ANIGBOGU, IFEOMA VERONICA PG/M.Sc/08/49193 DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY, FACULTY OF PHYSICAL SCIENCES UNIVERSITY OF NIGERIA, NSUKKA.

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1 PRECIPITATION OF ASPHALTENES, QUANTIFICATION OF MALTENES, UV AND FTIR SPECTROSCOPIC STUDIES OF C 7 AND C 5 + C 7 ASPHALTENES FROM 350 O C ATMOSPHERIC RESIDUUM CRUDES. BY ANIGBOGU, IFEOMA VERONICA PG/M.Sc/08/49193 DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY, FACULTY OF PHYSICAL SCIENCES UNIVERSITY OF NIGERIA, NSUKKA. NOVEMBER, 2011

2 i PRECIPITATION OF ASPHALTENES, QUANTIFICATION OF MALTENES, UV AND FTIR SPECTROSCOPIC STUDIES OF C 7 AND C 5 + C 7 ASPHALTENES FROM 350 O C ATMOSPHERIC RESIDUUM CRUDES. BY ANIGBOGU, IFEOMA VERONICA PG/M.Sc/08/49193 DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY, FACULTY OF PHYSICAL SCIENCES UNIVERSITY OF NIGERIA, NSUKKA. NOVEMBER, 2011.

3 ii CERTIFICATION Anigbogu, Ifeoma V. a postgraduate student of the Department of Pure and Industrial Chemistry with registration number, PG/M.Sc/08/49193 has satisfactorily completed the requirements for the course and research work for the award of the degree of Master of Sciecne (M.Sc) in Fossil Fuel (Petroleum and Coal) Chemistry. This research project has been approved for the Department of Pure and Industrial Chemistry, Faculty of Physical Sciences, University of Nigeria, Nsukka. By Prof. C.A. Nwadinigwe Project Supervisor Dr. P.A. Obuasi Head of Department

4 iii DEDICATION I dedicate this work firstly, to the saviour of my life, Jesus Christ, whom by His grace, favour and help kept me alive after the terrible sickness that befell me, and helped me to be able to complete this programme. Even when the going was tough, he encouraged me and taught me that only the tough gets going. Secondly, I dedicate this work to my beloved husband Mr. Emmanuel Anigbogu, who has always been a source of great support and inspiration all through the cause of this programme.

5 iv ACKNOWLEDGEMENTS I wish to acknowledge the assistance of some individuals who have contributed to the success of this work. First and foremost, my appreciation goes to my project supervior Prof. C.A. Nwadinigwe whose advice and helpful suggestion and support have directed the progress of this programme especially this project work from its insception to the conclusion. His instructions, criticisms and contributions greatly improved this work both in scope and in quality. My appreciation also goes to my Head of Department Dr. P.A. Obuasi. I will ever remain grateful to my beloved husband, who is God s gift to me. God will not disappoint us in Jesus name. I appreaciate my father Mwogeoffery Ugwu (Rtd), my sliblings, Obinna Ugwu, Mrs. Ngene Chizoba, Mr. Valentime Ugwu and Ejike for their prayers. My special thanks go to the senior laboratory technician Mr. Cliford Ezeugwu (Food Chemistry Department), Mr. Uba and Mr. Menakaya (Laboratory Attendants), Dr. Parka E. Joshua (Biochemistry Department), who were instrumental to the success of this work. Also to my special friends. Mrs. Ngozi Alumona, Obiageli Egbu, Amara Chukwuneke, Mr. Emmanuel Okon, Mr. Alifa David, Ikenna, Adika, C.C., Madam Gloria, Mr. Oformater, Mrs. Vivian Okonkwo and others. I say thank you. I have learnt so much from you all collectively and individually. I will not forget Jesus Reigns Catholic Charismatic Renewal UNN, a place where I encountered God as God. I express my thanks to the members of the singing ministry. I express my gratitude to all my roommates in 330 Odili (PG) Hall, You all have been like sisters to me. Lastly my special appreaciation goes to my Darling friend and sister Dr. to be Miss Phidelia Waziri who with perseverance carefully typed my work. ANIGBOGU IFEOMA VERONICA

6 v TABLE OF CONTENTS Approval page i Certification ii Dedication iii Acknowledgements iv Abstract v Table of contents vi List of figures x List of tables xi Chapter one 1.0 Introduction Background of the study Types of crude oil Fractions of crude oil Origin of asphaltene from petroleum/crude oil Statement of the Asphaltenes/Resins problem Aims and objectives Scope of the study CHAPTER TWO 2.0 Literature review Occurrence and nature of asphaltenes and resins

7 vi 2.2 Composition of asphaltenes and resins Structure and chemistry of asphaltenes and other heavy organic deposits Asphaltene chemical structure under pyrolysis condition Molecular weight of asphaltene particles Influence of resins constituents on asphaltene constituent Causes of asphaltene problem, asphaltene self-association and micelle / colloid concept Economic effect and relevance / significance of asphaltene precipitation Prevention and remedies of asphaltene precipitation CHAPTER THREE 3.0 Materials and Methods (Methodology) Experimental Methods Materials and Methods Distillation of Each of the Three Crudes Precipitation of Asphaltene, Purification of Asphaltenes and Various Analysis Carried out on the Pure Asphaltenes Precipitate Precipitation of Asphaltenes Purification of Extracted Asphaltenes Fractionation of Maltenes Activation of the Silica Gel Chromatography Procesure

8 vii Extraction of Each of the Saturates, Aromatics and Resins (Maltene) from their various Effluents Physical Methods for Analysing the Asphaltene Fraction Infrared Spectra Analysis The Ultraviolet Visible Spectra Analysis UV Specroscopic Procedure Melting Point Analysis CHAPTER FOUR 4.0 Results and Discussions Results of Bonny Export, Bodo and Mogho Crude oils before and after Distillation at 350 O C Results from Asphaltene Precipitation Comparism of the weight of the Precipitated Asphaltene with Time Using N-heptane and n-pentane + n- heptanes Mixed Solvent Summary of the results of FTIR Spectrophotometric analysis Summary of the results of UV/Visible Spectrophotometric Analysis Results of the chemical physical properties Result of the effect of resins on asphaltene precipitation

9 viii CHAPTER FIVE Conclusion New knowledge arising from this work Literature Citied Appendix

10 ix LIST OF TABLES 2.2a Elemental Composition of Asphaltenes from World Sources [23] b Elemental Composition of Various Asphaltenes [23] c Elemental Composition of Petroleum Resins [10] a Total nc 5, nc 7, nc 9 Asphaltene Content of Crude oil [75] Optimizing Asphaltene Dispersant Dosage in the Adiatic Sea. [8] Physical properties of Bonny Export, Bodo and Mogho crude oils before and after distillation at 350 O C a Composition of Asphaltenes in Bonny Export Crude b Physical Properties of Maltenes (filtrate) from Bonny Export Crude a Composition of the Asphaltenes from Bodo Crude b Physical Properties of Maltenes from Bodo Crude a Composition of the Asphaltenes in Mogho Crude b Physical properties of Maltenes from Mogho Crude a Summary of the results of IR Analysis of Asphaltenes from Single Solvent System (Sample A) a Summary of the results of IR Analysis of Asphaltenes from Mixed Solvent System (Sample B) UV Spectra of the Asphaltene Fractions of Crude Oil

11 x 4.7 Chemical and Physical Properties of Crude oils as obtained from n-heptane single solvent (80mins) Effect of resins on asphaltene precipitation

12 xi LIST OF FIGURES 1: Classification Procedure for Heavy Crude oil Fractions (>350 boiling Fraction) [3,8,13] a: Examples of Some organic Compounds in Petroleum (organic origin of Petroleum) [26] b: Continued Buried of Sediment and Rock layers in Subsiding Basin a: Asphaltene Precipitation and Deposition in Subsec Flowing, near Wellbore Region, Seperators e.t.c. [8] b Deposition and plugging of petroleum flow conduits due to streaming potential generated and sticking of asphaltene particules to the walls [34] (a) Simplified Petroleum Fractionation Method [9] b Continum of Aromatics, Resins and Asphaltenes in Petroleum [37] a Molecular Structure of Asphaltene Proposed for Maya crude (Mexico) by Altamirano, et al IMP Bulletin. [46] b Molecular Struture of Asphaltene Proposed for 510c Residue of venezuelian Crude by Carbognani [46] c Resin fraction with two subgroups (i,ii) a Proposed Asphaltene Struture Model: Condensed Aromatic Cluster Model [ b Proposed Asphaltene Struture based on Bridged Aromatic Model [47] a The Molecular Weight of this Asphaltene [45]

13 xii 2.5b Long Diagram shows that the Asphaltene include the Crude Oil Material Highest in Molecular Weight, Polarity, and/or Aromaticity [13] Schematic Illustration of Archipelago Model of Asphaltene Monomers, Asphaltene Aggregate in absence of Resins, and Asphaltemic Aggregate in presence of Resins [64] a: Formation of Asphaltene Micelles in the Presence of Excess Amounts of Aromatic Solvent [34] b: Asphaltene Flocculation due to Excess Amount of Paraffins in the Solution [34] c: Steric-colloid Formation of Flocculated Asphaltene with Resins [34] Flow chart of the separation scheme of the atmospheric residuum : Flow Chart of the Seperation Scheme of Maltenes a % Weight of Asphaltenes from Bonny Export Crude (Single Solvent) with stirring time b: % Weight of Asphaltenes from Bonny Export Crude (mixed Solvent) with stirring time a: % Weight of Asphaltenes from Bodo Crude (Single Solvent) with stirring time

14 xiii 4.3b: % Weight of Asphaltenes from Bodo Crude (mixed Solvent system) with stirring time a: % Weight of Asphaltenes from Mogho Crude (Single Solvent) with stirring time b: % Weight of Asphaltenes from Mogho Crude (mixed Solvent) with stirring time : Effect of n heptane Single Solvent with stirring time for Bonny, Bodo and Mogho Crudes (mixed graph) : Effect of n pentane + n - heptane mixed Solvents with stirring time for Bonny, Bodo and Mogho Crudes (mixed graph) Bodo and Mogho Crudes (mixed graph) : Barchart of the Weight of Heavy Fractions of each of the Three Crude Oils Studied and their Asphaltene content : Effects of Resin on Asphaltenes Stabilization

15 xiv LIST OF PICTURES 3.1 Weighing balance(model:adventurers) Magnetic stirrer Centrifuging apparatus Oven/Incubator (Model mini/50 Genlab limited Fractionation setup (column chromatography) Water bath with resin + dichloromethane + methanol effluent during evaporation of Bonny Export maltenes UV/Visible Spectrophotometer Melting point analyser

16 xv ABSTRACT Asphaltenes behave like blood cholesterol in that they deposit on the inner walls of crude oil transportation pipes thereby narrowing the internal diameters. This poses great dangers, including pipe bursts. This work aims at removing asphaltenes from light crudes by solvent precipitation. Three different Nigerian crude oils sourced from Bonny Export and Mogho in Rivers State and Bodo in Delta State were studied. The crude oils were first distilled at 350 O C to remove the lighter fractions leaving behind a dead crude known as 350 O C atmospheric residuum which consist mainly of high molecular weight saturates, aromatics, resins and asphaltenes. Asphaltenes were precipitated from each of these atmospheric distillation residues at different stirring time intervals using n-heptane (single solvent) and n-pentane + n-heptane (mixed solvent system). The corresponding yields of asphaltenes were determined for each time duration. It was found that asphaltene precipitation was more in Mogho crude oil for both n-hepane single solvent and n-pentane + n-heptane mixed solvent system and least in Bonny Export crude oil. Physical parameters such as FTIR (Fourier transform infrared spectroscopy), uv-vis spectroscopy and melting point analysis were used to characterize the precipitated asphaltenes while the maltenes (i.e. crude oil minus the asphaltenes: saturates, aromatics, and resins) were fractionated in other to quantify the ratio of aromatics to saturates and resins to asphaltenes as parameters that control the stability of asphaltenes in crude oils. From the results, the FTIR data revealled that the asphaltene fraction of crude oils was made up of both saturated (cyclic aliphatic hydrocarbons etc.) and unsaturated (e.g. substituted aromatic hydrocarbon etc) parts as supported by our uv/vis spectra on the asphaltene precipitates. Also the ratio of aromatics to saturates and resins to asphaltenes was higher in Bonny Export crude and lower in Mogho crude. This indicated that Bonny Export crude has the lowest asphaltene precipitation risk while Mogho crude had the highest asphaltene precipitation risk. Addition of resins (extracted from each of the crudes) to a mixture of 1ml crude + 40ml n-heptane brought about a reduction in asphaltene precipitate for all the crudes. This indicated that resins solubilize asphaltenes in crude oil. On the basis of these findings: it was shown that asphaltenes precipitated more using the mixed solvent system. These findings also showed that asphaltenes precipitate in crude oil, but other constituents of crude oil especially the resins, influence asphaltene precipitation.

