Ion Exchange This chapter discusses the unit process of ion exchange, including topics such as ion exchange reactions, unit operations of ion exchange, sodium and hydrogen cycles, regeneration, and design of ion exchangers. Some of these topics have already been discussed in the various sections of the unit operations in Part II and will only be incorporated into this chapter by reference. 16.1 ION EXCHANGE REACTIONS Ion exchange is the displacement of one ion by another. The displaced ion is originally a part of an insoluble material, and the displacing ion is originally in solution. At the completion of the process, the two ions are in reversed places: the displaced ion moves into solution and the displacing ion becomes a part of the insoluble material. Two types of ion exchange materials are used: the cation exchange material and the anion exchange material. The cation exchange material exchanges cations, while the anion exchange material exchanges anions. The insoluble part of the exchange material is called the host . If R − n represents the host part and C + m the exchangeable cation, the cation exchange material may be represented by ( R − n ) r ( C + m ) rn / m , where r is the number of active sites in the insoluble material, rn / m is the number of charged exchangeable particles attached to the host material, − n is the charge of the host, and + m is the charge of the exchangeable cation. On the other hand, if R + o represents the host part of the anion exchange material and A − p its exchangeable anion, the exchange material may be represented by ( R + o ) r ( A − p ) ro / p , where the subscripts and superscripts are similarly defined as those for the cation exchange material. Letting be the displacing cation from solution, the cation exchange reaction is (16.1) Also, letting be the displacing anion from solution, the anion exchange reaction may be represented by (16.2) As shown by the previous equations, ion exchange reactions are governed by equilibrium. For this reason, effluents from ion exchange processes never yield pure water. 16 C s +q R −n () r C +m () rn/m rn q C s +q + R −n () r C s +q () rn/q rn m C +m + A s −t R +o () r A −p () ro/ p ro t A s −t + R +o () r A s −t () ro/t ro p A −p + TX249_frame_C16.fm Page 719 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero 720 Table 16.1 shows the displacement series for ion exchange materials. When an ion species high in the table is in solution, it can displace ion species in the insoluble material below it in the table and, thus, be removed from solution. As noted in this table, to remove any cation in solution, the displaceable cation must be the proton H + ; and to remove any anion, the displaceable anion must be the hydroxyl ion OH − . Originally, natural and synthetic alumino silicates, called zeolites , were the only ones used as exchange materials. Presently, they have been largely replaced by synthetic resins. Synthetic resins are insoluble polymers to which are added, by certain chemical reactions, acidic and basic groups called functional groups . These groups are capable of performing reversible exchange reactions with ions in solution. The total number of these groups determines the exchange capacity of the exchange material, while the type of functional group determines ion selectivity. When the exchange capacity of the exchange material is exhausted, the exchanger may be regenerated by the reverse reactions above. The principles of regeneration are discussed in the section on “Sodium, Hydrogen Cycle, and Regeneration.” 