'Sloughing Behaviour' of Wax Deposit from Paraffin Mixtures in Pipe Flow

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2016 Investigation of 'Sloughing Behaviour' of Wax Deposit from Paraffin mIxtures in Pipe Flow

Sinha, Chandni

Sinha, C. (2016). Investigation of 'Sloughing Behaviour' of Wax Deposit from Paraffin mIxtures in Pipe Flow (Unpublished master's thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/26661 http://hdl.handle.net/11023/3362 master thesis

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Investigation of ‘Sloughing Behaviour’ of Wax Deposit from Paraffinic Mixtures in Pipe

Flow

Chandni Sinha

A THESIS

SUBMITTED TO THE FACULTY OF GRADUATE STUDIES

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF SCIENCE

GRADUATE PROGRAM IN CHEMICAL ENGINEERING

CALGARY, ALBERTA

SEPTEMBER, 2016

Sloughing of wax deposit from ‘waxy’ crude oils has been described in the literature whereby the deposit is postulated to dislodge from the pipe-wall due to changes in flow parameters. A bench scale flow loop apparatus was designed to carry out three sets of experiments wherein the deposit-layer at steady-state was subjected to step changes in the oil flow rate, the oil inlet temperature, and the coolant inlet temperature. Each of these parameters was varied at several levels, while allowing the deposition process to reach thermal steady-state at each level. It was observed that the deposit-layer thickness decreased gradually with an increase in each parameter. However, a complete or sudden dislodging of the deposit-layer was not observed at any point. A steady-state heat-transfer model was used to predict changes in the deposit mass as a function of variation in the selected parameters, and the predictions were found to match the experimental results adequately.

ii Acknowledgements

I would like to express my sincere gratitude and respect towards my supervisor, Dr.

Anil K. Mehrotra, for providing an opportunity to work with him on this project. His constant support and guidance, invaluable advice, immense encouragement and support helped me through every stage during the course of my program.

I would like to extend my sincere appreciation towards Mr. Jean-Marc Labonté,

Ms. Paige Deitsch, Mr. Mike Grigg, Ms. Lucila Molinos and Mr. Andrew Sutton for their assistance in various aspects of the project.

I would like to thank Dr. Adebola Kasumu for his tremendous support and helpful suggestions throughout my project. I would also like to acknowledge Ms. Samira Haj-Shafiei and Dr. Hamid Bidmus for their informative discussions and useful advice. The help and assistance offered by all the faculty members and fellow graduate students is also highly appreciated. I am grateful to the Natural Sciences and Engineering Research Council of

Canada (NSERC), the Centre for Environmental Engineering Research and Education

(CEERE), the Responsible Development of Oil Sand Resources Graduate Scholarship and the

Department of Chemical and Petroleum Engineering for the financial support.

Lastly, I would like to thank my parents for their unconditional love and support, their constant encouragement and their true faith in me throughout the course of this degree.

iii Table of Contents

Abstract ...... ii Acknowledgements ...... iii Table of Contents ...... iv List of Tables ...... vii List of Figures and Illustrations ...... viii

CHAPTER ONE: INTRODUCTION ...... 1 1.1 Introduction ...... 1 1.2 Objectives and Scope of the Study ...... 3

CHAPTER TWO: LITERATURE REVIEW ...... 6 2.1 Paraffin Wax ...... 6 2.1.1 Classification ...... 6 2.1.2 Physical and Thermal Properties ...... 7 2.1.2.1 Crystal Structure ...... 8 2.1.2.2 Heat Capacity ...... 9 2.1.2.3 Thermal Conductivity ...... 10 2.1.3 Crystallization ...... 11 2.1.3.1 Nucleation ...... 11 2.1.3.2 Crystal Growth ...... 12 2.1.4 Rheology ...... 13 2.2 Wax Deposition ...... 14 2.2.1 Wax Appearance Temperature (WAT) ...... 14 2.2.2 WAT Measurement Techniques ...... 15 2.2.3 Mechanisms of Wax Deposition ...... 18 2.2.3.1 Molecular Diffusion ...... 18 2.2.3.2 Heat Transfer ...... 19 2.2.4 Factors Affecting Wax Deposition ...... 20 2.2.4.1 Effect of Composition ...... 20 2.2.4.2 Effect of Temperature ...... 21 2.2.4.3 Effect of Flow Rate and Shear Rate ...... 22 2.2.4.4 Effect of Deposition Time and Aging ...... 22 2.2.4.5 Effect of Surface Properties ...... 23 2.2.5 Wax Deposition Experimental Apparatus ...... 25 2.2.6 Wax Deposition Modeling ...... 27 2.3 Control and Remediation ...... 29 2.3.1 Mechanical Methods ...... 30 2.3.2 Thermal Methods ...... 30 2.3.3 Chemical Methods ...... 31 2.3.4 Biological Methods ...... 32 2.3.5 Cold Flow of “Waxy” Crude Oils ...... 32 2.3.6 Sloughing of Wax Deposition ...... 33

CHAPTER THREE: EXPERIMENTAL ...... 35 3.1 Materials ...... 35

iv 3.1.1 Paraffin Wax ...... 35 3.1.2 Solvent ...... 36 3.2 Sample Preparation ...... 41 3.3 Experimental Apparatus ...... 41 3.3.1 Heating Bath and Associated Apparatus ...... 41 3.3.2 Cooling Bath and Associated Apparatus ...... 42 3.3.3 Flow Loop ...... 42 3.3.4 Wax – Solvent Mixture Reservoir ...... 47 3.3.5 Wax Mixture Centrifugal Pump ...... 47 3.3.6 Wax Deposition Section ...... 49 3.3.7 Wax Mixture Flow Regulator ...... 53 3.3.8 Flow Meter and Data Logger ...... 53 3.3.9 Thermocouple Data Acquisition System ...... 53 3.3.10 Pressure Drop Measurements ...... 54 3.3.11 GC Analysis of Samples ...... 56 3.4 Flow Loop experiments ...... 57 3.4.1 Experimental Procedure ...... 57 3.4.1.1 Deposition experiments ...... 57 3.4.1.2 Sloughing Experiments ...... 60 3.4.1.3 Calibration Experiments ...... 61 3.4.2 Experimental Design for flow Loop Experiments ...... 61 3.4.2.1 Deposition Experiments ...... 61 3.4.2.2 Sloughing Experiments ...... 63 3.4.2.3 Calibration Experiments ...... 67

CHAPTER FOUR: EXPERIMENTAL RESULTS ...... 69 4.1 Steady State Heat Transfer Model ...... 69 4.2 Results of Deposition Experiments ...... 73 4.3 Results of Calibration Experiments ...... 76 4.3.1 Estimation of Liquid-Deposit Interface Temperature (Td) and Thermal Conductivity (kd) ...... 76 4.3.2 Effect of Coolant Temperature (Tc), Wax-solvent Temperature (Th) and Reynolds Number (Re) ...... 77 4.4 Results of Sloughing Experiments ...... 82 4.4.1 Effect of Change in Coolant Inlet Temperature (Tci ) ...... 83 4.4.2 Effect of Change in Inlet Wax-Mixture Temperature (Thi) ...... 88 4.4.3 Effect of Change in Reynolds Number (Re) or Shear Rate ...... 92 4.5 Results of GC Analysis ...... 98 4.5.1 Aging of Wax Deposit ...... 98 4.5.2 Effect of Change in Tci, Thi and Re on Wax Deposit Composition ...... 101

CHAPTER FIVE: DEPOSIT THICKNESS PREDICTIONS FROM HEAT TRANSFER MODEL ...... 107 5.1 Equations for Deposit Thickness Predictions ...... 107 5.2 Model Predictions for Sloughing Experiments ...... 108 5.2.1 Effect of Increases in Tci ...... 108 5.2.2 Effect of Increases in Thi ...... 113

v 5.2.3 Effect of Increases in Re ...... 118

CHAPTER SIX: CONCLUSIONS AND RECOMMENDATIONS ...... 123 6.1 Conclusions ...... 123 6.2 Recommendations for Future Work ...... 127

REFERENCES ...... 128 APPENDIX A: EXPERIMENTAL DATA AND CALCULATIONS...... 138 APPENDIX B: RESULTS FROM GC ANALYIS ...... 143

vi List of Tables

Table 3.1 Composition of wax sample ...... 37

Table 3.2 Composition of solvent sample ...... 39

Table 3.3 Physical and chemical properties of Linpar 1416V ...... 40

Table 3.4 Conditions for flow loop deposition experiments (Thi = WAT+7, WAT=28 C) ...... 62

Table 3.5 Conditions for sloughing experiments with change in Tci (WAT=28 C) ...... 64

Table 3.6 Conditions for sloughing experiments with change in Thi (WAT=28 C) ...... 65

Table 3.7 Conditions for sloughing experiments with change in Re (WAT=28 C) ...... 66

Table 3.8 Conditions for calibration experiments (WAT=28 C) ...... 68

vii List of Figures and Illustrations

Figure 3.1 Composition of pure wax and solvent samples ...... 38

Figure 3.2 Bench scale flow loop setup: (a) front, (b)back ...... 45

Figure 3.3 Schematic diagram of the flow loop setup ...... 46

Figure 3.4 Centrifugal Pump...... 48

Figure 3.5 Cross Section of Aluminum deposition tube (Fong, 2007) ...... 49

Figure 3.6 Entrance flange: (a) inner side, (b) outer side (Fong, 2007) ...... 51

Figure 3.7 Plexiglass body of the wax deposition section (a) side view, (b) front view: entrance section (Fong, 2007) ...... 51

Figure 3.8 Cross sectional view of wax deposition section (Fong, 2007) ...... 52

Figure 3.9 Differential pressure gauge...... 55

Figure 3.10 ‘Speed bump’ observed due to delayed draining of deposition section ...... 59

Figure 3.11 Blockage of tube due to extended delay in draining of deposition section .. 59

Figure 4.1 Deposit mass per unit area measured at Tc = WAT-20, WAT-15 and WAT-10 ...... 75

Figure 4.2 Variation in extent of deposition with step changes in Tci ...... 79

Figure 4.3 Variation in extent of deposition with step changes in Thi ...... 80

Figure 4.4 Variation in extent of deposition with step changes in Re ...... 81

Figure 4.5 Effect of variation in (a) Tci on (b) Tc and (c) dP with time for Re=11,000 ...... 85

Figure 4.6 Effect of variation in (a) Tci on (b) Tc and (c) dP with time for Re = 25,000 ...... 86

Figure 4.7 Effect of variation in (a) Tci on (b) Tc and (c) dP with time for Re = 45,000 ...... 87

Figure 4.8 Effect of variation in (a) Thi on (b) Tc and (c) dP with time for Tci = WAT-20, Re = 11,000 ...... 90

Figure 4.9 Effect of variation in (a) Thi on (b) Tc and (c) dP with time for Tci = WAT-10, Re = 11,000 ...... 91

viii Figure 4.10 Effect of variation in (a) Thi on (b) Tc and (c) dP with time for Tci = WAT-20, Re = 45,000 ...... 92

Figure 4.11 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+7, Tci = WAT-20 ...... 94

Figure 4.12 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+7, Tci = WAT-10 ...... 95

Figure 4.13 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+12, Tci = WAT-20 ...... 96

Figure 4.14 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+12, Tci = WAT-10 ...... 97

Figure 4.15 Change in deposit wax composition with time at Thi = WAT+7, Tci = WAT-20 and Re = 11,000 ...... 100

Figure 4.16 Change in deposit wax composition with step changes in Tci at Thi = WAT+7 and Re = 11,000 ...... 104

Figure 4.17 Change in deposit wax composition with step changes in Thi on at Tci = WAT-20 and Re = 11,000 ...... 105

Figure 4.18 Change in deposit wax composition with step changes in Re on at Thi = WAT+7 and Tci = WAT-20 ...... 106

Figure 5.1 Variation in predicted xd/ri with changes in Tci at Re = 11,000, 27,000 and 44,000 ...... 110

Figure 5.2 Variation in predicted xd/ri with changes in Tci over time at Thi=WAT+7 and Re = 11,000 ...... 111

Figure 5.3 Variation in predicted xd/ri with changes in Tci over time at Thi=WAT+7 and Re = 25,000 ...... 111

Figure 5.4 Variation in predicted xd/ri with changes in Tci over time at Thi=WAT+7 and Re = 45,000 ...... 112

Figure 5.5 Variation in predicted xd/ri with changes in Thi at (a)Tci = WAT-20 and WAT- 10 (Re = 11,000) and at (b) Re = 11,000 and Re = 44,000 (Tci = WAT- 20) ...... 115

Figure 5.6 Variation in predicted xd/ri with changes in Thi over time at Tci = WAT-20 and Re= 11,000 ...... 116

Figure 5.7 Variation in predicted xd/ri with changes in Thi over time at Tci = WAT-10 and Re = 11,000 ...... 116

ix Figure 5.8 Variation in predicted xd/ri with changes in Thi over time at Tci = WAT-20 and Re =45,000 ...... 117

Figure 5.9 Variation in predicted xd/ri with changes in Re at Tci = WAT-20 and WAT-10 for (a) Thi = WAT+7 and (b) Thi = WAT+12 ...... 120

Figure 5.10 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+7 and Tci = WAT-20 ...... 121

Figure 5.11 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+7 and Tci = WAT-10 ...... 121

Figure 5.12 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+12 and Tci = WAT-20 ...... 122

Figure 5.13 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+12 and Tci = WAT-10 ...... 122

x List of Symbols, Abbreviations and Nomenclature

Symbol Definition a1, a2 regression constants in equation 4.10 b1, b2 regression constants in equation 4.11 c1, c2 regression constants in equation 4.12 d1, d2, d3 regression constants in equation 4.13 c empirical constant in equation 4.9 A surface area (m2) 2 Ai inside surface area of tube or deposition surface area (m ) –1 –1 Cc average specific heat capacity of coolant (J kg °C )

Ch average specific heat capacity of wax–solvent (hot) mixture (J kg–1 °C–1) –1 –1 Cp, L paraffin liquid heat capacity (J K kmol ) CH –1 Cp, L 2 empirical specific heat capacity methylene contribution (J K kmol–1) CH –1 Cp, L 3 empirical specific heat capacity methyl contribution (J K kmol–1) D inside tube diameter (m) 2 –1 Dm, diffusion coefficient (m s ) dP pressure drop across deposition tube –2 –1 hh oil–side heat transfer coefficient (W m K ) –2 –1 hc heat transfer coefficient for coolant (W m K ) –2 hh heat transfer coefficient for wax–solvent (hot) mixture (W m K–1) –1 –1 kd average thermal conductivity of deposit (W m K ) –1 –1 km average thermal conductivity of aluminum (W m K ) –1 –1 kw paraffin thermal conductivity by Warth (W m K ) L length of aluminum tube (m) M molar mass (kg kmol–1)

xi –1 mc mass rate of coolant (kg s ) md mass of deposited wax (kg) –1 mh mass rate of wax–solvent mixture (kg s ) n carbon number P pressure (Pa) q rate of heat transfer at steady-state (W) qgain rate of heat gained by the coolant from the surrounding at steady-state, (W) –1 Rc thermal resistance of coolant (K W ) –1 Rd thermal resistance of deposit layer (K W ) –1 Rh thermal resistance of wax–solvent mixture (K W ) –1 Rm thermal resistance of aluminum tube wall (K W ) Re Reynolds number ri inside aluminum tube radius (m) ro outside aluminum tube radius (m) t time T temperature (°C or K)

Tc average temperature of coolant ≡ 0.5 (Tci+Tco) (°C) * T c critical temperature (K)

Tci inlet temperature of coolant (°C)

Tco outlet temperature of coolant (°C)

Td average temperature at the interface of deposit and wax–solvent mixture (°C)

Tdavg average deposit temperature ≡ 0.5 (Td+Twi) (°C)

Th average temperature of wax solvent mixture ≡ 0.5 (Thi+Tho) (°C)

Thi inlet temperature of wax–solvent mixture (°C)

Tho outlet temperature of wax–solvent mixture (°C)

Twi temperature at the inside aluminum tube surface (°C)

Two temperature at the outside aluminum tube surface (°C)

Ui overall heat transfer coefficient based on inside tube surface area (W m–2 K–1)

xii + wd mass fraction of C20 n-alkane in the deposit + Wd normalized (solvent-free) mass fraction of C20 n-alkane in the deposit + wh mass fraction of C20 n-alkane in the wax–solvent mixture + Wh normalized (solvent-free) mass fraction of C20 n-alkane in the wax–solvent mixture xd deposit thickness (m)

Greek Symbols

α, β empirical constants in equation 4.8

λ thermal conductivity (W m–1 K–1) μ viscosity of wax–solvent mixture (Pa s) –3 ρh density of wax–solvent mixture (kg m ) –3 ρd density of deposit (kg m )

θc ratio of coolant (convective) thermal resistance and total thermal resistance

θd ratio of deposit (conductive) thermal resistance and total thermal resistance

θh ratio of wax–solvent (convective) thermal resistance and total thermal resistance

θm ratio of tube wall (conductive) thermal resistance and total thermal resistance Ω deposit mass per unit deposition area (kg m–2) τ Jamieson factor

xiii Acronyms

GC gas chromatography PPT pour point temperature (°C) WAT wax appearance temperature (°C) WDT wax disappearance temperature (°C)

xiv

Chapter One: Introduction

1.1 Introduction

Wax deposition from crude oils in transportation pipelines is a severe issue faced by the petroleum industry. Crude oil is mainly comprised of complex hydrocarbon compounds such as asphaltenes, paraffins, aromatics and resins. This thesis mainly focuses on studying the behaviour of crude oils having higher percentage of paraffin waxes (C20 –

C60+), which are referred to “waxy” crude oils. During the transportation of hot crude oil through pipelines, it gets exposed to colder environment like sub-sea pipelines or cold countries. In this situation, the wax constituents in the crude oil having lower solubility precipitate out, agglomerate and form a layer of wax deposit on the inside surface of the pipelines.

The highest temperature at which the first crystals of wax begin to appear and nucleate, upon cooling, is called the Wax Appearance Temperature (WAT). The crude oil behaves like a Newtonian fluid as long as it is above WAT. Below WAT, it is subjected to crystallization and displays non-Newtonian behaviour (Ronningsen et al., 1991; Bhat and

Mehrotra, 2004; Bidmus and Mehrotra, 2004; Kane et. al., 2004, Tiwary and Mehrotra,

2004). Many researches have also shown that even a concentration of wax as low as 1-2 % is sufficient to cause wax deposition (Holder and Winkler 1965).

