Solvation of asphaltenes in supercritical water: A molecular dynamics study

Dissolution Behavior of Polycyclic Aromatic Hydrocarbons in Heavy Oil in the Presence of Supercritical Cyclohexane

Supercritical cyclohexane (SC-cyclohexane) shows significant advantages in mild operating conditions and the modulation of product distribution. To gain insights into the upgrading process of heavy oil in SC-cyclohexane, the dissolution process of polycyclic aromatic hydrocarbons (PAHs) contained in heavy oil was simulated based on molecular dynamics with the use of naphthalene, benzopyrene, and mixtures of naphthalene and benzopyrene as the model compounds. As indicated by the radial distribution function results, in SC-cyclohexane exhibiting low density, cyclohexane formed a solvent shell around PAHs such that the local concentration was reduced and the aggregation of PAHs was inhibited. The results of the solvation free energy suggested that van der Waals forces between PAHs and cyclohexane were mainly dominant. As revealed by the dissolution process of the model compounds in SC-cyclohexane, a low density and a suitable temperature contributed to the solubilization of PAHs. An appropriate temperature and a low density can be selected for the upgrading reaction to limit coke formation.

Molecular Dynamics Investigation of Wettability Alteration of Quartz Surface under Thermal Recovery Processes

One of the primary methods for bitumen and heavy oil recovery is a steam-assisted gravity drainage (SAGD) process. However, the mechanisms related to wettability alteration under the SAGD process still need to be fully understood. In this study, we used MD simulation to evaluate the wettability alteration under a steam injection process for bitumen and heavy oil recovery. Various oil droplets with different asphaltene contents were considered to determine the effect of an asphaltene content on the adsorption of the oil droplets onto quartz surfaces and wettability alteration. Based on the MD simulation outputs, the higher the asphaltene content, the higher the adsorption energy between the bitumen/heavy oil and quartz surfaces due to coulombic interactions. Additionally, the quartz surfaces became more oil-wet at temperatures well beyond the water boiling temperature; however, they were extremely water-wet at ambient conditions. The results of this work provide in-depth information regarding wettability alteration during in situ thermal processes for bitumen and heavy oil recovery. Furthermore, they provide helpful information for optimizing the in situ thermal processes for successful operations.

Essential role of structure, architecture, and intermolecular interactions of asphaltene molecules on properties (self-association and surface activity)

One of the important challenges of the oil industry is the formation of asphaltene deposits and emulsions, which cause many operational and economic problems. Asphaltenes are heavy and polar fractions of petroleum with a mixture of diverse molecules. Their structural complexity makes the understanding of their properties puzzling. The purpose of this review is to understand the self-association and surface activity properties of asphaltenes. There are some popular models for the mechanism of asphaltene aggregation; each alone is not complete and without defects. Experimental studies and molecular dynamics demonstrate that the mechanism of aggregation is influenced by asphaltene' structure, architecture, and intermolecular forces. Factors such as oil composition, temperature, and pressure affect its intensity. In this article, these issues and their impact on the self-assembly of asphaltenes and ways to prevent it, especially chemical inhibitors, have been discussed in detail.

Precipitation Behavior of Salts in Supercritical Water: Experiments and Molecular Dynamics Simulations

Supercritical water desalination (SCWD) shows great potential in the treatment of high-salt wastewater with zero liquid discharge. To investigate the salt precipitation behavior and mechanism in supercritical water, experiments and molecular dynamics simulations (MDs) were used to study the salting-out process of different salts in supercritical water. The equilibrium concentrations of NaCl, KCl, CaCl2, Na2SO4, and Na2CO3 in supercritical water were experimentally measured. When the temperature exceeded 693 K, the salt equilibrium concentration measured in the experiment was less than 130 mg/L. The solubility decreased in the order of KCl > NaCl > CaCl2 > Na2SO4 > Na2CO3. To elucidate the effects of different cations and anions in supercritical water on salt dissolution and precipitation behavior, the potential energy, radial distribution function (RDF) and coordination number in the system were obtained via molecular dynamics simulation. Experimental and MD results showed that salt solubility has significant positive correlation with systemic potential energy and hydration number. MD results indicated that a small ionic radius, large ionic charge, and low hydration coordination number are favorable for inorganic salts to precipitate and crystallize since these factors can strengthen the interaction between free ions and salt clusters. Moreover, due to the formation of multilayer coordination structure, polyatomic ions can achieve a lower equilibrium concentration than that of the corresponding monatomic ions.

Molecular dynamics simulation study used in systems with supercritical water

Supercritical water (SCW) is a green solvent. The supercritical fluids have been increasingly concerned and studied in many areas such as SCW gasification, biofuel production, SCW hydrothermal conversion, organic wastes treatment and utilization, nanotechnology, etc. Because of the severe circumstances and rapid reactions in supercritical water, it is difficult for experimental researchers to disentangle various fundamental reaction steps from the intermediate and product distributions. From this perspective, molecular dynamics (MD) simulation based on quantum chemistry is an efficient tool for studying and exploring complex molecular systems. In recent years, molecular simulations and quantum chemical calculations have become powerful for illustrating the possible internal mechanism of a complex system. However, now there is no literature about the overview of MD simulation study of the system with SCW. Therefore, in this paper, an overview of MD simulation investigation applied in various systems with SCW is presented. In the current review we explore diverse research areas. Namely, the applications of MD simulation on investigating the properties of SCW, pyrolysis/gasification systems with SCW, dissolution systems and oxidation systems with SCW were summarized. And the corresponding problems in diverse systems were discussed. Furthermore, the advances and problems in MD simulation study were also discussed. Finally, possible directions for future research were outlined. This work is expected to be one reference for the further theoretical and molecular simulation investigations of systems involving SCW.

Enhanced Computational Sampling of Perylene and Perylothiophene Packing with Rigid-Body Models

Molecular simulations have the potential to advance the understanding of how the structure of organic materials can be engineered through the choice of chemical components but are limited by computational costs. The computational costs can be significantly lowered through the use of modeling approximations that capture the relevant features of a system, while lowering algorithmic complexity or by decreasing the degrees of freedom that must be integrated. Such methods include coarse-graining techniques, approximating long-range electrostatics with short-range potentials, and the use of rigid bodies to replace flexible bonded constraints between atoms. To understand whether and to what degree these techniques can be leveraged to enhance the understanding of planar organic molecules, we investigate the morphologies predicted by molecular dynamic simulations using simplified molecular models of perylene and perylothiophene. Approximately, 10 000 wall-clock hours of graphics processing unit-accelerated simulations are performed using both rigid and flexible models to test their efficiency and predictive capability with the two chemistries. We characterize the 1191 resulting morphologies using simulated X-ray diffraction and cluster analysis to distinguish structural transitions, summarized by four phase diagrams. We find that the morphologies generated by the rigid model of perylene and perylothiophene match with those generated by the flexible model. We find that ordered, hexagonally packed columnar phases are thermodynamically favored over a wide range of densities and temperatures for both molecules, in qualitative agreement with experiments. Furthermore, we find the rigid model to be more computationally efficient for both molecules, providing more samples per second and shorter times to equilibrium. Owing to the structural accuracy and improved computational efficiency of modeling polyaromatic groups as rigid bodies, we recommend this modeling choice for enhancing the sampling in polyaromatic molecular simulations.