Activated carbon adsorption treatment of VOCs processing techniques and problems
Abstract: Volatile organic compounds (VOCs) are an important class of atmospheric pollutants, it brings environmental pollution problems have attracted worldwide attention. Activated carbon adsorption method is an effective means to control VOCs pollution. This article describes the VOCs from treatment technology, outlines the current situation in the use ofVOCs governance, outlines the governance of VOCs and activated carbon adsorption technology problems, pointed out that variable temperature – pressure swing adsorption, absorption with its energy-efficient substation environmental advantages, has good prospects for development in VOCs governance. Analysis of the surface chemistry of the activated carbon, adsorbate properties, operating conditions, influence of adsorption of VOCs treatment activated carbon as VOCs and the Improvement and development of new products of activated carbon, provides a theoretical basis. On the basis of existing research summary of progress on the possible trend of VOCs control technology of activated carbon adsorption, coupling the use of the proposed process improvements and other VOCs exhaust gas treatment technologies, the development of different varieties of activated carbon and VOCs emissions of VOCs recovery for different places the device will be an important direction of future research. Volatile organic compounds (volatileorganiccompounds, VOCs) refers to the saturated vapor pressure of not less than 20 ℃ 0.13kPa organic compounds . Which mainly from the use of volatile and paints, coatings, packaging, printing, adhesives, cosmetics and other industries organic solvent petrochemical industry, oil exhaust emissions, oil storage facilities, gas stations and vehicles. According to statistics, in 2009 China\’s industrial source of VOCs emissions of about 12.06 million tons, and showed an increasing trend of about 8.6% per year . By 2030, only emissions of VOCs gas station up to 1271.03 tons, nearly one billion yuan of economic losses . Most VOCs toxic, and because the saturated vapor pressure is high, in the natural state can be volatilized into the air, into the human body through the respiratory tract, cause a variety of diseases. VOCs is one of the leading culprit fog and haze, the VOCs generated by chemical conversion of particulate matter in some areas may account for 21% PM2.5 sources. Secondary airgel formed by the photochemical reaction VOCs PM10 is from 25% to 35% , it is an important part of PM10. With the continued emergence of a wide range of fog and haze, VOCs governance issues have attracted great attention around the world, if cost-effectively recover VOCs, especially high concentration, high-value VOCs, have environmental, health, economic triple benefit. In order to better cope with the current form of air pollution, reducing emissions and control of VOCs, in September 2013, the State Council issued the \”Air Pollution Prevention Action Plan\” which called for promoting VOCs pollution control, particularly in the petrochemical, organic chemicals, surface coating, packaging and printing industry to implement comprehensive treatment of VOCs. In the same year, the State Environmental Protection issued a \”volatile organic compounds (VOCs) pollution control technology policy\” announcement, for products containing VOCs pollution in all sectors of production, storage and transportation sales, use and other proposed prevention strategies and methods. VOCs emission reduction and governance has become the focus of air pollution control. 1, VOCs control technology Treatment VOCs should first source of production and process control begins, clean production technologies using, feedstock containing less VOCs, the development of new alternative materials can be well prevented contamination. Second, we must strengthen the work of VOCs end of treatment, exhaust gas recycling process of economic value, handling and tank breathing exhaust gas and other; in accordance with laws and regulations, it is difficult for exhaust gas recycling processing. VOCs control technology currently dominated by the end of waste gas treatment. Conventional emission control technology has an end absorption method , a combustion method [6-7], the condensation method [8-9] and adsorption. Emerging technologies biological method, a low temperature plasma, membrane separation method, photocatalytic oxidation, etc. [10-13]. VOCs concentration generally between 100 ~ 2000mg / m3 of industrial waste gas emissions, such low concentrations of VOCs, by adsorption, absorption of the organic solvent recovery discharge standards; When not recycling, combustion may be employed, biological discharge standards methods, such as purification photocatalytic oxidation. Adsorption is the most common method of treatment of VOCs, especially for low concentrations of VOCs process . VOCs compared to other treatment techniques, adsorption processes can be difficult to distinguish from other selectively separating a mixture of high concentrations of hazardous substances of low removal efficiency, simple, safe operation, no secondary pollution, and after treatment can be achieved an organic solvent recovery the purpose of the adsorbent recycled. The