Enhanced heavies removal/LPG recovery process for LNG facilities

Abstract

A LNG facility, and operating method therefor, capable of more efficiently and/or effectively removing heavy hydrocarbon components from the processed natural gas stream and recovering the removed heavy components as LPG.

1 . A process for liquefying a natural gas stream, said process comprising: 
 (a) using a first distillation column to separate at least a portion of said natural gas stream into a first relatively more volatile fraction and a first relatively less volatile fraction;    (b) using a second distillation column to separate at least a portion of said first relatively less volatile fraction into a second relatively more volatile fraction and a second relatively less volatile fraction, said second distillation column operating at a lower pressure than said first distillation column;    (c) using a reflux portion of said second relatively more volatile fraction as reflux in said first and/or second distillation columns; and    (d) cooling at least a portion of said reflux portion via indirect heat exchange with at least a portion of said first relatively less volatile fraction.    
   
   
       2 . The process of  claim 1 , 
 said second distillation column being operated at a pressure at least about 25 psi less than the operating pressure of said first distillation column.    
   
   
       3 . The process of  claim 1 , 
 said second distillation column being operated at a pressure at least about 100 psi less than the operating pressure of said first distillation column.    
   
   
       4 . The process of  claim 1 , 
 said at least a portion of said natural gas stream separated in said first distillation column having a vapor fraction greater than about 0.7 on a molar basis when introduced into said first distillation column.    
   
   
       5 . The process of  claim 1;  and 
 (e) prior to step (a), condensing a portion of said natural gas stream;    (f) prior to step (a) and subsequent to step (e), separating said natural gas stream into a predominately vapor stream and a predominately liquid stream; and    (g) introducing at least a portion of said predominately vapor stream into said first distillation column.    
   
   
       6 . The process of  claim 5 , 
 said first distillation column having at least 5 theoretical stages,    said at least a portion of said predominately vapor stream introduced into said first distillation column in accordance with step (g) entering said first distillation column in or below one or more of the bottom 3 theoretical stages of said first distillation column.    
   
   
       7 . The process of  claim 5;  and 
 (h) expanding said predominately vapor stream prior to introduction into said first distillation column,    said expanding of step (h) causing at least a portion of said predominately vapor stream to condense.    
   
   
       8 . The process of  claim 5;  and 
 (i) combining at least a portion of said first relatively less volatile fraction with at least a portion of said predominately liquid stream to thereby form a combined stream; and    (j) introducing at least a portion of said combined stream into said second distillation column.    
   
   
       9 . The process of  claim 8 , 
 said combining of step (i) occurring subsequent to using said first relatively less volatile fraction for said cooling of step (d).    
   
   
       10 . The process of  claim 1;  and 
 (k) separating said reflux portion into a first reflux stream and a second reflux stream;    (l) introducing said first reflux stream into an upper section of said first distillation column; and    (m) introducing said second reflux stream into an upper section of said second distillation column.    
   
   
       11 . The process of  claim 10 , 
 said separating of step (k) including introducing at least a portion of said second relatively more volatile fraction into a reflux accumulator vessel, removing a liquid stream from said reflux accumulator, and removing a vapor stream from said reflux accumulator,    said liquid stream removed from said reflux accumulator being said reflux portion.    
   
   
       12 . The process of  claim 11;  and 
 (n) combining at least a portion of said vapor stream removed from said reflux accumulator with at least a portion of said first relatively more volatile stream.    
   
   
       13 . The process of  claim 12;  and 
 (o) cooling the combined stream resulting from step (n) in a methane refrigeration cycle employing a predominately methane refrigerant.    
   
   
       14 . The process of  claim 10;  and 
 (p) prior to introduction into said second distillation column, cooling said second reflux stream via indirect heat exchange with a mechanical refrigeration cycle.    
   
   
       15 . The process of  claim 14 , 
 said mechanical refrigeration cycle employing a predominately ethylene or ethane refrigerant.    
   
   
       16 . The process of  claim 1;  and 
 (q) cooling at least a portion of said first relatively more volatile fraction in a methane refrigeration cycle employing a predominately methane refrigerant.    
   
   
       17 . The process of  claim 1 , 
 said process being a cascade-type LNG process.    
   
   
       18 . The process of  claim 17 , 
 said cascade-type LNG process including an open methane refrigeration cycle.    
   
   
       19 . The process of  claim 1;  and 
 (r) vaporizing liquefied natural gas produced by the process of  claim 1 .    
   
   
       20 . A liquefied natural gas product produced by the process of  claim 1 .  
   
   
       21 . A computer-implemented simulation process comprising: using a computer to simulate the process of  claim 1 .  
   
   
       22 . A process for liquefying anatural gas stream, said process comprising: 
 (a) using a vapor/liquid separator to separate at least a portion of said natural gas stream into a predominately vapor separated portion and a predominately liquid separated portion;    (b) using a first distillation column to separate at least a portion of said predominately vapor separated portion into a first relatively more volatile fraction and a first relatively less volatile fraction;    (c) introducing a first reflux stream into an upper section of said first distillation column; and    (d) cooling at least a portion of said first reflux stream via indirect heat exchange with at least a portion of said predominately liquid separated portion.    
   
   
       23 . The process of  claim 22;  and 
 (e) using at least a portion of said first relatively less volatile fraction as said first reflux stream.    
   
   
       24 . The process of  claim 22;  and 
 (f) using at least a portion of said first relatively more volatile fraction as said first reflux stream.    
   
   
       25 . The process of  claim 22;  and 
 (g) cooling at least a portion of said first reflux stream via indirect heat exchange with at least a portion of said first relatively less volatile fraction.    
   
