Mixing intensity per volume — In which bioreactor is mixing per unit volume generally the poorest?

Introduction:
Mixing quality affects mass and heat transfer, gradients, and overall performance. Different reactor types provide different hydrodynamic environments. This question asks which configuration delivers the weakest mixing per unit volume under typical operating conditions.

Given Data / Assumptions:

• Packed bed with fixed particles and interstitial flow.
• Fluidized bed with particle motion and enhanced mixing.
• Airlift with gas-induced circulation providing moderate mixing.

Concept / Approach:
In a packed bed, fluid flows through narrow channels with limited transverse mixing and low turbulence levels; axial dispersion is minimized (approaching plug flow), but overall bulk mixing is inherently poor. Fluidized beds improve mixing through particle motion and bubbling, and air-lift reactors provide recirculation loops that enhance mixing relative to packed beds. Mechanically stirred tanks provide the strongest mixing among listed configurations.

Step-by-Step Solution:
Compare hydrodynamics: fixed packing restricts eddy formation and cross-mixing.Recognize that fluidization introduces vigorous mixing via particle–fluid interactions.Airlift circulation produces bulk loops, better than packed-bed interstitial flow.Therefore, packed bed has the poorest mixing per volume.

Verification / Alternative check:
Tracer studies show the smallest degree of back-mixing and the most pronounced concentration gradients in packed beds under laminar/transition flow, confirming limited mixing.

Why Other Options Are Wrong:

• (b) Fluidized beds are designed to enhance mixing.
• (c) Air-lift reactors generate circulation that improves mixing over packed beds.
• (d) Not applicable because option (a) is correct.
• (e) Agitated tanks have the highest mixing intensity among these.

Common Pitfalls:
Equating “less back-mixing” with “better mixing”; they are different metrics tied to different performance objectives.

Final Answer:
Continuous packed-bed reactor