Gases the cleaning

The permissible standards of the harmful substances, which are contained in the waste gas, strictly regulate by Sov. legislation from the first years of existence of the Soviet regime, from the end the 20's operates all-Union organization for purification and dust collection, to which is entrusted the scientific organizational development of the questions, connected with g. Is., design and production of the corresponding equipment. In the number of the branches of industry are created the trusts, the institutes, the laboratories, intended be occupied to constantly by questions of cleaning gases. The developed methods g. Is. allow in the whole series of the cases with the the correct of tekhnolo-gich. process and the correct organization of production to vent to the atmosphere the gases, which do not practically contain harmful substances.

Sources and the form of the pollution of industrial gases. Large of industr. enterprise, RR and avtomob. transport vent to the atmosphere an enormous quantity of gases, which carry diverse, in Vol. h. and harmful, admixtures. For example, thermal power station by power 2400 metal, that works at the angle sr. of ash content, vents to the atmosphere ok. 9 million. m³/h of the flue gases, which contain 180 t of ashes. Are especially contaminated waste gas of metallurg. enterprises, tsem. of plants, thermal power stations, chem. and oil- chem. plants.

The outgoing industr. gases contain admixtures in the form of solid particles, droplets of liquid, and also harmful gaseous products.

Solid admixtures in the industr. gases are finely-crushed and find in the form of dust or smoke. Sizes of the particles of the dust - from hundred m to the portions m; the sizes of the particles of the smoke are usually less than 1 m, but in the section the cases reach 2-3 m. The particles of dust are differed from the particles of the smoke not only in terms of size, but also by chem. composition. Comparatively large particles of dust present the ground material, processed in this enterprise (for example, the component of metallurg. charge). The particles of smoke sharply differ in composition from the source material, from which they were formed. In particular, during the melting, with the kilning of ores and with other metallurg. processes the evaporation of flying metals and their connections with the subsequent condensation and the formation of smoke occurs. As a result the thin dust being contained in the waste gas is frequently enriched in these metals so, that their extraction becomes advantageous. This side-line concentrate in the form of dust is only industr. raw material for obtaining mn. of rare elements (selenium, tellurium, indium, etc.), since their straight extraction is economically disadvantageous with the very low content of these elements in the poly-metallic ores. With the incomplete combustion of fuel into the composition of smoke enters also the soot.

Solid particles fall out from the waste gas, they clog air, detrimentally they act on the human organism, the vegetation, they soil soil.

Liquid admixtures be present in the industr. gases in the form of drops or fogs, i.e. suspension in the gas of the very small droplets (usually less than 1 m and to the thousandths m), which were formed as a result of the condensation of the substances, which were being been in gaseous state. Gases of the sulfuric acid production, which contain drops and fog of sulfuric acid, are a characteristic example of industr. gases with the admixture of the droplets of liquid; its catching from these gases composes the necessary stage of tech. process, and ejection into the atmosphere entails the loss of vegetation in the surrounding locality. Generator and coke gases contain the droplets of resin and oils; their extraction makes it possible to obtain valuable products and it is necessary it will prepare, by the stage before further use of gas.

Gaseous admixtures (usually harmful or undesirable) in the industr. gases are formed, as a rule, in the course of the production of these gases. Thus, for example, generator and coke gases contain hydrogen sulfide, carbon disulfide and other organic connection of sulfur (thiophene, mercaptans, etc.), which always is present in the source material - rock angle. Gases of metallurg. furnaces and products of fuel combustion - flue gases almost always contain in one or other quantity or another sulfurous anhydride. In connection with appearance and increase in the number of the branches of the industry of the syn. materials (ammonia, alcohols, etc.), which consume gases as raw material, won acceptance the fine purification of gases from different, in Vol. h. of gaseous, admixtures. Wide use of natural gases as fuels for by prom. and everyday needs necessitates for a number of cases the subjecting of to their cleaning from hydrogen sulfide to those established the dignity of standards.

Methods of cleaning gases. In the industry uses mech., elektrich. and the phys.-chem. methods of cleaning gases. Mechanical. and elec. cleaning is used for the catching from the gases of solid and liquid admixtures, and gaseous admixtures recover by phys.-chem. methods.

The mechanical cleaning of gases is performed with the precipitation of the particles of the admixtures under the action of the force of gravity or of centrifugal force, with filtration through the fibrous and porous materials, with washing of gas by water or by other liquid. Simplest, but little effective and by rarely used is the method of precipitating the large dust under the action of gravitational force in Vol. n. dust cameras. Inerts. the method of precipitating the particles of the dust (or the drops of liquid) is based on a change in the direction of the motion of gas with the particles weighed in it. Since particle density is approximately 1-3 thousand times more than gas density, they, continuing to move by the inertia in the previous direction, are separated from the gas. Inerts. by the snatchers of dust serve as Vol. n. dust bags, jalousied lattices, zigzag separators and the like in the some apparatuses is used the force of impact of particles. They use all such apparatuses for the catching of comparatively large particles; of the high degree of cleaning gases these methods do not give.

