Radial-Flow Impellers in Bioreactors — Which statements correctly describe their hydrodynamics and typical application?

Introduction:
Impeller selection determines mixing patterns, gas dispersion, and shear environment in bioreactors. Radial-flow turbines (for example, Rushton disc turbines) are ubiquitous in gas–liquid systems because their discharge pattern and turbulence help disperse bubbles and enhance mass transfer.

Given Data / Assumptions:

• Baffled, cylindrical stirred-tank reactor.
• Comparing radial-flow turbines with axial-flow impellers (for example, pitched-blade, hydrofoils).
• Goal is to understand flow direction and shear characteristics.

Concept / Approach:

Radial-flow turbines eject fluid perpendicular to the shaft, striking baffles and the tank wall, creating strong turbulence, secondary flows, and high local shear. This is useful for breaking up bubbles and droplets, increasing interfacial area for mass transfer. In contrast, axial-flow impellers promote bulk circulation with lower shear, often preferred for shear-sensitive cultures.

Step-by-Step Solution:

Verification / Alternative check:

Power number and gas-handling characteristics of Rushton turbines reflect higher shear rates than axial-flow designs, matching their use in oxygen transfer–limited operations.

Why Other Options Are Wrong:

D and E describe axial-flow or low-shear impellers, which are different from radial turbines in hydrodynamics and application.

Common Pitfalls:

Assuming more shear is always beneficial; delicate cells may require axial-flow or marine impellers to limit damage.

Final Answer:

Both (a) and (b).