Extractive Distillation

Extractive distillation can be labeled as distillation in the present of a miscible, high boiling, relatively nonvolatile element and the solvent which creates no azeotropes with the other elements in the blend. The prominent use of this type occurs in chemical and petrochemical industries for separating azeotropic, close boiling and other low relative volatility blend. 

This operates due to the solvent which is particularly selected to interact diversely with the elements of the original blend, thereby their altering relative volatilities. This is because of the interactions happens predominantly in the liquid phase, the solvent is constantly added near the top of the extractive distillation column so that an appreciable amount is present in the liquid phase on all of the trays below. The blend is separated is attached through second feed point further down the column.

Preliminary work and the categories

In the extractive column, the element having the superior volatility, not necessarily the element having the lowest boiling point, is taken overhead as a relatively pure distillate. The other remaining element leaves with the solvent through the column bottoms. The solvent is divided from the residual elements in a second distillation column and then recycled back to the first column.

And extractive distillation can be classified into three categories, each relate to the various residue curve maps, the minimum boiling azeotropes, maximum boiling azeotropes and the non-azeotropes blends. As in azeotropic distillation, design of extractive distillation system will need notable preliminary duty consists such as choosing the solvent, developing or finding the necessary data like azeotropic condition or residue curve, preliminary screening, computer simulation and small scale testing.

Greater volatility and top products

And this method extractive distillation employs a separation solvent that is commonly nonvolatile has a high boiling point and is miscible with the blend but do not form an azeotropic blend. The solvent interacts differently with the elements of the blend thereby causing their relative volatilities to change. This enables the new three-part blend to be separated by usual distillation. The original element with the greatest volatility divides out as the top product. 

The bottom product comprises of a blend of the solvent and other element which can be again be separated easily because the solvent doesn’t form an azeotropic with it. The bottom product can be divided by any of the techniques available. It is essential to pick a suitable division solvent for this kind of distillation. The solvent should change the corresponding volatility by a wide enough margin for a successful result.

Questions:

• What is extractive distillation?
• State the significance of extractive distillation.