Following are the common wet collection devices used for removing dust: 1. Cyclonic Scrubbers 2. Spray Chambers 3. Venturi Scrubbers 4. Packed Towers.
Device # 1. Cyclonic Scrubbers:
These are also known as wet cyclones. In these cyclones there is usually tangential entry of the aerosol and the particulate matter is commonly separated from the aerosol by using centrifugal force and impingement of water at the entrance.
This cyclone may be provided with a moisture elimination section consisting of zig-zag plates. The design of these cyclonic scrubbers is usually done upto 200 l/min and with water requirements ranging from 2 litres to 50 litres per 40 litres of gas. Dust particles of size 5 m and above arc separated with 90% efficiency and 40-70 mm of H2O of pressure drop.
Device # 2. Spray Chambers:
Fig. 6.5 shows a spray chamber dust remover. It has a simple gravity settling chamber with sprays of liquid. In this device, the fine water spray wash the gas and settle the dust as sludge in sludge tank. Like other settling chambers, some coarse dust will settle on to the surface of the water pool under gravitational force.
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Major removal is due to collision between a dust particle and water droplets resulting in the capture of the former by the latter. This causes increase in size and weight of the dust particles and helps the quick collection.
Device # 3. Venturi Scrubbers:
Fig. 6.6 shows venturi scrubber. These can clean about 400 litres of gas per minute. To remove mist usually the venturi scrubbers are followed by cyclonic separators.
Venturi scrubber mainly consists of a venturi throat through which the carrier gas pushes at a linear velocity of 3400 to 12,600 m/min. Water as the usual scrubbing liquor is added in the direction of the flow of the gas at a rate of 0.3-1.5 litres/min of gas. The introduction of the water is done at the throat with nozzles or at the entrance by means of nozzles or weir type construction.
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The efficiency of the venturi scrubber varies with the input of power and can be extremely high upto 99% even for submicron range particles. At high velocity, water atomizes at the throat and the turbulence created leads to high collection efficiencies for submicron size particles.
If the proper materials are used for the manufacture of venturi scrubbers, they can work for abrasive, corrosive and high temperature gases.
Device # 4. Packed Towers:
Fig. 6.7 shows packed tower for cleaning air and gases. In this tower the dust laden gas is passed upward through the bed of the collecting material. The liquid flows down through the bed, keeping it clean and preventing re-entertainment of the deposited particles. The collecting surface area should be relatively large. The materials used in the collecting surface should have low density, resistant to corrosion and chemically inert.
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Usually ceramic saddles, stone ware shapes, coke, reaching rings etc., are used as packing materials in these towers. Coarse packing materials are used for removing particles above size 10 micron, and fine packing materials for removing particles below 10 micron size.
The velocity of the gases through coarse packing bed is kept about 100 ml sec. The gas velocity in fine packing bed is kept between 18-20 m/min. In the wet collection the main advantage is that the pressure drop remains constant.
The sludge collected through the wet collecting devices is disposed off after required sedimentation, flocculation, thickening and chemical treatments. The treatments and disposal of sludge is an additional process which is required in this system.