23-Chem-A5 Chemical Plant Design and Economics · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 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. Question 1 is a conceptual flowsheet-synthesis question (hydrodealkylation of toluene to benzene) answered with a process flow sheet and organised prose; questions 2, 3 and 6 are numerical (capacity-scaled and index-escalated plant cost, yield-improvement rate of return, and evaporator heat-transfer area); questions 4 and 5 are qualitative essays on materials selection against the common corrosion mechanisms and on process-hazard classification.
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 profitability Ch. 7–10, materials of construction Ch. 12, plant safety and loss prevention Ch. 3); J.M. Douglas, Conceptual Design of Chemical Processes (McGraw-Hill) — the hydrodealkylation (HDA) flowsheet-synthesis case study used in Question 1; R. Turton et al., Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — flowsheet synthesis and equipment cost correlations; AIChE, Dow’s Fire & Explosion Index Hazard Classification Guide (7th ed.) — the process-hazard checklist behind Question 5; supporting Canadian practice from CCOHS/WHMIS 2015, the Canadian Environmental Protection Act (CEPA), and CSA/ASME materials and pressure-vessel codes.
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.
Materials selection is the art of matching an alloy’s failure resistance to the specific corrosion mechanism the service imposes. Each of the six mechanisms below calls for a distinct materials response.
(a) General (uniform) wastage. Metal is lost evenly over the whole wetted surface at a predictable rate. It is the most benign form because it is measurable: one either specifies a material whose uniform rate is acceptably low (< 0.5 mm/yr is usually tolerable) or accepts a cheap material such as carbon steel and adds a corrosion allowance (typically 2–4 mm of extra wall thickness) to the pressure-vessel design. Where the rate is too high, the answer is an upgrade to a more resistant alloy (stainless steel, nickel alloys) or a lining.
(b) Galvanic corrosion. When two dissimilar metals are electrically coupled in an electrolyte, the more active (anodic) metal corrodes preferentially. The materials response is to avoid dissimilar-metal couples — select metals close together in the galvanic series, electrically insulate the junction, apply coatings, and design a favourable area ratio (a large anode, small cathode) so attack is not concentrated. Deliberately coupling a sacrificial anode (zinc, magnesium) turns the effect into cathodic protection for the item to be preserved.
(c) Pitting (localized). A local breakdown of the passive film — most often by chloride ions — drives deep, narrow pits that can perforate a wall while the bulk surface stays bright. The materials answer is molybdenum-bearing stainless steels (Type 316, or duplex 2205) or high-nickel alloys whose passive film resists chloride attack, together with designs that avoid stagnant pockets and crevices where pitting initiates.
(d) Stress corrosion (SCC). Cracking occurs only under the simultaneous action of a tensile stress and a specific environment — chlorides on austenitic stainless steels, caustic or nitrates on carbon steel. Materials selection targets one leg of that triangle: choose a resistant alloy (duplex or high-nickel grades, or Monel for chlorides), stress-relieve welds to remove residual tension, and exclude the offending ion. Ferritic and duplex stainless steels are far less susceptible to chloride SCC than the austenitic 300 series.
(e) High-temperature oxidation and sulfidation. At elevated temperature the metal reacts directly with oxygen or sulfur-bearing gases to form a scale; protection depends on that scale being adherent and self-healing. Materials selection uses chromium-bearing alloys (stainless steels, Incoloy, Inconel) that grow a protective Cr2O3 scale; where sulfidation is severe, high-chromium and aluminized surfaces are used, and refractory linings shield the metal in fired equipment. Aluminium and silicon additions further improve scale stability.
(f) Intergranular corrosion. Preferential attack along grain boundaries occurs when austenitic stainless steel is sensitized — heating (e.g. during welding) precipitates chromium carbides at the boundaries and depletes the adjacent metal of chromium. The materials answers are low-carbon grades (304L, 316L) that starve carbide formation, stabilized grades (Type 321 with titanium or 347 with niobium) that tie up carbon preferentially, and solution annealing after welding to redissolve the carbides.
