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23-Chem-A5 Chemical Plant Design and Economics · May 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 — 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 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.

Given / Find. A recall question on the selection criteria for four classes of process equipment and the design workflow for a reactor. Each is answered as a compact list of the governing factors.

(a) Seven factors in dryer selection

  1. Physical form/state of the feed (wet solid, paste, slurry, granular, liquid) and of the desired dry product.
  2. Moisture content in and out, and whether the moisture is surface or bound (drying-rate behaviour).
  3. Heat sensitivity of the material (maximum allowable temperature; risk of degradation or discolouration).
  4. Throughput and whether batch or continuous operation is required.
  5. Flammability, toxicity and dust-explosion hazard (need for inert atmosphere / closed system).
  6. Product-quality requirements (particle size, bulk density, appearance, residual moisture spec).
  7. Heat source, thermal efficiency and operating/energy cost.

(b) Eight factors in filtration-equipment selection

  1. Slurry characteristics — solids concentration and particle-size distribution.
  2. Filtration objective — is the valuable product the cake, the filtrate, or both?
  3. Cake properties — compressibility and specific cake resistance (ease of filtration).
  4. Required throughput / production rate.
  5. Batch versus continuous operation.
  6. Filtration driving force — gravity, pressure or vacuum.
  7. Cake washing and dewatering (dryness) requirements.
  8. Corrosiveness/temperature of the slurry and hence materials of construction, plus capital and operating cost.

(c) Six factors in crushing/grinding-equipment selection

  1. Feed size (largest lump) and required product size / size distribution.
  2. Reduction ratio needed.
  3. Hardness and abrasiveness of the material.
  4. Required capacity (throughput).
  5. Moisture content and stickiness of the feed (wet vs. dry grinding).
  6. Heat sensitivity / any tendency to degrade, plus dust-explosion hazard.

(d) Five factors in liquid-mixing-equipment selection

  1. Viscosity of the liquids (sets impeller type — turbine, anchor, helical).
  2. Miscibility and density/viscosity difference of the phases.
  3. Degree/objective of mixing required (blending, suspension, dispersion, heat transfer, reaction).
  4. Batch or continuous operation and required residence time.
  5. Corrosiveness and other fluid properties (materials of construction) and scale/power cost.

(e) General procedure for designing a chemical reactor

  1. Collect reaction data. Establish the stoichiometry, thermodynamics (heat of reaction, equilibrium) and the reaction kinetics (rate law, activation energy) from data or the literature.
  2. Choose the reactor type. Select batch/CSTR/PFR (or fixed/fluidised bed) and phase configuration on the basis of the kinetics, required conversion/selectivity and heat effects.
  3. Set operating conditions. Fix temperature, pressure, feed composition, and conversion per pass to optimise yield/selectivity within safe and economic limits.
  4. Size the reactor. Apply the material and energy balances with the rate law to compute the required volume/residence time (design equation).
  5. Design heat transfer. Provide the heating/cooling (jacket, coils, external exchanger) needed to hold temperature and avoid runaway for exothermic reactions.
  6. Mechanical and materials design. Specify vessel geometry, agitation/catalyst arrangement, materials of construction (corrosion, pressure rating) and internals.
  7. Evaluate, cost and safety-check. Assess capital and operating cost, controllability and safety (relief, runaway scenarios), then iterate to an optimum design.
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