MINISTRY OF HIGHER EDUCATION AND SCINTIFIC RESEARCH

BRIGHT STAR UNIVERSITY, EL -BREGA

FACULTY OF TECHNICAL ENGINEERING

DEPARTMENT OF CHEMICAL ENGINEERING

MANUFACTURE OF UREA (Stamicarbon Technology)

Submitted in partial fulfillment of the requirement for (B.Sc.) degree in chemical engineering

By:

1) Ibrahim Ismael Ibrahim Milad 21141067 2) Omran Mohammed Abdalah Khamis 13236 3) Faiz Edris Jumaa Faiz 21141065 4) Waleed Salem Mathkour Shalash 21141063

Supervised by: Mr. Abdelhamid El Masry

FALL – 2017

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ACKNOWLEDGEMENT

We dedicate this research project to our dear parents who are the source of our happiness, progress and success in our life.

We are fully aware that there are many who joined us, helped us and

stood with us in shaping this into a final form. We acknowledge our

sincere gratitude to all of them who have encouraged and advised us in

this attempt.

First and the foremost we would like to express our sincere gratitude to

our supervisor, Mr.Abdelhamid El Masry Khan for the valuable

guidance and advice he has given while doing this research. We would

like to acknowledge our sincere gratitude to the Head of the

Department of Chemical Engineering.

We take the opportunity to thank our teachers, for their all-valuable

assistance in completing our graduation studies.

Finally we heartily thank our family and friends who inspired,

encouraged and fully supported us in every trial that came our way.

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VI

الشكر والتقدير

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VII

CONTENTS CHAPTER TITLE . PAGE

اآليةACKNOWLEDGEMENT

االهداء

التقديرشكر و 1 INTRODUCTION

1.1 Background 1

1.1.1 History of Libyan oil production. 3

1.2 The Study Problem 3

1.3 The research Questions 4

1.4 Objectives of the Study 4

1.5 Hypothesis 5

1.6 Methodology: 5

1.7 Study Environment 6

1.8 What is urea? 6-12

2 PRODUCTION PROCESS OF UREA 2.1 Manufacture Introduction 14

2.1.1 Manufacture Introduction 15

2.1.2 Raw materials for manufacturing Urea 19

2.1.2.1 Ammonia 20

2.1.2.2 Carbon dioxide 22

2.1.3 Sources and availability of raw materials 21

2.1.3.1 Hydrogen Production 21

2.1.3.2Nitrogen 21

2.2Process Selection 22

2.2.1 Stripping Processes 22

2.2.2 Reactor Conditions 23

2.2.3 Isobaric double recycle process 24

2.2.4 Granulation 25

2.2.5 Desorption hydrolysis system 25

2.3 Conventional Process of Urea manufacturing 26

2.3.1Once-through processes 26

2.3.2 Partial recycle processes 26

VIII

CONTENTS CHAPTER TITLE . PAGE

2.3.3 Total recycle processes 27

1.3.3.1 Gas recycles process 30

2.3.3.2 Liquid recycle process 32

2.3.3.3 Gas/Liquid recycle process 32

2.4 Supplying the manufacture of the urea 34

2.4.1MONTEDISON TOTAL RECYCLE 34

2.4.2MITSUI-TOATSU C IMPROVED PROCESS: 35

2.4.3SNAMPROGRETTI PROCESS 36

2.4.4STAMICARBON PROCESS 37

2.4.5ACES(Advanced Cost and Energy Savings) PROCESS 38

2.4.6MONTEDISON ISOBARIC DOUBLE RECYCLE(IDR) PROCESS: 39

3 STAMICARBON UREA TECHNOLOGY 3.1 What is Stamicarbon? 41

3.1.1 Stamicarbon history 41

3.1.2Modifications of the Stamicarbon CO2-stripping process: 42

3.2The original stamicarbon CO2-Stripping 43

3.2.1 Stripping Urea Process 43

3.2.2Main Reactions 44

3.2.3Side reactions 44

3.2.4Process Operating Variables Reactions 45

3.2.5 Urea manufacturing plant 46

3.2.6 Products Urea 46

3.2.7 Utility plant 47

3.3 Urea 2000 plus 47

3.3.1 Introduction 47

3.3.2Stamicarbon’s Avancore urea processes 48

3.3.3Synthesis : Avancore Urea Process 48

3.4 Synthesis : Urea 2000plus Pool Condenser Concept 50

3.5Synthesis : Urea 2000plus Pool Reactor Concept 52

3.5Evaporation section 55

3.6 Waste-water treatment section 55

3.7Finishing technology 1: Fluid-bed granulation 56

IX

CONTENTS CHAPTER TITLE . PAGE

3.8Finishing technology 2: Prilling 58

3.9UAN Process 59

3.10Mega Plant Concept 60

3.11Full life cycle support 62

3.12The Pool condenser 63

3.12.1The Optimum reactor Design 64

3.12.2The pool condenser design 65

3.12.3Advantages 65

3.13Corrosion 66

3.14The innovative Pool Reactor at the DSM urea plant in The Netherlands 68-69

4 CALCULATION 4.1 Material Balance 71

4.1.1Reactor 73

4.1.2 Stripper 74

4.1.3 Medium Pressure Separator 75

4.1.4 Low Pressure Separator 76

4.1.5 Vacuum Evaporator 77

4.1.6 Prilling Tower 79

4.2Energy Balance 82

4.2.1 Reactor 82

4.2.2 Stripper 83

4.2.3 Carbamate Condenser 84

4.2.4 Medium Pressure Separator 85

4.2.5 Low Pressure Separator 87

4.2.6 Evaporator 89

4.2.7 Prilling Tower 91

4.3Equipment Design 93

4.3.1 Reactor Design 93

4.3.2 Thickness Of Shell 95

4.3.3 Head Design 97

4.3.4 Diameter Of Pipes 97

4.3.5 Skirt Support For Reactor 98

X

CONTENTS CHAPTER TITLE . PAGE

4.3.6 Design 102

4.3.7 Wall Thickness Calculation 103

4.3.8 Separator 104

4.3.9 Bottom Head Design 105

4.4 Cost Estimation 106

4.4.1Cost Estimation 106

4.4.2Estimation of total capital investment : 107

4.4.2.1 I.Direct cost 107

4.3.2.2 II. Indirect cost 107

4.4.3 Estimation of total product cost: 108

44.3.1 I.Manufacturing cost 108

4.4.4 II.General Expenses 109

5 CONTROL AND SAFETY 5.1Control And Instrumentation 112

5.1.1 Instruments 112

5.1.2 Instrumentation and control objectives 112

5.1.3 Guide rules 113

5.1.4 Typical control systems 113

5.2 Safety 118

5.2.1 Industrial Safety 118

5.2.2Safety factors for industrial plants 118

5.2.3 Safety and security rules and regulations 119

5.2.4 Industrial safety objective 120

5.2.5Personal Protective Equipment (PPE) 120

5.2.6.Fire Extinguishers 120

5.2.7 Fire Protection Systems 120

5.2.8 Safety Awareness training obligations 121

6 CONCLUSIONS 123

REFERENECES 125 APPENDICES – I-II

XI

CONTENTS of Tablels TITLE .

Tablel.1 Chemical characteristics of urea

Table l.2 Physical characteristics of urea

Table 4.1.1.1 Flow of material across reactor

Table 4.1.2 Flow of material across stripper

Table 4.1.3 Flow of material across medium pressure separator

Table 4.1.4 Flow of material across low pressure separator

Table 4.1.5 Flow of material across vacuum evaporator

Table 5.1 Symbols control systems

XII

CONTENTS of Figure's TITLE .

Figure1.1 Stamicarbon urea plant (LIFECO Company-Braga-Libya).

Figure1.2 Chart Libyan urea products from value yearly from 1982 to 2008

Figure. 2.1 Schematic representation of urea synthesis.

Figure 2.2 Block Diagram of total recycle CO2 stripping urea process

Figure 2.2 Block Diagram of total recycle NH3 stripping urea process.

Figure2.3. Schematic representation of granulation.

Figure2.3. Once– through processes

Figure2.4. partial recycle processes

Figure2.5. Gas recycles process

Figure2.6. CPI-Allied gas recycle urea process

Figure 2.7. Inventa gas recycle urea process

Figure2.8. Liquid recycle process

Figure2.9. Stamicarbon CO2 stripping urea process

Figure2.10. Montecatini complete recycle urea process

Figure2.11. Inventa liquid recycle urea process

Figure2.12. Gas / liquid recycle process

Figure2.13. Stamicarbon total recycle process

Figure2.14. SNAM PROGETTI ammonia stripping urea process

figure2.15: digram decribing montedison process briefly.

Figure2.16 A brief descriptive diagram of Mitsui-Toatsu C improved Process.

Figure2.17: Brief block diagram of Stamicarbon process

Figer(3.1) block diagram for Stamicarbon CO2stripping Urea Process

figure 3.2 .stamecarbone processes diagram

chart shows 3.3the development of the Stamicarbon urea process.

Fig3.4 Urea 2000plus Pool Condenser Concept

Fig 3.5Urea 2000plus Pool Reactor Concept

Fig3.6 Urea 2000pluslow-pressure recirculation section

Fig 3.7Urea 2000plusEvaporation section

Fig 3.8Urea 2000plusWaste-water treatment section

Figure 3.9Stamicarbon’s fluid-bed granulation process works

Fig3.10 Urea 2000plus Prillingsection

XIII

Figure 3.11UAN process

Figure 3.12.Mega Plant Concept

Figure3.13 Urea 2000plus Pool Condenser urea plant

Figure3.13 Urea 2000plus Pool Reactor at the DSM urea plant

Fig 4.1.1Flow of material across reactor

Fig 4.1.2 Flow of material across stripper

Fig 4.1.3 Flow of material across medium pressure separator

Fig 4.1.4 Flow of material across low pressure separator

Fig 4. 1.5 Flow of material across vacuum evaporator

Fig 4.1.6 Flow of material across prilling tower

Fig 4.2.1 Energy flow across reactor

Fig 4.2.2 Energy flow across stripper

Fig 4.2.3 Energy flow across carbamate condenser

Fig 4.2.4 Energy flow across medium pressure separator

Fig 4.2.5 Energy flow across low pressure separator

Fig 4.2.6 Energy flow across evaporator

Fig 4.2.7 Energy balance across prilling tower

Fig 4.3.1 Urea reactor

Fig 4.3.2 Graph of % urea yield Vs molar ratio of NH3 Vs CO2

Fig 4.3.3 Graph of % urea yield Vs residence time.

Fig 4.3.4 Urea evaporator (climbing film long tube vertical evaporator)

Fig 4.3.5 Graph to find out heat transfer co-efficient

Fig 5.1 Urea reactor control temperature

Figure 5.1 Personal Protective Equipment (Ppe)

Figure 5.2 Fire Extinguishers

Figure 5.3 safety awareness training obligations

XIV

ABSTRACT Urea is one of the impotent chemical industrial in the world, as we are

Chemical Engendering we have to know how to manufacture it .so we

manufactured urea product as target for the year 300000 ton and for the month

25000 ton.

Urea is in many ways the most convenient form for fixed nitrogen. It is easy

to produce as prills or granules and easily transported in bulk or bags with no

explosive hazard. It leaves no salt residue after use on crops. Its specific gravity

is 1.335, decomposes on boiling and is fairly soluble in water.

The principal raw materials required for this purpose are NH3 & CO2.Two

reactions are involved in the manufacture of urea. First, ammonium carbamate

is formed under pressure by reaction between CO2 & NH3.

CO2 + 2NH3 → NH2COONH4 H= -37.4 Kcal This highly exothermic reaction is followed by an endothermic

decomposition of the ammonium carbamate.

NH2COONH4 ↔ NH2CONH2 + H2O H= + 6.3 Kcal We selected the Stamicarbon CO2 stripping process for the manufacture of

urea .In this process ammonia & CO2 are compressed & fed to the reactor. The

unconverted carbamate is stripped and recovered from the urea synthesis

reactor effluent solution at reactor pressure, condensed to an aqueous solution

in a steam producing high-pressure condenser & recycled back to the reactor

by gravity. Part of the liquid NH3 reactor feed , vaporized in a steam-heated

exchanger, is used as inert gas to decompose & strip ammonium carbamate in

the steam heated high pressure stripper.

Energy balance & material balance of the plant is done. The selected

capacity of the plant is 3,00,000 tons/year of urea with 98 % purity. Urea reactor

& vacuum evaporator are designed. Stamicarbon CO2 stripping process urea

process is selected because it involves a high NH3to CO2 ratio in the reactor,

ensuring the high conversion of carbamate to urea. The highly efficient

XV

ammonia stripping operation drastically reduces the recycling of carbamate and

the size of equipment in the carbamate decomposition. Stamicarbon technology

differs from competitors in being based on the use of excess ammonia to avoid

corrosion as well as promote the decomposition of unconverted carbamate into

urea at LIFCO.

This research comprises 6 Chapters, where each chapter has different

sections as follows:

• Chapter 1 is the Introduction. It is divided into sections that include the

background of urea, history of urea in Libya, properties of urea; uses and

application of urea, the basic concepts in urea plant the problem statement,

project and design objectives, the intended outcomes and deliverables, and

a summary of report structure.

• Chapter 2 is about urea product process. This chapter consists sections,

which includes a properties of manufacturing sources and availability of

raw material urea, plants shapes, technical process discretion, how we can

product urea, urea process.

• Chapter 3 in this chapter we went to the basic objective of this project was

known of the Stamicarbon technologies, the old technology and the new

technology (Urea 2000plus).

• Chapter 4 in this chapter we did the material, energy balances to study the

line of 1000 ton/day to product 300000 ton/year and 25000 ton /month in

2018 so we investigated about the problem of study how to became later

for the balance, design and cost.

• Chapter 5 addresses Safety, control, and Environmental Issues. It

discusses the ethical issues related to the selected process, and the relevant

safety and environmental considerations.

• Chapter6is the Conclusions summary of the urea process. It has

summarized in brief the results and the main findings, and the results of the

investigate product from urea manufacture.

XVI

Chapter one

Introduction

0

INTRODUCTION Cultural growth and industrial progress, witnessed by societies lad To, Urea is the most

popular form of solid nitrogen fertilizer, particularly in the developing regions of the world,

and is traded widely on the international market. Urea prices can fluctuate markedly and

frequently, depending on crop prices, which affect demand. Around 80-85% of the

production is used as a fertilizer. More than 40% of all food grown in the world is fertilized

by urea.

Urea is also used increasingly in the industrial sector to make urea-formaldehyde resins,

melamine, diesel exhaust fluids, and livestock feeds. It is also used to make adhesives and

paints, laminates, molding compounds, paper, and textiles .Fig 1.1Stamicarbon urea plant

(LIFECO Company-Braga-Libya).

Fig 1.1Stamicarbon Urea plant (LIFECO Company-Braga-Libya).

1

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.

1.1 Background

The discovery of natural urea preceded that of synthetic urea but the who and when seem

not clearly established. The French chemist Hilaire Marin Rouelle (1718-1779) is commonly

cited as the one who discovered urea. Accordingly, he isolated those colorless, odorless,

crystalline substance in 1773 by boiling urine (Myers 2007). In 1797, French chemists

Antoine François de Fourcroy (1755-1809) and Louis Nicolas Vauquelin (1763-

1829) named the substance “urea” (Richet 1988).

However, Dutch physician-chemist Hermann Boerhaave (1668–1738) seems to have

described urea much earlier. According to Rosenfeld (2003), even before 1727 Boerhaave

already obtained a crystalline residue from urine by heating, filtering, washing, and

evaporating. He called it “the native salt of urine.” He noted that it differed from the sea salt

(sodium chloride) which is also present in urine. Further, according to the same author,

Rouelle’s extract was impure and that it was British physician-chemist William Prout (1785-

1850) who, in 1817, isolated pure urea from urine.

The Beginning of Synthetic Urea. In 1828, German chemist Friedrich Wöhler (1800–

1882) produced a synthetic urea in the laboratory from inorganic compounds. He first heated

a solution consisting of a mixture of silver cyanate (Ag OCN) and ammonium chloride

(NH4Cl). As shown below, this formed ammonium cyanate, also an inorganic compound

(does not originate from living organism).

2

1.1.1 History of Libyan oil production. Libyan oil production began in 1961 by producing approximately 18 thousand barrel/day.

In early seventies, the oil production became the major industry in Libya, where the average

production was about 1.5 million barrels/day. Urea production began in Libya in 1979. It is

a joint venture between Sirte and Carbon Estimate. The company is located geographically

in the eastern part of the country in the city of Braga.Fig 1.2 Chart Libyan urea products

from value yearly from 1982 to 2008.

Fig 1.2 Chart Libyan urea products from value yearly from 1982 to 2008.

The main objective of the company was to produce petrochemicals from organic

fertilizers and urea. This was successful through the use of gas associated with the lean of

the wild. Marketing products at the national and international levels.The first urea plant was

founded by the Libyan oil company Sirte by Stamicarbon in 1979 and production started on

1981/7/3, with a design card of 1000 metric tons / day. The plant of urea II libyan in the oil

company cert by the Stamicarbon. was established in 1981 and production began on 1983

design card of 1750 m t per day.

In 2009, the Libyan Norwegian Fertilizer Company was established as a partnership

between the Libyan industry and the world's leading fertilizer company in Yara to develop

NOC’s long existing fertilizer complex at Marsa el Brega.

1.2 The Study Problem Through the exploratory study that will be conducted by the researcher on this issue, it

in the survey conducted by the research group on this question found a set of manufacturing models, we found that the special manufacturing model for the production

0100,000200,000300,000400,000500,000600,000700,000800,000900,000

value

3

of urea in the Norwegian company Brega has two production lines, the first with a

capacity of 1000 tons per day and the second line of 1750 ton/day, which includes the process

of manufacturing according to the Italian model.

