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Oxidative procedures for cleaning waste gases in the chemical industry
OXIDATIVE PROCEDURES FOR CLEANING WASTE GASES IN THE CHEMICAL 1NDUSTRY K. TROBISCIt
Farbwerke Hoechst A G, Frankfurt, Germany A source of air pollution is the waste gases formed in technical processes. Physical, chemical, and biological procedures have been developed for cleaning such gases. Their application depends primarily on the amount and temperature of the waste gases as well as the nature and concentration of the pollutants they contain. A survey of chemical and biological procedures, in which oxidation reactions are used for cleaning waste gases from chemical plants, is presented. In the Hoechst factory of Farbwerke Hoechst AG, for example, hypochlorite solution has proved suitable for the oxidative washing of thiosalicylic acid-containing waste gases formed during the production of an organic intermediate. In another case, involving the cleaning of unpleasantly smelling vapors from a drier for the fungal mycelium formed during penicillin manufacture, on the other hand, oxidizing wash solutions failed. Here, deodorization was successful with sodium bisulfite solution. The chemical reactions involved are not known to us.
1. Chemical Oxidation at Normal Temperature Waste gases containing low concentrations of readily oxidizable substances sometimes may be suitably cleaned by scrubbing with solutions of powerful oxidizing agents. This procedure is used, for example, for deodorizing waste gases which contain mercaptans or hydrogen sulfide whose intensive odor might become a considerable nuisance. Solutions of hypochlorites, chlorites, and permanganates are used as oxidizing agents. The waste gases are scrubbed with these solutions in packed towers of conventional design. Oxidation with ozone and nascent oxygen in the gas phase at room temperature has also proved suitable for cleaning waste gases. In this procedure, the waste gases are either mixed with ozone-containing air or exposed to ultraviolet radiation. The procedures mentioned so far serve mainly for the removal of odorous substances from wastegas streams. I t is often not known what substances cause the odor in a particular waste gas. It is necessary in most cases, therefore, to determine by experiment which procedure will make it possible to obtain adequate, and economically justifiable deodorization. Methodical investigations for optimizing the aforementioned deodorizing procedures on a physicochemical basis have not been described. All publications available reflect the difficulties which arise in the cleaning of waste gases where mixtures of unknown composition are concerned. However, in some cases it is possible by empirical means to find suitable wash solutions that are economically superior to any oxidation procedure operating at elevated temperatures.
2. Chemical Oxidation at Elevated Temperature 2.1 Thermal Combustion Gas mixtures whose recovery is not expedient and whose calorific value is sufficient for combustion with an open flame ( ~ 4 0 0 kcal/m 3 at STP) are mostly flared. I t is also possible to a certain extent to feed intermittently formed waste gases into the heating-gas grid, if adequate buffer volumes are available. In refineries for example, storage capacities of 500-1000 m 8 are employed. However, a flare must always be available if large amounts of ignitible gases are formed from time to time; this occurs during the start-up and shut-down of petroleum refineries and petrochemical plants, as well as during failures in such plants. 2.1.1 Flares To ensure an operation of flares that does not constitute a hazard, and causes as little nuisance
645
646
COMBUSTION PROBLEMS IIELATED TO AIR POLLUTION
as possible to the neighborhood, the following demands must be met: (a) (b) (c) (d) (e) (f)
(g) (h)
safe ignition stability of the flame sufficiently large safety zones for preventing fires at leaks, and for avoiding damage due to radiant heat flexibility in take-up as regards the amount and composition of the gases smokeless combustion adequate degree of combustion for avoiding a nuisance due to troublesome combustion products (e.g., unsaturated compounds, aldehydes, acids) low glare from the flame burning as noiselessly as possible, even when extremely large amounts of gas are charged.
