PRODUCTION OF N-PENTANAL FROM LOW-BUTENE FEEDSTOCK MIXTURES

Abstract

The invention is concerned with the issue of how to produce n-pentanal by hydroformylation from feedstock mixtures comprising a small proportion of n-butene and a large proportion of n-butane. Specifically, solutions for further optimizing established processes for hydroformylation of such low-butene mixtures in terms of material utilization are sought. The present invention has for its object to enhance the material utilization of the feedstock mixture in the production of n-pentanal from feedstock mixtures having a small proportion of n-butene and a large proportion of n-butane. The process shall be capable of economic operation on an industrial scale. In particular an existing oxo plant shall be honed to achieve better raw material utilization. This object is achieved by a combination of a hydroformylation and a dehydrogenation, wherein said combination has the special feature that the dehydrogenation is arranged after the hydroformylation in the downstream direction and is thus markedly smaller than conventional dehydrogenations provided upstream. A skillful product removal effectively removes contaminants formed in the process.

Claims

1. A process for producing n-pentanal comprising the steps of: a) providing a feedstock mixture having the following composition which sums to 100 wt %: n-butane: 70 wt % to 90 wt %; n-butene: 10 wt % to 30 wt %; 1-butene: 0 wt % to 3 wt %; isobutene: 0 wt % to 3 wt %; isobutane: 0 wt % to 3 wt %; 1,3-butadiene: 0 wt % to 1 wt %; other substances: 0 wt % to 1 wt %; b) mixing the feedstock mixture with a recyclate to obtain a feed; c) treating the feed with carbon monoxide and hydrogen in the presence of a first catalyst system to convert at least a portion of the n-butene present in the feed into aldehydes by hydroformylation to obtain a hydroformylation mixture; d) recovering a primary product fraction from the hydroformylation mixture, wherein the primary product fraction has the following composition which sums to 100 wt %: n-pentanal: 90 wt % to 98.5 wt %; 2-methylbutanal: 0 wt % to 5 wt %; 3-methylbutanal: 0 wt % to 3 wt %; other substances: 0 wt % to 2 wt %; e) recovering a subsidiary fraction from the hydroformylation mixture, wherein the subsidiary fraction has the following composition which sums to 100 wt %: n-butane: 80 wt % to 92 wt %; n-butene: 8 wt % to 20 wt %; other substances: 0 wt % to 1 wt %; f) subjecting the subsidiary fraction to a dehydrogenation in the presence of a second catalyst system to obtain a dehydrogenation mixture having the following composition which sums to 100 wt %: n-butene: 50 wt % to 60 wt %; n-butane: 40 wt % to 50 wt %; methane: 0 wt % to 4 wt %; ethene: 0 wt % to 3 wt %; propene: 0 wt % to 2 wt %; 1,3-butadiene: 0 wt % to 3 wt %; other substances: 0 wt % to 1 wt %; g) subjecting the dehydrogenation mixture to a selective hydrogenation in the presence of a third catalyst system to obtain a hydrogenation mixture having the following composition which sums to 100 wt %: n-butene: 50 wt % to 60 wt %; n-butane: 40 wt % to 50 wt %; 1,3-butadiene: 0 ppm by weight to 500 ppm by weight; other substances: 0 wt % to 5 wt %; h) direct use of the hydrogenation mixture as recyclate or purification of the hydrogenation mixture to obtain the recyclate.

2. The process according to claim 1, wherein the second catalyst system is a solid comprising at least platinum, tin and aluminum oxide and that the dehydrogenation is effected in the gas phase at a pressure of 0.8*10.sup.5 Pa to 1.2*10.sup.5 Pa and a temperature of 450° C. to 700° C.

3. The process according to claim 1, wherein at least two reactors, each heated and each filled with the second catalyst system, are provided for the dehydrogenation and the reactors are chargeable with subsidiary fraction individually or simultaneously in parallel and/or serially as desired.

4. The process according to claim 3, wherein the reactors are electrically heated.

5. The process according to claim 1, wherein the dehydrogenation mixture is liquefied by compression and cooling and the selective hydrogenation is effected in the liquid phase at a pressure of 18*10.sup.5 Pa to 22*10.sup.5 Pa and a temperature of 40° C. to 80° C.

6. The process according to claim 5, wherein the heat recovered during cooling is used for preheating the subsidiary fraction.

7. The process according to claim 5, wherein the compression is effected in two successive compression stages and that the cooling provided is an intercooling arranged between the compression stages.

