Configurations and methods for small scale lng production

Abstract

An LNG plant comprises a cold box and a refrigeration unit fluidly coupled with a plurality of heat exchanger passes in the cold box. The refrigeration unit is configured to provide a first refrigerant stream to a first heat exchanger pass of the plurality of heat exchanger passes at a first pressure, a second refrigerant stream to a second heat exchanger pass at a second pressure, and a third refrigerant stream to a third heat exchanger pass at a third pressure. The second refrigerant stream comprises a first portion of the first refrigerant stream, and the third refrigerant stream comprises a second portion of the first refrigerant stream. The second pressure and the third pressure are both below the first pressure. The cold box is configured to produce LNG from a natural gas feed stream to the cold box using a refrigeration content from the refrigeration unit.

What is claimed is: 
     
         1 . An LNG plant comprising:
 a cold box comprising a plurality of heat exchanger passes; and   a refrigeration unit comprising a closed refrigeration cycle, wherein the cold box is fluidly coupled with the refrigeration unit, wherein the cold box is configured to receive a natural gas feed stream and produce LNG from the feed stream using a refrigeration content from the refrigeration unit, wherein the refrigeration unit comprises:
 a first compressor unit configured to compress a refrigerant to produce a compressed refrigerant at a first pressure; 
 a first heat exchanger pass of the plurality of heat exchanger passes, wherein the first heat exchanger pass is configured to pass the compressed refrigerant through the cold box to cool the compressed refrigerant; 
 a splitter configured to separate the cooled, compressed refrigerant into a first portion and a second portion; 
 a first expander configured to receive the first portion from the splitter and expand the first portion to a second pressure, wherein the second pressure is less than the first pressure; 
 a second expander configured to receive the second portion from the splitter and expand the second portion to a third pressure, wherein the third pressure is less than the second pressure; 
 a second heat exchanger pass of the plurality of heat exchanger passes configured to pass the first portion at the second pressure through the cold box; 
 a third heat exchanger pass of the plurality of heat exchanger passes configured to pass the second portion at the third pressure through the cold box to provide at least a portion of the refrigeration content in the cold box; 
 at least one second compressor, wherein the at least one second compressor is configured to receive the second portion downstream of the third heat exchanger pass and compress the second portion to the second pressure; and 
 a mixer, wherein the mixer is configured to combine the compressed second portion downstream of the at least one second compressor and the first portion downstream of the second heat exchanger pass to form the refrigerant upstream of the first compressor. 
   
     
     
         2 . The LNG plant of  claim 1 , wherein the first compressor unit comprises a plurality of compressors arranged in series and an intercooler disposed between consecutive compressors. 
     
     
         3 . The LNG plant of  claim 1 , wherein the at least one second compressor comprises a plurality of second compressors, and wherein at least one second compressor of the plurality of second compressors is mechanically coupled to the first expander or the second expander. 
     
     
         4 . The LNG plant of  claim 1 , wherein the second pressure is between about 20% and about 50% of the first pressure on an absolute scale. 
     
     
         5 . The LNG plant of  claim 1 , wherein the third pressure is between about 3% and about 20% of the first pressure on an absolute scale. 
     
     
         6 . The LNG plant of  claim 1 , further comprising a heat exchanger fluidly coupled between the first compressor and the first heat exchanger pass, wherein the heat exchanger is configured to cool the compressed refrigerant prior to the compressed refrigerant passing to the first heat exchanger pass. 
     
     
         7 . An LNG plant comprising:
 a cold box comprising a heat exchanger, wherein the heat exchanger comprises a plurality of heat exchanger passes;   a refrigeration unit fluidly coupled with the plurality of heat exchanger passes, wherein the refrigeration unit is configured to provide:
 a first refrigerant stream to a first heat exchanger pass of the plurality of heat exchanger passes, wherein the first refrigerant stream is at a first pressure; 
 a second refrigerant stream to a second heat exchanger pass of the plurality of heat exchanger passes, wherein the second refrigerant stream comprises a first portion of the first refrigerant stream downstream of the first heat exchanger pass, and wherein the second refrigerant stream is at a second pressure; and 
 a third refrigerant stream to a third heat exchanger pass of the plurality of heat exchanger passes, wherein the third refrigerant stream comprises a second portion of the first refrigerant stream downstream of the first heat exchanger pass, and wherein the third refrigerant stream is at a third pressure, wherein the second pressure and the third pressure are both below the first pressure, 
   wherein the cold box is configured to receive a natural gas feed stream and produce LNG from the natural gas feed stream using a refrigeration content from the refrigeration unit in the plurality of heat exchanger passes.   
     
