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Ministry of Higher Education & scientific research University of Technology Chemical Engineering Department

ISOBARIC VAPOR-LIQUID EQUILIBRIA OF ENILOSAG

TA SMETSYS SEVITIDDA 101.3 aPk

A Thesis

Submitted to the Department of Chemical Engineering of The University of Technology

in Partial Fulfillment of the Requirements for the Degree of Master of Science

in Chemical Engineering

By

RAWA ABD AL RAZAQ KHAMAS

(B. Sc. 2003) 1429 2008

(501 )

Certificate

We certify, as an examining committee, that we have read the thesis titled

Isobaric Vapor-Liquid Equilibria of Gasoline additives Systems at 101.3kPa

and examined the student RAWA ABD AL RAZAQ KHAMAS and found that the

thesis meets the standard for the degree of Master of Science in Chemical

Engineering.

Signature : Signature:

Name : Dr. Khalid F. Chasib Name :prof. Dr. Balasim A. Abid

(Supervisor) (Member)

Date : / / 2008 Date : / / 2008

Signature :

Name : Dr.Adnan A. abdul Razak

(Member)

Date : / / 2008

Signature : Prof. Dr. Mahmoud O. Abdullah

Name :

(Chairman)

Date : / / 2008

Approval by the University of technology

Signature :

Name :

Date : / / 2008

Certification

I certify that this thesis titled Isobaric Vapor-Liquid Equilibria of

Gasoline Additives Systems at 101.3 kPa was prepared by RAWA

ABD AL RAZAQ KHAMAS, under my supervision at the Department of

Chemical Engineering, University of technology, in partial fulfillment of

the requirements for the degree of Master of science in Chemical

Engineering.

Signature:

Name : Dr. Khalid F. Chasib Data : / / 2008

ACKNOWLEDGMENT

i

ACKNOWLEDGMENT

Praise be to God for His entire blessing during the pursuit of

my academic study.

I would like to express my sincere thanks and gratitude to my

supervisor Dr. Khalid Farhod Chasib for his help throughout the

preparation of this thesis.

I would like to extend my acknowledgment to the Head of

Chemical Engineering Department for his help in providing

facilities.

Thanks are due to the rest of the staff of Chemical

Engineering Department of University of technology for their help

during my work.

I would like to express my gratitude and appreciation to my

family specially my father and my mother for their love, sacrifices

and patience during my all years of study.

Last but not least special thanks and appreciation go to my husband Abdullah Hamdan alkenzawi for his love and his help.

Abstract

ii

ABSTRACT

Obtaining accurate values for vapor-liquid equilibrium (VLE) is very

important in the design of separation equipment and some industrial applications

in chemical engineering. So in order to obtain accurate properties of VLE

attention has been turned to calculate them from equations of state (EOS) and

activity coefficient models , noting that those equations of state are strongly

nonlinear as well as inaccurate results may by obtained when they are applied to

nonideal and polar systems. To overcome these problems efforts were directed

to modify or improve EOS and their mixing and combining rules.

In this study, isobaric vapor-liquid equilibrium of gasoline additives for

three ternary systems:MTBE+Ethanol+2-Methyl-2-propanol,Ethanol+2-

Methyl-2- propanol+Octane, and MTBE+Ethanol+Octane at 101.3kPa are

studied. furthermore three binary systems: ethanol+2-Methyl-2-propanol, MTBE+Ethanol, and MTBE + Octane at 101.3 kPa have been studied.

The binary system MTBE + ethanol forms minimum boiling azeotrope.

The azeotrope data are xR1R(AZ) =0.955 mole fraction and T (AZ) =327.94 K. The

other ternary systems and the other binary systems do not form azeotrope.

All the data used from [3] and [69] passed successfully the test for

thermodynamic consistency using McDermott-Ellis test method.

In this study the calculation of VLE Kvalues is done by using three

methods, the first method uses modified Soave Redlich and Kwong (SRK), modified Peng and Robinson (PR) equations of state for two phases. The second

method uses SRK-EOS for vapor phase with (NRTL, UNIQUAC and UNIFAC

activity coefficient models for liquid phase) and using PR-EOS for vapor phase

with (NRTL, UNIQUAC and UNIFAC activity coefficient models for liquid

phase). The third method uses the Wong- Sandler mixing rules and the PRSV-

EOS based on GPE P of (NRTL and UNIQUAC activity coefficient models).

Abstract

iii

The non ideality of both vapor and liquid phases for the literature data for

the ternary and binary systems have been accounted for in developing of the

computer program for predicting VLE Kvalues using the maximum likelihood

principle for parameter estimation which provides a mathematical and

computational guarantee of global optimality in parameters estimation.

The Wong- Sandler mixing rules and the PRSV- EOS based on excess Gibbs

free energy GPE P of NRTL activity coefficient model give more accurate results

for correlation and prediction of the K-values than other methods for the ternary

and binary systems which contain asymmetric and polar compounds.

a) For the three binary systems of 71 data points the results obtained for VLE

K-values use theWong- Sandler mixing rules and the PRSV- EOS based on

GPE P of NRTL activity coefficient models , the overall average absolute

deviations (AAD) and overall percentage mean deviations are 0.0232 and

0.8387 respectively.

b) For the three ternary systems of 298 data points the results for VLE K-values

use theWong- Sandler mixing rules and the PRSV- EOS based on GPE P of

NRTL activity coefficient models, the overall average absolute deviations

(AAD) and overall percentage mean deviations are 0.011 and 1.387

respectively.

TNETNOC FO TSIL

iv

ULIST OF CONTENTS

Subject page Acknowledgment i Abstract ii List of Contents iv Nomenclature x

Chapter One

Introduction

Introduction 1 Chapter Two

Literature Survey

2.1 0BGasoline 5

2.2 Octane rating 5 2.3 lead 6

2.4 Oxygenates 7 2.5 Historical Perspective 7 2.6 Description of Oxygenates used in gasoline

2.6.1 Properties of MTBE

2.6.1.1 Physical Properties

2.6.1.2 Blending Characteristics

2.6.2 Properties of Ethanol



2.6.3 Other Oxygenates

2.6.3.1 2-methyl-2-propanol

10

13

14

14

15

16

17

18

18

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v

2.7 Environmental impact of using oxygenates in gasoline 19

2.8 Methods of Calculation of Vapor-Liquid Equilibria by Means

of an Equation of State and activity coefficient models

20

1BCHAPTER THREE

2BTHE THEORY

3.1 Phase Equilibrium

28

3.2 Equation of State

3.2.1 Properties from Equation of State

3.2.1.1 Fugacity and Fugacity Coefficients

3.2.2 Redlich - Kwong Equation of State (RK EOS)

3.2.3 Soave Redlich - Kwong Equation of State (SRK

3.2.3.1 SRK-EOS Parameters

3.2.3.2 SRK-EOS Mixing Rules

3.2.3.3 SRK-EOS Fugacity Coefficient

3.2.4 Peng-Robinson Equation of State (PR EOS)

3.2.4.1 PR-EOS Parameters

3.2.4.2 PR-EOS Mixing Rules

3.2.4.3 PR-EOS Fugacity Coefficient

3.2.5 SRK and PR Equations of State Failure and

Improved Points

29

32

32

35

37

39

39

40

40

41

41

42

42

3.3 Activity Coefficient Models

3.3.1 Activity and Activity Coefficients

43

43

3.4 Vapor Pressure Equations 44

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vi

3.5 Prediction and Correlation of Vapor Liquid Equilibrium

Data

3.5.1 Non-Random Two-Liquid (NRTL) Model

3.5.2 The UNIQUAC Equations

3.5.3 3BPrinciple of the Group Contribution Methods

3.5.3.1 UNIFAC model

45

46

48

51

52

3.6 Calculations with Mixing Rules Based on G PE P Models.

3.6.1 Wong-Sandler mixing rules

3.6.2 Peng-Robbinson-Strryjek-Vera Equation of state(PRSV-

EOS)

3.6.3 Using NRTL expression

3.6.4 Using UNIQUAC expression

54

54

57

58

58

3.7 Phase Equilibrium Calculations (K-Values) 59

3.8 Modification of Mixing and Combining Rules

3.8.1 First Correlation Approach Method

3.8.2 Second Correlation Approach Method

61

62

63

3.9 Equation of State Roots 65

3.10 Parameter Estimation

3.10.1 Maximum-Likelihood Principle

3.10.2 Effects of Estimated Variances

3.10.3 Systematic Error

67

67

70

70

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vii

3.11 Thermodynamic Consistency Test

70

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 System Selection

4.1.1 Binary systems

4.1.2 Ternary systems

73

73

75

4.2 Thermodynamic Consistency Test 81

4.3 Statistical Measurement and Analysis of Dispersion 82

4.4 Improvement of Equation of State 83

4.5 Computer Implementation

4.5.1 Estimation of Equilibrium Constant

4.5.2 Solution of Cubic Equation of State

4.5.3 Interaction Coefficient

85

85

86

90

4.6 Discussion of Results

4.6.1 Binary systems 4.6.2 Ternary systems

91

92

92

CHAPTER five

Conclusion and Recommendation for future work

5.1Conclusion 107

5.2 Recommendation for Future Work 112

REFERENCES

113

APPENDIX A TABLES OF DATA FOR BINARY & TERNARY SYSTEMS

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viii

A.1 Binary systems A.1.1 binary system ()

