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 :
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 endpoint 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 demineralized 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 demineralize 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 demineralized system cannot be counted on to deliver quality feed water.
One of the troublesome constituents in water used as a source of makeup for boiler systems is alkalinity. The so-called total alkalinity of water is the sum of the CO3 and OH+ found in that solution. The CO3 portion of the total alkalinity is especially troublesome. As raw water is processed through a strong cation unit in a demineralize train, the Ca or Mg normally associated with the CO3 is exchanged for H+ and the cation effluent contains H2CO3.
This acid, called carbonic acid, is very unstable. It disassociates into carbon dioxide (CO2) and water very rapidly. For this reason, many strong acid cation units are followed by a mechanical device called a decarbonate. A decarbonate is nothing more than a vessel filled with pall or Raschig rings supported on a grid over a plenum. A fan blows atmospheric air up through the fill and out a vent at the top of the tank or vessel. Carbon dioxide, which breaks out of the strong acid cation effluent stream, exists at the top. The effluent from the decarbonate normally contains 10 ppm CO2.
Decarbonization of the strong acid cation effluent can also be accomplished by passing the acidic solution through a strong base anion resin that has been regenerated with caustic. A more complete discussion of this process follows.
Weak Acid Cation
Certain waters that contain a high percentage of hardness associated with alkalinity can be economically treated by passage through a weak acid cation resin. By definition, the weak acid resin will remove Ca++, Mg++, and Na+ which enters the bicarbonate (HCO3)- form. Because most industrial water sources contain some noncarbonate hardness (CaSO4, etc.), it is necessary to follow the weak acid cation unit with a strong acid cation unit to achieve truly demineralized water.
Weak and strong acid cation resins can be placed in different vessels or they can be placed in two distinct layers in the same vessel. The regeneration efficiency of a weak acid resin is very high compared to that of a strong acid resin. Therefore, it is possible to utilize the regenerated acid stream from the strong acid unit to regenerate the weak acid unit. When weak and strong acid cation resins are loaded into the same vessel, the strong acid resin settles on the bottom of the unit after backwash because of the density difference between the two resins.
Because the weak acid resin contains some strong acid sites, after regeneration with sulfuric acid, a 10% brine solution must be passed through the unit. The brine solution exhausts any strong acid sites in the weak acid resin and regenerates the strong acid resin in the sodium form. If this is not done when raw water enters the weak acid resin, noncarbonate hardness exchanges at the strong acid sites. FMA exits the weak acid resin and prevents the exchange of residual noncarbonate hardness in the strong acid resin. Normally, a weak acid resin produces FMA for 40-60% of its service cycle. This combination would not be suitable for higher pressure boiler applications because of the presence of excess sodium in the effluent from the sodium-form strong acid resin.
Regeneration of a weak acid cation resin with sulfuric acid must be carefully monitored to insure that the acid concentration during the regeneration does not exceed 0.7%. Higher concentrations of sulfuric acid can react with the Ca++ in the exchange sites of the exhausted resin and result in the precipitation of calcium sulfate (CaSO4). Calcium sulfate, or gypsum, is insoluble even in the concentrated form of many acids. Often, mechanical removal is the only satisfactory way to rid the resin of this contaminant. From an operational standpoint, it is objectionable because it produces a pressure drop across the unit.