TECHNOLOGICAL UNIVERSITY OFTHE PHILIPPINES- TAGUIG CAMPUS

Km. 14 East Service Road Western Bicutan, Taguig City

Prduction Of Urea By Ammonium Carbamate Dehydration

First Draft

Aug 27, 2015

Bryan Jesher S. Dela Cruz

3B1

Ria Rose Fundimera

3B2

Submitted to:

Dr. Emmanuel Ferrer

RESEARCH REPORT OUTLINE / CONTENT

Title Of the Report: Production Of Urea By Ammonium Carbamate Dehydration

Legend : n – 1st Accomplished n -2nd Accomplished

i. TABLE of CONTENTSii. LIST of FIGURES

a.) Figure 1 : Schematic representation of the ammonia synthesis processb.) Figure 2 : Detailed plant layout model of process plant used for production of ammonia and

urea from natural gas, along with utilities and treatment sections.c.) Figure 3 : Percentage Conversion Vs. Temperature Graph For the yield Uread.) Figure 4 : Percentage Conversion Vs. Pressure Graph For the yield Ureae.) Figure 5 : Block Diagram of a Total Recycle CO2 Stripping Urea Processf.) Figure 6 : Figure 2 – Block Diagram of a Total Recycle NH3 Stripping Urea Processg.) Figure 7 : Block Diagram for Urea Granulation and Prilling Processes.

iii. LIST of TABLES

a.) Table 1 - Composition of the gas stream after each process step (Ammonia Production)b.) Table 2: Ammonia Specification

i. Table Of Contents:

1. BACKGROUND

1.1. Significance of the study

1.2. Timeline of Development

1.2.1. 1727 – Noticed amount of Urea from Human Urine by Herman Boerhaave1.2.2. 1773 – Obtained Crystals from urine assumpted as Urea by HM Rouelle1.2.3. 1778 - Rediscovery of Urea by H.M. Rouelle 1.2.4. 1799 – Invention of the term Urea by Antoine François, comte de Fourcroy and Louis

Nicolas Vauquelin1.2.5. 1817 – Determination of the chemical composition of the pure substance1.2.6. 1828 - Accidental synthesis of Urea from an inorganic Material by Friedrich Wohler1.2.7. 1828 - Theory of Vitalism, derived from the discovery of Urea1.2.8. 1870 – Production of Urea by heating ammonium carbamate in a seal vessel1.2.9. 1922 – Standard method developed for the production of Urea by Bosch-Meiser process1.2.10. 1932 –Reaction in the mammalian liver for Urea synthesis by Hans A. Krebs and Kurt

Henseleit1.2.11. 1950 – Urea application as an Osmotic agent for the reduction of brain volume1.2.12. 1960 – Replacement of Urea by Mannitol asthe hypersmolar agent

2. PROPERTIES

2.1. Physical Properties2.2. Chemical Properties2.3. Health Hazards

2.3.1. Oral Exposure2.3.2. Inhalation Exposure2.3.3. Dermal Exposure2.3.4. Chronic Effects on Animals2.3.5. Effect on Reproduction2.3.6. Carcinogenicity

2.4. Environmental Hazards

3. USES and APPLICATIONS

3.1. Commercial Purposes

3.1.1. For Agriculture3.1.2. Niche3.1.3. Laboratory Uses3.1.4. Medical Uses3.1.5. Pharmaceutical

3.2. Industrial Purposes

3.2.1. For the Manufacture of Resins for Marine Plywood Production3.2.2. For Explosives3.2.3. For Automobile Systems3.2.4. For separation of racemic mixture

4. DETAILED MANUFACTURING PROCESS or PROCESS OPERATION

4.1. Urea Plant Installation

4.1.1. Urea Plant Layout Description4.1.2. Factors which Influence the Plant Layout

4.2. Step by Step or Stage by Stage Detailed Process Description

4.2.1. Urea Process Description4.2.2. The Variables That Affect The Autoclave Reactions

4.2.2.1. Temperature4.2.2.2. Pressure4.2.2.3. Concentration4.2.2.4. Residence time

4.2.3. First Process: Production Of Feed For Urea Plant

4.2.3.1. Production Of Ammonia

4.2.3.1.1. Haber Process Schematic Representation (Ammonia Synthesis )4.2.3.1.2. Hydrogen Production

a.) Natural gas in Desulfurizerb.) Desulfurized Gas in Steam Reformer

4.2.3.1.3. Nitrogen Additiona.) Air Reformer

4.2.3.1.4. Removal Of Carbon Monoxide4.2.3.1.5. Water Removal4.2.3.1.6. Removal Of Carbon Oxides4.2.3.1.7. Synthesis Of Ammonia4.2.3.1.8. Material Balance

a.) Material Balance around Desulfurizerb.) Material Balance around Steam Reformerc.) Material Balance around Air Reformerd.) Material Balance around CO2 Strippere.) Material Balance around Decompressor

4.2.3.1.9. Energy Balancea.) Energy Balance around Desulfurizerb.) Energy Balance around Steam Reformerc.) Energy Balance around Air Reformerd.) Energy Balance around CO2 Strippere.) Energy Balance around Decompressor

4.2.3.2. Recycling Carbon dioxide from Ammonia Production Plant

4.2.4. Second Process: Storage and Transfer Equipment

a.) Ammoniab.) Carbon Dioxidec.) Conditioning Agent

4.2.5. Third Process: Urea Synthesis Tower

4.2.5.1. Ammonia and Carbondioxide Reaction4.2.5.2. Material Balance at Reactor4.2.5.3. Energy Balance at Reactor

4.2.6. Fourth Process: Stripping4.2.6.1. Options:

a.) Carbon dioxide Stripping Processb.) Ammonia Stripping Processc.) Advance Cost and Energy Saving (ACES ) Processd.) Isobaric Double Recycle ( IDR ) Process

4.2.6.2. Selection Of the Process4.2.6.3. Material Balance at Stripper4.2.6.4. Energy Balance at Stripper

4.2.7. Fifth Process: Decomposition / Purification and Recovery4.2.7.1. Stages Of Purification and Recovery

a. First Stage: Purification and Recovery Stage at 18 ata Medium Pressure Decomposer/Separator

b. Second Stage: First Stage: Purification and Recovery Stage at 4.5 ata Low Pressure Decomposer/Separator

4.2.7.2. Material Balancea. Material Balance at Medium Pressure Decomposer / separatorb. Material Balance at Low Pressure Decomposer / separator

