31 research outputs found
MOLECULAR SIMULATION STUDIES OF DIESEL AND DIESEL ADDITIVES
As diesel fuel is cooled down, waxes are deposited, which are made up from crystals of long chain n-alkanes. Wax depositions are undesirable, since they can block anything from filters in diesel engines to pipelines. It is already known that wax formation can be inhibited by the addition of wax crystal modifiers to diesel fuel. This thesis em- ploys computational models at atomistic and coarse-grained levels to investigate the crystallisation of diesel fuel and the effect of additives upon the crystallisation process.
In the first results section, a model for diesel fuel is introduced and a strategy for investigating its crystallisation is developed. Crystallisation was observed from pure n-tricosane, binary and tertiary mixtures of paraffins of similar chain lengths. These systems were found to crystallise into hexagonally arranged lamellae. The presence of different length alkanes was found to create gauche disorders, leading to the formation of lamellar layers with softer edges. It was also found that crystal growth could be simulated more efficiently in the presence of a positionally restrained crystal, acting as a nucleation centre.
Subsequently, crystallisation of paraffins, and the solvent effect upon it, was studied. This allowed to establish behavioural trends characteristic for aromatic and aliphatic solvents. Finally, paraffin crystallisation in the presence of four common additives was investigated. A common mode of action for these additives was identified, based upon partial co-crystallisation of additive alkyl chains and paraffin molecules.
The main drawback of atomistic simulation is the computational cost, which limits both the time and length scales accessible on modern computers. In order to overcome these inherent limitations, a coarse grained model was developed for a range of n-alkanes. Remarkably, the model shows transferability over 120 K, preserving thermodynamic and structural properties of both melt and crystal.
In summary, this thesis provides a detailed picture of diesel crystallisation at a molecular level, and provides new insights into the mechanism of action of a number of common diesel additives
Molecular dynamics simulation of realistic biochar models with controlled porosity
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Developing a molecular-level understanding of biochar materials using public characterization data
Biochars are black carbonaceous solids produced through biomass pyrolysis under conditions of little or no oxygen. While their properties and applications are well studied, the underlying molecular structures are poorly defined. Consequently, there has been limited computational study of biochars, despite the advantages of such techniques. In part one of this two-part study, we review the experimental techniques to characterize biochar and biochar-like materials and discuss the ambiguities, errors, and uncertainties inherent to each technique. We focus on techniques that provide chemical information and molecular-level insights, thereby adding to our understanding of the molecular structures comprising biochars. We also collect publicly available characterization data for woody biochars across a range of the highest treatment temperatures of pyrolysis. These collected data provide a quantitative description of the changes in biochar properties with increasing pyrolysis temperature. The collected data, shared as an open database, support the further development of biochar molecular models, reported in part two of this work
Biochars at the molecular level. Part 1 -- Insights into the molecular structures within biochars
Biochars are black carbonaceous solids produced through biomass pyrolysis
under conditions of little or no oxygen. Whilst their properties are well
studied, and their applications numerous, the underlying molecular structures
within biochars still need to be defined. This raises a substantial barrier to
the molecular modelling of biochars and has limited computational study of
these materials, despite the advantages of such techniques. In this work, we
critically assess the analytical techniques used to characterise biochars and
use this information to gain molecular-level insights into biochars' molecular
compositions and nanostructures. We focus on properties present at the
nanoscale and which provide atomic-resolution insights into the molecular
structures within these materials. Our goal is to create a holistic
understanding of biochars' chemical, physical and molecular properties and to
lay the foundation for future work focused on developing realistic molecular
models of these materials
Organic-mineral interactions under natural conditions -- a computational study of flavone adsorption on smectite clay
Interactions between organic species and natural minerals are fundamental to
the processes around us. With the aid of molecular dynamics simulations, we
identify key adsorption mechanisms of apigenin on smectite clay minerals. The
mechanism is highly sensitive to the pH -- changing from co-crystallisation in
acidic-to-neutral solutions to the ion-bridging in mild-alkaline. The ionic
species play a significant role in alkaline environments: the deprotonated
apigenin species chelate metals, which, in turn, leads to the formation of a
stable organic-metal-mineral complex and stronger adsorption in the presence of
divalent cations.Smectite clays buffer the solution to mildly alkaline; hence,
the type of exchangeable cations in the clay will be critical in determining
the adsorption mechanism and organic retention capacity. Overall, our study
showcases a computational strategy that can be transferred to a wide variety of
organic-mineral systems in the natural environment
Biochars at the molecular level. Part 2 -- Development of realistic molecular models of biochars
Biochars have been attracting renewed attention as economical and
environmentally friendly carbon sequestration materials with a diverse range of
applications. However, experimental developments may be limited by the lack of
molecular-level knowledge of the key interactions driving these applications.
