Description of existing methods of air purification from harmful gaseous impurities.
At present, a large number of different methods for purifying gases from technical pollution: NOx, SO2, H2S, NH3, carbon monoxide, various organic and inorganic substances have been developed and tested in industry.
We describe these basic methods and indicate their advantages and disadvantages.
Absorption is the process of dissolving a gaseous component in a liquid solvent. Absorption systems are divided into water and non-aqueous. In the second case, usually low volatile organic liquids are used. The liquid is used for absorption only once, or it is regenerated, releasing the contaminant in its pure form. Schemes with a single use of the absorber are used in cases where absorption leads directly to the production of a finished product or a semiproduct.
- production of mineral acids (absorption of SO3 in the production of sulfuric acid, absorption of nitrogen oxides in the production of nitric acid);
- obtaining salts (absorption of nitrogen oxides by alkaline solutions to obtain nitrite-nitrate liquors, absorption by aqueous solutions of lime or limestone to obtain calcium sulfate);
- other substances (absorption of NH3 water to produce ammonia water, etc.).
Schemes with multiple use of the absorber (cyclic processes) are more common. They are used for trapping hydrocarbons, cleaning SO2 from the flue gas of thermal power plants, cleaning gas from hydrogen sulphide using the iron-soda method to produce elemental sulfur, monoethanolamine treatment of gases from CO2 in the nitrogen industry.
Depending on the method of creating the contact surface of the phases, surface, bubbling and spraying absorption devices are distinguished.
In the first group of apparatus, the contact surface between the phases is a fluid mirror or the surface of a fluid fluid film. Here also include packed absorbers, in which the liquid flows over the surface of the nozzle loaded from them in bodies of various shapes.
In the second group of absorbents, the contact surface is increased due to the distribution of gas flows into the liquid in the form of bubbles and jets. Barbotage is carried out by passing gas through a liquid-filled apparatus or in apparatuses of column type with plates of various shapes.
In the third group, the contact surface is created by spraying the liquid in the gas mass. The contact surface and the efficiency of the process as a whole are determined by the dispersion of the atomized liquid.
The most widespread were packed (surface) and bubbler disk absorbers. For effective use of aqueous absorption media, the component to be removed must be readily soluble in the absorption medium and often chemically interact with water, as, for example, in the purification of gases from HCl, HF, NH3, NO2. To absorb gases with a lower solubility (SO2, Cl2, H2S), alkaline solutions based on NaOH or Ca (OH) 2 are used. Additions of chemical reagents in many cases increase the efficiency of absorption due to the occurrence of chemical reactions in the film. For the purification of gases from hydrocarbons, this method is used in practice much less often, which is due, first of all, to the high cost of absorbents. Common drawbacks of absorption methods are the formation of liquid effluents and the cumbersomeness of instrumental design.
Adsorption method is one of the most common means of protecting the air pool from pollution. Only in the USA tens of thousands of adsorption systems have been introduced and are successfully operating. The main industrial adsorbents are activated carbons, complex oxides and impregnated sorbents. Activated carbon (AU) is neutral with respect to polar and nonpolar molecules of adsorbed compounds. It is less selective than many other sorbents, and is one of the few that can be used in wet gas streams. Activated carbon is used, in particular, for cleaning gases from foul-smelling substances, solvent recovery, etc.
Oxide adsorbents (OA) have a higher selectivity for polar molecules due to the intrinsic non-uniform distribution of the electric potential. Their disadvantage is a decrease in efficiency in the presence of moisture. To class OA include silica gels, synthetic zeolites, aluminum oxide.
It is possible to single out the following basic methods for carrying out adsorption purification processes:
After adsorption, desorption is carried out and the recovered components are recovered for reuse. In this way various solvents, carbon disulfide in the production of artificial fibers and a number of other impurities are trapped.
After adsorption, the impurities are not disposed of, but are subjected to thermal or catalytic afterburning. This method is used for purification of waste gases of chemical-pharmaceutical and paint-and-lacquer enterprises, food industry and a number of other industries. This type of adsorption purification is economically justified at low concentrations of pollutants and (or) multi-component pollutants.
After purification, the adsorbent is not regenerated, but is subjected, for example, to burial or incineration together with a strongly chemisorbed contaminant. This method is suitable when using cheap adsorbents.
For desorption of impurities, heating of the adsorbent, evacuation, purging with an inert gas, displacement of impurities by an easily adsorbed substance, for example, water vapor, is used. Recently, special attention has been paid to the desorption of impurities by evacuation, and they are often easily disposed of.
