Fluidized bed drying of wood particles

In the field of wood drying, most efforts have been concentrated on the drying of veneer and lumber. A limited amount of research has been reported on the drying principles of particles. Most of the work on wood drying is concerned with mass transfer in porous material. Previous studies were restricted to softwood with a moisture content not greater than the fiber saturation point.

In this study, a boundary shrinking model is developed to describe the moisture transport phenomena for small wood particles during the drying period with an initial moisture content greater than the fiber saturation value. The particle is assumed to be divided into two zones -- an oversaturation zone and an undersaturation zone. A cylindrical boundary surface with the moisture content equal to the fiber saturation value exists between these two zones which shrinks toward the center during drying. A finite difference approximation technique is used to formulate a solution for the two-dimensional transport equation. The Crank-Nicolson scheme is used in this study.

The reliability of this model was tested by comparing computed and experimental data for wood particles in a shallow fluidized bed dryer. The particle size used was between 0,2-0,4 cm. The drying temperatures used were 40 and 60°C. The gas flow was assumed to be perfectly mixed in the shallow fluidized bed. Comparisons of calculated values and experimental data showed good agreement for both the moisture content profiles and drying rates.

Stamm's diffusion model was used to estimate the moisture diffusivity during the drying process. An experiment was devised and run to determine the sorption isotherm for wood. This was used in calculating the moisture diffusivity and the driving force for convective mass transfer in the wood drying model. The effects of density and sorption isotherm on the moisture diffusivity were studied.

Based upon the heat and mass transfer model for small particles in the dense phases developed by Nelson and Galloway, the experimental data obtained with large particles in fixed and fluidized beds were correlated with a new formula in this study. This correlation was used for estimating the convective heat transfer coefficient in the fluidized bed dryer. It was found that convective heat and mass transfer in packed and fluidized beds is a function of bed voidage rather than bed height. The minimum fluidization velocity for large particles was also studied in this work.