Microstructural study of asphaltene precipitated with methylene chloride and n-hexane

This work presents a study of the microstructure, molecular structure and elemental composition of asphaltene precipitated from vacuum residue using a solvent mixture. The infrared spectroscopy (FTIR) results show vibration bands of functional groups such as aromatic C-H (4 adj H and 1 adj H) and sulfoxide (C₂SO). Low vacuum scanning electron microscopy (LV-SEM) reveals a highly porous and a smooth surface asphaltene. C, S and V are the main elements identified by energy dispersive X-ray spectroscopy (EDS) analysis, though some variations in concentration were observed between the porous and the smooth surface asphaltene. Transmission electron microscopy (TEM) analysis shows that asphaltenes are constituted by nanometric particles (micelles) of ∼50 nm in diameter. These particles conform agglomerates (flocs) from ∼350 to ∼550 nm in size, some of them in layer arrangement with a tendency to graphitize. These results reveal important information about flocculation processes of the asphaltene.     

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.     

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.     

The effect of asphaltene on thermal properties of bitumen

Chemical composition and any variation of four main components of bitumen (asphaltene, saturated, naphthene aromatic and polar aromatic) have effective impact on its properties and especially, thermal properties. In other words, in order to have asphalt pavements with reasonable temperature susceptibility and thermal stability, first of all it is essential to know the effect of these components on bitumen properties. Therefore, in this research these effects were investigated and it was found that penetration index (PI) and penetration viscosity number (PVN) of bitumen were increased with increasing in asphaltene content.Thermal behavior of bitumen in an oxidizing environment was also studied using thermogravimetric analysis (TGA) technique. The results showed that increasing the asphaltene content of bitumen decreases the temperature susceptibility and improves the resistance of samples to thermal decomposition.     

Depolymerization and demetallation treatments of asphaltene in vacuum residue

The asphaltene fraction [hexane insoluble (HI)] of a vacuum residue (VR) was treated under ultrasonic irradiation at 40°C in THF or 150°C in 1-methyl naphthalene (1-MN) in the presence of an adsorbent composed of modified macro-reticular polystyrene resin. Such a treatment was found effective to convert the asphaltene into the hexane soluble (HS: maltene) without any hydrogen consumption. 61 and 72% of the HI was converted by the adsorption treatment at 40°C in THF and 150°C in 1-MN, respectively, to HS materials having lower molecular weights. About 65% of the metal contaminants in the original asphaltene remained with the newly formed maltenes after this treatment. Structural analyses of the asphaltene and maltene fractions before and after the treatment suggests decoagulation and/or depolymerization of the asphaltene into maltene, while the porphyrin moiety becomes soluble, being transformed to the maltene fraction. The roles of polar solvent, ultrasonic irradiation, and adsorbent are discussed based on the above results.     

Phase separation kinetics of maya asphaltene emulsion and free-to-bound water transformation

Differential scanning calorimetry (DSC) and inelastic neutron scattering were applied to study the phase separation kinetics of Maya asphaltene water-in-oil emulsion by following the water separation from the upper phase. In addition, transformation of the separated water from the free state to the bound and/or restricted state was also investigated. Maya asphaltene in toluene with 0.1 M HCl forms emulsion following ultrasonic or high speed mechanical emulsification. Initially, water molecules in the emulsion are largely free water. The emulsion gradually separates into two phases and at the 7th day over 90% of water molecules have situated at the bottom phase. In the mean time water molecules at the free state initially slowly transform into bound state in a much slower pace (over 100 days) than the phase separation kinetics.     

Study of self-aggregation of coal derived asphaltene in organic solvents: A fluorescence approach

Fluorescence spectra of coal derived asphaltenes sourced from Barari coke plant situated at Dhanbad, India, have been studied as a function of concentration in benzene, toluene and carbon tetrachloride with an eye to study the self-aggregation of the solute. Aggregation has been found to be a gradual process and the concentration characterizing onset of aggregation process coincide for the solvents. The critical aggregation constants (CAC) appear to be ca 90-100 and 180-200 for all the solvents. A small red shift of fluorescence band is observed when traces of water (∼0.05%, v/v) are present in carbon tetrachloride solution.     

