Scrubber system
Scrubber system Packing material of the packed bed wet scrubber is the most important component of the tower. The key to designing a packed column for air pollution control is the tower packing. It provides the surface over which the scrubbing liquid flows, presenting a large area for mass transfer to occur. It should be very open in design to minimize pressure drop while still maintaining a high surface where the absorption of the gas will occur. Packing material represents the largest material cost of the packed tower.
Scrubber system basically, the selected packing materials should
1. Provide a large surface area – resulting high gas liquid interface
2. Have an open structure – resulting low resistance to gas flow
3. Promote uniform liquid distribution
4. Promote uniform gas flow
Scrubber system and the type of packing most suitable for any application will vary with temperature, pressure, gas concentration, and efficiency requirements. Careful consideration is given to the various alternatives before selecting the packing for each application. These materials were originally made of stoneware, porcelain, or metal, but presently, a large majority are made of high-density thermoplastics (polyethylene and polypropylene).
View ports and access doors are needed for every scrubber for inspection and maintenance of packing. The access doors facilitate the scrubber cleaning process and replacement of new packing. Therefore, suitable selection of packing material reduces the problem and cost of maintenance besides enhancing the scrubber operation. Packing is the critical media for pollutant removal in packed bed scrubber and therefore it needs to be selected based on properties of the system, pollutant, scrubbing liquid, etc.
The following factors provide a general guide for selecting packing materials:
Criteria 1: Corrosion is usually resulting at areas which have wet dry interface. Packing acts as the contact media for gas separation has high potential for corrosion. Consequently, packing material is normally chosen based on the degree of corrosiveness of the air stream. Packing which made from material such as ceramic has low strength and brittle. Material such as metal is inappropriate to be used for highly corrosive gas due to decomposition.
Scrubber system Packing that made from plastic material such as polypropylene has greater resistance to corrosion and it is always used nowadays as the contact media for gases which are highly corrosive.
Criteria 2: Cost – Generally, plastic packing is less expensive than metal packing, with ceramic packing being the most expensive. Packing costs are expressed in peso per cubic meter.
Criteria 3: Low pressure drop – Pressure drop is a function of the volume of void space in a tower when filled with packing: generally, the larger the packing size for a given bed size, the smaller the pressure drop becomes.
Also, higher the gas velocity, the greater the resistance that will be encountered by the down-flowing liquid and the higher the pressure drop across the packings. Too high a gas velocity will lead to a condition known as flooding whereby the liquid filled the entire column and the operation became difficult to carry out. High pressure will crush and damage the packings in the column. Each packing has its own characteristics pressure drop chart as reported by the manufacturer
Points to note:
at constant liquid rate, gas pressure drop increases with gas velocity.
at constant gas velocity, the gas pressure drop is higher at larger liquid rate.
each liquid rate has its own loading and flooding points.
at higher liquid rate, the loading and flooding points occur at lower gas pressure drop.
Criteria 4: Large specific area – A large surface area per cubic foot of packing, m2/m3 (ft2/ft3), is desirable for mass transfer. A specific packing is described by its trade name and overall size. For example, a column can be packed with 5-cm (2-in.) Raschig rings or 2.5-cm (1-in.) Tellerette packing. The overall dimensions of packing materials normally range from 0.6 to 10 cm (0.25 to 4 in.)
In general in the Scrubber system , the largest size of packing that is suitable for the size of column should be used, up to 50mm. Small sizes are appreciably more expensive than the larger sizes. Above 50 mm the lover cost per cubic meter does not normally compensate for the lower mass transfer efficiency. Use of too larger a size in a small column can cause poor liquid distribution.
Recommended size range are:
Column diameter Use packing size
<0.3 m < 25 mm 0.3 to 0.9 m 25 to 38 mm >0.9 m 50 to 75 mm
Criteria 5: Structural strength – Packing must be strong enough to withstand normal loads during installation, service, physical handling, and thermal fluctuations. Ceramic packing may crack under sudden temperature changes.
Criteria 6: Weight – Heavier packing may require additional support materials or heavier tower construction. Plastics have a big advantage in this area since they are much lighter than either ceramic or metal packings.
Criteria 7: Design flexibility – The efficiency of a scrubber changes as the liquid and gas flow rates vary. Packing material must be able to handle the process changes without substantially affecting removal efficiency.
Criteria 8: Arrangement – Packing material may be arranged in an absorber in one of two ways. The packing may be dumped into the column randomly or, in certain cases, systematically stacked, as bricks are laid atop each other. Randomly packed towers provide a higher surface area, m2/m3 (ft2/ft3), but also cause a higher pressure drop than stacked packing. In addition to the lower pressure drop, the stacked packing provides better liquid distribution over the entire surface of the packing. However, the large installation costs required to stack the packing material usually make it impractical.
Avlon Inc, a Philippine based company leading in manufacturing and supplying best wet scrubbers such as packed bed scrubbers and venturi type scrubbers.
Avlon promises best customer service and quality assurance. To know more visit www.avlon-php.com
