Operation strategies for simulated moving bed in the presence of adsorbent ageing

The effect of adsorbent ageing on the operation of Simulated Moving Bed (SMB) units is addressed, for two different systems (linear and non-linear isotherms). Two different consequences of adsorbent deactivation are considered, one involving the loss of adsorption equilibrium capacity and the other related to the increase of mass transfer resistance. For each of these cases one direct and straightforward compensation measure is presented: the increase of the solid velocity (decrease of switching time) to compensate the adsorbent capacity decline and the decrease of solid and internal flow rates to compensate the mass transfer resistances increase. It is possible to compensate the adsorbent capacity decline, maintaining all SMB performance parameters, limited to the reduced contact time when high solid velocities are used. In the second case, when mass transfer resistance increases, it has been shown that the direct compensation strategy would lead to a decrease of productivity, to keep the same purity and recovery as with fresh adsorbent. Another possible corrective method to the ageing problem is the use of variable switching times in the non-conventional "modus operandi"-Varicol, a sort of re-arrangement of the SMB columns distribution by sections, here tested for the case of adsorbent capacity decline case as a consequence of adsorbent ageing mechanisms.