[Ethanol (1) +2-methyl-2-propanol (2)] A.1.2 binary system() [MTBE (1) + Ethanol (2)] A.1.3 binary system() [MTBE (1) +Octane (2)]

A-1 A-1 A-11 A-21

A.2 Ternary systems A.2.1 ternary system () [MTBE (1) +Ethanol (2) + 2-methyl-2-propanol(3) ] A.2.2 ternary system () [Ethanol (1) +2-methyl-2-propanol (2) +Octane (3)] A.2.3 ternary system () [MTBE (1) +Ethanol (2) + Octane (3)]

A-26 A-26 A-46 A-66

APPENDIX B TABLES OF EQUILIBRIUM CONSTANT FOR BINARY

&TERNARY SYSTEMS K-VALUE

B.1 BINARY SYSTEMES B.1.1 binary system ()

[Ethanol (1) +2-methyl-2-propanol (2)] B.1.2 binary system()

[MTBE (1) + Ethanol (2)] B.1.3 binary system()

[MTBE (1) +Octane (2)]

B-1 B-1 B-11 B-16

B.2 Ternary SYSTEMES: B.2.1 ternary system ()

[MTBE (1) +Ethanol (2) + 2-methyl-2-propanol (3) ] B.2.2 ternary system ()

[Ethanol (1) +2-methyl-2-propanol (2) +Octane (3)] B.2.3 ternary system () [MTBE (1) +Ethanol (2) + Octane (3)]

B-21 B-21 B-41 B-61

TNETNOC FO TSIL

ix

APPENDIX C CALCULATED RESULTS

C-1

4BAppendix D

5BCOMPUTER PROGRAM

D.1 Finding the Roots of a Polynomial D-1

D.2 Optimization Technique for Parameters Estimation D-2

D.3 NRTL activity Coefficient model program D-6

D.4 BASIC program for UNIFAC model

D-7 D.5 Prediction of Phase Equilibria K-Values D-23

6BAppendix E

7BExample of calculations

E-1

Note: all appendixes are shown in CD

Nomenclature

x

NOMENCLATURE

UAbbreviations

UAbbreviationU UMeaning U

AAD Average Absolute Deviations

CEOS Cubic equation of state

EOS Equation of State

EOS-GE Excess Gibbs free energy (G PE P) equation of state model

K-Value Equilibrium Constant

LLE Liquid-Liquid equilibrium

Mean D % Percentage of Mean Overall Deviation

NRTL Non-Random Two Liquid activity coefficient model

OF Objective Function

PR Peng and Robinson equation of state

PRSV Stryjek and Vera modification of Peng and Robinson

Equation of state

SRK Soave Redlich and Kwong equation of state

UNIFAC UNIQUAC Functional Group Activity Coefficients model

UNIQUAC Universal Quasi-Chemical activity coefficient model

VdW Van der Waals equation of state

VLE Vapor Liquid Equilibrium

WS Wong and Sandler mixing rules

Nomenclature

xi

Symbols

Symbol Meaning Unit

a Attractive term (parameter) of the cubic equation of state

a PE PREOSR Excess molar Helmoltz energy kJ / kmol aRiR Combining rule coefficient

aRijR Mixing rule coefficienamn

aRmn RGroup interaction parameter of equation (3.76) K Oija Mixing rule coefficient

A Cohesion parameter of cubic equation of state

A Vapor pressure coefficient of Antoine equation, equation C.14

ARiR Combining rule coefficient

b Covolume term (parameter) of the cubic equation of state

bRiR Combining rule coefficient

B Covolume term (parameter) of the cubic equation of state

B Vapor pressure coefficient of Antoine equation, equation C.14

BRiR Combining rule coefficient

C Vapor pressure coefficient of Antoine equation, equation C.14

CR REmpirical constant of equation (3.77)

D Local deviation, equation (4.2)

DRcdR Deviation of pair of points c and d

DRmaxR Local maximum deviation

fRiR Fugacity of pure component i

f P` PRiR Fugacity of component i in the solution

f Pig P Fugacity of ideal gas

gRijR Energy parameter of equation (3.52) J/mol GPE P Excess Gibbs free energy model kJ / kmol

Nomenclature

xii

hRijR Binary interaction coefficient of equation (3.116)

h PE PMolar excess enthalpy

kRijR Binary interaction parameter for components i and j

K1cal Calculated K-value of component 1

ijka Interaction coefficient of equation (3.109)

ijkb Interaction coefficient of equation (3.110)

KRiR K-values or equilibrium constant of component i

lRi RConstant defined on equation (3.65)

m Emprical constant for pure component of equation (3.106)

M Intensive property; M = K-value or T

n Emprical constant for pure component of equation (3.106)

nRiR Number of moles of component i

N Number of compounds in the mixture

P Equilibrium pressure of the system MPa

PRCR Critical Pressure MPa

PRiR Vapor pressure of pure component i MPa

PRiRPsP Vapor pressure at saturation of component i MPa

PRrR Reduce Pressure

PRlitR Literature Pressure MPa

PRcalR Calculated Pressure MPa

qRiR Pure component parameter of equation (3.71)

QRk RGroup surface parameter of equation (3.72)

r Pure component parameter of equation (3.71)

R Universal gas constant, its value equal to 8.314 Kmol

MPacm

3

Rv Ratio of volumes

S Objective function, equation (3.117)

tRaR, tRbR Temperature of point a and b , equation 4.3 K

T Temperature K

Nomenclature

xiii

TRCR Critical temperature K

TRrR Reduced temperature

uRij REnergy parameter of equation (3.63)

v Pure component molar volume cmP3P/kmole vRguessR Initial guess of molar volume calculation cmP3P/kmole V Volume cmP3 VPE PExcess molar volume cmP3P/kmole xRiR Mole fraction of component i in liquid phase

XRm R Group mole fraction for group m

yRiR Mole fraction of component i in vapor phase

z Compressibility factor

zRcR Critical compressibility factor

Greek Litters Symbol Meaning

Temperature dependency of the attractive term of equation of state

Activity coefficient

RR Activity coefficient of group k in equation (3.73)

Poynting factor of equation (3.47) Partial differentiation vP(i)PRk RNumber of group of type k in molecule i

Standard deviation

P2P Estimated variance

Fugacity coefficient

Rij REmpirical constant of equation (3.55)

RmnR Group interaction parameter of equation (3.76)

Acentric factor

Nomenclature

xiv

Subscript

Symbol Meaning

c Critical property

cal. Calculated value

cd Pair of points c and d

lit. Literature value

guess Initial guess

i Component i in the mixture

ii interaction between the i molecules

ij Binary interaction between i and j

j Component j in the mixture

m Mixture

max Maximum value

Superscript

Symbol Meaning

(1), (2), (M) Equilibrium phases

o Pure compound

c Calculated value

c Configurational

cal Calculated value

E Excess property

ig Ideal gas

lit. Literature value

L Liquid phase

Nomenclature

xv

o Estimated true value

obs Observed value

r residual

s Saturation condition

V Vapor phase

Chapter one: Introduction

1

CHAPTER ONE

INTRODUCTION Phase equilibria and fluid properties calculations are required in the design

of separation processes and analysis of petroleum and gas production operations.

The separation of multicomponent mixtures into pure components or fractions of

desired composition is of great interest in the petroleum industry [1].

The study of gasoline + alcohol and ethers mixtures using the methods of

physical-chemical analysis is considered at the present time as a difficult goal as

gasoline is an extremely complex mixture of hydrocarbons of varying composition.

Accordingly, a more appropriate approach would seem to be to study model

hydrocarbon + alcohol and ethers mixtures composed of a small number of

individual compounds [2].

The reasons for studying mixtures of hydrocarbons and oxygen-containing compounds relating to the use of oxygen-containing compounds in motor fuels.

Ethers and alcohols used as gasoline additives have excellent antiknock

properties and are environmentally acceptable substances. Gasoline blended with

about 7-15 % 2-methoxy-2-methyl propane (MTBE) has been used for high-

performance premium gasoline. On the other hand, recommendations for gasoline

additives include not only pure MTBE but also mixtures with alcohols for high-

octane gasoline [3].

