Dual mixed refrigerant system

Abstract

Processes and systems are provided for recovering a liquefied natural gas (LNG) stream from a hydrocarbon-containing feed gas stream using dual closed-loop mixed refrigerant cycles. In particular, the processes and systems described herein can be used to efficiently and effectively liquefy methane from a hydrocarbon-containing feed gas stream with the use of a first refrigeration system and a second refrigeration system utilizing a first mixed refrigerant and a second mixed refrigerant. Furthermore, the first refrigeration system and second refrigeration system may be in fluid communication with a turboexpander and separator.

1 . A process for liquefying a hydrocarbon-containing gas, the process comprising:
 (a) introducing a fully condensed first mixed refrigerant, a compressed second mixed refrigerant, and a feed stream comprising the hydrocarbon-containing gas into a first refrigeration system;   (b) cooling at least a portion of the feed stream, the fully condensed first mixed refrigerant, and the compressed second mixed refrigerant in the first refrigeration system via indirect heat exchange with an expanded first mixed refrigerant to form a first cooled feed stream, a cooled fully condensed first mixed refrigerant, and a fully condensed second mixed refrigerant;   (c) expanding at least a portion of the cooled fully condensed first mixed refrigerant to form the expanded first mixed refrigerant, wherein the cooled fully condensed first mixed refrigerant is not subjected to phase separation prior to the expanding step, wherein the expanded first mixed refrigerant is not subjected to phase separation following the expanding step;   (d) expanding at least a portion of the first cooled feed stream in a turboexpander to form an expanded feed stream;   (e) separating at least a portion of the expanded feed stream in a separator to form an overhead vapor fraction and a liquid bottom fraction;   (f) cooling at least a portion of the overhead vapor fraction in the first refrigeration system to form a cooled vapor fraction;   (g) cooling at least a portion of the cooled vapor fraction and the fully condensed second mixed refrigerant in a second refrigeration system via indirect heat exchange with an expanded second mixed refrigerant to form a second cooled feed stream and a cooled fully condensed second mixed refrigerant;   (h) expanding at least a portion of the cooled fully condensed second mixed refrigerant to form the expanded second mixed refrigerant, wherein the cooled fully condensed second mixed refrigerant is not subjected to separation prior to the expanding step, wherein the expanded second mixed refrigerant is not subjected to separation following the expanding step; and   (i) driving a compressor with the turboexpander.   
     
     
         2 . The process of  claim 1 , further comprising subcooling at least a portion of the cooled fully condensed first mixed refrigerant in step (b) to form a subcooled first mixed refrigerant, wherein the cooled fully condensed first mixed refrigerant in step (c) comprises the subcooled first mixed refrigerant. 
     
     
         3 . The process of  claim 1 , further comprising subcooling at least a portion of the cooled fully condensed second mixed refrigerant in step (g) to form a subcooled second mixed refrigerant, wherein the cooled fully condensed second mixed refrigerant in step (h) comprises the subcooled second mixed refrigerant. 
     
     
         4 . The process of  claim 1 , wherein the first mixed refrigerant comprises at least two compounds selected from the group consisting of hydrocarbons containing from 2 to 4 carbon atoms. 
     
     
         5 . The process of  claim 1 , wherein the second mixed refrigerant comprises at least two compounds selected from the group consisting of nitrogen and hydrocarbons containing from 1 to 3 carbon atoms. 
     
     
         6 . (canceled) 
     
     
         7 . (canceled) 
     
     
         8 . The process of  claim 1 , wherein the compressor compresses at least a portion of the expanded first mixed refrigerant after the cooling of step (b). 
     
     
         9 . The process of  claim 1 , wherein the compressor compresses at least a portion of the expanded second mixed refrigerant after the cooling of step (g). 
     
     
         10 . The process of  claim 1 , wherein the compressor compresses at least a portion of the overhead vapor fraction prior to the cooling of step (f). 
     
