This paper applies local equilibrium theory to analyze
pressure-swing adsorption with short
dimensionless contact times for systems with two adsorbable components
present in trace
amounts in an inert carrier gas. We are interested in finding
optimal operating conditions for
both separation and purification problems with nonlinear isotherms.
We consider Langmuirian
systems only; however, several concepts illustrated in this paper apply
irrespective of the type
of isotherm. By minimizing appropriate objective functions, at the
periodic state and for the
purification process, the purge step is found to be of short duration,
corresponding to low
volumetric purge-to-feed ratios and a high degree of incomplete
regeneration. In the separation
process, higher or lower degrees of regeneration may be appropriate
depending on whether the
goal is to enrich the purge in the impurities or to recover the light
component as product.
The periodic behavior of an adsorption bed that is
alternatively fed with a binary mixture in
inert gas and purged countercurrently, but incompletely, with pure
inert gas is analyzed using
local equilibrium theory. The bed is isothermal with the two
adsorbable components present in
trace amounts and adsorption equilibrium described by a binary Langmuir
isotherm. The purge
step ends as soon as one of the components has been completely removed
from the bed. For
this nonlinear system, the periodic state is found analytically,
without iteration, and feed and
purge times are obtained directly. Both separation and
purification problems are analyzed. A
system is completely defined by three parameters: the constant
separation factor for each
component and the ratio of partition ratios of the components as pure
feeds. Different
combinations of these parameters yield wave interactions among simple
waves and shocks, which
are resolved numerically, and corresponding bed profiles that are quite
dissimilar.
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