     
         8 . The LNG plant of  claim 7 , wherein the first pressure is between about 1,000 psia and 2,000 psia. 
     
     
         9 . The LNG plant of  claim 7 , wherein the second pressure is between about 20% and about 50% of the first pressure on an absolute scale. 
     
     
         10 . The LNG plant of  claim 7 , wherein the third pressure is between about 3% and about 20% of the first pressure on an absolute scale. 
     
     
         11 . The LNG plant of  claim 7 , wherein a ratio of the second pressure to the third pressure is between about 10:1 and about 2:1. 
     
     
         12 . The LNG plant of  claim 7 , wherein a molar ratio of a flowrate of the second refrigerant stream to a flowrate of the first refrigerant stream is between about 0.5 and about 0.75. 
     
     
         13 . The LNG plant of  claim 7 , wherein the refrigeration unit is configured to provide the LNG at an energy of between about 320 kW/ton and about 425 kW/ton. 
     
     
         14 . An LNG plant comprising:
 a cold box comprising a plurality of heat exchanger passes; and   a refrigeration unit comprising a closed refrigeration cycle, wherein the cold box is fluidly coupled with the refrigeration unit, wherein the cold box is configured to receive a natural gas feed stream and produce LNG from the natural gas feed stream using a refrigeration content from the refrigeration unit in the plurality of heat exchanger passes, wherein the refrigeration unit comprises:
 a first compressor unit configured to compress a refrigerant to produce a compressed refrigerant at a first pressure; 
 a first heat exchanger pass of the plurality of heat exchanger passes, wherein the first heat exchanger pass is configured to pass the compressed refrigerant through the cold box to cool the compressed refrigerant; 
 a splitter configured to separate the cooled, compressed refrigerant into a first portion and a second portion; 
 a second heat exchanger pass of the plurality of heat exchanger passes configured to pass the first portion at a second pressure through the cold box, wherein the second pressure is less than the first pressure; 
 a third heat exchanger pass of the plurality of heat exchanger passes configured to pass the second portion at a third pressure through the cold box to provide at least a portion of the refrigeration content in the cold box, wherein the third pressure is less than the second pressure; 
 at least one second compressor, wherein the at least one second compressor is configured to receive the second portion downstream of the third heat exchanger pass and compress the second portion to the second pressure; and 
 a mixer, wherein the mixer is configured to combine the first portion downstream of the second heat exchanger pass and the second portion downstream of the third heat exchanger pass or the compressed second portion downstream of the at least one second compressor to form the refrigerant upstream of the first compressor. 
   
     
     
         15 . The LNG plant of  claim 14  further comprising a first expander configured to receive the first portion from the splitter and expand the first portion to the second pressure; and/or a second expander configured to receive the second portion from the splitter and expand the second portion to the third pressure. 
     
     
         16 . The LNG plant of  claim 14 , wherein the at least one second compressor includes a two-stage compressor. 
     
     
         17 . The LNG plant of  claim 14 , wherein the mixer is downstream from the at least one second compressor, and is configured to combine the first portion downstream of the second heat exchanger pass and the compressed second portion to form the refrigerant upstream of the first compressor. 
     
     
         18 . The LNG plant of  claim 14 , wherein the refrigerant consists of one or more non-hydrocarbon refrigerants. 
     
     
         19 . The LNG plant of  claim 14 , wherein the second pressure is between 20% and 50% of the first pressure on an absolute scale, and wherein the third pressure is between 3% and 20% of the first pressure on an absolute scale. 
     
     
         20 . The LNG plant of  claim 14 , wherein a ratio of the second pressure to the third pressure is between about 10:1 and about 2:1.