A New Approach to the Study of Drying Diffusion Coefficients of Wood

The effect of temperature, density, and bound-water diffusion coefficients on the combined diffusion coefficient are considered. The activation energies are calculated and compared, and a theory that the combined diffusion is almost entirely controlled by its bound-water diffusion is proposed. The diffusion coefficients of free water vapor, bound water, and the combined effect of both will increase exponentially with an increase in temperature. Activation energy for the diffusion of moisture through wood is the energy required for the water molecule to jump from one site to another, and the activation energy for diffusion would be an average of all possible jumping energies. Since moisture diffuses through wood as a combined-bound-water and water-vapor diffusion, the activation energy should be somewhere between the values, for free water-vapor diffusion and bound-water diffusion. Calculating the values indicates that the diffusion coefficient of moisture is almost entirely affected by bound-water diffusion. The slow bound-water movement appears to control the overall rate of the combined diffusion. Finding the constant factor between the combined diffusion coefficient, Dc and its corresponding bound-water-diffusion coefficient, Dbm, further confirms that bound water controls the overall coefficient. The diffusion coefficients calculated by using the equation derived were compared with measured values and showed good agreement. A straight-line relationship was found between the logarithms of the diffusion coefficients and the corresponding wood specific gravity values.

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