Effects of temperature, relative humidity, and sludge thickness on sludge drying characteristics and kinetic investigation

Abstract

In order to analyze the drying and kinetic characteristics of sludge, drying experiments were designed and carried out in this paper. The changes of each key indicator in the drying process were investigated under drying temperature of 40–80 ℃, relative humidity of drying air of 7∼35 %, and sludge thickness of 1–9 mm, respectively. The results showed that the drying time was shortened by 12.63 % for each 10 ℃ increase in the range of 40–70 ℃. However, when the temperature was increased to 80 ℃, the drying time reduced significantly, and the drying rate of the sludge increased by 25.53 %. Reducing relative humidity shortened the drying time, and when the relative humidity was below 14 %, the relative humidity had no significant effect on the drying time. The reduction of sludge thickness decreased the drying time significantly, and the maximum drying rate of 1 mm thick sludge was 86.73 % higher than that of 9 mm. When the sludge thickness was 5 mm, the drying energy consumption per unit mass reached the minimum value of 49.63 kJ/g. The effective diffusion coefficient was positively correlated with temperature and thickness, while the opposite existed with relative humidity. The activation energy of drying under 3 mm thickness is 15.293 kJ/mol, and the pre-exponential factor is 8.951E-08 m²/s. The sludge drying followed the Midilli model, where the model parameters a and n were mainly affected by constants, and the R² of model parameters b and k were 0.628 and 0.891, respectively. The results can provide reliable parameter guidance for strengthening sludge drying performance and reducing drying energy consumption.