Page 178 - Adsorption Technology & Design, Elsevier (1998)

166 Design procedures

combinations in which the isotherm is unfavourable and special considera-
tion may need to be given if the isotherm is linear. For the case of a
favourable isotherm, the validity of the method is very much dependent on
the conditions used in the small-scale laboratory column being similar to
those in the full-scale unit. Factors which must be taken into account include
the diameter of columns and axial dispersion. Large diameter columns tend
to operate adiabatically and thus it is important that the laboratory column is
well insulated. If the heat loss from a small-scale laboratory column is
significant, then the apparent LUB will be erroneously low and will lead to
underdesign of the full-scale column. Differences in the extent of axial
dispersion in small-scale laboratory and full-size columns may also cause
discrepancies. The small-scale laboratory column should therefore be
designed to simulate as closely as possible the axial dispersion conditions of
the larger unit (see Sections 6.8.4 and 6.9).

6.7.2 Mass transfer zone length (MTZL)
The length of the bed, L - Le, shown in Figure 5.6 (b) is known as the mass
transfer zone length or MTZL. The adsorption literature contains some data
for the correlation of MTZL with flowrate or velocity. However, care should
be exercised when using this data because the experimental conditions may
be far removed from those for the intended design. It is far preferable to
obtain the MTZL from small-scale laboratory experiments on the
adsorbate-adsorbent system under consideration and with temperatures
and velocities which are representative of those to be used in the full-size
unit. A practical disadvantage of the MTZL method is that if a bed length is
obtained by adding together the equilibrium zone length and the MTZ
length, then more adsorbent will be installed than is necessary because some
adsorption actually occurs in the mass transfer zone.

6.7.3 Empty bed contact time (EBCT)
The empty bed contact time (EBCT) method is primarily used by the water
industry for the design of large-scale adsorbers from pilot-scale data. There
is no reason why the method should not be applied, however, to other
industrial situations, in particular to liquid phase separations.
As shown in Figure 6.7 a number of pilot-scale columns in series are used
to obtain breakthrough curves with flow at the desired velocity. Three or
more columns are usually used to represent different bed depths and
different contact times. The column diameters may range from 2.5 to 15 cm
in water treatment applications. As shown in Figure 6.8 a horizontal line that