Control of Gas and Vapour Emissions — Adsorption/Absorption, Cyclones and Incineration: 18-Env-A5 Air Quality and Pollution Control Engineering, May 2016, Q7

Question 7: Control of Gas and Vapour Emissions — Adsorption/Absorption, Cyclones and Incineration (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) Adsorption and Absorption Design Principles

Adsorption (e.g. activated-carbon bed). (1) Select an adsorbent with a pore-size distribution and surface area matched to the target VOC molecule, maximizing equilibrium capacity for that species; (2) provide sufficient empty-bed contact time and manage bed regeneration (steam or thermal-swing) so the bed is taken off-line and regenerated before the mass-transfer (adsorption) zone reaches the outlet face and breakthrough occurs, since adsorptive capacity is finite and the bed saturates progressively from the inlet.

Absorption (e.g. packed scrubber tower). (1) Maximize gas–liquid interfacial contact area (packed-bed or spray-tower internals) to enhance the rate of mass transfer of the soluble/reactive pollutant into the liquid phase; (2) select and maintain a scrubbing liquid matched to the pollutant's solubility/reactivity (e.g. water for HCl, a caustic or limestone slurry for SO₂) with an adequate liquid-to-gas ratio and countercurrent flow, sustaining the concentration driving force along the full height of the column.

(ii) Cyclone Cut Diameter and Collection Efficiencies

Given. A large-diameter cyclone removing grain dust:

Given data
Quantity	Symbol	Value
Particle diameters	$d_p$	20, 40, 60, 80 µm
Inlet width	$B_c$	0.5 m
Inlet gas velocity	$v_i$	30 m/s
Particle density	$\rho_p$	1300 kg/m³
Gas viscosity	$\mu_g$	$1.9\times10^{-5}$ kg/(m·s)

Find. The theoretical cut diameter $[d_p]_{cut}$, and the fractional collection efficiency at each of the four particle sizes from the printed "50% η" curve.

Approach. Compute $[d_p]_{cut}$ directly from the supplied formula, then read the collection efficiency at each of the four given particle sizes off the specified "50% η" curve (the chart's x-axis is the actual particle size, log scale 10–100 µm, not a normalized ratio).

Compute the theoretical cut diameter. $$[d_p]_{cut}=\sqrt{\frac{9\mu_g B_c}{2\pi v_i\rho_p}}=\sqrt{\frac{9(1.9\times10^{-5})(0.5)}{2\pi(30)(1300)}}=\boxed{18.7\ \mu\text{m}}.$$
Read the "50% η" curve at each given particle size. Reading the dotted, rightmost curve at each given particle size gives: $$\eta(20\ \mu\text{m})\approx4\%,\quad \eta(40\ \mu\text{m})\approx8\%,\quad \eta(60\ \mu\text{m})\approx24\%,\quad \eta(80\ \mu\text{m})\approx\boxed{70\%}.$$

[Figure not reproduced: Cyclone collection efficiency chart, 50% eta curve. See the official exam paper or the cited reference text.]

Printed cyclone collection-efficiency chart (curves C1, C2 and the specified "50% η" curve vs. actual particle size, log scale). The dotted "50% η" curve is read directly at $d_p=20,40,60,80\ \mu\text{m}$.

Quantity	Value
Theoretical cut diameter, $[d_p]_{cut}$	18.7 µm
Efficiency at $d_p=20\ \mu\text{m}$	≈ 4%
Efficiency at $d_p=40\ \mu\text{m}$	≈ 8%
Efficiency at $d_p=60\ \mu\text{m}$	≈ 24%
Efficiency at $d_p=80\ \mu\text{m}$	≈ 70%

Check: the printed "50% η" curve is read directly against actual particle size, as the question instructs, with no rescaling by $[d_p]_{cut}$. Note the printed curve's OWN 50%-efficiency crossing (log-interpolating between the 60 and 80 µm reads, ≈70 µm) is far coarser than the theoretical $[d_p]_{cut}=18.7\ \mu\text{m}$ computed above — expected, since this is explicitly described as a "large diameter" (i.e. lower-efficiency) cyclone, and the printed curve is an empirical performance curve for a specific real unit rather than the idealized theoretical cut-point.

(iii) Incineration Example and Design Principles

A representative example is a thermal (direct-flame) oxidizer treating solvent-laden exhaust from a printing or coating line: because the VOC-laden air stream is too dilute to support its own flame and too toxic/regulated to vent untreated, raising it to a controlled combustion temperature destroys the organics directly rather than trying to recover them, and applies well where the pollutant is a mix of many different VOCs unsuited to a single adsorbent.

Two key design principles/operating conditions, the first two of the classical "three T's": (1) sufficient temperature (typically 760–870°C for most VOCs) to drive the oxidation reaction to completion; (2) sufficient residence time (typically ≥0.5–1 s) combined with adequate turbulence (mixing) so that every parcel of waste gas actually reaches that temperature for the required dwell time — a chamber whose bulk average temperature is high enough can still under-perform if cold or poorly mixed pockets bypass destruction.

Incineration of waste materials results in: (1) conversion of the organic pollutant primarily to CO₂ and H₂O, with recoverable thermal energy (often reclaimed via a heat exchanger to preheat the incoming waste stream); (2) any halogenated content converted to acid gases (e.g. HCl from chlorinated solvents), requiring downstream acid-gas scrubbing; (3) a small residual ash/particulate fraction needing further PM control; and, if combustion is incomplete or the flue gas is quenched too quickly through the 200–450°C window, products of incomplete combustion (CO, soot, and in the presence of chlorine, trace dioxins/furans).

Concept & Theory

Topic. Adsorption/absorption design fundamentals, Lapple-type cyclone cut-diameter theory vs. an empirical performance curve, and thermal-oxidizer destruction efficiency.

Key relations. Lapple-type cut diameter $[d_p]_{cut}=\sqrt{9\mu_g B_c/(2\pi v_i\rho_p)}$; thermal DRE governed by the "three T's" (temperature, time, turbulence) with Arrhenius-type kinetics.

Why this works. A cyclone separates by centrifugal force overcoming drag, exactly analogous in form to gravity settling (Question 5's Stokes balance) but with a much larger effective "g" from the swirling flow, which is why cyclones handle far smaller particles than a gravity chamber of comparable size; the theoretical cut diameter is an idealized inertial-separation limit, while the printed efficiency curve is the real, empirically measured performance of an actual unit and is always somewhat worse.

Common pitfall. Assuming a higher chamber temperature alone guarantees higher destruction/removal efficiency — without adequate residence time and turbulence to bring every parcel of waste gas to that temperature, cold or unmixed pockets can still bypass destruction even when the bulk average temperature is well above the target.

Source. Cooper & Alley, Air Pollution Control, ch. 6 (cyclones), ch. 9 (adsorption/absorption) and ch. 7 (thermal incineration, the three T's).

Question 7 of 7: Control of Gas and Vapour Emissions — Adsorption/Absorption, Cyclones and Incineration

Question 7: Control of Gas and Vapour Emissions — Adsorption/Absorption, Cyclones and Incineration (20 marks)

(i) Adsorption and Absorption Design Principles

(ii) Cyclone Cut Diameter and Collection Efficiencies

(iii) Incineration Example and Design Principles