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 wa ter can be discharged into the environment. Conventional equipment to achie ve separation includes: settling tanks, hydrocyclones and centrifuges. Howe ver, these devices are not effective at removing oil drops with small diame ters and filtration may have to be used. The rejection (retention by the fi lter) of oil drops dispersed in water by an asymmetric surface filter has b een measured. The filter medium has conical pores that pass from the filter ing 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 pit ch 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 (filt rate almost clear of oil). The oil emulsion was stabilised by polyvinyl alc ohol at a concentration of 0.2 g/l to give drops of between 1-40 mu m in di ameter. This is the emulsion size associated with produced water from oil r eservoirs. The percentage retention of the oil drops by the filter, at a gi ven 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 r ate 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 r equired an increase in the transmembrane pressure from 0.18 to 0.2 bar, thi s pressure remained constant throughout the experiments. Thus, the microfil trations were performed under conditions of constant flux and pressure. Usi ng a feed of larger oil drops significantly changed the rejection: a coarse emulsion had a rejection of 80% at 2 mu m whilst a finer emulsion gave a 5 0% cut-off at 5 mu m. (C) 1999 Elsevier Science Ltd. All rights reserved.