effect of oil compatibility and resins / asphaltenes ratio

Effect of Oil Compatibility and Resins / Asphaltenes Ratio on Heat Exchanger Fouling of Mixtures Containing Heavy Oil

By

E M A N A L - A T A R

B.A.Sc, The University of British Columbia, 1997

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E

REQUIREMENTS FOR T H E DEGREE OF

MASTER OF APPLIED SCIENCE

in

T H E F A C U L T Y OF G R A D U A T E STUDIES

DEPARTMENT OF CHEMICAL AND BIO-RESOURCE ENGINEERING

We accept this thesis as conforming to the required standard

T H E UNIVERSITY OF BRITISH COLUMBIA

February 2000

© Eman Al-Atar, 2000

In presenting this thesis in partial fulfilment of the requirements for an advanced

degree at the University of British Columbia, I agree that the Library shall make it

freely available for reference and study. 1 further agree that permission for extensive

copying of this thesis for scholarly purposes may be granted by the head of my

department or by his or her representatives. It is understood that copying or

publication of this thesis for financial gain shall not be allowed without my written

permission.

Department of ( I L m rJL 9 r \ g i r\QOJn

The University of British Columbia ^ Vancouver, Canada

Date

DE-6 (2/88)

A B S T R A C T

Fouling o f heat transfer equipment due to unwanted deposition of solids during

heating remains a major cost penalty in oi l refineries. Severe fouling is encountered

during the processing o f asphaltene-containing oils, and with increased reliance on heavy

oils the situation has been exacerbated.

Petroleum oils can be separated by solvent fractionation into saturates, aromatics,

resins and asphaltenes with the latter having the highest molecular weight. Asphaltene

precipitation from oils depends on the concentration of solvent components such as resins

and aromatics. The available literature suggests that resins stabilize asphaltenes,

minimizing their tendency to flocculate. This work was undertaken to determine how the

asphaltene-resin interactions affect fouling. Fouling of asphaltenes from a heavy oil in

mixtures of fuel oi l and de-asphalted vacuum bottoms ( D A O ) was studied at asphaltene

concentration o f 0.04 - 3.4 %, and resin concentrations of 3.1 - 4.9 %. Experiments were

performed at a bulk temperature of 85 °C, fluid velocity o f 0.75 m/s and pressure of 410

kPa. Fluids under nitrogen were recirculated through an annular test section with initial

surface temperature of 230 °C for periods up to 30 hours, and the fouling monitored by

thermal measurement.

The effects of concentration of heavy oil and de-asphalted oil are explored. High

fouling rates were encountered at high pentane insolubles (asphaltene) concentrations.

Fouling rates are correlated with the ratio o f resins / asphaltenes. A t a fixed D A O

concentration, the fouling rate first increased, and then decreased as the H O concentration

was raised from zero to 20 % and as the Re/As ratio decreased. The maximum initial

fouling rate occurred at a ratio of = 2.5 and dropped to essentially zero for Re/As ratio >

ii

5.8. The initial fouling rate, hot filtration insolubles concentrations and pentane

insolubles concentrations were found to increase as DAO concentration was raised at a

fixed Re/As ratio. This was somewhat unexpected. Pentane insolubles concentrations

also increased as D A O concentration was increased at a constant asphaltene

concentration, which suggests that it is not only the asphaltenes in the heavy oil that

precipitate in fuel oil / D A O mixtures.

The relationship of fouling to oil compatibility as determined by the method of

Wiehe, was explored. Fouling rates of mixtures containing DAO did not correlate with

the colloidal instability index. A fouling regime map indicated that low fouling rates

were dependent on both the colloidal instability index and the resin/asphaltene ratio. Oil

Co mpatibility Model predictions correlated well with the colloidal instability index and

therefore were unable to predict the fouling behaviour of the mixtures. However, the Oil

Compatibility Model was found to be very sensitive to small errors in titrations.

Oil Compatibility Model titrations showed that the addition of D A O to a heavy oil

sample resulted in asphaltene precipitation at a lower heptane concentration and required

a higher toluene concentration in a toluene-heptane mixture to keep asphaltene in

solution. This finding was consistent with the measured effect of DAO on fouling.

iii

Table of Contents

Abstract ii

Table of Contents iv

List of Tables vi

List of Figures viii

Acknowledgment xi

1.0 INTRODUCTION 1

2.0 LITERATURE REVIEW 3

2.1 Heat Exchanger Fouling of Asphaltene-Containing Oils 4 2.1.1 Petroleum Oils and Solvent Fractionation 5 2.1.2 Petroleum Asphaltenes 6 2.1.3 Formation of Micelles, Colloids, and Flocculates by

Petroleum Asphaltenes 11 2.1.4 Chemistry of Resins 14 2.1.5 Role of Resins in Asphaltenes Stabilization 16

2.2 Deposit Formation by Petroleum Asphaltenes 19 2.2.1 Mechanisms of Deposit Formation 20 2.2.2 Modeling of Deposit Formation 23

2.3 Modeling Oil Compatibility and it Relation to Deposit Formation 26 2.3.1 Colloidal Instability Index 26 2.3.2 Oil Compatibility Model 27

2.4 Aims and Objectives of Work 31

3.0 EXPERIMENTAL MATERIALS AND APPARATUS 32

3.1 Experimental Materials 3 2 3.1.1 Properties of Heavy Oil 32 3.1.2 Properties of De-Asphalted Oil 33 3.1.3 Properties of Fuel Oil 37 3.1.4 Properties of Test Solutions 3 7

3.2 Experimental Apparatus 41 3.2.1 Thermal Fouling Test Apparatus 41 3.2.2 The Annular Test Section 42

4.0 EXPERIMENTAL PROCEDURES 46

4.1 Procedure for Thermal Fouling Runs 46 4.2 Determination of Pentane Insolubles and Hot Filtration Insolubles 47

iv

4.3 Measurement of Test Fluid Properties 48 4.4 Procedure for Oil Compatibility Tests 49

4.4.1 Heptane Dilution Test 49 4.4.2 Toluene Equivalence Test 50 4.4.3 Nonsolvent Oil Dilution Test 51 4.4.4 Solvent Oil Equivalence Test 51

5.0 RESULTS AND DISCUSSION 53

5.1 Typical Thermal Fouling Run 53 5.2 Effect of Resins to Asphaltenes Ration on Heavy Oil Fouling 53 5.3 Effect of Resins to Asphaltenes Ratio on Hot Filtration and Pentane

Insolubles 63 5.4 Deposit Characterization 72 5.5 Colloidal Instability Index 79 5.6 Oil Compatibility Model Relation to Asphaltenes Fouling 82

5.6.1 Heavy Oil as Reference Oil 82 5.6.1.1 Test Results 82 5.6.1.2 Model Prediction 84

5.6.2 Heavy Oil - DAO Blend as Reference Oil 85 5.6.2.1 Test Results 85 5.6.2.2 Model Prediction 88

6.0 CONCLUSIONS AND RECOMMENDATIONS 89

6.1 Conclusions 89

6.2 Recommendations 90

Abbreviations 92

Nomenclature 92

References 95

Appendices 100

Appendix A l : Summary of Fouling Runs 100 Appendix A2: Sample Calculations 101 Appendix A3: Reproducibility of Thermal Fouling Experiments 109 Appendix A4: Viscosity Data 112

V

List of Tables

Table 2.1: Analysis of Fractions of Cold Lake Vacuum Resid 5

Table 2.2: Molecular Weights and Average Molecular Formulae of Cold Lake Bitumen and its Fractions 8

Table 2.3: Yields of Asphaltenes Precipitated from Western Canadian

Bitumen Using Various Solvents 10

Table 2.4: Resin Fractions for Cold Lake Heavy Oil 15

Table 2.5: Summary of Resin Fractions Analyses of Study Done by

Hammami and Co-workers 19

Table 3.1: Properties of Cold Lake Heavy Oil 34

Table 3.2: Generalized Ranges for the Bulk Fractions in Crude Petroleum,

Heavy Oil, and Residua 35

Table 3.3: Properties of De-asphalted Oil 36

Table 3.4: Properties of Fuel Oil 38

Table 3.5: Composition and Properties of Test Solutions 39

Table 5.1: Thermal Fouling Parameters for Experiments of 5 wt % DAO in HO/FO Mixtures at T b of 85 °C, T s o of 230 °C and U bof0.75m/s 56

Table 5.2: Thermal Fouling Parameters for Experiments of 10 wt % DAO in HO/FO Mixtures at T b of 85 °C, T s o of 230 °C and U b

of 0.75 m/s 58 Table 5.3: Thermal Fouling Parameters for Experiments of 15 wt % DAO

in HO/FO Mixtures at T b o f 85 °C, T s o of 230 °C and U b

of 0.75 m/s 59

Table 5.4: Properties of Test Fluids 65

Table 5.5: Elemental Analysis of Hot Filtration Insolubles 71

Table 5.6 Chemical Characteristics of Probe Deposits of Some Runs 72

Table 5.7: Compositions of Test Fluids 81

vi

Table 5.8: Oil Compatibility Test Results using HO as The Reference Oil 84

Table 5.9: Calculated Oil Compatibility Model Parameters Using HO as

The Reference 85

Table 5.10: Oil Compatibility Model Prediction for Test Fluids 86

Table 5.11: Oil Compatibility Test Results Using HO-D AO as The Reference Oil 88

Table 5.12: Calculated Oil Compatibility Model Parameters Using

HO-D AO Blend as Reference 88

Table A l . 1: Summary of Fouling Runs 100

Table A2.1: Modeling Values of Initial Fouling Rates of All Mixtures 105

Table A2.2: Average SARA Analysis of Working Fluids. 106

Table A3.1 Test of Reproducibility of Data 109

Table A4.1 Kinematic Viscosity of a Mixture of 10% DAO, 10% HO and 80% FO at Various Temperatures 112

vii

List of Figures

Figure 2.1: Clay-Gel Percolating Column 7

Figure 2.2: A Hypothetical Asphaltene Molecule 9

Figure 2.3: Asphaltene Micelle Formation 13

Figure 2.4: Structures of Resins 15

Figure 2.5: Physical Model of Petroleum 16

Figure 2.6: Dependence of Asphaltene Solubility on Temperature 21

Figure 2.7: Oil Compatibility Numbers for Souedie and Forties Crudes 30

Figure 3.1: Dynamic Behavior of 15% DAO - 10% HO - 75% FO 40

Figure 3.2: Viscosity of 10% DAO - 10% HO - 80% FO at Different

Temperatures 40

Figure 3.3: Schematic of Fouling Apparatus 43

Figure 3.4: Heat Exchanger Fouling Probe 44

Figure 5.1: Surface Temperature and Heat Flux for a Typical Fouling Run 54

Figure 5.2: Overall Heat Transfer Coefficient and Thermal Resistance for a Typical Fouling Run 54

Figure 5.3: Fouling Resistance over Time of 5 wt % DAO in HO/FO Mixture 56



Figure 5.6: Relationship of Initial Fouling Rate with Calculated Asphaltene Content for 0, 5, 10 and 15 wt % DAO in HO/FO Mixture 61

Figure 5.7: Relationship of Initial Fouling Rate with Calculated Resins Content at Constant Asphaltenes Content 61

viii

Figure 5.8: Relationship of Initial Fouling Rate with Re/As ratio for 0, 5, 10 and 15 wt % DAO in HO/FO Mixture 62

Figure 5.9: Relationship of Initial Fouling Rate with (Re + Ar)/As Content in Mixture for 0, 5, 10 and 15 wt % DAO in HO/FO Mixture 64

Figure 5.10: Relationship of Properties of Mixtures with Re/As ratio for all oil Mixtures 66

Figure 5.11: Pentane Insolubles Variation with Re/As Ratio for Various DAO Concentrations 67

Figure 5.12: Measured Pentane Insloluble Concentration Variation with Calculated Asphaltene Contents for Various DAO Concentrations 68

Figure 5.13: Measured Hot Filtration Insoluble Concentration Variation with Calculated Resins Contents for Various Asphaltene Concentrations 68

Figure 5.14: Hot Filtration Insolubles Variation with Re/As Ratio for Various DAO Concentrations 69

Figure 5.15: Initial Fouling Rate Dependence on Solids Concentrations

(T b 85° C, T s o 230° C, V 0.75 m/s) 70

Figure 5.16: Relationship of % DAO and % HO to Hot Filtration Insolubles 71

Figure 5.17: Deposit Formed on Probe Surface for Run with 15 % DAO -3.5 % HO - 81.5 % FO with Low Fouling Rate. 73

Figure 5.18: Deposit Formed on Probe Surface for Run with 10 % D A O -10 % HO - 80 % FO with High Fouling Rate. 73

Figure 5.19: Close-Up of Deposit Formed on Probe Surface for Run with

10 % DAO - 10 % HO - 80 % FO with High Fouling Rate. 74

Figure 5.20: Deposit Characteristics Variation with DAO Content in Mixture 75

Figure 5.21: SEM Micrograph of Fouling Deposit for Run with 5 % DAO -5 % HO and 90 % FO 76

Figure 5.22: E D X Plot of Fouling Deposit for Run with 5 % DAO - 5 % HO and 90 % FO 76

ix

Figure 5.23:

Figure 5.24:

Figure 5.25:

Figure 5.26:

Figure 5.27:

Figure 5.28:

Figure A3.1

Figure A3.2

Figure A4.1

SEM Micrograph of Fouling Deposit for Run with 15 % DAO -5 % HO and 80 % FO 77

E D X Plot of Fouling Deposit for Run with 15 % DAO - 5 % HO and 80 % FO 77

SEM Micrograph of Fouling Deposit for Run with 5 % DAO -15%HOand80%FO 78

E D X Plot of Fouling Deposit for Run with 5 % DAO - 15 % HO and 80 % FO 78

Fouling Regime Map 80

Relationship of Oil Compatibility Model Index to Colloidal Instability Index 87

Fouling Resistance over Time for A Repeat Run with 5% DAO -15% HO - 80% FO Oil Mixture 110

Fouling Resistance over Time for A Repeat Run with 15% DAO -10% HO - 75% FO Oil Mixture 111

Fitting Kinematic Viscosity Variation with Temperature of 10 % DAO, 10 % HO and 80 % HO Oil Mixture into a First Order Exponential Decay Function 112

A C K N O W L E D G E M E N T

I wish to express my gratitude to Professor A. P. Watkinson, my research

supervisor, for his guidance, encouragement, support and utmost patience during the

course of this work without which this work would have not been possible.

I would like to thank Dr. J. J. Dusseault and Dr. A. Uppal of Imperial Oil Ltd. for

providing the oil samples, performing the HPLC SARA analysis on the samples and for

their technical support. The financial support of Imperial Oil Ltd. is gratefully

acknowledged.

The support and helpful suggestions of Dr. S. Asomaning, B. Sundaram, Dr. I.

Rose and Dr. D. Posarac are greatly appreciated. I would like to thank all members of the

chemical engineering faculty, staff, workshop, stores and graduate students for their

assistance.

I wold like to dedicate this thesis to my family for their love and support.

xi

1.0 INTRODUCTION

Fouling is the deposition of unwanted materials on equipment surfaces such as

heat exchangers while processing. The presence of these deposits represents a resistance

to the transfer of heat and therefore reduces the efficiency of the particular heat

exchanger. Therefore, when deposits accumulate, removing the deposits becomes very

necessary to maintain desired process conditions.

Fouling remains a major cost penalty in oil refineries. Bott [1995] reported that

fouling costs in a typical US refinery in 1993 is about $ 20 - 30 million per year for

processing 100000 barrels/day. This figure is based on extrapolation of results reported

by Van Nostrand in 1981. A number of factors contribute to this cost of fouling such as

increased capital investment, additional operating costs and loss of production. In order

to make allowance for potential fouling, in the design stage the area for a given heat

transfer is always increased. Operating costs result from cleaning the heat exchanger,

and involve both labour costs and the costs of cleaning chemicals. Cleaning processes

require shutdown of the heat exchanger, which result in severe cost penalties due to loss

of production.

Canadian petroleum resources include conventional light crude oils, heavy oils

and oil sands. The latter two contain high percentages of heavy oil fractions such as

asphaltenes, Speight [1991]. It is of great interest to the Canadian oil industry to process

these heavier fractions at a minimal expense. Severe fouling is encountered during the

processing of asphaltene-containing oils which increases the interest in understanding

asphaltene fouling.

1

Chapter 1: Introduction 2

Petroleum oils may be characterized by solvent fractionation into saturates,

aromatics, resins and asphaltenes. "Saturates" contain mainly paraffin and some olefin

compounds having low boiling points and molecular weights. Aromatics include

benzene-like compounds with higher boiling point and molecular weight compared to

that of saturates. Resins and asphaltenes on the other hand are the heavier fractions of

oils with high boiling points and molecular weights.

Asphaltene precipitation from oils depends on the concentration of solvent

components such as resins and aromatics. This thesis investigates the effect of varying

oil composition on fouling of asphaltene-containing oils. The compatibility of oil

mixtures and its relation to heat exchanger fouling is also studied.

