(186n) Flow Characteristics Across Hydrophobic/Hydrophilic Packed Beds in Hele-Shaw Machine

A modified Hele-Shaw machine was used to study bi-dimensional flow characteristics in a packed bed with dimensions 50 cm x 14 cm x 6.5 mm ± 0.5 mm, comprising either randomly packed super-hydrophilic or super-hydrophobic glass spheres. 6 mm glass beads were etched with 0.5 M aqueous HF solution for 2 hours at room temperature, flushed with de-ionized water and dried with N₂ to clean, modify the surface morphology and enhance the effect of subsequent chemical treatments without changing the apparent roundness, as verified by SEM imaging. The glass spheres were then subjected to treatment with Piranha solution (3:1 mixture of 98% H₂SO₄ and 30% H₂O₂) for 30 min at 85°C, or otherwise coated with a commercial ?water-repellent? composition (based on alkylpolysiloxanes, aliphatic hydrocarbons and inorganic acids), so as to alter the packing's wettability and thereby substantially modifying the glass-water contact angles. Image analysis using Image J and Matlab show that typical glass-water contact angles for hydrophobic and hydrophilic spheres are 77±4° and 30±4°, respectively. Water was fed into the flow visualization region at a rate of 7 ml/s and tracer ink was injected upstream to monitor flow patterns and residence times under laminar regime in the bed packed with the functionalized glass beads mentioned hereinbefore. The flow visualization region was registered at a frame rate of 15 photograms/second and the resulting images were processed and analyzed using Image J. The results indicate that flow paths across the hydrophobic bed are essentially following a straight narrow line and are considerably slow as compared to the rather diffuse, tortuous and fast flow paths found in beds packed with hydrophilic spheres. It is apparent from the results that the hydrophilic spheres induce the formation of a thick boundary layer wherein a considerable amount of stagnant fluid is held, i.e. driving the adjacent flow lines in a direction perpendicular to the flow and enhancing the mixing of the liquid in the bulk; whereas the hydrophobic spheres ?due to their non-wetting characteristics- generate a thin boundary layer that enables the liquid to flow freely in a more straightforward manner. Interestingly enough, the results seem to challenge the belief that equilibrium wall flow is almost independent of the wettability of the packing as it has been argued in previous studies under similar conditions. It is thus the objective of the work at hand to contribute to the understanding of the effect of water-glass contact angles on the laminar regime boundary layer at the solid-liquid interfaces. Elaborating on the mechanisms and dynamics of fluid flow in flooded packed beds could help gain some insight into the performance of PBR's, distillation columns and other separation processes.