Where we will study the first production line capacity of 1000 tons to know what are the

problems that prevent the production of high-quality urea and the most important questions

of the study :-

1- Is this design the first model for obtaining typical urea is 98% urea, 1% water and

1% biuret?

2- - Is the mass and energy of the plant in the plant are right to determine the raw

materials to enter in order to give 98% urea and 1% water and 1% biuret?

1.3 The research Questions 1- Is the design within the factory is producing high-quality urea free from

impurities?

2- Is the result of production abroad few waste, where the design depends on the

technique of the Stamicarbon?

3- Is this design the first model in obtaining the typical urea is 98% urea, 1% water

and 1% biuret?

4- Is the luminous process and design of Stamicarbonideal for this result?

5- What are the procedures for controlling and operating in this model and whether

emergency procedures and safety are fairly ideal?

6- Is not there any future plans to development the plant in the short term.

1.4 Objectives of the Study In this search I plan to introduce the production of urea, how it is produced and

manufactured at Lifeco Company. I will look at the total process involved in producing

urea production, both physically and the chemical processes involved in manufacturing

urea and ammonia at an industrially scale. We will look at the different stages involved

in producing and how different problems are overcome .

1- Aims to conduct a theoretical and field study to assess the environmental effects of

urea plants in the region of Braga.

4

2- To identify the view of the study community by geographical location of the urea

factories in Braga.

3- Study environmental damage in quantity and quality depending on the extent of and

natural resources design reaction of plant caused by these factories.

4- To have the desired value of study to design the first model in obtaining the typical

urea is 98% urea, 1% water and 1% biuret.

1.5 Hypothesis

Based on the study problem, the following hypotheses were formulated:

1. parameter that effect on process and they are:-

A- Temperature.

B- Pressure.

C- Resident time.

D- Concentration.

E- Biuret formation.

2. The manufacturing volume of the plant II in Lifeco varies.

3. Calculation of the process balances and design, to make the investigation of

production value.

1.6 Methodology This study includes two aspects, theoretical and practical:

1- Theoretical side: The relevant secondary data were collected fromthrough relevant

sources and references.

2- Practical (applied): In which the field study and analysis of data,

Through the following:

A. Society and Study Sample: We visited the urea plant to collect quantities of raw

material, diagrams of plant and another information from the plant.

B. Data collection tool: We divided into two groups going sequentially and

continuously to collect these information from LIFECO. To make investigate about

urea product process .

5

1.7 Study Environment The study research will be in plant 2 at LIFECO Company-Braga.

1.8 What is urea?

It’s a fertilizer and a chemical compound with the formula

CO(NH2)2 (or H2N.CO.NH2). The molecule has two –NH2

groups joined by a C=O or carbonyl functional group. It is also

called carbamide. It is very soluble in water but insoluble in ether, with a melting point at

132°C.

NH3 & CO2.Two reactions are involved in the manufacture of urea. First, ammonium

carbamate is formed under pressure by reaction between CO2 & NH3.

CO2 + 2NH3 ↔ NH2COONH4 H= -37.4 Kcal/gm mol

This highly exothermic reaction is followed by an endothermic decomposition of the

ammonium carbamate.

NH2COONH4 ↔ NH2CONH2 + H2O H= + 6.3 Kcal/gm mol

That it is a fertilizer should be the common reply to the question. Synthetic urea is largely

used as such in crop agriculture to supplement the essential major element nitrogen (N).

According to Soh (2006), the usage of urea as fertilizer accounts for more than 90% of total

production.

But there are plenty more as to what is urea. It is both a biosynthetic compound and an

industrial product with various applications.

Urea is a naturally occuring, colorless and odorless nonprotein nitrogenous compound

found in urines of humans and most other mammals. It is an organic compound both in

accordance with Jöns Jacob Berzelius’ 1807 original definition (Carey 1992) that organic

chemistry is the study of compounds produced by living organisms; and based on the current

definition that organic compounds are those that contain the element carbon. It is likewise

a synthetic or manufactured compound, as in the granular urea fertilizer which is

commercially produced through industrial processing. In other words, urea is a carbon-

containing compound that is both naturally and artificially produced.

6

1.8.1 Commercial production of urea

Urea is commercially produced from two raw materials. ammonia, and carbon

dioxide. 1_arge quantities of carbon dioxide are produced during the manufacture of

ammonia from coal or from indrocarhons such as natural gas and petroleum-derived raw

materials.

This allows direct synthesis of urea from these raw materials. The of urea from ammonia

and carbon dioxide takes place in an equilibrium reaction, with incomplete Conversion of

the reactants. The various urea processes are characterized by the conditions under which

urea formation takes place and the way in which unconverted reactants are further processed.

Unconverted reactants can be used for the manufacture of other products.

for example ammoniurn nitrate or sulfale. or they can be recycled for complete conversion

to urea in a total- recycle process. Two principal reactions take place in the formation ol urea

from ammonia and carbon dioxide. The first reaction is exothermic:

NH3 & CO2.Two reactions are involved in the manufacture of urea. First, ammonium

carbamate is formed under pressure by reaction between CO2 & NH3.

CO2+2NH3→NH2COONH4 H= -37.4 Kcal

This highly exothermic reaction is followed by an endothermic decomposition of the

ammonium carbamate.

NH2COONH4 ↔ NH2CONH2 + H2O H= + 6.3 Kcal

7

1.8.2 Chemical characteristics of urea

The urea molecule is planar and retains its 11111 molecular point symmetry, due to

conjugation of one of each nitrogen's P orbital to the carbonyl double bond. Each carbonyl

oxygen atom accepts four N-H-O hydrogen bonds, a very unusual feature ftr such a bond

type. This dense (and energetically favorable) hydrogen bond network is probably

established at the cost of molecular packing: The structure is quite open, the ribbons forming

tunnels with sciuare cross-section. Urea is stable under normal conditions.

IUPAC name Diamninomnetbanal

Chemical formula (𝑁𝑁𝑁𝑁2)2𝐶𝐶𝐶𝐶

Molecular mass 66.07 𝑔𝑔/𝑚𝑚𝑚𝑚𝑚𝑚 (approximate)

pole moment 4.56 p/D

Tablel.1 chemical characteristics of urea

1.8.3 Physical characteristics of urea Urea is a white odorless solid. Due to extensive hydrogen bonding with water (UP to six

hydrogen bonds may form - two Iron the oxygen atom and one from each hydrogen) urea is

very soluble.

Density 1 .33• 1O kg/rn3, solid

Melting point 132.7 °C (406 K) decomposes

Boiling point NA

Solubility in water

108 g/100 ml (20 °C)

167 g/100 ml (40 °C)

251 g/100 ml (60 °C)

400 g/100 ml (80 °C)

733 g/100ml (100 °C)

Vapor pressure <10 Pa

Bulk density 0.8 kg/m3

Table l.2Physical characteristics of urea

8

1.8.4 Advantages and disadvantages of Urea Fertilizer: A. Advantages

1. Urea has the highest nitrogen content, equal to 46%. This percentages much higher

than other nitrogenous fertilizers available in the market.

2. Urea production cost is relatively low since carbon dioxide required for its

manufacture is easily obtained.

3. Urea is not subject to fire or explosion hazards, and hence there is no risk in the

storage of urea.

4. Urea can be used for all types of crops and soils. After its assimilation by plants, urea

leaves behind only carbon dioxide in the soil through the interaction of nitrifying

bacteria. This carbon dioxide does not harm the soil.

B. Disadvantages:

1. Urea is very soluble in water, and hygroscopic water; hygroscopic water creates a

thin layer surrounding individual soil particles, which makes water unavailable to

plants, and hence requires better packaging quality.

2. Urea contains impurities more than 2%, it cannot be used as a fertilizer, since the

impurities are toxic to certain crops, particularly citrus.

1.9 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 unease, 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 unease, some ammonia-oxidizing bacteria (AOB),

such as species of Nitrosamines, 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 unease) to nitrite, a process termed nitrification.

9

Nitrite-oxidizing bacteria, especially Nitrobacteria, 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.

- About56%ofUreamanufacturedisusedinsolidfertilizer.

- About31%ofUreamanufacturedisusedinliquidfertilizer.

- Urea-formaldehyde resins have large use as a plywood adhesive. - Melamine-formaldehyde resins are used as dinnerware &for making extra hard

surfaces

1.9.1 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.

1.9.2 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.

10

1.10 The basic Concepts:- Raw material feed:-

The raw material feed to urea plant is ammonia (NH3) and CO2.amonia (NH3) is feed at

temperature of 20C0 and pressure of 17 kg/cm2.CO2 is feed at a temperature of about

49C0 and pressure of about 1.05kg/cm2, and compressed to about 143kg/cm2 .

Balance in manufacturing:-

The basic formula for balance in chemical engendering is INPUT (Feed) =OUTPUT

(Product) that means the production process is in safe side and stable .Material and

energy balances are very important in the food industry. Material balances are

fundamental to the control of processing, particularly in the control of yields of the

products. The first material balances are determined in the exploratory stages of a new

process, improved during pilot plant experiments when the process is being planned and

tested, checked out when the plant is commissioned and then refined and maintained as

a control instrument as production continues. When any changes occur in the process,

the material balances need to be determined again.

Reactor:-

The reactors, in which chemicals are made in industry, vary in size from a few cm3 to

the vast structures that are often depicted in photographs of industrial plants.

The design of the reactor is determined by many factors but of particular importance are

the thermodynamics and kinetics of the chemical reactions being carried out.

Strieber

The stripper is possibly the most critical equipment in the process. Different process

designs have different requirements; that is why so many materials have been used in

the past. The equipment itself is generally a falling evaporator, and different details

generally related to the different material requirements. Tube material is the most

important choice. Reliability is mainly connected to the specific choice that each basic

material selection involves. Notwithstanding the general basic material selection, from

stainless steel to duplex to titanium imply a specific detailed design, where specific

knowledge and experience is the key for a trouble free life of the equipment.

Evaporator:

Evaporator is an important component together with other major components in a

refrigeration system such as compressor, condenser and expansion device. The reason

for refrigeration is to remove heat from air, water or other substance.

11

prilling tower:

Prilling towers must be of sufficient height for the particles to be strong enough not to

break on impact. Latent heat is transferred from the drop to the air as it falls, and if

significant amounts of water are present evaporation also occurs, increasing the cooling

effect on the drop. It is important for the temperature of the feed liquor to be as low as

possible, just a degree or two above its solidification point. Higher temperatures require

taller towers, as do larger particle sizes. Prilling towers in the fertilizer industry are

typically over 50 m high for a mean particle size of about 2 mm.

In the explosives industry the particle size is smaller, the feed wetter and towers of

about 10 m are used.

12

Chapter Tow

Production process of Urea

13

Manufacturing Process of Urea

2.1 Manufacture Introduction

Urea (NH2CONH2) is produced at industrial scale by the reaction between ammonia

and carbon dioxide at high pressure (13–30MPa) and high temperature (170–2000C). There

are different types of processes to produce urea in the commercial units. These processes are

typically called once through, partial recycle and total recycle. In the total recycle process,

which is employed widely, all the ammonia leaving the synthesis section is recycled to the

reactor and the overall conversion of ammonia to urea reaches 99%. Stamicarbon and

Snamprogetti processes are the most common examples of such process.

Since urea has become almost the most widely used fertilizer and its production is

important in the petrochemical industry, there has been many attempts to model and simulate

the reactor of urea production as the heart of the process.

Inthe present work the entire urea synthesis section based on the of stamicarbon process (in

cludingurea reactor, stripper, scrubber, rectifying column and flash separator)

is modeled. Urea production consists of reaction between ammonia and carbon dioxide

react to form urea and water .The urea synthesis is considered to occur in heterogeneous

phase. In Stamicarbon, process compressed carbon dioxide feed passes through the stripper

along which ammonia and carbon dioxide are stripped off from the liquid phase to the gas

phase. The gas flow from the rectifying column which carries the stripped off ammonia and

carbon dioxide is mixed with pumped ammonia feed and gas flow from scrubber and on

further heating , compression and cooling is fed to the reactor. The liquid mixture in the

reactor overflows into the stripper. The gas phase exiting the reactor contains free ammonia

and carbon dioxide as well as inert gas and is discharged into the scrubber. In the scrubbing

part, remaining gases are scrubbed with effluent from flash separator. This stream is an

aqueous solution of unreached carbon dioxide and originates from flash separation of urea

from liquid outlet of CO2 stripper.

2.1.1 How is Manufacturing of Urea ?

14

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

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

the application of heat to form urea and water.

1 2

2NH3 + CO2 NH2COONH4 CO(NH2)2 + H2O

Ammonia Carbon Ammonium Urea Water Dioxide Carbamate Reaction 1 is fast and

exothermic and essentially goes to completion under the reaction conditions used

industrially. Reaction 2 is slower and endothermic and does not go to completion. The

conversion (on a CO2 basis) is usually in the order of 50-80%. The conversion increases

with increasing temperature and NH3/CO2 ratio and decreases with increasing H2O/CO2

ratio. The design of commercial processes has involved the consideration of how to separate

the urea from the other constituents, how to recover excess NH3 and decompose the

carbamate for recycle. Attention was also devoted to developing materials to withstand the

corrosive carbamate solution and to optimize the heat and energy balances. The structure of

these compounds is shown in Figure (2.1).

Figure.(2.1) Schematic representation of urea synthesis.

The simplest way to decompose the carbamate to CO2 and NH3 requires the reactor

effluent to be depressurized and heated. The earliest urea plants operated on a “Once

15

Through” principle where the off-gases were used as feedstock's for other products.

Subsequently “Partial Recycle” techniques were developed to recover and recycle some of

the NH3 and CO2 to the process. It was essential to recover all of the gases for recycle to

the synthesis to optimize raw material utilization and since recompression was too

expensive, an alternative method was developed. This involved cooling the gases and re-

combining them to form carbamate liquor which was pumped back to the synthesis. A series

of loops involving carbamate decomposers at progressively lower pressures and carbamate

condensers were used.

This was known as the “Total Recycle Process”. A basic consequence of recycling the gases was that the NH3/CO2 molar ratio in the reactor increased thereby increasing the urea yield. Block flow diagrams for CO2 and NH3 stripping total recycle processes are shown in figures 1 and 2.

Figure 2.2 Block Diagram of total recycle CO2 stripping urea process.

Significant improvements were subsequently achieved by decomposing the carbamate

in the reactor effluent without reducing the system pressure. This “Stripping Process”

dominated synthesis technology and provided capital/energy savings. Two commercial

16

stripping systems were developed, one using CO2 and the other using NH3 as the stripping

gases.

Since the base patents on stripping technology have expired, other processes have

emerged which combine the best features of Total Recycle and Stripping Technologies. For

convenience total recycle processes were identified as either “conventional” or “stripping”

processes. The urea solution arising from the synthesis/recycle stages of the process is

subsequently concentrated to a urea melt for conversion to a solid prilled or granular product.

Figure 2.2 Block Diagram of total recycle NH3 stripping urea process.

Improvements in process technology have concentrated on reducing production costs

and minimizing the environmental impact. These included boosting CO2 conversion

efficiency, increasing heat recovery, reducing utilities consumption and recovering residual

NH3 and urea from plant effluents. Simultaneously the size limitation of prills and concern

about the prill tower off-gas effluent were responsible for increased interest in melt

granulation processes and prill tower emission abatement. Some or all of these

17

improvements have been used in updating existing plants and some plants have added

computerized systems for process control.

New urea installations vary in size from 800 to 2,000t.d-1 and typically would be

1,500t.d-1 units. Modern processes have very similar energy requirements and nearly 100%

material efficiency. There are some differences in the detail of the energy balances but they

are deemed to be minor in effect.

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 un reacted (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. Un reacted 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

𝑁𝑁𝑁𝑁2𝐶𝐶𝐶𝐶𝐶𝐶𝑁𝑁𝑁𝑁4 2𝑁𝑁𝑁𝑁3 + 𝐶𝐶𝐶𝐶2

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 rediscover these crystals

18

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

140oC.Granulation. Figure (2.2):-

Figure(2.3). 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.

2.1.2 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.

2.1.2.1 Ammonia

19

Ammonia is a colorless gas with a characteristic pungent smell. It is lighter than air, its

density being 0.589 times that of air. It is easily liquefied due to the strong hydrogen

bonding between molecules; the liquid boils at −33.3 °C (−27.94 °F), and freezes at

−77.7 °C (−107.86 °F) to white crystals. Ammonia may be conveniently deodorized by

reacting it with either sodium bicarbonate or acetic acid. Both of these reactions form an

odorless ammonium salt.

Solid

The crystal symmetry is cubic, Pearson symbol cP16, space group P213 No.198, lattice

constant 0.5125 nm.[19]

Liquid

Liquid ammonia possesses strong ionizing powers reflecting its high ε of 22. Liquid

ammonia has a very high standard enthalpy change of

vaporization (23.35 kJ/mol, cf. water 40.65 kJ/mol, methane

8.19 kJ/mol, phosphine 14.6 kJ/mol) and can therefore be used in laboratories in un

insulated vessels without additional refrigeration. See liquid ammonia as a solvent.

Solvent properties

Ammonia is miscible with water. In an aqueous solution, it can be expelled by boiling.

The aqueous solution of ammonia is basic. The maximum concentration of ammonia in

water (a saturated solution) has a density of 0.880 g/cm3 and is often known as '.880

ammonia'. Ammonia does not burn readily or sustain combustion, except under narrow

fuel-to-air mixtures of 15–25% air.

