Design Of Stirring Dryer For Sewage Sludge With Superheated Steam And Two Stage Combined Drying System

With the development of China’s economy and society and the acceleration of urbanization,the amount of sewage disposal and sludge treatment increases gradually year by year.As the by-product after sewage disposal,sludge enriches a large number of heavy metals,pathogens,organic pollutants and other toxic substances.As a result,if not effectively treated,the sludge will brings hazards towards the econogical environment,animals and human beings.Sludge has the characteristics of large volume,wide source and high water content.Drying the sludge is an effective way to realize its goals of harmlessness,stabilization and resourse utilization.After the drying process,sludge volume is significantly reduced and the trait get improved,which has multiple ways for resource utlization.The stickness of sludge during the drying process is an effect of its adhesion and cohesion,which is one of the main factors affecting the drying effect.The stickness is manifested as the cohesion of the sludge itself and the adhesion of the sludge to the drying equipment wall.The stickness of sludge during the drying process is the result of a variety of factors,including organic matter,moisture content,drying temperature and heating methods.There are many technical solutions to solve the stickness of sludge drying: pretreatment of conditioner can be taken to destroy the inner structure of sludge,which as a result changes the distribution of sludge moisture,Besides,some drying processes and equipments can be designed to confront the problem,which include applying a polarized electric field with cathodic heat wall,electroosmotic dewatering,back mixing process,self-cleaning shaft and so on.In order to achieve the purposes of high efficiency and energy saving for sludge drying,a stirring dryer is designed,which conbines the advantage of mixing with the characteristics of superheated steam.Firstly,the basic paramaters and design requirements are introduced,and the overall structure design is made.Secondly,the mixing shaft assembly is designed,which include the analysis of the angle of the blade,the design and verification of the mixing shaft,the comparative analysis and selection of the double stirring shaft assembly and the single stirring shaft assembly.Thirdly,the structure and function of the shell and coupling device of the mixer are analyzed and designed,including the cylinder,the cover,the support,the end plate,the inlet and outlet and the shaft end sealing device.Finally,the bearing configuration is analysed and selected,and the transmission type is selected and designed.Considering the sludge drying rate,energy efficiency,the adhesive properties of the sludge and treatment of gas emission and so on,A two sludge combined drying system is designed,which mainly contain the steam generator,temperature control system,sludge feed device,mixing dryer,squeeze crushing machine,counter-current devices,heat exchangers,belt dryer and tail gas scrubber.The process and the various parts of two stage combined drying system are described.In the primary drying system,the sludge is stirred and dried to a lower water content and then fed into a crush forming machine to make pellets,and some of the steam is transported to the heat exchanger.In the secondary drying system,the sludge pellets contact and adsorp the dry sludge powder in the countercurrent device and form sludge particles which are wet inside while dry outside,and in this way avoiding adhesion and cohesion.The hot air formed in the heat exchanger cools the sludge particles in the belt dryer and the sludge moisture is reduced.The exhaust gas generated in the heat exchanger and the secondary drying system is discharged into the tail gas scrubber.