Methods and means of protecting the atmosphere Basic methods of protecting the atmosphere from chemical impurities. What are the ways to protect the atmosphere? Basic methods of protecting the atmosphere from pollution
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6.5. ATMOSPHERE PROTECTION MEANS.
The air of industrial premises is polluted by emissions from technological equipment or during technological processes without localization of waste substances. Ventilation air removed from the premises can cause air pollution in industrial sites and populated areas. Moreover, the air
polluted by technological emissions from workshops, such as forging and pressing shops, shops for thermal and mechanical processing of metals, foundries and others, on the basis of which modern mechanical engineering is developed. In the production process of machinery and equipment, welding operations, mechanical processing of metals, processing of non-metallic materials, paint and varnish operations, etc. are widely used. Therefore, the atmosphere needs protection.
Atmospheric protection means must limit the presence of harmful substances in the air of the human environment at a level not exceeding the maximum permissible concentration. This is achieved by localizing harmful substances at the point of their formation, removing them from the premises or from equipment and dispersing them into the atmosphere. If the concentrations of harmful substances in the atmosphere exceed the maximum permissible concentration, then the emissions are purified from harmful substances in cleaning devices installed in the exhaust system. The most common are ventilation, technological and transport exhaust systems.
In practice, the following options for protecting atmospheric air are implemented:
removal of toxic substances from the premises by general ventilation;
ventilation, purification of contaminated air in special devices and
its return to the production or domestic premises if the air
after cleaning in the device meets the regulatory requirements for
supply air,
localization of toxic substances in the zone of their formation local
ventilation, purification of contaminated air in special devices,
release and dispersion into the atmosphere,
purification of process gas emissions in special devices,
release and dispersion into the atmosphere; in some cases before release
the exhaust gases are diluted with atmospheric air.
To comply with the maximum permissible concentrations of harmful substances in the atmospheric air of populated areas, maximum permissible emissions (MAE) of harmful substances from exhaust ventilation systems, various technological and energy installations are established.
In accordance with the requirements of GOST 17.2.02, for each designed and operating industrial enterprise, a maximum permissible limit for harmful substances into the atmosphere is established, provided that emissions of harmful substances from a given source in combination with other sources (taking into account the prospects for their development) do not create a ground concentration exceeding the maximum permissible concentration .
Devices for cleaning ventilation and process emissions into the atmosphere are divided into:
dust collectors (dry, electric filters, wet filters);
mist eliminators (low-speed and high-speed);
apparatus for collecting vapors and gases (absorption,
chemisorption, adsorption and neutralizers);
multi-stage cleaning devices (dust and gas collectors,
mists and solids traps, multi-stage
dust collectors).
Electrical cleaning (electric precipitators) is one of the most advanced types of gas purification from suspended dust and fog particles. This process is based on impact ionization of gas in the corona discharge zone, transfer of ion charge to impurity particles and deposition of the latter on the collection corona electrodes. For this purpose, electric precipitators are used.
Electrostatic precipitator circuit.
1-corona electrode
2-precipitating electrode
Aerosol particles entering the zone between the corona 1 and precipitation 2 electrodes adsorb ions on their surface, acquiring an electrical charge, and thereby receive acceleration directed towards the electrode with a charge of the opposite sign. Considering that the mobility of negative ions in air and flue gases is higher than that of positive ones, electrostatic precipitators are usually made with a corona of negative polarity. The charging time of aerosol particles is short and measured in fractions of seconds. The movement of charged particles towards the collecting electrode occurs under the influence of aerodynamic forces and the force of interaction between the electric field and the particle charge.
The filter is a housing 1, divided by a porous partition (filter element) 2 into two strips. Contaminated gases enter the filter and are cleaned as they pass through the filter element. Impurity particles settle on the inlet part of the porous partition and are retained in the pores, forming layer 3 on the surface of the partition. For newly arriving particles, this layer becomes part of the filter partition, which increases the cleaning efficiency
filter and pressure drop across the filter element. The precipitation of particles on the surface of the pores of the filter element occurs as a result of the combined action of the touch effect, as well as diffusion, inertial and gravitational effects.
Wet dust collectors include bubbling-foam dust collectors with failure and overflow grids.
Scheme of bubbling-foam dust collectors with failure (a) and (b)
overflow grates.
3-lattice
In such devices, the gas for cleaning enters under the grid 3, passes through the holes in the grid and, bubbling through a layer of liquid and foam 2, is cleaned of dust by depositing particles on the inner surface of gas bubbles. The operating mode of the devices depends on the speed of air supply under the grille. At speeds up to 1 m/s, a bubbling mode of operation of the apparatus is observed. A further increase in gas velocity in the body 1 of the apparatus to 2...2.5 m/s is accompanied by the appearance of a foam layer above the liquid, which leads to an increase in the efficiency of gas purification and splash removal from the apparatus. Modern bubbling-foam devices provide an efficiency of gas purification from fine dust of -0.95...0.96 at a specific water consumption of 0.4...0.5 l/m. The practice of operating these devices shows that they are very sensitive to uneven gas supply under the failure gratings. An uneven supply of gas leads to local blowing off of the liquid film from the grate. In addition, the grilles of the devices are prone to clogging.
To clean the air from mists of acids, alkalis, oils and other liquids, fiber filters - mist eliminators - are used. The principle of their operation is based on the deposition of droplets on the surface of the pores, followed by the flow of liquid along the fibers into the lower part of the mist eliminator. The deposition of liquid droplets occurs under the influence of Brownian diffusion or an inertial mechanism for separating pollutant particles from the gas phase on filter elements depending on the filtration speed W. Mist eliminators are divided into low-speed (W< 0,15 м/с), в которых преобладает механизм диффузного осаждения капель, и высокоскоростные (W=2...2,5 м/с), где осаждение происходит главным образом под воздействием инерционных сил.
Felts made of polypropylene fibers are used as filter packing in such mist eliminators, which work successfully in an environment of dilute and concentrated acids and alkalis.
In cases where the diameters of fog droplets are 0.6...0.7 microns or less, to achieve acceptable cleaning efficiency it is necessary to increase the filtration speed to 4.5...5 m/s, which leads to noticeable spray entrainment from the outlet side of the filter element (splash entrainment usually occurs at speeds of 1.7...2.5 m/s), splash entrainment can be significantly reduced by using splash eliminators in the mist eliminator design. To capture liquid particles larger than 5 microns in size, splash traps made from mesh packages are used, where the capture of liquid particles occurs due to the effects of touch and inertial forces. The filtration speed in splash traps should not exceed 6 m/s.
Diagram of a high-speed mist eliminator.
1 - splash trap
3-filter element
High-speed mist eliminator with a cylindrical filter element 3, which is a perforated drum with a blind lid. Coarse fiber felt 2 with a thickness of 3...5 mm is installed in the drum. Around the drum on its outer side there is a splash trap 1, which is a set of perforated flat and corrugated layers of vinyl plastic tapes. The splash trap and filter element are installed with the lower part into the liquid layer.
Low-velocity mist eliminator filter element diagram
3-cylinders
4-fiber filter element
5-bottom flange
6-tube water seal
In the space between 3 cylinders made of meshes,
place a fibrous filter element 4, which is secured using
flange 2 to the mist eliminator body 1. Liquid deposited on
filter element; flows onto the lower flange 5 and through the tube
water seal 6 and glass 7 are drained from the filter. Fibrous
low-velocity mist eliminators provide high
gas purification efficiency (up to 0.999) from particles smaller than 3 microns and completely captures large particles. Fibrous layers are formed from glass fiber with a diameter of 7...40 microns. The layer thickness is 5... 15 cm, the hydraulic resistance of dry filter elements is 200... 1000 Pa.
High-speed mist eliminators are smaller in size and provide cleaning efficiency equal to 0.9...0.98 at Ap=1500...2000 Pa, from fog with particles less than 3 microns.
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INTRODUCTION
The revival of Russian industry is the primary task of strengthening the country's economy. Without a strong, competitive industry, it is impossible to ensure the normal life of the country and people. Market relations, the independence of factories, and the departure from a planned economy dictate that manufacturers produce products that are in global demand and at minimal cost. The engineering and technical personnel of the factories are entrusted with the task of producing these products at minimal cost in the shortest possible time, with guaranteed quality.
This can be achieved by using modern technologies for processing parts, equipment, materials, production automation systems and product quality control. The reliability of the manufactured machines, as well as the economics of their operation, largely depend on the adopted production technology.
The urgent task is to improve the technological support for the quality of manufactured machines, and first of all their accuracy. Precision in mechanical engineering is of great importance for improving the operational quality of machines and for their production technology. Increasing the accuracy of manufacturing workpieces reduces the labor intensity of machining, and increasing the accuracy of machining reduces the labor intensity of assembly as a result of eliminating fitting work and ensuring the interchangeability of product parts.
Compared to other methods for producing machine parts, cutting provides the greatest accuracy and the greatest flexibility of the production process, creating the possibility of the fastest transition from processing workpieces of one size to processing workpieces of a different size.
