vocs waste gas treatment technology

After the paint mist exhaust gas from the spraying station is treated by the dry spraying cabinet, a small part of the solid particles remains. The relatively clean organic waste gas is then adsorbed by the catalytic combustion pretreatment, and then enters the adsorption unit for adsorption and concentration. The unit is heated to desorb the organic waste gas, and the activated carbon of the adsorption unit can thus also be regenerated. The desorbed exhaust gas enters the catalytic oxidation bed for combustion. Due to the action of the catalyst, the combustion process is low-temperature, fast, and flameless, and the exhaust gas is purified. The hot gas generated by combustion can be recycled and reused for desorption and combustion oxidation. The specific flow of the treatment method includes three processes of pretreatment, adsorption, desorption-catalytic combustion, and the process flow is shown in the figure.

Process flow diagram of adsorption concentration + catalytic combustion

1. Preprocessing

During the painting process, a small amount of paint is taken away by the exhaust gas and condensed by the air to form paint mist and dust. These dusts have the characteristics of small particle size and low content in the exhaust gas, and most of them are below 10um. If these exhaust gases directly enter the activated carbon for adsorption, the activated carbon micropores will be blocked, and the activated carbon will eventually be deactivated. Therefore, the exhaust gas must be filtered before it can enter the subsequent adsorption stage.

2. Adsorption device (adsorption unit)

The core of the adsorption unit is activated carbon. The company uses high-quality waterproof activated carbon with an iodine value of 900-1200, thus ensuring the stability of the adsorption unit.

The pretreated organic waste gas is introduced into the adsorption unit under the action of the fan, and it is evenly distributed on the surface of the activated carbon. Relying on the complex internal structure system and super strong surface area of the activated carbon, the activated carbon adsorbs the organic waste gas on its surface. The process takes less time, but the longer the time, the more thorough the adsorption (design wind speed does not exceed 0.8m/s). And there is no chemical reaction between the two, so the organic waste gas can achieve the effect of purification. The purified clean gas can meet the relevant legal standards for the emission of air pollutants. Each exhaust gas purification and treatment system is equipped with multiple sets of adsorption units, one of which is used for desorption and the rest for adsorption. Multiple adsorption units work in turn and are automatically controlled by plc.

3. Desorption-catalytic combustion

The catalytic combustion method uses a catalyst as an intermediate, so that the organic gas becomes harmless water and carbon dioxide gas at a lower temperature, namely:

CnHm+(n+1/4m)O2catalyst 200～300℃nCO2+1/2mH2O

When the activated carbon of the adsorption unit is adsorbed to the saturation level, the adsorption unit is switched to the desorption unit. The desorption requires external heating. The heating device is set in the combustion furnace. After it is turned on, the catalyst is preheated at the same time, and the combustion furnace reaches the set temperature. Then the hot air is introduced into the desorption bed, and the organic waste gas is desorbed from the surface of the activated carbon under the action of heating. Since the temperature will change the internal structure of activated carbon, thermocouple temperature sensors are installed in the adsorption and desorption units. When the temperature is too high, the supplementary cooling air system can be adjusted in time, which can not only ensure the optimal desorption effect, but also provide activated carbon with a safe In the working environment, even if the temperature sensor is abnormal, the adsorption unit is also equipped with physical fire protection facilities.

The high-concentration organic waste gas enters the combustion furnace under the action of the desorption fan. Under the catalytic action of the precious metal platinum alloy, it is burned and decomposed into water and carbon dioxide, and the waste gas is purified. The combustion process is low-temperature, fast, flameless, and accompanied by a large amount of heat, which can be reused in the desorption process and combustion oxidation process of organic waste gas, so the energy consumption cost can be significantly reduced.

When the concentration of organic waste gas is large, the excessive heat generated by combustion will lead to excessively high temperature of the catalytic oxidation bed, which will affect the safety of the entire waste gas treatment system. Therefore, the system is equipped with a cold air supplementary device to introduce fresh air to reduce the reaction temperature and ensure the equipment security.

4. Activated carbon adsorption purification device

The adsorption box is made of carbon steel, the surface is painted, a certain amount of activated carbon is installed inside, and a high-temperature detection device is installed. When the exhaust gas containing organic matter passes through the action of the fan and the activated carbon adsorption layer (tidy stacking), the organic matter is absorbed by the unique effect of activated carbon. The force is trapped in its interior, and the clean gas is discharged; after a period of time, when the activated carbon reaches a saturated state, the adsorption is stopped, and the organic matter has been concentrated in the activated carbon. As shown in Figure 5.4.

