Gas liquefaction system and methods

Abstract

A liquefaction system that is configured to use a single methane expander to provide primary refrigeration duty. The liquefaction system can include a heat exchanger and a fluid circuit coupled with the heat exchanger, the fluid circuit configured to circulate a process stream derived from an incoming feedstock of natural gas. The fluid circuit can comprise a methane expander coupled with the heat exchanger, a sub-cooling unit coupled with the methane expander, the sub-cooling unit configured to form a liquid natural gas (LNG) product from the process stream, and a first throttling device interposed between the heat exchanger and the sub-cooling unit. The methane expander and the first throttling device can be configured to expand the process stream to a process pressure that is between a first pressure of the incoming feedstock and a second pressure of the process that exits the sub-cooling unit.

What is claimed is: 
     
         1 . A liquefaction system, comprising:
 a first heat exchanger; and   a fluid circuit coupled with the first heat exchanger, the fluid circuit configured to circulate a process stream derived from an incoming feedstock of natural gas through the first heat exchanger, the fluid circuit comprising:
 a methane expander coupled with the first heat exchanger; 
 a sub-cooling unit coupled with the methane expander, the sub-cooling unit configured to form a liquid natural gas (LNG) product from the process stream; and 
 a first throttling device interposed between the first heat exchanger and the sub-cooling unit, 
 wherein the first throttling device is configured to expand the process stream to an intermediate pressure that is between a first pressure of the incoming feedstock and a second pressure of the process stream that exits the sub-cooling unit. 
   
     
     
         2 . The liquefaction system of  claim 1 , wherein the second pressure is in a range of from 1 psig to 30 psig. 
     
     
         3 . The liquefaction system of  claim 1 , wherein the methane expander expands the process stream to the intermediate pressure. 
     
     
         4 . The liquefaction system of  claim 1 , wherein the fluid circuit is configured to receive and mix a stream with the process stream downstream of the sub-cooling unit and upstream of the first heat exchanger. 
     
     
         5 . The liquefaction system of  claim 5 , wherein the fluid circuit comprises a compression circuit configured to receive the process stream from the sub-cooling unit via the first heat exchanger. 
     
     
         6 . The liquefaction system of  claim 1 , wherein the fluid circuit comprises a first vessel interposed between the first throttling device and the sub-cooling unit, wherein the first vessel is configured to form a first stream and a second stream from the process stream, and wherein the second stream forms the LNG product. 
     
     
         7 . The liquefaction system of  claim 1 , wherein the sub-cooling unit comprises a second heat exchanger that is configured to cool the process stream from a first temperature to a second temperature that is less than the first temperature. 
     
     
         8 . The liquefaction system of  claim 7 , wherein the second heat exchanger has a first pass and a second pass, and wherein the fluid circuit couples the first pass to the second pass. 
     
     
         9 . The liquefaction system of  claim 8 , wherein the fluid circuit comprises a second throttling device interposed between the first pass and the second pass, and wherein the second throttling device is configured to reduce pressure of the process stream to the second pressure. 
     
     
         10 . The liquefaction system of  claim 1 , further comprising a second vessel coupled with the first heat exchanger to separate the incoming feedstock into vapor and liquid, wherein the fluid circuit directs the vapor from the second vessel to the first throttling device via the first heat exchanger. 
     
     
         11 . The liquefaction system of  claim 10 , further comprising a third vessel coupled with the second vessel and the first heat exchanger to separate the liquid from the second vessel into vapor and liquid, wherein the fluid circuit directs the vapor to from the third vessel to the first throttling device via the first heat exchanger. 
     
     
         12 . An apparatus, comprising:
 a first heat exchanger; and   a second heat exchanger coupled with the first heat exchanger,   wherein said apparatus is configured to circulate fluid derived from incoming natural gas so that fluid that enters the second heat exchanger is at an intermediate pressure that is less than a first pressure of the incoming natural gas and greater than a second pressure of fluid that exits the second heat exchanger.   
     
     
         13 . The apparatus of  claim 12 , further comprising:
 a first throttling device coupled downstream of the first heat exchanger and upstream of the second heat exchanger, wherein the first throttling device is configured to expand fluid to the intermediate pressure.   
     
     
         14 . The apparatus of  claim 12 , further comprising:
 a second throttling device coupled downstream of the second heat exchanger, wherein the second throttling device is configured to expand fluid to the second pressure.   
     
     
         15 . The apparatus of  claim 14 , wherein the second heat exchanger comprises a first pass and a second pass, and wherein the second throttling device is disposed between the first pass and the second pass. 
     
     
         16 . A liquefaction process, comprising:
 flashing a vapor stream derived from an incoming feedstock to a mixed-phase stream;   separating the mixed-phase stream into a first stream and a second stream;   passing the second stream through a heat exchanger;   directing a first portion of the second stream to form a liquid natural gas (LNG) product; and   mixing a second portion with boil-off gas,   wherein the mixed-phase stream is at an intermediate pressure that is between a first pressure of the incoming feedstock and a second pressure for the boil-off gas.   
     
     
         17 . The liquefaction process of  claim 16 , further comprising:
 compressing the first stream and the second portion with the boil-off gas to a third pressure that is greater than the first pressure.   
     
     
         18 . The liquefaction process of  claim 17 , further comprising:
 expanding the first stream and the second portion with the boil-off gas from the third pressure to the intermediate pressure.   
     
     
         19 . The liquefaction process of  claim 17 , further comprising:
 bleeding-off part of the first stream and the second portion with the boil-off gas at the third pressure;   flashing the part to the intermediate pressure; and   mixing the part with the mixed-phase stream at the intermediate pressure.   
     
     
         20 . The liquefaction process of  claim 19 , further comprising:
 separating the incoming feedstock into the vapor stream and a liquid petroleum (LPG) product.