Page 229 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 209

bed retains the heat of adsorption while the mass transfer zone travels
through the bed, the adsorbent solid could be at a sufficiently high
temperature to initiate regeneration of the bed immediately following the
adsorption step. The ratio R has also been shown to depend on the initial bed
temperature, total pressure and type of adsorbent. By the correct choice of
cycle conditions, the heat of adsorption can be stored within or behind the
mass transfer zone such that heat is retained in the bed before breakthrough
of the adsorbate. However, because of the increase in temperature of the
bed the capacity of the adsorbent is lower than that for corresponding
isothermal conditions. Nevertheless, efficient heat recovery is possible
utilizing the AHR drying method. Garg and Yon (1986) showed that a lower
volume and temperature of purge gas and only a fraction of the energy is
used for the AHR system than for conventional cycles employed to dry gas
streams with a high water content.
Figure 7.10 illustrates the AHR process. Two adsorbent beds are
employed, each acting alternately as an adsorber and regenerator. The feed
is first superheated sufficiently to prevent formation of liquid within the
adsorber and the capillary condensation in the adsorbent pore structure.
Flow of feed to the adsorber is continued until the capacity of the bed is
nearly exhausted but certainly stopped before either any breakthrough of
water or any temperature rise of the effluent. The time required to complete
the adsorption step is typically 10 to 60 minutes which is much shorter than in
conventional thermal swing processes. Regeneration is achieved within a
closed loop utilizing a non-condensable gas such as N2, CO2 or natural gas in
countercurrent flow. The time required for regeneration is equal to that for
adsorption in the two-bed configuration.

Drying liquid streams of alcohol-water mixture
Drying of liquid streams is also accomplished using two packed adsorbent
beds, each one acting alternately as a dehydrator or a regenerator. The feed
to the dehydrator should be near to the azeotrope composition to obtain the
best economic advantage and to avoid physical deterioration of the zeolite
adsorbent. The direction of the feed flow is normally upward in adsorption
and downward during regeneration. Cooling the adsorbent column with
inert gas between the adsorption and desorption stages should be upward if
the cooling gas is carrying moisture, but if a dry inert gas is used for cooling,
upward or downward flow may be chosen. Liquid stream velocity through
the bed should not exceed about 7.5 cm min -1 according to the recom-
mendations of the adsorbent manufacturers W.R. Grace and Company.
It is important to consider the process economics carefully before
selecting a cycle time. The longest time which is physically and economically