Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils and bitumen

The molecular weight of asphaltenes has been a controversy for several decades. In recent years, several techniques have converged on the size of the fused ring system; indicating that chromophores in virgin crude oil asphaltenes typically have 4-10 fused rings. Consequently, the molecular weight debate is equivalent to determining whether asphaltenes are monomeric (one fused-ring system per molecule) or whether they are polymeric. Time-resolved fluorescence depolarization (FD) is employed here to interrogate the absolute size of asphaltene molecules and to determine the relation of the size of the fused ring system to that of the corresponding molecule. Coal, petroleum and bitumen asphaltenes are compared. Molecular size of coal asphaltenes obtained here by FD-determined rotational diffusion match closely with Taylor-dispersion-derived translational diffusion measurements with UV absorption [1]. Coal asphaltenes are smaller than petroleum asphaltenes. N-methyl pyrrolidinone (NMP) soluble and insoluble fractions are examined. NMP soluble and insoluble fractions of asphaltenes are monomeric. It is suggested that the ‘giant’ asphaltene molecules reported from SEC studies using NMP as the eluting solvent may actually be the expected flocs of asphaltene which are not soluble in NMP. Data is presented that intramolecular electronic relaxation in asphaltenes does not perturb FD results.     

Molecular size of asphaltene fractions obtained from residuum hydrotreatment

Previously, fluorescence depolarization techniques (FD) have been shown to measure asphaltene molecular size, thereby establishing the substantial difference between asphaltenes derived from crude oil vs from coal. Here, FD is used to track the changes of the asphaltenes from a petroleum atmospheric resid feedstock that has been subjected to increasing thermal severity of catalytic hydrothermal cracking. Changes in asphaltene properties with increasing cracking are readily observed and understood. In addition, asphaltene molecular size is measured for various asphaltene solubility fractions in binary solvent mixtures of toluene with either n-heptane or acetone; a strong dependence is found of asphaltene properties on the particular solvent mixtures in accord with recent publications.     

DIFFUSION-CONTROLLED ADSORPTIVE UPTAKE OF COAL AND PETROLEUM ASPHALTENES IN A NiMo/Al2O3 HYDROTREATING CATALYST

In this work, hindered diffusion of one coal and two petroleum asphaltenes was studied by adsorptive uptake in THF from a bath surrounding a NiMo/Al₂O₃ catalyst. A mathematical model for the adsorption-diffusion of asphaltenes was developed. The model parameters were obtained by simulating the experimental data with the model solution. Several asphaltene fractions were defined via SEC (Size Exclusion Chromatography), with the molecular weight of each fraction being determined by its elution characteristics. It was found that both the coal and petroleum asphaltenes have very broad molecular weight distributions; however, the molecular weights of the coal asphaltenes (50-2000) were much smaller than those of the petroleum asphaltenes (200-30000). The uptake rates for asphaltene fractions with different molecular weights varied, depending on their diffusion rates and adsorption constants. Simulation results showed that even though the properties of coal and petroleum asphaltenes were quite different, the values of model parameters for the fractions of the three asphaltenes had the same trend and could be estimated by same numerical expressions; with increasing molecular weight of the fraction, the adsorption constant monotonically increased, and the effective diffusivity decreased. The experimental uptake data for the three asphaltenes as a function of molecular weight were well represented by same mathematical model.     

Carbonization of petroleum feedstocks II: Chemical constitution of feedstock asphaltenes and mesophase development

The characterization of the asphaltene fractions of a range of petroleum feedstocks by FT-IR, ¹H and ¹³C NMR spectroscopy indicated distinct differences in the molecular structure of the asphaltenes. Some of these differences could be related to the variations in the size of the principal optical texture of the semi-cokes produced by carbonization. The principal optical texture size was observed to increase steadily with the increasing hydrogen aromaticity of the asphaltenes over the whole range of the feedstocks used. There was no consistent correlation, however, between the carbon aromaticity of the asphaltenes and the optical texture size. The correlation between the hydrogen aromaticity and the principal optical texture size was attributed to structural differences among the asphaltenes that are critically important for the mesophase development.     

