The key role of entrainer inventory for operation and control of heterogeneous azeotropic distillation towers

Abstract

Heterogeneous azeotropic distillation is a continuous process in which entrainer inventory and composition profiles are critical elements to ensure stable control and to maintain product quality. In order to achieve the previous goals through multi-loop control systems, many aspects of control loop pairing and closed-loop quality control need to be considered. The objective of this paper is to study ternary heterogeneous azeotropic distillation and to develop a control scheme which will reduce energy consumption and, at the same time, will be able to ensure precise composition control of the temperature break inside the column. A new control structure based on a nonlinear feedforward strategy has been developed on the basis of studies of the dynamic and steady state behavior with a rigorous dynamic model in the case of ethanol dehydration using benzene as an entrainer, but the same principles can be applied to many other heterogeneous azeo-columns. The study is not restricted to a single heterogeneous column, but extends to a sequence of towers, including the entrainer recovery column with the corresponding recycle stream. The results here reported seem to demonstrate that the presence of possible multiple steady-state solutions can be related to a numerical problem caused by the high parametric sensitivity of the column profiles to small changes in the convergence criteria of the adopted algorithm. Finally, comparisons among traditional and modified control structures are presented and discussed in terms of quality products and plant operability.