Method for producing liquefied natural gas

Abstract

A method for liquefying natural gas using the pressure energy of the natural gas is provided, wherein flash vapor from natural gas depressurization is employed in a refrigeration cycle for subcooling high pressure natural gas prior to flashing.

1 . A method for producing liquefied natural gas comprising:
 (A) compressing a first portion of a high pressure natural gas, bringing it to a temperature above its dew point and turbo-expanding such first portion to produce at least a part of the refrigeration required to liquefy the remaining second portion of the natural gas;   (B) liquefying the remaining second portion of the natural gas and subcooling said liquefied second portion and flashing the subcooled natural gas to produce flash vapor and liquefied natural gas at low pressure;   (C) compressing a refrigeration gas and turbo-expanding the compressed refrigeration gas to produce cooled refrigeration gas; and   (D) warming the flash vapor and the cooled refrigeration gas by indirect heat exchange with the high pressure natural gas to produce subcooled liquefied natural gas.   
   
   
       2 . The method of  claim 1  wherein the compressed first portion of the natural gas is cooled to bring it to a temperature above its dew point. 
   
   
       3 . The method of  claim 1  wherein the compressed first portion is cooled and expanded in a first expansion to produce at least a part of the refrigeration required to liquefy the remaining second portion of the natural gas, and heating such first portion to bring it to a temperature above its dew point. 
   
   
       4 . The method of  claim 1  wherein the high pressure natural gas is purified in a first purification system. 
   
   
       5 . The method of  claim 4  wherein a part of the first portion of the natural gas is used as regeneration gas for the first purification system. 
   
   
       6 . The method of  claim 1  wherein the second portion of natural gas is purified in a second purification system and said purified second portion is cooled. 
   
   
       7 . The method of  claim 6  wherein a part of the second portion of natural gas is flashed to a pressure marginally above letdown pressure and such part of the second portion is used as regeneration gas for the second purification system, after recovering the contained refrigeration by indirect heat exchange with incoming streams. 
   
   
       8 . The method of  claim 1  wherein the flash vapor and the cooled refrigeration gas are combined prior to warming by indirect heat exchange with the subcooling liquid natural gas. 
   
   
       9 . The method of  claim 1  wherein a portion of the refrigeration gas is withdrawn prior to turbo-expansion. 
   
   
       10 . The method of  claim 1  wherein the flash vapor and the cooled refrigeration gas, after the said warming, are compressed to form the said compressed refrigeration gas. 
   
   
       11 . The method of  claim 1  wherein the subcooled liquid natural gas has a pressure within the range of from 400 to 1500 psia, and the flash vapor and liquefied natural gas resulting from the flashing of the subcooled natural gas have a pressure within the range of from 14.7 to 40 psia. 
   
   
       12 . A method for producing liquefied natural gas comprising:
 (A) purifying a high pressure natural gas in a first purification system;   (B) compressing a first portion of a high pressure natural gas, cooling it to a temperature above its dew point and turbo-expanding such first portion to produce at least a part of the refrigeration required to liquefy the remaining second portion of the natural gas;   (C) using a part of the first portion as regeneration gas for the first purification system;   (D) purifying the second portion of natural gas in a second purification system and cooling said purified second portion;   (E) flashing a first part of the second portion of natural gas to a pressure marginally above letdown pressure and using it as regeneration gas for the second purification system;   (F) recovering contained refrigeration of such first part of the second portion by indirect heat exchange with the remaining second portion;   (G) compressing a refrigeration gas and turbo-expanding the compressed refrigeration gas to produce cooled refrigeration gas; and   (H) warming the flash vapor and the cooled refrigeration gas by indirect heat exchange with the liquefied natural gas to effect the sub-cooling of the liquefied natural gas.   
   
   
       13 . The method of  claim 12  wherein the flash vapor and the cooled refrigeration gas are combined prior to warming by indirect heat exchange with the subcooled liquefied natural gas. 
   
