Cairo University Faculty of Engineering Chemical Engineering Department
Urea Ammonium Nitrate Project 1st Technical Report
Submitted to; Prof.\ Mohamed Ali Submitted by; Group 2
rd
3 Nov, 2011
UAN Project Urea Ammonium Nitrate
Abstract
This report is represented from group 2 working on Urea Ammonium Nitrate production project, The report give a general overview on the different aspects of the project, starting from the history of product and how it was discovered going through its physical and chemical properties that is really important to know the appropriate way of handling and using of the both the raw materials and the product. The report also discuss the main theory of producing Urea which is the main product we focus on through our report and also the different processes and technologies used in its production, where so many processes were developed with different technologies (different conditions of reaction) which are discussed in a general overview in this report, the processes are Stamicarbon process, Snamprogitti process and ASEC process. Different aspects like economical and environmental are also covered.
II
UAN Project Urea Ammonium Nitrate
Abstract
This report is represented from group 2 working on Urea Ammonium Nitrate production project, The report give a general overview on the different aspects of the project, starting from the history of product and how it was discovered going through its physical and chemical properties that is really important to know the appropriate way of handling and using of the both the raw materials and the product. The report also discuss the main theory of producing Urea which is the main product we focus on through our report and also the different processes and technologies used in its production, where so many processes were developed with different technologies (different conditions of reaction) which are discussed in a general overview in this report, the processes are Stamicarbon process, Snamprogitti process and ASEC process. Different aspects like economical and environmental are also covered.
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Contents Abstract
II
Table of contents
III
Table of Figures
V
Introduction
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1. Fertilizers 1.1 Forms 1.2 Main elements used in in-organic (manmade) fertilizers industry 1.3 Problems with in-organic fertilizers 1.4 Urea 2. History
3. 4. 5.
6.
2.1 production of Ammonia 2.2 Production of Urea 2.3 Production of Ammonium nitrate Uses of Urea Physical Properties Propert ies Production 5.1 General theory 5.2 Block flow diagram of Urea production 5.3 Principle 5.4 Challenges in Urea production process 5.4.1 Recycle of unconverted Ammonia and Carbon dioxide 5.4.2 Corrosion 5.4.3 Material selection select ion 5.4.4 Side reactions 5.5 Description of urea production process 5.5.1 conventional Processes 5.5.2 Stripping processes 5.6 Urea Ammonium nitrate (UAN) production 5.6.1 Overview of UAN process technology 5.6.2 Typical UAN solution analysis 5.6.3 Description of production product ion process 5.6.4 Urea Ammonium nitrate 5.6.5 Pool condenser process Environmental aspects 6.1 Environmental issues: Spills along pathway
III
6 6 6 6 6 7 7 7 8 9 11 13 13 14 15 19 19 21 22 22 23 23 24 30 30 30 30 31 31 33 33
6.2 Emissions, Effluents & solid wastes 6.2.1 Air Emissions 6.2.2 Effluents 6.2.3 Solid wastes 7. Economic aspects 7.1 Economic Analysis 7.2 Commercial Analysis 8. Quality Specifications Specificat ions 8.1 Urea quality specifications specificat ions 8.1.1 Prilled urea 8.1.2 Granulated urea 8.2 Ammonium nitrate specifications 9. References
IV
33 33 36 37 39 39 39 40 40 40 40 41 42
Table of Figures
Page
Fig 0
Synthesis gas
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Fig 1
Block flow diagram of Urea Production
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Fig 2
Physical and Chemical Equilibrium in urea production
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Fig 3
Carbon dioxide conversion with ratio …..
16
Fig 4
Ammonia conversion with ratio …
16
Fig 5
Carbon dioxide conversion with ratio …
16
Fig 6
Ammonia conversion with ratio …
16
Fig 7
Carbon dioxide conversion ...
17
Fig 8
Ammonia conversion with ratio ….
17
Fig 9 Fig 10
Urea yield in the liquid phase at chemical equilibrium as a function of Temperature Urea yield in the liquid phase at chemical equilibrium as function of H2O:CO2 ratio
17 18
Fig 11
Urea yield in the liquid phase at chemical equilibrium of NH3:CO2 ratio
18
Fig 12
Typical flow sheet of conventional urea plant
20
Fig 13
Conceptual diagram of the heat balance of conventional urea process heat balance
21
Fig 14
Conceptual diagram of heat balance of stripping plant
21
Fig 15
UTI heat recycle urea process
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Fig 16
Schematic of Stamicarbon process
25
Fig 17
Schematic of Snamprogetti self stripping process
27
Fig 18
Schematic of ACES process
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Fig 19
Block flow diagram of UAN process
31
Fig 20
Pool condenser process (UAN process)
32
Fig 21
Mixing and cooling process of UAN production
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Tables Table 1 Table 2 Table 3 Table 4
Specific heat vs. Temperature Properties of saturated aqueous solution of urea Solubility of Urea in various solvents (solubility in wt% of urea) Ammonium nitrate specifications
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11 11 12 41
Introduction 1. Fertilizer
Fertilizer (or fertiliser) is any organic or inorganic material of natural or synthetic origin (other than liming materials) that is added to a soil to supply one or more plant nutrients essential to the growth of plants .A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use. Mined inorganic fertilizers have been used for many centuries, whereas chemically synthesized inorganic fertilizers were only widely developed during the industrial revolution. 1.1 Forms Fertilizers come in various forms. The most typical form is granular fertilizer (powder form). The next most common form is liquid fertilizer; some advantages of liquid fertilizer are its immediate effect and wide coverage. There are also slow-release fertilizers (various forms including fertilizer spikes, tabs, etc.) which reduce the problem of "burning" the plants due to excess nitrogen. More recently, organic fertilizer is on the rise as people are resorting to environmental friendly (or 'green') products. Although organic fertilizer usually contain less nutrients, some people still prefer organic due to natural ingredients 1.2 Main Elements used in in-organic (manmade) Fertilizers industries;
Phosphorus
Nitrogen
Calcium
Potassium
Magnesium
Sulphur
1.3 Problems with inorganic fertilizer
1) 2) 3) 4)
Trace mineral depletion Over fertilization High energy consumption Long-Term Sustainability
1.4 Urea;
The highest fixed nitrogen-containing fertilizer 46.7 wt %, urea is a white solid that is soluble in water and alcohol. It is usually sold in the form of crystals, prills, flakes, or granules. Urea is an active compound that reacts with many reagents. It forms adducts and clathrates with many substances such as phenol and salicylic acid. Urea is gaining greater acceptance as a slow rate fertilizer.
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2. History 2.1 Production of ammonia: Ammonia was made for the first time in 1774 and by 1785 its exact composition was determined. Nitric acid, known earlier, was synthesized for the first time and by 1771 it was being produced commercially. Urea was identified in 1773 and in 1775 the presence of large amounts of calcium phosphate in bones was confirmed. In 1784, the chemical nature of water was discovered.
The Haber process is a method of producing ammonia developed in WWI. The Germans needed nitrogen to for making their explosives. When the Allies blocked off all trade routes going to and from Germany, they lost all source of sodium nitrate and potassium nitrate, their source of nitrogen. They found their source of nitrogen in the air, which was 80% nitrogen. The chemist Fritz Haber developed the Haber Process in WWI, which takes molecular nitrogen from the air and combines it with molecular hydrogen to form ammonia gas, which the chemical formula is NH3.The equation for the reversible reaction; N2(g) + 3H2(g) 2NH3(g) + 92 kJ.
2.2 Production of Urea: Urea was first discovered in human urine by H.M. Rouelle in 1773. It was synthesized in 1828 by Friedrich Wohler and was the first organic compound to be synthesized from inorganic starting materials. It was found when Wohler attempted to synthesis ammonium cyanate, to continue a study of cyanates which he had been carrying out for several years. On treating silver cyanate with ammonium chloride solution he obtains organic synthesis dealt a severe blow to a widespread belief called “vitalism” which maintained that organic chemicals could be modified by chemistry but could only be produced through the agency of a vital force present in living plants and animals. In 1870 urea was produced by heating ammonium carbamate in a sealed vessel. This provided the basis of the current industrial process for its production. The present commercial method of synthesizing urea through the reaction of ammonia and carbon dioxide was first introduced in Germany in 1920. In the U.S., commercial synthesis of urea was first achieved in 1932. Use of urea, including calcium nitrate-urea as a fertilizer in this country, started in the early 1920s with imported material mainly from Germany. Production of urea in the U.S. for fertilizer purposes increased dramatically from 47,000 tons in 1948 to 549,000 tons
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in 1960. Expansion in urea capacity slowed between 1960 and 1975, eventually reaching 3 million tons at the end of this period. By 1980, production of fertilizer urea was 7.2 million tons, 60 percent of it in solid form. In 1960, 75 percent of the total urea cont’ primary urea solutions produced at anhydrous ammonia plants was utilized as fertilizer and by 1982 this usage had increased to 92 percent. Urea consumption in 1994 accounted for 14.7 percent of the 12.6 million tons of nitrogen applied to U.S. croplands.
