Natural Gas Liquefaction by a High Pressure Expansion Process

Abstract

A method and system for liquefying a methane-rich high-pressure feed gas stream using a first heat exchanger zone and a second heat exchanger zone. The feed gas stream is mixed with a refrigerant stream to form a second gas stream, which is compressed, cooled, and directed to a second heat exchanger zone to be additionally cooled below ambient temperature. It is then expanded to a pressure less than 2,000 psia and no greater than the pressure to which the second gas stream was compressed, and then separated into a first expanded refrigerant stream and a chilled gas stream. The first expanded refrigerant stream is expanded and then passed through the first heat exchanger zone such that it has a temperature that is cooler, by at least 5° F., than the highest fluid temperature within the first heat exchanger zone.

What is claimed is: 
     
         1 . A method for liquefying a feed gas stream rich in methane using a first heat exchanger zone and a second heat exchanger zone, where the method comprises:
 (a) providing the feed gas stream at a pressure less than 1,200 psia;   (b) providing a refrigerant stream at near the same pressure of the feed gas stream;   (c) mixing the feed gas stream with the refrigerant stream to form a second gas stream;   (d) compressing the second gas stream to a pressure of at least 1,500 psia to form a compressed second gas stream;   (e) cooling the compressed second gas stream by indirect heat exchange with ambient temperature air or water, to form a compressed, cooled second gas stream;   (f) directing the compressed, cooled second gas stream to a second heat exchanger zone, to additionally cool the compressed, cooled second gas stream below ambient temperature, thereby producing a compressed, additionally cooled second gas stream;   (g) expanding the compressed, additionally cooled second gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the second gas stream was compressed, to thereby form an expanded, cooled second gas stream;   (h) separating the expanded, cooled second gas stream into a first expanded refrigerant stream and a chilled gas stream;   (i) expanding the first expanded refrigerant stream in at least one work producing expander, thereby producing a second expanded refrigerant stream;   (j) passing the second expanded refrigerant stream through the first heat exchanger zone to form a first warm refrigerant stream such that the first warm refrigerant stream has a temperature that is cooler, by at least 5° F., than the highest fluid temperature within the first heat exchanger zone;   (k) passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the second expanded refrigerant stream, thereby forming a liquefied gas stream;   (l) directing the first warm refrigerant stream to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled second gas stream, thereby forming a second warm refrigerant stream; and   (m) compressing the second warm refrigerant stream to produce the refrigerant stream.   
     
     
         2 . The method of  claim 1 , wherein the second gas stream is compressed to a pressure of at least 2,800 psia. 
     
     
         3 . The method of  claim 1 , wherein the second gas stream is compressed to a pressure equal to or greater than 2,000 psia and equal to or less than 3,500 psia. 
     
     
         4 . The method of  claim 1  wherein the second gas stream is compressed in at least two serially arranged compressors to a pressure of at least 1,500 psia. 
     
     
         5 . The method of  claim 4 , wherein at least one of the at least two serially arranged compressors is driven solely by the shaft work produced by the at least one work producing expander used to expand the compressed, additionally cooled second gas stream. 
     
     
         6 . The method of  claim 1 , wherein each of the first and second heat exchanger zones comprises one or more heat exchangers, and wherein the one or more heat exchangers of the first heat exchanger zone are of a different type than the one or more heat exchangers of the second heat exchanger zone. 
     
     
         7 . The method of  claim 6 , wherein the heat exchangers of the second heat exchanger zone comprise printed circuit heat exchangers. 
     
     
         8 . The method of  claim 1 , wherein the first warm refrigerant stream has a temperature that is cooler, by at least 10° F., than the highest fluid temperature within the first heat exchanger zone. 
     
     
         9 . The method of  claim 1 , further comprising:
 further cooling the liquefied gas stream within the first heat exchanger zone using a sub-cooling refrigeration cycle, to thereby form a sub-cooled gas stream.   
     
     
         10 . The method of  claim 1 , further comprising:
 expanding the sub-cooled gas stream in a hydraulic turbine to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream.   
     
     
         11 . The method of  claim 9 , wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant. 
     
     
         12 . The method of  claim 9 , wherein the sub-cooling refrigeration cycle comprises:
 withdrawing a portion not to exceed 50% of the expanded, sub-cooled gas stream and reducing its pressure in a pressure reduction valve to a range of about 30 to 300 psia to produce one or more reduced pressure gas streams; and   passing the one or more reduced pressure gas streams through the first heat exchanger zone as the sub-cooling refrigerant.   
     
     
         13 . The method of  claim 9 , wherein the one or more reduced pressure gas streams comprise two or more reduced pressure gas streams having different pressures from each other. 
     
