A simple design method for multicomponent distillation columns D.P.Rao*, Amit Kumar, Ashok Kumar, Devendra Agarwal, Abhishek Sinha Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India Abstract: Multicomponent-distillation calculation methods are classified as rating or design methods. A few rating methods are widely used but none of the design methods has found general acceptance. Recently, a few design methods have been proposed using the stage-to- stage calculations and starting from one of the ends of the column, but they require tedious calculations. We present here a simple design method in which the stage-to-stage calculations are initiated from the feed stage with the feed composition. The method permits the use of component or matrix tray efficiencies or the nonequilibrium-stage model to estimate the number of stages. We have studied the convergence behavior of the proposed method for several systems. The convergence was achieved in all the cases within 3-20 iterations taking CPU time in the order of 100 th of a second on a PC with a Pentium 4 processor. ------------------------------------------------------------------------- * Corresponding author. Tel.:+91-512-2597873 Fax: +91-512-2590104 E-mail address: dprao@iitk.ac.in (D.P.Rao) 1. Introduction Multicomponent distillation calculations are classified as design or rating methods. In the former, the number of stages have to be determined given the recovery of key components and in the latter the composition of products have to be found given the number stages in the enriching and stripping sections. Lewis-Matheson (1932) proposed a method of the stage-to- stage calculations for multicomponent distillation. To determine an approximate distillate composition, they considered that the components lighter than light key will present only in the distillate and the heavier than heavy in the bottom product. Assigning arbitrarily small amounts for the missing components, they proposed to compute the product compositions to initiate the stage-to-stage calculation and match the feed stage compositions computed from both ends. The discrepancy between the feed stage compositions thus computed is used to revise the products compositions. However, this method did not gain acceptance and currently there are no simple and robust design methods Fidkowski et al. (1991). To avoid the arbitrary assignment of the missing components in the products, Thiele- Geddes(1933) proposed a method in which they preassigned the number stages in each sections and determined the product compositions. This method lends itself well to cast the component and enthalpy balances around the stages into elegant tridiagonal or block diagonal matrix equation(s). Then, the Newton-Raphson method is used to find the compositions of products and liquid and vapor streams in the column. The developments in this direction led to the well-known Napthali-Sandholm method and its variants. To extend the domain of convergence and to find multiple solutions, physical and mathematical homotopy-continuation methods are used. Ketchum (1979) integrated the relaxation method, proposed by Rose et al. (1958), into a variant of the Napthali-Sandholm method. These can be adopted for equilibrium