SIZE EXCLUSION CHROMATOGRAPHY AND VAPOR PRESSURE OSMOMETRY IN THE DETERMINATION OF ASPHALTENE MOLECULAR WEIGHT.

Abstract

The problem concerning the determination of asphaltene molecular weight has been critically investigated. A size exclusion chromatography (SEC) analytical method has been developed using innovative calibration standards. The obtained results have been compared with those derived from vapor pressure osmometry (VPO). All the experimental data indicate that the number-average molecular weights (Mn) determined either by SEC or VPO are always different Nevertheless such discrepancies are less pronounced using the new calibration curve instead of either the conventional polystyrene standards or other polymers such as poly(ethylene glycol). Furthermore it has been observed that VPO measurements, considered to give the most reliable Mn values when performed in a highly dissociating solvent, at high temperature and low concentration, fails when strong interactions among single molecules are present.

SEC and VPO have also been employed to study association phenomena undergone by asphaltenes in solution. A correlation between molecular structure and degree of aggregation has been recognized indicating that aromaticity is primarily responsible for the importance of the observed intermolecular associations.     

DIBSOLUTION OF SOLID BOBCAN ASPHALTENES IN MIXED SOLVENTS.

ABSTRACT

Solid petroleum asphaltenes have been fractionated according to solubility in toluene/n-heptane mixtures of increasing toluene content. A large hysteresis was observed between this dissolution and the precipitation from the crude oil. In order to shed light on the solution mechanism, the fractions obtained have been analyzed using size exclusion chromatography (SEC-HPLC-UV-vis), VPO, elemental analysis, UV-vis adsorption spectroscopy and phenol interaction values and methylene content by FTir. Less polar non-associating low molecular weight species are dissolved and a specific extraction of porphyrins is observed. An increased association in the insolubles is indicated. More basic interaction sites are available on the asphaltenes in both fractions relative to the native asphaltene. From the SEC chromatograms it was seen that the soluble fractions did not associate as the insoluble fractions even when making up more than 60 % of the total asphaltenes.     

STUDIES ON LARGE CRUDE OIL ALKANES. I. HIGH TEMPERATURE LIQUID CHROMATOGRAPHY

Abstract

Isolation of very large alkane components from heavy oil fractions and solid deposits was achieved by high temperature medium pressure liquid chromatography (MPLC–. n-Heptane and silicagel were the selected solvent and adsorbent employed for routine analysis. Preadsorption of the sample proved to be the most convenient injection technique. Porosity of adsorbents and stepwise temperature gradients were assessed to influence the generation of diverse-multimodal solid phases. Total elution of complex mixtures was possible if high temperature was adopted during the entire experiment and, this behavior was rationalized on the basis of mutual cosolvency of very broad alkane distributions.

Isolated fractions were characterized by high temperature size exclusion chromatography (SEC), technique that allowed to detect very large alkanes preconcentrated during the stepwise separations. Porosity of solid asphaltenes was inferred and diffusional plus crystallization effects were proposed to explain all these findings.     

Behavior and Role of Asphaltenes in a Two-stage Fixed Bed Hydrotreating Process

Abstract

Residue upgrading processes are very important for the production of distillates and low sulfur fuel oils. Among those, fixed bed technologies are very efficient for deep desulfurization of petroleum residue heavy oils, even for highly asphaltenic feeds. This work analyzes the effects of the operating conditions on the evolution of asphaltenes and on their inhibition effect during the hydrodesulfurization reactions. Residue hydrotreating experiments were performed on a pilot plant and asphaltene fractions were investigated using size exclusion chromatography (SEC), ¹³C nuclear magnetic resonance (NMR), liquid chromatography, and elemental analyses. Besides the overall decrease in asphaltenes yield, significant changes in the average structure of the asphaltenes were also observed.     

