Method and system for treating and liquefying natural gas

Abstract

A method for liquefying natural gas comprises: condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and separating the partially condensed stream into a first liquid fraction and a first vapour fraction; wherein the first liquid fraction comprises liquefied natural gas.

We claim: 
     
         1 . A method for liquefying natural gas, the method comprising:
 condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and   separating the partially condensed stream into a first liquid fraction and a first vapour fraction;   wherein the first liquid fraction comprises liquefied natural gas.   
     
     
         2 . The method of  claim 1 , wherein separating the partially condensed stream into the first liquid fraction and the first vapour fraction comprises directing the partially condensed stream into a first flash separator to concentrate nitrogen in the first vapour fraction and reduce the concentration of nitrogen in the first liquid fraction. 
     
     
         3 . The method of  claim 1 , further comprising:
 condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction; and   separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction;   wherein the second liquid fraction comprises liquefied natural gas.   
     
     
         4 . The method of  claim 3 , wherein condensing at least a portion of the first vapour fraction comprises heating the first vapour fraction in a first indirect heat exchanger with the natural gas feed stream, thereby providing a portion of the cooling and condensing duty and reducing the refrigeration duty, compressing, after-cooling in a second indirect heat exchanger, and then cooling and partially condensing the first vapour fraction in the first indirect heat exchanger. 
     
     
         5 . The method of  claim 3 , wherein separating the partially condensed first vapour fraction into the second liquid fraction and the second vapour fraction comprises directing the partially condensed first vapour fraction into a second flash separator to concentrate nitrogen in the second vapour fraction and reduce the concentration of nitrogen in the second liquid fraction, thereby reducing methane component loss to the fuel gas product stream and increasing the liquefied natural gas (LNG) product yield. 
     
     
         6 . The method of  claim 5 , further comprising compressing, after-cooling in a separate exchanger, and then cooling and partially condensing the second vapour fraction to produce a partially condensed second vapour fraction, directing the partially condensed second vapour fraction into a third flash separator to concentrate nitrogen in a third vapour fraction and reduce the concentration of nitrogen in a third liquid fraction, thereby further reducing methane component loss to the fuel gas product stream and further increasing the liquefied natural gas (LNG) product yield. 
     
     
         7 . The method of  claim 1 , further comprising condensing heavy hydrocarbon components in the natural gas feed stream and separating the natural gas feed stream into a heavy hydrocarbon liquid fraction and a residual vapour fraction, wherein at least a portion of the residual vapour fraction is condensed and directed to the partially condensed stream. 
     
     
         8 . The method of  claim 7 , further comprising partially flashing the heavy hydrocarbon liquid fraction and distilling heavy hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a residual fraction, wherein the residual fraction is directed to the natural gas feed stream for further processing. 
     
     
         9 . The method of  claim 1 , further comprising directing at least a portion of the first vapour fraction to one or more refrigerant cycles for use as a refrigerant medium. 
     
     
         10 . The method of  claim 9 , wherein the one or more refrigerant cycles comprises a first methane refrigerant cycle that provides a portion of the duty required to cool and condense at least one of:
 the natural gas feed stream into the partially condensed stream;   the first vapour fraction into the partially condensed first vapour fraction;   the second vapour fraction into the partially condensed second vapour fraction; and   the heavy hydrocarbon components in the natural gas feed stream; and/or to pre-cool the methane refrigerant stream.   
     
     
         11 . The method of  claim 10 , wherein the first methane refrigerant cycle comprises the following steps:
 compressing at least a portion of the first vapour fraction, which is a low pressure methane refrigerant vapour stream, to produce an outlet stream;   compressing and pre-cooling the outlet stream to produce a pre-cooled high pressure methane refrigerant stream;   expanding the pre-cooled high pressure methane refrigerant stream to produce expanded and cooled methane refrigerant stream;   heating the expanded and cooled methane refrigerant stream to produce the low pressure methane refrigerant vapour stream, thus completing the first methane refrigerant cycle.   
     