8. The process according to claim 1, wherein the second catalyst system is a solid comprising aluminum oxide and chromium oxide and that the dehydrogenation is effected in the gas phase at a pressure of 0.8*10.sup.5 Pa to 1.2*10.sup.5 Pa and a temperature of 600° C. to 700° C.

9. The process according to claim 1, wherein the second catalyst system is a solid comprising aluminum oxide and magnesiochromite and that the dehydrogenation is effected in the gas phase at a pressure of 0.8*10.sup.5 Pa to 1.2*10.sup.5 Pa and a temperature of 600° C. to 700° C.

10. The process according to claim 1, wherein the dehydrogenation is effected without addition of an oxidant.

11. The process according to claim 1, wherein the dehydrogenation is effected with addition of oxygen, wherein the added amount of oxygen based on the mass of the n-butane present in the subsidiary fraction is 1.4 wt % to 14 wt %.

12. The process according to claim 1, wherein the hydrogenation mixture is mixed with the feedstock mixture as a recyclate without purification.

13. The process according to claim 1, wherein the hydroformylation mixture is exclusively separated into the primary product fraction and the subsidiary fraction.

14. The process according to claim 1, wherein the hydroformylation mixture is separated into a low boiler fraction, the subsidiary fraction and the primary product fraction.

15. The process according to claim 14, wherein the hydroformylation mixture is separated into the low boiler fraction, the subsidiary fraction, the primary product fraction and into a secondary product fraction, wherein the secondary product fraction has the following composition which sums to 100 wt %: propanal: 50 wt % to 70 wt %; n-butanal: 30 wt % to 50 wt %; other substances: 0 wt % to 10 wt %.

16. The process according to claim 1, wherein the mass flow of the subsidiary fraction is less than 4 kg/s and the apparatus of the dehydrogenation is of a size configured for continuous processing of this mass flow.

17. The process according to claim 1, wherein the subsidiary fraction is recovered by distillation with subsequent hydrogenation.

18. A method of a plant for dehydrogenation of alkanes comprising at least a heated reactor filled with a second catalyst system for retrofitting an existing plant for producing n-pentanal from feedstock mixtures comprising the step of hydroformylating of n-butene and n-butane where the plant for dehydrogenation is arranged downstream of the plant for hydroformylation, and feeding said plant for dehydrogenation with a subsidiary fraction from the hydroformylation and the effluent from the dehydrogenation is recycled into the hydroformylation with or without purification.

19. The process according to claim 2, wherein at least two reactors, each heated and each filled with the second catalyst system, are provided for the dehydrogenation and the reactors are chargeable with subsidiary fraction individually or simultaneously in parallel and/or serially as desired.

20. The process according to claim 2, wherein the dehydrogenation mixture is liquefied by compression and cooling and the selective hydrogenation is effected in the liquid phase at a pressure of 18*10.sup.5 Pa to 22*10.sup.5 Pa and a temperature of 40° C. to 80° C.

Description

[0100] FIG. 1: shows a process flow diagram of the basic concept;

[0101] FIG. 2: is as FIG. 1, additionally showing removal of secondary product;

[0102] FIG. 3: is as FIG. 1, additionally showing hydrogenation before dehydrogenation.

[0103] The basic concept of the process according to the invention is depicted in FIG. 1. A feedstock mixture 1 obtained from outside the process and comprising predominantly n-butane and a residual amount of n-butene is mixed with a recyclate 2 to afford a feed 3. The recyclate originates from the process itself, more about that later.

[0104] Feed 3 is run into hydroformylation 4 and there reacted together with synthesis gas 5 (a mixture of carbon monoxide and hydrogen) in customary fashion. Withdrawn from the hydroformylation 4 is a hydroformylation mixture 6 which comprises the desired n-pentanal (formed from the reaction of n-butene with synthesis gas), further byproducts, unconverted n-butene and, especially, unconverted n-butane. The necessary separation of the homogeneous first catalyst system used in the hydroformylation 4 is not depicted here.

[0105] In a separation sequence comprising three distillation columns 7, 8, 9 the hydroformylation mixture is fractionated by distillation. To this end the hydroformylation mixture 6 is run into the first column 7 and separated into tops product 10 and bottoms product 11. The bottoms product 11 from the first column 7 is used to feed the second column 8. Obtained at the bottom of the second column is a primary product fraction 12 which comprises the purified n-pentanal.

[0106] The tops product 13 from the second column 8 is mixed with the tops product 10 from the first column 7 and run into the third column 9. At the top thereof a low boiler fraction 14 is withdrawn and at the bottom a subsidiary fraction 15.

[0107] The subsidiary fraction essentially comprises the non-hydroformylatable n-butane and a significant proportion of n-butene not converted in the hydroformylation 4.

