CH 4202 Comprehensive Design Project

Assignment 1

Design of an Urea Manufacturing Plant

Group Members: - Wellappili A.Y.G 090556N

Wijesinghe C.D 090577E

Silva G.G.S.N 090488G

Pathiraja P.D.T.P 090361L

Ariyaratne V.T 090029d

Rashara G.A.D.D 090428B

Gamage G.P.S 090147L

Abeywickrama T.D 090010L

Dayananda L.K 090079E

Date of Submission:-

Department of Chemical and Process Engineering

University of Moratuwa

Table of Contents1) INTRODUCTION...................................................................................................................................4

1.1) Importance of Nitrogen for plant growth....................................................................................4

1.2) Nitrogen sources and fertilizers...................................................................................................4

1.3) About Urea..................................................................................................................................6

1.4) Importance of Urea as a fertilizer................................................................................................7

1.5) Other applications.......................................................................................................................8

2) PRODUCTION PROCESS OF UREA......................................................................................................10

2.1) Raw materials for manufacturing Urea.......................................................................................10

2.1.1) Ammonia...........................................................................................................................10

2.1.2) Carbon dioxide...................................................................................................................11

2.2) Sources and availability of raw materials...................................................................................11

2.2.1) Hydrogen Production..............................................................................................................11

2.2.2) Nitrogen...................................................................................................................................12

2.3) Synthesis of Ammonia...............................................................................................................12

2.3.1) Steam Reforming method........................................................................................................12

2.4) Process in general production of Urea.......................................................................................19

2.5) Process variation of Urea manufacturing..................................................................................22

2.5.1) Once-through processes..........................................................................................................22

2.5.2) Partial recycle processes..........................................................................................................23

2.5.3) Total recycle processes............................................................................................................24

3) ECONOMIC ASPECTS OF UREA PRODUCTION....................................................................................33

4) ENVIRONMENTAL AND SAFETY ISSUES..............................................................................................34

4.1) Health and Safety...........................................................................................................................34

4.1.1) Health......................................................................................................................................34

4.1.2) Safety.......................................................................................................................................35

4.2) Safety Procedures...........................................................................................................................37

4.3) Inherent Safe Design......................................................................................................................37

4.4) Layers of Process Safety.................................................................................................................38

4.5) conclusions.....................................................................................................................................39

4.6) recommendations..........................................................................................................................39

4.7) Environment...................................................................................................................................39

5) GREEN CONCEPTS USED IN PLANT DESIGNING.................................................................................40

5.1) why should we care about green buildings?..................................................................................40

5.2) Current methods used in industries...............................................................................................41

6) HISTORY OF UREA PRODUCTION IN SRI LANKA.................................................................................44

7) PROCESS SELECTION AND DESIGN DECISIONS...................................................................................46

REFERENCES..............................................................................................................................................58

LIST OF TABLES

LIST OF FIGURES

1) INTRODUCTION

1.1) Importance of Nitrogen for plant growth

Nitrogen is considered the most important component for supporting plant growth. While

nitrogen is a natural element existing as N2 (gas) accounting for 78% of the earth’s atmosphere,

plants cannot absorb it in this natural form (N2 molecules are inert under normal environmental

conditions due to their high bond energy). The nitrogen in the environment is synthesized into

fertilizers which are readily available to plants. Nitrogen is the main nutrient and regulator of

plant growth – it supplements and promotes all of a plant’s growth processes.

Nitrogen is a part of all living cells and an integral component of all chemical

compounds-proteins, enzymes, hormones and metabolic processes involved in the

synthesis and transfer of energy.

Nitrogen is a constituent of chlorophyll, the green pigment present in chloroplasts in

certain plant cells (esp. in green leaves-green leaves get their color due to chlorophyll) of

the plant that is responsible for photosynthesis. 

Helps accelerate plant growth, increasing seed and fruit production and improving the

quality of leaf and forage crops. 

1.2) Nitrogen sources and fertilizers

Nitrogen often comes from fertilizer application and from the air (legumes get their N from the

atmosphere, water or rainfall contributes very little nitrogen)

Nitrogen in the air is the ultimate source of all soil nitrogen.

Nitrogen may enter the soil through rainfall, plant residues, fixation by soil organisms,

animal manures and organic fertilizers (i.e. compost) and commercial fertilizers.

Nitrogen may be lost from the soil by plant removal, volatilization, leaching or erosion.

The earth's atmosphere is the ultimate source of nitrogen. In most areas of the world, the nitrogen

found in soil minerals is negligible. Nitrogen may be added to or lost from soil by a number of

processes. In the soil, nitrogen can undergo a number of transformations.

Rainfall adds about 10 lbs. of nitrogen to the soil per acre per year. The nitrogen oxides

and ammonium that are washed to earth are formed as a result of lightning during storms,

by internal combustion engines and through oxidation by sunlight. Crop residues

decompose in the soil to form soil organic matter. This organic matter contains about 5%

nitrogen. An acre-foot of soil having 2 percent organic matter would contain about 3,500

lbs. of nitrogen. Generally, about 1- 3% of this organic nitrogen is annually converted by

microorganisms to a form usable by plants.

Legumes fix atmospheric nitrogen through their symbiotic association with Rhizobium

bacteria. If plant roots are well modulated, the legume plant does not benefit from the

addition of fertilizer nitrogen. Perennial legumes, such as alfalfa, can fix several hundred

pounds of nitrogen per acre per year.

Manure contains an appreciable amount of nitrogen. Most of this nitrogen is present as

organic compounds. Cattle manure contains about 10-40 lbs. of nitrogen per ton. About

half of this nitrogen is converted to forms available to plants during the first growing

season. Lesser amounts are converted during succeeding seasons. Each ton of applied

manure is equal to about 5-20 pounds of commercial fertilizer nitrogen.

Commercial fertilizer nitrogen comes in three basic forms: gas, liquid and dry. All forms

are equally effective when properly applied. Once applied, fertilizer nitrogen is subject to

the same transformations as other sources of nitrogen. There is no difference between the

ammonium (NH4+) or nitrate (NO3

-) which are absorbed by the plant from commercial

fertilizers and that supplied by natural substances.

Nitrogen exists in a number of chemical forms and undergoes chemical and biological reactions.

1. Organic nitrogen to ammonium nitrogen (mineralization). Organic nitrogen comprises

over 95% of the nitrogen present in soil. This form of nitrogen cannot be used by plants but is

gradually transformed by soil microorganisms to ammonium (NH4+). Ammonium is not leached

to a great extent. Since NH4+ is a positively charged ion (cation), it is attracted to and held by the

negatively charged soil clay.

2. Ammonium nitrogen to nitrate nitrogen (nitrification). In warm, well-drained soil,

ammonium transforms rapidly to nitrate (NO3-). Nitrate is the principle form of nitrogen used by

plants. It leaches easily, since it is a negatively charged ion (anion) and is not attracted to soil

clay.

3. Nitrate or ammonium nitrogen to organic nitrogen (immobilization). Soil microorganisms

use nitrate and ammonium nitrogen when decomposing plant residues. These forms are

temporarily "tied-up" (incorporated into microbial tissue) in this process. This can be a major

concern if crop residues are high in carbon relative to nitrogen. Examples are wheat straw, corn

stalks and sawdust. The addition of 20 to 70 lbs. of nitrogen per ton of these residues is needed to

prevent this transformation. After the residues are decomposed, the microbial population starts

dying out and processes 1 and 2 take place.

4. Nitrate nitrogen to gaseous nitrogen (de-nitrification). When soil does not have sufficient

air, microorganisms use the oxygen from NO3- in place of that in the air and rapidly convert NO3

-

to nitrogen oxide and nitrogen gases (N2). These gases escape to the atmosphere and are not

available to plants. This transformation can occur within two or three days in poorly aerated soil

and can result in large losses of nitrate-type fertilizers.

5. Ammonium nitrogen to ammonia gas (ammonia volatilization). Soils that have a high pH

(> 7.5) can lose large amounts of NH4+ by conversion to NH3 gas. To minimize these losses,

incorporate solid ammonium-type fertilizers, urea- CO(NH2)2 and anhydrous ammonia below the

surface of a moist soil.

1.3) About Urea

Urea (also known as carbamide) is an organic compound with the chemical formula CO(NH2)2.

The molecule has two —NH2 (amine) groups linked to a carbonyl (C=O) functional group.

