Title of dissertation Steady State Modeling of Urea Synthesis Loop

I. ROZANA AZRINA BINTI SAZALI

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The undersigned certifythat they have read, and recommend to The Postgraduate Studies

Programme for acceptance, a dissertation entitled

Steady State ModeIin£ of Urea Synthesis Loop

submitted by

Rozana Azrina binti Sazali

for the fulfilment of the requirements for the degree of

Masters of Science in Process Integration

IB.oi.o1

Date

Signature

Main Supervisor

Date

Co-Supervisor

sAP. Dr. Suzana Yusuy

/f' 9'°^

UNIVERSITI TEKNOLOGI PETRONAS

Steady State Modeling of Urea Synthesis Loop

By

Rozana Azrina binti Sazali

A DISSERTATION PROJECT

SUBMITTED TO THE POSTGRADUATE STUDIES PROGRAMME

AS A REQUIREMENT FOR THE

DEGREE OF MASTERS OF SCIENCE IN PROCESS INTEGRATION

Chemical Engineering

BANDAR SERIISKANDAR,

PERAK

June, 2007

in

J

DECLARATION

I hereby declare that the dissertation is based on my original work except for

quotations and citations which have been duly acknowledged. I also declare that it has

not been previously or concurrently submitted for any other degree at UTP or other

institutions.

Signature: (/^'

Name : Rozana Azrina binti Sazali

Date : I'S-i^ool

IV

ACKNOWLEDGEMENT

Firstly, I would like to thank Allah S.W.T for His blessings which enable me

to complete this dissertation project. I would like also to express my sincere gratitude

to my supervisor, Associate Professor Dr Suzana binti Yusup for her dedication,

support, and invaluable guidance throughout this project and beyond.

Many thanks to the department of chemical engineering staff for their help

during the project.

Last but not least, I would like to express my sincere thanks to my beloved

parents and the closest friend of mine for the support and love throughoutthe project.

ABSTRACT

Urea, which is known to be an important petrochemical product, is mainly

used as fertilizer. Urea (NH2CONH2) is produced commercially by reaction of

ammonia (NH3) and carbon dioxide (C02), under conditions depending on each

particular plant technology. There are a lot of urea synthesis technologies available

such as Snamprogetti process, Stamicarbon process and etc. In most operating process

the synthesis reaction is carried out in the liquid phase, at pressure from 13 to 25 MPa

and at temperature between 170°C and 200°C. In this study, a simulation is developed

specifically for the highpressure ureasynthesis section of the ABF Plant; which adopt

Stamicarbon process. In this study, the formation of ammonium carbamate is

considered to occur through the heterogeneous reaction of carbon dioxide and

ammonia. In present study, the urea reactor is divided into three reactors namely the

equilibrium reactor where the ammonium carbamate formation take place, and two

continuous stirred tank reactors in which the urea formation and biuret formation are

taking place respectively. The validity of the proposed simulation was demonstrated

using the actual plant data from ABF Plant. From the Aspen HYSYS simulation

result, it shows that the simulation could predict the behavior of the urea reactor as

well as the urea synthesis section as per literature but could not give the accurate

value. The CO2 conversion in the first equilibrium reactor is calculated by Aspen

HYSYS to be approximately similar as found in the ABF Plant; that is 60% (with

5.2% error) while Polymath 5.1 calculates the C02 conversion in the equilibrium

reactor to be approximately 60% which also agrees with the Aspen HYSYS and the

ABF Plant data. For the second CSTR reactor, HYSYS and Polymath 5.1 calculate

the conversion of ammonium carbamate to urea to be 92.75%) and 94% respectively

which agreed with each other. The overall conversion of CO2 to urea in the high

pressure urea synthesis loop with the recycle stream is calculated by HYSYS to be

approximately 89.73%) which is 10% higher than is found in the ABF Plant. The urea

yield formed in the high pressure urea synthesis loop is calculated by HYSYS to be

90.86%) which is 5.5%) higher than is found in the ABF Plant. From the study

conducted, it is found that this simulation could be used to predict the CO2 conversion

as well as urea yield obtained while the throughput is varied within 10%o error.

VI

ABSTRAK

Urea dikenali sebagai produk daripada petrokimia yang sangat penting dan

digunakan terutamanya sebagai baja. Urea (NH2CONH2) dihasilkan secara komersial

melalui tindak balas kimia di antara ammonia (NH3) dan karbon dioksida (C02), pada

keadaan operasi bergantung kepada teknologi yang digunapakai oleh sesebuah loji

pengeluaran. Terdapat pelbagai teknologi yang digunakan dalam penghasilan urea

seperti proses Snamprogetti, proses Stamicarbon dan sebagainya. Di dalam

kebanyakan proses operasi, tindak balas sintesis dijalankan di dalam fasa cecair pada

tekanan di antara 13 hingga 25MPa dan pada suhu di antara 170°C hingga 200°C. Di

dalam kajian ini, sebuah simulasi dibangunkan khusus untuk bahagian sintesis urea

bertekanan tinggi di loji ABF; yang menggunakan teknologi proses Stamicarbon. Di

dalam projek ini, pembentukan ammonium karbamat berlaku melalui tindak balas

heterogenus di antara karbon dioksida dan ammonia. Di dalam kajian ini, reaktor urea

dibahagikan kepada tiga reaktor iaitu reaktor keseimbangan di mana pembentukan

ammonium karbamat berlaku, diikuti dengan dua reaktor kinetik di mana

pembentukan urea danbiuret berlaku. Simulasi yang dibangunkan diuji menggunakan

data daripada loji ABF. Daripada simulasi yang dibangunkan menggunakan Aspen

HYSYS 2004.2, didapati bahawa simulasi ini mampu untuk menjangkakan kesan

perubahan yang berlaku kepada reaktor dan bahagian sintesis urea bersama suapan

semula tetapi tidak dapat memberikan nilai yang tepat. Pertukaran C02 di dalam

reaktor keseimbangan yang didapati daripada simulasi HYSYS bersamaan dengan

kuantiti yang didapati daripada loji ABF iaitu 60% (dengan sisihan 5.2%>). Manakala

Polymath 5.1 turut memberikan keputusan yang sama iaitu 60%) dimana bertepatan

dengan keputusan yang diperolehi daripada simulasi HYSYS dan juga data daripada

loji ABF. Untuk reaktor kinetik CSTR-Urea; pertukaran ammonium karbamat kepada

urea adalah 92.75% yang dikira oleh HYSYS dan 94%> apabila dikira menggunakan

Polymath 5.1. Pertukaran keseluruhan C02 kepada urea di dalam bahagian sintesis

urea bertekanan tinggi dengan suapan semula yang dikira daripada HYSYS adalah

kira-kira 89.73%> iaitu 10% lebih tinggi daripada yang diperolehi daripada loji ABF.

Hasil urea yang terbentuk di dalam bahagian sintesis urea bertekanan tinggi pula

dikira oleh HYSYS sebagai 90.86%> iaitu 5.5%) lebih tinggi daripada yang diperolehi

vii

di dalam loji ABF. Daripada kajian yang telah dijalankan, didapati bahawa simulasi

yang dibangunkan berupaya untuk menjangkakan pertukaran C02 dan juga hasil urea

apabila kuantiti input berubah.