POLYOXYALKYLENATED AMINES FOR BREAKING OF WATER-IN-OIL EMULSIONS STABILIZED BY ASPHALTENES AND CLAY

ABSTRACT

The main objective of this paper is lo emphasize the efficiency of synthesized polyo.xyalkylenated amines (PPPEA) having different hydiophilic-lipophilic balance (HLB) values in breaking synthetic water-iu-benzene emulsions stabilized by both pelroleum asphaltenes and clay (EAC). It was found that the presence of clay and asphaltenes in a certain ratio (1:1 summing 0 5 wt / vol. % with respect to the total emulsion volume ) offers maximum emulsion stability. This was attributed to the effect of asphaltenes in converting water-wet particles (such as clay) into oil-wet panicles, which then act as emulsifying agents for waler-in-oil emulsions. This explains the synergistic effect resulting from the mutual interactions between asphaltenes and clay in stabilizing Eac. It was found that the most efficient demulsifiers for breaking Eac are the aromatic PPPEA having low HLB. This is attributed to the capability of the aromatic PPPEA demulsifiers lo form well developed interactions with the aromatic asphaltenes. This enhanced interaction will drag the asphaltenes and clay from the interlace increasing the demulsification efficiency.     

EFFECTS OF ASPHALTENE PRECIPITATION AND REPRECIPTION ON THE METAL-CONTAINING COMPOUNDS IN HEAVY RESIDUA

ABSTRACT

Boscan vacuum residuum (VR) has been separated into isooctane insoluble asphaltenes and isooctane soluble maltenes. The asphaltenes were dissolved in a minimum of toluene and were further separated by two additional reprecipitations using isooctane as the precipitating solvent. We examined the fractions, including the recovered isooctane soluble material, by size exclusion chromatography (SEC) with inductively coupled plasma (ICP) emission spectroscopy to determine the effects, if any, the reprecipitations have on the size distribution of the metal-containing compounds.

The asphaltene fractions show little change in size upon reprecipitation, but removal of a small amount of the metal components does occur. The first reprecipitation produces isooctane soluble material which is very similar in size to the original maltene fraction. However, the second reprecipitation removes small amounts of material in the size range of the asphaltenes. When the amounts were weight balanced, the resulting separation did little to effect the overall maltene and asphaltene size profiles.     

CHARACTERIZATION OF ASPHALTENES FROM SAUDI ARABIAN CRUDE OILS BY NONAQUEOUS POTENTIOMETRIC TITRATION AND ELEMENTAL ANALYSIS

ABSTRACT

Nonaqueous poientiometric titration was employed for the estimation of basic functionalities in the asphaltanes. A series of model basic compounds were titrated in a solvent mixture containing chlorobenzene and acetic anhydride. Perchloric acid in dioxane was used as the titrant. The model basic compounds were classified into very weak bases, weak bases and strong bases, based on their pKa (H₂O) values. Pure asphaltenc samples were titrated under similar titration conditions. The titration curves for the pure asphaltenes showed two end points corresponding to HNP values of about 350 mV and 600 mV. The first end point was due to strong bases (HNP : 350 mV) and the second end point was due to weak bases (HNP : 600 mv). Asphaltene samples spiked with known amounts of model basic compounds were also titrated. The spiking titrations showed three end points corresponding to storng bases, weak bases and very weak bases (HNP : 700 mV). The nonaqueous potentiometric titrations suggested the presence of very weak, weak and strong basic functionalities in the asphaltenes which were due to indole, phenazine and pyridine groups, respectively. The proportions of these functionalities were higher in the higher alkane precipitated asphaltenes. The amounts of total titratable bases were 0.49 mM/g for nC5-precipitated asphaltenes and 0.60 mM/g for the nC10-precipitated asphaltenes from Arabian Heavy Crude Oil. The quantities of very weak bases of the indole type were high in these asphaltenes and represented about 50% of the total titratable bases.     

Interfacial and Thermal Characterization of Asphaltenes Separated from Crude Oils Having Different Geological Origins

Abstract

Three crude oils having different geological origins, namely, DK from Eocene (Sylhet limestone and Langpar), SL from Barail (Oligocene), and JN from Tipam (lower Miocene), were selected. Asphaltenes were separated and characterized. Fourier transform infrared (FTIR) spectroscopy showed the presence of polar groups. Interfacial tension studies of the asphaltenes between benzene–water, toluene–water, and xylene–water showed the lowest interfacial tension with SL asphaltenes in a toluene–water system. Thermogravimetric analysis of asphaltenes indicates greater thermal degradation for DK asphaltenes. Asphaltenes were pyrolyzed and the pyrolyzed products were analyzed by gas chromatography/mass spectrometry (GC/MS). Based on the distribution pattern of n-alkanes in pyrolyzed product of asphaltenes, crude oils, and kerogens, it was concluded that crude oil and asphaltenes originate from the same source and asphaltenes are the unconverted parts of kerogens.     

