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Droplets banding is critical to emulsion separation under ultrasonic irradiation as it can greatly improve the separation efficiency. In this paper, the formation process of droplets banding under ultrasonic standing waves was precisely captured by high-speed microscopic photography; by processing the images, the droplets banding characteristics, including the banding formation time and banding interval, were extracted. Then the effects of acoustic intensity, frequency, droplet size, and physical properties of oil and water on the droplets banding characteristics were discussed in details. The results show that the range of acoustic intensities, within which the droplets banding can form, increases with the increase of the frequency; a maximum allowable acoustic intensity exists for banding formation, which also increases with the frequency. The banding formation time, which increases with increasing oil viscosity but decreases with droplet size, is found to be hardly affected by the oil-water interfacial tension. In addition, the banding interval is only related to the frequency, which closely corresponds to the half wavelength. [ABSTRACT FROM AUTHOR]

• Optimal acoustic parameters highly depend on emulsion properties. • Moderate acoustic cavitation facilitates droplet coalescence at low frequencies. • Droplets aggregate into bandings to enhance separation in high frequency USWs. • Optimization rules for acoustic parameters of ultrasonic separation are given. Ultrasound is an emerging and promising method for demulsification, which is highly affected by acoustic parameters and emulsion properties. Herein, a series of microscopic and dehydration experiments are carried out to investigate the parameter optimization of ultrasonic separation. The results show that the optimal acoustic parameters highly depend on the emulsion properties. For low frequency ultrasonic standing waves (USWs), mechanical vibrations not only facilitate droplet collision and coalescence, but also disperse the surfactant absorbed on the interface to decrease the interfacial strength. Therefore, low frequency ultrasound is suitable for separating emulsions with high viscosity and high interfacial strength. Increasing the energy density to produce moderate cavitation can increase demulsification efficiency. However, excessive cavitation results in secondary emulsification. In high frequency USWs, the droplets migrate directionally and form bandings, thereby promoting droplet coalescence. Therefore, high frequency ultrasound is favorable for separating emulsions with low dispersed phase content and small droplet size. Increasing the energy density can accelerate the aggregation of droplets, however, excessive energy density causes acoustic streaming that disturbs the aggregated droplets, resulting in reduced demulsification efficiency. This work presents rules for acoustic parameter optimization, further advancing industrial applications of ultrasonic separation. [ABSTRACT FROM AUTHOR]

Highlights • Effect of droplet banding on demulsification process is analyzed. • Optimal acoustic intensity and treatment time are obtained. • Acoustic cavitation threshold in emulsions is achieved by spectral analysis. • Range of oil-water properties suitable for ultrasonic separation is determined. Abstract This paper focuses on the application range and optimal parameters for separating water-in-oil (W/O) emulsions using ultrasonic standing waves (USWs). Firstly, the effect of droplet bandings on the demulsification process is theoretically analyzed. The formation of droplet bandings coincides with droplet sedimentation when horizontal irradiation is applied, which improves the separation efficiency. On this basis, the effects of acoustic parameters and oil-water properties on demulsification are analyzed in detail. It is shown that ultrasonic treatment is an efficient demulsification method, and that maximum demulsification efficiency can be achieved by optimizing the acoustic intensity and treatment time. Excessive acoustic intensity causes acoustic cavitation in low-frequency USWs and acoustic streaming in high-frequency USWs, resulting in a low separation efficiency. Acoustic cavitation also causes secondary emulsification, dispersing tiny oil droplets into separated water. In addition, ultrasonic separation is significantly affected by oil-water properties. The demulsification efficiency of W/O emulsions decreases with increasing oil viscosity and decreasing oil-water interfacial tension. Ultrasonic treatment has significant advantages for separation of emulsions with low water content and interfacial tension. This study presents an in-depth analysis of ultrasonic separation. [ABSTRACT FROM AUTHOR]

The separation characteristics of W/O emulsion were investigated under ultrasonic standing wave field in this paper. The effects of the irradiation time, acoustic intensity, frequency, oil viscosity, and oil-water interfacial tension on the separation characteristics of emulsion were studied in details. It was proved that there existed an optimal irradiation time, acoustic intensity and oil-water interfacial tension to achieve the maximum separation efficiency in ultrasonic treatment process. Excessive long irradiation time had little effect on improving the dehydration rate of emulsion but rather increased unnecessary processing time. Excessive high acoustic intensity caused the occurrence of acoustic cavitation phenomenon. The acoustic cavitation would cause the aggregated droplets to be dispersed by oscillating bubbles or even emulsified again by high speed micro jets. The acoustic cavitation threshold is effectively and accurately obtained by a spectral analysis method. The micro acoustic streaming which resulted from the attenuation of acoustic energy occurred with the increase of frequency. It sheared the aggregated droplets, destroyed the stable droplets banding and reduced the separation efficiency of emulsion. The excess surfactant adsorbed on the oil-water interface resulted in the increase of the interfacial film strength, which hindered the coalescence process of droplets and reduced the separation efficiency. [ABSTRACT FROM AUTHOR]