1. Conceptual Framework & Dual-Control Mechanism

A. Definition & Functional Role

• Scrubber System (Gas Scrubber / Wet Collector): An air pollution control device that removes gaseous pollutants (such as \(\text{SO}_2\), \(\text{HCl}\), \(\text{HF}\), \(\text{NH}_3\), \(\text{H}_2\text{S}\), and VOCs) and/or suspended particulate matter (PM) from industrial flue gas streams by bringing the gas into intimate contact with a scrubbing liquid (usually water or a chemical reagent slurry).
• Dual-Action Capability: Unlike cyclones, ESPs, or fabric filters—which exclusively collect dry particulate matter—wet scrubbers are capable of simultaneously absorbing soluble toxic/acidic gases and capturing fine particulates in a single unit.
• Classification in Air Pollution Control: Classified as a Gas Absorption & Wet Mechanical Particulate Collector.

AIR POLLUTION CONTROL EQUIPMENT FUNCTION MATRIX

System Type                   Particulate Matter (PM)    Gaseous Pollutants (SO₂, HCl, NH₃)
---------------------------   -----------------------    ---------------------------------
Cyclone Separator             Coarse PM (≥ 10 µm)        NO Removal
Baghouse Fabric Filter        Fine PM (PM₂.₅ / PM₁₀)     NO Removal (Requires Dry Sorbent)
Electrostatic Precipitator    Fine PM (PM₂.₅ / PM₁₀)     NO Removal
Scrubber Systems (Wet / FGD)  Fine & Coarse PM           HIGH REMOVAL (Gas Absorption)

2. Physical & Chemical Principles of Wet Scrubbing

Wet scrubbing relies on three main physical and chemical mechanisms:

WET SCRUBBING OPERATIONAL MECHANISMS

Contaminated Industrial Flue Gas + Spray Liquid
                       │
                       ├─► 1. Gaseous Absorption (Mass Transfer of Soluble Acid Gases into Liquid)
                       ├─► 2. Inertial Impaction & Interception (PM Collides with Liquid Droplets)
                       └─► 3. Condensation & Agglomeration (Moisture Humidification Enlarges Fine PM)

1. Gas Absorption (Mass Transfer): Dissolving soluble gas molecules into the scrubbing liquid. Acid gases (e.g., \(\text{SO}_2, \text{HCl}\)) react with alkaline scrubbing reagents (e.g., limestone \(\text{CaCO}_3\), lime \(\text{Ca(OH)}_2\), or caustic soda \(\text{NaOH}\)) to form dissolved or insoluble neutral salts.
2. Inertial Impaction & Interception: High-velocity particulate matter cannot follow the gas streamlines around atomized liquid droplets. Instead, particles collide with and become trapped inside the liquid droplets, which then settle out under gravity.
3. Condensation & Humidification: Cold scrubbing liquid cools hot flue gas below its dew point, causing water vapor to condense around fine sub-micron particles, increasing their size and mass so they can be captured easily.

3. Classification & Working Mechanisms of Scrubbing Systems

SCRUBBER SYSTEM ARCHITECTURE

1. Wet Scrubbing Systems
   - Spray Towers (Low pressure drop; simple counter-current gas-liquid spray)
   - Packed Bed Scrubbers (High surface area packing rings; high gas absorption)
   - Venturi Scrubbers (High velocity throat 60–120 m/s; atomizes liquid; fine PM capture)

2. Dry & Semi-Dry Scrubbing Systems
   - Spray Dryer Absorber (SDA) (Atomized lime slurry evaporates, leaving dry salt powder)
   - Dry Sorbent Injection (DSI) (Pneumatic injection of dry hydrated lime / sodium bicarbonate)

A. Spray Tower / Spray Chamber

• Mechanism: Contaminated flue gas flows upward through a vertical cylindrical tower while rows of spray nozzles distribute liquid droplets downward (counter-current flow).
• Characteristics: Low pressure drop (\(\Delta P = 100 \text{ to } 500 \text{ Pa}\)); highly effective for coarse particles (\(> 10\ \mu\text{m}\)) and highly soluble gases (\(\text{HCl}, \text{NH}_3\)). Resistant to clogging.

