Bioreactor Hydrodynamics — During aerobic culture in stirred tanks, which flow regime is generally preferred to maximize gas–liquid mass transfer and mixing without excessive stratification?

Introduction:
Aerobic bioprocesses require efficient oxygen transfer and uniform nutrient distribution. The hydrodynamic regime strongly influences kLa, bubble dispersion, and mixing time. This question targets the preferred regime in stirred tank reactors (STRs) for aerobic cultures.

Given Data / Assumptions:

• STR with properly sized impellers and baffles.
• Viscosity within typical microbial broth ranges (not extreme non-Newtonian gel).
• Shear sensitivity is considered but not overriding.

Concept / Approach:

Turbulent flow enhances eddy transport and bubble breakup, increasing interfacial area and oxygen transfer rates. Baffles suppress vortexing and improve axial and radial mixing. Operating in a turbulent regime (high Reynolds number) shortens mixing times and reduces concentration gradients compared to laminar or poorly mixed transient regimes.

Step-by-Step Solution:

Verification / Alternative check:

Empirical correlations for kLa increase with power input per volume and superficial gas velocity, both facilitated under turbulent flow. Many industrial STRs target turbulent conditions while limiting excessive shear to protect sensitive cells.

Why Other Options Are Wrong:

A: Laminar flow yields poor gas dispersion. B: Intermittent mixing cannot maintain uniform oxygen levels. D: Regimes are not equally suitable. E: Plug flow does not describe STR mixing and is unsuitable for uniform aeration in tanks.

Common Pitfalls:

Equating “more turbulence” with “better” without considering shear damage; optimization balances mass transfer and cell viability.

Final Answer:

Turbulent flow with adequate impeller design and baffles