4.2.7.3. Energy Balancea. Energy Balance at Medium Pressure Decomposer / separatorb. Energy Balance at Low Pressure Decomposer / separator

4.2.8. Sixth Process: Urea Concentration

4.2.8.1. Evaporation 4.2.8.2. Material Balance at Vacuum Evaporator4.2.8.3. Energy Balance at Vaccum Evaporator

4.2.9. Seventh Process: Final Processing

a.) For Urea Production

1.) Options for final product

1.1. Prilling1.2. Granulation

2.) Material Balance at Prilling tower3.) Energy Balance at Prilling tower

b.) For Water Treatment

5. PRODUCT QUALITY MONITORING/ TESTING / CONTROL / ASSURANCE

5.1. Alkalinity5.2. Biuret Analysis5.3. Chloride Analysis5.4. Formaldehyde5.5. Free Ammonia Content5.6. Water Analysis (Moisture Content)5.7. Total Nitrogen Analysis5.8. pH analysis5.9. Screening5.10. Borden Test

6.FUTURE DEVELOPMENTS and RECOMMENDATIONS7. REFERENCES

I. Background

1.2. Timeline of Development

1727 – Notice Urea from Human Urine by Herman Boerhaave

Before Wohler, it was still possible to get hold of urea but it doesn't sound pleasant Herman Boerhaave purified urea from urine 100 years earlier. Boerhaave, from Leiden in the Netherlands, is best known now as a physician and the founder of clinical teaching, and at the time he did not consider himself to be much of a chemist. In fact, he only published his chemistry work because he was forced to when his students published it on his behalf. He even wrote in his book: 'Nothing was formerly further from my thoughts than that I should trouble the world with anything in chemistry.'

1773 – Obtained Crystals from urine assumpted as Urea by HM Rouelle French chemist Hilaire Rouelle discovered urea crystals from the urine of several animals, including humans. Accordingly, he isolated that colorless, odorless, crystalline substance in 1773 by boiling urine (Myers 2007).

In 1799, French chemists Antoine François de Fourcroy (1755-1809) and Louis Nicolas Vauquelin (1763-1829) named the substance “urea” (Richet 1988).

1817 – Determination of the chemical composition of the pure substance

According to Rosenfeld (2003), even before 1727 Boerhaave already obtained a crystalline residue from urine by heating, filtering, washing, and evaporating. He called it “the native salt of urine.” He noted that it differed from the sea salt (sodium chloride) which is also present in urine. Further, according to the same author, Rouelle’s extract was impure and that it was British physician-chemist William Prout (1785-1850) who, in 1817, isolated pure urea from urine.

1828 – Accidental synthesis of Urea from aninorganic Material by Friedrich Wohler 1828 – Theory of Vitalism, derived from the discovery of Urea

Vitalism

Until the early 19th century, people - including many scientists - believed in a theory called vitalism. Those who believed in this theory held that life was not subject to the laws of physics and chemistry. They believed that there was an unknown, even divine principle, that governed living organisms, called the 'life spark'.

Because of this belief, it was thought that chemicals found in plant and animal bodies - like proteins and carbohydrates - were completely different from other chemicals like salts, acids and gases. Therefore, people thought that 'organic' chemicals (because they came from organs) could not be made artificially, but had to be extracted from living animals. This theory also stopped people from using inorganic chemicals to treat diseases.

The breakthrough came in 1828. And like many major discoveries in science, it was accidental. Friedrich Wohler (pictured) was a scientist at the polytechnic school of Berlin, who was then famous as the discoverer of aluminium. He was trying to make ammonium cyanate by mixing ammonium chloride with silver cyanate.

AgNCO + NH4Cl → NH4NCO + AgCl

But when he examined the resulting crystals closely, the compound he got did not behave like ammonium cyanate should. The crystals behaved like the ones Hilaire Rouelle had got - urea. Therefore Wohler came to a radical conclusion, as he wrote in a letter to the famous chemist Berzelius - I... must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea.

In 1870 urea was produced by heating ammonium carbamate in a sealed vessel. This provided the basis of the current industrial process for its production.

1922 – Standard method developed for the production of Urea by Bosch-Meiser process 1932 – Reaction in the mammalian liver for Urea synthesis by Hans A. Krebs and Kurt

Henseleit

The basic process, developed in 1922, is also called the Bosch–Meiser urea process after its discoverers. The various commercial urea processes are characterized by the conditions under which urea formation takes place and the way in which unconverted reactants are further processed. The process consists of two main equilibrium reactions, with incomplete conversion of the reactants.

The first is carbamate formation: The fast exothermic reaction of liquid ammonia with gaseous carbon dioxide (CO2) at high temperature and pressure to form ammonium carbamate (H2N-COONH4)

2NH3 + CO2 H2N-COONH4

The second is urea conversion: the slower endothermic decomposition of ammonium carbamate into urea and water:

H2N-COONH4 (NH2)2CO + H2O

In 1828, Friedrich Wöhler, a German physician and chemist by training, published a paper that describes the formation of urea, known since 1773 to be a major component of mammalian urine, by combining cyanic acid and ammonium in vitro. In these experiments the synthesis of an organic compound from two inorganic molecules was achieved for the first time. These results weakened significantly the vitalistic hypothesis on the functioning of living cells, although Wöhler, at that time, was more interested in the chemical consequences of isomerism than in the philosophical implications of his finding. However, the chemical synthesis observed by Wöhler does not represent the reaction which is employed in the mammalian liver for urea synthesis. The mechanism of this process was

elucidated by the German physician Hans A. Krebs and his medical student Kurt Henseleit in 1932 and was shown to include the ornithine cycle.

1950 –Urea application as an Osmotic agent for the reduction of brain volume 1960’s – Replacement of Urea by Mannitol asthe hypersmolar agent

Urea was first used as an osmotic agent for the reduction of brain volume in 1950. It was associated with greater efficacy and consistency than alternatives such as hyperosmolar glucose. Its use became the standard of clinical practice by 1957, in both the intensive care unit and operating room, to reduce intracranial pressure and brain bulk and was the first hyperosmolar compound to have widespread use. However, the prime of urea was rather short lived. Reports of side effects and complications associated with urea emerged. These included coagulopathy, hemoglobinuria, electrocardiography changes, tissue necrosis with extravasation, and a significant potential for rebound intracranial hypertension. Mannitol was introduced in 1961 as a comparable and potentially superior alternative to urea. However, mannitol was initially purported to be less effective at rapidly reducing intracranial pressure. The debate over the two compounds continued for a decade until mannitol eventually replaced urea by the late 1960s and early 1970s as the hyperosmolar agent of choice due to the ease of preparation, chemical stability, and decreased side effect profile. Although urea is not currently the standard of care today, its rise and eventual replacement by mannitol played a seminal role in both our understanding of cerebral edema and the establishment of strategies for its management.