2.3 Production of Ammonium Nitrate: This nitrogen compound was first employed as a fertilizer in Europe following World War I when large stocks not utilized for explosives were released for agricultural use. Because of explosions in Germany in 1920 and 1921, it was usually dry-mixed with limestone, gypsum, chalk or ammonium sulfate to avoid such danger and to also serve as conditioning agents. The first use of ammonium nitrate in the United States occurred in 1926 with product imported from Germany. It was not until 1943 that significant amounts of this nitrogen source were available to American farmers when supplies exceeded the needs for munitions. Additional surplus quantities of this material were released for fertilizer use in 1944 and 1945 In the 1970’s ammonium nitrate was produced at 29 locations separate from ammonia plant sites. Production of solid and solution ammonium nitrate fertilizer increased continuously from 383,000 tons in 1943 to 7.3 million tons in 1980. Manufacture of solid product was greater than the solution form until 1978, after which it accounted for less than half of the total produced. Ammonium nitrate accounted for about one-third of all the nitrogen applied directly to U.S. cropland in 1955. By 1980, this use had declined to 10 percent. The tonnage of directly applied nitrogen as ammonium nitrate was exceeded by anhydrous ammonia in 1960, by nitrogen solutions in 1970 and by urea in 1978. Ammonium nitrate supplied 5.3 percent of the total 12.6 million tons of nitrogen consumed nationally in 1994.
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3. Uses of Urea 1. The major use of urea is the fertilizer field, which accounts for approximately 80% of its production (about 16.2 billion pounds were produced during 1994 in U.S.). About 10% of urea is used for the production of adhesives and plastics (Urea formaldehyde). 2. Urea possesses a unique property of forming adducts with n-paraffins. This is used in separating C12-C14 n-paraffins from kerosene for detergent production 3. The nucleophilic addition reaction of urea to formaldehyde produces mainly monomethylol urea and some dimethylol urea. When the mixture is heated in presence of an acid, condensation occurs, and water is released, it produces an important commercial polymer (urea formaldehyde resins) that is used as glue for particle board and plywood. 4. Explosive; Urea can be used to make urea nitrate, a high explosive which is used industrially and as part of some improvised explosive devices 5. Automobile systems; Urea is used in SNCR and SCR reactions to reduce the NO x pollutants in exhaust gases from combustion from diesel, dual fuel, and lean-burn natural gas engines. The BlueTec system, for example, injects water-based urea solution into the exhaust system. The ammonia produced by the hydrolysis of the urea reacts with the nitrogen oxide emissions and is converted into nitrogen and water. 6. Also used as a stabilizer in nitrocellulose explosives 7. A component of animal feed, providing a relatively cheap source of nitrogen to promote growth. 8. A non-corroding alternative to rock salt for road de-icing, and the resurfacing of snowboarding half pipes and terrain parks 9. A flavor-enhancing additive for cigarettes 10. A main ingredient in hair removers such as Nair and Vet. 11. A browning agent in factory-produced pretzels 12. An ingredient in some skin creams, moisturizers, hair conditioners 13. A reactant in some ready-to-use cold compresses for first-aid use, due to the endothermic reaction it creates when mixed with water. 14. A cloud seeding agent, along with other salts. 15. A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as the urea-potassium bicarbonate mixture. 16. An ingredient in many tooth whitening products. 17. Along with ammonium phosphate, as a yeast nutrient, for fermentation of sugars into ethanol. 18. A nutrient used by plankton in ocean nourishment experiments for geo-engineering purposes. 19. As an additive to extend the working temperature and open time of hide glue 20. As a solubility-enhancing and moisture-retaining additive to dye baths for textile dyeing or printing 21. Urea can be used to make urea nitrate, a high explosive which is used industrially and as part of some improvised explosive devices.
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22. Why is Urea mentioned in the Petrochemical Industry?! The Answer is SYNTHESIS GAS;
Fig 0 Synthesis Gas
Synthesis gas is a major source of hydrogen, which is used for producing ammonia.
Ammonia is the host of many chemicals such as urea, ammonium nitrate and hydrazine. Carbon dioxide, a by-product from synthesis gas reacts with ammonia to produce urea.
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4. Physical Properties of Urea Pure urea forms white, odorless, long, thin needles, but it can also appear in the form of rhomboid prisms. The crystal lattice is tetragonal; the axis ratio a: c=1: 0.833. The urea crystal is anisotropic (non-cubic) and thus shows birefringence. At20 ◦C the refractive indices are 1.484 and1.602. Urea has an mp of 132.6 ◦C; its heat of fusion is 13.61kJ/mol. Urea is contains 46.65 percent nitrogen. It is not flammable, but it will melt and decompose in a fire to give off ammonia. When it is dissolved in water, it hydrolyzes very slowly to ammonium carbamate and eventually decomposes to ammonia and carbon dioxide Chemical formula NH2CONH2 Molecular weight 60.06 O Freezing/melting point 132.6 C decomposes Boiling point 2O 3 Density, d4 1.3230 g/cm Heat of solution in water +251J/g (60 cal/g) 3 Bulk density 0.74g/cm Specific heat at different temp(J/(kg.K) Table 1; Specific heat vs. Temp 0°C 50°C 100°C 150°C
1.439 1.661 1.887 2.109
Table 2; Properties of Saturated Aqueous Solutions of Urea Temperature ©
0 20 40 60 80 100 120
Solubility in Water (g/100 g Solution 41.0 51.6 62.2 72.2 80.6 88.3 95.5 130 99.2
Density (g/cm3)
Viscosity (mPa-s = cP)
1.120 1.147 1.167 1.184 1.198 1.210 1.22 1
2.63 1.96 1.72 1.72 1.93 2.35 2.93 1.226 ◦
Water Vapor Pressure (kPa) 0.53 1.73 5.33 12.00 2 1.33 29.33 18.00 3.25 0.93
Some other Physical properties of the melt at 135 C follow: 3 Molecular volume =48.16m /kmol 2 Kinematic viscosity =2.42×10−6 m /s 1 1 Molar heat capacity(Cp) =135.2 Jmol K −
11
−
Specific heat capacity(cp) =2.25 kJ kg Surface tension = 66.3×10−3 N/m
1
−
1
−
K
Table 3; Solubility of urea in various solvents (solubility in wt% of urea)
Solvent Ammonia Methanol Ethanol
0 34.9 13.0 2.5
Temperature 40 60 67.2 78.7 26.1 38.6 8.5 13.1
20 48.6 18.0 5.1
Physical and Chemical properties of Urea Ammonium Nitrate Physical Form: Liquid Color: Colorless Odor: Slight ammonia odor (pungent) Boiling Point: Approximately 225o F (107 o C) Melting/Freezing Point: 28% UAN salts out at 0o F (-18o C) 30% UAN salts out at 16o F (-9o C) 32% UAN salts out at 32o F (0o C) PH: 6.8 – 7.5 Solubility: 100% Specific Gravity: 28% 1.281 30% 1.304 32% 1.330 Vapor Density: Approximately 1.07 (@ 60o F) Vapor Pressure: 8.6 – 17.5 mmHg (@ 100o F) % Volatile by Volume: No test results Molecular Weight: Not applicable Density: 28% 10.67 lb/gal at 60o F 30% 10.86 lb/gal at 60o F 32% 11.07 lb/gal at 60o F Critical Temperature: No test results Critical pressure: No test results
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80 84.5
100 90.4
5. Production 5.1 General Theory; Step 1 – Synthesis A mixture of compressed CO2 and ammonia is reacted to form ammonium carbamate. This is an exothermic reaction, and heat is recovered by a boiler which produces steam. The first reactor achieves 78% conversion of the carbon dioxide to urea and the liquid is then purified. The second reactor receives the gas from the first reactor and recycles solution. Step 2 – Purification The major impurities in the mixture at this stage are water from the urea production reaction and unconsumed reactants (ammonia, carbon dioxide and ammonium carbamate). The unconsumed reactants are removed in three stages3. Firstly, the pressure is reduced and then the solution is heated, which causes the ammonium carbamate to decompose to ammonia and carbon dioxide: NH2COONH4 2NH3 + CO2 At the same time, some of the ammonia and carbon dioxide flash off. The pressure is then reduced on two other stages, with more ammonia and carbon dioxide being lost at each stage. By the time the mixture is at -0.35 bar gauge a solution of urea dissolved in water and free of other impurities remains. At each stage the unconsumed reactants are absorbed into a water solution which is recycled to the secondary reactor. The excess ammonia is purified and used as feedstock to the primary reactor. Step 3 – Concentration 75% of the urea solution is heated under vacuum, which evaporates off some of the water, increasing the urea concentration from 68% w/w to 80% w/w. At this stage some urea crystals also form. The solution is then heated again to dissolve these crystals prior o to evaporation. In the evaporation stage molten urea (99% w/w) is produced at 140 C. Step 4 – Granulation Urea is sold for fertilizer as 2 – 4 mm diameter granules. These granules are formed by spraying molten urea onto seed granules which are supported on a bed of air. This occurs in a granulator which receives the seed granules at one end and discharges enlarged granules at the other as molten urea is sprayed through nozzles. Dry, cool granules are classified using screens. Oversized granules are crushed and combined with undersized ones for use as seed. All dust and air from the granulator is removed by a fan into a dust scrubber, which removes the urea with a water solution then discharges the air to the atmosphere. The final product is cooled in air, weighed and conveyed to bulk storage ready for sale.