When thermodynamics is applied to vapor-liquid equilibrium, the goal is to

find by calculation the temperatures, pressures, and compositions of phases in

equilibrium. Indeed, thermodynamics provides the mathematical framework for the

systematic correlation, extension, generalization, evaluation, and interpretation of

data. Moreover, it is the means by which the predictions of various theories of


2

molecular physics and statistical mechanics may be applied to practical purposes.

None of this can be accomplished without models for the behavior of systems in

vapor-liquid equilibrium [4].

The quantitative prediction of phase behavior is a central problem in

chemical engineering thermodynamics and a very important consideration in the

design of chemical process plant.

The most infinite number of possible mixture and wide range of temperature

and pressure encountered in process engineering is such that no single

thermodynamic model is ever likely to be applicable in all cases.

The basic problem to deal with thermodynamic modeling of petroleum fluids

is the calculation of high-pressure vapor-liquid equilibria. This is usually done by

means of equation of state (EOS) among which the most common are Redlich-

Kwong-Soave (SRK) and Peng-Robinson (PR). Nevertheless, those equations

when applied to mixtures require mixing rules in which molecular interactions are

accounted for by means of a binary interaction parameter, the so-called kRijR, whose

choice is difficult even for the simplest systems [5].

The literature data were correlated using activity coefficient models for the

liquid phase and equation of state (EOS) for the vapor phase and some time with

the liquid phase too.

Many proposed activity coefficient models can be used to correlate the

literature data for the liquid phase for binary and ternary systems. Among these

models are Non-Random Two-Liquid (NRTL) model, the Universal quasi-chemical

equations (UNIQUAC), and the Uniquac functional group activity coefficients

(UNIFAC).

An important advance in the description of phase equilibria is to combine the

strengths of both EOS and activity coefficient approaches by forcing the mixing

rule of an EOS to behave with composition dependence like the G PE P model. These


3

are called GPE P mixing rules and generally include the direct use of activity

coefficient parameters fitted to VLE data [6].

In common practice, a chemical engineer is concerned with models for

predicting vapor-liquid equilibria from a minimum amount of data, with a

minimum set of parameters [7].

In the past five decades the researchers tried to calculate the properties of

vapor-liquid equilibria by making a mathematical model, which is a function of

composition and some constants, which fit the experimental data. This

mathematical model is not supported by any theoretical base. With the

development of computer and computer programs the use of analytical expressions

to interpolate, extrapolate and even predict thermodynamic information has become

of increasing importance for process design and for modeling of process operation

[8].

The scope of this work encompasses the following objectives:

1- Studying the vapor-liquid equilibrium data found in literature for Three

ternary systems: MTBE+Ethanol+2-Methyl-2-propanol, Ethanol+2-

Methyl-2-propanol+Octane, and MTBE+Ethanol+Octane And three binary

systems: ethanol+2-Methyl-2-propanol, MTBE+Ethanol, and MTBE +

Octane at 101.3 kPa.

2- Studying the most important equations of state and activity coefficient

models for their abilities to predict vapor-liquid equilibria K-values for binary

and ternary systems accurately.

3- Testing main existing correlations against values found in literature. This test

includes the effectiveness of interaction coefficients on the accuracy of


4

predictions of vapor-liquid equilibria K-values for the ternary systems and

the binary systems.

4- Developing of a computer program for correlation of vapor-liquid equilibria

K-values for the given mixtures.

Chapter two: Literature survey

5

CHAPTER TWO

LITERATURE SURVEY

The principal focus of this survey is on the following:

1- Definition of gasoline additives and the usage history and the properties of

gasoline additives.

2- Definition of the nature of problems associated with vapor-liquid equilibria.

3- Description of the methods for prediction of the properties of vapor-liquid

equilibria.

2.1 Gasoline Gasoline is a mixture of about 150 chemicals refined from crude oil. Its

usually a colorless, light brown or pink liquid. Gasoline is used in cars, boats,

motorcycles and other engines. Gasoline usually contains additives affecting the

way it burns.

In order to improve gasoline performance, many gasoline additives have been

developed. Starting in the 1920s, organo-lead compounds (e.g. tetraethyl-lead)

were added to gasoline to increase octane levels and gasoline performance [9].

2.2 Octane Rating

An important characteristic of gasoline is its octane rating, which is a

measure of how resistant gasoline is to the abnormal combustion phenomenon

known as detonation (also known as knocking, pinging, spark knock, and other

names). Deflagration is the normal type of combustion.


6

Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane (an

isomer of octane) and n-heptane. There are a number of different conventions for

expressing the octane rating; therefore, the same fuel may be labeled with a

different numbers, depending upon the system used.

2.3 Lead

The mixture known as gasoline, when used in high compression internal

combustion engines, has a tendency to auto ignite (detonation) causing a damaging

engine knocking (also called pinging or pinking) noise. Early research into

this effect was led by A.H. Gibson and Harry Ricardo in England and Thomas

Midgley and Thomas Boyd in the United States.The discovery that lead additives

modified this behavior led to the widespread adoption of the practice in the 1920s

and therefore more powerful higher compression engines. The most popular

additive was tetraethyl lead. However, with the discovery of the environmental and

health damage caused by lead, and the incompatibility of lead with catalytic

converters found on virtually all newly sold US automobiles since 1975s, this

practice began to wane (encouraged by many governments introducing differential

tax rates) in the 1980s. Most countries are phasing out leaded fuel; different

additives have replaced the lead compounds. The most popular additives include

aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).

In the U.S., where lead was blended with gasoline (primarily to boost octane

levels) since the early 1920s, standards to phase out leaded gasoline were first

implemented in 1970.

Many classic cars engines have needed modification to use lead-free fuels

since leaded fuels became unavailable. However, lead substitute products are

also produced and can sometimes be found at auto parts stores.


7

Gasoline, as delivered at the pump, also contains additives to reduce internal

engine carbon buildups, improve combustion, and to allow easier starting in cold

climates [10].

2.4 Oxygenates

Oxygenates are hydrocarbons that contain one or more oxygen atoms.

Oxygenates are usually employed as gasoline additives to reduce carbon monoxide that is created during the burning of the fuel.

The oxygenates commonly used are either alcohols or ethers:

Alcohols: Methanol (MeOH) Ethanol (EtOH) Isopropyl alcohol (IPA) n-butanol (BuOH) Gasoline grade t-butanol (GTBA)

Ethers: Methyl tert-butyl ether (MTBE) Tertiary amyl methyl ether (TAME) Tertiary hexyl methyl ether (THEME) Ethyl tertiary butyl ether (ETBE) Tertiary amyl ethyl ether (TAEE) Diisopropyl ether (DIPE)

2.5 Historical Perspective Gasoline refiners began using oxygenates in the U.S. principally as octane

boosters. Due to the lead phase out which began in 1973, refiners had to replace the

octane loss in gasoline. Oxygenates were a natural choice to replace the octane loss

because they have relatively high octane ratings.

Adding a small volume of oxygenate into gasoline offers significant gains in

the octane of the gasoline blend. Oxygenates such as ethanol were also used as

volume extenders in gasoline during the 1974 energy crisis. Refiners blended


8

gasoline with ethanol (gasohol) up to 10 volume percent to increase gasoline

supplies.

In California, most refiners have designed their refineries around the ability

to use MTBE to meet state and federal requirements for oxygenated and

reformulated gasoline and to provide the desired gasoline volumes.

In the early 20th century, automotive engineers discovered that engines with

no knocks would operate in smoother and more efficient way. In 1916, Thomas

Midgely[11] a research scientist working for the Dayton Research Laboratories of

Dayton, Ohio discovered that addition of iodine to gasoline substantially reduced

engine knocks. Engine knocks related to low quality of fuel combustion later

known as octane. Iodine raised octane and eliminated the knocks. Iodine has two

major drawbacks; it is corrosive and prohibitively expensive. In a joint research

work in 1917, Charles Kettering (inventor of electric self-starter) and Midgley

blended ethyl alcohol (grain alcohol) with gasoline and concluded that alcohols

mixed with gasoline could produce a suitable motor fuel [12].

During his search for chemicals that could be added to gasoline and reduce

engine knocks, Midgley [11] discovered the antiknock properties of tetraethyl lead

(TEL) in December 1921. Manufacturing of TEL began in 1923 with a small

operation in Dayton, Ohio that produced about 600 liter of TEL per day. One liter

of TEL was enough to treat 1150 liter of gasoline. The Research on ethanol-

blended gasoline continued until August of 1925, when Kettering announced a new

fuel called Synthol a mixture of alcohol and gasoline that would double gas

mileage. Oil companies preferred TEL to ethanol because addition of ethanol to

gasoline would have reduced vehicle's use of gasoline by 20-30%, thus making cars

less dependable on petroleum products.

