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.     

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.     

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.     

Hindered diffusion of asphaltenes through microporous membranes

The relationship between the diffusion coefficient of asphaltenes and pore size was studied experimentally using track-etched mica membranes. The asphaltenes were extracted from Kuwait atmospheric bottoms using criteria of n-pentane insolubility and tetrahydrofuran solubility at 25°C. Data were obtained for asphaltene fractions of narrow size range by using gel permeation chromatography to analyze the diffusion samples. The results show a very strong pore size effect on diffusion coefficient in the range 70–500 Å pore radius for asphaltenes whose GPC-equivalent polystyrene molecular weight is 3000 or greater. By properly accounting for the molecular size of the asphaltene fractions, we are able to obtain good agreement between the measured diffusion coefficients and those predicted from a hydrodynamic theory of hindered diffusion.     

Evaluation of nanoemulsions based on silicone polyethers for demulsification of asphaltene model emulsions

Asphaltenes are considered the main agents responsible for stabilizing petroleum emulsions. However, due to the complex chemical nature of crude oil, it is necessary to extract these molecules and prepare model solutions to investigate the effects of the various asphaltenes separately. In this study, the demulsification efficiency of oil‐in‐water (O/W) nanoemulsions based on silicone polyethers was evaluated using asphaltene model emulsions. The interfacial properties of the model emulsions were evaluated, with and without the presence of the nanoemulsions, by interfacial tension and inerfacial rheology measurements and correlating them with the ability and/or speed of diffusion to the interface. Dispersion/flocculation tests of the asphaltenes were performed to assess whether the nanoemulsions were modifying the aggregation state of the asphaltenes during the process of destabilizing the model emulsions. Through the interfacial rheology tests of the model asphaltene/saltwater system, with or without addition of the systems used in the demulsification tests, it was possible to determine the influence of the nanoemulsions on the mechanical properties of the interfacial film. The results of the water/oil gravitational separation tests showed that the nanoemulsions had separation efficiency between 80 and 95%, depending on the composition of the water/surfactant/oil/asphaltene system. The nanoemulsions containing xylene as the oil phase destabilized the emulsions the fastest. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44174.     

Fullerenic structures derived from oil asphaltenes

The handling and properties of heavy oils are becoming an increasingly important problem. Even though petroleum is the widest used source of fuels and it has been studied for decades, its complex nature is still an enigma in several ways. A reasonable approach to a definition of a crude oil is a colloidal fluid formed by several dispersed phases from gases (light hydrocarbons) to solids (heavy paraffins and asphaltenes). Through all these years of research, applications have been found for almost all classes of components in crude oil except for some of the solid phases such as asphaltenes. Very heavy petroleum is a non-newtonian liquid with a viscosity of ≈10⁶ Poise, and an average molecular weight of 600 amu. The solids that are toluene soluble but heptane insoluble are called asphaltenes and are the most aromatic fraction with the highest molecular weight of unconverted petroleum. In the present work, we applied high resolution transmission electron microscopy (HREM) and energy dispersive spectrometry (EDS) in the study of asphaltenes. It was found that when the asphaltenes are well separated from the resins the sample consists of a carbon structure containing S, V, Si, related to fullerenic carbon. During observation in the microscope it was possible to see the formation of fullerenes such as onions and C₂₄₀ @ C₆₀ structures. The fact that they decomposed under further irradiation suggests that they are metastable structures. Since the heteroatoms are still present they are likely to cause instability to the structure. Not only does our result indicate the possibility of obtaining fullerenes from crude oil but it also suggests the asphaltene molecule, when it is resin free, might be a precursor of fullerenic structures.     

