Carbon dioxide reduction scheme for NGL processes Number:6,823,692 from the United States Patent and Trademark Office (PTO) owispatent An ethane recovery process resulting in a reduction in the amount of carbon dioxide in the recovered NGL stream is provided. To reduce the amount of carbon dioxide, a fractionation tower is provided with a bottom reboiler, and possibly one or more side reboilers higher up in the tower that are integrated with the cooling train. The process disclosed modifies the flow of cold liquid from the separator and tower reboiling to reject some of the more volatile components such as carbon dioxide from the less volatile fraction while maintaining recovery of desirable components such as C2+ hydrocarbon components. The modification includes heating a part of the separator liquid and introducing it at least one stage below the remaining liquid.<H processes, reduction, Carbon, dioxide, , 6823692.html, Patent, pto, 6823692

Title: Carbon dioxide reduction scheme for NGL processes

Abstract: An ethane recovery process resulting in a reduction in the amount of carbon dioxide in the recovered NGL stream is provided. To reduce the amount of carbon dioxide, a fractionation tower is provided with a bottom reboiler, and possibly one or more side reboilers higher up in the tower that are integrated with the cooling train. The process disclosed modifies the flow of cold liquid from the separator and tower reboiling to reject some of the more volatile components such as carbon dioxide from the less volatile fraction while maintaining recovery of desirable components such as C2+ hydrocarbon components. The modification includes heating a part of the separator liquid and introducing it at least one stage below the remaining liquid.

Patent Number: 6,823,692 Issued on 11/30/2004 to Patel, et al.

Inventors: Patel; Sanjiv (Sugar Land, TX); Pan; Justin (Houston, TX); Foglietta; Jorge (Missouri City, TX)
Assignee: ABB Lummus Global Inc. (Houston, TX)

Appl. No.: 364073

Filed: February 11, 2003

Current U.S. Class: 62/620 ; 62/929

Current International Class: F25J 3/02 (20060101)

Field of Search: 62/620,617,929

References Cited [Referenced By]

U.S. Patent Documents


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4185978	January 1980	McGalliard et al.
4278457	July 1981	Campbell et al.
4851020	July 1989	Montgomery, IV
5291736	March 1994	Paradowski
5335504	August 1994	Durr et al.
5555748	September 1996	Campbell et al.
5566554	October 1996	Vijayaraghavan
5615561	April 1997	Houshmand et al.
5771712	June 1998	Campbell et al.
5799507	September 1998	Wilkinson et al.
5953935	September 1999	Sorensen
5960644	October 1999	Nagelvoort et al.
5992175	November 1999	Yao et al.
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6182469	February 2001	Campbell et al.
6425266	July 2002	Roberts

Primary Examiner: Doerrler; Willaim C.
Attorney, Agent or Firm: Bracewell & Patterson LLP

Parent Case Text

RELATED APPLICATIONS

This application claims the benefit of a provisional application having U.S. Ser. No. 60/356,102, filed on Feb. 11, 2002, which hereby is incorporated by reference in its entirety.

Claims

We claim:

1. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons into a volatile gas fraction containing substantially all the methane and a less volatile hydrocarbon fraction containing a large portion of the C2+ components, the process comprising the steps of: supplying and cooling an inlet gas stream having a quantity of CO2 such that at least a portion of the inlet gas stream is condensed to produce a first vapor stream and a first liquid stream; expanding the first vapor stream to a lower pressure, and then supplying a fractionation tower with the vapor stream as a tower feed stream so that the fractionation tower produces a tower bottoms stream containing a less volatile hydrocarbon fraction and a tower overhead stream containing a volatile gas fraction; and the improvement comprising:

splitting the first liquid stream into at least a second liquid stream and a third liquid stream;

supplying the fractionation tower with the second liquid stream as a second upper tower food stream; and

heating the third liquid stream and supplying the fractionation tower with the third liquid stream at a return location at least one theoretical stage below the second upper tower feed stream, the third liquid stream providing stripping vapors to remove CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction.

2. A process for separating as inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons into a volatile gas fraction containing substantially all the methane end a less volatile hydrocarbon fraction containing a large portion of the C2+ components, the process comprising the steps of: supplying and cooling an inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; expanding the first vapor stream to a lower pressure, and then supplying a fractionation tower with the vapor stream as a tower feed stream so that the fractionation tower produces a tower bottoms stream containing a less volatile hydrocarbon fraction and a tower overhead stream containing a volatile gas fraction; and the improvement comprising: splitting the first liquid stream into at least a second liquid stream and a third liquid stream; supplying the fractionation tower with the second liquid stream as a second upper tower feed stream; heating the third liquid stream and supplying the fractionation tower with the third liquid stream at a return location at least one theoretical stage below the second upper tower feed stream, the third liquid stream providing stripping vapors capable of removing CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction; and maintaining feed stream conditions including maintaining an adequate quantity and temperature of the third liquid stream and an amount of reboiling for the functionation tower so that a quantity of carbon dioxide in the tower bottoms stream is substantially reduced.

3. The process of claim 1, wherein the step of supplying and cooling the inlet gas stream includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream.

