The main ways to protect the atmosphere from pollution. Methods and means of protecting the atmosphere Basic methods of protecting the atmosphere from chemical impurities Methods and means of protecting the atmosphere
Atmospheric pollution is the introduction of chemical, physical and biological substances that are not characteristic of it into the air or a change in their natural concentration. In the conditions of active technogenesis, this problem has become extremely acute and necessitated the development of a set of measures to reduce its versatile negative impact.
At present, the following groups of measures aimed at preventing atmospheric 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 Chap. ten.
technological events, aimed primarily at the implementation of one of the principles of rational environmental management, which consists in the greening of production. This means the assimilation of 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. Actually wasteless technology is impossible in principle due to the law of conservation of matter. Of course, in natural biogeochemical cycles, a part of the substance is also constantly excluded from the circulation, but there is a fundamental difference between these processes and resource cycles: in nature, the substance does not pollute the environment and goes not to waste, but to the reserve.
This group also includes the replacement of harmful substances in production with less harmful or harmless ones, the purification of raw materials from harmful impurities (fuel desulfurization before burning it), the replacement of dry methods for processing dusty materials with wet ones, the replacement of flame heating with electric heating, the sealing of processes, the use of hydro- and pneumatic transport when transporting dusty 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, organizing sanitary protection zones, planting greenery in 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, as well as the speed of the winds of individual points.
The shielding function of the building is known, in connection with which the zoning of the building blocks bordering the main streets is being developed. It is recommended to build up the zone closest to the highway with public utility buildings, the next zone - with low-rise buildings, the third zone - with high-rise buildings, and the fourth - with children's, medical institutions, i.e. buildings with high air quality requirements. To combat air pollution in residential areas by exhaust gases of vehicles, the type of development is also important. It is advisable to use closed building 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-quarter green spaces and landscaping of main streets.
In cases where environmental and hygienic indicators exceed the standards, it becomes necessary to sanitary measures:, consisting in the inclusion in the system of removal of technological and ventilation emissions of devices for their purification from impurities.
Devices for cleaning emissions into the atmosphere are divided into: dust collectors (dry, wet, filters, etc.); mist eliminators (low and high speed); devices for capturing vapors and gases (absorption, chemisorption, adsorption and neutralizers); multi-stage cleaning devices (dust and gas traps, mists and solid impurities traps, multi-stage dust traps). 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, for dusts, the concept of fractional cleaning efficiency is used:
where from in j and from the output, - mass concentrations i-Pi dust fractions before and after the dust collector, respectively.
To assess the effectiveness of the cleaning process, the breakthrough coefficient of substances is also used To through the cleaning machine:
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 is defined as the pressure difference of the air flow at the inlet of the apparatus r in and output /; get out of it. Meaning Ar found experimentally or calculated by the formula
where?, - coefficient of hydraulic resistance of the apparatus; p and W- air density and velocity, respectively, in the calculated section of the apparatus.
During the cleaning process, the hydraulic resistance of the apparatus increases, therefore, upon reaching a certain regulated value, the cleaning process must be stopped and the apparatus regenerated or replaced.
Power N air movement stimulator is determined by hydraulic resistance and volumetric flow Q purified gas:
where k- power factor, usually k = 1.1-2-1.15; g|m - efficiency of power transfer 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 air purification from impurities is very extensive, which is explained by the diversity and complexity of modern technologies. A well-deserved recognition among devices for cleaning air from particulate matter has received dry dust collectors - cyclones (Fig. 5.2) of various types (cylindrical and conical). Polluted air is introduced into the cyclone through a nozzle 2 tangential to the inner surface of the body 1 and performs a rotational-translational movement along the body to the bunker 4. Under the action of centrifugal force, dust particles form a layer on the cyclone wall, 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 to clean large volumes battery cyclones, consisting of a large number of cyclone elements installed in parallel. Structurally, they are combined into one building and have a common gas supply and discharge. Operating experience with battery cyclones has shown that the cleaning efficiency of such cyclones is slightly 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 dropping liquid, various methods are used. filters. The filtration process consists in retaining particles of impurities on porous partitions when dispersed media move through them (Fig. 5.3).
Rice. 53.
The filter is a body 1 , separated by a porous partition (filter element) 2 into two cavities. Contaminated gases enter the filter and are cleaned when passing through the filter element. Particles of impurities settle on the inlet part of the porous partition, forming a layer on the surface of the partition 3> and stay in the pores. For newly arriving particles, this layer becomes part of the filter wall, 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: type of filter element, filter design and purpose, cleaning, etc.
According to the type of filter element, they are: with granular layers (fixed, freely poured, fluidized); with flexible porous partitions (fabrics, felts, fibrous mats, sponge rubber, polyurethane foam, etc.); with semi-rigid porous partitions (knitted and woven nets, pressed spirals, etc.); with rigid porous partitions (porous ceramics, porous metals, etc.).
Electrical cleaning(electrostatic precipitators) - one of the most advanced types of air purification from dust and fog particles suspended in it. This process is based on air ionization, ion charge transfer to impurity particles and their deposition on collecting and corona electrodes.
Wet gas scrubbers - wet dust collectors- are widely used, as they are characterized by high cleaning efficiency from fine dust with d4> 0.3 microns, as well as the possibility of cleaning dust from heated air. The scope of their application is limited by a number of disadvantages: the formation of sludge in the process of cleaning, which requires special systems for its processing; removal of moisture into the atmosphere and the formation of deposits in the exhaust gas ducts 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 on the surface of either drops or liquid films under the action of inertia forces and Brownian motion.
Among the wet cleaning devices with the deposition of dust particles on the surface of the drops, in practice the most applicable 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 - irrigation fluid. In the kopfusoria part of the nozzle, the air is accelerated from the input velocity (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 drops is due to the mass of the liquid, the developed surface of the drops, and the high relative velocity of the liquid and dust particles in the confusing part of the nozzle. The cleaning efficiency largely depends on the uniformity of the distribution of the liquid over the cross section of the confuser part of the nozzle. In the diffuser part of the nozzle, the flow is decelerated to a speed of 15-20 m/s and fed into the drop catcher 3, performed usually in the form of a once-through cyclone.
Rice. 5.4.
Venturi scrubbers provide high efficiency of aerosol removal at initial impurity concentration up to 100 g/m 3 . They are also widely used in air mist removal 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, fibrous filters are used - mist eliminators, the principle of operation of which is based on the deposition of drops on the surface of the pores, followed by the flow of liquid along the fibers into the lower part of the mist eliminator. Settling of liquid droplets occurs under the action of Brownian motion or an inertial mechanism for separating pollutant particles from the gas phase on the filter elements.
Absorption - purification of emissions from gases and vapors, based on the absorption of the latter by liquid in special apparatus - absorbers. The most important condition for the applicability of the method is the solubility of vapors or gases in the absorbent, estimated 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 oncoming movement of polluted air from below and the absorbent coming 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 scheme:
1 - nozzle; 2 - sprinkler
Chemisorption is based on the absorption of gases and vapors by liquid or solid absorbers with the formation of poorly soluble or low-volatile chemical compounds. The reactions occurring in this case are mainly exothermic and reversible, therefore, with an increase in the temperature of the solution, the resulting chemical compound decomposes with the release of the initial elements.