The quality and durability of the tool largely determine the productivity and efficiency of the processing process, and in some cases, the general ability to obtain parts of the required shape, quality and accuracy. Improving the quality and reliability of cutting tools contributes to increasing the productivity of metal cutting.
A reamer is a cutting tool that allows you to obtain high precision of machined parts. It is an inexpensive tool, and labor productivity when working with a reamer is high. Therefore, it is widely used in the finishing of various holes of machine parts. With the modern development of the mechanical engineering industry, the range of parts produced is enormous and the variety of holes requiring processing with reamers is very large. Therefore, designers are often faced with the task of developing a new development. They can be helped in this by a package of application programs on a computer, which calculates the geometry of the cutting tool and displays the working drawing of the development on the plotter.
The design sequence and calculation methods for cutting tools are based both on the general principles of the design process and on the specific features characteristic of the cutting tool. Each type of tool has design features that must be taken into account during design.
Specialists who will work in the metalworking industries must be able to competently design various designs of cutting tools for modern metalworking systems, effectively using computer technology (computers) and advances in the field of tool production.
To reduce time and increase the efficiency of cutting tool design, automated computer calculations are used, the basis of which is software and mathematics.
Creating application software packages for calculating the geometric parameters of complex and particularly complex cutting tools on a computer can dramatically reduce the cost of design labor and improve the quality of cutting tool design.
Places, %; Totd - time for rest and personal needs, %; K - coefficient taking into account the type of production; Кз - coefficient taking into account assembly conditions. For the general assembly of the hydraulic lock, the standard time is: = 1.308 min. Calculation of the required number of assembly stands and its load factors Let's find the estimated number of assembly stands, pcs. =0.06 pcs. Round up CP=1. ...
Ways to protect the atmosphere from pollutants?
Atmosphere- this is the gas shell of planet Earth, which rotates with it. The mixture of atmospheric gases is called air.
Pollution can be primary or secondary. Primary pollution occurs when substances released into the atmosphere have an adverse effect on living organisms. For example, phosgene gas is poisonous to all living things. Secondary pollution occurs when a relatively harmless substance in the atmosphere turns harmful. Thus, freon is a low-active chemical, but under the influence of ultraviolet radiation it decomposes, releasing harmful chlorine.
Pollutants entering the atmosphere come in solid, liquid and gaseous aggregate states. A significant contribution to the emission of harmful substances is made by household heating systems, or more precisely, solid fuel stoves. Also, a large number of pollutants enter the atmosphere with exhaust gases from various types of transport. All types of industry are responsible for air pollution with the most toxic substances. Livestock farms play a significant role in air pollution.
- Methods for cleaning up pollutants industrial emissions:
- Gravity. Used to settle large dust particles.
- Filtration. Suitable for separating substances in a solid state of aggregation with different particle diameters, this occurs in special devices: cyclones, scrubbers, filters, dust precipitators.
- Sorption. It is used to purify emissions from liquid and gaseous substances. It involves the absorption of pollutant molecules by special substances. It is carried out in adsorbers or absorbers.
- Condensation. Used to separate liquid or gaseous pollutants. It is carried out in special reactors or capacitors.
- Oxidation-reduction. The method is suitable for neutralizing substances in various states of aggregation by chemically converting them into harmless ones. It is carried out in special reactors under the influence of catalysts or in burners for thermal transformation.
- Protecting the atmosphere from exhaust gases transport:
- Changing the quality or type of fuel, for example, converting cars to liquefied gas, alcohol, etc.
- Installation of catalytic, flame or liquid converters on the exhaust system of cars.
- Transition to electric vehicles.
- Protecting the atmosphere from pollutants livestock complexes:
- physical and chemical methods, trapping and neutralizing harmful substances occurs in various filters, scrubbers, dust settling chambers;
- biological - extraction of carbon dioxide and hydrogen sulfide from the air using specially grown plants.
- Ways to reduce air pollution from solid fuel stoves:
- the use of modern catalytic and non-catalytic furnaces, the design of which promotes complete combustion of fuel and combustion of flue gases;
- use pellets or fuel briquettes for heating, the combustion of which produces almost half as many harmful substances as coal or firewood;
- switching to gas or electric heating.
Emissions from industrial enterprises are characterized by a wide variety of dispersed composition and other physicochemical properties. In this regard, various methods for their purification and types of gas and dust collectors - devices designed to purify emissions from pollutants - have been developed.
Methods for cleaning industrial emissions from dust can be divided into two groups: dust collection methods "dry" method and dust collection methods "wet" method. Gas dust removal devices include: dust settling chambers, cyclones, porous filters, electric precipitators, scrubbers, etc.
The most common dry dust collection installations are cyclones various types.
They are used to capture flour and tobacco dust, ash generated when burning fuel in boiler units. The gas flow enters the cyclone through pipe 2 tangentially to the inner surface of housing 1 and performs a rotational-translational motion along the housing. Under the influence of centrifugal force, dust particles are thrown to the wall of the cyclone and, under the influence of gravity, fall into the dust collection hopper 4, and the purified gas exits through the outlet pipe 3. For normal operation of the cyclone, its tightness is necessary; if the cyclone is not sealed, then due to suction outside air, dust is carried out with a flow through the outlet pipe.
The tasks of cleaning gases from dust can be successfully solved by cylindrical (TsN-11, TsN-15, TsN-24, TsP-2) and conical (SK-TsN-34, SK-TsN-34M, SKD-TsN-33) cyclones, developed by the Research Institute for Industrial and Sanitary Gas Purification (NIIOGAZ). For normal operation, the excess pressure of gases entering the cyclones should not exceed 2500 Pa. In this case, in order to avoid condensation of liquid vapors, the temperature of the gas is selected to be 30 - 50 o C above the t dew point, and according to the conditions of structural strength - no higher than 400 o C. The productivity of the cyclone depends on its diameter, increasing with the growth of the latter. The cleaning efficiency of cyclones of the TsN series decreases with increasing angle of entry into the cyclone. As the particle size increases and the cyclone diameter decreases, the cleaning efficiency increases. Cylindrical cyclones are designed to collect dry dust from aspiration systems and are recommended for use for pre-cleaning of gases at the inlet of filters and electric precipitators. Cyclones TsN-15 are made of carbon or low-alloy steel. Canonical cyclones of the SK series, designed for cleaning gases from soot, have increased efficiency compared to cyclones of the TsN type due to greater hydraulic resistance.
To purify large masses of gases, battery cyclones are used, consisting of a larger number of parallel installed cyclone elements. Structurally, they are combined into one housing and have a common gas supply and outlet. Experience in operating battery cyclones has shown that the cleaning efficiency of such cyclones is somewhat lower than the efficiency of individual elements due to the flow of gases between the cyclone elements. The domestic industry produces battery cyclones such as BC-2, BTsR-150u, etc.
Rotary Dust collectors are centrifugal devices that, while moving air, clean it from dust fractions larger than 5 microns. They are very compact, because... the fan and dust collector are usually combined in one unit. As a result, during the installation and operation of such machines, no additional space is required to accommodate special dust collection devices when moving a dusty flow with an ordinary fan.
The design diagram of the simplest rotary type dust collector is shown in the figure. When the fan wheel 1 operates, dust particles, due to centrifugal forces, are thrown towards the wall of the spiral casing 2 and move along it in the direction of the exhaust hole 3. The dust-enriched gas is discharged through a special dust receiving hole 3 into the dust bin, and the purified gas enters the exhaust pipe 4 .
To increase the efficiency of dust collectors of this design, it is necessary to increase the portable speed of the purified flow in the spiral casing, but this leads to a sharp increase in the hydraulic resistance of the device, or to reduce the radius of curvature of the casing spiral, but this reduces its productivity. Such machines provide a fairly high efficiency of air purification while capturing relatively large dust particles - over 20 - 40 microns.
More promising rotary dust separators, designed to clean air from particles > 5 µm in size, are counter-flow rotary dust separators (RPD). The dust separator consists of a hollow rotor 2 with a perforated surface built into the casing 1 and a fan wheel 3. The rotor and fan wheel are mounted on a common shaft. When the dust separator operates, dusty air enters the housing, where it swirls around the rotor. As a result of the rotation of the dust flow, centrifugal forces arise, under the influence of which suspended dust particles tend to separate from it in the radial direction. However, aerodynamic drag forces act on these particles in the opposite direction. Particles whose centrifugal force is greater than the aerodynamic drag force are thrown toward the walls of the casing and enter hopper 4. The purified air is thrown out through the perforation of the rotor using a fan.
The efficiency of PRP cleaning depends on the selected ratio of centrifugal and aerodynamic forces and theoretically can reach 1.
A comparison of PDPs with cyclones demonstrates the advantages of rotary dust collectors. Thus, the overall dimensions of the cyclone are 3–4 times, and the specific energy consumption for cleaning 1000 m 3 of gas is 20–40% higher than that of the PRP, all other things being equal. However, rotary dust collectors are not widely used due to the relative complexity of the design and operating process compared to other devices for dry gas purification from mechanical contaminants.
