There are four major classes of resins used in industrial water treatment: strong acid cation, weak acid cation, strong base anion, and weak base anion. Each of these major resin classes has several physical or chemical variations within the class. The variations impart different operating properties to the resin. A good ion exchange system designer not only will design the system to meet all design specifications but also will utilize resins that will allow the system to operate at peak efficiency and maximum cost effectiveness.Strong acid cation resins probably are the most common resins in use today. They are used in softening and demineralization applications. Weak acid cation resins can be used in demineralization and dealkalization systems. Strong base anion resins are used in ion exchange demineralization processes. They also are used in dealkalization, desilicization and organic trap applications. Weak base anion resins actually are acid absorbers as much as they are ion exchange resins. They remove only the anions of the strong mineral acids (sulfate, chloride and nitrate).
It is important to take resin samples and to send them out periodically for analysis. This will enable the plant to follow the normal degradation of the resins and to catch fouling problems before they become serious. For laboratory purposes, a quart size sample is usually sufficient, two quarts for mix bed samples.The recommended frequency of testing is once per year for softener resin more than five years old. Demineralizer resins should be samples and tested every six months or annually, once it is two or three years old. Some plants retain samples of unused or original resin to use for comparison purposes later on.The best way to get a sample of resin is with a thin-walled plastic tube or PVC pipe, about 1 inch in diameter. A recommended sampling procedure is as follows:
- Before taking the sample, drain the bed (preferably a freshly regenerated bed) until the water level drops just beneath the resin level. Slowly force the tube through the resin bed, taking care not to damage the distributor or gravel subfill. When the bottom of the vessel is reached, stopper or cap the tube and withdraw it slowly.
- The sample should be wet or moist with the free liquid poured out.
- The sample container should be clearly marked with as many of the following important bits of information as possible.
- Name and address of the plant
- Name and telephone number of the person in charge
- Number of the unit samples and the date the sample was taken
- Condition of resin: Exhausted or Regenerated
- Type of service (softening, two-bed deionization, mixed-bed polishing)
- Resin type and the manufacturer’s designation, if know
- Date the resin was installed or rebedded
- Whether or not resin had been added as makeup for losses
- Nature of the plant problem