Benzene series compounds are among the most common pollutants in industrial exhaust gases, and they primarily include benzene, toluene, xylene, ethylbenzene, styrene, and others. These substances are characterized by high volatility, high toxicity, and a significant carcinogenic risk, making them among the VOCs subject to key national control measures. Benzene series waste gas is widely produced in chemical, pharmaceutical, coating, printing, rubber, shoemaking and other industries. Improper handling not only faces severe environmental penalties, but also poses a serious threat to the health of operators and surrounding residents. So how to deal with the benzene series waste gas thoroughly and economically? The membrane condensation recovery scheme provides an efficient and reliable answer.
The membrane condensation recovery scheme was optimized for the physical and chemical properties of the benzene series. The boiling point of benzene series is high, benzene is 80 degrees Celsius, toluene is 110 degrees Celsius, xylene is 138 to 144 degrees Celsius, this high boiling point characteristics make condensation recovery has a natural advantage. At relatively low condensation temperatures, the benzene series can be efficiently converted from a gaseous state to a liquid state. Cooperationgas separation membraneThe overall recovery rate can reach more than 98%, and the export concentration is far lower than the national emission standard.
Compared with the traditionalactivated charcoalAdsorption orRTOCompared with combustion, the membrane condensation recovery scheme has significant advantages. Activated carbon adsorption of benzene series has the problems of limited adsorption capacity, frequent replacement, waste activated carbon belongs to dangerous waste and so on. Although RTO combustion has high processing efficiency, it consumes huge energy and burns valuable benzene series in vain. Membrane condensation recovery directly liquefies the benzene series in the exhaust gas into liquid recovery, and the recovered benzene series can be reused for production or external sales, resulting in direct economic benefits. Taking the treatment of waste gas containing toluene as an example, the recovered toluene is calculated at a market price of 5,000 to 7,000 yuan per ton, and the equipment investment can usually be recovered within one to two years, and then a continuous net income will be generated every year.
Product details
Benzene series compoundswaste gas treatmentThe special membrane condensation recovery equipment is specially designed for the characteristics of benzene series in product details. Benzene has a swelling effect on ordinary rubber and plastics, so all sealing materials in contact with benzene in the equipment are selected from fluorine rubber or polytetrafluoroethylene, and avoid the use of nitrile rubber, ethylene propylene diene rubber and other non-resistant materials. Pipes and valves are made of stainless steel, and oil-resistant and benzene-resistant gaskets are used for flange connections. The recovery solvent storage tank is made of stainless steel, equipped with liquid level gauge and sealing cover to prevent volatilization of benzene series.
The condensing unit adopts a three-stage heat exchange structure, and the temperature gradient is optimized for the boiling point range of the benzene series. The first-stage condensation is cooled by circulating water, and the temperature is set to ten to fifteen degrees Celsius. It mainly removes water vapor and a small amount of high-boiling impurities carried in the exhaust gas. The secondary condensation uses chilled water or refrigeration unit, the temperature is set to zero to five degrees Celsius, and 50% to 70 benzene series can be recovered. The three-stage condensation adopts direct evaporative refrigeration, the temperature is set to minus ten to minus twenty degrees Celsius, and more than 95% of the benzene series can be recovered. For substances with higher boiling points such as styrene, the condensation temperature can be appropriately increased; for substances with relatively low boiling points such as benzene, the tertiary condensation temperature needs to be as low as minus fifteen degrees Celsius.
The membrane separation unit selects a special composite membrane with high selectivity to aromatic hydrocarbons. The membrane material was modified on the basis of the common VOCs recovery membrane, and the benzene ring affinity group was introduced to enhance the adsorption and permeability of benzene, toluene, xylene and other aromatic hydrocarbons. The membrane module adopts spiral winding structure, with a single membrane area of ten to fifty square meters. The vacuum pump maintains an absolute pressure of five to twenty kPa on the permeate side of the membrane, and the benzene series that permeates through the membrane are continuously pumped and recycled back to the inlet of the condensation unit. This cycle design allows the benzene series in the tail gas to be continuously enriched and recovered, and the final emission concentration can be reduced to less than ten milligrams per cubic meter, which is far below the national limit.