Within the scope of this paper, the last four points are of particular interest. Experience has shown that the tendency of a flame to form smoke increases with higher molecular weights of the gas molecules to be burned and with higher degrees of unsaturation. From this knowledge, it has been deduced that during combustion of gases, generally, the carbon-rich particles producing the smoke are formed by cracking and polymerization reactions in the unburned portion of the gas. These particles leave the flame in the form of smoke if the gas temperature drops below the glowing temperature in an environment low in oxygen. Therefore, smoke formation can be prevented if sufficient amounts of oxygen are supplied to every part of the flame and if polymerization and cracking reactions are prevented, as far as possible, by adequate dilution and cooling of the gas. The necessary dilution and cooling of the flame and, consequently, the elimination of soot can be achieved by means of blowing in compressed air or--more economically in industrial plants--by steam injection which aspirates air at the same time. Steam is able to react with carbon that has already formed, producing carbon monoxide and hydrogen (water-gas reaction), which subsequently burn. It is possible, finally, with very large amounts of steam, to render the flame nonluminous. The following figures give the technical and economical problems that arise when large amounts of gas have to be flared: If more than 1 ton of hydrocarbons per hour are to be flared without air or steam injection, a luminous, often very sooty flame 3 to 4 m high is formed, which is visible over a wide area. This causes a nuisance, especially in densely populated
areas. If large amounts of steam are injected into the flame, the outlet speed of the steam is of necessity high and a nuisance is caused by the noise. For the smokeless combustion of hydrocarbons, each ton of gas--depending on its composition-requires 1 to 2 tons of steam. There are considerable costs, therefore, in the case of large amounts of waste gas. Refinery flares are often designed for gas quantities of more than 50 tons/h. It was necessary, therefore, to examine the formation of soot in more detail in order to find means of reducing costs. In Germany, Hess and StickeP have worked on this problem. The air and steam requirements of the readily sooting acetylene flame were determined theoretically from the balances for the Boudouard reaction and water gas reaction and for the formation of methane (from the elements) and, on the other hand, experimentally from the luminosity and soot limit of the flame of a test burner. The influence of the flow rate on soot formation and on the flame stability was also investigated. Knowledge of these quantitative interrelations is valuable for the design and operation of flares. At present, high-level flares, ground-level flares, and incinerators are used for the thermal combustion of waste gases. Of the flares, the high-level are preferred. They consist of steel pipes, 30 to 100 m high, fitted with a flare tip. Steam, at a pressure of 12 to 15 atm, is injected into the flame in a range of adjustment of about 1:10. Smokeless burning of waste gases is achieved with this equipment, largely independent of the gas composition. As yet, the nuisance caused by the glare and the noise of the steam injection cannot be avoided to the desired degree. Ground-level flares consist of burners and a brick-lined steel shell up to 10 m high, built on supports so that the air for combustion can enter from below. Various types of burner have proved suitable, into some of which water is injected instead of steam. The ground-level flare is less suitable than the high-level flare if highly fluctuating quantities of gas have to be burned. 2.1.2 Incinerators As stated, flares are suitable for the elimination of gases with energy concentrations sufficient for burning with an open flame. Nonflammable waste gases are burned in incinerators, which can be heated by means of gas or oil burners, and, if designed accordingly, can also serve for the combustion of liquid waste. The efficiency of incinerators depends essentially on the time during which the substances to
