Critical electric field strength for partial coalescence of droplets on oil–water interface under DC electric field

Abstract

Water droplets dispersed in crude oil have to be separated before processing and this is most commonly done by electrical dehydration. Under high strength electric fields, partial coalescence may occur. The critical electric field strength for partial coalescence occurance depends on several factors. In this paper, the effects of droplet diameter, conductivity, permittivity and other physical properties, such as viscosity, density and interfacial tension, on critical electric field strength have been studied experimentally, based on which a formula to predict the electric field strength is proposed. The critical electric field strength is shown to vary with the inverse of the square root of droplet radius, R-0.5. Its value is lower for droplets with high surfactant concentration. As the surfactant concentration increases, the slope Ecrit/ R-0.5 (k) decreases. The critical electric field strength increases as the concentration of alkali is increased. The slope k increases with the increase of conductivity and alkali concentration. On adding polymer, the critical electric field intensity is much higher than that of distilled water, and this suggests that droplets containing polymer do not * Viscosity ratio, Pa•s Interfacial tension, N•m-1 Induced charge, C Angle between the electric field direction and the droplet radius, ° Relative permittivity 0 Permittivity of free space F•m-1

Electrohydrodynamics of droplets and jets in multiphase microsystems

“…The critical strength of the electric field was found to depend on the surface tension, radius, conductivity, permittivity, density of the droplet, and the frequency of the AC or pulsed electric field. 67,128 In the absence of an electric field, the Ohnesorge number indicates the transition from complete to partial coalescence, 122,123,127 while under an electric field, partial coalescence can occur at a critical electrocapillary number, reportedly around 0.055 and regardless of the Ohnesorge number. 68 The partial coalescence under a strong electric field may result from charge transfer during the process.…”

Effect of hydrolyzed polyacrylamide used in polymer flooding on droplet–interface electro-coalescence: Variation of critical electric field strength of partial coalescence

“…The literature search revealed that both electric field and physical factors of liquids influence the occurrence of partial coalescence [16,[22][23][24][25][26][27][28]. In particular, higher electric field strengths are conducive to form larger secondary droplets [25,26].…”

“…The increase in radii of initial droplets may promote the occurrence of partial coalescence [16,23,28]. Aryafar et al [23] found that the use of a high viscosity oil or coalescing droplet with a low permittivity is not conducive to induce partial coalescence.…”

“…It has been reported that the formation of secondary droplets can be avoided if the water conductivity is increased within a certain range [27]; moreover, Ecrit also increases at elevated conductivities [28]. The increase in droplet viscosity ( 1) can lead to two competing effects: damping of the rate of drainage of the aqueous liquid into the continuous aqueous phase and prolonging the coalescence time [53].…”

Optimum Operating Frequency for Electrocoalescence Induced by Pulsed Corona Discharge

“…However, by increasing the strength of the electric field after some threshold, not only there is no increase in the coalescence event but also the breakup mechanisms are activated. 32 , 33 There are three main mechanisms of water droplet breakup that are introduced by Williams and many other researchers. 20 , 34 In the first one, the water droplet is polarized and elongated in the presence of the effect of the electric field and starts to disintegrate into smaller droplets (when the electric field is strong enough in that region).…”