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5.2 Block flow diagram of Urea p roduction process;
Fig 1 Block flow diagram of Urea production process
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5.3 Principles Chemical Equilibrium (1); In all commercial processes, urea is produced by reacting ammonia and carbon dioxide at elevated temperature and pressure according to the Basaroff reactions:
2NH3 (l) +CO2 (l)
NH2COONH4
(1) Δ H =−117 kJ/mol
NH2COONH4
NH2CONH2 +H2O (2) Δ H =+ 15.5 kJ/mol
In the first reaction, carbon dioxide and ammonia are converted to ammonium carbamate; the reaction is fast and exothermic. In the second reaction which is slow and endothermic, ammonium carbamate dehydrates to produce urea and water. Since more heat is produced in the first reaction than consumed in the second the overall reaction is exothermic.
-A schematic of the overall process and the physical and chemical equilibrium involved is shown followed;
Fig 2 Physical and chemical equilibrium in urea production
Processes differ mainly in the conditions (composition, temperature, and pressure) at which these reactions are carried out. Traditionally, the composition of the liquid phase in the reaction zone is expressed by two molar ratios: usually, the molar NH 3: CO 2 and the molar H2O: CO2 ratios. Both reflect the composition of the so-called initial mixture [i.e., the hypothetical mixture consisting only of NH 3, CO2, andH2Oif both Reactions (1) and (2) are shifted completely to the left]. First attempts to describe the chemical equilibrium of Reactions (1) and (2) were made by Frejacques. Later descriptions of the chemical equilibrium can be divided into regression analyses of measurements and thermodynamically consistent analyses of the equilibrium .As far as the most important consequences of these equilibrium on urea process design are concerned, the methods correspond closely to each other: The achievable conversion per pass, dictated by the chemical equilibrium as a function of temperature, goes through 15
a maximum (Figs 3 and 4). This effect is usually attributed to the fact that the ammonium carbamate concentration as a function of temperature goes through a maximum. This maximum in the ammonium carbamate concentration can be explained, at least qualitatively, by the respective heat effects of Reactions (1) and (2).However, this mechanism cannot explain the observed conversion maximum fully and quantitatively; other contributing mechanisms have been suggested.
Fig 3 Carbon dioxide conversion
Figure 4 Ammonia conversion
Figure 3&4 Ammonia and carbon dioxide conversion at chemical equilibrium as a function of temperature NH 3:CO2 ratio = 3.5 mol/mol (initial mixture); H 2O:CO2 ratio = 0.25 mol/mol (initial mixture) The influence of the composition of the initial mixture on the chemical equilibrium can be explained qualitatively by Reactions (1) and (2) and the law of mass action: 1- Increasing the NH 3:CO2 ratio (increasing the NH3 concentration) increases CO 2 conversion, but reducesNH 3 conversion (Figs. 5 and 6).
Fig 5 Carbon dioxide conversion
Fig 6 Ammonia conversion
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Figures 6 & 5 carbon dioxide and Ammonia conversion at chemical equilibrium respectively at chemical equilibrium as a function of NH 3:CO2 ratio T=190 ◦C; H2O: CO2 ratio = 0.25 mol/mol (initial mixture) 2) Increasing the amount of water in the initial mixture (increasing the H 2O: CO2 ratio) results in a decrease in both CO 2 and NH3 conversion (Figs. 6and 7).
Fig 7 Carbon dioxide conversion
Fig 8 Ammonia conversion
Figures 6 & 7 Ammonia conversion at chemical equilibrium as a function of H 2O: CO2 ratio T = 190 ◦C; NH3: CO2 ratio = 3.5 mol/mol (initial mixture). In these cases full quantitative description cannot be derived simply from the law of mass action and Reactions (1) and (2). Other, not yet fully understood reaction mechanisms probably contribute to the chemical equilibrium to a minor extent. In Figures 3, 5, and 7, the conversion at chemical equilibrium is expressed as CO 2 conversion, that is, the amount of CO 2 in the initial mixture converted into urea (plus biuret), if no changes occur in overall NH 3, CO2, and H2O concentrations in the liquid phase. This way of representing the chemical equilibrium is consistent with the presentation usually found in the traditional urea literature. However, it is based on the arbitrary choice of CO 2 as the key component. Historically, this may be justified by the fact that in early urea processes, CO 2 conversion was more important than NH 3 conversion. For the present generation of stripping processes, however, giving a higher weight to CO2 conversion is not justified. Comparing, e.g., Figs. 4 and 5, shows that an arbitrary choice of one of the two feedstock components as yardstick to evaluate optimum reaction conversion can easily lead to faulty conclusions
Fig 9 Urea yield in the liquid phase at chemical equilibrium as a function of temperature NH3: CO2 ratio = 3.5 mol/mol (initial mixture); H2O: CO2 ratio = 0.25 mol/mol (initial mixture)
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Fig 10 Urea yield in the liquid phase at chemical equilibrium as a function of H 2O:CO2 ratio T=190 ◦C NH3:CO2 ratio = 3.5 mol/mol (initial mixture)
Ultimately, project economics (investment and consumptions) will dictate the choice of process parameters in the reaction section. Without going into such time- and place-dependent economic considerations, one can argue that the urea yield (i.e., the concentration of urea in the liquid phase) is a better tool for judging optimum process parameters than CO 2 or NH3 conversion. Figure 8 illustrates that urea yield as a function of temperature also goes through a maximum; the location of this maximum is of course composition dependent. Figure 9 again shows the detrimental effect of excess water on urea yield; thus, one of the targets in designing a recycle system must be to minimize water recycle. Figure 10 shows that the urea yields as a function of NH3:CO2 ratio reaches a maximum somewhat above the stoichiometric ratio (2:1). This is one of the reasons that all commercial processes operate at NH 3:CO2 ratios above the stoichiometric ratio. Another important reason for this can be found from the physical phase equilibrium in the NH3 – CO2 – H2O – urea system
Fig 11 Urea yield in the liquid phase at chemical equilibrium as a function of NH3:CO2 ratio T=190 ◦C; H2O: CO2 ratio = 0.25 mol/mol (initial mixture)
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5.4 Challenges in Urea Production Process Design; Since the urea process already has half a century of commercial scale history, it will be clear that compromises between the aforementioned partly conflicting criteria are well established. Also resulting from the age of urea process design is the observation that a process can only be successful if acceptable and competing solutions to all of these criteria can be combined into one process design. Apart from applying straightforward normal engineering approaches, the challenge of finding an optimum synergy between partly conflicting criteria, focuses in urea plant design essentially on a few peculiarities:
1) The thermodynamic limit on the conversion per pass through the urea reactor, combined with the azeotropic behavior of the NH 3 – CO2 system, necessitates a cunning recycle system design. 2) The intermediate product ammonium carbamate is extremely corrosive. A proper combination of process conditions, construction materials, and equipment design is therefore essential. 3) The occurrence of two side reactions – hydrolysis of urea and biuret formation – must be considered.
