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 containing 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 at least two of which 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 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   wherein the three or more turbine/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 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 the expansion ratio of the cold turbine/expander and lower than the expansion ratio of the first warm turbine/expander.   
     
     
         2 . The natural gas liquefaction system of  claim 1 , wherein the cold turbine exhaust is at a temperature colder than -145° C. and the first warm exhaust is at a temperature colder than -90° C. 
     
     
         3 . The natural gas liquefaction system of  claim 2 , wherein the second warm exhaust is at a temperature above the critical point temperature of the compressed natural gas containing feed stream. 
     
     
         4 . The natural gas liquefaction system of  claim 3 , wherein the temperature of the second warm exhaust is colder than about -15° C. 
     
     
         5 . The natural gas liquefaction system of  claim 1 , wherein the natural gas containing feed stream is a natural gas feed stream derived from a biogas source. 
     
     
         6 . 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. 
     
     
         7 . 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 45% of the refrigeration for the natural gas liquefaction system. 
     
     
         8 . 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 25% of the refrigeration for the natural gas liquefaction system. 
     
     
         9 . 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 refrigeration for the natural gas liquefaction system. 
     
     
         10 . The natural gas liquefaction system of  claim 1 , further comprising an integral gear machine comprising a drive assembly, a bull gear, and a plurality of pinions arranged to drive the two or more refrigerant compression stages and/or for receiving work produced by the at least three turbine/expanders. 
     
     
         11 . The natural gas liquefaction system of  claim 10 , wherein the second warm turbine/expander and the upstream compression stage are operatively coupled to a first pinion of the plurality of pinions, and the first warm turbine/expander and the downstream compression stage are operatively coupled to a second pinion of the plurality of pinions. 
     
     
         12 . The natural gas liquefaction system of  claim 11 , wherein the cold turbine/expander is operatively coupled to a third pinion of the plurality of pinions. 
     
     
         13 . The natural gas liquefaction system of  claim 10 , 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. 
     
     
         14 . The natural gas liquefaction system of  claim 13 , wherein the second warm turbine/expander and the upstream refrigerant compression stage are operatively coupled to a first pinion of the plurality of pinions, and the first warm turbine/expander and the downstream refrigerant compression stage are operatively coupled to a second pinion of the plurality of pinions. 
     
     
         15 . The natural gas liquefaction system of  claim 14 , wherein the cold turbine/expander and the second upstream refrigerant compression stage are operatively coupled to a third pinion of the plurality of pinions. 
     
     
         16 . The natural gas liquefaction system of  claim 15 , wherein at least two of the plurality of pinions are a net absorber of power from the drive assembly. 
     
     
         17 . The natural gas liquefaction system of  claim 15 , wherein the driver assembly is an electric motor, a steam turbine, or a gas turbine. 
     
     
         18 . The natural gas liquefaction system of  claim 15 , wherein the power imparted to the two of the plurality of pinions does not differ by more than 10%. 
     
     
         19 . 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. 
     
     
         20 . 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). 
     
     
         21 . The natural gas liquefaction system of  claim 1 , wherein the refrigerant stream comprises more than about 80% nitrogen by volume. 
     
     
         22 . 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. 
     
     
         23 . A method to produce liquefied natural gas comprising the steps of:
 (a) receiving a purified, compressed natural gas containing feed stream;   (b) liquefying and subcooling a purified, compressed natural gas containing feed stream in at least one heat exchanger via indirect heat exchange with one or more refrigerant streams to produce one or more lower pressure recycle streams and a higher pressure recycle stream;   (c1) compressing the one or more lower pressure recycle streams in an upstream refrigeration compression stage to produce a compressed refrigerant stream;   (c2) compressing the higher pressure recycle streams and the compressed refrigerant stream in a downstream refrigeration compression stage to produce a further compressed refrigerant stream;   (d) cooling the further compressed refrigerant stream in the at least one heat exchanger;   (e1) extracting a cold portion of the further compressed refrigerant stream from the at least one heat exchanger;   (e2) extracting a first warm portion of the further compressed refrigerant stream from an intermediate location of the at least one heat exchanger;   (e3) extracting a second warm portion of the further compressed refrigerant stream from a second intermediate location of the at least one heat exchanger;   (f1) expanding the cold portion of the compressed refrigerant stream in a cold turbine/expander and produce a cold exhaust at a temperature colder than -145° C.;   (f2) expanding the first warm portion of the compressed refrigerant stream in a first warm turbine/expander to produce a first warm exhaust at a temperature colder than about -90° C. and warmer than the cold exhaust;   (f3) expanding the second warm portion of the compressed refrigerant stream in a second warm turbine/expander to produce a second warm exhaust at a temperature above the critical point temperature of the compressed natural gas containing feed stream and colder than about -15° C. and at an outlet pressure higher than outlet pressures of the cold turbine/expander and the first warm turbine/expander;   (g1) directing the cold exhaust and the first warm exhaust to the at least one heat exchanger as a refrigeration source to liquefy and subcool the purified, compressed natural gas containing feed stream, and yield one or more lower pressure recycle streams;   (g2) directing the second warm exhaust to the at least one heat exchanger as a refrigeration source to fore-cool the purified, compressed natural gas containing feed stream and yield a higher pressure recycle stream;   (h1) recycling the one or more lower pressure recycle streams to one or more upstream refrigerant compression stages in the plurality of compression stages; and   (h2) recycling the higher pressure recycle stream to a downstream refrigerant compression stage in the plurality of compression stages.   
     
     
         24 . The method of  claim 23 , 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 45% of the refrigeration for the step of liquefying and subcooling the purified, compressed natural gas containing feed stream. 
     
     
         25 . The method of  claim 23 , 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 25% of the refrigeration for the step of liquefying and subcooling the purified, compressed natural gas containing feed stream. 
     
     
         26 . The method of  claim 23 , 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 refrigeration for the step of liquefying and subcooling the purified, compressed natural gas containing feed stream. 
     
     
         27 . The method of  claim 23 , wherein the cold turbine/expander, first warm turbine/expander, the second warm turbine/expander, the upstream refrigeration compression stages and the downstream refrigeration compression stages are operatively coupled via an integral gear machine. 
     
     
         28 . The method of  claim 27 , wherein the cold turbine/expander is operatively coupled to a third pinion of the integral gear machine. 
     
     
         29 . The method of  claim 23 , wherein the purified, compressed natural gas containing feed stream is a natural gas feed stream derived from a biogas source . 
     
     
         30 . The method of  claim 23 , wherein the purified, compressed natural gas containing feed stream is at a pressure greater than the critical pressure of natural gas. 
     
     
         31 . The method of  claim 23 , wherein the purified, compressed natural gas containing feed stream is at a pressure between about 50 bar(a) and 80 bar(a). 
     
     
         32 . The method of  claim 23 , wherein the refrigerant stream comprises more than about 80% nitrogen by volume.