Method of controlling secondary condenser duty

Abstract

A portion of the overhead vapor from a distillation column is used to heat a process stream. The temperature and flow rate of the stream out of a heat exchanger are controlled so that the vapor is condensed.

What is claimed is: 
     
         1 . A process for condensing an overhead vapor stream from a distillation column comprising:
 dividing the overhead stream into two portions;   introducing the first portion of the overhead stream into a first heat exchanger to exchange heat with a process stream thereby increasing a temperature of the process stream and reducing the temperature of the first portion of the overhead stream to condense the first portion of the overhead stream;   controlling a flow rate of the first portion of the condensed overhead stream from the first heat exchanger;   introducing the first portion of the condensed overhead stream into a receiver;   introducing the second portion of the overhead stream into a second heat exchanger to reduce a temperature of the second portion of the overhead stream to condense the second portion of the overhead stream; and   introducing the second portion of the condensed overhead stream into the receiver and combining it with the first portion of the condensed overhead stream to form a condensed liquid.   
     
     
         2 . The process of  claim 1  wherein controlling the flow rate of the first portion of the condensed stream comprises:
 measuring a temperature of the first portion of the condensed overhead stream; 
 reducing the flow rate if the temperature of the first portion of the condensed overhead stream is above a predetermined temperature, wherein the predetermined temperature is below a dew point for the overhead vapor stream; and 
 increasing the flow rate if the temperature of the first portion of the condensed overhead stream is below the predetermined temperature. 
 
     
     
         3 . The process of  claim 2  wherein the predetermined temperature is at least about 1° C. less than a bubble point of the first portion of the overhead stream. 
     
     
         4 . The process of  claim 1  further comprising introducing the condensed liquid from the receiver into a dehydrogenation reactor. 
     
     
         5 . The process of  claim 1  wherein the first heat exchanger is positioned above the receiver. 
     
     
         6 . The process of  claim 1  wherein the second heat exchanger is positioned below the receiver. 
     
     
         7 . The process of  claim 1  wherein the overhead stream comprises at least one C 2  to C 6  alkane. 
     
     
         8 . The process of  claim 1  wherein the process stream is a feed to the distillation column. 
     
     
         9 . The process of  claim 8  wherein the overhead stream comprises propane and wherein the temperature of the feed to the distillation column is increased to at least 0° C. 
     
     
         10 . A separation process comprising:
 separating a product stream in a distillation column into a liquid bottoms stream and an overhead vapor stream;   dividing the overhead stream into two portions;   introducing the first portion of the overhead stream into a first heat exchanger to exchange heat with a process stream thereby increasing a temperature of the process stream and reducing the temperature of the first portion of the overhead stream to condense the first portion of the overhead stream;   controlling a flow rate of the first portion of the condensed overhead stream from the first heat exchanger;   introducing the first portion of the condensed overhead stream into a receiver;   introducing the second portion of the overhead stream into a second heat exchanger to reduce a temperature of the second portion of the overhead stream to condense the second portion of the overhead stream; and   introducing the second portion of the condensed overhead stream into the receiver and combining it with the first portion of the condensed overhead stream to form a condensed liquid.   
     
     
         11 . The process of  claim 10  wherein controlling the flow rate of the first portion of the condensed stream comprises:
 measuring a temperature of the first portion of the condensed overhead stream; 
 reducing the flow rate if the temperature of the first portion of the condensed overhead stream is above a predetermined temperature, wherein the predetermined temperature is below a dew point for the overhead vapor stream; 
 increasing the flow rate if the temperature of the first portion of the condensed overhead stream is below the predetermined temperature. 
 
     
     
         12 . The process of  claim 11  wherein the predetermined temperature is at least 1° C. less than the bubble point. 
     
     
         13 . The process of  claim 10  further comprising introducing the condensed liquid from the receiver into the dehydrogenation reaction zone. 
     
     
         14 . The process of  claim 10  wherein the first heat exchanger is positioned above the receiver. 
     
     
         15 . The process of  claim 10  wherein the second heat exchanger is positioned below the receiver. 
     
     
         16 . The process of  claim 10  wherein the overhead stream comprises at least one C 2  to C 6  alkane. 
     
     
         17 . The process of  claim 1  wherein the process stream is the product stream. 
     
     
         18 . The process of  claim 17  wherein the overhead stream comprises propane and wherein the temperature of the product stream is increased to at least 0° C.