Advances in adsorption and recovery technologies for volatile organic compounds (VOCs)
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
Volatile organic compounds (VOCs) not only have strong toxicity, but also are important precursors and participants affecting regional air pollution in China. Therefore, the control of VOCs has received more and more attention. Adsorption is currently the most widely used and most mature method for recovering VOCs. The common adsorbent, adsorbent regeneration technology, adsorption equipment, main adsorption technology and adsorption-related combination treatment technology are introduced. The problems and development trends of adsorption recovery technology in the treatment of VOCs are discussed.
In recent years, large-scale smog weather has occurred in many parts of China, and the problem of regional atmospheric composite pollution characterized by ozone, fine particulate matter (PM:) and acid rain has become increasingly prominent. As an important precursor of ozone and secondary organic particles, Volatile Organic Compounds (VOCs) play an extremely important role in the atmospheric chemical reaction process. At the same time, most VOCs are highly irritating and toxic. If benzene is listed as the first class of carcinogens, n-hexane, heptane and octane affect the human central nervous system.
In May 2010, the General Office of the State Council officially proposed the requirements for strengthening the prevention and control of VOCs pollution from the national level for the first time. VOCs, SO, NO and particulate matter were listed together as key pollutants for improving the quality of the atmospheric environment. China's first comprehensive air pollution prevention and control plan issued at the end of December 2012, the “Twelfth Five-Year Plan for Air Pollution Prevention and Control in Key Areas” requires the improvement of VOCs emission project construction requirements, the implementation of key industry governance, and the improvement of VOCs pollution prevention and control system.
The use of alternative products, improved processes and equipment replacement in the production process is the preferred measure to reduce the generation and emission of VOCs. However, for some processes and production, the road to cleaner production is still very long. In a short period of time, the end-control technology is still An indispensable means. At present, the governance technologies of VOCs mainly include recycling and destruction methods. Whether it is environmentally friendly or economically, recycling is a VOCs governance technology that deserves to be promoted.
VOCs recycling technology includes adsorption, condensation, absorption, membrane separation, etc. Among them, the adsorption method is simple, wide in application range and high in purification efficiency. It is a traditional VOCs treatment technology and is the most widely used treatment technology. Xi Jinjun and others found that the adsorption technology has the highest market share (38%) in China by investigating a large number of industrial VOCs processing technology projects. In the case of suitable VOCs, adsorption technology is an economical and clean production concept. Therefore, it is widely used at home and abroad.
1. Adsorption treatment technology
The adsorption method is a method for adsorbing and purifying pollutants in exhaust gas by using various solid adsorbents. Generally, adsorption is divided into physical adsorption and chemical adsorption, and the purification of VOCs is mainly carried out by physical adsorption. The adsorption method is suitable for treating low-concentration and high-volume organic waste gas, and is mainly used for adsorbing and recovering fat and aromatic hydrocarbons, most chlorine-containing solvents, common alcohols, some ketones and esters. The key technologies of the adsorption method are the determination of adsorbents, adsorption equipment and processes, regeneration media, post-treatment processes, and the like.
1.1 commonly used adsorbents
At present, the adsorbents commonly used in the purification of VOCs are inorganic and organic adsorbents. The inorganic adsorbents are mainly used, mainly activated carbon (including granular activated carbon, honeycomb activated carbon and activated carbon fiber), molecular sieve (including granular molecular sieve, molecular sieve molding). Bulk honeycomb and molecular sieve coating materials), zeolite, granular silica gel, activated alumina, porous clay ore, etc., and organic adsorbents mainly refer to high polymer adsorption resins.
The most inorganic inorganic adsorbent is activated carbon. Compared with granular activated carbon, honeycomb activated carbon has the advantage of low bed resistance. At present, the adsorbent for treating high-volume and low-concentration VOCs in China is mainly composed of honeycomb activated carbon. Activated carbon fiber has the advantages of large specific surface area, abundant micropores and uniform distribution, fast adsorption and desorption rate, high adsorption efficiency and easy regeneration. Activated carbon materials have very good adsorption capacity for non-polar substances such as organic solvents; on the contrary, for polar substances such as water, the adsorption is poor, so it is possible to regenerate with water vapor conveniently.
The main components of zeolite and molecular sieve are all aluminosilicates, which have good thermal stability and are safe to regenerate when using hot gas flow. Different types of molecular sieves have different adsorption effects on VOCs, so the molecular sieve can be chemically modified and modified to improve the removal effect of VOCs.
1.2 adsorbent regeneration
The adsorbent regeneration methods commonly used in the treatment of VOCs are low pressure steam displacement regeneration, hot gas purge regeneration and pressure reduction or vacuum desorption regeneration. Low-pressure steam displacement regeneration, hot gas stream purge regeneration is suitable for desorption of low molecular weight hydrocarbons and aromatic organic compounds with lower boiling points.
At present, some new energy-saving adsorbent regeneration technologies have been developed, such as microwave desorption, electric coke desorption, solvent replacement, and ultrasonic regeneration. These new desorption technologies have good energy-saving effects and high efficiency, but they are still in the research stage and have few practical applications.
1.3 commonly used adsorption device
In the field of organic waste gas treatment, the adsorption equipment commonly used in the industry has a fixed bed, a moving bed, a fluidized bed and a zeolite honeycomb wheel adsorption device, and the most classic and commonly used fixed bed. The zeolite honeycomb wheel adsorption device is a relatively newly developed rotary adsorption system in the field of organic waste gas purification, also known as a rotor adsorber. The exhaust gas can be purified radially or axially through the rotor containing the adsorbent and through most of the rotating bed. The advantage of the rotor adsorber is that the equipment is small in volume, convenient in operation and low in pressure, and the large-flow exhaust gas can be continuously processed into a low-concentration purified gas, and the desorbed gas has a high concentration and a low flow rate, and the general enrichment ratio can be reached. 1O ~ 15 times.
