UNSTEADY-STATE DIFFUSION OF METHANOL IN DOUGLAS-FIR HEARTWOOD AT HIGH-TEMPERATURES

The unsteady-state diffusion of methanol in the tangential direction o f water saturated Douglas fir heartwood was studied in an experimental analysis cell at temperature of 90, 120 and 150 degrees C. Concentrat ion profiles of methanol in wood were obtained fur times between 30 an d 480 minutes and sample thicknesses of 10 and 15mm. From the measured concentration profiles, diffusion coefficients were calculated using a special form of Fick's second law which rakes into account changes i n the methanol concentration at the boundary layer. The measured diffu sion coefficients of methanol in Douglas fir lie between 1.11510(-9) and 1.75510(-9)m(-2)s(-1), increasing with temperature. Data of the s ame order of magnitude are found as diffusion coefficients for alkalin e solutions in Kraft pulping. The resistance of the wood to the diffus ion of methanol, expressed as the relative diffusion coefficient, is i nversely proportional to the temperature. This might be attributed to increased permeability of the cell wall by softening of the Lignin or opening of aspirated pits. A deviation of Ficks second law was found a t low methanol concentrations. The activation energy of the diffusion process was calculated to be 9.8 kJ mole(-1).