Natural Gas Liquefaction Process

Abstract

The described invention relates to processes and systems for treating a gas stream, particularly one rich in methane for forming liquefied natural gas (LNG), said process including: (a) providing a gas stream; (b) providing a refrigerant; (c) compressing said refrigerant to provide a compressed refrigerant; (d) cooling said compressed refrigerant by indirect heat exchange with a cooling fluid; (e) expanding the refrigerant of (d) to cool said refrigerant, thereby producing an expanded, cooled refrigerant; (f) passing said expanded, cooled refrigerant to a first heat exchange area; (g) compressing the gas stream of (a) to a pressure of from greater than or equal to 1,000 psia to less than or equal to 4,500 psia; (h) cooling said compressed gas stream by indirect heat exchange with an external cooling fluid; and heat exchanging the compressed gas stream with the expanded, cooled refrigerant stream.

We claim: 
     
         1 . A process for liquefying a gas stream, said process comprising:
 (a) providing said gas stream at a pressure of from 600-1,000 psia (4137-6895 kPa) as a feed gas stream;   (b) providing a refrigerant at a pressure of less than 1,000 psia (6895 kPa);   (c) compressing said refrigerant to a pressure greater than or equal to 1500 to 5000 psia (10352 to 34474 kPa) to produce a compressed refrigerant;   (d) cooling said compressed refrigerant by indirect heat exchange with a cooling fluid;   (e) expanding the compressed refrigerant of (d) to cool said compressed refrigerant, to produce an expanded, cooled refrigerant at a pressure of from greater than or equal to 100 psia (689 kPa) to less than or equal to 1,000 psia (6895 kPa);   (f) passing said expanded, cooled refrigerant to a first heat exchange area;   (g) compressing the feed gas stream of (a) to a pressure of from greater than or equal to 1,200 psia (8,274 kPa) to less than or equal to 4,500 psia (31026 kPa) to produce a compressed feed gas stream;   (h) cooling said compressed feed gas stream by indirect heat exchange with an external cooling fluid; and,   (i) passing said compressed feed gas stream through the first heat exchange area to cool at least a part thereof by indirect heat exchange, to produce a compressed, further cooled feed gas stream.   
     
     
         2 . The process of  claim 1  wherein the feed gas stream of (a) is compressed in step (g) to 1,500 to 3,500 psia (10342 to 24132 kPa). 
     
     
         3 . The process of  claim 1  wherein the feed gas stream of (a) is compressed in step (g) to 2,500 to 3,500 psia (17237 to 24132 kPa). 
     
     
         4 . The process of  claim 3 , further comprising (j) passing the compressed, further cooled feed gas stream of (i) through a second heat exchange area for extra cooling. 
     
     
         5 . The process of  claim 4 , further comprising (k) expanding said compressed, further cooled feed gas stream of (j) to reduce the pressure of said stream to a pressure of from greater than or equal to 50 psia (345 kPa) to less than or equal to 450 psia (3103 kPa) to produce an expanded, cooled gas stream. 
     
     
         6 . The process of  claim 5 , further comprising withdrawing a portion not to exceed 50% of said expanded, cooled gas stream of (k) and reducing its pressure in a reduction valve to a range of about 30-200 psia (207-1379 kPa) to produce a reduced pressure gas stream and passing the reduced pressure gas stream through the second heat exchange area of (j) as a cooling gas stream; 
     
     
         7 . The process of  claim 6 , further comprising passing the cooling gas stream through the first heat exchange area to assist cooling of said compressed feed gas stream. 
     
     
         8 . The process of  claim 7 , further comprising subsequently compressing and cooling the cooling gas stream by indirect heat exchange with an external cooling unit, one or more times, and adding the cooling gas stream to the feed gas stream of 1(a) prior to the compressing of said feed gas stream in 1(g). 
     
     
         9 . The process of  claim 5 , further comprising expanding at least a portion of said expanded, compressed gas stream; and
 passing the expanded, compressed gas stream to a separation tank from which liquid natural gas is withdrawn and remaining gaseous vapors are withdrawn as flash gas.   
     