   
       14 . The method of  claim 12  wherein the flash vapor and the cooled refrigeration gas, after the said warming, are compressed to form the said compressed refrigeration gas. 
   
   
       15 . The method of  claim 12  wherein the subcooled liquefied natural gas has a pressure within the range of from 400 to 1500 psia, and the flash vapor and liquefied natural gas resulting from the flashing of the subcooled natural gas have a pressure within the range of from 14.7 to 40 psia. 
   
   
       16 . A method for producing liquefied natural gas comprising:
 (A) purifying a high pressure natural gas in a first purification system;   (B) compressing and cooling a first portion of a natural gas and expanding such first portion in a first expansion to produce at least a part of the refrigeration required to liquefy the remaining second portion of the natural gas;   (C) heating such first portion to bring it to a temperature above its dew point and turbo-expanding such heated first portion;   (D) using a part of the first portion as regeneration gas for the first purification system;   (E) purifying the second portion of natural gas in a second purification system and cooling said purified second portion;   (F) flashing a first part of the second portion of natural gas to a pressure marginally above letdown pressure and using it as regeneration gas for the second purification system;   (G) recovering contained refrigeration of such first part of the second portion by indirect heat exchange with the remaining second portion;   (H) compressing a refrigeration gas and turbo-expanding the compressed refrigeration gas to produce cooled refrigeration gas; and   (I) warming the flash vapor and the cooled refrigeration gas by indirect heat exchange with the cooled second portion of natural gas to produce sub-cooled liquefied natural gas.   
   
   
       17 . The method of  claim 16  wherein the flash vapor and the cooled refrigeration gas are combined prior to warming by indirect heat exchange with the subcooled liquefied natural gas. 
   
   
       18 . The method of  claim 16  wherein a portion of the refrigeration gas is withdrawn prior to turbo-expansion. 
   
   
       19 . The method of  claim 16  wherein the flash vapor and the cooled refrigeration gas, after the said warming, are compressed to form the said compressed refrigeration gas. 
   
   
       20 . The method of  claim 16  wherein the subcooled liquefied natural gas has a pressure within the range of from 400 to 1500 psia, and the flash vapor and liquefied natural gas resulting from the flashing of the subcooled natural gas have a pressure within the range of from 14.7 to 40 psia. 
   
   
       21 . A method for producing liquefied natural gas comprising:
 (A) purifying a high pressure natural gas in a purification system;   (B) compressing a first portion of a high pressure natural gas, cooling it to a temperature above its dew point and turbo-expanding such first portion to produce at least a part of the refrigeration required to liquefy the remaining second portion of the natural gas;   (C) using a part of the first portion as regeneration gas for the purification system;   (D) liquefying and subcooling the remaining second portion of the natural gas and flashing the subcooled natural gas to produce flash vapor and liquefied natural gas at low pressure;   (E) compressing a refrigeration gas and turbo-expanding the compressed refrigeration gas to produce cooled refrigeration gas; and   (F) warming the flash vapor and the cooled refrigeration gas by indirect heat exchange with the liquefied natural gas to effect the sub-cooling of the liquefied natural gas.   
   
   
       22 . The method of  claim 21  wherein the flash vapor and the cooled refrigeration gas are combined prior to warming by indirect heat exchange with the subcooled liquefied natural gas. 
   
   
       23 . The method of  claim 21  wherein a portion of the refrigeration gas is withdrawn prior to turbo-expansion. 
   
   
       24 . The method of  claim 21  wherein the flash vapor and the cooled refrigeration gas, after the said warming, are compressed to form the said compressed refrigeration gas. 
   
   
       25 . The method of  claim 21  wherein the subcooled liquefied natural gas has a pressure within the range of from 400 to 1500 psia, and the flash vapor and liquefied natural gas resulting from the flashing of the subcooled natural gas have a pressure within the range of from 14.7 to 40 psia.