Page 118 - Adsorption Technology & Design, Elsevier (1998)

114 Processes and cycles

5.4.4 Rotary beds
Two processes have been devised to combine the advantage of the fixed
bed in which attrition losses are small and the moving bed in which the ad-
sorbent is more effectively utilized. Both processes have the adsorbent in
a fixed position relative to the wall of the containment vessel. In order
for the process to operate on a continuous and steady state basis therefore,
it is necessary both to move the position of the fixed bed relative to the
feed and product lines and to incorporate a desorption or regeneration
section.
Two types of rotary bed adsorber exist. Both are used for removing
and/or recovering solvents from air streams. Figure 5.10 shows the rotary
bed adsorber which comprises a rotating drum containing the adsorbent in
several sections. It is used generally for solvent recovery from air streams.
The air enters the drum circumference to pass inwards through the
adsorbent. Cleaned air then leaves via a duct which is connected along the
drum's rotational axis. One or more of the adsorbent sections is regener-
ated by passing steam in the reverse direction from the central axis to
the circumference whence it leaves and passes to condensers. After
the regeneration step the adsorbent is not cooled because the proportion
of the adsorbent annulus which is cooling at any one time is relatively
small and the effect of a warm sector on the overall efficiency is relatively
small.
The adsorbent wheel shown schematically in Figure 5.11 is used particu-
larly for the removal of volatile organic compounds (VOCs) from vent
streams. Whether the removed VOCs can be recovered depends upon the
magnitude of the increase in concentration from the adsorption step to the
desorption step. Solvent-laden air passes via a duct through one side of
the wheel which rotates slowly. On the desorption side a lower flowrate of
heated air is used to desorb the VOCs. The two gas ducts do not have to be of
the same size but it is obvious that the time for desorption must be less than
or equal to the time for adsorption.

5.5 FIXED BEDS USED TO SIMULATE MOVING BEDS
Two basic approaches can be adopted for using fixed beds to simulate the
operation of moving beds. In the first, multiple fixed beds are used in
cascade, as shown in Figure 5.1.2 (and described later in Section 7.7.1) to
gain most of the benefit of a continuous steady state countercurrent process.
The concept is similar to that used in the pulsed bed. At each switch in the
cascade a fully regenerated bed is added to the outlet end of a sequence of
beds in series when breakthrough is about to occur. At the same time the