Natural gas liquefaction process

• US 8,616,021 B2
• Filed: 03/04/2008
• Issued: 12/31/2013
• Est. Priority Date: 05/03/2007
• Status: Active Grant

• Current Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Original Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Inventors: Minta, Moses
• Primary Examiner(s): Jules, Frantz
• Assistant Examiner(s): RAYMOND, KEITH MICHAEL
• Application Number:
  US12/527,539
• Publication Number:
  US 20100107684A1
• Time in Patent Office: 2,128 Days
• Field of Search: 626/11, 626/12, 626/13, 626/06, 621/13, 626/14
• US Class Current: 62/613
• 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 1/005 by expansion of a gaseous refrigerant stream with extraction of work
• CPC Class Codes: F25J 1/0072 Nitrogen
• CPC Class Codes: F25J 1/0082 Methane
• CPC Class Codes: F25J 1/0092 Mixtures of hydrocarbons comprising possibly also minor amounts of nitrogen
• CPC Class Codes: F25J 1/0095 Oxides of carbon, e.g. CO2
• CPC Class Codes: F25J 1/0218 with one or more SCR cycles, e.g. with a C3 pre-cooling cycle
• 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 1/0268 using a dedicated refrigeration means F25J1/0296 takes precedence
• CPC Class Codes: F25J 1/0278 Unit being stationary, e.g. on floating barge or fixed platform
• CPC Class Codes: F25J 1/0288 using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
• CPC Class Codes: F25J 1/0292 Refrigerant compression by cold or cryogenic suction of the refrigerant gas
• CPC Class Codes: F25J 1/0297 using an externally chilled fluid, e.g. chilled water
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2220/62 Separating low boiling components, e.g. He, H2, N2, Air
• CPC Class Codes: F25J 2230/04 Compressor cooling arrangement, e.g. inter- or after-stage cooling or condensate removal
• CPC Class Codes: F25J 2230/30 Compression of the feed stream
• CPC Class Codes: F25J 2270/90 : External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration

Abstract

The invention relates to a process for liquefying a gas stream rich in methane, said process comprising: (a) providing said gas stream; (b) withdrawing a portion of said gas stream for use as a refrigerant; (c) compressing said refrigerant; (d) cooling said compressed refrigerant with an ambient temperature cooling fluid; (e) subjecting the cooled, compressed refrigerant to supplemental cooling; (f) expanding the refrigerant of (e) to further cool said refrigerant, thereby producing an expanded, supplementally cooled refrigerant; (g) passing said expanded, supplementally cooled refrigerant to a heat exchange area; and, (h) passing said gas stream of (a) through said heat exchange area to cool at least part of said gas stream by indirect heat exchange with said expanded, supplementally cooled refrigerant, thereby forming a cooled gas stream. In further embodiments for improved efficiencies, additional supplemental cooling may be provided after one or more other compression steps.

Claims

1. A process for liquefying a gas stream rich in methane, said process comprising:

(a) providing said gas stream at a pressure less than 1,000 pounds per square inch absolute (psia);

(b) withdrawing a portion of said gas stream for use as a refrigerant;

(c) compressing said refrigerant to a pressure greater than 3,000 pounds per square inch absolute (psia) to provide a compressed refrigerant;

(d) cooling said compressed refrigerant by indirect heat exchange with an ambient temperature air or water to a process temperature above about 50 degrees Fahrenheit (°

F.) (10°

C.);

(e) subjecting the cooled, compressed refrigerant to supplemental cooling so as to reduce further its temperature thereby producing a supplementally cooled, compressed refrigerant, wherein the supplementally cooled, compressed refrigerant of (e) is from 10°

F. to 70°

F. (6°

C. to 39°

C.) cooler than said process temperature resulting in a supplementally cooled, compressed refrigerant temperature from −

35°

F. to 60°

F. (−

37.2 to 15.6°

C.);

(f) expanding the supplementally cooled, compressed refrigerant of (e) to further cool said refrigerant, thereby producing an expanded, supplementally cooled refrigerant;

(g) passing said expanded, supplementally cooled refrigerant to a heat exchange area; and

(h) passing said gas stream through said heat exchange area to cool at least part of said gas stream by indirect heat exchange with said expanded, supplementally cooled refrigerant, thereby forming a cooled fluid stream,

(i) passing said cooled fluid stream of (h) to a further heat exchange area for further cooling;

(j) withdrawing said cooled fluid stream after cooling in (i) and expanding said fluid stream for even further cooling, thereby producing an expanded, cooled fluid stream;

(k) passing said expanded, cooled fluid stream in (j) to a separator where a cooled liquid portion is withdrawn as liquefied natural gas and a vapor portion is withdrawn as a cooled vapor stream;

(l) passing said cooled vapor stream as a supplemental refrigerant back through the heat exchange areas of (i) and (g),

wherein

a portion of the cooled vapor stream from (k) is withdrawn prior to passing through the heat exchange area of (i) for use as a supplemental refrigerant by providing the portion of the cooled vapor stream to a secondary expansion loop which passes through the heat exchange areas of (i) and (h), is compressed after exiting heat exchange area of (h), subjected to ambient temperature cooling, optionally cooled by passing back through the heat exchange area of (h), then expanded for further cooling and re-introduction into the heat exchange areas of (i) and (g), and

at least a 10% saving in net horsepower or fuel usage is provided by;

(i) high pressure heat exchange of steps (c), (d), and (e), and (ii) utilizing the cooled vapor stream from step (k) as a supplemental refrigerant, when compared to a similar processes that does not utilize (i) and (ii).

2. The process of claim 1 wherein the ambient temperature in (d) is greater than 60°

F. (15.6°

C.).

3. The process of claim 1 wherein the ambient temperature in (d) is greater than 70°

F. (21.1°

C.).

4. The process of claim 1 wherein additional supplemental cooling is applied to the refrigerant prior to the compressing in (c), or at least prior to one stage of compressing where the compressing of (c) comprises more than one compressing stage.

5. The process of claim 1 wherein the portion of the cooled vapor stream is subjected to supplemental cooling after being subjected to ambient temperature cooling but prior to being passed back through the heat exchange area of 1(h).

6. The process of claim 1 wherein the expanded, supplementally cooled refrigerant is

compressed after exiting heat exchange area of 1(h),

subjected to ambient temperature cooling,

optionally cooled by passing back through the heat exchange area of 1(h), then

expanded for further cooling and re-introduction into heat exchange areas 6(a) and 1(g).

7. The process of claim 5, wherein the expanded, supplementally cooled refrigerant consists essentially of nitrogen or a nitrogen-rich gas.

8. The process of claim 1, wherein said gas stream of 1(a) is compressed, cooled by subjecting to one or more ambient temperature cooling units, and then further cooled in a supplemental cooling unit, all before introduction into the heat exchange area of 1(h).

9. The process of claim 1, wherein the supplemental cooling unit is an external refrigeration unit utilizing external refrigerants, wherein the external refrigerants are substantially independent of the portion of said gas stream for use as a refrigerant of 1(b).

10. The process of claim 1, wherein the only external refrigerant utilized is the indirect heat exchange with an ambient temperature air or water in step (d).