B. Packed Bed Scrubber (Packed Tower)

• Mechanism: The chamber is filled with random packing media (e.g., Raschig rings, Pall rings, Berl saddles) or structured corrugated sheets. Scrubbing liquid flows down over the packing, forming a thin liquid film with a large surface area for mass transfer with ascending gas.
• Characteristics: Highest efficiency for gaseous absorption (\(\text{SO}_2, \text{HCl}, \text{Cl}_2\)); susceptible to clogging if gas contains heavy particulate loads.

C. Venturi Scrubber (High-Energy Collector)

• Mechanism: Features a converging section, a narrow throat, and a diverging section. Flue gas accelerates to extreme velocities (60 to 120 m/s) inside the narrow throat. Scrubbing liquid injected at the throat is atomized into millions of microscopic droplets by the high-speed gas stream.
• Impaction Dynamics: The high relative velocity between fine dust particles and atomized droplets drives inertial impaction, capturing sub-micron particulates down to \(< 0.5\ \mu\text{m}\).
• Characteristics: Extremely high pressure drop (\(\Delta P = 2,000 \text{ to } 10,000 \text{ Pa}\) / 20 to 100 cm \(W.G.\)), requiring high fan power.

                   VENTURI SCRUBBER CROSS-SECTION

                      Flue Gas Inflow
                           │
                    ┌──────┴──────┐  Converging Duct
                    │             │  (Gas Accelerates)
               ─────┼─────────────┼─────
                 ▼  │   Throat    │  ▼
               Liquor Injected    │  Liquor Injected
               ─────┼─────────────┼─────  Atomization Zone (60–120 m/s)
                    │             │
                    └──────┬──────┘  Diverging Duct
                           │         (Pressure Recovery)
                           ▼
                 To Cyclonic Mist Eliminator

D. Dry & Semi-Dry Scrubbers (Spray Dryer Absorber & DSI)

• Spray Dryer Absorber (SDA): Lime slurry (\(\text{Ca(OH)}_2\)) is atomized into hot flue gas (\(150^\circ\text{C} \text{ to } 200^\circ\text{C}\)). The alkaline slurry neutralizes \(\text{SO}_2\) and acid gases while the heat completely evaporates the water, producing a dry solid salt powder collected downstream in a baghouse or ESP.
• Dry Sorbent Injection (DSI): Dry powdered hydrated lime or sodium bicarbonate (\(\text{NaHCO}_3\)) is injected directly into the hot flue gas ductwork to react with \(\text{SO}_2\) and \(\text{HCl}\).

4. Flue Gas Desulphurization (FGD) Technologies

Flue Gas Desulphurization (FGD) systems are specialized chemical scrubbers installed on fossil-fuel power plants to remove Sulfur Dioxide (\(\text{SO}_2\)) emissions.

FLUE GAS DESULPHURIZATION (FGD) PROCESSES

1. Wet Limestone-Gypsum FGD (Most Common Worldwide)
   - Reagent: Crushed Limestone (CaCO₃) slurry.
   - Reaction: CaCO₃ + SO₂ ──► CaSO₃ ──(Forced Aeration)──► CaSO₄·2H₂O (Commercial Gypsum).

2. Seawater FGD
   - Reagent: Natural alkalinity (HCO₃⁻ / CO₃²⁻) of seawater.
   - Reaction: SO₂ + Seawater ──► Dissolved Sulfate Ions (SO₄²⁻) (Discharged safely to ocean).

3. Regenerative Alkali FGD (Wellman-Lord Process)
   - Reagent: Sodium Sulfite (Na₂SO₃).
   - Advantage: Produces pure elemental Sulfur or concentrated liquid SO₂; recovers reagent.

