Semi-open loop liquefaction process

Abstract

Methods and systems for liquefying natural gas by: cooling and liquefying a natural gas feed stream via indirect heat exchange with at least a first cold refrigerant stream to form a first liquefied natural gas stream and a warmed gaseous refrigerant stream; flashing and separating the first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream; combining and compressing the first flash gas stream and the warmed gaseous refrigerant stream to form a compressed refrigerant stream; and expanding at least a first portion of the compressed refrigerant stream to form the first cold refrigerant stream; wherein the natural gas feed stream is kept separate from and is not combined with either the first flash gas stream or the compressed refrigerant stream.

1 . A method of liquefying natural gas, the method comprising the steps of:
 (a) cooling and liquefying a natural gas feed stream via indirect heat exchange with at least a first cold refrigerant stream to form a first liquefied natural gas stream and a warmed gaseous refrigerant stream;   (b) flashing and separating the first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream and a second flash gas stream;   (c) combining the first and second flash gas streams and compressing the combined first and second flash gas streams in a flash gas compressor with the second flash gas stream being routed to the inlet of the flash gas compressor and the first flash gas stream being routed to an intermediate stage of the flash gas compressor, and combining and compressing the warmed gaseous refrigerant stream and the combined first and second flash gas streams to form a compressed refrigerant stream; and   (d) expanding at least a first portion of the compressed refrigerant stream to form the first cold refrigerant stream;   wherein the natural gas feed stream is kept separate from and is not combined with either the first flash gas stream or the compressed refrigerant stream.   
     
     
         2 . The method of  claim 1 , wherein step (a) takes place in one or more coil-wound heat exchanger sections of a main coil-wound heat exchanger unit or set of units. 
     
     
         3 . The method of  claim 1 , wherein the method further comprises the steps of:
 (e) withdrawing a first auxiliary stream of natural gas from the natural gas feed stream prior to the natural gas feed stream being cooled and liquefied in step (a); and   (f) cooling and liquefying the first auxiliary natural gas stream via indirect heat exchange with the first flash gas stream to form a second liquefied natural gas stream;   wherein the first flash gas stream is warmed in step (f) before being compressed and combined with the second flash gas stream and the warmed gaseous refrigerant stream in step (c), and   wherein step (b) comprises combining, flashing and separating the second liquefied natural gas stream and the first liquefied natural gas stream to form the liquefied natural gas product stream and at least the first flash gas stream and second flash gas stream.   
     
     
         4 . The method of  claim 3 , wherein step (f) takes place in one or more coil-wound heat exchanger sections of a first flash gas heat exchanger unit or set of units. 
     
     
         5 . The method of  claim 4 , wherein the first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing containing one or more coil-wound heat exchanger sections located above a phase separator section, and wherein said phase separator section is used in step (b) to separate the first flash gas stream from the first and second liquefied natural gas streams. 
     
     
         6 . The method of  claim 3 , wherein step (a) comprises precooling, cooling and liquefying the natural gas feed stream via indirect heat exchange with at least the first cold refrigerant stream to form the first liquefied natural gas stream and the warmed gaseous refrigerant stream,
 wherein step (f) comprises precooling, cooling and liquefying the first auxiliary natural gas stream via indirect heat exchange with the first flash gas stream to form a second liquefied natural gas stream, and   wherein the method further comprises withdrawing a first side-stream of natural gas from the first auxiliary natural gas stream after precooling and prior to liquefaction of the first auxiliary natural gas stream, and introducing the first side-stream of natural gas into the natural gas feed stream after precooling of the natural gas feed stream and prior to liquefaction of the natural gas feed stream in step (a).   
     
     
         7 . The method of  claim 1 , wherein step (c) comprises compressing the second flash gas stream in one or more flash gas compression stages of the flash gas compressor prior to combining the second flash gas stream and the first flash gas stream and then compressing the combined first and second flash gas streams in one or more further flash gas compression stages of the flash gas compressor prior to combining the combined first and second flash gas streams and the warmed gaseous refrigerant stream and compressing the combined first and second flash gas streams and the warmed gaseous refrigerant stream in one or more refrigerant compression stages to form the compressed refrigerant stream. 
     
     
         8 . The method of  claim 1 , wherein the method further comprises the steps of:
 (e) withdrawing a first auxiliary stream of natural gas and a second auxiliary stream of natural gas from the natural gas feed stream prior to the natural gas feed stream being cooled and liquefied in step (a);   (f) cooling and liquefying the first auxiliary natural gas stream via indirect heat exchange with the first flash gas stream to form a second liquefied natural gas stream; and   (g) cooling and liquefying the second auxiliary natural gas stream via indirect heat exchange with the second flash gas stream to form a third liquefied natural gas stream   wherein the first flash gas stream is warmed in step (f) before being compressed and combined with the second flash gas stream and warmed gaseous refrigerant stream in step (c),   wherein the second flash gas stream is warmed in step (g) before being compressed and combined with the first flash gas stream and warmed gaseous refrigerant stream in step (c), and   wherein step (b) comprises combining, flashing and separating the second liquefied natural gas stream and the first liquefied natural gas stream to form a fourth liquefied natural gas stream and the first flash gas stream, and then combining, flashing and separating the fourth liquefied natural gas stream and the third liquefied natural gas stream to form the liquefied natural gas product stream and at least the second flash gas stream.   
     
     
         9 . The method of  claim 8 , wherein step (f) takes place in one or more coil-wound heat exchanger sections of a first flash gas heat exchanger unit or set of units, and step (g) takes place in one or more coil-wound heat exchanger sections of a second flash gas heat exchanger unit or set of units. 
     
