Extractive distillation is a powerful technique utilized to separate close-boiling and azeotropic mixtures. As it gains wider consideration within industrial chemical processes, it is important to understand the inner workings and possible applications.  

In the pharmaceutical and specialty chemical industries, extractive distillation may further play a role in solvent wastewater treatment. For example, isopropyl alcohol (IPA), one of the most widely used solvents in the manufacturing of pharmaceuticals, can be obtained from an IPA/water mixture as the top product of an extractive distillation column after the addition of ethylene glycol.6 The recovery of solvents such as IPA have many benefits, including waste reduction, cost savings and meeting stringent EPA requirements for wastewater disposal.

• Extractive distillation

• Azeotropic distillation

• Pressure swing distillation

These three methods are especially useful for “breaking” azeotropes. These mixtures are comprised of components that have constant boiling temperatures at which the liquid and the vapor compositions are equal at a given pressure, posing separation challenges not addressable by conventional distillation methods.

Extractive distillation works by introducing a solvent to modify the molecular interactions of a mixture. The solvent alters relative volatilities by changing the intermolecular interactions of the components within the mixture. This ultimately allows one of the other components to be driven overhead as a distillate product with high purity.2 Typically, the solvent added in extractive distillation has a higher boiling point than either of the feed components, and thus, is easier to recover for reuse. In azeotropic distillation, the solvent forms a new azeotrope with one of the components. Given the similarity with azeotropic distillation, extractive distillation was previously considered to be a special case of azeotropic distillation in a double-feed column, deemed suitable for the separation of close-boiling mixtures by using a solvent that would not form any new azeotrope.1 However, the two processes are now considered distinct since they obey different feasibility rules and operate using different column configurations. In addition, extractive distillation is easier to model via process simulations due to the absence of two liquid phases, normally present in azeotropic distillation. In this way, extractive distillation is often regarded as a preferable and easier method than azeotropic distillation.

There are other distillation processes used to break azeotropes that do not involve the introduction of solvent into the process, such as pressure swing distillation, which achieves separation by taking advantage of a shift in composition of an azeotropic mixture with pressure.2 Nonetheless, there are many reasons why pressure swing distillation may not be the ideal process for “breaking” azeotropes. For example, some systems do not exhibit a significant variation in azeotropic composition over the pressure range. This can become a cost-prohibitive solution due to the high energy usage required for a minimum-boiling azeotrope or the need for larger diameter columns to accommodate high flow rates. Also, operating at elevated pressure, which also increases the operating temperature, can result in thermal instability in some cases.