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Equilibrium studies of phenylalanine and tyrosine on ion-exchange resins
Chemical Engineering Science 60 (2005) 5022 – 5034 www.elsevier.com/locate/ces
Equilibrium studies of phenylalanine and tyrosine on ion-exchange resins Maria João A. Moreira, Licínio Manuel G.A. Ferreira∗ Department of Chemical Engineering, University of Coimbra, Pólo II-Pinhal de Marrocos, 3030 Coimbra, Portugal Received 22 July 2004; received in revised form 18 March 2005; accepted 30 March 2005 Available online 31 May 2005
Abstract The ion exchange equilibria of amino acids (phenylalanine and tyrosine) on a strong-acid cation exchanger resin (PK220) and on a strong-base anion exchanger resin (PA316) has been investigated. Based on experimental fixed-bed saturations at constant pH, each of the resins was previously screened of a set of three commercial lots with different physical properties. Overall mass balances from the saturation curves enabled to evaluate the linear isotherm constants and its dependence with pH. An equilibrium model that takes into account energetic heterogeneities in ion exchangers was successfully used to correlate binary and multicomponent data. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Ion-exchange; Amino acid; Equilibrium; Resin screening
1. Introduction Amino acids are molecules of biological interest that are often used in the food and pharmaceutical industries. They are usually produced by extraction, chemical synthesis, enzymatic catalysis and fermentation. Ion-exchange resins are widely used in the recovery and purification on a large scale of amino acid mixtures. Because of the amphoteric nature of these compounds, the net charge of their molecules may vary in magnitude and signal with the pH solution, resulting in the adsorption or in the desorption from the resins. The optimal design of fixed-bed exchangers, among other things, requires an accurate modelling of the ion-exchange equilibrium. The models proposed in the literature can be divided in two main groups: those describing the ion-exchange process in terms of the law of mass action and those regarding the ion exchange as a phase equilibrium. Several models of the first group have been proposed in the literature, which differ in the approach followed to describe the nonideal behavior of the liquid and solid phases. A rigorous approach based on thermodynamics was developed by Jansen (1996) ∗ Corresponding author. Tel.: +35 1239 798700; fax: +35 1239 798703.
E-mail address: [email protected] (L.M.G.A. Ferreira). 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.03.039
and Bellot et al. (1999). In the models that belong to the second group the ion exchange is treated as an adsorption process and the deviations from the ideal behavior are explained in terms of the energetic heterogeneity of the functional groups of the ion exchanger. A model of this group which has been used in several applications as reported by the literature (Dye et al., 1990; Jones and Carta, 1993; Melis et al., 1995; Saunders et al., 1989) was originally proposed by Myers and Byington (1986). The model uses a binomial distribution to describe the energetic variance of the functional groups allowing to predict the variability of the selectivity coefficients. Thus, the non-ideal behavior of the selectivity should be attributed to the solute–adsorbent rather than solute–solute interactions. This is the main difference between the models that take into account the heterogeneous nature of the solid and the ones of the first group based on the mass action law. In this work, the Myers and Byington model is tested by using binary and multicomponent exchange equilibria data of the amino acids phenylalanine (Phe) and tyrosine (Tyr) on a strong-acid cation exchanger resin (PK220) and on a strong-base anion exchanger resin (PA316). It is worth mentioning that while the application of this model to ionexchange data with cation exchanger resins is better understood, fewer studies have been done on the ion-exchange equilibrium using anion exchanger resins, involving amino
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 Table 1 Equilibrium parameters
1.2
CPheZ/CPhe
1.0
Phe±
0.8 0.6
-
Phe
Phe+
Amino acid
pK1
pK2
pK3
pI
L-Phe L-Tyr
2.11 2.20
9.13 9.11
— 10.10
5.48 5.64
0.4 0.2 0.0 0
2
4
8
6
10
12
14
where CAi is the total concentration of amino acid and CH+ is the concentration of hydrogen ion. The negatively charged form is given by
pH
CAi − =
Fig. 1. Calculated ionic species of Phenylalanine as function of pH.
1.0
Tyr±
0.8 Tyr-
CAi . 1 + (CH+ /K2 ) + (CH2 + /K1 K2 )
(4)
The amino acid Tyr, have a dissociable phenolic side chain. Thus, in addition to reactions (1) and (2) the following dissociation reaction occurs at high pH with the equilibrium constant K3
1.2
CTyrZ/CTyr
5023
Tyr2-
K3
− − − + NH+ 2 CHRCOO ←→ NH2 CR COO + H .
0.6 Tyr+ 0.4
(5)
The concentration of the chemical species CAi 2− is given by
0.2
CAi 2−
0.0 0
2
4
6
8
10
12
14
pH
=
CAi . 2 1 + (CH+ /K3 ) + (CH+ /K2 K3 ) + (CH3 + /K1 K2 K3 ) (6)
Fig. 2. Calculated ionic species of Tyrosine as function of pH.
The total concentration of amino acid Ai in solution is acid solutions in particular. This work offers the possibility of comparing the performance of the two types of ionexchange resins, structurally based on the same matrix of cross-linked polystyrene but with different exchange functionalities for the uptake of amino acids. 2. Theory 2.1. Dissociation equilibria of the amino acids in solution Amino acids are amphoteric molecules. In aqueous solution they dissociate into ionic species depending on the pH and on the nature of the amino acid, as is shown in Figs. 1 and 2. For neutral amino acids, as Phe, the following dissociation equilibrium takes place: K1
+ − + NH+ 3 CHRCOOH ←→ NH3 CHRCOO + H , K2
− − + NH+ 3 CHRCOO ←→ NH2 CHRCOO + H ,
(1)
CAi , 1 + (K1 /CH+ ) + (K1 K2 /CH2 + )
(3)
(7)
The values of equilibrium constants and isoelectric points (pI ’s) for the two amino acids under study are shown in Table 1. If the concentration of amino acid Ai in a solution containing NaCl is known, the pH solution may be calculated from the electroneutrality condition that can be expressed as follows: CAi + + CNa+ + CH+ = CAi − + CCl− + COH− + 2 CAi 2− . (8) 2.2. The ion-exchange equilibrium model This model assumes that the effectiveness of the global exchange process can be quantified by the selectivity coefficient. This coefficient extended to multicomponent systems is defined as
(2)
where K1 and K2 are the dissociation constants of the amino acid. The positively charged amino acid may be calculated from CAi + =
CAi = CAi 2− + CAi + + CAi ± + CAi − .
Si,j =
Y i Xj , Yj X i
(9)
where Si,j is the selectivity coefficient for ion i relative to ion j (in which the counter ion j is chosen as the reference ion), Yi is the equivalent ionic fraction of ion i in the resin calculated as Yi = qi zi /q0 and Xi is the equivalent ionic
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
fraction of ion i in solution given by
Table 2 Physical properties of the resins
C i zi X i = N . j C j zj N is the number of counter ions in solution, zj is the charge of counter ion j and q0 is the ion-exchange capacity of the resin. The selectivity coefficient for the ion preferred by the resin generally decreases as the ionic fraction of that species is increased. Myers and Byington (1986) attributed this variation to the energetic heterogeneity of exchange sites. High energetic sites are more selective than weak energetic sites, giving a high-initial selectivity coefficient. Because of its flexibility and mathematical convenience, Myers and Byington chose the discrete binomial distribution of n + 1 site types having a characteristic energy level Ei,j of the adsorption of an ion j on a site i. The probability of finding a site of type i is n Pi = pi (1 − p)n−i , 0 < p < 1, (10) i where p is the skewness parameter. The energy of an ion j adsorbed on site i is given by Ei,j = Ej + √
i − np j , np(1 − p)
(i = 0, 1, 2, . . . , n),
(11)
where Ej and j are the mean and the standard deviation of the energy distribution. The adsorption of ion j on site i is represented by qi,j
qi0 Ci,j Xj = N , j Ci,j Xj
(12)
where qi0 is the concentration of sites i and the Ci,j constant is given by Ei,j − Ej Ci,j = Cj exp . (13) RT Assuming a two site symmetrical distribution (n = 1) and summing all sites and replacing the separation factor, the selectivity coefficient for ion i relative to ion j is given by Si,j = S i,j
N
f 0+f 1 f0 f1 ((1−p)Wi,k +pW i,k )] k=1 [S k,j Xk Wk,j , f 0+f 1 f0 f1 ((1−p)Wj,k +pW j,k )] k=1 [S k,j Xk Wk,j
× N
(14) where S i,j
Ei − Ej = exp RT
Wi,j = exp
i − j RT
,
(15)
,
(16)
Resin
Wet particle porosity
Apparent density (gdryr /mlwetr )
Water content (%)
PA306 PA316 IRA900Cl PK228 PK220 Amberlite
0.74 0.54 0.70 0.47 0.60 0.66
0.31 0.54 0.37 0.90 0.60 0.51
70 49 64 35 49 56
−p , p(1 − p)
(17)
1−p . p(1 − p)
(18)
f0 = √ f1 = √
For the binary exchange, three parameters should be determined: the mean separation factor, Si,j , the skewness parameter, p, that indicates the probability of finding one type of sites, and the heterogeneity parameter, Wi,j , that indicates the variance of energy distribution on exchange sites. For multicomponent systems, the selectivity coefficient depends only on the binary parameter values. Once the entire Si,j are determined, the uptake of ion i may be computed from Xi Si,j . Yi = N k=1 Xk Sk,j 3. Experimental 3.1. Resins and chemicals In our studies we used strong cation and anion exchanger resins, in the macroreticular form and based on cross-linked polystyrene matrix. The physical properties of these resins are summarized in Table 2 . The dry weight fraction of the wet resins (w, g of dry resin/g of wet resin) was obtained by determining the weight loss at 110 ◦ C of the hydrated samples (see Dye et al., 1990). These samples were previously vacuum filtered in a Buchner funnel to remove the interstitial water. The apparent density, ap , was determined from the real density of the wet resin sample, rh , multiplied by w. rh was evaluated by the displacement of n-heptane in a picnometer. The wet particle porosity was obtained from the real and apparent densities. The cation exchanger resins, Amberlite 252 (Rohm & Haas SA.), PK220 and PK228 (Mitsubishi Chemical Corporation), have sulfonic acid (–SO3 H) groups as exchange groups and a degree of cross-linking of 8%, 10% and 14%, respectively. The total exchange capacity of PK220, q0 , was determined by equilibrating a sample of the resin in the hydrogen form with an excess of volume of 0.1 M NaOH containing 50 g of NaCl. At equilibrium, the excess NaOH was titrated with 0.1 M HCl and the capacity determined
