Preventing surface vortex formation: Which measures effectively prevent vortexing in stirred tank reactors during bioprocess agitation?

Introduction / Context:
Vortexing wastes energy, entrains air, and can damage cells. Engineers use geometric features and mounting choices to suppress free-surface swirl and maintain efficient mixing patterns in bioreactors.

Given Data / Assumptions:

• Cylindrical tank with free liquid surface.
• Mechanical agitator present.
• Goal: minimize central vortex formation and air entrainment.

Concept / Approach:
Four equally spaced baffles (≈10% of Dt) are the standard solution for eliminating swirl by interrupting tangential flow. If baffles are not feasible (e.g., single-use bags), moving the impeller off-centre or tilting it can break rotational symmetry and reduce vortex depth. Axial-flow impellers help circulation but typically still require baffles in open-surface tanks to fully prevent vortexing.

Step-by-Step Solution:
Add baffles to convert swirl into axial/radial circulation.Alternatively, use off-centre mounting to disrupt coherent swirl structures.Recognize that axial-flow impellers alone may not suppress vortex without baffles.Therefore, the effective measures are (a) and (b) together.

Verification / Alternative check:
Mixing studies show dramatic reduction in surface depression and improved mixing time when baffles or off-centre placement are used versus centered, unbaffled configurations at the same speed.

Why Other Options Are Wrong:
Option (d): axial impellers improve axial flow but do not guarantee vortex suppression; baffles are still standard practice.

Either (a) or (b) alone can work, but the combined option best reflects accepted solutions and the question stem.

Common Pitfalls:

• Attempting to fix vortexing by speed increases, which often worsens air entrainment.
• Ignoring headspace and foaming tendencies that exacerbate surface instability.

Final Answer:
Both (a) and (b)