P • A • R • T
•
8
PERSPECTIVES ON SPECIFIC CHEMICALS
CHAPTER 32
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA Richard Lawuyi and Merv Fingas Emergencies Science Division, Environment Canada, Environmental Technology Centre, River Road, Ottawa, Ontario
32.1
OVERVIEW OF PRODUCT AND INDUSTRIAL USES More than ever, the government and the chemical industry are committed to the prevention of accidental spills and the management of hazards and risks associated with these substances throughout their life cycles. Increasing awareness of the deleterious effects of hazardous chemicals has reduced the environmental load of pollutants, and this trend will no doubt continue. Reduction of chemical releases into the environment is a shared responsibility that we should all participate in and that will be of great benefit to us all. More than a decade ago, the Emergencies Branch of Environment Canada prepared a chemical spill priority list of over 500 chemicals, each chemical being ranked according to its supply volume, reported spill frequency, historical spill volume, and toxicity. Our main objective then was to identify the minimum number of chemicals that would account for the maximum number of spills. The use of the list would be to act as a focus for the development of countermeasures, analytical methods, and spill manuals. This function has been well served. The list has since been reviewed, in 1990 (Fingas et al., 1991). One other approach that was considered was ranking chemicals by groups according to their physical and chemical properties. A number of properties were selected, for example, LD50, bioaccumulation, and persistence as the main criteria for toxicity to the environment and humans. Substances were then classified according to the range they fell within (Ministers’ Advisory Panel, 1995). This approach was used for the Canadian Environmental Protection Act (CEPA) assessments. For example, polychlorinated dibenzodioxins, polycyclic aromatic hydrocarbons, inorganic cadmium compounds, benzidine, trichloroethylene, and a host of others were concluded to be toxic. Others, such as chlorobenzene, toluenes, xylenes, and dibutyl pthalate, were concluded to be nontoxic. Others, such as aniline, styrene, crankcase oils, and pentachlorobenzene, do not have sufficient information for classification. Ammonia is a rather ubiquitous substance that occurs naturally but can also be manufactured quite readily. Demand for ammonia has peaked since the late 1980s, and total supply both in Canada and the United States has leveled throughout the 1990s simply because no new uses have been found for this product (CIS, 1998). Ammonia has also been found to be a widespread contaminant in sewers and aquatic environments and has often been a nuisance around wastewater outfalls and in agricultural 32.3
32.4
CHAPTER THIRTY-TWO
TABLE 32.1 Priority List Ranking of the Top Hazardous
Chemicals
Chemical
Ranking
Spill number
Spill volume
Supply volume
Ammonia Chlorine Tetraethyllead Styrene PCBs Sulfuric acid Sodium cyanide Hydrochloric acid Potassium chloride Pentachlorophenol Phenol Zinc sulfate Phosphorus Toluene
1 2 3 4 5 6 7 8 9 10 11 12 13 14
107 36 4 24 334 155 3 123 31 19 10 3 16 13
470 120 72 5,000 89 13,000 83 3,300 12,000 110 14 68 46 110
3,700 1,700 26 630 – 3,700 12 170 – 1.5 68 1,500 68 430
wastes and industrial effluents. The ammonia produced in most of these cases is mostly nonanthropogenic. The main natural source is the decomposition of nitrogenous substances by bacteria. In Canada and the United States, some criteria and guidelines have been determined for ammonia in all kinds of water uses. Ammonia concentrations in air and some water columns have been shown to be well above the guideline values (Lusis and Phillips, 1990; NAQUADAT, 1991; Saskatchewan Environment Data, 1991; Fenske, 1993; Williamson, 1990). Large-scale ammonia releases can result in gross water, soil, and atmospheric contamination and hence constitute risk to public health and safety. Enormous fish kills and horrific human injuries and fatalities have also been reported during accidental spills (Environment Canada, 1999; Markham, 1987). While there have been many accidents involving ammonia at industrial facilities all over the world, very few have occurred during transportation, such as by rail, road, or water (Environment Canada, 1999; Markham, 1987). The reason for this is quite clear— transporters and distributors are better trained in handling dangerous goods than in the past. Handlers are now aware of the dangerous consequences of chemical spills and hence more respectful. Another reason is that there are more guidelines and regulations laid down by governments and corporations on handling and transportation of chemicals apart from emergency response planning activities that are often required. Significant progress has also been made in the safe storage of these chemicals. The phrase ‘‘responsible care’’ has been reverberating across North America for the last decade in meetings, lecture halls, conferences, and symposiums. Research on ammonia toxicity has been slowing down considerably since the early 1990s; only about 458 relevant papers could be found in the last decade. The most significant areas of immediate concern are: (1) human inhalation effects; (2) effects on aquatic biota; (3) effects on land fauna and microflora; and (4) effects and extent of water pollution. Of lesser importance is human ingestion since this is an improbable route of exposure. The concentration / exposure effects of ammonia in many areas such as odor recognition, respiratory and eye irritation, and death have not been clearly defined and tend to vary with the researcher. Markham (1987), in his review, discussed the absence in the literature of basic data on exposure times / concentrations of ammonia that affect aquatic biota, terrestrial animals, and humans as well as occasions where little or no damage has occurred or those that
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.5
show a threshold below which any effects are unlikely to occur. There has also been very little published on potential effects of low-level, long-term exposure and the sensitivity of certain high-risk sectors of the population, especially the sick, the very old, and infants. These sectors of the population are often ignored for most studies. How best to determine hazardous and lethal exposures for humans of hazardous chemicals is still a formidable problem. 32.1.1
Modern Industrial Uses
Since the late 1960s, many technological advances and easy access to abundant natural gas have made the production of ammonia cheaper and the chemical itself more readily available. The resulting rapid consumption of ammonia fertilizers has resulted in a green revolution. For example, the annual consumption of ammonia-based fertilizer grew by 11.5% in 1960– 1970, 6.7% in 1970–1980, 3.8% in the 1980s, and less than 3.0% in the 1990s. A slump in the world market in the 1980s led to downsizing and reduction in production in the late 1980s. Over 85% of ammonia produced worldwide is used as nitrogen-based fertilizers, with the remaining 15% going into industrial products such as pharmaceuticals, various fibers, animal feeds, and explosives. Aside from its extensive use as a fertilizer, other main uses of ammonia include: 1. Starting materials for the production of chemicals such as nitrates, ammonium salts, urea, amines, and nitrocompounds 2. As an industrial refrigerant 3. As a reactant in many industrial processes, e.g., nylons, plastics, isocyanates, pesticides, detergents, and neutralization of acids 4. In water disinfectant to supplement the chlorination process 5. In bleaching, removing stains, extraction, printing, and photocopying 6. In various metal-treating processes where ammonia atmosphere is required, such as metal-nitriding, carbo-nitriding, bright annealing, sintering, furnace-brazing, and atomic hydrogen welding 7. As a source of hydrogen for the hydrogenation of fats and oils 8. As a source of pure nitrogen 9. As a common household cleaning agent 10. For equipment protection such as in condensers, storage tanks, heat exchangers, and bubble plate towers 11. In the rubber industry as a stabilizer of raw latex to prevent coagulation 12. In mining to extract metals such as copper, nickel, and molybdenum from their ores 13. In the pulp industry as a bisulphite to improve the quality of pulp 14. As a catalyst in the manufacture of phenol-formaldehyde and urea-formaldehyde resins 15. In fruits for preservation during warehousing About 20% of total ammonia produced in Canada, the United States, and Western Europe is used for industrial purposes, but the figure represents only about 10% in Russia, 5% in Latin America, and 1 to 10% in Asia. While the largest exporter of ammonia is Russia (34%), the largest importer is the United States (34%). Ammonia is an ideal fertilizer and has many benefits over others in that: 1. It is easy to apply. 2. It has the highest nitrogen content (82%).
