Introduction to urea plant design – preliminary design...
German University of technology in Oman Department of Engineering – Process Engineering Plant Engineering Semester VII
Project 1 Introduction to urea plant design – preliminary design
Done by
ID
E-mail
Ali Sabt
11-0156
[email protected]
Abdullah Al Riyami
11-0069
[email protected]
Faisal Al Busaidi
12-0004
[email protected]
Supervisor: Dr Ayham Al Rahawi Date: 3 October 2014
Summary
Summary Urea is one of the most essential fertilizers used in agricultural that is efficient and relatively cheap. Being first discovered in Urine of mammals in 1773 by Rouelle, urea went for mass production on an industrial scale by hydration of Cyanamid process in the early 1900s. Major urea exporters produce it commercially from two raw materials, ammonia and carbon dioxide. Carbon dioxide is produce during the manufacture of ammonia from coal or from hydrocarbons such as natural gas. The Sultanate of Oman has already begun to manufacture urea for the local use and export to other countries like India and Australia. The plant located in Sohar, is under the Suhail Bahwan group and it operate according to the Snamprogetti process. The location of any manufacturing plant is dependent on many factors including the availability of resources and facilities, market accessibility, availability of labor, security of location and governmental influence. For a urea manufacturing plant, it is best to locate it near an ammonia manufacturing plant where ammonia (product) and carbon dioxide (byproduct from burning fuel) could be easily delivered and used as the main raw materials in producing urea. The basic chemistry behind urea manufacturing is relatively simple. The reactants under high pressure undergo two main reactions before urea is produced; the first one is exothermic while the other is endothermic. The difficult part is the design of a total recycle urea manufacturing process under carefully controlled operating conditions and in the same time keeping the manufacturing costs to minimum. The total recycle process is the most efficient and effective method to manufacture urea which could achieve a purity over 98% when it is compared to once through recycle and partial recycle processes. The total recycle process is classed to three main sections; the gas recycle, liquid recycle and gas/liquid recycle processes. Modern plants nowadays operate according to the gas/liquid recycle process. This concept has been developed to suit industrial scale by several people like Stamicarbon and Snamprogetti which caused design differences. The process in this report to be discussed in more detail is the Snamprogetti ammonia stripping process which our selected process. We have chosen this process mainly because the same concept has been used in the urea plant in Sohar. Our aim is to produce urea prills of 98% purity. Further details on mass and energy balance in addition to equipment, location and prices will be discussed in the following projects.
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Table of Contents
Table of Contents
1.
Introduction .......................................................................................................................................... 4 1.1 Properties of urea ............................................................................................................................... 4 1.1.1 Chemical properties ..................................................................................................................... 4 1.1.2 Physical properties ....................................................................................................................... 5 1.1.3 Uses of urea ................................................................................................................................. 5 1.2 World production................................................................................................................................ 5 1.3 World demand .................................................................................................................................... 6 1.4 Oman urea demand/Production/Prices.............................................................................................. 7
2.
Plant location ........................................................................................................................................ 8
3.
Raw materials........................................................................................................................................ 9 3.1 Ammonia ............................................................................................................................................. 9 3.2 Carbon dioxide .................................................................................................................................... 9
4.
Urea manufacturing processes ........................................................................................................... 10 4.1 Total recovery urea manufacturing process block diagram ............................................................. 10 4.2 Urea processes .................................................................................................................................. 12 4.3 Once through recycle process........................................................................................................... 13 4.4 Partial recycle process ...................................................................................................................... 14 4.5 Total recycle processes ..................................................................................................................... 14 4.5.1 Gas-recycle processes ................................................................................................................ 15 4.5.2 Liquid recycle processes............................................................................................................. 17 4.5.3 Gas/Liquid recycle processes ..................................................................................................... 19
5.
Selected process: Snamprogetti ammonia stripping process ............................................................. 22
6.
