Page 259 - Chiral Separation Techniques

9.5 The Steady State TMB Model 237

Fig. 9-10. Effect of the switch time interval on the purity.

Increasing the switch time interval is equivalent to decrease the solid flow rate and
the net fluxes of components in all sections of the TMB unit will be pushed in the
same direction of the liquid phase. This implies that, first, the more retained species
will move upwards in section III and will contaminate the raffinate stream; and the
less retained species will move upwards in section IV, will be recycled to section I,
and will contaminate also the extract stream. The decrease of the switch time inter-
val will have similar consequences. The equivalent solid flow rate will increase and
the net fluxes of component in all four sections of the TMB unit will be pushed in
the opposite direction of the liquid phase. This implies that, first, the less-retained
species will move downwards in section II and will contaminate the extract stream;
and the more retained component will also move downwards in section I, will be
recycled with the solid to the section IV, and will contaminate the raffinate stream.
It is possible to obtain simultaneously high purities and recoveries in a SMB, but the
tuning must be carefully carried out.

9.5.1.2 Effect of the Mass Transfer Resistance on the SMB Performance

The influence of the mass transfer resistance on the purity and on the steady state
internal concentration profiles are shown in Figs. 9-11 and 9-12. A higher value for
the mass transfer coefficient corresponds to a situation where mass transfer resis-
tance is less important, and a better performance of the SMB will be obtained with
sharper internal concentration profiles.