Natural gas processing method

Abstract

A natural gas processing method includes: a pretreatment process of obtaining a treated natural gas; an NGL recovery process of cooling the treated natural gas obtained in the pretreatment process to recover an NGL product composed of ethane and heavier hydrocarbons and to separate a lean gas; a sales gas production process of compressing a part of the lean gas separated in the NGL recovery process to obtain a high-pressure gas; and a natural gas liquefying process of cooling the high-pressure gas after the high-pressure gas is obtained by compressing the lean gas separated in the NGL recovery process, to thereby obtain the LNG product. In the natural gas processing method, a gas compressor used in the sales gas production process is used as a gas compressor in the natural gas liquefying process.

1 . A natural gas processing method of recovering a natural gas liquid (NGL) and a high-pressure sales gas (sales gas) as major products and recovering a liquefied natural gas (LNG) as a co-product from a natural gas, the method comprising:
 a pretreatment process of removing impurities from a feed natural gas to obtain a treated natural gas;   an NGL recovery process of cooling the treated natural gas obtained in the pretreatment process to a temperature at which ethane and heavier hydrocarbons are condensed and liquefied to recover an NGL product and separating a lean gas which mainly contains methane;   a sales gas production process of compressing a part of the lean gas separated in the NGL recovery process by a gas compressor to obtain a high-pressure gas which is delivered as a sales gas; and   a natural gas liquefying process of compressing the remaining part of the lean gas separated in the NGL recovery process by a gas compressor to obtain a high-pressure gas which exceeds critical pressure, and cooling the high-pressure gas up to a temperature at which methane is condensed and liquefied to thereby obtain an LNG product,   the gas compressor used in the sales gas production being used in the natural gas liquefying process as the gas compressor.   
     
     
         2 . The natural gas processing method according to  claim 1 , wherein the natural gas liquefying process comprises:
 a process A of cooling a part of the LNG feed gas, which is the high-pressure lean gas being supplied in the natural gas liquefying process and exceeding the critical pressure, by heat exchanging with a low-temperature gas generated when the LNG product is produced, and adiabatically expanding the part by a liquid expander;   a process B of cooling the remaining part of the LNG feed gas supplied in the process A by heat exchanging with a propane refrigerant, and adiabatically expanding the remaining part by a gas expander to liquefy the remaining part; and   merging LNG produced in the process A with LNG produced in the process B to obtain an LNG product.   
     
     
         3 . The natural gas processing method according to  claim 2 , wherein the low temperature gas generated when the LNG product is produced, and used to perform the cooling in the process A is returned to the gas compressor in the sales gas production process. 
     
     
         4 . The natural gas processing method according to  claim 1 , wherein the LNG feed gas supplied in the natural gas liquefying process is pressurized to a pressure higher than in the sales gas production process to be used in a state where the LNG feed gas exceeds the critical pressure. 
     
     
         5 . The natural gas processing method according to  claim 1 , wherein the natural gas liquefying process comprises:
 a process A 1  of cooling a part of the high-pressure lean gas being supplied in the natural gas liquefying process and exceeding the critical pressure by heat exchanging with a low temperature gas generated when the LNG product is produced to perform adiabatic expansion by a liquid expander;   a process C of cooling some of the remaining part of the high-pressure lean gas supplied in the process A 1  by heat exchanging with a propane refrigerant, cooling the some of the remaining part by heat exchanging with a self-cooling medium which is formed by cooling a fluid comprised of an LNG feed gas containing the remainder of the remaining part of the high-pressure lean gas supplied to the process A 1 , and liquefying the remainder of the remaining part by adiabatic expansion by the gas expander to obtain LNG; and   merging LNG produced in the process A 1  with LNG produced in the process C to obtain an LNG product.   
     
     
         6 . The natural gas processing method according to  claim 5 , wherein the self-cooling medium comprises a part of a gas which is cooled by heat exchanging between the remainder of the remaining part of the high-pressure lean gas supplied in the process A 1  and a propane refrigerant, and
 wherein the remainder of the remaining part is cooled by heat exchanging, merged with the part of the gas, followed by adiabatically expanded by the gas expander, and the resulting expanded fluid is heat-exchanged as the self-cooling medium with the remaining part of the gas cooled by the heat exchanging with a propane refrigerant.   
     
     
         7 . The natural gas processing method according to  claim 6 , wherein the self-cooling medium is returned to the gas compressor in the sales gas production process after the remainder of the remaining part of the LNG feed gas supplied in the process A 1  is cooled by heat exchanging and compressed by a gas compressor. 
     
     
         8 . The natural gas processing method according to  claim 5 , wherein the low temperature gas generated when the LNG product used to perform the cooling in the process A 1  is returned to the gas compressor in the sales gas production process. 
     
     
         9 . The natural gas processing method according to  claim 1 , wherein the natural gas liquefying process comprises:
 a process A 1  of cooling a part of an LNG feed gas which is the high-pressure lean gas being supplied in the natural gas liquefying process and exceeding the critical pressure, by heat exchanging with a low-temperature gas generated when the LNG product is produced, liquefying the part of the LNG feed gas, followed by adiabatic expansion by a liquid expander;   a process D of cooling the remaining part of the LNG feed gas supplied in the process A 1  by heat exchanging with a propane refrigerant, separating the gas into a gas for an LNG feed gas and a cooling medium gas, cooling the LNG feed gas by heat exchanging with a self-cooling medium obtained by adiabatically expanding the cooling medium gas, and liquefying the LNG feed gas by adiabatic expansion by a gas expander to obtain LNG; and   merging LNG produced in the process A 1  with LNG produced in the process D to obtain an LNG product.   
     
     
         10 . The natural gas processing method according to  claim 9 , wherein the cooling medium gas is adiabatically expanded by a gas expander and the resulting adiabatically expanded fluid is heat-exchanged with the LNG feed gas. 
     
     
         11 . The natural gas processing method according to  claim 9 , wherein the self-cooling medium forms a circulation cycle in which a part of a gas obtained by compressing the self-cooling medium by gas compressors is cooled by heat exchanging and adiabatically expanded by a gas expander, and the resulting adiabatically expanded fluid is heat-exchanged with the LNG feed gas, followed by heating by heat exchanging, and compressing by the gas compressors. 
     
     
         12 . The natural gas processing method according to  claim 11 , wherein the part of the gas compressed by the gas compressors is heat-exchanged with the cooling medium gas and merged with the remaining part of the gas cooled by the heat exchanging with the propane refrigerant, and the remaining part is merged with the remaining part of the LNG feed gas supplied in the process A 1 . 
     
     
         13 . The natural gas processing method according to  claim 5 , wherein the LNG feed gas supplied in the natural gas liquefying process is pressurized to a pressure higher than in the sales gas production process to be used in a state where the LNG feed gas exceeds the critical pressure. 
     
     
         14 . The natural gas processing method according to  claim 1 , wherein the pressure of the high-pressure gases supplied in the natural gas liquefying process is in the range of 40 bara to 150 bara.