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
from a material balance. The resin capacity was found to be 5.04 mmol/g dry resin. The anion exchanger resins used may be classified into two types, PA306 and PA316 from Mitsubishi belongs to type I, with a trimethylammonium exchange group (R–N + (CH3 )3 ) and having degree of cross-linking of 3% and 8%, respectively. IRA900Cl from Rohm & Haas belongs to type II with a dimethylethanolammonium exchange group (R − N + (CH3 )2 CH2 CH2 OH), and a degree of cross-linking of 8%. Both types I and II are strongly base, but as basicity of type II is slightly lower, it is much easier to regenerate. The total exchange capacity of PA316 was determined by equilibrating a sample of the resin in the hydroxide form with an excess of volume of 0.1 mol/L HCl containing 50 g/L of NaCl. At equilibrium, the excess HCl was titrated with 0.1 M NaOH and the capacity determined from a material balance. The resin capacity was found to be 3.145 mmol/g dry resin. Prior to any experiments, the resins were pretreated with by repeated washings with 2 M HCl and 2 M NaOH solutions, converted to the desired ionic form, and then thoroughly rinsed with deionized water. Pure crystalline forms of L-phenylalanine and L-tyrosine from Riedel–deHaen were used as solutes in all the experiments. The ionization constants of the amino acids used are given in Table 1. The other chemicals NaOH, HCl and NaCl were obtained from Merck. 3.2. Analysis The concentrations of amino acids were determined with an Absorbance detector (model-229, ISCO). For solutions containing both species, the concentrations of Phe and Tyr were determined by HPLC analysis with a Hewlett Packard series 1050 apparatus. A reverse-phase column (Amino Quant) was used to perform the analysis. The mobile phase was a 20 mM sodium acetate solution. The concentration of sodium was determined by atomic absorption, and the chloride concentration by ionic chromatography. The solution pH was recorded with a pH meter (WTW 540 GLP) by using a glass-combined electrode (Mettler Toledo). 3.3. Fixed-bed experiments Saturations experiments were carried out using an Omnifit glass column (10×50 mm) filled with resin particles with an average diameter of 0.05 cm. A peristaltic pump (Ismatec, model ISM 832) was used to percolate the solution through the column. Before starting each experiment, the resin in H/OH form was equilibrated with a HCl/NaOH solution to yield a desired pH and after this, a solution containing a given initial concentration of amino acid and the same pH was introduced at the entrance of the column as a step input. The concentration of Phe along the time, at the outlet column, was measured as described above.
5025
3.4. Equilibrium experiments Ion exchange isotherms for the two types of resins were determined from batch experiments, at constant chloride and sodium concentration, respectively. Varying amounts of dry resin, in hydrogen form (cation exchanger resins) or hydroxide form (anion exchanger resins), were placed in Erlenmeyer flasks in contact with solutions (75 ml) containing known initial concentrations of amino acids, sodium and chloride ions. The flasks were sealed and shaken for 48 h (time required to reach the equilibrium conditions) in a constant-temperature bath (Julabo SW-21C) at 25±1 ◦ C. After equilibrium was reached, solution in the flasks was sampled in order to determine the concentrations of amino acids, sodium and chloride and the pH. During the experiments, the concentration of Cl− and Na+ kept approximately constant for the experiments with cation and anion exchanger resins, respectively. The equilibrium concentrations of amino acids in the resin phase were determined from material balances.
4. Results and discussion 4.1. Fixed-bed saturations These saturations were performed to determine equilibrium constants (or the slopes of the linear isotherm) at constant pH. Based on the effect of the pH on the breakthrough curves of Phe it was possible to screen the resins tested. The experimental conditions for these runs are shown in Table 3. In all experiments, a flow rate of 10 ml/min was used to percolate through the anionic column the amino acid solution with initial concentration of 1.5 and 2 mM for the cationic column. For these concentrations of the amino acid Phe, it is a reasonable assumption to consider the validity of the linear isotherm in the description of the adsorption equilibrium. Overall mass balances from the saturation curves enabled to evaluate the equilibrium constants, keq , shown in Table 3. The experimental results are illustrated in Figs. 3–5 for anion resins and Figs. 6–8 for cation exchanger resins. 4.2. Determination of separation factors and resin screening From the equilibrium constants obtained previously by mass balances from the fixed-bed saturations it is possible now to determine separation factors defined by Pigford et al. (1969) for each resin. Linear equilibrium isotherms of Phe on PK220 for two specified pH values of the solution (adsorption favorable, pH = 5.5 and adsorption unfavorable pH = 2) are shown in Fig. 9. The relative gap between these two isotherms can be measured by the parameter b defined as b=
a , 1+m
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
Table 3 Experimental conditions and calculated equilibrium constants keq
Experimental conditions RUN
pH
Resin
(s)
ε
(dm3solution /dm3particle )
1 2 3 4 5 6 7 8 9 10 11 12
12.5 13.0 12.5 13.0 12.5 13.0 pI 2.0 pI 2.0 pI 2.0
PA306
4.35
0.42
PA316
3.98
0.42
IRA900Cl
4.06
0.42
PK228
4.35
0.42
PK220
4.35
0.42
Amberlite
4.23
0.39
253 107 352 105 221 95 689 529 935 562 655 468
1.0
0.8
C/CE
0.6
0.4
Exp. (pH=12.5) 0.2
Exp. (pH=13.0)
0.0 0
25
50
75
100
125
150
Time (min) Fig. 3. Experimental results of breakthrough curves for Phe. Runs 1 and 2.
1.0
0.8
C/CE
0.6
0.4
Exp. (pH=12.5) Exp. (pH=13.0)
0.2
0.0 0
25
50
75
100
125
Time (min) Fig. 4. Experimental results of breakthrough curves for Phe. Runs 3 and 4.
150
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=12.5) 0.2
Exp. (pH=13.0)
0.0 0
25
50
75
100
125
150
Time (min) Fig. 5. Experimental results of breakthrough curves for Phe. Runs 5 and 6. 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=pI) Exp. (pH=2.0) 0.2
0.0 0
100
200
300
400
500
Time (min) Fig. 6. Experimental results of breakthrough curves for Phe. Runs 7 and 8. 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=pI) Exp. (pH=2.0) 0.2
0.0 0
100
200
300
400
Time (min) Fig. 7. Experimental results of breakthrough curves for Phe. Runs 9 and 10.
500
5027
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0.8
C/CE
0.6
0.4
Exp. (pH=pI) Exp. (pH=2.0)
0.2
0.0 0
100
200
300
400
500
Time (min) Fig. 8. Experimental results of breakthrough curves for Phe. Runs 11 and 12.
Table 4 Pigford separation parameters
4.0
pH=pI
Resin
a
m
b
PA306 PA316 IRA900Cl PK228 PK220 Amberlite 252
101.957 170.745 87.101 110.721 257.837 145.748
248.113 315.834 218.097 841.130 1033.865 878.466
0.405 0.539 0.398 0.131 0.249 0.166
q (mmol/gwet resin)
3.0
∆q
2.0 ∆C
pH=2 1.0
4.3. Cation-exchange equilibrium on PK220
0.0 0.0
0.5
1.0
1.5
2.0
CPhe (mmol/dm3) Fig. 9. Scheme illustrating the effect of pH on the linear equilibrium isotherm of Phe on PK220.
where m=
m(pH1 ) + m(pH2 ) , 2
a=
m(pH1 ) − m(pH2 ) , 2
and pH1 and pH2 are the favorable and unfavorable pH, respectively; m(pH) is the capacity parameter defined as m(pH) =
(1 − ε)keq . ε
Table 4 shows the b parameters calculated for the six resins tested. The resins that exhibit a larger b parameter value, the PK220 for the cation exchanger resins and the PA316 for the anion exchanger resins, were screened as the most suitable for chromatographic separations of amino acids on the basis on the pH change.
4.3.1. Amino acid uptake equilibrium The equilibrium uptake of Phe and Tyr by the resin PK220 is shown in Figs. 10 and 11. qA is the total amount of amino acid taken up by the resin, and CA is the total concentration of amino acid in solution at equilibrium. Since it was our purpose to obtain equilibrium data in the limiting cases of low pH and high pH values, the experiments were carried out at two concentrations of the co-ion Cl− . This species is excluded from the resin due to the Donnan potential effect, and as consequence its concentration was kept approximately constant during the experiment. At high chloride concentration the pH is low and the hydrogen ions compete with the amino acid ions in its protonated form; in this case, the resin takes up lower amount of the solute. The opposite effect is observed at lower chloride concentration in which the pH is higher and a much larger amount of the amino acid is up taken by the resin. When the Cl− concentration is zero, the solution pH is near the isoelectric point of the amino acid and thus its uptake by the resin would be maximum as demonstrated by Dye et al. (1990). Indeed, regarding the behavior of Phe (see Fig. 10), it is visible that the plateau value for those conditions is close to the ion-exchange capacity of the resin. Data for Tyr shown in Fig. 11 at Cl−
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
5029
0.5
Exp. (CI=0.0 mM) Exp. (CI=100.0 mM) Model
qPhe (mmol/gdry resin)
0.4
0.3
0.2
0.1
0.0 0
5.0
10.0
20.0
15.0
CPhe (mM) Fig. 10. Experimental and calculated equilibrium data for Phe/H+ on PK220 at two chloride ion concentrations. 0.5
Exp. (CI=0.0 mM) Exp. (CI=52.5 mM)
qTyr (mmol/gdry resin)
0.4
Model 0.3
0.2
0.1
0.0 0
0.5
1.0
2.0
1.5
CTyr (mM) Fig. 11. Experimental and calculated equilibrium data for Tyr/H+ on PK220 at two chloride ion concentrations.