Molecular Formula CO(NH2)2

Molec

Physical properties

Molar mass: 60.06 gmol−1

Density: 1.32 g /cm3

Solubilty -

Melting point: 133−135℃

Urea serves an important role in the metabolism of nitrogen-containing compounds by animals

and is the main nitrogen-containing substance in the urine of mammals. It is a colorless, odorless

solid, although the ammonia that it gives off in the presence of water, including water vapor in

the air, has a strong odor. It is highly soluble in water and practically non-toxic. Dissolved in

water, it is neither acidic nor alkaline. Urea would hydrolyze in both acidic and basic aqueous

media. The body performs nitrogen excretion by means of urea. Urea is widely used in fertilizers

as a convenient source of nitrogen. It is also an important raw material for the chemical industry.

Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave. In 1828,

the German chemist Friedrich Wöhler was the first to artificially synthesize urea from an

inorganic precursor-this was an important breakthrough in organic chemistry since it

demonstrated that an organic compound present in living organisms could be obtained from

inanimate materials. It was done by treating silver isocyanate (AgNCO) with ammonium

chloride (NH4Cl).

AgNCO+N H 4 Cl→(N H2)2 CO+ AgCl

1.4) Importance of Urea as a fertilizer

More than 90% of world production of urea is destined for use as a nitrogen-release

fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in

common use. Therefore, it has the lowest transportation costs per unit of nitrogen

nutrient. The standard crop-nutrient rating of urea is 46-0-0.

Many soil bacteria possess the enzyme urease, which catalyzes the conversion of the urea

molecule to two ammonia molecules and one carbon dioxide (CO2) molecule, thus urea

fertilizers are very rapidly transformed to the ammonium form in soils. Among soil

bacteria known to carry urease, some ammonia-oxidizing bacteria (AOB), such as species

of Nitrosomonas, are also able to assimilate the carbon dioxide released by the reaction to

make biomass via the Calvin Cycle, and harvest energy by oxidizing ammonia (the other

product of urease) to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria,

especially Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile in soils and is

a major cause of water pollution from agriculture. Ammonia and nitrate are readily

absorbed by plants, and are the dominant sources of nitrogen for plant growth.

Urea is also used in many multi-component solid fertilizer formulations. Urea is highly

soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in

combination with ammonium nitrate: UAN), e.g., in 'foliar feed' fertilizers.

1.5) Other applications

Chemical Industry-Urea is an important raw material in the manufacture of several chemical

compounds. They include: plastics (such as urea-formaldehyde resins), adhesives (urea

formaldehyde or urea melamine formaldehyde) and Potassium Cyanate. Chemical Formulae and

relevant reactions.

Explosives-Urea is also used to produce urea nitrate which is a highly explosive substance.

Automobiles-Urea is used in Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic

Reduction (SCR) reactions to reduce nitrous oxides (NOx) in exhaust fumes from combustion of

diesel.

Other commercial uses

Stabilizer in nitrocellulose explosives

A component of animal feed, providing a relatively cheap source of nitrogen to promote

growth

A flavor-enhancing additive for cigarettes

A main ingredient in hair removers

An ingredient in skin cream, moisturizers and hair conditioners

A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as

the urea-potassium bicarbonate mixture

An ingredient in many tooth whitening products

Along with ammonium phosphate, as a yeast nutrient, for fermentation of sugars into ethanol

As a solubility-enhancing and moisture-retaining additive in dye baths for textile dyeing or

printing

2) PRODUCTION PROCESS OF UREA

2.1) Raw materials for manufacturing Urea

The industrial process of manufacturing urea basically involves two main steps. The first step is

the formation of ammonium carbamate (NH2COONH4) which is done by reacting ammonia

(NH3) and gaseous carbon dioxide (CO2). The second step is dehydration of ammonium

carbamate to produce molten urea (NH2CONH2). So main reactants needed for the process would

be ammonia and carbon dioxide.

2.1.1) Ammonia

Ammonia is a colourless gas having density 0.589 times that of air, with a sharp unpleasant

smell. At atmospheric pressure the boiling point of ammonia is -33.34 0C. So the storage must be

done under high pressure at low temperature. Due to the presence of strong hydrogen bonds

between molecules it can be easily liquefied and well miscible in water.

Natural occurrence of ammonia happens due to decay process of nitrogenous animal and

vegetable matter. In addition ammonia salts such as ammonium chloride, ammonium sulfate and

ammonium bicarbonate are available in soil and rain water. Biosynthesis of ammonia from

atmospheric nitrogen by enzymes called nitrogenase happens in certain organism which is called

nitrogen fixing. Ammonia is also produced through amino acid metabolism and is converted to

urea through series of reactions inside the liver.

The commercial production of ammonia is very important since it is not available as a natural

resource and it is essential in large quantities for urea manufacturing. Worldwide, the annual

production of synthetic ammonia is about 120 million tones, of which about 85% is used in

fertilizers, including urea [1]. Most of the industrial processes for synthesis of ammonia are

based on Haber Bosch process, developed in Germany 1904-1913. In that process ammonia is

produced by the reaction between gaseous hydrogen and nitrogen under high temperature and

pressure in the presence of iron based catalyst. Formation of ammonia from nitrogen and

hydrogen is basically a reversible reaction which the yield depends on the conditions employed.

Unreacted hydrogen and nitrogen are usually separated and recycled.

2.1.2) Carbon dioxide

Carbon dioxide is available in atmospheric air in trace amount which is known as a pollutant gas.

In most of industrial plants carbon dioxide is emitted as a result of burning fossil fuels. Carbon

dioxide is produces as a byproduct of ammonia synthesis itself. So carbon dioxide which is

produced as a byproduct can be removed and used in the production of urea.

When overall production process is consider primary raw materials needed to manufacture urea

are

Hydrogen (H2)

Nitrogen (N2)

2.2) Sources and availability of raw materials

2.2.1) Hydrogen Production

There are many sources that hydrogen can be obtained. The process of ammonia synthesis

basically depends on the hydrogen source used for the process.

Hydrogen can be produced from natural gas i.e. methane, liquid petroleum gases such as

propane and butane or petroleum naphtha. When light hydrocarbons mentioned above are

used as the source for hydrogen the production process used for ammonia synthesis is

known as “Steam Reforming”.

Heavy fuel oil or vacuum residue can be used to obtain hydrogen when partial oxidation

process for ammonia synthesis is used.

Although coal gasification and electrolysis of water can be used to produce hydrogen

those methods are no longer used in industrial scale ammonia production. In addition

hydrogen gas emitted as a byproduct of petroleum cracking can also be used to produce

hydrogen to manufacture urea.

Since different process techniques should be employed depending on the feedstock, energy

consumption, investment cost and production cost will be vary depending on the feedstock.

Following table gives an approximate comparison of the energy consumption, investment cost

and production cost for main three sources of hydrogen production [2]

Natural Gas Heavy oil Coal

Energy consumption 1.0 1.3 1.7

Investment cost 1.0 1.4 2.4

Production cost 1.0 1.2 1.7

In addition availability of raw materials is also important in deciding the production process.

2.2.2) Nitrogen

The most abundant source for nitrogen is atmospheric air. Dry air contains roughly 78% of

gaseous nitrogen (N2) by volume. Pure nitrogen needed for the Haber process can be easily

extracted by removing oxygen, carbon dioxide and other gases by liquefaction or law

temperature distillation. But if steam reforming process is used, no such method is needed. In

that process, oxygen is removed by simple combustion and carbon dioxide is removed by using

absorption process.

2.3) Synthesis of Ammonia

There are two methods available for synthesis of ammonia. Steam reforming method can be used

for natural gas and other light hydrocarbons such as liquid petroleum gas and naphtha. If the

feedstock is residual heavy oil or vacuum residue from a refinery, then the process will be partial

oxidation. Steam reforming process of natural gas is identified as the most efficient method for

synthesis of ammonia.

2.3.1) Steam Reforming method

This method is known as the best available technique for the synthesis of ammonia. Steam

reforming method can be divided into three as mentioned below [3].

Conventional steam reforming with fired primary reformer and stoichiometric air

secondary reforming (stoichiometric H/N- ratio)

Steam reforming with mild conditions in fired primary reformer and excess air in

secondary reformer (Under-stoichiometric H/N ratio)

Heat exchange auto thermal reforming, with a process gas heated steam reformer (heat

exchange reformer) and a separate secondary reformer, or in a combined auto thermal

reformer using excess or enriched air (under- stoichiometric or stoichiometric H/N-ratio)

Among these three techniques conventional steam reforming method is the oldest and most

widely used technique.

