(107a) Minimum Agitation Speed for Solid Suspension in a Torispherical-Bottomed Pharmaceutical Mixing Tank Agitated with a Retreat-Blade Impeller Under Different Baffling Conditions

The pharmaceutical and specialty chemicals industries often utilize glass-lined reactors in the manufacturing of Active Pharmaceutical Ingredients (API) because of their material compatibility with most reactants. These reactors are typically equipped with a retreat-blade impeller placed close to the bottom of the tank a single baffle mounted from the top. The mixing performance of such reactors has not received significant attention in the literature, despite their ubiquitous presence in the pharmaceutical industry. In this work the minimum agitation speed, Njs, required to just suspend solid particles dispersed in water was experimentally determined for a 60 L, scale-down version of a glass-lined pharmaceutical vessel provided with a torispherical bottom and agitated with a retreat-blade impeller under different baffling configurations, i.e., unbaffled, partially baffled, and fully baffled tank. Njs for the same tank but equipped with a different agitation system, namely an axial impeller, was also experimentally determined for the purpose of comparing of performances of the two systems. The effect of impeller off-bottom clearance and the vessels liquid level on the minimum agitation speed were also experimentally studied. Njs was experimentally determined using Zwietering's method, requiring that the solids do not rest on the tank bottom for more than 1-2 seconds. The value of Njs was found to depended strongly on the type of baffling, and was highest in the unbaffled tank and with solid particles 150µm in size and lowest in the partial baffled system.