2.0 L I T E R A T U R E R E V I E W

Heat exchanger fouling has been divided into five primary categories including

precipitation, particulate, chemical reaction, corrosion and biological fouling, Epstein

[1983]. Precipitation fouling, sometimes referred to as scaling, may be caused by

crystallization of dissolved inorganic salts present at the heat exchanger surface under

supersaturation conditions, Hasson [1981]. Solidification fouling is another subdivision

of this category which involves the freezing of a pure component, or of a high melting

temperature component such as hydrocarbon wax. Particulate fouling is the

accumulation of finely divided solids suspended in the process fluid on the heat transfer

surface. In some cases, the equipment is run vertically to avoid sedimentation fouling

that is caused by gravity. Chemical reaction fouling is defined as a deposition process in

which a chemical reaction either forms the deposit directly on a surface, or is involved in

forming foulants which become deposited, Watkinson [1992]. Reaction does not take

place with the surface material itself. Corrosion fouling refers to the formation or

accumulation of corrosion products on the heat transfer surface. Biological fouling

involves the attachment of macro or micro-organisms to a heat transfer surface.

Heat exchanger fouling is a complex process and in many practical situations,

more than one type of fouling may be present. Petroleum fouling is an example of such a

practical case in which many steps are involved. Therefore, this chapter will review

some aspects of heat exchanger fouling while processing heavy oil fractions that will lead

to the objectives of this work.

3

Chapter 2: Literature Review 4

2.1 Heat Exchanger Fouling of Asphaltenes-Containing Oils

Crude oil heat exchanger fouling is a major problem facing the oil industry.

Fouling occurs as a result of a combination of chemical reactions and physical changes

that occur when crude oil is exposed to high metal surface temperatures in an exchanger.

Analysis of these deposits indicates that they are composed primarily of infusable coke,

asphaltenes, and inorganic materials.

Many variables play a role in crude oil fouling in heat exchangers such as crude

oil composition, inorganic contaminants, process conditions, and metal surface

temperatures. Several mechanisms for fouling have been postulated including inorganic

compounds deposition and corrosion of metal surfaces, oxygen induced polymerization,

free radicals opening double bonds and initiating polymerization, and asphaltene

precipitation, Murphy and Campbell [1992]. Although there may be cases where one or

more other fouling mechanisms may predominate, both laboratory studies and analyses of

actual exchanger deposits point to asphaltene precipitation and subsequent carbonization

as the most significant mechanism.

Further studies have found that the presence of asphaltene does not necessarily

mean a crude oil will foul. Asphaltenes that are incompatible with the crude oil

chemistry and composition have a much greater tendency to precipitate and foul. The

unique chemistry of a particular crude oil, and the types and quantities of asphaltenes

present, determine the potential for a particular crude to foul, Dickakian and Seay [1988].

Therefore, it is very important to understand the chemistry and the compatibility of a

certain crude to be able to predict its tendency to foul.


The following sections will be devoted to understanding the chemistry of

asphaltenes and the role of other constituents in keeping asphaltenes in solution and

therefore preventing fouling.

2.1.1 Petroleum Oils and Solvent Fractionation

Petroleum consists of hydrocarbon compounds with a wide rage of boiling points

and carbon numbers, and other heteroatomic organic compounds containing nitrogen,

sulfur and oxygen as well as heavy metals such as vanadium and nickel. Oils are

classified based on their viscosities, densities and API gravities into light crude oils

(viscosity < 100 mPa.s, density < 934 kg/m3 and API > 20°), heavy crude oil (viscosity

100-10,000 mPa.s, density 934-1000 kg/m3 and API > 10° - 20°), and tar sand bitumen

(viscosity > 10,000 mPa.s, density > 1000 kg/m3 and API < 10°), Speight [1991].

Petroleum oils can be separated by solvent fractionation into four constituents

namely saturates, aromatics, resins and asphaltenes. Table 2.1 lists the properties of these

fractions given by Wiehe [1999] for a western Canadian vacuum resid. Asphaltenes have

the highest molecular weight being the heaviest fraction among the four constituents and

are commonly defined as the portion of petroleum which is insoluble in low-boiling

liquid hydrocarbon alkanes but soluble in benzene, Ferworn and co-workers [1993].

Table 2.1 Analysis of Fractions of Cold Lake Vacuum Resid, Wiehe [19991

Fraction Yield wt %

C Wt%

H wt%

H/C Atomic

S ' Wt%

N wt%

VPO M W

Saturates 18 84.54 12.31 1.73 2.74 0.03 920

Aromatics 17 81.87 10.00 1.46 5.56 0.12 613

Resins 40 82.08 9.50 1.38 6.09 0.77 986

Asphaltenes 25 81.93 7.94 1.15 7.50 1.15 2980


Solvent fractionation is used for classifying oil into the hydrocarbon types of

polar compounds, aromatics and saturates, and recovery of representative fractions of

these types. Asphaltenes and resins comprise the polar fraction of petroleum. The A S T M

D 2007-93 is a standard solvent fractionation method used for samples of initial boiling

point of at least 260°C. Asphaltene fraction is separated from the sample by precipitation

using n-pentane at a ratio of 1:40. Precipitated asphaltenes are removed by filtration. The

oil sample, diluted with n-pentane, is then charged to a glass percolation column

containing Attapulgus clay in the upper section and silica gel in the lower section. The

saturate fraction, on percolation in a n-pentane eluent, is not adsorbed on either the clay

or silica gel and therefore collects at the bottom of the columns. The resin fraction is

adsorbed on the clay and subsequently desorbed with a mixture of toluene and acetone.

Aromatics, on percolation, pass through the adsorbent clay but adsorb on the silica gel

and are later desorbed by recirculation of toluene. Solvents are evaporated off the

collected samples and the oil fractions are recovered. This procedure requires large

solvent quantities and therefore, is unfeasible for use to recover large samples of oil

fractions. The apparatus used in this test method is shown in Figure 2.1.

2.1.2 Petroleum Asphaltenes

Asphaltenes are dark brown amorphous solids which consist of highly

polydisperse macromolecules containing a broad distribution of polar groups in their

structure. The published molar mass data for petroleum asphaltenes range from 500 to

500,000 g/mol, Long [1981]. Current research suggests a much narrower range of 1000

to 10,000, Thawer et al. [1990]. The measured molar mass is dependent on the source of

the crude oil and the type of solvent used to precipitate the asphaltenes. Vapor pressure


Figure 2.1: Clay-Gel Percolating Column, A S T M D 2007-93

osmometry (VPO) is one of the most common methods of determining the molar mass of

asphaltenes. The color of dissolved asphaltenes in benzene is deep red at low

concentrations, Kawanaka and co-workers [1991], On heating to temperatures above

400°C, asphaltene molecules decompose forming carbonacious coke and volatile

products.

The complexity of asphaltene fractions has made it very challenging to determine

a definite structure; however, efforts have been made to describe asphaltenes in terms of


chemical structure or elemental analysis for the past six decades. An example of these

attempts is the work carried out by Suzuki and co-workers [1982], in which they were

able to obtain chemical structure of tar-sand bitumen. Table 2.2 represents the molecular

weights and average molecular formulae of Cold Lake bitumen and its fractions.

Table 2.2 Molecular Weights and Average Molecular Formulae of Cold Lake Bitumen and its Fractions, Suzuki et al. [19821

Bitumen and Fractions

wt% M W

(VPO) H / C

Atomic Average Molecular

Formula

Cold Lake Bitumen - 500 1.55 C34.5H53.5N0.11 S0.72O0.47

Saturate fr. 30.3 331 1.83 C24.0H44.0

Aromatic fr. 40.6 517 1.46 C34.9H52.3N0.09S 1.04

Resin fr. 13.5 1010 1.38 C68.8H94.9N0.95S 1.7601.18

Asphaltene 15.6 2030 1.19 C 139H165N2.33 S4.74O0.6O

More recent study was carried out by Strausz [1992] and co-workers where they

were able to obtain a hypothetical asphaltene molecule as shown in Figure 2.2. This

hypothetical molecule was obtained using Athabasca asphaltenes and incorporates in its

structure condensed aromatic clusters with side chains and heteroatoms. It has an

elemental formula of C420FI496N6S14O4V, and a H/C atomic ratio of 1.18.


Figure 2.2: A Hypothetical Asphaltene Molecule, Strausz et al. [19921

Asphaltenes solubility varies dramatically in different solvents. These differences

can be explained considering the solvent power of the precipitating liquids, which can be

related to molecular properties using the solubility parameter given by Hildebrand and

co-workers [1970].

The solubility parameter of nonpolar solvents can be related to the heat of

vaporization AH V and the molar volume V,

/ v \ 1 / 2

A H - R T

V

(2.1)

R is the gas constant and T is the absolute temperature. Rogel [1998] classified

asphaltenes according to their solubility parameters as being highly soluble if they are in

the 17.0 - 22.5 M P a 0 5 range, fairly soluble within the range 22.5 - 25.5 M P a 0 5 , and

difficult to dissolve beyond 25.5 MPa 0' 5. The solubility parameter of Cold Lake


asphaltenes was reported, by Rogel [1997], to be 24.3 MPa 0' 5 lying in the fairly soluble

range. The yield of precipitate of each solvent depends on the difference between the

solubility parameter of the asphaltenes and the solvent. The solubility parameters and the

asphaltene precipitate yield at 21°C for a variety of solvents are presented in Table 2.3. It

is apparent that asphaltenes are soluble in hydrocarbon solvents with solubility

parameters greater than or equal to 8.4 (cal/cm3)0'5, while they precipitate in solvents with

solubility parameters less than or equal to 8.2 (cal/cm3)05. In general, asphaltenes are

more soluble in aromatics than straight and branched chain paraffins.

Table 2.3 Yields of Asphaltenes Precipitated from Western Canadian Bitumen Using Various Solvents fSpeight, 19911

Hydrocarbon Solvent Solubility Parameter Precipitate

(wt. % Asphaltenes) Hydrocarbon Solvent (cal/ml)0 5

(MPa) 0 5

Isopentane 6.8 13.9 17.6

N-Pentane 7.0 14.3 16.9

Isohexane 7.1 14.5 15.3

N-Hexane 7.3 14.9 13.5

Isoheptane 7.2 14.7 12.8

N-Heptane 7.5 15.3 11.4

Isodecane 7.6 15.6 9.8

N-Decane 7.7 15.8 9.0

Cyclopentane 8.2 16.8 1.0

Cyclohexane 8.2 16.8 0.7

Benzene 9.2 18.8 0

Toluene 8.9 18.2 0

Xylene 8.8 18.0 0

Isopropylbenzene 8.6 17.6 0

Isobutylbenzene 8.4 17.2 0


2.1.3 Formation of Micelles, Colloids, and Flocculates by Petroleum Asphaltenes

Asphaltene particles are believed to exist in oil partly dissolved and partly in

colloidal and/or micellar form. Whether the asphaltene particles are dissolved in crude

oil, in colloidal state or in micellar form, depends to a large extent, on the presence of

other species (saturates, aromatics, resins) in the crude oil. The existence of various states

of asphaltenes in crude oil has been extensively discussed in literature, Yen [1974];

Mansoori [1996].

Small asphaltene particles can be dissolved in a petroleum fluid, whereas

relatively large asphaltene particles may flocculate out of the solution and then can form

colloids in the presence of excess amounts of resins and hydrocarbon saturates.

Flocculation of asphaltene in paraffinic crude oils is known to be irreversible. Asphaltene

and its flocculates are known to be surface-active agents. The flocculated asphaltene will

precipitate out of the solution unless there is enough resins in the solution so that they can

cover the surface of asphaltene particles by adsorption and form colloids. Asphaltene

flocculates will also precipitate upon any actions of a chemical, electrical, or mechanical

nature (such as the addition of a n-alkane) that would upset the colloidal balance of the

flocculates.

Various investigators have established the existence of asphaltene micelles when

an excess of aromatic hydrocarbons is present in a crude oil, Pfeiffer and Saal [1940];

Dickie and Yen [1967]; Galtsev and co-workers [1995]; Mansoori [1996]. Several

investigators have performed experimental measurements of critical micelle

concentration for solutions of asphaltene in aromatic solvents, Sheu [1996]; Andersen

and Birdi [1991]; Ravey and co-workers [1988]. Furthermore, the phenomenon of self-


association in asphaltene/toluene systems has been confirmed through measurements of

surface tension, Sheu [1996]. Sheu has shown that at low concentrations, below the

critical micelle concentration (CMC), the asphaltenes in solution are in a molecular state,

whereas, above the CMC, asphaltene micelle formation occurs in a manner similar to that

in surfactant systems. However, Andersen and Speight [1993], consider that there is an

alternative method of data interpretation that will add another dimension to the

determination of the critical micelle concentration. This alternative analysis is based on

the Gibbs excess adsorption equation:

- C ^ y _ a RT dC a

where T a is the Gibbs surface excess, C a the concentration of compound a, and y the

surface tension of the solution. According to this equation, the determination of a critical

micelle concentration would be better served by an examination of surface tension y

versus In C a . Espinat and Ravey [1993] with the use of scattering techniques have shown

that the best model to describe the morphology of asphaltene micelle in solution is a disk.

However, several other experimental investigations have shown that asphaltenes could be

of spherical-like, cylindrical-like, or disk-like form, Ravey and co-workers [1988]. All

these investigations are indicative of the fact that asphaltene particles may self-associate,

but not flocculate, and form micelles in the presence of excess amounts of aromatic

hydrocarbons as shown in Figure 2.3, Lian et al. [1994].


1 MOHOMERICSHEET }-2asa

2 - $ nm

REVERSED MICELLE i; HARTLEY MECFJXE

SUP5RR MICEULE

GIANT SUPER MTCELLE 200-2000 ran

10-20 wa

MULTT-LAMi.Lr.-AR VESICLE 1000 - iO,OGO am

UK ft?

FLOC 20,000 run

:w -GEL l O O M t t M

Figure 2.3 Asphaltene Micel le Formation, L i a n et al. [19941.

While the mechanisms of asphaltene flocculation and colloid formation are

relatively well understood and modeled [Mansoori, 1996], the phase behavior of

asphaltene micelle formation is not well characterized. In many cases, in recent years,

micelle formation is confused with the asphaltene colloids. Despite the experimental

http://MULTT-LAMi.Lr.-AR


evidence on the micellization of asphaltenes, little or no theoretical and modeling

research has been performed to explain and quantify this phenomenon.

The process of colloid formation of asphaltenes with resins follows an irreversible

process of flocculation, Leontaritis and Mansoori [1988]; Park and Mansoori [1988],

Kawanaka co-workers [1988]. When asphaltenes form micelles, a reversible self-

association process is recognized in which resins have no role.

2.1.4 Chemistry of Resins

Resins are defined as the polar fraction of petroleum that is soluble in n-alkanes

and aromatic solvents, and insoluble in ethyl acetate. Table 2.1 shows that the resin

fraction has properties intermediate between those of the asphaltene and the aromatic and

saturates fractions. There is a notable decrease in the H/C atomic ratio of the asphaltenes

relative to that of resins which indicates that aromatization is more advanced in the

asphaltenes than in the resins. It also indicates that if the asphaltenes are maturation

products of the resins, one of the maturation processes involves aromatization of the non-

aromatic portion of resins, Koots and Speight [1975]. Resins typically contain the

heteroatoms nitrogen, sulfur and oxygen in moderate amounts, Clark and Pruden [1997],

and can be found to be neutral, basic or acidic as shown by Strausz and Rubinstein [1980]

for Cold Lake heavy oil (Table 2.4).

Initial postulates of resin structure invoked the concept of long paraffinic chains

with naphthenic rings interspersed throughout as shown in Figure 2.4. Other structures

used the idea of condensed aromatic and naphthenic ring systems and allowed the

interspersion of heteroatoms throughout the molecule, Speight [ 1991 ]. The average


molecular formula of resins as found by Suzuki and co-workers [1982] in Cold Lake

Bitumen is given in Table 2.2.

Table 2.4 Resin Fractions for Cold Lake H ^ v y Oil, Strausz anH Rnbinste.n HMOl

Component Cold Lake Heavy Oil

Hydrocarbons: Saturates Aromatic

40 21 19

Asphaltenes 16

Resins: Acidic Basic Neutral

44 15 7

22

CH, {CH )-

CM-l

C H ,

"C?i,

cm,

Cl-i .£ *t

CM •3

CH...

CH.

C M ,

CH

5 CH: 3 C i ,

3 C H 3

Figure 2.4 Structures nf Resins. Speight 11991]


2.1.5 Role of Resins in Asphaltenes Stabilization

The activity of resins to prevent asphaltene flocculation is still not well known.