The absorption capacity of the chemisorbent is almost independent of pressure, so chemisorption is more beneficial at low concentrations of harmful substances in the exhaust gases.
The main apparatus for the implementation of the process are packed towers, bubbling-foam apparatuses, Venturi scrubbers, etc. Chemisorption is one of the common methods for cleaning polluted air from nitrogen oxides (the efficiency of cleaning from nitrogen oxides is 0.17-0.86) and acid vapors (the cleaning efficiency is 0.95).
Adsorption is based on the ability of some fine solids ( adsorbents) selectively extract and concentrate individual components of the gas mixture on its surface. As adsorbents, or absorbers, substances are used that have a large surface area per unit mass (activated carbons, as well as simple and complex oxides - activated alumina, silica gel, activated alumina, synthetic zeolites or molecular sieves).
Adsorbers are used to purify 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 stream of gas to be purified is filtered. Adsorbent cartridges are widely used in respirators and gas masks.
Thermal neutralization is based on the ability of combustible gases and vapors in the composition of 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 combustion.
Direct combustion is used in cases where the gases to be treated have significant energy sufficient to sustain combustion (flare combustion of combustible waste in petrochemistry).
Thermal oxidation is used when the gases to be treated have a high temperature, but do not contain enough oxygen, or when the concentration of combustible substances is insignificant and insufficient to maintain a flame.
In the first case, the thermal oxidation process is carried out in a chamber with the supply of fresh air (afterburning of carbon monoxide and hydrocarbons), and in the second case, when additional natural gas is supplied.
Catalytic afterburning is used to convert toxic components contained in exhaust gases into non-toxic or less toxic components by contact 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 efficient purification of multicomponent emissions (with simultaneous purification from solid and gaseous impurities, during purification from solid impurities and dropping liquid, etc.), it is necessary to use multi-stage cleaning devices. In this case, the gases to be purified sequentially pass through several autonomous purification apparatuses or one unit, which includes several purification stages.
In a system of series-connected devices, the overall cleaning efficiency d) is determined by the expression
where gr, r| 2 ,G| n - cleaning efficiency 1, 2 and P th devices.
Lecture 11
The atmospheric air surrounding a person is constantly exposed to pollution. The air of the production premises is polluted by emissions from process equipment or during technological processes without the localization of waste substances. Ventilation air removed from the premises can cause air pollution in industrial sites and populated areas. In addition, the air of industrial sites and populated areas is polluted by technological emissions from workshops, emissions from thermal power plants, and vehicles.
The air in residential premises is polluted by products of combustion of natural gas and other fuels, fumes of solvents, detergents, wood-shaving structures, etc., as well as toxic substances entering residential premises with an influx of ventilation air. In summer, at an average outdoor temperature of 20 0 С, about 90% of outdoor air impurities penetrate into living quarters, during the transition period at t = 25 0 С - 40%, in winter - up to 30%.
Sources of atmospheric air pollution in industrial premises are:
1. In foundries, these are dust and gas emissions from cupolas, electric arc and induction furnaces, areas for storing and processing charge (casting components) and molding materials, areas for knocking out and cleaning castings.
2. In the forging and pressing shops - dust, carbon monoxide, sulfur oxide and other harmful substances.
3. In electroplating shops - these are harmful substances in the form of fine mist, vapors and gases. The most intensively harmful substances are released in the processes of acid and alkali etching. When applying galvanic coatings, this is hydrogen fluoride, etc.
4. When machining metals on machine tools - dust, fog, oils and emulsions.
5. In the areas of welding and cutting of metals - dust, gases (hydrogen fluoride, etc.).
6. In soldering and tinning areas - toxic gases (carbon monoxide, hydrogen fluoride), aerosols (lead and its compounds).
7. In painting shops - toxic substances during degreasing and aerosols from varnish and paints.
8. From the operation of various power plants (ICE, etc.)
To remove and clean the air in industrial premises, various systems for cleaning and localizing harmful substances are used.
1. Removal of toxic substances from the premises by general ventilation;
2. Localization of toxic substances in the zone of their formation by local ventilation with purification of polluted air in special devices and its return to the industrial or domestic premises, if the air after cleaning in the device meets the regulatory requirements for supply air;
3. Localization of toxic substances in the zone of their formation by local ventilation, purification of polluted air in special devices, release and dispersion in the atmosphere.
Figure 3
1 - sources of toxic substances;
2 - devices for localization of toxic substances (local suction);
3 - cleaning apparatus.
4. Purification of technological gas emissions in special devices; in some cases, exhaust gases are diluted with atmospheric air before being released;
5. Purification of exhaust gases from power plants (for example, internal combustion engines) in special units, and release into the atmosphere or production area (mines, quarries, storage facilities, etc.).
In cases where real emissions exceed the maximum allowable emissions (MAE), taking into account already existing atmospheric pollution or, more precisely, its components already existing in the atmosphere, it is necessary to use devices for cleaning gases and impurities in the emission system.
Figure 4
1 – source of toxic substances and process gases;
2 - cleaning apparatus;
3 - pipe for dispersion of emissions;
4 - device (blower for supplying air to dilute emissions).
Devices for cleaning ventilation and technological emissions into the atmosphere are divided into:
Dust collectors (dry, electric, wet filters);
Mist eliminators (low speed and high speed);
Apparatus for capturing vapors and gases (absorption, chemisorption, absorption and neutralizers);
Multi-stage cleaning devices (dust and gas traps, mists and solid impurities traps, multi-stage dust traps).
Dry dust collectors - cyclones - have been widely used for cleaning gases from particles.
Electrostatic precipitators are the most perfect way to purify gases from dust particles and mists suspended in them.
Various filters are used to finely purify gases from particles and dropping liquid.
Wet gas scrubbers are widely used and are used to remove fine dust with d 2 ≥ 0.3 µm, as well as to remove dust from heated and explosive gases.
To clean the air from mists of acids, alkalis, oils and other liquids, fibrous filters are used.
Absorption method - purification of gas emissions from gases and vapors - is based on the absorption of the latter by liquid. The decisive condition for the application of this method is the solubility of gases and vapors in water. These can be, for example, technological emissions of ammonia, chlorine or hydrogen fluoride.
The work of chemisorbers is based on the absorption of gases and vapors by liquid or solid absorbers with the formation of poorly soluble and low-volatile chemical compounds (gases from nitrogen oxides and acid vapors).
The absorption method is based on the ability of some fine solids as an absorbent (activated alumina, silica gel, activated alumina, etc.) to extract and concentrate individual components of gas mixture emissions on their surface. They are used to clean the air from the vapors of solvents, ether, acetone, various hydrocarbons, etc. Absorbents are widely used in respirators and gas masks.