Infrared Study of Hydrogen Bond Types in Asphaltenes

Abstract

The structural characteristics of asphaltenes are important to refiners for determining the yields from the residual fraction of crude oils and the operating parameters of the process units for deep conversion processes. In this article, we studied the hydrogen bonding distribution of four Saudi Arabian crude-oil-derived asphaltenes against the phenol solutions of various concentrations in carbon tetrachloride. In an attempt to explain relative hydrogen bonding capabilities of asphaltenes, OH peak intensity of phenol and enthalpies of hydrogen bond formation were used in combination with the content of heteroatoms (N, O, and S) in asphaltenes. The OH peak intensity of phenol depends on the presence of acidic and basic moieties or functional groups on the surface of asphaltenes. The results obtained show that the asphaltenes with high oxygen and low nitrogen contents have poor interaction with phenol, which indicates that oxygen might be incorporated as acidic hydroxyl groups in asphaltenes. In a similar way, asphaltenes with low oxygen and high nitrogen contents give high phenol interaction values. Piperidine was also tried to evaluate the hydrogen bonding capabilities of NH group with the asphaltenes. The trace metal and GPC molecular weight measurements showed that the Arab heavy asphaltenes possess highest molecular weights and maximum V and Ni contents.     

Asphaltene Self-Association: Modeling and Effect of Fractionation With A Polar Solvent

Abstract

The self-association of asphaltenes in toluene is believed to occur step-wise, rather than by the formation of micelles. A number of step-wise models have been used to fit the calorimetric titration of asphaltenes in dried toluene solutions, with excellent results. All the models are based on chemical reactions equivalent to the ones found in polymerization. The study shows that the choice of the average properties of asphaltenes, such as the molecular weight, is critical in the final value of the parameter of interest, namely the average heat of self-association ΔH _a . The low values of ΔH _a obtained suggest that a fraction of asphaltenes is not active in the calorimetric experiments. Asphaltenes from Venezuela (LM1) and Mexico (KU) have been fractionated by precipitation with a mixture of acetone and toluene. It is considered that the most polar compounds are collected in the soluble fraction. A calorimetry study was performed on the two fractions, and the results show that the soluble fraction (SOL) has a much higher heat developed than the insoluble fraction (INS). This suggests again that a fraction of asphaltenes is not active in the calorimetric experiments, either because it does not self-associate or because the dilution effect is not strong enough to break the aggregates. Fluorescence and IR spectroscopy experiments confirm there is self-association in INS fraction, leading to the conclusion that asphaltene aggregates are formed by bonds of different strengths. The stronger aggregates would be predominantly in INS fraction and would be inactive in the calorimetric experiments.     

Adsorption of asphaltenes onto CaO,Co3O4, Fe3O4 and ZnO supported on 13X zeolite - An isothermal study

Abstract

Asphaltene has the highest molecular weight and the most polar organizers from petroleum. In this paper, the content of the asphaltenes from a crude oil was extracted and the amount of asphaltene in crude oil was measured to be 3.88%. Four metal oxides were added to 13X zeolite and 13X,10%CaO, 13X,10%Co₃O₄, 13X,10%Fe₃O₄ and 13X,10%ZnO were prepared. The adsorption behavior of the adsorbents was evaluated in asphaltenes adsorption. The capacity of the adsorbent for asphaltenes adsorption is as follows: 13X,10%CaO> 13X,10%Co₃O₄> 13X,10%Fe₃O₄> 13X,10%ZnO. The isotherm of adsorption of the asphaltenes over the adsorbent was studied and resulted that the experimental data could best fit with the Langmuir model, indicating that adsorption is as a monolayer.     

Structural Studies on Residual Fuel Oil Asphaltenes by RICO Method

Abstract

Structural characterization of asphaltenes isolated from Saudi Arabian heavy and medium crude oils was undertaken by using ruthenium ion catalyzed oxidation (RICO) method. The RICO method was capable to convert aromatic carbons selectively into carbon dioxide and carboxylic acids and esters group while leaving aliphatic and naphthenic structures of asphaltenes essentially unaffected. Detailed analyses of RICO products of both Arab heavy and Arab medium asphaltenes were conducted using FT-IR, ¹³C-NMR, IC, GPC, and GC-MS techniques. These analyses indicate that the aqueous phase fraction (water-soluble products) obtained from RICO reaction of asphaltenes consists of aliphatic dicarboxylic acids and aromatic poly carboxylic acids with longer alkyl chains. The ¹³C-NMR and GC-MS analyses of organic phase products of asphaltenes indicate that this fraction contains large amount of aliphatic carboxylic acids with longer alkyl groups. The oxidation products of both Arab heavy and Arab medium asphaltenes were found to be dominated by a homologous series of straight chain monocarboxylic acids suggesting that the normal alkyl chains are major and important constituents of the chemical structure of both asphaltenes.     

