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Purification and treatment of organic waste gas

TIME：

2019-03-22 15:34

1. Adsorption method
 

The adsorption method is to pass the organic waste gas through the packed bed filled with adsorbent to adsorb organic substances to reduce air pollution. The key lies in the performance of the adsorbent used, and high-performance adsorbents should have characteristics such as large adsorption capacity, uniform adsorption pore size, and easy regeneration. Common industrial adsorbents mainly include activated carbon, activated carbon fibers, coke powder particles, molecular sieve zeolites, etc. Activated carbon is suitable for the adsorption and purification of VOCs in paint exhaust gas due to its dense microporous structure, large internal surface area, good adsorption performance, and stable chemical properties; However, when dealing with waste gas with a humidity greater than 60%, the adsorption effect will be significantly reduced; If there is no regeneration device, replacing activated carbon increases operating costs; If hot air regeneration is used, it is easy to cause ignition. Molecular sieves have characteristics such as high temperature resistance, non flammability, and strong hydrophobicity compared to activated carbon. They can be regenerated through hot air and also exhibit good adsorption performance for exhaust gases with a humidity not exceeding 90%.

2 absorption method
 
The absorption method is a purification method that utilizes the high solubility of VOCs in certain solvents in paint exhaust gas, and uses oil soluble solvents with high boiling points and low vapor pressure to absorb VOCs. The absorption method is divided into physical absorption and chemical absorption. In practical industrial treatment processes, physical absorption is generally used, and whether absorbents are cheap, easy to obtain, and harmless is a key issue to consider. He Luhong et al. used non ionic surfactant Tween-20 as the main surfactant, added auxiliary surfactant sodium dodecylbenzenesulfonate (SDBS) and auxiliary sodium chloride to form a composite aqueous solution absorbent for treating toluene based VOCs waste gas. The removal rate of toluene can reach 77%. Xiao Xiao et al. [26] compared the absorption effect of absorbents such as diethyl hydroxylamine, polyethylene glycol 400, silicone oil, edible oil, waste engine oil, and 0 # diesel on toluene exhaust gas through experiments, and found that under the same experimental conditions, diethyl hydroxylamine had a large absorption amount of toluene. Li Jialiang et al. [Experimental research found that 4% 1,4-butanediol (BDO) has a good absorption effect on toluene waste gas, with a high absorption concentration of up to 43.87mg/L. In addition to physical absorption, alkali or acid solutions such as sodium hydroxide or sodium hypochlorite are also used in industrial production as absorbents for chemical absorption of waste gas.].
 
3 Combustion method
 

The combustion method is to burn and oxidize organic compounds in paint exhaust gas, converting them into harmless substances such as CO2 and H2O to achieve exhaust gas purification. The combustion method can be divided into direct combustion method, thermal combustion method, catalytic combustion method, thermal storage combustion method, and other types.

(1) The direct combustion method should be used for high concentration combustible organic waste gas. The direct combustion method requires a sufficiently high temperature and sufficient oxygen in the combustion space. If the oxygen content is insufficient, the combustion is incomplete; If there is too much oxygen, the concentration of combustible materials will not be within the ignition limit, leading to incomplete combustion. To prevent gas explosion, waste gas is generally burned in boilers or open burners, with a combustion temperature of 1100 ℃ higher; But when combustion is incomplete, it can cause some pollutants and smoke to be discharged into the atmosphere, and the heat generated by combustion cannot be recovered, resulting in fuel energy loss.

(2) The thermal combustion method can be used to treat low concentration combustible organic waste gas. The low concentration of flammable substances results in insufficient energy release to support the entire combustion process. Therefore, auxiliary fuel needs to be added as a combustion gas to increase heat by burning the combustion gas, so that the exhaust gas reaches the reaction temperature and is fully burned. The temperature of thermal combustion method is generally within the range of 500~900 ℃, which is lower than the temperature of direct combustion method.

 
4 condensation method
 
The condensation recovery method is to introduce organic waste gas into the condenser and use VOCs with different vapor partial pressures at different temperatures to gradually condense VOCs into a liquid state for recovery [33]. The condensation method is suitable for treating high concentration and low flow organic waste gases, mainly used in industries such as pharmaceuticals and chemicals; If condensation method is used in industries such as spray painting and printing and dyeing, lower concentrations of spray painting waste gas are usually compressed before treatment. The process involves transferring organic waste gas to the pre cooling stage unit for pre-treatment, with a general temperature controlled at 5 ℃ to remove any water vapor and avoid frost formation in the cooling stage evaporator, which may affect heat exchange [33]; The pre cooled organic waste gas is cooled by a cooling stage evaporator, with a temperature controlled at around -30 ℃, to condense VOCs into a liquid state. The condensation recovery operation is simple, the effect is stable, and its sealing performance is less affected by external temperature and pressure changes. Its working temperature is lower than the liquid flash ignition after condensation, which is safer. However, the low condensation temperature leads to high energy consumption, and the performance requirements, equipment investment, and operating costs of the condensation equipment are also high. The selection of refrigerants has different effects on refrigeration efficiency and recovery efficiency.

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