ABSTRACT

Mathematical Models describing the variations in the volume of the system, concentration of
reactant (s) yet to react, temperature of the system, and the temperature of the cooling jacket
over time in a non-isothermal CSTR that handles a simple, irreversible, first order or second
order exothermic reaction in liquid phase were formulated. This work is with a particular
reference to the synthesis of propylene from cyclopropane and that of cumene (isopropyl
benzene) from benzene and propylene. The models were solved simultaneously by analytical
approach rather than the normal numerical approach employed for solving non-linear
differential equations. We noticed that the major determinants of the reactants conversion
level and the extent of reaction are the feed concentrations, feed temperature and the cooling
jacket inlet temperature. The system is found to have a single, locally stable, steady state with
periodic (underdamped) behaviors due to the existence of both inherent negative and positive
feedback in it. Nonlinear feedforward control equations show that feed flowrate does not
have to be changed when feed temperature Ti changes, rather its changes inversely with feed
concentration CAi. Again, the cooling -jacket temperature Tc changes linearly with feed
temperature Ti and nonlinearly (inversely) with feed concentration CAi.
The models were utilized to explore the dynamic response and the controllers design
equations of the system. We noticed from the dynamic response that the system is self
regulatory. Also feed forward controller is physically realizable and has two (Gc1 and Gc3)
lead elements and one gain-only element (Gc2) controller for the control of concentration
CAO and CBO, and a lag element (GC1), gain-only element (GC2) and a lead-lag element (GC3)
controller for the control of temperature, To.

 

 

TABLE OF CONTENTS

 

TITLE PAGE i
CERTIFICATE OF APPROVAL ii
DEDICATION ACKNOWLEDGEMENT iv
ABSTRACT vi
TABLE OF CONTENTS vii
LIST OF FIGURES xi
LIST OF TABLES xii
LIST OF APPENDICES xiii
CHAPTER ONE
INTRODUCTION 1
1.0 Mathematical Modeling And Control 1
1.1 Aims / Objectives 2
1.2 Significance Of The Study 2
1.3 Scope Of The Study 3
1.4 Limitations Of The Study 4
1.5 Definitions Of Terms 5
CHAPTER TWO
LITERATURE REVIEW 7
CHAPTER THREE
THE THEORY 12
3.1 Chemical Reactions 12
3.1.1 Types Of Chemical Reactions 12
3.1.2 Phase Criterion 12
3.1.3 Reaction Mechanism Criterion 12
3.1.4 Molecularity of Reactions 14
3.1.5 Order of Reaction Criterion 14
3.1.6 Temperature Conditions 15
3.1.7 Heat Energy Requirement 15
3.1.8 Catalysis Criterion 15
3.2 Reaction Progress Variables 15
3.2.1 The Molar Extent Of Reaction 16
3.2.2 Fractional Conversion 16
vii
3.2.3 Rate Of Reaction 16
3.3 Factors That Affect Rate Of Chemical Reactions 17
3.3.1 Effect Of Concentration 17
3.3.2 Effect Of Temperature 18
3.3.3 Effect Of Surface Area Of Reactants 18
3.3.4 Effect Of Pressure 18
3.3.5 Effect Of Catalyst 18
3.4 Chemical Reactors 19
3.4.1 Types Of Chemical Reactors 19
3.4.2 Batch Reactors 19
3.4.3 Steady State Flow Reactors 19
3.4.4 Semi-Batch Reactors 19
3.4.5 Isothermal Reactors 19
3.4.6 Nonisothermal Reactors 20
3.4.7 Continuous Stirred Tank Reactors (CSTR) 20
3.4.8 Plug Flow Reactor (PFR) 20
3.4.9 Fixed Bed Reactors (FBR) 20
3.4.10 Packed Bed With Counter-Current Flow Reactors (PBCCFR) 20
3.4.11 Fluidized Bed Reactors (FLBR) 20
3.4.12 The Case Study 20
3.5 The Principles Of Conservation Of Fundamental Quantities 20
3.5.1 Total Continuity Equation 21
3.5.2 Component Continuity Equation 21
3.5.3 The Equations Of Motion 22
3.5.4 The Energy Equation 23
3.6 Constitutive Balance Equations For Fundamental Quantities 23
3.6.1 Transport Equations 24
3.6.2 Equations Of State 25
3.6.3 Chemical And Phase Equilibrium 25
3.6.4 Chemical Kinetics Rate 26
3.6.5 Dead Time 27
3.6.6 The Case Study 27
viii
CHAPTER FOUR
THE MODELS AND SOLUTIONS 29
4.1 The Models 29
4.1.1 Assumptions 29
4.2 First Order, Simple, Irreversible, Exothermic Reactions 30
4.2.1 Total Mass Balance 30
4.2.2 Mass Balance On Components 31
4.2.3 Total Energy Balance 31
4.3 Characterization of Its State Variables 32
4.4 Second Order, Simple, Irreversible, Exothermic Reactions 36
4.5 Characterization of Its State Variables 37
4.6 Empirical Nth Order Reactions 39
4.7 Solution of The Models 40
4.7.1 Solution of First Order Reaction Models 40
4.7.2 Solution of Second Order Reaction Models 47
CHAPTER FIVE
APPLICATIONS, ANALYSIS, DISCUSSIONS AND CONCLUSIONS 55
5.1 Applications of the Models 55
5.2 Transfer Function of the Linearized Models of The CSTR 55
5.2.1 Transfer Function of First Order Reaction Models 55
5.2.2 Transfer Function of Second Order Reaction Models 57
5.3 The Response of the CSTR System 61
5.4 Steady State Techniques 61
5.4.1 Steady State Techniques for First Order, Nonlinear Models 61
5.4.2 Steady State Techniques for Second Order Nonlinear Models 63
5.5 Dynamic Behaviour of the Linearized Nonisothermal CSTR 64
5.5.1 Dynamic Response for the First Order Reaction Systems 66
5.5.2 Dynamic Response for the Second Order Reaction Systems 69
5.5.3 Characteristics of an Underdamped Response for First and SecondOrderReactions 73
5.6 Design of Feed Forward for the Nonisothermal CSTR 74
5.6.1 Design of Steady State Nonlinear Feed forward Controllers 74
5.6.2 Design of Dynamic Feed forward controllers for the CSTR 76
5.7 Analysis and the Method of Analysis 80
ix
5.8 Discussion of Results 83
5.9 Conclusions 86
5.10 Appendices 87
Appendix A 87
Appendix B 106
References 121