Asphaltene molecules (micelles) are believed to be surrounded by resins that act as

peptizing agents as shown by Wiehe [1997] in Figure 2.5. The resins maintain the

asphaltenes in a colloidal dispersion (as opposed to a solution) within the crude oil. The

peptization model was first proposed by Pfeiffer and Saal [1940]. This concept is often

repeated, Dickie and Yen [1967], Koots and Speight [1975], Speight [1982], Leontaritis

and co-workers [1988], Wiehe and Liang [1996] and Barre [1997]. Experimental

evidence, Lichaa [1977], suggests that for an oil mixture there is a critical concentration

of resins below which the asphaltene flocculates may precipitate and above which they

cannot precipitate.

s a a a s s a R R R a s

s a IR'AA R a s s a B A A ' R . a s

s a R R R a s s a ,a.as

s s

A s Asphaltenes R s: Restns a - Aromatics s = Saturates

Figure 2.5 Physical Model of Petroleum, Wiehe [1996].

The resins are attracted to the asphaltene micelles through their end group. This

attraction is a result of both hydrogen bonding through the heteroatoms and dipole-dipole

interactions arising from the high polarities of the resin and asphaltene molecules. The


paraffinic component of the resin molecule acts as a tail making the transition to the

relatively non-polar bulk of the oil where individual molecules also exist in true solution,

Hunt [1996].

Koots and Speight [1975], studied the chemical and structural analyses of a series

of petroleum resins and concluded that a crude oil is a complex system whereby each

constituent fraction depends upon others for complete mobility; and it is not necessarily

achieved by interchanging fractions from one crude oil to another. It is presumed that the

resins associate with the asphaltenes in the manner of an electron donor-acceptor system

and that there could well be several points of structural similarity between the asphaltenes

and resins. In that case, this may well have an adverse effect on the ability of resins to

associate with asphaltenes from any other crude oil. Lian and co-workers [1994] reached

another interesting conclusion stating that the resins are better solubilizers of asphaltenes

from the same crude oil.

Extensive research is directed to evaluate the asphaltene-solvating power of

various nonconventional solvents for asphaltenes and the effect of adding resin-

containing compounds on the stability of asphaltenes in the oil phase. Jamaluddin et al.

[ 1996] systematically evaluated the asphaltene-solvating power of deasphalted oil (DAO)

using a light-scattering technique. DAO was prepared using Lindbergh oil and

asphaltenes were precipitated using n-pentane and a rotary evaporator was used to

remove the solvents. Asphaltenes were removed and dried in an oven then added at a

concentration of 4.8 % to 25 g of DAO. N-pentane was slowly added to the mixture and

the onset of asphaltene precipitation was determined using a light scattering technique.

Experimental results suggested that DAO is a strong asphaltene solvent presumably


because of its native resin and aromatics contents. In general it was concluded that a

saturates content of less than 30 - 35 wt. % as determined by SARA analysis represents a

good solvent.

Work of Clarke and Pruden [1997], was carried out to test the ability of a number

of aromatic compounds, chosen for their similarity to resins, to delay the onset of

asphaltene precipitation. Indole and quinoline were used in their study to represent the

non-basic and basic nitrogen that was found in the resin fraction. Quinoline also contains

an aromatic core. These two compounds were added separately to a sample of Cold Lake

bitumen and the onset of asphaltene precipitation was detected using heat transfer

analysis. Results indicated that quinoline was effective in delaying the onset of

asphaltene precipitation while indole enhanced the precipitation of asphaltenes.

Hammami and co-workers [1998] conducted a study to evaluate the effect of

different resin fractions on the onset of asphaltene precipitation. They separated the resin

fraction of a North Sea stock tank oil sample and classified it into resin (1) and resin (2)

fractions. They further classified the resin fractions in terms of their neutral, basic,

pyrrolic and acidic components as shown in Table 2.5. The onset of asphaltene

precipitation of a North Sea stock tank oil sample was determined upon addition of resins

(1) and (2) fractions using light transmission technique. Experimental results showed

that the addition of resins (2) fraction increased the stability of the asphaltenes by

increasing the minimum n-pentane concentration required to induce asphaltenes

precipitation. However, the addition of resins (1) fraction, was found to have no obvious

effect on asphaltenes stability over the range of concentrations tested. These results

imply that particular components or group of components present in the resins (2)


fraction are responsible for stabilizing asphaltenes in the mixture. As can seen in Table

2.5, resins (2) fraction is more polar in nature than resins (1). In addition, Resin (2)

fraction is more basic than resin (1) fraction which supports the results obtained in the

study by Clarke and Pruden [1997] where basic quinoline was effective in delaying

asphaltene precipitation.

Table 2.5 Summary of Resin Fractions Analyses of Study Done by Hammami and co-workers [19981.

Fraction Aromatics Polars Basic Others

Resins (1) 63.3 36.7 1.2 98.8

Resins (2) 48.9 51.1 10.3 89.7

Storm and co-workers [1998], performed a study where they observed a rise in

insoluble particles upon heating a heptane soluble oil sample (DAO) to 285°C. This

indicated that not all micelle-forming molecules are in the asphaltenes. They also found

that the presence of asphaltic (asphaltenes plus resins) micelles in the crude appears to

accelerate the conversion of pentane insolubles (asphaltenes and resins) to heptane

insolubles (asphaltenes) during the induction period to form coke as given by Wiehe's

model for coke formation, [1993].

2.2 Deposit Formation by Petroleum Asphaltenes

Asphaltene deposit formation is a complex process and it has been a great

challenge to try to postulate the mechanism of the process and develop a model to predict

it. Asphaltene precipitation is described in detail by Asomaning, [1997]. It has been

found that asphaltene precipitation during crude oil production is a physical process;

however, it is not yet clear if deposit formation on heat transfer surfaces is a physical


process, a chemical reaction, or a combination of both. There are many factors that play

a role in controlling deposit formation on heat transfer surface such as velocity, bulk

temperature and surface temperature, which makes model development a very

challenging task. The following sections will describe commonly developed mechanisms

and models for asphaltene deposit formation on heat exchanger surfaces.

2.2.1 Mechanisms of Deposit Formation

Deposit formation mechanisms proposed by Dickakian and Seay [1988], and

Lambourn and Durrieu [1983] are based on the incompatibility between asphaltenes and

other components of crude oil and therefore suggested to be a physical process.

Dickakian and Seay summarized the mechanism in the following steps:

1. Precipitation of asphaltenes initiated due to incompatibility between asphaltenes and

the oil.

2. Adherence of precipitated asphaltenes to the hot heat transfer surface.

3. Coking of asphaltenes on the heat transfer surface.

This mechanism suggests that the critical step in this process is the first step of

asphaltene precipitation. The precipitation step is linked to the solubility of asphaltenes

in the oil. However, the mechanism does not state the exact method by which the

asphaltene precipitate or asphaltene micelle gets destroyed. The mechanism suggests the

possible role of chemical reaction in adhesion and coke formation stages; however, this

role is minor compared to the physical process of asphaltene precipitation.

Lambourn and Durrieu studied fouling in crude oil heat exchangers and found that

asphaltene precipitation is the major cause of fouling. They performed microscopic


examination of colloidal asphaltenes in the crude. These studies revealed that the

asphaltenes, upon precipitation, coated droplets of water left in the crude oil after

desalting and formed an emulsion. The emulsion incorporated particulates, mainly

oxides and sulphides of iron, to form stable entities that were insoluble in the crude oils.

These emulsions deposited on the heat transfer surface where it aged to form coke. This

proposed mechanism requires a threshold concentration of asphaltenes of 1.3 wt %.

The study performed by Lambourn and Durrieu revealed another interesting

behavior for asphaltenes at elevated temperatures. The study showed that suspended

asphaltenes have a complex relationship with the bulk temperature, that asphaltenes

dissolve in the temperature range 100-140 °C, but these dissolved asphaltenes re-

100 203 330

Figure 2.6 Dependence of Asphaltene Solubility on Temperature TLambourn and Durrieu, 19831.

precipitate when the temperature is raised above 200 °C as shown in Figure 2.6.


The mechanism proposed by Lambourn and Durrieu included an asphaltene precipitation

step followed by an interaction of the asphaltenes with water and particulates to form an

emulsion which deposits on the heated surface to age and form coke-like materials.

These mechanisms suggest that asphaltene fouling of heat exchangers is mainly a

physical process, however Eaton and Lux [1984] proposed that fouling by asphaltenes is

principally via chemical reaction. They have suggested that saturated hydrocarbons

convert to unsaturated hydrocarbons, organic acids, resins and asphaltenes that are finally

converted to coke as follows:

Inorganic Acids 0 2

Saturated hydrocarbon > Unsaturated hydrocarbon > Organic

Metals Wall acids > Resins and Asphaltenes > Coke-like deposits.

A A

It is widely accepted that the quantity of asphaltenes in virgin crude oil is relatively low

and it increases upon processing and exposure to high temperatures as resins and

aromatics are converted to asphaltenes, Blazek and Sebor [1993]. This fact supports the

mechanism proposed by Eaton and Lux.

There are two processes that may be responsible for this increase in asphaltene

concentrations. The first is the hydrocracking of maltenes (saturates, aromatics and

resins) to asphaltenes as supported by Blazek and Sebor [1993] and Savage and Klein

[1987]. The second process is the dealkylation of resins and aromatics followed by

condensation reactions that result in the formation of asphaltenes of varying molecular

weights, and the production of lower molecular weight compounds.


Mechanisms proposed in the literature for asphaltene fouling are not consistent, which

suggests that additional efforts are necessary to understand and control the fouling

process.

2.2.2 Mode l ing of Deposit Format ion

Organic fluid fouling of heat exchangers has been the focus of many researchers

over the years and there have been many attempts to model this behavior to enable its

accurate prediction. However these models are based on many assumptions, some of

which are not valid in reality, that limit their predictive accuracy. The modeling of

asphaltene fouling has been a greater challenge due to numerous reasons such as the high

complexity of asphaltenes, inconsistency in the properties of asphaltenes from different

oil samples, variation in asphaltene precipitation detection methods encountered by

different researchers, and the lack of accurate thermodynamic properties for asphaltenes.

These difficulties have made it hard to develop an accurate model for asphaltene fouling

under conditions encountered in heat exchangers. However, there are several models

proposed in the literature for chemical reaction and particulate fouling of heat

exchangers. There have also been attempts to model asphaltene precipitation, mostly

under ambient temperatures and high pressures, but there has been no attempt to link

such models, or precipitation data to thermal fouling. This section will refer to some heat

exchanger fouling and asphaltene precipitation models.

A simple model was developed by Kern and Seaton [1959] to predict asymptotic

fouling. This model consists of a deposition term which is a function of the concentration

of foulants and the mass flow rate, and a removal term which is a function of the deposit


thickness and the wall shear stress. The model equation beyond the induction period is

given by:

m = m (l-exp(-bt)) (2.3)

where m is the mass of deposit per unit surface area (kg/m2), m* is the asymptotic mass of

deposit per unit area and "b" is the time constant taken to be the time when "m" reaches

63 % of its asymptotic value. The thermal fouling resistance is related to the mass of

deposit per unit surface area and the deposit thickness as follows:

R f = ~ = — (2.4) k f p f k f

where x is the deposit thickness, pf is the density of the foulant and kf is the thermal

conductivity of the foulant. Assuming that the density and the thermal conductivity are

constant with time, the fouling resistance is given by

R f =R/(l-exp(-bt)) (2.5)

where Rf* is the asymptotic fouling resistance. The initial fouling rate is given by

R f = b x R f * (2.6)

Taborek [1982] and Pinhero [1981] have modified the basic Kern and Seaton

model to incorporate other terms such as the shear strength of the deposit. Watkinson

and Epstein [1969] introduced an Arrhenius type sticking probability while Paterson and

Fryer [1988] introduced reaction kinetics to heat exchanger fouling modeling. Crittenden

et al. [1987] presented a general model that took into account the transport of fouling


precursors as well as chemical reaction. Epstein [1994] developed a modified version of

the Crittenden model by treating the attachment to the surface as a process in series with

mass transfer. This modified model is based on the assumption that the reaction and

attachment constants are proportional to the residence time of the fluid at the surface.

Therefore, the longer the fluid stayed at the surface the bigger the chance that reaction

would occur.

Zhang and Watkinson [1991] and Panchal and Watkinson [1993] modeled

autoxidation reaction fouling by assuming a two step process resulting in foulant

generation. The first step involves the reaction of a soluble reactant to form a sparingly

soluble foulant precursor which then reacts to form an insoluble foulant.

All the models described above were able to describe a certain type of fouling

mechanism with varying degrees of success. There have been great advancements in

modeling heat exchanger fouling over the years; however there is still a need to improve

the prediction accuracy of these models and enable their application more to the general

case.

Asphaltene precipitation models proposed in the literature are based on the

thermodynamic phase behaviour of asphaltenes and the use of equations of state to

predict the volume fractions of asphaltenes in different phases. Models available are

mostly developed for processes occurring during oil production, recovery and transport

with conditions different from those in heat exchangers. Hirshberg et al. [1984]

approached asphaltene precipitation by using a bulk phase equation of state to describe

asphaltene solubility, neglecting its colloid nature assuming it is a homogeneous solid. A

thermodynamic colloid model of asphaltene precipitation was proposed by Leontaritis


and Mansoori [1987]. This model does not explicitly deal with the dependence of the

micellization process on the characteristics of the micelles.

These models are based on many assumptions that are not valid in reality, such as

the elimination of asphaltenes micellization and the reversibility of the process of

asphaltene precipitation. In addition, the high complexity of some of these models limits

their use due to the lack of reliable thermodynamic properties for asphaltenes. However,

these models provide the first steps in future attempts in predicting heat exchanger

fouling of asphaltene-containing oils.

2.3 Modeling Oil Compatibility and Its Relation to Deposit Formation

The problems associated with crude oil fouling have made it desirable to develop

techniques that will enable the prediction of the fouling behavior of a certain oil mixture

prior to processing. It has been shown in previous sections that crude oil fouling is

caused mainly by asphaltenes that are incompatible with the crude oil chemistry.

Therefore, attempts have been made to predict the instability of oil blends. This section

will discuss some oil compatibility models used for stability prediction.

2.3.1 Colloidal Instability Index

The stability of crude oils has been expressed using the colloidal instability index

(CH) by Gastel, [1971]. The CII bases instability or incompatibility of the petroleum on

its composition of oil fractions, saturates, aromatics, resins and asphaltenes. The index is

an expression of the colloidal nature of petroleum fractions and is a ratio of the

asphaltenes and saturates which precipitate asphaltenes, to the sum of the aromatics and

resins that peptize asphaltenes as follows:


CII = (Saturates + Asphaltenes)

(Aromatics + Resins) (2.3)

At fixed temperature and velocity, the fouling rate of heavy oil-fuel oil blends

including mixtures with added pentane and xylene was previously correlated by

Asomaning and Watkinson [1997] via the colloidal instability index. In their

experiments, concentrations of asphaltene varied between 1.4 and 3.7 wt. %, resins

between 2.2 and 4.4 %, whereas saturates were 30 - 67 % and aromatics 29 - 66 %. The

ratio of Resins/Asphaltenes covered the narrow range of 1.2-2.6. It is evident that the

colloidal instability index alone can not predict fouling over a full range of composition,

since for a given sum of the peptizing agents (Resins + Aromatics), one should expect

greatly different behavior if either the one or the other of the two terms in the numerator,

the saturates or the asphaltenes, went to zero. Furthermore, low concentrations (ca. 100

ppm) of amphiphiles can greatly affect asphaltene solubility, and would not be reflected

in this stability ratio. Hence, additional factors beyond the CII will be necessary to

characterize the fouling potential of asphaltene-containing systems.

2.3.2 Oil Compatibility Model

A solubility parameter based model has been developed by Wiehe [1999] to

determine the correct order and proportions of blending petroleum oils to prevent rapid

fouling and coking from the precipitation of asphaltenes. The basic hypothesis of the Oil

Compatibility Model (OCM) is that the asphaltene/resin dispersion has the same

flocculation solubility parameter, whether the oil is blended with nonpolar liquids or

other oils.


Model parameters include the insolubility number, IN, which measures degree of

insolubility of the asphaltenes present in the oil, and the solubility blending number, S B N ,

which measures the solvency of the oil for asphaltenes. They are defined as follows,

I N = I O O I L Z H ( 2 4 )

IP T -0 H )

S b n = 1 0 O | ° " " 5 h ) (2.5)

where 5f is the flocculation solubility parameter, 8H is the solubility parameter of n-

heptane, 8T is the solubility parameter of toluene and 50ii is the solubility parameter of the

oil. Therefore, if the oil is completely soluble in n-heptane and thus, contains no

asphaltenes, the insolubility number is 0 but if the asphaltene-resin dispersion is barely

soluble in toluene, the insolubility number is 100. Likewise, an oil that is as poor a

solvent as n-heptane has a solubility blending number of 0 and an oil that is as good a

solvent as toluene has solubility blending number of 100. The model parameters are

calculated as follows,

T E I N - r -. ( 2 6 )

25p

C — T BN A N

1 + — 5

(2.7)

where T E is the minimum percentage of toluene required in a toluene-heptane mixture to

keep asphaltenes in solution at a concentration of 2 grams of oil and 10 ml of test liquid

(Toluene Equivalence Test), V H is the maximum volume in ml of n-heptane that can be

added to 5 ml of oil without precipitating asphaltenes (Heptane Dilution Test) and p is

the density of the oil in g/L.