Wax deposition leads to severe blockage of transportation pipelines leading to flow assurance problems. It decreases the effective cross sectional area of the pipeline which increases the pumping requirements. This blockage in pipelines also requires an additional capital investment caused due to increase in maintenance and remediation costs,

replacement of parts etc. Wax deposition in pipelines also leads to frequent shutdown of the operation for maintenance and removal of deposit, which compromises the production capacity of the plant. One of the examples of the impact of wax deposition on industry can be seen in the case of the Lasmo company (U.K.) where they had to abandon a platform at the cost of $100 million due to persistent wax deposition problems in the pipelines (Singh et al., 2000).

Various techniques pertaining to mechanical, chemical, thermal or a combination of the three methods have been used to circumvent the problem of wax deposition (Bern et al, 1980; McClain and Whitfill, 1984; Ferworn, 1997; Pederson and Ronningsen, 2002;

Paso et. al, 2009). These methods include using plungers and scrapers, chemical modifiers to change the behaviour of wax in crude oil, insulation of pipelines, providing electrical heating and so on. Some other unconventional methods for prevention of wax deposition included use of microbial treatment for reduction in wax precipitation (Etoumi, 2006) or using electromagnetic energy (Balakirev et al, 2001). These methods could not be commercialized due to the disadvantage of increase in production and operation costs.

The process of wax deposition can be well explained as a factor of various processes: mass transfer, heat transfer, fluid dynamics and transport properties (Burger et al, 1981; Azvedo and Teixeira, 2003; Mehrotra and Bidmus, 2004; Mehrotra and Fong,

2007). More recently, research has shown that wax deposition is majorly dependent on heat transfer mechanism (Bott and Gudmundsson, 1977; Bidmus and Mehrotra, 2004; Bhat and

Mehrotra, 2005; Parthasarthi and Mehrotra, 2005; Fong and Mehrotra, 2007; Tiwary and

Mehrotra, 2009; Bidmus and Mehrotra, 2009; Kasumu and Mehrotra, 2014; Shafiei et al,

2014). It has been shown that an increase in the differential temperature between the hot

crude oil and the wax does not particularly lead to an increase in wax deposition as suggested by Cole and Jessen (1960) and Agarwal et al (1990); instead it is dependent on the differences between the hot oil temperature and WAT and WAT and coolant temperature (Haj-Shafiei, 2014). An increase in shear rate would lead to a decrease in wax deposition (Jennings and Weispfennig, 2005; Tiwary and Mehrotra, 2008).

An interesting aspect of wax deposition process is ‘sloughing’. Sloughing has been defined as the phenomenon wherein the wax deposit gets dislodged from the inner wall surface and begins to flow with the crude oil due to sudden changes in hydrodynamic and thermal parameters (Hsu and Santamaria, 1994; Solaimany Nazar et al, 2005; Kok and

Saracoglu, 2007). One of the main purposes of this study is to observe if there is indeed a removal of wax deposit from the wall surface due to changes in temperatures and flow rates.

1.2 Objectives and Scope of the Study

The process of wax deposition is complex and dependent on a number of variables.

Studying a simple well-defined system would help in understanding the mechanisms in a better way. Wax deposition is dependent on six major variables: crude oil temperature, coolant temperature, flow rate of the crude oil, wax concentration in crude oil, tube size and residence time. Of these six parameters, only three will be studied in detail: crude oil temperature, coolant temperature and flow rate of the crude oil; the other three parameters will be held constant. The objectives of this study are as follows:

 To prepare simple model wax-solvent mixtures to simulate the crude oil in fields.

 To carry out deposition experiments at base conditions to estimate and confirm the time

required for the deposition process to reach a steady state.

 To investigate sloughing characteristics of the wax deposit as a function of coolant

temperature, wax-solvent mixture temperature and wax-solvent mixture flow rate.

 To study heat transfer effects during the sloughing phenomena.

 To design a suitable flow loop apparatus to achieve the above objectives.

 To confirm if heat transfer principles can be effectively applied to study sloughing.

Chapter 2 discusses a critical review of the existing literature pertaining to wax deposition from paraffinic mixtures. This review includes the classification and properties of paraffin waxes, their crystal structures and rheological behaviour of paraffinic crude oils. It also describes the various mechanisms of wax deposition, methods identified for study of this process and possible ways to eliminate wax deposition in pipelines.

Chapter 3 presents the details of the materials used, experimental procedure, apparatus and instrumentation, and analysis techniques. This chapter also includes the experimental protocol defined for each set of experiments.

Chapter 4 discusses the equations and steady state heat transfer model developed to represent the mechanism of wax deposition and the effect of the parameters: inlet coolant temperature, inlet wax mixture temperature and the flow rate of the wax mixture on it. It also explains the results obtained for all the three sets of experiments conducted with a detailed review of the reasoning behind these outcomes. The results obtained from the GC analysis of deposit samples, and the inferences drawn from them, are also examined in this chapter.

Chapter 5 explains the model predictions for the sloughing experiments carried out. The steady state heat transfer model is used to predict the thickness of deposit layer during each of the experimental run. These predictions are discussed so as to support the results obtained from the experiments and to justify the trends observed.

Chapter 6 presents a summary of the all the conclusions drawn in this study along with providing recommendations for future work.

Chapter Two: Literature Review

Crude oil is comprised of several hydrocarbon compounds which include alkanes, aromatics, naphthalenes, waxes, resins and asphaltenes. A crude oil is described as “waxy” or paraffin-based if it has a significant amount of paraffin content in it while it is termed as asphalt-based if it has higher asphaltene content in it (Singh et al., 1999). The high molecular weight components are generally soluble in the crude oil at certain high temperatures and pressure During extraction of crude oil and its flow through pipelines, the pipe wall temperature can drop below the wax appearance temperature (WAT) leading to precipitation and deposition of the paraffinic wax on the inner wall surface (Chen et. al.,

1997).

2.1 Paraffin Wax

2.1.1 Classification

Petroleum crude oil consists of two types of waxes, namely, macrocrystalline paraffinic waxes and microcrystalline waxes (Srivastava et al., 1993). The macrocrystalline waxes consist majorly of n-alkanes ranging from C18 to C65. Under certain conditions, they tend to precipitate out and agglomerate to form wax solids. The microcrystalline waxes mainly consist of a mixture of iso-alkanes, cyclo-alkanes and n-alkanes. These waxes also tend to precipitate and cluster under certain conditions but they usually slow down the deposition process due to their branched structures, hence producing unstable waxes.

Naphthalenes, or cyclo-alkanes, are stiff and bulky in nature and tend to disrupt wax nucleation process during wax deposition (Hammami and Raines, 1999). Macrocrytalline

waxes form large, flat platelets when they crystallize while microcrystalline waxes form tiny microscopic needles (Srivastava et al., 1993). Research has shown that the wax deposited during transportation of crude oil mainly consist of n-paraffins (Jorda, 1966).

2.1.2 Physical and Thermal Properties

Paraffins consist of n-alkanes which belong to a homologous series wherein each successive member has an additional -CH2 group attached to it. These hydrocarbons are non-volatile, non-polar, have very low electrical conductivity and low solubility in water.

(Turner, 1971). The molecules of an alkane are held together by covalent bonds and are aligned in a symmetric way such that the slight bond polarities cancel out resulting in non- polarity or very weak polar forces. These non-polar molecules are held together by weak

Van der Waal forces which act only on the surface of the molecules. Thus, the larger the molecule, the greater would be the surface area and hence larger intermolecular forces

(Morrison and Boyd, 1992).

At about room temperature and atmospheric pressure, the lower alkanes like methane, ethane and propane are in gaseous form, alkanes ranging from C5 to C17 are in liquid state while alkanes above C17 are in solid state. The density of each successive alkane initially increases rapidly but becomes constant at about 800 kg/m3 for higher alkanes. The boiling point also increases with increase in the carbon number of the alkanes, but the rate of increase in boiling point keeps on decreasing with each addition of a -CH2 group. Due to this difference in boiling point, the lower alkanes can be easily separated by fractional distillation as compared to higher alkanes. Branched or iso-alkanes do not show the same

trends in boiling point due to smaller surface area and complex structures. They generally have lower boiling and melting points when compared to their straight chain alkanes.

2.1.2.1 Crystal Structure

Paraffin wax usually forms a crystalline structure below its melting point, either from its individual components or from mixtures with other. These crystals are generally rhombic or monoclinic in shape and display low order of symmetry (Mozes et al., 1982).

Crystallization starts with nucleation when paraffins are cooled below their melting point.

The structure of paraffin wax is determined by both the rate of nucleation and crystal growth. This structure further changes when the temperature drops to the equilibrium transition temperature, which is below the melting point.

Paraffins ranging from C21 to C36 display a well-defined transition point below their melting point where the -phase, which is stable below the melting point, changes into the

-phase along with release of a large amount of heat (Mazee, 1949). Studies have shown that n-alkanes between C19 and C29 having odd number of carbon generally have an orthorhombic structure at ambient temperature. However, alkanes between C18 and C26 with even carbon numbers have a triclinic structure while those between C28 and C36 have a monoclinic structure. These different structures are a factor of carbon number, thermal history, temperature and purity of the sample (Turner, 1971; Srivastava et al., 1993).

Paraffin wax crystals appear in different forms: plates, needles and mal-crystalline shapes. The crystals with mal-crystalline shape are small and underdeveloped crystals that tend to agglomerate. Fast cooling rates produce needles and mal-crystalline shapes while slow cooling rates tend to yield growth of plates (Turner, 1971). It is quite likely that all forms of waxes are produced during a single crystallization process, however, one

particular structure dominates at a given set of conditions. It was also found that the crystal size varies with composition of the system (Anderson, 2001).

2.1.2.2 Heat Capacity

It is necessary to determine the heat capacity of paraffins in order to understand change in thermal energy associated with temperature changes in the system. The heat capacity values for paraffins up to tritriacontane (C33) were obtained using a calorimeter

(Finke, 1954; Huffman, 1931, Parks, 1930; Spaght, 1932). Heat capacities of paraffins up to polyethylene, both in solid and liquid state, have been expressed in terms of empirical equations proposed by various researches (Wunderlich and Dole, 1957; Broadhurst, 1962;

Karasz and Hamblin, 1963; Richardson, 1965; and Pan et al., 1986). Jin and Wunderlich

(1991) developed relations between heat capacity and carbon number (n), temperature and the empirical contributions from CH2 and CH3 groups (Equations 2.1a – 2.1c). It was reported that the heat capacities in the liquid state could be evaluated within the rms error of 1.7%.

퐶퐻3 퐶퐻2 퐶푝,푙 = 2퐶푝,1 + (푛 − 2)퐶푝,1 2.1a

퐶퐻3 2퐶푝,푙 = 17.33 + 0.04551푇 2.1b

퐶퐻3 2퐶푝,푙 = 30.41 + 0.01479푇 2.1c -1 -1. where, Cp,l is the specific heat capacity of the pure liquid component in J mol K , T is the temperature in K.

2.1.2.3 Thermal Conductivity

Since wax deposition from crude oil is believed to be a thermally driven process, thermal conductivity is considered as an important parameter to be studied (Kasumu and

Mehrotra, 2013; Fong and Mehrotra, 2007; Parthasarthi and Mehrotra, 2005; Bidmus and

Mehrotra, 2004; Gutheric et al., 2004; Cordoba and Schall, 2001a; Riberto et al., 1997;

Brown et al., 1993, Khan et al., 1993; Sharma et al., 1982).

Filipov (1968) measured the thermal conductivity of 83 organic liquids over a large temperature range. Dick and McCready (1954) also measured the thermal conductivity of around 19 organic liquids. They observed that the thermal conductivity increased with increase in chain length while it decreased for molecules with side chains, both having the same carbon number. The results obtained by Filippov (1968) and Wada et al. (1985) were in agreement with that of Dick and McCready’s (1954) on terms of general trends for organic liquids. Wada et. al. (1985) concluded that the thermal conductivity of n-alkanes such as m-undecane (n-C11), n-tetradecane (n-C14), n-pentadecane (n-C15) and n-

-1 -1 hexadecane (n-C16) ranged from 0.12-0.15 W m K . Skyler and Sparrow (1990) found

-1 -1 that the thermal conductivity of solid n-cicosane (n-C20) was 0.38-0.42 W m K depending on the temperature and sample preparation.

Various correlations have been developed for calculating thermal conductivity of paraffins. Warth (1956) provided the following relation for estimation of thermal conductivity of paraffins (kw) in terms of their molar mass M:

−4 푘푤 = 2.4 × 10 푀 2.2

Wada et al. (1985) gave a relationship for thermal conductivity as a function of temperature as follows:

휆 = 퐴푛2 + 퐵 푛 + 퐶 − [퐷(1⁄푛)2 + 퐸(1⁄푛) + 퐹]푇 2.3 where,  is the thermal conductivity (W m-1 K-1), A to F are constants, n is the carbon number, and T is the temperature (range: 20-90 C). Jameison (1979) also developed a correlation:

휆 = 퐴(1 + 퐵휏1⁄3 + 퐶휏2⁄3 + 퐷휏) 2.4

-1 -1 * * where, ,  is the thermal conductivity (W m K ),  = 1 – T/ Tc , Tc is the critical temperature, A is the pseudocritical thermal conductivity, B is a constant, C = 1 – 3B, and

D = 3B. Equation 2.3 is valid for n-paraffins with carbon number ranging up to 25 and for

* a temperature range of melting point to 0.9 Tc .

2.1.3 Crystallization

When the temperature of liquid crude oil is decreased, the energy of molecules reduces, and the molecules come together. With increase in time duration, the molecules start to have a more ordered arrangement. The degree of order is determined by the shapes of molecules and the way they fit together in adjacent positions (Turner, 1971). A degree of super-saturation is essential prior to the beginning of precipitation. When WAT is reached, the intermolecular attractive forces are much higher than the energy of molecular motion causing the molecules to come together to form a crystal. The two main steps involved in crystallization are nucleation and crystal growth.

2.1.3.1 Nucleation

The process of crystallization originates around a nucleus, which is the smallest stable part of the crystal under the given conditions. When the temperature is reduced, the 11

molecules attach and detach from the ordered crystal clusters until they reach a stable state.

This process of attaching and detaching is called nucleation and the stable cluster is the nuclei. Nucleation can be of two forms: homogenous or heterogeneous. Homogenous nucleation is a spontaneous thermal process that usually occurs from a pure sample with nucleation sites that are time dependent. Heterogeneous nucleation is an artificially induced process which may or may not be thermally driven. In this case, the nucleation sites are activated instantaneously (Turner, 1971). This process of nucleation usually occurs either on the surface of a wall or as a result of foreign particles present in the solution.

2.1.3.2 Crystal Growth

After the formation of a stable nucleus, the surrounding molecules attach themselves to the nucleation sites. This takes place as a result of intermolecular forces which draw the molecules towards the nucleus. A site with maximum number of neighboring molecules would be favourable due to the higher magnitude o attractive forces occurring there

(Keating, 1964). Once the molecules attach themselves to the sites, they themselves serve as suitable sites for other molecules to attach. When a n-alkane crystallizes, a one- molecule-thick layer is formed first with the long chain alkanes adding adjacent to each other, at a relatively fast pace. The addition of new layers is usually slower. The new layers generally begin to grow at the edges of the partially completed layers (Keating, 1964). The molecules get added or detached from the growing layers until a stable crystal form is attained.

Solid forms of n-alkanes (> n-C9) are found to be hexagonal, orthorhombic, triclinic or monoclinic. (Turner, 1971). Dorset (1995), Dirand et al (1998), Chevalier et al (2000)

and Dorset (2000) found that the crystal structure of paraffinic wax crystals in solution was orthorhombic when crystallized from a standing solution and under slow flow. Singh et al.

(1999) found that the crystals are generally monoclinic or triclinic when they crystallize on slow cooling under static conditions.

2.1.4 Rheology

At reservoir temperatures (70-200 C) and pressures (110 MPa), the paraffin content in crude oil remains dissolved in the mixture and the mixture is a Newtonian liquid

(Ungerer et al., 1995). When the crude oil is pumped out of the reservoir, it is exposed to colder surrounding temperatures (~ 4C) and the wax content precipitates out, resulting in wax deposition and complex flow behaviour. As the temperature drops below WAT, the viscosity rapidly increases, consequently causing non-Newtonian flow (Wardhaugh and

Boger, 1991). Dirand et al. (1998) identified the orthorhombic structure of wax crystal as the reason for the non-Newtonian behaviour. These wax crystallites flocculate to form a gel like mixture and hence, cause increased viscosity. The most important factors for determining flow properties are the thermal and shear histories of the crude oil.

Gelled oils present a yield behaviour, and high pressures need to be applied to restart their flow (Vignati et al., 2005). Yield stress of a mixture is majorly dependent on gel composition, thermal and shear histories (Venkatesan et al., 2005). Below the pour point (PP), the oil displays rheological properties of a viscoelastic solid (Silva and

Coutinho, 2004; Visintin et al., 2005). After wax precipitation, the rheology of the crude oil becomes complicated due to thixotropic behaviour, that is, time dependent change in

viscosity of mixture (Cawkwell and Charles, 1989; Fusi, 2003; Tiwary and Mehrotra,

2004).

2.2 Wax Deposition

Crude oils are complex mixtures of higher n-alkanes. When exposed to colder surroundings, the heavy paraffinic fractions can deposit and lead to plugging of pipelines rendering them unfit for transportation of crude oil. Immense research has been done for understanding the problem of wax deposition.

2.2.1 Wax Appearance Temperature (WAT)

Wax appearance temperature is an important parameter when discussing wax deposition. WAT is the highest temperature at which the first crystals of wax begin to precipitate out of the solution on cooling. The WAT can also be termed as cloud point temperature (CPT). Crude oils having higher WAT are more susceptible to wax deposition.

Wax deposition begins only when the wall temperature drops below WAT.

There is an important difference between the liquidus temperature and the experimentally determined WAT. The liquidus temperature is the true solid-liquid phase boundary temperature, whereas the experimentally determined WAT is the temperature at which the first crystals appear on cooling. The WAT can be very sensitive to the experimental method used for measurement. The experimental WAT should always be lower than the liquidus temperature (Bhat and Mehrotra, 2004).

