Dorota Antos

Comparative modeling of breakthrough curves of bovine serum albumin in anion-exchange chromatography.

The experimental results of a previous study of the mass transfer kinetics of bovine serum albumin (BSA) in ion-exchange chromatography, under nonlinear conditions, were reevaluated using the general rate model of chromatography. Solutions of this model were obtained numerically. The influences of axial dispersion, the resistance to mass transfer from the bulk mobile phase to the surface of the packing particles, and the intraparticle mass transfer resistances on the profiles of the breakthrough curves of BSA were investigated. The results obtained are compared to those of a previous investigation of the same data, using the simple transport-dispersive model and the lumped pore diffusion model. The results obtained show that the use of an oversimplified model for the analysis of chromatographic data can lead to erroneous interpretations of the experimental data and to misunderstandings of the fundamentals of the processes involved. Finally, a theoretical comparison between the properties and the range of application of the three models is provided.

Application of gradients in the simulated moving bed process

In the last decade the simulated moving bed (SMB) process was implemented successfully in the pharmaceutical industry to perform difficult separations of valued added products. Mainly liquid solvent mixtures with adjusted but constant composition were applied as the mobile phases (isocratic operation). Only recently the potential of modulating the solvent strength during the process was realised (gradient operation). The purpose of this paper is to present the results of studying a two-step gradient SMB process using a weaker solvent to introduce the feed and a stronger solvent for efficient adsorbent regeneration. A mathematical model for the simulation of this process was proposed and solved numerically. Systematic calculations were performed in order to identify suitable operating conditions as a function of averaged desorbent concentrations taking into account the locally changing nonlinear adsorption equilibria. Based on the predictions and on preliminary experimental investigations devoted to determine the required model parameters the separation of a mixture of two cycloketons using normal-phase silica was designed. A two-step gradient SMB process was applied using pure n-hexane as the weak solvent in the feed stream and pure ethyl acetate as the strong solvent in the desorbent stream. The achieved purities and concentrations in the raffinate and extract streams confirmed the potential of the gradient technique applied. In agreement with the theoretical results a significant improvement in the process performance compared to the common isocratic SMB separation was found.

Two-step solvent gradients in simulated moving bed chromatography. Numerical study for linear equilibria.

The application of gradients in simulated moving bed (SMB) chromatography has recently attracted interest as a method for further improving the performance of this continuous separation process. One possible implementation of gradients consists in setting the solvent strength in the desorbent stream higher than that in the feed stream. As a result, the components to be separated are more retained in the zones upstream of the feed position and more easily eluted in the zones downstream of the feed position. If a liquid mobile phase is used, gradients can be created by dosing different solvents into the feed and desorbent ports. In a closed-loop gradient SMB arrangement the solvent strength within the unit will depend on the two feed compositions and on the characteristic flow-rates of the process. In this work an equilibrium stage model describing a true moving bed process is used to analyze numerically the main features of a two-step gradient SMB process. The adsorption isotherms are assumed to be always linear under isocratic conditions. The relevant Henry constants depend in a nonlinear manner on the composition of the solvent. Based on numerical simulations the impact of the two inlet solvent compositions is demonstrated in terms of the size and shape of regions of applicable flow-rates. Different strategies of designing the process are discussed and compared with respect to maximizing productivities and minimizing desorbent requirements.

Concentration dependence of lumped mass transfer coefficients: Linear versus non-linear chromatography and isocratic versus gradient operation

The general rate model provides a reliable platform to predict elution bands in both linear and non-linear chromatography provided the required equilibrium functions and the coefficients quantifying the mass transfer in and around the particles are available. If further the variation of the equilibrium functions with changes in the mobile phase composition is known, this model is also able to predict gradient elution chromatography. Significant disadvantages of the model are the need to specify three kinetic coefficients and the amount of computing time required for the numerical solution of the underlying equations. Thus, several simplified models have been suggested lumping mass transfer resistances together. In this work the accuracy of predicting chromatographic bands based on the numerical solution of two lumped models has been analyzed. Elution profiles calculated by (a) the transport-dispersive and (b) the equilibrium-dispersive models were compared between each other and with the solution of the more detailed general rate model. In the analysis performed both linear and non-linear chromatography was considered under isocratic and gradient conditions.

Calculation of chromatographic band profiles with an implicit isotherm.