For the adsorption processes, a variety of apparatus has been developed. The most common adsorbers with a fixed layer of granular or honeycomb adsorbent. Continuity of the processes of adsorption and regeneration of the adsorbent is provided by the use of apparatus with fluidized bed.
In recent years, fibrous sorption-active materials have been increasingly used. Not very different from granular adsorbents in terms of their capacitive characteristics, they significantly exceed them for a number of other indicators. For example, they are distinguished by higher chemical and thermal stability, homogeneity of the porous structure, a significant volume of micropores and a higher mass transfer coefficient (10-100 times greater than sorption materials). Installations in which fibrous materials are used occupy a much smaller area. The weight of the adsorbent with the use of fibrous materials is less than when using AU in 15-100 times, and the mass of the apparatus is 10 times. The resistance of the layer does not exceed 100 Pa.
It is also possible to improve the technical and economic parameters of existing processes through the optimal organization of the desorption stage, for example, by means of a programmed temperature rise.
It should be noted the efficiency of cleaning on activated charcoal honeycomb (cellular) structure, which have improved hydraulic characteristics. Such sorbents can be obtained by applying certain compositions with powder AU to a foamed synthetic resin or by foaming a mixture of a given composition containing AU, as well as burning the filler from a mixture comprising AU along with the binder.
Another area of improvement in adsorption purification methods is the development of new modifications of adsorbents – silica gels and zeolites, which have increased thermal and mechanical strength. However, the hydrophilicity of these adsorbents makes their use difficult.
Adsorption methods for extracting solvents from flue gases, including organochlorine ones, have become most widespread. This is due to the high efficiency of the gas purification process (95-99%), the absence of chemical reactions to the formation of secondary pollutants, the quick payback of recuperation plants (usually 2-3 years) due to repeated use of solvents and long service life (up to 10 years). Active work is carried out on the adsorption of sulfur and nitrogen oxides from gases.
Adsorption methods are one of the most common methods in the industry for cleaning gases. Their use allows you to return to production a number of valuable compounds. At concentrations of impurities in gases more than 2-5 mg / m2, Cleaning is even cost-effective. The main disadvantage of the adsorption method lies in the high energy intensity of the desorption stages and the subsequent separation, which considerably complicates its use for multicomponent mixtures.
Afterburning is a method of neutralizing gases by thermal oxidation of various harmful substances, mainly organic, in practically harmless or less harmful, preferably CO2 and H2O. Typical post-combustion temperatures for most compounds lie in the range of 750-1200 ° C. The use of thermal methods of afterburning allows to achieve 99% purification of gases.
When considering the feasibility and feasibility of thermal disinfection, it is necessary to take into account the nature of the combustion products formed. Products burning gases containing sulfur compounds, halogens, phosphorus, can exceed the toxicity of the original gas release. In this case, additional cleaning is necessary. Thermal afterburning is very effective in neutralizing gases containing toxic substances in the form of solid inclusions of organic origin (soot, carbon particles, wood dust, etc.).
The most important factors determining the feasibility of thermal detoxification are energy (fuel) costs for ensuring high temperatures in the reaction zone, caloric content of detoxified impurities, and the possibility of preheating the gases to be purified. An increase in the concentration of after-burnable impurities leads to a significant reduction in fuel consumption. In some cases, the process can take place in an autothermal regime, that is, the operating regime is maintained only by the heat of the reaction of deep oxidation of harmful impurities and the preheating of the initial mixture by the exhausted neutralized gases.
The principal difficulty in the use of thermal afterburning is the formation of secondary pollutants, such as nitrogen oxides, chlorine, SO2, etc.
Thermal methods are widely used for purification of waste gases from toxic combustible compounds. The afterburning units developed in recent years differ in their compactness and low energy consumption. The use of thermal methods is effective for afterburning dust of multi-component and dusty off-gases.
Catalytic methods of gas cleaning are versatile. With their help it is possible to release gases from oxides of sulfur and nitrogen, various organic compounds, carbon monoxide and other toxic impurities. Catalytic methods allow to convert harmful impurities into harmless, less harmful and even useful. They make it possible to process multicomponent gases with small initial concentrations of harmful impurities, to achieve high purification rates, to conduct the process continuously, to avoid the formation of secondary pollutants. The use of catalytic methods is most often limited to the difficulty of finding and manufacturing suitable for long-term operation and reasonably cheap catalysts. The heterogeneously catalytic conversion of gaseous impurities is carried out in a reactor charged with a solid catalyst in the form of porous granules, rings, balls or blocks with a structure close to cellular. Chemical transformation occurs on the developed internal surface of catalysts, reaching 1000 m / g.