In 1923, some well-known public health and medical authorities at leading

universities, including Reid Hunt of Harvard, Yandell Henderson of Yale and Erik


9

Krause of the Institute of Technology, Postdam, and Germany wrote letters to

Midgley, expressing grave concerns over TEL and its poisonous characteristics

[12].

They chose MTBE because of its favorable blending properties and lower

cost. In California, over 95 percent of the oxygenated gasoline and reformulated

gasoline today is blended with MTBE. Also most refiners outside California use

MTBE to comply with government oxygenates requirements [13].

MTBE was first used as a gasoline additive in the U.S. in 1979, primarily as an

octane booster (3-8% by volume). Its use was infrequent and predominantly in

higher-grade (i.e. higher octane) gasolines. MTBE use was apparently most

common in New England, New Jersey and the eastern seaboard from 1979 through

the mid-1980s [14, 15].

In the late 1980s, MTBE usage for octane boosting increased steadily,

followed by its additional application in making cleaner burning gasoline in some

states.

Currently, 19 states in USA use regulatory-driven ''reformulated gasoline''

(RFG) containing MTBE up to 11% by volume. Other states use ''Oxy-fuel'' which

contains up to 15% MTBE, while even more states use MTBE as an octane booster

at up to 8% by volume [16]. In addition to varying with application (RFG, Oxy-

fuel, or octane booster), MTBE percentages in gasoline also vary with locale,

season, regional availability, economics, and the open market sale or trading of

gasoline and oxygenate additives. MTBE is the most popular gasoline additive used

today, with approximately 8 billion kg produced in the U.S.A. in 1997 [16].

Essentially all the MTBE produced is used in gasoline.

EPA (U. S. Environmental Protection Agency) attribute marked

improvements in air quality to the use of fuels containing MTBE (2-methoxy-2-

methylpropane) and other oxygenates. In Los Angeles, which had the worst air


10

quality in the USA, the use of reformulated gasoline was credited with reducing

ground-level ozone by 18% during the 1996 smog season, compared to weather-

adjusting data for the same period in 1994 and 1995 [17]. Therefore, MTBE

production has increased continuously during the last decade. It is used as an

octane-enhancing component and it enabled the ban of lead in gasoline. It also

enhances gasoline burning and this improves air quality in areas where car density

is high. However, leaks in gasoline storage have produced smell and contamination

of ground water in some area, particularly in California [18], where MTBE was

banned by the end of 2003. The originally planned ban at the end of 2002 was

postponed. New initiatives are required for replacement of MTBE. One option is to

transform old MTBE plants to reduce isooctane. It is made by dimerization of

isobutylene in the presence of alcohol and then hydrogenate isooctane to iso-

octane. The formation of azeotropes between iso-octane and alcohol is possible

[19].

Furthermore, ETBE, when compared to MTBE, has a higher octane rating

and a lower volatility and is less hydrophilic to permeate and pollute groundwater

supplies as well as being chemically more similar to hydrocarbons [19].

Ethanol can be used as such for gasoline or as a raw material for gasoline

component production. Ethanol is produced in vast amounts by fermentation.

2.6 DESCRIPTION OF OXYGENATES USED IN GASOLINE Oxygenates are chemical compounds that contain oxygen. There are several

oxygenates that can be blended into gasoline. Oxygenates contribute to improved

combustion, thereby reducing CO and hydrocarbon emissions in motor vehicle

exhaust. Oxygenates generally have a lower volumetric energy content than

gasoline. Thus, oxygenates reduce the energy content of the gasoline blend to


11

which they are added. This reduction in the gasoline's energy content results in

about 1 to 3 percent reduction in a vehicle's fuel economy [20].

Benefits and problems associated with gasoline additives in the United States

are reviewed by Nadim et al. [21]

Stephens [22] studied the comparison of Ethanol and MTBE and their use as

additives to Gasoline. The primary objective was to investigate the different

qualities of two different oxygenate additives. His goal was to recommend if

MTBE or Ethanol would be a better additive when looking at the impacts that each

has on the environment and human health. The results aim to provide evidence of

not only the benefits of oxygenating fuel but also to determine which of the two

primary oxygenate candidates (ethanol and MTBE) should be used as the standard

additive to reformulated gasoline. He discovered from this research , ethanol seems

to be the better additive based on oxygenates P Pdynamics regarding air and water

quality as well as human health.

Table (1) summarizes key properties of various oxygenates. Some of these

oxygenates can not be blended into gasoline at the present time because they are

not registered by the U.S. EPA as gasoline additives. However, at its discretion, the

U.S. EPA can register an additive based on emissions and health studies of other

similar additives which it has already registered.


12

TABLE(1)


13

2.6.1 Properties of MTBE

MTBE also known as: methyl t-butyl ether, tert-butyl methyl ether and 2-

methoxy-2-methylpropane.

Most MTBE is currently manufactured by a process which involves

isobutylene and methanol. Isobutylene feed for ether production is primarily

derived from steam cracking during olefin production and fluid catalytic cracking

during gasoline production. Normal butane can also be isomerized and

dehydrogenated to produce additional isobutylene feed. Methanol is produced

primarily through the synthesis of a mixture of carbon and hydrogen. The main

feed stock for methanol production is natural gas, although other light petroleum

distillates and coal can also be used. MTBE is used primarily as a gasoline additive

[13].

Using gasoline additive MTBE in forensic environmental investigations is

reviewed by Davidson and Creek [23]. The objective of this paper was to review

how MTBE data could be used when conducting forensic investigations of

subsurface gasoline spill. By carefully studying the occurrence and movement of

MTBE, it was possible to determine critical information about the gasoline spill

history at a site including the source, timing, and number of spills.

MTBE is generally blended in gasoline in volumes up to 15 percent. At a

refinery with a ether facility, MTBE and TAME are produced by mixing a

feedstock of isobutylene (alkanes and alkenes) with methanol in a controlled

optimized reaction. The quantities of MTBE and TAME produced will vary

depending on the ratio of the isobutenes and isoamylenes (2 ethyl 1 butene) in the

feed stock from the refinery. Controlling reaction temperature and optimizing the

ratio of methanol to isobutenes and isoamylenes is critical because in addition to

the reaction being reversible and controlled by equilibrium, a low temperature

favors MTBE while a higher temperature favors TAME.


14

The process is usually optimized to maximize MTBE, however production of

some TAME can not be avoided. Side reactions of isobutylenes and isoamylenes

also produce a small amount of tertiary butyl alcohol (TBA) and diisobutylene

(DIB). These by-products remain in the MTBE/TAME product, but do not

adversely affect the product since they are also acceptable gasoline components.


MTBE (CHR3ROC(CH R3R) R3R) is a synthetic chemical used as a blending component in

gasoline. It is a colorless liquid at room temperature and is flammable. MTBE is

relatively nonvolatile and has a high octane value. MTBE is soluble in gasoline

which means it disperses evenly and stays suspended without requiring physical

mixing. MTBE has a water solubility of 4.3 percent and a boiling point of 131 PoPF.

The energy content of MTBE is about 93,000 British thermal units (BTU) per

gallon.


MTBE has an octane rating of 110 which makes it particularly useful as a

gasoline octane booster because aromatic compounds such as benzene, traditionally

used to increase gasoline octane, are limited under the reformulated gasoline

regulations. These regulations required that aromatic and benzene levels of gasoline

be reduced, thus refiners have relied more heavily on oxygenates to meet the octane

requirements.

MTBE also provides good dilution of undesirable components (aromatics,

sulfur, olefin and benzene). The MTBE molecule contains 18.2 percent oxygen by

weight. The dilution effect of oxygenates, including MTBEs, is directly

proportional to the volume used in gasoline, thereby playing an important role in

meeting reformulated gasoline requirements for these compounds.


15

Blending MTBE also makes it easier for refiners to meet distillation

temperature requirements for reformulated gasoline, due to its relatively low

boiling point. MTBE depresses the distillation temperature of the gasoline blend.

The blending of 11 percent MTBE into gasoline reduces the gasolines TR50R (the

temperature at which 50 percent of the gasoline evaporates) by 10 to 20 F, and the

T90 (the temperature at which 90 percent of the gasoline evaporates) by 2 to 6 P PF.

The favorable properties of MTBE make it a useful blending component.

MTBE readily mixes with other gasoline components and can be readily

transported through the existing gasoline distribution system. Refiners can easily

integrate blending, shipping and, in some cases, production of MTBE into their

existing operations.

2.6.2 Properties of Ethanol

Ethanol has been used in gasoline blends (known as "gasohol") in the U.S. for

many years. The most common blends historically contained 10 volume percent

ethanol. Ethanol can also be used as pure ethanol (E100) and as a blend of 85

volume percent ethanol mixed with15 volume percent gasoline (E85). Ethanol used

in the U.S. is produced domestically primarily from corn. There is ongoing research

to commercialize production of ethanol from other crops, as well as from cellulosic

materials such as wood or paper wastes. Since ethanol is produced from plants that

harness sunlight, ethanol is also considered a renewable fuel.