16.2 UNIT OPERATIONS OF ION EXCHANGE Figure 16.1 shows the schematics of the unit operations of ion exchange. Figure 16.1a shows a cation exchanger and Figure 16.1b shows an anion exchanger. In both units, the influent is introduced at the top of the vessel. The bed of ion exchanger materials would be inside the vessels, where, as the water to be treated passes through, exchange TABLE 16.1 Displacement Series for Ion Exchange La 3 + Y 3 + Ba 2 + Pb 2 + Sr 2 + Ca 2 + Ni 2 + I − Cd 2 + Br − Cu 2 + Cl − Zn 2 + F − Mg 2 + OH − Ag + — Cs + — Rb + — K + — — Na + — Li + — H + — SO 4 2− CrO 4 2− NO 3 − AsO 4 −3 PO 4 −3 MoO 4 −2 NH 4 + TX249_frame_C16.fm Page 720 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero 721 of ions takes place. This exchange of ions is the chemical reaction of the unit process of ion exchange; the mere physical passing through of the water with the attendant head loss and pumping consideration is the unit operation of ion exchange. The unit operations of head losses are similar to those of granular filtration discussed in Part II. The unit operation of pumping was also discussed in Part II. After ion exchange, product waters are withdrawn at the respective bottoms of the vessels. 16.3 SODIUM, HYDROGEN CYCLE, AND REGENERATION As shown in Table 16.1, sodium, lithium, and hydrogen are the logical choices for the exchangeable ions. In practice, however, sodium and hydrogen are the ions of choice. The cation exchange resin using sodium may be represented by ( R − n ) r (Na + ) rn . Its exchange reaction with Ca +2 and similar cations is shown below: (16.3) As shown, Ca +2 has become embedded in the resin, thus removed from solution, and Na + has become solubilized. Similar reactions may be formulated for the rest of the ions in Table 16.1. As soon as the resin is exhausted, it may be regenerated. As shown in Equation (16.3), by the Law of Mass Action , the reaction may be driven to the left by increasing the concentration of the sodium ion on the right. In practice, this is what is actually done. The resin is regenerated by using a concentration of NaCl of about 5 to 10%, thus, driving the reaction to the left. Operations where regeneration is done using NaCl is said to run on the sodium cycle . Regeneration may also be made using acids, FIGURE 16.1 Unit operations of ion exchange. Cation exchanger Anion exchanger Cation waste stream Regenerant Backwash waste (Rinsing waste) Regenerant waste stream (cation salts) Regenerant Backwash waste (Rinsing waste) NaCI (sodium cycle) or HCI; H 2 SO 4 (acid cycle) NaOH or NH 4 OH Anionic waste Backwash (and rinsing) Sodium salts (sodium cycle) or Acids (hydrogen cycle) Regenerant waste Backwash (and rinsing) (a) Cation exchanger (b) Anion exchanger R −n () r Na + () rn rn 2 Ca +2 + R −n () r Ca +2 () rn/2 rnNa + + TX249_frame_C16.fm Page 721 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero 722 such as H 2 SO 4 . When regeneration is through the use of acids, the cycle is called the hydrogen cycle (from the proton or hydrogen ion content of acids). Table 16.2 shows approximate exchange capacities and regeneration require- ments for ion exchangers. As shown, the values have great ranges. Thus, in practice, one must have to perform an actual experiment or obtain data from the manufacturer for a particular ion exchanger to determine the exchange capacity and regeneration requirement. The capacity of an ion exchanger in terms of volume of influent treated varies with the nature and concentration of ions in solution. This is much the same as the characteristics of activated carbon. Hence, the experimental procedure is practically the same as that of activated carbon. Tables 16.3 and 16.4 show some additional properties of exchangers. The acidic exchangers are cationic exchangers. They are called acidic because their exchange sites are negatively charged to which the H + ion can attach, hence, acidic. The strongly acidic cation exchangers readily remove cations from solutions, while the weakly acidic exchangers will remove ions such as calcium and magnesium but have limited ability to remove sodium and potassium, which are way down the table in the dis- placement series. The basic exchangers, on the other hand, are anionic. They have positively charged exchange sites to which the hydroxyl ion can attach and other basic species such the quaternary and amine groups. The strongly basic exchanger can readily remove all anions. The weakly basic ones remove mainly the anions of strong acids such as , Cl − , and . 