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Title of dissertation Steady State Modeling of Urea Synthesis Loop
I. ROZANA AZRINA BINTI SAZALI
hereby allow my dissertation to be placed at the Information Resource Center (IRC)
of Universiti Teknologi PETRONAS (UTP) with the following conditions:
1. The dissertation becomes the property of UTP.
2. The IRC of UTP may make copies of the dissertation for academic purposes only.
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If this dissertation is confidential, please state the reason:
The contents of the dissertation will remain confidential for
Remarks on disclosure:
-^Signature of Author
Permanent: No. 66, Tmn Puteri Lindungan Bintang,Address 31650, Ipoh, Perak
Date:/s- j. o.oo~f
years.
Endorsed by
Signature o/f- Supervisor
' Name of SupervisorAP.Dr.Suzana Yusup
Date: ^' 7-
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UNIVERSITI TEKNOLOGI PETRONAS
Approval by Supervisor
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'°^
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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
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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
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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.
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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.
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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
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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
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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
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CHAPTER 2
Page
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iii
iv
v
vi
ix
xiii
xv
xvii
1
2
3
3
3
3
4
4
5
7
7
8
8
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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Subscript
/ chemical reactions defined by equations 3.5 to 3.9
o initial condition
xix
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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.
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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.
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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.
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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.
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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)
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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.
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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
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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).
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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.
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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.
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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.
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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.
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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
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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.
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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
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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.
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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.
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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.
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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.
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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
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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
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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
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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.
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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.
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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)
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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.
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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.
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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.
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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
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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.
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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