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.     

Cracking behavior of asphaltene in the presence of iron catalysts supported on mesoporous molecular sieve with different pore diameters

Cracking of a mixture of petroleum asphaltene and 10 wt% Fe catalyst supported on mesoporous molecular sieve (SBA-15) possessing a hexagonal array of uniform mesopores has been studied with a fixed bed reactor at 573 K under atmospheric He. When average pore diameter of Fe/SBA-15 is varied between 4.5 and 15 nm, asphaltene conversion increases almost linearly with increasing pore diameter up to 12 nm and reaches 65%, though the increment is small beyond this value. On the other hand, yield of maltene formed is almost independent of the diameter and less than 15% but greatly improved by using pressurized H₂ in place of He. Although pore volumes of all Fe/SBA-15 catalysts decrease by mixing with feed asphaltene, the extent of the decrease is larger for the catalyst with a larger pore diameter, which shows that higher asphaltene conversion may arise from the presence of larger amounts of asphaltene molecules held inside the larger mesopores. The N₂ adsorption measurements reveal that pore structures of Fe/SBA-15 catalysts are almost unchanged after cracking and subsequent re-calcination to remove deposited coke. The X-ray diffraction analysis and temperature programmed oxidation after cracking suggest that Fe species are highly dispersed inside the mesopores and present as the sulfided phases at the outermost layer.     

沥青质和胶质对辽河稠油乳状液破乳的影响

针对辽河曙光稠油乳状液粘度大、破乳难、破乳温度高等问题,以曙光稠油为研究对象,从中分离出沥青质和胶质,用傅里叶变换红外光谱法分析其主要官能团,考察沥青质和胶质对稠油乳状液粘度及破乳效果的影响。测定了不同含量胶质、沥青质对稠油油／水界面张力的影响。结果证实,沥青质比胶质含有羟基多,分子间氢键作用强烈,更容易造成原油粘稠。随着叫（胶质）、训（沥青质）升高,稠油乳状液脱水率降低,w（沥青质）超过2．1％,或w（胶质）超过32％时,稠油乳状液的破乳十分困难。w（胶质）、w（沥青质）的增高会使破乳剂水溶液与原油的界面张力增大,w（沥青质）增加0．7％比叫（胶质）增加7％原油的界面张力升高还要大,所以训（沥青质）的增加对油水界面张力影响更大。     

The study on composition changes of heavy oils during steam stimulation processes

Using the heavy oils obtained from Liaohe oilfields in China, we have conducted the aquathermolysis reaction in laboratory at 240 °C. The results showed that Liaohe heavy oils have been undergoing visbreaking in the process of steam-drive and steam stimulation. After reaction with steam, the viscosity of the heavy oil was reduced by 28-42% and the amount of the saturated and aromatic hydrocarbons increased, while resin and asphaltene decreased. The gas partition chromatography showed that the accumulated amount of carbon numbers increased, after reaction, the accumulated amount of carbon numbers less than C₂₀ are 38.79-53.92%, and before reaction they are 13.30-20.92%. The results provided the basic data for heavy oil recovery by in situ catalytic method in production of heavy oil in oilfields.     

Modification of a thermodynamic model and an equation of state for accurate calculation of asphaltene precipitation phase behavior

In this study, DPTG (Dashtizadeh-Pazuki-Taghikhani-Ghotbi) equation of state has been modified for calculation of phase behavior of fluids and solubility parameter. The accuracy of the modified EOS has been proved by estimation of the properties of some hydrocarbons such as densities of methane and condensate gases, vaporization enthalpy, sublimation pressure, compressibility factor and comparison of the obtained results with the results of the present equations of state such as NJ (Nasrifar-Jalali), ZMJL (Zhi-Meiren-Jun-Lee) and PR (Peng-Robinson). Then, the Flory-Huggins model has been modified and asphaltene precipitation phase behavior at different ratios of solvents in the crude oil has been predicted by the modified EOS and the developed Flory-Huggins model. Comparison of the obtained results with the experimental data of asphaltene precipitation and the calculated ones by the main Flory-Huggins model shows the accuracy of the developed model.     