17 1 CHAPTER ONE 3.0 INTRODUCTION 1.1 BACKGROUND OF THE STUDY Crude oil is a complex mixture of hydrocarbons and other compounds of varying molecular weight and polarity. [1] [2] Alternatively crude oil can be said to be a collective term used to described a hydrocarbon rich mixture of compounds that is usually found as a subterranean deposit that accumulated over millions of years. The physical and chemical characteristics of crude oil vary widely from one production field to another and even within the same field. [3] TYPES OF CRUDE OILS (i) LIGHT CRUDE AND VERY LIGHT CRUDE OIL: This is liquid petroleum that has low density and that flows freely at room temperature. It has low viscosity, low specific gravity and high API gravity (34 39 O API) due to the presence of a high proportion of light hydrocarbon fractions. It generally has a low wax content. [4a] very light crude is defined with API gravity above 40 O API. [4b] (ii) MEDIUM CRUDE OIL: This is any liquid petroleum with an API gravity between O API. [4b] (iii) HEAVY CRUDE OIL AND EXTRA HEAVY CRUDE OIL: is any type of crude oil which does not flow easily. It is refered to as heavy because its density or specific gravity is higher than that of light crude oil. Heavy crude oil is defined as any petroleum with an API gravity less than 20 [5] Heavy oil is asphaltic and contains asphaltenes and resins. It is heavy (dense and viscous) due to the high ratio of aromatics and napthenes to paraffins and

18 2 high amounts of NSO s (Nitrogen, Sulfur, Oxygen and heavy metals). Heavy oil has a higher percentage of compounds with over 60 carbon atoms and hence a high boiling point and molecular weight. [5] Heavy oil typically contains very little paraffin and may or may not contain high levels of asphaltenes. Extra heavy crude oil is defined with API gravity below 10.0 API (i.e. with density greater than 1000kg/m 3 or, equivalently, a specific gravity greater than 1), with a specific gravity of greater than one, extra heavy crude oil is present as a dense non-aqueous phase liquid when spilled in the environment. [6] FRACTIONS OF CRUDE OIL LIGHTER FRACTIONS: The compound and compound classes present in the lighter fractions of crude, which is typically the fraction that can be recovered by atmospheric distillation, can be identified by chromatographic and spectroscopic techniques. [7] HEAVY FRACTION OF CRUDE OIL: This fraction is classified based on solubility, [7] with maltenes (i.e. saturates, resins and aromatics) being soluble in n-alkanes such as n-heptane [8] and asphaltenes being soluble in benzene/toluene and insoluble in n-alkanes such as n-heptane or n- pentane. [8] This heavy fraction is non-distillable and remain in the residue fuels as the distillable fractions (lighter fractions) are removed. [9] The addition of a low boiling point alkanes example n-pentane, n-heptane or other alkanes to crude oils originates the selective precipitation of the most aromatic and highest molecular weight compounds present in the crude oils. [10][11] The crude oil fractions that precipitates under such conditions is known as asphaltenes. Being a complex mixture of a wide array of different molecular types. [12] The amount and characteristics of the asphaltene constituents in crude oil depends to a greater or lesser extent on the source of the crude oil. [9]

19 3 The asphaltene content of crude oil varies from 0.1% to more than 20% depending on the production field. A convenient laboratory method has been developed to quantify the asphaltene fraction. This technique separates dead oil that has lost its gaseous components, into saturates, aromatics, resins and asphaltenes (SARA) depending on their solubility and polarity, (figure 1.) [8] Atmospheric residue Extract with n heptanes (C 7 : oil = 30:1) Maltenes Adsorbed on silica, elute with Precipitation n-alkane Toluene Toluene/methanol Saturate Aromatics Resins Asphaltenes Figure 1: Classification procedure for heavy crude oil fractions (>360 C boiling fraction) [3],[8],[13] This SARA method is a reasonable first step for categorizing dead oil, it is also a simple procedure that can be performed in many laboratory because of its simplicity, SARA analysis has become a widespread means for comparing oils. However, SARA analysis has several disadvantages that becomes apparent when it is used for purpose beyond its original intent. [8] In addition laboratory methods varies greatly and the yield of asphaltenes varies with the type of n-

20 4 alkane (i.e. precipitant) used, liquid precipitant to oil volume ratio, contact time, pressure and temperature. [8,14,15,16] A single oil could have two or more SARA results depending on precipitant used. [8] A commonly accepted view in petroleum industry is that asphaltenes form micelles which are stabilized by adsorbed resins kept in solution by aromatics. Two key parameters that control the stability of asphaltene micelles in a crude oil are the ratio of aromatics to saturates and that of resins to asphaltenes. When these ratios decrease, asphaltene micelles will flocculate and form larger aggregates. [17][18] This fraction is well known for its tendency to precipitate during production and refining operations causing significant losses to the oil industry every year.[19-20] The asphaltene precipitation depends mainly on the stability of the asphaltenes and stability depends not only on the properties of the asphaltenes fraction but on how good a solvent the rest of the oil is for its asphaltene. As recognized by de Boer et al. (1995), light oils with small amounts of asphaltenes are more likely to cause problems during production than heavy oil with larger amounts of material in the asphaltene fraction. The heavier oil also contains plenty of intermediate components that are good asphaltene solvents whereas the light oil may consist largely of paraffinic materials in which, by definition, asphaltenes have very limited solubility. Asphaltenes in heavier oils can also cause problems if they are destabilized by mixing with another crude oil during transportation or by other steps in oil processing. [12,13] In particular, the characteristics of the disperse phase and the peptizing power of the resins [18,19] are considered fundamental factors for stabilization of asphaltenes in crude oil. [12] Crude oil can be classified by chemical compositon, density, viscosity and distillation characteristics to name a few. [23] The classification system based on composition refers to only

21 5 the hydrocarbon nature of oils namely paraffin (alkanes) naphthenic or cycloalkanes and aromatic hydrocarbons namely Sulphur containing compounds, Nitrogen containing compounds, organomettallic compounds, oxygen containing compounds. [24][25] 1.2 Origin of asphaltene from petroleum/crude oil Crude oil is a naturally occurring substance consisting of organic compounds in the form of gas, liquid, or semisolid. The simplest of these compounds is methane. Figure 1.2(a) Examples of some organic compounds in petroleum Figure 1.2(a) shows some examples of organic compounds in petroleum, from the simplest (methane) to the most complex (asphaltene). Asphaltenes are the most complex and most polar fractions found in crude oil, with more than 36 carbon atoms bound to more than 167 hydrogen atoms, three nitrogen atoms, two oxygen atoms, and two sulphur atoms. [26][27] Semisolid petroleum is tar, which is dominated by larger complex hydrocarbons and asphaltenes (Figure 1.2a). [25] Petroleum formation takes place in sedimentary basins, which are

22 6 areas where the earth crust subsides and sediments accumulate within the resulting depression. Throughout geologic time, the world oceans have expanded and receded over the earth s land surfaces and contributed sediment layers to subsiding sedimentary basins. [26] Development of stagnant water conditions in some of the expanded oceans caused the bottom waters to be depleted in oxygen (anoxic), which allowed portion of the decaying plankton to be preserved as a sediment layer enriched in organic matter. Methane producing microorganisms referred to as methanogens may thrive under certain favorable condition within the organic rich sediment layer during its early burial. There microorganisms consume portions of the organic matter as food source and generate methane as a byproduct. This methane, which is typically the main hydrocarbon in natural gas, has a distinct neutron deficiency in its carbon nuclei which allows microbial natural gas to be readily distinguished from methane generated by thermal processes later in the basin s subsidence history. The microbial methane may bubble up into the overlying sediment layers and escape into the ocean waters or atmosphere. If impermeable sediment layers, called seals, hinder the upward migration of microbial gas, the gas may collect in underlying porous sediments, called reservoirs. Burial of the organic-rich rock layer may continue in some subsiding basins to depths of 6,000 to 18,000 feet, exposing the rocks to temperatures of 150 to 350 F (66 to 177 C) for a few million to tens of billions of years. The organic matter within the organic rich rock layer begins to cook during this period of heating and portions of it thermally decompose into crude oil and natural gas. If the original source of the organic matter is plankton (i.e. algae, bacteria e.t.c) crude oil will be the dorminant petroleum generated with lesser amounts of natural gas generation.

23 7 Petroleum has a lower density, than the water that occupies pores, voids and cracks in the source rock and the overlying rock and sediment layers. This density difference forces the generated petroleum to migrate upwards by buoyancy until sealed reservoirs in the proper configurations serve as traps that concentrate and collect the petroleum Figure 1.2(b). Figure 1.2(b): Continued buried of sediment and rock layers in subsiding basin. In some basins, petroleum may not encounter a trap and continue migrating upwards into overlying water or atmosphere as petroleum seeps. Crude oil that migrates to or near the surface of a basin will lose a considerable amount of its hydrocarbons to evaporation, water washing, and microbial degradation leaving a residual tar enriched in large complex hydrocarbons and asphaltenes. [25] Asphaltene is an important constituents in crude oils. While it is also a major factor that causes difficulties in oil recovery. [28][29] During the evolution and migration of oil reservoirs, the

24 8 asphaltenes may be flocculated or precipitated out from crude oils due to the changes of pressure, temperature and/or the composition of reservoir fluid. [30] Owing to the alteration of ambient conditions, asphaltenes are liable to be precipitated out during oil recovery, transportation and post-processing. It can make oil production more arduous and costly because of the partially plugging in oil well- and pipeline by asphaltenes. It may further decrease recovery efficiency or even stop oil production due to the shutoff of oil pore throat or even of the whole oil well. [30] 1.3 STATEMENT OF THE ASPHALTENES/RESINS PROBLEMS Asphaltenes are best known for the problems they cause as solid deposit that obstruct flow in the production system. [31] In asphaltene self associate and/or precipitate, the self association and precipitation is mediated by other solubility fractions particularly the resins. [32] Hence asphaltenes and their related compounds resins have often been lumped together as residue in crude oil [8] causing reduction in crude oil production as they can block the pores of reservoir rocks and can also plug the wellbore tubing, flowlines, separators, pumps, tanks and other equipment and as a result, causing barrier to the flow of oil as shown schematically below: [33][34]

25 9 Figure 1.3(a): Asphaltene precipitation and deposition in subsea flowlines, near wellbore region, Seperators. e.t.c. [8] Not only do asphaltenes increase fluid viscosity and density, they also have the potentials to derail upstream activities, and can also cause downstream disruptions, such as adhering to hot surfaces in refineries. [8] As mentioned earlier asphaltene precipitation can make oil production more arduous and costly because of the partially plugging in oil well and pipeline by asphaltene. It may further decrease recovery efficiency or even stop oil production due to the shut off of oil pore throat or even of the whole oil well. [30] At reservoir conditions, the adsorption of asphaltene to mineral surfaces causes a reversal in wettability of the reservoir from water wet to oil wet and also results in insitu permeability reductions. Both factors also reduce oil production. Apart from the production loss, the cost of removing precipitated asphaltene from equipment and flowlines can be very expensive and significantly alter the economics of a project. Examples of this cases have been reported in the prinos field, Greece, Hansimessaoud field, Algeria, Ventura Avenue field, California, and other places throughout the world. [35][33]

26 10 Furthermore, flocculation of asphaltene was found to reduce the effectiveness of wax inhibitors, due to the formation of complex asphaltene paraffin solid aggregate. [36] Asphaltene precipitation can cause major problems during the transportation of bitumen and heavy oil. The flow of paraffin diluted bitumen through transportation pipelines and processing equipment can result in deposition of precipitated asphaltenes. This deposition causes higher pumping rates and can lead to a build up of internal pipeline pressure. [37] as shown in figure 1.3b below. Figure 1.3b: Deposition and plugging of petroleum flow conduits due to streaming potentail generated and sticking of asphaltene particles to the walls. [34] Some other examples of problems that arise due to asphaltene flocculation and/or sedimentation are: Destabilization of asphaltene constituent as a result of the change in medium during fuel oil-heavy crude oil blending. Ignition delay and poor combustion (often caused by high content of asphaltene constituent ( 6%) in crude oil) leading to boiler fouling, diminished heat transfer, stack (particulate) emissions, and corrosion. [9] e.t.c. Thus, with all these and other problems that arise as a result of asphaltene precipitation, it can be seen that there is need for predicting the conditions for asphaltene precipitation. 1.4 AIMS AND OBJECTIVES The definition of the non-volatile constituents of petroleum (i.e., the asphaltene constituents, the resin constituents and to some extent, part of the oils fraction, insofar as

27 11 nonvolatile oils occur in residue and other heavy feed stocks) is an operational aid. It is difficult to base such separations on chemical or structural features. This is particularly true for the asphaltene constituents and the resin constituents, for which the separation procedure not only dictates the yield but can also dictate the quality of the fraction. The technique employed also dictates whether or not the asphaltene contains coprecipitated resins. This is based on the general definition that asphaltene constituents are insoluble in n-pentane (or in n-heptane) but resins are soluble in n-pentane (or n-heptane). To date, little or no effort has been made to study asphaltene precipitation from crude oil using mixed n-pentane/n-heptane solvent system. Since the use of different hydrocarbon liquids influences the yield of asphaltenes as well as resins fraction, The objectives of this present work are as follows: To investigate the effect of the pure solvent i.e., n-heptane and also the mixed n- pentane/n-heptane solvent systems, on asphaltene precipitation. To investigate the effect of stirring time on asphaltene precipitate. To fractionate the resulting maltenes obtained after precipitation of asphaltenes and compare their ratios (i.e. of aromatics to saturates and resins to asphaltenes) with the extent of precipitation of asphaltene. Determine the melting point of the asphaltene precipitate obtained. To ascertain the functional group properties (i.e., using IR and UV) of each asphaltene precipitate, in other to elucidate if the compound is truly asphaltene. To determine the role resins play, if any, in asphaltene stability which may help chemists develop better methods for preventing and remediating asphaltene problems.

28 Scope of the Study Three different crude oil samples will be used for this work. The samples will be collected and distilled at 350 C to strip off lighter fractions. For each oil sample (350 0 C atmospheric residuum), asphaltene precipitation reaction will be carried out, keeping crude oil/solvent constant, varying stirring time and also weighing asphaltene yield in each case. Functional group properties of each asphaltene precipitation will be ascertained (IR and UV). Also melting point analysis will be carried out on the asphaltenes precipitated. To fractionate the resulting maltenes obtained after precipitation of asphaltenes and compare their ratios (i.e. aromatics to saturates and resins to asphaltenes ) with the extent of asphaltenes obtained.

