Angled-mounted impellers are commonly used in a number of pharmaceutical industry applications, from laboratory reactors to full-scale tanks. In these systems, the impeller is not centrally and vertically mounted, as in most stirred vessels, but it enters the vessel, and is therefore immersed in the fluid, at an angle from the vertical. This arrangement provides some baffling effects that would not otherwise exist if the impeller was centrally mounted.
One common requirement for such systems is that they are capable of suspending small solids dispersed in the liquid. Therefore, it is critical to know the value of the minimum agitation speed at which solids become just suspended, Njs. Despite their common use in industry, and especially the pharmaceutical industry, these systems have received little attention in the literature in general and as far as their ability to solid suspension is concerned in particular.
Therefore, the focus of this work was to determine under what conditions fine divided solids become suspended in a liquid-filled, unbaffled stirred vessel provided with angled-mounted impellers. A small laboratory vessel with a hemispherical bottom was used here. Different types of axial and radial impellers mounted at different angles (0°, 5° and 10°) at different off-bottom impeller clearances were tested. The value of Njs was experimentally obtained by visually inspecting the tank bottom and determining the impeller agitation speed at which the solids were observed to rest on the tank bottom for no more than 1-2 seconds before being swept away. In addition, a novel observer-independent criterion was developed, relying on the experimental determination of the fraction of solids remaining on the vessel bottom at increasing agitation speeds. This approach results in values of Njs that are close agreement with the visually determined Njsvalues.
This approach was used to determine Njs for different impellers under a number of operating conditions. In general, mounting the impeller at an angle resulted in Njs values lower than those obtained with centrally mounted impellers. In addition, it was found that lower impeller clearances resulted in lower Njs values, although the minimum for Njswas obtained when the impeller clearance was about 20% of the vessel diameter.
The results obtained in this work are expected to be applicable to a number of systems currently used in the pharmaceutical industry.
See more of this Group/Topical: North American Mixing Forum