Ion Exchange Resin Characteristics

Resin Specification Explanation
This section covers several important topics regarding the characteristics of ion exchange resin. The topics include a resin specification explanation, resin life expectancy, resin degradation process, resin structure and particle size distribution.

Process performance and operating life of resins are optimized through the specification of the resin characteristics which affect them. They cannot, however, be independently optimized. Resin characteristics having a positive effect on operating capacity can have a negative effect on the useful life of the resin. Additionally, a set of characteristics optimized for one particular syrup refining application may not perform effectively for the purification of another type of syrup solution. Optimization of resin characteristics for each type of application is achieved through the specification of resin manufacturing variables. These variables include total capacity, salt splitting capacity, moisture, macroporosity, microporosity, average particle size and particle size uniformity. These characteristics affect resin performance in the following ways:

Total Capacity
Total capacity is a general indicator of operating capacity, but the two are not directly proportional. For a new resin in a syrup demineralization application, the operating capacity is typically 50-60% of the total theoretical capacity. The resins are limited by equilibrium and kinetic constraints from achieving their total theoretical capacity in typical syrup refining operations.

Salt Splitting
Also known as strong base capacity, salt splitting affects the physical stability of weak base anion resins by limiting the amount of reversible swelling which occurs upon exhaustion and also affects the resin’s moisture level.

Moisture Level
Moisture levels inside the resin beads affect the rate of diffusion of soluble ions and molecules into the resin bead where exchange or adsorption can occur. Higher moisture content in a resin improves the rate of diffusion.

Macroporosity
Macroporosity also facilitates diffusion into and out of the interior of the resin bead. Large organic molecules have greater difficulty diffusing out of gel resins, thus resulting in a higher degree of fouling. Additionally, macroporosity imparts flexibility to the resin bead which gives it greater physical durability when subjected to osmotic shock or mechanical stresses. Gel resins, not having macroporosity, are more susceptible to osmotic shock and mechanical attrition.

Microporosity
The microporosity of the structure is determined by the degree of crosslinking and affects both ionic and molecular selectivity due to steric hindrance and water adsorption.

Particle Size
Particle size affects the kinetics of ion exchange. Smaller resin beads have shorter film diffusion and particle diffusion path lengths for ions and molecules to travel.

Uniformity of Particle Size
Uniformity affects both pressure drop through the bed and the sharpness of the adsorption and desorption wavefronts. As the particle size distribution widens, the adsorption and desorption bandwidths also increase.

Life Expectancy
The useful operating life of resin is limited by the physical and chemical degradation and fouling which occurs as a result of the particular operating conditions it is subjected to. These conditions include service and regeneration temperatures, syrup dry solids concentration, organic load, cleanup regeneration frequency, type of regenerant chemical and selection of the criteria for the point of replacement. 

Particle Size Distribution of Ion Exchange Resins
Particle-size distribution of ion exchange resins is determined by putting a representative sample through a series of standard sieves. The results are usually expressed as percent of the entire sample which is retained by or allowed to pass through specified openings in the sieves. The most useful data are obtained from resins in their fully swollen states. Wet-screen analyses are generally preferred to and are more consistent than dry-screen analyses. Since the swelling of ion exchange resins can be considerable, any report of particle-size distribution from screen analysis should be accompanied by a statement indicating whether the data were obtained wet or dry.

Particle size and size distribution are sometimes expressed in terms of “effective size” and “uniformity coefficient,” both of which may be obtained from screen analyses. Effective size is defined as that opening in millimeters that retains 90% (or passes 10%) of the total resin sample. Uniformity coefficient is the numerical value obtained by dividing the sieve opening (in millimeters) which retains 40% of the sample by that which retains 90%.