As effective catalysts that are used in practice, there are a variety of substances – from minerals, which are used almost without any pretreatment, and simple massive metals to complex compounds of a given composition and structure. Usually, the catalytic activity is manifested by solids with ionic or metallic bonds, which have strong interatomic fields. One of the basic requirements for the catalyst is the stability of its structure under the reaction conditions. For example, metals should not be converted into inactive compounds during the reaction.
Modern decontamination catalysts are characterized by high activity and selectivity, mechanical strength and resistance to the action of poisons and temperatures. Industrial catalysts, made in the form of rings and blocks of honeycomb structure, have low hydrodynamic resistance and high external specific surface.
Catalytic methods for neutralizing exhaust gases in a fixed catalyst bed were most widely used. It is possible to distinguish two fundamentally different methods for the gas cleaning process – in stationary and in artificially created non-stationary regimes.
1. Stationary method.
Practically acceptable rates of chemical reactions are achieved on most cheap industrial catalysts at a temperature of 200-600 ° C. After preliminary cleaning of dust (up to 20 mg / m2) And various catalytic poisons (As, Cl2, etc.), gases usually have a much lower temperature.
Heating of the gases to the required temperatures can be carried out by introducing hot flue gases or by means of an electric heater. After passing through the catalyst bed, the purified gases are released into the atmosphere, which requires considerable energy consumption. Achieving a reduction in energy costs is possible if the heat of the waste gases is used to heat the gases coming into the purification. For heating, usually recuperative tubular heat exchangers are used.
Under certain conditions, when the concentration of combustible impurities in the waste gases exceeds 4-5 g / m 2, the process according to the scheme with a heat exchanger makes it possible to do without additional costs.
Such devices can effectively work only at constant concentrations (costs) or using perfect automatic process control systems.
These difficulties can be overcome by conducting a gas purification in an unsteady regime.
2. Nonstationary method (reverse-process).
The reverse process provides for periodic changes in the direction of filtration of the gas mixture in the catalyst bed by means of special valves. The process proceeds in the following way. The catalyst bed is preheated to a temperature at which the catalytic process proceeds at a high rate. After this, a purified gas with a low temperature is supplied to the apparatus, at which the rate of chemical transformation is negligible. From direct contact with the solid material, the gas is heated, and catalytic reaction starts to flow at a noticeable rate in the catalyst bed. The layer of solid material (catalyst), giving off the heat of the gas, is gradually cooled to a temperature equal to the temperature of the gas at the inlet. Since heat is generated during the reaction, the temperature in the bed can exceed the initial heating temperature. A thermal wave is formed in the reactor, which moves in the direction of filtration of the reaction mixture, i.e. in the direction of exit from the layer. Periodic switching of the direction of gas flow to the opposite allows you to keep the heat wave within the layer as long as you want.
The advantage of this method in the stability of work with fluctuations in the concentrations of combustible mixtures and the absence of heat exchangers.
The main direction of the development of thermocatalytic methods is the creation of cheap catalysts that efficiently work at low temperatures and are resistant to various poisons, as well as the development of energy-saving technological processes with low capital outlays for equipment. The most common application of thermocatalytic methods is in the purification of gases from nitrogen oxides, neutralization and utilization of a variety of sulfur compounds, neutralization of organic compounds and CO.
For concentrations below 1 g / m2 and large volumes of cleaned gases, the use of a thermocatalytic method requires high energy inputs, as well as a large amount of catalyst.
Ozone methods are used to neutralize flue gases from SO2 (NOx) and deodorize gas emissions from industrial enterprises. The introduction of ozone accelerates the oxidation of NO to NO2 and SO2 to SO3. After the formation of NO2 and SO3, ammonia is introduced into the flue gases and a mixture of complex fertilizers (sulphate and ammonium nitrate) is formed. The contact time of the gas with ozone, necessary for purification from SO2 (80-90%) and NOx (70-80%) is 0.4-0.9 sec. Energy costs for gas cleaning by the ozone method are estimated at 4-4.5% of the equivalent capacity of the power unit, which is probably the main reason restraining the industrial application of this method.
The use of ozone for the deodorization of gas emissions is based on the oxidative decomposition of foul-smelling substances. In one group of methods, ozone is injected directly into the gases to be purified, in another, the gases are washed with pre-ozonated water. The subsequent passage of the ozonated gas through the activated carbon layer or feeding it to the catalyst is also applied. With the introduction of ozone and the subsequent passage of gas through the catalyst, the conversion temperature of such substances as amines, acetaldehyde, hydrogen sulphide, etc. is reduced to 60-80 ° C. Both Pt / Al2O3 and copper, cobalt and iron oxides on the support are used as a catalyst. The main application of ozone deodorization methods is in the purification of gases that are released when processing raw materials of animal origin for meat (fat) plants and in everyday life.
