Iterative dynamic programming employing randomly chosen candidates for admissible control is applied to the minimization of distillation startup time. The solution of this high-dimensional problem was facilitated mainly by avoiding iterative algebraic equation solving inside a rigorous dynamic distillation model, thereby reducing the computation time. Several illustrative applications of various complexity degrees are presented. The control variables are the reflux ratio, the reboiler heat duty, or both. Both piecewise constant control and piecewise linear control strategies were employed. The decrease of the startup times and heat consumption is high, having an appreciable economic significance.

The paper presents the analysis of the thermal efficiency of a dividing wall distillation column and the influence of the feed composition on the reduction of energy consumption compared to a classical scheme of multicomponent mixture separation. The study relays on rigorous simulations in HYSYSTM using thermodynamically equivalent schemes. A case study is presented for the separation of a hydrocarbon mixture: benzene, toluene, ethylbenzene, o-xylene separated in three products in a dividing wall column. The dividing wall column solution led to about 40 % energy saving. The efficiency of a dividing wall column increases when the middle component is in large amount in the feed.

Time-optimal control of startup traditional distillation columns by iterative programming proposed by Woinaroschy for ideal [Ind. Eng. Chem. Res. 2008, 47, 4158] and nonideal mixtures [Ind. Eng. Chem. Res. 2009, 48, 3873] is extended to the case of dividing-wall distillation columns. The minimization of distillation startup time is performed by iterative dynamic programming employing randomly chosen candidates for admissible control. The control variables are the reflux ratio, the reboiler heat duty, and the side-draw flow rate. The dynamic distillation model proposed by the author in the previous papers is applied. Two illustrative case studies for the separation in a dividing-wall column with sieve trays and lateral downcomers are presented as follows: the separation of an ideal benzene−toluene−ethylbenzene ternary mixture and the separation of a nonideal methanol−ethanol−1-propanol mixture. In another case study, a conventional two-column system is presented in comparison to the dividing-wall column. As in the cases of traditional distillation columns, the startup time decrease and the corresponding reboiler energy savings are significant for each of the control variables.

In this work, the time-optimal control of startup distillation columns by iterative programming proposed by Woinaroschy [Ind. Eng. Chem. Res.2008474158] is extended to the case of nonideal mixtures. The minimization of distillation startup time is performed by iterative dynamic programming employing randomly chosen candidates for admissible control. The control variables are the reflux ratio and/or the reboiler heat duty. The dynamic distillation model proposed by the author in the mentioned work is applied with a preliminary computation of suitable correlation relations for equilibrium constants. An illustrative application for separation of a propene−propane mixture at plant scale is presented. Because of the small relative volatility of this mixture and respective high reflux ratio, the decrease of the reboiler heat consumption corresponding to the minimization of the startup time is significant.

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