Cairo University

Faculty of Engineering

Chemical Engineering Department

Urea Ammonium Nitrate Project

1st Technical Report

Submitted to; Prof.\ Mohamed Ali

Submitted by; Group 2

3rd

Nov, 2011

II

UAN Project

Urea Ammonium Nitrate

Abstract

This report is represented from group 2 working on Urea Ammonium Nitrate production

project, The report give a general overview on the different aspects of the project, starting

from the history of product and how it was discovered going through its physical and

chemical properties that is really important to know the appropriate way of handling and

using of the both the raw materials and the product.

The report also discuss the main theory of producing Urea which is the main product we

focus on through our report and also the different processes and technologies used in its

production, where so many processes were developed with different technologies

(different conditions of reaction) which are discussed in a general overview in this report,

the processes are Stamicarbon process, Snamprogitti process and ASEC process.

Different aspects like economical and environmental are also covered.

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Contents Abstract II

Table of contents III

Table of Figures V

Introduction 6

1. Fertilizers 6

1.1 Forms 6

1.2 Main elements used in in-organic (manmade) fertilizers industry 6

1.3 Problems with in-organic fertilizers 6

1.4 Urea 6

2. History 7

2.1 production of Ammonia 7

2.2 Production of Urea 7

2.3 Production of Ammonium nitrate 8

3. Uses of Urea 9

4. Physical Properties 11

5. Production 13

5.1 General theory 13

5.2 Block flow diagram of Urea production 14

5.3 Principle 15

5.4 Challenges in Urea production process 19

5.4.1 Recycle of unconverted Ammonia and Carbon

dioxide 19

5.4.2 Corrosion 21

5.4.3 Material selection 22

5.4.4 Side reactions 22

5.5 Description of urea production process 23

5.5.1 conventional Processes 23

5.5.2 Stripping processes 24

5.6 Urea Ammonium nitrate (UAN) production 30

5.6.1 Overview of UAN process technology 30

5.6.2 Typical UAN solution analysis 30

5.6.3 Description of production process 30

5.6.4 Urea Ammonium nitrate 31

5.6.5 Pool condenser process 31

6. Environmental aspects 33

6.1 Environmental issues: Spills along pathway 33

IV

6.2 Emissions, Effluents & solid wastes 33

6.2.1 Air Emissions 33

6.2.2 Effluents 36

6.2.3 Solid wastes 37

7. Economic aspects 39

7.1 Economic Analysis 39

7.2 Commercial Analysis 39

8. Quality Specifications 40

8.1 Urea quality specifications 40

8.1.1 Prilled urea 40

8.1.2 Granulated urea 40

8.2 Ammonium nitrate specifications 41

9. References 42

V

Table of Figures Page

Fig 0 Synthesis gas 10

Fig 1 Block flow diagram of Urea Production 14

Fig 2 Physical and Chemical Equilibrium in urea production 15

Fig 3 Carbon dioxide conversion with ratio ….. 16

Fig 4 Ammonia conversion with ratio … 16

Fig 5 Carbon dioxide conversion with ratio … 16

Fig 6 Ammonia conversion with ratio … 16

Fig 7 Carbon dioxide conversion ... 17

Fig 8 Ammonia conversion with ratio …. 17

Fig 9 Urea yield in the liquid phase at chemical equilibrium as a function of

Temperature 17

Fig 10 Urea yield in the liquid phase at chemical equilibrium as function of

H2O:CO2 ratio 18

Fig 11 Urea yield in the liquid phase at chemical equilibrium of NH3:CO2 ratio 18

Fig 12 Typical flow sheet of conventional urea plant 20

Fig 13 Conceptual diagram of the heat balance of conventional urea process heat

balance 21

Fig 14 Conceptual diagram of heat balance of stripping plant 21

Fig 15 UTI heat recycle urea process 24

Fig 16 Schematic of Stamicarbon process 25

Fig 17 Schematic of Snamprogetti self stripping process 27

Fig 18 Schematic of ACES process 29

Fig 19 Block flow diagram of UAN process 31

Fig 20 Pool condenser process (UAN process) 32

Fig 21 Mixing and cooling process of UAN production 32

Tables

Table 1 Specific heat vs. Temperature 11

Table 2 Properties of saturated aqueous solution of urea 11

Table 3 Solubility of Urea in various solvents (solubility in wt% of urea) 12

Table 4 Ammonium nitrate specifications 41

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Introduction

1. Fertilizer

Fertilizer (or fertiliser) is any organic or inorganic material of natural or synthetic origin

(other than liming materials) that is added to a soil to supply one or more plant nutrients

essential to the growth of plants .A recent assessment found that about 40 to 60% of crop

yields are attributable to commercial fertilizer use. Mined inorganic fertilizers have been

used for many centuries, whereas chemically synthesized inorganic fertilizers were only

widely developed during the industrial revolution.

1.1 Forms

Fertilizers come in various forms. The most typical form is granular fertilizer (powder

form). The next most common form is liquid fertilizer; some advantages of liquid

fertilizer are its immediate effect and wide coverage. There are also slow-release

fertilizers (various forms including fertilizer spikes, tabs, etc.) which reduce the problem

of "burning" the plants due to excess nitrogen.

More recently, organic fertilizer is on the rise as people are resorting to environmental

friendly (or 'green') products. Although organic fertilizer usually contain less nutrients,

some people still prefer organic due to natural ingredients

1.2 Main Elements used in in-organic (manmade) Fertilizers industries;

Phosphorus

Nitrogen

Calcium

Potassium

Magnesium

Sulphur

1.3 Problems with inorganic fertilizer

1) Trace mineral depletion

2) Over fertilization

3) High energy consumption

4) Long-Term Sustainability

1.4 Urea;

The highest fixed nitrogen-containing fertilizer 46.7 wt %, urea is a white solid that is

soluble in water and alcohol. It is usually sold in the form of crystals, prills, flakes, or

granules. Urea is an active compound that reacts with many reagents. It forms adducts

and clathrates with many substances such as phenol and salicylic acid. Urea is gaining

greater acceptance as a slow rate fertilizer.

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2. History

2.1 Production of ammonia:

Ammonia was made for the first time in 1774 and by 1785 its exact composition was

determined. Nitric acid, known earlier, was synthesized for the first time and by 1771 it

was being produced commercially.

Urea was identified in 1773 and in 1775 the presence of large amounts of calcium

phosphate in bones was confirmed. In 1784, the chemical nature of water was discovered.

The Haber process is a method of producing ammonia developed in WWI. The Germans

needed nitrogen to for making their explosives. When the Allies blocked off all trade

routes going to and from Germany, they lost all source of sodium nitrate and potassium

nitrate, their source of nitrogen. They found their source of nitrogen in the air, which was

80% nitrogen. The chemist Fritz Haber developed the Haber Process in WWI, which

takes molecular nitrogen from the air and combines it with molecular hydrogen to form

ammonia gas, which the chemical formula is NH3.The equation for the reversible

reaction;

N2(g) + 3H2(g) 2NH3(g) + 92 kJ.

2.2 Production of Urea:

Urea was first discovered in human urine by H.M. Rouelle in 1773.

It was synthesized in 1828 by Friedrich Wohler and was the first organic compound to

be synthesized from inorganic starting materials. It was found when Wohler attempted to

synthesis ammonium cyanate, to continue a study of cyanates which he had been carrying

out for several years. On treating silver cyanate with ammonium chloride solution he

obtains organic synthesis dealt a severe blow to a widespread belief called “vitalism”

which maintained that organic chemicals could be modified by chemistry but could only

be produced through the agency of a vital force present in living plants and animals.

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.

The present commercial method of synthesizing urea through the reaction of ammonia

and carbon dioxide was first introduced in Germany in 1920. In the U.S., commercial

synthesis of urea was first achieved in 1932.

Use of urea, including calcium nitrate-urea as a fertilizer in this country, started in the

early 1920s with imported material mainly from Germany. Production of urea in the U.S.

for fertilizer purposes increased dramatically from 47,000 tons in 1948 to 549,000 tons

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in 1960. Expansion in urea capacity slowed between 1960 and 1975, eventually reaching

3 million tons at the end of this period. By 1980, production of fertilizer urea was 7.2

million tons, 60 percent of it in solid form. In 1960, 75 percent of the total urea cont’

primary urea solutions produced at anhydrous ammonia plants was utilized as fertilizer

and by 1982 this usage had increased to 92 percent. Urea consumption in 1994 accounted

for 14.7 percent of the 12.6 million tons of nitrogen applied to U.S. croplands.

