I. Introduction to Wet Scrubbers
Wet scrubbers are systems designed to remove harmful substances from industrial exhaust gases (flue gas) by using a liquid medium. They were the first type of scrubber developed to combat air pollution.

The primary purpose of a wet scrubber is to remove acidic gases, which contribute to acid rain, before they are emitted into the atmosphere. The system works by directing exhaust gas into a chamber where it is sprayed with a liquid such as water or a chemical solution. This liquid captures and neutralizes acidic gases and particulate matter.

II. Types of Wet Scrubbers
Wet scrubbers come in various types, each tailored for specific pollutants and applications:

1. Spray Tower Scrubbers

• Design and Function:
  • An open vessel or tower equipped with spray nozzles to distribute the scrubbing liquid.
  • Exhaust gas enters from the bottom and flows upward, counter to the liquid spray.
  • Pollutants are captured as the gas interacts with liquid droplets.
• Advantages:
  • Simple construction
  • Low pressure drop
  • Energy-efficient
• Disadvantages:
  • Less effective for small particles
  • Risk of gas bypassing the liquid
• Applications:
  • Control of dust, mists, and fumes
  • Particulate matter removal

2. Venturi Scrubbers

• Design and Function:
  • High-velocity gas flow is directed into a narrow throat where liquid is injected.
  • The high speed shatters the liquid into tiny droplets, increasing pollutant capture efficiency.
  • A diverging section reduces gas velocity post-scrubbing.
• Advantages:
  • High removal efficiency for fine particles
  • Effective for submicron particulates
• Disadvantages:
  • Higher energy usage and pressure drop
  • Noisy operation
  • Potential for erosion
• Applications:
  • Industrial exhaust gas cleaning
  • Fine particulate and acid gas removal

3. Packed-Bed Scrubbers

• Design and Function:
  • Use packing material (e.g., rings, saddles) to increase surface area for gas-liquid contact.
  • Liquid flows over the packing, creating a film to absorb pollutants from the gas.
  • Gas flows vertically upward in a counter-current arrangement.
• Advantages:
  • High removal efficiency
  • Moderate pressure drop
  • Compact design
• Disadvantages:
  • Limited to low dust loads
  • Risk of clogging and channeling
• Applications:
  • Removal of inorganic gases, VOCs, and some particulate matter

III. How Wet Scrubbers Work

Wet scrubbers operate by ensuring intimate contact between the polluted gas and a scrubbing liquid to remove contaminants effectively.

Gas-Liquid Contact Methods

1. Spraying Method:
   • Liquid is sprayed as fine droplets over the exhaust gas.
   • The liquid captures particulate matter and reacts chemically with acidic gases.
2. Bubbling Method:
   • Exhaust gas is passed through a bath of scrubbing liquid.
   • Pollutants are absorbed as bubbles rise through the liquid, maximizing contact time.

Scrubbing Mechanisms

• Particle Impaction: Solid particles collide and stick to liquid droplets.
• Diffusion: Gas molecules diffuse into the liquid and are absorbed.
• Chemical Reaction: Acidic gases react with neutralizing agents in the liquid.

Scrubbing Liquids

• Water: Effective for dust and particulate removal.
• Alkaline Solutions: Neutralize acidic gases (e.g., sodium hydroxide, calcium hydroxide).
• Specialized Solvents: Target specific gases (e.g., glycols).