First, the research status and development trend at home and abroad

Organic waste gas has many kinds, wide sources, difficult treatment, high one-time investment and operation cost, and basically has no recycling value. Organic waste gas with complex components is more difficult to purify, separate and recover.

Volatile organic compounds (VOCs), as the main branch of organic compounds, refer to organic compounds with saturated vapor pressure greater than 70Pa at room temperature and boiling point less than 260℃ at atmospheric pressure. From the point of view of environmental monitoring, it refers to the general name of non-methane hydrocarbons detected by hydrogen flame ion detector, including hydrocarbons, oxygen hydrocarbons, halogen-containing hydrocarbons, nitrogen hydrocarbons and sulfur hydrocarbons. VOCs are various and widely distributed. According to the list of some major foreign environmental priority pollutants, VOCs account for more than 80%. According to the environmental survey of Japan 1974- 1985, among the chemical poisons detected, there are at most 52 kinds of halogenated hydrocarbons, followed by 43 kinds of ordinary hydrocarbons and 40 kinds of nitrogen-containing organic compounds (mainly nitrobenzene and aniline compounds), accounting for 70% of the total detected poisons. VOCs are seriously polluted, and react with nitrogen oxides and CnHm under the action of sunlight, absorbing infrared radiation on the surface and causing greenhouse effect. The ozone hole is formed by destroying the ozone layer, which leads to human carcinogenesis and animal and plant poisoning.

With the expanding scope of VOCs pollution and people's gradual understanding of its harm, the United Nations Economic Commission for Europe held a conference on transnational air pollution in Geneva in 199 1, and adopted the Protocol on Transnational Air Pollution of VOCs, requiring signatory countries to adopt 1938. 1990, the United States revised the Clean Air Act (CAA), requiring that VOCs emissions be reduced by 70% by the year 2000. Therefore, it has become the only way to solve VOCs pollution to develop VOCs substitute products and find the best VOCs control technology.

With the increasing attention of countries around the world to VOC pollution and the strict requirements of environmental regulations on VOC emission standards, its treatment technology is gradually improving and perfecting.

(A) organic waste gas treatment technology

As early as 1925, Europe developed a fixed-bed activated carbon adsorption device, and Japan began to use this technology in 1958. This is a very classic and mature method, which can treat any concentration of organic waste gas at room temperature, but when dealing with low concentration and large air volume of organic waste gas, the equipment is huge and uneconomical. For the treatment of high-concentration organic waste gas with high exhaust temperature, the United States successfully developed the direct combustion technology with natural gas as fuel in 1950. 1965 Japan cooperated with the United States to introduce this technology into Japan. This method needs to heat the organic waste gas to 760℃ before the organic solvent can be oxidized and decomposed into harmless CO2 and H2O. Its disadvantage is high fuel cost, so it is widely used in Europe and America where natural gas is cheap. Later, people developed catalytic combustion technology. Because the organic solvent can be oxidized and decomposed at the low temperature of 300-350℃, the fuel cost is greatly reduced and the amount of nitrogen oxides produced is very small. Its disadvantage is that it is necessary to pretreat the substances and dust in the exhaust gas that are easy to cause catalyst poisoning. In addition, the heat exchanger used in the catalytic combustion device has a low heat exchange efficiency of about 50%. In order to improve thermal efficiency and reduce operating cost, the United States developed a regenerative combustion device with heat exchange efficiency over 90% in 1975. As the operation cost is reduced, it can be used to treat medium-concentration organic waste gas. Subsequently, Europe also developed this technology. In view of the regenerative combustion mode in the United States, Japan has developed an improved catalytic combustion device-regenerative catalytic oxidation method, and the product was first sold by Nippon Steel Corporation in 1977. This product can economically treat organic waste gas with high, medium concentration and high temperature.

Generally speaking, according to the treatment methods, there are two main treatment methods for organic waste gas: one is recovery method, and the other is elimination method. The recovery methods mainly include activated carbon adsorption, pressure swing adsorption, condensation and membrane separation technology. The recovery method is to separate VOC by physical methods, such as temperature, pressure, selective adsorbent, selective permeable membrane, etc. The elimination methods include thermal oxidation, catalytic combustion, biological oxidation and integrated technology; The elimination method mainly uses heat, catalysts and microorganisms to convert organic matter into CO2 and water through chemical or biochemical reactions.

