No abstract

The Hansen solubility parameters (HSPs) of asphaltenes extracted from oil sand bitumen samples produced at Athabasca in Canada and also from a vacuum residue fraction (VR) produced in the Middle East were determined by the Hansen solubility sphere method. For calculation of HSPs, the solubilities of asphaltenes were determined using a dynamic light scattering (DLS) method by dissolving or dispersing the asphaltenes in various solvents and measuring the particle size distributions thereof. The particle diameters of asphaltenes in good solvents were lower than its detection limit (<1 nm). It was demonstrated in the present study that asphaltenes differing in elemental composition had different HSP values corresponding to dispersion, dipole interaction, and hydrogen-bonding forces (δd, δp, and δh, respectively). Experimental results suggested that the differences in HSP values of the asphaltenes were influenced by the H/C ratio, oxygen content, and average asphaltene molecular weight.

We constructed a molecular model (digital oil model) for heavy crude oil based on analytical data and our newly developed method. Crude oil was separated into four fractions: saturates, aromatics, resins, and asphlatenes (SARA). Although it is classified as a heavy crude oil, the asphaltenes turned out to be at very low weight concentration (~0.4 wt. %), and were ignored in our study. The digital oil was constructed as a mixture of representative molecules of four fractions: saturates, aromatics, resins, and lost components (which resulted from our SARA analysis). Representative molecules were generated by quantitative molecular representation (QMR), a technique that provides a set of molecules consistent with analytical data, such as elemental composition, average molecular mass, and the proportions of structural types of hydrogen and carbon atoms, as revealed by 1 H and 13 C nuclear magnetic resonance. To enable the QMR method to be applicable to saturates, we made two developments: the first was the generation of non-aromatic molecules by a new algorithm that can generate a more branched structure by separating the chain bonding into main and subsidiary processes; the second was that the molecular mass distribution of the model could be fitted to that obtained from experiments. To validate the digital oil thus obtained, we first confirmed the validity of the model for each fraction in terms of plots of double-bond equivalent as a function of carbon number. We then calculated its density and viscosity by molecular dynamics simulations. The calculated density was in good agreement with experimental data for crude oil. The calculated viscosity was higher than experimental values; however, the error appeared systematic, being a factor of ~1.5 higher than that of experiments. Moreover, the calculated viscosity as a function of temperature was well described by the Vogel-Fulcher-Tammann equation. Digital oil will be a powerful tool to analyze both macroscopic properties and microscopic phenomena of crude oil under any thermodynamic conditions.

Summary Digital oil, a realistic molecular model of crude oil for a target reservoir, opens a new door to understand properties of crude oil under a wide range of thermodynamic conditions. In this study, we constructed a digital oil to model a light crude oil using analytical experiments after separating the light crude oil into gas, light and heavy fractions, and asphaltenes. The gas and light fractions were analyzed by gas chromatography (GC), and 105 kinds of molecules, including normal alkanes, isoalkanes, naphthenes, alkylbenzenes, and polyaromatics (with a maximum of three aromatic rings), were directly identified. The heavy fraction and asphaltenes were analyzed by elemental analysis, molecular-weight (MW) measurement with gel-permeation chromatography (GPC), and hydrogen and carbon nuclear-magnetic-resonance (NMR) spectroscopy, and represented by the quantitative molecular-representation method, which provides a mixture model imitating distributions of the crude-oil sample. Because of the low weight concentration of asphaltenes in the light crude oil (approximately 0.1 wt%), the digital oil model was constructed by mixing the gas, light-, and heavy-fraction models. To confirm the validity of the digital oil, density and viscosity were calculated over a wide range of pressures at the reservoir temperature by molecular-dynamics (MD) simulations. Because only experimental data for the liquid phase were available, we predicted the liquid components of the digital oil at pressures lower than 16.3 MPa (i.e., the bubblepoint pressure) by flash calculation, and calculated the liquid density by MD simulation. The calculated densities coincided with the experimental values at all pressures in the range from 0.1 to 29.5 MPa. We calculated the viscosity of the liquid phase at the same pressures by two independent methods. The calculated viscosities were in good agreement with each other. Moreover, the viscosity change with pressure was consistent with the experimental data. As a step for application of digital oil to predict asphaltene-precipitation risk, we calculated dimerization free energy of asphaltenes (which we regarded as asphaltene self-association energy) in the digital oil at the reservoir condition, using MD simulation with the umbrella sampling method. The calculated value is consistent with reported values used in phase-equilibrium calculation. Digital oil is a powerful tool to help us understand mechanisms of molecular-scale phenomena in oil reservoirs and solve problems in the upstream and downstream petroleum industry.

Simultaneous hydrothermal degradation and extraction at around 350 °C using flowing solvent as a reaction/extraction medium were proposed for upgrading brown coal, more specifically, for converting brown coal into several fractions having different molecular weight and chemical structure under mild conditions. When an Australian brown coal, Loy Yang coal, was treated by water at 350 °C under 18 MPa, the coal was separated into four fractions: gaseous product by 8% yield, water-soluble extract at room temperature (soluble) by 23% yield, extract precipitates as solid at room temperature (deposit) by 23% yield, and residual coal (upgraded coal) by 46% yield on daf basis. The separation was found to be realized by in situ extraction of low-molecular-weight substances released from coal macromolecular structure and/ or those generated by hydrothermal decomposition reactions at 350 °C. The solid products obtained, deposit and upgraded coal, were characterized in detail to examine the possibility of their effective utilization as solid fuel and chemical feed stock. The upgraded coal showed higher heating value and higher gasification reactivity than the parent coal, indicating that the upgraded coal can be a better solid fuel than the parent coal. The solid extract, deposit, was found to show thermoplasticity at less than 200 °C, suggesting the possibility of utilizing the deposit as a raw material of high performance carbon materials. Several variables affecting the performance of the proposed method are also examined in detail in this paper.