29 13 CHAPTER TWO 2.0 Literature Review Crude oils can be fractionated and classified in a number of ways. Standard laboratory methods have been defined for the fractionation of petroleum. The older ASTM D 2006 method and ASTM D-2007 method are no longer in official use but may still find use in private laboratories. Indeed, these methods found such wide use that many modifications have been proposed that are still in use. [32] The overall product of these fractionation methods, which with the ensuing sub-fractionation, provides the representation of petroleum a composite of the four fractions (saturates, aromatics, resins and asphaltenes). [9][2] Fig 2.0(a) below: Feedstock n - heptane Asphaltenes (insoluble) Deasphalted oil percolate through alumina Asphaltenes (soluble in toluene) Carbenes / carboids (insoluble in toluene) Resins (pyridine wash) Aromatic (Toluene wash) Saturates (n heptanes wash) Carbenes soluble in CS 2 Carboids insoluble in CS 2 Figure 2.0 (a): Simplified petroleum fractionation method [9]

30 14 Figure 2.0(a) above shows the four major solubility fractions (i.e. Asphaltenes, resins, aromatics and saturates). However, the heavier components, asphaltenes and their related compounds resins have often been lumped together as residue and deemed unworthy of or too challenging for further examination. [8] Details of the methods used to separate them are markedly different from the other three fractions as they mainly contain paraffins and naphthene therefore, are termed non polar. While, aromatics, resins and asphaltenes form a continuum with increasing polarity, molar mass and heteroatom content (Figure 2.0(b) below. Figure 2.0(b): Continuum of Aromatics, Resins and Asphaltenes in Petroleum [37]

31 15 Asphaltenes are the heaviest and most complex fraction in a crude oil sample, which appears as brown or black solid particles precipitated from a crude oil by using a low boiling point alkane e.g. n-pentane or n-heptane. The asphaltene yield decreases as the carbon number of an alkane increases, while it increases monotonically and finally reaches a plateau if the liquid precipitant to oil volume ratio increases up to for n-pentane and n-heptane, respectively. Some would argue that the n-c 7 asphaltenes are the real asphaltenes, whereas the n-c 5 material is a mixture of asphaltenes and resins [13]. Resins are primarily good arphaltene solvents, and are not known to deposit on their own, but they deposit with asphaltenes. [34],[39] The reasons for the asphaltene deposition (precipitation) can be many factors including variations of temperature, pressure, composition, flow regime, and wall and electrokinetic effect. [34] The precipitation is mediated by other solubility fractions. Therefore it is evident that petroleum is a delicately balanced physical system where the asphaltenes depend on the other fractions for complete mobility and phase stability. [9] Considering that the major barrier in a profitable deposition free oil production scheme is the presence of asphaltene, this literature review focuses on what follows the role of other solubility fractions such as aromatics, saturates and resins in crude oil and most expecially the role resins play if any in solubilizing asphaltene in petroleum fluids. 2.1 Occurrence and Nature of Asphaltenes and Resins Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons and alkanes (i.e., saturated hydrocarbons). [40],[41] Asphaltenes are today widely recognised as soluble chemically altered fragments, of kerogen which migrated out of the source rock for the oil, during oil catagenesis. Asphaltenes

32 16 had been taught to be held in solution in oil by resins (similar structure and chemistry, but smaller). [42] but this (i.e. the role that the resin fraction plays in stabilizing asphaltene) has been a major topic of debate. A contrary view holds that specific interactions between asphaltenes and resins are not required to explain asphaltene stability, [43] that resins are primarily good solvents for asphaltenes and that non vanderwalls forces are primarily responsible for flocculation of asphaltenes. [44] Another reference [45] state that there is no implied genetic relationship between asphaltenes and resins, that resins may polymerize to form asphaltenes and asphaltenes may break down into resins. However, no matter how resins are defined, they still include species that contribute to the overall solvent quality of the oil with respect to its asphaltenes. [13] Asphaltenes appear as brown or black solid particles precipitated from crude by using a low boiling point alkane. [4] The colour of dissolved asphaltene is deep red at very low concentration in benzene as 0.003% makes the solution distinctly yellowish. The colour of crude oils and residues is due to the combined effect of neutral resins and asphalteness. [46]. Heavier, black oil crudes will typically have higher asphaltene content. The black colour of some crude oils, and residues is related to the presence of asphaltenes which are not properly peptized [46],[47] In nature, asphaltenes are hypothesized to be formed as a result of oxidation of neutral resins. On the contrary, the hydrogenation of asphaltic compound products containing neutral resins and asphaltene produces heavy hydrocarbon oils, i.e. neutral resins and asphaltenes are hydrogenated into polycyclic aromatic or hydroaromatic hydrocarbons. They differ, however, from polycyclic aromatic hydrocarbons by presence of oxygen and sulfur in varied amounts. On heating above o c, asphaltenes are not melted but decompose leaving a carbonaceous residue [9] (or carbon and volatile products). [46] While the resin fraction becomes

33 17 quite fluid on heating but often show pronounced brittleness when cold. Being a polar molecule, asphaltenes adsorb to formation surfaces, especially clay. They can oil wet formations, which will increase water flow. With their aromatic ring structure, asphaltenes are not soluble in straight chain alkanes (hexane, heptanes, pentane). They are soluble in aromatic solvents like xylene and toluene. [47] Heavy oils, tar sands and biodegraded oils (as bacteria cannot assimilate asphaltenes, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers enzymatically controlled) contain much higher proportions of asphaltenes than do medium API oils or light oils. Condensate are virtually devoid of asphaltenes. [42] 2.2 Composition of asphaltenes and resins. Asphaltenes constitutents isolated from different sources are remarkably constant in terms of ultimate composition, although careful inspections of the date shows extreme ranges for the composition. Asphaltene constitutents from different sources have never before been compared with any degree of consistency. The composition of the resins fraction can vary considerably and is dependent on the kind of precipitating liquid and on the temperature of the liquid system. [9] Asphaltene and resins consist primarily of carbon, hydrogen, Nitrogen, oxygen, sulphur as well as trace amounts of vanadium and nickel, including condensed polynuclear aromatics and other metallic elements. [10],[42,[47] There are indication which shows that the condensed aromatic nuclei carry alkyl, and alicyclic systems with heteroatoms (that is N, O, and S) scattered throughout in various, aliphatic

34 18 and heterocyclic locations. With the increasing molar mass of the asphaltene fractions, both aromaticity and proportion of the heteroelement increases. [18] The elemental composition of asphaltene constituents isolated by use of excess (greater than 40) volumes of n-pentane as the precipitating medium show that the amounts of carbon and hydrogen usually vary over only a narrow range. These values corresponds to a hydrogen-tocarbon atomic ratio of % (as shown in figure 5 below) [32], although values outside this range are sometimes found. [9] Furthermore, it is still believed that asphaltene constituents, are precipitated from petroleum by hydrocarbons solvents because of this composition, not only because of solubility properties. [9]

35 19 Table 2.2a Elemental composition of asphaltenes from world sources (Speight, 1999) Source Composition (wt %) Atomic ratios C H N O S H/C N/C O/C S/C Canada Iran Iraq Kuwait Mexico Sicily USA Venezuela In contrast to the carbon and hydrogen contents of asphaltenes, notably variations occur in the proportions of the hetero elements, in particular in the propotions of oxygen and sulfur. Oxygen contents vary from 0.3 to 10.3%. On the otherhand, the nitrogen content of the asphaltenes has a somewhat lesser degree of variation ( %). [37] The use of n-heptane as the precipitating medium yields a product that is substantially different from the n-pentane insoluble material as shown below [32]

36 20 Table 2.2b Elemental composition of various asphaltenes Source Solvent medium Composition (wt %) Atomic ratios C H N O S H/C N/C O/C S/C Canada n-pentane n-heptane Iran n-pentane n-heptane Iraq n-pentane n-heptane Kuwait n-pentane n-heptane For example, the hydrogen-to-carbon atomic ratio of the n-heptane precipitate is lower than that of the n-pentane precipitate. This indicates a higher degree of aromaticity in the n- heptane precipitate. Nitrogen - to carbon, oxygen to carbon, and sulfur - to carbon ratios are usually higher in the n-heptane precipitate, indicating higher proportions of the hetero elements in this material. [32][37] Elemental constituents of a suite of petroleum resins isolated by the same procedure, and therefore comparable, [32] show that the proportions of carbon and hydrogen, like those of the asphaltenes, vary over a narrow range: 85± 3% carbon and 10.5 ± 1% hydrogen. The proportions

37 21 of nitrogen (0.5 ± 0.15%) and oxygen (1.0 ± 0.2%) also appear to vary over a narrow range, but the amount of sulfur (0.4 to 5.1%) varies over a much wider range. [9][32] There are notable increases in the H/C ratios of the resins relative to those of the asphaltenes. Indeed, where as the asphaltenes may have in excess of 50% of the total carbon as aromatic carbon, in the resins the proportion of the total carbon occurring as aromatic carbon is significantly lower. [18][48] Presumably this indicates that aromatization is less advanced in the resins than in the asphaltenes. There is also a tendency to decreased proportions of nitrogen, oxygen, and sulfur in the resins relative to the asphaltenes. [37] Table 2.2(c) Elemental composition of petroleum resins [10] Source Composition (wt%) Atomic ratios C H O N S H/C O/C N/C S/C Canada Iraq Italy Trace Kuwait USA Venezuela

38 STRUCTURE AND CHEMISTRY OF ASPHALTENES AND OTHER HEAVY ORGANIC DEPOSITS. Asphaltenes and resins are two of the several, but important, heavy organics present in petroleum fluids. There exact molecular structure are not generally known in a particular oil field and they could vary from well to well. [34][46] The molecular nature of the asphaltene fractions of petroleum and bitumens has been subject to numerous investigations. However, determining the actual structure of the constituents of the asphaltene fractions has proven to be difficult because they are a mixture of many thousands of molecular species. [37] Asphaltenes are lyophilic with respect to aromatics, in which they form highly scattered colloidal solutions. Specifically asphaltenes of low molecular weight are lyophobic with respect to paraffins like pentanes and petroleum crudes. There have been considerable efforts by analytic chemists to characterize the asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous sources. [46] Attempts have been made to describe the total structure of asphaltenes, resins and other heavy fractions based on physical and chemical methods. [34][37][46] Physical methods include IR, NMR, ESR, mass spectrometry, x-ray, ultracentrifugation, electron microscopy, VPO, GPC, e.t.c. Chemical methods involves oxidation, hydrogenation, elemental and pyrolysis GC FID GC MS. The chemical structures of asphaltenes, are difficult to ascertain due to the complex nature of the asphaltenes. [34][42][46] Nevertheless, the various investigations have brought to light some significant facts about asphaltene structure. [37] It is undisputed that the asphaltenes are composed mainly of polyaromatic carbon i.e., polycondensed aromatic benzene units with oxygen, nitrogen and sulfur, (NSO) compound

39 23 combined with minor amounts of a series of heavy metals, particularly vanadium and nickel which occur in porphyrin structures. Further more, asphaltene rotational diffusion measurements show that small PAH (polycyclic aromatic hydrocarbon), chromophores (blue fluoreseing) are in small asphaltene molecules while big PAH chromophores (red fluoreseing) are in big molecules. This implies that there is only one fused polycyclic aromatic hydrocarbon (PAH) ring system per molecule. [42] The various figures below shows some of the representative structures of asphaltenes: Figure 2.3a: molecular structure of asphaltene proposed for Maya crude (Mexico) by Altamirano, et al IMP Bulletin, 1986 [46]

40 24 Figure 2.3b: Molecular structure of asphaltene proposed for 510c residue of Venezuelan crude by carbognani [INTEVEP S.A Tech. Rept., 1992) [46] Petroleum asphaltene have a varied distribution of heteroatom (N, O, S) functionality. Nitrogen exist as varied heterocyclic types but the more conventional primary, secondary and tertiary aromatic amines have not been established as being present in petroleum asphaltenes. [50] There are also reports in which the organic nitrogen has been defined in terms of basic and nonbasic types. [37] Spectroscopic investigations suggest that carbazoles occur in asphaltenes, which supports, earlier mass spectroscopic evidence for the occurrence of carbazole nitrogen in asphaltenes. [37] The application of x-ray absorption near-edge structures (XANES) spectroscopy to the study of asphaltenes has led to the conclusion that a large portion of the nitrogen is present in aromatic systems, but in pyrrolic rather than pyridinic form. [51] Other studies have brought to light the occurrence of four ring aromatic nitrogen species in petroleum. Evidence for the presence and nature of oxygen functions in asphaltenes has been derived from infrared spectroscopic examination of the products after interaction of the asphaltenes with

41 25 acetic anhydride. Thus, when asphaltenes are, heated with acetic anhydride in the presence of pyridine, the infrared spectrum of the product exhibits prominent absorptions at 1680, 1730, and 1760cm -1. These observations suggest acetylation of free and hysdro-bonded phenolic hydroxyl groups present in asphaltenes. [50][52] Oxygen has been identified in carboxylic, phenolic and ketonic locations but is not usually regarded as being located primarily in heteroaromatic ring systems. [37] Sulfur occurs as benzothiophenes. More highly condensed thiophene types may also exist but are precluded from identification by low volatility. Other forms of sulfur that occur in asphaltenes include the alkyl alkyl sulfides, alkyl aryl sulfides and aryl aryl sulpfides. [37] Nickel and vanadium occur as porphyrins but whether or not these are an integral part of asphaltene structure is not known. Some of the porphyrins can be isolated as a separate stream from petroleum. In accordance with the Nuclear Magnetic Resonance (NMR) data and results of chemical analysis, attempts have been made to describe the total structure of asphaltenes. [37] Strausz et al identified a host of structural units in Alberta asphaltenes from detailed chemical and degradation studies. He also showed that the extent of aromatic condensation is low and that highly condensed pericyclic aromatic structures are present in very low concentrations. From his work he concluded that petroleum asphaltenes were mainly derived through the catalytic cyclization, aromatization and condensation of n alkanoic (probably fatty acids) precursors. He came up with a hypothetical asphaltene molecule consisting of large aromatic clusters. [54] It must be remembered that asphaltene constituents are a solubility class and, as such may be an accumulation of life (literally) thousands of structural entities. Hence caution is advised

42 26 against combining a range of identified products into one (albeit hypothetical) structure. An often ignored but extremely important aspect of asphaltene chemistry and physics, is the micelle structure, which represents the means by which asphaltene constituents exists in crude oil. The large variety of functional groups and heteroatom content in the asphaltenes indicates that asphaltene molecules have the potential to form links with other similar molecules in a number of ways. Their links may be formed through acid-base interactions, aromatics (П-П) stacking, hydrogen bonding, dipole dipole interactions or even weak van der waals interactions. However, П-П bonding is considered the prevalent theory. [55] Investigations have shown that a variety of hydrocarbon types and functional types occur in resin fractions. [18][32][56][57] In addition, the resin constituents contain a variety of functional groups, including thiophene, benzothiophene and dibenzothiophene systems, hydrogen bonded hydroxyl groups, pyrrole (and indole) N H functions, ester functions, acid functions, carbonyl (ketone or quinine) functions, and sulfur oxygen functions. [9][52] Figure 2.3c below shows a representation of a resin structure. Figure 2.3c: Resin fraction with two subgroups (i, ii)

43 Asphaltene chemical structure under pyrolysis condition Here, two different views of the asphaltene structure have been proposed. The first, following Yen, T,F [47] who assumes extensive condensation of the aromatic rings into large sheets. These sheets are assumed to be soluble, because of the saturated rings and side chains around the molecular periphery as shown in figure 2.4a below Figure 2.4a: Proposed asphaltene structural model: condensed aromatic cluster model [47] This type of structure, with lower molecular weight, has been proposed by groenzin and Mullins, [58] on the basis of spectroscopic studies. In a complex mixture, structure such as this cannot be left out, particularly when the fraction of aromatic carbon approaches 70%, however, such a chemical structure cannot account for the average amount of volatile product evolution under the pyrolysis conditions from a range of asphaltenes.

