Effect of pressure on urea formation step:\nAs reactor pressure increases, how does the equilibrium conversion of ammonium carbamate to urea change, all else equal?

Introduction / Context:
Urea synthesis proceeds via two reactions: rapid formation of ammonium carbamate (favoured by high pressure and excess NH3) and slower dehydration of carbamate to urea and water. Understanding how pressure influences the second, equilibrium-limited step helps explain why process licensors optimise not only pressure but also temperature and NH3/CO2 ratio.

Given Data / Assumptions:

• Key step: NH2COONH4 ⇌ (NH2)2CO + H2O.
• Number of moles increases from 1 to 2 in the dehydration step.
• Temperature and composition effects are held constant for this comparison.

Concept / Approach:
By Le Chatelier’s principle, increasing pressure favours the side with fewer total moles. Since the carbamate-to-urea step increases the number of moles, higher pressure shifts equilibrium back toward ammonium carbamate, decreasing urea formation at fixed temperature and composition. In practice, high pressure is still used overall to facilitate carbamate formation and maintain a liquid phase, but the intrinsic equilibrium of the dehydration step itself is disfavoured by pressure.

Step-by-Step Solution:

Verification / Alternative check:
Thermodynamic analyses and design texts discuss opposing effects: pressure aids carbamate formation but disfavors dehydration equilibrium. Process schemes manage this with temperature control and NH3 stripping/recirculation.

Why Other Options Are Wrong:

• Increase/unaltered: contradict equilibrium mole-count logic.
• “Depends only on biuret”: biuret is a side reaction at high T; not the governing factor for pressure dependence.
• “Increases then decreases”: not the standard monotonic equilibrium response to pressure for this stoichiometry.

Common Pitfalls:
Conflating overall plant pressure benefits with the specific equilibrium of the dehydration step; overlooking mole-count effects.

Final Answer:
It decreases