A review of the treatment techniques of VOC

Absorption methods can usually be classified into physical absorption and chemical absorption methods. It is well known that different types of gases show distinct solubility in the same absorbent. The physical absorption method takes advantage of this fact to selectively absorb harmful gases, thereby purifying the exhaust gas. The chemical method uses the chemical reaction between the waste gas and the absorbent to absorb the waste gas. Comparatively, the physical absorption method is more extensive.

The physical absorption method requires a number of factors to be taken into consideration, such as the type of absorbent and the choice of absorption device. There are three main types of general absorbents: oil-based absorbents (e.g. non-polar mineral oils such as diesel, oil wash etc.), water complex absorbents (e.g. water-oil wash, water-alkali etc.) and high boiling point organic solvents (e.g. phthalates, adipates etc.) [5, 6]. Some special adsorbents are also being investigated. Briefly, the choice of absorbent should be considered in terms of safety, effectiveness, cost and environmentally friendly performance. Common absorption devices include plate columns, packed columns and spray towers.

Absorption is a more mature technology, which has the advantages of simple operation, low investment and low maintenance costs. It is suitable for the treatment of high concentration of organic exhaust gas at room temperature. The method still faces many problems in the selection and development of absorbent and the optimisation of absorption equipment, which needs to be further explored.

Adsorption can adsorb VOC gas molecules selectively, using the inherent characteristics of the adsorbents to purify and treat the waste gas containing VOC. Depending on the characteristics of adsorbent regeneration, it can be classified into two types: regenerated adsorption and non-regenerated adsorption; in terms of adsorption principles, these two types cover physical adsorption and chemical adsorption, respectively. The former is mainly deployed in treatment with organic waste gas, while the latter is mostly used for wastewater treatment. Based on the process conditions, adsorption methods can be categorised into temperature swing adsorption (TSA); pressure swing adsorption (PSA); temperature and PSA; and electric swing adsorption [7].

The choice of the adsorbent is crucial to the adsorption effect. The adsorbent usually selects materials with large specific surface areas, high selectivity, loose structure and long service life, which are beneficial in terms of enhancing the adsorption effect. Most of the commonly used adsorbents are porous materials, such as activated carbon, zeolite molecular sieves, metal–organic framework materials, alumina and resins [8, 9]. Currently, activated carbon is the most effective adsorbent in organic waste gas treatment [1], which is attributed to its advantages such as strong adsorption capacity, large surface area and easy access. However, it has little effect in environments with high humidity or high temperature, and even some safety hazards. Molecular sieves also have certain advantages in exhaust gas treatment, such as good stability, high adsorption activity and hydrophobicity. It is a porous crystalline aluminosilicate material with pore structures [10]. Its adsorption process is mostly physical.

The adsorption method is simple and has a good purification effect. However, there are shortcomings such as adsorbent poisoning and secondary pollution. This method is effective for low concentration and high flow rate organic exhaust gas but is not suitable for high temperature, high humidity and high concentration exhaust gas cleaning.

Condensation is one of the simplest methods of exhaust gas treatment. Condensation converts VOC to liquid at lower temperatures or higher pressures, thereby recovering large amounts of VOC [11]. The saturated vapour pressure is related to the type of gas and the temperature at which it is applied. Industries usually use lower system temperature or higher system pressure to separate and purify organic exhaust gas.

Cooling units and refrigerants are critical to the treatment of exhaust gases by condensation. Currently, condensation is mainly divided into surface condensation and contact condensation – and the corresponding condensing units are surface condensers and contact condensers. Refrigerant is an industrial substance used for heat exchange. Refrigerants are usually selected from working fluids having high refrigeration efficiency, high thermal conductivity and high exothermic coefficient.

Condensation is simple when treating high concentrations and high boiling point organic exhaust gas. Theoretically, it is highly effective. If the method is used alone to treat components with low freezing points and low concentration, the system pressure needs to be significantly increased or the system temperature needs to be drastically reduced. The great change in the operating environment is a challenge to the equipment. The method has a high cost and consumes a lot of energy. Therefore, the condensation method is commonly used to treat high concentrations of valuable VOC [11].