Mass balance figure and plant capacity .............................................................................................. 24 6.1 Selected plant capacity ..................................................................................................................... 25 6.2 Composition of the final product: ..................................................................................................... 25
References .................................................................................................................................................. 26
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Introduction
1. Introduction Urea is an organic compound, chemical formula CO(NH2)2, that is essential to the agriculture industry due to it nitrogen- rich fertilizer. Its utilization is increasing being the best nitrogen fertilizer worldwide. It is used in solid fertilizer, liquid fertilizer, formaldehyde resins, and adhesives. Urea was first discovered in Urine of mammals in 1773 by Rouelle. After this discovery in 1828, Woehler was first to synthesize it from ammonia and cyanic acid which is considered to be the first synthesis of an organic compound from an inorganic compound. In 1870, Bassarow produced urea by heating ammonium carbamate in a sealed tube in what was the first synthesis of urea by dehydration. In the early 1900s, urea went for mass production on an industrial scale by hydration of Cyanamid process. When the process of ammonia production was developed by Haber and Bosch in 1913, urea production grew rapidly from ammonia and CO2. Urea, also known as carbamide, is an organic compound consisting of two –NH2 linked to a carbonyl (C=O) functional group giving it a chemical formula of CO(NH2)2. The chemical formula of urea indicates that it can be considered to be the amide of carbamic acid NH 2COOH, or the diamide of carbonic acid CO(OH)2. Urea has the highest nitrogen content between all of solid nitrogenous fertilizers and more the 90% of urea production, worldwide, is used for nitrogen fertilizer. Therefore the cost of transportation of urea might be the cheapest in the industry.
1.1 Properties of urea This section will cover some physical and chemical properties of urea in addition to some of its uses. 1.1.1 Chemical properties
Molecular weight…………………….60.05 Maximum Nitrogen content……46.6% Specific gravity………………………..1.335 Heat of fusion………………………….60 Cal/gm (endothermic) Heat of solution (in water)……….58 Cal/gm (endothermic) Bulk density……………………………..0.74 gm/cc Relative Humidity……………………..60% Specific heat of urea at different temperatures in the table below
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Introduction
Temperature (oC) 0 50 100 150
Specific Heat (KJ/kg oC) 1.398 1.66 1.89 2.11 Table 1: Specific heat of urea
1.1.2 Physical properties
Urea is a white, hygroscopic solid, odorless, and it is non-corrosive. 1.1.3 Uses of urea
Almost 90% of urea manufacture is used as fertilizer. Urea –formaldehyde resins are widely used as adhesive for plywood. Melamine-formaldehyde resins are also used to make extra hard surfaces.
1.2 World production Urea is a very important chemical in the agriculture industry. It is the world most regularly used nitrogen fertilizer. Urea has the highest mass manufacture than any other organic chemical. It contains 46% nitrogen, which ranked it as the most concentrated nitrogen fertilizer. The world produces in excess 151,000,000 tons of Urea on an annually basis, and 90% of it is used as a fertilizer. World
151 million tons
Europe
10 million tons
North America 10 million tons Asia
94 million tons
FSU
11 million tons
Table 2: Annual production of urea
Major urea exporters produce it commercially from two raw materials, ammonia and carbon dioxide. Carbon dioxide is produce during the manufacture of ammonia from coal or from hydrocarbons such as natural gas. This allows direct synthesis of urea from these materials. China and India are the world’s leading countries that produce Urea. In Asia itself produces about 94 million tons of Urea.
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Introduction
1.3 World demand
Table 3: World demand of urea
According to the chart above, the demand of urea has increased from 2013 to 2014, and predicted to increase at the same rate till the year 2017. There are many socio-economic reasons to this demand. First of all, since the global population is increasing, the agriculture industry grows along. Hence more demand of fertilizers. In addition, although arable land has been increasing, the amount of arable land per capita has been decreasing because of the increase in population. As a result, more fertilizer will be needed to meet the growing need for food. Urea enjoys a high reputation of having the highest nitrogen content makes it by default the number 1 fertilizer in the agriculture industry, particularly in the developing countries, and is traded in a wide perspective in the international market. More than 40% of all food grown in the world is fertilized by urea.
