Adsorption - Principles of Chemical Separations with Environmental Applications - page 184

7.3.3 Criteria for use

Distillation is usually the first choice for bulk separation of components in a liquid phase.
Another option often used is air stripping. In both cases, the ease of separation is based
on the relative volatility of the various components in the feed mixture. In comparison,
adsorption is a good choice when [4]:
1 The relative volatility between the key components to be separated is in the range of
1.2 to 1.5 or less. In this case, distillation is not an obvious choice and the separation
factor for adsorption could be very large based on criteria other than relative volatility.
2 The bulk of the feed is a relatively low value, more volatile component, and the product
of interest is in relatively low concentration. For this case, large reflux ratios (and large
energy consumption) would be required if distillation is considered. This situation is
often found in environmental applications where one has dilute concentrations of a
pollutant in an air or water stream.
3 The two groups of components to be separated have overlapping boiling ranges. Again,
compared to distillation, several distillation columns are required even if the relative
volatilities are large. For adsorption, this separation can be done effectively if the two
groups contain chemically or geometrically dissimilar molecules so as to obtain a high
separation factor.
4 A low temperature and a high pressure are required for liquefaction (for gases).
5 Components in the feed stream can be damaged or altered by heating. These alterations
can include chemical degradation, increases in chemical reactivity and polymerization
reactions. This chemical change can cause fouling in the equipment which reduces the
effective lifetime as well as the production of undesirable components.
6 Chemical corrosion, precipitation, and/or undesirable chemical reactions (explosive
conditions are an example) are problems using operating conditions for distillation.
7 Factors favorable for separation by adsorption exist. Adsorption selectivity between the
key components should be greater than 2, the sorbent can be easily regenerated, does not
easily foul for the given feed mixtures, and does not act as a catalyst or other reaction
medium to promote the production of undesirable side products. The feed throughput
and product purity are also important considerations. The costs for adsorption separation
are generally lower than distillation for small to medium throughputs (less than a few
tons per day), and when high-purity products are not required (function of selectivity
of the sorbent with respect to the feed components). The major cost for pressure-swing
adsorption (discussed in the next section) is the compressor costs. If a feed gas mixture
is available at an elevated pressure, the separation costs are substantially reduced.

7.4 Sorbent selection

Since almost all adsorptive separation processes are based on equilibrium partitioning,
the most important factor to consider initially is the adsorption isotherm (equilibrium