most commonly used adsorbents activated carbon, silica gel, activated alumina, zeolite molecular sieve . In recent years, with activated carbon fibers, carbon nanotubes activity The method of the carbide-derived carbon, activated carbon cloth treated VOCs also attracted attention. Activated carbon adsorbents have a number of advantages over other: its wide distribution of pore size, pore developed fast adsorption process, capable of adsorbing different molecular sizes substance adsorption recovery of benzene, ethyl acetate, chloroform and other VOCs are very effective, non- polar, hydrophobic surface characteristics, it has a good selectivity for the adsorption of non-polar substances; inexpensive and abundant raw material of activated carbon, the preparation process is simple, easy to desorption regeneration, based on this, the activated carbon has been widely used as adsorbent VOCs agent for treatment of low concentration, medium-molecular-mass air quantity (usually about 45 to 130) , in particular wood particles of activated carbon prepared by Method phosphate, having a large adsorption capacity and desorption of residual small, the surface functional groups rich, preparation of economic and environmental advantages, at home and abroad are used extensively for control of VOCs. In order to improve the purification efficiency, activated carbon adsorption method is often used in conjunction with other treatments, commonly used methods are adsorbed and concentrated – concentrated condensate recovery and adsorption – catalytic combustion. Adsorption Concentration – condensation method is recovered by hot gas VOCs adsorbed activated carbon desorption process and then desorbed with a high concentration of VOCs recovered condensing device. The method is suitable for a single treatment of high concentration VOCs of exhaust gas components, it is not suitable for multi-component treatment, low concentration. Adsorption Concentration – catalytic combustion method is a method of hot gas desorbed VOCs concentrated sent to catalytic combustion bed catalytic combustion process. Activated carbon as a carrier, carrying a transition metal (Cu, Co, Fe, Ni, etc.) of the catalyst may be at a lower temperature (200 ~ 250 ℃), low oxygen conditions, the catalytic combustion of VOCs into CO2 and H2O [ 23] this method is particularly suitable for exhaust VOCs benzenes, aldehydes, alcohols and other properties of the gas concentration low content stable process . 2. Treatment of VOCs Activated Carbon Adsorption Technology Treatment temperature swing adsorption of activated carbon with a pressure swing adsorption technology VOCs (pressureswingadsorption, PSA), temperature swing adsorption (thermalswingadsorption, TSA), two associated with – a pressure swing adsorption (thermalpressureswingadsorption, TPSA) and substationAdsorption (electricswingadsorption, ESA). 2.1, pressure swing adsorption a pressure swing adsorption (PSA) means at a constant temperature or pyrogen-free conditions, the system pressure by varying periodically the adsorbate is adsorption and desorption cycles at different pressures. In different modes of operation, it can be divided into pressure swing adsorption using the difference between the van der Waals forces generally used for separation of activated carbon adsorption equilibrium type, and using the speed difference between the speed of adsorption of molecules using specific activated carbon molecular sieves for separating separation type  . Adsorption is generally carried out at atmospheric pressure, desorption is achieved by reducing the operating pressure or vacuum suction method, and the greater the degree of vacuum more easily desorbed during desorption. In practice, however, the high degree of vacuum adsorption apparatus demanding and energy intensive, and comprehensive cost considerations adsorption, desorption pressure is generally used 8 ~ 10kPa  industrially. PSA technology can achieve a high degree of automation cycle operation, but during operation requires constant reduced pressure, high equipment requirements, a huge energy consumption for recovering the multi-grade solvents. 2.2, temperature swing adsorption temperature swing adsorption (TSA) adsorption using the adsorbent equilibrium decreases with increasing temperature characteristics, adsorption at ordinary temperature, the desorption process after the operation. Activated carbon desorption process is an endothermic process, contribute to desorption heating using steam, hot gas desorption, the desorption temperature is generally 100 ~ 200 ℃. Adsorption of VOCs, if the higher adsorption capacity, the adsorbate is a lower boiling aromatic hydrocarbons and small organic molecules, after desorption of water vapor condensation and collection can be used; if the amount of adsorption was low, such as toluene, dimethylacetamide amides of VOCs and ethyl acetate, the other available hot gas (hot air, hot N2, etc.) by the second purge burned or recycled after adsorption desorption . RAMALINGAM et al  using the TSA technique, three common indoor VOCs (acetone, methylene chloride and ethyl) recycling have been studied, found that the optimum operating conditions of three kinds of hot nitrogen regeneration VOCs is: T = 170 ℃, V = 0.17m / s. SHAH et al  using a temperature swing adsorption properties The thermal air fresheners acetone and methyl ethyl ketone, and acetone was found that the regeneration cycle at 80 ℃, the adsorption capacity of nearly 95% recovery after 8 consecutive cyclesRing remains substantially constant; for methyl ethyl ketone, the adsorption ability significantly. 