Thermal neutralization is based on the ability of combustible gases and vapors that are part of ventilation and process emissions to burn to form less toxic substances.
Passive methods are divided into:
1) emission control:
The sanitary protection zone is a strip of land that separates the enterprise from residential development. The width depends on the power, the volume of emissions, the concentration of emissions, the noise generated. The territory of sanitary protection zones must be landscaped (>
Air dedusting methods. Main technical indicators of dust collectors.
For dust removal, dry and wet dust collectors, as well as dry and wet electrostatic precipitators are used. The choice of method and apparatus for capturing aerosols depends on the disperse composition (particle size in the air), efficiency, consumption or performance of the apparatus.
Capture efficiency or degree of purification - is expressed by the amount of captured material that entered the gas cleaning apparatus with a gas stream over a certain period of time. (G 1 , G 2 - mass flow (concentration) of dust particles contained in the gas at the inlet and outlet of the apparatus [kg/h]).
The operation of dry apparatuses is based on gravitational, inertial and centrifugal mechanisms of sedimentation or filtration mechanisms. The main dry cleaning devices include: dust settling chambers, cyclones, filters, electrostatic precipitators.
"+" - the temperature of emissions after cleaning reaches up to 50 () ° С (there is a possibility of recycling):
When hot gases are released, their dispersion in the atmosphere improves;
No water consumption and wastewater generation;
Possibility to return the captured dust back to production.
"-" - possible condensation of vapors on the walls of the apparatus, which leads to corrosion of the walls and the formation of dust deposits that are difficult to catch;
Difficulty in removing trapped dust (possibility of secondary air pollution).
Centrifugal dust collectors.
These include various types of cyclones and swirl dust collectors.
Cyclone. They are most widely used in industry (for catching ash at thermal power plants, at woodworking plants). η=90%, d>10µm.
"+" - absence of moving parts in the device;
Reliable operation at high temperatures (up to 500°C) - when working with higher ° t are made of special. materials;
Ability to capture abrasive materials (the inner surface of the cyclone is treated with a special coating);
Constant hydraulic resistance;
Good performance at high gas pressures;
Ease of manufacture.
"-" - low efficiency when capturing particles less than 5 microns;
High hydraulic resistance (1.2-1.5 kPa).
1 inlet
In the cyclone, a spiral swirling of the flow takes place, as a result of which the particles are thrown to the walls and gradually descend into the hopper 2. The OM is ejected into the atmosphere through the outlet 3. Aerosol particles move along the resulting force Fp and are pressed against the inner surfaces of the body (pipe) and slide down along this surface and fall into the dust collector. Periodically, the lower part of the dust collector opens and thus dust is removed, for this time the damper on the nozzle is closed. The efficiency of trapping dust particles in a cyclone is directly proportional to the gas velocity to the power of ½ and inversely proportional to the diameter of the device.
To increase the centrifugal force Fц it is necessary (to increase efficiency):
Increase the speed of the dusty jet;
Reduce the diameter of the cyclone.
It is known from practice that the jet velocity should be from 15 to 18 m/s. The ratio of the height of the cyclone to D b.b. 2/3.
At high flow rates of purified gases, group / battery cyclones are used - this allows not to increase the D of the cyclone. The dusty gas enters the common collector and is distributed among the cyclones (they work in parallel).
Vortex dust collectors.Η<90%, d>2 µm.
The main difference from cyclones is the presence of an auxiliary swirling flow. In a nozzle-type apparatus, a dusty gas flow is fed from the bottom of the apparatus and swirled with a vaned swirler. The swirling gas flow moves upward, while being exposed to several jets of secondary gas. Secondary gas is supplied from tangential nozzles at the top of the apparatus. Under the action of centrifugal forces, the particles are thrown to the periphery of the body of the apparatus, and from there into the secondary gas flow created by the jets, which directs them down into the annular annulus. The annular annular space around the inlet pipe is equipped with a retaining washer, which ensures the descent of dust into the hopper.
1-camera; 2-outlet pipe; 3 nozzles;
4-vane swirler; 5-inlet pipe; 6-retaining washer;
7-dust bunker.
Electrostatic precipitators.
The electrostatic precipitator is the most modern dust collecting apparatus. η=99-99.5%, d=0.01-100µm. cleaning gas temperature up to 450°C.
The electrostatic precipitator uses a high-voltage electrostatic field. The voltage on the electrodes is up to 50 kV. Particles pass through 2 zones. In the 1st zone, the particle acquires El. potential (is being charged), in the 2nd zone the charged dust moves towards the opposite electrostatic charge and settles on it. Therefore, 3 types of forces are used to clean the air from dust: gravity; air force and electrostatic force.
By design, they can. vertical and horizontal.
1 - corona electrode
2 - collecting electrode
3 - bunker
4 - voltage source
When a high voltage is applied between the corona and collecting electrodes, an electrostatic field of high intensity is created. When polluted air enters through the nozzle, a laminar jet (flow) is formed, which moves vertically upward through an electrostatic field. In this case, the following forces act on the particle: G, Fh, and Rel.st. In this case, Fh exceeds G by several percent. With such a scheme of forces, the particle deviates from the vertical axis and moves towards the collecting electrode and sticks to the inner surface of the pipe. There is a transfer of a negative charge to dust particles and their deposition on the collecting electrodes. The filter is regenerated by shaking.
"-" high energy consumption (0.36-1.8 MJ per 1000 m 3 of gas).
The higher the field strength and the lower the gas velocity in the apparatus, the better the dust collection.
Straining and defending.
Straining- this is the process of passing wastewater through grates and sieves before finer cleaning
The gratings trap impurities of at least 10-20 mm, the gratings are periodically cleaned;
Work efficiency no more than 70%
Straining is used only for pre-treatment of wastewater
In some areas, sieves with a mesh size of up to 1 mm are used, which allow the removal of substances of 0.5-1 mm.
With the help of the calculation, the grate is selected, and the pressure loss in it is determined.
settling- this is the sedimentation of coarse impurities under the action of gravity.
Are used:
1) sand traps, used to remove mineral particles and sand (0.15-0.25 mm). A sandbox is a tank with a tropezoidal or triangular base (<0,3м/с, эффективность не более 95%).
There are: - vertical (movement from bottom to top); - horizontal; - aerated.
H \u003d 0.25 - 2 m
v = 0.15 -0.3 m/s
H \u003d 3 - 4.5 m
Working part length:
L = (1000*k s *H s *υ s)/ u s, where:
H s - estimated depth of the sand trap, k s - set, taken depending on the type of sand trap, υ s - speed of water in the sand trap, u s - hydraulic fineness (14 - 24 mm / s)
2) settling tanks.
By design: horizontal, vertical, radial, tubular and with inclined plates. By appointment: primary, - secondary.
Horizontal - rectangular tanks with 2 or more simultaneously operating compartments.
1 - input patch;
2 - output tray;
3 – settling chamber;
4 – a tray for removal of the floated impurity.
