Transport Properties in Drying of Solids

A. Sander, J. Prlić Kardum and D. Skansi

Abstract
The transport properties in convection drying of industrially prepared roof tile clay slab were investigated on a laboratory scale in a temperature interval from 30 °C to 70 °C. The velocity and relative humidity of the drying air was kept constant. Obtained drying data were approximated with different mathematical models. Applied models analysis enables evaluation of the main transport properties:ef fective diffusion coefficient, mass and heat transfer coefficients, thermal conductivity, drying constant, and exponential model parameters. Based on Biot number value, the main transport mechanism was defined. Very small Biot numbers, less than 0.1, for both heat and mass transfer, indicates that the heat and mass transfer are externally controlled. Evaluated parameters were connected with process condition and the drying constant. The air temperature strongly influences the drying kinetics, transport properties and exponential model parameters. Increase of temperature cause exponential increase of effective diffusion coefficient, thermal diffusivity and heat transfer coefficient, while values of parameter k and drying constant increases linearly. Exponential model parameter, n, is independent of temperature. It was found that the drying constant is essentially a combination of transport properties, transfer coefficients, geometry of the sample and experimental conditions.

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Keywords
Convection drying, drying constant, heat transfer, mass transfer, transport properties