Stirred-tank fermenter geometry: what is the typical height-to-diameter (H/D) ratio range used in tank fermenters for bioprocessing?

Introduction / Context:
Reactor geometry influences mixing, gas dispersion, power input, and scale-up rules. For stirred-tank fermenters, the height-to-diameter (H/D) ratio is a simple yet powerful design descriptor. This question asks for the typical H/D range used in practice.

Given Data / Assumptions:

• We consider conventional mechanically agitated, baffled tank fermenters.
• Designs target good gas–liquid contact and manageable shaft lengths.
• Viscosities and foaming can vary with the organism and medium.

Concept / Approach:
Most production-scale stirred tanks use H/D between roughly 2 and 3 to balance mixing time, oxygen transfer, and structural practicality. Ratios substantially below 1.5 can impair gas dispersion and circulation; much greater than 3 can cause mechanical issues and non-uniformities unless multiple impellers and special internals are used.

Step-by-Step Solution:
1) Recall common design: H/D about 2–3 for many fermenters.2) Compare options: “>1.5” captures this typical range without overstatement, whereas “>3” overshoots and “<1.5” undershoots.3) Therefore, the best general statement is H/D > 1.5.

Verification / Alternative check:
Vendor specifications for industrial glass-lined or stainless fermenters frequently list H/D near 2–3 for aerobic cultures with multiple impellers.

Why Other Options Are Wrong:
“<3” is true but too loose and includes unsuitable low ratios; “>3” is atypical; “<1.5” and “≈1” do not reflect standard fermenter practice.

Common Pitfalls:
Assuming a single “magic” ratio works for all processes; actual choice depends on power number, gas rates, and rheology.

Final Answer:
>1.5