Reactor selection — maximizing gas-phase conversion For a positive-order gas-phase catalytic reaction, which reactor type generally achieves the highest conversion per unit volume under comparable conditions?

Introduction / Context:
For most irreversible, positive-order reactions, reactor choice profoundly affects conversion for a given volume. Ideal plug-flow behavior avoids back-mixing and maintains high reactant concentration near the inlet, improving overall integral conversion.

Given Data / Assumptions:

• Single-pass, steady operation.
• Comparable catalyst activity and temperature control.
• Reaction of positive overall order.

Concept / Approach:
The space-time–conversion relationship shows that, for positive-order kinetics, the ideal PFR requires less volume than the ideal CSTR to reach a specified conversion. Back-mixing in fluidised or mixed reactors lowers the average driving force for reaction, reducing conversion for the same volume.

Step-by-Step Solution:
Compare ideal models: PFR vs CSTR for positive-order reaction.Note that PFR experiences highest concentration at inlet, maximizing reaction rate initially.Conclude: plug-flow catalytic reactor gives the highest conversion per unit volume.

Verification / Alternative check:
Volume–conversion graphs (Levenspiel plots) confirm the superior performance of PFR over CSTR for n > 0.

Why Other Options Are Wrong:

• Fixed-bed mixed-flow and fully back-mixed fluidised beds emulate CSTR behavior → lower conversion for same volume.
• Semi-fluidised beds fall in between and still involve mixing penalties.

Common Pitfalls:

• Ignoring heat/mass-transfer limits; the comparison assumes comparable effectiveness factors.

Final Answer:
Plug-flow catalytic reactor