Forward- and Reverse-Flow Regeneration

In a practical adsorption system the adsorption cycle is generally terminated prior to breakthrough in order to ensure purity of the raffinate product. 

When forward-flow regeneration is employed it is essential that the adsorption cycle should not be terminated prematurely otherwise desorption will be very inefficient since the sorbate must first be pushed through the unsaturated region of the bed before it can be removed from the outlet. The volume of purge required to desorb the partially saturated bed is therefore essentially the same as for a fully saturated bed. By contrast, with reverse-flow regeneration premature termination of the adsorption cycle means only that the full capacity of the adsorbent is not utilized and the purge requirement/mole desorbed is not increased by operating with an incompletely saturated bed. 

A full discussion of these factors has been given by Cartei who has also presented a numerical simulation of the regeneration of an adiabatic air drier showing clearly the advantage of reverse flow. In Carter s simulations the initial moisture profile through the bed was taken as the profile at the end of the adsorption cycle, which was terminated just prior to breakthrough. The 

FIGURE U.4. Simulated desorption curves for regeneration of a partially saturated bed (HjO-AI2O3) in forward flow (--) and with reverse flow (—). (a) Effluent concentration, (b) effluent temperature, and (c) rale of removal of moisture. (After Carter. ) 

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