II. Properties

Chemical Properties

Molecular Formula CH4N2OMolecular Weight 60.06 g/moleMelting Point 133 ° CDensity 1.335 at room temperatureBoiling point Decomposes

Physical Properties

Physical state CrystalsAppearance WhiteOdor Odorless solidSolubility in water Soluble

Health Hazards

Eye: Causes eye irritation

Skin: Causes skin irritation

Ingestion: Causes gastrointestinal irritation with nausea, vomiting and diarrhea. May cause cardiac

disturbances, May cause disturbed blood electrolyte balance

Inhalation: Inhalation of dust causes irritation of the nose and throat, coughing and sneezing

Chronic: Prolonged or repeated exposure may cause adverse reproductive effects. Laboratory

experiments have resulted in mutagenic effects. Prolonged exposure or exposure to high concentrations

may cause eye damage

First Aid measures

Eyes: Immediately flush eyes with plenty of water for at least 15 minutes, occasionally lifting the

upper and lower eyelids. Get medical aid. 

Skin: Get medical aid. Immediately flush skin with plenty of soap and water for at least 15 minutes

while removing contaminated clothing and shoes. Wash clothing before reuse. 

Ingestion: If victim is conscious and alert, give 2-4 cupfuls of milk or water. Never give anything by

mouth to an unconscious person. Get medical aid. 

Inhalation: Remove from exposure to fresh air immediately. If not breathing, give artificial

respiration. If breathing is difficult, give oxygen. Get medical aid. 

Notes to Physician: Treat symptomatically and supportively. Weak acids such as acetic acid and

propionic acid can be used as chemical antidotes, demulcents and stimulants.

Handling and Storage

Handling: Wash thoroughly after handling. Remove contaminated clothing and wash before reuse. Use with adequate ventilation. Minimize dust generation and accumulation. Avoid breathing dust, vapor, mist, or gas. Avoid contact with eyes, skin, and clothing. Keep container tightly closed. Avoid ingestion and inhalation.

Storage: Store in a tightly closed container. Store in a cool, dry, well-ventilated area away from incompatible substances.

Environmental hazards

No environmental hazard assessment was conducted as urea is not a hazardous substance

III. Uses and Applications

3.1 Commercial Purposes

Fertilizers

The most common application of this compound outside of metabolic functioning is as a type of fertilizer. Over 90% of the world's production of the substance is done for fertilizer-related products. When used in this way, it usually takes the form of granules or crystals. These may be manually distributed by farmers or scattered with the aid of farming equipment. It is also often used in fertilizing solutions, since it is highly water soluble, and often comes packed within soil and potting mixes.

Resins and Plastics (Urea Formaldehyde)

This compound is also frequently used as a base product in the manufacture of resins and commercial adhesives. The nitrogen bonds that it contains tend to be very strong, and can really help strengthen a number of glues and tapes. Manufacturers often activate these bonds by dissolving the crystals in formaldehyde. The resulting mixture can be used as an industrial adhesive, as in the production of cardboard boxes; it also features as an ingredient in many poured plastics. In some cases it could also be used as a coating for materials like textiles and paper.

Fig.1 Urea Fertilizer

Consumer Products

A number of consumer-oriented and cosmetics products also incorporate this substance. Hair conditioners or tooth-whitening products often use it, for instance, usually as a way to help the product stay thick in the tube or bottle. Dish soaps sometimes also include it in at least in trace amounts to help keep emulsified ingredients from separating.

Facial cleaners sometimes incorporate the substance, too, since it can help in hydrating the skin. Certain makeup products blend it in to help achieve a creamier, glossier finish once applied. Some cigarette brands have, from time to time, added it as a flavor enhancer. Environmental activists in many places are often quick to point out that it can be used in an eco-friendly way to reduce fuel emissions from power plants and diesel engines, too.

Livestock Feed

Urea is sometimes also used in cattle and livestock feed, particularly in the developing world. It is usually considered to be an effective feed since it contains high concentrations of nitrogen, which can generally aid animal growth. The relatively cheap price of urea-based products also makes this feed a popular choice for many farmers. The compound is relatively inexpensive to make and doesn’t cost much to transport, two factors that boost its popularity in many parts of the world. It isn’t always the best choice for a given set of circumstances, but it will work in a great many different ways.

Fig. 2.1 Urea Formaldehyde Resin Powder

Fig. 2 Urea Formaldehyde fused plugs

3.2 Industrial Purposes

For the Manufacture of Resins for Marine Plywood Production

Plywood is made of three or more thin layers of wood bonded together with an adhesive. Each layer of

wood, or ply, is usually oriented with its grain running at right angles to the adjacent layer in order to

reduce the shrinkage and improve the strength of the finished piece. Most plywood is pressed into

large, flat sheets used in building construction. 

Raw Materials

The outer layers of plywood are known respectively as the face and the back. The face is the surface

that is to be used or seen, while the back remains unused or hidden. The center layer is known as the

core. In plywoods with five or more plies, the inter-mediate layers are known as the crossbands.

Plywood may be made from hardwoods, softwoods, or a combination of the two. Some common

hardwoods include ash, maple, mahogany, oak, and teak. The most common softwood used to make

plywood in the United States is Douglas fir, although several varieties of pine, cedar, spruce, and

redwood are also used.

Composite plywood has a core made of particleboard or solid lumber pieces joined edge to edge. It is

finished with a plywood veneer face and back. Composite plywood is used where very thick sheets are

needed.

The type of adhesive used to bond the layers of wood together depends on the specific application for

the finished plywood. Softwood plywood sheets designed for installation on the exterior of a structure

usually use a phenol-formaldehyde resin as an adhesive because of its excellent strength and resistance

to moisture. Softwood plywood sheets designed for installation on the interior of a structure may use a

blood protein or a soybean protein adhesive, although most softwood interior sheets are now made

with the same phenol-formaldehyde resin used for exterior sheets. Hardwood plywood used for interior

applications and in the construction of furniture usually is made with a urea-formaldehyde resin.

Some applications require plywood sheets that have a thin layer of plastic, metal, or resin-impregnated

paper or fabric bonded to either the face or back (or both) to give the outer surface additional resistance

to moisture and abrasion or to improve its paint-holding properties. Such plywood is called overlaid

plywood and is commonly used in the construction, transportation, and agricultural industries.

