Natural Gas Liquefaction Process for Ling

Abstract

Embodiments of this invention relate to a process for liquefaction of natural gas and other methane-rich gas streams, and more particularly to a process for producing liquefied natural gas (LNG). In a first step of the process, a first fraction of the feed gas is withdrawn, compressed to a pressure greater than or equal to 1500 psia, cooled and expanded to a lower pressure to cool the withdrawn first fraction. The remaining fraction of the feed stream is cooled by indirect heat exchange with the expanded first fraction in a first heat exchange process. In a second step a separate stream comprising flash vapor is compressed, cooled and expanded to a lower pressure providing another cold stream. This cold stream is used to cool the remaining feed gas stream in a second indirect heat exchange process. The expanded stream exiting from the second heat exchange process is used for supplemental cooling in the first indirect heat exchange step. The remaining feed gas is subsequently expanded to a lower pressure, thereby partially liquefying this feed gas stream. The liquefied fraction of this stream is withdrawn from the process as LNG having a temperature corresponding to the bubble point pressure.

1 . A process for liquefying a gas stream rich in methane, said process comprising:
 providing said gas stream at a pressure less than 1,000 psia;   providing a refrigerant at a pressure of less than 1,000 psia;   compressing said refrigerant to a pressure greater than or equal to 1500 psia to provide a compressed refrigerant;   cooling said compressed refrigerant by indirect heat exchange with a cooling fluid;   expanding said compressed refrigerant to further cool said compressed refrigerant, thereby producing an expanded, cooled refrigerant;   passing said expanded, cooled refrigerant to a heat exchange area; and   passing said gas stream through said heat exchange area to cool at least part of said gas stream by indirect heat exchange with said expanded, cooled refrigerant, thereby forming a cooled gas stream.   
   
   
       2 . The process of  claim 1  wherein providing said refrigerant at a pressure of less than 1,000 psia comprises withdrawing a portion of said gas stream for use as said refrigerant. 
   
   
       3 . The process of  claim 2  wherein said portion of said gas stream is withdrawn before said gas stream is passed to said heat exchange area. 
   
   
       4 . The process of  claim 2  wherein said portion of said gas stream is withdrawn from said heat exchange area. 
   
   
       5 . The process of  claim 1  further comprising providing at least a portion of the refrigeration duty for said heat exchange area using a closed loop charged with a flash vapor produced in said process for liquefying a gas stream rich in methane. 
   
   
       6 . The process of  claim 5  further comprising:
 expanding at least a portion of said cooled gas stream to produce an expanded, cooled gas stream; and   further cooling said expanded, cooled gas stream by indirect heat exchange with said closed loop charged with the flash vapor.   
   
   
       7 . The process of  claim 1  further comprising:
 expanding at least a portion of said cooled gas stream to produce an expanded, cooled gas stream; and   further cooling said expanded, cooled gas stream by indirect heat exchange in one or more additional heat exchange areas.   
   
   
       8 . The process of  claim 1  further comprising:
 cooling said gas stream using a plurality of work expansion devices, each of said work expansion devices expanding a portion of the feed gas stream and thereby cooling said portion to form one or more expanded, cooled side streams, wherein each of said portions of the feed gas stream expanded in said work expansion devices is withdrawn from said feed gas stream at a different stage of feed gas stream cooling; and   cooling said feed gas stream by indirect heat exchange with said one or more expanded, cooled side streams.   
   
   
       9 . The process of  claim 1  further comprising:
 withdrawing one or more portions of said gas stream;   passing each of said one or more portions of said gas stream to one or more work expansion devices and expanding each of said one of more portions of said gas stream to expand and cool said one or more portions, thereby forming one or more expanded, cooled side streams;   passing said one or more expanded, cooled side streams to at least one heat exchange area;   passing said gas stream through said at least one heat exchange area; and   at least partially cooling said gas stream by indirect heat exchange with said one or more expanded, cooled side streams.   
   
   
       10 . The process of  claim 6 ,  7 ,  8 , or  9  wherein said gas stream is first compressed to a pressure above the gas supply pressure. 
   
   
       11 . The process of  claim 1  further comprising an expansion stage of said cooled gas stream before a final heat exchange step and prior to expansion to produce LNG. 
   
   
       12 . The process of  claim 1  further comprising:
 expanding at least a portion of said cooled gas stream before a final heat exchange step to produce an expanded, cooled gas stream;   passing a portion of said expanded, cooled gas stream to a work-producing expander and further expanding said portion of said expanded, cooled gas stream in said work-producing expander; and   passing the stream emerging from said work-producing expander to a heat exchange area to further cool the balance of said expanded, cooled gas stream by indirect heat exchange in said heat exchange area.   
   
   
       13 . The process of  claim 1  wherein said refrigerant is compressed to a pressure greater than or equal to 3,000 psia to provide a compressed refrigerant. 
   
   
       14 . The process of  claim 1  wherein said heat exchange area comprises multiple heat exchange chambers. 
   
   
       15 . The process of  claim 1  further comprising:
 a sub-cooling heat exchange area receiving said gas stream and cooled by expansion of a second refrigerant to provide a sub-cooled gas stream;   followed by final expansion of said sub-cooled gas stream and recovery of LNG.   
   
   
       16 . The process of  claim 15  wherein said second refrigerant is a portion of said gas stream rich in methane. 
   
   
       17 . The process of  claim 15  wherein said second refrigerant is sub-cooled in said sub-cooling heat exchange area prior to expansion of said second refrigerant. 
   
   
       18 . The process of  claim 16  wherein said gas stream rich in methane is re-pressurized before passing through said heat exchange area, said cooled gas stream is expanded, and a portion of said expanded, cooled gas stream is further expanded and used as said second refrigerant in said sub-cooling heat exchange area. 
   
   
       19 . The process of  claim 15  wherein a portion of said sub-cooled gas stream is expanded and a portion thereof is said second refrigerant. 
   
   
       20 . The process of  claim 19  wherein said portion of said sub-cooled gas stream is split into two partial streams, one of said partial streams is further expanded, and both of said partial streams comprise said second refrigerant. 
   
   
       21 . The process of  claim 1  further comprising rejecting nitrogen with LNG recovery. 
   
   
       22 . A process for liquefying a gas stream rich in methane, said process comprising:
 providing said gas stream at a pressure less than 1,000 psia;   providing a refrigerant in a closed loop;   compressing said refrigerant to a pressure greater than or equal to 1500 psia to provide a compressed refrigerant;   cooling said compressed refrigerant by indirect heat exchange with a cooling fluid;   expanding said compressed refrigerant to further cool said compressed refrigerant, thereby producing an expanded, cooled refrigerant;   passing said expanded, cooled refrigerant to a heat exchange area; and   passing said gas stream through said heat exchange area to cool at least part of said gas stream by indirect heat exchange with said expanded, cooled refrigerant.