Cascade refrigeration process for liquefaction of natural gas

• US 6,016,665 A
• Filed: 06/18/1998
• Issued: 01/25/2000
• Est. Priority Date: 06/20/1997
• Status: Expired due to Term

• Current Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Original Assignee: Exxon Production Research Company (Exxon Mobil Corporation)
• Inventors: Bowen, Ronald R., Cole, Eric T.
• Primary Examiner(s): DOERRLER, WILLIAM CHARLES
• Application Number:
  US09/099,590
• Time in Patent Office: 586 Days
• Field of Search: 62/612, 62/613, 62/335, 62/606, 62/619, 62/611, 62/618
• US Class Current: 62/612
• CPC Class Codes: B23K 35/3066 with Ni as next major constituent
• CPC Class Codes: B23K 9/173 and of a consumable electrode
• CPC Class Codes: B60K 15/03006 Gas tanks B60K15/07 takes precedence
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/08 containing nickel C22C38/105 takes precedence
• CPC Class Codes: C22C 38/12 containing tungsten, tantalum, molybdenum, vanadium, or niobium
• CPC Class Codes: C22C 38/14 containing titanium or zirconium
• CPC Class Codes: C22C 38/16 containing copper
• CPC Class Codes: F17C 1/00 Pressure vessels, e.g. gas cylinder, gas tank, replaceable cartridge pressurised apparatus for purposes other than storage, see the relevant subclasses such as A62C, B05B; associated with vehicles, see the appropriate subclass of classes B60 - B64; pressure vessels in general F16J12/00 ; autoclaves B01J3/04; tank vehicles B60P3/22; railway tank wagons for carrying fluent materials B61D5/00; accumulators for supplying fluid under pressure F15B1/04; liquefied gas stoves F24C3/00
• CPC Class Codes: F17C 1/002 Storage in barges or on ships
• CPC Class Codes: F17C 1/14 constructed of aluminium; constructed of non-magnetic steel
• CPC Class Codes: F17C 13/001 Thermal insulation specially adapted for cryogenic vessels vessels not under pressure with insulation F17C3/02; thermal insulation in general F16L59/00
• CPC Class Codes: F17C 2221/033 Methane, e.g. natural gas, CNG, LNG, GNL, GNC, PLNG
• CPC Class Codes: F17C 2223/0153 Liquefied gas, e.g. LPG, GPL
• CPC Class Codes: F17C 2227/0337 by cooling
• CPC Class Codes: F17C 2227/0355 in a closed loop
• CPC Class Codes: F17C 3/00 Vessels not under pressure
• CPC Class Codes: F17C 3/025 Bulk storage in barges or on ships constructive aspects B63B25/16
• CPC Class Codes: F17C 7/00 : Methods or apparatus for discharging liquefied, solidified, or compressed gases from pressure vessels, not covered by another subclass
• CPC Class Codes: F17C 7/02 : Discharging liquefied gases
• CPC Class Codes: F17D 1/082 : for cold fluids, e.g. liquefied gas F17D1/086 takes precedence
• CPC Class Codes: F25J 1/0022 : Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/004 : by flash gas recovery F25J1/0267 takes precedence
• CPC Class Codes: F25J 1/0042 : by liquid expansion with extraction of work
• CPC Class Codes: F25J 1/0205 : as a dual level SCR refrigeration cascade
• CPC Class Codes: F25J 1/0231 : for the working-up of the hydrocarbon feed, e.g. reinjection of heavier hydrocarbons into the liquefied gas
• CPC Class Codes: F25J 1/025 : Details related to the refrigerant production or treatment, e.g. make-up supply from feed gas itself
• CPC Class Codes: F25J 1/0254 : controlling particular process parameter, e.g. pressure, temperature
• CPC Class Codes: F25J 2205/04 : in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2205/50 : using absorption, i.e. with selective solvents or lean oil, heavier CnHm and including generally a regeneration step for the solvent or lean oil
• CPC Class Codes: F25J 2220/64 : Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
• CPC Class Codes: F25J 2230/08 : Cold compressor, i.e. suction of the gas at cryogenic temperature and generally without afterstage-cooler
• CPC Class Codes: F25J 2230/30 : Compression of the feed stream
• CPC Class Codes: F25J 2230/60 : the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2245/90 : the recycled stream being boil-off gas from storage
• CPC Class Codes: F25J 2290/62 : Details of storing a fluid in a tank

