Competitive adsorption isotherm

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

Fig. 9-16. Competitive adsorption isotherms experimental (points) and model (lines) results.

The steps when designing a SMB which would allow one to process a given amount of feed per unit time have been described in detail [46, 57]. The procedure described was based on modeling of nonlinear chromatography. The procedure is rigorous, versatile and mainly requires the determination of competitive adsorption isotherms. If the adequate tools and methods are used, the procedure is not tedious and requires less work than is sometimes claimed. The procedure is briefly described below. 

The precise measurement of competitive adsorption isotherms not only of theoretical importance but may help the optimization of chromatographic processes in both analytical and preparative separation modes. The methods applied for the experimental determination of such isotherms have been recently reviewed [90], Frontal analysis using various flow rates can be successfully applied for the determination of competitive adsorption isotherms [91]. 

A. Seidel-Morgenstem, Experimental determination of single solute and competitive adsorption isotherms. J. Chromatogr.A 1037 (2004) 255-272. 

However, when substances A and B adsorb on two different active sites of the adsorbent surface, the competitive adsorption isotherm is not valid. 

FIGURE 1.10 Model calculations of ion exchange and competitive adsorption isotherm. 

Novasep, Champigneulles, etc., or with the SMB guide software from Knauer, Berlin, etc.) along the columns on the basis of known competitive adsorption isotherms or even easier empirically with the help of a chart that depicts visually the interrelationships between the system flow rates and the SMB design criteria as described by Antia [94], The steady-state concentration profile in such a SMB system is schematically illustrated in Figure 21-17 for an artificial two-component mixture with linear conditions assumed. 

Because of the limited surface capacity of the stationary phase, the column often operates under overloaded conditions. This is why a further increased sample load results in a smaller amount of the fraction adsorbed. Thus, nonlinear conditions prevail. A further complication in the preparative case is that the different types of components compete with each other for the same binding site, an effect that ultimately results in strong band interactions and band contaminations. The functions describing this complex behavior are called competitive adsorption isotherm parameters. Because of... 

Single-component isotherm parameters cannot always predict elution profiles with satisfied accuracy [122, 123], Therefore, to be able to predict accurate overloaded multi-component elution profiles where competition occurs competitive adsorption isotherm parameters are often necessary. Measurement of isotherms from a mixture is also often necessary because the pure enantiomers are not always accessible in large quantities. However, there exist only a small number of reports on the determination of multi-component adsorption isotherm parameters. FA can be used to determine binary isotherm data but it is time-consuming. The PP method is an alternative method to determine isotherm parameters from binary mixtures. It has been reported that the PP method works well up to weakly non-linear conditions [118, 119],... 

Preparative chromatography is widely used for the purification of different compounds, but this procedure needs to be optimized to achieve the minimum production costs. This can be done by computer-assisted modeling. However, this approach requires a priori determination of accurate competitive adsorption isotherm parameters. The methods to determine this competitive information are poorly developed and hence often a time limiting step or even the reason why the computer-assisted optimization is still seldom used. In this thesis in papers IV-VI, a new injection method was developed that makes it possible to determine these competitive adsorption isotherm parameters more easily and faster than before. The use of this new... 

I hope that my thesis can be used as a contribution in the future for analysis and validation in biotechnology, and also for the rapid determination of competitive adsorption isotherm parameters so that computer-assisted simulations may be used more extensively, in scaling-up and optimization of large scale chromatography. 

Figure 4.2 illustrates the best competitive adsorption isotherm model for benzyl alcohol and 2-phenylethanol [16]. The whole set of competitive adsorption data obtained using Frontal Analysis was fitted to obtain the Langmuir parameters column saturation capacity qs =146 g/1), equilibrium constant for benzyl alcohol bsA = 0.0143) and the equilibrium constant for 2-phenylethanol (bpE = 0.0254 1/g). The quality of the fit obtained with this simple model is in part explained by the small variation of the activity coefficients of the two solutes in the mobile phase when the solute concentrations increased from 0 to 50 g/1. The Langmuir competitive adsorption isotherm simplifies also in the case where activity coefficients are of constant value in both phases over the whole concentration range [17]. 

This isotherm model has been used successfully to accoimt for the adsorption behavior of numerous compounds, particularly (but not only) pairs of enantiomers on different chiral stationary phases. For example, Zhou et ah [28] foimd that the competitive isotherms of two homologous peptides, kallidin and bradyki-nine are well described by the bi-Langmuir model (see Figure 4.3). However, most examples of applications of the bi-Langmuir isotherm are found with enantiomers. lire N-benzoyl derivatives of several amino acids were separated on bovine serum albumin immobilized on silica [26]. Figure 4.25c compares the competitive isotherms measured by frontal analysis with the racemic (1 1) mixture of N-benzoyl-D and L-alanine, and with the single-component isotherms of these compounds determined by ECP [29]. Charton et al. foimd that the competitive adsorption isotherms of the enantiomers of ketoprofen on cellulose tris-(4-methyl benzoate) are well accounted for by a bi-Langmuir isotherm [30]. Fornstedt et al. obtained the same results for several jS-blockers (amino-alcohols) on immobilized Cel-7A, a protein [31,32]. 

The IAS theory was later extended to account for the adsorption of gas mixtures on heterogenous surfaces [52,53]. It was also extended to calculate the competitive adsorption isotherms of components from hquid solutions [54]. At large solute loadings, the simplifying assumptions of the LAS theory must be relaxed in order to account for solute-solute interactions in the adsorbed phase. The IAS model is then replaced by the real adsorbed solution (RAS) model, in which the deviations of the adsorption equilibrium from ideal behavior are lumped into an activity coefficient [54,55]. Note that this deviation from ideal beha dor can also be due to the heterogeneity of the adsorbent surface rather than to adsorbate-adsorbate interactions, in which case the heterogeneous IAS model [55] should be used. 

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