4. The process of claim 3, wherein the step of cooling at least a portion of the inlet gas stream further includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream and at least one tower reboiler stream, the at least one tower reboiler stream being removed from the fractionation tower at a removal location and being returned at a return location located at essentially a same theoretical stage within the fractionation tower as the removal location.

5. The process of claim 1, wherein the step of expanding the first vapor stream to a lower pressure further includes the steps of: supplying an expander outlet separator with the first vapor stream and the tower overhead stream thereby forming a separator bottoms stream and a separator overhead stream; supplying the fractionation tower with the separator bottoms stream as the tower feed stream; and heating and boosting in pressure the separator overhead stream to form a residue gas stream.

6. The process of claim 5, wherein the step of healing and boosting in pressure the separator overhead stream to form a residue gas stream includes heating the separator overhead stream by heat exchange contact with at least a portion of the inlet gas stream.

7. The process of claim 1, further comprising the steps of: heating and boosting in pressure the tower overhead stream to form a residue gas stream; removing at least a portion of the residue gas stream and cooling at least a portion of the residue gas stream to substantially condense at least a portion of the tower overhead stream; and supplying the fractionation tower with the at least a portion of the substantially condensed tower overhead stream as a top tower feed stream.

8. The process of claim 7, wherein the step of heating and boosting in pressure the tower overhead stream to form a residue gas stream includes heating the tower overhead stream by heat exchange contact with a stream selected from the group consisting of at least a portion of the inlet gas stream, at least a portion of the residue gas stream, and combinations thereof.

9. The process of claim 7, wherein the step of cooling at least a portion of the inlet gas stream further includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream and at least one tower reboiler stream, the at least one tower reboiler stream being removed from the fractionation tower at a removal location and being returned at a return location located at essentially a same theoretical stage within the fractionation tower as the removal location.

10. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling an inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; splitting at least a portion of the first vapor stream into a second vapor stream a third vapor stream; cooling, expanding, and then supplying the fractionation tower with the second vapor stream as a first upper tower feed stream; expanding the third vapor stream and supplying the fractionation tower with the third vapor stream as a second upper tower feed stream so that the fractionation tower produces a tower bottoms stream containing a less volatile hydrocarbon stream and a tower overhead stream containing a more volatile fraction; and an improvement to the process comprising the steps of:

splitting the first liquid stream into at least a second liquid stream and a third liquid stream;

supplying the fractionation tower with the second liquid stream as a middle feed stream; and

heating the third liquid stream and supplying the third liquid stream to the fractionation tower at a location at learnt one theoretical stage below the middle feed stream, the third liquid stream providing stripping vapors to remove CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 as significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction.

11. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling an inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; splitting at least a portion of the first vapor stream into a second vapor stream and a third vapor stream; cooling, expanding, and then supplying the fractionation tower with the second vapor stream as a first upper tower feed stream; expanding the third vapor stream and supplying the fractionation tower with the third vapor stream as a second upper tower feed stream so that the fractionation tower produces a tower bottoms stream containing a less volatile hydrocarbon stream and a tower overhead stream containing a more volatile fraction; and an improvement to the process comprising the step of: splitting the first liquid stream into at least a second liquid stream and a third liquid stream; supplying the fractionation tower with the second liquid stream as a middle feed stream; heating the third liquid stream and supplying the third liquid stream to the fractionation tower at a location at least one theoretical stage below the middle feed stream, the third liquid stream providing stripping vapors capable of removing CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction; and maintaining feed stream conditions including maintaining an adequate quantity and temperature of the third liquid stream and an amount of reboiling for the fractionation tower so that a quantity of carbon dioxide in the tower bottoms stream is substantially reduced.

12. The process of claim 10, further including heating and boosting in pressure the tower overhead stream to form a residue gas stream.

13. The process of claim 10, wherein the step of supplying and cooling the inlet gas stream includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid steam.

14. The process of claim 13, wherein the step of cooling at least a portion of the inlet gas stream further includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream and at least one tower reboiler stream, least one tower reboiler stream being removed from the fractionation tower at a removal location and being returned at a return location located at essentially a same theoretical stage within the fractionation tower as the removal location.

15. The process of claim 10, further including the step of cooling the second vapor stream to partially condense at least a portion of the second vapor stream to form a separator overhead steam and a separator bottoms stream.

16. The process of claim 11, further comprising the steps of: heating and boosting in pressure the tower overhead stream to form a residue gas stream; removing and cooling at least a portion of the residue gas stream to substantially condense the at least a portion of the residue gas stream; and supplying the fractionation tower with the at least a portion of the residue gas stream as a top tower food stream.

17. The process of claim 16, wherein the step of heating and boosting in pressure the tower overhead stream to form a residue gas stream includes heating the tower overhead stream by heat exchange contact with a stream selected from the group consisting of the second vapor stream, at least a portion of the inlet gas stream, and combinations thereof.

18. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling an inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; splitting at least a portion of the first vapor stream into a second vapor stream and a third vapor stream; cooling the second vapor stream to substantially condense the second vapor stream and supplying the second vapor stream to an absorber tower as an absorber top feed stream; expanding the third vapor stream to a lower pressure and then supplying the absorber tower with the third vapor stream as an absorber bottoms feed stream so that the absorber tower produces an absorber overhead stream containing a more volatile fraction of the second and third vapor streams and an absorber bottoms stream containing a less volatile fraction of the second and third vapor streams; supplying a fractionation tower with the absorber bottoms stream as a top tower feed stream so that the fractionation tower produces a tower bottom stream containing a less volatile hydrocarbon fraction of the inlet gas stream and a tower overhead stream containing a more volatile fraction of the inlet gas stream; supplying the absorber tower with the tower overhead stream; and an improvement to the process comprising the steps of:

dividing the first liquid stream into at least a second liquid stream and a third liquid stream;

supplying the fractionation tower with the second liquid stream as a lower tower feed stream; and

healing and then supplying the fractionation tower with the third liquid stream at a location at least one theoretical stage below the lower tower feed stream, the third liquid stream providing stripping vapors to remove CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction.

19. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon strewn containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling an inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; splitting at least a portion of the first vapor stream into a second vapor stream and a third vapor stream; cooling the second vapor stream to substantially condense the second vapor stream and supplying the second vapor stream to an absorber tower as an absorber top feed stream; expanding the third vapor stream to a lower pressure and then supplying the absorber tower with the third vapor stream as an absorber bottoms feed stream so that the absorber tower produces an absorber overhead stream containing a more volatile function of the second and third vapor streams and an absorber bottoms stream containing a less volatile fraction of the second and third vapor streams; supplying a fractionation tower with the absorber bottoms stream as a top tower feed stream so that the fractionation tower produces a tower bottoms stream containing a less volatile hydrocarbon fraction of the inlet gas stream and a tower overhead stream containing a more volatile fraction of the inlet gas stream; supplying the absorber tower with the tower overhead stream; and an improvement to the process comprising the steps of: dividing the first liquid stream into at least a second liquid stream and a third liquid stream; supplying the fractionation tower with the second liquid stream as a lower tower feed stream; heating and that supplying the fractionation tower with the third liquid stream at a location at least one theoretical stage below the lower tower feed stream, the third liquid stream providing stripping vapors capable of removing CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction; and maintaining feed stream conditions including maintaining an adequate quantity and temperature of the third liquid stream and an amount of reboiling for the fractionation tower so that a quantity of carbon dioxide in the tower bottoms stream is substantially reduced.

20. The process of claim 18, wherein the step of supplying the absorber tower with the tower overhead stream includes supplying the absorber tower with the tower overhead stream at a lower absorber feed position.

21. The process of claim 18, wherein the step of supplying the absorber tower with the tower overhead stream includes cooling and at least partially condensing the tower overhead stream and supplying the absorber tower at a second absorber top feed position.

22. The process of claim 18, further including heating and boosting in pressure the absorber overhead stream to form a residue gas stream.

23. The process of claim 22, wherein the step of heating and boosting in pressure the absorber overhead stream to form a residue gas stream includes the step of heating the absorber overhead stream by heat exchange contact with a stream selected from the group consisting of the tower overhead stream, the second vapor stream, at least a portion of the inlet gas stream, and combinations thereof.

24. The process of claim 18, wherein the step of supplying and cooling the inlet gas stream includes the step of cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream and at least one tower reboiler stream, the at least one tower reboiler stream being removed from the fractionation tower at a removal location and being returned at a return location located at essentially a same theoretical stage within the fractionation tower as the removal location.

25. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling the inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; expanding the first vapor stream to a lower pressure, and then supplying an absorber tower with the first vapor stream as an absorber bottom feed stream so that the absorber tower produces an absorber overhead stream and an absorber bottoms stream; supplying the absorber bottoms stream to a fractionation tower as top tower feed stream so that the fractionation tower produces a tower overhead stream and a tower bottoms stream; heating and boosting in pressure the absorber overhead stream to form a residue gas stream; removing and then cooling at least a portion of the residue gas stream so that the at least a portion of the residue gas stream is substantially condensed; supplying the absorber tower with the at least a portion of the residue gas stream as a top absorber feed stream; supplying the absorber tower with the tower overhead stream; and an improvement to the process comprising the steps of:

dividing the first liquid stream into at least a second liquid stream and a third liquid stream;

supplying fractionation tower with the second liquid stream as a first lower tower feed stream; and

heating and then supplying the fractionation tower with the third liquid stream at a location at least one theoretical stage below the first lower tower feed stream, the third liquid stream providing stripping vapors to remove CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction.

26. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling the inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; expanding the first vapor stream to a lower pressure, and then supplying an absorber tower with the first vapor stream as an absorber bottom feed stream so that the absorber tower produces an absorber overhead stream and an absorber bottoms stream; supplying the absorber bottoms stream to a fractionation tower as top tower feed stream so that the fractionation tower produces a tower overhead stream and a tower bottoms stream; heating and boosting in pressure the absorber overhead stream to turn a residue gas stream; removing and then cooling at least a portion of the residue gas stream so that the at least a portion of the residue gas stream is substantially condensed; supplying the absorber tower with the at least a portion of the residue gas stream as a top absorber feed stream; supplying the absorber tower with the tower overhead stream; and an improvement to the process comprising the steps of: dividing the first liquid stream into at least a second liquid stream and a third liquid stream; supplying fractionation tower with the second liquid stream as a first lower tower feed stream; heating and than supplying the fractionation tower with the third liquid stream at a location at least one theoretical stage below the first lower tower feed stream, the third liquid stream providing stripping vapor capable of removing CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction; and maintaining feed stream conditions including maintaining an adequate quantity and temperature of the third liquid stream and an amount of reboiling for the fractionation tower so that a quantity of carbon dioxide in the tower bottoms stream is substantially reduced.

27. The process of claim 25, wherein the step of supplying the absorber tower with the tower overhead stream includes supplying the absorber tower with the tower overhead stream at a lower absorber feed position.

28. The process of claim 25, wherein the step of supplying the absorber tower with the tower overhead stream includes cooling and at least partially condensing the tower overhead stream and supplying the absorber tower at an upper absorber feed position.

29. The process of claim 25, wherein the step of supplying the cooling the inlet gas stream includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream and at least one tower reboiler stream, the at least one tower reboiler stream being removed from the fractionation tower at a removal location and being returned at a return location located at essentially a same theoretical stage within the fractionation tower as the removal location.

30. The process of claim 25, wherein the step of heating and boosting in pressure the absorber overhead stream to form a residue gas stream includes heating the absorber overhead stream by heat exchange contact with a stream selected from the group consisting of the tower overhead stream, the at least a portion of the residue gas stream, the at least a portion of the inlet gas stream, and combinations thereof.

31. The process of claim 25, wherein the step of supplying an absorber tower with the first vapor stream includes supplying an absorber tower having at least one mass transfer zone contained therein with the first vapor stream.

32. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling the inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; dividing the first vapor stream into a second vapor stream and a third vapor stream; cooling and at least partially condensing the second vapor stream thereby forming a flash separator bottoms stream and a flash separator overhead stream and expanding the third vapor stream; cooling the flash separator overhead stream and supplying an absorber tower with the flash separator overhead stream as a first upper absorber feed stream, the flash separator bottoms stream a first lower absorber feed stream, and the third vapor stream as a second lower absorber feed stream to thereby produce an absorber overhead stream and an absorber bottoms stream; supplying a fractionation tower with the absorber bottoms stream as an upper tower feed stream to thereby produce a tower overhead stream and a tower bottoms stream; heating and boosting in pressure the absorber overhead stream to form a residue gas stream; removing and cooling at least a portion of the residue gas stream so that the at least a portion of the residue gas stream is substantially condensed; supplying the absorber tower with the tower overhead stream and the at least a portion of the residue gas stream as a second upper absorber feed stream; and an improvement to the process comprising the steps of:

dividing the first liquid stream into at last a second liquid stream and a third liquid stream;

supplying the fractionation tower with the second liquid stream; and

heating and supplying the fractionation tower with the third liquid stream at a location at least one theoretical stage below the second liquid stream, the third liquid stream providing stripping vapors to remove CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction.

33. A process for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the process comprising steps of: supplying and cooling the inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; dividing the first vapor stream into a second vapor stream and a third vapor stream; cooling and at least partially condensing the second vapor stream thereby forming a flash separator bottoms stream and a flash separator overhead stream and expanding the third vapor stream; cooling the flash separator overhead stream and supplying an absorber tower with the flash separator overhead stream as a first upper absorber feed stream, the flash separator bottoms stream as a first lower absorber feed stream, and the third vapor stream as a second lower absorber feed stream to thereby produce an absorber overhead stream and an absorber bottoms stream; supplying a fractionation tower with the absorber bottoms stream as an upper tower feed stream to thereby produce a tower overhead stream and a tower bottoms stream; heating and boosting in pressure the absorber overhead stream to form a residue gas stream; removing and cooling at least a portion of the residue gas stream so that the at least a portion of the residue gas stream is substantially condensed; supplying the absorber tower with the tower overhead stream and the at least a portion of the residue gas stream as a second upper absorber feed stream; and an improvement to the process comprising the steps of; dividing the first liquid stream into at least a second liquid stream and a third liquid stream; supplying the fractionation tower with the second liquid stream; heating and supplying the fractionation tower with the third liquid stream at a location at least one theoretical stage below the second liquid stream, the third liquid stream providing stripping vapors capable of removing CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction; and maintaining feed stream conditions including maintaining an adequate quantity and temperature of the third liquid stream and an amount of rebelling for the fractionation tower so that a quantity of carbon dioxide in the tower bottoms stream is substantially reduced.

34. The process of claim 32, wherein the step of supplying the absorber tower with the tower overhead stream includes supplying the absorber tower with the tower overhead stream at a tower absorber feed position.

35. The process of claim 32, wherein the step of supplying the absorber tower with the tower overhead stream includes cooling and at least partially condensing the tower overhead stream and supplying the absorber tower at an upper absorber feed position.