CLEANING WASTE GASES be burned are exposed, in the presence of excess oxygen, to a temperature above the ignition temperature. At 800 ~ to 1000~ residence times of 0.5 to 2 seconds are necessary if an efficiency of more than 90% is to be achieved. The higher the combustion temperatures, the shorter the residence time may be, and the smaller the combustion chamber required. The temperature load of the combustion chamber is limited by the resistance of the material of which it is made. In a combustion plant for liquid waste at the Hoeehst factory, an interesting, new method has been used in which the brickwork is protected from the heat by means of a boiler-tube cage. The "34 rule" summarizes interrelations important for combustion: t i m e - - t e m p e r a t u r e - turbulence have to be adapted to one another for optimum conditions in the combustion plant. As the interaction of these three factors is decisive for the quality of an incinerator, the individual factors may be varied within wide limits. Thus, it is not practical to lay down fixed standards for the design of incinerators. As far as possible, and within economical bounds, the heat content of the combustion gases is utilized for preheating the untreated waste gas or fresh air, or for the production of steam. Because of the inherent danger of explosion, great experience is required for designing and operating incinerators. When waste gases containing fluorine or large amounts of chlorine compounds are burned, the combustion products--hydrogen fluoride and hydrogen chloride--generally have to be removed by scrubbing with water. In the ease of waste gases containing sulfur, the combustion of which produces sulfur dioxide, the flue gases have to be emitted through a suitably high stack. 2.2 Catalytic Oxidation If large amounts of waste gas are to be freed from combustible substances whose calorific value is insufficient for combustion without additional burning of fuel, it is advantageous to employ catalytic oxidation. By this method, it is possible to achieve oxidation down to an energy content of about 40 keal/m ~ at STP without additional energy, or to reduce the additional energy requirements drastically if the calorific value of the waste gas is less than 40 kcal/m ~ at STP. This is possible because the oxidation temperatures of 800 ~ to 1000~ which are required for thermal combustion, can be reduced to 250 ~ to 500~ Details of the fundamentals of the process are contained in the paper by Vollheiin in This Symposium. The operating temperatures of the catalysts
647
vary in accordance with the type and concentration of the substances to be oxidized. While hydrogen, for example, can be oxidized in the presence of base metal oxides at about 250~ considerably higher catalyst temperatures are required for methane or aromatic compounds. The gases to be cleaned have to be as free of mineral dust as possible so that the catalyst does not become fouled; nor should they contain any catalyst poisons. Metal catalysts are available, depending on the intended use--for example, in the form of chrome-nickel tapes coated with noble metal, or the usual catalyst bases with coatings of noble metal or base metal oxide. The 3-t rule applies also to catalytic combustion. Turbulence is imparted to the gas on passing over the catalyst. The residence time is governed by the effective catalyst surface available to the gas. For technical purposes, the residence time for a gi~'en catalyst with a defined surface is related to the so-called space velocity, the quotient of the waste gas amount [m~/hr at STP], and the amount of catalyst [m3-]. The catalyst temperature required for sufficient oxidation is determined by experiment. Individual catalysts withstand permanent loads up to 1200~ The usual "volume loads" of the catalysts are between 10,000 and 50,000 m 3 at STP waste gas per cubic meter of catalyst per hour. The heat of reaction in the catalyst may reach 4 to 5X 106 kcal/m 3 of catalyst. Particularly at high reaction speeds, the temperature of the catalyst material may rise noticeably above the temperature of the waste gas, as the heat dissipation from the catalyst is limited. This effect is especially important for temperature-sensitive catalysts whose service life may be considerably reduced due to overheating. I t is also possible that flashbacks occur above the lower ignition limit, due to unexpected temperature increases which lead to explosions. Where necessary, waste gases rich in energy have to be cooled by means of secondary air or circulating air. A new course for cooling the catalyst material has been pursued in a plant built in the Hoechst factory. In this system, the catalyst, a palladium catalyst with aluminium oxide base, is cooled in pipe coils through which water flows. 24,000 m 3 of waste gas per hour is cleaned which, apart fl'om air, contains aboat 2000 m 3 of steam, 4000 m :~of carbon dioxide, as well as organic impurities, the waste gas stemming from the stripping of the waste water from a high-temperature pyrolysis plant with a mixture of air and regenerated gas from a carbon dioxide scrubber. During the stripping process, the gas absorbs benzene, toluene, and other aromatic and aliphatie hydrocarbons. Their concentration is equivalent to
648
COMBUSTION PROBLEMS RELATED TO AIR POLLUTION
Hydrogen? ~ Steam - Air~ rt
I_ Condensate
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-7
!
