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Lit. survey urea

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this is about literature review of urea production

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    Lit. survey  urea Lit. survey urea Document Transcript

    • CH 4202 Comprehensive Design Project Assignment 1 Design of an Urea Manufacturing Plant Group Members: - Wellappili A.Y.G 090556N Wijesinghe C.D 090577E Silva G.G.S.N 090488G Pathiraja P.D.T.P 090361L Ariyaratne V.T 090029d Rashara G.A.D.D 090428B Gamage G.P.S 090147L Abeywickrama T.D 090010L Dayananda L.K 090079E Date of Submission:- Department of Chemical and Process Engineering University of Moratuwa
    • Table of Contents 1) INTRODUCTION ..................................................................................................................................... 4 1.1) 1.2) Nitrogen sources and fertilizers .................................................................................................... 4 1.3) About Urea .................................................................................................................................... 6 1.4) Importance of Urea as a fertilizer ................................................................................................. 7 1.5) 2) Importance of Nitrogen for plant growth ..................................................................................... 4 Other applications......................................................................................................................... 8 PRODUCTION PROCESS OF UREA........................................................................................................ 10 2.1) Raw materials for manufacturing Urea ....................................................................................... 10 2.1.1) Ammonia ............................................................................................................................. 10 2.1.2) Carbon dioxide .................................................................................................................... 11 2.2) Sources and availability of raw materials ................................................................................... 11 2.2.1) Hydrogen Production ............................................................................................................... 11 2.2.2) Nitrogen .................................................................................................................................... 12 2.3) Synthesis of Ammonia ................................................................................................................ 12 2.3.1) Steam Reforming method ......................................................................................................... 12 2.4) Process in general production of Urea ....................................................................................... 19 2.5) Process variation of Urea manufacturing ................................................................................... 22 2.5.1) Once-through processes ........................................................................................................... 22 2.5.2) Partial recycle processes ........................................................................................................... 23 2.5.3) Total recycle processes ............................................................................................................. 24 3) ECONOMIC ASPECTS OF UREA PRODUCTION ..................................................................................... 33 4) ENVIRONMENTAL AND SAFETY ISSUES............................................................................................... 34 4.1) Health and Safety............................................................................................................................. 34 4.1.1) Health........................................................................................................................................ 34 4.1.2) Safety ........................................................................................................................................ 35 4.2) Safety Procedures ............................................................................................................................ 37 4.3) Inherent Safe Design ........................................................................................................................ 37 4.4) Layers of Process Safety................................................................................................................... 38 4.5) conclusions ...................................................................................................................................... 39 4.6) recommendations............................................................................................................................ 39 4.7) Environment .................................................................................................................................... 39
    • 5) GREEN CONCEPTS USED IN PLANT DESIGNING .................................................................................. 40 5.1) why should we care about green buildings? ................................................................................... 40 5.2) Current methods used in industries ................................................................................................ 41 6) HISTORY OF UREA PRODUCTION IN SRI LANKA .................................................................................. 44 7) PROCESS SELECTION AND DESIGN DECISIONS.................................................................................... 46 REFERENCES ................................................................................................................................................ 58 LIST OF TABLES LIST OF FIGURES
    • 1) INTRODUCTION 1.1) Importance of Nitrogen for plant growth Nitrogen is considered the most important component for supporting plant growth. While nitrogen is a natural element existing as N2 (gas) accounting for 78% of the earth’s atmosphere, plants cannot absorb it in this natural form (N2 molecules are inert under normal environmental conditions due to their high bond energy). The nitrogen in the environment is synthesized into fertilizers which are readily available to plants. Nitrogen is the main nutrient and regulator of plant growth – it supplements and promotes all of a plant’s growth processes. Nitrogen is a part of all living cells and an integral component of all chemical compounds-proteins, enzymes, hormones and metabolic processes involved in the synthesis and transfer of energy. Nitrogen is a constituent of chlorophyll, the green pigment present in chloroplasts in certain plant cells (esp. in green leaves-green leaves get their color due to chlorophyll) of the plant that is responsible for photosynthesis. Helps accelerate plant growth, increasing seed and fruit production and improving the quality of leaf and forage crops. 1.2) Nitrogen sources and fertilizers Nitrogen often comes from fertilizer application and from the air (legumes get their N from the atmosphere, water or rainfall contributes very little nitrogen) Nitrogen in the air is the ultimate source of all soil nitrogen. Nitrogen may enter the soil through rainfall, plant residues, fixation by soil organisms, animal manures and organic fertilizers (i.e. compost) and commercial fertilizers. Nitrogen may be lost from the soil by plant removal, volatilization, leaching or erosion. The earth's atmosphere is the ultimate source of nitrogen. In most areas of the world, the nitrogen found in soil minerals is negligible. Nitrogen may be added to or lost from soil by a number of processes. In the soil, nitrogen can undergo a number of transformations.
    • Rainfall adds about 10 lbs. of nitrogen to the soil per acre per year. The nitrogen oxides and ammonium that are washed to earth are formed as a result of lightning during storms, by internal combustion engines and through oxidation by sunlight. Crop residues decompose in the soil to form soil organic matter. This organic matter contains about 5% nitrogen. An acre-foot of soil having 2 percent organic matter would contain about 3,500 lbs. of nitrogen. Generally, about 1- 3% of this organic nitrogen is annually converted by microorganisms to a form usable by plants. Legumes fix atmospheric nitrogen through their symbiotic association with Rhizobium bacteria. If plant roots are well modulated, the legume plant does not benefit from the addition of fertilizer nitrogen. Perennial legumes, such as alfalfa, can fix several hundred pounds of nitrogen per acre per year. Manure contains an appreciable amount of nitrogen. Most of this nitrogen is present as organic compounds. Cattle manure contains about 10-40 lbs. of nitrogen per ton. About half of this nitrogen is converted to forms available to plants during the first growing season. Lesser amounts are converted during succeeding seasons. Each ton of applied manure is equal to about 5-20 pounds of commercial fertilizer nitrogen. Commercial fertilizer nitrogen comes in three basic forms: gas, liquid and dry. All forms are equally effective when properly applied. Once applied, fertilizer nitrogen is subject to the same transformations as other sources of nitrogen. There is no difference between the ammonium (NH4+) or nitrate (NO3-) which are absorbed by the plant from commercial fertilizers and that supplied by natural substances. Nitrogen exists in a number of chemical forms and undergoes chemical and biological reactions. 1. Organic nitrogen to ammonium nitrogen (mineralization). Organic nitrogen comprises over 95% of the nitrogen present in soil. This form of nitrogen cannot be used by plants but is gradually transformed by soil microorganisms to ammonium (NH4+). Ammonium is not leached to a great extent. Since NH4+ is a positively charged ion (cation), it is attracted to and held by the negatively charged soil clay. 2. Ammonium nitrogen to nitrate nitrogen (nitrification). In warm, well-drained soil, ammonium transforms rapidly to nitrate (NO3-). Nitrate is the principle form of nitrogen used by
    • plants. It leaches easily, since it is a negatively charged ion (anion) and is not attracted to soil clay. 3. Nitrate or ammonium nitrogen to organic nitrogen (immobilization). Soil microorganisms use nitrate and ammonium nitrogen when decomposing plant residues. These forms are temporarily "tied-up" (incorporated into microbial tissue) in this process. This can be a major concern if crop residues are high in carbon relative to nitrogen. Examples are wheat straw, corn stalks and sawdust. The addition of 20 to 70 lbs. of nitrogen per ton of these residues is needed to prevent this transformation. After the residues are decomposed, the microbial population starts dying out and processes 1 and 2 take place. 4. Nitrate nitrogen to gaseous nitrogen (de-nitrification). When soil does not have sufficient air, microorganisms use the oxygen from NO3- in place of that in the air and rapidly convert NO3to nitrogen oxide and nitrogen gases (N2). These gases escape to the atmosphere and are not available to plants. This transformation can occur within two or three days in poorly aerated soil and can result in large losses of nitrate-type fertilizers. 5. Ammonium nitrogen to ammonia gas (ammonia volatilization). Soils that have a high pH (> 7.5) can lose large amounts of NH4+ by conversion to NH3 gas. To minimize these losses, incorporate solid ammonium-type fertilizers, urea- CO(NH2)2 and anhydrous ammonia below the surface of a moist soil. 1.3) About Urea Urea (also known as carbamide) is an organic compound with the chemical formula CO(NH2)2. The molecule has two —NH2 (amine) groups linked to a carbonyl (C=O) functional group. Molecular Formula CO(NH2)2 Molec Physical properties
    • Molar mass: Density: Solubilty Melting point: Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. It is a colorless, odorless solid, although the ammonia that it gives off in the presence of water, including water vapor in the air, has a strong odor. It is highly soluble in water and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline. Urea would hydrolyze in both acidic and basic aqueous media. The body performs nitrogen excretion by means of urea. Urea is widely used in fertilizers as a convenient source of nitrogen. It is also an important raw material for the chemical industry. Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave. In 1828, the German chemist Friedrich Wöhler was the first to artificially synthesize urea from an inorganic precursor-this was an important breakthrough in organic chemistry since it demonstrated that an organic compound present in living organisms could be obtained from inanimate materials. It was done by treating silver isocyanate (AgNCO) with ammonium chloride (NH4Cl). 1.4) Importance of Urea as a fertilizer More than 90% of world production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has the lowest transportation costs per unit of nitrogen nutrient. The standard crop-nutrient rating of urea is 46-0-0.
