| Rape(Brassica campestris L.) is planted for the oil production, protein, forage, nectar, and energy, with characteristics of high economic value, great potential for development, and high values of comprehensive process and utilization. Rapeseed is the third-leading source of vegetable oil in the world and the world’s second-leading source of protein. China is the major country of rapeseed production and consumption, and the Yangtze River region is the main region for rapeseed production, and rape(Brassica napus L.) is the principal species with planting area and production accounting for about70%of China.The newly harvested rapeseed contains high moisture content. The harvest period often runs into rainy season of high temperature, and it is difficult to control the harvest and storage conditions. Then, the rapeseed is often deteriorated because of overheat, acidification and mildew, which affects physiological characteristics of the rapeseed for seeding purpose, and the process features and oil quality of the rapeseed for oil purpose. It is necessary to dry the rapeseed to keep seeding seed safe, to ensure the oil quality, and to meet the storage requirements of the rapeseed. Hot-air drying is currently the most widely employed method to dry rapeseed in China. Rapeseed is an unsaturated porous medium with sorptivity, and its kernel owns complexed porous media structure with biomass features. The heat&mass transfer process of rapeseed hot-air drying consists of multi-phase coupling and wet-phase transition, and it is sensitive to hot-air temperature.By taking hot-air drying of seed of rape(Brassica napus L.) as the object of study, focused on the moisture change during drying process, the heat&mass transfer characteristics and its model of rapedeed hot-air drying were studied in this paper. The heat&mass transfer characteristics of thin layer hot-air drying (including sorption) were experimentally studied. The effects of length to diameter ratio and tortuosity of the micro pores on the heat&mass transfer characteristics of the pore scale of the rapeseed kernel were obtained by means of mathematic method. The study conducted can contribute an overall description of the heat&mass transfer of rapeseed hot-air drying, which can provides basis for optimization of drying technique, drying parameters and process control of rapeseed hot-air drying. The main conclusions from the study are as follows: Firstly, by means of thin layer hot-air drying, the effects of initial moisture content, hot-air temperature, and air velocity on the hot-air drying characteristics and the drying process of rapeseed were experimentally studied. By applying heat&mass transfer method of porous media, the reasons of findings and rules observed during the rapeseed drying were analyzed. No obvious constant rate drying stage was observed during the thin layer rapeseed hot-air drying. The drying is mainly carried out during falling rate drying stage. The curves of drying characteristics of rapeseed under different drying conditions have largely a same trend, and follow rules of an expotional change. Fitting was applied to the experiment data by means of10mathematic models, and fitting analysis results showed that Page Model is the best model to describe the rapeseed hot-air drying. The predicted drying characteristics curves fit well to the curves from experiment, and Page Model has strong applicability to rapeseed hot-air drying. The air temperature is the main factor that affects the rapeseed hot-air drying. With the increase of air temperature, the effective moisture diffusitivity increase, and while air temperature increases from45℃to65℃, the effective moisture diffusitivity increases from3.835×10-10m2/s to7.666×10-10m2/s. The activation energy of rapeseed is29.26kJ/mol. The moisture removed during hot-air drying is mainly distributed in the rapeseed by physical adsorption.Secondly, by means of sorption experiment, the sorption characteristics of rapeseed under conditions of different air temperature and relative humidity were obtained, and the effects of the temperature and relative humidity on the sorption characteristics were obtained, and the reasons of findings and rules observed during the sorption were analyzed as well. Temperature and relative humidity are the main factors that affect the moisture sorption of rapeseed. With the increase of the temperature, the time needed for sorption equilibrium decreases, and with the increase of relative humidity, the equilibrium moisture content and time needed for sorption equilibrium increase. Peleg Model is the best model to describe the moisture sorption of rapeseed. By means of Peleg Model, the effects of air temperature on the critical relative humidity were calculated and analyzed. With the increase of air temperature, the critical relative humidity of rapeseed increases, and while the temperature exceeds20℃, the change of critical relative humidity tends to be slow. With the increase of equilibrium moisture content, the net isosteric heat of sorption of rapeseed exponentially decreases. Therefore, as to the storage of rapeseed with low moisture content, while the ambitus temperature is quite high, special cautions should be payed to the heat emission that accompanies by moisture sorption which easily leads to overheat and acidification of the rapeseed in a short period.Then, based on the thought of2-D regular Network Model, the pore model of different length to diameter ratio and tortuosity of the micro pores of rapeseed kernel was built. Mixture Model in the ANSYS Fluent was adopted to mathematically study the heat&mass transfer process in the micro pores of rapeseed kernel, and the mass transfer between liquid water and vapor was described by Evaporation-Condensation Model. The heat&mass transfer in the micro pores of the rapeseed hot-air drying, and the effects of length to diameter ratio and tortuosity of the micro pores of rapeseed kernel on the drying process were obtained. The moisture discharge from the micro pores is mainly progressed during the initial stage of drying process, expressed by the change of density of mixture level in the pores. With the progress of drying, the moisture discharge rate exponentially decreases, which is consistent with the simplified drying model of rapeseed kernel and the trend of the thin layer rapeseed hot-air drying. Under condition of constant diameter of micro pores, with the increase of linar length, namely length to diameter ratio, or with the decrease of tortuosity of micro pores, the remaining moisture in the pores increases, and with the process of drying, the remaining moisture in the pores tends to be close in the later drying stage.Finally, data of activation energy during the rapeseed hot-air drying and data of effective moisture diffusitivity at different temperature were obtained by thin layer hot-air drying experiment, and data of net isosteric heat of sorption at different air temperature and equilibrium moisture content levels were obtained by rapeseed sorption experiment. These data are important to evaluate technique, efficiency and energy consumption of hot-air drying, and the data can provide reference for their evaluation. |
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