反应时间对加氢沥青质结构变化的影响

在高压反应釜中对煤焦油进行催化加氢反应,利用XPS和元素分析对不同加氢反应时间下提取的煤焦油沥青质进行表征,系统分析加氢油品四组分变化、沥青质转化率、化学组成和杂原子赋存形态的变化。结果表明,随着反应时间的延长,加氢油品中饱和分和芳香分收率增大,胶质和沥青质收率减小。加氢后沥青质转化率逐渐增大,分子量先减小后增大,沥青质中氧原子主要以羰基氧和碳氧单键形式存在,氮原子主要以吡咯氮、吡啶氮和质子化吡啶形式存在。     

Viscosity of coal-derived liquids

A coal-derived liquid was separated into toluene insolubles, asphaltene and pentane soluble oil by conventional solvent analysis. The asphaltene was separated further into an acid-neutral and a basic component by a previously reported procedure from our laboratory. The viscosities of liquids reconstituted from toluene insolubles, asphaltenes, acid-neutral asphaltene or basic asphaltene in the pentane soluble oil were determined. The logarithm of the viscosity ratio is a linear function of concentration for each of the solutes. On a weight basis, the toluene insolubles produce about twice the viscosity as the asphaltenes. The viscosity of mixtures of acid-neutral and basic asphaltene in pentane soluble oil is greater than either component determined separately, presumably due to complex formation. The variation of the reduced specific viscosity with concentration indicates that aggregation of the asphaltene and toluene insoluble fractions plays a significant role in the viscosity of coal-derived liquids.     

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.     

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.     

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.     

Pyrolysis of petroleum asphaltene in tetralin

The pyrolysis of a petroleum asphaltene in tetralin donor solvent at 450 °C has been examined, and the products characterized by elemental, molecular weight, n.m.r. and g.c.-m.s. analysis. Degradation to mainly lower molecular weight products takes place, the residual asphaltene having a smaller average cluster size, higher aromaticity and a heteroatom content resistant to further reaction. β-bond scission is important in the early stages of reaction.     

Aggregation thermodynamics for asphaltene precipitation

Asphaltene precipitation has been a major concern for petroleum industry due to its adverse effect on upstream production, midstream transportation, and downstream refining. As a complex phenomenon involving solubility, aggregation, and clustering, asphaltene precipitation has been extensively investigated and correlated with empirical models and equations. Based on the insight regarding hierarchical structure of asphaltenes recently elucidated by Mullins, we present a thermodynamic formulation for asphaltene aggregation, the onset of asphaltene precipitation. The thermodynamic formulation accounts for asphaltene aggregation driving force as a two‐step process: (1) molecular asphaltene forming imaginary “nanocrystals”, and (2) “nanocrystals” re‐dissolving as colloidal nanoaggregates. Applying UNIFAC with this thermodynamic formulation, we show semi‐quantitative predictions of asphaltene precipitation in 13 binary solvents with wide varieties of chemical structures and solvent combinations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1254–1264, 2016     

Effect of heavy asphaltene on stability of residual oil

An asphaltene fractionation method was developed in order to investigate the effect of heptane (n-C7) insoluble asphaltene (C7-asphaltene) on residual oil stability. C7-asphaltene was separated into heavy and light fractions by a new method using a binary solvent system of toluene and heptane (n-C7). It was found that the heavy fraction of C7-asphaltene in residual oil, extracted by this method, consisted of highly condensed polynuclear aromatics. Our new fractionation method and the Heithaus stability evaluation method were applied to hydroprocessed residual oils. The peptizability of heavy fraction of C7-asphaltene defined by the Heithaus method decreased in accordance with structural condensation of that fraction. On the other hand, light fraction of C7-asphaltene was considered to influence the peptizing power. We proposed a new conceptual model: light asphaltenes would perform as peptizing material as well as resin, and heavy asphaltene would be peptized in oil. This model introduced from our new asphaltene fractionation method could be more effective for understanding the destabilization phenomenon of residual oil.     