32.6
CHAPTER THIRTY-TWO
FIGURE 32.1 Fertilizer nitrogen consumption, ⫻106 t.
3. Because of its anhydrous nature, it can increase the soil capacity to hold water and also the soil tilth. 4. It reacts with soil, organic, inorganic, and crop constituents, thus enhancing its fertilizing power, and retards leaching. 5. When applied below the soil, it spreads under its own pressure over a wide area, thus reaching more plant roots.
32.2
INTRODUCTION The main differences in the fate and behavior of various chemicals, both organic and inorganic, are due to their physical and chemical properties, such as solubility in various media, octanol / water partition coefficient, vapor pressure, biodegradation, chemical reactivity, and persistence. The need for physical properties data on behavior and environmental fate of chemicals in scientific research, emergency response, including firefighting, and government regulations and guidelines cannot be overemphasized. While the data in this chapter came from different sources, as shown in the references, the most reliable authors have been included. It is also recommended that the reader consult the original reference in case of updates. Some of these data have also been generated or verified in our laboratory. Some estimates have been made where necessary as well as comparisons between different chemicals when such exist.
32.3
PHYSICAL AND CHEMICAL PROPERTIES AND GUIDELINES At room temperature, ammonia is one of the most stable compounds in commerce (Czuppon et al., 1992), but it decomposes only at very high temperatures (above 450⬚C). One of its main hazards arises from the fact that it is transported as a liquefied gas under pressure (cryogenic) and on release quickly evaporates and dissipates. It has also been known to explode or catch fire. Many models have treated ammonia as a heavy gas because the vapor
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.7
cloud often absorbs moisture and eventually moves close to the ground. When released as a vapor from a gaseous source, it behaves as a lighter-than-air vapor, with its density a function of its vapor temperature only. When released as a liquid from a pressurized source, it will atomize and droplets will autorefrigerate, resulting in a cold mixture containing droplets of ammonia, water vapor, and water vapor with dissolved ammonia. This mixture is generally heavier than air. Ammonia will also undergo a number of reactions with other chemicals, including oxidation and neutralization of acidic compartments on release. Its main properties are as shown below (Compressed Gas Association, 1990; RTECS On-Line, 2000; Transport Canada, 1985; Braker and Mossman, 1980; AIHA, 1988). Common name: Ammonia, anhydrous or aqueous ammonia Molecular formula: NH3 Molecular weight: 17.031 CAS number: 7664-41-7 UN number: 1005 (⬎50%) 2672 (⬎10% ⱕ 50%) RTECS number: Anhydrous: BO0875000 10–35%: BO0876000 35–50%: BO0877000 Labels: Compressed gases, corrosive gases Specific hazards: Alkali—ALK Corrosive—COR Use no water with liquefied gas Synonyms and trade names: Ammonia gas: Ammonia anhydrous Ammoniac (French) Ammonia gas Ammoniak (German) Spirit of hartshorn Anhydrous ammonia (DOT) Nitrosil Ammonia solutions: Ammonium hydroxide (⬍28% aqueous ammonia) Ammonia water Aqueous ammonia Grades: Weight % minimum Grade Commercial Agricultural Refrigeration Anhydrous Metallurgical Electronic
99.5 99.7 99.95 99.99 99.995 99.998
Impurities are usually water, oil, and noncondensable gases or inerts such as hydrogen, argon, nitrogen, and methane carried over usually from the different steps.
32.8
CHAPTER THIRTY-TWO
Physical data: Anhydrous ammonia (NH3): State (room temp., 1 atm)
Gas
Boiling point (101.325 kPa) ⫺33.4⬚C ⫺77.72⬚C Melting point (1 atm.) ⫺78.85⬚C (NH3 H2O) ⫺78.68⬚C (2NH3 H2O) 0.6828 kg / L Density, liquid (⫺33.7⬚C) 888.0 kPa Vapor pressure (21.1⬚C) Autoignition temperature 651⬚C Vapor density (⫺33.4⬚C, 1 atm) 0.88983 kg / m3 Flammability limits in air 15 to 28% by volume 0.771 kg / m3 Gas density (gas, 0⬚C, 1 atm) Odor threshold range 2–10 mg / m3 Ignition temperature 850.0⬚C Ammonium hydroxide (ammonia water, aqua ammonia, about 25% ammonia): State (room temp., 1 atm)
Gas
Boiling point Melting point / freezing point Relative density (water ⫽ 1) Relative vapor density (air ⫽ 1) Vapor pressure (20⬚C) Solubility in water Relative molecular mass Heat of solution (28% / w) Heat of solution (0% / w)
36⬚C ⫺77⬚C 0.9 0.6 153 mbar Infinity 35.1 4.999 ⫻ 105 J / kg 8.081 ⫻ 105 J / kg
Hazards and toxicity: Mixtures of ammonia and air in certain proportions (15 to 28% ammonia) can ignite or even explode at high temperatures (above 648.9⬚C) if a source of ignition is present. Ammonia can react to form explosives with some chemicals. Human health: Anhydrous ammonia is very corrosive, with high affinity for water. In high concentration, inhalation can cause paralysis of the respiratory system and death. Environment: Ammonia can adversely affect many microorganisms, plants, and aquatic life. Ammonia toxicity to the biota is often due to its severe systemic and metabolic disruptions. Behavior in air: Anhydrous ammonia will react with water vapor in the air to form dense fog. Ammonia will burn with a yellow flame when exposed to an open flame in air or oxygen. Ammonia–air mixtures can also explode. Behavior in water: Anhydrous ammonia dissolves readily and reversibly in water to produce aqueous ammonia, ionized and un-ionized ammonia. The ratio of ionized and un-ionized ammonia depends on the pH. High concentrations in water will adversely affect aquatic organisms.
32.3.1
Emergency Response
Victims should be quickly removed from spill site to an uncontaminated area where there is an abundant supply of fresh air. A doctor should be called.
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.9
Inhalation: Give artificial respiration if breathing is difficult or has ceased. Skin: Wash and flush with water immediately. Remove clothing. Keep on flushing for at least another 15 minutes. Eyes: Eyes should be well rinsed with large quantities of water or 5% boric acid. Nose and mouth: Should be flushed with large quantities of water continuously. Ingestion: This is very rare. Victims should be given copious amounts of water to drink. Seek medical attention immediately. Spill control: Self-contained breathing apparatus should be worn for all emergency measures involving anhydrous ammonia and full protective clothing for liquefied ammonia. Leaks: Most leaks can be detected by smell, wet red litmus paper, or an open bottle of hydrochloric acid. Try to seal leak if possible. Fires: Anhydrous ammonia can burn but is very difficult to ignite. In the presence of air it can also explode when exposed to a flame or a source of ignition. Use dry chemicals or carbon dioxide to put out small fires. Use cold spray to cool container. Spills: Evacuate spill site. Eliminate ignition sources. Provide ventilation and stay upwind if possible. Neutralize ammonia spills with carbon dioxide. Special foams are also available. Soils: Flood site with plenty of cold water. Build barriers (dikes and lagoons) to contain large spills. Allow small spills to vaporize and absorb with sand or vermiculite. Use carbon dioxide foams to neutralize spills. Water: Contain by damming or water diversion. Use carbon dioxide to neutralize spills. Air: Let spill evaporate. Follow the emergency response planning guidelines (ERPG) of the American Industrial Hygiene Association (AIHA, 1988): ERPG-3. 1,000 ppm. This is the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing life-threatening health effects. ERPG-2. 200 ppm. This is the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual’s ability to take protective action. ERPG-1. 25 ppm. This is the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing other than mild, transient adverse health effects or without perceiving a clearly defined objectionable odor.