5.4.1 Recycle of unconverted Ammonia and Carbon Dioxide; -From a chemical equilibrium date it was found that indicates that the conversion of the feed stocks NH3 and CO2 to urea is limited. An important differentiator between processes is the way these unconverted materials are handled i.
Once through Process; In this process non converted ammonia was neutralized with acid such as nitric acid to produce ammonium salt such as ammonium nitrate as co products of urea production. In this way, a relatively simple urea process scheme was realized. The main disadvantages of this process are the large quantity of ammonia salt formed as co product and the limited amount of overall carbon dioxide conversion that can be achieved. But there is a partial “revival” of this combined urea – ammonium nitrate production facilities
ii.
Conventional Recycle Processes; Once through processes were soon replaced by total recycle processes, where essentially all of the unconverted ammonia and carbon dioxide were recycled to the urea reactor. In the first generation of total-recycle processes, several licensors developed schemes in which the recirculation of unconverted NH3 and CO2 was performed in two stages. Figure 11 is a typical flow sheet of these, now called conventional, processes. The first recirculation stage was operated at medium pressure (18 – 25 bars); the second, at low pressure (2 – 5 bar). The first recirculation stage comprises at least a decomposition heater (d), in which carbamate decomposes into gaseous NH 3 and CO 2, while excess NH 3 evaporates 19
simultaneously. The off-gas from this first decomposition step was subjected to rectification, from which relatively pure NH 3 (at the top) and a bottom product consisting of an aqueous ammonium carbamate solution were obtained. Both products are recycled separately to the urea reactor. In these processes, all unconverted CO 2 was recycled as an aqueous solution, whereas the main portion of unconverted NH3 was recycled without an associated water recycle. Because of the detrimental effect of water on reaction conversion, achieving a minimum CO2 recycle (and thus maximum CO 2 conversion per reaction pass) was much more important than achieving a low NH3 recycle. All conventional processes therefore typically operate at high NH 3:CO2 ratios (4 – 5 mol/mol) to maximize CO2 conversion per pass. Although some of these conventional processes, partly equipped with ingenious heat-exchanging networks, have survived until now (see Section 4.3.1), their importance decreased rapidly as the so-called stripping processes were developed.
Figure 12 Typical flow sheet of a conventional urea plant a)CO2 compressor; b) High-pressure ammonia pump; c) Urea reactor; d) Medium-pressure decomposer; e) Ammonia – carbamate separation column; f) Low-pressure decomposer; g) Evaporator; h) Prilling; i) Desorber (wastewater stripper); j) Vacuum condensation section
iii.
Stripping Processes.
In the 1960s, the Stamicarbon CO 2-stripping process was developed, followed later by other processes (in Section 3.3.2). Characteristic of these processes is that the major part of the recycle of both unconverted NH 3 and unconverted CO 2 occurs via the gas phase; such that none of these recycles is associated with large water recycles to the synthesis zone. Another characteristic difference between conventional and stripping processes in terms of the recycle scheme can be found in the way of which heat is supplied to the recirculation zones. The energy balance of the conventional processes is shown in Figure 12. In this first-generation urea process, the heat supplied to the urea synthesis solution was used only once; therefore, this type of process can be referred to as an N = 1 process. Such a process required about 1.8 t of steam per ton of urea. 20
Fig 13 Conceptual diagram of the heat balance of a conventional urea process Heat to each subsequent heater is supplied in the form of steam; the heat is used only once ( N = 1).
The energy balance of a stripping plant is shown in Figure 13. As in conventional plants, heat must be supplied to the urea synthesis solution to decompose unconverted carbamate and to evaporate excess ammonia and water. However, a distinct difference in the heat balance with respect to the conventional process is that only the heat in the first heater (the high-pressure stripper) is imported. This heat is recovered in a high pressure carbamate condenser (unconverted ammonia and carbon dioxide are condensed to form ammonium carbamate) and reused in the low pressure heaters. The heat supplied is effectively used twice; thus, the term N =2 process is used. The average energy consumption of the stripping process is 0.8 – 1.0 t of steam per ton of urea.
Fig 14 Conceptual diagram of the heat balance of a stripping plant
5.4.2. Corrosion Urea synthesis solutions are very corrosive. Generally, ammonium carbamate is considered the aggressive component. This follows from the observation that carbamatecontaining product streams are corrosive whereas pure urea solutions are not. The corrosiveness of the synthesis solution has forced urea manufacturers to set very strict demands on the quality and composition of construction materials. Awareness of the important factors in material selection, equipment manufacture and inspection, technological design and proper operations of the plant, together with periodic inspections and nondestructive testing are the key factors for safe operation for many years Corrosion protection is achieved by the use of well proven design principles, stringent material and fabrication specifications, complemented by detailed codes of 21
practice for operating, monitoring and inspecting equipment. The corrosiveness at a given point in the plant is determined by the temperature, the process components, the concentration of dissolved oxygen and the presence of contaminants that may accelerate corrosion. The formation on start-up and maintenance of a passive oxide layer on stainless steel surfaces is of the utmost importance. Stainless steel lined carbon steel vessels are usually used in the HP synthesis section for economic reasons, including leak detection units to protect the integrity of the vessels and avoid a potentially hazardous situation 5.4.3. Material Selection Corrosion resistance is not the only factor determining the choice of construction materials. Other factors such as mechanical properties, workability, and weld ability, as well as economic considerations such as price, availability, and delivery time, also deserve attention. Stainless steels that have found wide use are the austenitic grades AISI 316L and 317 L. Like all Cr-containing stainless steels, AISI 316L and 317 L are not resistant to the action of sulfides. Hence it is imperative in plants using the 316L and 317 L grades in combination with CO 2 derived from sulfur-containing gas, to purify this gas or the CO 2 thoroughly. In stripping processes, the process conditions in the high-pressure stripper are most severe with respect to corrosion. In the Stamicarbon CO 2-stripping process, a higher-alloyed, but still fully austenitic
stainless steel (25 Cr 22 Ni 2 Mo) was chosen as construction material for the stripper tubes. This choice ensures better corrosion resistance than 316L or 317L types of material but still maintains the advantages of workability, weld ability, reparability, and the cheaper price of stainless steel-type materials. In the Snamprogetti stripping processes, titanium usually is chosen for this critical application, although mechanically bonded bimetallic 25 Cr 22 Ni 2 Mo – zirconium tubes have also been suggested to improve corrosion resistance In the ACES process, duplex alloys (ferrite – austenitic) are used as construction material for the stripper tubes. 5.4.4. Side Reactions Three side reactions are of special importance in the design of urea production processes: 1. Hydrolysis of urea CO(NH2)2 +H2O→NH2COONH4→2NH3 +CO2 (3)
2. Biuret formation from urea: 2 CO(NH2)2→NH2CONHCONH2 +NH3
(4)
3. Formation of iso-cyanic acid from urea: CO(NH2)2→NH4NCO→NH3 +HNCO
(5)
All three side reactions have in common the decomposition of urea; thus, the extent to which they occur must be minimized. The hydrolysis reaction (3) is nothing but the reverse of urea formation. Whereas this reaction approaches equilibrium in the reactor, in all downstream sections of the plant the NH 3 and CO2 concentrations in urea-containing solutions are such
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that Reaction (3) is shifted to the right. The extent to which the reaction occurs is determined by temperature (high temperatures favor hydrolysis) and reaction kinetics; in practice, this means that retention times of urea-containing solutions at high temperatures must be minimized. The biuret reaction (4) also approaches equilibrium in the urea reactor the high NH3 concentration in the reactor shifts Reaction (4) to the left, such that only a small amount of biuret is formed in the reactor. In downstream section of the plant, NH3 is removed from the urea solutions, thereby creating a driving force for biuret formation. The extent to which biuret is formed is determined by reaction kinetics; therefore, the practical measures to minimize biuret formation are the same as described above for the hydrolysis reaction. The iso-cyanic acid reaction (5) is also favored by low NH 3 concentrations. This reaction is especially relevant in the evaporation section of the plant. Here, low pressures are applied, resulting in a transfer of NH 3 and HNCO into the gas phase and, consequently, low concentrations of these constituents in the liquid phase. Together with the relatively high temperatures in the evaporators, this shifts Reaction (5) to the right. The extent to which this reaction occurs is again determined by kinetics. The HNCO removed via the gas in the evaporators collects in the process condensate from the vacuum condensers, where low temperatures shift Reaction (5) to the left, again forming urea. As a result of this mechanism of chemical entrainment, attempts to minimize entrainment from evaporators with physical (liquid – gas) separation devices are destined to be unsuccessful.