1, recovery technology

(1) carbon adsorption method

Activated carbon adsorption is the most widely used recovery technology at present, and its principle is to capture VOC in waste gas by using the porous structure of adsorbents (granular activated carbon and activated carbon fiber). The organic waste gas containing VOC passes through the activated carbon bed, where the VOC is adsorbed by the adsorbent, and the waste gas is purified and discharged into the atmosphere.

When the carbon adsorption reaches saturation, the saturated carbon bed desorbs and regenerates; Introducing steam to heat the carbon layer, VOC is blown off and released, and forms a steam mixture with steam, which leaves the carbon adsorption bed together. The steam mixture is cooled by a condenser to condense the steam into liquid. If VOC is water-soluble, the liquid mixture is purified by rectification; If it is insoluble in water, VOC can be directly recovered by precipitator. Because the "triphenyl" used in the coating is immiscible with water, it can be directly recovered.

Carbon adsorption technology is mainly used in the case of simple components in waste gas and high recovery value of organic matter. The size and cost of waste gas treatment equipment are directly proportional to the VOC content in the gas, but relatively independent of the waste gas flow. Therefore, carbon adsorption beds tend to dilute the atmospheric mass flow, and are generally used when the VOC concentration is less than 5000PPM. It is suitable for paint, printing, adhesives and other occasions with low temperature, low humidity and large exhaust volume, especially for the purification and recovery of halides.

(2) condensation method

Condensation method is the simplest recovery technology, which cools the waste gas below the dew point temperature of organic matter, so that the organic matter condenses into droplets, which are separated from the waste gas and directly recovered. However, in this case, the exhaust gas leaving the condenser still contains a relatively high concentration of VOC, which cannot meet the environmental emission standards. In order to obtain high recovery rate, the system needs high pressure and low temperature, and the equipment cost increases significantly.

Condensation method is mainly used to recover VOC with high boiling point and high concentration, and the suitable concentration range is > 5% (volume).

(3) Membrane separation technology

Membrane separation system is an efficient new separation technology, which has the advantages of simple process, high recovery rate, low energy consumption and no secondary pollution.

The basis of membrane separation technology is to use a polymer membrane with selective permeability to organic matter, and its permeability to organic vapor is 10- 100 times that of air, thus realizing the separation of organic matter.

The simplest membrane separation is a single-stage membrane separation system, which directly allows compressed gas to pass through the membrane surface to realize the separation of VOC. However, single-stage membrane is difficult to meet the separation requirements because of its low separation degree, while multi-stage membrane separation system will greatly increase equipment investment.

MTR has developed a new integrated membrane system, which can greatly improve the recovery rate and reduce the system cost by using only a single-stage membrane.

This technology combines the characteristics of compression condensation and membrane separation to realize separation. Firstly, the feed gas is boosted to a certain pressure by a compressor, and then sent to a cooler for condensation, so that part of VOC is condensed, and the condensate is directly put into a storage tank. The non-condensable gas leaving the condenser still contains a considerable amount of organic matter, and its pressure is high, which can be used as the driving force of membrane permeation, so that membrane separation does not need additional power. Non-condensable gas is sent to the membrane system, and the gas is divided into two streams by the organic selective permeable membrane, and the purified gas on the non-permeable side from which VOC is removed is discharged; The permeate flow is steam rich in organic matter, which circulates to the inlet of the compressor. The system can usually remove more than 99% VOC from raw gas, so that VOC in waste gas can meet the environmental protection emission standards.

The characteristic of the system is that the concentration of the final permeate stream has nothing to do with the concentration of the feed gas, which is determined by the pressure and temperature of the condenser.

(4) Pressure swing adsorption technology

The technology firstly adsorbs organic matter with adsorbent under a certain pressure. When the adsorbent is saturated, the adsorbent is regenerated. Regeneration does not use steam, but desorbs organic matter through pressure shift. When the pressure decreases, the organic matter desorbs from the surface of the adsorbent. It is characterized by no pollutants, high recovery efficiency and recovery of active organic matter. However, the operation cost of this technology is high, adsorption needs pressurization, desorption needs decompression, and it is rarely used in environmental protection.

