23-Chem-A5 Chemical Plant Design and Economics · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2019 — 16-Chem-A5 Chemical Plant Design and Economics. Three-hour closed-book examination; one aid sheet (both sides) and an approved calculator are permitted. Six questions are printed and any five constitute a complete paper (each worth 20 marks); all six are solved below for completeness. Three questions carry numbers (Q1 route economics, Q3 production cost, Q4 depreciation); the other three (Q2 supercritical extraction, Q5 the design hierarchy intrinsic to a chemical process, Q6 VOC-abatement P&ID) are answered as structured description with a supporting diagram where the paper asks for one.
Reference texts: M. S. Peters, K. D. Timmerhaus & R. E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill) — total-product-cost anatomy, straight-line depreciation, after-tax cash flow, profitability; R. Turton, R. C. Bailie, W. B. Whiting & J. A. Shaeiwitz, Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — the economic-potential screen, reaction-path selection and the process flow diagram; J. M. Douglas, Conceptual Design of Chemical Processes (McGraw-Hill) — the level-2 economic-potential hierarchy and the balanced vinyl-chloride process; G. Towler & R. Sinnott, Chemical Engineering Design (Coulson & Richardson Vol. 6, 2nd ed.) — utilities, VOC control and product recovery; R. H. Perry & D. W. Green, Perry’s Chemical Engineers’ Handbook (9th ed.) — supercritical-fluid extraction. Depreciation is worked in the U.S. MACRS/straight-line framework the question specifies; the Canadian CCA declining-balance analogue is noted where relevant.
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.
A supercritical fluid is held above its critical temperature and pressure, where it has no distinct vapour–liquid boundary; carbon dioxide (critical point $31\,{}^{\circ}\text{C}$, $73.8\,\text{bar}$) is by far the most common. In this state the fluid has a liquid-like density (so it dissolves solutes like a solvent) but a gas-like viscosity and diffusivity (so it penetrates a matrix and equilibrates quickly). Crucially, the solvent power of an SCF is a strong function of pressure, because near the critical point the density — and hence the solubility of a solute — changes steeply with modest pressure changes. This tunability is the source of most of its advantages over a conventional liquid solvent.
Advantages. (1) Solvent-free product. On expansion to atmospheric pressure the CO₂ simply flashes off as a gas, leaving essentially no solvent residue in the product — decisive for food, flavour, nutraceutical and pharmaceutical products where trace organic solvent is unacceptable (decaffeination and hops extraction are classic examples). (2) Tunable, selective separation. Because solubility tracks density, the extraction and the subsequent solute recovery can both be driven by pressure (and temperature) alone; dropping the pressure precipitates the solute, so no energy-intensive distillation of a high-boiling solvent is needed. (3) Mild temperatures. Operating near ambient temperature protects heat-sensitive and oxidisable products that would degrade in a distillation reboiler. (4) Favourable transport properties. The low viscosity and high diffusivity give fast mass transfer, better matrix penetration and lower stage heights than a viscous liquid solvent. (5) A benign solvent. CO₂ is non-toxic, non-flammable, cheap, abundant and easily recycled in a closed loop, avoiding the toxicity, flammability and disposal burdens of benzene or chloroform.
Disadvantages. (1) High-pressure equipment. Operating at 100–400 bar demands thick-walled vessels, high-pressure pumps/compressors and stringent safety systems, so both the capital cost and the mechanical complexity are high. (2) Compression energy. Recompressing the recycled CO₂ consumes significant power, which partly offsets the distillation energy that was saved. (3) Limited solvent strength and polarity. Supercritical CO₂ is essentially non-polar, so it extracts lipophilic species well but dissolves polar or high-molecular-weight solutes poorly; a co-solvent (entrainer) such as ethanol is often needed, which reintroduces a separation step. (4) Scarce, non-linear design data. Phase behaviour near the critical point is highly non-ideal and process-specific, so reliable solubility data and models are limited and pilot work is usually required. (5) Throughput and economics. The technology is generally justified only for higher-value, lower-tonnage products; for bulk commodities a conventional solvent train is usually cheaper. On balance, supercritical CO₂ extraction wins where product purity, thermal sensitivity and solvent-residue limits matter more than capital cost.