1. The shell of the adsorption box is made of Q235 t=2mm steel plate, and the exterior is continuously welded, without bubbles, slag inclusions, etc., and the overall appearance is beautiful;

2. The internal circulation pipeline is made of t=1.2mm galvanized steel plate, connected by folded bayonet, the overall appearance is beautiful, the sealing performance is good, and the flange is connected by bolts;

3. The exhaust gas collection pipe is made of t=1.2mm galvanized steel plate, connected by folded bayonet, the overall appearance is beautiful, the sealing performance is good, and the flange is connected by bolts (between the export of your fan and the processing equipment of our company);

4. The performance parameters of honeycomb activated carbon are shown in Table 5.5.

Table 5.5 Performance parameters of honeycomb activated carbon

Specification

body density

specific surface area

Benzene absorption

Desorption temperature

service life

100*100*100mm

380～450Kg/m³

＞700㎡/g

≥25%

<120℃

≥2 years

5. Catalytic combustion device

The interior of the catalytic combustion furnace is mainly composed of a high-efficiency heat exchanger, a stainless steel hearth, an isolated heater and a catalytic combustion chamber loaded with catalysts. The catalytic combustion device is shown in Figure 5.6.

picture 

Figure 5.6 Catalytic combustion device

If the incineration temperature of organic waste gas exceeds 800°C, the waste gas will be completely decomposed into CO2 and H2O, but at high temperature, the equipment is easily deformed, the heat loss is high, and the equipment loss is large. When the precious metals platinum and palladium are used as catalysts, when the catalysts participate in the reaction, the catalytic oxidation reaction temperature of organic waste gas can be reduced from 800 °C to about 250 °C, and a large amount of heat energy is emitted at the same time. The performance parameters of the ceramic honeycomb supported catalyst are shown in Table 5.7.

Table 5.7 Catalyst performance parameters

Specification

Bulk density

design airspeed

specific surface area

Active temperature

service life

100*100*40mm

600Kg/m³

1.5×104/h

43㎡/g

210℃

≥10000h

First turn on the electric heater of the catalytic combustion bed to heat the air and send it to the activated carbon adsorption bed. The desorbed organic solvent is then heated in the heating chamber, gradually rising to the reaction temperature of 250-300 °C, and then enters the catalytic combustion chamber. Under ideal conditions, the catalytic reaction temperature can reach 500-550 °C, and the hot gas flows through the upper heat exchanger to release the heat to the desorption gas flow that has just entered the catalytic bed, so that it can achieve catalytic At this time, the electric heating device can be turned off to achieve thermal balance and save energy.

The furnace is equipped with a temperature control probe and a special explosion-proof vent on the top. When the temperature in the combustion furnace is too high, the explosion-proof vent can be automatically ruptured, the gas pressure will be released, and the cooling air will be supplemented to maintain the safety of the workshop production equipment.

1. The external frame of the equipment is made of Q235 t=2mm carbon steel plate, and the surface is painted, and the overall appearance is beautiful;

2. The equipment panel is made of Q235 t=1.5mm carbon steel plate, and the surface is beautiful as a whole;

3. The equipment heat preservation adopts 80kg/m³ rock wool heat preservation, the heat preservation thickness is 80mm, and the heat preservation effect is excellent;

4. The catalytic combustion chamber is made of Q235 t=10mm carbon steel plate, with good overall temperature resistance;

5. The heat exchanger is made of Q235 t=1.2mm carbon steel plate, with continuous external welding, good internal sealing performance and high heat exchange efficiency;

6. The equipment connecting air duct is made of Q235 t=1.2mm carbon steel plate, and the exterior is continuously welded, and the overall appearance is beautiful;

6. Main equipment technical parameters

The exhaust gas filtered by the dry filter enters the adsorption unit with honeycomb activated carbon. After a period of use, the activated carbon of one of the adsorption units reaches saturation, the system automatically switches to the standby unit, and the saturated adsorption unit is closed and isolated. , and start the regeneration process of the adsorption unit, desorb the organic waste gas in the activated carbon of the saturated adsorption unit, and burn and convert it into carbon dioxide and water vapor under the action of the catalyst. The activated carbon adsorption unit after regeneration is used as a spare bed, waiting for the next After the adsorption bed is saturated, it is rotated to ensure the continuous operation of the system. The system is equipped with flame arresters, temperature monitors, etc., and the adsorption and catalytic combustion processes of each set are automatically controlled by PLC. The technical parameters of its organic waste gas purification device are as follows.

7. Electrical control

The fully automatic control adopts PLC programming logic control, supporting electric (pneumatic) stepless limit regulating valve, touch screen, etc., as shown in Figure 6.1.