To separate the gas flow into purified gas and dust-enriched gas, use louvered dust separator On the louvre grille 1, the gas flow with flow rate Q is divided into two flow paths with flow rates Q 1 and Q 2. Usually Q 1 = (0.8-0.9)Q, and Q 2 = (0.1-0.2)Q. The separation of dust particles from the main gas flow on the louvre grille occurs under the influence of inertial forces that arise when the gas flow turns at the entrance to the louvre grille, as well as due to the effect of reflection of particles from the surface of the grille upon impact. The dust-enriched gas flow after the louvered grille is directed to a cyclone, where it is cleaned of particles, and is reintroduced into the pipeline behind the louvered grille. Louvre dust separators are simple in design and are well arranged in gas ducts, providing a cleaning efficiency of 0.8 or more for particles larger than 20 microns. They are used to clean flue gases from coarse dust at temperatures up to 450 – 600 o C.
Electric precipitator. Electrical cleaning is one of the most advanced types of gas purification from suspended particles of dust and fog. This process is based on impact ionization of gas in the corona discharge zone, transfer of ion charge to impurity particles and deposition of the latter on collecting and corona electrodes. Precipitation electrodes 2 are connected to the positive pole of the rectifier 4 and grounded, and the corona electrodes are connected to the negative pole. The particles entering the electrostatic precipitator are connected to the positive pole of the rectifier 4 and are grounded, and the corona electrodes are charged with ion impurity ions. Usually they already have a small charge obtained due to friction against the walls of pipelines and equipment. Thus, negatively charged particles move towards the collection electrode, and positively charged particles settle on the negative discharge electrode.
Filters widely used for fine purification of gas emissions from impurities. The filtration process consists of retaining impurity particles on porous partitions as they move through them. The filter consists of housing 1, separated by a porous partition (filter-
element) 2 into two cavities. Contaminated gases enter the filter and are cleaned as they pass through the filter element. Impurity particles settle on the inlet part of the porous partition and are retained in the pores, forming layer 3 on the surface of the partition.
According to the type of partitions, filters are: - with granular layers (stationary, freely poured granular materials) consisting of grains of various shapes, used to purify gases from large impurities. To purify gases from dust of mechanical origin (from crushers, dryers, mills, etc.), gravel filters are often used. Such filters are cheap, easy to operate and provide high cleaning efficiency (up to 0.99) of gases from coarse dust.
With flexible porous partitions (fabrics, felts, sponge rubber, polyurethane foam, etc.);
With semi-rigid porous partitions (knitted and woven mesh, pressed spirals and shavings, etc.);
With rigid porous partitions (porous ceramics, porous metals, etc.).
The most widely used in industry for dry purification of gas emissions from impurities are bag filters. The required number of hoses 1 is installed in the filter housing 2, into the internal cavity of which dusty gas is supplied from the incoming pipe 5. Due to sieve and other effects, particles of contaminants settle in the pile and form a dust layer on the inner surface of the hoses. Purified air leaves the filter through pipe 3. When the maximum permissible pressure drop across the filter is reached, it is disconnected from the system and regeneration is carried out by shaking the hoses and blowing them with compressed gas. Regeneration is carried out by a special device 4.
Dust collectors of various types, including electric precipitators, are used at elevated concentrations of impurities in the air. Filters are used for fine air purification with impurity concentrations of no more than 50 mg/m 3; if the required fine air purification occurs at high initial concentrations of impurities, then the purification is carried out in a system of series-connected dust collectors and filters.
Devices wet cleaning gases are widespread, because are characterized by high cleaning efficiency from fine dust with d h ≥ (0.3-1.0) microns, as well as the ability to clean hot and explosive gases from dust. However, wet dust collectors have a number of disadvantages that limit their scope of application: formations during the cleaning process sludge, which requires special systems for its processing; removal of moisture into the atmosphere and formation of deposits in exhaust flues when gases are cooled to the dew point temperature; the need to create circulating systems for supplying water to the dust collector.
Wet cleaning devices operate on the principle of deposition of dust particles onto the surface of either liquid droplets or a liquid film. The deposition of dust particles onto the liquid occurs under the influence of inertial forces and Brownian motion.
Among wet cleaning devices with the deposition of dust particles on the surface of droplets, in practice they are more applicable Venturi scrubbers. The main part of the scrubber is Venturi nozzle 2, into the confuser part of which a dusty gas flow is supplied and liquid is supplied through centrifugal nozzles 1 for irrigation. In the confuser part of the nozzle, the gas accelerates from an input speed of 15-20 m/s to a speed in the narrow section of the nozzle of 30-200 m/s, and in the diffuser part of the nozzle the flow is decelerated to a speed of 15-20 m/s and fed into the droplet eliminator 3. The droplet eliminator is usually made in the form of a direct-flow cyclone. Venturi scrubbers provide high efficiency in cleaning aerosols with an average particle size of 1-2 microns with an initial impurity concentration of up to 100 g/m 3 .
Wet dust collectors include bubbling foam dust collectors with failure and overflow grilles. In such devices, the gas for cleaning enters under the grid 3, passes through the holes in the grid and, passing through a layer of liquid or foam 2, under pressure, is cleaned of part of the dust due to the deposition of particles on the inner surface of gas bubbles. The operating mode of the devices depends on the speed of air supply under the grille. At speeds up to 1 m/s, a bubbling mode of operation of the apparatus is observed. A further increase in gas velocity in the apparatus body from 1 to 2-2.5 m/s is accompanied by the appearance of a foam layer above the liquid, which leads to an increase in the efficiency of gas purification and splash removal from the apparatus. Modern bubbling-foam devices provide an efficiency of gas purification from fine dust of ≈ 0.95-0.96 at a specific water consumption of 0.4-0.5 l/m 3 . But these devices are very sensitive to uneven gas supply under the failure grates, which leads to local blowing off of the liquid film from the grate. Grates are prone to clogging.
Methods for purifying industrial emissions from gaseous pollutants, based on the nature of the physical and chemical processes, are divided into five main groups: washing emissions with solvents of impurities (absorption); washing emissions with solutions of reagents that bind impurities chemically (chemisorption); absorption of gaseous impurities by solid active substances (adsorption); thermal neutralization of waste gases and the use of catalytic conversion.
Absorption method. In gas emissions purification technology, the absorption process is often called scrubber process. Purification of gas emissions by the absorption method involves separating a gas-air mixture into its component parts by absorbing one or more gas components (absorbates) of this mixture with a liquid absorbent (absorbent) to form a solution.
The driving force here is the concentration gradient at the gas-liquid phase boundary. The component of the gas-air mixture (absorbate) dissolved in the liquid penetrates into the internal layers of the absorbent due to diffusion. The process proceeds faster, the larger the phase interface, flow turbulence and diffusion coefficients, i.e. in the process of designing absorbers, special attention should be paid to organizing the contact of the gas flow with the liquid solvent and the selection of the absorbing liquid (absorbent).
The decisive condition when choosing an absorbent is the solubility of the extracted component in it and its dependence on temperature and pressure. If the solubility of gases at 0°C and a partial pressure of 101.3 kPa is hundreds of grams per 1 kg of solvent, then such gases are called highly soluble.
The organization of contact of the gas flow with the liquid solvent is carried out either by passing the gas through a packed column, or by spraying the liquid, or by bubbling the gas through a layer of absorbent liquid. Depending on the implemented method of gas-liquid contact, the following are distinguished: packed towers: nozzle and centrifugal scrubbers, Venturi scrubbers; bubbling foam and other scrubbers.
The general structure of the counterflow packed tower is shown in the figure. Contaminated gas enters the lower part of the tower, and purified gas leaves it through the upper part, where with the help of one or more sprinklers 2
A clean absorbent is introduced, and the waste solution is taken from the bottom. The purified gas is usually released into the atmosphere. 
The liquid leaving the absorber is regenerated, desorbing the contaminant, and returned to the process or removed as a waste (by-product). The chemically inert nozzle 1, filling the internal cavity of the column, is designed to increase the surface of the liquid spreading over it in the form of a film. As a nozzle, bodies of different geometric shapes are used, each of which is characterized by its own specific surface area and resistance to the movement of the gas flow.
The choice of purification method is determined by technical and economic calculations and depends on: the concentration of the pollutant in the gas being purified and the required degree of purification, depending on the background air pollution in a given region; volumes of purified gases and their temperatures; the presence of accompanying gaseous impurities and dust; the need for certain recycling products and the availability of the required sorbent; the size of the areas available for the construction of a gas treatment plant; availability of the necessary catalyst, natural gas, etc.
When choosing equipment for new technological processes, as well as when reconstructing existing gas purification installations, it is necessary to be guided by the following requirements: maximum efficiency of the purification process in a wide range of load characteristics at low energy costs; simplicity of design and maintenance; compactness and the ability to manufacture devices or individual units from polymer materials; possibility of working with circulation irrigation or self-irrigation. The main principle that should be the basis for the design of treatment facilities is the maximum possible retention of harmful substances, heat and their return to the technological process.
Task No. 2: At the grain processing enterprise, equipment is installed that is a source of grain dust. To remove it from the working area, the equipment is equipped with an aspiration system. In order to clean the air before releasing it into the atmosphere, a dust collection unit consisting of a single or battery cyclone is used.
