ANS:B - high cost of the high pressure vessel used for the reactor.

Multistage operation, as in the case of catalytic oxidation of SO2, is not typically carried out for NH3 synthesis primarily due to the chances of entrainment and disturbance of the catalyst bed. The catalytic synthesis of ammonia (NH3) from nitrogen (N2) and hydrogen (H2) gases occurs in a single pass through a fixed-bed reactor containing an iron-based catalyst. This process, known as the Haber-Bosch process, operates at high pressure (usually around 150-300 atmospheres) and moderate temperatures (around 400-550°C). Here's why multistage operation is not favored for NH3 synthesis:

1. Entrainment and Catalyst Bed Disturbance: In multistage operations, where the reactants pass through multiple reactor beds, there is a risk of entrainment, which refers to the carrying away of catalyst particles by the flowing gases. This can lead to the disturbance of the catalyst bed, reducing its effectiveness and causing operational issues.
2. Complexity and Cost: Implementing a multistage operation for NH3 synthesis would increase the complexity and cost of the process. It would require additional equipment, such as separators and intercoolers, to separate and cool the gas streams between stages. Additionally, maintaining multiple reactor beds and controlling the flow of gases between them would add to the operational complexity and cost.
3. Efficiency of Single-Stage Operation: The Haber-Bosch process has been optimized for single-stage operation, where the nitrogen and hydrogen gases react once over a fixed-bed catalyst to produce ammonia. This configuration has been proven to be efficient and reliable for large-scale ammonia production.

While high pressure drop, the cost of high-pressure vessels, and pumping costs are factors that influence the design and operation of NH3 synthesis plants, they are not the primary reasons for avoiding multistage operation. Instead, the risk of entrainment and disturbance of the catalyst bed is the key consideration in favor of single-stage operation for NH3 synthesis.