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23-Chem-B2 Environmental Engineering · May 2013

Question 4 of 7: Fabric filters, absorption, and catalytic reactors for particulates, gases and vapours

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. EGBC 04-Chem-B2 Environmental Engineering, May 2013, 3 hours, closed-book with a candidate-prepared double-sided 8½×11-inch aid sheet. Seven problems, each worth 20 marks; candidates attempt any five, and only the first five answers in the workbook are marked. All seven problems are solved below as a complete study resource.

Reference texts: G. Tchobanoglous, F. L. Burton & H. D. Stensel (Metcalf & Eddy), Wastewater Engineering: Treatment and Reuse (4th ed., McGraw-Hill) — BOD kinetics, dissolved air flotation, activated-sludge design, phosphorus removal; M. L. Davis & D. A. Cornwell, Introduction to Environmental Engineering (5th ed., McGraw-Hill) — air pollution control, ion exchange, reverse osmosis, soil remediation; L. Theodore & A. J. Buonicore / C. D. Cooper & F. C. Alley, Air Pollution Control: A Design Approach — fabric filtration, absorption, catalytic oxidation, odour control; S. P. Turner, Workbook of Atmospheric Dispersion Estimates (2nd ed., CRC Press) — the Gaussian plume model and Pasquill–Gifford stability classes. Canadian context follows the Canadian Environmental Protection Act (CEPA 1999), the Canadian Council of Ministers of the Environment (CCME) Municipal Wastewater Effluent guidelines, and provincial air/water permitting practice (e.g. BC Environmental Management Act and Metro Vancouver air-quality bylaws), which govern effluent/emission limits, monitoring frequency, and buffer-strip / best-management-practice programs referenced throughout.

Question 4: Fabric filters, absorption, and catalytic reactors for particulates, gases and vapours (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.

TechnologyMain design principleAdvantages (2)Limitations (2)Example industrial process
(i) Fabric filter (baghouse)Dust-laden gas is forced through a woven or felted fabric; particles are captured by sieving, impaction and interception on the fibres and, once a dust cake builds, by the cake itself acting as the primary filtering medium (which is periodically shaken, reverse-air or pulse-jet cleaned off).Very high efficiency across the full size range including sub-micron PM2.5 (>99.9% typical); collected dust is dry, simplifying disposal/recovery of valuable product compared with a wet scrubber's sludge.Pressure drop rises as the cake builds, so cleaning cycles are an energy/maintenance burden; unsuitable for hot, moist, or sticky gas streams that blind or chemically attack the fabric (temperature limit typically <260 °C for common synthetic fabrics).Cement-kiln or grain-elevator dust collection, where a dry, high-value, fine product must be recovered at very high capture efficiency.
(ii) Absorption (gas scrubbing)The gas stream is contacted with a liquid (often chemically reactive) in a packed or tray column; soluble/reactive gas-phase pollutant transfers across the gas–liquid interface into the liquid, driven by the concentration gradient described by Henry's law, and the pollutant leaves in the liquid effluent (often after a subsequent regeneration or neutralization step).Effective for highly soluble or reactive acid/base gases (SO₂, HCl, NH₃) even at trace concentrations; can simultaneously remove some particulate and recover a saleable by-product (e.g. gypsum from flue-gas desulfurization).Generates a liquid waste/wastewater stream requiring its own treatment or disposal; poor removal of gases with low solubility/reactivity unless a specific reactive scrubbing liquid is engineered for that pollutant.Flue-gas desulfurization (lime/limestone scrubbing of SO₂) at a coal-fired power plant or smelter.
(iii) Catalytic (thermal) oxidationThe odorous/VOC-laden gas is preheated and passed over a catalyst bed (typically a noble-metal or metal-oxide catalyst) that lowers the activation energy for oxidation, allowing near-complete combustion of the organic/odorous compounds to CO₂ and H₂O at a much lower temperature (300–450 °C) than thermal (non-catalytic) oxidation (700–850 °C).Large fuel/energy saving relative to thermal oxidation because of the lower operating temperature; very high destruction efficiency (>95–99%) for odorous VOCs, eliminating rather than just diluting or masking the odour.Catalyst is vulnerable to poisoning (sulfur, halogens, particulate fouling) and to thermal degradation, requiring periodic replacement; not suitable for streams carrying particulate loads that foul the catalyst bed without upstream filtration.Rendering-plant or coating/printing-facility exhaust odour control, where VOC/odour destruction (not just capture) is required to satisfy a nuisance-odour bylaw.