Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Process Engineering, Biotechnology, Nutrition Technology
A model for a pneumatic-conveying dryer is presented, with the focus on the superheated steam drying of wood chips, although it can also be used for other porous materials and drying media. It includes a comprehensive 2-D model for the drying of single wood chips, which accounts for the main physical mechanisms occurring in wood during drying, including coupled transport of water, air, vapor and heat. This model allows 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 calculated by applying mass, heat and momentum equations for each incremental step in dryer length. A plug-flow assumption is made for the dryer model, and single-particle and dryer models were solved iteratively. The irregular movement and nonspherical shape of wood chips are accounted for by measuring drag and heat-transfer coefficients.Model calculations illustrate the complex interactions among steam, particles, and walls that occur in a flash dryer. The drying rate, the slip velocity, and temperature vary in a complex manner through the dryer, necessitating the use of a comprehensive single-particle model, as in this case. Previous 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, agreed well with the measurements. Thus, the model provides a useful tool for the design and scale-up of pneumatic-conveying dryers. Effects of steam and material properties on the drying were investigated with different design parameters.
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