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.  

One of the major applications of extractive distillation in the petrochemical and refining industry is the recovery of aromatic hydrocarbons.

Advanced solvent technology has increased the feasibility and profitability of using extractive distillation, resulting in benefits such as lower capital cost, lowered risk of product contamination with solvent carryover, and high product recovery and purity.3 As a result, this industry currently uses extractive distillation processes for the recovery of pygas byproducts such as benzene, toluene and xylene (BTX) from naphtha-based steam crackers, and in the desulfurization of FCC gasoline and styrene recovery from pyrolysis gasoline.3,5

• 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.