Emissions and treatment during coating production and coating

0 Foreword
There have been several incidents of smog poisoning in London, United Kingdom, since 1880, consisting of soot from heating coal and winter morning fog, the main pollutants being sulfur dioxide and coal dust (primary pollutant), sulfate mist and sulphate aerosols (secondary pollutants). When the inverse layer appears at high altitude, the pollutant is enclosed under the inverse temperature layer, which can not be effectively spread, resulting in accumulation and formation of haze. The London smog incident was mainly caused by reductive air pollution caused by burning coal
the
. Photochemical smog in Los Angeles is an oxidizing air pollution caused by vehicle exhaust emissions. Photochemical fumes are formed by chemical reactions between hydrocarbons and nitrogen oxides under the action of sunlight. Once pollutants such as hydrocarbons and nitrogen oxides in the atmosphere are exposed to sunlight, photochemical reactions occur, resulting in secondary pollutants: ozone, aldehyde, ketones, acids, peroxyacetyl nitrates, aerosols, etc., a mixture of reactants (primary pollutants) and generators (secondary pollutants) is called photochemical fumes. Photochemical smoke can be seen in height blue, brown or white and has a special odor, will significantly reduce atmospheric visibility, the human eye, nasal mucosa, etc. have a stimulating effect, causing inflammation and varying degrees of headache and vomiting symptoms, and even life-threatening.
recently, Beijing and many areas of the haze weather is serious, volatile organic matter (VOCs) on air pollution, VOCs in the haze content and its prevention and control measures are of concern to all walks of life. In 2002, the antho-human source of VOCs in the environment and atmosphere was studied and analyzed, and the results showed that the average annual contribution rate of each man-made pollution source was 62% of automobile exhaust gas, 9% of gasoline volatilization, 10% of petroleum liquefied gas, 6% of paint, 6% of petrochemical industry and 6% of unknown source. Although the proportion of VOCs contributed by coatings in that year is not too high, by 2011 China's total production of coatings more than 10 million t, is nearly 5 times that of 2002, so the contribution of coatings to VOCs man-made pollution sources can not be underestimated, should cause concern.1 Emissions and emissions of exhaust gases in coating production
the production of VOCs in the production process of coatings is relatively small, the amount of production depends on the type of solvent, production equipment and processes. Solvent boiling point is high, the production process is closed, the temperature is low, the time is short, the degree of automation is high, the variety type is small (less cleaning frequency), then the amount of VOCs is less; Obviously, by using closed equipment, improve the level of automation, optimize the production process, can effectively control the production of VOCs from the source.2 Production of waste gas dust treatment
paint production of dust is mainly pigments and fillers, removal methods have dry and wet method 2 kinds, dry separation of dust can be used back, wet method out of the dust can not be used back. Dry method dust removal equipment is: electric dust remover, cyclone dust remover, cloth bag duster. The efficiency of electric dust remover is high, the particle size can be removed is small, but there are high cost, high operating cost and high management cost. Cyclone dust remover cost is low, processing capacity is large, but the dust removal efficiency of small particle size is low, cloth bag dust remover dust removal efficiency is high, the price is moderate, but there are easy to clog the shortcomings. At present, most enterprises are using cloth bag dust remover.
method dust remover is the use of water as washing liquid, through the contact between the two phases of the gas liquid, the dust transferred to the water phase, the method of dust removal efficiency is higher, lower operating costs, but increased the amount of wastewater. The main equipments for wet dust removal are: spray absorption tower, liquid column absorption tower (improved spray absorption tower), power wave washer, etc.
3 Treatment of VOCs in the production of exhaust gases
3.1 adsorption separation technology
adsorption separation technology is the use of solid adsorption agent to adsorption of VOCs in mixed gases, desorption of VOCs under heating conditions, recovery of VOCs by cooling condensation or heat decomposition of VOCs using incineration equipment, as shown in Figure 1.
active carbon adsorption recovery VOCs process flow chart
currently commonly used in industry adsorption agents in addition to activated carbon, there are activated carbon fiber, molecular sieve, activated carbon according to different raw materials, and coal activated carbon, coconut shell activated carbon and so on. The price of activated carbon is relatively low and the adsorption of VOCs is less selective, so most enterprises generally use activated carbon adsorbent. The types of adsorption equipment include fixed beds, mobile beds and adsorption wheels, etc., and the source of desorption is water vapor or hot air. Water vapor regeneration desorbed gas containing VOCs for cooling condensation can be recovered solvents, hot air regeneration de-attached gas containing VOCs can also be condensed to recover organic solvents, can also be used incineration equipment incineration decomposition VOCs and recovery of heat energy.
water vapor regeneration method will result in a significant reduction in the adsorption capacity of regenerated activated carbon, thus reducing the adsorption separation time, resulting in a shorter adsorption-desorption cycle, thereby increasing energy and water consumption. Hot air regeneration has less effect on activated carbon adsorption capacity, but more equipment for condensation separation recovery voCs.
activated carbon adsorption separation only under suitable process conditions, may have a higher effect of removing VOCs, first of all, the installation position of the suction cover to the exhaust gas production site distance of about 1 m, catch point suitable wind speed from 0.5 to 1 m/s. The operating parameters of the adsorption system should meet the indicators in Table 1.
