Gas Purification

the removal of the impurities contained in industrial gases. Gases are purified for the purpose of further using the gases themselves or the impurities contained in them; industrial gases discharged into the atmosphere are purified to prevent pollution of the air by noxious substances. Until the second half of the 19th century the struggle against the harmful effect of discharging industrial gases into the atmosphere consisted only of banning or restricting the construction of certain enterprises; however, these measures were rendered ineffective by the growth of industry, transportation, and large cities. The problem of purification of industrial gases arose precisely because of rapid industrial development, the concentration of industries, and increased scales of production. In industrially developed countries, the saturation of areas with industries and transportation was such that local pollution of the atmosphere became universal and led to the pollution of the entire air basin, or at least a huge part of it.

Standards governing the permissible content of noxious substances in waste gases have been strictly regulated under Soviet law since the very first years of the existence of Soviet power; an all-Union organization for gas purification and dust elimination, charged with the scientific management of problems associated with gas purification and the design and manufacture of the necessary equipment, has existed since the late 1920’s. Trusts, institutes, and laboratories, which are called upon constantly to handle gas-purification problems, have been created in various branches of industry. Gas-purification methods that in a number of instances allow the discharge into the atmosphere of gases practically free of noxious substances with correct manufacturing processes and under normal production management have been developed.

Sources and forms of industrial gas pollution. Large-scale industrial enterprises, railroads, and motor-vehicle transportation discharge into the atmosphere a huge quantity of gases bearing various impurities, including noxious substances. For example, a steam power plant rated at 2,400 megawatts and operating on coal of average ash content discharges into the atmosphere approximately 9 million cu m of stack gases per hour, containing 180 tons of ash. The waste gases of metallurgical enterprises, cement factories, steam power plants, chemical enterprises, and petrochemical refineries are especially contaminated.

Industrial waste gases contain impurities in the form of solid particles, droplets of liquid, and harmful gaseous products.

Solid impurities in industrial gases are finely granulated and occur in the form of dust or smoke. The sizes of dust particles range from hundreds of microns (μ,m) to fractions of a micron; smoke particles are usually less than 1 μm in size, although in some instances they may be as large as 2-3 μm. Dust particles differ from smoke particles not only in size but also in chemical composition. Granulated material treated at a particular enterprise (for example, components of a metallurgical furnace charge) are relatively large dust particles. Smoke particles are clearly differentiated in composition from the material from which they were formed. The evaporation of volatile metals and their compounds, with subsequent condensation and formation of smoke, takes place particularly during smelting, roasting of ore, and other metallurgical processes. As a result, the fine dust contained in the waste gases is often so enriched with these metals that it becomes profitable to recover them. Such a by-product concentrate in the form of dust is the only industrial raw material for obtaining many rare elements (such as selenium, tellurium, and indium), since the very low content of these elements in complex ores makes their direct extraction economically unfeasible. Soot also becomes part of smoke during incomplete combustion of fuel.

Solid particles precipitate out of the waste gases, dirty the air, have a harmful effect on humans and vegetation, and contaminate the soil.

Liquid impurities are present in industrial gases in the form of sprays or mists—that is, extremely fine droplets (usually less than 1 μm, and down to thousandths of a micron) suspended in the gas—that formed as a result of the condensation of substances that were in the gaseous state. Gases from the production of sulfuric acid, which contain sprays and mists of sulfuric acid, are a typical example of industrial gases mixed with liquid droplets; trapping the acid from these gases is a necessary phase of the manufacturing process, whereas emission into the atmosphere brings about the destruction of vegetation in the surrounding region. Generator and coke gases contain droplets of resins and oils; their recovery makes possible the production of valuable products and is a necessary preliminary stage before further utilization of the gas.

Gaseous impurities (usually harmful or undesirable) in industrial gases are generally formed during production of these gases. For example, generator and coke gases contain hydrogen sulfide, carbon disulfide, and other organic compounds of sulfur (thiophene, mercaptans, and such), which is always present in the original raw material, coal. Metallurgical furnace gases and products of the combustion of fuels (flue gases) almost always contain some amount of sulfur dioxide. The fine removal of various contaminants—including gaseous impurities—from gases became commonplace in connection with the origin and growth of a number of branches of industry in which synthetic materials (ammonia, alcohols, and so on), which require gases as a raw material, are used. The widespread use of natural gases as fuels for industrial and domestic needs gave rise in a number of instances to the necessity of treating them to remove hydrogen sulfide in accordance with specified sanitary standards.

Methods of gas purification. Mechanical, electrical, and physicochemical methods are used in industry for purifying gases. Mechanical and electrical purification is used for recovering solid and liquid impurities from gases, and physicochemical methods are used for removing gaseous impurities.