Figure 1: World price of Urea from Aug-2009 to Aug-2014 (5 years)
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Introduction
1.4 Oman urea demand/Production/Prices With a production capacity of over 1.2 million tonnes per annum of granular urea, the fertilizer project is one of the largest industrial investments by an individual within the GCC countries. “The Sohar Fertilizer Project is one of the largest ammonia/urea projects in the world, which uses best quality equipment from Japan and Europe, state-of-the-art instrumentation and control technology as well as advanced pollution control and environmental monitoring system. MHI is fully committed to the success of the Project and will provide all technical support required during the life of the Project.”1 The fertilizer plants use natural gas as feedstock. It is extracted and supplied via an overlaid pipeline from Oman. The Omani Ministry of Oil and Gas will supply natural gas to the SIUCI ammonia and urea project at reduced prices for 25 years under an agreement signed in February 2005. In June 2009, the first shipment of the urea products was made to Australia from the SIUCI Terminal, a dedicated berth at the Port of Sohar. The granular urea products will be exported to the Asian and western countries which have huge market.2 The country had imported 8.04 million tons (MT) of urea in the entire 2012-13 fiscal, out of which 1.83 MT of urea was from Oman India Fertilizer Company (OMIFCO), a joint venture between Oman Oil Company, Indian Farmers Fertilizer Cooperative Ltd and Krishak Bharati Cooperative Ltd. The government in the first 10 months of 2013-14 imported 1.83 million tonnes of urea from Omifco.3 India, one of the largest consumers of fertilizer in the world, imported 1.67million tons of urea from Oman during the eight-month period from April to November this year, up 36 per cent from 1.23million tons of imports in the same period of 2012. According to data from India's fertilizer ministry, Oman was the second biggest supplier of urea to India with the sultanate accounting for over 29 per cent of India's total urea imports during the eight-month period. China was the largest urea supplier to India with exports of 2.52million tons.4 1
Suhail Bahwan group. URL: http://www.suhailbahwangroup.com/index.php?option=com_content&view=article&id=116%3Asohar-fertilizerproject-commences-urea-production&catid=17%3Anews-a-events&Itemid=6&lang=en 2
3
Retrieved from: http://www.chemicals-technology.com/projects/soharinternationalur/
Retrieved from:http://www.thehindu.com/business/Industry/india-imported-192billion-urea-duringaprjan/article5713649.ec 4 Retrieved from: http://www.muscatdaily.com/Archive/Business/Oman-s-urea-exports-to-India-up-36-2svg
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Plant location
2. Plant location Plant location is an important aspect in plant design. It refers to the choice of region and selection of a certain site for setting up a business. The choice should be made only after taking into consideration the cost and benefits of different alternatives. Once implemented, the strategic decision cannot be undone or altered. An ideal location of the plant is where the cost of the product is set at minimum, minimum risk, and maximum social advantage. The location should be a place of maximum net advantage or which gives the lowest unit cost of production and distribution. Selection criteria for location:
Availability and nearness to the sources of raw materials is a major aspect. As said the ideal location should give an advantage of the plant. Where we try to keep the production cost at minimum, hence the transportation cost of the raw materials should be kept a minimum. The location should have an access to the market in order to introduce the plant’s production to the world. Availability of infrastructure facilities and accessibility of the plant. Availability of skilled and non-skilled labour, and qualified managers. Banking and financial institution are located in the region. Region should be safe and secure. Government influence.
We have thought of all the possible aspects of the location where the production cost is kept at minimum. However, we have to take into consideration of the environmental aspect for the well-being of the community of the region and to avoid danger to natural habitat. Therefore, it is desirable to have the plant located far from residential areas.
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Raw materials
3. Raw materials The process of manufacturing urea based on ammonia and carbon dioxide, which are the main reactants needed for producing urea. The first reaction is 2NH3 + CO2 → NH2COONH4 (ammonium carbamate). This reaction is done by reacting ammonia (NH3) and gaseous carbon dioxide (CO2). The second reaction is needed for dehydration of NH2COONH4 (ammonium carbamate) to produce Urea. NH2COONH4 → H2O + NH2CONH2 (urea)
3.1 Ammonia Ammonia is colorless gas; it boils at −33.34°C under atmospheric pressure. The storage should be under high pressure and low temperature, because of strong hydrogen bonds among molecules will easily liquefied and well miscible in water. Ammonia gas is lighter than air, with a density of approximately 0.6 times than air at equal temperature. Ammonia is not available as a natural resource, so the commercial production of ammonia is very important and it is the main component that used to manufacturing urea. Yearly production of ammonia around the world is about 120 million tones, and about 85% of this amount is used in fertilizers including urea. A lot of industrial process of synthesis of ammonia is based on Haber Bosch Process, which developed in Germany 1904-19135.
