Zeolite runner-catalytic oxidation VOCs treatment device in the packaging and printing industry

Due to the use of a large number of solvent-based inks and organic solvents for dilution in the color printing and plastic film composite process, the annual emissions of volatile organic compounds (VOCs) from the packaging and printing industry in my country can reach about 2 million to 3 million tons. VOCs exhaust gas is usually treated by activated carbon adsorption, photocatalysis, plasma, catalytic oxidation/regenerative catalytic oxidation (CO/RCO), regenerative incineration (RTO) and other methods, of which catalytic oxidation and incineration are the most popular . Packaging and printing waste gas has the following characteristics: (1) The composition of the waste gas is complex and contains a variety of organic substances; (2) When the ink is drying, because the mass concentration of waste gas in the production workshop needs to be strictly controlled, a large amount of air is usually introduced for ventilation, so the generated The VOCs exhaust gas has large air volume and low mass concentration. Traditional catalytic oxidation or incineration equipment is suitable for treating VOCs with different air volumes of medium and high concentration. The size of the equipment mainly depends on its own maximum processing air volume. However, when processing VOCs with high air volume and low mass concentration, the use of a single catalytic oxidation or incineration method requires a huge device, which not only requires a high investment cost for primary equipment, but also greatly increases the operating cost of subsequent fuels. Therefore, the actual treatment needs to introduce the zeolite runner technology to first adsorb the high-volume low-mass concentration VOCs exhaust gas, and then concentrate it into a small-volume high-quality concentration gas before performing catalytic oxidation treatment.
 
  With the introduction of new environmental regulations and strict control of VOCs emission restrictions in various places, the industry has put forward higher requirements for VOCs treatment equipment. Compared with a single VOCs exhaust gas treatment equipment, the zeolite runner-catalytic oxidation combined device has multiple advantages of small equipment size, high removal efficiency, safety and good economy. This will also be the mainstream development direction of VOCs exhaust gas treatment devices in the future.
 
1 The principle of zeolite runner-catalytic oxidation device
 
  At present, the exhaust gas of the domestic packaging and printing industry has the characteristics of large exhaust air volume, low mass concentration, and complex exhaust gas composition, and is generally discharged in an organized manner. For the large air volume and low mass concentration VOCs exhaust gas, when only the catalytic oxidation or incineration device is used for separate treatment, the investment cost of the primary equipment is large, and the later operation cost is high; the VOCs exhaust gas treatment device using the zeolite runner-catalytic oxidation technology can be The waste gas with high air volume and low mass concentration is separated and concentrated first to form a gas with high mass concentration and small air volume before being subjected to catalytic oxidation treatment.
 
1.1 Zeolite runner-process flow of catalytic oxidation unit
 
  The VOCs zeolite runner-catalytic oxidation device adopts the combined technology of zeolite concentration and catalyst oxidation, and consists of multi-stage filters, zeolite runners, adsorption fans, desorption fans, heat exchangers, catalytic oxidation devices and other segmented equipment.
 
  The organic waste gas containing VOCs is first subjected to preliminary multi-stage filtration, and then sent by the blower to the adsorption zone (A zone) of the zeolite runner segmentation device for adsorption treatment, and the generated clean air is directly discharged. With the continuous rotation of the zeolite runner, the saturated runner adsorption zone is partially transferred to the regeneration zone (R zone), where it is purged from the reverse high temperature regeneration air and desorbed. The desorbed high-quality organic waste gas directly enters the catalytic oxidation device for oxidative decomposition. The VOCs exhaust gas passing through the desorption zone is then rotated into the cooling zone (P zone), and then returned to the adsorption zone for circulation operation after cooling. Since the air volume in the desorption regeneration zone is generally only 1/10 of the air volume in the treatment zone, the mass concentration of VOCs in the exhaust gas after regeneration is about 10 times that of before concentration.
 
  The high-quality organic waste gas after the regeneration and concentration of the zeolite runner is blown into the downstream catalytic oxidation device, and is heated by the burner, and the catalytic oxidation reaction is carried out after preheating to 350°C. The whole process of catalytic oxidation uses honeycomb platinum (Pt) to contact coal, and the VOCs in the exhaust gas undergo a catalytic oxidation reaction to generate non-toxic and harmless carbon dioxide and water.
 
  After passing through the catalytic oxidation section, the temperature of the purified gas discharged is about 360 ℃; in order to make full use of the waste heat, the gas purified by the catalytic oxidation equipment is heat exchanged with the regeneration exhaust gas, and the warmed regeneration exhaust gas is used for the desorption of the zeolite runner Zone desorption.
 
1.2 Structure and principle of zeolite runner thickening and segmentation device
 
  1.2.1 Structure and composition of zeolite runner
 
  When the exhaust gas has the characteristics of large air volume and low mass concentration, the characteristics of low temperature high adsorption and high temperature high desorption of molecular sieve inside the zeolite runner can be used to absorb and desorb the organic waste gas. The mass concentration of the generated exhaust gas is about 10-20 times the mass concentration of the original gas, which saves equipment and operating costs for subsequent catalytic oxidation treatment.
 
