Page 188 - Adsorption Technology & Design, Elsevier (1998)

174 Design procedures

fraction, of adsorbate retained by an initially adsorbate-free bed up to the
breakthrough point. It has been found that the parameter does not vary
appreciably with operating conditions other than temperature and thus can
be used for design.
In the adsorptive drying of gases, several researchers (Miller and Roberts
1958, Bowen and Donald 1963, 1964) have found that the capacity at
breakthrough depends linearly on the relative concentration at the inlet, this
parameter being defined as the ratio of adsorbate (water) concentration in
the feed to the concentration of adsorbate (water) that would saturate the
feed at the mean temperature of the bed during operation. The capacity at
breakpoint is a useful concept when the overall energy requirements of a
cyclic operation are being assessed. Bowen (1971) provides details for
estimating the times for adsorption, bed heating and bed cooling of a two-
bed system.

6.8 HYDRODYNAMICS

Four major considerations must be made when selecting the size of
adsorbent particles, namely the availability of commercial materials, the
effect of size on mass transfer and adsorption rate, the effect of size on
pressure drop, and the effect of size on axial dispersion.
Pressure drops in fixed beds can be reduced by selecting larger particle
sizes. However, large particle sizes can give rise to low mass transfer rates,
which in turn would lead to increased bed sizes. Small particles can increase
the effect of axial dispersion. A balance therefore needs to be struck
between the need to keep operating costs low by keeping the pressure drop
low and the need to keep capital investment low by ensuring a high
adsorption rate without high axial dispersion. The pressure drop must not be
too low, otherwise poor flow distribution in the bed may occur. In some
applications it might become necessary to use beds which contain two sizes
of adsorbent in order to avoid to some extent the trade-off that must be
made between pressure drop and mass transfer rate. The highest pressure
drop is likely to occur during the regeneration step, since in this step the fluid
is usually at its highest temperature and/or its lowest pressure. For cyclic
fixed bed processes therefore it is important that a pressure drop analysis is
carried out for each step in the cycle.

6.8.1 Pressure drop
The particle Reynolds number is generally required in the several correla-
tions which are available for estimating pressure drop: