Progress in Volatile Organic Compounds (VOCs) Adsorption Technology
Volatile Organic Compounds (VOCs) not only have strong toxicity themselves, but also are important precursors and participants in regional composite air pollution in China. Therefore, the control of VOCs has received increasing attention. The adsorption method is currently the most widely used and technologically mature method for recovering VOCs. This paper introduces common adsorbents, adsorbent regeneration technologies, adsorption equipment, main adsorption processes, and related combined treatment technologies for adsorption recovery. It also discusses the urgent problems and development trends in VOCs treatment through adsorption recovery technology.
In recent years, multiple regions in China have experienced large-scale haze weather, characterized by ozone, fine particulate matter (PM), and acid rain. Regional composite atmospheric pollution problems have become increasingly prominent. As important precursors and participants of ozone and secondary organic particles, volatile organic compounds (VOCs) play a crucial role in atmospheric chemical reactions. Moreover, most VOCs have strong irritancy and toxicity. For example, benzene is classified as a first-class carcinogen, and n-hexane, heptane, and octane can affect the central nervous system of humans.
In May 2010, the State Council's Office first formally proposed requirements for strengthening VOCs pollution prevention and control at the national level, listing VOCs, SO, NO, and particulate matter as priority control pollutants for improving air environmental quality. The first comprehensive air pollution prevention and control plan in China, "The 12th Five-Year Plan for Key Regional Air Pollution Prevention and Control," was released at the end of December 2012, requiring enhanced requirements for VOCs emission projects, conducting targeted industry governance, and improving the VOCs pollution prevention and control system.
In the production process, using alternative products, improving processes, and replacing equipment are the preferred measures to reduce VOCs generation and emissions. However, for some processes and production, the path of clean production is still long, and end-point control technologies remain an indispensable means in a short period. Currently, VOCs treatment technologies mainly include recovery methods and destruction methods. From both environmental protection and economic perspectives, the recovery method is a recommended VOCs treatment technology.
VOCs recovery technologies include adsorption, condensation, absorption, membrane separation, etc. Among them, the adsorption method has simple equipment, wide application range, and high purification efficiency, and is a traditional VOCs treatment technology, which is also the most widely used treatment technology at present. Shi Jingying et al. found through a large number of industrial VOCs treatment technology engineering cases that the adsorption technology has the highest market share (38%) in China. When it is suitable for VOCs recovery, the adsorption technology is an economic and in line with the concept of clean production choice, and thus has been widely applied both domestically and internationally.
1. Adsorption Method Treatment Technology
The adsorption method is a method that uses various solid adsorbents to adsorb and purify pollutants in the emitted gas. Adsorption is divided into physical adsorption and chemical adsorption, and the purification of VOCs emissions mainly adopts the physical adsorption method. The adsorption method is suitable for treating low-concentration, high-flow organic gases and is mainly used for adsorbing and recovering aliphatic and aromatic hydrocarbons, most chlorinated solvents, common alcohols, some ketones, and esters, etc. The key technologies of the adsorption method are the determination of adsorbents, adsorption equipment and processes, regeneration media, and post-treatment processes.
The most representative inorganic adsorbent is activated carbon. Compared with granular activated carbon, honeycomb activated carbon has the advantage of lower bed layer resistance. Currently, in China, honeycomb-shaped activated carbon is mainly used as the adsorbent for equipment that treats high volume and low concentration VOCs. Activated carbon fibers have the advantages of large specific surface area, rich and evenly distributed micropores, fast adsorption and desorption rates, high adsorption efficiency, and easy regeneration. Activated carbon materials have excellent adsorption capacity for non-polar substances, such as organic solvents; conversely, they have poor adsorption for polar substances such as water, so it is possible to regenerate with steam vapor conveniently.
Zeolites and molecular sieves are mainly composed of aluminum silicate and have good thermal stability, with good safety when using hot air flow for regeneration. Different types of molecular sieves have different adsorption effects on VOCs, so chemical modification and modification of molecular sieves can be carried out to improve their removal efficiency of VOCs.
1.2 Adsorbent Regeneration
The commonly used regeneration methods for adsorbents in VOCs treatment include low-pressure steam replacement regeneration, hot air flow sweeping regeneration, and pressure reduction or vacuum desorption regeneration. Low-pressure steam replacement regeneration and hot air flow sweeping regeneration are suitable for desorbing low-molecular carbon hydrocarbons and aromatic organic compounds with lower boiling points.
Some new energy-saving adsorbent regeneration technologies have also been developed, such as microwave desorption, Joule heating 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 less practical applications.
1.3 Commonly Used Adsorption Devices
In the treatment of organic waste gas, industrial commonly used adsorption devices include fixed beds, moving beds, fluidized beds, and zeolite honeycomb rotary wheel adsorption devices. The most classic and commonly used one is the fixed bed. The zeolite honeycomb rotary wheel adsorption device is a relatively new rotary adsorption system in the field of organic waste gas purification, also known as a rotor adsorber. The waste gas can pass radially or axially through the adsorbent rotor and be purified after passing through most of the rotating bed layer. The advantages of the rotor adsorber are small equipment volume, convenient operation, low pressure, and the ability to continuously treat large volumes of waste gas into low-concentration purified gas, while the desorbed gas has a high concentration and low flow rate, and the concentration enhancement ratio can reach 10 to 15 times.