2.3.1.1) Conventional Steam Reforming method

Desulphurization

Sulphur and sulphorous compounds which contain in natural gas poison the catalyst used in

ammonia synthesis process. The feed gas is pre heated up to 350-400 0C in the primary reformer

convection section and then passed to the desulphurization vessel. In that section sulphur

compounds are hydrogenated to hydrogen sulphide (H2S) using cobalt molybdenum catalyst and

then H2S is removed by reacting with Zinc Oxide (ZnO).

R-SH + H2 H2S + RH

ZnO + H2S ZnS + H2O

Hydrogen gas needed for hydrogenation of sulphur compounds is usually recycled from the

synthesis section of the plant and consumed zinc oxide or zinc sulphide remains in the adsorption

bed.

Primary Reforming

Primary reformer consists of nickel or iron containing catalyst. Desulphurized gas reacts with

superheated steam which is heated up to 500-600 0C in the convection section before entering the

primary reformer. Following reactions take place in the primary reformer.

CH4 (g) + H2O (g) CO (g) + 3H2 (g) H=206 kJ.mol-1

CO (g) + H2O (g) CO2 (g) + H2 (g) H=-41 kJ.mol -1

The overall reaction would be

CH4 (g) + 2H2O (g) 4H2 (g) + CO2 (g)

In modern plants preheated steam and gas mixture is passed through an adiabatic pre reformer

which uses pre reformer catalyst to reheat the mixture in the convection section. In some plants

part of process steam is obtained from feed gas saturation. The amount of steam that should be

supplied is given by steam to carbon molar ratio. It is usually maintained at 3.0 for an efficient

process. The optimum ratio depends on factors such as feedstock quality, purge gas recovery,

primary reformer capacity, shift operation, and the plant steam balance.

The reaction between methane and steam takes place between 780-830 °C. At this temperature

the above equilibrium reaction is driven to the right, giving high yield of hydrogen, carbon

dioxide and small quantities of carbon monoxide. Since the overall reaction is highly

endothermic additional heat should be supplied to maintain optimum temperature by burning

natural gas or other gaseous fuel. The output from the primary reformer is commonly known as

synthesis gas.

Secondary Reforming

The synthesis gas, small amount of carbon monoxide and unreacted methane leaving the primary

reformer enters the secondary reformer where it is mixed with calculated amount of air. At actual

operating conditions only 30-40% of hydrocarbons are reformed and temperature must be

increased to increase the conversion. Addition of air converts methane molecules that are not

reacted during primary steam reforming in to synthesis gas.

Following reaction takes place in the secondary reforming process.

2CH4 (g) + O2 (g) + 4N2 (g) 2CO (g) + 4H2 (g) + 4N2 (g)

CO produced in the above reaction is converted to CO2 as mentioned in the following reaction.

CO (g) + H2O (g) CO2 (g) + H2 (g)

So the overall reaction would be

Ni Catalyst

2CH4 (g) + O2 (g) + 4N2 (g) + 2H2O (g)

2O2 (g) + 6H2 (g) + 4N2 (g)

This internal combustion is highly exothermic and supplies necessary heat and also gives high

yield of hydrogen. The temperature at the outlet of secondary reformer is around 1000 °C and

99% of hydrocarbons coming from primary reformer is converted leading to reduced methane

fraction of 0.2-0.3% on dry gas basis. This reaction also removes oxygen from air and provides

nitrogen for final synthesis of ammonia. In the conventional reforming process the degree of

primary reforming is adjusted so that air supplied to the secondary reformer balances both heat

requirement and stoichiometric nitrogen gas requirement. Finally gas leaving is cooled to 350-

400 °C in a waste heat steam boiler downstream from the secondary reformer.

Shift Conversion

In order to increase the efficiency of the process there should be a very lower amount of carbon

monoxide (CO) content where as the process gas leaving the secondary reformer contains 12-

15% of CO. It is reduced up to 3% on dry base in high temperature shift (HTS) conversion. For

that gas is passed through a bed of iron oxide or chromium oxide catalyst at around 4000C. The

use of copper containing catalyst increases the conversion. The gas from HTS converter is

cooled and passed through a Low temperature shift(LTS) converter, which has a bed of copper

oxide or zinc oxide-based catalyst and operates at about 200-220 °C. The final amount of CO

content is about 0.2-0.4% on dry base. Following reaction takes place in the shift conversion.

CO (g) + H2O (g) CO2 (g) + H2 (g) H=-41 kJ.mol -1

Water Removal

Iron catalyst used in ammonia synthesis can be oxidized due to the presence of water, carbon

dioxide and carbon monoxide. Because of that those compounds must be removed using above

mentioned shift conversion and following water removal and carbon dioxide removal methods.

Water is removed by cooling the mixture up to 400C and allowing it to condense. The condensate

may contain some amount of ammonia, methanol, and minor amounts of amines, formic acid and

Iron Oxide Catalyst

acetic acid. These compounds can be stripped and recycled for more efficient process. The heat

released during condensation can be used for the stripping of carbon dioxide, driving an

absorption refrigeration unit and boiler water preheating.

Carbon Dioxide Removal

The process gas now contains mainly H2, N2 and CO2. CO2 can be easily removed by absorption

process. Mono ethanol Amine (MEA), Activated Methyl Di Ethanol Amine (AMDEA) or

potassium carbonate solutions can be used for chemical absorption. Glycol Di Methyl Ethers

(Selexol) and propylene carbonate can be considered as physical absorption solutions. When

MEA is used it needs high amount of regeneration energy and because of that it is not

considered as an efficient process.

Finally absorbed carbon dioxide is stripped and recycled back to use as a raw material for

manufacturing of urea.

Methanation

Even after the shift conversion and CO2 removal, still there can be a trace amount of CO and

CO2 which is poisonous for the ammonia synthesis catalyst. So in methanation process they are

converted to methane by reacting with hydrogen. Following reactions take place in the reactor

filled with nickel catalyst at about 3000C.

CO (g) + 3H2 (g) CH4 (g) + H2O (g)

CO2 (g) + 4H2 (g) CH4 (g) + 2H2O (g)

Although methane is an inert gas in ammonia synthesis, water produced in the reaction oxidizes

the iron catalyst used in ammonia synthesis. So water is removed in two stages at the

downstream of the methanator and in ammonia synthesis loop by condensation.

Synthesis of ammonia

Synthesis of ammonia is done by Haber-Bosch process. Following reaction takes place in the

reactor when incoming gas (N2 and H2) is passed over iron catalyst at a pressure usually in the

range 100-250 bar and at the temperature range of 350-5500C.

N2 (g) + H2 (g) 2NH3 (g)H=-46 kJ.mol -1 NH3

The required pressure is achieved by centrifugal compression which is driven by a steam

turbine. The required amount of steam for the turbine can be obtained from the steam produced

in the ammonia plant itself.

At this equilibrium conditions, the conversion of reactants are only 20-30% per pass. So

unreacted gas is separated and recycled. The synthesis is typically carried out in a loop as

shown in the diagram. Ammonia formed is separated from the recycle gas by condensation. The

refrigeration compressor needed for this task is also driven by a steam turbine that uses steam

produced in the plant itself. In order to maintain the pressure and to shift the reaction forward,

fresh make up synthesis gas is supplied to the reactor.

Synthesis loop can be designed in many different ways with respect to the points which make

up gas is added and ammonia and purge gas is removed. . The best arrangement is to add the

make-up gas after ammonia condensation and ahead of the converter. The loop purge should

be taken out after ammonia separation and before make-up gas addition. This configuration is

dependent on the make-up gas being treated in a drying step before entering the loop. [4].

since the reaction is exothermic there should be a method to remove excess heat to maintain

equilibrium conditions. Ammonia produced in this process can be used as raw material for

urea manufacturing process.

2.3.1.2) Steam reforming with excess air secondary reforming

In conventional process the marginal efficiency of primary reforming is very low. So the

process designed to reduce the primary reforming and to move some of duty to secondary

reforming is known as Steam reforming with excess air secondary reforming. Following

features can be observed in this process compared to a conventional process.