     
         10 . The process of  claim 9  wherein said first heat exchange area and said second, heat exchange area are provided with a sub-cooling expander loop cooling stream comprising nitrogen gas, nitrogen-containing gas, or said flash gas from the final separation of the liquefied feed gas stream. 
     
     
         11 . The process of  claim 10  wherein said sub-cooling expander loop cooling stream flows in a closed loop comprising compressing said sub-cooling expander loop cooling stream after passing through said first heat exchange area and said second heat exchange area, cooling with at least one external refrigerant cooling unit, and expanding said sub-cooling expander loop cooling stream prior to providing to the first and second heat exchange areas. 
     
     
         12 . The process of  claim 11  wherein said sub-cooling expander loop cooling stream comprises nitrogen or nitrogen-containing gas. 
     
     
         13 . The process of  claim 11  wherein said sub-cooling expander loop cooling stream comprises a portion of said flash gas and the remaining portion is passed through one or both of the first and second heat exchange areas as a cooling fluid stream before being routed for use as a fuel source. 
     
     
         14 . The process of  claim 1 , wherein the refrigerant is comprised of a side stream from the feed gas stream. 
     
     
         15 . A system for treating a gaseous feed stream, comprising:
 (a) a gaseous feed stream;   (b) a first refrigeration loop having a refrigerant stream, a first compression unit, and a first cooler configured to produce a compressed, cooled refrigerant stream;   (c) a second compression unit configured to compress the gaseous feed stream to greater than 1,000 psia (6895 kPa) to form a compressed gaseous feed stream;   (d) a second cooler configured to cool the compressed gaseous feed stream to form a compressed, cooled gaseous feed stream, wherein the second cooler utilizes an external cooling fluid; and   (e) a first heat exchange area configured to further cool the compressed, cooled gaseous feed stream at least partially by indirect heat exchange with the compressed, cooled refrigerant stream to produce a sub-cooled, compressed, cooled gaseous feed stream.   
     
     
         16 . The system of  claim 15 , further comprising:
 (f) a first expander configured to expand the sub-cooled, compressed, cooled gaseous feed stream to form a product stream having a liquid fraction and a remaining vapor fraction; and   (g) a liquid separation vessel configured to separate the liquid fraction and the remaining vapor fraction.   
     
     
         17 . The system of  claim 16 , wherein the gaseous feed stream is compressed in step (c) to from 1,500 to 3,500 psia (10342 to 24132 kPa). 
     
     
         18 . The system of  claim 16 , wherein the gaseous feed stream is compressed in step (c) to 2,500 to 3,000 psia (17237 to 20684 kPa). 
     
     
         19 . The system of  claim 16 , wherein the first cooler utilizes an external cooling fluid. 
     
     
         20 . The system of  claim 16 , wherein the refrigerant stream is comprised of a side stream from the gaseous feed stream. 
     
     
         21 . The system of  claim 16 , further comprising a second heat exchange area configured to further cool at least the sub-cooled, compressed, cooled gaseous feed stream. 
     
     
         22 . The system of  claim 21 , further comprising:
 (h) a sub-cooling expander loop cooling stream comprising a gas selected from the group consisting of: nitrogen gas, nitrogen-containing gas, and the remaining vapor fraction, wherein the sub-cooling expander loop cooling stream flows in a closed loop, the closed loop comprising:   (i) a third compressor configured to compress the sub-cooling expander loop cooling stream after passing it through the first heat exchange area and said second heat exchange area to form a compressed sub-cooling expander loop cooling stream,   j) a third cooler configured to further cool the compressed sub-cooling expander loop cooling stream to form a further cooled, compressed sub-cooling expander loop cooling stream; and   (k) a second expander configured to expand the further cooled, compressed sub-cooling expander loop cooling stream prior to providing it to the first and second heat exchange areas.   
     
     
         23 . The system of  claim 22 , wherein each of the first, second, and third compressor and each of the first, second, and third cooler are configured to be compact devices sufficient for utilization in an offshore environment.