Selection Guide
Benzene series compoundsWaste gas treatment equipmentThe selection needs to be based on accurate working condition parameters. The customer needs to provide the specific composition of the benzene series in the exhaust gas and the concentration of each component, total air volume, exhaust gas temperature, relative humidity, whether it contains particulate matter, emission standard requirements, etc. The boiling points of different components of benzene series are quite different, and the condensation temperature needs to be determined according to the main components when selecting the type.
For benzene-based waste gas, the boiling point of benzene is 80 degrees Celsius, and the saturated vapor pressure is high. It is necessary to reduce the three-stage condensation temperature to minus fifteen to minus twenty degrees Celsius to achieve a recovery rate of more than 98%. For toluene-based exhaust gas, the boiling point of toluene is 110 degrees Celsius, and the tertiary condensation temperature is set to minus 10 to minus 15 degrees Celsius. For the exhaust gas dominated by xylene, the boiling point of xylene is higher, and the tertiary condensation temperature is set to minus five to minus ten degrees Celsius to achieve a good recovery effect. For the mixed benzene series exhaust gas, the condensation temperature is set based on the component with the lowest boiling point.
The air volume determines the specifications of the equipment. For small scenes with an air volume of 500 to 5,000 cubic meters per hour, a skid-mounted all-in-one machine is adopted, which covers a small area and is convenient to install. For medium-sized scenes with an air volume of 5,000 to 20,000 cubic meters per hour, modular design is adopted, and the condensation unit and membrane separation unit can be arranged separately. For large-scale scenes with an air volume of more than 20,000 cubic meters per hour, a combination scheme of zeolite runner concentration and membrane condensation can be adopted to concentrate first and then recover, thus reducing equipment investment and operating costs.
If the exhaust gas contains particulate matter, it is necessary to install a high-efficiency filter before the inlet of the equipment to prevent the particulate matter from blocking the heat exchanger and the membrane module. If the exhaust gas contains high boiling point substances such as resin monomer, it is necessary to increase the cooler in the pretreatment stage to condense and remove these substances first to prevent them from condensing on the surface of the heat exchanger to form dirt that is difficult to remove. LuoyangWohong PetrochemicalEquipment Co., Ltd. provides free working condition analysis and selection services. Customers only need to provide exhaust gas parameters to obtain targeted program design and investment recovery period calculation.
Customization Case
A chemical enterprise produces benzene series intermediates. The reaction kettle and drying section produce mixed waste gas containing toluene and xylene. The air volume is 3,000 cubic meters per hour, the inlet toluene concentration is 15 grams per cubic meter, and the xylene concentration is 5 grams per cubic meter, totaling 20 grams per cubic meter. The enterprise originally adopted activated carbon adsorption treatment, replacing about 1.5 tons of activated carbon per month, purchasing about 10,000 yuan per month, and disposing of hazardous waste of waste activated carbon about 4,500 yuan per month, totaling 14,005 yuan per month. At the same time, the adsorption efficiency of activated carbon is unstable, and the outlet concentration is high and low, and there is a risk of exceeding the standard.
The enterprise has introduced the membrane condensation recovery equipment of Luoyang Wo Hong Petrochemical Equipment Co., Ltd., which adopts the combined process of three-stage condensation and gas separation membrane. The three-stage condensation temperature is set to ten degrees Celsius, zero degrees Celsius, and minus twelve degrees Celsius, respectively. After the equipment was put into operation, the recovery rate of toluene and xylene in the tertiary condensation reached 96%, and the outlet concentration dropped to 0.8 grams per cubic meter. The tail gas enters the membrane separation unit for further treatment, and the selectivity of the membrane module to aromatic hydrocarbons reaches more than 40. The benzene series in the tail gas is enriched and recycled to the condensation inlet, and the final overall recovery rate reaches 98.5%. The concentration of total benzene series exported dropped to fifteen milligrams per cubic meter, far below the national emission standards.