     
         10 . The method of  claim 9 , wherein the first flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing containing one or more coil-wound heat exchanger sections located above a phase separator section, wherein said phase separator section is used in step (b) to separate the first flash gas stream from the first and second liquefied natural gas streams, and
 wherein the second flash gas heat exchanger unit is an integrated heat exchanger and phase separator comprising a shell casing containing one or more coil-wound heat exchanger sections located above a phase separator section, wherein said phase separator section is used in step (b) to separate the second flash gas stream from the third and fourth liquefied natural gas stream.   
     
     
         11 . The method of  claim 8 , wherein step (a) comprises precooling, cooling and liquefying the natural gas feed stream via indirect heat exchange with at least the first cold refrigerant stream to form the first liquefied natural gas stream and the warmed gaseous refrigerant stream,
 wherein step (f) comprises precooling, cooling and liquefying the first auxiliary natural gas stream via indirect heat exchange with the first flash gas stream to form a second liquefied natural gas stream,   wherein step (g) comprises precooling, cooling and liquefying the second auxiliary natural gas stream via indirect heat exchange with the second flash gas stream to form a third liquefied natural gas stream, and   wherein the method further comprises withdrawing a first side-stream of natural gas from the first auxiliary natural gas stream after precooling and prior to liquefaction of the first auxiliary natural gas stream, withdrawing a second side-stream of natural gas from the second auxiliary natural gas stream after precooling and prior to liquefaction of the second auxiliary natural gas stream, and introducing the first side-stream of natural gas and the second side-stream of natural gas into the natural gas feed stream after precooling of the natural gas feed stream and prior to liquefaction of the natural gas feed stream in step (a).   
     
     
         12 . The method of  claim 1 , wherein the method further comprises the steps of:
 (h) introducing the liquefied natural gas product stream into and storing the liquefied natural gas product in a liquefied natural gas storage tank; and   (i) withdrawing a boil-off gas stream from the liquefied natural gas storage tank;   wherein step (c) comprises combining the first and second flash gas streams and compressing the combined first and second flash gas streams in a flash gas compressor with the second flash gas stream being routed to the inlet of the flash gas compressor and the first flash gas stream being routed to an intermediate stage of the flash gas compressor, and combining and compressing the boil-off gas stream, the warmed gaseous refrigerant stream and the combined first and second flash gas streams to form the compressed refrigerant stream, and   wherein the natural gas feed stream is also kept separate from and is not combined with the boil-off gas stream.   
     
     
         13 . The method of  claim 12 , wherein step (c) comprises compressing the boil-off gas stream in one or more boil-off gas compression stages prior to combining the boil-off gas stream, the warmed gaseous refrigerant stream and the combined first and second flash gas streams and compressing said combined boil-off gas stream, warmed gaseous refrigerant stream and combined first and second flash gas streams in one or more refrigerant compression stages to form the compressed refrigerant stream. 
     
     
         14 . The method of  claim 1 , wherein step (d) comprises expanding a first portion of the compressed refrigerant stream to form the first cold refrigerant stream,
 wherein step (b) comprises combining, flashing and separating the first liquefied natural gas stream and a second cold refrigerant stream to form the liquefied natural gas product stream and at least the first flash gas stream and second flash gas stream, and   wherein the method further comprises the step of:   (j) cooling a second portion of the compressed refrigerant stream, via indirect heat exchange with the first cold refrigerant stream, to form the second cold refrigerant stream.   
     
     
         15 . The method of  claim 14 , wherein the method further comprises the step of:
 (k) expanding a third portion of the compressed refrigerant stream to form a third cold refrigerant stream; and   wherein step (a) comprises precooling the natural gas feed stream via indirect heat exchange with the first and third cold refrigerant streams and further cooling and liquefying a natural gas feed stream via indirect heat exchange with the first cold refrigerant stream to form the first liquefied natural gas stream from the natural gas feed stream and the warmed gaseous refrigerant stream from the first and third cold refrigerant streams.   
     
     
         16 . The method of  claim 15 , wherein the method further comprises the step of:
 (l) precooling the first and second portions of the compressed refrigerant stream via indirect heat exchange with the first and third cold refrigerant streams prior to the first portion of the compressed refrigerant stream being expanded in step (d) and prior to the second portion of the compressed refrigerant being stream being further cooled in step (j).   
     
     
         17 . The method of  claim 1 , wherein the first cold refrigerant stream is a gaseous refrigerant stream. 
     
     
         18 . A system for liquefying natural gas, the system comprising:
 one or more heat exchanger sections arranged and configured to receive a natural gas feed stream and at least a first cold refrigerant steam and to cool and liquefy the natural gas feed stream via indirect heat exchange with at least the first cold refrigerant stream to form a first liquefied natural gas stream and a warmed gaseous refrigerant stream;   one or more expansion and separation devices arranged and configured to receive, flash and separate the first liquefied natural gas stream to form a liquefied natural gas product stream and at least a first flash gas stream and a second flash gas stream;   a flash gas compressor configured to receive and combine the first and second flash gas streams and compress the combined first and second flash gas streams with the second flash gas stream being routed to the inlet of the flash gas compressor and the first flash gas stream being routed to an intermediate stage of the flash gas compressor;   one or more conduits and refrigerant compression stages arranged and configured to receive, combine and compress the warmed gaseous refrigerant stream and the combined first and second flash gas streams to form a compressed refrigerant stream; and   an expansion device arranged and configured to receive and expand at least a first portion of the compressed refrigerant stream to form the first cold refrigerant stream;   wherein the system is arranged and configured such that the natural gas feed stream is kept separate from and is not combined with either the first flash gas stream or the compressed refrigerant stream.