Optimized thermal separation by prior gas exchange
A process can be used for preparing aldehydes from C2 to C20 olefins with a subsequent thermal separation for removal of the aldehyde formed. The process involves a membrane separation, which is preceded by performance of a gas exchange by which the proportion of the partial pressure represented by carbon monoxide or hydrogen is increased in order to reduce catalyst losses.
1. A process for preparing an aldehyde, the process comprising at least:
a) reacting, in a hydroformylation, at least one C 2 to C 20 olefin with synthesis gas in the presence of a homogeneously dissolved catalyst system comprising at least one metal from the group consisting of Co and Rh, and a phosphorus ligand; wherein the hydroformylation is conducted in at least one reaction zone and at a pressure of 10 to 350 bar absolute, which affords a first liquid reaction output containing at least the aldehyde and unconverted olefins, and wherein a partial pressure of CO or H 2 accounts for at most 70% of a total gas pressure of all gaseous substances present;
b) performing a gas exchange with the first liquid reaction output, which affords a second liquid reaction output in which the partial pressure of CO or H 2 accounts for more than 90% of the total gas pressure;
c) expanding the second liquid reaction output obtained from b) in a single-stage or multistage expansion, to a pressure of less than 1 bar absolute, which affords a third liquid reaction output; and
d) feeding the third liquid reaction output obtained from c) to a thermal separation in order to separate the third liquid reaction output into a low-boiling fraction and a high-boiling fraction, with the low-boiling fraction containing at least a portion of the aldehyde.
2. The process according to claim 1 , wherein the hydroformylation in a) is conducted at a pressure of 15 to 350 bar absolute.
3. The process according to claim 1 , wherein the homogeneously dissolved catalyst system in a) comprises at least Rh and the phosphorus ligand.
4. The process according to claim 1 , wherein the gas exchange in b) is effected by expanding the first liquid reaction output and subsequently injecting CO or H 2 until a desired partial pressure of CO or H 2 is attained.
5. The process according to claim 4 , wherein the expansion of the first liquid reaction output and the subsequent injection of CO or H 2 are effected in repeated succession.
6. The process according to claim 1 , wherein the hydroformylation is conducted at a temperature of 80° C. to 250° C.
7. The process according to claim 1 , wherein the total gas pressure after the gas exchange is 1 to 70 bar absolute.
8. The process according to claim 1 , wherein the thermal separation is conducted at a temperature of 20° C. to 200° C.
9. The process according to claim 1 , wherein the thermal separation comprises at least one thermal separation process selected from the group consisting of thin-film evaporation, distillation, flash evaporation, falling-film evaporation, and short-path evaporation.
10. The process according to claim 1 , wherein the process is conducted continuously.
11. The process according to claim 1 , wherein the at least one reaction zone comprises at least one reactor.
12. The process according to claim 1 , wherein the thermal separation is preceded or followed by performance of at least one membrane separation.
13. The process according to claim 12 , wherein the thermal separation is followed by injection of CO or H 2 onto a liquid reaction output depleted of the aldehyde until the partial pressure of CO or H 2 accounts for more than 90% of the total gas pressure.
14. The process according to claim 12 , wherein a membrane material in the at least one membrane separation is selected from the group consisting of poly(dimethylsiloxane)s, polyimides, partially fluorinated polymers, completely fluorinated polymers, perfluorinated polymers, amorphous fluoropolymers, amorphous or partially crystalline perfluoroalkoxy polymers, block copolymers of the abovementioned materials, polymers having intrinsic microporosity, poly(aryletherketone)s, polybenzimidazoles, and polyethers.
15. The process according to claim 14 , wherein the membrane material is intrinsically stable in a reaction output or in a mixture present after the thermal separation.
16. The process according to claim 1 , wherein in b), the partial pressure of CO or H 2 accounts for more than 98% of the total gas pressure.
17. The process according to claim 7 , wherein the total gas pressure after the gas exchange is 4 to 20 bar absolute.
18. The process according to claim 8 , wherein the thermal separation is conducted at a temperature between 50° C. and 160° C.
19. The process according to claim 13 , wherein the partial pressure of CO or H 2 accounts for more than 98% of the total gas pressure.
20. The process according to claim 14 , wherein the membrane material is a poly(etheretherketone) or a poly(etherketoneketone).