Application of electrical capacitance tomography to the fluidized bed drying of pharmaceutical granule

A novel calibration technique that accounts for the presence of moisture has allowed for the application of electrical capacitance tomography (ECT) to a fluidized bed dryer containing wet pharmaceutical granule. A statistical attractor comparison test denoted as the S-statistic has been used to analyze both the reconstructed and non-reconstructed ECT images to determine radial variations in hydrodynamic behavior at an axial location 10.5 cm above the distributor. This analysis has shown that variations in the bed dynamics associated with the loss of moisture during the drying process are most intense near the walls of the dryer, while the central region is characterized by a more consistent dynamic behavior. Investigation of the voidage distribution in the individual tomograms over this period in the drying process shows that the changes in the S-statistic correspond to a reduction in area of the region involved in bubbling behavior. Tomograms from early in the drying process indicate a central core of gas channeling through the very wet granule. This is due to the high cohesive forces present at high moisture. At moisture contents below 5-wt%, near the end of drying, significant divergence in dynamic behavior is identified in the S-statistic. This divergence is associated with the appearance of asymmetric bubbling behavior in the individual tomograms. (c) 2005 Elsevier Ltd. All fights reserved.