report_1 - 12.12.2011
TRANSCRIPT
C Cairo University
Faculty of Engineering
Chemical Engineering Department
Senior year 2012
Urea Ammonium Nitrate Fertilizer (UAN)
Submitted to: Dr.Mohammed Ali
Prepared by:
Alaa Mahmoud Samy
Amira Mahmoud Badran
Rasha Maher Edward
Nihad Nasr El-Din Soliman
Nehal Rizk Ahmed
Yasmine Ossama El-Sergany
December, 2011
I
Abstract:
This report will cover the organic and inorganic fertilizers types, also it will
cover fertilizers‘ advantages and disadvantages .we are concerned with
nitrogen fertilizers and its types and importance and the main project topic is
talking about urea fertilizer.
The raw materials used for urea production are ammonia and carbon dioxide,
by knowing their conditions and how ammonia is synthesized from natural
gas, and carbon dioxide is obtained as a by-product from any other plants so,
urea is produced by optimizing reaction conditions.
There are different types of technologies to produce urea, this report is
focusing on the Stami-carbon technology and its modifications, as Stami-
carbon is the most commonly used technology in Egypt. Other technologies
for urea production also mentioned such as; TOYO‘s technology for urea
production and Snamprojectti‘s technology, those types of technologies are
commonly used world-wide.
The project main concern is urea ammonium nitrate fertilizer (UAN) which
could be considered as a modification for the urea fertilizer itself. This type of
fertilizer is produced directly by mixing urea solution with ammonium nitrate
solution.
II
Contents:
Chapter One: Introduction .............................................................................. 1
Chapter Two: Fertilizers ................................................................................. 3
I. History: ................................................................................................... 3
II. Advantages and disadvantages of organic fertilizers: ............................... 3
III. Advantages and disadvantages of Inorganic fertilizers: ........................ 4
IV. Nitrogen fertilizers: .............................................................................. 4
A. Importance of nitrogen fertilizers: ..................................................... 4
B. Advantages and disadvantages of nitrogen fertilizers: ....................... 5
V. Organic and inorganic chemical nitrogen fertilizers types: ...................... 5
A. Sodium Nitrate: ................................................................................ 5
B. Ammonium Sulfate: ......................................................................... 5
C. Ammonium Nitrate:.......................................................................... 6
D. Ammonium Sulfate Nitrate: .............................................................. 6
E. Ammonium Chloride: ....................................................................... 6
F. Urea: .................................................................................................... 6
G. Ammonia: ........................................................................................ 7
H. Organic Nitrogen Fertilizers: ............................................................ 7
VI. Future of fertilizers: ............................................................................. 7
Chapter Three: Raw Material of Urea production............................................ 8
I. Ammonia: ............................................................................................... 8
A. History: ............................................................................................ 8
B. Occurrence: ...................................................................................... 8
C. Physical Properties of ammonia: ....................................................... 8
D. Importance of ammonia: ................................................................... 9
E. Process Steps of Ammonia Production: .......................................... 10
F. Synthesis Gas Production:.................................................................. 11
G. Feedstock Pre-treatment and Raw Gas Production: ......................... 11
H. Carbon Monoxide Shift Conversion:............................................... 12
I. Gas Purification: ................................................................................ 12
J. Methanation: ...................................................................................... 13
K. Ammonia Synthesis: ....................................................................... 13
L. Safety Features &health aspects of ammonia: ................................. 14
II. Carbon Di-oxide: .................................................................................. 14
A. History: .......................................................................................... 14
B. Carbon dioxide in the gas form: ...................................................... 14
III
C. Carbon Dioxide in the liquid form: ................................................. 15
D. Gasification of coal: ....................................................................... 15
E. Properties of carbon dioxide: .......................................................... 16
F. Environmental hazards for CO2: ......................................................... 16
1. CO2 Emissions: ........................................................................... 16
2. Greenhouse Effect:...................................................................... 16
Chapter Four: Urea Fertilizer ........................................................................ 17
I. History: ................................................................................................. 17
II. Importance of urea: ............................................................................... 17
III. Physical Properties of pure urea: ........................................................ 17
IV. Advantages and disadvantages of Urea Fertilizer: .............................. 17
V. Modifications of the Stami-carbon CO2-stripping process: .................... 18
A. The Original Stamicarbon CO2-Stripping Process: .......................... 19
1. Main Reactions: .......................................................................... 21
2. Side reactions:............................................................................. 21
B. Urea 2000 plus: .............................................................................. 22
1. Urea 2000 plus with pool condenser: ........................................... 22
2. Urea 2000 plus with pool reactor:................................................ 23
C. The Avancore process: ................................................................... 24
VI. Corrosion: .......................................................................................... 26
A. Role of Oxygen Content: ................................................................ 26
B. Role of Temperature: ...................................................................... 27
C. Material Selection: ......................................................................... 27
Chapter Five: Other technologies for urea production ................................... 28
I. Technology for urea production (TOYO): ............................................. 28
A. History: .......................................................................................... 28
B. Process description: ........................................................................ 28
1. Ground Level Reactor: ................................................................ 28
2. Vertical Submerged Carbamate Condenser: ................................ 29
3. Optimum Selection of Synthesis Condition: ................................ 29
C. TOYO‘S Process Performance: ...................................................... 30
II. Snamprogetti Urea Technology: ............................................................ 30
A. Process description: ........................................................................ 30
1. Synthesis and High Pressure (HP) recovery: ............................... 30
2. Medium Pressure (MP) purification and recovery: ...................... 31
3. Low Pressure (LP) purification and recovery: ............................. 31
Chapter six: Urea-Ammonium Nitrate (UAN) .............................................. 33
IV
I. Overview of UAN Process Technology:................................................ 33
II. physical properties of Urea ammonium nitrate solution: ........................ 33
III. Description of Production Processes: ................................................. 33
References: ................................................................................................... 35
V
Lists of Figures:
Fig.1 Block flow diagram for ammonia production
9
Fig.2 Process flow diagram for ammonia production
10
Fig.3 Block diagram for Stami-carbon CO2 stripping Urea Process 19
Fig.4 Flow diagram of Stami-carbon CO2 stripping urea process 21
Fig.5 Process flow diagram for Stami-carbon urea 2000plus with pool
condenser
22
Fig.6 Process flow diagram for Stami-carbon urea 2000plus with pool
reactor
24
Fig.7 Process flow diagram for Avancore Urea Process 25
Fig.8 Process flow diagram for TOYO Urea Process 29
Fig.9 Block flow diagram for Snamprojectti urea process 32
Fig.10 Process flow diagram for Snamprojectti urea process 32
Fig.11 Block diagram for Urea ammonium nitrate process 34
Fig.12 Block diagram for partial recycle CO2 stripping Urea Process for
Urea ammonium nitrate production
34
VI
List of Tables:
Table.1 Physical properties of ammonia
8
Table.2 Feedstock distribution of world ammonia production capacity
11
Table.3 Properties of Carbon dioxide
16
Table.4 Physical properties of pure Urea
17
Table.5 TOYO ‗s process performance 30
Table.6 Physical properties of Urea ammonium nitrate solution 33
1
Chapter One: Introduction
Fertilizer is any organic or inorganic material of natural or synthetic origin. It
is added to a soil to supply one or more plant nutrients essential to the growth
of plants. Soil amendments are made by adding fertilizer to the soil. There are
different types of fertilizers; bulky organic fertilizer such as cow manure, bat
guano, bone meal, and organic compost and green manure crops. There is also
chemical fertilizer which is inorganic fertilizer and is made up with different
formulations to suit a variety of specified uses. Many governments and
agricultural departments want to increase the supply of organic fertilizers, as
they are not enough to meet the existing and future fertilizer needs. Compared
to organic compost, chemical or inorganic fertilizers also have the added
advantage of being less bulky which makes chemical fertilizer easier to
transport, because they are available to the plant relatively quickly when
incorporated as part of the plant-food constituents. The different types of
chemical fertilizers are usually classified according to the three principal
elements, namely Nitrogen (N), Phosphorous (P) and Potassium (K).we are
concerned with nitrogen fertilizers and its types and importance, and urea is
one of the most important nitrogen fertilizers.
