,β-ethylene unsaturated aldehyde.Essentially, there are no gaps between the blocks. If necessary, any gaps can be sealed gas-tight, e.g., with a fiberglass fabric. Preferably, the blocks completely fill the cross-sectional area of ​​the replaceable structure.

[0065] In one embodiment, the exchangeable structure comprises channels. The channels can be formed, in particular, in the aforementioned blocks. The channels can have different cross-sections, for example, round, honeycomb-shaped, rectangular, and square. Channels with a square cross-section are preferred. The longitudinal direction of the channels is aligned with the flow direction of the gas stream containing at least one α,β-ethylene unsaturated aldehyde. The surfaces of the channels provide the surface of the exchangeable structure on which the discharged catalyst components desublimate, thereby reducing the maintenance requirements of the system.

[0066] The cross-sectional area of ​​the connecting pipe is completely filled by the exchangeable structure. The gas stream containing at least one α,β-ethylene unsaturated aldehyde is guided through the channels of the exchangeable structure and thus across its surface, and cannot bypass the exchangeable structure. Particularly preferably, the gas stream containing at least one α,β-ethylene unsaturated aldehyde cannot reach the second reactor without first flowing through the channels of the exchangeable structure. This means that the cross-sectional area of ​​the connecting pipe is free of gaps. The channels in the blocks of the exchangeable structure are not considered "gaps" in this context. "Gaps" are understood to be, for example, gaps between the outer end of the exchangeable structure and the casing, or gaps between two blocks, provided they are not positioned so close together that their outer walls are in contact.

[0067] In one embodiment, the connecting line has a built-in housing in which the replaceable structure is arranged. The housing is gas-tight. The replaceable structure is, in particular, detachably mounted in the housing so that it can be replaced. For example, the replaceable structure can be removed and replaced with a new one.

[0068] Alternatively or additionally, the housing can be detached from the remaining sections of the connecting pipe and removed. For this purpose, the housing has a connection fitting at each end, through which the housing is detachably connected to the connecting pipe via screws and can be secured with flanges. In this case, the housing, together with the replaceable structure, can be replaced with a new housing and a new replaceable structure. The replaceable structure can then be permanently mounted in the housing. The arrangement of the replaceable structure in a housing integrated into the connecting pipe allows for a simple and quick replacement of the replaceable structure within the housing by replacing a worn-out replaceable structure with a new one or a new housing containing the replaceable structure.

[0069] The replaceable structure typically rests on a support element located beneath it within the housing. Common support elements include, for example, grids and grates. The openings of the support elements have larger cross-sectional areas than the channels of the structured packing. The support element serves to prevent displacement of the replaceable structure within the housing or along the connecting line, for example, by sliding down.

[0070] In one embodiment, the cross-sectional area of ​​the housing of the replaceable structure is larger than the cross-sectional area of ​​the connecting line. The "cross-sectional area" of the housing is defined as an area of ​​the housing perpendicular to the flow direction of the gas stream containing at least one α,β-ethylene unsaturated aldehyde. Preferably, the cross-sectional area of ​​the housing of the replaceable structure is at least 25% larger than the cross-sectional area of ​​the connecting line, and particularly preferably at least 150% larger. In this embodiment, where the cross-sectional area of ​​the housing of the replaceable structure is larger than the cross-sectional area of ​​the connecting line, the pressure loss in the method according to the invention can be kept low.Embodiments in which the pressure loss can be minimized result in a higher selectivity of acrylic acid during the oxidation of propylene to acrylic acid and are therefore particularly preferred.

[0071] Furthermore, the larger cross-sectional areas increase the surface area available for the sublimation of the catalyst components. In particular, the surfaces are arranged parallel to each other, so that even if catalyst components accumulate on the surface, no pressure drop occurs. Consequently, the cross-section remains sufficient for gas flow over a longer period. This advantageously extends maintenance intervals and reduces the maintenance effort required for the system.

[0072] In one embodiment, the system includes a cooler for cooling the gas stream containing at least one α,β-ethylene unsaturated aldehyde after it leaves the first reactor and before passing through the exchangeable structure. Cooling the gas stream containing at least one α,β-ethylene unsaturated aldehyde, which may contain catalyst components of the first catalyst carried out of the first reactor with the reaction gas, facilitates partial or complete desublimation on the large specific surface area of ​​the exchangeable structure.

[0073] Unless otherwise stated, the embodiments, explanations and preferences described above apply equally to the method and the plant according to the invention.

[0074] The present invention will now be explained in detail with reference to the attached drawings, using the example of the production of acrylic acid from propylene. Figure 1 schematically shows an embodiment of the inventive apparatus for the production of acrylic acid by two-stage catalytic gas-phase oxidation of propylene. Figure 2 shows a schematic, perspective view of a cross-section of the exchangeable structure with a large specific surface area.

