Treatment of volatile organic waste gas from pesti

Treatment of volatile organic waste gas from pesticide industry by regenerative thermal oxidation furnace

taking volatile organic waste gas from pesticide industry as the research object, the material selection of thermal storage ceramics, switching valves, burners, safety control and dioxin prevention and control design parameters were optimized, and the inlet and outlet gas concentration and removal efficiency of characteristic pollutants were analyzed, This paper discusses the practical operation effect of regenerativethermaloxidizer (RTO) technology in the treatment of volatile organic waste gas. The results show that the emission limits of toluene and non methane total hydrocarbons meet the secondary standard in Table 2 of the integrated emission standard of air pollutants (GBL), the emission limits of dichloroethane meet the calculated values of the EPA Industrial Environment Laboratory and the technical methods for formulating local emission standards of air pollutants (GB/T), and the emission limits of dioxin meet the standard in Table 4 of the pollution control standard for domestic waste incineration (gbl8485-2014). The total investment of RTO system is 1.5 million yuan, and the annual operation cost is 499800 yuan, which does not constitute an economic burden on the enterprise. RTO is suitable for the treatment of volatile organic waste gas in pesticide industry, especially for waste gas containing low concentration halogen, which can not only ensure the purification effect, but also inhibit the production of dioxin

as of December 2015, there were 28 Enterprises above Designated Size in the pesticide industry in Jiangsu Province, with a total output value of 43billion yuan. According to the national economic industries (GB/t4754-2011), pesticide production belongs to the category of chemical raw materials and chemical products manufacturing. There are many kinds of raw and auxiliary materials, large consumption of organic solvents, and many processes and products, which are easy to cause environmental pollution. Commonly used organic solvents in this industry include aromatic hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons, halogenated hydrocarbons, alcohols, ethers, esters, ketones, etc. VOCs (volatileorganiccompounds) formed by volatilization during the use of these organic solvents are important precursors of atmospheric ozone and secondary organic aerosol pollution, and are highly toxic and carcinogenic, and direct discharge will cause certain harm to human health

according to the national VOCs emission base by industry in the 13th five year plan. Jiangsu Province, at present, the total VOCs emission in Jiangsu Province is 2.385 million tons, and the emission from industrial sources is 1.0807 million tons, accounting for 46.81%, which is far higher than that from transportation sources, living sources or agricultural sources. Among the industrial sources, the VOCs emission of chemical raw materials and chemical products manufacturing industry is 27500 tons, accounting for 2.54% of the industrial source emissions, becoming one of the current VOCs sources in our province

at present, the commonly used methods for recovering VOCs include absorption, adsorption, biological purification, boiler thermal incineration, low-temperature plasma, photocatalytic oxidation, regenerative thermal oxidation, etc. The purification efficiency of absorption method depends on the water solubility of VOCs. The overall purification efficiency is low, and it is easy to produce secondary pollution; Adsorption regeneration method can be used for substances that are easy to desorb and have utilization value. If adsorption method is used for substances that are non renewable or have no recovery value, the operation cost is high; Low temperature plasma, photocatalytic oxidation and biological methods are generally only suitable for the treatment of low concentration and large air volume organic waste gas; The boiler thermal incineration purification efficiency is high, but it needs to rely on the boiler, the operation cost is high, and the use occasions are limited; Compared with the traditional catalytic combustion and direct combustion thermal oxidation furnace, RTO has the characteristics of high heat recovery efficiency (95%) and purification efficiency (98%), low operation cost, strong change of pollutant concentration, and can handle large air volume and low concentration industrial waste gas. It is gradually applied to the pollution prevention and control of volatile organic waste gas in chemical industry, painting, printing and other industries

combined with the production and emission characteristics and engineering experience of organic solvent containing waste gas in a pesticide production enterprise in Yancheng, Jiangsu Province, this paper proposes the application of three chamber RTO in the end treatment of medical and chemical waste gas, which has achieved better environmental and social benefits