The numerical method for solving the mathematical model of chromatography process coupled with implicit isotherm has been proposed. The exemplary predictions of elution band profiles were performed for competitive adsorption data of 2-phenylethanol and 3-phenylpropanol on ODS-silica with methanol-water as the mobile phase. The simulations of chromatography process with various isotherm models taking into account lateral interactions in adsorbed phase and surface heterogeneity were discussed.

Modified Rouchon and Rouchon-like algorithms for solving different models of multicomponent preparative chromatography

Modified Rouchon and Rouchon-like algorithms were used to solve multicomponent equilibrium-dispersive chromatographic models as well as a more general lumped pore diffusion model. The modified algorithms enable remarkable reduction of computation time and avoid computation errors that result from the original Rouchon approach for some cases of multicomponent chromatography. A comparison is given between the general chromatographic model, the lumped pore diffusion model and the equilibrium-dispersive model.

Determination of mobile phase effect on single-component adsorption isotherm by use of numerical estimation.

Numerical estimation was used to determine adsorption isotherm parameters of a single-component in a normal-phase system. The distribution isotherm of methyl deoxycholate was described between the mobile phase containing hexane, ethyl acetate, methanol with varied concentration and a silica gel adsorbent. The effect of the mobile phase composition on the isotherm parameters and the band profiles was investigated. The results obtained were used to simulate the overload gradient elution. The validity of the method proposed was verified by comparison of the computer simulations with the experimental band profiles.

Theoretical study of using simulated moving bed chromatography to separate intermediately eluting target compounds.

This study deals with the separation of ternary mixtures based on Simulated Moving Bed chromatography to isolate target components with intermediate adsorption strength. To overcome the limitations of conventional SMB systems, which are designed for binary separations and unable to perform center-cut separations, several modifications have been proposed. The purpose of this study was to provide a theoretical comparison of several of advanced SMB configurations capable to separate ternary mixtures. Emphasis is given to those techniques, which have already been used in practice, and to those having potential for future industrial application. SMB cascades connected in series via the extract or raffinate ports of the first unit are analyzed and compared are as well as an integrated 8-zone SMB unit with internal recycle. Additionally, the commercialized pseudo SMB process (JO process) was evaluated. The performance of these modified SMB systems was investigated based on the assumption of linear adsorption isotherms for all three components considering three separation problems characterized by different separation difficulty. Besides the influence of separation factors, the concentrations of the impurities in the feed mixture and the purity requirements for the target product were studied systematically.

Influence of the sample-solvent on protein retention, mass transfer and unfolding kinetics in hydrophobic interaction chromatography.

Typical mobile phase employed in hydrophobic interaction chromatography contains cosmotropic salts, which promote retention and simultaneously reduce the protein solubility in the mobile phase. To increase mass overloading in the separation process the protein can be dissolved in a sample-solvent with concentration of salt lower than that in the mobile phase or in salt free solutions. However, this methodology may cause band splitting and band deformation, which results in yield losses. In this study, these phenomena were analyzed based on the retention behavior of two model proteins, i.e., lysozyme and bovine serum albumin. Retention of these proteins was accompanied by strong band broadening originated from slow rates of mass transfer and/or of adsorption-desorption process involving the protein conformational changes. The mass transport resistances and unfolding kinetics were found to contribute to the sample-solvent effects. To avoid band deformations the process variables such as the salt concentration and temperature were adjusted in such a way that complete resolution between band profile of the sample-solvent and the protein was achieved. For the process simulation a dynamic model, which accounted for underlying kinetics was used. General guidelines of the process design were developed.

Study of the competitive isotherm model and the mass transfer kinetics for a BET binary system

The competitive adsorption behavior of the binary mixture of phenetole (ethoxy-benzene) and propyl benzoate in a reversed-phase system was investigated. The adsorption equilibrium data of the single-component systems were acquired by frontal analysis. The same data for binary mixtures were acquired by the perturbation method. For both compounds, the single-component isotherm data fit best to the multilayer BET model. The experimental overloaded band profiles are in excellent agreement with the profiles calculated with either the general rate model or the modified transport-dispersive models. The competitive adsorption data were modeled using the ideal adsorbed solution (IAS) theory. The numerical values of the coefficients were derived by fitting the retention times of the perturbation pulses to those calculated using the IAS theory compiled with the coherence conditions. Finally, the elution profiles of binary mixtures were recorded. They compared very well with those calculated. As a characteristic feature of this case, an unusual retainment effect of the chromatographic band of the more retained component by the less retained one was observed. The combination of the General Rate Model and the adsorption isotherm model allowed an accurate prediction of the band profiles.