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23-Chem-A5 Chemical Plant Design and Economics · May 2016

Question 4 of 6: Selection of Materials of Construction

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams — May 2016 — 04-Chem-A5 Chemical Plant Design and Economics. Three-hour, closed-book exam; one two-sided aid sheet and an approved 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 1–3 are numerical (distillation heat integration, after-tax net present worth, and a coagulant-dosage cost optimisation); questions 4–6 are qualitative essays on materials of construction, plant startup/shutdown safety, and equipment-selection factors.

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 (interest and profitability Ch. 7–10, materials of construction Ch. 12, plant safety and loss prevention Ch. 3, equipment selection throughout Ch. 14–22); R. Smith, Chemical Process Design and Integration (2nd ed., Wiley) and B. Linnhoff et al., A User Guide on Process Integration (IChemE) — pinch analysis and column heat integration behind Question 1; R.K. Sinnott & G. Towler, Chemical Engineering Design (Coulson & Richardson vol. 6) — materials selection and equipment sizing; supporting Canadian practice from CCOHS, provincial OH&S process-safety-management regulations and the CSA Z767 (PSM) framework.

Question 4: Selection of Materials of Construction (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.

Given. A low-pressure (2 bar), small-bore pipe; mechanical loading is trivial, so selection is governed entirely by corrosion resistance to each fluid at the stated concentration and temperature.

Find. A suitable material for each service, with the controlling corrosion mechanism.

Concentration and temperature both switch the controlling mechanism, so the choices are not a single "acid-proof" alloy but a service-by-service match. The key facts: concentrated (>70 %) sulfuric acid passivates plain steel, whereas dilute sulfuric is aggressively reducing; hydrochloric acid attacks almost all common metals and demands linings or Hastelloy B; nitric acid is oxidising and is handled in stainless steel until it becomes hot and concentrated; caustic is mild to steel when dilute but embrittles it when hot and concentrated; chlorides pit austenitic stainless, and cupric/cuprous ions rule out copper-bearing alloys.

ServiceRecommended materialControlling mechanism / rationale
(a) 98 % H2SO4, 70 °CCarbon steel (or cast iron)Concentrated acid forms a protective iron-sulfate film; steel is standard for >70 % at moderate temperature (keep velocity low).
(b) 5 % H2SO4, 30 °CRubber-lined steel (or high-silicon iron / lead)Dilute acid is strongly reducing and destroys steel; an inert lining or Duriron is required.
(c) 30 % HCl, 50 °CRubber-lined steel (or Hastelloy B)HCl attacks all common metals; non-metallic lining or a Ni-Mo alloy resists the reducing chloride.
(d) 5 % NaOH, 30 °CCarbon steelSteel resists dilute caustic at low temperature with negligible attack.
(e) Conc. NaOH, 50 °CNickel (or Monel)Hot concentrated caustic risks caustic embrittlement of steel; nickel is the preferred material.
(f) 5 % HNO3, 30 °CStainless steel (18-8 / type 304)Oxidising acid passivates chromium; austenitic stainless is the classic nitric-acid material.
(g) Boiling conc. HNO3Titanium (or high-silicon iron / tantalum)Hot concentrated nitric over-oxidises stainless (grain-boundary attack); titanium/Duriron survive.
(h) 10 % NaCl solutionRubber- or plastic-lined steel (or Monel)Neutral chloride pits austenitic stainless; use a lining or a chloride-tolerant Ni-Cu alloy.
(i) 5 % CuCl in HClHastelloy B (or tantalum)Reducing chloride plus cuprous ions rules out copper alloys and stainless; Ni-Mo or Ta resists.
(j) 10 % HFMonel (Ni-Cu)Aqueous HF attacks steel and stainless; Monel is the standard hydrofluoric-acid material.
Check: these are first-choice materials for the stated conditions; a final selection would confirm the corrosion rate against an isocorrosion chart (e.g. the NACE/Peters materials tables) for the exact concentration, temperature and velocity, and would specify the lining grade or alloy UNS number. Where two options are shown, the parenthetical is an equally acceptable alternative used in Canadian practice.