Maltenes and Asphaltenes of Petroleum Vacuum Residues: Physico-Chemical Characterization

Solvent separation is frequently applied to petroleum vacuum residues to reduce the coke-forming tendencies of these materials. This process is capable of removing all or a substantial amount of asphaltenes from feedstocks that are destined for further processing and thus applied as the first step of refining. Maltenes and asphaltenes obtained from vacuum residues of Heera (HVR) and Jodhpur (JVR) Indian crude oils using n-hexane, n-heptane, and soluble and insoluble fractions obtained using ethyl acetate, were characterized for elemental analysis, molecular weight, conradson carbon residue (CCR), specific gravity, and pour points. The resulting degree of removal of asphaltenes ranged from 10-28 wt% of the HVR and 25-50 wt% of the JVR. The increasing trend of the American Petroleum Institute (API) gravity and the decreasing trend of CCR and pour point are observed with the increase in removal of asphaltenes.     

The Evaluation of Two Local Petroleum Residues

Abstract

Two vacuum residues were delivered from two different petroleum refineries, one from Suez Petroleum Company and the second from Alexandria Petroleum Company. They were subjected to solvent extraction using n-pentane, n-heptane, and ethyl acetate. The process of solvent extraction aims to separate maltenes and asphaltenes. The maltenes were further subjected to liquid chromatography (column chromatograph) in order to separate them into saturates and aromatics (mono-, di-, and poly-) and resins. The saturates that were separated from the maltenes of the two vacuum residues with different solvents were studied by gas chromatography in order to determine how much n-paraffins and cyclo-paraffins they contained.     

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.     

Resolution and Quantification of Ring Type Aromatics by HPLC Method Using n -Hexane Elution

Abstract

A method is described for the ring type separation of aromatic fraction of crude oils. The crude oils were dissolved in n-hexane to precipitate asphaltenes. The n-hexane dissolved portion of the crude oil was separated into saturates, aromatic types, and polars using high performance liquid chromatography. The aromatics were differentiated into monoaromatics (1R), diaromatics (2R), triaromatics (3R), tetraaromatics (4R), and polyaromatics (5R+). This method was applied to four commercial Saudi Arabian crude oils namely Arab Berri, Arab Light, Arab Medium, and Arab Heavy having API gravity in the range 28.0–38.5. The HPLC separation was carried out by n-hexane elution on a amino propylsilane silica column. The hydrocarbon group types were determined on weight percent basis. The saturates and total aromatics were found decreasing with decreasing API gravity while polars and asphaltenes increased. This trend may be due to the reason that the condensed polycyclic compounds and heteroatomic contents increase with decreasing API gravity in crude oils. This method may be applied to other crude oils for aromatic types determination.     

CATALYTIC HYDROTREATMENT OF PETROLEUM RESIDUE

ABSTRACT

Iraqi reduced crude (350°C+) with a sulfur content of 4.3 wt% and a total metal content (Ni+V) of 141 WPPM was n-heptane deasphalted at specified conditions. The deasphalted oil (97.2 wt% of original residue) contains 4.1 wt% of sulfur and 103 ppm of metal. The original reduced crude and deasphalted oil were hydrotreated on a commercial Ni-Mo-alumina catalyst presulfided at specified conditions in a laboratory trickle-bed reactor. The reaction temperatures varied from 300 to 420°C with the liquid hourly space velocity (LHSV) ranging from 0.37 to 2.6 h^?1. Hydrogen pressure was kept constant throughout the experiments at 6.1 MPa, with a hydrogen/oil ratio of about 300 NLL^?1 (normal liters of hydrogen per liter of feedstock). Analysis for sulfur, nickel, vanadium and n-pentane asphaltenes were carried out for hydrotreated products from both the original residue and the deasphalted oil. The comparison of the results obtained for the hydrotreatment of deasphalted oil and original reduced crude indicates that the removal of sulfur, nickel and vanadium was higher for the deasphalted oil than those obtained for the non-deasphalted residue over the entire range of conversion. The exclusion of extremely high molecular weight asphaltenes by n-heptane deasphalting seems to improve the access of oil into catalyst pores resulting in higher desulfurization and conversion of the lower molecular weight asphaltenes. The sulfur content of n-pentane precipitated asphaltenes remained unchaneed with LHSV for various temperature for hydrotreated products produced from both deasphalted oil and original reduced crude.     

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.     

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.     

