Air Pollution Control by Wet Scrubbing

Summary

This video provides an in-depth explanation of wet scrubbers, which are used in air pollution control to capture particles from gas streams. It covers the working principles, types of scrubbers (such as spray towers, tray towers, and venturi scrubbers), and their design features. The video also explores the operating conditions, challenges, and performance monitoring of wet scrubbers. Key points include the importance of liquid flow, pressure drops, and the role of mist eliminators in ensuring efficient operation. Additionally, it discusses safety considerations and the need for proper system inspection to maintain optimal performance.

Takeaways

• 😀 Wet scrubbers are devices used to capture particles and pollutants from gas streams by transferring them into a liquid, typically water or an aqueous chemical solution.
• 😀 Wet scrubbers are categorized by their energy usage levels: low, medium, and high energy, which impacts their efficiency in collecting particles of different sizes.
• 😀 Low-energy wet scrubbers can efficiently remove particles larger than 5 micrometers in diameter, while high-energy scrubbers are capable of capturing submicrometer particles with significant energy input.
• 😀 The spray tower is a low-energy wet scrubber that uses nozzles to distribute liquid over a gas stream, capturing particles when they collide with droplets.
• 😀 Tray towers and packed bed scrubbers are medium-energy scrubbers that use trays or packing materials to increase the surface area for particle collection and are suitable for handling gas flows of varying rates.
• 😀 Venturi scrubbers are high-energy devices that accelerate gas streams through a narrow throat, where liquid streams collide with the gas to break it into droplets that capture particles.
• 😀 Mist eliminators, such as chevrons, mesh pads, and cyclones, are used in all wet scrubbers to remove entrained droplets from the gas stream before it exits the system.
• 😀 The performance of a wet scrubber is influenced by factors such as particle size, pressure drop, liquid flow rate, and pH levels, which must be monitored to maintain efficiency.
• 😀 Common issues that affect scrubber performance include inadequate liquid flow, improper liquid pH, nozzle plugging, corrosion, and decreased gas-liquid contact.
• 😀 In-field inspections of wet scrubbers involve evaluating parameters like gas stream opacity, temperature differences between inlet and outlet, liquid flow rate, and pressure drops to assess performance and ensure compliance with regulations.

Q & A

What is the primary purpose of wet scrubbers in air pollution control?

Wet scrubbers are used to capture particles and various pollutants from gas streams by transferring them to a liquid, usually water or a chemical solution. This process helps reduce air pollution by efficiently removing contaminants from the air.

How do wet scrubbers remove particles from the gas stream?

Wet scrubbers remove particles from a gas stream by transferring them to a liquid. The liquid typically interacts with the gas stream to absorb or react with the contaminants, thereby capturing the particles.

What are the different types of wet scrubbers based on energy usage?

Wet scrubbers are classified into three types based on energy usage: low energy (pressure drop <5 inches of water), medium energy (pressure drop 5–25 inches of water), and high energy (pressure drop >25 inches of water).

What are spray towers, and how do they work in wet scrubbing?

Spray towers are a low-energy scrubber type where the gas stream enters from the bottom and passes upward through spray nozzles distributing the scrubbing liquid. Particles are collected when they impact the liquid droplets in a countercurrent or cross-current arrangement.

What is the difference between countercurrent and cross-current spray towers?

In a countercurrent spray tower, the gas stream flows upward while the liquid flows downward, maximizing contact for particle collection. In a cross-current spray tower, the gas flows horizontally while the liquid flows downward, but these are generally less efficient than countercurrent designs.

What is a packed bed scrubber, and how does it function?

A packed bed scrubber is a medium-energy wet scrubber that uses packing material to create a large surface area for gas-liquid contact. The liquid is introduced at the top, flowing downward over the packing, while the gas flows upward, enabling particles to be captured by impaction.

What are some common mist eliminators used in wet scrubbers?

Common mist eliminators in wet scrubbers include chevrons (zigzag baffles), mesh pads (interlaced fibers that trap droplets), and cyclones (which use centrifugal force to remove droplets). These devices help prevent entrained droplets from exiting the scrubber with the gas stream.

What performance monitoring methods are used for wet scrubbers?

Performance monitoring methods include evaluating the opacity of the gas stream (to check for particulate presence), measuring the temperature difference between the inlet and outlet, monitoring liquid flow rates, pH levels, and pressure drops, and observing mist eliminator efficiency.

What operating problems are commonly encountered in wet scrubbers?

Common issues in wet scrubbers include inadequate liquid flow, improper pH levels, nozzle or bed clogging, poor gas-liquid contact, corrosion, and equipment failure, all of which can reduce the collection efficiency.

How can field inspections help in maintaining wet scrubber performance?

Field inspections help by assessing gas stream opacity, liquid flow rates, pH levels, temperature differences, pressure drops, and mist eliminator performance. Inspectors can identify deviations from baseline values that may indicate reduced collection efficiency or other issues requiring maintenance.