     
         12 . The method of  claim 10 , wherein the one or more refrigerant cycles comprises a second methane refrigerant cycle that provides a portion of the duty required to pre-cool the first vapour fraction for use in the first methane refrigerant cycle. 
     
     
         13 . The method of  claim 12 , wherein the second methane refrigerant cycle further provides a portion of the duty required to cool and condense at least one of:
 the natural gas feed stream into the partially condensed stream;   the first vapour fraction into the partially condensed first vapour fraction;   the second vapour fraction into the partially condensed second vapour fraction; and   the heavy hydrocarbon components in the natural gas feed stream.   
     
     
         14 . The method of  claim 12 , wherein the second methane refrigerant cycle comprises the following steps:
 compressing at least a portion of the first vapour fraction, which is a low pressure pre-cooling methane refrigerant vapour stream, to produce an outlet stream;   compressing the outlet stream to produce a pre-cooled methane refrigerant stream;   expanding the pre-cooled methane refrigerant stream to produce expanded and cooled pre-cooling methane refrigerant stream;   heating the expanded and cooled pre--cooling methane refrigerant stream to produce the low pressure pre-cooling methane refrigerant vapour stream, thus completing the second methane refrigerant cycle.   
     
     
         15 . The method of  claim 12 , wherein the second methane refrigerant cycle is used in place of a propane refrigeration cycle. 
     
     
         16 . The method of  claim 1 , Wherein at least a portion of the duty required to cool at least one of:
 the first vapour fraction for use in the first methane refrigerant cycle;   the natural gas feed stream into the partially condensed stream;   the first vapour fraction into the partially condensed first vapour fraction;   the second vapour fraction into the partially condensed second vapour fraction; and   the heavy hydrocarbon components in the natural gas feed stream is provided by a two-stage closed loop propane refrigeration cycle.   
     
     
         17 . The method of  claim 16 , wherein the propane refrigeration cycle comprises a high pressure cycle and a low pressure cycle, linked through a region of intermediate pressure. 
     
     
         18 . The method of  claim 17 , wherein the propane refrigeration cycle comprises the following steps:
 reducing the pressure on a high pressure saturated liquid phase propane refrigerant stream to produce an intermediate pressure stream;   separating the intermediate pressure stream into an intermediate pressure propane refrigerant liquid fraction and an intermediate pressure propane vapour fraction;   heating a first portion of the intermediate pressure propane refrigerant liquid fraction to produce a first vapour-liquid stream;   combining the first vapour-liquid stream with the intermediate pressure stream;   reducing the pressure on a second portion of the intermediate pressure propane refrigerant liquid fraction to produce a low pressure stream;   separating the low pressure stream into a low pressure propane refrigerant liquid fraction and a low pressure propane refrigerant vapour fraction;   heating the low pressure propane refrigerant liquid fraction to produce a second vapour-liquid stream;   combining the second vapour-liquid stream with the low pressure stream,   compressing the low pressure propane refrigerant vapour fraction to produce a stream that is combined with the intermediate pressure propane refrigerant vapour fraction to produce a combined intermediate pressure propane refrigerant vapour fraction;   compressing the combined intermediate pressure propane refrigerant vapour fraction to produce a high pressure propane refrigerant vapour stream; and   condensing the high pressure propane refrigerant vapour stream to produce the high pressure saturated liquid phase propane refrigerant stream, thus completing the propane refrigeration cycle.   
     
     
         19 . A system for liquefying natural gas, The system comprising:
 a first condensing system for condensing at least a portion of a natural gas feed stream to produce a partially condensed stream; and   a first flash separator for separating the partially condensed stream into a first liquid fraction and a first vapour fraction;   wherein the first liquid fraction comprises liquefied natural gas.   
     
     
         20 . The system of  claim 19 , wherein the first flash separator concentrates nitrogen in the first vapour fraction and reduces the concentration of nitrogen in the first liquid fraction. 
     
