Page 186 - Adsorption Technology & Design, Elsevier (1998)

172 Design procedures

Faust and Aly (1987) also describe how the BDST model can be modified
to account for changes in feed concentration from co to c~ such that the
original and new effluent concentrations are Cb and cf, respectively:
co
new slope a' = a-- (6.60)
c1

new inter ept, : b (6.61)

Of course it may be desirable that Cb and Cf should be equal. This method for
estimating the effect of changing feed concentration appears to work well
when single-component impurities are removed from water on carbon.
However, further validation is required before the method can be applied to
multicomponent and non-dilute systems.
The BDST equation describes how the mass transfer zone progresses
through a single fixed bed of adsorbent. The equation can be adapted to include
series of fixed beds and moving bed systems in which either a bed is removed or
fresh adsorbent is added, respectively, when breakthrough from one bed
occurs. These methods of operation are popular in the water industry. Clearly,
the most reliable method of determining the speed of the MTZ is to conduct
an experimental test. However, a good estimate of the speed by which the
MTZ moves through an adsorbent bed can be made by applying the BDST
equation with an assumed average concentration of adsorbable species in the
feed and the appropriate factors shown in Table 6.5 (Faust and Aly 1987). The
factors in this table were developed by assuming that the moving bed is pulsed,
or a column is removed, just as the wave front begins to exit from the system.
This is the point at which the adsorbent is exhausted, i.e. the adsorbent phase
is in equilibrium with the feed concentration.
The factors in Table 6.5 are the fractions of the original impurity
concentration that is assumed to be the average fed to freshly added
adsorbent. If it can be assumed that the intercept of the BDST equation is
not significant in a series or moving bed system, then the slope of a BDST
equation describing the movement of a wave front through fresh or
regenerated adsorbent can be estimated from equation (6.62) in which f is
the factor from Table 6.5.
a
a' = - (6.62)
f