Research on membranes has been performed since the 18th century and has been gradually applied in the industrial field since the 1960s of the last century. With the development of science and technology as well as the continuous improvement of the research level, membrane separation technology has been applied more and more widely. It has made significant contributions to wastewater treatment, food processing, pharmaceutical production and petrochemical refining.

The membrane has selective permeability. Under the push of external force, membrane separation technology can separate and purify organic waste gas. It is well known that the diffusion rate in the same solvent is related to the type of gas; membrane separation technology mainly makes use of this feature and the inherent characteristics of membranes to realise the needed purification. Exhaust gas emission standards are met and then discharged. There are various types of membranes. Generally, based on the separation mechanism and application scope, they can be grouped into microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes, pervaporation membranes and ion exchange membranes [12].

Corresponding to the classification of membranes, relatively mature membrane separation technologies include microfiltration, ultrafiltration, nanofiltration, reverse osmosis and electrodialysis. The driving force of microfiltration, ultrafiltration, nanofiltration and reverse osmosis is the pressure difference. Driven by pressure, the solvent is facilitated to flow smoothly. Electrodialysis is driven by a voltage differential and allows only ions to pass through.

Membrane separation technology is a relatively advanced treatment technology. It possesses the significant advantages of simple operation, high efficiency and wide application range. It is energy efficient because no thermal driving force is required to separate the mixture [13, 14]. However, the cost of the membrane is high. The membrane separation method has poor stability and can cause membrane contamination problems. The technology is appropriate for treating VOC with low flow rates and high concentrations.

Biodegradation is a new technology. Research on the use of this technology to treat VOC began as early as the mid-to-late 20th century. By taking advantage of the unique physiological functions and characteristics of living organisms, harmful substances in VOC are degraded into inorganic substances such as water and carbon dioxide, thereby purifying exhaust gases.

Biological methods can be divided into four types: biofiltration, bio-trickling filtration, bio-scrubber and membrane bioreactors [15].

The main equipment of the biological filtration method is the biological filter tower. It is based on the principle that pre-treated exhaust gases can be transported to a biological bed containing packing materials, where the organic exhaust gases would be broken down into harmless substances.

The biological drip filter method is equipped with a biological drip filter tower, where organic waste gases are purified after contact with a moist biofilm. The organic waste gases enter the tower from the bottom for purification and eventually release from the top. Metabolic wastes are eliminated with the waste liquid [16].

The important equipment of the biological scrubbing method is a biological scrubbing tower, which is sprayed with a circulating scrubbing solution to absorb the organic waste gases. After absorption, organic waste gas is introduced into the regeneration tank. Then the activated sludge degrades it [17], resulting in the purification of organic waste gas.

Membrane bioreactor is a new process that combines membrane separation technology with biological methods. It is composed of three parts mainly: a bioreactor, a membrane module and a control system [18].

The biodegradation has simple equipment. Its operation as well as the management cost is low. There will be no secondary pollution during the treatment. However, this technology is greatly influenced by temperature and humidity. The biodegradation method has a high removal efficiency when treating low concentrations of VOC.

Photochemical water-cracking reactions were discovered in the early 1970s, after which photocatalysis was widely used in water treatment and degradation of VOC [19]. The basic principle is that the photocatalyst produces electron-hole pairs under the action of light. By using the strong oxidation of electron holes, VOC will be oxidised into carbon dioxide and water, and other substances. Thus, the purpose of eliminating pollutants can be achieved. The method also includes processes such as adsorption, chemical degradation and desorption of products or intermediates of VOC [20]. Catalytic efficiency is related to the type of catalyst. Commonly used photocatalysts include TiO₂, ZnO, WO₃, ZnS and CdS [20]. Among these, the most extensively studied catalytic material is TiO₂, which is non-toxic and has the advantages of simple preparation, stability, low cost and strong ability to degrade VOC pollutants [21]. TiO₂ is capable of being not only used alone but also making the modified TiO₂ photocatalysts. Both of these can be utilised for the purification of VOC.

