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 containing feed stream 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:
 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 a purified, compressed natural gas containing 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 four pinions and configured for driving the plurality of refrigerant compression stages and at least one natural gas compression stage and for receiving work produced by the two or more turbines;   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 four pinions and the at least one natural gas compression stage is configured to receive the purified, compressed natural gas containing stream and produce a further compressed natural gas stream at a pressure above the critical pressure of natural gas, and a portion of which is directed to one or more of the plurality of cooling passages in the at least one heat exchanger to be liquefied; and   wherein the plurality of primary refrigerant compression stages are operatively coupled to the integral gear machine by one or more of the at least four pinions.   
     
     
         2 . The natural gas liquefaction system of  claim 1 , wherein the two or more turbines further comprise at least one natural gas expander that is paired with at least one compression stage from the first refrigeration circuit and at least one primary refrigerant turbine that is paired with at least one primary refrigerant compression stage from the second refrigeration circuit on the integral gear machine. 
     
     
         3 . The natural gas liquefaction system of  claim 1 , wherein the plurality of primary refrigerant compression stages comprise a first refrigerant compression stage, a second refrigerant compression stage, and a third refrigerant compression stage and wherein at least one of the primary refrigerant compression stages is operatively coupled to the integral gear machine by a second pinion, at least one of the refrigerant compression stages is operatively coupled to the integral gear machine by a third pinion, and at least one of the refrigerant compression stages is operatively coupled to the integral gear machine by a fourth pinion. 
     
     
         4 . The natural gas liquefaction system of  claim 1 , further comprising a conditioning circuit configured to receive a natural gas containing feed stream and produce the purified, compressed natural gas containing stream. 
     
     
         5 . The natural gas liquefaction system of  claim 4 , wherein the natural gas containing feed stream is a methane containing biogas feed stream. 
     
     
         6 . The natural gas liquefaction system of  claim 4 , 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 feed stream to produce the purified natural gas stream as an overhead stream and an impure bottoms liquid stream. 
     
     
         7 . The natural gas liquefaction system of  claim 6 , wherein the pressure of the natural gas feed stream is between about 20 bar(a) and 40 bar(a) and the natural gas compression stage is configured to further compress the purified natural gas feed stream and produce the purified, compressed natural gas stream at a pressure between about 50 bar(a) and 80 bar(a). 
     
     
         8 . The natural gas liquefaction system of  claim 1 , wherein one of the refrigerant streams is a nitrogen-based refrigerant stream and the other refrigerant streams is a second portion of the purified, further compressed natural gas. 
     
     
         9 . The natural gas liquefaction system of  claim 8 , wherein one of the two or more turbines is a natural gas expander configured to expand the 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 feed stream. 
     
     
         10 . The natural gas liquefaction system of  claim 8 , 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 10 , 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 , further comprising a liquid turbine configured to expand the subcooled liquified natural gas. 
     
     
         13 . The natural gas liquefaction system of  claim 1 , wherein the integral gear machine further comprises a bull gear, a driver assembly and four pinions, and wherein the driver assembly is an electric motor, a steam turbine, or a gas turbine. 
     
     
         14 . The natural gas liquefaction system of  claim 13 , wherein each pinion of the four pinions of the integral gear machine is a net absorber of power.