Determine: 1. Maximum permissible emission of grain dust.
2. Select the design of a dust collection installation consisting of cyclones from the Scientific Research Institute for Industrial and Sanitary Gas Purification (NII OGAZ), determine its efficiency according to the schedule and calculate the dust concentration at the inlet and outlet of the cyclone.
Emission source height H = 15 m,
The speed of release of the gas-air mixture from the source w o = 6 m/s,
Source mouth diameter D = 0.5 m,
Release temperature Тg = 25 о С,
Ambient air temperature Тв = _ -14 о С,
Average dust particle size d h = 4 µm,
MPC of grain dust = 0.5 mg/m 3,
Background concentration of grain dust C f = 0.1 mg/m 3,
The company is located in the Moscow region,
The terrain is calm.
Solution.1. Determine the maximum permissible value of grain dust:
M pdv = 
, mg/m 3
from the definition of the maximum permissible value we have: C m = C maximum permissible concentration – C f = 0.5-0.1 = 0.4 mg/m 3 ,
Gas-air mixture flow rate V 1 = 
,
DT = Тg – Тв = 25 – (-14) = 39 о С,
determine the emission parameters: f =1000 
, Then
m = 1/(0.67+0.1 + 0.34) = 1/(0.67 + 0.1 +0.34) = 0.8.
V m = 0.65 
, Then
n = 0.532V m 2 – 2.13V m + 3.13= 0.532×0.94 2 – 2.13×0.94 + 3.13 = 1.59, and
M pdv = 
g/s.
2. Selection of a treatment plant and determination of its parameters.
a) The selection of a dust collection unit is made according to catalogs and tables (“Ventilation, air conditioning and air purification at food industry enterprises” E.A. Shtokman, V.A. Shilov, E.E. Novgorodsky et al., M., 1997). The selection criterion is the performance of the cyclone, i.e. the flow rate of the gas-air mixture at which the cyclone has maximum efficiency. To solve the problem, we will use the table:
The first line provides data for a single cyclone, the second - for a battery cyclone.
If the calculated productivity is in the range between the table values, then choose the design of the dust collection installation with the next higher productivity.
We determine the hourly productivity of the treatment plant:
V h = V 1 × 3600 = 1.18 × 3600 = 4250 m 3 / h
According to the table, according to the nearest larger value V h = 4500 m 3 / h, we select a dust collection unit in the form of a single cyclone TsN-11 with a diameter of 800 mm.
b) According to the graph in Fig. 1 of the appendix, the efficiency of the dust collection installation with an average diameter of dust particles of 4 microns is hp = 70%.
c) Determine the dust concentration at the exit from the cyclone (at the mouth of the source):
From out = 
The maximum concentration of dust in the purified air Cin is determined:
C in = 
.
If the actual value of Cin is more than 1695 mg/m 3, then the dust collection installation will not give the desired effect. In this case, more advanced cleaning methods must be used.
3. Determine the pollution indicator
P = 
,
where M is the mass of pollutant emission, g/s,
The pollution indicator shows how much clean air is needed to “dissolve” the pollutant emitted by the source per unit of time to the maximum permissible concentration, taking into account the background concentration.
P = 
.
The annual pollution indicator is the total pollution indicator. To determine it, we find the mass of grain dust emissions per year:
M year = 3.6 × M MPE × T × d ×10 -3 = 3.6 × 0.6 × 8 × 250 × 10 -3 = 4.32 t/year, then
åР = 
.
The pollution indicator is necessary for the comparative assessment of different emission sources.
For comparison, let’s calculate åP for sulfur dioxide from the previous problem for the same period of time:
M year = 3.6 × M MPE × T × d × 10 -3 = 3.6 × 0.71 × 8 × 250 × 10 -3 = 5.11 t/year, then
åР = 
And in conclusion, it is necessary to draw a sketch of the selected cyclone according to the dimensions given in the appendix, on an arbitrary scale.
Pollution control. Payment for environmental damage.
When calculating the amount of pollutant, i.e. ejection mass is determined by two values: gross emissions (t/year) and maximum single emissions (g/s). The gross emission value is used for a general assessment of air pollution by a given source or group of sources, and is also the basis for calculating payments for environmental pollution.
The maximum single emission makes it possible to assess the state of atmospheric air pollution at a given time and is the initial value for calculating the maximum surface concentration of a pollutant and its dispersion in the atmosphere.
When developing measures to reduce emissions of pollutants into the atmosphere, it is necessary to know what contribution each source makes to the overall picture of air pollution in the area where the enterprise is located.
TSV – temporarily coordinated release. If at a given enterprise or group of enterprises located in the same area (Normal Physics is large), the MPE value for objective reasons cannot be achieved at the present time, then, in agreement with the body exercising state control over the protection of the atmosphere from pollution, the user of natural resources is assigned an ELV with adoption of a gradual reduction of emissions to MPE values and the development of specific measures for this.
Payments are collected for the following types of harmful effects on the environment: - emission of pollutants into the atmosphere from stationary and mobile sources;
Discharge of pollutants into surface and underground water bodies;
Waste disposal;
Dr. types of harmful effects (noise, vibration, electromagnetic and radiation effects, etc.).
Two types of basic payment standards have been established:
a) for emissions, discharges of pollutants and waste disposal within acceptable standards
b) for emissions, discharges of pollutants and waste disposal within established limits (temporarily agreed standards).
Basic payment standards are established for each pollutant ingredient (waste), taking into account their degree of danger to the environment and public health.
The rates of payment for pollution of hazardous pollutants are indicated in the Decree of the Government of the Russian Federation of June 12, 2003. No. 344 “On payment standards for emissions of pollutants into the atmospheric air from stationary and mobile sources, discharges of pollutants into surface and underground water bodies, disposal of industrial and consumer waste” for 1 ton in rubles:
Payment for emissions of pollutants that do not exceed the standards established for the user of natural resources:
П = С Н × М Ф, with М Ф £ М Н,
where М Ф – actual emission of pollutant, t/year;
МН – maximum permissible standard for this pollutant;
С Н – rate of payment for the emission of 1 ton of a given pollutant within the limits of permissible emission standards, rubles/t.
Payment for emissions of pollutants within established emission limits:
P = S L (M F – M N) + S N M N, with M N< М Ф < М Л, где
S L – rate of payment for the emission of 1 ton of pollutant within the established emission limits, rub/t;
M L – established emission limit for a given pollutant, t/year.
Payment for excess emissions of pollutants:
P = 5× S L (M F – M L) + S L (M L – M N) + S N × M N, with M F > M L.
Payment for the emission of pollutants when the user of natural resources has not established standards for the emission of pollutants or a fine:
P = 5 × S L × M F
Payments for maximum permissible emissions, pollutant discharges, waste disposal are made at the expense of the cost of products (works, services), and for exceeding them - at the expense of the profit remaining at the disposal of the natural resource user.
Payments for environmental pollution are received:
19% to the Federal Budget,
81% to the budget of the subject of the Federation.
Task No. 3. “Calculation of technological emissions and payment for environmental pollution using the example of a bakery”
The bulk of pollutants, such as ethyl alcohol, acetic acid, acetaldehyde, are formed in baking chambers, from where they are removed through exhaust ducts due to natural draft or emitted into the atmosphere through metal pipes or shafts at least 10 - 15 m high. Emissions of flour dust , mainly occur in flour warehouses. Oxides of nitrogen and carbon are formed when natural gas is burned in baking chambers.
Initial data:
1. Annual production of the Moscow bakery is 20,000 tons/year of bakery products, incl. bakery products from wheat flour - 8,000 t/year, bakery products from rye flour - 5,000 t/year, bakery products from mixed rolls - 7,000 t/year.
2. Roll recipe: 30% - wheat flour and 70% - rye flour
3. The storage condition for flour is bulk.
4. Fuel in furnaces and boilers is natural gas.
I. Technological emissions from the bakery.
II. Payment for air pollution, if the maximum permissible limit is:
Ethyl alcohol – 21t/year,
Acetic acid – 1.5 t/year (VSV – 2.6 t/year),
Acetaldehyde – 1 t/year,
Flour dust – 0.5 t/year,
Nitrogen oxides – 6.2 t/year,
Carbon oxides – 6 t/year.
1. In accordance with the methodology of the All-Russian Research Institute of HP, technological emissions when baking bakery products are determined by the method of specific indicators:
M = B × m, where
M – amount of pollutant emissions in kg per unit of time,
B – production output in tons for the same period of time,
m – specific indicator of pollutant emissions per unit of output, kg/t.
Specific emissions of pollutants in kg/t of finished products.
1. Ethyl alcohol: bakery products made from wheat flour – 1.1 kg/t,
bakery products made from rye flour – 0.98 kg/t.
2. Acetic acid: bakery products made from wheat flour – 0.1 kg/t,
bakery products made from rye flour – 0.2 kg/t.
