Ton Exchange 387 Table 1 Distribution Coefficients[17l Resin Ethylene Glycol Dowex 50-X8.H Dowex 50-X8.H' d-Glucose Dowex 50-X8.H 22 Glycerine Dowex 50-X8, H Triethylene glycol Dowex 50-X8.H+ Dowex 50-X8.H 3.08 Acetic Acid Dowex 50-X8.H Acetone Formaldehyde Dowex 50-X8.H Methanol Dowex 50-X8.H Dowex 1-X75 CI Dowex 1-X75 cI. Glycerine Dowex 1-X7,5. CI 12 Methanol Dowex 1-X7,5. CI Phenol Dowex 1-X75 CI Formaldehyde Dowex I-8,SO4=,50-1001.02 Acetone Dowex 1-X8, SO4=, 50-100 Xylose Dowex 50-X8 Na Glycerine Dowex 50-X8 Na+ Pentaerythritol Dowex 50-X8 Na+ Ethylene Glycol Dowex 50-X8 Na Diethylene glycol Dowex 50-X8. Nat Triethylene Glycol Dowex 50-X8, Na Ethylene Diamine Dowex 50-X8 Na+ Dowex 50-X8, Na Triethylene Tetramine Dowex 50-X8, Na Tetraethylene Pentamine Dowex 50-X8, Na 66 The acetone-formaldehyde separation would be an example of affinity difference chromatography in which molecules of similar molecular weight or isomers of compounds are separated on the basis of differing attractions or distribution coefficients for the resin. The largest industrial chromatogra- phy application of this type is the separation of fructose from glucose to produce 55% or 90% fructose corm sweetener
Ion Exchange 387 Table 1 Distribution Coefficients["1 Solute Resin Kd Ethylene Glycol Sucrose d-Glucose Glycerine Triethylene Glycol Phenol Acetic Acid Acetone Formaldehyde Methanol Formaldehyde Acetone Glycerine Methanol Phenol Formaldehyde Acetone Xylose Glycerine Pentaerythntol Ethylene Glycol Diethylene Glycol Triethylene Glycol Ethylene Diamine Diethylene Triamine Triethylene Tetramine Dowex 50-X8, H' .67 Dowex 50-X8, H' .24 Dowex 50-X8, H' .22 Dowex 50-X8, H' .49 Dowex 50-X8, H' .74 Dowex 50-X8, H' 1.20 Dowex 50-X8, H' .59 Dowex 50-X8, H' 3.08 Dowex 50-X8, H' .71 Dowex 50-X8, H' .6 1 Dowex 1-X7.5, C1- 1.06 Dowex 1-X7.5, C1- 1.08 Dowex 1-X7.5, C1- 1.12 Dowex 1-X7.5, C1- .61 Dowex 1-X7.5, Cl- 17.70 Dowex 1-X8, SO4=, 50-100 .66 Dowex 50-X8, Na' -56 Dowex 50-X8, Na' .63 Dowex 50-X8, Na' .67 Dowex 50-X8, Na' .61 Dowex 1-X8, SO,=, 50-100 1.02 Dowex 50-X8, Na' .45 Dowex 50-X8, Na' .39 Dowex 50-X8, Na' .57 Dowex 50-X8, Na' .57 Dowex 50-X8, Na' .64 Tetraethylene Pentamine Dowex 50-X8, Na' .66 The acetone-formaldehyde separation would be an example of affinity difference chromatography in which molecules of similar molecular weight or isomers of compounds are separated on the basis of differing attractions or distribution coefficients for the resin. The largest industrial chromatography application of this type is the separation of fructose from glucose to produce 55% or 90% fructose corn sweetener
388 Fermentation and Biochemical Engineering Handbook Ion exclusion chromatography involves the separation of an ionic component from a nonionic component. The ionic component is excluded from the resin beads by ionic repulsion, while the nonionic component will be distributed into the liquid phase inside the resin beads. Since the ionic solute travels only in the interstitial volume, it will reach the end of the column before the nonionic solute which must travel a more tortuous path through the ion exchange beads. a major industrial chromatography application of this type is the recovery of sucrose from the ionic components of molasses In size exclusion chromatography, the resin beads act as molecular sieves, allowing the smaller molecules to enter the beads while the larger molecules areexcluded. Figure 3ns shows the effect of molecular size on the elution volume required for a given resin. The ion exclusion technique has been used for the separation of monosodium glutamate from other neutral 9 ETHYLENE GLYCOL IETHYEE GLYCOL AR TETRAETHYLENE GLYCOL 3 POLYETHYLBE GLYCOL M400 7 VoiD VOUME 10020050100 MLECULAR WEIGHT OF SOLUTE Figure 3. Effect of molecular weight on the elution volume required for glycol Ion retardation chromatography involves the separation of two ionic solutes with a common counter ion. Unless a specific complexing resin is used, the resin must be placed in the form of the common counter ion. The other solute ions are separated on the basis of different affinities for the resin Ion retardation chromatography is starting to see use in the recovery of acids from waste salts following the regeneration of ion exchange columns