44 28 A very different structural organization, illustrated in figure 2.4b below was proposed by Speight and by Strausz and co workers (e.g. Murgich et al) on the basis of pyrolytic and selective oxidation studies to determine the building blocks of asphaltenes. Figure 2.4b Proposed asphaltene structure based on bridged aromatic model [47] A structure of the type in figure 2.4a cannot give significant mass yields of volatile products in a pyrolysis experiment. The side chains would crack off readily, and the naphthenic rings then would undergo a combination of dehydrogenation (to form aromatics) and cracking

45 29 (to give mainly light ends). On the contrary, a structure of the type in figure 2.4b can give a wide range of product sizes, from methane to toluene insoluble carbon residue, depending on the balance between cracking, product release, and molecular rearrangements. Data on the nature of product from the cracking of asphaltenes give further support to the diversity of asphaltenes from different crude oils and indicate that the chemical structures in asphaltenes must be consistent with the evolution of a significant yield of volatile products during pyrolysis. [59] Chemical characterization of the products from the pyrolysis of asphaltenes depends strongly on the analytical methods selected and, possibly on the chemistry of the asphaltene selected. When the cracked products are analysed by gas chromatography (GC), the dominant components in the cracked products are n alkanes and n- alkenes for a wide range of asphaltenes. For example, Arkok et al. [33] used curic point pyrolysis to analyse products from asphaltenes from an Arabian crude. Paraffins, Olefins, and aromatics were identified by gas chromatography mass spectroscopy (GC-MS), but only up to C 21. In this range, n alkane and n alkene products dominated. [60][47] 2.5 MOLECULAR WEIGHT OF ASPHALTENE AND RESIN PARTICLES The physical and physico-chemical properties of asphaltenes are different from those of neutral resins. The reported molecular weight of asphaltenes varies considerably depending upon the method and conditions of measurement. These methods include ultracentrifuge, vapour pressure Osmometry (VPO), electron microscope, solution viscometry, cryoscopic methods, e.t.c. Reported molecular weight from ultracentrifuge and electron microscope studies are high. To the contrary, those from solution viscometry and cryoscopic methods are low. The prevalent method for determining asphaltene molecular weights has become vapour pressure 0smometry

46 30 (VPO). However, the value of the molecular weight from VPO must be weighed carefully, since in general, the measured value of the molecular weight is a function of temperature, the solvent molecular properties. Asphaltene particles can assume various forms when mixed with other molecules depending on the relative sizes and polarities of the particles present. The molecules of asphaltene constituents span a wide range from a few hundred to several million leading to speculation about self-association. [9][34][46] Figure 2.5a: The molecular weight of this asphaltene is 7819 [45] Asphaltene molecular weights are variable because of the tendency of the asphaltene constituents to associate even in dilute solution in nonpolar solvents. However, data obtained using highly polar solvents indicate that the molecular weights, in solvents that prevent association, usually fall in the range 2000 ± 500. [9][55] It should be noted that although the results with asphaltene constituents available from several crude oils suggest that molecular weight varies with the dielectric constant of the solvent, there may be other factors which may in part also be responsible for this phenomenon. The final phenomenon that influences the molecular weight of

47 31 the asphaltene is the relative polarity of the solvent used in the precipitation technique. Figure 2.5b below. n-c 5 asphaltene Molecular weight Polarity = N,S,O Aromaticity = n-c 7 asphaltene Polarity and aromaticity Figure 2.5b: Long diagram shows that the asphaltenes include the crude oil material highest in molecular weight, polarity and/or aromaticity. [13] Both polarity and molecular weight of aspahltene constituents in a solvent define the solubility boundaries and explains conceptually how asphaltene constituents are precipitated from the mixture in crude oils that can be considered a type of continuum of molecular weight and polarities. Also, Auflem 2002 observed that the molecular weight, polarity and aromaticity of precipitated asphaltene generally decrease with increasing carbon number of n-alkane precipitant. He compared the wt % of Asphaltene precipitates of the n-c 5, n-c 7 and nc 9 fractions of crude oil.

48 32 Table 2.5a: Total nc 5, nc 7, nc 9 asphaltene content of crude oil [75] Paraffin solvent Wt % Asphaltene n-pentane (C 5 ) ± n- heptane (C 7 ) ± n-nonane (C 9 ) This is an additional confirmation that asphaltene precipitation decreases with increase in carbon chain length of precipitating solvent. The molecular weights of resin fractions in benzene are substantially lower than the molecular weights of the corresponding asphaltenes in benzene. Compared to the molecular weights of the asphaltenes, the molecular weight of the resins do not vary, except for the limits of experimental error, with the nature of the solvent or the temperature of the determination indicating that there is no association in non polar solvents such as benzene. The molecular weights of resin fractions, as determined by various methods, are true molecular weights and that forces that result in intermolecular association contribute very little, if anything to their magnitude. [9] One area of investigation that has shed some light on the behaviour of resins during refining is the construction of molecular weight and polarity maps based on gel permeation chromatographic data. [61]

49 INFLUENCE OF RESIN CONSTITUENTS ON ASPHALTENE CONSTITUENTS The behaviour of asphaltene in petroleum has been complicated by another solubility class called the resins [9] which have similar structure and chemistry like asphaltene. [42] There is evidence that the structural aspects of the constituents of the resin fraction may differ very little from those of the corresponding asphaltene fraction, the main difference being the proportion of aromatic carbon within each fraction. [18][32][62] It has also been postulated that resin constituents and asphaltene constituents are small fragments of kerogen [63] or atleast have the same origins as the kerogen and therefore, a relationship might be anticipated. The analogy is to lock and key mechanism in which the asphaltene constituent and resin constituents with similar structural features form a bonding union. [9]][42] As mentioned earlier, resins are structurally very similar to asphaltenes but have a higher hydrocarbon ratio and lower heteroatom content, polarity and molar mass. Hence, the number of links they can form through hydrogen bonding, aromatic stacking or acid base interactions is lower than those formed by asphaltenes. [37] It has been suggested that resins contributes to the enhanced solubility of asphaltenes in crude oil by solvating the polar and aromatic portions of the asphaltenic molecules and aggregates. The solubility of asphaltenes in crude oil is mediated largely by resin salvation and these resins play a critical role in precipitation, and emulsion stabilization phenomena [64] Asphaltenes may be dispersed in the crude oils by the action of resins. The polar resin molecules may form micelles with asphaltene molecules as the nucleus. Figure 2.6

50 34 Figure 2.6: Schematic illustration of archipelago model of asphaltene monomers, asphaltene aggregate in absence of resins, and asphaltenic aggregate in presence of resins. [64] (a) In a resin poor environment micelles may form from multiple asphaltene molecules. (b) As a result of these physico-chemical shifts the chemistry of asphaltenes is extremely difficult to establish as it changes with the composition of the crude oil. [45] In addition when resins and asphaltenes are present together, resin-asphaltene interactions appear to be preferred over asphaltene asphaltene interactions, resin resins interactions appear to be inconsequential in petroleum. Numerous analytical techniques have been employed to work on asphaltene and oil fraction of the crude oil, while only few studies to determine the role resins play in asphaltene stability has only been briefly documented in the literature. For instance, Chang and Fogler [65] studied the interactions between asphaltenes and resins. In their study, two types of oil soluble polymers, dodecylphenolic resin and poly (octadecene maleic anhydride)

51 35 were synthesized and used to prevent asphaltenes from flocculating in heptanes media through the acid-base interactions with asphaltenes. The results indicated that there polymers could associate with asphaltenes to either inhibit or delay the growth of asphaltene aggregates in alkane media. However, multiple polar groups on a polymer molecule make it possible to associate with more than one asphaltene molecule, resulting in hetero coagulation between asphaltenes and polymers. It was found that the size of the asphaltenepolymer aggregates was strongly affected by the polymer-to-asphaltene weight ratio. At low polymer-to-asphaltene weight ratios, asphaltenes were found to flocculate among themselves and with polymers until the flocs precipitated out of solution. On the other hand, at high polymer-to-asphaltene weight ratios, small asphaltene polymer aggregates formed that remained fairly stable in solution. Moschopedia and Speight showed that dilute solutions ( % w/w) of Athabasca asphaltenes in a variety of non-polar organic solvents exhibit the free hydroxyl absorption band (c. 3585cm -1 ) in the infrared. At higher concentration (> 1% w/w) this band becomes less distinquishable, with concurrent onset of the hydrogen bonded hydroxyl absorption (c cm -1 ). Upon addition of a dilute solution (0.1 1%) of the corresponding resins to the asphaltene solutions, the free hydroxyl absorption was reduced markedly or disappeared, indicating the occurrence of intermolecular hydrogen bonding between the asphaltenes and resins. Therefore, hydrogen bonding may be one of the mechanism by which resin-asphaltene interactions are achieved. Also resin asphaltene interactions appear to be stronger than asphaltene asphaltene interactions. Thus, in petroleums and bitumen s it is believed that asphaltenes exist not as agglomerations but as single entities that are dispersed by resins. [50]

52 36 In a recent work by Murgich et al (1999), the conformation of lowest energy of an asphaltene molecule of the Athabasca sand oil was calculated through molecular mechanics. Molecular aggregates formed from the asphaltene with nine resins from the same oil, in an n- octane and toluene medium were studied. The resins showed higher affinities for the asphaltene than toluene and n-octane and also exhibited a noticeable selectivity for some of the external sites of the asphaltene. This showed that this selectivity depended on the structural fit between the resins and the site of the asphaltene. The selectivity explains why resins of one oil may not solubilize asphaltenes from other crudes. [66] 2.7 Causes of asphaltene problem, asphaltene self-association and micelle/colloid concept The mere presence of asphaltenes in crude oil does not portend asphaltene-related production problems. [8] As mentioned earlier, what is important is the stability of those asphaltene and stability depends not only on the properties of the asphaltene fraction, but on how good a solvent the rest of the oil is for its asphaltenes. As recognized by de Boer et al (1995), light oils with small amounts of asphaltenes are more likely to cause problems during production than heavy oil with large amount of material in the asphaltene fraction. [8][9][13] The heavier oils also contains plenty of intermediate components that are good asphaltene solvents whereas the light oil may consist largely of paraffin materials in which, by definition, asphaltene have very limited solubility. Asphaltenes in heavier oil can also cause problems if they are destabilized by mixing with another crude oil during transportation or by other steps in oil processing.[ 13] The problem appears to be that asphaltene self associate and form aggregates. [37]

53 37 Measurement of asphaltene molar mass was the first indication of asphaltene self association [37] Leon et al performed surface tension and stability measurements to study the self association behaviour of two different association samples, one from a stable crude oil (non-precipitation) and the other from an unstable crude oil were characterized by high aromaticity. Low hydrogen content, and high condensation of the aromatic rings. Asphaltenes from stable crude oils showed low aromaticity, high hydrogen content, and low condensation of their aromatic rings. They showed that these structural and compositional characteristics of the asphaltenes strongly influence their self-association behaviour. They found that asphaltenes from unstable oil begin to aggregate at lower concentrations than asphaltenes from stable oils. Self association appears to be related to a high content of condensed aromatics, which supports a П П bonding mechanism. However, the role of heteroatoms in asphaltene self association was not investigated by this group of researchers. [67] As mentioned earlier, asphaltene self-associate and other constituents especially resins, influence the association. The associated asphaltenes can be considered as micelles, colloidal particles and / or macromolecules. [37] An early hypothesis of the physical structure of petroleum indicated that asphaltenes are the centre of micelles or colloids formed by association or possibly adsorption of part of the maltenes (i.e., resins) onto the surfaces or into the interiors of the asphaltene aggregates. [68] It is widely accepted that at low concentrations asphaltene appear as non associated molecules (monomers), but with changes in temperature, pressure, or concentration, monomers associate to form aggregates or micelles. At higher concentrations, asphaltenes aggregate and

