TWO-STAGE PREPARATION PROCESS FOR ?,?-ETHYLENICALLY UNSATURATED CARBOXYLIC ACIDS AND PLANT FOR THE PURPOSE

Abstract

The invention relates to a process for preparing ,-ethylenically unsaturated carboxylic acids by two-stage catalytic gas phase oxidation of alkenes, in which a gas stream (1) containing 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) containing at least one ,-ethylenically unsaturated aldehyde, the gas stream (2) containing the at least one ,-ethylenically unsaturated aldehyde is guided through a connecting conduit (V) into a second reactor (B) and the gas stream (2) containing the at least one ,-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) containing at least one ,-ethylenically unsaturated carboxylic acid. In the process according to the invention, the gas stream (2) containing the at least one ,-ethylenically unsaturated aldehyde is guided through an exchangeable structure (S) having high specific surface area which is disposed within the connecting conduit (V). The invention further relates to a corresponding plant.

Claims

1.-15. (canceled)

16. A process for producing ,-ethylenically unsaturated carboxylic acids by two-stage catalytic gas phase oxidation of alkenes, in which a) a gas stream comprising at least one alkene, in a first reactor in the presence of oxygen, is subjected to a first catalytic oxidation reaction over a first catalyst in the form of a multimetal oxide of molybdenum to obtain a gas stream comprising at least one ,-ethylenically unsaturated aldehyde, b) the gas stream comprising at least one ,-ethylenically unsaturated aldehyde is directed through a connecting conduit into a second reactor and c) the gas stream comprising at least one ,-ethylenically unsaturated aldehyde, in the second reactor in the presence of oxygen, is subjected to a second catalytic oxidation reaction over a second catalyst to obtain a gas stream comprising at least one ,-ethylenically unsaturated carboxylic acid, which comprises directing the gas stream comprising at least one ,-ethylenically unsaturated aldehyde through an exchangeable structure with high specific surface area which is disposed in the connecting conduit.

17. The process according to claim 16, in which the flow rate of the gas stream comprising at least one ,-ethylenically unsaturated aldehyde is slowed in the region of the exchangeable structure.

18. The process according to claim 16, in which the gas stream comprising the at least one ,-ethylenically unsaturated aldehyde, after leaving the first reactor and before passing through the exchangeable structure, is cooled by 80 to 160 C.

19. The process according to claim 16, in which the exchangeable structure is exchanged once or periodically while the first reactor and/or second reactor is not being cooled below 150 C.

20. The process according to claim 16 for production of acrylic acid by two-stage catalytic gas phase oxidation of propylene.

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

22. The plant according to claim 21, in which the exchangeable structure has a specific surface area of at least 400 m.sup.2/m.sup.3.

23. The plant according to claim 21, in which the exchangeable structure has a specific surface area per unit gas volume of at least 600 m.sup.2/m.sup.3.

24. The plant according to claim 21, in which the exchangeable structure is a structured packing.

25. The plant according to claim 21, in which the exchangeable structure consists of a ceramic.

26. The plant according to claim 21, in which the exchangeable structure comprises channels.

27. The plant according to claim 21, in which the connecting conduit has an installed housing, and the exchangeable structure is disposed within the housing.

28. The plant according to claim 27, in which the cross-sectional area of the housing of the exchangeable structure is greater than the cross-sectional area of the connecting conduit.

29. The plant according to claim 21, in which the first reactor and/or second reactor is a fixed bed shell-and-tube heat exchange reactor.

30. The plant according to claim 21, also comprising a cooler for cooling of the gas stream comprising at least one ,-ethylenically unsaturated aldehyde after it leaves the first reactor and before it passes through the exchangeable structure.

Description

[0178] The present invention is now elucidated in detail by the appended drawings using the example of the production of acrylic acid from propylene.

[0179] FIG. 1 shows a schematic of a working example of the plant of the invention for production of acrylic acid by two-stage catalytic gas phase oxidation of propylene.

[0180] FIG. 2 shows a schematic perspective diagram of a cross section of the exchangeable structure with high specific surface area.

[0181] With reference to FIGS. 1 and 2, a working example of the plant of the invention is elucidated:

[0182] FIG. 1 shows a plant of the invention for catalytic gas phase oxidation of propylene to acrylic acid. The plant comprises a salt bath-moderated first shell-and-tube heat exchange reactor A, which has reaction tubes R1 filled with a first catalyst K1. A connecting conduit V connects the first shell-and-tube heat exchange reactor A to a salt bath-moderated second shell-and-tube heat exchange reactor B. The second shell-and-tube heat exchange reactor B comprises reaction tubes R2 filled with a second catalyst K2.

[0183] 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 an exchangeable structure S disposed therein. The housing G is part of the connecting conduit V. It 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 comprising the exchangeable structure S has a diameter of 2 m.

[0184] The exchangeable structure S disposed in the housing G has blocks U and channels C formed in flow direction of the acrolein-comprising gas stream 2 that flows through it. A schematic diagram of a cross section of the exchangeable structure S is shown in FIG. 2. The channels C are indicated as dotted lines in FIG. 2. The surfaces of the channels provide a high specific surface area of the exchangeable structure by comparison with the surface area provided by the connecting conduit on its own. This high specific surface area is 600 m.sup.2/m.sup.3. The specific surface area per unit gas volume is 800 m.sup.2/m.sup.3. The specific surface area is the geometric surface area neglecting the increases in surface area on account of roughness. The average roughness Ra of the structure is within a range from 2 m to 2.5 m.

[0185] The blocks of the exchangeable structure S lie on a grid Y in the housing G. They essentially completely fill the cross section of the housing G and hence of the connecting conduit V. Any gaps are filled, for example, by glass weave. The housing G has one connection port at one end and one at the other end, via which the housing G is releasably connected to other sections of the connecting conduit V via screws and is secured by flanges.

