Hydrodynamic Design, Part 8: Flow Through Ion Exchange Beds

By C.F. ‘Chubb’ Michaud, CWS-VI

There is probably no media filter that has a more critical hydraulic design than does an ion exchange (IEx) system. Distribution is key. Service quality and capacity depend upon how well the beds are regenerated and regeneration is all about flow distribution. Part 8 of this series looks in depth at the highs and lows of suggested flowrates.

Sizing an ion exchange (softener) system
Ion exchange systems are very fast kinetically, which is why softeners can be run at 10 to 15 gpm/ft³ without producing ‘blow by’ (meaning that the system is run so fast it doesn’t have time to remove all the hardness). Why don’t we design commercial and industrial filters this way? The answer is simply the consideration of  cycle time, which is the length of the run between regenerations. Besides, industrial softeners often have to reduce hardness to fractional ppm levels of leakage and that requires optimized flow characteristics. Let’s start at the back and work forward.

If we feed 20 gpg (grains per gallon) hard water to a one-cubic-foot softener system, how long would it take to exhaust the unit at 10 gpm? Assuming a normal capacity of 24,000 grains/ cu ft, that works out to (24,000/20) = 1,200 gallons or (1,200/10) = 120 minutes. Can you build a system to regenerate every two hours? Yes and it’s called twin alternating but it takes four hours to make brine (so maybe you will install a twin two-cubic-foot system). I bet you don’t sell those to a residential user very often. Fortunately, the homeowner does not need 10 gpm continuously 24/7. With intermittent flows, you can get by with a one-cubic-foot system regenerating every 3-4 days. How about a commercial or large industrial unit?

Let’s design a system for 100 gpm on a 24/7 basis to feed a medium pressure boiler. This makes it an automatic twin system with one unit always in service. How long do we want to run between regeneration cycles? There is no fixed rule here but it is prudent to have a long enough run that if work needs to be done on the off-line unit, there is at least time for the service. Commercial and industrial systems usually are sized for between 8 and 24 hours. Let’s go with a 12- hour cycle. The burn rate for 12 hours at 100 gpm on 20 gpg water is: (20gpg)(100gpm) (60min/hr)(12hr) = 1,440,000 grains/cycle. With a nominal capacity of 24 kilograins/ft³, our burn rate is a 5 ft³/hour or 60 ft³/ 12-hour cycle.

This fits perfectly into a 63-inch-diameter fiberglass tank or 60-inch steel with a three-foot bed depth. But what happens when oxidation breaks up a few cubic feet of resin and we lose it through backwash? Or the hardness goes up a grain? Or a little iron sneaks in and fouls the top of the bed? Or the water gets colder during  the winter? Do we still have a 12-hour system? No, we do not. So we can either add another six cubic feet of resin or downgrade the system to a 10.5-hour cycle. I  like to take at least a 10-percent engineering factor (downgrade) on softeners to allow for such events. So we build this one with 66 ft³.

A system with 66 cubic feet in a 63-inch tank will flow at 100 gpm (1.5 gpm/ft³ and 5 gpm/ ft²), which works well. The acceptable range is 1 to 3 gpm/ft³ and 4 to 10 gpm/ft². We will regenerate it at 0.25 to 0.5 gpm/ft³, which is 16.5 to 33 gpm (total brine and dilution). The service-to-regeneration flow ratio is 3 on up to 6:1. Try to stay under 7:1.  ΔP in service will go up by the square of this ratio. At 7:1, the service ΔP is 49 times (7×7) higher in service than in regeneration. If the lateral was designed at 0.15 psi ΔP, the service ΔP will be 7.4 psi. If you try to run the system to an arbitrary maximum rating of 15 gpm/ft² (300 gpm) this raises the service/regeneration flow ratio to 10:1 and the resulting ΔP in service increases by a factor of 100 (15 psi in this example). Figure 13 illustrates the hydraulic design flows.

