Stirred-tank design practice — In a standard baffled, cylindrical stirred-tank bioreactor, how many baffles are typically installed?

Introduction:
Baffles suppress vortex formation, enhance mixing, and improve gas dispersion in stirred-tank bioreactors. The standard number of baffles is a basic design fact referenced in textbooks and vendor guidelines.

Given Data / Assumptions:

• Cylindrical vessel with flat bottom/standard aspect ratio.
• Radial-flow impellers (e.g., Rushton) or mixed-flow impellers.
• Standard baffle width on the order of 1/10 of tank diameter.

Concept / Approach:
Four equally spaced baffles (90 degrees apart) are standard, providing symmetric flow interruptions and preventing swirl. This configuration balances cost, manufacturability, and performance.

Step-by-Step Solution:
1) Identify conventional practice: 4 baffles at 90° spacing.2) Recognize that too few baffles (e.g., 2) allow swirling; too many (e.g., 6–8) provide marginal gains with higher cost/complexity.3) Conclude that 4 is the accepted standard for most designs.

Verification / Alternative check:
Vendor drawings and standard design handbooks (Perry’s, bioreactor design texts) consistently show four baffles in typical tanks.

Why Other Options Are Wrong:

• (a), (b) Excessive and rarely standard.
• (d) Insufficient to break swirl effectively.
• (e) Three baffles can work in constrained retrofits but are not the standard choice.

Common Pitfalls:
Confusing baffle count with special geometries (e.g., draft-tube reactors) or small bench-top vessels where inserts substitute baffles.

Final Answer:
4