Application of Chebyshev polynomials to predict phase behavior of fluids containing asphaltene and associating components using SAFT equation of state

In this work, a thermodynamic model based on statistical association fluid theory (SAFT) is developed to predict the phase behavior of mixtures containing asphaltene contents. The SAFT equation of state is a good candidate for closing that gap between statistical mechanic models and the classical models dominated by cubic equation of state. A robust, fast and accurate computational algorithm based on Chebyshev polynomial approximation is developed to calculate the density and hence fugacity using SAFT equation of state in order to perform phase equilibrium calculations. Application of Chebyshev polynomials to approximate pressure-density function leads to an interpolation error of degree 10⁻¹³. Application of the proposed algorithm to calculate density of binary systems composed of ethanol and toluene shows an average relative deviation of 0.143% in the temperature range 283.15-353.15 K and for pressures up to 45 MPa. The proposed model is developed to predict the precipitation behavior of petroleum fluids containing asphaltene. The effect of pressure, temperature and solvent concentration on the amount of asphaltene precipitation is investigated. A good agreement with an AAD of 2.593% is observed between experimental and predicted amount of asphaltene precipitate. The model is also tested to investigate the effect of temperature and solvent concentration on asphaltene onset pressures (upper and lower). Again, an excellent agreement is observed between experimental and predicted values of the asphaltene onset pressure at different temperatures and solvent concentrations with an average 0.705% relative error. The accuracy of the proposed model is compared with WinProp software using Peng-Robinson equation of state with average 53.132% and 8.657% relative errors for the amount of asphaltene precipitate and onset pressure, respectively.     

Interfacial film properties of asphaltenes and resins

Interfacial film properties of asphaltenes and resins have been studied by interfacial shear viscosity measurements. The results show that the structure of the asphaltene film at the interface between oil and water is changed from two-dimensional to three-dimensional network as the concentration of the asphaltene at the interface increased. The film can be divided into three types namely expanded liquid film, condensed liquid film and solid-like three-dimensional network film. Furthermore, the structures of the interfacial films formed by asphaltene molecules and asphaltene particles are different and the strengths of these films are also different. The adsorption and migration processes of asphaltene molecules and migration process of asphaltene particles at the interface are different.     

Effect of hydrate formation/dissociation on emulsion stability using DSC and visual techniques

A key factor in hydrate risk management for an oil-dominated system is the stability of the emulsified water with gas hydrate formation. We show via differential scanning calorimetry (DSC) that gas hydrate formation and dissociation has a destabilizing effect on water-in-oil (W/O) emulsions, and can lead to a free water phase through agglomeration and coalescence of dissociated hydrate particles. High asphaltene content crude oils are shown to resist hydrate destabilization of the emulsion. Span80 was successfully used as an analog to asphaltene surface activity. Based on our experimental results, a new conceptual hydrate-induced destabilization model is proposed.     

Contribution of organic and mineral compounds to the formation of solid deposits inside petroleum wells

Deposit samples collected from three Mexican wells, their asphaltene and residue fractions as well as the sediment and asphaltene fractions separated from the corresponding crude oils were characterized. An appreciable amount of minerals such as CaCO₃, BaSO₄, NaCl, quartz and iron compounds were detected in the deposit compositions. Additionally, aromatic factors and structural parameters such as interlayer distance and crystallite diameters were calculated from the (γ) and (002) characteristic bands from the X-ray diffractograms of the deposits, deposit-asphaltene, oil-asphaltenes as well as oil-sediments. FTIR, elemental analysis and thermal techniques were also used to elucidate the sample compositions. The stages of the deposit formation inside petroleum wells include steel surface corrosion products formed in situ by the presence of brine and sulfur bearing compounds and adsorption or chemisorption of organic compounds on the modified tubing steel surface and mineral pores. Moreover, the high amount of vanadium and nickel indicates a time-dependent process of accumulation of some organometallic compounds independent of the oil-asphaltene amount.     

