In industrial urea manufacture (NH3 + CO2 → ammonium carbamate → urea), what is the effect of increasing the NH3/CO2 molar ratio in the synthesis section?

Introduction / Context:
Urea plants first form ammonium carbamate from ammonia and carbon dioxide, then dehydrate it to urea. Manipulating the NH3/CO2 ratio affects equilibria, corrosion, and physical properties. Understanding how excess ammonia influences conversion is central to process optimization and energy balance in high-pressure synthesis loops.

Given Data / Assumptions:

• Operating at high pressure and moderate temperature in the synthesis zone.
• Carbamate formation is favored by excess NH3.
• Downstream recovery strips/recycles unconverted NH3 and CO2.

Concept / Approach:
Raising the NH3/CO2 ratio drives formation of ammonium carbamate and subsequently favors the dehydration route to urea, increasing the single-pass conversion of CO2. Although excess ammonia is later recovered, within the synthesis section this ratio shift improves conversion and can also beneficially affect melt properties (e.g., reduce specific volume/viscosity), but the primary textbook takeaway is the conversion gain for CO2.

Step-by-Step Solution:

Verification / Alternative check:
Process design literature cites higher NH3/CO2 improving CO2 conversion at the expense of larger recycle duties—managed by stripper/condenser sections.

Why Other Options Are Wrong:

• Decreased NH3 conversion or overall yield: inconsistent with equilibrium shift toward carbamate/urea.
• Decreased specific volume only: while melt properties may change, the dominant effect tested is conversion increase.
• Ammonia-free operation: not applicable.

Common Pitfalls:
Ignoring the recycle loop: higher NH3/CO2 aids conversion but increases ammonia recovery load; the question focuses on conversion effect within synthesis.

Final Answer:
Increased degree of conversion of CO2 to urea