Periodic Countercurrent Sorption in Multiple-Column Systems

PERIODIC COUNTERCURRENT SORPTION IN MULTIPLE-COLUMN SYSTEMS 

FIGURE 12.9, Schematic diagram showing the sequence of column interchange in a periodic countercurrent adsorption system. Shading indicates concentration profile at the end of the adsorption cycle. 

Periodic countercurrent systems are widely used in ion exchange and water purification systems for the removal of trace concentrations of organic components with beds of activated carbon.At low sorbate concentrations the adsorption isotherms for many organic pollutants are linear or only slightly favorable so that the mass transfer zone is wide, and in a simple two-bed cyclic batch system the LUB would therefore be uneconomically large. For such systems, in order to obtain an economic process, some form of counter-current operation is necessary in order to reduce the adsorbent inventory. 

FIGURE 12.10. Comparison of continuous countercurrent, periodic countercurrent, and cyclic batch adsorption systems showing effectiveness of adsorbent utilization as a function of bed length. (From ref. 15, reprinted with permission.) 

It is evident that in going from a single-column to a two-column system the utilization of the adsorbent increases dramatically from about 33% to 78%. Further subdivision gives a less dramatic improvement to about 85% utilization with a six-column system while in a true countercurrent system with a sufficiently long column the utilization approaches 100%. 

---