Urea yield in synthesis can be increased by using excess ammonia and higher pressure/temperature. In practice this is avoided primarily because of which combined drawbacks?

Introduction / Context:
Urea manufacture from ammonia and carbon dioxide is equilibrium-limited and sensitive to operating conditions. While excess ammonia and higher pressure/temperature can push conversion, plant designers balance yield against materials, by-product formation, and economics. Understanding these trade-offs is essential in fertiliser process design.

Given Data / Assumptions:

• Raising pressure and temperature and using excess NH3 can increase apparent conversion.
• Biuret forms more readily at higher temperature and longer residence.
• Corrosion in the urea carbamate environment is severe and worsens under harsher conditions.
• Higher P/T demands thicker, costlier equipment.

Concept / Approach:
Process intensification must consider product quality (biuret limits for agriculture), equipment life (corrosion/embrittlement), and capital/operating costs. The cumulative penalty often outweighs incremental yield gains, so industry uses optimized, not extreme, conditions and employs stripper/recirculation schemes instead of simply forcing equilibrium with P/T and NH3 excess.

Step-by-Step Solution:

Verification / Alternative check:
Vendor literature and design handbooks specify biuret limits and corrosion-resistant alloys (e.g., 25-22-2) to manage these risks, confirming the multi-factor constraint.

Why Other Options Are Wrong:

• Any single factor alone understates the practical limitation.
• None of these: contradicted by known issues.

Common Pitfalls:
Focusing solely on equilibrium without considering kinetics, corrosion, and specification limits for product biuret.

Final Answer:
all (a), (b) & (c)