Rheological Properties and Composition of Some Vacuum Distillates Derived from Indian Crudes

Abstract

Three vacuum distillates boiling range 400°C–530°C derived from Bakrola, Dholka, and PY-3 crudes of Gujarat region (India) were subjected to urea adduction to isolate n-paraffin components from them. The deparaffinized base oils obtained after removal of n-paraffin components from the distillate fractions were further fractionated into various hydrocarbon-type constituents viz iso + cyclo paraffins, aromatics, and polar components by column chromatography. The influence of various hydrocarbon-type components thus separated on rheological characteristics (pour point and viscosity temperature behavior) of deparaffinized base oil was investigated. This study was done by preparing base oil blends by adding separated hydrocarbon-type constituents in various concentrations to different base oils and characterizing them. The effect of change of solvent matrix on the rheological characteristics was also determined. The study demonstrated the role of solid n-paraffins for variation in pour point and viscosity temperature behavior of the waxy distillate fractions.     

THE COLLOIDAL STRUCTURE OF CRUDE OILS AND SUSPENSIONS OF ASPHALTENES AND RESINS

ABSTRACT

A better understanding of colloidal macrostructure of the heavy petroleum products and their complex fractions is of great importance in the context of industrial problems that arise during the crude oil production, refining and transport. Much effort has been devoted to the chemical structure studies, but there is a need for more precise data regarding parameters that characterize those complex systems. For instance, the molecular weight of heavy molecules, the composition and size of aggregates formed during the industrial processing and their evolution upon the variation of temperature, pressure and with the addition of solvent have not been well known. In this paper we present new results obtained using several powerful techniques. Scattering methods (using X-rays and neutrons) are applied to study both the fractionated products (asphaltene and resin solutions in more or less good solvents) and the real systems (Safaniya vacuum residue). The lamellar structural model for asphaltenes and resins is confirmed and the molecular weight of these species determined using a polydisperse size distribution. Discussion is presented concerning the specificity of X-ray and neutron scattering : X-ray experiments are more sensitive to the aromatic-rich regions, whereas the neutron scattering data provide information about all the particle volume. Viscosimetry measurements provide information on the molecular shape of asphaltene and confirm the disk-like model. Critical micellar concentration has been obtained using Vapour Pressure Osmometry (VPO) for asphaltene suspensions in toluene and in pyridine. The resin molecules are smaller than asphaltenes, and appear to be a good solvent for asphaltenes. One of the major conclusions of this work is the wide-spread presence of density heterogeneities in diluted solutions of asphaltenes and resins as well as in the pure product (Safaniya vacuum residue). This was deduced from the scattering experiments and cryo-scanning electron microscopy data. The heating effects. were studied: a temperature increase leads to the decrease of molecular weight, but heterogeneities remain present. The structure of vacuum residue exhibits large density fluctuations which are thermally stable. These dense regions remain connected into a network up to 393°K and determine the yield value of the rheological behaviour.     

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.     

CHANGES IN THE COMPOSITION AND PROPERTIES OF THE VACUUM RESIDUES AS A RESULT OF VISBREAKING

ABSTRACT

Two vacuum residues of heavy petroleum blends and residues of the products of their visbreaking, on a commercial unit, were investigated by means of column chromatography, cryoscopic analysis, elemental analysis, NMR spectroscopy, etc. An increase in the contents of asphaltenes and saturated, increase in asphaltenes/polar components ratio, and decrease in the aromatics contents during visbreaking were observed for both kinds of feed. The N/C ratio increased, the S/C ratio did not change significantly. Most of the nickel and vanadium were concentrated in the asphaltenes and aromatics. The length of the paraffinic chains of the saturated decreases during visbreaking from about 50 to 30 carbon atoms. The number of carbon atoms per alkyl chain and the number of naphthenic rings in the average molecule of the aromatics decreased, whereas the number of aromatic rings increased. About 2/3 of the total aromatic carbon in the aromatics are non-bridged. The average asphaltenes molecule contains about 36-38 aromatic rings (mostly condensed), 5-7 naphthenic rings and 12-16 alkyl substituents having relatively short (n = 5-6) chains. Two contrary processes, relating asphaltenes, take place during visbreaking: 1) cracking of asphaltenes, which results in a decrease in their content and molecular weight, decrease in the number of the side chains and their length, and increase in asphaltenes aromaticity; 2) asphaltenes formation from the polar components of the feed. The resulting process may be expressed both in a decrease and in an increase in the asphaltenes content. Reactions of condensation of the asphaltenes precursors (resins, etc.) prevail for the light stocks. For heavier stocks, cracking of the asphaltenes plays a more significant role, and the increase in asphaltenes content takes place mainly due to concentrating them in the visbreaking residue as a result of the distillation of the visbroken product.     

Molecular Weight Determination of Asphaltenes from Colombian Crudes by Size Exclusion Chromatography (SEC) and Vapor Pressure Osmometry (VPO)

Abstract

Characterization of Colombian asphaltenes isolated from feedstocks to refining units was carried out during this study, emphasizing the determination of their average molecular weights (MW). Experimental conditions used for vapor pressure osmometry (VPO) and size exclusion chromatography (SEC) were investigated aiming to select the most suitable set up that affords the best representative values. SEC was observed to provide the most reasonable MW values, after analysis improvements were achieved by using realistic calibration curves got with phthalocyanines, compounds that closely mimic component structures within oil and asphaltenes fractions. MWs displayed by Colombian vacuum residua asphaltenes fall within the range 1,700 ± 200 Daltons, reported previously for asphaltenes from different reservoirs worldwide.     