3.2 Carbon dioxide Carbon dioxide (CO2) is present in atmospheric air. In most of industrial plant, carbon dioxide is result of burning the fuel. It is produce as byproduct of ammonia synthesis. Anyhow, carbon dioxide can be removed and used in production of urea.
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http://www.arabianoilandgas.com/article-8283-technical-challenges-of-fertiliser-production/
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Urea manufacturing processes
4. Urea manufacturing processes The main principle of manufacture of urea is under two main reactions 2NH3 + CO2
NH2COONH4
NH2COONH4
NH2CONH2 + H2O
∆H= -37.4 Kcal/gm mol ∆H= +6.3 Kcal/gm mol
While going main reactions, the undesirable side reaction taking place is 2NH2CONH2
NH2CONHCONH2 (Biuret) + NH3
This process is further complicated by the formation of a substance called biuret, NH2CONHCONH2, which should be kept low because it adversely affects the growth of some plants.
4.1 Total recovery urea manufacturing process block diagram
Figure 2: Schematic representation of urea synthesis Reference no. 7
We can consider that the total recycle process can be divided to four sub processes. 1- Synthesis Ammonia and CO2 are compressed separately and fed at high pressure (180 atm) to the reactor vessel, then a mixture of urea, ammonium carbamate and H2O is produced along with the
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Urea manufacturing processes
unreacted feed (NH3+CO2). Both first and second reactions are equilibrium reactions. The first reaction almost goes to completion at 185-190oC and 180-200 atm. The second reaction (decomposition reaction) is slow and determines the rate of the reaction. 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). This liquid effluent is let down to 27 atm and fed to a special flash-evaporator containing a gas-liquid separator and condenser. Unreacted NH3, CO2 and H2O are thus removed and 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 and 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. 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 from 80 to 110 oC to re-dissolve these crystals prior to evaporation. In the evaporation stage molten urea (99% w/w) is produced at 140 oC.
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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Urea manufacturing processes
Figure 3: Schematic representation of granulation Reference no. 7
Summary of typical optimum operating conditions to maximize the production of urea are in the table below. Operating Condition
Range
Temperature
180-210oC
Pressure
140-250 atm
NH3:CO2
3.1 - 4.1
Retention time
20-30 min
Table 4: Typical optimum operating conditions
4.2 Urea processes The basic process chemistry behind urea manufacturing is relatively simple. Nonetheless, because the operating parameters vary, the concentration of the urea solution formed initially will be different. Design differences happen in the separation and recycle of component streams. Urea manufacturing processes are mainly divided into three classes; the once through, the partial recycle and total recycle processes. All modern plants operate by the full recycle systems.
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Urea manufacturing processes
4.3 Once through recycle process
Figure 4: Once through process Reference no. 7
The once through process is the simplest and the least expansive one (both capital and operating cost) among the three process. Excess liquid ammonia is pumped through a high pressure plunger pump and CO2 is compressed at the compressor and passes up to the urea reactor. The feed mole ratio NH3:CO2 is 2:1 or 3:1. The reactor operates at temperature range of 175-190oC and pressure of 200 atm. The reactor effluent pressure is let down to almost 2 atm. Ammonium carbamate is then decomposed and stripped form the urea solution in a steam heated shell and a tube heat exchanger. The moist gases separated from the 80% product urea solution contain about 0.6 tons of NH3 gas per ton urea produced. It usually is forwarded to other plants that manufacture ammonium nitrate or ammonium sulfate for recovery. Average conversion of 60% of ammonium carbamate to urea is achieved. Excess heat from reactor is removed and utilized to generate a low-pressure steam.