Combustion

When mixed with oxygen, it burns with a pale yellowish-green flame. At high

temperature and in the presence of a suitable catalyst, ammonia is decomposed into its

constituent elements. Ignition occurs when chlorine is passed into ammonia, forming

nitrogen and hydrogen chloride; if chlorine is present in excess, then the highly

explosive nitrogen trichloride (NCl3) is also formed. 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

20

ammonia from nitrogen and hydrogen is basically a reversible reaction which the yield depends on

the conditions employed. Un reacted hydrogen and nitrogen are usually separated and recycled.

2.1.2.2 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.

Carbon dioxide is able to be converted from a gas to a solid to a liquid. It has almost

twice the density of air , making it able to be poured from a container.

- gas density (natural state): 1.87 kg/m3

- liquid density: 1032 kg/m3

- solid density: 1562 kg/m3

- molecular mass: 44.01 g/mol

- colorless because it is a gas,

- odorless

- melting point: -55.6 deg C

- boiling point: -78.5 deg C

- Is not flammable

2.1.3 Sources and availability of raw materials

In an aqueous environment, carbon dioxide becomes carbonic acid. Carbon dioxide is

not reactive with water. When overall production process is consider primary raw materials

needed to manufacture urea are:- ( Hydrogen (H2)- Nitrogen (N2) )

2.1.3.1 Hydrogen Production

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

synthesis 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.

21

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 .

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.

2.1.3.2 Nitrogen

2.2 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 Process Selection

2.2.1 Stripping Processes

• Carbon dioxide stripping process

NH3 and CO2 are converted to urea via ammonium carbamate. The molar NH3/CO2

ratio applied in the reactor is 2.95. This results in a CO2 conversion of about 60% and an

NH3 conversion of 41%. The reactor effluent, containing unconverted NH3 and CO2 is

subjected to a stripping operation at essentially reactor pressure, using CO2 as stripping

agent. The stripped off NH3 and CO2 are then partially condensed and recycled to the

reactor. The heat evolving from this condensation is used to produce 4.5bar steam some of

which can be used for heating purposes in the downstream sections of the plant. Surplus

4.5bar steam is sent to the turbine of the CO2 compressor.

22

The NH3 and CO2 in the stripper effluent are vaporized in a 4bar decomposition stage

and subsequently condensed to form a carbamate solution, which is recycled to the 140bar

synthesis section. Further concentration of the urea solution leaving the 4bar decomposition

stage takes place in the evaporation section, where a 99.7% urea melt is produced.

• Ammonia stripping process

NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of 150bar

and a temperature of 180°C. A molar ratio of 3.5 is used in the reactor giving a CO2

conversion of 65%. The reactor effluent enters the stripper where a large part of the

unconverted carbamate is decomposed by the stripping action of the excess NH3. Residual

carbamateand CO2 are recovered downstream of the stripper in two successive stages

operating at 17and 3.5bar respectively. NH3 and CO2 vapors from the stripper top are mixed

with there covered carbamate solution from the High Pressure (HP)/Low Pressure (LP)

sections ,condensed in the HP carbamate condenser and fed to the reactor. The heat of

condensation is used to produce LP steam. The urea solution leaving the LP decomposition

stage is concentrated in the evaporation section to a urea melt.

2.2.2 Reactor Conditions

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 counter current 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 resulting CO2 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 condenser. Here 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 condenser is used to

generate low pressure steam. The gas and liquid from the carbamate condensers are recycled

to the reactor by gravity flow. The urea solution from the stripper, with a typical NH3 content

of 15 wt%, is purified further in the subsequent medium and low pressure decomposers,

23

operating at 17.5 and 2.5 bars, respectively. Ammonia and carbon dioxide separated from

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

pressure absorbers. Condensation heat in the medium pressure absorber is transferred

directly to the aqueous urea solution feed in the final concentration section; the purified urea

solution is concentrated further either by two stage evaporation up to 99.7 % for urea prill

production or by a single evaporation 98.5 % for urea granule production. Water vapour

formed in the final concentrating section is condensed in surface condensers to form process

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

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

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

is finally processed either through the prilling tower or via the urea granulator. Instead of

concentration via evaporation, the ACES process can also be combined with a crystallization

section to produce urea with low biuret content.

2.2.3 Isobaric double recycle process

In this process the urea synthesis takes place at 180-200bar and 185-190°C. The

NH3/CO2 ratio is approximately 3.5-4, giving about 70% CO2 conversion per pass. Most

of the unconverted material in the urea solution leaving the reactor is separated by heating

and stripping at synthesis pressure using two strippers, heated by 25bar steam, arranged in

series. In the first stripper, carbamate is decomposed/stripped by ammonia and the remaining

ammonia is removed in the second stripper using carbon dioxides as stripping agent.

Whereas all the carbon dioxide is fed to the plant through the second stripper, only 40% of

the ammonia is fed to the first stripper. The remainder goes directly to the reactor for

temperature control. The ammonia-rich vapors from the first stripper are fed directly to the

urea reactor. The carbon dioxide-rich vapors from the second stripper are recycled to the

reactor via the carbamate condenser, irrigated with carbamate solution recycled from the

lower-pressure section of the plant. The heat of condensation is recovered as 7bar steam

which is used down-stream in the process. Urea solution leaving the IDR loop contains

unconverted ammonia, carbon dioxide and carbamate. These residuals are decomposed and

vaporized in two successive distillers, heated with low pressure recovered steam. After this,

the vapors are condensed to carbamate solution and recycled to the synthesis loop.

The urea solution leaving the LP decomposition for further concentration, is fed to two

vacuum evaporators in series, producing the urea melt for prilling and granulation.

24

2.2.4 Granulation

Depending on the process a 95-99.7% urea feedstock is used. The lower feedstock

concentration allows the second step of the evaporation process to be omitted and also

simplifies the process condensate treatment step. The basic principle of the process involves

the spraying of the melt onto recycled seed particles or prills circulating in the granulator. A

slow increase in granule size and drying of the product takes place simultaneously. Air

passing through the granulator solidifies the melt deposited on the seed material. Processes

using low concentration feedstock require less cooling air since the evaporation of the

additional water dissipates part of the heat which is released when the urea crystallizes from

liquid to solid. All the commercial processes available are characterized by product recycle,

and the ratio of recycled to final product varies between 0.5 and 2.5. Prill granulation or

fattening systems have a very small recycle, typically 2 to 4%. Usually the product leaving

the granulator is cooled and screened prior to transfer to storage. Conditioning of the urea

melt prior to spraying may also be used to enhance the storage/handling characteristics of

the granular product.

2.2.5 Desorption hydrolysis system

Heated process water is fed to the top of Desorber 1 where it is stripped of NH3 and

CO2 by gas streams from Desorber 2 and the hydrolyser. The liquid leaving Desorber 1

bottom is preheated to 190°C and fed at 17bar pressure to the top of the hydrolyser. 25bar

steam is introduced to the bottom of the hydrolyser and under these conditions the urea is

decomposed and the gases are counter currently stripped. The vapours go to Desorber 1. The

urea free liquid stream leaving the desorber is used to heat the hydrolyser feed stream and is

fed after expansion to Desorber 2 where LP steam counter currently strips the remaining

NH3 and CO2 and the off-gases pass to Desorber 1. The off-gases from Desorber 1 are

condensed in a cooled reflux condenser/separator. Part of the separated liquid is pumped

back to Desorber 1 and the remainder is returned to the LP recirculation section of the urea

plant. Residual NH3 in the separator off-gas is recovered in an atmospheric absorber and

returned to the LP recirculation section also. The treated water which leaves Desorber 2 is

cooled and concentrations of 5mg.

2.3 conventional Process of Urea manufacturing

2.3.1 Once-through processes

25

Figure(2.3) below is generalized flow diagram of a once- urea process

Figure2.3. 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 CO2 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.

2.3.2 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.

26

Figure2.4. 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 K Pa 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, carbamate 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.

2.3.3 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.

27

Figure2.5. 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 un reacted 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 bediscussed; the CPI – Allied and inventa

processes

CPI-Allied gas recycle urea process Higher operating temperature (1940C -2330C) at 30.3 M Pa 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 monoethanol amine

(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

28

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

less than 0.7% biuret and 0.20% water.

Figure2.6. CPI-Allied gas recycle urea process

• Inventa gas recycle urea process

The Inventa process utilizes a reactor operating at 20.2 M Pa 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 ammonia cal solution of ammonium carbamate goes to a desorber to

remove ammonia for recycle

Figure 2.7. Inventa gas recycle urea process

29

2.3.3.1 Liquid recycle process

Figure2.8. 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 M Pa to 15.1 M Pa . The

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

Operating at 14.1 M Pa 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 k pa 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 k pa 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.

30

Figure2.9. 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 M Pa 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 M Pa 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 carbamate

condenser which operates at 8.1 M Pa 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 M Pa, and finally to a third stage operating

at 202kpa to 303 k pa 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.

31

Figure2.10. Montecatini complete recycle urea process

Inventa liquid recycle urea process

Figure2.11. Inventa liquid recycle urea process

2.3.3.2 Gas/Liquid recycle process

Figure2.12. 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.

32

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

M Pa and 1700C to 1900C. The reactor effluent is lowered to approximately 505 K Pa 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

Figure2.13. Stamicarbon total recycle process

Snamprogetti ammonia stripping urea process The Snamprogetti 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 Snamprogetti 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.

33

.

Figure2.14. SNAM PROGETTI ammonia stripping urea process

2.5 Various Process manufacturing of Urea

2.5.1MONTEDISON TOTAL RECYCLE (Revamped by Urea Casale)

PROCESS:

The process involves two reactors, a single once through reactor (R2) and a reactor

(R1) which handles the carbamate recycle. A total Carbamate condenser is installed before

Reactor R1. There are three decomposition stages, followed by two Vacuum concentration

stages, to give a Urea melt of 99 % concentration. A natural draft prilling tower is

installed. Low pressure adsorbate is used to absorb gases in high pressure stages.

Typical process parameter used in plant operation:

- N/C ratio = 3 to 4

- Temperature of reactor = 200 o C

- pressure inside reactor = 200 to 350 ATA

Compared to other Urea manufacturing processes this process has a slightly higher

energy consumption, due to several stages of ammonia recovery and recycle. Water

content in recycle is comparatively high.

34

figure2.15: diagram describing montedison process briefly.

2.5.2MITSUI-TOATSU C IMPROVED PROCESS:

The process involves three decomposition stages or less, followed by a crystallisation

section of urea melt, the off gases from the units are absorbed in the mother liquor from the

crystaliser and recycled back to the reactor. The urea melt from subsequent stages is

stripped in next decomposition stage from the vapors. The main focus of the process is to

reduce water content in the reactor recycle stream, this is achieved by absorbing the gases

in urea mother liquor from the crystalizer, which is heated and then used for absorbing the

gases in high pressure decomposition sections instead of using low pressure stage absorb

ate. The overall conversion of the reactor is thus maintained.

Typical parameters are:

- N/C = 3 to 5

- Reactor temperature = 250 OC

- Reactor pressure = 200 to 400 ATA

This process is little better in terms of yield and biuret content in Urea product, the

energy consumption is more or less same as the Montedison process.

35

Figure2.16 A brief descriptive diagram of Mitsui-Toatsu C improved Process.

2.5.3SNAMPROGRETTI PROCESS:

This is a total recycle process based on stripping, ammonia is used generally as

stripping agent but CO2 can also be used. The plant operates at a high N/C ratio, thus

ammonia acts as stripping agent.

SAIPEM recommends use of Urea grade steel for reactor section and offers different

options for the stripper material, namely Zirconium and Urea grade steel material. Now a

days SAIPEM has launched Omega Bond TM material compromising Titanium tube with

extruded bonded Zirconium inner tube.

The need for high pressure pumping equipment is eliminated as the stripper and

carbamate condenser operate at the same pressure as the synthesis section. Conversion in

reactor is around 60% , overall conversion on the basis of CO2 fed can be as high as 75-

80% due to carbamate decomposition in HP section. The plant layout is horizontal.

Typical Operating Parameters:

- N/C=3.3-5

- Reactor temperature= 190 o C Stripper Pressure= 160 ATA

- Stripper temperature= 210 oC Reactor Pressure= 160 ATA

36

The Energy savings in this process compared to the previous two process is

substantially high, nowadays new plants of more capacity are being installed using

Snamprogetti. Capacity ranging from 3000 to 3500 MTPD Urea..

Figure:2.16 Block diagram describing in brief the Snamprogretti process.

2.5.4STAMICARBON PROCESS:

Stamicarbon process consists of a reactor, stripper , pool condenser and scrubber in the

synthesis section.

The design has been named LAUNCH MELTTM. The N//C ratio is maintained

around 3, stripping agent is CO2,, and mateial used in the stripper is reactor recommended

by Stamicarbon is Safurex R..

Typical parameters are:

- Reactor temperature: 185 oC

- Reactor pressure : 140 ATA

- N/C = 3

overall conversion can be as high as 80% of fed carbon dioxide. Stripper pressure is

around same as that of reactor. Stamicarbon uses a vertical plant layout hence uses gravity

for driving the carbamate solution. Stamicarbon has introduced Technology to increase the

plant capacity to 6000 MTPD. The single stream process does not have MP decomposition

section.

37

The process requires less energy, and is comparable to Saipem technology. The

stripper tubes consists of ferrules and top section of stripper has a packed bed, followed by

a distributor to evenly distribute the urea solution coming from reactor over the tubes,

heating is achieved by condensing

steam.

The pool reactor has a pool of condensing liquid, which has u tube bundles immersed

in it. Steam in generated in the tube side and the steam thus produced used in LP

recirculation section and vacuum section.

Figure2.17: Brief block diagram of Stamicarbon process.

2.5.5ACES(Advanced Cost and Energy Savings) PROCESS:

The main idea of the process is to reduce steam consumption of the plant. A high N/C

ratio is used, as a result of which high single pass conversion is achieved in the reactor.

Two parallel carbamate condensers are used which give maximum heat recovery in form

of steam generated, one used for MP decomposition and other for generating steam to be

used in LP section.

Toyo Engineering Corporation along with PUSRI developed the ACES21 process.

The process has synthesis in two stages, a vertical submerged scrubber functioning as

carbamate condenser, HP scrubber and the primary urea reactor followed by a secondary

vertical reactor. Different values of N/C values are used for reactor and carbamate

condenser in ACES21 process, and layout is horizontal.

38

- Typical parameters of the process

- Reactor temperature = 200 o C

- reactor pressure = 180 ATA

- N/C(ACES) = 4

2.5.6MONTEDISON ISOBARIC DOUBLE RECYCLE(IDR) PROCESS

The process involves heat treatment of reactor effluent in two isobaric stages. By heat

treatment is meant stripping, employing excess ammonia in first stage and excess carbon

dioxide in second stage. The first stage effluent is recycled directory to reactor, the second

stage effluent is condensed and recycled as liquid. A high N/C ratio is used.

Typical parameters are

- Reactor temperature = 200 o C

- reactor pressure = 200 ATA

- N/C= 4.25(3333)

39

Cheater Three

Stamicarbon Urea Technology

40

Stamicarbon Urea Technology 3.1 What is Stamicarbon?

Stamicarbon was the original pioneer of the stripping concept, in which one of the

input materials - carbon dioxide - is used to strip free ammonia from the urea synthesis

reactor effluent, promoting decomposition of its residual ammonium carbamate content and

allowing it to be reconstituted at the full system pressure. This had a profoundly beneficial

effect on the economics of urea production by simplifying and reducing the cost of the plant

and raising both energy and material conversion efficiencies. Today all the most modern

urea processes embody the stripping principle.

3.1.1 Stamicarbon history In 1947, DSM (then known as Dutch State Mines) formed the subsidiary Stamicarbon

to obtain and exploit coal-washing inventions, patents, and know-how. The name

Stamicarbon is a contraction of the words “State Mines” and “Carbon”. The 70-year history

of Stamicarbon shows its constant ability to respond and adapt to a wide range of market

developments around the world

Stamicarbon was founded in The Netherlands in 1947 as licensing subsidiary of

DSM. Since October 2009 it is part of the Italian Maire Tecnimont Group. The Stamicarbon

headquarters is in Sittard, in the south of The Netherlands, with representative offices in

Russia and China.

Nowadays the company is a global leader in the design, licensing and development of

urea plants and a supplier of high-end equipment and services for the petrochemical industry,

with more than 50% market share in urea synthesis and about 30% market share in urea

granulation technology.

A pioneering company specialized in the fertilizer industry with the vision to help

enable the world to feed itself and improve the quality of life

During its 70 years history, Stamicarbon has licensed over 250 urea plants located in

over 50 countries across the globe. Furthermore it has completed over 90 revamp projects in

both Stamicarbon and non-Stamicarbon plants. Currently, the company is involved in 15

urea melt plants, 10 urea granulation plants and 5 revamping projects.

41

3.1.2Modifications of the Stamicarbon CO2-stripping process: Throughout the study of many technologies concerning urea production the most

commonly used technology in Libya is Stamicarbon technology, so were focusing now on

the Stamicarbon technology

Finger (3.1) block diagram for Stamicarbon CO2stripping Urea Process

42

3.2The original Stamicarbon CO2-Stripping

3.2.1 Stripping Urea Process In this first-generation CO2-stripping plant, the high-pressure carbamate condenser

was of the vertical falling-film type. The condensed liquid carbamate is flowing down along

the inside wall of the (vertical) heat exchanger tubes. Condensation of ammonia and carbon

dioxide gases occurs in the high-pressure carbamate condenser at synthesis pressure. Besides

condensation, also chemical formation of ammonium carbamate from ammonia and carbon

dioxide takes place in this condenser. Because of the high pressure, the heat liberated from

the condensation and subsequent ammonium carbamate formation is at a high temperature.