2.3, temperature – pressure swing adsorption variable temperature – pressure swing adsorption (the TPSA) combines the advantages of temperature swing adsorption and pressure swing adsorption two technologies, based on PSA technology basis in the pressure swing desorption desorption efficient technology. By increasing the bed temperature and lower pressure column so that the desorption performed more thoroughly, to improve the efficiency of the regeneration of activated carbon . RAMALINGAM etc.  The binding studies thermal desorption of nitrogen under reduced pressure and vacuum desorption has been shown that the two technologies, the recovery rate of 82% methylene chloride. Further, after the desorption in vacuo under reduced pressure, activated carbon bed temperature decreased from 93 ℃ to 63 ℃, it can significantly reduce the cooling time until the next cycle. 2.4, transforming adsorbed variable adsorption (ESA) is a method for gas purification and separation of the emerging technology, it is the essence of temperature swing adsorption. And various conventional temperature swing adsorption, the adsorption and desorption process variable, the heat generated by Joule effect heating power to achieve the saturated adsorbent causes release adsorbate. Adsorption substation has many advantages: a simple heating system, the energy is directly transferred to the adsorbent, high heating efficiency, can significantly reduce energy consumption ; rate of temperature rise can be controlled independently of the flow rate of the gas and the adsorbent; heat flow and mass flow with the more conducive desorption ; low cost, the cost of using the variable desorbed using superheated steam recycling rate lower than 50% ; good reproduction performance, SNYDER et al  found that the use of 12 cycles after the adsorption capacity of the adsorbent retains 97% to 100%. 2.5 Summary adapted for pressure swing adsorption purification of exhaust gas of high concentration VOCs and recovering high-grade organic solvent, with a high degree of automation, environmental benefits, and inlet gas concentration can be adjusted flexibly, etc., but due to the initial investment high cost, need to keep adsorption and desorption pressure, reduced pressure or vacuum, enormous energy consumption, but also pay attention to the gas pressure in the dead space, there are some limitations in use. Currently VOCs governance multi-use temperature swing adsorption 36], the majority of temperature swing adsorption bed again. However, heating and cooling temperature swing adsorption the adsorbent takes a long time during use, the adsorbent will appear due to heat aging performance degradation problems after multiple cycles, and forIn trichloroethane, styrene Thermosensitive VOCs does not apply, so the researchers also developed a variable adsorbed on the basis of a temperature swing adsorption. Variable absorption with high heating efficiency, fast heating, solvent and high recovery rate, in VOCs governance has been the concern of many scholars at home and abroad, as an emerging technology, has good prospects for development. Variable Temperature – combines the advantages of a pressure swing adsorption temperature swing adsorption and pressure swing adsorption two technologies, can significantly improve the recovery rate of regeneration of activated carbon and an organic solvent, a shortening of the cycle time, but still can not get rid of the limitation of the best of both sexual, currently seldom used, but the couple use a variety of technologies, development of composite gas separation technology is still important to the future development direction of VOCs governance. To select a different actual use of adsorption recovery process according to different working conditions and environmental requirements, and to strengthen the development and promotion of new equipment, actively seeking efficient and environmentally friendly economic governance of VOCs new technology. 3, factors govern VOCs activated carbon adsorption and solution conditions, activated carbon adsorption performance of VOCs in addition related to the activated nature of its own, but also with the physical properties of adsorbate, the adsorption operation related to . Reforming process for certain types of activated carbon to meet the governance requirements of VOCs, activated carbon, or to match the appropriate operating conditions for certain species and VOCs are research hotspot. 3.1 Effect of chemically modified activated carbon and surface chemical properties of the surface of the surface chemistry of the activated carbon is determined by the kind and amount of surface functional groups of the activated carbon, activated carbon adsorption properties affect the chemical properties of the difference in surface chemistry. By chemical modification of the surface of the activated carbon, activated carbon adsorption capacity can be varied selective adsorption of VOCs.  