Flash Distillation Control Using a Feasible Operating Region: A Sliding Mode Control Approach

The aim of this work is to develop a sliding mode control scheme to keep the operation of a flash distiller in the feasible operating region. Flash distillation is characterized as a simple single-phase separation system but operationally complex because it has a strong nonlinear behavior due to the thermodynamic equilibrium, obligatory to reach the required component separation. Control of flash distillation processes must guarantee a good performance and robustness to achieve preseparation quality before entering to next fractional distillation processes. A phenomenological-based semiphysical model is used here to represent the flash distillation process. The proposed control structure tries to avoid the biggest problem of the flash distiller: its recurrent restart due to temperature disturbances in the feed flow. The suggested control schemes, applied to a given separation, allow regulation of the pressure and the liquid level of the flash drum, keeping also the concentration of propylene glycol at the flash vapor outlet near its set point, rejecting temperature disturbances. The simulation results show the features of the proposed controllers, which overcome some of the disadvantages of typical flash distiller controllers operating in design conditions.