This heat, there fore, can effectively be used for the production of 4.5bar steam for further

use in the urea plant itself. The condensation in the high-pressure carbamate condenser is

not effected completely. Remaining gases are condensed in the reactor and provide the heat

required for the dehydration of carbamate. Ammonia and carbon dioxide are introduced to

the reactor with a molar ratio3:1.The operating conditions of the reactor are 190 o C and 140

bar. There fore ,maximum urea yield per pass is achieved. Ammonia and carbon dioxide are

stripped off. The stripper is realized in the form of a falling-film evaporator, 20where the

urea synthesis solution flows as a falling film along the inside of the vertical heat-exchanging

tubes. Heat, in the form of medium pressure steam, is supplied to the outside of these tubes.

The supply of heat at this place results in decomposition of unconverted ammonium

carbamate into ammonia and carbon dioxide. Moreover, the heat supplied in this way will

transfer ammonia and carbon dioxide from the liquid phase into the gaseous phase. Fresh

carbon dioxide and air are supplied to the bottom of the tubes flows counter-currently to the

urea solution from top to bottom. Addition of air and lowering the temperature are important

to maintain a corrosion-resistant layer. To avoid the formation of explosive hydrogen oxygen

mixtures in the tail gas of the plant, hydrogen is catalytically removed from the carbon

dioxide feed. The carbon dioxide acts as a stripping agent, enhancing the transfer of ammonia

from the liquid phase into the gaseous phase. Stripping with carbon dioxide not only recycles

ammonia, but also effectively reduces the carbon dioxide content of the urea synthesis

solution flowing down the heat exchanger tubes. Low ammonia and carbon dioxide

concentrations in the stripped urea solution are obtained, such that the recycle from the low-

pressure recirculation stage is minimized. Before the inert gases, mainly oxygen and

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

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

43

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

performed in a low-pressure absorber to obtain a purge gas that is practically ammonia free.

3.2.2Main Reactions: The commercial processes in current use are based on two reactions:

CO2+2NH3 NH4CO2NH2(1)∆H=67,000BTU/ Ib mole

NH4CO2NH2 N2CONH2+ H2O (2) ∆H=+18,000BTU/ Ib mole

Reaction (1) is fast and exothermic and essentially goes to completion under the

reaction conditions used industrially

.Reaction (2) is slower and endothermic and does not go to completion. The conversion

(on a CO2 basis) is usually in the order of 50 80%. The conversion increases with increasing

temperature and NH 3 /CO2 ratio and decreases with increasing H2O/CO2ratio

3.2.3Side reactions: Hydrolysis of urea:

CO(NH2)2+H2O NH2COONH4 2NH3+CO2 (3)

Biuret formation from urea:

2CO(NH 2)2 NH2CONHCONH2+ NH3 (4)

Formation of is ocyanic 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 (1) is nothing but the reverse

of urea formation. Whereas this reaction approaches equilibrium in the reactor, in all down

stream sections of the plant the NH3 and CO2 concentrations in urea-containing solutions

are such that Reaction (1) is shifted to the right. The extent to which the reaction occurs is

determined by temperature (high temperatures favor hydrolysis) and reaction kinetics; in

practice, this means that retention times of urea-containing solutions at high temperatures

must be minimized. The biuret reaction (2) also approaches equilibrium in the urea reactor

.The high NH3 concentration in the reactor shifts Reaction (2) to the left, such that only a

small amount of biuret is formed in the reactor. In downstream sections of the plant, NH 3

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 minim izebiuret formation are the same as described above for the

hydrolysis reaction.

44

3.2.4Process Operating Variables Reactions: Temperature:

• Rate of Carbamate decomposition reaction increases with temperature. It is slow at < 150oC

(NH3:CO2, stoichiometric) and quite rapid at 210oC.

• 180-210 o C in 0.3 to 1.0 hr is optimum for most process. At high temperature, corrosion

rate is high.

Pressure:

• Preferred pressure is 140 – 250 atm.

Mole ratio of NH3: CO2

• Excess ammonia above the stoichiometric ratio favors the rate of reaction. (3:1 = NH3:

CO2)

Other factors:

• The presence of water decreases conversion.

• The presence of small amount of O2, decreases corrosion.

Optimum Conditions

• Maximize the production of urea

per unit time with due regard to cost of recycling un reacted NH3 and

CO2, the cost increase of reactor size, corrosion difficulties.

NOT to increase the percentage of conversion.

Typical Operating Conditions:

T: 180 – 210oC NH3:CO2 = 3.1 - 4.1

P: 140 – 250 atm Retention time: 20-30 min

3.2.5 Urea manufacturing plant

Urea plant based on CO2 Stripping process.

The Urea process consists of the following steps.

45

1. CO2 supply and compression

2. NH3 supply and pumping

3. Reaction/High pressure synthesis

4. Low pressure/Recirculation system

5. Evaporation and Prilling System

6. Urea hydrolyser and Desorber

7. Steam and condensation

Figure 3.2 . The Original Stamicarbon processes diagram

3.2.6 Products Urea Product properties :

- State : Crystalline solid prills

- Chemical formula : NH2-CO-NH2

- Molecular weight : 60

- Melting point : 132.7 C

- Specific gravity : 1.33

- Hydrolyses very slowly in ammonia and carbon dioxide.

- Absorbs moisture from the air.

- Urea undergoes a number of reactions of heating about its boiling point.

- At 160 C, it decomposes to yield ammonium biuret and higher condensation products.

- Longer the urea is held above its melting point, the further reaction proceeds.

46

Urea specifications :

Total nitrogen : 46.3% by wt.

Moisture : 0.3% by wt.

Biuret : 0.9% by wt.

Fe – content : up to 1.5 ppm

3.2.7 Utility plant Utility plant consists of mainly:-

1. Steam Generation Plant (Boiler)

2. Inert gas plant/Plant & Instrument Air

3. Cooling Tower

4. Demineralization plant (DM plant)

3.3 Urea 2000 plus 3.3.1 Introduction

Back in the late sixties, the development of the Stamicarbon CO2 stripping process

revolutionized urea process technology. The introduction of Stamicarbon's Urea

2000plusTM in 1996, was yet another milestone. By combining equipment, Their

engineers have been able to reduce the number of high pressure vessels in the process,

thereby also simplifying the overall design, piping and construction.

The result: a considerable lowering of investments and operating costs. The Pool

Condenser concept was the first step in the development of this technology. A second step

was the introduction of the Urea 2000plus Pool Reactor concept, which has been built at

DSM in Geleen, The Netherlands, with a capacity of 1150 m t/d.

Stamicarbon can offer this process in its pool condenser alternative in single train

capacities up to 3,250 mt/d. This capacity range again signifies an improvement over

today's plants, which have typically been designed for around 2000 m t/d.

The Pool Condenser concept was the first step in the development of this technology.

The second step was the introduction of the Urea 2000plus Pool Reactor concept.

47

The following chart shows 3.3the development of the Stamicarbon urea process over the last

decades and the consequences for the plant height.

3.3.2Stamicarbon’s Avancoreurea processes It reduces the required plant height to just 25m. This obviously has brought down

investment costs considerably. However the Avancoreprocess is also available with a pool

reactor for urea plants up-to medium capacities.

Customers can now choose among basic synthesis concepts that use either Avancore or

the Urea 2000plus technology.

3.3.3Synthesis : Avancore Urea Process

Section, hydrogen or any other combustibles present in the feed to the urea plant no longer

pose any risk of explosion. The ammonia emissions will also be kept to an absolute minimum

thanks to the absence of passivation air.

• Revamps

Experiences gained in revamp projects have also led to the incorporation of the following

innovations into the Avancore urea process

48

• : Low elevation lay-out of the synthesis section

While the urea synthesis loop still relies on gravity flow, thus offering maximum

reliability, the equipment elevation has been reduced, allowing for lower investment.

• Reduced-pressure inert washing system

The vapor leaving the urea synthesis sections treated in a scrubber operating at a reduced

pressure. Most of the ammonia and carbon dioxide left after this scrubbing are absorbed in

acarbamate solution coming from the downstream

low pressure recirculation section. As a result, The Avancore urea process is a new urea

synthesis concept that incorporates all the benefi ts of Stamicarbon’s earlier proven

innovations. The Avancore urea process combines the advantages of Urea 2000plus

technology, Safurex and innovations and experiences gained from revamp projects.

• Urea 2000plus

The Urea 2000plus technology already provided the technological advantage of

improving heat transfer in the condensing part of the urea synthesis, achieved by the

application of pool condensation, and increasing the available temperature difference over

the condenser by combining carbamate condensation and urea reaction in one vessel.

• Safurex

The excellent corrosion-resistant properties of the Safurex material in an oxygen-free

carbamate environment, eliminates the need to use the passivation air required in urea

processes.Because of the absence of oxygen in the synthesis

• Features Avancore

• No high-pressure scrubber

• Pool condensation

• Synthesis options: pool condenser or pool reactor

• All gravity fl ow

• Low elevation

• Zero oxygen

• Low maintenance synthesis

• Lower investment

49

no additional water needs to be recycled to the synthesis section, meaning that the urea

reaction is therefore not affected. The Avancore urea process works as follows: Ammonia

and carbon dioxide are introduced to the high-pressure synthesis using a high-pressure

ammonia pump and a carbon-dioxide compressor. The ammonia, together with the

carbamate solution from the downstream recirculation section, enters the pool condenser.

The major part of the carbon dioxide enters the synthesis through the high-pressure stripper

counter-current to the urea/carbamate solution leaving the reactor. On the shell side, the

high-pressure stripper is heated with steam. The off-gas of the high-pressure stripper,

containing the carbon dioxide, together with the ammonia and carbon dioxide resulting from

dissociated carbamate, is fed into the pool condenser. low-pressure steam. Downstream from

the pool condenser, the urea-carbamate liquid enters the vertical reactor, if required, located

at ground level. Here, the fi nal part of the urea conversion takes place. The urea solution

then leaves the top of the reactor, all by gravity flow (via an overflow funnel) before being

introduced into the high-pressure stripper. Gases leaving the urea reactor are directed to the

pool condenser. Gases leaving the pool condenser are fed into the scrubber operating at a

reduced pressure. Here, the gases are washed with the carbamate solution from the low-

pressure recirculation stage. The enriched carbamate solution is then fed into the pool

condenser. This enriched carbamate fl ow contains no more water than in earlier generations

of Stamicarbon CO2-stripping plants, meaning that the conversions in the synthesis section

are as high as ever. Inert gases leaving the scrubber at reduced pressure containing some

ammonia and carbon dioxide are then released into the atmosphere after treatment in a 4-bar

absorber.

50

3.4 Synthesis : Urea 2000plus Pool Condenser Concept

Fig3.4 Urea 2000plus Pool Condenser Concept

The Urea 2000plus Pool Condenser process works as follows: Ammonia and carbon

dioxide are introduced to the high-pressure synthesis using a high-pressure ammonia pump

and a carbon dioxide compressor.

The ammonia then drives an ejector, which conveys a carbamate solution into the pool

condenser.

The carbon dioxide feed usually originates from an associated ammonia plant, and

therefore always contains hydrogen. This hydrogen is removed by catalytic combustion that

uses air. Air also used to be supplied as a passivating agent to minimize corrosion in the

synthesis. When using Safurex material, this is in essence no longer necessary.

In the high-pressure stripper, the carbon dioxide, entering the synthesis as a feed, flows

countercurrent to the urea solution leaving the reactor. On the shell side, the high-pressure

stripper is heated with steam. The off-gas of the high-pressure stripper, containing the carbon

dioxide, together with the dissociated carbamate, is then fed into the pool condenser.

51

In the pool condenser, ammonia and carbon dioxide are condensed to form carbamate,

and a substantial part of the conversion to urea is already established here.

The heat released by condensation and subsequent formation of carbamate is used to

produce re-usable low-pressure steam.

After the pool condenser, the remaining gases and a urea-carbamate liquid enter the

vertical reactor. Here, the final part of the urea conversion takes place. The urea solution

then leaves the top of the reactor (via an overflow funnel) before being introduced into the

high-pressure stripper. Ammonia and carbon dioxide conversions in the synthesis section of

a Stamicarbon carbon dioxide stripping plant are high, reducing the need for a medium

pressure stage to recycle any unconverted ammonia and carbon dioxide. As a result, the

Stamicarbon CO2 stripping process is the only commercial available process that does not

require a medium-pressure recirculation stage downstream from the HP stripper.

Gases leaving the reactor are fed into the high-pressure scrubber. Here, the gases are

washed with the carbamate solution from the low-pressure recirculation stage. The enriched

carbamate soluion is then fed into the high-pressure ejector and, subsequently, to the pool

condenser. Inert gases and some ammonia and carbon dioxide are then released into the 4-

bar absorber.

3.5 Synthesis : Urea 2000plus Pool Reactor Concept

52

Fig 3.5Urea 2000plus Pool Reactor Concept

The Urea 2000plus Pool Reactor process works as follows:

Unlike the Pool Condenser concept, the Pool Reactor concept combines the condenser

and reactor within a single pool reactor. This is achieved by enlarging the horizontal

condenser so as to incorporate additional reactor volume. As a

result, it becomes possible to achieve sufficiently high residence times, eliminating the

need for a separate vertical reactor, while creating the conditions that will allow the reaction

to reach its optimum condition. The high-pressure scrubbing operation can also be simplified

in the Pool Reactor concept by placing the scrubber sphere above the pool reactor and adding

the ammonia to the synthesis via this scrubber. This ensures that no separate heat exchanging

section in this scrubbing operation is required.

In the Pool Reactor concept, carbamate from the low-pressure recirculation section

flows together with the absorbed gases and the ammonia via a sparger into the pool reactor.

As the static liquid height ensures gravity flow, no high-pressure ejector is needed.

53

Fig3.6 Urea 2000pluslow-pressure recirculation section.

This stage recovers the ammonia and carbon dioxide still present in the urea solution coming

from the high-pressure stripper. Thanks to the low ammonia and carbon dioxide

concentrations in the stripped urea solutions, the Stamicarbon CO2 stripping process is the

only process that requires just one single low-pressure recirculation stage.

Coming out of the stripper, the urea solution is fed into the dissociation heater, where most

of the ammonia and carbon dioxide are removed. T he ammonia and carbon dioxide are then

fed into the low-pressure carbamate condenser, where they are condensed. Because the ratio

between ammonia and carbon dioxide in the recovered gases is optimal, the quantity of water

needed to dilute the resultant ammonium carbamate solution can be kept to a minimum,

maximizing conversion figures for the urea plant. The resultant carbamate solution is fed,

via a high-pressure carbamate pump, back to the synthesis as a scrubbing agent in the high-

pressure scrubber.

Before entering the urea solution tank, part of the water present in the urea solution is

evaporated by further pre-flashing. The vent gas from the recirculation stage is practically

free from ammonia because it is scrubbed in an atmospheric absorber. The heat required for

this section is derived from the condensation of the low-pressure steam produced in the urea

synthesis.

54

3.5Evaporation section

Fig 3.7Urea 2000plusEvaporation section

Before the entire urea production process is complete, the urea solution present in the

urea solution tank must be concentrated. The urea solution is therefore sent to an evaporation

section. The topology of this evaporation section depends on the applied finishing section

(prilling, granulation or rotoform pastillation). Depending on the requirements of the

finishing section, the evaporation section may, for example, consist of two consecutive

evaporators, where the water in the urea solution is evaporated under vacuum conditions.

The remaining urea melt has a urea

concentration varying from 96 to 99.7wt%, depending on the requirements of the

downstream finishing section.

3.6 Waste-water treatment section

The process condensate coming from the evaporation section, together with other

process effluents such as sealing water from stuffing boxes, contains ammonia and urea. All

of the process condensate is collected in the ammonia water tank.

From this tank, the water is fed to the first desorber. In the first desorber, the bulk of

ammonia and carbon dioxide are stripped off from the water phase by using the off-gas from

the second desorber as a stripping agent. The descending effluent still contains urea and

some ammonia. To remove this urea, this effluent is then fed to the hydrolyzer. The

hydrolyzer is a liquid-filled column. In the hydrolyzer, the urea, at elevated pressure and

55

temperature, is dissociated into ammonia and carbon dioxide by the application of heat

(steam) and retention time. The process condensate feed is kept in counter-current contact

with the steam in order to obtain extremely low urea content in the hydrolyzer effluent. The

remaining ammonia and carbon dioxide in the effluent of the hydrolyzer are stripped off with

steam at a reduced pressure in the second desorber. The off-gases leaving the first desorber

are recycled to the synthesis section after being condensed in the reflux condenser. The

purity of the remaining water satisfies requirements for boiler feed water make-up or cooling

water make-up - which means that Stamicarbon urea plants do not produce a waste-water

stream.

Fig 3.8Urea 2000plusWaste-water treatment section

3.7Finishing technology 1: Fluid-bed granulation

Today, the most commonly used finishing technology is fluid-bed granulation, which was

commercialized by Stamicarbon in response to changing market needs. Our patented fluid-

bed urea technology is in use in several urea plants.

Stamicarbon fluid-bed granulation technology offers:

• Large reductions in formaldehyde content compared to other fluid-bed granulation

technologies.

• Unprecedented uninterrupted run times, which can exceed 100 days before washing is

required.

• Excellent product quality (round and uniform, with a smooth surface) More stable

operation conditions.

56

• Low urea dust formation, resulting in a lower recycle of urea solution to the urea synthesis

plant.

• Low opacity at outlet granulation vent stack.

• Substantial savings on operational costs when compared to other fluid-bed granulation

technologies.

• Excellent properties for downstream coating (for specialty fertilizers).

In just five years, Stamicarbon fluid-bed granulation technology has been licensed over 10

times for commercial scale plants, including capacities exceeding 3500 m t p d. The plants

using this technology are operating at or above their original design capacity, producing

superior-quality products that meet all required product quality standards.