Abstract

This invention relates to a process for liquefying a pressurized gas stream rich in methane in which the liquefication of the gas stream occurs in a heat exchanger being cooled by a cascade refrigeration system to produce a methane-rich liquid product having a temperature above about -112° C. (-170° F.). In this process, a pressurized gas stream is introduced into heat exchange contact with a first refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the gas stream is reduced by heat exchange with a first portion of a first refrigerant to produce a cooled gas stream. The cooled gas stream is then introduced into heat exchange contact with a second refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the cooled gas stream is further reduced by heat exchange with a second refrigerant to produce a liquefied methane-rich stream having a temperature above about -112° C. (-170° F.) and a pressure sufficient for the liquefied stream to be at or below its bubble point.

Claims

1. A process for liquefying a pressurized methane-rich gas stream comprising the steps of:

(a) introducing the gas stream into heat exchange contact with a first refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the gas stream is reduced by heat exchange with a first portion of a first refrigerant to produce a cooled gas stream;

(b) introducing the cooled gas stream into heat exchange contact with a second refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the cooled gas stream is further reduced by heat exchange with a second refrigerant to produce a liquefied methane-rich stream having a temperature above about -112°

C. (-170°

F.), said second refrigerant having a boiling point lower than the boiling point of the first refrigerant and the second refrigerant being partially cooled and condensed by heat exchange with a second portion of the first refrigerant; and

(c) introducing the liquefied stream to a storage means at a temperature above about -112°

C. (-170°

F.) and a pressure sufficient for the liquefied stream to be at or below its bubble point.

2. The process of claim 1 further comprises passing to said process boil-off gas resulting from evaporation of liquefied natural gas having a temperature above about -112°

C. (-170°

F.), the boil-off gas being at least partially liquefied by the liquefaction process.

3. The process of claim 2 further comprises separating the boil-off gas into a first stream and a second stream, compressing the first stream and passing the compressed first stream to the liquefaction process prior to at least the last cooling stage of the second refrigeration cycle, said second stream being passed to a heat exchanger to warm the second boil-off stream and to cool the natural gas stream, and using the warmed second boil-off stream as fuel.

4. The process of claim 3 comprises introducing the first stream of the boil-off gas to the gas stream before the last stage of the second refrigeration cycle.

5. The process of claim 2 further comprises separating the boil-off gas into a first stream and a second stream, compressing the first stream and passing the compressed first stream to a heat exchanger, passing the second stream through the heat exchanger to heat the second stream and to cool the compressed first stream, and introducing the cooled compressed first stream to the natural gas stream before at least the last stage of the second refrigeration cycle.

6. The process of claim 1 wherein the gas stream contains methane and hydrocarbon components heavier than methane, which further comprises removing a predominant portion of the heavier hydrocarbons to produce a vapor stream rich in methane and a liquid stream rich in the heavier hydrocarbons, the vapor stream then being liquefied by the process of claim 1.

7. The process of claim 6 wherein the liquid stream rich in the heavier hydrocarbons is further fractionated producing vapor rich in ethane which is combined with the methane-rich stream of claim 1.

8. The process of claim 1 wherein the liquefaction of the gas stream is performed using only two closed refrigeration cycles in cascade arrangement.

9. The process of claim 1 wherein the gas stream contains methane and hydrocarbon components heavier than methane, which further comprising after step (a) the additional steps of removing a predominant portion of the heavier hydrocarbons to produce a gas stream substantially free of hydrocarbons having three or more carbon atoms, compressing the vapor stream, again cooling the gas stream in at least one refrigeration stage with a third portion of the refrigerant of the first refrigerant cycle, and thereafter continuing with step (b) of claim 1.

10. The process of claim 1 wherein the pressurized methane-rich gas stream has a pressure above 3,103 kPa (450 psia).

11. A process for liquefying a natural gas stream which comprises the steps of:

(a) cooling the natural gas stream by one or more heat exchangers by means of a first refrigeration cycle of cascade refrigeration system having two cycles;

(b) passing the cooled natural gas to a phase separator to produce a first vapor stream and a liquid stream;

(c) passing the liquid natural gas stream to a demethanizer to produce a second vapor stream and a second liquid stream;

(d) passing the second liquid stream to a fractionation plant to produce condensate product, make-up refrigerant and a third vapor stream;

(e) combining the first vapor stream, the second vapor stream, and the third vapor stream and passing the combined vapor stream to one or more heat exchangers cooled by a second refrigeration cycle of the cascade refrigeration system to at least partially liquefy the combined vapor stream; and

(f) passing the combined vapor stream of step (e) to an expansion means to produce liquefied natural gas having a temperature above about -112°

C. (-170°

F.) and a pressure sufficient for the liquid product to be at or below its bubble point.