Photocatalysis is a new treatment method with high efficiency in the treatment of low concentration organic waste gases. The reaction conditions are mild. It has good green environmental performance and low energy consumption. The operation and management costs are also low. However, there are problems such as catalyst deactivation. Therefore, in order to enhance the photocatalytic efficiency or activate the photocatalyst, it is usually necessary to develop new catalytic materials [22].

In 1928, the concept of plasma was first proposed to describe a collection of charged particles [23, 24]. It is also the fourth state of matter. Depending upon thermodynamic equilibrium, plasma can be divided into thermodynamic equilibrium and non-thermodynamic equilibrium [25]. Non-thermodynamic equilibrium plasma is also called low-temperature plasma. Low-temperature plasma has been increasingly studied and applied in the treatment of organic waste gas. Low-temperature plasma technology is a method that uses high-energy electrons to cause inelastic collisions with gas molecules or atoms to cause free radicals, and the free radicals react with organic gas molecules to degrade VOC [26]. The main generation mechanism of low-temperature plasma is discharge, including corona discharge [27], dielectric barrier discharge [28, 29] and glow discharge [30].

The low-temperature plasma method applies to processing large flows of low concentrations of organic waste gas. It is a simple energy-efficient treatment method. In addition, low operating and maintenance costs are another advantage. However, the generation of by-products, safety and severe condition controls are the issues to be considered when applying this method.

The combustion method is also known as the thermal oxidation method. It means that under certain conditions, the combustible organic exhaust gases are burned (or oxidised) into CO₂, H₂O and other harmless substances, achieving the purpose of purifying exhaust gases. The combustion method can be further sorted into direct combustion, catalytic combustion, porous media combustion, regenerative combustion and so on.

The components of the RTO burner include a regenerative body, combustion chamber, rotary valve, fan, control system etc. The regenerative body is the core of the RTO system and plays a key role in storing and releasing heat. Spherical and honeycomb are the main varieties widely used in the domestic and international industries [38]. Ceramic honeycomb regenerative bodies are widely used in RTO systems due to their advantages such as high thermal conductivity, high-temperature resistance and corrosion resistance. The combustion chamber is the main place where the combustible components of the organic exhaust gases are burned and purified. The exhaust gases preheated by the regenerative body are combusted and decomposed after entering the combustion chamber to produce carbon dioxide and water. The rotary valve is set up to help direct the gas distribution. It is also gas-tight, which greatly reduces the possibility of gas leakage.

According to the different airflow switching methods, rotary RTOs can be broadly classified into rotary valve type and multi-valve switching type [39]. Based on the sealing method of the rotary valve, there are three types: gas seal, mechanical flexible seal and the combination of the two. In terms of different purging media, it can be divided into two types: air purge and purge gas purge.

The regenerative body is preheated to 700–1000°C. The rotary valve is driven by the fan. Under the guidance of the fan and the rotary valve, the VOC flow through the regenerative body and absorb the stored heat. When the gas reaches a certain temperature, it is sent to the combustion chamber for combustion, generating carbon dioxide and water. Meanwhile, it releases a large amount of reaction heat. The purified gas carries a large amount of heat. Heat transfer occurs when it flows through the regenerative body. Most of the heat is transferred to the regenerative body by heat conduction and radiation. The temperature of the purified exhaust gas drops significantly. Finally, it is discharged from the bottom outlet. At the same time, the temperature of the regenerative body increases, storing a large amount of heat. The task of the stored heat is to preheat the organic waste gas that has just entered. In this way, the continuous and efficient treatment of the organic waste gas is realised.

Fig. 1

The rotating thermal storage oxidation technology has three advantages: First, multiple valves are simplified into one valve, which greatly simplifies the complexity of the device and reduces the flow resistance of gas. Therefore, the intake, exhaust and scavenging process is smoother. Second, the oxidation of the intake air and the exothermic heat of the exhaust occur simultaneously in the oxidation bed. The heat transfer intensity and efficiency are higher. The heat exchange area is saved and the structure is more compact. Third, the device can rely on the oxidative heat release of the fuel to realise self heating to maintain operation, which greatly saves energy.

