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

Question 3 of 6: Design Procedures — Heat Exchanger and Cyclone

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 3: Design Procedures — Heat Exchanger and Cyclone (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.

(a) Design procedure for a (tubular, S&T) heat exchanger [11 points]

Heat-exchanger design is inherently iterative: an overall coefficient is assumed to size the unit, then checked against the coefficient the chosen geometry actually delivers, and the geometry is revised until the two agree and the pressure drops are acceptable. A typical procedure runs:

  1. Specify the duty. Fix the heat-transfer rate $Q$, the two stream flow rates and their inlet/outlet temperatures and operating pressures (complete any missing terminal temperature from an energy balance).
  2. Collect physical properties. Density, viscosity, thermal conductivity and specific heat of both fluids at their mean bulk temperatures.
  3. Select the exchanger type and a trial overall coefficient $U$. Choose the configuration (fixed-tube, U-tube or floating-head tubular unit, or a compact type) and take a trial $U$ from tables of typical values.
  4. Calculate the mean temperature difference. Compute the log-mean $\Delta T_{lm}$ and, for a multi-pass unit, the correction factor $F_t$; select the number of tube/pass arrangements so that $F_t \gtrsim 0.75$.
  5. Calculate the provisional area. $A = Q/(U\,\Delta T_m)$ using the trial $U$.
  6. Decide the layout. Tube diameter, length, pitch and number; number of tube passes; bundle and casing diameters.
  7. Estimate the tube-side film coefficient from the appropriate correlation (e.g. Dittus–Boelter / Sieder–Tate).
  8. Estimate the outer-side (bundle/annulus) film coefficient by Kern's or Bell's method.
  9. Calculate the overall coefficient including fouling. Combine the film coefficients with the wall and dirt (fouling) resistances and compare with the trial $U$: if the calculated value is not within about 0 to +30 % of the trial, revise the trial $U$ and repeat from the area step.
  10. Calculate the pressure drops on both streams. If either exceeds its allowable value, revise the layout (passes, tube length, velocities) and iterate.
  11. Optimise and finalise. Trade capital against pumping cost, then complete the mechanical design and issue the specification sheet.

(b) Design procedure for a cyclone separator [9 points]

Cyclone design starts from a standard geometry whose proportions are all fixed relative to the barrel diameter, so the only real decisions are the diameter (set by the inlet velocity and the number in parallel) and whether the resulting collection efficiency and pressure drop meet the spec:

  1. Select a standard cyclone geometry. Choose a high-efficiency (Stairmand) or a high-throughput design; this fixes every dimension ratio to the barrel diameter.
  2. Collect the required data. Gas volumetric flow rate, gas density and viscosity, solids density, the particle-size distribution, and the required collection efficiency or emission limit.
  3. Choose the number of units in parallel and hence the gas flow handled by each cyclone.
  4. Set the inlet velocity and size the unit. Take an inlet velocity of about 15 m/s, size the rectangular inlet, and thereby fix the barrel diameter for the chosen standard geometry.
  5. Scale the standard performance curve to the chosen diameter and gas conditions using the flow-rate, density and viscosity scaling factors.
  6. Calculate the grade (collection) efficiency for each particle-size fraction and integrate over the actual size distribution to get the overall efficiency; compare with the requirement.
  7. Revise if necessary. If efficiency is inadequate, reduce the diameter (more units in parallel) and repeat; balance efficiency against the resulting pressure drop.
  8. Calculate the pressure drop and confirm it (and the fan power) is within the allowable limit.
  9. Finalise the dimensions and check the design for solids loading, erosion and re-entrainment (dip-leg / dust-hopper arrangement).