3. Acetaldehyde – 0.04 kg/t.
4. Flour dust – 0.024 kg/t (for bulk storage of flour), 0.043 kg/t (for containerized storage of flour).
5. Nitrogen oxides - 0.31 kg/t.
6. Carbon oxides – 0.3 kg/t.
I. Calculation of process emissions:
1. Ethyl alcohol:
M 1 = 8000 × 1.1 = 8800 kg/year;
M 2 = 5000 × 0.98 = 4900 kg/year;
M 3 = 7000(1.1×0.3+0.98×0.7) = 7133 kg/year;
total emission M = M 1 + M 2 + M 3 = 8800 + 4900 + 7133 = 20913 kg/year.
2. Acetic acid:
Bakery products made from wheat flour
M 1 = 8000 × 0.1 = 800 kg/year;
Bakery products made from rye flour
M 2 = 5000 × 0.2 = 1000 kg/year;
Mixed roll baked goods
M 3 = 7000(0.1×0.3+0.2×0.7) = 1190 kg/year,
total emission M = M 1 + M 2 + M 3 = 800 + 1000 + 1190 = 2990 kg/year.
3. Acetaldehyde M = 20000 × 0.04 = 800 kg/year.
4. Flour dust M = 20000 × 0.024 = 480 kg/year.
5. Nitrogen oxides M = 20000 × 0.31 = 6200 kg/year.
6. Carbon oxides M = 20000 × 0.3 = 6000 kg/year.
II. Calculation of fees for pollution of hazardous pollutants.
1. Ethyl alcohol: M H = 21 t/year, M F = 20.913 t/year Þ P = S H × M f = 0.4 × 20.913 = 8.365 rub.
2. Acetic acid: M H = 1.5 t/year, M L = 2.6 t/year, M F = 2.99 t/year Þ P = 5 S L (M F – M L) + S L ( M L – M N)+S N × M N =
5 × 175 × (2.99-2.6) + 175 × (2.6 – 1.5) + 35 × 1.5 = 586.25 rub.
3. Acetic aldehyde: M H = 1 t/year, M F = 0.8 t/year Þ P = S H × M F = 68 × 0.8 = 54.4 rub.
4. Flour dust: M N = 0.5 t/year, M F = 0.48 t/year Þ P = S N × M F = 13.7 × 0.48 = 6.576 rubles.
5. Nitrogen oxide: M N = 6.2 t/year, M F = 6.2 t/year Þ P = S N × M F = 35 × 6.2 = 217 rub.
6. Carbon oxide: M H = 6 t/year, M F = 6 t/year Þ
P = S N × M F = 0.6 × 6 = 3.6 rub.
The coefficient taking into account environmental factors for the Central region of the Russian Federation = 1.9 for atmospheric air, for the city the coefficient is 1.2.
åП = 876.191 · 1.9 · 1.2 = 1997.72 rubles
CONTROL TASKS.
Exercise 1
| Option No. | Boiler room productivity Q about, MJ/hour | Source height H, m | Mouth diameter D, m | Background concentration of SO 2 C f, mg/m 3 |
| 0,59 | 0,004 | |||
| 0,59 | 0,005 | |||
| 0,6 | 0,006 | |||
| 0,61 | 0,007 | |||
| 0,62 | 0,008 | |||
| 0,63 | 0,004 | |||
| 0,64 | 0,005 | |||
| 0,65 | 0,006 | |||
| 0,66 | 0,007 | |||
| 0,67 | 0,008 | |||
| 0,68 | 0,004 | |||
| 0,69 | 0,005 | |||
| 0,7 | 0,006 | |||
| 0,71 | 0,007 | |||
| 0,72 | 0,008 | |||
| 0,73 | 0,004 | |||
| 0,74 | 0,005 | |||
| 0,75 | 0,006 | |||
| 0,76 | 0,007 | |||
| 0,77 | 0,008 | |||
| 0,78 | 0,004 | |||
| 0,79 | 0,005 | |||
| 0,8 | 0,006 | |||
| 0,81 | 0,007 | |||
| 0,82 | 0,008 | |||
| 0,83 | 0,004 | |||
| 0,84 | 0,005 | |||
| 0,85 | 0,006 | |||
| 0,86 | 0,007 | |||
| 0,87 | 0,004 | |||
| 0,88 | 0,005 | |||
| 0,89 | 0,006 |
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Ministry of Education and Science of the Russian Federation
Federal State Budgetary Educational Institution
higher professional education
"Don State Technical University" (DSTU)
Methods and means of protecting the atmosphere and assessing their effectiveness
Performed:
student of MTS group IS 121
Kolemasova A.S.
Rostov-on-Don
Introduction
2. Mechanical gas purification
Sources used
Introduction
The atmosphere is characterized by extremely high dynamism, due to both the rapid movement of air masses in the lateral and vertical directions, and high speeds and the variety of physical and chemical reactions occurring in it. The atmosphere is considered as a huge “chemical cauldron”, which is influenced by numerous and variable anthropogenic and natural factors. Gases and aerosols emitted into the atmosphere are characterized by high reactivity. Dust and soot arising from fuel combustion and forest fires absorb heavy metals and radionuclides and, when deposited on the surface, can pollute large areas and enter the human body through the respiratory system.
Atmospheric pollution is the direct or indirect introduction of any substance into it in such a quantity that affects the quality and composition of the outside air, causing harm to people, living and inanimate nature, ecosystems, building materials, natural resources - the entire environment.
Air purification from impurities.
To protect the atmosphere from negative anthropogenic impacts, the following measures are used:
Greening of technological processes;
Purification of gas emissions from harmful impurities;
Dispersion of gas emissions in the atmosphere;
Construction of sanitary protection zones, architectural and planning solutions.
Waste-free and low-waste technology.
Greening technological processes is the creation of closed technological cycles, waste-free and low-waste technologies that prevent the release of harmful pollutants into the atmosphere.
The most reliable and most economical way to protect the biosphere from harmful gas emissions is the transition to waste-free production, or to waste-free technologies. The term “waste-free technology” was first proposed by academician N.N. Semenov. It means the creation of optimal technological systems with closed material and energy flows. Such production should not have wastewater, harmful emissions into the atmosphere and solid waste and should not consume water from natural reservoirs. That is, they understand the principle of organization and operation of production, with the rational use of all components of raw materials and energy in a closed cycle: (primary raw materials - production - consumption - secondary raw materials).
Of course, the concept of “waste-free production” is somewhat conditional; This is an ideal production model, since in real conditions it is impossible to completely eliminate waste and get rid of the impact of production on the environment. More precisely, such systems should be called low-waste, producing minimal emissions, in which the damage to natural ecosystems will be minimal. Low-waste technology is an intermediate step in creating waste-free production.
1. Development of waste-free technologies
Currently, several main directions for protecting the biosphere have been identified, which ultimately lead to the creation of waste-free technologies:
1) development and implementation of fundamentally new technological processes and systems operating in a closed cycle, allowing to eliminate the formation of the main amount of waste;
2) processing of production and consumption waste as secondary raw materials;
3) creation of territorial-industrial complexes with a closed structure of material flows of raw materials and waste within the complex.
The importance of economical and rational use of natural resources does not require justification. The world's demand for raw materials is constantly growing, the production of which is becoming more and more expensive. Being an intersectoral problem, the development of low-waste and non-waste technologies and the rational use of secondary resources requires the adoption of intersectoral solutions.
The development and implementation of fundamentally new technological processes and systems operating in a closed cycle, eliminating the formation of the bulk of waste, is the main direction of technical progress.
Purification of gas emissions from harmful impurities
Gas emissions are classified according to the organization of removal and control - into organized and unorganized, by temperature - into heated and cold.
Organized industrial emissions are emissions entering the atmosphere through specially constructed flues, air ducts, and pipes.
Unorganized refers to industrial emissions that enter the atmosphere in the form of undirected gas flows as a result of equipment leakage. Absence or unsatisfactory operation of gas suction equipment in places of loading, unloading and storage of the product.
To reduce air pollution from industrial emissions, gas purification systems are used. Gas purification refers to the separation from gas or transformation into a harmless state of a pollutant coming from an industrial source.
2. Mechanical gas purification
It includes dry and wet methods.
Gas purification in dry mechanical dust collectors.
Dry mechanical dust collectors include devices that use various deposition mechanisms: gravitational (dust settling chamber), inertial (chambers in which dust deposition occurs as a result of changing the direction of gas flow or placing an obstacle in its path) and centrifugal.
Gravity sedimentation is based on the sedimentation of suspended particles under the influence of gravity when dusty gas moves at low speed without changing the direction of flow. The process is carried out in settling flues and dust settling chambers (Fig. 1). To reduce the height of particle deposition in the settling chambers, many horizontal shelves are installed at a distance of 40-100 mm, breaking the gas flow into flat jets. Gravity sedimentation is effective only for large particles with a diameter of more than 50-100 microns, and the degree of purification is no higher than 40-50%. The method is suitable only for preliminary, rough purification of gases.
Dust settling chambers (Fig. 1). The sedimentation of particles suspended in the gas flow in dust settling chambers occurs under the influence of gravity. The simplest designs of devices of this type are settling flues, sometimes equipped with vertical partitions for better sedimentation of solid particles. Multi-shelf dust settling chambers are widely used for cleaning hot furnace gases.
