(569d) Mass Transfer Coefficient for Drying of Moist Particulate in Bubbling Fluidised Bed

Experiments on drying of moist particles by ambient air were carried out to measure the mass transfer coefficient in a bubbling fluidised bed. Fine glass particles of mean diameter 125 ?Ým were used as the bed material. Throughout the drying process, the dynamic material distribution was recorded for 60 seconds, at intervals of 5 minutes, using Electrical Capacitance Tomography (ECT). Simultaneously, the exit air condition was recorded at intervals of 2 minutes using a temp/humidity probe. The ECT data were used to obtain qualitative and quantitative information on the bubble characteristics. The exit air moisture content was used to determine the water content in the bed. The measured overall mass transfer coefficient within the first drying period (fast drying rate) was in the range of 0.0145-0.021 m/s, which is close to the literature values for free water surface diffusion into slow ambient air stream. Because of the change in the drying mechanism during the second drying period (slow drying rate), the values obtained falls sharply in the range of 0.0002-0.0037 m/s. A simple model based on bubble-cloud and cloud-dense interchange (two-region model) was used to predict the overall mass transfer coefficient. Comparison between the measured and predicted mass transfer coefficient has shown a reasonable agreement within the range of the first drying period, while a clear disagreement was evident during the second drying period. The results were also used to determine the relative importance of the two transfer regions and to identify the mass transfer controlling region at various operating condition.