[0075] With reference to the Figure 1 and 2 An exemplary embodiment of the system according to the invention is explained: Figure 1Figure 1 shows a system according to the invention for the catalytic gas-phase oxidation of propylene to acrylic acid. The system comprises a salt bath-moderated, first shell-and-tube heat exchanger reactor A, which has reaction tubes R1 filled with a first catalyst K1. The first shell-and-tube heat exchanger reactor A is connected via a connecting line V to a second, salt bath-moderated, shell-and-tube heat exchanger reactor B. The second shell-and-tube heat exchanger reactor B contains reaction tubes R2 filled with a second catalyst K2.

[0076] The connecting line V includes a cooler K' between the first shell-and-tube heat exchanger reactor A and the second shell-and-tube heat exchanger reactor B. The connecting line V also includes a casing G in which an exchangeable structure S is located. The casing G is part of the connecting line V and is inserted into the connecting line V between the cooler K' and the second shell-and-tube heat exchanger reactor B. The connecting line V between the first shell-and-tube heat exchanger reactor A and the casing G has a diameter of 1.4 m. The connecting line V between the casing G and the second shell-and-tube heat exchanger reactor B has a diameter of 1.6 m. The casing containing the exchangeable structure S has a diameter of 2 m.

[0077] The replaceable structure S arranged in the housing G has blocks U and channels C that are oriented in the direction of flow of the acrolein-containing gas stream 2 passing through it. A schematic representation of a cross-section of the replaceable structure S is shown in Figure 2 shown. Channels C are in Figure 2 The channels are indicated by dashed lines. Compared to the surface area provided by the connecting pipe alone, the channels offer a large specific surface area for the exchangeable structure. This large specific surface area is 600 m² / m³. The specific surface area per gas volume is 800 m² / m³. The specific surface area is the geometric surface area, neglecting surface area increases due to roughness. The mean roughness Ra of the structure is in the range of 2 µm to 2.5 µm.

[0078] The blocks of the replaceable structure S rest on a grid Y within the housing G. They essentially fill the cross-section of the housing G and thus of the connecting line V completely. Any gaps are filled, for example, with fiberglass mesh. The housing G has a connection fitting at one end and at the other end, via which the housing G is detachably connected to other sections of the connecting line V using screws and secured with flanges.

[0079] An embodiment of the process according to the invention is described below, with further details of the embodiment of the apparatus according to the invention being explained: A gas stream 1 containing propylene is introduced into the first shell-and-tube heat exchanger reactor A and flows through the reaction tubes R1 filled with the first catalyst K1. In the first shell-and-tube heat exchanger reactor A, propylene is reacted with atmospheric oxygen at the first catalyst K1 in a first catalytic oxidation reaction to form acrolein. A multimetal oxide of molybdenum serves as the first catalyst K1. A resulting gas stream 2, containing essentially acrolein in addition to unreacted atmospheric oxygen, exits as described in Figure 1The first tube bundle heat exchange reactor A is represented by the arrow at the lower end, via connecting line V. Under the prevailing reaction conditions of 300 to 400 °C and 2 to 3 bar absolute in the first tube bundle heat exchange reactor A, components of the first catalyst K1 partially sublimate and are carried into connecting line V by the acrolein-containing gas stream 2.

[0080] The acrolein-containing gas stream 2 passes through cooler K' before reaching the second tube bundle heat exchange reactor B and is thereby cooled from 350 °C to 240 °C. After leaving cooler K' and before reaching the exchangeable structure S, the acrolein-containing gas stream 2 cools further from 240 °C to 150 °C due to heat loss in the pipeline. The acrolein-containing gas stream 2 is then, as described in the Figure 1The gas stream 2, represented by the arrows, is guided through the exchangeable structure S by flowing through the channels C of the blocks U of the exchangeable structure S. The sublimited components of the first catalyst K1 contained in the acrolein-containing gas stream 2 are subsequently partially or completely desublimated on the surface of the channels C in the blocks U of the exchangeable structure S. The resulting gas stream 2, containing acrolein and essentially free of sublimated components of the first catalyst K1, then leaves the exchangeable structure S via connecting line V (in Figure 2 not shown).

[0081] The gas stream 2 containing acrolein is then introduced into the second tube bundle heat exchange reactor B and flows through it, as described in Figure 1This is indicated by the arrow at the reactor inlet of the second shell-and-tube heat exchange reactor B. In the second shell-and-tube heat exchange reactor B, the acrolein undergoes a second oxidation reaction in the presence of atmospheric oxygen at the second catalyst K2, producing acrylic acid. The second catalyst K2 is a multimetal oxide of molybdenum. A resulting gas stream 3 containing acrylic acid, along with propylene, acrolein, and oxygen, consists primarily of acrylic acid and is discharged at the outlet of the second shell-and-tube heat exchange reactor B.

[0082] As sublimited components of the first catalyst K1 are progressively deposited in the channels C of the exchangeable structure S, the cross-section of the channels C is reduced by the deposition of the undublimited components of the first catalyst K1. This can lead to a pressure increase in the first shell-and-tube heat exchanger reactor A, necessitating replacement of the exchangeable structure (maintenance). During the lifetime of the first catalyst 1 and / or the second catalyst 2, the exchangeable structure can be replaced once or periodically. The replacement takes place after interrupting the synthesis in the first shell-and-tube heat exchanger reactor A and the second shell-and-tube heat exchanger reactor B by shutting down the system (standby). For this purpose, the reactors are typically maintained at 1 bar absolute and approximately 150 °C. The specified temperature is higher than the melting point of the salt bath.By keeping the salt bath in a molten state, the reaction conditions are quickly restored after restarting the synthesis. Cooling the molten salt to room temperature and re-tempering it to at least 150 °C is significantly more time-consuming and resource-intensive in comparison.

