11.4 Gaseous Emission Control and Air Cleaner Selection

Gaseous Emission Control and Air Cleaner Selection

Particulate collectors work on inertia, interception, and electrostatic attraction. None of those mechanisms touches a gas or vapour molecule, which is why gaseous control uses an entirely different family of devices — absorption, adsorption, and oxidation.

1. Gaseous Emission Control: Absorption, Adsorption & Oxidation

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   |                  GASEOUS EMISSION CONTROL TECHNOLOGIES                  |
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   |                                                                         |
   |  A. ABSORPTION (Gas-to-Liquid Mass Transfer):                           |
   |     - Soluble gases (HCl, NH3, SO2, Cl2) dissolve into liquid solvent.  |
   |     - Packed towers with random/structured packing provide high surface.|
   |                                                                         |
   |  B. ADSORPTION (Gas-to-Solid Surface Condensation):                     |
   |     - VOCs captured in micropores of activated carbon (1,000 m²/g).     |
   |     - Characterized by S-shaped breakthrough curves and bed life.       |
   |                                                                         |
   |  C. THERMAL / CATALYTIC OXIDATION (Combustion Destruction):             |
   |     - VOC + O2 + Heat ===> CO2 + H2O                                    |
   |     - Direct Thermal: 1,400 - 1,800°F (High fuel, RTO has 95% recovery) |
   |     - Catalytic (Pt/Pd): 600 - 800°F (Lower energy; poison sensitive)   |
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1. Packed Bed Absorption Towers (Scrubbers)

Absorption involves transferring a soluble gaseous pollutant from a gas stream into a liquid solvent (physical absorption) followed by chemical neutralization (chemical absorption):

• Governing Law: Governed by Henry's Law (pi = Hi · xi, where pi is partial pressure and Hi is Henry's constant).
• Column Hydrodynamics: Counter-current packed beds operate between the loading point (where liquid holdup begins to impede gas flow) and the flooding point (where upward gas velocity completely backs up liquid, causing column failure). Towers are designed to operate at 50% to 70% of flooding velocity.
• Common Reagents: Sodium hydroxide (NaOH) for acid gases (HCl, SO2, HF); Sulfuric acid (H2SO4) for basic gases (ammonia, amines); Sodium hypochlorite (NaOCl) for odors and sulfur compounds.

2. Adsorption Systems (Activated Carbon)

Adsorption is a surface phenomenon where gas molecules (adsorbate) are attracted and held to the porous solid surface (adsorbent) by physical van der Waals forces (physisorption) or covalent chemical bonding (chemisorption).

• Adsorbent Characteristics: Granular Activated Carbon (GAC) possesses an extraordinary specific surface area of 800 to 1,500 m²/g packed into micro- and mesopores (< 2 to 50 nm).
• Breakthrough Curve: As gas passes through the carbon bed, an adsorption zone (mass transfer zone, MTZ) moves progressively downstream. When the leading edge of the MTZ reaches the bed exit, the exit concentration abruptly spikes—termed the breakthrough point (Cb, typically defined at 5% of inlet concentration C0).

   +-------------------------------------------------------------------------+
   |                  CARBON BED BREAKTHROUGH DYNAMICS                       |
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   |                                                                         |
   |  Exit Concentration (C / C0)                                            |
   |    1.0 |                                                  ...---* Saturated
   |        |                                            ..-''               |
   |        |                                         .-'                    |
   |        |                                      .-' (Mass Transfer Zone)  |
   |    0.05|-------------------------------------* Breakthrough Point (tb)  |
   |    0.0 +-------------------------------------+-------------------> Time |
   |        0                                    tb                          |
   +-------------------------------------------------------------------------+

• Wheeler-Jonas Bed Depth Service Life (BDSL) Model: The breakthrough time (tb) is calculated from carbon bed mass (W), bulk density (ρb), airflow (Q), adsorptive capacity (We), and adsorption rate constant (kv):

$t_b = \frac{W_e \cdot W}{C_0 \cdot Q} - \frac{W_e \cdot \rho_b}{k_v \cdot C_0} \ln\left(\frac{C_0 - C_b}{C_b}\right)$

• Regeneration: Saturated carbon beds are desorbed using superheated steam or hot inert gas (extN2 at 250--300°F), recovering solvent and restoring bed capacity.

3. Thermal and Catalytic Oxidizers

Thermal destruction of VOCs converts hydrocarbons into carbon dioxide and water vapor via high-temperature oxidation (CxHy + (x + y/4)O2 → x CO2 + (y/2) H2O):

• The 3 T's of Combustion: Complete destruction (> 99%) requires optimizing Time (residence time 0.5 to 2.0 seconds), Temperature, and Turbulence (Reynolds number > 10,000).
• Direct Flame Thermal Oxidizers (DFTO): Operate at 1,400 to 1,800°F (760 to 980°C). High auxiliary fuel consumption unless equipped with recuperative heat exchangers.
• Regenerative Thermal Oxidizers (RTO): Utilize multiple ceramic honeycomb/saddle beds to preheat incoming process air with outgoing combustion exhaust, achieving 95%+ thermal energy recovery and allowing self-sustaining (autothermal) operation at VOC concentrations > 1.5--2.0 g/m³.
• Catalytic Oxidizers: Pass preheated VOC gas across a noble metal catalyst bed (platinum, palladium, rhodium on ceramic honeycomb). The catalyst lowers the required activation energy, allowing complete oxidation at 600 to 800°F (315 to 425°C)—saving massive auxiliary fuel.