The main raw materials used for urea production are ammonia and carbon
dioxide.
The complete process of industrial ammonia production may be subdivided
into the following sections: Synthesis gas production including; Feedstock
pre-treatment and gas generation, Carbon monoxide conversion, and Gas
purification. The remaining sections are Compression and Synthesis and purge
gas management. Carbon dioxide enters the reaction in the gas form, CO2 is
produced as a by-product from many industries such as; the production of
ethanol by fermentation and manufacture of ammonia.
Urea fertilizer is also known as carbamide. It is organic chemical compound
containing about 46% nitrogen. Urea is widely used in the agriculture sector
both as a fertilizer and animal feed additive; this makes the production of urea
considerably high in comparison to other fertilizers. The consumption of
fertilizers in Egypt is around 9.2 million tons per year and the local production
of fertilizers is estimated at 8.3 million tons. This means that there are annual
imports of around 900,000 tons. Urea is the world‘s most produced chemical
at around 140 million tons per year. Demand for urea is growing at 3.7%
globally – higher even than the rise in population. In fact, around 90% of the
world‘s urea is used to fertilize crops and more than 40% of all food grown in
the world is fertilized by urea.
There are different types of technologies producing urea; one of these
technologies is the Stamicarbon technology and its modifications as
Stamicarbon is the most commonly used technology in Egypt, NH3 and
CO2 are converted to urea via ammonium carbamate at a pressure of
approximately 140 bar and a temperature of 190° C. The molar NH3/CO2 ratio
applied in the reactor is 3:1. This results in a CO2 conversion of about 60%
2
and an NH3 conversion of 41%. The reactor effluent, containing unconverted
NH3 and CO2 is subjected to a stripping operation at essentially reactor
pressure, using CO2 as stripping agent. The stripped-off NH3 and CO2 are then
partially condensed and recycled to the reactor. The heat evolving from this
condensation is utilized to produce 4.5 bar steam, some of which can be used
for heating purposes in the downstream sections of the plant and urea is
produced.
The other technologies concerning urea production are TOYO and
Snamprojectti; they are used world-wide. TOYO‘s process description
includes ground level reactor, vertical submerged carbamate condenser
(VSCC) and optimum selection of synthesis condition.
Snamprojectii‘s Urea Technology process description includes Synthesis and
High Pressure (HP) recovery, Medium Pressure (MP) purification and
recovery, Low Pressure (LP) purification and recovery. The product is urea
ammonium nitrate solution. It‘s produced via direct mixing of urea solution,
and ammonium nitrate solution.
3
Chapter Two: Fertilizers
I. History:
The process of adding substances to soil to improve its growing capacity was
developed in the early days of agriculture. Ancient farmers knew that the first
yields on a plot of land were much better than those of subsequent years. This
caused them to move to new, uncultivated areas, which again showed the same
pattern of reduced yields over time. Eventually it was discovered that plant
growth on a plot of land could be improved by spreading animal manure
throughout the soil.
Over time, fertilizer technology became more refined. New substances that
improved the growth of plants were discovered. The Egyptians are known to
have added ashes from burned weeds to soil. Ancient Greek and Roman
writings indicate that various animal excrements were used, depending on the
type of soil or plant grown. It was also known by this time that growing
leguminous plants on plots prior to growing wheat was beneficial. Other types
of materials added include sea-shells, clay, vegetable waste, waste from
different manufacturing processes, and other assorted trash.
Organized research into fertilizer technology began in the early seventeenth
century. Early scientists such as Francis Bacon and Johann Glauber describe
the beneficial effects of the addition of saltpeter to soil. Glauber developed the
first complete mineral fertilizer, which was a mixture of saltpeter, lime,
phosphoric acid, nitrogen, and potash. As scientific chemical theories
developed, the chemical needs of plants were discovered, which led to
improved fertilizer compositions. Organic chemist Justus von Liebig
demonstrated that plants need mineral elements such as nitrogen and
phosphorous in order to grow. The chemical fertilizer industry could be said to
have its beginnings with a patent issued to Sir John Lawes, which outlined a
method for producing a form of phosphate that was an effective fertilizer. The
synthetic fertilizer industry experienced significant growth after the First
World War, when facilities that had produced ammonia and synthetic nitrates
for explosives were converted to the production of nitrogen-based fertilizers.
II. Advantages and disadvantages of organic fertilizers:
Advantages:
Improve biodiversity (soil life).
Provide long-term productivity of soil.
Prove a large depository for excess carbon dioxide.
Increase the abundance of soil organisms by providing organic matter
and micronutrients for organisms.
Disadvantages:
Organic fertilizers may contain pathogens and other disease causing
death of organisms if not properly composted.
4
Organic fertilizers‘ nutrient contents are very variable and their release
to available forms that the plant can use may not occur at the right
plant growth stage.
Organic fertilizers are comparatively too bulky to deploy the right
amount of nutrients that will be beneficial to plants
III. Advantages and disadvantages of Inorganic fertilizers:
Advantages:
Inorganic fertilizers are usually given as a ―rescue treatment‖ to plants
that are dying. They are appropriate in this situation because the
nutrients needed by the plants are readily available.
Inorganic fertilizers are relatively cheap.
Inorganic fertilizers are easily used and prepared.
Disadvantages:
Inorganic fertilizers contain nutrients that have been broken down
already into the most basic of its components for easy absorption by
the plants. It can also be washed away easily when watering or
irrigating the plants, and this is called leaching.
Inorganic fertilizers contain salts and other compounds. These are not
absorbed by the plants so they are left behind in the soil and build up
over time. This can alter the chemistry of the soil that makes it less
ideal for planting.
Inorganic fertilizers application with great amount may burn the
delicate plant structures such as the roots. This could affect the over-all
development of the plant.
IV. Nitrogen fertilizers:
Nitrogen fertilizer is an inorganic fertilizer consisting of nitrous
compounds such as ammonium nitrate. It is available in different forms:
liquid, which must be injected into the ground, or dry pellets, which can be
applied to the surface of the soil by hand or by machine. In some cases,
industrial by-products such as pure ammonia can be used as a nitrogen
fertilizer, but these should be avoided as they are toxic and pose health
problems in terms of handling and exposure.
A. Importance of nitrogen fertilizers:
Fertilizers are used to maintain the correct level of nitrogen in the soil,
ensuring strong, green plants with a healthy growth rate. Nitrogen
fertilizers can be used on a wide range of flora, from the household garden
to commercial crops. They are currently used in one-third of the world's
total crop production, including species such as maize, barley and soybean.
Nitrogen fertilizers are also extensively used on commercial lawns,
including a large proportion of the world's golf courses.
5
B. Advantages and disadvantages of nitrogen
fertilizers:
Advantages:
Nitrogen fertilizers are able to make up the deficiency when the soil
has become depleted of its natural nitrogen stores.
The use of nitrogen fertilizers helps to keep nutrient levels at an
optimum level, protect against disease and control weeds, resulting in
healthier crops and consistent quality and quantity of yields.
Disadvantages:
Excess nitrogen not absorbed by the plants has been shown to leach
into the groundwater and nearby rivers. High levels of nitrogen in the
water can create algal blooms, large growths of algae that imbalance
the delicate ecosystem to the detriment of other aquatic species.
V. Organic and inorganic chemical nitrogen fertilizers
types:
This type of fertilizer is divided into different groups according to the manner
in which the nitrogen combines with other elements.
