Dealkalizer Performance Calculations

We recently had questions about a perceived problem with a dealkalizer. This customer believed the capacity of the dealkalizer was lower than originally specified and was unhappy with its performance. The customer was regenerating this new system more often than they regenerated the older system. We reviewed the system design and operation and found the problems.

Determining the Dealkalizer Capacity:
In order for us to know how long a dealkalizer system will produce dealkalized water between regenerations, we need a complete water analysis. There are a number of factors in determining the capacity of Dealkalizer Systems. The two most important things used to determine resin capacity are: Influent TDS and Alkalinity (as a percentage of the TDS). Once these are known, the resin capacity can be determined by using established resin manufacturers' charts, or by doing calculations based on total exchangeable anions and percentages of alkalinity and chlorides.

Based on using the charts and info from Purolite and Rohm & Haas

the capacity they should be getting is approximately 2,900 grains/cu ft x 15 cu ft = 43,500 grains removal. With 50 ppm (2.93 grains/gallon) alkalinity, this equates to 14,846 gallons between regenerations.

Minimum Flow Rate:
This capacity is further based on the flow rate of the system. Ideally, the unit should run @ 2 gpm/cu ft of resin or 30 gpm. Further, the minimum flow rate to insure proper kinetics and to prevent channeling is 2 gpm/sq ft of tank surface area. In their case, with a 30" diameter fiberglass tank, which has a surface area of 4.6 sq ft, the minimum flow rate should be 9.2 gpm.

We advised the customer that their average real time flow rate is 271 gph, which is 4.5 gpm. This is well below the recommended flow rate of 9.2 gpm. At this rate, the water is definitely channeling resulting in premature alkalinity breakthrough.

Premature alkalinity breakthrough would necessitate regenerating the system more often. If the dealkalizer was being run at the design rate of 9.2 gpm it would regenerate less often.

Resin Regeneration Frequency and Resin Life:
Before this customer understood the cause of the dealkalizer problem he was concerned the dealkalizer resin had lost its original capacity. Furthermore he was concerned that regenerating the dealkalizer more often would affect the life of the resin. The answer in both cases was no. The resin was in good shape and regenerating the system more often would not be detrimental to it.

Their very old dealkalizer did not appear to regenerate as often as the new system. There could have been any number of reasons it didn't.

• Was it actually producing dealkalized water in the 5 ppm range?
• Was the resin broken down to the point where it could operate at low flows and still work?
• Is the water analysis from 2008 the same as now?

We don't know the answers to the first two questions but it's very doubtful that the analysis is the same, as water in most areas can change dramatically from summer to winter and from drought to rainy times. We asked them to check the water in the winter when there is a lot of snow and/or ice. The salt on the roads increases the TDS of the water, not to mention the alkalinity and chlorides; all of which will have a significant affect on the performance of a dealkalizer.

Solutions and Recomendations:

Adding Caustic to Salt During Regeneration Increases Resin Capacity
They may be able to increase the capacity of the dealkalizer somewhat by increasing the amount of the salt and caustic. However, it may not be worth it given the low flow rate of operation.

We don't feel making changes to the dealkalizer will enable it to produce more treated water between regenerations. If the real time flow will continue at 4.5 gpm (or less), we suggested they installation of a recirculation pump on the softener/dealkalizer system. This will insure there is enough water going through the units and prevent channeling.

Short Ion Exchange Resin Life-What's Happening?

Lately I've been hearing our sales people commenting that customers don't think their cation softening resin is lasting as long as it used to. This is a general comment, not something we're surveying. Here is some food for thought:

Has the manufacturing process changed?
Yes. One of the relatively recent changes to the process came with the non-solvent resins. The greatest motivating factor behind the non-solvent resin came from the EPA. Simply put, the resin manufacturers had to stop manufacturing the resin using solvents because they couldn't put the by-products down the drain. So the resin we used to clean up the water had a manufacturing process that potentially contaminated the water. Makes sense!

Are the non-solvent cation resins lasting as long as the solvent based resins?
According to a major manufacturer non-solvent ion exchange resins are manufactured to meet the same standards of the solvent type. That is, both the solvent and non-solvent resins are 8% crosslink and will react similarly under the same set of circumstances. For instance, both resins, in the presence of 2 ppm chlorine, will react and break down.

Should the solvent free ion exchange resins be used in industrial applications?
According to the data sheets for Sybron C-249 NS and Purolite C-100, the answer to this is yes.

So what are some of the reasons we are seeing shortened life/capacity?
Is there chlorine in the feed water?
In the presence of chlorine or any oxidant, cation ion exchange resins will breakdown prematurely. You say - yes, but the chlorine has always been there. I agree. However, as our water infrastructure has aged have the municipalities been forced in some instances to add more chlorine to compensate?

Are there higher levels of iron in the water?
As we know cation resin will remove ferrous iron but regenerating the iron off of the resin is challenging. Over time there is a loss of capacity as a result of the iron being embedded into the cation bead.

Is the cation resin seeing higher temperatures?
Gel cation resin can tolerate high temperatures but the combination of higher temperatures and an oxidant such as chlorine will dramatically lessen the life of the resin.

We've been supplying ion exchange resins for over 25 years now. We have customers who call every three years, most commonly replacement of anion resin in a demineralizer application and other customers we hear from every 5 or even 7 years - softener applications. I can't say as I've seen a trend that points to bad manufacturing practices by manufacturers.

In my opinion, the overwhelmingly majority of the problems result from oxidants such as chlorine and chloramine and my favorite - Operator Error - Oops ... we just backwashed all the resin out!