    • Many soil bacteria possess the enzyme urease, which catalyzes the conversion of the urea molecule to two ammonia molecules and one carbon dioxide (CO2) molecule, thus urea fertilizers are very rapidly transformed to the ammonium form in soils. Among soil bacteria known to carry urease, some ammonia-oxidizing bacteria (AOB), such as species of Nitrosomonas, are also able to assimilate the carbon dioxide released by the reaction to make biomass via the Calvin Cycle, and harvest energy by oxidizing ammonia (the other product of urease) to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile in soils and is a major cause of water pollution from agriculture. Ammonia and nitrate are readily absorbed by plants, and are the dominant sources of nitrogen for plant growth. Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g., in 'foliar feed' fertilizers. 1.5) Other applications Chemical Industry-Urea is an important raw material in the manufacture of several chemical compounds. They include: plastics (such as urea-formaldehyde resins), adhesives (urea formaldehyde or urea melamine formaldehyde) and Potassium Cyanate. Chemical Formulae and relevant reactions. Explosives-Urea is also used to produce urea nitrate which is a highly explosive substance. Automobiles-Urea is used in Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) reactions to reduce nitrous oxides (NOx) in exhaust fumes from combustion of diesel. Other commercial uses Stabilizer in nitrocellulose explosives
    • A component of animal feed, providing a relatively cheap source of nitrogen to promote growth A flavor-enhancing additive for cigarettes A main ingredient in hair removers An ingredient in skin cream, moisturizers and hair conditioners A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as the urea-potassium bicarbonate mixture An ingredient in many tooth whitening products Along with ammonium phosphate, as a yeast nutrient, for fermentation of sugars into ethanol As a solubility-enhancing and moisture-retaining additive in dye baths for textile dyeing or printing
    • 2) PRODUCTION PROCESS OF UREA 2.1) Raw materials for manufacturing Urea The industrial process of manufacturing urea basically involves two main steps. The first step is the formation of ammonium carbamate (NH2COONH4) which is done by reacting ammonia (NH3) and gaseous carbon dioxide (CO2). The second step is dehydration of ammonium carbamate to produce molten urea (NH2CONH2). So main reactants needed for the process would be ammonia and carbon dioxide. 2.1.1) Ammonia Ammonia is a colourless gas having density 0.589 times that of air, with a sharp unpleasant smell. At atmospheric pressure the boiling point of ammonia is -33.34 0C. So the storage must be done under high pressure at low temperature. Due to the presence of strong hydrogen bonds between molecules it can be easily liquefied and well miscible in water. Natural occurrence of ammonia happens due to decay process of nitrogenous animal and vegetable matter. In addition ammonia salts such as ammonium chloride, ammonium sulfate and ammonium bicarbonate are available in soil and rain water. Biosynthesis of ammonia from atmospheric nitrogen by enzymes called nitrogenase happens in certain organism which is called nitrogen fixing. Ammonia is also produced through amino acid metabolism and is converted to urea through series of reactions inside the liver. The commercial production of ammonia is very important since it is not available as a natural resource and it is essential in large quantities for urea manufacturing. Worldwide, the annual production of synthetic ammonia is about 120 million tones, of which about 85% is used in fertilizers, including urea [1]. Most of the industrial processes for synthesis of ammonia are based on Haber Bosch process, developed in Germany 1904-1913. In that process ammonia is produced by the reaction between gaseous hydrogen and nitrogen under high temperature and pressure in the presence of iron based catalyst. Formation of ammonia from nitrogen and hydrogen is basically a reversible reaction which the yield depends on the conditions employed. Unreacted hydrogen and nitrogen are usually separated and recycled.
    • 2.1.2) Carbon dioxide Carbon dioxide is available in atmospheric air in trace amount which is known as a pollutant gas. In most of industrial plants carbon dioxide is emitted as a result of burning fossil fuels. Carbon dioxide is produces as a byproduct of ammonia synthesis itself. So carbon dioxide which is produced as a byproduct can be removed and used in the production of urea. When overall production process is consider primary raw materials needed to manufacture urea are Hydrogen (H2) Nitrogen (N2) 2.2) Sources and availability of raw materials 2.2.1) Hydrogen Production There are many sources that hydrogen can be obtained. The process of ammonia synthesis basically depends on the hydrogen source used for the process. Hydrogen can be produced from natural gas i.e. methane, liquid petroleum gases such as propane and butane or petroleum naphtha. When light hydrocarbons mentioned above are used as the source for hydrogen the production process used for ammonia synthesis is known as ―Steam Reforming‖. Heavy fuel oil or vacuum residue can be used to obtain hydrogen when partial oxidation process for ammonia synthesis is used. Although coal gasification and electrolysis of water can be used to produce hydrogen those methods are no longer used in industrial scale ammonia production. In addition hydrogen gas emitted as a byproduct of petroleum cracking can also be used to produce hydrogen to manufacture urea. Since different process techniques should be employed depending on the feedstock, energy consumption, investment cost and production cost will be vary depending on the feedstock.
    • Following table gives an approximate comparison of the energy consumption, investment cost and production cost for main three sources of hydrogen production [2] Natural Gas Heavy oil Coal Energy consumption 1.0 1.3 1.7 Investment cost 1.0 1.4 2.4 Production cost 1.0 1.2 1.7 In addition availability of raw materials is also important in deciding the production process. 2.2.2) Nitrogen The most abundant source for nitrogen is atmospheric air. Dry air contains roughly 78% of gaseous nitrogen (N2) by volume. Pure nitrogen needed for the Haber process can be easily extracted by removing oxygen, carbon dioxide and other gases by liquefaction or law temperature distillation. But if steam reforming process is used, no such method is needed. In that process, oxygen is removed by simple combustion and carbon dioxide is removed by using absorption process. 2.3) Synthesis of Ammonia There are two methods available for synthesis of ammonia. Steam reforming method can be used for natural gas and other light hydrocarbons such as liquid petroleum gas and naphtha. If the feedstock is residual heavy oil or vacuum residue from a refinery, then the process will be partial oxidation. Steam reforming process of natural gas is identified as the most efficient method for synthesis of ammonia. 2.3.1) Steam Reforming method This method is known as the best available technique for the synthesis of ammonia. Steam reforming method can be divided into three as mentioned below [3].
