Effect of ultrasonic treatment on hot air drying process of sludge

Abstract: Solid-liquid separation is crucial for reduction and recycling disposal of sludge, and ultrasonic treatment is regarded as one of the most potential and effective method for improving dewaterability. In order to investigate the effect of ultrasonic treatment on hot air drying process of sludge, an experimental apparatus for ultrasound-assisted hot air drying of sludge was designed and built. The experimental apparatus were composed of hot air generator, ultrasonic generator and transducer，data acquisition system and drying chamber. The ultrasonic apparatus operated at a frequency of 20 kHz and supplied a continuous adjustable power of 0-600 W. The hot air temperature was respectively set at 60, 80, 100, 120℃, a drying experiment was performed to analyze hot air drying characteristic of sludge under different acoustic parameters and to assess the effects of ultrasonic energy density, irradiation time and ultrasonic thermal effect on hot air drying process of sludge. Results showed that ultrasonic treatment can accelerate drying process, and the ultrasonic enhancing effect on sludge drying process gradually strengthened with the increase of ultrasonic energy density. When sludge sample thickness was 15 mm, hot air temperature and velocity was 80℃ and 1.0 m/s, the dry basis moisture content of sample reduced to 100% after drying 4.8 hours without ultrasonic treatment. When hot air temperature was 80℃ and ultrasound energy density was 0.2, 0.4, 0.6, 0.8 and 1.0 W/mL, respectively, the drying time of moisture content of sludge decreasing to 100% were shortened by 8.3%, 22.9%, 33.3%, 37.5% and 39.5%, respectively, compared with that under untreated ultrasound. The ultrasonic treatment has no obvious effect on sludge drying process after 2 hours at ultrasonic energy density of 0.8 W/mL and hot air temperature of 80℃. Thus there is no need to keep on the ultrasonic treatment anymore after this time. Moisture content of sample has an important influence on ultrasonic enhancement effect on sludge drying process. In the later stage of drying, moisture content of sludge becomes very low, attenuation of acoustic energy is serious in sample and ultrasonic cavitation and mechanical effect is extremely weak. So it is better that ultrasonic treatment is applied in constant rate drying period of sludge. In addition, ultrasonic energy density remains the same, but the sludge drying rate is varying degrees of increase under different hot air temperature. When the acoustic energy density was 0.4 W/mL for arriving to the same drying destination , the dry time was shortened by 19.2%, 22.9%, 20% and 18.2% respectively at the hot air temperature of 60, 80, 100, 120℃ compared with that untreated with ultrasound. That is because the drying process is determined by both internal and external transport capacity. The ultrasonic irradiation is preferable to enhance the internal moisture transfer process, but its superiority can be confined under an inapposite exterior evaporation capability such as hot air temperature. So the reasonable combination between ultrasonic energy and external transport capability is essential to optimize ultrasound-assisted hot air drying process of sludge. However, the inner temperature of sample increases was caused by ultrasonic thermal effect during sludge drying process, but temperature rising value reduced gradually along the direction of acoustic propagation. So temperature gradient of sludge inner with ultrasonic irradiation was slightly lower than that without it. Therefore, the moisture diffusion caused by the temperature gradient of sludge inner couldn't be enhanced by ultrasonic effect. The study in this paper can provide theoretical guidance and experience for practical application of ultrasonic technology in the field of sludge thermal drying.