Optimized model of sludge drying characteristics based on an experimental study of thickness, temperature, and humidity

Abstract

Heat pump drying is an energy-saving method for sludge drying. However, current research on sludge drying characteristics is inconsistent with the working conditions of a heat pump. To provide an energy-saving working condition for heat pump drying, the effects of temperature, relative humidity, and sludge thickness on sludge drying characteristics were comprehensively studied. Results showed that under the conditions of 2–6 mm thickness, 40 °C–60 °C temperature, and 30%–60% relative humidity, drying time decreased by 15%–25% for every 10 °C increase in temperature when thickness and relative humidity were fixed. When thickness and temperature were fixed, relative humidity decreased from 60% to 30% and drying time decreased by about 50%. When temperature and relative humidity were fixed, drying time was reduced by one-third for every 2 mm decrease in thickness. The effects of thickness, temperature, and relative humidity on the effective diffusion coefficient were fitted and exhibited a linear experimental relationship (R2 = 0.944). Based on the Midilli model, the parameters were quantitatively processed, and the model for sludge drying under low temperature conditions was optimized based on the effective diffusion coefficient. The accuracy of the simulation was similar to that of the logarithmic model. The maximum relative error between the fitting results and experimental results was 9.8%, effectively describing the drying characteristics of sludge at low temperatures.