    • Conventional steam reforming with fired primary reformer and stoichiometric air secondary reforming (stoichiometric H/N- ratio) Steam reforming with mild conditions in fired primary reformer and excess air in secondary reformer (Under-stoichiometric H/N ratio) Heat exchange auto thermal reforming, with a process gas heated steam reformer (heat exchange reformer) and a separate secondary reformer, or in a combined auto thermal reformer using excess or enriched air (under- stoichiometric or stoichiometric H/N-ratio) Among these three techniques conventional steam reforming method is the oldest and most widely used technique. 2.3.1.1) Conventional Steam Reforming method Desulphurization Sulphur and sulphorous compounds which contain in natural gas poison the catalyst used in ammonia synthesis process. The feed gas is pre heated up to 350-400 0C in the primary reformer convection section and then passed to the desulphurization vessel. In that section sulphur compounds are hydrogenated to hydrogen sulphide (H2S) using cobalt molybdenum catalyst and then H2S is removed by reacting with Zinc Oxide (ZnO). R-SH + H2 H2S + RH ZnO H2S ZnS + H2O + Hydrogen gas needed for hydrogenation of sulphur compounds is usually recycled from the synthesis section of the plant and consumed zinc oxide or zinc sulphide remains in the adsorption bed. Primary Reforming Primary reformer consists of nickel or iron containing catalyst. Desulphurized gas reacts with superheated steam which is heated up to 500-600 0C in the convection section before entering the primary reformer. Following reactions take place in the primary reformer. CH4 (g) + H2O (g) CO (g) + 3H2 (g) H=206 kJ.mol-1
    • CO (g) + H2O (g) CO2 (g) + 4H2 (g) + H=-41 kJ.mol -1 H2 (g) CO2 (g) The overall reaction would be CH4 (g) + 2H2O (g) In modern plants preheated steam and gas mixture is passed through an adiabatic pre reformer which uses pre reformer catalyst to reheat the mixture in the convection section. In some plants part of process steam is obtained from feed gas saturation. The amount of steam that should be supplied is given by steam to carbon molar ratio. It is usually maintained at 3.0 for an efficient process. The optimum ratio depends on factors such as feedstock quality, purge gas recovery, primary reformer capacity, shift operation, and the plant steam balance. The reaction between methane and steam takes place between 780-830 °C. At this temperature the above equilibrium reaction is driven to the right, giving high yield of hydrogen, carbon dioxide and small quantities of carbon monoxide. Since the overall reaction is highly endothermic additional heat should be supplied to maintain optimum temperature by burning natural gas or other gaseous fuel. The output from the primary reformer is commonly known as synthesis gas. Secondary Reforming The synthesis gas, small amount of carbon monoxide and unreacted methane leaving the primary reformer enters the secondary reformer where it is mixed with calculated amount of air. At actual operating conditions only 30-40% of hydrocarbons are reformed and temperature must be increased to increase the conversion. Addition of air converts methane molecules that are not reacted during primary steam reforming in to synthesis gas. Following reaction takes place in the secondary reforming process. Ni Catalyst 2CH4 (g) + O2 (g) + 4N2 (g) 2CO (g) + 4H2 (g) + 4N2 (g) CO produced in the above reaction is converted to CO2 as mentioned in the following reaction. CO (g) + H2O (g) So the overall reaction would be CO2 (g) + H2 (g)
    • 2CH4 (g) + O2 (g) + 4N2 (g) + 2H2O (g) 2O2 (g) + 6H2 (g) + 4N2 (g) This internal combustion is highly exothermic and supplies necessary heat and also gives high yield of hydrogen. The temperature at the outlet of secondary reformer is around 1000 °C and 99% of hydrocarbons coming from primary reformer is converted leading to reduced methane fraction of 0.2-0.3% on dry gas basis. This reaction also removes oxygen from air and provides nitrogen for final synthesis of ammonia. In the conventional reforming process the degree of primary reforming is adjusted so that air supplied to the secondary reformer balances both heat requirement and stoichiometric nitrogen gas requirement. Finally gas leaving is cooled to 350400 °C in a waste heat steam boiler downstream from the secondary reformer. Shift Conversion In order to increase the efficiency of the process there should be a very lower amount of carbon monoxide (CO) content where as the process gas leaving the secondary reformer contains 1215% of CO. It is reduced up to 3% on dry base in high temperature shift (HTS) conversion. For that gas is passed through a bed of iron oxide or chromium oxide catalyst at around 400 0C. The use of copper containing catalyst increases the conversion. The gas from HTS converter is cooled and passed through a Low temperature shift(LTS) converter, which has a bed of copper oxide or zinc oxide-based catalyst and operates at about 200-220 °C. The final amount of CO content is about 0.2-0.4% on dry base. Following reaction takes place in the shift conversion. Iron Oxide Catalyst CO (g) + H2O (g) CO2 (g) + H2 (g) H=-41 kJ.mol -1 Water Removal Iron catalyst used in ammonia synthesis can be oxidized due to the presence of water, carbon dioxide and carbon monoxide. Because of that those compounds must be removed using above mentioned shift conversion and following water removal and carbon dioxide removal methods. Water is removed by cooling the mixture up to 400C and allowing it to condense. The condensate may contain some amount of ammonia, methanol, and minor amounts of amines, formic acid and
    • acetic acid. These compounds can be stripped and recycled for more efficient process. The heat released during condensation can be used for the stripping of carbon dioxide, driving an absorption refrigeration unit and boiler water preheating. Carbon Dioxide Removal The process gas now contains mainly H2, N2 and CO2. CO2 can be easily removed by absorption process. Mono ethanol Amine (MEA), Activated Methyl Di Ethanol Amine (AMDEA) or potassium carbonate solutions can be used for chemical absorption. Glycol Di Methyl Ethers (Selexol) and propylene carbonate can be considered as physical absorption solutions. When MEA is used it needs high amount of regeneration energy and because of that it is not considered as an efficient process. Finally absorbed carbon dioxide is stripped and recycled back to use as a raw material for manufacturing of urea. Methanation Even after the shift conversion and CO2 removal, still there can be a trace amount of CO and CO2 which is poisonous for the ammonia synthesis catalyst. So in methanation process they are converted to methane by reacting with hydrogen. Following reactions take place in the reactor filled with nickel catalyst at about 3000C. CO (g) + 3H2 (g) CH4 (g) + H2O (g) CO2 (g) + 4H2 (g) CH4 (g) + 2H2O (g) Although methane is an inert gas in ammonia synthesis, water produced in the reaction oxidizes the iron catalyst used in ammonia synthesis. So water is removed in two stages at the downstream of the methanator and in ammonia synthesis loop by condensation. Synthesis of ammonia
    • Synthesis of ammonia is done by Haber-Bosch process. Following reaction takes place in the reactor when incoming gas (N2 and H2) is passed over iron catalyst at a pressure usually in the range 100-250 bar and at the temperature range of 350-5500C. N2 (g) + H2 (g) 2NH3 (g) H=-46 kJ.mol -1 NH3 The required pressure is achieved by centrifugal compression which is driven by a steam turbine. The required amount of steam for the turbine can be obtained from the steam produced in the ammonia plant itself. At this equilibrium conditions, the conversion of reactants are only 20-30% per pass. So unreacted gas is separated and recycled. The synthesis is typically carried out in a loop as shown in the diagram. Ammonia formed is separated from the recycle gas by condensation. The refrigeration compressor needed for this task is also driven by a steam turbine that uses steam produced in the plant itself. In order to maintain the pressure and to shift the reaction forward, fresh make up synthesis gas is supplied to the reactor. Synthesis loop can be designed in many different ways with respect to the points which make up gas is added and ammonia and purge gas is removed. . The best arrangement is to add the make-up gas after ammonia condensation and ahead of the converter. The loop purge should be taken out after ammonia separation and before make-up gas addition. This configuration is dependent on the make-up gas being treated in a drying step before entering the loop. [4]. since the reaction is exothermic there should be a method to remove excess heat to maintain equilibrium conditions. Ammonia produced in this process can be used as raw material for urea manufacturing process. 2.3.1.2) Steam reforming with excess air secondary reforming In conventional process the marginal efficiency of primary reforming is very low. So the process designed to reduce the primary reforming and to move some of duty to secondary reforming is known as Steam reforming with excess air secondary reforming. Following features can be observed in this process compared to a conventional process. Decreased firing in the primary reformer
