Separations as unit operations - Principles of Chemical Separations with Environmental Applications - page 26

K h = equilibrium constant for reaction

k h = forward reaction rate constant for reaction (2.11) .

+ ↔ ,

A B AB (2.12)

where A = solute

B = complexing agent in the low-affinity state

K = equilibrium constant for low-affinity reaction

.
A change in solute concentration (partial pressure, for example), a change in temperature
that changes the equilibrium constant of the reversible reaction, or a change in the oxidation
state of the MSA, usually accomplishes the reversibility. For example, in pressure-swing
adsorption, a change in system pressure alters the binding affinity of the solute to the
solid sorbent (Le Chatelier's Principle). In the high-affinity state, the solute adsorbs onto
the sorbent and is thus removed from the process fluid. Upon lowering the adsorbent
bed pressure, the solute desorbs from the sorbent. This allows the solid sorbent to be
regenerated for continued use without interrupting the overall system.
Reversible chemical complexation processes can be either equilibrium or mass transfer
(rate) limited. For those in which equilibrium is the controlling, or design, mechanism, it
is important that

k = forward reaction rate constant for reaction (2.12)

K h / K 1 . (2.13)

For those in which rate of formation of the complex is the limiting factor,

k h / k 1 . (2.14)

The preferred usage is when the complexing agent interacts with the solute of interest
and has little or no interaction with the other components of the feed stream. Separation
processes based on reversible chemical complexation provide an enhancement in the
solubility of the selected solute through the complexation reaction. This approach can
provide high enhancement of capacities and selectivities for dilute solutes, especially
when the solute feed concentration is below 10%.
There are several characteristics of a good complexing agent. First, for the reversibility
of a complexation reaction to be easily accomplished, the bond energy for the association
should be in the range of 7-70 kJ

mol. Second, there must be no side reactions. The
complexation reaction must only take place with the solute of interest. Third, there can
also be no irreversible or degradation reactions. Any reaction which decreases the amount
of complexing agent available for the separation reduces the capacity and selectivity of the
separation process. Thermal instability, oxidations, polymerizations, and reactions with the
separation process materials of construction are all examples of potential problems. Fourth,
a good complexing agent has no co-extraction of solvent from the feed phase. One example
of an undesirable co-extraction is the case of liquid-phase metal extractions where water
is extracted with the metal ion. This co-extracted water dilutes the metal concentration in
the receiving phase. Fifth, it is important for a complexing agent to provide rapid kinetics.

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