The dust settling chamber consists of: 1 - inlet pipe; 2 - outlet pipe; 3 - body; 4 - suspended particles bunker.
Inertial sedimentation is based on the tendency of suspended particles to maintain their original direction of movement when the direction of the gas flow changes. Among inertial devices, louvered dust collectors with a large number of slits (louvres) are most often used. Gases are dedusted, leaving through the cracks and changing the direction of movement; the gas speed at the entrance to the apparatus is 10-15 m/s. The hydraulic resistance of the device is 100-400 Pa (10-40 mm water column). Dust particles with d< 20 мкм в жалюзийных аппаратах не улавливаются. Степень очистки в зависимости от дисперсности частиц составляет 20-70%. Инерционный метод можно применять лишь для грубой очистки газа. Помимо малой эффективности недостаток этого метода - быстрое истирание или забивание щелей.
These devices are easy to manufacture and operate; they are widely used in industry. But the capture efficiency is not always sufficient.
Centrifugal methods of gas purification are based on the action of centrifugal force that occurs when the gas flow being purified rotates in the purification apparatus or when parts of the apparatus itself rotate. Cyclones (Fig. 2) of various types are used as centrifugal dust cleaning devices: battery cyclones, rotating dust collectors (rotoclones), etc. Cyclones are most often used in industry for the sedimentation of solid aerosols. Cyclones are characterized by high gas productivity, simplicity of design, and operational reliability. The degree of dust removal depends on the particle size. For high-performance cyclones, in particular battery cyclones (with a capacity of more than 20,000 m 3 /h), the degree of purification is about 90% with a particle diameter d> 30 microns. For particles with d = 5-30 µm, the degree of purification is reduced to 80%, and for d == 2-5 µm it is less than 40%.
atmosphere industrial emissions cleaning
In Fig. 2, air is introduced tangentially into the inlet pipe (4) of the cyclone, which is a twisting apparatus. The rotating flow formed here descends through the annular space formed by the cylindrical part of the cyclone (3) and the exhaust pipe (5), into its conical part (2), and then, continuing to rotate, exits the cyclone through the exhaust pipe. (1) - dust release device.
Aerodynamic forces bend the trajectory of particles. During the rotationally downward movement of the dusty flow, dust particles reach the inner surface of the cylinder and are separated from the flow. Under the influence of gravity and the entraining effect of the flow, the separated particles fall and pass through the dust outlet into the hopper.
A higher degree of air purification from dust compared to a dry cyclone can be obtained in wet-type dust collectors (Fig. 3), in which dust is captured as a result of contact of particles with a wetting liquid. This contact can occur on wetted walls flowing around air, on drops or on the free surface of water.
In Fig. Figure 3 shows a cyclone with a water film. Dusty air is supplied tangentially through the air duct (5) to the lower part of the apparatus at a speed of 15-21 m/s. The swirling air flow, moving upward, encounters a film of water flowing down the surface of the cylinder (2). Purified air is discharged from the upper part of the apparatus (4) also tangentially in the direction of rotation of the air flow. A cyclone with a water film does not have an exhaust pipe, which is typical for dry cyclones, which makes it possible to reduce the diameter of its cylindrical part.
The inner surface of the cyclone is continuously irrigated with water from nozzles (3) located around the circumference. The film of water on the inner surface of the cyclone must be continuous, so the nozzles are installed so that the water jets are directed tangentially to the surface of the cylinder along the direction of rotation of the air flow. Dust captured by the water film flows together with water into the conical part of the cyclone and is removed through a pipe (1) immersed in the water of the settling tank. The settled water is fed back into the cyclone. The air speed at the cyclone inlet is 15-20 m/s. The efficiency of cyclones with a water film is 88-89% for dust with particle sizes up to 5 microns, and 95-100% for dust with larger particles.
Other types of centrifugal dust collector are the rotoclone (Fig. 4) and the scrubber (Fig. 5).
Cyclone devices are the most common in industry, since they have no moving parts in the device and high reliability of operation at gas temperatures up to 500 0 C, dry dust collection, almost constant hydraulic resistance of the device, ease of manufacture, and a high degree of purification.
Rice. 4 - Gas washer with a central lower pipe: 1 - inlet pipe; 2 - reservoir with liquid; 3 - nozzle
The dusty gas enters through the central pipe, hits the surface of the liquid at high speed and, turning 180°, is removed from the apparatus. Upon impact, dust particles penetrate into the liquid and are periodically or continuously removed from the apparatus in the form of sludge.
Disadvantages: high hydraulic resistance 1250-1500 Pa, poor capture of particles smaller than 5 microns.
Hollow nozzle scrubbers are columns of round or rectangular cross-section in which contact occurs between gases and droplets of liquid sprayed by nozzles. According to the direction of movement of gases and liquids, hollow scrubbers are divided into counter-flow, direct-flow and with a transverse liquid supply. For wet dust removal, devices with counter-directional movement of gases and liquids are usually used, less often - with a transverse supply of liquid. Once-through hollow scrubbers are widely used in evaporative cooling of gases.
In a countercurrent scrubber (Fig. 5.), droplets from the nozzles fall towards the dusty gas flow. The drops must be large enough so as not to be carried away by the gas flow, the speed of which is usually vg = 0.61.2 m/s. Therefore, coarse spray nozzles operating at a pressure of 0.3-0.4 MPa are usually installed in gas scrubbers. At gas velocities of more than 5 m/s, a droplet eliminator must be installed after the gas scrubber.
Rice. 5 - Hollow nozzle scrubber: 1 - body; 2 - gas distribution grid; 3 - nozzles
The height of the apparatus is usually 2.5 times its diameter (H = 2.5D). Nozzles are installed in the apparatus in one or several sections: sometimes in rows (up to 14-16 in a cross-section), sometimes only along the axis of the apparatus. The spray pattern of the nozzles can be directed vertically from top to bottom or at a certain angle to the horizontal plane. When nozzles are arranged in several tiers, a combined installation of sprayers is possible: part of the torches is directed along the flow of gases, the other part - in the opposite direction. For better distribution of gases across the cross section of the apparatus, a gas distribution grid is installed in the lower part of the scrubber.
Hollow nozzle scrubbers are widely used for collecting coarse dust, as well as in gas refrigeration and air conditioning. The specific liquid consumption is small - from 0.5 to 8 l/m 3 of purified gas.
Filters are also used to purify gases. Filtration is based on the passage of the purified gas through various filter materials. Filter partitions consist of fibrous or granular elements and are conventionally divided into the following types.
Flexible porous partitions - fabric materials made of natural, synthetic or mineral fibers, non-woven fibrous materials (felt, paper, cardboard) cellular sheets (sponge rubber, polyurethane foam, membrane filters).
Filtration is a very common technique for fine gas purification. Its advantages are the comparatively low cost of equipment (with the exception of metal-ceramic filters) and high efficiency of fine cleaning. Disadvantages of filtration are high hydraulic resistance and rapid clogging of the filter material with dust.
3. Purification of emissions of gaseous substances from industrial enterprises
At present, when waste-free technology is in its infancy and there are no completely waste-free enterprises yet, the main task of gas purification is to bring the content of toxic impurities in gas impurities to the maximum permissible concentrations (MPC) established by sanitary standards.
Industrial methods for purifying gas emissions from gas and vapor toxic impurities can be divided into five main groups:
1. Absorption method - consists of the absorption of individual components of a gaseous mixture by an absorbent (absorber), which is a liquid.
Absorbents used in industry are assessed according to the following indicators:
1) absorption capacity, i.e. solubility of the extracted component in the absorber depending on temperature and pressure;
2) selectivity, characterized by the ratio of the solubilities of the separated gases and the rates of their absorption;
3) minimum vapor pressure to avoid contamination of the purified gas with absorbent vapors;
4) low cost;
5) no corrosive effect on the equipment.
Water, solutions of ammonia, caustic and carbonate alkalis, manganese salts, ethanolamines, oils, suspensions of calcium hydroxide, manganese and magnesium oxides, magnesium sulfate, etc. are used as absorbents. For example, for purifying gases from ammonia, hydrogen chloride and hydrogen fluoride as an absorbent They use water, sulfuric acid to capture water vapor, and oil to capture aromatic hydrocarbons.
Absorption purification is a continuous and, as a rule, cyclic process, since the absorption of impurities is usually accompanied by the regeneration of the absorption solution and its return at the beginning of the purification cycle. During physical absorption, the regeneration of the absorbent is carried out by heating and reducing the pressure, resulting in desorption of the absorbed gas impurity and its concentration.
To implement the cleaning process, absorbers of various designs are used (film, packed, tubular, etc.). The most common is a packed scrubber used to purify gases from sulfur dioxide, hydrogen sulfide, hydrogen chloride, chlorine, carbon monoxide and dioxide, phenols, etc. In packed scrubbers, the rate of mass transfer processes is low due to the low-intensity hydrodynamic regime of these reactors, operating at a gas speed of 0.02-0.7 m/s. The volumes of the apparatus are therefore large and the installations are cumbersome.
