Volatile organic compounds (VOCs) are organic compounds whose saturated vapor pressure is greater than or equal to 0.13 kPa at 20°C. It mainly comes from the emission of exhaust gas in the petrochemical industry, the volatilization of oil products such as oil storage depots, gas stations, and vehicles, and the use of organic solvents in paint, coating, packaging, printing, adhesives, and cosmetics industries. According to statistics, in 2009, China's industrial VOCs emissions were about 12.06 million tons, and the annual increase was about 8.6%. By 2030, VOCs emissions from gas stations alone will reach 1271.03 kilotons, with an economic loss of nearly one billion yuan. Most VOCs are toxic, and due to the high saturated vapor pressure, they can volatilize into the air in a natural state, enter the human body through the respiratory tract, and induce a variety of diseases. VOCs are also one of the main culprits of haze weather. The particulate matter generated by chemical conversion of VOCs can account for 21% of the source of PM2.5 in some areas. The secondary aerogel formed by the photochemical reaction of VOCs accounts for 25% to 35% of PM10 and is an important part of PM10.
With the continuous emergence of haze weather on a large scale, the problem of VOCs governance has attracted great attention from countries all over the world. If VOCs can be recycled economically and effectively, especially high-concentration, high-value VOCs, there will be triple benefits of environment, health, and economy. In order to better respond to China’s current forms of air pollution and promote the emission reduction and control of VOCs, in September 2013, the State Council issued the "Air Pollution Prevention and Control Action Plan", which required the promotion of VOCs pollution control, especially in petrochemical, organic chemical, and chemical industries. The surface coating, packaging and printing industries implement comprehensive rectification of VOCs. In the same year, the Ministry of Environmental Protection issued the "VOCs Pollution Prevention and Control Technology Policy" announcement, which proposed prevention and control strategies and methods for the pollution problems of VOCs-containing products in production, storage, transportation and sales, and use. The reduction and treatment of VOCs has become the current focus of air pollution prevention and control.
1VOCs governance technology
The treatment of VOCs should first start from the source of production and process control, adopting cleaner production technology, using raw materials with less VOCs, and developing new alternative raw materials to prevent pollution. Secondly, it is necessary to strengthen the end-of-line treatment of VOCs, recycling economically valuable process waste gas, loading and unloading waste gas and storage tank breathing gas, etc.; in accordance with laws and regulations, the waste gas that is difficult to recycle should be treated. At present, the treatment technology of VOCs is mainly based on the treatment of end exhaust gas. The traditional end gas treatment technologies include absorption method, combustion method, condensation method and adsorption method. Emerging technologies include biological methods, low-temperature plasma methods, membrane separation methods, and photocatalytic oxidation methods. Generally, the concentration of VOCs in industrial exhaust gas is between 100～2000mg/m3. For this kind of medium and low concentration VOCs, adsorption method and absorption method are adopted to discharge the organic solvent after reaching the standard; when it is not suitable for recycling, combustion method and biological method can be used. After purification, such as method, photocatalytic oxidation method, etc., meet the emission standards.
The adsorption method is currently the most common method for processing VOCs, and is especially suitable for processing low-concentration VOCs. Compared with other VOCs treatment technologies, the adsorption method can selectively separate mixtures that are difficult to separate in other processes, has high removal efficiency of low-concentration toxic and harmful substances, is simple and safe to operate, has no secondary pollution, and can achieve organic solvent recovery after treatment , The purpose of adsorbent recycling. At present, the commonly used adsorbents are activated carbon, silica gel, activated alumina, and zeolite molecular sieves [15]. In recent years, the methods of treating VOCs with activated carbon fibers, activated carbon nanotubes, carbonized-derived carbon, activated carbon cloth, etc. have also attracted people's attention. Compared with other adsorbents, activated carbon has many advantages: its pore size distribution is wide, the micropores are developed, the adsorption process is fast, and it can adsorb substances with different molecular sizes. It is very effective in the adsorption and recovery of VOCs such as benzene, ethyl acetate, and chloroform. The surface characteristics of polarity and hydrophobicity make it have good selectivity for the adsorption of non-polar substances; and the activated carbon is cheap and sufficient, the preparation process is simple, and it is easy to desorb and regenerate. Based on this, activated carbon has been widely used for adsorption It is used to treat VOCs with low concentration and large air volume with medium relative molecular mass (usually about 45-130), especially wood granular activated carbon prepared by phosphoric acid method, which has large adsorption capacity, small desorption residue, and rich surface functional groups. The advantages of process economy and environmental protection are widely used in the treatment of VOCs at home and abroad.