Chemical Reaction Pathway of Wet Limestone FGD:

1. \(\text{SO}_2\) Absorption: \[\text{SO}_{2\text{ (gas)}} + \text{H}_2\text{O} \rightarrow \text{H}2\text{SO}{3\text{ (aqueous)}} \rightarrow \text{H}^+ + \text{HSO}_3^-\]

2. Limestone Dissolution & Neutralization: \[\text{CaCO}{3\text{ (slurry)}} + 2\text{H}^+ \rightarrow \text{Ca}^{2+} + \text{CO}{2\text{ (gas)}} + \text{H}_2\text{O}\] \[\text{Ca}^{2+} + \text{HSO}3^- \rightarrow \text{CaSO}{3\text{ (Calcium Sulfite)}} + \text{H}^+\]

3. Forced Oxidation to Commercial Gypsum: Air is bubbled through the reaction tank to convert calcium sulfite into commercial-grade Gypsum: \[\text{CaSO}_3 + \frac{1}{2}\text{O}_2 + 2\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}2\text{O}{\text{ (Synthetic Gypsum)}}\] (Note: Synthetic gypsum is widely recycled in manufacturing plasterboard and Portland cement).

5. Key Operating Parameters & Design Indicators

• Liquid-to-Gas (L/G) Ratio: Expresses the volume of scrubbing liquid injected per unit volume of treated gas (typically 1.0 to 3.0 \(\text{L/m}^3\) for particulate scrubbers and 10 to 20 \(\text{L/m}^3\) for wet FGD absorbers).
• Gas-to-Liquid Contact Area: Maximized using fine spray atomization, packed beds, or high-velocity Venturi throats.
• Mist Eliminator (Demister Pad): Installed at the scrubber outlet (vane-type or mesh pad) to remove entrained liquid droplets from the cleaned gas stream, preventing liquid carryover and stack corrosion.

6. Advantages, Limitations, and Environmental Issues

A. Major Advantages

• Simultaneous Removal: Removes both fine particulate matter and acid gases (\(\text{SO}_2, \text{HCl}, \text{HF}\)) in a single system.
• Handles Hot & Moist Gases: Cools hot flue gases without risk of thermal damage; insensitive to high gas moisture (unlike baghouses, which suffer bag blinding).
• Explosion Protection: Wet environment neutralizes sticky, flammable, or explosive dust risks (e.g., aluminum or magnesium powder).

B. Major Limitations & Environmental Drawbacks

• Wastewater & Sludge Generation: Produces secondary acidic liquid effluent and toxic heavy-metal sludge that require dedicated wastewater treatment plants (WWTP) or solid waste disposal.
• High Operating Energy (Venturi): High pressure drop across Venturi scrubbers dramatically increases fan power requirements.
• Corrosion & Scaling: Acidic liquid streams (\(\text{H}_2\text{SO}_3, \text{HCl}\)) cause severe corrosion on metal walls, while calcium salts cause scaling and nozzle clogging.

7. Comparative Evaluation: Air Pollution Control Equipment

Quick Revision

• Scrubber Definition: Device using a liquid stream to capture particulate matter and absorb gaseous pollutants (\(\text{SO}_2, \text{HCl}\)).
• Dual Function: Simultaneously removes fine PM and acid gases in a single installation.
• Venturi Scrubber: High-velocity throat (\(60\text{--}120\text{ m/s}\)) atomizes liquid, capturing sub-micron PM (\(< 0.5\ \mu\text{m}\)) at a high pressure drop (\(2,000\text{--}10,000\text{ Pa}\)).
• Packed Bed Scrubber: Filled with packing media to maximize liquid-gas contact area for gas absorption.
• Wet FGD (Limestone): Reacts \(\text{SO}_2\) with limestone (\(\text{CaCO}_3\)) slurry; forced oxidation yields commercial Gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)).
• Spray Dryer Absorber (SDA): Semi-dry FGD that atomizes lime slurry into hot gas; water evaporates completely, leaving dry salt powder.
• Mist Eliminator: Mesh pad at scrubber outlet that traps entrained liquid droplets to prevent stack corrosion.
• Main Disadvantages: Produces liquid wastewater/toxic sludge, experiences corrosion and nozzle scaling, and incurs high operating energy in Venturi systems.

