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characteristics of the activated carbon produced vary depending on the type of material used. In general, activated carbon made from coconuts performs better at higher humidity than those made from coal. Activated carbon can also be modified to improve adsorption of other pollutants, such as hydrogen sulfide, mercury and ammonia. [18] Activated carbon can adsorb a wide range of VOCs; however, there are some limitations. First, activated carbon is less effective for compounds that are highly polar, volatile or have small diameters. For example, vinyl chloride, methanol, and formaldehyde are not adsorbed well by activated carbon. Second, activated carbon is less effective in situations where the waste gas has high relative humidity as the water molecules readily adsorb to the activated carbon reducing the number of available absorption sites. [18] Some studies have shown a 30% reduction in removal efficiency when the relative humidity of the waste gas exceeds 75%. [20] Moisture in the bed can also promote biological growth on the carbon surface. [18] Third, carbon bed fires and explosions can occur with activated carbon adsorbers. Wastes with oxygen bearing compounds, such as peroxides, ketones, organic acids, aldehydes and organic sulfur compounds should be avoided as activated carbon can cause exothermic reactions with these compounds. The heat from exothermic reactions can ignite any flammable compounds present in the waste stream. In such cases, fire suppression measures can be used, such as increasing flow rate or using nitrogen or water to reduce the oxygen level. [18] Lastly, some VOCs may undergo chemical reactions to produce contaminants that are difficult to remove during desorption. For example, styrene monomers have been shown to polymerize to polystyrene making desorption possible only at very high temperatures. [18] Activated carbon is generally regenerated by heating to temperatures of between 250 and 350oF. Steam is often used for this process, followed by a drying cycle to remove moisture. However, some molecules remain after regeneration. The adsorption capacity of regenerated carbon is typically about 50% of that for virgin material. The adsorption capacity declines during subsequent regeneration cycles, until eventually the activated carbon must be replaced. [21] The typical cost for virgin activated carbon is between $1.90/lb and $6.50.2 Costs for reactivated carbon are slightly lower and range from $0.95/lb to 1.55/lb.3 Zeolites: Zeolites have a uniform crystalline structure with high specific surface areas and small, uniformly sized pores. Zeolites are sometimes called “molecular sieves” because their pores trap molecules of specific sizes, while allowing other molecules to pass through. For a given application, the zeolite must be carefully selected to match the molecular diameter of the VOCs to be removed. Zeolites have specific surface areas that are similar to activated carbon. Naturally occurring zeolites include aluminosilicate minerals that are hydrophilic. However, synthetic zeolites are also available with characteristics tailored to a particular application. For example, some synthetic zeolites are hydrophobic with greater affinity for non-polar VOCs and low affinity for water molecules. Unlike activated carbon, zeolites are effective for removing VOC 2 Based on 2018 costs for virgin activated carbon provided by Carbtrol and Calgon Carbon Corporation. 3 Based on 2018 costs for reactivated carbon provided by Calgon Carbon Corporation. 1-6PDF Image | Carbon Adsorbers
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