2.3 Production of Ammonium Nitrate:

This nitrogen compound was first employed as a fertilizer in Europe following World

War I when large stocks not utilized for explosives were released for agricultural use.

Because of explosions in Germany in 1920 and 1921, it was usually dry-mixed with

limestone, gypsum, chalk or ammonium sulfate to avoid such danger and to also serve as

conditioning agents.

The first use of ammonium nitrate in the United States occurred in 1926 with product

imported from Germany. It was not until 1943 that significant amounts of this nitrogen

source were available to American farmers when supplies exceeded the needs for

munitions. Additional surplus quantities of this material were released for fertilizer use in

1944 and 1945

In the 1970’s ammonium nitrate was produced at 29 locations separate from ammonia

plant sites. Production of solid and solution ammonium nitrate fertilizer increased

continuously from 383,000 tons in 1943 to 7.3 million tons in 1980. Manufacture of solid

product was greater than the solution form until 1978, after which it accounted for less

than half of the total produced.

Ammonium nitrate accounted for about one-third of all the nitrogen applied directly to

U.S. cropland in 1955. By 1980, this use had declined to 10 percent. The tonnage of

directly applied nitrogen as ammonium nitrate was exceeded by anhydrous ammonia in

1960, by nitrogen solutions in 1970 and by urea in 1978. Ammonium nitrate supplied 5.3

percent of the total 12.6 million tons of nitrogen consumed nationally in 1994.

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3. Uses of Urea

1. The major use of urea is the fertilizer field, which accounts for approximately 80% of

its production (about 16.2 billion pounds were produced during 1994 in U.S.). About

10% of urea is used for the production of adhesives and plastics (Urea formaldehyde).

2. Urea possesses a unique property of forming adducts with n-paraffins. This is used in

separating C12-C14 n-paraffins from kerosene for detergent production

3. The nucleophilic addition reaction of urea to formaldehyde produces mainly

monomethylol urea and some dimethylol urea. When the mixture is heated in presence of

an acid, condensation occurs, and water is released, it produces an important commercial

polymer (urea formaldehyde resins) that is used as glue for particle board and plywood.

4. Explosive; Urea can be used to make urea nitrate, a high explosive which is used

industrially and as part of some improvised explosive devices

5. 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 BlueTec system, for example, injects 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.

6. Also used as a stabilizer in nitrocellulose explosives

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

promote growth.

8. A non-corroding alternative to rock salt for road de-icing, and the resurfacing of

snowboarding half pipes and terrain parks

9. A flavor-enhancing additive for cigarettes

10. A main ingredient in hair removers such as Nair and Vet.

11. A browning agent in factory-produced pretzels

12. An ingredient in some skin creams, moisturizers, hair conditioners

13. A reactant in some ready-to-use cold compresses for first-aid use, due to the

endothermic reaction it creates when mixed with water.

14. A cloud seeding agent, along with other salts.

15. A flame-proofing agent, commonly used in dry chemical fire extinguisher charges

such as the urea-potassium bicarbonate mixture.

16. An ingredient in many tooth whitening products.

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

ethanol.

18. A nutrient used by plankton in ocean nourishment experiments for geo-engineering

purposes.

19. As an additive to extend the working temperature and open time of hide glue

20. As a solubility-enhancing and moisture-retaining additive to dye baths for textile

dyeing or printing

21. Urea can be used to make urea nitrate, a high explosive which is used industrially and

as part of some improvised explosive devices.

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22. Why is Urea mentioned in the Petrochemical Industry?! The Answer is SYNTHESIS GAS;

Fig 0 Synthesis Gas

Synthesis gas is a major source of hydrogen, which is used for producing ammonia.

Ammonia is the host of many chemicals such as urea, ammonium nitrate and

hydrazine. Carbon dioxide, a by-product from synthesis gas reacts with ammonia to

produce urea.

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4. Physical Properties of Urea

Pure urea forms white, odorless, long, thin needles, but it can also appear in the form of

rhomboid prisms. The crystal lattice is tetragonal; the axis ratio a: c=1: 0.833. The urea

crystal is anisotropic (non-cubic) and thus shows birefringence. At20 ◦C the refractive

indices are 1.484 and1.602. Urea has an mp of 132.6 ◦C; its heat of fusion is 13.61kJ/mol.

Urea is contains 46.65 percent nitrogen. It is not flammable, but it will melt and

decompose in a fire to give off ammonia. When it is dissolved in water, it hydrolyzes

very slowly to ammonium carbamate and eventually decomposes to ammonia and carbon

dioxide

Chemical formula NH2CONH2

Molecular weight 60.06

Freezing/melting point 132.6OC

Boiling point decomposes

Density, d42O

1.3230 g/cm3

Heat of solution in water +251J/g (60 cal/g)

Bulk density 0.74g/cm3

Specific heat at different temp(J/(kg.K)

Table 1; Specific heat vs. Temp

0°C 1.439

50°C 1.661

100°C 1.887

150°C 2.109

Table 2; Properties of Saturated Aqueous Solutions of Urea

Water

Vapor

Pressure

(kPa)

Viscosity

(mPa-s =

cP)

Density

(g/cm3)

Solubility

in Water

(g/100 g

Solution

Temperature

©

0.53

1.73

5.33

12.00

2 1.33

29.33

18.00

0.93

2.63

1.96

1.72

1.72

1.93

2.35

2.93

3.25

1.120

1.147

1.167

1.184

1.198

1.210

1.22 1

1.226

41.0

51.6

62.2

72.2

80.6

88.3

95.5

99.2

0

20

40

60

80

100

120

130

Some other Physical properties of the melt at 135 ◦C follow:

Molecular volume =48.16m3/kmol

Kinematic viscosity =2.42×10−6 m2/s

Molar heat capacity(Cp) =135.2 Jmol−1

K−1

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Specific heat capacity(cp) =2.25 kJ kg−1

K−1

Surface tension = 66.3×10−3 N/m

Table 3; Solubility of urea in various solvents (solubility in wt% of urea)

Temperature

100 80 60 40 20 0 Solvent

90.4 84.5 78.7 67.2 48.6 34.9 Ammonia

38.6 26.1 18.0 13.0 Methanol

13.1 8.5 5.1 2.5 Ethanol

Physical and Chemical properties of Urea Ammonium Nitrate

Physical Form: Liquid

Color: Colorless

Odor: Slight ammonia odor (pungent)

Boiling Point: Approximately 225o F (107 o C)

Melting/Freezing Point:

28% UAN salts out at 0o F (-18o C)

30% UAN salts out at 16o F (-9o C)

32% UAN salts out at 32o F (0o C)

PH: 6.8 – 7.5

Solubility: 100%

Specific Gravity:

28% 1.281

30% 1.304

32% 1.330

Vapor Density: Approximately 1.07 (@ 60o F)

Vapor Pressure: 8.6 – 17.5 mmHg (@ 100o F)

% Volatile by Volume: No test results

Molecular Weight: Not applicable

Density:

28% 10.67 lb/gal at 60o F

30% 10.86 lb/gal at 60o F

32% 11.07 lb/gal at 60o F

Critical Temperature: No test results

Critical pressure: No test results

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5. Production

5.1 General Theory;

Step 1 – Synthesis

A mixture of compressed CO2 and ammonia is reacted to form ammonium carbamate.

This is an exothermic reaction, and heat is recovered by a boiler which produces steam.

The first reactor achieves 78% conversion of the carbon dioxide to urea and the liquid is

then purified. The second reactor receives the gas from the first reactor and recycles

solution.

Step 2 – Purification

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

reaction and unconsumed reactants (ammonia, carbon dioxide and ammonium

carbamate). The unconsumed reactants are removed in three stages3. Firstly, the pressure

is reduced and then the solution is heated, which causes the ammonium carbamate to

decompose to ammonia and carbon dioxide:

NH2COONH4 2NH3 + CO2

At the same time, some of the ammonia and carbon dioxide flash off. The pressure is then

reduced on two other stages, with more ammonia and carbon dioxide being lost at each

stage. By the time the mixture is at -0.35 bar gauge 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.

Step 3 – Concentration

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

water, increasing the urea concentration from 68% w/w to 80% w/w. At this stage some

urea crystals also form. The solution is then heated again to dissolve these crystals prior

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

Step 4 – Granulation

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

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

in a granulator which receives the seed granules at one end and discharges enlarged

granules at the other as molten urea is sprayed through nozzles. Dry, cool granules are

classified using screens. Oversized granules are crushed and combined with undersized

ones for use as seed.

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

removes the urea with a water solution then discharges the air to the atmosphere. The

final product is cooled in air, weighed and conveyed to bulk storage ready for sale.