2.2.2 WAT Measurement Techniques

Various equipment and methods have been developed for determining the WAT of

crude oils. The temperature measured depends on various factors: oil composition,

measurement technique, thermal history, residence time of measurement and the fluid

properties relating to crystal nucleation and growth (Hammammi et al., 2003). Generally,

more sensitive methods of measurement detect higher cloud point temperatures. Also,

increase in system pressure can lead to a decrease in the measured CPT, especially if the

sample contains solution gas (Monger-McClaire et al., 1999). The various techniques are

as follows:

(1) Visual Methods (ASTM Standard D 2500-09)

The ASTM standard test is a visual method for determining the WAT. In this method, the

sample is cooled down from a temperature at least 14 C above the expected WAT of the

sample, The temperature at which the first crystals appear is noted as the WAT. This

method is specifically used for petroleum products and biodiesel fuels that are transparent

with 40 mm thickness layers and a CPT below 49 C. Tiwary (2002) altered this method

of measurement by cooling down the sample in steps of 1C and allowing the sample to

stay at that temperature for about 15 minutes. At each step change, the sample was checked

visually for any appearance of wax crystals.

(2) Plugging (FP)

In this method, a solution of preheated and pre-filtered oil is passed through a capillary to a filter. The filter and the oil sample are both submerged in a programmable temperature bath. With the cooling of the oil at a constant rate, the pressure drop across the capillary and the filter is noted. This value of pressure is then compared to the pressure

drops used for determination on CPT (Monger-McClaire et al., 1999). This method is suitable for measuring the WAT for live oils, but not for viscous oils.

(3) Viscometry

Wax precipitation from crude oil changes its behaviour from Newtonian to non-

Newtonian type. When the crude oil is at a temperature above WAT, it behaves like a

Newtonian fluid and its viscosity is majorly a function of temperature (Tiwary, 2002).

When temperature falls below WAT, the precipitation of wax crystals causes the rheology of the sample to becomes dependent on shear rate as well. By using a rheometer for measuring viscosity of the sample during cooling, the temperature at which the viscosity- temperature relationship starts to change can be recorded as WAT (Ronnningsen et al.,

1991)

(4) Solids Deposition System (SDS)

In this method of WAT measurement, a beam of light is transmitted through the sample. The presence of wax crystals causes an appreciable change in the intensity of light.

When the sample is cooled isobarically along with constant mixing, the average transmitted light power and the corresponding temperature are recorded as a function of time using computerized data acquisition system. The test is stopped after a few degrees below the temperature at which the drop in intensity of light is observed (Hammami and Raines,

1999)

(5) Cross Polar Microscopy (CPM)

The CPM is considered to be the most accurate method for measurement of WAT and gives the highest WAT value (Ronningsen et al., 1991). The method is based on the fact that all crystalline structures rotate the plane of polarized light while liquid

hydrocarbons do not. The equipment comprises of a light source, an infrared filter, a polarizer, a temperature controller and a microscope. The sample is placed on a glass cover slide which is placed on the variable temperature microscope stage and viewed through the crossed polarizer. When the sample is cooled, the appearance of wax crystals is observed as isolated points of light using a video camera visually (Monger-McClure, 1999).

(6) Differential Scanning Calorimeter (DSC)

This method utilizes the heat released from the sample during crystallization. The heat released or absorbed and the variable specific heats of the sample during cooling or heating is determined with changes in temperature. Since the heat released during the onset of wax crystallization is comparatively small, a stable baseline must be obtained and as large a sample volume as possible should be used without distorting the DSC signal. The temperature at which a melting peak occurs in the heat flow-temperature curve

(thermogram) is considered as the WAT (Tiwary, 2002).

(7) Fourier Transform Infrared (FTIR)

For FTIR, the increase in energy scattering due to solid formation when wax

crystallization takes place is used for detecting the WAT. The mid-infrared spectrum

between 650 and 40000 cm-1 contains wavelengths wherein the energy absorbed by

hydrocarbons is relatively small. This particular spectrum is used since a wavelength that

indicates wax crystal formation in this region can be easily detected by spectral subtraction

(Monger-McClure et al., 1999). The FTIR is suitable for live oils.

2.2.3 Mechanisms of Wax Deposition

A number of mechanisms have been proposed for describing the process of wax deposition and estimation of amount of deposit in a system under given set of conditions.

Molecular diffusion, shear dispersion, Brownian diffusion, gravity settling and heat transfer are the mechanisms proposed in the literature. Of these, molecular diffusion and heat transfer are the most commonly accepted mechanisms and will be discussed in this section.

2.2.3.1 Molecular Diffusion

When the pipe carrying the crude oil is exposed to lower surrounding temperature, it causes the pipe wall temperature to fall below WAT of the oil due to which a radial temperature gradient is created, which gives rise to a concentration gradient that causes the diffusion of wax from higher concentration region in the bulk towards the wall where the concentration of dissolved wax is lower. A pseudo steady state mathematical model is considered to explain the phenomena. The amount of deposit is calculated from the rate of mass transfer at the liquid-deposit interface. The liquid-deposit interface temperature is obtained from an energy balance over this boundary. An assumption made in this mechanism is that the interface temperature is variable, which is predicted to increase with increase in deposit layer from an initial value close to pipe-wall temperature to the WAT at steady state. This mechanism has been reported as the dominant process for wax deposition by various researchers (Burger et al., 1981; Weungarten and Euchner, 1986;

Svendson, 1993; Brown et al., 1993; Frickson et al., 19993; Hsu and Brubaker, 1995;

Creek et al., 1999; Singh et al., 2000, 2001). It is also the basis for most commercial software available for wax deposition.

2.2.3.2 Heat Transfer

In this mechanism, the deposit formation and its growth is considered to be a partial solidification or freezing process involving crystallization (Ghedamu et al., 1997; Cordoba and Schall, 2001a; Bidmus and Mehrotra, 2004, 2008a, 2008b, 2009, 2012; Mehrotra and

Bidmus, 2005; Bhat and Mehrotra, 2005, 2006, 2008; Mehrotra and Bhat, 2007, 2010;

Parthasarthi and Mehrotra, 2005; Fong and Mehrotra, 2007; Tiwary and Mehrotra, 2009;

Arumugam et al., 2012, 2013; Kasumu and Mehrotra, 2013, 2014; Haj-Shafiei et al., 2014).

The rate of heat transfer across the deposit layer is a function of temperature gradient between the bulk “waxy” oil/mixture and the cooler pipe wall. The overall heat transfer is a result of the convective heat transfer from the hot crude oil and colder surroundings and the conductive heat transfer across the pipe wall and the deposit layer. All these thermal resistances are in series with each other.

Mathematical models incorporating heat transfer associated with phase transformation have been developed based on moving boundary formulation (Bhat and

Mehrotra, 2005, 2006, 2008; Mehrotra and Bhat, 2007, 2010; Arumugam et al., 2012,

2013). An assumption made in the heat transfer approach for wax deposition is that the liquid-deposit interface temperature is equal to the WAT of the crude oil, or the mixture, throughout the deposition process. Various batch cooling experiments under static and sheared conditions have been performed to confirm this fact (Bidmus and Mehrotra, 2008a,

2008b). The heat transfer mechanism can explain wax deposition both in “hot” flow (when

wax mixture or crude oil temperature is above the WAT) and “cold” flow (when wax mixture or crude oil temperature is lower than the WAT) conditions (Bidmus and Mehrotra,

2009, 2012; Arumugam et al., 2013; Haj-Shafiei et al., 2014).

2.2.4 Factors Affecting Wax Deposition

This section describes the various factors which affect the amount of wax deposited from crude oils in pipelines when exposed to colder surroundings.

2.2.4.1 Effect of Composition

The wax deposition will be lower for crude oils having lower paraffinic content in them. For single phase wax deposition, it was observed that the amount of deposit was the same when different compositions of wax-solvent mixtures were exposed to the same relative temperature with respect to their WAT (Bidmus and Mehrotra, 2004; Parthasarthi and Mehrotra, 2005; Fong and Mehrotra, 2007). But, when these mixtures were exposed to identical conditions, the wax deposit from the mixture having higher wax concentration is relatively more when compared to those with lower wax concentrations.

Hammani and Raines (1999) have shown that while both n-paraffins and iso- paraffins tend to agglomerate and precipitate from crude oil, the iso-paraffins tend to delay the formation of wax nuclei, thus forming unstable solids. The presence of impurities such as asphaltenes reduces wax deposition from crude oils (Woo et al., 1984; Misra et al.,

1995). Other impurities such as naphthalene, i.e., cyclo-paraffins, also tend to disturb the nucleation process and hence delay wax deposition process. Addition of lighter fractions to the crude oil decreases the WAT by almost 15 C, hence lowering the possibility of wax

deposition. Along with these impurities, the presence of water has been reported to reduce the amount of wax deposited, especially on a water-wet surface (Cole and Jessen, 1960;

Beyer and Osborn, 1969; Li et al., 1997).

2.2.4.2 Effect of Temperature

Some researchers believed that the overall temperature difference between the hot crude oil and the colder surrounding is the governing factor for wax deposition (Agarwal et al., 1990; Creek et al., 1999; Wu et al., 2002). However, it has been proved that a higher overall temperature difference does not necessarily account for wax deposition. In fact, wax deposition is governed by the temperature difference between the hot crude oil temperature and WAT and that between WAT and the colder surroundings (or the coolant temperature) (Bidmus and Mehrotra, 2005; Mehrotra and Bidmus, 2005; Parthasarthi and

Mehrotra, 2005). Mehrotra and Bidmus (2005) also showed that the wax deposition could be prevented when crude oil flows through a highly conductive pipe which would be maintained at a temperature given by the following formula:

ℎ푐푟표 2.5 푇ℎ = 푊퐴푇 + (푊퐴푇 − 푇푐) ℎℎ푟𝑖 where, hc and hh are coolant and hot crude oil heat transfer coefficients, respectively ro and ri are outside and inside pipe radii respectively, and Th and Tc are crude oil and coolant temperatures, respectively. The drawback of using this formula was that the temperature calculated would be very high for sub-sea pipelines and hence, would be more energy- intensive and less economical.

2.2.4.3 Effect of Flow Rate and Shear Rate

Wax deposit layer thickness reduces with increase in flow rate of the crude oil or waxy mixture, either in laminar or turbulent region (Patton and Casad, 1970; Bott and

Gudmunsson, 1977; Creck et al., 1999; Wu et al., 2002; Bidmus and Mehrotra, 2004;

Jennings and Weispfeng, 2005; Parthasarthi and Mehrotra, 2005; Fong and Mehrotra,

2007; Tiwary and Mehrotra, 2009; Kasumu and Mehrotra, 2014). The wax deposit is believed to begin to shear off when the cohesive and adhesive forces of molecules of the paraffin wax and the deposition surface are overcome by the shear rate (Bott and

Gudmunsson, 1977). Deposits obtained under increased flow rate conditions were found to be harder, with higher solid content and lower solvent or oil content (Jessen and Howell,

1958; Hsu and Bubaker, 1995; Creek et al., 1999; Singh et al., 2000-2001; Bidmus and

Mehrotra, 2004; Fong and Mehrotra, 2007; Tiwary and Mehrotra, 2009, Kasumu and

Mehrotra, 2014).

Bhat and Mehrotra (2007) explained the effect of shear rate by considering the wax deposit to be comprised of individual cubical cages made of solid wax, with embedded liquid oil. When a shear stress is applied to this cubical cage, it is postulated to result in the tilting of the cubical structure, which causes a part of the liquid oil to be “squeezed” out of the deposit. It was observed that an increase in shear stress caused an enrichment in the solid wax phase in the deposit.

2.2.4.4 Effect of Deposition Time and Aging

As time passes, the rate of wax deposition reduces due to increase in thermal insulation provided by the existing deposit layer (Cole and Jessen, 1960). The amount of

deposit increases with time until it reaches an asymptotic value at steady state conditions.

A number of bench-scale experiments have been carried out to establish the time required to attain a steady state and an optimum value of 30 minutes was observed (Bidmus and

Mehrotra, 2004; Parthasarthi and Mehrotra, 2005; Fong and Mehrotra, 2007; Tiwary and

Mehrotra, 2009; Kasumu and Mehrotra, 2014). The increase in deposit mass becomes negligible after a duration of 4 hours.

With time, the wax content in the deposit also increases, causing hardening of the deposit layer, which is termed as “aging” (Creek et al., 1999; Singh et al., 1999-2001; Wu et al., 2002; Bidmus and Mehrotra, 2004; Parthasarthi and Mehrotra, 2005; Fong and

Mehrotra, 2007; Tiwary and Mehrotra, 2009; Kasumu and Mehrotra, 2014). The deposit consists of a 3-dimendional network of solid wax with oil entrapped within it. During aging, the deposit becomes richer in the heavier paraffin or wax content and leaner in the lighter paraffin content or entrapped oil simultaneously. Singh et al. (2000) proposed a counter diffusion process in which the wax molecules having carbon number below a critical value would get diffused out and those with carbon number above the critical value would diffuse into the deposit. It was observed that the aging process depended on process conditions and it was mainly dependent on the temperature difference across the deposit layer rather than the compressive force due to flow rates. Fong and Mehrotra (2007) also observed that the aging process was more prominent at higher Reynolds number.

2.2.4.5 Effect of Surface Properties

The process of wax deposition is greatly affected by the surface properties of the pipe wall. The adhesion of deposit on the inner pipe wall surface is a string function of

either wettability (free surface energy) and/or surface roughness. Patton and Casad (1970) supported the wettability theory and suggested that wax crystals are held together by adsorption forces of both the paraffin molecules and the surface. It was observed that the wax deposition from single-phase mixtures reduced with reduction in free surface energy for a constant temperature difference (Cole and Jessen, 1960). As the free surface energy of the deposition surface decreases, the adsorption forces also become less strong. This leads to a reduction in wax deposition over the surface (Bott and Gudmundsson, 1977). A series of experiments showed that less deposition took place when pipelines were lined with polypropylene when compared to those lined with high-density polyethylene or vinyl acetate copolymer. This was a result of higher contact angle, thus, lower wettability between the crude oil and the pipe wall surface (Quintella et al., 2006).

The other surface property – surface roughness, can also be a factor in wax deposition on the pipe wall surface. The higher the roughness of the surface, the higher will be the frictional forces acting on the deposit molecules, and hence, this could prevent the paraffin wax from being sheared off. Hunt (1962) and Jorda (1966) concluded from a series of experiments using a cold spot test apparatus that the amount of wax deposit accumulating on the deposition surface increases as surface roughness increases. Hunt

(1962) suggested that surface roughness is more dominant than the composition of the deposition surface. Jorda (1966) stated that a lower amount of wax was deposited on plastic coated surfaces as compared to metallic surfaces. The lower amount of wax deposit on plastic surfaces was attributed to the thermal insulation provided by plastic layer. Contrary to these researches, Patton and Casad (1977) observed that surface roughness had no effect

on the amount of deposit. The wax deposit having lower molecular weight was flaked off smooth surfaces while heavier paraffin waxes did not.

2.2.5 Wax Deposition Experimental Apparatus

Various experimental apparatus have been developed and fabricated to carry out laboratory scale experiments to study wax deposition. The main principle behind these setups is to provide a temperature difference between a colder surface and a hot crude oil mixture or sample which would facilitate wax deposition. In each apparatus, there is a means of measuring and monitoring the amount of wax being deposited under different process conditions. This section describes the various setups used for studying wax deposition.

Flow Loop Apparatus

The flow loop setup is most widely used apparatus for studying wax deposition. It consists of a reservoir where the oil mixture is heated up and then pumped through the pipeline which is arranged in the form of a loop. The loop also incorporates a double pipe heat exchanger where the deposition takes place. The hot oil mixture flows through the inside of inner pipe while the coolant flows through the annulus between the two pipes. A large volume of oil is required to maintain the flow in this setup. Also, the initial composition of the oil changes gradually as deposition commences, but it is not significant if the oil reservoir is large and the deposition section is small.

Cold Spot or Finger

The cold finger is the next most common apparatus used for studying wax deposition.

The setup is basically a temperature controlled cold deposition surface in the form of a

metal finger that is submerged in a sample of oil mixture, held at a temperature above its

WAT. In the case of a cold spot, the only difference is that it has a flat disk which is used as the deposition surface. The oil mixture is continuously agitated using a stirrer to simulate shear stress on the surface of cold finger. The oil mixture close to the finger surface gets cooled and, thus, wax gets deposited on it. The main advantages of cold finger are that the oil mixture requirement is much less, it is more economical and easy to set up.

Draft Tube Assembly

The draft tube assembly is similar to a cold finger. It comprises of an oil reservoir with a concentric heat exchanger inserted within it at the centre (Bidmus and Mehrotra, 2004).

The coolant flows through the annulus of the draft tube while the deposition takes place on the inner wall surface of the inner tube. An impeller placed at the beginning of the draft tube is used to provide shearing for the setup.

Co-Axial Shearing Cell

This setup consists of an outer stationary cylinder and a central rotating cylinder, having the oil mixture in the annulus between the two cylinders. The deposition surface could either be the outer surface of the rotating cylinder or the inner surface of the stationary surface. The drawback of the former approach is that it becomes difficult to separate the coolant flowing into the inner cylinder from the oil mixture on the outside with the cylinder rotating simultaneously. The latter approach is much easier to set up.

2.2.6 Wax Deposition Modeling

As discussed earlier, the two main mechanisms to describe wax deposition are the molecular diffusion approach and the heat transfer approach.

For the molecular diffusion approach, the rate of wax deposition is estimated using a modified relation of the Fick’s law of diffusion. Burger et al. (1981) calculated wax deposition in the form of flux and correlated it to wax solubility coefficient for the oil, dC/dT, and the radial temperature gradient, dT/dr as follows:

푑푚푑 푑퐶 푑푇 2.6 = 퐷 퐴 ( ) ( ) 푑푡 푚 푑푇 푑푟 where md is the mass of deposit, C is the wax concentration, and Dm is the mass diffusivity.

Singh et al. (1999, 2000, 2001) used a modified form of equation 2.8 to model wax deposition and aging from wax solvent mixtures. In the case of molecular diffusion, the liquid-interface temperature is considered to be a variable, which is predicted from back calculation from an energy balance. This value of temperature gradually increases from an initial value, ranging from pipe wall temperature to the WAT of the sample. Once the interface temperature reaches the WAT, the deposition process ceases and it is taken to be the steady state.

Agarwal et al. (1990) formulated a mathematical model to determine the amount of wax deposited at equilibrium conditions by relating the flow rate and the oil and wall temperatures from the deposition experiments. Mehrotra (1996) pointed out the limitations of interpolating or extrapolating an empirical model to fit experimental data for a similar relation used by Khan et al. (1995).