vui

CONTENTS

STATUS OF THESIS

CERTIFICATION OF APPROVAL

TITLE PAGE

DECLARATION

ACKNOWLEDGEMENT

ABSTRACT

CONTENTS

LIST OF FIGURES

LIST OF TABLES

NOMENCLATURE

CHAPTER 1 INTRODUCTION

1.1 Problem Statement

1.2 Significant of the Project

1.3 Aim, Objective and Scope of the Project

1.3.1 Aim

1.3.2 Objectives

1.3.3 Scope

1.4 Assumptions

1.5 Limitation of the Model Developed

LITERATURE REVIEW

2.1 Introduction to Urea

2.2 History of Urea

2.3 Application of Urea

ix

CHAPTER 2

Page

u

iii

iv

v

vi

ix

xiii

xv

xvii

1

2

3

3

3

3

4

4

5

7

7

8

8

2.4 Properties of Urea Production Materials 9

2.4.1 Chemical Properties of Urea 9

2.4.2 Chemical Properties of Ammonia 12

2.4.3 Chemical Properties of Carbon Dioxide 13

2.4.4 Chemical Properties of Water 15

2.4.5 Chemical Properties of Ammonium Carbamate 17

2.4.6 Chemical Properties of Biuret 18

Chemistry of Urea Reaction 21

Review and Screening of Alternative Design Process 24

Conventional Process- Once-Through Process 26

Stamicarbon Process 27

Summary on Literature Review 29

THEORY 31

Process Description 32

Thermodynamic Modeling 36

Effects of Process Variables on the Degree of Conversion 39

3.3.1 Effect of Temperature on the Conversion of 39

Carbamate to Urea

3.3.2 Effect of Pressure on the Conversion of Carbamate 40

to Urea

3.3.3 Effect of Excess of Ammonia on the Yield of Urea 42

3.3.4 Effect of Temperature on the Formation of Biuret 44

METHODOLOGY 45

4.1 Property Package Selection 45

4.2 Creating the Hypothetical Components 46

4.3 Modeling of Reactor 48

4.3.1 Calculation of Amount of Ammonium Carbamate 49

2.5

2.6

2.7

2.8

2.9

3.1

3.2

3.3

CHAPTER 3

CHAPTER 4

CHAPTER 5

5.1

5.2

5.3

5.4

CHAPTER 6

6.1

6.2

REFERENCES

APPENDIX A

A.l

A.2

A.3

APPENDIX B

Formation

4.3.2 Calculation of Amount of Urea Formation 52

4.3.3 Calculation of Amount of Biuret Formation 53

RESULTS AND DISCUSSION 55

Aspen HYSYS Simulation Results 55

5.1.1 Inlet and Outlet Temperature of the Individual 57

Reactor And Overall Reactor

5.1.2 Urea Yield and C02 and NH3 Conversion 60

5.1.3 Comparison on Several Parameters when the Inlet 62

Ratio (NH3/CO2) into the Reactor is varied

Analysis of Equilibrium Reactor for Ammonium 72

Carbamate Formation by Using Polymath 5.1

Analysis of Continuous Stirred Tank Reactor (CSTR) for 75

Urea Formation (CSTR-Urea) by Using Polymath 5.1

Analysis of Continuous Stirred Tank Reactor (CSTR) for 80

Biuret Formation (CSTR-Biuret) by Using Polymath 5.1

CONCLUSION AND RECOMMENDATIONS 84

Conclusions 84

Recommendations 87

88

CALCULATION USING POLYMATH 5.1 91

Formation of Ammonium Carbamate in Equilibrium 91

Reactor

Formation of Urea in CSTR-Urea

Formation of Biuret in CSTR-Biuret

ASPEN HYSYS 2004.2 SIMULATION RESULTS

93

97

101

B.l Available UNIFAC Group Stucture in the Aspen HYSYS 101

XI

B.2

B.3

B.4

APPENDIX C

C.l

C.2

APPENDIX D

Library

Physical Properties of Ammonium Carbamate and Biuret 106

Calculated by HYSYS

Aspen HYSYS Simulation Results 109

Aspen HYSYS Simulation Process Flow Diagram 111

ABF PLANT 112

ABF Plant Process Flow Diagram 112

ABF Plant Data 113

ANALYSIS OF UREA CONCENTRATION 116

XII

LIST OF FIGURES

Page

CHAPTER 2

2.1 Process Reaction of Urea 22

2.2 Flow Diagram of Once-Through Urea Process 27

2.3 Schematic Diagram of Stamicarbon Process 29

CHAPTER 3

3.1 High Pressure Urea Synthesis Loop of ABF Plant 32

3.2 Relationship between Temperature and the Conversion of Carbamate 40

to Urea

3.3 Relationship between Pressure and the Conversion of Carbamate 41

to Urea

3.4 Effect of Excess Ammonia on Conversion of Carbamate 43

3.5 Effect of Temperature on Biuret Formation 44

CHAPTER 4

4.1 Impact of the Number of CSTRs on the Model Prediction of Conversion 48

to Urea

4.2 Equilibrium Constant Kp as a Function of Temperature 51

CHAPTER 5

5.1 Aspen HYSYS 2004.2 Simulation Process Flowsheet Diagram 55

5.2 C02 Conversion to Urea in Synthesis Loop (%>) as a Function of Inlet 63

Ratio (NH3/CO2) into the Reactor

5.3 NH3 Conversion to Urea in Synthesis Loop (%>) as a Function of Inlet 65

Ratio (NH3/CO2) into the Reactor

5.4 Urea Yield (%>) as a Function of Inlet Ratio (NH3/C02) into the Reactor 66

5.5 Reactor Outlet Temperature (°C) and CO2 Conversion in Synthesis 68

xiii

Loop (%) as a Function of Inlet Ratio (NH3/C02) into the Reactor

5.6 C02 Conversion in Reactor (%) as a Function of Inlet Ratio (NH3/CO2) 70

into the Reactor

5.7 NH3 Conversion in Reactor (%) as a Function of Inlet Ratio (NH3/CO2) 71

into the Reactor

5.8 Equilibrium Conversion of C02 to Ammonium Carbamate(%) as a 72

Function of Temperature (°C)

5.9 The Conversion of Ammonium Carbamate, XA(%) and Xa,eb (%) 75

as a Function of OutletTemperature, Tout (°C)

5.10 Effect of Temperature (°C) on the Equilibrium Conversion of C02 to 77

Ammonium Carbamate (%) and Conversion of Ammonium Carbamate

to Urea (%) for a Typical Condition

5.11 The Conversion of Ammonium Carbamate, XA (%>) and the 79

Reaction Temperature, Tout (°C) as a Function ofInlet Temperature, T0 (°C)

5.12 The Conversion of Urea to Biuret, XB (%) and Xb,eb (%) as a Function 80

of Outlet Temperature, Tout (°C)

5.13 Partial Conversion of Urea to Biuret, XB (%>) and Outlet Reactor 82

Temperature, Tout (°C) as a Function ofInlet Temperature, T0 (°C)

APPENDIX

B.4 Aspen HYSYS 2004.2 Simulation Process Flow Diagram 111

C.1 ABF Process Flow Diagram 112

D.1 Urea Concentration at the Liquid Stream Reactor Outlet (wt%>) as a 116

Function of Inlet Ratio (NH3/CO2) into the Reactor.

D.2 Urea Concentrationat the Liquid Stream Reactor Outlet (mole%>) as a 117

Function of Inlet Ratio (NH3/CO2) into the Reactor.

D.3 Urea Flow Rate at the Liquid Stream Reactor Outlet (kgmole/hr) as a 118

Function of Inlet Ratio (NH3/CO2) into the Reactor.

xiv

LIST OF TABLES

Page

CHAPTER 2

2.1 Physical Properties of Urea 11

2.2 Physical Properties of Ammonia 13

2.3 Physical Properties of Carbon Dioxide 14

2.4 Physical Properties of Water 16

2.5 Physical Properties of Ammonium Carbamate 17

2.6 Physical Properties of Biuret 20

CHAPTER 4

4.1 Temperature Independent Interaction Parameters of UNIQUAC Equation 45

4.2 Information Required Creating the Hypothetical Components 47

4.3 Typical Industrial Feed Conditions of a Urea Production Plant 49

CHAPTER 5

5.1 Comparison between Plant Data and Simulation Results 55

5.2 Comparison on the Inlet and Outlet Temperature between HYSYS 57

Simulation Results and ABF Plant Data

5.3 Comparison between the Actual Component Structure and UNIFAC 60

Structure

5.4 Comparison on the Reactor/Loop Conversion and Urea Yield 60

5.5 Comparison on Several Parameters when the Inlet Ratio (NH3/CO2) 62

into the Reactor is Varied

5.6 Comparison on Heat of Reactions and the Inlet and Outlet Temperature 67

of the Reactor

5.7 Comparison on Equilibrium Conversion between Polymath 5.1 with 73

Aspen HYSYS Results, ABF Plant Data and Literature

xv

5.8 Comparison onOutlet Temperature ofCSTR-Urea Reactor and 76

Conversion of Ammonium Carbamate to Urea inside the CSTR-Urea Reactor

5.9 Comparison on Outlet Temperature ofCSTR-Biuret Reactor and 83

the Conversion of Urea to Biuret in CSTR-Biuret Reactor

CHAPTER 6

6.1 Comparison between ABF Plant Data HYSYS Simulation Results 84

6.2 Comparison between HYSYS with Polymath 5.1 Results 85

APPENDIX

B.l Available UNIFAC Group Structure in the Aspen HYSYS Library 101

B.2 Physical Properties ofAmmonium Carbamate and Biuret Calculated by 106

HYSYS

B.3 Aspen HYSYS Simulation Results 109

C.2 ABF Plant Data 113

xvi

Symbols

cB

cc

cf

cb

Q

CPi

• Ao

'Bo

'Co

F Bo

FCo

F Do

F Eo

Fo

K

Kp

*,

Q

NOMENCLATURE

Descriptions

concentration of ammonium carbamate at t (mol 1" )

concentration of urea at t (mol 1" )

concentration of water at t (mol 1" )

initial concentration of ammonium carbamate (mol 1" )

initial concentration of urea (mol 1" )

initial concentration of water (mol 1" )

heat capacity of species i between temperature To and temperature T

(kJ/ kg mol.K)

initial molar flow rate of ammonium carbamate (kg mol h")

initial molar flow rate of urea (kg mol h" )

initial molar flow rate ofwater (kg mol h"1)

initial molar flow rate of carbon dioxide (kg mol h")

initial molar flow rate of biuret (kg mol h" )

initial molar flow rate ofammonia (kg mol h"1)

equilibrium constants

equilibrium constant of Reaction 1

rate constant of urea production (h")

rate constant of biuret production (1 mol" h" )

rate of flow of heat to the system from the surroundings

•I i

reaction rate of ammonium carbamate (kg mol m" h" )

reaction rate of urea (kg mol m" h")

xvn

T liquid phase temperature (K)

T.10

initial temperature

T1 R reference temperature (K)

V volume (m3)

v stoichiometric coefficient for each of the components present in

reaction /'

•

Ws rate of work done by the system on the surroundings; often referred as

shaft work

X partial conversion of ammonium carbamate to urea calculated from

mass balance

X . partial conversion of urea to biuret calculated from mass balance

calculated from mass balance

X partial conversion of carbon dioxide to ammonium carbamate

calculated from mass balance

XA FB partial conversion of ammonium carbamate to urea calculated from

energy balance

XB EB partial conversion of urea to biuret calculated from energy balance

XD EB partial conversion of carbon dioxide to ammonium carbamate

calculated from energy balance

x composition vector in the liquid phase

ACp overall change in the heat capacity per mole of A (i.e. base component)

reacted; —CpD + —Cpc —CpB -CpAa a a .

AH"fx heat of reaction at the reference temperature Tr

di ratio of the number of moles of species i initially (entering) to the

number of moles of base component initially (entering)

y activity coefficient

xviii

Subscript

/ chemical reactions defined by equations 3.5 to 3.9

o initial condition

xix

1.0 CHAPTER 1: INTRODUCTION

Mathematical model can be useful in all phases of chemical engineering, from

research and development to plant operations, and even business and economic studies.