54 38 form non dissolved larger particles that at a later stage agglomerate and precipitate, forming the undesirable deposits. [17] The term micelle, colloid and aggregate are often used interchangeably in the literature. A micelle is an aggregate that remains constant in size and aggregation number for a given set of environmental constraints. The concentration at which asphaltene molecules start to aggregate is called the critical micelle concentration (CMC). In the micellar view of asphaltenes, asphaltene-monomers form micelles above a CMC. Researchers have focused on identifying a CMC with interfacial tension measurements [69][70] However, Yarranton et al (2000) demonstrated that asphaltene self-association occurs in the absence of any evidence of micelle formation. [71] Recent work by Alboundoware et al (2001) suggested that apparent asphaltene CMC s may result simply from a change in asphaltene molar mass, without involving the micelle model. Hence, the micelle model is not supported by strong experimental evidence. [72] A better supported model of asphaltene structure is the colloidal model. According to the colloidal view (Leontaritis and Mansoori, 1998), a crude oil is composed of asphaltene molecules (colloids with their surface covered by resin molecules) suspended in the crude oil. Figure 2.6 above. The adsorbed resins prevent aggregates and disperse the asphaltene. The colloids can aggregate upon a change in the system temperature, pressure, concentration and composition that causes resins to desorb from the asphaltenes. The colloidal view is consistent with small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) evidence of asphaltene aggregate in the nanometer size range. The colloidal model is the prevalent view of asphaltenes in crude oils. [64][73]

55 39 According to this alternative school of thought, asphaltenes exist as free molecules in a non-ideal solution (Hirschberg et al,). Hirschberg et al assumed that pure asphaltenes aggregate by a linear polymerization process. The asphaltene monomer they considered corresponded to asphaltene sheet defined by Yen. They proposed that in crude oil the polymerization is blocked (reduced) by the association of asphaltenes with similar but less polar hetero-components, the resins. [74] The greatest difference between the polymer/macromoleculer view and micelle/colloidal view of asphaltene is the fact that the latter considers asphaltene aggregates to be solid particles. There is no convincing evidence to explain which if any of the views correctly describes the nature of the asphaltenes. [37] The different views of an asphaltene aggregate have led to two types of asphaltene solubility models: the continous thermodynamic models and colloidal models [37] (i) The continous thermodynamic models: this is based on the assumption that asphaltene precipitation process is thermodynamically reversible. This may happen since it is assumed that the asphaltene particles can be dispersed and stabilized in the oil. The complete dissolution of asphaltenes in some organic solvents such as toluene supports this assumption. (ii) Thermodynamic colloidal model: this approach is based on the assumption that asphaltenes are solid particles colloidally suspended in crude oil. Asphaltene particles may undergo aggregation to form larger flocculation under Vander waals attraction forces. This concept is based mainly on titration experiments,, which demonstrate that once the adsorption equilibrium of resins between solid (asphaltene)

56 40 and liquid phases is disturbed by adding paraffin solvents, the asphaltene particles flocculate irreversibly. [75] (iii) Reversibility of asphaltene aggregation: Hirseberg et al assumed that aggregation was reversible, but probably very slow, Joshi et al found the precipitation from a live crude oil to be reversible in a matter of minutes, except for a subtle irreversibility observed for the first depressurization of crude oil. Hammani et al also found that the aggregation was generally reversible, but that the kinetics of the reticulation varied significantly depending on the physical state of the system. Peramary et al reported differences in the reversibility of solvent and temperature induced aggregation. [75] Finally, whether the asphaltene particles are dissolved in crude oil, in steric colloidal state or in micellar form, depends to a large extent, on the presence of the other particles (parafins, aromatics, resins e.t.c) in the crude oil. [34] Various investigators, have established the existence of asphaltene micelles when an excess of aromatic hydrocarbons is present in a crude oil as shown below + + Figure 2.7a: formation of asphaltene micelles in the presence of excess amounts of aromatic solvents. [34]

57 41 The figure above shows three forms of asphaltene micelle and this indicates the fact that in the presence of excess amounts of aromatic hydrocarbon, asphaltene particles cannot flocculate but self associate, and form micelle. [34] In a petroleum fluid, due to excess amounts of paraffins in the solution, small asphaltene particles can be dissolved while relatively large particles of asphaltene may flocculate out of the solution and then form steric colloids. Flocculation of asphaltene in paraffinic crude oil are known to be irreversible, because of their large size and their adsorption affinity to solid surfaces. Figure 2.7b : Asphaltene flocculation due to excess amount of paraffins in the solution. [34] Asphaltene and its flocculates are said to be surface active agents. However, if there is enough resin in the solution so that they can cover the surface of the asphaltene particles by adsorption, asphaltene steric colloids are formed. Figure 2.7c below.

58 42 Figure 2.7c: Steric colloid formation of flocculated asphaltenes with resins. [34] 2.8 ECONOMIC EFFECT AND RELEVANCE / SIGNIFICANCE OF ASPHALTENE PRECIPITATION Heavy organic deposition during oil production and processing is a very serious problems in many areas including Venezuela, the Persian Gulf, the Adriatic Sea and the Gulf of Mexico and other areas throughout the world; [8][34] causing several undersea pipeline plugging with substantial economic loss to the oil production operation. [76] In one example from eastern Venezuala, severe asphaltene deposition problems caused a high volume production well to plug within seven months of treatment. [77] Several cleaning methods has been attempted, including physically scraping the wellbore and injecting xylene down the tubing. Each cleaning event cost approximately US $50,000 and two days of shut-in production. After squeeze treatment (2.9) with activator and inhibitor, the oil production rate increased and frequency of well cleaning decreased to every eight months. The combination of increased production and less frequent cleaning generated an annualized gain of 60,882 barrels (9,674m 3 ), and a return on investment of more than 3,000%. [8] Also in the prinos field in North Aegean Sea, there were wells that, especially at the start of production, would completely cease flowing in a matter of few days after an initial production rate of up to 3,000 BPD. [34] Heavy organic deposition in the North Sea

59 43 and in the Gulf of Mexico oil fields, in recent years have caused several under sea pipeline plugging with substantial economic loss to oil production, operation. [76] The economic implications of this problems were tremendous considering the fact that a problem well workover cost could get as high as a quarter of a million dollars. [32] Although asphaltenes are a major concern in production operations, because of their role in emulsion stabilization and fouling, this fraction of crude oil is also important [47] in the following ways: The conversion of vacuum residues by processes such as coking, catalytic cracking and hydrogenation. [32] Asphaltene in the form of distillation products from oil refineries are used as tar mats on roads. [42] Asphaltene materials are used for water proofing and roofing. [42] 2.9 Prevention and remedies of asphaltene precipitation Asphaltenes can deposit anywhere in the production system, but perhaps the most damaging place is in the near wellbore region where asphaltene blocked pores are difficult to access for remediation. [8] Flocculation and deposition of asphaltenes can be controlled through better knowledge of the mechanisms that cause its flocculation in the first place. [34] It can also be controlled using various methods such as:

60 44 1. Production techniques 2. Chemical treating techniques. 1. Production techniques includes: i. Reduction of shear ii. iii. iv. Elimination of incompatible materials from asphaltic crude streams Minimization of pressure drops in the production facility and Minimization of mixing of lean feed stock liquids into asphaltic crude streams 2. Chemical treating techniques i. Solvents ii. iii. Dispersants/solvents Oil/dispersant/solvents The dispersant/solvent approach is used for removing asphaltenes from formation minerals. Continuous treating may be required to inhibit asphaltene deposition in the tubing. Batch treatments are common for dehydration equipment and tank bottoms. There are also asphaltenes precipitation inhibitors that can be used by continuous treatment or squeeze treatments. [34][42] Conventional asphaltene flocculation inhibitor treatments involve either periodic intervention with solvent soaks or continuous injection of chemicals into wellbore. These methods are effective at presenting agglomeration and deposition of asphaltenes in flowlines and tubular, but they do not protect the producing formation, because the chemicals interact with the oil after it has left the formation, potentially leaving asphaltenes behind. An improved method developed by Nalco Energy Services adds chemicals to the crude oil while it is still in the formation. [78] The method entails squeezing as asphaltene deposition inhibitor into the formation to stabilize the asphaltenes before flocculation occurs. However,

61 45 tests have shown that squeezing inhibitors alone does not, produce long-term benefits; formations do not absorb inhibitors adequately, allowing inhibitors to be quickly released from the formation as oil is produced. Pretreating the formation with an activator chemical enhances absorbtion of the inhibitor into the formation without changing formation wettablity. The general squeeze procedure includes cleaning out and flowing back the well, pumping in activator, a spacer of crude oil, inhibitor, and then more crude oil, and shutting in the well for 12 to 24 hours before resuming production. [77] The activator prepares the formation and reacts with the inhibitor to make a complex that remains in place for a prolonged period as the well produces oil. [8] Asphaltene dispersants are substitutes for the natural resins and works much in the same way as resins. That is, dispersants will keep the asphaltenes well dispersed (peptized) to prevent their flocculation / aggregation. Dispersants will also clean up sludge in the fuel system and they have the ability to adhere to surface of materials that are insoluble in the oil and convert them into stable colloidal suspensions. [27] Laboratory analysis of fluid samples indicated that asphaltene deposition could be controlled only by continuous downhole injection of asphaltene dispersant. The appropriate treatment program was designed and initiated with the desired results. Once a successful treatment program was underway, additional laboratory work on samples collected as part of a monitoring program helped the operator optimize dispersant dosage. It was clear from surface samples analysis that as dosage increased, the volume of

62 46 stable asphaltene dispersed in the crude increased. This indicated that fewer asphaltenes were available to deposit in the well. Asphaltene deposition Condition of crude 0 to 1% Crude strongly stabilized; dosage reduction indicated 1 to 2% Crude well stabilized; treatment adequate; no dosage change indicated. 2 to 3.5% Crude not perfectly stabilized; small increase in dosage indicated > 3.5% Crude not stabilized; insufficient dosage Table 2.9: Optimizing asphaltene dispersant dosage in the Adriatic Sea. [8] The volume of asphaltene deposition decreased as dispersant dosage increased. However, over treatment with dispersant increases cost. Optimization requires a compromise that allows a tolerable level that allowed deposition of only 1% to 2% of the asphaltene volume enabled the wells to operate for several years without asphaltene deposition problems. [77] A treatment level that optimized cost and sufficiently stabilized the asphaltene was shown to provide a protection level that was 98% to 100% effective. Continuous treatment at this level has enabled the wells to operate for several years without any plugging problems. [8] Cleaning the pipeline of asphaltene required a technique that would be environmentally acceptable, cost effective and successful in the complex pipeline geometry. [79]

63 47 CHAPTER THREE MATERIALS AND METHOD (METHODOLOGY) 3.0 Sample Collection In this present work, a total of three Nigerian crude oils were studied. The crude oils were sourced from Bodo in Delta State, Bonny Export and Mogho (Port Harcourt) in Rivers State as shown in the map in appendix Sample Treatment The three crude oils were employed for precipitation of asphaltenes using pure n-heptane solvent and n-pentane + n-heptane mixed solvent with respect to stirring time, in a solvent to oil volume ratio of 40:1 and also for fractionation of maltenes. Detailed results of this work are provided in chapter four. 3.2 Materials and reagents All the analysis in this work was performed using high purity solvents and chemicals that were of analytical reagent grade. For distillation of crude oil at 350 c an appropriately calibrated thermometer(360 0 c), a heating mantle (that heats more than360 c) with thermal regulator, cotton wool to lag exposed portion of flask, a round bottom flask (250ml), antibumping chip (porcelane chips) to avoid spilling of the crude, distillation chamber, water for cooling the distillation chamber, analytical balance (weighing with an accuracy of 0.001g and of maximum capacity 310g), reagent bottle (for storing crude oil before distillation and atmospheric residuum after distillation), conical flask, a stopper.

64 48 For asphaltene precipitation and Fractionation of the maltenes, reagent grade solvents and materials used were n-pentane (95% purity, sigma Aldrich laborchemical), n-heptane (99.5% FSA Laboratory supplies meadow road, Lough borough, LEII ORG, England), Toluene (99.96% purity BDH laboratory supplies, chemical Ltd poole BH15 17D, England), methanol (99% purity Fisher Chemicals), Silica gel (99.5% purity, BDH Laboratory supplies, chemical ltd poole BH D England). Dichloromethane (99% purity, BDH Laboratory supplies), Magnetic Stirrer (constant temperature magnetic stirrer 78 HW - 1). Spatula, weighing balance (capable of weighing with an accuracy of 0.001g and a maximum capacity of 310g), filter paper (Whatman number 2 with pore size of 2µm), graduated cylinders, Centrifuge (model no. 80-2B with maximum speed 4000rpm serviced and maintained by Finlab Nig. Limited), beakers, petridish, oven (model-mini/50 Genlab Limited), 50ml standard laboratory burrette with a glass stopper. For melting point analysis, the material and apparatus used were capillary tubes and the melting point analyser (electrothermal melting point analyser, melts with a maximum capacity of c). For UV/vis spectrophotometer the reagent grade solvent and apparatus used were Toluene and UV spectrophotometer (Jenway England, model 6405 UV/vis spectrometer), and printer. For Ir the apparatus used was FTIR Spectrophotometer.

65 49 Picture 3.1: Weighing balance (Ohaous; Model: Advanturers) 3.3 DISTILLATION OF EACH OF THE THREE CRUDES The simple distillation process was conducted at the Department of Food Science and Technology, Post - Graduate laboratory, University of Nigeria Nsukka. For each distillation process that took place, the set up was first made.then 200mls of the particular crude to be distilled was poured into a 250ml round bottom flask containing porcelain chip and corked with a stopper to prevent bubbling off of the crude. The heating mantle was then plugged to the source of electricity and set at c, with a thermometer inserted to the stopper. As the temperature rises, the lighter fractions of the crude oil distilled off and was collected in the receiver (conical flask with cotton wool to lag off exposed portion of the flask). At 350 C the lighter fractions of the crude had been distilled off leaving a heavy organic rich residue known as the dead crude oil (Atmospheric residuum). The whole system was allowed to cool, and the dead crude was stored in the reagent bottle. The process was repeated until a reasonable quantity for each of the crude oil was obtained. The results of this process are shown in chapter four.