    • Since the amount of heat supplied to the primary reformer is comparatively lower the process outlet temperature is also generally lower. This will increase the firing efficiency, reduce the size and the cost of primary reformer and prolong catalyst due to mild operating conditions. But the degree of primary reforming is reduced due to law supply of heat which results lower temperatures. Increased process air supply to the secondary reformer Since the degree of reforming is lower in the primary reformer there should be an increased amount of internal firing or combustion in the secondary reformer in order to achieve the same degree of overall conversion at the outlet of secondary reformer. The process air requirement is 50% higher than in the conventional method. Cryogenic final purification In this process all the methane and excess nitrogen is removed from the synthesis gas using a cryogenic purifier. The purified gas is very pure except a trace amount of argon. Purge gas from the synthesis section is also sent to this unit where it delivers an off gas for fuel. Lower synthesis gas inert level In the traditional process impurities such as CO and CO2 are removed by methanation. In this process a significant improvement of purity level can be achieved in synthesis gas. This leads to a higher conversion per pass and reduce purge gas flow which results a more efficient process than the conventional process. 2.3.1.3) Heat exchange auto-thermal reforming From a thermodynamic point of view it is wasteful to use the high-level heat of the secondary reformer outlet gas and the primary reformer flue-gas, both at temperatures around 1,000°C, simply to raise steam. Recent developments are to recycle this heat to the process itself, by using the heat content of the secondary reformed gas in a newly-developed primary reformer (gas heated reformer, heat exchange reformer), thus eliminating the fired furnace. Surplus air or oxygen-
    • enriched air is required in the secondary reformer to meet the heat balance in this auto thermal concept. When auto thermal reforming is used nitrogen dioxide emissions to the atmosphere is significantly reduced due to the elimination of flue gas in the primary reformer. 2.4) Process in general production of Urea The main principle of manufacture of urea has two main reactions. 2NH3 + CO2 NH2COONH4 -37.4 Kcal/gm mol NH2COONH4 NH2CONH2 + H2O + 6.3 Kcal/gm mol While going main reactions undesirable side reaction taking place is 2NH2CONH2 NH2CONHCONH2 (Biuret) + NH3 The synthesis is further complicated by the formation of a dimmer called biuret, NH2CONHCONH2, which must be kept low because it adversely affects the growth of some plants. The structure of these compounds is shown in Figure
    • Figure. Schematic representation of urea synthesis We can consider that the totally process which can be divided to fore sub processes. Synthesis Ammonia & CO2 are compressed separately and fed to the high pressure (180 atms) then a mixture of urea, ammonium carbamate, H2O and unreacted (NH3+CO2) is produced. Both 1st & 2nd reactions are equilibrium reactions. The 1st reaction almost goes to completion at 185-190 oC & 180-200 atms. The 2nd reaction (decomposition reaction) is slow and determines the rate of the reaction. 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). This liquid effluent is let down to 27 atms and fed to a special flash-evaporator containing a gas-liquid separator and condenser. Unreacted NH3, CO2 & H2O are thus removed & recycled. An aqueous solution of carbamate-urea is passed to the atmospheric flash drum where further decomposition of carbamate takes place NH2COONH4 2NH3 + CO2 The pressure is then reduced 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. 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 from 80 to 110oC to redissolve these crystals prior to evaporation. In the evaporation stage molten urea (99% w/w) is produced at 140oC. Granulation
    • Figure. Schematic representation of 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.
    • 2.5) Process variation of Urea manufacturing Once– through processes Process partial recycle processes Total recycle processes 1 Gas recycles process  CPI-Allied gas recycle urea process  Inventa gas recycle urea process 2 Liquid recycles process  Stamicarbon CO2 stripping urea process  Montecatini complete recycle urea process  Pechiney total recycle urea process  Inventa liquid recycle urea process 3 Gas / liquid recycle process  Mitsui Toatsu total recycle D improved urea process  Stamicarbon total recycle process  SNAM PROGETTI ammonia stripping urea process  Chemico total recycle urea process  Lonza- Lummus urea process The basic process chemistry of urea manufacturing is relatively simple. However, because operating parameters vary; particularly in the initial formation of the urea solution, numerous process designs have been utilized. Design differences occur in the separation and recycle of component streams. There are three major classes of urea processes, based on the type or quantity of recycle: once– through processes, partial recycle processes and total recycle processes. At least 75% of the urea produced today is by total recycle systems. 2.5.1) Once-through processes Figure below is generalized flow diagram of a once- urea process.
    • Figure. Once– through processes In this process liquid NH3 is pumped through a high pressure plunger pump and gaseous CO2 is compressed through a compressor up to the urea synthesis reactor pressure at an NH3 to CO2 feed mole ratio of 2/1 or 3/1. The reactor usually operates in a temperature range from 175 to 190 0C. The reactor effluent is let down in pressure to about 2 atm and the carbamate decomposed and stripped from the urea-product solution in a steam heated shell & tube heat exchanger. The moist gas, separated from the 85-90 % urea product solution, & containing about 0.6 tons of gaseous NH3 per ton of urea produced is usually sent to an adjacent ammonium nitrate or ammonium sulfate producing plant for recovery. An average conversion of carbamate to urea of about 60 % is attained. Excess heat is removed from the reactor by means of a low pressure steam-producing coil in an amount of about 280,000 cal/Kg urea produced. 2.5.2) Partial recycle processes Partial recycle processes as shown in figure bellow. This process is termed partial recycle because only excess ammonia is recovered and recycles to the reactor.
    • Figure. partial recycle processes The synthesis is carried out with as much as 200% excess ammonia. This process is also similar to that of the once through process, with one additional step : the reactor effluent contacting urea ammonium carbamate, water, and excess ammonia. Passes though an expansion valve reducing the pressure to a few hundred KPa depending on the particular process design. The steam goes to an ammonia separator where excess ammonia is removed, condensed, and recycled to the reactor. This is necessary to recover some of the cost of using excess ammonia. Also if passed directly to the carbamate decomposer, the excess ammonia could hinder the decomposition of the carbamate. The steam containing urea, cabamate and water goes to a carbamate decomposer which dissociates the cabamate to ammonia and carbon dioxide. The aqueous urea solution is separated and goes to further processing or shipment. 2.5.3) Total recycle processes The total recycle process is most widely used process in the urea manufacturing industry. There are three variations of the total recycle process. 1. Decomposed carbamate gases are separated and recycled in their pure states. 2. Carbamate solution is recycled to the reactor. 3. A combination gas/ liquid recycle may occur. 2.5.3.1) Gas recycles process
    • Figure. Gas recycles process The material leaving the reactor is a mixture of urea, ammonium carbamate, water, and excess ammonia. This stream goes to a decomposer which separates the carbamate into ammonia and carbon dioxide. The separated gases may both be recycled, or one may be purified at the expense of the other and returned to the process. In the partial recycle process, however, the excess ammonia is recovered and the ammonia or carbon dioxide in the unreacted carbamate is lost to the process. In the total recycle process, the entire quantity of ammonia is reused; i.e., excess plus decomposed carbamate ammonia. Two examples of the gas recycle process will be discussed; the CPI – Allied and inventa processes.  CPI -Allied gas recycle urea process Higher operating temperature (1940C -2330C) at 30.3 MPa and Conversion rate is 80%-85%. Ammonia and carbon dioxide feed entering with ration ratio- 4:1. The reactor products pass through an expansion valve to primary carbamate decomposer where 90% of the carbamate is flashed and stripped along with water vapor. The urea solution contains approximately 1.5% of the initial carbon dioxide feed. This stream is sent to an ammonia separator, where excess ammonia is stripped, and on to secondary decomposer where any remaining carbamate dissociates at atmospheric pressure.The overheads from both decomposers are passed through a two – unit series of absorbers where monoethanolamine (MEA) selectively absorbs carbon dioxide and water, leaving ammonia for recycle to the reactor. The carbon dioxide – rich solvent is sent to a stripper which thermally regenerates the MEA creating a rich carbon dioxide stream which is recycled to the reactor. The urea solution leaving the secondary
    • decomposer passes through a centrifugal Min-film evaporator unit. The product contains less than 0.7% biuret and 0.20% water. Figure. CPI-Allied gas recycle urea process  Inventa gas recycle urea process The Inventa process utilizes a reactor operating at 20.2 MPa and 180 0C to 2000C. The molar feed ratio of ammonia to carbon dioxide is 2:1 with a maximum carbon dioxide conversion to urea of 50%. The reactor effluent containing excess ammonia, ammonium carbamate, urea, and water passes through an expansion valve where it is lowered to 549 kpa and heated to 1200C in the carbamate decomposer. The ammonia and carbon dioxide go to an absorber where the ammonia is selectively absorbed and the carbon dioxide exits for recycle. The resulting ammoniacal solution of ammonium carbamate goes to a desorber to remove ammonia for recycle
    • Figure. Inventa gas recycle urea process 2.5.3.2) Liquid recycle process Figure. Liquid recycle process This process is similar to the gas recycle process except that the gases are condensed with the addition of water when needed, to form a carbamate solution for recycle. Those processes which will be discussed in this category are the Stamicarbon CO2 Stripping, Montecatini, Pechiney, and inventa processes.  Stamicarbon CO2 stripping urea process In the Stamicarbon CO2 Stripping urea process ammonia and carbon dioxide are reacted in the molar ratio of 2.4:1 to 2.9:1 at 1700C to 1900C and 12.1 MPa to 15.1 MPa . The reaction product (1850C, 14.1 MPa) goes immediately to a high pressure stripper.
