Method for the production of liquefied natural gas

Abstract

A method for the production of liquefied natural gas is provided. The method may include providing a high pressure natural gas stream, splitting the high pressure natural gas stream into a first portion and a second portion, and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream. The refrigeration needed for cooling and liquefaction of the natural gas can be provided by a closed nitrogen refrigeration cycle and letdown of the second portion of the high pressure natural gas stream.

We claim: 
     
         1 . A method for the production of liquefied natural gas (“LNG”), the method comprising the steps of:
 a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle is configured to provide refrigeration within a heat exchanger; 
 b) purifying a first natural gas stream in a first purification unit to remove a first set of impurities to produce a purified first natural gas stream; 
 c) cooling and liquefying the first natural gas stream in the heat exchanger using the refrigeration from the nitrogen refrigeration cycle to produce an LNG stream, wherein the first natural gas stream has an LNG refrigeration requirement, wherein the LNG stream is liquefied at a first pressure P H ; 
 d) purifying a second natural gas stream in a second purification unit to remove a second set of impurities to produce a purified second natural gas stream; 
 e) partially cooling the second natural gas stream in the heat exchanger; 
 f) withdrawing the partially cooled second natural gas stream from an intermediate section of the heat exchanger; 
 g) expanding the partially cooled second natural gas stream to a medium pressure P M  in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P M  is at a pressure lower than the first pressure P H ; and 
 h) warming the cold natural gas stream in the heat exchanger by heat exchange against the first natural gas stream to produce a warm natural gas stream at the warm end of the heat exchanger, 
 wherein the natural gas expansion turbine drives a first booster, 
 wherein the LNG refrigeration requirement is supplied by a combination of refrigeration from the nitrogen refrigeration cycle and step h). 
 
     
     
         2 . The method as claimed in  claim 1 , wherein the first booster is configured to compress the second natural gas stream or a stream derived from the second natural gas stream. 
     
     
         3 . The method as claimed in  claim 1 , wherein the first booster is configured to compress a stream selected from the group consisting of the first natural gas stream, the first purified natural gas stream, the second natural gas stream, the purified second natural gas stream, the partially cooled natural gas stream, the warm natural gas stream, and a nitrogen fluid within the nitrogen refrigeration cycle. 
     
     
         4 . The method as claimed in  claim 1 , wherein the first set of impurities has a freezing point at or above the liquefaction temperature of methane at the first pressure P H . 
     
     
         5 . The method as claimed in  claim 1 , wherein the second set of impurities comprises water. 
     
     
         6 . The method as claimed in  claim 1 , wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster. 
     
     
         7 . The method as claimed in  claim 1 , wherein the first natural gas stream and the second natural gas stream come from the same natural gas source. 
     
     
         8 . The method as claimed in  claim 7 , wherein the natural gas source is a natural gas pipeline having a pressure between 15 and 100 bara. 
     
     
         9 . The method as claimed in  claim 1 , wherein the first natural gas stream comes from a first natural gas source, and the second natural gas stream comes from a second natural gas source, wherein the first and second natural gas sources are different sources. 
     
     
         10 . The method as claimed in  claim 9 , wherein the first natural gas source comprises a natural gas pipeline. 
     
     
         11 . The method as claimed in  claim 10 , wherein the natural gas pipeline has a pressure between 15 and 100 bara. 
     
     
         12 . The method as claimed in  claim 1 , wherein the first purification unit and the second purification unit are the same unit. 
     
     
         13 . The method as claimed in  claim 1 , wherein the first purification unit and the second purification unit are separate units, wherein the first purification unit is configured to remove at least water and carbon dioxide, and wherein the second purification unit is configured to remove at least water. 
     
     
         14 . A method for the production of liquefied natural gas (“LNG”), the method comprising the steps of:
 a) providing a nitrogen refrigeration cycle; 
 b) cooling and liquefying a first natural gas stream in a heat exchanger by heat exchange with nitrogen from the nitrogen refrigeration cycle to produce an LNG stream, wherein the LNG stream is liquefied at a first pressure; 
 c) expanding a second natural gas stream to a second pressure to produce an expanded natural gas stream; and 
 d) warming the expanded natural gas stream in the heat exchanger to produce a warmed natural gas stream, 
 wherein step d) provides a portion of the refrigeration used to cool and liquefy the first natural gas stream. 
 
     
     
         15 . The method as claimed in  claim 14 , wherein the first natural gas stream comes from a first natural gas source, and the second natural gas stream comes from a second natural gas source, wherein the first and second natural gas sources are different sources. 
     
     
         16 . The method as claimed in  claim 14 , wherein the first natural gas liquefied in step b) is derived from the expanded natural gas stream, wherein the first pressure and the second pressure are about the same. 
     
     
         17 . A method for the production of liquefied natural gas (“LNG”), the method comprising the steps of:
 a) providing a high pressure natural gas stream; 
 b) splitting the high pressure natural gas stream into a first portion and a second portion; 
 c) cooling and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream; 
 d) providing a first portion of refrigeration via a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster; 
 e) providing a second portion of refrigeration by expanding the second portion of the high pressure natural gas; and 
 f) using the first portion of refrigeration and the second portion of refrigeration to achieve the cooling and liquefaction of the first portion of the high pressure natural gas stream in step c).