Methane and ChloroflorocarbonsHydrocarbons are a class of compounds primarily composed of carbon and hydrogen, and they are major components of oil, natural gas and pesticides. These substances contribute to the greenhouse effect and global warming, deplete the ozone, increase occurrences of cancer and respiratory disorders, reduce the photosynthetic ability of plants and, in the notorious form of oil spills, do untold damage to ecosystems.
Methane and chloroflorocarbons (CFCs) are two hydrocarbons that can drastically alter our atmosphere. Methane is oxidized into carbon dioxide (CO2), which increases the amount of this substance in the atmosphere and adds to the greenhouse effect and global warming. CFCs are hydrocarbons used in refrigeration and aerosol cans. High up in the atmosphere, they produce chlorine and reduce the ozone layer, which protects the earth from ultraviolet radiation.
Adsorption:
This is one of the most commonly used methods, especially for controlling emissions from small sources. It can be physical adsorption or chemisorptions. The later is rarely used for the VOC emission control because, it involves a less-reversible chemical bonding of the adsorbate (pollutant) and the adsorbing solid ( packing) and is relatively expensive. Physical adsorption uses the Van der Waals force, giving the advantage of reversibility and regeneration due to the weaker bonding of the gas and adsorbent material. The adsorbed material can be either recovered or incinerated. Regeneration is usually accomplished by heating or extraction/displacement.
Activated carbon is a commonly used adsorbent because of its high surface area and material hardness. It has between 800 and 1200 m2/g of surface area. In general, activated carbon and other adsorbents such as hollow aluminum spheres coated with a catalyst can be employed in a fixed, moving or fluidized bed system.
Fluidized bed systems, though more expensive to build and operate, yield high contacting with low pressure loss and regeneration can be accomplished within the system. The fixed beds are less expensive and provide longer packing life, but provide less contacting per unit length and require a larger pressure loss; because they are regenerated individually.
Moving beds have properties between fixed and fluidized beds. The useful life of activated carbon can be determined using break through curves.
Regeneration can be achieved by contact with a hot, inert gas, contact with a low pressure gas stream and pressure reduction over the bed.
Steam desorption is the most commonly used process for regeneration.
Incineration:
Incineration or combustion is another common VOC control technology. Complete combustion or oxidation of pure hydrocarbons produces carbon dioxide and water. Sulfur and nitrogen compounds produce acid gases and limited air supply results in the formation of carbon monoxide.
Complex organic compounds may not oxidize completely in the residence time and ash may form. Most VOC oxidation must be done at high temperature, unless catalysts are involved.
Flares, thermal oxidizers and catalytic converters all use oxidation chemistry to treat VOC emissions. Flares mostly treat moderate to high temperature concentrations. All of the heat produced by the combustion process is lost when the flares are used. Most thermal oxidizers treat emission streams with maximum VOC concentrations of 25% of the LEL ( lower explosive limit). Catalyst beds, especially when used to enhance the oxidation of VOCs (usually noble metals like platinum and palladium) must be able to withstand high temperatures and must be designed so that a minimum pressure drop is created when the gas passes through the bed. For example by using catalytic converters, thermal oxidation of the by-products of the incomplete engine combustion can be safely accomplished at temperatures much lower than would be required without the aid of catalysis.
Condensation:
Condensation and gas absorption are most commonly used for highly concentrated VOC streams that are advantageous to recover and the relatively large expense is warranted. It employs a drop in temperature and/ or increase in pressure to cause the VOCs in the emission stream to condense. The cleaned air stream is separated from the condensate containing target pollutants. In many cases, very large temperature drops are required to achieve effective condensation, requiring significant energy investment to accomplish cooling.
Condensation is used to recover gasoline and fuel vapors at gasoline loading terminals and in gasoline dispensing facilities. It is also used in the adsorbent regeneration process to separate solvents from the stream to separate solvents from the stream used to regenerate the activated carbon.
Gas Absorption:
Gas absorption involves the absorption of a gas into a liquid. Water can be used for recovery of water-soluble compounds such
as acetone and low molecular weight alcohols, which can later be separated from water using distillation. Additives are often used to increase the effective mass transfer rate of the pollutant from the gas phase into the liquid phase, affecting the surface tension, reducing interfacial resistance and increasing the apparent solubility.
