Heat and mass transfer characteristics and model of rapeseed fluidized-bed drying with constant drying rate

Abstract: The rapeseed oil is one of the main edible vegetable oils in China and the third largest edible vegetable oil all over the world. It is easier to mold under the high temperature and humidity because the rapeseed has more protein and very small porosity. Therefore, the fresh rapeseed is hard to be safely stored. At present, the traditional solar drying method of rapeseed can hardly meet the requirements of rapeseed drying in China. For the timely safe storage, artificial drying methods must be employed, among which the hot-air fluidized-bed drying is commonly applied. The principle of the rapeseed fluidized-bed drying is extremely sophisticated because it is often associated with impaired heat transfer, mass transfer, and momentum transfer. The convective heat and mass transfer coefficient are key influence factors to the drying efficiency and the drying quality of rapeseed fluidized-bed drying. In brief, the changing regularity of convective heat and mass transfer coefficient, and the measures on improving convective heat and mass transfer have become the vital problems concerning the timely safe storage of fresh rapeseed. In order to investigate the changing regularity of convective heat and mass transfer of the rapeseed fluidized-bed drying, the influence of the initial moisture content, the hot air temperature, and the hot air velocity on the convective heat and mass transfer coefficient was analyzed by using experiment equipment of rapeseed fluidized-bed drying. The results showed that the convective heat and mass transfer coefficient increased with the increase of the initial moisture content, the hot air temperature, and the hot air velocity. And the primary and secondary order of the factor influencing the convective heat and mass transfer coefficient was: the initial moisture content, the hot air temperature, and the hot air velocity. The convective heat transfer coefficient and the convective mass transfer coefficient of the rapeseed with 29.72% initial moisture content were both 1.9 times that with 14.41% initial moisture content. The convective heat transfer coefficient and the convective mass transfer coefficient of the rapeseed with 2.25 m/s hot air velocity were both 1.2 times that with 1.75 m/s hot air velocity. At the same time, the convective heat transfer coefficient and the convective mass transfer coefficient of the rapeseed with 65 ℃ hot air temperature were 1.2 and 1.4 times that with 45 ℃ hot air temperature, respectively. On the basis of these, a second orthogonal rotation combination test was executed through the Design-Expert 8.0.6 software. The initial moisture content, the hot air temperature, and the hot air velocity were selected as influencing factors. The convective heat and mass transfer coefficient were response indices. Then the regression models of the convective heat and mass transfer coefficient were established. For the 3 groups of experiment results, the maximum relative errors of the convective heat and mass transfer coefficient between the model prediction values and the corresponding experimental values were 4.83% and 4.79%, respectively. It was proved that the regression models had higher precision. The results may provide the theoretic basis for improving convective heat and mass transfer, which effectively improves the drying efficiency and the drying quality of rapeseed fluidized-bed drying.