Scope of application of recycling technology:

Granular activated carbon is mainly used to recover fats and aromatics, most chlorine-containing solvents, commonly used alcohols, some ketones and esters. Common ones are benzene, toluene, xylene, hexane, heptane, methyl ethyl ketone, acetone, carbon tetrachloride, ethyl acetate and so on. Reactive monomers such as styrene and acrylonitrile can be recovered by activated carbon fiber adsorption, but the cost is much higher than that by granular activated carbon adsorption. Adsorption method has been widely used to recover triphenyl and ethyl acetate in paint industry, triphenyl in shoemaking industry, toluene and ethyl acetate in printing industry, dichloromethane and trichloroethane in electronics industry. Carbon adsorption method requires that VOC in waste gas should not exceed 5000PPM and humidity should not exceed 50%. When the concentration is greater than 5000PPM, it needs to be diluted before adsorption, which is not suitable for some active substances such as ketones, aldehydes and esters. This VOC will react with activated carbon or its surface, blocking its pores and deactivating it.

Condensation method has good effect on high boiling point organic matter, but not on medium and high volatile organic matter. This method is suitable for VOC concentration > 5% and low recovery rate. However, most of the waste gas contains water, which will freeze when the temperature is lower than 0℃, which reduces the reliability of the system, so it is rarely used alone.

The membrane separation method is suitable for treating concentrated material flow, that is, 0. 1% < VOC concentration < 10%. The cost of the membrane system is directly proportional to the inlet velocity, but has little to do with the concentration. It is suitable for the recovery of high-concentration and high-value organic matter with high equipment cost.

In industry, olefin monomer and helium are recovered from the flushing gas of polyolefin plant. In the field of environmental protection, hydrocarbons are recovered from gas stations; CFC is recovered from the production and use of refrigeration equipment, aerosol and foam plastics, and vinyl chloride monomer is recovered from PVC processing. This technology is very promising. With the appearance of new high-efficiency membrane and the reduction of system cost, it will become an important means of recovery.

2. Eliminate technology

(1) thermal oxidation

Thermal oxidation system is a flame oxidizer, which can eliminate organic matter by combustion, and its working temperature is as high as 700℃- 1000℃. This inevitably has a high fuel cost, and in order to reduce the fuel cost, it is necessary to recover the heat in the exhaust gas leaving the oxidizer. There are two ways of heat recovery, traditional zonal heat exchange and new unsteady heat storage heat exchange technology.

Wall thermal oxidation is to capture the heat of purified waste gas with a tube or plate wall heat exchanger, which can recover 40%-70% of heat energy and use the recovered heat to preheat the organic waste gas entering the oxidation system. After preheating, the exhaust gas reaches the oxidation temperature through the flame and is purified. The disadvantage of heat exchange between walls is low heat recovery efficiency.

Regenerative thermal oxidation (RTO) uses a new unsteady heat transfer method to recover heat. The main principle is that organic waste gas and purified waste gas circulate alternately, and heat can be captured to the maximum extent by changing the flow direction for many times. The heat storage system provides extremely high heat recovery.

In a certain cycle, the organic waste gas containing VOC enters the RTO system, first enters the refractory regenerative bed 1 (which has been heated by the purified gas in the previous cycle), and the waste gas absorbs heat energy from the bed 1 and then enters the oxidation chamber; VOC is oxidized into CO2 and H2O in the oxidation chamber, and the waste gas is purified; The oxidized high-temperature purified gas leaves the combustion chamber and enters another cold regeneration bed 2, which absorbs heat from the purified waste gas and stores it (for preheating the organic waste gas entering the system in the next cycle) and lowers the temperature of the purified waste gas. When this process lasts for a certain period of time, the direction of air flow is reversed and organic waste gas enters the system from bed 2. This cycle constantly absorbs and releases heat. As a heat sink, the heat storage bed constantly changes in the operation mode of inlet and outlet, resulting in efficient heat recovery, with the heat recovery rate as high as 95% and VOC elimination rate as high as 99%.

(2) Catalytic combustion

Catalytic combustion is a method similar to thermal oxidation to treat VOC. It uses precious metal catalysts such as platinum and palladium and transition metal oxide catalysts instead of flame, and its working temperature is half lower than that of thermal oxidation, usually 250℃-500℃. Because of the low temperature, standard materials are allowed to be used instead of expensive special materials, which greatly reduces the equipment cost and operation cost. Similar to thermal oxidation, the system can still be divided into two types of heat recovery methods: zonal and regenerative.

