Determination of Minimum Reflux ratio of Simple Distillation Columns using Underwood’s Equation: Influence of non-key Distributions in Products

Abstract

Minimum reflux ratio is an important parameter in distillation column design. It can be used for evaluating a column in terms of energy demands and determining the optimum operating conditions. Underwood’s equation is a shortcut method that is commonly used for determining the minimum reflux ratio of simple columns separating ideal mixtures. Underwood’s equation is widely used because it requires simple calculations. To apply the equation, the mole fractions of all components in the products must be specified; unfortunately, at the preliminary stage of column design, the distributions of other components besides the key components are unknown. Those values may be specified arbitrarily or estimated by using Fenske’s equation. However, specifying arbitrarily or estimating the distributions of other components using the Fenske’s equation may lead to an inaccuracy of the minimum reflux ratio estimation by Underwood’s equation, particularly for cases with high distribution of non-key components in products. In this article, the principle of Underwood’s method is reviewed. The effect of the distribution of non-key components in products on the accuracy of Underwood’s equation is demonstrated. In addition, this article presents a method for evaluating the distribution of non-key components in products.