Currently, nearly a billion gallons of ethanol per year are added to U.S.

gasoline stocks to create gasohol, a 90 percent gasoline/10 percent ethanol [24].

In looking to ethanol use as an aid to reducing automotive air pollution, the

sought-after benefits are quite different for blends and neat ethanol use. The

addition of small quantities of ethanol to gasoline as in gasohol is viewed primarily

as a means to reduce carbon monoxide emissions; use of neat ethanol is viewed


16

primarily as a means to reduce concentrations of urban ozone, by reducing the

reactivity of the organic component of vehicle emissions [24].

Ethanol is made by the fermentation of sugars. More than half of industrial

ethanol is still made by this process. The ethanol produced by fermentation ranges

in concentration from a few percent up to about 14 percent. Ethanol is normally

concentrated by distillation of aqueous solutions, but the composition of the vapor

from aqueous ethanol is 96 percent ethanol and four percent water. Commercial

ethanol typically contains 95 percent by volume ethanol and five percent water.

Most industrial ethanol is denatured to prevent its use as a beverage. Denatured

ethanol contains small amounts, 1 or 2 percent each, of several different unpleasant

or poisonous substances [13].

Ethanol as lead replacement was proposed by Thomas and Kwong [25].

When lead is phased out of gasoline in Africa, the rising cost of lead additives and

of gasoline, and the falling cost of ethanol and sugarcane, have created favorable

economic condition for fuel ethanol production. Lead additives are used in

gasoline because they have been the cheapest source of octane. The most common

ways to eliminate the need for lead additives have been to upgrade refineries to

increase production of aromatic and alkylated hydrocarbons, or to use the additive

MTBE.


Ethanol is an alcohol, a group of chemical compounds whose molecules

contain a hydroxyl group, -OH, bonded to a carbon atom. Ethanol (CHR3RCHR2ROH) is

a clear, colorless liquid with a characteristic agreeable odor. Ethanol is miscible

(mixable) in all proportions with water and with most organic solvents. Ethanol has

a boiling point of 173 PoPF and an energy content of 76,000 BTU per gallon [13].


17


Ethanol has an octane rating of 115 and a blending RVP of 18. Ethanols

blending RVP is significantly higher than most of the ethers, including MTBE. The

ethanol molecule contains 34.8 percent oxygen by weight, almost twice as much as

MTBE. Thus, ethanol will provide less dilution than the other oxygenates when

blended at the same oxygen level.

The use of ethanol presents some blending and distribution-related problems

not seen with MTBE and the other ethers. Ethanol has a high affinity for water. If a

gasoline containing ethanol comes into contact with water, the water and ethanol

will combine into a separate phase and separate from the gasoline. Because of the

potential for phase separation, ethanol and gasoline containing ethanol are not

transported through pipelines where there is a possibility of water being present.

Therefore, when ethanol is blended into gasoline, the blending must be done at the

terminals. Terminals in California are not currently equipped to handle large

amounts of ethanol blending. A further complication is that most ethanol is

currently produced in the Midwest which adds additional costs to import it into

California.

Another important limitation to the use of ethanol in gasoline, specifically

during the summer months, is its effect on the RVP of gasoline. Ethanol will

increase the RVP of the blend by about 1 pound per square inch (psi) at the

required levels. Prior to the introduction of reformulated gasoline, ethanol blends

had been granted a 1 psi RVP allowance. The federal government does not provide

ethanol blends an RVP allowance where reformulated gasoline is required.

California also does not provide an RVP allowance for ethanol blends since the

Introduction of cleaner-burning gasoline [13].


18

2.6.3 Other Oxygenates

The process for producing ETBE is similar to that of MTBE. The refinery

feedstock is combined with ethanol in a controlled and optimized ratio to

isobutylene in a catalytic reaction. Side reactions of the ETBE process will produce

small amounts of TBA, DIB, and diethyl ether. Because of the azeotropic

properties of ethanol, the ETBE system differs markedly from that used for

MTBE/TAME. Significant amount of ethanol is present in the affluent from the

reactor which must be further fractionated to produce high purity ETBE.

ETBE and TAME are similar to MTBE in terms of the benefits they provide.

ETBE has a slightly higher and TAME slightly lower octane rating than MTBE.

Both ETBE and TAME have significantly lower blending RVP than MTBE.

However, ETBE and TAME provide less TR50R depression benefits than MTBE and

as a result they provide less flexibility to refiners in meeting the cleaner-burning

gasoline requirement for TR50R.

Tertiary butyl alcohol (TBA) has an octane rating of about 100, about 10

numbers less than MTBE but still higher than most gasoline blending components.

However, TBA, like ethanol, increases the volatility of gasoline or its tendency to

evaporate, so that the mixture of gasoline and TBA will not comply with

reformulated gasoline specifications for gasoline volatility, unless steps are taken to

adjust the volatility of the blend [13].

2.6.3.1 2-methyl-2-propanol

Also known as: t-butyl alcohol; 1,1-Dimethylethanol; Trimethylcarbinol; 2-

Methylpropan-2-ol; tert-Butanol; TBA; t-butyl hydroxide; trimethyl methanol;

Dimethylethanol; Methyl-2-propanol; tertiary-butyl alcohol .


19

tert-Butyl alcohol is used as a solvent, as a 0Tdenaturant 0T for ethanol, as an

ingredient in 0Tpaint removers 0T, as an 0Toctane0T booster for 0Tgasoline0T, as an 0Toxygenate0T

0Tgasoline additive0T, and as an intermediate in the synthesis of other chemical

commodities such as flavors and perfumes .

Tert-butyl alcohol (TBA) is a clear, noncorrosive liquid. It is miscible with

water as well as most common organic solvents and forms azeotrope. The

satirically hindered tertiary butyl group imparts stability compared to primary and

secondary alcohols. In result, the solubility and oxidative stability characteristics

provide many industrial applications as a reaction and process solvent and chemical

intermediate. It is used as a non-reactive solvent for chemical reactions, a non-

surfactant compatibilizer for many solvent blends, and a non-corrosive solvent. It is

used in free radical polymerizations to dissolve monomers. TBA is the main raw

material of tert-butyl functional group in organic synthesis. It is used as a coupling

aid for formulating pesticides and fertilizers in aqueous solutions without

generating an emulsion. TBA was used as a fuel additive for high octane

component to replace tetraethyl lead. TBA is used as a reaction solvent as well as

an intermediate for the product of organic peroxides, metal alkoxides. Tert-butyl

group is readily cleaved under strongly acidic conditions. TBA is a source to

produce alkylated aromatic and nitrogen compounds in mid acidic conditions.

Dehydration reaction of TBA yields isobutylene which is the main raw material of

MTBE, the mostly used fuel oxygenates.

2.7 ENVIRONMENTAL IMPACTS OF USING OXYGENATES

IN GASOLINE

The primary emission benefits of oxygenate use in gasoline is a reduction in

CO and hydrocarbon emissions from motor vehicles. Also, as reformulated


20

gasoline regulations have required the reduction of aromatic and benzene levels in

gasoline, oxygenates have played an important role in reducing these toxic

compounds. Although the use of oxygenates reduces the overall toxicity of vehicle

emissions, there are some potential adverse effects from the oxygenates

themselves.

By the late 1970s, lead emissions from automobiles were recognized as a

serious problem, and so a new class of octane-boosting additives emerged: alcohols

and ethers. Alcohol and ether additives provide several benefits including higher

octane, favorable cost-effectiveness, and lower air pollution emissions (by adding

oxygen to the gasoline). . Because of their physicochemical properties, these

``oxygenate'' additives move somewhat differently than other gasoline compounds

when spilled into the subsurface [26, 27, 28].

2.8 Methods of Calculation of Vapor-Liquid Equilibria by

Means of an Equation of State and Activity Coefficient

Models

The calculations of thermodynamic properties (especially vapor-liquid

equilibria) of mixture have been investigated by many authors using different

approaches.

The main procedure to improve the results obtained from EOS is to improve

the mixing rules. They generally give satisfactory results, but suffer from a

common weakness: they fail to describe asymmetric, mixture, namely mixture

constituted by molecules differing very much in size and shape, but especially in

intermolecular energy. As a consequence the parameters in the combining rules lose their physical significance. To overcome these problems, many researchers

have turned their attention toward development of new mixing rules. All these


21

attempts can be roughly classified in two categories empirical mixing rules and

statistical mechanics mixing rules [29].

Gibbs energy models, such as NRTL and UNIQUAC have typically been

used to correlate the VLE of ethanol with hydrocarbons [30]. As these models

require binary interaction parameter for each pair, their use can be problematic in

modeling mixtures with a large number of components, such as gasoline.

