Shortcut methods are a prerequisite for the fast evaluation of alternatives in process design. The rectification body method (RBM) for calculating the minimum energy demand of homogeneous, azeotropic multicomponent distillation processes is presented. The new method incorporates both the classical Underwood technique and certain preuiously proposed techniques as special cases. It employs all pinch point branches of both column sections and thus requires no a priori selection of active pinch points. It is entirely general and can be applied to any type of split of nonideal and azeotropic mixtures irrespective of the number of components. Hence, it significantly extends previous results and practical applicability. Its features are highlighted through examples of nonideal separations with three, four, and five components.

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In conceptual process design many alternatives can arise for the separation of multicomponent mixtures. Shortcut methods can be used to evaluate and compare all variants in a first step. In this work shortcut methods for hybrid membrane/distillation processes are presented. The distillation is addressed by the rectification body method recently developed by the authors, providing the minimum energy demand of a separation and the optimal sidestream composition. The membrane unit is evaluated using the minimum membrane area. It is a lower bound for the actual membrane area and can be used to estimate the real membrane area and thus the investment cost of the membrane process. The novel method overcomes limitations of previous methods:  It can be applied to multicomponent nonideal mixtures and to general flux laws for any membrane. The stated methodology is applied to the purification of ethanol and to the separation of the reactor output stream in methyl tert-butyl ether production.

An experimental study of ethanol dehydration with cyclohexane as the entrainer has been carried out in order to verify the existence of multiple steady states in heterogeneous azeotropic distillation. As a basis, the bifurcation diagram for the ethanol product purity is determined from thermodynamic considerations using the distillate flow rate as the bifurcation parameter. The predicted multiple steady states are then verified by rigorous simulation and by experiments on a laboratory tray column with a decanter. The hysteresis behavior due to multiple steady states is also verified in a dynamic experiment.

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