Discussion on VOC efficient treatment scheme of th

Discussion on VOC efficient treatment scheme for bicycle painting

discussion on VOC efficient treatment scheme for bicycle painting

May 23, 2019

discussion on VOC efficient treatment scheme for bicycle painting

Dai Jizhu

(giant (China) Co., Ltd., Kunshan 215300, Jiangsu)

0 Introduction

in recent years, the increase of haze weather and its harm to people's health have increased, People have higher and higher requirements for air quality, and the concept of not only golden mountains and silver mountains, but also green rivers and mountains is deeply rooted in the hearts of the people. As an economically developed area, Kunshan Development Zone has proposed that VOC emission enterprises within its jurisdiction should not only refer to the requirements of Tianjin local emission standard db12/emission control standard of volatile organic compounds for industrial enterprises, but also challenge 99% on the basis of meeting the treatment efficiency of 90% in order to significantly reduce the total VOC emission. This requirement poses a great challenge to us. In the face of such requirements, we actively study and take measures to analyze and improve the coating at the same time from the source and the end

1 source of VOC from bicycle painting

bicycle painting has high requirements for decoration. In the past, most of them used solvent based coatings. The VOC content of solvent based coatings is generally between 30~50%, and reaches 50~70% after dilution. The general process flow of bicycle coating is 4 spraying and 5 baking, that is, primer, intermediate coating, surface coating, labeling, gloss varnish. Complex coating will have 5 spraying and 6 baking, or more coatings. The process of each layer is basically pre correction, electrostatic spraying, post correction, leveling and baking. VOC volatilizes from these processes. Each bicycle coating contains about 200g of organic solvent on average, which is released into the atmosphere during the spraying and baking process, creating a great burden on the environment

2 there are two main ways to reduce VOC emission from bicycle coating

one is to use low VOC content coatings from the source, the other is to discharge VOC after terminal treatment

2.1 use low VOC content coatings

low VOC coatings, which are mainly powder coatings, UV coatings, high solid component coatings and water-based coatings at present, and the VOC treatment processes include adsorption, biological treatment, photocatalysis Combustion method, etc.

for powder coatings that basically do not contain VOC, due to the characteristics of poor leveling, poor decoration and not suitable for frequent color change in small batches, only some parts with low decorative requirements and less color will adopt powder coating. Although more than 10 years ago, international well-known brands such as DuPont and AkzoNobel developed some acrylic resin transparent powder, and its leveling, fullness and weather resistance reached or even exceeded that of paint, it has not been promoted because of its high price and high requirements for temperature resistance and toughness of color paint and stickers, and few manufacturers use it on bicycles

UV paint, due to the use of UV light, but due to the structure of bicycles, some dead corner light is not easy to shine. In addition, different colors have different absorption of light, and there are weather resistance and other problems, which also restrict the use of UV paint on bicycles

in fact, high solid coating has been applied by many manufacturers in production, and VOC under construction viscosity can be controlled below 50%. However, according to the current situation, the environmental protection department has not listed it as the direction of promotion, but still classified it as solvent based coating, and solvent based coating is not approved in many places when new coating projects are approved

in this way, water-based coatings are the only ones that can be focused on. Although there are still many items of water-based coatings that cannot be compared with solvent based coatings, especially the arrangement of metal powder is quite different during disc spraying, which needs us to make a breakthrough. Water based coatings can reduce VOC emissions by at least 80% or more, and indeed make a great contribution to exhaust emissions. After several years of efforts, we have more than 50% of the coatings are water-based, and we will continue to strive for the full water-based coating of bicycles as soon as possible

2.2 method of VOC emission from bicycle coating for end treatment

the wind speed of manual spraying room is generally 0.5~1.0 m/s, the static coating is 0.25~0.5 m/s, and the air volume of a four spraying and five baking coating line is 250000m ³/H, about 85mg/m ³ Concentration, significantly low concentration of exhaust gas with large air volume. For low concentration VOC treatment processes, there are activated carbon adsorption method, biological treatment method, photocatalysis method, low-temperature plasma, combustion method, etc. (see Figure 1)

2.2.1 activated carbon adsorption

traditional activated carbon adsorption, the ratio of activated carbon demand to solvent is generally solvent: activated carbon =1:3, the activated carbon adsorption bed occupies a large space, and it cannot be adsorbed after adsorption saturation, and it will not work if it is not replaced in time. For workshops with mass production, activated carbon needs to be replaced frequently, and a few tons of activated carbon need to be replaced each time, which not only has a large amount of work and a long shutdown time, but also has a high cost of treating the adsorbed activated carbon as solid waste. On the whole, the operation cost is high, and this method has been less and less used in recent years

2.2.2 activated carbon adsorption desorption regeneration

in order to reduce the use of activated carbon, the process of desorption and regeneration after adsorption saturation of activated carbon has been applied. Desorption is carried out through time control or concentration control, and the process of catalytic combustion or RTO of high concentration VOC after desorption. The activated carbon can be regenerated to restore the adsorption capacity, so that the service life of activated carbon can be extended to about 2 years. Due to the flammability of activated carbon, the desorption temperature cannot be too high. If it is too high, there is an unsafe hidden danger of spontaneous combustion. Generally, the desorption temperature is about 120 ℃, and there is incomplete desorption. The efficiency will decline from 80% in the initial stage to gradually reduce, and the efficiency may be less than 50% by the time of replacement

