Diffusion Model for the Chemical Impregnation of Hardwoods and Its Significance for Rapid Steam Treatments

Through impregnation it is possible to bring the chemically reactive agents uniformly throughout the lignocellulosic matrix at a relatively low temperature. These preimpregnated chips can then be treated at higher temperatures (>200 degrees C) for a short period (2 - 4 minutes) via a rapid treatment process and a homogeneous product can be obtained. The impregnation of hardwood chips can bemodeled by a diffusional mechanism. The diffusion coefficient is a function of temperature and pressure. It increases with pressure at a power of 0.38 in the radial direction and 0.27 in the axial direction. It increases exponentially with temperature following and Arrhenius type correlation: the activation energies for radial and axial thermal diffusion have been found to be 2.7 and 1.2kJ/gmol, with diffusivity constants of 5.8x10-7 and 2.0x10-4 cm2/sec/K(1/2) respectively.