5.5 Description of Processes 5.5.1 Conventional Processes As explained in Section 3.2, conventional processes have generally been replaced by stripping processes. Only two conventional processes may still have some importance in the near future; 1) Urea Technologies Inc. (UTI) Heat Recycle Process (Fig 15). Ammonia (containing passivating air), recycled carbamate, and about 60%of the feed CO2 are charged to the top of an open-ended coil reactor (c) operating at 210 bars. Ammonium carbamate is formed within the coil, exits the coil at the bottom, and then flows up and around it – the exothermic heat of carbamate formation in the coil driving the endothermic dehydration of carbamate to urea outside the coil. The reactor is claimed to achieve a uniform temperature profile in this way. In the reactor, a relative high NH 3:CO2 ratio (4.2:1) is applied. The reactor effluent is depressurized and sub cooled, and the flashed gases are released before the first decomposer (f ). Gases leaving the first decomposer separator (g) are mixed with about 40% of the feed carbon dioxide and partially condensed in the heat recovery section (i – k).The two combined gas streams are then further condensed and form the carbamate recycle flow. Urea as an 86 – 88% solution is concentrated by evaporation (i) before granulation or prilling. The process is applied in eight small-scale and two medium -scale plants.
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Fig 15 UTI heat recycles urea process a) CO2 compressor; b) High-pressure ammonia feed pump; c) Urea reactor with internal coil; d) Air compressor; e) Liquid distributor (used as first flash vessel); f ) First decomposer; g) First separator; h) Second decomposer; i) Urea concentrator; j) Carbamate heater; k) Ammonia heater; l) High-pressure carbamate recycle pump; CW= Cooling water
2) MTC (Mitsui Toatsu Corporation) Conventional Processes of Toyo Engineering Corporation. The conventional processes developed by Toyo Engineering Corporation (TEC) were successfully commercialized until the mid- 1980s. The continuous evolution of these processes is reflected in their sequential nomenclature: TR – A Total-Recycle A Process TR – B Total-Recycle B Process TR – C Total-Recycle C Process TR – CI Total-Recycle C Improved Process TR – D Total-Recycle D Process Partial-recycle versions of these processes were also realized. These TEC MTC conventional processes were applied in more than 70 plants. 5.5.2 Stripping Processes 5.4.2.1 Stamicarbon CO2-Stripping Process (Figs. 16) The synthesis stage of the Stamicarbon process consists of a urea reactor (c), a stripper for unconverted reactants (d), a high-pressure carbamate condenser (e), and a highpressure reactor off-gas scrubber (f). To realize maximum urea yield per pass through the reactor at the stipulated optimum pressure of 140 bar, an NH 3:CO2 molar ratio of 3:1 is applied. The greater part of the unconverted carbamate is decomposed in the stripper, where ammonia and carbon dioxide are stripped off. This stripping action is effected by countercurrent contact between the urea solution and fresh carbon dioxide at synthesis pressure. Low ammonia and carbon dioxide concentrations in the stripped urea solution are obtained, such that the recycle from the low-pressure recirculation stage (h, j) is minimized. These low concentrations of both ammonia and carbon dioxide in the stripper effluent can be obtained at relatively low temperatures of the urea solution because carbon dioxide is only sparingly soluble under such conditions.
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Fig 16 Stamicarbon CO2-stripping urea process (The process suitable for combination with a granulation plant is shown here; combination with prilling is also possible.) a) CO2 compressor; b) Hydrogen removal reactor; c) Urea reactor; d) High-pressure stripper ; e) Highpressure carbamate condenser; f) High-pressure scrubber; g) Low-pressure absorber; h) Low-pressure decomposer and rectifier; i) Pre-evaporator; j) Low-pressure carbamate condenser; k) Evaporator; l) Vacuum condensation section; m) Process condensate treatment CW= Cooling water; TCW= Tempered cooling water
Condensation of ammonia and carbon dioxide gases, leaving the stripper, occurs in the high pressure carbamate condenser (e) at synthesis pressure. As a result, the heat liberated from ammonium carbamate formation is at a high temperature. This heat is used for the production of 4.5-bar steam for use in the urea plant itself. The condensation in the high pressure carbamate condenser is not effected completely. Remaining gases are condensed in the reactor and provide the heat required for the dehydration of carbamate, as well as for heating the mixture to its equilibrium temperature. In an improvement to this process, the condensation of off-gas from the stripper is carried out in a pre-reactor, where sufficient residence time for the liquid phase is provided. As a result of urea and water formation in the condensing zone, the condensation temperature is increased, thus enabling the production of steam at a higher pressure level. The feed carbon dioxide, invariably originating from an associated ammonia plant, always contains hydrogen. To avoid the formation of explosive hydrogen – oxygen mixtures in the tail gas of the plant, hydrogen is catalytically removed from the carbon dioxide feed (b). Apart from the air required for this purpose, additional air is supplied to the fresh carbon dioxide input stream. This extra portion of oxygen is needed to maintain a corrosion-resistant layer on the stainless steel in the synthesis section. Before the inert gases, mainly oxygen and nitrogen, are purged from the synthesis section, they are washed with carbamate solution from the low pressure recirculation stage in the high-pressure scrubber (f) to obtain a low ammonia concentration in the subsequently purged gas. Further washing of the off-gas is performed in a low pressure absorber (g) to obtain a purge gas that is practically ammonia free. Only one low-pressure recirculation stage is required due to the low ammonia and carbon dioxide concentrations in the stripped urea solution. Because of the ideal ratio between ammonia and carbon dioxide in the recovered gases in this section, water dilution of the resultant ammonium carbamate is at a minimum despite the low
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pressure (about 4 bars). As a result of the efficiency of the stripper, the quantities of ammonium carbamate for recycle to the synthesis section are also minimized, and no separate ammonia recycle is required. The urea solution coming from the recirculation stage contains about 75 wt% urea. This solution is concentrated in the evaporation section (k). If the process is combined with a prilling tower for final product shaping, the final moisture content of urea from the evaporation section is ca. 0.25 wt %. If the process is combined with a granulation unit, the final moisture content may vary from 1 to 5 wt %, depending on granulation requirements. Higher moisture contents can be realized in a single-stage evaporator, whereas low moisture contents are economically achieved in a two-stage evaporation section. When urea with an extremely low biuret content is required (at a maximum of 0.3 wt %), pure urea crystals are produced in a crystallization section. These crystals are separated from the mother liquor by a combination of sieve bends and centrifuges and are melted prior to final shaping in a prilling tower or granulation unit. The process condensate emanating from water evaporation from the evaporation or crystallization sections contains ammonia and urea. Before this process condensate is purged, urea is hydrolyzed into ammonia and carbon dioxide (l), which are stripped off with steam and returned to urea synthesis via the recirculation section. This process condensate treatment section can produce water with high purity, thus transforming this “wastewater” treatment into the production unit of a valuable process condensate, suitable for, e.g., cooling tower or boiler feed water makeup. Since the introduction of the Stamicarbon CO 2-stripping process, some 125 units have been built according to this process all over the world. 5.5.2.2. Snamprogetti Ammonia-and Self-Stripping Processes In the first generation of NH 3 and self strip ping processes, ammonia was used as stripping agent. Because of the extreme solubility of ammonia in the urea containing synthesis fluid, the stripper effluent contained rather large amounts of dissolved ammonia, causing ammonia overload in downstream section of the plant. Later versions of the process abandoned the idea of using ammonia as stripping agent; stripping was achieved only by supply of heat .Even without using ammonia as a stripping agent, the NH3:CO2ratio in the stripper effluent is relatively high. So the recirculation section of the plant requires an ammonia-carbamate separation section the process uses a vertical layout in the synthesis section. Recycle within the synthesis section, from the stripper via the high pressure carbamate condenser, through the carbamate separator back to the reactor, is maintained by using an ammonia-driven liquid-liquid ejector. In the reactor, which is operated at 150 bars, NH 3:CO2 molar feed ratio of 3.5 is applied .The stripper is of the falling film type? Since stripping is achieved thermally, relatively high temperatures (200-2100C) are required to obtain a reasonable stripping efficiency. Because of this high temperature, stainless steel is not suitable as a construction material for the stripper from a corrosion point of view; titanium and bimetallic zirconium – stainless steel tubes have been used Off gas from the stripper is condensed in a kettle type boiler. At the tube side of this condenser the off gas is absorbed in recycled liquid carbamate from the medium pressure recovery section. The heat of absorption is removed through the tubes, which are cooled by the production of low pressure steam at the shell side. The steam produced is used effectively in the back end of the process .In the medium pressure decomposition and recirculation section, typically operated at 18 bar, the urea solution from the high 26
