Urea process chemistry:\nThe dehydration of ammonium carbamate to yield urea in high-pressure synthesis loops is best characterized as which type of reaction step?

Introduction / Context:
Modern urea plants produce urea via a two-step sequence: rapid formation of ammonium carbamate from ammonia and carbon dioxide, followed by its dehydration to urea with release of water. Distinguishing the thermal nature of each step is essential for energy integration and equipment design in high-pressure synthesis sections.

Given Data / Assumptions:

• Step 1: 2 NH3 + CO2 → NH2COONH4 (exothermic, equilibrium-limited).
• Step 2: NH2COONH4 → NH2CONH2 + H2O (dehydration to urea).
• No solid catalyst is used in standard high-pressure urea synthesis.

Concept / Approach:
The dehydration of ammonium carbamate to urea requires heat input and proceeds more readily at elevated temperatures under high pressure to keep reactants in the liquid phase. This step is endothermic, in contrast to the preceding carbamate formation which is exothermic. Plants therefore recover heat from exothermic sections and supply heat to drive dehydration, while carefully controlling pressure and residence time to achieve the desired conversion before downstream recovery and recycling.

Step-by-Step Solution:

Verification / Alternative check:
Plant heat balances show duty added in the synthesis section for dehydration, with coolers handling the earlier exotherm from carbamate formation.

Why Other Options Are Wrong:

• Exothermic: applies to carbamate formation, not dehydration.
• Autocatalytic: no product-driven catalysis is involved.
• Catalytic: standard designs do not rely on solid catalysts for this step.
• Athermal: contradicted by measured heat duties.

Common Pitfalls:
Confusing the thermodynamics of the two sequential steps; remembering “first exothermic, then endothermic” helps avoid mistakes.

Final Answer:
Endothermic.