66 PRECIPITATION OF ASPHALTENES, PURIFICATION OF ASPHALTENES AND VARIOUS ANALYSIS CARRIED OUT ON THE PURE ASPHALTENES PRECIPITATE PRECIPITATION OF ASPHALTENES The precipitation of asphaltenes was conducted at the Energy Commission Research Centre, University of Nigeria Nsukka. In this present work 40mls of n-heptane single solvent was mixed with 1ml of each of the crudes also 40mls of n-pentane + n-heptane (mixed solvents in the ratio 20:20) was added to 1ml of each of the crudes. The mixture of dead oil (i.e. atmospheric residium) and the n-heptane solvent and also the mixture of the dead crude and the n-pentane + n-heptane mixed solvent was agitated by using a magnetic stirrer for 20mins, 40mins, 60mins and 80mins respectively and allowed to equilibrate for 48hrs. The stirring time was varied with same quantity of liquid precipitant to oil volume ratio for the n-heptanes solvent and the n-pentane+n-heptane solvent in other to study the stirring time effect on the asphaltene yield. Picture 3.4: Magnetic Stirrer (Constant temperature magnetic stirrer 78 HW-1) After 48hrs equilibration, the mixture was centrifuged for 30mins at 2000rpm using a centrifuging apparatus. After this procedure the supernatant (maltene) was decanted and kept separately while the solid residue, which was mainly composed of precipitated asphaltenes, was kept rinsing with the liquid precipitant (about 40mls) until a clear solvent was observed.

67 51 Picture 3.5: Centrifuging apparatus (model no. 80-2B with maximum speed 4000rpm) The precipitated asphaltenes were slowly dried in a vacuum oven at about 80 c (as shown below) until no change in weight was observed. Picture 3.6: Oven/incubator (model mini/50 Genlab Limited) These asphaltenes are refered to as C 7 asphaltenes since n-heptane was used for the precipitation and C 5+ C 7 asphaltenes since C 5 +C 7 mixed solvent system (i.e. in the ratio 20:20) was used for the precipitation PURIFICATION OF EXTRACTED ASPHALTENES The dried C 7 and C 5 +C 7 asphaltenes were purified to remove any non asphaltenic solids (i.e clay, sand, some adsorbed hydrocarbons e.t.c.) that co-precipitated along with the asphaltenes. To remove these solids, each of the asphaltenes were dissolved in 10mls of toluene in which asphaltene is soluble, this mixture was filtered and to the filterate (composed mainly of asphaltenes) 20mls of n-alkanes was added to reprecipitate asphaltenes. The fractions of the C 7 and C 5 +C 7 asphaltenes that did not desolve in toluene were discarded (i.e. non asphaltenic

68 52 solids). Finally, the reprecipitated asphaltenes were dried for 80mins in an oven, the dried asphaltene from the oven was washed and dried until no change in weight was observed. The melting point, ultraviolet and infrared analyses were carried out. The UV and IR spectroscopy were used for final assessment of the precipitated asphaltenes as shown in subsequent sub topics below. The maltenes were further separated into saturates, aromatics and resins (by a process termed SAR method) by first exposing the maltenes at room temperature to evaporate the n- alkane that was used to precipitate asphaltene, the volume, weight and density of the maltenes were obtained and the results are as shown in chapter 4. Figure 3.4: below shows the flow chart of the separation of the atmospheric residuum from Bonny Export, Bodo and Mogho crudes. Atomospheric residuum Extract with C 5 and C 7 C 7 : oil = 40:1 & C 5 + C 7 : oil = 40:1 Extract with pentane Insoluble Soluble Malteness Insoluble Soluble Non aspaltenic solids Asphaltenes Figure 3.4: Classification procedure for the various heavy crude oil fractions (350 O C boiling fraction) This is refered to as NAMAL method for the separation of atmospheric residuum into asphaltenes and maltenes.

69 FRACTIONATION OF MALTENES The fractionation of maltenes was conducted at the department of Pure and Industrial Chemistry. The maltenes obtained from the n-heptane single solvent (ie 80mins only for each of the crudes) were fractionated into saturates, aromatics and resins. The separation into these petroleum fractions was performed using chromatographic method. This technique is described in detail below ACTIVATION OF THE SILICA GEL Approximately 200g of silica (granulated) gel was spread evenly on a tray and dried in an oven for 24hrs at a temperature of 105 C. Aproximately 40.34g of the dried silica gel was packed in a 50ml standard laboratory burette and used to carry out the column chromatography and 50mls of dichloromethane was added gradually in other to prepare the slurry and also to wash off any trace of impurity in the silica gel. After this procedure, the silica gel was ready for use in chromatography CHROMATOGRAPHY PROCEDURE 5ml of maltene sample was added to the top of the activated silica gel (granulated) in the standard laboratory burette with the help of a funnel and allowed to percolate as shown below.

70 54 Picture 3.7: Fractionation setup (Column chromatography) After the entire sample has entered the gel, 30mls of n-heptane was added to maintain a liquid level well above the silica gel until saturates were washed off from the adsorbent. Approximately 30mls of heptane effluent (i.e. n-heptane + saturates) was collected from the column. After the collection, the flask was replaced with another flask for the collection of aromatics. Immediately after all the heptane effluent has eluted toluene in the amount of 200mls (for aromatics from Mogho crude) and 150mls (for aromatics from Bodo Crude) and 130mls (for aromatics from Bonny crude) was added to the column through a separatory funnel. The column was allowed to drain and approximately 200mls, 150mls and 130mls of toluene effluent was collected for Mogho aromatics, Bonny export aromatics and Bodo aromatics respectively. At this point, resins have adsorbed on the gel. To recover the resins, a solvent mixture of dichloromethane and methanol (in the ratio 50:50) in the amount of 100ml was charged slowly to the top of the gel column. At this point

71 55 approximately 100ml of dichloromethane + methanol effluent (resin + solvent) was collected. After each effluent has been collected for a particular crude, the silica gel was removed and replaced with a fresh activated silica gel. This procedure was carried out for the maltenes in the three crudes studied. Figure 3.5: below shows the flow chart of the separation of maltenes into saturates, aromatics and resins This is referred to as SAR method for the recovery of the components of the maltenes in the three crude oil studied. Figure 3.5: Flow chart of the separation scheme EXTRACTION OF EACH OF THE SATURATES, AROMATICS AND RESINS (MALTENE) FROM THEIR VARIOUS EFFLUENTS The drying of the various affluents were conducted at the department of Biochemistry University of Nigeria Nsukka. The n-heptane effluent (saturates + n heptane) was evaporated at room

72 56 temperature, while the dichloromethane + methanol effluent (resin + dichloromethane + methanol) and the tolune effluents (aromatic + toluene)were evaporated at 65 c in a water bath as shown below. After solvent evaporation each fraction was weighed until no change in weighed was observed.. Picture 3.8: Water bath (Model DK) with resin + dichloromethane + methanol effluent during evaporation of Bonny Export crude 3.6 PHYSICAL METHODS FOR ANALYSING THE ASPHALTENE FRACTION INFRARED SPECTRA ANALYSIS: The FTIR analysis was conducted at the National Research Institute for Chemical Technology (NARICT), Federal Institute for Science and Technology, Zaria. The infrared spectra analysis of asphaltenes (from 80mins stirring time) were obtained by carrying out the analysis without interacting the asphalting with any solvent (termed neat). The procedure was carried out using an FTIR Spectrophotometer. Specify the model of the Spec THE ULTRAVIOLET VISIBLE SPECTRA ANALYSIS: This analysis was conducted at the department of Pure and Industrial Chemistry. The ultraviolet visible Spectra analysis of the

73 57 precipitated asphaltene fractions were recorded on a Jenway England, model 6405 UV/Vis as shown in picture 3.9 below UV SPECTROSCOPIC PROCEDURE 0.01g of each of the precipitates from Bonny Export, Bodo and Mogho crude (ie 80mins only for both n-heptane and n-pentane+n-heptane mixed solvent ) was dissolved in 3mls of toluene and poured into a cuvet then placed in the sample compartment of the UV/vis spectrophotometer as shown below. The sample was then scanned in the UV region. Details of this report is shown in chapter 4. Picture 3.9: UV Visible spectrophotometer (Jenway England, model 6405 UV/vis spectrometer) MELTING POINT ANALYSIS: This analysis was carried out in Pharma-chem laboratory (U.N.N). It was done by putting a pinch of the asphaltene precipitate in a capillary tube and dropped in one of the three compartments in the melting point analyser (electrothermal melting point analyser, which heats up to the range of C) as shown in picture 3.10 below

74 58 Picture 3.10: Melting point analyser (Electrothermal melting point analyser) Cat no.: 1A6304, For all these analysis, the samples decomposed between the range of O C to a darker material (carbonaceous material). EXPERIMENTAL PROCEDURE TO DETERMINE THE ROLE OF RESINS IN STABILISING (SOLUBILISING) ASPHALTENES IN CRUDE OIL USING n-heptane SINGLE SOLVENT ONLY. To 1ml of each of Bonny Export, Bodo and Mogho crudes (atmospheric residium) 40mls of n- heptane was added and to this mixture, the same quantity of resins extracted from the fractionation of n-heptane maltenes (that was obtained from 80mins asphaltenes precipitation)from Bonny Export, Bodo, and Mogho crudes respectively were added to the same crude from which they were extracted and stirred with a magnetic stirrer for 80minutes and the mixtures (resins+1ml of crudes + 40mls of n-heptane) were allowed to age (ie equilibrate) for 2 days (48hours). After 48hours equilibration (aging) the mixture was Centrifuged for 30minutes at 2000rpm using a Centrifuging apparatus (model no: 80 2B). after this procedure the Supernatant (maltenes) was decanted and kept seperately while the solid residue composed

75 59 mainly of asphaltenes was kept rinsing with the liquid precipitant (about 40mls) until a clear solvent was observed. The precipitated asphaltenes were slowly dried in a vacuum oven/incubator (model; mini/50) at about 80 C until no change in weight was observed.details of this result is as shown in chapter 4. PURIFICATION OF PRECIPITATED C 7 ASPHALTENES WHEN RESINS WAS ADDED TO THE CRUDE OIL: The dried C 7 asphaltenes were purified to remove any non asphaltic materia that co-precipitated along with the asphaltenes. To remove this solids the asphaltenes were dissolved in 10ml of toluene and filtered to remove any solid particles, the fraction of the C 7 asphaltenes that did not desolve in toluene (non aspaltenic) was discarded. To the soluble part (composed of asphaltenes) 20ml of n-heptane was added, and then dried in a vacuum oven at 80 C until no change in weight was observed. Detailed result of this experiment is shown in chapter 4.

76 60 CHAPTER FOUR 4.0: RESULTS AND DISCUSSIONS Asphaltenes has been precipitated from Bonny, Bodo and Mogho (Porth Harcourt) crudes using n-heptane and n-pentane+n-heptane mixed solvent at various reaction time (20mins, 40mins, 60mins and 80mins). The results of the distillation and composition of the asphaltenes and maltenes in each of the three different crudes and the role resins play in solubilising asphaltenes in crude oils are shown in table 4.1, 4.2a, 4.2b. 4.1 RESULTS OF THE PHYSICAL PROPERTIES OF BONNY EXPORT, BODO AND MOGHO CRUDE OILS BEFORE AND AFTER DISTILLATION AT 350 O C Table 4.1: PHYSICAL PROPERTIES OF BONNY EXPORT, BODO AND MOGHO CRUDE OILS BEFORE AND AFTER DISTILLATION Source of the crude oils Bonny Export Bodo Mogho Weight of crude used (g) Weight of atmospheric residuum (g) Volume of crude (ml) Volume of atmospheric residuum (ml) Density of crude (g/ml) Density of atmospheric residuum (g/ml) API gravity of crude o o o API gravity of atmospheric residuum 45.3 o o o

77 61 COMMENTS As accepted generally, there are various types of crudes, the extra heavy crude oil, the heavy crude oil, the medium crude oil, the light crude oil and the very light crude oil. Extra heavy crude oil is any liquid petroleum with an API gravity less than 10 API. Heavy crude oil is defined as any liquid petroleum with an API gravity less than 20 API. Medium crude oil is any liquid petroleum with an API gravity between API. Light crude oil is any liquid petroleum with an API gravity between API. Very light crude oil is defined as any liquid petroleum with an API gravity above 40 API. This shows that from the results obtained for API gravity in table 4.1 above Bonny Export is a very light crude i.e. with an API gravity of 49.91, Bodo crude is a light crude because its API gravity is API also Mogho crude is a light crude because its API gravity is API. However, after distillation when all the lighter fractions of the various crudes has been removed, Bonny Export (atmospheric residuum) still behaved like a very light crude but its density became slightly higher than normal. Bodo (atmospheric residuum) behaved like a light crude with a slightly raised density compared to its original density but in the case of Mogho crude though a light crude but after distillation it behaved more like a medium crude and its density increased greatly compared to its original density. This shows that the heavy organics in Mogho crude oil will be more compared to that in Bonny Export and Bodo crudes, indicating the presence of a very high paraffinic material which light crude oils are known for meaning that Mogho crude may probably have the highest asphaltene content compared to Bodo and Bonny Export crude, because asphaltene have very limited solubility in paraffinic materials

78 62 It is a well known fact that density measurement is the simplest way to estimate the cohesive forces and, therefore, the interaction energies of a particular material. The density is also a measurement of the molecular parking of the solid, and in the case of aromatic compounds, this parking strongly depends on the structural molecular topology of the molecules. This indicates that Mogho crude and Bodo crude with higher densities are likely to have higher aromaticity, therefore higher asphaltene precipitates and more complex structures than Bonny Export crude. [80] 4.2: RESULTS FROM ASPHALTENE PRECIPITATION Detailed result of the compositions of asphaltene and the physical properties of the maltenes from each of the three crude oils are as shown in the tables below. Table 4.2a: Composition of Asphaltenes in Bonny Export crude for both n-heptane single solvent and n-pentane + n-heptane mixed solvent. Stirring Time Solvent Weight of % weight of asphaltenes after asphaltenes (%) (solvent+ crude) drying (g) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Table 4.2(a): Shows clearly that the weight / weight 0 / 0 of asphaltenes increased with increase in stirring time for both single n-heptane solvent and the mixed n-pentane + n-heptane solvents. This is also shown in figure 4.2a and b.

