Capacity. Ion exchange capacity may be expressed in a number of ways. Total capacity, i.e., the total number of sites available for exchange, is normally determined after converting the resin by chemical regeneration techniques to a given ionic form. The ion is then chemically removed from a measured quantity of the resin and quantitatively determined in solution by conventional analytical methods. Total capacity is expressed on a dry weight, wet weight or wet volume Figure 1. Cation Exchange Resin Schematic Showing Negatively Charged Matrix and Exchangeable Positive Ions basis. The water uptake of a resin and therefore its wet weight and wet volume capacities are dependent an the nature of the polymer backbone as well as an the environment in which the sample is placed.
Operating capacity is a measure of the useful performance obtained with the ion exchange material when it is operating in a column under a prescribed set of conditions. It is dependent on a number of factors including the inherent (total) capacity of the resin, the level of regeneration, the composition of solution treated, the flow rates through the column, temperature, particle size and distribution. An example is shown in Figure 3 for the case of water softening with a standard sulfonic resin at several regenerant levels. Swelling.
Water swelling of an ion exchanger is primarily a hydration of the fixed ionic groups and increases with an increase in capacity to the limits imposed by the polymer network. Resin volumes change with conversion to ionic forms of differing degrees of hydration; thus, for a cation exchanger, there is a volume change with the monovalent ion species, Li + > Na + > K + > Cs + > Ag + . With polyvalent ions, hydration is reduced by the cross-linking action; therefore, Na + > Ca2 + > Al 3+ . In more concentrated solutions, less water is taken up owing to greater osmotic pressure. Selectivity. Ion exchange reactions are reversible. By contacting a resin with an excess of electrolyte (B + in the following reaction), the resin can be converted entirely to the desired salt form: RA + + B + ! RB + +A + However, with a limited quantity of B + in batch contact, a reproducible equilibrium is established which is dependent an the proportions of A + and B + and on the selectivity of the Figure 2. Total Capacity vs. Cross-Linkage (Percent DVB) Polystyrene Sulfonic Acid Resin, H + Form Figure 3. Operating Capacity vs. Regenerant Level for Sodium-Cycle Operation, Sulfonic Acid Resin resin. The selectivity coefficient, K B A , for this reaction is given by: where m and refer to ionic concentrations in solution and resin phase, respectively. Resin selectivity coefficients have been determined for a range of ionic species and related to H + for cations and OH – for anions, which are assigned selectivity values of 1.00. Kinetics. The speed with which ion exchange takes place. The ion exchange process involves diffusion through the film of solution that is in close contact with the resins and diffusion within the resin particle. Film diffusion is rate-controlling at low concentrations and particle diffusion is rate-controlling at high concentrations. Whether film diffusion or particle diffusion is the rate- controlling mechanism, the particle size of the resin also is a determining factor. Uniform particle sized resins exhibit enhanced kinetic performance compared to conventional polydispersed resins due to the absence of kinetically slow larger beads.
Stability. Strong oxidizing agents, such as nitric or chromic acid, rapidly degrade ion exchange resins. Slower degradation with oxygen and chlorine may be induced catalytically. For this reason, certain metal ions, for example, iron, manganese and copper, should be minimized in an oxidizing solution. With cation exchangers, attack is principally an the polymer backbone. Highly cross- linked cation resins have an extended useful life because of the great number of sites that must be attacked before swelling reduces the useful volume based capacity and produces unacceptable physical properties, for example, crush strength reduction and pressure drop increase. With anion exchangers, attack first occurs on the more susceptible functional groups, leading to loss of total capacity and/or conversion of strong base to weak base capacity. The limits of thermal stability are imposed by the strength of the carbon-nitrogen bond in the case of anion resins. This strength is sensitive to pH and low pH enhances stability. A temperature limitation of 60°C (140°F) is recommended for hydroxide cycle operations. Cation resin stability also is dependent on pH; the stability to hydrolysis of the carbon-sulfur bond diminishes with a lowering of pH. They are much more stable than anions however and can be operated up to 150°C (300°F). Resin Structure and Manufacture The manufacture of ion exchange resins involves the preparation of a cross-linked bead copolymer followed by sulfonation in the case of strong acid cation resins, or chloromethylation and the amination of the copolymer for anion resins. Cation Exchange Resins.
Weak acid cation exchange resins are based primarily an acrylic or methacrylic acid that has been cross- linked with a di-functional monomer (usually divinylbenzene [DVB]). The manufacturing process may start with the ester of the acid in suspension polymerization followed by hydrolysis of the resulting product to produce the functional acid group. Weak acid resins have a high affinity for the hydrogen ion and are therefore easily regenerated with strong acids. The acid-regenerated resin exhibits a high capacity for the alkaline earth metals associated with alkalinity and a more limited capacity for the alkali metals with alkalinity. No significant salt splitting occurs with neutral salts. However, when the resin is not protonated (e.g., if it has been neutralized with sodium hydroxide), softening can be performed, even in the presence of a high salt background. Strong acid resins are sulfonated copolymers of styrene and DVB. These materials are characterized by their ability to exchange cations or split neutral salts and are useful across the entire pH range. Anion Exchange Resins. Weak base resins do not contain exchangeable ionic sites and function as acid adsorbers. These resins are capable of sorbing strong acids with a high capacity and are readily regenerated with caustic. They are therefore particularly effective when used in combination with a strong base anion by providing an overall high operating capacity and regeneration efficiency.
Ionic Systems PC-200 Resin is tested and certified by WQA under Pure Resin Company, LTD. against NSF/ANSI 44 & 61.