The other mechanism of heat transfer has also been studied quite widely to understand the phenomenon of wax deposition. Mehrotra (1990) developed a heat transfer model for which he assumed the thermal resistance offered by the pipe wall thickness and the coolant flow were negligible as can be seen in equation 2.6.

ℎ푥푑 2푥푑 퐷 (푇푑 − 푇푐) 2.7 = (푙푛 ) 푘푑 퐷 − 2푥푑 퐷 − 2푥푑 (푇ℎ − 푇푑)

The (hxd/kd) parameter was plotted against temperature difference and flow rate results from experiments carried out by Agarwal et al. (1990) and the average value was estimated to be around 0.29  0.19. This value was suggested as a parameter for scale up.

Bidmus and Mehrotra (2004) suggested a dimensionless parameter, d, which is defined as the ratio of thermal resistance offered by the deposit layer to the overall thermal resistance. It can also be represented as the ratio of temperature difference across the deposit layer to the overall temperature difference.

푅푑 푇푑 − 푇푤𝑖 2.8 휃푑 = = 푅ℎ + 푅푑 + 푅푚 + 푅푐 푇ℎ − 푇푐 where, Td is the liquid-deposit interface temperature, Twi is the inner wall temperature, Th is the average hot oil temperature and Tc is the average coolant temperature. d has been correlated to the amount of wax deposited for various process variables (Bidmus and

Mehrotra, 2004; Parthasarthi and Mehrotra, 2005; Fong and Mehrotra, 2007; Tiwary and

Mehrotra, 2009; Kasumu and Mehrotra, 2014).

Bhat and Mehrotra (2005, 2006) developed a transient heat transfer mathematical model which took into consideration the deposition in both radial and axial directions for a developed laminar flow in a pipeline. The moving boundary formulation was used to model the solid deposition process. It was concluded that the heat transfer at the liquid-

deposit interface, the liquid region as well as the deposit layer, influenced the growth of the deposit layer. Arumugam et al. (2013) further modified the model formulated by Bhat and Mehrotra (2005) by incorporating a correlation developed by Kasumu et al. (2013) for prediction of wax deposition under both hot flow and cold flow conditions, individually.

Their predictions adequately matched the experimental trends observed by Bidmus and

Mehrotra (2009). Haj-Shafiei et al. (2014) developed a steady state heat transfer model for single-phase mixtures where they modeled the wax deposition as the crude oil regime changed from hot flow (where the average oil temperature is above WAT) to cold flow

(where the average oil temperature is below WAT). They observed that the predicted deposit thickness axially increased when in the hot flow regime and reached a maximum when the liquid temperature approached WAT of the mixture, after which it began to decrease gradually to zero as it moved through the cold regime.

2.3 Control and Remediation

Wax deposition from crude oils is a major problem during production, transportation, storage and processing of oil. The recommended approach in sequence would be: predict/diagnose, prevent and mitigate/remediate the solid deposition

(Leontaritis, 1996). When evaluating wax problems, various factors should be considered like the concentration of the n-paraffins, the carbon number distribution, concentration of branched paraffins, naphthalenes, aromatics, resins, asphaltenes and the temperature of the geographical area (Misra et al., 1995). Different methods such as chemical, mechanical, thermal and biological methods will be discussed in this section. Cold flow has recently

been identified as an alternate method for controlling wax deposition. Some researchers also state ‘sloughing’ mechanism of wax deposit as an effective method of wax removal in pipelines.

2.3.1 Mechanical Methods

The crude oil industry most generally uses various mechanical equipment to scrape off wax deposits from pipelines such as, rod scrapers, paraffin cutters, plunger lifts and flowline pigs. Flowline pigs or pigging is most common method for removing solid deposits. However, there are a few drawbacks associated with the usage of these devices such as stuck pigs in the pipeline, wearing of the tube due to continuous use of these tools, flowline excavation, capital investment and maintenance cost for these devices (Aiyejina et al., 2011). Using pigs regularly can help in avoiding large wax deposition in pipelines.

More recent methods include using of remote controlled tools that reduce use of wirelines thus preventing severed lines.

2.3.2 Thermal Methods

The application of external heat on deposition surface or minimizing heat loss form pipelines can serve to prevent wax deposition. McClain and Whitfill (1984) proposed the insulation of pipeline to minimize heat loss or maintain a higher pressure in flow lines to reduce cooling due to dissolved gas expansion. Becker (2000) suggested the injection of hot water or use of hot oil and providing electrical heating element as methods of application of external heat to mitigate wax deposition. Hot oil acts both as a heater as well as a solvent for wax deposits but the drawback is that it could plug pumps or separators due to eventual cooling (Newberry et al., 1986). Usage of electrical heating elements

requires a lot of power and hence, a large investment when applied over long pipelines. A method to accomplish this could be to use electrical heating intermittently over the length of the pipeline. Some research using electromagnetic radiation and inductive heating have also been carried out to eliminate wax deposition (Balakirev et al., 2001; Sarmento et al.,

2004). A 50 % decrease in oil viscosity and 87.5% decrease in wax deposition rate was observed in experiments wherein magnetic paraffin control (MPC) was studied (Zhang et al., 2013).

2.3.3 Chemical Methods

Chemical methods include use of solvents, surfactants, pour point depressants, wax crystal modifiers, anti-sticking agents or a combination of one or more of these chemicals.

One of the commonly used methods for elimination of wax deposition in crude oil is to add crystal modifiers to the crude oil. These substances essentially change the lattice structure of paraffin wax thus preventing the agglomeration of wax crystals and formation of a massive crystal lattice structure. A research was carried out by Pederson and Ronningsen

(2003) in which they treated the waxy North Sea crude oil samples with 12 different commercial wax-crystal modifiers. The results showed that this method was applicable only till a certain molecular weight of compounds. Higher weight fractions of hydrocarbons

(>C30) remained unaffected. Thus, this method cannot be used for elimination of deposition of heavier hydrocarbons. Another method to curb this problem is the addition of detergents and dispersants to the waxy crude oil. Groffe et al. (2001) suggested the use of a chemical dispersant with anti-sticking properties could considerably reduce the amount of wax deposition by lowering the WAT along with creating less adhesion between wax deposit

and the metal surface. The major drawback of this method is that it is temperature dependent; the inhibition activity reduces with decrease in temperature. Certain coatings on the pipe could help in reducing wax deposition as well. Paso et al. (2009) suggested the use of nanocomposite polymers such as fluoro-siloxanes, fluoro-urethanes, oxazolane- based polymers, and DLC-polymer hybrids which exhibit low surface energy properties.

2.3.4 Biological Methods

The use of enzymes has been studied as a method to prevent wax deposition

(Braden, 1997). These enzymes were injected into a well bore and allowed to soak. The enzymes then start to break down the wax deposit and thus, form shorter chained alkanes.

Brown (1992) suggested the use of bacteria for prevention of wax deposition. The metabolic activity of bacteria causes the alkanes to break down and form organic acids and alcohols. The drawback with this method is that the bacteria blend and treatment volume needs to be determined for different crude oils and reservoir surroundings.

2.3.5 Cold Flow of “Waxy” Crude Oils

The methods discussed above have limitations with respect to wearing of equipment and cost and power requirements, especially for longer pipelines. The cold flow approach overcomes these disadvantages and can serve as a useful method to curb the issue of wax deposition. ‘Cold flow’ takes place when the crude oil being transported through pipelines is in the form of a slurry, having solid wax suspended in it and no deposition takes place under stable conditions. The cold flow takes place when the temperature of the crude oil falls between its WAT and PPT. In order to apply cold flow method as a way of preventing wax deposition, the precipitated solids in the oil should serve only as nucleation

sites and not deposit on the colder walls. Hence, the challenges for maintaining cold flow are to create od a stable slurry and to be able to maintain the ‘waxy’ crude oil at a temperature below its WAT without having any deposition on the inner pipe walls (Merino-

Garcia and Correra, 2008).

2.3.6 Sloughing of Wax Deposition

Sloughing can be described as the phenomenon wherein the wax deposit dislodges from the pipe wall and begins to flow along with the oil mixture, either due to thermal or hydrodynamic fluctuations in the system. Some researchers have shown that sloughing is an important mechanism when the oil is subjected to turbulent flow (Hsu and Santamaria,

1994; Hamouda and Davidsen, 1995; Creek et al., 1999). It has been stated that wax deposition increases with increase in flow rate in laminar flow, which was contradicted by

Parthasarthi and Mehrotra (2005) who concluded from experimental results that in fact the wax deposit thickness reduces with increase in laminar flow, which closely matched with model predictions. In turbulent flow, a similar trend was observed, i.e., the deposition reduced (Hsu and Santamaria, 1994; Creek et al., 1999). This reduction in wax deposition in turbulent flow regime has been termed as ‘sloughing’ and hence considered a wax deposition removal method.

Solaimany Nazar et al. (2007) formulated a model based on the mathematical model presented by Svendsen (1993) to predict wax deposition rate as a result of molecular diffusion by incorporating sloughing effects in it. The sloughing mechanism was directly related to aging effect on the deposit layer. From model predictions for turbulent flow conditions, it was observed that sloughing was only a function of hydrodynamic changes.

They reported that the total mass of deposition at high flow rates reduced sharply in the turbulent regime because of sloughing effect as the dominant mechanism. Thus, sloughing was suggested as an effective method of wax removal. The predictions also showed that the sloughing effect was relatively higher at the inlet of the pipe due to presence of more wax deposit and decreased through the length of the pipe due to decrease in deposit mass by molecular diffusion. Although this method poses an interesting aspect for wax removal, no laboratory experiments have been carried out in order to simulate the transportation pipelines and observe various trends related to different parameters.

Chapter Three: Experimental

This chapter describes the experimental procedures for this project. Three sets of experiments were carried out: estimation of deposit mass at base conditions, investigation of sloughing, and a set of calibration experiments. The materials used, preparation of sample, experimental setup and procedure and data collection will be explained in detail in this chapter. A bench-scale flow loop apparatus was used to study sloughing as a function of variation in coolant temperature, inlet hot wax-mixture temperature and flow arte of the wax-mixture under hot flow conditions.

3.1 Materials

3.1.1 Paraffin Wax

The wax used for all the experiments was Bernardin Parowax, which was obtained locally. The wax was in the form of rectangular blocks and mainly consisted of n-alkanes

ᵒ ranging between C20 to C50. The melting point was in the range of 57-61 C and the density was 912 kg/m3. Bernardin Parowax has an average molar mass of 420.9 kg kmol-1. The wax was characterized using HP 6890 series GC system in the In Situ Combustion

Laboratory at the University of Calgary. The details of the equipment and procedure are explained in detail in section 3.3.11. Table 3.1 and Figure 3.1 provide the carbon distribution for the wax sample.

3.1.2 Solvent

The solvent used for preparation of wax–solvent mixture was Linpar 1416V. which was obtained from APCO Industries Ltd. (Ontario, Canada). Linpar 1416 V was an oily, water-white liquid which mainly comprised of n-alkanes in the range of C7 – C19. The flash point of this solvent was 118 ᵒC which made it easy to heat the mixture up to 70ᵒC without the danger of ignition. Table 3.1 and Figure 3.1 provide the composition and carbon number distribution of the solvent. Some of the properties of the solvent are presented in

Table 3.2 as well.

Table 3.1 Composition of wax sample

Component Bernardin Parowax

(mass %)

C20 0.02 C21 0.52 C22 1.24 C23 2.47 C24 4.31 C25 6.46 C25 8.12 C27 8.99 C28 9.40 C29 10.23 C30 9.88 C31 8.89 C32 7.31 C33 4.72 C34 3.72 C35 2.17 C36 1.94 C37 1.12 C38 1.03 C39 0.59 C40 0.55 C41 0.47 C42 0.42 C43 0.20 C44 0.20 C45 0.24 C46 0.2 C47 0.13 C48 0.02 C49 0.02 C50 0.02

Figure 3.1 Composition of pure wax and solvent samples

Table 3.2 Composition of solvent sample

Component Linpar 1416 V

(mass %)

C7 0.05

C8 0.09

C9 0.08

C10 0.08

C11 0.07

C12 0.07

C13 0.06

C14 1.48

C15 58.86

C16 28.99

C17 7.07

C18 0.43

C19 0.35

Table 3.3 Physical and chemical properties of Linpar 1416V

Property Value

Appearance Water-White Liquid Average Molar Mass (kg kmol–1) 201 Flash Point (C) 118 Boiling Point (C) 248 - 284 Melting Point (C) 4 Auto Ignition Temperature (C) 204 Vapour Pressure at 20 C (kPa) 0.01 Vapour Density 7.1 Specific Gravity at 16 C 0.768 Viscosity at 40 C (cSt) 2.3 - 2.5 Water Solubility Negligible

3.2 Sample Preparation

A 6 mass% wax–solvent mixture was prepared for all the experiments. The use of a model mixture provided the ease of carrying out deposition experiments by eliminating the complexity due to the vast number of components present in field crude oils and the difficulty in procuring these samples due to the variability in properties, related costs of acquiring the crude oil and confidentiality issues. For this, the required amount of solvent was weighed and an amount of wax equal to 6% of the weight of mixture was added to it.

This was heated up to 70 ᵒC on a hot plate for 2-3 hours until a homogenous mixture was formed. The mixture was then allowed to cool down to the required wax-solvent mixture temperature.

3.3 Experimental Apparatus

3.3.1 Heating Bath and Associated Apparatus

The prepared wax–solvent mixture was contained in a 24 L reservoir made of aluminum, which was placed in a water bath. The internal dimensions of the water bath were 19 x 11.5 x 8 inch (L x W x H). Two immersion circulators (Model: LX Immersion

Circulator), obtained from VWR, were placed on either sides of the bath to heat up the water which would consequently heat up the wax–solvent mixture in the reservoir. A VWR recirculating chiller with positive displacement pump (Model No: 13271-214) was connected to the water bath to further regulate the temperature of the bath. The inlet and outlet of the chiller were fitted with 77 X 0.375 x 0.25 inch Nylaflow® nylon pressure tubing (GE Polymershapes Plastic, Calgary, AB) which were then dropped in to the bath to form a loop through which water could be circulated.

3.3.2 Cooling Bath and Associated Apparatus

The cooling unit comprised of a Haake D8 immersion circulator in conjunction with a Haake DC1-V refrigerated bath, obtained from Fischer Scientific. A submersible pump

(Model PE-2F-PW) purchased from The Little Giant Pump Company (Oklahama, USA) was used for pumping coolant water at a flow arte of 0.0082 L/s from the refrigerated bath to the coolant inlet of the deposition section. The pump was connected to the inlet of the section by a 50 x 0.25 x 0.1562 inches (L x OD x ID) Nylaflow® nylon pressure tubing attached to a Swagelok® female connector (B-400-7-8). A similar tubing was used to connect the coolant outlet back to the refrigerated bath.

3.3.3 Flow Loop

A bench scale flow loop apparatus was designed and fabricated for carrying out all the experiments. This design was adapted from the flow loop used by Kasumu (2014) for his experiments.

The outlet of the wax-mixture reservoir was connected to a hole drilled in the heating bath using a rubber hose and metal clips. This ensured that there was no leakage while pumping the mixture. The hole drilled in the bath was connected to a 6 x 1.25 inch

(L x ID) flanged stainless steel pipe. The other end of the flanged pipe was connected to a

1.25 x 0.75 x 1.25-inch copper Tee and secured using reinforced rubber and metal clips.

The smaller end of the copper Tee was connected to s a valve which was used for draining the mixture after each batch was used. This eliminated the need to remove the wax- mixture reservoir each time the mixture batch needed to be replaced. The other end of the copper

Tee was attached to s 1.5” male NPT which was further connected to the inlet of the pump.

The other end of the pump was joined to a 1” female NPT which was attached to a

1” ID brass union via a 1”ID, 2 inches in length copper pipe. The other end of the brass union was attached to a 1” ID copper pipe which in turn was connected to a 1” 45ᵒ elbow.

The elbow was further connected to a 1” ID, 6.5 inches in length of copper pipe which was connected to a second 1” 45ᵒ elbow. The second elbow was joined to a third 1” 45ᵒ elbow via a 1” ID, 2.5 inches in length of copper pipe. The third elbow was further attached to a

1” ID, 2.5 inches in length of copper tube which in turn was connected to a fourth 1” 45ᵒ elbow. This fourth elbow was connected to a 1” ID, 24.5 inches in length of copper pipe.

This length of pipe was connected to a 1” ID, 8.5 inches in length of copper pipe via a 1”

ID brass union. The total length of 33 inches of copper pipe was the ‘entrance length’ for the flow to be developed to completely turbulent flow before entering the deposition section. The end of the 1” ID, 8.5 inches in length of pipe was flanged and connected to the plexiglass part of the deposition section.

The other end of the deposition section was connected to a flanged 1” ID, 2 inches in length of copper pipe which was joined to another 1”ID, 2 inches in length of copper pipe using a 1”ID brass union. This was further connected to a fifth 1” 45ᵒ elbow which attached to a 1”ID, 2.5 inches in length of copper pipe. This length of pipe was connected to a sixth 1” 45ᵒ elbow which further connected to a fourth 1” ID brass union through a 1” id, 2 inches in length of copper pipe. The brass union was attached to a 1” ID, 4.25 inches in length of copper pipe which was connected to a 1” brass gate valve for regulating the flow of mixture. The valve further connected to a 1” ID, 5.5 inches in length of copper tube which was attached to a seventh 1” 45ᵒ elbow.

The seventh elbow was affixed to a series of connections as follows: 1” ID, 2.5 inch copper pipe, 1” 45ᵒ elbow, 1” ID, 2.25 inch copper pipe, 1” ID brass union, 1” ID, 1.5 inch copper pipe, 1”-0.75” copper reducer, 0.75” ID, 4 inch copper pipe, 0.75” threaded brass union, flow sensor with 0.75” ID inlet and outlet, 0.75” ID threaded brass union connected to outlet of flow sensor, 0.75” ID, 1.5 inch copper pipe, 0.75”-1” copper reducer, 1” ID,

1.5 inch copper pipe, 1” ID brass union, 1” ID, 1.5 inch copper pipe, 45ᵒ elbow, I” ID, 2 inch pipe, 45ᵒ elbow and finally connected to a 8 inch length Tygon® discharge tube.

The part before deposition section is inclined upwards and the part after the deposition section is inclined downwards to provide additional discharging from the section after the run is stopped. The use of 45ᵒ elbow instead of a 90ᵒ was done in order to reduce pressure drop in the flow loop. The entire flow loop was insulated with foam in order to minimize heat exchange with the surroundings during the experiments.