388 Fermentation and Biochemical Engineering Handbook Ion exclusion chromatography involves the separation of an ionic component from a nonionic component. The ionic component is excluded from the resin beads by ionic repulsion, while the nonionic component will be distributed into the liquid phase inside the resin beads. Since the ionic solute travels only in the interstitial volume, it will reach the end ofthe column before the nonionic solute which must travel a more tortuous path through the ion exchange beads. A major industrial chromatography application of this type is the recovery of sucrose from the ionic components of molasses. In size exclusion chromatography, the resin beads act as molecular sieves, allowing the smaller molecules to enter the beads while the larger molecules are excluded. Figure 31181 shows the effect ofmolecular size on the elution volume required for a given resin. The ion exclusion technique has been used for the separation of monosodium glutamate from other neutral amino acid~.I’~] 1Do 90 ;. 9” Figure 3. compounds. [*I Effect of molecular weight on the elution volume required for glycol Ion retardation chromatography involves the separation of two ionic solutes with a common counter ion. Unless a specific complexing resin is used, the resin must be placed in the form of the common counter ion. The other solute ions are separated on the basis of different affinities for the resin. Ion retardation chromatography is starting to see use in the recovery of acids from waste salts following the regeneration of ion exchange columns
Ton Exchange 389 2.0 THEORY The important features of ion exchange reactions are that they are stoichiometric, reversible and possible with any ionizable compound. The reaction that occurs in a specific length of time depends on the selectivity of he resin for the ions or molecules involved and the kinetics of that reaction he stoichiometric nature of the reaction allows resin requirements to be predicted and equipment to be sized The reversible nature of the reaction, Eq,(3) R-H+NaC动R-Na++HCl allows for the repeated reuse of the resin since there is no substantial change in its structure The equilibrium constant, K, for Eq. (1), is defined for such mono- valent exchange by the equation Eq(4) K R-H+Na+CI In general, if K is a large number, the reverse reaction is much less efficient and requires a large excess of regenerant chemical, HCI in this instance, for moderate regeneration levels be. With proper processing and regenerants, the ion exchange resins may be selectively and repeatedly converted from one ionic form to another. The definition of the proper processing requirements is based upon the selectivity and kinetic theories of ion exchange reactions 2.1 Selectivity When ion B, which is initially in the resin, is exchanged for ion A in solution, the selectivity is represented by Eq(5) InKa n(2A5-) where Zi is the charge and v is the partial volume of ion i. The selectivity which a resin has for various ions is affected by many factors. The factors include the valence and size of the exchange ion, the ionic form of the resin
Ion Exchange 389 2.0 THEORY The important features of ion exchange reactions are that they are stoichiometric, reversible and possible with any ionizable compound. The reaction that occurs in a specific length of time depends on the selectivity of the resin for the ions or molecules involved and the kinetics of that reaction. The stoichiometric nature of the reaction allows resin requirements to be predicted and equipment to be sized. The reversible nature of the reaction, illustrated as follows: Eq. (3) R - H' + Na'Cl- C= R - Na' + H'Clallows for the repeated reuse of the resin since there is no substantial change in its structure. The equilibrium constant, K, for Eq. (l), is defined for such monovalent exchange by the equation: IR-Na+] [H+Cl-l Eq. (4) [R-H+] [Na+Cl-] K= In general, if K is a large number, the reverse reaction is much less efficient and requires a large excess of regenerant chemical, HCl in this instanc,e, for moderate regeneration levels. With proper processing and regenerants, the ion exchange resins may be selectively and repeatedly converted from one ionic form to another. The definition of the proper processing requirements is based upon the selectivity and kinetic theories of ion exchange reactions. 2.1 Selectivity When ion B, which is initially in the resin, is exchanged for ion A in solution, the selectivity is represented by: where Zi is the charge and y. is the partial volume of ion i. The selectivity which a resin has for various ions is affected by many factors. The factors include the valence and size of the exchange ion, the ionic form of the resin
