Mechanical Characteristics And Drying Kinetics During Superheated Steam Drying Of Rice Kernel

For the problem of long drying time and high energy consumption for early indica rice with high moisture, studies on the mechanical characteristics of biological material and drying dynamic of rice kernel during superheated steam (SHS) drying could provide a basis for study the application of SHS drying technique to high moisture paddy dehydration. The main research contents and conclusions of this paper are as follows:1. A series thin layer drying experiments on SHS drying paddy with high moisture were conducted on a self built SHS dryer. And the drying characteristics was analyzed and the experimental coefiBcient of moisture transfer in paddy Deff during SHS drying was obtained from experiments, with the magnitude10-9m2s.2. The force-deformation curves and elastic modulus of paddy subjected to SHS drying were gotten through mechanical experiments. Results showed that, when moisture content is beyond0.235kg/kg wet basis (w.b.), the force rice kernel subjected linear increases with the increase of the deformation, and rice can withstand large deformation; when the moisture content ranges from0.16to0.235kg/kg w.b., the force-deformation curve shows the characteristics as that of elastic-plastic material; when the moisture content is lower than0.16kg/kg w.b., rice kernel becomes brittle and just withstands small deformation; the elastic modulus of rice is below20MPa when the moisture content is higher than0.25kg/kg dry basis (d.b.), however, it dramatically increases and beyond100MPa while the moisture content is lower than0.25kg/kg d.b. There is no significant difference among elastic modulus of rice kernels dried with SHS with temperature range from135to180℃. At last, the relation between elastic modulus and moisture content of rice kernel was determined.3. Paddy was viewed as an elastic ellipsoid with two layers:hull and brown rice. Base on the principle of heat and mass transfer and the theory of elasticity, three-dimensional mathematical models of heat and mass transfer and stress were built.. Soft ware COMSOL Multiphysics was employed to solve the mathematical equations.4. Take the simulation results from drying condition at150℃at1atm for example. Simulation results were generally consistent with experiment results. Simulated results showed that, temperature in rice kernel becomes uniform and reaches to steam temperature in about two minutes, so thermal stress can be neglected; moisture in rice kernel is not uniform that induces moisture content gradient (MCG); the max of MCG appears in the short axis in direction of the thickness of rice kernel, followed by that in the width, and the minimum appears in the long axis in direction of the length; MCGs in rice kernel decreases from surface to center, and they firstly increase and then decrease during SHS drying; outer layer of rice kernel is subjected tensile stress while the inner is subjected compressive stress, and the tensile stress (the first principle stress) decreases with the decrease of moisture content while the compressive stress firstly increases and then reduces as the moisture content decreases during SHS drying; magnitude of stress is106Pa, and maximum compressive stress is higher than maximal tensile stress in rice kernel during SHS drying; there are two stress concentration zones in rice kernel:the tensile stress concentration zone in surface in direction of thickness and the compressive stress concentration zone in center.5. Compared to the additional breakage ratio33%of paddy dried with continuous hot air drying(temperature50℃, relative humidity22%, velocity0.5m/s), the additional breakage ratio of paddy subjected to SHS drying with the temperature of140～180℃at latm is reduced sharply, less than12%. Base on this study, SHS drying paddy with high moisture, which is as feed grain, has an application value.