Gas absorption can be expensive, however it is generally used only to recover VOCs that have a secondary market value. Gas absorption techniques are used for the recovery of a variety of chemicals in the coke manufacturing industry. They are often called scrubbers.
Control and Treatment of VOC and Hydrocarbons
Control and treatment of VOC and organic hazardous air pollutant emissions are generally accomplished by adsorption, incineration, condensation and gas absorption. The methodology is usually chosen depending upon the temperature, composition and volumetric flow rate of the emission stream, space constraints and allowable installation and operational costs. A brief description of each method is given below:Adsorption:
This is one of the most commonly used methods, especially for controlling emissions from small sources. It can be physical adsorption or chemisorptions. The later is rarely used for the VOC emission control because, it involves a less-reversible chemical bonding of the adsorbate (pollutant) and the adsorbing solid ( packing) and is relatively expensive. Physical adsorption uses the Van der Waals force, giving the advantage of reversibility and regeneration due to the weaker bonding of the gas and adsorbent material. The adsorbed material can be either recovered or incinerated. Regeneration is usually accomplished by heating or extraction/displacement.
Activated carbon is a commonly used adsorbent because of its high surface area and material hardness. It has between 800 and 1200 m2/g of surface area. In general, activated carbon and other adsorbents such as hollow aluminum spheres coated with a catalyst can be employed in a fixed, moving or fluidized bed system.
Fluidized bed systems, though more expensive to build and operate, yield high contacting with low pressure loss and regeneration can be accomplished within the system. The fixed beds are less expensive and provide longer packing life, but provide less contacting per unit length and require a larger pressure loss; because they are regenerated individually.
Moving beds have properties between fixed and fluidized beds. The useful life of activated carbon can be determined using break through curves.
Regeneration can be achieved by contact with a hot, inert gas, contact with a low pressure gas stream and pressure reduction over the bed.
Steam desorption is the most commonly used process for regeneration.
Incineration:
Incineration or combustion is another common VOC control technology. Complete combustion or oxidation of pure hydrocarbons produces carbon dioxide and water. Sulfur and nitrogen compounds produce acid gases and limited air supply results in the formation of carbon monoxide.
Complex organic compounds may not oxidize completely in the residence time and ash may form. Most VOC oxidation must be done at high temperature, unless catalysts are involved.
Flares, thermal oxidizers and catalytic converters all use oxidation chemistry to treat VOC emissions. Flares mostly treat moderate to high temperature concentrations. All of the heat produced by the combustion process is lost when the flares are used. Most thermal oxidizers treat emission streams with maximum VOC concentrations of 25% of the LEL ( lower explosive limit). Catalyst beds, especially when used to enhance the oxidation of VOCs (usually noble metals like platinum and palladium) must be able to withstand high temperatures and must be designed so that a minimum pressure drop is created when the gas passes through the bed. For example by using catalytic converters, thermal oxidation of the by-products of the incomplete engine combustion can be safely accomplished at temperatures much lower than would be required without the aid of catalysis.
Condensation:
Condensation and gas absorption are most commonly used for highly concentrated VOC streams that are advantageous to recover and the relatively large expense is warranted. It employs a drop in temperature and/ or increase in pressure to cause the VOCs in the emission stream to condense. The cleaned air stream is separated from the condensate containing target pollutants. In many cases, very large temperature drops are required to achieve effective condensation, requiring significant energy investment to accomplish cooling.
Condensation is used to recover gasoline and fuel vapors at gasoline loading terminals and in gasoline dispensing facilities. It is also used in the adsorbent regeneration process to separate solvents from the stream to separate solvents from the stream used to regenerate the activated carbon.
Gas Absorption:
Gas absorption involves the absorption of a gas into a liquid. Water can be used for recovery of water-soluble compounds such
as acetone and low molecular weight alcohols, which can later be separated from water using distillation. Additives are often used to increase the effective mass transfer rate of the pollutant from the gas phase into the liquid phase, affecting the surface tension, reducing interfacial resistance and increasing the apparent solubility.
Gas absorption can be expensive, however it is generally used only to recover VOCs that have a secondary market value. Gas absorption techniques are used for the recovery of a variety of chemicals in the coke manufacturing industry. They are often called scrubbers.
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