The Study On Drying Characteristics Of Wet-Dry-Mixed Sludge

At the certain range of moisture content,the sludge with high viscosity will appear the phenomenon of wall sticking during drying,which will corrode and block drying equipment,affecting the safe and economical operation of drying equipment.However,the mixed sludge can reduce its moisture content and avoid the region with high viscosity,which is beneficial to the safe operation of drying equipment for sludge.By comparison test between the mixed and non-mixed sludge at the constant temperature drying,it was confirmed that the mixed sludge was beneficial of the sludge drying and further researches were made to study the different mixedways how to and mixing ratios on the effect of drying efficiency of mixed sludge.The way of mixed sludge with wet sludge covered with dry sludge(MSWD)could speed up the drying rate,and on the other hand the way of mixed sludge with dry sludge covered with wet sludge(MSDW)can reduce the drying rate.During the mixing of sludge,some mixed sludge would form the core of dry sludge and the wet sludge covered with dry sludge granules as.The granules expanded the contact area of the wet sludge and dry media may reduce the resistance of drying,which could speed up the drying efficiency.For different kinds of mixing ratios,the drying effect is different for MSWD,and the difference of drying efficiency between MSWD and MSDW was most obvious in the ratio between dry sludge and wet near 1:5.In the meanwhile,the gap of drying rate was biggest at this ratio.At the ratio of 1:4,the drying rate of MSWD is the fastest,which is most advantageous for drying,that is to sayIn order to avoid the region of high viscosity of sludge and search fast and low power consumption drying method of mixed sludge,further researches were made to study different kinds of mixing ratios(1:1,1:1/2,1:1/3,1:1/4)between wet sludge and dry sludge of the municipal sludge at the constant heat flux,getting the drying curve as well as analyzing its energy consumption.The results show that the mixed sludge made up of the same quality of wet sludge mixed with different proportion of dry sludgeexperienced the three stages:accelerated stage,constant rate stage and decelerated stages during drying,and the duration of constant stage was different under different mixing ratio of sludge.The laws of drying rate and the amount of energy consumption under different mixing ratio were different,and considering the viscosity of sludge,the optimal mixing ratio of the sludge at the constant heat flux drying should take drying rate and the amount of energy consumption into account.When the heat flux is 1140W/m2 and 310 W/m2,the drying rate at the mixing ratio of 1:1/3 is better than other ratio under the synthetic effect of heat and mass transfer because the mixing ratio was moderate among all mixing ratios.However,when the heat flux is 690 W/m2 and 430 W/m2,the drying rate at the mixing ratio of 1:1/4 was better,which is related to the change of the internal structure after wet-dry-mixed sludge and the process of the heat transfer and the mass transfer of water evaporation and diffusion in sludge.The structural information was obtained through the image of the sludge and the internal structure of the sludge was measured by the direct instrument(scanning electron microscope),by contrast,finding that the comparative study that the internal structure of the sludge changed with the change of water during the drying process.As the drying process went on,the pore of sludge firstly increased and then decreased.When the pore size was the biggest,the crack of the sludge was biggest,then crack shrank and the pore size also began to decrease.Subsequently,the related research results of isothermal drying and constant heat flux drying were explained by the void structure.The above research will provide certain help for the drying application of wet-dry-mixed sludge.