By C.F. “Chubb” Michaud, MWS
Prior to the development of the modern ion-exchange processes, the only path to “purified” water was distillation. But contrary to popular belief, distillation does not produce pure water. In fact, it would take more than 20 distillations in series using inert quartz vessels to equal the purity of water produced by ion exchange.
Although the ion exchange concept had existed for centuries, it wasn’t until 1905 that Robert Gans applied the technology to the softening process using natural zeolites (sodium aluminates). But the invention of the modern styrene-divinylbenzene (S-DVB) polymer by Gaetano F. D’Alelio in 1944 narrowed the path for the development of the strong base anion (SBA) by Charles H. McBurney in 1946 and the strong acid cation (SAC), first reported in 1947. Today, 95 percent of modern ion exchangers are based on S-DVB.[1]
Ion Exchange Process
The ion exchange process can be easily visualized. An insoluble plastic bead (S-DVB resin) containing a reactive site with an exchangeable ion is contained in a vessel (bed) through which water is passed. Ions in the water are selectively attracted to the reactive sites on the resin in exchange for the ions that are on the resin, which then pass through the bed. The result of the process is that the new composition results in a more desirable chemistry for the intended use of the water. The softener is one example:
O represents the ion exchange resin bead, Ca is water hardness, and Na is the exchangeable ion on the strong acid cation bead. The process removes the hardness and exchanges it for a sodium, which is a more favorable ion for many water processes.
Boiler feed water containing bicarbonate ions will decompose on heating to produce volatile carbon dioxide, which produces a corrosive steam. A de-alkalizer exchanges the bicarbonate for chloride, which does not boil off with the vapor:
The bicarbonate is exchanged for a chloride using a Type II strong base anion with an exchangeable chloride. A similar reaction occurs with treatment to reduce materials such as fluorides, sulfates, and uranium.
Deionization (DI) by ion exchange is a two-step process. This can be done with a two-bed system of cation followed by anion resin:
This reaction converts the anions to acids.
This reaction removes the anions, including silica, and the byproduct is water.
Two-bed DI can be replaced with an intimate mix of the two into what is called a mixed bed. Mixed beds were first used in 1949 and, to everyone’s delight, were found to produce a much higher quality of water. The downside was that the resins had to be carefully separated prior to regeneration, which required more complicated equipment, more complex controls, and more skilled operators. For this reason, many portable exchange deionization (PEDI) plants opted to stick with the two-bed concept, in which the resins could be regenerated without removing them from their service vessels. This feature enabled the building of smaller plants at far lower costs. If a three-bed system of cation/anion/cation was used, there was a considerable improvement in product quality that could boost properties near the 1 megohm (meg) range without the complicated mixed bed.
Ion exchange is an equilibrium reaction. If the product of decationization is an acid, and acid is used for the regeneration, then there is a bit of an uphill run for the cation as the total dissolved solids (TDS) of the feed water increases. The higher the TDS, the higher the amount of acid in the product and the lower the pH. At some point, the pH of the product could be low enough to start regenerating the cation resin, which would result in very high leakage. This leakage, primarily sodium, would convert to sodium hydroxide through the anion, giving a very high pH (above pH 10) and greatly limiting the use for the water. If we look at the equilibrium relationship for cation exchange, we can readily see what is happening:
This equation states that the relationship for a cation exchanger in H+ form will depend on the level of sodium in the water [Na+]W and the level of hydrogen on the resin [H+]R. So the higher the water TDS and the better the regeneration, the more this equation is driven to the right. As the bed starts to exhaust and the level of sodium on the resin increases while the level of hydrogen in the water also increases, the numbers get larger on the bottom and the reaction is then driven to the left. In other words, there will come a limit, after which the leakage, or bypass, will increase.
Higher Purity Without Mixed Beds
As the pH of the product water (from either cation or anion bed) goes away from a pH of 7, the equilibrium reaction gets slower and pushes back against a favorable reaction. Therefore, as the TDS increases, the sodium leakage increases. And as the sodium leakage increases, the pH of the anion product also increases, which, in turn, increases silica leakage. With the pH of both products going away from pH 7, the two-bed DI train will be limited in TDS. Experience tells me that this limit will be around 2,000 parts per million (ppm) TDS. However, putting two or more two-bed trains in a row will solve the problem.
In general, a cation DI train will reduce the cation content of the water by 99.5 percent. That means a 1,000 ppm feed will have a leakage of about 5 ppm of sodium. When that converts through the anion to sodium hydroxide, the resulting pH will be around 10 and the resistivity will reach about 60,000 ohms. This might work for a portable carwash, but that’s about it.
However, if that same product water is fed to another cation, the high pH being reduced towards pH 7 will be very favorable. If we see an additional 99 percent reduction of cation, we exit the bed at less than 0.1 ppm. That alone can bump the resistivity to near 1 meg. Follow that with another anion, and more than 1 meg is achievable without the use of the complex mixed bed.
