Mixed-reflux for heavies removal in lng processing

Abstract

Systems and methods for removing heavy hydrocarbons are provided. Methods for liquefying a natural gas stream include: (a) cooling at least a portion of the natural gas stream in an upstream refrigeration cycle of a liquefaction process to produce a cooled natural gas stream; (b) separating via a first distillation column the cooled natural gas stream into a first top fraction and a first bottom fraction, wherein the first fraction does not freeze in a subsequent downstream step of the liquefaction process; (c) separating via a second distillation column the first bottom fraction into a second top fraction and a second bottom fraction, wherein the second top fraction at least a portion of a reflux stream; (d) optionally separating via a third distillation column the second bottom fraction into a third top fraction and a third bottom fraction, wherein the third top fraction forms a portion of the reflux stream; and (e) introducing the reflux stream into the first distillation column.

1 . A method of liquefying a natural gas stream, comprising:
 (a) cooling at least a portion of the natural gas stream in an upstream refrigeration cycle of a liquefaction process to produce a cooled natural gas stream;   (b) separating via a first distillation column the cooled natural gas stream into a first top fraction and a first bottom fraction, wherein the first distillation column is a heavies removal column;   (c) separating via a second distillation column the first bottom fraction into a second top fraction and a second bottom fraction;   (d) separating via a third distillation column the second bottom fraction into a third top fraction and a third bottom fraction;   (e) combining at least a portion of the second top fraction and a portion of the third top fraction to form a mixed-reflux stream; and   (f) introducing the mixed-reflux stream into the first distillation column.   
     
     
         2 . The method of  claim 1 , wherein the mixed-reflux comprises one or more fraction originating from the first distillation column, the second distillation column, and the third distillation column 
     
     
         3 . The method of  claim 2 , wherein the one or more fraction originating from the first distillation column is the first bottom fraction. 
     
     
         4 . The method of  claim 1 , wherein the mixed-reflux is routed to a reflux drum prior to step (f). 
     
     
         5 . The method of  claim 4 , further comprising: separating the mixed-reflux is separated into a top mixed-reflux portion and a bottom mixed-reflux portion, wherein the top mixed-reflux portion is routed to fuel. 
     
     
         6 . The method of  claim 1 , further comprising: reboiling the first distillation column prior to step (c). 
     
     
         7 . The method of  claim 1 , further comprising: partially condensing the second top fraction prior to step (e). 
     
     
         8 . The method of  claim 1 , further comprising: condensing the third top fraction prior to step (e). 
     
     
         9 . The method of  claim 1 , wherein an uncondensed portion of the second top fraction is routed to a methane compressor. 
     
     
         10 . The method of  claim 1 , further comprising: cooling the mixed-reflux stream with one or more indirect heat exchangers. 
     
     
         11 . The method of  claim 1 , wherein the second distillation column is a nominal debutanizer. 
     
     
         12 . The method of  claim 1 , wherein the third distillation column is a condensate stabilizer. 
     
     
         13 . The method of  claim 1 , further comprising: cooling the natural gas stream into liquefied natural gas. 
     
     
         14 . A method of liquefying a natural gas stream, comprising:
 (a) cooling at least a portion of the natural gas stream in an upstream refrigeration cycle of a liquefaction process to produce a cooled natural gas stream;   (b) separating via a first distillation column the cooled natural gas stream into a first top fraction and a first bottom fraction, wherein the first fraction does not freeze in a subsequent downstream step of the liquefaction process or does not result in a liquefied natural gas product that does not meet selected specifications;   (c) separating via a second distillation column the first bottom fraction into a second top fraction and a second bottom fraction, wherein the second top fraction forms at least a portion of a reflux stream;   (d) optionally separating via a third distillation column the second bottom fraction into a third top fraction and a third bottom fraction, wherein the third top fraction forms a portion of the reflux stream; and   (e) introducing the reflux stream into the first distillation column.   
     
     
         15 . The method of  claim 14 , further comprising: reboiling the first distillation column prior to step (c). 
     
     
         16 . The method of  claim 14 , wherein heat duty for the first distillation column is provided by a stripping gas. 
     
     
         17 . The method of  claim 14 , further comprising: partially condensing the second top fraction prior to step (e). 
     
     
         18 . The method of  claim 14 , further comprising: condensing the third top fraction prior to step (e). 
     
     
         19 . The method of  claim 14 , wherein an uncondensed portion of the second top fraction is routed to a methane compressor. 
     
     
         20 . The method of  claim 14 , further comprising: cooling the mixed-reflux stream with at least two separate indirect heat exchangers, each utilizing a refrigerant selected from the group consisting of: propane, propylene, ethylene, and any combination thereof. 
     
     
         21 . The method of  claim 14 , wherein the second distillation column is a nominal debutanizer. 
     
     
         22 . The method of  claim 14 , wherein the third distillation column is a condensate stabilizer. 
     
     
         23 . The method of  claim 14 , further comprising: cooling the natural gas stream into liquefied natural gas. 
     
     
         24 . The method of  claim 14 , wherein the first distillation column is a heavies removal column, the second distillation column is a nominal debutanizer, and the third distillation column is a condensate stabilizer. 
     
     
         25 . The method of  claim 14 , wherein the reflux stream is a mixed-reflux stream.