Investigation Of The Behavior Of Single Droplet And Multiple Droplets In Water-in-Oil Emulsions Under Pulsatile Electric Fields

With the oilfield exploitation having entered later period and the polymer flooding method having been widely utilized,the crude oil becomes more and more inferior and emulsified.Consequently,the separation of water phase from oil phase gets harder and harder.Salt is soluble in water,which causes equipment and pipeline corrosion.The separation of water is essential before transpiration and delivery to refineries.The existing technology for de-watering relies heavily on the slow migration of water droplets,requiring a huge volume for the separator vessel making it uneconomic and undesirable.By applying electric fields,small water droplets in the oil phase can coalescence more easily and,as they become bigger,they also settle faster.Based on Electrohydrodynamics(EHD)theory,this paper conducts multi-scale experiments,numerical research and theoretical analysis on the high-voltage pulsed electrocoalescence of water-in-oil(W/O)emulsion.Firstly,microscopic experiments and simulations are conducted on the mechanism of droplet deformation,breakup,drop-drop coalescence and drop-interface coalescence.The results show that the process of droplet deformation,breakup and coalescence is heavily influenced by electric parameters(electric field strength,frequency and waveforms),physical parameters(initial droplet size,interfacial tension,bulk phase viscosity,salt content,permittivity and solid particles)and operating parameters(droplet angle and droplet distance).The deforming process is a competing process between the electrostatic stress(driving force)and interfacial tension(drag force),and its deformation rate is affected by the bulk viscosity.Therefore,the ratio of Weber Number(describing electric field effects)and Ohnesorge Number(describing physical properties)is found to describe the droplet deformation well.Two mechanisms exist,i.e.complete coalescence and partial coalescence,during drop-interface coalescence.The dominant factor is the competition of pumping and necking process.Secondly,microscopic experiments on static electrocoalescence of W/O emulsions are carried out to investigate the droplet migration,water chain formation and dissipation,and coalescence conditions.The results show that during the electrocoalescence process,the dominant mechanism is drop-drop coalescence.Under excessively high electric fields,some droplets arrange to form water chains rather than coalescence.With increasing electric field strength,water content,salt content and SiO2 content,the droplet coalescence efficiency increases first and then decreases.The rank of the waveforms is as follows: square wave≥half-sinusodial wave>sawtooth wave.Two water chain formation mechanisms exist as droplet middle-part breakup and water droplet group coalescence.Also,two water chain dissipation mechanisms exist as water chain coalescence and the charge effect of extern droplets coalescence.Finally,this paper proposes a novel “V” electrode design.The comparison experiments are conducted with plat electrode and Calgavin electrode,and much higher efficiency is observed with the novel “V” electrodes.The influence of electric parameters,physical parameters and residence time is investigated experimentally thoroughly.The advantages of the novel “V” electrode are as follows.The water-in-oil emulsion goes through the electrodes rather than flowing over them and the direction of the flow is aligned with that of the external electric field.This ensures the most effective electrostatic effect,while the high electrode surface area maximizes the interaction of the water droplets with the electrodes.Furthermore,thanks to the inclination of the electrodes,water droplets roll on the electrodes and tend to directionally flow to the central region of the coalesce,as observed by high speed videos,thus enhancing phase separation.The structure of the electrodes can also hold up large droplets,at least temporarily,to form films.Therefore,both droplet-droplet and droplet-interface coalescence are promoted in this design.Both coalescence mechanisms due to dipole-dipole interaction and electrophoresis are present.As this configuration allows nesting consecutive electrode elements in a compact configuration,a significant reduction of the electrocoalescer volume can be expected.The experimental and numerical work and theory analysis work above lays a good foundation for further research on demulsification mechanism of W/O emulsion.The outcome of this work is potentially useful for optimizing the design of compact and efficient oil-water separator.