Urea manufacturing chemistry (biuret control): In molten urea processing, what is the effect of operating at a lower temperature with a large excess of ammonia on biuret formation?

Introduction:
Biuret is a by-product formed when two urea molecules condense, releasing ammonia. Excessive biuret in urea fertilizer is undesirable because it can harm sensitive crops, especially at seedling stages. Process conditions strongly influence the extent of biuret formation.

Given Data / Assumptions:

• Molten urea stage where side reactions can occur.
• Variables: temperature and ammonia (NH3) excess.
• Goal: minimize biuret content in the product.

Concept / Approach:
Biuret forms via thermal condensation: 2 urea → biuret + NH3. The reaction is favored at higher temperatures and when free ammonia is scarce. Operating at lower temperatures suppresses the endothermic condensation. A high NH3 partial pressure shifts equilibria to favor urea and discourage further condensation to biuret and higher condensates (e.g., triuret, cyanuric species).

Step-by-Step Solution:
Reduce temperature of the urea melt to slow thermal condensation kinetics.Maintain a large excess of ammonia to shift the reaction away from biuret formation.Result: biuret concentration decreases in the final urea product.

Verification / Alternative check:
Plant guidelines specify temperature control and NH3 excess in the synthesis/finishing sections to keep biuret within specification (often below ~1% for prilled/granular product depending on grade).

Why Other Options Are Wrong:

• Increases or no effect: Opposite to established kinetics and equilibrium.
• “Is undesirable”: Condition itself is actually desirable for minimizing biuret.
• Cyanuric acid comment: Not the primary controlled pathway in this context.

Common Pitfalls:
Allowing hot holdup and long residence times; both raise biuret. Also, insufficient NH3 recycle can worsen condensation products.

Final Answer:
decreases biuret formation