Heat And Mass Transfer Characteristics & Distributor Structure-parameter Optimization Of Rapeseed (Brassica Rapus) Fluidized Bed Drying

The rapeseed oil is the third largest edible vegetable oils after soybean oil and palm oil. However, it is easier to mold under the condition of high temperature and high humidity. Moreover, the rapeseed could cause rancidity if their drying time is too long. For timely and safe storage of the rapeseed, the hot-air fluidized bed drying must be employed. Therefore, it is very necessary that the fluidized-bed drying characteristics and laws of gas-solid heat and mass transfer were analysed through many experiments. And it was confirmed which drying dynamic model was suitable for the rapeseed fluidized-bed drying. Then the drying process conditions were optimized. Basing on these, the opening ratio and hole arrangement of distributor is reasonably designed to confirm the optimal distributor structure through theoretical analysis, numerical simulation and experimental research. Finally, the laws of gas-solid heat and mass transfer of the optimal distributor were revealed. These results can provide theoretical basis for the new-type distributor development and distributor’s performance improvement of rapeseed fluidized-bed drying equipment. The main research contents are as follows:(1) The influence of the initial drying basis moisture content, the hot-air temperature and the hot-air velocity on rapeseed fluidized bed drying was studied through many experiments. The results showed that the drying rate, the drying thermal efficiency, the heat and mass transfer coefficient & the effective moisturediffusion coefficients increased when the initial drying basis moisture content, the hot-air temperature and the hot-air velocity increased. The descending order of sensitivity of 3 influence factors which influencing on the drying rate & the average convection heat and mass transfer coefficient were the initial drying basis moisture content, the hot-air temperature and the hot-air velocity. At the same time, their descending order of influencing on the effective moisturediffusion coefficients were the hot-air temperature, the hot-air velocity and the initial drying basis moisture content.(2) The drying rate of the rapeseed fluidized-bed drying was optimized through the objective function and the nonlinear programming optimization calculation of MATLAB7.0 software. The optimization results showed that the best level combination of influence factors was 23.44% initial drying basis moisture content, 2.25m/s hot-air velocity and 65℃ hot-air temperature.(3) Among the four mathematical models used to fit drying process, through comparing with the determination coefficient R2,Chi square χ2, and RMSE, it was found that the Page model was the best one for all the data points (R2=0.9993). On the basis of these, the quadratic regression model equations of the model parameters n and k were achieved. Then the drying dynamic model of the rapeseed fluidized-bed drying was set up. The experimental results showed that this drying dynamic model can well predict the water losses rule of the rapeseed fluidized-bed drying because the predicted moisture ratio were in good agreement with experimental data (the maximum relative error is only 3.2%).(4)The standard regression models of the average convection heat and mass transfer coefficient & the effective moisturediffusion coefficients were set up through response surface analysis of the Design-Expert 8.0.6 software. The maximum relative errors of the predicted vaule and experimental data for the average convection heat and mass transfer coefficient & the effective moisturediffusion coefficients respectively were 4.4%,3.3% and 2.8% through direct error. At the same time, their no difference probabilities respectively were 0.9238,0.8633 and 0.9126 (>0.05) through paired double sample t-test analysis. Therefore, the predicted values of 3 standard regression models were in good agreement with experimental data.(5) The interaction influence of two factors on the average convection heat and mass transfer coefficient & the effective moisturediffusion coefficients were analyzed through response surface analysis. The analysis results are as follows:① the interaction influence of the initial drying basis moisture content and the hot-air velocity maked the most impact on the average convection heat and mass transfer coefficient, but which affected the effective moisturediffusion coefficients least. ② the interaction influence of the initial drying basis moisture content and the hot-air temperature were medium.③the interaction influence of the hot-air temperature and the hot-air velocity had the least influence on the average convection heat and mass transfer coefficient, but which was the most influential factor of the effective moisturediffusion coefficients. Moreover, the interaction influence of any two factors on the average convection heat transfer coefficient was less than that of average convection mass transfer coefficient.(6) The effective moisturediffusion coefficients and the average activation energy were calculated on basis of Fick’ ssecond law. The results showed that the magnitudes order of the effective moisturediffusion coefficients are 10-10～10-9 m2/s which is in the range of the normal levels of dehydrated foods (10-12～108m2/s). The effective moisturediffusion coefficients of 14.41%～29.72% initial drying basis moisture content varied in the range of 6.485×10-10～10.133×10-10 m2/s. The effective moisturediffusion coefficients of 1.75-2.25m/s hot-air velocity varied in the range of 7.296×10-10～9.525×10-10 m2/s. The effective moisturediffusion coefficients of 45～65℃ hot-air temperature varied in the range of 5.269×10-10～8.917×10-10m2/s. At the same time, the average activation energy of the rapeseed fluidized-bed drying was 22.84kJ/mol.(7) The five distributors (including 12.27% opening ratio triangular-even hole arrangement,14.10% opening ratio triangular-even hole arrangement,15.84% opening ratio triangular-even hole arrangement,15.84% opening ratio triangular-uneven hole arrangement and 15.84% opening ratio, round-uneven hole arrangement) were designed based on the open porosity ratio and the arrangement holes mode. Then their drying characteristics (such as moisture ratio and drying rate) were analyzed through numerical simulation and experimental research. The results showed that the distribution of 15.84% opening ratio and round-uneven holes arrangement was an ideal distribution for the rapeseed fluidized bed drying. This distribution might be attributed to enhancement of liquidity of hot air, reduction of the hot air accumulated in localized areas, and making gas-particle two phase flows to normally fluidize. At the same time, the moisture ratio and specific energy consumption for the distribution of 15.84% opening ratio and round-uneven holes arrangement was minimum, drying rate and thermal efficiency was maximum among five types of distributors. Thus its drying time was shortest.(8) The laws of gas-solid heat and mass transfer of the ideal distribution (the distribution of 15.84% opening ratio and round-uneven holes arrangement) were analyzed through numerical simulation software of Ansys Fluent 15.0 based on Euler two-fluid model and standard k-ε turbulence model. The results showed that the hot-air temperature firstly decreased by degrees, and then quickly decreased to stabilize in the upper part of fluidized-bed dryer. The hot-air temperature, the hot-air velocity and the rapeseed temperature all significantly increased and reached the maximum at the nearly area of distribution, and then declined rapidly to stabilize. Then the gauge pressure sharply decreased to zero with rising of the fluidized bed height. In addition, the main driving forces of gas-solid heat transfer were gas-soild velocity difference, gas-soild temperature difference and rapeseed volume fractions, among which gas-soild temperature difference was main controlling factors at the nearly area of distribution, and gas-soild velocity difference and rapeseed volume fractions were main controlling factors in other areas of the fluidized bed. At the same time, the main driving forces of gas-solid mass transfer were gas-soild velocity difference, pressure difference and rapeseed volume fractions, among which gas-soild velocity difference was main controlling factors at the nearly area of distribution, and pressure difference and rapeseed volume fractions were main controlling factors in other areas of the fluidized bed.(9) The variation of the heat and mass transfer coefficient of the ideal distribution was analyzed through numerical simulation. The results showed that the gas-soild heat and mass transfer coefficient increased, and then gradually decreased, after again slightly rising with rising of the fluidized bed height. In addition, the gas-soild heat and mass transfer coefficients were maximum and remained relatively constant in the center of the distribution, and decreased gradually on both sides of fluidized bed wall.