For this study, it is done specifically to get an accurate modeling of the high pressure urea

synthesis loop and followed by analyzing the effects of different operating conditions for

optimization and aiding scale up calculations. It is also useful in control studies which are

not covered in this thesis. In this project, a simulation is developed for the high pressure

synthesis section of an industrial urea plant, specifically the ABF Plant. The simulation

developed is based on the Stamicarbon process which includes the urea reactor, high

pressure carbamate condenser, high pressure heat exchanger which acts as a stripper, and

low pressure carbamate condenser (consists of a heat exchanger and an absorber).

The main objective of this project is to produce a simulation of the high pressure

urea synthesis section by using Aspen Tech's process simulator; Aspen HYSYS 2004.2

and comparing the findings with the actual plant data; i.e. the ABF Plant data and

available literature.

Urea production consists of two main reactions. In the first reaction, ammoniaand

carbon dioxide reacts to form ammonium carbamate which decomposes to urea and water

in the next step. Since both carbon dioxide and ammonia are in the gas phase in the

reactor and reaction between these two results in production of ammonium carbamate

which is in the liquid phase, in this study this reaction is considered to be heterogeneous.

1.1 Problem Statement

This project is focusing only on the high pressure urea synthesis loop which

consists of the urea reactor itself, the high pressure heat exchange as stripper, the high

pressure carbamate condenser, and the low pressure carbamate condenser which consist

of absorber and heat exchanging section. Therefore, the simulation developed is only

done on these four equipments leaving the urea granulation section.

The simulation is only focusing on the main variables that can be manipulated in

the urea reactor such as the ratio of ammonia to carbon dioxide in the feed stream and the

temperature.

This project is done to develop tools to help to predict the process performance

(i.e. yield, selectivity) which ultimately will increase profit. The simulation developed can

be used to find the optimum condition for the urea rector so that feed and utility can be

fully utilized. The simulation can also be used to increase the capacity of urea production

(can predict the quality and quantity of the urea product and the byproduct from the urea

process).

The scope of this project is focused on the high pressure urea synthesis loop which

consists of urea reactor, stripper (high pressure heat exchanger), high pressure carbamate

condenser, low pressure carbamate condenser (consisting of two parts; the shell and tube

section and the absorbing section).

The efficiency of urea reactor is determined from CO2 conversion into urea in the

reactor. Conversion value can be calculated by knowing the important components of the

reactor outlet stream; i.e. CO2, NH3 and urea.

1.2 Significant of the Project

The project allows the development of a tool to predict the urea reactor behavior

as well as the high pressure urea synthesis loop when the inlet ratio of NH3 to CO2 into

the reactor and temperature is varied. By using the simulation developed, it enables the

prediction of the quality and quantity of the urea product as well as the byproduct from

the urea process when needs to increase the plant capacity arise. Therefore, the plant can

be operated at the optimum condition and hence the feed and utility can be fully utilized.

1.3 Aim, Objective and Scope of the Project

1.3.1 Aim

The project aim is to simulate the high pressure urea synthesis loop using Aspen

HYSYS 2004.2 simulator and compare the result to the real plant data; specifically to be

ABF Plant.

1.3.2 Objectives

The objectives of this study are to:

1. Establish a simulation of high pressure urea synthesis loop by using Aspen

Tech's process simulator Aspen HYSYS 2004.2

2. Enable to simulate the urea reactor and the urea synthesis loop and comparing it

to the real plant data; i.e. ABF plant data.

3. Compare findings with available literature

4. Manipulate process variables and unit operation topology to find the optimum

condition for the urea reactor in order to maximize profit.

5. Prediction of the quality and quantity of the urea product as well as the byproduct

from the urea synthesis loop.

1.3.3 Scope

This project focused on studying the behavior and the effect of varying process

conditions such as varying the inlet ratio of ammonia to carbon dioxide and the

temperature to the inlet of urea reactor on the high pressure urea synthesis section

1.4 Assumptions

Simulation of the syntheses section of the urea plant in this study was done by the

following assumptions:

1. Ammonium carbamate and biuret which are not available in the HYSYS

component library are created using the hypo manager. A wide selection of

estimation methods are provided for the various Hypo groups (hydrocarbons,

alcohols, etc.) to ensure the best representation of behavior for the Hypothetical

component in the simulation. In addition, methods are provided for estimating the

interaction binaries between hypothetical and library components.

2. Ammonium carbamate which is actually an ionic substance (NH/ andNH2COO")

is represented as a liquid hypo whiles the biuret as a solid hypo.

3. The formation of carbamic acid from dissolved CO2 and NH3 will be appreciably

only in dilute solutions, with concentrations far from those found in the synthesis

section1. Therefore, the following reaction will not be considered;

Hatch, T.F. and Pigford, R.L., 1962. Simulataneous Absorption of Carbon Dioxide and Ammonia in

Water. Ind. Eng. Chem. Fundam. 1 (3), p. 209-214 via CrossRef.

Buckingham, A.D., Handy, N.C., Rice, J.E., Somasundram, K. and Dijgraaf, C, 1986. Reactions Involving

C02, H20 andNH3: The Formation ofCarbamic Acid. J. Comput. Chem, 7 (3), p. 283-293 via CrossRef.

C02 (1) + NH3 (1) ~ NH2COOH (1)

4. The ionic dissociation of ammonia and water has very small extents of reaction

In this project, this reaction is not considered for modeling simplicity purposes

NH3 (1) + H20 (1) «-• NH4+ + HO"

H20 (1)

In Aspen Plus, all unit operation models can handle electrolyte reactions. Solution

chemistry also impacts physical property calculations and phase equilibrium calculations.

The presence of ions in the liquid phase causes highly nonideal thermodynamic behavior.

Aspen Plus provides specialized thermodynamic models and built-in data to represent the

nonideal behavior of liquid phase components in order to get accurate results.

Therefore, while this project is done using Aspen HYSYS, the data calculated is

expected to deviate from the ABF Plant data. Due to unavailability of Aspen PLUS,

Aspen HYSYS was used that has limitation on prediction of the highly non ideal ionic

system. In addition, some of the components present in this study are not available in

HYSYS library (ammonium carbamate and biuret). Thus, hypothetical components were

assumed.

2.0 CHAPTER 2: LITERATURE REVIEW

2.1 Introduction to Urea3

Urea is a white, crystalline, water-soluble compound and with melting point

132.7°C. The chemical formula, NH2CONH2, indicates that urea can be considered to be

the amide of carbamic acid NH2COOH, or the diamide of carbonic acid CO(OH)2. At

room temperature urea is white, odorless, and tasteless. When it is dissolved in water it

hydrolyzes very slow to ammonium carbamate and eventually decomposes to ammonia

and carbon dioxide. This is the basis for the use of urea as a fertilizer.

Urea has a formula weight of 60.06 and a nitrogen content of 46.7% by weight.

Urea is very soluble in water, and it is less soluble in methanol and ethanol. Urea is

commercially produced by the direct dehydration of ammonium carbamate NH2COONH4

at elevated temperature and pressure. Ammonium carbamate is obtained by direct

reaction of ammonia, NH3, and carbon dioxide, CO2. The two reactions are usually

carried out simultaneously in a high-pressure reactor.

Urea is a nitrogen-containing chemical product which is produced on a scale of

some 100,000,000 tonnes per year worldwide. More than 90% of world production is

destined for use as a fertilizer. Urea has the highest nitrogen content of all solid

nitrogeneous fertilizers in common use (46.7%N.) It therefore has the lowest

transportation costs per unit of nitrogen nutrient.

In the soil, urea is converted into the ammonium ion form of nitrogen. For most

floras, the ammonium form of nitrogen is just as effective as the nitrate form. The

ammonium form is better retained in the soil by the clay materials than the nitrate form

and is therefore less subject to leaching. Urea is highly soluble in water and is therefore

also very suitable for use in fertilizer solutions, e.g. in "foliar feed' fertilizers. Solid urea

is marketed as prills or granules. The advantage of prills is that in general they can be

http://www.fertilizer.org

produced more cheaply than granules which, because of their narrower particle size

distribution have an advantage over prills if applied mechanically to the soil. Properties

such as impact strength, crushing strength and free-flowing behavior are particularly

important in product handling, storage and bulk transportation. Urea contains a small

percentage of biuret, which is normally not a problem in soil fertilization. With foliar

fertilization however, this biuret content must not exceed 0.3%.

2.2 History of Urea4

Urea, NH2CONH2, first discovered in urine by Rouelle in 1773 and identified as a

pure crystalline organic compound in 1822, became famous in 1828, when Woehler

synthesized it from the inorganic compounds lead cyanate and ammonium hydroxide,

thus proving for the first time that an organic compound could be produced outside a

living organism. In 1870, Bassarow produced urea by heating ammonium carbamate in a

sealed tube in what was the first synthesis of urea by dehydration. However, nearly 100

years passed before urea was manufactured in substantial quantities.

2.3 Applications of Urea4

Urea reacts with formaldehyde to form compounds that are used as slow-release

fertilizers, adhesives, and plastics. Urea also reacts with H2O2 to yield a crystalline

oxidizing agent.

Urea forms crystalline complexes with straight-chain alkanes, which has led to its

use in the petroleum industry for separating straight- and branched-chain hydrocarbons.

