Air Pollutants: What are they, why do we abate them, where are they found and how do we address them?
Air pollutants which are also commonly referred to as Hazardous Air Pollutants, HAPs and air toxics are those pollutants known to cause serious health impacts or adverse environmental effects. The Clean Air Act requires the EPA to regulate toxic air pollutants from categories of industrial facilities. This is done in two phases, the first being technology-based, where standards for controlling the emissions of air toxics from industry are created. The top of these standards are referred to as BACT, Best Available Control Technologies and this is where GCES excels as a pollution control solution provider.
With the EPA focused on reducing 187 toxic air pollutants from being emitted there is a never ending list of hazardous air pollutants we are tasked with abating for our customers, the environment and the future. In our series on air pollutants we will review what air pollutants are, why we abate them, where they are found and how they are destroyed.
Part 1 of Abating Hazardous Air Pollutants: BTEX
BTEX: BTEX is an acronym that stands for Benzene, Toluene, Ethylbenzene and Xylenes. These compounds are volatile organic compounds (VOCs) that are found in petroleum and petroleum products such as gasoline. Let’s break down each of these VOCs and explore why we abate them.
What is Benzene? Benzene, C6H6, is considered one of the elementary petrochemicals with a molecule combination of six carbon atoms and one hydrogen atom. Because its contents are only carbon and hydrogen atoms it is classified as a hydrocarbon. Benzene is colorless and highly flammable with a sweet odor.
Where is Benzene found? Benzene’s primary uses are in the fabrication of other chemicals including ethylbenzene, cumene, cyclohexane, nitrobenzene and alkylbenzene with ethylbenzene being the most common. Ethylbenzene is used to make polymers and plastics such as polystyrene and EPS. Benzene can also be found in phenol, acetones for resins and adhesives, nylon, rubbers, lubricants, dyes, detergents drugs, explosives and pesticides. With all of these uses we must further explore the need for abatement.
Why do we abate Benzene? We destroy Benzene before exhausting it into the air because despite its many uses it is a human carcinogen. Carcinogens are defined as any substance that is an agent directly involved in causing cancer. Benzene has also been linked to bone marrow failure, aplastic anemia, acute leukemia, myelodysplastic syndrome, acute lymphoblastic leukemia and chronic myeloid leukemia. In 1948 the American Petroleum Institute stated that “it is generally considered that the only absolutely safe concentration for benzene is zero”. Because Benzene is found in gasoline and hydrocarbon fuels that are in use virtually everywhere, exposure to it has become a global health problem and at GCES we are committed to being a part of the solution.
Also in BTEX is Toluene. Toluene, also know as Toluol, is a VOC that is in the aromatic hydrocarbon category.
What is Toluene? Toluene, CH3, is a water-insoluble liquid that is colorless and has a smell that is associated with that of paint thinner. A mono-substituted derivative of benzene which has worldwide sales of over 25 billion in US dollars.
Where is Toluene found? Widely used as an industrial feedstock as well as a solvent Toluene is a highly flammable chemical. Toluene is found naturally at low levels in crude oil and is a byproduct of the production of gasoline as well as the production of coke from coal. Purification is done during a final separation by any distillation or solvent extraction process used for BTEX aromatics. Toluene which is used in the manufacturing of polyurethane foam, trinitrontoluene aka TNT and a number of synthetic drugs is feedstock for toluene diisocyanate and a precursor to other chemicals. Also used as a solvent for paints, paint thinners, silicone sealants, rubber, printing ink, adhesives, glues, lacquers, leather tanners and disinfectants. As a fuel Toluene is used as an octane booster in gasoline for internal combustion engines, as a fuel for two-stroke and four-stroke engines, jet fuel surrogate blends and perhaps most famously in Honda’s Formula One cars. Alternative applications range from breaking red blood cells open in order to extract hemoglobin in biochemistry experiments and as a cement in polystyrene kits as it can be applied with precision by brush without the bulk of adhesives, to name a few.
Why do we abate Toluene? When inhaled Toluene causes tiredness, weakness, confusion, memory loss, loss of appetite, nausea, hearing loss, vision loss including that of color and drunken like behavior and actions. When exposure is stopped these symptoms often disappear although high levels of inhalation cause light-headedness, nausea, sleepiness, unconsciousness and even death. Exposure can be recreational as an inhalant and has been known to cause severe neurological harm and as of 2007 it was illegal to use, possess and distribute in 24 states for such uses. Unlike similar VOCs such as Benzene the EPA states that the carcinogenic potential of toluene cannot be evaluated due to insufficient information.
Ethylbenzene, the third part of BTEX, is also known as Phenylethane and Methyltoluene. It is a highly flammable liquid that is colorless and has a gasoline like odor that is often described as slightly sweet.