Therefore, various versions of the UNIFAC group contribution approach have been

used in most studies of ethanol-gasoline volatility. However, the accuracy of

UNIFAC predictions for ethanol-gasoline volatility remains a concern [31, 32].

Phase equilibria of ethanol fuel blends studied by French and Malone [30]

illustrate the impact of ethanol used on the transportation fuel industry. They used

thermodynamics as a unifying theme to understand these phenomena and they

reviewed available data and the state of the-art in modeling these effects.

Isobaric vapor-liquid equilibria were measured by Hiaki et al. [3] for 2-

Methoxy-2-methylpropane +Ethanol +Octane ternary system and for the

constituent binary systems MTBE+ Ethanol and MTBE +Octane at 101.3KPa. the

measurements were made in an equilibrium still with circulation of both the vapor

and liquid phases. The experimental data of binary system were correlated with the

nonrandom two liquid (NRTL) equations. The NRTL equation yields a good

prediction of activity coefficients for the ternary system from the parameter of

correlated binary systems.

VaporLiquid equilibrium of binary mixtures Ethanol + 1-Butanol, Ethanol +

Octane, and 1-Butanol + Octane was calculated by Hull et al. [2]. The activity

coefficients of these binary mixtures were determined at temperatures of (308.15,

313.15, and 318.15) K. the determination of the vapor phase composition at

equilibrium was carried out using headspace gas chromatography analysis.

Multiple headspace extraction was used to calibrate the headspace gas


22

chromatograph. Composition of the phase diagrams produced using standard

Legendre orthogonal polynomial techniques with phase diagrams from the

literature showed good agreement. The composition of the azeotropes was reported,

where applicable.

Hull et al. [33] measured the vapor-liquid equilibrium of binary mixtures.

Ethanol + 2,2,4-Trimethylpentane, 1-Butanol+2,2,4-Trimethylpentane, and ethanol

+ o-Xylene. The activity coefficients of these binary mixtures were determined at

temperatures of (308.15, 313.15, and 318.15) K. the molar Gibbs energy of mixing

is reported for all mixtures studied. The infinite dilution activity coefficients are

reported for all components of all mixtures. Some thermodynamic models (those of

Wilson, NRTL,and UNIQUAC) have been compared with regard to their suitability

for modeling the experimental data.

Menezes at el. [34] proposed an azeotropic ETBE/ethanol mixture as possible

oxygenated additives for the formulation of eurosuper-type gasolines. Two

eurosuper gasolines with different chemical compositions and well defined

characteristics of density, volatility and octane numbers were used.

In spite of the success of some researchers in describing mixtures of real

fluids, the rigorous statistical mechanics treatment of complex systems for which

excess Gibbes free energy (GPE P) models have customarily been used is not near. On

the other hand, empiricism should be introduced at some point in the development.

This theoretical approach, however, will be very useful in developing more

theoretically based functional relationship for treatment of real fluids [29].

Park and Lee [35] reported the isothermal vapor-liquid equilibrium (VLE)

data, the excess molar enthalpy (hPE P) and the excess molar volume (VPE P) of binary

liquid mixtures of methyl tert-butyl ether (MTBE) with methanol and ethanol. The

VLE data were measured at 313.15 K. The experimental data were correlated with

some GPE P models or empirical polynomial.


23

Hiaki at el. [36] measured isobaric vapor liquid equilibrium (VLE) for two

binary systems of 1,1,1-Trifluoro-2-(2,2,2-trifluoroethoxy)ethane+2-methyl-2-

propanol and pentane+1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane system at

101.3 KPa. The measurements were made in an equilibrium still, with circulation

of both the vapor and liquid phases. The binary isobaric systems exhibit minimum

boiling azeotropes. The thermodynamic consistencies of the experimental data

were good by the Van Ness et al. and Herington methods. The experimental data

for the binary systems investigated were correlated with activity coefficient

equations. The nonrandom two-liquid (NRTL) equation yielded a good correlation

of activity coefficients.

Isobaric vapor-liquid equilibrium (VLE) of four binary systems methyl

tertiary butyl ether (MTBE) +methanol, MTBE +heptane, MTBE +octane and

MTBE +i-octane were measured by Watanabe at el. [37] at atmospheric pressure.

The VLE of a ternary system, MTBE +methanol + heptane, were also measured at

atmospheric pressure. These VLE data were predicted by ASOG and correlated by

Wilson equation, and the prediction and correlation performances were discussed.

Watanabe at el. [38] measured the isobaric vapor-liquid equilibria (VLE) of

two ternary systems (methyl tertiary butyl ether + methanol + octane and methyl

tertiary butyl ether + methanol + i-octane) at atmospheric pressure by using a

modified Othmer-type circulation apparatus. The VLE data of these ternary

systems can be predicted by Wilson equation with the parameters determined by

VLE data of constituent binary systems.

Consistent vapor-liquid equilibrium data for the binary systems ethyl 1,1-

dimethylethyl ether (ETBE) + 2,2,4-trimethylpentane and ethyl 1,1- dimethylethyl

ether +octane at 94 kPa, including pure component vapor pressures of 2,2,4-

trimethylpentane and octane, have been experimentally determined by Wisniak at

el. [39]. The measured systems deviate slightly from ideal behavior, can be


24

described as symmetric regular solutions and present no azeotropes. Russinyol at

el. [40] reported The consistent vaporliquid equilibrium (VLE) data for the ternary

systems 3-methylpentane + ethanol + methylcyclohexane and ethanol + tert-butyl

alcohol (TBA) + methylcyclohexane at 101.3 kPa. The VLE data have been

correlated by Wilson, UNIQUAC and NRTL equations. The ternary systems do not

present azeotrope and are well predicted from binary interaction parameters.

Abu Al-Rub and Datta [41] measured the isobaric vapor-liquid equilibrium

(VLE) using the modified Othmer circulation still for the 2-methoxy-2-

methylpropane (MTBE)-ethanol mixtures at 90.0 and 101.3 kPa. Such oxygenate

mixtures are being considered for use as octane enhancers in gasoline. The

experimental data showed that at 101.3 kPa, this system has an azeotropic point at

95.02 mol% MTBE and 328.06 K. Reducing the pressure to 90.0 kPa did not result

in a significant alteration of the VLE of this system. the experimental data were

correlated to the Wilson and the NRTL models. The parameters obtained for these

models were used to calculate the bubble point temperature and the vapor phase

composition. The calculated data are in a good agreement with the experimental

results.

New consistent vaporliquid equilibrium (VLE) data for the binary system

methyl 1,1-dimethylpropyl ether (TAME)+2,2,4-trimethylpentane (isooctane) and

the ternary system 2-methyl-2-propanol (TBA)+methyl 1,1-dimethylpropyl ether

(TAME)+2,2,4-trimethylpentane (isooctane) are reported by Loras at el. [42] at

101.3 kPa. In the binary system, the results indicate a positive deviation from

ideality and no azeotrope is present. The ternary system presents a saddle point

azeotrope that can be predicted from binary data. The activity coefficients and

boiling points of the solutions were correlated with their composition by Wilson,

UNIQUAC and NRTL equations.


25

Vaporliquid equilibrium (VLE) data were reported by Aucejo at el. [43] for

the binary systems, 2-methylpentane+ethanol and 2-methylpentane+2-methyl-2-

propanol (TBA), at 101.3 kPa, including pure component vapor pressures. The

systems deviate remarkably from ideal behaviour presenting one positive

azeotrope. The activity coefficients and boiling points of the solutions were

correlated with its composition by Wilson, UNIQUAC, and NRTL equations.

Vapor-Liquid Equilibria of the Blended Octane Enhancers in a Gasoline

Hydrocarbon studied by Chen Ku [44] illustrated the fuel oxygenates such as ethers

and alkanols which are frequently added to reformulated gasoline to improve the

octane rating and decrease air pollution. The blended octane enhances considered

are acetone + ethanol and acetone + diisopropyl ether, and gasoline hydrocarbon is

2,2,4- trimethylpentane (isooctane). The vapor-liquid equilibrium (VLE) was

measured at 101.3 kPa for five binary systems (T=329~372K) and two ternary

systems (T=329~360K) formed by acetone, ethanol, diisopropyl ether, and 2,2,4-

trimethyl pentane (isooctane). The activity coefficients of binary and ternary liquid

mixtures were calculated by both the equation with fugacity coefficients and the

equation based on modified Raoults law. The fugacity coefficients were calculated

according to the SRK equation of state. Minimum boiling azeotropes were

exhibited in three binary systems of acetone + 2,2,4-trimethylpentane, diisopropyl

ether + ethanol, and ethanol + 2,2,4-trimethylpentane. Azeotropic behavior was

not found for the ternary mixtures. The thermodynamic consistency of binary

systems was treated by the point test, area test, and infinite dilution test from

Kojima et al., and the direct test from Van Ness. The thermodynamic consistency

of the ternary data was tested by a modified McDermott-Ellis method. The activity

coefficients of binary systems were correlated by using the Wilson, NRTL, and

UNIQUAC models. The models with their best-fitted parameters were used to


26

predict the ternary vapor-liquid equilibrium. The excess molar Gibbs free energy,

equilibrium boiling temperature deviation, and vapor-phase composition deviation

of binary mixtures were correlated by using the Redlich-Kister equation.

