Extractive distillation

Extractive distillation separates close-boiling or azeotropic mixtures by adding a relatively nonvolatile solvent that changes component volatilities without forming a new azeotrope.

Relative volatility: A measure of how readily one component vaporizes relative to another in a mixture. Extractive solvents work by changing this separation measure between the mixture’s components.
Distillation: A separation process that uses differences in volatility to distribute components between vapor and liquid. Extractive distillation modifies ordinary distillation by changing mixture volatility with a solvent.
Ethanol dehydration: The removal of water from ethanol to produce ethanol at a higher concentration. Extractive distillation can separate ethanol from water beyond the ordinary distillation limit.
Azeotropic distillation: A distillation method that adds an entrainer to create or modify azeotropic behavior for separation. Unlike extractive distillation, it relies on forming or exploiting an azeotrope.
Vapor–liquid equilibrium: The equilibrium distribution of components between vapor and liquid phases at specified conditions. The column’s separation depends on how solvent addition reshapes this equilibrium.
Raoult's law: An ideal-solution relation connecting a liquid component’s mole fraction to its partial vapor pressure. Departures from ideal behavior help explain why solvent addition changes component vapor pressures.
Benzene–cyclohexane separation: The separation of benzene and cyclohexane, whose boiling points are close. A selective solvent can increase their relative volatility enough for practical column separation.
Pressure-swing distillation: A process that separates an azeotropic mixture by distilling it at different pressures. It changes operating pressure rather than adding a high-boiling selective solvent.
Solvent selectivity: A solvent’s ability to interact differently with two or more mixture components. Selectivity determines which component’s volatility the added solvent suppresses most.
Activity coefficient: A factor that expresses how a component’s chemical potential differs from ideal-solution behavior. Solvent–solute interactions change activity coefficients and therefore relative volatility.

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