Process for liquefying a hydrocarbon-rich fraction

Abstract

A process is proposed for liquefying a hydrocarbon-rich fraction (A), especially natural gas, by a) liquefying the hydrocarbon-rich fraction (A) against the coolant mixture of a cooling circuit, b) compressing the coolant mixture in at least two stages (C 1, C 2 ), c) partially condensing (E 1 ) the compressed coolant mixture ( 2 ) at least downstream of the penultimate compressor stage (C 1 ), d) compressing (C 2 ) the lower-boiling gas fraction ( 2′ ) obtained to the final pressure, e) while cooling (E) the first higher-boiling liquid fraction ( 3 ) obtained, expanding it (a) to perform cooling and vaporizing it (E) against the hydrocarbon-rich fraction (A) to be cooled, f) partially condensing (E 2 ) the coolant mixture fraction ( 4 ) compressed to the final pressure and separating the first lower-boiling gas fraction ( 5 ) obtained, after partial condensation (E), into a second lower-boiling gas fraction ( 7 ) and a second higher-boiling liquid fraction ( 6 ), and g) liquefying and subcooling (E) the second lower-boiling gas fraction ( 7 ), sub-cooling (E) the second higher-boiling liquid fraction ( 6 ) and expanding the two fractions to different temperature levels to perform cooling (b, c), and partly heating and at least partly vaporizing them (E) against the hydrocarbon-rich fraction (A) to be cooled. According to the invention, the composition of the coolant mixture is selected such that the final boiling point (dew point) of the second lower-boiling gas fraction ( 7 ) is at a lower temperature than the initial boiling point of the first higher-boiling liquid fraction ( 3 ).

1 . A process for liquefying a hydrocarbon-rich fraction (A) comprising:
 a) liquefying hydrocarbon-rich fraction (A) against a coolant mixture of a cooling circuit,   b) compressing said coolant mixture in at least two stages (C 1 , C 2 ),   c) partially condensing (E 1 ) the compressed coolant mixture ( 2 ) at least downstream of the penultimate compressor stage (C 1 ), thereby obtaining a lower-boiling gas fraction ( 2 ′) and a first higher-boiling liquid fraction ( 3 ),   d) compressing (C 2 ) said the lower-boiling gas fraction ( 2 ′) to a final pressure,   e) cooling (E) said the first higher-boiling liquid fraction ( 3 ), expanding said first higher-boiling liquid fraction ( 3 ) (a) to perform cooling, and vaporizing said first higher-boiling liquid fraction ( 3 ) (E) against the hydrocarbon-rich fraction (A) to be cooled,   f) partially condensing (E 2 ) said coolant mixture fraction ( 4 ) compressed to the final pressure in b) to obtain a first lower-boiling gas fraction ( 5 ), and separating the first lower-boiling gas fraction ( 5 ), after partial condensation (E), into a second lower-boiling gas fraction ( 7 ) and a second higher-boiling liquid fraction ( 6 ), and   g) liquefying and subcooling (E) said second lower-boiling gas fraction ( 7 ), subcooling (E) said second higher-boiling liquid fraction ( 6 ) and expanding the two fractions to different temperature levels to perform cooling (b, c), and partly heating and at least partly vaporizing the two fractions (E) against the hydrocarbon-rich fraction (A) to be cooled,   
       wherein the composition of the coolant mixture is selected such that the final boiling point of said second lower-boiling gas fraction ( 7 ) is at a lower temperature than the initial boiling point of said first higher-boiling liquid fraction ( 3 ). 
     
     
         2 . A process according to  claim 1 , wherein the temperature difference between the final boiling point of said second lower-boiling gas fraction ( 7 ) and the initial boiling point of said the first higher-boiling liquid fraction ( 3 ) is at least 5 K. 
     
     
         3 . A process according to  claim 2 , wherein the temperature difference between the final boiling point of said second lower-boiling gas fraction ( 7 ) and the initial boiling point of said first higher-boiling liquid fraction ( 3 ) is at least 10 K. 
     
     
         4 . A process according to  claim 1 , wherein said second higher-boiling liquid fraction ( 6 ,  6 ′) is vaporized (E) separately from said first higher-boiling liquid fraction ( 3 ,  3 ′) and said second lower-boiling gas fraction ( 7 ). 
     
     
         5 . A process according to  claim 4 , wherein said first higher-boiling liquid fraction ( 3 ) and said second lower-boiling gas fraction ( 7 ) are not combined until after they have been vaporized with said second higher-boiling liquid fraction ( 6 ). 
     
     
         6 . A process according to  claim 1 , wherein at least a substream ( 9 ) of the cooled second lower-boiling gas fraction ( 7 ) is added to the expanded (b) second higher-boiling liquid fraction ( 6 ,  6 ′). 
     
     
         7 . A process according to  claim 1 , wherein expanded first higher-boiling liquid fraction ( 3 ,  3 ′) and vaporized second lower-boiling gas fraction ( 7 ,  8 ) are mixed outside the heat exchanger(s) (E) required for the heat exchange between the hydrocarbon-rich fraction (A) to be liquefied and the cooling circuit. 
     
     
         8 . A process according to  claim 7 , wherein the expanded first higher-boiling liquid fraction ( 3 ,  3 ′) and vaporized second lower-boiling gas fraction ( 7 ,  8 ) are mixed in a separator, and the vaporized second lower-boiling gas fraction ( 7 ,  8 ) is supplied to the separator in monophasic form. 
     
     
         9 . A process according to  claim 1 , wherein the liquid fraction ( 4 ′) obtained in the partial condensation (E 2 ) of the coolant mixture fraction ( 4 ) compressed to the final pressure is used to subcool (E 3 ) said first higher-boiling liquid fraction ( 3 ). 
     
     
         10 . A process according to  claim 1 , wherein said hydrocarbon-rich fraction (A) is natural gas.