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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
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 5.4: Fouling Resistance over Time of 10 wt % DAO in HO/FO Mixture 57
Figure 5.5: Fouling Resistance over Time of 15 wt % DAO in HO/FO Mixture 59
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.
Chapter 2: Literature Review 5
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
Chapter 2: Literature Review 6
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
Chapter 2: Literature Review 7
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
Chapter 2: Literature Review 8
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
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
Chapter 5: Results and Discussion 57
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
Chapter 5: Results and Discussion 58
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)
Range of Initial Rate Calculation
(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.
Chapter 5: Results and Discussion 59
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)
Range of Initial Rate Calculation
(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
Chapter 5: Results and Discussion 60
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.
Chapter 5: Results and Discussion 61
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
Chapter 5: Results and Discussion 62
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
Chapter 5: Results and Discussion 63
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
Chapter 5: Results and Discussion 64
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
Chapter 5: Results and Discussion 66
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
Chapter 5: Results and Discussion 67
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.
Chapter 5: Results and Discussion 68
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
Chapter 5: Results and Discussion 69
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
Chapter 5: Results and Discussion 70
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.
Chapter 5: Results and Discussion 71
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.
Chapter 5: Results and Discussion 72
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
Chapter 5: Results and Discussion 73
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.
Chapter 5: Results and Discussion 74
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.
Chapter 5: Results and Discussion 75
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.
Chapter 5: Results and Discussion 76
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
Chapter 5: Results and Discussion 77
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
Chapter 5: Results and Discussion 78
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
Chapter 5: Results and Discussion 79
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.
Chapter 5: Results and Discussion 80
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
Chapter 5: Results and Discussion 82
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.
Chapter 5: Results and Discussion 83
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
Chapter 5: Results and Discussion 84
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
Chapter 5: Results and Discussion 85
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
Chapter 5: Results and Discussion 86
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.
Chapter 5: Results and Discussion 87
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
Chapter 5: Results and Discussion 88
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
Range of Initial Rate Calculation
(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
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)
Range of Initial Rate Calculation
(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