Q - more than 15,000 m 3 / day
H \u003d 1.5 - 4 m, L \u003d 8 -27 m, V \u003d 3-6 m, v \u003d 0.01 m / s.
Vertical - tanks round in plan, with a diameter of 4, 6, 9 m with a conical bottom. Waste water is brought in the center to the pipe, and after entering it moves from bottom to top.
1- central pipe;
2- chute for the hole;
3- cylindrical part;
4 - conical part.
Q - less than 20000 m 3 / day;
Diameter - 4, 6, 9; height - 4 -5 m, speed - 0.5 - 0.6 m / s.
Radial - round in terms of tanks, water enters through the center of the pipe and moves from the center to the periphery.
2- switchgear;
3- scraper mechanism;
Q - more than 20000 m 3 / day;
Height - 1.5-5 m, diameter - 16 - 60 m.
The calculation of the sump is carried out according to the kinetics of precipitation of suspended solids, taking into account the required effect of clarification. The calculation determines the hydraulic fineness, according to which the parameters of the sump are calculated.
You can increase the efficiency of deposition by:
By increasing particle size by coagulation; - reducing the viscosity of water (for example, by heating); - increasing the area of settling.
3) oil trap
1- case;
2- layer of oil;
3- pipe for collecting oil (fat);
4 - partition to hold the emerging oil products;
5- pit for precipitation
The degree of purification is less than 70%. To increase efficiency, air is supplied from below. They are calculated as settling tanks, taking into account the hydraulic size of floating particles.
Clarifiers are used for purification of natural waters and for preliminary clarification of wastewater. in clarifiers, a suspended layer of sediment is created through which WW is filtered.
The settling process is also used to clean particles with a density less than the density of water, such particles float up and are removed from the surface of the settling tank (grease traps and oil traps). Efficiency for oil 96-98% for fat no more than 70%.
Methods of protection of the atmosphere, their classification.
Active - they provide for the greening of technological processes, i.e. creation of non-waste technologies, creation of closed technological cycles (rarely).
Passive methods are divided into:
1) emission control:
Improvement of fuel and replacement with another type;
Ensuring more complete combustion of fuel;
Preliminary purification of raw materials from volatile impurities;
Increasing the role of non-waste energy sources (nuclear power plants, solar, wind).
2) dispersal, localization and dispersion of emissions
The choice is made at the stage of design, construction of the emission facility;
You can not build in places of stagnant air;
At a certain distance from residential areas, taking into account the wind rose;
D. b. the minimum number of days in a year in which the wind blows from the enterprise to the city;
The location of industrial and residential buildings should contribute to cross-ventilation;
When arranging buildings near the highway, it should be: in the center of the hospital, det. gardens...
Containment is a fume hood device for removing pollutants. Centralization - several small sources are combined into one large source for the most efficient operation of treatment facilities (low cost of air purification). Dispersion - the release of pollutants into the upper layer of the atmosphere through pipes and its further dilution with pure (the most dangerous of the low pipes). Dispersal - the location of enterprises on the territory, taking into account the location of the city, the wind rose (at the design stage).
3) arrangement of sanitary protection zones:
To reduce the impact of enterprises on the environment, sanitary protection zones are made around them;
The sanitary protection zone is a strip of land that separates the enterprise from residential development. The width depends on the power, the volume of emissions, the concentration of emissions, the noise generated. The territory of the sanitary protection zones must be landscaped (>=60% of the area) and landscaped (except for hospitals, parks, stadiums...)
4) purification of emissions - this is the capture of pollutants from exhaust gases.
All emissions are divided into combined-cycle and aerosol emissions, the production is always cleaned from dust and then from gases.
Dust removal: -dry methods (dust collection chambers, dust collectors (inertial, dynamic, vortex), cyclones, filters (fibrous, fabric, granular, ceramic)); - wet methods (gas scrubbers (hollow, packed, disk-shaped, impact-inertial, centrifugal, mechanical, high-speed)); - electrical methods (dry and wet electrostatic precipitators).
Cleaning from fogs and splashes: - mist filters; - mesh sprinklers.
Emission requirements. Means of protection of the atmosphere should limit the presence of harmful substances in the air of the human environment at a level not exceeding the MPC. In all cases, the condition
C+c f £ MPC (6.2)
for each harmful substance (c - background concentration), and in the presence of several harmful substances of unidirectional action - condition (3.1). Compliance with these requirements is achieved by localization of harmful substances at the place of their formation, removal from the room or equipment and dispersion in the atmosphere. If at the same time the concentration of harmful substances in the atmosphere exceeds the MPC, then the emissions are cleaned from harmful substances in the cleaning devices installed in the exhaust system. The most common are ventilation, technological and transport exhaust systems.
Rice. 6.2. Schemes for the use of atmospheric protection means:
/- source of toxic substances; 2- device for localization of toxic substances (local suction); 3- cleaning apparatus; 4- a device for taking air from the atmosphere; 5- emission dissipation pipe; 6- device (blower) for supplying air to dilute emissions
In practice, the following options for protecting atmospheric air are implemented:
Removal of toxic substances from the premises by general ventilation;
Localization of toxic substances in the zone of their formation by local ventilation, purification of polluted 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 (Fig. 6.2, a);
Localization of toxic substances in the zone of their formation by local ventilation, purification of polluted air in special devices, emission and dispersion in the atmosphere (Fig. 6.2, b );
Purification of technological gas emissions in special devices, emission and dispersion in the atmosphere; in some cases, exhaust gases are diluted with atmospheric air before being released (Fig. 6.2, c);
Purification of exhaust gases from power plants, for example, internal combustion engines in special units, and release into the atmosphere or production area (mines, quarries, storage facilities, etc.) (Fig. 6.2, d).
To comply with the MPC of harmful substances in the atmospheric air of populated areas, the maximum allowable emission (MAE) of harmful substances from exhaust ventilation systems, various technological and power plants is established. The maximum allowable emissions of gas turbine engines of civil aviation aircraft are determined by GOST 17.2.2.04-86, emissions of vehicles with internal combustion engines-GOST 17.2.2.03-87 and a number of others.
In accordance with the requirements of GOST 17.2.3.02-78, for each designed and operating industrial enterprise, the MPE of harmful substances into the atmosphere is set, provided that emissions of harmful substances from this source in combination with other sources (taking into account the prospects for their development) will not create a Rizem concentration, exceeding the MPC.
Dissipation of emissions in the atmosphere. Process gases and ventilation air, after exiting pipes or ventilation devices, obey the laws of turbulent diffusion. On fig. 6.3 shows the distribution of the concentration of harmful substances in the atmosphere under the torch of an organized high emission source. As you move away from the pipe in the direction of the spread of industrial emissions, three zones of atmospheric pollution can be conventionally distinguished:
flare transfer B, characterized by a relatively low content of harmful substances in the surface layer of the atmosphere;
smoke AT with the maximum content of harmful substances and a gradual decrease in the level of pollution G. The smoke zone is the most dangerous for the population and should be excluded from residential development. The dimensions of this zone, depending on meteorological conditions, are within 10 ... 49 pipe heights.
