Membrane emulsification with vibrating membranes: A numerical study

Membrane emulsification of oil in water may be enhanced by mechanically exciting the membrane, thereby enabling the formation of smaller droplets of a narrower size distribution, combined with higher specific production rate. To evaluate this potential, a force balance model was developed that includes the additional forces induced by the transversal membrane movement. This model yielded the ranges of interest of the excitation amplitude A and frequency f. In these ranges of interest, 3D transient simulations were carried out to predict the 3D droplet formation and detachment on a single pore under constant cross-flow. For conditions without membrane excitation, the force balance model agreed satisfactorily with experimental results reported in the literature and with the current 3D computations. The model also predicts a non-linear dependence of the droplet size on the pore diameter. However, under membrane excitation the extended force balance model does not seem to give reliable results. This is probably due to the simplifications in this model, which does not include the effects of the dispersed phase flux and viscosity, and the strongly non-spherical droplet geometry upon detachment. Moreover, for large vibration-induced forces coalescence occurred in the 3D model, which leads to much larger droplets. Thus, fully transient 3D CFD simulations appear to be required for reliable predictions. Ideally these should account for surfactant dynamics and a variable surface tension coefficient. The simulations show that membrane excitation potentially has a strong effect on the average droplet size in membrane emulsification, but that successful exploitation will require careful design of membrane and process. First estimates seem to indicate that systems with lower excitation frequency and larger excitation amplitude may perform better, but this will require experimental verification. (c) 2007 Elsevier B.V. All rights reserved.