Ion Exchange Processes

TECHNIFAX® Ion exchange processes TF Ion exchange is an effective, Table 1 — Types of ion exchange ION EXCHANGE -24 versatile means of conditioning MATERIALS materials boiler feedwater. The term “ion Ion exchange processes are Capacity exchange” describes the process: Exchangers kgr/ft3 versatile — specific types of ions as water flows through a bed of can be removed from water CATION ion exchange material, undesir- Inorganic (zeolites) able ions are removed and depending on the choice of Natural (greensand) 3–5 replaced with less objectionable exchange material and regenerant Synthetic 12–16 used. ones. Organic Sulfonated coal — 5–7 For example, in softening pro- (carbonaceous) cesses, calcium and magnesium Development Synthetic – (phenolic types) 6–18 ions (hardness) are exchanged (styrene base) 20–30 Slightly more than 100 years ago, for sodium ions. In dealkali- two English agricultural chemists, ANION zation, the ions contributing to H. S. Thompson and J. Thomas Inorganic Not widely alkalinity (carbonate, bicarbon- Metallic oxides used Way, noted that certain soils had ate, etc.) are removed and a greater ability than others to Organic replaced with chloride ions. absorb ammonia from fertilizers. Synthetic resins 10–22 Other dealkalization processes They found that complex silicates utilizing weak acid cation resin in the soil performed an ion or strong acid cation resin in a exchange function. They were split stream process, exchange Cation exchangers able to prepare materials of this cations with hydrogen. This There has been constant improve- type in the laboratory from forms carbonic acid which can ment in ion exchange materials solutions of sodium aluminate be removed in a decarbonator since the first use of natural and and sodium silicate. In 1906, tower. synthetic inorganic products. Robert Gans used materials of this Sulfonated coal, styrene-base Demineralization is simply type for softening water. The early resins, phenolic resins and acrylic replacing all cations with hy- materials used were slow in resins are some that have been drogen ions (H+) and all anions regenerating and lacked physical developed. Exchange capacities with hydroxide ions (OH–). Ion stability. These first synthetic, were greatly increased with the exchange materials are like inorganic exchangers were called development of the styrene-base storage batteries; they must be zeolites. Today, zeolites are exchangers. These resins are recharged (regenerated) periodi- almost totally replaced by syn- manufactured in spherical, stress cally to restore their exchange thetic ion exchange materials. The and strain-free form to resist capacity. With proper design basic types of ion exchangers in physical degradation. They are and operation, ion exchange use for water conditioning are stable at temperatures as high as processes are capable of remov- listed in Table 1. 300°F and are applicable over a ing selected ions almost com- wide pH range. More dense pletely (in some cases to a fraction of a part per million).

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Registered Trademarks of Nalco Chemical Company ©1978, 1998 Nalco Chemical Company All Rights Reserved Printed in U.S.A. 1-98 resins, those having greater Table 2 — Types of ion exchange processes degrees of copolymer cross- linking, were specially developed Typical Minerals Types of Minerals in Influent Exchanger Converted to for heavy duty industrial applications. These products are more Cation (A) Ca(HCO ) → Na+ → NaHCO resistant to degradation by 3 2 3 CaSO → Exchanger → Na SO oxidizing agents such as chlorine, 4 2 4 and withstand physical stresses that fracture lighter duty materials. Cation (B) Ca(HCO ) → H+ → H CO Weakly acidic cation exchange 3 2 2 3 CaSO → Exchanger → H SO resins contain carboxylic and 4 2 4 phenolic groups. They remove Cation alkalinity by exchanging their → → (C) Ca(HCO3)2 H+ H2CO3 hydrogen ions for the cations Exchanger associated with the bicarbonate (Weak Acid) ion (calcium, magnesium, and sodium bicarbonates). Being Anion weakly acidic, they will not affect → – → (D) Na2SO4 Cl NaCl → → the cations associated with the NaHCO3 Exchanger NaCl anions of strong acids (chlorides or sulfates). Because of almost Anion 100% utilization of the regenerant → – → (E) H2CO3 OH H2O → → acid, chemical operating costs H2SO4 Exchanger H2O will be at a minimum, and there Conventional softening — process (A) will be little excess acid to produce Dealkalization by split stream softening — blending effluents from (A) and (B) objectionable waste effluents. Dealkalization by anion exchange — process (D) proceded by (A) Dealkalization by weak acid cation exchanger followed by conventional softening process (C) followed by (A) Anion exchangers Demineralizing — combination of (B) and (E) Anion exchange materials are classified as either weak base or strong base depending on the type of exchange group. Weak more economical removal of exchanger is more than 25% of base resins act as acid adsorbers, sulfates and chlorides. the total anions. When there is efficiently removing strong acids contamination of the water with These are two general classes of organic matter, a more porous such as sulfuric and hydrochloric. strong base anion exchangers, However, they will not remove form of Type I resin is recom- Types I and II, denoting differ- mended. The Type II anion carbon dioxide or silica. They are ences in chemical nature. Both used in systems where strong material is used in treating waters remove silica and carbon dioxide where the combined carbon acids predominate, where silica as well as other anions. Type I is reduction is not required, and dioxide and silica content is less more effective in removing silica, than 25% of the total anions. This where carbon dioxide is removed and is used when the combined in degasifiers. Preceding strong is often the case when carbon silica and carbon dioxide content dioxide is taken out in a degasifier base units in demineralizing of the water contacting the processes, weak base resins give ahead of the anion exchanger unit.

