Resin Performance

Variances in resin performance and capacities can be expected from normal annual attrition rates of ion-exchange resins. Typical attrition losses that can be expected include: (1) Strong cation resin: 3 percent per year for three years or 1,000,000 gals/ cu.ft; (2) Strong anion resin: 25 percent per year for two years or 1,000,000 gals/ cu.ft; (3) Weak cation/anion: 10 percent per year for two years or 750,000 gals/ cu. ft. A steady falloff of resin-exchange capacity is a matter of concern to the operator and is due to several conditions:

Improper backwash. Blowoff of resin from the vessel during the backwash step can occur if too high a backwash flow rate is used. This flow rate is temperature dependent and must be regulated accordingly. Also, adequate time must be allotted for backwashing to insure a clean bed prior to chemical injection.

Channeling. Cleavage and furrowing of the resin bed can be caused by faulty operational procedures or a clogged bed or underdrain. This can mean that the solution being treated follows the path of least resistance, runs through these furrows, and fails to contact active groups in other parts of the bed.

Incorrect chemical application. Resin capacities can suffer when the regenerant is applied in a concentration that is too high or too low. Another important parameter to be considered during chemical application is the location of the regenerant distributor. Excessive dilution of the regenerant chemical can occur in the vessel if the distributor is located too high above the resin bed. A recommended

Mechanical strain. When broken beads and fines migrate to the top of the resin bed during service, mechanical strain is caused which results in channeling, increased pressure drop, or premature breakthrough. The combination of these

Resin Fouling. In addition to the physical causes of capacity losses listed previously, there are a number of chemically caused problems that merit attention, specifically the several forms of resin fouling that may be found.

Organic fouling occurs on anion resins when organics precipitate onto basic exchange sites. Regeneration efficiency is then lowered, thereby reducing the exchange capacity of the resin. Causes of organic fouling are fulvic, humic, or tannic acids or degradation products of DVB (divinylbenzene) cross linkage material of cation resins. The DVB is degraded through oxidation and causes irreversible fouling of downstream anion resins.

Iron fouling is caused by both forms of iron ions; the insoluble form will coat the resin bead surface and the soluble form can exchange and attach to exchange sites on the resin bead. These exchanged ions can be oxidized by subsequent cycles and

Silica fouling is the accumulation of insoluble silica on anion resins. It is caused by improper regeneration which allows the silicate (ionic form) to hydrolyze to soluble silicic acid which in turn polymerizes to form colloidal silicic acid with the beads. Silica fouling occurs in weak-base anion resins when they are regenerated with silica-laden waste caustic from the strongbase anion resin unless intermediate partial

Microbiological fouling (MB) becomes a potential problem when microbic growth is supported by organic compounds, ammonia, nitrates, and so on which are concentrated on the resin. Signs of MB fouling are increased pressure drops, plugged distributor laterals, and highly contaminated treated water.

Calcium sulfate fouling occurs when sulfuric acid is used to regenerate a cation exchanger after exhaustion by a water high in calcium. The precipitate of calcium sulfate (gypsum) that forms can cause calcium and sulfate leakage during subsequent service runs. Given a sufficient calcium input in the water to treat, calcium sulfate fouling is especially prevalent when the percent solution of regenerant is greater than 5 percent, or the temperature is greater than 100*F, or when the flow rate is less than 1 gpm/cu ft. Stepwise injection of sulfuric acid during regeneration can help prevent fouling.

Aluminum fouling of resins can appear when aluminum floe from alum or other coagulants in pretreatment are encountered by the resin bead. This floe coats the resin bead and in the ionic form will be exchanged. However, these ions are not efficiently removed during regeneration so the available exchange sites continuously decrease in number.

Copper fouling is found primarily in condensate polishing applications. Capacity loss is due to copper oxides coating the resin beads.

Oil fouling does not cause chemical degradation but gives loss of capacity due to filming on the resin beads and the reduction of their active surface. Agglomeration of beads also occurs causing increased pressure drop, channeling, and premature breakthrough. The oil-fouling problem can be alleviated by the use of surfactants.

OPERATIONAL SEQUENCING CONSIDERATIONS

The mode of operation for ion-exchange units can vary greatly from one system to the next, depending on the user's requirements. Service and regeneration cycles can be fully manual to totally automatic, with the method of regeneration being cocurrent, countercurrent, or external. The exhaustion phase is called the service run. This is followed by the regeneration phase which is necessary to bring the bed back to initial conditions to cycle. The regeneration phase includes four steps: backwashing to clean the bed, introduction of the excess regenerant, a slow rinse or displacement step to push the regenerant slowly through the bed, and finally a fast rinse to remove the excess regenerant from the resin and elute the unwanted ions to waste.

Service Cycle - The service cycle is normally terminated by one or a combination of the following criteria:

• High effluent conductivity.