In order to improve the purification efficiency, the activated carbon adsorption method is often combined with other treatment methods. The commonly used methods are adsorption concentration-condensation recovery method and adsorption concentration-catalytic combustion method. The adsorption concentration-condensation recovery method uses hot gas to desorb activated carbon that has adsorbed VOCs, and then recovers the desorbed high-concentration VOCs with a condensing device. This method is suitable for the treatment of high-concentration VOCs exhaust gas with a single component, but is not suitable for the treatment of multi-component, low-concentration situations. The adsorption concentration-catalytic combustion method refers to the method in which the concentrated VOCs desorbed from the hot gas are sent to the catalytic combustion bed for catalytic combustion treatment. Using activated carbon as a carrier and supporting a catalyst with transition metals (Cu, Co, Fe, Ni, etc.), it can catalyze the combustion of VOCs into CO2 and H2O at a lower temperature (200-250°C) and lower oxygen content. This method is particularly suitable for the exhaust gas treatment of VOCs with relatively stable gas concentrations such as benzene, aldehydes, and alcohols.
2 Process technology of activated carbon adsorption method to treat VOCs
Activated carbon adsorption method to treat VOCs technology includes pressure swing adsorption (PSA), thermal swing adsorption (thermalswing adsorption, TSA), the combination of the two-thermal pressure swing adsorption (TPSA) and electro-swing adsorption (Electricswing adsorption, ESA).
2.1 Pressure swing adsorption
Pressure swing adsorption (PSA) refers to a cyclic process in which adsorbates are adsorbed and desorbed under different pressures by periodically changing the system pressure under constant temperature or no heat source conditions. According to different operation methods, PSA can be divided into equilibrium adsorption type that uses general activated carbon for separation using the difference between van der Waals forces and speed separation type that uses special activated carbon molecular sieves for separation using the difference between molecular adsorption speeds. Adsorption is usually carried out under normal pressure, and the desorption process is achieved by reducing the operating pressure or vacuuming, and the greater the vacuum during desorption, the easier it is to desorb. However, in actual operation, high vacuum requires a high degree of adsorption equipment and consumes huge energy. Considering the overall cost and adsorption effect, the industry generally uses a desorption pressure of 8-10kPa. The high degree of automation of PSA technology can realize cyclic operation, but it needs to be continuously pressurized and decompressed during the operation, which requires high equipment and huge energy consumption, and it is mostly used for the recovery of high-grade solvents.
2.2 Temperature swing adsorption
Temperature swing adsorption (TSA) is an operation process that uses the characteristic that the equilibrium adsorption capacity of the adsorbent decreases with the increase of temperature, adsorbs at room temperature, and desorbs after heating. The desorption process of activated carbon is an endothermic process, and heating can help desorption. When water vapor or hot gas is used for desorption, the desorption temperature is usually 100-200°C. When adsorbing VOCs, if the adsorption capacity is high, and the adsorbate is small molecular hydrocarbons and aromatic organics with lower boiling points, it can be recovered by condensation after desorption of water vapor; if the adsorption capacity is low, such as toluene, dimethyl ethyl VOCs such as amide and ethyl acetate can be purged with other hot gases (hot air, hot N2, etc.) for desorption and then burned or recovered after secondary adsorption [27]. RAMALINGAM et al. used TSA technology to study the recycling of three common indoor VOCs (acetone, dichloromethane and ethyl formate) and found that the best operating conditions for the hot nitrogen regeneration of the three VOCs are: T=170℃, V=0.17m/s. SHAH et al. used temperature swing adsorption to study the hot air regeneration performance of acetone and methyl ethyl ketone, and found that the adsorption capacity of acetone was restored to nearly 95% after one cycle of regeneration at 80°C, and remained basically unchanged after 8 consecutive cycles; while for methyl ethyl ketone, The adsorption capacity drops significantly after regeneration.
