OIL AND WATER SEPARATION IN MARINE OIL-SPILL CLEANUP OPERATIONS

This paper discusses the changes in spilled oil properties over time a nd how these changes affect differential density separation. It presen ts methods to improve differential density, and operational effectiven ess when oil-water separation is incorporated in a recovery system. Se parators function because of the difference in density between oil and seawater. As an oil weathers this difference decreases, because the o il density increases as the lighter components evaporate. The density also increases as the oil incorporates water droplets to form a water- in-oil emulsion. These changes occur simultaneously during weathering and reduce the effectiveness of separators. Today, the state-of-the-ar t technologies have limited capabilities for separating spilled marine oil that has weathered. For separation of emulsified water in an emul sion, the viscosity of the oil will have a significant impact on drag forces, reducing the effect of gravity or centrifugal separation. Sinc e mater content in an emulsion greatly increases the clean up volume ( which can contain as much as two to five times as much water as the vo lume of recovered oil), it is equally important to remove water from a n emulsion as to remove free water recovered owing to low skimmer effe ctiveness. Removal of both free water and water from an emulsion, has the potential to increase effective skimming time, recovery effectiven ess and capacity, and facilitate waste handling and disposal. Therefor e, effective oil and water separation in marine oil spill clean-up ope rations may be a more critical process than credited because it can me an that fewer resources are needed to clean up an oil spill with subse quent effects on capital investment and basic stand-by and operating c osts for a spill response organization. A large increase in continuous skimming time and recovery capacity has been demonstrated for total w ater (free and emulsified water) separation. Assuming a 200 m(3) stora ge tank, 100 m(3) h(-1) skimmer capacity, 25% skimmer effectiveness, a nd 80% water content in the emulsion, the time of continuous operation (before discharge of oil residue is needed), increases from 2 to 40 h and recovery of oil residue from 10 to 200 m(3). Use of emulsion brea kers to enhance and accelerate the separation process may, in some cas es, be a rapid and cost effective method to separate crude oil emulsio ns. Decrease of water content in an emulsion, by heating or use of emu lsion breakers and subsequent reduction in viscosity, may improve pump ability, reduce transfer and discharge time, and can reduce oily waste handling, and disposal casts by a factor of 10. However, effective us e of emulsion breakers is dependant on the effectiveness of the produc t, oil properties, application methods and time of application after a spill. (C) 1997 Elsevier Science Ltd.