Chemical Engineering Journal 84 (2001) 173–192 Review Electrostatic enhancement of coalescence of water droplets in oil: a review of the current understanding John S. Eow a , Mojtaba Ghadiri a,∗ , Adel O. Sharif a , Trevor J. Williams b a Department of Chemical and Process Engineering, University of Surrey, Surrey, Guildford GU2 7XH, UK b Electrical Power Engineering Research Group, Department of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Received 5 May 2000; accepted 21 October 2000 Abstract This paper reviews the current understanding of electrocoalescence of water droplets in oil, highlighting particularly the mechanisms proposed for droplet–droplet and droplet–interface coalescence under the influence of an applied electric field, as well as various factors influencing the electrocoalescence phenomenon. Generally, the coalescence behaviour can be described in three stages: droplets approaching each other, the process of film thinning/drainage, and film rupture leading to droplet–droplet coalescence. Other possible mechanisms, such as droplet chain formation, dipole–dipole coalescence, electrophoresis, dielectrophoresis and random collisions, are also presented. Experimental work and mathematical modelling of the coalescence process are both reviewed, including various models, such as molecular dynamic simulation, random collision/coalescence modelling, and linear condensation polymerisation kinetics. The type of electric field, such as alternating, direct and pulsed direct current, plays a significant role, depending on the design and set-up of the system. The concept of an optimum frequency is also discussed here, relating to the electrode design and coating. Other factors, such as the average droplet size and the residence time of the liquid mixture exposed to the electric field, are highlighted relating to coalescence efficiency. The characteristics of the emulsion system itself determine the practicality of employing a high electric field to break the emulsion. Emulsions with high aqueous phase content tend to short-circuit the electrodes and collapse the electric field. Type and concentration of surface-active components have been shown to impart considerably stability and rheological property changes to the interfacial films, thus making the coalescence mechanism more complicated. More investigations, both experimental and by computer simulation, should be carried out to study the electrocoalescence phenomenon and to contribute to the design and operation of new electrocoalescers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrostatic separation; Coalescence; Droplet–droplet interaction; Film drainage; Film rupture; Water-in-oil emulsion; Collision; Surfactants 1. Introduction To date, there exist several techniques for enhancing the separation of water-in-oil emulsions, such as the addition of chemical demulsifier [1], pH adjustment [2], gravity or cen- trifugal settling [3], filtration [2], heat treatment and electro- static demulsification [4,5]. From the viewpoints of energy efficiency, electrical demulsification is considered to be the best among the above methods [4]. The electrical phase separation concept has been used in the petroleum industry for separating water-in- crude oil dispersions by applying a high electric field onto the flowing emulsion to effect flocculation and coalescence of dispersed water droplets [6,7]. Bailes and Larkai [8,9] ∗ Corresponding author. Tel.: +44-1483-879-470; fax: +44-1483-876-581. E-mail address: m.ghadiri@surrey.ac.uk (M. Ghadiri). applied this technique to promote phase separation of an aqueous dispersion from an organic phase, and developed an effective separator for solvent extraction. Some coales- cence can occur due to Brownian motion and differential sedimentation, but these effects are insignificant compared to electrocoalescence [10]. Generally, an irreversible rup- turing of the emulsions can occur in an electric field due to the coalescence of droplets [11]. In low electric fields, however, water droplets attain a linear chain-like configura- tion, though the electric field is not high enough to induce coalescence. When the field is switched off, the droplets return to a random distribution [12,13]. The concepts here are believed to be the interaction be- tween the drops and the externally applied electrostatic field, resulting in drop charging and agglomeration, and eventu- ally coalescence. Generally, external electric fields can cause the coalescence of drops at an interface, and drop–drop co- alescence in a dielectric fluid. When two drops approach 1385-8947/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S1385-8947(00)00386-7