The maximum concentration of impurities in the surface zone is directly proportional to the productivity of the source and inversely proportional to the square of its height above the ground. The rise of hot jets is almost entirely due to the lifting force of gases that have a higher temperature than the surrounding air. An increase in temperature and momentum of the emitted gases leads to an increase in lift and a decrease in their surface concentration.
Rice. 6.3. The distribution of the concentration of harmful substances in
atmosphere near the earth's surface from an organized high
emission source:
A - zone of unorganized pollution; B - flare transfer zone; AT - smoke zone; G - gradual reduction zone
The distribution of gaseous impurities and dust particles with a diameter of less than 10 μm, which have an insignificant settling rate, obeys general laws. For larger particles, this pattern is violated, since the rate of their sedimentation under the action of gravity increases. Since large particles tend to be more easily captured during dedusting than small particles, very small particles remain in the emissions; their dispersion in the atmosphere is calculated in the same way as gaseous emissions.
Depending on the location and organization of emissions, air pollution sources are divided into shaded and non-shaded, linear and point sources. Point sources are used when the removed pollution is concentrated in one place. These include exhaust pipes, shafts, roof fans and other sources. The harmful substances emitted from them during dispersion do not overlap one another at a distance of two building heights (on the windward side). Linear sources have a significant extent in the direction perpendicular to the wind. These are aeration lights, open windows, closely spaced exhaust shafts and roof fans.
Unshaded, or tall springs are loosely positioned in a deformed wind current. These include high pipes, as well as point sources that remove pollution to a height exceeding 2.5 N zd. Shaded or low sources are located in the zone of backwater or aerodynamic shadow formed on the building or behind it (as a result of wind blowing it) at a height h £ , 2.5 N zd.
The main document regulating the calculation of dispersion and determination of surface concentrations of emissions from industrial enterprises is the “Method for calculating the concentrations in the atmospheric air of harmful substances contained in emissions from enterprises OND-86”. This technique makes it possible to solve the problems of determining the MPE in case of dispersion through a single non-shaded chimney, in case of emission through a low shaded chimney, and in case of emission through a lantern from the condition of ensuring the MPC in the surface air layer.
When determining the MPE of an impurity from a calculated source, it is necessary to take into account its concentration c f in the atmosphere, due to emissions from other sources. For the case of dissipation of heated emissions through a single unshaded pipe
where N- pipe height; Q- the volume of the consumed gas-air mixture ejected through the pipe; ΔT is the difference between the temperature of the emitted gas-air mixture and the temperature of the ambient atmospheric air, equal to the average temperature of the hottest month at 13:00; BUT - a coefficient that depends on the temperature gradient of the atmosphere and determines the conditions for vertical and horizontal dispersion of harmful substances; kF- coefficient taking into account the settling rate of suspended particles of the emission in the atmosphere; m and n are dimensionless coefficients that take into account the conditions for the exit of the gas-air mixture from the mouth of the pipe.
Emission Treatment Equipment. In cases where real emissions exceed the maximum allowable values, it is necessary to use devices for cleaning gases from impurities in the emission system.
Devices for cleaning ventilation and technological emissions into the atmosphere are divided into: dust collectors (dry, electric, filters, wet); mist eliminators (low and high speed); devices for capturing vapors and gases (absorption, chemisorption, adsorption and neutralizers); multi-stage cleaning devices (dust and gas traps, mists and solid impurities traps, multi-stage dust traps). Their work is characterized by a number of parameters. The main ones 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 before and after the apparatus.
In some cases, for dusts, the concept of fractional cleaning efficiency is used.
where C in i and C in i are the mass concentrations of the i-th fraction of dust before and after the dust collector.
To assess the effectiveness of the cleaning process, the breakthrough coefficient of substances is also used To through the cleaning machine:
As follows from formulas (6.4) and (6.5), the breakthrough coefficient and cleaning efficiency are related by the relation K = 1 - h|.
The hydraulic resistance of the cleaning apparatus Δp is determined as the difference in the pressures of the gas flow at the inlet of the apparatus p in and outlet p out of it. The value of Δp is found experimentally or calculated by the formula
where ς - coefficient of hydraulic resistance of the device; ρ and W - density and velocity of the gas in the calculated section of the apparatus.
If during the cleaning process the hydraulic resistance of the apparatus changes (usually increases), then it is necessary to regulate its initial Δp start and final value Δp end. Upon reaching Δр = Δр con, the cleaning process must be stopped and regeneration (cleaning) of the device should be carried out. The latter circumstance is of fundamental importance for filters. For filters Δbright = (2...5)Δр initial
Power N gas movement exciter is determined by hydraulic resistance and volumetric flow Q purified gas
where k- power factor, usually k= 1.1...1.15; h m - efficiency of power transfer from the electric motor to the fan; usually h m = 0.92 ... 0.95; h a - fan efficiency; usually h a \u003d 0.65 ... 0.8.
Widespread use for the purification of gases from particles received dry dust collectors- cyclones (Fig. 6.4) of various types. The gas flow is introduced into the cyclone through pipe 2 tangentially to the inner surface of the housing 1 and performs a rotational-translational movement along the body to the bunker 4. Under the action of centrifugal force, dust particles form a dust layer on the cyclone wall, which, together with part of the gas, enters the hopper. The separation of dust particles from the gas entering the hopper occurs when the gas flow in the hopper is rotated by 180°. Freed from dust, the gas flow forms a vortex and exits the hopper, giving rise to a gas vortex leaving the cyclone through the outlet pipe 3. The tightness of the hopper is necessary for the normal operation of the cyclone. If the hopper is not hermetic, then due to the suction of friendly air, dust is carried out with the flow through the outlet pipe.
Many problems of cleaning gases from dust are successfully solved by cylindrical (TsN-11 TsN-15, TsN-24, TsP-2) and conical (SK-Tsts 34, SK-TsN-34M and SDK-TsN-33) cyclones of NIIOGAZ. Cylindrical cyclones of NIIO-GAZ are designed to capture dry dust from aspiration systems. They are recommended to be used for pre-treatment of gases and installed in front of filters or electrostatic precipitators.
The conical cyclones of NIIOGAZ of the SK series, designed for gas purification from soot, have an increased efficiency compared to cyclones of the TsN type, which is achieved due to the greater hydraulic resistance of the SK series cyclones.
To clean large masses of gases, battery cyclones are used, consisting of a large number of cyclone elements installed in parallel. Structurally, they are combined into one building and have a common gas supply and discharge. Operating experience with battery cyclones has shown that the cleaning efficiency of such cyclones is slightly lower than the efficiency of individual elements due to the flow of gases between the cyclone elements. The method for calculating cyclones is given in the work.
