Analysis of moisture diffusion and activation energy in superheated steam and hot air sludge thin layer drying

Abstract: Sewage sludge is generated in wastewater treatment processes. It has a solids content of about 1-2% typically. The key step to treating sludge is dewatering.. Dewatering of sludge by belt presses, filters, and centrifuges can lead to dry solids contents in the range of 15-25%. This step can substantially reduce the volume of the sludge. Characteristics of sludge include high water content, bulk mass, and containment of pathogenic microorganisms. Landfilling of sludge has the disadvantages of occupying land and causing secondary pollutions, especially to groundwater. Thermal drying of dewatered sludge is another step to reduce the volume of dewatered sludge. The drying process consists of complex mechanisms such as molecular diffusion, capillary flow, Knudsen flow, water uptake kinetics flow, and surface diffusion.In order to study the effective diffusion coefficient and the activation energy characteristics of the sludge layer in the process of superheated steam drying and hot air drying, an internal-circulation drying test-bed under normal pressure was built to carry out superheated steam drying and hot air drying tests on sludge layers with thicknesses of 4 mm and 10 mm respectively at the temperature range of 160-280℃. The linear relationship between effective diffusion coefficient and drying time was established through the Fick diffusion model. It was found that the effective diffusion coefficients for the 4 mm sludge layer ranged 7.1515×10-9-2.4852×10-8m2/s and 1.2414×10-8-2.2769×10-8 m2/s for superheated steam drying and hot air drying respectively. The effective diffusion coefficients for the 10 mm sludge layer ranged 1.9659×10-8-5.8811×10-8 m2/s and 2.8042×10-8- 5.6095×10-8 m2/s for superheated steam drying and hot air drying respectively. The linear relationship between effective diffusion coefficient and temperature was established based on the Arrhenius empirical formula. Thus, the average activation energies of 4 and 10 mm sludge layers can be obtained respectively as 21.173 and 18.085 kJ/mol by superheated steam drying and 9.485, 11.191 kJ/mol by hot air drying. These values are mostly in conformity with the effective diffusion coefficient and activation energy obtained by the Midilli thin layer drying model This test showed that when temperature exceeds 260℃, the effective diffusion coefficient of a sludge layer by superheated steam drying is greater than that created by hot air drying. Values obtained showed a linear increase in diffusion coefficients to temperature by superheated steam drying but a curve in hot air drying, suggesting the possibility of oxidation and combustion of the sludge layer by hot air drying.