1.0
0.8
0.6
YPhe
concentration equal to zero, reveal only an uptake maximum of around 40% of the resin capacity corresponding to 2 mM Tyr solution at equilibrium. For more concentrated solutions it is not possible to obtain uptake values because of low solubility of the Tyr in aqueous solutions. In Figs. 12 and 13, the data are re-plotted in the form YA —ionic fraction of amino acid cations in the resin vs. XA —ionic fraction of amino acid cations in the solution. All the experimental points obtained at two Cl− concentrations and at different pH values fall on a single line. This shows that the uptake depends only on the ionic fraction of amino acids cations and not upon the total amino acid or co-ion (Cl− ) concentration. The Y –X diagrams shows also that the selectivity coefficient is not constant; it decreases with the increasing ionic fraction. Their values calculated in all of the X range are greater than unity, showing a favorable behavior for the binary exchange systems studied Phe+ /H+ and Tyr + /H+ . Similar results were obtained by Saunders et al. (1989) for the uptake of Phe and Tyr by the resin Amberlite 252.
0.4
Exp. (CI=0.0 mM) Exp. (CI=100.0 mM)
0.2
Model 0.0 0.0
0.2
0.4
0.6
0.8
1.0
XPhe Fig. 12. Experimental and calculated ion-exchange equilibrium for Phe/H+ on PK220.
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0.4
0.8
0.3
0.6
YCl-
YTyr
0.5
0.2
0.4 Exp. (CI=0.0 mM) Exp. (CI=52.5 mM)
0.1
Exp. Model
0.2
Model 0.0 0
0.1
0.2
0.3
0.4
0.0
0.5
0.0
XTyr
0.4
0.6
0.8
1.0
XCl-
Fig. 13. Experimental and calculated ion-exchange equilibrium for Tyr/H+ on PK220.
Fig. 14. Experimental and calculated ion-exchange equilibrium for Cl− /OH− on PA316.
experimental and calculated results was obtained and thus the assumption of the heterogeneous distribution of the functional groups for explaining the variability of the selectivity in the systems studied appears to be consistent.
The equilibrium model (Eq. (14)) was used to fit experimental data obtained for each binary Phe+ /H+ and Tyr + /H+ . The best values for the parameters p, S¯ and W were calculated by using a nonlinear optimization routine GREG (Stewart et al., 1993) and are given in Table 5. As these parameters will be used to make multicomponent predictions, one single value of p was determined for the two binary systems. A good agreement between all the
4.3.2. Multicomponent ion-exchange equilibrium Multicomponent equilibrium measurements were also carried out for the system Phe+ /Tyr + /H+ using the batch technique described earlier, and the results are shown in Table 6. Calculations were based on the Eq. (14) using the parameters given in Table 5. Theoretical predictions are in satisfactory agreement with experiments, with errors within in the ±18% range.
Table 5 Equilibrium parameters for PK220
Phe+ Tyr +
0.2
S
W
p
4.4. Anion-exchange equilibrium on PA316
3.26 1.85
3.01 3.52
0.41 0.41
4.4.1. Cl− /OH exchange equilibrium Experimental data for the exchange of Cl− and OH− ions are shown in Fig. 14. The data are given in terms of ionic
Reference ion: H+ . Table 6 Phe+ /Tyr + /H+ equilibrium on PK220 CPhe
CTyr
pHcal
XPhe
XTyr
YPhe
exp
calc YPhe
YTyr
exp
calc YTyr
0.022 0.030 0.058 0.073 0.104 0.175 0.242 0.322 0.537 0.745 0.926 1.352
0.033 0.048 0.095 0.123 0.174 0.283 0.373 0.497 0.764 0.982 1.187 1.542
6.64 6.58 6.45 6.40 6.34 6.24 6.18 6.12 6.04 5.99 5.95 5.90
0.003 0.004 0.007 0.009 0.013 0.021 0.029 0.037 0.058 0.077 0.091 0.123
0.005 0.007 0.015 0.019 0.026 0.042 0.054 0.070 0.102 0.124 0.144 0.172
0.011 0.022 0.036 0.055 0.071 0.104 0.119 0.153 0.199 0.236 0.259 0.314
0.015 0.020 0.037 0.044 0.058 0.085 0.105 0.125 0.167 0.199 0.220 0.265
0.011 0.022 0.035 0.053 0.069 0.097 0.110 0.137 0.168 0.191 0.196 0.222
0.019 0.026 0.048 0.060 0.078 0.109 0.129 0.152 0.185 0.202 0.217 0.230
CCl = 0 mmol/l.
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
5031
3.0
qPhe (mmol/gdry resin)
2.5
2.0
1.5
Exp. (Na=0.0 mM) Exp. (Na=10.0 mM)
1.0
Exp. (Na=100.0 mM) 0.5
Model
0.0 0
5.0
10.0
15.0
ConcPhe (mM) Fig. 15. Experimental and calculated equilibrium data for Phe/OH− on PA316 at different sodium hydroxide concentration.
1.0
0.8
YPhe
0.6
0.4 Exp. (Na=0.0 mM) Exp. (Na=10.0 mM) Exp. (Na=100.0 mM)
0.2
Model
0.0 0.0
0.2
0.4
0.6
0.8
1.0
XPhe Fig. 16. Experimental and calculated ion-exchange equilibrium for Phe/OH− on PA316.
fraction of chloride in the resin, YCl− = qCl /q0 and in the . = C − /(C − + C − ) where q − is the total solution, XCl Cl Cl Cl OH amount of Cl− taken up by the resin. The values of selectivity coefficient in the entire X range are greater than unity, showing a favorable behavior for the binary Cl− /OH− . 4.4.2. Amino acid uptake equilibrium The equilibrium data of Phe (neutral amino acid) on PA316 are illustrated in Figs. 15 and 16. The general behavior of the uptake of Phe by the resin at different sodium hydroxide concentration can be explained in a similar way to that of the amino acid in its cationic form as previously mentioned. In this case, the amount of Phe anions taken up
by the resin decreases as the pH is increased (see Fig. 15), due to the competition of the hydroxide ions. The uptake is maximum at hydroxide concentration equal to zero in which the solution pH is near the isoelectric pH of the amino acid. The isotherm curve for these conditions exhibits the shape of an isotherm irreversible corresponding to the maximum of highly favorable ion-exchange equilibrium. The diagram Y –X represented in Fig. 16, where the data are expressed in terms of ionic fraction of Phe in the resin, YPhe− =qPhe− /q0 , and in the solution XPhe− = CPhe− /(CPhe− + COH− ), shows once again, data at different co-ion concentration and pH’s fall along the same line. It can also be seen that YA initially increases as function of XA with selectivity coefficient values greater than unity, and then approaches a plateau at approximately 60% of the resin ion-exchange capacity for intermediate XA values, and it finally increases again at higher ionic fractions accompanied by selectivity values above unity. Despite this inversion of selectivity, the model still gives a good agreement between experimental and calculated uptakes, which proves its great flexibility. The equilibrium uptake of Tyr is shown in Fig. 17 at different sodium hydroxide concentration. Here the uptake process of Tyr is also dependent on the solution pH as result of the amount of NaOH added. Depending on the hydroxide concentration, Tyr may present the ionic forms: Tyr − and Tyr 2− . The variation of the equivalent ionic fraction of Tyr − and Tyr 2− with pH is represented Fig. 2. Note that for pH values approximately between 8 and 11, the two forms anionic of Tyr are present in solution. Thus, the description of the equilibrium of this amino acid is more complex since we have the exchange equilibrium involving three species Tyr − /Tyr 2− /OH− . To quantify the equilibrium of this ternary system, data of each binary pair Tyr − /OH− and Tyr 2− /OH− are needed. Similar systems in which the amino acid exhibits two ionic forms, univalent and divalent, are reported in the literature. Zammouri et al. (2000)
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 2.5
qTyr (mmol/gdry resin)
2.0
1.5
1.0
Exp. (Na=0.0 mM) Exp. (Na=10.0 mM) Exp. (Na=100 mM) Model
0.5
0.0 0
0.5
1.0
1.5
2.0
2.5
3.0
ConcTyr (mM) Fig. 17. Experimental and calculated equilibrium data for Tyr/OH− on PA316 at different sodium hydroxide concentration.