Rice. 6.4. Cyclone scheme
Electrical cleaning(electrostatic precipitators) - one of the most advanced types of gas purification from dust and fog particles suspended in them. This process is based on the impact ionization of gas in the zone of the corona discharge, the transfer of the ion charge to impurity particles and the deposition of the latter on the collecting and corona electrodes. For this, electrofilters are used.
Aerosol particles entering the zone between the corona 7 and the precipitation 2 electrodes (Fig. 6.5), adsorb ions on their surface, acquiring an electric charge, and thereby receive an acceleration directed towards the electrode with a charge of the opposite sign. The particle charging process depends on the mobility of the ions, the trajectory of motion, and the residence time of the particles in the zone of the corona charge. Considering that the mobility of negative ions in air and flue gases is higher than positive ones, electrostatic precipitators are usually made with a corona of negative polarity. The charging time of aerosol particles is short and is measured in fractions of a second. The movement of charged particles to the collecting electrode occurs under the action of aerodynamic forces and the force of interaction between the electric field and the charge of the particle.
Rice. 6.5. Scheme of the electrostatic precipitator
Of great importance for the process of dust deposition on electrodes is the electrical resistance of dust layers. According to the magnitude of electrical resistance, they distinguish:
1) dust with low electrical resistivity (< 10 4 Ом"см), которые при соприкосновении с электродом мгновенно теряют свой заряд и приобретают заряд, соответствующий знаку электрода, после чего между электродом и частицей возникает сила отталкивания, стремящаяся вернуть частицу в газовый поток; противодействует этой силе только сила адгезии, если она оказывается недостаточной, то резко снижается эффективность процесса очистки;
2) dust with electrical resistivity from 10 4 to 10 10 Ohm-cm; they are well deposited on the electrodes and are easily removed from them when shaken;
3) dust with a specific electrical resistance of more than 10 10 Ohm-cm; they are most difficult to capture in electrostatic precipitators, since particles are discharged slowly at the electrodes, which largely prevents the deposition of new particles.
Under real conditions, the electrical resistivity of dust can be reduced by moistening the dusty gas.
Determination of the efficiency of cleaning dusty gas in electrostatic precipitators is usually carried out according to the Deutsch formula:
where W E - velocity of a particle in an electric field, m/s;
F sp is the specific surface of the collecting electrodes, equal to the ratio of the surface of the collecting elements to the flow rate of the gases being cleaned, m 2 s/m 3 . From formula (6.7) it follows that the efficiency of gas purification depends on the exponent W e F sp:
| W e F beats | 3,0 | 3,7 | 3,9 | 4,6 |
| η | 0,95 | 0,975 | 0,98 | 0,99 |
The design of electrostatic precipitators is determined by the composition and properties of the gases being cleaned, the concentration and properties of suspended particles, the parameters of the gas flow, the required cleaning efficiency, etc. The industry uses several typical designs of dry and wet electrostatic precipitatorsused to treat process emissions (Fig. 6.6) .
The operational characteristics of electrostatic precipitators are very sensitive to changes in the uniformity of the velocity field at the filter inlet. To obtain high cleaning efficiency, it is necessary to ensure a uniform gas supply to the electrostatic precipitator by properly organizing the supply gas path and using distribution grids in the inlet part of the electrostatic precipitator
Rice. 6.7. Filter scheme
For fine purification of gases from particles and dropping liquid, various methods are used. filters. The filtration process consists in retaining particles of impurities on porous partitions when dispersed media move through them. A schematic diagram of the filtration process in a porous partition is shown in fig. 6.7. The filter is a body 1, separated by a porous partition (filter element) 2 into two cavities. Contaminated gases enter the filter, which are cleaned when passing through the filter element. Particles of impurities settle on the inlet part of the porous partition and linger in the pores, forming a layer on the surface of the partition 3. For newly arriving particles, this layer becomes part of the filter wall, 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 diffusion, inertial and gravitational.
The classification of filters is based on the type of filter partition, the design of the filter and its purpose, the fineness of cleaning, etc.
According to the type of partition, filters are: with granular layers (fixed, freely poured granular materials, pseudo-fluidized layers); with flexible porous partitions (fabrics, felts, fibrous mats, sponge rubber, polyurethane foam, etc.); with semi-rigid porous partitions (knitted and woven nets, pressed spirals and shavings, etc.); with rigid porous partitions (porous ceramics, porous metals, etc.).
Bag filters are the most widely used in the industry for dry cleaning of gas emissions (Fig. 6.8).
Wet gas scrubbers - wet dust collectors - are widely used, as they are characterized by high cleaning efficiency from fine dust with d h > 0.3 microns, as well as the possibility of cleaning dust from heated and explosive gases. However, wet dust collectors have a number of disadvantages that limit the scope of their application: 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 the outlet gas ducts when the gases are cooled to the dew point temperature; need Editing circulation systems for supplying water to the dust collector.
Rice. 6.8. Bag filter:
1 - sleeve; 2 - frame; 3 - outlet pipe;
4 - device for regeneration;
5- inlet pipe
Wet cleaning devices work on the principle of deposition of dust particles on the surface of either drops or liquid films. The sedimentation of dust particles on the liquid occurs under the action of inertia forces and Brownian motion.
Rice. 6.9. Scheme of a venturi scrubber
Among wet cleaning devices with the deposition of dust particles on the droplet surface, Venturi scrubbers are more applicable in practice (Fig. 6.9). The main part of the scrubber is a Venturi nozzle 2. A dusty gas flow is fed into its confuser part and through centrifugal nozzles 1 irrigation fluid. In the confuser part of the nozzle, the gas is accelerated from the input velocity (W τ = 15...20 m/s) up to speed in the narrow section of the nozzle 30...200 m/s and more. The process of dust deposition on liquid drops is due to the mass of the liquid, the developed surface of the drops, and the high relative velocity of the liquid and dust particles in the confusing part of the nozzle. The cleaning efficiency largely depends on the uniformity of the liquid distribution over the cross section of the confuser part of the nozzle. In the diffuser part of the nozzle, the flow is decelerated to a speed of 15...20 m/s and fed into the drop catcher 3. The droplet eliminator is usually made in the form of a once-through cyclone.
Venturi scrubbers provide high efficiency of aerosol purification at initial impurity concentration up to 100 g/m 3 . If the specific water consumption for irrigation is 0.1 ... 6.0 l / m 3, then the purification efficiency is equal to:
| d h, µm. ……………. η ……………………. | 0.70...0.90 | 5 0.90...0.98 | 0.94...0.99 |
Venturi scrubbers are widely used in systems for cleaning gases from fogs. The efficiency of air purification from fog with an average particle size of more than 0.3 microns reaches 0.999, which is quite comparable with high-efficiency filters.