The hydrocarbon is recovered from the complex by adding water to dissolve the urea.

http://en.wikipedia.org/wiki/Urea

Urea hydrolyzes in acids or bases, yielding NH3 and C02. Hydrolysis in aqueous

solution is induced quantitatively by an enzyme (urease) obtained from jack beans or

soybeans. Analysis ofthe NH3 or C02 produced is used for the quantitative determinationof urea. Other important applications for urea are the manufacture of resins, glues,

solvents, and some medicines.

Nowadays, the main use for Urea is Fertilizers, including solid and nitrogen

solutions, 86 percent; livestock feed, 7 percent; urea-formaldehyde resins, 5 percent;

melamine, 1 percent; miscellaneous, including cyanuric acid for chlorinated

isocyanurates, crystalline adducts, deicing agents, pharmaceutical intermediates, and

sulfamic acid and its ammonium salt, 1 percent.

2.4 Properties ofUrea Production Materials5

2.4.1 Chemical Properties of Urea, NH2CONH2

The chemical formula, NH2CONH2, indicates as urea. At room temperature urea

is white, odorless, and tasteless. When it is dissolved in water it hydrolyzes very slowly to

ammonium carbamate and eventually decomposes to ammonia andcarbon dioxide.

At atmospheric pressure and at its melting point urea decomposes to ammonia,

biuret, HN(CONH2)2, cyanuric acid, C3N3(OH)3, ammelide, NH2C3(OH)2, and triuret,

NH(CONH)2CONH2. Biuret is in practice the main and the least desirable by-product

present in the commercially synthesized urea. An excessive amount (more than 2 wt %)of biuret in fertilizer-grade urea is detrimental to plant growth. Solid urea is rather stable

5Physical and chemical properties for every material that involved in production ofurea are necessary to know theirconditions. These properties are obtained from certain sources such as Lange 's Handbook of Chemistry, Perry'sChemical Engineering Handbook and Chemical Engineering Volume 6and through internet (http://www.nist.com).

10

at room temperature and atmospheric pressure. Heated under vacuum and at its melting

point, urea sublimes without change. At 180-190°C urea will sublime, under vacuum, and

be converted to ammonium cyanate, NH4OCN.

When solid urea is rapidly heated in a stream of gaseous ammonia at elevated

temperature and at a pressure of several atmospheres it sublimes completely anddecomposes partially to cyanic acid, HNCO, and to ammonium cyanate. Also solid ureawill dissolve in liquid ammonia and will form the very unstable compound urea-

ammonia, CO(NH2)2NH3, which decomposes above 45°C.

An aqueous urea solution slowly hydrolyses to ammonium carbamate at room

temperature or at its boiling point. Traces of cyanate are found in solution. Prolongedheating of aqueous urea solutions will cause the formation ofbiuret;

2NH2CONH2 4 » NH2CONHCONH2 +NH3

Urea Biuret Ammonia

This reaction is promoted by low pressure, high temperature, and prolonged

heating time. At pressure of 100-200 atm, biuret will revert to urea when heating in the

presence of ammonia.

11

Table 2.1: Physical Properties of Urea

UREA

Physical Properties ValueMolecular Formula NH2CONH>Structural Formula R. JL H

i TH H

"•""': " "" Synonyms Carbamide; Carbonyidiamide iMolecular weight, g/mole 60.06

Specific Gravity 1-.32Normal Freezing Point, °C 132.7

Normal Boiling Point. °C 191.85Critical Temperature, K 705

Critical Pressure, bar 90.5

Critical Volume, m3/mole 2.18e-4Density, kg/m3 ~ (T=133 to 150oC) "

138.5-0.96T;solid (20°C)

_ _ ___ 1335 iHeat of Vaporization, kJ/mole 54.57457

Heat of Formation, kJ/mole -245.59 . jSolubility in water 119 g/100 g@25°C

Heat Capacity ConstantCp = A + BT+ CT2 (kJ/kmole.K) @ 25"C

A 0.287231

B 3.8597e-3 i

C 1.3e-6 iAntoine Constant

In P* (kPa) = A+B/{T(K) + C} +DlnT + ETFA 2.13316e+l

B -1.05011e+4

C 0.0

D 9.7941e-2

E 6.34741e-9

F 2.0

These properties are essential for reference to know about phases that exist at

certain temperature, critical pressure and critical temperature. Consequently, it is

important to choose the suitable equipment and certain condition for the operation. This

data also required calculating mass balance and the data such as enthalpy and heat of

formation are required to calculate energy balance.

12

From this, we can make a little conclusion, where the physical and chemical

properties are very important in designed a chemical plant no matter what is the size of

plant.

2.4.2 Chemical Properties of Ammonia, NH35

Ammonia is a colorless and pungent gas which is highly soluble in water. A

saturated aqueous (water) solution of ammonia contains 45 percent ammonia by weight at

0° C (32° F) and 30 percent at ordinary room temperatures. On solution in water,

ammonia becomes ammonium hydroxide, NH4OH, which is strongly basic and similar in

chemical behavior to the hydroxides of the alkali metals. Compared to water, liquid

ammonia is less likely to release protons (H+ ions), is more likely to take up protons (to

form NH4+ ions), and is a stronger reducing agent.

Moreover, it becomes highly reactive when dissolved in water and readily

combines with many chemicals. Ammonia is easily liquefied by compression or by

cooling to about -33°C (-27.4°F). In returning to the gaseous state, it absorbs substantial

amounts of heat from its surroundings. Because of this property, it is frequently employed

as a coolant in refrigerating and air-cooling equipment. Ammonia is alkaline and caustic,

and is a powerful irritant. It is incompatible or reactive with strong oxidizers, acids,

halogen, salts of silver, and zinc. It is corrosive to copper and galvanized surfaces; liquid

ammonia will attack some forms of plastics, rubber, and coatings. It is highly water-

soluble and soluble in chloroform and ether. It is easily liquefied under pressure.

Ammonia forms a minute proportion of the atmosphere; it is prepared

commercially in vast quantities. The major method of production is the Haber process, in

which nitrogen is combined directly with hydrogen at high temperatures and pressures in

the presence of a catalyst. Representative properties of ammonia are shown in Table 2.2.

13

Table 2.2: Physical Properties of Ammonia5

AMMONIA

Physical Properties Value

Molecular Formula NH3Structural Formula H N H

H

Synonyms Anhydrous AmmoniaMolecular weight, g/mole 17.03

Specific Gravity 0.6818Normal Freezing Point, °C -77.73

Normal Boiling Point, °C -33.34

Critical Temperature, K 405.5Critical Pressure, bar 112.8

Critical Volume, m3/mole 0.0725Liquid Density, kg/m3 639

Heat of Vaporization, kJ/mole 23.351 iHeat of Formation, kJ/mole -67.20(1)

-46.19(g)Solubility in water 54 g/lOOml !

Flammable limit in air % by volumeLower explosive limit 15Upper explosive limit 28

Vapor PressureIn P* (kPa) =Pc 10A{C5(Tr-l)/Tr}

C, 0.0C2 1.5714C3 0.48316C4 0.0C5 2.9562C6 0.0

2.4.3 Chemical Properties of Carbon Dioxide, CO2'

It is a gas approximately heavy as air, odorless, colorless, and slightly acidic and it

will not support combustion. Alone carbon dioxide is not chemically reactive, although

aqueous solutions of CO2 are acidic and many reactions including the corrosion of carbon

steel occur readily. The reaction with hydrogen is an endothermic and is reversible over a

suitable catalyst at proper condition of temperature and pressure. The reversed reaction is

important in the production of hydrogen.

14

C02 + H2->CO + H20

The reduction CO2 with carbon occurs in the stage of some coal gasification

process in accordance with equation below:

C02 + C -> 2CO

At temperature about 1500 °C CO2 almost completely dissociates in CO and O2.

In interesting characteristic of CO2 is that forms a solid when its liquid form is

"flashed" to a low pressure with consequent of a cooling resulting from evaporator of a

portion of the liquid CO2. This characteristic enables the CO2 to be reduced to a solid dry

ice, which is a useful low temperature refrigerant.

Carbon dioxide occurs in nature both free and in combination (e.g., in carbonates).

It is part of the atmosphere, making up about 1% of the volume of dry air. It is a product

of combustion of carbonaceous fuels (e.g., coal, coke, fuel oil, gasoline, and cooking gas).

For commercial use it is available as a liquid under high pressure in steel cylinders, as a

low-temperature liquid at lower pressures, and as the solid dry ice. Table 2.3 shows the

physical properties of carbon dioxide.

Table 2.3: Physical Properties of Carbon Dioxide5

CARBON DIOXIDE

Physical Properties Value

Molecular Formula C02Structural Formula O = C = O

Synonyms Carbonic acid gas; Dry ice;Carbonic anhydride

Molecular weight, g/mole 44.01Specific Gravity

Normal Freezing Point, °C -56.6 @ 5.2atmNormal Boiling Point, °C -78.5 (sublimes) -

Critical Temperature, K 304.2

.. _ - .__

Critical Pressure,_bar 73-?3 _ ICritical Volume, m3/mole 0.094

Density, kg/mJ 1.527(vapor, air=l) !777 (liquid) |

Heat of Vaporization, kJ/mole 17.166

Heat of Formation, kJ/mole -412.9(1)-393^5(g).

Solubility in water 88g/100mlVapor Pressure

In P* (kPa) = Pc 10A{C5(Tr-l)/Tr}c. 0.0

c2 1.732

c3 0.56907

c4 0.0

c5 3.3097

c6 0.0

15

2.4.4 Chemical Properties ofWater, H2O 5

Water means different things to different people. It has unique physical and

chemical properties; it can be freezing, melting, evaporating, heating, and combine with

other process.

