System and method for the production of liquefied natural gas

Abstract

A method for producing liquefied natural gas (LNG) and separating natural gas liquids (NGLs) from the LNG is provided. The method may include compressing natural gas to compressed natural gas, removing a non-hydrocarbon from the compressed natural gas, and cooling the compressed natural gas to a cooled, compressed natural gas. The method may also include expanding a first portion and a second portion of the cooled, compressed natural gas in a first expansion element and a second expansion element to generate a first refrigeration stream and a second refrigeration stream, respectively. The method may further include separating a third portion of the cooled, compressed natural gas into a methane lean natural gas fraction containing the NGLs and a methane rich natural gas fraction. The methane rich natural gas fraction may be cooled in a liquefaction assembly with the first and second refrigeration streams to thereby produce the LNG.

We claim: 
     
         1 . A method for producing liquefied natural gas from a natural gas source and separating natural gas liquids from the liquefied natural gas produced, comprising:
 compressing natural gas from the natural gas source in a compression assembly fluidly coupled with the natural gas source to produce a compressed natural gas;   removing a non-hydrocarbon from the compressed natural gas in a separator fluidly coupled with the compression assembly;   cooling the compressed natural gas to a cooled, compressed natural gas with a cooling assembly in thermal communication with the compression assembly;   expanding a first portion of the cooled, compressed natural gas in a first expansion element to generate a first refrigeration stream;   expanding a second portion of the cooled, compressed natural gas in a second expansion element to generate a second refrigeration stream;   feeding a third portion of the cooled, compressed natural gas to a distillation column of a distillation assembly;   separating the third portion of the cooled, compressed natural gas into a methane lean natural gas fraction and a methane rich natural gas fraction in the distillation column of the distillation assembly, the methane lean natural gas fraction containing the natural gas liquids;   feeding the methane rich natural gas fraction to a liquefaction assembly; and   cooling at least a portion of the methane rich natural gas fraction with the first refrigeration stream and the second refrigeration stream in the liquefaction assembly to thereby produce the liquefied natural gas.   
     
     
         2 . The method of  claim 1 , wherein the third portion of the cooled, compressed natural gas is flowed to a middle zone of the distillation column. 
     
     
         3 . The method of  claim 2 , further comprising storing the liquefied natural gas in a storage tank. 
     
     
         4 . The method of  claim 3 , wherein separating the third portion of the cooled, compressed natural gas into the methane lean natural gas fraction and the methane rich natural gas fraction comprises:
 at least partially cooling the third portion of the cooled, compressed natural gas in a top zone of the distillation column.   
     
     
         5 . The method of  claim 4 , wherein at least partially cooling the third portion of the cooled, compressed natural gas comprises:
 pumping a portion of the liquefied natural gas from the liquefaction assembly to the top zone of the distillation column.   
     
     
         6 . The method of  claim 5 , wherein separating the third portion of the cooled, compressed natural gas into the methane lean natural gas fraction and the methane rich natural gas fraction further comprises:
 at least partially condensing the third portion of the cooled, compressed natural gas to thereby produce the natural gas liquids; and   collecting the natural gas liquids in a bottom zone of the distillation column.   
     
     
         7 . The method of  claim 4 , wherein separating the third portion of the cooled, compressed natural gas into the methane lean natural gas fraction and the methane rich natural gas fraction comprises:
 at least partially heating the third portion of the cooled, compressed natural gas in a bottom zone of the distillation column.   
     
     
         8 . The method of  claim 7 , wherein at least partially heating the third portion of the cooled, compressed natural gas comprises:
 feeding a heat transfer medium to a heat exchanger fluidly coupled with the bottom zone of the distillation column.   
     
     
         9 . The method of  claim 7 , wherein at least partially heating the third portion of the cooled, compressed natural gas produces methane vapors. 
     
     
         10 . The method of  claim 9 , wherein separating the third portion of the cooled, compressed natural gas into the methane lean natural gas fraction and the methane rich natural gas fraction further comprises:
 collecting the methane vapors in the top zone of the distillation column.   
     
     
         11 . The method of  claim 1 , further comprising precooling the third portion of the cooled, compressed natural gas with the methane rich natural gas fraction in a heat exchanger of the distillation assembly before feeding the third portion of the cooled, compressed natural gas to the distillation column of the distillation assembly. 
     
     
         12 . The method of  claim 1 , further comprising separating a portion of the natural gas liquids from the first portion of the cooled, compressed natural gas in a liquid separator before expanding the first portion of the cooled, compressed natural gas in the first expansion element. 
     
     
         13 . The method of  claim 12 , further comprising pumping the portion of the natural gas liquids from the liquid separator to the distillation column of the distillation assembly. 
     