     
         14 . The method of  claim 9 , further comprising:
 compressing the sub-cooling refrigerant stream exiting the first heat exchanger zone; and   cooling the sub-cooling refrigerant stream by indirect heat exchange with an ambient temperature air or water and then adding the sub-cooling refrigerant to the gas stream.   
     
     
         15 . The method of  claim 7 , wherein at least a portion of the expanded, sub-cooled gas stream is further expanded and then directed to a separation tank from which liquid natural gas is withdrawn and remaining gaseous vapors are withdrawn as a flash gas stream. 
     
     
         16 . A system for liquefying a feed gas stream rich in methane, the feed gas stream having a pressure of less than 1,200 psia, the system including a first heat exchanger zone and a second heat exchanger zone, the system comprising:
 a refrigerant stream having a pressure near the same pressure of the feed gas stream;   a compressor that compresses the combined refrigerant stream and feed gas stream to a pressure of at least 1,500 psia, thereby forming a compressed second gas stream;   a cooler that cools the compressed second gas stream by indirect heat exchange with ambient temperature air or water, to thereby form a compressed, cooled second gas stream;   wherein the compressed, cooled second gas stream is directed to the second heat exchanger zone, to additionally cool the compressed, cooled second gas stream below ambient temperature, thereby producing a compressed, additionally cooled second gas stream;   at least one work producing expander that expands the compressed, additionally cooled second gas stream to a pressure that is less than 2,000 psia and no greater than the pressure to which the second gas stream was compressed, to thereby form an expanded, cooled second gas stream;   wherein the expanded, cooled second gas stream is separated into a first expanded refrigerant stream and a chilled gas stream;   an additional at least one work producing expander that expands the first expanded refrigerant stream, thereby producing a second expanded refrigerant stream;   wherein the second expanded refrigerant stream is passed through the first heat exchanger zone to form a first warm refrigerant stream such that the first warm refrigerant stream has a temperature that is cooler, by at least 5° F., than the highest fluid temperature within the first heat exchanger zone;   wherein the chilled gas stream is passed through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the second expanded refrigerant stream, thereby forming a liquefied gas stream;   wherein the first warm refrigerant stream is directed to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled second gas stream, thereby forming a second warm refrigerant stream; and   an additional compressor that compresses the second warm refrigerant stream to produce the refrigerant stream.   
     
     
         17 . The system of  claim 16 , wherein the second gas stream is compressed to a pressure of at least 2,800 psia. 
     
     
         18 . The system of  claim 16 , wherein the second gas stream is compressed in at least two serially arranged compressors to a pressure of at least 1,500 psia. 
     
     
         19 . The system of  claim 18 , wherein at least one of the at least two serially arranged compressors is driven solely by the shaft work produced by the at least one work producing expander used to expand the compressed, additionally cooled second gas stream. 
     
     
         20 . The system of  claim 16 , wherein each of the first and second heat exchanger zones comprises one or more heat exchangers, and wherein the one or more heat exchangers of the first heat exchanger zone are of a different type than the one or more heat exchangers of the second heat exchanger zone. 
     
     
         21 . The system of  claim 20 , wherein the heat exchangers of the second heat exchanger zone comprise printed circuit heat exchangers. 
     
     
         22 . The system of  claim 16 , wherein the first warm refrigerant stream has a temperature that is cooler, by at least 10° F., than the highest fluid temperature within the first heat exchanger zone. 
     
     
         23 . The system of  claim 16 , wherein the first heat exchanger zone includes a sub-cooling refrigeration cycle to further cool the liquefied gas stream, to thereby form a sub-cooled gas stream. 
     
     
         24 . The system of  claim 16  further comprising an expander configured to expand the sub-cooled gas stream to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream, and wherein the expander comprises a hydraulic turbine. 
     
     
         25 . The system of  claim 23 , wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant. 
     
     
         26 . The system of  claim 23 , further comprising:
 a pressure reduction valve configured to reduce the pressure of a portion, not to exceed 50%, of the expanded, sub-cooled gas stream, to a range of about 30 to 300 psia, thereby producing one or more reduced pressure gas streams;   wherein the one or more reduced pressure gas streams is passed through the first heat exchanger zone as the sub-cooling refrigerant stream.   
     
     
         27 . The system of  claim 26 , wherein the one or more reduced pressure gas streams comprise two or more reduced pressure gas streams having different pressures from each other. 
     
     
         28 . The system of  claim 23 , further comprising:
 a sub-cooling compressor configured to compress the sub-cooling refrigerant stream exiting the first heat exchanger zone; and   to an external cooling unit configured to cool the sub-cooling refrigerant stream by indirect heat exchange with an ambient temperature air or water.   
     
     
         29 . The system of  claim 24 , further comprising:
 an additional expander configured to further expand at least a portion of the expanded, sub-cooled gas stream; and   a separation tank to which the expanded, sub-cooled gas stream is directed after passing through the additional expander.