NGL recovery from natural gas using a mixed refrigerant

• US 10,139,157 B2
• Filed: 02/22/2012
• Issued: 11/27/2018
• Est. Priority Date: 02/22/2012
• Status: Active Grant

• Current Assignee: Black & Veatch International Company (Black & Veatch Corporation)
• Original Assignee: Black & Veatch International Company (Black & Veatch Corporation)
• Inventors: Currence, Kevin L.
• Primary Examiner(s): Jules, Frantz
• Assistant Examiner(s): Mengesha, Webeshet
• Application Number:
  US13/402,349
• Publication Number:
  US 20130213087A1
• Time in Patent Office: 2,470 Days
• Field of Search: 62612, 62614, 62618, 62620, 62621, 62623
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• 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 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/66 Closed external refrigeration cycle with multi component refrigerant [MCR], e.g. mixture of hydrocarbons
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• 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

Abstract

An NGL recovery facility for separating ethane and heavier (C₂ ) components from a hydrocarbon-containing feed gas stream that utilizes a single, closed-loop mixed refrigerant cycle. The vapor and liquid portions of the feed gas stream are isenthalpically flashed and the resulting expanded streams are introduced into the NGL recovery column. Optionally, a second vapor stream can be flashed and then introduced into the recovery column at the same or lower separation stage than the flashed liquid stream. As a result, the NGL recovery facility can optimize C₂ recovery with compression costs.

Claims

1. A process for recovering natural gas liquids (NGL) from a hydrocarbon-containing feed gas stream, said process comprising:

(a) cooling and at least partially condensing a hydrocarbon-containing feed gas stream to thereby provide a cooled feed gas stream, wherein at least a portion of said cooling is carried out via indirect heat exchange with a mixed refrigerant stream in a first heat exchanger of a closed-loop mixed refrigerant refrigeration cycle;

(b) separating said cooled feed gas stream into a first vapor stream and a first liquid stream in a vapor-liquid separator;

(c) splitting said first vapor stream into a first vapor portion and a second vapor portion;

(d) cooling at least a portion of said first vapor portion in said first heat exchanger to thereby provide a cooled first vapor stream, wherein at least a portion of said cooling is carried out via indirect heat exchange with said mixed refrigerant stream used to perform said cooling of step (a);

(e) flashing said cooled first vapor stream to thereby provide a first flashed stream;

(f) flashing said second vapor portion to thereby provide a second flashed stream;

(g) flashing the entire first liquid stream to provide a two-phase fluid stream having a vapor fraction of at least 0.10;

(h) introducing said first flashed stream, the entire two-phase fluid stream, and said second flashed stream into a distillation column via respective first, second, and third fluid inlets of said distillation column, wherein said first fluid inlet is located at a higher separation stage than said second and said third fluid inlets, and wherein said third fluid inlet is located at the same separation stage or at a lower separation stage than said second fluid inlet; and

(i) recovering an overhead residue gas stream and a liquid bottoms product stream from said distillation column, wherein said recovering step includes operating said distillation column in a C₂ recovery mode or a C₂ rejection mode,

wherein, when said distillation column is operated in said C₂ recovery mode, said liquid bottoms product stream comprises at least 90 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger and said overhead residue gas stream comprises less than 10 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger, and

wherein, when said distillation column is operated in a C₂ rejection mode, said liquid bottoms product stream comprises less than 30 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger and said overhead residue gas stream comprises at least 70 percent of the total moles of C₂ components in said hydrocarbon containing feed gas stream introduced into said first heat exchanger,

wherein prior to step (a), said mixed refrigerant stream is compressed, cooled, and expanded to thereby generate refrigeration, wherein at least a portion of said refrigeration is used to accomplish at least a portion of said cooling of step (a) and at least a portion of said cooling of step (d).

2. The process of claim 1, wherein at least a portion of said cooling of step (d) is carried out via indirect heat exchange with said overhead residue gas stream recovered from said distillation column.

3. The process of claim 1, wherein the temperature of said cooled first vapor stream is reduced by less than 75°

F. during said flashing of step (e).

4. The process of claim 1, wherein said distillation column includes a total number of separation stages and wherein said third fluid inlet is located in the lower two-thirds of the total number of separation stages of said distillation column.