Rice. 6 - Packed scrubber with transverse irrigation: 1 - body; 2 - nozzles; 3 - irrigation device; 4 - support grid; 5 - nozzle; 6 - sludge collector
Absorption methods are characterized by the continuity and versatility of the process, efficiency and the ability to extract large quantities of impurities from gases. The disadvantage of this method is that packed scrubbers, bubbling and even foam devices provide a fairly high degree of extraction of harmful impurities (up to the maximum permissible concentration) and complete regeneration of absorbers only with a large number of purification stages. Therefore, technological schemes for wet cleaning are usually complex, multi-stage, and treatment reactors (especially scrubbers) have large volumes.
Any process of wet absorption purification of exhaust gases from gas and vapor impurities is advisable only if it is cyclical and waste-free. But cyclic wet cleaning systems are competitive only when they are combined with dust cleaning and gas cooling.
2. Chemisorption method - based on the absorption of gases and vapors by solid and liquid absorbers, resulting in the formation of slightly volatile and slightly soluble compounds. Most chemisorption gas purification processes are reversible, i.e. When the temperature of the absorption solution increases, the chemical compounds formed during chemisorption decompose with the regeneration of the active components of the absorption solution and with the desorption of impurities absorbed from the gas. This technique forms the basis for the regeneration of chemisorbents in cyclic gas cleaning systems. Chemisorption is especially applicable for fine purification of gases with a relatively low initial concentration of impurities.
3. Adsorption method - based on the capture of harmful gas impurities by the surface of solids, highly porous materials with a developed specific surface area.
Adsorption methods are used for various technological purposes - separation of vapor-gas mixtures into components with the separation of fractions, drying of gases and for sanitary cleaning of gas exhausts. Recently, adsorption methods have come to the fore as a reliable means of protecting the atmosphere from toxic gaseous substances, providing the possibility of concentrating and recycling these substances.
Industrial adsorbents most often used in gas purification are activated carbon, silica gel, aluminum gel, natural and synthetic zeolites (molecular sieves). The main requirements for industrial sorbents are high absorption capacity, selectivity of action (selectivity), thermal stability, long service without changing the structure and properties of the surface, and the possibility of easy regeneration. Activated carbon is most often used for sanitary gas purification due to its high absorption capacity and ease of regeneration. Various designs of adsorbents are known (vertical, used at low flow rates, horizontal, used at high flow rates, annular). Gas purification is carried out through fixed layers of adsorbent and moving layers. The gas to be purified passes through the adsorber at a speed of 0.05-0.3 m/s. After cleaning, the adsorber switches to regeneration. An adsorption plant, consisting of several reactors, operates generally continuously, since at the same time some reactors are at the purification stage, while others are at the stages of regeneration, cooling, etc. Regeneration is carried out by heating, for example, by burning out organic substances, passing hot or superheated steam, air , inert gas (nitrogen). Sometimes an adsorbent that has lost activity (shielded by dust, resin) is completely replaced.
The most promising are continuous cyclic processes of adsorption gas purification in reactors with a moving or suspended layer of adsorbent, which are characterized by high gas flow rates (an order of magnitude higher than in batch reactors), high gas productivity and work intensity.
General advantages of adsorption methods of gas purification:
1) deep purification of gases from toxic impurities;
2) the relative ease of regenerating these impurities with their transformation into a commercial product or return to production; In this way, the principle of waste-free technology is implemented. The adsorption method is especially rational for removing toxic impurities (organic compounds, mercury vapor, etc.) contained in low concentrations, i.e. as the final stage of sanitary cleaning of waste gases.
The disadvantage of most adsorption plants is the frequency.
4. Catalytic oxidation method - based on removing impurities from the gas being purified in the presence of catalysts.
The action of catalysts is manifested in the intermediate chemical interaction of the catalyst with the reacting substances, resulting in the formation of intermediate compounds.
Metals and their compounds (oxides of copper, manganese, etc.) are used as catalysts. Catalysts have the form of balls, rings or other shapes. This method is especially widely used for exhaust gas purification. As a result of catalytic reactions, impurities in the gas are converted into other compounds, i.e. in contrast to the methods considered, impurities are not extracted from the gas, but are transformed into harmless compounds, the presence of which is acceptable in the exhaust gas, or into compounds that are easily removed from the gas flow. If the formed substances must be removed, then additional operations are required (for example, extraction with liquid or solid sorbents).
Catalytic methods are becoming increasingly widespread due to the deep purification of gases from toxic impurities (up to 99.9%) at relatively low temperatures and normal pressure, as well as at very low initial concentrations of impurities. Catalytic methods make it possible to utilize reaction heat, i.e. create energy technology systems. Catalytic treatment plants are easy to operate and small in size.
The disadvantage of many catalytic purification processes is the formation of new substances that must be removed from the gas by other methods (absorption, adsorption), which complicates the installation and reduces the overall economic effect.
5. The thermal method involves purifying gases before releasing them into the atmosphere by high-temperature afterburning.
Thermal methods for neutralizing gas emissions are applicable at high concentrations of flammable organic pollutants or carbon monoxide. The simplest method, flaring, is possible when the concentration of flammable pollutants is close to the lower flammability limit. In this case, the impurities serve as fuel, the process temperature is 750-900°C and the combustion heat of the impurities can be utilized.
When the concentration of flammable impurities is less than the lower flammability limit, it is necessary to supply a certain amount of heat from the outside. Most often, heat is supplied by adding combustible gas and burning it in the purified gas. Combustible gases pass through a heat recovery system and are released into the atmosphere.
Such energy technology schemes are used when the content of flammable impurities is sufficiently high, otherwise the consumption of added combustible gas increases.
Sources used
1. Environmental doctrine of the Russian Federation. Official website of the State Service for Environmental Protection of Russia - eco-net/
2. Vnukov A.K., Protection of the atmosphere from emissions from energy facilities. Directory, M.: Energoatomizdat, 2001
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Atmospheric pollution is the introduction into the air of chemical, physical and biological substances that are not characteristic of it or a change in their natural concentration. In conditions of active technogenesis, this problem has become extremely acute and has necessitated the development of a set of measures to reduce its diverse negative impact.
Currently, the following groups of measures aimed at preventing air pollution can be distinguished: technological, planning and sanitary-technical. As a special group, measures of a legal and economic nature should be noted, which will be discussed in Chapter. 10.
Technological activities, aimed primarily at the implementation of one of the principles of rational environmental management, which is the greening of production. This means similarizing production processes, i.e. resource cycles, natural closed cycles of substances in the biosphere. The basis of greening is the development and implementation of low-waste, energy- and resource-saving technologies. In fact, waste-free technology is impossible in principle due to the law of conservation of matter. Of course, in natural biogeochemical cycles, part of the substance is also constantly excluded from the circulation, however, there is a fundamental difference between these processes and resource cycles: in nature, the substance does not pollute the environment and does not go into waste, but into reserve.
This group also includes the replacement of harmful substances in production with less harmful or harmless ones, purification of raw materials from harmful impurities (desulfurization of fuel before combustion), replacement of dry methods of processing dust-producing materials with wet ones, replacement of flame heating with electric heating, sealing of processes, use of hydraulic and pneumatic transport when transporting dust-producing materials, replacing intermittent processes with continuous ones.
To the group planning activities includes a set of techniques, including taking into account the wind rose, zoning the city territory, organizing sanitary protection zones, landscaping populated areas, and planning residential areas.
Typically, industrial zones are located in well-ventilated areas of the city downwind of residential areas. They take into account not only the average annual wind rose, but also seasonal ones, as well as wind speeds of individual directions.
The shielding function of the building is known, and therefore zoning for the development of blocks bordering main streets is being developed. It is recommended to build up the zone closest to the highway with buildings for public utility purposes, the next with low-rise buildings, the third zone with high-rise buildings, and the fourth with children's and medical institutions, i.e. developments with increased requirements for air quality. To combat air pollution in residential areas from vehicle exhaust gases, the type of development also matters. It is advisable to use closed development methods only in cities where high-speed winds (above 5 m/s) prevail. Also of great importance in reducing air pollution in populated areas are intra-block green spaces and landscaping of main streets.
In cases where environmental and hygienic indicators exceed standards, there is a need for sanitary measures:, consisting of including devices in the system for removing technological and ventilation emissions to clean them from impurities.
Air emission purification devices are divided into: dust collectors (dry, wet, filters, etc.); mist eliminators (low-speed and high-speed); devices for collecting vapors and gases (absorption, chemisorption, adsorption and neutralizers); multi-stage cleaning devices (dust and gas collectors, mists and solid impurities collectors, multi-stage dust collectors). The operation of such devices is characterized by a number of parameters, the main of which are cleaning efficiency, hydraulic resistance and power consumption.
Cleaning efficiency
where c in and c out are the mass concentrations of impurities in the gas, respectively, before and after the apparatus.
In some cases, the concept of fractional cleaning efficiency is used for dusts:
where c input j and from out, - mass concentrations i-Pi dust fractions before and after the dust collector, respectively.
To assess the effectiveness of the cleaning process, the substance breakthrough coefficient is also used TO via cleaning device:
As follows from formulas (5.2) and (5.3), the breakthrough coefficient and cleaning efficiency are related by the relation K = 1 - G.
