Coalescence of freely moving bubbles in water by the action of suspended hydrophobic particles

The presence of (catalyst) particles in slurry columns may induce significant changes in the overall column hydrodynamics. This is commonly attributed to changes in the apparent viscosity and density of the slurry phase as a result of the presence of particles. However, in case of solids demonstrating a strong repulsive S-L interaction, other factors come into play. Presently, we address the behaviour of hydrophobic particles in aqueous slurry bubble columns, using an air-water-carbon model system. Hydrophobic particles, as most carbons, tend to segregate from an aqueous phase, thus giving rise to particle-particle cohesion in the liquid phase and to particle-bubble adhesion. Cohesion or agglomeration is important, for instance in relation to the filtration behaviour of catalysts. In this study, however, the focus is particle-to-bubble adhesion. From antifoaming studies it is known that particles adhering to bubbles may promote bubble coalescence. Whereas in foams particles are forced in the G-L interlayer and bubbles are stagnant, in slurry bubble columns both the solid phase and the bubbles move freely. As is currently demonstrated, adhering particles do nevertheless promote bubble coalescence also under flow conditions. By consequence the bubble-size distribution is markedly shifted to larger bubbles. As these latter govern the overall flow pattern and back-mixing behaviour of a bubble column, this is crucial to practical operation.