In bioreactor design and scale-up, what is correct regarding the use of off-centre impellers (i.e., impellers mounted with a deliberate offset from the vessel axis)?

Introduction / Context:
Large bioreactors present mixing and gas–liquid dispersion challenges that are not as prominent in small lab vessels. One practical strategy is to mount the impeller off the vessel centreline. This question probes when off-centre impellers are appropriate and why they are often chosen during scale-up to industrial sizes (greater than 10 m^3).

Given Data / Assumptions:

• Cylindrical, baffled or partly baffled vessels typical of fermentation service.
• Industrial volumes may exceed 10 m^3 with tall aspect ratios.
• Process objectives include good bulk mixing, gas dispersion, and reduced vortexing.

Concept / Approach:
Off-centre (eccentric) mounting disrupts swirl and large coherent vortices, improves top-to-bottom circulation, and can enhance gas dispersion when multiple impellers are impractical. At very large scales, perfect baffling and ideal shaft alignment are difficult; a deliberate offset can mitigate maldistribution and dead zones without excessive power demand.

Step-by-Step Solution:

Verification / Alternative check:
Mixing audits and tracer tests in large tanks often show improved circulation times with slight eccentricity or tilted shafts, confirming the engineering rationale.

Why Other Options Are Wrong:

• Any size reactor: not necessary in small, well-baffled vessels; centreline mounting suffices.
• Less than 10 m^3: contradicts typical practice; benefits are most pronounced at larger scales.
• Never off-centre: overly absolute and ignores proven designs.
• Banned by sanitary codes: no universal prohibition; cleanability depends on detailed design.

Common Pitfalls:
Confusing off-centre mounting with a lack of baffles; both can be used together if hygienic design and validation are maintained.

Final Answer:
Off-centre impellers can be used when volume is greater than 10 m^3