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Urea manufacturing processes
4.4 Partial recycle process
Figure 5: Partial recycle process Reference no. 7
The process is called partial since only excess ammonia is recovered and recycled to the reactor. The main reaction is carried out with excess ammonia at similar conditions as the once through process. The reactor effluent containing 80% urea passes through a high-pressure separator where excess ammonia is stripped using steam. The ammonia is then recovered in the absorption tower and pumped back to the reactor. It is necessary to recover ammonia before passing it directly to the carbamate stripper because excess ammonia will hinder the decomposition of ammonium carbamate. In addition, recovering ammonia covers some of the production costs. The stream containing the carbamate, urea and water goes to the high- then to the low-pressure carbamate decomposers where the carbamate decomposes to NH3 and CO2. Also, the urea solution gets separated and goes to the finishing for further processing. The investment and operating costs are still lower than the total recycle process.
4.5 Total recycle processes It is the most expensive process in investment and operating and most complicated one. All unconverted feed (NH3 and CO2) is recycled back to the reactor.
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Urea manufacturing processes
The total recycle process is widely used by most industries. There are several methods of to execute this process. 1. Decomposed carbamate gas are separated and recycled in their pure state 2. Carbamate solution is recycled to the reactor 3. A combination of gas/liquid recycle 4.5.1 Gas-recycle processes
Figure 6: Gas-recycle process Reference no. 7
The materials leaving the reactor (urea, ammonium carbamate, ammonia, CO2, water) go to a decomposer which decomposes the ammonium carbamate to NH3 and CO2. The separated gases may both be recycled or are purified first before feeding them to the reactor.
CPI-Allied gas recycle urea process
The reactor products (urea, ammonium carbamate, ammonia, CO2, water) pass through an expansion valve to a primary carbamate decomposer, in which about 90% of the carbamate is flashed and stripped along with the water moisture. The remaining stream which contains the urea solution is then sent to an ammonia separator where the excess NH 3 is stripped. Then it goes on to the secondary decomposer to remove all traces of the carbamate at atmospheric pressure. The overheads from both decomposer units are passed through a series of absorption towers where MEA (monomethylamine) selectively absorbs CO2 and water, leaving ammonia to be pumped to the reactor. The CO2 rich solvent is sent to stripper where MEA is regenerated by using heat, leaving CO2 behind to be recycled.
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Urea manufacturing processes
Figure 7: CPI-allied gas-recycle urea process Reference no. 7
Inventa gas-recycle urea process
The Inventa process uses a reactor that operate at 20 MPa and 180-200oC temperature range. The molar feed ratio NH3:CO2 is 2:1 with an upper limit of 50% CO2 conversion to urea. The reactor products pass through an expansion valve where its pressure is lowered to several hundred KPa. Then it is heated in the carbamate decomposer up to 120oC to disintegrate the carbamate. The NH3 and CO2 go to the absorber unit where ammonia will be selectively absorbed and the CO2 exits for the recycle. The ammonia rich solution will be sent to the desorption tower for solvent recovery and NH3 removal for recycle.
Figure 8: Inventa gas-recycle urea process Reference no. 7
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Urea manufacturing processes
4.5.2 Liquid recycle processes
Figure 9: Liquid recycle process Reference no. 7
The concept here is similar to the gas recycle process, except the fact that the gases are condensed (with an additional liquid if necessary) to form a carbamate solution for the recycle.
Stamicarbon CO2 stripping urea process
Ammonia and carbon dioxide are reacted in the molar ratio NH3:CO2 of 2.4:1 to 2.9:1 at 170oC190oC temperature range. The product, at 185oC and 14 MPa, goes immediately to the high pressure stripper where the stream is stripped by the incoming CO 2. The stream containing 15% of unconverted ammonium carbamate passes through the expansion valve to reduce its pressure to 300 kPa then is sent to the low-pressure decomposer which operates at 120oC. The carbon dioxide and ammonia are then condensed in the low pressure up to the high condensers along with the off-gas from the high pressure stripper and some ammonia from the feed line. The condensed stream is the recycled back to the reactor. Conversion efficiencies in this process for carbon dioxide and ammonia are 70-85% and 65-85%, respectively.