32.4
INDUSTRIAL ASPECTS AND PRODUCTION IN THE UNITED STATES, CANADA, AND WORLDWIDE The name ‘‘ammonia’’ derives ultimately, through the Latin sal ammoniacum [salt of ammonia], from the Egyptian god Amon, or Ammon. It was near an oasis beside a temple to the god that ammonia is said to have first been used, several thousand years ago, by Arab farmers spreading burned camel dung on their fields. Ammonium salts were observed in Persia about 900 A.D. Ammonia gas was first produced in pure form by J. B. Priestley in 1774, followed by C. L. Berthollet in 1784, who identified its elements. In 1809, W. B. Henry determined its composition and the formula to be NH3. It wasn’t until the end of the last century that fertilization of the soil with ammonia became a widespread farming practice. Ammonia gas
32.10
CHAPTER THIRTY-TWO
was also produced in 1828 by heating animal refuse with lime. In the same year, Wohler observed that the evaporation of a solution of ammonium cyanate when heated produced urea. Ammonia is the largest-volume basic, inorganic substance in use today and is generally considered the most important agricultural chemical. Anhydrous ammonia is usually stored and transported as a liquefied gas under pressure. As the use of ammonia grows and gains general acceptance, so grows the number of accidents. An examination of the origin and trend of these accidents will be useful in understanding and preventing further accidents.
32.4.1
Manufacture of Ammonia
The basic synthetic process is the Haber-Bosch method. As presented in Fig. 32.2, ammonia is produced by the reversible reaction of hydrogen with nitrogen. The reactants mixture, often called the synthesis gas, consists of hydrogen and nitrogen in roughly 3:1 molar ratio and impurities such as argon, methane, carbon dioxide, carbon monoxide, and water vapor. Nitrogen is separated from the air. Hydrogen, on the other hand, can be obtained from a variety of sources, such as hydrocarbons, coal, water, natural gas, coke oven gas, and mixtures of these. However, the most common and cheapest source is by steam reforming of natural gas, in which case the overall reaction is: 7CH4 ⫹ 10H2O ⫹ 8N2 ⫹ 2O2 ⇒ 16NH3 ⫹ 7CO2
(32.1)
Table 32.2 shows the different types of feedstocks and their contributions to the world manufacture of ammonia (Czuppon et al., 1992). The exothermic reversible reaction of hydrogen and nitrogen to form ammonia occurs according to the following equation:
FIGURE 32.2 Manufacturing of ammonia.
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.11
TABLE 32.2 Raw Materials for Hydrogen
Production Feedstock Natural gas Naphtha and fuel oil Naphtha and fuel oil condensate Coke oven, refinery gas, hydrogen Coal Total
2NH3 N2 ⫹ 3H2
1987 as % 69.8 5.8 10.5 3.9 10 100
⌬H ⫽ ⫺91.44 kJ / mol
(32.2)
The product yield depends on the pressure, temperature, reactant ratios, and type of catalyst used. Canada has an abundant supply of the most important raw material for the manufacture of ammonia-natural gas. In Canada, ammonia is manufactured by the highpressure catalytic steam reforming of natural gas. The main source of nitrogen is air, while hydrogen is also obtainable from a variety of other sources, such as crude oil, coal, and other hydrocarbons. The crude gas is processed in a series of operations consisting of steam regeneration, washing, compression, sulfur removal, shift conversion, carbon dioxide removal, nitrogen scrubbing, compression, and synthesis. The most important stages are as follows: 1. 2. 3. 4. 5. 6.
Purification of feedstock Primary and secondary reforming Shift conversion Carbon dioxide removal Synthesis gas purification Ammonia synthesis and recovery
The reaction proceeds at very low temperatures or high pressures, but the use of catalysts produces satisfactory yields at approximately 350⬚C. The rate of conversion per pass is about 25 to 35% and much greater in some cases. Coke was used as the main source of hydrogen in the original Haber-Bosch process. Today coal is still being used in some processes, such as the Lurgi process and the KoppersTotzek process. However, with the discovery of large gas reserves, coal use is rapidly declining. Over 80% of ammonia produced today is obtained by steam-reforming, and about 70% uses natural gas feedstock. Purification of Feedstock. The feedstock is first desulfurized or purified. This is a very crucial step in ammonia manufacture. It is required in order to maintain the life of the catalyst. Impurities such as chlorine, arsenic, and nickel used in the manufacturing process tend to shorten the life of the catalyst. The usual methods for desulfurization are by activated carbon adsorption at about 15 to 50⬚C or oxidation with zinc oxide or both. The use of zinc oxide is often preferred when large quantities of mercaptans and highly condensable hydrocarbons are present, which may quickly saturate the catalysts. The main function of the zinc oxide is to remove the hydrogen sulfide, mercaptans and chlorine. Combining activated carbon and zinc oxide is very effective in removing the different types of sulfide compounds.
32.12
CHAPTER THIRTY-TWO
Desulfurization
Hydrocarbon feed
Steam added Primary
Air added Secondary reforming
CO2 removal
Shift conversion
Compression, Ammonia synthesis
Methanation
Ammonia
FIGURE 32.3 A typical scheme for the production of ammonia.
