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The purpose of this paper is to study the effects of steam temperature, sludge thickness, and steam flow on the drying characteristics and drying kinetics of sludge superheated steam and optimize the process parameters of sludge superheated steam drying by using the response surface method and taking relative unit energy consumption as the response value. Single-factor test of superheated steam drying of sludge was conducted under different drying conditions of steam temperature (180, 195, 220, and 260°C), sludge thickness (2, 3.5, 6, and 10 mm), and steam flow rate (22, 29, 36, and 40 m3/h). Taking relative unit energy consumption as the response value and steam temperature, sludge mass (corresponding to sludge thickness), and steam flow as the test factors, the three-factor and five-level response surface test was conducted on the sludge. The experimental results show that steam temperature and sludge thickness had a significant influence on the drying characteristics of superheated sludge steam (p < 0.05). With the increase of steam temperature, the decrease of sludge thickness, and the increase of steam flow, the drying time of sludge will be shortened, the drying efficiency of sludge will be improved, and the maximum condensation amount and recovery time will be reduced. The quadratic polynomials, linear equation, and logarithmic model can better fit the change law of water variation of the sludge of superheated steam condensation section, recovery section, and drying section. With the increase of steam temperature, sludge thickness, and steam flow rate, the effective diffusion coefficient also increases. The activation energy of sludge superheated steam drying was 13.42 kJ/mol at a sludge thickness of 6 mm, steam flow rate of 22 m3/h, and different steam temperatures. The order of influence degree of three test factors on relative unit energy consumption is as follows: sludge mass (corresponding to sludge thickness) > steam flow rate > steam temperature. According to response surface test analysis, the optimal drying process with relative unit energy consumption is steam temperature of 215°C, sludge mass of 25 g (corresponding sludge thickness of 8 mm), and steam flow of 30 m3/h. Under this drying process parameter, the minimum energy consumption of sludge superheated steam drying is 284.61 kJ/g. Compared with the relative unit energy consumption value obtained in the verification test, the relative error is 1.65%. [ABSTRACT FROM AUTHOR]

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]