A. Sodium Nitrate:
Sodium nitrates are also known as Chilean Nitrate. The nitrogen contained in
sodium nitrate is refined to 16%. This means that the nitrogen is immediately
available to plants and is a valuable source of nitrogen in this type of fertilizer.
When one makes a soil amendment using sodium nitrates as a type of fertilizer
in the garden, it is usually as a top- and side-dressing. Particularly when
nursing young plants and garden vegetables. Sodium nitrate is quite useful as a
type of fertilizer in an acidic soil. However, the excess use of sodium nitrate
may cause de-flocculation.
B. Ammonium Sulfate:
This fertilizer type comes in a white crystalline salt form. It is easy to handle
and it stored well under dry conditions. However, during the rainy season, it
sometimes forms lumps. Though this fertilizer type is soluble in water, its
nitrogen is not readily lost in drainage, because the ammonium ion is retained
by the soil particles. Ammonium sulfate may have an acidic effect on garden
soil. Over time, the long-continued use of this type of fertilizer will increase
soil acidity and thus lower the yield. The application of ammonium sulfate
fertilizer can be done before sowing, at sowing time, or even as a top-dressing
to the growing crop.
6
C. Ammonium Nitrate:
This fertilizer type also comes in white crystalline salts. Ammonium nitrate
salts contain 33 to 35% nitrogen, of which half is nitrate nitrogen and the other
half is in the ammonium form. As part of the ammonium form, this type of
fertilizer cannot be easily leached from the soil. This fertilizer is quick-acting,
but highly hygroscopic thus making it unfit for storage. Ammonium nitrate
also has an acidic effect on the soil, in addition this type of fertilizer can be
explosive under certain conditions.
D. Ammonium Sulfate Nitrate:
This fertilizer type is available as a mixture of ammonium nitrate and
ammonium sulfate. It is recognizable as a white crystal or as dirty-white
granules. This fertilizer contains 26% nitrogen, three-fourths of it in the
ammonium form and the remainder (i.e. 6.5%) as nitrate nitrogen. Ammonium
sulfate nitrate is non-explosive, readily soluble in water and is very quick-
acting. Because this type of fertilizer keeps well, it is very useful for all crops.
Though it can also render garden soil acidic, the acidifying effect is only one-
half of that of ammonium sulfate on garden soil. Application of this fertilizer
type can be done before sowing, at sowing time or as a top-dressing, but it
should not be applied along the seed.
E. Ammonium Chloride:
This fertilizer type comes in a white crystalline compound, which has a good
physical condition and 26% ammoniac nitrogen. It is not recommended to use
this type of fertilizer on crops such as tomatoes because the chlorine may harm
the crop.
F. Urea:
This type of fertilizer usually is available to the public in a white, crystalline,
organic form. It is a highly concentrated nitrogen fertilizer and fairly
hygroscopic. This also means that this fertilizer can be quite difficult to apply.
Urea is also produced in granular or pellet forms and is coated with a non-
hygroscopic inert material. It is highly soluble in water and therefore, subject
to rapid leaching. It produces quick results. When applied to the soil, its
nitrogen is rapidly changed into ammonia. Urea supplies nothing but nitrogen
and the application of urea as fertilizer can be done at sowing time or as a top-
dressing, but should not be allowed to come into contact with the seed.
7
G. Ammonia:
This fertilizer type is a gas that is made up of about 80% of nitrogen and
comes in a liquid form as well because under the right conditions of
temperature and pressure, ammonia becomes liquid (anhydrous ammonia).
Another form, aqueous ammonia, results from the absorption of ammonia gas
into water, in which it is soluble. Ammonia is used as a fertilizer in both
forms. The anhydrous liquid form of ammonia can be applied by introducing it
into irrigation water, or directly into the soil from special containers.
H. Organic Nitrogen Fertilizers:
Organic nitrogen fertilizer is the type of fertilizer that includes plant and
animal by-products. These by-products can be anything from oil cakes, to fish
manure and even to dried blood. The nitrogen available in organic nitrogen
fertilizer types first has to be converted before the plants can use it. This
conversion occurs through bacterial action which is a slow process. The upside
of this situation is that the supply of available nitrogen lasts so much longer.
This type of fertilizer may contain small amounts of organic stimulants that
contain other minor elements that might also be needed by the plants that are
being fertilized. Furthermore, they may also contain small amounts of organic
stimulants, or some of the minor elements needed by plant. Oil-cakes contain
not only nitrogen but also some phosphoric and potash, besides a large
quantity of organic matter. This type of fertilizer is used in conjunction with
quicker-acting chemical fertilizers.
VI. Future of fertilizers:
Fertilizer research is currently focusing on reducing the harmful
environmental impacts of fertilizer use and finding new, less expensive
sources of fertilizers. Such things that are being investigated to make
fertilizers more environmentally friendly are improved methods of application,
supplying fertilizer in a form which is less susceptible to runoff, and making
more concentrated mixtures. New sources of fertilizers are also being
investigated. It has been found that sewage sludge contains many of the
nutrients that are needed for a good fertilizer. Unfortunately, it also contains
certain substances such as lead, cadmium, and mercury in concentrations
which would be harmful to plants. Efforts are underway to remove the
unwanted elements, making this material a viable fertilizer. Another source
that is being developed is manures. The first fertilizers were manures;
however, they are not utilized on a large scale because their handling has
proven to be too expensive. When technology improves and costs are reduced,
this material will be a viable new fertilizer.
8
Chapter Three: Raw Material of Urea production
I. Ammonia:
A. History:
The name ammonia is derived from ―sal ammoniacum‖ (Oasis Ammon in
Egypt, today Siwa). Sal ammoniac was known to the Ancient Egyptians. Free
ammonia was prepared for the first time in 1774 by J. B. Priestley. In 1784, C.
L. Berthollet recognized that ammonia was composed of the elements nitrogen
and hydrogen. W. Henry, in 1809, determined the volumetric ratio of the
elements as 1:3, corresponding to the chemical formula NH3.
Following the discovery of the nature and value of mineral fertilization by
Liebig in 1840, nitrogen compounds were used in increasing quantities as an
ingredient of mineral fertilizers. At the end of the last century ammonia was
recovered in coke oven plants and gas works as a by-product of the destructive
distillation of coal. The produced ammonium sulfate was used as fertilizer.
Another nitrogen fertilizer was calcium cyanamid. Since both sources of
nitrogen were limited in quantity they did not suffice for fertilization. In 1913,
the first Haber – Bosch plant went on stream, representing the first
commercial synthesis of ammonia from the elements. Subsequently, other
ammonia plants were started up. Today ammonia is a commodity product of
the chemical industry.
B. Occurrence:
Ammonia, NH3, occurs in nature almost exclusively in the form of ammonium
salts. Natural formation of ammonia is primarily by decomposition of
nitrogen-containing organic materials or through volcanic activity. Ammonia
and its oxidation products, which combine to form ammonium nitrate and
nitrite, are produced from nitrogen and water vapor through electrical
discharges in the atmosphere. Ammonia and its salts are also by-products of
commercial processing (gasification, coking) of fossil fuels such as coal,
lignite, and peat.
C. Physical Properties of ammonia:
Property Value
Melting point -77.7°C
Boiling point 33.4°C
Specific gravity 0.771 at 1bar (temperature
unspecified)
Table.1 Physical Properties of ammonia
9
D. Importance of ammonia:
1. In the manufacture of nitric acid.
2. In manufacture of explosives.
3. In manufacture of synthetic fibers and fertilizers.
4. In refrigeration.
5. As a chemical intermediate in the production of cyanides, amides,
nitrates and dyestuffs.