Active carbon system operating parameters indicators
activated carbon adsorption method low energy consumption, small equipment investment, suitable for low concentrations of a variety of organic matter, and the possibility of recovery of solvents, but not suitable for the treatment of high concentrations of organic matter, and water or particulate matter exhaust gas needs to be pre-treated.
3.2 Catalytic combustion technology
The biggest difference between catalytic combustion technology and adsorption separation technology is the conditions for desorption and the different treatment methods for desorption VOCs. The catalytic combustion technology process flow chart is shown in Figure 2.
Catalytic combustion technology process flow chart
Because catalytic combustion technology in the desorption phase of the use of higher temperature, the use of activated carbon adsorbent there is a fire safety hazard, so the use of molecular sieve as adsorbent. Because the adsorption capacity of the molecular sieve is smaller than that of activated carbon, the adsorption-desorption cycle is shortened. The operating parameters of the catalytic combustion system are set out in Table 2.
the gas temperature after catalytic combustion is higher, generally through the heat exchanger to heat the imported gas, part of the ADS for adsorption, to achieve waste heat utilization and energy-saving purposes. Catalytic combustion technology for high concentration of organic exhaust gas treatment efficiency is high, catalytic combustion chamber will be organic gas into harmless CO2 and H2O, and maintain a stable state of self-combustion, treatment is more complete, after a start-up without external heating, after combustion of the thermal exhaust gas is used for adsorption regeneration of adsorbents, to achieve the purpose of waste heat utilization, organic matter treatment. However, catalytic combustion has relatively high requirements for catalysts, in the process of catalytic oxidation will also produce some secondary pollutants, sulfur and chlorine-containing VOCs easily poisoning catalysts, etc. ;
3.3 Biological purification technology
biological method mainly uses the metabolic action of microorganisms to convert VOCs into inorganic objects such as CO2 and H2O. Biofiltration towers, bio-washing towers and biodropping towers are the three main bioprocessing processes, as shown in Figure 3.
of VOCs bioprocessing processes
compared to three typical VOCs bioprocessing processes (see Table 3).
3 voCs biological processing process comparison
bioprocessing advantages are simple process equipment, easy to operate, less investment, low operating costs, no secondary pollution, can be treated with different properties of the mixture of gases and so on. Due to the slow rate of biodegradation, the concentration of VOCs in the exhaust gas is less than 5,000 mg/m3, the load can not be too high, the amount of exhaust gas treated is limited, the exhaust gas can not contain biotoxic substances.
3.4 Low-temperature plasma decomposition technology
low-temperature plasma technology, also known as unbalanced plasma technology, is under the role of analectonic field, through the medium discharge to produce a large number of high-energy particles, high-energy particles and organic pollutant molecules a series of complex plasma physical-chemical reactions, so that organic pollutants degrade into non-toxic harmless substances. Low temperature plasma is characterized by low energy density, heavy particle temperature close to room temperature and electron temperature is very high, the macro temperature of the whole system is not high, its electrons and particles have a high reactive activity. The advantage of low temperature plasma technology is suitable for all kinds of VOCs treatment, high processing efficiency, easy to operate, more suitable for low concentration of VOCs treatment. 4 VoCs in
coatings are basically emitted during the coating process, but China does not have VOCs emission standards for various industries. The national mandatory standard, the Comprehensive Emission Standard for Atmospheric Pollutants (GB 16297-1996), sets emission limits for characteristic pollutants benzene, toluene, xylene, formaldehyde, methanol and non-methane total hydrocarbons. At present, coating production and coating enterprises are reference to the implementation of the above-mentioned national standards.
