Environmental Engineering Reference
In-Depth Information
Capacity is the quantity of the counterion that the resin can exchange. It is a critical
factor in evaluating a resin for a given application. The total capacity is determined by the
number and charge on the fixed ion-exchange groups in the resin. The dry-weight capacity
is then determined as the milliequivalents per gram of dry resin (meq
/
g). This quantity is
a measure of the loading capacity of the resin and is a constant.
The wet-exchange capacity accounts for the fact that the resin will swell or shrink
during operation. This quantity will vary with moisture content. It is usually reported as
equivalents per liter of resin (eq
ft
3
)
/
L). Units of kilograms of CaCO
3
per cubic foot (kg
/
are also used. It is this term that is typically reported.
Selectivity is primarily dictated by ionic charge and size, with charge having the most
significant effect. Kunin [5] proposed the following empirical points to approximate
selectivities.
1 At low aqueous concentrations and ambient temperatures, the extent of exchange
increases with increasing valence of the exchanging ion:
Th
4
+
>
Al
3
+
>
Ca
2
+
>
Na
+
PO
3
4
SO
2
4
Cl
−
.
>
>
2 At low aqueous concentrations, ambient temperatures and constant valence, the extent
of exchange increases with increasing atomic number (decreasing hydrated radius) of
the exchanging ion:
Cs
+
>
Rb
+
>
K
+
>
Na
+
>
Li
+
Ba
2
+
>
Sr
2
+
>
Ca
2
+
>
Mg
2
+
>
Be
2
+
.
3 At high ionic concentrations, the difference in exchange “potentials” of ions of different
valence (Na
+
vs Ca
2
+
or NO
3
vs SO
2
4
) diminish and, in some cases, the ion of lower
valence has the higher exchange potential.
It is important to note that these are “rules-of-thumb” and exceptions do occur.
Another aspect of selectivity is ion exclusion. Large organic ions or inorganic complexes
can be excluded from the resin pore structure. Obviously, the smaller the pore size, the
larger the potential for this effect.
Particle size affects the pressure drop through the column. Smaller particle size leads
to higher pressure drops for a given flowrate. Often, hydraulic limitations are the most
important consideration in design. Particle size also affects the relative magnitude of
transport Steps 2 and 3 listed above. This is analogous to the discussion on this point in
Chapter 7: Adsorption.
Stability is directly related to the lifetime of the resin. This, in turn, directly affects
the cost of the process. Physical stresses can occur through swelling and shrinking cycles
due to osmotic pressure changes. Mechanical forces, such as static pressure load, and
abrasion can cause breakage. Operation outside the normal temperature range will also
add to particle degradation.
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