Other methods include hydrodesulfurization and absorption-stripping followed by zinc oxide treatment. ZnO ⫹ H2S → ZnS ⫹ H2O
(32.3)
Natural gas containing large amounts of hydrogen sulfide is often subjected to an absorption-stripping operation followed by zinc oxide treatment. Organic sulfur compounds are often removed by hydrodesulfurization, in which the organic sulfur is converted to hydrogen sulfide in the presence of hydrogen nickel molybdate and cobalt catalysts at 340 to 400⬚C. The process also includes some zinc oxide as a supporting catalyst. Reforming. Reforming of the natural gas feedstock takes place in two distinct catalytic reaction steps. The first step is the primary reforming and is carried out in a furnace in the presence of steam to produce a partially reformed gas. The second step constitutes the secondary reforming, in which the reaction is carried out in a refractory-lined pressure vessel to produce a low-methane product. Primary reformer CH4 ⫹ H2O → CO ⫹ 3H2
⌬H ⫽ 206.08 kJ / mol at 25⬚C
(32.4)
⌬H ⫽ 206.08 kJ / mol at 25⬚C
(32.5)
Secondary reformer CO ⫹ H2O → CO2 ⫹ H2
The primary reformer reaction is largely endothermic, and considerable heat must be supplied to this process. Nickel supported on alumina is generally the primary reformer
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.13
catalyst, the reaction temperature being in the 750 to 850⬚C range at a pressure of 2,860 to 3,550 kPa. Calcium aluminate is often the catalyst used at higher operating temperatures and is often supported by sodium or potassium oxides. The following side reactions will lead to the deposition of carbon. The presence of these oxides leads to reduced carbon deposition and increased life of the catalysts. CH4 → C ⫹ 2H2
(32.6)
2CO → C ⫹ CO2
(32.7)
CO ⫹ H2 → C ⫹ H2O
(32.8)
CO2 ⫹ 2H2 → C ⫹ 2H2O
(32.9)
The catalyst often used in the secondary reformer is nickel supported on alumina. Air is added in adequate quantities to supply the required amount of nitrogen. Shift Conversion. Byproducts such as carbon oxides that deactivate the catalysts used in the synthesis have to be removed or converted to the inert species before the actual synthesis occurs. In the shift conversion process, carbon monoxide is converted to carbon dioxide. The reaction is exothermic and reversible. CO ⫹ H2O → CO2 ⫹ H2
⌬H ⫽ ⫺41.17 kJ / mol at 25⬚C
(32.10)
This reaction equilibrium is favored at low temperatures, and most of the carbon monoxide is converted to carbon dioxide in a high-temperature shift furnace (HTS) operating at 350 to 450⬚C. This step is followed by low-temperature conversion of the remaining carbon monoxide to carbon dioxide in a low-temperature shift converter (LTS) after cooling. The usual catalyst for the low-temperature shift converter is copper oxide supported on zinc oxide and alumina. Removal of Carbon Dioxide and Methanation. About 17 to 19% of the effluent gases is carbon dioxide. This undesirable byproduct is usually removed by a number of methods, such as physical absorption, chemical reaction, or a combination of both. Removal methods include monoethanolamine process for removing acid gases, activated carbonate process based on absorption of carbon dioxide by potassium carbonate, and physical absorption systems in which solvents and agents that remove carbon dioxide are used. Generally, the choice of carbon dioxide removal depends on the overall plant design and the integration procedures needed. The final purification step employs the methanation process to convert carbon oxides to methane. In the very final cleaning up, the cryogenic purifier system is often used. In the cryogenic process, the gas from methanation is dried to a low dew point, cooled, and then expanded in a turbine to liquefy some portion of the gas followed by further cooling. The vapor emerging after this step is scrubbed in a rectifying column to remove most of the argon and almost all of the methane. Methanation reactions are as follows: CO ⫹ 3H2 → ← CH4 ⫹ H2O
⌬H ⫽ ⫺206 kJ / mol
(32.11)
→ CH4 ⫹ 2H2O CO2 ⫹ 4H2 ←
⌬H ⫽ ⫺165 kJ / mol
(32.12)
and
Synthesis of Ammonia and Recovery. After the various purification steps, the synthesis gas, consisting of hydrogen and nitrogen, is ready for compression and ammonia manufacture usually in a synthesis loop. The operating pressure is normally in the range of 13,785 to 34,475 kPa. Most modern large-scale ammonia plants use centrifugal compression for the synthesis. The synthesis loops fall into two main groups: those that recover ammonia after
32.14
CHAPTER THIRTY-TWO
make-up gas-recycle compression and those that recover ammonia before the recycle compression. The synthesis gas is then delivered to the converter. There are two types of converters: tubular and multiple bed. The feed gas is fed at a 3:1 ratio, hydrogen to nitrogen. The synthesis is carried out on a catalyst surface based on metallic iron (magnetite) that has been promoted with iron oxides. An increase in pressure increases the equilibrium percentage of ammonia as well as the reaction rate. An increase in temperature accelerates the reaction, decreases the equilibrium amount of ammonia, and also degrades the catalyst. The ammonia product is usually recovered by condensation, and any unreacted feed gas is recycled through the synthesizer. Modern ammonia plants are designed and built to reflect some degree of integration between the process and energy systems. Any waste heat in the process is used to provide energy for the boiler-feed water heating and steam generation. Thus, in modern plants it is not difficult to run all the processes without employing external power. Environmental Concerns. Natural gas contains low quantities of sulfur that are easily removed by stripping in a conventional adsorption / absorption system. Estimates of effluent emissions from ammonia plants have revealed the results shown in Table 32.3. (Committee on Medical and Biological Effects, 1979). Other feedstocks, such as coal and naphthas, may present some environmental problems. For example, coal contains significant quantities of sulfur and metals requiring extra steps for removal and safe disposal. Byproducts such as tars, phenols, and naphthas must also be safely disposed of. Apart from feedlots and other natural sources, which account for 95.5% of total fugitive emissions of ammonia, anthropogenic sources of ammonia emitted into the environment during the production of fertilizers and industrial chemicals are shown in Table 32.4. 32.4.2
Production in Canada, the United States, and Worldwide
Because of the cheap and abundant natural gas, most of the ammonia produced in Canada comes from western Canada and some from Ontario (CIS, 1998). Plants are located in Calgary, Carseland, Joffre, Redwater, Sombra, Trail, Niagara Falls, Maitland, Belle Plaine, Hamilton, Nanticoke, Medicine Hat, Fort Saskatchewan, Redwater, Belle Plaine, Brandon, Kitimat, and Courtright. The major market for anhydrous ammonia is in western Canada. Ammonia is shipped from different manufacturing sites by rail to other manufacturing facilities, atmospheric storage tanks or rail transload locations; by truck or road transport to atmospheric storage tanks and retail storage locations; and by retail truck to farm wagons. About 5,000 kilotons of anhydrous ammonia were produced in Canada in 1997, of which 3,500 were utilized as fertilizer products and 2,000 applied directly to the soil as urea. About 14 kilotons were imported mostly from the United States (CIS, 1998). Ammonia prices and
TABLE 32.3 Pollutants from Ammonia Production
Pollutant
Emission factor (kg / t)
Total emissions (t / year)a
Sulfur dioxide Nitrogen dioxide Carbon monoxide Ammonia
0.4 0.6 6.0 1.3
5,900 8,900 89,200 19,300
a The abbreviation ‘‘t’’ is used to denote the metric ton of 1,000 kg (2204.0 lb).
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.15
TABLE 32.4 Ammonia Emissions from
Production of Fertilizers and Industrial Chemicals
Source
Emission rate (tons / year)
Direct application Ammonium nitrate Petroleum refineries Sodium carbonate (Solvay) Diammonium phosphate Ammoniator-granulators Urea Miscellaneous Beehive ovens Total
168,000 59,000 32,000 14,000 10,000 10,000 4,000 2,000 1,000 300,000
sales reached a record high in 1996. Ammonia plants continue to expand as a result of a cheap supply of natural gas. The price range per ton in western Canada was $350 to 420 in 1997. The major Canadian producers are Agrium, Beker Industries, Canadian Fertilizers, Cominco, Cyanamid, Nitrochem, Pacific Ammonia, Saskferco Products, Sherritt International, Simplot Chemical, Stelco, Terra International, and Western Cooperative. The total production of anhydrous ammonia in the United States in 1997 was about 17,000 kilotons, in addition to around 3,000 kilotons that was imported. Prices at the U.S. Gulf Coast ranged from $225 to 230 in 1997. Ammonia plants are located in several cities including Borger, Pace Junction, Hopewell, Fortier, Geismar, Walla Walla, Donaldsonville, Girard, Cheyenne, Freeport, St. Helens, Beulah, Beaumont, Beatrice, Coffeyville, Dodge City, Enid, Fort Dodge, Lawrence, Pollock, Port Comfort, Creston, Faustina, East Dubuque, Pocatello, Wells, Sterlington, Cherokee, Yazoo City, Dumas, Augusta, Clinton, Geismar, Laplatte, Lima, Memphis, Gordon, Luling, Beaumont, Blytheville, Port Neal, Verdigris, Woodward, Finley, Kenai, and Pryor. The major U.S. producers are Agrium, Air Products, Allied Signal, Avondale Ammonia, Borden Chemicals, BTU Energy, CF Industries, COGA Industries, Coastal Chem, Coastal Refining, Coastal St. Helens Chemical, Dakota Gasification, Dupont, Farmland Industries, Formica Plastics, Green Valley Chemical, IMC-Agrico, IMC AgriBusiness, J R Simplot, Koch Industries, LaRoche Industries, Mississippi Chemical, Nitromite Fertilizer, PCS Nitrogen, Shoreline Chemical, Solutia, Terra Nitrogen, Triad Nitrogen, Unocal, and Wil-Grow Fertilizer. Other countries actively producing anhydrous ammonia in North America are Mexico, which produces 26,995 kilotons, and Trinidad, which produces about 2,370 kilotons. On a global scale, ammonia consumption as a fertilizer has been growing steadily since 1960, as shown in Table 32.5 (Lancaster, 1989; IFDC, 1989). The use of ammonia fertilizers is steadily growing in developing countries as the soil quality deteriorates over time. Urea is the most common form of ammonia fertilizer that is exported by the Commonwealth of Independent States (CIS) and Romania (43%). While the United States is the largest importer of ammonia (34%), CIS is the world’s largest producer and the largest exporter of ammonia (34%), followed by China, India, Mexico, Indonesia, and the Middle East. There are still large variations in ammonia supply, use, and distribution in different regions of the world. This probably depends on affordability, agricultural practice, and the industrial base in various parts of the globe. The demand for ammonia in different parts of the world is shown in Table 32.6 (Lancaster, 1989).
