System and method to produce liquefied natural gas using two distinct refrigeration cycles with an integral gear machine

Abstract

A system and method for liquefaction of natural gas using two distinct refrigeration circuits having compositionally different working fluids and operating at different temperature levels is provided. The turbomachinery associated with the liquefaction system are driven by a single three-pinion or four-pinion integral gear machine with customized pairing arrangements. The system and method of natural gas liquefaction further includes the conditioning of a lower pressure natural gas containing feed stream to produce a purified, compressed natural gas containing stream at a pressure equal to or above the critical pressure of natural gas and substantially free of heavy hydrocarbons to be liquefied.

What is claimed is: 
     
         1 . A natural gas liquefaction system configured for producing a stream of liquefied natural gas, comprising:
 at least one heat exchanger having one or more cooling passages and a plurality of warming passages wherein the at least one heat exchanger is configured to liquefy a natural gas containing feed stream traversing the one or more cooling passages via indirect heat exchange with a primary refrigerant stream traversing the one or more of the plurality of warming passages to produce the stream of liquefied natural gas;   a refrigeration circuit comprising at least two distinct refrigeration cycles with at least two compositionally different working fluids operating at different temperature levels;   wherein the refrigeration circuit further comprises a plurality of primary refrigerant compression stages and one or more turbines disposed in operative association with the at least one heat exchanger and configured for recirculating a primary refrigerant stream therethrough to provide refrigeration as a first refrigeration cycle of the at least two distinct refrigeration cycles;   wherein the refrigeration circuit further comprises at least one natural gas compression stage and one or more natural gas turbines disposed in operative association with the at least one heat exchanger and configured for recirculating a secondary refrigerant stream therethrough to provide refrigeration as a second refrigeration cycle of the at least two distinct refrigeration cycles;   an integral gear compressor machine having at least three pinions and configured for driving the plurality of refrigerant compression stages and at least one natural gas compression stage and receiving work produced by at least one of the one or more primary refrigerant turbines and the one or more natural gas turbines;   wherein the at least one natural gas compression stage is operatively coupled to the integral gear compressor machine by a first pinion of the at least three pinions and the at least one natural gas compression stage is configured to receive a portion of the natural gas containing feed stream and produce a further compressed natural gas stream that is directed to one or more of the plurality of cooling passages in the at least one heat exchanger; and   wherein the plurality of refrigerant compression stages are operatively coupled to the integral gear compressor machine by a second pinion or a third pinion of the at least three pinions or both the second pinion and the third pinion; and   wherein the one or more natural gas turbines further comprise a warm natural gas turbine configured to expand a portion of the further compressed natural gas stream; and   wherein the one or more primary refrigerant turbines further comprise a cold turbine configured to expand a cold portion of the primary refrigerant stream.   
     
     
         2 . The natural gas liquefaction system of  claim 1 , wherein the natural gas containing feed stream is derived from a biogas source. 
     
     
         3 . The natural gas liquefaction system of  claim 1 , wherein the natural gas compression stage and the warm natural gas turbine are operatively coupled to the integral gear compressor machine by the first pinion of the at least three pinions. 
     
     
         4 . The natural gas liquefaction system of  claim 1 , wherein the plurality of refrigerant compression stages comprise a first refrigerant compression stage and a second refrigerant compression stage and wherein one of the first refrigerant compression stage or the second refrigerant compression stage and the cold turbine are operatively coupled to the integral gear compressor machine by a common pinion. 
     
     
         5 . The natural gas liquefaction system of  claim 1 , wherein:
 the one or more primary refrigerant turbines further comprise a warm primary refrigerant turbine configured to expand a warm portion of the primary refrigerant stream and the cold turbine configured to expand a cold portion of the primary refrigerant stream, and   the plurality of refrigerant compression stages comprise a first refrigerant compression stage and a second refrigerant compression stage and wherein the first refrigerant compression stage and the warm primary refrigerant turbine are operatively coupled to the integral gear compressor machine by the second pinion of the at least three pinions and the second refrigerant compression stage and the cold turbine are operatively coupled to the integral gear compressor machine by the third pinion of the at least three pinions.   
     
     
         6 . The natural gas liquefaction system of  claim 5 , wherein the first refrigerant compression stage and the warm primary refrigerant turbine are configured to operate at rotational speeds of between about 20,000 rpm and 50,000 rpm. 
     
     
         7 . The natural gas liquefaction system of  claim 6 , wherein the second refrigerant compression stage and the cold turbine are configured to operate at a rotational speed greater than the rotational speed of the first refrigerant compression stage and the warm primary refrigerant turbine. 
     
     
         8 . The natural gas liquefaction system of  claim 1 , wherein the primary refrigerant stream is a nitrogen-based refrigerant stream. 
     
     
         9 . The natural gas liquefaction system of  claim 8 , wherein the plurality of refrigerant compression stages comprise a first nitrogen compression stage, a second nitrogen compression stage, and a third nitrogen compression stage and wherein at least two of the nitrogen compression stages are operatively coupled to the integral gear compressor machine by a common pinion. 
     
     
         10 . The natural gas liquefaction system of  claim 1 , further comprising a scrubbing column or phase separator configured to receive the further compressed natural gas stream and strip out impurities to produce an overhead stream of purified natural gas and an impure bottoms liquid stream. 
     
     
         11 . The natural gas liquefaction system of  claim 1 , further comprising a liquid turbine configured to expand the subcooled liquid natural gas to provide a supplemental source of refrigeration. 
     
     
         12 . The natural gas liquefaction system of  claim 3 , wherein the warm natural gas turbine is a high speed turbine configured with an expansion ratio approximately equal to or comparable to a compression ratio of the natural gas compression stage.