Experimental Study On Kinetics Of Sewage Sludge Thin-layer During Ultrasonic-assisted Hot Air Convective Drying

Scientific and rational disposal of municipal sewage sludge is beneficial for environment protection and has important significance in the field of energy development.The enormous energy consumption is still required for sludge drying,which hampers the wider use of sludge drying.Ultrasound can reduce energy consumption for sludge drying processes to overcome the disadvantage of traditional hot air convection drying.So ultrasound-assisted convection drying is meaningful in resource utilization of sewage sludge.In this paper,the effects of hot air temperature,ultrasonic power,ultrasonic frequency,sludge additive and back mixing dry sewage sludge on the drying characteristics of the sludge were studied based on an ultrasound assisted convection drying experimental setup.Based on experimental research and moisture migration dynamics theory,the influences of hot air temperature(70-130?),ultrasound powers(30,90,150W)and ultrasound frequencies(21,25,28kHz)on the drying kinetics of sewage sludge was investigated under a certain wind speed.The results showed that the influences of the temperature and ultrasonic power on the drying rate were significant(P<0.05).The drying effect was better when ultrasonic frequency was 21kHz.The average drying rates within whole drying temperature range at ultrasound powers of 30,90 and 150W increased by about 22.6%,27.8%and 32.2%compared with the convective drying alone(without ultrasound).The values of the apparent activation energy in the first falling rate period were down from 14.21kJ/mol to 13.52kJ/mol,12.80kJ/mol and 12.78kJ/mol,and from 18.84kJ/mol to 17.21kJ/mol,16.68kJ/mol and 15.10kJ/mol for the second falling rate period.By visual experiments,ultrasound could aggravate the morphology(crack and contraction)of the sludge was observed.Based on the experimental study,the effects of sludge additive(lime,sodium carbonate,sodium bicarbonate)and back mixing dry sewage sludge on the drying characteristics of the sludge were obtained.The influences of sludge additive(lime,sodium carbonate,sodium bicarbonate)and back mixing dry sewage sludge on the drying rate were significant(P<0.05).Under the same proportion(5%)of lime,sodium carbonate and sodium bicarbonate,the average drying time within whole drying temperature was shorten by 10min,42min and 47min.The values of the apparent activation energy in the first falling rate period decreased by 0.73kJ/mol,0.84kJ/mol and 2.05kJ/mol,and by 3.97kJ/mol,3.46kJ/mol and 4.97kJ/mol for the second falling rate period.The ultrasonic could be available for further prompting the dry characterics of the sludge from lime additive.Dry sludge backmixing also was an effective way to the drying process.The average drying rate increased by 10.92%and 18.98%,respectively,with 5%and 10%dry sludge backmixing.The Midilli models could well describe the ultrasonic-assisted air convection drying sludge samples under the condition of the relationship between the moisture ratio over time and could give good agreement between experimental and predicted moisture ratios of sewage sludge/additive or sewage sludge/back mixing dry sewage sludge.