By C.F. ’Chubb’ Michaud, CWS-VI

Definitions
In general terms, the capacity of an ion exchange resin can be expressed as the quantity of ions that can be taken up by a specific volume of the resin. This would be expressed in quantity per unit volume such as kilograins per cubic foot (Kgr/ft3), milli-equivalents per milliliter (meq/ml), which also equals equivalents per liter (eq/l). For the record, for a softener, 1 eq/l = 21.87 Kgr/ft3. 1.0 grains per gallon = 17.1 ppm of substance expressed as calcium carbonate. Equivalents refers to the equivalent weight (EW) of the substance expressed in grams (or meq in milligrams [mg], which is the molecular weight (MW) divided by the valence. Calcium (Ca), for instance has a molecular weight of 40 and is divalent (2) so the EW is 20 (40/2 = 20). An ion exchanger with a capacity of 1.95 eq/l would therefore have a capacity to remove 1.95 x 20 = 39 grams of calcium per liter of resin or 1.1 kilograms (k) or 2.43 lbs of calcium/ft3.

Technically, resin capacity is a measurement of total capacity, as determined by a test performed in the lab by a titration methodology. A measured quantity of cation resin, for example, is fully converted to the hydrogen (H) form with an excess of strong acid and then well rinsed. A measured quantity of caustic (NaOH) is then passed through the resin so as to totally exhaust the resin. The effluent is captured. The NaOH that passed through the column represents the sodium (Na) ions that were not captured by the resin. This solution is then titrated with acid to neutralize it and the amount of acid required is expressed in equivalents. The difference between the total equivalents of NaOH passed through the column and the NaOH exiting the column represents the total equivalents of Na captured by the resin. The capacity of the resin is then determined. Similarly, anion resin would be fully regenerated with NaOH and exhausted with acid to make the capacity measurement.

Total capacity can be expressed as meq/ml (volumetric) or meq/gm (based on weight). The lab determinations are actually in meq/ dry gm but once the moisture content of the resin is measured, it is converted to meq/ml. If you were curious, by now you have probably converted the typical cation resin capacity of 1.95 meq/ml to the more familiar Kgr/ft3 and arrived at 42.65 Kgr. How come no one rates their softener at 42,650 grains per cubic foot? You are probably more familiar with the rating values of 24 Kgr/ft3 or 30 Kgr, or 32 Kgr or 36 Kgr! Believe it or not, these numbers are all commonly used to describe a 1 cubic foot softener (28.32 liters for those scientifically advanced enough to be using the metric system—which is about 97 percent of the planet). Figure 1 shows how this is possible.

Figure 1. Run Length

Graphically, the total capacity value for a resin is represented by the gold-shaded area above the curve in Figure 1, between the vertical lines from A to C. We designate the silver-shaded area below the curve as leakage.” Note that leakage is very low in the beginning of the run and remains pretty much constant until the exhaustion zone of the resin approaches the bottom of the column. Leakage then increases to some predetermined shut off point (in the case of residential softeners this may be 1 gpg but for industrial systems, it may be 0.1 gpg [or about 2 ppm or even 0.01 gpg] which is about 0.2 ppm). When leakage crosses the limit, that is the end of the run (signified by line B). The run length is represented by the horizontal line x-y and would be expressed in gallons or liters. The total operating capacity would be the rectangle ABMN. This represents the usable capacity where the effluent is within the leakage specifications. The capacity curve for any resin will show the expected capacity versus the regeneration dosage as seen in Figure 2.

Figure 2. Capacity versus regenerant dose

Total capacity versus operating capacity
Generally speaking, when we refer to the capacity of resin, we are referring to operating capacity. Operating capacity must specify the exact feed water challenge and flow rates as well as the end point and other performance specifications. Technically, the stated capacity will only be correct when run under those exact conditions. As the ad says,your results may vary.

There are many factors which influence the operating capacity of any given resin – cation or anion, and subsequent performance. Achieving those very low leakages (red line, Figure 2) referenced above requires very high regenerant doses. Regenerant levels are never increased to increase capacity. They are increased to decrease leakage. Capacity is what capacity is based on that particular chemical dosing level. If you need higher capacity, increase the amount of resin. The closer you are to the theoretical efficiency line (green line, Figure 2), the more economical your system will be.

When softening resin is brand new, it is essentially 100 percent regenerated into the Na+ form. You will get a tremendous capacity on this run and essentially no leakage until the very end of the run. It is not unusual to register 36 to 38 Kgr/ft3 on the virgin run. There are some that will actually rate their softener at this value since it would be achievable every time you regenerate to the 100-percent level. This will take a full 40 lb. bag of salt/cu. ft. (640 gms/l).

Others are more realistic and will rate their softeners at 35 Kgr (with 20 lbs. of salt = 320 gm/l) or 32 Kgr (with 16 lbs. of salt = 250 gm/l). This is all part of the marketing hype you see on line. Water softeners are manufactured and sold by the cubic foot or liter volume capacity but they are marketed and purchased by the grain capacity. There is a lot of misrepresentation there to show a higher rating than the competition. (One big box store whose web site I visited rated their softener at 20 lbs. salt/cu. ft. dose for capacity but rated their brine efficiency at the 2.5 lb level.) Likewise, they claimed maximum flow rates of 14 gpm at 15-psi pressure drop but run certification testing at a more reasonable 4 gpm. Entire books have been written on how to massage results with statistic and are apparently well read. Read the fine print and ask questions. The bottom line is that most softeners are factory set to regenerate at the 6 to 8 lb,/cu, ft, level (100 – 125 gms/l) and will produce 22-24 Kgr/ft3, which is about 50-55 percent efficient. If applications require very low leakage, crank up the salt. See the reference section for more articles written on the subject. 1, 2