     
         11 .- 20 . (canceled) 
     
     
         21 . The process of  claim 1 , wherein the second cooled feed stream comprises a liquefied natural gas. 
     
     
         22 . The process of  claim 1 , wherein the second refrigeration system consists essentially of a single heat exchange zone, wherein the cooling of step (g) occurs in the single heat exchange zone. 
     
     
         23 . A process for liquefying a hydrocarbon-containing gas, the process comprising:
 (a) introducing a fully condensed first mixed refrigerant, a compressed second mixed refrigerant, and a feed stream comprising the hydrocarbon-containing gas into a first refrigeration system;   (b) cooling at least a portion of the feed stream, the fully condensed first mixed refrigerant, and the compressed second mixed refrigerant in the first refrigeration system via indirect heat exchange with an expanded first mixed refrigerant to form a first cooled feed stream, a cooled fully condensed first mixed refrigerant, and a fully condensed second mixed refrigerant;   (c) expanding at least a portion of the cooled fully condensed first mixed refrigerant to form the expanded first mixed refrigerant, wherein the cooled folly condensed first mixed refrigerant is not subjected to phase separation prior to the expanding step, wherein the expanded first mixed refrigerant is not subjected to phase separation following the expanding step;   (d) expanding at least a portion of the first cooled feed stream in a turboexpander to form an expanded feed stream;   (e) separating at least a portion of the expanded feed stream in a separator to form an overhead vapor fraction and a liquid bottom fraction;   (f) cooling at least a portion of the overhead vapor fraction in the first refrigeration system to form a cooled vapor fraction;   (g) cooling at least a portion of the cooled vapor fraction in a first cooling pass of the second refrigeration system and the fully condensed second mixed refrigerant in a second cooling pass of the second refrigeration system via indirect heat exchange with an expanded second mixed refrigerant in a single refrigerant warming pass of the second refrigeration system to form a second cooled feed stream, a cooled fully condensed second mixed refrigerant, and a gaseous second mixed refrigerant, wherein the second cooled feed stream from the first cooling pass comprises a liquefied natural gas, wherein the stream exiting the single refrigerant warming pass consists of the gaseous second mixed refrigerant;   (h) expanding at least a portion of the cooled fully condensed second mixed refrigerant to form the expanded second mixed refrigerant, wherein the cooled fully condensed second mixed refrigerant is not subjected to phase separation prior to the expanding step, wherein the expanded second mixed refrigerant is not subjected to phase separation following the expanding step; and   (i) driving a compressor with the turboexpander.   
     
     
         24 . The process of  claim 23 , wherein the second refrigeration system consists essentially of a single heat exchange zone, wherein the single heat exchange zone consists of the first cooling pass, the second cooling pass, and the single refrigerant warming pass. 
     
     
         25 . The process of  claim 23 , further comprising subcooling at least a portion of the cooled fully condensed first mixed refrigerant in step (b) to form a subcooled first mixed refrigerant, wherein the cooled fully condensed first mixed refrigerant in step (c) comprises the subcooled first mixed refrigerant. 
     
     
         26 . The process of  claim 23 , further comprising subcooling at least a portion of the cooled fully condensed second mixed refrigerant in step (g) to form a subcooled second mixed refrigerant, wherein the cooled fully condensed second mixed refrigerant in step (h) comprises the subcooled second mixed refrigerant. 
     
     
         27 . The process of  claim 23 , wherein the first mixed refrigerant comprises at least two compounds selected from the group consisting of hydrocarbons containing from 2 to 4 carbon atoms. 
     
     
         28 . The process of  claim 23 , wherein the second mixed refrigerant comprises at least two compounds selected from the group consisting of nitrogen and hydrocarbons containing from 1 to 3 carbon atoms. 
     
     
         29 . The process of  claim 23 , wherein the compressor compresses at least a portion of the overhead vapor fraction prior to the cooling of step (f).