CHAPTER 7 CLEARING GAS EMISSIONS
Purification of industrial gaseous emissions containing toxic substances is an indispensable requirement in all industries. Dispersed and gas pollutants are often the result of the same production processes, move together in communications, work closely in the treatment apparatus and atmosphere, together cause damage to the environment and man. Therefore, it is necessary to take into account the entire complex of pollutants present in the technological emission. It is not possible to take as a means of cleaning dusty gases a dusting device that emits harmful gaseous substances into the atmosphere. It is also unacceptable to use such means in which neutralization of the initial gas pollutants is accompanied by the formation and release of toxic fogs and fumes of other substances.
7.1. Methods for cleaning gas emissions
Methods for purifying gas emissions take, depending on the physico-chemical properties of the pollutant, its aggregate state, concentration in the environment being cleaned, etc.
When cleaning emissions from gas pollution, a number of problems have to be solved simultaneously related to the fact that in emissions containing harmful vapors and gases there are also aerosols - dust, soot; emissions are in some cases heated to high temperatures, the contaminants contained in them are multicomponent, and they need to be subjected to various methods of cleaning, the discharge of emissions is unstable over time, the concentration of various harmful substances varies, etc.
Judging by the composition of real waste gases and the scale of environmental pollution, it is possible to develop dust-cleaning devices without taking into account gaseous pollutants only for ventilation discharges of mechanical shops. Emissions from all other industries require the removal of both dispersed and gas pollutants, sometimes this can be done by an aqueous cleaning device.
To neutralize emissions by the principle of eliminating toxic impurities, chemical processes are used along with physical processes by which the physical properties of impurities can be varied widely (for example, to convert the initial gaseous pollutants into compounds with a high boiling point) in order to facilitate their further capture.
To implement the second principle of neutralization - the transformation of pollutants into harmless substances - a combination of chemical and physical processes is necessary. To this end, the processes of thermal destruction and thermal oxidation are most often used. They are applicable to pollutants of all aggregate states, but are limited by the composition of the substance being treated. Thermal treatment for the purpose of neutralization can only be subjected to substances whose molecules consist of carbon, hydrogen and oxygen atoms. Otherwise, the heat-setting facilities become a source of atmospheric pollution, and are often extremely dangerous.
The classification of means for neutralizing gaseous pollutants consists in the separation according to the applied processes. Mainly for gas purification use means of chemical technology. Therefore, the classification of the means for neutralizing emissions practically coincides with the classification of processes and apparatuses of the chemical industry that produce harmful emissions as wastes of the main production.
For the purpose of trapping gaseous impurities, condensation, sorption (absorption and adsorption), and chemisorption processes are used, and pollutants are converted into harmless compounds by means of thermochemical (thermal destruction, thermal and catalytic oxidation) and chemical processes (see Figure 6.1).
Methods of absorption, adsorption, catalytic purification, heat removal and condensation of gas impurities are most often used to purify emissions from gaseous pollutants.
Absorption purification of emissions into the atmosphere is used both for extraction of a valuable component from gas, and for sanitary cleaning of gas. Absorption treatment is subjected to emissions, the contaminants of which are readily soluble in the absorbent. Absorption is advisable to use if the concentration of this component in the gas stream is more than 1%. If the pollutant concentration in the emissions exceeds (1 ... 2) · 10 kg/m, then it is technically possible to achieve a purification degree of more than 90%. Absorption is the most common process for purifying gas mixtures in many industries. It is used to purify emissions from hydrogen sulphide, other sulfur compounds, hydrochloric, sulfuric acid, cyanide compounds, organic substances (phenol, formaldehyde, etc.). As an absorbent, water or organic liquids boiling at high temperature are most often used.
In devices with organic absorbents, it is possible to process emissions that do not contain solid impurities, which practically can not be separated from the absorption liquid. Some chemical contaminants are successfully removed by chemical absorption (chemisorption), a process in which the pollutant to be removed reacts chemically with the absorber and forms a neutral or easily removable compound from the process. Such processes are specific and are developed specifically for each type of emissions and a set of pollutants.