Figure 3.2 (a) and (b) represent the flow loop setup used in the laboratory while

Figure 3.3 represents the schematic diagram of the flow loop for better understanding.

(a)

(b)

Figure 3.2 Bench scale flow loop setup: (a) front, (b)back

Thi Air Valve Deposition Section

Coolant Coolant Tci In Out Tco

Coolant Bath Flow Regulating Valve

Flow Meter

Wax-Solvent Mixture Reservoir

Heating Bath Centrifugal Pump

Figure 3.3 Schematic diagram of the flow loop setup

3.3.4 Wax – Solvent Mixture Reservoir

The reservoir was an aluminum container with a diameter of 12 inches and a height of 15.5 inches. An aluminum container was chosen in order to provide high heat transfer

(due to high thermal conductivity) between the water in the heating bath and the wax mixture in the reservoir. A 1.2-inch diameter, 2-inches deep cut was made at the top of the container in order to allow the outlet of the flow loop to reach the reservoir.

3.3.5 Wax Mixture Centrifugal Pump

A centrifugal pump (Model No: COMSV33) obtained from Cole Palmer Instrument

Inc. (Chicago IL, USA) was used for pumping wax-solvent mixture from the reservoir to the flow loop. The pump has a maximum flow rate of 80 GPM, maximum operating pressure of 120 psi and maximum operating temperature of 250 ᵒF. The pump was positioned in such a way that the inlet of the pump was in line with the outlet of the wax mixture reservoir to provide self-priming by gravity. A gate valve placed downstream of the flow loop was used to control the flow of the mixture from the pump to the flow loop.

Figure 3.4 shows the placement of the pump and its connection to the wax mixture reservoir.

Figure 3.4 Centrifugal Pump

3.3.6 Wax Deposition Section

The wax deposition section was a co-current double pipe heat exchanger, wherein the coolant flowed through the annulus and the wax-solvent mixture flowed through the inner pipe. The deposition section used in this project is similar to the one used by Fong and Mehrotra (2007) and Kasumu and Mehrotra (2014). The deposition basically took place on the inside of a 1.0 x 1.3 x4.0 inch (ID x OD x L) aluminum tube (6061 grade) which formed the inner tube of the heat exchanger. Figure 3.5 shows the dimensions of the

Aluminum tube used.

Figure 3.5 Cross Section of Aluminum deposition tube (Fong, 2007)

The outer tube was a flanged plexiglass measuring 4.0 x 1.3 x 1.0 inches (L x OD x ID). The entrance flange measured 3.5 x 0.865 inches (OD x Thickness). On the outer side of the flange of the plexiglass section, a groove measuring 1.132 x 0.4 inches (OD x

Depth) was made for fitting an O–ring. The centre hole of the inside flange was machined with two grooves (for setting O-rings), outer one measuring 1.5 x 0.125 (OD x Depth) and

the inner one measuring 1 x 0.335 (OD x Depth). The entrance flange was sealed with 3

O-rings: one O-ring of dimension 1.125 x 1.3125 x 0.0937 (ID x OD x Thickness, #122) and two O-rings of dimension 2.0 x 2.1825 x 0.0937 (ID x OD x Thickness, #136). The first mentioned O-ring (#122) was used to seal the entrance copper pipe to the deposition section to avoid any leaks. A second O-ring (#136) was used to hold the Aluminum tube in place in the section while the third O-ring (#136) was used to seal the annulus of the deposition section which ensured no mixing between the wax-mixture and coolant water.

The top of the entrance flange was drilled with a 0.125 inch FNPT threaded hole to which a Swagelok® 1/16” tube – 1/8” connector (Cat No. B-100-1-2) was attached for fitting in the thermocouple to measure the inlet wax-mixture temperature. Four 0.266-inch diameter holes were drilled to a depth of 0.3” (edge to centre) on the entrance flange. These holes were used to connect the flange to the inlet copper pipe using stainless steel socket head cap screws,

The exit flange was designed similar to the entrance flange. In case of the exit flange, the top hole was used to provide an inlet for pressurized air into the deposition section (used for drainage after each experiment). A Swagelok® plug valve (Cat No. B-

4TA-1-2) connected to a ¼” tube – 1/8” NPT male adaptor (Cat. No. B-4TA-1-2) worked as the inlet for air. Figure 3.6 represents the outside and inside of the entrance flange while

Figure 3.7 shows the plexiglass body of the deposition section.

(a) (b)

Figure 3.6 Entrance flange: (a) inner side, (b) outer side (Fong, 2007)

(a) (b)

Figure 3.7 Plexiglass body of the wax deposition section (a) side view, (b) front view: entrance section (Fong, 2007)

The outside of the plexiglass was covered with a 2-cm thickness of Styrofoam to minimize heat loss to the surroundings. Figure 3.8 shows the schematic of the cross section of the deposition section without the end flanges.

Figure 3.8 Cross sectional view of wax deposition section (Fong, 2007)

3.3.7 Wax Mixture Flow Regulator

The flow rate of the mixture from the reservoir to the flow loop was controlled using a gate valve. A gate valve was chosen in order to provide flexibility in changing flow rates as required and least restriction to the flow. The valve was positioned between the wax deposition section and the outlet of the flow sensor.

3.3.8 Flow Meter and Data Logger

A 6000 Series Flow Meter (Model No.: D60168N60QC) was used in conjunction with a Florite 900 Series Data Logger (Model No. DLM920B4A0L) to measure the flow rate of the wax mixture in the flow loop. The flow meter had a pulsed output and it could handle flow rates ranging between 0.06 to 60 GPM. The data logger had a digital output to display the flow rate measured. The data logger along with the flow meter was precalibrated by Proteus Industries as per the density and viscosity of the wax mixture.

Thus, the output of the data logger was used as displayed for all experiments, without the use of any further correlations.

3.3.9 Thermocouple Data Acquisition System

To measure the temperature at various points in the system, T type thermocouples

(Cat No. TMQSS-062G-6) purchased from Omega (Stanford, Connecticut, USA) were used. Five thermocouples were placed to measure temperatures at various points as follows: (1) heating bath (2) wax mixture reservoir (3) inlet of wax mixture at the deposition section (4) coolant inlet and (5) coolant outlet. A Swagelok® male connector

(Cat No: B -100-1-2) was used to connect the thermocouple for measuring the wax mixture

inlet temperature to the deposition section and Swagelok® nylon male run tees (Cat. No:

NY-200-3TMT) and Swagelok® reducing unions (Cat No: NY -200-6-1) were used to connect the thermocouples measuring coolant inlet and outlet temperatures.

The thermocouples were connected to a FP-TC-120 module attached to a FP-TB-1 terminal base acquired from National Instruments (Austin, Texas, USA). The module has

8 input channels for connecting the thermocouples. The module along with the terminal base was attached to a FP-1000 network module that transmitted the signals from the thermocouples to the computer. LabView, a National Instruments data acquisition software, was used to record the temperatures from the thermocouples.

All the five thermocouples were calibrated in a mixture of water and ice and boiling water. The calibration was done by noting the deviation of temperatures at 0 ᵒC and 100 ᵒC.

The deviation was accounted for in a calibration file in the software.

3.3.10 Pressure Drop Measurements

A PX5200 Differential Pressure Gauge was connected across the deposition section to measure the pressure drop across the deposition tube. The pressure drop was measured continuously through every run which helped in indicating the build-up or removal of the deposit layer. The 1-inch outlets (marked high and low) of the pressure gauge were connected to a 1”-0.5” brass reducer which was further connected to two Tygon® tubings measuring 18 x 0.75 x0.5 inch (L x OD x ID). The tubings were consequently connected to the high and low pressure ends of the depositions section. It was required to bleed the air from the device before start of every run so that excessive pressure is not applied to the pressure sensor. This was done by opening the equalizing valve provided on the high

pressure side of the device. The pressure gauge was mounted at a desired level using a laboratory clamp and stand.

The differential pressure had a digital read out which displayed the pressure drop being measured in units of inch of H2O. The device was also connected through a port to the same data acquisition system (explained in section 3.3.9) which was used for connecting the thermocouples. The data was continuously recorded onto the LabView software along with the temperature measurements which helped in constantly monitoring the pressure drop. Figure 3.9 represents the DPG cell along with its connections to the wax deposition section.

Figure 3.9 Differential pressure gauge

3.3.11 GC Analysis of Samples

The GC analysis was done to provide data on composition of pure wax and solvent, wax-solvent mixture and deposit samples after selected runs. The GC analysis was performed in an In-Situ Combustion laboratory in the Department of Chemical engineering at the University of Calgary (Calgary, AB). A HP 6890 series Gas Chromatography (GC) system was used for the analysis. The system basically used the simulated distillation method. It had a fused-silica non-polar column with dimensions as 0 m x 0.53 mm x 0.88

µm film obtained from Separations Systems Inc. (Florida, USA). The hydrocarbon content was identified using a flame ionization detector (FID) and the data was collected using HP

ChemStaion software. Each hydrocarbon component was eluted on the basis of increasing boiling point in a capillary column. The GC data was analyzed using SimDist Expert V6.3 software. An n-alkane standard (C5 – C66) SD-SS3E-5 (Separations Systems Inc.) was used for the calibration of the GC system using ASTM D2887 extended method. The sample was prepared by dissolving in carbon disulphide to obtain ~2% sample solution.

3.4 Flow Loop experiments

3.4.1 Experimental Procedure

3.4.1.1 Deposition experiments

A set of deposition experiments were carried out at base conditions to reaffirm the time required for deposition process to reach a thermal steady state.

Before each experimental run, the immersion circulators in the water bath and the refrigerated bath were tuned on to reach the desired temperature. After a few trial

ᵒ experiments, it was observed that the inlet wax-mixture temperature (Thi) increased by 5 C due to additional heat input from the pump. Thus, the bath temperature was set 5 ᵒC below the required temperature to accommodate for this increase. For similar reasons, the

ᵒ refrigerated bath was set at 0.2 C below the desired coolant inlet temperature (Tci). The weight of the empty deposition tube was measured using Santorius BP210S balance which had a precision of ± 0.1 mg. Once the desired bath temperatures were achieved, the wax- solvent mixture pump and the data acquisition system were turned on. The required flow rate was set using the gate valve on the downstream of the system.

The deposition process was started when the coolant pump was tuned on; this marked as t = 0s for the deposition process. The values of Tbulk, Troom, Thi, Tci and Tco were constantly monitored throughout the run using the LabView software.

After the set duration for each run, the wax-mixture pump was tuned off and subsequently the deposition section was drained of any wax-mixture remaining by turning on the pressurized air inlet. The pressure was set at 5 psig which was enough to drain any oil left. Following this, the coolant pump was tuned off and the coolant was drained. After this, the deposition section was dismantled and the deposition tube was taken out and

weighed on the balance. By subtracting the mass of empty tube from the mass of the tube after the experiment, the actual deposit mass was obtained. After each run, a small part of the deposit was saved as a sample for GC analysis. It was observed by Fong and Mehrotra

(2007) and Tiwary and Mehrotra (2009) that deposition continues if section is not drained immediately and leads to blobs at the end of the deposition tube which was termed as ‘speed bumps’. This phenomenon was observed during certain experimental runs when the deposition section was not drained quickly enough (Figure 3.10). If the section was not drained for a couple of hours, the entire tube was blocked with wax deposit which simulates the wax deposition in transportation pipelines during shutdowns (Figure 3.11).

Speed Bump

Figure 3.10 ‘Speed bump’ observed due to delayed draining of deposition section

Figure 3.11 Blockage of tube due to extended delay in draining of deposition section

3.4.1.2 Sloughing Experiments

The basic procedure for sloughing experiments was similar to that of the deposition experiments. Based on the deposition experiments, it was determined that the deposition process takes about 1 hour to reach its thermal steady state. For the sloughing experiments, the parameters to be varied were: coolant inlet temperature (Tci), wax-mixture inlet temperature (Thi) and the wax-mixture flow rate (mh) or its Reynolds Number (Re).

The deposition at the base condition was allowed to commence for 1 hour. After the system reached a thermal steady state, one of the parameters was changed to the next set level. As an example, consider the experiments concerning change in Tci. The system was allowed to reach thermal steady state at the base condition of Tci = WAT–20 for one hour. After this, it was changed to the next level of Tci = WAT–10. The system was allowed to stabilize till this value of coolant temperature was attained. After this, the system was allowed to reach thermal steady state at that temperature for about 15–20 minutes which was observed in the form of stability in temperatures. The temperature was changed again to the next level and the same process was repeated. During each run, the coolant temperature difference was constantly monitored. A removal of deposit from the tube would lead to a decrease in the thermal resistance offered by the deposit layer thus leading to an increase in heat transfer from hot wax-mixture to the coolant. The increase in heat transfer should be marked by an increase in the coolant temperature difference, which would then confirm sloughing phenomenon. Thus, recording the coolant temperature difference was an essential part of the sloughing experiments.

At the end of each run, the same steps were followed as that for deposition experiments for dismantling the section and measuring the deposit remaining.

3.4.1.3 Calibration Experiments

A set of calibration experiments were carried out to obtain deposit mass at various levels of the sloughing experiments. The experimental procedure was similar to that of the sloughing experiments. In each run the deposition process was allowed for one hour followed by change in selected parameter. After each step change in the variable, the experiment was terminated and the deposition section dismantled to weigh the tube for deposit mass. The deposit mass was used for calculation of various properties like thermal conductivity of deposit, liquid-interface temperature, and extent of deposition using a steady state heat transfer model (discussed in Chapter 4). A small portion of the deposit sample was also saved for compositional analysis.

3.4.2 Experimental Design for flow Loop Experiments

3.4.2.1 Deposition Experiments

The deposition experiments were carried out to study deposition as a function of time, flow rate (mh) or Reynolds Number and coolant inlet temperature (Tci). Table 3.4 shows the experimental protocol followed for the deposition experiments. The duration for deposition was studied at 4 levels: 30 minutes, 1 hour, 2 hours and 4 hours. These extended experiments helped in determining the time required for deposition to reach steady state.

The flow rate of the wax-mixture was varied between 0.5 to 2 L/s corresponding to

Reynolds Number between 11,000 to 45,000. The coolant temperature was varied at three levels: WAT-20, WAT-15 and WAT-10. The coolant flow rate was held constant at

0.00852 L/s while the inlet wax-mixture temperature was constant at WAT+7 ᵒC throughout all experiments.

Table 3.4 Conditions for flow loop deposition experiments (Thi = WAT+7, WAT=28 C)

Run No Time Reynolds Number Inlet Coolant (Re) Temperature (Tci) (°C)

1 30 min 11,000 WAT-20

2 1 h 11,000 WAT-20

3 2 h 11,000 WAT-20

4 4 h 11,000 WAT-20

5 1 h 25,000 WAT-20

6 1 h 45,000 WAT-20

7 50 min 11,000 WAT-15

7R 1 h 11,000 WAT-15

8 1 h 11,000 WAT-10

3.4.2.2 Sloughing Experiments

The sloughing experiments constituted of three parts with variation in inlet coolant temperature (Tci), inlet wax-mixture temperature (Thi) and the flow rate (mh) of the wax-

ᵒ ᵒ mixture respectively. Tci was varied from WAT-20 C to WAT+2 C at six levels. Three experiments were carried out to study effect on deposit mass due to change in Tci at flow

ᵒ rates equal to 0.5 L/s, 1.7 L/s and 2.0 L/s, while keeping Thi constant at WAT+7 C (Table

3.5).

For sloughing experiments concerning the change in Thi, the temperature was varied

ᵒ ᵒ ᵒ ᵒ between WAT+1 C to WAT+7 C at six levels, at Tci = WAT-20 C and WAT-10 C and flow rate of wax-mixture at 0.5 L/s and 2.0 L/s (Table 3.6).

The sloughing experiments with change in flow rate of wax-mixture were carried out between mh = 0.5 L/s to 2.0 L/s or Re = 11,000 to 45,000 and at six levels for Tci =

ᵒ ᵒ ᵒ ᵒ WAT-20 C WAT-10 C and Thi = WAT+7 C and WAT+15 C. Table 3.7 explains the levels chosen for this set of experiments.

Table 3.5 Conditions for sloughing experiments with change in Tci (WAT=28 C)

Run No Inlet Wax-Mixture Reynolds Number Inlet Coolant

Temperature (Thi) (Re) Temperature (Tci) (C) (C)

WAT – 20 1 WAT-10 WAT-5 WAT+7 11,000 WAT WAT+1 WAT+2

WAT – 20 WAT-10 WAT-5 2 WAT + 7 25,000 WAT WAT+1 WAT+2

WAT – 20 WAT-10 WAT-5 3 WAT + 7 45,000 WAT WAT+1 WAT+2

Table 3.6 Conditions for sloughing experiments with change in Thi (WAT=28 C)

Run No Reynolds Number Inlet Coolant Inlet Wax-Mixture

(Re) Temperature (Tci) Temperature (Thi) (C) (C)

WAT + 1 WAT + 2 1 11,000 WAT - 20 WAT + 3 WAT + 4 WAT + 5 WAT + 6

WAT + 1 WAT + 2 WAT + 3 2 11,000 WAT - 10 WAT + 4 WAT + 5 WAT + 6

WAT + 1 WAT + 2 WAT + 3 3 45,000 WAT - 20 WAT + 4 WAT + 5 WAT + 6

Table 3.7 Conditions for sloughing experiments with change in Re (WAT=28 C)

Run No Inlet Wax-Mixture Inlet Coolant Reynolds Number

Temperature (Thi) Temperature (Tci) (Re)

(C) (C)

11,000 20,000 26,000 35,000 1 WAT + 7 WAT – 20 40,000 45,000

11,000 20,000 26,000 35,000 2 WAT + 7 WAT - 10 40,000 45,000

11,000 20,000 26,000 35,000 3 WAT + 15 WAT – 20 40,000 45,000

11,000 20,000 26,000 35,000 4 WAT + 15 WAT – 10 40,000 45,000

3.4.2.3 Calibration Experiments

In order to establish certain properties such as thermal conductivity of deposit sample (kd) and the liquid-deposit interface temperature (Td), three sets of calibration experiments, each with variation in one parameter (inlet coolant temperature, inlet wax- mixture temperature or flow rate), were carried out. Each parameter was studied in four different runs. For each parameter, the first run constituted the measurement of deposit mass at the base condition. In the next run, the system was first allowed to stabilize at the base condition for an hour, and then subjected to a step change in the parameter being varied and again allowed to reach thermal steady state at this new value. After this, the run was terminated and the deposit mass was measured. This procedure was repeated for the next two runs with increasing step changes in each run.

