M13.8 Other Issues in Distillation Column Control

Distillation is probably the most widely studied multiloop control problem, with many papers and books published on the topic. In this chapter, we have focused on "conventional control," where reflux flow rate is manipulated to control distillate composition, and vapor boil-up (related to reboiler duty) is manipulated to control bottoms composition. The pressure and level loops are assumed to be perfectly controlled. An alternative control structure, often used on high-purity columns, is known as material balance control. In material balance control, distillate flow rate is manipulated to control distillate composition. (See Additional Exercise 8 for a study using material balance control.) In practice, for this strategy to work, the distillate level must be very tightly controlled by manipulation of the reflux flow rate.

In this chapter we have assumed that the pressure and level loops are perfectly controlled. In Figures M13-1 and M13-2 we used a simplified representation for the pressure control loop, where the pressure was controlled by manipulating the condenser cooling water flow rate. In practice, there are many different ways to control column pressure. (See the references section for articles discussing various pressure control methods.)

The multivariable interactions, particularly in high-purity distillation, make it very difficult to control the distillate and bottoms compositions simultaneously. More often, the important product is under feedback control, while the less important product is manually controlled. Alternatively, the controller for the most important product could be tightly tuned, while the other composition loop is loosely tuned.