Where is Ethylbenzene found? An important component of the petrochemical industry, Ethylbenzene is an intermediate in production of styrene, one of the most common plastic materials. Although 99% of Ethylbenzene is used in the production of plastics there are other uses for the monocyclic aromatic hydrocarbon, C6H5CH2CH3, including fuel, solvent in ink, rubber adhesive, varnish and paints. Other uses includes as an ant-knock agent in gasoline to reduce engine knocking and increase the octane rating.
Why do we abate Ethylbenzene? To answer this question we must look at both the effects on human health as well as the environment. Let’s start with human health and finish with environmental protection which is where GCES experts have engineered solutions. While long term exposure is not expected to have an adverse effect, The US Occupation Safety and Health Administration (OSHA) limits exposure to workers to an average of 100 ppm for an 8-hour work day with a 40-hour work week. The International Agency for Research on Cancer has classified Ethylbenzene as a possible carcinogen. Some studies have found that exposure to ethylbenzene in rats and mice resulted in an increased incidence of tumors. Short term effects in humans includes throat and eye sensitivity as well as dizziness.
In the environment Ethylbenzene can easily move from water to soil and is mostly found as a vapor in air. Often released into the air through the burning of coal, gas and oil, Ethylbenzene reacts to sunlight and assists in the production of smog. As it does not eagerly bind to soil it moves quickly into groundwater and can often be found in waterwells that are near waste sites or underground fuel storage tanks that have been found to have leaks and landfills.
What is Xylene? Perhaps the safest of the four parts of BTEX, Xylene is a petrochemical produced by catalytic reforming as well as coal carbonization during the manufacturing of fuel, specifically coke fuel. Found in small quantities in gasoline and aircraft fuels, Xylene is produced at a rate of several million tons per year with the largest manufacturing facilities in Singapore.
Where is Xylene found? Xylene is the main procurer to terephthalic acid and dimethyl terephthalate both of which are monomers used in the production of polyethylene terephthalate plastic bottles and polyester clothing. 98% of the production of Xylene is for the before mentioned products with other applications including as a solvent in the printing, rubber and leather industries. Often a component of ink, rubber, adhesives, paint thinners, varnishes, as a cleaning agent and even as a root canal treatment in dentistry or the active ingredient in products used for the cleaning of ear wax the uses for Xylene are wide reaching. Historically Xylene was used as a precursor chemical compound for tear gas used in World War I.
Why is Xylene abated? While Xylene has acute toxicity one of its more troubling factors is it is highly flammable. The main physical effects of exposure to Xylene vapor is its effect on the central nervous system. Short term symptoms include headaches, dizziness, nausea, vomiting, weakness, irritability and reductions in reaction times. Low concentration exposure is reversible and is not known to cause permanent damage. However, long term exposure to low levels of Xylene led to decrease in balance, coordination and reaction times. Long term exposure may also lead to headaches, irritability, insomnia, agitation, depression, tiredness, impaired concentration, tremors and even short term memory loss. It has also been known to effect skin as an irritant that strips skin of oils and for that reason it is recommended that individuals facing occupational exposure to Xylene must wear gloves in addition to masks. When Xylene enters the environment it is usually through evaporation which often leaks into soil, surface water and groundwater.
How does Gulf Coast Environmental Systems abate BTEX? Since the structure of BTEX compounds are only carbon and hydrogen atoms, a simple combustion reaction will abate BTEX compounds. Thermal oxidation is the primary method that GCES will recommend to abate BTEX. If there needs to be a higher efficiency of destruction and lower cost of ownership, regenerative thermal oxidation may also be an abatement choice. Additional options can include a scrubber system.
THEORY OF OPERATION
The method of reduction of Volatile Organic Compounds (VOCs) in a Thermal Oxidizer revolves around thermal destruction. The chemical process of thermal oxidation is quite simple; the exhaust stream temperature is raised to a point that the chemical bonds that hold the molecules together are broken. The VOCs in the process exhaust stream are converted to various combinations of carbon dioxide (CO2), water (H2O), and thermal energy by the high temperature of the combustion chamber. A standard Direct Fire Thermal Oxidizer is effective in high destruction scenarios.
For situations requiring lower efficiency of destruction and a lower cost of ownership, regenerative thermal oxidation may also be an abatement choice. In this situation the destruction method is still thermal oxidizations but with a Regenerative Thermal Oxidizer (RTO) two energy recovery canisters are included in the system which are used as housings for ceramic heat recovery media. The ceramic heat recovery media acts as a heat exchanger for the system. The two canisters operate under a “swing bed” absorption principle: which is the principle of transfer through two beds by the use of flow reversal. In the use of this principle with ceramic stoneware, the process is called regeneration.
Additional articles in the GCES series ‘Abating Hazardous Air Pollutants’ include:
Part 2: Chlorine Abatement
Part 6: SOx, the compounds of sulfur and oxygen molecules including Sulfur Monoxide, Sulfur Dioxide and Sulfur Trioxide
Part 11: Sulfuric Acid – H2SO4
Part 12: Ethylene Oxide – EtO