Lng system with enhanced pre-cooling cycle

Abstract

A natural gas liquefaction system employing a high pressure pre-cooling refrigeration cycle. The natural gas liquefaction system can be advantageously employed in cold weather regions and/or in regions that exhibit large variations in ambient temperature.

1 . A process for liquefying natural gas, said process comprising:
 (a) cooling a natural gas stream in a high pressure pre-cooling cycle via indirect heat exchange with a pre-cooling refrigerant, said pre-cooling cycle employing a pre-cooling compressor that discharges said pre-cooling refrigerant at a discharge pressure of at least 225 pounds per square inch atmospheric (psia), said pre-cooling refrigerant having a boiling point temperature lower than −43° C. (−45° F.) at one atmosphere; and   (b) further cooling and at least partly condensing at least a portion of said natural gas stream in a subsequent cooling cycle via indirect heat exchange with a subsequent refrigerant having a lower boiling point temperature than said pre-cooling refrigerant.   
   
   
       2 . The process of  claim 1 , said pre-cooling refrigerant having a latent heat of vaporization in the range of from about 75 to about 205 British thermal units per pound (btu/lb) at one atmosphere and boiling point temperature. 
   
   
       3 . The process of  claim 1 , said pre-cooling refrigerant having a boiling point temperature at least about 10 percent lower than the boiling point temperature of propane at one atmosphere, on a Fahrenheit temperature scale. 
   
   
       4 . The process of  claim 1 , said pre-cooling refrigerant having a vapor pressure in the range of from about 130 to about 180 psia at 20° C. (68° F.). 
   
   
       5 . The process of  claim 1 , said pre-cooling refrigerant having a boiling point temperature at one atmosphere within 66° C. (150° F.) of the boiling point temperature at one atmosphere of said subsequent refrigerant. 
   
   
       6 . The process of  claim 1 , said discharge pressure being at least 250 psia. 
   
   
       7 . The process of  claim 1 , said pre-cooling refrigerant having a boiling point temperature in the range of from about −54° C. (−65° F.) to about −46° C. (−50° F.) at one atmosphere, said pre-cooling refrigerant having a latent heat of vaporization in the range of from about 180 to about 195 btu/lb at one atmosphere and boiling point temperature. 
   
   
       8 . The process of  claim 7 , said pre-cooling refrigerant having a vapor pressure in the range of from about 140 to about 170 psia at 20° C. (68° F.). 
   
   
       9 . The process of  claim 7 , said pre-cooling refrigerant having a boiling point temperature at one atmosphere within 43° C. (110° F.) of the boiling point temperature at one atmosphere of said subsequent refrigerant. 
   
   
       10 . The process of  claim 1 , said subsequent refrigerant comprising ethane and/or ethylene. 
   
   
       11 . The process of  claim 1 , said pre-cooling refrigerant comprising predominately propylene. 
   
   
       12 . The process of  claim 11 , said subsequent refrigerant comprising predominately ethylene. 
   
   
       13 . The process of  claim 1 , said pre-cooling compressor providing a maximum inlet-to-discharge pressure increase in the range of from about 200 to about 350 psi. 
   
   
       14 . The process of  claim 1 , said pre-cooling compressor comprising low-stage, intermediate-stage, and high-stage inlets each receiving at least a portion of said pre-cooling refrigerant. 
   
   
       15 . The process of  claim 14 , said pre-cooling compressor operating at a low-stage inlet pressure of at least about 15 psia, an intermediate-stage inlet pressure of at least about 40 psia, and a high-stage inlet pressure of at least about 80 psia. 
   
   
       16 . The process of  claim 15 , said pre-cooling refrigerant having a temperature in the range of from about −73° C. (−100° F.) to about 10° C. (−50° F.) at said low-stage inlet, said pre-cooling refrigerant having a temperature in the range of from about −46° C. (−50° F.) to about 38° C. (100° F.) at said intermediate-stage inlet, said pre-cooling refrigerant having a temperature in the range of from about −18° C. (0° F.) to about 93° C. (200° F.) at said high-stage inlet. 
   
