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, three-turbine 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 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:
 a conditioning circuit configured to receive a natural gas containing feed stream and produce a purified, compressed natural gas stream at a pressure equal to or above the critical pressure of natural gas, the conditioning circuit having at least one natural gas compression stage;   a refrigeration circuit comprising: (i) at least two compositionally different working fluids operating at different temperature levels thereby forming a first refrigeration circuit and a second refrigeration circuit; (ii) 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 and subcool a first portion of the purified, compressed natural gas stream traversing the one or more cooling passages via indirect heat exchange with two or more different refrigerant streams traversing the one or more of the plurality of warming passages to produce the stream of liquefied natural gas; and (iii) two or more turbines configured to expand the at least two compositionally different working fluids to provide refrigeration; and (iv) one or more recycle compression stages configured to recycle the expanded and warmed working fluids; and   an integral gear machine having at least three pinions and configured for driving the one or more recycle compression stages and at least one natural gas compression stage and receiving work produced by at least one of the one or more turbines of the first and second refrigeration circuits;   wherein at least one turbine from the first refrigeration circuit is paired with at least one compression stage from the first refrigeration circuit and at least one turbine from the second refrigeration circuit is paired with at least one compression stage from the second refrigeration circuit on the integral gear machine.   
     
     
         2 . The natural gas liquefaction system of  claim 1 , wherein natural gas containing feed stream is a methane containing biogas feed stream. 
     
     
         3 . The natural gas liquefaction system of  claim 1 , wherein the integral gear machine further comprises a bull gear, a driver assembly and at least three pinions, and wherein the driver assembly is an electric motor, a steam turbine, or a gas turbine. 
     
     
         4 . The natural gas liquefaction system of  claim 3 , wherein the work produced by each of the turbines on each of the at least three pinions of the integral gear machine is less than the power required by the one recycle compression stage or one natural gas compression stage on the same pinion such that each pinion is a net absorber of power. 
     
     
         5 . The natural gas liquefaction system of  claim 1 , wherein the pressure of the natural gas containing feed stream is between about 20 bar(a) and 40 bar(a) and the at least one natural gas compression stage in the conditioning circuit is configured to further compress the natural gas containing feed stream and produce the purified, compressed natural gas stream at a pressure between about 50 bar(a) and 80 bar(a). 
     
     
         6 . The natural gas liquefaction system of  claim 1 , wherein one of the two or more turbines is a natural gas expander configured to expand a second portion of the purified, compressed natural gas stream to produce a natural gas exhaust stream having a pressure less than or equal to the pressure of the natural gas containing feed stream. 
     
     
         7 . The natural gas liquefaction system of  claim 6 , wherein the natural gas expander is a high speed turbo-expander configured with an expansion ratio approximately equal to or comparable to a compression ratio of the natural gas compression stage. 
     
     
         8 . The natural gas liquefaction system of  claim 7 , wherein the compression ratio of the natural gas compression stage is less than about 3.0. 
     
     
         9 . The natural gas liquefaction system of  claim 6 , wherein the natural gas exhaust stream is directed to warming passages in the at least one heat exchanger to warming passages to cool the first portion of the purified, compressed natural gas stream or other natural gas streams traversing the one or more cooling passages via indirect heat exchange and is then recycled to the at least one natural gas compression stage. 
     
     
         10 . The natural gas liquefaction system of  claim 6 , wherein the flow of second portion of the purified, compressed natural gas stream is at least 2.0 times greater than the flow of first portion of the purified, compressed natural gas stream. 
     
     
         11 . The natural gas liquefaction system of  claim 6 , wherein the flow of the second portion of the purified, compressed natural gas stream is at least 2.5 times greater than the flow of first portion of the purified, compressed natural gas stream. 
     
     
         12 . The natural gas liquefaction system of  claim 1 , wherein the conditioning circuit further comprises a phase separator or a scrubbing column or both a phase separator and a scrubbing column configured to strip out heavy hydrocarbons and other impurities from the natural gas containing feed stream to produce an overhead stream of purified natural gas and an impure bottoms liquid stream. 
     
     
         13 . The natural gas liquefaction system of  claim 1 , wherein one of the two or more turbines is a cold turbine configured to expand a cold portion of the primary refrigerant stream to produce a cold exhaust stream that is recycled to the one or more primary refrigerant compression stages via one or more of the plurality of warming passages in the at least one heat exchanger and another of the two or more turbines is a warm turbine configured to expand a warm portion of the primary refrigerant stream to produce a warm exhaust stream that is also recycled to the one or more primary refrigerant compression stages via one or more of the plurality of warming passages in the at least one heat exchanger. 
     
     
         14 . The natural gas liquefaction system of  claim 13 , wherein at least one of the plurality of primary refrigerant compression stages and the cold turbine are operatively coupled to the integral gear machine by a first pinion of the at least three pinions. 
     
     
         15 . The natural gas liquefaction system of  claim 13 , wherein at least one of the plurality of primary refrigerant compression stages and the warm turbine are operatively coupled to the integral gear machine by a second pinion of the at least three pinions. 
     
     
         16 . The natural gas liquefaction system of  claim 1 , wherein the at least one natural gas compression stage and the natural gas expander are operatively coupled to the integral gear machine by a third pinion of the at least three pinions. 
     
