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) Applying Cyclone Efficiency Curves (Laminar, Turbulent, Leith–Licht)

All three theoretical curves plot fractional collection efficiency $\eta$ rising monotonically with particle diameter $D_p$ from near zero to near 100%, but they differ in the physical model used for the swirling flow inside the cyclone and therefore in how steep and how far to the left (i.e., toward smaller $D_p$) the efficiency curve sits. The laminar (Lapple-type) model idealizes the gas as undergoing solid-body-like rotation with a single characteristic number of effective turns, giving the simplest closed-form cut-diameter expression and the most optimistic (steepest, left-shifted) efficiency curve. The turbulent model accounts for the fact that real cyclone flow is turbulent, which re-entrains and redisperses partially-separated particles back toward the core, so the turbulent curve sits to the right of (predicts lower efficiency than) the laminar curve at the same $D_p$. The Leith–Licht model is a more rigorous, semi-empirical treatment that additionally accounts for the actual (non-uniform) velocity profile and particle-loading effects, and is generally regarded as the most realistic predictor of field performance among the three. In practice, an engineer applies these curves by selecting the model appropriate to the design confidence needed: the laminar curve gives a fast, conservative-optimistic first sizing estimate; the turbulent and Leith–Licht curves are then used to check that the predicted efficiency at the target particle size still meets the emission limit under realistic (non-ideal) flow conditions, and to decide whether cyclone diameter, inlet velocity, or a downstream polishing device (fabric filter, ESP) is needed to close the gap between the optimistic and realistic efficiency predictions at the size fraction that actually drives compliance.

(ii) Design Principles of Absorption Control Equipment

Selecting absorption (e.g. a packed-bed or spray-tower scrubber, the same unit-operation family as the FGD absorber in Question 6(ii)): the first fundamental design principle is maximizing gas–liquid interfacial area and contact time — achieved with packing media (structured or random packing) or fine liquid droplets in a spray tower — because the mass-transfer rate of the target gas into the liquid is directly proportional to the interfacial area available and the driving-force gradient sustained across it (per two-film theory). The second fundamental principle is choosing a liquid and operating chemistry that keeps the equilibrium driving force favourable throughout the column, either by using a large excess of liquid (dilute absorption), by chemically reacting the absorbed species so its gas-phase equilibrium partial pressure stays near zero (as the limestone slurry does for SO₂), or by countercurrent flow arrangement so the least-contaminated liquid meets the least-contaminated gas at the top of the tower, maximizing the average driving force across the whole bed rather than letting it collapse to near-equilibrium partway through.

(iii) Combustion/Incineration System Design — Example

A representative example is a thermal oxidizer (afterburner) for VOC-laden process exhaust, of the type commonly applied downstream of a solvent-coating or printing operation, typically achieving 95–99% destruction efficiency of the VOC loading when properly operated. Its performance is governed by the classic "three T's" of combustion, of which two are the key design principles here: (1) adequate residence time at temperature — the combustion chamber is sized so that, at the design flow rate, the exhaust gas spends at least the 0.5–1 second dwell time typically needed to complete oxidation of the VOC mixture; (2) sufficient temperature and turbulent mixing — the chamber is operated at 760–870 °C (well above the autoignition temperature of the VOC mixture, with margin for incomplete mixing) with a burner and chamber geometry designed to promote turbulent mixing of the fuel-assisted flame with the VOC-laden stream, since poor mixing leaves cold, unreacted pockets that escape oxidation regardless of how hot the bulk gas is. Operating conditions that maximize efficiency in practice include maintaining excess O₂ (to avoid a fuel-rich, incomplete-combustion regime that itself generates CO and partial-oxidation products), and — where economics allow — recovering the flue-gas heat through a regenerative or recuperative heat exchanger to preheat incoming process air, which both reduces auxiliary fuel demand and, by raising the effective inlet temperature, helps sustain the required chamber temperature at lower fuel firing rate.