Heterogeneous azeotropic distillation columns exhibit complexities in their steady state and dynamic behaviour due to the existence of multiple states and possible occurrence of phase splits in the internal stages. Through simulation of an ethanol dehydration column using rigorous steady state and dynamic models, differences in steady state and dynamic behaviour for operation around the three steady states in the column are demonstrated. In the case of those variables for which the column demonstrates high sensitivity, there is transition in the character of the steady state profile from one stable steady state to the other stable one. Such transitions are shown to exhibit slow and complex dynamic behaviour because of the presence of the intermediate unstable steady state which the changing profiles have to cross. These complex dynamic characteristics inherent in the column can generate operability and controllability problems. This aspect has been investigated through a detailed controllability analysis, the results of which have been used in the synthesis of a dual point control structure. The performance of the column under dual point control has been compared with that of single point control.

Key words: Heterogeneous azeotropic distillation, controllability, dynamic simulation

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