• Total gallons throughput.

• Termination of the service cycle can be manually or automatically initiated.

Backwash Cycle - Normally, the first step in the regeneration sequence is designed to reverse flow from the service cycle using sufficient volume and flow rate to develop proper bed expansion for the purpose of removing suspended material (crud) trapped in the ion-exchange bed during the service cycle. The backwash waste water is collected by the raw water inlet distributor and diverted to waste via value sequencing. Backwash rate and internal design should avoid potential loss of whole bead resin during the backwash step. (Lower water temperature means more viscous force and more expansion.)

Regenerant Introduction - This introduction of regenerant chemicals can be cocurrent or countercurrent depending on effluent requirements, operating cost, and so on. Regenerant dosages (pounds per cubic foot), concentrations, flow rate, and contact time are determined for each application. The regenerant distribution and collection system must provide uniform contact throughout the bed and should avoid regenerant hideout. Additional effluent purity is obtained with countercurrent systems since the final resin contact in the service will be the most highly regenerated resin in the bed, creating a polishing effect.

Displacement Slow Rinse Cycle - The final steps in the regeneration sequence are generally terminated on acceptable quality. Displacement, which precedes the rinse step, is generally an extension of the regenerant introduction step. The displacement step is designed to give final contact with the resin, removing the bulk of the spent regenerant from the resin bed.

Fast Rinse Cycle - The fast rinse step is essentially the service cycle except that the effluent is diverted to waste until quality is proven. This final rinse is always in the same direction as the service flow. Therefore, in countercurrent systems the displacement flow and rinse flows will be in opposite directions.

SEQUENCE OF OPERATION: MIXED BED UNITS

In mixed-bed units, both the cation and the anion resins are mixed together thoroughly in the same vessel by compressed air. The cation and the anion resins being next to each other constitute an infinite number of cation and anion exchangers. The effluent quality obtainable from a well-designed and operated mixed-bed exchanger will readily produce demineralized water of conductivity less than 0.5 mmho and silica less than 10 ppb.

Service Cycle - As far as the mode of operation is concerned, the service cycle of a mixedbed unit is very similar to a conventional two-bed system, in that water flows into the top of the vessel, down through the bed, and the purified effluent comes out the bottom. It is in the regeneration and the preparation of it that the mixed-bed differs from the two-bed equipment. The resins must be separated, regenerated separately, and remixed for the next service cycle.

Backwash Cycle - Prior to regeneration, the cation and the anion resins are separated by backwashing at a flow rate of 3.0 to 3.5 gpm/ft. The separation occurs because of the difference in the density of the two types of resin. The cation resin, being heavier, settles on the bottom, while the anion resin, being lighter, settles on top of the cation resin. After backwashing, the bed is allowed to settle down for 5 to 10 minutes and two clearly distinct layers are formed. After separation, the two resins are independently regenerated.

Regenerant Introduction - The anion resin is regenerated with caustic flowing downward from the distributor placed just above the bed, while the cation resin is regenerated with either hydrochloric or sulfuric acid, usually flowing upward. The spent acid and caustic are collected in the interface collector, situated at the interface of the two resins. The regenerant injection can be carried out simultaneously as described or sequentially. In sequential regeneration, the cation-resin regeneration should precede the anion-resin regeneration to prevent the possibility of calcium carbonate and magnesium hydroxide precipitation, which may occur because of the anion-regeneration waste coming in contact with the exhausted cation resin. If this precipitation occurs, it can foul the resins at the interface. This becomes very critical when only the mixed-bed exchanger is installed to demineralize the incoming raw water.

In the case of sequential regeneration, during the caustic and acid injection period, a blocking flow of the demineralized water is provided in the opposite direction of the regenerant injection. This is required to prevent the caustic from entering the cation resin and acid from entering the anion resin. When regeneration is carried out simultaneously, acid and caustic injection flows act like blocking flows to each other and no additional blocking flow with water is needed. In a few sequential-type regeneration systems, acid is injected to flow downward through the central interface collector which now also acts as an acid distributor.

Rinsing and Air Mix Cycles - After completion of the acid and caustic injection, both the cation and anion resins are rinsed slowly to remove the majority of the regenerant, without attempting to eliminate it completely. After the use of 7 to 10 gallons of slow rinse volume per cubic foot of each type of resin, the unit is drained to lower the water to a few inches above the resin bed. The resins are now remixed with an upflow of air. After remixing, the unit is filled completely with water flowing slowly from the top, to prevent anion-resin separation in the upper layers. The mixed-bed exchanger is then rinsed at fast flow rates. The conductivity of the effluent water may be very high for a few minutes and will then drop suddenly to the value usually observed in the service cycle. This phenomenon is characteristic of mixed beds and is due to the absorption of the remaining acid or caustic in different parts of the bed, by one or the other resin. This, no doubt, results in the loss of resin capacity, but this loss is negligible as compared to the length of the service cycle and the savings in the overall time required for regeneration.

