Constraint control of a fractional distillation process

A method for operating and controlling a fractional distillation process, said method comprising the steps of:

• providing a feed stream containing components to be separated to a fractional distillation column;
• withdrawing an overhead stream from said fractional distillation column and providing said overhead stream to the suction inlet of a compressor;
• providing fluid from the discharge outlet of said compressor through a first control valve to a reboiler associated with said fractional distillation column to thereby supply heat to said fractional distillation column;
• establishing a first signal representative of the actual position of said first control valve;
• establishing a second signal representative of the desired position of said first control valve;
• using computing means to compare said first signal and said second signal and establish a third signal which is responsive to the difference between said first signal and said second signal, wherein said third signal is scaled so as to be representative of the speed of said compressor required to maintain said first signal substantially equal to said second signal;
• establishing a fourth signal representative of the actual suction pressure for said compressor;
• establishing a fifth signal representative of the maximum allowable suction pressure for said compressor;
• 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, wherein said sixth signal is scaled so as to be representative of the speed of said compressor required to maintain the magnitude of said fourth signal substantially equal to the magnitude of fifth signal;
• establishing a seventh signal representative of the actual discharge pressure for said compressor;
• establishing an eighth signal representative of the minimum allowable discharge pressure for said compressor;
• using computing means to compare said seventh signal and said eighth signal and establish a ninth signal which is responsive to the difference between said seventh signal and said eighth signal, wherein said ninth signal is scaled so as to be representative of the speed of said compressor required to maintain the magnitude of said seventh signal substantially equal to the magnitude of said eighth signal;
• using high select means to establish a tenth signal representative of the highest compressor speed represented by said third, sixth and ninth signals;
• and
• controlling the speed of said compressor in response to said tenth signal.