Decreased firing in the primary reformer

Since the amount of heat supplied to the primary reformer is comparatively lower the process

outlet temperature is also generally lower. This will increase the firing efficiency, reduce the

size and the cost of primary reformer and prolong catalyst due to mild operating conditions. But

the degree of primary reforming is reduced due to law supply of heat which results lower

temperatures.

Increased process air supply to the secondary reformer

Since the degree of reforming is lower in the primary reformer there should be an increased

amount of internal firing or combustion in the secondary reformer in order to achieve the same

degree of overall conversion at the outlet of secondary reformer. The process air requirement is

50% higher than in the conventional method.

Cryogenic final purification

In this process all the methane and excess nitrogen is removed from the synthesis gas using a

cryogenic purifier. The purified gas is very pure except a trace amount of argon. Purge gas from

the synthesis section is also sent to this unit where it delivers an off gas for fuel.

Lower synthesis gas inert level

In the traditional process impurities such as CO and CO2 are removed by methanation. In this

process a significant improvement of purity level can be achieved in synthesis gas. This leads to

a higher conversion per pass and reduce purge gas flow which results a more efficient process

than the conventional process.

2.3.1.3) Heat exchange auto-thermal reforming

From a thermodynamic point of view it is wasteful to use the high-level heat of the secondary

reformer outlet gas and the primary reformer flue-gas, both at temperatures around 1,000°C,

simply to raise steam. Recent developments are to recycle this heat to the process itself, by using

the heat content of the secondary reformed gas in a newly-developed primary reformer (gas

heated reformer, heat exchange reformer), thus eliminating the fired furnace. Surplus air or

oxygen-enriched air is required in the secondary reformer to meet the heat balance in this auto

thermal concept.

When auto thermal reforming is used nitrogen dioxide emissions to the atmosphere is

significantly reduced due to the elimination of flue gas in the primary reformer.

2.4) Process in general production of Urea

The main principle of manufacture of urea has two main reactions.

2NH3 + CO2 NH2COONH4 -37.4 Kcal/gm mol

NH2COONH4 NH2CONH2 + H2O + 6.3 Kcal/gm mol

While going main reactions undesirable side reaction taking place is

2NH2CONH2 NH2CONHCONH2 (Biuret) + NH3

The synthesis is further complicated by the formation of a dimmer called biuret,

NH2CONHCONH2, which must be kept low because it adversely affects the growth of some

plants. The structure of these compounds is shown in Figure

Figure. Schematic representation of urea synthesis

We can consider that the totally process which can be divided to fore sub processes.

Synthesis

Ammonia & CO2 are compressed separately and fed to the high pressure (180 atms) then a

mixture of urea, ammonium carbamate, H2O and unreacted (NH3+CO2) is produced. Both 1st &

2nd reactions are equilibrium reactions. The 1st reaction almost goes to completion at 185-190

oC & 180-200 atms. The 2nd reaction (decomposition reaction) is slow and determines the rate

of the reaction.

Purification

The major impurities in the mixture at this stage are water from the urea production reaction and

unconsumed reactants (ammonia, carbon dioxide and ammonium carbamate). This liquid

effluent is let down to 27 atms and fed to a special flash-evaporator containing a gas-liquid

separator and condenser. Unreacted NH3, CO2 & H2O are thus removed & recycled. An

aqueous solution of carbamate-urea is passed to the atmospheric flash drum where further

decomposition of carbamate takes place NH2COONH4 2NH3 + CO2 The pressure is then

reduced a solution of urea dissolved in water and free of other impurities remains. At each stage

the unconsumed reactants are absorbed into a water solution which is recycled to the secondary

reactor. The excess ammonia is purified and used as feedstock to the

primary reactor.

Concentration

75% of the urea solution is heated under vacuum, which evaporates off some of the water,

increasing the urea concentration from 68% w/w to 80% w/w. At this stage some urea crystals also form.

The solution is then heated from 80 to 110oC to redissolve these crystals prior to evaporation. In the

evaporation stage molten urea (99% w/w) is produced at 140oC.

Granulation

Figure. Schematic representation of granulation

Urea is sold for fertilizer as 2 - 4 mm diameter granules. These granules are formed by spraying

molten urea onto seed granules which are supported on a bed of air. This occurs in a granulator

which receives the seed granules at one end and discharges enlarged granules at the other as

molten urea is sprayed through nozzles. Dry, cool granules are classified using screens.

Oversized granules are crushed and combined with undersized ones for use as seed.

All dust and air from the granulator is removed by a fan into a dust scrubber, which removes the

urea with a water solution then discharges the air to the atmosphere. The final product is cooled

in air, weighed and conveyed to bulk storage ready for sale.

2.5) Process variation of Urea manufacturing

Once– through processes

Process partial recycle processes

Total recycle processes 1 Gas recycles process

CPI-Allied gas recycle urea process Inventa gas recycle urea process

2 Liquid recycles process

Stamicarbon CO2 stripping urea process Montecatini complete recycle urea

process Pechiney total recycle urea process Inventa liquid recycle urea process

3 Gas / liquid recycle process

Mitsui Toatsu total recycle D improved urea process

Stamicarbon total recycle process SNAM PROGETTI ammonia stripping

urea process Chemico total recycle urea process Lonza- Lummus urea process

The basic process chemistry of urea manufacturing is relatively simple. However, because

operating parameters vary; particularly in the initial formation of the urea solution, numerous

process designs have been utilized. Design differences occur in the separation and recycle of

component streams. There are three major classes of urea processes, based on the type or

quantity of recycle: once– through processes, partial recycle processes and total recycle processes. At

least 75% of the urea produced today is by total recycle systems.

2.5.1) Once-through processes

Figure below is generalized flow diagram of a once- urea process.

Figure. Once– through processes

In this process liquid NH3 is pumped through a high pressure plunger pump and gaseous CO2 is

compressed through a compressor up to the urea synthesis reactor pressure at an NH3 to CO 2

feed mole ratio of 2/1 or 3/1. The reactor usually operates in a temperature range from 175 to

190 0C. The reactor effluent is let down in pressure to about 2 atm and the carbamate

decomposed and stripped from the urea-product solution in a steam heated shell & tube heat

exchanger. The moist gas, separated from the 85-90 % urea product solution, & containing about

0.6 tons of gaseous NH3 per ton of urea produced is usually sent to an adjacent ammonium

nitrate or ammonium sulfate producing plant for recovery. An average conversion of carbamate

to urea of about 60 % is attained. Excess heat is removed from the reactor by means of a low

pressure steam-producing coil in an amount of about 280,000 cal/Kg urea produced.

2.5.2) Partial recycle processes

Partial recycle processes as shown in figure bellow. This process is termed partial recycle

because only excess ammonia is recovered and recycles to the reactor.

Figure. partial recycle processes

The synthesis is carried out with as much as 200% excess ammonia. This process is also similar

to that of the once through process, with one additional step : the reactor effluent contacting urea

ammonium carbamate, water, and excess ammonia. Passes though an expansion valve reducing

the pressure to a few hundred KPa depending on the particular process design. The steam goes to

an ammonia separator where excess ammonia is removed, condensed, and recycled to the

reactor. This is necessary to recover some of the cost of using excess ammonia. Also if passed

directly to the carbamate decomposer, the excess ammonia could hinder the decomposition of the

carbamate. The steam containing urea, cabamate and water goes to a carbamate decomposer

which dissociates the cabamate to ammonia and carbon dioxide. The aqueous urea solution is

separated and goes to further processing or shipment.

2.5.3) Total recycle processes

The total recycle process is most widely used process in the urea manufacturing industry. There

are three variations of the total recycle process.

1. Decomposed carbamate gases are separated and recycled in their pure states.

2. Carbamate solution is recycled to the reactor.

3. A combination gas/ liquid recycle may occur.

2.5.3.1) Gas recycles process

Figure. Gas recycles process

The material leaving the reactor is a mixture of urea, ammonium carbamate, water, and excess

ammonia. This stream goes to a decomposer which separates the carbamate into ammonia and

carbon dioxide. The separated gases may both be recycled, or one may be purified at the expense

of the other and returned to the process. In the partial recycle process, however, the excess

ammonia is recovered and the ammonia or carbon dioxide in the unreacted carbamate is lost to

the process. In the total recycle process, the entire quantity of ammonia is reused; i.e., excess

plus decomposed carbamate ammonia. Two examples of the gas recycle process will be

discussed; the CPI – Allied and inventa processes.