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5.2 Block flow diagram of Urea production process;

Fig 1 Block flow diagram of Urea production process

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5.3 Principles Chemical Equilibrium (1);

In all commercial processes, urea is produced by reacting ammonia and carbon dioxide at

elevated temperature and pressure according to the Basaroff reactions:

2NH3 (l) +CO2 (l) NH2COONH4 (1)

ΔH =−117 kJ/mol

NH2COONH4 NH2CONH2 +H2O (2)

ΔH =+ 15.5 kJ/mol

In the first reaction, carbon dioxide and ammonia are converted to ammonium

carbamate; the reaction is fast and exothermic.

In the second reaction which is slow and endothermic, ammonium carbamate

dehydrates to produce urea and water.

Since more heat is produced in the first reaction than consumed in the second the

overall reaction is exothermic.

-A schematic of the overall process and the physical and chemical equilibrium

involved is shown followed;

Fig 2 Physical and chemical equilibrium in urea production

Processes differ mainly in the conditions (composition, temperature, and pressure) at

which these reactions are carried out. Traditionally, the composition of the liquid phase in

the reaction zone is expressed by two molar ratios: usually, the molar NH3: CO2 and the

molar H2O: CO2 ratios. Both reflect the composition of the so-called initial mixture [i.e.,

the hypothetical mixture consisting only of NH3, CO2, andH2Oif both Reactions (1) and

(2) are shifted completely to the left].

First attempts to describe the chemical equilibrium of Reactions (1) and (2) were made by

Frejacques. Later descriptions of the chemical equilibrium can be divided into regression

analyses of measurements and thermodynamically consistent analyses of the equilibrium

.As far as the most important consequences of these equilibrium on urea process design

are concerned, the methods correspond closely to each other: The achievable conversion

per pass, dictated by the chemical equilibrium as a function of temperature, goes through

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a maximum (Figs 3 and 4). This effect is usually attributed to the fact that the ammonium

carbamate concentration as a function of temperature goes through a maximum. This

maximum in the ammonium carbamate concentration can be explained, at least

qualitatively, by the respective heat effects of Reactions (1) and (2).However, this

mechanism cannot explain the observed conversion maximum fully and quantitatively;

other contributing mechanisms have been suggested.

Fig 3 Carbon dioxide conversion Figure 4 Ammonia conversion

Figure 3&4 Ammonia and carbon dioxide conversion at chemical equilibrium as a

function of temperature NH3:CO2 ratio = 3.5 mol/mol (initial mixture); H2O:CO2 ratio =

0.25 mol/mol (initial mixture)

The influence of the composition of the initial mixture on the chemical equilibrium can

be explained qualitatively by Reactions (1) and (2) and the law of mass action:

1- Increasing the NH3:CO2 ratio (increasing the NH3 concentration) increases CO2

conversion, but reducesNH3 conversion (Figs. 5 and 6).

Fig 5 Carbon dioxide conversion Fig 6 Ammonia conversion

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Figures 6 & 5 carbon dioxide and Ammonia conversion at chemical equilibrium

respectively at chemical equilibrium as a function of NH3:CO2 ratio T=190 ◦C; H2O:

CO2 ratio = 0.25 mol/mol (initial mixture)

2) Increasing the amount of water in the initial mixture (increasing the H2O: CO2 ratio)

results in a decrease in both CO2 and NH3 conversion (Figs. 6and 7).

Fig 7 Carbon dioxide conversion Fig 8 Ammonia conversion

Figures 6 & 7 Ammonia conversion at chemical equilibrium as a function of H2O: CO2

ratio T = 190 ◦C; NH3: CO2 ratio = 3.5 mol/mol (initial mixture).

In these cases full quantitative description cannot be derived simply from the law of mass

action and Reactions (1) and (2). Other, not yet fully understood reaction mechanisms

probably contribute to the chemical equilibrium to a minor extent.

In Figures 3, 5, and 7, the conversion at chemical equilibrium is expressed as CO2

conversion, that is, the amount of CO2 in the initial mixture converted into urea (plus

biuret), if no changes occur in overall NH3, CO2, and H2O concentrations in the liquid

phase. This way of representing the chemical equilibrium is consistent with the

presentation usually found in the traditional urea literature. However, it is based on the

arbitrary choice of CO2 as the key component. Historically, this may be justified by the

fact that in early urea processes, CO2 conversion was more important than NH3

conversion. For the present generation of stripping processes, however, giving a higher

weight to CO2 conversion is not justified. Comparing, e.g., Figs. 4 and 5, shows that an

arbitrary choice of one of the two feedstock components as yardstick to evaluate

optimum reaction conversion can easily lead to faulty conclusions

Fig 9 Urea yield in the liquid phase at chemical equilibrium as a function of temperature NH3: CO2 ratio = 3.5 mol/mol (initial

mixture); H2O: CO2 ratio = 0.25 mol/mol (initial mixture)

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Fig 10 Urea yield in the liquid phase at chemical equilibrium as a function of H2O:CO2 ratio T=190 ◦C NH3:CO2 ratio

= 3.5 mol/mol (initial mixture)

Ultimately, project economics (investment and consumptions) will dictate the choice of process

parameters in the reaction section. Without going into such time- and place-dependent economic

considerations, one can argue that the urea yield (i.e., the concentration of urea in the liquid

phase) is a better tool for judging optimum process parameters than CO2 or NH3 conversion.

Figure 8 illustrates that urea yield as a function of temperature also goes through a maximum; the

location of this maximum is of course composition dependent. Figure 9 again shows the

detrimental effect of excess water on urea yield; thus, one of the targets in designing a recycle

system must be to minimize water recycle. Figure 10 shows that the urea yields as a function of

NH3:CO2 ratio reaches a maximum somewhat above the stoichiometric ratio (2:1). This is one of

the reasons that all commercial processes operate at NH3:CO2 ratios above the stoichiometric

ratio. Another important reason for this can be found from the physical phase equilibrium in the

NH3 –CO2 –H2O– urea system

Fig 11 Urea yield in the liquid phase at chemical equilibrium as a function of NH3:CO2 ratio T=190 ◦C; H2O: CO2

ratio = 0.25 mol/mol (initial mixture)

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5.4 Challenges in Urea Production Process Design; Since the urea process already has half a century of commercial scale history, it will be

clear that compromises between the aforementioned partly conflicting criteria are well

established. Also resulting from the age of urea process design is the observation that a

process can only be successful if acceptable and competing solutions to all of these

criteria can be combined into one process design. Apart from applying straightforward

normal engineering approaches, the challenge of finding an optimum synergy between

partly conflicting criteria, focuses in urea plant design essentially on a few peculiarities:

1) The thermodynamic limit on the conversion per pass through the urea reactor,

combined with the azeotropic behavior of the NH3 –CO2 system, necessitates a

cunning recycle system design.

2) The intermediate product ammonium carbamate is extremely corrosive. A proper

combination of process conditions, construction materials, and equipment design

is therefore essential.

3) The occurrence of two side reactions – hydrolysis of urea and biuret formation –

must be considered.

5.4.1 Recycle of unconverted Ammonia and Carbon Dioxide;

-From a chemical equilibrium date it was found that indicates that the conversion of the

feed stocks NH3 and CO2 to urea is limited. An important differentiator between

processes is the way these unconverted materials are handled

i. Once through Process;

In this process non converted ammonia was neutralized with acid such as nitric

acid to produce ammonium salt such as ammonium nitrate as co products of urea

production. In this way, a relatively simple urea process scheme was realized.

The main disadvantages of this process are the large quantity of ammonia salt

formed as co product and the limited amount of overall carbon dioxide

conversion that can be achieved. But there is a partial “revival” of this combined

urea – ammonium nitrate production facilities

ii. Conventional Recycle Processes;

Once through processes were soon replaced by total recycle processes, where

essentially all of the unconverted ammonia and carbon dioxide were recycled to

the urea reactor. In the first generation of total-recycle processes, several

licensors developed schemes in which the recirculation of unconverted NH3 and

CO2 was performed in two stages. Figure 11 is a typical flow sheet of these, now

called conventional, processes. The first recirculation stage was operated at

medium pressure (18 – 25 bars); the second, at low pressure (2 – 5 bar). The first

recirculation stage comprises at least a decomposition heater (d), in which

carbamate decomposes into gaseous NH3 and CO2, while excess NH3 evaporates

20

simultaneously. The off-gas from this first decomposition step was subjected to

rectification, from which relatively pure NH3 (at the top) and a bottom product

consisting of an aqueous ammonium carbamate solution were obtained. Both

products are recycled separately to the urea reactor. In these processes, all

unconverted CO2 was recycled as an aqueous solution, whereas the main portion

of unconverted NH3 was recycled without an associated water recycle. Because

of the detrimental effect of water on reaction conversion, achieving a minimum

CO2 recycle (and thus maximum CO2 conversion per reaction pass) was much

more important than achieving a low NH3 recycle. All conventional processes

therefore typically operate at high NH3:CO2 ratios (4–5 mol/mol) to maximize

CO2 conversion per pass. Although some of these conventional processes, partly

equipped with ingenious heat-exchanging networks, have survived until now (see

Section 4.3.1), their importance decreased rapidly as the so-called stripping

processes were developed.