5. The process of claim 1, wherein said mixed refrigerant stream comprises two or more components selected from the group consisting of methane, ethylene, ethane, propylene, propane, isobutane, normal butane, isopentane, and normal pentane.

6. The process of claim 1, wherein said recovering step includes operating said distillation column in said C₂ rejection mode wherein said liquid bottoms product stream comprises less than 10 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger.

13. The process of claim 1, further comprising, warming said overhead residue gas stream recovered from said distillation column in said first heat exchanger via indirect heat exchange with at least one of said feed gas stream, said mixed refrigerant stream, and said first vapor stream.

16. The process of claim 1, wherein said third fluid inlet is located at the same separation stage as said second fluid inlet.

7. A process for recovering natural gas liquids (NGL) from a hydrocarbon-containing feed gas stream, said process comprising:

(a) cooling a hydrocarbon-containing feed gas stream in a first heat exchanger via indirect heat exchange with a mixed refrigerant stream to thereby provide a cooled feed gas stream and a warmed refrigerant stream;

(b) separating said cooled feed gas stream into a first vapor stream and a first liquid stream in a vapor-liquid separator;

(c) splitting said first vapor stream into a first vapor portion and a second vapor portion;

(d) cooling said first vapor portion to thereby provide a cooled vapor portion, wherein at least a portion of said cooling is carried out in said first heat exchanger via indirect heat exchange with said mixed refrigerant stream used to perform said cooling of step (a);

(e) flashing said cooled vapor portion to thereby provide a first flashed stream;

(f) flashing said second vapor portion to thereby provide a second flashed stream;

(g) introducing said first and said second flashed streams into a distillation column at respective first and second fluid inlets;

(h) flashing the entire first liquid stream to thereby provide a third flashed stream and introducing the entire third flashed stream into said distillation column via a third fluid inlet, wherein said third flashed stream is a two-phase stream having a vapor fraction of at least 0.10; and

(i) recovering an NGL-enriched liquid product stream and an overhead residue gas stream from said distillation column, wherein said recovering includes operating said distillation column in a C₂ recovery mode or a C₂ rejection mode,

wherein said second fluid inlet is and said third fluid inlet are located at lower separation stages than said first fluid inlet, wherein said second fluid inlet is located at the same separation stage as or at a lower separation stage than said third fluid inlet,

wherein, when said distillation column is operated in said C₂ recovery mode, said liquid product stream comprises at least 90 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger and said overhead residue gas stream comprises less than 10 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger, and

wherein, when said distillation column is operated in a C₂ rejection mode, said liquid product stream comprises less than 30 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first heat exchanger and said overhead residue gas stream comprises at least 70 percent of the total moles of C₂ components in said hydrocarbon containing feed gas stream introduced into said first heat exchanger.

8. The process of claim 7, wherein said distillation column has a total number of separation stages, and wherein said second fluid inlet is located in the lower two-thirds of the total number of separation stages of said distillation column.

9. The process of claim 7, wherein said mixed refrigerant stream comprises two or more components selected from the group consisting of methane, ethylene, ethane, propylene, propane, isobutane, normal butane, isopentane, and normal pentane.

10. The process of claim 7, wherein said mixed refrigerant stream is compressed, subcooled, and expanded prior to said cooling of step (a), wherein the minimum temperature of said mixed refrigerant stream during each of the compression, subcooling, and expansion steps is not less than 175°

F.

14. The process of claim 7, further comprising recovering an overhead residue gas stream from said distillation column and warming said overhead residue gas stream via indirect heat exchange with at least one other stream in said first heat exchanger.

17. The process of claim 7, wherein said second fluid inlet is located at the same separation stage as said third fluid inlet.

