Treatment of organic waste gases generated in chemical process production

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Treatment of organic waste gases Organic waste gases are one of the main air pollutants; they are harmful to human health, and the common symptoms they cause include fatigue, dizziness and headaches, nausea and vomiting, palpitations and shortness of breath, as well as changes in blood counts. Substances such as aldehydes and amines also have a foul smell. Organic waste gases can also cause photochemical smog, leading to secondary pollution. The fundamental approach to treating organic waste gases is to use pollution-free processes, minimize the use of toxic materials, and control the emission of toxic waste gases. 1 Treatment of pollutants in organic vapor emissions There are mainly two types of methods for treating organic vapor pollutants: one is the recovery method, and the other is the elimination method. The main recovery methods include carbon adsorption, pressure swing adsorption, absorption, condensation, and membrane separation technology ； The general recovery method involves physical techniques such as changing temperature and pressure, or using selective adsorbents and selective permeable membranes to concentrate and separate organic vapor pollutants. The elimination methods include direct combustion, thermal oxidation, catalytic combustion, biological oxidation, plasma treatment, ultraviolet photocatalytic oxidation, and their integrated technologies ； The elimination method primarily involves using chemical or biochemical reactions, along with heat, light, catalysts, and microorganisms, to convert organic vapor pollutants into non-toxic inorganic small molecule compounds such as CO2 and water. Based on these principles, traditional methods for treating organic vapor pollutants include adsorption or absorption, as well as combustion. In recent years, biooxidation and semiconductor photocatalyst technologies have seen rapid development. 1.1 Traditional treatment technologies for organic vapor pollutants 　　1.1.1 Adsorption or absorption methods 　　The adsorption method involves using substances with adsorption capabilities, such as activated carbon, silica gel, zeolite molecular sieves, and activated alumina, to absorb harmful components thereby eliminating these toxic pollutants. The adsorption method is suitable for almost all gaseous pollutants, generally those with moderate to low concentrations ； The adsorption efficiency depends on factors such as the properties of the adsorbent, the types of gaseous pollutants, as well as the operating temperature, humidity, and pressure of the adsorption system. It offers the advantage of high removal efficiency, which makes it a commonly used method for removing gaseous pollutants; however, it has drawbacks such as high operating costs after initial investment and the potential for generating secondary pollution. Adsorption devices include fixed-bed and fluidized-bed types, among others. Fixed-bed operation is simple, has low costs, and is widely used. In a fixed-bed system, the empty-bed filtration velocity is controlled at 0.2–0.5 meters per second, the contact time is at least 0.5 seconds, and the resistance is around 100 millimeters of water column. Fluidized beds can be automatically controlled and operated continuously, but they are costly and are thus used less frequently at present. 　　The absorption method is a gas-phase pollutant control technique that involves using low-volatility or non-volatile solvents to absorb gas-phase pollutants, and then separating them based on the differences in the physical properties of organic molecules and the absorbents. This method is applicable to the treatment of gaseous pollutants under conditions of high concentration, low temperature, and high pressure. Luo Jiaosheng chose “water-washed oil” as an absorbent to treat benzene-containing waste gas. The absorption mechanism of this “water-washed oil” absorbent is as follows: Generally, organic compounds are substances with weak polarity or no polarity at all; therefore, they are insoluble or barely soluble in water, but they can dissolve in certain organic solvents ； Benzene derivatives are non-polar substances; therefore, they are insoluble in water but soluble in non-polar mineral oils such as washing oils ； According to his experiments, the ratio of water to cleaning oil, pH value, and the type of benzene derivatives all have an impact on the absorption efficiency, and the maximum absorption capacity of this combination of absorbents was determined. However, there are also issues of complex post-treatment processes and secondary pollution. 　　1.1.2 Combustion method The combustion method is a treatment technique that takes advantage of the flammability of organic vapor pollutants. Among them, the direct combustion method, also known as the flame combustion method, is a technique in which combustible organic vapor pollutants are used as fuel for combustion. This method is suitable for treating high-concentration organic vapor pollutants; with a combustion temperature maintained above 1100°C, the removal efficiency exceeds 95%. 　　