In biochemical engineering, a stirred-tank reactor (STR) mixes its contents primarily by a powered impeller system. Which device is chiefly responsible for bulk mixing and suspension of cells/particles in an STR?

Introduction / Context:
A stirred-tank reactor (STR) is the workhorse of bioprocessing. Its hallmark is an internal mechanical agitation system that provides bulk mixing, solid suspension, and gas–liquid mass transfer when aerated. This question asks which device is chiefly responsible for mixing inside a standard STR.

Given Data / Assumptions:

• Vessel equipped with one or more impellers and baffles.
• Objective is to mix liquid phases and, when present, disperse gas and suspend cells/solids.
• Focus on the primary, built-in mixing mechanism of an STR, not on alternative recirculation schemes.

Concept / Approach:
Mechanical agitation driven by a motor–shaft–impeller assembly generates turbulence and macro-circulation loops. Baffles suppress vortex formation and enhance axial and radial flow, producing uniform composition and temperature. Compressed air may be introduced through a sparger in aerobic cultures, but in an STR it is the impeller that distributes bubbles and closes the mixing loop.

Step-by-Step Solution:

Verification / Alternative check:
Power number correlations (Np), tip speed, and Reynolds number (Re) for impellers are standard tools to predict STR mixing performance—further evidence that agitation is the principal mixing driver.

Why Other Options Are Wrong:

• Compressed air alone: in bubble columns it mixes; in STRs, it is secondary.
• External pumping: adds complexity and contamination risk.
• Any of these equally: incorrect; the impeller is primary.
• Static mixers: not typical inside an STR for bulk mixing.

Common Pitfalls:
Confusing air-lift/bubble column mixing with STR mixing; assuming aeration alone ensures homogeneity in viscous broths.

Final Answer:
A mechanical agitator (impeller with baffles)