Abstract: This paper focuses on the demonstration project as an example to discuss the feasibility of applying strong oxygen catalytic oxidation technology to plastic granulation waste gas treatment. The system adopts the combination process of “water spray absorption pretreatment + strong oxygen catalytic oxidationâ€, which utilizes the strong oxidizing property exhibited by ozone under the action of catalyst, and realizes deep purification treatment of plastic waste gas. The successful application of strong oxygen catalytic oxidation technology to the treatment of plastic waste gas by 21 companies fully demonstrates that the strong oxygen catalytic oxidation technology can achieve deep and thorough treatment of plastic waste gas.
introduction
Plastics are widely used in people's daily life because of their light weight, high strength, easy processing and low price. The output and consumption of plastic products are also increasing year by year. From January to September 2014, China's plastic products industry completed a total output of 53.395 million tons, an increase of 7.8%. However, while the plastics industry is developing rapidly, the environmental pollution problems in the production and processing of plastics are also becoming more serious.
After the plastic is heated and melted under high temperature conditions, on the one hand, the plastic polymer is heated and volatilized by the free hydrocarbon monomer, and on the other hand, various plastic additives are thermally decomposed and volatilized to produce organic pollutants. The plastic exhaust gas is a high temperature malodorous exhaust gas containing soot. At present, plastic waste gas treatment technologies include filtration, absorption, adsorption, low temperature plasma, combustion, and catalytic methods. The catalytic method includes photocatalytic oxidation, ozone catalytic oxidation and other technologies, and has the advantages of less secondary pollution, safety and reliability, and stable treatment efficiency, and is an emerging technology developed in recent years. At present, the application of strong oxygen catalytic oxidation technology in the treatment of plastic waste gas has not been reported in the literature.
The plastics industry is a pillar industry of the private economy in Taizhou, a coastal city in Zhejiang Province. Plastic processing and manufacturing has brought huge economic benefits to the local area. There are hundreds of waste plastic granulation processing enterprises in Jiaojiang District of Taizhou City. For historical reasons, these plastics companies are in residential-industrial mixed areas. The large amount of pungent and odorous exhaust gas produced by plastic granulation seriously affects the surrounding ambient air quality and interferes with the lives and work of surrounding residents. Residents around the factory have complained constantly. We creatively applied the strong oxygen catalytic oxidation technology to the waste gas treatment project of plastic granulation enterprises in Jiaojiang District, Taizhou City. This paper takes one of the plastic granulation enterprises as the representative, and comprehensively discusses the feasibility of the strong oxygen catalytic oxidation technology to control the odor of waste gas from plastic granulation.
1 demonstration project application
1.1 Process ideas and routes
1.1.1 analysis of exhaust gas source and exhaust gas characteristics
The demonstration engineering company is a company engaged in the production of polypropylene (PP) plastic particles. Plastic waste gas pollutants are mainly derived from the compound of plastic base material and auxiliary materials which are volatilized or decomposed by heat melting. The plastic waste gas is obviously visible smoke, high temperature, irritating odor, and obvious irritation to eyes and respiratory tract. The main components of the plastic waste gas are hydrocarbons such as propylene and benzene, as well as oils and fats and soot. The exhaust gas at the gas outlet of the granulation line is collected, and the average odor concentration of the plastic waste gas is determined to be about 5000 (dimensionless).
1.1.2 Process Route
Plastic waste gas is obviously visible smoke, high temperature, and irritating odor. In order to completely solve the problem of odor and nuisance of plastic waste gas, plastic odorous waste gas needs to be effectively collected and thoroughly purified for deep treatment. In view of the characteristics of plastic granulated waste gas, the demonstration project chose to adopt the combination process of “wet spray absorption pretreatment + strong oxygen catalytic oxidation and deodorizationâ€. Wet spray absorption pretreatment reduces the temperature and dust pretreatment of the plastic waste gas to prevent high temperature from reducing the catalytic efficiency of strong oxygen oxidation, and the soot blocks the catalytic packing layer.
In the demonstration project, a gas collecting hood is arranged at each odor generating point of the plastic granulator to collect plastic waste gas, and the fan negative pressure induces air. The collected plastic waste gas is passed to a strong oxygen catalytic oxidation treatment system. The exhaust gas first enters the first-stage wet spray absorption pretreatment tank, and after cooling and dust removal, it enters the multi-stage strong oxygen catalytic oxidation reaction tank, and finally the purified exhaust gas is discharged through the 5m exhaust cylinder at low altitude (required by the local environmental protection department). The specific process flow is shown in Figure 1.
