Design methods mass transfer zone length

A quick way to design an adsorber is by the length of unused bed (LUB) approach suggested by Collins (1967). It involves observations about the mass transfer zone (MTZ), so experimental data are reqnired. A precnrsor was snggested by Michaels (1952) as a method of understanding ion exchange colnmn performance. The concept assumes that the transition of concentration from that of the initial contents of the bed to that of the feed is due solely to mass transfer resistance. That is a misconception, but in many cases it sufQces. 

SCALE-UP. The width of the mass-transfer zone depends on the mass-transfer rate, the flow rate, and the shape of the equilibrium curve. Methods of predicting the concentration profiles and zone width have been published, but lengthy computations are often required, and the results may be inaccurate because of uncertainties in the mass-transfer correlations. Usually adsorbers are scaled up from laboratory tests in a small-diameter bed, and the large unit is designed for the same particle size and superficial velocity. The bed length need not be the same, as shown in the next section. 

This design procedure is widely used and its validity depends on the conditions in the laboratory column beilig similar to those for the full-scale unit. The small-diameter unit must be well insulated to be similar to the large-diameter tower, which operates adiabatically. The mass velocity in both units must be the same and the bed of sufficient length to contain a steady-state mass transfer zone (LI). Axial dispersion or axial mixing may not be exactly the same in both towers, but if caution is exercised, this method is a useful design method. 

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