I1
i
Strata
J
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,..L.
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Wastewater
a
Air Strippedwastewater FIG. 1. Catalytic combustion unit for petrochemical waste gases. Key: a, Regeneration of CO~-absorbing solution; b, Condensed water separator; c, Air blower; d, Stripping column; e, Heat exchanger; f, Reactor; g, Water distributor; h, Water collector; i, Expansion; k, Feedwater container; l, Feedwater pump; m, Stack; n, Start-up reactor; o, Start-up blower.
0.8% by volume of benzene or a calorific value of 280 kcal/m a at STP. Figure 1 shows a diagram of the plant. After passing through the heat exchanger, in which the waste gas is heated to 170~ the stream of gas eaters the reactor. Here, the hydrocarbons are oxidized in three series-connected catalyst layers, each approximately 10 cm high. The catalyst is started up stepwise with hot air, which is produced in a separate start-up reactor by means of catalytic oxidation of a hydrogen-air mixture. Pipe coils, embedded in the individual catalyst layers of the main reactor, make it possible to limit the catalyst temperature, provided that the cooling surfaces in the catalyst bed are correctly designed. The heat transfer of an embedded pipe system is 6 to 10 times greater than that of the same pipe system around which, under otherwise equal conditions, a gas flows freely. Consequently, the cooling surfaces may be relatively small. The
cooling system supplies 4 tons of low-pressure steam per hour. It is not possible to regulate the temperature adequately by chai~ging the flow rate of the cooling water in the pipe coils, as the heat trans[er on the gas side determines the heat abstraction. The amount of combustible substance which has to be fed into the plant, therefore, has to be as uniform as possible with respect to time. This is done by changing the water feed to the stripping column. Regulation of the water supply depends on the measurement of the hydrocarbon concentration of the gas to be cleaned. The measurements are conducted by means of two analytical instruments working independently of each other. These instruments also actuate an alarm and close the stripping-water inlet when the maximum permissible hydrocarbon concentration is exceeded. In addition, flashback safety devices (on the basis of lamellae) and bursting disks are fitted.
CLEANING WASTE GASES In addition to the cleaning of petrochemical waste gases, catalytic oxidation is utilized in the chemical industry for the treatment of waste gases from the production of phthalic anhydride, maleie anhydride, and acrylonitrile. It also serves for example, to eliminate formaldehyde and amines in waste gases. Catalytic oxidation procedures are also gaining importance in the cleaning of sulfur dioxidecontaining waste gases formed in chemical processes or in incinerators. In the "Sulfacid process" the sulfur dioxide is reacted to sulfuric acid on activated charcoal that contains catalyst additions, and is sprinkled with water. Cokes on the bases of peat, lignite, or hard coal, oxidize the sulfur dioxide in the "Reinluft process" and adsorb the sulfuric acid formed, which is reduced again in a desorption zone yielding a sulfur dioxide-containing gas of high percentage. In the "Wickert process," catalytic oxidation is carried out with iron oxide-containing dolomite dust, which is injected into steam boiler plants and binds sulfur dioxide as sulfate. Other processes are based on the oxidation of sulfur dioxide on metal oxides or metal carbonates.
649
3. Biological Oxidation Biological procedures have been suggested recently for eliminating foul-smelling organic substances---e.g., protein decomposition products from waste gases. The gas to be cleaned is conducted through a moist filter mass, arranged in several layers, which contains bacteria in high concentration. The bacteria ingest as food the organic substances contained in the waste gas. There has not been much experience with this method so far and it remains to be seen whether microbial oxidation, which has been tried and proved for years in organically loaded waste water, will also be suitable for cleaning waste gases. Together with the intensified endeavors of the chemical industry to prevent air pollution, increasing importance will attach to oxidative processes as well. 1{EFE RENCE 1. HEss, K. AND STICKEL, I)t.: Ctmm. Ing. Teeh. 2,9, a34 (1967).