Other plywood sheets may be coated with a liquid stain to give the surfaces a finished appearance, or

may be treated with various chemicals to improve the plywood's flame resistance or resistance to

decay.

For explosives

Urea Nitrate

Urea Nitrate is a high explosive produced by combining dissolved urea fertilizer with nitric acid. Urea nitrate is formed as odorless crystals that are colorless to off-white, although additives and or metal from the mixing container may alter the compound’s appearance. Urea nitrate is used as a secondary explosive/main charge

Automobile systems

Urea is used in SNCR and SCR reactions to reduce the NOx pollutants in exhaust

gases from combustion from Diesel, dual fuel, and lean-burn natural gas engines. The BlueTecsystem,

for example, injects a water-based urea solution into the exhaust system. The ammonia produced by

the hydrolysis of the urea reacts with the nitrogen oxide emissions and is converted into nitrogen and

water within the catalytic converter. Trucks and cars using these catalytic converters need to carry a

supply of diesel exhaust fluid (DEF, also known as AdBlue), a mixture of urea and water.

IV. DETAILED MANUFACTURING PROCESS or PROCESS OPERATION

IV.1. Urea Plant Installation

IV.1.1. Urea Plant Layout Description

Figure 2 : Detailed plant layout model of process plant used for production of ammonia and urea from natural gas, along with utilities and treatment sections.

A simple plant layout model explains how to design a process plant effectively with all the facilities required for smooth running of production of desired product, for example to produce ammonia from natural gas and air and then urea by ammonia and carbon dioxide, a prefect design can be made so that it operates with high performance and less maintenance with negligible shut downs. Start up of plant after periodic maintenance should not be much difficult to handle and it should come in line as quickly as possible with appropriate controlling system.Coming to the description of plant layout designing a process plant requires enough area so that it can adjust some of the sections like:

Process unit Off-sites Cooling towers Electrical stations Effluent treatment plant Demineralisation plant Pre-treatment plant Raw material and product storage facility Power generator unit Boiler section Control room, administration block and canteen Transport station Maintenance and workshop section

The above are some of the main section to present in any process plant by default and of course based on product and chemical reaction mechanism some of other section can be included for example in case of pharmaceutical product separate quarantine and packing section are included. As per know let see what are the operation of each section and their role in industry would be. By design a plant layout before construction of the plant will help to place the section in available are so that plant can operate economically and obtain profits. During the design some of the factors really help a design engineer to give his best output.

IV.1.2. Factors which Influence the Plant Layout:

Integration of section Minimum man and material movements Smooth running of process Safe and secured system Flexibility

IV.2. Step by Step or Stage by Stage Detailed Process Description:

IV.2.1. Urea Process Description:

The commercial synthesis of urea involves the combination of ammonia and carbon dioxide

at high pressure to form ammonium carbamate which is subsequently dehydrated by the

application of heat to form urea and water.

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 conversionincreases with increasing temperature and NH3/CO2 ratio and decreases with increasingH2O/CO2 ratio.

The design of commercial processes has involved the consideration of how to separate theurea from the other constituents, how to recover excess NH3 and decompose the carbamatefor recycle. Attention was also devoted to developing materials to withstand the corrosivecarbamate solution and to optimise the heat and energy balances. The simplest way to decompose the carbamate to CO2 and NH3 requires the reactor effluent to be depressurised and heated. The earliest urea plants operated on a “Once Through” principle where the off-gases were used as feedstocks for other products.

Subsequently “Partial Recycle” techniques were developed to recover and recycle some ofthe NH3 and CO2 to the process. It was essential to recover all of the gases for recycle to thesynthesis to optimise raw material utilisation and since recompression was too expensive analternative method was developed. This involved cooling the gases and re-combining them toform carbamate liquor which was pumped back to the synthesis. A series of loops involvingcarbamate decomposers at progressively lower pressures and carbamate condensers wereused. This was known as the “Total Recycle Process”. A basic consequence of recycling thegases was that the NH3/CO2 molar ratio in the reactor increased thereby increasing the ureayield.Significant improvements were subsequently achieved by decomposing the carbamate inthe reactor effluent without reducing the system pressure. This “Stripping Process” dominatedsynthesis technology and provided capital/energy savings. Two commercial stripping systemswere developed, one using CO2 and the other using NH3 as the stripping gases.Since the base patents on stripping technology have expired, other processes have emergedwhich combine the best features of Total Recycle and Stripping Technologies. For conveniencetotal recycle processes were identified as either “conventional” or “stripping” processes.

The urea solution arising from the synthesis/recycle stages of the process is subsequentlyconcentrated to a urea melt for conversion to a solid prilled or granular product.

Improvements in process technology have concentrated on reducing production costs andminimising the environmental impact. These included boosting CO2 conversion efficiency,increasing heat recovery, reducing utilities consumption and recovering residual NH3 andurea from plant effluents. Simultaneously the size limitation of prills and concern about theprill tower off-gas effluent were responsible for increased interest in melt granulation processesand prill tower emission abatement. Some or all of these improvements have been used inupdating existing plants and some plants have added computerised systems for process control.New urea installations vary in size from 800 to 2,000t.d-1 and typically would be1,500t.d-1 units.

IV.2.2. The Variables That Affect The Autoclave Reactions

IV.2.2.1. Temperature

Process temperature (185 oC) favours equilibrium yield at a given pressure (180atm). The conversion of ammonium carbamate to urea gradually increases as thetemperature increases. However, after a particular temperature, depending uponthe pressure, the conversion suddenly drops with further increase in temperature.The pressure corresponding to this temperature which is usually in the range of175-185oC, is known as the decomposition pressure which is about 180 atm.

IV.2.2.2. Pressure

The main reaction is sufficiently slow at atmospheric pressure. However, it startsalmost instantaneously at pressure of the order of 100 atm and temperature of 150oC. There is reduction in volume in the overall reaction and so high pressurefavors the forward reaction. This pressure is selected according to the temperatureto be maintained & NH3:CO2 ratio.

IV.2.2.3. Concentration

Higher the concentration of the reactants, higher will be the forward reactionaccording to the law of mass action. CO2 being the limitimg reagent higher

NH3:CO2 ratio favors conversion. Since, dehydration of carbamate results in ureaproduction, lesser H2O:CO2 ratio favors conversion. Water intake to the reactorshould therefore be minimum.

IV.2.2.4. Residence time

Since, urea reaction is slow and takes about 20 mins to attain equilibrium,sufficient time is to be provided to get higher conversion. Reactor is designed toaccommodate this with respect to the other parameters of temperature, pressureand concentration.

IV.2.3. First Process: Production Of Feed For Urea Plant

IV.2.3.1. Production Of AmmoniaIV.2.3.1.1. Haber Process Schematic Representation (Ammonia Synthesis )

IV.2.3.1.2. Hydrogen Production

Hydrogen is produced by the reaction of methane with water. However, before this can becarried out, all sulfurous compounds must be removed from the natural gas to preventcatalyst poisoning. These are removed by heating the gas to 400oC and reacting it with zincoxide:

Following this, the gas is sent to the primary reformer for steam reforming, where superheatedsteam is fed into the reformer with the methane. The gas mixture heated with naturalgas and purge gas to 770oC in the presence of a nickel catalyst. At this temperature thefollowing equilibrium reactions are driven to the right, converting the methane to hydrogen,carbon dioxide and small quantities of carbon monoxide:

This gaseous mixture is known as synthesis gas.

IV.2.3.1.3. Nitrogen Addition

The synthesis gas is cooled slightly to 735oC. It then flows to the secondary reformer whereit is mixed with a calculated amount of air. The highly exothermic reaction between oxygenand methane produces more hydrogen. Important reactions are:

1Air is approximately 78% nitrogen, 21% oxygen and 1% argon with traces of other gases.

In addition, the necessary nitrogen is added in the secondary reformer.As the catalyst that is used to form the ammonia is pure iron, water, carbon dioxide andcarbon monoxide must be removed from the gas stream to prevent oxidation of the iron. Thisis carried out in the next three steps.

IV.2.3.1.4. Removal Of Carbon Monoxide

Here the carbon monoxide is converted to carbon dioxide (which is used later in the synthesisof urea) in a reaction known as the water gas shift reaction:

This is achieved in two steps. Firstly, the gas stream is passed over a Cr/Fe3O4 catalyst at360oC and then over a Cu/ZnO/Cr catalyst at 210oC. The same reaction occurs in both steps,but using the two steps maximizes conversion.

IV.2.3.1.5. Water Removal

The gas mixture is further cooled to 40oC, at which temperature the water condenses out andis removed.

IV.2.3.1.6. Removal Of Carbon Oxides

The gases are then pumped up through a counter-current of UCARSOL solution (anMDEA solution, see article). Carbon dioxide is highly soluble in UCARSOL, and more than99.9% of the CO2 in the mixture dissolves in it. The remaining CO2 (as well as any CO thatwas not converted to CO2 in Step 3) is converted to methane (methanation) using a Ni/Al2O3catalyst at 325oC: 2

The water which is produced in these reactions is removed by condensation at 40oC as above.The carbon dioxide is stripped from the UCARSOL and used in urea manufacture. TheUCARSOL is cooled and reused for carbon dioxide removal.

IV.2.3.1.7. Synthesis Of Ammonia

The gas mixture is now cooled, compressed and fed into the ammonia synthesis loop (seeFigure 1). A mixture of ammonia and unreacted gases which have already been around theloop are mixed with the incoming gas stream and cooled to 5oC. The ammonia present isremoved and the unreacted gases heated to 400oC at a pressure of 330 barg and passed overan iron catalyst. Under these conditions 26% of the hydrogen and nitrogen are converted toammonia. The outlet gas from the ammonia converter is cooled from 220oC to 30oC. Thiscooling process condenses more the half the ammonia, which is then separated out. The2These reactions are the reverse of the primary reformer reactions seen in Step 1. Thecatalyst in both

cases

is nickel, illustrating the fact that a catalyst accelerates both the forward and back reactions of an equilibrium system.

At reforming temperatures (~850oC) the methane is almost completely converted to carbon oxides and hydrogen as the reaction is endothermic and favoured by the high temperature. However, at the much lower temperature used for methanation (~325oC), the equilibrium lies to the right and practically complete conversion of the carbon oxides to methane is obtained.

remaining gas is mixed with more cooled, compressed incoming gas. The reaction occuringin the ammonia converter is:

The ammonia is rapidly decompressed to 24 barg. At this pressure, impurities such asmethane and hydrogen become gases. The gas mixture above the liquid ammonia (whichalso contains significant levels of ammonia) is removed and sent to the ammonia recoveryunit. This is an absorber-stripper system using water as solvent. The remaining gas (purgegas) is used as fuel for the heating of the primary reformer. The pure ammonia remaining ismixed with the pure ammonia from the initial condensation above and is ready for use in ureaproduction, for storage or for direct sale. Ammonia product specifications are given inTable 2.

†Water is not listed among the gases considered because its levels are highly variable. All water is eliminated

after step 4.‡All figures are given in mol % (i.e. the percentage of the total number of moles of gas present that are

due tothis gas).

*The gaseous composition after Step 4 is the same as that after Step 3 as Step 4 is simply the removal of water.

IV.2.3.1.8. Material BalanceIV.2.3.1.9. Energy Balance

IV.2.3.2. Recycling Carbon Dioxide from Ammonia Production Plant

4.2.4. Second Process: Storage and Transfer Equipment

DESCRIPTION OF STORAGE AND TRANSFER EQUIPMENT

Liquefied ammonia from production plants is either used directly in downstream plants or transferred to storage tanks. From these the ammonia can be transferred to road tankers, rail tank cars or ships. Ammonia is usually stored by using one or other of three methods:

– Fully refrigerated storage in large tanks with a typical capacity of 10,000 to 30,000 tonnes (up to 50,000) –

Pressurised storage spheres or cylinders up to about 1,700 tonnes

– Semi-refrigerated tanks Emissions during normal operation are negligible. Major leaks of ammonia from storage tanks are almost unknown with most of the leaks which do occur being during transport or transfer. A well designed, constructed, operated and maintained installation has a very low

probability of an ammonia leak of hazardous proportions. However, even though the residual risk is small, the effects of a major leak on areas of high population density could be very serious. It is therefore good practice to build ammonia storage and handling installations at a sufficient distance from domestic housing, schools, hospitals or any area where substantial numbers of people may assemble. In some countries there are planning procedures or regulations which control the siting of ammonia storage installations and similar establishments. Where there are no formal controls, the siting of ammonia storage facilities should be given serious consideration at the design stage. It is undesirable for ammonia storage tanks to be sited close to installations where there is a risk of fire or explosion, since these could increase the possibility of a release of ammonia.

