However, emulsified water can be very stable and may not settle out from the oil, especially in the presence of surfactants. Dissolved water is chemically absorbed in solution with the oil. Vacuum dehydration and absorption are only two techniques to remove free, emulsified and dissolved water. The advantage and disadvantage of these technologies will be discussed as follows. Gravity settling can eliminate free water in large vessels at very low flow rates, but is ineffective at removing emulsified and dissolved water.
Centrifuges can separate water at much higher process rates than gravity settling.
However they cannot remove all free water and particulate, and they are expensive to operate and maintain. In coalescers, tiny water droplets adhere to fibres. Flow pushes the droplets to the fibre intersections where they are coalesced to form larger droplets. Coalescers offer high process rates and low energy cost, but are not as effective in high viscosity oils. Many coalescers are also disarmed by surfactants.