12. A process for liquefying a pressurized methane-rich gas stream comprising the steps of:

(a) introducing the gas stream into heat exchange contact with a first refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the gas stream is reduced by heat exchange with a first portion of a first refrigerant to produce a cooled gas stream;

(b) introducing the cooled gas stream into heat exchange contact with a second refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the cooled gas stream is further reduced by heat exchange with a second refrigerant, thereby condensing at least a portion of the gas stream to produce a liquefied methane-rich stream having a temperature above -112°

C., said second refrigerant having a boiling point lower than the boiling point of the first refrigerant and the second refrigerant being partially cooled and condensed by heat exchange with a second portion of the first refrigerant; and

(c) introducing the liquefied stream to a transportation means at a temperature above about -112°

C. (-170°

F.) and a pressure sufficient for such liquefied stream to be at or below its bubble point.

13. The process of claim 12 comprising an additional step of expanding the liquefied portion of the gas stream produced by step (b) to lower the temperature and pressure of said liquefied portion.

14. A process for liquefying a pressurized methane-rich gas stream comprising the steps of:

(a) introducing the gas stream into heat exchange contact with a first refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the gas stream is reduced by heat exchange with a first portion of a first refrigerant to produce a cooled gas stream;

(b) introducing the cooled gas stream into heat exchange contact with a second refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the cooled gas stream is further reduced by heat exchange with a second refrigerant, said second refrigerant having a boiling point lower than the boiling point of the first refrigerant and the second refrigerant being partially cooled and condensed by heat exchange with a second portion of the first refrigerant;

(c) expanding the gas stream cooled by step (b) to produce a liquid stream at a temperature above about -112°

C. (-170°

F.) and a pressure sufficient for the liquefied stream to be at or below its bubble point; and

(d) introducing the liquefied stream to a storage or transportation means at a temperature above about -112°

C. (-170°

F.).

15. The process of claim 14 further comprises passing to said process boil-off gas resulting from evaporation of liquefied natural gas having a temperature above about -112°

C. (-170°

F.), the boil-off gas being at least partially liquefied by the liquefaction process.

16. The process of claim 15 further comprises separating the boil-off gas into a first stream and a second stream, compressing the first stream and passing the compressed first stream to the liquefaction process prior to at least the last cooling stage of the second refrigeration cycle, said second stream being passed to a heat exchanger to warm the second boil-off stream and to cool the natural gas stream, and using the warmed second boil-off stream as fuel.

17. The process of claim 16 further comprising introducing the first stream of the boil-off gas to the gas stream before the last stage of the second refrigeration cycle.

18. The process of claim 16 further comprises separating the boil-off gas into a first stream and a second stream, compressing the first stream and passing the compressed first stream to a heat exchanger, passing the second stream through the heat exchanger to heat the second stream and to cool the compressed first stream, and introducing the cooled compressed first stream to the natural gas stream before at least the last stage of the second refrigeration cycle.

19. The process of claim 14 wherein the gas stream contains methane and hydrocarbon components heavier than methane, which further comprises removing a predominant portion of the heavier hydrocarbons to produce a vapor stream rich in methane and a liquid stream rich in the heavier hydrocarbons, the vapor stream then being liquefied by the process of claim 14.

20. The process of claim 19 wherein the liquid stream rich in the heavier hydrocarbons is further fractionated producing vapor rich in ethane which is combined with the methane-rich stream of claim 14.

21. The process of claim 14 wherein the liquefaction of the gas stream is performed using only two closed refrigeration cycles in cascade arrangement.

22. The process of claim 14 wherein the gas stream contains methane and hydrocarbon components heavier than methane, which further comprising after step (a) the additional steps of removing a predominant portion of the heavier hydrocarbons to produce a gas stream substantially free of hydrocarbons having three or more carbon atoms, compressing the vapor stream, again cooling the gas stream in at least one refrigeration stage with a third portion of the refrigerant of the first refrigerant cycle, and thereafter continuing with step (b) of claim 14.

23. The process of claim 14 wherein the pressurized methane-rich gas stream has a pressure above 3,103 kPa (450 psia).