Study On Thin Layer Sludge Drying With Superheated Steam And Chaotic Characteristics Of Evaporation Curve

Thermal drying is one of effective ways to reduce the moisture content of sludge todisposal requirements. The traditional hot air drying has the problems of high energyconsumption, huge exhaust gas processing capacity and so on. Superheated steamdrying as a new drying technology, with advantages of low energy consumption, highefficiency, no combustion and explosion risks and environmentally friendly, particularlysuitable for sludge, coal, and other flammable or high moisture content and low addedvalue product drying.According to the test objectives and test conditions, the major equipment andinstrumentation was designed and selected. Installation and debugging of sludgesuperheated steam drying experimental setup was done. The experimental setup toachieve real-time test data transmission, flexible adjustment for superheated steam flowrate and temperature. The drying system included strong stability and continuity.In order to investigate the drying characteristics of sludge thin layer by superheatedsteam drying. The thin layer experiment of sludge with thickness of2mm and4mmunder superheated steam temperature of160,200,240,280℃had been carried out. Thesludge drying to moisture content was20%(wet basis) with the dense crack and roughsurface which were beneficial to drying process. The sludge samples were not oxidizedand burnt after superheated steam drying because of absence of oxygen even in hightemperature. The effect of condensation of superheated steam drying had beenresearched. When the temperature was higher, the influence of superheated steamcondensation could be ignored. Quadratic equations, linear equation and Midilli modelcan well describe the moisture change of thin layer sludge drying in the process ofcondensation, restoration and drying, respectively. The Effective moisture diffusivityand activation energy of2and4mm sludge thin layer by superheated steam drying atthe temperature160~280℃.The experiment of2mm、4mm thin layer sludge and sand under160℃、200℃bysuperheated steam drying was done to study characteristics of evaporation rate curves of material with different water signature. Evaporation rate curves under differentcondition were determined have fractal characteristics by using principal componentanalysis method and power spectral analysis. Fractal dimension of evaporation rate wascalculated by using method of variation, the dimension of the evaporation rate of thesludge was greater than sand under the same drying conditions, which drew aconclusion that the fluctuation of moisture evaporation speed of sludge was bigger thanthe sand during drying process. The change of fluctuation degree of water evaporationrate could be reflected by the variation of Hausdorff dimension with drying time.Hausdorff dimension of sludge and sand increased first and then decreased, indicatingthe active degree of drying also been strong to weak.