LNG system with high pressure pre-cooling cycle

Abstract

A natural gas liquefaction system employing a high pressure pre-cooling refrigeration cycle.

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 −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 68° F.    
   
   
       5 . The process of  claim 1 , 
 said pre-cooling refrigerant having a boiling point temperature at one atmosphere within 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 −65° to about −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 68° F.    
   
   
       9 . The process of  claim 7 , 
 said pre-cooling refrigerant having a boiling point temperature at one atmosphere within 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 −250° to about −75° F. at said low-stage inlet,    said pre-cooling refrigerant having a temperature in the range of from about −50° to about 100° F. at said intermediate-stage inlet,    said pre-cooling refrigerant having a temperature in the range of from about 0° to about 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/ft 3  at said intermediate-stage inlet,    said pre-cooling refrigerant having a density of at least about 0.9 lb/ft 3  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 −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 68° F.    
   
   
       27 . The apparatus of  claim 23 , 
 said pre-cooling refrigerant having a boiling point temperature at one atmosphere within 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 −65° to about −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.