[0083] The housing G containing the replaceable structure S is then removed. For this purpose, the screws of the flanges connecting the housing G to the connecting line V are loosened. The connecting line V is then opened at the connecting ports located at the ends of the housing G. The housing G, containing the spent replaceable structure S, is lifted off by crane. A new housing G with a new replaceable structure is then inserted, and the connecting line V is resealed and screwed gas-tight to the flanges. In an alternative embodiment, a bypass or two structures are connected in parallel, with only one operating at a time and the system being switched to the other during operation. Synthesis can then be restarted in the first shell-and-tube heat exchanger reactor A and the second shell-and-tube heat exchanger reactor B.

[0084] The arrangement of the replaceable structure in a housing integrated into the connecting pipe enables simple and quick replacement of the replaceable structure within the housing. Furthermore, the replacement process is significantly simpler than in previously described methods. Previously, the discharged catalyst components accumulated on surfaces (e.g., of packed beds) in the second tube bundle heat exchange reactor B. Consequently, when the pressure drop increased, the process had to be interrupted to cool the reactor, open it, and perform maintenance in the form of cleaning the individual reaction tubes. The maintenance effort of the system according to the invention is therefore many times lower. Reference symbol list

[0085] 1 Gas stream containing at least one alkene 2 Gas stream containing at least one α,β-ethylene unsaturated aldehyde 3 Gas stream containing at least one α,β-ethylene 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 first shell-and-tube heat exchange reactor A) R2 Reaction tube (in second shell-and-tube heat exchange reactor B) S Replaceable structure U Block V Connecting line Y Grate

Claims

1. A process for producing α,β-ethylenically unsaturated carboxylic acids by two-stage catalytic gas phase oxidation of alkenes, in which a) a gas stream (1) comprising at least one alkene, in a first reactor (A) in the presence of oxygen, is subjected to a first catalytic oxidation reaction over a first catalyst (K1) in the form of a multimetal oxide of molybdenum to obtain a gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde, b) the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde is directed through a connecting conduit (V) into a second reactor (B) and c) the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde, in the second reactor (B) in the presence of oxygen, is subjected to a second catalytic oxidation reaction over a second catalyst (K2) to obtain a gas stream (3) comprising at least one α,β-ethylenically unsaturated carboxylic acid, wherein the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde is directed through an exchangeable structure (S) with high specific surface area of at least 400 m2 / m3 which is disposed in the connecting conduit (V).

2. The process according to claim 1, in which 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. The process according to claim 1 or 2, in which 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), is cooled by 80 to 160°C, preferably 110 to 145°C.

4. The process according to any of claims 1 to 3, in which the exchangeable structure (S) is changed once or periodically while the first reactor (A) and / or second reactor (B) is not cooled below 150°C.

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

6. A plant for producing α,β-ethylenically unsaturated carboxylic acids by two-stage catalytic gas phase oxidation of alkenes, comprising a) a first reactor (A) designed for performance of a first catalytic oxidation reaction of a gas stream (1) comprising at least one alkene in the presence of oxygen over a first catalyst (K1) in the form of a multimetal oxide of molybdenum to obtain a gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde, b) a second reactor (B) designed for performance of a second catalytic oxidation reaction of the 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) which is disposed between the first reactor (A) and the second reactor (B) in order to direct the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde into the second reactor (B), and d) an exchangeable structure (S) with high specific surface area of at least 400 m2 / m3 which is disposed in the connecting conduit (V), and through which the gas stream (2) comprising at least one α,β-ethylenically unsaturated aldehyde can be passed.

7. The plant according to claim 6, in which the exchangeable structure (S) has a specific surface area of at least 600 m2 / m3.

8. The plant according to claim 6, in which the exchangeable structure (S) has a specific surface area per unit gas volume of at least 600 m2 / m3, preferably at least 800 m2 / m3.

9. The plant according to any of claims 6 to 8, in which the exchangeable structure (S) is a structured packing.

10. The plant according to any of claims 6 to 9, in which the exchangeable structure (S) consists of a ceramic, preferably of silicon dioxide and / or aluminum oxide.

11. The plant according to any of claims 6 to 10, in which the exchangeable structure (S) comprises channels (C) .

12. The plant according to any of claims 6 to 11, in which the connecting conduit (V) has an installed housing (G), and the exchangeable structure (S) is disposed within the housing (G).

13. The plant according to claim 12, in which the cross-sectional area of the housing (G) of the exchangeable structure (S) is greater than the cross-sectional area of the connecting conduit (V), preferably at least 25% greater, more preferably at least 150% greater.

14. The plant according to any of claims 6 to 13, in which the first reactor (A) and / or second reactor (B) is a fixed bed shell-and-tube heat exchange reactor.

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