2.2.3 molecular sieve zeolite runner adsorption desorption regeneration

in order to solve the problem of adsorption efficiency after safe desorption and regeneration, molecular sieve zeolite runner technology has been developed in recent years, and the adsorption efficiency can reach 90%~95%. Due to the high temperature resistance and non flammability of the zeolite runner, the conventional desorption temperature of the runner can be set above 200 ℃, and most of the organic solvents used in the coating process can be desorbed, maintaining the adsorption efficiency of the zeolite runner at the same level. Due to this feature, its service life is longer, which can reach 10 years. Moreover, because the zeolite runner rotates continuously, and the adsorption, desorption and cooling are carried out continuously, the amount of adsorption materials is greatly reduced. The weight of adsorption materials required for the treatment of the same waste gas is only about 5% of that of activated carbon, which not only reduces the cost of solid waste treatment, but also greatly reduces the occupation of space

2.2.4 VOC treatment methods after concentration - condensation method, RCO, RTO

high concentration VOC desorbed, some adopt condensation method, and some adopt combustion method. The condensation method is mostly used for the treatment of VOC waste gas with high concentration, simple composition and recycling value. The condensed liquid needs to be treated again, and it is rarely used in the treatment of coating waste gas [1]

combustion method is more used to oxidize and burn VOC in organic waste gas directly into carbon dioxide and water and discharge it into the atmosphere except for the first verification. The commonly used oxidation combustion method is catalytic combustion or direct combustion. In order to save energy, ceramic heat storage is generally used for insulation 10 Input power: 1 kW, basically using RTO and RCO processes

rco is a method to reduce energy consumption by adding precious metal catalysts to reduce the combustion temperature to about 300 ℃. However, the catalysts generally need to be replaced after 8000 hours, which is prone to poisoning and failure. The replacement cost is high, and the combustion efficiency is 95%

rto is a method of directly burning VOC above about 760 ℃, with high combustion efficiency, while the microcomputer controlled electronic control experimental machine uses force sensor as the force measuring device to reach 99%

2.2.5 other treatment methods

in addition to the adsorption, desorption, concentration and oxidation processes introduced above, there are also low-temperature plasma + catalytic combustion, UV photocatalysis technology, etc. the efficiency of low-temperature plasma is about 90%, and there are also fire hazards. The efficiency of UV photocatalysis technology is about 85%. The lamp life and wavelength attenuation also affect the efficiency

3 discussion on the method of bicycle VOC treatment efficiency of 99%

comparing the above treatment processes, the efficiency of activated carbon is about 80% and will gradually decrease after desorption and regeneration. The efficiency of zeolite runner of molecular sieve is about 90~95%, and the highest combustion efficiency of RCO is about 95%. Only RTO combustion can reach the efficiency of more than 99%. However, the emission of VOC from bicycle coating is the standard high air volume and low concentration, which is far from reaching the concentration that can be directly burned. If there is no concentrated combustion, it will only burn a large amount of natural gas and a small amount of VOC, which will cause a great waste of energy. Emission reduction without energy conservation is not the result that enterprises want to see nor the government wants. The activated carbon mentioned above is about 80% and will gradually decrease. The molecular sieve zeolite runner is about 90~95%. If the adsorption efficiency is less than 99%, then the exhaust gas passing through the adsorption material cannot be removed by 99%. When it is combined with the exhaust gas after RTO combustion to the chimney, it still cannot achieve the effect of comprehensive treatment of 99%. The design that challenges 99% of the processing efficiency fell into a situation of no solution for a time

after searching various resources, we failed to find a suitable filter material that can achieve 99% efficiency. Finally, we still consider finding a way on the zeolite runner. After many discussions with Qingdao huashijie company, we have designed a two-stage filtration treatment scheme, that is, the coating waste gas is filtered by two zeolite runners, the first treatment is 90-95%, the remaining 5-10%, and the second treatment is 90% of the remaining 5-10%, finally achieving the effect of filtration of 99%. Finally, it is combined with the waste gas burned by RTO in the chimney, and the overall design reaches 99% efficiency (see Figure 2)

the process of molecular sieve zeolite runner secondary filtration +rto has VOC concentration and high-temperature combustion, so the safety design is very important. A number of safety measures have been designed in the whole system, including the interlocking design of the system and the spray booth, the emergency evacuation design, the spray design of the molecular sieve runner, the periodic high-temperature desorption of the molecular sieve, the secondary alarm interlocking control system for VOC detection, the fresh air dilution protection measures, the safety control of the burner, the low-pressure bursting disc, the flame arrester (flashback preventer), the pressure relief valve and other UV absorbers, ensuring the abnormal concentration, abnormal temperature When abnormal pressure occurs, the equipment can alarm in time, shut down when necessary, and take emergency measures to protect the safety of the equipment

4 conclusion

VOC emissions from bicycle coating can be reduced by using water-based coatings at the source, and VOC emissions from oil-based coatings can also be reduced by end treatment. Through analysis and comparison, for the exhaust gas with large air volume and low concentration of bicycle coating, the process of secondary molecular sieve runner +rto can be adopted to challenge the treatment efficiency of 99%