In zonal catalytic combustion, the heat exchanger is installed behind the catalytic bed. The heat exchanger not only reduces the temperature of waste gas, but also preheats the organic waste gas containing VOC, and its heat recovery reaches 60%-75%. This oxidant has been used in industrial processes for a long time.

Regenerative catalytic combustion is a new catalytic technology. It not only has the characteristics of RTO efficient energy recovery, but also has the advantages of low temperature operation and catalytic reaction energy efficiency. The catalyst is placed on the top of the heat storage material to optimize purification, and its heat recovery rate is as high as 95%-98%.

The key to the performance of RCO system is to use special catalyst, precious metal or transition metal catalyst impregnated on saddle or honeycomb ceramics, allowing oxidation at half the temperature of RTO system, which not only reduces fuel consumption, but also reduces equipment cost.

At present, some countries have begun to use RCO technology to eliminate organic waste gas, and many RTO devices have begun to convert to RCO, which can reduce the operating cost by 33%-75% and increase the discharge gas flow by 20%-40%.

(3) Integrated technology (carbon adsorption+catalytic oxidation)

For organic waste gas with large flow and low concentration, the cost of adopting the above method alone is too high and uneconomical. Using the advantages of low concentration and atmospheric volume of activated carbon adsorption treatment, the organic matter in waste gas is first captured by activated carbon and then desorbed by hot air with much smaller flow rate, which can enrich VOC 10- 15 times, greatly reducing the waste gas volume and the scale of post-treatment equipment. The concentrated gas is sent to a catalytic combustion device, and VOC is eliminated by using the characteristic that catalytic combustion is suitable for treating high concentration. The heat released by catalytic combustion can preheat the desorbed gas entering the carbon adsorption bed through the baffle heat exchanger, thus reducing the energy demand of the system.

This technology is a very effective integration technology, which takes advantage of carbon adsorption to treat low concentration and atmospheric pressure retention, and uses catalytic bed to treat medium flow and high concentration. In China, this technology is also used to deal with industries with large flow and low concentration of organic waste gas, such as painting, printing, shoemaking and so on.

Scope of application of elimination technology:

(1) thermal oxidation

The working temperature of thermal oxidation system is 700℃- 1000℃, which is suitable for the conditions of flow rate of 2000-50000m3/h and VOC concentration of 100-2000ppm.

Compared with regenerative type, the advantage of baffle type is that it can capture heat with a simple metal heat exchanger, and it only takes a few minutes to reach the required operating conditions, so it is most suitable for cyclic operation.

Regenerative thermal oxidation has a very high oxidation temperature, which can treat organic matter that is difficult to decompose. The VOC elimination rate of 98%-99% in this system is very common. The heat recovery efficiency is 85%-95%. It can operate with little or no fuel, especially for gases with relatively low VOC content, which is lower than thermal oxidation of the partition wall.

The disadvantages of thermal oxidation are as follows: ① Nitrogen oxides produced by high-temperature combustion are also dangerous emissions and need further treatment; ② Slow thermal reaction; (3) The halide treatment is not ideal, so it is necessary to increase the washing tower of post-treatment device to treat acid gas; (4) Intake air concentration shall not be greater than 25% LEL；; ⑤ The equipment investment cost is high.

(2) Catalytic oxidation

Catalytic oxidation is carried out at a lower temperature than thermal oxidation, usually 250℃-500℃, with a treatment capacity of 2000-20000 m3/h, which is suitable for VOC concentration of 100-2000 ppm, and the elimination efficiency is as high as 95%. Low operating temperature combined with baffle heat exchanger can reduce the fuel required for startup.

Catalytic combustion has several advantages over thermal oxidation: ① the reaction temperature is half lower than that of thermal oxidation, saving fuel; (2) the residence time is short, which reduces the size of the equipment; (3) Due to the decrease of fuel, less CO is produced, and CO and VOC are transformed together; ④ The start-up and cooling time is shorter than that of thermal oxidation system; ⑤ Lower working temperature eliminates the generation of nitrogen oxides; ⑥ Due to the temperature drop, it is allowed to replace expensive special materials with standard materials, and the overall mechanical life of RCO system will be increased.