2.3 Temperature-swing-pressure swing adsorption
Temperature swing-pressure swing adsorption (TPSA) combines the advantages of two technologies: temperature swing adsorption and pressure swing adsorption. It is an efficient process technology that performs temperature-rising desorption after pressure swing desorption based on the pressure swing adsorption technology. By increasing the bed temperature and lowering the column pressure, the desorption is carried out more thoroughly and the regeneration efficiency of activated carbon is improved [30]. The research of RAMALINGAM et al. combined hot nitrogen desorption and vacuum decompression desorption. It has been shown that the recovery rate of methylene chloride can reach 82% after the two technologies are combined. In addition, after vacuum decompression desorption, the activated carbon bed temperature is reduced from 93°C to 63°C, which can significantly reduce the cooling time before the next cycle.
2.4 Electric swing adsorption
Electric swing adsorption (ESA) is an emerging process for gas purification and separation. Its essence is temperature swing adsorption. Different from the traditional temperature swing adsorption, the desorption process of the electricity swing adsorption is realized by heating the saturated adsorbent with electricity, and the heat generated by the Joule effect promotes the release of the adsorbate. Electric variable adsorption has many advantages: the heating system is simple, the energy is directly transferred to the adsorbent, the heating efficiency is high, and the energy consumption can be significantly reduced; the gas flow rate and the heating rate of the adsorbent can be independently controlled; the heat flow and mass flow are in the same direction, and more Conducive to desorption; low cost, the cost of using substation desorption can be 50% lower than the cost of using hot steam regeneration; good regeneration performance, SNYDER and other research found that after 12 cycles of use, the adsorption capacity of the adsorbent retains 97% to 100 %.
2.5 Summary
Pressure swing adsorption is suitable for the purification of high-concentration VOCs exhaust gas and the recovery of high-end organic solvents. It has the advantages of high automation, good environmental benefits, and flexible adjustment of imported gas volume and concentration. However, due to the high initial investment cost, adsorption and desorption need to be continuously increased. Pressure, decompression or vacuuming, energy consumption is huge, but also pay attention to the pressure of the gas in the dead space, there are certain limitations in use. At present, the treatment of VOCs mostly adopts temperature swing adsorption [36], and the fixed bed is mostly used for temperature swing adsorption. However, it takes a long time for the temperature swing adsorption to heat and cool the adsorbent during use. After multiple cycles, the adsorbent will have the problem of reduced performance due to thermal aging, and the temperature-sensitive VOCs such as trichloroethane and styrene are not Not applicable, so the researchers developed electro-swing adsorption based on temperature swing adsorption. Electric swing adsorption has the advantages of high heating efficiency, fast heating speed, and high solvent recovery rate. It has attracted the attention of many scholars at home and abroad in the treatment of VOCs. As a new technology, it has a good development prospect. Temperature swing-pressure swing adsorption combines the advantages of two technologies: temperature swing adsorption and pressure swing adsorption, which can significantly increase the regeneration rate of activated carbon and the recovery rate of organic solvents, shorten the time of a cycle process, but still cannot get rid of the limitations of the two technologies. Currently, there are few applications, but the combined use of multiple technologies and the development of composite gas separation technologies are still an important development direction for VOCs governance in the future. In actual use, different adsorption and recovery processes should be selected according to different working conditions and environmental protection requirements. At the same time, the research and development and promotion of new equipment should be strengthened, and new technologies for VOCs treatment with high efficiency, environmental protection and economy should be actively sought.
3 Influencing factors and solutions for the treatment of VOCs by activated carbon adsorption
The adsorption performance of activated carbon for VOCs is not only related to the properties of activated carbon itself, but also related to the physical properties of the adsorbate and the conditions of the adsorption operation. Modification treatment of activated carbon to meet the treatment requirements of a certain type of VOCs, or matching of suitable activated carbon varieties and operating conditions for a certain type of VOCs is a hot research topic at present.