For cleaning of gases widely use the cyclones, in which separation from gas of solid and liquid particles occurs under the action of centrifugal force (during the rotation of gas flow). Since centrifugal force many times exceeds gravitational force, in the cyclones precipitates out a comparatively fine dust, with the size of particles approximately 10-20 m.

Woven and boom filters, and also filters in the form of the layer of coke breeze, gravel or some of porous materials (for example, porous ceramics) use for cleaning of gases by means of the filtration. The most common scrubbers of such type are woven sacking, or sleeve, filters. Depending on the nature of dust and composition of gas bags manufacture from the woolen, the xl.- boom or special (for example, glass) cloths. Gas is passed through the cloth, and the particles of dust stay in the bags (sleeves). Bag-type filters serve mainly for the catching of very thin dust; for example, with cleaning of the gases, which go out from strip aglomerats. of machines or from the shaft furnaces, in the bag-type filters it is recovered by 98-99% of entire dust.

Removal of dust from gases by washing by water uses in the apparatuses of different types. The widest acceptance obtained scrubbers, wet cyclones, high-speed dust catchers and foam dust catchers. In the high-speed (turbulent) dust catchers the water, introduced into the flow of the dust-laden gas, which moves with the high speed, is crushed to the small drops. The high degree of the agitation of gas flow with this speed contributes to the confluence of the particles of the dust with the drops of water. The relatively major drops of water together with the particles of dust easily are separated then in the simplest snatchers (for example, in the wet cyclones). Apparatuses of this type widely adapt for the catching of very fine dust (sublimates) and they can ensure the high degree of cleaning gases. In the foam dust catchers the dust-laden gas in the form of small phials penetrates the layer of liquid with the definite speed, in consequence of which the foam with the highly developed contact surface between the liquid and the gas is formed. In the foam layer the moistening and the catching of the particles of the dust occurs. Because of the high degree of the catching of dust with the sizes of particles it is more than 2-3 m and the low hydraulic resistance (on the order of 80-100 mm of waters. cm.) foam dust catchers received wide distribution.

The electrical cleaning of gases is based on the action of the forces of the heterogeneous elec. field of high voltage (to 80 000 V. Apparatuses for cleaning of gases by this method NAZ. by electric filters. With the transmission through such filters of the contaminated gas its ionization occurs, the charged particles are carried along k it will precipitate, to electrode they precipitate out on it. The application of elec. filters for g. Is. is extremely extended, especially for the fine purification of flue gases of thermal power stations, into tsem. of industry, ferrous and nonferrous metallurgy.

The methods of physical chemistry cleaning use for the removal of gaseous admixtures. To such methods they relate the washing of gases by solvents (absorption); the washing of gases by the solutions of the reagents, which connect admixtures chemically (chem. absorption); the absorption of admixtures by the solid active materials (adsorption); physical separation (for example, the condensation of components), the catalytic transformation of admixtures into the harmless connections. The absorption of gaseous admixtures by solvents is produced via the washing of gases in the watered apparatuses of the type of scrubbers or in the bubblers (see bubbling); in the latter the gas passes through the liquid solvent, which well dissolves gaseous admixtures and it is very bad - remaining components of gas mixture. So is produced, for example, catching by water of ammonia from the coke gas, catching by different oils of aromatic hydrocarbons from the coke gas, the extraction of carbon dioxide from different gases, etc. In such a case, when it is necessary to use the grasped products, they are extracted from the solvent saturated by them via desorption. Cleaning gases by the means of chem. absorption is produced in analogous type apparatuses. The extracted gas admixtures are chemically connected by reagents. Then solutions frequently regenerate, i.e. as a result of various operations the connected admixtures are separated, and the properties of solutions are restored.

The adsorption of gaseous admixtures is produced with the aid of different porous active materials: activated carbon, silica gel, bauxites and other harmful impurities are adsorbed on the surface of absorber, and after its saturation they are driven off by scavenging by hot air, by gas or by overheated vapor.

The some being contained in the gases harmful gaseous admixtures can be catalytically converted into others, easily recovered, substance; sometimes transformation and catching are combined in one process. So is produced, for example, removal of organic connections of sulfur (carbon disulfide, carbon oxysulfide, thiophene, mercaptans) from gases; these connections with 300-400 °S in the presence of hydrogen or water vapor are converted on the catalysts into hydrogen sulfide, which then is extracted from the gas and it can be decomposed with the utilization of sulfur.

Lit.: Gordon g. m., Peysakhov i. l., dust collection and cleaning gases, 2 publ., M., 1968; Uzho in v. n., cleaning industrial gases by cottrell filters, 2 publ., M., 1967; Koul' A. l., Rizenfel'd F. S., cleaning gas, translated from English, M., 1968; Removal of sulfur of coking and other combustible gases, 2 publ., M., 1960. A. p. andrianov.