The resolution of water-in-oil emulsions by the application of an external electric field

Over the years, the electrical treatment of W/O emulsions has established itself as the most important method of separation, though the process is usually enhanced by heating and chemical addition. It is practised at the oil field, in the refinery and on site by the user. Water becomes mixed with the oil in several ways, e.g. it may be produced with the oil or contamination during shipping may occur. Emulsification of the two immiscible liquids results from the agitation induced by pumping. Application of an electric field induces droplet coalescence leading to increased size and settling rate. Electrophoretic and dielectrophoretic forces may be established by the field, which induce droplet coagulation by migratory coalescence and dipole coalescence respectively. These processes may be modelled using a mechanistic approach or by considering their coagulation kinetics. They are investigated experimentally using turbidimetric, laser diffraction and microscopic techniques.The effects of different electric excitations are considered, as is the use of insulated electrodes. Interfacial polarization is important in the latter case and time-varying electric fields are required for efficient phase separation. Though separation efficiency initially increases with electric field intensity, a limit is eventually reached when droplet dispersion occurs; the various electrostatic and hydrodynamic dispersion mechanisms are therefore examined. The design of electric treaters (electric-coalescers) has continued to improve since they were first used commercially in 1909. However, as oil supplies diminish, user specifications become increasingly stringent, and more sensitive refinery processes are introduced, the requirement to improve their efficiency will persist.