Method and Apparatus for Liquefying a Natural Gas Stream

Abstract

The present invention relates to a method of liquefying a natural gas stream, wherein the natural gas stream ( 10 ) is provided at a pressure of 30-80 bar, expanded to a pressure <35 bar, supplied to a gas/liquid separator ( 31 ) and therein into a vaporous stream ( 40 ) and a liquid stream ( 30 ). The pressure of the vaporous stream is increased to a pressure of at least 70 bar and the pressurized vaporous stream ( 90 ) is liquefied to obtain a liquefied natural gas stream.

1 . A method of liquefying a natural gas stream, the method comprising the steps of:
 (a) providing a feed stream containing natural gas at a pressure of 30-80 bar;   (b) expanding the feed stream of step (a), thereby obtaining an expanded feed stream having a pressure <35 bar;   (c) supplying the expanded feed stream to a gas/liquid separator;   (d) separating the expanded feed stream in the gas/liquid separator into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream;   (e) increasing the pressure of the vaporous stream obtained in step (d) to a pressure of at least 70 bar;   (f) liquefying the pressurized vaporous stream obtained in step (e) thereby obtaining a liquefied natural gas stream;   wherein the pressure of the feed stream provided in step (a) is not increased until the increase of pressure in step (e).   
   
   
       2 . The method according to  claim 1 , wherein in step (e) the pressure is increased to at least 86 bar. 
   
   
       3 . The method according to  claim 1 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.5 mol %. 
   
   
       4 . The method according to  claim 1 , wherein the pressure in step (e) is increased by compressing the vaporous stream, thereby obtaining a compressed stream. 
   
   
       5 . The method according to  claim 1 , wherein the vaporous stream obtained in step (e) is cooled. 
   
   
       6 . The method according to  claim 4 , wherein the compressed stream, before it is liquefied in step (f), is heat exchanged against the vaporous stream obtained in step (d). 
   
   
       7 . The method according to  claim 1 , wherein an expander for expanding the feed stream in step (b) is functionally coupled to a compressor for compressing the vaporous stream. 
   
   
       8 . An apparatus for liquefying a natural gas stream, the apparatus at least comprising:
 means for providing a feed stream containing natural gas at a pressure of 30-80 bar;   an expander for expanding the feed stream thereby obtaining an expanded feed stream having a pressure of <35 bar;   a gas/liquid separator for separating the expanded feed stream into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream;   a pressurizing unit for increasing the pressure of the vaporous stream obtained in the gas/liquid separator to a pressure of at least 70 bar, and   a liquefaction unit for liquefying the vaporous stream having a pressure of at least 70 bar, the liquefaction unit comprising at least one cryogenic heat exchanger.   
   
   
       9 . The apparatus according to  claim 8 , wherein the pressurizing unit comprises a compressor. 
   
   
       10 . The apparatus according to  claim 9 , wherein the apparatus further comprises a heat exchanger for heat exchanging an effluent from the compressor against the vaporous stream obtained in the gas/liquid separator. 
   
   
       11 . The apparatus according to  claim 9 , wherein the compressor and expander are functionally coupled. 
   
   
       12 . The apparatus according to  claim 8 , wherein no further pressurizing unit is present between the means (for providing the feed stream at a pressure of 30-80 bar and the pressurizing unit  52 . 
   
   
       13 . The method according to  claim 1 , wherein in step (e) the pressure is increased to at least 84 bar. 
   
   
       14 . The method according to  claim 1 , wherein in step (e) the pressure is increased to at least 90 bar. 
   
   
       15 . The method according to  claim 2 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.5 mol %. 
   
   
       16 . The method according to  claim 13 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.5 mol %. 
   
   
       17 . The method according to  claim 14 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.5 mol %. 
   
   
       18 . The method according to  claim 1 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.1 mol %. 
   
   
       19 . The method according to  claim 2 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.1 mol %. 
   
   
       20 . The method according to  claim 13 , wherein the vapour stream obtained in step (d) has a C 5 + content of below 0.1 mol %.