16.4 PRODUCTION OF “PURE WATER” Theoretically, it would seem possible to produce pure water by combining the cation exchanger operating on the hydrogen cycle and the anion exchanger operating on the OH cycle. This is shown in the following discussions. Let Equation (16.1) be written specifically for the hydrogen cycle. The resulting equation is (16.4) TABLE 16.2 General Properties of Ion Exchangers Exchanger, cycle Exchange Capacity, Regenerant Regenerant Requirement, Cation exchangers: Natural zeolite, Na 175–350 NaCl 3–6 Synthetic zeolite, Na 350–700 NaCl 2–3 Resin, Na 350–1760 NaCl 1.8–3.6 Resin, H 350–1760 H 2 SO 4 2–4 Anion exchanger: Resin, OH 700–1050 NaOH 5–8 geq m 3 geq m 3 SO 4 2− NO 3 − R −n () r H + () rn rn q C s +q R −n () r C s +q () rn/q rnH + ++ TX249_frame_C16.fm Page 722 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero From this equation, the number of reference species is rn/q(q), based on the cation in solution; and the equivalent mass of species is TABLE 16.3 Some Additional Properties of Cation Exchangers Material Exchange Capacity, , Average Packed Density, , Average Particle Shape Strongly Acidic: Sulfonated polystyrene: Homogeneous resin: 1% cross-linked 5.4 750 Spherical 2% cross-linked 5.5 720 Spherical 4% cross-linked 5.2 800 Spherical 5–6% cross-linked 5.0 810 Spherical 8–10% cross-linked 4.9 855 Spherical 12% cross-linked 5.1 840 Spherical 14% cross-linked 4.6 940 Spherical 16% cross-linked 4.9 860 Spherical 20% cross-linked 3.9 840 Spherical Macroporous 4.8 790 Spherical Sulfonated phenolic resins 2.4 800 Granular Resins from phenol methylene sulfuric acid 2.9 730 Granular Sulfonated coal 1.6 430 Granular Weakly Acidic: Acrylic or meta acrylic: Homogeneous resin: 5% cross-linked 10.0 720 Spherical 10% cross-linked 6.5 750 Spherical Macroporous 8.0 745 Spherical Phenolic and related condensation products 2.5 720 Granular Polystyrene phosphonic acid 6.6 735 Granular Polystyrene iminodiacetate 2.9 735 Spherical Inorganic materials: Greensand 0.14 1280 Granular Aluminum silicate 1.4 800 Granular Celluloses: Phosphonic, low capacity 1.0 — Fiber Phosphonic, high capacity 7.0 — Granular Methyl carboxylic 0.7 — Fiber dry meq gm kg m 3 C s +q rn q C s +q rn q q() C s +q q = TX249_frame_C16.fm Page 723 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero Letting the molar concentration of be gmol/L, the corresponding con- centration in geq/L is Note: From , the units of q is equivalents per mole. Therefore, the total concentration in gram equivalents per liter of removable cations in solution, [CatT] eq , is the sum of all the cations. Let there be a total of i cations. Thus, (16.5) As [CatT] eq of cations is removed from solution, a corresponding number of equiv- alent concentrations of anions pair with the H + ions displaced from the cation bed. TABLE 16.4 Some Additional Properties of Anion Exchangers Material Exchange Capacity, , Average Packed Density, , Average Particle Shape Strongly Basic: Polystyrene matrix: Trimethyl benzene ammonium: 1% cross-linked 3.2 700 Spherical 2% cross-linked 3.5 700 Spherical 4% cross-linked 4.0 670 Spherical 8% cross-linked 3.5 720 Spherical Dimethyl hydroxyethyl benzyl ammonium 1–4% cross-linked 3.2 705 Spherical 6% cross-linked 3.1 705 Spherical 8% cross-linked 3.4 705 Spherical 10–12% cross-linked 3.0 705 Spherical Condensation products with pyridium quaternary amine 4.0 800 Spherical Weakly Basic: Aminopolystyrene 5.6 690 Spherical Mixed aliphatic amine and quaternary ammonium 3.7 900 Granular Epoxy polyamine 8.5 740 Spherical dry meq gm kg m 3 C s +q C s +q [] C s +q []C s +q C s +q q qC s +q []= qC s +q [] CatT[] eq q