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Graphical abstract Highlights • Critical electric field strength (E crit) of partial coalescence is investigated. • Increasing initial droplet size or addition of non-ionic surfactant reduces E crit. • Droplets are periodically deformed under pulsatile electric fields. • Increasing frequency inhibits the periodical deformation of droplets. • The optimal waveform that inhibits partial coalescence can be changed. Abstract Electro-coalescence technology is commonly used for separation of water from oil. However, the coalescence may not be complete at high field strengths, forming secondary droplets. This process is, hence, undesirable for separation due to the challenging removal of fine droplets. In this study, the effects of non-ionic surfactant (Tween 80) on the critical electric field strength (E crit) of partial coalescence of droplets present at oil–water interface were investigated. Different waveforms and frequencies were employed to gain better understanding of the phenomena. The results showed that the increase in initial droplet radius (R) or surfactant concentration may reduce E crit. Also, E crit was found to vary linearly with R −0.5 and the slope E crit / R −0.5 was proportional to the interfacial tension. Compared to DC electric fields, pulsatile electric field (PEF) can affect E crit by periodically deforming the droplet. The deformation of droplets was found to be suppressed with elevated frequencies, resulting in an increase in E crit. Generally, E crit of DC and bipolar square waveforms were the highest, indicating they were more effective in suppressing formation of secondary droplets. However, E crit of pulse AC may exceed that of sinusoidal AC and DC at high frequencies. These findings are useful for optimizing electro-coalescence systems. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Addition of silica nanoparticles affects droplet–interface electrocoalescence. • Reduced interfacial tension, increased water conductivity are responsible for effect. • Effect of field strength is pronounced compared to that of initial droplet size. • Formation of a ring by the separation of vortices is visualized by nanoparticles. • A group (electric We × Oh) is not suitable for coalescence containing nanoparticles. Abstract Electric dehydration method is widely used in oil industry to separate water from oil. Droplets resting on the oil–water interface discharge their liquids into the bulk phase, and the process can be altered by the change of electric field strength (E) or properties of water. Phenomenon of silica (SiO 2) nanoparticle-laden droplets coalescing with a planar interface under direct current electric field was observed using a high-speed digital video camera. The effects of initial droplet diameter (D), E , and weight percent of nanoparticles in deionized water (C) were studied experimentally. The results showed that increasing D or E contributed to the formation of secondary droplets. Addition of SiO 2 nanoparticles to water produced two competing effects: reduction in oil–water interfacial tension and increase in water conductivity, and a shift in the dominant effect occurred with the increase of E. Furthermore, formation of a ring by the separation of vortices was visualized by movement of nanoparticles in experiments, and its downward moving velocity decreased with time, which is small compared to that reported in literature. Finally, it was confirmed that the dimensionless WO number is not suitable for describing the volume fraction of secondary droplets for droplet–interface coalescence containing nanoparticles. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Forces acting on the droplet during droplet–interface coalescence are analyzed. • Using pulsatile electric fields (PEF) affects the formation of secondary droplets. • Increasing the frequency of PEF is conducive to droplet's vertical collapse. • Critical electric field strength increases with the increase in frequency. Abstract Droplet–interface electrocoalescence may be partial, which is undesirable for separating water from oil. This paper focused on the coalescence characteristics of water droplets at oil–water interface, and the influences of droplet diameter, electric field strength (E), waveform and frequency were experimentally evaluated. The results showed that the normalized volume of secondary droplets (V nor) increased as E rose, and it was larger for large initial droplet. Increasing the frequency of pulsatile electric fields (PEF) reduced V nor. The droplet was periodically stretched under PEF according to the waveform's periodic characteristic. It was proved that the increase of frequency suppressed the deformation of the droplet in vertical direction, which was conducive to droplet's vertical collapse, and therefore inhibited the formation of secondary droplets. Critical electric field strength (E crit) for the occurrence of partial coalescence also increased with the elevated frequency, and the increase became relatively slow at high frequencies. E crit of square waveform was the largest in experiments, indicating it was most effective in suppressing partial coalescence; moreover, E crit of PUAC could exceed that of sine AC and DC waveforms at high frequencies. As the initial droplet radius (R) reduced, E crit increased and showed a linear correlation with R −0.5. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Existence of SiO 2 nanoparticles affects water droplet-interface coalescence. • Reduced interfacial tension, increased conductivity are responsible for effects. • Increasing frequency inhibits the formation of secondary droplets. • Optimal waveform inhibiting partial coalescence changes with increasing frequency. Abstract Nano-fluid flooding can enhance oil recovery, but nanoparticles may have impacts on the subsequent electric dehydration, which seldom did people lay emphasis on. This work focus on the influence of SiO 2 nanoparticles and electric field factors on the droplet-interface coalescence, and the physical mechanism is studied. The effects of mass concentration of nanoparticles, electric field strength, waveform and frequency have been investigated experimentally. Results show that using high electric field strength promotes forming secondary droplets while increasing the mass concentration of nanoparticles inhibits that. SiO 2 nanoparticles mainly affect electrocoalescence by increasing conductivity of water and reducing oil–water interfacial tension irregularly, and the dominant role changes from interfacial tension to conductivity with increasing applied electric field strength. In addition, pulsed DC and pulsed AC waveforms are more likely to form secondary droplets. Sinusoidal AC waveform is optimal in suppressing the secondary droplets formation when the electric field strength is low; however, bipolar square waveform becomes the best when the strength is high. Finally, increasing frequency also suppresses the formation of secondary droplets and the optimal waveform changes as the frequency increases. The outcome of this work is useful for optimizing the electric dehydration process containing nanoparticles. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Additives used in ASP flooding greatly affect critical electric field strength. • The effects of oil and water physical properties on critical electric field strength were analysed. • A formula expressing the critical electric field strength is given in an empirical way. • The formula takes account of the conductivity of water which is neglected in the past. Abstract Water droplets dispersed in crude oil have to be separated and this is most commonly done by electrical dehydration. Under high strength electric fields, partial coalescence may occur and leave fine secondary droplets, which reduce the separation efficiency. The critical electric field strength ( E crit ) for partial coalescence occurrence depends on several factors. In this paper, the effects of droplet radius, conductivity, interfacial tension, viscosity (changed by adding alkali, surfactant, and polymer respectively) and oil density on E crit have been studied experimentally. E crit increases linearly with the inverse of the square root of droplet radius, R −0.5, but the slope E crit / R −0.5 ( k ) can be changed. Increasing surfactant concentration reduces E crit and the slope k decreases, which indicates reducing interfacial tension promotes partial coalescence. Whereas, adding alkali or polymer improve E crit and the slope k increases with the increase of its concentration, because of the changes in water conductivity or viscosity. In addition, E crit is proportional to the product of density difference and oil viscosity. A proposed formula expressing the E crit , albeit in an empirical way, was given which takes account of the relevant parameters. These results will be of guiding significance to the choice of electrical field strength for electro-dehydration. [ABSTRACT FROM AUTHOR]