Farbwerke Hoechst A G, Frankfurt, Germany A source of air pollution is the waste gases formed in technical processes. Physical, chemical, and biological procedures have been developed for cleaning such gases. Their application depends primarily on the amount and temperature of the waste gases as well as the nature and concentration of the pollutants they contain. A survey of chemical and biological procedures, in which oxidation reactions are used for cleaning waste gases from chemical plants, is presented. In the Hoechst factory of Farbwerke Hoechst AG, for example, hypochlorite solution has proved suitable for the oxidative washing of thiosalicylic acid-containing waste gases formed during the production of an organic intermediate. In another case, involving the cleaning of unpleasantly smelling vapors from a drier for the fungal mycelium formed during penicillin manufacture, on the other hand, oxidizing wash solutions failed. Here, deodorization was successful with sodium bisulfite solution. The chemical reactions involved are not known to us.
1. Chemical Oxidation at Normal Temperature Waste gases containing low concentrations of readily oxidizable substances sometimes may be suitably cleaned by scrubbing with solutions of powerful oxidizing agents. This procedure is used, for example, for deodorizing waste gases which contain mercaptans or hydrogen sulfide whose intensive odor might become a considerable nuisance. Solutions of hypochlorites, chlorites, and permanganates are used as oxidizing agents. The waste gases are scrubbed with these solutions in packed towers of conventional design. Oxidation with ozone and nascent oxygen in the gas phase at room temperature has also proved suitable for cleaning waste gases. In this procedure, the waste gases are either mixed with ozone-containing air or exposed to ultraviolet radiation. The procedures mentioned so far serve mainly for the removal of odorous substances from wastegas streams. I t is often not known what substances cause the odor in a particular waste gas. It is necessary in most cases, therefore, to determine by experiment which procedure will make it possible to obtain adequate, and economically justifiable deodorization. Methodical investigations for optimizing the aforementioned deodorizing procedures on a physicochemical basis have not been described. All publications available reflect the difficulties which arise in the cleaning of waste gases where mixtures of unknown composition are concerned. However, in some cases it is possible by empirical means to find suitable wash solutions that are economically superior to any oxidation procedure operating at elevated temperatures.
2. Chemical Oxidation at Elevated Temperature 2.1 Thermal Combustion Gas mixtures whose recovery is not expedient and whose calorific value is sufficient for combustion with an open flame ( ~ 4 0 0 kcal/m 3 at STP) are mostly flared. I t is also possible to a certain extent to feed intermittently formed waste gases into the heating-gas grid, if adequate buffer volumes are available. In refineries for example, storage capacities of 500-1000 m 8 are employed. However, a flare must always be available if large amounts of ignitible gases are formed from time to time; this occurs during the start-up and shut-down of petroleum refineries and petrochemical plants, as well as during failures in such plants. 2.1.1 Flares To ensure an operation of flares that does not constitute a hazard, and causes as little nuisance
645
646
COMBUSTION PROBLEMS IIELATED TO AIR POLLUTION
as possible to the neighborhood, the following demands must be met: (a) (b) (c) (d) (e) (f)
(g) (h)
safe ignition stability of the flame sufficiently large safety zones for preventing fires at leaks, and for avoiding damage due to radiant heat flexibility in take-up as regards the amount and composition of the gases smokeless combustion adequate degree of combustion for avoiding a nuisance due to troublesome combustion products (e.g., unsaturated compounds, aldehydes, acids) low glare from the flame burning as noiselessly as possible, even when extremely large amounts of gas are charged.
