Fractional distillation column control

A method for maintaining a desired cloud point for a side draw product stream which is formed from at least a portion of a side draw stream withdrawn from an intermediate portion of a fractional distillation column, said method comprising the steps of:

• using computer means to establish a first signal representative of the actual internal reflux to vapor ratio in said intermediate portion of said fractional distillation column;
• using computing means to establish a second signal representative of the calculated actual cloud point of said side draw product stream in response to said first signal;
• measuring the cloud point of said side draw product stream and establishing a third signal representative of the measured actual cloud point of said side draw product stream;
• utilizing said third signal to correct any error in the cloud point represented by said second signal to thereby establish a fourth signal representative of a corrected actual cloud point of said side draw product stream;
• establishing a fifth signal representative of the desired cloud point of said side draw product stream;
• using computing means to compare said fourth signal and said fifth signal and establish a sixth signal which is responsive to the difference between said fourth signal and said fifth signal;
• and
• controlling the cloud point of said side draw product stream in response to said sixth signal.