ABSTRACT
Separations1 use 6% of the total energy consumed by industry in the United States.2 They can account for up to 70% of the plant costs. The many types include (a) removal of a catalyst, (b) removal of unchanged starting material, (c) removal of by-products, and (d) recovery of solvent or others from a reaction mixture. Common separations involve (a) aromatic from aliphatic, (b) substituted from unsubstituted, (c) linear from branched, (d) positional isomers, (e) diastereoisomers and enantiomers, (f) small molecules from polymers, (g) one polymer from another, (h) charged from uncharged, (i) one metal ion from another, ( j) one anion from another, and so forth. The traditional methods have not been entirely satisfactory. Distillation becomes more dif - cult with heat-sensitive materials, close-boiling mixtures, and azeotropes. Crystallization may not be as selective as desirable and may leave part of the product in the mother liquor as waste.3 Extraction may not be completely selective and may involve the use of large volumes of solvents, frequently with pH changes that result in waste salts. Chromatography4 is ne for small amounts and analytical purposes, but it becomes cumbersome on an industrial scale. However, simulated moving bed chromatography (a method in which much of the solvent is recirculated and the inlets and outlets are moved as the pro le of the material moves through the column) is making the technique more practical and economical for ne chemicals production.5 Supercritical uid chromatography is also being scaled up for the separation of optical isomers.
