Emulsification with microfiltration ceramic membranes: A different approach to droplet formation mechanism

Oil-in-water (O/W) emulsions were prepared by membrane emulsification (ME) in a large scale stirred tank with a submerged cell using flat membranes, and in a cross-flow tubular unit. Results obtained with flat and tubular membranes devices were compared. Common ceramic microfiltration (MF) membranes (ZrO2 supported on TiO2) were used in both cases. The main advantage of these membranes is their low cost compared to others frequently used in ME (SPG membranes), which implies a reduction up to 60%. The effect of operation parameters on droplet size distributions was studied to understand the droplet formation mechanism that takes place in both devices. Monodisperse O/W emulsions were obtained using flat membranes with a droplet-to-pore diameter ratio (Dd/Dp) in the range 2.1–2.9. Span values of 0.58, 0.66 and 0.81 corresponded to membrane pore diameters of 0.45, 0.80, and 1.4 μm, respectively, being the active pores fraction in the range of 27–36%. The cross-flow tubular unit allowed for production of monodisperse O/W emulsions but with larger droplet sizes, having droplet-to-pore diameter ratios (Dd/Dp) in the range 3.9–4.7. It was observed a major influence of pore activation by dispersed phase pressure on droplet size distribution. Shear stress had little influence on the droplet size, with both flat and tubular membranes. This suggested that droplet formation mechanism was not shear stress-based. A spontaneous emulsification mechanism was proposed.