Strong Acid Cation

As mentioned previously, the strong acid cation unit contains a zeolite resin that is regenerated with sulfuric acid (HCI can also be used, but it is more expensive than H₂SO₄). As the untreated water passes through the strong acid cation unit, the hydrogen ions that occupy the exchange sites in the resin are replaced by Ca, Mg, Fe, etc., ions. The companion ions of the cations removed-CO₃, SO₄, Cl, N0₃, PO₄, etc.-pass through the resin and link up with the rejected hydrogen to form the strong acids H₂CO₃, H₂SO₄, HCI, HNO₃, H₃PO₄, etc. Obviously, the effluent from a strong acid cation unit is very acidic-, it often has a 2.0-3.0 pH. For this reason, the cation vessel and the interconnecting piping and valves are lined with rubber.

The order of ionic preference for a strong acid resin is:

1. Hydrogen

2. Calcium

3. Magnesium

4. Potassium

5. Sodium

As one might expect, sodium, the least preferred cation, is also the most weakly bound. As a strong acid cation unit approaches the limit of its capacity, the ions shown in the list above begin to leak through in reverse order, i.e., sodium will leak first, followed in order by potassium, magnesium and calcium. In fact, a strong acid cation resin's affinity for sodium is so low that some sodium will always leak through, even when the resin is freshly regenerated.

In actual operation, if a cation unit is to be run to near its "break" point for economic reasons, a parameter known as free mineral acidity is monitored to determine when exhaustion is approaching. Free mineral acidity, or FMA, is present when the water pH is less than 4.3 (the methyl purple or methyl orange end point in the total alkalinity test). By definition, FMA = the sum of the sulfuric, nitrate, phosphoric and hydrochloric acid in the water sample. The FMA of the effluent from a strong acid cation unit, therefore, is proportional to the level of the total exchangeable cations in the raw water.

Monitoring the strength of the acid in the cation effluent by FMA analysis is therefore a good indicator of the performance of the unit. As the rate of cation exchange decreases due to the decrease in available exchange sites in the resin, the amount of acid in the effluent (FMA) decreases. Therefore, decreasing FMA heralds the end of the service cycle in a strong acid cation unit. Cation units are not run to exhaustion because of the need to double regenerate them to get back the total capacity and because high-pressure systems using this water could not tolerate the hardness associated with the end of the run.

In practice, most demineralizer trains are designed to produce the maximum amount of water per desired service cycle. Invariably, the limiting resin volume is that of the anion unit. Strong base resins, even when preceded by a weak base resin, can process less water than a strong acid resin. Frequently, the anion vessel is designed to accommodate the resin volume necessary to treat the desired amount of water and the strong acid cation unit dimensions are duplicated from the anion unit design. This, in effect, insures that when a train breaks, it will break on silica first. In a high-pressure system, silica intrusion is more easily handled than hardness intrusion.

A boiler system can function properly or cease operation as a result of the quality of demineralized water that is used for makeup. Even the best internal treatment programs have their limits. This is why a thorough understanding of the owner's demineralizer system is so important to the water treatment consultant. With excellent feed water, even a mediocre water treatment program can be made to work in a high pressure system. Conversely, even the best chemicals and the most carefully thought-out treatment program can fail miserably when the demineralizer system cannot be counted on to deliver a quality feed water.