New insights on mass transfer kinetics in chromatography

The mass transfer kinetics of thiourea, phenol, ethylbenzene, propylbenzene, butylbenzene, and amylbenzene were studied on a Gemini‐C₁₈ (5 μm, 110 A?, 375 m²/g) column (150 mm × 4.6 mm) eluted with methanol/water solutions (100, 90, and 20% v/v). Each of the successive steps of the mass transfer of these solutes (axial diffusion, eddy dispersion, film mass transfer resistance, and transparticle mass transfer resistance) was unambiguously measured, using a combination of the peak parking method, the total pore blocking method, and moment analysis, in a wide range of reduced linear velocities. The results obtained offer new insights on the mass transfer kinetics in chromatographic columns. They show first that the eddy dispersion A‐term is strongly correlated with the particle porosity. The complex, anastomosed transcolumn flow pattern causes extra band broadening. This transcolumn effect was found to be markedly smaller with porous particles than with nonporous particles of the same size. Second, the film mass transfer coefficient of retained compounds is smaller for porous than for nonporous particles, a result consistent with concentration gradients being steeper at the wall of solid particles than across the entrance surface of pores. The external mass transfer coefficient decreases with increasing fraction of the surface area of the particles that is open to pores, e.g., with increasing particle porosity. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Importance of sample intraparticle diffusivity in investigations of the mass transfer mechanism in liquid chromatography

The effective diffusivity of a nonretained (thiourea) and of a strongly retained (phenol) compounds were measured with the peak parking method in two different columns (both 150 × 4.6 mm) packed with two types of porous particles having different mesopore sizes [5 μm Jupiter‐C₁₈, 320 Å and Luna(2)‐C₁₈, 100 Å]. The eluent was a methanol–water mixture (10/90 v/v) and the temperature 294 K. The effective diffusivity data acquired were used to determine the intraparticle diffusivity, D_p, based on two different diffusion models. The first one assumes that the diffusion fluxes across the particles and in the interparticle volume are additive (parallel diffusion model). The second model was rigorously derived on the basis of the effective medium theory of diffusion (diffusion model) in a binary composite medium (particles + interparticle volume). In both models, it was assumed that the rate of equilibrium between the liquid and the solid phases was infinitely faster than the rate of axial diffusion along the column at zero flow rate. Both models provide physically meaningful intraparticle diffusivity coefficients that take into account the average mesopore size of the particles, their specific surface area, and the retention factor of the analyte. Although the actual effective intraparticle diffusivity remains unknown, these result confirm that the mass transfer resistance due to diffusion through the porous particles has almost negligible effects in reversed phase liquid chromatography due to the importance of surface diffusion. Combining the results of the peak parking method with the h data measured at high linear velocities allows the unambiguous measurement of the film mass transfer and the surface diffusion coefficients. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Use of axial dispersion and plug flow models for determination of axial mixing and mass transfer coefficient in an l‐shaped pulsed packed extraction column

In this study, the volumetric overall mass transfer and phases axial mixing coefficients have been investigated in a pilot plant of an L‐shaped pulsed packed extraction column by using two liquid systems of toluene/acetone/water and n‐butyl/acetone/water. The mass transfer performance has been evaluated using two methods of axial dispersion and a plug flow model. The effect of the operational variables and physical properties, including the dispersed and continuous phases flow rates, pulsation intensity, and interfacial tension, on mass transfer and phases axial mixing coefficients have been considered. It has been found that the pulsation intensity and the continuous phase flow rate seriously affect the mass transfer coefficient, however, the dispersed phase flow rate has a weaker effect. Also, the axial mixing of a phase is strongly affected by the pulsation intensity and the flow rate of the phase itself and it is not affected by the second phase flow rate. Finally, new correlations are proposed to accurately predict the mass transfer and axial mixing coefficients.     

Mixing and solid‐liquid mass transfer characteristics in a three phase pulsed plate column with packed bed of solids in interplate spaces—a novel aerobic immobilized cell bioreactor

BACKGROUND: The pulsed plate column (PPC) with packed bed of solids in the interplate spaces finds use as a three phase aerobic bioreactor and is a potential heterogeneous catalytic reactor. Good knowledge of the extent of mixing in the liquid phase and solid‐liquid mass transfer coefficient are essential for modeling, design and optimization of these columns. The present work aims at the study of liquid phase mixing and solid–liquid mass transfer characteristics in a three phase PPC. RESULTS: Residence time distribution studies were performed. Dispersion number was found to increase with increase in liquid superficial velocities, frequency of pulsation, amplitude of pulsation and the vibrational velocities. Increase in frequency and amplitude of pulsation, and hence increase in vibrational velocity, resulted in increase of the solid–liquid mass transfer coefficient. CONCLUSIONS: The mixing behaviour in this contactor approximated a mixed flow behaviour. The three phase PPC was found to outperform many other kinds of three phase contactors in terms of solid liquid mass transfer characteristics. Empirical correlations developed can be used for the determination of solid–liquid mass transfer coefficients for three phase PPC and hence can facilitate the design, scale‐up and modeling of these columns, when used as chemical or biochemical reactors. Copyright © 2011 Society of Chemical Industry