Investigation of the Molecular Structure of Turkish Asphaltenes

Abstract

Molecular structure of asphaltenes prepared from four Turkish crude oils with different origin were characterized by elemental analysis, proton nuclear magnetic resonance (¹H NMR), gel permeation chromatography (GPC) by X-ray diffraction (XRD) and by Fourier transform infrared spectroscopy (FTIR). The X-ray diffraction method was used to investigate the crystallite and aromaticity parameters of the asphaltenes. Average distance between the aromatic sheets, average distance between the aliphatic chains, average diameter of the cluster, and average number of aromatic sheets per stack parameters were calculated for the asphaltenes. The combined NMR, FTIR, molecular weight, elemental content, and XRD results have been used to calculate hypothetical structure of the Turkish asphaltenes.     

Spectrophotometric Measurement of Asphaltene Concentration

Abstract

Typically, when ultraviolet and visible absorbance of asphaltenes is employed to measure asphaltene concentration, linear calibrations of absorbance vs. asphaltene concentration are prepared from a sample of asphaltenes in a given solvent. This calibration is shown to be sensitive to: (a) the inorganic solids content of the asphaltenes; (b) physical–chemical differences between asphaltenes from different sources or extracted with different methods; and (c) selective adsorption of asphaltenes on liquid–liquid or solid–liquid interfaces. Calibration constants were determined at wavelengths of 288 and 800 nm for samples of Athabasca and Cold Lake asphaltenes obtained using different extraction methods, from precipitation experiments, and from adsorption experiments on water-in-hydrocarbon emulsions and on powdered metals. It was found that the inorganic solids content did not affect absorbance but the asphaltene concentrations must be corrected to a solids-free basis for accurate results. Calibration constants were found to correlate to the average associated molar masses of the asphaltenes. Therefore, any change in molar mass of asphaltenes during the course of an experiment may change the calibration constant. Partial precipitation and the selective adsorption of asphaltenes can lead to a change in the molar mass of asphaltenes left in solution. The corresponding change in the calibration constants can lead to errors of 5–25% in the estimated concentration.     

Comparisons Between Asphaltenes from the Dead and Live-Oil Samples of the Same Crude Oils

Abstract

Asphaltenes precipitated from pressure-preserve bottomhole oil samples have been obtained for three oils at different pressures, using a bulk high-pressure filtration apparatus. The precipitates captured on the filter were recovered, the asphaltenes defined by the n-heptane insolubility were extracted and analyzed. These pressure-driven asphaltenes found on the filter were found to make up in the range between 50 and 100 ppm of the whole crude oil. Opening of the cell did not reveal asphaltenes retained due to wall adhesion. Size exclusion chromatography tests performed on both the live-oil-derived asphaltenes and the standard asphaltenes as precipitated by atmospheric titration on the same crude oil, revealed that the live-oil asphaltenes had apparent smaller hydrodynamic volume and narrower distributions than the standard asphaltenes for two oils. Further FTIR tests also showed large differences between standard asphaltenes and the asphaltenes obtained at high pressure filter. The latter appeared to contain more functional groups and be less saturated. Implication of these structural differences on precipitation modeling is discussed.     

APPLICATION OF HIGH PERFORMANCE LIQUID CHROMATOGRAPHY FOR HYDROCARBON GROUP TYPE ANALYSIS OF CRUDE OILS

ABSTRACT

High performance liquid chromatography (HPLC) was applied to four commercial grade Saudi Arabian crude oils having API gravity in the range 28 to 38 for the determination of hydrocarbon group types namely asphaltenes, saturates, aromatics and polars. Each of these crude oils was separated into asphaltenes and maltenes using n-hexane as the precipitating solvent. The maltenes (n-hexane soluble) were fractionated into saturates, aromatics and polars fractions by n-hexane elution on a column packed with amino propylsilane chemically bonded to porous silica particles.