Normally, water is a liquid substance made of molecules containing one atom of

oxygen and two atoms of hydrogen (H2O). Pure water has no color, no taste, no smell,

turns to a solid at 0°C and a vapor at 100°C. Its density is 1 gram per cubic centimeter

(1 g/cm3), and it is an extremely good solvent.

Water has a covalent bonds hold two hydrogen atoms to one oxygen atom and

dipole moment-an asymmetric of charge across the molecule hydrogen bonds generated

by electrostatic nature of the molecule hold molecules together. Pure water boiled at

100°C and melt at 0°C. At 0°C, the bonds will rupture and the lattice disorganization will

thus increase the density. From 4°C on, increased energy leads to increasing inter-atomic

distances. Latent heat of melting (fusion)-amount of heat required to change ice to liquid

water it is very large-79.72 cal/g and latent heat of fusion (79.72 cal/g). So that large

energy input required to melt ice and large energy loss required to form ice. Water has

775 times the density of air and therefore is much more viscous effects on organisms. The

second value of our example is that is points rather directly to the cause of water's

16

peculiar behavior. The anomalously high transition temperature clearly indicated that the

water molecules are 10 atm to separate to another.

Water is polar liquid with high dielectric constant (81 at 17°C); it is weak

electrolyte. The aspect presented by the electronic cloud of the water molecular is that of

a sort of truncated, distorted jack. The oxygen atom occupied the center of the not-quite

cube; the hydrogen, opposite corner of one face. Direct toward the corner of the opposite

face, most removed from the hydrogen, are two arms of negative electrification-possibly

the most important chemical feature in all of creation-for these arms are responsible for

the hydrogen bounding which in turn makes the phenomenon of life possible. Physical

properties of water are shown in Table 2.4.

Table 2.4: Physical Properties ofWater5

WATER

Physical Properties ValueMolecular Formula 02Structural Formula O - O

Synonyms Dihydrogen oxideMolecular weight, g/mole 18.015

Specific Gravity '1.0Normal Freezing Point, °C 0.0

Normal Boiling Point, °C 100Critical Temperature, K 647.3

Critical Pressure, bar 220.5

Critical Volume, m3/mole 0.056 ~"Liquid Density, kg/m3 997.97 (25°C) •

??9.87._(0°C).Heat of Vaporization, kJ/mole 40.683

Heat of Formation, kJ/mole -242.0Solubility in Water Miscible

Antoine Constant

P* (Pa) =exp[C,+(C2/T)+C3lnT+C4lnTC5] ijC, 73.649C2 -7258.2 i

1

C3 -7.3037 ItC4 4.17E-06

i

i

C5 2.0C6 0.0 ;

17

2.4.5 Chemical Properties of Ammonium Carbamate, NH4NH2C02

Ammonium carbamate is a salt, a carbonate of ammonium, NH4NH2C02, which is

an intermediate in the manufacture of urea and a component of smelling salts. It is a

colorless and rhombic crystal in shape. It smells like ammonia odor and its main use are

as ammoniating agent and in fertilizer manufacturing.

6

Table 2.5: Physical Properties of Ammonium Carbamate

Ammonium Carbamate

Physical Properties Value

Molecular Formula H2NCOONH4

Structural Formula NH_+ NHo

Synonyms Ammonium aminoforniate, Ammonium

carbonate anhydrate6, Carbamic acid

. amLmo.niuml_Ammoni.um_S.alt!!Chemical class Inorganic7

Molecular Weight, g/mole 78.076Specific Gravity NA

Normal Freezing Point, oC NA

Normal Boiling Point, oC 60(sublimes)10

Critical Temperature, K NA

Critical Pressure, bar NA

Critical Volume, m3/mole NAoy^csVapor Pressure 492 mmHg (51°C)

,o>~.\982 mbar (20°C)

Pradyot Patnaik, 2002. Handbook ofInorganic Chemicals. McGraw Hill.http://www.pesticideinfo.org/List_Chemicals.jsp

8http://www.toxnet.nlm.nih.gOv/cgi-bin/sis/search/r7./temp/~yHr67S:l

9http.7/physchem.ox.ac.uk/MSDS/AM/ammonium_carbamate.html

Density, g/cm3 1.38 (20°C)8

1.6(20°C)10

Bulk Density, kg/m3 780-850"

Heat of Vaporization, kJ/mole NA

Heat ofT?o7mation7kj7m^e~ ~^38000nHeat of Combustion, kJ/mole -469

Specific Heat ofSolid Ammonium Carbamate 1.67 J/(g.K) at 20"C8

18

Heat of Fusion, kJ/mole 16.74S^ubiTiVyTn"Water Soluble10 I

Antoine Constant NA. — . ~ -i

P* (Pa) =exp[C1+(C2/T)+C3lnT+C4lnTL5]CI

C2

C3

C4

C5

C6

2.4.6 Chemical Properties of Biuret, NH2-CO-NH-CO-NH2

Biuret is a common by-product (NH2-CO-NH-CO-NH2) formed in the industrial

production ofurea fertilizers. It is a phytotoxic substance affecting plants in the very early

stages of growth. To prevent plant injury, the biuret content in urea should not exceed 1-

1.2% in soil dressing, and commonly only 0.25 - 0.5% in foliar treatment.

Biuret is a condensation compound of urea, equivalent to two molecules of urea

less one of ammonia. It is a white solid soluble in hot water and decomposes at 186-189

°C. The parent compound can be prepared by heating urea above the melting point at

which temperature ammonia is expelled.

2 CO(NH2)2 -> H2N-CO-NH-CO-NH2 + NH3 T

10 http://ull.chemistry.uakrom.edu/erd/chemicals/7000/6014.html

"Urea Shift Superintendent, 1999. Urea Plant Process Description. PFK Resource Centre.

19

A biuret is also a functional group and a class of organic compounds with the

general structure RHN-CO-NR-CO-NHR where R is an organic residue. Biuret can be

prepared by trimerization of isocyanates. For example the trimer of 1, 6-hexamethylene• 17

diisocyanate is also known as HDI-biuret.

Biuret chelated with Cu2+ in alkaline solution to form an intense violet-red color.

The reaction also occurs with polypeptides, but not with dipeptides or amino acids. It is

used in colorimetric methods for total proteins. Carbamic acid is a compound of chemical

formula H2NCOOH. But it exists only in the form of carbamate (its salt or ester),

carbamide (amide) and carbamoyl (acyl radical). Carbamate structure inhibits

cholinesterase and many insecticides and parasiticides contain carbamate functional

group. It is highly toxic to human also. Heavy exposure to IT can cause carbamate

poisoning. Natural carbamide (urea) can be found in protein metabolism in urine.

Carbamoyl is the radical NH2CO-, also called carbamyl. It is involved in the biosynthesis

of the pyrimidine ring. Carbamoyl compounds, such as salicylamides, are important for

the preparation of pharmaceutical products, pesticides, dyes, and biosynthesis

researches.13

At atmospheric pressure and at its melting point urea decomposes to ammonia,

biuret, HN(CONH2), cyanuric acid, C3N3(OH)3, ammelide, NH2C3(OH)2, and triuret,

NH(CONH)2CONH2. Biuret is in practice the main and the least desirable by-product

present in the commercially synthesized urea. An excessive amount (more than 2 wt %)

of biuret in fertilizer-grade urea is detrimental to plant growth. Solid urea is rather stable

at room temperature and atmospheric pressure. Heated under vacuum and at its melting

point, urea sublimes without change. At 180-190°C ureawill sublime, under vacuum, and

be converted to ammonium cyanate, NH4OCN.

When solid urea is rapidly heated in a stream of gaseous ammonia at elevated

temperature and at a pressure of several atmospheres it sublimes completely and

decomposes partially to cyanic acid, HNCO, and to ammonium cyanate. Also solid urea

12 http://en.wikipedia.org/biuret

13 http://www.chemicalland21.com/industrialchem/inorganic/BITJRET.htm

20

will dissolve in liquid ammonia and will form the very unstable compound urea-

ammonia, CO(NH2)2NH3, whichdecomposes above 45°C.

An aqueous urea solution slowly hydrolyses to ammonium carbamate at room

temperature or at its boiling point. Traces of cyanate are found in solution. Prolonged

heating of aqueous urea solutions will cause the formation of biuret;

2NH2CONH2 » NH2CONHCONH2 + NH3

Urea Biuret

This reaction is promoted by low pressure, high temperature, and prolonged

heating time. At pressure of 100-200 atm, biuret will revert to urea when heating in the

presence of ammonia.

Table 2.6: Physical Properties of Biuret

Biuret

Physical Properties Value

Molecular Formula H2NCONHCONH2

Structural Formula O O

X XH2N N NH2

H

Synonyms 2-imidodicarbonic diamide, Allophanamide,

Allophanic acid amide, Allophanimidic acid

Allophanimidic acid (VAN), Biuret,

Carbamoylurea, Carbamylurea,

Dicarbamylamine, IMIDODICARBONIC

DIAMIDE, Isobiuret, Urea, (aminocarbonyl)-,

Ureidoformamide14

Chemical class Organic compound

Molecular Weight, g/mole 103.08

Specific Gravity NA

http://books.elsevier.com/bookscat/samples/9780444824783/Sample_Chapters/02~chapter_l.pdf

Normal Freezing Point, oC NA

Normal Boiling Point, oC NA

Melting Point, oC 187 -190 C (Decomposes)14Critical Temperature, K NA

Critical Pressure, bar NA

itical Volume, m3/mole NAVapor Pressure NA

Density, g/cm3 NA

Bulk Density, kg/m3 NA

Heat of Vaporization, kJ/mole NA

Heat of Formation, kJ/mole NA

Heat of Combustion, kJ/mole NA

Specific Heat of Solid Ammonium Carbamate NA

Heat of Fusion, kJ/mole NA

Solubility in Water NA

Antoine Constant NA

P* (Pa) =exp[C,+(C2/T)+C3lnT+C4lnTcsiCI

C2

C3

C4

C5

C6

21

2.5 Chemistry of Urea Reaction

The process reactions occurring in urea processes are illustrated in the diagram of

reaction sequences shown below. Two principal reactions take place in the formation of

urea from ammonia and carbon dioxide. The first reaction is exothermic and the second

reaction is endothermic. Both reactions combined are exothermic.