Catalytic combustion is a method similar to thermal oxidation for treating organic vapor pollutants; it uses precious metal catalysts such as platinum and palladium, as well as transition metal oxide catalysts, to replace flames in order to purify organic substances. The operating temperature is half that of thermal oxidation, typically ranging from 250°C to 500°C. Due to the lower temperature, it is possible to use standard materials in place of expensive special materials, **thereby reducing equipment costs and operating expenses. In fact, catalytic combustion is an oxidation reaction of fuel under near-stoichiometric conditions, releasing a large amount of heat. Therefore, the primary requirement for a catalyst to be used in catalytic combustion is that it must exhibit high activity under high-temperature oxidation reactions; in addition, it needs to have high thermal stability, high mechanical strength, and high resistance to corrosion caused by the toxins present in the fuel. In the catalytic combustion method, organic waste gases are oxidized into carbon dioxide and water at temperatures of 200–400°C, with the help of a catalyst; simultaneously, heat released from the combustion is generated. The catalytic combustion method is mainly used for the purification of organic waste gases with high temperature, high concentration, and continuous emission. Direct combustion method: Organic waste gases are generally combustible gases; under conditions of a temperature of 600–800°C and a residence time of 0.3–0.5 seconds, they can be completely burned to form carbon dioxide and water. The direct combustion method is suitable for the purification of organic waste gases at high temperatures and high concentrations, and it allows for heat recovery. The direct combustion method consumes a lot of energy; heavy oil and natural gas are suitable as preheating fuels. During preheating, the temperature inside the combustion chamber should be uniform, so that all exhaust gases are heated to the desired temperature; otherwise, foul-smelling intermediate products will be formed. If there is a boiler with a matching air volume, the exhaust gas can be used as draft air to be burned in the boiler. The concentrated combustion method involves first purifying the exhaust gases using adsorption, and then treating the desorbed organic substances through combustion. Fibrous activated carbon developed in recent years features fast adsorption speed and easy regeneration; rotary honeycomb adsorbers made from it can generally concentrate waste gas to 1/20 to 1/10 of its original volume. This method is suitable for the purification of low-concentration, high-volume organic waste gases. 　　1.2 New technologies for removing organic vapor pollutants 　　1.2.1 Biological methods 　　This method was first applied in deodorization. In recent years, as research on technologies for controlling organic vapor pollutants has progressed, this method has been gradually applied to the control of such pollutants. Compared with conventional treatment technologies, it has advantages such as simple equipment, low investment and operating costs, and no secondary pollution. However, it is only economical when treating low-concentration, biodegradable organic vapor pollutants, meaning its applicability is limited. 　　Research and development on the use of biological filters to treat volatile organic waste gases abroad has a history of over 30 years, while research in this area in China is still in its initial stages. However, practical application reports show that several main problems may arise during operation, leading to poor treatment efficiency and failures: (1) When the flow rate and concentration of exhaust gases fluctuate significantly, the designed load of the biological filter does not match the actual load, which can result in insufficient residence time for the exhaust gases and a decline in treatment effectiveness ； 　　(2) The accumulation of particulates in the exhaust gas within the biofilter bed causes blockage of the bed, leading to increased resistance ； 　　(3) Improper control of the filter bed humidity can lead to drying and cracking, resulting in airflow short circuits ； 　　(4) Improper pH adjustment, with a significant drop in pH, leads to a decrease in the number of microorganisms, thereby reducing the effectiveness of the treatment. 　　Improving the biodegradation rate of organic pollutants by the microorganisms used in biological filters, particularly by cultivating superior strains for substances that are difficult to biodegrade and optimizing their growth conditions, is the main direction of development for this technology at present. As a result, this approach allows the volume of the biological filter to be significantly reduced, making it more competitive compared to other air pollution control technologies. Organic waste gases are flammable and explosive; during the purification process, the concentration of these gases should be kept at no more than 1/4 of their lower explosion limit. The system should also be equipped with emergency devices such as flame arresters and rupture disks.