pressure stripper is subjected to the decomposition of carbamate and evaporation of ammonia. The off gas from this medium pressure decomposer is rectified. Liquid ammonia reflux is applied to the top of this rectifier; in this way a top product consisting of pure gaseous ammonia and a bottom product of liquid ammonium carbamate are obtained. The pure ammonia off gas is condensed and recycled to the synthesis section. To prevent solidification of ammonium carbamate in the rectifier, some water is added to the bottom section of the column to dilute the ammonium carbamate below its crystallization point. The liquid ammonium carbamate-water mixture obtained in this way is also recycled to the synthesis section. The purge gas of the ammonia condenser is treated in a scrubber prior to being purged to the atmosphere .The urea solution from the medium pressure decomposer is subjected to a second low pressure decomposition step. Here further decomposition of ammonium carbamate is achieved, so that a substantially carbamate – free aqueous urea solution is obtained. Off gas from this low pressure decomposer is condensed and recycled as an aqueous ammonium carbamate solution to the synthesis section via the medium pressure recovery section. Concentrating the urea water mixture obtained from the low pressure decomposer is preformed in a single or double evaporator depending on the requirement of the finishing section. Typically, if prilling is chosen as the final shaping procedure, a two stage evaporator is required, whereas in the case of a fluidized bed granulator a single evaporation step is sufficient to achieve the required final moisture content of the urea melt. In some versions of the process, heat exchange is applied between the off gas from the medium pressure decomposer and the aqueous urea solution to the evaporation section. In this way, the consumption of low pressure steam by the process is reduced. The process condensate obtained from the evaporation section is subjected to adsorption hydrolysis operation to recover the urea and ammonia contained in the process condensate. In this way, the consumption of low-pressure steam by the process is reduced. The process condensate obtained from the evaporation section is subjected to desorption – hydrolysis operation to recover the urea and ammonia contained in the process condensate. Up to now, about 70 plants have been designed according to the Snamprogetti ammonia and self-stripping processes
Fig 17 Schematic of the Snamprogetti self-stripping process a) CO2 compressor; b) Urea reactor; c) Ejector; d) High-pressure ammonia pump; e) Carbamate separator; f) Highpressure carbamate condenser; g) High-pressure carbamate pump; h) High-pressure stripper; i) Medium-pressure decomposer and rectifier; j) Ammonia – carbamate separation column; k) Ammonia condenser; l) Ammonia receiver; m) Low-pressure ammonia pump; n) Ammonia scrubber; o) Low-pressure decomposer and rectifier; p) Low-pressure
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carbamate condenser; q) Low-pressure carbamate receiver; r) Low-pressure off-gas scrubber; s) First evaporation heater; t) First evaporation separator; u) Second evaporation heater; v) Second evaporation separator; w) Wastewater treatment; x) Vacuum condensation section CW= Cooling water.
5.5.2.3 Isobaric Double-Recycle Process This process is developed by Montedison, is characterized by recycle of most of the un reacted ammonia and ammonium carbamate in two decomposer in series, both operating at the synthesis pressure. A high molar NH 3:CO2 ratio (4:1 to 5:1) in the reactor is applied. As a result of this choice ratio, the reactor effluent contains a relatively high amount of non converted ammonia. In the first, steam heated, high pressure decomposer, this large quantity of free ammonia is mainly removed from the urea solution. Most of the residual solution, as well as some ammonium carbamate, is removed in the second high pressure decomposer where steam heating and CO 2 stripping are applied. The high pressure synthesis section is followed by two low pressure decomposing stages of traditional design, where heat exchange between the condensing off gas of the medium pressure decomposition stage and the aqueous urea solution to the final concentration section improves the overall energy consumption of the process. Probably because of the complexity of this process, it has not achieved great popularity so far. This process or parts of the process are used in four revamps of older conventional plant 5.5.2.4 ACES Process The ACES (i.e., Advanced Process for Cost and Energy Saving) process has been developed by Toyo Engineering Corporation. Its synthesis section consists of the reactor (a), stripper (d), two parallel carbamate condensers (e), and a scrubber (f ) – all operated at 175 bar (see Fig. 18). The reactor is operated at 190 ◦C and an NH 3:CO2 molar feed ratio of 4:1. Liquid ammonia is fed directly to the reactor, whereas gaseous carbon dioxide after compression is introduced into the bottom of the stripper as a stripping aid. The synthesis mixture from the reactor, consisting of urea, unconverted ammonium carbamate, excess ammonia, and water, is fed to the top of the stripper. The stripper has two functions. Its upper part is equipped with trays where excess ammonia is partly separated from the stripper feed by direct countercurrent contact of the feed solution with the gas coming from the lower part of the stripper. This pre-stripping in the top is said to be required to achieve effective CO 2 stripping in the lower part. In the lower part of the stripper (a falling film heater), ammonium carbamate is decomposed and the resultingnCO 2 and NH3 as well as the excess NH3 are evaporated by CO 2 stripping and steam heating. The overhead gaseous mixture from the top of the stripper is introduced into the carbamate condensers (e). Here, two units in parallel are installed, where the gaseous mixture is condensed and absorbed by the carbamate solution coming from the medium-pressure recovery stage. Heat liberated in the high-pressure carbamate condensers is used to generate low-pressure steam in one of the condensers and to heat the urea solution from the stripper after the pressure is reduced to about 19 bars in the shell side of the second carbamate condenser. The gas and liquid from the carbamate condensers are recycled to the reactor by gravity 28
flow. The urea solution from the stripper, with a typical NH3 content of 12 wt %, is purified further in the subsequent medium- and low-pressure decomposers (i, j), operating at 19 and 3 bar, respectively. Ammonia and carbon dioxide separated from the urea solution here are recovered through stepwise absorption in the low- and mediumpressure absorbers (h, k). Condensation heat in the medium-pressure absorber is transferred directly to the aqueous urea solution feed in the final concentration section. In this final concentration section (l), the purified urea solution is concentrated further either by two-stage evaporation up to 99.7% for urea prills production or by a single evaporation up to 98.5% for urea granule production. Water vapor formed in the final concentrating section is condensed in surface condensers (q) to form process condensate. Part of this condensate is used as an absorbent in the recovery sections, whereas the remainder is purified in the process condensate treatment section by hydrolysis and steam stripping, before being discharged from the urea plant. The highly concentrated urea solution is finally processed either through the prilling tower (o) or via the urea granulator (p). Instead of concentration via evaporation, the ACES process can also be combined with a crystallization section to produce urea with low biuret content. Until now, the ACES process has been used in seven urea plants.
Fig 18 Schematic of the ACES process a) Urea reactor; b) High-pressure ammonia pump; c) CO2 compressor; d) Stripper; e) High-pressure carbamate condensers; f) High-pressure scrubber; g) High-pressure carbamate pump; h) Medium-pressure absorber; i) Mediumpressure decomposer; j) Low-pressure decomposer; k) Low-pressure absorber; l) Evaporators; m) Process condensate stripper; n) Hydrolyser; o) Prilling tower; p) Granulation section; q) Surface condensers CW= Cooling water
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5.6 UREA-AMMONIUM NITRATE (UAN) PRODUCTION 5.6.1 Overview of UAN Process Technology Ammonium nitrate (AN) and urea are used as feed stocks in the production of ureaammonium nitrate (UAN) liquid fertilizers. Most UAN solutions typically contain 28, 30 or 32% N but other customized concentrations (including additional nutrients) are produced. Plant capacities for the production of UAN solutions range between 200 and 2,000t.d-1. Most of the large scale production units are located on complexes where either urea or ammonium nitrate or both are produced. In some of the European UAN plants, ammonium nitrate is being synthesized directly from nitric acid and ammonia. In some cases carbamate solution from the urea reactor outlet is being used as feedstock for the production of UAN. In those plants the UAN technology is an integral part of the fertilizer complex. UAN from scrubbing systems, urea from sieving machines, etc. are fed to a central UAN system, where quality adjustments can be done. The addition of corrosion inhibitors or the use of corrosion resistant coatings allows carbon steel to be used for storage and transportation equipment for the solutions. West European consumption of UAN in 1998/1999 was 3.72 x 106 t of solution, 41% of which was imported. 5.6.2 Typical UAN solution analysis;
N content 28-32% by weight, pH 7 to 7.5, density 1,280-1,320kg.m-3, salt-out temperature – 18 to – 2°C, depending on the N content and at its lowest when the Urea N/Ammonium Nitrate N ratio is about 1:1. 5.6.3 Description of Production Processes;
Continuous and batch type processes are used and in both processes concentrated urea and ammonium nitrate solutions are measured, mixed and then cooled. Block diagrams for UAN production are shown in Figures 19. In the continuous process the ingredients of the UAN solution are continuously fed to and mixed in a series of appropriately sized static mixers. Raw material flow as well as finished product flow, PH and density are continuously measured and adjusted. The finished product is cooled and transferred to a storage tank for distribution. In the batch process the raw materials are sequentially fed to a mixing vessel fitted with an agitator and mounted on load-cells. The dissolving of the solid raw material(s) can be enhanced by recirculation and heat exchange as required. The PH of the UAN product is adjusted prior to the addition of the corrosion inhibitor. A partial recycle CO 2 stripping urea process is also suitable for UAN solution production. Unconverted NH3 and CO2 coming from the stripped urea solution, together with the gases from the water treatment unit, are transferred for conversion into UAN solutions.
