Energy-efficient process for removing butenes from C4-hydrocarbon streams and subsequent n/iso separation
A process for removing butenes from C4-hydrocarbon streams containing butanes and butenes involves extractive distillation with a suitable solvent. The process also involves heat integration, which allows utilization of the heat of the solvent for heating and/or at least partly evaporating various streams.
1. A process for separating butenes from a C4-hydrocarbon stream which contains at least butenes, n-butane, and isobutane, by extractive distillation with a solvent, the process comprising:
a. at least partially evaporating a liquid C4-hydrocarbon stream in a feed evaporator, to obtain a gaseous C4-hydrocarbon stream, and supplying the solvent at a location spatially above the gaseous C4-hydrocarbon stream to an absorber in which the gaseous C4-hydrocarbon stream and the solvent are contacted with one another to transfer predominantly butenes from the gaseous C4-hydrocarbon stream to the solvent, to obtain a laden solvent, wherein the laden solvent is collected in a liquid collector of the absorber, passed through an absorber evaporator, and then passed into the bottom of the absorber below the liquid collector to outgas predominantly butanes from the laden solvent, and wherein the laden solvent is subsequently passed to a separate desorber as a bottoms stream and wherein a stream enriched in butanes relative to the C4-hydrocarbon stream is obtained at the top of the absorber;
b. supplying the laden solvent to the desorber, the bottom of which is at an elevated temperature relative to the bottom of the absorber, and in which the butenes are separated from the solvent to obtain at the top of the desorber a stream enriched in butenes, wherein the solvent, at least partially freed of butenes, is collected in a liquid collector of the desorber, passed through a desorber evaporator, and then passed into the bottom of the desorber below the liquid collector to outgas any butenes remaining in the solvent, and wherein the solvent is subsequently recycled to the absorber as a stream;
c. supplying the stream enriched in butanes to an n/iso splitter in which n-butane and isobutane are separated from one another to obtain a stream enriched in isobutane at the top of the n/iso splitter and a stream enriched in n-butane at the bottom of the n/iso splitter;
wherein heat of the solvent withdrawn as a bottoms stream of the desorber is used for heat integration by employing the heat of the solvent in at least one respective heat exchanger for preheating the laden solvent passed to the desorber, for evaporation in the absorber evaporator, and for evaporation of the liquid C4-hydrocarbon stream, and
wherein the stream enriched in butenes is withdrawn at the top of the desorber and subjected to a condensation, wherein heat of condensation is at least partially used for heating the n/iso splitter.
2. The process according to claim 1 , wherein the solvent is N-methyl-2-pyrrolidone (NMP).
3. The process according to claim 1 , wherein the solvent contains water and a water content is between 1% and 10% by weight.
4. The process according to claim 1 , wherein the stream enriched in butenes obtained at the top of the desorber additionally contains water originating from the solvent.
5. The process according to claim 4 , wherein the stream enriched in butenes is withdrawn at the top of the desorber and subjected to a condensation, wherein a water-containing stream and a butene-containing product stream are condensed out and separated from one another.
6. The process according to claim 5 , wherein the butene-containing product stream obtained from the condensation has a butene content of at least 70% by weight, based on a total composition of the butene-containing product stream.
7. The process according to claim 1 , wherein the stream enriched in butanes withdrawn at the top of the absorber is subjected to a hydrogenation before entry into the n/iso splitter to convert any remaining butenes into butanes.
8. The process according to claim 7 , wherein at least a portion of heat of reaction formed during the hydrogenation is used for heating the n/iso splitter.
9. The process according to claim 1 , wherein the stream enriched in isobutane obtained at the top of the n/iso splitter and/or the stream enriched in n-butane obtained at the bottom of the n/iso splitter are/is subjected to a hydrogenation to convert any remaining butanes into butanes.
10. The process according to claim 9 , wherein at least a portion of the heat of reaction formed during the hydrogenation or at least a portion of the heat of reaction formed during two hydrogenations is used for heating the n/iso splitter.
11. The process according to claim 1 , wherein the preheating of the laden solvent passed to the desorber is performed in two stages, wherein a first heat transfer to the solvent is effected in a heat exchanger and a second heat transfer to the solvent is effected in a kettle evaporator.
12. The process according to claim 1 , wherein the heat for evaporation in the desorber evaporator is introduced in a heat exchanger by heat transfer with a suitable heat transfer medium.
13. The process according to claim 12 , wherein the suitable heat transfer medium is heating steam, wherein the heating steam undergoes at least partial condensation in the heat exchanger, thus generating a hat condensate at a pressure of 10 to 20 bar absolute, and a temperature of 150° C. to 200° C., which is passed to a condensate container.
14. The process according to claim 13 , wherein the pressure in the condensate container is lower than in a heating steam side of the heat exchanger, thus causing a portion of the heating condensate to be re-evaporated, as a result of which the combined steam is obtained as low pressure steam.
15. The process according to claim 1 , wherein the heating steam for the desorber evaporator is provided using a steam ejector supplied with high pressure or medium pressure steam and the low pressure steam obtained in the condensate container.
16. The process according to claim 1 , wherein the bottom of the desorber is at a lower pressure relative to the bottom of the absorber.
17. The process according to claim 3 , wherein the water content is between 4% and 9% by weight.
18. The process according to claim 6 , wherein the butene-containing product stream has a butene content of at least 86% by weight, based on the total composition of the butene-containing product stream.
19. The process according to claim 12 , wherein the suitable heat transfer medium is heating steam.
20. The process according to claim 13 , wherein the hot condensate is generated at a pressure of 12 to 17 bar absolute and a temperature of 160° C. to 190° C.