Part 2 of Abating Hazardous Air Pollutants: Chlorine Abatement
What is Chlorine? Chlorine is the second most abundant halogen and the 21st most abundant chemical found in the Earth’s crust in the form of ionic chloride. A yellow-green gas, Chlorine is in the halogen group and is one of the VOC’s (Volatile Organic Compounds) that GCES has spent nearly two dozen years abating.
When someone hears the word Chlorine there are many immediate associations that come to mind. For children Chlorine is memories of summer, long days in the pool and red, burning eyes. Moms hear the word Chlorine and immediately think of stains and laundry. In our industry Chlorine means CI and we think of thermal oxidization and highly acid gas. Chlorine is used in commercial bleaches, disinfectants and as a reagent for many processes in the chemical industry as a result of the high oxidizing potential of elemental chlorine. Also used to manufacture a wide range of consumer products including polyvinyl chloride and many intermediates for the production of plastics, chlorines usage fall into three main categories:
- Production of industrial and consumer products
- Sanitation and disinfection
- As a weapon on the form of chlorine gas aka bertholite
Where is chlorine found? Chlorine is found on the earth primarily in the form of chlorine ion in minerals. In oceans chlorine is a component of the salt dissolved in seawater and accounts for about 1.9% of seawaters mass. In areas such as the Dead Sea and in underground brine deposits higher concentrations of chlorine are found. Interestingly, more than 2,000 natural occurring organic chlorine compounds are known to date.
So, why must we abate something found in simple salt which sits on nearly every kitchen table in the world? Chlorine-containing organic molecules such as chlorofluorocarbons have been responsible for ozone depletion in the upper atmosphere. A single chlorine atom reacting to an ozone molecule can create a catalyst that causes ozone destruction. On average a single chlorine atom is able to react with 100,000 ozone molecules before it is removed from the catalytic cycle. With the growing human uses for Chlorine the need for abatement before release into the atmosphere is also growing in order to provide atmospheric protection and VOC reduction.
Chlorine gas health effects vary based on exposure and sensitivity. Chlorine gas is considered a toxic substance that attacks the repertory system, eyes and skin. It can cause vomiting, coughing and lung damage at lower parts per million. At higher parts per million breathing the gas can be fatal. It is recommended to exercise caution when using chlorine even in household applications as chlorine is known to have reactions to certain chemicals and additives which can result in the production of additional toxic chemicals.
How does GCES achieve Chlorine Abatement? A complete pollution control solution of Thermal Oxidizers (Also known as TOs or TOX units) and Scrubbers are often used to dispose of chlorinated hydrocarbons (hydrocarbon molecules which contain at least one atom of chlorine). Hydrogen chlorine or chlorine gases are abated through the use of a scrubber. In the scrubber unit contaminated gas (in this case hydrogen chlorine or chlorine gases) flow through a specially designed packing media that is wetted with recirculated liquid. The liquid solvent absorbs the gas pollutants by physical or chemical reactions. Through a process the scrubber then removes contaminant products before they precipitate. For each application, the scrubber is sized and designed to meet specific customer process and business requirements.
The simplest chlorinated hydrocarbon is Methyl Chloride, which consists of one atom of carbon, three of hydrogen and one of chlorine. The formula for Methyl Chloride is CH3Cl. The next simplest chlorinated hydrocarbon is Methylene Chloride, which contains two atoms of chlorine (formula CH2Cl2).
When Methylene Chloride is destroyed in a Thermal Oxidizer, as oxygen from the combustion air is added, it is broken down by heat. The carbon atom combines with oxygen to form carbon dioxide (CO2) as well as a small amount of carbon monoxide (CO). There is always some CO present in the TO flue gas because CO2 and CO exist in a “chemical equilibrium”, which can be described by the equation: CO + ½ O2 <–> CO2
A simple description of the above equation is, carbon dioxide can be converted to a mixture of carbon monoxide and oxygen, while carbon dioxide when added to oxygen can convert into carbon dioxide. CO2 and CO exist in equilibrium with each other when occurring naturally, as a result that equilibrium will favor CO2. The result being, at high temperature and with the passage of time, any mixture of CO2 and CO will end up with mostly CO2 and very little CO left. Testing has been used to determine the “equilibrium constant” for this reaction so chemists can calculate how much CO to expect in any particular situation. At GCES chemical engineers use this information to calculate abatement needs to provide environmental protection solutions through the implementation of thermal oxidization technologies. The equilibrium constant for this reaction at 1800°F is 15,848,932 – such a large number indicates that the final mixture will contain much more CO2 than CO. The equilibrium constant is not infinite, which means there will always be at least a trace of CO remaining in the output gas from the Thermal Oxidizer. Note that the source of the carbon and oxygen atoms does not affect the equilibrium reaction, so burning natural gas and burning fuel oil both result in the same CO2/CO ratio, everything else being equal. The ratio can be changed by changing the reaction temperature or by changing the amount of O2 present.
Effect of Oxygen
If oxygen is added to the reaction, less CO will remain. If we go back to the equilibrium equation, as long as the temperature does not change then balance is always maintained. If you increase the amount of O2, you must decrease the amount of CO at the same measurement in order for both sides of the equation to remain equal.
Effect of Temperature
At 3000°F the equilibrium constant is 1000, indicating that more CO (and less CO2) will exist at higher temperature.
The hydrogen atoms in Methylene Chloride react according to this equation: H2 + ½ O2 <–> H2O
The equilibrium constant for this reaction is also a very large number, indicating that almost all of the hydrogen present will exist as H2O in the TOX flue gas.
The chlorine atoms in Methylene Chloride react according to this equation: Cl2 + H2O <–> 2HCl + ½ O2
At 1700°F, the equilibrium constant for this reaction is 9.5 which means that most of the chlorine atoms will leave the thermal oxidizer as HCl (hydrogen chloride), although a considerable amount will also be in the form of Cl2 (chlorine). As above, the source of the chlorine atoms does not matter – the thermal oxidizer flue gas will always rearrange itself to agree with the equilibrium equation. Also as above, adding water vapor (H2O) will reduce the amount of Cl2 present although adding just oxygen (02) will shift the balance back towards Cl2. In this case hydrogen has a large impact on the balance and reaction of chlorine.
Other solutions for chlorine abatement include the utilization of Wet Scrubber Systems. Wet scrubbers remove VOCs, chlorine in this case, by injecting a liquid (a catalyst) into the gas stream. Contaminated gas flows through packing media that is specially designed and wetted with recirculated liquid. This liquid solvent absorbs the gas pollutant by either a physical or chemical reaction. The liquid then blows down from the tank or sump section of the scrubber to remove contaminated products prior to their precipitation.
For each application, the scrubber is sized and designed to meet specific customer requirements. The tail gas to be cleaned is carefully analyzed to determine the optimum design parameters and to allow for the best operating solution for each installation. After thorough analysis, GCES offers several options of fume or exhaust scrubber equipment packages based on plant equipment, local air regulations, plant locations, and other factors.
Our experience in designing pollution control systems for many different chemicals provides our customers with the assurance that the plant’s air quality will have a minimal impact on the environment. Our reputation provides our customers with the peace of mind associated with the reliability of our products.
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