23-Chem-A5 Chemical Plant Design and Economics · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 04-Chem-A5 Chemical Plant Design and Economics. Three-hour, closed-book exam; any non-communicating calculator permitted. Six equally weighted (20-mark) questions are posed and the candidate answers any five; only the first five are marked. All six are answered below for completeness. Questions 1, 5 and 6 are conceptual design / management / safety questions answered as organised prose; questions 2, 3(i) and 4 contain the numerical work (cost–capacity scaling of a heat exchanger, sinking-fund depreciation, and simple/compound loan interest), 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 Ch. 6, interest and investment Ch. 7, depreciation Ch. 9, profitability Ch. 10, optimum design Ch. 11, plant safety and loss prevention); R. Turton et al., Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — flowsheet synthesis and process development; T.M. Duncan & J.A. Reimer, Chemical Engineering Design and Analysis (Cambridge, 1998) — the source of the boiling-point data used in Question 1; 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.
Large chemical plants concentrate energy and hazardous material, so their principal safety risk areas follow directly from where that energy and material are stored or released:
| Risk area | Why it is hazardous | Mitigation |
|---|---|---|
| Fire & explosion (flammable gases/liquids) | Leaks of fuel-range material can ignite or form vapour clouds — the ammonia synthesis loop handles flammable H2. | Area electrical classification, gas detection, elimination of ignition sources, deluge/foam systems, blast-resistant layout and spacing. |
| Toxic release | Many process chemicals (ammonia itself) are acutely toxic or asphyxiating on release. | Containment and secondary containment, scrubbers/flares, toxic-gas detection, emergency isolation and shelter/evacuation plans. |
| High-pressure / high-temperature equipment | Stored mechanical and thermal energy can cause vessel rupture or severe burns (the ammonia converter runs at high T and P). | Design to ASME BPVC, pressure-relief valves and rupture disks, regular inspection, over-pressure and over-temperature trips. |
| Reactive / runaway reactions | Exothermic reactions can lose thermal control and over-pressure the system. | Reaction-hazard (calorimetry) studies, adequate cooling and quench, interlocks, emergency relief and dump systems. |
| Hazardous storage & material handling | Large inventories of feedstock/product magnify the consequence of any loss of containment. | Minimise inventory (inherently safer design), dyked storage, level and leak monitoring, safe transfer procedures and permit-to-work. |
(Rotating equipment, confined-space entry and corrosion/erosion are further recognised risk areas managed by guarding, entry permits, and corrosion-allowance plus inspection, respectively.)
Optimum economic design is a design-stage optimization. Here the design variables — equipment sizes, configuration, operating targets — are still free to choose, and the objective is to minimise the total annual cost (or maximise profitability) by trading capital cost against operating cost. The classic examples are optimum pipe diameter, optimum reflux ratio, optimum insulation thickness and optimum heat-exchanger area: a larger, more expensive unit (higher fixed cost) reduces the ongoing operating cost, and the optimum is the size where the sum of the two is least. The outcome fixes the plant that gets built.
Optimum operation design (optimum operating design) applies to an existing, already-built plant whose equipment is fixed. The capital is now sunk, so only the operating variables — throughput, temperature, pressure, reflux, recycle, feed split — can be adjusted, and the objective is to maximise profit (or minimise operating cost) within the constraints of the installed equipment. In short, economic (design) optimization chooses the best plant to build by balancing capital and operating cost; operation optimization chooses the best way to run the plant you already have, with capital cost no longer a variable.
