Laboratory fume hood exhaust gas treatment method

The waste gas treatment of laboratory fume hood includes destructive and non-destructive methods, and the combination of these two methods Destructive methods include combustion, biological oxidation, thermal oxidation, photocatalytic oxidation, low-temperature plasma and its integrated technology, mainly including chemical or biochemical reactions, conversion of VOCs into non-toxic inorganic small molecular compounds such as CO2 and H2O by light, heat, microorganism and catalyst The non-destructive method, i.e recovery method, is mainly carbon adsorption, absorption, condensation and membrane separation technology Through physical methods, control temperature and pressure or use selective osmosis membrane and selective adsorbent to enrich and separate volatile organic compounds In recent years, the technology of semiconductor photocatalyst, low temperature plasma, has developed rapidly Adsorption process (1) introduction of adsorption process adsorption method is mainly applicable to the purification of low concentration gaseous pollutants For high concentration organic gases, it is usually necessary to reduce the concentration through condensation and other processes before adsorption purification Adsorption technology is a classic and commonly used gas purification technology, and is also one of the mainstream technologies of industrial VOCs treatment The key technologies of adsorption are adsorbent, adsorption equipment and process, regeneration medium, post-treatment process, etc Activated carbon is widely used in adsorption and recovery of organic gases because of its large specific surface area and microporous structure At present, the researches on the adsorption of organic gases by activated carbon mainly focus on the prediction of adsorption equilibrium, the modification of activated carbon materials and the influence of the physical and chemical properties of organic matters on the adsorption performance of activated carbon (2) the principle and process of activated carbon adsorption process There is an activated carbon treatment method for the waste gas treatment of the laboratory ventilation cabinet The adsorption of organic waste gas by activated carbon fiber is one of the advanced technologies in the world Compared with granular activated carbon, activated carbon fiber has larger adsorption capacity and faster adsorption kinetics performance See Figure 1 for the process flow of activated carbon adsorption and desorption (3) factors affecting the adsorption process of activated carbon molecular structure diagram of activated carbon (4) physical adsorption of purified air of activated carbon, as shown in Figure 2, there are four situations: molecular diameter is larger than the diameter of the hole, because of the steric hindrance, molecules cannot enter the hole, so they are not adsorbed The molecular diameter is equal to the diameter of the pore, and the adsorbent has a strong capture force, which is very suitable for low concentration adsorption; the molecular diameter is smaller than the diameter of the pore, and capillary condensation occurs in the pore, so the adsorption capacity is large; the molecular diameter is far smaller than the diameter of the pore, so the adsorption molecules are easy to desorb, with high desorption rate and small adsorption capacity at low concentration (5) advantages and disadvantages of activated carbon adsorption process: it is suitable for all kinds of pollutants with low concentration; the price of activated carbon is not high, the energy consumption is low, and the application is relatively economic; the solvent organics can be recovered through desorption condensation; the application is convenient, and it can play a role only in contact with the air Activated carbon has good acid and alkali resistance, heat resistance and high chemical stability Disadvantages: the adsorption capacity is small and the physical adsorption is saturated With the consumption of adsorbent, the adsorption capacity becomes weak After a period of use, the adsorption capacity may be small or lose the adsorption function During adsorption, there is the problem of adsorption specificity For the mixed gas, the adsorption may be weakened, and there is also a mismatch between the molecular diameter and the pore diameter of the activated carbon, resulting in the phenomenon of desorption; 2 Absorption process (1) introduction to absorption process: use solution, solvent or clear water to absorb the volatile gas in the industrial waste gas, so as to separate it from the waste gas is called absorption method Solution, solvent and clear water are called absorbents Different absorbents can absorb different harmful gases Absorption equipment used in absorption method is called absorber, purifier or scrubber The absorption process is similar to the wet dedusting process, except that the wet dedusting process uses clear water, while the absorption process uses solvents or solutions to purify harmful gases (2) absorption process principle and process There is a method of container absorption for the treatment of exhaust gas from the laboratory fume hood Taking the recovery of oil and natural gas as an example, the recovery of oil and natural gas should include the oil and gas from the loading and unloading of oil refineries, chemical plants and oil and natural gas stations Oil and gas is a complete system from the factory to the sales terminal In the United States and European countries, oil and gas recovery measures are usually adopted in the gas stations in the first and second stages, i.e closed oil unloading and refueling, oil and gas in the tank are returned to the tank car, and vacuum auxiliary devices or internal pressure in the tank are used to return to the tank when refueling Oil and gas recovery units shall be set up in oil depots, refineries and other distribution places of petroleum products to recover oil and gas Absorption is usually used for oil and gas recovery When loading and unloading oil products, the oil gas enters into the absorption tower and exhausts the lean oil air from the outlet The vacuum desorption of the absorption liquid is carried out in the desorption tower, and the desorption absorption liquid is recycled The gasoline in the recovery tower will be used to recycle the entered desorption gas, and the tail gas will return to the absorption tower to repeat the process The absorption solution of VOC recovery by solution absorption method is usually a special absorption solution, and the selection of absorption solution will affect the recovery effect (3) advantages and disadvantages of the absorption process: the absorption process is relatively simple, the equipment investment is low, and the operation and maintenance costs are basically equivalent to that of the carbon absorption process Because the absorption medium is kerosene and absorption liquid, there is no secondary pollution Disadvantages: the recovery efficiency of this process is low When the environmental protection requirements are high, it is difficult to meet the allowable oil and gas emission standards; the equipment occupies a large space; the energy consumption is high; the absorbent consumption is large, which needs to be supplemented continuously 3 Condensation process (1) brief introduction of condensation process: during the storage, transportation and sales