36. The process of claim 32, wherein the step of supplying and cooling the inlet gas stream includes cooling at least a portion of the inlet gas stream by heat exchange contact with the third liquid stream and at least one tower reboiler stream, the at least one tower reboiler stream being removed from the fractionation tower at a removal location and being returned at a return location located at essentially a same theoretical stage within the fractionation tower as the removal location.

37. The process of claim 32, wherein the step of heating and boosting in pressure the absorber overhead stream to form a residue gas stream includes heating the absorber overhead stream by heat exchange contact with a stream selected from the group consisting of the tower overhead stream, the at least a portion of the residue gas stream, the at least a portion of the inlet gas stream, the second vapor stream, the flash separator overhead stream, and combinations thereof.

38. The process of claim 32, wherein the step of supplying absorber tower with the flash separator overhead stream includes supplying an absorber tower having at least one mass transfer zone contained therein with the flash separator overhead stream.

39. An apparatus for separating an inlet gas stream containing methane, C2 components, C3 components and heavier hydrocarbons, into a residue gas stream containing substantially all the methane and more volatile components and a less volatile hydrocarbon stream containing C2 components, C3 and heavier components, the apparatus comprising: an inlet heat exchanger for cooling an inlet gas stream having a quantity of CO2 to partially condense at least a portion of the inlet gas stream to produce a first vapor stream and a first liquid stream; an expander for expending the first vapor stream to a lower pressure; a fractionation tower for receiving a tower feed stream and producing a tower bottoms stream containing a less volatile hydrocarbon fraction and a tower overhead stream containing a more volatile gas fraction; at least one side reboiler that removes that returns a tower reboiler stream from essentially a same theoretical stage with the fractionation tower, the side reboiler heats the more volatile gas fraction higher in the fractionation tower thereby preventing the more volatile gas fraction from reaching a bottom of the fractionation tower and reducing an amount of the more volatile gas fraction recovered in the tower bottoms stream; and a splitter for splitting the first liquid stream into at least a second liquid stream and a third liquid stream, the second liquid stream being supplied to the fractionation tower as a second upper tower feed stream and the third liquid stream being heated and supplied to the fractionation lower at a return location at least one theoretical stage below the second upper tower feed stream, the third liquid stream providing stripping vapors to remove CO2 from the liquid descending down the fractionation tower such that the quantity of CO2 is significantly reduced in the less volatile hydrocarbon fraction or ethane is significantly increased while the quantity of CO2 is substantially maintained in the less volatile hydrocarbon fraction.

40. The apparatus of claim 39, further including an absorber tower for receiving a first vapor stream and producing an absorber overhead stream and an absorber bottoms stream.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to natural gas liquid (NGL) processes. More particularly, this invention relates to reducing the amount of carbon dioxide (CO.sub.2) recovered with NGL during cryogenic processing.

2. Description of Prior Art

Natural gas and refinery off gas streams generally contain more volatile components such as hydrogen, methane, carbon monoxide, CO.sub.2, nitrogen and heavier hydrocarbon components such as ethane, ethylene, propane, propylene, and other heavier components. The amount of these components present depends on the source of the feed gas. Recovery of ethane and ethylene from natural gas and refinery off gas streams is a common hydrocarbon recovery process. However, along with the ethane and ethylene, a significant amount of the more volatile components, such as CO.sub.2, in the feed stream will also be recovered with NGL in this process. Pipelines generally have a maximum allowable amount of CO.sub.2 that is permissible in NGL. As a result of this, recovered CO.sub.2 may need to be removed with downstream equipment to meet the CO.sub.2 specification limits in the NGL. The additional equipment required to remove the CO.sub.2 adds considerable capital and operating costs to the process.

In order to reduce the amount of CO2 contained in the NGL stream, CO2 needs to be extracted from the NGL stream by treating it with an amine. Once the CO2 is removed, it is typically vented to the atmosphere. The amine system needed to treat the NGL stream will need a significant amount of fuel to regenerate itself, which sends even more CO2 to be vented to the atmosphere. If the NGL recovery plant is producing liquid hydrocarbon that is to be used by a petrochemical plant, the ethane from the NGL is fractionated out and treated for CO2 removal. Again, the treating is done by amines, and leads to significant excess CO2 venting to atmosphere.

As an alternative method of CO2 reduction, the feed gas can be treated to reduce the amount of CO2 in the feedstream, which will in turn reduce the amount that is recovered with the NGL during cryogenic processing. However, pretreating the feed gas stream also adds considerable costs to the overall NGL process.

In many NGL recovery processes, there is little control over the amount of CO.sub.2 that is recovered with the NGL. If higher C2 recovery is desired, NGL will contain more CO2. In order to reduce the amount of CO2 in NGL, the fractionation tower used in the process needs to be reboiled more. The increased reboiler activity in turn will lead to some loss of desirable components, such as ethane and ethylene, or a loss of process efficiency if the same recovery is maintained.