    • Operating at 14.1 MPa and 1900C (1), where the reactor stream is stripped by incoming carbon dioxide. The stream containing 15% unconverted carbamate is then let down for further decomposition in the low pressure decomposer operating at 300 kpa and 1200C. the ammonia and carbon dioxide are condensed in the low pressure to the high pressure condenser where it combines with the off-gas from the high pressure stripper and a split from the ammonia feed line. The condensed stream from the high pressure condenser operating at 1700C and 14.1MPa, goes to the reactor. An equivalent amount of 345 kpa steam is produced in the high pressure condenser and is used in other sections of the plant. This process claims ammonia and carbon dioxide consumption of 0.57 metric ton and 0.755 metric ton per metric ton of urea produced, respectively. Conversion efficiencies for ammonia and carbon dioxide are 65% to 85% and 70% to 85%, respectively. Figure. Stamicarbon CO2 stripping urea process  Montecatini complete recycle urea process Montecatini process ( Montedison) in which preheated liquid ammonia and carbon dioxide are compressed to 20.2 MPa and enter the reactor operating at 1950C (12,16). The reactor mole ratio for NH3:CO2 is 3.5.1;for H2O : CO2 it is 0.6.1 (17).the effluent containing urea, excess
    • ammonia, ammonium carbamate, and water enters a first – stage decomposer/separator operating at 8.1 MPa and 1850C. In this decomposer/ separator most of the ammonia is driven off along with the carbamate decomposition products. This stream, along with 20% to 30% of the carbon dioxide feed stream, is fed to the first – stage carbvamate condenser which operates at 8.1 MPa and 1450C The effluent form the first – stage condenser passes to an auxiliary condenser operating at the same pressure but at 1150C so that condensation is completed. The gas leaving this condenser is washed to remove ammonia. The liquid stream is recycled to the reactor. The liquid stream leaving the first – stage decomposer/ separator proceeds to a second- stage unit operating at the same temperature as stage one and 1.2 MPa, and finally to a third stage operating at 202kpa to 303 kpa before leaving the facilities. The gaseous effluents from the stage two and three decomposer/ separators are condensed in carbamate condensers three and four, respectively. In condenser three the gas stream is mixed with liquid effluent from both wash vessels and the liquid carbamate is sent to the first stage condenser. In condenser four an ammonia bearing gas stream from the solidification section is washed with cold ammonium carbamate, and the resulting effluent is cycled through a wash vessel before going to condenser three.
    • Figure. Montecatini complete recycle urea process  Inventa liquid recycle urea process Figure. Inventa liquid recycle urea process 2.5.3.3) Gas/Liquid recycle process Figure. Gas / liquid recycle process
    • It is characterize by ammonia recycle with carbon dioxide being recycled in the form of carbamate. The following processes of this type will be considered: Mitsui Toatsu (Total Recycle D Improved) , Stamicarbon, SNAM PROGETTI ,chemico, and Lonza-lummus.  Stamicarbon total recycle process The stamicarbon total recycle process is shown in below the reaction take place at 20.2 MPa and 1700C to 1900C. The reactor effluent is lowered to approximately 505 KPa before going to the preseparater. The liquid stream form the preseparater passes through to additional separation steps before finally leaving the process. The various ammonia and carbon dioxide streams are condense, and the carbomate formed is recycle to the reactor. A wet scrubber is used on the gas stream to recover ammonia for recycle. Figure. Stamicarbon total recycle process  SNAM PROGETTI ammonia stripping urea process The SNAM PROGETTI urea process ,as shown in below is similar to the stamicarbon CO2 stripping process , but the stripping is done by ammonia rather than carbon dioxide. The process
    • as shown can operate at two different reactor pressure, 13MPa to 16MPa or 20.2MPa to 25 MPa. Operating temperature is 180C to 190C.malar ratio is 3.5:1. The effluent leaving the reactor is passed to a stripping operating at 10 MPa to 15MPa and 160C to200C. Most (>90%) of the ammonia and carbon dioxide are removed in the stripper with the remainder being removed in the flash separator. These overheads are collected and the cabomate is recycled; excess ammonia is also recycled to storage. The unique feature in the SNAM PROGETTI process is the cabomate ejector which introduces carbomate and ammonia to the reactor. The ammonia pressure drop through the ejector of 41MPa supplies the necessary driving force . Figure. SNAM PROGETTI ammonia stripping urea process
    • 3) ECONOMIC ASPECTS OF UREA PRODUCTION
    • 4) ENVIRONMENTAL AND SAFETY ISSUES 4.1) Health and Safety 4.1.1) Health Ammonia is a material that is widely used in chemical industry and it has its own toxicity, health issues and safety measures that require attention. Therefore it is required for the designers of ammonia and derivatives plant to have a clear understanding about these aspects of ammonia. In analyzing the effects of releases of ammonia the toxicity of ammonia should be considered. The threshold limit value (TLV) for ammonia is 25 PPM (TWA), short term exposure limit (STEL) 35 PPM and initialism for Immediately Dangerous to Life or Health (IDLH) 300 PPM [51]. Ammonia gas is irritating to skin, nose, eyes, throat, respiratory tract and mucous membranes. Even though at lower concentrations below TLV there has been no evidence of affect to lung functioning. Various concentrations of ammonia gas cause following health concerns. Concentration 400-700 PPM Affect Severe eye and respiratory irritation. Potential Permanent damage 1700 PPM Convulsive coughing and bronchial spasms Exposure more than half an hour can be fatal 2500 PPM Can be life threatening 5000-10000 PPM Death by suffocation [36], [52] Ammonia vapor can be extremely irritant to eyes and mists or liquid ammonia can course permanent blindness. Cryogenic burns can be coursed by liquid ammonia. Ammonia has not been found to be carcinogenic or has the ability to reproductive or developmental toxicity in human body. [36], [52]
    • 4.1.2) Safety When the safety of ammonia and its derivatives plant is considered, a common accident that has occurred over the years is accidental releases of ammonia. Metals and alloys of particularly copper and zinc are extremely vulnerable to corrosion when exposed to ammonia. In general ammonia can be stored in iron or steel containers, transferred through iron and steel piping and fittings. [36]. It should be noted that under certain conditions with few specific steels ammonia can produce embrittlement [35]. Therefore it is required to follow strict guidelines when choosing materials to avoid releases of ammonia liquid or gasses to the atmosphere. Ammonia can flash in case of loss of containment when ammonia is stored under refrigerated conditions or liquefied gas under pressure. Quantity flashed will depend on the temperature conditions [35]. Even though ammonia has lower density than air, the resultant clouds formed by flashing are found to have higher densities than air. This will result in ground level ammonia clouds [54]. Past events In order to get an idea about the safety measures that should be taken when designing an ammonia and derivatives plant, it is required to have an understanding about the incidents that have occurred with the presence of ammonia and in ammonia production. In 1976, a release of 19 tons of ammonia from a road tanker in Texas, USA caused 6 deaths. It should be noted that just after 2.5 hours the levels of ammonia has returned to the background levels.[39] [40] Even though there were no human fatalities, a massive 600 ton spill of ammonia to a watercourse, caused a severe environmental hazard in Arkansas, USA in 1971 killing thousands of fish [59]. The reason for the spill was accidently filling a tank with warm ammonia. The warm ammonia transferred to the tank from the bottom, formed a layer at the bottom. Afterwards this layer suddenly rose to the surface due to higher vapor pressure of the warmer ammonia causing the tank to burst. A very important fact to note is that with ammonia, brittle fractures can occur in metal containments. Brittle fractures are catastrophic since the fracture can propagate at a velocity