Each type of resin may express capacity in a different light in order to reflect what the user may require. Cation resins used for softening are called strong acid cation (SAC) resins. They are widely used in both sodium or potassium forms for softening and the hydrogen (H) form for deminerization. Capacity stated in the literature is for the Na form of the resin (even though the resin may be run in the H form). Some of the newer literature will give the capacity in H form and it is always lower than Na form, because the resin swells about 5-6 percent in transition from the Na to the H form. There is more water and less resin in that cubic foot and therefore, fewer reactive sites. If a resin manufacturer (or the literature) indicates that 35 cu. ft. of resin is needed for the process, capacity of 35 cu. ft. of resin in the Na form is actually needed and then converted via double regeneration to the H form for service (you will end up with 37 cu. ft. of H form resin. OR…purchase 37 cu. ft. of H form resin in the first place.

Figure 3. Reactions

Different resins have different definitions for capacity
The capacity expressed for SAC may also be referred to as cation salt splitting capacity (CSSC). Strong acid and strong base (anion or SBA) resins are so called because of their chemical nature (equivalent to sulfuric acid or sodium hydroxide in chemical strength) and the ability to split a neutral salt. For SBA resins, this is the anion salt splitting capacity (ASSC) often referred to as simply strong base capacity. This is shown in Reactions 1 and 2.

These resins can be regenerated with common salt (NaCl) or potassium salts (KCl) and restored to their respective Na (or K) and chloride (Cl) forms. This property gives them great utility for the removal of cation and anion contaminants from residential, industrial or wastewater steams. They can also be regenerated with acid (such as hydrochloric (HCl)) or caustic (NaOH) and put into respective H and OH forms for deionization applications. This is shown in Reactions 3 and 4.

Strong versus weak resins
These functions differ from that of weak acid cation (WAC) and weak base anion (WBA) resins (so called because of their chemical similarity to acetic acid and ammonium hydroxide). These resins do not split neutral salts but will neutralize high and/or low pH feed streams as shown in Reactions 5 and 6. Note that WBA resin is never in the OH form when regenerated but is in what is called the free base form (FB). WBA acts as adsorbants for acid and pick up the entire molecule, as shown in Reaction 6.

WAC and WBA have a higher selectivity difference when converted to the salt forms than do their strong chemical counter parts. This gives them the ability to then split neutral salts where the challenge levels are very low or the total dissolved solids (TDS) of the feed stream are too high for the stronger counter parts. This is shown in Reactions 6, 7 and 8.

WAC and WBA do not split neutral salts unless they are first converted to their salt forms using Reaction 5 and 6, respectively. This also means that one cannot regenerate these resins with salt (NaCl or KCl). Nonetheless, they enjoy great utility in high solids softening3 and ground water remediation (Reaction 8 shows hexavalent chromium? reduction using WBA). To use these resins, they must first be regenerated with acid or caustic and then converted to salt form using caustic (or other base) and acid, respectively. Note that Reaction 5 for WAC can simultaneously remove both hardness AND alkalinity.

Selecting the right form of the resin
When stating or interpreting the capacity of any ion exchange resin, it is extremely important that the ionic form of the resin be stated. Resins change size when they are regenerated or exhausted so the volumetric requirements for sizing will be greatly influenced by the form you order and the capacity you obtain. As a guide, please take note. SBC and SBA shrink when exhausting. WAC and WBA swell. Plan your freeboard accordingly.

When we talk capacity for SBA resins, we have to point out that SBA resins do not have 100-percent strong base functionality; there is some weak base activity. In addition, the ratio of SB to WB will vary with the type of resin (Type I or Type II) and will change with age.4 As SBA resins get older, or if they are exposed to high operating temperatures, there will be either a loss of ASSC or conversion to weak base through a partial decomposition of the functional sites. It is important to understand what happens here and plan carefully. In a salt-regenerated application such as nitrate removal, it is only the strong base capacity that comes into play. In an acid-neutralization application (de-ionization), both the strong and weak base activity are utilized. Silica removal is not accomplished with weak base functionality nor is it possible with a salt regenerated form of BA resin. Silica removal can only be accomplished with the OH form of the SBA resin. Older resin may still have a great acid neutralizing capacity for DI needs but will slowly lose its ability to remove silica. This is the number one reason SBA resins are replaced for DI applications. Anion resin capacity that is stated as total capacity will include BOTH strong base and weak base capacities. A second data point is the pure strong base or ASSC, which is very good to know when troubleshooting issues with performance.

Conclusions
The ion exchange capacity stated in the literature usually refers to the total capacity of the resin. Operating capacity, which is the usable design capacity of the system, will be lower because there are performance criteria on leakage that will terminate the run long before total capacity is attained. SAC and SBA resins have salt-splitting capacity and can be regenerated with salt for many residential, industrial and waste treatment needs. It is the ASSC that dictates silica removal when operated in OH form. WAC and WBA are not salt splitters in their normal regenerated form (and cannot be regenerated with salt) but can be converted for special needs. WAC resins (H form) have a greater capacity than SAC for neutralizing alkalinity. WBA (FB form) generally have a much higher capacity than do SBA for neutralizing acids.

References

1. Michaud, C. F., Factors Affecting the Brine Efficiency of Softeners-Part I, WC&P, August 1999.
2. Michaud, C. F., Factors Affecting the Brine Efficiency of Softeners-Part 2, WC&P, September 1999
3. Michaud, C.F., IEx-treme Softening, WC&P, June 2010.
4. Michaud, C.F. and Brodie, D. F., Ion Exchange-Methods of Degradation, WC&P, January 1990.