Desulphurisation
Primary reformer
Secondary reformer
Carbon monoxide Removal
Carbon dioxide Removal
Methanation
Synthesis gas compression
Ammonia synthesis
chilling
Natural gas
steam
Fuel
Air
Combustion air
Carbon dioxide To urea plant
Liquid ammonia
Fig.1 Block flow diagram for ammonia production
10
Fig. 2 Process flow diagram for ammonia production
E. Process Steps of Ammonia Production:
The complete process of industrial ammonia production may be subdivided
into the following sections:
Synthesis gas production
1. Feedstock pre-treatment and gas generation
2. Carbon monoxide conversion
3. Gas purification
Compression
Synthesis and purge gas management
The most fundamental changes over the years have occurred in synthesis gas
production and gas compression. In the synthesis section itself, some progress
has been made in converter design and optimization of heat recovery.
11
Table.2 feedstock distribution of world ammonia production capacity
F. Synthesis Gas Production:
The goal is preparing a pure mixture of nitrogen and hydrogen in the
stoichiometric ratio of 1: 3. The raw materials are water, air, and a carbon-
containing reducing medium that, for its part, may contain hydrogen (natural
gas, CH4; naphtha, ≈ CH2; petroleum, ≈ CH) and nitrogen.
G. Feedstock Pre-treatment and Raw Gas Production:
The chemical reaction of hydrocarbons with water, oxygen, air, or any
combination of these is generally referred to as gasification. It yields a gas
mixture made from CO and H2 in various proportions along with carbon
dioxide and where air is used, some nitrogen. Any carbon containing
feedstock will undergo a reaction:
CH4+H2O CO+3H2 ∆ H= 88563.678 BTU/Ibmole
Natural gas consists predominantly of methane and is therefore the most
hydrogen-rich and energetically the best raw material for the steam-reforming
route.
To introduce nitrogen to achieve the required stoichiometric
hydrogen/nitrogen ratio for ammonia synthesis, the reforming reaction is split
into two sections. In the first section, the primary reformer, the reaction
proceeds in indirectly heated tubes filled with nickel-containing reforming
catalyst and is controlled to achieve a partial conversion only [in conventional
plants 65 % based on methane feed, leaving around 14 mol % methane (dry
basis) in the effluent gas]. In the following secondary reformer — a refractory-
lined vessel filled with nickel catalyst — the gas is mixed with a controlled
amount of air introduced through a nozzle (burner). By combustion of a
quantity of the gas the temperature is raised sufficiently (to about 1200 °C)
that the endothermic reforming reaction is completed with the gas
adiabatically passing the catalyst layer.
A concept developed by Uhde goes a step further in this direction: exchanger
reforming and subsequent non-catalytic partial oxidation, which provides the
12
reaction heat, are accommodated in a single vessel. This combined auto
thermal reformer (CAR) design, was operated in a demonstration unit
producing 13 000 m3/h of synthesis gas.
H. Carbon Monoxide Shift Conversion:
As ammonia synthesis needs only nitrogen and hydrogen, all carbon oxides
must be removed from the raw synthesis gas of the gasification process. In the
water gas shift reaction, traditionally known as carbon monoxide shift
conversion, the carbon monoxide serves as reducing agent for water to yield
hydrogen and carbon dioxide. In this way not only is the carbon monoxide
converted to readily removable carbon dioxide but also additional hydrogen is
produced:
CO+H2O CO2 + H2 ∆H= -177127.355E-1 BTU/Ibmole
I. Gas Purification:
In further purification, carbon dioxide, residual carbon monoxide, and sulfur
compounds (only present in the synthesis gas from partial oxidation) have to
be removed as they are not only useless ballast but above all poisons for the
ammonia synthesis catalyst.
In contrast steam reforming requires removal of sulfur from the natural gas
and light hydrocarbon feed stocks upstream of gasification to avoid poisoning
of the sensitive reforming catalysts. This is usually performed by hydro
desulfurization and adsorption of the H2S by ZnO. As this is an essential part
of the steam reforming process it is treated in Section.
The classical method for CO2 removal is to scrub the CO2 containing synthesis
gas under pressure with a solvent capable of dissolving carbon dioxide in
sufficient quantity and at sufficient rate, usually in counter current in a column
equipped with trays or packing .
Nowadays, various chemical and physical absorption systems are available for
CO2 removal, aMEDA®,
Benefield, Amine Guard and selexol.
Uhde has used all these processes in the past, the lowest energy consumption
process is achieved by using the activated aMEDA®,
process .The key to these
energy savings is that the solution is primarily regenerated by flashing rather
than steam stripping.
In addition, the process offers the following advantages:
High CO2 recovery rate (<96%) and CO2 purity (<99%by volume).
No need for corrosion inhibitors as the solution is not corrosive to
carbon steel
13
Minimization of solution losses because aMEDA®,
has a low vapour
pressure and does not degrade during operation .No reclaiming of the
solution is required.
No toxic solvents.
No crystallization problems.
J. Methanation:
Methanation is a physical-chemical process to generate Methane from a
mixture of various gases out of biomass fermentation or thermo-chemical
gasification. The main components are carbon monoxide and hydrogen.
The following main process describes the methanation:
CO+3H2 CH4 +H2O
Methanation is the reverse reaction of steam methane reforming, which
converts methane into synthesis gas. It‘s the simplest method to reduce the
concentrations of the carbon oxides well below 10 ppm and is widely used in
steam reforming plants.
K. Ammonia Synthesis:
Under the conditions practical for an industrial process ammonia formation
from hydrogen and nitrogen is limited by the unfavorable position of the
thermodynamic equilibrium, so that only partial conversion of the synthesis
gas (25 – 35 %) can be attained on its passage through the catalyst. Ammonia
is separated from the unreacted gas by condensation, which requires relatively
low temperatures for reasonable efficiency. The unconverted gas is
supplemented with fresh synthesis gas and recycled to the converter. The
concentration of the inert gases (methane and argon) in the synthesis loop is
controlled by withdrawing a small continuous purge gas stream.
N2+3H2 2NH3 ∆H= -397418.756E-1 BTU/Ibmole
The main feature of this unit is its high conversion rate which is achieved by a
large catalyst volume. In order to minimize reactor size and cost while keeping
the pressure drop low, the large catalyst volume requires:
The use of small grain-size catalyst
Application of the radial-flow concept in the ammonia reactor
Uhde‗s ammonium synthesis unit is based on a three-bed reactors system, each
bed with a radial flow.
14
L. Safety Features &health aspects of ammonia:
In ammonia production three potential hazard events can be identified:
fire/explosion hazard from the hydrocarbon feed system; fire/explosion hazard
due to leaks in the synthesis gas purification, compression, or synthesis section
(75 % hydrogen); and toxic hazard from release of liquid ammonia from the
synthesis loop. In addition there is also a potential toxic hazard in handling
and storing of liquid ammonia. The long history of ammonia production since
1913 has demonstrated this production technology is a very safe operation.
In ammonia production, storage, and handling the main potential health hazard
is the toxicity of the product itself. For this reason this section concentrates on
ammonia only. Other toxic substances such as carbon monoxide or traces of
nickel carbonyl (which may be formed during shut down in the methanation
stage) may be only a risk in maintenance operations and need appropriate
protection provisions as well as blanketing or flushing with nitrogen.
As about 85-87 % of the ammonia consumption goes into the manufacture of
fertilizers, it is obvious that the future of the ammonia industry is very closely
bound up with future fertilizer needs and the pattern of the world supply.
II. Carbon Di-oxide:
A. History:
CO2 discovered in 1750s, with temperature range (20-250C), carbon dioxide is
an odorless, colorless gas; it is faintly acidic and non-flammable. Its molecular
formula (O=C=O). CO2 exists commonly in the gaseous phase, exists in solid
phase (at temperature below 780C), and in liquid phase. Liquid carbon dioxide
exists as a dissolved form with water. CO2 is only water soluble when pressure
is maintained but after pressure drops carbon dioxide gas will try to escape to
air. Carbon dioxide can be found mainly in air, but also in water as a part of
the carbon cycle.
