Drying kinetics and parameters optimization of sludge drying at low temperature

Abstract: The production of sewage sludge in China is increasing daily. Such a large amount of sewage sludge has a tremendous pressure on environment. The moisture content of sewage sludge is greatly high. Decreasing the moisture content of sludge is a critical step in harmlessness, bulkiness reduction, and resource utilization. Thermal drying has been proven to be an effective method to reduce solid volumes. The traditional sludge drying is often carried out by drum dryer, rotary dryer, fluidized dryer, and so on. The drying temperature is set higher (above 100 ℃), resulting in high energy consumption and producing secondary pollution. However, sludge drying at low temperature can not only save energy, but also reduce the emissions of malodorous gases and have a safer drying process. In order to study the drying kinetics of sludge drying at low temperature, the effects of drying temperature, thin-layer thickness and air speed on sludge moisture ratio and drying rate were examined. The optimization of the low temperature drying parameters was also studied. The results showed that the drying rate and drying time were influenced significantly by drying temperature, thickness and air speed. The low temperature drying process could be mainly divided into 2 periods: the acceleration period and falling rate period. The falling rate period could be also divided into 2 periods: the first falling rate period and the second falling rate period. The critical water contents under different drying conditions were about 0.5 g/g (dry basis) between the first falling rate period and the second falling rate period. According to statistical parameters, i.e. coefficient of determination (R2), chi-square (χ2), and root mean square error (RMSE), the Midilli model was the most suitable to describe the relationship between sewage sludge moisture and drying time among 6 common thin-layer drying models. The effective diffusion coefficients of sludge having thicknesses of 5, 10, and 15 mm were 8.72×10-10-3.61×10-9, 1.41×10-9-5.22×10-9 and 2.46×10-9-9.69×10-9 m2/s respectively under the temperature range of 50-90 ℃ at the air speed of 0.6 m/s. Besides, the activation energy values were 25.25, 25.28 and 23.96 kJ/mol respectively. The effective diffusion coefficients of sludge with thickness of 10 mm was 2.46×10-9 and 4.48×10-9 m2/s under the air speed of 0.4 and 0.8 m/s at the temperature of 70 ℃. An orthogonal experiment was designed with the drying temperatures of 50, 70 and 90 ℃, the thicknesses of 5, 10 and 15 mm and the air speeds of 0.4, 0.6 and 0.8 m/s. The effect of drying temperature on relative unit energy consumption was extremely significant. The effect of air speed and thickness on average drying intensity was significant. The order of the factors for decreasing relative unit energy consumption was drying temperature > air speed > thickness, and for increasing average drying intensity was drying temperature > thickness > air speed. In terms of relative unit energy consumption, the optimal technological parameters were presented as follows: drying temperature of 90 ℃, thickness of thin layer of 10 mm, and air speed of 0.8 m/s. In terms of average drying intensity, the optimal technological parameters were presented as follows: drying temperature of 90 ℃, thickness of thin layer of 5 mm, and air speed of 0.8 m/s. The results of this study provide a reference for the subsequent study of the heat pump drying or combined drying of solar and heat pump for sewage sludge.