Urea plant front end: What is the composition and physical state of the fresh feed entering the high-pressure urea synthesis autoclave (reactor)?

Introduction / Context:
Commercial urea synthesis operates at high pressure and uses two steps: formation of ammonium carbamate from NH3 and CO2, followed by its dehydration to urea and water. The entering feed composition and phase are deliberately chosen to favor carbamate formation and control corrosion and heat balance. Recognizing the correct feed state helps interpret process flow diagrams and operating procedures.

Given Data / Assumptions:

• High-pressure synthesis loop operating typically at 130–180 bar and elevated temperature.
• NH3 is fed in excess to push equilibrium toward carbamate formation.
• CO2 is available from the upstream ammonia plant and is densified.

Concept / Approach:
Industry practice is to feed excess liquid ammonia together with liquefied (dense phase) carbon dioxide into the autoclave. At these pressures and moderate temperatures, CO2 can be handled in a liquid/dense phase, and ammonia is kept as a liquid as well. The excess of NH3 improves kinetics and conversion to carbamate and moderates corrosion. Therefore, “excess liquid ammonia and liquefied CO2” characterizes the fresh feed correctly.

Step-by-Step Solution:
Identify objective: efficient carbamate formation in HP section.Set NH3 to excess to drive equilibrium and stabilize operation.Feed CO2 in liquefied form under HP to facilitate mixing and reaction.Hence, choose “excess liquid ammonia and liquefied CO2.”

Verification / Alternative check:
Licensor documentation (e.g., Stamicarbon/Saipem concepts) illustrates liquid NH3 feed and dense CO2 feed lines merging prior to or inside the autoclave/reactor, with interstage heat recovery and HP carbamate condensers.

Why Other Options Are Wrong:
Compressed CO2 gas: technically CO2 can be gaseous, but plants densify it; the textbook answer emphasizes liquefied CO2 handling.Options with “compressed ammonia gas” misrepresent the usual NH3 liquid feed regime in HP urea synthesis.

Common Pitfalls:
Confusing the downstream stripper/recycle sections with the HP feed point; overlooking why excess NH3 is used (conversion and material compatibility).

Final Answer:
Excess liquid ammonia and liquefied CO2