Molecular modelling techniques, such as molecular dynamics simulations, offer a
systematic and reproducible alternative and yield atomistic insights into
physicochemical processes, allowing the identification of adsorption mechanisms
and, through this, informing and guiding experimental development. In this
work, on the basis of the critical assessment of the analytical techniques for
characterisation of biochars and collation of a large volume of experimental
data, we develop molecular models of three woody biochar materials,
representative of those produced under low-, medium-, and high-temperature
treatments. We characterise these models, validating them against experimental
data, and share them with the research community. Furthermore, we detail our
iterative approach to the design of these biochar models, discuss what we have
learned about the relationship between biochar composition and its morphology,
and finally share all of the building blocks used to create these biochar
models. With this work, we hope to speed up the uptake of molecular dynamics
simulations for the study and development of biochar materials and, to this
end, we distribute our easy-to-use surface-exposed biochar models ready for the
adsorption studies
Molecular Dynamic Simulations of Montmorillonite-Organic Interactions under Varying Salinity: An Insight into Enhanced Oil Recovery
Enhanced oil recovery is becoming commonplace in order to maximize recovery from oil fields. One of these methods, low-salinity enhanced oil recovery (EOR), has shown promise; however, the fundamental underlying chemistry requires elucidating. Here, three mechanisms proposed to account for low-salinity enhanced oil recovery in sandstone reservoirs are investigated using molecular dynamic simulations. The mechanisms probed are electric double layer expansion, multicomponent ionic exchange, and pH effects arising at clay mineral surfaces. Simulations of smectite basal planes interacting with uncharged nonpolar decane, uncharged polar decanoic acid, and charged Na decanoate model compounds are used to this end. Various salt concentrations of NaCl are modeled: 0â°, 1â°, 5â°, and 35â° to determine the role of salinity upon the three separate mechanisms. Furthermore, the initial oil/water-wetness of the clay surface is modeled. Results show that electric double layer expansion is not able to fully explain the effects of low-salinity enhanced oil recovery. The pH surrounding a clayâs basal plane, and hence the protonation and charge of acid molecules, is determined to be one of the dominant effects driving low-salinity EOR. Further, results indicate that the presence of calcium cations can drastically alter the oil wettability of a clay mineral surface. Replacing all divalent cations with monovalent cations through multicomponent cation exchange dramatically increases the water wettability of a clay surface and will increase EOR
Revealing crucial effects of reservoir environment and hydrocarbon fractions on fluid behaviour in kaolinite pores
The adsorption interactions of hydrocarbons and clay surfaces are crucial to understanding fluid behaviour within shale reservoirs and to mediating organic pollutants in soils. These interactions are affected by the diversity of complex hydrocarbon components and the variations in environmental conditions. This study examines the interactions between kaolinite clay, featuring two distinct basal surfaces, and an array of hydrocarbons. We assess the impact of various molecular structures, functional groups, and environmental conditions (focusing on the reservoir temperature and pressure ranges) on the adsorption selectivity, surface packing, molecular alignment and orientation, and diffusion of hydrocarbons. Analyses of molecular interaction energies provide a quantitative elucidation of the adsorption mechanisms of hydrocarbons on the different kaolinite surfaces. Our findings suggest that molecular configuration, functional group properties, and spatial effects dictate the distribution patterns of hydrocarbons for the different kaolinite surfaces. The differences in the interaction energy between various hydrocarbons with kaolinite reveal the adsorption strength of different hydrocarbons in the order of asphaltenes > heteroatomic hydrocarbons > saturated hydrocarbons > aromatic hydrocarbons. Furthermore, we observe that the adsorptive characteristics of hydrocarbons on kaolinite are highly temperature-sensitive, with increased temperatures markedly reducing the adsorption amount. Beyond a certain threshold, the effect of pressure rise on the fluid behaviour of hydrocarbons is non-negligible and is related to molecular packing and reduced mobility. Simulation results based on actual geological characteristics demonstrate notable adsorption disparities among hydrocarbon components on different kaolinite surfaces, influenced by competitive adsorption and clay surface interactions. Polar surfaces are predominantly occupied by heteroatomic hydrocarbons, whereas on non-polar surfaces, asphaltenes and heavy saturated hydrocarbons develop multi-layer adsorption structures, with molecules aligned parallel to the surface
A computational study of the mechanism of the unimolecular elimination of α, ÎČ - unsaturated aldehydes in the gas phase
The 13th International Electronic Conference on Synthetic Organic Chemistry session Computational Chemistr