Figure 12 Typical flow sheet of a conventional urea plant

a)CO2 compressor; b) High-pressure ammonia pump; c) Urea reactor; d) Medium-pressure decomposer; e) Ammonia – carbamate separation column; f) Low-pressure decomposer; g) Evaporator; h) Prilling; i) Desorber (wastewater

stripper); j) Vacuum condensation section

iii. Stripping Processes.

In the 1960s, the Stamicarbon CO2-stripping process was developed, followed later by

other processes (in Section 3.3.2). Characteristic of these processes is that the major part

of the recycle of both unconverted NH3 and unconverted CO2 occurs via the gas phase;

such that none of these recycles is associated with large water recycles to the synthesis

zone. Another characteristic difference between conventional and stripping processes in

terms of the recycle scheme can be found in the way of which heat is supplied to the

recirculation zones. The energy balance of the conventional processes is shown in Figure

12. In this first-generation urea process, the heat supplied to the urea synthesis solution

was used only once; therefore, this type of process can be referred to as an N = 1 process.

Such a process required about 1.8 t of steam per ton of urea.

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Fig 13 Conceptual diagram of the heat balance of a conventional urea process Heat to each subsequent heater is

supplied in the form of steam; the heat is used only once (N = 1).

The energy balance of a stripping plant is shown in Figure 13. As in conventional plants,

heat must be supplied to the urea synthesis solution to decompose unconverted carbamate

and to evaporate excess ammonia and water. However, a distinct difference in the heat

balance with respect to the conventional process is that only the heat in the first heater

(the high-pressure stripper) is imported. This heat is recovered in a high pressure

carbamate condenser (unconverted ammonia and carbon dioxide are condensed to form

ammonium carbamate) and reused in the low pressure heaters. The heat supplied is

effectively used twice; thus, the term N =2 process is used. The average energy

consumption of the stripping process is 0.8 – 1.0 t of steam per ton of urea.

Fig 14 Conceptual diagram of the heat balance of a stripping plant

5.4.2. Corrosion

Urea synthesis solutions are very corrosive. Generally, ammonium carbamate is

considered the aggressive component. This follows from the observation that carbamate-

containing product streams are corrosive whereas pure urea solutions are not. The

corrosiveness of the synthesis solution has forced urea manufacturers to set very strict

demands on the quality and composition of construction materials. Awareness of the

important factors in material selection, equipment manufacture and inspection,

technological design and proper operations of the plant, together with periodic

inspections and nondestructive testing are the key factors for safe operation for many

years Corrosion protection is achieved by the use of well proven design principles,

stringent material and fabrication specifications, complemented by detailed codes of

22

practice for operating, monitoring and inspecting equipment. The corrosiveness at a given

point in the plant is determined by the temperature, the process components, the

concentration of dissolved oxygen and the presence of contaminants that may accelerate

corrosion. The formation on start-up and maintenance of a passive oxide layer on

stainless steel surfaces is of the utmost importance. Stainless steel lined carbon steel

vessels are usually used in the HP synthesis section for economic reasons, including leak

detection units to protect the integrity of the vessels and avoid a potentially hazardous

situation

5.4.3. Material Selection

Corrosion resistance is not the only factor determining the choice of construction

materials. Other factors such as mechanical properties, workability, and weld ability,

as well as economic considerations such as price, availability, and delivery time, also

deserve attention. Stainless steels that have found wide use are the austenitic grades

AISI 316L and 317 L. Like all Cr-containing stainless steels, AISI 316L and 317 L

are not resistant to the action of sulfides. Hence it is imperative in plants using the

316L and 317 L grades in combination with CO2 derived from sulfur-containing gas,

to purify this gas or the CO2 thoroughly. In stripping processes, the process

conditions in the high-pressure stripper are most severe with respect to corrosion.

In the Stamicarbon CO2-stripping process, a higher-alloyed, but still fully austenitic

stainless steel (25 Cr 22 Ni 2 Mo) was chosen as construction material for the stripper

tubes. This choice ensures better corrosion resistance than 316L or 317L types of

material but still maintains the advantages of workability, weld ability, reparability,

and the cheaper price of stainless steel-type materials.

In the Snamprogetti stripping processes, titanium usually is chosen for this critical

application, although mechanically bonded bimetallic 25 Cr 22 Ni 2 Mo – zirconium

tubes have also been suggested to improve corrosion resistance

In the ACES process, duplex alloys (ferrite – austenitic) are used as construction

material for the stripper tubes.

5.4.4. Side Reactions Three side reactions are of special importance in the design of urea production processes:

1. Hydrolysis of urea

CO(NH2)2 +H2O→NH2COONH4→2NH3 +CO2 (3)

2. Biuret formation from urea:

2 CO(NH2)2→NH2CONHCONH2 +NH3 (4)

3. Formation of iso-cyanic acid from urea:

CO(NH2)2→NH4NCO→NH3 +HNCO (5)

All three side reactions have in common the decomposition of urea; thus, the extent to

which they occur must be minimized.

The hydrolysis reaction (3) is nothing but the reverse of urea formation. Whereas

this reaction approaches equilibrium in the reactor, in all downstream sections of

the plant the NH3 and CO2 concentrations in urea-containing solutions are such

23

that Reaction (3) is shifted to the right. The extent to which the reaction occurs is

determined by temperature (high temperatures favor hydrolysis) and reaction

kinetics; in practice, this means that retention times of urea-containing solutions at

high temperatures must be minimized.

The biuret reaction (4) also approaches equilibrium in the urea reactor the high

NH3 concentration in the reactor shifts Reaction (4) to the left, such that only a

small amount of biuret is formed in the reactor. In downstream section of the

plant, NH3 is removed from the urea solutions, thereby creating a driving force for

biuret formation. The extent to which biuret is formed is determined by reaction

kinetics; therefore, the practical measures to minimize biuret formation are the

same as described above for the hydrolysis reaction.

The iso-cyanic acid reaction (5) is also favored by low NH3 concentrations. This

reaction is especially relevant in the evaporation section of the plant. Here, low

pressures are applied, resulting in a transfer of NH3 and HNCO into the gas phase

and, consequently, low concentrations of these constituents in the liquid phase.

Together with the relatively high temperatures in the evaporators, this shifts

Reaction (5) to the right. The extent to which this reaction occurs is again

determined by kinetics. The HNCO removed via the gas in the evaporators

collects in the process condensate from the vacuum condensers, where low

temperatures shift Reaction (5) to the left, again forming urea. As a result of this

mechanism of chemical entrainment, attempts to minimize entrainment from

evaporators with physical (liquid – gas) separation devices are destined to be

unsuccessful.

5.5 Description of Processes 5.5.1 Conventional Processes

As explained in Section 3.2, conventional processes have generally been replaced by

stripping processes. Only two conventional processes may still have some importance in

the near future;

1) Urea Technologies Inc. (UTI) Heat Recycle Process (Fig 15).

Ammonia (containing passivating air), recycled carbamate, and about 60%of the

feed CO2 are charged to the top of an open-ended coil reactor (c) operating at 210

bars. Ammonium carbamate is formed within the coil, exits the coil at the bottom,

and then flows up and around it–the exothermic heat of carbamate formation in

the coil driving the endothermic dehydration of carbamate to urea outside the coil.

The reactor is claimed to achieve a uniform temperature profile in this way. In the

reactor, a relative high NH3:CO2 ratio (4.2:1) is applied. The reactor effluent is

depressurized and sub cooled, and the flashed gases are released before the first

decomposer (f ). Gases leaving the first decomposer separator (g) are mixed with

about 40% of the feed carbon dioxide and partially condensed in the heat recovery

section (i – k).The two combined gas streams are then further condensed and form

the carbamate recycle flow. Urea as an 86– 88% solution is concentrated by

evaporation (i) before granulation or prilling. The process is applied in eight

small-scale and two medium-scale plants.