1.4 main adsorption process
1.4.1 Fixed bed adsorption, water vapor replacement, regeneration, condensation recovery process
The process usually uses granular activated carbon, activated carbon fiber or zeolite as an adsorbent, and mainly recovers the solvent in a lower concentration of organic waste gas. After the adsorption of the adsorbent in the fixed bed reaches saturation, the high temperature water vapor is passed to cause the adsorbed organic matter to leave the adsorption bed along with the water vapor, and then the vapor mixture is cooled by a condenser to be condensed into a liquid.
1.4.2 fixed bed adsorption-vacuum desorption regeneration-absorption recovery process
The process usually adopts granular activated carbon with well-developed mesopores as adsorbent, and then uses the vacuum to reduce the adsorbent, and the extremely high concentration of exhaust gas extracted by the vacuum pump is usually absorbed by a low volatility organic solvent. This process is suitable for the recovery of high concentration organic waste gas.
1.4.3 Zeolite runner adsorption and concentration process
In the current organic waste gas pollution in China, the concentration of VOCs with low concentration and high air volume accounts for a considerable proportion. Therefore, adsorption and concentration technology is the most economical and effective technical way to treat low-concentration waste gas. In recent years, China has introduced a number of zeolite wheel adsorption and concentration devices from Japan, and many companies in the industry are actively developing and applying this technology. Compared with fixed bed adsorption concentration technology, zeolite rotary adsorption and concentration technology has many advantages: using zeolite zeolite as adsorbent, high temperature desorption can be used, high regeneration efficiency, good safety performance, and a wide range of suitable VOCs for processing; The resistance is small, the utilization rate of the adsorbent is high, the running cost is low, the concentration of the organic pollutant in the exhaust gas is stable, and the control is convenient; the structure is compact, and the volume and the occupied area of the device are small.
1.4.4 adsorption-condensation combination process
The adsorption-concentration-condensation recovery process is generally used for the purification of low-concentration, high-volume, high-recovery VOCs. Currently, fixed-bed or zeolite runners are mainly used as adsorption devices in the industry. After the adsorption of the adsorbent reaches saturation, the water vapor or hot gas stream is selected for desorption according to the organic waste gas and the adsorbent property used, and vacuum desorption regeneration can also be performed, and the desorbed high-temperature and high-concentration mixed steam is then sent to the condenser for condensation recovery. .
Lu Shuhua et al. The traditional process of condensation and adsorption in integrated organic waste gas recovery integrates four processes of pre-cooling, condensation, adsorption and separation into the same flow channel, and designs a new type of plate structure core with organic exhaust gas passage and refrigerant. The channels are separated by sheet metal with triangular fins. The adsorbent is filled between the fins of the organic waste gas passage, and the refrigerant flows through the passage on the other side of the metal sheet. The organic waste gas is adsorbed in the opposite direction of the refrigerant while being adsorbed by the adsorbent, which reduces the temperature rise of the adsorbent and prolongs the life of the adsorbent. The low-temperature exhaust gas after condensation adsorption is led to the core of the upper part of the device for pre-cooling, thereby achieving secondary utilization of energy and improving energy utilization. The device is divided into upper and lower parts and flanged for easy disassembly of the device and replacement of the adsorbent. The device has the advantages of high organic waste gas recovery rate, small space occupation and low manufacturing cost, and is suitable for recycling industrial organic waste gas.
2. Conclusion
(1) From the perspective of resource utilization, recycling technology is an effective method for managing VOCs. The US EPA pointed out that activated carbon adsorption is the best technology that can be used to remove VOCs, and the adsorption method is also the most widely used and mature technology in China. In particular, activated carbon fixed bed adsorption and temperature change technology is suitable for China's existing economy. techinque level.
(2) The complexity of the composition, composition and nature of VOCs leads to the failure of a single treatment technology to solve the problem of unsatisfactory exhaust emissions, high operating costs and low economic efficiency of enterprises and chemical parks in most cases. . Combined methods based on different process integration, such as absorption-adsorption, condensation-adsorption, adsorption-membrane separation, etc., can further improve the removal rate of VOCs, reduce costs and reduce secondary pollution.
(3) Develop adsorbents with better adsorption performance such as catalytic oxidation or meet specific requirements, find practical and feasible methods for surface modification of adsorbents; explore the mechanism of heat and mass coupling transfer of VOCs during adsorption and desorption, and analyze the heat transfer of VOCs Mass transfer characteristics; strengthen the research on the factors affecting the adsorption desorption process, improve the adsorption efficiency, prolong the life of the adsorbent; accurately predict the phase change and adsorption desorption process of VOCs, establish the VOCs recovery process prediction method and recovery device design method; Purification equipment for adsorption efficiency. These issues need further research and discussion.
(4) At present, the adsorption and condensation in the common adsorption-condensation combination process are two independent processes of the same process, which is a weak coupling, which simply connects the two devices of condensation and adsorption, which will cause a large area of equipment. Insufficient energy utilization and high cost. Condensation and adsorption coupling simultaneously in the same channel can shorten the process, extend the life of the adsorbent, and improve energy efficiency and VOCs recovery efficiency. Therefore, the systematic study of the adsorption and condensation coupling process and behavior of Shenren is expected to break through the limitations of adsorption and condensation technology in the traditional sense, which has important scientific significance and engineering application value.