Distillation column heat pump with compressor inlet superheater

Abstract

Light naphtha distillation columns with heat pumps and methods of operating them are described. The overhead stream is compressed to increase temperature so that it can be used both to heat the reboiler and to superheat the overhead stream before it enters the heat pump compressor.

What is claimed is: 
     
         1 . A process for light naphtha distillation comprising:
 providing a light naphtha feed stream to a fractionation zone at a first position;   fractionating the light naphtha feed stream into an overhead stream and a bottoms stream;   heating a first portion of the overhead stream to a temperature above a dew point temperature of the overhead stream;   compressing the heated first overhead stream portion;   removing a portion of a stream from the fractionation zone at a second position below the first position;   heating the removed stream portion by indirectly contacting the removed stream portion with the compressed first overhead stream portion;   returning the heated removed stream portion to the fractionation zone at a third position above the second position and below the first position;   reducing the pressure of the compressed first overhead stream portion to form a reduced enthalpy overhead stream;   returning a portion of the reduced enthalpy overhead stream to the top of the fractionation zone;   wherein heating the first overhead stream portion comprises indirectly contacting the first overhead stream portion with the compressed first overhead stream portion after indirectly contacting the removed stream portion with the compressed first overhead stream portion.   
     
     
         2 . The process of  claim 1  wherein the removed stream portion is a side stream or the bottoms stream. 
     
     
         3 . The process of  claim 1  further comprising removing a side cut from the fractionation zone at a fourth position below the first position, the side cut having a boiling point between a boiling point of the overhead stream and a boiling point of the bottoms stream. 
     
     
         4 . The process of  claim 1  further comprising combining the reduced enthalpy overhead stream with a second overhead stream portion. 
     
     
         5 . The process of  claim 3  further comprising recycling the side cut to a reactor. 
     
     
         6 . The process of  claim 1  wherein indirectly contacting the removed stream portion with the compressed first overhead stream portion comprises heating the removed stream portion in a heat exchanger. 
     
     
         7 . The process of  claim 1  wherein a temperature difference between the overhead stream and the bottoms stream is at least about 38.9° C. (70° F.). 
     
     
         8 . The process of  claim 1  wherein the light naphtha feed stream contains primarily C 5 -C 6  hydrocarbons, wherein the overhead stream contains primarily di-methyl butane and lighter hydrocarbons, and wherein the bottoms stream contains primarily C 7+  hydrocarbons. 
     
     
         9 . The process of  claim 8  wherein the removed stream portion is a side stream. 
     
     
         10 . The process of  claim 8  further comprising removing a side cut from the fractionation zone at a fourth position below the first position and above the third position, the side cut having a boiling point between a boiling point of the overhead stream and a boiling point of the bottoms stream, and wherein the side cut contains primarily methyl pentanes, normal hexane, and C 6  naphthenes. 
     
     
         11 . The process of  claim 1  wherein the light naphtha feed stream contains primarily C 5 -C 6  hydrocarbons with little to no C 7+  hydrocarbons, wherein the overhead stream contains primarily di-methyl butane and lighter hydrocarbons, and wherein the bottoms stream contains primarily methyl pentanes, normal hexane, and C 6  naphthenes. 
     
     
         12 . The process of  claim 1  wherein the removed stream portion is a side stream and wherein a temperature difference between the removed stream portion before heating and the first portion of the overhead stream before heating is less than about 38.9° C. (70° F.). 
     
     
         13 . The process of  claim 1  further comprising providing additional heat to the first overhead stream portion during a start-up period using steam, and stopping the additional heat after the start-up period. 
     
     
         14 . The process of  claim 1  further comprising condensing the reduced enthalpy overhead stream before returning the portion of the reduced enthalpy overhead stream to the top of the fractionation zone. 
     
     
         15 . A process for light naphtha distillation comprising:
 providing a light naphtha feed stream containing primarily C 5  to C 6  hydrocarbons to a fractionation zone at a first position;   fractionating the light naphtha feed stream into an overhead stream containing primarily di-methyl butane and lighter hydrocarbons, and a bottoms stream containing primarily C 7+  hydrocarbons wherein a temperature difference between the overhead stream and the bottoms stream is at least about 38.9° C. (70° F.);   heating a first portion of the overhead stream to a temperature above a dew point temperature of the overhead stream;   compressing the heated first overhead stream portion;   removing a portion of a side stream from the fractionation zone at a second position below the first position;   heating the side stream portion by indirectly contacting the side stream portion with the compressed first overhead stream portion;   returning the heated side stream portion to the fractionation zone at a third position above the second position and below the first position;   reducing the pressure of the compressed first overhead stream portion to form a reduced enthalpy overhead stream;   returning a portion of the reduced enthalpy overhead stream to the top of the fractionation zone;   wherein heating the first overhead stream portion comprises indirectly contacting the first overhead stream portion with the compressed first overhead stream portion after indirectly contacting the side stream portion with the compressed first overhead stream portion.   
     
     
         16 . The process of  claim 15  further comprising removing a side cut from the fractionation zone at a fourth position below the first position, the side cut having a boiling point between a boiling point of the overhead stream and a boiling point of the bottoms stream and wherein the side cut contains primarily methyl pentanes, normal hexane, and C 6  naphthenes. 
     
     
         17 . The process of  claim 15  further comprising condensing the reduced enthalpy overhead stream before returning the portion of the reduced enthalpy overhead stream to the top of the fractionation zone. 
     
     
         18 . A distillation column and heat pump comprising:
 the distillation column having a feed inlet at a first position, an overhead outlet, and a bottoms outlet;   a reboiler having an inlet and an outlet, the reboiler inlet in fluid communication with a second position below the first position, and the reboiler outlet being in fluid communication with a third position on the distillation column, the third position being above the second position and below the first position;   a heat exchanger in heat exchange communication with at least a portion of the distillation column overhead outlet;   a compressor having a compressor inlet in fluid communication with at least the portion of the distillation column overhead outlet and a compressor outlet in heat exchange communication with the reboiler and the heat exchanger; and   an expansion valve having an expansion valve inlet in fluid communication with the compressor outlet and an expansion valve outlet in fluid communication with an inlet at a position above the first position and below the overhead outlet.   
     
     
         19 . The distillation column of  claim 18  further comprising a side cut outlet at a fourth position below the first position. 
     
     
         20 . The distillation column of  claim 18  wherein the reboiler is a side reboiler and further comprising a bottoms reboiler having an inlet and an outlet, the bottoms reboiler inlet being in fluid communication with the bottoms outlet, the bottoms reboiler outlet being in fluid communication with a fifth position on the distillation column, the fifth position being above the bottoms outlet and below the second position.