By C.F. “Chubb” Michaud, CWS-VI
Ion exchange is a water purification process whereby various ions in a solution can be attracted to and adsorbed by chemically treated resins and effectively replaced (or exchanged) by ions that are less offending. Hardness (calcium and magnesium), for instance, can be exchanged for less objectionable sodium or potassium as a treatment for boiler feed and thus prevent scale formation. In designing or specifying these ion exchange systems, they are rated in order to predetermine their throughput capacity and set up their regeneration frequency. The capacity (rate) is based on a complex set of equilibrium relations, the water analysis and the desired quality to be achieved. This article discusses various factors affecting the total capacities and the operating capacities of ion exchange resins.
Ion exchange is an equilibrium reaction that is dependent on the ionic concentrations (of various ions) both inside and outside the resin bead. A water softener, for instance, responds both to how much hardness is in the water (and how much sodium is left on the bead) and to how much sodium is in the water (and how much hardness is on the bead). Simply stated, the more hardness you have on the resin, the more difficult it is to put even more hardness onto that resin. At the same time, the more sodium you have in the water, the more difficult it becomes to put more sodium into that water. Equilibrium, in turn, determines capacity for a given quality.
When resins are manufactured, their capacity is measured and reported as total capacity. Quantitatively, this is an absolute measure of the number of reactive ionic sites per unit volume contained within the resin. This can be expressed as milli-equivalents per milliliter (meq/ml), equivalents per liter (eq/L) or kilograins per cubic foot (Kgr/cu.ft.); where 1 meq/ml = 1 eq/L = 21.87 Kgr/ft3.
A typical strong acid cation exchanger will have a total capacity of 1.95 meq/ml or 42.7 Kgr/cu.ft. of resin. To get an idea of how this capacity equates to ions, one equivalent of sodium is equal to Avogadro’s Number (6.02 x 1023 ions). A liter of resin would therefore have 1.17 x 1024 (6.02 x 1023 x 1.95) reactive sites per liter of resin or 3.32 x 1025 sites per cubic foot. A single exchange bead would thus contain some 1.7 x 1017 reactive sites (that’s 170,000,000,000,000,000 or one hundred seventy quadrillion reactive sites). If the total capacity is 42.7 kilograins/cu.ft., why do we typically rate a softener at only 24.0 kilograins? The explanation lies in the difference between the total capacity and the operating capacity.
Capacity and efficiency
Observe new resin (100 percent sodium form) in a softener in service for the first time and note that even after it starts to load up (at point AB, resulting in breakthrough), it can still remove hardness (see Figure 1). If this softener runs until the effluent residual hardness is equal to the influent hardness (run length = AC), it has completely exhausted the resin, having used all of its capacity. Therefore, the shaded area, CT, represents the total amount of hardness removed and can be considered as total capacity. In the lab, this value is determined by fully converting the resins with acid or caustic and then running a known but excess amount of a salt, acid or base solution through it.1 The resin converts the salt to either an acid or base. This solution can be titrated against standards to determine just how much of the salt was converted. The moisture content of the resin is determined and the capacity per dry gram of resin is calculated. The results are reported as meq/gm which can then be converted to meq/ml which can then be expressed as Kgr/cu.ft.
In Figure 1, the point AB will also be high (30 to 32 kilograins) because the resin was 100 percent regenerated to start. Some manufacturers report this number as the capacity of the system because it represents the throughput one could obtain if salt was free and efficiency did not matter. Therefore, when you hear of a 96,000-grain system, they are referring to a three cu.ft. unit. Unfortunately, you will never realize this capacity again unless you restore the resin to its original 100 percent sodium form (which would require 50 to 60 pounds of salt).
In reality, it is impractical to operate a softener in a manner similar to Figure 1. Nearly a third of the run contains a considerable amount of hardness (run length = BC) and it would, as previously mentioned, require impractical quantities of salt (see Figure 2). Instead, pick a regeneration salt dose that provides good recovery economics (five to 10 lbs/ft3) and observe the curve shown in Figure 3. The line xy represents the maximum level of hardness residual it was chosen to tolerate. Let’s say this is four to six parts per million (ppm) of hardness. The run length, represented by the interval AB, is the time it takes to rinse down to quality (fast rinse) and the length BC represents the usable or operating capacity (C0). An increase in the salt dosage increases the length of C0 but the return on salt expenditure decreases rapidly (see Figure 2)—resulting in less efficiency. Given these facts, note that the operating capacity of any resin system can only be defined if the final end point quality is specified. Without a quality cut-off point, all softeners would have a capacity of 42.7 Kgr/cu.ft. The lower the leakage tolerances, the lower the capacities.
