Oxidation, condensation and adsorption technologies for gases, vapours and aerosols: 23-Chem-B2 Environmental Engineering, May 2015, Q2

Question 2: Oxidation, condensation and adsorption technologies for gases, vapours and aerosols (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 comparative matrix, as recommended by the question, is the clearest way to present the three control-technology families side by side.

Technology	Main technology principle	Advantages (2)	Limitations (2)	Example application
(i) Oxidation-based — toxic gases	The toxic gas stream (e.g. H₂S, CO) is thermally or catalytically oxidized to a less hazardous species (H₂S→SO₂, CO→CO₂), with sufficient temperature/residence time/turbulence ("three T's") to complete the reaction.	Achieves near-complete destruction (not just phase transfer) of the toxic species regardless of its identity; catalytic oxidation can operate at a much lower temperature than thermal, cutting fuel cost.	Produces its own combustion by-products (SO₂ from H₂S oxidation may itself require downstream scrubbing); catalyst beds are vulnerable to poisoning (particulates, certain sulfur/metal species) that progressively degrades conversion efficiency.	Catalytic oxidation of H₂S-laden vent gas from a sour-gas processing or pulp-mill recovery operation.
(ii) Condensation-based — odorous vapours	The odorous vapour-laden gas is cooled (directly or via a refrigerated/chilled-water surface condenser) below the dew point of the target compound, condensing it out of the gas phase into a liquid that is collected and separately treated or recovered.	Can recover the condensed compound for reuse/disposal rather than destroying it, valuable for solvent-recovery applications; effective as a pretreatment stage ahead of a smaller downstream polishing unit (adsorption or scrubbing), reducing that unit's loading.	Only effective for compounds with a sufficiently high boiling point/low vapour pressure at an economically achievable condensing temperature — highly volatile odorants pass through largely uncondensed; refrigeration energy cost rises sharply as the required condensing temperature drops.	Refrigerated condensation recovery of solvent vapour ahead of a carbon-adsorption polishing stage at a chemical/coatings plant vent.
(iii) Adsorption-based — aerosols	The aerosol-laden gas passes through a packed/granular sorbent bed (e.g. activated carbon or a fibrous adsorptive media); the bed acts simultaneously as a depth filter capturing aerosol droplets/particles by impaction and interception on the packing surfaces, while any co-adsorbable vapour phase associated with the aerosol is captured by physisorption on the sorbent's internal pore surface.	Simultaneously addresses an aerosol's particulate and any accompanying semi-volatile vapour fraction in a single unit, unlike a pure-particulate device (e.g. a baghouse) which passes vapour straight through; effective even at the relatively low mass loadings typical of a fine mist/aerosol stream.	Aerosol droplets can progressively wet/blind the sorbent bed, raising pressure drop and shortening the effective service life compared with a dry-gas adsorption duty; regeneration/replacement is more frequent and costly than for a dedicated mist eliminator when the stream is aerosol-dominated rather than truly vapour-phase.	Activated-carbon adsorber polishing an oil-mist/solvent-aerosol vent from a metalworking or degreasing operation, downstream of a coarse mist eliminator.

Concept & Theory

Topic. Oxidation, condensation, and adsorption as complementary gas/vapour/aerosol control mechanisms.

Key relations. Oxidation destruction efficiency governed by the "three T's"; condensation recovery set by the vapour-liquid equilibrium (compound partial pressure vs. saturation pressure at the condensing temperature); adsorption bed life set by capacity divided by combined particulate+vapour mass loading.

Why this works. Each technology targets a different physical mechanism — oxidation destroys the molecule chemically, condensation exploits a phase-equilibrium boundary, and adsorption exploits surface capture — which is why an aerosol (a two-phase mist, not a pure gas) is often best handled by a technology, like a sorbent bed, that can act on both its particulate and vapour character at once.

Common pitfall. Specifying condensation alone for a highly volatile odorant that will not condense at an economical temperature; under-estimating how quickly an aerosol-wetted adsorption bed blinds compared with the same bed in dry vapour-phase service.

Source. Cooper & Alley Ch. 6–8 (adsorption, condensation, thermal/catalytic oxidation).

Question 2 of 7: Oxidation, condensation and adsorption technologies for gases, vapours and aerosols

Question 2: Oxidation, condensation and adsorption technologies for gases, vapours and aerosols (20 marks)