CPI-Allied gas recycle urea process

Higher operating temperature (1940C -2330C) at 30.3 MPa and Conversion rate is 80%-85%.

Ammonia and carbon dioxide feed entering with ration ratio- 4:1.

The reactor products pass through an expansion valve to primary carbamate decomposer where

90% of the carbamate is flashed and stripped along with water vapor. The urea solution contains

approximately 1.5% of the initial carbon dioxide feed. This stream is sent to an ammonia

separator, where excess ammonia is stripped, and on to secondary decomposer where any

remaining carbamate dissociates at atmospheric pressure.The overheads from both decomposers

are passed through a two – unit series of absorbers where monoethanolamine (MEA) selectively

absorbs carbon dioxide and water, leaving ammonia for recycle to the reactor. The carbon

dioxide – rich solvent is sent to a stripper which thermally regenerates the MEA creating a rich

carbon dioxide stream which is recycled to the reactor. The urea solution leaving the secondary

decomposer passes through a centrifugal Min-film evaporator unit. The product contains less

than 0.7% biuret and 0.20% water.

Figure. CPI-Allied gas recycle urea process

Inventa gas recycle urea process

The Inventa process utilizes a reactor operating at 20.2 MPa and 180 0C to 2000C. The molar

feed ratio of ammonia to carbon dioxide is 2:1 with a maximum carbon dioxide conversion to

urea of 50%. The reactor effluent containing excess ammonia, ammonium carbamate, urea, and

water passes through an expansion valve where it is lowered to 549 kpa and heated to 1200C in

the carbamate decomposer. The ammonia and carbon dioxide go to an absorber where the

ammonia is selectively absorbed and the carbon dioxide exits for recycle. The resulting

ammoniacal solution of ammonium carbamate goes to a desorber to remove ammonia for recycle

Figure. Inventa gas recycle urea process

2.5.3.2) Liquid recycle process

Figure. Liquid recycle process

This process is similar to the gas recycle process except that the gases are condensed with the

addition of water when needed, to form a carbamate solution for recycle. Those processes which

will be discussed in this category are the Stamicarbon CO2 Stripping, Montecatini, Pechiney, and

inventa processes.

Stamicarbon CO2 stripping urea process

In the Stamicarbon CO2 Stripping urea process ammonia and carbon dioxide are reacted

in the molar ratio of 2.4:1 to 2.9:1 at 1700C to 1900C and 12.1 MPa to 15.1 MPa . The

reaction product (1850C, 14.1 MPa) goes immediately to a high pressure stripper.

Operating at 14.1 MPa and 1900C (1), where the reactor stream is stripped by incoming carbon

dioxide. The stream containing 15% unconverted carbamate is then let down for further

decomposition in the low pressure decomposer operating at 300 kpa and 1200C. the ammonia

and carbon dioxide are condensed in the low pressure to the high pressure condenser where it

combines with the off-gas from the high pressure stripper and a split from the ammonia feed line.

The condensed stream from the high pressure condenser operating at 1700C and 14.1MPa, goes

to the reactor. An equivalent amount of 345 kpa steam is produced in the high pressure

condenser and is used in other sections of the plant.

This process claims ammonia and carbon dioxide consumption of 0.57 metric ton and 0.755

metric ton per metric ton of urea produced, respectively. Conversion efficiencies for ammonia

and carbon dioxide are 65% to 85% and 70% to 85%, respectively.

Figure. Stamicarbon CO2 stripping urea process

Montecatini complete recycle urea process

Montecatini process ( Montedison) in which preheated liquid ammonia and carbon dioxide are

compressed to 20.2 MPa and enter the reactor operating at 1950C (12,16). The reactor mole ratio

for NH3:CO2 is 3.5.1;for H2O : CO2 it is 0.6.1 (17).the effluent containing urea, excess

ammonia, ammonium carbamate, and water enters a first – stage decomposer/separator operating

at 8.1 MPa and 1850C. In this decomposer/ separator most of the ammonia is driven off along

with the carbamate decomposition products. This stream, along with 20% to 30% of the carbon

dioxide feed stream, is fed to the first – stage carbvamate condenser which operates at 8.1 MPa

and 1450C

The effluent form the first – stage condenser passes to an auxiliary condenser operating at the

same pressure but at 1150C so that condensation is completed. The gas leaving this condenser is

washed to remove ammonia. The liquid stream is recycled to the reactor.

The liquid stream leaving the first – stage decomposer/ separator proceeds to a second- stage unit

operating at the same temperature as stage one and 1.2 MPa, and finally to a third stage operating

at 202kpa to 303 kpa before leaving the facilities. The gaseous effluents from the stage two and

three decomposer/ separators are condensed in carbamate condensers three and four,

respectively. In condenser three the gas stream is mixed with liquid effluent from both wash

vessels and the liquid carbamate is sent to the first stage condenser. In condenser four an

ammonia bearing gas stream from the solidification section is washed with cold ammonium

carbamate, and the resulting effluent is cycled through a wash vessel before going to condenser

three.

Figure. Montecatini complete recycle urea process

Inventa liquid recycle urea process

Figure. Inventa liquid recycle urea process

2.5.3.3) Gas/Liquid recycle process

Figure. Gas / liquid recycle process

It is characterize by ammonia recycle with carbon dioxide being recycled in the form of

carbamate. The following processes of this type will be considered: Mitsui Toatsu (Total Recycle

D Improved) , Stamicarbon, SNAM PROGETTI ,chemico, and Lonza-lummus.

Stamicarbon total recycle process

The stamicarbon total recycle process is shown in below the reaction take place at 20.2 MPa and

1700C to 1900C. The reactor effluent is lowered to approximately 505 KPa before going to the

preseparater. The liquid stream form the preseparater passes through to additional separation

steps before finally leaving the process. The various ammonia and carbon dioxide streams are

condense, and the carbomate formed is recycle to the reactor. A wet scrubber is used on the gas

stream to recover ammonia for recycle.

Figure. Stamicarbon total recycle process

SNAM PROGETTI ammonia stripping urea process

The SNAM PROGETTI urea process ,as shown in below is similar to the stamicarbon CO2

stripping process , but the stripping is done by ammonia rather than carbon dioxide. The process

as shown can operate at two different reactor pressure, 13MPa to 16MPa or 20.2MPa to 25 MPa.

Operating temperature is 180C to 190C.malar ratio is 3.5:1.

The effluent leaving the reactor is passed to a stripping operating at 10 MPa to 15MPa and 160C

to200C. Most (>90%) of the ammonia and carbon dioxide are removed in the stripper with the

remainder being removed in the flash separator. These overheads are collected and the cabomate

is recycled; excess ammonia is also recycled to storage.

The unique feature in the SNAM PROGETTI process is the cabomate ejector which introduces

carbomate and ammonia to the reactor. The ammonia pressure drop through the ejector of

41MPa supplies the necessary driving force

.

Figure. SNAM PROGETTI ammonia stripping urea process

3) ECONOMIC ASPECTS OF UREA PRODUCTION

4) ENVIRONMENTAL AND SAFETY ISSUES

4.1) Health and Safety

4.1.1) Health

Ammonia is a material that is widely used in chemical industry and it has its own toxicity, health

issues and safety measures that require attention. Therefore it is required for the designers of

ammonia and derivatives plant to have a clear understanding about these aspects of ammonia. In

analyzing the effects of releases of ammonia the toxicity of ammonia should be considered.

The threshold limit value (TLV) for ammonia is 25 PPM (TWA), short term exposure limit

(STEL) 35 PPM and initialism for Immediately Dangerous to Life or Health (IDLH) 300 PPM

[51]. Ammonia gas is irritating to skin, nose, eyes, throat, respiratory tract and mucous

membranes. Even though at lower concentrations below TLV there has been no evidence of

affect to lung functioning. Various concentrations of ammonia gas cause following health

concerns.

Concentration Affect

400-700 PPM Severe eye and respiratory irritation.

