GATE Environmental Engineering Air and Noise Pollution Syllabus

Describe the operating principle and effective size range of a fabric (bag-house) filter.

Gas passes through woven or felted fabric bags; particles are captured by sieving, impaction, interception, and diffusion. A 'dust cake' builds up and enhances filtration. It is very efficient (>99%) even for fine particles down to ~$0.1$–$1\ \mu\mathrm{m}$.

Define the air-to-cloth ratio (filtration velocity) for a bag-house filter.

The air-to-cloth ratio is the volumetric gas flow rate per unit of fabric area: $$\text{A/C} = \frac{Q}{A_{cloth}}$$ It has units of velocity (e.g. m/min) and is the superficial face velocity of gas through the fabric. Lower ratios give better collection and lower pressure drop.

Name the three common bag-cleaning methods used in fabric filters.

Shaker (mechanical shaking), reverse-air (reverse gas flow collapses the bag), and pulse-jet (a burst of compressed air dislodges the cake). Pulse-jet allows online cleaning at higher A/C ratios.

On what principle does an electrostatic precipitator (ESP) operate?

An ESP charges particles using a high-voltage corona discharge from discharge electrodes, then the charged particles migrate to oppositely-charged (grounded) collecting plates under the electric field, where they are deposited and periodically rapped off.

State the Deutsch-Anderson equation for ESP collection efficiency.

$$\eta = 1 - e^{-\frac{w\,A}{Q}}$$ where $w$ = drift (migration) velocity of charged particle, $A$ = total collecting-plate area, $Q$ = volumetric gas flow rate.

What is the drift (migration) velocity in an ESP and on what does it depend?

The drift velocity $w$ is the terminal velocity of a charged particle moving toward the collecting plate under the electric field. It increases with particle charge, field strength, and particle size, and decreases with gas viscosity: $$w = \frac{q\,E}{3\pi\,\mu\,d_p}$$

Why is fly-ash resistivity important to ESP performance, and what is the ideal range?

Particle resistivity controls charge dissipation at the plate. The ideal range is about $10^{4}$–$10^{11}\ \Omega\cdot\mathrm{cm}$. Too low: particles lose charge and are re-entrained. Too high: charge builds up causing 'back corona' and reduced efficiency.

Compare the four major particulate control devices by effective minimum particle size (largest to smallest captured).

Gravitational settling chamber: $> 50\ \mu\mathrm{m}$; cyclone: $> 5$–$10\ \mu\mathrm{m}$; wet scrubber (venturi): down to ~$0.5\ \mu\mathrm{m}$; fabric filter and ESP: high efficiency down to ~$0.1\ \mu\mathrm{m}$ (submicron).

Which particulate control device has the lowest collection efficiency and lowest pressure drop, and why is it used?

The gravitational settling chamber has the lowest efficiency (only coarse particles) and very low pressure drop. It is used as a pre-cleaner to remove large/abrasive particles and reduce load on downstream high-efficiency devices.

What are the three principal methods for controlling gaseous pollutants (besides combustion/condensation)?

Absorption (gas dissolves into a liquid), adsorption (gas adheres to a solid surface), and chemical conversion. Combustion (incineration) and condensation are additional methods for destroying or recovering gaseous contaminants.

Define absorption as a gas-control method and name a common application.

Absorption is the transfer (dissolution) of a soluble gaseous pollutant from the gas phase into a contacting liquid (solvent). Example: removing $\ce{SO2}$ from flue gas by scrubbing with a lime/limestone slurry (flue-gas desulphurization).

State Henry's law as applied to gas absorption.

$$p_A = H\, x_A$$ where $p_A$ = partial pressure of gas A in the gas phase, $x_A$ = mole fraction of A dissolved in liquid, and $H$ = Henry's law constant. It defines the vapour-liquid equilibrium for dilute solutions.

Distinguish physical adsorption from chemisorption.

Physical adsorption (physisorption) involves weak van der Waals forces, is reversible, has low heat of adsorption, and can form multilayers. Chemisorption involves chemical bonds, is often irreversible, has high heat of adsorption, and forms a single monolayer.

Name common adsorbents used for gaseous pollutant control and a key selection property.

Activated carbon (for organic vapours/VOCs), silica gel, activated alumina, and molecular sieves (zeolites). Key property: high specific surface area (large internal porosity) per unit mass to maximize adsorption capacity.

When is condensation an appropriate gas-control technique?

Condensation is suitable for recovering vapours present at high concentration and with high boiling points (low volatility). It works by cooling and/or compressing the gas below its dew point; it is often used as a pre-treatment before adsorption or incineration.

Distinguish thermal incineration from catalytic incineration for gaseous contaminant control.

Thermal incineration (direct flame) oxidizes combustible gases at high temperature (~$700$–$1000\,^{\circ}\mathrm{C}$). Catalytic incineration uses a catalyst (e.g. Pt, Pd) to oxidize at lower temperature (~$300$–$500\,^{\circ}\mathrm{C}$), saving fuel but susceptible to catalyst poisoning.

