Solvent Based Separation

Mole% ethanol (liquid)

Extract

(2) Ethyl acetate

Feed Water

Extract

Feed Water

Phenol

Figure 1 (A) VLE phase diagrams for ethanol-water (pressure 1 atm) with and without solvents (plotted on a solvent-free basis). (B) Ternary LLE diagram for acetone-water-ethyl acetate. (C) Process flowsheet for separation of phenol from wastewater.

Phenol

Figure 1 (A) VLE phase diagrams for ethanol-water (pressure 1 atm) with and without solvents (plotted on a solvent-free basis). (B) Ternary LLE diagram for acetone-water-ethyl acetate. (C) Process flowsheet for separation of phenol from wastewater.

Table 1 Classification of important solvent-based separation techniques

Separation Solute property Number and identity Separation barrier Separation technique ofphase phenomena

Liquid-liquid extraction Extractive distillation

Azeotropic distillation

Absorption

Stripping

Leaching

Totally miscible solutes Solutes from azeotrope or are close boiling

Solutes from azeotrope or are close boiling

Absorbed gases in liquid

Entrained liquids in gases

Solid particles

Two liquid phases Total miscibility

Vapour and liquid phases

Vapour and two liquid phases

Vapour and liquid phases

Vapour and liquid phases

Solid(s) and liquid phase

Azeotropes or relative volatilities

Azeotropes or relative volatilities

Solubility of gases Solubility of liquids Solubility of solids

Property differences in liquid phases Property differences in vapour and liquid phases

Property differences in vapour and two liquid phases

Differences in solubility

Differences in solubility

Differences in solubility

Solvent function

Addition of solvent causes phase split Addition of solvent breaks the azeotrope but does not cause liquid phase split Addition of solvent breaks the azeotrope but also causes liquid phase split

Solvent must be able to dissolve the solute (gas) Solvent must be able to dissolve the solute (liquid) Solvent must be able to dissolve the solute (solid)

or removal of a chemical species (undesirable byproduct or raw material) from a wastewater stream through solvent-based separation are typical examples of industrial application. Figure 1C illustrates the removal of phenol from water through solvent based liquid-liquid extraction. An important feature in this and most other vapour-liquid and/or liquid-liquid solvent-based separation techniques is that the solvent is recovered and recycled back to the solvent-based separation unit.

A logical criterion for classification of solvent-based separation techniques is the number and identities of the coexisting phases and the function of the solvent. Table 1 gives a list of some of the well-known solvent-based separation techniques, classified in terms of the number and identities of the coexisting phases and function of the solvent. It can be noted from Table 1 that the selected solvent is directly related to the separation task and the separation technique and indirectly related to factors such as cost of operation, the efficiency of separation and the environmental impact. Therefore, solvent selection plays an important role in solvent-based separation. While solvents and solvent-based separation techniques have been known for a very long time, use of efficient search techniques, such as computer-aided molecular design (CAMD) and computer-aided database search, are fairly new. This article highlights the computer-aided methods and tools related to solvent selection.

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