Potential Permanent damage

1700 PPM Convulsive coughing and bronchial

spasms

Exposure more than half an hour can be

fatal

2500 PPM Can be life threatening

5000-10000 PPM Death by suffocation

[36], [52]

Ammonia vapor can be extremely irritant to eyes and mists or liquid ammonia can course

permanent blindness. Cryogenic burns can be coursed by liquid ammonia. Ammonia has not

been found to be carcinogenic or has the ability to reproductive or developmental toxicity in

human body. [36], [52]

4.1.2) Safety

When the safety of ammonia and its derivatives plant is considered, a common accident that has

occurred over the years is accidental releases of ammonia. Metals and alloys of particularly

copper and zinc are extremely vulnerable to corrosion when exposed to ammonia. In general

ammonia can be stored in iron or steel containers, transferred through iron and steel piping and

fittings. [36]. It should be noted that under certain conditions with few specific steels ammonia

can produce embrittlement [35]. Therefore it is required to follow strict guidelines when

choosing materials to avoid releases of ammonia liquid or gasses to the atmosphere. Ammonia

can flash in case of loss of containment when ammonia is stored under refrigerated conditions or

liquefied gas under pressure. Quantity flashed will depend on the temperature conditions [35].

Even though ammonia has lower density than air, the resultant clouds formed by flashing are

found to have higher densities than air. This will result in ground level ammonia clouds [54].

Past events

In order to get an idea about the safety measures that should be taken when designing an

ammonia and derivatives plant, it is required to have an understanding about the incidents that

have occurred with the presence of ammonia and in ammonia production.

In 1976, a release of 19 tons of ammonia from a road tanker in Texas, USA caused 6 deaths. It

should be noted that just after 2.5 hours the levels of ammonia has returned to the background

levels.[39] [40]

Even though there were no human fatalities, a massive 600 ton spill of ammonia to a

watercourse, caused a severe environmental hazard in Arkansas, USA in 1971 killing thousands

of fish [59]. The reason for the spill was accidently filling a tank with warm ammonia. The warm

ammonia transferred to the tank from the bottom, formed a layer at the bottom. Afterwards this

layer suddenly rose to the surface due to higher vapor pressure of the warmer ammonia causing

the tank to burst.

A very important fact to note is that with ammonia, brittle fractures can occur in metal

containments. Brittle fractures are catastrophic since the fracture can propagate at a velocity

closer to the velocity of sound. What appears to be the worst accident involving ammonia is the

release of approximately 38 tons of ammonia which caused 18 deaths. The release was due to a

brittle fracture in an ammonia tank. The accident occurred in Potchefstroom, South Africa in

1973. The post incident investigation revealed neither over pressure nor over temperature nor

other triggering event. The fracture started in a carbon steel dished end. The minimum transition

temperatures were 200C for fragment and 1150C for remaining part of the dished end. The

investigation showed that the operating conditions were below these temperatures which made

the material brittle. A gas cloud of 150m in diameter and 20m deep was resulted from this

failure. [39] [58].

Another type of accidents that has occurred in ammonia production plants over the years is vapor

cloud explosions. This is due to sudden releases of hydrogen rich synthesis gases being ignited

after a while. The flammable region of these clouds is spread from 4%-74% v/v. Due to the

diffusivity, these vapor clouds are not often formed in unconfined or semi-confined areas. But in

confined spaces, (eg: compressors) ignition of such a vapor cloud is catastrophic.

Another possible, yet uncommon type of accident is ammonia explosions. Ammonia explosions

are rare because of the unusually high, lower explosive limits (LEL-16%, HEL- 25%). The auto

ignition temperature of ammonia is about 6500C. There are no reports of ammonia explosions in

unconfined spaces due to the fact that it is not easier to get a 16% concentration in open air. In

Oklahoma, in 1978, a refrigeration system failed causing the ammonia storage to warm up. The

pressure eventually rose and the pressure relief valves discharged ammonia, which was ignited

by a nearby flare. In New Zealand in 1991, a welder was killed due to an explosion occurred

when he was welding a tank which supposed to be empty, yet which was later found out to have

a flammable mixture of ammonia vapor and air.[16]

Common avoidable mistakes

In the famous book “What Went Wrong” by Trevor Kletz, it has been identified that many of the

failures have occurred due to the use of wrong material other than the one that has been

specified. There has been one incident where the converter has been pushed over due to the

reaction forces from a hydrogen leak out. The reason for the leak out is the use of a carbon steel

exit pipe for the converter instead of 1.25% Cr, 0.5% Mo alloy, in which a hole was created due

to the hydrogen attack [16]

An interesting investigation was done to check the materials delivered to a new ammonia plant.

5480 items (1.8% of the total) was found to delivered in the wrong material. This included 2750

furnace roof hangers. Had this investigation been not done, the roof would probably have failed.

[16]. A specific reason has not been found for the delivery of wrong materials (Can be cost

cutting or unavailability). Therefore the engineers are advised to check the items before they are

used in the plant to avoid severe accidents in future. [16]

4.2) Safety ProceduresIt should be noted that not all the failures, problems or hazards can be anticipated. Events such as

those are eye openers for everyone, reminding that never be complacent with safety and the

importance of following the procedures to prevent accidents.

One such widely accepted procedure is “Inherent safety design”.

4.3) Inherent Safe DesignInherent safety is an approach to process design and operation which builds in safety, health and

environmental considerations at the start. This is done to ensure that even if something goes

wrong, the level of danger is minimized. Practically it is impossible to have an inherently safe

design. But one can have an inherently safer design.

“What you don’t have can’t leak!”

- Trevor Kletz, (Author: What Went Wrong)

There are four guidewords that drive inherent safety.

Substitute: Replace hazardous substances and procedures with less hazardous substances and

procedures

Minimize: Use as less as possible hazardous materials when it is not avoidable. Perform

hazardous procedures as few times as possible.

Moderate: Use hazardous materials in the least hazardous forms and identify processing

options that involve less severe process conditions.

Simplify: Finish the design, processing equipment and procedures to eliminate the chances

of errors by eliminating excessive use of safety features and protective devices.

Less equipment of any kind means that there is less to go wrong.

4.4) Layers of Process Safety

Inharent Safety: Elimination of hazards from the design

Passive Safety: Protection to the design so that it cannot be easily changed

Eg: process conditions

Active Safety: Prevent. Eg: high level trip isolates flow into a tank before it can overfill

Control. Eg: A restrictive orifice plate limits the rate of loss of

containment if a line fails.

Mitigate. Eg: heat activated links open deluge valves to spray water in

case of fire

Procedural Safety: Risk management systems

Eg: company policies, site rules, operating procedures, training,

maintenance, test procedures, emergency response plans

Inharren

t

Passive

Active

Procedur

al

There will be a conflict of interest when implementing inherent safety concepts due to many

reasons like workload, motivation, complexity, time, communications, organizational structure,

cost cutting, poor working environment etc.

It should be strictly noted that alarm systems are less complicated and work properly to make

sure that alarms are not ignored due to the fact that they are often activated due to

malfunctioning.

4.5) conclusions

4.6) recommendations

4.7) Environment

5) GREEN CONCEPTS USED IN PLANT DESIGNING

Green concepts are used frequently in industrial plant designing recently, as the whole world is

aware of the sustainability of the environment and the energy sources. These concepts mainly

addresses the issues with renewable energy sources, minimizing the pollutants which affects the

environment, Life cycle management of each raw material used, Efficient usage of natural

resources and protecting occupational health of the employees.

In a green building design, application of green concepts comes throughout the designing stage

to maintenance stage. This focuses on energy and resources efficiency, overall impact to the

environment, indoor environment quality and any other sustainable concepts (1). Life cycle of

each material used will be focused to make its’ use maximum and impact to the environment

minimum. Green building has the potential to save 30%-40% of energy while reducing the

operating cost and improving good health and a comfortable environment to work in (2).

5.1) why should we care about green buildings?For last 20-30 years, mankind faced bad experiences with global warming, ozone depletion,

resource depletion, energy scarcity, ecological toxicity, humantoxicity, acid rains etc. (1). These

have made them think about how they use the environment for their uses. Although it’s really

hard to stop the environment impact under current circumstances, idea of green industries is to

minimize the effect to the environment done by human activities.

There are lots of tangible and intangible benefits that an industry would get by applying green

concepts into its design (3).

Tangible benefits

o Sustained savings

Energy saving- 30%-40%

Water saving-20%-30%

Reduction in initial investments

o Reduce operational costs

o Optimize life cycle

o Waste minimization

Intangible benefits

o Reduce the impact to the environment

o Enhance occupant comfort

o Improve productivity of occupants

There are several aspects of green building concepts which can be taken to our consideration.

