LNG Liquefaction System and Process

Abstract

The present invention comprises systems and methods for natural gas liquefaction. In embodiments, the systems comprise a dual turbo-expander, methane-based refrigeration system that also uses a slip stream of LNG for additional cooling.

1 . A method for natural gas liquefaction, comprising:
 providing a clean gas stream and a recycled gas stream at a first pressure;   mixing the clean gas stream and the recycled gas stream to form a mixed gas stream;   splitting the mixed gas stream into at least a first stream and a second stream;   passing the first stream and the second stream through a heat exchanger;
 wherein the heat exchanger cools the first stream to form a first liquefied stream by cross exchanging with one or more refrigeration streams, wherein the one or more refrigeration streams comprise:
 a warm expander refrigeration stream; 
 a cold expander refrigeration stream; and 
 a secondary refrigeration stream; and 
 
   cooling the second stream by passing it through the heat exchanger to form a cooled gas stream;   splitting the cooled gas stream into a first split stream and a second split stream;   passing the first split stream through a warm turbo-expander to form the warm expander refrigeration stream;   passing the second split stream through the heat exchanger and a cold turbo-expander to form a cooled split stream;   passing the cooled split stream through a cold separator to separate the cooled split stream into a second liquefied stream and the cold expander refrigeration stream;   combining the second liquefied stream with the first liquefied stream to form a third liquefied stream;   passing the warm expander refrigeration stream and the cold expander refrigeration stream through the heat exchanger;
 wherein the warm expander refrigeration stream and the cold expander refrigeration stream are combined in or after exiting the heat exchanger to form a combined stream; 
   generating a slipstream from the first liquefied stream;   combining the slipstream with a boil off gas stream from liquid natural gas storage to form the secondary refrigeration stream;   passing the secondary refrigeration stream through the heat exchanger to form a secondary refrigeration return gas stream;   compressing the secondary refrigeration return gas stream using a first compressor to form a compressed secondary refrigeration return gas stream;   combining the compressed secondary refrigeration return gas steam with the combined stream to form a second combined stream;   compressing and cooling the second combined stream using one or more additional compressor and one or more cooler to form the recycled gas stream at pressure P recycle ;   reducing a pressure of the third liquefied stream to form a liquefied product stream; and   recycling the one or more refrigeration streams through the system until a desired cryogenic liquid storage temperature is reached.   
     
     
         2 . The method of  claim 1 , wherein the clean gas stream is free of or reduced in impurities that tend to freeze at cryogenic temperatures. 
     
     
         3 . The method of  claim 1 , wherein the first stream is a product gas stream. 
     
     
         4 . The method of  claim 1 , wherein the second stream is an expander gas stream. 
     
     
         5 . The method of  claim 1 , wherein the first stream is cooled by the heat exchanger to a cryogenic temperature. 
     
     
         6 . The method of  claim 1 , wherein the first liquefied stream is a liquefied natural gas product. 
     
     
         7 . The method of  claim 1 , wherein the cooled gas stream is a cooled expander gas stream. 
     
     
         8 . The method of  claim 1 , wherein the first split stream is a warm expander split stream. 
     
     
         9 . The method of  claim 1 , wherein the second split stream is a cold expander split stream. 
     
     
         10 . The method of  claim 1 , wherein passing the warm expander refrigeration stream and the cold expander refrigeration stream through the heat exchanger results in cooling of other gases in the heat exchanger. 
     
     
         11 . The method of  claim 1 , wherein one or more of the compressing steps is performed using work extracted at the warm turbo-expander and the cold turbo-expander. 
     
     
         12 . The method of  claim 1 , further comprising monitoring one or more of flow rate, flow volume, gas temperature, gas composition, or gas pressure. 
     
     
         13 . The method of  claim 1 , further comprising adjusting one or more of flow rate, flow volume, and/or flow ratio of one or more of the clean gas stream, the first stream, the second stream, the first split stream, the second split stream, the warm refrigeration stream, the cold refrigeration stream, and/or the secondary refrigeration stream based on the monitoring. 
     
