In recent years, with the economic development, a large number of new chemical companies, coupled with insufficient investment in environmental protection, have led to the discharge of a large number of industrial organic waste gas, which has caused the decline in the quality of the atmospheric environment, severely harming human health, and causing serious harm to the national economy. The huge losses, therefore, need to increase the treatment of organic waste gas. The treatment of organic waste gas has been studied for a long time, and some effective control technologies have been developed, such as the widely used and more studied heat destruction method, condensation method, absorption method, etc.The new control technology formed in recent years has Biofilm method, corona method, plasma decomposition method, etc. This article will give a more detailed introduction to the above methods.
1. Thermal Destruction
Thermal destruction is a widely used organic waste gas treatment method that has been widely studied , especially for low-concentration organic waste gas . Thermal destruction of organic compounds can be divided into direct flame combustion and catalytic combustion. Direct flame combustion is a method in which organic matter is directly burned in the air stream and auxiliary fuel is burned. In most cases , the concentration of organic matter is low , not enough to burn without auxiliary fuel. Flame directly at an appropriate temperature and retention time conditions , it can reach 99% of the heat efficiency.
Catalytic combustion is the heating of organic matter in the gas stream . Under the action of the catalytic bed , the chemical reaction of organic matter is accelerated ( or the method of destroying efficiency ) .The presence of the catalyst makes the organic matter require less retention time and more than the direct combustion method when it is thermally destroyed. Low temperature. The catalyst plays an important role in the catalytic combustion system. The catalysts used for purification of organic waste gas are mainly metals and metal salts , and metals include precious metals and non-precious metals. Currently metal catalyst used mainly of Pt , Pd, mature technology , and high catalytic activity , but are expensive and the process halo organics , containing N , S , P when the other elements , organic matter oxidative like prone to catalyst deactivation. Non-metallic catalysts include transition group elements cobalt and rare earth. In recent years, more and more catalysts have been developed both domestically and abroad , and they have focused on non-noble metal catalysts and obtained many results. For example , a catalyst made of V2O5 +MOX (M: transition group metal ) + noble metal is used to treat methanethiol waste gas , and Pt + Pd +Cu catalyst is used to treat nitrogen-containing organic alcohol waste gas.
Because impurities often appear in organic waste gas , it is easy to cause catalyst poisoning and poisons that cause catalyst poisoning ( inhibitors are mainly phosphorus, lead, bismuth arsenic, tin, mercury, ferrous ion zinc, halogen, etc.. The catalyst carrier saves catalyst and increases large effective area of the catalyst , the catalyst having certain mechanical strength , to reduce the sintering , improving effect of the catalytic activity and stability. can be used as carrier materials are AL2O3 , ferrovanadium, asbestos, clay, activated carbon, metal or the like , the most commonly used is a ceramic reticulated vectors typically, spherical, cylindrical, honeycomb-shaped. Further research in recent years and many successful mordenite and the like. for catalytic combustion , the focus of future research will continue to be the focus efficient exploration of high activity catalyst And its carrier , catalytic oxidation mechanism.
2. Liquid absorption method
The liquid absorption method utilizes the similar compatibility principle of the liquid absorption liquid and the organic waste gas to achieve the purpose of treating the organic waste gas. Generally, a liquid mixture of liquid petroleum substances, surfactants and water is used as the absorption liquid to enhance the absorption effect . In recent years , Japanese studied cyclodextrin absorbing material using an organic halide , in accordance with the principles of the cyclodextrin strong affinity for the organic halide , the aqueous cyclodextrin as an absorbent for an organic halide gas Be absorbed. The absorbent has the advantages of non-toxicity, no pollution , high desorption rate after collection , energy saving by recycling , and repeated use.
3. Adsorption method
The adsorption method is widely used , has the advantages of low energy consumption, mature process, high removal rate, thorough purification, easy to promote, and has good environmental and economic benefits. The disadvantage is that the equipment is huge and the process is complicated . When there are colloidal particles or other impurities in the exhaust gas , the adsorbent is easily poisoned. The adsorption method is mainly used for the treatment of low concentration and high flux volatile organic compounds (VOCs) . The key to determining the adsorption method for processing VOCs is the adsorbent . The adsorbent should have a dense pore structure, large internal surface area, good adsorption performance, stable chemical properties, not easy to break, and low resistance to air . Commonly used are activated carbon and alumina , Silica gel, artificial zeolite, etc.
At present , most of them use activated carbon , which has high removal efficiency. There are two types of activated carbon, granular and fibrous. The granular activated carbon structure has uniform pores , in addition to small pores , there are 10 to 100nm mesopores and 1.5 to 5um macropores , the treatment gas diffuses from the outside to the inside , and the adsorption and desorption are slow ; while the pore size distribution of the fiber activated carbon is uniform The pore size is small and the vast majority are micropores of 1.5 ~ 3nm . Because the pores are outwards , the gas diffusion distance is short , so the adsorption and desorption are fast. Activated carbon treated with iron oxide or sodium hydroxide or ozone tends to have better adsorption performance.
4. Condensation method
Condensation method is the use of a substance having different saturation vapor pressure at different temperatures of this nature , using the system to reduce or increase the temperature of the system pressure , the process of the condensed contaminants in a vapor state and separated from the exhaust gas. Condensation process can be used to increase the pressure of the approach to achieving at constant temperature , it can be used to reduce the temperature approaches implemented at constant pressure , more commonly the latter. Using condensation approach , enables to obtain a high degree of purification of exhaust gases , but requires high purification , often at room temperature of the cooling water can not be achieved. Purification higher requirements , the required cooling lower the temperature , have increased pressure, if necessary , this will increase the difficulty and cost of processing. Thus , condensation is often associated with adsorption, combustion and other purification methods in combination , to recover valuable products.
5. Biological Law
Biological purification is essentially an oxidative decomposition process : the active microorganisms attached to the porous and humid media use the organic components in the exhaust gas as energy or nutrients for their life activities , and convert them into simple inorganic substances (CO2 , H2O) or cell composition substance. The main processes at this stage include : biological filter bed, biological trickle filter bed and biological washing bed.
5.1 Filter bed
A biological filter bed is a purification device filled with adsorbent filter materials ( such as peat, soil, activated carbon, etc. ) . Before hanging the biofilm , pH buffer and N , P , K and other nutrients ( such as NH4NO3 and K2HPO3) are mixed into the filter bed . When the exhaust gas with a certain humidity enters the biological filter bed , it passes about 0.5 ~