1 project overview

a pesticide enterprise in Yancheng, Jiangsu Province mainly produces fluconazole, cyhalothrin, pyrfluramide, diazinon, imipramine, dimethylmorpholine, herbicide, anti inversion ester and other products. In the normal production process, various reactors, rectification towers, vacuum pumps, centrifuges, centrifugal mother liquor collection tanks, dryers, raw materials and product storage tanks and other equipment will produce waste gas, which mainly contains methanol, isopropanol, toluene, xylene, acetone, phenol, diethylamine, triethylamine, thionyl chloride, acetic acid, dichloroethane, petroleum ether, n-butanol Volatile organic compounds such as dimethyl sulfoxide, such as spring steel, and a small amount of inorganic pollutants such as NOx, SO2, HC1, HBr, Br2, C12. The original waste gas treatment methods of the enterprise include condensation, water washing, alkali washing, sodium hypochlorite absorption, activated carbon adsorption, etc

according to the on-site investigation, the waste gas collection and treatment of the enterprise mainly has the following problems: ① the condensation method can recover a large amount of organic materials, and water washing and alkali washing have a certain effect on inorganic pollutants, but the effect on most VOCs is not good; ② Sodium hypochlorite absorption can remove reductive VOCs to a certain extent, but there is a problem of secondary pollution; ③ The existing activated carbon adsorption and purification device has no desorption and regeneration system, which is very easy to saturate. It is difficult to ensure up to standard discharge only by using activated carbon adsorption, and at the same time, it can produce a large number of secondary pollutants such as waste activated carbon. Therefore, combined with the actual situation of the pesticide company, the author proposes to use RTO purification process to treat VOCs containing waste gas

2 process flow

the VOCs process flow of the pesticide enterprise by three bed RTO is shown in Figure 1. The VOCs containing waste gas generated in the workshop is sent to the first two stages of water washing tower by the front blower after pretreatment to remove inorganic waste gas and a small amount of water-soluble organic waste gas, and at the same time, it plays the role of dust removal and cooling, so as to reduce the RTO processing load; Then, the water brought into the water washing tower is removed through the gas-water separator to avoid safety accidents; Then the waste gas is sent to RTO for high-temperature incineration through the main fan; The burned waste gas passes through the mixing tank, water cooling tower and post alkali washing tower. After cooling and removing the acid gas generated by incineration, it is discharged through the exhaust funnel to meet the standard

3 design points

3.1 design parameters

according to the existing waste gas collection system of the enterprise, the measured waste gas flow is q--8000m3/L1, and the exhaust temperature is 30-c. considering that the treatment system has 20% of the operating margin, the waste gas treatment capacity entering the RTO device is determined to be q=10000m/h. Other design parameters are shown in Table 1

3.2 fire prevention spacing

code for fire protection design of buildings (gb50016-2014) clearly puts forward the minimum fire prevention spacing between plant, warehouse, storage tank and combustible material storage yard and open fire or spark emitting place (IO incinerator), that is, the fire prevention spacing between RTO incinerator and class A and class B plant should not be less than 30m,

the fire prevention spacing between RTO incinerator and class a warehouse should be at least 25m, and the fire prevention spacing between RTO incinerator and class A, class B and class C liquid storage tank should be at least 25m, The fire separation distance with the wet combustible gas storage tank shall be at least 20m, the fire separation distance with the wet oxygen storage tank shall be at least 25m, and the fire separation distance with the combustible material storage yard shall be at least 12.5m. The fire protection distance between the site of RTO incinerator of the project and class a plant is 35m, which meets the requirements of gb50016.2014

3.3 principle of material selection

the waste gas of the pesticide enterprise contains halogen, nitrogen, sulfur and other elements. After high-temperature incineration, these organic substances produce acid gases such as hydrogen halide, which cause serious corrosion to the RTO furnace body, thus affecting the normal operation of the equipment. Therefore, corrosion prevention must be considered in the selection of RTO. In order to slow down the corrosion of RTO and auxiliary equipment, the system has done the following work in material selection: 1) the regenerator grate is made of 316L stainless steel; 2) The inner wall of RTO shell is coated with temperature resistant and anti-corrosion pouring materials (such as acid resistant glue); 3) 316L stainless steel is also used for the auxiliary equipment such as mixing box, water cooling tower and post alkali washing tower. Anti corrosion and explosion-proof fans are used for the forced draft fan and main fan

