Advances in Extractive Distillation

This contribution provides an update on extractive distillation topics of the reference module “Extractive Distillation” written by Kiss7 (https://doi.org/10.1016/B978-0-12-409547-2.05949-7) that covered basic principles of process design and industrial applications, with a focus on extractive dividing wall column. Aiming at separating non-ideal minimum or maximum boiling azeotropic and low relative volatility mixtures, extractive distillation requires the feeding of a solvent, so-called entrainer, at another location than the main feed. The resulting occurrence of an extractive section implies design rules that are different from classical azeotropic distillation ones. In particular, solvent selection rules rely upon selectivity ratios and residue curve map (rcm) topology including univolatility curves. A general feasibility criterion shows that any kind of solvents, intermediate, light, heterogeneous and the well know heavy solvents give rise to a feasible process with restrictions about the achievable products and the limiting values of solvent-to-feed flow rate ratio and reflux ratio. A classification of all extractive separation classes has been published in a review paper (https://doi.org/10.1016/j.cherd.2018.09.020) by Gerbaud et al.8 Batch operation mode differences with respect to continuous operation are highlighted, namely that the batch process is feasible under infinite reflux and can drive the still path through a change of the reflux policy. Necessity of considering solvent regeneration column when carrying out the extractive process optimization is discussed, as well as several energy saving modifications of the process like pressure change to increase relative volatility, heat integration, extractive divided wall column or use of a preconcentrator.