| The production of sewage sludge in our country is increasing daily.As a residual product of sewage treatment,the moisture content of sewage sludge is greatly high and it contains a large number of pathogenic microorganisms,parasites,and heavy metals.If the disposal of sewage sludge is unreasonable,it will brings serious secondary pollution to environment.The common disposal methods like land filling,composting,land utilization,building materials and incineration are all required to reduce the water content of sludge.Hot drying is an effective way to reduce the moisture content.The traditional sludge drying technology has the disadvantages of large investment in equipment,high energy consumption,high running cost,the risk of explosion and secondary pollution.However,the sludge low temperature drying technology can not only save energy and reduce the odor gas,but also saves the calorific value and has a safer drying process.In order to study the low temperature drying characteristics of sludge within 100?,a low temperature hot air drying test device was set up.The whole device mainly includes drying box,heating pipe,temperature control system,data acquisition device,power control cabinet,high-pressure fan and other components.After the redesign of some devices and the selection of related devices,the test device has the characteristics of compact structure,simple operation and cost saving.The low temperature drying characteristics of different temperatures,thin layer thicknesses and air speeds were studied.The main results were as following: the shrinkage and cracks of sludge was significant in the low temperature drying process.The higher the drying temperature,the smaller the thickness of the thin layer and the greater the hot air speed,the shorter the drying time was.The low temperature drying process could be divided into three periods: the acceleration period,constant rate period and falling rate period.The whole drying process was mainly in the falling rate period,accounting for about 2/3 of the total drying process.The falling rate period could be divided into two periods: the first falling rate period and the second falling rate period.The first critical moisture content and the second critical moisture content were about2.0g/g(d.b.)and 0.5g/g(d.b.).Compared with the traditional thin-layer drying models,the new model could describe the low temperature drying curves with the best accurate.The Weibull distribution function could have a good fit to the low temperature drying process.The drying temperatures,the thicknesses of thin layer and the wind speeds hada significant effect on scale parameter ?,but the shape parameter ? was only affected by the drying temperatures.The effective diffusion coefficients of 5,10,and 15 mm thicknesses of sludge were 8.72×10-10~3.61×10-9,1.41×10-9~5.22×10-9,2.46×10-9~9.69×10-9m2/s respectively under the temperature range of 50~90? at the air speed of 0.6m/s.Besides,the activation energy values of 5,10,15 mm thicknesses of sludge were 25.25,25.28 and 23.96kJ/mol respectively.The effective diffusion coefficients of sludge with thickness of 10 mm was 2.46×10-9,4.48×10-9m2/s under the air speed range of 0.4 and 0.8m/s at the temperature of 70?.An orthogonal experiment was designed with the influential factors of drying temperatures,thicknesses and air speeds.In terms of relative unit energy consumption,the optimal technological parameters were presented as following: drying temperature being 90?,thickness of thin layer being 10 mm,air speed being 0.8m/s.In terms of average drying intensity,the optimal technological parameters were presented as following: drying temperature being 90?,thickness of thin layer being 5mm,air speed being 0.8m/s.The results of this study provide a reference for the subsequent study of the sludge combined drying of solar and heat pump. |
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