The solubility blending number of a mixture of oils from the mixing rule for solubility

parameters is the volumetric average calculated as follows,

_ V , S N > . „ + V T S D M , + V 5 S „ M 7 + ... 3 BNmix

C M ° B N 1 ^ V 2 ° B N 2 ^ Y 3 | J B N 3 ^ ••• fn Q\

V l + V 2 + V 3 + • •

The compatibility criterion for a mixture of oils can be defined as,

^BNmix > ^Nmax (2-9)

The nonsolvent oil dilution test is used for a sample oil that is a nonsolvent for

asphaltenes but contains no asphaltenes itself. For such oil the insolubility index is set to

0 and the solubility blending number is calculated as follows,

S-ro [ V N S D

— V H ] 'NSO _ r „ i (210) c -

v V N S D

1 + ^ 5

where STO is the solubility blending number for the test oil, VNSD is the maximum volume

in ml of nonsolvent oil that can be blended with 5 ml of test oil without precipitating

asphaltenes from the nonsolvent dilution test, and V R is the maximum volume in ml of n-

heptane that can be blended with 5 ml of test oil without precipitating asphaltenes from

the heptane dilution test.

The solvent oil equivalence test is used for a sample oil that is a solvent for

asphaltenes but contains no asphaltenes itself. The insolubility number is also set to 0,

and solubility blending number is calculated as follows,

S s o =100 T E

SOE (2.11)


where T E is the toluene equivalence of the test oil and SOE is the minimum percentage

of sample oil required in a sample oil-heptane mixture to keep asphaltenes in solution at a

concentration of 2 grams of test oil and 10 ml of test liquid.

An example is shown in Figure 2.7 given by Wiehe [1999] to explain this model.

Figure 2.7 shows the insolubility number for a mixture of Souedie and Forties crudes It

shows that for a compatible mixture, the volume percentage of Forties crude must be less

than 67 % to obtain an insolubility number for the mixture lower than that of Souedie

crude.

Blends of Souedie arid Forties 1 0 O T

A $ 0 -

3 8 0 -mi 7 0 -

S 6 0 *

S S O & ®

4 0

3 0 -

I 2 0 -3 1 0 -0 » 0 -

F o r t i e s ; S B K = 2 7 , X N = 11

S o u e d i e : 8 M * 6 3 , 1 N « 3 9

I N « 3 9 f o r S o u e d i e •

-J F

0 10 20 30 40 W ^ Volume % Forties

Figure 2.7: Oil Compatibility Numbers for Souedie and Forties Crudes Wj^Hp M9991. "


2.4 Aims and Objectives of Work

The role of resins in asphaltene stability in crude oil has been documented in the

literature. It is of interest to examine the role of resins on the fouling of asphaltene-

containing oils.

The objectives of this work include the following:

1. Investigate the effect of varying the resins to asphaltenes ratio on the asphaltene

fouling rate, hot filtration insolubles present in the sample at bulk temperature and

pentane insolubles.

2. Study the effect on asphaltene fouling of adding de-asphalted oil to heavy oil blends.

3. Investigate the asphaltene-crude oil incompatibility and its relation to asphaltene

fouling. This will aim at investigating oil compatibility models and their ability to

predict fouling behavior.

4. Characterize deposits formed at heat exchanger surface and investigate their relations

to the insoluble species in the fluid.

3.0 E X P E R I M E N T A L M A T E R I A L S AND A P P A R A T U S

This chapter describes the fluids used in this study and the properties of test

solutions. It also describes the experimental apparatus used to carry out the thermal

fouling experiments.

3.1 Experimental Materials

Cold Lake heavy oil (HO) supplied by Imperial Oil Resources Ltd. was used as

the source for asphaltenes. Fuel oil (FO) taken from a crude unit vacuum-top-side cut

supplied by Chevron Canada Ltd., was used as the carrier solvent. A sample of de-

asphalted vacuum bottoms (DAO), made available by Imperial Oil Ltd., was added to the

mixture to vary the resins/asphaltenes ratio.

3.1.1 Properties of Heavy Oil

Cold Lake heavy oil is a black viscous liquid with high density and low API

gravity compared to crude oil. It contains a significant fraction of high molecular weight

hydrocarbons with carbon number greater than C25. It has an asphaltenes content of

about 16.6 %, which is much higher than that of conventional crude oils. Heavy oil is

known to have a sulphur content higher than 2 %, which gives it a rotten-egg smell, and it

also includes heteroatoms and heavy metals. Properties of heavy oil used in this work are

shown in Table 3.1.

Saturates, aromatics, resins and asphaltenes contents of test oils used in this study

were based on an average of two analyses done at two different research centers to ensure

accuracy. One analysis was performed at the Imperial Oil Research Laboratory in Sarnia,

Ontario, using a high precision liquid chromatography method designed for oils with a

32

Chapter 3: Experimental Materials and Apparatus 33

boiling point greater than 300°C. This method separates the oil sample into saturates,

aromatics and polars. The total asphaltenes content was found by n-pentane precipitation

and the resins content was found by the difference between the polar and asphaltene

contents. Another analysis was performed at the National Center for Upgrading

Technology according to A S T M D 2007M dividing the oil sample into the four oil

solvent fraction constituents. Results of both analyses were similar; therefore, the

average was used in this study. Results obtained using the A S T M D 2007M showed

higher polar content and lower aromatics content than that of the H.P.L.C. SARA

analysis shows that heavy oil has high aromatics content of 50 % followed by saturates at

23 %, asphaltenes at 17 % and resins at 10 %. Elemental analysis of heavy oil was

carried out by Canadian Mircroanalytical Service Ltd. of Delta, B.C. Results of

elemental analysis indicate high sulphur content of 4.51 %, and low H/C atomic ratio of

1.57.

SARA analysis results obtained in this study are consistent with values reported in

literature by Speight (1991) as shown in Table 3.2. These literature values are

generalized ranges for the bulk fractions in crude petroleum, heavy oil, and residua.

3.1.2 Properties of De-asphalted Oil

Amounts of resins needed in this study made it impractical to recover a pure resin

stream from the heavy oil. De-asphalted oil was therefore used as the source of natural

resins to vary the Re/As ratio of mixtures. It is a sample of de-asphalted vacuum bottoms

that was made available by Imperial Oil. Oil de-asphalting is a common process in

refineries where propane is used to precipitate asphaltenes out of the oil. Propane is then

separated from the oil in evaporators heated by steam (Speight 1991).


Table 3.1: Properties of Cold Lake Heavy Oil

Test Description Value

SARA Analysis (HPLC)

Saturates (wt. %) 21.94

1 Ring Aromatics (wt. %) 12.4




Aromatics (total wt. %) 54.1

Resins (wt. %) 8.4

Asphaltenes (wt. %) 15.6

SARA Analysis (ASTM D 2007M )


Aromatics (wt. %) 45.58

Resins (wt. %) 12.39


Average SARA Analysis



Resins (wt. %) 10.4


Elemental Analysis

Carbon 80.27

Hydrogen 10.52

Nitrogen 0.41

Sulphur 4.51

H/C atomic ratio 1.57

Specific Gravity (@15°C) 1.038

API Gravity 10.1

Kinematic Viscosity (@ 80°C, m2/s) 4.25E-03

350-525°C 23.75 %

525°C + 76.25 % ~1


Table 3.2: Generalized Ranges for the Bulk Fractions in Crude Petroleum, Heavy Oil, and Residua. Speight H9911

Range of Composition (wt/wt%)

Asphaltenes Resins Oils

Carbon Residue (wt/wt%)

Petroleum <0.1-12 3-22 67-97 0.2-10.0

Heavy oil 11-45 14-39 24-64 10.0-22.0

Residua 11-29 29-39 ?-49 10.0-32.0

De-asphalted oil is a brown viscous semi-solid liquid at room temperature. It has

a smell similar to that of heavy oil that is caused by its sulphur content. Table 3.3 shows

some of the properties of DAO.

SARA analyses results obtained by the two methods gave almost identical results

for DAO. It was found that DAO has a high aromatics content of 68.45 %, followed by a

saturates content of 20.72 % and a resins content of 10.08 %. It has an asphaltene content

of 0.76 %. The DAO and HO contain roughly the same amount of saturates (22 %), and

resins (10 %), but strikingly different asphaltenes levels of 17 % for HO and < 1 % for

the DAO. Therefore DAO is used to vary the Re/As ratio of the mixture. However, it

should be noted that adding DAO to the mixture increases total aromatics and saturates

content of the mixture since they make up about 90 % of DAO.

DAO has a sulphur content of 3.5 % compared to 4.5 % for heavy oil. DAO and

HO have similar H/C ratio of 1.57 and 1.58, respectively, indicating that both oils may

contain heavy fractions of similar compositions.

Chapter 3: Experimental Materials and Apparatus

Table 3.3: Properties of De-asphalted Oil









Resins (wt. %) 10.1


SARA Analysis (ASTM D 2007M)



Resins (wt. %) 10.05





Resins (wt. %) 10.0


Elemental Analysis

Carbon 86.71

Hydrogen 11.15

Nitrogen 0.28

Sulphur 3.54

H/C atomic ratio 1.58


3.1.3 Properties of Fuel Oil

Fuel oil is used as a diluent for heavy oil and de-asphalted oil to reduce the

viscosity of the mixture to make pumping of the test fluid easier. It is also used to vary

the composition of test fluid and therefore test for a wider range of Re/As ratio.

Fuel oil is yellowish brown in colour. It is semi-solid similar to paraffin wax at

room temperature, however, upon heating it turns into a viscous fluid similar to lube oil.

Table 3.4 shows the properties of fuel oil.

Fuel oil is very high in saturates content followed by aromatics. Resin content is

low, at 2.7 %, and there are only trace amounts of asphaltenes present. Its sulphur

content is much less than that of heavy oil. However, the H/C atomic ratio is higher in

fuel oil since it is a lighter oil than heavy oil. SARA analysis results using A S T M

D2007M method showed higher aromatics content and lower polars content compared to

that of H P . L . C .

3.1.4 Properties of Test Solutions

The compositions of the mixtures for the thermal fouling runs are shown in Table

3.5. The table also includes some of the properties of these test solutions such as

kinematic viscosity and density. Results show that increasing heavy oil and DAO

concentrations increases the viscosity and density of the test solution.

Viscosity measurements are obtained using the Haake V T 500 Rotovisco which

gives information about the dynamic behavior of the fluid. Test fluids are found to

exhibit Newtonian behaviour when the shear stress is plotted against the shear rate.

Results for 15% DAO - 10% HO - 75% FO are shown in Figure 3.1.


Table 3.4: Properties of Fuel Oil









Resins (wt. %) 3.7

Asphaltenes (wt. %) Trace

SARA Analysis (ASTM D 2007M)



Resins (wt. %) 1.74





Resins (wt. %) 2.7


Elemental Analysis

Carbon 86.41

Hydrogen 12.76

Nitrogen 0.21

Sulphur 0.56

H / C atomic ratio 1.77

Density (@ 25°C, kg/m 3) 851

Kinematic Viscosity (@ 85°C, m 2/s) 2.15E-06


Table 3.5: Composition and Properties of Test Solutions

Weight Percent Kinematic Viscosity

at 85°C x 10 6 (m2/s)

Density at 85°C (kg/m 3)

Kinematic Viscosity at 158°C

x 10 6 (m2/s)

0% D A O - 5% H O - 95% F O 5.38 848 3.93

5% D A O - 0% H O - 95% F O 6.15 847 4.39

5% D A O - 5% H O - 90% F O 5.92 850 4.25

5% D A O - 10% H O - 85% F O 7.81 858 5.33

5% D A O - 15% H O - 80% F O 7.50 863 5.16

5% D A O - 20% H O - 75% F O 8.60 867 5.76

10% D A O - 0% H O - 90% F O 6.40 848 4.54

10% D A O - 3% H O - 87% F O 6.94 852 4.85

10% D A O - 5% H O - 85% F O 7.11 855 4.94

10% D A O - 10% H O - 80% F O 7.40 861 5.11

15% D A O - 2% H O - 83% F O 7.60 851 5.22

15% D A O - 3.5% H O - 87% F O 7.70 855 5.27

15% D A O - 5% H O - 80% F O 8.16 858 5.53

15% D A O - 10% H O - 75% F O 10.2 860 6.61

15% D A O - 15% H O - 70% F O 9.60 865 6.30

* Estimated at the film temperature using Puttagunta equation

The kinematic viscosity of 10% D A O - 10% H O - 80% F O was measured at

different temperatures and the results are plotted in Figure 3.2. The viscosity

exponentially decreased with increasing temperature. Viscosity values at each

temperature along with an exponential fit to the data is given in Appendix A 4 .


10

200 — I — 400 600 800 1000

Shear Rate (1/s)

Figure 3.1: Dynamic Behavior of 15% D A O - 10% H O - 75% F O at 85°C

Temperature (°C)

Figure 3.2: Viscosity of 10% D A O - 10% H O - 80% F O at Different Temperatures


3.2 Experimental Apparatus

3.2.1 Thermal Fouling Test Apparatus

Fouling runs were carried out in an existing UBC fouling loop, which is equipped

with an electrically heated annular probe supplied by Ashland Chemical Company, Drew

Divisions. The fouling loop is a recirculation system which consists of a supply tank for

holding the liquid, a centrifugal pump, an orifice meter for measuring flow rates, the

annular fouling probe, two pressure relief valves and a host of regulating valves.

A schematic of the apparatus is shown in Figure 3.3. All surfaces in contact with

the liquid other than the pump, were constructed from stainless steel. Liquid is pumped

by a 2.2 kW centrifugal pump from a 9.45 L holding tank through the flow control valve,

the orifice and the fouling probe before being returned to the tank. The fluid flows

through two mixing chambers (MC) one upstream and another downstream of the test

section where thermocouples measure the entry and exit bulk temperatures respectively.

Most experiments were performed at a constant holding tank pressure of 411 kPa (45

psig).

The flow rate is set using a control valve and it is measured using an orifice

meter. The pressure drop across the orifice meter is obtained and the following equation

is used to calculate the flow rate,

(3.1)

where V is the volume flow rate in m3/s, Ca is the orifice discharge coefficient found by

Asomaning [1990], A o r is the cross sectional area of the orifice in m 2, A P is the pressure


drop across the orifice meter in Pa, p is the density of the fluid at bulk temperature in

kg/m3, P = di/d2 where di and 62 are the diameters of the orifice plate and pipe

respectively.

3.2.2 Annular Test Section

The annular test section consists of the probe and an outer annular assembly. The design

of the probe is shown in Figure 3.4. Liquid flows upwards through the annulus between

the metal core and the outside wall. The heated section consists of a 32 ohm nichrome

electric heater embedded in a ceramic matrix and sheathed with a stainless steel tube.

The length of the heated section is 0.102 m. The surface temperature (Ts) is calculated

from the temperatures, T m , measured by four thermocouples embedded in the sheath, a

distance xs, below the surface, using the calibrations supplied by the supplier and the

following relationship

T s = T m - - ^ q (3.2) A m e t

where q = Q/A = (power input to the probe)/(probe heat transfer area). The diameter of

the probe and outer annulus are 0.011 m and 0.0254 m respectively. The probe is

designed for a maximum power input of 1920 Watts. It operates at a constant heat flux

with time, hence the fluid/deposit interface is assumed to be at a constant temperature.

The reciprocal of clean overall heat transfer coefficient is calculated as follows


Air

Rotameter

RBPC

Cooling -Water out

STT

Immersion Heater

h - f X r -

Supply Tank

Bypass Valve

Hx3-

SS Q

DPM

Annular Test Section

MC L J - " T b , i n

Orifice Plate

SS - Syringe Sampling (Septum) DPM RBPC - Rotameter Back Pressure Control p V - Outlet for Venting Gas STT MC - Mixing Chamber TC T b

- Bulk Temperature Thermocouples TC

- Pressure Gauge

Figure 3.3 Schematic of Fouling Apparatus


To Datalogger

To 240V Power Supply

294 mm

Fi2ure 3.4: Heat Exchanger Fouline Probe

The fouling reciprocal overall heat transfer coefficient is give by

v s b ' t

U(t) (3.4)


The fouling resistance could then be obtained using the expression

I T - T . 1 - I T - T , I

R f ( t ) = J — b / t V s b *> (3.5) f U(t) U(0) q V '

This fouling resistance was plotted against time to determine the thermal fouling profiles.

The fouling rate can then be calculated as

dR f _ d

dt dt

f 1 ^

v U ( t ) , (3.6)

In some cases where the fouling rate exhibited an asymptotic behaviour the initial fouling

rates were determined from fits of the fouling data to the Kern-Seaton equation, for times

beyond the induction period

R f = R f * ( l - e x p ( - b t ) )

(3.7)

Whence R f o =bxR* f

4.0 E X P E R I M E N T A L P R O C E D U R E S

This chapter will explain the procedures followed for thermal fouling runs, test fluid

properties and oil compatibility tests.

