Page 116 - Adsorption Technology & Design, Elsevier (1998)

112 Processes and cycles

5.4.2 Pulsed flow
As shown schematically in Figure 5.8 the feed enters the pulsed flow
adsorber at the bottom and flows upwards. The adsorber is designed such
that the effluent carries some adsorbent out from the top of the bed and this
loss is made up at the bottom with freshly regenerated adsorbent. In this way
the mass transfer zone can be retained inside the column. In practice the
removal and addition of adsorbent is not generally carried out continuously.
Instead it is more common for the column to be operated on a semi-
continuous basis in which a predetermined amount of adsorbent is removed
and added periodically. The column can therefore be likened to a series of
fixed beds stacked on top of each other with the top one being removed and a
new one being added at the bottom. The adsorbent flow should be as close as
possible to plug flow. The column is normally full with adsorbent so that no
expansion can occur which would cause mixing, lengthening of the mass
transfer zone and hence reduced efficiency.

5.4.3 Fluidized bed
The literature contains many references to the use of fluidized bed
adsorption processes. Applications have included the removal of organic
compounds from air and vent streams (Avery and Tracey 1968, Rowson
1963) and the drying of air with silica gel (Ermenc 1961, Cox 1958).
Fluidized beds are attractive because, when fully fluidized, the pressure
drop is independent of flowrate and heat and mass transfer processes
external to the adsorbent particles are very good. The main problems lie, as
with many moving bed processes, with the mechanical strength of the
adsorbent particles.
The Purasiv HR fluidized bed/moving bed process shown schematically in
Figure 5.9 became technically feasible following the development of a hard
microspherical activated carbon known as bead activated carbon (BAC).
The principal application has been the removal of small amounts of solvent
vapour, typically 100-10 000 ppm, from air and vent streams with flowrates
in the range 2-50 m3/s (Anon 1977). Feed gas is passed upwards through
trays which are similar to those used in distillation and on which the BAC is
fluidized. The BAC passes down through the column via downcomers, again
in a manner similar to that in distillation. The BAC is passed ultimately into
a heated moving bed regeneration zone in which the adsorbed species are
desorbed for recovery. An air lift is used to transport the adsorbent to the
top tray in the adsorption section. The process has been adapted for use with
polymeric adsorbents, again for the removal of volatile organic compounds
from air streams.