B. Carbon dioxide in the gas form:
Carbon dioxide gas is uniformly distributed over the earth‘s surface at a
concentration of about 330 ppm, as the concentration of the CO2 is low in
nature so it‘s not practical to extract the gas from air, so it‘s commercially
obtained as a by-product from other industries such as the production of
ethanol by fermentation and manufacture of ammonia. So, if we are building
ammonia plant fertilizer next to urea plant fertilizer we can get both raw
materials from the neighborhood plant.
Also, we can get CO2 from combustion of coke or other carbon containing
fuels
C (coke) + O2 CO2 (g)
15
Carbon dioxide is released into our atmosphere when carbon containing fossil
fuels such as oil, natural gas, and coal are burned in air. As a result of the
tremendous world-wide consumption of such fossil fuels, the amount of CO2
increased with a rising rate 1ppm/year.
C. Carbon Dioxide in the liquid form:
in addition to being a component of the atmosphere, carbon dioxide also
dissolves in water of the oceans. At room temperature, the solubility of CO2 is
about 90 cm3 of CO2 per 100 ml of water. In aqueous solution, CO2 exists in
many forms first is simply dissolves.
CO2 (g) CO2(aq)
Then an equilibrium is established between the dissolved CO2 and carbonic
acid H2CO3
CO2 (aq) +H2O (l) H2CO3 (aq)
Only about 1% of the dissolved CO2 exists as H2CO3. Carbonic acid is weak
acid which dissociates producing water and CO2.
D. Gasification of coal:
Coal gasification can be used to produce synthesis gas, a mixture of carbon
monoxide (CO) and hydrogen (H2) gas.
(Coal) + O2 + H2O H2 + CO
The synthesis gas is fed into the water gas shift reaction where CO2 is evolved
CO + H2O CO2 + H2
However, the process main concern is H2 but H2 will also serves in the
fertilizer plant to produce ammonia from H2 and urea from the liberated CO2 .
16
E. Properties of carbon dioxide:
Property Value
Molecular weight 44.01
Specific gravity 1.53 at 21 oC
Critical density 29.216 Ib/ft3
Concentration in air 370,3E7 ppm
Stability High
Liquid Pressure < 4.158 bar
Solid Temperature < -78 oC
Henry constant for solubility 0.29815 Ibmol/ Ib . bar
Water solubility 0.9 volume/volume at 20 oC
Table.3 Properties of carbon dioxide
F. Environmental hazards for CO2:
1. CO2 Emissions:
Due to human activities, the amount of CO2 released into the atmosphere has
been rising during the last 150 years. As a result, it has exceeded the amount
sequestered in biomass, the oceans, and other sinks.
Carbon dioxide concentrations are climbed in the atmosphere of about 280
ppm in 1850 to 364 ppm in 1998, mainly due to human activities during and
after the industrial revolution, which began in 1850.
Humans have been increasing the amount of carbon dioxide in air by burning
of fossil fuels, by producing cement and by carrying out land clearing and
forest combustion. About 22% of the current atmospheric CO2 concentrations
exist due to these human activities, considered that there is no change in
natural amounts of carbon dioxide.
2. Greenhouse Effect:
The troposphere is the lower part of the atmosphere, of about 10000- 15000 m
thick. Within the troposphere there are gasses called greenhouse gasses. When
sunlight reaches the earth, some of it, are converted to heat. Greenhouse
gasses absorb some of the heat and trap it near the earth's surface, then earth is
warmed up. This process, commonly known as the greenhouse effect, has been
discovered many years ago and was later confirmed by means of laboratory
experiments and atmospheric measurements.
17
Chapter Four: Urea Fertilizer
I. History:
Urea was first discovered by a French scientist, named Hillaire Rouelle in
1773.The synthesis of urea by Wohler in 1828 by heating ammonium cyanate
had a profound influence upon chemistry and upon civilization. It was the first
time that a substance produced by life had been prepared in the laboratory,
thus opening up the entire of the synthetic organic chemistry. For 100 years,
the epoch-making compound was relatively un-important industrially and was
made by the acid hydrolysis of calcium cyanamide. In 1933 Du Pont began
production at Belle, W. Va., and remained the sole domestic producer until
1950 when Solvay opened their plant at South Point, Ohio.
II. Importance of urea:
1- Used in nitrogen fertilizers.
2- Used in plastics in combination with formaldehyde and furfural.
3- Used in adhesives.
4- Used in coatings, textile anti shrink compounds, Ion-exchange resins.
5- Used as an intermediate for ammonium sulfamate, sulfumic acid, and
the phthalocyanine pigments.
III. Physical Properties of pure urea:
Color Colorless
Odor Odorless
Melting point 132.7 oC
Table.4 Physical Properties of pure urea
IV. Advantages and disadvantages of Urea Fertilizer:
Advantages:
Urea has the highest nitrogen content, equal to 46%. This percentage
is much higher than other nitrogenous fertilizers available in the
market.
Urea production cost is relatively low since carbon dioxide required for
its manufacture is easily obtained.
Urea is not subject to fire or explosion hazards, and hence there is no
risk in the storage of urea.
Urea can be used for all types of crops and soils. After its assimilation
by plants, urea leaves behind only carbon dioxide in the soil through
the interaction of nitrifying bacteria. This carbon dioxide does not
harm the soil.
18
Urea can be applied to soil as a solid, or solution, or to certain crops as
a foliar spray.
Urea manufacture releases few pollutants to the environment.
Disadvantages:
Urea is very soluble in water, and hygroscopic water; hygroscopic
water creates a thin layer surrounding individual soil particles, which
makes water unavailable to plants, and hence requires better packaging
quality.
Urea contains impurities more than 2%, it cannot be used as a
fertilizer, since the impurities are toxic to certain crops, particularly
citrus.
V. Modifications of the Stami-carbon CO2-stripping
process:
Throughout the study of many technologies concerning urea production the
most commonly used technology in Egypt is Stami-carbon technology, so we
are focusing now on the Stami-carbon technology
19
A. The Original Stamicarbon CO2-Stripping Process:
Fig.3 block diagram for Stamicarbon CO2 stripping Urea Process
In this first-generation CO2-stripping plant, the high-pressure carbamate
condenser was of the vertical falling-film type. The condensed liquid
carbamate is flowing down along the inside wall of the (vertical) heat
exchanger tubes. Condensation of ammonia and carbon dioxide gases occurs
in the high-pressure carbamate condenser at synthesis pressure. Besides
condensation, also chemical formation of ammonium carbamate from
ammonia and carbon dioxide takes place in this condenser. Because of the
high pressure, the heat liberated from the condensation and subsequent
ammonium carbamate formation is at a high temperature. This heat, therefore,
can effectively be used for the production of 4.5bar steam for further use in the
urea plant itself. The condensation in the high-pressure carbamate condenser is
not effected completely. Remaining gases are condensed in the reactor and
provide the heat required for the dehydration of carbamate.