Hiaki at el. [45] measured isothermal vapor-liquid equilibria for

ethanol+octane at 343.15 K and 1-propanol+octane at 358.15 K. The measurements

were made in a still with the aid of a computer for control of the temperature and

measurement of the total pressure. The results were best correlated with the Wilson

equation for the system of ethanol+octane and with the nonrandom two-liquid

(NRTL) equations for the 1-propanol+octane system

Aucejo at el. [46] reported the consistent vaporliquid equilibrium data for

the binary and ternary systems ethanol+2-methyl-2-propanol (TBA) and 2-

methylpentane+ethanol+TBA at 101.3 kPa. In the binary system, the results

indicate a negative deviation from ideality and no azeotrope is present. The ternary

system shows negative and positive deviations from ideality, does not present

azeotrope, and is well predicted from binary data. The activity coefficients and

boiling points of the solutions were correlated with its composition by using

Wilson, UNIQUAC and NRTL equations.

Vaporliquid equilibrium of octane-enhancing additives in gasolines were

studied by Alonso at el. [47] who reported the experimental isothermal Px data at

T=313.15 K for the binary systems 1,1-dimethylethyl methyl ether (MTBE)+n-

hexane and methanol+n-hexane, and the ternary system MTBE+methanol+n-

hexane. Wilson, NRTL and UNIQUAC models have been applied successfully to

both the binary and the ternary systems. Moreover, they compare the experimental

results for these binary mixtures to the prediction of the UNIFAC (Dortmund)

model. Experimental results have been compared to predictions for the ternary


27

system obtained from the Wilson, NRTL, UNIQUAC and UNIFAC models; for the

ternary system, the UNIFAC predictions seem poor. The presence of azeotropes in

the binary systems has been studied.

A total pressure measurement approach leading to pTz data is presented by

Pokki at el. [48] who determined isothermal vapourliquid equilibria for five

binary systems 1-butene+methanol, + ethanol, + 2-propanol, + 2-butanol and + 2-

methyl-2-propanol at 326 K. Error analysis of the measured results has been

presented. All systems exhibit positive deviation from Raoults law and 1-

butene+methanol showed azeotropic behaviour.

Fischer and Gmehling [6] give a comparison of several mixing rules in the

Soave-Redlich-Kwong EoS based on predictive GPE P models. Their results for nine

systems with a great variety of substances divide the methods in to two groups:

older data with ~3.5% error in pressure and ~2% error in vapor mole fraction, and

new data with ~2.5% error in P and 1% error in yR1R.

CHAPTER THREE: THE THEORY

28

CHAPTER THREE

THE THEORY

3.1 Phase Equilibrium The term phase equilibrium implies a condition of equilibrium between

two or more phases. By equilibrium we mean no tendency toward change with

time; the term phase denotes any quantity of matter that is either

homogeneous throughout or contains no discontinuity of properties within the

space it occupies. The term homogeneous is intended to include portions of

material that may have been physically separated from one another but are

identical in properties and composition; the alternate specification of no

discontinuity of properties throughout allows us to consider as a phase a portion

of matter that is undergoing a dynamic change with property gradients existing

within it but having no boundary across whose properties are discontinuous. In

certain applications it is also convenient to consider physically separated regions

of material, each of which has similar property values and gradients as a single

phase.

The term Vapor-Liquid Equilibrium (VLE) refers to systems in which a

single liquid phase is in equilibrium with its vapor, schematic diagram of the

vapor-liquid equilibrium is illustrated in Figure 3.1. In studies of phase

equilibrium, however, the phase containing gradients is not considered.

Wherever gradients exist there is a tendency for change with time; hence there is

no equilibrium. On the other hand, there can be two or more phases, each of

which is homogenous throughout, with no tendency for any change in properties

with time, even though the phases are in intimate physical contact with one

another. The latter is the condition that we denote by the term phase


29

equilibrium. In a condition of phase equilibrium there are some properties that

are drastically different between the phases and others that must be identical for

all phases to prevent a change in properties within individual phases from

occurring [49]. The thermodynamic equilibrium determines how components in

a mixture are distributed between phases.

liquid vapor

Figure 3.1 Schematic diagram illustrating the vapor-liquid equilibrium of binary system.

3.2 Equation of State Equations of state are widely used in the prediction of thermodynamic

properties of pure fluids and fluid mixtures, in part because they provide a

thermodynamically consistent route to the configurational properties of both

gaseous and liquid phases. Consequently, equation of state method may be used

to determine phase equilibrium conditions as well as other properties. Many

equations of state have been proposed in the technical literature, but almost all

of them are essentially empirical in nature. A few (e.g. the equation of Van der

Waals) have at least some theoretical basis, but all empirical equations of state

for a pure gas have at best only approximate physical significance. It is very

difficult (and frequently impossible) to justify mixing rules for expressing the

constants of the mixture in terms of the constants of the pure components which

comprise the mixture. As a result, such relationship introduces further arbitrary

empirical equation of state; one set of mixing rules may work for one or several

mixtures but work poorly for others.

The term equation of state is used to describe an empirically-derived

function which provides a relation between pressure, density, temperature and

(for a mixture) composition, such a relation provides a prescription for the


30

calculation of all of the configurational and residual thermodynamic properties

of the system within some domain of applicability. The functional form of most

equations of state has been arrived at by combining some semi-rigorous

theoretical analysis of molecular interactions with a parameterization that

permits satisfactory reproduction of certain experimental data such as vapor

pressures and compression factors. Among all types of equation of state, the

most popular class is formed by the so-called cubic equation of state so named

because, although usually written in pressure-explicit form (P is given by a

polynomial in 1/V with temperature and composition dependent coefficients)

[50].

P = P (T, V) (3.1)

When solved with respect to volume they give rise to a cubic equation,

which can be solved by fast and accurate method, with no ambiguity in the

choice of the roots, so avoiding one of the main problems posed by other, more

complex equations of state.

There are two common types of equations of state: one of these expresses

the volume as a function of the temperature, pressure, and mole numbers

(volume-explicit equation) while the other expresses the pressure as a function

of the temperature, volume, and mole numbers (pressure-explicit equation). The

latter form is more common but, since pressure (rather than volume) is a

preferred independent variable, it is more convenient, whenever possible, to use

a volume-explicit equation of state.

The oldest, simplest and best known cubic equation of state is that

conceived by Van der Waals in the 19 Pth P century, his equation accounts for two

effects:

1. The volume of the molecules reduces the amount of free volume in the

fluid, and therefore the molar volume the fluid is V b.


31

2. The attraction of the molecules produces an additional pressure The

pressure of the fluid is therefore corrected by a term related to the attraction

parameter a of the molecules: P + a/V P2P

The resulting equation is:

Va

bVRTP 2=

(3.2)

Here P is the pressure, T the temperature and V the molar volume. In this

equation, the first term on second member expresses (in a somewhat simplified

form) the effect of repulsion between molecules, assimilated to hard spheres,

while the second term expresses the attraction forces due to the Van der Waals

forces. After some manipulation one can show that this equation is cubic in the

volume. This property is the minimum requirement for an equation that shall

exhibit phase separation. It should be mentioned that the van der Waals equation

is not accurate but is historically important and useful for qualitative

investigations. Today there are hundreds of different equations of state out there

which have been developed theoretically or empirically for lots of substances or

substance classes.

The traditional way of calculating phase equilibria features the use of

activity coefficient model to describe liquid-phase non-idealities and an equation

of state to describe vapor phase non-idealities. This method has proven to be

accurate in the low-to moderate pressure regions and thus can be used to

represent highly non-ideal systems. However, activity coefficient models have

some limitations. Difficulties and inaccuracies arise when calculating high-

pressure equilibria and equilibria for mixtures that contain supercritical

compounds. In addition, the number of parameters used in the activity

coefficient models ranges from two to four parameters per binary, a

characteristic that leads to errors when extrapolating experimental data. Cubic

equations of state do not inherently suffer from these limitations. They present a


32

simple, robust, and computationally economical method of correlating phase

equilibria for hydrocarbons and slightly polar systems. Their only limitation lies

in the prediction of phase equilibria for polar and highly polar mixtures. Much

research has been aimed at overcoming this deficiency through the development

of new mixing rules.