Memory Map

1. Flue Gas Entry & Scrubbing Fluid Inflow ──► Dirty Gas + Soluble Acid Gases (\(\text{SO}_2, \text{HCl}\)) + Fine Dust.

2. Contact & Reaction Mechanics ──► Venturi Throat (High Speed Atomization) ──► Inertial Impaction Captures Sub-Micron PM. ──► Packed Bed / Spray Tower ──► Alkaline Slurry (\(\text{CaCO}_3\)) Absorbs \(\text{SO}_2\).

3. Chemical Oxidation & Output ──► Forced Aeration converts Calcium Sulfite ──► Synthetic Gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). ──► Demister Pad removes Liquid Drops ──► Clean Gas to Stack.

4. Secondary Waste Handling ──► Acidic Liquid Effluent & Sludge ──► Directed to Wastewater Treatment Plant (WWTP).

Must Remember

• Dual PM & Gas Removal: Wet scrubbers are the only standard primary air pollution control devices that remove both particulate matter and gaseous pollutants (\(\text{SO}_2, \text{HCl}\)) simultaneously.
• Venturi Throat Velocity: Flue gas reaches \(60\text{--}120\text{ m/s}\) in a Venturi throat, atomizing the scrubbing liquid into micro-droplets.
• Limestone FGD By-product: Forced oxidation in wet limestone FGD converts \(\text{SO}_2\) into synthetic Gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) used in building construction.
• Demister Pad Function: Traps liquid droplets from the outlet gas stream to prevent stack corrosion and rainout.
• Wastewater Trade-Off: Air pollution is converted into water pollution / sludge, requiring secondary effluent management.

Key Terms

• Scrubber: A control device that uses liquid contact to remove particulates and gaseous pollutants from exhaust gases.
• Venturi Scrubber: A high-energy wet scrubber featuring a constricted throat that atomizes liquid droplets for sub-micron particulate capture.
• Flue Gas Desulphurization (FGD): A chemical technology used to remove sulfur dioxide (\(\text{SO}_2\)) from flue gases.
• Spray Dryer Absorber (SDA): A semi-dry scrubbing process where an atomized lime slurry evaporates completely while absorbing acid gases.
• Demister Pad (Mist Eliminator): A device installed at the scrubber outlet to remove entrained liquid droplets from the gas stream.
• Synthetic Gypsum: Calcium sulfate dihydrate (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) produced as a commercial by-product of wet limestone FGD.

1-Line Bits / Short Points

• Wet scrubbers remove both particulate matter and gaseous pollutants in a single system.
• Acidic gases like \(\text{SO}_2\) and \(\text{HCl}\) are absorbed using alkaline scrubbing liquid streams.
• Gas scrubbers use water or liquid solutions to collect dust and acid gas pollutants.
• Flue gas reaches velocities of 60 to 120 m/s inside a Venturi scrubber throat.
• Venturi scrubbers capture sub-micron particles down to less than 0.5 micrometers.
• The pressure drop across a Venturi scrubber ranges from 2,000 to 10,000 Pa.
• Packed bed scrubbers use packing media to provide a high surface area for gas absorption.
• Wet limestone FGD reacts \(\text{SO}_2\) with calcium carbonate to form calcium sulfite.
• Forced oxidation converts calcium sulfite slurry into commercial-grade synthetic gypsum.
• Synthetic gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) is used to manufacture plasterboard and cement.
• Spray dryer absorbers evaporate liquid slurry completely, yielding a dry byproduct powder.
• Demister pads remove entrained liquid droplets from exhaust gas to prevent stack corrosion.
• Wet scrubbers handle hot, humid, or explosive gas streams without risking bag blinding or fires.
• A major disadvantage of wet scrubbers is the generation of acidic wastewater and heavy-metal sludge.