In a typical turbo expander plant, feed gas is treated to remove impurities such as water, mercury, etc. and then sent for hydrocarbon recovery. If the feed gas pressure is not high enough, compression of the feed gas may be utilized. Gas entering the cryogenic section of the plant is first cooled in one or more exchangers to at least partially condense the gas. The two-phase stream is then sent to a cold separator to separate the vapor from the liquid. For an ethane and heavier compound ("C2+") recovery process, the liquid stream is expanded and sent to a fractionation tower, while the vapor stream is expanded with a work expansion device, such as a turbo expander, and sent to the fractionation tower as an upper tower feed stream. A bottom reboiler is provided for the fractionation tower to control the amount of lighter components exiting the bottom of the fractionation tower with desirable C2+ components. One or more side reboilers are added to the fractionation tower to increase efficiency of cross exchange. The overhead of the fractionation tower is the cold residue gas, which essentially contains the lighter components in the feed gas. Residue stream is preheated in the cross exchanger train and then sent for further processing. Further processing could include compression and cooling of the gas to the desired pressure and temperature.

For a high C2 recovery process, a reflux stream is required above the expander outlet feed location in the fractionation tower to recover some of the C2+ components that are leaving the top of the tower. Several sources for a reflux stream can be used. One source can be at least a portion of the warmed and compressed residue gas. A part of this high-pressure residue gas is cooled in the chilling train and substantially condensed. This condensed stream, which is lean in C2+ components, is fed above the expander outlet feed of the fractionation tower. Such a process is able to recover well in excess of 95% of the C2+ components. An alternate source of a reflux stream can be at least a portion of the vapor stream being sent to the expander. This stream is condensed under pressure and sent as a top feed stream to the fractionation tower. Such a process can produce C2+ recovery in the low to middle 90's %. Yet another source for a reflux stream is to take at least a portion of the expander feed gas and partially condense it. This condensed stream is sent at a lower location in the fractionation tower, while the vapor stream that is leaner in C2+ components than the expander feed stream is condensed under pressure and sent as top feed to the fractionation tower. Such a process can produce C2 recovery in the middle 90's %.

Several new processes have been developed in recent years that use multiple reflux streams above the expander outlet feed location in the fractionation tower. These processes generate streams of various C2+ richness levels and use them at different locations in the fractionation tower to increase ethane recovery and efficiency of the process. These multiple reflux processes are capable of C2 recovery well in excess of 95%.

Not only is recovery of NGL an issue, but the removal of other components from either the NGL stream or the residue gas is also important. An example process in which CO2 is removed from the residue gas stream can be found in U.S. Pat. No. 5,960,644 issued to Nagelvoort et al. In Nagelvoort, natural gas is condensed and then separated into a liquid stream and a vapor stream. The vapor stream is sent to a fractionation tower and the liquid stream is also sent to the tower below the vapor stream. A stream taken from the tower, reboiled, and returned to the tower at location below the liquid stream feed location. The tower produces an overhead stream, which is condensed and separated. The resulting liquid stream is refluxed back to the tower at a higher location than the vapor stream feed location. The resulting vapor is condensed and separated again. The resulting liquid stream is refluxed back to the tower as a second reflux stream at a higher location than the first reflux stream. This process removes the CO2 from vapors and refluxes the CO2 back into the column. Ultimately, the tower bottoms liquid stream contains the majority of CO2, which has to be removed with further processing, and the residue gas stream is relatively free of CO2.

Others have developed processes to try to reduce the amount of CO2 contained within the NGL liquids that are recovered from natural gas streams. An example can be found in U.S. Pat. No. 4,185,978 issued to McGalliard et al. In this process, a hydrocarbon feed gas is expanded, separated, and sent to a demethanizer tower. The demethanizer tower produces an overhead stream containing essentially all of the methane and gaseous CO2 and a bottoms stream containing essentially all of the liquid ethane and heavier components, along with non-gaseous CO2 dissolved in the liquid stream. To remove the CO2 from the liquid stream, an external inert sweep gas is injected into the liquid stream as a stripping gas. This stripping gas helps regulate the reboiler temperatures to reduce temperature fluctuations within the tower that can lead to significant swings in the amount of CO2 that is recovered in the NGL liquid streams.

A need exists for a more economical and efficient method of reducing the amount of CO.sub.2 that is recovered in the NGL cryogenic processes. A need also exists for a process to NGL streams with reduced amounts of CO2 in the NGL stream, as opposed to processing the stream further to remove CO2. A further need exists for a method of reducing CO2 in NGL streams without having to add additional chemicals, which increases the operating costs of the process. It is an object and goal to provide a process and apparatus to reduce the amount of CO.sub.2 recovered in the NGL product. It is an additional object and goal to improve ethane recovery in the NGL product when CO.sub.2 recovery is maintained.

SUMMARY OF THE INVENTION

The present invention includes a process and apparatus for reducing the amount of CO.sub.2 that is recovered in a NGL product stream. The invention can also be used to increase the amount of ethane and ethylene recovery in the NGL product stream, while maintaining the same amount of CO.sub.2 in the NGL product stream. In this process, a cold separator is used to separate the feed into a first liquid stream and a first vapor stream. The first liquid stream is then divided into two streams, a second liquid stream and a third liquid stream. The third liquid stream is heated and supplied to a fractionation tower as a stripping gas at a point below the other feed streams. The stripping gas strips the CO.sub.2 from the liquids falling down the tower. The result of this stripping mechanism is reduced CO.sub.2 in the NGL product stream or increased ethane and ethylene recovery with maintained CO.sub.2 recovery levels.

