Optimization of heterogeneous batch extractive distillation with non negligible heat effects
Résumé
Optimization of batch distillation has been treated extensively in the literature. However, batch heterogeneous distillation has been less considered and even less in extractive mode. The performance of the optimization scheme is illustrated through the separation of chloroform – methanol mixture with water considering non negligible heat effects. The resulting design is compared to the corresponding case in which heat effect is ignored.
The objective function maximizes the overall profit and the optimization variables are the entrainer flowrate, temperature of entrainer feeding, the reflux ratio composed by an optimal proportion of both decanted phases during the chloroform distillation stage and the reflux ratio during the methanol distillation stage. Recovery and purity of chloroform methanol and water are set as constraints during the calculation. The optimization relis upon a genetic algorithm procedure and the simulation of the process is coded in two different modes: considering a short-cut model with constant molar overflow assumption for the negligible heat effect process and taking into account a differential mass balance model computing the molar overflow variation inside the column through the energy balance.
Two different alternatives are compared, taking into account negligible (case I) and not heat effects (case II). Profitability for case II is about 35 % higher compared to case I. The optimal temperature for feeding the heterogeneous entrainer (water) was around 28°C allowing a reduction of 18.4% of entrainer, a lower chloroform-entrainer reflux to the column top (14%) during the chloroform withdrawal and lesser reflux ratio for the recovery of methanol. Besides, better recovery (+3.4%, +1.3%) and purity (+0.2%, +0.5%) of chloroform and methanol were obtained, respectively. This optimal policy allows the feeding of heterogeneous entrainer near to the normal environmental temperature reducing the extra energy spending to heat the entrainer until its boiling temperature.
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