Pressure drop differential through a resin bed is proportional to the square of the differences in flow. If we double the flow, we increase the ΔP by four times. If we triple it, we increase the ΔP by nine times. To regenerate an ion exchange system at 0.5 gpm/ft³ and run it at 5 gpm/ft³ in service is an increase of 10 times. That means the ΔP will increase by 100 x (102). This would pose quite a challenge.

If we assume that we are going to regenerate with brine at the 8 lb/cu ft level, how do we determine the proper flowrate and choose the proper eductor for the system? Saturated brine contains roughly 2.6 lbs of NaCl per gallon at about 24 to 25 percent NaCl (100-percent saturated) and that is diluted to 40 percent saturated (10 to 11 percent NaCl) via the eductor. This diluted brine will contain approximately one pound of NaCl/gal so an eight-pound regeneration will involve about eight gallons of total brine/ft³. We would like to see a brine contact time of 20 to 30 minutes. To put eight gallons of brine through the bed in 20 minutes equates to a flow of 0.4 gpm/ft³ and for 30 minutes, 0.27 gpm/cu ft. We have to select an eductor that will deliver that flow as in the earlier examples. With softeners, it is the total amount of salt that impacts the capacity, more so than the contact time during regeneration. Obtaining proper distribution is more important than a long, slow regeneration with poor flow. To gain minimal leakage, industrial systems generally require designed laterals so that the distribution during regeneration is uniform.

Figure 13. Measures of dynamic flow

Dealkalizers and other salt-regenerated anion systems
Anion exchangers used as dealkalizers, nitrate, fluoride, arsenic and uranium removal systems can operate pretty much like a softener, although resin manufacturers caution against flowrates higher than 5 gpm/ft³. Anion systems often do not deliver the very high capacities experienced with softeners, so it really is best to limit the flow to under 5 gpm/ft³ in order to meet any reasonable cycle time. In addition, anion units are generally used to remove dangerous contaminants with MCLs in the ppb range so conservative and redundant designs are frequently warranted. Two to three gpm/ft³ is a suitable flow range for any anion system.

If you are treating well water containing 80 or 90 ppm of nitrates, there may be serious consequences if you fail. This is not a situation where you put two scoops of anion on top of a softener bed and walk away. Serious anion contaminants should have a dedicated and properly sized filter with its own cycle time, brine supply and internals design. The same could be said of a cation unit used to reduce metals or radium.

Sizing an ion exchange (deionizer) system
Demineralizers remove all of the ions from water, not just the hardness or a selected anion or two. As such, they are generally larger in order to provide the  appropriate cycle times. In addition, the cation is used to regenerate the anion and the regeneration times are much longer for the combined systems. Cycle lengths are set to coincide with a specific day of the week, a particular shift or hour of the day. Past articles¹¹ have offered instruction on the rating of ion exchange beds; they may differ in size with a change in the water analysis, even at the same flowrate. A brief explanation of sizing is offered here.

A rule I generally use in the initial design (knee jerk) of an ion exchange system is 3 gpm/ft³ for the cation and 2 gpm/ft³ for the anion. This also puts the ratio of anion to cation at 3:2, which is in keeping with their capacity ranges. Regeneration is usually figured at 0.33 to 0.5 gpm/ft³ for the cation and 0.25 to 0.33 gpm/ft³ for the anion. The flow is presented as a range with the end target of 25 to 40 minutes for the cation and 40- to 60-minute contact time for the anion. Once the water analysis is balanced, exact system specifications can be drawn based on quality and quantity demands.

The second rule on hydraulic flow is that we try to achieve 1 to 3 gpm/ft³ and 4 to 10 gpm/ft² to maintain flow distribution. The minimum flows represent considerations to avoid channeling. Running too slowly induces poor flow distribution. The upper values represent considerations for cycle time and pressure drop. As an example, let’s size a two-bed demineralizer to provide eight hours of continuous supply at 20 gpm from a feed of 350 ppm (as CaCO3) with 25 ppm of silica. If we have a balanced analysis, we have (375/17.1 =) 21.9 gpg for the anion and (350/17.1 =) 20.5 gpg for the cation. Based on the quality needed, we have selected a strong acid cation and strong base anion Type I.

