Equilibrium zone |

High Imeä r flow rale Low linear flow rite

Length of column

Figure 10. MTZ plot showing equilibrium zones.

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed.

To best understand adsorptive solvent recovery we have to consider some fundamentals of adsorption and desorption. In a very general sense, adsorption is the term for the enrichment of gaseous or dissolved substances (the adsórbate) on the boundary surface of a solid (the adsorbent). On their surfaces adsorbents have what we call active centers where the binding forces between the individual atoms of the solid structure are not completely saturated. At these active centers an adsorption of foreign molecules takes place.

The adsorption process generally is of an exothermal nature. With increasing temperature and decreasing adsórbate concentration the adsorption capacity decreases. For the design of adsorption processes it is important to know the adsorption capacity at constant temperature in relation to the adsórbate concentration. Figure 11 shows the adsorption isotherms for several common solvents.

Remember that this technology is versatile, and is applied equally well to solvent recovery and pollution control applications in gas as well as liquid systems. Let's now focus attention on the applications in water treatment.

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