4860
Ind. Eng. Chem. Res. 1999, 38, 4860-4867
Displacement Chromatography of Amino Acids by Carbon Dioxide Dissolved in Water Amel Zammouri,* Simone Chanel, Laurence Muhr, and Georges Grevillot Laboratoire des Sciences du Ge´ nie Chimique, CNRS, 1 Rue Grandville, 54001 Nancy, France
This work is a study of the behavior of amino acids on columns of strong anion-exchange resins in displacement chromatography. A new technology allowing the minimization of pollutant reagents was applied. It consists of using dissolved carbon dioxide in water as a displacer. The emphasis was also placed on the use of unbuffered solutions of amino acids in water. A mathematical model for the displacement process was constructed. Ion-exchange equilibrium relations based on variable selectivity coefficients and dissociation reactions of amino acids were incorporated in the model. It is shown that, in the case of negligible hydrodynamic and kinetic resistances, pure and concentrated products could be obtained. Introduction Because amino acids are used in the food and pharmaceutical industries, they must be produced at great purity. Ion exchange is a powerful technique for amino acids separations and purification because of their amphoteric nature. By adjustment of the solution pH, they can be positively or negatively charged and they can thus be fixed or eluted from ion-exchange resins. Several processes were developed for amino acids separations on the basis of ion-exchange chromatography. De Carli et al.5 used a continuous annular chromatograph for amino acids purifications. Maki9 applied a moving simulated bed to glutamic acid and glutionine separation. Casillas et al.2 used modified polystyrene divinyl benzene resins for amino acid separations. Carta et al.1 and Saunders et al.14 studied displacement chromatography of unbuffered solutions of amino acids. With large-scale processes to separate a multicomponent amino acid mixture, use arrangements of different types of resins and buffers, acids, and salts. By choosing the proper resin and adjusting the pH, it is possible to separate the various charge groups through selective uptake on the various ion-exchange resins. Several successive binary cuts are thus able to separate a multicomponent mixture in the same way as binary distillation for the separations based on liquid-vapor equilibria. Figure 1 shows an example of a pattern used to separate several acidic, neutral, and basic amino acids. The first bed, a weakly basic anion-exchange resin, uptakes the acidic amino acids which are subsequently desorbed using an acetate buffer. The neutral and basic amino acids are fed on a strong cation exchanger buffered at pH 7. Basic amino acids are fixed and after they are eluted with HCl, it is apparent that large amounts of various chemicals (acids, bases, buffers, etc.) are needed in the process and will appear in the products, thus leading to a new purification step, or in the wastes. In fact, ion exchange is intrinsically pollutant. It consumes and produces pollutant reagents. Because ecological exigencies become more and more severe, in this work a nonpollutant material, CO2 dissolved in water, was used as a displacer of amino acids. This idea was inspired from the CARIX process (carbonate regeneration of ion exchangers) developed by
Figure 1. Separation of amino acids with ion exchange.
Ho¨ll and Hagen6 for partial regeneration of a mixed anionic and cationic resins bed where the regenerant was CO2 dissolved in water under 5 bar. In the case of amino acids the method was first proposed for the recovery, elution, and concentration of individual amino acids.12 In this study, it is extended to displacement chromatography. Three amino acids have been considered: two of the neutral type with very close pI, phenyalanine (pI ) 5.48), and glycine (pI ) 5.97). This binary mixture represents a rather difficult separation. Histidine (pI ) 7.59) is of the basic type. The displacement effect using carbon dioxide solutions being obtained by fixation of the bicarbonate or carbonate anions, an anion-exchange resin is required. Two very
10.1021/ie990302d CCC: $18.00 © 1999 American Chemical Society Published on Web 11/11/1999
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4861
different types of resins have been used, gel and macroporous material, the latter with smaller particle sizes. Different mass-transfer behaviors are thus expected.
The negatively charged species concentrations are obtained as functions of their total concentrations and hydrogen ion concentration by arranging equations 5-11:
Theory
CA- ) i
CO2 Solubility in Water. In our experiments, CO2 pressure does not exceed 5 bar, so the concentration of CO2 dissolved in water can be calculated by Henry’s law:
CCO2 ) HcpCO2
1140 - 5.3 Te
(2)
He in mol/(L atm) and Te in K. Liquid-Phase Dissociation Equilibria. At CO2 dissociation in water, the following reactions take place in the liquid phase: K1
CO2 + H2O 798 HCO3- + H+, K2
HCO3- 798 CO32- + H+,
(4)
In addition to carbonic acid dissociation in solution, the neutral amino acids Phe, Gly, and the basic amino acid His dissociate to give the following products: +
(
+
(
-
+
Phe T Phe + H , Phe T Phe + H , Gly+ T Gly( + H+, (
-
+
CHis CH+ CH+2 CH+3 1+ + + K3His K2HisK3His K1HisK2HisK3His
pK1,Phe ) 1.83 pK2,Phe ) 9.13 pK1,Gly ) 2.34
(5) (6)
∑i CA
-
i
+ COH- + CHCO3- + 2CCO32- )
∑i CA
Xi )
(8)
His2+ T His+ + H+,
pK1,His ) 1.82
(9)
His+ T His( + H+,
pK2,His ) 6.00
(10)
His( T His- + H+,
pK3,His ) 9.17
(11)
The total concentration of Phe or Gly is given by
CAi ) C(Ai + C+Ai + C-Ai
∑k
|zk|Czkk
(16)
+ COH-
zi is the anionic valence of species i. Ion-Exchange Equilibria. The binary exchange of amino acids or carbonic acid with an anionic resin in the hydroxyl form can be formulated by
|zA|R - OH + AzA T R|zA| - AzA + |zA|OH- (17) This equilibrium is described quantitatively by means of the separation factor:
YAXOHYOH-XA
(18)
This coefficient is rarely constant and generally varies with ionic fraction in solution. Myers and Byington10 attributed this variation to the energetic heterogeneity of exchange sites. For low ionic fractions, exchange takes place on the more energetic sites, leading to a high initial selectivity. When these sites become saturated, exchange takes place on less energetic sites and hence the selectivity decreases with the ionic fraction. To represent energetic heterogeneity, Myers and Byington selected a discrete binomial distribution of (n + 1) sites. The probability of finding a site of type i is
pi ) Cni pi(1 - p)n-i
0
