Study On Drying Process In The Spray-Smtd Combined Dryer

Combined drying is a energy-saving technology which based on the characteristics of the materials to be dried, will combine two or more drying methods together to complete the drying process,Spray-Cyclone combined drying is one of styles. Spray drying may be remove a large number of moisture of materials(usually water) in a few seconds to tens of seconds to yield solid products. Cyclone drying with long resident time could be dehydrate to the lower level. Spray-Cyclone combined drying organic combination of the two drying methods, reducing energy consumption and saving equipment investment based on accomplishing the drying tasks.Experiments in two parts:1,in a spray dryer,diameter of 340mm, tower height of 1.5m, with external mixing air nozzles, experimental on the lotus powder's suspension for a single factor experiment, study on the solid content of initial solution, inlet gas temperature, spray rate, gas velocity effect on the product's moisture content,and more significant factors on moisture content of the product were selected to design a orthogonal regression experiments and establishing a mathematical model of solid content of initial solution, inlet gas temperature, spray rate and lotus powder's final moisture content. Analyzing the interaction between these factors and determining optimization of operating parameters under the condition of minimum moisture content.2,In a cyclone dryer, cylinder diameter of 400mm, center hole diameter of 180mm, tower height of 0.95m, experimental on lotus root powder for drying kinetics, studying on inlet gas temperature, feed rate, gas velocity effect on drying rate and establishing drying kinetics model of lotus root powder in the cyclone dryer. The results as following: (1) On Spray Drying, Lotus root powder's final moisture content decrease with inlet gas temperature increase,gas velocity increase,spray rate decrease and the moisture content of initial solution decrease.(2) On Spray Drying, the regression equation for drying lotus root powder's moisture content and inlet gas temperature, spray rate, the moisture content of initial solution is:(3) On Cyclone Drying, As inlet gas temperature increase, gas velocity increase and feed rate decrease, drying rate becomes faster. At temperature lower than 80℃, the temperature's effect on the drying rate is remarkable; in the higher gas velocity range, constant drying rate is fast. The critical moisture content of material is 0.09-0.11 kg/kgdb.(4) The expression of cyclone drying kinetics model is:(5) At 60℃~ 80℃, effective diffusion coefficient of lotus root powder distribute in m2/s~ m2/s range. The Arrhenius relationship between effective diffusion coefficient and the gas temperature is:Base on the drying kinetics model, establishing an optimization model for annual cost as objective function , particle middle moisture content as decision variables. Using the minimum annual cost as the target , optimizing the middle moisture content in order to determine the optimal middle moisture content under the different operating parameters, the results show that: (1) When the required moisture content of material is low, the optimal material middle moisture content is the critical moisture content ,may be 0.1 kg/kgdb; when the required moisture content of materials is high, the selection criterion of the optimal middle moisture content is for the spray drying as less as possible to complete drying tasks, when the droplets become solid particles, the cyclone dryer can complete the remain drying task. (2)at the same drying task, comparing spray-cyclone combined drying with the single-stage spray drying, equipment investment decreased 21.7%, 25.8% reduction in energy cost, annual cost reduction of 24.5%.