Use of the capillary number for the prediction of droplet size in membrane emulsification

In order to optimise the membrane emulsification process, practical decision tools are needed to select the correct process parameters. The aim of this work is to further the understanding of how the main parameters – transmembrane pressure, wall shear stress and interfacial tension – influence the dispersed phase flux and the droplet size and size distribution. The influence of transmembrane pressure has been analysed by the point of view of the fraction of active pores. The higher the transmembrane pressure, the higher the fraction of active pores: this analysis agrees with our experimental results. We have obtained up to 60% of active pores with dp = 0.8 μm at Ptm = 2 bar and up to 47% of active pores with dp = 0.5 μm at Ptm = 2 bar. The influence of wall shear stress and interfacial tension have been studied in parallel with the objective to group these two parameters in a single non-dimensional number, the capillary number. The higher the capillary number, the smaller the drops formed: the mean Sauter diameter decreases rapidly when increasing the capillary number from 0.11 to 0.25 and then the mean Sauter diameter reaches a plateau above a capillary number of 0.5. Our results show that to produce small droplets a capillary number above Cac = 0.5 is necessary, this critical capillary number being independent of the emulsifier used.