Agitation energy demand: On what does the power consumption of an agitated bioreactor primarily depend?

Introduction / Context:
Power input drives mixing, suspension, and mass transfer in bioreactors. Engineers must anticipate how fluid properties, hardware geometry, and operating parameters combine to determine the required shaft power and energy per unit volume.

Given Data / Assumptions:

• Mechanically agitated tank with baffles.
• Possibility of gas sparging affecting power draw.
• Newtonian or non-Newtonian broths depending on biomass.

Concept / Approach:
Power correlations use the power number Po and Reynolds number Re (and sometimes gas flow number Fl). For a given geometry, P = Po * ρ * N^3 * D^5 in ungassed conditions; Po depends on Re (via viscosity μ) and geometry, and gas sparging typically reduces P at constant N. Hence power depends on physical properties, operating conditions, and vessel/impeller geometry.

Step-by-Step Solution:
Relate P to Po, ρ, N, and D for ungassed operation.Include viscosity via Re = ρ * N * D^2 / μ.Account for geometry: D/T, blade type (Rushton vs. pitched–blade), clearance, and baffles set Po–Re trends.Consider operating conditions: speed N, gas flow, and baffling modify effective Po and thus P.

Verification / Alternative check:
Scale-up rules (constant P/V or constant tip speed) explicitly reference these variables, demonstrating that all three categories influence power consumption.

Why Other Options Are Wrong:
Any single category alone omits critical determinants. Accurate predictions require fluid properties, operating parameters, and geometric factors together.

Common Pitfalls:

• Extrapolating Po from water-like fluids to viscous, shear-thinning broths.
• Ignoring gas-induced power reduction when sparging at fixed N.

Final Answer:
All of the above