Characterization And Numerical Simulation Of Low Temperature Hot Drying Process Of Sewage Sludge

The continuous development of China's industry and the speed of urbanization are accelerating,and sludge produced by sewage treatment plants is increasing rapidly.Sludge drying is an important pre-treatment step before sludge disposal or recycling.Thermal drying is a common method of drying sludge.In order to improve the efficiency of dry sludge.It is of great scientific significance to study the effects of heat and mass transfer characteristics and shrinkage characteristics of sludge drying process.(1)This paper is based on the self-designed low-temperature hot air convection drying test bench to analyze the heat and mass transfer characteristics of the sludge thin layer drying process.The effects of hot air temperature,air flow rate and sludge thickness on sludge moisture ratio,moisture migration rate and temperature within the thin layer,and the mathematical model of sludge drying was proposed.Based on heat and mass transfer theory,Fick's second law of liquid diffusion theory and Arrhennius' s law,the wettability kinetics and the shrinkage mechanism of sludge drying process are discussed.Finally,the ANSYS FLUENT software was used to simulate the heat and mass transfer characteristics of the sludge drying process.(2)The research results of sludge moisture ratio and moisture migration rate show that the sludge wet migration process is roughly divided into the rising speed section,the constant speed section,the first speed reduction section and the second speed reduction section.The higher the temperature and wind speed of the dry air,the thinner the sludge thickness,the faster the sludge drying.The higher the temperature and the wind speed,the thinner the thickness,the greater the moisture migration rate.And as the water ratio decreases,the influence of temperature,wind speed and sludge thickness on the moisture migration rate becomes smaller and smaller.The sludge drying model was predicted by the data of sludge moisture content change with time.It was found that the Modified Page model can reflect the change of water ratio with time.Then calculate and analyze the effective moisture diffusion coefficient.When the sludge is 10 mm thick and the wind speed is 4 m/s,the drying temperature rises from 40 °C to 60 °C,and the effective moisture in the first and second stages of the sludge drying process.Diffusion coefficient increased by 33.4% and 54% respectively.Other drying conditions remain unchanged,Calculate the variation range of the effective moisture diffusion coefficient when changing the thickness of the sludge and the flow rate of the hot air,respectively.The results show that hot air temperature,wind speed and sludge layer thickness have significant influence on the effective moisture diffusion coefficient of sludge,and the influence of sludge thickness is the greatest,and the influence of hot wind speed is the least.(3)This section writes a program for grayscale and binarization of sludge image,and uses the MATLAB software to process the apparent image of sludge drying process to study the shrinkage phenomenon of sludge drying process.The results show that: The sludge shrinkage is large in the middle stage of the drying process,Drying shrinkage of the latter was not significant.Taking sludge 10 mm thick and drying temperature 40 °C as an example,the wind speed is changed to 4 m/s,5 m/s,6 m/s,and the volume ratios at the end of drying are 0.44,0.48,and 0.46,respectively.When the sludge is 10 mm thick and 5 m/s wind speed,the hot air temperatures were 40 °C,50 °C and 60 °C,respectively.The sludge volume ratios at the end of drying were 0.48,0.56 and 0.68,respectively.The same wind speed and thickness conditions.The higher the hot air temperature,the larger the volume ratio at the end of drying and the smaller the amount of sludge shrinkage;The same hot air temperature and sludge thickness conditions,the wind speed change has no effect on the shrinkage of the sludge.(4)Based on 3D ICEM CFD software,the physical model of the drying box and sludge thin layer was established.Self-written UDF(User Defined Function)program for sludge source items.the numerical simulation of sludge drying process was carried out by ANYSYS FLUENT simulation analysis software.Comparing the experimental results with the simulation results,it is found that the simulated value of the water change in the sludge is in good agreement with the experimental value before the sludge is cracked,the error is about 5%.As the drying process progresses,the sludge begins to shrink and crack.At this time,the simulated value and the experimental value begin to produce errors and increase;The simulated value of the average temperature distribution inside the thin layer of sludge is basically the same as the experimental value.It is concluded that the mathematical model and sludge physical property parameters selected in the calculation process of AnsysFluent software are suitable for the simulation calculation of the sludge drying process.Through simulation calculations,it is also found that the shrinkage crack phenomenon generated by the sludge drying process has a great influence on the sludge mass transfer phenomenon,but has little effect on the heat transfer of the sludge.