The equipment recovers about 50 kilograms of mixed benzene series every day. Based on the calculation of 5 yuan per kilogram of mixed solvent, the daily value is 250 yuan, 7,500 yuan per month, and 90,000 yuan per year. The electricity charge for equipment operation is about 2,000 yuan per month and 24,000 yuan per year. After deducting electricity charges, the annual net income is 66,000 yuan. At the same time, companies no longer need to replace activated carbon and deal with hazardous waste every month, saving about 174,000 yuan in activated carbon-related costs each year. Taken together, the total value saved and created for the enterprise each year is about $240,000. The equipment investment is about 350,000 yuan and the payback period is about 18 months. The person in charge of the enterprise said that the recovered toluene and xylene mixture was directly used for reactor cleaning, further reducing the solvent procurement cost.
Another case comes from a painting company. The spray booth produces exhaust gas containing xylene, with an air volume of 15,000 cubic meters per hour and an inlet concentration of two grams per cubic meter. The enterprise originally used RTO incineration, with an annual gas cost of 600,000 yuan and an electricity cost of 150,000 yuan. After switching to the zeolite runner and membrane condensation combination scheme, the concentration ratio is 15 times, the concentration after concentration is 30 grams per cubic meter, the membrane condensation recovery rate is 98%, and about 40 tons of xylene are recovered every year, with a recovery value of 240,000 yuan calculated at 6,000 yuan per ton. The electricity charge for equipment operation is about 200,000 yuan per year, and the comprehensive calculation saves about 790,000 yuan per year, with a payback period of two years.
Operation procedure
The operation process of benzene series waste gas treatment equipment is similar to other membrane condensation equipment, but due to the toxicity and flammability of benzene series, more attention should be paid to safety protection in operation. Equipment adopts PLC automatic control, normal operation without manual intervention.
Perform a pre-startup inspection. The operator must wear a gas mask or organic vapor filter box, because the benzene series is harmful to human body. Check whether the power supply, cooling water, and compressed air are functioning properly. Check the liquid level in the solvent recovery tank and confirm that there is sufficient capacity remaining. Check that all manual valves are in the correct position. Check whether the combustible gas detector works normally. The lower explosive limit of benzene series is low, benzene is 1.2%, toluene is 1.1%, and xylene is 1.0%. Therefore, the alarm value should be set at 10% of the lower explosive limit, which is more stringent than general solvents. After confirming that everything is correct, close the main power supply.
Start running. Set the process parameters on the touch screen. For the mixed benzene series exhaust gas, the three-stage condensation temperature is recommended to be set to ten degrees Celsius, zero degrees Celsius, and minus twelve degrees Celsius. If benzene is dominant, the tertiary temperature should be set to minus eighteen degrees Celsius. After the parameter setting is completed, start the refrigeration compressor, vacuum pump and circulating fan in turn. The system automatically cools down to the working point, and the cooling process takes about 20 to 30 minutes. After the equipment is started, the operator should stay away from the equipment area to reduce the risk of contact with benzene series.
Normal operation monitoring. The operator checks the operating parameters through the touch screen every two hours, including condensation temperature, membrane pressure difference, recovery liquid level, etc. Under normal circumstances, the condensation temperature at all levels should be stable within the set value of plus or minus one degree, and the membrane pressure difference should be less than 30 kPa. If the tertiary condensation temperature is found to be too high, it may be a refrigeration system failure or insufficient refrigerant; if the membrane pressure difference increases, it may be the surface pollution of the membrane module, and cleaning needs to be arranged. When the liquid level of the recovered solvent storage tank reaches the high level, the system will automatically alarm. After the operator wears protective equipment, open the drain valve and transfer the recovered benzene series to the special container.
Shutdown operation. Press the stop button, the system automatically executes the shutdown procedure. Stop the fan first, then stop the vacuum pump, the refrigeration system continues to run for ten to fifteen minutes after the automatic stop. After the shutdown, the operator should check whether the recovery solvent storage tank has been emptied to prevent the benzene series from volatilizing to the environment. If the machine is shut down for a long time, it is recommended to purge the inside of the system with nitrogen and keep it dry. All tools and containers that have been exposed to benzene series shall be disposed of in accordance with hazardous waste management requirements.
benefit analysis
Benzene exhaust gas treatment using membrane condensation recovery scheme, its economic and environmental benefits are very significant. The following analysis is based on the three dimensions of recovery revenue, operating costs, and environmental compliance.
