7.1 Gaseous Pollutant Control Technologies

Gaseous Pollutant Control Technologies

Controlling industrial air emissions—specifically Volatile Organic Compounds (VOCs), Hazardous Air Pollutants (HAPs), and acid gases ($HCl, HF, SO_x$)—is required under the Clean Air Act to meet Maximum Achievable Control Technology (MACT) and National Emission Standards for Hazardous Air Pollutants (NESHAP). Selecting the appropriate gaseous control technology requires evaluating gas stream volumetric flow rate ($Q$), pollutant concentration, solubility, molecular weight, thermodynamic properties, and required destruction or removal efficiency (DRE). Environmental engineers utilize four primary unit operations: gas absorption (scrubbing), solid adsorption, thermal incineration, and catalytic oxidation.

Absorption & Packed-Bed Wet Scrubber Engineering

Absorption is a mass transfer operation where soluble gaseous contaminants dissolve into a liquid solvent (typically water or a chemical reagent solution like $NaOH$). Gas absorption is executed in counter-current packed-bed wet scrubbers to maximize gas-liquid contact area.

Equilibrium & Henry's Law

The maximum theoretical concentration of gas that can dissolve in the liquid phase is governed by Henry's Law:

$$P_A = H \cdot x_A$$

Where:

• $P_A$ = Partial pressure of pollutant $A$ in the gas phase ($atm$)
• $H$ = Henry's Law constant for the specific chemical system ($atm/mole \ fraction$)
• $x_A$ = Equilibrium mole fraction of pollutant $A$ in the liquid phase

High $H$ values ($> 100 \ atm/mole \ fraction$) indicate low aqueous solubility (difficult to absorb in water alone), requiring chemical scrubbing reagents to react with and consume the dissolved gas.

Column Sizing: HTU and NTU Method

The total height ($Z$) of packing required in a packed tower is calculated by multiplying the Height of a Transfer Unit ($HTU$) by the Number of Transfer Units ($NTU$):

$$Z = HTU \times NTU$$

• Height of a Transfer Unit ($HTU$): Measures the physical mass transfer efficiency of the packing material: $$HTU = \frac{G}{K_G a \cdot P_T}$$ Where $G$ is gas molar flux ($kmol/m^2\cdot s$), $K_G a$ is overall volumetric mass transfer coefficient ($kmol/m^3\cdot s \cdot atm$), and $P_T$ is total pressure.
• Number of Transfer Units ($NTU$): Represents the thermodynamic difficulty of the separation. For dilute gas streams with a linear equilibrium line ($y^* = m x$), $NTU$ is calculated as: $$NTU = \int_{y_2}^{y_1} \frac{dy}{y - y^*} = \frac{\ln\left[ \left(\frac{y_1 - m x_2}{y_2 - m x_2}\right)(1 - \frac{1}{A_{abs}}) + \frac{1}{A_{abs}} \right]}{1 - \frac{1}{A_{abs}}}$$ Where $y_1, y_2$ are inlet/outlet gas mole fractions, and $A_{abs} = \frac{L}{m G}$ is the Absorption Factor.

Hydraulics & Flooding Limits

Scrubber diameter is sized based on gas velocity relative to the flooding limit. Passing gas upward against descending liquid creates a pressure drop across the packing. If gas velocity is too high, liquid is held up in the packing (flooding), causing massive pressure spikes and liquid entrainment. Scrubbers are designed to operate at $50 - 70\%$ of the flooding velocity, determined via the Sherwood-Leva-Eckert generalized pressure drop correlation.

Solid Adsorption & Fixed-Bed Carbon Systems

Adsorption is a physical surface phenomenon (physisorption) where VOC molecules (adsorbates) are captured within the micropores of a porous solid (adsorbent), primarily Granular Activated Carbon (GAC) ($surface \ area \ 800 - 1500 \ m^2/g$). Adsorption is ideal for dilute VOC streams ($< 1000 \ ppmv$) and solvent recovery.

Adsorption Isotherms

The equilibrium capacity of carbon ($q_e$, $g \ VOC / g \ carbon$) at a constant temperature is modeled using isotherms:

1. Freundlich Isotherm (Multi-layer / Heterogeneous surface): $$q_e = K_f P_A^{1/n}$$ Where $K_f$ and $n$ are empirical constants.
2. Langmuir Isotherm (Monolayer / Homogeneous surface): $$q_e = \frac{q_{max} K_L P_A}{1 + K_L P_A}$$ Where $q_{max}$ is maximum monolayer capacity and $K_L$ is the Langmuir affinity constant.