Since the influent water has a known grain per gallon loading we can calculate the operating capacity by measuring the run (BC) in gallons and multiplying by the loading. For example, if BC is 1,000 gallons to a specified quality break and the feed is 23.5 grains, the C0 becomes 23,500 grains, or 23.5 Kgr. With normal salt doses of six to eight lbs/ft3, C0 will be 50 to 60 percent of CT .
Different types of resin have different terminologies for stating their respective capacity. Strong acid cation (SAC) and strong base anion (SBA) resins have ‘salt splitting’ capacity. When a softener exchanges a sodium for a calcium (see Reaction 1) or a dealkalizer exchanges a chloride for a bicarbonate, we literally split the salt and reform a less objectionable one. In deionizers, the SAC resin must split a neutral salt such as sodium chloride and exchange the sodium with a hydrogen ion. The acid formed (HCl), does not require salt splitting for its removal since it is an acid and not a salt. The acid can be removed with either a SBA or weak base (WBA). WBA resins do not split salts but they can neutralize acids. Therefore, there are two parameters for measuring anion capacity: salt splitting and acid neutralizing. WBA have only neutralizing capacity. SBA have both and the ratios change with age (salt splitting declines). Furthermore, WBA resins only neutralize strong acids such as HCl and H2SO4. They do not neutralize weak acids such as silicic or carbonic. Anion capacities must, therefore, be defined and carefully stated when designing DI systems. There are equally complex determinations when WBA resins are run in the acid form.
There are also weak acid resins (WAC) that have the ability to neutralize alkaline salts such as sodium bicarbonate but do not react with neutral salts such as sodium chloride. WAC have utility in the DI arena, however, when a partial demineralization is acceptable. The resin will convert neutral salts if alkalinity is present. WAC can also be run in the sodium form which will exchange for ions of higher selectivity (hardness) and have great utility for softening in high TDS backgrounds.
It takes several runs to establish an equilibrium condition with a new ion exchange system. Because the first run is with highly regenerated resin, that run will be long and the capacity will be high. The subsequent regeneration will result in a short run because the equilibrium condition created by the first run will technically be an overrun and the normal regenerant level will not strip all of the exhausting ions. The third run will approach normal and the next three runs should agree on capacity within five percent or so. This average value becomes the operating capacity for that system, run at that flow rate, on that particular water and using that regenerant dosage. Operating capacity will, therefore, vary with the location and conditions of use, even for identical systems. Our ‘32,000-grain’ softening unit becomes a 21,000-grain unit at the seven lb. salt setting. That is its rating.
As noted in Figure 1, the resin bed still has capacity at the breakthrough cutoff point B. In fact, 20 to 25 percent of the bed is still in the sodium form. At that point, however, equilibrium does not allow for the complete removal of hardness. Leakage increases and we terminate the run. Regenerating with six to eight pounds of salt restores 50 to 60 percent of the total capacity, which means that our resin bed still contains about 20 percent hardness after regeneration. It is this hardness that, when surrounded by soft water in the next run, leaches out (see Reaction 1) and produces background leakage. Running a softener at exorbitantly high flow rates does not produce leakage. It will, however, shorten the run but the quality will be the same.
Total capacity measurement is a laboratory tool to determine if newly manufactured resins meet specifications. It usually falls within a narrow range for any given composition and doesn’t really vary from one manufacturer to another. ‘Good’ resin and ‘bad’ resin generally does not refer to capacity but to other physical factors related to bead integrity and cleanliness. It must be kept in mind that total capacity is not the same as operating capacity. One cannot always compare data sheets to determine the proper choice of resin but must rely on knowledge, experience and the help of one’s resin supplier.
Operating capacity is a performance criteria. It is what you sell and what you buy when ion exchange equipment is employed. There are many factors affecting the operating capacity of a resin system.2 The total capacity of the resin is but one of them. Operating capacity will change with the water analysis and with the age of the resin.3 It may change seasonally with water temperature. Residential installations for softeners and such are generally designed with a large cushion to override those variables and pretty much keep with the ‘one size fits all’ approach. However, industrial equipment is not designed or built that way and must be constantly monitored for performance. The major variable in industrial equipment will be the length of the service run.