1. Site design, Preparation and development

2. Resource efficiency

a. Energy efficiency

b. Water efficiency

c. Other resources efficiency

3. Impact to the environment

a. Carbon foot print

b. Waste minimization

c. Life cycle management

4. Indoor environment quality

5.2) Current methods used in industriesMain areas addressed in the effort are

Search for renewable energy sources

Main renewable sources considered currently are

o Solar power

PV cells are used in different scales to create electricity from solar rays. Usage of

these has being increasing, but still the initial investment and the payback period

are problems related with this option. Still this stands as one of the main

renewable options to go with mainly for countries as Sri Lanka.

o Wind power

This is the other main energy source considered as a renewable option for

industries. Companies have widely tried to get wind power to their plant designs.

Optimize current energy consumption

This is the best starter for a current industry to go green. Most of the factories have their

problems in consuming energy in an optimum way, so that they will surely find their

energy demand decreased by concerning on the current energy plan.

Break-up of energy consumption in an industrial building(3)

Most of the energy consumed in industrial buildings are for ventilation and lighting. So,

most common way to save electricity is through upgrades to more energy-efficient

lighting systems, the use of energy management systems and upgrade to their HVAC

system.

Minimize greenhouse gas emission

Minimizing the carbon foot print on earth is the other main area stressed with green

concepts. For this, most industries have started acting on minimizing the emission of

greenhouse gasses. As CO2 being the main greenhouse gas produced in industries the

impact you do to the environment is measured as CO2.

Optimizing the usage of natural resources, waste treatment and life cycle management

Usage of natural resources as water and other raw materials takes a major role.

Management of life cycle of each raw material will help to minimize the waste and

improve the productivity of natural sources.

6) HISTORY OF UREA PRODUCTION IN SRI LANKA

There many views being aired about the Fertilizer and the Fertilizer plant that was closed down

in 1986. The plant was constructed by British Company to manufacture 980 tons of urea per day

at a cost 250 million US $.

Feedstock used for the process was Naphtha (fuel even lighter than petrol), which is a byproduct

of the oil refinery at Sapugaskanda. Naphtha has a severe demand in the world market for

making many petroleum products. CPC invariable preferred to sell naphtha to foreign markets

for obvious reason mentioned below.

In Middle East and in most countries Urea is manufactured using Natural gas, which can be

tapped easily in oil producing countries. Therefore it is a known fact that urea manufacture using

Natural gas is much more economical than using Naphtha. Under these circumstances Naphtha

based plants could not compete with natural gas plants particularly in a country like Sri Lanka

where there was only one oil refinery. Since our cost of production was extremely high treasure

could no settle the feedstock bills to refinery. CPC in return curtailed the live wire to Urea plant

as the Government was of the view that it would be cheaper to import urea and sell naphtha to

foreign market thereby eliminating the burden on the treasury.

The other factor that contributed to uneconomical operating cost was the regular breakdown in

the machinery and the lack of trained staff. Engineers and the Technicians who were trained

abroad for this specialized industry were gradually leaving for stabilized industries with the

development of uncertainty in the organization. Liberalized policy of the then government did

not want the staff to be retained by force. For practically all the spare parts and equipment we

had to depend purely on foreign suppliers. Probably a country like India could manage Naphtha

based plant because of the many refineries that they possess and also because of their well-

developed infrastructure. India not only could manufacture their own spares but also they could

manufacture all the required plant equipment.

Chlorophyl – C55H54N4Mg - Photosynthesis

Mol Wt – 55x12 +54x1+14x4+24x1 = 794

N – Wt % = 56/794 = 7.05%

C - = 660/794 = 83%

Mg - = 24/794 = 3.02%

Cellulose – (C12H10O5)n = 234 : C – 61%

2NH3 + CO2 ------- (NH2)2CO + H2O

At STP 1 gmol of a gas occupies 22.4 litres of volume

At STP 3 gmol (78 g) of a gas occupies 67.2 litres of volume

At 600 Bar pressure 67.2 litres will contain - 600x3gmols (23,400g)

Atmosphere – 0.03% CO2

Urea – CO(NH2)2

N – 28/60 – 46.7%

C – 12/60 – 20%

(NH4)2SO4 – 28/132 – 21%

NH4NO3 – 28/80 – 35%

MSDS of Methane, Ammonia, Urea, CO, CO2

Demand for Natural Gas – CH4 + 2O2 ---- CO2 + 2 H2O 16 44Cal Value – 16,000 + kcals/kg

Other hydrocarbons – LPG - 12,000 + kcals/kg

Gasoline - 12,000 + kcals/kg

7) PROCESS SELECTION AND DESIGN DECISIONS

Cost effective study of raw materials for production of Hydrogen

Considering the literature review done on existing processes to produce Hydrogen, following raw materials are considered as possible major options for the production process of Hydrogen for manufacturing of Ammonia for Urea.

1. Natural Gas2. Heavy oil 3. Coal4. Water/Alkaline electrolysis

1.Natural Gas

Currently natural gas is nor produced, imported or consumed in Sri Lanka. Although there are new prospects of natural gas wells in Sri Lanka, this unavailability of natural gas really is the main challenge with Natural gas. So we assume importing of natural gas which is the only option if we are going with this option.

CH4(g) + H2O(g) CO(g) + 3H2(g)

Production cost

Price of natural gas in international market = Rs 53,300/103m3

Density =0.5kg/m3

Price to produce 1kg of H2 = (Rs 53,300/103m3)*6/(16*0.5)

= Rs.79.50/kg of H2

Shipping cost

Shipping cost of avg 2.8kg of natural gas= Rs. 39/2.8kg of natural gas

(which is equalant to 1kg of H2 )

Total cost for 1kg of H2 = Rs.118.50/=

Heavy fuel oil

Heavy fuel will be bought from petroleum corperation.

C50H102(l) + 25O2 (g) 5CO(g)+51H2(g)

Price per 1liter of Heavy fuel oil in Sri Lanka = Rs. 90/=

Avg. specific gravity of heavy oil = 0.95

Price to produce 1kg of H2 = Rs.(90*702)/(102*0.95)

= Rs. 648.88/=

Coal

Although producing hydrogen from coal is a matured industry, it is not considered as a cost effective option for the countries which don’t have coal.

C(s)+ H2O(g)+heat CO(g)+H2(g)

This process need very high temperatures and considering the unavailability of coal in Sri Lanka and the electricity consumption, this is not considered as a favorable option.

4.Water/Alkaline electrolysis

H2O + electricity H2 + 1/2O2

With non-renewable fuels been decaying, cost forecast for electrolysis has shown good signs with the statistics.

With above factors, this is can also been be considered as a possible candidate.But the high electricity consumption and lack of R&D in industrial scale have hold this option from been being a cost effective option for the process, specially for in Sri Lanka.

When above analysis is considered, the best cost effective option seems to be natural gas. Natural gas has a high calorific value, which attracts the attention as a potential fuel. This leads the designers to be considerate about the future prices of natural gas.

In this regard, there were two forecasts referred. Both of the forecasts are done by United States Energy Information Administration (EIA). Forecasts were done from 2009 – 2030 time period which is likely to be the core operating time period of the proposed plant.

The first graph is of the price forecast of crude oil until 2030. Heavy oil is the heavy facture of the crude oil. If the liquid fuel consumption is broken down to sectors it can be clearly seen that the major consumption is for transportation (i.e. the consumption of gasoline and diesel). Therefore it is safe to assume that the world will focus on producing light fuels even from the heavier fractions.

When taking a look at the current price of a barrel of crude oil (2013 ≈$95) it is clear that the predictions are almost at reference prediction. Furthermore, the forecasted price varies from $50-$200.

In contrary the natural gas price tend to increase by maximum of $4 and seems very stable.

Therefore the overall conclusion is to use natural gas as a raw material.

According to the economical analysis mentioned above, it can be seen that using natural gas as

the feed is more cost effective than using heavy oils or coal. Because of that “Steam reforming”

process should be used for production of ammonia. New technologies can be used along with the

conventional steam reforming technology in order to increase the efficiency and thereby to

reduce the overall operating cost. Although the process layout is identical to the process that has

been used for decades, the performance is expected to be significantly improved due to

incorporation of modern technologies with the proposed plant.

Two main areas that have significant impact on the performance and the cost of an ammonia

production section are the reforming section and the synthesis section. Improvements for those

sections have been a major concern in this process design. Following improvements are basically

considered in designing the process for ammonia synthesis.

