Does the fluid elasticity influence the dispersion in packed beds?

Modeling of flow in a polymeric chromatographic monolith

The flow behavior of a commercial polymeric monolith was investigated by direct numerical simulations employing the lattice-Boltzmann (LB) methodology. An explicit structural representation of the monolith was obtained by serial sectioning of a portion of the monolith and imaging by scanning electron microscopy. After image processing, the three-dimensional structure of a sample block with dimensions of 17.8 μm × 17.8 μm × 14.1 μm was obtained, with uniform 18.5 nm voxel size. Flow was simulated on this reconstructed block using the LB method to obtain the velocity distribution, and in turn macroscopic flow properties such as the permeability and the average velocity. The computed axial velocity distribution exhibits a sharp peak with an exponentially decaying tail. Analysis of the local components of the flow field suggests that flow is not evenly distributed throughout the sample geometry, as is also seen in geometries that exhibit preferential flow paths, such as sphere pack arrays with defects. A significant fraction of negative axial velocities are observed; the largest of these are due to flow along horizontal pores that are also slightly oriented in the negative axial direction. Possible implications for mass transfer are discussed.

A computational study of the porosity effects in silica monolithic columns

We report on a theoretical study of the influence of the through-pore porosity on the main chromatographic performance parameters (reduced theoretical plate height, flow resistance, and separation impedance) of silica monoliths. To investigate this problem devoid of any structural uncertainties, computer-generated structural mimics of the pore geometry of silica monolithic columns have been studied. The band broadening in these synthetic monoliths was determined using a commercial Computational Fluid Dynamics (CFD) software package. Three widely differing external porosities (epsilon = 0.38, epsilon = 0.60, and epsilon = 0.86) are considered and are compared on the basis of an identical intra-skeleton diffusivity (Ds = 5 x 10(-10)m2/s), internal porosity (epsilon(int) = 0.5), and for the same phase retention factor (k' = 1.25). Since the data are obtained for perfectly ordered structures, the calculated plate heights and separation impedances constitute the ultimate performance ever to be expected from a monolithic column. It is found that, if silica monoliths could be made perfectly homogeneous, domain size-based reduced plate heights as small as h(min) approximately 0.8 (roughly independent of the porosity) and separation impedances as small as Emin approximately 130 (epsilon = 0.60) and Emin approximately 40 (epsilon = 0.86) should be achievable with pure water as the working fluid. The data also show that, although the domain size is a much better reduction basis than the skeleton size, the former is still not capable of bringing the van Deemter curves of different porosity columns into perfect agreement in the C term dominated velocity range. It is found that, in this range, large porosity monoliths can be expected to yield smaller domain size-based reduced plate heights than small porosity monoliths.

Model column structure for the analysis of the flow and band-broadening characteristics of silica monoliths

We report on the use of commercial computational fluid dynamics software to study the band broadening in a perfectly ordered three-dimensional model structure, the so-called tetrahedral skeleton column (TSC), selected for its close geometrical resemblance to the specific pore network topology of silica monoliths. Van Deemter plots are presented for the case of a species flow through a non-porous skeleton and for the case of a retained component (k' = 1) in a porous skeleton (mesopore porosity epsilon = 0.6 in both cases). Using the flow domain as the characteristic scaling dimension, the TSC model yields reduced plate heights as small as h(min) = 0.8 and separation impedances as small as Emin = 120 for a retained component with k' = 1. The very small reduced plate heights for the TSC model can without any doubt largely be attributed to the perfect homogeneity of the considered model structure: the B and C terms are similar to those obtained in real silica monoliths with similar external porosity, whereas the A term is significantly (about a factor of 10) smaller. The present study hence suggests that further experimental work to obtain more homogeneous silica networks could yield large gains in reduced plate height and separation impedance. Comparing the three-dimensional TSC model with a 2D array of cylinders, it was found that the use of the domain size as the characteristic dimension in the reduced plate height expression is much more appropriate than the use of the skeleton size, hence validating earlier approaches adopted in the literature.

A correlation for the pressure drop in monolithic silica columns

To gain insight into how the pressure drop in monolithic silica columns is determined by the microscopic details of the pore structure, a series of well-validated computational fluid dynamics simulations has been performed on a simplified model structure, the so-called tetrahedral skeleton column. From these simulations, a direct correlation between the pressure drop and two main structural properties (skeleton thickness and column porosity) of the monolithic skeleton could be established. The correlation shows good agreement with the experimental pressure-drop data available from the literature on silica monoliths, especially when a correction for the flow-through pore size heterogeneity is made. The established correlation also yields a much more accurate representation of the relation between the flow resistance and the bed porosity than does the Kozeny-Carman model, making it much better suited for porosity optimization calculations.