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Urea manufacturing processes
Figure 10: Stamicarbon CO2 stripping urea process Reference no. 7
Montecatini complete recycle urea process
Preheated carbon dioxide and liquid ammonia are compressed to a pressure of 20 MPa and fed to the reactor which operates at 195oC, at molar ratio NH3:CO2 of 3.5:1 and H2O:CO2 0.6:1. The reactor products enters a first stage decomposer/separator operating at 8.1 MPa and 185 oC, where most excess ammonia is driven along with ammonium carbamate decomposition gaseous products. This stream along is then fed to the first stage carbamate condenser which operate at 8.1 MPa and 145oC. The effluent from the first stage condenser is taken to the second stage condenser operating at 115oC and sane pressure as the first one. The gas leaving this condenser is then washed to remove the ammonia, while the liquid stream is recycled to the reactor. The liquid stream leaving the first stage decomposer/separator will be sent to the second decomposer/separator unit operating at pressure of 1.2 MPa and same temperature as stage one, then finally to the third stage which operate at 200-300 kPa. The liquid stream leaving the last one is discharge as a product, urea solution. The gaseous effluents from the second and third decomposer/separator units are condensed in the carbamate condensers number three and four, respectively. Carbamate condenser three receive the gas stream combined with two liquid streams from the washed vessels and the liquid carbamate solution will be taken back to the first carbamate condenser. The fourth carbamate condenser gets ammonia bearing gas
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Urea manufacturing processes
stream from the solidification section which will get washed with cold carbamate, before proceeding it to the second washing vessel.
Figure 11: Montecatini complete recycle urea process Reference no. 7
4.5.3 Gas/Liquid recycle processes
Figure 12: Gas / liquid recycle process Reference no. 7
The ammonia and the carbon dioxide recycle streams are mixed and fed to the reactor in the form of carbamate.
Stamicarbon total recycle process
The urea synthesis takes place in the reactor at 20.2 MPa and 170-190oC temperature range. The reactor effluent pressure is lowered to about 500 kPa before it proceeds to the preseparator. The liquid stream from the pre-separator will then go further to several separation steps before finally leaving the process as the product. The various CO2 and NH3 streams are
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Urea manufacturing processes
condensed together, and the formed carbamate is fed to the reactor. A wet scrubber unit is utilized to recover ammonia from the gas for recycle.
Figure 13: Stamicarbon total recycle process Reference no. 7
Snamprogetti ammonia stripping process
This process is similar to the Stamicarbon CO2 stripping process, however the stripping is done using the ammonia gas rather than CO2. Reactor operating temperature range is 180-190oC and it could operate at two different pressure ranges 13-16 MPa and 20.2-25 MPa. The molar ratio NH3:CO2 is 3.5:1. The reactor products will be passed through a stripper where most of the NH 3 and CO2 will be removed, which operates at pressure 10-15 MPa and temperature 160-200oC ranges. These overhead gases are collected along with the carbamate and are recycled to the reactor as one stream. The liquid stream from the stripper will proceed to a flash separator which will separate the product urea solution from ammonia, CO2 and water traces which will concentrate our product for prilling.
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Urea manufacturing processes
Figure 14: Snamprogetti ammonia stripping process Reference no. 7
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Selected process: Snamprogetti ammonia stripping process
5. Selected process: Snamprogetti ammonia stripping process It is widely used in urea industries and it the process used in Sohar urea manufacturing plant. The sequence of operations could be divided to five main steps.
Synthesis and high-pressure recovery
High pressure ammonia and compressed CO2 are fed to the reactor and urea is formed at pressure range of 15.8-16 MPa and temperature of 190oC. The molar ratio of NH3:CO2 is around 2:1 and the molar ratio of H2O:CO2 is around 0.67:1. Passivation air is added to the CO2 stream in order to prevent corrosion of the reactor and compressor. The reactor effluent is then sent to the stripper for high-pressure decomposition and the released gases are recycled. The CO2 will be stripped out using NH3. The liquid stream from the stripper will be sent to the medium pressure units. 2NH3 + CO2
NH2COONH4
NH2COONH4
NH2CONH2 + H2O
∆H= -37.4 Kcal/gm mol ∆H= +6.3 Kcal/gm mol
Medium-pressure purification and recovery.
