Drop–drop coalescence in an electric field: the effects of applied electric field and electrode geometry

Separation methods utilising high electric fields have been used extensively in the oil and petroleum industries, where the occurrence of water-in-oil dispersions is highly unwelcome due to physical constraints, as well as high maintenance costs required to treat these dispersions. Most of the conventional electro-separators are huge and bulky, thus having high capital and operating costs. There is, therefore, a great scope to optimise the design and operation of these separators based on a fundamental understanding of the role of high electric field in the coalescence of aqueous drops in oil. This paper reports the studies of the effects of the direction of the applied electric field as well as the geometry of the electrodes. The angle between the electric field and the centre line of two drops, θ, should be zero for the electrically induced force to attain its maximum attractive value. The maximum induced force is large enough to deform the adjacent surfaces of the drops prior to drop–drop coalescence. It has been shown experimentally and theoretically that drop–drop attraction can also occur when θ is less than 54.7° or more than 125.3°. Several two-dimensional electrode designs have been shown to conform to this theory. When a pulsed electric field is applied to a drop, the drop vibrates with a frequency following the applied pulse frequency until a limit, beyond which the observed frequency of drop vibration will not follow the applied pulse frequency linearly. The limit depends on the continuous liquid phase. Above this limit, the drop has also a very small magnitude of vibration. This is believed to have an influence on the optimum pulse frequency for liquid–liquid separation in a particular physical system. The premature drop–drop coalescence in an electric field is believed to be influenced by the natural mechanical vibration and cavitation within the drops. Therefore, the magnitude of the applied electric field and the pulsing frequency can be optimised to suit the physical liquid–liquid system, and, therefore, giving a better design of the electrocoalescer.