Microfiltration of saline crude oil emulsions: Effects of dispersant and salinity.

Dispersants reduce oil-water interfacial tension making the separation of oil-water emulsions challenging. In this study, crude oil stabilized by the dispersant, Corexit EC9500A, was emulsified in synthetic sea water using a range of Corexit/crude oil concentration ratios (up to 10% by volume). With an interfacial tension of only 8.0 mJ/m2 at 0.5 mL(Corexit)/L, approximately 50% of the crude was dispersed into droplets <10 µm. Near complete rejection of oil in crossflow separation tests was accompanied by a precipitous flux decline attributable in part to dispersant- and salinity-induced decrease in membrane's oleophobicity (4.2 mJ/m2 decrease in surface energy). Screening of electrostatic interactions prompted oil coalescence that occurred at the membrane surface but not in the bulk of the emulsion. Real-time in situ visualization by Direct Observation Through Membrane gave direct evidence of surface coalescence pointing to both its detrimental effects (spread of contiguous films) and possible advantages (removal of large droplets by crossflow shear).

Oil droplet behavior on model nanofiltration membrane surfaces under conditions of hydrodynamic shear and salinity.

HYPOTHESES: Oil droplet stability and electrical charge, and membrane's affinity for oil govern droplet attachment to a membrane surface. Moderate droplet-surface affinity encourages surface coalescence and removal of droplets to help maintain the membrane relatively oil-free. EXPERIMENTS: Droplet attachment onto model nanofiltration membranes was studied, in situ and in real time, using the Direct Observation Through the Membrane method. Optically transparent nanofiltration membranes were designed by forming polyelectrolyte multilayer films, with either positively or negatively charged surfaces, on Anopore ultrafilters. Crossflow across the membrane surface employed hexadecane-in-water emulsions stabilized by an anionic surfactant (sodium dodecylsulfate) in model sea water or aqueous solutions containing NaCl or MgSO4. FINDINGS: Moderate affinity between oil and the polyelectrolyte-coated surface promotes crossflow controlled coalescence to remove droplets larger than a critical size, ddropcrit, in the crossflow shear. The torque balance on a sessile oil droplet in a linear shear field overpredicted ddropcrit pointing to a need for more accurate estimates of lift and drag forces on a droplet. In the presence of divalent cations, lower electrostatic repulsion between droplets facilitated droplet-droplet adhesion and led to rapid coalescence that resulted in membrane fouling. The most significant fouling appeared in tests with positively charged and less oleophobic coatings.

Membrane-based separation for oily wastewater: A practical perspective.

The large volumes of oily wastewater generated by various industries, such as oil and gas, food and beverage, and metal processing, need to be de-oiled prior to being discharged into the environment. Compared to conventional technologies such as dissolved air flotation (DAF), coagulation or solvent extraction, membrane filtration can treat oily wastewater of a much broader compositional range and still ensure high oil removals. In the present review, various aspects related to the practical implementation of membranes for the treatment of oily wastewater are summarized. First, sources and composition of oily wastewater, regulations that stipulate the extent of treatment needed before discharge, and the conventional technologies that enable such treatment are appraised. Second, commercially available membranes, membrane modules, operation modes and hybrids are overviewed, and their economics are discussed. Third, challenges associated with membrane filtration are examined, along with means to quantify and mitigate membrane fouling. Finally, perspectives on state-of-the-art techniques to facilitate better monitoring and control of such systems are briefly discussed.