Age and capacity
With time, resins oxidize, moisture content increases and the beads swell.4 Since the resin now occupies a larger space, the relative volumetric capacity has decreased. However, when the lab determines capacity, the actual function per dry gram solids may not have changed substantially. When resins swell up they do not necessarily loose functionality although a capacity loss is associated with aging of resin, which can stem from several factors:
- Aged resins can de-crosslink, swell, break up and get washed out of the column resulting in a capacity loss due to resin loss.
- Aged resins that have swollen will be less selective in the ion exchange process and will give higher background leakage, thus requiring more regenerant and more frequent regenerations to try to keep up.
- Aged resins can pick up foulants that will slow the kinetics of the resin and occupy sites that may otherwise be useable for the ion exchange process.
- Aged resins can lose reactivity due to partial solubility of the resin and functional groups.
- Aged resins, particularly strong base anions, may undergo an internal conversion from strong base to weak base and lose the ability to remove silica, thus losing capacity.
Fouling and kinetics
In the course of use, resin may pick up something that coats its surface and interferes with its ability to do its job. Oil, for example, will cause this fouling, which may show up as a loss of operating capacity and a general ‘slowness’ to react. Such resins are often referred to as being kinetically impaired. The capacity of the resin is essentially the same; however, the rate at which it can react has slowed due to ions now having to diffuse through a layer of oil. The resins still work, but they have to be run more slowly. Regeneration is less efficient for the same reason. Leakage goes up and capacities go down. The definition of (C0) a resin over a flow rate range, is its kinetic capacity. Keeping resins clean avoids a loss of kinetic capacity, which is real capacity.
Should something exchange onto the resin that doesn’t want to come off in normal regeneration, a serious fouling problem can occur, resulting in a near complete loss of capacity. Iron, aluminum and lead can cause this. The resins appear normal although certain types of foulants (like iron) may be visible. Moisture may test low because the foulant makes the bead heavier. Normal regeneration will not restore the bead. In addition to iron, hardness, organics, silica and heavy metals may come into play. The specific problem must be diagnosed before treatment can be prescribed. This is often difficult or nearly impossible to determine in the field and a complete lab analysis may be more costly than replacement. Often there is some permanent loss of operating capacity and a loss of kinetic capacity as well—even with successful diagnosis and treatment. Proper pretreatment to prevent fouling is a better strategy.
Even though ion exchange resins will have fairly consistent total capacity (as supplied), the operating capacity for the system they are in will vary with changes in feed water composition and flow rate at any given regenerant level. For all practical purposes, the kinetic capacity represents the true capacity of the resin you buy and use.
On the contrary, resins with very high ‘total’ capacities may not be the best choice. Higher total capacity usually indicates higher cross-linking and lower moisture. This means a tighter structure which may make high operating capacity difficult to realize without running very slowly (one to two gpm/ft3)1 and using high salt doses (15+ lbs/ft3). Ease of regeneration, which gives high recovery, is more important.
Salt splitting capacity is important for full deionization as well as for softening, dealkalization, nitrate removal and all other applications using brine as a regenerant. Total capacity becomes more important in neutralization reactions and DI applications. As always, keeping resins clean and free from foulants is the best way to prolong the useful life of any resin system.
- Kunin, Robert, Ion Exchange Resins, Robert E. Krieger Publishing Co., Huntington, N.Y., 1972.
- Michaud, C.F., Factors Affecting the Brine Efficiency of Softeners, Part I, WC&P, Aug, 1999
- Michaud, C.F., Factors Affecting the Brine Efficiency of Softeners, Part II, WC&P, Sept, 1999
- Michaud, C.F., Oxidation: Age and Softening, WC&P, Aug, 2000
About the author
C.F. “Chubb” Michaud is the CEO and technical director of Systematix Company, of Buena Park, CA, which he founded in 1982. An active member of the Water Quality Association, Michaud is a member of the Board and of the Board of Governors and chairs the Commercial/Industrial Section (since 2001). He is a Certified Water Specialist Level VI. He has served on the Board of Directors of the Pacific WQA since 2001 and chairs its technical committee. He was a founding member of (and continues to serve on) the technical review committee for WC&P and has authored or presented over 100 technical publications and papers. He can be reached at Systematix Inc., 6902 Aragon Circle, Buena Park CA 90620, telephone (714) 522-5453 or via email at [email protected]