1.1.3 Process principle
In the process of strong oxygen catalytic oxidation of organic pollutants, ozone can be decomposed to produce OH ̇ radicals with higher oxidation-reduction potential in the presence of a catalyst. The ozone molecule and the oxygen-containing group on the surface of the catalyst form a five-membered ring through hydrogen bonding, electrostatic force, etc., and then decompose by electron transfer to form an OH ̇ radical. OH ̇ is a highly oxidizing free radical with a high redox potential and can rapidly oxidize pollutants in wastewater with little selectivity. The O3 molecule is easily converted into O2 under high temperature conditions, and the flue gas contains dust, which easily blocks the filler in the catalytic reaction tank. Therefore, the plastic waste gas needs to be subjected to cooling and dust removal pretreatment. This project uses a first-level water spray for pretreatment. The pretreated exhaust gas enters a strong oxygen catalytic oxidation reaction system. The key treatment medium for strong oxygen catalytic oxidation is high concentration modified oxygenated water. The reaction process is divided into two phases: an absorption contact phase and a catalytic oxidation phase.
1) Absorption contact stage: The pretreated exhaust gas enters from the bottom of the catalytic oxidation tank, and is in intense collision contact with the high-concentration modified strong oxygen water sprayed downward from the upper high-speed high pressure. The oxygenated water in which a large amount of O2/O3 molecules are dissolved forms bubbles during the high-pressure high-speed spraying. The contaminant molecules in the exhaust gas are "adhered" to the bubbles of the strong oxygen water or directly dissolved in the strong oxygen water to be purified. Contaminants that are not absorbed by the strong oxygen water continue to advance with the gas flow.
2) Catalytic oxidation stage: O3 molecules in the strong oxygen water decompose on the surface of the stationary phase catalytic packing to produce a large amount of OH ̇. The resulting OH å‘生 undergoes a series of chain oxidative decomposition reactions with the contaminant molecules. The oxidation reaction is carried out simultaneously in the liquid phase and the gas phase: a part of the pollutant undergoes a liquid phase catalytic oxidation reaction in the strong oxygen water; another part of the pollutant that is not absorbed by the strong oxygen water contacts the OH 飘 which flows into the gas phase, and catalyzes Oxidation reaction. The pollutants in the final exhaust gas are fully degraded, and the exhaust gas is effluxed. The oxygenated water has regained its "absorption contact" ability and continues to act as a circulating fluid to absorb contaminants in the exhaust gas. The absorption contact phase and the catalytic oxidation phase are carried out almost simultaneously, mutually promoting and complementing each other.
1.2 Main equipment and facilities and economic and technical indicators
1.2.1 System air volume
The demonstration engineering company has a total of 9 plastic granulation lines. Each of the three granulation lines is a treatment unit, and the design air volume of each unit plastic waste gas treatment system is 6000m3/h. Therefore, the whole plant has set up 3 sets of strong oxygen catalytic oxidation deodorization system. The air volume of a single system is 6000m3/h, and the total design air volume is 18000m3/h.
1.2.2 Main equipment and facilities
The plastic exhaust gas strong oxygen catalytic oxidation deodorization system mainly consists of pretreatment system, strong oxygen source generation system, strong oxygen water preparation system, catalytic oxidation reaction system, power system and control system. The specific equipment and facilities are shown in Table 1.
1.2.3 Economic and technical indicators
The technical and technical indicators of this project are detailed in Table 2.
1.3 Governance effectiveness
The three sets of strong oxygen catalytic oxidation treatment system (single set of treatment air volume 6000m3/h) of the demonstration project was successfully installed and commissioned in mid-April 2014, and the test operation was half a year. During operation, the system is stable and there are no major faults or overhauls. In November 2014, the demonstration project was monitored for acceptance. The specific test results are shown in Tables 3 and 4. It can be seen from Table 3 that during the monitoring period, the odor concentration of the plant in the southeast and northwest of the enterprise is below the unorganized monitoring concentration of the plant boundary 20 (dimensionless), and the concentration of the boundary is up to standard. This shows that the company's plastic waste gas collection and treatment effect is good. It can be seen from Table 4 that the average odor concentration of the intake of the three sets of strong oxygen catalytic oxidation deodorization devices of the enterprise is 1091 (dimensionless), and the average odor concentration of the export exhaust gas is 217 (dimensionless). This indicates that the system purifies the odorous exhaust gas more thoroughly and can achieve an outlet odor concentration below 300 (dimensionless). The average concentration of non-methane total hydrocarbons in the system's imported exhaust gas is 8.76 mg/m3, the average concentration of non-methane total hydrocarbons in the export exhaust gas is 2.18 mg/m3, and the average removal rate of non-methane total hydrocarbons in the system is 74.9%.