24

Fig 15 UTI heat recycles urea process

a) CO2 compressor; b) High-pressure ammonia feed pump; c) Urea reactor with internal coil; d) Air compressor; e) Liquid distributor (used as first flash vessel); f ) First decomposer; g) First separator; h) Second decomposer; i) Urea

concentrator; j) Carbamate heater; k) Ammonia heater; l) High-pressure carbamate recycle pump; CW= Cooling water

2) MTC (Mitsui Toatsu Corporation) Conventional Processes of Toyo

Engineering Corporation.

The conventional processes developed by Toyo Engineering Corporation (TEC)

were successfully commercialized until the mid- 1980s. The continuous evolution

of these processes is reflected in their sequential nomenclature:

TR –A Total-Recycle A Process

TR –B Total-Recycle B Process

TR –C Total-Recycle C Process

TR –CI Total-Recycle C Improved Process

TR –D Total-Recycle D Process

Partial-recycle versions of these processes were also realized. These TEC MTC

conventional processes were applied in more than 70 plants.

5.5.2 Stripping Processes

5.4.2.1 Stamicarbon CO2-Stripping Process (Figs. 16)

The synthesis stage of the Stamicarbon process consists of a urea reactor (c), a stripper

for unconverted reactants (d), a high-pressure carbamate condenser (e), and a high-

pressure reactor off-gas scrubber (f). To realize maximum urea yield per pass through the

reactor at the stipulated optimum pressure of 140 bar, an NH3:CO2 molar ratio of 3:1 is

applied. The greater part of the unconverted carbamate is decomposed in the stripper,

where ammonia and carbon dioxide are stripped off. This stripping action is effected by

countercurrent contact between the urea solution and fresh carbon dioxide at synthesis

pressure.

Low ammonia and carbon dioxide concentrations in the stripped urea solution are

obtained, such that the recycle from the low-pressure recirculation stage (h, j) is

minimized. These low concentrations of both ammonia and carbon dioxide in the stripper

effluent can be obtained at relatively low temperatures of the urea solution because

carbon dioxide is only sparingly soluble under such conditions.

25

Fig 16 Stamicarbon CO2-stripping urea process (The process suitable for combination with a granulation plant is shown here; combination with prilling is also possible.)

a) CO2 compressor; b) Hydrogen removal reactor; c) Urea reactor; d) High-pressure stripper; e) High-pressure carbamate condenser; f) High-pressure scrubber; g) Low-pressure absorber; h) Low-pressure decomposer and rectifier; i) Pre-evaporator; j) Low-pressure carbamate condenser; k) Evaporator; l) Vacuum condensation section; m) Process condensate treatment CW= Cooling water; TCW= Tempered cooling water

Condensation of ammonia and carbon dioxide gases, leaving the stripper, occurs in the

high pressure carbamate condenser (e) at synthesis pressure. As a result, the heat

liberated from ammonium carbamate formation is at a high temperature. This heat is used

for the production of 4.5-bar steam for use in the urea plant itself. The condensation in

the high pressure carbamate condenser is not effected completely. Remaining gases are

condensed in the reactor and provide the heat required for the dehydration of carbamate,

as well as for heating the mixture to its equilibrium temperature. In an improvement to

this process, the condensation of off-gas from the stripper is carried out in a pre-reactor,

where sufficient residence time for the liquid phase is provided. As a result of urea and

water formation in the condensing zone, the condensation temperature is increased, thus

enabling the production of steam at a higher pressure level. The feed carbon dioxide,

invariably originating from an associated ammonia plant, always contains hydrogen. To

avoid the formation of explosive hydrogen – oxygen mixtures in the tail gas of the plant,

hydrogen is catalytically removed from the carbon dioxide feed (b). Apart from the air

required for this purpose, additional air is supplied to the fresh carbon dioxide input

stream. This extra portion of oxygen is needed to maintain a corrosion-resistant layer on

the stainless steel in the synthesis section. Before the inert gases, mainly oxygen and

nitrogen, are purged from the synthesis section, they are washed with carbamate solution

from the low pressure recirculation stage in the high-pressure scrubber (f) to obtain a low

ammonia concentration in the subsequently purged gas. Further washing of the off-gas is

performed in a low pressure absorber (g) to obtain a purge gas that is practically

ammonia free. Only one low-pressure recirculation stage is required due to the low

ammonia and carbon dioxide concentrations in the stripped urea solution. Because of the

ideal ratio between ammonia and carbon dioxide in the recovered gases in this section,

water dilution of the resultant ammonium carbamate is at a minimum despite the low

26

pressure (about 4 bars). As a result of the efficiency of the stripper, the quantities of

ammonium carbamate for recycle to the synthesis section are also minimized, and no

separate ammonia recycle is required. The urea solution coming from the recirculation

stage contains about 75 wt% urea. This solution is concentrated in the evaporation section

(k). If the process is combined with a prilling tower for final product shaping, the final

moisture content of urea from the evaporation section is ca. 0.25 wt %. If the process is

combined with a granulation unit, the final moisture content may vary from 1 to 5 wt %,

depending on granulation requirements. Higher moisture contents can be realized in a

single-stage evaporator, whereas low moisture contents are economically achieved in a

two-stage evaporation section. When urea with an extremely low biuret content is

required (at a maximum of 0.3 wt %), pure urea crystals are produced in a crystallization

section. These crystals are separated from the mother liquor by a combination of sieve

bends and centrifuges and are melted prior to final shaping in a prilling tower or

granulation unit. The process condensate emanating from water evaporation from the

evaporation or crystallization sections contains ammonia and urea. Before this process

condensate is purged, urea is hydrolyzed into ammonia and carbon dioxide (l), which are

stripped off with steam and returned to urea synthesis via the recirculation section. This

process condensate treatment section can produce water with high purity, thus

transforming this “wastewater” treatment into the production unit of a valuable process

condensate, suitable for, e.g., cooling tower or boiler feed water makeup. Since the

introduction of the Stamicarbon CO2-stripping process, some 125 units have been built

according to this process all over the world.

5.5.2.2. Snamprogetti Ammonia-and Self-Stripping Processes

In the first generation of NH3 and self strip ping processes, ammonia was used as

stripping agent. Because of the extreme solubility of ammonia in the urea containing

synthesis fluid, the stripper effluent contained rather large amounts of dissolved

ammonia, causing ammonia overload in downstream section of the plant. Later versions

of the process abandoned the idea of using ammonia as stripping agent; stripping was

achieved only by supply of heat .Even without using ammonia as a stripping agent, the

NH3:CO2ratio in the stripper effluent is relatively high. So the recirculation section of the

plant requires an ammonia-carbamate separation section the process uses a vertical layout

in the synthesis section. Recycle within the synthesis section, from the stripper via the

high pressure carbamate condenser, through the carbamate separator back to the reactor,

is maintained by using an ammonia-driven liquid-liquid ejector. In the reactor, which is

operated at 150 bars, NH3:CO2molar feed ratio of 3.5 is applied .The stripper is of the

falling film type? Since stripping is achieved thermally, relatively high temperatures

(200-2100C) are required to obtain a reasonable stripping efficiency. Because of this high

temperature, stainless steel is not suitable as a construction material for the stripper from

a corrosion point of view; titanium and bimetallic zirconium – stainless steel tubes have

been used Off gas from the stripper is condensed in a kettle type boiler. At the tube side

of this condenser the off gas is absorbed in recycled liquid carbamate from the medium

pressure recovery section. The heat of absorption is removed through the tubes, which are

cooled by the production of low pressure steam at the shell side. The steam produced is

used effectively in the back end of the process .In the medium pressure decomposition

and recirculation section, typically operated at 18 bar, the urea solution from the high

27

pressure stripper is subjected to the decomposition of carbamate and evaporation of

ammonia. The off gas from this medium pressure decomposer is rectified. Liquid

ammonia reflux is applied to the top of this rectifier; in this way a top product consisting

of pure gaseous ammonia and a bottom product of liquid ammonium carbamate are

obtained. The pure ammonia off gas is condensed and recycled to the synthesis section.

To prevent solidification of ammonium carbamate in the rectifier, some water is added to

the bottom section of the column to dilute the ammonium carbamate below its

crystallization point. The liquid ammonium carbamate-water mixture obtained in this

way is also recycled to the synthesis section. The purge gas of the ammonia condenser is

treated in a scrubber prior to being purged to the atmosphere .The urea solution from the

medium pressure decomposer is subjected to a second low pressure decomposition step.