   
       17 . The process of  claim 14 , said pre-cooling refrigerant having a density of at least about 0.18 pounds per cubic foot (lb/ft 3 ) at said low-stage inlet, said pre-cooling refrigerant having a density of at least about 0.5 lb/ft3 at said intermediate-stage inlet, said pre-cooling refrigerant having a density of at least about 0.9 lb/ft3 at said high-stage inlet. 
   
   
       18 . The process of  claim 1 , and (c) cooling at least a portion of said subsequent refrigerant via indirect heat exchange with said pre-cooling refrigerant. 
   
   
       19 . The process of  claim 1 , and (d) further cooling at least a portion of said natural gas stream in a final cooling cycle via indirect heat exchange with a final refrigerant having a lower boiling point temperature than said subsequent refrigerant. 
   
   
       20 . The process of  claim 19 , and (e) separating a portion of said natural gas stream and employing the separated portion as said final refrigerant. 
   
   
       21 . The process of  claim 19 , said final refrigerant comprising predominately methane. 
   
   
       22 . The process of  claim 19 , and (f) cooling at least a portion of said final refrigerant via indirect heat exchange with said pre-cooling refrigerant. 
   
   
       23 . An apparatus for liquefying a natural gas stream, said apparatus comprising:
 a pre-cooling refrigeration cycle for cooling said natural gas stream, said pre-cooling refrigeration cycle including a pre-cooling compressor, a pre-cooling chiller, and a pre-cooling refrigerant circulating through said pre-cooling compressor and pre-cooling chiller, said pre-cooling compressor being configured to discharge said pre-cooling refrigerant at a discharge pressure of at least 225 pounds per square inch atmospheric (psia), said pre-cooling refrigerant having a boiling point temperature lower than −43° C. (−45° F.) at one atmosphere; and   a subsequent refrigeration cycle for cooling at least a portion of said natural gas stream downstream of said pre-cooling refrigeration cycle, said subsequent refrigeration cycle including a subsequent compressor, a subsequent chiller, and a subsequent refrigerant circulating through said subsequent compressor and subsequent chiller, said subsequent refrigerant having a lower boiling point temperature than said pre-cooling refrigerant.   
   
   
       24 . The apparatus of  claim 23 , said pre-cooling refrigerant having a latent heat of vaporization in the range of from about 75 to about 205 British thermal units per pound (btu/lb) at one atmosphere and boiling point temperature. 
   
   
       25 . The apparatus of  claim 23 , said pre-cooling refrigerant having a boiling point temperature at least about 10 percent greater than the boiling point temperature of propane at one atmosphere, on a Fahrenheit temperature scale. 
   
   
       26 . The apparatus of  claim 23 , said pre-cooling refrigerant having a vapor pressure in a range of from about 130 to about 180 psia at 20° C. (68° F.). 
   
   
       27 . The apparatus of  claim 23 , said pre-cooling refrigerant having a boiling point temperature at one atmosphere within 66° C. (150° F.) of the boiling point temperature at one atmosphere of said subsequent refrigerant. 
   
   
       28 . The apparatus of  claim 23 , said discharge pressure being at least 250 psia. 
   
   
       29 . The apparatus of  claim 23 , said pre-cooling refrigerant having a boiling point temperature in the range of from about −54° C. (−65° F.) to about −46° C. (−50° F.) at one atmosphere,
 said pre-cooling refrigerant having a latent heat of vaporization in the range of from about 180 to about 195 btu/lb at one atmosphere and boiling point temperature.   
   
   
       30 . The apparatus of  claim 23 , said pre-cooling refrigerant comprising predominately propylene. 
   
   
       31 . The apparatus of  claim 30 , said subsequent refrigerant comprising predominately ethylene. 
   
   
       32 . The apparatus of  claim 23 , and a final refrigeration cycle for cooling at least a portion of said natural gas stream downstream of said subsequent refrigeration cycle, said final refrigeration cycle including a final compressor, a final chiller, and a final refrigerant circulating through said final compressor and final chiller, said final refrigerant having a lower boiling point than said subsequent refrigerant. 
   
   
       33 . The apparatus of  claim 32 , said final refrigerant comprising predominately methane. 
   
