Six common types of volatile organic compounds (VOCs) and their treatment technologies
I. Common Classifications of Organic Waste Gases
VOCs (Volatile Organic Compounds) are a class of common air pollutants, generated in industries such as paint production, chemical fiber manufacturing, metal coating, chemical coatings, shoemaking, leather tanning, plywood manufacturing, and tire manufacturing. Harmful VOCs mainly include acetone, toluene, phenol, dimethylaniline, formaldehyde, n-hexane, ethyl acetate, and ethanol.
In industrial enterprises, VOCs are mainly classified according to their source as follows:
1. Spray Painting Waste Gas: The main components are acetone, butanol, xylene, toluene, ethyl acetate, butyl acetate, and other VOCs. This is mainly generated in surface treatment enterprises such as paint spraying. Common treatment methods include oil curtain absorption, water curtain absorption, combined with secondary and tertiary activated carbon adsorption.
2. Plastic and Rubber Waste Gas: The main components are polymer monomers volatilized during the thermal processing of plastic and rubber particles. Due to the complex composition of plastics and rubber, the waste gas mainly contains olefin polymer monomers such as ethylene, propylene, styrene, acrylonitrile, and butadiene, but the concentration is generally low while the volume is large. Enterprises involved mainly include plastic granulation companies, chemical fiber production companies, injection molding companies, and rubber production companies. Treatment methods mainly include activated carbon absorption and plasma purification.
3. Sterilization Waste Gas: The main components are aldehydes, ketones, hydrocarbons, fatty acids, alcohols, esters, lactones, heterocyclic compounds, and aromatic compounds. Enterprises involved mainly include dyeing and finishing companies and chemical fiber production companies. Water spray treatment and electrostatic adsorption treatment processes are commonly used.
4. Chemical Organic Waste Gas: Mainly generated by chemical enterprises. The composition of the waste gas is closely related to the types of chemical products designed and produced by the chemical enterprise. Common purification and collection methods include condensation recovery and catalytic combustion technology.
5. Printing Exhaust Gas: The main components are toluene, non-methane hydrocarbons, ethyl acetate, ethanol, etc., volatilized from ink. Enterprises involved are mainly those with ink printing processes, such as packaging and printing companies, and generally use activated carbon adsorption.
II. Summary of Common VOC Organic Waste Gas Purification Methods
Prioritize waste gas purification methods that are low-cost, low-energy-consumption, and do not cause secondary pollution, and fully utilize the waste heat of the waste gas to achieve resource recycling. Generally, petrochemical enterprises, due to the special nature of their production activities, have high exhaust gas concentrations and often use condensation, absorption, and combustion methods for waste gas purification. Industries such as printing have lower exhaust gas concentrations and often use adsorption and catalytic combustion methods. The following is a brief overview of these methods:
1. Condensation Recovery Method
The condensation method involves directly introducing industrial waste gas into a condenser. Through adsorption, absorption, desorption, and separation, valuable organic matter is recovered, waste heat is recovered, and the waste gas is purified to meet emission standards. 1. **Condensation Method:** When organic waste gas concentration is high, temperature is low, and air volume is small, condensation purification can be used. This method is commonly used in pharmaceutical and petrochemical enterprises. Usually, one or more other organic waste gas purification devices are added after the condensation recovery unit to achieve emission standards.
2. **Absorption Method:** Physical absorption is commonly used in industrial production. Waste gas is introduced into an absorbent liquid for absorption and purification. After the absorbent liquid is saturated, it undergoes heating, desorption, and condensation to recover waste heat. This method can be used when concentration is low, temperature is low, and air volume is large, but it requires a heating, desorption, and recovery device, resulting in a high investment. Oil curtains and water curtains are commonly used in paint coating operations to absorb paint mist, which is a common organic waste gas absorption method.
3. **Direct Combustion Method:** Direct combustion uses auxiliary materials such as fuel gas to ignite the waste gas, causing harmful substances to transform into harmless substances under high-temperature combustion. This method requires less investment, is simple to operate, and is suitable for waste gas with high concentration and low air volume, but it has higher safety requirements.
4. Catalytic Combustion Method
Catalysis involves heating the waste gas and then catalytically combusting it into harmless carbon dioxide and water. This method is suitable for purifying high-temperature, high-concentration organic waste gases. It has advantages such as low combustion temperature, energy saving, high purification rate, and small footprint, but the investment is relatively large.
5. Adsorption Method
The adsorption method can be divided into three types:
1. Direct Adsorption Method: This method uses activated carbon to adsorb and purify organic waste gases, achieving a purification rate of over 95%. The equipment is simple and the investment is low, but frequent replacement of activated carbon increases operating costs due to the frequent loading, unloading, and replacement procedures.
2. Adsorption-Recovery Method: This method uses fiber activated carbon to adsorb organic waste gases, then uses superheated steam backflushing to achieve desorption and regeneration when the carbon is near saturation.
3. Novel Adsorption-Catalytic Combustion Method: This method combines the advantages of adsorption and catalytic combustion, offering advantages such as stable operation, low investment, low operating costs, and simple maintenance. This method utilizes novel adsorption materials to adsorb and treat organic waste gas. Under near-saturation conditions, the gas undergoes adsorption, desorption, and desorption in the presence of hot air. The waste gas is then introduced into a catalytic combustion bed for flameless combustion, achieving complete purification. This method is suitable for treating waste gas with low concentrations and high airflow, and is currently the most widely used waste gas purification method in China.
4. Low-Temperature Plasma Purification Method
Low-temperature plasma is the fourth state of matter after solid, liquid, and gas. When the applied voltage reaches the gas's discharge voltage, the gas breaks down, producing a mixture including electrons, various ions, atoms, and free radicals.
Although the electron temperature is very high during the discharge process, the temperature of the heavy particles is very low, resulting in a low-temperature state for the entire system, hence the name low-temperature plasma. Low-temperature plasma degrades pollutants by utilizing these high-energy electrons, free radicals, and other active particles to decompose pollutant molecules in a very short time, leading to various subsequent reactions that degrade the pollutants.
Traditional methods for treating volatile organic compounds (VOCs), such as absorption, adsorption, condensation, and combustion, are difficult to implement for low-concentration VOCs. Photocatalytic degradation of VOCs suffers from catalyst deactivation. Low-temperature plasma treatment of VOCs, however, is not limited by these conditions and has potential advantages.
However, plasma technology is an interdisciplinary field encompassing fundamental disciplines such as discharge physics, discharge chemistry, chemical reaction engineering, and vacuum technology. Therefore, very few organizations currently possess mature expertise in this technology. Most advertisements promoting low-temperature plasma technology for waste gas treatment do not truly represent the technology.
In summary, different organic waste gas compositions and concentrations require different treatment methods. Currently, considering factors such as technological maturity, economics, and equipment maintenance, activated carbon adsorption remains the most widely used method. However, activated carbon adsorption suffers from drawbacks such as high costs for washing and recycling spent activated carbon after its service life and the risk of pollution transfer. Therefore, the novel adsorption-catalytic combustion method is widely used in technological upgrades and new projects.
Low-temperature plasma purification is gaining popularity among businesses due to its low maintenance costs, but it also faces challenges such as high equipment investment costs. It is believed that with technological and industrial advancements, low-temperature plasma purification technology will become increasingly mature, reducing equipment investment and leading to its widespread adoption.