By Mike Keller
Summary: Ion exchange resins, like anything associated with water treatment equipment, are bound to wear down over time. Once the problem has been identified, steps can be taken to return the system back to an efficient mode. However, it’s important to know what to look for and how to get accurate data.
If an ion exchange resin unit isn’t performing up to expectations, the system must be thoroughly checked. Once it’s determined the equipment is functioning properly, the focus will generally turn to the media contained within the unit. In the case of ion exchange resin, an analysis can be performed to assess the condition of the resin.
The economics of having a resin analysis performed should be established. For example, the resin from a one cubic water softener, if suspect, should simply be replaced, because the analysis cost will be far greater than the cost of replacing one cubic foot of resin. A resin analysis will usually be economically feasible in commercial and industrial systems when the units contain 10 cubic feet (ft3) or more of resin. The resin manufacturer may perform free analysis or low cost analysis for their customers and should be contacted. There are also laboratories that specialize in these types of testing procedures. Check with yours.
The first step in a resin analysis is obtaining the sample. The unit should be regenerated before the sample is taken. This will clean the resin bed. After the valve or “manway” is removed, the height of the resin bed should be examined. If the resin volume is low, the backwash flow rate may be excessive and should be checked. If the backwash flow rate is low, resin fines, dirt and debris will accumulate and eventually cause pressure drop and channeling in the unit.
A dirty unit should be flushed to eliminate suspended material. This can be accomplished by increasing the backwash flow until resin beads are seen coming out the drain line. At the first sign of resin coming out of the drain line, reduce the backwash flow so there isn’t excessive resin loss. The resin sample should be representative of the whole resin bed. If the sample is taken from the top of the resin bed, it will generally be in poor condition. The top of the bed will be made up of fine and broken resin beads, as well as accumulated dirt and debris. Foulants will also tend to be more prevalent in this area. The sample can be obtained using a simple rigid plastic tube that can be inserted into the resin bed as deeply as possible.
The top of the tube should be plugged up and the tube removed from the unit. The resin trapped in the tube should be placed into a container and completely mixed. A “grain thief” used by grain elevators for this purpose can also be used to sample resin. A visual inspection of the resin should be made at this time. If the sample is still dirty, further flushing is required. One quart (or liter) is sufficient to run most analytical tests. Check with the laboratory for volume requirements. The samples should be labeled with the type of resin, age and a description of the problem.
The laboratory will generally have a set of standard tests performed on cation or anion resin samples. The cation resin can have tests like total capacity, water retention, microscopic examination (bead count) and fouling to check the condition. The anion will have similar tests performed, plus an evaluation of salt-splitting capacity. When the resin is used in a deionization process, a conversion test can be run on the resin. An explanation of these tests follows below.
The water retention measures the amount of water associated within the resin bead, and is expressed as a percentage. The resin is first drained of any excess moisture before drying the sample. An increase in water retention indicates the resin is being oxidized. Substances like chlorine will attack and break down the skeletal structure and reduces crosslinking of resin (see Figure 1), which can cause a decrease in total capacity (see Figure 2), operating capacity and produce pressure drop and channeling. If the resin can be crushed into a powder when rubbed between the thumb and forefinger, oxidation is generally considered extreme. Such degradation is irreversible.
A decrease in water retention is rare and would most likely be caused by heavy metal fouling. A standard 8-percent-crosslinked cation softening resin will have a water retention of 45-to-48 percent. Once the water retention exceeds 55 percent, beads can be crushed to a powder resulting in excessive pressure drop and replacement should be considered. Different resins have different water retention levels. Contact the resin supplier for more information.