 

CHAPTER ONE

 

INTRODUCTION
1.0 MATHEMATICAL MODELING AND CONTROL
This research work is based on the characterization of a processing system, non-isothermal
continuous stirred tank reactor (CSTR) and its behaviour using a set of fundamental
dependent quantities (mass, energy, and momentum) whose values describe the natural state
of the system, and the modeling of a set of equations in the dependent variables which
describe how the natural state of the system changes with time.
The study is carried out by the use of mathematical models which are built based on the
knowledge of the constitutive equations namely; transport rate equations, equations of state,
chemical and phase equilibrium, kinetic rate equations, and dead-time. These were used in
characterizing the conservation balances on mass, energy, and momentum. This is done with
particular interest on a non-isothermal CSTR reactor for simple irreversible, exothermic
reactions in the same phase. Every physical and chemical phenomenon applied, and the
balance equations developed were all from the macroscopic viewpoint so as to moderate the
size and complexity of the emerging models.
Since the fundamental variables cannot be measured conveniently and directly, other
variables which can be measured conveniently, and when grouped appropriately determine
the values of the fundamental variables, were selected. Thus mass, energy, and momentum
can be characterized by variables (state variables) such as density, concentration,
temperature, pressure and flow rate which define the state of the system. The equations that
relate the state variables (dependent variables) to the various independent variables are
derived from application of the conservation principle on the fundamental quantities and are
called state equations.
The study was considered for a single, irreversible, unimolecular first order reaction,
bimolecular second order reaction, and the empirical nth order reaction occurring
exothermically in the same phase. The developed models were solved simultaneously by
numerical and analytical approach employed for solving non-linear differential equations.
The dynamic responses of the system were analyzed and the steady state and the dynamic
feed forward controllers design equations were derived. Such other information that may be
very necessary for the proper understanding of our mathematical models was also treated.
2
1.1 AIMS/ OBJECTIVES OF THIS STUDY
This work is aimed at the formulation of a mathematical representation of the physical
and chemical phenomena (temperature, density, pressure, concentration, flow rate, etc).
taking place in a non isothermal continuous stirred tank reactor, CSTR which handles a
simple, irreversible first or second order exothermic reaction in same phase. The
mathematical model is to describe the variations in the volume of the system, concentrations
of the reactants, temperature of the system and the temperature of the coolant over time.
Other objectives that this model is called on to satisfy or perform are to ensure the
stability in the operation of the chemical reactor, and to suppress the influence of external
disturbances on the reactor. The purpose at this stage is to translate all the important
phenomena occurring in the physical and chemical processes into quantitative mathematical
equations. The models give the understanding of what really make the process “tick”, enable
one get to the core of the system to see clearly the cause-and-effect relationships between the
variables. The work also gives a physical application of the solution of the models obtained.
1.2 SIGNIFICANCE OF THE STUDY
Mathematical models can be useful in all area of life, and as in chemical engineering,
it is useful in all phases of chemical engineering, from research and development to plant
operations, and even in business and economic studies. Most often the physical equipment of
chemical process we want to design and control has not been constructed. Consequently, we
cannot experiment to determine how the process reacts to various inputs and therefore we
cannot deign them and their appropriate control system. But even if the process equipment is
available for experimentation, the procedure is usually very costly.
Therefore, we need a simple description of how the process reacts of various inputs,
and this is what the mathematical models can provide to the process and control engineer.
Uses Of This Mathematical Model Are As Follows.
(1) Research and development
(2) Design of chemical processing equipment and their control.
(3) Plant operation and optimization is cheaper, safer, and faster done on a mathematical
model then experimentally on an operating unit.
3
1.3 SCOPE OF THE STUDY