i C s i +q i [] i=1 i=m ∑ = TX249_frame_C16.fm Page 724 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero Let [AnionT] eq and [HT ] eq be the total anions and the hydrogen ions displaced, respectively. Since the number of equivalents of one substance in a reaction is equal to the number of equivalents of all the other substances participating in the reaction, (16.6) Let the [AnionT ] eq from the effluent of the cation exchanger be introduced into an anion exchanger. For the anion exchanger operating under the OH cycle, the total equivalents of OH − released from the anion bed is equal to that of the anions, [AnionT] eq , removed from solution. Let [OHT ] eq be this total OH − . Since [AnionT ] eq is equal to [HT] eq , [OHT ] eq must be equal to [HT ] eq . This means that all the acids produced in the cation exchanger are neutralized in the anion exchanger, and all ions in the water have been removed by using the combination of cation exchanger followed by anion exchanger. On the surface, the combination of cation exchanger and anion exchanger would mean that pure water is produced. As shown in Equations (16.1) and (16.2), however, the unit process of ion exchange is governed by equilibrium constants. The values of these constants depend upon how tightly the removed ions from solution are bound to the bed exchanger sites. In general, however, by the nature of equilibrium constants, the concentrations of the affected solutes in solution are extremely small. Practically, then, we may say that “pure water” has been produced. By analogy with Equation (16.5), (16.7) As with q i , the units of t i are equivalents per mole. Example 16.1 A wastewater contains the following ions: = 120 mg/L, Cu 2+ = 30 mg/L, Zn 2+ = 15 mg/L, and . Calculate the total equiv- alents of cations and anions, assuming the volume of the wastewater is 450 m 3 . Solution: Total equivalents of cations = 2.395(450) = 1077.75 Ans Total equivalents of cations = 2.069(450) = 931.05 Ans Ions (mg/L) Equiv. Mass Cations (meq/L) Anions (meq/L) 58 a — 2.069 b 31.75 0.945 — 32.7 0.469 — 29.35 0.681 — ∑ = 2.395 ∑ = 2.069 Note: The ions are not balanced, meaning error in analysis. a Equiv. mass . b 120/58 = 2.069 AnionT[] eq HT[] eq CatT[] eq == AnionT[] eq t i A s i −t i [] i=1 i=m ∑ = CrO 4 2− Ni 2+ 20 mg/L= CrO 4 2− 120= Cu 2+ 30= Zn 2+ 15= Ni 2+ 20= CrO 4 2− 52 4 16()+[]/258== TX249_frame_C16.fm Page 725 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero 16.5 ACTIVE OR EXCHANGE ZONE Figure 16.2 is the same figure illustrated in a previous chapter under carbon adsorp- tion. The length of the active zone was derived in that chapter and is reproduced next. (16.8) where δ = length of active zone = total volume of water or wastewater treated at complete exhaustion of bed = volume treated at breakthrough [C o ] = influent concentration to δ = total volume treated at time t n+1 = total volume treated at time t n [C n+1 ] = concentration of solute at effluent of δ at time t n+1 [C n ] = concentration of solute at effluent of δ at time t n A s = surficial area of exchanger bed FIGURE 16.2 Active zones at various times during adsorption and the breakthrough curve. δ 2 V x V b –()C o []∑ V n+1 V n –() C n+1 []C n []+ 2 – A s ρ p X M ult = V x V b V n+1 V n C C C C C C V V V L Clean water TX249_frame_C16.fm Page 726 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero ρ p = pack density of ion exchange material (X/M) ult = ultimate exchange capacity of the bed or simply, the exchange capacity of the bed. It should be emphasized that to use the equation [C o ], [C n+1 ], and [C n ] should be the total concentration of ions. For example, if the influent is composed of the ions Ca 2+ = 50 mg/L, Mg 2+ = 60 mg/L, and Zn 2+ = 2 mg/L, then [C o ] meq in meq/L is 50/(Ca/2) + 60/(Mg/2) + 2/(Zn/2). Example 16.2 A breakthrough experiment is conducted for a wastewater pro- ducing the results below. Determine