Reactor classifications by agitation method: Which of the following are non-mechanically agitated bioreactors (i.e., mixing is achieved without a rotating impeller)?

Introduction / Context:
Bioreactors can be mixed by mechanical impellers or by gas-driven circulation. Non-mechanical systems avoid moving shafts and seals, often reducing shear and maintenance while relying on gas flow for mixing and mass transfer.

Given Data / Assumptions:

• Bubble columns rely on rising gas bubbles to induce circulation.
• Airlift reactors use riser and downcomer sections (with or without a draft tube) to create loop flow.
• Stirred tank reactors employ powered impellers.

Concept / Approach:
Non-mechanical agitation is defined by the absence of rotating mechanical elements inside the vessel. Bubble columns and airlifts satisfy this definition; their hydrodynamics arise from gas hold-up differences and buoyancy-driven circulation, not impeller rotation.

Step-by-Step Solution:
Identify STR as mechanically agitated (reject option a).Recognize bubble column mixing via gas sparging-induced eddies (accept b).Recognize airlift loop circulation via density differences (accept c).Therefore, choose Both (b) and (c).

Verification / Alternative check:
Design literature classifies bubble columns and airlifts as gas–liquid contactors without internal moving parts; they are widely used for shear-sensitive cultures and large-scale oxidations.

Why Other Options Are Wrong:
STR: explicitly mechanical, with impellers and seals.

Either single choice (b) or (c) alone omits the other non-mechanical class.

Common Pitfalls:

• Assuming airlift reactors contain impellers; the “draft tube” guides flow but is not a rotating agitator.
• Underestimating shear levels; gas-driven systems can still be shearing near spargers.

Final Answer:
Both (b) and (c)