Table 7 Equilibrium parameters for PA316
1.0
0.8
Cl− Phe− Tyr −
0.6
YTyr2-
Tyr 2−
S
W
p
13.35 0.43 0.22 113.89
1.65 45.27 73.12 6.05
0.60 0.60 0.60 0.60
Reference ion: OH− . 0.4 Exp. (Na=10.0 mM) Exp. (Na=100.0 mM) 0.2
Model
0.0 0.0
0.2
0.4
0.6
0.8
1.0
XTyr2Fig. 18. Experimental and calculated ion-exchange equilibrium for Tyr 2 − /OH− on PA316.
studied the exchange of glutamic acid into strong anion exchanger resins as a multicomponent system by considering the species Glu− , Glu2− and OH− . They have obtained data in all the XGlu (ionic fraction of Glu in solution) range at a fixed concentration of NaOH. In the case of tyrosine, equilibrium data at high concentration cannot be measured due to its low solubility. In this study, we first modelled the equilibrium Tyr 2− /OH− directly from the isotherms obtained at NaOH concentration of 10 and 100 mM. Under these conditions the ionic fraction of Tyr − is negligible. The results in Y –X diagram are shown in Fig. 18). Regarding the system Tyr − /OH− ,
we have carried out experiments at CNaOH = 0 in order to have the tyrosine predominantly in the form Tyr − . Results given in terms of the equivalent ionic fraction of Tyr − in the resin YTyr− = qTyr− /q0 vs. equivalent ionic fraction of Tyr − in the solution, XTyr− = CTyr− /(CTyr− + COH− ), were located near X = 1. This is so because for the conditions used in the experiments the OH− concentration is quite small; therefore, these data do not allow to determine accurately the equilibrium parameters relative to the exchange Tyr − /OH− , S¯Tyr−,OH− and WTyr−,OH− . Then, we fitted other set of experimental data based on conditions that led to the presence of the two species Tyr − and Tyr 2− in solution and by considering the parameters already determined from the data of the binary Tyr 2− /OH− (see Table 7). Table 8 shows experimental and calculated values of the fraction ZTyr = YTyr− + YTyr2− /2, where YTyr− and YTyr2− are resin compositions given by YTyr− = =
qTyr− q0
XTyr− STyr− ,OH−
XTyr− STyr− ,OH− + XTyr2− STyr2− ,OH− + XOH−
, (19)
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
5033
Table 8 Tyr − /Tyr 2− /OH− equilibrium on PA316 CTyr (mmol/I)
pHexp
pHcal
XTyr−
XTyr2−
ZTyr
exp
cal ZTyr
2.570 2.212 1.667 1.656 0.775 0.778 0.256 0.608 0.196
8.52 8.30 8.75 8.10 8.96 8.58 9.56 8.22 9.25
8.85 8.94 9.13 9.14 9.83 9.83 10.73 10.11 10.81
0.893 0.870 0.811 0.809 0.448 0.451 0.048 0.287 0.032
0.100 0.121 0.175 0.177 0.484 0.482 0.414 0.585 0.327
0.558 0.519 0.592 0.442 0.563 0.480 0.496 0.383 0.399
0.688 0.673 0.640 0.639 0.543 0.543 0.487 0.522 0.477
CNal+ = 1 mmol/l. Table 9 Phe− /Tyr − /Tyr 2− /OH− equilibrium on PA316 CNa+ (mmol/l)
pHcal
XPhe−
XTyr−
XTyr2−
YPhe
exp
calc YPhe
ZTyr
exp
calc ZTyr
1.597 1.103 0.675 0.447 0.188 0.110 0.044 0.008 0.010 0.003
5.87 5.90 5.93 5.98 6.05 6.13 6.26 6.56 6.51 6.71
0.541 0.616 0.701 0.745 0.834 0.842 0.861 0.788 0.824 0.674
0.456 0.380 0.293 0.248 0.155 0.142 0.108 0.083 0.075 0.069
0.00005 0.00005 0.00004 0.00004 0.00003 0.00003 0.00003 0.00005 0.00004 0.00006
0.081 0.112 0.141 0.200 0.245 0.278 0.293 0.274 0.266 0.239
0.189 0.210 0.240 0.251 0.305 0.292 0.308 0.301 0.327 0.296
0.533 0.571 0.528 0.482 0.449 0.385 0.342 0.278 0.273 0.237
0.582 0.551 0.507 0.478 0.397 0.380 0.334 0.304 0.283 0.296
1.618 1.302 0.762 0.635 0.326 0.084 0.054 0.037 0.022
11.69 11.74 11.82 11.84 11.88 11.92 11.94 11.95 11.96
0.196 0.194 0.189 0.186 0.176 0.146 0.126 0.104 0.088
0.0041 0.0029 0.0014 0.0011 0.0005 0.0001 0.0001 0.0001 0.0000
0.315 0.255 0.149 0.125 0.064 0.017 0.011 0.007 0.004
0.047 0.038 0.048 0.043 0.060 0.111 0.132 0.149 0.153
0.007 0.009 0.014 0.017 0.030 0.081 0.099 0.110 0.130
0.388 0.392 0.490 0.383 0.401 0.382 0.339 0.301 0.268
0.474 0.466 0.438 0.428 0.382 0.275 0.242 0.215 0.177
CPhe (mmol/l)
CTyr (mmol/l)
0
1.985 1.870 1.693 1.403 1.057 0.684 0.371 0.083 0.110 0.035
10
1.969 1.948 1.895 1.862 1.765 1.458 1.260 1.043 0.883
YTyr2− = =
2qTyr2− q0 X
Tyr −
XTyr2− STyr2− ,OH− S
Tyr − ,OH−
+ XTyr2− STyr2− ,OH− + XOH−
.
are in reasonable agreement with the experiments and some more significant deviations appear to occur at low concentrations of the amino acids caused by inaccuracies of the measurements.
(20) The values of p, S¯ and W determined for all the binaries studied are shown in Table 7. 4.4.3. Ion-exchange equilibrium of the system P he− /T yr − /T yr 2− /OH − Experimental multicomponent equilibrium measurements of the quaternary system involving Phe− , Tyr − , Tyr 2− and OH− are presented in Table 9. In this table, the model predictions are also tabulated by using the parameter values given in Table 7. It can be observed that these predictions
5. Conclusions We have studied the equilibrium sorption of amino acids by two strong ion-exchange resins, one cationic acid (PK220) and the other anionic base (PA316). Each of the resins was previously screened of a set of three commercial lots with different physical properties. From the equilibrium constants obtained by overall mass balances from experimental fixed-bed saturations at constant pH, it was possible to calculate the b parameter separation defined by Pigford
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for each resin. The most suitable resin, among the three samples with either cation or anion exchange functionalities, for the study of the ion-exchange equilibrium was chosen as the one that presented a larger b value. Experimental data obtained show that the two amino acids (Phe and Tyr) are adsorbed in lower or higher extension on both the resins dependent on the solution pH. The maximum adsorption takes place near the isoelectric point. In these conditions, the ion-exchange process on the anion exchanger resin exhibits a much more favorable behavior, corresponding to an isotherm irreversible, than those verified on the cation exchanger resin. It is worth also mentioning that exchange data for Phe in terms of Y –X diagram illustrate a selectivity reversal when the ion-exchanger is the anion exchanger resin demonstrating thus an behavior highly nonideal. A model that takes into account energetic heterogeneities in ion exchangers has been proved to be very flexible and to give good agreement with experimental amino acid uptakes on strong cation and anion exchanger resins. Even the predictions of multicomponent equilibrium measurements using model parameters p, S¯ and W determined for the binaries were reasonably good, with errors generally in the ±20% range. For PK220, the value of p obtained indicates a probability of 41% of finding one kind of site while the other type of resin PA316 has 60% of probability to be found. The model used may be included in a more general model to simulate ion-exchange chromatographic separations. Notation C CE Ci,j Ei,j E Ki N p q q0 Si,j S i,j T Wi,j X
liquid-phase solute concentration, mol/L feed concentration, mol/L equilibrium constant adsorption energy for ion j on site i, J/mol mean of the energy deviation dissociation equilibrium constant of component i, mol/L number of sites-1 skewness parameter resin equilibrium uptake, mmol/gdry resin resin ion exchange capacity, mmol/gdry resin separation factor for ion i relative to ion j average binary separation factor for ion i relative to ion j temperature, K heterogeneity parameter liquid-phase equivalent ionic fraction
Y zi
resin-phase equivalent ionic fraction valence of ion i
Greek letters ε j
bed void fraction bed resin density, gdry resin /cm3bed standard deviation of energy distribution function of ion j , J/mol residence time, s
Superscripts calc exp
calculated value experimental value
Acknowledgements The work of Maria João Moreira was supported by FCT: Grant BD/9052/96. The research was also supported by funds from Project PBIC/C/QUI/2416/95. References Bellot, J.C., Tarantino, R.V., Condoret, J-S., 1999. Thermodynamic modelling of multicomponent ion-exchange equilibira of amino acids. A.I.Ch.E. Journal 45, 1329–1341. Dye, S.R., DeCarli, J.P., Carta, G., 1990. Equilibrium sorption of amino acids by a cation-exchange resin. Industrial & Engineering Chemistry Research 29, 849–857. Jansen, M.L., Straathof, A.J.J., van der Wielen, L.A.M., Luyben, K.Ch.A.M, van den Tweel, W.J.J., 1996. Rigorous model for ion exchange equilibria of strong and weak electrolytes. A.I.Ch.E. Journal 42, 1911–1924. Jones, I.L., Carta, G., 1993. Ion exchange of amino acids and dipeptides on cation resins with varying degree of cross-linking, 1: equilibrium. Industrial & Engineering Chemistry Research 32, 107–117. Melis, S., Cao, G., Morbidelli, M., 1995. A new model for the simulation of ion exchange equilibria. Industrial & Engineering Chemistry Research 34, 3916–3924. Myers, A.L., Byington, S., 1986. Thermodynamics of Ion Exchange: Prediction of Multicomponent Equilibria From Binary Data. Ion Exchange Science and Technology. In: Rodrigues, A.E. (Ed.), NATO ASI, Series E. vol. 107, pp. 119–145. Pigford, L.R., Baker, B., Blum, D.E., 1969. An equilibrium theory of the parametric pumping. Industrial & Engineering Chemistry Fundamentals 8, 144–149. Saunders, M.S., Vierow, J.B., Carta, G., 1989. Uptake of phenylalanine and tyrosine by a strong-acid cation exchanger. American Institute for Chemical Engineers Journal 35 (1), 53–68. Stewart, W.E., Caracotsios, M., SZrensen, J.P., 1993. Software documentation of GREG-general regression software package for nonlinear parameter estimation (Version of July 1993). University of Wisconsin-Madison, Madison, WI. Zammouri, A., Chanel, S., Muhr, L., Grevillot, G., 2000. Ion-exchange equilibria of amino acids on strong anionic resins. Industrial & Engineering Chemistry Research 39, 1397–1408.