79 63 FILTRATE (MALTENES) FROM BONNY EXPORT CRUDE AS SHOWN BELOW: Table 4.2b: Physical Properties of maltenes (filtrate) from Bonny Export crude Stirring Time Solvent Weight of Volume of Density of (solvent+ crude) maltenes maltenes maltenes (g/ml) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Table 4.2 (b): Shows that the densities of the maltenes from Bonny Export crude increased with increase in asphaltene yield.

80 % weight of asphaltene vs time mins 40mins 60mins 80mins Fig. 4.2a(i): % weight of asphaltens for bonny Export Crude (n-heptane solvent) with time mins 40mins 60mins 80mins % weight of asphaltenes vs time Fig. 4.2a(ii): % weight of asphaltenes from Bonny Export Crude (n-pentane + n-heptane solvent system) versus time

81 65 Table 4.3a: Composition of the asphaltenes from Bodo Crude for both n-heptane single solvent and n-pentane + n-heptane mixed solvent. Stirring Time Solvent weight of % weight of (solvent+ crude) asphaltene asphaltene after drying (g) ( 0 / 0 ) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Table 4.3 (a) : shows that the weight / weight 0 / 0 of the asphaltenes increased from 20mins to 60 mins but decreased slightly at 80mins stirring time for both the single n-heptane solvent and mixed n-pentane + n-heptane solvent system. This is also shown in Figure 4.3 a and b.

82 66 FILTRATE (MALTENES) FROM BODO CRUDE AS SHOWN BELOW: Table 4.3b: Physical Properties of maltenes (Filtrate) from Bodo Crude for both n-heptane single solvent and n-pentane + n-heptane mixed solvent. Stirring Time Solvent Weight of Volume of Density of (solvent+ crude) maltenes maltenes maltenes (g/ml) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-hepta ne Table 4.3(b), shows clearly that the densities of the maltenes from Bodo crude increased with its resulting asphaltenes (Table 4.3a).

83 % weight of asphaltene vs time mins 0 20mins 40mins 60mins 80mins Fig. 4.3a(i) % weight of asphaltenes from Bodo Crudes (n-heptane single solvent) mins 20mins 40mins 60mins 80mins % weight of asphaltene vs time Fig. 4.3a(ii) % weight of asphaltenes from Bodo Crudes (n-pentane + n-heptane mixed solvent)

84 68 Table 4.4a: Composition of the asphaltenes in Mogho crude for both n-heptane single solvent and n-pentane + n-heptane mixed solvent. Stirring Time Solvent Weight of asphaltene after % weight of asphaltene drying (g) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Table 4.4(a), shows clearly that the weight / weight 0 / 0 of the asphaltenes decreased from 20mins to 60mins but increased slightly with 80mins stirring time giving a precipitate slightly higher than the precipitate obtained from 60mins stirring time for the n-heptane single solvent and the n-pentane + n-heptane mixed solvent. This is also shown in Figure 4.4 a and b.

85 69 FILTRATE (MALTENES) FROM MOGHO CRUDE AS SHOWN BELOW: Table 4.4b: physical properties of maltenes from Mogho crude for both n-heptane single solvent and n-pentane + n-heptane mixed solvent. Stirring Time Solvent Weight of Volume of Density of maltenes maltenes maltenes (g/ml) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Table 4.4b, shows clearly that the densities of the maltenes increased along side with their corresponding asphaltenes, but decreased at 60mins stirring time for n-heptane single solvent and also decreased at 80mins stirring time for n-pentane + n-heptane mixed solvent.

86 % weight of asphaltenes vs time mins 20mins 40mins 60mins 80mins Fig. 4.4a(i) % weight of asphaltenes from Mogho Crude (n-heptane single solvent) vs time mins 20mins 40mins 60mins 80mins % weight of asphaltenes vs time Fig. 4.4(ii) % weight of asphaltenes from Mogho Crude (n-heptane + n-pentane mixed solvent) vs time

87 COMPARISM OF THE WEIGHT OF THE PRECIPITATED ASPHALTENE WITH STIRRING TIME USING N-HEPTANE SINGLE SOLVENT AND N- PENTANE + N-HEPTANE MIXED SOLVENT. It is generally accepted that asphaltene precipitation depends mainly on the stability of the asphaltenes and stability depends not only on the properties of the asphaltene fraction but on how good a solvent the rest of the oil is for its asphaltenes. In comparism of the weight of the asphaltene precipitated using n heptane single solvent and n pentane + n-heptane mixed solvent with respect to stirring time for Bonny Export, Bodo and Mogho crudes: Table 4.2 (a-b), table 4.3 (a-b), and table 4.4 (a-b) shows the percentage weight of asphaltenes precipitated from Bonny Export, Bodo and Mogho crudes and their resulting maltenes using n-heptane single solvent and n-pentane + n-heptane mixed solvent for 20, 40, 60 and 80 minutes stirring time for each of the crudes. Table 4.2 (a-b) from Bonny Export crude shows that the weight of asphaltenes precipitate increased with increase in stirring time, for both the C 7 asphaltene and the C 5 + C 7 asphaltenes (Figure 4.2 a - b), also the densities of their maltenes increased with increase in stirring time. In the case of Bodo crude (Table 4.3 a-b) and Mogho crude (Table 4.4 a-b), their asphaltene precipitate do not follow the same trend as in Bonny Export crude (Figure 4.3 a-b and Figure 4.4 a-b), but the densities of their maltenes increased alonge with their corresponding asphaltene precipitate. According to these results, it is possible to suppose that asphaltene precipitation depends on stirring time but due to the presence of other solubility fractions (i.e. the saturates, aromatics and resins) which may not be present in the right ratios, Bodo and Mogho

88 72 crudes did not depend on stirring time. It may also be because the other solubility fractions (such as saturates, aromatics and resins) in Bodo and Mogho crude oils are not good solvents for their asphaltenes, therefore making their asphaltenes unstable and so precipitation in these crudes were more and did not depend on stirring time. It was also noticed that Moghoo crude (though a light crude) precipitated more asphaltenes followed by Bodo crude and then Bonny Export crude for both single and mixed solvent system (Figure 4.5 and 4.6 below). This indicates that the amount and characteristics of the asphaltene constituents in crude oil depends to a greater extent on the source of the crude. [9] MIXED GRAPH OF ASPHALTENE PRECIPITATE FROM BONNY EXPORT, BODO AND MOGHO CRUDES USING N-HEPTANE SINGLE SOLVENT WITH RESPECT TO TIME mins 40mins 60mins 80mins % weight of asphaltenes for Bonny Export crude using single solvent % weight of asphaltenes for Bodo crude using single solvent % weight of asphaltenes for Mogho crude using single solvent Figure 4.5: Effect of n-heptane single solvent with stirring time on Bonny Export, Bodo and Mogho Crudes

89 73 MIXED GRAPH OF ASPHALTENE PRECIPITATE FROM BONNY EXPORT, BODO AND MOGHO CRUDES USING N-PENTANE + N-HEPTANE MIXED SOLVENTS WITH RESPECT TO STIRRING TIME. % weight of asphaltenes for Bonny Export Crude using mixed solvent % weight of asphaltenes for Bodo Crude using mixed solvent % weight of asphaltenes for Mogho Crude using mixed solvent mins 40mins 60mins 80mins Figure 4.6: Effect of n-heptane + n-pentane mixed solvent precipitant on Bonny Export, Bodo and Mogho Crudes COMMENT For both Bonny Export, Bodo and Mogho crudes as shown in figures 4.5 and 4.6 above, the mixed solvent precipitant (n-pentane + n-heptane) precipitated more asphaltenes than single solvent (n-heptane) precipitant. This is due to the addition of n pentane to n heptane which made up the mixed solvent system as in agreement with the generally accepted fact that asphaltene precipitation increases with decrease in the carbon chain length of the precipitating solvent. [75] This increase in asphaltene yield for precipitation using mixed solvent system is also indicative of the fact that for a given crude oil sample, the yield and properties of the precipitated

90 74 asphaltenes strongly depend on the specific precipitation method and precipitant used. This means that a single oil could have two or more results depending on the precipitant used. [8] 4.4 SUMMARY OF THE RESULT OF FTIR SPECTROPHOTOMETRIC ANALYSIS Table 4.5a: RESULT OF IR ANALYSIS OF ASPHALTENES OBTAINED USING SINGLE N-HEPTANE SOLVENT Samples (A) Asphaltenes Precipitation using Single Solvent (nheptane) Bonny Export Crude Mogho (Port Harcourt) Crude Approximate characteristic frequencies (cm -1 ) Bonds Substituted aromatic hydrocarbon. C H bending. C H of aromatics. Substituted aromatic hydrocarbon. C H bending. C H bending. Cyclic aliphatic hydrocarbon. C-H of aromatics. Bodo Crude Substituted aromatics hydrocarbon C = C H bending out of plane. C H bending. C-H bending. C H bending. C = C of aromatic C = O (acid, aldehydes, ketones and esters Cyclic aliphatic hydrocarbon. C-H of aromatics.

91 75 TABLE 4.5b: RESULTS OF IR ANALYSIS OF ASPHALTENES OBTAINED USING N- PENTANE + N-HEPTANE MIXED SOLVENT SYSTEM. Samples B Asphaltenes Precipitated using mixed Solvent (npentane+n-heptane) Bonny Export Crude at Mogho (Port Harcourt) Crude Bodo Crude characteristic frequencies (cm -1 ) Bonds Substituted aromatic hydrocarbon C H bending Cyclic aliphatic hydrocarbon. Substituted aromatic hydrocarbon. C H bending. C H bending. Cyclic aliphatic hydrocarbon. Substituted aromatic hydrocarbon. C H bending. C H bending. C = O (acid, aldehydes, ketones and esters Cyclic aliphatic hydrocarbon. IR INTERPRETATION Data from IR as shown in figure I-VI in the appendice is summarized in table 4.5a and b, obtained from Bonny Export, Bodo and Mogho crudes shows characteristic frequencies at ,3060,3080 that are due to C-H stretch for aromatic hydrocarbon. This is supported by the absorptions at , , that are due to substituted aromatic hydrocarbon. This confirms the same class of crude oil composition, this class of crude oil composition consist of the unsaturated part of asphaltenes, that is that part of asphaltenes that consist of fused benzene rings. However, absorption frequencies at , , , and are

92 76 due to cyclic aliphatic hydrocarbon. This is supported by the absorptions at , , , , that are due to C-H bending. These suggest the same class of crude oil composition. These classes of crude oil consist of the saturated part of asphaltenes structure. The IR reveals that asphaltenes fraction of crude oil is made up of both saturated and unsaturated part as supported by our UV spectra on the asphaltene precipitates. 4.5 RESULTS OF UV/VISIBLE SPECTROPHOTOMETRIC ANALYSIS. Table 4.6: UV Spectra of the Asphaltene Fractions of Crude Oil. Samples UV Spectra Data Asphaltenes precipitated using single solvent (n-heptane) for 80 minutes λ (nm) A λ (nm) A Bonny Export Crude (C 7 asphaltenes) Bodo Crudes (C 7 asphaltenes) Mogho Crude (C 7 asphaltenes) Asphaltenes precipitated using mixed solvent (n-pentane+n-heptane) for 80 minutes Bonny Export Crude (C 5 + C 7 asphaltenes) Bodo Crudes (C 5 + C 7 asphaltenes) Mogho Crude (C 5 + C 7 asphaltenes) UV/VISIBLE INTERPRETATION Table 4.6 shows the summary of the UV-visible spectra data of asphaltene fractions from Bonny Export, Bodo and Mogho crudes using n-heptane single solvent and n-pentane + n- heptane mixed solvent system for 80mins stirring time as shown in figure vii - xii (Appendix 14 19). Each of the asphaltene fractions shows absortion maxima in the visible region of the

93 77 electromagnetic spectrum, indicating that the C 7 and C 5 + C 7 asphaltenes are largely unsaturated as supported by the asphaltene structure in figures 2.3a, 2.3b, 2.4a and 2.4b. Similar absoptions at the following wavelengths: 389.8, 388.9, and 388.6nm which are supported by absorptions at 418.2, 509.6, 510.0, 449.4, and 432.8nm suggest the same class(es) of crude oil composition which is the presence of fused benzene rings or polynuclear aromatics in asphaltenes which indicate unsaturated and highly conjugated systems. These are similar to the absorptions found in the literature. The range of the absorption bands for both the C 7 and C 5 + C 7 asphaltenes are about nm which corresponds to benzenoide band for highly polynuclear aromatics. This is in agreement with the asphaltene structure. In addition the uv visible absorption bands are similar to those shown else where (Evdokimov and Losev). [81] This range ( ) of absorption band for all the asphaltene precipitates obtained from Bonny Export, Bodo, and Mogho crudes, which corresponds to the benzenoid bands for polynuclear aromatics, implies the existence of chromophores in the asphaltenes. These asphaltenes as a consequence are coloured the chromophores very likely available in these asphaltenes are: conjugated double bonds involving aromatic hydrocarbons as supported by the infrared spectra on the asphaltenes from Bonny Export,Bodo and Mogho crudes. From molecular orbital theory, the allowed possible transitions are: σ σ*, n σ*, П П*, and n П* where n= non bonding, σ = sigma, П = pie and those with asterisk are antibonding. The wavelength absorption range is consistent with fundamental molecular orbital theoretical assumption or specification. This assumptions is that the energy difference E = HOMO LUMO is small, and also the longer the wavelength of absorption, the smaller the energy of irradiation. Thus, exposure of these compounds with small E values and long