In terms of recovery income, the market value of benzene series is higher. The market price of benzene is about 5,000 to 6,000 yuan per ton, toluene is about 6,000 to 7,000 yuan per ton, xylene is about 7,000 to 8,000 yuan per ton, and mixed benzene series solvents are about 4,000 to 6,000 yuan per ton. Taking the waste gas with an air volume of 5,000 cubic meters per hour and an inlet toluene concentration of 10 grams per cubic meter as an example, the annual operation is 8,000 hours, and about 360 tons of toluene can be recovered each year, which is calculated at 6,000 yuan per ton. The annual recovery value is 2.16 million yuan. Even after deducting equipment depreciation and operating expenses, the net income is still very substantial. For lower concentrations of exhaust gas, the recovery benefits will be correspondingly reduced, but most of the operating costs can still be offset.
In terms of operating costs, the main consumption of membrane condensation equipment is electricity. Under the same air volume, the annual electricity charge of membrane condensation equipment is usually 30% to 50 of the sum of RTO gas charge and electricity charge. Taking the equipment with air volume of 10,000 cubic meters per hour as an example, the annual electricity charge for membrane condensation is about 200,000 to 300,000 yuan, while the annual gas charge and electricity charge for RTO are about 800,000 to 1.2 million yuan. In addition, the membrane condensation equipment does not have the replacement cost of consumables like the activated carbon adsorption method, and the annual maintenance cost is only one to twenty thousand yuan.
In terms of environmental compliance, membrane condensation equipment can stably control the emission concentration of benzene series below 10 to 20 milligrams per cubic meter, which is far lower than the limit of benzene series in the national comprehensive emission standard of air pollutants. The emission limit is twelve milligrams per cubic meter for benzene, forty milligrams per cubic meter for toluene and seventy milligrams per cubic meter for xylene. Membrane condensation equipment is fully capable of meeting the most stringent standards. At the same time, the equipment does not produce secondary pollutants such as nitrogen oxides and dioxins, nor does it produce hazardous wastes such as waste activated carbon, so the environmental benefits are very prominent.
On the whole, the membrane condensation recovery scheme is one of the most economical and environmentally friendly choices for the treatment of waste gas containing recoverable value benzene series. The payback period is usually between one and two years, after which it produces a continuous net income each year. For areas with strict environmental protection requirements, the membrane condensation recovery scheme is also a reliable choice to ensure stable discharge standards.
Security features
Benzene exhaust gas treatment equipment has higher requirements in safety design than ordinary VOCs equipment, because benzene series is toxic, flammable and carcinogenic. The equipment integrates multiple safety protection measures to ensure the safety of operators and the environment.
Explosion-proof safety is the primary design principle. All electrical components, including compressor motor, fan motor, vacuum pump motor, relays and contactors in the control cabinet, are explosion-proof products, explosion-proof grade is not less than ExdIIBT4. The control cabinet adopts positive pressure ventilation type structure, and maintains micro positive pressure inside to prevent combustible gas from entering. All cable entry devices adopt explosion-proof glands to ensure reliable sealing. The equipment shell and all metal parts are reliably grounded to prevent static electricity accumulation. Install jumpers between pipe flanges to ensure electrical communication throughout the system.
Toxic gas monitoring is an important part of active safety protection. The equipment inlet and outlet pipes are equipped with special gas detectors for benzene series, which adopt PID light ion detection principle and have high sensitivity and selectivity for benzene series. The range of the detector is zero to five hundred milligrams per cubic meter, and the alarm value can be set to fifty milligrams per cubic meter or less. When it is detected that the concentration exceeds the alarm value, the control system immediately sends out an audible and visual alarm, and automatically increases the air volume of the fume hood or exhaust system, and quickly discharges the leaked gas to the outside. In the area around the equipment, it is recommended to install a fixed benzene detector to monitor the environmental concentration and ensure the safety of operators.
