Process Design: 23-Chem-A5 Chemical Plant Design and Economics, December 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.

As in every plant-design question, the answer has two distinct layers that must be kept apart: (a) the process flow sheet — the physical sequence of unit operations that turns the CO/H₂ syngas into pure methanol — and (b) the project-development flow sheet — the ordered engineering stages that carry 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 $\mathrm{CO+2H_2\rightarrow CH_3OH}$ fixes the feed as a 2:1 H₂:CO syngas, and the boiling-point data supplied are the key to the separation, because the one split in this process is a difference-in-volatility split:

Feed conditioning and compression. The 2:1 H₂:CO makeup gas is purified of catalyst poisons (sulphur, chlorides, carbonyls) and compressed to the reaction pressure of 80 bar. Compression dominates the utility cost of a methanol loop.
Synthesis in a slurry reactor. The gas is passed over the catalyst suspended in an inert oil (slurry-phase) at 80 bar and 180°C. The slurry's high heat capacity absorbs the strongly exothermic heat of reaction almost isothermally, which is the reason a slurry reactor is specified over a fixed bed. Because the reaction is equilibrium-limited at these conditions, single-pass conversion of CO is modest (typically 20–60 %).
Product recovery by cooling. This is where the boiling points pay off: methanol boils at $+65\,{}^{\circ}\mathrm{C}$, hundreds of degrees above the unreacted CO (b.p. $\approx\!-191\,{}^{\circ}\mathrm{C}$, close to N₂) and H₂ (b.p. $-253\,{}^{\circ}\mathrm{C}$). A single refrigerated condenser / separator therefore drops liquid crude methanol out of the reactor effluent while the CO and H₂ stay gaseous.
Recycle and purge. The uncondensed CO/H₂ is recycled to the loop to lift the overall conversion, with a small purge to bleed inerts (CO₂, CH₄, N₂) that would otherwise accumulate.
Purification. The crude methanol is finished in a distillation column that strips dissolved light ends and rejects water and higher alcohols as bottoms, delivering pure product methanol overhead.

Figure 1.1 — Qualitative process flow sheet for methanol from 2:1 H₂:CO syngas. The gas is compressed to 80 bar, reacted over a slurry catalyst at 180°C, and a refrigerated condenser recovers crude methanol (b.p. $+65\,{}^{\circ}\mathrm{C}$) while the far-lower-boiling CO/H₂ are recycled (with a small inert purge); a distillation column finishes the product.

(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	Methanol demand and price, plant capacity, syngas source (reformed natural gas vs. purchased CO/H₂), siting near feedstock and market; go / no-go on order-of-magnitude economics.
2. Laboratory & thermodynamic feasibility	Reaction equilibrium and kinetics, Cu/ZnO catalyst selection and life, single-pass conversion, physical-property and phase-behaviour data (the boiling points here).
3. Process synthesis / flow-sheet development	Choose and sequence unit operations (feed compression, slurry reactor, condenser/separator, recycle, distillation); fix the block flow diagram.
4. Material & energy balances	Close mass and energy balances around every unit; set stream flows, compositions, temperatures and pressures, and the recycle ratio.
5. Equipment sizing & mechanical design	Size the reactor, column, compressors and exchangers; select materials of construction; apply codes (ASME BPVC for the 80-bar 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, CO-toxicity and flammability controls, 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 and reduction, controlled start-up, performance test, then steady operation and continual optimization.

Check: the answer takes the CO/H₂ syngas as a purchased/upstream-supplied feed, exactly as the question states ("a gas mixture containing CO and hydrogen"). If the syngas were instead generated on-site by steam–methane reforming, an additional reforming and gas-conditioning section would precede the compressor, but the synthesis-and-recovery logic (react at 80 bar/180°C, condense methanol, recycle CO/H₂) is unchanged. The N₂/O₂ boiling points supplied are reference values for the volatility scale; they enter only as potential inert-purge components, since the specified feed is CO/H₂, not air.

Concept & Theory

Topic. Process synthesis by volatility differences and equilibrium-limited recycle, plus the structured development of a design project.

Key relations. Methanol synthesis $\mathrm{CO+2H_2\rightleftharpoons CH_3OH}$ (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; overall conversion is lifted above the single-pass value by the recycle ratio.

Why this works. The single product/reactant split is a boiling-point split: methanol at $+65\,{}^{\circ}\mathrm{C}$ condenses out cleanly while CO ($\approx\!-191\,{}^{\circ}\mathrm{C}$) and H₂ ($-253\,{}^{\circ}\mathrm{C}$) remain gaseous, so one refrigerated condenser suffices and the gas is recycled to overcome the equilibrium limit. The slurry reactor is chosen because its liquid heat sink removes the exotherm nearly isothermally, protecting the catalyst and the equilibrium.

Common pitfall. Answering only the chemistry and forgetting the project-development sequence the question explicitly asks for (or vice-versa); and omitting the inert purge, without which CO₂/CH₄/N₂ accumulate in the loop and choke 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