    • closer to the velocity of sound. What appears to be the worst accident involving ammonia is the release of approximately 38 tons of ammonia which caused 18 deaths. The release was due to a brittle fracture in an ammonia tank. The accident occurred in Potchefstroom, South Africa in 1973. The post incident investigation revealed neither over pressure nor over temperature nor other triggering event. The fracture started in a carbon steel dished end. The minimum transition temperatures were 200C for fragment and 1150C for remaining part of the dished end. The investigation showed that the operating conditions were below these temperatures which made the material brittle. A gas cloud of 150m in diameter and 20m deep was resulted from this failure. [39] [58]. Another type of accidents that has occurred in ammonia production plants over the years is vapor cloud explosions. This is due to sudden releases of hydrogen rich synthesis gases being ignited after a while. The flammable region of these clouds is spread from 4%-74% v/v. Due to the diffusivity, these vapor clouds are not often formed in unconfined or semi-confined areas. But in confined spaces, (eg: compressors) ignition of such a vapor cloud is catastrophic. Another possible, yet uncommon type of accident is ammonia explosions. Ammonia explosions are rare because of the unusually high, lower explosive limits (LEL-16%, HEL- 25%). The auto ignition temperature of ammonia is about 6500C. There are no reports of ammonia explosions in unconfined spaces due to the fact that it is not easier to get a 16% concentration in open air. In Oklahoma, in 1978, a refrigeration system failed causing the ammonia storage to warm up. The pressure eventually rose and the pressure relief valves discharged ammonia, which was ignited by a nearby flare. In New Zealand in 1991, a welder was killed due to an explosion occurred when he was welding a tank which supposed to be empty, yet which was later found out to have a flammable mixture of ammonia vapor and air.[16] Common avoidable mistakes In the famous book ―What Went Wrong‖ by Trevor Kletz, it has been identified that many of the failures have occurred due to the use of wrong material other than the one that has been specified. There has been one incident where the converter has been pushed over due to the reaction forces from a hydrogen leak out. The reason for the leak out is the use of a carbon steel
    • exit pipe for the converter instead of 1.25% Cr, 0.5% Mo alloy, in which a hole was created due to the hydrogen attack [16] An interesting investigation was done to check the materials delivered to a new ammonia plant. 5480 items (1.8% of the total) was found to delivered in the wrong material. This included 2750 furnace roof hangers. Had this investigation been not done, the roof would probably have failed. [16]. A specific reason has not been found for the delivery of wrong materials (Can be cost cutting or unavailability). Therefore the engineers are advised to check the items before they are used in the plant to avoid severe accidents in future. [16] 4.2) Safety Procedures It should be noted that not all the failures, problems or hazards can be anticipated. Events such as those are eye openers for everyone, reminding that never be complacent with safety and the importance of following the procedures to prevent accidents. One such widely accepted procedure is ―Inherent safety design‖. 4.3) Inherent Safe Design Inherent safety is an approach to process design and operation which builds in safety, health and environmental considerations at the start. This is done to ensure that even if something goes wrong, the level of danger is minimized. Practically it is impossible to have an inherently safe design. But one can have an inherently safer design. ―What you don’t have can’t leak!‖ - Trevor Kletz, (Author: What Went Wrong) There are four guidewords that drive inherent safety. Substitute: Replace hazardous substances and procedures with less hazardous substances and procedures Minimize: Use as less as possible hazardous materials when it is not avoidable. Perform hazardous procedures as few times as possible. Moderate: Use hazardous materials in the least hazardous forms and identify processing options that involve less severe process conditions.
    • Simplify: Finish the design, processing equipment and procedures to eliminate the chances of errors by eliminating excessive use of safety features and protective devices. Less equipment of any kind means that there is less to go wrong. 4.4) Layers of Process Safety Inharrent Passive Active Procedural Inharent Safety: Elimination of hazards from the design Passive Safety: Protection to the design so that it cannot be easily changed Eg: process conditions Active Safety: Prevent. Eg: high level trip isolates flow into a tank before it can overfill Control. Eg: A restrictive orifice plate limits the rate of loss of containment if a line fails. Mitigate. Eg: heat activated links open deluge valves to spray water in case of fire Procedural Safety: Risk management systems Eg: company policies, site rules, operating procedures, training, maintenance, test procedures, emergency response plans
    • There will be a conflict of interest when implementing inherent safety concepts due to many reasons like workload, motivation, complexity, time, communications, organizational structure, cost cutting, poor working environment etc. It should be strictly noted that alarm systems are less complicated and work properly to make sure that alarms are not ignored due to the fact that they are often activated due to malfunctioning. 4.5) conclusions 4.6) recommendations 4.7) Environment
    • 5) GREEN CONCEPTS USED IN PLANT DESIGNING Green concepts are used frequently in industrial plant designing recently, as the whole world is aware of the sustainability of the environment and the energy sources. These concepts mainly addresses the issues with renewable energy sources, minimizing the pollutants which affects the environment, Life cycle management of each raw material used, Efficient usage of natural resources and protecting occupational health of the employees. In a green building design, application of green concepts comes throughout the designing stage to maintenance stage. This focuses on energy and resources efficiency, overall impact to the environment, indoor environment quality and any other sustainable concepts (1). Life cycle of each material used will be focused to make its’ use maximum and impact to the environment minimum. Green building has the potential to save 30%-40% of energy while reducing the operating cost and improving good health and a comfortable environment to work in (2). 5.1) why should we care about green buildings? For last 20-30 years, mankind faced bad experiences with global warming, ozone depletion, resource depletion, energy scarcity, ecological toxicity, humantoxicity, acid rains etc. (1). These have made them think about how they use the environment for their uses. Although it’s really hard to stop the environment impact under current circumstances, idea of green industries is to minimize the effect to the environment done by human activities. There are lots of tangible and intangible benefits that an industry would get by applying green concepts into its design (3).  Tangible benefits o Sustained savings  Energy saving- 30%-40%  Water saving-20%-30%  Reduction in initial investments o Reduce operational costs o Optimize life cycle o Waste minimization
    •  Intangible benefits o Reduce the impact to the environment o Enhance occupant comfort o Improve productivity of occupants There are several aspects of green building concepts which can be taken to our consideration. 1. Site design, Preparation and development 2. Resource efficiency a. Energy efficiency b. Water efficiency c. Other resources efficiency 3. Impact to the environment a. Carbon foot print b. Waste minimization c. Life cycle management 4. Indoor environment quality 5.2) Current methods used in industries Main areas addressed in the effort are Search for renewable energy sources Main renewable sources considered currently are o Solar power PV cells are used in different scales to create electricity from solar rays. Usage of these has being increasing, but still the initial investment and the payback period are problems related with this option. Still this stands as one of the main renewable options to go with mainly for countries as Sri Lanka. o Wind power
    • This is the other main energy source considered as a renewable option for industries. Companies have widely tried to get wind power to their plant designs. Optimize current energy consumption This is the best starter for a current industry to go green. Most of the factories have their problems in consuming energy in an optimum way, so that they will surely find their energy demand decreased by concerning on the current energy plan. Break-up of energy consumption in an industrial building(3) Most of the energy consumed in industrial buildings are for ventilation and lighting. So, most common way to save electricity is through upgrades to more energy-efficient lighting systems, the use of energy management systems and upgrade to their HVAC system. Minimize greenhouse gas emission
    • Minimizing the carbon foot print on earth is the other main area stressed with green concepts. For this, most industries have started acting on minimizing the emission of greenhouse gasses. As CO2 being the main greenhouse gas produced in industries the impact you do to the environment is measured as CO2. Optimizing the usage of natural resources, waste treatment and life cycle management Usage of natural resources as water and other raw materials takes a major role. Management of life cycle of each raw material will help to minimize the waste and improve the productivity of natural sources.
