Monitoring fluidized bed dryer hydrodynamics using pressure fluctuations and electrical capacitance tomography

As part of the production of certain solid-dosage pharmaceuticals, granulated ingredients are dried in a batch fluidized bed dryer. Currently, the determination of the completion of the drying process is accomplished through measurements of product or outlet air temperatures. No quantitative measurement of hydrodynamic behaviour is employed. Changes in bed hydrodynamics caused by variations in fluidization velocity may lead to increased particle attrition. In addition, excessive desiccation of the granules caused by inaccurate determination of the drying endpoint may lead to an increase in the thermal and mechanical stresses within the granules. The activity of future high-potency or peptide based drug products may be influenced by these effects. Therefore, the quantification of hydrodynamic changes may be a key factor in the tighter control of both fluidization velocity and product moisture, which are critical for maintaining product quality.; High-frequency measurements of pressure fluctuations in a batch fluidized bed dryer containing pharmaceutical granulate have been used to provide a global, non-intrusive indication of the hydrodynamic changes occurring throughout the drying process. A chaotic attractor comparison statistical test known as the S-statistic, has been applied to quantify these changes in drying and a related unit operation, fluidized bed granulation. The S-statistic showed a sensitivity to moisture which is not seen with frequency and amplitude analysis. In addition, the S-statistic has been shown to be useful in identifying an undesirable bed state associated with the onset of entrainment in a bed instrumented for the collection of both pressure fluctuation and entrainment data.; Thus, the use of the S-statistic analysis of pressure fluctuations may be utilized as a low-cost method for determining product moisture or changes hydrodynamic state during fluidized bed drying.; Electrical capacitance tomography (ECT) has also been applied in this study to image the flow structure within a batch fluidized bed used for the drying of pharmaceutical granulate. This represents the first time that ECT has been applied to a bed of wet granulate material. This was accomplished through the use of a novel dynamic correction technique which accounts for the significant reduction in electrical permittivity occurring as moisture is lost during the drying process. The correction has been independently verified using x-ray tomography.