Strongly acidic cation resins derive their exchange activity from sulfonic functional groups (HSO). The major cations in water are calcium, magnesium, sodium, and potassium and they are exchanged for hydrogen in the strong acid cation exchanger when operated in the hydrogen cycle. The following stoichiometric equation represents the exhaustion phase, and is written in the molecular form (as if the salts present were undissociated). It shows the cations in combination with the major anions, the bicarbonate, sulfate, and chloride anions:
Ca 2HC02 Ca 2H2C03
Na 2C1 Na 2HC1
Note that R represents the complex resin matrix. Because these equilibrium reactions are reversible, when the resin capacity has been exhausted it can be recovered through regeneration with a mineral acid. The strong-acid exchangers operate at any pH, split strong or weak salts, require excess strong-acid regenerant (typical regeneration efficiency varies from 25 percent to 45 percent in concurrent regeneration), and they permit low leakage. In addition, they have rapid exchange rates, are stable, exhibit swelling less than 7 percent going from Na+ to H+ form, and may last 20 years or more with little loss of capacity.
These resins have found a wide range of application, being used on the sodium cycle for softening, and on the hydrogen cycle for softening, dealkalization, and demineralization.
Weakly acidic cation-exchange resins have carboxylic groups (COOH) as the exchange sites. When operated on the hydrogen cycle, the weakly acidic resins are capable of removing only those cations equivalent to the amount of alkalinity present in the water, and most efficiently the hardness (calcium and magnesium) associated with alkalinity, according to these reactions:
These reactions are also reversible and permit acid regeneration to return the exhausted resin to the hydrogen form. The resin is highly efficient, for it is regenerated with 110 percent of the stoichiometric amount of acid as compared to 200 to 300 percent for strong-acid cation-exchange resins. It can be regenerated with the waste acid from a strong-acid cation exchanger and there is little waste problem during the regeneration cycle. In order to prevent calcium sulfate precipitation when regenerated with H2S04, it is usually regenerated stepwise with initial H2S04 at 0.5 percent. The resins are subject to reduced capacity from increasing flow rate (above 2 gpm/ft), low temperatures, and/or a hardness-alkalinity ratio especially below 1.0.
Weakly acidic resins are used primarily for softening and dealkalization, frequently in conjunction with a strongly acidic polishing resin. Systems which use both resins profit from the regeneration economy of the weakly acidic resin and produce treated water of quality comparable to that available with a strongly acidic resin.
Strongly basic anion-exchange resins derive their functionality from the quaternary ammonium exchange sites. All the strongly basic resins used for demineralization purposes belong to two main groups commonly known as type I and type 11. The principal difference between the two resins, operationally, is that type I has a greater chemical stability, and type II has a slightly greater regeneration efficiency and capacity. Physically, the two types differ by the species of quaternary ammonium exchange sites they exhibit. Type I sites have three methyl groups, while in type 11, an ethanol group replaces one of the methyl groups. In the hydroxide form, the strongly basic anion will remove all the commonly encountered
Like the cation resin reactions, the anion-exchange reactions are also reversible and regeneration with a strong alkali, such as caustic soda, will return the resin to the
The strong-base exchangers operate at any pH, can split strong or weak salts, require excess high-grade NaOH for regeneration (with the typical efficiency varying from 18 percent to 33 percent), are subject to organic fouling from such compounds when present in the raw water and to resin degradation due to oxidation and chemical breakdown. The strong-base anion resins suffer from capacity decrease and silica leakage increase at flow rates above 2 gpm/ft3 of resin, and cannot operate over 130° to 150° F depending on resin type. The normal maximum continuous operating temperature is 120° F, and to minimize silica leakage, warm caustic (up to 120° F) should be used. Type I exchangers are for maximum silica removal. They are more difficult to regenerate and swell more (from CI to OH form) than type II. The major case for selecting a type I resin is where high operating temperatures and/or very high silica levels are present in the influent water or superior resistance to oxidation or organics is required.
Type II exchangers remove silica (but less efficiently than type I) and other weak anions, regenerate more easily, are less subject to fouling, are freer from the odor of amine, and are cheaper to operate than type I. Where free mineral acids are the main constituent to be removed and very high silica removal is not required, type
384 WATER AND WASTEWATER TREATMENT TECHNOLOGIES Weakly Basic Anion Resins
Weakly basic anion resins derive their functionality from primary (R-NH), secondary (R-NHR'), tertiary (R-N-R'2), and sometimes quaternary amine groups. The weakly basic resin readily absorbs such free mineral acids as hydrochloric and sulfuric, and the reactions may be represented according to the following:
Because the preceding reactions are also reversible, the weakly basic resins can be regenerated by applying caustic soda, soda ash, or ammonia. The weak-base exchanger regenerates with a nearly stoichiometric amount of base (with the regeneration efficiency possibly exceeding 90 percent) and can utilize waste caustic following strong-base anion-exchange resins. Weakly basic resins are used for high strong-acid waters (CI, S04, N03), and low alkalinity, do not remove anions satisfactorily above pH 6, do not remove CO or silica, but have capacities about twice as great as for strong-base exchangers. Weak-base resins can be used to precede a strong-base anion resin to provide the maximum protection of the latter against organic fouling and to reduce regenerant costs. Make note of the sidebar discussions. These summarize for you the important sodium cation-exchanger reactions. Note in the sidebar discussions that Z denotes the sodium cation exchanger. An important term we should make note of in reviewing these stocihiometric relations is Compensated Hardness. What this refers to is that the hardness of the water for softening by the zeolite process should be compensated when: (1) the total hardness (TH) is greater than 400 ppm as CaC03, or (2) the sodium salts (Na) are over 100 ppm as CaC03. Compensated hardness can be calculated from the following formula:
Compensated Hardness (ppm) =
Express compensated hardness according to the following:
Next higher tenth of a grain up to 0.5 grains per gallon; Next higher half of grain from 5.0 to 10.0 grains per gallon; Next higher grain above 10.0 grains per gallon.
The salt consumption with a sodium 0.275 and 0.533 Lbs of salt per 1,000 grains of hardness, expressed as calcium carbonate, removed. This range is attributed to two factors: (1) the water composition, and (2) the operating exchange value at which the exchange resin is to be worked. The lower salt consumption may be attained with waters that are not excessively hard nor high in sodium salts, and where the exchange resin is not worked at its maximum capacity.
cation exchange water softener ranges between
The next group of sidebar discussion summarize for you the important reactions for hydrogen cation exchange resins. The symbol Z denotes the hydrogen cation-exchanger resin. Three groups of reactions are summarized, these are the reactions with bicarbonates, the reactions with sulfates or chlorides, and finally regeneration reactions. The reactions are fairly straightforward, and if you work regularly with water softening systems, it's best to try and memorize these reactions.
There also exists a type of resin with no functional groups attached. This resin offers no capacity to the system but increases regeneration efficiency in mixed-bed exchangers. These inert resins are of a density between cation and anion resins and when present in
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