  The structure of the zeolite concentration runner is divided into adsorption zone (A zone), regeneration zone (R zone) and cooling zone (P zone). The processed corrugated and flat ceramic fiber paper is made into a honeycomb runner by inorganic bonding, and then a hydrophobic zeolite molecular sieve is smeared on the runner channel to make it adsorbable. The general chemical formula of zeolite molecular sieve is Mx/m[(AlO2)x·(SiO2)y]·zH2O, which is a kind of porous crystal of crystalline aluminum silicate metal salt, in which silicon oxygen tetrahedron and aluminum oxygen tetrahedron are shared by The oxygen atoms are connected to each other to form a skeleton structure. The inside of the molecular sieve crystal has pores of different sizes, which can adsorb molecules smaller than its own pore size and discharge molecules larger than its pore size. The relative humidity of waste gas in the packaging and printing industry is generally less than 70%, and the adsorption rate of VOCs by the zeolite runner can reach more than 90%. As the relative humidity of the exhaust gas increases, the adsorption efficiency will decrease. Therefore, if necessary, the exhaust gas can be heated and dehumidified before entering the zeolite runner. According to the air volume, set the zeolite runner to rotate at a rate of 1~6 r/h.
 
  1.2.2 Applicable air volume and VOCs mass concentration of zeolite runner
 
  For the exhaust gas with different mass concentrations of VOCs, the treatment methods used are different, and the zeolite runner is often used for the concentration treatment of high-volume low-mass concentration organic waste gas.
 
  For large-volume exhaust gas with a mass concentration of VOCs less than 600 mg/m3, the use of a zeolite runner concentrator can achieve the purpose of subsequent energy-saving treatment. According to the current diameter and thickness of the runner, when the mass concentration is lower than 600 mg/m3, the air volume can be handled in the range of 0.4-18m3/h.
 
 1.2.3 The adsorption curve of VOCs in packaging and printing exhaust gas by zeolite runner
 
  The waste gas of the packaging and printing industry mainly contains o-xylene, isopropanol, ethyl acetate, adipic acid and other benzene series, alcohols and esters. Therefore, it is necessary to evaluate the adsorption efficiency of the hydrophobic molecular sieve on the zeolite runner. According to the relationship between adsorption efficiency and time, related experiments were carried out on the adsorption performance of zeolite runner molecular sieves. The mass concentration of 500 mg/m3 benzene series, 400 mg/m3 alcohols and 300 mg/m3 esters were used. As a component of exhaust gas.
 
  For the VOCs contained in the exhaust gas of the packaging and printing industry (ie, benzene series, alcohols and esters), hydrophobic molecular sieves can effectively adsorb.
 
1.3 Structure and principle of catalytic oxidation segmented device
 
  The catalytic oxidation segmentation device uses precious metal Pt as a catalyst to preheat and catalyze and oxidize and decompose the high-quality concentration exhaust gas treated by the zeolite runner. The principle is to reduce the activation energy of the reaction with the help of a catalyst, so that the oxidation reaction occurs at a lower ignition temperature (250~400 ℃). Since the exhaust gas to be treated may contain substances that poison the catalyst (compounds containing sulfur, phosphorus, silicon and other elements), it is necessary to set up a pretreatment process at the front end, that is, use ceramic as a carrier pretreatment agent (see Table 2) The catalyst poisoned material is intercepted. When the temperature of the high mass concentration exhaust gas entering the catalyst chamber is low, it can be preheated by the burner to increase the temperature to 350 ℃; since this temperature is the optimal active temperature of the oxidation catalyst, the treatment efficiency of VOCs can reach above 95.
 
2 The composition and test emission standards of exhaust gas in the packaging and printing industry
 
  2.1 The main components of VOCs exhaust gas in the packaging and printing industry
 
  The VOCs exhaust gas produced by the packaging and printing industry mainly contains o-xylene, isopropanol, methoxypropanol, ethyl acetate, propyl acetate, adipic acid and other benzene series, esters and alcohols.
 
  2.2 Test emission standards for packaging and printing industry
 
  At present, the VOCs control standards adopted by various provinces in my country are not the same. The most commonly used standard is Tianjin landmark DB 12/524-2014 "Volatile Organic Compound Emission Control Standards for Industrial Enterprises". The emission requirements for benzene, toluene, xylene and VOCs are shown in table 3. The corresponding test method adopts HJ 734-2014 "Determination of Volatile Organic Compounds in Exhaust Gases from Stationary Pollution Sources: Solid Phase Adsorption-Thermal Desorption/Gas Chromatography-Mass Spectrometry", in which VOCs test content is 24 items: acetone, isopropanol, n-hexane, acetic acid Ethyl, benzenehexamethyldisiloxane, 3-pentanone, n-heptane, toluene, cyclopentanone, ethyl lactate, butyl acetate (butyl acetate), propylene glycol monomethyl ether acetate, ethylbenzene , P/m-xylene, 2-heptanone, styrene, o-xylene, anisole, benzaldehyde, 1-decene, 2-nonanone, 1-dodecene, etc.
 
  For the treatment of packaging and printing waste gas with high air volume and low VOCs mass concentration, the zeolite runner-catalytic oxidation integrated purification device has the characteristics of high efficiency, safety and economy, and the adsorption efficiency of benzene series, esters, and alcohols in the exhaust gas can reach 90%~97%. The molecular sieve for the zeolite runner is non-flammable and safe. It can be desorbed and regenerated at high temperature and has a service life of 5-10 years. The oxidation catalyst VOCs used in the catalytic oxidation section has a high treatment efficiency (95% to 98%). For intermittent working conditions, catalytic oxidation is more energy-efficient than regenerative catalytic combustion. The catalyst used in the oxidation reaction has a long service life. It can be replaced every 5 years on average and can be regenerated. The whole device adopts low temperature combustion, which saves energy and has extremely high safety. However, it is necessary to prevent the occurrence of smoldering due to the accumulation of high-quality VOCs in the roller during use, so the equipment needs to be monitored and maintained.
 
  Today, with the rapid development of industry and the increasingly severe environmental protection situation, the zeolite runner-catalytic oxidation device will be more widely recognized and applied.