Control of a fractional distillation process

Apparatus comprising:

• a fractional distillation column;
• means for passing a feed mixture containing at least first and second components into said fractional distillation column;
• means for passing a heating fluid to said fractional distillation column;
• means for withdrawing an overhead vapor stream, having a substantially increased concentration of said first component with respect to the concentration of said first component in said feed mixture, from an upper portion of said fractional distillation column;
• means for condensing at least a portion of said overhead vapor stream;
• an accumulator;
• means for passing the resulting at least partially condensed overhead stream into said accumulator;
• means for withdrawing condensate from said accumulator and for passing a first portion of the thus withdrawn condensate into an upper portion of said fractional distillation column as an external reflux stream therefor and for passing a second portion of the thus withdrawn condensate as an overhead product stream;
• means for withdrawing a bottoms product stream, having a substantially increased concentration of said second component with respect to the concentration of said second component in said feed mixture, from a lower portion of said fractional distillation column;
• means for establishing a first signal which is representative of the concentration of said first component in said bottoms product stream;
• means for establishing a second signal representative of the concentration of said second component in said overhead product stream;
• means for establishing a first control signal representative of the desired flow rate of said external reflux stream in response to said first signal and said second signal, wherein said first control signal is periodically increased or decreased by a fixed increment or left unchanged depending upon the relationship between the actual concentration of said first component in said buttoms product stream as represented by said first signal and a high limit and low limit for the actual concentration of said first component in said bottoms product stream, depending upon the relationship between the actual concentration of said second component in said overhead product stream as represented by said second signal and high and low limits for the actual concentration of said second component in said overhead product stream and depending on the relationship between the magnitude of said first control signal and a high limit for the flow rate of said external reflux stream;
• means for manipulating the flow rate of said external reflux in response to said first control signal so as to maintain the actual external reflux flow rate substantially equal to the desired external reflux flow rate as represented by said first control signal;
• means for establishing a second control signal representative of the desired temperature at a selected point in said fractional distillation column in response to said first signal and said second signal, wherein said second control signal is periodically increased or decreased by a fixed increment or is left unchanged depending upon the relationship between the actual concentration of said first component in said bottoms product stream as represented by said first signal and said high and low limits for the actual concentration of said first component in said bottoms product stream, depending upon the relationship between the actual concentration of said second component in said overhead product stream as represented by said second signal and said high and low limits for the actual concentration of said second component in said overhead product stream and depending on the relationship between the magnitude of said first control signal and a high limit for the flow rate of said external reflux stream;
• and
• means for manipulating the flow rate of heat to said fractional distillation column in response to said second control signal so as to maintain the actual temperature at said selected point substantially equal to the desired temperature represented by said second control signal.