Effect of frequency on ultrasound-assisted centrifugal dewatering of petroleum sludge

In this work, petroleum sludge has been treated with ultrasound at 20, 25, 30, 35, and 40 kHz to study the effects of frequency on dewatering during pretreatment process. Various parameters, including water content, droplet size distribution, viscosity, and temperature, were analyzed to quantify the improvement of dewaterability which will affect the subsequent centrifugal dehydration. The results showed that 25 and 30 kHz ultrasound could directly remove water by 26.3 and 28.1 wt%, respectively, but resulted in decrease in droplet diameter in remaining water. Although there is little or no separation of water by high-frequency ultrasounds, 35 and 40 kHz strongly affected the emulsified water and increased the mean droplet size. These differences could be primarily explained by acoustic resonance and cavitation effects. In addition, the centrifugal results showed that dewatering ratio up to 78.5 wt% was achieved for sludge with viscosity of 6 Pa · s after treated at 40 kHz. Through analysis and comparison of different-frequency ultrasonic treatments, a unified characterization method is developed to provide quantitative evaluation for selecting the proper pretreatment.