the length δ of the active zone. The diameter of the column used is 2.5 cm, and the packed density of the bed is 750 kg/m 3 . [C o ] is equal to 2.2 meq/L. (X/M) ult = 6.5 meq/g. Solution: C, meq/L , L 0.06 1.0 0.08 1.20 0.09 1.30 0.10 1.40 0.20 1.48 0.46 1.58 1.30 1.70 1.80 1.85 2.10 2.00 C, meq/L , L () 0.06 1.0 0.20 0.07 0.014 0.08 1.20 0.10 0.085 0.0085 0.09 1.30 0.10 0.095 0.0095 0.10 1.40 0.08 0.15 0.012 0.20 1.48 0.10 0.33 0.033 0.46 1.58 0.12 0.88 0.1056 1.30 1.70 0.15 1.55 0.2325 1.80 1.85 0.15 1.95 0.2925 2.10 2.00 ∑ = 0.7076 V δ 2 V x V b –()C o []∑ V n+1 V n –() C n+1 []C n []+ 2 – A s ρ p X M ult = V V n+1 V n – C n +1 []C n []+ 2 ( V n+1 V n – ) C n++ ++ 1 []C n []+ 2 A s π 0.025() 2 4 0.00049 m 2 == TX249_frame_C16.fm Page 727 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero Therefore, 16.6 DESIGN OF ION EXCHANGERS Generally, designs of ion exchangers should include the following: quantity of exchange materials and regenerants; dimension of the bed (volume of bed); interval of bed regeneration, backwash, and rinse water requirements. The amount of exchange materials determines the dimension of the bed. The interval of regeneration may be arbitrarily set from which the quantity of exchange bed material may be calculated. Regeneration, backwash, and rinse waters may pose pollution problems. 16.6.1 QUANTITY OF EXCHANGE MATERIALS Before discussing quantities of exchange materials, a method of expressing exchange capacity in terms of calcium carbonate is addressed. This method of expressing capacity is very troublesome, and it should not have been adopted; nonetheless, it is used and we must know it. As shown in Tables 16.2, 16.3, and 16.4, equivalents, among other units, are used to express exchange capacities. This is appropriate because reactants react in equivalent amounts; but to express this in terms of calcium carbonate is a bit unusual. As addressed in previous chapters, however, arbitrarily adopt CaCO 3 /2 = 50 as the equivalent mass of calcium carbonate. From this, the exchange capacity, expressed in equivalents, may be obtained by dividing the exchange capacity expressed in calcium carbonate by 50. Let us first derive the formula for the exchange materials for the cation bed. The amount of exchange bed materials required can be determined by calculating first the amount of displacing ions in solution to be removed. Let the exchange capacity of the bed be (X/M ) ult meq/g of bed. The equivalents of ion displaced from the bed is equal to the equivalents of displacing ion in solution; therefore, the mass of bed material CatTBedMass in kilograms is (16.9) Q is the m 3 /d of flow and t int is the interval of regeneration in hours. In concept, the interval of regeneration may be arbitrarily set. A value of 8 hours is not unreasonable. The factor (1000/24) is used so that the unit of CatTBedMass will be in kilograms. δ 221–()2.2[]0.7076–{} 0.00049()750()1000()6.5() 0.0012 m 1.2 mm=== CatTBedMass CatT[] eq ()Q()t int () X M ult 1000 24 = ∑ i=m i=1 q i C s i +q i []()Q()t int () X M ult 1000 24 = TX249_frame_C16.fm Page 728 Friday, June 14, 2002 4:47 PM © 2003 by A. P. Sincero and G. A. Sincero [...]... exchanger, a volume of water is treated for hardness removal between regenerations having intervals of 8 h The raw water contains 400 mg/L of hardness as CaCO3 The exchanger is a resin of exchange 3 capacity of 1412.8 geq/m Assume that the packed density of the resin 3 is 720 kg/m The mass of exchanger material is 51.02 kg and its volume 3 is 0.13 m at a swell of 0.8 Calculate the volume of water treated... exchanger, a volume of water is to be treated for hardness removal between regenerations having intervals of 8 h The raw water 2+ 2+ contains 80 mg/L of Ca and 15 mg/L of Mg The exchanger is a resin 3 of exchange capacity of 1412.8 geq/m Assume that the packed density 3 of the resin is 720 kg/m The kilograms of sodium chloride