   
       26 . The process of  claim 25;  and 
 (h) combining at least a portion of said predominately liquid separated portion with at least a portion of said first relatively less volatile fraction.    
   
   
       27 . The process of  claim 26 , 
 said combining of step (f) occurring subsequent to using said predominately liquid separated stream and said first relatively less volatile fraction for said cooling of steps (d) and (g).    
   
   
       28 . The process of  claim 22;  and 
 (i) using a second distillation column to separate at least a portion of said first relatively less volatile fraction into a second relatively more volatile fraction and a second relatively less volatile fraction.    
   
   
       29 . The process of  claim 28;  and 
 (j) using at least a portion of said second relatively more volatile fraction as said first reflux stream.    
   
   
       30 . The process of  claim 28;  and 
 (k) introducing a second reflux stream into an upper section of said second distillation column.    
   
   
       31 . The process of  claim 30;  and 
 (l) using a first portion of said second relatively more volatile fraction as said first reflux stream and a second portion of said second relatively more volatile fraction as said second reflux stream.    
   
   
       32 . The process of  claim 28 , 
 said second distillation column being operated a pressure that is at least about 25 psi less than the operating pressure of said first distillation column.    
   
   
       33 . The process of  claim 22;  and 
 (m) prior to step (a), condensing at least a portion of said natural gas stream;    (n) prior to step (a), separating said natural gas stream into a predominately vapor stream and a predominately liquid stream; and    (o) feeding at least a portion of said predominately vapor stream to said first distillation column.    
   
   
       34 . The process of  claim 33;  and 
 (p) expanding said predominately vapor stream prior to introduction into said first distillation column,    said expanding of step (p) causing at least a portion of said predominately vapor stream to condense.    
   
   
       35 . The process of  claim 22 , 
 said process being a cascade-type LNG process.    
   
   
       36 . The process of  claim 22;  and 
 (q) vaporizing liquefied natural gas produced by the process of  claim 22 .    
   
   
       37 . A liquefied natural gas product produced by the process of  claim 22 .  
   
   
       38 . A computer-implemented simulation process comprising: using a computer to simulate the process of  claim 22 .  
   
   
       39 . A process for liquefying a natural gas stream, said process comprising: 
 (a) using a distillation column to separate at least a portion of said natural gas stream into a relatively more volatile fraction and a relatively less volatile fraction;    (b) cooling at least a portion of said relatively more volatile fraction in a first heat exchange pass via indirect heat exchange with a first refrigerant;    (c) flashing at least a portion of the cooled relatively more volatile fraction to thereby produce a flash gas;    (d) compressing at least a portion of said flash gas; and    (e) cooling at least a portion of the compressed flash gas in a second heat exchange pass via indirect heat exchange with a second refrigerant, said first and second heat exchange passes being separate from one another.    
   
   
       40 . The process of  claim 39 , 
 said first and second refrigerants comprising predominately propane, propylene, carbon dioxide, ethane, ethylene, or methane.    
   
   
       41 . The process of  claim 39 , 
 said first and second refrigerants comprising predominately ethylene or ethane.    
   
   
       42 . The process of  claim 39 , 
 said first and second refrigerants being the same refrigerant.    
   
   
       43 . The process of  claim 39 , 
 said cooling of steps (b) and (e) being carried out in a common heat exchanger.    
   
   
       44 . The process of  claim 43 , 
 said common heat exchanger being a core-in-kettle type heat exchanger,    said first heat exchange pass being defined by a first core of said common heat exchanger,    said second heat exchange pass being defined by a second core of said common heat exchanger.    
   
   
       45 . The process of  claim 39 , 
 (f) prior to step (a), cooling at least a portion of said natural gas stream via indirect heat exchanger with a second refrigerant; and    (g) prior to step (e) cooling at least a portion of said compressed flash gas via indirect heat exchange with said second refrigerant.    
   
   
       46 . The process of  claim 45 , 
 said second refrigerant comprising predominately propane, propylene, or carbon dioxide.    
   
   
       47 . An apparatus for liquefying natural gas, said apparatus comprising: 
 a vapor/liquid separator having a vapor outlet and a liquid outlet;    a first distillation column fluidly coupled to said vapor outlet and having a first overhead outlet and a first bottom outlet;    a heat exchanger having a heating pass and a cooling pass, said heating and cooling passes being configured to facilitate indirect heat exchange between fluids flowing therethrough, said heating pass being fluidly coupled to said first bottom outlet; and    a second distillation column fluidly coupled to said heating pass and having a second overhead outlet and a second bottom outlet, said second overhead outlet being fluidly coupled to said cooling pass.    
   
   
       48 . The apparatus of  claim 47;  and 
 an expander fluidly coupled between said vapor outlet and said first distillation column.    
   
   
       49 . The apparatus of  claim 47 , 
 said first distillation column including a first reflux inlet,    said cooling pass being fluidly coupled between said first reflux inlet and said second overhead outlet.    
   
   
       50 . The apparatus of  claim 49 , 
 said second distillation column including a second reflux inlet,    said cooling pass being fluidly coupled between said second reflux inlet and said second overhead outlet.    
   
   
       51 . The apparatus of  claim 50;  and 
 a reflux accumulator fluidly disposed between said cooling pass and said first and second reflux inlets.    
   
   
       52 . The apparatus of  claim 47 , 
 said second heating pass being fluidly coupled to said first bottom outlet.    
   
   
       53 . The apparatus of  claim 47 , 
 said heat exchanger having a second heating pass,    said second heating pass being fluidly coupled to said first bottom outlet.