Here further decomposition of ammonium carbamate is achieved, so that a substantially

carbamate – free aqueous urea solution is obtained. Off gas from this low pressure

decomposer is condensed and recycled as an aqueous ammonium carbamate solution to

the synthesis section via the medium pressure recovery section. Concentrating the urea

water mixture obtained from the low pressure decomposer is preformed in a single or

double evaporator depending on the requirement of the finishing section. Typically, if

prilling is chosen as the final shaping procedure, a two stage evaporator is required,

whereas in the case of a fluidized bed granulator a single evaporation step is sufficient to

achieve the required final moisture content of the urea melt. In some versions of the

process, heat exchange is applied between the off gas from the medium pressure

decomposer and the aqueous urea solution to the evaporation section. In this way, the

consumption of low pressure steam by the process is reduced. The process condensate

obtained from the evaporation section is subjected to adsorption hydrolysis operation to

recover the urea and ammonia contained in the process condensate. In this way, the

consumption of low-pressure steam by the process is reduced. The process condensate

obtained from the evaporation section is subjected to desorption– hydrolysis operation to

recover the urea and ammonia contained in the process condensate. Up to now, about 70

plants have been designed according to the Snamprogetti ammonia and self-stripping

processes

Fig 17 Schematic of the Snamprogetti self-stripping process

a) CO2 compressor; b) Urea reactor; c) Ejector; d) High-pressure ammonia pump; e) Carbamate separator; f) High-pressure carbamate condenser; g) High-pressure carbamate pump; h) High-pressure stripper; i) Medium-pressure

decomposer and rectifier; j) Ammonia – carbamate separation column; k) Ammonia condenser; l) Ammonia receiver; m) Low-pressure ammonia pump; n) Ammonia scrubber; o) Low-pressure decomposer and rectifier; p) Low-pressure

28

carbamate condenser; q) Low-pressure carbamate receiver; r) Low-pressure off-gas scrubber; s) First evaporation heater; t) First evaporation separator; u) Second evaporation heater; v) Second evaporation separator; w) Wastewater

treatment; x) Vacuum condensation section CW= Cooling water.

5.5.2.3 Isobaric Double-Recycle Process This process is developed by Montedison, is characterized by recycle of most of the un

reacted ammonia and ammonium carbamate in two decomposer in series, both operating

at the synthesis pressure. A high molar NH3:CO2 ratio (4:1 to 5:1) in the reactor is

applied. As a result of this choice ratio, the reactor effluent contains a relatively high

amount of non converted ammonia. In the first, steam heated, high pressure decomposer,

this large quantity of free ammonia is mainly removed from the urea solution. Most of the

residual solution, as well as some ammonium carbamate, is removed in the second high

pressure decomposer where steam heating and CO2 stripping are applied. The high

pressure synthesis section is followed by two low pressure decomposing stages of

traditional design, where heat exchange between the condensing off gas of the medium

pressure decomposition stage and the aqueous urea solution to the final concentration

section improves the overall energy consumption of the process. Probably because of the

complexity of this process, it has not achieved great popularity so far. This process or

parts of the process are used in four revamps of older conventional plant

5.5.2.4 ACES Process The ACES (i.e., Advanced Process for Cost and Energy Saving) process has been

developed by Toyo Engineering Corporation. Its synthesis section consists of the reactor

(a), stripper (d), two parallel carbamate condensers (e), and a scrubber (f ) – all operated

at 175 bar (see Fig. 18).

The reactor is operated at 190 ◦C and an NH3:CO2 molar feed ratio of 4:1. Liquid

ammonia is fed directly to the reactor, whereas gaseous carbon dioxide after compression

is introduced into the bottom of the stripper as a stripping aid. The synthesis mixture from

the reactor, consisting of urea, unconverted ammonium carbamate, excess ammonia, and

water, is fed to the top of the stripper. The stripper has two functions. Its upper part is

equipped with trays where excess ammonia is partly separated from the stripper feed by

direct countercurrent contact of the feed solution with the gas coming from the lower part

of the stripper. This pre-stripping in the top is said to be required to achieve effective CO2

stripping in the lower part. In the lower part of the stripper (a falling film heater),

ammonium carbamate is decomposed and the resultingnCO2 and NH3 as well as the

excess NH3 are evaporated by CO2 stripping and steam heating. The overhead gaseous

mixture from the top of the stripper is introduced into the carbamate condensers (e). Here,

two units in parallel are installed, where the gaseous mixture is condensed and absorbed

by the carbamate solution coming from the medium-pressure recovery stage. Heat

liberated in the high-pressure carbamate condensers is used to generate low-pressure

steam in one of the condensers and to heat the urea solution from the stripper after the

pressure is reduced to about 19 bars in the shell side of the second carbamate condenser.

The gas and liquid from the carbamate condensers are recycled to the reactor by gravity

29

flow. The urea solution from the stripper, with a typical NH3 content of 12 wt %, is

purified further in the subsequent medium- and low-pressure decomposers (i, j),

operating at 19 and 3 bar, respectively. Ammonia and carbon dioxide separated from the

urea solution here are recovered through stepwise absorption in the low- and medium-

pressure absorbers (h, k). Condensation heat in the medium-pressure absorber is

transferred directly to the aqueous urea solution feed in the final concentration section. In

this final concentration section (l), the purified urea solution is concentrated further either

by two-stage evaporation up to 99.7% for urea prills production or by a single

evaporation up to 98.5% for urea granule production. Water vapor formed in the final

concentrating section is condensed in surface condensers (q) to form process condensate.

Part of this condensate is used as an absorbent in the recovery sections, whereas the

remainder is purified in the process condensate treatment section by hydrolysis and steam

stripping, before being discharged from the urea plant. The highly concentrated urea

solution is finally processed either through the prilling tower (o) or via the urea

granulator (p). Instead of concentration via evaporation, the ACES process can also be

combined with a crystallization section to produce urea with low biuret content. Until

now, the ACES process has been used in seven urea plants.

Fig 18 Schematic of the ACES process

a) Urea reactor; b) High-pressure ammonia pump; c) CO2 compressor; d) Stripper; e) High-pressure carbamate condensers; f) High-pressure scrubber; g) High-pressure carbamate pump; h) Medium-pressure absorber; i) Medium-pressure decomposer; j) Low-pressure decomposer; k) Low-pressure absorber; l) Evaporators; m) Process condensate

stripper; n) Hydrolyser; o) Prilling tower; p) Granulation section; q) Surface condensers CW= Cooling water

30

5.6 UREA-AMMONIUM NITRATE (UAN) PRODUCTION

5.6.1 Overview of UAN Process Technology

Ammonium nitrate (AN) and urea are used as feed stocks in the production of urea-

ammonium nitrate (UAN) liquid fertilizers. Most UAN solutions typically contain 28, 30

or 32% N but other customized concentrations (including additional nutrients) are

produced. Plant capacities for the production of UAN solutions range between 200 and

2,000t.d-1. Most of the large scale production units are located on complexes where

either urea or ammonium nitrate or both are produced. In some of the European UAN

plants, ammonium nitrate is being synthesized directly from nitric acid and ammonia. In

some cases carbamate solution from the urea reactor outlet is being used as feedstock for

the production of UAN. In those plants the UAN technology is an integral part of the

fertilizer complex. UAN from scrubbing systems, urea from sieving machines, etc. are

fed to a central UAN system, where quality adjustments can be done. The addition of

corrosion inhibitors or the use of corrosion resistant coatings allows carbon steel to be

used for storage and transportation equipment for the solutions. West European

consumption of UAN in 1998/1999 was 3.72 x 106 t of solution, 41% of which was

imported.

5.6.2 Typical UAN solution analysis;

N content 28-32% by weight, pH 7 to 7.5, density 1,280-1,320kg.m-3, salt-out

temperature –18 to –2°C, depending on the N content and at its lowest when the Urea

N/Ammonium Nitrate N ratio is about 1:1.

5.6.3 Description of Production Processes;

Continuous and batch type processes are used and in both processes concentrated urea

and ammonium nitrate solutions are measured, mixed and then cooled. Block diagrams

for UAN production are shown in Figures 19.

In the continuous process the ingredients of the UAN solution are continuously fed to and

mixed in a series of appropriately sized static mixers. Raw material flow as well as

finished product flow, PH and density are continuously measured and adjusted. The

finished product is cooled and transferred to a storage tank for distribution.

In the batch process the raw materials are sequentially fed to a mixing vessel fitted with

an agitator and mounted on load-cells. The dissolving of the solid raw material(s) can be

enhanced by recirculation and heat exchange as required. The PH of the UAN product is

adjusted prior to the addition of the corrosion inhibitor.

A partial recycle CO2 stripping urea process is also suitable for UAN solution production.

Unconverted NH3 and CO2 coming from the stripped urea solution, together with the

gases from the water treatment unit, are transferred for conversion into UAN solutions.

