The S-statistic as an early warning of entrainment in a fluidized bed dryer containing pharmaceutical granule

The chaotic attractor comparison technique known as the S-statistic has been applied to pressure fluctuation data collected in a lab-scale fluidized bed dryer containing pharmaceutical granule. Along with pressure fluctuations, this unit has also been instrumented for measurements of bed temperature and fines entrainment. It has been determined that an increase in bed temperature, corresponding to the loss of granule surface moisture, occurs simultaneously with the onset of entrainment at a granule moisture content of approximately 11 wt.%. The S-statistic analysis is able to give an early warning of the undesirable hydrodynamic state where entrainment occurs provided an appropriate reference state is chosen. When a reference state of 10 wt.% moisture was chosen, a statistically similar hydrodynamic state was identified by the S-statistic occurring 8-9 min prior to the onset of entrainment, thus providing an early warning of this change. The monitoring of bed temperature alone provides no such early warning that entrainment is imminent. With further development, the S-statistic could become a valuable monitoring and control tool for the drying of granular material. (C) 2004 Elsevier B.V. All rights reserved.