Pyrolysis of SCG extract coal asphaltenes: Gas chromatographic-mass spectroscopic study of the n-pentane soluble fraction

The asphaltene fraction from a super-critical gas (SCG) extract of bituminous coal was pyrolysed under nitrogen at temperatures from 200 to 450 °C, and the resultant pyrolysate re-fractionated into four conventional solubility classes (n-pentane-soluble, benzene-soluble, pyridine-soluble and pyridine-insoluble). The present work describes the characterization of the n-pentane-soluble fraction by combined gas chromatography-mass spectrometry. Preliminary studies indicated that asphaltenes, conventionally isolated, occlude significant amounts of pentane-soluble material and a multiple precipitation procedure for generating a ‘clean’ asphaltene was accordingly adopted. A very complex mix of pentane-soluble material was generated from the asphaltenes at the ≈2–4% level throughout the entire pyrolysis temperature range. The products identified included (1) n-alkanes (2) alkylbenzenes (3) alkylnaphthalenes (4) indanes (5) biphenyls (6) phenols and (7) other minor constituents such as fluorene and phenanthrenes. They were separated by capillary g.c. and identified either by co-injection of standards or from their mass spectral properties using comparison with a computer accessed library of mass spectral data. The major products of pyrolysis of SCG asphaltenes are benzene-insoluble fractions (including pyridine-insolubles); the formation of these is associated with the reduction of phenolic content of the asphaltene pyrolysand.     

Comparison of g.p.c. elution characteristics and diffusion coefficients of asphaltenes

A theoretical analysis of configurational effects on exclusion chromatography (g.p.c.) and diffusion rates of macromolecules is used to interpret data for the diffusion coefficients of petroleum-derived asphaltenes in tetrahydrofuran at 25 °C. If the asphaltene constituents are considered to be rigid, disc-like molecules, the theory and experiments are consistent in that both show the asphaltene diffusion coefficients to be smaller than those of g.p.c.-equivalent polystyrenes. Two conclusions result from this work: (1) polystyrene is not a good molecular model for predicting the diffusional characteristics of asphaltenes; and (2) there is a broad variation (by a factor of ten) of diffusion coefficients among the constituents of one asphaltene extract.     

Extraction of victorian brown coals with supercritical water

Extraction of five Victorian brown coals with supercritical water in a semi-continuous reactor was investigated. At 380°C and 22 MPa conversions of 42–54% and extract yields of 20–26% were obtained, significantly higher than with toluene under the same conditions. The extracts were separated into oils, asphaltenes and pre-asphaltenes and these were studied by NMR spectroscopy combined with elemental, OH and molecular weight analyses. The extacts have a high asphaltene and pre-asphaltene content, together with a high oxygen content, especially in the asphaltene and pre-asphaltene fractions. Long alkyl chains were present in the oils and asphaltenes. Temperature had little effect on the extraction, whereas pressure has a marked effect on both the conversion and the composition of the extract. The major difference between the toluene and water extracts is the higher oxygen content of the latter. There was a considerable improvement in the conversion using 0.5 M NaOH rather than water. Analyses of some of the chars were also carried out.     

Diffusivity of coal and petroleum asphaltene monomers by fluorescence correlation spectroscopy

Using fluorescence correlation spectroscopy (FCS) we measure the translational diffusion coefficient of Iino coal asphaltenes and UG8 petroleum asphaltenes dispersed in toluene at extremely low concentrations (0.03–3.0 mg/L). Using a simple scaling argument we compare them with several molecules of known size and molecular weight. We find that Iino coal asphaltene molecules – which have a comparatively small alkane fraction – have a diffusion constant of 1.5 × 10⁻⁵ cm²/s and corresponding hydrodynamic radius close to 0.25 nm: making them significantly smaller than petroleum asphaltenes.     

Studies on the evolution of asphaltene structure during hydroconversion of petroleum residues

The evolution of asphaltenes has been studied under hydroconversion conditions with an ebullated bed on a Buzurgan (Middle East) feedstock. A bench unit was used to produce effluents in residue conversion conditions ranging from 55 to 85 wt.%. In those conditions, asphaltene conversion ranged 62–89 wt.%. Asphaltenes from the feedstock and unconverted asphaltenes have been characterized using Size Exclusion Chromatography to evaluate asphaltene size, and ¹³C Nuclear Magnetic Resonance to evaluate the evolution of the average molecular structure parameters of asphaltenes. Results obtained were compared to results obtained at moderate residue conversion levels in a fixed bed.

