Chapter 2 - Extractive distillation

Abstract

Extractive distillation is commonly applied in industry, and is becoming an increasingly important separation method in petrochemical engineering The prominent characteristics of extractive distillation is that one new solvent with a high boiling point, is added to the components to be separated so as to increase their relative volatility. Selection of a suitable solvent is fundamental to ensure an effective and economical design. Computer-aided molecular design (CAMD) is a useful tool and is applied for screening the solvents and thus reducing the experimental work. There are two theories related to extractive distillation: Prausnitz and Anderson theory, which gives semi-quantitative explanations, and scaled particle theory, which gives complete quantitative results. The two theories provide a molecular basis for explaining why some solvents can increase the relative volatility of the components to be separated and why more selective than others. Prausnitz and Anderson theory explains the solvent selectivity in extractive distillation of hydrocarbons from the viewpoint of molecular thermodynamics and intermolecular forces. The interaction forces between the solvent and the component are broadly divided into two types: physical force and chemical force. Scaled particle theory is extended to solve the problem of extractive distillation with the combination of solvent and salt, and is expected to promote the development of extractive distillation. For design of distillation process, two types of modeling approaches are developed: the equilibrium (EQ) stage model and the non-equilibrium (NEQ) stage model.