Abstract
Fluidised beds have not yet been widely utilised for processing of vegetable products. The main reason is that the materials often possess physical properties that render them difficult to handle by existing fluidisation techniques. The vegetable product chosen for this work is cut lamina tobacco provided by British American Tobacco Company Limited. The objectives of this project are to develop a fluidised bed device capable of handling cut lamina tobacco and to investigate the use of the device for heat and mass transfer operations. It is shown that conventional technology, such as spouted beds or slot spouted beds, cannot cope with cut lamina tobacco because of its entangled nature. A novel design of contacting device is developed, suitable for maintaining controlled inventory of cut lamina tobacco. The device uses relatively high velocity gas jets to create a well defined flow pattern which the solid follows, thereby disentangling the tobacco fibres. Heat transfer experiments show that heat transfer coefficients in the mobilised bed are sufficiently high to permit rapid equilibration of the bed mass with the gas. The high recirculation rate of the solid also ensures that the temperature is uniform everywhere in the bed. Drying tests in the mobilised bed show that the overall mass transfer is not gas phase limited but is controlled by internal migration of moisture within the tobacco particles. The effective diffusion coefficients calculated from the drying curves showed an Arrhenius-type relationship with temperature, with values in the range 2.7X10[-6] to 17.5X10[-6] cm[2]/s for a temperature range of 40°-85°C. Mobilisation of the tobacco results mainly from momentum transfer from the gas jets to the plug of tobacco. The maximum bed pressure drop is below that calculated to support the bed weight (or DeltaP[mf]), due to the contribution of the inlet momentum flux of the mobilising gas jets. A model is developed which adequately predicts the maximum mobilising flowrate at incipient mobilisation. The heat and mass transfer equations combined with the analysis of incipient mobilisation are used in a demonstration design of a complete continuous mobile bed dryer for a primary tobacco process.