In industrial urea manufacture for fertilizers, which specific pair of feedstocks directly react in the high-pressure synthesis section to form urea (after ammonium carbamate formation)?

Introduction / Context:
Urea is the world’s most widely used nitrogenous fertilizer. In plant flowsheets, urea is produced by reacting ammonia and carbon dioxide at high pressure and temperature. Although the ammonia itself typically originates from synthesis gas made using nitrogen (N2) and hydrogen (H2), the immediate, direct reactants in the urea synthesis loop are ammonia (NH3) and carbon dioxide (CO2). Understanding this distinction helps avoid common exam traps that conflate upstream ammonia production with the downstream urea synthesis step.

Given Data / Assumptions:

• We are concerned with the direct chemistry inside the urea reactor/stripper section.
• Standard high-pressure urea process conditions apply (formation of ammonium carbamate followed by dehydration).
• Feed purification, recycles, and off-gas treatments are in place but not directly relevant to the identity of reactants.

Concept / Approach:
Urea synthesis proceeds in two steps. First, NH3 reacts with CO2 to form ammonium carbamate. Then, ammonium carbamate dehydrates to urea and water. Symbolically: 2 NH3 + CO2 → NH2COONH4 → NH2CONH2 + H2O. Therefore, the essential immediate reactants are NH3 and CO2, not N2 or H2, which are associated with the prior ammonia plant.

Step-by-Step Solution:
Recognize urea section chemistry is separate from ammonia synthesis.Write the overall urea reaction: 2 NH3 + CO2 → urea + H2O (via carbamate).Identify the direct feed pair: NH3 and CO2.Exclude species used only upstream (N2/H2) or in other fertilizer routes (HNO3 with CaCO3).

Verification / Alternative check:
Standard process descriptions for Stamicarbon, Snamprogetti/Saipem, and other licensors all specify NH3 and CO2 as the feed to the urea synthesis loop. PFDs show CO2 from the ammonia plant’s shift/CO2 removal section entering the urea unit to react with liquid ammonia.

Why Other Options Are Wrong:
CO2 and N2: N2 is used earlier to make NH3, not in the urea reactor.NH3 and CO: CO is an impurity/poison, not a urea feed.HNO3 and CaCO3: those relate to nitrate/limestone chemistries, not urea synthesis.

Common Pitfalls:
Confusing the ammonia plant’s N2/H2 chemistry with the subsequent urea loop, and assuming nitrogen gas participates directly in the urea reactor.

Final Answer:
Ammonia (NH3) and carbon dioxide (CO2)