     
         21 . The system of  claim 19 , further comprising:
 a second condensing system for condensing at least a portion of the first vapour fraction to produce a partially condensed first vapour fraction; and   a second flash separator for separating the partially condensed first vapour fraction into a second liquid fraction and a second vapour fraction;   wherein the second liquid fraction comprises liquefied natural gas.   
     
     
         22 . The system of  claim 21 , wherein the second flash separator concentrates nitrogen in the second vapour fraction and reduces the concentration of nitrogen in the second liquid fraction. 
     
     
         23 . The system of  claim 22 , further comprising a third condensing system and a third flash separator for concentrating nitrogen in a third vapour fraction and reducing the concentration of nitrogen in a third liquid fraction. 
     
     
         24 . The system of  claim 19 , further comprising:
 a heavy hydrocarbon condensing system for condensing heavy hydrocarbon components in the natural gas feed stream; and   a flash separator for separating the natural gas feed stream into a heavy hydrocarbon liquid fraction and a residual vapour fraction and directing at least a portion of the residual vapour fraction to the partially condensed stream.   
     
     
         25 . The system of  claim 24 , wherein the heavy hydrocarbon condensing system comprises at least one of an inlet gas compressor, an inlet gas compressor after-cooler, and an indirect heat exchanger. 
     
     
         26 . The system of  claim 24 , further comprising a heavy hydrocarbon distiller for distilling, heavy hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a residual fraction, and directing the residual fraction to the natural gas feed stream for further processing. 
     
     
         27 . The system of  claim 19 , further comprising a valve for directing at least a portion of the first vapour fraction to one or more refrigerant cycles for use as a refrigerant medium. 
     
     
         28 . The system of  claim 27 , wherein the one or more refrigerant cycles comprises a first methane refrigerant cycle that provides a portion of the duty required to cool and condense at least one of:
 the natural gas feed stream into the partially condensed stream;   the first vapour fraction into the partially condensed first vapour fraction;   the second vapour fraction into the partially condensed second vapour fraction; and   the heavy hydrocarbon components in the natural gas feed stream; and/or to pre-cool the methane refrigerant stream,   wherein the first methane refrigerant cycle comprises at least one of an expander brake compressor, a first heat exchanger, a multi-stage methane refrigerant compressor, a second heat exchanger, an indirect heat exchanger, a methane refrigerant expander, and an expander brake compressor.   
     
     
         29 . The system of  claim 27 , wherein the one or more refrigerant cycles comprises a second methane refrigerant cycle that provides a portion of the duty required to pre-cool the first vapour fraction for use in the first methane refrigerant cycle. 
     
     
         30 . The system of  claim 29 , wherein the second methane refrigerant cycle further provides a portion of the duty required to cool and condense at least one of:
 the natural gas feed stream into the partially condensed stream;   the first vapour fraction into the partially condensed first vapour fraction;   the second vapour fraction into the partially condensed second vapour fraction; and   the heavy hydrocarbon components in the natural gas feed stream,   wherein the second methane refrigerant cycle comprises at least one of a pre cooling expander brake compressor, a first heat exchanger, a multi-stage pre-cooling methane refrigerant compressor, a second heat exchanger, a methane expander, and an indirect heat exchanger.   
     
     
         31 . The system of  claim 27 , wherein the second methane refrigerant cycle is used in place of a propane refrigeration cycle. 
     
     
         32 . The system of  claim 19 , wherein at least a portion of the duty required to cool at least one of:
 the first vapour fraction for use in the first methane refrigerant cycle;   the natural gas feed stream into the partially condensed stream;   the first vapour fraction into the partially condensed first vapour fraction;   the second vapour fraction into the partially condensed second vapour fraction; and   the heavy hydrocarbon components in the natural gas feed stream is provided by a two-stage closed loop propane refrigeration cycle.   
     
     
         33 . The system of  claim 32 , wherein the propane refrigeration cycle comprises a high pressure cycle and a low pressure cycle, linked through a region of intermediate pressure. 
     
     
         34 . A methane refrigerant system for use in liquefying natural gas, the system comprising a methane gas source derived at least in part from a vapour fraction of the natural gas feed stream.