Mass Transfer Zone (MTZ) & Breakthrough Curves

As VOC-laden gas flows through a fixed carbon bed, an active Mass Transfer Zone (MTZ) forms and migrates downstream. Plotting effluent VOC concentration ($C$) versus time yields a Breakthrough Curve:

• Breakthrough Point ($t_b$): Time when effluent concentration reaches the allowable limit ($C/C_0 \approx 0.05$). The bed must be taken offline.
• Saturation Point ($t_s$): Time when carbon is fully saturated ($C/C_0 \approx 0.95$).
• Carbon Usage Rate (CUR): Mass of carbon consumed per unit volume of gas treated: $$CUR = \frac{\text{Mass of Carbon in Bed}}{\text{Volume of Gas Treated until } t_b}$$

Regeneration: Thermal Swing (TSA) vs. Pressure Swing (PSA)

Dual-bed adsorbers run in parallel. While Bed A treats gas, Bed B is regenerated:

• Thermal Swing Adsorption (TSA): Injected live steam ($212 - 300^\circ F$) heats the carbon, desorbing VOCs into a concentrated vapor stream that is condensed and recovered.
• Pressure Swing Adsorption (PSA): Reduces vessel pressure under vacuum to desorb gases, common in gas purification.

Thermal Incineration & Oxidizer Thermodynamics

Thermal oxidation destroys organic pollutants by high-temperature combustion, converting VOCs and HAPs into $CO_2$ and $H_2O$:

$$C_x H_y + \left(x + \frac{y}{4}\right) O_2 \xrightarrow{\Delta} x CO_2 + \frac{y}{2} H_2O + \text{Heat}$$

The 3 Ts of Combustion & Destruction Efficiency

Complete destruction ($DRE \ge 99\%$) requires strict adherence to the 3 Ts of Combustion:

1. Temperature: High combustion chamber operating temperature ($1400 - 1800^\circ F$ / $760 - 980^\circ C$).
2. Time: Adequate residence time ($t = V_{chamber} / Q_{actual}$) inside the reactor, typically $0.5 - 1.0 \text{ seconds}$.
3. Turbulence: High Reynolds number mixing of VOC vapors with combustion air to prevent localized unburned fuel pockets.

Energy Recovery: Recuperative vs. Regenerative (RTO)

Because thermal oxidizers consume significant natural gas, heat recovery is essential:

• Recuperative Thermal Oxidizers: Use metallic shell-and-tube heat exchangers to preheat incoming gas using hot exhaust ($50 - 70\%$ thermal efficiency).
• Regenerative Thermal Oxidizers (RTO): Utilize multiple ceramic media beds to capture waste heat. Flow direction is periodically reversed via valves, achieving $95 - 97\%$ thermal energy recovery and drastically reducing auxiliary fuel costs.

Catalytic Oxidation & Catalyst Degradation

Catalytic oxidizers insert a noble metal catalyst bed (Platinum (Pt), Palladium (Pd), or Rhodium (Rh)) into the gas stream. The catalyst accelerates the oxidation reaction by significantly lowering the required activation energy ($E_a$).

Operational Parameters & Advantages

• Operating Temperature: Operates at $500 - 900^\circ F$ ($260 - 480^\circ C$), saving up to $75\%$ in supplemental natural gas compared to thermal oxidizers.
• Space Velocity ($SV$): Defines reactor loading rate ($h^{-1}$): $$SV = \frac{Q_{gas} (m^3/h)}{V_{catalyst} (m^3)}$$

Catalyst Poisoning & Deactivation

Catalytic systems are vulnerable to performance loss from:

• Poisoning: Irreversible chemical bonding of silicon compounds, heavy metals (Pb, As), or phosphorus to active catalyst sites.
• Inhibition / Masking: Reversible masking by sulfur oxides ($SO_x$), halogens, or soot/particulate deposits.
• Thermal Sintering: Permanent loss of catalyst surface area due to localized overheating ($> 1200^\circ F$).