Method for liquefying a methane-rich feed gas, and corresponding facility

Abstract

The invention relates to a method for liquefying methane-rich feed gas ( 12 ), the method comprising: mixing the feed gas with recycled gas ( 18 ), compressing the gas to a pressure of between 19 and 70 bar absolute, and purifying the gas, a first pre-cooling of the gas to a temperature of between −40° C. and −15° C., and second pre-cooling of the gas by heat exchange with the recycled gas, liquefying the gas by heat exchange only with a liquefaction refrigeration cycle ( 46 ), sub-cooling the gas by heat exchange with at least the recycled gas, and expanding the sub-cooled liquid ( 52 ) to obtain a liquefied gas ( 14 ) and the recycled gas representing a mole fraction of less than 35%.

1 . A method for liquefying a feed gas comprising at least 40% by volume of methane, the method comprising the following steps:
 mixing the feed gas with a first stream of recycled gas to obtain a gas-to-be-treated, and compressing the gas-to-be-treated to a treatment pressure comprised between 19 and 70 bar absolute,   purification of the gas-to-be-treated to obtain a purified gas,   first pre-cooling of the purified gas to obtain a first pre-cooled gas having a temperature of less than or equal to −15° C. and greater than or equal to −40° C.,   second pre-cooling of the first pre-cooled gas by heat exchange with at least one second stream of recycled gas to obtain a second pre-cooled gas and the first stream of recycled gas,   liquefaction of the second pre-cooled gas to obtain a stream of liquid, by heat exchange only with a liquefaction refrigeration cycle,   sub-cooling of the stream of liquid by heat exchange with at least one third stream of recycled gas to obtain a stream of sub-cooled liquid and the second stream of recycled gas, the stream of sub-cooled liquid-being at a sub-cooling temperature, and   expansion of the stream of sub-cooled liquid to obtain a liquefied gas and the third stream of recycled gas, said expansion and the sub-cooling temperature being such that the third stream of recycled gas-represents a mole fraction, relative the stream of sub-cooled liquid, of less than 35%.   
     
     
         2 . The method according to  claim 1 , wherein the sub-cooling of the stream of liquid comprises:
 a first sub-cooling of the stream of liquid by heat exchange with the third stream of recycled gas to obtain an intermediate stream of sub-cooled liquid and the second stream of recycled gas, and   a second sub-cooling of the intermediate stream of sub-cooled liquid to obtain the stream of sub-cooled liquid.   
     
     
         3 . The method according to  claim 2 , wherein the second sub-cooling of the intermediate stream of sub-cooled liquid is achieved by exchange with a stream of liquid nitrogen, the second sub-cooling producing the stream of sub-cooled liquid and a stream of vaporized nitrogen, the second pre-cooling of the first pre-cooled gas being performed by heat exchange with the stream of vaporized nitrogen. 
     
     
         4 . The method according to  claim 1 , wherein the purified gas undergoes the first pre-cooling in a first pre-cooling unit including a pre-cooling refrigeration cycle, by exchanging heat with a refrigerant to form the first pre-cooled gas without exchanging heat with the first stream of recycled gas, the refrigerant being produced by the pre-cooled refrigerant cycle, the pre-cooling refrigeration cycle and the liquefaction refrigeration cycle being disjoint. 
     
     
         5 . The method according to  claim 1 , wherein the treatment pressure is lower than 45 bar absolute. 
     
     
         6 . The method according to  claim 1 , wherein the stream of liquid at the outlet of the liquefaction unit has a temperature comprised between −113° C. and −90° C. 
     
     
         7 . The method according to  claim 1 , wherein the second pre-cooled gas is liquefied by the liquefaction unit with a sub-cooling of less than or equal to 5° C. 
     
     
         8 . The method according to  claim 1 , wherein the liquefaction refrigeration cycle is a Stirling cycle or a reversed Brayton cycle. 
     
     
         9 . The method according to  claim 1 , wherein said expansion of the stream of sub-cooled liquid is performed in at least one Joule-Thomson valve or using an expansion turbine. 
     
     
         10 . A facility suitable for implementing a method according to  claim 1 , comprising:
 a mixer for mixing the feed gas with the first stream of recycled gas and obtaining the gas-to-be-treated, and at least one compressor suitable for compressing the gas-to-be-treated to the treatment pressure,   a purification unit suitable for purifying the gas-to-be-treated and for obtaining the purified gas,   a first pre-cooling unit-suitable for pre-cooling the purified gas and for obtaining the first pre-cooled gas,   a second pre-cooling unit suitable for pre-cooling the first pre-cooled gas by heat exchange with at least the second stream of recycled gas and for obtaining the second pre-cooled gas and the first stream of recycled gas,   a liquefaction unit for liquefying the second pre-cooled gas and for obtaining the stream of liquid, the liquefaction unit including the liquefaction refrigeration cycle,   a sub-cooling unit suitable for sub-cooling the stream of liquid to the sub-cooling temperature by heat exchange with at least the third stream of recycled gas and for obtaining the stream of sub-cooled liquid and the second stream of recycled gas,   an expansion unit for expanding the stream of sub-cooled liquid to obtain the liquefied gas and the third stream of recycled gas, the sub-cooling unit and the expansion unit being configured so that the third stream of recycled gas represents a mole fraction, with respect to the stream of sub-cooled liquid, of less than 35%.