Stamicarbon fluid-bed granulation process works as follows:

A urea melt stream with a urea concentration of 98.5wt% is introduced into the fluid-bed

granulator through the injection headers, which are connected to the urea melt line and the

secondary air system. Each injection header comprises

vertically placed risers fitted with spray nozzles that spray the urea melt onto the seed

particles. The secondary air, required to transport the granules through the urea melt film, is

provided by a secondary air blower. Urea formaldehyde is added to the urea melt as a

granulation additive and anti-caking agent. This also improves the granule crushing strength.

Figure 3.9Stamicarbon’s fluid-bed granulation process works

The granulator is divided into a granulation section and a cooling/conditioning section.

In both sections, fluidization air is evenly distributed to fluidize and cool the granules. Seed

57

(recycled) material is introduced into the first chamber of the granulation section. The urea

melt is then sprayed onto this seed material. As the granules move through the granulation

section, their size is steadily increased by layering until they reach the required granule

diameter.

At this point, the product finally flows out of the granulator. The granules fl ow from

the granulation section to the cooling section (without spray nozzles), where they cool down

and harden. Fluidization air and secondary air are exhausted from the top of the granulator

by means of an off-gas fan in the off-gas line of the granulator scrubbers. In the scrubbers,

the air is cleaned using a scrubbing solution, and the cleaned air is exhausted into the

atmosphere. The scrubbing solution (a dilute urea solution) is partly recycled to the scrubber

as a scrubbing solution. A purge stream is also pumped to the urea-dissolving vessel and

recycled to the urea melt plant.

The product from the granulator flows through a screen to prevent any lumps from

reaching the granulate cooler. The fluidization/cooling air, which contains some dust, is

exhausted from the top of the granulate cooler and is combined with the air from the product

cooler and the de-dusting air. This combined stream is cleaned in the cooler scrubber system.

3.8 Finishing technology 2: Prilling The Prilling process works as follows:

The urea solution is concentrated to 99.7% urea in two steps under vacuum. The resultant

molten stream is prilled with the aid of a rotating prilling bucket, designed by Stamicarbon.

Using an optional technique of seeding when prilling, impact-resistant prills are obtained.

These prills are very resistant to degradation during product handling.

58

Fig3.10 Urea 2000plus Prilling section

3.9 UAN Process The Stamicarbon partial-recycle CO2-stripping process is eminently suitable for the

manufacture of Urea Ammonium Nitrate (UAN) solutions. The ratio of unconverted

ammonia to urea is such that the required ratio between urea and ammonium nitrate for the

production of UAN solutions can be achieved directly. Ammonia still present in the stripped

urea solution, together with the ammonia in the reactor’s off-gases, is converted into

ammonium nitrate in a neutralization reactor using nitric acid. UAN solution product is

obtained by mixing the urea and ammonium nitrate solutions. The Avancore and Urea

2000plus process can both be applied for the Stamicarbon UAN process.

The UAN process works as follows:

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

urea in very much the same way as in the previously described processes.

The urea formation is an equilibrium reaction, so the urea solution formed contains

unconverted ammonia and carbon dioxide. Stripping with carbon dioxide causes the greater

part of these components to evaporate from the solution. Evaporated ammonia and carbon

dioxide, together with fresh ammonia and carbon dioxide, are condensed in the pool reactor,

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

59

dissociation section, the stripped urea solution is almost entirely freed from ammonia and

carbon dioxide.

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.

Figure 3.11UAN process.

The neutralization section comprises a neutralizer, an (optional) ammonium nitrate

storage tank, a mixing pipe, a UAN storage tank and off-gas purification equipment.

3.10Mega Plant Concept Large urea plants require large high-pressure equipment that is difficult and costly to

manufacture and transport. To reduce urea fabrication costs, Stamicarbon has developed a

Mega Plant concept for single-line urea plants that produce capacities of 5000 m t p d. In the

Stamicarbon Mega Plant concept, a proportion of the liquid effluent from the reactor is

60

diverted to a medium-pressure recycling section, thereby reducing the size of the high-

pressure vessels needed. In fact, thanks to the Mega Plant concept, the size of the required

high-pressure equipment and lines will not exceed the size of equipment needed for a 3250

m t p d pool condenser type CO2-stripping urea plant! A Mega Plant can be built with both

the Avancore or the Urea 2000plus technology.

The Mega Plant process works as follows:

About 70% of the urea solution leaving the urea reactor flows to the high-pressure CO2

stripper, while the remainder is fed into a medium-pressure recirculation section. This

reduced liquid feed to the stripper in turn reduces not only the size of the stripper needed,

but also the heat exchange area of the pool condenser. The degree of stripping efficiency is

adjusted to ensure that as much low-pressure steam is produced by the carbamate reaction

in the pool condenser as is needed in the downstream sections of the urea plant. About

30% of the urea solution that leaves the reactor is expanded and enters a gas/liquid

separator in a recirculation stage operating at a reduced pressure. After expansion, the urea

solution is

Figure 3.12.Mega Plant Concept

heated by medium-pressure steam. By heating the urea solution, the unconverted

carbamate is dissociated into ammonia and carbon dioxide.

Our Mega Plant Concept does not need the ammonia recycle section or the ammonia

holdup steps that are commonly seen in competitors total recycle urea plants. This is because

the low ammonia-to-carbon dioxide molar ratio in the

separated gases allows for easy condensation as carbamate only.

61

The operating pressure in this medium-pressure recirculation stage is about 20 bars.

After the urea solution leaves the medium-pressure dissociation separator, it fl ows into an

adiabatic CO2 stripper, which uses carbon dioxide to strip the solution.

As a result of this process, the ammonia-tocarbon dioxide molar ratio in the liquid

leaving the medium-pressure recirculation section is reduced, facilitating the condensation

of carbamate gases in the next step. The vapors leaving the medium pressure dissociation

separator, together with the gases leaving the adiabatic CO2 stripper, are condensed on the

shell side of the evaporator.

The carbamate formed in the low-pressure recirculation stage is also added to the shell

side of this evaporator. The heat released by condensation is used to concentrate the urea

solution. Further concentration of the urea solution is achieved using low-pressure steam

produced in the pool condenser.

The remaining uncondensed ammonia and carbon dioxide leaving the shell side of the

evaporator are sent to a medium-pressure carbamate condenser. The heat released by

condensation in this condenser is dissipated into a tempered cooling water system. This

process forms medium pressure carbamate that contains only 20-22wt% water. The

carbamate is transferred via a high pressure carbamate pump to the high-pressure scrubber

in the urea synthesis section. The urea solution leaving the adiabatic CO2 stripper and the

high-pressure stripper are expanded together in the low-pressure recirculation section.

3.11 Full life cycle support Successful urea plants comply with all applicable rules and regulations and produce

products efficiently and effectively with maximum on stream times. However, as plants get

older, it may become increasingly difficult to maintain efficient production and full

compliance. Many urea plants have been designed for a service life not greatly exceeding

twenty years. However, such plants can remain competitive due to the depreciated initial

capital investment. Well maintained and upgraded with state-of-the-art technology, these

plants succeed in producing at competitive cost prices.

To ensure our customers’ urea plants remain competitive, we offer a range of activities

and services covering the complete life cycle of a plant:

• Plant operation.

• Plant maintenance .

• Plant improvement, including capacity increase.

62

Examples of Stamicarbon’s full life cycle support are:

Corrosion inspection of the critical urea Equipment.

Process analyses and plant optimization.

Debottlenecking ideas and life study.

Simplifi cation of the urea process steps for easeof operation.

Supply of critical equipment items.

Schemes for sustained maximum output.

3.12The Pool condenser In the effort to develop the pool condenser, use was made of extensive experience gained

in urea plants of our licensees in which similar condensers are used for different purposes

and from the extensive process database available at Stamicarbon.

Basically, the pool condenser is a horizontal vessel with a submerged U-tube bundle. It

is fabricated from carbon steel with internals and lining in stainless steel.

Figure3.13 Urea 2000plus The Pool condenser .

The operating principles are:

Strip-gases are condensed in a pool of liquid on the shell side, with low pressure steam

being generated on the tube side.

Adequate residence time allows the reaction of ammonium carbamate to urea and water

to proceed to up to 60 % of equilibrium.

63

The condensation temperature on the shell side is high as a result of the formation of high

boiling components (urea and water), resulting in a higher delta T for the exchanger

part. The formation of gas bubbles ensures a high degree of turbulence and provides a

large area for mass and heat transfer. Both phenomena contribute to a higher heat

transfer.

3.12.1The Optimum reactor Design

Figure3.13 The optimum reactor design.

The optimum reactor design curve indicates that as much as 60% of the fractional

approach to equilibrium (F.A.E.) is reached already within the pool condenser whose

capacity is only 30 to 40 % of the required reaction volume. As a direct consequence, the

actual reactor volume can be considerable reduced.

Figure3.14 Optimum reactor design curve indicates that as much as 60%

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3.12.2The pool condenser design

Process data: Mechanical design data:

• Shell side: - - 140 bar synthesis pressure - 175 degree C at reactor outlet

• Tube side: 4.5 bar steam/BFW • Gas dividing system with flow

deflector plates

• Stainless steel vessel • U-tube bundle • Internal bore welding • Tube supports

3.12.3Advantages

The pool condenser configuration offers the following technological advantages:

• The urea reactor volume is reduced by some 40 %.

• The pool condenser requires some 45 % less heat exchange area.

• The height of the steel structure is reduced by some 10 meter.

• No stress corrosion can take place, because of the absence of crevices between tube

and tube sheet.

• The operational flexibility has improved immensely.

• The start-up of the synthesis section is very easy.

The Avancore process:

The Avancore urea process was introduced by Stamicarbon in 2009. It comprises a new

urea synthesis concept that incorporates the benefits of Stamicarbon's earlier proven

innovations. The Avancore Urea process material of construction is Safurex, and includes a

low-elevation layout of the synthesis section.The excellent corrosion resistant properties of

the Safurex material in anoxygen-free carbamate environment eliminate the need of using

passivationair in the urea processes. Because of the absence of oxygen in the synthesis

section, hydrogen or any other combustibles present in the feed no longer poses any risk of

explosion for the urea plant. The ammonia emissions are also kept to a minimum because of

the absence of passivation air. In the Avancore process, Stamicarbon has introduced a low-

level arrangement of the synthesis section, where the reactor is located on ground level,

which allows less investment and easier maintenance. The concept still makes use of a

gravity flow in the synthesis recycle loop However, the low-level arrangement of the reactor

necessitates another heat source for the endothermic dehydration reaction taking place in the

reactor because the poolcondenser off-gas cannot flow into this low-level reactor any more.

Most of the urea formation, however, already takes place in the pool condenser and

65

,therefore, only a minor amount of CO2 supplied to the reactor is sufficient to close the heat

balance around it

3.13Corrosion:

Urea synthesis solutions are very corrosive. Generally, ammonium carbamateis

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

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

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

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

important factors in material selection, equipment manufacture and inspection, technological

design and proper operations of the plant, together with periodic inspections and non-

destructive testing are the key factors for safe operation for many years.

Role of Oxygen Content:

Since the liquid phase in urea synthesis behaves as an electrolyte, it causes

electrochemical nature corrosion. Stainless steel in a corrosive medium owesits corrosion

resistance to the presence of a protective oxide layer on the metal. As long as this layer is

intact, the metal corrodes at a very low rate. Passive corrosion rates of austenitic urea-grade

stainless steels are generally between

0.01 and (max.) 0.10 mm/day. Upon removal of the oxide layer, activation corrosion set

in unless the medium contains sufficient oxygen or oxidation agent to build a new layer.

Active corrosion rates can reach values of 50 mm/day. Stainless steel exposed to carbamate

containing. Solutions involved in urea synthesis can be kept in a passivity (non-corroding)

state by a given quantity of oxygen. If the oxygen content drops below this limit, corrosion

starts after some time

–its onset depending on process conditions and the quality of the passive layer. Hence,

introduction of oxygen and maintenance of sufficiently high oxygen content in the various

process streams are prerequisites to preventing corrosion of the equipment. From the point

of view of corrosion prevention, the condensation of NH3

– CO2 – H2O gas mixtures to carbamate solutions deserves great attention. This is

necessary because not withstanding the presence of oxygen in the gas phase –

an oxygen-deficient corrosive condensate is initially formed on condensation. In this

condensate the oxygen is absorbed only slowly. This accounts for the severe corrosion

sometimes observed in cold spots in side gas lines. The trouble can be remedied by adequate

isolation and tracing of the lines. When condensation constitutes an essential process

step– for example, in high-pressure and low-pressure carbamate condensers

66

special technological measures must be taken. These measures can involve ensuring that

an oxygen rich liquid phase is introduced into the condenser, while appropriate liquid

gas distribution devices ensure that no dry spots exist on condensing surfaces. Not only

condensing but also stagnant conditions are dangerous, especially where narrow crevices are

present, into which hardly any oxygen can penetrate and oxygen depletion may occur.

Role of Temperature:

Temperature is the most important technological factor in the behavior of the steels

employed in urea synthesis. An increase in temperature increases active corrosion, but more

important, above a critical temperature it causes spontaneous activation of passive steel. The

higher-alloyed austenitic stain Urea 11 less steel (e.g., containing 25 wt% chromium (Cr),

22 wt% nickel(Ni), and 2wt%molybdenum(Mo)) appear to be much less sensitive to this

critical temperature than 316L types of steel. Sometimes, the NH3: CO2 ratio in synthesis

solutions is also claimed to have an influence on the corrosion rate of steels under urea

synthesis conditions. Experiments have showed that under practical conditions this influence

is not measurable because the steel retains passivity. Spontaneous activation did not occur.

Only with electrochemical activation could 316L types of steel be activated at intermediate

NH3:CO2 ratios. At low and high ratios, 316L stainless steel could not be activated. The

higher-alloyed steel type25wt% Cr, 22 wt %Ni, 2wt%Mo showed stable passivity,

irrespective of theNH3: CO2

ratio, even when activated electrochemically. Of course, these results depend on the

specific temperature and oxygen content during the experiments.

Material Selection:

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

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

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

attention. Stainless steels that have found wide use are the austenitic grades 316L and 317

L. Like all Cr-containing stainless steels, 316L and 317 L are not resistant to the action of

sulfides. Hence it is imperative in plants using the 316L and 317 L grades in combination

with CO2 derived from sulfur-containing gas, to purify this gas or the CO2 thoroughly. In

stripping processes, the process conditions inthe high-pressure stripper are most severe with

respect to corrosion. In the Stami-carbon CO2-stripping process, a higher-alloyed, but still

fully austenitic stainless steel was chosen as construction material for the stripper tubes. This

67

choice ensures better corrosion resistance than 316L or 317 Types of material but still

maintains the advantages of workability, weld-ability, reparability, and the cheaper price of

stainless steel-type materials

3.14The innovative Pool Reactor at the DSM urea plant in The Netherlands

The stripper off-gases, the recycle carbamate solution and the ammonia feedstock are

introduced into the pool reactor. The liquid phase is thoroughly agitated by the gases from

the stripper. The heat of condensation is utilized for aiding the dehydration reaction and for

generating steam in the tube bundle. The rate of condensation is controlled by the pressure

of the generated steam.

The part of the pool reactor that is equipped with the tube bundle is the condensing part

of the reactor while the other section can be called the reaction part.

Figure3.15 Urea 2000 plus the Pool Reactor .

commissioned at DSM's Geleen site, in the Netherlands, in March 1998.

3.15 Stamicarbon urea process data

68

Typical consumption figures of a Stamicarbon urea plant per ton of urea:

69

Chapter Four

Calculation

70

calculation of Balance

4.1 material Balance

SELECT OF PROCESSE :STAMECARBON CO2 STRIPPING PROCESSE

TARGET FOR THE YEAR:300000 TARGET FOR THE MONTH:25000

TARGET FOR THE DAY:1OOO UREA:41667 kg/hr OF 98% PURITY

We will investigate that the product :-

urea 98% (40834 Kg/hr)

Biuret 1% (417 Kg/hr)

water 1% (417 Kg/hr)

assumption : overall conversion to urea is assumed to be 95%

Main Reactions:

1) CO2 + 2NH3 ==== NH2COONH4 ………………………………………….. (1)

(44) (17) (78)

2) NH2COONH4 ==== NH2CONH2 + H2O …………………………..….…… (2)

(60) (18)

3)CO2 + 2NH3 ==== NH2CONH2 + H2O (Overall reaction) …………….. (3) Side reaction:

4)2NH2CONH2 ==== NH2CONHCONH2 + NH3 ……………………...……. (4)

(120) (103)

- 417 Kg/hr of Biuret produced by = (120/103)*417 = 486Kg/hr of urea (reaction 4)

- So, urea produced by reaction (2) = 40834 + 486 = 41320 Kg/hr

- 41320 Kg/hr of urea produced by = (34/60)*41320 = 23414 Kg/hr NH3

- Similarly, CO2 reacted in reaction (1) = (44/60)*41320 = 30301 Kg/hr

- Assuming 95 % conversion we get

- NH3 actually required = 23414/0.95 = 24646 Kg/hr

- CO2 actually required = 30301/0.95 = 31896Kg/hr

71

Now, considering reaction (4) :

If reaction (3) is 100 % complete then,

- Urea produced = (60/44)*31896 = 43495 Kg/hr

But, for 95 % conversion

- Urea produced = 0.95*43495 = 41320 Kg/hr

Therefore, Urea converted to Biuret and NH3 = 41320 – 40834 = 486 Kg/hr

So, from reaction (4)

- Biuret produced = (103/120)*487 = 417 Kg/hr

- Water produced in reaction (2) = (18/60)*41319 =12396 Kg/hr

At reactor’s exit (Urea = 34 %)

- Flow rate of stream = 40834/0.34 = 120100 Kg/hr

- NH3 reacted in reaction (1) = (34/60)*41320 = 23415 Kg/hr

- NH3 produced in reaction (4) = (17/120)*487 = 69 Kg/hr

- So, NH3 unreacted = 24646 – 23414 +69 = 1301Kg/hr

- CO2 reacted in reaction (1) = (44/60)*41320 = 30301 Kg/hr

- Therefore, CO2 unreacted = 31896 – 30301 = 1595 Kg/hr

Now,

Flow rate of stream at reactor’s exit (flow rate of urea+CO2+NH3+water+biuret)

= Flow rate of carbamate

- 120100- (40834 + 1595 + 1301 +12396 +417) = 63557 Kg/hr

72

4.1.1reactor

Fig 4.1.1 .1Flow of material across reactor

Table 4.1.1Flow of material across reactor

Input Output

Material Flow Rate (Kg/hr) % Material Flow Rate (Kg/hr) Bottom Product %

Feed (liq) 3NH 24646 43.59 Unreacted

(liq) 3NH 1301 1.08

(gas) 2CO 31896 56.41 (gas) 2CO 1595 1.32

TOTAL 56542 100 Recycle

Carbamate 63557

Urea 40834 34

Water 12396 10.32

Biuret 417 0.36

Carbamate 63557 52.92

Total 120099 100 120099 100

Carbamate =63557kg/hr

= 24646 Kg/hr 3 NH

= 31896 2 CO

=1301 kg/hr 3 NH

=1595 Kg/hr 2CO

Urea = 40834 Kg/hr [34%]

Carbamate = 17651 Kg/hr

Water = 12396 Kg/hr

Biuret = 417 kg/hr

RE

AC

TO

R

73

4.1.2 Stripper

Fig 4.1.2 Flow of material across stripper

Since, no reaction takes place in the stripper and only carbamate gets recycled back to

the reactor. Therefore, the amount of ammonia, carbon-dioxide ,water and biuret in the outlet

stream of stripper will be same as it was in the inlet stream.

