Kinetics of Sludge Superheated Steam Drying and Optimization of Process Parameters.

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]