Equilibrium studies of phenylalanine and tyrosine on ion-exchange resins Maria João A. Moreira, Licínio Manuel G.A. Ferreira∗ Department of Chemical Engineering, University of Coimbra, Pólo II-Pinhal de Marrocos, 3030 Coimbra, Portugal Received 22 July 2004; received in revised form 18 March 2005; accepted 30 March 2005 Available online 31 May 2005
Abstract The ion exchange equilibria of amino acids (phenylalanine and tyrosine) on a strong-acid cation exchanger resin (PK220) and on a strong-base anion exchanger resin (PA316) has been investigated. Based on experimental fixed-bed saturations at constant pH, each of the resins was previously screened of a set of three commercial lots with different physical properties. Overall mass balances from the saturation curves enabled to evaluate the linear isotherm constants and its dependence with pH. An equilibrium model that takes into account energetic heterogeneities in ion exchangers was successfully used to correlate binary and multicomponent data. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Ion-exchange; Amino acid; Equilibrium; Resin screening
1. Introduction Amino acids are molecules of biological interest that are often used in the food and pharmaceutical industries. They are usually produced by extraction, chemical synthesis, enzymatic catalysis and fermentation. Ion-exchange resins are widely used in the recovery and purification on a large scale of amino acid mixtures. Because of the amphoteric nature of these compounds, the net charge of their molecules may vary in magnitude and signal with the pH solution, resulting in the adsorption or in the desorption from the resins. The optimal design of fixed-bed exchangers, among other things, requires an accurate modelling of the ion-exchange equilibrium. The models proposed in the literature can be divided in two main groups: those describing the ion-exchange process in terms of the law of mass action and those regarding the ion exchange as a phase equilibrium. Several models of the first group have been proposed in the literature, which differ in the approach followed to describe the nonideal behavior of the liquid and solid phases. A rigorous approach based on thermodynamics was developed by Jansen (1996) ∗ Corresponding author. Tel.: +35 1239 798700; fax: +35 1239 798703.
E-mail address: [email protected] (L.M.G.A. Ferreira). 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.03.039
and Bellot et al. (1999). In the models that belong to the second group the ion exchange is treated as an adsorption process and the deviations from the ideal behavior are explained in terms of the energetic heterogeneity of the functional groups of the ion exchanger. A model of this group which has been used in several applications as reported by the literature (Dye et al., 1990; Jones and Carta, 1993; Melis et al., 1995; Saunders et al., 1989) was originally proposed by Myers and Byington (1986). The model uses a binomial distribution to describe the energetic variance of the functional groups allowing to predict the variability of the selectivity coefficients. Thus, the non-ideal behavior of the selectivity should be attributed to the solute–adsorbent rather than solute–solute interactions. This is the main difference between the models that take into account the heterogeneous nature of the solid and the ones of the first group based on the mass action law. In this work, the Myers and Byington model is tested by using binary and multicomponent exchange equilibria data of the amino acids phenylalanine (Phe) and tyrosine (Tyr) on a strong-acid cation exchanger resin (PK220) and on a strong-base anion exchanger resin (PA316). It is worth mentioning that while the application of this model to ionexchange data with cation exchanger resins is better understood, fewer studies have been done on the ion-exchange equilibrium using anion exchanger resins, involving amino
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 Table 1 Equilibrium parameters
1.2
CPheZ/CPhe
1.0
Phe±
0.8 0.6
-
Phe
Phe+
Amino acid
pK1
pK2
pK3
pI
L-Phe L-Tyr
2.11 2.20
9.13 9.11
— 10.10
5.48 5.64
0.4 0.2 0.0 0
2
4
8
6
10
12
14
where CAi is the total concentration of amino acid and CH+ is the concentration of hydrogen ion. The negatively charged form is given by
pH
CAi − =
Fig. 1. Calculated ionic species of Phenylalanine as function of pH.
1.0
Tyr±
0.8 Tyr-
CAi . 1 + (CH+ /K2 ) + (CH2 + /K1 K2 )
(4)
The amino acid Tyr, have a dissociable phenolic side chain. Thus, in addition to reactions (1) and (2) the following dissociation reaction occurs at high pH with the equilibrium constant K3
1.2
CTyrZ/CTyr
5023
Tyr2-
K3
− − − + NH+ 2 CHRCOO ←→ NH2 CR COO + H .
0.6 Tyr+ 0.4
(5)
The concentration of the chemical species CAi 2− is given by
0.2
CAi 2−
0.0 0
2
4
6
8
10
12
14
pH
=
CAi . 2 1 + (CH+ /K3 ) + (CH+ /K2 K3 ) + (CH3 + /K1 K2 K3 ) (6)
Fig. 2. Calculated ionic species of Tyrosine as function of pH.
The total concentration of amino acid Ai in solution is acid solutions in particular. This work offers the possibility of comparing the performance of the two types of ionexchange resins, structurally based on the same matrix of cross-linked polystyrene but with different exchange functionalities for the uptake of amino acids. 2. Theory 2.1. Dissociation equilibria of the amino acids in solution Amino acids are amphoteric molecules. In aqueous solution they dissociate into ionic species depending on the pH and on the nature of the amino acid, as is shown in Figs. 1 and 2. For neutral amino acids, as Phe, the following dissociation equilibrium takes place: K1
+ − + NH+ 3 CHRCOOH ←→ NH3 CHRCOO + H , K2
− − + NH+ 3 CHRCOO ←→ NH2 CHRCOO + H ,
(1)
CAi , 1 + (K1 /CH+ ) + (K1 K2 /CH2 + )
(3)
(7)
The values of equilibrium constants and isoelectric points (pI ’s) for the two amino acids under study are shown in Table 1. If the concentration of amino acid Ai in a solution containing NaCl is known, the pH solution may be calculated from the electroneutrality condition that can be expressed as follows: CAi + + CNa+ + CH+ = CAi − + CCl− + COH− + 2 CAi 2− . (8) 2.2. The ion-exchange equilibrium model This model assumes that the effectiveness of the global exchange process can be quantified by the selectivity coefficient. This coefficient extended to multicomponent systems is defined as
(2)
where K1 and K2 are the dissociation constants of the amino acid. The positively charged amino acid may be calculated from CAi + =
CAi = CAi 2− + CAi + + CAi ± + CAi − .
Si,j =
Y i Xj , Yj X i
(9)
where Si,j is the selectivity coefficient for ion i relative to ion j (in which the counter ion j is chosen as the reference ion), Yi is the equivalent ionic fraction of ion i in the resin calculated as Yi = qi zi /q0 and Xi is the equivalent ionic
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
fraction of ion i in solution given by
Table 2 Physical properties of the resins
C i zi X i = N . j C j zj N is the number of counter ions in solution, zj is the charge of counter ion j and q0 is the ion-exchange capacity of the resin. The selectivity coefficient for the ion preferred by the resin generally decreases as the ionic fraction of that species is increased. Myers and Byington (1986) attributed this variation to the energetic heterogeneity of exchange sites. High energetic sites are more selective than weak energetic sites, giving a high-initial selectivity coefficient. Because of its flexibility and mathematical convenience, Myers and Byington chose the discrete binomial distribution of n + 1 site types having a characteristic energy level Ei,j of the adsorption of an ion j on a site i. The probability of finding a site of type i is n Pi = pi (1 − p)n−i , 0 < p < 1, (10) i where p is the skewness parameter. The energy of an ion j adsorbed on site i is given by Ei,j = Ej + √
i − np j , np(1 − p)
(i = 0, 1, 2, . . . , n),
(11)
where Ej and j are the mean and the standard deviation of the energy distribution. The adsorption of ion j on site i is represented by qi,j
qi0 Ci,j Xj = N , j Ci,j Xj
(12)
where qi0 is the concentration of sites i and the Ci,j constant is given by Ei,j − Ej Ci,j = Cj exp . (13) RT Assuming a two site symmetrical distribution (n = 1) and summing all sites and replacing the separation factor, the selectivity coefficient for ion i relative to ion j is given by Si,j = S i,j
N
f 0+f 1 f0 f1 ((1−p)Wi,k +pW i,k )] k=1 [S k,j Xk Wk,j , f 0+f 1 f0 f1 ((1−p)Wj,k +pW j,k )] k=1 [S k,j Xk Wk,j
× N
(14) where S i,j
Ei − Ej = exp RT
Wi,j = exp
i − j RT
,
(15)
,
(16)
Resin
Wet particle porosity
Apparent density (gdryr /mlwetr )
Water content (%)
PA306 PA316 IRA900Cl PK228 PK220 Amberlite
0.74 0.54 0.70 0.47 0.60 0.66
0.31 0.54 0.37 0.90 0.60 0.51
70 49 64 35 49 56
−p , p(1 − p)
(17)
1−p . p(1 − p)
(18)
f0 = √ f1 = √
For the binary exchange, three parameters should be determined: the mean separation factor, Si,j , the skewness parameter, p, that indicates the probability of finding one type of sites, and the heterogeneity parameter, Wi,j , that indicates the variance of energy distribution on exchange sites. For multicomponent systems, the selectivity coefficient depends only on the binary parameter values. Once the entire Si,j are determined, the uptake of ion i may be computed from Xi Si,j . Yi = N k=1 Xk Sk,j 3. Experimental 3.1. Resins and chemicals In our studies we used strong cation and anion exchanger resins, in the macroreticular form and based on cross-linked polystyrene matrix. The physical properties of these resins are summarized in Table 2 . The dry weight fraction of the wet resins (w, g of dry resin/g of wet resin) was obtained by determining the weight loss at 110 ◦ C of the hydrated samples (see Dye et al., 1990). These samples were previously vacuum filtered in a Buchner funnel to remove the interstitial water. The apparent density, ap , was determined from the real density of the wet resin sample, rh , multiplied by w. rh was evaluated by the displacement of n-heptane in a picnometer. The wet particle porosity was obtained from the real and apparent densities. The cation exchanger resins, Amberlite 252 (Rohm & Haas SA.), PK220 and PK228 (Mitsubishi Chemical Corporation), have sulfonic acid (–SO3 H) groups as exchange groups and a degree of cross-linking of 8%, 10% and 14%, respectively. The total exchange capacity of PK220, q0 , was determined by equilibrating a sample of the resin in the hydrogen form with an excess of volume of 0.1 M NaOH containing 50 g of NaCl. At equilibrium, the excess NaOH was titrated with 0.1 M HCl and the capacity determined
