Page 232 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 211

feasible should be chosen for the adsorption stage. For ethanol dehydration
employing a 3A zeolite sieve, W.R. Grace and Company recommend a
period not less than 24 hours so that the energy required for heating and
cooling the beds of adsorbent and associated vessel and piping is minimized.
Hot carbon dioxide or natural gas, depending on the particular process
arrangement, may be used for regenerating the bed, the inert gas being
passed through a furnace prior to the adsorbent bed. CO2 from a
fermentation unit, or natural gas utilized for firing furnaces, could be
employed for regeneration and subsequently used for cooling (after removal
of moisture in a typical gas-liquid separator). In this manner a closed loop,
containing appropriate heat exchangers, could be usefully employed were
CO2 the inert gas. A semi-closed loop could be used if natural gas were
employed, the gas bypassing the furnace during cooling.

7.6 DISPLACEMENT PURGE CYCLES

When regeneration of the adsorbent is not feasible by either a PSA or a TSA
process and the relative volatilities of the components militate against
distillation, the method of displacement purge regeneration is employed.
Neither the pressure nor the temperature of the bed is varied from that
during adsorption, regeneration being dependent solely upon the ability of
an adsorbable purge to cleanse the bed in readiness for the next adsorption
stage. The principle upon which a displacement purge cycle operates
(illustrated by means of Figure 7.11) is the reduction of partial pressure of
the adsorbate by the displacing purge gas and competitive adsorption of the
adsorbate and purge. In Figure 7.11, A is the most strongly adsorbed
component of the binary component feed mixture of (A + B) while D is the
displacement purge gas. The feed mixture of (A + B) is passed through the
bed acting as the adsorber, already loaded with D from the previous cycle
(when the column was the regenerator), and a mixture of (B + D) emerges
from the top of the column. (B + D) are easily separated by distillation so
that the raffinate (B) is collected in a fairly pure state. The displacement
purge gas D then enters the second column acting as regenerator and from
which emerges a mixture of (A + D) separated without difficulty in a
separate distillation column. In effect the original mixture of (A + B), which
would have been difficult to separate by PSA or TSA, is separated by the
intervention of another strongly adsorbed component D. The ease of
separation of A from D and B from D in additional distillation stages is,
however, crucial in determining the economics of displacement purge cycle
operation.
Commercial processes for the separation of linear paraffins from mixtures