3.1 The influence of surface chemical properties of activated carbon and surface chemical modification
The surface chemistry of activated carbon is determined by the type and number of functional groups on the surface of activated carbon, and the difference in surface chemistry affects the chemisorption performance of activated carbon. By chemically modifying the surface of activated carbon, the adsorption capacity and adsorption selectivity of activated carbon for VOCs can be changed. Research by SHEN et al. showed that ammoniation can increase the basic functional groups on the surface of activated carbon, and oxidation can increase the acidic functional groups on the surface of activated carbon. KIM et al. studied the adsorption performance of different acid and alkali impregnated coconut shell activated carbons for a variety of VOCs, and found that phosphoric acid impregnated modified activated carbons improved the adsorption performance of benzene, toluene, xylene and other VOCs. Liu Yaoyuan and others used H2SO4/H2O2 and NaOH to modify activated carbon from corn stalks, and found that the activated carbon modified with H2SO4/H2O2 reduced its adsorption of weakly polar and non-polar substances such as toluene, and improved its performance with NaOH. Its adsorption capacity for polar substances such as formaldehyde. LI et al. impregnated modified activated carbon with ammonia water and found that the modified activated carbon has stronger adsorption capacity for o-xylene and other hydrophobic VOCs than acid modification. Loaded metal modification is a method to improve the adsorption and separation performance of activated carbon through the strong binding force between the metal element or metal ion and the adsorbate supported on the activated carbon. It is generally believed that the metal-loaded performance changes the chemical properties of the activated carbon surface, which in turn changes the polarity of the activated carbon, so that the adsorption of activated carbon is dominated by chemical adsorption, which increases the selectivity of adsorption. The modified activated carbon was impregnated with Co under low oxygen conditions at 200°C, and it was found that the modified activated carbon had significantly improved toluene adsorption performance. The metal-loaded modified activated carbon technology is currently mainly used in the treatment of low molecular weight pollutants such as formaldehyde and toluene. The application of some large molecular weight VOCs needs further study.
3.2 The influence of adsorbate physical properties
Whether adsorbate molecules can enter the pores of activated carbon is related to its own dynamic diameter. According to the size exclusion theory, only when the pore diameter of the activated carbon is larger than the diameter of the adsorbate molecules, the adsorbate molecules can enter the pores of the activated carbon. The study found that when the adsorption efficiency of the adsorbent is the highest, the ratio of the pore size of the adsorbent to the molecular diameter of the adsorbate is 1.7 to 3.0. The molecular size of most gaseous pollutants is less than 2 nm, so the internal pores of activated carbon suitable for VOCs adsorption should be mainly micropores, and pores larger than the effective pore size have little adsorption effect. Research by LILLO-RDENAS et al. found that micropores smaller than 0.7nm have a strong adsorption capacity for benzene and toluene. Research by Ji Youjun et al. found that the micropores in the range of 0.60～1.15nm are the effective range of CH4 adsorption, and the pores larger than this range mainly act as channels in the adsorption process. The influence of adsorbate physical properties is also manifested in molecular weight, saturated vapor pressure, boiling point and so on. Activated carbon itself has a limited number of effective adsorption sites. When activated carbon adsorbs different substances with a similar number of molecules, the high molecular weight shows that the activated carbon has a large saturated adsorption capacity. Since gaseous substances with high boiling points are prone to capillary condensation during the adsorption process, they are easy to be adsorbed. Saturated vapor pressure is significantly related to the saturated adsorption capacity of activated carbon. At a certain temperature, the higher the saturated vapor pressure, the easier it is for VOCs to be desorbed. Chen Liangjie et al. studied the relationship between the saturated vapor pressure of six VOCs and the saturated adsorption capacity of activated carbon, and found that the higher the saturated vapor pressure of VOCs, the smaller the saturated adsorption capacity of activated carbon. Li Liqing et al. studied the effects of the physical properties of toluene, acetone, and xylene on the adsorption behavior of three kinds of VOCs on activated carbon. The results showed that the saturated adsorption capacity of activated carbon for organic gases increased with the molecular dynamics diameter, molecular weight, and boiling point of the adsorbate. Larger increases, and decreases with the increase of adsorbate polarity and vapor pressure.
