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 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 refrigeration circuit comprising with at least two compositionally different refrigerants operating at different temperature levels and thereby forming a first refrigeration circuit and a second refrigeration circuit, the refrigeration circuit having at least one heat exchanger and a plurality of recycle compression stages and a plurality of turbines;   wherein the at least one heat exchanger further comprises 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 refrigerant stream in the first refrigeration circuit and the second refrigeration circuit traversing the one or more of the plurality of warming passages to produce the stream of liquefied natural gas;   wherein the plurality of recycle compression stages and the plurality of turbines further comprises a plurality of primary refrigerant compression stages and one or more primary refrigerant turbines disposed in the first refrigeration circuit and configured for recirculating a primary refrigerant stream through the at least one heat exchanger to provide primary refrigeration; and   wherein the plurality of recycle compression stages and the plurality of turbines further comprises at least one natural gas compression stage and one or more natural gas expanders disposed in the second refrigeration circuit and configured for recirculating a secondary refrigerant stream through the at least one heat exchanger to provide auxiliary refrigeration;   an integral gear machine having at least three pinions and configured for driving the plurality of recycle compression stages and receiving work produced by at least one of the one or more turbines of the first and second refrigeration cycles;   wherein the at least one natural gas compression stage is operatively coupled to the integral gear machine by a third pinion of the at least three pinions and wherein the plurality of primary refrigerant compression stages are operatively coupled to the integral gear machine by a first pinion or a second pinion of the at least three pinions or both the first pinion and the second pinion;   wherein the one or more natural gas expanders further comprise a natural gas expander configured to expand a first portion of the further compressed natural gas stream and produce a natural gas exhaust stream that is directed to a natural gas warming passage in the at least one heat exchanger and combined with the natural gas containing feed stream; and   wherein the one or more primary refrigerant turbines further comprise a warm turbine configured to expand a warm portion of the primary refrigerant stream and produce a warm exhaust that is recycled to the one or more refrigerant compression stages via one of the plurality of warming passages in the at least one heat exchanger; and a cold turbine configured to expand a cold portion of the primary refrigerant stream and produce a cold exhaust 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.   
     
     
         2 . The natural gas liquefaction system of  claim 1 , wherein the natural gas containing feed stream is a methane containing biogas feed stream. 
     
     
         3 . The natural gas liquefaction system of  claim 1 , wherein the at least one natural gas compression stage and the one or more natural gas expanders are configured to operate at a rotational speed of greater than 20,000 rpm. 
     
     
         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 the first refrigerant compression stage and the warm turbine configured to expand a warm portion of the primary refrigerant stream are operatively coupled to the integral gear machine by the first pinion of the at least three pinions and the second refrigerant compression stage and the cold turbine configured to expand a cold portion of the primary refrigerant stream are operatively coupled to the integral gear machine by the second pinion of the at least three pinions. 
     
     
         5 . The natural gas liquefaction system of  claim 1 , wherein the first refrigerant compression stage and the warm turbine are configured to operate at rotational speeds of between 20,000 rpm and 50,000 rpm. 
     
     
         6 . The natural gas liquefaction system of  claim 5 , 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 turbine. 
     
     
         6 . The natural gas liquefaction system of  claim 1 , wherein the plurality of refrigerant compression stages comprise a first refrigerant compression stage, a second refrigerant compression stage, and a third refrigerant compression stage and wherein at least two of the refrigerant compression stages are operatively coupled to the integral gear machine by a common pinion. 
     
     
         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 1 , further comprising a purification system wherein the purification system 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 feed stream to produce an overhead stream of purified natural gas and an impure bottoms liquid stream. 
     
     
         10 . The natural gas liquefaction system of  claim 1 , wherein the natural gas exhaust stream is combined with the purified natural gas 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 . 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;   liquefying and subcooling a first portion of the purified, compressed natural gas stream in a refrigeration circuit comprising at least two distinct refrigeration cycles with at least two compositionally different refrigerants operating at different temperature levels, the refrigeration circuit having at least one heat exchanger and a plurality of recycle compression stages and a plurality of turbines;   wherein the plurality of recycle compression stages and the plurality of turbines further comprises a plurality of primary refrigerant compression stages, at least one natural gas compression stage, one or more primary refrigerant turbines, and one or more natural gas expander disposed in operative association with the at least one heat exchanger;   wherein the process to produce liquefied natural gas further comprises the steps of: (i) expanding a second portion of the purified, compressed natural gas stream in the at least one 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 at least one natural gas compression stage; (iv) compressing a primary refrigerant stream in a plurality of primary refrigerant recycle compression stages; (v) expanding the compressed primary 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, and the one or more primary refrigerant turbines are operatively coupled to an integral gear machine having at least three pinions and wherein one turbine is paired with one recycle compression stage or one natural gas compression stage on each of the three pinions of the integral gear machine.   
     
     
         13 . The process to produce liquefied natural gas of  claim 12 , wherein the natural gas containing feed stream is a methane containing biogas feed stream. 
     
     
         14 . The process to produce liquefied natural gas of  claim 12 , 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. 
     
     
         15 . The process to produce liquefied natural gas of  claim 12  wherein the at least one natural gas compression stage is operatively coupled to the integral gear machine by a third pinion of the at least three pinions and wherein the plurality of primary refrigerant compression stages are operatively coupled to the integral gear machine by a first pinion or a second pinion of the at least three pinions or both the first pinion and the second pinion. 
     
     
         16 . The process to produce liquefied natural gas of  claim 12 , wherein the one or more primary refrigerant turbines further comprise a cold turbine and a warm turbine;
 wherein the step of expanding the compressed primary 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; and   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.   
     
     
         17 . The process to produce liquefied natural gas of  claim 16 , 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. 
     
     
         18 . The process to produce liquefied natural gas of  claim 16 , 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. 
     
     
         19 . The process to produce liquefied natural gas of  claim 12 , wherein the primary refrigerant stream is a nitrogen based refrigerant stream. 
     
     
         20 . The process to produce liquefied natural gas of  claim 12 , 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). 
     
     
         21 . The process to produce liquefied natural gas of  claim 20 , wherein the step of expanding the second portion of the purified, compressed natural gas stream further comprises expanding the second portion of the purified, compressed natural gas stream at an expansion ratio approximately equal to or comparable to a compression ratio of the natural gas compression stage. 
     
     
         22 . The process to produce liquefied natural gas of  claim 21 , wherein the at least one natural gas compression stage has a compression ratio less than about 3.0. 
     
     
         23 . The process to produce liquefied natural gas of  claim 12 , 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. 
     
     
         24 . The process to produce liquefied natural gas of  claim 12 , 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 24 , 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 12 , 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.