Surface colour change in wood during drying above and below fibre saturation point

A technique, useful for studying the formation of kiln brown stain in wood drying, has been developed to measure the surface colour change in a single board wood sample during drying.  The wood sample is planed carefully in the green state to remove any surface wood that was damaged during cutting.  The intact tracheids at the surface cause the evaporative front to remain at the surface during drying and therefore colour formation also occurs right at the surface. In this way, the colour can be measured using a spectrophotometer at various stages during drying without having to slice the sample.Experiments were carried out to measure the change in colour of wood from green to EMC corresponding to the drying schedule used.  At the end of each schedule the boards were held at the EMC to determine how the colour changed below fibre saturation point.  The results show that the colour of the wood continues to change below the fibre saturation point and the nature of the colour change indicates an increase in the complexity of the coloured compounds present.Further experiments were done to measure the rate of colour development at different temperatures using the technique developed.  The results have shown a correlation between temperature and colour development over the range 50ºC to 70ºC with the rate increasing significantly above 60ºC

Numerical study on hygroscopic material drying in packed bed

The paper addresses numerical simulation for the case of convective drying of hygroscopic material in a packed bed, analyzing agreement between the simulated and the corresponding experimental results. In the simulation model of unsteady simultaneous one-dimensional heat and mass transfer between gas phase and dried material, it is assumed that the gas-solid interface is at thermodynamic equilibrium, while the drying rate of the specific product is calculated by applying the concept of a "drying coefficient". Model validation was clone on the basis of the experimental data obtained with potato cubes. The obtained drying kinetics, both experimental and numerical, show that higher gas (drying agent) velocities (flow-rates), as well as lower equivalent grain diameters, induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity

Isothermal Moisture Transfer Coefficients in Pinus Radiata Above the Fiber-Saturation Point, Using the Moment Method

The "moment method" was used to measure the transfer coefficients for moisture movement above fiber-saturation in Pinus radiata under isothermal conditions using moisture concentration as the driving force. Measurements were made at 5 C, 17 C and 30 C, and in the longitudinal, radial and tangential directions. Results obtained lie between 1.6 x 10-10 m2/s and 1.24 x 10-7 m2/s. Despite scatter in the results, certain important trends are detectable, viz: (1) there is roughly an order of magnitude difference between the longitudinal results, and the results in the radial and tangential directions at 30 C; (2) there is an apparent increase in transfer rate as the temperature rises; (3) the transfer coefficient tends to increase with moisture content for most results

BIOLOGICAL CONTROL OF KILN BROWN STAIN IN RADIATA PINE

Kiln brown stain in radiata pine (Pinus radiata) is a chocolate-brown discolouration that forms near the surface of boards dried at high temperature. This stain is the result of a thermochemical reaction between compounds in the sap that accumulate at the surface of the boards during drying. In this study, an attempt was made to remove the compounds that cause the stain, using bacteria in water sprinkled over the boards. Boards were stored in a tank, which was inoculated with rotted woodchips, with a recirculating overhead sprinkling buffer solution. Boards were periodically removed from the tank and dried under conditions that normally produce severe kiln brown stain (120/70°C). Samples were then removed to determine the level of stain by image analysis and the ease of moisture movement by neutron radiography. The bacterial treatment modified the extent of the stain layer so that after about 8 days of treatment there was no noticeable stain. The drying characteristics of the wood were modified so that the boards dried more slowly above fibre-saturation point, although the permeability of the dried wood increase