Question

The power required by an impeller in a gas sparged system compared to the power required by impeller operating at same speed in a gas free liquid is usually

• A. \( \text{lesser} \)
• B. \( \text{higher} \)
• C. \( \text{same} \)
• D. \( \text{data insufficient} \)

Hint:

The ratio \(P_g/P_u\) is a critical parameter for designing motors to ensure they don't burn out when aeration is turned off.

Answer 1

The Correct Option is A

Concept: Aeration (sparging) significantly affects the power consumption of an impeller in a bioreactor.
• \(P_u\) is the power required for ungassed liquid.
• \(P_g\) is the power required for gassed liquid.

Step 1: Effect of Gas Bubbles on Fluid Density.
When gas is sparged into a liquid, the effective density of the gas-liquid mixture decreases. Since the power requirement is directly proportional to density, a lighter mixture requires less work to move.

Step 2: Formation of Gas Cavities.
Gas bubbles tend to accumulate in the low-pressure regions behind the impeller blades. These "gas cavities" reduce the drag force on the impeller blades. Consequently, the gassed power (\(P_g\)) is usually 30% to 50% lower than the ungassed power (\(P_u\)). Final Answer: The power required in a gas-sparged system is generally lesser than in a gas-free liquid.