32.16
CHAPTER THIRTY-TWO
TABLE 32.5 Global Ammonia Fertilizer
Use
32.4.3
Year
Ammonia fertilizer use (kilotons)
1960 1970 1980 1990
9.54 28.17 57.19 80.3
Production and Transportation Costs
The total production of ammonia worldwide in 1990–1991 was 98.84 megatons, even though the nominal capacity was 119.18 megatons. Industrial activities utilized 10.93 (11%) of this amount. Production costs and profitability largely depend on the type of feedstock being used, labor, capital investment costs, and energy prices (Sheldrick, 1987; Green Markets, 1990). Profitability also depends on the selling price of ammonia, which in turn is a function of supply and demand dynamics. Estimation of the total production costs in a 1000 metric tons per day plant at the United States Gulf Coast was $106.38 / ton. This rises to $150 / ton when taxes, storage, and investment costs are included. Transportation costs are separate from the costs of manufacturing. The transportation and use of ammonia are at their peak during the spring fertilizer season. In North America, ammonia is usually transported for long distances by rail, barge, and pipeline or by truck for very short distances from local dealers or distributors to farm locations. Long-distance transport is generally cheaper than for shorter distances. For example, costs estimates for long-distance hauling (1,600 km) by barge, pipeline, and rail are $0.0153, $0.0161, and $0.0215 per ton per kilometer, respectively, while for short distances (160 km), it is about $0.0365 per ton per kilometer. Shipping liquefied ammonia across the Atlantic costs about $35 a ton.
32.5
CHEMISTRY Due to its reactivity, ammonia will take part in a variety of reactions. Only those of some environmental significance or relevance to spill situations will be discussed here. The main driving force for the reactions of ammonia is the lone pair of electrons on the nitrogen atom, which is so oriented that it adds to the nitrogen-hydrogen dipole moment. Hence, ammonia is an extremely polar molecule.
TABLE 32.6 Global Demand for Ammonia in Different Regions in 1990
Region
Percentage of global distribution
Africa Asia Latin America North America Eastern Europe Western Europe CIS
3 35.4 5.3 13.8 9.7 11.5 21.5
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.5.1
32.17
Reactions
Water. The reaction between ammonia and water is a reversible one in which very small amounts of the hydroxide are formed. This is clearly evident from the equilibrium constant (Keq) of the reaction (Jolly, 1964): NH3 ⫹ H2O NH⫹ 4 ⫹ OH Keq ⫽
⫺ (NH⫹ 4 )(OH ) ⫽ 1.75 ⫻ 10⫺5 at 18⬚C (NH3)(H2O)
(32.13) (32.14)
Spectroscopic data provide evidence for the presence of the dihydrate NH3 2H2O, the monohydrate NH3 H2O, the hemihydrate 2NH3 H2O, and unhydrated ammonia in aqueous ammonia solutions. This is also shown in the water–ammonia (Eq. 32.15). Ammonia and water will mix in all proportions above 0⬚C. The maxima shown in the diagrams indicate the existence of these hydrates. The monohydrate melts at ⫺79.0⬚C, the hemihydrate at ⫺78.8⬚C, and the dihydrate at ⫺98⬚C. NH3 H2O 2NH3 H2O NH3 2H2O NH4⫹OH⫺ NH3 ⫹ H2O
(32.15)
Oxidation. When ammonia is ignited, it burns to produce water and nitrogen. Powerful oxidizing agents such as potassium permanganate will also oxidize ammonia to elemental nitrogen. In the presence of catalysts, ammonia is used in the production of nitric acid. 2NH3 ⫹ O2 → N2 ⫹ 3H2O
(32.16)
Decomposition. Even though quite stable at high temperatures, ammonia will decompose to hydrogen and nitrogen. The decomposition range is about 300 to 600⬚C. 2NH3 N2 ⫹ 3H2
(32.17)
Neutralization. The acid-base properties of ammonia are very useful in spill situations. This is an important reaction often used in spill response and disposal. Ammonia will react with acids to give salts and water. The product, an ammonium salt, often makes a good fertilizer. NH3 H2O ⫹ HNO3 → NH4NO3 ⫹ H2O
(32.18)
Another reaction of great importance in spill response is of ammonia with carbon dioxide to yield ammonium carbamate, which then decomposes to urea. 2NH3 ⫹ CO2 → NH2CO2NH4
(32.19)
NH2CO2NH4 → NH2CONH2 ⫹ H2O
(32.20)
Reactions with Metals. Liquid ammonia possesses a remarkable ability to dissolve some electropositive metals, mostly the alkali and the alkali earths metals such as lithium, sodium, potassium, cesium, and copper. It is therefore advisable not to store these chemicals together. For example, sodium dissolves in excess ammonia to produce sodamide, a white solid, while magnesium, when heated in ammonia, produces magnesium nitride. 2Na ⫹ 2NH3 → 2NaNH2 ⫹ H2
(32.21)
3Mg ⫹ 2NH3 → Mg3N2 ⫹ 3H2
(32.22)
32.18
CHAPTER THIRTY-TWO
Reactions with Nonmetals Halogens. Halogens react with ammonia to yield haloamines. The eventual trihalides formed can react with excess ammonia to produce unstable ammoniates, which can decompose to give ammonium salt and nitrogen. This reaction has some implication for water purification using chlorine as disinfectant. Aqueous solutions of chloramine, NH2Cl, are formed by the reaction of aqueous ammonia and chlorine or hypochlorite solutions: 2NH3 ⫹ Cl2 → NH2Cl ⫹ NH4Cl NH3 ⫹ OCl⫺ → NH2Cl ⫹ OH⫺ NH3 ⫹ Cl → NH2Cl → NHCl2 → NCl3 NCl3 ⫹ NH3 → NCl3 NH3 NCl NH3 ⫹ 3NH3 → N2 ⫹ 3NH4Cl
(32.23) (32.24) (32.25) (32.26) (32.27)
Reaction with Phosphorus. Ammonia will react with phosphorus vapor at red heat to produce nitrogen and phosphine. 2NH3 ⫹ 2P → 2PH3 ⫹ N2
(32.28)
Reaction with Sulfur. Sulfur reacts with anhydrous ammonia to form nitrogen sulfide. 10S ⫹ 4NH3 → 6H2S ⫹ N4S4
(32.29)
Reaction with Metallic Salts. Ammonia solutions will react with some metallic salts to form complexes. CuSO4 ⫹ 2NH3 H2O → Cu(OH)2 ⫹ (NH4)2SO4 Cu(OH)2 → Cu2⫹ ⫹ 2OH⫺ [Cu(NH3)4]2⫹ 4NH3 ⫹ Cu2⫹
32.6
(32.30) (32.31) (32.32)
ENVIRONMENTAL FATE Over 99.5% of atmospheric and aquatic ammonia is from natural sources, namely from biodegradation or decomposition of organic matter. Ammonia also enters the atmosphere from anthropogenic sources such as during production and use. Other sources include waste incineration, domestic heating, internal combustion engines, and other industry-related sources.