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
from a material balance. The resin capacity was found to be 5.04 mmol/g dry resin. The anion exchanger resins used may be classified into two types, PA306 and PA316 from Mitsubishi belongs to type I, with a trimethylammonium exchange group (R–N + (CH3 )3 ) and having degree of cross-linking of 3% and 8%, respectively. IRA900Cl from Rohm & Haas belongs to type II with a dimethylethanolammonium exchange group (R − N + (CH3 )2 CH2 CH2 OH), and a degree of cross-linking of 8%. Both types I and II are strongly base, but as basicity of type II is slightly lower, it is much easier to regenerate. The total exchange capacity of PA316 was determined by equilibrating a sample of the resin in the hydroxide form with an excess of volume of 0.1 mol/L HCl containing 50 g/L of NaCl. At equilibrium, the excess HCl was titrated with 0.1 M NaOH and the capacity determined from a material balance. The resin capacity was found to be 3.145 mmol/g dry resin. Prior to any experiments, the resins were pretreated with by repeated washings with 2 M HCl and 2 M NaOH solutions, converted to the desired ionic form, and then thoroughly rinsed with deionized water. Pure crystalline forms of L-phenylalanine and L-tyrosine from Riedel–deHaen were used as solutes in all the experiments. The ionization constants of the amino acids used are given in Table 1. The other chemicals NaOH, HCl and NaCl were obtained from Merck. 3.2. Analysis The concentrations of amino acids were determined with an Absorbance detector (model-229, ISCO). For solutions containing both species, the concentrations of Phe and Tyr were determined by HPLC analysis with a Hewlett Packard series 1050 apparatus. A reverse-phase column (Amino Quant) was used to perform the analysis. The mobile phase was a 20 mM sodium acetate solution. The concentration of sodium was determined by atomic absorption, and the chloride concentration by ionic chromatography. The solution pH was recorded with a pH meter (WTW 540 GLP) by using a glass-combined electrode (Mettler Toledo). 3.3. Fixed-bed experiments Saturations experiments were carried out using an Omnifit glass column (10×50 mm) filled with resin particles with an average diameter of 0.05 cm. A peristaltic pump (Ismatec, model ISM 832) was used to percolate the solution through the column. Before starting each experiment, the resin in H/OH form was equilibrated with a HCl/NaOH solution to yield a desired pH and after this, a solution containing a given initial concentration of amino acid and the same pH was introduced at the entrance of the column as a step input. The concentration of Phe along the time, at the outlet column, was measured as described above.
5025
3.4. Equilibrium experiments Ion exchange isotherms for the two types of resins were determined from batch experiments, at constant chloride and sodium concentration, respectively. Varying amounts of dry resin, in hydrogen form (cation exchanger resins) or hydroxide form (anion exchanger resins), were placed in Erlenmeyer flasks in contact with solutions (75 ml) containing known initial concentrations of amino acids, sodium and chloride ions. The flasks were sealed and shaken for 48 h (time required to reach the equilibrium conditions) in a constant-temperature bath (Julabo SW-21C) at 25±1 ◦ C. After equilibrium was reached, solution in the flasks was sampled in order to determine the concentrations of amino acids, sodium and chloride and the pH. During the experiments, the concentration of Cl− and Na+ kept approximately constant for the experiments with cation and anion exchanger resins, respectively. The equilibrium concentrations of amino acids in the resin phase were determined from material balances.
4. Results and discussion 4.1. Fixed-bed saturations These saturations were performed to determine equilibrium constants (or the slopes of the linear isotherm) at constant pH. Based on the effect of the pH on the breakthrough curves of Phe it was possible to screen the resins tested. The experimental conditions for these runs are shown in Table 3. In all experiments, a flow rate of 10 ml/min was used to percolate through the anionic column the amino acid solution with initial concentration of 1.5 and 2 mM for the cationic column. For these concentrations of the amino acid Phe, it is a reasonable assumption to consider the validity of the linear isotherm in the description of the adsorption equilibrium. Overall mass balances from the saturation curves enabled to evaluate the equilibrium constants, keq , shown in Table 3. The experimental results are illustrated in Figs. 3–5 for anion resins and Figs. 6–8 for cation exchanger resins. 4.2. Determination of separation factors and resin screening From the equilibrium constants obtained previously by mass balances from the fixed-bed saturations it is possible now to determine separation factors defined by Pigford et al. (1969) for each resin. Linear equilibrium isotherms of Phe on PK220 for two specified pH values of the solution (adsorption favorable, pH = 5.5 and adsorption unfavorable pH = 2) are shown in Fig. 9. The relative gap between these two isotherms can be measured by the parameter b defined as b=
a , 1+m
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
Table 3 Experimental conditions and calculated equilibrium constants keq
Experimental conditions RUN
pH
Resin
(s)
ε
(dm3solution /dm3particle )
1 2 3 4 5 6 7 8 9 10 11 12
12.5 13.0 12.5 13.0 12.5 13.0 pI 2.0 pI 2.0 pI 2.0
PA306
4.35
0.42
PA316
3.98
0.42
IRA900Cl
4.06
0.42
PK228
4.35
0.42
PK220
4.35
0.42
Amberlite
4.23
0.39
253 107 352 105 221 95 689 529 935 562 655 468
1.0
0.8
C/CE
0.6
0.4
Exp. (pH=12.5) 0.2
Exp. (pH=13.0)
0.0 0
25
50
75
100
125
150
Time (min) Fig. 3. Experimental results of breakthrough curves for Phe. Runs 1 and 2.
1.0
0.8
C/CE
0.6
0.4
Exp. (pH=12.5) Exp. (pH=13.0)
0.2
0.0 0
25
50
75
100
125
Time (min) Fig. 4. Experimental results of breakthrough curves for Phe. Runs 3 and 4.
150
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=12.5) 0.2
Exp. (pH=13.0)
0.0 0
25
50
75
100
125
150
Time (min) Fig. 5. Experimental results of breakthrough curves for Phe. Runs 5 and 6. 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=pI) Exp. (pH=2.0) 0.2
0.0 0
100
200
300
400
500
Time (min) Fig. 6. Experimental results of breakthrough curves for Phe. Runs 7 and 8. 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=pI) Exp. (pH=2.0) 0.2
0.0 0
100
200
300
400
Time (min) Fig. 7. Experimental results of breakthrough curves for Phe. Runs 9 and 10.
500
5027
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 1.0
0.8
C/CE
0.6
0.4
Exp. (pH=pI) Exp. (pH=2.0)
0.2
0.0 0
100
200
300
400
500
Time (min) Fig. 8. Experimental results of breakthrough curves for Phe. Runs 11 and 12.
Table 4 Pigford separation parameters
4.0
pH=pI
Resin
a
m
b
PA306 PA316 IRA900Cl PK228 PK220 Amberlite 252
101.957 170.745 87.101 110.721 257.837 145.748
248.113 315.834 218.097 841.130 1033.865 878.466
0.405 0.539 0.398 0.131 0.249 0.166
q (mmol/gwet resin)
3.0
∆q
2.0 ∆C
pH=2 1.0
4.3. Cation-exchange equilibrium on PK220
0.0 0.0
0.5
1.0
1.5
2.0
CPhe (mmol/dm3) Fig. 9. Scheme illustrating the effect of pH on the linear equilibrium isotherm of Phe on PK220.
where m=
m(pH1 ) + m(pH2 ) , 2
a=
m(pH1 ) − m(pH2 ) , 2
and pH1 and pH2 are the favorable and unfavorable pH, respectively; m(pH) is the capacity parameter defined as m(pH) =
(1 − ε)keq . ε
Table 4 shows the b parameters calculated for the six resins tested. The resins that exhibit a larger b parameter value, the PK220 for the cation exchanger resins and the PA316 for the anion exchanger resins, were screened as the most suitable for chromatographic separations of amino acids on the basis on the pH change.
4.3.1. Amino acid uptake equilibrium The equilibrium uptake of Phe and Tyr by the resin PK220 is shown in Figs. 10 and 11. qA is the total amount of amino acid taken up by the resin, and CA is the total concentration of amino acid in solution at equilibrium. Since it was our purpose to obtain equilibrium data in the limiting cases of low pH and high pH values, the experiments were carried out at two concentrations of the co-ion Cl− . This species is excluded from the resin due to the Donnan potential effect, and as consequence its concentration was kept approximately constant during the experiment. At high chloride concentration the pH is low and the hydrogen ions compete with the amino acid ions in its protonated form; in this case, the resin takes up lower amount of the solute. The opposite effect is observed at lower chloride concentration in which the pH is higher and a much larger amount of the amino acid is up taken by the resin. When the Cl− concentration is zero, the solution pH is near the isoelectric point of the amino acid and thus its uptake by the resin would be maximum as demonstrated by Dye et al. (1990). Indeed, regarding the behavior of Phe (see Fig. 10), it is visible that the plateau value for those conditions is close to the ion-exchange capacity of the resin. Data for Tyr shown in Fig. 11 at Cl−
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
5029
0.5
Exp. (CI=0.0 mM) Exp. (CI=100.0 mM) Model
qPhe (mmol/gdry resin)
0.4
0.3
0.2
0.1
0.0 0
5.0
10.0
20.0
15.0
CPhe (mM) Fig. 10. Experimental and calculated equilibrium data for Phe/H+ on PK220 at two chloride ion concentrations. 0.5
Exp. (CI=0.0 mM) Exp. (CI=52.5 mM)
qTyr (mmol/gdry resin)
0.4
Model 0.3
0.2
0.1
0.0 0
0.5
1.0
2.0
1.5
CTyr (mM) Fig. 11. Experimental and calculated equilibrium data for Tyr/H+ on PK220 at two chloride ion concentrations.
