Molecular Structure Of Asphaltenes Research Articles

Small angle scattering and asphaltenes

Petroleum is a mixture of organic material consisting of a series of molecules withincreasing molecular weight but with decreasing carbon to hydrogen ratios. This monotonictrend leads to distinctive properties of each class, cut by solvents. Asphaltenes are a classsoluble in toluene but not in heptane. The importance of asphaltenes lies in their relevanceto petroleum operations. Many properties of petroleum liquids are due to the interplaybetween asphaltenes and other co-existing components. These complex interactions impacton petroleum phases, and thus the operations. So-called petroleomics is a scheme to linkthe molecular structures of the most relevant components in the petroleum liquid to itsoverall properties, similar to the proteomics widely accepted in biological sciences.However, though the asphaltene molecular structure and compositions are relevant to themacroscopic properties of petroleum liquids, their aggregates on the colloidal lengthscale could be the most relevant elementary unit that dictates the properties ofthe petroleum mixtures. In this regard, it is legitimate to use small angle x-rayscattering (SAXS) and small angle neutron scattering (SANS) techniques to bridgethe molecular structures of asphaltenes and the operational parameters that arecommonly applied in the field. In this review, the linkages between asphaltenemolecules and their aggregates and the asphaltene aggregates and the macroscopicproperties are described. Applications of small angle x-ray and neutron scattering forcharacterizing asphaltene aggregates and asphaltene emulsions are also discussed.

Asphaltene Molecular Structure and Chemical Influences on the Morphology of Coke Produced in Delayed Coking

The average chemical structure of asphaltenes present in a vacuum resid feed is related to the morphology of the coke that is produced in a delayed coker (shot coke vs sponge coke). A combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS), and elemental abundance was used to characterize the average chemical structure of several n-heptane asphaltenes from shot-coke- and sponge-coke-producing vacuum resid feeds. The chemical structural properties of the asphaltenes are discussed in relation to the coke morphology produced from the parent resid. The average asphaltene aromatic carbon per cluster size is between 14 and 22 carbon atoms, which corresponds to three-to-five-ring average clusters. When the ratio of aromatic carbon to unreactive (i.e., heterocyclic aromatic) nitrogen and sulfur in asphaltenes is <16, the feed tendency is to produce shot coke. Representative chemical structural models of asphaltenes reveal significant differences (1.5 cal/cm3)1/2 in the calculated solubility parameter of the PNA core structure. Feeds with larger-solubility-parameter asphaltene PNA cores tend to produce a shot coke morphology. The differentiation appears despite much smaller differences (<0.3 cal/cm3)1/2 in the solubility parameter for the full non-thermally treated asphaltene average structure. The quantity of asphaltenes and the asphaltene:concarbon ratio are not reliable predictors of coke morphology produced in delayed cokers. Microcarbon residue (MCR) tests on the vacuum resid feeds were performed, followed by cross-polarized light optical microscopy on the resultant cokes. The micrographs show a small (2−10 μm) domain size/anisotropy in the optically anisotropic coke produced from shot-coke-forming feeds. This is comparable to the mosaic size of commercial shot cokes. In contrast, larger flow domains of more highly anisotropic appearance are characteristic of sponge-coke-forming feeds. The approach provides a method of assessing the morphology of the coke that will be produced from a given feed.

Nanoaggregates and Structure−Function Relations in Asphaltenes

The goal of predictive science is to establish structure−function relations for a system of interest. The obvious first step is to determine the structure of the system. Predictions in asphaltene science have been greatly inhibited, because of disagreement regarding molecular weight and molecular structure. With substantial progress on both structural fronts, structure−function relationships can be explored. Here, high quality factor (high-Q) ultrasonics is used to demonstrate asphaltene nanoaggregation at ∼100 mg/L. Fluorescence quenching measurements corroborate these results. Simple concepts regarding asphaltene molecular structure can be used to understand asphaltene nanoaggregation. These relations are seen to apply for asphaltene samples of very different origin. The implications of this understanding on larger-scale asphaltene aggregation and solubility are discussed.