Catalytic oxidation also has some disadvantages: ① the catalyst is easily covered by heavy metals or particles and becomes inactive; (2) When treating halides and sulfides, acid gas will be generated, which needs further treatment in the washing tower; (3) Waste catalyst that cannot be recovered should be treated; (4) The intake air concentration should not be greater than 25%.

(3) Integrated technology (carbon adsorption+catalytic combustion)

VOC recovery by activated carbon adsorption has been widely used in paint, printing, electronics and other industries, and the elimination rate can reach 90%-95%. However, for low-concentration waste gas, the recovery is not economical, so the elimination technology is adopted.

The advantage of integrated technology is to treat low-concentration and atmospheric waste gas at low cost. By concentrating the waste gas, the volume of the waste gas to be treated is reduced, and the small-volume catalytic combustion oxidizer is used to treat the large-flow waste gas, which reduces the equipment cost and operation cost.

This method also has some disadvantages. This technology is not suitable for the situation that the waste gas contains VOC with high activity and easy reaction and the relative humidity is more than 50%. Exhaust gas containing halogen compounds still needs post-treatment equipment.

It can be seen that the above methods have their own advantages and disadvantages and applicable objects. The advantages and disadvantages of several common methods are summarized and compared as follows.

Main advantages and disadvantages of governance methods

hot

force

burn

burn

Method is converted to 1. High purification efficiency

2. Various organic waste gases can be purified without pretreatment, with few unstable factors and high reliability.

3. The heat energy can be recovered to 1 when the waste gas concentration is high and the design is reasonable. High treatment temperature and high energy consumption.

2. There is secondary pollution.

3. Combustion device, combustion chamber and heat recovery device are expensive and difficult to maintain.

4. Treating large flow and low concentration waste gas consumes too much energy and has high operating cost.

RTO 1。 It has all the advantages of TO, but it needs to pretreat the complex organic waste gas.

2. The energy consumption is much lower than TO, and it can handle large flow and low concentration of 1. The treatment temperature is lower than TO, but it is still high, so there is still a little secondary pollution.

2. The cost is very high

It occupies a large area.

be on an impulse

change

burn

burn

Co 1。 High purification efficiency and no secondary pollution.

2. The energy consumption is low, which is about 50% lower than TO under the same conditions, so the operating cost is 1. When preheating with electric energy, low-concentration waste gas cannot be treated.

2. The catalyst has high cost and limited service life.

3. Complex waste gas needs pretreatment.

RCO 1。 High purification efficiency and no secondary pollution.

2. The energy consumption is the lowest among all combustion modes, and it can run without consumption when the waste gas concentration is 1- 1.5g/m3.

3. It can treat all kinds of organic waste gas 1. The integral type occupies a small area, but it is difficult to maintain.

2. Split type covers a large area.

3. The monolithic type is not suitable for high concentration (4g/m3), otherwise the catalytic bed will overheat.

4. Complex waste gas needs pretreatment.

Adsorption method 1. It can purify waste gas with large flow and low concentration.

2. The solvent can be recovered from a single kind of waste gas.

3. Low operating cost, 1. The adsorbent needs to be replenished and regenerated.

2. The waste gas with higher temperature needs to be cooled first.

3. Complex waste gas needs pretreatment.

4. Management inconvenience

5. There is secondary pollution.

6. Poor security

Absorption method 1. When hydrophilic solvent vapor is used as adsorbent, the equipment cost is low, the operation cost is low and it is safe.

2. Benzene waste gas can be absorbed by grease, and the purification rate is high.

3. It is suitable for large-flow low-concentration waste gas 1. When water is used as adsorbent, the generated wastewater needs to be treated.

2. The control and management of absorption and desorption are complicated.

(2) Low concentration and large air volume organic waste gas treatment technology

In industries that use organic solvents, such as automobile painting and printing, organic waste gas has the characteristics of low concentration of organic solvents and large air volume. If the above method is adopted, huge equipment will be used and a lot of money will be consumed. At present, the following methods are mainly used to treat this low concentration and large air volume organic waste gas in the world.

