Temperature control strategy – highly exothermic solid-catalysed reactions For achieving high conversion in a highly exothermic, solid-catalysed reaction, which reactor sequence is preferred?

Introduction / Context:
Highly exothermic catalytic reactions pose hot-spot and runaway risks. Proper temperature management improves selectivity, avoids catalyst sintering, and enables high overall conversion. Combining different reactor types can exploit their strengths in heat removal and approach to equilibrium.

Given Data / Assumptions:

• Gas-phase, solid-catalysed, strongly exothermic reaction.
• Objective: high overall conversion with safe temperature control.
• Industrial-scale heat removal considerations are critical.

Concept / Approach:
Fluidised beds provide excellent heat transfer and near-isothermal operation, limiting hot spots during the most vigorous part of the reaction (high driving force). After the major exotherm is tamed and reactant concentration drops, a polish fixed bed can drive conversion higher with lower heat release rates per volume, simplifying temperature control.

Step-by-Step Solution:

Verification / Alternative check:
Analogous industrial strategies include multi-bed adiabatic reactors with intercooling or fluidised-bed front-ends followed by fixed beds for finishing steps.

Why Other Options Are Wrong:

• Fixed bed only: Prone to hot spots at the inlet where reaction rate is highest.
• Fixed → fluidised: Places the more temperature-sensitive step first; less optimal for hot-spot control.
• Fluidised only: Good temperature control but may not maximize final conversion economically without a polishing stage.

Common Pitfalls:
Ignoring heat removal limits and focusing solely on kinetics; for highly exothermic reactions, thermal management is often the bottleneck.

Final Answer:
Fluidised bed reactor followed by a fixed bed reactor