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

Question 6 of 6: Equipment-Selection Factors

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 6: Equipment-Selection Factors (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.

Selecting solids- and liquor-handling equipment turns on the physical nature of the material, the throughput, the required product quality, and the practical constraints of corrosion, hazard and cost. The governing factors for each duty are listed below.

(a) Selecting screening / sieving equipment — five factors [4.5 points]

  1. Particle-size range and the cut size(s) required (the separation the screen must make).
  2. Throughput / capacity to be handled.
  3. Nature of the feed — dry or wet, moisture content, stickiness/cohesiveness, tendency to blind the screen.
  4. Material characteristics — bulk density, abrasiveness, friability/fragility.
  5. Required separation sharpness / efficiency and product-purity specification (plus dust/hazard considerations).

(b) Using a centrifugal filter — six factors [5.5 points]

  1. Feed slurry characteristics — solids concentration, particle size and cake permeability (filterability).
  2. Required throughput / filtration rate.
  3. Cake dryness (residual moisture) and washing requirements.
  4. Batch or continuous operation needed.
  5. Solids handling — cake-discharge method and solids friability/degradation.
  6. Centrifugal (G) force required, and the corrosiveness/abrasiveness of the slurry (materials of construction).

(c) Selecting an evaporator — seven factors [6.5 points]

  1. Throughput / evaporation capacity.
  2. Heat-sensitivity of the product — thermal degradation dictates short residence time / low temperature (e.g. falling-film).
  3. Fouling, scaling or salting tendency — may require forced circulation or scraped-surface units.
  4. Liquor viscosity and how it rises with concentration.
  5. Foaming tendency of the liquor.
  6. Corrosiveness — materials of construction.
  7. Solids content / crystallisation, and energy economy (single- vs multiple-effect, thermal or mechanical vapour recompression).

(d) Movement and storage of solids — four factors [3.5 points]

  1. Solids properties — particle size, bulk density, flowability, angle of repose, abrasiveness.
  2. Throughput and the distance / height over which the solid must be moved.
  3. Moisture, stickiness and cohesiveness — tendency to bridge, cake or segregate.
  4. Hazard and condition — dustiness/explosivity, toxicity, temperature, and sensitivity to attrition/degradation.
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