The rejection of oil using an asymmetric metal microfilter to separate an oil in water dispersion

Dispersed oil drops need to be reduced to very low concentrations before water can be discharged into the environment. Conventional equipment to achieve separation includes: settling tanks, hydrocyclones and centrifuges. However, these devices are not effective at removing oil drops with small diameters and filtration may have to be used. The rejection (retention by the filter) of oil drops dispersed in water by an asymmetric surface filter has been measured. The filter medium has conical pores that pass from the filtering side to the filtrate side with no internal tortuosity. The filter tube was cylindrical with a central rod supporting an annular helix of 14 mm pitch and the oil in water emulsion was pumped through the tube giving rise to a retentate (concentrated suspension of oil in water) and a permeate (filtrate almost clear of oil). The oil emulsion was stabilised by polyvinyl alcohol at a concentration of 0.2 g/l to give drops of between 1–40 μm in diameter. This is the emulsion size associated with produced water from oil reservoirs. The percentage retention of the oil drops by the filter, at a given drop diameter, was determined using a Coulter Multisizer in conjunction with a metal tracer added to the oil. The filters were tested at a fixed rate of permeate flux and gave similar rejections whether the more open end or tighter end of the pores faced the challenge emulsion. The latter case required an increase in the transmembrane pressure from 0.18 to 0.2 bar, this pressure remained constant throughout the experiments. Thus, the microfiltrations were performed under conditions of constant flux and pressure. Using a feed of larger oil drops significantly changed the rejection: a coarse emulsion had a rejection of 80% at 2 μm whilst a finer emulsion gave a 50% cut-off at 5 μm.