Mixing patterns — In which reactor configuration are mixing profiles closest to ideal plug flow (minimal back-mixing)?

Introduction:
Reactor performance depends on hydrodynamics. Ideal plug flow has negligible axial mixing and a narrow residence-time distribution, while stirred tanks approach complete back-mixing. This question identifies which configuration best approximates plug flow in practice.

Given Data / Assumptions:

• Steady operation with constant flow.
• Non-channeling, well-designed packed bed with uniform packing.
• Comparable scales across options.

Concept / Approach:
In a packed bed, fluid moves through tortuous interstitial pathways with limited axial dispersion, approaching plug-flow behavior at suitable Reynolds and Peclet numbers. Fluidized beds suspend particles and increase mixing; stirred tanks with or without recycle are highly back-mixed. Therefore, packed beds most closely approach plug flow among the listed choices.

Step-by-Step Solution:
Compare dispersion: packed bed has low axial dispersion when designed properly.Stirred tank (with recycle) intentionally promotes mixing, broadening residence times.Fluidized bed adds particle motion, enhancing axial mixing relative to packed bed.Conclude packed bed is closest to plug flow.

Verification / Alternative check:
Tracer tests yield residence-time distributions: packed beds show narrow E(t) functions; CSTRs show exponential decay; fluidized beds lie in between. Modeling with dispersion coefficients confirms smallest axial dispersion for packed beds under non-channeling conditions.

Why Other Options Are Wrong:

• (b) and (e) are mixing-dominant systems; recycle further increases back-mixing.
• (c) More mixed than packed beds due to solids motion.
• (d) Not equal; designs differ fundamentally.

Common Pitfalls:
Assuming all packed beds behave ideally; ignoring channeling or maldistribution that can degrade plug-flow performance.

Final Answer:
Continuous packed-bed reactor