Hydrodynamic and Disperson behaviour of an analytical silica monolith reconstructed from Sub-microtomographic scans using computational fluid dynamics

Abstract

Downstream separation of mixtures in a variety of fields such as protein purification, quality control of drugs, pharmacokinetic studies, and determination of pollutants or food additives has traditionally been carried out using particulate HPLC columns where the separation efficiency increases with decreasing particle size, at the cost of higher operating pressures. Monoliths are a class of chromatographic columns cast in the form of tubes, rods or disks as a single and co-continuous block that is porous and permeable. A high external porosity resulting from a regular network of through-macropores and a mesoporous skeleton network provide a combination of low hydraulic resistance to the mobile phase and enhanced mass transfer rates of sample molecules through the column, respectively. In this research, an analysis of the transport properties of the bulk homogeneous core of a silica monolith is presented via direct numerical simulations in a topological model reconstructed from 3D nanotomographic scans. A commercially available silica monolith (Chromolith?) was scanned at three isotropic resolutions to investigate the resolution required to adequately capture the throughpore and skeleton-surface heterogeneity. Hydrodynamic behaviour of the macropore space in domains representative of the bulk porosity was analysed via computational fluid dynamics. A 30?m cubic unit cell at 190nm scanning resolution was found to be representative of the Darcy permeability, with a ?6% deviation from experimental and reported literature data. Transcolumn eddy dispersion, reported to be the single-most dominant contributor of inefficiency in the first generation of silica monoliths, was estimated from the deviation of axial dispersion simulations under non-porous, porous/non-retained and retained simulations from experiments using appropriate molecular probes. A phenomenological approach was developed to estimate the transcolumn eddy dispersion contribution from the simulated transverse dispersion coefficients at all ranges of superficial velocities and retention factors. Comparison of simulations with experimental dispersion also helped estimate the contribution of external-film mass transfer resistance to the overall dispersion. The simulation resources utilized to study the hydrodynamic and dispersion phenomena were substantially lower than those reported in literature. The effect of external porosity on the hydrodynamic and dispersion characteristics of the silica monolith was theoretically investigated by manual segmentation of the scanned images so as to obtain unit cells of different porosities, but identical domain-sizes. Characteristic lengths that describe the hydrodynamic and dispersion behaviour under various conditions of retention were identified through a scaling analysis. Monoliths with higher external porosity were found to be more efficient than lower porosity ones, albeit at the cost of a reduced capacity. Availability of high performance computing resources and rapid improvements in non-invasive 3D scanning technology has enabled realistic microscopic insight into the transport properties of porous media at the pore level. The advent of a second generation of silica monolithic columns in 2011, with a more radially homogeneous structure, calls for an urgent need to perform a similar morphology-structure analysis to study the source of various dispersion phenomena, and thereby to recommend improvements in the morphology. Similar analysis can also be performed in the other two dominant stationary phases, viz. solid core-porous shell 3?m particles and sub-2?m particles, as also in processes that involve transport through porous media such as catalytic bed reactors, gas-liquid absorption columns, GC columns, multiphase flow in reservoir rocks, etc.