Superheated Steam Drying of Wood Chips in Pneumatic Conveying Dryers
This thesis contains a fundamental investigation of a pneumatic conveying dryer. The model presented can be used both for design and rating purposes and, although the main emphasis is put on the drying of wood chips in superheated steam, the results can be used for other systems as well. In order to simulate the present type of dryer, both the interactions between the gas and particles as well as the internal characteristics of the wood chips were taken into account. The complete dryer model consists of two sub-models: one for the single particle and one for the hydrodynamics of gas and particles in the dryer. The single particle model is two-dimensional in order to account for the strong anisotropicity of wood and contains the main mechanisms of heat and mass transport which occur in wood during drying. The hydrodynamic model is one-dimensional and calculates the profiles of temperature, pressure and slip-velocity along the dryer. The non-spherical shape and irregular motion of the chips in the dryer are accounted for by measuring the drag and heat transfer coefficients.
An experimental device which enabled measurements of the average moisture content, centre temperature and pressure of a single wood chip was developed in order to confirm the single particle model. The complete dryer model was validated by a comparison with actual running data from a present conveying dryer. All experimentally observed features, both for the single particle model and the complete dryer model, were well predicted by the simulations.
Model calculations illustrate the complex interactions between the particulate and gas phases which occur in this type of dryer. The effects of steam properties such as temperature, pressure and steam velocity on drying were investigated. The chip size and wood species also affect drying and were therefore varied. Different design parameters such as height and diameter of the dryer were also varied in order to understand their effects on drying performance. Important features which are typical for steam drying, such as initial condensation and flashing at the outlet, were also studied.
Some conclusions which can be drawn from the present work are:
-Preheating of the wood chips before entering the dryer is necessary to avoid initial condensation.
-Flashing at the outlet decreases the final moisture content significantly.
-Zones with low drying rates can be avoided by the use of the present model.
-The resistance to mass transfer inside the wood chips becomes very important in the present type of dryer.
-The single particle model must be two-dimensional in order to predict correct drying rates. This is due to the strong anisotropicity of wood.