Coalescence mechanisms of dispersions in glass fibre beds

Abstract

The separation of secondary liquid-liquid dispersions in fibrous glass packing has been investigated. A novel technique was developed to study drops within the packing by matching the refractive indices of the packing, its holder and the continuous and dispersed phases. This rendered the packing transparent and a soluble phototropic dye was used to colour the drops by illumination with ultraviolet light. A second technique developed was the use of ultrasonic probes, to detect drop collection and coalescence within the packing. Observation of droplet collection, coalescence and travel within the packing indicated that captured drops moved along the fibres and coalesced at intersections. Droplets exceeding the equilibrium size moved in a tortuous path with velocities far greater than the continuous phase superficial velocity. A novel knit-fibre glass packing was subsequently selected and utilized to collect and coalesce droplets in the range of 1-100 µm with both oil/water and water/oil disper­sions. This packing proved to be 100% efficient for super­ficial velocities substantially higher than critical velocities for other packings. Four different oil/water and water/oil dispersion systems were studied giving an interfacial tension range of 8.6 x 10¯³ Nm¯¹ to 29.2 x 10¯³ Nm¯¹. Reproducible results were obtained from coalescers made-up from different numbers of layers. Single and two phase flow pressure drops across the packing were correlated by modified Blake-Kozeny type equations. Six different mechanisms of droplet release were identified viz drip-point, jetting, pointing, graping, foaming and chaining.