The effects of air velocity, temperature and particle size on low-temperature bed drying of wood chips

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This paper describes a mathematical model for wood chip packed bed drying process with the effects of hot air flow velocity, temperature and particle size. A single-particle drying model was developed by considering impacts of external and internal parameters. External parameters are hot air flow velocity and air temperature. Internal parameters are porosity and particle size. These parameters are incorporated to the present model by introducing two mass transfer coefficients. The model was fine-tuned by comparing simulation results and experimental data. Effects of a factor relating to internal mass transfer coefficient were found for three wood types, and a functional dependence of internal mass transfer on temperature was suggested in this study. The model was implemented in computational fluid dynamics (CFD) to evaluate spatial variation of moisture in the packed bed drying process. The CFD model was validated by results of lab-scale packed bed. Drying performance of the packed bed was estimated by CFD simulations for variations of external hot air flow velocity, flow temperature and particle size. Sensitivity of these parameters for dying performance was evaluated by design of experiment (DOE) method. It was clarified that air temperature is most critical for the drying process. Interaction of between external hot air flow velocity and particle size for dying performance is also significant.