[0186] There follows a description of a working example of the process of the invention, wherein further details of the working example of the plant of the invention are elucidated:

[0187] A propylene-comprising gas stream 1 is introduced into the first shell-and-tube heat exchange reactor A and flows through the reaction tubes R1 filled with the first catalyst K1. In the first shell-and-tube heat exchange reactor A, propylene is reacted with atmospheric oxygen over the first catalyst K1 in a first catalytic oxidation reaction to give acrolein. The first catalyst K1 used is a multimetal oxide of molybdenum. A resulting gas stream 2 that comprises essentially acrolein as well as unconverted atmospheric oxygen, as shown in FIG. 1 by the arrow at the lower end of the first shell-and-tube heat exchange reactor A, leaves the first shell-and-tube heat exchange reactor A via connecting conduit V. 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, constituents of the first catalyst K1 sublime to some degree and are discharged into the connecting conduit V by the acrolein-comprising gas stream 2.

[0188] The acrolein-comprising gas stream 2, before reaching the second shell-and-tube heat exchange reactor B, passes through the cooler K, which cools it down from 350 C. to 240 C. In addition, the acrolein-comprising gas stream 2, after leaving the cooler K and before reaching the exchangeable structure S, is cooled down by heat loss in the pipeline from 240 C. to 150 C. The acrolein-comprising gas stream 2, as illustrated in FIG. 1 by the arrows, is guided through the exchangeable structure S in that it flows through the channels C of the blocks U of the exchangeable structure S. The sublimed constituents of the first catalyst K1 that are present in the acrolein-comprising gas stream 2 are subsequently partly or fully desublimed at the surface of the channels C in the blocks U of the exchangeable structure S. The resulting acrolein-comprising gas stream 2 that has been essentially freed of sublimed constituents of the first catalyst K1 then leaves the exchangeable structure S via connecting conduit V (not shown in FIG. 2).

[0189] Subsequently, the acrolein-comprising gas stream 2 is introduced into and flows through the second shell-and-tube heat exchange reactor B, as indicated in FIG. 1 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 is subjected to a second oxidation reaction in the presence of atmospheric oxygen over the second catalyst K2, forming acrylic acid from acrolein. The second catalyst K2 used is a multimetal oxide of molybdenum. A resulting acrylic acid-comprising gas stream 3 comprises essentially acrylic acid as well as propylene, acrolein and oxygen, and is removed at the outlet from the second shell-and-tube heat exchange reactor B.

[0190] With increasing deposition of sublimed constituents of the first catalyst K1 in the channels C of the exchangeable structure S, the cross section of the channels C is reduced by the accumulation of the desublimed constituents of the first catalyst K1. This can lead to a rise in pressure in the first shell-and-tube heat exchange reactor A, which necessitates an exchange of the exchangeable structure (maintenance). During the lifetime of the first catalyst 1 and/or of the second catalyst 2, the exchangeable structure may be exchanged once or periodically. The exchange follows interruption of the synthesis in the first shell-and-tube heat exchange reactor A and second shell-and-tube heat exchange reactor B by shutdown of the plant (standby). For this purpose, the reactors are typically kept at 1 bar absolute and about 150 C. The stated temperature is higher than the melting temperature of the salt bath. By keeping the salt bath in the molten state, the reaction conditions are very rapidly reattained after the synthesis is restarted. Cooling of the salt melt to room temperature and reheating to at least 150 C., by comparison, is significantly more time-consuming and resource-intensive.

[0191] Subsequently, the housing G comprising the exchangeable structure S is removed. For this purpose, the screws of the flanges that connect the housing G to the connecting conduit V are released. Subsequently, the connecting conduit V is opened at the connection ports mounted at the ends of the housing G. The housing G comprising the used exchangeable structure S is lifted away with a crane. Subsequently, a new housing G with a new exchangeable structure is inserted, and the connecting conduit V is sealed gastight again with the flanges and screwed in. In an alternative working example, a bypass or two structures are connected in parallel, in which case only one at a time is in operation and it is possible to switch to the other during operation. Subsequently, the synthesis in the first shell-and-tube heat exchange reactor A and second shell-and-tube heat exchange reactor B can be restarted.

[0192] The arrangement of the exchangeable structure in a housing installed in the connecting conduit enables simple and rapid exchange of the exchangeable structure disposed in the housing. Moreover, the exchange is very much simpler than in the processes described to date in the prior art. To date, the discharged catalyst constituents have been deposited on surfaces (for example of beds) in the second shell-and-tube heat exchange reactor B. When there was a rise in pressure drop, it was consequently necessary to stop the process in order to cool the reactor, to open it, and to be able to conduct maintenance in the form of cleaning of the individual reaction tubes. The need for maintenance of the plant of the invention is consequently many times smaller.

LIST OF REFERENCE NUMERALS

[0193] 1 gas stream comprising at least one alkene [0194] 2 gas stream comprising at least one ,-ethylenically unsaturated aldehyde [0195] 3 gas stream comprising at least one ,-ethylenically unsaturated carboxylic acid [0196] A first shell-and-tube heat exchange reactor [0197] B second shell-and-tube heat exchange reactor [0198] C channel [0199] G housing [0200] K cooler [0201] K1 first catalyst [0202] K2 second catalyst [0203] R1 reaction tube (in the first shell-and-tube heat exchange reactor A) [0204] R2 reaction tube (in the second shell-and-tube heat exchange reactor B) [0205] S exchangeable structure [0206] U block [0207] V connecting conduit [0208] Y grid