Research On Shrinkage Characteristics Of Sludge In Superheated Steam And Hot Air Drying

With the unceasing acceleration of the pace of industrialization and urbanization in our country, sewage treatment plants and sludge production are increasing rapidly.Sludge contains a large number of pathogenic microorganisms, volatile substances,heavy metals, other toxic and harmful substances. If the sludge does not be carried on rational disposal, the survival of plants, animals and humans is bound to be jeopardized.The notable feature of the sludge is the high moisture content. Reducing the moisture content of sludge is a prerequisite for its treatment and disposal, and it is the key. As an important way to reduce the moisture content, thermal drying is widely used in volume and mass reduction of sludge disposing process.Sludge drying is a complex heat and mass transfer process and the sludge produces shrinkage in the drying process. The sludge shrinkage has an important impact on heat and mass transfer, so a set of superheated steam and hot air drying test bench was set up to study the sludge shrinkage characteristics in contrast by experiments. The entire test bench mainly consists of steam generation apparatus, heating device, drying oven,temperature control system, data acquisition system, image capture device,high-pressure fan and other components. According to the test requirements, the design of relevant parts and selection of equipments had been completed. The overall structure of test apparatus is compact and it can be operated conveniently.The existing models mostly ignore materials’ shrinkage during drying process, and the finite regular materials were considered as an infinite one to simplify. This may lead to erroneous results in the calculation process of drying parameters such as the effective diffusion coefficient and so on. The moisture diffusion theory model of Fick’s second law based on infinite slab model and infinite cylinder model concerning finite round slab was also built, which considered the shrinkage phenomenon, by superimposition technique. Combining the Henderson and Pabis model, the expression of calculating effective diffusion coefficients was elicited.According to the pre-experiments and the structure dimensions of the test apparatus, the sludge samples’ demensions of 10 mm in thickness, 50 mm in diameter and temperatures of 160 and 200℃ were determined. The superheated steam and hot air drying experiments were carried out with aforementioned conditions, respectively.The image processing technique was used to analyze the shrinkage phenomena and the characteristics of caky sludge. The effective diffusion coefficients were calculated bythe expression elicited in the former part. 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 0.3and 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 to0.94×10-8, 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.