Investigation Of The Movement And Coalescence Characteristics Of Droplets Under The Effect Of Ultrasonic

Water-oil emulsion is a very complex liquid-liquid dispersion system, the stability of which mainly depends on the oil-water interfacial film strength. Ultrasonic demulsification mainly uses the mechanical vibration and thermal effect of ultrasonic to strengthen the collision and coalescence of water droplets in oil, thus accelerating the separation of oil and water. Until now, there have been not enough studies of ultrasonic coalescence mechanism, neither a theoretical system. In this paper, by micro study and macro experiment, the movement and coalescence characteristics of water droplets under the effect of ultrasonic were studied. And the influence of ultrasonic and oil physical parameters on the movement and coalescence characteristics of water droplets and the dehydration effectiveness of emulsions was studied. The main conclusions are as following.Ultrasonic has an obvious effect on the movement trajectory, the speed and the oscillation amplitude and frequency of water droplets. The water droplet does similar sine oscillation movement. With the increment of ultrasonic intensity, the oscillation amplitude of water droplets increases to the maximum and then decreases. There exists an ultrasonic intensity under which the water droplet does the most severe oscillation movement. That is the critical ultrasonic intensity. On this experimental conditions, the critical value of ultrasonic intensity is 4.89 W·cm-2. With the increment of ultrasonic frequency, the oscillation movement of water droplet weakens. The optimum frequency is 20 kHz. The water droplet whose diameter is between 200 and 400 microns does the most severe oscillation movement. The oscillation amplitude is large and the frequency is high. While the diameter exceeds this range, the water droplet suffers bigger viscous resistance and its oscillation amplitude decreases. While the diameter is less than this range, the oil-water interface slip velocity decreases and the oscillation frequency of water droplet decreases. The interfacial tension has not so obvious influence on the oscillation frequency of water droplets. While the surfactant attains its critical micelle concentration of 400 ppm, the oscillation frequency of water droplet decreases and the oscillation movement weakens. With the decrement of oil viscosity, the oscillation amplitude and frequency of water droplet increases. Under the effect of ultrasonic, the oil-water interface is periodically stretched and compressed, which leads to the water droplet’s periodic deformation, and the deformation changing period is just the same with the oscillation period. The deformation degree of droplet is mainly affected by ultrasonic intensity, ultrasonic frequency, droplet size, oil viscosity and interfacial tension. With the increment of ultrasonic intensity and with the decrement of ultrasonic frequency, droplet size, oil viscosity and interfacial tension, the maximum deformation degree of water droplet increases.Ultrasonic mechanical vibration effect cuts the oil-water interfacial film strength and accelerates the liquid film drainage, thus promoting the coalescence of droplets. The water droplets do similar sine oscillation movement before coalescing. Under experimental conditions of ultrasonic intensity of 4.89 W·cm-2, ultrasonic frequency of 20 kHz and surfactant concentration of 70 ppm, the water droplets whose diameters are between 200 and 400 microns do severe oscillation movement and coalescence effect is the best. The oscillation frequency and coalescence time of water droplets are mainly affected by ultrasonic intensity, ultrasonic frequency, droplet size, oil viscosity and interfacial tension. With the increment of ultrasonic intensity, the coalescence time of water droplets decreases. With the increment of ultrasonic frequency, the coalescence time of water droplets increases. With the decrement of droplet size and oil viscosity, the oscillation frequency of water droplets increases and the coalescence time decreases. With the decrement of interfacial tension, the oscillation frequency of water droplets increases to the maximum and then decrease, while the coalescence time is inversely.Ultrasonic thermal effect makes the temperature of emulsions rise significantly after a period of irradiation. With the increase of ultrasonic intensity and irradiation time, the temperature of emulsions increases. Under the same ultrasonic intensity, with the increment of ultrasonic irradiation time, the dehydration rate of emulsions increases to the maximum and then decreases. The greater the ultrasonic intensity is, the shorter the ultrasonic irradiation time to attain the maximum dehydration rate of this ultrasonic intensity is. Under short ultrasonic irradiation time, the dehydration rate of emulsions increases with the increment of ultrasonic intensity. While the ultrasonic irradiation time becomes longer, the dehydration rate of emulsions increases to the maximum and then decreases with the increment of ultrasonic intensity. The longer the ultrasonic irradiation time is, the smaller the ultrasonic intensity to attain the maximum dehydration rate of this ultrasonic irradiation time is. With the increment of water content, the average particle size of emulsions after ultrasonic irradiation decreases and the percentage of small particle size droplets increases. Thus the dehydration rate of ultrasonic demulsification decreases. With the increment of ultrasonic frequency, the optimal ultrasonic demulsification intensity and irradiation time decrease and the dehydration rate of emulsions decreases. With the decrement of oil viscosity, the dehydration rate of emulsions increases. With the decrement of interfacial tension, the dehydration rate of emulsions increases to the maximum and then decreases.