11. A facility for recovering natural gas liquids (NGL) from a hydrocarbon-containing feed gas stream using a single closed-loop mixed refrigeration cycle, said facility comprising:

a primary heat exchanger having a first cooling pass disposed therein, wherein said first cooling pass is operable to cool said hydrocarbon-containing feed gas stream;

a vapor-liquid separator fluidly coupled to said first cooling pass for receiving the cooled feed gas stream, said vapor-liquid separator comprising a first vapor outlet for discharging a first vapor stream, and a first liquid outlet for discharging a first liquid stream;

a vapor splitter defining a single vapor inlet and two vapor outlets for splitting said first vapor stream into a first vapor portion and a second vapor portion, wherein said single vapor inlet is fluidly coupled to said vapor outlet of said vapor-liquid separator;

a second cooling pass disposed within said primary heat exchanger and fluidly coupled to one of said vapor outlets of said vapor splitter, wherein said second cooling pass is configured for cooling at least a portion of said first vapor portion;

a first expansion device fluidly coupled to said second cooling pass for flashing at least a portion of the cooled first vapor portion to provide a first flashed stream;

a second expansion device fluidly coupled to said first liquid outlet of said vapor-liquid separator for flashing the entire first liquid stream to provide a second flashed stream comprising two phases and having a vapor fraction of at least 0.10;

a third expansion device for flashing the second vapor portion to provide a third flashed stream, wherein the other of said vapor outlets of said splitter is fluidly coupled to said third expansion device;

a distillation column comprising a first fluid inlet for receiving said first flashed stream from said first expansion device, a second fluid inlet for receiving the entire second flashed stream from said second expansion device, a third fluid inlet for receiving said third flashed stream from said third expansion device, an upper vapor outlet for discharging an overhead residue gas stream, and a lower liquid outlet for discharging a liquid product stream, wherein said distillation column is configured to operate in a C₂ recovery mode or a C₂ rejection mode, wherein said first fluid inlet of said distillation column is positioned at a higher separation stage than said second fluid inlet and said third fluid inlet of said distillation column, wherein and said third fluid inlet is positioned at the same separation stage or at a lower separation stage than said second fluid inlet; and

a single closed-loop mixed refrigeration cycle, said cycle comprising—

a refrigerant compressor defining a suction inlet for receiving a mixed refrigerant stream and a discharge outlet for discharging a stream of compressed mixed refrigerant;

a first refrigerant cooling pass disposed within said primary heat exchanger, wherein said first refrigerant cooling pass is fluidly coupled to said discharge outlet of said refrigerant compressor and is configured to subcool the compressed mixed refrigerant stream;

a refrigerant expansion device fluidly coupled to said first refrigerant cooling pass for expanding the subcooled mixed refrigerant stream and generating refrigeration; and

a first refrigerant warming pass disposed within said primary heat exchanger, wherein said first refrigerant warming pass fluidly coupled to said refrigerant expansion device, wherein said first refrigerant warming pass is configured to cool the feed gas stream in said first cooling pass and the vapor stream in said second cooling pass, wherein said first refrigerant warming pass is fluidly coupled to said suction inlet of said refrigerant compressor,

wherein, when said distillation column is operated in said C₂ recovery mode, said liquid product stream comprises at least 90 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first cooling pass of said primary heat exchanger and said overhead residue gas stream comprises less than 10 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first cooling pass of said primary heat exchanger, and

wherein, when said distillation column is operated in a C₂ rejection mode, said liquid product stream comprises less than 30 percent of the total moles of C₂ components in said hydrocarbon-containing feed gas stream introduced into said first cooling pass of said primary heat exchanger and said overhead residue gas stream comprises at least 70 percent of the total moles of C₂ components in said hydrocarbon containing feed gas stream introduced into said first cooling pass of said primary heat exchanger.

12. The facility of claim 11, wherein said distillation column has a total number of separation stages, wherein said first fluid inlet is located in the upper one-third of the total number of separation stages of said distillation column and said second and said third fluid inlets are located in the lower two-thirds of the total number of separation stages of said distillation column.

15. The facility of claim 11, wherein said distillation column further comprises an upper vapor outlet for discharging an overhead gas stream;

and further comprising a residue gas warming pass disposed within said primary heat exchanger for heating the overhead residue gas stream withdrawn from said distillation column, wherein said residue gas warming pass is coupled in fluid flow communication with said upper vapor outlet of said distillation column.

18. The facility of claim 11, wherein said third fluid inlet is positioned at the same separation stage as said second fluid inlet.