Analysis of effective diffusivity of sludge in superheated steam drying based on Fourier number method and optimization method.

The production of sewage sludge from wastewater treatment plants has been continuously increasing in China. One way to manage sewage sludge is drying, which yields a solid with low humidity. This solid can be easily stored, recycled and transported to other facilities. The drying of sewage sludge has been proved to considerably reduce solid volumes. Using superheated steam in the drying process can lead to substantial energy, if the vapour evaporating from the materials is used elsewhere in overall process. Therefore, superheated steam drying is an attractive drying process for industrial use especially if the cost energy for the drying is a major part of the cost of manufacture such as sewage sludge drying process. In order to further study the moisture diffusion mechanism and effective diffusivity of thin layer of sludge in superheated steam drying, this paper ignoring the influence of sample's moisture content change on effective diffusivity when determining the effective diffusivity through drying method, and 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°, respectively. 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 the thickness at 4 mm were 2.52×10-10-2.93×10-9m2/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 2, 4, 6 and 8 mm were 3.85×10-10-2.28×10-9 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 a linear relationship with the two parameters. By using the 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. [ABSTRACT FROM AUTHOR]