Method and system for liquefying a natural gas feed stream

Abstract

The invention relates to a method of liquefying a natural gas feed stream. A first split-off stream from a compressed process stream is expanded. A remainder of the compressed process stream is cooled against the expanded first split-off stream. A second split-off stream from the precooled process stream is expanded, while a remainder of the precooled compressed process stream is cooled against a vapour stream obtained from the second split-off stream. The further cooled process stream is expanded, thereby obtaining a liquid natural gas stream. The first split-off stream and the vapour stream are passed to a recompression stage to obtain a recycle stream to be combined with a natural gas feed stream to form the process stream.

1 . A method of liquefying a natural gas feed stream, the method comprising at least the steps of:
 (a) providing a process feed stream by mixing the natural gas feed stream with a recycle stream,   (b) compressing the process feed stream and cooling the process feed stream against ambient in a compressor stage, thereby obtaining a compressed process stream having a pressure of at least 120 bar and a first temperature below 40° C.,   (c1) obtaining a first split-off stream from the compressed process stream and expanding the first split-off stream in a precool expander, thereby obtaining an expanded first split-off stream, having a second temperature below the first temperature,   (c2) cooling a remainder of the compressed process stream in a first heat exchanger against the expanded first split-off stream, thereby obtaining a precooled process stream and a warmed first split-off stream,   (d1) obtaining a second split-off stream from the precooled process stream and expanding the second split-off stream in an expander, thereby obtaining an expanded and cooled multiphase second split-off stream having a third temperature below the second temperature,   (d2) splitting the expanded and cooled multiphase second split-off stream in a phase separator to obtain a vapour stream and a liquid stream,   (d3) cooling a remainder of the precooled compressed process stream in a second heat exchanger against the vapour stream, thereby obtaining a further cooled process stream and a warmed vapour stream,   (e) expanding the further cooled process stream thereby obtaining a liquid natural gas stream,   (f) passing the warmed first split-off stream and the warmed vapour stream to a recompression stage, the recompression stage generating the recycle stream.   
     
     
         2 . The method according to  claim 1 , wherein the mass flow rate of the natural gas feed stream (MF 1 ) and the mass flow rate of the recycle stream (MF 105 ) is in the range MF 1 :MF 105 =1:2-1:4. 
     
     
         3 . The method according to  claim 1 , wherein the method further comprises the step of passing the liquid natural gas stream to a flash vessel and obtaining a liquid natural gas product stream as bottom stream from the flash vessel. 
     
     
         4 . The method according to  claim 3 , wherein the method comprises the step of obtaining a flash gas stream as top stream from the flash vessel, passing the flash gas stream to the recompression stage. 
     
     
         5 . The method according to  claim 17 , wherein the method comprises the steps of:
 (e1) splitting the liquid stream obtained in (d2) into a first liquid portion and a second liquid portion,   (e2) expanding the first liquid portion in a first pressure reduction device to obtain a second liquid natural gas stream, and   (e3) cooling the second liquid portion by passing the second liquid portion through the third heat exchanger and a second pressure reduction device to obtain a third liquid natural gas stream,   (e4) collecting the liquid natural gas stream obtained in (e), the second liquid natural gas stream obtained in (e2) and the third liquid natural gas stream obtained in (e3) in the flash vessel.   
     
     
         6 . The method according to  claim 17 , wherein the method comprises the steps of:
 (e1′) passing the liquid stream obtained in (d2) through the third heat exchanger and a valve or an expander to obtain a further liquid natural gas stream,   (e2′) collecting the liquid natural gas stream obtained in (e) and the further liquid natural gas stream obtained in (e1′) in the flash vessel.   
     
     
         7 . The Method according to  claim 1 , wherein the warmed vapour stream obtained from the second heat exchanger in (d3) is passed through the first heat exchanger to provide cooling to the remainder of the compressed process stream thereby obtaining a further warmed vapour stream before being passed to the recompression stage. 
     
     
         8 . The method according to  claim 1 , wherein (f) comprises separately passing the warmed first split-off stream and one of the warmed vapour stream and the further warmed vapour stream to the recompression stage to obtain the recycle stream. 
     
