Unlike catalytic SO2 oxidation, ammonia synthesis is not implemented as multiple separate reactor stages in series. What is the principal reason for avoiding true multistage operation in NH3 synthesis?

Introduction / Context:
Ammonia synthesis is equilibrium-limited and exothermic. Modern converters use internal multibed arrangements with interbed cooling or quenching within a single pressure shell. The question contrasts this with processes like SO2 oxidation that can deploy multiple separate beds in separate vessels.

Given Data / Assumptions:

• Ammonia reactors operate at high pressures (often >100 bar).
• Catalyst is iron-based with heat management via interbed exchangers or quench gas.
• Objective is to understand why not use multiple separate reactors in series.

Concept / Approach:
While pressure drop and pumping are relevant, the dominant constraint is economics: duplicating thick-walled high-pressure reactor shells is extremely costly. Therefore, designers place multiple catalyst beds inside one shell to achieve quasi-multistage temperature control and conversion while paying for only one pressure boundary and one set of costly high-pressure internals.

Step-by-Step Solution:
List potential drawbacks of true multistage vessels: more pressure boundaries, interconnecting high-pressure piping, additional safety systems.Identify the largest cost driver: multiple HP shells with thick walls dominate CAPEX.Recognize engineering practice: multibed internals inside one shell mitigate this.Select high capital cost of multiple HP shells as primary reason.

Verification / Alternative check:
Converter designs (e.g., three-bed radial-flow internals) achieve desired temperature profiles with minimal additional shell resources, confirming the economic rationale against separate staged vessels.

Why Other Options Are Wrong:
Pressure drop exists but is handled by internal design; it is not the primary barrier.Pumping cost is not a central factor in a loop dominated by compressor duty.Entrainment/bed disturbance are engineering issues but not the decisive reason versus shell cost.

Common Pitfalls:
Confusing internal multibed staging (common) with multiple separate pressure vessels (rare due to cost).

Final Answer:
High capital cost of multiple high-pressure reactor shells