    • 6) HISTORY OF UREA PRODUCTION IN SRI LANKA There many views being aired about the Fertilizer and the Fertilizer plant that was closed down in 1986. The plant was constructed by British Company to manufacture 980 tons of urea per day at a cost 250 million US $. Feedstock used for the process was Naphtha (fuel even lighter than petrol), which is a byproduct of the oil refinery at Sapugaskanda. Naphtha has a severe demand in the world market for making many petroleum products. CPC invariable preferred to sell naphtha to foreign markets for obvious reason mentioned below. In Middle East and in most countries Urea is manufactured using Natural gas, which can be tapped easily in oil producing countries. Therefore it is a known fact that urea manufacture using Natural gas is much more economical than using Naphtha. Under these circumstances Naphtha based plants could not compete with natural gas plants particularly in a country like Sri Lanka where there was only one oil refinery. Since our cost of production was extremely high treasure could no settle the feedstock bills to refinery. CPC in return curtailed the live wire to Urea plant as the Government was of the view that it would be cheaper to import urea and sell naphtha to foreign market thereby eliminating the burden on the treasury. The other factor that contributed to uneconomical operating cost was the regular breakdown in the machinery and the lack of trained staff. Engineers and the Technicians who were trained abroad for this specialized industry were gradually leaving for stabilized industries with the development of uncertainty in the organization. Liberalized policy of the then government did not want the staff to be retained by force. For practically all the spare parts and equipment we had to depend purely on foreign suppliers. Probably a country like India could manage Naphtha based plant because of the many refineries that they possess and also because of their welldeveloped infrastructure. India not only could manufacture their own spares but also they could manufacture all the required plant equipment.
    • Chlorophyl – C55H54N4Mg - Photosynthesis Mol Wt – 55x12 +54x1+14x4+24x1 = 794 N – Wt % = 56/794 = 7.05% C- = 660/794 = 83% Mg - = 24/794 = 3.02% Cellulose – (C12H10O5)n = 234 : C – 61% 2NH3 + CO2 ------- (NH2)2CO + H2O At STP 1 gmol of a gas occupies 22.4 litres of volume At STP 3 gmol (78 g) of a gas occupies 67.2 litres of volume At 600 Bar pressure 67.2 litres will contain - 600x3gmols (23,400g) Atmosphere – 0.03% CO2 Urea – CO(NH2)2 N – 28/60 – 46.7% C – 12/60 – 20% (NH4)2SO4 – 28/132 – 21% NH4NO3 – 28/80 – 35% MSDS of Methane, Ammonia, Urea, CO, CO2 Demand for Natural Gas – CH4 + 2O2 ---- CO2 + 2 H2O 16 44 Cal Value – 16,000 + kcals/kg Other hydrocarbons – LPG - 12,000 + kcals/kg Gasoline - 12,000 + kcals/kg
    • 7) PROCESS SELECTION AND DESIGN DECISIONS Cost effective study of raw materials for production of Hydrogen Considering the literature review done on existing processes to produce Hydrogen, following raw materials are considered as possible major options for the production process of Hydrogen for manufacturing of Ammonia for Urea. 1. 2. 3. 4. Natural Gas Heavy oil Coal Water/Alkaline electrolysis 1.Natural Gas Currently natural gas is nor produced, imported or consumed in Sri Lanka. Although there are new prospects of natural gas wells in Sri Lanka, unavailability of natural gas is the main challenge with Natural gas. So we assume importing of natural gas which is the only option if we are going with this option. CH4(g) + H2O(g) CO(g) + 3H2(g) Production cost Price of natural gas in international market Density = Rs 53,300/103m3 =0.5kg/m3 = (Rs 53,300/103m3)*6/(16*0.5) Price to produce 1kg of H2 = Rs.79.50/kg of H2 Shipping cost Shipping cost of avg 2.8kg of natural gas = Rs. 39/2.8kg of natural gas (which is equalant to 1kg of H2 ) Total cost for 1kg of H2 = Rs.118.50/=
    • Heavy fuel oil Heavy fuel will be bought from petroleum corperation. C50H102(l) + 25O2 (g) 5CO(g)+51H2(g) Price per 1liter of Heavy fuel oil in Sri Lanka = Rs. 90/= Avg. specific gravity of heavy oil = 0.95 Price to produce 1kg of H2 = Rs.(90*702)/(102*0.95) = Rs. 648.88/= Coal Although producing hydrogen from coal is a matured industry, it is not considered as a cost effective option for the countries which don’t have coal. C(s)+ H2O(g)+heat CO(g)+H2(g) This process need very high temperatures and considering the unavailability of coal in Sri Lanka and the electricity consumption, this is not considered as a favorable option. 4.Water/Alkaline electrolysis H2O + electricity H2 + 1/2O2 With non-renewable fuels been decaying, cost forecast for electrolysis has shown good signs with the statistics.
    • With above factors, this can also be considered as a possible candidate.But the high electricity consumption and lack of R&D in industrial scale have hold this option from being a cost effective option for the process, for Sri Lanka. When above analysis is considered, the best cost effective option seems to be natural gas. Natural gas has a high calorific value, which attracts the attention as a potential fuel. This leads the designers to be considerate about the future prices of natural gas. In this regard, there were two forecasts referred. Both of the forecasts are done by United States Energy Information Administration (EIA). Forecasts were done from 2009 – 2030 time period which is likely to be the core operating time period of the proposed plant.
    • The first graph is of the price forecast of crude oil until 2030. Heavy oil is the heavy facture of the crude oil. If the liquid fuel consumption is broken down to sectors it can be clearly seen that the major consumption is for transportation (i.e. the consumption of gasoline and diesel). Therefore it is safe to assume that the world will focus on producing light fuels even from the heavier fractions.