Storage Tanks Anhydrous ammonia is stored in three types of tank as outlined above:

– Fully refrigerated at a temperature of about –33°C, these tanks are provided with refrigeration equipment – Non-refrigerated tanks in which the ammonia is stored at ambient emperature – Semi-refrigerated spheres Refrigerated storage is preferred for storage of large quantities of liquid ammonia. The initial release of ammonia in the case of a line or tank failure is much slower than with pressurized ammonia. There are several construction types for the storage of refrigerated liquid products. The most important types are:

– Single containment: a single-wall insulated tank, normally with a containment bund around it

– Double containment: this type of storage tank has two vertical walls, both of which are designed to contain the stored amount of liquid and withstand the hydrostatic pressure of the liquid. The roof rests on the inner wall

– Full containment: the two walls of this closed storage tank are also designed to contain the stored amount of liquid, but in this case the roof rests on the outer wall. The tank must be constructed in conformity with an agreed code for the construction of pressure vessels or storage tanks and taking account of its pressure and operating temperature.

The design and materials of construction of the tank should be checked by consulting an appropriate national, or recognised international, standard. These could make demands on the blast resistance of storage tanks in some cases. The storage tank must be safeguarded against high pressure and in the case of refrigerated liquid ammonia also against a pressure below the minimum design pressure.

The ingress of warm ammonia into cold ammonia must be avoided to eliminate risk of excessive evaporation and the “roll-over” phenomenon. All storage tanks should be equipped with two independent level indicators, each having a high level alarm. An automatic cut-off valve, operated by a very high level alarm instrument, should be installed on the feeding line. In cases of refrigerated liquid ammonia, storage tanks must be equipped with a recompression installation to liquefy the boil-off. There should be at least two refrigeration units to allow proper maintenance and to prevent the emission of ammonia via the relief valves. Furthermore, an installed alternative power supply may be necessary. An automatic discharge system to a flare may be provided in case of failure of the

refrigeration equipment. The flare must be located at a suitable distance from the tanks. Relief valves should be provided, appropriate for the duty using an adequate margin between operating and relief pressure.

Urea (NH2CONH2) is of great importance to the agriculture industry as a nitrogen-richfertiliser. In Kapuni, Petrochem manufacture ammonia and then convert the majority of itinto urea. The remainder is sold for industrial use.

Storage and Transfer Equipment in the Planta.) Ammonia

NH3 is pumped to the urea plant at 25bar pressure and 27°C. It is then supplied to a high pressure reciprocating pump for discharge to the urea synthesis section of the plant and the flow is regulated by a speed controller at a discharge pressure of 150-200bar depending on the process applied

b.) Carbon Dioxide

CO2 is supplied to the CO2 compressor and discharged at high pressure to the synthesis section of the urea plant.

c.) Conditioning Agent

An aqueous solution of urea-formaldehyde resin containing 50-60%wt formaldehyde and 20-25%wt urea is supplied by tanker and off-loaded to a buffer storage tank. It is injected by pump into the urea melt prior to prilling or granulation. In some modern urea granulation plants continuous urea-formaldehyde resin production units are an integral part of the granulation technology.

Feedstocks are aqueous formaldehyde, molten urea and ammonia.

The liquid ammonia coming directly from battery limits is collected in the ammonia receiver tank from where it is drawn to & compessed at about 23 ata pressure by means of centrifugal pump. Part of this ammonia is sent to medium pressure absorber & remaining part enters the high pressure synthesis loop . The NH3 of this synthesis loop is compressed to a pressure of about 240 ata . Before entering the reactor it is used as a driving fluid in the carbamate ejector, where the carbamate coming from carbamate separator is compressed upto synthesis pressure . The liquid mixture of ammonia & carbamate enters the reactor where it reacts with compressed CO2.

4.2.5) Third Process: Urea Synthesis in Synthesis Tower

4.2.5.1) Urea Synthesis

Urea synthesis

Urea is made from ammonia and carbon dioxide. The ammonia and carbon dioxide arefed into the reactor at high pressure and temperature, and the urea is formed in a two stepreaction

2NH3 + CO2→ NH2COONH4 (ammonium carbamate)NH2COONH4→H2O + NH2CONH2 (urea)

The urea contains unreacted NH3 and CO2 and ammonium carbamate. As the pressure is reduced and heat applied the NH2COONH4 decomposes to NH3

and CO2. The ammonia and carbon dioxide are recycled.

The urea solution is then concentrated to give 99.6% w/w molten urea, and granulated for use as fertiliser and chemical feedstock.

In the reactor the NH3& gaseous CO2 react to form amm. Carbamate , a portion of which dehydrates to form urea & water . The fraction of carbamate that dehydrates is determined by the ratios of various reactants , operating temp , the residence time in the reactor & reaction pressure . The mole ratio of NH3 / CO2 is around 2:1 , the mole ratio of water to CO2 is around 0.67 : 1 .

In the synthesis conditions ( T= 190°C , P= 154 atm) , the 1st reaction occurs rapidly & is completed . The 2nd reaction occurs slowly & determines the reactor volume . Urea reactor is a plug flow type with 10 no.s of sieve trays to avoid back mixing & to avoid escape of gaseous CO2 which must react in the lower part of the reactor . Stagewise decomposition is carried out to reduce water carry over to the reactor which could adversely affect conversion .

4.2.5.2.) Material Balance at Reactor

Selected capacity : 4,50,000 tons/year No. of working days: 300 Daily production : 4,50,000/300 = 1500 tons/day Urea : 62,500 Kg/hr of 98 % purityComposition of the final product : Urea : 98 % (61,250 Kg/hr) Biuret : 1 % (625 Kg/hr) Water : 1 % (625 Kg/hr)Assumption : Overall conversion to urea is assumed to be 95 %.

