(372z) A Shortcut Model for Multicomponent Azeotropic Distillation Column Design

Chemical, pharmaceutical, and agrochemical industries frequently face the challenge of separating multicomponent mixtures exhibiting one or more azeotropes using distillation. To design a distillation column that is energy efficient and cost-effective, key parameters such as the minimum reflux ratio are generally required. Therefore, design engineers need to understand and be able to determine the minimum reflux condition of a distillation column when separating a multicomponent azeotropic mixture.

Due to its practical importance, this problem has attracted great attention from researchers and industrial practitioners over the past decades. However, it remains challenging to solve due to high non-idealities of the multicomponent system. Most existing approaches, including the well-known boundary value method¹ and the rectification body method², perform some sort of relaxation to the set of rigorous but highly nonlinear MESH equations in order to obtain an approximated solution to the minimum reflux ratio. Nevertheless, in their inner core, these methods still rely on tray-by-tray calculations and/or iterative guessing of reflux ratio, which can be too computationally expensive and unstable to perform, especially when the problem size becomes large. As a result, they cannot be easily implemented into a global optimization framework that identifies the optimal distillation column sequence.

Here, we present a simple and easy-to-use shortcut method to analytically determine the minimum reflux ratio of a distillation column for multicomponent homogeneous azeotropic mixture separations. We treat each azeotrope as a pseudocomponent after exploiting the physical and mathematical properties of azeotropes and applying the proper linear transformation. Such transformation preserves the mathematical simplicity of the resulting system and allows us to derive the minimum reflux condition for multicomponent azeotropic distillation.

Through case studies, we demonstrate the accuracy and effectiveness of our new approach. We also show that the classic Underwood’s method used for ideal multicomponent distillation turns out to be a special case of this general approach. Our method does not require any tray-by-tray calculation and is iteration free. Thus, it can be easily incorporated into a global optimization framework that will allow industrial practitioners to, for the first time, quickly identify a handful of attractive distillation configurations from the immense configuration search space.

References

1. Levy GS, Van Dongen DB, Doherty MF. Design and synthesis of homogeneous azeotropic distillations. 2. Minimum reflux calculations for nonideal and azeotropic columns. Ind. Eng. Chem. Fund. 1985;24(4):463-474.
2. Bausa J, Watzdorf Rv, Marquardt W. Shortcut methods for nonideal multicomponent distillation: 1. Simple columns. AIChE J. 1998;44(10):2181-2198.