Critical CIH Exam Concept — Catalyst Poisoning & Deactivation: Catalytic oxidizers cannot be used if the exhaust gas contains catalyst poisons or masking agents:

• Irreversible Chemical Poisons: Heavy metals (lead, arsenic, mercury), phosphorus, silicones (form glass-like SiO2 crusts over active sites).
• Catalyst Inhibitors / Deactivators: Sulfur compounds (SO2, H2S), halogens (chlorinated solvents like TCE or methylene chloride form acidic HCl and corrode/poison noble metals).

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2. Comprehensive Air Cleaner Comparison Matrix

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3. Worked Step-by-Step Calculation Examples

Worked Example 10.2.1: Sizing a Pulse-Jet Baghouse Filter Area

Problem: An industrial woodworking facility generates fine sawdust and sander dust collected by an LEV system exhausting Q = 18,000 cfm. The engineering design standard specifies a pulse-jet baghouse with needle-punched polyester felt media operating at an Air-to-Cloth ratio (A/C) of 6.0 ft/min. Cylindrical filter bags with a diameter D = 6.0 inches (0.50 ft) and length L = 10.0 feet are specified.

1. Calculate the total active cloth filtration area (Acloth) required.
2. Calculate the surface area of an individual filter bag (Abag).
3. Determine the minimum number of filter bags required in the baghouse.

Solution Steps:

1. Calculate total required cloth area (Acloth = Q / [A/C]): $A_{\text{cloth}} = \frac{18,000\text{ cfm}}{6.0\text{ ft/min}} = 3000\text{ ft}^2$

2. Calculate the surface area of one cylindrical bag (Abag = π · D · L): $D = 6.0\text{ in} = 0.50\text{ ft}$ $A_{\text{bag}} = \pi \times 0.50\text{ ft} \times 10.0\text{ ft} = 3.14159 \times 5.0 = 15.708\text{ ft}^2$

3. Determine the number of bags required (Nbags = Acloth / Abag): $N_{\text{bags}} = \frac{3000\text{ ft}^2}{15.708\text{ ft}^2/\text{bag}} = 190.98 \implies 191\text{ bags}$ (In standard practice, this is rounded up to a symmetrical structural grid, e.g., a 14 × 14 = 196 bag housing).

Result: The baghouse requires a total active cloth area of 3,000 ft² and a minimum of 191 filter bags.

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Worked Example 10.2.2: Electrostatic Precipitator Efficiency via Deutsch-Anderson Equation

Problem: An industrial coal-fired utility boiler utilizes an electrostatic precipitator to clean Q = 120,000 cfm (2000 ft³/sec) of flue gas. The ESP has a total collection plate surface area A = 40,000 ft². Field testing confirms an average particle migration velocity w = 0.35 ft/sec (21.0 ft/min).

1. Calculate the fractional collection efficiency (η) and percentage efficiency of the ESP.
2. If the plate area is increased by 25% to Anew = 50,000 ft², calculate the new collection efficiency.

Solution Steps:

1. Calculate the exponent term ((w · A)/Q): $\text{Using consistent units in seconds: } w = 0.35\text{ ft/s}, \quad A = 40,000\text{ ft}^2, \quad Q = 2000\text{ ft}^3/\text{s}$ $\frac{w \cdot A}{Q} = \frac{0.35 \times 40,000}{2000} = \frac{14,000}{2000} = 7.0$ (Using minutes: (21.0 ft/min × 40,000 ft²)/(120,000 cfm) = 840,000/120,000 = 7.0)

2. Calculate collection efficiency using Deutsch-Anderson equation: $\eta = 1 - e^{-7.0} = 1 - 0.0009118 = 0.999088 \implies \mathbf{99.91\%}$

3. Calculate new efficiency with A = 50,000 ft²: $\frac{w \cdot A_{\text{new}}}{Q} = \frac{0.35 \times 50,000}{2000} = 8.75$ $\eta_{\text{new}} = 1 - e^{-8.75} = 1 - 0.0001584 = 0.999842 \implies \mathbf{99.98\%}$

Result: The initial collection efficiency is 99.91%; expanding the plate area by 25% increases efficiency to 99.98% (reducing particulate emissions by over 82%).

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Worked Example 10.2.3: Activated Carbon Adsorber Bed Service Life

Problem: An exhaust stream conveying Q = 2,000 cfm containing C0 = 150 ppm of toluene vapor enters an activated carbon adsorption vessel containing W = 1,200 lbs of virgin GAC.

• Toluene molecular weight = 92.14 g/mol
• Carbon equilibrium capacity (We) = 0.15 lb toluene / lb carbon (15% by weight)
• Calculate the operating bed service life in hours before total saturation, assuming ideal sharp breakthrough.

Solution Steps:

1. Calculate total toluene adsorptive capacity of the carbon bed: $\text{Total Capacity} = W \times W_e = 1,200\text{ lbs carbon} \times 0.15\text{ lb toluene/lb carbon} = 180\text{ lbs of toluene}$

2. Calculate the mass emission rate of toluene entering the bed (ER(lb/hr)): $Q = \frac{387 \times 10^6 \cdot ER_{\text{lb/hr}}}{60 \cdot \text{MW} \cdot C_{\text{ppm}}} \implies ER_{\text{lb/hr}} = \frac{Q \cdot 60 \cdot \text{MW} \cdot C_{\text{ppm}}}{387 \times 10^6}$ $ER_{\text{lb/hr}} = \frac{2000 \times 60 \times 92.14 \times 150}{387,000,000} = \frac{1,658,520,000}{387,000,000} = 4.2856\text{ lbs of toluene/hr}$

3. Calculate bed service life (tservice): $t_{\text{service}} = \frac{\text{Total Capacity}}{ER_{\text{lb/hr}}} = \frac{180\text{ lbs}}{4.2856\text{ lbs/hr}} = 42.0\text{ hours}$

Result: The carbon bed will reach saturation after 42.0 hours of continuous operation.