As one of the five key industries in the VOCs governance objectives, the printing industry is facing unprecedented environmental pressure for a time. From source, process control to end treatment, how to carry out VOCs treatment and how to select treatment equipment have become the focus of printing enterprises.
Reducing the use and emission of organic solvents from the source has great potential for emission reduction. In the long run, it is the fundamental way to realize the emission reduction of VOCs in the industry. However, at this stage, it is not possible to treat the waste gas completely from the source. For the production process links with VOCs as raw materials and the use process links of products containing VOCs, only waste gas collection measures can be taken to improve the waste gas collection efficiency and reduce the unorganized emission and escape of waste gas, which is the prerequisite for follow-up treatment. In the production process of products containing VOCs and the production process with VOCs as raw materials, VOCs emission is inevitable, and terminal treatment must be carried out according to the existing national laws, regulations and standards.
Generation of VOCs
Printing is a process technology that uses printing plate or other means to transfer the graphic information on the original to the substrate. According to classification, it mainly includes publication printing, packaging printing and other printed matter printing. According to the carrier, it is divided into paper and Paperboard Printing, metal printing, plastic printing, etc.
The printing process mainly includes lithography, relief printing (such as flexographic printing), gravure printing and hole printing. The sources and emission methods of VOCs in different printing processes are basically the same: VOCs come from the used ink and diluent (diluent needs to be added to the airtight substrate for printing, such as metal printing and plastic printing), composite adhesive (only limited to some printing enterprises with composite process) and equipment cleaning agent. The possible emission ways include solvent volatilization in the ink mixing process, ink solvent volatilization in the printing process, drying stage Compound process and equipment cleaning process, etc.
The printing production process and main VOCs generation links are shown in the figure.
1. Lithographic printing process and VOCs emission
Lithography (lithography), also known as offset printing (offset printing), is characterized by the fact that the inked part and the blank part of the printing plate are almost on the same plane. The inks used by lithographic printing enterprises include solvent based inks, plant soybean inks, UV curing inks and water-based inks. Solvent based inks have a high content of volatile organic compounds and are the main VOCs emission source of lithographic printing enterprises. In addition, the organic solvent car wash water and fountain solution used in the production process of lithography are also one of the VOCs emission sources.
2. Relief printing process and VOCs emission
The graphic part of relief printing (embossing) is in a plane, which is significantly higher than the blank part. When the printing plate is inked, the ink is attached to the convex part of the printing plate and transferred to the substrate under pressure. The traditional relief printing adopts copper zinc plate. At present, it is gradually replaced by flexographic printing (flexographic printing), using soft resin plate.
Flexographic printing is usually used for product packaging and printing. For corrugated box products with low color requirements, water-based ink is generally used, and there is almost no VOCs emission; For brightly colored film products, alcohol soluble ink is generally used, and VOCs Pollution is produced in the printing process.
3. Gravure printing process and VOCs emission
The blank part on the plate cylinder of gravure printing (gravure printing) is higher than the printed picture and text part, and the height is very different. The blank part is on the same plane or the same curved surface. The concave graphic part on the printing plate forms a net hole to accommodate the ink, and the graphic imprint on the printing plate cylinder is transferred to the surface of the substrate through roller imprinting.
Gravure printing is widely used in the field of packaging and special product printing. It is suitable for film, composite materials, paper and other media. Ink with low viscosity and high VOCs content is usually used. A large number of VOCs are produced in the printing process, and the composition is complex.
4. Hole printing process and VOCs emission
Hole printing (also known as screen printing and silk screen printing) is to weave real silk, nylon or metal wire into a net, tighten it on the net frame, and make a screen by manual film engraving or photochemical plate making. The non graphic part of the screen is sealed by the coated photosensitive coating, leaving only the mesh of the graphic part to pass through the ink. When printing, first apply ink on the screen, and then gently scrape it with a rubber scraper. The ink passes through the screen and is transferred to the printing material placed under the screen.
VOCs in hole printing mainly come from ink and cleaning agent. When solvent ink is used, the emission concentration of VOCs is relatively high.
5. Composite process and VOCs emission
Composite process refers to a post press processing method in which different substrates are bonded by adhesive to form a combination of two or more materials, including dry composite, wet composite, extrusion composite, hot melt composite and other processes. The dry composite process requires a large amount of adhesive and diluent, with large VOCs emission and single composition. The printing process, raw and auxiliary materials containing VOCs and VOCs emission characteristics are shown in the table.
Due to the complexity of VOCs waste gas composition and properties and the limitations of a single treatment technology, it is often difficult to meet the treatment requirements by using a single technology in most cases. Using the advantages of different treatment technologies, the combined process of two or more single purification technologies has received great attention. The combined treatment process can not only meet the emission requirements, but also reduce the operation cost of purification equipment.