(1) Honeycomb wheel concentrating system

This system was successfully developed in 1977- 1979 in Japan, and also developed and sold in 1985- 1986 in Sweden. After the stricter total emission control of organic solvents was implemented around 1990, Europe and America also introduced this technology from Japan, and the market expanded rapidly. In this system, a honeycomb wheel is used to continuously adsorb and separate organic solvents from low-concentration and high-volume tail gas. Then desorbing by hot air with small air volume to obtain gas containing high concentration and small air volume organic solvent. Then the concentrated gas is combined with a small catalytic combustion or activated carbon recovery device to form an economical treatment system. The key component of the system is a cylindrical adsorption wheel, which is made of activated carbon or hydrophobic zeolite and corrugated, and then rolled into a honeycomb structure. The whole honeycomb wheel is divided into adsorption zone and regeneration zone, which continuously rotates at a very low speed during operation. When the waste gas containing organic solvent passes through the adsorption zone, the organic solvent is adsorbed and the purified gas is discharged. The organic solvent adsorbed by the runner is sent to the regeneration zone along with the rotation of the runner, heated and desorbed by hot air at 120- 140℃, and discharged with the hot air. Because the desorption air volume is much smaller than the adsorption air volume, the concentration of organic solvent in the desorbed gas can be increased by 10-20 times. After desorption, the waste gas can be treated by a device that absorbs more than one tenth of the air volume. The system is small in size and low in cost, and has become the first choice for treating low-concentration and large-volume organic waste gas abroad, and has been widely used. However, its introduction is expensive, which is unbearable in the promotion economy of China. Some domestic research institutions have taken advantage of their purification process to transform the main equipment to suit the national conditions of China. For example, several fixed adsorption and concentration devices filled with honeycomb activated carbon are used to replace the honeycomb wheel concentration device, and the function of rotating the honeycomb wheel is completed by switching the adsorption and desorption processes between several fixed beds. Because this method has no rotating parts, there is no dynamic sealing problem, so the equipment is simple to manufacture, convenient to maintain, low in price and has the advantages of centralized original technology. The Stamp Printing Bureau of the Ministry of Posts and Telecommunications has completed the industrial test of microcomputer automatic control with the treatment air volume of 2 1000-30000 m3/h in the waste gas treatment of six-color printing machines imported from France. After two years of operation, satisfactory results have been achieved. It provides a suitable method for treating low concentration and large air volume organic waste gas in China.

(2) liquid absorption method

In this method, the organic waste gas is contacted with a liquid absorbent, so that the organic solvent is absorbed by the absorbent, and then desorbed, removed or recovered, so that the absorbent can be regenerated and reused. Because tower absorption equipment which is several times larger than the gas handling capacity of adsorption and catalytic combustion device can be used in this process, the volume of the equipment can be made much smaller and the cost of the equipment is low. However, it is difficult to find an ideal absorbent, because organic solvents are generally nonpolar substances, and they will repel each other with polar water molecules, so it is difficult to dissolve. However, oil or aromatic extractants with high solubility in organic solvents are generally expensive, and some of them have peculiar smell. Some people in China have studied the method of adding active ingredients such as surfactants to water to improve the solubility of organic solvents. The research shows that it is feasible to treat benzene-containing spray paint tail gas with this absorbent, but this laboratory research result has not been popularized, which may be related to the unresolved problem of absorbent regeneration with limited absorption capacity. In the past few years, domestic organic waste gas absorption devices with diesel oil and aromatic hydrocarbon extractant as absorption liquid have been applied in some industrial applications, but all of them have been dismantled because of the great loss of absorbent itself or the inability to treat saturated absorbent. Liquid absorption method is rarely used abroad, and there are not many reports. It is reported that Japanese printing houses use liquid absorption method. The absorbent used is liquid containing catalyst, which has low operating cost, but the efficiency needs to be further improved. There are still many problems to be solved in liquid absorption, which limit its application.

