ABSTRACT
Environmental disturbances such as day and night temperature changes, or a rain storm, are shed reasonably well because the primary effect is on internal reflux in the column. Similarly, a slave flow control loop can be used for the reflux flow rate and another for the bottoms flow rate. However, this situation can be handled with computer control by using a low-level constraint control that will constrain the reflux flow rate to maintain a low-level constraint setpoint until the column temperature is low enough, so the temperature controller calls for less reflux. The origins of the boilup scheme may go back to the days when the feed flow rate and reflux flow rate to a distillation column were controlled by manual valves and glass rotameters in the control room. The side draw flow rate can be manipulated by the second temperature controller for the most responsive temperature point below the side draw.
