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 by utilizing a first refrigeration system and a second refrigeration system in fluid communication with a turboexpander and separator.

1 . A process for liquefying a hydrocarbon-containing gas, the process comprising:
 (a) introducing a first 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 in the first refrigeration system via indirect heat exchange with the first mixed refrigerant to form a first cooled feed stream;   (c) cooling at least a portion of the first cooled feed stream in a second refrigeration system via indirect heat exchange with a second mixed refrigerant to form a second cooled feed stream;   (d) expanding at least a portion of the first cooled feed stream or the second 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 or the second refrigeration system; and   (g) driving a compressor with the turboexpander.   
     
     
         2 . The process of  claim 1 , further comprising expanding at least a portion of the first mixed refrigerant to form an expanded first mixed refrigerant, wherein the expanded first mixed refrigerant is used as the first mixed refrigerant during the cooling of step (b). 
     
     
         3 . The process of  claim 2 , further comprising subcooling at least a portion of the first mixed refrigerant in step (b) to form a subcooled first mixed refrigerant, wherein the subcooled first mixed refrigerant is the first mixed refrigerant subjected to expansion. 
     
     
         4 . The process of  claim 1 , further comprising expanding at least a portion of the second mixed refrigerant to form an expanded second mixed refrigerant, wherein the expanded second mixed refrigerant is used as the second mixed refrigerant during the cooling of step (c). 
     
     
         5 . The process of  claim 4 , further comprising subcooling at least a portion of the second mixed refrigerant in step (c) to form a subcooled second mixed refrigerant, wherein the subcooled second mixed refrigerant is the second mixed refrigerant subjected to expansion. 
     
     
         6 . The process of  claim 1 , wherein the first cooled feed stream is expanded during the expanding of step (d), wherein the overhead vapor fraction is cooled in the first refrigeration system. 
     
     
         7 . The process of  claim 1 , wherein the second cooled feed stream is expanded during the expanding of step (d), wherein the overhead vapor fraction is cooled in the second refrigeration system. 
     
     
         8 . The process of  claim 1 , wherein the compressor compresses at least a portion of the first mixed refrigerant. 
     
     
         9 . The process of  claim 1 , wherein the compressor compresses at least a portion of the second mixed refrigerant. 
     
     
         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 . A process for liquefying a hydrocarbon-containing gas, the process comprising:
 (a) introducing a first 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 in the first refrigeration system via indirect heat exchange with the first mixed refrigerant to form a first cooled feed stream;   (c) cooling at least a portion of the first cooled feed stream in a second refrigeration system via indirect heat exchange with a second mixed refrigerant to form a second cooled feed stream;   (d) separating at least a portion of the second cooled feed stream in a separator to form an overhead vapor fraction and a liquid bottom fraction; and   (e) cooling at least a portion of the overhead vapor fraction in the first refrigeration system or the second refrigeration system.   
     
     
         12 . The process of  claim 11 , further comprising expanding at least a portion of the first mixed refrigerant to form an expanded first mixed refrigerant, wherein the expanded first mixed refrigerant is used as the first mixed refrigerant during the cooling of step (b). 
     
     
         13 . The process of  claim 12 , further comprising subcooling at least a portion of the first mixed refrigerant in step (b) to form a subcooled first mixed refrigerant, wherein the subcooled first mixed refrigerant is the first mixed refrigerant subjected to expansion. 
     
     
         14 . The process of  claim 11 , further comprising expanding at least a portion of the second mixed refrigerant to form an expanded second mixed refrigerant, wherein the expanded second mixed refrigerant is used as the second mixed refrigerant during the cooling of step (c). 
     
     
         15 . The process of  claim 14 , further comprising subcooling at least a portion of the second mixed refrigerant in step (c) to form a subcooled second mixed refrigerant, wherein the subcooled second mixed refrigerant is the second mixed refrigerant subjected to expansion. 
     
     
         16 . The process of  claim 11 , further comprising expanding at least a portion of the second cooled feed stream in a turboexpander to form an expanded feed stream, wherein the expanded feed stream is the second cooled feed stream in the separating of (d). 
     
     
         17 . The process of  claim 16 , further comprising a compressor at least partially driven by the turboexpander, wherein the compressor at least partially compresses the first mixed refrigerant, the second mixed refrigerant, or the overhead vapor fraction. 
     
     
         18 . The process of  claim 17 , wherein the compressor at least partially compresses the first mixed refrigerant. 
     
     
         19 . The process of  claim 17 , wherein the compressor at least partially compresses the second mixed refrigerant. 
     
     
         20 . The process of  claim 17 , wherein the compressor at least partially compresses the overhead vapor fraction. 
     
     
         21 . The process of  claim 11 , cooling at least a portion of the overhead vapor fraction in the second refrigeration system. 
     
     
         22 . A system for liquefying a hydrocarbon-containing gas, the system comprising:
 (a) a first refrigeration system comprising a first cooling zone disposed therein, wherein the first cooling zone is configured to cool a feed stream comprising the hydrocarbon-containing gas via indirect heat exchange with a first mixed refrigerant to form a first cooled feed stream;   (b) a first closed-loop mixed refrigeration cycle at least partially disposed within the first refrigeration system, wherein the first closed-loop mixed refrigeration cycle comprises the first mixed refrigerant;   (c) a second refrigeration system in fluid communication with the first refrigeration system, wherein the second refrigeration system comprises a second cooling zone disposed therein, wherein the second cooling zone is configured to cool the first cooled feed stream via indirect heat exchange with a second mixed refrigerant to form a second cooled feed stream;   (d) a second closed-loop mixed refrigeration cycle at least partially disposed within the second refrigeration system, wherein the second closed-loop mixed refrigeration cycle comprises the second mixed refrigerant;   (e) a turboexpander in fluid communication with the first refrigeration system or second refrigeration system, wherein the turboexpander is configured to expand the first cooled feed stream or the second cooled feed stream into an expanded stream;   (f) a separator in fluid communication with the turboexpander, wherein the separator is configured to separate the expanded stream into an overhead vapor fraction and a liquid bottom fraction;   (g) a conduit for returning at least a portion of the overhead vapor fraction to the first refrigeration system or second refrigeration system; and   (h) a compressor at least partially driven from work derived from the turboexpander, wherein the compressor is configured to at least partially compress the first mixed refrigerant, the second mixed refrigerant, or the overhead vapor fraction.   
     
     
         23 . The system of  claim 22 , wherein the turboexpander is in fluid communication with the first refrigeration system and the conduit returns at least a portion of the overhead vapor fraction to the first refrigeration system. 
     
     
         24 . The system of  claim 22 , wherein the turboexpander is in fluid communication with the second refrigeration system and the conduit returns at least a portion of the overhead vapor fraction to the second refrigeration system.