The work clearly shows that unconverted asphaltene evolution is continuous. Unit size decreases with increasing conversion while the aromaticity of the asphaltenes increases due to dealkylation. Our results suggest that the remaining asphaltenes are dissociated in smaller aggregates around 50% conversion. Below this level, asphaltene evolution may be related to dissociation mechanisms. Above 50% conversion, the chemical structure of asphaltene units still significantly evolves through dealkylation mechanisms, leading to fairly condensed structures that remain at high conversion. Based on these results, an attempt was conducted to interpret the evolution of unconverted asphaltenes as a function of residue conversion level using a simple molecular reconstruction method.     

The effect of bitumen molecular fractions on diffusivity and rheology of bitumen under high-temperature conditions: Molecular dynamics (MD) simulation study

Heavy oil and bitumen play an incredible role in Canada's energy resources. The main processes that have already been applied to produce heavy oil and bitumen are in-situ thermal methods. The primary mechanism of production in these reservoirs is a reduction in heavy oil and bitumen viscosities via heat transfer. Having deep knowledge about the rheological behaviour of heavy oil and bitumen is crucial to designing a more accurate and efficient in-situ thermal recovery method. In this work, molecular dynamics (MD) simulation was used to model the rheological behaviour of bitumen under different temperatures. According to MD outputs, the highest diffusion coefficient between bitumen fractions belongs to saturate fractions. On the other hand, the lowest diffusion coefficient belongs to asphaltene fractions. The size of asphaltene, its polarity, and the polarity of a resin fraction affect the diffusion coefficient of asphaltene in a bitumen sample and its rheological behaviour. The MD simulation aims to provide molecular insights and essential information about the rheological trend of bitumen under different thermodynamic conditions. The results of the current work provide essential information about the effect of bitumen fractions on its rheological behaviour.     

Asphaltene analysis using size exclusion chromatography

A common technique used in coal liquefaction investigations is the asphaltene analysis, whereby products are separated into four major fractions (insolubles, preasphaltenes, asphaltenes and oils) by solvent extraction techniques. The fractions are defined by their solubility in organic solvents such as tetrahydrofuran, toluene and pentane. An instrumental method of analysis, using gel permeation liquid chromatography, was developed to reduce the problems with the conventional solvent separation. The technique relies upon the fact that the fractions differ in molecular size as well as solubility. The method uses one solvent (THF) and a single 100 Å microstyragel column to separate the three soluble fractions. The trends in the data obtained with the molecular size separation agree with the trends obtained by conventional solvent separations on the same samples.     

The zeta potential and surface properties of asphaltenes obtained with different crude oil/n-heptane proportions

We have investigated some surface properties of asphaltenes precipitated from crude oil with different volumes of n-heptane. According to the crude oil/n-heptane proportions used, asphaltenes are identified as 1:5, 1:15 and 1:40. Zeta potential results indicate that the amount of n-heptane determines the electrokintetic behaviour of asphaltenes in aqueous suspensions. Asphaltene 1:5 exhibits an isoelectric point (IEP) at pH 4.5 whereas asphaltenes 1:15 and 1:40 show an IEP at about pH 3. Surface charge on asphaltenes arises from the dissociation of acid functionalities and the protonation of basic functional groups. The presence of resins remaining on the asphaltene molecules may be responsible for the different IEP of asphaltene 1:5. Both sodium dodecyl sulfate (an anionic surfactant) and cetylpyridinium chloride (a cationic surfactant) were found to adsorb specifically onto asphaltenes. They reverse the sign of the zeta potential under certain conditions. These surfactants may be potential candidates to aid in controlling the stability of crude oil dispersions. Critical micelle concentration, interfacial tension measurements, and Langmuir isotherms at the air-water interface confirm the different nature of asphaltene 1:5, which also showed more solubility and a larger molecular size.     