3.4 heat storage ceramic body

the ceramic heat storage body plays the function of heat absorption and heat release during the regular conversion of air flow, so that the average temperature difference between the inlet and outlet exhaust gases of RTO is controlled at 30-c100-c, and the heat exchange efficiency is greater than 95%, reducing the energy consumption of RTO to reduce the operating cost. The ceramic regenerator of this project adopts the patented product of lantec

mlm180, which is characterized by large specific surface area of 680m2/m3, low resistance, 11 enterprises, large heat capacity of 0.22btu/lb*f (2.326j/kg*f), high temperature resistance of 1200-c, acid resistance of 99.5%, water absorption of less than 0.5%, crushing force of more than 4kgf/cm2, small thermal expansion and cold contraction coefficient of 4.7x10-c, good crack resistance and long service life

3.5 switching valve

switching valve is one of the key components of heat storage ceramic body to realize the functions of heat storage, combustion and purging. Because the exhaust gas contains corrosive media and dust particles, the frequent action of switching valve will cause corrosion and wear, and then the problems of loose valve sealing and slow action speed will occur, resulting in the instantaneous concentration exceeding the standard of exhaust gas, which greatly affects the purification effect, How to solve the contradiction between flexible rotation and sealing under high temperature is very important. For this reason, all switching valves in this system are imported high-quality pneumatic butterfly valves. The selected switching valves have high precision, small leakage (≤ 1%), long service life (up to 1million times), rapid opening and closing (≤ LS), and reliable operation

3.6 burner

the main purpose of the burner is to ensure that the fuel burns in a low oxygen environment and avoid the formation of local high temperature or insufficient combustion. This requires taking into account the diffusion between the fuel and gas, the mixing with the exhaust gas in the furnace, and the angle and depth of the jet. Then select the most appropriate burner according to the actual process needs, otherwise it will directly affect the incineration effect of RTO. The system adopts American na5424-5 (20 × 104kcal/h) fuel oil proportioning burner is characterized by continuous proportioning (adjustment range 10:1) and high-pressure ignition, which can adapt to a variety of situations

3.7 control system

DCS system is used to automatically control RTO, and computer is equipped to monitor the operating conditions of the whole system in real time. DCS system mainly includes combustion and explosion detection and control system, furnace temperature control system and negative pressure control system

1) combustion and explosion detection and control system: in this system, VOCs combustible gas detector is set on the waste gas main pipe at the corresponding position between the front stage water washing tower and RTO incinerator to measure the concentration of VOCs combustible gas in the waste gas, leaving enough switching time for the valve before RTO to ensure that the concentration of VOCs combustible gas entering

RTO is less than 25% of the lower explosive limit of mixed gas

2) furnace temperature control system: when the furnace temperature exceeds the upper limit temperature of 920 ℃, the system will automatically open the fresh air valve; When the furnace temperature exceeds the upper limit temperature of 970-c, the system will automatically open the overtemperature discharge valve; When the furnace temperature exceeds the upper limit temperature L050 ℃, the system will automatically alarm and shut down, and open the bypass discharge valve at the same time

3) negative pressure control system: in this system, a pressure sensor is set at the corresponding position between the front water washing tower and RTO incinerator, and the negative pressure controls the frequency converter of the forced draft fan to control and adjust the air volume of the forced draft fan; The negative pressure sensor of the furnace controls the frequency converter of the exhaust fan to control and adjust the air volume of the exhaust fan

3.8 prevention and treatment of dioxin

based on the mechanism of dioxin production [1 introduction, this system has done the following work: 1) strengthen the condensation recovery of dichloroethane containing waste gas generated by reactors, vacuum pumps and other equipment; 2) collect the dichloroethane containing waste gas separately and use activated carbon adsorption steam desorption recovery; 3) under the condition of minimizing the dichloroethane containing waste gas entering RTO, reasonably design the size of the regenerator, shorten the extremely cold time of high-temperature waste gas after combustion, and ensure that the waste gas is in the medium temperature zone (300-c-5o0 ℃) residence time is less than 2S, thus reducing the production of dioxin

4 operation effect

rto treatment the VOCs of the pesticide enterprise has been running stably for two years, and the third party is entrusted