Method for liquefying a methane-rich gas to be processed, and corresponding facility

Abstract

Method for liquefying a gas (12) to be processed comprising at least 50% by volume of methane, comprising the following steps:purification of the gas to be processed,precooling of the purified gas (22) to a temperature less than or equal to −15° C. by heat exchange with a precooling refrigeration cycle (40),liquefaction of the precooled gas (26), in a liquefaction unit (30), into a liquid stream (32), with subcooling less than or equal to 5° C., the liquefaction unit including a Stirling refrigeration cycle (50) distinct from the precooling refrigeration cycle, this cycle using a first refrigerant (42) precooled by a precooling refrigeration cycle (40) distinct from a Stirling cycle,subcooling of the liquid stream, andexpansion of the subcooled liquid stream (36) to obtain a liquefied gas (14).

1 . A method for liquefying a gas to be processed comprising at least 50% by volume of methane, the method comprising the following steps:
 purification of the gas to be processed to obtain a purified gas,   precooling of the purified gas to obtain a precooled gas having a temperature less than or equal to −15° C., the precooling being performed by heat exchange with a precooling refrigeration cycle,   liquefaction of the precooled gas, in a liquefaction unit, to obtain a liquid stream, with a subcooling of the liquid stream of less than or equal to 5° C. at the outlet of the liquefaction unit, the liquefaction unit including a Stirling refrigeration cycle distinct from said precooling refrigeration cycle and implementing a first refrigerant, the Stirling refrigeration cycle comprising a precooling of the first refrigerant by heat exchange with a second refrigerant of a precooling refrigeration cycle distinct from a Stirling cycle,   subcooling of the liquid stream to obtain a subcooled liquid stream, and   expansion of the subcooled liquid stream to obtain a liquefied gas.   
     
     
         2 . The method according to  claim 1 , wherein the precooling refrigeration cycle implemented in the precooling step of the purified gas and the precooling refrigeration cycle implemented for the precooling of the first refrigerant form a single precooling refrigeration cycle. 
     
     
         3 . The method according to  claim 2 , wherein the single precooling refrigeration cycle is a glycol water cycle, a CO 2  cycle, an ammonia cycle, a freon cycle or a propane cycle. 
     
     
         4 . The method according to  claim 2 , wherein the single precooling refrigeration cycle comprises:
 cooling the second refrigerant to produce a cooled second refrigerant stream, and   dividing the cooled second refrigerant stream into at least two flows used respectively to perform the precooling of the first refrigerant and the precooling of the purified gas.   
     
     
         5 . The method according to  claim 1 , wherein the temperature of the precooled gas is higher than or equal to −50° C. 
     
     
         6 . The method according to  claim 1 , wherein the subcooling comprises:
 a heat exchange with an open cycle implementing liquid nitrogen, and/or   a heat exchange with a vapor produced by the expansion of the subcooled liquefied gas stream.   
     
     
         7 . The method according to  claim 1 , wherein the expansion of the subcooled liquid stream produces a vapor, the method further comprising mixing the gas to be processed with at least a portion of the vapor to obtain a mixture. 
     
     
         8 . The method according to  claim 1 , further comprising, prior to the liquefaction of the precooled gas, an expansion of the precooled gas. 
     
     
         9 . The method according to  claim 1 , wherein said liquid stream obtained presents a temperature of between −115° C. and −90° C. at the outlet of the liquefaction unit. 
     
     
         10 . A facility adapted to implement the method according to  claim 1 , comprising:
 a purification unit able to purify a gas to be processed comprising at least 50% by volume of methane and to obtain a purified gas,   a precooling unit able to precool the purified gas and to obtain a precooled gas at a temperature less than or equal to −15° C.,   at least one precooling refrigeration cycle for implementing a heat exchange with the purified gas,   a liquefaction unit able to liquefy the precooled gas and to obtain a liquid stream, with a subcooling of the liquid stream less than or equal to 5° C. at the outlet of the liquefaction unit, the liquefaction unit including a Stirling refrigeration cycle distinct from said precooling refrigeration cycle and being able to implement a first refrigerant, the Stirling refrigeration cycle being able to perform a precooling of the first refrigerant,   at least one precooling refrigeration cycle distinct from the Stirling refrigeration cycle and able to implement a second refrigerant and to perform a heat exchange with the first refrigerant in order to obtain said precooling of the first refrigerant, the precooling refrigeration cycle being distinct from a Stirling cycle,   a subcooling unit for subcooling the liquid stream to obtain a subcooled liquid stream, and   an expansion unit for expanding the subcooled liquid stream to obtain a liquefied gas.