Ammonia and carbon dioxide are introduced to the reactor with a molar ratio
3:1.The operating conditions of the reactor are 190oC and 140 bar. Therefore,
maximum urea yield per pass is achieved. Ammonia and carbon dioxide are
stripped off. The stripper is realized in the form of a falling-film evaporator,
20
where the urea synthesis solution flows as a falling film along the inside of the
vertical heat-exchanging tubes. Heat, in the form of medium-pressure steam, is
supplied to the outside of these tubes. The supply of heat at this place results
in decomposition of unconverted ammonium carbamate into ammonia and
carbon dioxide. Moreover, the heat supplied in this way will transfer ammonia
and carbon dioxide from the liquid phase into the gaseous phase. Fresh carbon
dioxide and air are supplied to the bottom of the tubes flows counter-currently
to the urea solution from top to bottom. Addition of air and lowering the
temperature are important to maintain a corrosion-resistant layer. To avoid the
formation of explosive hydrogen–oxygen mixtures in the tail gas of the plant,
hydrogen is catalytically removed from the carbon dioxide feed. The carbon
dioxide acts as a stripping agent, enhancing the transfer of ammonia from the
liquid phase into the gaseous phase. Stripping with carbon dioxide not only
recycles ammonia, but also effectively reduces the carbon dioxide content of
the urea synthesis solution flowing down the heat exchanger tubes. Low
ammonia and carbon dioxide concentrations in the stripped urea solution are
obtained, such that the recycle from the low-pressure recirculation stage is
minimized. Before the inert gases, mainly oxygen and nitrogen, are purged
from the synthesis section, they are washed with carbamate solution from the
low-pressure recirculation stage in the high-pressure scrubber to obtain a low
ammonia concentration in the subsequently purged gas. Further washing of the
off-gas is performed in a low-pressure absorber to obtain a purge gas that is
practically ammonia free.
21
Fig.4 Flow diagram of Stamicarbon CO2-stripping Urea Process.
1. Main Reactions:
The commercial processes in current use are based on two reactions:
CO2 + 2NH3 NH4CO2NH2 (1) ∆H= -67,000 BTU/Ibmole
NH4CO2NH2 NH2CONH2 + H2O (2) ∆H=+18,000 BTU/Ibmole
Reaction (1) is fast and exothermic and essentially goes to completion under
the reaction conditions used industrially.
Reaction (2) is slower and endothermic and does not go to completion. The
conversion (on a CO2 basis) is usually in the order of 50-80%. The conversion
increases with increasing temperature and NH3/CO2 ratio and decreases with
increasing H2O/CO2 ratio.
2. Side reactions:
Hydrolysis of urea:
CO(NH2)2+H2O NH2COONH4 2NH3+CO2 (3)
Biuret formation from urea:
2CO(NH2)2 NH2CONHCONH2 + NH3 (4)
Formation of isocyanic acid from urea:
CO(NH2)2 NH4NCO NH3+HNCO (5)
22
All three side reactions have in common the decomposition of urea; thus, the
extent to which they occur must be minimized.
The hydrolysis reaction (1) is nothing but the reverse of urea formation.
Whereas this reaction approaches equilibrium in the reactor, in all downstream
sections of the plant the NH3 and CO2 concentrations in urea-containing
solutions are such that Reaction (1) is shifted to the right. The extent to which
the reaction occurs is determined by temperature (high temperatures favor
hydrolysis) and reaction kinetics; in practice, this means that retention times of
urea-containing solutions at high temperatures must be minimized.
The biuret reaction (2) also approaches equilibrium in the urea reactor .The
high NH3 concentration in the reactor shifts Reaction (2) to the left, such that
only a small amount of biuret is formed in the reactor. In downstream sections
of the plant, NH3 is removed from the urea solutions, thereby creating a
driving force for biuret formation. The extent to which biuret is formed is
determined by reaction kinetics; therefore, the practical measures to minimize
biuret formation are the same as described above for the hydrolysis reaction.
B. Urea 2000 plus:
1. Urea 2000 plus with pool condenser:
In the 1990s, Stamicarbon introduced a new synthesis concept under the name
―Urea 2000plus‖. The key difference with respect to the previous Stami-
carbon processes is the application of pool condensation in the condensing
step in the synthesis recycle loop.
Fig.5 process flow diagram for Stamicarbon Urea
2000plus Process with pool condenser
23
Pool condensation is a technology where in a condensing operation; the liquid
phase is the continuous phase, whereas the gases to be condensed are present
as bubbles, rising through the liquid phase. As compared to the technique of
falling-film condensation pool condensation offers some considerable
advantages:
for pool condensation in any application in the process industry:
1- The turbulence that is introduced into the liquid phase by the
rising bubbles enhances the heat transfer from the liquid phase
to the cooling surfaces.
2- The contact area between the gaseous phase and the liquid
phase in pool condensation is considerably larger than in
falling-film condensation.
This advantage is specific for urea production since the liquid phase now is the
continuous phase, in pool condensation the residence time of the liquid phase
in the condenser is considerably longer. As the formation of urea from
ammonia and carbon dioxide basically goes through two steps: first the
chemical reaction to form ammonium carbamate from ammonia and carbon
dioxide which is fast and exothermic. The second step is the formation of urea
and water as a result of dehydration of ammonium carbamate which is slow
and endothermic. The urea and water formed during the dehydration in the
pool condenser have a higher boiling temperature than ammonia and
ammonium carbamate. This leads to a higher net boiling temperature of the
liquid mixture in the condensation step which also gives rise to a higher
temperature difference between the process side and the cooling side. This
increase in temperature difference can advantageously been applied for further
reduction in investment (smaller heat-exchanging area required).
2. Urea 2000 plus with pool reactor:
In a first variant of the Urea 2000plus technology, the pool condenser simply
replaced the falling-film condenser provided the technological advantage of
improving heat transfer in the condensing part of the urea synthesis. In a later
variant of this process, the pool condenser and the urea reactor were combined
into one single high-pressure vessel, called the pool reactor (Fig.6) By this
combination of high-pressure equipment items, the available temperature
difference over the condenser has increased by combining carbamate
condensation and urea reaction in one vessel, a further investment reduction
could be realized, especially for small- and medium-size production plants.
24
Fig.6 Process flow diagram for Stamicarbon Urea 2000plus process with
pool reactor.
In the pool-reactor concept, a further simplification of the process was realized
by deletion of the heat-exchanging part of the high-pressure scrubber. The
heat-exchange
step was replaced by a process step where cooling of the reactor off-gases
takes place through their direct contact with the relatively cold fresh ammonia
.The synthesis section of the Urea 2000plus plant is completed with a single
low-pressure recirculation stage. These subsequent process steps are similar to
the ones in the original Stamicarbon CO2-stripping process.
C. The Avancore process:
The Avancore urea process was introduced by Stamicarbon in 2009. It
comprises a new urea synthesis concept that incorporates the benefits of
Stamicarbon's earlier proven innovations. The Avancore Urea process material
of construction is Safurex, and includes a low-elevation layout of the synthesis
section.
The excellent corrosion resistant properties of the Safurex material in an
oxygen-free carbamate environment eliminate the need of using passivation
air in the urea processes. Because of the absence of oxygen in the synthesis
section, hydrogen or any other combustibles present in the feed no longer
poses any risk of explosion for the urea plant. The ammonia emissions are also
kept to a minimum because of the absence of passivation air.
In the Avancore process, Stamicarbon has introduced a low-level arrangement
of the synthesis section, where the reactor is located on ground level, which
allows less investment and easier maintenance. The concept still makes use of
a gravity flow in the synthesis recycle loop (Fig.7). However, the low-level
arrangement of the reactor necessitates another heat source for the
25
endothermic dehydration reaction taking place in the reactor because the pool
condenser off-gas cannot flow into this low-level reactor any more. Most of
the urea formation, however, already takes place in the pool condenser and,
therefore, only a minor amount of CO2 supplied to the reactor is sufficient to
close the heat balance around it.
Fig.7 Process flow diagram for Avancore urea process
All Stamicarbon CO2 stripping processes have some common features:
The use of carbon dioxide as stripping agent in the high-pressure
stripper.
The use of gravity flow to maintain the main recycles flow in the
high-pressure loop.
The use of an azeotropic N/C ratio (3:1) in the reactor.
