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

Question 5 of 6: Inherently Safer Design

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

Notes on this paper

National Exams — December 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. Question 1 combines conceptual process-synthesis (reactor and recycle structure for a series-reaction chlorination) with a short economic-potential calculation; Question 2 is a numerical retrofit-costing problem (replacing distillation trays with structured packing); Questions 3–6 are qualitative essays on equipment design procedures, materials of construction, inherently safer design, and equipment-selection factors.

Reference texts: R. Smith, Chemical Process Design and Integration (2nd ed., Wiley) — reaction path, reactor conversion and the recycle structure of the flowsheet, and the economic-potential screen behind Question 1 (the monochlorodecane example is worked there); R.K. Sinnott & G. Towler, Chemical Engineering Design (Coulson & Richardson vol. 6, 6th ed., Butterworth-Heinemann) — the equipment cost correlations and retrofit factors of Question 2, the heat-exchanger and cyclone design procedures (Ch. 12, Ch. 10), materials of construction (Ch. 7), and equipment selection (Ch. 10, 18); T.A. Kletz & P. Amyotte, Process Plants: A Handbook for Inherently Safer Design (2nd ed., CRC) — the inherently-safer-design changes of Question 5; M.S. Peters, K.D. Timmerhaus & R.E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill). Canadian practice per CCOHS and CSA Z767 (Process Safety Management) where jurisdiction matters.

Question 5: Inherently Safer 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.

Inherently safer design removes or reduces a hazard at source rather than controlling it with added protective systems. The recurring strategies are to minimise (intensify) inventory, substitute a less hazardous material or route, moderate (attenuate) conditions so a release is less severe, and simplify so there is less to go wrong. Applied to each operation:

(a) Reactors — seven changes [9 points]

  1. Reduce the reactor inventory — intensify to a smaller reactor for the same output (higher temperature/pressure/concentration or a better catalyst).
  2. Change from batch to continuous operation, so far less hazardous material is present at any instant.
  3. Use a low-hold-up reactor type (tubular/plug-flow or a small CSTR) in place of a large stirred batch vessel.
  4. Add reactants gradually (semi-batch) so the accumulated unreacted hazardous inventory stays small and the reaction can be stopped by cutting feed.
  5. Substitute a safer reaction route or chemistry — less hazardous raw materials, solvents or intermediates.
  6. Moderate the conditions (attenuation) — run at lower temperature and pressure, e.g. with a more active catalyst or a diluent, so a release is less energetic.
  7. Improve heat removal (more cooling area, dilution, better mixing) to prevent a runaway, and generate or consume hazardous intermediates in–situ rather than storing them, so they never accumulate.

(b) Distillation — three changes [4 points]

  1. Reduce column and sump liquid hold-up — use low-hold-up internals (structured packing rather than trays) and minimise base, reflux-drum and reboiler inventories.
  2. Lower the operating temperature (operate under vacuum) so the inventory is not a superheated liquid that would flash on release.
  3. Simplify and integrate — fewer/smaller columns (heat integration), thermosyphon or once-through reboilers to cut the hot-liquid inventory.

(c) Heat-transfer operations — four changes [4 points]

  1. Reduce the hazardous-fluid inventory in the exchanger — use compact (plate/plate-fin) units and put the hazardous fluid on the low-hold-up side.
  2. Use an intermediate heat-transfer medium so the hazardous process fluid cannot contact the utility (and to avoid direct fired heating).
  3. Limit the heating-medium temperature to below the decomposition/auto-ignition temperature of the process fluid, so overheating cannot initiate a hazard.
  4. Design against tube failure consequences — keep the higher pressure on the safe side and provide adequate relief so a tube rupture cannot over-pressure the low-pressure side or mix incompatible fluids.

(d) Storage — three changes [3 points]

  1. Reduce the stored inventory — hold only what is needed, with just-in-time supply or production on demand.
  2. Store under safer conditions — refrigerated at atmospheric pressure rather than pressurised, so a leak of a flashing liquid is avoided.
  3. Store as a safer form — a less hazardous chemical, a dilute solution, or below its atmospheric boiling point; use several small bunded tanks rather than one large one.