Experiment on influence of distributor structure on drying characteristics of rapeseed (Brassica rapus) fluidized bed

The rapeseed is the second largest oil crop next to soybean. And 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 because the rapeseeds have more protein and very small porosity for aeration when stock pile in storage. Moreover, the rapeseed could cause rancidity if their drying time is too long. However, the traditional solar drying method of rapeseed in China can hardly meet the requirements of commercial rapeseed oil production. For the timely safe storage, artificial drying methods must be employed, among which the hot-air fluidized-bed drying is commonly applied. It is known that the distributor is one of the key parts of the fluidized-bed dryer. The distributor can uniformly distribute air flow and fluidize rapeseed. And its specific structure and size significantly influenced on properties of heat and mass transfer of the fluidized-bed dryer. The resistance must be provided to uniformly distribute air flow when the hot air flowed through the distributor. However, the resistance can't be too large in order to reduce energy consumption. Therefore, the resistance is one of the most important factors for distributor designing. Then the resistance of air flow comes mainly from the opening ratio and hole arrangement of the distributor which effect on airflow structure and solid materials particles fluidized state. Thus it is very necessary that the opening ratio and hole arrangement of distributor is reasonably designed to improve the quality and efficiency of rapeseed drying. Then drying characteristics analysis is the basis of design and development of large drying equipment. In this paper, we analyzed drying characteristics of 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) for their moisture ratio and drying rate. The results showed that the moisture ratio of rapeseed for five types of distributors all decreased exponentially with drying time. And 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 of fluidized drying was maximum among five types of distributors. Thus the distribution of 15.84% opening ratio and round-uneven holes arrangement was ideal for distribution. Such 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. 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. The effective moisture diffusion coefficients for five types of distributors varied in the range of 1.0133×10^-9~1.0943×10^-9 m2/s on basis of Fick's second law, and its maximum relative error was only 7.4%. Its average valve was 1.0538×10^-9 m2/s. The results can provide the theoretic basis for distributor design, new-type distributor development and distributor's performance improvement of rapeseed fluidized-bed drying equipment.