regenerant required is 0.26 Calculate the volume of water treated assuming... m of water per day is to be treated for hardness removal between regenerations having intervals of 8 hours The raw 2+ 2+ water contains 80 mg/L of Ca and 15 mg/L of Mg The exchanger is a resin of 3 exchange capacity of 1412.8 geq/m Assume that the packed density of the resin is 3 720 kg/m Calculate the total volume of rinse and backwash requirement if the 3 3 backwash and rinse per unit volume of. .. time tn Total volume treated at time tn+1 Total volume of water or wastewater treated at complete exhaustion of bed Ultimate exchange capacity of the bed or simply, the exchange capacity of the bed For ion exchangers, (X/M)ult has the units of milligram equivalents of solute exchanged per gram of exchanger material Length of active zone Pack density of ion exchange material, kilograms per cubic meter... exchanger, 75 m of water per day is to be treated for hardness removal between regenerations having intervals of 8 hours The raw 2+ 2+ water contains 80 mg/L of Ca and 15 mg/L of Mg The exchanger is a resin of 3 exchange capacity of 1412.8 geq/m Assume that the packed density of the resin is 3 720 kg/m Calculate the kilograms of sodium chloride regenerant required assuming R = 2 and that all of the cations... of water is treated for hardness removal The raw water contains 400 mg/L of hardness as CaCO3 Assume that the 3 packed density of the resin is 720 kg/m The mass of exchanger material 3 is 51.02 kg and its volume is 0.13 m at a swell of 0.8 The exchange capacity 3 3 is 1412.8 geq/m and the volume of water treated is 75 m /d Determine the interval of regeneration 3 16.7 Using a bed exchanger, 75 m of. .. concentration of NaCl is about 5 to 10% for an average of 7.5% This means that, for example, if the quantity of regenerant required is 0.26 kg, the volume of wastewater produced from regeneration can be calculated as follows: the total mass of regenerant solution is 0.26/0.075 = 3 3.47 kilograms; the corresponding volume is 3.47/1000 = 0.0035 m For an interval 3 of regeneration of 8 h and assuming a rate of. .. that all of the cations were removed 3 16.9 Using a bed exchanger, 75 m of water per day is treated for hardness 2+ 2+ removal The raw water contains 80 mg/L of Ca and 15 mg/L of Mg 3 The exchanger is a resin of exchange capacity of 1412.8 geq/m Assume 3 that the packed density of the resin is 720 kg/m The kilograms of sodium chloride regenerant required is 0.26 Calculate the interval of regeneration... = 2 and that all of the cations were removed 16.10 Using a bed exchanger, a volume of water is to be treated for hardness removal between regenerations having intervals of 8 hours The raw water 2+ 2+ contains 80 mg/L of Ca and 15 mg/L of Mg The exchanger is a resin 3 of exchange capacity of 1412.8 geq/m Assume that the packed density 3 of the resin is 720 kg/m The total volume of the rinse and backwash... of exchange capacity of 1412.8 geq/m The total 3 volume of the rinse and backwash requirement is 1.66 m , the interval of regeneration is 8 h, and the backwash and rinse per unit volume of the 3 3 bed is 18 m /m Calculate the packed density of the bed 3 16.14 Using a bed exchanger, 75 m of water per day is to be treated for hardness 2+ 2+ removal The raw water contains 80 mg/L of Ca and 15 mg/L of . 0.0035 m 3 . For an interval of regeneration of 8 h and assuming a rate of flow for the water treated of 75 m 3 /d, the volume of water treated is 75/24(8) = 25 m 3 . Thus, the wastewater produced. exchanger, 75 m 3 of water per day is treated for hardness removal. The raw water contains 80 mg/L of Ca 2+ and 15 mg/L of Mg 2+ . The exchanger is a resin of exchange capacity of 1412.8 geq/m 3 volume of water is to be treated for hardness removal between regenerations having intervals of 8 hours. The raw water contains 80 mg/L of Ca 2+ and 15 mg/L of Mg 2+ . The exchanger is a resin of