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Wet Scrubbers Principle Wet scrubbers are used for removal of particles which have a diameter of the order of 0.2 mm or higher. Wet scrubbers work by spraying a stream of fine liquid droplets on the incoming stream. The droplets capture the particles The liquid is subsequently removed for treatment.

Wet Scrubbers Construction and Operation A wet scrubber consists of a rectangular or circular chamber in which nozzles are mounted. The nozzles spray a stream of droplets on the incoming gas stream The droplets contact the particulate matter, and the particles get sorbed. The droplet size has to be optimized.

Wet Scrubbers (contd.) Construction and Operation (contd.) Smaller droplets provide better cleaning, but are more difficult to remove from the cleaned stream. The polluted spray is collected. Particles are settled out or otherwise removed from the liquid. The liquid is recycled. Wet scrubbers are also used for the removal of gases from the air streams.

Types of scrubbers Spray towers Venturi scrubbers Cyclone scrubbers Packed scrubbers Mechanical scrubbers Impingement Plate Scrubbers

Spray Tower Scrubbers This is the simplest type of particulate wet scrubber in commercial service. Sets of spray nozzles located near the top of the scrubber vessel generate water droplets that impact with particles in the gas stream as the gas stream moves upwards.

Spray towers

Venturi Scrubbers The dirty gas is led in to the chamber at high inlet velocities. At the inlet throat, liquid at low pressure is added to the gas stream This increases the relative velocity between the gas and the droplets, thus increasing the efficiency of removal. Efficiencies of the range of 95% for particles larger than 0.2 mm have been obtained.

Venturi Scrubber Absolute Pressure Drop Δp = pressure drop ( cm of water) ug = gas velocity (cm/s) Qt = liquid volume flow rate Qg = gas volume flow rate

Problem Water is introduced into the throat of a venturi scrubber. The air velocity through the scrubber is 550 fps and the liquid to gas ratio is 8.5 gal/1000 actual ft3. Determine the pressure drop? Δp = 4.8

Venturi Scrubbers

Cyclone Spray Chambers These scrubbers combine a cyclone with a spray nozzle. The added centrifugal force permits good separation of the droplets, hence a smaller droplet size can be used. Cyclone spray chambers provide up to 95% removal of particles > 5 micron.

Cyclone scrubber

Mechanical scrubbers

Packed scrubbers

Impingement Plate Scrubbers These scrubbers usually have one to three horizontal plates, each of which has a large number of small holes. The gas stream accelerating through the holes atomizes some water droplets in the water layer above the plate. Particles impact into these water droplets.

Efficiency where, k = Scrubber coefficient (m3 of gas/ m3 of liquid) R = Liquid-to-gas flow rate (QL/QG) ψ = internal impaction parameter Internal impaction parameter c = Cunningham correction factor ρp = particle density (kg/m3) Vg = speed of gas at throat (m/sec) dp = diameter of particle (m) dd = diameter of droplet (m) μ = dynamic viscosity of gas, (Pa-S)

Collection Efficiency of Wet Scrubbers The velocities of the particle-laden gas stream and the liquid targets vary substantially. There are substantial differences in the ability of particulate wet scrubbers to collect particles less than approximately 5 micrometers. If a significant portion of the particulate matter mass is composed of particles < 5 micrometers, care is needed to select the type of scrubber that is effective in this size range. It should be noted that some types of wet scrubbers have limited capability to remove particles in the less than 0.3-micrometer range. These particles are so small that their movement is influenced by collisions with individual molecules in the gas stream.

Scrubber Operating Principles The ability of a particulate wet scrubber to remove particles depends on two or more of the following variables: The size (aerodynamic diameter) of the particle The velocity of the particle The velocity of the droplet.

Advantages of Wet Scrubbers Wet Scrubbers can handle incoming streams at high temperature, thus removing the need for temperature control equipment. Wet scrubbers can handle high particle loading. Loading fluctuations do not affect the removal efficiency. They can handle explosive gases with little risk. Gas adsorption and dust collection are handled in one unit. Corrosive gases and dusts are neutralized. Hot gases can be cooled down. Corrosive gases can be recovered and neutralize.

Disadvantages of Wet Scrubbers High potential for corrosive problems Effluent scrubbing liquid poses a water pollution problem. Poses freezing problem in cold countries. Maintenance cost is high when corrosive materials are collected. they require make-up water to replace the water vaporized into the gas stream and lost to purge liquid and sludge removed from the scrubber system.