Spill protection is equally important. The recycled solvent storage tank adopts a double-layer structure, the inner layer is stainless steel, the outer layer is carbon steel protective layer, and the middle is filled with thermal insulation material. The top of the storage tank is equipped with a breather valve and a nitrogen seal interface. During normal operation, a small amount of nitrogen is introduced to maintain a slight positive pressure to prevent the volatilization of benzene series. The bottom of the storage tank is provided with a leakproof cofferdam. Once leakage occurs, the leakage liquid is confined in the cofferdam and will not spread. Anti-seepage treatment shall be conducted on the ground of equipment area, and emergency materials for leakage such as absorbent cotton and sand shall be provided.
Operator safety is the last line of defense. Operation and maintenance personnel must be specially trained to understand the hazard characteristics and protective measures of benzene series. Gas masks, protective glasses, chemical protection gloves and protective clothing must be provided in the operation area. The equipment area shall be provided with eyewash and emergency shower device, which can be washed immediately in case of skin or eye contact. Luoyang Wo Hong provides detailed safety operation procedures and emergency response plans when the equipment is delivered, and trains operators to ensure the safe operation of the equipment.
FAQ
What emission standards can benzene exhaust gas treatment meet?
The membrane condensation recovery equipment can stably control the emission concentration of benzene series at a low level. Taking benzene as an example, after three-stage condensation and membrane separation treatment, the export concentration can usually be reduced to 5 to 10 mg per cubic meter, which is far lower than the 12 mg limit of benzene per cubic meter in the GB16297 of the National Comprehensive Emission Standard of Air Pollutants. The toluene outlet concentration can be reduced to ten to twenty milligrams per cubic meter, below the limit of forty milligrams per cubic meter. The xylene outlet concentration can be reduced to ten to twenty milligrams per cubic meter, below the limit of seventy milligrams per cubic meter. For more stringent local standards, such as Beijing, Shanghai and other places require benzene to be less than one milligram per cubic meter, which can be achieved by increasing the number of condensation stages or using two-stage membrane separation.
ask benzene can be used directly after recycling?
Answer: Yes. The purity of the recovered benzene series is usually 90% to ninety-five, and the main impurities are water and a small amount of air dissolved. For most industrial applications, such as cleaning agents, diluents, and reaction solvents, this purity can be used directly. If higher purity is desired, it can be processed by simple distillation or drying. Taking the recovery of toluene as an example, the water content of toluene obtained through three-stage condensation is usually less than 0.5%, which can be directly reused for coating dilution or reaction medium. If used for moisture-sensitive reactions, it can be dried with molecular sieves before use. The use of recycled benzene series reduces the purchase of fresh solvents and reduces the amount of hazardous waste, which has double economic benefits.
ask benzene series of waste gas containing resin monomer how to do?
A benzene series exhaust gas often contains a small amount of resin monomer, plasticizer and other high boiling point substances, these substances in the condensation process will be preferred condensation, attached to the surface of the heat exchanger to form difficult to remove dirt. In view of this situation, it is recommended to add a pre-condenser before the exhaust gas enters the main condensing unit, and the temperature is set at 20 to 30 degrees Celsius to specifically remove high boiling point substances. The pre-condenser adopts a detachable structure for regular cleaning. The pre-treated exhaust gas enters the three-stage main condensation unit, which can significantly extend the equipment cleaning cycle and the life of the membrane module. Luoyang Wo Hong can design a special pretreatment scheme according to the composition of the exhaust gas.
How long is the payback period of the equipment?
A The payback period depends on the exhaust gas concentration, air volume and the market value of the benzene series. For scenarios with high concentration, large air volume and high value of benzene series, such as waste gas from chemical plant reactor and paint shop, the payback period is usually between six months and one year. For scenarios with medium concentration and medium air volume, such as printing and pharmaceutical industries, the payback period is between one and two years. For lower concentration scenarios, the payback period may exceed two years, but it is still economical because the operating cost of membrane condensation is lower than that of activated carbon adsorption and RTO. Luoyang Wo Hong will provide detailed investment payback period calculations during the design phase to help customers make decisions.
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