     
         9 . The method according to  claim 4 , wherein (f) further comprises passing the flash gas stream or the warmed flash gas stream to the recompression stage. 
     
     
         10 . The method according to  claim 1 , wherein the method further comprises:
 (g) obtaining a fuel stream from an intermediate position of the recompression stage.   
     
     
         11 . A system for liquefying a natural gas feed stream, the system comprising:
 a compressor stage being arranged to receive a process feed stream comprising the natural gas feed stream and a recycle stream, the compressor stage further being arranged to compress the process feed stream and cool the process feed stream to obtain a compressed process stream having a pressure of at least 120bar and a first temperature below 40° C.,   a first splitter arranged to receive the compressed process stream and output a first split-off stream and a remainder of the compressed process stream,   a precool expander arranged to receive and expand the first split-off stream to obtain an expanded first split-off stream, having a second temperature below the first temperature,   a first heat exchanger arranged to receive the expanded first split-off stream and the remainder of the compressed process stream thereby cooling the remainder of the compressed process stream against the expanded first split-off stream obtaining a precooled process stream and a warmed first split-off stream,   a second splitter arranged to receive the precooled process stream and discharge a second split-off stream and a remainder of the precooled compressed process stream,   an expander arranged to receive and expand the second split-off stream thereby obtaining an expanded and cooled multiphase second split-off stream, having a third temperature below the second temperature,   a phase separator arranged to receive the expanded and cooled multiphase second split-off stream and discharge a vapour stream and a liquid stream,   a second heat exchanger arranged to receive the vapour stream and the remainder of the precooled compressed process stream thereby cooling the remainder of the precooled compressed process stream against the vapour stream obtaining a further cooled process stream and a warmed vapour stream,   a liquid expander arranged to receive the further cooled process stream obtaining a liquid natural gas stream, and   a recompression stage arranged to receive, combine and recompress at least the warmed first split-off stream and the warmed vapour stream to obtain the recycle stream.   
     
     
         12 . The system according to  claim 11 , wherein the system further comprises a flash vessel arranged to receive the liquid natural gas stream, the flash vessel further being arranged to discharge a liquid natural gas product stream. 
     
     
         13 . The system according to  claim 12 , wherein the flash vessel is arranged to discharge a flash gas stream, the system comprising a flash gas conduit arranged to pass the flash gas stream to the recompression stage. 
     
     
         14 . The system according to  claim 19 , comprising:
 a further splitter arranged to receive the liquid stream and split the liquid stream into a first liquid portion and a second liquid portion,   a first pressure reduction device arranged to receive and expand the first liquid portion to obtain a second liquid natural gas stream,   the third heat exchanger being arranged to receive the flash gas stream and the second liquid portion thereby cooling the second liquid portion and forwarding the second liquid portion to a second pressure reduction device to obtain a third liquid natural gas stream,   wherein the flash vessel is further arranged to receive the second liquid natural gas stream and the third liquid natural gas stream.   
     
     
         15 . The system according to  claim 19 , wherein the third heat exchanger is arranged to receive the flash gas stream and at least part of the liquid stream, thereby cooling the at least part of the liquid stream against the flash gas stream, the system further comprising a valve or an expander positioned downstream of the third heat exchanger arranged to receive the at least part of the liquid stream from the third heat exchanger to expand the at least part of the liquid stream thereby obtaining a further liquid natural gas stream,
 wherein the flash vessel is further arranged to receive the further liquid natural gas stream.   
     
     
         16 . The method according to  claim 1 , wherein the mass flow rate of the natural gas feed stream (MF 1 ) and the mass flow rate of the recycle stream (MF 105 ) is MF 1 :MF 105 =1:3. 
     
     
         17 . The method according to  claim 4 , wherein passing the flash gas stream to the recompression stage comprises passing the flash gas stream at least partially through a third heat exchanger to provide cooling to at least part of the liquid stream obtained in (d2). 
     
     
         18 . The method according to  claim 10 , wherein the fuel stream is obtained upstream of the position at which the warmed first split-off stream is fed to the recompression stage. 
     
     
         19 . The system according to  claim 13 , further comprising a third heat exchanger arranged to receive the flash gas stream and at least part of the liquid stream thereby cooling the at least part of the liquid stream against the flash gas stream.