Within the scope of this paper, the last four points are of particular interest. Experience has shown that the tendency of a flame to form smoke increases with higher molecular weights of the gas molecules to be burned and with higher degrees of unsaturation. From this knowledge, it has been deduced that during combustion of gases, generally, the carbon-rich particles producing the smoke are formed by cracking and polymerization reactions in the unburned portion of the gas. These particles leave the flame in the form of smoke if the gas temperature drops below the glowing temperature in an environment low in oxygen. Therefore, smoke formation can be prevented if sufficient amounts of oxygen are supplied to every part of the flame and if polymerization and cracking reactions are prevented, as far as possible, by adequate dilution and cooling of the gas. The necessary dilution and cooling of the flame and, consequently, the elimination of soot can be achieved by means of blowing in compressed air or--more economically in industrial plants--by steam injection which aspirates air at the same time. Steam is able to react with carbon that has already formed, producing carbon monoxide and hydrogen (water-gas reaction), which subsequently burn. It is possible, finally, with very large amounts of steam, to render the flame nonluminous. The following figures give the technical and economical problems that arise when large amounts of gas have to be flared: If more than 1 ton of hydrocarbons per hour are to be flared without air or steam injection, a luminous, often very sooty flame 3 to 4 m high is formed, which is visible over a wide area. This causes a nuisance, especially in densely populated
areas. If large amounts of steam are injected into the flame, the outlet speed of the steam is of necessity high and a nuisance is caused by the noise. For the smokeless combustion of hydrocarbons, each ton of gas--depending on its composition-requires 1 to 2 tons of steam. There are considerable costs, therefore, in the case of large amounts of waste gas. Refinery flares are often designed for gas quantities of more than 50 tons/h. It was necessary, therefore, to examine the formation of soot in more detail in order to find means of reducing costs. In Germany, Hess and StickeP have worked on this problem. The air and steam requirements of the readily sooting acetylene flame were determined theoretically from the balances for the Boudouard reaction and water gas reaction and for the formation of methane (from the elements) and, on the other hand, experimentally from the luminosity and soot limit of the flame of a test burner. The influence of the flow rate on soot formation and on the flame stability was also investigated. Knowledge of these quantitative interrelations is valuable for the design and operation of flares. At present, high-level flares, ground-level flares, and incinerators are used for the thermal combustion of waste gases. Of the flares, the high-level are preferred. They consist of steel pipes, 30 to 100 m high, fitted with a flare tip. Steam, at a pressure of 12 to 15 atm, is injected into the flame in a range of adjustment of about 1:10. Smokeless burning of waste gases is achieved with this equipment, largely independent of the gas composition. As yet, the nuisance caused by the glare and the noise of the steam injection cannot be avoided to the desired degree. Ground-level flares consist of burners and a brick-lined steel shell up to 10 m high, built on supports so that the air for combustion can enter from below. Various types of burner have proved suitable, into some of which water is injected instead of steam. The ground-level flare is less suitable than the high-level flare if highly fluctuating quantities of gas have to be burned. 2.1.2 Incinerators As stated, flares are suitable for the elimination of gases with energy concentrations sufficient for burning with an open flame. Nonflammable waste gases are burned in incinerators, which can be heated by means of gas or oil burners, and, if designed accordingly, can also serve for the combustion of liquid waste. The efficiency of incinerators depends essentially on the time during which the substances to
