Research On Characteristics Of Heating Surface Coupled With Rapid Heating For Viscosity Reduction In Indirect Sludge Drying

The moisture content of the sludge after being crushed by the plate frame is still as high as about 80%,which makes transportation and subsequent treatment difficult.At present,the main solution is still to stabilize the sludge by thermal drying.The heat drying method is divided into direct and indirect drying,but the high energy consumption,large dust and difficult disposal of tail gas result in high direct drying cost and low environmental benefit.Indirect drying avoids the above disadvantages and becomes a more reasonable method for sludge thermal drying.Indirect sludge drying sticky wall phenomenon not only reduces drying efficiency,but also causes blockage of drying equipment,increases maintenance cost and reduces the service life of drying equipment.The possible factors of sludge sticking wall were carefully analyzed by adsorption theory,and improved methods were proposed:low surface energy treatment of indirect drying heating surface and self-desorption using rapid heating sludge shrinkage characteristics.Based on the adhesion theory,several sets of representative comparative tests were used to verify the adhesion of ordinary304 stainless steel and low surface energy Teflon(Poly tetra fluoroethylene)heating surface at different heating temperatures of 100℃,120℃and 140℃.The effects of heating temperature on sludge wall adhesion were investigated under different indirect heating temperatures and surface characteristics of heating surfaces.According to the experimental results:(1)According to all the test results,the drying curve,drying rate curve and sticky wall volume curve of sludge on 304 stainless steel heating plate showed similar trends at 100,120 and 140℃.The change rule of sludge wall strength is not only related to temperature,but also related to the time when the sludge contacts the heating surface.(2)When sludge is indirectly heated and dried by PTFE heating plate at 140℃,the drying curve and drying rate are similar to those of 304 stainless steel.The PTFE sticky wall content is always about zero,and it can slide freely below 60%moisture content.The viscosity reduction characteristics of PTFE improve the heat transfer efficiency of indirect thermal drying of sludge.(3)Sludge water content is about 60%.Sludge wall strength of 304 stainless steel is 140 g/m~2,215 g/m~2 and 160 g/m~2 at different temperatures,while that of PTFE heating plate is 0 g/m~2.(4)The sludge shrinkage is isotropic.At the end of hot air drying at 160℃,the sludge volume shrinkage ratio is about 0.7 and the volume shrinkage coefficient is0.3.According to adsorption theory and experimental results,the overall structural parameters of the sludge viscosity reduction indirect dryer were preliminarily designed,and the annular groove heating roller of the core part of the dryer was designed and calculated according to the container design instruction manual.