1.0
0.8
0.6
YPhe
concentration equal to zero, reveal only an uptake maximum of around 40% of the resin capacity corresponding to 2 mM Tyr solution at equilibrium. For more concentrated solutions it is not possible to obtain uptake values because of low solubility of the Tyr in aqueous solutions. In Figs. 12 and 13, the data are re-plotted in the form YA —ionic fraction of amino acid cations in the resin vs. XA —ionic fraction of amino acid cations in the solution. All the experimental points obtained at two Cl− concentrations and at different pH values fall on a single line. This shows that the uptake depends only on the ionic fraction of amino acids cations and not upon the total amino acid or co-ion (Cl− ) concentration. The Y –X diagrams shows also that the selectivity coefficient is not constant; it decreases with the increasing ionic fraction. Their values calculated in all of the X range are greater than unity, showing a favorable behavior for the binary exchange systems studied Phe+ /H+ and Tyr + /H+ . Similar results were obtained by Saunders et al. (1989) for the uptake of Phe and Tyr by the resin Amberlite 252.
0.4
Exp. (CI=0.0 mM) Exp. (CI=100.0 mM)
0.2
Model 0.0 0.0
0.2
0.4
0.6
0.8
1.0
XPhe Fig. 12. Experimental and calculated ion-exchange equilibrium for Phe/H+ on PK220.
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 1.0
0.4
0.8
0.3
0.6
YCl-
YTyr
0.5
0.2
0.4 Exp. (CI=0.0 mM) Exp. (CI=52.5 mM)
0.1
Exp. Model
0.2
Model 0.0 0
0.1
0.2
0.3
0.4
0.0
0.5
0.0
XTyr
0.4
0.6
0.8
1.0
XCl-
Fig. 13. Experimental and calculated ion-exchange equilibrium for Tyr/H+ on PK220.
Fig. 14. Experimental and calculated ion-exchange equilibrium for Cl− /OH− on PA316.
experimental and calculated results was obtained and thus the assumption of the heterogeneous distribution of the functional groups for explaining the variability of the selectivity in the systems studied appears to be consistent.
The equilibrium model (Eq. (14)) was used to fit experimental data obtained for each binary Phe+ /H+ and Tyr + /H+ . The best values for the parameters p, S¯ and W were calculated by using a nonlinear optimization routine GREG (Stewart et al., 1993) and are given in Table 5. As these parameters will be used to make multicomponent predictions, one single value of p was determined for the two binary systems. A good agreement between all the
4.3.2. Multicomponent ion-exchange equilibrium Multicomponent equilibrium measurements were also carried out for the system Phe+ /Tyr + /H+ using the batch technique described earlier, and the results are shown in Table 6. Calculations were based on the Eq. (14) using the parameters given in Table 5. Theoretical predictions are in satisfactory agreement with experiments, with errors within in the ±18% range.
Table 5 Equilibrium parameters for PK220
Phe+ Tyr +
0.2
S
W
p
4.4. Anion-exchange equilibrium on PA316
3.26 1.85
3.01 3.52
0.41 0.41
4.4.1. Cl− /OH exchange equilibrium Experimental data for the exchange of Cl− and OH− ions are shown in Fig. 14. The data are given in terms of ionic
Reference ion: H+ . Table 6 Phe+ /Tyr + /H+ equilibrium on PK220 CPhe
CTyr
pHcal
XPhe
XTyr
YPhe
exp
calc YPhe
YTyr
exp
calc YTyr
0.022 0.030 0.058 0.073 0.104 0.175 0.242 0.322 0.537 0.745 0.926 1.352
0.033 0.048 0.095 0.123 0.174 0.283 0.373 0.497 0.764 0.982 1.187 1.542
6.64 6.58 6.45 6.40 6.34 6.24 6.18 6.12 6.04 5.99 5.95 5.90
0.003 0.004 0.007 0.009 0.013 0.021 0.029 0.037 0.058 0.077 0.091 0.123
0.005 0.007 0.015 0.019 0.026 0.042 0.054 0.070 0.102 0.124 0.144 0.172
0.011 0.022 0.036 0.055 0.071 0.104 0.119 0.153 0.199 0.236 0.259 0.314
0.015 0.020 0.037 0.044 0.058 0.085 0.105 0.125 0.167 0.199 0.220 0.265
0.011 0.022 0.035 0.053 0.069 0.097 0.110 0.137 0.168 0.191 0.196 0.222
0.019 0.026 0.048 0.060 0.078 0.109 0.129 0.152 0.185 0.202 0.217 0.230
CCl = 0 mmol/l.
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
5031
3.0
qPhe (mmol/gdry resin)
2.5
2.0
1.5
Exp. (Na=0.0 mM) Exp. (Na=10.0 mM)
1.0
Exp. (Na=100.0 mM) 0.5
Model
0.0 0
5.0
10.0
15.0
ConcPhe (mM) Fig. 15. Experimental and calculated equilibrium data for Phe/OH− on PA316 at different sodium hydroxide concentration.
1.0
0.8
YPhe
0.6
0.4 Exp. (Na=0.0 mM) Exp. (Na=10.0 mM) Exp. (Na=100.0 mM)
0.2
Model
0.0 0.0
0.2
0.4
0.6
0.8
1.0
XPhe Fig. 16. Experimental and calculated ion-exchange equilibrium for Phe/OH− on PA316.
fraction of chloride in the resin, YCl− = qCl /q0 and in the . = C − /(C − + C − ) where q − is the total solution, XCl Cl Cl Cl OH amount of Cl− taken up by the resin. The values of selectivity coefficient in the entire X range are greater than unity, showing a favorable behavior for the binary Cl− /OH− . 4.4.2. Amino acid uptake equilibrium The equilibrium data of Phe (neutral amino acid) on PA316 are illustrated in Figs. 15 and 16. The general behavior of the uptake of Phe by the resin at different sodium hydroxide concentration can be explained in a similar way to that of the amino acid in its cationic form as previously mentioned. In this case, the amount of Phe anions taken up
by the resin decreases as the pH is increased (see Fig. 15), due to the competition of the hydroxide ions. The uptake is maximum at hydroxide concentration equal to zero in which the solution pH is near the isoelectric pH of the amino acid. The isotherm curve for these conditions exhibits the shape of an isotherm irreversible corresponding to the maximum of highly favorable ion-exchange equilibrium. The diagram Y –X represented in Fig. 16, where the data are expressed in terms of ionic fraction of Phe in the resin, YPhe− =qPhe− /q0 , and in the solution XPhe− = CPhe− /(CPhe− + COH− ), shows once again, data at different co-ion concentration and pH’s fall along the same line. It can also be seen that YA initially increases as function of XA with selectivity coefficient values greater than unity, and then approaches a plateau at approximately 60% of the resin ion-exchange capacity for intermediate XA values, and it finally increases again at higher ionic fractions accompanied by selectivity values above unity. Despite this inversion of selectivity, the model still gives a good agreement between experimental and calculated uptakes, which proves its great flexibility. The equilibrium uptake of Tyr is shown in Fig. 17 at different sodium hydroxide concentration. Here the uptake process of Tyr is also dependent on the solution pH as result of the amount of NaOH added. Depending on the hydroxide concentration, Tyr may present the ionic forms: Tyr − and Tyr 2− . The variation of the equivalent ionic fraction of Tyr − and Tyr 2− with pH is represented Fig. 2. Note that for pH values approximately between 8 and 11, the two forms anionic of Tyr are present in solution. Thus, the description of the equilibrium of this amino acid is more complex since we have the exchange equilibrium involving three species Tyr − /Tyr 2− /OH− . To quantify the equilibrium of this ternary system, data of each binary pair Tyr − /OH− and Tyr 2− /OH− are needed. Similar systems in which the amino acid exhibits two ionic forms, univalent and divalent, are reported in the literature. Zammouri et al. (2000)
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034 2.5
qTyr (mmol/gdry resin)
2.0
1.5
1.0
Exp. (Na=0.0 mM) Exp. (Na=10.0 mM) Exp. (Na=100 mM) Model
0.5
0.0 0
0.5
1.0
1.5
2.0
2.5
3.0
ConcTyr (mM) Fig. 17. Experimental and calculated equilibrium data for Tyr/OH− on PA316 at different sodium hydroxide concentration.
