Research On Effective Diffusivity Of Sludge In Superheated Steam Drying Based On Fourier Number Method And Optimization Method

Data from the Department of housing construction revealed that, by the end of 2014, the number of sewage treatment plants and the sewage treatment capacity increased 82.7% and 53.9% respectively compared to the year 2009. Consequently, the annual yield of sludge increased dramatically with the increased amount of sewage. Sludge drying is a good way to reduce the mass and volume of sludge, which would promote its stability, and as a result provide great convenience for later treatment and disposal. Superheated drying is a new drying technology which have the advantages of energy-saving, low gas emission and effective sterilization. With these characteristics, it was suitably introduced for the high energy consumptive sludge drying.The effective diffusivity is a key parameter which reflects moisture diffusion mechanism during drying process. Also, it is a important transfer characteristic for drying parameter optimization and drying equipment design. In order to further study the moisture diffusion mechanism and effective diffusivity of thin layer of sludge in superheated steam drying, atmospheric convection superheated steam drying experiment platform was established.According to the principle of ignoring the influence of sample’s moisture content change on effective diffusivity when determining the effective diffusivity through drying method, and based on the premise that the effective diffusivity relies on the moisture content, This paper adopts Fourier number method and optimization method respectively to calculate the effective diffusivity of sludge thin layers with the thickness of 2, 4, 6 and 10 mm at superheated steam temperature from 120 to 280℃ ℃. Relations of the effective diffusivity with superheated steam temperature and the thickness of thin layer were analyzed. The results showed that the effective diffusivity values derived from Fourier number method and optimization method were basically identical, and the effective diffusivity values and change characteristics of thin layer sludge in superheated steam drying were more accurately reflected based on these two methods. Calculation results derived from the Fourier number method and optimization method showed that the effective diffusivity of sludge with thickness of 4 mm were 2.52×10-10~ 2.93×10-9 m2/s and 2.75×10-10~ 3.32×10-9 m2/s respectively at the temperature 120-280℃,and the effective diffusivity of sludge with thicknesses of 2mm, 4mm, 6mm and 8mm were 3.84×10-10~2.28×10-9 m2/s and 4.40×10-10~2.72×10-9 m2/s respectively at the temperature of 160℃. The effective diffusivity increased gradually with the increase of superheated steam temperature and the thickness of thin layer, and had linear relationship with the two parameters. By using multiple linear regression method, simplified expressions that reflect the relationship between effective diffusivity and drying temperature and thickness of layer were established respectively according to Fourier number method and optimization method, with the corresponding determination coefficient of 0.9982 and 0.9956. The calculation results can provide a valuable reference for the analysis of moisture diffusion process and effective diffusion coefficient of thin sludge layer in superheated steam drying.