Semi-closed loop lng process

Abstract

A semi-closed loop system for producing liquefied natural gas (LNG) that combines certain advantages of closed-loop systems with certain advantages of open-loop systems to provide a more efficient and effective hybrid system. In the semi-closed loop system, the final methane refrigeration cycle provides significant cooling of the natural gas stream via indirect heat transfer, as opposed to expansion-type cooling. A minor portion of the LNG product from the methane refrigeration cycle is used as make-up refrigerant in the methane refrigeration cycle. A pressurized portion of the refrigerant from the methane refrigeration cycle is employed as fuel gas. Excess refrigerant from the methane refrigeration cycle can be recombined with the processed natural gas stream, rather than flared.

1 . A method of liquefying natural gas, said method comprising the steps of:
 (a) cooling the natural gas by indirect heat exchange with a predominantly methane refrigerant, thereby providing liquefied natural gas, wherein said cooling reduces the temperature of the natural gas by at least 100° F.;   (b) flashing at least a portion of the liquefied natural gas to thereby provide a predominantly vapor fraction and a predominantly liquid fraction; and   (c) combining at least a portion of the predominantly vapor fraction with the predominantly methane refrigerant used to cool the natural gas in step (a),   wherein the pressure of the natural gas prior to the cooling of step (a) within about 50 psi of the liquefied natural gas stream subsequent to step (a).   
     
     
         2 . A method according to  claim 1 , further comprising the step of combining at least a portion of the predominantly methane refrigerant with the natural gas stream upstream of the cooling carried out in step (a). 
     
     
         3 . A method according to  claim 1 , said predominately methane refrigerant comprising less than 10 mole percent nitrogen. 
     
     
         4 . A method according to  claim 1 , said cooling of step (a) being carried out in a series of at least two separate methane heat exchangers. 
     
     
         5 . A method according to  claim 1 , further comprising the steps of separating the predominantly vapor fraction and the predominantly liquid fraction in a separator prior to step (c) and subsequent to step (b); and conducting the predominantly liquid fraction from the separator to a liquefied natural gas storage tank. 
     
     
         6 . A method according to  claim 1 , further comprising the step of compressing the combined predominantly methane refrigerant and predominantly vapor fraction in a methane compressor, thereby providing a compressed refrigerant stream. 
     
     
         7 . A method according to  claim 6 , further comprising the step of using a first portion of the compressed refrigerant stream as the predominantly methane refrigerant. 
     
     
         8 . A method according to  claim 7 , further comprising the step of using a second portion of the compressed refrigerant stream as fuel gas. 
     
     
         9 . A method according to  claim 1 , further comprising the step of cooling at least a portion of the natural gas via indirect heat exchange with a first refrigerant comprising predominantly propane, propylene, or carbon dioxide. 
     
     
         10 . A method according to  claim 9 , further comprising the step of cooling at least a portion of the predominantly methane refrigerant via indirect heat exchange with the first refrigerant. 
     
     
         11 . A method according to  claim 9 , further comprising the step of cooling at least a portion of the natural gas via indirect heat exchange with a second refrigerant comprising predominantly ethane, ethylene, or carbon dioxide. 
     
     
         12 . A method according to  claim 11 , further comprising the step of cooling at least a portion of the predominantly methane refrigerant via indirect heat exchange with the second refrigerant, thereby providing a cooled predominantly methane refrigerant. 
     
     
         13 . A method according to  claim 12 , further comprising the step of combining a first portion of the cooled predominantly methane refrigerant with the natural gas. 
     
     
         14 . A method according to  claim 13 , further comprising the steps of, removing heavy hydrocarbon components from the natural gas in a heavies removal column prior to step (a), thereby providing a removed heavies stream and a heavies-reduced natural gas stream, and combining said first portion of the cooled predominantly methane refrigerant with the heavies-reduced natural gas stream. 
     
     
         15 . A method according to  claim 13 , further comprising the step of cooling a second portion of the cooled predominantly methane refrigerant via indirect heat exchange with the second refrigerant, thereby providing a further cooled predominantly methane refrigerant. 
     
     
         16 . A method according to  claim 15 , wherein step (a) includes using at least a portion of said further cooled predominantly methane refrigerant as the predominantly methane refrigerant to cool the natural gas via indirect heat exchange. 
     
     
         17 . A method according to  claim 1 , further comprising the step of cooling the natural gas via indirect heat exchange with a first refrigerant comprising less than 50 mole percent methane prior to step (a). 
     
     
         18 . A method according to  claim 17 , said first refrigerant comprising predominantly propane, propylene, ethane, ethylene, or carbon dioxide. 
     
     
         19 . A method according to  claim 18 , further comprising the step of separating the natural gas into a first lights stream and a first heavies stream in a first column prior to step (a) and subsequent to the step of  claim 17 . 
     
     
         20 . A method according to  claim 19 , further comprising the step of separating the first heavies stream into a second lights stream and a second heavies stream in a second column. 
     
     
         21 . A method according to  claim 20 , further comprising the step of cooling the second lights stream via indirect heat exchange with the predominantly methane refrigerant. 
     
