Analysis of shrinkage characteristics of sludge in superheated steam and hot air drying processes based on image processing

Abstract: Large amounts of sewage sludge with high moisture content are generated every year. There are varieties of methods to manage sewage sludge such as land filling, composting, pyrolysis, incineration and thermal drying. Decreasing the moisture content of sludge is a critical step in harmlessness, bulkiness reduction, and resource utilization. Thermal drying has been proven to be an effective approach to reduce solid volumes compared with other ways. Drying is a complex process which may consist of molecular diffusion, capillary flow, Knudsen flow, surface diffusion and so on. Generally, the phenomena of shrinkage crack and skin layer formation can be observed during the drying process and each of them occurs can modify the drying kinetics. Currently, there are many studies on sludge shrinkage in hot air drying but lack of studies in superheated steam drying which has the advantages of energy saving, high efficiency and great heat and mass transfer coefficients. Furthermore, most of moisture diffusion models proposed by researchers are not consider the shrinkage and regard the finite regular geometry as an infinite one when they deal with the drying process, it may result in large error or erroneous results. The purpose of this paper is to investigate the shrinkage characteristics of sludge in superheated steam and hot air drying processes and their influences on the effective diffusion coefficients. The experimental devices for superheated steam drying under atmospheric pressure and hot air drying were built. The drying experiments were carried out with caky sludge samples of 10 mm in thickness and 50 mm in diameter at the temperature of 160 and 200 ℃. During the drying process, the photographs of sludge samples were obtained with digital camera by taking out the sludge samples from the drying chamber every tem minutes. In order to study the shrinkage phenomenon and its characteristic of caky sludge, the image processing technique was used to processing and analyze the photographs. In addition, the moisture diffusion theory model based on Fick's second law, infinite slab model and infinite cylinder model concerning finite round slab was also built, which considered the shrinkage phenomenon, by superimposition technique. Combining the empirical model, the expression of calculating effective diffusion coefficients was elicited. The results showed that the shrinkage phenomena can be observed remarkably during the drying process and the variation law was corresponding with the moisture that decreased sharply in great range at the forepart and reduced slowly in narrow range after that. The volume ratio is about 0.3 and the volume shrinkage coefficient equal to 0.7 at the end of superheated steam and hot air drying at the temperature of 160 and 200℃.The influences on sludge shrinkage were consistent between superheated steam and hot air drying. The change of effective diffusion coefficients was coincident with the moisture ratio. Considering the shrinkage of sludge samples, the average effective diffusion coefficients of superheated steam drying were equal to 1.92×10-8, 3.75×10-8 m2/s at the temperature of 160 and 200℃, respectively. The average effective diffusion coefficients of hot air drying were equal to 0.94×10-8, 1.31×10-8 m2/s in the same conditions, respectively. The average effective diffusion coefficients were half compared with those without shrinkage. A valuable reference for analysis of drying process mechanism, process parameter optimization of sludge drying and design of drying devices can be provided from the experimental results.