94 78 wavelengths of absorptions to high irradiation energies will destroy these compounds. Hence, under these conditions, they can decompose. Our UV and IR spectra are consistent and reveal the presence of some Island and Archipelago architectures (Sabbah et al, 2011) [82] : 2,3,7,8,12,13,17,18 octaethyl - 21H, 23H porphine; 5,10,15,20 Tetra p- tolyl 21H, 23H-porphine; 5,10,15,20 Tetrakis (4- methoxyphenyl) -21H, 23H porphines; 5,10,15,20 Tetrakis [4 (allyloxy) phenyl] 21H, 23H porphine; phenanthrene; 1,3,6,8 tetradecyl pyrene; 2,7 Bis (2-pyren-1-yl-ethyl) -9, 9 diethyl 9H fluorene; 1,4 Bis (2-pyren-1-yl-ethyl) benzene; 1,4 dipyren 1 yl- butane. 4.6 RESULT OF THE CHEMICAL AND PHYSICAL PROPERTIES OF CRUDE OILS OBTAINED FROM N-HEPTANCE MALTENES Table 4.7: Chemical and Physical Properties of Crude oils as obtained from n-heptane maltenes ( 80minutes). Source API Density of Wt of Wt of Ratio of Wt Wt of Ratio of resins of crude gravity atmospheric saturates aromatics aromatics of asphalt to asphaltenes of crude residuum to resin enes saturates s (g) Bonny Export Bodo crude Mogho crude Table 4.7 shows the physical and chemical properties of the studied crudes. As mentioned earlier, it is generally accepted that a high ratio of resins to asphaltenes and aromatics to saturates is indicative of low asphaltene precipitation risk. [2] As recognized by De Boer et al,

95 79 the heavier oil also contains plenty of intermediate component that are good asphaltene solvents whereas the light oil may consist largely of paraffinic materials in which, by definition, asphaltenes have very limited solubility. [13] In this present study, the composition factor mentioned were examined and shown in Table 4.7. The three crudes shows very high values of saturates, indicating the presence of paraffinic material. Also, according to these results, it is possible to suppose that the crude oils with higher densities also show the higher cohesive energies and, therefore, the lower solubility in the crude oil generating unstable crude oils. This result also shows that the density and aromaticity of the crudes increases simultaneously as the asphaltene precipitates increases from Bonny Export, Bodo and Mogho crudes. This shows that crude oil with higher aromaticity, higher saturates and higher density like in Bodo and Mogho crude oils precipitate more asphaltenes, and are likely to be problematic, most times such crudes are termed unstable crudes. Infact, it was observed that the ratio of saturates to aromatics and the ratio of resins to asphaltenes decreases as the asphaltene precipitate increases. This can be shown in the chart below (figure 4.9).

96 wt of saturates (g) wt of aromatics (g) 0.3 wt of resins (g) wt of asphaltenes (g) 0 Bonny Export Bodo Crude Mogho Crude Figure 4.9: Weight of heavy fractions of each of the three crude oils studied and their various asphaltene content. Figure 4.9 shows a bar chart which can be interpreted in terms of asphaltene stability and stabilization properties of the maltenes. From the bar chart above Mogho crude has the highest precipitation of asphaltenes because of the presence of saturate which indicates high paraffinic material in which by definition asphaltenes has limited solubility also the weight of aromatics is higher in Mogho crude indicating higher asphaltene precipitation than in Bonny Export and Bodoo crudes. Also as mentioned earlier the ratio of saturates to aromatics and resins to asphaltenes is highest for Bonny Export crude indicating the lowest asphaltene precipitation risk and lowest for Mogho crude, indicating the highest asphaltene precipitation risk. these ratios are All these shows that Mogho crude is likely to be more problematic followed by Bodo crude.

97 RESULT OF THE EFFECT OF RESINS ON ASPHALTENE PRECIPITATION Addition of resins to crude oil reduced the precipitation of asphaltenes in the crude oil as shown in Table 4.8 below. Table 4.8: Effect of Resins on Asphaltene Precipitation. Weight % of asphaltenes without resins Weight % of asphaltenes with resins Source of Final weight Weight % of Final weight Weight % of crude of asphaltenes asphaltenes of asphaltenes after purification asphaltenes Bonny Export Bodo crude Mogho Crude In the experiment reported in table 4.8 Above, resins separated from Bonny Export crude oil were added to the n heptanes solution (i.e. 1ml crude + 40ml of n heptane) of Bonny Export crude oil, also the resins separated from Bodo crude and Mogho crudes were added to their various n-heptane solutions (ie 1ml crude + 40ml n heptane). From table 4.8 shown above, 3.2% of asphaltene was precipitated from Bonny Export crude, 6.5% from Bodo crude and 9.877% from Mogho crude before addition of the resins extracted from each of the crudes but after the addition of resins to these crude oils, asphaltene precipitation in Bonny Export crude, Bodo crude and Mogho crude reduced to 1.663%, 3.035% and 5.425% respectively indicating that resins stabilize (solubilize) asphaltenes in crude oil.

98 82 Figure 4.10 shows the effectiveness of resins to stabilize their corresponding asphaltenes. Mogho Crude Bodo crude Bonny Export Weight % of asphaltenes with resins Weight % of asphaltenes without resins To estimate the relative contribution of the resins to the stability of the crude oil, a plot of the effectiveness of the resins and the asphaltene precipitation reduction as a function of the stability of the crude oils is shown in Figure 4.10 above. Figure 4.10 shows that the percentage weight of asphaltenes in Mogho, Bodo and Bonny Export crudes (when resins extracted from the crude was added) reduced from / 0, / 0 and / 0 respectively to / 0 ( for Mogho crude), / 0 ( for Bodo), and / 0 (for Bonny Export). Indicating that resins stabilize (solubilize) asphaltenes in crude oil.

99 83 CHAPTER FIVE Conclusion Asphaltene precipitation with stirring time using n heptane single solvent and n-pentane + n-heptane mixed solvent from 350 O C atmospheric residuum (dead crude oils) is a useful method to study the stability of crude oils. It was also found to be an important technique for the study of the chemical factors that affect asphaltene precipitation. As said earlier De Boer et al, recognized that light crude oils may consist largely of paraffinic material in which by definition asphaltenes has very limited solubility. [13] This indicates why the weight of saturates (composed mainly of paraffinic material) obtained from these studied crude oils were very high compared to the weights of their aromatics, resins and asphaltenes. In this present study, it was also found that the density and the aromaticity of the atmospheric residium increases simultaneousely with increase in asphaltenes from Bonny Export, Bodo to Mogho crudes. There was a noticeable decrease in the ratio of aromatics to saturates and resins to asphaltenes with Bonny Export having the highest ratio and Mogho the least, there was also a noticeable increase in the ratio of asphaltenes to maltenes with Bonny Export having the least ratio and Mogho the higest ratio. This shows that Mogho crude has the highest asphaltene precipitate and Bonny Export the least. Therefore, Mogho crude with the highest amount of density, saturate, aromaticity and asphaltenes is likely to be more problematic. Further more, the drastic reduction of asphaltene precipitate in Bonny Export, Bodo and Mogho crudes with additional resins extracted from the same crudes showed that resins from one crude oil solubilise asphaltenes from the same crude oil.

100 84 This study also shows generally that the maltenes from Bonny Export crude oil exhibits higher asphaltene stabilization effectiveness compared to the maltenes from Bodo and Mogho crude oils indicating that Bonny Export crude is a more stable crude compared to Bodo and Mogho crudes (unstable crudes). Finally, this study shows clearly that asphaltene precipitation occurs in crude oil but other constituents of crude oil especially resins, influence this precipitation. Thus resins play a critical role in asphaltene precipitation.

101 85 NEW KNOWLEDGE ARISING FROM THIS RESEARCH WORK For the first time precipitation of asphaltenes from C atmospheric residuum (dead crude oil) using mixed solvent system is reported. This research work confirmed that the length of stirring time affect the yield of asphaltenes. This has not been reported before. Apart from the generally accepted fact that high ratio of aromatic to saturates and resins to asphaltenes is indicative of low asphaltene precipitation risk. It was found from this work that high ratio of asphaltenes to maltenes is indicative of high asphaltene precipitation risk. This work showed that NAMAL method was an effective method for the separation of atmospheric residuum into asphaltenes and maltenes. This work showed that SAR method was an effective and cheap method for the separation of maltenes into saturates, aromatics, and resins. It was found that increasing resin content of one crude oil solubilizes the asphaltenes from the same crude oil.

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112 96 APPENDIX 1 MAP OF THE SOURCE OF BONNY EXPORT, BODO AND MOGHO CRUDE OILS

113 97 APPENDIX 2 DENSITY OF CRUDE OIL IN g/ml Bonny Export Bodo Mogho Density of crude (g/ml) = ( ) ( ) Bonny Export crude =. =0.78 / Bodo crude =. =0.84 / Mogho crude =. =0.83 / DENSITY OF ATOMPHERIC RESIDUUM Bonny Export Bodo Mogho Density of atmospheric residuum in g/ml = ( ) ( ) Bonny export =. =0.81 / Bodo =. =0.85 / Mogho =. =0.90 /

114 98 APPENDIX 3 API GRAVITY OF THE CRUDE OILS Bonny Export Bodo Mogho O O O O API gravity = Bonny Export =.. Bodo =.. Mogho = = = = API GRAVITY OF ATMOSPHERIC RESIDUUM Bonny Export Bodo Mogho 45.3 O O O O API gravity = Bonny Export = = Bodo = = Mogho = =25.72

115 99 APPENDIX 4 PERCENTAGE WEIGHT OF ASPHALTENES FROM BONNY EXPORT ATMOSPHERIC RESIDUUM Stirring time Solvent Weight of asphaltenes before drying (g) Weight of asphaletenes after drying (g) Percentage weight of asphaltenes (%) 20mins n-heptane % 40mins n-heptane % 60mins n-heptane % 80mins n-heptane % 20mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Percentage weight of Bonny export asphaltenes = 100 Density of Bonny Export maltenes = ( ) ( )

116 100 APPENDIX 5 PERCENTAGE WEIGHT OF ASPHALTENES FROM BODO ATMOSPHERIC RESIDUUM Stirring time Solvent Weight of asphaltenes before drying (g) Weight of asphaletenes after drying (g) Percentage weight of asphaltenes (%) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Percentage weight of Bodo asphaltenes = 100 Density of Bodo maltenes = ( ) ( )

117 101 APPENDIX 6 PERCENTAGE WEIGHT OF ASPHALTENES FROM MOGHO ATMOSPHERIC RESIDUUM Stirring time Solvent Weight of asphaltenes before drying (g) Weight of asphaletenes after drying (g) Percentage weight of asphaltenes (%) 20mins n-heptane mins n-heptane mins n-heptane mins n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane mins n-pentane + n-heptane Percentage weight of Mogho asphaltenes = 100 Density of Mogho maltenes = ( ) ( )

118 102 APPENDIX 7 WEIGHT OF MALTENES AND RATIO OF ASHALTENES TO MALTENES Source of crudes Weight of saturate (g) Weight of aromatics (g) Weight of resins (g) Weight of asphaltenes (g) Weight of maltenes (g) Ratio of asphaltenes to maltenes Bonny Export Bodo Mogho Weight of maltenes (g) = wt of saturates + wt of aromatics + wt of resins Ratio of asphaltenes to maltenes = PERCENTAGE WEIGHT OF ASPHALTENE FROM 350 O C ATMOSPHERIC RESIDUUM WITH ADDITIONAL RESINS EXTRACTED FROM EACH OF THE CRUDES Source of crudes Solvent Initial weight of asphaltenes before drying (g) Final weight of asphaltenes after drying (g) Bonny Export n-heptane Percentage weight of asphaltenes (%) Bodo n-heptane Mogho n-heptane Percentage of weight of asphaltenes when resins was added to the atmospheric residuum = ( ) 100

119 103 APPENDIX 8 Figure 1: C 7 asphaltene from Bonny Export Crude

120 104 APPENDIX 9 ANIGBOGU IFEOMA V. Figure II: C 7 asphaltene from Bodo Crude

121 105 APPENDIX 10 Figure III: C 7 asphaltene from Mogho Crude

122 106 APPENDIX 11 Figure IV: C 5 + C 7 asphaltene from Bonny Export Crude

123 107 APPENDIX 12 ANIGBOGU IFEOMA V. Figure V: C 5 + C 7 asphaltene from Bodo Crude

124 108 APPENDIX 13 Figure VI: C 5 + C 7 asphaltene from Mogho Crude

125 109 APPENDIX 14 Figure VII: C 7 asphaltene from Bonny Export Crude (UV)

126 110 APPENDIX 15 Figure VIII: C 7 asphaltene from Bodo Crude (UV)

127 111 APPENDIX 16 Figure IX: C 7 asphaltene from Mogho Crude (UV)

128 112 APPENDIX 17 Figure X: C5 + C7 asphaltene from Bonny Crude (UV)

129 113 APPENDIX 18 Figure XI: C5 + C7 asphaltene from Bodo Crude (UV)

130 114 APPENDIX 19 Figure XII: C5 + C7 asphaltene from Mogho Crude (UV)