DETERMINATION OF DEIONIZER SPECIFICATION REQUIREMENTS

SIZING A DEIONIZER:
In terms of effluent water quality, the performance of a deionizer is determined by the suspended or dissolved minerals present in the raw water supply, various types of exchange resins employed, the regenerant dosage and to a certain extent, the flow rate through the deionizer.
The upcoming reasoning and calculations are based on the use of strong base anion resin; as a result the types of exchange resins employed will not a factor. Before going through a sample design calculation, let us first elaborate the remaining three parameters.

ANALYSING THE RAW WATER SUPPLY:
The very first step is to obtain an accurate water analysis. This can be obtained from the water department, if municipal water is used or from a private lab or state water testing facility if well supply is used. However, the analysis must include several items of information that are obtainable only from on-site determinations. Some of these are: chlorine (Cl2), CO2, iron, pH and turbidity. It must be noted that full emphasis should be on clean, filtered water, which must be introduced into the deionizer. The presence of iron, organics or filterable material will tend to foul the exchange resin and reduce the capacity of deionizer. Chlorine concentration in excess of 0.20 PPM can be detrimental to exchange resins. Pre-filtration with carbon for chlorine removal and organics reduction must be used with waters containing these contaminants.
After the water analysis has been completed and the customer’s water quality requirements determined you can proceed with selection of the proper system.

REGENERANT POSOLOGY:
The quantity of chemical used to regenerate the resin columns will have a direct effect on the exchange capacity of each cubic foot of resin, as well as on the quality of water that can be produced by the system. In most cases, a chemical dosage of 8 lbs. of 100% hydrochloric acid (HCl) for each cubic foot of cation resin and 8 lbs. of 100% sodium hydroxide (NaOH) for each cubic foot of Anion Exchange Resin will provide good system performance. In cases where raw water having high sodium content is preventing the system from providing water of acceptable quality, the acid dosage may be increased. Where silica leakage from the Anion Exchange Resin is unacceptably high, the caustic dosage may be increased.

CATION EXCHANGE RESIN

Cation Exchange Resins used in deionization are operated in the hydrogen form and are typically regenerated with hydrochloric acid (HCl). Figure 1.1 shows the theoretical capacities at several Regeneration levels (these capacities are based on a calcium chloride standard having zero percent sodium and alkalinity content). For example, at 3 lbs. of 100% HCl per cubic foot of resin the theoretical capacity is 17,500 grains (17.5 kgr) as CaCO3 per cubic foot of resin. At 8lbs. of 100% HCl per cubic foot of resin, the theoretical capacity if 29700 grains (29.7 kgr) as CaCO3 per cubic foot of resin.

Exhausting Solution (ppm as CaCO3
Regeneration Level lbs. of 100% HCl/ft3 of Resin
Capacity kgr. As CaCO3
Kgr. Capacity per lb. of HCl
500 ppm CaCl2
3
17.5
5.8
500 ppm CaCl2
4.5
22.7
5.0
500 ppm CaCl2
6
26.6
4.4
500 ppm CaCl2
7.5
28.9
3.8
500 ppm CaCl2
8
29.7
3.7
500 ppm CaCl2
9
30.7
3.4

Figure 1.1 Theoretical Exchange Capacities

However, sodium and alkalinity in the raw water will affect the actual capacity of the cation resin as well as the quality of the water produced by the system.

For sequential regeneration systems, in which the anion resin column is regenerated using decationized water from the cation column, the cation resin must be the total cation content (in PPM as CaCO3) of the raw water in order to determine the ultimate water quality that can be expected from the system.
For example, if water analysis shows:
Total cation = 500 PPM (as CaCO3),
Total anion (less silica and CO2)=500 PPM,
Sodium content = 250 PPM = 50% of total cations,
Alkalinity =250 PPM = 50% of total anions,
Then the average leakage determined for the two regeneration levels will be:
At 8lbs. HCl/ft3, Average leakage = 2.6% (0.026) =500 PPM *0.026 = 13 PPM
At 8lbs. HCl/ft3, Average leakage = 0.5% (0.005) =500 PPM *0.005 = 2.5 PPM