[0108] In order to make the carbon atoms present in the subsidiary fraction 15 usable for the process, the subsidiary fraction 15 is initially preheated in a first heat exchanger 16 and then catalytically dehydrogenated in a dehydrogenation 17. The dehydrogenation 17 is effected in the gas phase in the presence of a heterogeneous second catalyst system, optionally with addition of small amounts of oxygen 18.

[0109] The dehydrogenation requires thermal energy which is preferably electrically generated. It will be appreciated that traditional heating with fuel gas is also possible.

[0110] In the course of the dehydrogenation the n-butane present in the subsidiary fraction 15 is converted into n-butene. Further substances are formed, such as 1,3-butadiene, methane, ethene, propene for instance. The dehydrogenation mixture 19 comprising these substances is withdrawn from the dehydrogenation in gaseous form and then compressed in a first compressor stage 20. The heat from the compressor thus generated is removed by a second heat exchanger 21 and the dehydrogenation mixture 19 is thus intercooled. The heat generated in the intercooling is utilized for preheating the subsidiary fraction 15 before entry into the dehydrogenation 17. To this end the first heat exchanger 16 and the second heat exchanger 21 are interconnected via a circuit 22 which contains a heat transfer medium. The ultimate liquefaction of the dehydrogenation mixture 19 is effected in a second compressor stage 23.

[0111] The now liquid dehydrogenation mixture is now subjected to a selective hydrogenation 24 in the presence of a heterogeneous third catalyst system with addition of hydrogen 25 and carbon monoxide 26 as moderator. The selective hydrogenation 24 hydrogenates and thus neutralizes undesired polyunsaturated compounds such as 1,3-butadiene. The alkenes, by contrast, are preserved.

[0112] The hydrogenation mixture 27 withdrawn from the selective hydrogenation 24 is mixed as recyclate 2 with the feedstock mixture 1 and thus ultimately made available to the process again.

[0113] The hydrogenation mixture 27 may optionally also be purified and then mixed as recyclate 2 with the feedstock mixture 1. However, this is not preferred and therefore not depicted.

[0114] The inventive dehydrogenation and recycling of the recyclate 2 has the effect that the butanes present in the subsidiary fraction 15 reenter the hydroformylation in the form, thanks to the dehydrogenation, of butenes and can there be converted into the primary product n-pentanal. The material efficiency of the process is thus enhanced compared to hydroformylation without dehydrogenation.

[0115] As previously mentioned the dehydrogenation 17 produces not only n-butene but also ethene and propene—both hydroformylatable substrates. Provided that the rate of formation of ethene and propene is high enough a fourth column 28 may be provided in the separation sequence, as depicted in FIG. 2. The fourth column 28 is fed with the tops product 13 from the second column 8. A secondary product fraction 29 comprising propanal and n-butanal, both formed from ethene and propene in the hydroformylation 4, may then be withdrawn from the bottom of the fourth column 28. The tops product 30 from the fourth column 28 is mixed with the tops product 10 from the first column 7 and run into the third column 9.

[0116] A further alternative embodiment is shown in FIG. 3. Here, the subsidiary fraction 15 is obtained when the bottoms product from the third column 9 is hydrogenated with hydrogen 25 in a hydrogenation 31. This measure is necessary when the content of high-reactivity substances in the bottoms product from the third column 9 would be too high to run said product directly into the dehydrogenation 17. However, such a procedure is not preferred.

LIST OF REFERENCE SYMBOLS

[0117] 1 Feedstock mixture [0118] 2 Recyclate [0119] 3 Feed [0120] 4 Hydroformylation [0121] 5 Synthesis gas [0122] 6 Hydroformylation mixture [0123] 7 First column [0124] 8 Second column [0125] 9 Third column [0126] 10 Tops product from first column [0127] 11 Bottoms product from first column [0128] 12 Primary product fraction [0129] 13 Tops product from second column [0130] 14 Low boiler fraction [0131] 15 Subsidiary fraction [0132] 16 First heat exchanger [0133] 17 Dehydrogenation [0134] 18 Oxygen [0135] 19 Dehydrogenation mixture [0136] 20 First compressor stage [0137] 21 Second heat exchanger [0138] 22 Circuit [0139] 23 Second compressor stage [0140] 24 Selective hydrogenation [0141] 25 Hydrogen [0142] 26 Carbon monoxide [0143] 27 Hydrogenation mixture [0144] 28 Fourth column [0145] 29 Secondary product fraction [0146] 30 Tops product from fourth column [0147] 31 Hydrogenation