     
         14 . The method of  claim 1 , further comprising expanding, decreasing the pressure of, and/or cooling one or more stream by way of one or more Joule-Thompson valve(s). 
     
     
         15 . The method of  claim 1 , further comprising after passing the first liquefied stream through the Joule-Thompson valve(s):
 i) providing the first liquefied stream that is an LNG to storage stream; and   ii) generating the slipstream that mixes with the boil off gas stream to form the secondary refrigeration stream.   
     
     
         16 . The method of  claim 1 , further comprising delivering the liquefied product stream to a storage container once the desired cryogenic liquid storage temperature is reached. 
     
     
         17 . The method of  claim 1 , wherein the warm turbo-expander, cold turbo-expander, and one or more compressor are part of a single system coupled via a bull gear and pinions. 
     
     
         18 . The method of  claim 17 , wherein a single motor provides all external power required to perform the method. 
     
     
         19 . The method of any of  claim 1 , wherein work extracted at the warm turbo-expander and/or the cold turbo-expander are used in compressing the secondary refrigeration return gas stream. 
     
     
         20 . The method of any of  claim 1 , wherein the secondary refrigeration return gas stream is boosted in pressure by way of a low-pressure compressor. 
     
     
         21 . The method of  claim 1 , wherein the secondary refrigeration stream passes through the heat exchanger to provide cooling for the process. 
     
     
         22 . A method for natural gas liquefaction, comprising:
 providing a gas stream and a recycled gas stream;   mixing the gas stream and the recycled gas stream to form a mixed gas stream;   splitting the mixed gas stream into at least a first stream and a second stream;   passing the first stream and the second stream through a heat exchanger comprising:
 a warm expander refrigeration stream; 
 a cold expander refrigeration stream; and 
 a secondary refrigeration stream; and 
 wherein the heat exchanger cools the first stream to form a first liquefied stream, which is split to provide the secondary refrigeration stream and a stream of liquefied natural gas; 
 wherein the heat exchanger cools the second stream by passing it through the heat exchanger to form a cooled gas stream, which is split into a first and second split stream:
 the first split stream is optionally passed through a heat exchanger and is passed through a cold turbo-expander to provide the cold expander refrigeration stream, which is optionally split to provide a second liquefied stream; and 
 the second split stream is passed through a warm turbo-expander to provide the warm expander refrigeration stream; 
 
   wherein one or more of the warm expander refrigeration stream, the cold expander refrigeration stream, and/or the secondary refrigeration stream are optionally passed through a heat exchanger and are compressed one or more times, individually or together, to provide a portion or all of the recycled gas stream.   
     
     
         23 . A system for natural gas liquefaction, comprising:
 one or more heat exchanger comprising:
 a warm expander refrigeration stream; 
 a cold expander refrigeration stream; and/or 
 a secondary refrigeration stream; and 
   wherein one or more of the heat exchangers comprise one or more inputs to receive one or more mixed gas streams from a natural gas stream and a recycled gas stream;   wherein one or more of the heat exchangers is configured to cool the mixed gas streams and provide a first liquefied stream and a cooled gas stream therefrom;   at least one warm turbo-expander configured to receive a portion of the cooled gas stream and to provide the warm expander refrigeration stream for input into one or more of the heat exchangers;   at least one cold turbo-expander configured to receive another portion of the cooled gas stream and to provide the cold expander refrigeration stream for input into one or more of the heat exchangers;   storage configured to receive all or a portion of the first liquefied stream, which stream optionally provides for the secondary refrigeration stream;   wherein one or more of the heat exchangers comprises one or more inputs to receive one or more or all of the warm expander refrigeration stream, the cold expander refrigeration stream and/or the secondary refrigeration stream;   one or more compressors with one or more inputs for receiving one or more or all of the warm expander refrigeration stream, the cold expander refrigeration stream and/or the secondary refrigeration stream, which compressor(s) provide the recycled gas stream as an output.