(360a) Experimental and Computational Investigation of the Hydrodynamics of Partially Baffled and Unbaffled Stirred Tank Reactor Systems Equipped with a Retreat-Blade Impeller

In this work, the velocity distribution inside the typical glass-lined vessel/impeller system was experimentally and computationally quantified using Laser Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD). Two different reactor configurations were investigated, i.e., a flat-bottom tank and a hemispherical-bottom tank. In each case, two baffling configurations were studied, i.e., a partially baffled tank with a single beaver-tail baffle (the most common baffled configuration used in the pharmaceutical industry), and an unbaffled system. The three velocity components (tangential, axial, and radial) at 8 to 13 radial locations on 7 horizontal planes for the flat-bottom tank case and 5 horizontal planes for the hemispherical-bottom tank case were experimentally determined by LDV for both the partially baffled and unbaffled configuration. Numerical simulations of the velocity distribution inside each vessel were conducted using a commercial mesh generator (Gambit) coupled with a computational fluid dynamic (CFD) package (Fluent). The full 360°-tank geometry was incorporated in the simulations. Agreement between the experimental and simulation results was found to be favorable.

In the unbaffled flat-bottom reactor case, the tangential component of the velocity appears to dominate over the other velocity components at nearly every location, with tangential velocity typically on the order of 40% to 50% of the impeller tip speed. The radial and axial velocities, especially in the region just below the impeller, were found to be very small, with magnitudes typically smaller than 15% for the axial component and 5% to 10% for the radial component. In general, the presence of a hemispherical bottom did not alter significantly the magnitude of the velocity components except in the lower portion of the tank, where the hemispherical bottom generated a stronger axial and radial recirculation pattern. The velocity distribution in the single-baffle case was found to be only partially different from the unbaffled case, and primarily in the upper portion of the tank, where the baffle is. The velocity distribution in the lower portion of these vessels was not significantly affected by the presence of the baffle.

The dominance of the tangential velocity and the small value of the radial and especially axial velocity in all the system investigated here indicate a poor vertical recirculation of the fluid inside the tank and imply that these systems may have a limited mixing performance.