4.1 Procedure for Thermal Foul ing Runs

Cold Lake heavy oil was received from Imperial Oil Resources Ltd. in 5 gallon

containers. The contents of the containers were all mixed together prior to the fouling

runs to ensure uniformity. The bottom third of each container was first discarded to

avoid the water that was found at the bottom of the container being mixed with the oil

sample.

This research is built on methodology developed previously by Asomaning

(1997). Heavy Oil is weighed in a flask and heated on a hot plate to 30 - 40°C. De-

asphated oil is added to the flask and mixed while heating. Fuel oil is added gradually to

the flask to dissolve the heavy oil and de-asphalted oil. The mixture is then mixed

thoroughly to ensure a uniform test solution. After filling the supply tank, the test fluid is

purged with nitrogen for one hour under 410 kPa absolute pressure to eliminate any

dissolved oxygen. After purging, the control panel is activated along with the pump and

tank heater. If pressure in the system rises due to the presence of volatiles upon heating,

the system is vented to achieve the desired pressure. Once the desired pressure is

reached, the vent is closed completely for the experiment. When the bulk temperature

reaches 35°C, the flow control valve is set to achieve the desired bulk velocity. Before

starting the probe heater, a sample of the test fluid is taken for analysis. The power to the

heat transfer probe is adjusted to achieve the desired initial surface temperature of 230°C.

46

Chapter 4: Experimental Procedure 47

The datalogger and the PC are activated to record system variables at a scanning interval

of 10 minutes. A small flow of cooling water is used to maintain a constant bulk

temperature throughout the run.

The end of the experiment is identified when the maximum probe temperature is

reached or a constant surface temperature is obtained. The power to the probe and the

pump are switched off. A sample of test fluid is taken once again for analysis and the

cooling water flow rate is increased to cool the system. The pressure of the system is

released and the fluid is drained. The probe is taken out of the test section and rinsed

with varsol, a paint thinner sold by Esso Chemicals, to dissolve the oil, followed by

acetone to evaporate varsol. The probe is photographed and the deposits are removed

mechanically, with care, and stored for further analysis.

The rig is cleaned by pumping 9 liters of varsol for 30 minutes. This is followed

by 8 liters of a 50-50 % acetone-toluene mixture that is circulated for only 15 minutes

due to the corrosiveness of acetone and the toxicity of toluene. Finally, the rig is rinsed

with 6 liters of acetone to help evaporate the cleaning solvents. The probe is cleaned

with varsol and acetone prior to each run.

4,2 Determination of Pentane Insolubles and Hot Filtration Insolubles

The test sample is analyzed for pentane insolubles, filtration insolubles, and

viscosity at the beginning and end of each run. Pentane insolubles content is determined

by measuring 2 ml of test sample into a flask and adding 80 ml of n-pentane. In the case

of heavy oil, an equal volume of benzene is added to the sample prior to the addition of

n-pentane. The sample is well mixed and placed in the dark for several hours with

intermittent shaking. The content of the flask is then filtered at room temperature using a


3 micron Millipore filter paper and a Millipore filter funnel holding 47 mm diameter

filters. The precipitate is washed with n-pentane until the filtrate is clear. The filter with

the precipitate is placed in an aluminum dish and dried over night in an oven at 100°C.

Hot filtration insolubles are determined at the bulk temperature. The sample is

heated in a beaker on a hot plate to 85°C. Ten ml of sample are measured into a

graduated cylinder and filtered through a pre-weighed 3 micron membrane filter that has

been washed with n-pentane. After filtration, the precipitate is washed with n-pentane

until the filtrate is clear. The filter and the precipitate are placed in an aluminum dish and

dried over night in an oven at 100°C.

4.3 Measurement of Test Fluid Properties

The density and viscosity of mixtures are measured prior to each run. These

properties are required to set the desired flow rate for each run. The viscosity of mixtures

is determined using the Haake V T 500 Rotovisko, a computer controlled rotary

viscometer. A N V sensor system consisting of a cup and a bell-shaped rotor is used. The

sample is introduced into the space between the coaxial concentric cylinders. The outer

hollow cylinder is stationary while the inner solid bob rotates. The bob is motor driven

and its torque is measured by a force sensor. The viscosity of the fluid is a measure of

the resistance of the fluid to the rotation of the inner bob. The connected computer uses

the Haake V T 500 software to record the data and compute the variables.

The shear stress of Newtonian fluids is directly proportional to the shear rate in

the absence of turbulence. In this case, the absolute viscosity is given by Newton's

equation:

(4.1)


where x is the shear stress and D is the shear rate.

The densities of the mixtures are measured using a density bottle. The density

bottle is rinsed with a solvent, dried and weighed. The sample is heated to the bulk

temperature and charged to the density bottle. The bottle is then weighed and the density

is determined as the ratio of the mass to the volume of the sample.

4.4 Procedure for Oil Compatibility Tests

Oil compatibility tests were developed by Exxon Ltd. Detailed procedures were

supplied by Wiehe (1999). The four tests outlined below permit one to determine

compatibility of oils upon mixing.

4.4.1 Heptane Dilution Test

This test method is used to determine the onset of precipitation during heptane

dilution of oils that contain heptane insolubles (asphaltenes). N-heptane is added to 5 ml

of sample until insoluble asphaltenes are detected. In case of viscous oils, the sample is

heated to 70°C for half an hour at least with intermediate shaking using an ultrasonic bath

to assure that the sample is well mixed.

The point of precipitation is detected using a spot test, where a drop of the sample

is placed on a 5 micron filter membrane using a medical dropper and examined. The

point of asphaltene precipitation is taken where a definite ring of darker color is observed

in the center of the spot with a lighter surrounding color. The point is always tested with

a further dilution. If asphaltenes were present, the color change gets more definite.

In the first trial, n-heptane is added in 5 ml increments until a definite change in

the spot is observed. If asphaltenes are detected, the test is repeated with an amount of n-


heptane equal to 4 ml less than the amount when asphaltenes were first detected in the

first trial. In the second trial, n-heptane is added in 1 ml increments until asphaltenes are

detected and confirmed with an extra dilution. The test is then repeated for a third time

with an amount of n-heptane equal to 0.8 ml less than the amount when asphaltenes were

first detected in the second trial. n-Heptane is added in 0.2 ml increments in this case

until asphaltene detection is confirmed with an extra dilution.

The heptane dilution is taken as the average of the total ml of n-heptane added

when asphaltenes were first detected in the third trial and the highest total ml of n-

heptane added without detecting asphaltenes. The results are reported as the volume of

n-heptane in ml. added to 5 ml. of oil at the point just before insoluble asphaltenes first

appear, V H . The range of the heptane dilution is from 0 to 25 ml. If no insolubles are

detected at a heptane dilution of 25 ml., the oil is declared heptane soluble.

4.4.2 Toluene Equivalence Test

This test method is used to determine the toluene equivalence of oils that contain

heptane insolubles (asphaltenes). Ten ml. of a mixture of toluene and n-heptane are

added to 2 grams of the sample oil and the presence or not of insoluble asphaltenes is

detected using the spot test. In the case of viscous oils, the sample is heated to 7 0 ° C for

half an hour at least with intermediate shaking using an ultrasonic bath to assure that the

sample is well mixed. If insoluble asphaltenes are detected, the test is repeated but with a

higher percentage of toluene in the n-heptane-toluene mixture. On the other hand, if no

insoluble asphaltenes are detected in the first test, the second test is done with a lower

percentage of toluene. This procedure is continued until the minimum percentage of

toluene in the n-heptane-toluene mixture to keep asphaltenes in solution is determined to


the desired accuracy. This minimum percentage of toluene is reported as the toluene

equivalence, TE. The range of the toluene equivalence is from 0 to 100. If no insolubles

are detected at a toluene equivalence of 0, the oil is declared heptane soluble.

4.4.3 Nonsolvent Oil Dilution Test

This test method is used for an oil sample that is a nonsolvent for asphaltenes but

contains no asphaltenes itself. In the present case, this is done on the fuel oil. The

nonsolvent oil is blended with an asphaltene containing test oil that has been previously

subjected to the Toluene Equivalence and Heptane Dilution tests. Heavy oil is used as

the test oil in this case. Basically, for the Nonsolvent Oil Dilution test, the sample oil (or

nonsolvent oil) is added to the test oil until insoluble asphaltenes are detected. The test is

repeated to obtain the desired accuracy. The results are reported as the volume of

nonsolvent oil in ml. added to 5 ml. of test oil at the point just before insoluble

asphaltenes first appear, VNSD- The range of the nonsolvent oil dilution is from 0 to 25

ml.

4.4.4 Solvent Oil Equivalence Test

This test method is used for an oil sample that is solvent for asphaltenes but

contains no asphaltenes (soluble in heptane). In the present case, this is applied to the

DAO. A test oil containing asphaltenes is needed that has been previously put through

the Toluene Equivalence and Heptane Dilution tests. In this case, heavy oil is used as the

test oil. Basically, for the Solvent Oil Equivalence test, toluene in the Toluene

Equivalence Test is replaced by the sample oil (or solvent oil) and the equivalence test is

rerun on the test oil. That is the ratio of solvent oil to n-heptane is varied and mixed with

the test oil in the ratio of 10 ml. of solvent oil-heptane mixture per 2 grams of test oil.


The minimum volume percent of solvent oil in the solvent oil-heptane mixture to keep

the asphaltenes in solution is the Solvent Oil Equivalence, SOE. The range of values is

from 0 to 100.

5.0 RESULTS AND DISCUSSION

Experiments were carried out in this research to examine the effect of resins to

asphaltenes ratio on heat exchanger fouling of heavy oil. A sample of de-asphalted oil

was used to vary the Re/As ratio as it was difficult to separate large quantities of resins

required for this study. Blends of varying concentrations of HO, DAO and FO were used

in thermal fouling runs at similar conditions to examine their fouling behaviour. The

results along with the discussion of these results will be presented in this chapter.

5.1 Typical Thermal Fouling Run

Fouling runs are carried out for periods up to 30 hours. Examples of typical results

obtained in a thermal fouling run are shown in Figures 5.1 and 5.2. The surface

temperature starts rising as the deposit builds up beyond the induction period. The heat

flux drops slightly in the beginning of the experiment, but it stays almost constant over

the course of the experiment. The overall heat transfer coefficient decreases as deposit

builds up on-heat exchanger surface and the fouling resistance increases correspondingly.

Results shown in Figures 5.1 and 5.2 are for the experiment carried out with 5 % DAO -

15 % HO and 80 % FO. A test of reproducibility is given in Appendix A3.

5.2 Effect of Resins to Asphaltenes Ratio on Heavy Oil Fouling

The role of resins on asphaltene stability is not very well understood. There are

views in support of the idea that resins keep asphaltenes in solution and form a stable

mixture. However, others noted that not all types of resins have such an effect and it is

important that the resins come from the same crude as the asphaltenes. Therefore, the

effect of resins to asphaltenes ratio on the stability of asphaltenes in solution is examined

53

Chapter 5: Results and Discussion 54

Figure 5.1: Surface Temperature and Heat Flux for a Typical Fouling Run

Figure 5.2: Overall Heat Transfer Coefficient and Thermal Resistance for a Typical Fouling Run


in this work through investigating the fouling behaviour of oil samples of varying resins

to asphaltenes ratio.

Blends of HO/FO/DAO were prepared with 5 wt % of DAO and different

amounts of HO to test the effect of HO concentration and resins/asphaltenes ratio on the

fouling rate. The fouling resistance over time for these runs is shown in Figure 5.3.

There appears to be an induction period of 2 - 5 h, followed by fouling with a falling rate

with time. For zero percent heavy oil, fouling is negligible. Thus on its own DAO does

not cause fouling. As the HO concentration was raised from 0 to 20 %, the extent of

fouling increased. The corresponding asphaltene concentration rose from 0.04 % to 3.36

%, and the Re/As ratio decreased from 81.3 to 1.37. The run at 10 % HO showed a

shorter induction time, and had the most rapid rise in fouling resistance with time. At 15

% HO and 20 % HO, fouling behaviour was similar and the extent of fouling exceeded

that at 10 % HO after about 14 hours, although the rate was lower.

Table 5.1 lists the thermal fouling parameters for the series of experiments of 5 wt

% DAO in HO/FO mixtures. The reciprocal of the clean overall heat transfer coefficient

decreased with increasing percentage of HO in mixture. The final fouling resistance was

consistent with the initial fouling rate.

The initial fouling rates of these runs were determined by fitting the data to the

Kern-Seaton [1959] asymptotic fouling model except for the run with 10 % HO where

the slope was used to obtain the initial fouling rate. A program in M A T L A B was

prepared using Marquardt's method to fit experimental data to the asymptotic fouling

model. The parameters obtained along with the standard deviation of the fit can be found

in Appendix A2.


-0.04 - | 1 1 1 1 • 1 • 1 > 1 r 1 0 5 10 15 20 25 30

Time (h)

Figure 5.3: Fouling Resistance over Time of 5 wt % D A O in H O / F O Mixture

Table 5.1: Thermal Fouling Parameters for Experiments of 5 wt % D A O in H O / F O Mixtures at T h of 85 °C, T«„ of 230 ° C and Uh of 0.75 m/s

Heavy Oil

(wt %)

Re/As Heat Flux

(kW/m2)

Initial Fouling

Rate (m 2 K/k\\h)

1/U„ (m 2K/k\V)

Range of Initial Rate Calculation

(h)

Final R f

(m 2K/k\V)

0 81.3 250 Neg. Fouling 0.57 0-26 0.008

5 4.0 366 0.020 0.385 3 - 22.5 0.164

10 2.3 365 0.039 0.395 1.1-6.5 0.272

15 1.7 411 0.026 0.354 3 - 19 0.305

20 1.4 451 0.026 0.301 3 - 16.5 0.269

Figure 5.4 shows the fouling resistance over time for 10 wt % DAO with different

HO (and hence asphaltene) concentrations. Fouling was negligible in the absence of HO.

It is evident in this case as well, that the extent of fouling increased with increasing HO


or asphaltene concentration, which corresponded to decreasing Re/As ratio. As in Figure

5.3, fouling was rapid and severe, reaching Rf values of 0.1-0.2 m 2K/kW in less than ten

hours. Thermal fouling parameters of these runs are listed in Table 5.2. Heat flux

increased while the reciprocal of the clean overall heat transfer coefficient decreased with

increasing percentage heavy oil. Initial fouling rates were estimated using the slope of

the curve in these runs except for the 10 % HO where the fouling model was used to

obtain the initial fouling rate.

Figure 5.4: Fouling Resistance over Time of 10 wt % D A O in H O / F O Mixture

A series of experiments was performed at 15 wt % of DAO in which the effect of

addition of low concentrations of HO was also examined. Results shown in Figure 5.5,

indicate a dramatic increase in fouling as the HO concentration was increased from 3.5 to


5 wt %, which corresponded to a decrease in Re/As ratio from 5.9 to 4.5. As in Figure

5.3, fouling was most rapid at the intermediate concentration of 10 % HO.

Table 5.2: Thermal Fouling Parameters for Experiments of 10 wt % D A O in H O / F O Mixtures at T h of 85 ° C T.» of 230 °C and U h of 0.75 m/s

Heavy Oil

(wt %)

Re/As Heat Flux

(kW/m2)

Initial Fouling

Rate (m 2K/kWh)

1/Uo (m 2K/k\V)


(h)

Final R f

(m 2K/kW)

0 45.5 243 Neg. Fouling 0.587 0-21 0.0049

3 6.4 238 Neg. Fouling 0.596 2-25 0.007

5 4.2 353 0.025 0.392 0- 18 0.183

10 2.4 452 0.045 0.324 1 - 11 0.279

Parameters of the thermal fouling runs for 15 % DAO series runs are available in

Table 5.3. Results show that as percentage of heavy oil increased the heat flux required

to achieve the desired initial surface temperature is increased. The reciprocal of the clean

overall heat transfer coefficient decreased as heavy oil concentration increased in the

mixture. The final fouling resistance values are consistent with the initial fouling rate

calculated. The 5 % HO and 15 % curves were fitted to the Kern-Seaton model to obtain

the initial fouling rate while the slope of the fouling resistance versus time plot was used

for the 10 % HO concentration curve.


Figure 5.5: Fouling Resistance over Time of 15 wt % DAO in HO/FO Mixture

Table 5.3: Thermal Fouling Parameters for Experiments of 15 wt % DAO in HO/FO Mixtures at T h of 85 °C, T«n of 230 °C and U h of 0.75 m/s

Heavy Oil

(wt %)

Re/As Heat Flux

(kW/m2)

Initial Fouling

Rate (m2K/k\\h)

1/U„ (m2K/k\V)


(h)

Final R f

(m2K/kW)

2 8.9 237 Neg. Fouling 0.587 0-27 0.017

3.5 5.9 237 Neg. Fouling 0.600 0-27 0.020

5 4.5 389 0.036 0.353 0-19 0.257

10 2.6 431 0.054 0.317 1 -6.3 0.305

15 1.9 516 0.039 0.231 0-11 0.271

Initial fouling rates for 0 % DAO, obtained by Asomaning [1997], 5 % DAO, 10

% DAO and 15 % DAO mixtures were plotted against their calculated asphaltene content


as shown in Figure 5.6. The graph indicates that for each feed mixture, the initial fouling

rate increases with asphaltene content, reaches a maximum, and then decreases. The

value of the maximum rate increases as the DAO and hence resin content in the mixture

increases for a given percentage of asphaltene. The maximum occurs at an asphatlene

content of = 1.7 % corresponding to a Re/As ratio of = 2.5. At asphaltene contents below

0.75 %, negligible fouling is observed.

