The Study On Vacuum Drying Characteristics Of Maize

Maize is mainly produced by northeast region in China. Every winter it has a great deal of maize need to be drying. The vacuum drying of maize has a lot of benefit, including good quality, lower drying temperature, economic, saving energy, and environment friendly etc. The continuous vacuum drying can raise the drying efficiency and production. Now, the vacuum drying technique of maize has been obtained the better result, but it still has the weakness and shortage.In this paper, the research is based on National Key Technology R&D Program (2006BAD08B06) supported by State Administration of Grain, P.R.China. Aimed at the maize vacuum drying process, the static and dynamic drying characteristics were studied with theory and experiments. The main research contents are as follows:The static vacuum drying experiments were carried out on box vacuum dryer. The effect of temperature, vacuum degree, initial moisture and bed thickness on vacuum drying rate, crack percent and germination percent were studied. Through the orthogonal experiment of temperature, vacuum degree and bed height, the optimal drying process was gotten. The appearance and shape of vacuum drying, natural drying and not-drying was compared with each other. The maize has the obvious for bulge in soft endosperm.The maize was considered as porous medium. The moisture transfer affected by pressure was adopted, and the heat and mass transfer inner maize was studied by heat and mass transfer theory for porous medium. The model was solved by FEM, and the distribution of temperature, moisture and pressure was gotten. The temperature stress, moisture stress and pressure gradient were studied by viscoelastic materials mechanics.The dynamic experiments were carried out by pilot scale continuous vacuum dryer. The maize was gotten from northeastern region of China. The drying characteristics of hot-water and hot-vapor was compared with each other through experiment results and temperature curve. When using hot-water supplying heat, the supplying heat was uniformity, the maize temperature of end-drying was about 38℃. The moisture was gotten 14% in 3 hours. The maize color was natural and the quality was better. Its bulk density was about 732 kg/m. The dryer performance was gotten and the improved method for dryer was given.The flowing characteristics of granular in tower vacuum dryer were studied by DEM method. Through the simulation, the relation between the biggest diameter of material and the distance of diamond pipes was gotten. The retention time of granular was studied and a mixture characteristic across the dryer was gotten. The guide plate was designed in order to improve the mixture characteristics.Because the gas flowing inner vacuum dryer was the important condition of mass transfer, the gas flowing field was studied by numerical simulation. CFD method was applied, and the model was adopted. The material zone was considered as porous zone. The Darcy’s law, Ergun equation and interface jumping condition was used. The mathematical model of 3D for gas flowing was built and it was solved by Fluent software. By analyzing the gas flowing direction, pressure and velocity, the pumping pipe position, the pumping pipe numbers and the vacuum degree were determined.