of oil products, some light hydrocarbon components volatilize into the atmosphere, causing resource waste and environmental harm At the same time, organic solvents are widely used in industrial production A large number of organic solvents are volatilized into the air every year, endangering human health and causing serious environmental pollution Adopting appropriate methods to recover these VOCs can not only reduce the production cost of enterprises, but also have great environmental benefits Condensation is an effective method to recover VOCs Its basic principle is to use gaseous pollutants with different saturated vapor pressures at different temperatures and pressures By reducing the temperature and increasing the pressure, some organic substances will condense out, so that VOCs can be purified and recovered (2) principle and process of condensation process; process diagram of waste gas treatment and absorption of ventilation cabinet in laboratory; multi-stage cascade or self cascade refrigeration technology is adopted for condensing oil and gas recovery equipment Although the system process is relatively complex, the key components of compressor and throttling mechanism have all realized localized production with low investment and operation cost According to the working principle of heat exchange tube, it can be divided into refrigerant circuit and gas circuit, and the heat exchange tube is connected with two parts In the gas circulation part, the low-temperature refrigerant exchanges heat with the hot organic solvent mixture in the heat exchanger, and the organic solvent is recovered after liquefaction, and the refrigerant flows into the liquid storage tank In the refrigerant circuit, the compressor compresses the refrigerant into high-temperature and high-pressure gaseous refrigerant, liquefies it through the air-cooled condenser, exchanges heat with the refrigerant through the drying filter in the refrigerant refrigerant refrigerant heat exchanger, the low-temperature refrigerant enters the liquid storage tank, and the refrigerant enters the compressor inlet through the suction filter, completing the whole refrigerant refrigerant refrigerant heat exchange process (3) the influence factor of condensation process is that the raw gas should be cooled and cooled to recover light hydrocarbon by condensation separation method According to the principle, it can be divided into throttling expansion refrigeration and expander expansion refrigeration According to the process, it can be divided into refrigerant refrigeration (such as propane refrigeration), throttling expansion refrigeration, expander expansion refrigeration, mixed refrigeration (on the basis of expander expansion refrigeration or process fluid throttling expansion refrigeration plus refrigerant refrigeration) The separation method includes distillation separation of distillation system and phase equilibrium separation of separator This process generally includes dehydration, pressurization (low pressure gas), distillation and refrigeration The selection of each part of the above condensation process will affect the final condensation effect (4) advantages and disadvantages of condensation process: condensation process is to use different boiling points of substances for recovery, which is suitable for organic substances with higher boiling points This method has the advantages of high purity recovery, simple equipment process and low energy consumption It also has the advantages of compact equipment, small occupation space, high automation, convenient maintenance, good safety and direct use of liquid oil output Disadvantages: the single condensation method needs to be lowered to a very low temperature to reach the standard, and the power consumption is huge, which is not really "energy saving and emission reduction" 4 Membrane separation process (1) brief introduction of membrane separation process: in the process of oil exploitation, storage and transportation, some oil volatilizes to the oil and gas formed in the atmosphere, except for the air, mainly C4-C5 and a small amount of aromatic hydrocarbon These organic vapor emissions not only cause serious waste of resources, but also have a great impact on air quality, and then affect human health At present, the separation and recovery methods of organic vapor are mainly condensation, activated carbon adsorption, membrane separation, solvent absorption Membrane separation technology is a high efficiency separation method (2) the principle and process of membrane separation There is a method of chemical separation in the exhaust gas treatment of the ventilation cabinet in the laboratory The organic vapor recovery system of membrane separation realizes the separation through the dissolution diffusion mechanism After contacting with the membrane, the gas molecules will dissolve on the surface of the membrane, and then a concentration gradient will be generated on both sides of the membrane Because the dissolution and diffusion speed of different gas molecules through the dense membrane are different, the gas molecules will diffuse from the membrane to the other side of the membrane, * and then desorb from the other side of the membrane, * finally achieving the purpose of separation The membrane separation device is located behind the high-pressure condenser and in front of the buffer tank Due to the insufficient capacity of the discharge gas compressor, only a part of the gas passes through the membrane separation device, and the other part directly enters the buffer tank The permeate gas returns to the front of the low-pressure cooler, and the tail gas enters the buffer tank (3) influencing factors of membrane separation process: the material of support layer has an important influence on the permeability rate and hydrocarbon VOCs recovery rate For the support layer of the same material, the permeability rate and hydrocarbon VOCs recovery rate increase with the decrease of pore diameter, but when the pore diameter decreases to a certain critical value, the permeability rate and hydrocarbon VOCs recovery rate will decrease with the decrease of pore diameter (4) advantages and disadvantages of membrane separation technology: membrane separation technology is the frontier technology of separation science in modern petrochemical discipline It has the characteristics of small investment, quick effect, simple process, high recovery rate, low energy consumption and no secondary pollution It has high scientific and technological content Disadvantages: large investment; low domestic production rate of membrane, high price, and short membrane life Membrane separation device requires stable flow, stable pressure gas and high operation requirements 5 Combustion process (1) brief introduction of combustion process a kind of VOCs treatment method is the so-called destructive technology, that is, through chemical or biological technology, VOCs is converted into carbon dioxide, water, hydrogen chloride and other non-toxic or less toxic inorganic substances Combustion is such a technology Combustion method includes direct combustion method and catalytic combustion method The direct combustion method is suitable for the treatment of high concentration VOCs waste gas, because its operating temperature is usually 800-1200 ℃, the process energy consumption cost is high, and dioxins are easy to appear in the combustion tail gas