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• The influence mechanism of E and D is revealed by experiments and simulations. • Increasing E strengthens the electric field distortion and charge polarization. • The high-pressure gradient region is easier to form in large D. • The vertical collapse evolution is suppressed with the increase of E and D. Under high-strength electric fields, partial coalescence may occur, thus producing fine secondary droplets, which are undesirable for separating water from oil. In this study, experimental and numerical methods are employed to investigate the coalescence behavior under a DC electric field to comprehensively understand the effect of electric field strength (E) and droplet diameter (D) on coalescence. The Level-set method is employed to capture the moving interface, and the numerical data are validated through experimental tests. The results reveal the easy formation of secondary droplets with increasing E and D values. In particular, the electric field strength suppresses the expansion of the liquid bridge and hinders the vertical collapse of the droplets. The distortion of the electric field at the liquid bridge and the polarized charge both increase with E , thus resulting in stronger electric field repulsion. Moreover, the enhancement in D significantly affects the shrinkage of the liquid bridge and prevents the vertical collapse of the droplet. The pressure gradient at the liquid bridge from the outside to inside decreases as a function of D , thereby inhibiting the contraction speed. Furthermore, the number of polarized charges concentrated on the top of the droplet increases, thus resulting in an enhanced electric field tension. The results obtained herein can potentially act as guide for the design and optimization of electric dehydrators. [ABSTRACT FROM AUTHOR]

Nano-fluid flooding is a new method capable of improving oil recovery; however, nanoparticles (NPs) significantly affect electric dehydration, which has rarely been investigated. The effect of silica (SiO2) NPs on the droplet–interface coalescence was investigated using a high-speed digital camera under an electric field. The droplet experienced a fall, coalescence, and secondary droplet formation. The results revealed that the oil–water interfacial tension and water conductivity changed because of the SiO2 NPs. The decrease of interfacial tension facilitated droplet deformation during the falling process. However, with the increase of particle concentration, the formed particle film inhibited the droplet deformation degree. Droplet and interface are connected by a liquid bridge during coalescence, and the NP concentration also resulted in the shape of this liquid bridge changing. The increase of NP concentration inhibited the horizontal contraction of the liquid bridge while promoting vertical collapse. As a result, it did not facilitate secondary droplet formation. Moreover, the droplet falling velocity decreased, while the rising velocity of the secondary droplet increased. Additionally, the inverse calculation of the force balance equation showed that the charge of the secondary droplet also increased. This is attributed to nanoparticle accumulation, which resulted in charge accumulation on the top of the droplet. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultra-high frequency with self-adaptive voltage transformer is developed. • The voltage gradually decreases to a very small value in the dehydration process. • Electric field strength is low when frequency is high. • Self-adaptive voltage technology is more efficient for complex emulsion dehydration. • Droplet chains attract surrounding small water droplets to form larger droplets. Electrical dehydrators are often used to separate water from oil emulsions. However, the frequency and electric field strength are fixed during separation and cannot change with the flow rate and emulsion properties. To solve this problem, ultra-high frequency and self-adaptive voltage transformer was developed to enhance the separation efficiency and compared with ordinary high frequency transformer. To this end, a small-scale evaluation test system was designed to investigate the influence of electric fields and operating parameters on water dehydration of different emulsions. The results showed there are different optimal electric field strengths for emulsions of various properties. Higher frequency values required lower electric field strengths. Under high frequencies, the formed water chains attracted surrounding small water droplets to form larger droplets. During the separation process, the frequency was kept stable but the electric field strength adaptively changed and gradually decreased, resulting in declined power consumption. Ordinary high-frequency electric fields illustrated low dehydration efficiencies on heavy oil emulsions, high-water concentration emulsions, and water-containing polymer emulsions. By comparison, ultra-high-frequency self-adaptive electric field strengths transformer showed superior separation efficiencies, low energy consumptions, and more stability. Besides, the separated water was cleaner and exhibiting superior adaptability to adverse conditions. In sum, these findings look promising for optimizing the operation of existing electrical dehydrators and promoting the use of ultra-high-frequency and self-adaptive electric fields. [ABSTRACT FROM AUTHOR]

Notes: A spherical water droplet oscillates in an alternating electric field and gradually falls to the oil–water interface to coalesce. • The shape and trajectory of a falling droplet oscillate in alternating electric fields. • Droplet deformation follows the change of electric fields while the migration lags behind that. • The deformation characteristics of droplets depend on the electric field waveform and frequency (felectric). • The increment of felectric inhibits the deformation degree of droplets and extends the falling time. • Pulsed DC electric fields with high felectric are optimal for inhibiting partial coalescence. Droplet–interface collision coalescence is an important pattern of phase separation, especially for removing water from water-in-oil emulsions in electrostatic dehydrators. Deformation, migration to the interface, and coalescence of droplets driven by electric fields are fundamental processes but have not been completely understood. In this work, we studied the influences of three typical alternating current or pulsed electric fields on these dynamic processes of a single water droplet falling to the planar oil-water interface. The results depicted that the falling droplet oscillated in both shape and trajectory in an electric field. Specifically, the deformation of droplets should be able to follow the change of electric fields, while the migration may lag behind that. The pattern of droplet deformation matched the electric field waveform, and the frequency had different multiple relationship with the electrical frequency in the electric field of different waveforms. Moreover, the increment of electrical frequency could reduce the degree of droplet deformation and extend the falling time required for droplets to reach the interface. When the droplet coalesced with the interface, secondary droplets were observed if the applied electric field strength exceeded a critical value (E crit), detrimental to dehydration efficiency. E crit varied from different electric field waveforms and increased with elevated electrical frequencies. Compared to sinusoidal AC and square electric fields, the values of E crit in a pulsed DC electric field were highest in the experiments which could be more effective in suppressing the formation of secondary droplets. These findings will be useful for designing high efficient electro-dehydration plants. [ABSTRACT FROM AUTHOR]