CLEANING WASTE GASES be burned are exposed, in the presence of excess oxygen, to a temperature above the ignition temperature. At 800 ~ to 1000~ residence times of 0.5 to 2 seconds are necessary if an efficiency of more than 90% is to be achieved. The higher the combustion temperatures, the shorter the residence time may be, and the smaller the combustion chamber required. The temperature load of the combustion chamber is limited by the resistance of the material of which it is made. In a combustion plant for liquid waste at the Hoeehst factory, an interesting, new method has been used in which the brickwork is protected from the heat by means of a boiler-tube cage. The "34 rule" summarizes interrelations important for combustion: t i m e - - t e m p e r a t u r e - turbulence have to be adapted to one another for optimum conditions in the combustion plant. As the interaction of these three factors is decisive for the quality of an incinerator, the individual factors may be varied within wide limits. Thus, it is not practical to lay down fixed standards for the design of incinerators. As far as possible, and within economical bounds, the heat content of the combustion gases is utilized for preheating the untreated waste gas or fresh air, or for the production of steam. Because of the inherent danger of explosion, great experience is required for designing and operating incinerators. When waste gases containing fluorine or large amounts of chlorine compounds are burned, the combustion products--hydrogen fluoride and hydrogen chloride--generally have to be removed by scrubbing with water. In the ease of waste gases containing sulfur, the combustion of which produces sulfur dioxide, the flue gases have to be emitted through a suitably high stack. 2.2 Catalytic Oxidation If large amounts of waste gas are to be freed from combustible substances whose calorific value is insufficient for combustion without additional burning of fuel, it is advantageous to employ catalytic oxidation. By this method, it is possible to achieve oxidation down to an energy content of about 40 keal/m ~ at STP without additional energy, or to reduce the additional energy requirements drastically if the calorific value of the waste gas is less than 40 kcal/m ~ at STP. This is possible because the oxidation temperatures of 800 ~ to 1000~ which are required for thermal combustion, can be reduced to 250 ~ to 500~ Details of the fundamentals of the process are contained in the paper by Vollheiin in This Symposium. The operating temperatures of the catalysts
647
vary in accordance with the type and concentration of the substances to be oxidized. While hydrogen, for example, can be oxidized in the presence of base metal oxides at about 250~ considerably higher catalyst temperatures are required for methane or aromatic compounds. The gases to be cleaned have to be as free of mineral dust as possible so that the catalyst does not become fouled; nor should they contain any catalyst poisons. Metal catalysts are available, depending on the intended use--for example, in the form of chrome-nickel tapes coated with noble metal, or the usual catalyst bases with coatings of noble metal or base metal oxide. The 3-t rule applies also to catalytic combustion. Turbulence is imparted to the gas on passing over the catalyst. The residence time is governed by the effective catalyst surface available to the gas. For technical purposes, the residence time for a gi~'en catalyst with a defined surface is related to the so-called space velocity, the quotient of the waste gas amount [m~/hr at STP], and the amount of catalyst [m3-]. The catalyst temperature required for sufficient oxidation is determined by experiment. Individual catalysts withstand permanent loads up to 1200~ The usual "volume loads" of the catalysts are between 10,000 and 50,000 m 3 at STP waste gas per cubic meter of catalyst per hour. The heat of reaction in the catalyst may reach 4 to 5X 106 kcal/m 3 of catalyst. Particularly at high reaction speeds, the temperature of the catalyst material may rise noticeably above the temperature of the waste gas, as the heat dissipation from the catalyst is limited. This effect is especially important for temperature-sensitive catalysts whose service life may be considerably reduced due to overheating. I t is also possible that flashbacks occur above the lower ignition limit, due to unexpected temperature increases which lead to explosions. Where necessary, waste gases rich in energy have to be cooled by means of secondary air or circulating air. A new course for cooling the catalyst material has been pursued in a plant built in the Hoechst factory. In this system, the catalyst, a palladium catalyst with aluminium oxide base, is cooled in pipe coils through which water flows. 24,000 m 3 of waste gas per hour is cleaned which, apart fl'om air, contains aboat 2000 m 3 of steam, 4000 m :~of carbon dioxide, as well as organic impurities, the waste gas stemming from the stripping of the waste water from a high-temperature pyrolysis plant with a mixture of air and regenerated gas from a carbon dioxide scrubber. During the stripping process, the gas absorbs benzene, toluene, and other aromatic and aliphatie hydrocarbons. Their concentration is equivalent to
648
COMBUSTION PROBLEMS RELATED TO AIR POLLUTION
Hydrogen? ~ Steam - Air~ rt
I_ Condensate
pl l , ,,f)
-7
!
I1
i
Strata
J
C9
@
|
,..L.
