Patent ID: 7096688

Claim:
A process for liquefying a natural gas ( 1 ), under pressure, containing methane and C 2 and higher hydrocarbons, said process comprising: (a) a first step I, in which the natural gas ( 1 ) is subjected to a first refrigerating cycle in order to obtain a cooled natural gas ( 4 ) and is taken to a temperature below −20° C. by a first coolant ( 201 ), said first refrigerating cycle comprising a succession of substeps (i) to (v) in which the first coolant ( 201 ) is (i) subcooled, (ii) expanded, (iii) vaporized, (iv) compressed and (v) at least partly condensed by cooling with a first external refrigerating fluid (E 201 ); (b) a second step II, in which the cooled natural gas ( 4 ) coming from step I is subjected to a second refrigerating cycle in which the cooled natural gas ( 4 ) is cooled and condensed by a second coolant ( 103 ) comprising methane, ethane, propane and nitrogen, said second refrigerating cycle comprising a succession of substeps (i) to (vi) in which the second coolant ( 103 ) is (i) subcooled, (ii) expanded, (iii) vaporized, (iv) compressed, (v) cooled with a second external refrigerating fluid (E 102 ) and (vi) at least partly condensed by cooling with said first coolant ( 201 ); the second coolant ( 103 ) furthermore containing ethylene, the total content in ethane and ethylene being close to 50 mol %, the second coolant ( 103 ) being subcooled without phase separation; characterized in that the ethylene and ethane proportions in the second coolant are adjusted according to the intake pressure of the natural gas, and in that the ratio of the ethylene content in the second coolant ( 103 ), expressed in mol %, and the total content in ethylene and ethane in this same second coolant, expressed in mol %, to the total content in ethylene and ethane in this same second coolant, expressed in mol %, is greater than 25% when the natural gas is available at a pressure below 35 bar.