Process for liquefaction of natural gas

Abstract

A process comprising: cooling natural gas with a heat exchanger and a first expander. The heat exchanger cools the feed natural gas to temperature higher than the outlet temperature of the expander, reheating the expander outlet stream in a first cold passage of the heat exchanger to slightly below the temperature of the feed natural gas to the heat exchanger, passing the cold outlet stream from the heat exchanger into a second expander wherein it is partly liquefied, separating the outlet stream of second expander into liquid and vapour fractions, collecting the liquid fraction for use as LNG product, reheating the vapour fraction in a second cold side passage of the heat exchanger to substantially the same temperature as the temperature of the feed natural gas to the heat exchanger, recycling the reheated vapour fraction partly as feed to the first expander and partly as feed to the heat exchanger.

1 . A process for liquefying natural gas or other methane-rich gases comprising:
 cooling feed natural gas at a pressure of from 40 to 100 bar (4 to 10 MPa) to a temperature of −50° to −80° C. by means of a heat exchanger and a first gas expander, the heat exchanger receiving the feed natural gas and having an outlet temperature higher than the outlet temperature of the expander;   reheating the expander outlet stream in a first cold passage of said heat exchanger to just below the inlet temperature of the feed natural gas to said heat exchanger, compressing and recycling;   passing part or all of the cold outlet stream from said heat exchanger into a second expander in which it is partly liquefied;   separating the outlet stream of said second expander into vapour and liquid fractions;   collecting the liquid fraction for use as LNG product,   reheating the vapour fraction in a second cold side passage of said heat exchanger to just below the inlet temperature of the feed natural gas to said heat exchanger;   recycling the said reheated vapour fraction after compression in part to the said first expander and in part to the said heat exchanger;   
       wherein
 the outlet stream of the second expander is at a pressure between 5 and 15 bar (0.5 and 1.5 MPa). 
 
     
     
         2 . The process as claimed in  claim 1  in which the heat exchanger receives all the feed natural gas. 
     
     
         3 . The process as claimed in  claim 1  in which the heat exchanger receives a large part, at least 30%, of the feed natural gas. 
     
     
         4 . The process as claimed in  claim 1  in which the feed natural gas is cooled to a temperature of −60° to −70° C. 
     
     
         5 . The modification of the process claimed in  claim 1  in which the said first and second gas expanders have essentially the same outlet pressure of between 5 bar and 15 bar (0.5 and 1.5 MPa), and the outlet streams from both expanders are combined prior to final reheating, compression and recycle. 
     
     
         6 . The process as claimed in  claim 1  in which any part or all of the feed and/or compressor discharge and/or recycle streams are cooled, typically by use of an absorption refrigeration cycles such as lithium bromide (LiBr). 
     
     
         7 . The process as claimed in  claim 1  in which the heat requirement for an absorption refrigeration system is supplied by gas engine or gas turbine exhaust heat, such gas engines or turbines which may be used for supplying power to the process compressors. 
     
     
         8 . The process as claimed in  claim 1  wherein such cooling of either feed and/or recycle streams is combined with removal of carbon dioxide and/or other impurities from the feed gas.