Combined with the characteristics of the industry and the analysis of raw and auxiliary materials, we found that the VOCs waste gas of printing industry contains toluene, ethyl acetate, methyl ethyl ketone and isopropanol (ethanol), with high emission intensity, while the emission intensity of general paper printing is low, but the composition is more complex. Combined with the most common VOCs treatment technology at present and the requirements of local governments that gas-related enterprises should not choose a single process when choosing treatment technology and process, enterprises are basically required to achieve more than 80% of the end treatment efficiency. The traditional treatment process can not meet this requirement at all, so they can only adopt combustion method for treatment. However, in the process of waste gas treatment by direct combustion method, there are problems such as high energy consumption, and it is basically impossible to burn when the waste gas concentration is not high. It is necessary to change the waste gas concentration through early technology and increase the waste gas concentration through physical means.
After the investigation of printing enterprises and the actual situation of VOCs Pollution in printing enterprises, this paper focuses on the activated carbon concentration catalytic combustion technology.
The essence of activated carbon adsorption is to use the characteristics of activated carbon adsorption to adsorb and concentrate the organic solvent in low concentration and large air volume waste gas into activated carbon, and the gas purified by activated carbon adsorption is directly evacuated. Its essence is a process of adsorption and concentration without removing the organic solvent. This is a physical process.
The essence of catalytic combustion desorption is to use the hot air of catalytic combustion to heat the adsorbed organic solvent in the activated carbon to reach the boiling point of the solvent, desorb the organic solvent from the activated carbon, and introduce the high concentration waste gas into the catalytic combustion reactor. At the catalytic ignition temperature of 250 ℃, the oxidation reaction is carried out through the action of catalyst, which is transformed into harmless water and carbon dioxide and discharged into the atmosphere. This is a secondary combustion process, which can not be completely solved by chemical desorption.
Activated carbon adsorption catalytic combustion desorption is an effective combination of the above two advantages. That is, first use activated carbon for adsorption and concentration. When the activated carbon adsorption reaches saturation, use electric heating to start the catalytic combustion equipment, and use hot air to locally heat the activated carbon adsorption bed. When the catalytic combustion reaction bed is heated to 250 ℃ and the activated carbon adsorption bed locally reaches 60 ~ 110 ℃, the high concentration waste gas desorbed from the adsorption bed can undergo oxidation reaction in the catalytic reaction bed. The high-temperature gas after reaction is heat exchanged by the heat exchanger. Part of the gas after heat exchange is reused and sent to the activated carbon adsorption bed for desorption, and the other part is discharged into the atmosphere. The temperature of the desorbed waste gas increases rapidly after heat exchange by the heat exchanger, so that the catalytic combustion device and desorption can operate at low power or no power.
Most printing enterprises have the characteristics of large exhaust air volume and low concentration in the collection process due to large equipment, large workshop space and other reasons. The activated carbon concentration catalytic combustion technology is a better treatment method. Its main features are:
1) The biggest feature of catalytic combustion is that high, low and medium exhaust gases can be used. When the exhaust gas concentration is high, the adsorption and desorption time is short, and it can become high concentration exhaust gas; When the concentration of waste gas is low, the adsorption and desorption time is longer, and it is also collected. When it reaches a certain concentration, the desorption treatment becomes high concentration waste gas, and then it is burned. Due to the rapid transformation of raw and auxiliary materials, enterprises can gradually replace them from the source and continuously reduce the VOCs concentration of raw and auxiliary materials. However, the treatment effect of waste gas treatment equipment will not change, but the operation cost is reduced, and the equipment does not need to be replaced.
2) It is more in line with the carbon neutralization mentioned in the 14th five year plan. Through the combustion device (start when in use and stop when not in use), it not only meets the governance objectives, but also helps enterprises achieve energy conservation and consumption reduction.
3) The activated carbon concentration catalytic combustion technology is mature and stable. It has been operated in the market for more than 4 years since 2017, with good stability. Through the promotion and use in previous years, the market price is relatively transparent and the overall cost is low.
4) The operation cost of the technology in the operation process is relatively simple, mainly including adsorbent (activated carbon), catalyst, collection of wind turbine energy and combustion of wind tower power. In these aspects, enterprises can adjust in time according to VOCs concentration and production orders without professionals. At the same time, activated carbon is cheap and there are many manufacturers. Enterprises can choose independent operation and maintenance or third-party operation and maintenance.
There are various VOCs waste gas treatment technologies. Enterprises need to choose after comprehensive analysis in technology and economy (technical feasibility and economic rationality). Through the detailed detection and analysis of pollutant components, and according to the necessary on-site tests, they need to carry out expert "consultation" on the recovery value of organic matter, the boiling point of pollutants, the level of water solubility, biodegradability and the emission mode of waste gas, and pass the technical scheme review, Choose the most appropriate way of governance.
The content of the article is extracted from: on VOCs management in the printing industry. If Beijing Xinhua Printing Co., Ltd. has infringement, please contact to delete it