23-Chem-A5 Chemical Plant Design and Economics · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 04-Chem-A5 Chemical Plant Design and Economics. Three-hour, open-book exam; any non-communicating calculator permitted. Six equally weighted questions are posed and the candidate answers any five; only the first five are marked. All six are answered below for completeness. Questions 1, 3 and 6 are conceptual design / management questions answered as organised prose; questions 2, 4 and 5 contain the numerical work (production capacity and pricing, simple- and compound-interest loan accounting, and sinking-fund depreciation) and every boxed figure.
Reference texts: M.S. Peters, K.D. Timmerhaus & R.E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill) — the exam's named primary text (cost estimation, interest and investment, depreciation, profitability, process synthesis, and plant safety); R. Turton et al., Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — flowsheet synthesis, separation selection, and safety; W.D. Seider et al., Product and Process Design Principles (3rd ed., Wiley) — separation-train synthesis; supporting Canadian tax practice from the Canada Revenue Agency Capital Cost Allowance classes and the half-year rule.
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.
1. Storage of flammable and toxic materials (tank farms and bulk storage). These hold the largest inventories on site, so a leak or fire has the greatest potential consequence — a boiling-liquid expanding-vapour explosion (BLEVE), a vapour-cloud explosion, or a large toxic release. The hazard scales with the quantity stored.
2. Reactors, especially exothermic ones. Reactors concentrate chemical energy; a loss of cooling or a runaway exothermic reaction can raise temperature and pressure faster than relief can cope, causing rupture or explosion. Most serious process incidents originate in the reaction step.
3. Fired equipment and hot surfaces (furnaces, fired heaters, boilers). These combine open flame or high surface temperatures with fuel and, often, flammable process streams — an ignition source next to combustibles, with firebox-explosion and tube-rupture potential.
4. High-pressure equipment and pressure vessels / gas holders. Stored mechanical (pressure) energy makes any failure violent; overpressure from a blocked outlet, external fire, or thermal expansion can burst a vessel or line if relief is inadequate.
5. Loading, unloading and transfer operations (and rotating equipment such as compressors/pumps handling flammables). Frequent connection and disconnection, human involvement, and the possibility of leaks, static discharge, or seal failure make these transfer points and machines statistically prone to releases and ignition.
(Utility failures — power, cooling water, instrument air — and confined spaces are further recognised hot spots because their failure can simultaneously disable several safeguards.)
The risks at these hot spots are minimised by a systematic, layered approach rather than any single fix. The steps, and the tools used at each, are:
Step 1 — Identify the hazards systematically. Use structured hazard-identification tools: HAZOP studies, what-if and checklist reviews, failure-modes-and-effects analysis (FMEA), and, where consequences warrant, quantitative risk assessment (QRA) with fault-tree and event-tree analysis. These surface the credible incident scenarios at each hot spot.
Step 2 — Design them out (inherently safer design). Apply the hierarchy of controls: first eliminate or substitute (reduce inventories, use less hazardous materials, milder conditions), then engineer safeguards — pressure-relief valves and rupture discs sized to the governing case (per API 520/521) and routed to flare or scrubber, emergency cooling and quench, dikes and bunds around storage, spacing and fire walls, interlocks and a safety-instrumented system (SIS) designed to the required SIL.
Step 3 — Add detection and protection layers. Gas, flame and fire detection; firewater, foam and deluge systems; blast-resistant control rooms; and emergency shutdown systems form independent protection layers, verified by layer-of-protection analysis (LOPA).
Step 4 — Control the residual risk administratively. Safe operating procedures, permit-to-work systems, management-of-change, operator training and drills, inspection and preventive-maintenance programmes, and personal protective equipment as the last line.
Step 5 — Verify and sustain. Pre-startup safety reviews, periodic re-HAZOP, mechanical-integrity inspection and testing, incident investigation and audit close the loop. In Canada this sits within the WHMIS, provincial OH&S Acts, and the process-safety-management framework. The tools, in short, are HAZOP/FMEA/QRA and LOPA for analysis; relief-sizing standards (API), the SIS/SIL methodology and detection/suppression hardware for engineering; and procedures, training, and audit for the administrative layers.
