Page 195 - Adsorption Technology & Design, Elsevier (1998)

Design procedures 181

MTZ has been achieved in the small-scale column. A constant pattern MTZ
can only be achieved if the equilibrium isotherm is favourable to adsorption.
The bed length for the full-size plant can then be obtained by adding to the
length of bed which contains the equilibrium section that part which must
contain the region in which mass transfer is occurring, that is the MTZL.
If the isotherm is unfavourable to adsorption then the full-scale bed length
should be set equal to the small-scale bed length which gives the required
performance. For multicomponent and/or non-adiabatic systems, more
than one mass transfer zone will occur for either favourable, unfavourable
or mixed isotherms. Again in such cases it would be prudent to ensure that
the bed lengths for both small-scale and full-scale plants were the same.
Hydrodynamic and dispersion effects should be taken into account from
the outset. The aim should always be to ensure that the adsorption bed is
well packed and that there is no flow maldistribution. While a high pressure
drop in a gas phase application might lead to a better bed loading near the
bed entrance it could possibly lead to condensation. Problems with axial
dispersion may be avoidable if empirical guidelines, available from reaction
engineering experience, are taken into account. Carberry (1976) suggests
that for isothermal operations the column length to diameter ratio should be
more than 20 and flow should be turbulent, i.e. Re > 10. The axial Peclet
number has been shown to be important only when the ratio of column
diameter to particle diameter is less than 12, and when the column length to
particle diameter is less than 50 (Gunn and Malik 1968, Carberry 1976).
Carberry suggests that in order to maintain plug flow for non-isothermal
operations the column length to particle diameter ratio should be more than
150.
LeVan and Vermeulen (1984) report that to avoid channelling, the ratio
of bed diameter to particle diameter should be greater than 20. On the other
hand, Carberry (1976) suggests that in order to avoid radial temperature
gradients, the ratio of bed diameter to particle diameter should be less than 5
or 6. Clearly it might be necessary to make a compromise in the design of
adsorption beds in which heat needs to be added or removed via the walls.
Scale-up could be achieved simply by retaining the same suitable
superficial velocity in the small-scale and full-size plants and increasing the
cross-sectional area for flow by increasing the number of beds which operate
in parallel and/or by increasing the diameter of a single bed. The first option
requires the highest capital investment but the advantage gained is that the
full-size facility should operate identically to the small-scale unit. The
second option leads to simpler and cheaper plant but it is necessary to ensure
that good flow distribution, and redistribution if necessary, is provided.
Care needs to be taken if data is taken from small-scale equipment which
is of a different configuration or design, or differs in any other important