By means of adsorption it is possible in principle to extract any pollutant from the emissions in a wide range of concentrations. However, highly concentrated pollutants (with approxi- mately concentrations of more than 5 · 10 kg/m) should be pre-treated (condensation, absorption) to reduce their concentrations. It is also necessary to pre-treat (dehydrate) highly humidified gases.
The catalytic purification process is based on the chemical transformation of toxic impurities into non-toxic on the surface of solid catalysts. As a result of reactions, the impurities in the gas are converted into other compounds that are less dangerous, or easily separated from the gas. Catalytic purification is used mainly at a small concentration of the component to be removed in the gas to be purified. It allows to neutralize nitrogen oxides, carbon monoxide, other harmful gas pollution. Thanks to the use of catalysts, it is possible to achieve a high degree of gas cleaning, which in some cases reaches 99.9%. At a temperature of 100 ... 150 ° C, the processes are considered as irreversible, which makes it possible to obtain a gas with a very low content of impurities.
Thermal decontamination is based on burning combustible harmful impurities in a flame or by afterburning impurities. Afterburning is a method for purifying gases by thermal oxidation of hydrocarbon components to CO 2 and H 2 О. During the post-combustion process, other components of the gas mixture, for example halogen- and sulfur-containing organic compounds, also undergo chemical changes and in a new form can be efficiently removed or extracted from gas streams. Thermo-isolation is often considered as a universal means of cleaning emissions, which it really is not. In thermo-oxidative processes, the quality of air used for combustion is irreversibly lost, and the oxidation products released into the atmosphere contain a certain amount of new toxic substances - carbon monoxide CO and nitrogen oxides NO x . The field of application of thermal de-etching is limited only by compounds in whose molecules there are no other elements except carbon C, hydrogen H and oxygen O. It is essentially impossible to obtain non-toxic reaction products of any other compounds with oxygen. For the same reason, the burning of organic compounds in an open flame can not be attributed to the method of thermal neutralization.
To perspective methods of processing large volumes of emissions with low concentrations of organic gaseous pollutants, one can include a thermal elimination scheme with preliminary concentration of pollutants through adsorption. Such a scheme can be technically and economically feasible with an initial pollutant concentration above 50 mg/m. The heat released during the combustion of contaminants can be easily disposed of. If the concentration of combustible pollutants can be brought to about (5 ... 6) · 10 kg/m, the heat treatment can be organized with little addition of fuel, and at higher concentrations, one can expect the economic efficiency of the installation.
Condensation of gas impurities is a promising method for treating waste gases, based on the transfer of vaporous contaminants to a condensed state and subsequent filtration of the aerosol formed. The condensation process is based on the phenomenon of a decrease in the pressure of a saturated vapor of a solvent when the temperature is lowered. If the pollutants have a low vapor pressure, condensation can be acceptable by increasing the pressure and lowering the temperature of the emissions. Pairs of contaminants of low-boiling substances are treated with chemical reagents in such a way that the reaction products have low saturated vapor pressures. In this case, the methods of chemical treatment must be selected so that recycling of the trapped product is possible. If the boiling point of the contaminants at atmospheric pressure is low (about 100 ° C), then deep cleaning by cooling and increasing pressure will require excessively high energy costs, and condensation cleaning can only be used as a preliminary.
Obviously, the possibility of further processing of waste by means of the main technology is very limited, since initially low quality of emission purification is predetermined. This approach to the problem requires a substantial revision. One of the effective steps could be the inclusion of waste disposal operations in the main technological process as limiting the quantity and quality of products.
Unlimited growth in the range and volume of products produced in the modern world leads to the complication and cost of technologies for processing gas emissions. It can be assumed that already in the near future the methods used today in low-tonnage production will be quite acceptable: gas separation through molecular sieve chromatography, centrifugation of heavy components, thermal diffusion, detoxification of contaminants by plasma destruction.
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