System and method to produce liquefied natural gas

Abstract

A small to mid-scale liquefied natural gas production system and method is provided. The disclosed liquefied natural gas production system employs a nitrogen-based refrigerant, at least one heat exchanger, three turbine/expanders and two or more refrigerant compression stages. The expansion ratio of one turbine/expander is appreciably lower than the expansion ratio of the other turbine/expanders such that the temperature of the exhaust stream from the turbine/expander with the lower expansion ratio is above the critical point temperature of the compressed natural gas feed stream but colder than −15° C.

What is claimed is: 
     
         1 . A natural gas liquefaction system, comprising:
 a refrigeration circuit comprising: (i) at least one heat exchanger configured to liquefy and subcool a compressed natural gas containing feed stream via indirect heat exchange with a refrigerant stream; (ii) three or more turbine/expanders configured to expand portions of the refrigerant stream to produce at least three exhaust streams that are directed to the at least one heat exchanger to liquefy and subcool the natural gas containing feed stream via indirect heat exchange and exit the at least one heat exchanger as one or more warmed recycle streams; and (iii) two or more refrigerant compression stages including an upstream refrigerant compression stage and a downstream refrigerant compression stage both configured to compress the warmed recycle streams; and   an integral gear machine comprising a drive assembly, a bull gear, and at least three pinions arranged to drive the two or more refrigerant compression stages and/or for receiving work produced by the at least three turbines/expanders;   wherein the three or more turbines/expanders further comprise: (i) a cold turbine/expander configured to expand a cold portion of the refrigerant stream and produce a cold exhaust that is also recycled to the upstream refrigerant compression stage of the two or more refrigerant compression stages; (ii) a first warm turbine/expander configured to expand a first warm portion of the refrigerant stream and produce a first warm exhaust that is recycled to the upstream refrigerant compression stage of the two or more refrigerant compression stages; and (iii) a second warm turbine/expander configured to expand a second warm portion of the refrigerant stream and produce a second warm exhaust that is recycled to the downstream refrigerant compression stage of the two or more refrigerant compression stages; and   wherein an expansion ratio of the secondary warm turbine/expander is lower than an expansion ratio of the cold turbine/expander and lower than an expansion ratio of the warm turbine/expander.   
     
     
         2 . The natural gas liquefaction system of  claim 1 , wherein the second warm exhaust is above the critical point temperature of the compressed natural gas containing feed stream. 
     
     
         3 . The natural gas liquefaction system of  claim 1 , wherein the second warm exhaust is less than about −15° C. 
     
     
         4 . The natural gas liquefaction system of  claim 1 , wherein an inlet pressure of the cold turbine/expander and an inlet pressure of the first warm turbine/expander are approximately equal and an outlet pressure of the cold turbine/expander and an outlet pressure of the first warm turbine/expander are approximately equal. 
     
     
         5 . The natural gas liquefaction system of  claim 1 , wherein the first warm turbine/expander is configured with an expansion ratio of between 4.0 and 5.0 and is further configured to produce over 50% of the turbine work used to produce refrigeration for the natural gas liquefaction system. 
     
     
         6 . The natural gas liquefaction system of  claim 1 , wherein the cold turbine/expander is configured with an expansion ratio of between 4.0 and 5.0 and is further configured to produce less than 20% of the turbine work used to produce refrigeration for the natural gas liquefaction system. 
     
     
         7 . The natural gas liquefaction system of  claim 1 , wherein the second warm turbine/expander is configured with an expansion ratio of between 1.5 and 2.5 and is further configured to produce between about 20% to 35% of the turbine work used to produce refrigeration for the natural gas liquefaction system. 
     
     
         8 . The natural gas liquefaction system of  claim 1 , wherein the second warm turbine/expander and the upstream compression stage are operatively coupled to a first pinion of the at least three pinions, and the first warm turbine/expander and the downstream compression stage are operatively coupled to a second pinion of the at least three pinions. 
     
     
         9 . The natural gas liquefaction system of  claim 8 , wherein the cold turbine/expander is operatively coupled to a third pinion of the at least three pinions. 
     
     
         10 . The natural gas liquefaction system of  claim 1 , wherein the two or more refrigerant compression stages further comprise at least three refrigerant compression stages and wherein the upstream refrigerant compression stage further comprises a first upstream refrigerant compression stage and a second upstream refrigerant compression stage arranged in parallel. 
     
     
         11 . The natural gas liquefaction system of  claim 10 , wherein the second warm turbine/expander and the first upstream refrigerant compression stage are operatively coupled to a first pinion of the at least three pinions, and the first warm turbine/expander and the downstream refrigerant compression stage are operatively coupled to a second pinion of the at least three pinions. 
     
     
         12 . The natural gas liquefaction system of  claim 8 , wherein the cold turbine/expander and the second upstream refrigerant compression stage are operatively coupled to a third pinion of the at least three pinions. 
     
     
         13 . The natural gas liquefaction system of  claim 1 , wherein the compressed natural gas containing feed stream is at a pressure greater than the critical pressure of natural gas. 
     
     
         14 . The natural gas liquefaction system of  claim 1 , wherein the compressed natural gas containing feed stream is at a pressure between about 50 bar(a) and 80 bar(a). 
     
     
         15 . The natural gas liquefaction system of  claim 1 , wherein the compressed natural gas containing feed stream is a methane containing biogas feed. 
     
     
         16 . The natural gas liquefaction system of  claim 1 , wherein the one or more refrigerant streams comprise more than about 80% nitrogen by volume. 
     
     
         17 . The natural gas liquefaction system of  claim 1 , wherein the driver assembly is an electric motor, a steam turbine, or a gas turbine. 
     
     
         18 . The natural gas liquefaction system of  claim 1 , wherein the at least one heat exchanger further comprises multiple heat exchangers or multiple heat exchange cores with a first heat exchanger or first heat exchange core configured for liquefying the natural gas containing feed stream and a second heat exchanger or second heat exchange core configured for either cooling the natural gas containing feed stream, cooling a warm portion of the refrigerant stream, or sub-cooling the liquefied natural gas stream via indirect heat exchange with one or more of the at least three exhaust streams. 
     
     
         19 . The natural gas liquefaction system of  claim 1 , further comprising a phase separator configured for separating nitrogen and other light gases from the liquefied and subcooled natural gas stream.