The achievement of high degree conversion of both feed stocks (NH3
and CO2) within the synthesis loop. As a result, only one small low-
pressure carbamate recycle loop is required.
26
VI. Corrosion:
Urea synthesis solutions are very corrosive. Generally, ammonium carbamate
is considered the aggressive component. This follows from the observation
that carbamate-containing product streams are corrosive whereas pure urea
solutions are not. The corrosiveness of the synthesis solution has forced urea
manufacturers to set very strict demands on the quality and composition of
construction materials. Awareness of the important factors in material
selection, equipment manufacture and inspection, technological design and
proper operations of the plant, together with periodic inspections and non-
destructive testing are the key factors for safe operation for many years.
A. Role of Oxygen Content:
Since the liquid phase in urea synthesis behaves as an electrolyte, it causes
electrochemical nature corrosion. Stainless steel in a corrosive medium owes
its corrosion resistance to the presence of a protective oxide layer on the
metal. As long as this layer is intact, the metal corrodes at a very low rate.
Passive corrosion rates of austenitic urea-grade stainless steels are generally
between <0.01 and (max.) 0.10 mm/day. Upon removal of the oxide layer,
activation corrosion set in unless the medium contains sufficient oxygen or
oxidation agent to build a new layer. Active corrosion rates can reach values
of 50 mm/day. Stainless steel exposed to carbamate containing. Solutions
involved in urea synthesis can be kept in a passivity (non-corroding) state by
a given quantity of oxygen. If the oxygen content drops below this limit,
corrosion starts after some time – its onset depending on process conditions
and the quality of the passive layer. Hence, introduction of oxygen and
maintenance of sufficiently high oxygen content in the various process
streams are prerequisites to preventing corrosion of the equipment.
From the point of view of corrosion prevention, the condensation of NH3 –
CO2 –H2O gas mixtures to carbamate solutions deserves great attention. This
is necessary because not withstanding the presence of oxygen in the gas
phase – an oxygen-deficient corrosive condensate is initially formed on
condensation. In this condensate the oxygen is absorbed only slowly. This
accounts for the severe corrosion sometimes observed in cold spots inside
gas lines. The trouble can be remedied by adequate isolation and tracing of
the lines. When condensation constitutes an essential process step – for
example, in high-pressure and low-pressure carbamate condensers – special
technological measures must be taken. These measures can involve ensuring
that an oxygen rich liquid phase is introduced into the condenser, while
appropriate liquid – gas distribution devices ensure that no dry spots exist on
condensing surfaces. Not only condensing but also stagnant conditions are
dangerous, especially where narrow crevices are present, into which hardly
any oxygen can penetrate and oxygen depletion may occur.
27
B. Role of Temperature:
Temperature is the most important technological factor in the behavior of the
steels employed in urea synthesis. An increase in temperature increases
active corrosion, but more important, above a critical temperature it causes
spontaneous activation of passive steel. The higher-alloyed austenitic stain
Urea 11 less steel (e.g., containing 25 wt% chromium (Cr), 22 wt%
nickel(Ni), and 2wt%molybdenum(Mo)) appear to be much less sensitive to
this critical temperature than 316L types of steel. Sometimes, the NH3: CO2
ratio in synthesis solutions is also claimed to have an influence on the
corrosion rate of steels under urea synthesis conditions. Experiments have
showed that under practical conditions this influence is not measurable
because the steel retains passivity. Spontaneous activation did not occur.
Only with electrochemical activation could 316L types of steel be activated
at intermediate NH3:CO2 ratios. At low and high ratios, 316L stainless steel
could not be activated. The higher-alloyed steel type
25wt% Cr, 22 wt %Ni, 2wt%Mo showed stable passivity, irrespective of the
NH3: CO2 ratio, even when activated electrochemically. Of course, these
results depend on the specific temperature and oxygen content during the
experiments.
C. Material Selection:
Corrosion resistance is not the only factor determining the choice of
construction materials. Other factors such as mechanical properties,
workability, and weld-ability, as well as economic considerations such as
price, availability, and delivery time, also deserve attention. Stainless steels
that have found wide use are the austenitic grades 316L and 317 L. Like all
Cr-containing stainless steels, 316L and 317 L are not resistant to the action
of sulfides. Hence it is imperative in plants using the 316L and 317 L grades
in combination with CO2 derived from sulfur-containing gas, to purify this
gas or the CO2 thoroughly. In stripping processes, the process conditions in
the high-pressure stripper are most severe with respect to corrosion. In the
Stami-carbon CO2-stripping process, a higher-alloyed, but still fully
austenitic stainless steel was chosen as construction material for the stripper
tubes. This choice ensures better corrosion resistance than 316L or 317 L
types of material but still maintains the advantages of workability, weld-
ability, reparability, and the cheaper price of stainless steel-type materials.
28
Chapter Five: Other technologies for urea production
I. Technology for urea production (TOYO):
A. History:
Since its establishment in 1961, Toyo engineering corporation has been a
leader in the urea industry. TOYO has designed engineered, constructed and
commissioned over 100 urea plants based on the TOYO urea process
including its urea synthesis technologies and urea granulation technologies.
TOYO has established the ACES21® process (advanced process for cost and
energy saving urea production). Which achieves energy saving and plant cost
reduction without sacrificing high performance and high efficiency of the
urea plant. ACES21®is advanced technology to realize low investment cost
and low energy consumption for urea production .A major feature of this
technology is that it reduces the number of components in the urea synthesis
loop to simplify the system. This lessens construction costs with the
installation of the reactor on the ground in the CO2 stripping process.
B. Process description:
ACES21® process synthesis section consists of a reactor, a stripper and a
carbamate condenser. Liquid ammonia is fed to the reactor via the HP
carbamate Ejector which provides the driving force for circulation in
synthesis loop instead of the gravity system of the original ACES. The
reactor is operated at an N/C (NH3 to CO2) ratio of 3.7, 182 °C and 152 bar.
The CO2 conversion to urea is high as 63% at the exit of the reactor. 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
(VSCC),operated at an N/C ratio of 3.0, 180°C and 152 bar .
Ammonia and CO2 Gases condense to form ammonium carbamate and
subsequently urea is formed by dehydration of the carbamate in shell side.
Reaction heat of carbmate formation is recovered to generate 5bar steam in
the tube side. A packed bed is provided at the top of the VSCC to absorb
uncondensed ammonia and CO2 gases into a recycle carbamate solution from
the MP (medium pressure) absorption stage. Inert gas from the top of the
packed bed is sent to the MP absorption stage.
1. Ground Level Reactor:
The two-stage synthesis concept employing a VSCC and an HP ejector
enables the HP equipment in the synthesis section to be laid-out very
compactly in low elevation. The highest level (the VSCC top) is about 30 to
35 m (Depending on the plant‘s capacity and configuration). This is
29
significantly lower than even the traditional solution recycle process in which
the reactor is installed on the ground.
2. Vertical Submerged Carbamate Condenser:
The Vertical Submerged Carbamate Condenser (VSCC) functions to
condense NH3 and CO2 gas from the stripper to form ammonium carbamate
and synthesize urea by dehydration of ammonium carbamate. In the shell
side, and to remove the reaction heat of ammonium carbamate. Formation by
generating 5 bar steam in boiler tubes.
The advantages of the vertical submerged configuration of the carbamate
condenser are summarized as follows:
- High gas velocity, appropriate gas hold up and sufficient liquid depth in the
bubble column promote mass and heat transfer.
- An appropriate number of baffle plates distribute gas bubbles in the column
effectively without pressure loss.
- A vertical design allows a smaller plot area.