Liquid ammonia enters into the top section of the medium-pressure absorber; then it is fed to the reactor and to the carbamate condenser. Solution product from the medium-pressure decomposer is fed to the low-pressure section.
Low-pressure purification and recovery
Urea purification in here takes place in two stages. The first stage recovery and purification takes place at 18 ata in a falling type MP decomposer which is divided into two parts; the decomposition section where the carbamate disintegrates to NH3 and CO2, and the top separator in which the gases are released. The second stage takes place at a lower pressure at 4.5 ata and the decomposer unit here is fed with the solution leaving the first stage.
Vacuum concentration
The urea solution (70% urea) leaving the low-pressure section which is almost free of any NH3, CO2 or carbamate is fed to a series of two vacuum concentrators, in order to remove the water content thus increasing the urea concentration in the solution. The objective is to achieve a urea solution of 99% purity. The pressure in the first vacuum concentrator is 0.23 ata and 0.03 ata in the second one.
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Selected process: Snamprogetti ammonia stripping process
Urea prilling
The molten urea leaving the vacuum concentration section is fed to the prilling tower on top by a centrifugal pump and where it is dripped. As the drops fall they encounter a cold air flow which causes the drops to solidify. The solid prills are then screened as to control product particle size. Big lumps are then melted and recycled.
Figure 15: Snamprogetti ammonia stripping urea manufacturing process steps Reference no. 8
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Mass balance figure and plant capacity
6. Mass balance figure and plant capacity For the sake of mass balance, we decided to simplify the process as an assumption to proceed with our calculations. The process will have six main units: 1. 2. 3. 4. 5. 6.
Reactor Stripper MP decomposer LP decomposer Vacuum evaporator Prilling tower
The aim is to reach a urea product of 98% purity.
Figure 10: Simplified Snampogetti process Reference no. 11
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Mass balance figure and plant capacity
6.1 Selected plant capacity Selected Capacity: 438,000 tons/year Number of working days: 365 days Daily production: [438,000 tons/day] / 365 days = 1200 tons Urea production: 50 tons/hr = 50,000 kg/hr of urea (98% purity)
6.2 Composition of the final product: Urea: 98% (49,000 Kg/hr) Biuret: 1% (500 Kg/hr) Water: 1% (500Kg/hr) Overall assumed conversion to urea: 95%
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References
References 1. http://www.chemicals-technology.com/projects/soharinternationalur/ 2. http://www.thehindu.com/business/Industry/india-imported-192billion-urea-duringaprjan/article5713649.ec 3. http://www.muscatdaily.com/Archive/Business/Oman-s-urea-exports-to-India-up-362svg 4. http://www.suhailbahwangroup.com/index.php?option=com_content&view=article&id =116%3Asohar-fertilizer-project-commences-urea-production&catid=17%3Anews-aevents&Itemid=6&lang=en 5. http://www.arabianoilandgas.com/article-8283-technical-challenges-of-fertiliserproduction/ 6. Toyo Enginnering corporation; http://www.toyo-eng.com/jp/en/products/petrochmical/urea/aces21/ http://www.toyo-eng.com/jp/en/products/environment/energy/ 7. University of Moratuwa, Department of Chemical and Process Engineering, Design of an urea manufacturing plant; Slideshare URL: http://www.slideshare.net/shenalozile/lit-survey-urea 8. Chauvel, Alain and Lefebvre, gilles. Petrochemical processes technical and economic characteristics. Synthesis gas derivatives and major hydrocarbons (Vol 1, 2nd edition). Editions Technip, Paris 1989. 9. Saipem, eni. The snamprogetti urea technology; URL: http://www.saipem.com/site/download.jsp?idDocument=1321&instance=2 10. Process Flowsheets. Urea fertilizer manufacturing process flowsheet design. Posted on August 13, 2014. URL: http://processflowsheet.com/urea-fertilizer-manufacturing-process-flowsheetdesign/ 11. Kumar Bhaskar and Pratap Chandra Das. National Institute of Technology. Department of chemical Engineering. Manufacture of urea. 2007. Retrieved from: http://ethesis.nitrkl.ac.in/4237/1/Manufacture_of_Urea.pdf
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