After treatment with a strong oxygen catalytic oxidation system, the odor concentration of plastic waste gas was reduced by an average of 79.8%. This indicates that the strong oxygen catalytic oxidation deodorization system has a remarkable effect on the removal of low-concentration plastic malodorous exhaust gas, and can achieve deep treatment of low-concentration malodorous exhaust gas.
2 promotion and application
After the successful application of the demonstration project, the strong oxygen catalytic oxidation treatment technology has been promoted and applied in the treatment of odorous waste gas in 21 plastic granulation enterprises in Taizhou. These companies are involved in the processing and production of various plastic particles such as PP, PE, PVC, ABS, and PC. A total of 28 sets of strong oxygen catalytic oxidation deodorization devices were applied to 21 plastic granulation enterprises. The air volume of one set of three facilities was 8000m3/h, and the air volume of one set of four sets of facilities was 4000m3/h. The air volume is 6000m3/h. According to the experience monitoring, the odor concentration of the plastic granulated exhaust gas can be reduced by up to 84.8%. After the high-oxygen catalytic oxidation treatment system was launched, the odor concentrations of the 21 plastic granulation enterprises were below 20 (dimensionless), and the exhaust gas odor concentrations were below 500 or 300 (dimensionless). This fully demonstrates that the strong oxygen catalytic oxidation deodorization system can achieve deep treatment of various plastic granulation waste gases, effectively solving the problem of the odorous exhaust gas interference to the living and production of surrounding residents.
3 process characteristics and advantages analysis
The treatment of plastic waste gas by the strong oxygen catalytic oxidation technology has the following technological features:
1) The system set pretreatment system, the strong oxygen source generation system, the strong oxygen water preparation system, the catalytic oxidation reaction system, the power system and the control system are integrated, the layout is compact, and the floor space is small. At present, the system has been completed.
2) The equipment and facilities of the whole system are installed in a movable chassis for easy installation and management.
3) The system adopts automatic control, and the whole system operation is one-button operation, which is convenient for operation and management, safe and reliable.
4) The plastic exhaust gas contains soot. After circulating the circulating liquid of the system for a period of time, it needs to be periodically discharged due to the accumulation of impurities. According to the actual operation, the hourly waste liquid discharge of a set of 6000m3/h strong oxygen catalytic oxidation complete system is 120L/h. The main pollutants of waste liquid are suspended solids and COD. The COD concentration of the waste liquid was determined to be 210 mg/L. Therefore, the waste liquid discharged from the system can be directly connected to the municipal sewage pipe network after a simple precipitation treatment. It can be seen that the wastewater pollution generated by the system is easy to control. If the wastewater is strictly controlled, it will not have a significant impact on the water environment.
5) The fan is installed in the FRP soundproof box. The system operates at low noise and hardly affects the surrounding environment.
6) The plastic waste gas belongs to low-concentration malodorous exhaust gas (the odor concentration is below 2000 (dimensionless)), and the odor concentration of the exhaust gas of the purification system is below 500 (dimensionless), and the effective reduction rate is more than 75%. This indicates that the strong oxygen catalytic oxidation system can achieve advanced treatment of plastic malodorous exhaust gas.
7) During the six-month trial run, the system is stable and there are no major faults or overhauls. This shows that the system is stable and reliable.
4 Conclusion
For the first time, the strong oxygen catalytic oxidation deodorization technology is applied to the treatment of plastic waste gas. The system adopts the combination process of “wet spray pretreatment + strong oxygen catalytic oxidationâ€. The system realizes complete set and automation, with small floor space, less secondary pollution, simple operation, stable and safe operation, high deodorization efficiency for plastic waste gas. . Successful application examples of plastic waste gas treatment in dozens of plastic enterprises fully demonstrate that the strong oxygen catalytic oxidation waste gas treatment technology can stably and effectively control various plastic waste gases.
The key role of the strong oxygen catalytic oxidation waste gas treatment technology is OH ̇ free radicals. OH ̇ free radicals can decompose pollutants without selective oxidation. In the future, the strong oxygen catalytic oxidation waste gas treatment technology can be applied in the treatment of other types of organic waste gas such as medical waste gas, rubber waste gas, and landfill leachate waste gas.
Original title: Application of strong oxygen catalytic oxidation technology in plastic waste gas treatment
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