22

15Figure 2.1: Process Reaction of Urea

The commercial production of urea is based on the reaction of ammonia and

carbon dioxide at high pressure and temperature to form ammonium carbamate, which in

turn is dehydrated into urea and water:

2NH3 + C02 NH2COONH, (2.1)

NH2COONH4 NH2CONH2+H20 (2.2)

Reaction (2.1) is fast, highly exothermic, and goes essentially to completion under

normal industrial processing conditions, while reaction (2.2) is slow, endothermic and

usually does not reach thermodynamic equilibrium under processing conditions. It is

common practice to report conversions in a CO2 basis. According to Le Chatellier's

principles, the conversion increases with an increasing NH3/C02 ratio and temperature,

and decreases with an increasing H2O/CO2 ratio 16

15 http://www.stamicarbon.com/urea/innovative_processes/en_/index.htm

16 Satyro, M. A., Yau-Kun Li, Agrawal, R.K. and Santollani, O. J., 2000. Modelling Urea Processes: A

New Thermodynamic Model and Software IntegrationParadigm. Virtual Material Group, Inc.

23

In most operating processes the synthesis reaction is carried out in the liquid

phase, at pressure from 13 to 25 MPa and at temperature between 170 and 200°C. The

synthesis proceeds via formation of ammonium carbamate as intermediate, which then

dehydrates to give urea. At pressures above its dissociation pressure the formation of

ammonium carbamate is fast and complete.

The carbamate dehydration reaction is slower and does not proceed to completion.

The equilibrium conversion usually reaches value greater than 80% on a CO2 basis.

The fraction of ammonium carbamate that dehydrates to form urea is determined

by the ratio of various reactants, the operating pressure, the operating temperature as well

as the residence time of the reactor. The rate of formation of ammonium carbamate is

highly dependent upon the pressure. When all other factors are constant, the rate of

formation increases proportionally to the square root of the pressure. The rate of

formation of ammonium carbamate is also increases with temperature, until a maximum

is reached, at which point the rate decreases sharply to zero at a temperature where the

dissociation pressure is equal to the reaction pressure. From this information, it is

advisable in industrial urea production to operate at the highest possible pressure and at

the highest temperature compatible with this pressure. The conversion of ammonia to

urea depends on the dehydration reaction, in which ammonium carbamate decomposes

into water and urea. This occurs when carbamate is heated in a closed vessel, part

dissociates into the vapor. When the degree of filling is sufficient the pressure prevents

other carbamate from dissociating and this part spontaneously decomposes into urea

releasing water vapor.

17 Isla, M.A. and Irazoqui, H.A., 1993. Simulation of Urea Synthesis Reactor. 1. Thermodynamic

Framework. Ind. Eng. Chem. Res. 32, p. 2662-2670.

18 Lemkowitz, S. M., de Cooker, M. G. R. and van den Berg, P. J., 1973. An Empirical Thermodynamic

Model for the Ammonia-Water-Carbon Dioxide System at Urea Synthesis Conditions. J. Appl. Chem.

Biotechnol 23, p. 63-76 via CrossRef.

24

2.6 Review and Screening of Alternative Design Process

There are three types of reaction path to produce urea based on different raw

materials. The first one is the starting point for the present industrial production of the

urea is the synthesis of BASAROFF [2], in which urea is obtained by dehydration of

ammonium carbamate at increased temperature and pressure shown by the below

equation:

NH2COONH4 CO(NH2)2 +H20

The second reaction started to be used in the beginning of this century where urea

was produced on an industrial scale by hydration ofcyanamide, which was obtained from

calcium cyanamide:

CaCN2 + H20 + C02 -+ CaC03 + CNNH2

CNNH2 +H20^r CO(NH2)2

The third and the ultimate reaction path used after development of the NH3

process (HABER and BOSCH. 1913) is the production ofurea from NH3 and C02, whichare both, formed in the NH3 synthesis developed rapidly:

2NH, +C02 NH2COONH4

NH2COONH, ^ CO(NH2)2 +H20

Nowadays, urea is prepared on an industrial scale exclusively by reaction based

on this reaction mechanism.

The former two reactions had been abandoned due to several drawbacks and its

inconvenience and thus after the 20th century, all urea is produced through the reaction

between NH3 and CO2.

25

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

dioxide. Large quantities of carbon dioxide are produced during the manufacture of

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

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

The production 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 ammonium nitrate or sulphate,

or they can be recycled for complete conversion to urea in a total-recycle process.

Basically, the production of urea involves the the reaction between ammonia and

carbon dioxide producing urea and water. All of the existing urea plants today adopt

chemical routes as their base. These plants differ only in terms of the operating

conditions, the arrangement of equipments, energy and material recovery and choice of

stripping agent. The alternative process ofurea production can beclassified into two main

groups, namely the conventional process and stripping process. The other processes are

basically modification of these two processes.

Today, there are many types of processes to produce urea using NH3 and CO2 in

the modern industrial scale. These processes include:

i. Conventional Process- One-through Urea Process

ii. Allied Chemical Corporation Solution Recycle Urea Process

iii. Chemical Construction Corporation (Chemico) Total Recycle Process

iv. Snamprogetti (Italy) Ammonia and Self-Stripping Urea Process

v. Conventional Recycle Processes

vi. Mitsui Toatsu Chemicals, Inc urea process

vii. Stamicarbon C02-Stripping Processes

26

2.7 Conventional Process- Once-Through Process1

The simplest step and using plant requiring the least capital is the so-called Once-

Through Process where the ammonia feedstock is reacted with compressed carbon

dioxide in the reactor. 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 2.0-3.0. The reactor usually operates in a

temperature range from 175°C to 190°C. The reactor effluent is let down in pressure to

about 2 atm and then will be passed to the medium-pressure decomposer, ammonia-

carbamate separation column and low pressure decomposer. The moist gas, separated

from the 85-90% urea-product solution, and 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. The urea solution produced will be evaporated to a desired concentration

and then sent to the prilling section where the final product will be in the granular form.

As the name dictates, no reactants will be recycled. The unconverted ammonia will be

neutralized with acids to form ammonium salt as the co-product.

Figure 2.2 is the flow diagram for the once-through urea process. The larger

licensing companies who employ this process are as follows: Chemical Construction

Corporation (USA); Mitsui Toatsu (Japan); Montecatini (Italy); Vulcan Copper and

Supply Co. (USA); and Lonza and Inventa (Switzerland). The drawbacks of this scheme

are the limited overall conversion of carbon dioxide and the large amount of co-products

(i.e. ammonium salts) formed.

Ullmann's Encyclopedia of Industrial Chemistry, Volume A27 via CrossRef.

NH3

CO,(auto clave') valve dissociator>ci

ACID

Reactor ^ Reducing ^Carbamate •Absorber •C°2 TO

UREA SOLUTION

AMMONIUM SALTS SOLUTION

WASTE

Figure 2.2: Flow Diagram for theOnce-Through Urea Process15

2.8 Stamicarbon Process15

27

In this project, the simulation process developed is entirely based on the

Stamicarbon process which is adopted by ABF Plant.

The Stamicarbon process was developed in 1960's. It is also known as CO2-

Stripping Process as it uses carbon dioxide as the stripping agent. The main

characteristics of this process are i) the recycle of nonconverted material from the high

pressure stripper in gas phase rather than aqueous and ii) the heat is recirculated rather

than being passed once as in the previous scheme. The average energy consumption in

this plant is estimated at 0.8-1.0 tonne of steam per tonne of urea compared to 1.8 tonne

of steam required in the original scheme-Once-Through Process (Refer Figure 2.2).

This scheme consists of a urea reactor, a stripper for unconverted reactants, a high

pressure carbamate condenser and a high pressure scrubber for the reactor off-gas. The

reactor operates in a temperature range of 170-190°C and 130-150 atm.g pressure. The

NH3:C02 molar ratios are between 2.8-2.9. In the reactor, the ammonia and carbon

dioxide reacts to form ammonium carbamate, which consequently turned into urea and

water. The unconverted carbamate will be sent to the stripper where it will be

decomposed by the stripping action of carbon dioxide contacted counter currently to the

28

urea solution at synthesis pressure and relatively low temperature. This stripper effluent

will be the urea solution containing low concentration of carbon dioxide and ammonia.

Because of the ideal ratio between ammonia and carbon dioxide concentrations in the

recovered gases in this section, water dilution of the resultant ammonium carbamate is at

a minimum despite the low pressure (about 4 bars). As a result of the efficiency of the

stripper, the quantities of ammonium carbamate for recycle to the synthesis section are

also minimized, and no separate ammonia recycle is required.

The gaseous ammonia and carbon dioxide coming out from the top of the stripper

will be condensed in the high pressure carbamate condenser, operating at the synthesis

pressure. The condensed carbamate is recycled back to the reactor for further conversion

to urea. About 75% by weight urea solution is produced at this stage. Since the urea is

marketed in solid form, further concentration is done in the evaporation stage where the

product leaving the evaporator should have certain moisture content depending on the

method of product shaping. Prilling tower requires a moisture content of 0.25 wt% while

a granulation unit requires moisture content of about 1-5% by weight. The number of

evaporation stage may vary depending on the moisture content requirement. This process

condensate treatment section can produce water with high purity, thus transforming this

"wastewater" treatment into the production unit of a valuable process condensate, suitable

for e.g., cooling tower or boiler feed water makeup.

