Membrane emulsification: process principles

With membrane emulsification in principle monodisperse emulsions can be produced, requiring a relatively low energy density which implies that the shear stress exerted on the ingredients is low. In membrane emulsification the to-be-dispersed phase is pressed through the membrane pores; under certain conditions droplets are formed at the membrane surface. In cross-flow membrane emulsification the droplets are detached by the continuous phase flowing across the membrane surface. A limiting factor for emulsion production on a commercial scale will be a low disperse phase flux. Better knowledge of how membrane parameters affect the disperse phase flux would enable the targeted development of membranes, optimal for the process of cross-flow membrane emulsification for a given application. Therefore, the objective of this research is to gain a fundamental understanding of the mechanism of droplet formation at the membrane surface and of the flow of the disperse phase through the membrane as a function of the membrane characteristics.Droplet formation was studied at a microscopic level with computational fluid dynamics (CFD) simulations and by microscopic experiments of droplet formation at a very thin microsieve with uniform pores. Since these membranes are extremely well defined, they are a good model system for detailed study. Results from both simulations and experiments indicate that to prevent coalescence and steric hindrance of droplets, the membrane porosity should be very low. Steric hindrance resulted in polydisperse emulsions and led to coupling of droplet detachment from neighboring pores. Furthermore, although the pores all had the same diameter, the number of pores at which droplets were formed only increased gradually with increasing transmembrane pressure. This effect was further studied with a scaled-up analogon and could be modeled by taking the resistance of the pores and the resistance of a membrane substructure into account. This model is compared with a model for flow through an isotropic membrane with interconnected uniform pores and extended to describe flow through a membrane with a pore size distribution. This model is used to show that in most cases the estimation of a membrane pore size distribution by using the liquid displacement method is not correct. Just as in membrane emulsification, pores become active at higher transmembrane pressures than expected. Finally, the effects of several membrane parameters on membrane emulsification performance are summarized. As an example, the membrane area required for a typical industrial application is estimated using the models mentioned above, for different types of membranes.