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 is compressed in at least two serially arranged compressors to a pressure of at least 2,000 psia and cooled to form a cooled compressed natural gas stream. The cooled compressed natural gas stream is additionally cooled to a temperature below an ambient temperature to form an additionally cooled compressed natural gas stream, which 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 by indirect heat exchange with a refrigerant to form liquefied natural gas and a warm refrigerant. The cooled compressed natural gas stream is additionally cooled using the warm refrigerant.

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 by indirect heat exchange with an ambient temperature air or water to form a cooled compressed natural gas stream;   additionally cooling the cooled compressed natural gas stream to a temperature below the ambient temperature to form an additionally cooled compressed natural gas stream;   expanding, in at least one work producing natural gas expander, the additionally cooled compressed natural gas stream 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; and   liquefying the chilled natural gas stream by indirect heat exchange with a refrigerant to form liquefied natural gas and a warm refrigerant;   wherein the cooled compressed natural gas stream is additionally cooled using the warm refrigerant.   
     
     
         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 to form a first warm cooling stream and a second warm 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 9 , wherein the first warm cooling stream is used as the warm refrigerant to additionally cool the cooled compressed natural gas stream to form the additionally cooled compressed natural gas stream. 
     
     
         15 . The method of  claim 9 , wherein the second warm cooling stream is used as the warm refrigerant to additionally cool the cooled compressed natural gas stream to form the additionally cooled compressed natural gas stream. 
     
     
         16 . The method of  claim 3 , wherein the expander-based liquefaction module comprises:
 a first expanded refrigerant within a first gas phase refrigeration cycle; and   a second expanded refrigerant within a second gas phase refrigeration cycle.   
     
     
         17 . The method of  claim 16 , wherein the first expanded refrigerant is feed gas. 
     
     
         18 . The method of  claim 16 , wherein the first gas phase refrigeration cycle is a closed loop refrigeration cycle. 
     
     
         19 . The method of  claim 16 , wherein the second expanded refrigerant is nitrogen. 
     
     
         20 . The method of  claim 16 , wherein the second gas phase refrigeration cycle is a closed loop refrigeration cycle. 
     
     
         21 . The method of  claim 1 , wherein the at least two compressors compress the natural gas stream to a pressure greater than 3,000 psia. 
     
     
         22 . The method of  claim 1 , wherein the natural gas expander is a work producing expander that expands the additionally cooled compressed natural gas stream to a pressure less than 2,000 psia. 
     
     
         23 . The method of  claim 1 , further comprising:
 performing the compressing, cooling, additionally cooling, expanding, and liquefying steps on a topside of a floating LNG structure.   
     
     
         24 . The method of  claim 1 , wherein the temperature of the additionally cooled compressed natural gas stream is less than 30° C. 
     
     
         25 . The method of  claim 1 , wherein the temperature of the additionally cooled compressed natural gas stream is less than 15° C. 
     
     
         26 . 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;   a heat exchanger configured to further cool the cooled compressed natural gas stream to a temperature below an ambient temperature to thereby produce an additionally cooled compressed natural gas stream;   at least one work-producing expander configured to expand the additionally 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;   wherein a warm refrigerant used by the liquefaction train is directed to the heat exchanger to further cool the cooled compressed natural gas stream.   
     
     
         27 . The apparatus of  claim 26 , 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, when the liquefaction train comprises an open loop feed gas expander-based module, a recycle refrigerant stream of the open loop feed gas expander-based module 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. 
     
     
         28 . The apparatus of  claim 27 , 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.   
     
     
         29 . The apparatus of  claim 26 , 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. 
     
     
         30 . The apparatus of  claim 26 , wherein the at least two serially arranged compressors, the cooling element, the heat exchanger, the at least one work-producing expander, and the liquefaction train are disposed on a floating LNG structure. 
     
     
         31 . The apparatus of  claim 30 , wherein the at least two serially arranged compressors, the cooling element, the heat exchanger, and the at least one work-producing expander are disposed within a single module on a topside of the floating LNG structure. 
     
     
         32 . 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;   a heat exchanger configured to further cool the cooled compressed natural gas stream to a temperature below an ambient temperature to thereby produce an additionally cooled compressed natural gas stream;   at least one work-producing expander configured to expand the additionally 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;   wherein a warm refrigerant used by the liquefaction train is directed to the heat exchanger to further cool the cooled compressed natural gas stream.