Pre-Cooling of Natural Gas by High Pressure Compression and Expansion

Abstract

A method of producing liquefied natural gas (LNG) is disclosed. A natural gas stream is provided from a supply of natural gas. The natural gas stream is compressed in at least two serially arranged compressors to a pressure of at least 2,000 psia to form a compressed natural gas stream. The compressed natural gas stream is cooled to form a cooled compressed natural gas stream. The cooled compressed natural gas stream is expanded in at least one work producing natural gas expander to a pressure that is less than 3,000 psia and no greater than the pressure to which the at least two serially arranged compressors compress the natural gas stream, to thereby form a chilled natural gas stream. The chilled natural gas stream is liquefied.

What is claimed is: 
     
         1 . A method of producing liquefied natural gas (LNG), the method comprising:
 providing a natural gas stream from a supply of natural gas;   compressing the natural gas stream in at least two serially arranged compressors to a pressure of at least 2,000 psia to form a compressed natural gas stream;   cooling the compressed natural gas stream to form a cooled compressed natural gas stream;   expanding, in at least one work producing natural gas expander, the cooled compressed natural gas stream to a pressure that is less than 3000 psia and no greater than the pressure to which the at least two serially arranged compressors compress the natural gas stream, to thereby form a chilled natural gas stream; and   liquefying the chilled natural gas stream.   
     
     
         2 . The method of  claim 1 , wherein liquefying the chilled natural gas stream is performed in one or more single mixed refrigerant (SMR) liquefaction trains. 
     
     
         3 . The method of  claim 1 , wherein liquefying the chilled natural gas stream is performed in one or more expander-based liquefaction modules, and wherein the expander-based liquefaction module is one of a nitrogen gas expander-based liquefaction module and a feed gas expander-based liquefaction module. 
     
     
         4 . The method of  claim 3 , wherein the feed gas expander-based liquefaction module is an open loop feed gas expander-based liquefaction module, and wherein a recycle refrigerant stream of the open loop feed gas expander-based process is combined with the natural gas stream prior to the compressing step. 
     
     
         5 . The method of  claim 4 , wherein the chilled natural gas stream is a first chilled natural gas stream, and further comprising:
 separating the first chilled natural gas stream into a second chilled natural gas stream, a first refrigerant stream, and a second refrigerant stream;   discharging a first cooling stream from a warm-end expander forming part of the feed gas expander-based liquefaction module, the first cooling stream having a first temperature; and   discharging a second cooling stream from a cold-end expander forming part of the feed gas expander-based liquefaction module, the second cooling stream having a second temperature;   wherein the first temperature is higher than the second temperature.   
     
     
         6 . The method of  claim 5 , further comprising:
 expanding the first refrigerant stream in the warm-end expander to produce the first cooling stream; and   expanding the second refrigerant stream in the cold-end expander to produce the second cooling stream.   
     
     
         7 . The method of  claim 4 , further comprising:
 discharging a first cooling stream from a warm-end expander forming part of the feed gas expander-based liquefaction module, the first cooling stream having a first temperature;   discharging a two-phase stream from a cold-end expander forming part of the feed gas expander-based liquefaction module, the two-phase stream having a second temperature, wherein the first temperature is higher than the second temperature;   expanding the first refrigerant stream in the warm-end expander to produce the first cooling stream;   expanding the second refrigerant stream in the cold-end expander to produce the two-phase stream; and   separating the two-phase stream into a second cooling stream and a first pressurized LNG stream.   
     
     
         8 . The method of  claim 5 , wherein a pressure of the first cooling stream is one of
 the same or similar to a pressure of the second cooling stream, or   higher than a pressure of the second cooling stream.   
     
     
         9 . The method of  claim 5 , wherein the liquefying step comprises cooling the second chilled natural gas stream to form a second pressurized LNG stream by exchanging heat with the first cooling stream and the second cooling stream. 
     
     
         10 . The method of  claim 9 , wherein the second pressurized LNG stream is mixed with the first pressurized LNG stream prior to expanding the second pressurized LNG stream. 
     
     
         11 . The method of  claim 9 , further comprising:
 reducing a pressure of the second pressurized LNG stream such that the second pressurized LNG stream undergoes at least one stage of pressure reduction;   separating the reduced-pressure second pressurized LNG stream into an end-flash gas stream and an LNG stream; and   cooling the second pressurized LNG stream and the second chilled natural gas stream using the end-flash gas stream.   
     