     
         14 . A system for producing liquefied natural gas and separating natural gas liquids from the liquefied natural gas produced, comprising:
 a compressor configured to compress a process stream containing natural gas to a compressed process stream;   a separator fluidly coupled with the compressor and configured to receive the compressed process stream and at least partially separate water and carbon dioxide from the natural gas contained in the compressed process stream;   a mechanical chiller in thermal communication with the compressor and configured to cool the natural gas in the compressed process stream to cooled, compressed natural gas;   a turbo-expander fluidly coupled with the compressor and configured to expand a first portion of the cooled, compressed natural gas to generate a first refrigeration stream;   an expansion valve fluidly coupled with the compressor and configured to expand a second portion of the cooled, compressed natural gas to generate a second refrigeration stream;   a first heat exchanger fluidly coupled with the turbo-expander and configured to cool the cooled, compressed natural gas with the first refrigeration stream;   a second heat exchanger fluidly coupled with the expansion valve and configured to cool the cooled, compressed natural gas to subcritical natural gas with the second refrigeration stream to thereby produce the liquefied natural gas;   a distillation column having a middle zone fluidly coupled with the compressor upstream of the expansion valve via a feed line and configured to receive a third portion of the cooled, compressed natural gas;   a third heat exchanger fluidly coupled with and disposed downstream from a bottom zone of the distillation column via a first discharge line; and   a second discharge line fluidly coupling a top zone of the distillation column with the second heat exchanger upstream of the expansion valve.   
     
     
         15 . The system of  claim 14 , further comprising a fourth heat exchanger fluidly coupled with and disposed upstream of the middle zone of the distillation column via the feed line, the fourth heat exchanger further fluidly coupled with and disposed downstream from the top zone of the distillation column via the second discharge line. 
     
     
         16 . The system of  claim 14 , further comprising a liquid separator fluidly coupled with and disposed upstream of the turbo-expander and configured to separate a portion of the natural gas liquids from the first portion of the cooled, compressed natural gas. 
     
     
         17 . The system of  claim 16 , wherein the liquid separator is fluidly coupled with the middle zone of the distillation column. 
     
     
         18 . The system of  claim 17 , further comprising a pump fluidly coupled with and disposed downstream from the liquid separator and configured to pump the portion of the natural gas liquids from the liquid separator to the middle zone of the distillation column. 
     
     
         19 . The system of  claim 14 , further comprising:
 a storage tank fluidly coupled with the second heat exchanger and configured to receive and store the liquefied natural gas; and   a cooling line fluidly coupling the storage tank with the top zone of the distillation column.   
     
     
         20 . A system for producing liquefied natural gas from a natural gas source and separating natural gas liquids from the liquefied natural gas produced, comprising:
 a compressor configured to compress a process stream containing natural gas to a compressed process stream;   a separator fluidly coupled with the compressor and configured to receive the compressed process stream and at least partially separate water and carbon dioxide from the natural gas contained in the compressed process stream;   a mechanical chiller in thermal communication with the compressor and configured to cool the natural gas in the compressed process stream to cooled, compressed natural gas;   a turbo-expander fluidly coupled with the compressor and configured to expand a first portion of the cooled, compressed natural gas to generate a first refrigeration stream;   an expansion valve fluidly coupled with the compressor and configured to expand a second portion of the cooled, compressed natural gas to generate a second refrigeration stream;   a first heat exchanger fluidly coupled with the turbo-expander and configured to cool the cooled, compressed natural gas with the first refrigeration stream;   a second heat exchanger fluidly coupled with the expansion valve and configured cool the cooled, compressed natural gas to the liquefied natural gas with the second refrigeration stream;   a storage tank fluidly coupled with the second heat exchanger and configured to receive and store the liquefied natural gas;   a distillation assembly configured to receive a third portion of the cooled, compressed natural gas and separate at least a portion the natural gas liquids from the third portion of the cooled, compressed natural gas, the distillation assembly comprising:
 a distillation column having a middle zone fluidly coupled with the compressor upstream of the expansion valve via a feed line and configured to receive the third portion of the cooled, compressed natural gas; 
 a third heat exchanger fluidly coupled with and disposed downstream from a bottom zone of the distillation column via a first discharge line; 
 a second discharge line fluidly coupling a top zone of the distillation column with the second heat exchanger; 
 a fourth heat exchanger fluidly coupled with and disposed upstream of the distillation column via the feed line and downstream of the distillation column via the second discharge line; 
 a cooling line fluidly coupling the storage tank with the top zone of the distillation column; and 
 a pump fluidly coupled with the cooling line and configured to pump a portion of the liquefied natural gas from the storage tank to the top zone of the distillation column.