Table 4.1.2 Flow of material across stripper

Input Output

Material Flow Rate (Kg/hr) % Material Flow Rate (Kg/hr) Bottom Product %

NH3 1301 1.08 NH3 1301 2.30

CO2 1595 1.32 CH2 1595 2.82

Carbamate 63557 52.92 Urea 40834 72.22

Urea 40834 34 Water 12396 21.9

Water 12396 10.32 Biuret 417 0.76

Biuret 417 0.36 Total 56543 100

Top Product Ammonium Carbamate

63557 100

Total 120099 100

Urea = 40834 Kg/hr [34%] Carbamate = 63557 Kg/hr

Water = 12396 Kg/hr Biuret = 417 kg/hr

= 1301 kg/hr 3NHCO 2 = 1595 Kg/hr

= 1301Kg/hr 3NH = 1595 Kg/hr 2CO

Carbamate = 63557 kg/hr

Urea = 40834 kg/hr [72.22%] Water = 12396 Kg/hr

Biuret = 417kg/hr St

ripp

er

74

4.1.3 Medium Pressure Separator

Fig 4.1.3 Flow of material across medium pressure separator

The amount of ammonia ,carbon-

dioxide ,water and biuret will remain constant as no

reaction is taking place.

50 % of ammonia and carbon

dioxide are assumed to escape from the top of the separator

and rest goes with the bottom product. Amount of water and biuret remains constant as

no reaction takes place.

Table 4.1.3 Flow of material across medium pressure separator

Input Output

Material Flow Rate (Kg/hr) % Material Flow Rate (Kg/hr) Bottom Product %

3NH 1301 2.3 3NH 651 1.18

2CO 1595 2.82 2CO 798 1.44

Urea 40834 72.22 Urea 40834 74.11

Water 12396 21.9 Water 12396 22.49

Biuret 417 Biuret 417 0.78

Total 55096 100

TOTAL LOSSES

3NH 651 44.91

2CO 798 55.09

Total 56543 100 1449 100

NH 3 = 651Kg/hr CO 2 = 798 Kg/hr Urea = 40834Kg/hr Water = 12396 Kg/hr Biuret = 417 kg/hr

= 1301 Kg/hr 3NH595 Kg/hr= 1 2CO

Urea=40834kg/hr [72.22%] Water = 12394 Kg/hr Biuret = 417 Kg/hr

= 651Kg/hr 3NH= 798 Kg/hr 2CO

M P

S

75

4.1.4 Low Pressure Separator

Fig 4.1.4 Flow of material across low pressure separator

Remaining ammonia and carbon dioxide are assumed to escape from the top.

Table 4.1.4 Flow of material across low pressure separator

Input Output

Material Flow Rate (Kg/hr) % Material Flow Rate (Kg/hr) %

3NH 651 1.44

2CO 798 74.11

Urea 40834 22.49 Urea 40834 76.11

Water 12396 0.78 Water 12396 23

Biuret 417 Biuret 417 0.79

Total 53647 100

TOTAL LOSSES

3NH 651 44.91

2CO 798 55.09

Total 55096 100 1449 100

NH3 = 651 Kg/hr CO2 = 798 Kg/hr Urea = 40834 Kg/hr Water = 12396 Kg/h Biuret = 417Kg/hr

NH3 = 651 Kg/hr CO2 = 798 Kg/hr

Urea = 40834Kg/hr [76.11%] Water = 12396 Kg/hr Biuret = 417 kg/hr

L P

S

76

4.1.5 Vacuum Evaporator

[76.11%] [97.88%]

Fig 4.1.5 Flow of material across vacuum evaporator

Let x and y be the mass fractions of Urea in feed (F) and product (P) resp.

x= 0.7611 (76.11 %)

y= 0.9788 (97.88 % )

Making urea balance:

F.x = P.y

53647*0.7611 = P*0.9788

P = 41715 Kg/hr

Overall material balance gives:

F=P+E

53647 = 41715 + E

E = 11932 Kg/hr

Urea = 40834 Kg/hr Water = 12396 Kg/hr

Biuret = 417 Kg/hr

Water = 11931 Kg/hr

Urea = 40834 Kg/hr Water= 465 Kg/hr

Biuret = 417 Kg/hr

EV

APO

RA

TO

R

77

Table 4.1.5 Flow of material across vacuum evaporator

Input Output

Material Flow Rate (Kg/hr) % Material Flow Rate (Kg/hr) %

Urea 40834 76.11 Urea 40834 97.88

Water 12396 23.10 Water 12396 1.11

Biuret 417 0.79 Biuret 417 1.01

Total 41716 100

LOSSES

Water 11931 100

Total 53647 100

78

4.1.6 Prilling Tower

Fig 4.1.6 Flow of material across Prilling tower

Air + 50 Kg/hr Of Water 26 CO

Urea = 40834Kg/hr [97.88%]

Water = 465 Kg/hr Biuret = 417 Kg/hr

Urae = 40834 Kg/hr [97.96 %] Water = 415 Kg/hr Biuret = 417Kg/hr

Air = 25c

79

Let x and y be the mass fractions of Urea in feed (F) and product (P) resp.

x= 0.9788 (97.88 %)

y= 0.9796 (97.96 %)

Making urea balance:

F.x = P.y

41715*0.9788 = P*0.9796

P = 41681Kg/hr

Table 4.1.6 Flow of material across Prilling tower

Input Output

Material Flow Rate (Kg/hr) % Material Flow Rate (Kg/hr) %

Urea 40834 97.88 Urea 40834 97.96

Water 12396 1.11 Water 12396 1.11

Biuret 417 1.01 Biuret 417 0.93

Total 53647 100

LOSSES

Water 50 100

Total 53647 100

80

4.2 Energy Balance

Assumption : Datum temperature = 0oC 4.2.1 Reactor

Fig 4.2.1 Energy flow across reactor Inlet Stream

Material specific heat at 40oC

NH3 0.53 cal/gm oC = 2.219 Kj/KgoC

CO2 0.22 cal/gm oC = 0.9211 Kj/Kg oC

specific heat at 180oC

Carbamate 0.62 cal/gm oC = 2.596 Kj/Kg oC

Heat input

NH3 : 2.4646 x 104 x 2.219 x 40 = 0.218 x 107 Kj/hr

CO2 : 3.1896 x 104 x 0.9211 x 40=0.117x 107 Kj/hr

Carbamate: 6.3557 x 104 x 2.596 x 180= 2.96 x 107 Kj/hr

Heat input = 3.295 x 107 Kj/hr

HR = - 31.32 Kcal/gm mol

= -0.013 x 107 Kj/Kmol of Urea formed

Amount of urea formed during the reaction = 1020.83 Kmol/hr

= 31896 kg/hr 2CO Reactor C0180

Urea = 40834 kg/hr [34%] Carbamate = 63557 kg/hr Water = 12396 kg/hr Biuret = 417 kg/hr

= 1301 kg/hr 3NH= 1595 2CO

kg/hr

= 24646 kg/hr 3NH

Carbamate = 63557 kg/hr

81

HR = 1020.83 x 0.013 x 107 Kj/hr

= 13.27 x 107 Kj/hr

OUTLET STREAM

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

(Kmol/hr)

NH3 0.55 cal/gm oC = 39.15 Kj/KmoloC 0.033 114.76

CO2 0.23 cal/gm oC = 42.37 Kj/Kmol oC 0.0158 54.36

Carbamate 0.62 cal/gm oC = 202.49 Kj/KmoloC 0.354 1222.3

Urea 0.4828 cal/gm oC=121.32 Kj/KmoloC 0.296 1020.83 o o

Biuret 183.8 Kj/KmoloC 0.002 6.07

Total = 3,451.3 Cp of mixture = ∑ xiCpi So, Cp= 0.033 x 39.15 + 0.0158 x 42.37 + 0.296 x 121.32 + 0.354 x 202.49 + 0.002 x

183.8 + 0.299 x 75.37 = 132.46 Kj/KmoloC So, heat carried by outlet stream = mCp t

= 3,451.3 x 132.46 x 180

= 8.229 x 107 Kj/hr

Heat input + HR - Heat output = rate of accumulation

3.295 x 107 + 13.27 x 107 -8.229 x 107 = rate of accumulation

rate of accumulation = 8.336 x 107 Kj/hr

Assumption : Cooling water at 25oC is used to remove heat from the reactor. The outlet is

steam at an absolute pressure of 4.5 bar (Ts = 147.9 oC).

So, heat gained by cooling water = 8.336x 107 Kj/hr

mCp t + mλ = 8.336 x 107 Kj/hr

or, m (Cp t + λ ) = 8.336x 107

m [ 4.187 x (147.9-25) + 2120.6 ] = 8.336 x 107 Kj/hr

( Here λ = 2120.6 kj/kg & =4.187 Kj/kg oC)

Water 1 cal/gm C = 75.37 Kj/Kmol C 0.299 1032.94

82

/2635.18 810 = m

m = 37,948 Kg/hr

4.2.2 Stripper

Fig 4.2.2 Energy flow across stripper

Total heat input = 8.229 x 107 Kj/hr Outlet Stream 1) Liquid Material specific heat at [ 185oC] mol fractions (x) Flow rate

(Kmol/hr)

NH3 0.58 cal/gm oC = 41.31 Kj/KmoloC 0.05 114.76 o o

Urea 0.5385 cal/gm oC=135.3 Kj/KmoloC 0.46 1020.83 Water 1 cal/gm oC = 75.37 Kj/KmoloC 0.463 1032.94

Biuret 183.8 Kj/KmoloC 0.003 6.07

Total = 2,228.96

Cp of mixture = ∑ xiCpi So, Cp= 0.05 x 41.31 + 0.024 x 44.22 + 0.46 x 135.3 + 0.003 x 183.8 + 0.463 x 75.37 =

100.81 Kj/KmoloC So, heat carried by outlet stream = mCp t

STRIPPER CO185

Urea = 40834 kg/hr [72.22%] Water = 12396 kg/hr

Biuret = 417 kg/hr

= 1301 kg/hr 3NH = 1595 kg/hr 2CO

= 1301 kg/hr 3NH = 1595 kg/hr 2CO

Urea = 40834 kg/hr [34%] Carbamate = 63557 kg/hr Water = 12396 kg/hr Biuret = 417 kg/Hr

Carbamate Vapours 63557 Kg/Hr

CO2 0.24 cal/gm C = 44.22 Kj/Kmol C 0.024 54.36

83

= 2228.96 x 100.81 x 185

= 4.157 x 107 Kj/hr

2) Vapour stream : Ammonium carbamate Material specific heat at [ 185oC] Flow rate (Kmol/hr)

Carbamate 0.62 cal/gm oC = 202.49 Kj/KmoloC 1222.3 For carbamate λ = 210 Kj/Kg

So, heat carried by carbamate = m Cp t + mλ = 1222.3 x 202.49 x 185 + 63557 x 210 =5.913 x 107 Kj/hr

Here, steam at 24 atm is used (Ts = 221.8 oC). λ of steam =1855.3 Kj/kg

Heat supplied by steam = Heat output – Heat input

= (6.581 + 4.157 – 8.229) x 107 Kj/hr

m λ = 1.841 x 107 Kj/hr

m = 1.841 x 107 /1855.3

m = 9.755 Kg/hr

4.2.3 Carbamate Condenser

Fig 4.2.3 Energy flow across carbamate condenser

Energy balance

mv λv = ms Cp ( Ts-25) + ms λs

Putting the values we get :

63557 x 210 = ms [4.187 x (147.9 – 25) + 2120.6] [ where λs =2120.6 kj/kg] So, ms = 7,597.5 kg/h

Carbamate Condenser

Steam c OTs = 147.9

4.5 bar

C O25 Water

Carbamate Vapour

(Liquid) Carbamate 63557

Kg/Hr

84

4.2.4 Medium Pressure Separator

Fig 4.2.4 Energy flow across medium pressure separator

Heat input = 4.157 x 107 Kj/hr 1) Liquid Outlet Stream Material specific heat at [ 140oC] mol fractions (x Flow rate (Kmol/hr)

NH3 0.54 cal/gm oC = 38.4 Kj/KmoloC 0.027 57.4

CO2 0.23 cal/gm oC = 42.37 Kj/Kmol oC 0.0127 27.182

Urea 0.493 cal/gm oC=123.84 Kj/KmoloC 0.476 1020.83

Water 1 cal/gm oC = 75.37 Kj/KmoloC 0.4815 1032.94

Biuret 170.92 Kj/KmoloC 0.0028 6.07 Total = 2144.42

Cp of mixture = ∑ xiCpi

So, Cp= 0.027 x 38.4 + 0.0127 x 42.37 + 0.476 x 123.84 + 0.0028 x 170.92 + 0.4815 x

75.37 Kj/KmoloC = 97.29 Kj/KmoloC

heat output = 2144.42 x 97.29 x 140 Kj/hr

= 2.921x 107 Kj/hr

2) For gases escaping from the top

M

P S

NH3 = 651 kg/hr CO2 = 798 kg/hr Urea = 40834 kg/hr [74.12%] Water = 12396 kg/hr Biuret = 417 kg/hr 140c

Urea = 40834 kg/hr [72.22%] Water = 12396 kg/hr Biuret = 417 kg/hr

r= 1301 kg/h 3NH = 1595 kg/hr 2CO

= 651 kg/hr 3NH= 798 kg/hr 2CO

CO185

M P S

85

Material λ at 140oC mol fractions (x) Flow rate

(Kmol/hr)

NH3 320 cal/gm oC = 22.777 x 103 Kj/KmoloC 0.6785 57.35

CO2 110 cal/gm oC = 20.265 x 103 Kj/Kmol oC 0.3215 27.182

Total = 84.53 λ of mixture = ∑ xiλi

So, λ = (0.6785 x 22.777 + 0.321 x 20.265) x 103 Kj/KmoloC

= 21.969 x 103 Kj/Kmol

Material specific heat at [ 140oC] mol fractions (x) Flow rate

(Kmol/hr)

NH3 0.54 cal/gm oC = 38.4 Kj/KmoloC 0.6785 57.35 o o

Total = 84.53

Cp of mixture = ∑ xiCpi

So, Cp = 0.6785 x 38.4 + 0.321 x 42.37 Kj/KmoloC

= 39.676 Kj/KmoloC

Heat escaping from the top = m ( Cp t + λ )

= 84.53( 39.676 x 140 + 21.969 x 103 )

= 0.2327 x 107 Kj/hr

Assumption : Cooling water enters at 25oC & leaves at 50oC.

So , heat gained by cooling water = Heat input – heat output

= ( 4.157– 2.921 – 0.2327) x 107 Kj/hr

mCp t = 1.00 x 107 Kj/hr

m = 1.00 x 107 / ( 4.187 x 25)

m = 95,533.8 Kg/hr

4.2.5 Low Pressure Separator

CO2 0.23 cal/gm C = 42.37 Kj/Kmol C 0.3215 27.182

= 651 kg/hr 3NH = 798 kg/hr 2CO

86

Fig 4.2.5 Energy flow across low pressure separator Heat input = 2.921 x 107 Kj/hr

1) Liquid Outlet Stream

Material specific heat at [ 80oC] mol fractions (x) Flow rate

(Kmol/hr)

Urea 0.429 cal/gm oC=107.76 Kj/KmoloC 0.496 1020.83

Water 1 cal/gm oC = 75.37 Kj/KmoloC 0.5 1032.94

Biuret 149 Kj/KmoloC 0.004 6.07

Total = 2059.8 Cp of mixture = ∑ xiCpi

So, Cp= 0.496 x 107.76 + 0.5 x 75.37 + 0.476 x 123.84 + 0.004 x 149 Kj/KmoloC

= 91.73 Kj/KmoloC

heat output = 2059.8 x 91.73 x 80

= 1.51 x 107 Kj/hr

2) For gases escaping from the top Material λ at 140oC mol fractions (x) Flow rate

(Kmol/hr)

NH3 260 cal/gm oC = 18.51 x 103 Kj/KmoloC 0.679 57.41

CO2 85 cal/gm oC = 15.66 x 103 Kj/Kmol oC 0.321 27.182

L

P S

87

Total = 84.59

λ of mixture = ∑ xiλi

So, λ = (0.679 x 18.51 + 0.321 x 15.66) x 103 Kj/KmoloC

= 17.6 x 103 Kj/Kmol Material specific heat at [ 140oC] mol fractions (x) Flow rate

(Kmol/hr)

NH3 0.52 cal/gm oC = 37.013 Kj/KmoloC 0.027 57.41

CO2 0.21 cal/gm oC = 38.69 Kj/Kmol oC 0.0127 27.182

Total = 84.59 Cp of mixture = ∑ xiCpi

So, Cp = 0.679 x 37.013 + 0.321 x 38.69 Kj/KmoloC

= 37.55 Kj/KmoloC

Heat escaping from the top = m ( Cp t + λ )

= 84.59 ( 37.55 x 80 + 17.7 x 103)

= 0.1743 x 107 Kj/hr

Assumption : Cooling water enters at 25oC & leaves at 50oC.