影响煤焦油沥青质测定量的工艺参数

煤焦油通过溶剂分析可定量分离为三部分：甲苯不溶物、沥青质和油相。本文主要研究了煤焦油在此分离过程中芳烃溶剂类型、萃取方法、甲苯溶液浓缩量、烷烃溶剂类型和正庚烷溶剂与甲苯溶液浓缩量的添加比例对煤焦油沥青质沉淀量的影响。结果表明,甲苯溶液浓缩量、正庚烷与甲苯溶液浓缩量的添加比例对沥青质沉淀量的影响较大,萃取方法次之,将苯替换成甲苯影响最小。当选择甲苯溶解煤焦油,超声萃取3h,甲苯溶液浓缩量9mL,正庚烷与甲苯溶液浓缩液的添加比例为20∶1时,煤焦油沥青质的沉淀量为13.4%。本实验表明煤焦油的分离工艺会影响沥青质沉淀量,故在研究煤焦油沥青质性质和结构时需注明沥青质的沉淀分离条件。     

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.     

微生物降解稠油中沥青质的研究进展

稠油因黏度高、流动性差,存在开采难度大、开发成本高的问题。沥青质等重组分含量高是造成稠油高黏度的主要原因。微生物通过降解重组分可降低稠油的平均分子量,从而降低稠油黏度。本文从沥青质的结构与组成出发,阐述了微生物对沥青质的降解机理,总结了近年来国内外最新研究进展,指出了目前微生物降解沥青质研究与应用所面临的挑战,并对其发展趋势做出了展望。微生物对沥青质的降解主要是通过将多环芳烃进行开环降解、将长链正构烷烃降解为短链、将杂环化合物进行开环除去杂原子三个方面。但因沥青质的分子量较大且组成有着不确定性,所筛选的微生物是否能够高效地降解储层中原油所含沥青质仍需进一步探究,因此高效沥青质降解菌株的筛选和利用基因工程等技术手段改造菌株应是未来工作的研究重点。此外,通过菌株的复配,利用菌株间的协同效应也可以达到更为高效降解沥青质的目的。     

Chromatographic separation and characterization of coal-derived asphaltenes

Coal-derived asphaltenes from five major demonstration processes in the United States (Synthoil, HRI H-Coal, FMC-COED, PAMCO SRC, and Catalytic Inc. SRC) have been separated by a simple, rapid chromatographic procedure, on silica gel, into a limited number of acid/neutral and basic fractions which contain high concentrations of major functional groups. Analytical, molecular weight, and nuclear magnetic resonance methods have been used to deduce features of the microstructures of these asphaltenes. They usually have number-average MWs in the 400–550 range, and are made up of cata-systems of small ring number (3–5), with 30–50% of the available aromatic edge atoms occupied by substituents, and joined by naphthenic, methylenic or etheric bridges. Chemical and instrumental methods were used to determine qualitatively and quantitatively the composition of the functional groups OH, NH, and N. The fraction of O as OH ranges from 0.59 to 0.83, and of N as NH from 0.53 to 0.74. The macrostructure of asphaltene crystallites was studied by X-ray diffraction methods and average dimensions deduced from an assumed model. The nature of the association between asphaltene molecules was studied by electron spin resonance; if charge-transfer interactions are present, they appear not to be significant as binding forces between the asphaltene molecules or nuclei.     

GPC Separation and Integrated Structural Analysis of Petroleum Heavy Ends

Abstract

Three hundred grams of a Venezuelan asphalt was separated on a 70-liter GPC column in one run. The 30 cuts collected were consolidated to 7 fractions. After separating the insoluble asphaltenes from the soluble maltenes by n-pentane extraction, the maltenes were chromatographed on deactivated alumina into oil and three different resin fractions. Selected fractions were analyzed by NMR, IR, density, and elemental analysis. The experimental results were evaluated by a novel method making use of an extensive scheme of structure relations. Percentages of aromatic, naphthenic, and paraffinic carbon; number of ring systems per molecule, their average size, compactness, composition, and substitution are given explicitly or implicitly.     

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.