Numerical and Experimental Studies on the Effects of Flow on Drop Formation in Cross-flow Membrane Emulsification

Sammanfattning: Membrane emulsification is a gentle process that produces emulsions with a narrow drop size distribution, which increases the stability of the emulsion. The dispersed phase is forced through the pores in a membrane and drops are formed at the other side of the membrane where a continuous phase, flowing along the membrane surface, sweeps away the forming drops. The growth and detachment of drops is a complex phenomenon that is dependent on the process conditions, which affect the size of the drops and the drop formation time. The main objective with the project work presented in this thesis was to develop experimental and numerical tools for investigation of the effects of selected process conditions. The particle image velocimetry technique was used to measure the velocity field inside and around a drop during formation from a single pore with a diameter of 200 µm. The system included a microscope and an objective with a narrow focal depth that made measurements possible in a cross-section of the drop formation volume when the two liquids had matching refractive indices. The drop formation process was also modeled with the computational fluid dynamics (CFD) program Fluent. The drop formation time and the size of the detached drop were used as validation parameters and the results from the two methods corresponded well, with a difference of less than 5% for the drop formation time and the drop diameter. The cross-flow velocity has a major impact on drop size, which decreases as the cross-flow increases. An increase in cross-flow, oil viscosity and pore pressure displace the position of necking and drop detachment away from the pore opening, which will have a decreasing effect on the final size of the drop. A force balance model that takes into account the drop deformation that occurs as the drop approaches detachment was developed. The results given by the model was compared with CFD simulations, and the drop diameters agreed within 10%, except at low wall shear stresses. The model was also compared with experimental results on drop formation using various membranes, cross-flow velocities and surfactants. The difference between the model and experimental results is mainly due to the adsorption of surfactants onto the drop interface and the shape of the membrane pores. The effects of hydrodynamics and drop interaction on drop size were determined by studying simultaneous drop formation using CFD. The conditions studied were pore spacing of 10, 15 and 20 times the pore diameter (20 µm) at a highly dispersed phase velocity of 0.18 m/s, and 10 times the pore diameter at a low velocity of 0.019 m/s. In the case of short pore separation and a low dispersed phase velocity, the drop formation process was uniform, resulting in an emulsion with a narrow drop size distribution. At the higher dispersed phase velocity, the shortest pore separation gave a polydispersed emulsion, whereas larger pore separations gave nearly monodispersed emulsions.