31

Fig 19 block flow diagram for UAN production

5.6.4 Urea – Ammonium Nitrate (UAN) process

Mixtures of urea (mp 133 ◦C) and ammonium nitrate (mp 169 ◦C) with water have a

eutectic point at −26.5 ◦C [51]. As a result solutions with high nitrogen content can be

made with solidification temperatures below ambient temperature. These mixtures, called

UAN solutions, are used as liquid nitrogen fertilizers. UAN solutions can be made by

mixing the appropriate amounts of solid urea and solid ammonium nitrate with water or,

alternatively, in a production facility specially designed to produce UAN solutions. In

this latter category the StamicarbonCO2-stripping technology is especially suitable, in a

partial-recycle version of this process.

5.5.5 Pool condenser process (stamicarbon) (fig 20&21)

Stamicarbon creates UAN by using the residual ammonia remaining in the stripped urea

solution, together with the ammonia in the high-pressure urea synthesis's off-gases. This

is converted into ammonium nitrate in a neutralization reactor using nitric acid. In the

high-pressure synthesis section, carbon dioxide and ammonia are converted into urea

which contains unconverted ammonia and carbon dioxide. Stripping with carbon dioxide

causes the greater part of these components to evaporate from the solution. From here,

evaporated ammonia and carbon dioxide, together with fresh ammonia and carbon

dioxide, are condensed in the pool reactor, with the heat from this condensation being

used to produce low-pressure steam. The condensed ammonia and carbon dioxide are

partly converted into urea and water. In the low-pressure dissociation section, the stripped

urea solution is almost entirely freed from ammonia and carbon dioxide. Then the

overhead vapors of the reactor, mixed with off-gases from the dissociation section and

the ammonia present in the urea solution from the urea solution tank, are all sent to the

neutralization section.

32

Fig 20 Pool condenser Process (UAN process)

Fig 21 Mixing and cooling Process of UAN production

33

6. Environmental Aspects

The benefits of using a nitrogenous fertilizers is obvious because the crops grow taller,

and are healthier therefore yielding a higher crop and therefore cheaper, more plentiful

food.

There are always disadvantages. If after applying the fertilizer and it rained or when

too much fertilizer is used it gets into the streams or rivers and pollutes them. In the

rivers, the fertilizer does the same as it would on land, the river plants grow and algae

grow rapidly because of the abundant food supply. The algae then die in large numbers.

The bacteria feeding on the dead plant material use up the oxygen in the water, Fish may

then die because of the lack of oxygen in the water. Also too high of nitrates in the

drinking water is a health hazard, particularly with infants. Nitrates can interfere with the

oxygen flow in the blood stream.

6.1 Environmental issues: spills along pathways;

• Given existing urea production facilities and extensive distribution pathways, SCR-

urea should not have significant incremental spill incidents or well-tank greenhouse

gas emissions.

• Urea is considered a stable and safely transportable product and degrades quickly in

soil, water, and air.

• However, ingested or absorbed in large quantities, it can be hazardous to plant and

animal life.

• Spill clean-up procedures are well-established and will have to be modified and/or

updated to accommodate new locations where urea will be used.

6.2 Emissions, Effluents and Solid Wastes

Fertilizers industry manufacturing processes originate is gas, contaminated liquid

effluents in addition to dust and solid by-products. These wastes pollute air, water as well

as impair soil. Pollution problems arise from low processes efficiency and/or inefficient

dust collecting systems, gas scrubbing (if it does exist) and most important is the lack of a

proven recycle of such recovered contaminants to the process streams

6.2.1 Air Emissions:

The major emission in phosphoric and nitrogenous fertilizers cited here below includes

particulates, sulphur oxides, hydrogen fluoride (and compounds Ca F2 SiF4) phosphor-

gypsum, phosphoric acid, ammonia, NOx, ammonium nitrate, sulphate, carbon oxides …

etc.

34

6.2.1.1 for Ammonium Nitrate Production

The primary air emissions from ammonium nitrate production plants are particulate

matter (ammonium nitrate and coating materials), ammonia and nitric acid. Ammonia

and nitric acid are emitted primarily from solution formation and granulators. Particulate

matter (largely as ammonium nitrate) can be emitted from most of the process operations.

The emission sources in solution formation and concentration processes are

neutralizers and evaporators, emitting droplets of ammonium nitrate and

ammonia. The vapor stream off the top of the neutralization reactor is primarily

steam with some ammonia and NH4NO3 particulates present. Specific plant

operating characteristics, however, make these emissions vary depending upon

use of excess ammonia or acid in the neutralizer. Particulate emissions from these

operations tend to be smaller in size than those from solids production and

handling processes and generally are recycled back to the process. Emissions of

very fine particles are very difficult to remove. A combination of droplet

separators and scrubbers can be used.

Contaminated steam from evaporator should be purified before discharge to the

environment.

Emissions from solids formation processes are ammonium nitrate particulate

matter and ammonia. The sources of primary importance are prilling towers (for

high density and low density prills) and granulators (rotary drum and pan).

Ammonia is normally removed by neutralization in a wet scrubber. Most

conventional scrubbers are less efficient for the removal of particles below 1

micron of prilling tower fume. Candle filters are normally required. Gases from

granulator and drier may be cleaned by combination of dry cyclones or bag filters

and wet scrubbers.

Emissions from prilling towers result from carryover of fine particles and fume by

the prills cooling air flowing through the tower. These fine particles are from

micro-prilling formation, attrition of prills colliding with the tower or one another,

and rapid transition of the ammonia nitrate between crystal states.

Micro-prilling formation resulting from partially plugged orifices of melt spray

devices can increase fine dust loading and emissions. Certain designs (spinning

buckets) and practices (vibration of spray plates) help reduce plugged orifices and

thus micro-prilling formation. High ambient air temperatures can cause increased

emissions because of entrainment as a result of higher air flow required to cool

prills and because of increased fume formation at the higher temperatures.

Emissions from screening operations are generated by the attrition of the

ammonium nitrate solids against the screens and against one another. Almost all

screening operations used in the ammonium nitrate manufacturing industry are

35

enclosed or have a cover over the uppermost screen. Emissions are ducted from

the process for recovery or reuse.

Bagging and bulk loading operations are also a source of particulate emissions.

Dust is emitted from each type of bagging process during final filling when dust

laden air is displaced from the bag by the ammonium nitrate. The potential for

emissions during bagging is greater for coated than for uncoated material. It is

expected that emissions from bagging operations are primarily the kaolin, talc or

diatomaceous earth coating matter. About 90 percent of solid ammonium nitrate

produced domestically is bulk loaded. While particulate emissions from bulk

loading are not generally controlled, visible emissions are within typical state

regulatory requirements (below 20 percent opacity).

6.2.1.2 for Urea Production:

Emissions from urea manufacture are mainly ammonia particulate matter.

Formaldehyde and methanol, hazardous air pollutants, may be emitted if additives are

used. Formalin TM, used as a formaldehyde additive, may contain up to 15 percent

methanol. Ammonia is emitted during the solution synthesis and solids production

processes. Particulate matter is emitted during all urea processes. In the synthesis process,

some emission control is inherent in the recycle process where carbamate gases and/or

liquids are recovered and recycled. Typical emission sources from the solution synthesis

process are non-condensable vent streams from ammonium carbamate decomposers and

separators. Emissions from synthesis processes are generally combined with emissions

from the solution concentration process and are vented through a common stack.

Combined particulate emissions from urea synthesis and concentration operations are

small compared to particulate emissions from a typical solids-producing urea plant. The

synthesis and concentration operations are usually uncontrolled except for recycle

provisions to recover ammonia.

Uncontrolled emission rates from prilling towers may be affected by the following

factors:

Product grade being produced

Air flow rate through the tower

Type of tower bed.

The total of mass emissions per unit is usually lower for feed grade prilling production

than for agricultural grade prills, due to lower airflows. Uncontrolled particulate emission

rates for fluidized bed prilling towers are higher than those for non-fluidized bed prilling

towers making agricultural grade prills, and are approximately equal to those for non-

fluidized bed feed grade prills.

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Ambient air conditions can affect prilling tower emissions. Available data indicate that

colder temperatures promote the formation of smaller particles in the prilling tower

exhaust. Since smaller particles are more difficult to remove, the efficiency of prilling

tower control devices tends to decrease with ambient temperatures. This can lead to

higher emission levels for prilling towers operated during cold weather. Ambient

humidity can also affect prilling tower emissions. Air flow rates must be increased with

high humidity, and higher air flow rates usually cause higher emissions. In the solids

screening process, dust is generated by abrasion of urea particles and the vibration of the

screening mechanisms. Therefore, almost all screening operations used in the urea

manufacturing industry are enclosed or are covered over the uppermost screen. Emissions

attributable to coating include entrained clay dust from loading, in-plant transfer, and

leaks from the seals of the coater.