A Geochemical Framework for Understanding Residue Properties

Crude oils, and the residual fractions thereof, vary widely in many relevant properties. Thus, a detailed characterization of a single residue stream or even a number of residue streams from one region may not be very relevant in a different part of the world. We have therefore attempted to develop a framework for understanding residue properties in a generic fashion. A detailed analysis of 11 vacuum residue fractions from a wide variety of crude oils resulted in a general model for understanding the composition and processability of residue streams. This model is based on the geochemical origin (kerogen type) and the maturity of the crude oil. The different origin of kerogen I (paraffinic) residues, compared with the more conventional kerogen II residues, is reflected in a large number of properties and also in a larger variability in these properties. However, upon maturation, the average properties of kerogen I residues and kerogen II residues converge so that mature residues of both kerogen types have much in common (low S content, low metals content, high H/C ratio etc). Maturation is found to have a negative effect on the stability and coking tendency of the residue fraction. NMR data reveal that the asphalthenes become more aromatic upon ageing, whereas the maltenes (non-asphalthenes) become less aromatic, thus causing an increasing gap in aromaticity between these fractions. Significant differences in asphaltene molecular structure between kerogen I and kerogen II residues were observed. The analytical data suggest that the concept of kerogen type and maturity may be a useful tool in describing and understanding residue characteristics and assist in optimizing feedstock selection for residue conversion processes and other residue applications (fuel oils, bitumen), as well as in understanding fouling and coking phenomena. Further research is required to establish the trends shown in this paper.

Ozonolysis of Asphaltenes from Semicoking Tar of G17 Coal

Liquid-phase ozonolysis of asphaltenes from the semicoking tar of G17 coal was studied. In structural studies of asphaltenes, ozone is of particular interest as a specific agent cleaving double bonds [139]. Identification of low-molecular-weight (water-soluble) and high-molecular-weight (waterinsoluble) products of asphaltene ozonolysis by 1H and 13C NMR spectroscopy, gas chromatography3 mass spectrometry (GC3MS), gas3liquid (GC) and thin-layer (TLC) chromatography, UV/Vis spectroscopy, and ESR allows detailed characterization of the molecular structure of asphaltenes of various origins, which, in turn, is necessary for their efficient application. In this work we studied liquid-phase ozonolysis of asphaltenes from the semicoking tar of G17 coal.

1-2. New insights into asphaltene molecular size and structure by combination of HRTEM and FD

1-2. New insights into asphaltene molecular size and structure by combination of HRTEM and FD

The Overriding Chemical Principles that Define Asphaltenes

Asphaltenes are defined in terms of their solubility classification. This operational definition combined with the previous controversy over asphaltene molecular weight have obscured the governing chemical and structural parameters that define the asphaltene fraction. Here, asphaltenes are investigated by several techniques to elucidate relations between structure and properties. In particular, the asphaltene molecular size is compared to the ratio of aromatic to saturated carbon. The conclusion is obtained that asphaltene molecular structure is governed by the balance between the propensity of fused aromatic ring systems to stack via π-bonding, reducing solubility, vs the steric disruption of stacking due alkane groups, increasing solubility.

Studies of transport of asphaltenes through porous membranes: statistical structural models and continuum hydrodynamic theories

The results of our ongoing studies of the transport os asphaltene molecules through model membranes are presented. The experimental results are briefly reviewed, and a model is presented which aims to capture the effect on the transport of the polydisperse nature of the asphaltene molecules. The asphaltene molecular structure is first generated stochastically using a Monte Carlo technique. The individual asphaltene molecules are then approximated as spheroids for the purpose of calculating their hindered diffusivities. Continuum hydrodynamic theories of hindered diffusion are then used to calculate the individual transport coefficients.

Imaging of Petroleum Asphaltenes Using Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) was used to image Ratawi (neutral zone) asphaltenes deposited on highly oriented pyrolytic graphite (HOPG). STM images revealed the formation of ordered arrays covering hundreds of angstroms of the surface. Samples deposited from pyridine solutions above and below the critical micelle concentration were quite similar. The individual features imaged in these arrays were attributed to the termini of the pendant aliphatic chains known to be a part of the asphaltene molecular structure. The ordered arrays imaged were extremely flat and uniform, in contrast to the assumed molecular complexity and the absence of unique structure of asphaltenes. The formation of these ordered surface arrays is further evidence of the tendency of asphaltenes to self-associate.