A New Batch Extractive Distillation Operational Policy for Methanol Recovery

Abstract

The recovery of methanol from a multicomponent aqueous waste solvent mixture was studied. The components form several minimum azeotropes, of which the methanol-THF and methanol-toluene azeotropes limit the recovery of methanol by traditional batch distillation (BD). THF and toluene are removed in the fore-cut, causing a significant loss of methanol. The addition of water decreases both methanol-THF and methanol-toluene relative volatility, and therefore by feeding water as entrainer, methanol loss can be decreased. A new BED operational policy is suggested, where water feeding is applied only during the heating-up of the column (BED1). Compared to BD, at the end of the heating-up, the concentration of organic pollutants is increased, and that of methanol is significantly decreased in the top of the column. Water feeding can continue during the fore-cut (BED2), but this increases the amount of fore-cut and dilutes the mixture from which methanol is recovered. Laboratory experiments were performed in a packed column to compare the BD and the two BED operational policies. The highest recovery was obtained by BED1, the lowest one by BD. The preliminary and posterior rigorous dynamic simulation of the experiments was made with a professional dynamic flowsheet simulator. Industrial-size pilot productions of BD and BED1 in a 50 bubble cap tray column also showed a 5% increase in methanol recovery.