Search

The sludge is known as the byproduct generated in the process of pollution control. It is featured by the high moisture content, complicated components and massive deadly microorganism, thus easily leading to secondary pollution. The dehydration is a necessary process to realize the sludge harmless reduce the sludge amount and improve the resource utilization. The thermal drying is a feasible method to reduce the moisture content. The process of hot air drying will cause a large amount of harmful exhaust gas, adding to the difficulty and cost of controlling exhaust gas. The sludge drying, which uses superheated steam as the drying medium, will further lower the risk of explosion and reduce the harmful gas account due to the absence of oxygen in the drying medium. However, the superheated steam drying of sludge is also faced with the problems such as energy consumption and long drying time. Hence, it has become a major task to dry the sludge in an economic and effective manner. The purpose of this research is to determine the technological parameters for optimal sludge superheated steam drying. Besides building the experimental device for superheated steam drying under atmospheric pressure and choosing the relative unit energy consumption and the average drying strength as the evaluation indices, this paper has also designed the experiment of three-factor quadratic orthogonal rotation combination. A mathematical model is also built, which adopts the technological parameters for sludge superheated steam drying including superheated steam temperature sludge quality and superheated steam flow. In addition, this paper has employed the single-factor and two-factor method to analyze the relationship between various factors and evaluation indices. According to the analysis on experimental data, the established regression equation of establishing is significant, and the coefficient of determination on the relative unit energy consumption and the average drying strength were 0.842 and 0.797, respectively. The 3 factors listed in the descending order of their importance to the 2 evaluation ind ices are sludge mass, steam flow and steam temperature. For the sludge superheated steam drying in terms of relative unit energy consumption, the optimal technological parameters are presented as follows, steam temperature being 215°C, superheated steam flow being 30 m³/h, sludge mass being 26 g (8 mm thickness approximately) and steam flow being 30 m³/h. Under these conditions, the predicted relative unit energy consumption is 281.313 kJ/g. The actual relative unit energy consumption gained from the validity experiment is 280 kJ/g, with the relative error being 0.467%. Hence, the experimental result can provide reference for parameter optimization of sludge superheated steam drying and the design of drying equipment. [ABSTRACT FROM AUTHOR]

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]