32.6.1
Terrestrial Fate
Anhydrous ammonia is an extremely volatile compound, and when released on land it does not stay in the same place for very long periods of time. It will tend to volatilize very rapidly, albeit some of it will be adsorbed onto the soil. The portion that is sorbed to the soil will undergo further reactions such as nitrification and eventual plant uptake. Because it is poorly adsorbed to the soil, leaching into underground water may also occur.
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.6.2
32.19
Atmospheric Fate
Comparatively, ammonia has a residence time of 5 to 10 days in the atmosphere, depending on the location and weather conditions. The main fraction that evaporates into the air can undergo a variety of reactions, such as (1) decomposition, thermal, and photochemical reactions, (2) aqueous phase reactions that produce aerosols, and (3) heterogeneous reactions with soot particles. 32.6.3
Photodecomposition
Ammonia is known to decompose when exposed to radiation in the far ultraviolet to produce the reactive amino and imino radicals. NH3 ⫹ h → NH2 ⫹ H
(32.33)
NH3 ⫹ h → NH ⫹ 2H
(32.34)
There is a theory that ammonia decomposes at high temperatures (above 450⬚C) to produce nitrogen and hydrogen, which often causes explosions during ammonia fires. With photochemically excited species: NH3 ⫹ OH → NH2 ⫹ H2O
(32.35)
NH3 ⫹ O → NH2 ⫹ OH
(32.36)
2NH3 ⫹ 4O3 → 4O2 ⫹ H2O ⫹ NH4NO3 32.6.4
(32.37)
Aqueous Phase Reactions
With acid aerosols, ammonia forms salt clusters: 8NH3 ⫹ 8HNO3 → 8NH4NO3
(32.38)
With sulfur dioxide, it initially forms amido sulfurous acid. nNH3
( g)
⫹ SO2
NH3SO2
( g)
⫹ H2O(g) → NH3SO2(aq)
(g)
(NH3)nSO2
(s)
→ → → (NH4)2SO4
32.6.5
(32.39) (32.40) (32.41)
Rainfall and Precipitation
Washout of ammonia by rainfall, precipitation, water vapor, and reactions with other reactive species reduces the amounts of ammonia in the air and also ensures quick return to the soil. Levels of ammonia in precipitation have been shown to be much higher in rural areas than in urban centers and have shown seasonal variation (Lusis and Phillips, 1990). 32.6.6
Aquatic Fate
If anhydrous ammonia is discharged into water, while most of it will volatilize into the air (35%) depending on the wind and mixing conditions, some will stay in the water. Ammonia
32.20
CHAPTER THIRTY-TWO
is extremely soluble in water. It will dissolve to produce un-ionized ammonia, such as ammonia (NH3), ammonia hydrates NH3 H2O, NH3 2H2O, 2NH3 2H2O, NH4OH, and the ⫺ ionized ammonia NH⫹ 4 OH . It is known that the un-ionized ammonia is toxic to fishes and aquatic organisms. ⫺ NH3 ⫹ H2O NH3 H2O; NH3 2H2O; 2NH3 H2O NH⫹ 4 ⫹ OH
(32.42)
The relative proportion of ionized and un-ionized ammonia formed depends on pH, temperature, the ionic strength of the receiving waters, and the prevailing weather conditions. One would therefore expect different fractions of these ammonia species when the same quantity of ammonia is released in sea water as in fresh water. Any of these solution products can react with chemical species already present in the water, such as humic substances, metals, and metal complexes. As for the rest of the dissolved ammonia, while some will bind to sediments, suspended particles, and organic matter available in the water, most will undergo nitrification by the Nitrosomonas and Nitrobacter bacteria species to yield nitrates that can be uptaken by aquatic plants and organisms. Many algae and phytoplankton have been found to utilize ammonia directly as their source of nitrogen. Because of its low octanol / water partition coefficient, ammonia is not expected to adsorb strongly to sediments.
32.7
BEHAVIOR Because of many variations in conditions such as cold products versus hot products, weather and wind conditions, various leak geometries, instantaneous releases, massive tank failures, continuous versus intermittent releases, and pipeline leaks versus railcar punctures, there is no universal model to make accurate quantitative predictions of dispersion of ammonia (Pederson and Selig, 1989; Kaiser and Walker, 1978). All models have described the behavior of ammonia on sudden release from pressurized tank as follows: 1. Liquefied ammonia will definitely flash off with simultaneous entrainment of liquid droplets depending on the pressure drop in the storage tank or size of leak. 2. There is a possibility of gravity-driven slumping, becoming very much broader than would be expected for a cloud that, e.g., is not denser than air. 3. There is likely to be a period of low-level ground hugging during which the vertical rate of growth of the cloud is less than that expected for a neutral cloud. 4. The cloud is not expected to become buoyant except when the wind speed is small.