So , heat gained by cooling water = Heat input – heat output

= ( 2.921– 1.51 – 0.1743) x 107 Kj/hr

mCp t = 1.2367 x 107 Kj/hr

m = 1.2367 x 107 / ( 4.187 x 25)

m = 1,18,146 Kg/hr

4.2.6 Evaporator

E2 = 5.300 kg/hr E1 = 12.593 kg/hr

C O80

88

Fig 4.2.6 Energy flow across evaporator

For product stream coming out of 1st evaporator:

Material specific heat at [ 85oC] mol fractions (x) Flow rate (Kmol/hr)

o o

Water 1 cal/gm oC = 75.37 Kj/KmoloC 0.245 333.33 Biuret 149 Kj/KmoloC 0.005 6.06

Total = 1360.223 Cp of mixture = ∑ xiCpi

So, Cp = 0.75 x 109.28 + 0.245 x 75.37 + 0.005 x 149 Kj/KmoloC

= 101.17 Kj/KmoloC

mCp t = 1360.223 x 101.17 x 85

= 1.17 x 107 Kj/hr

Heat balance

1st evaporator :

Urea 0.435cal/gm C=109.28 Kj/Kmol C 0.75 1020.83

89

Heat input (feed) + Heat input by steam = heat carried by water vapour + energy of the

bottom product

Heat input (feed) + S1 λs1 = E1HE1 + energy of the bottom product

1.537 x107 + S1 x 2123.2 = 12,593 x 2614.97 + 1.17 x 107

S1 = 13,781 Kg/hr 2nd evaporator :

Heat input (feed) + Heat input by steam = heat carried by water vapour + energy of the

bottom product

Heat input (feed) + S2 λs2 = E2HE2 + energy of the bottom product

1.17 x 107 + S2 x 2123.2 = 5,300 x 2545.7 + 1065.8 x 96.84 x 27

S2 = 2,157 Kg/hr

90

4.2.7 Prilling Tower

Fig 4.2.7 Energy balance across prilling tower

Heat input = 1065.8 x 96.84 x 27 = 0.279 x 107 Kj/hr Outlet Stream

Material specific heat at [ 30oC] mol fractions (x) Flow rate (Kmol/hr) Urea 0.3758 cal/gm oC=94.41 Kj/KmoloC 0.96 1020.83

Water 1 cal/gm oC = 75.37 Kj/KmoloC 0.034 36.11

Biuret 133.02 Kj/KmoloC 0.006 6.07

Total = 1063.01

AIR 26 OC

AIR CO25

Urea = 40834 kg/hr [97.88%]

Water = 465 kg/hr Biuret = 417 kg/hr

Water vapour = 50 kg/hr

CO27

Urea = 40834 kg/hr [97.96%] Water = 415 kg/hr Biuret = 417 kg/hr Co25

91

Cp of mixture = ∑ xiCpi

So, Cp = 0.96 x 94.41 + 0.034 x 75.37 + 0.006 x 133.02 Kj/KmoloC

= 93.99 Kj/KmoloC Heat output = 1063.01 x 93.99 x 25

= 0.250 x 107 Kj/hr

Assuming, humidity of air at 25oC = 0.01

Heat carried away by air = heat input – heat output

(mCp t)dry air = ( 0.279 – 0.250) x 107

m = .029 x 107 /(1.009 x 1)

m = 28,74,133 Kg/hr So,flow rate of air =28,74,133 Kg/h

92

4.3Equipment Design

4.3.1 Reactor Design

Fig 4.3.1 Urea reactor From fig-6.2, for NH3 to CO2 ratio of 2 corresponding yield of urea is 50 %.From fig- 6. 3,for 50 % yield of urea the residence time is 40 min.

t = 40 min

154

Carbamate = 63,557 KgQ

= 246463NH

= 318962CO

Urea =40,834

Carbamate = 63,557

Water = 12,396

Biuret = 417

1301= 3NH

=1595 2CO

SDFVSDVD

93

2Vs CO 3Fig 4.3.2 Graph of % urea yield Vs molar ratio of NH

Fig 4.3.3 Graph of % urea yield Vs residence time.

94

Now, t = V/F ( Ref : Chemical reactor design-Peter Harriott, Pg-90)

Where, t = residence time

F = Volumetric flow rate into the reactor in m3/hr.

V = Volume of the reactor in m3.

Now,

Density of liquid NH3 = 618 Kg/ m3 ( Ref: J H Perry)

Density of CO2 gas at 40 oC = 277.38 Kg/ m3 (density=PM/RT; P=162 atm,T=313 K)

Density of Carbamate = 1600 Kg/ m 3 (Ref: http://www.inorganics.basf.com)

So, NH3 flowing into the reactor = 36,968/618 = 59.82 m3 /hr

CO2 flowing into the reactor = 47,842/277.38 = 172.478 m3 /hr

Carbamate flowing into the reactor = 95,337/1600 = 59.59 m3 /hr

Total flow rate into the reactor = 59.82 + 172.478 + 59.59

= 291.89 m3 /hr

Since, t = V/F

Therefore, V=txF

= (40 x 291.89)/60

V = 194.59 m3

For design purpose V = 195 m3

Now, volume of the reactor = (Л D2/4)L = 195 [D = 2.5 m (given)]

(Ref : Equipment Design-Brownell and Young;Pg-80)

or, L = 195 x 4/(3.14 x 2.52)

= 39.75 m

or, L = 40 m

4.3.2 Thickness Of Shell

Data available :

Temperature inside the reactor = 180 oC

Pressure inside the reactor = 154 atm

Material of construction :

Low alloy carbon steel (Ref : Fertilizer manufacture- M E Pozin)

95

Material specification :

IS : 2002-1962 Grade 2B (Ref: B C Bhattacharya, Table-A1,Pg-261)

Allowable stress = 1.18 x 108 N/m2

Diameter of the reactor = 2.5 m

(Ref : Fertilizer manufacture- M E Pozin,Pg-263;for plants having capacity of 4,50,000

tons/yr)

Now, volume of the reactor = (Л D2/4)L = 195

(Ref : Equipment Design-Brownell and Young;Pg-80)

or, L = 195 x 4/(3.14 x 2.52)

= 39.75 m

or, L = 40 m

Also,L/D = 40/2.5 = 16 which is consistent with the actual ratio which is between 14 to

20.

Now,

t = pDi/(2fj – p) [Ref : Equipment design- M V Joshi,Pg-96 ]

where, t = thickness of the shell

Di = internal diameter

J = joint efficiency

p = design pressure

f = permissible stress

internal pressure = 154 atm = 1.56 x 107 N/m2

Design pressure p = (10 % extra)

= 1.1 x 1.56 x 107 N/m2

= 1.716 x 107 N/ m2

J=1 [ For class 1 pressure vessels , BIS-2825]

f = 1.18

Di = 2.5 m

So, t = 1.716 x 107 x 2.5/(2 x 1.18 x 108 x 1 – 1.716 x 107 )

t = 0.196 m

= 196 mm

or, t = 200 mm

96

4.3.3 Head Design

For 2 : 1 ellipsoidal dished head

th = pDV/2fJ [ ref : Equipment design- M V Joshi,Pg-106,Eq-5.24]

where, p = internal design pressure

D = major axis of ellipse

V = stress intensification factor = ( 2 + k2)/4

k = major axis/minor axis

So, th = 1.716 x107 x 2.5 x 1.5 /(2 x 1.18 x108 x 1)

t = 0.273 m

or, t = 273 mm

or, t = 300 mm

4.3.4 Diameter Of Pipes

We know that,

(Di)opt = 0.0144 x (m`)0.45/(ρ)0.32

For inlet pipes:

(Di)NH3 = 0.0144 x (36968)0.45/(618)0.32

= 0.2093 m

= 8.24 inch

Standardizing using Table-11,PHT,D Q Kern we get:

NPS = 10

Schedule no. = 60

OD = 10.75 inch

ID = 9.75 inch

(Di)CO2 = 0.0144 x (47842)0.45/(277.38)0.32

= 0.3037 m

= 11.95 inch

Standardizing using Table-11,PHT,D Q Kern we get:

NPS = 12

Schedule no. = 30

OD = 12.75 inch

ID = 12.09 inch

97

(Di)carbamate = 0.0144 x (95337)0.45/(1600)0.32

= 0.2364 m

= 9.307 inch

Standardizing using Table-11,PHT,D Q Kern we get:

NPS = 10

Schedule no. = 60

OD = 10.75 inch

ID = 9.75 inch

(Di)outlet stream = 0.0144 x (1,80,147)0.45/(1283.97)0.32

= 0.3378 m

= 13.29 inch

Standardizing using Table-11,PHT,D Q Kern we get:

NPS = 16

Schedule no. = 30

OD = 16 inch

ID = 15.25 inch

4.3.5 Skirt Support For Reactor

Wt. of the reactor = wt. of material of costruction + weight of the contents of the reactor

= ПDtLρ + weight of the contents of the reactor

= П x 2.5 x 0.2 x 44 x 7857 + 1,80,147 [ρ = 7857 kg/m3]

W = 722 tons

= 7.085 x 107 N

Material of construction :

IS : 2002-1962 Grade 2B (Ref: B C Bhattacharya, Table-A1,Pg-261)

Allowable tensile stress = 1.18 x 108 N/m2

Yield stress = 2.55 x 108 N/m2

Wind pressure upto = 1300 N/m2

Stress due to dead weight:

fd = ∑W/( П Dok tsk)

98

where,

fd = Stress

∑W = Dead wt of vessel

Dok = Outside diameter of the skirt

tsk = thickness of skirt

fd = 7.085 x 107/(3.14 x 2.5 x tsk)

= 9.025 x 106/tsk N/m2

Assuming height of skirt = 5 m

fwb = Mw/Z = 4Mw /( П D2ok tsk) [ Ref: equation 13.22,M V joshi]

Mw = Plw (h1/2) + Puw ( h1 + h2/2)

Plw = kP1h1Do [ where k = 0.7 ]

Puw =kP2h2Do

fwb = 0.7 x 1300 x 20 x 2.5 x (20/2) x 4/( 3.14 x 2.52 x tsk ) +

0.7 x 1300 x 20 x 2.5 x 30 x 4 / ( 3.14 x 2.52 x tsk )

fwb = 3.709 x 105 / tsk N/m2

Stress due to seismic load :

fsb = (2/3) x CWH/ (П Rok tsk) [ here C = 0.08 ]

fsb = (2/3) x (0.08 x 7.085 x 107x 40)/( ( 3.14 x (2.5/2) x tsk )

fsb = 3.85 x 107 / tsk N/m2

Maximum tensile stress = fsb -- fd

= 3.85 x 107/tsk -- 0.9025 x 107/tsk

(ft) Max = 2.9475 x 107/tsk

Now,permissible tensile stress = 1.18 x 108 N/m2

tsk = 2.9475 x 107/ ( 1.18 x 108)

= 0.2498 mm

Maximum compressive stress : [ Ref : Equation 13.29,Pg-326,M V Joshi]

(fc) Max = 3.85 x 107/tsk + 0.9025 x 107/tsk

(fc) Max = 4.7525 x 107/tsk

(fc) Permissible ≤ ⅓ Y.P

99

≤ ⅓ x 2.55 x 108 N/m2

or, tsk = 4.7525 x 107/ (0.85 x 108)

tsk = 559.1 mm

So, thickness to be used = 600 mm

Fig 4.3.4 Urea evaporator (climbing film long tube vertical evaporator)

Vapour space pressure = 0.23 atm

Vapour space temperature = 63.1 oC

BPR = 21.9 oC

[Ref : Kirk Othmer,Encyclopedia of chemical technology,Vol-21]

Boiling point of liquid = 85 oC

100

For product stream coming out of 1st evaporator:

Material C] mol fractions (x) Flow rateospecific heat at [ 85 (Kmol/hr)

Urea CoKj/Kmol C=109.28o 0.435cal/gm 0.75 1020.83

Water Co cal/gm 1 CoKj/Kmol 75.37 = 0.245 333.33

Biuret CoKj/Kmol 149 0.005 6.06

Total = 1360.223

Cp of mixture = ∑ xiCpi

So, Cp = 0.75 x 109.28 + 0.245 x 75.37 + 0.005 x 149 Kj/KmoloC

= 101.17 Kj/KmoloC

mCp t = 1360.223 x 101.17 x 85

= 1.17 x 107 Kj/hr

Heat balance

1st evaporator :

Heat input (feed) + Heat input by steam = heat carried by water vapour + energy of the

bottom product

Heat input (feed) + S1 λs1 = E1HE1 + energy of the bottom product

For steam at 147.165 oC, λs1 = 2123.2 kj/kg

Putting the values we get

1.537 x107 + S1 x 2123.2 = 12,593 x 2614.97 + 1.17 x 107

S1 = 13,781 Kg/hr

Economy = 12,593/13781 = 0.914

Now,

U1 value is obtained from fig-6.2.2.At 63.1oC ( 145.58oF) the value of U1 is 270

Btu/hr.sq.ft.oF. Multiplying this value by 5.6783 gives the value of U1 in W/m2K.

A1 = S1 λs1 / U1 T1

T1 = ( T)app – BPR1

= 147.165 – 63.1 – 21.9

= 62.165 oC

So, A1 = 13,781 x 2123.2 / 1533 x 62.165

= 307.03 m2

101

similarly,

A2 = S2 λs2 / U2 T2

T2 = ( T)app – BPR2

= 147.165 – 23.77– 3.48

= 119.915 oC

So, A2 = 2157 x 2123.2 / 738 x 119.915

= 51.75 m2

( Ref: values of U1 and U2 from Perry’s handbook,10-35)

Fig 4.3.5 Graph to find out heat transfer co-efficient

4.3.6 Design :

Assuming :

Length = 6 m [ Ref: M V Joshi,Pg-220]

Tube OD = 1 inch [ Table-10,PHT,D Q Kern]

Tube ID = 0.834 inch [14 BWG]

Minimum pitch = 1.25 x OD

= 1.25 x 25 = 31.25 mm [ Ref: M V Joshi,Pg-220]

Let pitch = 32 mm square pitch

Area = 307.03 m2

No. of tubes (N) :

307.03 = П x 0.025 x 6 x N

102

N = 651.8

or, N = 652

Let OTL = D

So, (П /4) x D2 = 652 x (0.032)2

D = 922 cm

Now, Ddi = OTL + 2C

= 0.922 + 2 x 0.075

Ddi = 1.072 m

Standardizing D using Table-B4,Pg271,B C Bhattacharya

Ddi = 1100 mm

4.3.7 Wall Thickness Calculation

Material of construction : Mild steel

Specification : IS 2002-1962 Grade-1 [Ref : B C bhattacharya,Pg-261,Appendix-A]

fall = 0.93 x 108 N/m2

C = 0 mm

J = 0.85

t = pdDi/(2fj – p) + C [Ref : Equipment design- M V Joshi,Pg-96 ]

where, t = thickness of the shell

Di = internal diameter

J = joint efficiency Hence, this thickness is also not acceptable.

Again, taking tstd = 7 mm [Ref: B C Bhattacharya, Pg-269]

We get,

Pc = 3.0387 Kg/cm2

Pall = 3.0387 /4 = 0.75 Kg/cm2

Which is less than 1 Kg/cm2

Hence, this thickness is also not acceptable.

Again, taking tstd = 8 mm [Ref: B C Bhattacharya, Pg-269]

We get,

Pc = 4.234 Kg/cm2

Pall = 4.234 /4 = 1.05 Kg/cm2

Which is greater than 1 Kg/cm2

103

Hence, this thickness is acceptable.

So, tmin = 8 mm

4.3.8 Separator [10]

Top head (Elliptical head)

For 2 : 1 ellipsoidal dished head

Di = 1 m [ Ref: Table B-4,Pg-271,B C Bhattachrya]

L =4m [ Ref: Table B-2,Pg-269,B C Bhattachrya]

th = pDV/2fJ [ ref : Equipment design- M V Joshi,Pg-106,Eq-5.24]

where, p = internal design pressure

D = major axis of ellipse Graph to find out heat transfer co-efficient

V = stress intensification factor = ( 2 + k2)/4

k = major axis/minor axis

p = 0.23 ata = 0.226 x 105 N/m2

j = 0.85

Di = 1.6 m

k =2

V = 1.5

For internal pressure :

th = (0.226 x 105 x 1.5 x 1) / (2 x 0.93 x108 x 0.85)

= 2.144 x 10-4 m

= 0.214 mm

For external pressure :

Pext = 1 Kg/cm2

Corresponding internal pressure to be used to calculate th = 1.67 x Pext

So, Pint = 1.67 Kg/cm2

So,

th = 4.4 x Rc[3 x (1 – µ2)]1/2 x (p/2E)1/2 [ Ref : M V Joshi ,eqn- 5.26,Pg-107]

where, p = Design external pressure

Rc = Crown radius for torispherical and hemispherical heads and

equivalent crown radius for elliptical head.

