Divided wall columns (DWCs) can save energy and capital costs compared with traditional distillation columns; however, the design of DWCs is more difficult because there are more degrees of freedom. This paper describes a novel short-cut method that can be used to rapidly determine near-optimal values of important design parameters, including the reflux ratio, number of stages in all sections, and split liquid and vapor ratios for the three most common types of DWCs. The method is based on the development of a rational and efficient net flow model and the application of the methods of Fenske, Underwood, and Gilliland and the Kirkbride equation. The method is applied to two real systems, and the results are compared with results from rigorous simulations and optimization. The results show that the shortcut method leads to a process similar to a feasible actual process, and the total annual cost (TAC) based on the design method is close to the minimum (optimum) total annual cost. The results also show that the method also provides good initial values for rigorous optimization. The method is also applied to a fictitious process consisting of three components with constant relative volatilities, for different values of the ease of separation index (ESI), overall split difficulty, and feed composition. The results indicate that the method works well for a variety of process conditions and that the minimum vapor flow rate is a good surrogate for the total cost of process operation.
This study focuses on the physical property model parameters estimation in order to accurately simulate separation processes for a given set of components. The non-random two-liquid (NRTL) model was chosen and parameters were calculated using different methods: experimental data regression and UNIFAC and COSMO-SAC (conductor-like screening model, segment activity coefficient) predictive models. The vapor-liquid equilibrium (VLE) obtained from these different models was compared and results showed that COSMO-SAC can be a reliable tool when data or functional groups are missing. Results also showed that the use of UNIFAC to estimate activity coefficients at infinite dilution can, in some cases, leads to inaccurate results and strongly impact process simulation results. 2009
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