32.8
HUMAN AND ENVIRONMENTAL TOXICITY It has been known for more than a century that ammonia is toxic. In 1893 the main effects of ammonium chloride salt on humans were described as convulsions, twitches, tremors, salivation, irregular respiration, somnolence, and lassitude. Toxic effects of other ammonia derivatives, such as urea, have been described as being characterized by restlessness, ataxia, dyspnea, collapse, edema, heart abnormalities, muscle spasms, tetany, and death. Because the odor threshold of ammonia gas is very low (approximately 5 ppm), it provides ample warning properties. Ammonia attacks wet tissue such as the eyes and mucous membranes of the respiratory system. Two types of injuries can result from ammonia spills. One is direct contact, in which the victim is sprayed or splashed resulting in skin burns or loss of vision. Such injuries are often restricted to the immediate surroundings of the release point. Ammonia can also be released as a gas or cloud, which can spread over a wide area
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.21
and affect many people adversely as it is inhaled. In this case, injuries are often confined to the respiratory system (Markham, 1987; Czuppon et al., 1992; Braker and Mossman, 1980; Pedersen and Selig, 1989). At very high concentrations, bronchial spasm or edema of the lungs can result in permanent lung damage or death in a few minutes. The range of effects of ammonia exposure to humans can be divided into three groups: 1. Mild effects: Exposure to concentrations lower than 5,000 ppm for a few minutes. • Symptoms: Restricted to the eyes and upper respiratory tract, smarting sensation in the eyes and mouth, pain when swallowing, marked hoarseness and tightness of the throat, slight cough. • Signs: Reddening of the conjunctivae, lips, mouth, and tongue with swelling of the eyelids and edema of the throat. No clinical signs of lung involvement. • Consequences: There may be spontaneous recovery without pulmonary complications ensuing. 2. Moderate effects: Exposure to concentrations from 5,000 to 10,000 ppm for a few minutes. There is evidence of the progression of effects deeper into the respiratory system with involvement of the bronchi and bronchioles. • Symptoms: Exaggeration of the symptoms described under mild effects. Feeling of tightness in the chest, difficulty in swallowing, sometimes complete loss of voice. Cough with copious sputum, sometimes bloodstained. • Signs: Distress, increase in pulse and respiration rates. Marked swelling of the eyelids with spasm and lacrimation. Moderate edema of the oropharynx with burning of the mucous membranes and resultant stripping of the epithelium to reveal dark red glazed patches. Examination of the chest reveals diminished air entry with the presence of moist sounds. • Consequences: Fatalities due to obstruction of airways or complications such as lung infection cannot be ruled out. 3. Severe effects: Exposure to concentrations in excess of 10,000 ppm for a few minutes. • Symptoms: Symptoms of oropharynx and eyes similar to those described under moderate effects. Persistent cough with copious frothy sputum. • Signs: Shock, restlessness, and obvious distress. Rapid pulse of poor volume. Cyanosis and great difficulty in breathing. Generalized moist sounds in the chest. Death as a result of asphyxiation may be expected. Survivors may die later as a result of complications, such as lung infections. 32.8.1
Environmental Toxicity
In dilute or moderate concentrations, ammonia is good for the soil and microorganisms. During a spill on the soil, all acid constituents are neutralized. Liquefied ammonia, on the TABLE 32.7 Typical Effects of Gaseous Ammonia on Humans
Concentration (ppm)
Exposure time (min)
Effects
72 330 600 1,000
5 30 1 to 3 1 to 3
1,500
1 to 3
Some irritation. Concentration tolerated. Eyes streaming within 30 seconds. Eyes streaming instantly, vision impaired but not lost. Breathing intolerable to most participants after exposure. Instant reaction is to get out.
32.22
CHAPTER THIRTY-TWO
other hand, is very corrosive, and excessive discoloration of plants has been observed during large spills of liquefied ammonia. Fatalities in ponies and hogs have been reported at concentrations of 5,000 to 20,000 ppm (assumed) and 700 to 10,000 ppm, respectively. Birds and small wildlife have also been reported dead at 2,300 ppm; trees and ground vegetation have withered. Ammonia is also produced from burning of coal and bacterial decomposition of proteinaceous organic matter. Conversion of ammonia to ammonium salts will occur rapidly both in the atmosphere and in the soil but will result in less acidity of the soil. It is not known to what extent ammonia reduces excess greenhouse gases such as carbon dioxide in the atmosphere to form ammonium carbamate. Microbial assimilation of ammonium compounds consumes alkalinity of the soil as well as converting ammonium ions to nitrate. Nitrification is carried out by many aerobic bacteria. Nitrification is pH sensitive. Denitrification, which is an anaerobic process, is also pH sensitive.
32.9
PAST SPILLS, LESSONS LEARNED, AND COUNTERMEASURES APPLIED Three well-researched accidents involving instantaneous releases of massive quantities of anhydrous ammonia from pressurized containers are discussed in this section.
32.9.1
Houston, Texas, May 11, 1976
A road tanker carrying approximately 19 tons of anhydrous ammonia crashed through a barrier at an elevated section of motorway near Houston, Texas (Pedersen and Selig, 1989; Kaiser and Walker, 1978). The pressurized tank burst on hitting the roadway below. Five people were killed and 178 people injured. High concentrations of ammonia were confined to within a few hundred meters of the source of the accident. All the fatalities and permanent injuries were within 70 m of the release source. The concentration contours predicted from calculations and models to account for the fatalities and injuries are shown in Table 32.8. Prevention may help in reducing road accidents of this nature. Although the cause of the crash was not determined, it was due either to mechanical failure or human error. Sound maintenance programs should be in place for these chemical tankers in accordance with the industry guidelines. Drivers should be given medical examinations and regularly tested for fitness and drug use. Long routes that could result in fatigue should also be prevented. Weather is also an important factor. Chemical tanker drivers should limit their activities on the road during inclement weather conditions.
TABLE 32.8 Predicted Concentration Contours for the Houston
Accident Isopleth (ppm)
Approximate length (m)
Approximate width (m)
1,200 2,500 5,000 10,000
1,130 875 835 600
400 420 430 350
Source: Pedersen and Selig, 1989.
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.9.2
32.23
Potchefstroom, South Africa, July 13, 1973
In the Potchefstroom accident, 18 people died and hundreds were injured as a result of the release of 38 tons of anhydrous ammonia following a sudden failure of a pressurized ammonia storage tank (Pedersen and Selig, 1989). The cloud was visible about 450 m downwind and was about 300 m wide. Most of the fatalities were within 200 m of the release point. Model-predicted concentration contours and calculations for this spill are shown in Table 32.9. Lessons Learned. After an inquiry and follow-up, it was determined that the accident happened as a result of equipment failure. It was apparent that proper guidelines and codes of conduct were not followed. For example, pressure vessels containing ammonia should have been subjected to periodic examination for strains and cracks. The following are the causes of the spill: 1. The vessel was not stress-relieved after manufacture. 2. The vessel material had been weakened by strain and aging. 3. The weld repairs as well as hydraulic pressure testing had induced more stresses on the vessel wall and were not followed by stress relief. 4. Failure of the tank could also have been caused by temperature fluctuations during decanting operations. The following recommendations were made. 1. Employees must have emergency response training and be taught to recognize emergency alarms and obey them. 2. It may be a lot safer to stay in a closed room if the route to a safe area is blocked or falls within a hot spot. Strategically located rooms should be set aside and equipped and clearly identified as gas-proof rooms. 3. Air supply and self-contained-breathing apparatus (SCBA) must be made available to every employee. 4. Driving through a dense ammonia cloud is very risky and should not be attempted. 5. Emergency plans must be in place for worst-case scenarios, not just for leaks. Communications between different departments and the outside world must be made possible during catastrophic accidents such as this, and plant control rooms must be habitable.
TABLE 32.9 Predicted Concentration Contours of the Potchefstroom
Accident Isopleth (ppm)
Approximate length (m)
Approximate width (m)
1,200 2,500 5,000 10,000
2,670 1,400 910 730
680 650 640 660
Source: Pedersen and Selig, 1989
32.24
CHAPTER THIRTY-TWO
32.9.3
Lithuanian Ammonia Accident, March 20, 1989
This accident started when a 10,000-tons unpressurized liquid ammonia tank failed. (Andersson, 1991). The ammonia gas released caught fire and ignited a fertilizer store containing 15,000 tons of fertilizer, NPK. The ammonia vapor and fertilizer decomposition products formed a 400-km2 cloud. After 12 hours, all of the ammonia had evaporated, but the fertilizer continued to burn (decompose) for three days, producing large quantities of nitrogen oxide fumes. There were 7 fatalities, 57 injured, and 32,000 people evacuated. The following are the causes of the failure according to the official Commission of the Commonwealth of Independent States (CIS):
• The tank experienced an overpressure due to delivery of 14 tons of warm ammonia (10⬚C) • • • • •
at the bottom of the tank because of an operating mistake in the plant. The warm ammonia accumulated at the bottom of the tank in the form of lenses or a layer of warm unstable ammonia. It did not evaporate at once because of the hydrostatic pressure. This layer or lenses tilted over the surface of the liquid. The warm liquid ammonia evaporated to lower its temperature to ⫺33⬚C, and this caused the overpressure. All refrigeration compressors were out of operation at the time and ammonia did catch fire.