The data obtained shows that the weight percent saturates increase whereas aromatics, polars and asphaltenes decrease from Arab Heavy to Arab Bern through Arab Medium and Arab Light crude oil. The results obtained from HPLC were in comparison with those obtained from ASTM method D2007. This method is easier, faster and offer good repeatability. This method can be applied to other crude oils.     

Determination by PIXE of the Metallic Content in Vacuum Residue,Asphaltenes and Maltenes

Abstract

Asphaltenes from Mexican Maya crude oil were precipitated during one agitation hour using n-C₅, n-C₆, n-C₇, and n-C₈ at room temperature. Later the asphaltenes were washed with a soxhlet extraction system during 24 hr to remove the maltenes. The characterization of the vacuum residue, asphaltenes, and maltenes was realized using proton induced x-ray emission (PIXE) for the direct determination of the distributions and abundances of metals in the vacuum residue and their respective fractions. The analysis revealed that vacuum residue contains Fe, Al, V, and Ni, while the asphaltenes and maltenes mainly contain V and Ni.     

Structural Characterization of Asphaltenes and Ethyl Acetate Insoluble Fractions of Petroleum Vacuum Residues

Asphaltenes and insoluble fractions of vacuum residues (VRs) of two Indian crude oils (viz. Heera and Jodhpur) of different specific gravity were obtained by precipitation of VRs in n-hexane, n-heptane, and ethyl acetate, and also by subsequent reprecipitation of n-heptane and ethyl acetate soluble fractions by n-pentane. The effect of various solvents on average molecular structure of asphaltenes and insolubles was studied using nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), and size exclusion chromatography (SEC). The asphaltenes and insolubles of Jodhpur VR have higher amounts of high molecular weight species with a high concentration of condensed and substituted aromatic rings, branched and/or short alkyl side chains, oxygen and nitrogen functionalities, compared to that of Heera VR. Ethyl acetate insolubles comprise a higher number of substituted aromatic structures, branched aliphatic structures, complex average unit structures, nitrogen and oxygen functionalities, and high molecular weight (MW) species as compared to hexane and heptane asphaltenes. Heptane insolubles consist of more naphthenic rings condensed with aromatic rings than C6A and EAI.     

Preparative Subfractionation of Petroleum Resins and Asphaltenes. II. Characterization of Size Exclusion Chromatography Isolated Fractions

Abstract

In the first study of this series, resins and asphaltenes from stable and unstable crude oils have been separated by size exclusion chromatography in three molecular mass ranges (MM: high, medium, and low). In the present study, the isolated fractions were further characterized in order to correlate several of their structural properties with crude oil tendency to solid deposition. Elemental analysis (C, H, S, N), infrared spectrophotometry (IR), and proton nuclear magnetic resonance (¹H NMR) were the characterization techniques performed in order to meet the objective. Hydrogen deficiency and aromaticity appear to be the main parameters governing the intrinsic stability for the studied samples. Oxygen compounds were observed to be more abundant within fractions isolated from unstable oils, particularly in resins and in the low MM range fractions both from resins and asphaltenes. However, causes for the presence of these oxygenates remain open to further study. Experimental evidence gathered during this work suggest the possibility of small compounds acting like natural dispersants of the larger components present in complex mixtures like asphaltenes.     

SEPARATION OF ASPHALTENES BY POLARITY USING LIQUID-LIQUID EXTRACTION

ABSTRACT

In order to investigate the nature of petroleum asphaltenes in terms of polarity a process was developed using initial liquid-liquid extraction of the oil phase followed by precipitation of the asphaltenes using n-heptane. The liquid-liquid extraction was performed using toluene-methanol mixtures with increasing content of toluene. Although large fractions of the crude oil (Alaska '93) was extracted in the higher polarity solvents (high concentration of methanol), the asphaltene content of the dissolved material was low As the toluene content increased more asphaltenes were transferred to the solvent phase. The asphaltenes were analysed using FTir, Elemental analysis, and HPLC-SEC with a diode array detector. With increasing content of toluene in the methanol the molecular weight distribution of the asphaltenes significantly move to higher molecular weights. The content of nitrogen and sulfur of the mallene phase also increase while H/C decreases. The content of heteroatoms in the asphaltenes are relatively higher and apparently increase with the polarity of the solvent. It is concluded that these asphaltenes are indeed dominated by high molecular weight substances that cannot be extracted in the high polarity solvents