Eric Y. Sheu

Asphaltenes, Heavy Oils, and Petroleomics

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Structures and dynamics of asphaltenes

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Aggregation and kinetics of asphaltenes in organic solvents

Asphaltenes in the Unifying Principles of Carbonaceous Sources T.F. Yen. The Structure and Dynamics of Aromatic Moieties in Crude Oils and Asphaltenes O.C. Mullins. Molecular Structure and Intermolecular Interaction of Asphaltenes by NMR, IR and ESR R. Scotti. Physics of Asphaltene Colloids E.Y. Sheu. Characterization of the Colloidal Structural Evolution from Stable to Flocculated State of Asphalthene and Resin Suspensions and Heavy Crudes D. Espinat, et al. The Colloidal Structure of Coal Asphaltenes M. Lino. Characterization and Phase Behavior of Asphaltenic Crude Oils K.A. Ferworn, W.Y. Svrcek. Conductivity of Asphaltenes P. Fotland. Asphaltene Colloids in Asphalt C. Glover. Characterization of Heavy Oils Using Hydrodynamic Property Measurements R. Baltus. Crude Oil Emulsions J. Sjoblom. Effect of Asphaltene Solvency and Emulsion Stability in Water in Crude Oil Emulsions P. Kilpatrick. Index.

Polydispersity analysis of asphaltene solutions in toluene

Abstract A series of surface tension measurements were performed to study the self-association of asphaltenes. A distinctive discontinuous point was observed for surface tension as a function of asphaltene concentration. This signifies the existence of a critical concentration, above which self-association occurs, similar to surfactant systems. The self-association kinetics was also studied by dynamic surface tension measurements. A parameter was established to qualitatively evaluate the association kinetics.

SANS STUDY OF ASPHALTENE AGGREGATION

Abstract We performed a series of small angle neutron scattering (SANS) measurements on asphaltene solutions in toluene at various temperatures to study the structure and polydispersity of asphaltene colloids. In order to extract the polydispersity unambiguously, we developed a method to self-consistently evaluate our analysis. From our polydispersity analysis we found that the SANS data were best interpreted using a spherical model for particle structure and a Schultz distribution for particle sizes. A possible thermodynamic origin of the polydispersity is suggested.

Macrostructure of asphaltenes in vacuum residue by small-angle X-ray scattering

ABSTRACT The colloidal properties of asphaltenes affects their solubility, reactivity and transport properties. We have been investigating the molecular basis for the aggregation of asphaltenes in toluene through the use of Small Angle Neutron Scattering (SANS), in which the hydrogen nucleus provides the strongest scatterer, and the contrast may be varied by deuterium replacement. The measured intensity curves can be fitted with a elongated particle. Both radius of gyration and molecular weight are significantly decreased upon increasing. temperature, indicating a disaggregation rather than a conformational change mechanism. Thus, the “true” molecular weight of asphaltenes is much less that that measured at room temperature, and appears to be ≤6000. The internal structure is probably complex enough so that a simple description in terms of colloidal or micellar structures is not granted.

Asphaltenes in polar solvents

Abstract Small-angle X-ray scattering was used to investigate the macrostructure of the asphaltenes (heptane-insolubles) in Duri, Ratawi, Oriente and Merey vacuum residues at 93 °C. Synthetic vacuum residues prepared from these materials with different amounts of asphaltenes were also studied. To determine the shape and average size of the colloidal particles, the measured scattering intensities were fitted by applying the constraint that the contrast must remain independent of shape and size distribution parameters as the asphaltene concentration is varied. According to this analysis the asphaltenic colloidal particles appear spherical. The average radii are in the range 30–60 A, depending on the crude oil; the radii obey the Schultz distribution. The degree of polydispersity is ~15%. The average particle size does not appear to depend on heteroatom content. Larger asphaltenic particles appear to dissociate when vacuum residue is diluted with the non-asphaltenic fraction.

Characterization of colloidal asphaltenic particles in heavy oil

Abstract Various techniques were employed to study the colloidal properties of asphaltenes in solutions. Based on surface tension, viscosity, dielectric relaxation, conductivity, and small angle neutron scattering measurements, we have found that asphaltene molecules exhibit a strong propensity for self-association, and that these aggregates are approximately spherical in shape. Unlike micelles, these aggregates do not grow with increasing concentration. This is mainly due to packing constraints, resulting from the complicated molecular structure of asphaltenes. The conductivity and dielectric relaxation measurements suggest that the electron transformation between asphaltene molecules is the main mechanism in forming aggregates, but these aggregates do not percolate at either high concentration, or high temperature (60° C).

Molecular Weight Measurement of UG8 Asphaltene Using APCI Mass Spectroscopy

Abstract Studies with the vacuum residue from Ratawi crude oil and the asphaltenes obtained from this residue support certain aspects of both the Pfeiffer-Saal and Yen models of the nature of asphaltenes in heavy oil. However, the asphaltenic molecules are not found to be macromolecular. Instead, ∼ 100 asphaltenic molecules, with molecular weights of ∼ 1000 u, spontaneously self-associate to form micelle-like particles. The average asphaltenic micelle-like particle is 66 A in diameter and has an apparent molecular weight of ∼ 100 000 u in toluene-pyridine mixtures or in vacuum residue. These asphaltenic micelles are solvated in the vacuum residue by non-asphaltenic molecules, as suggested by Pfeiffer and Saal, making their effective volumes and apparent molecular weights about three times larger, at 93°C. The volume of the average solvated particle in Ratawi vacuum residue is ∼450 000 A 3 and its apparent molecular weight is approximately 300 000 u, at 93°C.

Fractal structure of asphaltenes in toluene

Abstract Asphaltene molecular weight has been a controversial issue in the past several decades and continues on nowadays. From industrial application point of view, asphaltene molecular weight is important for setting up a heavy oil refining strategy so that the process is efficient and economically viable. If the measured average molecular weight of asphaltene is high and is the true molecular weight, then substantial amount of energy will be needed, in order to break the molecule into light products during refining process. This is likely not an economical option. On the other, if the measured high molecular weight is due to self-association and the true molecular weight is low (e.g., less than 1500 Da), it will be energetically attractive to refiners to develop heavy oil cracking technology. Vapor pressure osmometry (VPO) has been routinely used for measuring molecular weight. However, it measures the apparent molecular weight and is likely not the true molecular weight. In order to unambiguously measure the molecular weight, it is necessary to develop a convincing technology and a reliable experimental procedure that allows one to measure the molecular weight accurately and consistently. We chose the Atmospheric Pressure Chemical Ionization (APCI) technique and Atmospheric Pressure Photo Ionization (APPI) to measure UG8 asphaltene. Both APCI and APPI have mild ionization processes and have been applied to many unstable drug compounds such as proteins and peptides with reliable outcomes. In addition, we measured the sample on two APPI instruments to compare the results. We also demonstrated how one can choose wrong set of operating parameters and lead to erroneous results. The relevant parameters for APCI and APPI are temperature, voltage, and sample concentration. We chose 0.01 mg/mL as the concentration, much below any known critical aggregation concentration. As for temperature and ionization voltage, we varied systematically varied (T = 300–600°C; V = 30–150 V) in order to demonstrate the consistency of the methods and how one can easily make mistake. Through these measurements, an average molecular weight of 400 to 900 Da was obtained for UG8 asphaltene.