18-Env-A5 Air Quality and Pollution Control Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (the first five answers as they appear are marked); all seven are solved below for completeness. Each question is worth 20 marks with section marks shown in brackets.
Reference texts. Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Canadian Environmental Protection Act, 1999 (CEPA) and the Canadian Ambient Air Quality Standards (CAAQS) administered by Environment and Climate Change Canada.
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. A moderately efficient, large-diameter cyclone removing grain dust:
| Quantity | Symbol | Value |
|---|---|---|
| Particle diameters | $d_p$ | 5, 10, 15, 20 µm |
| Cyclone inlet width | $B_c$ | 0.4 m |
| Inlet gas velocity | $v_i$ | 20 m/s |
| Particle density | $\rho_p$ | 1200 kg/m³ |
| Gas viscosity | $\mu_g$ | $1.8\times10^{-5}$ kg/(m·s) |
Find. The cut diameter $[d_p]_{cut}$ and the collection efficiency of each of the four particle sizes.
Approach. Compute $[d_p]_{cut}$ from the supplied Lapple relation, form the particle-size ratio $d_p/[d_p]_{cut}$ for each size, then read the collection efficiency off the supplied efficiency-vs-ratio curve — anchored by the curve's own defining property that a ratio of exactly 1 (i.e., $d_p=[d_p]_{cut}$) always reads 50%, since the cut diameter is BY DEFINITION the size collected at 50% efficiency.
| $d_p$ (µm) | $d_p/[d_p]_{cut}$ | Collection efficiency, η |
|---|---|---|
| 5 | 0.24 | < 5% (off-chart extrapolation) |
| 10 | 0.48 | ≈ 27% |
| 15 | 0.72 | ≈ 47% |
| 20 | 0.97 | ≈ 49% |
The result reinforces the theme of Question 1(iii) and Question 2(ii): this "moderately efficient" cyclone captures the coarser half of the grain-dust size range reasonably well (47–49% at 15–20 µm) but collects less than a third of the 10 µm fraction and is essentially transparent to the 5 µm fraction — exactly the size range where a centrifugal device's finite (not infinite) force density stops being effective, and where a downstream baghouse or wet scrubber would be needed if a stricter overall removal efficiency were required.
Difference 1 — purpose and feed. Combustion-based control (thermal/catalytic oxidizers, flares) is applied to a gas or vapour stream that is itself treated as fuel value to be oxidized cleanly to CO2 and H2O (e.g., VOC-laden solvent exhaust); incineration is engineered specifically for waste destruction, typically of mixed or hazardous streams (solids, liquids, sludges as well as gas), often requiring auxiliary fuel because the waste itself may not sustain combustion.
Difference 2 — temperature/residence-time requirement and downstream treatment. Combustion control for a clean VOC stream typically needs only 1–2 seconds at 750–900 °C to achieve high destruction efficiency with little downstream treatment beyond heat recovery. Incineration of hazardous or mixed waste is held to a much stricter destruction-and-removal-efficiency (DRE, e.g., 99.99%) standard and correspondingly requires higher temperature, longer residence time, turbulent mixing, and comprehensive downstream air-pollution control (acid-gas scrubbing, particulate/ash collection) because the feed itself may contain chlorine, metals or other species that generate secondary pollutants (e.g., HCl, dioxins/furans) that a clean-VOC combustion process does not have to manage.
Example: fixed-bed activated-carbon adsorption for solvent-VOC recovery. Solvent-laden exhaust from a printing or coating operation is passed through a bed of granular activated carbon; non-polar organic vapour molecules are held on the carbon's internal micropore surface by van der Waals forces until the bed approaches saturation (breakthrough), at which point flow is switched to a fresh/regenerated bed while the spent bed is thermally regenerated with steam or hot inert gas, desorbing and typically recovering the solvent. Well-designed systems of this type routinely achieve 95–98% VOC removal efficiency.
Key design principles and operating conditions: (1) Empty-bed residence time / bed depth sized so the mass-transfer (adsorption) zone fully develops before the gas exits, avoiding premature breakthrough; (2) superficial gas velocity kept low enough (typically 0.3–0.5 m/s) to limit pressure drop and channelling while still giving practical throughput; (3) relative humidity control, since water vapour competes for adsorption sites on activated carbon and can cut VOC capacity substantially — feed streams above roughly 50% RH are often pre-dried; (4) regeneration cycle management (steam temperature, regeneration time, and number of parallel beds) sized so one bed can always regenerate while the other(s) remain in service, sustaining continuous removal efficiency rather than a periodic dip to zero during changeover.