Thermoplastic properties of coals carbonized at elevated pressures: effects of waste inorganic materials (GOB)

Thermal behaviour of coal-derived asphaltenes was investigated under atmospheric pressure by thermogravimetry (TG). The weight loss is rapid from 300 to 500°C and is slow above 500°C. The asphaltene comparatively low in molecular weight shows the greater weight loss. The temperature at which the differential thermogravimetry peak appears, T_m, correlates to the asphaltene aromaticity: the asphaltene comparatively high in aromaticity shows higher T_m. Of the residual asphaltene after heating up to 600°C, the obtained crystallite thickness through the c-axis direction (Lc₀₀₂) has a correlation with the molecular weight of the parent asphaltene: the parent asphaltene comparatively low in molecular weight produces residual asphaltene of larger Lc₀₀₂.     

Pressure and temperature effects on the hydrogenation of coal-derived asphaltene

Pressure and temperature effects on hydrogenation reactions were examined using coal-derived asphaltene at 390,420 and 450 °C, under 3 and 10 MPa of hydrogen partial pressure. Higher conversion was obtained at higher reaction temperatures. Benzene-insoluble material (Bl) was formed at higher temperatures especially at low hydrogen pressure, this Bl being one-third of the reaction product at 450 °C. From structural analysis of unreacted asphaltenes and product oils, at 390 °C, it was concluded that smaller molecular components convert to oil initially and the larger molecules remain as unreacted asphaltene. Under higher hydrogen pressure for all temperatures carbon aromaticity (f_a) and number of aromatic ring per structural unit (R_aus) in unreacted asphaltenes were lower than those under lower hydrogen pressure suggesting that hydrogenation of the aromatic nucleus was promoted by higher pressure. At lower hydrogen pressure, R_aus for asphaltenes at higher temperature is larger than that at lower temperature. This suggests that at lower hydrogen pressure, dehydrogenation or condensation reactions occur more easily. A large effect at higher hydrogen pressure was a reduction in the extent of condensation reactions. Higher reaction temperatures contribute to splitting of bridged linkages so reducing molecular size and degree of aromatization.     

Relationship between solvent-derived and compound-class fractions in coal-derived distillates and vacuum still bottoms

High-boiling SRC-1 process-derived distillable liquids and nondistillable vacuum still bottoms (VSB) from Wyodak and Kentucky 9/14 coals were separated into solvent-derived and compound-class fractions using Chromatographic techniques. The fractions were characterized using infrared spectrometry, proton nuclear magnetic resonance spectrometry, field ionization mass spectrometry, and elemental analysis. Emphasis was placed on the determination of the composition of oils and asphaltenes. Results showed that oils and asphaltenes consist of the same compound classes: hydrocarbons, nitrogen compounds, and hydroxyl aromatics. The main differences between the oil and asphaltene fractions are in concentrations of compound classes. It was found that oils are rich in hydrocarbons while asphaltenes are rich in hydroxyl aromatics. Also, oils and asphaltenes contain compounds of the same homologous series, and molecular weight is not a factor which differentiates oils and asphaltenes. Components in VSB oils have higher molecular weights than components in distillate asphaltenes. Molecular structure rather than molecular weight is a major parameter that determines solubility of coal-derived liquids.     

Thermal field-flow fractionation of asphaltenes

The thermal field-flow fractionation technique has been applied to the analysis of three samples of asphalt and asphaltenes. These samples were effectively retained in all three of the solvents used as eluants, with toluene giving the best retention level and solubility. The influence of eluant flow rate on peak retention and peak broadening was studied. It has been established, by means of the thermogravitational effect, that asphaltenes, like polymers, migrate toward decreasing temperatures under the influence of a thermal gradient. Asphaltene sample fractions were collected at the separator outlet and then reinjected into the channel, demonstrating an important fractionation effect. U.v.-visible spectrometric analysis of the fractions revealed a continuous modification of the spectra with the fraction collection order. The retention of asphaltenes was compared with that of polystyrenes. The sample molecular weight distribution and the average molecular weights were determined by assuming that the relation between thermal diffusion factor and molecular weight is the same for these two kinds of substances. The average diffusion coefficient of asphaltenes was independently determined by the chromatographic peak dispersion method; in terms of molecular weight, this provided a result coherent with the retention measurements. It is therefore proposed that polystyrene be used as a convenient reference material for the determination of molecular weights of heavy oil fractions and for the comparison of samples from different origins.     