1) Reducing the duty of primary reformer

In every reforming ammonia process the reformer furnace and the flue gas duct represents about

25% of total investment cost of ammonia production plant.[1] Because of that the concept of

reducing the size of primary reformer and shifting duty to secondary reformer or another heat

exchange reformer is very important. Although there are so many methods to achieve this such

as Braun Purifier Process, ICI AMV process and Foster Wheeler process etc, The Topsoe

Process is selected in this design which is considered as the latest and energy efficient process

[2]. According to that process the size of the primary reformer can be reduced by following steps

Installation of pre reformer upstream to the primary reformer

Natural gas basically contains methane. But it may contain varying amount of other

hydrocarbons depending on the source. In the pre reformer all higher hydrocarbons are converted

in to a carbon oxides, hydrogen and methane. When a pre reformer is installed the primary

reformer has to reform methane only. Adiabatic reforming can be used for steam reforming of

feed stocks ranging from natural gas to heavy naphtha. Because of that the process also becomes

more flexible. In case of shortage of natural gas in Sri Lanka, Naphtha can be used as the feed

stock which can be obtained as a byproduct of petroleum refinery. At the same time pre

reforming catalyst will pick up any sulphur components in the feed and will allow much higher

heat flux in the primary reformer. The pre reformed feed can be reheated to 650°C before

entering the primary reformer. This will result in reduced firing in the primary reformer, and

thereby reduced fuel consumption [3].

Figure 1- Installation of Pre reformer

Installation of heat exchange reformer downstream of the secondary reformer

The temperature of the flue gas emitted from a conventional primary reformer is usually about

9000C and the process gas at the outlet of the secondary reformer is also at around 1000 0C. From

a thermodynamic point of view it is waste of energy to use this high amount of heat simply for

raising steam and preheating process air for the secondary reformer. The boiling temperature in a

125bar main boiler on the secondary reformer outlet is only 3250C.

According to the Haldor Topsoe process a heat exchange reformer unit which is named as

HTER-p (Haldor Topsøe Exchange Reformer) can be installed at the downstream of the

secondary reformer. The HTER-p is heated by the process gas exit from the secondary reformer,

and thereby the waste heat normally used for High pressure steam production can be utilized for

the reforming process down to 750–850°C approximately. Operating conditions in the HTER-p

are adjusted independently of the primary reformer in order to get the optimum performance of

the overall reforming unit. As shown in the diagram around 20% of the natural gas feed can be

by-pass the primary reformer according to this concept. [3]

Figure 2- Arrangement of Heat exchanger reformer unit

2) Pre heat combustion air for the primary reformer and process air to the secondary

reformer.

Although the duty of primary reformer is reduced using new technologies, the flue gas from the

primary reformer is expected to be at a temperature higher than 6500C. This heat can be

recovered to preheat combustion air for the primary reformer up to about 4000C and to pre heat

the process air for the secondary reformer. Rest of the heat that is needed to heat process air up to

6000C can be recovered from the gas emitted from the secondary reformer. Any remaining heat

from the flue gas can be used to preheat fuel to 1000C and to pre heat boiler water. According to

the energy efficient Topsoe process permitted stack temperature is lowered to 1000C using about

energy recovery methods.

3) Raising steam using waste heat steam boiler at the downstream of the HTER-p unit

The process gas at the outlet of the HTER-p is around 750–850°C. Temperature should be

reduced up to 350-4000C to feed the High Temperature Shift converter unit. This heat can be

recovered to raise high pressure steam which can be utilized in other heating requirements of the

plant.

4) S-350 Ammonia Synthesis Loop produced by Haldor Topsoe

There are various typical arrangements of ammonia synthesis loop based on the point at which

the fresh make up gas is delivered and ammonia formed is condensed. Since make up gas is

absolutely free from catalyst poisoning agents such as sulphur, water and carbon dioxide can be

directly sent to the converter. After that ammonia leaves the converter can be condensed out by

cooling. This can be considered as the most favorable arrangement in the minimum energy point

of view because it results the lowest ammonia concentration at the converter and highest

ammonia content at the condensation.

S-350 ammonia synthesis loop is the latest development of ammonia synthesis technology with

related to the above configuration. As shown in the diagram it contains two ammonia converters

which are known as S-300 converter follows by S-50 converter. This will increase overall

ammonia conversion than conventional loops. S-300 converter is the most updated version of

radial flow converter which is designed to increase the conversion with reduced catalyst volume.

[4] In addition this increased conversion will results improved steam generation. Considering

above facts S-350 synthesis loop is proposed to incorporate in the plant design.

Reasons for selecting ACES 21 process

ACES21 Process

Advanced process for Cost and Energy Saving (ACES) urea production is one of the latest

technologies which were introduced by TOYO Engineering Corporation, Japan. Newest version

of ACES was currently used in many urea plants worldwide. (ACES 21)

ACES 21 advanced technology for urea production ascertain initial investment cost and energy

consumption and other operating costs are lower compared to other technologies. Operating

conditions of the urea synthesis reaction have been optimized under lower temperature and lower

pressure than normal conditions without reducing reaction rate significantly. As a result, energy

consumption and other related operating costs were reduced noticeably.

Synthesis of Urea by ACES 21 Advanced Process

ACES21 process synthesis section consists of three sections as follows,

1. Reactor

2. Stripper

3. Vertical Submerged Carbamate Condenser

Reactor

Liquid ammonia is fed to the reactor via the HP Carbamate Ejector which provides the driving

force for circulation in the synthesis loop instead of the gravity system of the original ACES. The

reactor is operated at an N/C ratio of 3.7, 182 °C and 152 bar. The CO2 conversion to urea is as

high as 63% at the exit of the reactor.

Following reactions were carried out in the reactor,

NH2COONH4 NH2CONH2 + H2O : ΔH = +23 kJ/mol

2NH3 + CO2 NH2COONH4 + heat : ΔH = -84 kJ/mol

Stripper

Urea synthesis solution leaving the reactor is fed to the stripper where unconverted carbamate is

thermally decomposed and excess ammonia and CO2 are efficiently separated by CO2 stripping.

The stripped off gas from the stripper is fed to the Vertical Submerged Carbamate Condenser.

Part of original CO2 is fed to the stripper as stripping agent. The rest carbon dioxide is supplied

to the synthesis reactor.

Vertical Submerged Carbamate Condenser

Vertical Submerged Carbamate Condenser (VSCC) operated at an N/C ratio of 3.0, 180°C and

152 bar. Ammonia and CO2 gas condense to form ammonium carbamate and subsequently urea

is formed by dehydration of the carbamate in the shell side. Reaction heat of carbamate

formation is recovered to generate 5 bar steam in the tube side. A packed bed is provided at the

top of the VSCC to absorb uncondensed ammonia and CO2 gas into a recycle carbamate solution

from the MP absorption stage. Inert gas from the top of the packed bed is sent to the MP

absorption stage.

Medium Pressure Absorber

Ammonia and carbon dioxide separated from urea solution in medium pressure decomposer are

recovered in medium pressure absorber. Then condensation heat is transferred to the aqueous

urea solution feed in the final concentration section.

2NH3 + CO2 ↔ NH2COONH4 ; ΔH = -84 kJ/mol

Low Pressure Absorber

Ammonia and carbon dioxide separated from urea solution in low pressure decomposer are

recovered in low pressure absorber. Heat is released from the reaction inside low pressure

absorber. That heat is used to produce steam at 2bar. This steam is used for the evaporation

process of lower and upper separator.

2NH3 + CO2 ↔ NH2COONH4 ; ΔH = -84 kJ/mol

Flash Separator

In this unit, water is evaporated by reducing pressure in order to concentrate the urea solution.

This unit is operated at 1.0 bar and 110 °C.

H2O(l) → H2O(g)

Lower Separator

In this unit, purified urea is further purified. The head required for this process is taken from

steam produced in low pressure absorber. This is operated at 0.55 bar vacuum pressure and at

110 °C. Calendria evaporator is used for this.

Upper Separator

Urea solution coming from lower separator is further concentrated in this unit. This is operated at

0.55 bar vacuum pressure and at 112 °C. 99.2% pure urea can be obtained from unit.

Granulation Plant

In here the basic principle of the process involves the spraying of the melt onto recycled seed

particles circulating in the granulator. These seed particles gradually increase in size as the

process continues. This is operated at 110-115oC and at slightly negative pressure. Then the heat

of solidification is removed by cooling air to about 90ºC.

REFERENCES