(498b) CFD Predictions of Power Dissipation and Related Hydrodynamic Effects for a Retreat-Bladed Impeller in a Stirred Vessel Under Different Baffling Configuration

The manufacture of active pharmaceutical ingredient (API) in the pharmaceutical industry typically involves a number of fluid mixing and reaction operations, many of which are conducted in mechanically stirred, glass-lined tanks and reactors provided with a torispherical bottom and equipped with a single baffle and a retreat-blade impeller (RBI). The use of glass lining is critical to provide corrosion resistance, allow ease of cleanliness, and reduce product contamination. Still it requires coating with glass not just the internal wall of the tank, but also all components that may come in contact with the liquid contents, including the impeller and the baffles. This often results in the use of partial baffling, typically a beavertail baffle, which is mounted from the top of the reactor and inserted between the shaft and the tank wall. However, in other industrially relevant systems the more conventional four vertical baffles are mounted on the vessel wall, although in some pharmaceutical product manufacturing and applications (e.g., crystallization, biopharmaceutical processes, solid suspensions, etc.) unbaffled stirred vessels can also be found.

In previous work by our group, the power dissipated by the impeller, P, and the impeller Power Number, Po, which arecritical for scale-up purposes, especially when mass transfer operations are involved, were experimentally determined for systems commonly found in the pharmaceutical industry. This was achieved by measuring the power dissipated by an RBI under fully baffled, partially baffled, and unbaffled conditions, with different liquids, and under different hydrodynamic regimes (1<Re< 300,000) using a 61-Liter vessel that was built as an actual scaled-down replica of the glass-lined vessels typically used for API manufacturing in the pharmaceutical industry.

In our present work, Computational Fluid Dynamics (CFD) was utilized to conduct detailed hydrodynamic simulations for the same systems, not only to determine the flow features of the different systems computationally but also to obtain relevant critical quantities such as the power dissipation and Power Number. Simulations were conducted for a variety of baffling configurations and operating conditions using different CFD approaches, turbulence models, and simulation software. The results indicate that the computational predictions typically compared well with the experimental data in all cases, thus validating the CFD approach. The information provided by this approach to understanding the different fluid-dynamics features of vessel-baffling configurations and the relevant effect of operating parameters is clearly beneficial for the selection, design and use of suitable vessels and reactors for industrial applications, and it is expected to be of value to the industrial practice.