The investigation reported in this project bothers on the formulation of a mathematical
representation of the physical and chemical phenomena occurring in the state system;
non-isothermal CSTR from a microscopic view point. In the work, the principles of
conservation of fundamental quantities (mass, energy, and momentum) were applied
using already well-developed constitutive models. We did not go into driving a reaction
mechanism, kinetic rate equations, transport rate equations, equations of stage, chemical
and phase equilibrium equations but the well developed models from a number of
postulated mechanisms were used to correlate the constitutive fundamental quantities.
The verified mathematical models for the temperature and concentration-dependent
terms of the rate equation for first order, second order, and nth order reactions were used
and not developed in this investigation Levenspiel (1972). The models were derived for a
unimolecular first order, bimolecular second order and nth order reactions respectively.
The emerged models were simultaneously solved by analytical and numerical approach
employed for solving nonlinear differential equations, and the comparison of the solutions
and the subsequent analysis were properly carried out. We considered the dynamic
responses of the system, developed its transfer functions (inputs-output interaction), and
subsequently the steady state and dynamic feed forward controllers design equations.
The research is particular to a non isothermal continuous stirred tank reactor, CSTR
that handles a simple, irreversible exothermic reaction in liquid phase to enable us predict
the rate mechanisms, rate equations and reaction occurring in it. The result obtained may
be extrapolated to cover a simple, irreversible endothermic reaction in same phase to a
good accuracy.
1.4 LIMITATIONS OF THE STUDY
There are series of difficulties that were encountered in an effort to develop a
meaningful and realistic mathematical description of this chemical process – a nonisothermal
continuous stirred tank reactor, CSTR. Serious difficulties occurred due to
incomplete knowledge of the physical and chemical phenomena taking place in the
reactor. Even an acceptable degree of knowledge is at times very difficult. How to
account for the effect of alteration of the value of the overall heat transfer co-efficient
caused by scaling, fouling etc during the operation of the reactor became a limitation.
Also we have considered only the first, second, and empirical nth order kinetics to
describe the reaction rate.
4
Again, imprecisely known parameters become an impediment. The dead time is a
critical parameter whose value is usually imprecisely known, varying and can lead to
serious stability problems. The availability of accurate value for the parameters of a
model is indispensable for any quantitative analysis of the process behaviour.
Unfortunately they are not always possible. Parameters such as densities r , heat of
reaction (- DHr), pre-exponential constant Ko, activation energy E, and overall heat
transfer coefficient U, of the jacketed reactor do not remain constant over long periods of
time but are in general functions of concentrations CA, CB, and CP, and the temperate To.
Hence, for effective modeling we need not only accurate values but also some
quantitative description of how the parametric values changes with time. How to decide
that this dependence is weak (as to use constant values) or strong (in which case the
modeling becomes very complicated) imposes a limitation. Determination of the values of
these parameters is difficult.
Also, the size and complexity of the model induces some problems. An effort to
develop as accurate and precise a model as possible, its size and complexity increase
significantly and exceed manageable levels, beyond which the model loses its value and
became less attractive.
1.5 DEFINITION OF TERMS
Fi and Fo = volumetric flow rates of the system’s inlet and outlet streams.
r i and r o=densities of the materials in the inlet and outlet streams.
Fci and Fco = volumetric flow rates of the coolant in the inlet and outlet streams.
r = Density of the material in the system.
nA and nB=number of moles of component A and B in the system.
np= number of moles of component P in the system.
CAi, CBi and CPi =molar concentration (moles/ volume) of A, B, and P in the
Inlet streams respectively.
CAo, CBo, and CPo = molar concentration (moles/ volume) of A, B, and P in the outlet
streams respectively
rA, rB, and rP = reaction rate per unit volume of components
A, B and P in the system.
hi and ho = specific enthalpy (enthalpy per unit mass) of the feed and
Outlet streams.
UE,KE,PE= internal, kinetic and potential energies of the system,

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