Figure 6.1 Electronic control system

(1) The operation process is fully automatic, which greatly reduces the labor intensity of operators;

(2) To realize automatic, continuous and stable operation of treatment facilities;

(3) The touch screen is used to make the control system have a good man-machine interface and real-time recording and storage functions of important working parameters;

(4) PLC control mode is adopted, which is convenient to adjust the working parameters of the facility;

(5) PLC and touch screen adopt well-known brands at home and abroad, and the system is equipped with a limitless regulating valve with high precision.

Figure 6.2 PLC automatic control flow chart

As shown in Figure 6.2, multiple temperature control points are set up in the system to ensure the operation of the equipment at a safe temperature. For example: T1 is the temperature of the desorption gas entering the adsorption unit, and the upper limit is set to 90°C. When T1 exceeds 90°C, an audible and visual alarm will be given first, the system will automatically stop the desorption fan, the electric heating source will be turned off, and the supplemental cooling fan will run normally. , and open the large heat exhaust valve and supplemental cooling valve. At this time, the system cools down with the maximum amount of cold air. When the temperature of T1 drops to <85°C, the desorption fan will be automatically turned on and the system will resume normal operation.

Features

1. The PLC fully automatic control method is adopted, and the computer touch screen is specially designed for real-time monitoring and recording. The system is equipped with an automatic monitoring and recording reading system. The relevant data of the equipment operation can be obtained at any time with a computer and connecting line, including time, desorption temperature, combustion temperature etc.

2. The complete set of equipment has compact structure, reasonable layout and beautiful appearance.

3. Pay attention to safe use performance, and adopt multiple safety facilities in the design to prevent safety accidents; the heater adopts far-infrared fin-type electric heating tube, which is safe and efficient.

4. Desorption-catalytic combustion system has a sophisticated structure and adopts a hot-air double-cycle heat storage system, which has high thermal efficiency and low energy loss. Very low consumption.

5. Adopt new adsorption material - honeycomb activated carbon, which has good kinetic performance, fast adsorption and desorption rate, complete desorption analysis, slow decay of regeneration adsorption rate and long service life.

6. High catalytic combustion efficiency and thorough purification. Due to the use of a new honeycomb precious metal catalyst, the light-off temperature is low, the combustion is thorough, safe and flameless, and the product is non-toxic and harmless.

7. The air distribution of the adsorption bed is uniform, and the carbon loading, maintenance and replacement are convenient. The air distributor is installed inside, so that the gas in the cross-sectional area of the carbon layer can pass through evenly, without short-circuiting or penetrating the carbon layer, so that the adsorption effect is better.

8. Safety measures

Since the purification system deals with flammable and explosive gases, the following safety measures are taken in this design to ensure the safe operation of the system:

1. Set up high-efficiency filter flame arrester, so that the equipment can play a role of fire-stop while high-efficiency filtration, and the double effect is integrated.

2. The desorption-catalytic combustion system is relatively independent from the adsorption system (the single bed can only be in the adsorption state or desorption state during the operation of the whole system), which completely avoids the need for the adsorption system and the pipe when the desorption-catalytic combustion system works. The impact of the road, thereby preventing the occurrence of the phenomenon of "backflow".

3. The combustion method is catalytic combustion, which is a low temperature flameless combustion, and absolutely no open flame is generated.

4. Strictly control the exhaust gas "VOC" concentration in the system to be lower than 1/4 of the lower explosion limit (about 10g/m3). When the exhaust gas concentration is too high, an alarm will sound and the fresh air valve will be opened (see the desorption process flow chart). Immediately Reduce the concentration to avoid potential safety hazards.

5. Both the catalytic device and the adsorption device are equipped with explosion-proof diaphragms.

6. The purification system is equipped with a bypass valve to ensure the ventilation effect of the workshop and facilitate emergency maintenance; the system is equipped with a manual system for starting debugging and emergency conditions to ensure normal production.

7. Multi-point temperature control points are set in the equipment, and an automatic alarm system is set at the same time.

8. The equipment is equipped with a safety fire damper. When the temperature is too high during the operation of the equipment, other air valves except the straight exhaust valve are closed to cut off the passage between the equipment and the workshop. The fan stops running and immediately fills in inert gas to prevent accidents.

9. Under the condition of high temperature in summer, ventilate and cool down and fill in inert gas to avoid the conditions of natural oxidation reaction of activated carbon.

10. The activated carbon is desorbed and regenerated by using heat exchange fresh hot air to enter the adsorption box evenly;

11. The equipment and air ducts of the whole system are well grounded to eliminate static electricity, and the lightning protection system is installed according to the relevant specifications.

12. The preheating tube adopts far infrared heating tube.

13. Both catalytic and adsorption devices have temperature alarm systems, and are equipped with bypass fresh air ducts to introduce air during "flying temperature".

14. Use refractory fiber material with good thermal insulation performance and light weight for thermal insulation