1.4 Main Adsorption Processes
1.4.1 Fixed Bed Adsorption - Steam Replacement Regeneration - Condensation Recovery Process This process usually uses granular activated carbon, activated carbon fibers, or zeolite as adsorbents, mainly for the recovery of solvents in low-concentration organic waste gas. After the adsorbent in the fixed bed reaches saturation, high-temperature steam is introduced to make the adsorbed organic substances leave the adsorption bed together with the steam vapor, and then the steam mixture is cooled by a condenser to condense into liquid.
1.4.2 Fixed Bed Adsorption - Vacuum Desorption Regeneration - Absorption Recovery Process This process usually uses granular activated carbon with well-developed mesopores as the adsorbent for adsorption, and then uses vacuum pressure reduction to regenerate the adsorbent. The extremely high-concentration waste gas extracted by the vacuum pump is usually absorbed and returned using low-volatile organic solvents. This process is suitable for the recovery of high-concentration organic waste gas.
1.4.3 Zeolite Rotary Wheel Adsorption Concentration Process
In current organic waste gas pollution in China, low-concentration and high-volume VOCs emissions account for a considerable proportion. Therefore, adsorption concentration technology is the most economical and effective technical approach for low-concentration waste gas treatment. In recent years, a batch of zeolite rotary wheel adsorption concentration devices have been introduced from Japan, and many enterprises in the industry are actively developing and applying this technology. Compared with the fixed bed adsorption concentration technology, zeolite rotary wheel adsorption concentration technology has many advantages: using honeycomb-shaped zeolite as the adsorbent, high-temperature desorption can be adopted, high regeneration efficiency, good safety performance, and a wider range of VOCs that can be treated; The equipment has low resistance and high utilization rate of the adsorbent, with low operating costs; the concentration of organic pollutants in the tail gas is stable, facilitating control; the structure is compact, and the volume and floor area of the equipment are small.
1.4.4 Adsorption-Cooling Combined Process
The adsorption-concentration-cooling recovery process is typically used for the purification of VOCs with low concentration, large air volume, and high recovery value. Currently, fixed beds or zeolite rotors are mainly used as adsorption devices. When the adsorbent reaches saturation, water vapor or hot air flow is selected for desorption based on the properties of the organic waste gas and the adsorbent. Vacuum desorption regeneration can also be carried out. The high-temperature and high-concentration mixed steam released by desorption then enters the condenser for condensation and recovery.
Lu Shuhua et al. integrated the traditional processes of condensation and adsorption in organic waste gas recovery, integrating the four processes of pre-cooling, condensation, adsorption, and separation into the same flow channel, and designed a new plate-type core body with a new structure. The organic waste gas channel and the refrigerant channel are separated by metal plates with triangular fins. Adsorbents are filled in the channels between the fins of the organic waste gas channel, and refrigerant flows through the channels on the other side of the metal plates. The organic waste gas is adsorbed while flowing in the opposite direction to the refrigerant, reducing the temperature rise of the adsorbent and extending the service life of the adsorbent. The low-temperature tail gas after condensation and adsorption is led to the upper core body of the device for pre-cooling, achieving secondary energy utilization and improving energy utilization efficiency. The device is divided into two parts and connected by flanges, facilitating the disassembly of the device and the replacement of the adsorbent. This device has high organic waste gas recovery rate, small floor space, and low manufacturing cost, and is suitable for the recovery and treatment of industrial organic waste gas.
2 Conclusion
(1) From the perspective of resource utilization, the recovery technology is an effective method for the treatment of VOCs. The US EPA pointed out that activated carbon adsorption is the "best technology that can be adopted" for removing VOCs, and the adsorption method is also the most widely used and mature technology in China at present, especially the fixed-bed adsorption temperature-changing technology is suitable for the current economic and technical level in China.
(2) The complexity of the composition, composition, and properties of VOCs leads to the fact that a single treatment technology cannot effectively solve the problems of non-compliance with tail gas emissions, high equipment operating costs, and low economic benefits that are commonly encountered by enterprises and chemical parks. Based on integrated combination methods such as absorption-adsorption, cooling-adsorption, and adsorption-membrane separation, the removal rate of VOCs can be further improved, costs can be reduced, and secondary pollution can be reduced.
(3) Developing adsorbents with better adsorption performance or meeting specific requirements, finding feasible surface modification methods for adsorbents; exploring the heat and mass coupling transfer mechanism of VOCs during adsorption and desorption, analyzing the heat and mass transfer characteristics of VOCs; strengthening the research on the influencing factors of the adsorption and desorption process, improving the adsorption efficiency, extending the service life of the adsorbent; accurately predicting the phase change and adsorption-desorption process, establishing prediction methods for VOCs recovery processes and design methods for recovery devices; developing purification equipment with higher adsorption efficiency. These issues require further research and discussion.
(4) In the commonly used adsorption-cooling combined process, adsorption and cooling are two independent processes of the same process, which is a weak coupling. It is simply formed by simply connecting the cooling and adsorption devices. This will result in large equipment floor space, low energy utilization efficiency, and high costs. Coupling cooling and adsorption in the same channel simultaneously can shorten the process, extend the service life of the adsorbent, improve energy utilization efficiency and VOCs recovery efficiency. Therefore, in-depth systematic research on the coupling process and behavior of adsorption-cooling is expected to break through the limitations of traditional adsorption and cooling technologies, with significant scientific and engineering application value.