At the end of each run, the deposition tube was taken out and deposit mass was measured. Table 3.8 explains the selected values at which the experiments were carried out. Block 1 2 and 3 represent the three sets of calibration experiments with variation in one parameter in each block. The deposit mass values were used to estimate kd and Td using heat transfer equations at steady state (discussed in chapter 4).

Table 3.8 Conditions for calibration experiments (WAT=28 C)

Block No. Run No Inlet Wax-Mixture Inlet Coolant Reynolds

Temperature (Thi) Temperature Number (Re)

(C) (Tci) (C) 1 WAT-20

2 WAT-20 WAR-15 1 3 WAT-20 WAT-15 WAT+7 WAR-10 11,000

4 WAT-20 WAT-15 WAT-10 WAT-5

1 WAT+2

2 WAT+2 WAT+4 2 3 WAT+2 WAT+4 WAT-20 11,000 WAT+5 4 WAT+2 WAT+4 WAT+5 WAT+6 1 11,000

2 11,000 20,000 3 3 WAT+7 WAT-20 11,000 20,000 36,800 4 11,000 20,000 36,800 43,000

Chapter Four: Experimental Results

This chapter presents and analyzes the results obtained from each of the three sets of experiments conducted with the simulated wax–solvent mixture described earlier in

Chapter 3. The observations discussed here are those obtained from the deposition, calibration and sloughing experiments. The heat transfer model used to estimate the thermal conductivity and liquid-deposit interface temperature will also be explained. This model was similar to the one used by Fong and Mehrotra (2007), Tiwary and Mehrotra

(2009), and Kasumu and Mehrotra (2014 for the turbulent flow of wax-mixture in a flow loop setup.

4.1 Steady State Heat Transfer Model

To explain the heat transfer model used for the flow loop experiments, consider the

“hot” wax-solvent mixture flowing through the tube, held at a temperature higher than

WAT (Th > WAT) and cooled by a coolant at a temperature below WAT (Tc

of heat transfer through each resistance (i.e., the wax-mixture, the deposit layer, the tube wall, and the coolant) becomes equal.

For the flow loop apparatus, a double pipe concentric tube heat exchanger was used with concurrent flow. For this configuration, at steady state, the rate of heat transfer is equal to the thermal energy released by the waxy mixture, and the rate of thermal energy accepted by the coolant:

푞 = 푚̇ ℎ퐶ℎ(푇ℎ𝑖 − 푇ℎ표) = 푚̇ 푐퐶푐(푇푐표 − 푇푐𝑖) − 푞𝑔푎𝑖푛 4.1

(푇ℎ𝑖 − 푇푐𝑖) − (푇ℎ표 − 푇푐표) = 푈𝑖퐴𝑖 [ ] 푙푛[(푇ℎ𝑖 − 푇푐𝑖)⁄(푇ℎ표 − 푇푐표)] where, 푚̇ ℎ and 푚̇ 푐 are mass flow rate of wax-mixture and coolant respectively, 퐶ℎ and 퐶푐 are specific heat capacities of waxy mixture and coolant, respectively. 푇ℎ𝑖 and 푇ℎ표 are the inlet and outlet temperatures of wax mixture respectively, 푇푐𝑖 and 푇푐표 are inlet and outlet temperatures of the coolant respectively, 푈𝑖 is the overall heat transfer coefficient, and 퐴 is the inside tube surface area. Radiation losses were neglected due to relatively low difference between room and coolant temperatures. Thus, conduction and convection are the dominant mechanisms for heat transfer in the system.

The combined thermal resistance is a sum of the four individual thermal resistances in series as expressed in equation 4.2:

1 1 ln (푟𝑖⁄(푟𝑖 − 푥푑) ln (푟표⁄푟𝑖) 1 4.2 = + + + 푈𝑖퐴𝑖 2휋(푟𝑖 − 푥푑)퐿ℎℎ 2휋푘푑퐿 2휋푘푚퐿 2휋푟표퐿ℎ푐

On equating the heat flux through each of the thermal resistances:

푞 ℎℎ(푇ℎ − 푇푑) 푘푑(푇푑 − 푇푤𝑖) 푘푚(푇푤𝑖 − 푇푤표) ℎ푐(푇푤표 − 푇푐) 4.3 = = = = 퐴𝑖 푟𝑖⁄(푟𝑖 − 푥푑) 푟𝑖ln (푟𝑖⁄(푟𝑖 − 푥푑)) 푟𝑖 ln(푟표⁄푟𝑖) 푟𝑖⁄푟표

where, hh and hc are the convective heat transfer coefficients for the wax mixture and the coolant, respectively, xd is the deposit layer thickness, kd and km are the thermal conductivities of the deposit layer and the pipe wall thickness respectively, Td is the liquid- deposit interface temperature, Twi and Two are the inner and outer wall temperatures respectively. The values for q, Ai, Th, Tc, hh, hc, ri, ro, xd and km, the four inequalities in equation 4.3 can be solved simultaneously to obtain values of Twi, Two, Td and kd.. An important assumption is that xd, Td and kd are considered constant throughout the length of deposition tube.

Bidmus and Mehrotra (2004) introduced a dimensionless ratio, d, which is the ratio of the thermal resistance offered by the deposit layer to the overall thermal resistance. It could also be represented as the temperature difference across the deposit layer to the overall temperature difference as explained in equation 4.4.

푅푑 푇푑 − 푇푤𝑖 4.4 휃푑 = = 푅ℎ + 푅푑 + 푅푚 + 푅푐 푇ℎ − 푇푐 where, Rh, Rd, Rm and Rc are the thermal resistances offered by the wax-mixture, deposit layer, wall thickness and the coolant respectively. Similar ratios can be obtained for the other three thermal resistances as well (Equations 4.5-4.7).

푅ℎ (푇ℎ − 푇푑) 4.5 휃ℎ = = 푅ℎ + 푅푑 + 푅푚 + 푅푐 (푇ℎ − 푇푐)

푅푚 (푇푤𝑖 − 푇푤표) 4.6 휃푚 = = 푅ℎ + 푅푑 + 푅푚 + 푅푐 (푇ℎ − 푇푐)

푅푐 (푇푤표 − 푇푐) 4.7 휃푐 = = 푅ℎ + 푅푑 + 푅푚 + 푅푐 (푇ℎ − 푇푐)

At steady state conditions, (h + d + m + c) = 1.

The steady state heat transfer model was used to estimate the average values of Td and kd. These equations were also modified to obtain deposit thickness for the three sets of sloughing experiments as will be explained in the following sections.

Kasumu (2014) developed various correlations required for calculations with a steady state heat transfer model. The correlations developed for convective heat transfer coefficients for the wax-mixture and the coolant are as presented in equations 4.8 and 4.9, respectively.

훼 −1 ℎℎ = [훽(푚ℎ) ] 4.8

푟𝑖 4.9 ℎ푐 = 푟𝑖 ln(푟표⁄푟𝑖) (푟표 (푐 − )) 푘푚 where,  = 1.960 ,  = 0.86 and c = 0.0003. These constants were determined experientially for a 6% wax-solvent mixture.

The properties of coolant water, such as density of water and specific heat capacity

Cc, were obtained from Perry’s handbook (Perry et al., 1997). Kasumu (2014) also carried out various experiments to develop relations for properties like density, viscosity and specific heat capacity of the wax-solvent mixture for various concentrations. Equations developed for 6% wax-solvent mixture were used for calculations. The density of the mixture is calculated using the empirical equation 4.10.

휌ℎ = 푎1 + 푎2(푇ℎ + 273.15) 4.10

-3 where, h (kg m ) is the density of the wax mixture, Th is the average mixture temperature

-3 -3 and a1 and a2 are constants which are equal to 779.30.1 kg m and -0.5350.003 kg m

K-1, respectively. The viscosity of the wax mixture was correlated as:

−3 휇 = 10 푒푥푝[푏1 + 푏2(푇ℎ + 273.15)] 4.11 where,  (Pa.s) is the viscosity of the wax mixture and b1 and b2 are constants which are equal to -4.530.02 and 15405, respectively. The specific heat capacity was estimated using equation 4.12.

퐶ℎ = 푐1 + 푐2(푇ℎ + 273.15) 4.12

-1 -1 where, Ch (J kg K ) is the specific heat capacity of the wax-mixture and c1 and c2 are regressions constants, which are equal to 2330 J kg-1 K-1 and 1.305 J kg-1 K-1.

Kasumu (2014) also proposed an empirical equation for the calculation of deposit density (d) used for calculation of deposit thickness (xd) as presented in equation 4.13.

−1 휌푑 = 푑1 + 푑2푅푒 − 푑3(푊퐴푇 − 푇ℎ) 4.13 where, d1, d2 and d3 are regression constants which are equal to 788.1, -358000 and 0.784, respectively.

4.2 Results of Deposition Experiments

As mentioned earlier, the deposition experiments were carried out to establish the time taken for the deposition process to reach a thermal steady state. Figure 4.1 shows the trend observed from all the seven runs carried out. The deposit mass per area (kg/m2) was plotted against the time duration (in hours) for each run in order to observe change in deposit mass with time.

Four runs were carried out at Tc = WAT-20 for various time durations. The trend shows that the deposition process reached a thermal steady state between 30 minutes to 1 hour, after which the deposit mass per unit area remains approximately the same for extended time durations. Another evident trend observed from this plot is the reduction in deposit mass per unit area with increase in coolant temperature. This is because an increase in the coolant temperature leads to a decrease in (Tc – WAT), which correlates to a decrease in deposit mass. This was in agreement with the observations made by Bidmus and

Mehrotra (2004), Parthasarthi and Mehrotra (2005), Fong and Mehrotra (2007), Tiwary and Mehrotra (2009) and Kasumu and Mehrotra (2014).

Time (h)

Figure 4.1 Deposit mass per unit area measured at Tc = WAT-20, WAT-15 and WAT-10

4.3 Results of Calibration Experiments

4.3.1 Estimation of Liquid-Deposit Interface Temperature (Td) and Thermal Conductivity (kd)

The results of the calibration experiments were used to estimate Td and kd values for the deposit mass. The steady state data were analyzed using equations 4.1 and 4.3. The heat flux equalities in equation 4.3 were used to calculate Twi, Two, Td and kd. The experimentally measured Tc was used to calculate Two, which was further used to calculate

Twi, Then, the experimental Th was used to calculate Td, which was in turn used to calculate

-1 thermal conductivity of the deposit kd. The ratio ri/(ri – xd) can be rewritten as (1 – xd/ri) .

Note that, when xd/ri <<1, the term (1-xd/ri) from the first inequality in equation 4.3 is close to 1 but the term –ln (1-xd/ri) from the second inequality in equation 5.3 still varies with xd/ri. Due to this, Td estimated from the first equality of equation 4.3 is less sensitive to xd, but kd calculated from the second inequality is much more sensitive to xd. That is, a small experimental uncertainty in xd causes a relatively larger fluctuation in the estimated kd than in Td.

The heat transfer calculations were applied for all 12 calibration experiments to estimate Td and xd. Since the materials used in all the experiments: paraffin wax and solvent

(Linpar 1416 V), and the composition of the wax-solvent mixture (6 %) was identical to the model mixture used by Kasumu (2014) for his flow loop experiments, the WAT experimentally reported by him has been used for all the calculations in this thesis. The average Td from heat transfer calculations was found to be 29  2.6 C, which was close to the measured WAT of 28.0 C reported by Kasumu (2014). This result was consistent with those presented by Bidmus and Mehrotra (2004), Parthasarthi and Mehrotra (2005), Fong

and Mehrotra (2007) and Tiwary and Mehrotra (2009) which stated that the liquid-deposit interface temperature is close to the WAT of the mixture.

The average thermal conductivity of the deposit was also calculated from the heat transfer calculations and was found to be 0.39  0.05 W m-1 K-1. These values compare well with those reported by Fong and Mehrotra (2007), Tiwary and Mehrotra (2009) and

Kasumu and Mehrotra (2014), This value is a little higher than those reported by Bidmus and Mehrotra (2004) and Parthasarthi and Mehrotra (2005) since they conducted all experiments in laminar flow, while these experiments were carried out in the turbulent flow.

4.3.2 Effect of Coolant Temperature (Tc), Wax-solvent Temperature (Th) and Reynolds Number (Re)

The three sets of calibration experiments with changes in Tc, Th and Re showed the effect of each parameter on the wax deposition process. To obtain trends to showcase these effects, the extent of deposition (), i.e., the mass deposited per unit area of the deposition section, was calculated.

Previous studies carried out by Bidmus and Mehrotra (2004), Parthasarthi and

Mehrotra (2005), Fong and Mehrotra (2007), Tiwary and Mehrotra (2009) and Kasumu and Mehrotra (2014) reported that the deposit thickness increases with a decrease in coolant temperature or an increase in (WAT–Tc). As seen from the calibration experiments, the extent of deposition reduced with an increase in the coolant temperature (Figure 4.2). These results were consistent with those reported for both laminar as well as turbulent flow deposition processes.

Figure 4.3 shows the effect of Th on extent of deposition () and it can be seen that

 decreased with increase in Th. This can also be interpreted as: deposition decreases with increase in (Th – WAT). These results are in agreement with those reported previously for laminar flow regime (Bidmus and Mehrotra, 2004; Parthasarthi and Mehrotra, 2005) and for turbulent flow (Fong and Mehrotra, 2007; Tiwary and Mehrotra, 2009; Kasumu and

Mehrotra, 2014). The observations for effect of Tc as well as that for Th are also consistent with the predictions from the mathematical model based on moving boundary formulation developed by Bhat and Mehrotra (2005, 2006, 2007, 2010).

The effect of Re on  showed that the deposition decreased with an increase in the

Reynolds number or flow rate of the wax mixture (Figure 4.4). This is due to the fact that an increase in Re results in an increase in hh and hence a lower convective thermal resistance offered by the wax mixture (Rh). For a constant value of Th and Tc, a lower value of Rh implies a lower value of Rd, i.e., the thermal resistance offered by the deposit layer, thus increasing the rate of heat transfer and hence, a decrease in deposit layer thickness and consequently, a decrease in .

Figure 4.2 Variation in extent of deposition with step changes in Tci

Figure 4.3 Variation in extent of deposition with step changes in Thi

Figure 4.4 Variation in extent of deposition with step changes in Re

4.4 Results of Sloughing Experiments

The sloughing experiments were carried out to verify experimentally if thermal and hydrodynamic variations in the system would lead to a complete removal, or even partial dislodging, of wax deposit from the inner tube surface, as postulated in the literature

(Hsu and Santamaria, 1994; Creek et al., 1999; Solaimany Nazar et al., 2007). Thus, the three sets of experiments carried out with changes in coolant temperature, wax mixture inlet temperature and flow rate would help in understanding the phenomenon of sloughing.

If there is any sloughing, it would be observed in sudden changes in the coolant temperature difference as well as pressure drop across the deposition section. That is, if sloughing were to occur (with the deposit layer removed from the tube surface), a sudden increase in coolant temperature difference (Tc) should be noticed. This is because the removal of the deposit layer would lead to a decrease in the thermal resistance offered by the deposit layer

(Rh), which would lead to an increase in the overall heat transfer coefficient (U). This would consequently lead to an increase in the rate of heat transfer from the wax mixture to the coolant thus, causing a higher rate of heat accepted by the coolant. Hence, the outlet temperature (Tco) would be much higher than the inlet temperature (Tci), consequently Tc would increase substantially at the point where the deposit is dislodged from the tube wall.

At this point, the pressure drop (dP) across the deposition section would also decrease considerably since a lower pressure or resistance to the flow would be experienced due to lower deposit thickness.

4.4.1 Effect of Change in Coolant Inlet Temperature (Tci )

Tci was varied at six levels ranging from WAT–20 to WAT+2 C and also at three levels of flow rate (0.5 L/s, 1.7 L/s and 2.0 L/s ) or Reynolds number (Re = 11,000, 25,000 and 45,000) as discussed in the previous chapter. Figures 4.5-4.7 show the change in Tci,

Tc and dP with time at different Re values. These figures show the variation in coolant temperature and pressure drop along the entire duration of the run. Consider the run at Re

= 10,000 (Figure 4.5). It was observed that Tc kept on gradually decreasing until Tc became equal to WAT of the mixture. After this point, a slight increase in Tc is noticed.

When Tci becomes equal to or higher than the WAT of the mixture, there can be no more deposit (by definition of WAT) and hence, the deposit gets removed at this point leading to an increase in Tc. The pressure drop increased with the build-up of deposit thickness and thereafter remained almost constant. After Tci was equal to the WAT of the mixture, a drop in dP was observed. This can be attributed to the fact that after Tci was equal to the

WAT, the deposit layer melts off and hence, the pressure drop reduces.

Similar trends were observed for Re = 23,000 and 44,000. For these two runs, a constant gradual decrease in Tc was observed without any sudden increase even after Tci was increased beyond WAT. A slight decrease in dP was noted for both of these runs after

Tci was above WAT. This could be due to the fact that the amount of wax deposit at these high Reynolds number was comparatively lower and hence, any significant changes over the thin deposit layer could not be observed when Tci was varied.

For all the three runs, it was observed that for every step change in Tci, Tc was initially high and then reduced to an almost constant value as the system approached steady

state at that particular step change. Another observation that could be made from these plots is that the value of Tc is lower for a lower Re of 11,000 while it increases with increase in Re for each run. This is so because with increase in Re, the convective heat transfer coefficient of the wax-mixture (hh) increases thus reducing its thermal resistance

(Rh). Also, an increase in Re results in a decrease in the deposit thickness which consequently results in the decrease of the deposit thermal resistance (Rd). The reduction in the thermal resistance leads to an overall increase in U, which ultimately leads to an increase in the heat transfer thus increasing the value of Tc.

The results of this set of experiments show that step changes in the coolant (or surroundings temperature) would not lead to the removal of the wax deposit from the tube, until Tci approaches WAT. Visual inspection of the deposition tube showed that there was no deposit left after each run, which was anticipated given that Tci was slightly higher than

WAT.