The initial fouling rates of mixtures containing similar asphaltene contents were

plotted against their resins content in Figure 5.7. As the resin concentration is increased,

higher fouling rates are encountered when it was expected that fouling would decrease

based on available literature. Figure 5.7 clearly indicates that at a given asphaltene

concentration, fouling rates increase with the resins content in this system. This suggests

that there is some interaction between the resins and the asphaltenes which promotes

fouling in this case.

The effect of resin and asphaltene contents on the initial fouling rates of mixtures

was examined further by plotting the initial fouling rates of the mixtures against Re/As

ratios in Figure 5.8. For a given percentage DAO, initial fouling rates go through a

maximum with increasing Re/As ratios. At Re/As < 2.5, the initial fouling rate increased

as the Re/As ratio increased. However, for values > 2.5 the initial fouling rate decreased

as the Re/As ratio increased with a dramatic decrease in the initial fouling rate at Re/As >

4. For mixtures containing DAO, fouling rates were essentially zero at Re/As > 6.


0.060

0.055 - • 0 % DAO 1 Re/As-2.5 0.050 - 9 5 % DAO /'j 0.045 - - - A 10% DAO / t 0.040 - ••• 15% DAO /I9

0.035 - y / j 0.030 - / / ' ' , * /

0.025 - / / \ e

0.020 - )"/ \ 0.015 - \ 0.010- :U 0.005 -

0.000 -

-0.005 -

// 8Sk-

— i 1 1 1 1 1 — -1 1 T I 1 | • | — i 1

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Calculated Asphaltene Content (wt. %)

4.0

Figure 5.6: Relationship of Initial Fouling Rate with Calculated Asphaltene Content for 0. 5.10 and 15 wt % D A O in H O / F O Mixture

0.010 -\ 1 1 i 1 • 1 > 1 ' r 2.5 3.0 3.5 4.0 4.5 5.0

Calculated Resins Content (wt. %)

Figure 5.7: Relationship of Initial Fouling Rate with Calculated Resins Content at Constant Asphaltenes Content


0.060

R e / A s

Figure 5.8: Relationship of Initial Fouling Rate with Re/As ratio for 0, 5, 10 and 15 wt % D A O in H O / F O Mixture

Results in Figure 5.8 showed that at a constant Re/As ratio, the initial fouling rate

increased as the DAO concentration increased in the mixture. This indicates that the

addition of DAO caused incompatibility in the mixture and therefore increased the

fouling rate. As noted previously the presence of DAO did not cause fouling in the

absence of heavy oil.

Examination of Figure 5.8 reveals the complexity of the role of resins in

asphaltenes stability. For Re/As ratios larger than 2.5, results show a decrease in initial

fouling rate indicating a possible role of resins in keeping asphaltenes in solution and

therefore increasing compatibility. On the other hand, at lower Re/As ratios it seems that


there are factors overcoming the role of resins in keeping asphaltenes in solution. It is

indicated by Leontaritis et al. [1988], that there is a threshold concentration of resins

required to keep asphaltenes in solution below which asphaltenes would precipitate. It

could be possible that a Re/As ratio 2.5 indicates the threshold- resins concentration ratio

required for this system to enable the positive role of resins in stabilizing the crude and

below which its role is eliminated.

On the other hand, asphaltenes concentration varied from 0.04 - 3.4 wt %, while

the resins content was limited to between 3.1 - 4.9 wt %. The wide variation in the

resulting Re/As ratio (from 1.4 to 81.3) was dictated primarily by the changing

asphaltene content. The narrow range of resin concentration variation in the mixtures

made it difficult to study its role as a peptizing agent in more detail. If the fuel oil

content was kept low, the viscosity of the mixture became too high for pumping. In

addition, the high saturates content of these mixtures (55.3 - 67.3 %) made it even harder

to examine the effect of other constituents. Aromatics varied between 28.7 and 37.1 %.

Therefore, when the initial fouling rates were plotted versus (Ar + Re)/As in Figure 5.9,

the graph was similar to that of Figure 5.8. A detailed list of constituent concentrations

of test fluids is available in a later section in Table 5.6.

5.3 Effect of Resins to Asphaltenes Ratio on Hot Filtration and Pentane Insolubles

Pentane insolubles basically provide a measure of the total asphaltenes content in

the mixture. Since hot filtrate insoluble material is asphaltene, its measurement

represents the flocculated asphaltene concentration. The hot filtration insolubles test

therefore gives a rough indication of the compatibility of the mixture. There is some

inaccuracy in measurements of hot filtration insolubles which is due to the low


concentrations of insolubles present at bulk conditions and in some cases samples were

not filtered directly after sampling which might have affected its concentration of

filterable solids. Properties of all test fluids are presented in Table 5.4. Results for

concentrations of hot filtration insolubles and pentane insolubles are plotted against

Re/As ratio for all test fluids in Figure 5.10.

. a

o

0.060

0.055 H

0.050

0.045

0.040 -\

0.035

0.030

0.025 H

0.020

0.015 H 0.010

0.005

0.000 ̂

-0.005 10

— • — 0 % DAO (Asomaning 1997) ® 5 %DAO

— A — io % DAO —v— 15 % DAO

- T 1 1 1—i—i—i—p

100

(Re+Ar)/As

- A

1000

Figure 5.9: Relationship of Initial Fouling Rate with (Re + Art/As Content in Mixture for 0. 5.10 and 15 wt % DAO in HO/FO Mixture

Figure 5.10 shows the variation of pentane insolubles and hot filtration solids with the

Re/As ratio for all the mixtures. Pentane insolubles and hot filtration insolubles

concentrations were measured at the beginning and end of each run and the average was

used in this study since there was a negligible difference in both measurements. The

pentane insolubles (and calculated asphaltene contents) decreased monotonically with the

increasing Re/As ratio. The hot filtration solids appeared to first increase and then to

Chapter 5: Results and Discussion

Table 5.4: Properties of Test Fluids

Test Fluid Hot Filtration

Insolubles (a/i)

Pentane Insolubles

(g/1)

Initial Fouling Rate

(m2K/kWh)

Reynolds Numbers*

Reynolds Numbers"1" Re/As

0 % DAO 5 %HO 95 %FO

2.7 7.3 0.010 1998 2734 3.7

5 %DAO 0 %HO 95 %FO

0.9 1.3 Neg. fouling 1748 2449 81.3

5 %DAO 5 %HO 90 % FO

4.0 8.4 0.020 1816 2527 4.0

5 %DAO 10 %HO 85 %FO

2.5 10.2 0.039 1376 2016 2.3

5 %DAO 15 %HO 80 %FO

3.6 14.2 0.026 1433 2083 1.7

5 %DAO 20 %HO 75 %FO

3.3 21.0 0.026 1250 1865 1.4

10 %DAO 0 %HO 90 %FO

1.6 1.6 Neg. fouling 1680 2370 45.5

10 % DAO 3 % HO 87 %FO

2.4 5.2 Neg. fouling 1549 2218 6.4

10 % DAO 5 %HO 85 %FO

4.2 9.4 0.025 1512 2175 4.2

10 %DAO 10 %HO 80 %FO

5.2 11.3 0.045 1453 2106 2.4

15 %DAO 2 %HO 83 %FO

0.9 5.0 0.001 1414 2060 8.9

15 %DAO 3.5 %HO 81.5 %FO

1.2 5.8 0.001 1396 2040 5.9

15 %DAO 5 % K O 80 %FO

5.2 10.9 0.036 1317 1945 4.4

15 %DAO 10 %HO 75 %FO

5.4 11.8 0.054 1054 1626 2.6

15 %DAO 15 %HO 70 %FO

7.2 18.6 0.039 1120 1707 1.9

* Calculated based on bulk temperature properties + Calculated based on estimated film temperature properties


decrease with the ratio Re/As. Comparing pentane insolubles values with the hot

filtration results suggests that on average, less than 50 % of the potential asphaltenes are

insoluble at the bulk temperature, whereas the rest remain in solution. At low Re/As

ratios, only 15 % of the asphaltenes are insoluble, whereas at the highest Re/As levels,

about 70 % of the asphaltenes are insoluble.

^ 25 •

-59 <D

£ 20 •

O

a is

12 10

S3 O

o

OH

• Hot Filtration Insolubles e Pentane Insolubles

- i 1 1 r -

10

Re/As

100

Figure 5.10: Relationship of Properties of Mixtures with Re/As ratio for all oil Mixtures

Pentane Insoluble contents of test fluids are plotted against their Re/As ratio for various

DAO concentrations in Figure 5.11. The graph shows the decrease in pentane insolubles

with increasing Re/As ratio. In addition, it is clear that at a constant Re/As ratio, the

pentane insoluble concentration increases with increasing DAO content in the mixture for

Re/As < 5. This result indicates that DAO addition to the mixture increases the content

of pentane insolubles and therefore affects the compatibility of the mixture. However, it


is necessary to make sure that this increase in pentane insolubles content is not due to an

increase in the total calculated asphaltene content of the mixture due to the addition of

DAO. Therefore, the pentane insoluble concentrations are plotted versus the calculated

asphaltene content for various DAO concentrations as shown in Figure 5.12.

Examination of Figure 5.12 shows that the pentane insoluble concentration increases with

increasing DAO content at a constant asphaltenes content. These findings lead to the

conclusion that the addition of DAO to the mixture may enhance the formation of

asphaltenes that were not present in the mixture and therefore affect the oil compatibility.

25

20

jE^ 15

o C O

I

10

OH

-•—5 %DAO ® 10 % DAO A -- 15 % DAO

A._ • - A

- T 1 1 — I — I — r I I I — I — r

10

Re/As

100

Figure 5.11: Pentane Insolubles Variation with Re/As ratio for Various DAO Concentrations

The effect of resins concentration on hot filtration insolubles at contant asphaltene

content is shown in Figure 5.13. As the resin concentration is raised at a given asphaltene

level, the hot filtration solids concentration generally increased indicating less

compatibility in the mixture. This result supports the fouling rate findings in Figure 5.7.


Figure 5.12: Measured Pentane Insloluble Concentration Variation with Calculated Asphaltene Contents for Various DAO Concentrations

7.5

2.0 H 1 1 1 1 1 1 1 1 r 3.0 3.5 4.0 4.5 5.0

Calculated Resins Content (wt. %)

Figure 5.13: Measured Hot Filtration Insoluble Concentration Variation with Calculated Resins Contents for Various Asphaltene Concentrations


Hot filtration insoluble concentrations are plotted against the Re/As ratios for

various DAO concentrations in Figure 5.14. Although there is considerable scatter in the

data, it appears that hot filtration insolubles generally decrease with increasing Re/As for

all DAO concentrations. Mixtures with higher DAO concentrations have a higher hot

filtration insoluble concentration at a constant Re/As ratio, with the exception of the data

at 15 % DAO (Re/As > 5). This figure shows that DAO is generally playing a negative

role in the compatibility of the mixture as noted previously with the pentane insolubles

concentrations.

7.5-7.0 H 6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.5-

• 5 %DAO a 10 % DAO A 15 % DAO

• i i i i i |

l 10

Re/As 100

Figure 5.14: Hot Filtration Insolubles Variation with Re/As ratio for Various D A O Concentrations

A rough correlation of the fouling rate with concentration of hot filtration solids is

shown in Figure 5.15, where the dashed line, and some of the data are taken from

Asomaning and Watkinson [1999]. The Asomaning data shows fouling rates increasing


by a factor of about one hundred as the insoluble solid concentration goes from about 2

g/L to 6 g/L. The present data shows as two horizontal clusters on the plot. The initial

fouling rate is about 0.001 m 2 K / k W h where the concentration of precipitated solids is

from about 0.7 to 1.5 g/L, and at insolubles concentrations of 3-7 g/L, the rate did not

change substantially from its average value of 0.03 m 2 K / k W h .

o.i-d

o o.oi-j

I? 1 fa 13 1E-3 ' 4 3 • Asomaning [1997]

e This Work

i l i I i l 0 1 2 3

T — 1 — r 4 5

Hot Filtration Insolubles (g/L)

Figure 5.15: Initial Fouling Rate Dependence on Solids ConcentrationstTh 85" C Tg,

230° C . V 0.75 m/s)

Hot filtration insolubles results are plotted in terms of percentages of H O and F O

in Figure 5.16. The plot shows general trends of increasing hot filtration insolubles with

increasing concentrations of both H O and D A O . Although there is some scatter, the

contours do show a general trend which suggests that increasing the D A O concentrations

makes the system less compatible at any level of H O . As well , where no heavy oil is

present, a small concentration of filterable solids exists.


Elemental analysis was performed on a sample of hot filtration insolubles

collected by fdtering an oil mixture of 15 % DAO - 15 % HO - 70 % FO. Results of

sample analyses are show in Table 5.5. The H/C ratio is lower in this sample of

precipitate than that found in the original test fluids, HO, DAO and FO as reported in

Tables 3.1, 3.3 and 3.4; and is very close to values of asphaltenes given by Suzuki et al.

[1982] as presented in Table 2.2, and by Strausz [1992].

Table 5.5: Elemental Analysis of Hot Filtration Insolubles

Oil Sample C H N S H/C atomic

15%DAO-15%HO-75%FO 77.38 7.09 1.36 8.91 1.1

•A

g

-•—0 %HO ® -5 %HO A 10% HO

- T - 15 % HO

10

% DAO

i 12 14 16

Figure 5.16: Relationship of % DAO and % HO to Hot Filtration Insolubles

As well, as expected for asphaltenes, the nitrogen content is found to be higher and the

sulfur content is much higher than that of the original test oils.


5.4 Deposit Characterization

Fouling occurs on the surface of the probe in the fouling rig. Deposit builds up as

the run proceeds appearing as a thin black layer on the surface, for low fouling rates as

shown in Figure 5.17, and a thick black layer in cases of high fouling rates as it appears

in Figure 5.18. Figure 5.19 shows a close-up of the thick deposit evenly distributed along

the heated length and the roughness is apparent at the surface. Deposits are collected at

the end of each run after photographing the probe. Elemental analyses were performed

for some deposit samples and these results are presented in Table 5.6.

Chemical characteristics of collected probe deposit show that the H/C atomic ratio

of the deposits is in the range 1.2-1.3, nitrogen content is 0.77- 1.1 %, and the sulphur

content is 4.5-5.8 %. These values are characteristic of asphaltenes as reported by Wiehe

[1999b]. On average, the H/C ratios and nitrogen contents of the deposits are similar to

that of the hot filtration insolubles as shown in Table 5.5 indicating that insolubles are

depositing on the heat exchanger surface causing fouling. However, the sulfur content of

the hot filtration insolubles is much higher than that of the deposits.

Table 5.6 Chemical Characteristics of Probe Deposits of Some Runs

C H N S . H/C

atomic

0%DAO-5%HO-95%FO 79.83 8.07 1.1 4.46 1.21

5%DAO-5%HO-90%FO 72.22 7.10 0.86 5.83 1.18

10%DAO-5%HO-85%FO 75.19 8.06 0.82 4.78 1.29

15%DAO-5%HO-80%FO 78.24 8.67 0.77 4.68 1.33


Figure 5.18: Deposit Formed on Probe Surface for Run with 10 % D A O - 1 0 % H O - 80 % F O with High Fouling Rate.


Figure 5.19: Close-Up of Deposit Formed on Probe Surface for R u n wi th 10 % D A O - 1 0 % H O - 80 % F O with H i g h Foul ing Rate.

The H/C atomic ratio and the nitrogen content are plotted versus the percentage of

DAO in the mixture in Figure 5.20. It is shown that the H/C atomic ratio of the deposits

increases and nitrogen content decreases with increasing DAO content. These results

show that DAO may contribute to precipitation as the properties of the deposits formed

are consistently changing with DAO concentrations.

Fouling deposits of selected runs were further studied using a scanning electron

microscope (SEM) along with an Energy-Dispersion X-ray (EDX). The SEM

micrographs of the deposits of these runs are shown in Figures 5.21, 5.23 and 5.25 while

the EDX plots are shown in Figures 5.22, 5.24 and 5.26.


1.35

—•— H/C Atomic Ratio

1.30

125 o

O 1.20

> 3 o' 70

0.7 1.15 0 2 4 6 8 10 12 14 16

Wt % DAO

Figure 5.20: Deposit Characteristics Var ia t ion with D A O Content in M i x t u r e

SEM micrographs of fouling deposit show clusters of agglomeration that could be

asphaltenes that have undergone some form of chemical change on the hot probe

surfaces. The E D X analyser is attached to the SEM to examine the deposits for the

presence of elements using carbon as the standard. The E D X analyses showed the

presence of sulphur, silicon, sodium, chlorine and trace quantities of sodium and copper.

