Investigation On The Drying Behaviors Of Typical Solid Waste During Microwave Combined Hot Air Drying

The safe and efficient minimization of compactable solid waste was a key step in the development of nuclear power industry.Drying pretreatment was an effective method to ensure the safety and the volume minimization of waste treatment.The long drying duration and high energy consumption of the traditional hot air convective indicated low drying efficiency.The assissted microwave drying could increase the efficiency of the convective drying for its high drying rate and low energy consumption.In this paper,the drying experiments of three kinds of typical compactable solid waste were carried out on a microwave combined hot air convective drying system.The effects of microwave power,hot air temperature and other factors on the drying characteristics of the solid waste were investigated.The effects of the hot air temperature(90-130 ?)and the microwave power(200-800 W)on the drying kinetics for the cotton fabrics and the paper towel were determined respectively by the separate and combined microwave and convective drying experiments.The results shown that the effective moisture diffusivities of the cotton fabric and the paper towel during the convective drying process were 2.86×10-8-4.58×10-8 m2/s and 6.07×10-9-1.31×10-8 m2/s,respectively,and the activation energy were 15.8 kJ/mol and 23.2 kJ/mol,respectively;their effective moisture diffusivities during the microwave drying process were 1.04×10-8-5.7×10-8 m2/s and 3.54×10-8-1.52×10-7 m2/s,respectively,and the activation energy were 25.2 W/g and 22.6 W/g,respectively;their effective moisture diffusivities during the microwave convective combined drying were 4.95×10-8-1.28×10-7 m2/s and 5.71×10-8-2.05x 10-7 m2/s,respectively.The energy efficiencies of the three samples were within 1-2%during the convective drying process.The energy efficienies of the cotton fabrics and the paper towel under microwave drying were 3%and 10%,respectively.During the combined drying process,the energy efficiencies of the plastic debris and the cotton fabrics were about 2%and 4%.The efficiency of the paper towel was about 8%,which was lower than that in the microwave drying alone.For the cotton fabric and the paper towel,the energy consumption decreased with the increasing microwave duration;while for the plastic,the lowest energy consumption was obtained by closing the microwave radiation at low moisture content,which was about 13%lower than the continuous microwave combined drying method.During the microwave drying process,the uniformity index of the microwave energy distribution on the cotton fabrics and the paper towel surface were 26%and 28%.The hot spot appeared on the sample surface at low moisture content.The uniformity index of the surface temperature distribution was 25-50%in the microwave drying alone.The surface temperature uniformity index decreased to 15-20%in the microwave convective combined method,which suggested that the microwave heating uniformity can be improved with assistance of hot air drying.With the application of the X-shaped and O-shaped mode stirrers,the uniformity index of the surface microwave energy distribution decreased by 8%and 13%,respectively Correspondingly,the uniformity index of the surface temperature distribution decreased by 5%and 8%,respectively.The O-shaped mode stirrer was more favorable than the X-shaped one on the heating uniformity.The results developed in the work can be used for the treatment of the solid wastes and the design of the new dryer.