6.2.2 Effluents; “Wastewater”;

Wastewater from the fertilizer industry can be classified into four groups:

Process effluents resulting from contact with gas, liquids, or solids

Dedicated effluents which may be separated for use in one process or for

recycling at a controlled rate

Effluents from general services such as cleaning or pretreatment

Occasional effluents such as leaks or spills

Wastewater is generated in any fertilizer production facility by leaks, spills, cleaning,

maintenance, and laboratory tests. Cooling water may contain ammonia, sulfate, chloride,

phosphate, chromate, and dissolved solids which become concentrated through

evaporation.

6.2.2.1 for Nitrogenous Fertilizers:

Wastewater is considered the major pollution problem in such plants. A number of

process wastewater streams from the nitrogenous fertilizer industry have been identified.

Wastewater is analyzed in terms of ammonia, nitrates, organic nitrogen and hydrogen

ions concentration (pH).

6.2.2.2 Ammonia Production:

In ammonia production, wastewater is generated from process condensate stripping.

Cooling the synthesis gas after low temperature shift conversion forms a condensate

containing NH3, CO2, and methanol (CH3OH), and trace metals. Condensate steam

strippers are used to remove NH3 and methanol from the water. Stripped condensate can

be reused as boiler feed water after treatment of ion exchange. In some cases, the process

condensate is used for feed gas saturation and thus recycled.

Wastewater is contaminated with traces of ammonia and oils from compressing gas unit.

Emissions to water may also occur due to the scrubbing of waste gases containing

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ammonia if gases are not recovered in a closed loop so that no aqueous ammonia

emissions occur.

6.2.2.3 Nitric Acid Production

Nitric acid production generates relatively little wastewater since there is no process

wastewater source. Steam condensate generated in nitrogenous fertilizer processing is

characterized by dissolved and suspended solids, alkalinity, and hardness. The ammonia

vaporizer blow-down should have the ammonia vaporized and recovered into the process.

The waste oil should be reprocessed.

6.2.2.4 Ammonium Nitrate Production

Wastewater effluent is discharged from cooling tower blow down, backwash from

multi-media filters and from equipment and floor washing. Loss of ammonium nitrate to

drain from a large number of sources is a potential problem for all ammonium nitrate

plants. Causes are different kinds of leaks and washings.

Ammonium nitrate manufacturing produces process wastewater in the neutralization

process, the evaporation unit, and air cooling equipment. The vacuum condenser in

ammonium nitrate plants is a source of wastewater. Most scrubbing operations are also a

source of wastewater. The steam condensate leaving the neutralizer can be purified.

Purification can be achieved by stripping, distillation and membrane separation processes

as reverse osmosis. Ion exchange can also be considered but there are some safety

concerns which must be addressed. The steam can be used in the evaporator or to preheat

and evaporate ammonia and to preheat nitric acid. The solution from wet scrubbers is

normally recycled.

6.2.3 Solid Wastes:

For all fertilizers plants, solid materials may be found in storage piles, settled dust and

other similar forms. The following are the major solid wastes in different production lines

of fertilizers.

6.2.3.1 For Nitrogenous Fertilizers

Ammonia Production;

Solid wastes are generated due to the usage of many catalysts types and

quantities. The spent catalysts cannot be purified or recycled, hence, they create a

disposal problem which should be checked. These catalysts are considered

hazardous, due to their heavy metals content (chromium and nickel). Another

solid waste problem is the filling material for absorption tower. They are

consumed and disposed off-site.

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Catalysts used in the steam reforming process need to be replaced every two to six

years. Spent catalysts contain oxides of hexa valent chromium, zinc, iron, and

nickel. They are typically returned to the manufacturer or other metal recovery

companies for recycling and reclamation of valuable materials.

Nitric Acid Production

Spent catalyst from the reactor and filter cake and cloth generated from the filter

are considered the major solid wastes generated from this unit. Usually spent

catalyst is sent to the suppliers and it must not be disposed in dumping sites due to

its hazardous nature.

Ammonium Nitrate Production

Dolomite and clay from the mixing and coating operations are the major solid

wastes from this unit as well dump product.

Urea Production

Prills coating material in the bagging section are collected and recycled.

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7. Economic Aspects

7.1 Economic Analysis:

Costs of production estimates for different urea processes in different parts of the world

should be carried out.

The major contributor to the urea cost is the cost of ammonia. Since all the urea plants are

situated next to ammonia facilities, the effect of transferring ammonia into the urea

process at cash cost and full cost should be examined. Additionally, a cost of production

estimate for the production of urea based on market price ammonia should be done.

7.2 Commercial Analysis:

Urea is used as a solid nitrogen release fertilizer, because it has a high nitrogen content

(46 percent), compared with 34 percent for ammonium nitrate and less for ammonium

sulfate.

The high nitrogen content of urea reduces transportation costs per ton of nutrient. This,

together with its low solubility and absence of safety considerations make it more

suitable for deep-sea expert and use in the tropical climates of Asia and Latin America.

As these are growing economies with high populations, urea is the most widely used

nitrogen fertilizer.

In addition to fertilizer uses, there are also chemical uses such as melamine, resins,

pharmaceutical and others.

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8. Quality Specifications

8.1 Urea Quality Specifications:

8.1.1 Prilled Urea:

1- White prilled

2- Mass part nitrogen: 46.2%

3- Biuret, not more than: 1.0%

4- Moisture not more than: 0.5%

5- Granule size: 1mm to 4mm 90% min.

6- Free ammonia: 160 ppm max.

7- Melting point: 32o C

8- Physical properties:

Non radioactive

White

Free flowing

Free from harmful substances

100% treated against caking

8.1.2 Granulated Urea:

1- White granules

2- Nitrogen recalculated to dry: 46.2%

3- Biuret, not more than: 1.0%

4- Water, not more than: 0.5%

5- Granules size: 2mm to 4mm 90% min.

6- Free ammonia: 160 pxt ppm max.

7- Melting point: 132 degrees cen.

8- Crumbliness: 100%

9- Physical properties:

Non radioactive.

White

Free flowing

Free from harmful substances

Coated

Spherical

Uniform in size

100% treated against coating

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8.2 Ammonium Nitrate Specifications: “Table 4”

GOST 2-85

A B

Higher Grade First Grade

Total mass portion of nitrate and

ammonium nitrogen based on:

NH4NO3 in dry matter,%, not less than 98 No standard No standard

Nitrogen in dry matter, %, not less than No standard 34.4 34.4

Water mass portion, %, not more than:

with sulphate and phosphate additives 0.2 0.2 0.2

with calcium/ magnesium nitrate

additives 0.3 0.3 0.3

PH of 10% water solution, not less than 5.0 5.0 5.0

..with sulphate & phosphate additives 4.0 4.0 4.0

Mass portion of substances insoluble in

10% nitric acid solution,%, not more than 0.2 No standard No standard

Grain Size Distribution

Mass portion of granule size from 1 to

3mm, %, not less than 93 No standard No standard

Mass portion of granule size from 1 to 4

mm, %, not less than No standard 95 95

Granule size from 2 to 4mm, %, not less

than No standard 80 50

Mass portion of granule size less than

1mm, %, not more than 4 3 3

Mass portion of granule size exceeding

6mm, % 0.0 0.0 0.0

Granule static strength, n/granule

(kg/granule), not less than 5(0.5) 7(0.7)

with sulphate and sulphate & phosphate

additives 10(1.0)

with calcium and magnesium nitrate

additives 8(0.8)

Friability, %, not less than 100 100 100

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9. References 1. Ullmann`s encyclopedia, “encyclopedia of industrial chemistry”.

2. Kent and Riegel's handbook, “industrial chemistry and biotechnology”.

3. “Encyclopedia of chemical processing”, volume 1, 2006.

4. http://www.agriservices.com/pdf/uan.pdf (Cited in October 2011)

5. http://www.chemsystems.com (Cited in October 2011)

6. http://www.valetimegroup.com (Cited in October 2011)

7. http://www.agrodev.com (Cited in October 2011)

8. http://www.efma.org/Product-Stewardship

programe10/images/EFMABATURE.pdf (Cited in October 2011)

9. http://www.cyberax.eu/book/657680/urea-final-report (Cited in October 2011)

10. http://www.stamicarbon.com/UAN.html (Cited in October 2011)