392 WATER AND WASTEWATER TREATMENT TECHNOLOGIES SEQUENCE OF OPERATION: SOFTENER UNITS

Following are the basic steps involved in a regeneration of a water softener.

Backwashing - After exhaustion, the bed is backwashed to effect a 50 percent minimum bed expansion to release any trapped air from the air pockets, minimize the compactness of the bed, reclassify the resin particles, and purge the bed of any suspended insoluble material. Backwashing is normally carried out at 5-6 gpm/ft. However, the backwash flow rates are directly proportional to the temperature of water.

Brine Iiyection - After backwashing, a 5 percent to 10 percent brine solution is injected during a 30-minute period. The maximum exchange capacity of the resin is restored with 10 percent strength of brine solution. The brine is injected through a separate distributor placed slightly above the resin bed.

Displacement or Slow Rinse - After brine injection, the salt solution remaining inside the vessel is displaced slowly, at the same rate as the brine injection rate. The slow rinsing should be continued for at least 15 minutes and the slow rinse volume should not be less than 10 gallons/cu ft of the resin. The actual duration of the slow rinse should be based on the greater of these two parameters.

Fast Rinse - Rinsing is carried out to remove excessive brine from the resin. The rinsing operation is generally stopped when the effluent chloride concentration is less than 5-10 ppm in excess of the influent chloride concentration and the hardness is equal to or less than 1 ppm as CaCO.

Each arrangement will vary substantially in both operating and installed costs. Important factors for selection are:

1. Influent water analysis.

3. Effluent quality.

4. Waste requirements.

5. Operating cost.

SELECTIVITY AND GENERAL CONSIDERATIONS

As noted, ion exchange reactions are stoichiometric and reversible, and in that way they are similar to other solution phase reactions. For example:

In this reaction, the nickel ions of the nickel sulfate (NiS04) are exchanged for the calcium ions of the calcium hydroxide [Ca(OH)2 ] molecule. Similarly, a resin with hydrogen ions available for exchange will exchange those ions for nickel ions from solution. The reaction can be written as follows:

R indicates the organic portion of the resin and S03 is the immobile portion of the ion active group. Two resin sites are needed for nickel ions with a plus 2 valence (Ni+2). Trivalent ferric ions would require three resin sites. As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference, or selectivity, for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient. K. which in its simplest form for the reaction

is expressed as: K = (concentration of B+ in resin/concentration of A+ in resin) x (concentration of A+ in solution/concentration of B+ in solution).

The selectivity coefficient expresses the relative distribution of the ions when a resin in the A+ form is placed in a solution containing B+ ions. Table 2 gives the selectivity's of strong acid and strong base ion exchange resins for various ionic compounds. It should be pointed out that the selectivity coefficient is not constant but varies with changes in solution conditions. It does provide a means of determining what to expect when various ions are involved. As indicated in Table 2, strong acid resins have a preference for nickel over hydrogen. Despite this preference, the resin can be converted back to the hydrogen form by contact with a concentrated solution of sulfuric acid (H2S04):

As we noted above, but a little differently, this step is known as regeneration. In general terms, the higher the preference a resin exhibits for a particular ion, the greater the exchange efficiency in terms of resin capacity for removal of that ion from solution. Greater preference for a particular ion, however, will result in increased consumption of chemicals for regeneration.

Resins currently available exhibit a range of selectivity's and thus have broad application. As an example, for a strong acid resin, the relative preference for divalent calcium ions (Ca+2) over divalent copper ions (Cu+2) is approximately 1.5

to 1. For a heavy-metal-selective resin, the preference is reversed and favors copper by a ratio of 2.300 to 1.

Table 2. Selectivity of ion Exchange Resins in Order of Decreasing Preference.

Strong acid cation exchanger

Strong base anion exchanger

Barium

Iodide

Lead

Nitrate

Calcium

Bisulfate

Nickel

Chloride

Cadmium

Cyanide

Copper

Bicarbonate

Zinc

Hydroxide

Magnesium

Fluoride

Potassium

Sulfate

Ammonia Sodium

Hydrogen

Ion exchange resins are classified as cation exchangers, which have positively charged mobile ions available for exchange, and anion exchangers, whose exchangeable ions are negatively charged. Both anion and cation resins are produced from the same basic organic polymers. They differ in the ionizable group attached to the hydrocarbon network. It is this functional group that determines the chemical behavior of the resin. Resins can be broadly classified as strong or weak acid cation exchangers or strong or weak base anion exchangers.

