Process Design: 23-Chem-A5 Chemical Plant Design and Economics, May 2014, Q1

Question 1: Process Design (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The question has two layers that a good answer must keep distinct: (a) the process flow sheet — the physical sequence of unit operations that turns air and water into pure ammonia — and (b) the project-development flow sheet — the sequence of engineering stages that carries the venture from a conceptual idea to a fully operational plant. Both are addressed below.

(a) Process flow sheet — how the chemistry drives the units

The stoichiometry $\mathrm{N_2+3H_2\rightarrow 2NH_3}$ says the plant must first manufacture the two reactant gases from the given feedstocks, react them, and then separate the product. The boiling-point table is the key to the separations, because every split in this process is a difference-in-volatility split:

Nitrogen from air. Air is a mixture of N₂ (b.p. −196°C) and O₂ (b.p. −183°C). These boil only 13°C apart, so the split needs a cryogenic air-separation unit (ASU): the air is compressed, dried, cooled to liquefaction and fractionally distilled, taking N₂ overhead and rejecting O₂ as a saleable by-product.
Hydrogen from water. With water as the only hydrogen source, H₂ is produced by electrolysis ($\mathrm{2H_2O\rightarrow 2H_2 + O_2}$), which also yields O₂ by-product.
Synthesis. The N₂ and H₂ are combined in the stoichiometric 1:3 ratio, compressed and passed over a promoted-iron catalyst in the ammonia converter at high temperature and pressure. Because the reaction is equilibrium-limited, single-pass conversion is low.
Product recovery and recycle. This is where the boiling points pay off: NH₃ boils at −33°C, far warmer than the unreacted H₂ (−253°C) and N₂ (−196°C). A single refrigerated condenser therefore drops liquid ammonia out of the reactor effluent while the H₂/N₂ stay gaseous and are recycled to the converter loop, with a small purge to bleed off inerts (argon carried in with the air).

Figure 1.1 — Qualitative process flow sheet for ammonia from air and water. Cryogenic distillation of air supplies N₂; electrolysis of water supplies H₂; the synthesis loop reacts them over an iron catalyst; a refrigerated condenser recovers liquid NH₃ (b.p. −33°C) while the far-lower-boiling H₂/N₂ are recycled (with a small inert purge).

(b) Project-development flow sheet — concept to operating plant

The project flow sheet is the ordered set of engineering stages, each with its own decision points, that P&T Ch. 1 calls the development of a design project. Written as a sequence:

Development stage	Key points to be considered
1. Conceptual idea / market survey	Product demand and price, plant capacity, competing routes, siting near feedstock/market; go / no-go on order-of-magnitude economics.
2. Laboratory & thermodynamic feasibility	Reaction equilibrium and kinetics, catalyst selection, yield, physical-property and phase-behaviour data (the boiling points here).
3. Process synthesis / flow-sheet development	Choose and sequence unit operations (ASU, electrolysis, converter, condenser, recycle); fix the block flow diagram.
4. Material & energy balances	Close the mass and energy balances around every unit; set stream flows, compositions, temperatures and pressures.
5. Equipment sizing & mechanical design	Size reactors, columns, compressors, exchangers; select materials of construction; apply codes (ASME BPVC for the high-pressure loop).
6. Cost estimation & profitability	Capital (fixed + working) and operating cost estimates; return on investment, payout, discounted-cash-flow analysis.
7. Detailed engineering (PFD → P&ID)	Piping, instrumentation and control, plot plan, utilities, electrical; issue for-construction drawings.
8. Safety, loss-prevention & environmental review	HAZOP, relief-system design, emissions and effluent permits (Canadian CEPA / provincial approvals).
9. Procurement & construction	Purchase long-lead equipment, fabricate, erect, inspect and test.
10. Commissioning, start-up & operation	Pre-commissioning checks, catalyst loading, controlled start-up, performance test, then steady operation and continual optimization.

Check: the answer assumes hydrogen is made on-site by water electrolysis, which is faithful to the stated feedstock ("air and liquid water"). A commercial ammonia plant would normally make H₂ by steam–methane reforming; if natural gas were an allowed feedstock the H₂ block would change, but the separation logic (condense NH₃, recycle H₂/N₂) is unchanged.

Concept & Theory

Topic. Process synthesis by volatility differences, and the structured development of a design project.

Key relations. Ammonia synthesis $\mathrm{N_2+3H_2\rightleftharpoons 2NH_3}$ (exothermic, mole-reducing — favoured by high pressure, limited by high temperature); separation feasibility is governed by relative volatility, i.e. the ratio of vapour pressures inferred from boiling points.

Why this works. Every split in the flow sheet is a boiling-point split: close-boiling O₂/N₂ demand cryogenic distillation, whereas the wide gap between NH₃ (−33°C) and the recycle gases lets a single refrigerated condenser recover product cheaply. Because single-pass conversion is equilibrium-limited, the recycle loop is essential to overall yield.

Common pitfall. Answering only the chemistry and forgetting the project-development sequence the question explicitly asks for — or vice-versa. A second trap is omitting the inert purge; without it argon accumulates in the loop and starves the reaction.

Source. Peters, Timmerhaus & West, Ch. 1 (development of a design project) and Ch. 3 (general design considerations); Turton et al., Ch. 1–2 (flowsheet synthesis).

Question 1 of 6: Process Design

Question 1: Process Design (20 marks)

(a) Process flow sheet — how the chemistry drives the units

(b) Project-development flow sheet — concept to operating plant