18-Env-A5 Air Quality and Pollution Control Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
04-Env-A5 / 18-Env-A5, Air Quality and Pollution Control Engineering — National Exam, December 2016. 3 hours, open book. Question 1 is compulsory; any other four (4) of Questions 2–7 complete the 100-mark paper (only the first five (5) answers in the work book are marked). All seven Problems are answered below.
Reference texts
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.
Part (i) — two selected control methods. Adsorption onto activated carbon and absorption into a scrubbing liquid are selected as the two methods here (both are effective for the dilute, moderate-solubility organic-vapour streams typical of solvent-use and process-vent applications; combustion/incineration, discussed in part (ii), is the higher-concentration/higher-destruction alternative).
Part (ii) — the four control mechanisms.
| Mechanism | Description | Typical application |
|---|---|---|
| Adsorption | Contaminant molecules bind to the internal surface of a porous solid (typically activated carbon) as the gas passes through a packed bed; the bed is regenerated by steam or hot-air desorption once saturated (breakthrough). | Solvent-vapour recovery (e.g. printing, dry cleaning) where the solvent is recovered and reused. |
| Absorption | The gas stream is contacted with a liquid (often water or a reactive aqueous solution) in a packed or spray tower; the pollutant dissolves or reacts across the gas–liquid interface and leaves with the liquid. | Wet scrubbing of acid gases (SO₂, HCl) or highly water-soluble VOCs. |
| Combustion (flaring/thermal oxidation) | The contaminant is oxidised at high temperature in an open or enclosed flame, converting organics to CO₂ and H₂O. | Refinery/landfill flares for intermittent, high-BTU waste gas streams. |
| Incineration (thermal/catalytic oxidiser) | Similar oxidation chemistry to combustion but in an enclosed, controlled chamber (thermal oxidiser) or over a catalyst bed at lower temperature (catalytic oxidiser), giving a defined, monitorable destruction efficiency. | Continuous dilute VOC/odour streams from process exhaust (e.g. the pet-food odour stream in Problem 4(ii)). |
Part (iii) — design principles for three of the mechanisms. Adsorption: (1) size the bed depth/contact time so the mass-transfer (and stoichiometric) zone does not reach the outlet before scheduled regeneration — i.e. stay within the bed's breakthrough capacity; (2) control regeneration temperature/steam rate to fully desorb the bed without damaging the carbon's pore structure, or capacity degrades over repeated cycles. Absorption: (1) maximise gas–liquid interfacial area (packing selection, spray nozzle design) since mass transfer scales with interfacial area; (2) maintain adequate liquid-to-gas ratio and liquid pH/reagent concentration so the driving-force gradient for absorption is not exhausted before the liquid leaves the tower. Combustion/Incineration: (1) maintain the "three T's" — sufficiently high temperature, residence time, and turbulence (good gas–air mixing) — to complete oxidation rather than partially combust to CO/soot; (2) supply adequate excess combustion air/oxygen while avoiding so much excess that flame temperature (and hence destruction efficiency) drops.
