The mechanism of a temperature decrement of porous materials immersed in a fluidized bed in drying

The temperature decrement in the inner parts of porous materials immersed in a gas-solid fluidized bed was observed in drying. To clarify the mechanism of this phenomenon, a theoretical calculation includes heat and mass transfers in a porous material (sample) was performed, and the heat balance in the sample was estimated. The effects of temperature of drying gas and the thermal conductivity of the sample on the temperature decrement were also examined. In the case of fluidized bed drying, the amount of heat now in a sample is very large at the earlier period of drying (surface evaporation period). After the sample surface dried, the amount of heat flow in the inner parts of the sample is governed by the thermal conductivity of the sample. During the period, the amount of heat consumption for evaporation increases and it becomes larger than that of heat conduction in the inner parts of the sample in drying. This inversion is the reason of the temperature decrement in the inner parts of the sample in fluidized bed drying. The decrement degree of temperature becomes larger as the temperature of drying gas increases. The temperature decrement occurs more easily as the thermal conductivity becomes lower.