Question 6 of 6: Control of Gaseous and Vapour Emissions

Problem 6: Control of Gaseous and Vapour Emissions (25 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.

Part (i) — absorption. Governing principle: a contaminated gas stream is contacted counter-currently with a liquid absorbent (typically water or a reactive aqueous solution) in a packed tower; the target gaseous pollutant dissolves (physically, or reacts chemically for enhanced capacity) into the liquid phase across the gas–liquid interface, driven by the concentration gradient between the bulk gas and the gas–liquid equilibrium (Henry's law for physical absorption). The liquid, now laden with pollutant, exits the bottom as spent liquor for treatment/regeneration, while the scrubbed gas exits the top.

Real application: flue-gas desulphurisation (FGD) on a coal-fired power plant, where flue gas is scrubbed with a limestone slurry that reacts with SO₂ to form gypsum (CaSO₄·2H₂O), routinely achieving >90% SO₂ removal. Limitations: absorption is only effective for soluble/reactive gases (it does not remove particulate matter or insoluble VOCs well without a reactive additive); it generates a liquid (or slurry) waste stream requiring further treatment or disposal, and performance is sensitive to liquid-to-gas ratio, packing surface area, and temperature (solubility generally falls as temperature rises).

Part (ii) — combustion/incineration. Governing principle: the contaminated gas stream (typically VOC-laden) is raised to a high temperature (commonly 750–1000 °C) in a combustion chamber, with sufficient residence time (≥0.5–1 s) and turbulent mixing (the classic "3 T's": Time, Temperature, Turbulence) to oxidise the organic contaminants completely to CO₂ and H₂O; supplemental fuel is added when the waste stream's own heating value is insufficient to sustain the target temperature, and a heat-recovery (recuperative or regenerative) exchanger typically preheats the incoming waste gas with the hot combustion products to reduce fuel demand.

Real application: a regenerative thermal oxidizer (RTO) on a printing, coating or automotive paint-booth exhaust, where VOC destruction efficiency routinely exceeds 95–99% and the ceramic heat-recovery bed can recover >90% of the combustion heat, making it economical even for dilute VOC streams. Limitations: fuel cost can be significant for dilute or low-heating-value streams; incomplete combustion (insufficient temperature/residence time) can generate products of incomplete combustion (CO, dioxins/furans from chlorinated feedstocks) rather than eliminating the hazard, and any halogenated or sulphur-bearing VOC content requires downstream acid-gas scrubbing of the combustion products.

Part (iii) — a third control mechanism: adsorption. (Not previously selected in (i)/(ii), which used absorption and combustion.) Governing principle: the contaminated gas is passed through a bed of high-surface-area solid adsorbent (most commonly activated carbon, ~500–1500 m²/g), and the target VOC/odour molecules are held on the adsorbent surface by van der Waals forces; once the bed approaches saturation ("breakthrough"), it is regenerated (steam or hot-air desorption, recovering the solvent) or the spent carbon is replaced. Performance efficiency: activated-carbon adsorption routinely achieves 90–98% VOC removal for well-designed systems treating dilute (typically <1000 ppm), non-humid streams; efficiency drops sharply for high-molecular-weight/high-boiling-point compounds that do not desorb cleanly (fouling the bed permanently), and high relative humidity competes for adsorption sites and can cut capacity by 50% or more, so a dehumidification pre-treatment step is often needed to sustain the higher end of that efficiency range.