Wet dust collectors include bubbling-foam dust collectors with a failure (Fig. 6.10, a) and overflow grates (Fig. 6.10, b). In such devices, gas for purification enters under the grate 3, passes through the holes in the grate and, bubbling through a layer of liquid and foam 2, is cleaned of dust by deposition of particles on the inner surface of the gas bubbles. The mode of operation of the devices depends on the speed of air supply under the grate. At a speed of up to 1 m/s, a bubbling mode of operation of the apparatus is observed. A further increase in the gas velocity in the body 1 of the apparatus up 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 spray entrainment from the apparatus. Modern bubbling-foam devices ensure the efficiency of gas purification from fine dust ~ 0.95 ... 0.96 at specific water flow rates of 0.4 ... 0.5 l / m. The practice of operating these devices shows that they are very sensitive to the uneven supply of gas under the failed gratings. Uneven gas supply leads to local blow-off of the liquid film from the grate. In addition, the grates of the apparatus are prone to clogging.
Fig. 6.10. Scheme of bubble-foam dust collector with
failed (a) and overflow (b) gratings
To clean the air from mists of acids, alkalis, oils and other liquids, fibrous filters are used - mist eliminators. The principle of their operation is based on the deposition of drops on the surface of the pores, followed by the flow of liquid along the fibers to the lower part of the mist eliminator. The precipitation of liquid droplets occurs under the action of Brownian diffusion or the inertial mechanism of separation of pollutant particles from the gas phase on the filter elements, depending on the filtration rate Wf. Mist eliminators are divided into low-speed ones (W f ≤d 0.15 m/s), in which the mechanism of diffuse droplet deposition prevails, and high-speed ones (W f = 2...2.5 m/s), where deposition occurs mainly under the influence of inertial forces.
The filter element of the low speed mist eliminator is shown in fig. 6.11. Into the space between two cylinders 3, made of nets, a fibrous filter element is placed 4, which is attached with a flange 2 to the body of the mist eliminator 7. Liquid deposited on the filter element; flows down to the lower flange 5 and through the water seal tube 6 and glass 7 is drained from the filter. Fibrous low-velocity mist eliminators provide high gas cleaning efficiency (up to 0.999) from particles smaller than 3 µm and completely trap larger particles. Fibrous layers are formed from fiberglass 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.
Rice. 6.11. Filter element diagram
low speed mist trap
High-speed mist eliminators are smaller and provide a cleaning efficiency equal to 0.9...0.98 at D/»= 1500...2000 Pa from mist with particles less than 3 µm. Felts made of polypropylene fibers are used as filter packing in such mist eliminators, which successfully operate in dilute and concentrated acids and alkalis.
In cases where the diameter of the mist droplets is 0.6...0.7 µm or less, in order to achieve an acceptable cleaning efficiency, it is necessary to increase the filtration rate to 4.5...5 m/s, which leads to a noticeable spray entrainment from the output side of the filter element (splash-drift usually occurs at speeds of 1.7 ... 2.5 m / s). It is possible to significantly reduce the spray entrainment by using spray eliminators in the design of the mist eliminator. To trap liquid particles larger than 5 microns, spray traps from mesh packages are used, where liquid particles are captured due to touch effects and inertial forces. The filtration speed in the spray traps must not exceed 6 m/s.
On fig. 6.12 shows a diagram of a high-speed fiber mist eliminator with a cylindrical filter element. 3, which is a perforated drum with a blind lid. Coarse-fiber felt 3...5 mm thick is installed in the drum. Around the drum on its outer side there is a spray trap 7, which is a set of perforated flat and corrugated layers of vinyl plastic tapes. The splash trap and the filter element are installed in the liquid layer at the bottom
Rice. 6.12. Diagram of a high speed mist eliminator
To clean the aspiration air of chromium plating baths, containing fog and splashes of chromic and sulfuric acids, fibrous filters of the FVG-T type are used. In the body there is a cassette with a filtering material - needle-punched felt, consisting of fibers with a diameter of 70 microns, a layer thickness of 4 ... 5 mm.
The absorption method - cleaning gas emissions from gases and vapors - is based on the absorption of the latter by liquid. For this use absorbers. The decisive condition for the application of the absorption method is the solubility of the vapors or gases in the absorbent. Thus, to remove ammonia, chlorine or hydrogen fluoride from technological emissions, it is advisable to use water as an absorbent. For a highly efficient absorption process, special design solutions are required. They are sold in the form of packed towers (Fig. 6.13), nozzle bubbling-foam and other scrubbers. The description of the cleaning process and the calculation of the devices are given in the work.
Rice. 6.13. Packed tower scheme:
1 - nozzle; 2 - sprinkler
Work chemisorbers is based on the absorption of gases and vapors by liquid or solid absorbers with the formation of poorly soluble or low-volatile chemical compounds. The main apparatus for the implementation of the process are packed towers, bubbling-foam apparatuses, Venturi scrubbers, etc. Chemisorption - one of the common methods for cleaning exhaust gases from nitrogen oxides and acid vapors. The efficiency of purification from nitrogen oxides is 0.17 ... 0.86 and from acid vapors - 0.95.
The adsorption method is based on the ability of some fine solids to selectively extract and concentrate individual components of a gas mixture on their surface. For this method use adsorbents. As adsorbents, or absorbers, substances are used that have a large surface area per unit mass. Thus, the specific surface of activated carbons reaches 10 5 ... 10 6 m 2 /kg. They are used to clean gases 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. Simple and complex oxides (activated alumina, silica gel, activated alumina, synthetic zeolites or molecular sieves) are also used as adsorbents, which have a greater selectivity than activated carbons.
Structurally, adsorbers are made in the form of containers filled with a porous adsorbent, through which the stream of the purified gas is filtered. Adsorbers are used to purify air from vapors of solvents, ether, acetone, various hydrocarbons, etc.
Adsorbers are widely used in respirators and gas masks. Cartridges with an adsorbent should be used strictly in accordance with the operating conditions specified in the passport of the respirator or gas mask. So, the RPG-67 filtering anti-gas respirator (GOST 12.4.004-74) should be used in accordance with the recommendations given in Table. 6.2 and 6.3.
6.5. MEANS OF PROTECTION OF THE ATMOSPHERE.
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. In addition, air
polluted by technological emissions from workshops, such as forging and pressing workshops, workshops for thermal and mechanical processing of metals, foundries and others, on the basis of which modern mechanical engineering develops. In the process of manufacturing machinery and equipment, welding, metal machining, processing of non-metallic materials, paint and varnish operations, etc. are widely used. Therefore, the atmosphere needs to be protected.
Means of protection of the atmosphere should limit the presence of harmful substances in the air of the human environment at a level not exceeding the MPC. This is achieved by localization of harmful substances in the place of their formation, removal from the room or equipment and dispersion in the atmosphere. If at the same time the concentration of harmful substances in the atmosphere exceeds the MPC, then the emissions are cleaned from harmful substances in the 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 polluted air in special devices and
its return to the production or household premises, if the air
after cleaning in the apparatus meets the regulatory requirements for
supply air,
localization of toxic substances in the zone of their formation local
ventilation, purification of polluted air in special devices,
release and dispersion in the atmosphere,
purification of technological gas emissions in special devices,
release and dispersion in the atmosphere; in some cases before release
exhaust gases are diluted with atmospheric air.