Hydraulic resistance of devices cleaning Ar determined as the difference in air flow pressure at the apparatus inlet r in and exit /; get out of it. Meaning Ar found experimentally or calculated using the formula
where?, is the coefficient of hydraulic resistance of the apparatus; r and W- density and air speed, respectively, in the design section of the apparatus.
During the cleaning process, the hydraulic resistance of the device increases, therefore, upon reaching a certain regulated value, the cleaning process must be stopped and the device must be regenerated or replaced.
Power N the stimulator of air movement is determined by hydraulic resistance and volumetric flow Q purified gas:
Where k- power reserve factor, usually k = 1.1 -2- 1.15; g|m - efficiency of power transmission from the electric motor to the fan, usually c m = = 0.92 0.95; g| c - fan efficiency, usually g| c = 0.65 -2- 0.8.
The range of devices for purifying air from impurities is very extensive, which is explained by the variety and complexity of modern technologies. Received well-deserved recognition among air purification devices for removing particulate matter. dry dust collectors - cyclones (Fig. 5.2) of various types (cylindrical and conical). Contaminated air is introduced into the cyclone through a pipe 2 tangential to the inner surface of the housing 1 and makes a rotational-translational movement along the body towards the hopper 4. Under the influence of centrifugal force, dust particles form a layer on the wall of the cyclone, which, together with part of the air, enters the hopper. Freed from dust, the resulting air vortex exits the hopper and leaves the cyclone through the outlet pipe 3.
Rice. 5.2.
Used for cleaning large masses battery cyclones, consisting of a large number of parallel installed cyclone elements. Structurally, they are combined into one housing and have a common gas supply and outlet. Experience in operating battery cyclones has shown that the cleaning efficiency of such cyclones is somewhat lower than the efficiency of individual elements due to the flow of gases between the cyclone elements.
For fine purification of air from particles and droplets, various filters. The filtration process consists of retaining impurity particles on porous partitions as dispersed media move through them (Fig. 5.3).
Rice. 53.
The filter is a housing 1 , separated by a porous partition (filter element) 2 into two cavities. Contaminated gases enter the filter and are cleaned as they pass through the filter element. Impurity particles settle on the inlet part of the porous partition, forming a layer on the surface of the partition 3> and linger in the pores. For newly arriving particles, this layer becomes part of the filter baffle, which increases the filter cleaning efficiency and the pressure drop across the filter element. The deposition of particles on the surface of the pores of the filter element occurs as a result of the combined action of the touch effect, as well as diffuse, inertial and gravitational effects.
Filters are classified according to various criteria: filter element type, filter design and purpose, cleaning, etc.
According to the type of filter element, they are: with granular layers (fixed, loosely poured, fluidized); with flexible porous partitions (fabrics, felts, fiber mats, sponge rubber, polyurethane foam, etc.); with semi-rigid porous partitions (knitted and woven meshes, pressed spirals, etc.); with rigid porous partitions (porous ceramics, porous metals, etc.).
Electrical cleaning(electric precipitators) are one of the most advanced types of air purification from suspended dust and fog particles. This process is based on the ionization of air, the transfer of ion charge to impurity particles and the deposition of the latter on precipitation and corona electrodes.
Wet gas cleaning devices - wet dust collectors- are widely used, as they are characterized by high cleaning efficiency from fine dust with d 4> 0.3 microns, as well as the ability to clean heated air from dust. The scope of their application is limited by a number of disadvantages: the formation of sludge during the cleaning process, which requires special systems for its processing; removal of moisture into the atmosphere and the formation of deposits in exhaust flues when the air is cooled to the dew point temperature; the need to create circulating systems for supplying water to the dust collector.
Wet cleaning devices operate on the principle of deposition of dust particles onto the surface of either droplets or a film of liquid under the influence of inertial forces and Brownian motion.
Among wet cleaning devices with the deposition of dust particles on the surface of droplets, the most applicable in practice are: Venturi scrubbers(Fig. 5.4). The main part of the scrubber is the Venturi nozzle 2. A dusty air flow is supplied to its confuser part and through centrifugal nozzles 1 - liquid for irrigation. In the exhaust part of the nozzle, air is accelerated from the input speed (W r= 15 -s- 20 m/s) to a speed in the narrow section of the nozzle of 80-200 m/s or more. The process of dust deposition on liquid droplets is determined by the mass of the liquid, the developed surface of the droplets and the high relative velocity of liquid and dust particles in the confuser part of the nozzle. The efficiency of cleaning largely depends on the uniformity of liquid distribution over the cross-section of the convergent part of the nozzle. In the diffuser part of the nozzle, the flow is slowed down to a speed of 15-20 m/s and fed into the droplet eliminator 3, usually performed in the form of a direct-flow cyclone.
Rice. 5.4.
Venturi scrubbers provide high cleaning efficiency from aerosols with an initial impurity concentration of up to 100 g/m 3 . They are also widely used in air de-mist systems, where their efficiency reaches 0.999, which is quite comparable to high-efficiency filters.
To clean the air from mists of acids, alkalis, oils and other liquids, fiber filters are used - mist eliminators, the operating principle of which is based on the deposition of droplets on the surface of the pores with the subsequent flow of liquid through the fibers into the lower part of the mist eliminator. The deposition of liquid droplets occurs under the influence of Brownian motion or an inertial mechanism for separating pollutant particles from the gas phase on filter elements.
Absorption - purification of emissions from gases and vapors, based on the absorption of the latter by liquid in special devices - absorbers. The most important condition for the applicability of the method is the solubility of vapors or gases in the absorbent, assessed by its absorption capacity. In most cases, water is used as an absorbent, but in some cases it is necessary to resort to special liquids of a rather complex composition. The absorption of gases and vaporous impurities occurs in the process of counter-movement of contaminated air from below and the absorbent entering from above through the sprinkler 2 on nozzles 1 (Fig. 5.5). Structurally, absorbers are implemented in the form of packed towers, bubbling-foam, spraying and other devices.
Rice. 5.5. Packed tower diagram:
1 - nozzle; 2 - sprinkler
Chemisorption is based on the absorption of gases and vapors by liquid or solid absorbers with the formation of slightly soluble or slightly volatile chemical compounds. The reactions occurring in this case are mainly exothermic and reversible, therefore, when the temperature of the solution increases, the resulting chemical compound decomposes with the release of the original elements.
The absorption capacity of a chemisorbent is almost independent of pressure, so chemisorption is more beneficial when the concentration of harmful substances in the exhaust gases is low.
The main devices for implementing the process are packed towers, bubbling-foam devices, Venturi scrubbers, etc. Chemisorption is one of the common methods of purifying polluted air from nitrogen oxides (purification efficiency of nitrogen oxides 0.17-0.86) and acid vapors (purification efficiency 0.95).
Adsorption based on the ability of some finely divided solids ( adsorbents) selectively extract and concentrate individual components of the gas mixture on its surface. Substances with a large surface area per unit mass (activated carbons, as well as simple and complex oxides - activated alumina, silica gel, activated aluminum oxide, synthetic zeolites or molecular sieves) are used as adsorbents or absorbers.
Adsorbers are used to clean air from organic vapors, remove unpleasant odors and gaseous impurities contained in small quantities in industrial emissions, as well as volatile solvents and a number of other gases.
Structurally, adsorbers are made in the form of containers filled with a porous adsorbent, through which the flow of the gas being purified is filtered. Cartridges with adsorbent are widely used in respirators and gas masks.
Thermal neutralization is based on the ability of flammable gases and vapors in ventilation or process emissions to burn to form less toxic substances. For this method, neutralizers are used that use various thermal neutralization schemes: direct combustion; thermal oxidation; catalytic afterburning.
Direct combustion is used in cases where the purified gases have significant energy sufficient to maintain combustion (flaring of combustible waste in petrochemicals).
Thermal oxidation is used in cases where the gases to be purified have a high temperature but do not contain enough oxygen or when the concentration of flammable substances is low and insufficient to maintain a flame.
In the first case, the thermal oxidation process is carried out in a chamber with fresh air supply (afterburning of carbon monoxide and hydrocarbons), and in the second case, with additional natural gas supplied.
Catalytic afterburning is used to convert toxic components contained in exhaust gases into non-toxic or less toxic ones by contacting them with catalysts. To implement the process, it is necessary, in addition to the use of catalysts, to maintain such gas flow parameters as temperature and gas velocity. Platinum, palladium, copper, etc. are used as catalysts.
Catalytic converters are used to neutralize carbon monoxide, volatile hydrocarbons, solvents, exhaust gases, etc.
For highly effective purification of multicomponent emissions (with simultaneous purification from solid and gaseous impurities, when purifying solid impurities and droplets, etc.) it is necessary to use multi-stage cleaning devices. In this case, the gases to be purified sequentially pass through several autonomous purification devices or one unit that includes several purification stages.
In a system of series-connected devices, the overall cleaning efficiency d) is determined by the expression
where gr, g| 2 ,G| n - cleaning efficiency 1, 2 and P th devices.