Temperature Based Inferential Control of a Methyl Acetate Reactive Distillation Column

The performance of two temperature based inferential control structures (CS1 and CS2) is evaluated for an example methyl acetate column. The fresh acetic acid and methanol feeds control a reactive and stripping tray temperature, respectively in CS1 (Roat et al., 1986) while in CS2, the reboiler duty replaces fresh acetic acid as the manipulation handle for reactive tray temperature control. High non-linearity in the form of process gain ‘sign reversal’ causing the steady-state reactive tray temperature to move in the same direction for large opposite changes in the control input is observed. The use of the difference between two suitably chosen reactive tray temperatures, also referred to as ΔT, as the controlled variable significantly mitigates this non-linearity. Closed loop results clearly show that controlling ΔT leads to a substantial improvement in the control system robustness in terms of its ability to handle large production rate changes for both the structures. Also, tight distillate and bottoms purity control is achieved. The results also establish the superiority of CS2 over CS1 in terms of response time and product purity control. The crucial role of the choice of the controlled and manipulated variables on control performance is highlighted.