Question 7 of 7: Control of Gaseous and Vapour Emissions

Question 7: Control of Gaseous and Vapour Emissions (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.

(i) Absorption Equipment for SO₂, and Physical vs. Chemical Adsorption

Absorption main principle. Absorption equipment (e.g. a spray tower, venturi scrubber or wet flue-gas desulfurization system) works by intimate gas–liquid contact: the flue gas is contacted with a liquid solvent — commonly a lime or limestone slurry, or seawater — so that the soluble gaseous pollutant, SO₂, mass-transfers across the gas–liquid interface from the gas phase into the liquid, driven by the difference between SO₂'s partial pressure in the gas and its equilibrium vapour pressure over the liquid (Henry's law). Once absorbed, the SO₂ reacts with the alkaline slurry (e.g. $\text{CaCO}_3+\text{SO}_2 \rightarrow \text{CaSO}_3$, further oxidized to gypsum, $\text{CaSO}_4\cdot 2\text{H}_2\text{O}$) to be permanently fixed as a solid byproduct rather than being able to re-evolve back into the gas.

Physical vs. chemical adsorption. These are a distinct removal mechanism from absorption, using a solid adsorbent rather than a liquid. Physical adsorption (physisorption) holds the gas molecule on the adsorbent surface through weak van der Waals forces; it has a low heat of adsorption, is readily reversible (the adsorbent can be regenerated by lowering pressure or raising temperature, e.g. thermal-swing regeneration of activated carbon for VOC recovery), and can build up in multiple molecular layers. Chemical adsorption (chemisorption) forms an actual chemical bond between the gas molecule and the surface; it has a much higher heat of adsorption, is essentially irreversible under normal operating conditions, is limited to a single molecular monolayer, and is the mechanism deliberately exploited when the adsorbent is chosen to react with and destroy the pollutant (e.g. sulfur- or metal-impregnated activated carbon for H₂S or mercury capture) rather than simply capture it for later release.

(ii) Gas Adsorption in Packed Towers and a Key Maintenance Item

In a packed adsorption tower, the polluted gas stream flows — typically upward, countercurrent to a trickling scrubbing liquid in an absorption duty, or through a fixed bed for an adsorption duty — through a bed of high-surface-area packing (structured or random packing wetted with liquid, or granular activated carbon for adsorption) contained between a support grid at the base and a mist eliminator near the top. As the gas passes through the bed, the pollutant transfers either into the counter-flowing liquid film (absorption) or onto the solid adsorbent surface (adsorption) until essentially the whole gas volume has been processed through the bed's active mass-transfer zone; clean gas exits the top through the mist eliminator, which removes any entrained liquid droplets before discharge, while the loaded adsorbent is periodically regenerated (steam or thermal-swing) or replaced, and spent scrubbing liquor is bled off and replenished with fresh reagent.

Key maintenance item. Preventing and managing packing fouling and channeling. Particulate buildup, scale formation (e.g. gypsum/CaSO₄ scale in a wet limestone FGD tower) or biological growth on the packing progressively reduces the available surface area and can create preferential flow channels through the bed — gas simply bypasses the packing through these channels rather than contacting it uniformly, causing a silent loss of removal efficiency long before any process alarm would trip. Regular monitoring of pressure drop across the bed is the practical early-warning indicator (a rising pressure drop signals fouling; an anomalously low pressure drop for the same flow signals channeling) that triggers a cleaning, descaling or repacking maintenance cycle before performance is compromised.

(iii) Incinerators: Operating Principles and Thermal vs. Catalytic Types

Operating principle 1 — the "3 T's". Complete destruction (oxidation) of toxic solvent vapours to CO₂ and H₂O requires sufficient Temperature, residence Time and Turbulence (mixing) simultaneously; falling short on any one of the three — for example, adequate temperature but insufficient residence time — allows unburned or partially-oxidized organics to pass through untreated.

Operating principle 2 — excess-air balance. Sufficient oxygen must be supplied for complete stoichiometric combustion of the solvent vapours, but excessive dilution air lowers the combustion-chamber temperature below the destruction threshold — the system must be operated with enough excess air to guarantee complete oxidation without so much that the flame or chamber temperature drops out of the effective destruction range.

Thermal vs. catalytic incineration. A thermal incinerator (afterburner) achieves destruction purely through elevated temperature and residence time alone, typically 750–1000 °C with 0.5–1+ second residence and no catalyst — simple and robust, but energy-intensive to reach and hold that temperature, and can itself generate thermal NO_x at those operating temperatures. A catalytic incinerator passes the vapour stream over a catalyst bed (commonly platinum or palladium on a ceramic substrate) that lowers the activation energy for oxidation, achieving equivalent destruction efficiency at a much lower operating temperature, typically 300–500 °C — substantially cutting fuel cost and thermal-NO_x formation, but the catalyst is vulnerable to poisoning or deactivation by compounds commonly present in curing-oven solvent streams (sulfur, halogens, particulate, silicone-containing solvents), requiring careful compatibility screening and periodic catalyst replacement — a maintenance liability thermal incineration does not have.