Ammonia synthesis operating temperature:\nAlthough equilibrium favours low temperature, industrial ammonia converters run at moderately high temperatures (around 400–500 °C). What is the main reason low temperature is avoided?

Introduction / Context:
The synthesis of ammonia (N2 + 3 H2 ⇌ 2 NH3) is exothermic and equilibrium-favoured at low temperature and high pressure. However, converters typically operate near 400–500 °C (not “very low” temperature) because the reaction is kinetically slow. This question targets the classic trade-off between thermodynamic equilibrium and reaction kinetics in reactor design and operation.

Given Data / Assumptions:

• Fixed-bed iron-based or promoted catalysts are used.
• Pressures are high (typically 100–250 bar), but finite.
• Single-pass conversion is enhanced by interbed cooling and recycle, not by running at arbitrarily low temperature.

Concept / Approach:
While lowering temperature increases equilibrium NH3 content, it also reduces the rate constant dramatically (Arrhenius relationship). Industrial design finds an optimum temperature window where the rate is high enough to produce practical space-time yields while maintaining acceptable equilibrium levels. At too low a temperature, even with high pressure and good mixing, the space-time yield collapses, making the process uneconomic despite favourable equilibrium.

Step-by-Step Solution:

Verification / Alternative check:
Performance curves show that decreasing bed temperature below the optimal region reduces per-pass production severely; plants use quench or heat-exchange between beds to manage exotherms while staying in the kinetic “sweet spot”.

Why Other Options Are Wrong:

• Very high pressure (b) does improve equilibrium but is constrained by mechanics; it is not specifically “required” by low T alone to make the process feasible.
• Space velocity (c) is an operational choice; the root cause at low T is slow intrinsic kinetics.
• Poisoning (e) is not specifically a low-temperature phenomenon; many poisons are problematic regardless of T.

Common Pitfalls:
Confusing equilibrium improvement with practical productivity; overlooking Arrhenius control of rates in heterogeneous catalysis.

Final Answer:
The intrinsic rate of reaction becomes very low at low temperature