Tong Yun

Effect of the homogeneity of the column set on the performance of a simulated moving bed unit: I. Theory

Although it is impossible to manufacture identical columns for use in a simulated moving bed (SMB) process, theoretical studies assume that all the columns in an SMB unit have identical characteristics. In practice, calculations in modeling and optimization studies are made with the average values of each column parameter set. In this report, the effects on SMB process performance caused by column-to-column fluctuations of the parameters are discussed. As a first step, we show how the differences in porosity of the columns may be taken into account with a revised set of separation conditions. Reductions in the purity of the extract and the raffinate streams are quantitatively related to the column-to-column fluctuations of the retention times of the two components arising from these porosity differences. For the sake of simplicity, the discussion first addresses the case of a four-column SMB operating under linear conditions. Then, the scope is extended to the cases of SMB units incorporating several columns in each section and to SMB units operating under nonlinear conditions.

Modeling of radial heterogeneity in chromatographic columns Columns with cylindrical symmetry and ideal model

Elution band profiles are calculated for radially heterogeneous cylindrical columns, assuming an infinite rate of mass transfer kinetics and no axial dispersion. Steady-state flow-rate is assumed, with a given, cylindrical radial distribution of the velocity. Parabolic, cuspal and cosinusoidal distributions were studied, all three with either the velocity maximum or minimum in the column center. In all cases, strong changes in the band profiles appear for velocity distributions in which the ratio of the maximum to the minimum velocity is as low as 1.05. Some of the profiles obtained appear to be similar to experimental profiles.

Visualization of the heterogeneity of column beds

Columns were packed with alternating layers of virgin Zorbax C18 and blue colored Zorbax, treated with the dye Cibacron Blue. After consolidation of the column packing, the beds were extruded off the column, examined and photographed as they were, then cut along an axial plane, and these sections were examined and photographed. Evidence of radial heterogeneity of the distribution of the layers of the two packing materials in the bed after consolidation was observed. Friction of the bed along the column wall during consolidation was demonstrated.

Modeling of radial heterogeneity in chromatographic columns. II. Separation of a two-component mixture on a column with cylindrical symmetry

The elution band profiles of two partially separated components were calculated for a radially heterogeneous cylindrical column, using a modified equilibrium-dispersive model, assuming no radial dispersion. Steady-state flow-rate is assumed with a given cylindrical radial distribution of the velocity. The results show a marked degradation of the column performance when the ratio of the mobile-phase velocities at the column center and at the wall differs from unity by more than a few percent.

Profiles of large-size system peaks and vacancy bands in liquid chromatography II. Comparison of experimental and calculated profiles

The profiles of large-size system peaks and vacancies have been studied andcompared. A reversed-phase system was used with analytical-size column packed with ZORBAX C18 spherical particles and methanol-water as the mobile phase. The analyte was 3-phenyl-1-propanol. The equilibrium-dispersive model of chromatography was used to calculate band profiles using the measured isotherm data and the column efficiency. The isotherm data were determined using frontal analysis and could be fitted to a simple Langmuir equation. Excellent agreement was found between experimental and calculated results.

Effect of column efficiency on the internal concentration profiles and the performance of a simulated moving-bed unit in the case of a linear isotherm

Abstract Simulated moving-bed (SMB) separators have been used successfully to separate or purify continuously two components with different adsorption selectivities. A detailed understanding of their behavior is still lacking, however. A numerical solution of the equilibrium—dispersive model allows the calculation of the concentration profiles along the columns, of their progressive shifts during a cycle, of the progressive build-up of these concentration profiles in each column during the start-up period, of the concentration profiles under steady-state and of the concentration histories at the extract and raffinate ports. These profiles are illustrated and discussed. The results obtained are compared to those of the exact analytical solution of the linear, ideal model previously developed. This comparison illustrates the influence of column efficiency on the time needed to reach steady-state operation.

Theoretical analysis of the behavior of a centrally injected band in a homogeneous chromatographic column

Abstract A new implementation of the equilibrium–dispersive model permits the study of problems requiring two-space dimensions for their modeling, such as a nonhomogeneous column with a cylindrical symmetry, a nonplanar injection with a cylindrical symmetry, or a cylindrical central injection. The migration and broadening of a cylindrical band coaxial with the column but narrower, moving along a homogeneous column is studied. The axial and radial profiles of the band inside the column and the elution profiles are calculated. These profiles depend on both the radial and the axial dispersion coefficients. The radial dispersion coefficient decreases with increasing mobile phase velocity and is usually low, much lower than the axial dispersion coefficient. Thus, the material in a relatively wide central injection made on a wide bore column may never have a chance to get close to the column wall. The clearance required at injection is estimated.

Influence of the heat of adsorption on elution band profiles in nonlinear liquid chromatography

Fundamental investigations of band profiles in nonlinear liquid chromatography have always assumed isothermal conditions. This is correct in linear, analytical chromatography, in which case the solute concentrations are very small and the influence of the heat of adsorption on the band profiles can always be neglected. However, this is no longer exact in nonlinear chromatography because the concentrations of the feed components are usually high. If the heat of adsorption is large enough, the chromatographic system cannot be considered as isothermal. The differential heat and mass balance equations are then coupled. Experimental data acquired in a typical case illustrate the effect. These data fit well to a lumped kinetic model with slow heat transfer kinetics. Further calculations suggest that the influence of the heat of adsorption on actual separations is small and could be negligible in almost all cases.

Shock layer analysis for a single-component in preparative elution chromatography

The onset of the nonlinear behavior of band profiles in elution chromatography is investigated by studying the profile of the shock layer caused by a finite mass transfer resistance in the absence of axial dispersion for a single component. A closed-form analytical expression of the shock layer is obtained for a parabolic isotherm. This solution depends on a dimensionless number which may be used to characterize the degree of nonlinear behavior of the band profiles and to select the model most appropriate for their accurate description. The profiles resulting from this solution are compared with those obtained by numerical calculations under different conditions to assess the influence of the assumptions made. The differences observed illustrate the sensitivity of the elution profiles to small changes in the equilibrium isotherm.

Study of the operating conditions of radial compression columns in preparative chromatography

Performance and stability were tested for radial compression columns packed with spherical particles of an octadecyl bonded silica. The performance of the radial columns remained unchanged after approximately 500 h of constant use, including a number of solvent gradient runs. The performance of analytical columns packed with the same material was comparable to that of the radial columns. The column permeability decreases and the inlet pressure required to maintain a constant flow-rate increases with increasing radial compression pressure. Maximum column efficiency was obtained for a value of radial compression pressure approximately equal to 25 bar.