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

approximately 140 bars and a temperature of 180-185°C. The molar NH3/C02 ratio

applied in the reactor is 2.95. This results in a C02 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 C02 are then partially condensed and recycled to the

reactor. The heat evolving from this condensation is utilized to produce 4.5 bar steam,

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

Surplus 4.5 bars steam is sent to the turbine of the CO2 compressor. The NH3 and CO2 in

the stripper effluent are vaporized in a 4 bar decomposition stage and subsequently

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

29

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

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

The process can be represented by the following block diagram:

NH3 1

' i '

co2

Synthesis 4-i

Carbamate Recyclei rLow pressurerecirculation

^ r

Evaporation —»Wastewater

treatment->

Purified processcondensate

1 r

Finishing

*

( Urea )

isFigure 2.3: Schematic Diagram of Stamicarbon Process

2.9 Summary on Literature Review

Isla et. al. have develop a thermodynamic model for the system NH3-C02-H20-

urea as a supporting program of a urea synthesis reactor simulation module. The model

covers a wide range of composition and temperature and can be used to predict the

behavior of the system at and removed from urea synthesis conditions. Irazoqui et. al.

concern with the physicochemical and mathematical modeling of a urea synthesis reactor

for simulation and optimization purposes, and with the computer implementation of a

reactor simulation module. This module allows comparison of the performance of

20 Production of Urea and Urea Ammonium Nitrate, 1995. European Fertilizer Manufacturer's Association,

via CrossRef.

30

reactors with different configurations and different degrees of concentration back-mixing.

Lemkowitz et. al. developed an empirical thermodynamic model for the ammonia-water-

carbon dioxide system at urea synthesis condition. They solved chemical and gas-liquid

equilibria using experimental values of the reaction equilibrium constants. Henry's

constants of NH3 and C02 were measured considering the reaction mixture as a solvent.

Bubble points of liquid reaction mixtures were predicted with the assumption of ideal

behavior ofgas and liquid phase. Dente et. al. have develop a simulation program for urea

plants of Snamprogetti and Stamicarbon process to improve the energy efficiency of theconventional urea processes. Apowerful convergence promoter based onthe evolutionary

approach has been used to solve the recycle problem in their work. Satyro et. al. modelthe urea processes by using modern software technology which allows the model to beused in process simulators or other applications such as spreadsheets or operator training

software. Their work is being done in further refining the low and medium pressure

thermodynamic models and in the creation ofa mass transfer based high- pressure steady

state decomposer model. Hamidipour et. el. model the synthesis section of an industrial

urea plant based on the Stamicarbon process. In the proposed model the urea reactor is

divided into several continuously stirred tank reactors (CSTRs) and considered the

formation of ammonium carbamate occurs through the heterogeneous reaction of carbon

monoxide and ammonia. They also consider the biuret formation in their work and the

dynamic behavior ofthe corresponding parameters was also investigated.

31

3.0 CHAPTER 3: THEORY

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

dioxide. Large quantities of carbon dioxide are produced during the manufacture of

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

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

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

ammonium nitrate or sulphate, or they can be recycled for complete conversion to urea in

a total-recycle process.

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

ammonia and carbon dioxide at high pressure (13-30MPa) and high temperature (170-

200°C). 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, 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

process21.3-

This chapter describe about the process adopted by ABF Plant, the

thermodynamic modeling framework discussing on the effects of process variables on the

degree of conversion.

21Dente, M., Pierucci, S., Sogaro, A., Carloni, G. and Rigolli, E., 1988. Simulation Program for Urea

Plants. Comput. Chem. Eng. 21, p. 389-400.

3.1 Process Description

NH3 in

Ammonium

Carbamate Gas•Bh™Liquid AmmoniumCarbamate

LOW

PRESSURE

CARBAMATE

CONDENSER

(HPCC) •' " u

207 vapor

REMOVAL

REACTORV

A /VAmmonium

Carbamate Gas

Ammonium Carbamate

Liquid

Urea Solution

Ammonium

Carbamate Gas

HIGH

PRESSURE D

?CARBAMATE

CONDENSER

(HPCC) ,

22Figure 3.1: High Pressure Urea Synthesis Loop of ABF Plant

The overall reaction of urea production is as follows:

2NH, + CO, NH.CONH, +H,0'3 r Cond.

Urea Solution

"HIGH

PRESSURE

HEAT

EXCHANGER

(3.1)

33

The process of urea is based on two sequential steps. In the first reaction,

ammonia and carbon dioxide is reacted to form ammonium carbamate:

2NH3(g) + C02(g) NH2COONH,(I) (3.2)

Reaction (3.2) is very exothermic and fast in both directions so that it could be

considered at equilibrium at the conditions found in industrial reactors where the

residence time is rather high.

Several literatures reported that value of the heat of reaction (3.2) as -117 kJ per

mole (AH = -117 kJ mol"1)15. Weast reported that the heat of reaction at 25°C to be -84 kJ

mol"1 (AHr° = -84kJ mol"1) which is calculated from the standard enthalpy of formation

data23. PFK also reported that the heat ofreaction is found to be -38000 kcal per mole24.

Then, the ammonium carbamate is dehydrated to form urea through the following

reaction:

NH2COONH4(/) NH2CONH2(/) + H20(l) (3.3)

This reaction is endothermic and slow as compared to the preceding reaction.

Therefore, it needs a long residence time to reach the equilibrium. It has been reported

that the heat of formation of reaction (3.3) is to be +15.5 kJ per mole15. Claudel reported

AHR° = 23 kJ mol"1.25 PFK reported the value of the heat of reaction 2 to be +3 to +6 kcal

per mole24. Therefore, the overall reaction is exothermic reaction.

In this project, the formation of biuret is also considered to take place. The

correspondence reaction is as follows:

23 Weast, R.C., 1980. Ed. CRC Handbook of Chemical and Physics, 60?h ed, CRC Press: Boca Raton, FL,

Section D via Cross-Ref.

24 Abu Hafsin bin Suja', 1999. Urea Synthesis Unit. PFKOperation Engineer LogBook.

25 Claudel, B., Brousse, E. and Shehadeh, G., 1986. Novel Thermodynamic and Kinetic Investigation of

Ammonium Carbamate Decomposition into Urea, Thermochim. Acta 102, p. 357-371.

34

27V//,CONH, o NH, CONHCONH, + NH, (3.4)

This reaction is slow and endothermic and is promoted when there is a high urea

concentration, low ammonia concentration and high temperature.

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

approximately 140bar and a temperature of 180-185°C. The molarNH3/CO2 ratio applied

in the reactor is 2.95. Satyro reported that this results in a CO2 conversion of about 60%

and an NH3 conversion of 41%.16

Compressed carbon dioxide feed is passing through the high pressure heat

exchanger (stripper) which acts as the stripping agent to strip the ammonia and carbon

dioxide (the ammonia and carbon dioxide are decomposed from ammonium carbamate

and due to the free excess ammonia) from the liquid phase to the gas phase. The reverse

reaction of (3.2) is considered to take place in stripper24. In stripper (C02 stripping),

unconverted NH3 and CO2 are stripped off, leaving carbamate, urea and water. This will

move carbamate back to NH3 and C02 (fast reaction) rather than urea and water

converted back to carbamate (slow reaction). Therefore only the reverse of reaction (3.2)

is considered to take place in the stripper. This results in recovery of NH3 and CO2 as well

as a higher purity of urea is obtained. The stripper is also a shell and tube heat exchanger

in which the non-reacted ammonium carbamate from the reactor is decomposed to

ammonia and carbon dioxide. The heat of reaction for this endothermic reaction is

supplied by condensation of steam in the shell. In practice, the conversion of ammonium

carbamate in the stripper is controlled by the amount of steam being consumed in the

shell side26.

Then, the stripped-off ammonia and carbon dioxide is then introduced into the top

of the high pressure carbamate condenser. Ammonia, together with the carbamate

overflow from the scrubber, is also introduced into the top of the carbamate condenser.

The carbamate condenser is in fact a heat exchanger in which the heat generated during

26 Mohsen Hamidipour, Navid Mustoufi and Rahmat Sotudeh-Gharebagh, 2005. Modelling the Synthesis

Section ofan Industrial UreaPlant. Chemical Engineering Journal Volume 106 Issue 3, p. 249-260.

35

the condensation of ammonia and carbon dioxide to form ammonium carbamate in the

tube side is used to produce low pressure steam in the shell side. Only part of ammonia

and carbon dioxide condense in the carbamate condenser and the rest react in the urea

reactor in order to supply the heat required for the urea production reaction. Formation of

ammonium carbamate in the synthesis section is mainly occurs in the tubes of the

carbamate condenser. The conversion of carbon dioxide to ammonium carbamate is

controlled with absorbing the heat released by the reaction by the water being evaporated

in the shell side. Since the reaction (3.2) is a fast and exothermic reaction, the conversion

of carbon dioxide in the carbamate condenser could be determined by the amount of heat

exchanged between the shell and tubes (i.e. the amount of the steam generated in the

shell). Only reaction (3.2) is considered to take place in the carbamate condenser.

Liquid and gas phases leave the carbamate condenser via two separate lines to

ensure a stable flow into the reactor. In this reactor, 11 trays are installed to improve the

contact between the two phases. The liquid mixture in the reactor overflows into the

stripper while the gas phase exiting the reactor which consists of free ammonia and

carbon dioxide (the unreacted reactants) as well as the inert gas is discharged into the

scrubber. In reactor, all three reactions (Reactions 3.2, 3.3 and 3.4) are considered to take

place.

