Where Adsorption is Important

Adsorption is important because it can be effective in dilute solution. Many other separations are not. The ability to treat dilute solutions easily is uncommon and is what makes adsorption especially valuable. To illustrate this, consider the costs of the enormous variety of solutes shown in Fig. 15.1-1. The prices of these solutes shown on the abcissca vary by over 10 orders of magnitude, from about 0.01/kg to S 1,000,000,000/kg. These prices are strikingly well correlated with the feed concentrations of the various products, shown on the ordinate. The correlation between feed concentration and price is almost perfectly inverse, i.e., the feed concentration varies with the product price to the (— 1.0) power. Therefore, separation processes which can concentrate valuable products from dilute solutions will have special value. 

Note By purification, we imply that only small solute concentrations are removed. By bulk separations, we imply high solute concentrations. 

Adsorption uses a bed of porous solid particles. The particles pores are small, giving a surface area of several hundred square meters per gram. When a solution of liquid or gas flows through the bed, solutes are adsorbed from the solution onto the surface of the particles. 

In addition to these three simple forms of adsorption, two other types of bonds are important but harder to explain. In some cases, adsorption depends on solute shape. One good example is adsorption of antibiotics on custom-synthesized ion-exchange resins. There, successful separation can involve solute adsorption on several adjacent, non-ionic sites. Such multiple site adsorption tends to reflect the solute s molecular shape and, hence, is of special value for expensive solutes. 

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