Properties of Ion Exchange Materials

This supplements the previous page with only a little duplication. Ion exchange operation is dependent upon certain properties of the system. Each property has an affect on the efficiency and productivity of an ion exchanger. Below are select properties which affect the system.

• The density of resin has an affect upon how the system performs. Properties of resin should be understood. For example, the density of the dry, water free resin is generally smaller for anion exchangers than cation exchangers. The density of water swollen resin depends on the type counter ion, swelling capacity and on the degree of crosslinking, besides the density of dry resin. Furthermore, it should be noted that bulk density is different than the density of the swollen resin. These densities are important because operation is dependent upon the resins.
• The mechanical resistance is a variable that is studied for ion exchangers. The mechanical resistance is found to vary with structure of the system. It should be noted that air dried resin is destroyed by certain friction. This needs to be thought of in design stages.
• The grain size is a major part of the fluid flow and effectiveness of seperation of systems. For example, condensation type resins are generally broken granules. On the contrary, polymerization-type resins are small beads that are uniformally packed. To measure the grain size a mesh is used to keep out larger particles. In addition, for certain processes grain size is extremely important to efficiency. One such process is seperations carried out by chromotography. The major point of study of grain size is that it determines the fluid resistance of an ion exchange column made from ion exchange resin. This can be the key to success of an industrial operation.
• The total capacity is a measurement tool used to rate an ion exchanger. The total capacity is the amount of exchangeable ions of unit weight of resin. The determination of such factor is done by acid-base titration. Another capacity is salt splitting. This is the amount of sodium ions absorbed by the cation exchanger in the hydrogen form from a sodium chloride solution or hydrogen released by unit weight or unit volume. For an anion exchanger the amount of base liberated from a salt by unit weight or unit volume of the hydroxyl-form anion-exchange resin. Dissociation constants of active groups of the resin is a major part of the salt splitting capacity. Further, noted is the rest capacity which consists of the difference in monofunctinal strongly acidic or basic resin of splitting capacity. Also, the apparent capacity can be defined as the affects of multivalent anions on anion exchanger. Further, the break-through capacity depends on the pH, grain size, column size and flowrate. These capacities are properties of a system. Knowing and understanding the capacities allows for proper design.
• The porosity of a system controls much of the capacity of the exchanger. The surface active groups and capillary groups take part in the characteristics of a ion exchanger. The pores of IERs are of variable size even for the same resin product. Due to porosity, the affects on capacity of greater sized ions is because of the sieve effect. The determination of porosity can be done by means of solution containing ions of known size and similarity by using capacity measurements. Also, the same measurement can be done by the use of vapor pressures. Although, these methods only measure mean particle size, it results in useful knowledge. In addition to the above, it should be noted that crosslinking affects mean pore size.
• The operating rate is essential to chemical engineers. Knowing the affects of controlling the flow is desireable also. For example, natural zeolite exchangeres operate slower and an ion exchanger of larger pores quicker. A cation exchanger is also knowed to set up equilibrium quicker. One may image that the process is controlled by the chemical reaction. But it is known that the diffusivity is a controlling factor. In addition, the rate depends on diffusivity constants of active groups of the resins. Other effects are on account of temperature and looseness of crosslinks.
   
• Recent Improvements on Uniformity of Beads and Selectivity

Ion Exchange Menu