HYDROCARBON GROUP TYPES ANALYSIS OF PETROLEUM PRODUCTS: A COMPARATIVE EVALUATION OF HPLC AND TLC ANALYTICAL PERFORMANCE

ABSTRACT

Four petroleum samples representing a range of hydrocarbon composition were analyzed using the HPLC and TLC–FID techniques that provided separation and quantitation of saturates, aromatics, polars (resins) and asphaltenes. The results obtained from the two methods, for residues and crude oil samples, were not comparable because these samples contain high amount of polars and asphaltenes whereas VGO shows a very good comparison between the results obtained from the two techniques because of its asphaltenes-free and low polar composition. TLC–FID separation required a number of solvents while in HPLC, only one type of solvent was used. In TLC method, a number of samples can be analyzed simultaneously while in HPLC, only one sample is analyzed at a time. HPLC system analyzes a large number of sample in a complete automated fashion without operator involvement while the TLC method needs more involvement and attention of the operators. In HPLC, the aromatics can be further classified based on number of rings while in TLC–FID, aromatics were detected as one peak. The reproducibility of TLC–FID analysis for individual fractions was found good.     

Test Methods for Determining Asphaltene Stability in Crude Oils

Abstract

The high cost of remediating asphaltene deposition in crude oil production and processing has necessitated the development of test methods for determining the stability of asphaltenes in crude oils. In the current work, the stability of asphaltenes in crude oils of varying API gravity is predicted using the Oliensis Spot Test, the Colloidal Instability Index, the Asphaltene–Resin ratio, and a solvent titration method with NIR solids detection. The test methods are described in detail and experimental data from them presented. The experimental stability data were validated via correlation with field deposition data. The effectiveness of the various tests as predictors of the stability of asphaltenes in oils is discussed. The Colloidal Instability Index and the solvent titration method were found to predict a crude oil's propensity towards asphaltene precipitation better than both the Asphaltene–Resin ratio and the Oliensis Spot Test. For oils with low asphaltene content where most stability tests fail, live oil depressurization is proposed as the test for predicting the stability of asphaltenes.     

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.     

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

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.     

CATALYTIC HYDROTREATMENT OF PETROLEUM RESIDUE

Iraqi reduced crude (350°C+) with a sulfur content of 4.3 wt% and a total metal content (Ni+V) of 141 WPPM was n-heptane deasphalted at specified conditions. The deasphalted oil (97.2 wt% of original residue) contains 4.1 wt% of sulfur and 103 ppm of metal. The original reduced crude and deasphalted oil were hydrotreated on a commercial Ni-Mo-alumina catalyst presulfided at specified conditions in a laboratory trickle-bed reactor. The reaction temperatures varied from 300 to 420°C with the liquid hourly space velocity (LHSV) ranging from 0.37 to 2.6 h^-1. Hydrogen pressure was kept constant throughout the experiments at 6.1 MPa, with a hydrogen/oil ratio of about 300 NLL^-1 (normal liters of hydrogen per liter of feedstock). Analysis for sulfur, nickel, vanadium and n-pentane asphaltenes were carried out for hydrotreated products from both the original residue and the deasphalted oil. The comparison of the results obtained for the hydrotreatment of deasphalted oil and original reduced crude indicates that the removal of sulfur, nickel and vanadium was higher for the deasphalted oil than those obtained for the non-deasphalted residue over the entire range of conversion. The exclusion of extremely high molecular weight asphaltenes by n-heptane deasphalting seems to improve the access of oil into catalyst pores resulting in higher desulfurization and conversion of the lower molecular weight asphaltenes. The sulfur content of n-pentane precipitated asphaltenes remained unchaneed with LHSV for various temperature for hydrotreated products produced from both deasphalted oil and original reduced crude.     

Effect of polar organic substances on the distillates yield in atmospheric and vacuum distillation of crude oil

The influence of admixtures of chemically different polar organic solvents on the yield of distillates in atmospheric and vacuum distillation of crude oil has been studied. A comparative assessment of the compositions of asphaltenes isolated from vacuum residues has been made with the use of IR spectroscopy. The distillate recovery efficiency has been found to increase to the greatest extent in the presence of N-methylpyrrolidone as an additive during atmospheric distillation and ?-caprolactam in the case of vacuum distillation. However, the admixture of ?-caprolactam to crude oil does not provide an increase in aromaticity of vacuum-residue asphaltenes.