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Fig 19 block flow diagram for UAN production
5.6.4 Urea – Ammonium Nitrate (UAN) process Mixtures of urea ( mp 133 ◦C) and ammonium nitrate ( mp 169 ◦C) with water have a eutectic point at −26.5 ◦C [51]. As a result solutions with high nitrogen content can be made with solidification temperatures below ambient temperature. These mixtures, called UAN solutions, are used as liquid nitrogen fertilizers. UAN solutions can be made by mixing the appropriate amounts of solid urea and solid ammonium nitrate with water or, alternatively, in a production facility specially designed to produce UAN solutions. In this latter category the StamicarbonCO 2-stripping technology is especially suitable, in a partial-recycle version of this process. 5.5.5 Pool condenser process (stamicarbon) (fig 20&21) Stamicarbon creates UAN by using the residual ammonia remaining in the stripped urea solution, together with the ammonia in the high-pressure urea synthesis's off-gases. This is converted into ammonium nitrate in a neutralization reactor using nitric acid. In the high-pressure synthesis section, carbon dioxide and ammonia are converted into urea which contains unconverted ammonia and carbon dioxide. Stripping with carbon dioxide causes the greater part of these components to evaporate from the solution. From here, evaporated ammonia and carbon dioxide, together with fresh ammonia and carbon dioxide, are condensed in the pool reactor, with the heat from this condensation being used to produce low-pressure steam. The condensed ammonia and carbon dioxide are partly converted into urea and water. In the low-pressure dissociation section, the stripped urea solution is almost entirely freed from ammonia and carbon dioxide. Then the overhead vapors of the reactor, mixed with off-gases from the dissociation section and the ammonia present in the urea solution from the urea solution tank, are all sent to the neutralization section.
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Fig 20 Pool condenser Process (UAN process)
Fig 21 Mixing and cooling Process of UAN production
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6. Environmental Aspects The benefits of using a nitrogenous fertilizers is obvious because the crops grow taller, and are healthier therefore yielding a higher crop and therefore cheaper, more plentiful food. There are always disadvantages. If after applying the fertilizer and it rained or when too much fertilizer is used it gets into the streams or rivers and pollutes them. In the rivers, the fertilizer does the same as it would on land, the river plants grow and algae grow rapidly because of the abundant food supply. The algae then die in large numbers. The bacteria feeding on the dead plant material use up the oxygen in the water, Fish may then die because of the lack of oxygen in the water. Also too high of nitrates in the drinking water is a health hazard, particularly with infants. Nitrates can interfere with the oxygen flow in the blood stream.
6.1 Environmental issues: spills along pathways;
• Given existing urea production facilities and extensive distribution pathways, SCRurea should not have significant incremental spill incidents or well-tank greenhouse gas emissions. • Urea is considered a stable and safely transportable product and degrades quickly in soil, water, and air. • However, ingested or absorbed in large quantities, it can be hazardous to plant and animal life. • Spill clean-up procedures are well-established and will have to be modified and/or updated to accommodate new locations where urea will be used. 6.2 Emissions, Effluents and Solid Wastes
Fertilizers industry manufacturing processes originate is gas, contaminated liquid effluents in addition to dust and solid by-products. These wastes pollute air, water as well as impair soil. Pollution problems arise from low processes efficiency and/or inefficient dust collecting systems, gas scrubbing (if it does exist) and most important is the lack of a proven recycle of such recovered contaminants to the process streams 6.2.1 Air Emissions: The major emission in phosphoric and nitrogenous fertilizers cited here below includes particulates, sulphur oxides, hydrogen fluoride (and compounds Ca F 2 SiF4) phosphorgypsum, phosphoric acid, ammonia, NO x, ammonium nitrate, sulphate, carbon oxides … etc.
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6.2.1.1 for Ammonium Nitrate Production The primary air emissions from ammonium nitrate production plants are particulate matter (ammonium nitrate and coating materials), ammonia and nitric acid. Ammonia and nitric acid are emitted primarily from solution formation and granulators. Particulate matter (largely as ammonium nitrate) can be emitted from most of the process operations.
The emission sources in solution formation and concentration processes are neutralizers and evaporators, emitting droplets of ammonium nitrate and ammonia. The vapor stream off the top of the neutralization reactor is primarily steam with some ammonia and NH 4NO3 particulates present. Specific plant operating characteristics, however, make these emissions vary depending upon use of excess ammonia or acid in the neutralizer. Particulate emissions from these operations tend to be smaller in size than those from solids production and handling processes and generally are recycled back to the process. Emissions of very fine particles are very difficult to remove. A combination of droplet separators and scrubbers can be used. Contaminated steam from evaporator should be purified before discharge to the environment. Emissions from solids formation processes are ammonium nitrate particulate matter and ammonia. The sources of primary importance are prilling towers (for high density and low density prills) and granulators (rotary drum and pan). Ammonia is normally removed by neutralization in a wet scrubber. Most conventional scrubbers are less efficient for the removal of particles below 1 micron of prilling tower fume. Candle filters are normally required. Gases from granulator and drier may be cleaned by combination of dry cyclones or bag filters and wet scrubbers. Emissions from prilling towers result from carryover of fine particles and fume by the prills cooling air flowing through the tower. These fine particles are from micro-prilling formation, attrition of prills colliding with the tower or one another, and rapid transition of the ammonia nitrate between crystal states. Micro-prilling formation resulting from partially plugged orifices of melt spray devices can increase fine dust loading and emissions. Certain designs (spinning buckets) and practices (vibration of spray plates) help reduce plugged orifices and thus micro-prilling formation. High ambient air temperatures can cause increased emissions because of entrainment as a result of higher air flow required to cool prills and because of increased fume formation at the higher temperatures. Emissions from screening operations are generated by the attrition of the ammonium nitrate solids against the screens and against one another. Almost all screening operations used in the ammonium nitrate manufacturing industry are
34
enclosed or have a cover over the uppermost screen. Emissions are ducted from the process for recovery or reuse.
Bagging and bulk loading operations are also a source of particulate emissions. Dust is emitted from each type of bagging process during final filling when dust laden air is displaced from the bag by the ammonium nitrate. The potential for emissions during bagging is greater for coated than for uncoated material. It is expected that emissions from bagging operations are primarily the kaolin, talc or diatomaceous earth coating matter. About 90 percent of solid ammonium nitrate produced domestically is bulk loaded. While particulate emissions from bulk loading are not generally controlled, visible emissions are within typical state regulatory requirements (below 20 percent opacity).
6.2.1.2 for Urea Production: Emissions from urea manufacture are mainly ammonia particulate matter. Formaldehyde and methanol, hazardous air pollutants, may be emitted if additives are used. Formalin TM, used as a formaldehyde additive, may contain up to 15 percent methanol. Ammonia is emitted during the solution synthesis and solids production processes. Particulate matter is emitted during all urea processes. In the synthesis process, some emission control is inherent in the recycle process where carbamate gases and/or liquids are recovered and recycled. Typical emission sources from the solution synthesis process are non-condensable vent streams from ammonium carbamate decomposers and separators. Emissions from synthesis processes are generally combined with emissions from the solution concentration process and are vented through a common stack. Combined particulate emissions from urea synthesis and concentration operations are small compared to particulate emissions from a typical solids-producing urea plant. The synthesis and concentration operations are usually uncontrolled except for recycle provisions to recover ammonia. Uncontrolled emission rates from prilling towers may be affected by the following
factors:
Product grade being produced Air flow rate through the tower Type of tower bed.