(3) Biological treatment method

Biological deodorization was successfully developed in Germany and the United States in the 1940s and 1950s. Japan started the research on soil deodorization and activated sludge deodorization around 1970, and various devices have been developed and put into practical use. This method decomposes organic solvents by microorganisms. Because of its low energy consumption and low operating cost, it has attracted people's attention, especially in Europe, where technology development is centered on Germany and application examples are gradually increasing. Its disadvantage is its selectivity to various organic solvents, which limits its application field. At present, it has been used in sewage treatment plants and feed processing plants to deodorize polar substances such as hydrogen sulfide, low molecular aldehydes, ethanol and organic acids. The treatment of methanol and ethyl acetate produced in the process of coating and drying color film emulsion has also achieved good results. Biological treatment technology for treating non-hydrophilic aromatic compounds such as toluene and xylene has also been successfully developed. Compared with other methods, this method occupies a large area, which is another disadvantage.

(4) Other methods

In addition to the above three industrialization methods, two others are still in the laboratory research stage.

A) solid membrane separation and purification method

This method uses membrane separation to purify organic waste gas, and the membrane separation process of gas is realized by using the difference of permeability of separated components to the membrane. Domestic scientists have studied the separation and treatment of benzene-containing waste gas by tubular silicone rubber membrane, measured the separation factor of xylene to air, and deduced the relationship between the separation factor and Reynolds number of gas flowing through tubular membrane separator. At present, the research on low-concentration organic waste gas by membrane separation and enrichment, and then recovery or catalytic combustion treatment is in the laboratory research stage. The results show that when removing p-toluene and xylene, the purification rate can reach 90% and the concentration multiple can reach 10-20 times, which can greatly reduce the cost of treating low concentration and large volume benzene series waste gas. Therefore, membrane separation technology is an economical and effective new way to treat benzene series waste gas with low concentration and large air volume.

B) photocatalytic oxidation technology

Foreign scientists have studied the photocatalytic oxidation of benzene and the treatment of waste gas containing benzene, toluene, xylene and ethylbenzene with ozone as auxiliary oxidant and various photocatalytic oxidation reactions as compensation technologies. The research shows that photocatalytic oxidation has economic potential compared with compensation technologies such as activated carbon adsorption and catalytic combustion.

No matter which method is used to treat organic waste gas with low concentration and large air volume, the capital consumption is high. In contrast, the current method of combining activated carbon adsorption concentration with catalytic combustion or the method of combining activated carbon adsorption concentration with activated carbon recovery of organic solvents is more economical, effective and widely used. The solid membrane separation method is still in the laboratory research stage. Because of its low energy consumption and low operating cost, biological treatment has been paid attention to by all countries, and its industrial application examples and application fields are expanding, which is a technology with great application prospects.

In view of this, Hangzhou Xizi Environmental Protection Equipment Factory has developed 1988 regenerative (reversing) catalytic burner to treat low concentration and large air volume organic waste gas. The burner adopts integral structure, and after two years' efforts, it was successful in 1990. 199 1 was rated as a provincial-level new product by Zhejiang science and technology commission and obtained a national patent. 1992 was rated as a national key new product, and 1996 was awarded the best practical technology for environmental protection (Class A) by the State Environmental Protection Bureau. This kind of burner uses ceramics as heat storage material. When the relative surface area reaches 150-200m2/m3, the heat exchange efficiency is 90-95%, which far exceeds that of the partition wall (tube or plate), so the energy consumption is obviously reduced. When the concentration of waste gas reaches 1- 1.5g/m3, it can run without consumption, so the operating cost is extremely low. Basically, this is a VOC pollution control device with advanced technology, novel structure, high purification rate and low energy consumption. However, monolithic RCO also has some major disadvantages. Among them, the residual gas cannot be treated when reversing, which is the problem of reversing equipment. Besides, it's hard to maintain. When the continuous concentration of waste gas is higher than 4 g/m3, the temperature of the catalytic bed will rise to 600-700℃, and if it works at high temperature for a long time, the service life of the catalyst will be affected. In addition, the heavy weight of the equipment is also its disadvantage. In order to solve the problems existing in the overall structure, the factory also successfully developed a catalytic purifier with split structure (this product passed the appraisal of Zhejiang Science and Technology Department in 2002), which better solved the adaptability problem of the burner when the exhaust gas concentration fluctuated. For example, when the exhaust gas concentration is high (more than 3 g/m3), the hot gas can be led out and discharged or reused in the upper space, which is difficult for the integrated RCO. In addition, the maintenance and reuse of the split structure, however, the equipment occupies a large area, and the main machine almost doubles the area, which is costly and complicated to control.