Controlling granule size by granulation liquid feed pulsing.

The effects of inlet air humidity, granulation liquid feed rate and granulation liquid feed pulsing on the particle-size distribution of final granules were studied in a laboratory scale fluid-bed granulator using a central composite study design. The factors examined were modelled using three different particle-size measurement techniques (sieve analysis, laser light diffraction and the spatial filtering technique). Best goodness of fit (R2=0.94) and goodness of prediction (Q2=0.90) values were obtained using particle-size results of the spatial filtering technique. Increasing inlet air humidity and granulation liquid feed rate resulted in greater median granule size, as expected. When pulsing (interruption of granulation liquid feed in predetermined sequences) was used, the median granule size decreased clearly. This effect was strong, especially with high inlet air humidity and rapid liquid feed rate processes. Granulation liquid feed pulsing is an effective way to modify the particle size of final granules. Pulsing can be used as a controlling tool to compensate for excessive moisture content in the granulation process.

Effect of fluidisation activity on end-point detection of a fluid bed drying process.

The influence of inlet air humidity variations on fluid bed drying end-point detection was the primary focus here. Various drying end-point criteria based on temperature and humidity measurements were compared. Seasonally changing inlet air humidity affects the moisture content of the finished granules, as long as the drying process remains unchanged. However, a specific moisture content of the finished granules is commonly desired after fluid bed drying. When experimental batches of varying inlet air humidity were compared at the beginning of the drying phase, the temperature of the granules increased linearly as the humidity of the inlet air increased. This effect causes variation in moisture contents of the final granules of different batches when the fixed temperature of the mass is used as an end-point criterion. With varying inlet air humidity, the often used DeltaT temperature difference method resulted in more precise estimation of the drying end-point than the constant temperature criterion. In this study new insights were found into the correlation between moisture content and temperature of the fluidising mass. Fluidisation activity greatly affected detection of drying end-point. Use of the DeltaT criterion requires proper fluidisation throughout the process.