The total of mass emissions per unit is usually lower for feed grade prilling production than for agricultural grade prills, due to lower airflows. Uncontrolled particulate emission rates for fluidized bed prilling towers are higher than those for non-fluidized bed prilling towers making agricultural grade prills, and are approximately equal to those for nonfluidized bed feed grade prills. 35
Ambient air conditions can affect prilling tower emissions. Available data indicate that colder temperatures promote the formation of smaller particles in the prilling tower exhaust. Since smaller particles are more difficult to remove, the efficiency of prilling tower control devices tends to decrease with ambient temperatures. This can lead to higher emission levels for prilling towers operated during cold weather. Ambient humidity can also affect prilling tower emissions. Air flow rates must be increased with high humidity, and higher air flow rates usually cause higher emissions. In the solids screening process, dust is generated by abrasion of urea particles and the vibration of the screening mechanisms. Therefore, almost all screening operations used in the urea manufacturing industry are enclosed or are covered over the uppermost screen. Emissions attributable to coating include entrained clay dust from loading, in-plant transfer, and leaks from the seals of the coater. 6.2.2 Effluents; “Wastewater”; Wastewater from the fertilizer industry can be classified into four groups:
Process effluents resulting from contact with gas, liquids, or solids Dedicated effluents which may be separated for use in one process or for recycling at a controlled rate Effluents from general services such as cleaning or pretreatment Occasional effluents such as leaks or spills
Wastewater is generated in any fertilizer production facility by leaks, spills, cleaning, maintenance, and laboratory tests. Cooling water may contain ammonia, sulfate, chloride, phosphate, chromate, and dissolved solids which become concentrated through evaporation. 6.2.2.1 for Nitrogenous Fertilizers: Wastewater is considered the major pollution problem in such plants. A number of process wastewater streams from the nitrogenous fertilizer industry have been identified. Wastewater is analyzed in terms of ammonia, nitrates, organic nitrogen and hydrogen ions concentration (pH). 6.2.2.2 Ammonia Production: In ammonia production, wastewater is generated from process condensate stripping. Cooling the synthesis gas after low temperature shift conversion forms a condensate containing NH3, CO2, and methanol (CH3OH), and trace metals. Condensate steam strippers are used to remove NH3 and methanol from the water. Stripped condensate can be reused as boiler feed water after treatment of ion exchange. In some cases, the process condensate is used for feed gas saturation and thus recycled.
Wastewater is contaminated with traces of ammonia and oils from compressing gas unit. Emissions to water may also occur due to the scrubbing of waste gases containing
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ammonia if gases are not recovered in a closed loop so that no aqueous ammonia emissions occur.
6.2.2.3 Nitric Acid Production Nitric acid production generates relatively little wastewater since there is no process wastewater source. Steam condensate generated in nitrogenous fertilizer processing is characterized by dissolved and suspended solids, alkalinity, and hardness. The ammonia vaporizer blow-down should have the ammonia vaporized and recovered into the process. The waste oil should be reprocessed.
6.2.2.4 Ammonium Nitrate Production Wastewater effluent is discharged from cooling tower blow down, backwash from multi-media filters and from equipment and floor washing. Loss of ammonium nitrate to drain from a large number of sources is a potential problem for all ammonium nitrate plants. Causes are different kinds of leaks and washings.
Ammonium nitrate manufacturing produces process wastewater in the neutralization process, the evaporation unit, and air cooling equipment. The vacuum condenser in ammonium nitrate plants is a source of wastewater. Most scrubbing operations are also a source of wastewater. The steam condensate leaving the neutralizer can be purified. Purification can be achieved by stripping, distillation and membrane separation processes as reverse osmosis. Ion exchange can also be considered but there are some safety concerns which must be addressed. The steam can be used in the evaporator or to preheat and evaporate ammonia and to preheat nitric acid. The solution from wet scrubbers is normally recycled. 6.2.3 Solid Wastes: For all fertilizers plants, solid materials may be found in storage piles, settled dust and other similar forms. The following are the major solid wastes in different production lines of fertilizers. 6.2.3.1 For Nitrogenous Fertilizers
Ammonia Production; Solid wastes are generated due to the usage of many catalysts types and quantities. The spent catalysts cannot be purified or recycled, hence, they create a disposal problem which should be checked. These catalysts are considered hazardous, due to their heavy metals content (chromium and nickel). Another solid waste problem is the filling material for absorption tower. They are consumed and disposed off-site.
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Catalysts used in the steam reforming process need to be replaced every two to six years. Spent catalysts contain oxides of hexa valent chromium, zinc, iron, and nickel. They are typically returned to the manufacturer or other metal recovery companies for recycling and reclamation of valuable materials.
Nitric Acid Production Spent catalyst from the reactor and filter cake and cloth generated from the filter are considered the major solid wastes generated from this unit. Usually spent catalyst is sent to the suppliers and it must not be disposed in dumping sites due to its hazardous nature. Ammonium Nitrate Production Dolomite and clay from the mixing and coating operations are the major solid wastes from this unit as well dump product. Urea Production Prills coating material in the bagging section are collected and recycled.
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7. Economic Aspects 7.1 Economic Analysis: Costs of production estimates for different urea processes in different parts of the world should be carried out. The major contributor to the urea cost is the cost of ammonia. Since all the urea plants are situated next to ammonia facilities, the effect of transferring ammonia into the urea process at cash cost and full cost should be examined. Additionally, a cost of production estimate for the production of urea based on market price ammonia should be done.
7.2 Commercial Analysis: Urea is used as a solid nitrogen release fertilizer, because it has a high nitrogen content (46 percent), compared with 34 percent for ammonium nitrate and less for ammonium sulfate. The high nitrogen content of urea reduces transportation costs per ton of nutrient. This, together with its low solubility and absence of safety considerations make it more suitable for deep-sea expert and use in the tropical climates of Asia and Latin America. As these are growing economies with high populations, urea is the most widely used nitrogen fertilizer. In addition to fertilizer uses, there are also chemical uses such as melamine, resins, pharmaceutical and others.
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8. Quality Specifications 8.1 Urea Quality Specifications: 8.1.1 Prilled Urea: 1- White prilled 2- Mass part nitrogen: 46.2% 3- Biuret, not more than: 1.0% 4- Moisture not more than: 0.5% 5- Granule size: 1mm to 4mm 90% min. 6- Free ammonia: 160 ppm max. o 7- Melting point: 32 C 8- Physical properties:
Non radioactive
White
Free flowing
Free from harmful substances
100% treated against caking
8.1.2 Granulated Urea: 1- White granules 2- Nitrogen recalculated to dry: 46.2% 3- Biuret, not more than: 1.0% 4- Water, not more than: 0.5% 5- Granules size: 2mm to 4mm 90% min. 6- Free ammonia: 160 pxt ppm max. 7- Melting point: 132 degrees cen. 8- Crumbliness: 100% 9- Physical properties:
Non radioactive.
White
Free flowing
Free from harmful substances
Coated
Spherical
Uniform in size
100% treated against coating
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8.2 Ammonium Nitrate Specifications:
“Table 4”
GOST 2-85
A Total mass portion of nitrate and ammonium nitrogen based on: NH4NO3 in dry matter,%, not less than Nitrogen in dry matter, %, not less than Water mass portion, %, not more than:
with sulphate and phosphate additives with calcium/ magnesium nitrate additives PH of 10% water solution, not less than ..with sulphate & phosphate additives Mass portion of substances insoluble in 10% nitric acid solution,%, not more than Grain Size Distribution Mass portion of granule size from 1 to 3mm, %, not less than Mass portion of granule size from 1 to 4 mm, %, not less than Granule size from 2 to 4mm, %, not less than Mass portion of granule size less than 1mm, %, not more than Mass portion of granule size exceeding 6mm, % Granule static strength, n/granule (kg/granule), not less than with sulphate and sulphate & phosphate additives with calcium and magnesium nitrate additives
B Higher Grade First Grade
98 No standard
No standard 34.4
No standard 34.4
0.2
0.2
0.2
0.3
0.3
0.3
5.0 4.0
5.0 4.0
5.0 4.0
0.2
No standard
No standard
93
No standard
No standard
No standard
95
95
No standard
80
50
4
3
3
0.0
0.0
0.0
5(0.5)
7(0.7) 10(1.0) 8(0.8)
Friability, %, not less than
100
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100
100