3.3 Influence of operating conditions
The temperature, inlet concentration, gas flow rate, pressure, moisture, gas composition, etc. during the adsorption operation will affect the adsorption performance of activated carbon. It is very important to select appropriate operating conditions for different VOCs. Temperature can affect the diffusion rate and adsorption equilibrium. Increasing the temperature can increase the diffusion rate and speed up the time to reach the adsorption equilibrium. However, increasing the temperature will cause the adsorption capacity to decrease. The temperature should be controlled within 40°C during the adsorption operation. Han Xu et al. studied the adsorption process of methyl methacrylate on activated carbon at different temperatures and found that as the temperature increases, the saturated adsorption capacity continues to decrease. For the adsorption of the same organic matter, the adsorption capacity increases with the increase of the inlet concentration, and decreases with the increase of the gas flow rate. The activated carbon adsorption method is most suitable for processing VOCs with a concentration of 300-5000μL/L. After studying the adsorption behavior of granular activated carbon on benzene and toluene, GUPTA established a mathematical model and found that the model can determine the breakthrough time by flow rate, bed height, and inlet concentration. Mei Lei et al. used a fixed-bed reactor experiment to investigate the adsorption behavior of low-concentration naphthalene on GH-8 activated carbon at different temperatures and superficial gas velocities. The Yoon-Nelson model can be used to describe the adsorption behavior of GH-8 activated carbon. Increasing the main pressure of the gas phase means increasing the partial pressure of the adsorbate, which is conducive to adsorption, and the decrease in pressure is conducive to desorption. Gas with low partial pressure is easier to adsorb than gas with high partial pressure. Humidity can significantly affect the adsorption performance of activated carbon for VOCs. Gao Huasheng et al. [58] found that when the gas humidity is greater than 50%, the inhibition of adsorption is significantly enhanced, especially for low-concentration VOCs. Research by Zhou Jianfeng and others found that when activated carbon is used to treat dichloromethane-based water-insoluble VOCs, the moisture content in the gas has a great influence on the adsorption effect, and can even desorb the dichloromethane; while for ethanol-based water-soluble VOCs, the moisture content The effect is not large, which is related to the greater polarity of ethanol and its miscibility with water. The organic waste gas discharged by industry often contains multiple components. When multi-component VOCs are adsorbed on activated carbon, competitive adsorption will occur among the components. The presence of one component often has side effects on another component, and there is a displacement effect in the adsorption process. TEFERA et al. established a two-dimensional mathematical model to study the adsorption competition of multi-component VOCs on a fixed bed adsorber. This model can accurately predict the adsorption competition and adsorption equilibrium of multi-component mixtures. Cao Li et al. studied the binary adsorption process of VOCs on activated carbon and found that high-boiling components can replace low-boiling components, and the adsorption capacity of the binary system is reduced to varying degrees compared with the single-component adsorption under the same conditions.
4 Conclusion
Activated carbon adsorption method is the most widely used VOCs treatment method in the industry, but activated carbon still has some problems in practical applications, such as low adsorption capacity, poor regeneration capacity of activated carbon after adsorption, and adsorption performance that is greatly affected by environmental factors such as moisture Wait. In order to further optimize the adsorption performance of activated carbon, it is necessary to strengthen the research on the factors affecting the adsorption process of activated carbon, find effective methods for adjusting the pore structure of activated carbon and surface modification, and develop efficient adsorption materials with better adsorption performance or to meet specific needs (such as Special purpose activated carbon, high-strength activated carbon fiber, activated carbon cloth, etc.). On the basis of comprehensive consideration of the influencing factors of activated carbon adsorption and treatment of VOCs, the improvement and development of VOCs recovery and comprehensive utilization equipment, and the design of optimal process operating conditions, enable activated carbon to be more widely used in the treatment of VOCs.
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