As a new technology for the treatment of VOC, many scholars and research departments have seen the huge potential for development, and have initiated a series of research and development.

Chen and Wei [40] introduced the governance technology of VOC in flexible packaging printing enterprises in recent years. The novel rotary combustion method was introduced in the novel VOC end treatment method, showing the structural principle of the method and the process route. A technical comparison between the traditional and new VOC terminal management technologies was also carried out. It was found that the new governance technology has high heat efficiency, low energy consumption and much lower operating costs. A detailed introduction of rotating RTO was carried out by Li et al. [39]. The significant advantages of rotary RTO were emphasised based on the comparison of RTO types and performance. And the rotary RTO were classified and introduced based on five aspects. Guo et al. [41] introduced the working principle and the main performance index of the rotary RTO. It showed that the thermal efficiency of the rotary RTO reaches 97%. It achieved the secondary recycling of heat energy while reducing heat loss in operation. Iloeje et al. [42] proposed an accurate simplified model of the rotating reactor, which greatly reduced the computational cost. And the optimal combination between the variables was determined to meet the needs of reactor design. Cheng et al. [43] proposed a rotating reactor for NO_x reduction, using a fixed bed reactor to simulate the dynamic adsorption-reduction process of the reactor. Compared with the conventional fixed bed reactor, it had a high removal efficiency of NO_x in an oxygen-rich environment. And it was adapted to a wide temperature range. A rotary regenerative catalytic combustion reactor (RRCCR) was proposed by Sang et al. [35]. The results of their study showed that both the inlet velocity and concentration had a large effect on the performance of this reactor. The reactor performance was found to be more sensitive to the increase in velocity and a decrease in methane concentration. Sun et al. [44] simulated the effect of reaction conditions such as oxygen concentration in the adsorption zone and adsorption to the reduction time ratio on the performance of the rotary reactor during CO reduction of NO_x. SO₂ and H₂O in the flue gas were found to have a severe influence on the NO adsorption-reduction dynamic process.

In the current rotating RTO system, the temperature field in the regenerative body is stable, and continuous gas input is achieved. However, most rotary reversing valves have a complex structure, and large flow losses may occur during gas transport. The friction of the plane seal is high, and the high energy consumption of the drive motor is a problem [45]. These problems can shorten the service life of rotary reversing valves and lead to unstable system operation. Therefore, Mao et al. [46] invented a new type of rotary directional valve for a rotary heat storage combustion device. An inlet air distribution plate, an exhausting return air distribution plate and a sweep air distribution plate were correspondingly installed at the top, middle and bottom of the rotary shaft in the valve. Each air distribution plate and the cylinder body were surrounded to form the intake chamber, exhaust chamber and air scavenging chamber. The simple structure of the reversing valve and the reasonable distribution of the three air distribution plates made the process of the air intake, exhaust and sweeping smoother; thus, the flow loss was minimised and the jamming situation could be avoided. The operation was stable and the energy consumption was low.

Current rotating regenerative combustion units are mostly constructed with a separate regenerative chamber and combustion chamber. The combustion chamber needs to rely on an igniter to ignite the inlet air that has been preheated to a high temperature (about 800°C), which makes it difficult for the combustion process to maintain self heating and requires the utilisation of an external heat source. Also, the regenerative body is bulky for a simple heat transfer process in the heat storage body. To remedy the above shortcomings, Mao et al. [45] developed a coal mine spent air rotary heat storage catalytic oxidation device with a combined oxidation bed. The oxidation bed consisted of a catalytic bed body and a regenerative body. There was a heater in the top air collection chamber. After the spent air entered the system, the inlet preheating combustion and exhaust heat storage occurred simultaneously in the combined oxidation bed. This design greatly improves heat transfer efficiency and makes the structure more compact. Under the combined effect of the low-temperature catalytic oxidation of the catalytic oxidation bed and the high heat accumulation and strong thermal feedback of the honeycomb ceramic oxidation bed, the device can achieve self heating to maintain operation and save energy.