(a)

(b)

(c)

Figure 4.5 Effect of variation in (a) Tci on (b) Tc and (c) dP with time for Re=11,000

(a)

(b)

(c)

Figure 4.6 Effect of variation in (a) Tci on (b) Tc and (c) dP with time for Re = 25,000

(a)

(b)

(c)

Figure 4.7 Effect of variation in (a) Tci on (b) Tc and (c) dP with time for Re = 45,000

4.4.2 Effect of Change in Inlet Wax-Mixture Temperature (Thi)

The effect of change in Thi was studied at two levels of the coolant temperature (Tci

= WAT–20 and WAT–10 C) and two levels of the flow rate (0.5 L/s and 2.0 L/s) or

Reynolds number (Re = 11,000 and 45,000). Figures 4.8 - 4.10 depict trends observed for

Thi, Tc and dP with time. For all the three runs, Tc was high for the first few seconds which indicates that the coolant just started circulating and the deposition started to commence, after which the value drops down and becomes constant at steady state conditions. Tc was found to be constant for lower Thi values, but there was a slight increase when Thi, was further increased. Consider equation 4.13.

푞 = ℎℎ퐴푑(푇ℎ − 푇푑) = 푚̇ 푐퐶푐(푇푐표 − 푇푐𝑖) 4.14

When Thi is increased it results in a decrease in deposit thickness, thus reducing Ad (area available for convective heat transfer from the wax mixture to the deposit layer) while (Th

– Td) increases proportionately. Since 푚̇ 푐 and Cc remain constant, it implies that (Tco – Tci) should increase as well as observed in the trends. As Thi does not vary over a large range in these experiments, there is not much fluctuation observed in Tc values. The pressure drop (dP) for each run increases initially as the deposit builds up and then remains almost constant throughout the run.

As seen from the plots, there were no sudden peaks or troughs observed during all the three runs. On visual inspection after the run, a layer of wax deposit was found for each of the runs and the deposit mass was measured. The deposit mass/thickness was found to be lower for the run with higher Tci and also for the run with higher Re, as was expected.

Thus, it is evident that an increase in Thi only reduces the wax deposit thickness; but it does not lead to sloughing of the deposit layer.

(a)

(b)

(c)

Figure 4.8 Effect of variation in (a) Thi on (b) Tc and (c) dP with time for

Tci = WAT-20, Re = 11,000

Time (s) (a)

(b)

(c)

Figure 4.9 Effect of variation in (a) Thi on (b) Tc and (c) dP with time for Tci = WAT-10, Re = 11,000

(a)

(b)

(c)

Figure 4.10 Effect of variation in (a) Thi on (b) Tc and (c) dP with time for Tci = WAT-20, Re = 45,000 4.4.3 Effect of Change in Reynolds Number (Re) or Shear Rate

The sloughing phenomena has been described to be mainly dependent on hydrodynamic variations in the system. This set of experiments was carried out to study the effect of step changes in Re on the wax deposit at two levels of the inlet wax mixture temperature (Thi = WAT+7 and WAT+15C) and two levels of the inlet coolant

For all the four runs, it was observed that Tc increased with each step increase in

Re. At the beginning of each step change, Tc increased and then stabilized till Re was varied again. When Re increases, the convective heat transfer coefficient of the wax mixture (hh) also increases. From equation 4.13, an increase in Re would result in consequent decrease in deposit thickness, thus increasing Ad, while (Th – Td) remains constant. Since 푚̇ 푐 and Cc remain constant, with each step increase in Re, both hh and Ad will increase, and these would result in a corresponding increase in Tc.

The trends for dP show that the pressure drop increased with each step increase in

Re; dP increased at each level of Re and remained constant at that level till Re was changed again. The increasing trend in dP is mainly due to the increase in flow rate or Re since pressure drop across the deposition section is dependent on the length of the tube and mass flow rate.

After each run, on visual inspection, a deposit layer was found on the tube wall surface. The deposit mass measured was found to be lower for higher Thi (WAT+12) and higher Tci (WAT-10) values, as expected. There was no complete removal of the wax

deposit at any point during the entire run; there was only a gradual decrease in the deposit mass or deposit thickness due to increase in Re. Hence, these results show that any sloughing or abrupt removal of the deposit did not occur as a result of change in the flow rate of the wax mixture.

(a)

(b)

(c)

Figure 4.11 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+7, Tci = WAT-20

(a)

(b)

(c)

Figure 4.12 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+7, Tci = WAT-10

(a)

(b)

(c)

Figure 4.13 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+12, Tci = WAT-20

(a)

(b)

(c)

Figure 4.14 Effect of variation in (a) Re on (b) Tc and (c) dP with time for Thi =WAT+12, Tci = WAT-10

4.5 Results of GC Analysis

The GC analysis of deposit samples from various experiments helped in observing trends in compositional changes in the solvent and wax fractions of samples as a function of various parameters. The sections below discuss the compositional analysis of samples as a function of variation in time, change in Tci, Thi and Re in the system.

4.5.1 Aging of Wax Deposit

Aging of the deposit has been defined in the literature as change in deposit composition with time. The effect of time duration on wax deposition was studied at 4 different levels: 30 minutes, 1 hour, 2 hours and 4 hours. The deposit samples collected at the end of each run of the deposition experiments were analyzed for the variation in the wax composition. Thi and Tci were held constant at WAT+7 and WAT-20, respectively, and the Re of the mixture was equal to 11,000 for all the four runs.

To understand the variation in carbon number distribution of the deposit sample, the mass fractions of C20 – C40 n-alkanes were normalized on a solvent-free basis. This yielded the compositional variation in the wax fraction of the sample. The normalized mass fractions of the n-alkanes in the deposit (Wd) and in the wax-mixture (Wh) were calculated as follows:

50 4.15 (푊푑)𝑖 = (푤푑)𝑖⁄ ∑ (푤푑)𝑖 1=20

50 4.16 (푊ℎ)𝑖 = (푤ℎ)𝑖⁄ ∑ (푤ℎ)𝑖 1=20

The difference between the two normalized mass fraction, [(Wd)I – (Wh)i], gives the change in composition of the ith carbon number of the n-alkane and helps to compare the change in the mass fraction as compared to the original wax-mixture composition. This difference was represented as percentage of deviation in the mass fractions.

Figure 4.15 shows the variation in wax composition as a function of time. As observed from the plot, there is a sharp minimum observed at C26 and a sharp maximum at about

C32. where the minimum and maximum values signify the deviation of composition in comparison to the original wax-solvent mixture. The plot also clarifies that the fraction of n-alkanes lighter than C28 decreased while the fraction of n-alkanes heavier than C28 increased as compared to the original wax-solvent mixture. For the shorter deposition time of 30 minutes, the minimum and maximum were noted to be about -3 and +7 mass% while for the longer deposition time of 4 hours, the minimum and maximum deviation were about

-5.5 and +9.5 mass%. These results show that with time, the deposit gets enriched in heavier alkanes (C30 – C35) and depleted in lighter alkanes (C25 – C28). This shows that the time duration of the deposition process is a strong factor for the aging of the wax deposit.

Figure 4.15 Change in deposit wax composition with time at Thi = WAT+7, Tci = WAT-20 and Re = 11,000

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4.5.2 Effect of Change in Tci, Thi and Re on Wax Deposit Composition

The deposit samples collected after each run of the calibration experiments were analyzed to establish trends in the change of composition of the wax deposit as the three parameters were varied. This analysis would help in understanding how the composition of wax and solvent vary with the step changes in each parameter. The plots are presented as deviation in wax fraction concentration of the deposit from the original wax-mixture concentration. The same procedure for calculation was used as explained in section 4.5.1 and the deviation in composition of ith carbon number was presented in terms of mass percent.

For the set of calibration experiments with change in Tci, the composition was analyzed at all the four step changes: WAT-20, WAT-15, WAT-10 and WAT-5 (Figure

4.16). With each step increase in Tci, it was observed that the deviation in wax concentration enhanced. A sharp minimum was observed at C26 and a sharp maximum was observed at

C32. For base condition at WAT-20, the maximum and minimum deviations were around -

6 and +9.5 mass % respectively, whereas that for step changes till WAT-5 the deviation were around -7 and +11 mass % respectively. The plot shows that the deposit thickness became enriched in heavier n-alkanes (C30 – C35) and leaner in lighter n-alkanes (C25 –

C29), indicating reduction in deposit thickness. On reduction in deposit thickness, the solvent is “squeezed” out from the cubical cage structure of the wax and solvent, thus increasing the heavier components of the wax fraction in the deposit (Mehrotra and Bhat,

2007; Bhat and Mehrotra, 2008).

For the set of experiments with change in Thi, the compositional analysis was carried out for step changes in Thi at WAT+2, WAT+4, WAT+5 and WAT+6. Similar to

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the previous trend explained, sharp minimum was observed at C26 and a sharp maximum was observed at C32. As seen from figure 4.17, the deviation in wax fraction was lowest for Thi = WAT+2, with minimum and maximum deviations at -5.5 and +9.0 mass % respectively, and it increased with each step increase in Thi. For the final run with maximum step changes till WAT+6, the minimum and maximum deviations were observed to be -7.5 and 11 mass % respectively. The plot shows that the deposit was enriched in heavier n-alkanes and depleted in lighter n-alkanes with each increasing step change in Thi.

This trend of enrichment of heavier components of wax is indicative of the fact that the deposit thickness decreased with each step increase in Thi.

The calibration experiments with change in Re were analyzed for compositional changes with step increase in Re at 11,000, 20,000, 36,800 and 43,000. Figure 4.18 shows the trends observed for these four runs. Similar trends were observed with the base condition of Re at 11,000 having the lowest deviation with -5.0 and +7.0 mass % as minimum and maximum mass % deviations while the run with highest step change at Re

= 43,000 having minimum and maximum deviations at -7.0 and +10.0 mass %. The plot was indicative of the fact that with increasing step changes in Re, the deposit layer was enriched in heavier n-alkanes (>C29) and depleted in lighter n-alkanes (

The trends observed for Tci, Thi and Re can be extended to the sloughing experiments carried out with change in each of these parameters. The wax fraction would become enriched in heavier n-alkanes and leaner in lighter n-alkanes with increase in each

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of the three parameters for all the sloughing runs. Thus, the GC analysis gives a clear picture of variation in composition as a factor of changes in the three parameters.

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Figure 4.16 Change in deposit wax composition with step changes in Tci at Thi = WAT+7 and Re = 11,000

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Figure 4.17 Change in deposit wax composition with step changes in Thi on at Tci = WAT-20 and Re = 11,000

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Figure 4.18 Change in deposit wax composition with step changes in Re on at Thi = WAT+7 and Tci = WAT-20

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Chapter Five: Deposit Thickness Predictions from Heat Transfer Model

The previous chapter discussed various trends that were observed during the sloughing experiments in terms of changes in Tc and dP. It is noted that the deposit mass or thickness could not be measured because it would have required stopping and discontinuing the sloughing experiment in order to extract the deposition section for mass measurements. The prediction of changes in the deposit mass during all the runs for the sloughing experiments would help in justifying the trends observed and the conclusions drawn in the previous sections.

5.1 Equations for Deposit Thickness Predictions

The steady state heat transfer model was used for predicting the deposit mass for all of the sloughing experiments. In Section 4.3.1, average values of the liquid–deposit interface temperature (Td) and the deposit thermal conductivity (kd) were established. These average values were used for the estimation of deposit mass. The second equality in equation 4.3 was rearranged as presented in equation 5.1 to obtain the deposit thickness:

5.1 푟𝑖 푥 = 푟 − [ ] 푑 𝑖 푘 퐴 (푇 − 푇 ) 푒푥푝 { 푑 𝑖 푑 푤𝑖 } 푞 푟𝑖

Here, q is calculated as qc, i.e., the heat absorbed by the coolant. Referring to equation 4.1, the quantities required for calculating qc are 푚̇ 푐, Cc, Tci and Tco , of which 푚̇ 푐 and Cc are constant and Tci and Tco are measured for each run. The limitation of using qh, alternatively instead of qc, to calculate xd is that the outlet wax mixture temperature, Tho, could not be measured experimentally but was estimated from the energy balance by equating qh and qc 107

(Equation 4.1). To estimate the value of Twi, it was first required to calculate Two. From the fourth equality in equation 4.3, Two was calculated by using values of qc and Tc as represented in equation 5.2. This was further used to calculate Twi as expressed in equation

5.3.

푞 푟표 5.2 푇푤표 = 푇푐 + ( ) 퐴𝑖 푟𝑖ℎ푐

푞 푟𝑖 푙푛(푟표⁄푟𝑖) 5.3 푇푤𝑖 = 푇푤표 + ( ) 퐴𝑖푘푚

All of the graphs for the variation of the deposit thickness in this section have been plotted in terms of xd/ri, which is a dimensionless ratio, against various other quantities.

The predictions for xd/ri were subject to uncertainties in Td and kd as these values were average values estimated from the steady state heat transfer model.

5.2 Model Predictions for Sloughing Experiments

5.2.1 Effect of Increases in Tci

The three experiments carried out at different Re were analyzed to predict deposit thickness change with each step change in Tci. For each step change, average values of Thi,

Tci and Tco were calculated from the recorded temperatures. The deposit mass was then calculated using these values in equation 5.1 for each step change.

Figure 5.1 represents the trends observed for the variation in deposit mass as a function of change in Tci. A decreasing trend in xd/ri was observed with increasing Tci indicating that the deposit thickness decreases with an increase in the coolant temperature.

Note that xd/ri approaches zero as the coolant temperature approaches WAT. This is indicative of the fact that the deposit is completely removed only when Tci = WAT, which

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is consistent with the definition of WAT. Another interpretation from this plot is the xd/ri is lower for the experiments at higher Re; that is, the deposit thickness decreases with increasing Re, which is consistent with previous studies ((Bidmus and Mehrotra, 2004;

Fong and Mehrotra, 2007; Tiwary and Mehrotra, 2009, Kasumu and Mehrotra, 2014).

The deposit thickness was also predicted for the entire run to study its variation as a function of time. It is pointed out that equations 4.3 and 5.1 have been derived for steady- state conditions; hence, their use while Tci was increased gradually to the next level can be questionable because these do not account for the latent heat of melting while the deposit thickness deceases gradually. The correct approach for predicting the deposit thickness under unsteady state conditions would be via the moving boundary formulation, as described by Bhat and Mehrotra (2005, 2006) and Arumugam et al. (2013). Hence, the deposit-thickness predictions in these sections, while Tci was allowed to change, should only be used for observing the trends, i.e. for only qualitative purposes.

In Figures 5.2 - 5.4, xd/ri as well as Tci were plotted against a common x-axis representing time for each run individually. It was observed that, for each of the three runs, xd/ri decreased gradually as Tci was increased over the entire duration. Also, xd/ri was noted to approach zero as Tci approached WAT. The trends in these predictions are in accordance with the conclusions drawn in the previous chapter. No sloughing of the deposit can be observed in any of the runs.

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Figure 5.1 Variation in predicted xd/ri with changes in Tci at Re = 11,000, 27,000 and 44,000

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Figure 5.2 Variation in predicted xd/ri with changes in Tci over time at Thi=WAT+7 and Re = 11,000

Figure 5.3 Variation in predicted xd/ri with changes in Tci over time at Thi=WAT+7 and Re = 25,000

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Figure 5.4 Variation in predicted xd/ri with changes in Tci over time at Thi=WAT+7 and Re = 45,000

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5.2.2 Effect of Increases in Thi

For each run in the set of sloughing experiments with changes in Thi, the average values of Thi, Tci and Tco at each step change were calculated and used along with equation

5.1 to predict the deposit thickness.

Figures 5.5 (a) and (b) compare xd/ri values with changes in Thi at different values of Tci (WAT-20 and WAT-10) and Re (11,000 and 44,000). In both the plots, it can be seen that, with a gradual increase in Thi, xd/ri decreases steadily, implying that the deposit thickness reduces. This is consistent with the fact that increasing Thi leads to a reduction in the deposit thickness. From these plots, it is observed that xd/ri values are higher for Tci =

WAT-20 as compared to those for Tci = WAT-10; these results confirm that an increase in

Tci decreases the deposit thickness. Similarly, it can be noted that xd/ri values are higher for a lower Re of 11,000 as compared to those for a higher Re of 45,000, thus implying that increasing Re reduces the deposit thickness.

The deposit thickness was also studied as a function of time for each of the runs.

For this, the values of xd/ri were calculated for the entire time duration using values of Thi,

Tci and Tco at each instant. A plot of Thi and xd/ri with a common x-axis of time was prepared to help understand the variation in the deposit thickness with Thi along the entire duration of the run (Figure 5.6 – 5.8). For all the three runs, it was observed that the value of xd/ri continuously decreased with increasing Thi, confirming that increasing Thi results in a decrease in the deposit thickness.

All the trends observed in this section are in accordance with the inferences drawn from the trends observed in the previous chapter. These predictions confirm that no

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sloughing occurred even when Thi was varied at six step increments; there was a deposit layer present at all instances.

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(a)

(b)

Figure 5.5 Variation(a) in predicted xd/ri with changes in Thi at (a)Tci = WAT-20 and WAT- 10 (Re = 11,000) and at (b) Re = 11,000 and Re = 44,000 (Tci = WAT-20)

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Figure 5.6 Variation in predicted xd/ri with changes in Thi over time at Tci = WAT-20 and Re= 11,000

Figure 5.7 Variation in predicted xd/ri with changes in Thi over time at Tci = WAT-10 and Re = 11,000

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Figure 5.8 Variation in predicted xd/ri with changes in Thi over time at Tci = WAT-20 and Re =45,000

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5.2.3 Effect of Increases in Re

The effect of change in Re on deposit thickness was studied by first calculating the average values of Thi, Tci and Tco for every step change in Re and then using these values to obtain xd from equation 5.1 at each step increase.

Figures 5.9 (a) and (b) show the trends observed for variation in xd/ri as a function of changes in Re for two levels of Thi (WAT+7 and WAT+12) and two levels of Tci (WAT-

20 and WAT-10). From figure 5.9(a), it can be interpreted that the deposit thickness continuously decreases with stepwise increases in Re. As seen from this plot, the decrease in xd/ri is much more pronounced for Tci = WAT-20 as compared to that for Tci = WAT-10.

This could be due to the fact that the deposit thickness would be larger at Tci = WAT-20; hence, a larger decrease is observed when compared to that at Tci = WAT-10. Figure 5.9(b) shows the trends observed for Thi = WAT+12 for two levels of Tci (WAT-20 and WAT-

10). Similar to the previous plot, it shows that xd/ri follows a decreasing trend with increase in Re. The curve for Tci = WAT-10 shows that the value of xd/ri became almost constant with increasing Re which could be due to the lower deposition at high temperature values of Thi and Tci. On comparing the plots in Figures 5.9(a) and (b), it can be observed that xd/ri was much smaller for Thi = WAT+12 when compared to the runs for Thi = WAT+5, which is supportive of our understanding that increasing Thi leads to decrease in wax deposition.

