Bubble column and airlift reactors achieve bulk mixing by gas introduction. Which mechanism primarily drives circulation and mixing in these non-mechanically agitated systems?

Introduction / Context:
Unlike stirred tanks, bubble columns and airlift reactors lack internal impellers. They rely on gas sparging to generate circulation, renew liquid at gas–liquid interfaces, and promote mass transfer—vital for aerobic cultures and gas-liquid reactions.

Given Data / Assumptions:

• Gas introduced at the bottom decreases local density.
• Rising bubbles induce liquid circulation.
• No mechanical agitators are present.

Concept / Approach:
Gas holdup gradients create buoyancy-driven flow: liquid escalates in gassed regions and descends in degassed regions (downcomer for airlifts). Mixing intensity depends on gas superficial velocity, distributor design, and column geometry. This mechanism replaces the role of impellers in STRs.

Step-by-Step Solution:

Verification / Alternative check:
Empirical correlations for kLa and circulation velocity scale with gas rate (Ug) in bubble columns/airlifts, confirming gas-driven mixing dominance.

Why Other Options Are Wrong:

• Mechanical moving parts: define a stirred system, not bubble columns/airlifts.
• External pumping: atypical and complicates sterility.
• Any of these equally: incorrect; gas sparging is primary.
• Vessel vibration: not a standard mixing mechanism.

Common Pitfalls:
Assuming identical mixing mechanisms across reactor types; ignoring the role of gas distributor design on circulation patterns.

Final Answer:
Compressed air (gas sparging and buoyancy-driven circulation)