Comparison between Azeotropic and Extractive Distillation

Many of the entrainers used in azeotropic distillation are either proven or suspected carcinogens or otherwise classified as hazardous pollutants. Using the example of ethanol-water from previous section, noted that benzene is the entrainer. Ethanol is removed as the bottoms product from the column. Benzene is too hazardous for various reasons ranging from workplace to product to environmental safety.

An alternative to recover ethanol is to use extractive distillation. The solvent used is Propylene Glycol. Recall also that ethanol forms a minimum-boiling azeotrope with water at approximately 89.4 mole% (96 wt%) ethanol.

A process schematic for the process is shown in the Figure below:

Extractive Distillation: Example 2

For this separation, propylene glycol meets all the requirements of an ideal extractive solvent:

BulletIt is miscible with water at all concentrations

BulletIt has a higher boiling point than water (187 degC at 1 atm)

BulletIt does not form an azeotrope with water

BulletIt has a molecular affinity for water (the hydroxyl -OH group forms a weak bond with water molecule)

BulletIt is a relatively safe workplace material

In the above system, the first column is the ordinary azeotropic distillation that produces an ethanol-water azeotrope as the distillate and nearly pure water as the bottoms. The distillate is fed to the second column for extractive distillation, where propylene glycol is added. Ethanol is produced as the distillate, leaving the top of the column.

This column can be conceptually divided into 3 sections. The middle section is the rectifying section where ethanol is purified by the removal of water. Bonding of the water molecules with glycol raises ethanol's relative volatility with respect to water, thus facilitating separation. The following table shows the change in relative volatility in the columns.

Separation by Extractive Distillation

The top section reduces the concentration of propylene glycol in the ethanol distillate to negligible level. The bottom section strips ethanol from water. The bottoms from the second column is sent to the third column, a glycol stripper, where the glycol is recovered. The propylene glycol leaves the stripper as a bottoms product and is recycled back to the extraction column as the source of solvent. The overhead from the glycol stripper (containing mainly water and some ethanol) is sent back to the first column where it combined with fresh feed.

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