An ammonia - liquefaction unit produces liquid NH₃ that is sent to storage at 298 K and 10 bar. The process sequence is simplified as follows:
Step
Chemical transformation
ΔH (kJ mol⁻¹)
1
N₂(g) + 3 H₂(g) → 2 NH₃(g)
–92.4
2
NH₃(g) → NH₃(l) (298 K)
–23.3
Additional information:
• The plant produces 1.00 × 10⁵ tonnes of liquid NH₃ per year.
• 90 % of the gaseous NH₃ leaving the synthesis reactor is sent to a refrigeration train where it condenses (Step 2). The remaining 10 % is recycled directly back to the reactor without condensation.
Neglect heat losses to the surroundings and assume steady-state operation.
(a) Derive the overall balanced chemical equation that represents the net production of liquid NH₃, taking the recycle stream into account.
(b) Calculate the net enthalpy change per mole of liquid NH₃ that actually leaves the plant as product.
(c) Determine the total annual thermal energy released or absorbed by the plant because of the reactions and phase change, expressing your answer in gigajoules (GJ) to three significant figures.
(d) The condensed NH₃ is stored in insulated tanks, but the recycle loop operates at 800 K. Propose one modification to the heat-exchange network that would improve the plant’s overall energy efficiency, and justify your suggestion based on your results from parts (b) and (c).