     
         22 . A method according to  claim 21 , further comprising the step of conducting the second lights stream from the second column to the cooling of the step of  claim 21  without compressing the second lights stream. 
     
     
         23 . A method according to  claim 1 , steps (a)-(c) being carried out in a cascade-type liquefied natural gas facility having at least three sequential cooling cycles, each employing a different refrigerant. 
     
     
         24 . A method according to  claim 1 , further comprising the step of vaporizing liquefied natural gas produced via steps (a)-(c). 
     
     
         25 . A method of liquefying natural gas, said process comprising the steps of:
 (a) cooling a natural gas stream with a first refrigeration cycle via indirect heat exchange with a first refrigerant comprising predominantly propane, propylene, or carbon dioxide to thereby provide a first cooled natural gas stream;   (b) downstream of the first refrigeration cycle, cooling at least a portion of the first cooled natural gas stream with a second refrigeration cycle via indirect heat exchange with a second refrigerant comprising predominantly ethane, ethylene, or carbon dioxide to thereby provide a second cooled natural gas stream;   (c) downstream of the second refrigeration cycle, introducing at least a portion of the second cooled natural gas stream into an open-loop methane refrigeration cycle;   (d) cooling at least a portion of the second cooled natural gas stream introduced into said open-loop methane refrigeration cycle by indirect heat exchange with a predominantly methane refrigerant; and   (e) cooling at least a portion of the predominantly methane refrigerant in the second refrigeration cycle via indirect heat exchange with the second refrigerant,   wherein said cooling of step (c) reduces the temperature of the natural gas stream by at least 40° F.,   wherein the portion of the second cooled natural gas stream introduced into the open-loop methane refrigeration cycle comprises greater than 25 mole percent vapor.   
     
     
         26 . A method according to  claim 25 , further comprising the step of cooling the predominantly methane refrigerant in the first refrigeration cycle via indirect heat exchange with the first refrigerant. 
     
     
         27 . A method according to  claim 25 , further comprising the steps of, flashing the natural gas stream downstream of the methane refrigeration cycle to thereby provide a predominantly vapor fraction and a predominantly liquid fraction; and combining the predominantly vapor fraction with the predominantly methane refrigerant in the methane refrigeration cycle. 
     
     
         28 . A method according to  claim 25 , further comprising the steps of compressing the predominantly methane refrigerant in a methane compressor, thereby providing a compressed predominantly methane refrigerant; using a first portion of the compressed predominantly methane refrigerant as a refrigerant in the methane refrigeration cycle; and using a second portion of the compressed predominantly methane refrigerant as fuel gas. 
     
     
         29 . A method according to  claim 25 , said cooling of step (c) being performed by a series of at least two methane heat exchangers, each of said methane heat exchangers facilitating indirect heat exchange between the natural gas and the predominantly methane refrigerant. 
     
     
         30 . A method according to  claim 29 , said methane heat exchangers being separate from one another. 
     
     
         31 . A method according to  claim 29 , said series of methane heat exchangers including at least three separate heat exchangers. 
     
     
         32 . A method according to  claim 25 , step (c) including cooling the natural gas stream at least 60° F. 
     
     
         33 . A method according to  claim 25 , further comprising the step of vaporizing liquefied natural gas produced via steps (a)-(d). 
     
     
         34 . An apparatus for liquefying natural gas, said apparatus comprising:
 a first refrigeration cycle comprising a first heat exchanger defining a first cooling pass, wherein said first cooling pass defines a first warm fluid inlet and a first cool fluid outlet;   a first distillation column defining a first fluid inlet, a first vapor outlet, and a first liquid outlet, wherein said first fluid inlet is in fluid flow communication with said first cool fluid outlet of said first cooling pass;   a second distillation column defining a second fluid inlet, a second vapor outlet, and a second liquid outlet, wherein said second fluid inlet is in fluid flow communication with said first liquid outlet of said first distillation column;   a second refrigeration cycle comprising a second heat exchanger and a second refrigerant compressor,   wherein said second heat exchanger defines a second cooling pass, a third cooling pass, and a refrigerant inlet, wherein said second and third cooling passes are parallel cooling passes; wherein said second cooling pass defines a second warm fluid inlet and a second cool fluid outlet, wherein said third cooling pass defines a third warm fluid inlet and a third cool fluid outlet, wherein said second warm fluid inlet is in fluid flow communication with said first vapor outlet of said first distillation column, wherein said third warm fluid inlet of said second heat exchanger is in fluid flow communication with said second vapor outlet of said second distillation column.   
     
     
         35 . An apparatus according to  claim 34 , wherein said first vapor outlet is not in fluid flow communication with said third warm fluid inlet, wherein said second vapor outlet is not in fluid flow communication with said second warm fluid inlet. 
     
     
         36 . An apparatus according to  claim 34 , wherein said second refrigeration cycle comprises an open-loop methane refrigeration cycle. 
     
     
         37 . An apparatus according to  claim 36 , wherein said first refrigeration cycle is a propane, propylene, ethane, ethylene, or carbon dioxide refrigeration cycle.