Introduction of the Mixed Bed
The idea of multiple trains led to the development of the mixed bed. If two trains can take us from 60,000 resistivity to 1 meg, what can a few hundred cation/anion cycles do? The short answer is we can produce more than 18 meg product water. It’s almost impossible to measure the resistivity of water beyond that because the very pure water becomes auto-ionic and begins to ionize into conductive components. The literature says the maximum resistivity for pure water is 18.3 meg. The corresponding TDS is less than 0.03 ppm. Even a poorly regenerated mixed bed can produce 10 meg.
Matching the Capacity Ratios
The ion exchange reaction is favored by dilute solutions. As feed water passes through a mixed bed and the conductive ions are replaced by H₂O, the TDS drops and the reaction proceeds favorably to completion. It is therefore possible to have an operating capacity of the resins very close to the total capacity of the respective resins. In a two-bed SAC/SBA system, the ratio of capacities is about 2:1 in favor of the cation. However, in a mixed bed, they are closer to the absolute total capacities of 1.95:1.2 (mEq/ml). Since the cation capacity is 50 percent higher than that of the anion, we compensate by making the mixed bed 60 percent anion and 40 percent cation. This allows a slight advantage in capacity for the cation. This helps us to avoid hardness fouling of the anion and gives us a pronounced pH drop as the bed nears exhaustion.
The advantage of the mixed bed is not only the quality that can be achieved, but the capacity as well. For example, a plating shop needs 1 meg water for rinsing critical parts and is using a feed water with a TDS of 500 ppm as calcium carbonate (CaCO3). That’s 29.2 grains per gallon (gpg). The shop has to make up rinse water at 10 gallons per minute and run continuously for six hours. That’s 3,600 gallons per day and 105,120 grains. The local PEDI service company has supplied the shop with four tanks of 3.6 cubic feet each: one cation, two anions, and one mixed bed. For simplicity, we are calculating the capacity of the cation at 26,000 grains per cubic foot and the anion at 13,000 grains per cubic foot.
So the two-bed portion of the train is balanced and will produce 93,600 grains of capacity. That leaves the mixed bed picking up 11,520 grains. With a rated capacity of 12,000 grains per cubic foot, the mixed bed should last for 3.75 days (12,000 x 3.6/11,520 = 3.75). Using four 3.6 cubic feet mixed beds would supply a total capacity of 172,800 grains and last 1.64 days. Operating 20 days per month, 20 cations, 40 anions, and six mixed beds per month would be replaced. Using mixed beds only, 49 mixed beds per month would be replaced.
SAC is strong acid cation, SBA is strong base anion, MB is mixed bed.
Figure 2. Both systems have the same footprint.
Economics
PEDI plants that produce mixed-bed exchanges have more costs invested in their equipment than those that operate two-bed only. It is not unexpected that the cost per cubic foot of an exchanged mixed bed is more than the equivalent in two-bed. Two-bed PEDI can be regenerated right in the service vessel. A typical setup is shown in Figure 3.
Contrast that to the more complex batch plant in Figure 4.
Cost Comparison
If we price out the plating shop’s needs with relative pricing for exchange tank service, and we assign a cost on SAC of $$1.0X/exchange, SBA of $$1.1X/exchange, and mixed bed at $$1.2X/exchange (X equals a cubic foot), we have a total of (20 x $$1.0) + (40 x $$1.1) + (6 x $$1.2) = $$71.2X/month versus (49 x $$1.2) = $$58.8X/month. The all-mixed-bed system saves the customer almost 20 percent per month.
Making the decision to advance to a mixed-bed batch plant suggests a strong commitment of resources. Upgrades are needed according to the operators’ skill level and the skill level of the sales force. However, for businesses, it opens doors of opportunity that didn’t exist with the in-tank regeneration plant.
Reference
1. De Dardel, François, “History of Ion Exchange,” Updated October 1, 2011. http://dardel.info/IX/other_info/history.html.
About the author
C.F. “Chubb” Michaud, MWS, 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 within the Water Quality Association (WQA), as well as served as a past director and governor. He served on the Pacific Water Quality Association (PWQA) board, chairing the Technical and Education Committees for 12 years. Michaud is a proud member of both the WQA and PWQA Halls of Fame, has been honored with the WQA Award of Merit, and is a two-time recipient of the PWQA Robert Gans Award. A frequent and well-published author and speaker, Michaud has contributed over 100 original papers on water treatment techniques and holds four U.S. patents on ion exchange technologies. He holds a BS and an MS degree from the University of Maine.
About the Company
Systematix Company, founded in 1982, is an innovative media supply company with a focus on ion exchange media, processes, and systems design. Expert advice is offered for the asking. The company can be reached at (714) 522-5453, or email [email protected].