The present invention is applicable for the separation of ethane, ethylene, propane propylene and other C3 components and heavier components from the above mentioned feed gases using cryogenic turbo expander process. The present invention can be modified to use two separate towers, an absorber tower and a fractionation tower. Other variations can be used, such as a split vapor feed stream and using a portion of a residue gas stream as a reflux stream in the fractionation tower.

The apparatus preferably includes an inlet heat exchanger, an expander, a fractionation tower, at least one side reboiler, and a splitter for splitting the first liquid stream to provide a stripping gas for the fractionation tower. An absorber tower can also be used, as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of the invention, as well as others that will become apparent, may be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of the invention's scope as it may admit to other equally effective embodiments.

FIG. 1 is a simplified flow diagram of a cryogenic gas separation utilizing a basic expander scheme without any reflux to a fractionation tower in accordance with prior art processes;

FIG. 2 is a simplified flow diagram illustrating a cryogenic gas separation process utilizing a two tower, multiple reflux scheme in accordance with copending U.S. Provisional Patent Application Ser. No. 60/440,538.

FIG. 3 is a simplified flow diagram of a cryogenic gas separation process utilizing an expander scheme according to an embodiment of the present invention;

FIG. 4 is a simplified flow diagram of a cryogenic gas separation process utilizing an expander with a split vapor stream and a single tower with a single reflux stream scheme according to an embodiment of the present invention;

FIG. 5 is a simplified flow diagram of a cryogenic gas separation process utilizing a single tower with a single reflux stream scheme, with the reflux stream being taken as a portion of a residue gas stream, according to an embodiment of the present invention;

FIG. 6 is a simplified flow diagram of a cryogenic gas separation process utilizing a single tower expander scheme with the tower having multiple reflux streams according to an embodiment of the present invention;

FIG. 7 is a simplified flow diagram of a cryogenic gas separation process that utilizes a dual tower, expander scheme with a split vapor stream and a single reflux stream according to an embodiment of the present invention;

FIG. 8 is a simplified flow diagram of a cryogenic gas separation process that utilizes a residue recycle stream as a single reflux stream and dual tower scheme according to an embodiment of the present invention; and

FIG. 9 is a simplified flow diagram of a cryogenic gas separation process that utilizes multiple reflux stream and dual tower scheme according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Numerous configurations exist for ethane recovery processes in the prior art. FIG. 1 is one such example of a moderate ethane recovery process. This process shown in FIG. 1 does not make use of a reflux stream for the fractionation tower. Feed gas to the plant is processed in the front end of the plant to remove water and other contaminants, such as mercury, that are detrimental to the performance of the cryogenic plant. Clean, dry, and filtered feed gas is split into two streams. The larger of the two streams cross exchanges with cold residue gas, while the smaller stream cross exchanges with cold liquid from the fractionation tower. Additional refrigeration may be used in the form of external mechanical refrigeration if required. The two partially condensed feed streams are mixed and sent to a cold separator for phase separation. Liquid from the cold separator is sent directly to the fractionation tower, while vapor is expanded through a turbo expander by isentropic expansion thereby cooling it. The cooled and partially condensed gas is sent to the expander outlet separator. Liquid from the expander outlet separator is pumped to the fractionation tower as a top tower feed stream. The fractionation tower produces a tower bottoms stream that contains a less volatile fraction of the inlet gas containing ethane, ethylene, propane, propylene and heavier hydrocarbon components. The overhead of the fractionation tower produces the more volatile components such as methane, CO2, etc. The more volatile gas leaving the tower is routed to the expander outlet separator. Gas leaving the expander outlet separator is residue gas that is preheated in the inlet gas exchanger and sent to the booster compressor where its pressure is raised. This gas is then compressed further to a pressure sufficient to inject it into a lean gas pipeline.

FIG. 3 illustrates an embodiment of the present invention that is similar to the prior art process of FIG. 1, but incorporates the present invention within the process. Table 1 lists some of the key parameters from a computer simulation comparing the two processes. The specifics of the process shown in FIG. 3 will be described in greater detail herein.

TABLE 1 SIMULATION RESULTS FOR FIGS. 1 AND 3 FIG. 1 FIG. 3 Plant Feed (MMSCFD) 1062 1062 Feed Temperature (.degree. F.) 68 68 Feed Pressure (psia) 607.7 607.7 Feed Composition (mol %) C1 93.68 93.68 C2 3.247 3.247 C3+ 1.069 1.069 CO2 1 1 N2 1.004 1.004 NGL Product (BPD) 25940 25310 NGL Composition (mol %) C1 1.22 1.29 C2 61.04 64.46 C3+ 26.84 28.25 CO2 10.9 6 C2 Recovery (%) 73.73 74.52 Ton/day 414.29 418.76 CO2 Recovery (%) 42.8 22.53 Ton/day 108.32 57.01 C3 Recovery (%) 97.71 98.78 Ton/day 185.82 187.85 C4+ Recovery Ton/day 112.42 112.51 Residue Compression, hp 40924 42573 Cold Separator Temp (.degree. F.) -110.6 -118.5 Frac. Tower Ovhd Temp (.degree. F.) -146.3 -139.3 Frac. Tower Ovhd Pressure (psia) 321 305

