Page 143 - Adsorption Technology & Design, Elsevier (1998)

132 Processes and cycles

Common examples of thermal swing adsorption include solvent recovery
with activated carbons, and drying of gases or liquids with type A zeolites.
Drying of organic vapours using type A zeolites can overcome the difficulty
in distillation of reaching high purities in chemical systems which contain
vapour-liquid azeotropes. All these processes can be carried out without the
need to remove the adsorbent from its vessel. In contrast, granular activated
carbon (GAC) which is used to adsorb high molecular weight, low volatility
or strongly adsorbed species, must usually be removed from the adsorption
vessel for regeneration in special furnaces. In this case the GAC needs to be
heated to over 800~ Multiple hearth, rotary kiln, fluised bed and electric
belt furnaces are available (Liu and Wagner 1985). The multiple hearth
furnace is shown in Figure 2.8. Further developments and examples of TSA
are provided in Sections 7.4 and 7.5.
A practical problem in thermal swing processes is the reduction in the
capacity or life of the adsorbent when it is subjected to repeated thermal
cycling. Another problem is the formation of coke in applications in which
reactive hydrocarbons are exposed to elevated temperatures during the
desorption/regeneration step.

5.9 DISPLACEMENT FLUID

Adsorbates can be removed from the adsorbent surface by replacing them
with a more preferentially adsorbed species. This displacement fluid, which
can be a gas, vapour or liquid, should adsorb about as strongly as the species
which are to be desorbed. If the displacement fluid is adsorbed too strongly
then there may be subsequent difficulties in removing it from the adsorbent.
The mechanism for desorption of the original adsorbate involves two
aspects. First, the partial pressure of the original adsorbate in the gas phase
surrounding the adsorbent is reduced (or the concentration is reduced in the
case of a liquid displacement fluid). Secondly, there is competitive
adsorption of the displacement fluid. The displacement fluid is present on
the adsorbent at the next adsorption step and thus it will contaminate the
product. A practical process therefore requires that the displacement fluid
must be recoverable from the products of the adsorptive separation by a
process such as simple distillation. This is the basis of the Sorbex type
process shown in Figure 5.13.
One advantage of the displacement fluid method of regeneration is that
the net heat generated or consumed in the adsorbent will be close to zero
because the heat of adsorption of the displacement fluid is likely to be close
to that of the original adsorbate. Thus the temperature of the adsorbent
should remain more or less constant throughout the cycle. In turn this leads