The large content of sulphur is noticeable in all samples. The presence of most of these

elements is believed to be due to heavy oil since DAO and fuel oil samples are processed

while heavy oil is not. However, it should be noted that the quantities of these elements

are very low and they are present in most fluid mixtures in similar quantities, therefore

their effect on the fouling behaviour of these mixture is believed to be negligible.


Figure 5.21: S E M Mic rog raph of Foul ing Deposit for R u n with 5 % D A O - 5 % H O and 90 % F O

Counts

1875 4

1500 4

1125 4

Figure 5.22: E D X Plot of Foul ing Deposit for R u n with 5 % D A O - 5 % H O and 90 % F O


H H S

Figure 5.23: S E M Micrograph of Foul ing Deposit for R u n wi th 15 % D A O - 5 % H O and 80 % F O

Counts

1065

852 4

639 - J

426

213

Figure 5.24: E D X Plot of Fouling Deposit for R u n with 15 % D A O - 5 % H O and 80 % F O


M l

Figure 5.25: S E M Mic rograph of Fouling Deposit for R u n with 5 % D A O - 15 % H O and 80 % F O

Counts

1025 4

820

615

410 J

205

. Cu

8 9 keV

Figure 5.26: E D X Plot of Foul ing Deposit for R u n with 5 % D A O - 15 % H O and 80 % F O


5.5 Colloidal Instability Index

Asomaning and Watkinson [1997] reported that their fouling rate data for HO-FO

mixtures, including those with pentane and xylene additions, could be correlated by the

Colloidal Instability Index. However the present data which involves HO-FO-DAO

mixtures could not. The reason for the failure of the CII to correlate the present data is

unclear; however, it is noted that in contrast to the present data, all of Asomaning's data

except one point was taken at Re/As less than 2.5, and hence below the condition for

maximum fouling rate shown in Figure 5.8. Both sets of results were explored through

the fouling regime map of Figure 5.27. The boundary between a "no-fouling" regime

(initial fouling rate < 0.001 m2K7kWh), and the "fouling" regime can be approximated by

the condition,

R f o - » 0 , for CH < (Re/As) 0 3 (5.1)

Additional data are required to fix this boundary with greater accuracy, however

Figure 5.27 strongly suggests that additional parameters beyond the CII are needed to

predict fouling rates in asphaltene-containing systems where a wide variation in Re/As

ratio exists. The Asomaning data showed measurable fouling rates once CII reached

about 1.2, whereas the present data suggest that CII values as high as 1.6 will not produce

fouling, if the ratio of Re/As is large. Calculated oil constituent contents of test fluids

along with their CII values are presented in Table 5.7.


10-

Fouling

• • • y

0.1 •

A

No Fouling

• Th i s W o r k (Fouling)

® Asoman ing 1997 (Fouling)

^ Th i s W o r k (No Foul ing)

• Asoman ing 1997 ( N o Foul ing)

10 100

R e / A s

Figure 5.27: Foul ing Regime M a p

( " N o Fou l ing" corresponds to R f < or equal to 0.001 m 2 K / k W h )

Chapter 5: Results and Discussion -81

Table 5.7: Compositions of Test Fluids

Test Fluid Calculated Saturates

Content (%)

Calculated Aromatics

Content (%)

Calculated Resins

Content (%)

Calculated Asphaltenes Content (%)

Re/As CII

4 % DAO 5 % HO 95 % FO

67.30 28.77 3.10 0.83 3.7 2.14

6 % DAO 7 % HO 95 % FO

67.17 29.70 3.09 0.04 81.3 2.05

8 % DAO 9 % HO 90 % FO

64.85 30.81 3.47 0.87 4.0 1.92

10 % DAO 11 %HO 85 % FO

62.53 31.92 3.86 1.70 2.3 1.80

12 %DAO 13 % HO 80 % FO

60.20 33.03 4.24 2.53 1.7 1.68

14 % DAO 20 %HO 75 % FO

57.88 34.14 4.62 3.36 1.4 1.58

10 %DAO 0 % HO 90 %FO

64.73 31.74 3.46 0.08 45.5 1.84

10 %DAO 3 % HO 87 % FO

63.34 32.45 3.69 0.57 6.4 1.77

10 %DAO 5 % HO 85 % FO

62.40 32.85 3.84 0.91 4.2 1.73

10 %DAO 10 %HO 80 %FO

60.08 33.96 4.22 1.74 2.4 1.62

15 % DAO 2 % HO 83 % FO

61.35 34.20 3.98 0.45 8.9 1.62

15 % DAO 3.5 %HO 81.5 %FO

60.66 34.60 4.09 0.70 5.9 1.59

15 %DAO 5 %HO 80 %FO

59.96 34.89 4.21 0.95 4.4 1.56

15 %DAO 10 %HO 75 % FO

57.64 36.0 4.59 1.78 2.6 1.46

15 %DAO 15 %HO 70 %FO

55.31 37.11 4.97 2.61 1.9 1.38


5.6 Oil Compatibility Model Relation to Asphaltenes Fouling

The Oil Compatibility Model as proposed by Wiehe [1999] is used to predict the

order of blending and the ratio of certain oils to ensure compatibility of the oil mixture. It

is assumed that if the fluid mixture is incompatible, fouling will occur. In this study, Oil

Compatibility tests were performed on test fluids HO, DAO and FO to obtain model

parameters required to enable correlation of model results with available fouling data.

Two sets of tests were performed. In the first set, heavy oil was used as the

reference oil and both DAO and FO were tested against it. A blend of 50-50 weight

percent of HO-D AO was used as the reference oil and FO was tested against it in the

second test set. Results obtained in both test sets are discussed in the following sections.

5.6.1 Heavy Oil as Reference Oil

Heavy oil contains 16 % of asphaltenes and therefore is defined as heptane

insoluble. However, DAO and FO have low asphaltene contents and therefore are

defined as heptane soluble. Heptane Dilution tests and Toluene Equivalence tests were

performed on HO. Since FO is not a good solvent for asphaltenes, the Nonsolvent

Dilution test was performed. The solvency of DAO for asphaltenes was not clearly

known, therefore both the Nonsolvent Oil Dilution and the Solvent Oil Dilution tests

were performed.

5.6.1.1 Test Results

Heavy oil and DAO are very viscous at room temperature. Therefore, the sample

was usually heated to 70°C for half an hour at least with intermediate shaking using an

ultrasonic bath to lower the viscosity of the mixture and assure adequate mixing.


The Heptane Dilution test was performed for heavy oil. It was found that 9.2 ml.

of n-heptane was needed to precipitate asphaltenes. The Toluene Equivalence test was

performed on heavy oil and it was found that a 23.5% toluene in a toluene-n-heptane test

sample is required to keep asphaltenes in solution at a concentration of 10 ml. of test

sample to 2 grams of oil.

Heptane Dilution test was also carried out for DAO to ensure that it is heptane

soluble. There were no asphaltenes detected after the addition of 25 ml. of n-heptane to 5

ml. of DAO, so it was confirmed that it is heptane soluble.

The Nonsolvent Oil Dilution test was performed on fuel oil using heavy oil as the

reference oil. It was found that 5.75 ml. fuel oil can be added to 5 ml. of heavy oil before

precipitating asphaltenes.

The Nonsolvent Oil Dilution test was performed on De-asphalted oil and it was

found to be asphaltene soluble at a concentration of 25 ml. DAO to 5 ml. heavy oil.

Therefore, the Solvent Equivalence test was performed and it was found that 37.50 % of

DAO is required in a DAO-n-heptane mixture to keep asphaltenes in solution at a

concentration of 10 ml. of DAO-n-heptane mixture to 2 grams of heavy oil. Results of

these tests are listed in Table 5.8.

It should be noted that major difficulties were encountered in trying to detect the

point of asphaltene precipitation. This was mainly due to the high viscosity of the

mixtures and the dark color of heavy oil that made it very hard to identify the end point

clearly. It is suspected that these results have low accuracy; however, the trend is

believed to be of importance here. This work was done to check whether the extent of


incompatibility from this model would correlate with the initial fouling rates obtained for

these mixtures.

Table 5.8: Oil Compatibility Test Results using H O as the Reference Oil

Heptane Dilution test

ml. of n-heptane added to 5 ml. HO 9.2

Toluene Equivalence test

% toluene in 10 ml. of toluene-n-heptane mixture added to 2 grams of HO

23.5

Nonsolvent Oil Dilution test (FO)

ml. of FO added to 5 ml. HO 5.75

Solvent Oil Equivalence test

% DAO in 10 ml. of DAO-n-heptane mixture added to 2 grams of HO

47.5

5.6.1.2 Model Prediction

Model parameters were calculated using equations 2.4 to 2.7. Results obtained

are shown in Table 5.9. Wiehe [1999c], had previously performed these tests on Cold

Lake HO and found that HO gave an IN of 30 and SBN of 81 compared to an IN of 37.1

and SBN of 105.5 obtained in this study. Results obtained by Wiehe give a V H of 8.5 ml

and T E of 30 % compared to a V H of 9.2 ml and T E of 23.5 %. Test results obtained in

this study are close to those obtained by Wiehe considering that both samples were not

identical. Therefore test results can be considered accurate within experimental error.

The solubility blending number of all the mixtures were calculated using equation

2.8 and the volume fraction along with the solubility blending numbers of individual oils.

Solubility blending numbers of all mixtures are listed in Table 5.10. The maximum


Table 5.9: Calculated O i l Compatibi l i ty M o d e l Parameters Using H O as Reference

IN SBN

HO 37.1 105.5

FO 0 -22.3 (calculated using eqn 2.10)

DAO 0 49.5 (calculated using eqn 2.11)

insolubility index in all mixtures is that of heavy oil being 37.1. Therefore, to achieve

compatibility for these mixtures, the solubility blending number should be greater than

37.1. As seen in table 5.10, all test fluids were found to be incompatible according to the

compatibility criterion, having solubility blending numbers between -18.3 and 7.6. The

results obtained from this model did not correlate with the initial fouling rate results

obtained for these mixtures. For example, the two mixtures with the most negative

blending numbers contained no heavy oil, and showed negligible fouling. The three

mixtures with the highest positive blending numbers, showed significant but not the

highest fouling rates. However, it was found that the compatibility results of this model

correlate well with the colloidal instability index CII as shown in Figure 5.28. This could

suggest that both tests give similar results therefore, it is preferred to use the CII since it

is based on more accurate test methods.

5.6.2 Heavy O i l - D A O Blend as Reference O i l

Oil compatibility tests were repeated using a 50-50 mixture of HO-D AO as the

reference oil instead of HO. The objective of these tests was to examine the role of DAO

on the stability of asphaltenes. It is reported in literature that resins and aromatics are

peptizing agents that keep asphaltenes in solution. DAO is low in asphaltenes and


Table 5.10: Oil Compatibility Model Prediction for Test Fluids

Test Fluid Solubility Blending Numbers

0 % DAO - 5 % HO - 95 % FO -15.6

5 % DAO - 0 % HO - 95 % FO -18.3

5 % DAO - 5 % HO - 90 % FO -12.0

5 % DAO - 10 % HO - 85 % FO -5.77

5 % DAO -15 % HO - 80 % FO 0.5

5 % DAO - 20 % HO - 75 % FO 6.88

10 % DAO - 0 % HO - 90 % FO -14.7

10 % DAO - 3 % HO - 87 % FO -11.0

10 % DAO - 5 % HO - 85 % FO -8.5

10 % DAO -10 % HO - 80 % FO -2.2

15 % DAO - 2 % HO - 83 % FO -8.7

15 % DAO - 3.5 % HO - 81.5 % FO -6.8

15 % DAO - 5 % HO - 80 % FO -4.9

15 % DAO -10 % HO - 75 % FO 1.4

15 % DAO -15 % HO - 70 % FO 7.6

contains high percentages of aromatics and resins. Therefore, it was blended with heavy

oil to observe its effect on the stability of asphaltenes.

5.6.2.1 Test Results

Heptane dilution test was performed on a mixture of 2.5 ml. of HO and 2.5 ml of

DAO. Insoluble asphaltenes were detected after adding 3.75 ml. of n-heptane to the oil

blend. This n-heptane volume is much less than 9.2 ml., which was added to 5 ml. of HO

before asphaltenes were detected. This shows that DAO is not helping in keeping

asphaltenes in solution, instead, it caused asphaltenes to precipitate more readily.


The Toluene Equivalence test was also performed using 1 gram of HO and

another gram of DAO. The oil mixture was heated to 70°C for half an hour with

intermediate shaking in the ultrasonic bath to ensure mixing. It was found that 37.5 % of

toluene in a toluene-n-heptane mixture was required to keep asphaltenes in solution

compared to 23.5 % which was required for the 2 grams of HO. This also indicates that

DAO is playing a negative role in stabilizing asphaltenes in solution.

Figure 5.28: Relationship of Oil Compatibility Model Index to Colloidal Instability Index

The Nonsolvent Oil Dilution test was performed. Fuel oil was added to a 5 ml. of

a 50-50 mixture of HO-DAO mixture. In this case, asphaltenes were detected after

adding 1.75 ml. of FO to 5 ml. oil mixture compared to 5.75 ml. when HO was used as

the reference oil. All the results obtained in this part of the study show that DAO is

playing a negative role in asphaltene stability and therefore causing more precipitation of

asphaltenes. Results are shown in Table 5.11


Table 5.11: Oil Compatibility Test Results Using H O - D A O as The Reference Oil

Heptane Dilution Test

ml. of n-heptane added to 5 ml. HO-DAO 3.75

Toluene Equivalence Test

% toluene in 10 ml. of toluene-n-heptane mixture added to 2 grams of HO-DAO

37.50

Nonsolvent Oil Dilution Test (Fuel Oil)

ml. of FO added to 5 ml. HO-DAO 1.75

5.6.2.2 Model Prediction

Model Parameters were calculated using equations 2.6, 2.7 and 2.10. The results

obtained are shown in Table 5.12. Results show a discrepancy between the SBN index

calculated for FO using heavy oil as the reference oil and that calculated using HO-DAO

as the reference oil. However, when calculations were carried out, it was found that the

difference in the indices is due to a volume difference of 0.75 ml. This indicates that the

model parameters are very sensitive to a small error in test results, Test results obtained

in this study are of greater importance at this stage of the research, than the calculated

model parameters, in explaining the fouling behavior observed for these oil mixtures.

Table 5.12: Calculated Oil Compatibility Model Parameters Using H O - D A O Blend as Reference

IN SBN

HO-DAO Blend 43.8 76.7

FO 0 -50.1 (calculated using eqn 2.10)

6.0 CONCLUSIONS AND R E C O M M E N D A T I O N S

6.1 Conclusions

This study of heat exchanger fouling of mixtures of heavy oil , de-asphalted oil and

fuel oi l at initial surface temperature of 230 °C and bulk temperature o f 85 °C, and a

fixed velocity, led to the following conclusions:

• A t a fixed D A O concentration, the fouling rate first increased, and then decreased as

the H O concentration was raised from 0 to 20 % and as Re/As ratio decreased. The

initial fouling rate passed through a maximum as the Re/As ratio was raised over the

range 1.3 to 81.3. The maximum fouling rate occurred at Re/As = 2.5 (which

corresponded to about 1.75 % asphaltenes), and decreased to essentially zero for

Re/As > 5.8.

• Fouling rate, pentane insolubles concentrations and hot filtration insolubles

concentrations all increased as D A O concentration was raised at a fixed Re/As ratio.

This suggests that resins from the D A O are involved in enhancing precipitation, and

fouling.

• High fouling rates were generally encountered at high pentane insolubles

concentrations, which increased as D A O concentration was raised at a fixed

Asphaltenes concentration.

89

Chapter 6: Conclusions and Recommendations 90

• Hot filtration insolubles and probe deposits have chemical properties similar to

asphaltenes. There was some indication of higher H/C ratios in deposits where D A O

concentrations were higher.

• Fouling rates for mixtures containing DAO, did not correlate with the colloidal

instability index alone. A fouling regime map indicated that low fouling rates were

dependent on both the colloidal instability index and the resins/asphaltene ratios.

High colloidal instability indexes could be tolerated, provided that Re/As ratio was

sufficiently large.

• Oil Compatibility Model predictions correlated well with the colloidal instability

index, and therefore also did not predict which mixture would foul most heavily.

• Oil compatibility model titrations showed that all mixtures used were incompatible to

some extent. This was also reflected by the presence of hot filtration insolubles in all

fluid mixtures. Addition of DAO to heavy oil caused asphaltenes to precipitate at a

lower concentration of n-heptane. Hence blending of DAO with heavy oil required a

higher percentage of toluene in a toluene-n-heptane mixture to keep asphaltenes in

solution. This result was consistent with the findings on the role of D A O in raising

hot filtration insolubles concentrations and fouling rates. Model parameters are very

sensitive to small errors in test method results.

