Electro-hydrodynamic separation of aqueous drops from flowing viscous oil

Abstract

In a number of industrial processes, gravitation usually plays a significant role in the separation of dispersed particles or drops from another immiscible liquid. This separation, taking place in large tanks, provides low operating cost when the residence time of the liquid–liquid system is long and the density difference between the two phases is large. However, the situation becomes complicated for small liquid drops dispersed in another liquid when the density difference is very small and the carrier liquid is at high velocity. In this paper, using a novel compact electrocoalescer-separator that has been developed recently, an externally applied electric field has been shown to significantly enhance the separation of aqueous drops in a flowing viscous oil, with low concentrations of the dispersed phase. The separation efficiency increases with the electric field strength until a limit, above which, drop deformation and break-up occur. Using pulsed direct current electric fields, an optimum electric field and frequency exist for the enhancement of drop–drop and drop–interface coalescence, thus producing an optimum separation efficiency for the system. The separation efficiency increases with drop size up to a certain diameter, as larger drops have been observed to deform and short-circuit the system. Short-circuiting can be avoided by optimising the electric field. From a force balance on a single sphere, a parameter IP is defined which describes the acceleration of the sphere under an electric field. The parameter IP can predict the behaviour of the system qualitatively up to the limit where drop break-up and electrical short-circuiting occur.