What is meant by vapour-solid equilibrium in the context of adsorption, and what describes it?

Vapour-solid equilibrium describes the relationship between the amount of gas adsorbed on a solid and the gas-phase partial pressure at constant temperature. It is described by adsorption isotherms such as the Freundlich and Langmuir isotherms.

Write the Freundlich adsorption isotherm equation.

$$\frac{x}{m} = K\, p^{1/n}$$ where $x/m$ = mass of adsorbate per unit mass of adsorbent, $p$ = equilibrium partial pressure (or concentration), and $K$, $n$ are empirical constants ($n > 1$).

Write the Langmuir adsorption isotherm equation and state its key assumption.

$$\frac{x}{m} = \frac{a\,b\,p}{1 + b\,p}$$ where $a$, $b$ are constants and $p$ is partial pressure. Key assumption: monolayer adsorption on a finite number of identical, energetically equivalent sites with no interaction between adsorbed molecules.

State Fick's first law of diffusion.

$$J_A = -D_{AB}\frac{dC_A}{dx}$$ where $J_A$ = molar flux of A (mol/m$^2$·s), $D_{AB}$ = diffusion coefficient (diffusivity), and $\dfrac{dC_A}{dx}$ = concentration gradient. The negative sign shows diffusion proceeds down the gradient.

State Fick's second law of diffusion (unsteady-state, one dimension).

$$\frac{\partial C_A}{\partial t} = D_{AB}\frac{\partial^{2} C_A}{\partial x^{2}}$$ It describes how concentration changes with time at a point due to diffusion.

What is the two-film (two-resistance) theory of interfacial mass transfer?

It assumes two stagnant films (one gas-side, one liquid-side) on either side of the interface where all resistance to mass transfer is concentrated. Equilibrium exists at the interface, and transfer occurs by molecular diffusion through each film, the rates being equal at steady state.

Write the overall gas-phase mass-transfer flux equation using an overall coefficient.

$$N_A = K_G\,(p_{A} - p_{A}^{*})$$ where $N_A$ = molar flux, $K_G$ = overall gas-phase mass-transfer coefficient, $p_A$ = bulk gas partial pressure, $p_A^{*}$ = partial pressure in equilibrium with bulk liquid.

How do the individual film coefficients combine into the overall gas-phase mass-transfer coefficient?

$$\frac{1}{K_G} = \frac{1}{k_G} + \frac{H}{k_L}$$ where $k_G$ and $k_L$ are gas- and liquid-film coefficients and $H$ is Henry's law constant. Total resistance is the sum of gas-film and liquid-film resistances.

What is an emission inventory in air-quality management?

An emission inventory is a comprehensive, quantitative database listing the amounts of each pollutant emitted by all sources (point, line, area, mobile) over a defined area and time period. It is the basis for dispersion modelling and control-strategy planning.

What is a wind rose diagram and what information does it convey?

A wind rose is a polar diagram showing the frequency (and often speed) of winds blowing from each compass direction over a period. It indicates the prevailing wind direction, which determines the direction pollutants are transported and where downwind impacts occur.

Define atmospheric stability and relate it to the environmental lapse rate.

Atmospheric stability describes the tendency of air to resist or promote vertical motion. Comparing the environmental lapse rate (ELR) with the dry adiabatic lapse rate (DALR $\approx 9.8\,^{\circ}\mathrm{C/km}$): ELR > DALR is unstable (superadiabatic, good dispersion); ELR < DALR is stable (subadiabatic); ELR = DALR is neutral. Inversions (ELR negative) are strongly stable.

Define mixing height and explain its significance for pollutant dispersion.

Mixing height (mixing depth) is the height of the atmospheric layer near the ground within which vigorous vertical mixing occurs. A greater mixing height gives a larger volume for dilution and lower ground-level concentrations; a low mixing height (e.g. during inversions) traps pollutants and raises concentrations.

Write the Gaussian plume equation for ground-level concentration along the plume centerline from an elevated point source.

$$C(x,0,0) = \frac{Q}{\pi\, u\, \sigma_y\, \sigma_z}\exp\!\left(-\frac{H^{2}}{2\sigma_z^{2}}\right)$$ where $Q$ = emission rate, $u$ = mean wind speed, $\sigma_y,\sigma_z$ = horizontal/vertical dispersion coefficients, $H$ = effective stack height. The full equation adds Gaussian terms in $y$ and $z$ with ground reflection.

Define effective stack height and write its components.

Effective stack height $H$ is the height at which the plume centerline levels off: $$H = h_s + \Delta h$$ where $h_s$ = physical stack height and $\Delta h$ = plume rise due to the exhaust gas momentum and buoyancy. Plume rise is estimated by formulas such as Holland's equation.