E = modulus of elasticity

104

µ = Poisson’s ratio

Putting the values we get

th = 0.0061 mm

So, tstd = 5 mm [ Ref: B C Bhattacharya,Pg-269]

4.3.9 Bottom Head Design

Assuming an apex angle of 90o For , conical head D=1m th = pDV/2fJCos α [ Ref : M V Joshi,Pg-106]

Here, α = Half the apex angle

For, internal pressure :

p = 0.226 x 105

th = (0.226x 105 x 1.5 x 1) / (2 x 0.93 x108 x 0.85x 0.707)

= 0.3032 mm

For, external pressure :

p = 1.67 x Pext

= 1.67 x 1 kg/cm2

So,

th = (1.67x 105 x 1.5 x 1) / (2 x 0.93 x108 x 0.85x 0.707)

= 2.241 mm

tstd = 5 mm [Ref: B C Bhattacharya,Pg-269]

Checking this thickness for critical buckling pressure :

Pc = [2.42E / (1-µ2)3/4 ] x [( t/Do)5/2/ { L/Do – 0.45 x (t/Do)1/2}]

[ Ref : M V Joshi ,eqn- 5.14,Pg-100]

Here, L = D/2

Putting the values we get, Pc = 19.11 kg/cm2 So, Pall = Pc/4

= 19.11 / 4

= 4.778 kg/cm2

Which is greater than 1 kg/cm2

So, this thickness is acceptable

Hence, tmin = 5 m

105

calculation of Cost 4.4.1Cost Estimation

• Costing Methods

• Capital Cost Factors:

– N = number of functional units

– Q = plant capacity, ton per year

• The correlation of Timms, IChemE (1988) gives a simple equation for gas

phase

processes (updated to 1999).

Considering year = 300 days

C = 8000*N*Q0.615

• Procedures:

– In our project there are 6 functional units

– C = 8000 * 6 * (1000*300)0.615

= 112116829.8 US dollar (in 1999)

dollar 175819574 112116829 * 176276

1998Cost * 1999Index Cost 2004Index Cost 2004Cost

==

=

– To update the plant cost to 2005, the rate of inflation for each year

(2%) must be applied

– Cost 2005 = 1758819574 * 1.02 =179335965.5 US Dollars

– Cost 2006 = 179335965.5 * 1.02 = 182922684.8 US Dollars

– Cost 2007 = 182922684.8* 1.02 = 186581388.5US Dollars

– Cost 2008 = 186581138.5* 1.02 = 190312761.3 US Dollars

– Cost 2009 = 190312761.3* 1.02 = 194119016.5 US Dollars

– Cost 2010 = 194119016.5 * 1.02 = 198001396.8US Dollars

– Cost 2011 = 98001396.8 * 1.02 = 201961924.8 US Dollars

– Cost 2012 = 201961924.8* 1.02 = 206000653.2US Dollars

– Cost 2013 = 206000653.2 * 1.02 = 210120666.3US Dollars

106

4.4.2Estimation of total capital investment :

4.4.2.1 I.Direct cost:

A. Equipment, installation, piping etc.

1. Purchased equipment (30% of fixed capital investment)

= 0.3 x 210120666.3 = 63036199.89Rs.

2.Installation, including insulation and painting (30% of purchased equipment)

= 0.3x63036199.89

= 18910859.97Rs.

3.Instrumentation and controls, installed ( 10% of purchased equipment)

= 0.1 x 63036199.89

= 6303619.98Rs.

4.Piping, installed ( 20% of purchased equipment)

= 0.2 x 63036199.89

= 12607239.98Rs.

5.Electrical, installed (15% of purchased equipment)

= 0.15 x63036199.89

= 9455429.98Rs.

B. Buildings ( 20% of purchased equipment cost )

= 0.2 x 63036199.89

= 12607239.98Rs.

C. Service facilities and yard improvements:(60% of purchased equipment)

= 0.6 x 63036199.89

= 37821719.93Rs.

D.Land (5% of purchased equipment)

= 0.05x 63036199.89

=3151809.99Rs.

Direct cost = 163894119.7Rs.

4.3.2.2 II. Indirect cost :-

1.Engineering and supervision ( 10% of direct cost)

= 0.1x163894119.7

= 16389411.9Rs.

2.Construction expense and contractor’s fee(11% of direct cost)

= 0.11x163894119.7

107

= 1802835.31Rs.

3. Contingency (6% of fixed capital investment)

= 0.06 x 210120666.3 = 12607239.98Rs

Indirect cost = 30799487.19Rs.

Total capital investment = fixed capital investment + working capital

Let working capital = 15% of total capital investment

Fixedcapital investment = 210120666.3Rs.

Totalcapital investment = 241638766.2Rs.

4.4.3 Estimation of total product cost:

4.4.3.1 I.Manufacturing cost

A.Fixed charges:

1. Depreciation (10% of fixed capital investment + 2% of building)

= 0.1 x210120666.3 + 0.02 x12607239.98

=21264211.43Rs.

2.Local taxes (3% of fixed capital investment)

= 0.03 x210120666.3

= 6303619.98Rs.

3.Insurance ( 0.8% of fixed capital investment )

= 0.008 x210120666.3

= 16809653.3Rs.

Fixed charges = 44377484.7Rs.

Let fixed charge be 15% of total product cost

Total product cost = 443774844.7/0.15

= 295849898Rs

B.Direct production cost:

1.Raw materials (15% of total product cost)

=0.15 x295849898

= 44377484.7Rs.

2.Operating labor ( 11% of total product cost) = 0.11x295849898 =32544348.79Rs.

3.Direct supervisory and clerical labor (15% of operating labor)

108

=0.15 x32544348.79

= 4881652.31Rs.

4.Utilities (15% of total product cost) = 0.15 x295849898 = 44377484.7Rs.

5.Maintenance and repairs (5% of fixed capital investment)

= 0.05 x210120666.3

= 10506033.32Rs.

6.Operating supplies (15% of maintenance and repairs)

= 0.15 x10506033.32

= 1575904.99Rs

7.Laboratory charges (15% of operating labor)

= 0.15 x32544348.79

= 4881652.31Rs.

8.Patents and royalties (3% of total product cost)

= 0.03 x295849898

= 8875496.94Rs.

C. Plant overhead costs (5% of total product cost)

= 0.05 x295849898

= 14792494.9Rs.

I . Manufacturing cost = Fixed charges + direct production cost + plant overhead cost

= 211190037.7Rs.

4.4.4 II.General Expenses:

A.Administrative costs (5% of total product cost)

= 0.05 x295849898

= 1479249898Rs.

B. Distribution and selling costs (14% of total product cost )

= 0.14 x295849898

= 41418985.72Rs.

C.Research and development costs (5% of total product cost)

= 0.05 x295849898

= 14792494.9Rs.

D.Financing (2% of total capital investment)

=0.02 x241638766.2

109

= 5916997.96Rs.

General expenses76920973.48Rs

Total product cost = manufacturing cost + general expenses

= 288111011.2Rs

Cost of the product =(288111011.2)/ (340000x105)

= 8.47Rs/Kg

With a profit margin of 20% = 1.2 x 3.27

= 10.16Rs/Kg

Gross annual earning = 3.67% of (10.16x340000 x 103)

= 0.0367 x3454400000

(GAE) = 126776480Rs.

Net annual earnings = GAE – Income tax

Income tax = 40% of GAE

Net annual earnings = 50710592Rs.

Payback period = (total capital investment) / (net annual earnings)

= 241638766.250710592

= 5 years

Rate of return =(net annual earnings) / (fixed capital investment)

= 50710592210120666.3

= 24%

110

Chapter five

Control And Instrumentation

and Safety

111

5.1Control And Instrumentation

5.1.1 Instruments :

Instruments are provided to monitor the key process variables during plant operation.

They may be incorporated in automatic control loops, or used for the manual monitoring of

the process operation. Instruments monitoring critical process variables will be fitted with

automatic alarms to alert the operators to critical and hazardous situations. Comprehensive

reviews of process instruments and control equipment are published periodically in the

journal chemical engineering . These reviews give details of all the instruments and control

hardware available commercially, including those for the on-line analysis of stream

compositions.

It is desirable that the process variable to be monitored be measured directly; often,

however, this is impractical and some dependent variable, that is easier to measure, is

monitored in its place. For example, in the control of distillation columns the continuous,

on-line, analysis of the overhead product is desirable but difficult and expensive to achieve

reliably, so temperature is often monitored as an indication of composition. The temperature

instrument may form part of a control loop controlling, say, reflux flow; with the

composition of the overheads checked frequently by sampling and laboratory analysis.

5.1.2 Instrumentation and control objectives :

The primary objectives of the designer when specifying instrumentation and control

schemes are:

i. Safe plant operation:

(a) To keep the process variables within known safe operating limits.

(b) To detect dangerous situations as they develop and to provide alarms and automatic

shut-down systems.

(c) To provide interlocks and alarms to prevent dangerous operating procedures.

ii. Production rate:

To achieve the design product output.

iii. Product quality:

To maintain the product composition within the specified quality standards.

iv. Cost:

To operate at the lowest production cost, commensurate with the other objectives.

These are not separate objectives and must be considered together. The order in which

they are listed is not meant to imply the precedence of any objective over another, other than

112

that of putting safety first. Product quality, production rate and the cost of production will

be dependent on sales requirements. For example, it may be a better strategy to produce a

better-quality product at a higher cost.

In a typical chemical processing plant these objectives are achieved by a combination of

automatic control, manual monitoring and laboratory analysis.

5.1.3 Guide rules :

The following procedure can be used when drawing up preliminary P and I diagrams:

i. Identify and draw in those control loops that are obviously needed for steady plant

operation, such as:

(a) level controls,

(b) flow controls,

(c) pressure controls,

(d) temperature controls.

ii. Identify the key process variables that need to be controlled to achieve the specified

product quality.

iii. Identify and include those additional control loops required for safe operation,

iv. Decide and show those ancillary instruments needed for the monitoring of the plant

operation by the operators.

v. Decide on the location of sample points.

vi. Decide on the need for recorders and the location of the readout points,

5.1.4 Typical control systems:

1. Level control

In any equipment where an interface exists between two phases (e.g. liquid-vapour),

some means of maintaining the interface at the required level must be provided. This may

be incorporated in the design of the equipment, as is usually done for decanters, or by

automatic control of the flow from the equipment. Figure shows a typical arrangement for

the level control at the base of a column. The control valve should be placed on the

discharge line from the pump.

113

LC

2. Pressure control

Pressure control will be necessary for most systems handling vapor or gas. The method

of control will depend on the nature of the process. Typical schemes are shown in Figure

The scheme would not be used where the vented gas was toxic, or valuable. In these

circumstances the vent should be taken to a vent recovery system, such as a scrubber.

3. Flow control

Flow control is usually associated with inventory control in a storage tank or other

equipment. There must be a reservoir to take up the changes in flow-rate.

To provide flow control on a compressor or pump running at a fixed speed and supplying

a near constant volume output, a by-pass control would be used, as shown in .

PC

114

4. Ratio controlRatio control can be used where it is desired to maintain two flows at a

constant ratio; for example, reactor feeds and distillation column reflux. A typical scheme

for ratio control is shown in Figure.

4. Temperature controlThe temperature being controlled by varying the flow of the

cooling or heating medium . A typical scheme for temperature control is shown in

Figure .

FC

FC

115

5.2.4 Industrial safety objective

The following table show some symbols that used in the control systems :

Table 5.1 : Symbols control systems

symbol Nomenclature

TI Temperature indicator

TIRC Temperature indicator

recorder controller

FC Flow controller

FRC Controller flow recorder

FI Flow indicator

PI Pressure indicator

PC Pressure controller

XC Ratio controller

FR Flow recorder

LIR Level indicator recorder

PRC Pressure recorder controller

CI Composition indicator

LA Level alarm

PA Pressure alarm

TC

116

Fig 5.1 Urea reactor control temperature

154

Carbamate = 63,557 kg Q

= 246463NH

= 318962CO

Urea =40,834

Carbamate = 63,557

Water = 12,396

Biuret = 417

NH3 =1232 CO2 =1595

T

SDFVSDVD

117

5.2Safety

5.2.1 Industrial Safety : The portion of internal security which refers to the protection of industrial installations,

resources, utilities, materials, labor , and classified information essential to protection from

loss or damage and to prevent accidents and create a safe environment.

All manufacturing processes are to some extent hazardous, but in chemical processes

there are additional, special, hazards associated with the chemicals used and the process

conditions. The designer must be aware of these hazards, and ensure, through the application

of sound engineering practice, that the risks are reduced to acceptable levels.

The more general, normal, hazards present in all manufacturing process such as, the

dangers from rotating machinery, falls, falling objects, use of machine tools, and of

electrocution will not be considered.And it also includes Protection and maintenance of

equipment from fire,damage ,and explosions.

• Protection of fired equipment (heaters, furnaces) against accidental explosion and fire.

5.2.2Safety factors for industrial plants 1. The safety of the plant can depend on the civil, structural, and architectural design.

Failures of foundations, walls or supporting structures can rupture piping and vessels

and lead to release of hazardous materials. Cracks of walls and wall painting must be

checked regularly and windows must be cleaned to provide a good quantity of

lightening

2. The arrangement of process units and buildings are crucial factors in the safety and

economics of a chemical plant. The plant layout (plot plan) should incorporate safety

while providing access for operations and maintenance.

3. A plant must be located near sources of workers, but not so close that neighbors can be

injured by gas release, fire, or explosion.

4. Plant layout is largely constrained by the need to observe minimum safe separation

distances, interunit spacing between units for oil and chemical plants = 3 M at least for

workers that allows access for fire fighting and fresh air

5. Provide and maintain adequate lighting for employees to see nearby objects that might

be potential hazards or to see to operate emergency controls or other equipment, and to

reduce uncomfortable glare .

118

6. Some form of ventilation should be provided to sweep out traces of dangerous fumes

,the removal of heat , air motion for cooling ,freshening and counteracting discomfort

due to humidity and to clear out an atmosphere which may be deficient in oxygen .

Fans , ejectors , or several windows could be used to provide a good ventilation

system.

7. Excessive noise is a hazard to health and safety. Long exposure to high noise levels can

cause permanent damage to hearing. At lower levels, noise is a distraction and causes

fatigue.

Permanent damage to hearing can be caused at sound levels above about 90 dB(A), and

it is normal practice to provide ear protection in areas where the level is above 80 dB(A).

equipment that is likely to be excessively noisy; such as, compressors, fans, burners and

steam relief valves.

8. Relative Humidity of air ranges must be within (40-50 %) because dry weather causes

dryness of throat and eyes and increases evaporation of perspiration , excessive relative

humidity leads to decreasing evaporation of perspiration and then brings discomfort to

the workers so the productivity will decrease.

9. Temperature rise or fall may affect the workers , because it leads to fast tiredness and

exhausting .

The best work environment is at 22 C and 30 c in summer at most , 15 C in winter at

least .

10. In the workplace, each container must be labeled, tagged, or marked with the identity

of the chemical it contains. Hazard warnings may be words, pictures, or symbols, , to

give a visual warning , to direct attention to exits and escape routes , and so on .

5.2.3 Safety and security rules and regulations : To avoid and prevent accidents and injures to happen some information should be given to

all workers and Employees like :

• take reasonable care of their safety and health and that of others

• follow all safety and health policies and procedures

• report all known or observed hazards, incidents and injuries.

• Concentrate on the task at hand.

• Learn and use the proper (and safe) work methods for all tasks.

119

5.2.4 Industrial safety objective :

Figure 5.2 Fire Extinguishers

5.2.5 Fire Protection Systems : 1 . Water

2 . Chemical and special agent extinguishing systems

a. Foam Systems

b. Dry Chemical Systems

c. Carbon Dioxide Systems

d. Inert Systems

e. Vaporizing liquids ( Halon systems )

Figure 5.3 safety awareness training obligations

120

5.2.6 safety awareness training obligations

1. Training must be provided to each employee assigned to a job and/or machine

2. Employees receive training in the usage of the required personal protective equipment

provided

3. Employees receive training on the chemicals they work with or may be exposed to as

part of their job duties.

4. Identification of hazardous gases, chemicals, or materials used on-the-job and

instruction about the safe use and emergency action to take after accidental exposure.

5. The use and care of required personal protective equipment (PPE) and fire

extinguishers.

6. All staff should be trained and know what to do in the event of an emergency.

7. Employees should be informed and familiar with the company’s emergency

evacuation plan and fire exit routes.

121

Chapter SIX

Conclusions

122

Conclusions

Result And Discussion

The selected capacity of the plant is 300,000 tons/year based on 300

working days (into urea at LIFCO .co). The product from the Prilling tower

contains 98 % urea.

Material and energy balance for each of the equipment has been done. The

reactor is designed and its volume is found to be 195 m3. The length and

diameter of the reactor has been found to be 27m and 2.5 m respectively. The

L/D ratio of the reactor is found to be16 which is consistent with the actual

plant data. The L/D ratio of the urea reactor according to the actual plant data

lies between 14 to 20.

Climbing-film, long-tube vertical evaporator is used for the concentration

of urea. The length of the heat exchanger is found to be 6 m. The diameter

and height of the separator is1 m and 4 m respectively. 652 number of tubes

of 1 inch OD, 14 BWG and 6 m long (Area =307 m2) have been used.

• Payback period = (total capital investment) / (net annual earnings)

= 241638766.250710592

= 5 years

• Rate of return = (net annual earnings) / (fixed capital investment)

= 50710592

210120666.3

= 24%

123

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124

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