With the development of computers, the equation of state route for

calculation of phase equilibria proved a powerful tool, thus great impulse was

given to the development of new, accurate equation of state. The use of a single

equation of state to reproduce the thermodynamic properties of both pure

compounds and mixture (in vapor or liquid phase) has been one of the most

elusive research goals of thermodynamicists for over a century.

3.2.1 Properties from Equation of State

A fundamental equation of state may be used to obtain all of the

thermodynamic properties of the fluid. The more usual P-V-T equation of state

permits calculation of all configurational and residual thermodynamic properties

of the fluid within that equation domain of applicability.

3.2.1.1 Fugacity and Fugacity Coefficients

When two or more phases are at equilibrium in direct contact across an

interface, the potentials for change must be zero. This means that there is no

tendency for mass or energy to cross the boundary, thus any transport of

material or energy from one phase to the other will be a reversible process. It is

evident that in order for a condition of equilibrium to exist the temperature of

the phases must be the same, otherwise an irreversible flow of heat will occur. It

is also evident that, if the phases are in direct contact with one another, the

pressure must be the same in all phases; otherwise the force imbalance would

tend to compress one phase and expand the other, with a net irreversible

exchange of energy. These two conditions are not sufficient to ensure phase


33

equilibrium, however, and the remaining conditions required are not quite so

self-evident. The classic work of Gibbs defined the remaining conditions that

ensure equilibrium between phases; his thermodynamic treatment is based on

the concept of the chemical potential. Two phases are in thermodynamic

equilibrium when the temperature of one phase is equal to that of the other and

when the chemical potential of each component present is the same in both

phases. For engineering purposes, the chemical potential is an awkward

quantity, devoid of any immediate sense of physical reality. Lewis [49] showed

that a physically more meaningful quantity, equivalent to the chemical potential,

could be obtained by a simple transformation, the result of this transformation is

a quantity called the fugacity, which has units of pressure. Physically, it is

convenient to think of the fugacity as a thermodynamic pressure since, in a

mixture of ideal gases, the fugacity ( f ) of each component is equal to its partial

pressure. In real mixture, the fugacity can be considered as a partial pressure,

corrected for nonideal behavior.

Pf ig = (3.3)

where f PigP is the fugacity of ideal gas

For a vapor phase (superscript V) and a liquid phase (superscript L), at the

same temperature, the equation of equilibrium for each component i is expressed

in term of the fugacity ( fRiR ):

ff VL ii = (3.4) This equation is of little practical use unless the fugacity can be related to

the experimentally accessible quantities x, y, T, and P, where x stands for the

composition (expressed in mole fraction) of the liquid phase, y for the

composition (also expressed in mole fraction) of the vapor phase, T for the

absolute temperature, and P for the total pressure, assumed to be the same for


34

both phases. The desired relationship between fugacities and experimentally

accessible quantities is facilitated by auxiliary function, which is the fugacity

coefficient () [49].

The ratio of fugacity to pressure is known as fugacity coefficient, and it is

a dimensionless ratio [4].

Pf

= (3.5)

For the vapor phase the fugacity coefficient RiR relates the vapor-phase

fugacity fRiRPV P to the mole fraction yRiR and to the total pressure P. it is defined by:

Py

fi

iV

i =

(3.6)

A rigorous relation exists between the fugacity coefficient of a component

in a phase and the volumetric properties of that phase; these properties are

conveniently expressed in the form of an equation of state. In order to obtain

fugacity coefficient by using an EOS the following expression is used:

zdVVRT

np

RTV ij

nVTii ln1ln

,,

=

(3.7)

where nRiR is the number of moles of component i [50].

A component in a mixture exhibits nonideal behavior as a result of

molecular interactions, only when these interactions are very weak or very

infrequent is ideal behavior approached. The fugacity coefficient RiR is a

measure of nonideality and the departure of RiR from unity is a measure of the

extent to which a molecule i interacts with its neighbors. The fugacity


35

coefficient depends on pressure, temperature, and composition, this dependence,

in the moderate pressure region is usually as follows:

a. At constant temperature and composition, an increase in pressure almost

always causes RiR to depart further from unity, usually in the direction

RiR


36

no particular theoretical basis for the equation; rather it is to be regarded as an

effective empirical modification of its predecessors.

The full form of the Redlich-Kwong equation of state is given in Table

3.1 together with relations for all parameters involved. The mixture combining

rules shown here are the conventional ones discussed above in connection with

Van der Waals equation but now incorporating binary interaction parameters in

the attractive term.

About 110 modifications to the Redlich-Kwong equation have been

proposed to date including the widely used equation of Soave (henceforth called

the SRK equation) which will be discussed in the next section. Although a great

improvement over Van der Waals equation, the basic Redlich-Kwong equation

gives useful results for only a few rather simple fluids. This is because the

parameters of the equation are based entirely on the two critical constant TRCR and

PRCR and do not incorporate the acentric factor, as well as the critical

compressibility factor (ZRCR) of RK-EOS is constant and its value equal 1/3 for all

fluids. Because of this shortcoming, the RK equation is inaccurate around

critical region [50].

Table (3.1) Redlich-Kwong Equation of State

)( bVVTa

bVRTP +

= (3.8)

( ) 0223 =+ ABZBBAZZ (3.9)

Where =

iiijji

jjAAkxxA 1 (3.10)

=i

ii BxB (3.11)


37

= i

iijjij

jaakxxa 1 (3.12)

=i

iibxb (3.13)

and TP

Ai

i

r

ri 5.242748.0= T

PB

i

i

r

ri 08664.0= (3.14)

PTR

ai

i

C

Ci

5.22

42748.0

= P

RTb

i

i

C

Ci 08664.0= (3.15)

Note: kRiiR=0

3.2.3 Soave Redlich - Kwong Equation of State (SRK EOS)

Soave defined which is made the parameter (a) function of reduced

temperature (TRrR) and acentric factor () [i.e., =f (TRrR, )] to overcome the

difficulty of inability of equations of state to represent the entire fluid range

which prevents their use for the vapor-liquid equilibria calculations.

Soave calculated the values of at a series of temperature of a number of

pure hydrocarbons, using the equality of vapor and liquid fugacities along the

saturation curve. The fugacity of each component in a mixture is identical in all

phases at equilibrium. This is equally true for a single component system having

vapor and liquid phases at equilibrium. In this case,

ff VL ii = (3.4) This equation is valid at any point on the saturation curve, where the

vapor and liquid coexist in equilibrium.


38

Soave calculates the values of over a temperature range of TRrR =0.4 to

1.5 for a number of light hydrocarbons and found that P0.5 P was linear function of

TRrRP0.5 P with a negative slope for each fluid studied. It is represented by the

following equation:

Tmc r

5.05.0= (3.16)

Because =1.0at TRrR =1.0, so, Eq.(3.16) may be written as follows:

( )Tm r5.05.0 11 += (3.17)

To obtain the value of m, it is calculated at TRrR = 0.7 since is calculated

at TRrR = 0.7 from definitions.

( ) 5.05.0

7.0117.0

= = rTm (3.18)

In this manner the values of m are calculated for a series of values from

0 to 0.5 with an interval of 0.05 and then correlated as a quadratic function of ,

as follows:

15613.055171.148506.0 2+=m (3.19)

Soave replaced a/TP0.5 P by a(T) so, that the equation of state becomes:

)()(bVV

TabV

RTP +=

(3.20)

Eq.(3.20) in polynomial form in Z factor will thus be

( ) ( ) 01 223 =+ ABZBBAZBZ (3.21)

The compressibility factor (Z) is determined from the cubic equation of

state by solving it with iterative procedure using Newton-Raphson method. One


39

or three real roots can be obtained; in the late case, the smallest root will be

taken for a liquid phase and the highest one for a vapor phase [51].

3.2.3.1 SRK-EOS Parameters

Soave predicates a new method for determining the new equation

parameters, as follows:

2

11

+= Tm

irii (3.17)

215613.055171.148508.0 iiim += (3.19)

TP

Ai

i

r

rii 242747.0 = T

PB

i

i

r

ri 08664.0= (3.22)

PRT

ai

i

C

Ci

2

42747.0

= P

RTb

i

i

C

Ci 08664.0= (3.23)

3.2.3.2 SRK-EOS Mixing Rules

In order to extend the application of SRK-EOS to mixtures,

=

iiijji

jjAAkxxA 1 (3.24)

=

iii BxB (3.25)

=

iiiiji

j

jjj aakxxa ))((1 (3.26)


40

=i

iibxb (3.27)

These mixing rules connect the mixture parameters to pure component

parameters and mixture composition.

Such rules are adequate for regular solutions, such as hydrocarbon

mixtures. In presence of polar component they must be improved by introducing

empirical corre

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