Guangdong Province in order to improve the regional atmospheric environmental quality, strengthen the automotive industry, packaging and printing industry, furniture industry and leather industry VOCs emission control and management, promote the above-mentioned industry process and pollution control technology progress, by the Guangdong Provincial Environmental Protection Agency and Guangdong Provincial Quality and Technical Supervision Bureau jointly issued four industries of Guangdong Province local standards: "furniture manufacturing industry volatile organic compound emission standards" (DB 44/8142010), "packaging and printing industry wave The Emission Standard for Volatile Organic Compounds (DB 44/815 2010), the Standard for The Emissions of Volatile Organic Compounds for Surface Coating (Automotive Manufacturing) (DB 44/816 2010), and the Standard for Volatile Organic Compounds for the Footwear Industry (DB 44/817 2010), two of which are related to coating. These standards were formally implemented on 1 November 2010. Among them, the Surface Coating (Automotive Manufacturing) Volatile Organic Compound Emissions Standard (DB 44/816 2010) stipulates that from 1 January 2013, the VOCs emission limits for the coating line unit coating area should meet the requirements of Table 4.
Coating production line unit coating area of VOCs emission limit
in various industries, the coating is mostly spraying process, so the resulting exhaust gas in addition to VOCs, but also contains fine particles of solid matter (resin and pigments), because it contains resin and other viscous substances, so the first step in the treatment of spray exhaust gas is to remove these sticky fine particulate matter. At present, there are 2 kinds of wet method and dry method, wet method is through the water curtain or spray tower to absorb the removal of fine particulate matter, fine particulate matter into the water to form lacquer slag, with flocculant condensation after filtration removal, water recycling. Starting fresh water begins to dissolve a small amount of solvent, a short cycle is saturated by the solvent, after which the water can no longer dissolve the solvent.
Water with the increase in the number of cycles, the amount of paint slag is constantly accumulating, in which the viscous material in the cycle process is easy to adhere to the pipe and equipment, will eventually lead to the blockage of the pipe, so the water cycle after a period of time, need to be replaced. Dry method is the use of filter cotton, powder and other adsorption exhaust gas solid substances, filter cotton and powder adsorption saturation saturation, re-replacement. The moisture contained in the exhaust gas after the use of wet depaint slag is absorbed by activated carbon, which occupies the capacity of the adsorbent, which is not conducive to the adsorption separation operation of organic solvents. If thermal oxidation is used to break down, the increase in moisture increases the energy consumption of the process. Due to the high price of filter cotton, the current dry method is more used in the spray exhaust gas generation is less industry or laboratory. The amount of VOCs in the filtered exhaust gas after coating has not been reduced, and further treatment is needed to reduce the emission concentration and total amount, in addition to the absorption, adsorption, condensation, catalytic combustion method discussed earlier, more is the use of heat storage exhaust incinerator, also known as heat storage thermal oxide (RTO).
RTO works by: organic exhaust gas after heating up through the preheat chamber, into the combustion chamber high temperature incineration, organic matter oxidized into carbon dioxide and water, and then after another heat storage chamber to store heat emissions, stored heat for preheat the newly entered organic exhaust gas, after a periodic change in the direction of the air flow to maintain the stability of the furnace temperature. RTO's equipment and processes are continuously improved to further reduce energy consumption, stabilize operations and increase VOC removal rates. Figure 4 is a three-room RTO process, the process has 3 heat storage rooms and 2 combustion chambers, organic exhaust gas from the fan introduced heat storage chamber A heat absorption heating, into the combustion chamber combustion, combustion gas into the heat storage chamber B storage heat after discharge, while the residual uncomposed organic exhaust gas in the heat storage room C is purified after the gas blowback combustion chamber combustion treatment
Into the combustion chamber combustion, the heat storage in the combustion chamber C after combustion is present in the heat storage chamber C after discharge, while the heat storage chamber A is in the reverse blow state, and after a certain period of time, the exhaust gas is introduced into the heat storage chamber C heating up, after combustion chamber combustion, heat storage in the heat storage chamber A after discharge, while the heat storage chamber B is in the reverse blow state;
three-room RTO process flow diagram
RTO's suitable VOC concentration ranges from 2 to 8 g/m3, which can be increased by up to 12 g/m3 for low thermal gases. The combustion chamber temperature needs to be stable at more than 800 degrees C, and when the VOC concentration in the exhaust gas is low, gas or fuel needs to be replenished. 5 Questions and recommendations
Most of the VOCs produced in the paint production process are active carbon adsorption treatment methods, not regenerative activated carbon but replacement, replacement of activated carbon is hazardous waste, need to be handed over to the hazardous waste treatment qualification units to deal with, which will increase the cost of enterprises, coupled with regulatory difficulties, so some enterprises rarely replace or even never replace activated carbon, some areas of the environmental protection departments on the new project VOCs using activated carbon adsorption process is not recognized. New treatment methods and processes for VOCs are being introduced, but many of the results are still in the laboratory stage, some processes are not clear about the intermediates of VOCs degradation, and there are few examples of engineering applications. All devices that handle VOCs have their requirements for the flow rate, temperature and concentration of exhaust gases, i.c. the above conditions