Study On Coalescence Criterion And Interface Characteristics Of Water Droplets In Oil Under DC Electric Field

Demulsification of crude oil is an important procedure in the oilfield production.The high-strength electric field is applied to promote the droplet coalescence suspended in oil and accelerate the separation of oil and water.However,the existence of the oil displacement agent can affect the dynamic response of droplet interface,which makes the droplets difficult to coalesce and reduces the dehydration efficiency of electric dehydrator.At present,the classical theory of electrostatic coalescence is difficult to clarify this scientific problem.In this paper,the coalescence criteria and interfacial characteristics of water droplets suspended in oil under DC electric field are studied by microscopic experiments.The micro-mechanism of non-coalescence of complex droplets and the interfacial response during coalescence are revealed.The main research contents and conclusions are as follows:The effects of physical parameters and electric field strength on the coalescence criteria of water droplets suspended in oil are studied.It is found that there are three kinds of behaviors of water droplets under DC electric field:coalescence,non-coalescence and coalescence-splitting.The coalescence angleβc of droplets is the key parameter that determines whether the droplets can coalesce or not.Based on a large number of microscopic experimental data,the phase diagram of the effects of conductivity,interfacial tension,particle size and electric field strength on the coalescence criteria of droplet were obtained.The results show that by adjusting the concentrations of alkali and surfactant,the droplets behavior can change from non-coalescence region to coalescence region,which can be used to guide the design and operation parameters optimization of electric dehydrator.The effects of interfacial tension and electric field strength on the interfacial characteristics of droplets with the same particle size are analyzed.It is proved that the internal flow field and pressure field distribute symmetrically during the coalescence process.The increase of the liquid bridge diameter follows the outer-viscous regime.For different surfactant concentrations,the dimensionless liquid bridge radius rb^*follows an identical linear dependence with（t/τb）^0.5.The migration and diffusion of surfactant molecules at droplet interface and bulk phase can increase the coalescence time of droplets.For different Ca,the dimensionless liquid bridge radius rb^*follows an identical linear dependence with（t/（τtanα0））^0.5,and the slope decreases with the increase of Ca.It is proved that the initial contact angle of the dropletα0 is the fundamental reason for the varying coalescence time of the droplet under different Ca.The coalescence process of droplets with different particle sizes under electric field is analyzed.It is found that the growth direction of the liquid bridge inclined to smaller droplets,accompanied by a"mushroom"jet flow.The increase of the droplet size ratio aggravates the asymmetry of droplet aggregation,and the relationship between the liquid bridge diameter and the liquid bridge angle gradually deviates from the linear relationship.The coalescence time of droplets increases with the increase of droplet size ratio and the decrease of surfactant concentration.In the process of droplet coalescence,the width and velocity of the"mushroom"jet flow increase gradually,but the flow rate of the jet basically remains constant.With the increase of particle size ratio and the decrease of surfactant concentration,the capillary pressure difference between droplets increases and the jet flow rate increases.The experimental results show that higher coalescence efficiency can be achieved by pretreatment of emulsion,optimization of particle size distribution and reduction of surfactant concentration.