On the stability of Würster fluid bed of pharmaceutical pellets
Mohammad Foroughi-Dahr, Rahmat Sotudeh-Gharebagh*, Navid Mostoufi
The flow of pharmaceutical pellets in a Würster fluid bed (WFB) was characterized by a frequency domain analysis of pressure fluctuations. Pellets with a diameter of 0.780 mm and density of 1.225 kg/m3 were used in the experiments. Different flow structures were identified in the bed, including bulk movement of pellets in the annulus (f < 5 Hz), bulk movement of pellets inside the draft tube and bulk horizontal movement of pellets through the entrainment zone (5 < f < 15 Hz), and clustering (15 < f < 145 Hz). The minimum spouting velocity was also measured experimentally. Effects of bed height, distance of the entrainment zone, and distributor hole pitch on minimum spouting velocity were investigated. It was found that the minimum spouting velocity increased with increasing bed height and distance of the entrainment zone while it decreased with increasing distributor hole pitch. A correlation was developed for estimating the minimum spouting velocity in WFBs containing pharmaceutical pellets. The correlation fit the experimental data satisfactorily. Studying the WFB hydrodynamics and determining the minimum spouting velocity provides information that can be used to properly design, operate, and scale up such systems.
Pharmaceutical pellets; Würster fluid bed; Draft tube; Minimum spouting velocity