Strong acid resins are so named because their chemical behavior is similar to that of a strong acid. The resins are highly ionized in both the acid (R-S03H) and salt (R-SOjNa) form. They can convert a metal salt to the corresponding acid by the reaction:

The hydrogen and sodium forms of strong acid resins are highly dissociated and the exchangeable Na+ and H+ are readily available for exchange over the entire pH range. Consequently, the exchange capacity of strong acid resins is independent of solution pH. These resins would be used in the hydrogen form for complete deionization; they are used in the sodium form for water softening (calcium and magnesium removal). After exhaustion, the resin is converted back to the hydrogen form (regenerated) by contact with a strong acid solution, or the resin can be convened to the sodium form with a sodium chloride solution. In the above, the hydrochloric acid (HC1) regeneration would result in a concentrated nickel chloride (NiCl,) solution.

In a weak acid resin, the ionizable group is a carboxylic acid (COOH) as opposed to the sulfonic acid group (S03H) used in strong acid resins. These resins behave similarly to weak organic acids that are weakly dissociated. Weak acid resins exhibit a much higher affinity for hydrogen ions than do strong acid resins. This characteristic allows for regeneration to the hydrogen form with significantly less acid than is required for strong acid resins. Almost complete regeneration can be accomplished with stoichiometric amounts of acid. The degree of dissociation of a weak acid resin is strongly influenced by the solution pH. Consequently, resin capacity depends in part on solution pH. Figure 1 shows that a typical weak acid resin has limited capacity below a pH of 6.0. making it unsuitable for deionizing acidic metal finishing wastewater.

Like strong acid resins, strong base resins are highly ionized and can be used over the entire pH range. These resins are used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid solution to pure water:

Regeneration with concentrated sodium hydroxide (NaOH) converts the exhausted resin to the hydroxide form.

Weak base resins are like weak acid resins, in that the degree of ionization is strongly influenced by pH. Consequently, weak base resins exhibit minimum exchange capacity above a pH of 7.0. These resins merely sorb strong acids: they cannot split salts.

In an ion exchange wastewater deionization unit, the wastewater would pass first through a bed of strong acid resin. Replacement of the metal cations (Ni+2. Cu+2) With hydrogen ions would lower the solution pH. The anions (S04"2. CI") can then be removed with a weak base resin because the entering wastewater will normally be acidic and weak base resins sorb acids. Weak base resins are preferred over strong base resins because they require less regenerant chemical. A reaction between the resin in the free base form and HC1 would proceed as follows:

The weak base resin does not have a hydroxide ion form as does the strong base resin. Consequently, regeneration needs only to neutralize the absorbed acid: it need not provide hydroxide ions. Less expensive weakly basic reagents such as ammonia (NH3) or sodium carbonate can be employed. Chelating resins behave similarly to weak acid cation resins but exhibit a high degree of selectivity for heavy metal cations. Chelating resins are analogous to chelating compounds found in metal finishing wastewater; that is, they tend to form stable complexes with the heavy metals. In fact, the functional group used in these resins is an EDTAa compound. The resin structure in the sodium form is expressed as R-EDTA-Na. The high degree of selectivity for heavy metals permits separation of these ionic compounds from solutions containing high background levels of calcium, magnesium, and sodium ions. A chelating resin exhibits greater selectivity for heavy metals in its sodium form than in its hydrogen form. Regeneration properties are similar to those of a weak acid resin; the chelating resin can be converted to the hydrogen form with slightly greater than stoichiometric doses of acid because of the fortunate tendency of the heavy metal complex to become less stable under low pH conditions. Potential applications of the chelating resin include polishing to lower the heavy metal concentration in the effluent from a hydroxide treatment process or directly removing toxic heavy metal cations from wastewaters containing a high concentration of nontoxic, multivalent cations. Table 3 shows the preference of a commercially available chelating resin for heavy metal cations over calcium ions. (The chelating resins exhibit a similar magnitude of selectivity for heavy metals over sodium or magnesium ions.) The selectivity coefficient defines the relative preference the resin exhibits for different ions. The preference for copper (shown in Table 3) is 2300 times that for calcium. Therefore, when a solution is treated that contains equal molar concentrations of copper and calcium ions, at equilibrium, the molar concentration of copper ions on the resin will be 2300 times the concentration of calcium ions. Or, when solution is treated that contains a calcium ion molar concentration 2300 times that of the copper ion concentration, at equilibrium, the resin would hold an equal concentration of copper and calcium.

Table 3. Chelating Cation Resin Selectivities for Metal Ions

Metal

KM/Caa

Hg+2

2800

Cu+2

2300

Pb+2

1200

Ni+2

57

Metal

KM/Caa

Zn+2

17

Cd+2

15

Co+2

6.7

Fe+2

4.7

Mn+2

1.2

Ca+2

1

a — Selectivity coefficient for the metal over calcium ions at a pH of 4.

a — Selectivity coefficient for the metal over calcium ions at a pH of 4.

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