    • When taking a look at the current price of a barrel of crude oil (2013 ≈$95) it is clear that the predictions are almost at reference prediction. Furthermore, the forecasted price varies from $50-$200. In contrary the natural gas price tend to increase by maximum of $4 and seems very stable. Therefore the overall conclusion is to use natural gas as a raw material. According to the economical analysis mentioned above, it can be seen that using natural gas as the feed is more cost effective than using heavy oils or coal. Because of that ―Steam reforming‖ process should be used for production of ammonia. New technologies can be used along with the conventional steam reforming technology in order to increase the efficiency and thereby to reduce the overall operating cost. Although the process layout is identical to the process that has been used for decades, the performance is expected to be significantly improved due to incorporation of modern technologies with the proposed plant. Two main areas that have significant impact on the performance and the cost of an ammonia production section are the reforming section and the synthesis section. Improvements for those sections have been a major concern in this process design. Following improvements are basically considered in designing the process for ammonia synthesis. 1) Reducing the duty of primary reformer In every reforming ammonia process the reformer furnace and the flue gas duct represents about 25% of total investment cost of ammonia production plant.[1] Because of that the concept of reducing the size of primary reformer and shifting duty to secondary reformer or another heat exchange reformer is very important. Although there are so many methods to achieve this such as Braun Purifier Process, ICI AMV process and Foster Wheeler process etc, The Topsoe Process is selected in this design which is considered as the latest and energy efficient process [2]. According to that process the size of the primary reformer can be reduced by following steps Installation of pre reformer upstream to the primary reformer Natural gas basically contains methane. But it may contain varying amount of other hydrocarbons depending on the source. In the pre reformer all higher hydrocarbons are converted in to a carbon oxides, hydrogen and methane. When a pre reformer is installed the primary
    • reformer has to reform methane only. Adiabatic reforming can be used for steam reforming of feed stocks ranging from natural gas to heavy naphtha. Because of that the process also becomes more flexible. In case of shortage of natural gas in Sri Lanka, Naphtha can be used as the feed stock which can be obtained as a byproduct of petroleum refinery. At the same time pre reforming catalyst will pick up any sulphur components in the feed and will allow much higher heat flux in the primary reformer. The pre reformed feed can be reheated to 650°C before entering the primary reformer. This will result in reduced firing in the primary reformer, and thereby reduced fuel consumption [3]. Figure 1- Installation of Pre reformer Installation of heat exchange reformer downstream of the secondary reformer The temperature of the flue gas emitted from a conventional primary reformer is usually about 9000C and the process gas at the outlet of the secondary reformer is also at around 10000C. From a thermodynamic point of view it is waste of energy to use this high amount of heat simply for
    • raising steam and preheating process air for the secondary reformer. The boiling temperature in a 125bar main boiler on the secondary reformer outlet is only 3250C. According to the Haldor Topsoe process a heat exchange reformer unit which is named as HTER-p (Haldor Topsøe Exchange Reformer) can be installed at the downstream of the secondary reformer. The HTER-p is heated by the process gas exit from the secondary reformer, and thereby the waste heat normally used for High pressure steam production can be utilized for the reforming process down to 750–850°C approximately. Operating conditions in the HTER-p are adjusted independently of the primary reformer in order to get the optimum performance of the overall reforming unit. As shown in the diagram around 20% of the natural gas feed can be by-pass the primary reformer according to this concept. [3] Figure 2- Arrangement of Heat exchanger reformer unit 2) Pre heat combustion air for the primary reformer and process air to the secondary reformer. Although the duty of primary reformer is reduced using new technologies, the flue gas from the primary reformer is expected to be at a temperature higher than 650 0C. This heat can be recovered to preheat combustion air for the primary reformer up to about 4000C and to pre heat the process air for the secondary reformer. Rest of the heat that is needed to heat process air up to
    • 6000C can be recovered from the gas emitted from the secondary reformer. Any remaining heat from the flue gas can be used to preheat fuel to 1000C and to pre heat boiler water. According to the energy efficient Topsoe process permitted stack temperature is lowered to 100 0C using about energy recovery methods. 3) Raising steam using waste heat steam boiler at the downstream of the HTER-p unit The process gas at the outlet of the HTER-p is around 750–850°C. Temperature should be reduced up to 350-4000C to feed the High Temperature Shift converter unit. This heat can be recovered to raise high pressure steam which can be utilized in other heating requirements of the plant. 4) S-350 Ammonia Synthesis Loop produced by Haldor Topsoe There are various typical arrangements of ammonia synthesis loop based on the point at which the fresh make up gas is delivered and ammonia formed is condensed. Since make up gas is absolutely free from catalyst poisoning agents such as sulphur, water and carbon dioxide can be directly sent to the converter. After that ammonia leaves the converter can be condensed out by cooling. This can be considered as the most favorable arrangement in the minimum energy point of view because it results the lowest ammonia concentration at the converter and highest ammonia content at the condensation. S-350 ammonia synthesis loop is the latest development of ammonia synthesis technology with related to the above configuration. As shown in the diagram it contains two ammonia converters which are known as S-300 converter follows by S-50 converter. This will increase overall ammonia conversion than conventional loops. S-300 converter is the most updated version of radial flow converter which is designed to increase the conversion with reduced catalyst volume. [4] In addition this increased conversion will results improved steam generation. Considering above facts S-350 synthesis loop is proposed to incorporate in the plant design.
    • Reasons for selecting ACES 21 process ACES21 Process Advanced process for Cost and Energy Saving (ACES) urea production is one of the latest technologies which were introduced by TOYO Engineering Corporation, Japan. Newest version of ACES was currently used in many urea plants worldwide. (ACES 21) ACES 21 advanced technology for urea production ascertain initial investment cost and energy consumption and other operating costs are lower compared to other technologies. Operating conditions of the urea synthesis reaction have been optimized under lower temperature and lower pressure than normal conditions without reducing reaction rate significantly. As a result, energy consumption and other related operating costs were reduced noticeably. Synthesis of Urea by ACES 21 Advanced Process ACES21 process synthesis section consists of three sections as follows, 1. Reactor 2. Stripper 3. Vertical Submerged Carbamate Condenser Reactor Liquid ammonia is fed to the reactor via the HP Carbamate Ejector which provides the driving force for circulation in the synthesis loop instead of the gravity system of the original ACES. The reactor is operated at an N/C ratio of 3.7, 182 °C and 152 bar. The CO2 conversion to urea is as high as 63% at the exit of the reactor.
    • Following reactions were carried out in the reactor, NH2COONH4 NH2CONH2 + H2O : ΔH = +23 kJ/mol 2NH3 + CO2 NH2COONH4 + heat : ΔH = -84 kJ/mol Stripper Urea synthesis solution leaving the reactor is fed to the stripper where unconverted carbamate is thermally decomposed and excess ammonia and CO2 are efficiently separated by CO2 stripping. The stripped off gas from the stripper is fed to the Vertical Submerged Carbamate Condenser. Part of original CO2 is fed to the stripper as stripping agent. The rest carbon dioxide is supplied to the synthesis reactor. Vertical Submerged Carbamate Condenser Vertical Submerged Carbamate Condenser (VSCC) operated at an N/C ratio of 3.0, 180°C and 152 bar. Ammonia and CO2 gas condense to form ammonium carbamate and subsequently urea is formed by dehydration of the carbamate in the shell side. Reaction heat of carbamate formation is recovered to generate 5 bar steam in the tube side. A packed bed is provided at the top of the VSCC to absorb uncondensed ammonia and CO2 gas into a recycle carbamate solution from the MP absorption stage. Inert gas from the top of the packed bed is sent to the MP absorption stage. Medium Pressure Absorber Ammonia and carbon dioxide separated from urea solution in medium pressure decomposer are recovered in medium pressure absorber. Then condensation heat is transferred to the aqueous urea solution feed in the final concentration section. 2NH3 + CO2 ↔ NH2COONH4 ; ΔH = -84 kJ/mol
    • Low Pressure Absorber Ammonia and carbon dioxide separated from urea solution in low pressure decomposer are recovered in low pressure absorber. Heat is released from the reaction inside low pressure absorber. That heat is used to produce steam at 2bar. This steam is used for the evaporation process of lower and upper separator. 2NH3 + CO2 ↔ NH2COONH4 ; ΔH = -84 kJ/mol Flash Separator In this unit, water is evaporated by reducing pressure in order to concentrate the urea solution. This unit is operated at 1.0 bar and 110 °C. H2O(l) → H2O(g) Lower Separator In this unit, purified urea is further purified. The head required for this process is taken from steam produced in low pressure absorber. This is operated at 0.55 bar vacuum pressure and at 110 °C. Calendria evaporator is used for this. Upper Separator Urea solution coming from lower separator is further concentrated in this unit. This is operated at 0.55 bar vacuum pressure and at 112 °C. 99.2% pure urea can be obtained from unit.
    • Granulation Plant In here the basic principle of the process involves the spraying of the melt onto recycled seed particles circulating in the granulator. These seed particles gradually increase in size as the process continues. This is operated at 110-115oC and at slightly negative pressure. Then the heat of solidification is removed by cooling air to about 90ºC. REFERENCES