SHEN other studies show that the amide carbon surface can increase a basic functional group, oxidation can increase the carbon surface acidic functional groups. KIM et al.  The effects of different acids and bases Impregnated coconut shell activated carbon adsorption performance of various VOCs found modified phosphoric impregnated activated carbon adsorption properties improve as benzene, toluene, xylene and the like VOCs. Liu Yaoyuan, respectively, using the H2SO4 / H2O2 , NaOH  Modified Corn stover activated carbon, activated carbon with H2SO4 / H2O2 modified found that the reduction of its weakly polar toluene and the like, a non-polarAdsorption substance, to improve the adsorption capacity of polar substances such as formaldehyde with NaOH reforming activity. LI et al  with ammonia Impregnated activated carbon, modified activated carbon was found on the adsorption of the hydrophobic VOCs stronger than the acid-modified or ortho-xylene. Supported by the metal-modified stronger binding force between the elemental metal or metal ion adsorbate supported on activated carbon, activated carbon adsorption method of improving the separation performance. Is generally believed, alter the chemical nature of the load the metal-modified carbon surface, thereby changing the polarity of activated carbon, the activated carbon such that the adsorption via chemical adsorption, the adsorption selectivity increased . LU et al  under hypoxic conditions 200 ℃ impregnated with Co-modified activated carbon, modified activated carbon found in Adsorption significantly improved. Modified activated carbon supported metal technology is mainly used in processing small molecular weight contaminants on formaldehyde, toluene and the like, for application of some of the large molecular weight VOCs further study . 3.2 Effect adsorbate physical properties whether the adsorbate molecules can enter its own relevant kinetic diameter mesoporous carbon. The size exclusion theory, only when the pore diameter of activated carbon is greater than the molecular diameter adsorbate, adsorbate molecules to enter into the pores of activated carbon . Study found that the most efficient adsorbent, the aperture ratio of the adsorbate and adsorbent molecules diameter 1.7 to 3.0 . Most of the molecular size of the gaseous pollutants is less than 2nm , and therefore suitable for the VOCs adsorbed to the activated carbon pore microporous based, is greater than the effective pore size of pores little adsorption. LILLO-RÓDENAS et al  found that the micropores of less than 0.7nm benzene and toluene has a strong adsorption capacity. Jiyou Jun et al  found that in the range of 0.60 ~ 1.15nm micropores is valid interval CH4 adsorption, the pores larger than this range play a major role in the adsorption process channels. Effect of adsorbate was also reflected in the molecular weight of the saturated vapor pressure, boiling point. A limited number of active charcoal adsorption body point, different substances when similar number of molecules activated carbon adsorption, exhibits a large molecular weight to its saturation adsorption capacity of activated carbon. Due to the high boiling gaseous material and capillary condensation phenomenon is likely to occur in the adsorption process , it is easy to be adsorbed. Saturated vapor pressure and the adsorption capacity of activated carbon significantly correlated, at a certain temperature, saturatedAnd a vapor pressure greater VOCs desorb more easily. Chenliang Jie et al  The relationship between the saturated vapor pressure of the saturated adsorption amount of activated carbon 6 kinds of VOCs, found that the larger the saturated vapor pressure VOCs, the smaller the adsorption amount of activated carbon. Lili Qing et al.  The effects of toluene, acetone and physical properties Xylene VOCs adsorbed on activated carbon its behavior, the results show that: the saturated activated carbon as the gas absorption amount of organic adsorbate molecules kinetic diameter, molecular weight, boiling point increases, with the polar adsorbate, decreasing the vapor pressure is increased. 3.3, the influence of operating conditions temperature during adsorption operation, inlet concentration, gas flow rate, pressure, moisture, gas composition will affect the activated carbon adsorption, select the appropriate operating conditions are very important for various VOCs. Temperature can affect the diffusion rate and the adsorption equilibrium, the diffusion rate can be improved to increase the temperature to accelerate the time to reach adsorption equilibrium, but the increase in temperature leads to decrease of adsorption capacity, the temperature should be controlled within 40 ℃ adsorption operation. Han Xu et al.  