Control Structure Selection for Reactor, Separator, and Recycle Processes

We consider control structure selection, with emphasis on “what to control”, for a simple plant with a liquid-phase reactor, a distillation column, and recycle of unreacted reactants. Plants of this kind have been studied extensively in the plantwide control literature. Our starting point is a clear definition of the operational objectives, constraints, and degrees of freedom. Active constraints should be controlled to optimize the economic performance. This implies for this case study that the reactor level should be kept at its maximum, which rules out many of the control structures proposed in the literature from being economically attractive. Maximizing the reactor holdup also minimizes the “snowball effect”. The main focus is on the selection of a suitable controlled variable for the remaining unconstrained degree of freedom, where we use the concept of self-optimizing control, which is to search for a constant setpoint strategy with an acceptable economic loss. Both for the case with a given feed rate where the energy costs should be minimized and for the case where the production rate should be maximized, we find that a good controlled variable is the reflux ratio L/F. This applies to single-loop control as well as multivariable model predictive control.