MAIN REACTIONS: 1) CO2 + 2NH3 ==== NH2COONH4

(44) (17) (78) 2) NH2COONH4 ==== NH2CONH2 + H2O (60) (18) 3)CO2 + 2NH3 ==== NH2CONH2 + H2O (Overall reaction)

Side

reaction: 4)2NH2CONH2 ==== NH2CONHCONH2 + NH3

(103)

INLET STREAMMaterial specific heat at 40 °C

NH3 0.53 cal/gm °C = 2.219 Kj/Kg°CCO2 0.22 cal/gm °C = 0.9211 Kj/Kg °Cspecific heat at 180 °CCarbamate 0.62 cal/gm °C = 2.596 Kj/Kg °C

Heat inputmCptNH3 : 3.6968 x 104 x 2.219 x 40 = 0.328 x 107 Kj/hrCO2 : 4.7843 x 104 x 0.9211 x 40=0.176 x 107 Kj/hrCarbamate: 9.5336 x 104 x 2.24 x 180= 4.455 x 107 Kj/hr

Heat input = 4.959 x 107 Kj/hr

Figure: Material Balance at Reactor in Urea Plant

Table: Flow of Materials across Reactor

OUTLET STREAMMaterial specific heat at 180oC mol fractions (x) Flow rate

(Kmol/hr)NH3 0.55 cal/gm oC 39.15 Kj/KmoloC 0.033 114.76CO2 0.23 cal/gm oC 42.37 Kj/Kmol oC0.0158 54.36Carbamate 0.62 cal/gm oC 202.49 Kj/KmoloC 0.354 1222.3Urea 0.4828 cal/gm oC 121.32 Kj/KmoloC0.296 1020.83Water 1 cal/gm oC 75.37 Kj/KmoloC 0.299 1032.94Biuret 183.8 Kj/KmoloC 0.002 6.07

Total = 3,451.3

4.2.5.3. Energy Balance at Reactor

Figure: Energy Balance at Reactor in Urea Plant

4.2.6) Fourth Process: Stripping

4.2.6.1. Options

a.) Carbon dioxide Stripping Process

NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of approximately 140bar and a temperature of 180-185°C. The molar NH3 /CO2 ratio applied in the reactor is 2.95. This results in a CO2 conversion of about 60% 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 NH3and CO2

are then partially condensed and recycled to the reactor. The heat evolving from this condensation is used to produce 4.5bar steam some of which can be used for heating purposes in the downstream sections of the plant. Surplus 4.5bar steam is sent to the turbine of the CO2 compressor.The NH3 and CO2 in the stripper effluent are vaporised in a 4bar decomposition stage and subsequently condensed to form a carbamate solution, which is recycled to the 140bar synthesis section. Further concentration of the urea solution leaving the 4bar decomposition stage takes place in the evaporation section, where a 99.7% urea melt is produced.

b.) Ammonia Stripping Process

NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of 150bar and a temperature of 180°C. A molar ratio of 3.5 is used in the reactor giving a CO2 conversion of 65%. The reactor

Figure: Block Diagram of a Total Recycle CO2 Stripping Urea Process.

effluent enters the stripper where a large part of the unconverted carbamate is decomposed by the stripping action of the excess NH3. Residual carbamateand CO2 are recovered downstream of the stripper in two successive stages operating at 17 and 3.5bar respectively. NH3 and CO2 vapours from the stripper top are mixed with the recovered carbamate solution from the High Pressure (HP)/Low Pressure (LP) sections, condensed in the HP carbamate condenser and fed to the reactor. The heat of condensation is used to produce LP steam.The urea solution leaving the LP decomposition stage is concentrated in the evaporation section to a urea melt.

c.) Advance Cost and Energy Saving (ACES ) Process

Figure: Block Diagram of a Total Recycle NH3 Stripping Urea Process.

In this process the synthesis section operates at 175bar with an NH3 / CO2 molar ratio of 4and a temperature of 185 to 190°C.13. The reactor effluent is stripped at essentially reactor pressure using CO2 as the stripping agent. The overhead gas mixture from the stripper is fed to two carbamate condensers in parallel where the gases are condensed and recycled under gravity to the reactor along with absorbent solutions from the HP scrubber and absorber. The heat generated in the first carbamate condenser is used to generate 5bar steam and the heat formed in the second condenser is used to heat the solution leaving the stripper bottom after pressure reduction.The inerts in the synthesis section are purged to the scrubber from the reactor top for recovery and recycle of NH3 and CO2 . The urea solution leaving the bottom of the stripper is further purified in HP and LP decomposers operating at approx. 17.5bar and 2.5bar respectively. The separated NH3 and CO2 are recovered to the synthesis via HP and LP absorbers. The aqueous urea solution is first concentrated to 88.7%wt in a vacuum concentrator and then to the required concentration for prilling or granulating.

d.) Isobaric Double Recycle ( IDR ) Process

In this process the urea synthesis takes place at 180-200bar and 185-190°C. The NH3 / CO2 ratio is approximately 3.5-4, giving about 70% CO2 conversion per pass. Most of the unconverted material in the urea solution leaving the reactor is separated by heating and stripping at synthesis pressure using two strippers, heated by 25bar steam, arranged in series. In the first stripper, carbamate is decomposed/stripped by ammonia and the remaining ammonia is removed in the second stripper using carbon dioxides as stripping agent.

Whereas all the carbon dioxide is fed to the plant through the second stripper, only 40%of the ammonia is fed to the first stripper. The remainder goes directly to the reactor for temperature control. The ammonia-rich vapours from the first stripper are fed directly to the urea reactor. The carbon dioxide rich vapours from the second stripper are recycled to the reactor via the carbamate condenser, irrigated with carbamate solution recycled fromthe lower-pressure section of the plant.

The heat of condensation is recovered as 7bar steam which is used down-stream in theprocess. Urea solution leaving the IDR loop contains unconverted ammonia, carbon diox-ide and carbamate. These residuals are decomposed and vaporised in two successive dis-tillers, heated with low pressure recovered steam. After this, the vapours are condensed tocarbamate solution and recycled to the synthesis loop.

The urea solution leaving the LP decomposition for further concentration, is fed to twovacuum evaporators in series, producing the urea melt for prilling and granulation.

4.2.6.2. Selection Of the Process

Snamprogetti ammonia-stripping urea process is selected because it involves a high NH3 to CO2 ratio in the reactor, ensuring the high conversion of carbamate to urea . The highly efficient ammonia stripping operation drastically reduces the recycling of carbamate and the size of equipment in the carbamate decomposition . Snamprogetti technology differs from competitors in being based on the use of excess ammonia to avoid corrosion as well as promote the decomposition of unconverted carbamate into urea. Formation of urea from ammonia & carbon di oxide takes place through

reversible reactions with formation of ammonium Carbamate as intermediateproduct . Now, success of any urea mfg process depends on how economically we can recycle carbamate to the reactor. Snamprogetti process of urea mnufacturing accomplishes the above task by stripping process.

