Dynamics of Capillary Electrochromatography : Experimental Study of Flow and Transport in Particulate Beds

The chromatographic performance with respect to the flow behavior and dispersion in fixed beds of nonporous and macroporous particles (having mean intraparticle pore diameters of 41, 105, and 232 nm) has been studied in capillary HPLC and electrochromatography. The existence of substantial electroosmotic intraparticle pore flow (perfusive electroosmosis) in columns packed with the macroporous particles was found to reduce stagnant mobile mass transfer resistance and decrease the global flow inhomogeneity over the column cross-section, leading to a significant improvement in column efficiency compared to capillary HPLC. The effect of electroosmotic perfusion on axial dispersion was shown to be sensitive to the mobile phase ionic strength and mean intraparticle pore diameter, thus, on an electrical double layer interaction within the particles. Complementary and consistent results were observed for the average electroosmotic flow through packed capillaries. It was found to depend on particle porosity and distinct contributions to the electrical double layer behavior within and between particles. Based on these data an optimum chromatographic performance in view of speed and efficiency can be achieved by straightforward adjustment of the electrolyte concentration and characteristic intraparticle pore size. Copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [accessed 2013 November 27th

Quantifying Morphology and Diffusion Properties of Mesoporous Carbon from High-Fidelity 3D Reconstructions

A reliable quantitative analysis in electron tomography, which depends on the segmentation of the three-dimensional reconstruction, is challenging because of constraints during tilt-series acquisition (missing wedge) and reconstruction artifacts introduced by reconstruction algorithms such as the Simultaneous Iterative Reconstruction Technique (SIRT) and Discrete Algebraic Reconstruction Technique (DART). We have carefully evaluated the fidelity of segmented reconstructions analyzing a disordered mesoporous carbon used as support in catalysis. Using experimental scanning transmission electron microscopy (STEM) tomography data as well as realistic phantoms, we have quantitatively analyzed the effect on the morphological description as well as on diffusion properties (based on a random-walk particle-tracking simulation) to understand the role of porosity in catalysis. The morphological description of the pore structure can be obtained reliably both using SIRT and DART reconstructions even in the presence of a limited missing wedge. However, the measured pore volume is sensitive to the threshold settings, which are difficult to define globally for SIRT reconstructions. This leads to noticeable variations of the diffusion coefficients in the case of SIRT reconstructions, whereas DART reconstructions resulted in more reliable data. In addition, the anisotropy of the determined diffusion properties was evaluated, which was significant in the presence of a limited missing wedge for SIRT and strongly reduced for DART

On the Parametrisation of Lattice Boltzmann Method in Pore-Scale Flow Simulations

We analyse impact of parametrisation in lattice Boltzmann simulations of flow in complex geometries. For the input geometries we use four sets of regularly and irregularly packed spheres (“packings”) with known accurate solution for the permeability or drag. All four geometries have porosity equal to 0.366 but different microstructure resulting in their different permeability values. We vary spatial resolution in the range between 5 and 750 lattice nodes per sphere diameter,observe different behaviour of the numerical error for several resolution sub-ranges and address them in detail providing practical guidelines for increasing accuracy in low-resolutions imulations, which are typical for practical problems

Int. J. Numer. Meth. Fluids

In this article we are concerned with an extension of the lattice-Boltzmann method for the numerical simulation of three-dimensional electroosmotic flow problems in porous media. Our description is evaluated using simple geometries as those encountered in open-channel microfluidic devices. In particular, we consider electroosmosis in straight cylindrical capillaries with a (non)uniform zeta-potential distribution for ratios of the capillary inner radius to the thickness of the electrical double layer from 10 to 100. The general case of hetergeneous zeta-potential distributions at the surface of a capillary requires solution of the following coupled equations in three dimensions: Navier-Stokes equation for liquid flow, Poisson equation for electrical potential distribution, and the Nernst-Planck equation for distribution of ionic species. The hydrodynamic problem has been treated with high efficiency by code parallelization through the lattice-Boltzmann method. For validation velocity fields were simulated in several microcapillary systems and good agreement with results predicted either theoretically or obtained by alternative numerical methods could be established. Results are also discussed with respect to the use of a slip boundary condition for the velocity field at the surface. Copyright © 2004 John Wiley & Sons, Ltd. [accessed 2013 November 27th

Stagnant mobile phase mass transfer in chromatographic media: Intraparticle diffusion and exchange kinetics

Pulsed field gradient nuclear magnetic resonance has been successfully applied to a direct and detailed experimental study of topological and dynamic aspects involved in the exchange of small, nonsorbed fluid molecules between the intraparticle pore network and the interparticle void space in chromatographic columns packed with spherical-shaped, porous particles. The approach provides quantitative data about the effective, intraparticle diffusion coefficients (and tortuosity factors) and about the associated, diffusion-limited mass transfer kinetics, including stagnant boundary layer contributions. In view of the recorded exchange kinetics, an analytical description for solute diffusion into/out of spherical particles is offered and addresses the influence of the particle size distribution and particle shape on the observed mass transfer rates and calculated diffusivities. The combined analyses of the steady-state intraparticle pore diffusion data and the associated exchange kinetics with Peclet numbers up to 500 reveals the existence of external stagnant fluid where all the interparticle fluid-side resistance to diffusion is localized. It is represented by a thin stagnant boundary layer around the particles and can be accounted for by the introduction of a hydrodynamically effective particle diameter which is found to depend on the Peclet number. The approach appears to be promising for a selective, detailed study of the boundary layer dynamics. Concerning the investigation of different chromatographic media and intraparticle morphologies, we demonstrate that the actual correlation (or randomness) of interconnection between intraparticle pores of different size has a profound effect on the observed tortuosity factors and the diffusion-limited stagnant mobile phase mass transfer kinetics. Compared to intraparticle pore networks with a random assignment of different pore sizes, hierarchically structured bidisperse porous particles offer a superior network topology, which can form the basis for an increased chromatographic performance

Indications du thalidomide en 2005 et applications dans le traitement de l'érythème noueux lépreux

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