2 Physical Characteristics of Resin Anion and cation resins can be obtained in several different physical forms. They can be obtained with different ions located on the exchange site. This has importance in applications such as mixed beds where minimal leakage rates are required even from a newly installed bed. Particle size can also be specified. Uniform particle sized (UPS) resins are now available where all beads fit into a very close particle size range. For practical purposes, all of the beads are the same size. Beds of UPS resins have some unique operating characteristics which offer advan- tages when they are used in mixed bed, layered bed, and packed bed applications. Figure 1 — Typical ion exchange unit Macroporous resins are highly porous which give them advan- Equipment operation Several newer designs for ion tages when used in processes that Ion exchange material is housed exchange are now coming into have high fouling potential. in specially constructed tanks common use. These are becoming (Figure 1) where it forms a bed, popular because of their advan- usually 30–60 inches deep. In tages of higher operating efficien- ION EXCHANGE some cases, it is supported by cies and lower leakage rates. In PROCESSES another bed of graded gravel or countercurrent regeneration Ion exchange processes fall into anthracite filter media. In other procedures, the regenerant flow is several categories: softening cases, some special methods of opposite in direction to the service (including removal of iron and support, without gravel or anthra- water flow. Therefore, the resin manganese), dealkalization, and cite, are used. During normal located in the bed where the demineralization. Examples of operation, water enters the top of finished water leaves the vessel, is these processes are listed in the tank through a pipe, which the most highly regenerated. This Table 2. distributes it over the surface of results in lower leakage rates and the exchanger bed. The treated slightly higher operating capaci- water is drawn off by collector ties at equal regenerant dosages to piping at the bottom. cocurrent operated vessels.

3 R–2 Ca+2 R– H+

R–2 Mg+2

R– Na+

R–2 Ca+2 –2 +2 R– H+ R Mg R– Na+

R–2 Mg+2 R–2 Ca+2 R–2 Ca+2 R– Na+

R–2 Mg+2

R– Na+

– + R– H+ R H

Cocurrent vs countercurrent: With downflow operation, ion distribution at end of operating cycle is the same for cocurrent and countercurrent operation. But at end of regenerating cycle, ion distribution is revised when cocurrent is compared with countercurrent regeneration. Key to optimum results is quality of rinse water used with countercurrent regeneration.

Figure 2 — Cocurrent vs. countercurrent regeneration

4 However, countercurrent vessels are more sensitive to operating problems. The bed must be immobilized during the regeneration process and the influent water must be very low in suspended solids. Figure 2 illustrates what happens in the two regeneration modes at exhaustion and after regeneration. Packed bed systems essentially fill the vessel with resin. The systems are countercurrent in design and offer the low leakage rate advantage. However, packed beds also offer the advantage of Figure 3 — Valve arrangements for regeneration with salt reduced waste generation. Since there is no space for proper backwash, packed bed systems usually are built with external Ion exchange units are usually density of the ion exchanger. backwash tanks which allow the installed in duplicate to permit Figure 5 shows the expansion resin to be backwashed after it is continuous service during characteristics of typical cation sluiced out of the operating regeneration. Some typical and anion resins. equipment arrangements are vessel. All countercurrent ion The capacity of ion exchange shown in Figure 4. exchange systems require material varies according to the feedwater that is very low in amount and concentration of suspended solids. regenerating chemical used and Regeneration procedures Regeneration of the exchange the time the chemical contacts the material involves three steps: When regeneration is ineffective, exchanger. Table 3 shows this backwash, introduction of the the bed is usually fouled with variation in capacity with regeneration chemicals, and suspended matter. This empha- regenerant dosages. rinse. sizes the importance of proper backwash procedures. During Selecting the optimum dosage Figure 3 shows valve arrange- backwash, the cation exchanger level depends mainly on the ments on a salt regenerated unit. bed should expand at least 50%, quality of finished water re- Backwash is simply a reversal of while the anion exchanger bed quired, considering both eco- the normal flow to wash out any should expand at least 75%. nomic and operating factors. suspended matter in the bed and to “fluff” the bed, to break up How much the bed expands packed areas. This is done just depends on the backwash water before the unit is regenerated. temperature, backwash rate and During regeneration, chemicals are introduced at the top surface of the bed and removed through the bottom outlet. The rinse washes out the last traces of regenerant chemical.

5 KEY: WAC – Weak acid cation WBA – Weak base anion SAC – Strong acid cation SBA – Strong base anion

Figure 4 — Typical equipment arrangements

6 CONCLUSIONS The use of ion exchange processes affords numerous efficient and effective means of conditioning feedwater. The proper selection of the specific ion exchange process depends on water quality needs, operating convenience, and economic considerations. For effective results, the system must be carefully selected, designed, Figure 5 — Expansion characteristics of exchange beds operated and maintained. Be- cause the decision is complex, an experienced ion exchange Table 3 — Effect of regenerant levels on exchange capacity engineer should be consulted to Regenerant Operating assist in selection and design. Exchanger Dosage Capacity Type Regenerant (lb/ft3) (kgr/ft3) Cation (high capacity) Salt 6 – 8 20 – 24 10 – 15 25 – 30 Cation (high capacity) Sulfuric acid 4 – 6 10 – 12 8 – 10 14 – 16 Anion (weak base) Ammonia 1.5 – 2 20 – 22 Caustic 3 – 4 20 – 22 Soda ash 3 – 5 12 – 15 Anion (strong base) Caustic 3.5 – 5 10 – 12

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