390 Fermentation and Biochemical Engineering Handbook of functional group and the nature of the non-exchanging iong n, the type he total ionic strength of the solution, the cross- linkage of th The ionic hydration theory has been used to explain the effect of some ofthese factors on selectivity. 20 According to this theory, the ions in aqueous lution are hydrated and the degree of hydration for cations increases with increasing charge and decreasing crystallographic radius, as shown in Table 2. 21] It is the high dielectric constant of water molecules that is responsible for the hydration of ions in aqueous solutions. The hydration potential of an ion depends on the intensity of the change on its surface. The degree of hydration of an ion increases as its valence increases and decreases as its hydrated radius increases. Therefore, it is expected that the selectivity of a resin for an ion is inversely proportional to the ratio of the valence/ionic radius for ions of a given radius. In dilute solution, the following selectivity series are followed Li< Na<K<Rb<o Mg Ca sr Ba F<Cl< Br <I Table 2. Ionic Size of Cations[21] Crystallographic Hydrated Ionization Radius(A) Radius(A) Potential 0.68 10.00 1.30 0.98 K 33 5.30 0.75 NH4 1.43 5.37 1.65 5.05 0.61 0.89 2.60 1.34 960 1.60 Ba 149 8.80 140 The selectivity of resins in the hydrogen ion or hydroxide ion form, however, depends on the strength of the acid or base formed between the functional group and the ion. The stronger the acid or base formed the lower is the selectivity coefficient. It should be noted that these series are not followed in nonaqueous solutions, at high solute concentrations or at high temperature
390 Fermentation and Biochemical Engineering Handbook the total ionic strength of the solution, the cross-linkage of the resin, the type of functional group and the nature of the nonexchanging ions. The ionic hydration theory has been used to explain the effect of some ofthese factors on selectivity.[20] According tothis theory, the ions in aqueous solution are hydrated and the degree of hydration for cations increases with increasing charge and decreasing crystallographic radius, as shown in Table 2.r2l] It is the high dielectric constant of water molecules that is responsible for the hydration of ions in aqueous solutions. The hydration potential of an ion depends on the intensity of the change on its surface. The degree of hydration of an ion increases as its valence increases and decreases as its hydrated radius increases. Therefore, it is expected that the selectivity of a resin for an ion is inversely proportional tothe ratio ofthe valencehonic radius for ions of a given radius. In dilute solution, the following selectivity series are followed: Li <Na < K < Rb < Cs Mg < Ca < Sr < Ba F < C1< Br < I Table 2. Ionic Size of Cations[21] Crystallographic Hydrated Ionization Ion Radius (A) Radius (A) Potential Li Na K NH4 Rb cs Mg Ca Sr Ba 0.68 0.98 1.33 1.43 1.49 1.65 0.89 1.17 1.34 1.49 10.00 7.90 5.30 5.37 5.09 5.05 10.80 9.60 9.60 8.80 1.30 1 .oo 0.75 0.67 0.61 2.60 1.90 1.60 1.40 - The selectivity of resins in the hydrogen ion or hydroxide ion form, however, depends on the strength of the acid or base formed between the functional group and the ion. The stronger the acid or base formed, the lower is the selectivity coefficient. It should be noted that these series are not followed in nonaqueous solutions, at high solute concentrations or at high temperature
Ton Exchange 39 The dependence of selectivity on the ionic strength of the solution has been related through the mean activity coefficient to be inversely proportional to the Debye-Huckel parameter, a [22 1+Ba° where, is the mean activity coefficient, A and B are constants, and u is the ionic strength of the solution. The mean activity coefficient in this instand represents the standard free energy of formation(-AF)for the salt formed by the ion exchange resin and the exchanged ion. Figure 4(231 shows this dependence as the ionic concentration of the solution is changed. As the concentration increases, the differences in the selectivity of the resin for ions ofdifferent valence decreases and, beyond certain concentrations, the affinity is seen to be greater for the lower valence ion 1.5 s1.2 MOLARITY OF SOLUTION
Ion Exchange 391 The dependence of selectivity on the ionic strength of the solution has been related through the mean activity coefficient to be inversely proportional to the Debye-Huckel parameter, where 'y* is the mean activity coefficient, A and B are constants, and ,u is the ionic strength of the solution. The mean activity coefficient in this instance represents the standard free energy of formation (-W) for the salt formed by the ion exchange resin and the exchanged ion. Figure 4[231 shows this dependence as the ionic concentration of the solution is changed. As the concentration increases, the differences in the selectivity of the resin for ions of different valence decreases and, beyond certain concentrations, the affinity is seen to be greater for the lower valence ion. f I I I I 0 0,4 008 la2 106 2,o MOLARITY OF SOLUTION Figure 4. Dependence of the activity coefficient on the ionic concentration of aqueous