As shown in Table 1, for a slight increase of 4% in the residue compression requirements, there is a 47.4% drop in the amount of CO2 that is recovered in the NGL stream. In addition, there is a slight increase in C2 and C3+ recovery. The increase in residue compression is well within the capabilities of the electric motor driven residue compressors. When the present invention is applied to existing plants, constraints may exist that limit the rejection of CO2. For a new plant, CO2 rejection could be higher. FIG. 3 shows the improvements of the new invention applied to the above process. The modification involves taking a part of the cold separator liquid, flashing it preferably across a valve, and then preheating it, preferably by heat exchange contact with at least a portion of the feed gas stream. The partially vaporized stream would normally be sent towards the bottom of the tower. However, in this case, the new process was being applied to an existing plant, where the flexibility of adding or moving feed locations or changing the diameter of the tower may not be possible. FIG. 3 shows the new routing of the feed streams on an existing facility. The improvement of the present invention used on an existing plant worked well enough to significantly reduce the amount of CO2 in the NGL product.

FIG. 2 illustrates a two-tower, multiple reflux process scheme that can be used in ethane recovery processes, with a recovery rate of about 85% ethane. The process shown in FIG. 2 has a potential recovery rate of about 95%. Such a scheme splits the conventional demethanizer tower into two separate vessels, an absorber tower and a fractionation tower. The advantage of such a scheme is that it maintains efficiency during ethane recovery mode of operation, but can easily by converted to high propane recovery operation while still maintaining efficiency. Use of two towers, along with the use of multiple reflux streams, is more fully described in copending U.S. Provisional Patent Application Ser. No. 60/440,538. In this two-tower process, liquid from the cold separator is expanded, preferably across a control valve, and sent to the fractionation tower, preferably as a middle feed stream. Vapor from the cold separator is split into two streams. The larger of the two streams is sent to a turbo expander where gas pressure is reduced by isentropic expansion. Such an expansion not only lowers the gas pressure, but also extracts work, thereby cooling and partially condensing the gas. This cooled and partially condensed gas is routed to the bottom of the absorber tower. The smaller cold separator vapor stream is partially condensed against cold residue gas and then sent to a flash separator for phase separation. The liquid separated from the flash separator is sent to the bottom of the absorber tower, while the lean gas leaving the flash separator is condensed under pressure expanded across a control valve and sent, preferably as a middle feed stream, to the absorber tower. The absorber tower preferably is a multi feed tower that produces a lean residue gas as an absorber overhead stream and a cold hydrocarbon liquid as an absorber bottoms stream. Liquid leaving the absorber is pumped to the fractionation tower, preferably as a top feed stream. Absorber overhead stream is preheated by cross exchange with warm streams and then boosted in pressure in the expander booster compressor to form a residue gas stream. The medium pressure residue gas is then sent to the residue compressors where its pressure is raised to the pipeline pressure. A part of this high-pressure residue gas is cooled, condensed under pressure and sent as top feed to the absorber.

The fractionation tower is a reboiled tower that produces a NGL product stream that contains C2+ components at the bottom of the fractionation tower. The overhead of the tower is lean gas, which is condensed as much as possible and sent as lower feed for the absorber or, alternatively, directly routed to the bottom of the absorber tower. The fractionation tower is preferably provided with a bottom reboiler and at least one side reboiler. The location and duties of these exchangers are selected to maximize heat integration with hot streams. Table 2 shown below lists simulation results.

TABLE 2 SIMULATION RESULTS FOR FIG. 2 NGL PRODUCT INLET GAS STREAM STREAM Flow 734.9 MMSCFD 73941 SBPD C2 (mol %) 7.633 43.35 (ton/day) 2016.07 1714.1 C3 (mol %) 3.968 26.41 (ton/day) 1537 1531.6 CO2 (mol %) 1.5 3.51 (ton/day) 580.03 203.06 Residue comp (hp) 36911 C3 refrig (mmbtu/hr) 21.6 Plate Fin UA (BtU/F-hr) 2.29E+07 C2 Recovery (%) 85 C3 Recovery (%) 99.65 CO2 Recovery (%) 35

As shown in Table 2, there is significant recovery of CO2 in the NGL product, which will need to be removed by downstream processing. The process of FIG. 2 can be modified in accordance with the present invention, as shown in FIG. 9, to recover less CO2 in the tower bottoms stream, while maintaining ethane and propane recovery in the residue gas stream.

FIG. 3 illustrates one embodiment of the CO.sub.2 reduction scheme for NGL processes 10 in accordance with the present invention. The feed gas stream 12 is first sent through dehydration and inlet processing (not shown). Feed gas stream 12 is then split into two streams, 12a and 12b. Stream 12a is cooled by heat exchange contact with other process streams in a front-end exchanger 14. In all embodiments of the present invention, front-end exchanger 14 can be a single multi-path exchanger, a plurality of individual heat exchangers, or combinations thereof. The cooled feed