~)anden~
Wastewater
a
Air Strippedwastewater FIG. 1. Catalytic combustion unit for petrochemical waste gases. Key: a, Regeneration of CO~-absorbing solution; b, Condensed water separator; c, Air blower; d, Stripping column; e, Heat exchanger; f, Reactor; g, Water distributor; h, Water collector; i, Expansion; k, Feedwater container; l, Feedwater pump; m, Stack; n, Start-up reactor; o, Start-up blower.
0.8% by volume of benzene or a calorific value of 280 kcal/m a at STP. Figure 1 shows a diagram of the plant. After passing through the heat exchanger, in which the waste gas is heated to 170~ the stream of gas eaters the reactor. Here, the hydrocarbons are oxidized in three series-connected catalyst layers, each approximately 10 cm high. The catalyst is started up stepwise with hot air, which is produced in a separate start-up reactor by means of catalytic oxidation of a hydrogen-air mixture. Pipe coils, embedded in the individual catalyst layers of the main reactor, make it possible to limit the catalyst temperature, provided that the cooling surfaces in the catalyst bed are correctly designed. The heat transfer of an embedded pipe system is 6 to 10 times greater than that of the same pipe system around which, under otherwise equal conditions, a gas flows freely. Consequently, the cooling surfaces may be relatively small. The
cooling system supplies 4 tons of low-pressure steam per hour. It is not possible to regulate the temperature adequately by chai~ging the flow rate of the cooling water in the pipe coils, as the heat trans[er on the gas side determines the heat abstraction. The amount of combustible substance which has to be fed into the plant, therefore, has to be as uniform as possible with respect to time. This is done by changing the water feed to the stripping column. Regulation of the water supply depends on the measurement of the hydrocarbon concentration of the gas to be cleaned. The measurements are conducted by means of two analytical instruments working independently of each other. These instruments also actuate an alarm and close the stripping-water inlet when the maximum permissible hydrocarbon concentration is exceeded. In addition, flashback safety devices (on the basis of lamellae) and bursting disks are fitted.
CLEANING WASTE GASES In addition to the cleaning of petrochemical waste gases, catalytic oxidation is utilized in the chemical industry for the treatment of waste gases from the production of phthalic anhydride, maleie anhydride, and acrylonitrile. It also serves for example, to eliminate formaldehyde and amines in waste gases. Catalytic oxidation procedures are also gaining importance in the cleaning of sulfur dioxidecontaining waste gases formed in chemical processes or in incinerators. In the "Sulfacid process" the sulfur dioxide is reacted to sulfuric acid on activated charcoal that contains catalyst additions, and is sprinkled with water. Cokes on the bases of peat, lignite, or hard coal, oxidize the sulfur dioxide in the "Reinluft process" and adsorb the sulfuric acid formed, which is reduced again in a desorption zone yielding a sulfur dioxide-containing gas of high percentage. In the "Wickert process," catalytic oxidation is carried out with iron oxide-containing dolomite dust, which is injected into steam boiler plants and binds sulfur dioxide as sulfate. Other processes are based on the oxidation of sulfur dioxide on metal oxides or metal carbonates.
649
3. Biological Oxidation Biological procedures have been suggested recently for eliminating foul-smelling organic substances---e.g., protein decomposition products from waste gases. The gas to be cleaned is conducted through a moist filter mass, arranged in several layers, which contains bacteria in high concentration. The bacteria ingest as food the organic substances contained in the waste gas. There has not been much experience with this method so far and it remains to be seen whether microbial oxidation, which has been tried and proved for years in organically loaded waste water, will also be suitable for cleaning waste gases. Together with the intensified endeavors of the chemical industry to prevent air pollution, increasing importance will attach to oxidative processes as well. 1{EFE RENCE 1. HEss, K. AND STICKEL, I)t.: Ctmm. Ing. Teeh. 2,9, a34 (1967).