MATHEMATICAL-MODEL OF A PNEUMATIC CONVEYING DRYER

A model for a pneumatic-conveying dryer is presented, with the focus o n the superheated steam drying of wood chips, although it can also be used for other porous materials and drying media. it includes a compre hensive 2-D model for the drying of single wood chips, which accounts for the main physical mechanisms occurring in wood during drying inclu ding coupled transport of water, air, vapor and heat. This model allow s for features such as initial condensation and flashing at the outlet , as well as the falling rate period when the drying is controlled by internal transport. External drying conditions in the dryer are calcul ated by applying mass, heat and momentum equations for each incrementa l step in dryer length. A plug-flow assumption is made for the dryer m odel, and single-particle and dryer models were solved iteratively. Th e irregular movement and nonspherical shape of wood chips are accounte d for by measuring drag and heat-transfer coefficients. Model calculat ions illustrate the complex interactions among steam, particles, and w alls that occur in a flash dryer The drying rate, the slip velocity, a nd temperature vary in a complex manner through the dryer necessitatin g the use of a comprehensive single-particle model, as in this case. P revious experimental data on the drying of bark chips in a pilot dryer was used to verify the model. The predicted temperature and pressure profiles, as well as the final moisture content of the material, agree d well with the measurements. Thus, the model provides a useful tool f or the design and scale-up of pneumatic-conveying dryers. Effects of s team and material properties on the drying were investigated with diff erent design parameters.