Introduction
This quiz package focuses on power requirements in fermentation systems, tailored for M.Pharm students studying Bioprocess Engineering and Technology. It covers fundamental concepts such as power number, impeller characteristics, scaling laws, power per unit volume, and the effects of aeration and viscosity on energy input. Questions probe relationships between agitation, oxygen transfer, shear, and microbial sensitivity, plus practical calculations using P = Np·ρ·N³·D⁵ and Reynolds number regimes. The set emphasizes interpretation of power measurements, selection of impellers for various broth rheologies, and scale-up strategies to preserve mixing and mass transfer. Use these MCQs to test and deepen your understanding of energy management in bioreactors.
Q1. What is the dimensional correlation commonly used to estimate power consumption for an agitated stirred tank?
- P = Np · ρ · N · D
- P = Np · ρ · N³ · D⁵
- P = Np · μ · N² · D³
- P = Np · ρ · g · D²
Correct Answer: P = Np · ρ · N³ · D⁵
Q2. The dimensionless power number (Np) for a given impeller depends primarily on which parameter?
- Absolute power P in watts
- Reynolds number (Re)
- Fluid density only
- Impeller shaft diameter
Correct Answer: Reynolds number (Re)
Q3. In which flow regime does the power number become essentially constant for most impellers?
- Laminar flow (Re < 10)
- Transitional flow (10 < Re < 10^4)
- Turbulent flow (Re > 10^4)
- Viscoelastic flow only
Correct Answer: Turbulent flow (Re > 10^4)
Q4. For a Newtonian fluid, doubling impeller speed (N) affects power consumption (P) by what factor, assuming constant Np and geometric similarity?
- 2 times
- 4 times
- 8 times
- 16 times
Correct Answer: 8 times
Q5. Which parameter is commonly used to express power input normalized for reactor size and biomass needs in fermentation: power per unit volume?
- P/V (W · m⁻³)
- Power number Np
- Tip speed (m · s⁻¹)
- Reynolds number
Correct Answer: P/V (W · m⁻³)
Q6. How does aeration (gas sparging) typically affect the power draw of an agitated bioreactor at moderate gas flow rates?
- Always increases power draw linearly
- Has no effect on power draw
- Can decrease impeller torque and therefore reduce power draw
- Replaces impeller power with gas power so net power equals zero
Correct Answer: Can decrease impeller torque and therefore reduce power draw
Q7. For shear-sensitive cultures, which strategy reduces shear while maintaining oxygen transfer?
- Increase tip speed drastically
- Use high-shear Rushton turbines
- Use low-shear axial-flow impellers and increase power moderately
- Decrease aeration to near zero
Correct Answer: Use low-shear axial-flow impellers and increase power moderately
Q8. Which impeller type is generally preferred for achieving high oxygen transfer with lower shear in viscous broths?
- Rushton disc turbine
- Pitched-blade axial flow impeller
- High-speed propeller with small diameter
- Flat-blade paddle
Correct Answer: Pitched-blade axial flow impeller
Q9. During scale-up using constant power per unit volume (P/V), which operational change is required when reactor volume increases?
- Keep impeller diameter constant and reduce speed proportionally
- Increase impeller speed to maintain P/V at larger volumes
- Reduce impeller diameter drastically
- Maintain identical tip speed as in lab scale
Correct Answer: Increase impeller speed to maintain P/V at larger volumes
Q10. Reynolds number for stirred tanks is defined using which characteristic length and velocity?
- Re = ρND²/μ using impeller diameter D and speed N
- Re = ρgD/μ using tank diameter D and gravity g
- Re = μ/ρND using viscosity μ and speed N
- Re = Np · ρ · N³ · D⁵
Correct Answer: Re = ρND²/μ using impeller diameter D and speed N
Q11. Which effect describes the reduction of power draw in presence of gas due to gas induced discharge and slip?
- Power augmentation
- Gas entrainment effect
- Gas blanketing or gas unloading
- Bubble cavitation
Correct Answer: Gas blanketing or gas unloading
Q12. The mass transfer coefficient kLa is often correlated empirically with power input. Which of the following trends is generally true?
- kLa decreases as P/V increases
- kLa is independent of agitation
- kLa increases with both P/V and superficial gas velocity
- kLa depends only on impeller material
Correct Answer: kLa increases with both P/V and superficial gas velocity
Q13. For highly viscous, non-Newtonian fermentation broths, which scale-up criterion is most appropriate to maintain mixing characteristics?
- Constant tip speed
- Constant Reynolds number
- Constant power per unit mass or P/V
- Constant impeller diameter
Correct Answer: Constant power per unit mass or P/V
Q14. Which measurement method is commonly used to estimate actual power consumption of a bioreactor drive?
- Direct calorimetry of broth
- Measure motor electrical input (voltage × current) and correct for efficiency
- Count bubbles per second
- Estimate from impeller color change
Correct Answer: Measure motor electrical input (voltage × current) and correct for efficiency
Q15. Tip speed is an important parameter for shear-sensitive organisms. Tip speed is defined as:
- π · D · N (where D is impeller diameter and N is revolutions per second)
- D · N²
- P/(ρ · V)
- μ · N/D
Correct Answer: π · D · N (where D is impeller diameter and N is revolutions per second)
Q16. If the power number Np for an impeller in turbulent regime is 5 and ρ = 1000 kg·m⁻³, N = 10 s⁻¹, D = 0.2 m, what is approximate power P?
- 0.16 W
- 16 W
- 160 W
- 1600 W
Correct Answer: 160 W
Q17. Which situation increases power requirement most significantly in a fermenter?
- Decreasing broth viscosity
- Decreasing impeller diameter
- Increasing gas flow while staying in gas–liquid mixing regime
- Changing impeller from radial to axial without speed change
Correct Answer: Increasing gas flow while staying in gas–liquid mixing regime
Q18. When scaling up between geometrically similar stirred tanks at constant tip speed, how does P/V scale with the characteristic linear dimension L (tank size)?
- P/V remains constant with scale
- P/V scales as L^0 (independent)
- P/V scales as L⁻¹ (decreases with size)
- P/V scales as L² (increases with size)
Correct Answer: P/V scales as L⁻¹ (decreases with size)
Q19. For low Reynolds number (laminar) stirring, power consumption scales with speed (N) approximately as:
- P ∝ N
- P ∝ N²
- P ∝ N³
- P ∝ N⁵
Correct Answer: P ∝ N²
Q20. In practical fermentation operation, which combination most directly improves oxygen transfer without excessively increasing shear?
- High-speed small-diameter Rushton turbine + high aeration
- Moderate-speed axial impeller + increased gas dispersion/microbubble sparging
- Stop agitation and rely on diffusion
- Use larger impeller at very low speed with no gas
Correct Answer: Moderate-speed axial impeller + increased gas dispersion/microbubble sparging