3. Optimum Selection of Synthesis Condition:
In the ACES21® Process, the VSCC is operated at an N/C ratio of 3.0 which
allows relatively high temperature operation of the VSCC, rendering efficient
heat transfer between the shell and the tube and higher reaction rate of
ammonium carbamate dehydration to form urea. The reactor N/C ratio is
selected at around 3.7 to maximize CO2 conversion with appropriate excess
pressure. Resultantly, a high CO2 conversion of 63% is achieved in the
reactor at relatively low temperature and pressure, i.e. 182 °C and 152 bar.
.
Fig.8 process flow diagram for TOYO urea Process
30
C. TOYO’S Process Performance: :OYO‘s Activities and Services
Product Quality Typical
Nitrogen (N) Content Content 46.4 wt%
Biuret Content 0.8 wt%
Water 0.2 wt%
Raw Material
NH3 (100%) 0.563 ton,CO2 (100%)
0.731 ton
Table.5 TOYO’S Process Performance
II. Snamprogetti Urea Technology:
A. Process description:
1. Synthesis and High Pressure (HP) recovery.
2. Medium Pressure (MP) purification and recovery.
3. Low Pressure (LP) purification and recovery.
1. Synthesis and High Pressure (HP) recovery:
In addition to the HP machinery required to feed ammonia and carbon
dioxide and to recycle ammonium carbamate solution, this section includes;
the reactor where urea is formed; the stripper necessary to strip out as vapors,
from the urea solution leaving the reactor, a large amount of ammonia and
carbon dioxide not converted to urea in the reactor; the carbamate condenser
that condenses these vapors; the ejector that recycles the ammonium
carbamate solution to the reactor. In this equipment the pressures are of a
similar level, 150 bar, while the temperatures of the outlet solutions are 188,
205 and 155oC for the reactor, the stripper and the carbamate condenser,
respectively. In the Snamprogetti technology, the urea reactors is
characterized by a high ammonia-carbon dioxide ratio (NH3/CO2) (3.2 – 3.4)
and a low water carbon dioxide ratio (0.4 – 0.6 molar). Inside the reactor a
matching number of trays of a very simple design are installed to improve the
conversion. Under these conditions 62-64% (conversion) of the total CO2
entering the reactor is converted to urea. The total carbon dioxide conversion
in the HP section (or loop) is 85-90%.
31
2. Medium Pressure (MP) purification and
recovery:
The purpose of this section is to partially strip out the reactants, ammonia
and carbon dioxide from the urea solution and, after their condensation in
water, to recycle the obtained solution to the reactor, together with the
ammonia and carbon dioxide aqueous solution resulting from the
downstream sections of the plant. The ammonia excess is separated in this
section and recycled to the reactor separately. A distillation column is
provided for this purpose. The operating pressure is 17 barg. A particular
feature is included in this section. Ammonia and carbon dioxide are partially
condensed in the shell of a preheater within the vacuum section, thus
recovering some energy in the form of 200kg of steam per ton of urea, with
an investment cost that, even in existing plants, has a pay-back time of less
than two years. Another particular characteristic of the MP section is the
washing of the inerts (CO, H2 and CH4) contained mainly in the carbon
dioxide and the passivation air. The quantity of passivation air in the
Snamprogetti technology is very small (one third compared with other
technologies). It is therefore easy to recover ammonia from the inerts without
the risk of explosion mainly due to H2/O2 mixtures. No hydrogen removal
from carbon dioxide is required. Upon special requests, different washing
systems have been designed by Snamprogetti and have already been installed
in industrial plants. To completely reduce the ammonia contained in the
inerts, in completely safe conditions with regard to explosions. It should be
emphasized that the presence of the MP section provides great plant
flexibility, which can be operated over a wide range of NH3/CO2 ratios, with
excess ammonia present in the urea stream from the stripper being recovered
and condensed by the MP section.
3. Low Pressure (LP) purification and
recovery:
Further stripping of ammonia and carbon dioxide is made in the LP section,
operating at 3.5 bar g. The vapors, containing ammonia and carbon dioxide,
are condensed and recycled to the reactor via the MP section. The urea
solution leaving the LP section is about 70% and contains small quantities of
ammonia and carbon dioxide.
32
Fig.9 Block flow diagram for snamprojectti Urea Process
UREA,
Fig.10 Process flow diagram for snamprojectti Urea Process
33
Chapter six: Urea-Ammonium Nitrate (UAN)
I. Overview of UAN Process Technology:
Ammonium nitrate (AN) and urea are used as feed stocks in the production
of urea-ammonium nitrate (UAN) liquid fertilizers. Most UAN solutions
typically contain 28, 30 or 32% nitrogen but other customized concentrations
(including additional nutrients) are produced. Plant capacities for the
production of UAN solutions range between 200 and 2,000 ton/day . Most of
the large scale production units are located on complexes where either urea
or ammonium nitrate or both are produced. In some of the European UAN
plants, ammonium nitrate is being synthesized directly from nitric acid and
ammonia. In some cases carbamate solution from the urea reactor outlet is
being used as feedstock for the production of UAN. In those plants the UAN
technology is an integral part of the fertilizer complex. UAN from scrubbing
systems, urea from sieving machines, etc. are fed to a central UAN system,
where quality adjustments can be done. The addition of corrosion inhibitors
or the use of corrosion resistant coatings allows carbon steel to be used for
storage and transportation equipment for the solutions.
II. physical properties of Urea ammonium nitrate solution:
Property Value
Nitrogen content 28-32% by weight
Ph 7 to 7.5
Density 80-82 Ib/ft3
Salt-out temperature –18 to –2°C depending on the N
content and at its lowest when the Urea
N/Ammonium Nitrate N ratio is about
1:1.
Table.6 physical properties of Urea ammonium nitrate solution
III. Description of Production Processes:
The Continuous and batch type processes are used and in both processes
concentrated urea and ammonium nitrate solutions are measured, mixed and
then cooled. Block diagrams for UAN production are shown in (Figures 1 -
2). In the continuous process the ingredients of the UAN solution are
continuously fed to and mixed in a series of appropriately sized static mixers.
Raw material flow as well as finished product flow, pH and density are
continuously measured and adjusted. The finished product is cooled and
transferred to a storage tank for distribution. In the batch process the raw
materials are sequentially fed to a mixing vessel fitted with an agitator and
mounted on load-cells. The dissolving of the solid raw material(s) can be
34
enhanced by recirculation and heat exchange as required. The pH of the
UAN product is adjusted prior to the addition of the corrosion inhibitor. A
partial recycle CO2stripping urea process is also suitable for UAN solution
production. Unconverted NH3and CO2 coming from the stripped urea
solution, together with the gases from the water treatment unit, are
transferred for conversion into UAN solutions. But nitrogen and the
application of Urea as fertilizer can be done at sowing time or as a top-
dressing, but should not be allowed to come into contact with the seed.
Fig.11 Block flow diagram for UAN process
Fig.12 Block diagram for a partial recycle CO2 stripping urea process
for UAN production
35
References:
1- Fertilizer manufacture, M.E.Pozin, 1990, MIR Publishers.
2- Ullman chemical encyclopaedia 4th
edition.
3- Rao, N. S. Bio fertilizers in Agriculture & Forestry.IBH, 1993.
4- Lowrison, George. Fertilizer Technology. John Wiley and Sons,
1989.
5- http://www.landscape-and-garden.com.
6- Uhde Engineering Egypt co. S.A.E (UEE).
Company contacts:
Internet: www.uhde-engineering-egypt.com
E-mail: [email protected]
7- MOPCO ( Misr fertilizers production Company).
Company contacts:
Internet: www.mopco-eg.com
8- Published document for Helwan fertilizers company December 2011
http://www.chemicals-technology.com/projects/helwanfertiliserco/
9- Published document for The urea technology Snamprojectti company
www.saipem.it
10. Published document for ACES21®
urea process by TOYO
engineering corporation
http://www.toyo-eng.co