First we calculate the burn rate on the anion:
20 gpm x 21.9 gpg x 60 min/hr x 8 hrs = 210,240 grains. Assuming the anion capacity is 12,500 grains/ft³, we would burn (210,240/12,500 =) 16.8 ft³. Using an engineering factor (downgrade) of 15 percent, we size our anion at (16.8/.85 =) 19.76 or 20 ft³.

Our cation burn rate is:
20 gpm x 20.5 gpg x 60 min/hr x 8 hrs = 196,800 grains. If we determine the cation capacity to be 26,500 grains/cu ft, we need 7.5 ft³ and here, we use a 10-percent engineering factor, which makes our bed 8.25 ft³. In addition, the cation must supply enough decationized water to regenerate the 20 cubic feet of anion. We estimate using 75 gallons of decationized water/cu ft of anion, so we have to add (20 x 75 =) 1,500 gallons for the cation: 1,500 gal x 20.5 gpg = 30,750 grains. At 26,500 gr/cu ft, we need an additional 1.2 cu ft. We end up with a 20-cubic-foot anion bed and 9.5-cubic-foot cation (installed as 10 cubic feet). With a suggested flowrate of 1 to 2.5 gpm/ft³, we are okay on both. Using a nominal three-foot-deep bed, we have a 24-inch cation tank and 30-inch anion tank.

In sizing any ion exchange system, we can follow the same steps:

1. Start with a complete and properly balanced water analysis.
2. Determine the ionic loading (in grains/gallon).
3. Determine the flow requirements (minimum and maximum in gpm).
4. What is the acceptable cycle time?
5. What quality is required (determines leakage)?
6. Determine the regeneration level based on quality.
7. Determine the resulting capacity.
8. Calculate the burn rate in ft³/cycle (for anion, then cation).
9. Adjust resin by applying an engineering factor (15-percent downgrade for anion, 10 percent for cation). This gives a ‘corrected’ burn rate.
10. Compare hydraulics (gpm/ft² and gpm/ft³).
11. Design tank with a bed depth of 30 to 60 inches (compare to #10).
12. Design distribution (laterals) system.
13. Select pipe and valve size based on accepted flow for pipe size (see Figure 2, Part 2).
14. Create valve nest layout based on flow configuration (up-flow, down-flow).
15. Design flow through the laterals based on ΔP during regeneration and service.

The second rule on hydraulic flow is that we try to achieve 1 to 3 gpm/ft³ and 4 to 10 gpm/ ft² to maintain flow distribution. The minimum flows represent considerations to avoid channeling. Running too slowly induces poor flow distribution. The upper values represent considerations for cycle time and pressure drop. As an example, let’s size a two-bed demineralizer to provide eight hours of continuous supply at 20 gpm from a feed of 350 ppm (as CaCO3) with 25 ppm of silica. If we have a balanced analysis, we have (375/17.1 =) 21.9 gpg for the anion and (350/17.1 =) 20.5 gpg for the cation. Based on the quality needed, we have selected a strong acid cation and strong base anion Type I.

Summary
To optimize capacity with minimum leakage, ion exchange systems require well-designed distribution systems. You are generally looking for 99-percent removal of hardness for softeners and 99.9+ percent of all ions for demineralizers. If, through faulty design, the regenerant solution fails to reach all of the resin, the ions left on the bed will produce excess leakage during the next service cycle and quality and cycle length will not be met.

References

1. Michaud, C.F., Ion Exchange Capacity and Systems Rating, WC&P International, November, 2005.

About the author
C.F. ’Chubb’ Michaud is the Technical Director and CEO of Systematix Company of Buena Park, CA, which he founded in 1982. He has served as chair of several sections, committees and task forces with WQA, is a Past Director and Governor of WQA and currently serves on the PWQA Board, chairing the Technical and Education Committees. Michaud is a past recipient of the WQA Award of Merit, PWQA Robert Gans Award and a member of the PWQA Hall of Fame. He can be reached at (714) 522-5453 or via email at [email protected]