In the low pressure carbamate condenser that consist of scrubber and the heat

exchanger, the off gas from the reactor enters the scrubber to reduce the amount of

ammonia by contacting with the lean carbamate solution. In the heat exchanger section,

ammonium carbamate is formed which operates similar to high pressure carbamate

condenser. The heat of condensation is partly removed by the cooling water. In the

scrubber, only reaction (3.2) is considered to occur.

36

3.2 Thermodynamic Modeling

Urea processes are challenging to model from a thermodynamic point of view.

From one side, accurate low pressure equilibrium thermodynamic equilibrium is

necessary to model aqueous urea solution, while accurate high pressure modeling is

necessary to properly model the high pressure synthesis reactor. The thermodynamic

package also has to properly take into account the formation of new chemical species,

some which are ionic. The effect of minute amounts of inert in the saturation bubble

pressure also has to be taken into account. In addition, the model has to provide

reasonable enthalpy and entropy values for flow sheeting calculations. Last but not least,

some operations in the urea process require special behavior from the property package

calculation engine and proper communication between the unit operation and the property

package system has to be implemented.

It is recommended to using Aspen Plus to simulate the ionic compound since

HYSYS will not be able to predict well for the ionic compound27. However, since we do

not have that package, the simulation is conducted using HYSYS and all the results is

expected to have some deviation.

According to Satyro, the high-pressure section was modeled using a full ionic

model. Albeit the model showed a good performance when used to model industrial units,

enhancements were possible in terms of computational speed and accuracy with respect to

ammonia and carbon dioxide vapor compositions at the outlet of the urea synthesis

reactor. The majority of the time spent in thermodynamic calculations was determined to

be in the convergence of the ionic chemical equilibrium, and any simplification in that

area would have significant impact in the calculation speed, and therefore would allow

27 http://support.aspentech.com

37

the use of the model not only for steady state calculations but also dynamic calculations

necessary for safety studies and operator training .

In this project, the reactive system was simplified by considering all the chemical

species in their molecular states which is not true from a purely, physical-chemical point

of view since the reactions happening in the liquid phase at high pressure are well

presented by the following reaction system as proposed by Satyro :

C02 (g) + 2NH3 (g) +-> H2NCOO" (1) + NH4+ (1) (3.5)

C02 (g) + NH3 (g) + H20 (1)

Ki,x - r

K;r =

( \

V •/-' J Products

\

IK\ k-\ J Re oc tan/j

( \

V -,~1 JProducts

^ A-l / Reoctan w

38

(3.11)

(3.12)

The calculation of ionic species activity coefficients is somewhat laborious and

the details can be found in Satyro. Since the chemical equilibrium has to be evaluated at

every iteration when calculating liquid phase fugacity coefficients, any reduction in10computational load while keeping accuracy will translate into substantial time saving .

In this project, the above equations (3.5) to (3.9) have been simplified to following

equations as mentioned previously in section 3.1 Process Description.

2NH3 (g) + C02 (g) «-• NH2COONH4 (1)

NH2COONH4 (1) «-• NH2CONH2 (1) + H20 (1)

By also considering the following reaction:

2NH2CONH2

39

3.3 Effects of Process Variables on the Degree of Conversion

There are three major factors that influence the rate of conversion of ammonium

carbamate into urea. That are; operating temperature, operating pressure, excess of one

reactant as well as the residence time in the reactor. When the residence time is longer,

more carbamate will convert into urea. But, the longer the period, the higher the capital

investment that needs to be invested (i.e. the reactor volume has to be increased in order

to provide a longer residence time). Normally, residence time should be more than 20

minutes but most of the reactors operate at 55 minutes residence time.

3.3.1 Effect of Temperature on the Conversion of Carbamate to Urea

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

carbamate; the reaction is fast and exothermic. In the second reaction, which is slow and

endothermic; ammonium carbamate dehydrates to produce urea and water. Since more

heat is produce in the first reaction than consumed in the second, the overall process is

exothermic. We want to maximize the first reaction to maximize the urea production in

the second reaction. According to Le Chatelier's principle, for a reversible exothermic

reaction; operation at low temperature will increases the maximum conversion but at the

same time it will decrease the rate of reaction and hence increases the reactor volume.

% yielduraa

es -

of

1 I ' I ' I ' I ' I ' I ' I ' I

19D 140 1M 1M 170 -HO 100 SOD CIO

T»mp*r»1ut« 0

40

Figure 3.2: Relationship between Temperature and the Conversion of Carbamate to29

Urea

Figure 3.2 shows that at pressure of 140 bars the conversion percentage does

gradually increase with increasing temperature until 190°C after which the yield

decreased again. This can be explained by the fact that as temperature increases so does

the dissociation of carbamate to CO2 and NH3. Therefore in order to keep yield high the

pressure has to be kept higher than the dissociation pressure of the carbamate.

3.3.2 Effect of Pressure on the Conversion of Carbamate to Urea

Operation in the liquid phase is preferred to reduce the reactor volume and hence

the capital cost. But, there is trade off between capital cost of the reactor and the capital

cost of the compressor required as well as the operating cost of the compressor.

'www.keele.ac.uk/depts/ch/resources/urea/ureahome.html

41

From the literature, the rate of formation of carbamate is highly dependent upon

pressure and when all other factors are constant the rate of formation increases

proportionally to the square root of the pressure. The rate of formation also increase with

temperature, until a maximum is reached, at which point the dissociation pressure is equal

to the reaction pressure.

% urM yalld

At -

46 -

44 -

43 -

// -

/

• //

4a -

41 -•/

40 -

3D -

30 -

37 -

aa -m

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

flO 70 00 00 MO 110 1S0 100 140

prtMlir* *tm

Figure 3.3: Relationship between Pressure and the Conversion of Carbamate29

to Urea

The graph shows that conversion percentage increases with increasing pressure.

The pressure and temperature are linked however, as the temperature is increased so must

the pressure, to keep the yield sufficiently high. For example at 160°C the pressure must

be higher than 130 atm for maximum yield, but at 180°C it must be above 210 atm.

From the graph, it is obvious that we have to operate our reactor at high possible

pressure. But, when pressure is very high, it will incur higher capital cost due to higher

42

cost of material. Therefore, there is trade off between reactor capital cost with capital cost

and operating cost of separation system.

3.3.3 Effect of Excess of Ammonia on the Yield of Urea

An excess of one feed component can force another component towards complete

conversion. In this system, ammonia has been chosen as the excess reactant to ensure

complete conversion of carbon dioxide to ammonium carbamate and hence to urea. From

the equation (3.2) it is obvious to see that ammonia has the stoichiometric coefficient of

2. Therefore, ammonia will give a bigger impact than C02 to the equilibrium conversion

of reaction (3.2) and (3.3) consequently. From Le Chatelier's principle, it shows that by

having NH3 in excess will shift the reaction (3.2) to the right and consequently will

increase the urea production. An excess of ammonia will also shift the reaction (3.4) to

the left, and hence reduce or eliminate the formation of biuret that is toxic to the plant.

The high concentration of ammonia also shifts the biuret formation reaction to the

left, such that only a small amount of, or no biuret is formed in the reactor.

V.

conversion

to urea

Excess ammonia X

29Figure 3.4: Effect of Excess Ammonia on Conversion of Carbamate

43

Excess C02, however, shows very little effect on the yield of urea produced; one

reason for this is that the C02 remains in the gaseous phase whilst the reaction is

occurring in the molten phase.

From figure 3.4, we can conclude that by increasing the NH3 to C02 ratio will

increase the conversion of carbamate to urea. However, as the ratio increases, the reactor

volume will also increases which lead to higher capital cost. And also, as the ratio

increases, the cost or recycling the unreacted reactant will increase i.e.; the capital cost

and the operating cost of the pump required will increase. Therefore, there is trade off

between reactor capital cost and recycling cost versus the separation cost.

3.3.4 Effect of Temperature on the Formation of Biuret

BO

dbfeOttftdto blvrwt

Time: 1 hour

-' 1 ' 1 ' 1 ' 1

19D «40 ISO 180 17D

T*np O

29Figure 3.5: Effect of Temperature on Biuret Formation

44

The above graph shows that there is a very important relationship between

temperature and the formation of biuret, this being that the higher the temperature is then

the more urea is converted to biuret. The data were obtained using an unstirred melt of

urea maintained at the given temperatures for 1 hour.

Biuret is impurity which has been shown to be toxic to crops, thus the levels of

this must be kept low in the marketed urea product. An excessive amount (more than 2 wt

%) of biuret in fertilizer-grade urea is detrimental to plant growth. Prolonged heating of

aqueous urea solutions will cause the formation of biuret. This reaction is promoted by

low pressure, high temperature and prolonged heating time. At pressure of 100 to 200

atm, biuret will revert to urea when heating in the presence of ammonia.

45

4.0 CHAPTER 4: METHODOLOGY

The methodology consists of three parts. The first one is data gathering of the

ABF plant. The second part is to prepare the simulation i.e. to select the appropriate

property package for this case, selecting the components from the HYSYS library, create

the hypothetical components for non-available components (ammonium carbamate and

biuret), creating the appropriate reactiontypes, selecting the equipments and installing the

streams, and finally but yet importantly is to attach the reactions with the appropriate

equipments. The details of the series of reactions occurring in each equipment have been

described in section 3.1 Process Description.

4.1 Property Package Selection - UNIQUAC-Ideal

In this study, the property pac