The value of xd/ri did not reach zero at any point thus indicating that there was wax deposit present even at the highest value of Thi used in these experiments.

To obtain a clearer picture of the variation in deposit thickness with changes in Re, a plot of xd/ri and Re against a common time axis was prepared for each of the runs. For these plots, the values of Thi, Tci and Tco for each time was used to calculate the value of

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xd/ri at these instants. Figures 5.11 - 5 .13 show the results for all four runs. These plots show that there is a considerable decrease in the deposit thickness with increasing Re values. The decreasing trend in deposit thickness proves the conclusions drawn for effect of increase in Re. Throughout the duration of each run, it can be seen that the value of xd/ri never reached zero indicating that there was deposit present at all instants.

All the trends observed for change in Re indicate that the deposit thickness only decreased with increases in Re, but there was never complete removal or dislodging of the deposit in any of the runs. That is, the results of this study do not show any sloughing of the deposit as a result of increases in the flow rate or Reynolds number of the wax mixture.

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(a)

(b)

(a) Figure 5.9 Variation in predicted xd/ri with changes in Re at Tci = WAT-20 and WAT-10 for (a) Thi = WAT+7 and (b) Thi = WAT+12

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Figure 5.10 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+7 and Tci = WAT-20

Figure 5.11 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+7 and Tci = WAT-10

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Figure 5.12 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+12 and Tci = WAT-20

Figure 5.13 Variation in predicted xd/ri with changes in Re over time at Thi = WAT+12 and Tci = WAT-10

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Chapter Six: Conclusions and Recommendations

6.1 Conclusions

A bench scale flow loop apparatus was designed and fabricated to investigate the phenomenon of sloughing, i.e., the dislodging and complete or even partial removal of wax deposit from the tube due to variations in thermal and hydrodynamic parameters. A prepared 6% wax-solvent mixture was used to simulate paraffinic crude oils. The first set of deposition experiments were carried out to establish the time required for the system to reach steady state. A set of calibration experiments were carried out at four levels each of the inlet coolant temperature, the inlet wax-mixture temperature and the flow rate of the wax mixture. Another set of sloughing experiments were performed to study the effect on wax deposit due to changes in the inlet coolant temperature, the inlet wax-mixture temperature and the flow rate of the wax mixture. The experiments for changes in Tci were carried out at six levels of Tci and three levels of Re of the wax-mixture. Experiments carried out to study effect of changes in Thi were carried out at six levels of Thi, two levels of Re of the mixture and two levels of Tci. To study effect of flow rate or Re of the mixture, experiments were carried out at six levels of Re, two levels of Thi and two levels of Tci.

The results from the calibration experiments were used to estimate the liquid- deposit interface temperature, Td, and the deposit thermal conductivity, kd. Td was found to be close to the WAT of the wax mixture for all experiments, which confirms the results of

- previous studies reported in the literature. Average kd was estimated to be 0.390.05 W m

1 K-1.

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The results of the calibration experiments were also utilized for studying the effect of various parameters on the wax deposit in the tube. It was concluded that the extent of wax deposition decreased with increases in the inlet coolant temperature (or, corresponding decreases in (WAT – Tc) and with increases in the inlet wax mixture temperature (or, corresponding increases in (Th – WAT). An increase in the flow rate or Reynolds number of the wax mixture also resulted in a decrease of the extent of wax deposition. These results were consistent with the trends previously reported in the literature.

A large set of sloughing experiments carried out to study the effect of Tci on the wax deposit showed that there was a gradual decrease in both Tc and dP with increases in

Tci in all cases. There was no sudden spike in Tc, which would be expected if any sloughing were to take place. A small increase in Tc and a sudden decrease in dP was observed only after Tci was equal to or greater than WAT, due to complete loss of the deposit, which is expected from the definition of WAT. In summary, the trends did not indicate any sudden removal or sloughing of wax deposit as a result of increases in the coolant temperature.

The sloughing experiments carried out to study the effect of Thi on wax deposit showed that Tc was almost constant for lower values of Thi but then increased slightly as

Thi was increased due to reduction in the deposit mass and the deposit thermal resistance, which would also result in an increased rate of heat transfer. A gradual decrease in dP was observed which could be attributed to decreasing deposit thickness with increasing Thi. A layer of deposit was also found on the tube wall at the end of each run. Since there were no

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sudden peaks observed either in Tc or dP values, it can be concluded that increases in Thi did not lead to any sloughing of the wax deposit on the tube surface.

The set of experiments carried out to study the sloughing behaviour as a function of flow rate or Re of the wax mixture showed step increases in both Tc and in dP corresponding to each step increase in Re, which was solely due to increasing flow rate of the mixture. Again, no sharp changes or peaks were observed throughout any of the runs with increases in Re. Visual inspections, at the conclusion of each run, showed a layer of deposit on the tube wall, indicating that the wax deposit was not completely removed even at high values of Re. Hence, increases in the wax mixture flow rate did not produce any sloughing of the wax deposit from the tube wall.

The GC analysis of the deposit samples provided results that were consistent with the observations made for each set of experiments. It was observed that with an increase in the deposition time, the deposit became depleted in the fractions of lighter alkanes (below

C28) and became enriched in the fractions of heavier n-alkanes (above C28). The above changes became intensified with deposition time, indicating the deposit aging effect. The

GC analyses of deposit samples from the calibration experiments supported the conclusions drawn for these experiments. An increase in each of the three parameters, namely the inlet coolant temperature, the inlet wax mixture temperature and the flow rate or Reynolds number of wax-mixture, resulted in an increase in the heavier fraction of deposit wax with each step increase.

In order to further confirm the experimental findings, a steady-state heat transfer model was utilized to predict the deposit thickness for each experimental run. For the case of changes in Tci, the predictions showed that the deposit thickness decreased with increases

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in Tci and the deposit thickness became zero only after Tci was equal to or higher than the

WAT of the mixture. This confirmed the lack of any deposit sloughing throughout the runs.

The decreasing trend in the deposit thickness also supported those observed for Tc and dP.

The deposit thickness predictions for increases in Thi indicated a decreasing trend with increasing values of Thi, but the deposit thickness did not become zero at any instant, thus implying that complete removal of wax deposit did not take place. That is, increasing

Thi did not cause any deposit sloughing.

For the increase in flow rate or Re of the wax mixture, the deposit thickness predictions showed a gradual decrease in the deposit thickness with increasing Re. Again, the deposit thickness never became zero in any of the experiments, run implying that there was deposit present at each instant, even at the highest Re at the end of each experiment.

These results were supported by the trends for Tc and dP.

Overall, on the basis of experimental measurements supported by the predictions for deposit thickness from a steady-state heat transfer model, it is concluded that

‘sloughing’ of wax deposit layer cannot take place due to variations in either the coolant temperature, the wax mixture temperature, or the flow rate of the wax mixture. Any increase in either one of these parameters can only result in a corresponding decrease in the deposit layer thickness, but not complete removal or even partial dislodging of the deposit layer as is implied by the “sloughing” term used in the literature.

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6.2 Recommendations for Future Work

This study has provided a good understanding of the ‘sloughing’ mechanism of wax deposit in pipe flow and the effect of various parameters on it. Sloughing was studied only for 6% wax solvent mixture composition. Experiments could be carried out with other wax concentrations to further generalize the effects of the same three parameters on sloughing.

Since this study only covered the flow rates in the turbulent regime, another study could be conducted wherein the flow of the wax mixture could be varied within the laminar regime and the transition from laminar to turbulent flow regimes. Also, the flow rates much higher than those covered in this study could be investigated, which might reveal some interesting results.

This study focused on investigating the deposit sloughing only under the ‘hot’ flow conditions. The phenomenon of sloughing could be studied during transition from ‘hot’ flow to ‘cold’ flow conditions, since it would entail a wider variation in temperatures and characteristics of the wax deposit. A mathematical model could be developed for transient heat transfer during transition from ‘hot’ flow to ‘cold’ flow which could support the mechanism of wax deposition under these conditions. Since sloughing was studied as a method of wax deposit removal, an interesting study would be to examine the material of the deposition tube. Wax deposition could be studied as a function of tube or pipe roughness. In addition, the effect of a film coating on the pipe surface to eliminate wax deposition could also be studied.

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APPENDIX A: EXPERIMENTAL DATA AND CALCULATIONS

138

Table A.1 Experimental Data for Deposition Experiments

Run Thi Tci (C) Tco (C) Re Deposit Mass Extent of No (C) (kg) Deposition (kg/m2) 1 35.1 8.0 11.0 11000 0.0039 0.458

2 34.9 7.8 11.3 11000 0.0043 0.503

3 34.9 7.8 11.0 11000 0.0043 0.505

4 35.1 7.9 12.0 11000 0.0043 0.506

5 34.9 7.8 12.2 25000 0.0032 0.369

6 34.5 7.9 10.6 45000 0.0022 0.255

7 34.7 12.9 14.9 11000 0.0032 0.379

8 34.8 17.8 20.1 11000 0.0023 0.271

139

Table A.2 Experimental Data for Calibration Experiments with Change in Tci

Run Thi Tci (C) Tco (C) Re Deposit Mass Extent of No (C) (kg) Deposition (kg/m2) 1 35.0 8.0 8.8 11000 0.0043 0.498

2 35.6 13.0 13.6 11000 0.0038 0.449

3 35.1 18.0 18.6 11000 0.0033 0.386

4 35.5 23.0 23.5 11000 0.0027 0.316

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Table A.3 Experimental Data for Calibration Experiments with Change in Thi

Run Thi Tci (C) Tco (C) Re Deposit Mass Extent of No (C) (kg) Deposition (kg/m2) 1 30.8 8.0 8.6 11000 0.0047 0.549

2 32.0 8.0 8.6 11000 0.0045 0.525

3 33.1 7.9 8.7 11000 0.0040 0.472

4 34.0 8.0 8.7 11000 0.0038 0.449

141

Table A. 3 Experimental Data for Calibration Experiments with Change in Re

Run Thi Tci (C) Tco (C) Re Deposit Mass Extent of No (C) (kg) Deposition (kg/m2) 1 35.4 8.0 8.6 11000 0.0041 0.473

2 35.6 8.0 8.6 20000 0.0038 0.448

3 35.9 8.0 8.6 36800 0.0038 0.440

4 35.6 8.0 8.7 43000 0.0028 0.332

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APPENDIX B: RESULTS FROM GC ANALYIS

143

Table B.1 Compositional Analysis of Bulk Wax-Solvent Mixture

Carbon Number Wax-Solvent Mixture Sample 13 1.45 14 54.71 15 26.25 16 7.00 17 0.29 18 0.21 19 0.20 20 0.20 21 0.18 22 0.26 23 0.26 24 0.25 25 0.24 26 0.49 27 0.49 28 0.50 29 0.49 30 0.51 31 0.50 32 0.24 33 0.18 34 0.20 35 0.18 36 0.18 37 0.08 38 0.01 39 0.01 40 0.01 41 0.01 42 0.01 43 0.01 44 0.01 45 0.01 46 0.01 47 0.01 48 0.01 49 0.01 50 0.01

144

Table B.2. Compositional Analysis for Deposition Experiments with Change in Time

Carbon Number Run 1 Run 2 Run 3 Run 4 13 1.64 2.00 2.06 1.99 14 52.81 49.46 51.09 47.12 15 26.95 25.35 26.24 24.33 16 6.23 6.66 6.12 6.04 17 1.01 0.17 0.97 0.17 18 0.14 0.16 0.14 0.16 19 0.13 0.15 0.14 0.15 20 0.13 0.15 0.14 0.16 21 0.12 0.14 0.13 0.14 22 0.12 0.13 0.12 0.14 23 0.11 0.14 0.12 0.25 24 0.11 0.36 0.11 0.41 25 0.37 0.34 0.16 0.40 26 0.4 0.42 0.55 0.56 27 0.5 0.88 0.60 0.73 28 0.72 0.95 0.82 1.62 29 0.92 1.01 0.97 1.16 30 1.00 1.82 1.33 1.99 31 1.01 1.22 1.53 2.18 32 1.02 1.85 1.10 1.89 33 0.47 0.71 0.54 1.23 34 0.48 0.63 0.55 0.96 35 0.16 0.43 0.26 0.65 36 0.03 0.36 0.26 0.23 37 0.02 0.03 0.22 0.17 38 0.03 0.03 0.22 0.20 39 0.02 0.03 0.01 0.17 40 0.02 0.03 0.01 0.17 41 0.02 0.03 0.01 0.12 42 0.02 0.03 0.01 0.03 43 0.02 0.02 0.01 0.02 44 0.02 0.02 0.01 0.02 45 0.02 0.03 0.01 0.03 46 0.02 0.02 0.01 0.02 47 0.02 0.02 0.01 0.02 48 0.01 0.02 0.01 0.02 49 0.02 0.02 0.01 0.02 50 0.02 0.02 0.01 0.02

145

Table B.3 Compositional Analysis of Deposit Sample from Calibration Experiments with Change in Tci Carbon Number Run 1 Run 2 Run 3 Run 4 13 1.47 1.45 1.46 1.38 14 49.15 44.97 44.99 36.07 15 24.76 22.04 22.9 18.01 16 6.05 5.98 5.13 4.97 17 0.21 0.20 0.19 0.19 18 0.14 0.14 0.18 0.13 19 0.13 0.13 0.17 0.12 20 0.13 0.13 0.17 0.12 21 0.12 0.12 0.16 0.11 22 0.12 0.12 0.15 0.11 23 0.11 0.11 0.27 0.1 24 0.11 0.11 0.26 0.1 25 0.48 0.49 0.25 0.09 26 0.47 0.48 0.31 0.93 27 0.89 0.95 0.94 1.03 28 0.99 1.04 1.81 1.97 29 1.04 1.97 1.97 3.85 30 1.95 2.90 2.19 4.49 31 2.00 3.01 3.26 5.57 32 2.08 3.11 3.26 5.76 33 1.51 2.64 2.34 4.27 34 1.21 2.00 2.00 3.63 35 0.58 1.17 0.99 2.27 36 0.42 0.58 0.6 1.23 37 0.36 0.39 0.45 0.66 38 0.03 0.29 0.14 0.49 39 0.02 0.03 0.04 0.22 40 0.02 0.03 0.04 0.22 41 0.02 0.03 0.04 0.22 42 0.02 0.03 0.04 0.2 43 0.02 0.02 0.04 0.02 44 0.02 0.02 0.04 0.02 45 0.02 0.03 0.04 0.03 46 0.02 0.02 0.04 0.02 47 0.02 0.02 0.04 0.02 48 0.01 0.02 0.03 0.02 49 0.02 0.02 0.04 0.02 50 0.01 0.02 0.03 0.02

146

Table B.4 Compositional Analysis of Deposit Sample from Calibration Experiments with Change in Thi Carbon Number Run 1 Run 2 Run 3 Run 4 13 1.46 1.48 1.49 1.48 14 54.69 54.9 51.97 50.69 15 25.03 26.27 27.00 25.95 16 6.95 7.00 7.03 6.98 17 0.24 0.54 0.23 0.26 18 0.14 0.49 0.16 0.14 19 0.13 0.17 0.15 0.13 20 0.14 0.16 0.16 0.13 21 0.12 0.15 0.14 0.12 22 0.12 0.14 0.13 0.12 23 0.11 0.13 0.19 0.11 24 0.13 0.13 0.36 0.11 25 0.34 0.47 0.47 0.12 26 0.40 0.46 0.47 0.38 27 0.49 0.49 0.50 0.39 28 0.54 0.49 0.49 0.47 29 0.59 0.53 0.55 0.65 30 0.54 0.51 0.55 0.47 31 0.50 0.58 0.80 0.41 32 0.32 0.54 0.48 0.37 33 0.24 0.29 0.34 0.22 34 0.27 0.33 0.37 0.25 35 0.24 0.29 0.19 0.22 36 0.04 0.10 0.19 0.22 37 0.02 0.09 0.16 0.02 38 0.02 0.10 0.19 0.02 39 0.02 0.09 0.16 0.02 40 0.02 0.09 0.09 0.02 41 0.02 0.09 0.03 0.02 42 0.02 0.09 0.03 0.02 43 0.02 0.07 0.02 0.02 44 0.02 0.07 0.02 0.02 45 0.02 0.09 0.03 0.02 46 0.02 0.07 0.02 0.02 47 0.02 0.03 0.02 0.02 48 0.01 0.00 0.02 0.01 49 0.02 0.00 0.02 0.02 50 0.01 0.00 0.02 0.01

147

Table B.5 Compositional Analysis of Deposit Sample from Calibration Experiments with Change in Re Carbon Number Run 1 Run 2 Run 3 Run 4 13 1.46 1.46 1.44 1.43 14 53.92 52.93 50.7 47.9 15 26.96 26.91 25.19 23.93 16 7.01 6.15 6.08 5.40 17 0.27 1.04 0.25 0.97 18 0.14 0.18 0.16 0.14 19 0.13 0.15 0.15 0.13 20 0.14 0.15 0.15 0.13 21 0.12 0.14 0.14 0.12 22 0.12 0.13 0.13 0.12 23 0.11 0.13 0.13 0.11 24 0.15 0.12 0.45 0.11 25 0.47 0.46 0.44 0.35 26 0.46 0.50 0.51 0.40 27 0.79 0.87 0.51 0.47 28 0.62 0.94 0.53 0.69 29 0.53 0.98 0.52 0.66 30 0.85 1.01 0.85 0.65 31 0.63 1.01 0.61 0.69 32 0.57 0.79 0.55 0.46 33 0.32 0.56 0.26 0.35 34 0.36 0.47 0.28 0.34 35 0.32 0.41 0.25 0.12 36 0.13 0.24 0.23 0.12 37 0.10 0.19 0.02 0.10 38 0.12 0.23 0.02 0.12 39 0.10 0.19 0.02 0.10 40 0.10 0.19 0.02 0.10 41 0.10 0.10 0.02 0.10 42 0.10 0.10 0.02 0.10 43 0.08 0.09 0.02 0.08 44 0.08 0.09 0.02 0.01 45 0.09 0.10 0.02 0.01 46 0.01 0.02 0.02 0.01 47 0.01 0.01 0.02 0.01 48 0.01 0.01 0.01 0.01 49 0.01 0.01 0.02 0.01 50 0.01 0.01 0.01 0.01

148