Testing for capacity
The total capacity is a measure of the ultimate capacity of an ion exchange resin. This test is usually expressed as meq/ml (milliequivalent per milliliter). Meq/ml can be converted to kilograins per cubic foot (kgr/ft3) by multiplying by 21.8. An 8-percent-crosslinked softening resin will have a total capacity of 2.0 meq/ml (43.6 kgr/ft3) when new. This is a theoretical capacity and will never be achieved in the field. Operating capacities will be lower depending on the regeneration dosage. For example, at 15 pounds per cubic foot (lbs/ ft3) of regenerant, the operating capacity will be approximately 30-to-32 kgr/ft3. The total capacity can be lost through aging, irreversible fouling and oxidation.
A salt-splitting capacity test can be run on the resin. This is recommended for strong base anion resins. A cation resin doesn’t need to have this procedure run, since the total capacity and salt-splitting capacity should be nearly identical. The salt-splitting capacity measures an anion’s ability to remove weak acids like silica and alkalinity. This test is especially important when the anion resin is used to deionize water. The salt-splitting capacity can be lost through aging, fouling, oxidation and elevated temperature.
Based on the analysis in Table 1, the cation has a water retention higher than standard 8-percent-crosslinked cation resin. Oxidants such as chlorine will attack the skeletal structure of the resin causing the water retention to increase. The total capacity has decreased 15 percent. The loss in capacity is probably due to oxidation and is unrecoverable. The cation unit should be backwashed for an extended period to reduce the broken bead concentration. If the unit isn’t producing the quality and quantity of water required, replacement should be considered.
Types of fouling
The microscopic examination or bead count provides the percentage of whole and broken resin beads. Ion exchange resin will generally have a 1-to-2 percent attrition rate over a year’s time. Depending on the stress put on the system, this attrition rate can be higher. High amounts of broken beads will lead to channeling and increased pressure drop, which will ultimately lead to higher ion leakage and lower run lengths.
Fouling tests should be run on both cation and anion resin samples. The cation will generally be checked for metal fouling while the anion will be checked for organic fouling. Iron fouling is common for cation softening resin, but aluminum and lead are also common. If a system is treating a waste stream, the chemical composition of the stream should be known. This information should be passed on to the laboratory analyzing the resin for possible foulants. The laboratory should also know what type of regenerant is being used.
The anion will generally be checked for organic fouling. Naturally occurring organics—also known as tannins—can foul the anion resin. Organic fouling tends to blind exchange sites, so a loss in total capacity and salt splitting capacity may be observed. Each laboratory will have a scale to identify resin that has low, moderate or heavy fouling. Fouled resin can usually be cleaned to some degree. The cleaning procedure will be dependent on the type of fouling. Check your ion exchange resin representative for proper cleaning procedures.
Ion exchange resins used to deionize water may need to have a conversion test run. The conversion will indicate the effectiveness of the regeneration. The test provides the percentage of cation resin converted to the hydrogen form and the percentage of anion resin to the hydroxide form. Conversions of 70-to-80 percent for the cation and 60-to-70 percent for the anion are considered good. It isn’t economically feasible to get the conversion much higher.
A screen analysis will indicate the size distribution of the resin beads. U.S. standard mesh screens are generally used in the analysis. The mesh sizes range from 16 (1.2 millimeters—or mm) to 50 mesh (0.3 mm) in this test. Normal cation resin will range predominantly in the 16 to 40 mesh screen with the peak at 30 mesh. Normal anion resin will usually have a finer bead size with a range of 20-to-50 mesh and a peak at 40 mesh. The screen analysis is especially important in mixed bed resin since it helps to determine how easily the cation and anion will separate when backwashed. Coarse resin will normally achieve lower pressure drop, while finer resin beads will promote better kinetics (faster rate of reaction).
Once these tests are run, a good conclusion can be drawn as to the condition of the resin and how best to attack a system that isn’t operating up to specification. The laboratory should be able to make recommendations to bring the system back into specification. These recommendations may be acid treatment, brine caustic treatment, better backwashing or resin replacement, just to name a few options.
About the author
Mike Keller is a marketing specialist for Sybron Chemicals Inc. of Birmingham, N.J. He can be reached at (800) 678-0020, (609) 894-8641 (fax) or email: http://email@example.com