Microfiltration membrane fouling by oil/water emulsions

Membrane fouling is a major challenge faced by almost all water purification membrane processes, especially when challenged with complex fouling mixtures. Realistic feed streams often contain both hydrocarbon foulants and ionic species. This dissertation focuses on understanding the influences of feed streams on membrane fouling propensity and providing a framework for constant flux fouling studies. A poly(vinylidene fluoride) (PVDF) microfiltration (MF) membrane was challenged with aqueous fouling mixtures, including a suspension of latex beads and soybean, motor and crude oil emulsions. The fouling propensity of each fouling mixture was evaluated based on their threshold fluxes and the extent of transmembrane pressure increase during constant permeate flux fouling experiments. Fouling media zeta potential was correlated with fouling propensity. The higher the zeta potential, the lower the fouling propensity. Their fouling propensity follows the order of: latex beads < soybean oil < crude oil < motor oil. Crude oil-in-water emulsions were formulated at three different salt concentrations. Surface properties, such as surface tension and surface charge, of these emulsions and the PVDF MF membrane were characterized. The Derjaguin-Landau-Verwey-Overbeek (DLVO) model was utilized to simulate membrane-oil droplet and oil layer-oil droplet surface interactions. The DLVO model predicted increasing fouling propensity with increasing emulsion salt concentration. The fouling propensities of the various emulsions demonstrated by crossflow constant permeate flux fouling test were consistent with the model predictions. The critical and threshold flux values were estimated using the flux-stepping technique. A constant mass transfer resistance below the threshold flux, R [subscript B] , was determined from linear regression of flux-stepping results. Constant flux crossflow fouling tests were performed at selected fluxes below and above the critical and threshold fluxes. Below the critical flux, mass transfer resistance remained constant at clean membrane resistance. Above the critical flux but below the threshold flux, mass transfer resistance approached a steady state resistance that coincided with R [subscript B] values determined from flux-stepping experiments. Above the threshold flux, TMP presented a three-stage profile, an initial gradual increase stage, a TMP jump stage, and a pseudo-steady stage. The pseudo-steady state TMP corresponded to the critical pressure of an oil layer.Chemical Engineerin