     
         17 . A process to produce liquefied natural gas comprising the steps of:
 receiving a natural gas containing feed stream;   conditioning the natural gas containing feed stream to produce a purified, compressed natural gas stream at a pressure equal to or above the critical pressure of natural gas, the step of conditioning further comprises compressing the natural gas containing feed stream in at least one natural gas compression stage;   liquefying and subcooling a first portion of the purified, compressed natural gas stream in at least one heat exchanger via indirect heat exchange with at least two compositionally different refrigerant streams operating at different temperature levels;   wherein a first refrigerant stream of the of the at least two compositionally different refrigerant streams is a second portion of the purified, compressed natural gas stream and a second refrigerant stream further comprises a primary refrigerant and wherein the process to produce liquefied natural gas further comprises the steps of: (i) expanding the first refrigerant stream in a natural gas expander to produce a natural gas exhaust stream having a pressure less than or equal to the pressure of the natural gas containing feed stream; (ii) directing the natural gas exhaust stream to the at least one heat exchanger to cool the first portion of the purified, compressed natural gas stream; (iii) recycling the warmed natural gas exhaust stream to the natural gas compression stage; (iv) compressing the second refrigerant stream in a plurality of primary refrigerant recycle compression stages; (v) expanding the compressed first refrigerant stream in one or more primary refrigerant turbines to produce one or more primary refrigerant exhaust streams; (vi) directing the one or more primary refrigerant exhaust streams to the at least one heat exchanger to cool the first portion of the purified, compressed natural gas stream; and (vii) recycling the warmed primary refrigerant exhaust streams to the plurality of primary refrigerant recycle compression stages; wherein the one or more recycle compression stages, the at least one natural gas compression stage, the natural gas expander and the one or more primary refrigerant turbines are operatively coupled to an integral gear machine having at least three pinions.   
     
     
         18 . The process to produce liquefied natural gas of  claim 17 , wherein natural gas containing feed stream is a methane containing biogas feed stream. 
     
     
         19 . The process to produce liquefied natural gas of  claim 17 , wherein one turbine is paired with one recycle compression stage or one natural gas compression stage on each of the at least three pinions of the integral gear machine. 
     
     
         20 . The process to produce liquefied natural gas of  claim 19 , wherein the work produced by each of the turbines on the at least three pinions of the integral gear machine is less than the power required by the one recycle compression stage or one natural gas compression stage on the same pinion. 
     
     
         21 . The process to produce liquefied natural gas of  claim 17 , wherein the pressure of the natural gas containing feed stream is between about 20 bar(a) and 40 bar(a) and the step of conditioning the natural gas containing feed stream further comprises compressing the natural gas containing feed stream to a pressure between about 60 bar(a) and 80 bar(a). 
     
     
         22 . The process to produce liquefied natural gas of  claim 17 , wherein the step of expanding the first refrigerant stream in the natural gas expander further comprises expanding the first refrigerant stream at an expansion ratio approximately equal to or comparable to a compression ratio of the natural gas compression stage. 
     
     
         23 . The process to produce liquefied natural gas of  claim 22 , wherein the step of compressing the natural gas containing feed stream in at least one natural gas compression stage further comprises compressing the natural gas containing feed stream at a compression ratio less than about 3.0. 
     
     
         24 . The process to produce liquefied natural gas of  claim 17 , wherein the flow of second portion of the purified, compressed natural gas stream is at least 2.0 times greater than the flow of first portion of the purified, compressed natural gas stream. 
     
     
         25 . The process to produce liquefied natural gas of  claim 17 , wherein the flow of second portion of the purified, compressed natural gas stream is at least 2.5 times greater than the flow of first portion of the purified, compressed natural gas stream. 
     
     
         26 . The process to produce liquefied natural gas of  claim 17 , wherein the step of conditioning the natural gas containing feed stream further comprises stripping out heavy hydrocarbons and other impurities from the natural gas containing feed stream. 
     
     
         27 . The process to produce liquefied natural gas of  claim 17 , wherein the one or more primary refrigerant turbines further comprise a cold turbine configured and a warm turbine and:
 wherein the step of expanding the compressed first refrigerant stream further comprises expanding a cold portion of the primary refrigerant stream to produce a cold exhaust stream and expanding a warm portion of the primary refrigerant stream to produce a warm exhaust stream;   wherein the step of directing the one or more primary refrigerant exhaust streams to the at least one heat exchanger further comprises directing the cold exhaust stream and the warm exhaust stream to the at least one heat exchanger to cool the first portion of the purified, compressed natural gas stream; and   wherein the step of recycling the warmed primary refrigerant exhaust streams further comprises recycling the warmed exhaust streams to the plurality of primary refrigerant recycle compression stages.   
     
     
         28 . The process to produce liquefied natural gas of  claim 27 , wherein at least one of the plurality of primary refrigerant compression stages and the cold turbine are operatively coupled to the integral gear machine by a first pinion of the at least three pinions. 
     
     
         29 . The process to produce liquefied natural gas of  claim 27 , wherein at least one of the plurality of primary refrigerant compression stages and the warm turbine are operatively coupled to the integral gear machine by a second pinion of the at least three pinions. 
     
     
         30 . The process to produce liquefied natural gas of  claim 17 , wherein the at least one natural gas compression stage and the natural gas expander are operatively coupled to the integral gear machine by a third pinion of the at least three pinions.