Page 253 - Chiral Separation Techniques

9.4 Simulation Results 231

Fig. 9-6. Steady state internal concentration profiles in the TMB approach and SMB cases at half-time
between switchings for the more retained component.

Nevertheless, near the feed point, there is a difference between TMB and all SMB
cases due to the fact that the internal flow rates in the TMB are smaller than in the
SMB, leading to a small dilution of the feed stream. As a consequence, near the feed
inlet, TMB concentrations will be higher than in the SMB operation. The raffinate
and extract purities in SMB units with four (95.2 % and 89.5 %), eight (98.7 % and
95.9 %) and 12 columns (99.1 % and 96.8 %) are increasing towards the one
obtained in the equivalent TMB unit (99.3 % and 97.7 %). The optimum degree of
subdivision of the SMB unit will depend of the difficulty of the separation and the
product purity requirements. Typically, systems for the pharmaceutical industry have
six to 16 columns.

9.4.2 Separation Regions

The design problem of a TMB consists on setting the flow rates in each section to
obtain the desired separation. Some constraints have to be met to recover the less-
adsorbed component A in the raffinate and the more retained component B in the
extract. These constraints are expressed in terms of the net fluxes of components in
each section (see Fig. 9-1). In section I, both species must move upwards, in sections
II and III the light species must move upwards, while the net flux of the more
retained component must be downwards, and in section IV the net flux of both
species have to be downwards, i.e.,