Distillation column pressure control

Abstract

Methods and systems for controlling the pressure of distillation columns, for example those operating under vacuum pressure and conventionally equipped with a steam ejector system, are described. Representative distillation columns are used in the separation of thermally unstable components, such as the physical solvent sulfolane, having relatively low volatility. Such columns are employed in aromatic hydrocarbon extraction processes for the recovery of purified C 6 -C 8 aromatic hydrocarbons from a hydrocarbon feed stream (e.g., obtained from the catalytic reforming of naphtha).

1 . A method for purifying aromatic hydrocarbons in a hydrocarbon feed stream, the method comprising:
 (a) selectively dissolving the aromatic hydrocarbons in a lean solvent, to provide a rich solvent, and   (b) distilling the rich solvent in a distillation column operating at sub-atmospheric pressure, optionally after subjecting the rich solvent to stripping,   
       wherein step (b) comprises (i) removing a net overhead vapor enriched in the aromatic hydrocarbons, relative to the rich solvent, from an upper section of the column, (ii) cooling and condensing a first part of the net overhead vapor with a cooler, and (iii) controlling the flow of a second part of the net overhead vapor, bypassing the cooler, to control a pressure measured at or above a top vapor-liquid contacting stage in the distillation column. 
     
     
         2 . The method of  claim 1 , wherein step (a) comprises contacting the lean solvent with the hydrocarbon feed stream in a countercurrent extraction or extractive distillation zone. 
     
     
         3 . The method of  claim 1 , wherein the lean solvent and the rich solvent comprise tetrahydrothiophene dioxide. 
     
     
         4 . The method of  claim 1 , wherein the aromatic hydrocarbons are C 6 -C 8  aromatic hydrocarbons. 
     
     
         5 . The method of  claim 4 , wherein at least a portion of the hydrocarbon feed stream is obtained from catalytic reforming of naphtha. 
     
     
         6 . The method of  claim 4 , further comprising introducing the first and second parts of the net overhead vapor into an overhead receiver vessel and removing an aromatic hydrocarbon product liquid from the overhead receiver vessel. 
     
     
         7 . The method of  claim 6 , further comprising separating the aromatic hydrocarbon product liquid into fractions enriched in benzene, toluene, and xylenes. 
     
     
         8 . A pressure control system for a distillation column operating at sub-atmospheric pressure, the system comprising:
 (a) first and second vapor fraction removal lines in fluid communication with an upper section of the distillation column;   (b) an overhead receiver in fluid communication with both the first and second vapor fraction removal lines;   (c) at least one indirect heat exchanger having a process side in fluid communication with the first vapor fraction removal line, between the distillation column and the overhead receiver; and   (d) a controller for regulating a flow through the second vapor fraction removal line in response to a measured pressure in the overhead receiver or in the distillation column, wherein the second vapor fraction removal line bypasses the indirect heat exchanger.