Research On Diffusion Coefficient、Inversion And Activation Energy In Superheated Steam And Hot Air Sludge Drying

For the high moisture content and high viscosity characteristics of sludge，whichis leading to difficult to handle and a series of problems. Through belt filter press,sludge dehydrating centrifuge and filter, the water content in the sludge will belowered down by20%to35%or so, but remains to dissatisfy the required level forsubsequent disposal of sludge (e.g. compost, sanitary landfill, etc.), so a furthertreatment of hot drying and capacity reduction must be conducted for sludge. First theprinciple and characteristics of the traditional sludge drying was introduced. Thechanges of temperature, speed, time and moisture content in sewage sludge drying arepresented. In addition, the principle and characteristic of improved sludge drying arediscussed. Finally, the superheated steam convection and conduction drying hasenergy saving and high temperature sterilization and so on, but it has somedisadvantages, for example parking condensation. The paper briefly describes thesuperheated steam convection and conduction drying development prospect in sludge.In the drying process of dewatered sludge, the sludge moisture diffusion is acomplex process, which may include molecular diffusion, capillary flow, Knudsenflow, water uptake kinetics flow and surface diffusion. The synthesis of all thephenomena is defined as effective diffusion coefficient by Fick’s second law. Dryingactivation energy refers to the starting energy required for evaporating the unit moleof water in the drying process.In order to study the effective diffusion coefficient and the activation energycharacteristics of the sludge thin layer in the process of superheated steam drying andhot air drying, the internal-circulation drying test-bed under normal pressure is built tocarry out the superheated steam drying and hot air drying for the sludge thin layerwith a thickness of4mm and10mm respectively at the temperature160-280℃. Thelinear relationship between effective diffusion coefficient and drying time isestablished through using the Fick diffusion model. It is found that the effectivediffusion coefficient variation range of4mm sludge thin layer by superheated steamdrying and hot air drying are7.1515×10-9-2.4852×10-8m2/s and1.2414×10-8-2.2769×10-8m2/s respectively. The effective diffusion coefficient variation range of10mm sludge thin layer by superheated steam drying and hot airdrying are1.9659×10-8-5.8811×10-8m2/s and2.8042×10-8-5.6095×10-8m2/srespectively. The linear relationship between effective diffusion coefficient andtemperature is built based on the Arrhenius empirical formula. Thus, the averageactivation energies of4,10mm sludge thin layer by the superheated steam drying andhot air drying can be obtained respectively as21.173,18.085and9.485,11.191kJ/mol.The test values are basically in conformity with the effective diffusion coefficient andactivation energy obtained by Midilli thin layer drying model. The results show that:At a temperature of260℃, the effective diffusion coefficient of sludge thin layer bysuperheated steam drying reverses, the degree of which is greater than that by hot airdrying. The effective diffusion coefficients increase with temperature trend almostinto an inclined straight line by superheated steam drying, but in hot air drying it likesa curve. So it is possible that the oxidization and combustion of sludge thin layer byhot air drying. The research results provided a method for effective diffusioncoefficient values strike and superheated steam drying reversal point temperaturedetermined in sludge thin layer drying.