6.2 Recommendations

The role of resins in asphaltene stability is not very clear in this study due to the

narrow range of resin concentrations covered. This was due to the presence of other oil

Chapter 6: Conclusions and Recommendations 91

constituents in large quantities in the test samples which might have overcome the effect

of the resin fraction. Therefore, it is recommended to isolate a resin fraction from a

heavy oil sample through precipitation or adsorption/desorption methods. This would

have the advantage of both the asphaltenes and the added resins originating from the

same source.

The concentrations of hot fdtration insolubles in the samples were too low to detect

with high accuracy levels, therefore it is recommended to use a larger oil sample in the

fdtration tests and use insulation around the filter funnel to reduce heat loss and keep the

sample at the desired bulk temperature.

In this study, experiments were performed at a constant bulk temperature. It is

recommended to examine this behaviour at higher bulk temperatures to examine the

effect of temperature on the fouling behaviour of these mixtures.

It was noted in this study that the pentane insolubles concentration increased as DAO

concentration was raised in the mixture. This issue can be investigated further by

preparing different oil mixtures and measuring their pentane insolubles.

The asphaltene precipitation detection method used in the oil compatibility testing

involves high levels of inaccuracy. A better method could provide an accurate measure

of oil compatibility and improve the chance of correlating oil properties to fouling

behaviour.

Abbreviations and Nomenclature 92

Abbreviations

API American Petroleum Institute

Ar Aromatics

As Asphaltenes

A S T M American Standard Test Methods

CII Colloidal Instability Index

C M C Critical Micelle Concentration

DAO De-Asphalted Oil

E D X Energy-Dispersion X-Ray

FO Fuel Oil

HO Heavy Oil

HPLC High Pressure Liquid Chromatography

O C M Oil Compatibility Model

Re Resins

Sa Saturates

SARA Saturates, Aromatics, Resins and Asphaltene Fractionation

SEM Scanning Electron Microscope

VPO Vapour Pressure Osmometry

Nomenclature

Aor Cross sectional area of the orifice m 2

b Constant in Kern-Seaton equation s"1

C Concentration mol/L, g/L

Abbreviations and Nomenclature

c d Orifice discharge coefficient -

d Diameter m

D Shear rate 1/s

d e q Equivalent diameter of annular test section m

H v Heat of vaporization J/mol, kJ/n

IN Insolubility number -

k f Thermal conductivity of deposit kW/m 2K

m Mass deposit per unit area kg/m2

P Pressure Pa

q Heat flux kW/m 2

Q Power input W

Pv Universal gas Constant J/mol.K

Rf Thermal fouling resistance m 2K/kW

Rr* Asymptotic fouling resistance m 2K/kW

Initial fouling rate m2K7kWh

SBN Solubility blending number -

SNSO Solubility blending number for non-solvent oils -

SOE Solvent oil equivalence %

Sso Solubility blending number for solvent oils -

t Time s

T Temperature ° C , K

T b Bulk temperature °C, K

T E Toluene equivalence %

94

T m

Thermocouple temperature reading ° C , K

T s

Surface temperature ° C , K

U Overall heat transfer coefficient kW/m 2K

V Molar volume m3/mol

V Volumetric flowrate m3/s

V H

Heptane dilution volume ml

VNSD Volume for non-solvent dilution test ml

X Deposit thickness m

X S

Thermocouple depth from surface m

Subscripts

a Component a

f Foulant

0 Initial

s Surface

Greek

P Ratio of orifice diameter to pipe diameter -

r Gibbs surface excess mol/m2

Y Suface tension of the solution N/m

5 Solubility parameter MPa 0' 5

^met Thermal Conductivity of metallic wall kW/m.

P Density kg/m3

Dynamic viscosity Pa.s

X Shear stress N / m 2

.3\0.5

References 95

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Appendices 100

Table A l . l : STIMMARY OF FOULING RUNS*

Test Fluid

5 95

% DAO %HO %FO % DAO %HO

95 %FO 0

90

% DAO %HO %FO

Tb,avg

(°C)

86.3

83.8

86.1

Ts,i

226

231

233

Heat Flux (kW/m2)

350

250

366

Initial Fouling Rate

(m2K/kWh)

0.013

Neg. Fouling

0.020

1/U. (m2K/kW)

0.40

0.57

0.385


(h)

4-32

0-26

3 - 22.5

Final R f

(m2K/kW)

0.141

0.008

0.164

5 % DAO 10 %HO 85 %FO 5 % DAO 15 %HO 80 %FO

85.7 228 365 0.039 0.395

86.1 233 411 0.026 0.354

1.1-6.5

3 - 19

0.272

0.305

5 % DAO 20 %HO 75 %FO

86.1 230 451 0.026 0.301 0-16.5 0.269

10 %DAO 0 % HO 90 %FO 10 %DAO 3 % HO 87 %FO 10 %DAO 5 % HO 85 %FO 10 10 80

% DAO %HO %FO

84.7 232 243 Neg. Fouling 0.587

85.1 230 238 Neg. Fouling 0.596

87 232 353 0.025 0.392

86.6 231 452 0.045 0.324

0-21

2-25

0-18

1 - 11

0.0049

0.007

0.183

0.279

15 2 83

% DAO % HO %FO

15 %DAO 3.5 %HO 81.5% FO 15 %DAO 5 % HO 80 %FO 15 %DAO 10 %HO 75 %FO 15 15 70

% DAO %HO %FO

86.6 230 237 Neg. Fouling 0.587

85.4 227 237 Neg. Fouling

86.3 231

85.5 222

86.3 214

389 0.036

431 0.054

516 0.039

0.600

0.353

0.317

0.231

0-27

0-27

0-19

1-6.3

0-11

* Detailed data points for all runs are available on diskette from A. P. Watkinson

0.017

0.020

0.257

0.305

0.271

Appendices 101

^2 Sample Calculations

1 Bulk Velocity and Reynolds Number

The flow rates are calculated using Equation 3.1

V = C d A o r

\ 2(AP)

C d = 0.6102 d 2 = 0.0158 m (pipe diameter) di = 0.0008 m (orifice diameter) fj = 0.5024 Aor= 4.7 x 10"5 m 2

p =851 kg/m3 (15% DAO - 2% HO - 83% FO)

For a velocity of 0.75 m/s, the volumetric flow rate is at follows,

V = u • ACT

where,

A = - ( d 2

0 -d 2 i )=- (o .025 2 -0.0103 2) 4 4

- 0.000401 m 2

where d 0 = annulus outer diameter di = annulus inner diameter

therefore, the volumetric flow rate is,

V = (0.000401) (0.75)

V = 0.0003 m3/s

using the calculated flow rate, then the required pressure drop across the orifice to obtain

the desired velocity is given by solving the following for P,

0.0003 = (0.6102)(4.7xl0-5)

P = 43592.62 Pa

2p 85l(l-0.50244)

Appendices 102

Since the mercury manometer is used, then the required manometer reading required is

given by,

AP = A z ( p H g - p f ) g

A z = 0.350m

A z = 13.78 inches

given P H G = 13543 kg/m3.

The bulk reynolds number is given by,

u d e q Re = 2_

where d e Q is the equivalent diameter of the annulus - (d0 - d;) q = 0.025-0.0103 =0.0143 m

and the viscosity of this fluid is 7.6 x 10"6 m2/s

(0.75X0-0147) R e - ,

7.6xl0 - 6

Re =1414

2. Test Fluid Concentration

Heavy oil and DAO are measured by mass while FO is measured by volume, using its

density. Normally for a 10 % heavy oil mixture, 860 g of heavy oil is utilized.

Calculating the required volume of FO for preparing 15% DAO - 2% HO - 83% FO

mixture is as follows,

0.17 = W H 0 + W D A 0

W H 0 + W D A 0 -1- HFO V FO + P F 0 V F

Appendices 103

0.172 kg+ 1.290 kg 0.17 = - ~ —

851*| V m y

V V F O

0.172 kg+ 1.290 kg +

V F O = 8.39 Liters of FO

3. Thermal Fouling Resistances

Sample calculation will be performed for run with 5 % DAO - 15 % HO - 80 % FO.

The data logger records the voltage and the current at each data point. The power

supplied to the probe is given by,

Q = V x l

Q = (213X6.83)

Q = 1455 W

the heat flux is given calculated as follows,

Q _ 1455̂ W q A / , 2 / l 0 0 0 W

(3.41xl0- 3m 2l v \ l k W

q = 426.7 kW/m 2

where A is the surface area of the heated section of the probe obtained as follows,

A = T C / D

A = 7t( 0.1016 m) (0.0107 m)

A = 3.41 x 10"3 m 2

The reciprocal of the clean overall heat transfer coefficient for T s o = 232.7 °C and Tb

85.9 °C is given by,

Appendices 104

_L - ZkzZk = = Q 3 5 4 m 2 K / k W U o q 0 426.7

The thermal resistance or the reciprocal of overall heat transfer coefficient at time t

(under fouled condition, in this case at 8 hours), is evaluated as follows,

J _ = T * - T b = 272.5-86 = Q _ 4 7 2 m 2 K y k W

U f q 409.4

The thermal fouling resistance is calculated as follows,

R f =— — = 0.472- 0.354 = 0.118m2K/kW U f U 0

4. Initial Fouling Rate

Table A2 lists the calculated initial fouling rates, method of determination and the

regression of the slope or the standard deviation of the model fitting. In cases where the

fouling resistance did not reach the asymptotic fouling value, the initial fouling rate is

obtained by determining the slope of the fouling resistance versus time curve as,

d R f _ r1 f 1 ^

dt dt vU( t )y

However, in cases where asymptotic behavior is observed, the Kern-Seaton asymptotic

fouling resistance model is used to fit the data and obtain the initial fouling rate as

follows,

R f = R f * ( l - e x p ( - b t ) )

Whence R f =bxR* f

Appendices 105

Table A2.1: Modeling Values of Initial Fouling Rates of A l l Mixtures

Test Fluid Initial

Fouling Rate (m2K/kWh)

Modeling/ Slope

R 2 for rates obtained

using slope

Fitted to Asymptotic Fouling Model Test Fluid

Initial Fouling Rate (m2K/kWh)

Modeling/ Slope

R 2 for rates obtained

using slope Standard

Deviation R / b

0 % DAO 5 % HO 95 %FO

0.013 Modeling - 4.40E-3 0.151 0.089

5 % DAO 0 % HO 95 %FO

Neg. Fouling - - . - - -

5 % DAO 5 % HO 90 %FO

0.020 Modeling - 3.81E-3 0.188 1.070

5 % DAO 10 %HO 85 %FO

0.039 Slope 0.988 - - -

5 % DAO 15 %HO 80 %FO

0.026 Modeling - 5.29E-3 0.578 0.047

5 % DAO 20 %HO 75 %FO

0.026 Modeling - 5.29E-3 0.578 0.047

10 %DAO 0 % HO 90 %FO

Neg. Fouling - - - - -

10 %DAO 3 % HO 87 %FO


10 %DAO 5 % HO 85 %FO

0.025 Slope 0.990 - - -

10 %DAO 10 %HO 80 %FO

0.045 Modeling - 5.90E-3 0.451 0.097

15 %DAO 2 % HO 83 %FO


15 %DAO 3.5 %HO 81.5% FO


15 %DAO 5 % HO 80 %FO

0.036 Modeling - 7.68E-3 0.281 0.127

15 %DAO 10 %HO 75 %FO

0.054 Slope 0.987 - - -

15 %DAO 15 %HO 70 %FO

0.039 Modeling - 4.00E-3 0.447 0.088

Appendices 106

5. Colloidal Instability Index

The colloidal instability index is calculated using the definition,

Q J _ (Saturates + Asphaltenes)

(Aromatics + Resins)

Table A2.2 Average SARA Analysis of Working Fluids.

Working Fluid Saturates

Weig Aromatics

ht % Resins Asphaltenes

Heavy Oil 23.1 49.8 10.4 16.6

DAO 20.7 68.5 10.1 0.8

Fuel Oil 69.6 27.7 2.7 Trace

The CII for mixtures are calculated as follows,

(0.166WHO +0.008WDAO)+(0.231WHO +0.207WDAO +0.696WFO)

(0.104WHO +0.101WDAO +0.027WFO)+(0.498WHO +0.685WDAO +0.277WFO)

80 % FO oil mixture, the CII works out to be,

(0.0253 + 0.6018) 6 g

(0.0423 + 0.329) ~

6. Oil Compatibility Model Parameters

Insolubility index for heavy oil is obtained as follows,

H

25p

For 5% DAO - 1 5 % H O -

CII =

Appendices 107

T E = 23.5 % of toluene required in a toluene-heptane mixture to keep asphaltenes in solution in the toluene equivalence test V H = 9.2 ml of heptane resulted in asphaltene precipitation in the heptane dilution test, p = (1.038)(0.997 g/L) which is the density of heavy oil at 25°C.

23.5

9.2

25 1.038x0.997 g

= 37.1

The solubility blending number of heavy oil is given by,

5

SB N=37.1x 1 + -9.2

The non-solvent oil dilution test is performed for fuel oil and its solubility blending

number is calculated as follows,

S-ro IVNSD V H ] JNSO

V NSD 1 + ^ 5

Sxo = Solubility blending number for heavy oil.

Y N S D = 5 75 m i of fuel oil that results in asphaltene precipitation in the non-solvent oil dilution test.

Appendices 108

105.5[5.75-9.2] 'NSO

5.75 1 + 9.2

-22.3

The solvent oil equivalence test is used for DAO and its solubility parameter is calculated

as follows,

S s o = 100 T E

SOE

T E = Tolene equivalence test result for heavy oil

SOE = 47.5 % of DAO required to keep asphaltene in solution in a heptane-DAO solution.

s s o = i o o | 23.5

47.5 = 49.5

The solubility blending number for test fluid mixtures are obtained as follows,

' BNmix V , + v 2 + v 3 + . . .

For a mixture of 5 % DAO - 15 % HO - 80 % FO, the solubility blending number is calculated as

' BNmix

5(49.5) +15(105.5) +80(-22.3)

100 0.50

Appendices 109

A3 Reproducibility of Thermal Fouling Experiments

It is critical to examine the reproducibility of certain sets of data. Therefore, the

experiments with 5% DAO -15% HO - 80% FO and 51% DAO -10% HO - 75% FO were

repeated. Table A3.1 lists the properties of these runs.

Table A3.1 Test of Reproducibility of Data

Trial No.

Test Fluid Tb,avg

(°C) Ts,i

Heat Flux

(kW/m2)

Initial Fouling

Rate (m2K/kWh)

1/Uo (m2K/kW)


(h)

Final R r

(m2K/kW)

1

5 % DAO 15 %HO 80 %FO

86.0 227.5 406 0.026 0.35 3 . 7 - 2 0 . 3 0.322

2

5 % DAO 15 %HO 80 %FO

86.1 232.7 411 0.026 0.35 3.1 - 19.3 0.305

1

15 %DAO 10 %HO 75 %FO

85.5 221.6 431 0.054 0.317 0 - 6 . 3 0.305

2

15 %DAO 10 %HO 75 %FO

85.3 228.0 411 0.050 0.35 0 - 1 0 . 5 0.312

The thermal fouling resistance results of both runs are shown in Figures A3.1 and

A3.2. Results in Figure A3.1 are very similar, however results in Figure A3.2 show some

discrepancy in both runs with time which may be due to small differences in variables.

Results indicate that the results of the thermal fouling experiments are reliable and

reproducible within experimental error.

Sources of error in thermal fouling experiment can be due to several factors.

These include consistency in test sample properties, cleaning of the fouling rig,

maintaining constant variables such as bulk temperature, pressure and heat flux over the

Appendices 110

course of the experiment. These sources of errors can be avoided by through mixing of

the test samples upon receiving to insure consistency of oil samples. In addition, the

fouling rig is cleaned rigorously prior to each experiment to avoid contamination of test

fluid. The probe is cleaned thoroughly prior to each experiment to ensure consistency in

the clean overall heat transfer coefficient. Great caution is taken through the course of

experiment to ensure constant variables through out the experiment.

0.35-, ,

-0.05 -| , 1 1 1 1 1 • r 0 5 10 15 20

Time (h)

Figure A3.1: Fouling Resistance over Time for A Repeat Run with 5% DAO -15% HO - 80% FO Oil Mixture.

Appendices 111

o°

CP

o.oo H

• First Trial o Second Trial

T • r

Time (h) 10 12

Figure A3.2: Fouling Resistance over Time for A Repeat Run with 15% D A O -10% H O - 75% F O Oil Mixture.

Appendices 112

A4 Viscosity Data

Table A4.1 Kinematic Viscosiv of a mixture of 10% DAO, 10% HO and 80% FO at Various Temperatures

Temperature (°C) Kinematic Viscoisty (m /s)

xlO 6 (m2/s)

25 76.894

40 31.609

60 14.429

70 10.387

85 6.905

92 5.705

Figure A4.1 Fitting Kinematic Viscosity Varation with Temperature of 10% DAO, 10% HO and 80% FO Oil Mixture into a First Order Exponential Decay Function

effect of oil compatibility and resins / asphaltenes ratio

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