Process Technology: An Introduction - Haan A.B. 2015

9 Adsorption and ion exchange
9.4 Principles of ion exchange

9.4.1 Ion-exchange resins

Naturally occurring inorganic aluminosilicates (zeolites) were the first ion exchangers used in water softening. Today, synthetic organic polymer resins based on styrene or acrylic acid type monomers are the most widely used ion exchangers. These resin particles consist of a three-dimensional polymeric network with attached ionic functional groups to the polymer backbone. Ion exchange resins are categorized by the nature of functional groups attached to a polymeric matrix, by the chemistry of the particular polymer in the matrix. Strong acid and strong base resins are based on the copolymerization of styrene and a crosslinking agent, divinylbenzene, to produce the three-dimensional cross-linked structure shown in Fig. 9.17. In strong acid cation exchange resins sulfonic acid groups in the hydrogen form exchange hydrogen ions for the other cations present in the liquid phase:

Image

(9.10)

Image

Fig. 9.16: Displacement purge cycle.

It is not always necessary for the resin to be in the hydrogen form for adsorption of cations. Softening of water is accomplished by displacing sodium ions from the resin by calcium ions, for which the resin has a greater affinity:

Image

(9.11)

In strong base anion-exchange resins quaternary ammonium groups are used as the functional exchange sites. They are used most often in the hydroxide form to reduce acidity:

Image

(9.12)

During the exchange the resin releases hydroxide ions as anions are adsorbed from the liquid phase. The effect is elimination of acidity in the liquid and conversion of the resin to a salt form. Ion exchange reactions are reversible. After complete loading a regeneration is used to restore the resin to its original ionic form. For strong cation and anion resins this is typically done with dilute (up to 4 %) solutions of hydrochloric acid, sulfuric acid, sodium hydroxide, or sodium chloride.

Weak acid cation exchanger resins have carboxylic acid groups attached to the polymeric matrix derived from the copolymerization of acrylic acid and methacrylic acid. Weak base anion exchanger resins may have primary, secondary, or tertiary amines as functional groups. The tertiary amine is most common. Weak base resins are frequently preferred over strong base resins for removal of strong acids in order to take advantage of the greater ease of regeneration.

Image

Fig. 9.17: Styrene and divinyl benzene based ion exchange resins: (a) sulfonated cation exchanger; (b) aminated anion exchanger.

9.4.2 Equilibria and selectivity

Ion exchange differs from adsorption in that one adsorbate (a counterion) is exchanged for a solute ion, and the exchange is governed by a reversible stoichiometric chemical reaction equation. The exchange equilibria depends largely on the type of functional group and the degree of crosslinking in the resin. The quantity of ions, acids, or bases that can be adsorbed or exchanged by the resin is called the operating capacity. Operating capacities vary from one installation to another, as a result of differences in composition of the stream to be treated.

In ion exchange significant exchange does not occur unless the functional group of the resin has a greater selectivity for the ions in solution than for ions occupying the functional groups, or unless there is excess in concentration as in regeneration. This is pictured in the following reaction where the cation exchange resin removes cation B+ from solution in exchange for A+ on the resin:

Image

(9.13)

For this case we can define a conventional chemical equilibrium constant, determining the selectivity for B over A:

Image

(9.14)

where m and q are the concentrations of the ions in the solution and the resin phase, respectively. A typical plot of Image for univalent exchange is given in Fig. 9.18. B is preferred by the exchanger if Image, while for Image B is less preferred and the ion exchanger preferentially absorbs species A. Selectivity for ions of the same charge usually increases with atomic weight (Li < Na < K < Rb < Cs), and selectivities for divalent ions are greater than for monovalent ions. For practical applications it is rarely necessary to know the selectivity precisely. However, knowledge of relative differences is important when deciding if the reaction is favorable or not. Selectivity differences are marginally influenced by the degree of cross-linking of a resin. The main factor is the structure of the functional groups.

Image

Fig. 9.18: Equilibrium plot for univalent-univalent ion exchange.