Comparison of Controllability Features of Extractive and Pressure Swing Distillations on the Example of Tetrahydrofuran Dewatering

Abstract

The controllability study is an integral part of chemical process design. In this work, the controllability of two special distillation techniques, extractive distillation and pressure swing distillation, designed for the separation of azeotropic mixtures is investigated with dynamic tools. The control design interface of Aspen Plus and Matlab are applied for the modeling and evaluation of the two systems. Dynamic controllability indices are determined and aggregated in a desirability function. The results are compared to obtain efficient help for process design activity. The pressure swing distillation shows significantly better controllability features than the extractive distillation. The reason can be the fact that in the case of the extractive distillation, a third compound, the extractive agent, is added to the system to carry out the separation, therefore making the system more complex. As far as the selection of manipulated variables is concerned, in the case of the extractive distillation, the reflux flows should be preferred to the reflux ratios but in the case of the pressure swing distillation, the reboiler heat loads are preferred to the reflux ratios since those are closer to the controlled compositions. Both separation systems show worse controllability features if the product purity requirement is approaching to the pure products, that is, close to 100%. Although the energy consumption of the pressure swing distillation is higher than that of the extractive distillation, it has the inherent feature that it can be automatically heat integrated due to a column operated at high pressure and, as a consequence, higher temperatures.

Effects of Energy Intensification of Pressure-Swing Distillation on Energy Consumption and Controllability

“…In this work, one maximum-boiling azeotrope and two minimum-boiling azeotropes are adopted from existing publications to represent the PSD for binary azeotrope separation. ,− For THF–water, acetone–water, and acetonitrile–water systems; binary feed flows, product purity levels, operating pressures of the columns, number of stages of the columns, and thermodynamic models are similar to the initial works. Sequential iterative optimization and heuristic optimization are methods commonly used in the optimization of PSD.…”

“…The design and control of both nonheat-integrated PSD − and HIPSD , for separating azeotropes have been investigated. For example, Mtogo et al worked on the THF–water system, designing a suitable control structure for a nonheat-integrated PSD using the desirability function.…”

“…The design and control of both nonheat-integrated PSD − and HIPSD , for separating azeotropes have been investigated. For example, Mtogo et al worked on the THF–water system, designing a suitable control structure for a nonheat-integrated PSD using the desirability function. The identification of a tight control strategy, particularly for fully heat integration processes, is an integral step toward achieving safe and optimum operation for PSD systems because the neat configuration requirement is much stricter than that for ordinary PSD systems.…”

Multi-objective Optimization and Control of Self-Heat Recuperative Azeoropic Distillation for Separating an Ethanol/Water Mixture

“…At this time, ethanol and water will form azeotrope, which is difficult to be further purified by ordinary distillation, and thus other dehydration methods need to be used. 3,4 Azeotropic distillation, 5,6 extractive distillation, 7,8 and pressure-swing distillation 9 are all effective methods for separating azeotropic mixtures, and lots of energy-saving technologies are proposed to improve these processes. 10,11 The introduction of process intensification and integration technology in distillation can effectively reduce the energy consumption and improve the energy efficiency, such as heatintegrated distillation, 12,13 dividing-wall column distillation, 14,15 and cyclic distillation.…”

“…The mass fraction of ethanol obtained from the ordinary distillation section can reach 92.4– 94 wt %. At this time, ethanol and water will form azeotrope, which is difficult to be further purified by ordinary distillation, and thus other dehydration methods need to be used. , Azeotropic distillation, , extractive distillation, , and pressure-swing distillation are all effective methods for separating azeotropic mixtures, and lots of energy-saving technologies are proposed to improve these processes. , …”