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23-Chem-A5 Chemical Plant Design and Economics · May 2018

Question 6 of 6: Pollution-Prevention Considerations in Unit-Operation Design

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

Notes on this paper

Closed-book exam, 3 hours; one aid sheet (both sides) permitted; approved calculator. Six questions of equal value (20 marks each); five constitute a complete paper — full solutions to all six are given here. Question 1 is process synthesis (draw a flowsheet), Question 2 is quantitative (separation-train economics), and Questions 3–6 are design-practice list/essay questions.

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 (process synthesis & flowsheet development Ch. 2–4, general design considerations incl. materials of construction Ch. 3–4, cost & depreciation Ch. 6–9); R. Turton et al., Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — separation sequencing heuristics and pollution-prevention hierarchy; R.K. Sinnott & G. Towler, Chemical Engineering Design (Coulson & Richardson Vol. 6) — distillation column design and column-internals selection; D.A. Crowl & J.F. Louvar, Chemical Process Safety (4th ed.) — batch-reactor procedures and inherently safer design. Canadian practice framed by CCOHS/WHMIS 2015 and provincial OH&S process-safety expectations.

Question 6: Pollution-Prevention Considerations in Unit-Operation Design (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.

Pollution prevention (P2) designs waste out at source rather than treating it downstream. Six considerations that guide unit-operation design are:

  1. Maximize raw-material conversion and yield. Higher single-pass conversion, recycle of unreacted feed, and selective catalysts turn more raw material into product and less into waste — the largest lever for both environmental and economic gain.
  2. Recover and recycle materials and energy. Recycle solvents, catalysts and unreacted reactants; use heat integration (a pinch analysis) so hot and cold streams exchange energy internally, cutting fuel and cooling-water use and their emissions.
  3. Substitute benign materials. Replace toxic or hazardous solvents, reagents and intermediates with safer, less volatile or aqueous alternatives, reducing fugitive emissions and hazardous-waste generation at the unit.
  4. Reduce and minimize waste streams at source. Choose reaction routes and separations that avoid stoichiometric by-products, minimize purge and vent flows, and prefer separations (e.g. distillation, membranes) that do not introduce a mass separating agent that itself becomes waste.
  5. Minimize fugitive and vent emissions. Specify low-leak equipment (welded connections, sealless pumps, better valve packing), vapour-recovery and closed-vent systems on tanks and reactors, and route vents to recovery rather than to atmosphere.
  6. Improve process control and operability. Tighter control of temperature, pressure and feed ratios keeps the unit near optimum, reducing off-spec product, upsets and the emissions that accompany start-ups, shutdowns and reprocessing.

(Two further considerations often cited are proper handling/segregation of waste for reuse and design for safe, low-emission start-up and shutdown.) Applied while the flowsheet is still fluid, these choices lower waste-treatment and compliance costs and raise the fraction of raw material sold as product — the profitability the question notes.

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