Separation of Binary Azeotropes by Hybrid Processes



This process is generally known as "extractive distillation", even though in reality it involves an absorption step and not an extraction step.

Example: separation of ethanol-water mixture using ethylene glycol as the absorbent. See Figure below.

Hybrid Processes: Distillation-Absorption

Distillation-Absorption: Ethanol, water, ethylene glycol

In column C1 most of the water is removed as bottoms. The overhead vapour with azeotropic composition of ethanol-water is fed into the absorber A1 where water is removed by absorption into ethylene glycol. Ethanol is recovered at the top product from absorber A1. The bottom product is mainly ethylene glycol and water, and is fed into column C2 where the ethylene glycol is regenerated by vacuum distillation. It appears as the bottoms of column C2 and is recycled. The overhead product from column C2 is essentially pure water.

NOTE: Often the vapour product from the absorber contains small amounts of the absorbent. Part of the vapour can be condensed and recycled as reflux at some stages above the absorbent entry point. Similarly, some of the ethanol is dissolved in the bottom liquid, which can be separated by reboiling.


The process is widely used not only for separation of azeotropic mixtures but also for the separation of close boiling mixtures (see later section on "Extractive Distillation".

Examples of Distillation-Absorption

From: Table 5.9, "Distillation - Principles and Practice", J.G. Stichlmair, J.R. Fair, p.243.

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