Effect of composition and configuration on hindered diffusion of residue fractions through polycarbonate membranes

Diffusion behavior of narrow fractions of three residues and subfractions of a residue through four polycarbonate membranes was investigated using a diaphragm cell at 308 K. The results show that the diffusivities of fractions with similar molecular weight behaved differently. Comparisons among SAR subfractions (saturates, aromatics, and resins) of the same fraction show that saturates has the largest diffusion coefficient, followed by aromatics, and then resins. The diffusion coefficients of fractions fall among that of their subfractions. The regular variation of diffusivity along with the properties for fractions with similar molecular weight is an indication of the difference in their diffusivities resulting from the difference in their compositions and structures. Hindrance factors of the subfractions through membranes with small pores have a similar variation trend as their diffusion coefficients. The hindrance factors of the feedstocks in 50 and 80 nm pore size membranes are mostly larger than 0.90, but the values in 15 nm membranes range from 0.55 to 0.81. The study not only indicates that the diffusional limitations are significant in pores with the typical hydrotreating catalyst size but composition and configuration have an effect on the hindered diffusion of residue molecules. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1369–1377, 2013     

TGA studies of asphaltenes derived from cold-lake (Canada) bitumen

The pyrolysis of the asphaltene fraction from Cold-Lake, Canada, bitumen in a nitrogen environment in the temperature range 20–845°C has been studied by TGA. A rate equation for the decomposition of asphaltenes was derived and a second order decomposition of asphaltenes was observed. The apparent activation energy and the Arrhenius constant for the decomposition of the asphaltenes were calculated and found to be 56.5 kcal/mol and 5.2 × 10¹⁸g^?1 min^?1, respectively.     

Coal-derived asphaltenes: effect of phenol content and molecular weight on viscosity of solutions

Phenol contents and molecular weights of coal-derived asphaltenes are shown to affect the viscosity of their solutions. Phenol contents were determined by non-aqueous titrimetry. Intermolecular aggregation probably involving hydrogen bonding is a prime factor in the increase in viscosity found with increased asphaltene concentration in a reference solvent system composed of an $\text{88}\text{12}$ ($\text{wt}\text{wt}$) mixture of 1-methylnaphthalene and o-cresol. Aggregation effects are greater for those asphaltenes with relatively higher phenol contents. Asphaltenes were separated into fractions of different polarity by adsorption chromatography. The more polar subfraction was found to increase viscosity in the reference solvent to a greater extent than the less polar subfraction. The logarithmic viscosity numbers of solutions of the asphaltenes and their subfractions are correlated by a linear combination of molecular weights and phenol contents. It is concluded that an effective means of reducing the viscosity of coal-derived liquids would be to reduce the phenol content of the asphaltene fraction.     

Heterocompounds present in asphaltenes from various products of coal hydrogenation

The asphaltenes from hydrogenated coal extract (product A) as well as from hydrogenated low-temperature coal tars (products B and C) obtained from three bituminous coals were investigated. The asphaltenes were separated into basic (B) and acidic/neutral (A/N) fractions by precipitation of HCl adducts of nitrogen bases. The B and A/N fractions were investigated by means of thin-layer chromatography and i.r. spectroscopy to assist functional-group detection. Moreover, their high-resolution and field-ionization mass spectra were determined. The molecular weights of the asphaltene fractions are in the 80–600 amu range, and a few hundred molecular peaks were observed on the FIMS spectra. Part of the components were identified by HRMS — 27 nitrogen-compound types were found in the B fractions, while 38 compound types of O, N and S were detected in the A/N fractions. Although the asphaltenes were derived from two different liquefaction processes of three different coals, no significant differences in their composition have been found. They are hydrogenbonded complexes of basic and acidic components that are essentially the same heterocompounds in products A, B and C.