Enhanced NGL recovery processes

• US 5,992,175 A
• Filed: 12/08/1997
• Issued: 11/30/1999
• Est. Priority Date: 12/08/1997
• Status: Expired due to Term

• Current Assignee: IPSI, LLC (Bechtel Group Incorporated)
• Original Assignee: IPSI, LLC (Bechtel Group Incorporated)
• Inventors: Yao, Jame, Elliot, Douglas G., Chen, Jong Juh
• Primary Examiner(s): DOERRLER, WILLIAM CHARLES
• Application Number:
  US08/987,183
• Time in Patent Office: 722 Days
• Field of Search: 62/620, 62/621, 62/628, 62/618
• US Class Current: 62/621
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/40 Features relating to the provision of boil-up in the bottom of a column
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2210/12 Refinery or petrochemical off-gas
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2245/02 Recycle of a stream in general, e.g. a by-pass stream
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/60 Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons
• CPC Class Codes: F25J 2270/88 Quasi-closed internal refrigeration or heat pump cycle, if not otherwise provided
• CPC Class Codes: F25J 2280/02 Control in general, load changes, different modes ("runs"), measurements
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0219 Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more
• CPC Class Codes: F25J 3/0242 separation of CnHm with 3 carbon atoms or more

Abstract

The present invention is directed to methods for improving the efficiency and economy of processes for the recovery of natural gas liquids (NGL) from a gas feed, e.g., raw natural gas or a refinery or petrochemical plant gas stream. These methods may be employed with most, if not all, conventional separation methods using a distillation tower, e.g., a demethanizer or deethanizer column. In the methods of the present invention, a portion of a hydrocarbon liquid condensed on a chimney tray disposed below the lowest feed tray of the column is withdrawn from the tower. This withdrawn liquid hydrocarbon is expanded and heated to produce a two-phase system for separation into a heavy, liquid hydrocarbon product and a vapor phase for recycle to the column, preferably as a stripping gas. The withdrawn hydrocarbon liquid is preferably heated by indirect heat exchange with the inlet gas, thus reducing or eliminating the external refrigeration requirements of the process. The expanded, heated vapor recycled to the column increases the ethane and propane concentration in the column, thus reducing the tray temperature profile and increasing the separation efficiency. Accordingly, the column may be operated at lower temperatures and higher pressures, resulting in significant energy savings and economies of operation.

Claims

1. A process for the separation of C₂₊ or C₃₊ hydrocarbons from a hydrocarbon-containing gas feed under pressure, comprising:

introducing a cooled gas feed condensate into a distillation column at one or more feed trays, said column having a plurality of liquid recovery trays;

condensing hydrocarbon liquids in said recovery trays;

withdrawing a condensed, hydrocarbon liquid from one or more recovery trays disposed below the lowest feed tray in said column;

reducing the pressure of at least a portion of said withdrawn liquid to preferentially vaporize some of said withdrawn liquid and produce a two phase stream;

separating said two phase stream into a vapor stream for use as an enhancement vapor and a liquid stream;

increasing the pressure of said enhancement vapor; and

reintroducing said pressurized enhancement vapor back into said distillation column.

2. The process of claim 1 further comprising passing said gas feed and said portion of said withdrawn liquid through a heat exchanger to reduce the temperature of said feed and increase the temperature of said portion of withdrawn liquid.

3. The process of claim 1 comprising reducing the temperature of said enhancement vapor before reintroducing said enhancement vapor into said column.

4. The process of claim 1 comprising increasing the temperature of said enhancement vapor before reintroducing said enhancement vapor into said column.

5. The process of claim 1 wherein said enhancement vapor is reintroduced into said column below the lowest tray of said column.

6. The process of claim 5 wherein said enhancement vapor increases the traffic of ethane and propane inside said column.

7. The process of claim 6 wherein said increased traffic of ethane and propane in said column reduces the tray temperature profile and enhances the separation efficiency in said column.

8. The process of claim 1 wherein said liquid is withdrawn from the lowest tray in said column.

9. The process of claim 1 wherein said liquid is withdrawn from the bottom of said column.

10. The process of claim 1 wherein said enhancement vapor is reintroduced into said column below the tray from which said liquid was withdrawn.

11. The process of claim 1 wherein said column is operated with a bottom temperature of about 0°

F. to about 350°

F. and a top temperature of about -160°

F. to about 0°

F.

12. The process of claim 1 wherein said column is operated at a pressure of about 150 psia to about 700 psia.

13. The process of claim 1 wherein said vapor stream mainly comprises a mixture of ethane and propane.

14. A process for the separation of C₂₊ or C₃₊ hydrocarbons from a hydrocarbon-containing gas feed under pressure, comprising:

introducing a cooled gas feed condensate into a distillation column at one or more feed trays, said column having a plurality of liquid recovery trays;

condensing hydrocarbon liquids in said recovery trays;

withdrawing a condensed, hydrocarbon liquid from one or more recovery trays disposed below the lowest feed tray in said column;

reducing the pressure and increasing the temperature of at least a portion of said withdrawn liquid to produce a two phase stream;

separating said two phase stream into a vapor stream and a liquid stream; and

reintroducing said vapor stream back into said distillation column.

15. The process of claim 14 comprising passing said gas feed and said portion of said withdrawn liquid through a heat exchanger to reduce the temperature of said feed and increase the temperature of said portion of withdrawn liquid.

16. The process of claim 14 comprising increasing the pressure and decreasing the temperature of said vapor stream before reintroducing said vapor stream into said column.

17. The process of claim 14 wherein said vapor stream is reintroduced into said column below the lowest tray to provide a stripping gas for said column.

18. The process of claim 17 wherein said stripping gas increases the traffic of ethane and propane inside said column.

19. The process of claim 18 wherein said increased traffic of ethane and propane in said column reduces the tray temperature profile and enhances the separation efficiency in said column.

20. The process of claim 14 wherein said liquid is withdrawn from the lowest tray in said column.

21. The process of claim 14 wherein said liquid is withdrawn from the bottom of said column.

22. The process of claim 14 wherein said vapor stream is reintroduced into said column below the tray from which said liquid was withdrawn.

23. The process of claim 14 wherein said column is operated with a bottom temperature of about 0°

F. to about 350°

F. and a top temperature of about -160°

F. to 0°

F.

24. The process of claim 14 wherein said column is operated at a pressure of about 150 psia to about 700 psia.

25. The process of claim 14 wherein said vapor stream mainly comprises a mixture of ethane and propane.

26. An apparatus for separating C₂₊ or C₃₊ hydrocarbons from a hydrocarbon-containing gas feed under pressure, comprising:

a distillation column having a plurality of liquid feed and recovery trays;

means for introducing a cooled gas feed condensate into said distillation column at one or more of said feed trays;

means for withdrawing a condensed, hydrocarbon liquid from one or more of said recovery trays disposed below the lowest feed tray in said column;

means for reducing the pressure of at least a portion of said withdrawn liquid to preferentially vaporize some of said withdrawn liquid and produce a two phase stream;

means for separating said two phase stream into a vapor stream for use as an enhancement vapor and a liquid stream;

means for increasing the pressure of said enhancement vapor; and

means for reintroducing said pressurized enhancement vapor back into said distillation column.

27. The apparatus of claim 26 further comprising means for increasing the temperature of said portion of said withdrawn liquid.

28. The apparatus of claim 27 wherein said means for increasing the temperature comprises one or more indirect heat exchangers in which the portion of said withdrawn liquid provides refrigeration to at least a portion of said gas feed.