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.

What is claimed is: 
     
         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) expanding at least a portion of the fully condensed first mixed refrigerant to form an expanded first mixed refrigerant, wherein the fully condensed first mixed refrigerant is not subjected to phase separation prior to or following the expanding step;   (c) cooling at least a portion of the feed stream and the compressed second mixed refrigerant in the first refrigeration system via indirect heat exchange with the expanded first mixed refrigerant to form a first cooled feed stream and a fully condensed second mixed refrigerant;   (d) expanding at least a portion of the fully condensed second mixed refrigerant to form an expanded second mixed refrigerant; and   (e) cooling at least a portion of the first cooled feed stream in a second refrigeration system via indirect heat exchange with the expanded second mixed refrigerant to form a second cooled feed stream, wherein the second refrigeration system comprises a single cooling zone.   
     
     
         2 . The process of  claim 1 , further comprising subcooling at least a portion of the fully condensed first mixed refrigerant in step (c) to form a subcooled first mixed refrigerant, wherein the fully condensed first mixed refrigerant in step (b) comprises the subcooled first mixed refrigerant. 
     
     
         3 . The process of  claim 1 , further comprising subcooling at least a portion of the fully condensed second mixed refrigerant in step (e) to form a subcooled second mixed refrigerant, wherein the fully condensed second mixed refrigerant in step (d) comprises the subcooled second mixed refrigerant. 
     
     
         4 . The process of  claim 1 , wherein the fully condensed second mixed refrigerant is not subjected to phase separation prior to or following the expanding of step (d). 
     
     
         5 . The process of  claim 1 , wherein the first refrigeration system comprises a plurality of cooling zones. 
     
     
         6 . 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. 
     
     
         7 . 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. 
     
     
         8 . The process of  claim 1 , wherein the second mixed refrigerant has a bubble point temperature that is lower than the bubble point temperature of the first mixed refrigerant at a given pressure. 
     
     
         9 . 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) expanding at least a portion of the fully condensed first mixed refrigerant to form an expanded first mixed refrigerant;   (c) cooling at least a portion of the feed stream and the compressed second mixed refrigerant in the first refrigeration system via indirect heat exchange with the expanded first mixed refrigerant to form a first cooled feed stream and a fully condensed second mixed refrigerant;   (d) expanding at least a portion of the fully condensed second mixed refrigerant to form an expanded second mixed refrigerant;   (e) cooling at least a portion of the first cooled feed stream in a second refrigeration system via indirect heat exchange with the expanded second mixed refrigerant to form a second cooled feed stream, wherein the first refrigeration system and the second refrigeration system are contained in the same heat exchanger; and   (f) expanding at least a portion of the second cooled feed stream to form an LNG stream.   
     
     
         10 . The process of  claim 9 , further comprising subcooling at least a portion of the fully condensed first mixed refrigerant in step (c) to form a subcooled first mixed refrigerant, wherein the fully condensed first mixed refrigerant in step (b) comprises the subcooled first mixed refrigerant. 
     
     
         11 . The process of  claim 9 , further comprising subcooling at least a portion of the fully condensed second mixed refrigerant in step (e) to form a subcooled second mixed refrigerant, wherein the fully condensed second mixed refrigerant in step (d) comprises the subcooled second mixed refrigerant. 
     
     
         12 . The process of  claim 9 , wherein the fully condensed first mixed refrigerant is not subjected to phase separation prior to or following the expanding of step (b). 
     
     
         13 . The process of  claim 9 , wherein the fully condensed second mixed refrigerant is not subjected to phase separation prior to or following the expanding of step (d). 
     
     
         14 . The process of  claim 9 , wherein the first mixed refrigerant comprises at least two compounds selected from the group consisting of hydrocarbons containing from 2 to 4 carbon atoms. 
     
     
         15 . The process of  claim 9 , 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. 
     
     
         16 . The process of  claim 9 , wherein the second mixed refrigerant has a bubble point temperature that is lower than the bubble point temperature of the first mixed refrigerant at a given pressure. 
     
     
         17 . A system for liquefying a hydrocarbon-containing gas, the system comprising:
 (a) a first refrigeration system comprising a plurality of cooling zones disposed therein, wherein each of the cooling zones are configured to cool a second mixed refrigerant and a feed stream comprising the hydrocarbon-containing gas via indirect heat exchange with a first mixed refrigerant to thereby form a first cooled feed stream and fully condensed second mixed refrigerant;   (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, wherein the first closed-loop mixed refrigeration cycle does not contain a phase separator;   (c) a second refrigeration system in fluid communication with the first refrigeration system, wherein the second refrigeration system comprises a single cooling zone disposed therein, wherein the cooling zone is configured to cool the first cooled feed stream via indirect heat exchange with the second mixed refrigerant to form a second cooled feed stream; and   (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, wherein the second closed-loop mixed refrigeration cycle does not contain a phase separator.   
     
     
         18 . The system of  claim 17 , wherein the plurality of cooling zones in the first refrigeration system are configured to subcool the first mixed refrigerant. 
     
     
         19 . The system of  claim 17 , wherein the single cooling zone in the second refrigeration system is configured to subcool the second mixed refrigerant. 
     
     
         20 . The system of  claim 17 , wherein the first closed-loop mixed refrigeration cycle comprises at least one expansion device configured to expand the first mixed refrigerant and the second closed-loop refrigeration cycle comprises at least one expansion device configured to expand the second mixed refrigerant.