Industrial urea synthesis — Urea is formed only under which physical condition of the reacting mixture (ammonia + carbon dioxide via ammonium carbamate)? Choose the most accurate statement about the phase in which urea actually forms in commercial high-pressure synthesis.

Introduction:
Modern urea plants synthesize urea from ammonia and carbon dioxide in two steps: fast formation of ammonium carbamate followed by its dehydration to urea and water. Understanding the phase in which urea actually forms is vital for reactor design, conversion control, and corrosion/energy considerations.

Given Data / Assumptions:

• Synthesis loop operates typically at 140–200 bar and about 170–200°C.
• Reaction mixture is a concentrated aqueous solution of NH3, CO2, and ammonium carbamate.
• No catalytic solid bed is employed; contacting is liquid-phase.

Concept / Approach:
Step 1: NH3 + CO2 → NH2COONH4 (ammonium carbamate) is rapid and exothermic in solution. Step 2: NH2COONH4 → NH2CONH2 + H2O (urea + water) is reversible and relatively slow, proceeding in the liquid medium. Gas-phase urea formation is negligible under practical conditions because the concentrations and collision efficiency required are not met; instead, the high-pressure liquid solution provides intimate molecular contact and solvates ionic intermediates.

Step-by-Step Solution:
Recognize the synthesis loop is a high-pressure liquid solution.Identify that both carbamate formation and dehydration proceed in this liquid phase.Conclude: urea forms only in the liquid phase in commercial practice.

Verification / Alternative check:
Process flow diagrams show liquid-phase reactors/strippers with recirculated aqueous ammonia and carbamate; no gas-phase reactor is used. Equilibrium/kinetic models are expressed for liquid compositions and activities.

Why Other Options Are Wrong:

• Vapour phase: impractical and not used; conversion would be negligible.
• Very high temperature alone does not ensure formation and can destabilize the system.
• Vacuum is used in downstream concentration, not in synthesis.
• Solid-state reaction does not describe the commercial route.

Common Pitfalls:
Confusing downstream evaporation/concentration (often under vacuum) with the synthesis step; assuming “gas synthesis” like ammonia applies here.

Final Answer:
In liquid phase (high-pressure solution)