Biochemical purification methods are based on the ability of microorganisms to destroy and transform various compounds. The decomposition of substances occurs under the action of enzymes produced by microorganisms in the environment of the gases being purified. With a frequent change in the composition of the gas, microorganisms do not have time to adapt to the development of new enzymes, and the degree of destruction of harmful impurities becomes incomplete. Therefore, biochemical systems are most suitable for the purification of gases of constant composition.
Biochemical gas cleaning is carried out either in biofilters or in bioscrubbers. In biofilters, the gas to be purified is passed through a nozzle layer irrigated with water, which creates a moisture sufficient to support the vital activity of the microorganisms. The surface of the nozzle is covered with biologically active biofilm (BP) from microorganisms.
Microorganisms BP in the course of its life absorb and destroy the substances contained in the gas medium, resulting in an increase in their mass. The efficiency of purification is largely determined by the mass transfer from the gas phase in the BP and by the uniform distribution of gas in the bed of the nozzle. Such filters are used, for example, for air deodorization. In this case, the gas stream to be cleaned is filtered under direct flow conditions with an irrigated liquid containing nutrients. After the filter, the liquid enters the sedimentation tanks and is then fed back to the irrigation.
At present, biofilters are used for purification of waste gases from ammonia, phenol, cresol, formaldehyde, organic solvents of paint and drying lines, hydrogen sulfide, methyl mercaptan and other organosulfur compounds.
The disadvantages of biochemical methods include:
- a low rate of biochemical reactions, which increases the size of the equipment;
- specificity (high selectivity) of strains of microorganisms, which complicates the processing of multicomponent mixtures;
- labor intensity of processing mixtures of variable composition.
The plasma-chemical method is based on the transmission through the high-voltage discharge of an air mixture with harmful impurities. Usually, ozonizers are used on the basis of barrier, crown or sliding discharges, or pulsed high-frequency discharges on electrostatic precipitators. Passing low-temperature plasma air with impurities is bombarded by electrons and ions. As a result, atomic oxygen, ozone, hydroxyl groups, excited molecules and atoms are formed in the gaseous medium, which participate in plasma-chemical reactions with harmful impurities. The main directions for the application of this method are to remove SO2, NOx and organic compounds. The use of ammonia, when neutralizing SO2 and NOx, gives powdered fertilizers (NH4) 2SO4 and NH4NH3 at the outlet of the reactor, which are filtered.
The disadvantage of this method are:
Insufficiently complete decomposition of harmful substances to water and carbon dioxide, in the case of oxidation of organic components, at acceptable energies of the discharge
The presence of residual ozone, which must be decomposed thermally or catalytically
significant dependence on dust concentration when using ozonizers with the use of barrier discharge.
Plasma catalytic method.
This is a fairly new method of purification, which uses two known methods – plasma-chemical and catalytic. The settings based on this method consist of two stages. The first is a plasma-chemical reactor (ozonator), the second is a catalytic reactor. Gaseous pollutants, passing the zone of high-voltage discharge in gas-discharge cells and interacting with the products of electrosynthesis, are destroyed and become harmless compounds, down to CO2 and H2O. The depth of conversion (purification) depends on the value of the specific energy released in the reaction zone. After the plasma-chemical reactor, the air is finely finely purified in the catalytic reactor. The ozone synthesized in the gas discharge of a plasma chemical reactor falls on a catalyst, where it immediately decomposes into active atomic and molecular oxygen. Remnants of pollutants (active radicals, excited atoms and molecules) that are not destroyed in the plasma-chemical reactor are destroyed on the catalyst due to deep oxidation with oxygen.
The advantage of this method is the use of catalytic reactions at temperatures lower (40-100 ° C) than in the thermocatalytic method, which leads to an increase in the life of the catalysts, as well as to lower energy costs (at concentrations of harmful substances up to 0.5 g / m2.).
The disadvantages of this method are:
- a large dependence on the concentration of dust, the need for preliminary cleaning to a concentration of 3-5 mg / m2,
- at high concentrations of harmful substances (over 1 g / m2), the cost of equipment and operating costs exceed the corresponding costs in comparison with the thermocatalytic method
Now the photocatalytic method of oxidation of organic compounds is widely studied and is developing. Basically, the catalysts based on TiO2 are used, which are irradiated with ultraviolet light. There are known household air purifiers of different firms using this method. The disadvantage of the method is the clogging of the catalyst by the products of the reaction. To solve this problem, the introduction of ozone into the cleared mixture is used, however this technology is applicable for a limited composition of organic compounds and at low concentrations.
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