Batch distillation models incorporating a complete set of hydraulic relations, energy balances, and tray efficiencies and simpler models based upon constant molar holdup and zero holdup (the Rayleigh equation) were used to obtain distillation curves and optimal reflux policies. The approach to hydraulic limitations was monitored in the complete hydraulic model. The results indicate that during a multicomponent batch distillation the combination of the changes in the liquid composition and liquid flow rates causes the molar holdup on the trays to vary with time and, consequently, the characteristic time constant of the trays changes. The predicted composition profiles for descriptions which incorporate tray hydraulics differ significantly from those obtained from simpler models. Optimal reflux policies generated from the complete model which includes tray hydraulics differ significantly from those obtained from simple models and may lead to lower product yield. The optimum reflux ratios obtained from the model which includes tray hydraulics and tray efficiencies may be greater than the reflux ratios obtained from a model without tray hydraulics, but which includes tray efficiencies by as much as a factor of 1.3. Tray hydraulics must be included in the process model to obtain a reliable and effective optimum operating strategy for batch distillation.


Chemical and Biochemical Engineering

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Article - Journal

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Publication Date

01 Jan 1997