To comply with the MPC of harmful substances in the atmospheric air of populated areas, the maximum permissible emission (MAE) of harmful substances from exhaust ventilation systems, various technological and power plants is established.
In accordance with the requirements of GOST 17.2.02, for each projected and operating industrial enterprise, the MPE of harmful substances into the atmosphere is set, provided that emissions of harmful substances from this source in combination with other sources (taking into account the prospects for their development) do not create a surface concentration exceeding the MPC .
Devices for cleaning ventilation and technological emissions into the atmosphere are divided into:
dust collectors (dry, electric filters, wet filters);
mist eliminators (low and high speed);
devices for capturing vapors and gases (absorption,
chemisorption, adsorption and neutralizers);
multi-stage cleaning devices (dust and gas traps,
mist and particulate traps, multi-stage
dust collectors).
Electric cleaning (electrostatic precipitators) is one of the most advanced types of gas cleaning from dust and fog particles suspended in them. This process is based on the impact ionization of gas in the zone of the corona discharge, the transfer of the ion charge to impurity particles and the deposition of the latter on the precipitation corona electrodes. For this, electrofilters are used.
Scheme of the electrostatic precipitator.
1-corona electrode
2-collecting electrode
Aerosol particles entering the zone between the corona 1 and collecting 2 electrodes adsorb ions on their surface, acquiring an electric charge, and thereby receive an 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 positive ones, electrostatic precipitators are usually made with a corona of negative polarity. The charging time of aerosol particles is short and is measured in fractions of seconds. The movement of charged particles to the collecting electrode occurs under the action of aerodynamic forces and the force of interaction between the electric field and the charge of the particle.
The filter is a housing 1, divided by a porous partition (filter element) 2 into two bands. Contaminated gases enter the filter, which are cleaned when passing through the filter element. Particles of impurities settle on the inlet part of the porous partition and linger 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 purification efficiency
filter and pressure drop across the filter element. 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 diffusion, inertial and gravitational.
Wet dust collectors include bubbling-foam dust collectors with failure and overflow grates.
Diagram of bubbling-foam dust collectors with failure (a) and (b)
overflow gratings.
3-lattice
In such devices, the gas for purification enters under the grate 3, passes through the holes in the grate and, bubbling through the layer of liquid and foam 2, is cleaned of dust by deposition of particles on the inner surface of the gas bubbles. The mode of operation of the devices depends on the speed of air supply under the grate. At a speed of up to 1 m/s, a bubbling mode of operation of the apparatus is observed. A further increase in the gas velocity in the body 1 of the apparatus up 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 spray entrainment from the apparatus. Modern bubbling-foam devices provide the efficiency of gas purification from fine dust -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 the uneven supply of gas under the failed gratings. Uneven gas supply leads to local blow-off of the liquid film from the grate. In addition, the grates of the apparatus are prone to clogging.
To clean the air from mists of acids, alkalis, oils and other liquids, fibrous filters - mist eliminators are used. The principle of their operation is based on the deposition of drops on the surface of the pores, followed by the flow of liquid along the fibers to the lower part of the mist eliminator. Deposition of liquid droplets occurs under the action of Brownian diffusion or the inertial mechanism of separation of pollutant particles from the gas phase on the filter elements, depending on the filtration rate W. The 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 successfully operate in dilute and concentrated acids and alkalis.
In cases where the diameter of the mist droplets is 0.6...0.7 µm or less, in order to achieve an acceptable cleaning efficiency, it is necessary to increase the filtration rate to 4.5...5 m/s, which leads to a noticeable spray entrainment from the output side of the filter element (spray usually occurs at speeds of 1.7 ... To trap liquid particles larger than 5 microns, spray traps from mesh packages are used, where liquid particles are captured due to touch effects and inertial forces. The filtration speed in the spray traps must not exceed 6 m/s.
Diagram of a high-speed fog eliminator.
1 - sprinkler
3-filter element
High-speed mist eliminator with a cylindrical filter element 3, which is a perforated drum with a blind cover. 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 spray trap 1, which is a set of perforated flat and corrugated layers of vinyl plastic tapes. The splash trap and the filter element are installed in the liquid layer at the bottom.
Diagram of the filter element of a low-velocity mist eliminator
3-cylinders
4 fiber filter element
5-bottom flange
6-pipe water seal
In the space between the cylinders 3, made of grids,
a fibrous filter element 4 is placed, which is attached with
flange 2 to the body of the mist eliminator 1. Liquid deposited on
filter element; flows down to the lower flange 5 and through the tube
water seal 6 and glass 7 is drained from the filter. fibrous
low speed mist eliminators provide high
the efficiency of gas purification (up to 0.999) from particles smaller than 3 microns and completely trap large particles. Fibrous layers are formed from fiberglass 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 and provide cleaning efficiency equal to 0.9...0.98 at Ap=1500...2000 Pa from mist with particles less than 3 microns.
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INTRODUCTION
The revival of Russian industry is the first 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, the departure from a planned economy dictate to manufacturers to produce products that are in world demand and at minimal cost. The engineering and technical personnel of the plants 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 applying modern technologies for processing parts, equipment, materials, production automation systems and product quality control. The reliability of the machines produced, as well as the economics of their operation, largely depend on the adopted production technology.
The task of improving the technological support of the quality of manufactured machines, and, first of all, their accuracy, is urgent. Precision in mechanical engineering is of great importance for improving the operational quality of machines and for the technology of their production. Increasing the accuracy of manufacturing blanks reduces the complexity of machining, and increasing the accuracy of machining reduces the complexity of assembly as a result of eliminating fitting work and ensuring interchangeability of product parts.
Compared with other methods of obtaining machine parts, machining provides the greatest accuracy and the greatest flexibility of the production process, creates the possibility of the fastest transition from processing workpieces of one size to processing workpieces of another size.
The quality and durability of the tool largely determine the productivity and efficiency of the machining process, and in some cases the general possibility of obtaining parts of the required shape, quality and accuracy. Improving the quality and reliability of the cutting tool contributes to an increase in 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 engineering industry, the range of manufactured parts is huge and the variety of holes requiring reaming is very large. Therefore, designers are often faced with the task of developing a new sweep. A package of applied programs on a computer can help them in this, which calculates the geometry of the cutting tool and displays a working drawing of the sweep on the plotter.
The design sequence and methods for calculating the cutting tool are based both on the general patterns 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 when designing.
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 tool manufacturing.
To reduce the time and increase the efficiency of designing a cutting tool, computer-aided calculations are used, the basis of which is software and mathematical software.
The creation of application software packages for calculating the geometric parameters of a complex and especially complex cutting tool on a computer makes it possible to drastically reduce the cost of design labor and improve the quality of designing a cutting tool.
Places, %; Todd - time for rest and personal needs, %; K - coefficient taking into account the type of production; Kz - coefficient taking into account the assembly conditions. For the general assembly of the hydraulic lock, the norm of time: \u003d 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. \u003d 0.06 pcs. Round up CP = 1. ...
