Simulation and Evaluation of Distillation Control Systems Which Control Pressure With Reboiler Heat.

A recent industrial development in distillation control strategy is to involve pressure control as an integral part of an overall control strategy, rather than as an external feature. This approach is not to simply maintain column pressure, but to manipulate other variables using the pressure as a controlled variable. In order to study control systems which utilizes this approach, a rigorous non-linear multicomponent dynamic distillation model which includes pressure dynamics was developed. The model was based on total mole, component, and enthalpy balances as well as phase equilibrium relationships and tray hydraulics on each stage. The differential equations of this model were integrated by an algorithm developed in this research which gives second-order truncation accuracy with only one functional evaluation per time step, making it roughly twice as fast as the equivalent unmodified predictor-corrector method. A simple control scheme which utilized pressure to manipulate reboiler heat was devised based on literature and tested. This control scheme included material balance control to maintain the top product purity. Open-loop and closed-loop responses to column upsets were examined and compared