The following recommendations were made: 1. Human error seems to have been the cause of the accident. Safe and standard operating procedures and practices should be established. In this case, the ammonia could have been added at the top of the tank and not at the bottom. 2. The fertilizer plant should be built at some distance from other facilities to avoid fire spread. 3. The tank should be surrounded by steel retainer walls. 4. Pressure tanks with higher relief valve specifications could have helped. 5. Frangible roof designs and continuous circulation of ammonia would reduce risks. 6. Emergency response planning must be in place.
32.10 32.10.1
ENVIRONMENTAL CONCENTRATIONS AND STANDARDS Ammonia Levels in Different Media
The highest levels of ammonia in the environment result from natural sources. The levels of ammonia in various media in Canada have been measured by Atmospheric Services, Environment Canada. These are shown in Table 32.10. Values obtained are generally very low for most environmental compartments except for spills, sewages, outfalls, and animal-rearing houses.
32.10.2
Threshold Limit Values
The American Conference Governmental Industrial Hygienists have determined the threshold limit values for TWA (time-weighted average) to be 25 ppm (17 mg / m3) and STEL (shortterm exposure limit) to be 35 ppm (24 mg / m3).
PERSPECTIVES ON SPECIFIC SUBSTANCES: AMMONIA
32.25
TABLE 32.10 Environmental Levels in Different Media
in Canada Compartment
Levels (mean in ppm)
Surface water Drinking water Precipitation Aerosols Food Soil
0.76 0.1 0.59 1.8 ⫻ 10⫺3 Varies extensively Varies extensively 478 g / g (0–18 cm) 413 g / g (75–112 cm)
When the threshold limit levels are compared with the levels found in the Canadian environment, one can only conclude that the Canadian public is not significantly exposed to dangerous levels of ammonia and is therefore not at great risk. High exposures are often occupational or purely accidental.
32.11
CONCLUSIONS Ammonia is one of the most beneficial industrial chemicals to mankind in its use as a fertilizer, or precursor, reactant, and intermediate in many syntheses. The manufacture of ammonia is a relatively clean process except for greenhouse gases and ammonia emissions. The driving force for most of its reactions is the lone pair of electrons on the nitrogen atom. The toxic component in water is the un-ionized ammonia. A person caught in a large spill should stay indoors with doors and windows closed and ventilation shut down. Only in circumstances where there is little alternative or where success is sure should one run away from the cloud. In any case, respiratory breathing apparatus is recommended. The most serious injuries often occur within a couple of hundred meters from the spill source. Most risks from ammonia exposure are accidental or occupational. Epidemiological studies with respect to ammonia exposures are still inadequate. Since the aquatic ecosystems are the most sensitive to high ammonia concentrations, long-term monitoring of ammonia is needed in underground sewage systems and outfalls. Effects of other chemical species in the presence of ammonia should also be studied.
32.12
REFERENCES American Industrial Hygiene Association (AIHA). 1988. Emergency Response Planning Guidelines: Ammonia, AIHA, Akron, OH. Andersson, B. O. 1991. ‘‘Lithuanian Ammonia Accident, March 20, 1989,’’ Icheme Symposium Series, no. 124, pp. 15–17. Braker, W. and A. L. Mossman. 1980. Matheson Gas Data Book, 6th ed., Matheson Gas Products, East Rutherford, NJ, pp. 23–33. Camford Information Services, Inc. (CIS). 1998. ‘‘Ammonia,’’ in CPI Product Profiles, CIS, Scarborough, ON. Committee on Medical and Biological Effects of Environmental Pollutants. 1970. ‘‘Chemical and Biological Effects of Pollutants,’’ in Ammonia, University Park Press, Baltimore, MD, pp. 115–176. Compressed Gas Association, Inc. 1990. Handbook of Compressed Gases, 3rd ed., Van Nostrand Reinhold, New York, NY, pp. 231–232.
32.26
CHAPTER THIRTY-TWO
Czuppon, T. A., S. A. Knez, J. M. Rovner, and M. W. Kellog Company. 1992. ‘‘Ammonia,’’ in Encyclopedia of Chemical Technology, ed. M. Howe-Grant. 4th ed., John Wiley & Sons, New York, pp. 638–688. Environment Canada. 1999. National Analysis of Trends in Emergency Situations (NATES): Chemical Accidents Reports Database, Hull, QC. Fingas, M., N. Laroche, G. Sergy, B. Mansfield, G. Clouthier, and P. Mazerolle. 1991. ‘‘A New Chemical Spill Priority List,’’ in Proceedings of the 8th Technical Seminar on Chemical Spills, Environment Canada, Ottawa, ON, pp. 223–237. Fenske, B. 1993. Manitoba Ambient Air Quality Annual Report—1989, Air Quality Report No. 93-02. Green Markets. 1990. McGraw-Hill, New York, NY. International Fertilizer Development Center (IFDC). 1989. Global Fertilizer Perspective, 1960–1995, IFDC, Muscle Shoals, AL. Jolly, W. L. 1964. The Inorganic Chemistry of Nitrogen, W. A. Benjamin, New York, NY, pp. 1–42. Kaiser, G. D., and B. C. Walker. 1978. ‘‘Releases of Anhydrous Ammonia from Pressurized Containers— The Importance of Denser than Air Mixture,’’ Atmospheric Environ., vol. 12, pp. 2289–2300. Lancaster, J. M. 1989. ‘‘ New Developments and Growth in World Fertilizer, Demand, Supply and Trade,’’ in Fertilizer Latin American International Conference, British Sulphur Corporation Ltd., Caracas, Venezuela. Lusis, M. A. and M. L. Phillips, eds. 1990. The 1990 Canadian Long-Range Transport of Air Pollutants and Acid Deposition Assessment Report, Part 3: Atmospheric Sciences, Federal / Provincial Research and Monitoring Coordinating Committee (RMCC), Environment Canada, Downsview, ON. Markham, R. S. 1987. ‘‘Review of Damages from Ammonia Spills,’’ Ammonia Plant Safety, vol. 27, pp. 137–149. Ministers’ Expert Advisory Panel on the PSL2. 1995. Report of the Ministers’ Expert Advisory Panel on the Second Priority Substances List, Environment Canada, Hull, QC. National Water Quality Data Bank (NAQUADAT). 1991. Water Quality Branch, Inland Waters Directorate, Environment Canada, Ottawa, ON. Pedersen, F. and R. S. Selig. 1989. ‘‘Predicting the Consequences of Short-term Exposure to High Concentrations of Gaseous Ammonia,’’ Journal of Hazardous Materials, vol. 21, pp. 143–159. RTECS On-Line. 2000. Registry of Toxic Effects of Chemical Substances, Department of Health and Human Services, Centers for Disease Control, National Institute for Occupational Safety and Health, Washington, DC. Saskatchewan Environment Data (WQB). 1991. Esquadat Summary Report, pp. 1–16. Sheldrick, W. L. 1987. World Nitrogen Survey, Technical Paper No. 59, World Bank, Washington, DC. Transport Canada. 1985. Transport of Dangerous Goods Act and Regulations, Ottawa, ON. Williamson, D. A. 1990. The Development and Use of Water Quality Objectives in Manitoba, Water Standards Section Report 90-2.