Heterogeneity of a vulcanized rubber by the formation of ZnS clusters

We investigated vulcanized rubber structures by energy-filtering transmission electron microscopy (EFTEM) and by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Both techniques revealed that numerous particles with the diameter of about 20 nm, which could not be observed by conventional TEM, were distributed in the rubber matrix. Further examination based on electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) analysis indicated that those particles are composed of ZnS clusters with the sizes of approximately 3-5 nm, which are produced as a by-product in the vulcanization. We believe that the formation of ZnS clusters in a rubber network is one of the origins of rubber heterogeneities.     

Particulate clusters and permeability in porous media

The permeability of particulate colloidal titanium dioxide, P25, was investigated during sedimentation, permeation and filtration when suspended in water at a consistent ionic strength similar to tap water. Happel's cell model of permeability was used to determine the apparent particle size during these processes, and compared with the size of particle clusters measured using laser diffraction under identical ionic conditions and varying degree of shear. The primary particle size of the P25 was determined to be 28 nm, from consideration of the surface area and density of the particles, and the cluster size during permeation and filtration was close to 100 nm. During sedimentation the cluster size was determined to be close to 10 μm, which is the same size obtained by laser diffraction when measuring under conditions of low shear. Using the above two sizes (28 nm and 10 μm) as limits in Happel's permeability model it was possible to determine an ‘operating envelope’ of permeability that matched the experimentally measured values for the sedimentation, permeation and filtration processes.     

Bifurcated correlations of the properties of crude oils with their asphaltene content

Asphaltene-based correlations of crude oil properties were analysed for more than 390 recovered (dead) crudes from a new database of the oils of the former USSR, as well as from databases by other authors. For densities, viscosities and pour points the analysis revealed a definite absence of frequently quoted strong linear correlations with asphaltene content and a persistent occurrence of non-linear anomalies for oils with ≈2.5-3 wt.% asphaltenes. As shown by selective filtering of databases, these anomalies most probably are due to the existence of two distinct types of world's crude oils, as classified with respect to their asphaltene-based correlations. Two separate branches also are observed in most correlations for SARA components, for contents of N, S, O, V and Ni. The analysed correlations show definite imprints of biodegradation and of source types. However, a review of various oil alteration processes shows that an apparent subdivision of crudes into two types is most probably due to deasphalting via a natural geochromatography of oils in course of migration, resulting in a selective removal of a well-defined polar asphaltene fraction, distinguished by its low solubility.     

Corrosion-protection properties of water-borne paint coatings as studied by electrochemical impedance spectroscopy and gravimetry

Water-borne paint coatings while used outdoors, show good protective behaviour on metals, even though they can take up and release much moisture. As experience shows for physically drying paint coatings, the changing wet-and-dry environment improve their corrosion protection properties. To find out the reasons, eight model coatings – built up from various combinations of layers made of the physically drying styrene-acrylate, and the oxidatively drying alkyd-acrylate based paints – were tested on metal substrate by electrochemical impedance spectroscopy; water uptake/release kinetics was followed by gravimetry on self-standing films made of the above resins. The results of experiments simulating the weathering effects (wet–dry cycling and heating) with the physically drying paint coatings lead us to the conclusions that: (i) water uptake and release removes the water-soluble components from the film and thereby contributes to the good or improving corrosion protection feature, and explains the observation that these coatings perform satisfactorily outdoors and (ii) water release is always much faster than the uptake which helps to keep the interior of the coating dry protecting the metal surface against moisture.