     
         12 . The method of  claim 11 , further comprising:
 after cooling the second pressurized LNG stream and the second chilled natural gas stream using the end-flash gas stream, compressing the end-flash gas stream and mixing the compressed end-flash gas stream with one or more recycling refrigerant streams.   
     
     
         13 . The method of  claim 11 , further comprising:
 after cooling the second pressurized LNG stream and the second chilled natural gas stream using the end-flash gas stream, compressing the end-flash gas stream and using the compressed end-flash gas stream as fuel.   
     
     
         14 . The method of  claim 1 , wherein the at least two compressors compress the natural gas stream to a pressure greater than 3,000 psia. 
     
     
         15 . The method of  claim 1 , wherein the natural gas expander is a work producing expander that expands the cooled compressed natural gas stream to a pressure less than 2,000 psia. 
     
     
         16 . The method of  claim 1 , wherein the natural gas expander is mechanically coupled to at least one compressor. 
     
     
         17 . The method of  claim 16 , wherein cooling the compressed natural gas stream comprises cooling the compressed natural gas stream in at least one heat exchanger that exchanges heat with the environment. 
     
     
         18 . The method of  claim 1 , wherein one of the at least two serially arranged compressors is driven by the natural gas expander. 
     
     
         19 . The method of  claim 1 , further comprising:
 performing the compressing, cooling, expanding, and liquefying steps on a topside of a floating LNG structure.   
     
     
         20 . The method of  claim 20 , wherein the compressing, cooling, and expanding steps are performed within a single module on the topside of the floating LNG structure. 
     
     
         21 . An apparatus for the liquefaction of natural gas, comprising:
 at least two serially arranged compressors configured to compress a natural gas stream to a pressure greater than 2,000 psia, thereby forming a compressed natural gas stream;   a cooling element configured to cool the compressed natural gas stream, thereby forming a cooled compressed natural gas stream;   at least one work-producing expander configured to expand the cooled compressed natural gas stream to a pressure less than 3,000 psia and no greater than the pressure to which the at least two serially arranged compressors compress the natural gas stream, to thereby form a chilled natural gas stream; and   a liquefaction train configured to liquefy the chilled natural gas stream.   
     
     
         22 . The apparatus of  claim 21 , wherein the liquefaction train comprises one of a nitrogen gas expander-based liquefaction module and an open loop feed gas expander-based liquefaction module, and further comprising a recycle refrigerant stream of the open loop feed gas expander-based process that is combined with the natural gas stream prior to the natural gas stream being compressed by the two or more serially-arranged compressors, wherein the chilled natural gas stream is a first chilled natural gas stream that is separated into a second chilled natural gas stream, a first refrigerant stream, and a second refrigerant stream. 
     
     
         23 . The apparatus of  claim 22 , wherein the feed gas expander-based liquefaction module comprises:
 a warm-end expander configured to expand the first refrigerant stream to form a first cooling stream discharged therefrom, the first cooling stream having a first temperature; and   a cold-end expander configured to expand the second refrigerant stream to form one of a second cooling stream and a two-phase stream discharged therefrom, the second cooling stream having a second temperature;   wherein the first temperature is higher than the second temperature.   
     
     
         24 . The apparatus of  claim 21 , wherein the natural gas expander is a work producing expander configured to expand the cooled compressed natural gas stream to a pressure less than 2,000 psia. 
     
     
         25 . The apparatus of  claim 21 , wherein the cooling element comprises a heat exchanger configured to cool the compressed natural gas stream by exchanging heat with the environment. 
     
     
         26 . The apparatus of  claim 21 , wherein the at least two serially arranged compressors, the cooling element, the at least one work-producing expander, and the liquefaction train are disposed on a floating LNG structure. 
     
     
         27 . The apparatus of  claim 26 , wherein the at least two serially arranged compressors, the cooling element, and the at least one work-producing expander are disposed within a single module on a topside of the floating LNG structure. 
     
     
         28 . A floating LNG structure, comprising:
 at least two serially arranged compressors configured to compress a natural gas stream to a pressure greater than 2,000 psia, thereby forming a compressed natural gas stream;   a cooling element configured to cool the compressed natural gas stream, thereby forming a cooled compressed natural gas stream;   at least one work-producing expander configured to expand the cooled compressed natural gas stream to a pressure less than 3,000 psia and no greater than the pressure to which the at least two serially arranged compressors compress the natural gas stream, to thereby form a chilled natural gas stream; and   a liquefaction train configured to liquefy the chilled natural gas stream.