Table 7 Equilibrium parameters for PA316
1.0
0.8
Cl− Phe− Tyr −
0.6
YTyr2-
Tyr 2−
S
W
p
13.35 0.43 0.22 113.89
1.65 45.27 73.12 6.05
0.60 0.60 0.60 0.60
Reference ion: OH− . 0.4 Exp. (Na=10.0 mM) Exp. (Na=100.0 mM) 0.2
Model
0.0 0.0
0.2
0.4
0.6
0.8
1.0
XTyr2Fig. 18. Experimental and calculated ion-exchange equilibrium for Tyr 2 − /OH− on PA316.
studied the exchange of glutamic acid into strong anion exchanger resins as a multicomponent system by considering the species Glu− , Glu2− and OH− . They have obtained data in all the XGlu (ionic fraction of Glu in solution) range at a fixed concentration of NaOH. In the case of tyrosine, equilibrium data at high concentration cannot be measured due to its low solubility. In this study, we first modelled the equilibrium Tyr 2− /OH− directly from the isotherms obtained at NaOH concentration of 10 and 100 mM. Under these conditions the ionic fraction of Tyr − is negligible. The results in Y –X diagram are shown in Fig. 18). Regarding the system Tyr − /OH− ,
we have carried out experiments at CNaOH = 0 in order to have the tyrosine predominantly in the form Tyr − . Results given in terms of the equivalent ionic fraction of Tyr − in the resin YTyr− = qTyr− /q0 vs. equivalent ionic fraction of Tyr − in the solution, XTyr− = CTyr− /(CTyr− + COH− ), were located near X = 1. This is so because for the conditions used in the experiments the OH− concentration is quite small; therefore, these data do not allow to determine accurately the equilibrium parameters relative to the exchange Tyr − /OH− , S¯Tyr−,OH− and WTyr−,OH− . Then, we fitted other set of experimental data based on conditions that led to the presence of the two species Tyr − and Tyr 2− in solution and by considering the parameters already determined from the data of the binary Tyr 2− /OH− (see Table 7). Table 8 shows experimental and calculated values of the fraction ZTyr = YTyr− + YTyr2− /2, where YTyr− and YTyr2− are resin compositions given by YTyr− = =
qTyr− q0
XTyr− STyr− ,OH−
XTyr− STyr− ,OH− + XTyr2− STyr2− ,OH− + XOH−
, (19)
Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
5033
Table 8 Tyr − /Tyr 2− /OH− equilibrium on PA316 CTyr (mmol/I)
pHexp
pHcal
XTyr−
XTyr2−
ZTyr
exp
cal ZTyr
2.570 2.212 1.667 1.656 0.775 0.778 0.256 0.608 0.196
8.52 8.30 8.75 8.10 8.96 8.58 9.56 8.22 9.25
8.85 8.94 9.13 9.14 9.83 9.83 10.73 10.11 10.81
0.893 0.870 0.811 0.809 0.448 0.451 0.048 0.287 0.032
0.100 0.121 0.175 0.177 0.484 0.482 0.414 0.585 0.327
0.558 0.519 0.592 0.442 0.563 0.480 0.496 0.383 0.399
0.688 0.673 0.640 0.639 0.543 0.543 0.487 0.522 0.477
CNal+ = 1 mmol/l. Table 9 Phe− /Tyr − /Tyr 2− /OH− equilibrium on PA316 CNa+ (mmol/l)
pHcal
XPhe−
XTyr−
XTyr2−
YPhe
exp
calc YPhe
ZTyr
exp
calc ZTyr
1.597 1.103 0.675 0.447 0.188 0.110 0.044 0.008 0.010 0.003
5.87 5.90 5.93 5.98 6.05 6.13 6.26 6.56 6.51 6.71
0.541 0.616 0.701 0.745 0.834 0.842 0.861 0.788 0.824 0.674
0.456 0.380 0.293 0.248 0.155 0.142 0.108 0.083 0.075 0.069
0.00005 0.00005 0.00004 0.00004 0.00003 0.00003 0.00003 0.00005 0.00004 0.00006
0.081 0.112 0.141 0.200 0.245 0.278 0.293 0.274 0.266 0.239
0.189 0.210 0.240 0.251 0.305 0.292 0.308 0.301 0.327 0.296
0.533 0.571 0.528 0.482 0.449 0.385 0.342 0.278 0.273 0.237
0.582 0.551 0.507 0.478 0.397 0.380 0.334 0.304 0.283 0.296
1.618 1.302 0.762 0.635 0.326 0.084 0.054 0.037 0.022
11.69 11.74 11.82 11.84 11.88 11.92 11.94 11.95 11.96
0.196 0.194 0.189 0.186 0.176 0.146 0.126 0.104 0.088
0.0041 0.0029 0.0014 0.0011 0.0005 0.0001 0.0001 0.0001 0.0000
0.315 0.255 0.149 0.125 0.064 0.017 0.011 0.007 0.004
0.047 0.038 0.048 0.043 0.060 0.111 0.132 0.149 0.153
0.007 0.009 0.014 0.017 0.030 0.081 0.099 0.110 0.130
0.388 0.392 0.490 0.383 0.401 0.382 0.339 0.301 0.268
0.474 0.466 0.438 0.428 0.382 0.275 0.242 0.215 0.177
CPhe (mmol/l)
CTyr (mmol/l)
0
1.985 1.870 1.693 1.403 1.057 0.684 0.371 0.083 0.110 0.035
10
1.969 1.948 1.895 1.862 1.765 1.458 1.260 1.043 0.883
YTyr2− = =
2qTyr2− q0 X
Tyr −
XTyr2− STyr2− ,OH− S
Tyr − ,OH−
+ XTyr2− STyr2− ,OH− + XOH−
.
are in reasonable agreement with the experiments and some more significant deviations appear to occur at low concentrations of the amino acids caused by inaccuracies of the measurements.
(20) The values of p, S¯ and W determined for all the binaries studied are shown in Table 7. 4.4.3. Ion-exchange equilibrium of the system P he− /T yr − /T yr 2− /OH − Experimental multicomponent equilibrium measurements of the quaternary system involving Phe− , Tyr − , Tyr 2− and OH− are presented in Table 9. In this table, the model predictions are also tabulated by using the parameter values given in Table 7. It can be observed that these predictions
5. Conclusions We have studied the equilibrium sorption of amino acids by two strong ion-exchange resins, one cationic acid (PK220) and the other anionic base (PA316). Each of the resins was previously screened of a set of three commercial lots with different physical properties. From the equilibrium constants obtained by overall mass balances from experimental fixed-bed saturations at constant pH, it was possible to calculate the b parameter separation defined by Pigford
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Maria João A. Moreira, L.M.G.A. Ferreira / Chemical Engineering Science 60 (2005) 5022 – 5034
for each resin. The most suitable resin, among the three samples with either cation or anion exchange functionalities, for the study of the ion-exchange equilibrium was chosen as the one that presented a larger b value. Experimental data obtained show that the two amino acids (Phe and Tyr) are adsorbed in lower or higher extension on both the resins dependent on the solution pH. The maximum adsorption takes place near the isoelectric point. In these conditions, the ion-exchange process on the anion exchanger resin exhibits a much more favorable behavior, corresponding to an isotherm irreversible, than those verified on the cation exchanger resin. It is worth also mentioning that exchange data for Phe in terms of Y –X diagram illustrate a selectivity reversal when the ion-exchanger is the anion exchanger resin demonstrating thus an behavior highly nonideal. A model that takes into account energetic heterogeneities in ion exchangers has been proved to be very flexible and to give good agreement with experimental amino acid uptakes on strong cation and anion exchanger resins. Even the predictions of multicomponent equilibrium measurements using model parameters p, S¯ and W determined for the binaries were reasonably good, with errors generally in the ±20% range. For PK220, the value of p obtained indicates a probability of 41% of finding one kind of site while the other type of resin PA316 has 60% of probability to be found. The model used may be included in a more general model to simulate ion-exchange chromatographic separations. Notation C CE Ci,j Ei,j E Ki N p q q0 Si,j S i,j T Wi,j X
liquid-phase solute concentration, mol/L feed concentration, mol/L equilibrium constant adsorption energy for ion j on site i, J/mol mean of the energy deviation dissociation equilibrium constant of component i, mol/L number of sites-1 skewness parameter resin equilibrium uptake, mmol/gdry resin resin ion exchange capacity, mmol/gdry resin separation factor for ion i relative to ion j average binary separation factor for ion i relative to ion j temperature, K heterogeneity parameter liquid-phase equivalent ionic fraction
Y zi
resin-phase equivalent ionic fraction valence of ion i
Greek letters ε j
bed void fraction bed resin density, gdry resin /cm3bed standard deviation of energy distribution function of ion j , J/mol residence time, s
Superscripts calc exp
calculated value experimental value
Acknowledgements The work of Maria João Moreira was supported by FCT: Grant BD/9052/96. The research was also supported by funds from Project PBIC/C/QUI/2416/95. References Bellot, J.C., Tarantino, R.V., Condoret, J-S., 1999. Thermodynamic modelling of multicomponent ion-exchange equilibira of amino acids. A.I.Ch.E. Journal 45, 1329–1341. Dye, S.R., DeCarli, J.P., Carta, G., 1990. Equilibrium sorption of amino acids by a cation-exchange resin. Industrial & Engineering Chemistry Research 29, 849–857. Jansen, M.L., Straathof, A.J.J., van der Wielen, L.A.M., Luyben, K.Ch.A.M, van den Tweel, W.J.J., 1996. Rigorous model for ion exchange equilibria of strong and weak electrolytes. A.I.Ch.E. Journal 42, 1911–1924. Jones, I.L., Carta, G., 1993. Ion exchange of amino acids and dipeptides on cation resins with varying degree of cross-linking, 1: equilibrium. Industrial & Engineering Chemistry Research 32, 107–117. Melis, S., Cao, G., Morbidelli, M., 1995. A new model for the simulation of ion exchange equilibria. Industrial & Engineering Chemistry Research 34, 3916–3924. Myers, A.L., Byington, S., 1986. Thermodynamics of Ion Exchange: Prediction of Multicomponent Equilibria From Binary Data. Ion Exchange Science and Technology. In: Rodrigues, A.E. (Ed.), NATO ASI, Series E. vol. 107, pp. 119–145. Pigford, L.R., Baker, B., Blum, D.E., 1969. An equilibrium theory of the parametric pumping. Industrial & Engineering Chemistry Fundamentals 8, 144–149. Saunders, M.S., Vierow, J.B., Carta, G., 1989. Uptake of phenylalanine and tyrosine by a strong-acid cation exchanger. American Institute for Chemical Engineers Journal 35 (1), 53–68. Stewart, W.E., Caracotsios, M., SZrensen, J.P., 1993. Software documentation of GREG-general regression software package for nonlinear parameter estimation (Version of July 1993). University of Wisconsin-Madison, Madison, WI. Zammouri, A., Chanel, S., Muhr, L., Grevillot, G., 2000. Ion-exchange equilibria of amino acids on strong anionic resins. Industrial & Engineering Chemistry Research 39, 1397–1408.