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 at least one heat exchanger, three turbine/expanders and at least three 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 containing feed stream but colder than about −15° C. The present system and method may be configured using either a single nitrogen-based expansion refrigerant circuit or two separate refrigerant circuits wherein the turbine/expander with the lowest expansion ratio is contained within a separate refrigeration circuit from the other two turbine/expanders with the higher expansion ratios.

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) at least three refrigerant compression stages including an upstream refrigerant compression stage and a pair of downstream refrigerant compression stages arranged in parallel, wherein the three refrigerant compression stages are configured to compress the warmed recycle streams;   an integral gear machine comprising a drive assembly, a bull gear, and at least three pinions arranged to drive the at least three 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; (ii) a first warm turbine/expander configured to expand a first warm portion of the refrigerant stream and produce a first warm exhaust to be recycled to the downstream 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 to be recycled to the downstream refrigerant compression stages;   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 and less than about −15° C. 
     
     
         3 . 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. 
     
     
         4 . 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. 
     
     
         5 . 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. 
     
     
         6 . The natural gas liquefaction system of  claim 1 , wherein the first warm turbine/expander and the upstream compression stage are operatively coupled to a first pinion of the at least three pinions, and the cold turbine/expander and one of the pair of downstream compression stages are operatively coupled to a second pinion of the at least three pinions, and the second warm turbine/expander and another of the pair of downstream compression stages are operatively coupled to a third pinion of the at least three pinions. 
     
     
         7 . The natural gas liquefaction system of  claim 1 , wherein all three pinions are net absorbers of power and the power is distributed to these three pinions in generally equal or roughly equal proportions. 
     
     
         8 . The natural gas liquefaction system of  claim 1 , wherein the compressed natural gas containing feed stream is a methane containing biogas feed stream. 
     
     
         9 . 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. 
     
     
         10 . 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). 
     
     
         11 . The natural gas liquefaction system of  claim 1 , wherein the one or more refrigerant streams comprise more than about 80% nitrogen by volume. 
     
     
         12 . The natural gas liquefaction system of  claim 1 , wherein the driver assembly is an electric motor, a steam turbine, or a gas turbine. 
     
     
         13 . 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. 
     
     
         14 . A natural gas liquefaction system, comprising:
 at least one heat exchanger configured to liquefy and subcool a compressed natural gas containing feed stream via indirect heat exchange with a nitrogen-based refrigerant stream and a secondary refrigerant stream;   a first refrigeration circuit comprising at least two turbine/expanders configured to expand portions of the nitrogen-based refrigerant stream to produce one or more 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 at least two primary refrigerant compression stages including an upstream refrigerant compression stage and a serially arranged downstream refrigerant compression stage, wherein the refrigerant compression stages are configured to compress the warmed recycle streams;   a second refrigeration circuit comprising at least one turbine/expander configured to expand portions of the secondary refrigerant stream to produce one or more secondary 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 a warmed secondary recycle stream; and at least one secondary refrigerant compression stage configured to compress the warmed secondary recycle stream; and   an integral gear machine comprising a drive assembly; a bull gear; and at least three pinions arranged to drive the at least two primary refrigerant compression stages, the at least one secondary refrigerant compression stage, and for receiving work produced by the turbines/expanders in the first refrigeration circuit and the second refrigeration circuit;   wherein the two or more turbines/expanders in the first refrigeration circuit further comprise: a cold turbine/expander configured to expand a cold portion of the nitrogen-based refrigerant stream and produce a cold exhaust that is also recycled to the upstream refrigerant compression stage; and a first warm turbine/expander configured to expand a first warm portion of the nitrogen-based refrigerant stream and produce a first warm exhaust to be recycled to the upstream refrigerant compression stage;   wherein the at least one turbine/expander in the second refrigeration circuit further comprises: a second warm turbine/expander configured to expand a second warm portion of the secondary refrigerant stream and produce a second warm exhaust to be recycled to the secondary refrigerant compression stage; 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.   
     
     
         15 . The natural gas liquefaction system of  claim 14 , wherein the second warm exhaust is above the critical point temperature of the compressed natural gas containing feed stream and less than about −15° C. 
     
     
         16 . The natural gas liquefaction system of  claim 14 , wherein the first warm turbine/expander and the cold turbine/expander are each configured with an expansion ratio of between 4.0 and 5.0 and wherein the second warm turbine/expander is configured with an expansion ratio of between 1.5 and 2.5. 
     
     
         17 . The natural gas liquefaction system of  claim 14 , wherein the first warm turbine/expander and the upstream compression stage are operatively coupled to a first pinion of the at least three pinions, and the cold turbine/expander and the downstream compression stage in the first refrigeration circuit are operatively coupled to a second pinion of the at least three pinions, and the second warm turbine/expander and the secondary refrigerant compression stage are operatively coupled to a third pinion of the at least three pinions. 
     
     
         18 . The natural gas liquefaction system of  claim 14 , wherein the compressed natural gas containing feed stream is at a pressure greater than the critical pressure of natural gas and between about 50 bar(a) and 80 bar(a). 
     
     
         19 . The natural gas liquefaction system of  claim 14 , wherein the nitrogen-based refrigerant comprise more than about 80% nitrogen by volume and the secondary refrigerant has a different composition than the nitrogen-based refrigerant.