The adsorption process on activated carbon (methyl methacrylate) at various temperatures, was found as the temperature rises, the saturated adsorption amount is decreasing. For the same adsorption of organic matter, the adsorption capacity with increasing inlet concentration increases with the increase of the gas flow rate is reduced, activated carbon adsorption method is most suitable for the treatment of VOCs concentration 300 ~ 5000μL / L. GUPTA et al  After the adsorption behavior of benzene and toluene, a mathematical model study by granular activated carbon, found that the model may be determined by the flow rate of the breakthrough time, and high bed inlet concentration. Mei Lei et al  using a fixed bed reactor experiments investigated Yoon-Nelson Model GH-8 is described activated carbon for low concentration naphthalene available under different temperature and superficial gas velocity. Increasing the gas pressure body, i.e. increasing the partial pressure of the adsorbate, is conducive to adsorption, the pressure is reduced in favor resolution, low partial pressure of the gas adsorbed more than the high-pressure gas . Moisture can significantly affect the performance of the activated carbon adsorption of VOCs, high Watson et al  found that when the air humidity is greater than 50% inhibition of the adsorption is significantly enhanced, particularly low concentration VOCs influence is very significant. Zhoujian Feng et al  found that in processing activated methylene chloride to non-water soluble VOCs, water content in the gas adsorptionEffective great influence, even desorbed methylene chloride; of VOCs and for alcohol based water-soluble, not big influence of moisture, which has a larger polarity and the ethanol is miscible with water related. Industrial organic waste discharged often contain a variety of components, multi-component VOCs adsorbed on activated carbon, competitive adsorption between the components may occur. The presence of one component, the other component will often have side effects, there metathesis adsorption process. TEFERA et al  to establish competitive adsorption of VOCs multicomponent two-dimensional mathematical model of a fixed bed adsorber, the model can accurately predict competitive adsorption between the multicomponent adsorption equilibrium mixture. CAO Li et al  studied VOCs binary adsorption process on activated carbon, found that high-boiling components can be replaced with low-boiling components, the amount of adsorption of the binary system of single component adsorption capacity than the same conditions have different degrees reduced. 4 Conclusion Activated carbon adsorption is VOCs treatment method most widely used in industry, but the activated carbon, there are some problems in practical applications, such as the adsorption capacity is not high, the difference between the activated carbon regeneration after adsorption, adsorption performance by environmental factors such as water, gas and other greater impact. To further optimize the adsorption properties of activated carbon, to strengthen the study of factors affecting adsorption process on activated carbon, the activated carbon to find effective pore structure control and surface modification methods, the development of efficient adsorbent (e.g., having specific needs or better adsorption properties satisfied special purpose activated carbon, high-strength carbon fiber, activated carbon cloth, etc.). After considering factors activated carbon adsorption of VOCs on the basis of governance, improvement and development of VOCs recovery and utilization of equipment, design the optimal process conditions, the activated carbon to be more broad application in the governance of VOCs. 5, prospects With the history of the most stringent implementation of \”environmental law\”, as well as the community\’s attention haze pollution, the environmental protection industry market, especially the market will benefit VOCs governance. VOCs can foresee the future governance work will be rapid development, VOCs control technology levels will be rising, VOCs governance development industry will enter a period of rapid growth. Activated carbon adsorption method is the most widely used treatment VOCs, future demand for activated carbon VOCs governance will continue to increase, activated carbon varieties, performance will be higherbegging. Compared to the conventional activated carbon treatment technology, using a variety of coupling VOCs control technology, a trend in the future will be the exhaust gas treatment VOCs. Such as by temperature – pressure swing adsorption, membrane separation – pressure swing adsorption, adsorption Concentration – condensation and adsorption recovery technology Concentration – VOCs catalytic combustion exhaust gas treatment technology, the energy consumption to achieve complete processing of VOCs; VOCs according to their solubility, boiling point other characteristics, selecting a pressure swing adsorption or temperature swing adsorption for recovery of the solvent, and the like. Key to the development of activated carbon adsorption-based VOCs recovery and utilization equipment is also of future research. Such as the improvement and development of small mobile vapor recovery system improvement and development of transport used oil storage facilities, gas stations and other places of activated carbon oil vapor recovery systems, tankers, oil tankers, etc., with mist recovery functions use catering services fume extraction system developed, will be the next hot application of activated carbon. While developing strong practical and efficient adsorption performance of activated carbon, to solve the problem of recovering gasoline, naphtha, kerosene and high fat organic liquids as benzene, toluene, xylene and other hazardous chemicals, still requires constant depth.