Gases are purified mechanically by the precipitation of foreign particles under the force of gravity or by centrifugal force, by filtration through fibrous and porous materials, or by scrubbing the gas with water or some other liquid. The simplest method, although inefficient and seldom used, is the precipitation of coarse dust particles under the force of gravity in so-called dust chambers. The inertial method of precipitation of dust particles (or drops of liquid) is based on changing the direction of the movement of the gas with the particles suspended in it. Since the density of the particles is 1,000-3,000 times greater than the density of the gas, they are separated from the gas when inertia causes them to continue to move in the original direction. Dust bags, louvered grilles, and zigzag separators serve as inertial traps. The force of particle impact is also used in some units. All of these units and devices are used for trapping relatively large particles; however, these methods do not provide a high degree of purification.

Cyclones, in which the solid and liquid particles are separated from the gas by centrifugal force (upon rotation of the gas flow), are widely used for purifying gases. Since centrifugal force is many times greater than gravitational force, even relatively fine dust particles, approximately 10-20 μm in size, are precipitated in cyclones.

Cloth and paper filters, as well as filters in the form of a layer of breeze, gravel, or porous materials (such as porous ceramics), are used for purifying gases by filtration. Cloth bag filters are the most common gas purifiers of this type. Depending on the nature of the dust and the composition of the gas, the bags are made of wool, cotton, or special (for example, glass) cloth. The gas passes through the cloth, and the dust particles are trapped in the bags. Filtering bags are used primarily for capturing extremely fine dust; for example, they collect 98-99 percent of all dust during the purification of waste gases from conveyer-type sintering machines or shaft furnaces.

There are various types of units that remove dust from gases by means of scrubbing (washing) with water. Scrubbers, wet cyclones, and high-velocity and foam dedusters are the most commonly used types. In high-velocity (turbulent) dedusters, the water injected into the flow of dust-laden gas moving at a high velocity is broken up into fine drops. The high degree of agitation of the gas flow at this high velocity enables the dust particles to blend with the drops of water. The relatively large drops of water, together with the particles of dust, are readily separated later in the simplest traps (for example, wet cyclones). Devices of this type are widely used for collecting very fine dust (sublimates) and can provide a high degree of gas purification. In foam dedusters, dust-laden gas in the form of small bubbles passes through a layer of liquid at a specific rate, resulting in the formation of a foam with a highly developed contact surface between the liquid and gas. Wetting and trapping of the dust particles occur in the foam layer. Foam dedusters are widely used because of the high degree of trapping of dust with particle sizes of more than 2-3 μm and the low hydraulic resistance (on the order of 80-100 mm of water).

Electrical purification is based on the action of the forces of a high-voltage (up to 80,000 volts), nonuniform electric field. Devices for purifying gases by this method are called electrical filters. The contaminated gas is ionized upon passing through these filters; the charged particles are attracted to a receiving electrode and settle on it. The use of electrical filters for gas purification is extremely common, especially for the fine purification of the flue gases of steam power plants and in the cement industry and ferrous and nonferrous metallurgy.

Physicochemical purification methods are used for the removal of gaseous impurities. These methods include scrubbing with solvents (absorption), scrubbing with solutions that bind the impurities chemically (chemical absorption), sorption of impurities by solid active substances (adsorption), physical separation (for example, condensation of the components), and catalytic transformation of impurities into harmless compounds. Gaseous impurities are absorbed by solvents by washing the gases in scrubber-type trickle units or in bubblers; in the latter case, the gas passes through a liquid solvent, which dissolves the gaseous impurities readily and the remaining components of the gas mixture very poorly. For example, ammonia is recovered from coke gas by water, aromatic hydrocarbons are removed from coke gas by various oils, and carbon dioxide is extracted from various gases in this manner. If the recovered products are to be utilized, they are extracted from the solvent, which is saturated with them, by means of desorption. The purification of gases by means of chemical absorption takes place in units of a similar type. The gaseous impurities to be recovered are bound chemically by reagent solutions. Later, the solutions are often recovered—that is, as a result of certain operations the combined impurities are separated and the properties of the solutions are restored.

Gaseous impurities are adsorbed by various porous active substances: active coal, silica gel, bauxities, and so on. Noxious impurities are adsorbed on the surface of a sorbent, and after its saturation they are eliminated by exhausting (scavenging) with hot air, gas, or superheated steam.

Some harmful gaseous impurities contained in gases can be catalytically transformed into other readily recoverable substances; the transformation and recovery are sometimes combined into one process. In this way, for example, organic compounds of sulfur (carbon disulfide, carbon oxysulfide, thiophene, and mercaptans) are removed from gases; these compounds are converted to hydrogen sulfide in catalyzers at temperatures of 300°-400° C in the presence of hydrogen or water vapor. The hydrogen sulfide is later recovered from the gas and can be decomposed, with recovery of the sulfur.