Many of the reactions carried out in the pharmaceutical industry to synthesize new chemical entities and their intermediates are conducted in stirred tank reactors equipped with a single retreat-blade impeller and a single baffle. These reactors and their internals, including the impeller and baffle as well as the vessel wall, are typically glass lined in order to prevent contamination of the products and reduce chemical attack.

Little information is available to date on the hydrodynamics of partially baffled, retreat-blade impeller systems, although these are ubiquitous in pharmaceutical production. Basic fluid dynamic knowledge of this kind is essential to determine how to operate processes conducted in these systems, such as solid suspension, crystallization and chemical reaction.

This research work is focused on the determination of the velocity distribution inside a scaled-down, retreat-blade impeller-vessel system using Laser-Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD). The experimental apparatus consisted of a 19-liter vessel along with a retreat-blade impeller and a single cylindrical baffle. All three velocity components and the turbulence intensity were measured at different radial positions on a number of horizontal planes inside the vessel. Numerical simulations of the velocity distribution and turbulence levels inside the vessel were conducted using a commercial mesh generator (Gambit) coupled with a computational fluid dynamic (CFD) package (Fluent). Both pseudo-steady state multiple reference frame (MRF) simulations and time dependent simulation were conducted.

The flow pattern in this partially baffled system is generally complex. It was found that the flow in the lower section of the vessel is dominated by the tangential component of the velocity as in unbaffled systems, although the presence of the baffle produced a stronger axial flow in the upper region. This indicates that phenomena that are dominated by the hydrodynamics near the vessel bottom, such as solid suspension, may be more difficult to carry out in such vessels as compared to fully baffled systems.