Cottrell-Moeller Method

Cottrell-Moeller Method (F. G. Cottrell, E. Moeller) for purifying gases from dust and mist (often called the Lodge-Cottrell method in English literature) consists of passing the gas to be purified through special apparatuses (electrostatic precipitators), where it is simultaneously subjected to ionization and the influence of a strong electric field. The essential parts of Cottrell-Moeller electrostatic precipitators are two systems of electrodes, one of which must have a significantly smaller surface area than the other (for example, a system of wires hanging between metal plates). The electrodes are connected to a high-voltage current source. To ionize the gas in the Cottrell-Moeller method, a so-called "corona" discharge is used, which is characterized externally by the appearance of a violet glow at the electrodes with the smaller surface area (the wire) and a characteristic hissing sound, and physically by the intense ionization of the gas. The gas to be purified is passed between the electrodes, whereby dust particles are deposited on the electrodes with the larger surface area (the plates), and the gas exits the apparatus purified. In accordance with their functions, one electrode system is called "ionizing" and the other "precipitating." The physical phenomena that make up the Cottrell-Moeller process are quite complex. Basically, the phenomenon consists of the fact that particles, once in the ionized gas, adsorb gas ions and acquire a charge of the same sign as the ionizing electrodes; while in the electric field, they begin to move toward the precipitating electrodes and are removed from the gas stream. The beginning of experiments on the electrostatic precipitation of gases dates back to 1824, when the mathematician M. Hohlfeld in Leipzig made an interesting observation: a vessel filled with tobacco smoke suddenly becomes transparent if a point connected to the pole of an electrostatic machine is introduced into it. In 1883, this phenomenon was investigated in more detail by O. Lodge, who precipitated the smoke of tobacco, camphor, turpentine, MgO, NH₄Cl, (NH₄)₂SO₃, P₂O₅, burning paper, lead, and zinc. However, the successful practical application of this method became possible only after the work of Cottrell, begun in 1905–06 in America, and Moeller, begun in 1907 in Germany. At present, three types of electrostatic precipitators have become widely used: 1. The tubular type, the design of which is shown in Figure 1 (Lurgi company, Germany). The precipitating electrodes are in the form of tubes, along the axis of which ionizing electrodes in the form of wires are stretched. The most common dimensions are: length 3–4 m, diameter 25–30 cm, wire diameter 1–2 mm.

Figure 1. The precipitating and ionizing electrodes are insulated from each other by means of special insulators. Normally, there are two groups of tubes (2 and 3). The contaminated gas enters the upper chamber (1), passes sequentially through the 2 groups of tubes (2 and 3), and exits purified through the discharge shaft (4). The dust that has settled on the walls of the tubes either falls off by itself and falls into the dust hoppers (5) or is removed by tapping on the walls of the tubes manually or with special knockers. It should be noted, however, that with some types of dust, the removal of already settled particles from the tube walls can present great difficulties. This type of electrostatic precipitator is used mainly for cleaning blast furnace gases. It is characterized by very complete gas purification (up to 99.9%). 2. The plate or sheet apparatus (Figure 2). The precipitating electrodes are in the form of planes, executed as solid plates (2), between which are placed ionizing electrodes in the form of a system of parallel wires (3). The electrodes are enclosed in a chamber which occupies a small area compared to the tubular apparatus. The contaminated gas enters opening 1 and exits purified through 4. The precipitated impurity exits through opening 5. Plate apparatuses are especially suitable in cases where it is desirable to avoid cooling the gas. The degree of purification is lower than in tubular apparatuses. Plate apparatuses have become especially widespread in the separation of liquid particles (acid mists, etc.). 3. The mesh filter is very similar to the plate apparatus. The precipitating electrodes are given the shape of flat or slightly wavy meshes. The gas to be purified is forced to move either parallel to the planes of the electrodes (as in the plate apparatus) or perpendicular to them. As a source of high-voltage current for industrial electrostatic precipitators, high-voltage transformers are used exclusively at the present time. Experience shows that when using direct current for electrostatic precipitation, a much higher degree of gas purification is obtained, and the danger of a complete breakdown of the gas in the electrostatic precipitator (the transition of the corona discharge into a spark or arc), which disrupts the electrostatic precipitation process, is reduced.

Figure 2.

Figure 3.

Therefore, a current rectifier must be introduced into the electrostatic precipitator circuit. At present, rotating mechanical rectifiers are used almost exclusively for this purpose, which have proven to be the most convenient and cheap for operation in a factory setting (Figure 3). Figure 3 shows the schematic diagram of the electrostatic precipitator connection. T—transformer, R—rectifier, E—electrostatic precipitator. To eliminate the danger to operating personnel associated with the use of high voltage, the casing of the electrostatic precipitator (and the precipitating electrodes) is grounded. Advantages and disadvantages of the Cottrell-Moeller method. The main advantage of the Cottrell-Moeller method is the possibility of very complete purification of gas from any dust-like impurity, which cannot be said of all other known methods of gas purification. Furthermore, electrostatic precipitators have very low resistance to gas movement and thus allow for a significant reduction in the power of the exhaust fans of the ventilation system. Among the known disadvantages, one should include the comparative complexity of the equipment, which requires systematic care and supervision (though very little), as well as rather high initial costs. In operation, in most cases, electrostatic precipitators turn out to be cheaper than other installations. The field of application of the Cottrell-Moeller method is extremely vast. In industry, the use of electrostatic precipitators is extremely important from a sanitary point of view. In a whole series of industries, many tons of various substances are carried away in the exhaust gases in a finely dispersed state, which, if not captured, contaminate the atmosphere, disrupt the proper operation of the ventilation system, pollute the surroundings, etc. In economic terms, the use of electrostatic precipitators makes it possible to utilize a very significant amount of products which, if not captured, are uselessly lost. With these goals in mind, the Cottrell-Moeller method is successfully used at the present time in the following branches of industry: in the chemical industry in the production of acids (purification of gases from pyrite furnaces, separation of arsenic and selenium in the contact process, purification of concentration gases); in the production of soda, potash, zinc white, artificial fertilizers, dry milk, paints, for the fractionated separation of resins and oils; in the pulp and paper industry; in the metallurgical industry—for the retention of valuable metal dust and metal oxides; in the cement, gypsum, and lime industry for the purification of flue gases; in the textile industry; in the briquetting of coal, etc. Thanks to the very complete purification of gas with electrostatic precipitators, it is also possible in many cases to implement a recirculation ventilation system, which leads to savings on the heating of industrial premises. The question of cleaning blast furnace and generator gases, for the purity of which very high requirements are imposed, by electrical means has acquired great independent importance. The Cottrell-Moeller method can also be used for the sterilization of gases. As experiments have shown, air that has passed through an electrostatic precipitator turns out to be completely freed from fungi and bacteria. In recent years, the Cottrell-Moeller method has begun to be successfully used in professional-hygienic practice for the purpose of measuring air dustiness. A miniature electrostatic precipitator, appropriately adapted (e.g., the design proposed by the State Scientific Institute for Labor Protection), thanks to its very perfect, practically 100% dust capture, proves to be very suitable for this purpose. In laboratory practice, the Cottrell-Moeller method proves in many cases to be convenient for studying the properties of aerosols.