   
       34 . In a process for producing liquefied natural gas at an LNG facility location where the average ambient temperature was less than 10° C. (50° F.) for at least two calendar months of at least one calendar year from 1995 to 2005, the improvement comprising:
 (a) cooling a natural gas stream in a first refrigeration cycle via indirect heat exchange with a first refrigerant having a boiling point temperature of less than −43° C. (−45° F.) at one atmosphere; and   (b) further cooling at least a portion of said natural gas stream in a second refrigeration cycle via indirect heat exchange with a second refrigerant having a lower boiling point temperature than said first refrigerant.   
   
   
       35 . The process of  claim 34 , wherein the yearly variation in ambient temperature extremes of said LNG facility location was at least 10° C. (50° F.) for at least one calendar year from 1995 to 2005. 
   
   
       36 . The process of  claim 34 , wherein the yearly average ambient temperature of said LNG facility location was less than 10° C. (50° F.) for at least one calendar year from 1995 to 2005. 
   
   
       37 . The process of  claim 34 , wherein the yearly variation in ambient temperature extremes of said LNG facility location was in the range of from about 23° C. (75° F.) to about 66° C. (150° F.) for at least one calendar year from 1995 to 2005. 
   
   
       38 . The process of  claim 37 , wherein the yearly average ambient temperature of said LNG facility location was in the range of from about −18° C. (0° F.) to about 7° C. (45° F.) for at least one calendar year from 1995 to 2005. 
   
   
       39 . The process of  claim 34 , said first refrigeration cycle employing a first compressor that discharges said first refrigerant at a discharge pressure less than about 240 psia. 
   
   
       40 . The process of  claim 39 , said first compressor including one or more first inlets for receiving said first refrigerant, wherein said inlets include a low-state inlet through which said first refrigerant enters said first compressor at a low-stage inlet pressure that is the lowest pressure of any of said first inlets, said low-stage inlet pressure being greater than 1 psia. 
   
   
       41 . The process of  claim 40 , said discharge pressure being in the range of from about 5 to about 60 psia. 
   
   
       42 . The process of  claim 34 , said first refrigerant having a vapor pressure in the range of from about 130 to about 180 psia at 20° C. (68° F.). 
   
   
       43 . The process of  claim 34 , said first refrigerant having a boiling point temperature in the range of from about 18° C. (−65°) to about −46° C. (−50° F.) at one atmosphere, said first refrigerant having a vapor pressure in the range of from about 140 to about 170 psia at 20° C. (68° F.). 
   
   
       44 . The process of  claim 34 , said subsequent refrigerant comprising ethane and/or ethylene. 
   
   
       45 . The process of  claim 34 , said pre-cooling refrigerant comprising predominately propylene. 
   
   
       46 . The process of  claim 45 , said subsequent refrigerant comprising predominately ethylene. 
   
   
       47 . The process of  claim 34 , said pre-cooling compressor comprising low-stage and high-stage inlets each receiving at least a portion of said pre-cooling refrigerant. 
   
   
       48 . The process of  claim 47 , said pre-cooling compressor operating at a low-stage inlet pressure in the range of from about 5 to about 60 psia and a high-stage inlet pressure in the range of from about 40 to about 80 psia. 
   
   
       49 . The process of  claim 48 , said pre-cooling refrigerant having a temperature in the range of from about −62° C. (−80° F.) to about −18° C. (0° F.) at said low-stage inlet, said pre-cooling refrigerant having a temperature in the range of from about −12° C. (10° F.) to about 38° C. (100° F.) at said hid-stage inlet. 
   
   
       50 . The process of  claim 34 ; and
 (c) cooling at least a portion of said second refrigerant via indirect heat exchange with said pre-cooling refrigerant.   
   
   
       51 . The process of  claim 50 ; and
 (d) further cooling at least a portion of said natural gas stream in a third cooling cycle via indirect heat exchange with a third refrigerant having a lower boiling point temperature than said second refrigerant.   
   
   
       52 . The process of  claim 51 ; and
 (e) separating a portion of said natural gas stream and employing the separated portion as said third refrigerant.   
   
   
       53 . The process of  claim 51 , said third refrigerant comprising predominately methane. 
   
   
       54 . The process of  claim 51 ; and
 (f) cooling at least a portion of said third refrigerant via indirect heat exchange with said first refrigerant.