Page 106 - Adsorption Technology & Design, Elsevier (1998)

102 Processes and cycles

A more efficient way of carrying out a multistage purification or
separation is to adopt a countercurrent configuration of fluid and adsorbent
flows as shown, for example, for a three-stage continuous process in Figure
5.4. Here, the feed is contacted initially with a quantity of adsorbent which
has passed through two other stages. As with the cross-current method, the
quantity of adsorbent required for a given separation can be reduced by
increasing the number of stages. Clearly an economic evaluation is required
to find the optimum number of stages but it should be noted that there is a
minimum amount of adsorbent in a countercurrent operation which will
cause the number of stages required to increase to infinity (Ruthven and
Ching 1989). This phenomenon is similar to the minimum solvent-to-feed
ratio which occurs in solvent extraction (Crittenden 1991) and to the
minimum reflux ratio which occurs in distillation.

5.3 FIXED BED PROCESSES

Separation in a fixed bed of adsorbent is, in virtually all practical cases, an
unsteady state rate controlled process. This means that conditions at any
particular point within the fixed bed vary with time. Adsorption therefore
occurs only in a particular region of the bed, known as the mass transfer
zone, which moves through the bed with time.

5.3.1 The mass transfer zone (MTZ)
Progress of the mass transfer zone (MTZ) through a fixed bed for a single
adsorbate in a diluent is shown schematically in Figures 5.1 and 5.5. In
practice, it is difficult to follow the progress of the MTZ inside a column
packed with adsorbent because it is difficult to make meaningful
measurements of parameters other than temperature. By following the
progress of the exotherm which accompanies the adsorption process it is
possible to gain an indication of the position of the MTZ. Methods have now
been devised for applications in which the temperature rise is small because
the adsorbing species are dilute (Lockett et al. 1992). It is, of course, much
easier to measure the concentration of an adsorbate as it leaves the fixed bed
but this clearly cannot be done routinely in an industrial process since
breakthrough of the species which is meant to be retained within the bed will
have occured. A detector could be placed within the adsorbent bed but there
is then the risk that uncertainties about the shape of the MTZ and the
possibility of channeling could lead to breakthrough earlier than anti-
cipated. In spite of such difficulties, much information valuable to the design