This reaction involves increase in volume & absorption of heat . Thus this reaction will be favored by decrease in pressure & increase in temp . Moreover decreasing the partial pressure of either of the products will also favor the forward reaction . Process based on first principle of decrease in pressure & decrease in temp is called conventional process , whereas process based on increase/decrease of partial pressures of NH3or CO2 is called stripping process. According to above equation we have : K= (pNH3)2*(pCO2) [where, K =equilibrium constant]

The stripping is effected at synthesis pressure itself using CO2 or NH3 as stripping agent . If CO2 is selected , it is to be supplied to the decomposers/stripper as in Stamicarbon CO2 stripping process. While if NH3 is selected , it is to be obtained from the system itself because excess NH3 is present in the reactor as in Snam’s process. CO2 stripping is advantageous because introducing CO2 increases pCO2. So pNH3 will be reduced to maintain P constant as P = pCO2+ pNH3.

At a particular temp K is constant so when pNH3 is reduced to keep K constant , carbamate will be reduced much faster by decomposition as pNH3 appears in the equilibrium equation with a power of two. Selection of 1st stage decomposition should be in such a way that min water evaporates because the recovered gases go alongwith the carbamate to reactor again & if water enters reactor producti

on will be affected adversely due to hydrolysis of urea . So , stagewise decomposition of carbamate

is done . Second consideration in favor of isobaric stripping is that higher carbamate

recycle pressure results in condensation at higher temp & that recovery in the form of f low pressure steam. This is why stagewise reduction in pressure is practiced.

4.2.6.3. Material Balance at Stripper

Since, no reaction takes place in the stripper & only carbamate gets recycled back to the reactor. Therefore, the amount of ammonia ,carbon-di-oxide ,water & biuret in the outlet stream of stripper will be same as it was in the inlet stream.

Figure: Material Balance at Stripper in Urea Plant

4.2.6.4. Energy Balance at Stripper

Table: Flow of Materials across Stripper

Figure: Energy Balance at Stripper in Urea Plant

4.2.7) Fifth Process: Decomposition / Purification and Recovery4.2.7.1. Stages Of Purification

Urea purification takes place in two stages at decreasing pressure as follows :

1st stage at 18 ata pressure, i.e, MP decomposer 2nd stage at 4.5 ata pressure ,i.e, LP decomposer

a.) First Stage:

1st stage purification & recovery stage at 18 ata:

It is falling film type MP decomposer . It is divided into 2 parts : Top separator, where the released flash gases , the solution enters the tube bundle & decomposition section where the residual carbamate is decomposed & required heat is supplied by means of 24 ata steam condensate flowing out of the stripper.

b.) Seond Stage:

2nd purification & recovery stage at 4.5 ata:

The solution leaving the bottom of MP decomposer is expanded at 4.5 ata pressure& enters the LP decomposer (falling film type). This is again divided into two parts : top separator where the released flash gases are removed before the solution enters the tube bundle . Decomposition section where the last residual carbamate is decomposed & the required heat is supplied by means of steam saturated at 4.5 ata.

4.2.7.2. Material Balancea.) Material Balance at Medium Pressure Decomposer

The amount of ammonia ,carbon-di-oxide ,water & biuret will remain constant as no reaction is taking place. 50 % of ammonia & carbon-di-oxide are assumed to escape from the top of the separator & rest goes with the bottom product. Amount of water & biuret remains constant as no reaction takes place.

c.) Material Balane at Low Pressure Decomposer

Remaining ammonia & carbon-di-oxide are assumed to escape from the top.

4.2.7.3. Energy Balancea.) Energy Balance at Medium Pressure Decomposer

b.) Energy Balane at Low Pressure Decomposer

Heat input = 2.921 x 107 Kj/hr

4.2.8) Sixth Process: Urea Concentration

4.2.8.1. Evaporation

Next section is urea concentration & objective is to reduce water content of urea to as low as 1 % . For the purpose a vacuum concentrator in two stages is provided . The solution leaving the LP decomposer bottom with about 72% urea is sent to the 1st vacuum concentrator operating at a pressure of 0.23 ata .The mixed phase coming out enters the gas liquid separator, wherefrom the vapours are extracted by the 1st vacuum system, while the solution enters the 2nd vacuum concentrator operating at a pressure of 0.03 ata . The two concentrators are fed by saturated steam at 4.5 ata . The mixed phase coming out enters the gas liquid separator , wherefrom the vapours are extracted by the 2nd vacuum system .

4.2.8.2. Material Balance

4.2.8.3. Energy Balance

4.2.9) Seventh Process: Final Processing

4.2.9.1. For UreaProduction:

4.2.9.1.1. Option for Final Product:

In urea fertilizer production operations, the final product is in either prilled or granular form.Production of either form from urea melt requires the use of a large volume of cooling airwhich is subsequently discharged to the atmosphere

a.) PrillingThe concentrated (99.7%) urea melt is fed to the prilling device (e.g. rotating bucket/shower type spray head) located at the top of the prilling tower. Liquid droplets are formed which solidify and cool on free fall through the tower against a forced or natural up-draft of ambient air. The product is removed from the tower base to a conveyor belt using a rotating rake, a fluidised bed or a conical hopper. Cooling to ambient temperature and screening may be used before the product is finally transferred to storage.The design/operation of the prilling device exerts a major influence on product size.

Figure: Block diagram for Urea Granullation and Prilling Process

Collision of the molten droplets with the tower wall as well as inter-droplet contact causing agglomeration must be prevented. Normally mean prill diameters range from 1.6-2.0mm for prilling operations. Conditioning of the urea melt and “crystal seeding” of the melt, may be used to enhance the anti-caking and mechanical properties of the prilled product during storage/handling.

b.) Granullating

Depending on the process a 95-99.7% urea feedstock is used. The lower feedstock concentration allows the second step of the evaporation process to be omitted and also simplifies the process condensate treatment step. The basic principle of the process involves the spraying of the melt onto recycled seed particles or prills circulating in the granulator. A slow increase in granule size and drying of the product takes place simultaneously. Air passing through the granulator solidifies the melt deposited on the seed material. Processes using low concentration feedstock require less cooling air since the evaporation of the additional water dissipates part of the heat which is released when the urea crystallizes from liquid to solid. All the commercial processes available are characterised by product recycle, and the ratio of recycled to final product varies between 0.5 and 2.5. Prill granulation or fattening systems have a very small recycle, typically 2 to 4%. Usually the product leaving the granulator is cooled and screened prior to transfer to storage. Conditioning of the urea melt prior to spraying may also be used to enhance the storage/handling characteristics of the granular product.

4.2.9.1.2. Material Balance

4.2.9.1.3. Energy Balance