1 H DOSY NMR experiments were used to investigate the macrostructure of the asphaltenes of Maya, Athabasca, and Buzurgan feedstocks in toluene-d 8 at 20°C. The influence of the concentration of asphaltenes on their diffusion coefficients is presented for the three asphaltenes. A separation between two classes of aggregates-one diffusing quickly and one diffusing more slowly-was observed at an onset concentration that is dependent upon the origin of the sample. This illustrates that the chemical interactions (and, hence, the chemical structures) are different for the three asphaltenes. Because asphaltenes are a continuum of both archipelago and continental asphaltenes, the interactions in the solutions differ, depending on the repartition between archipelago-and continental-type asphaltenes in the feed. Maya and Buzurgan asphaltenes show similar diffusion properties in the dilute regime, while Athabasca asphaltenes diffuse more slowly. Results obtained from DOSY experiments data seem to indicate that Buzurgan asphaltenes show a more continental character than the two other asphaltenes, while Athabasca asphaltenes seem to contain more archipelago asphaltenes. 1 H and 13 C NMR experiments were also performed to determine the average structural parameters of asphaltenes. Average sizes and molecular weights were determined from the 1 H-DOSY NMR diffusion coefficients and compared to size exclusion chromatography (SEC) data.

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-In this paper, kinetic modeling techniques for complex chemical processes are reviewed. After a brief historical overview of chemical kinetics, an overview is given of the theoretical background of kinetic modeling of elementary steps and of multistep reactions. Classic lumping techniques are introduced and analyzed. Two examples of lumped kinetic models (atmospheric gasoil hydrotreating and residue hydroprocessing) developed at IFP Energies nouvelles (IFPEN) are presented. The largest part of this review describes advanced kinetic modeling strategies, in which the molecular detail is retained, i.e. the reactions are represented between molecules or even subdivided into elementary steps. To be able to retain this molecular level throughout the kinetic model and the reactor simulations, several hurdles have to be cleared first: (i) the feedstock needs to be described in terms of molecules, (ii) large reaction networks need to be automatically generated, and (iii) a large number of rate equations with their rate parameters need to be derived. For these three obstacles, molecular reconstruction techniques, deterministic or stochastic network generation programs, and single-event micro-kinetics and/or linear free energy relationships have been applied at IFPEN, as illustrated by several examples of kinetic models for industrial refining processes.Résumé -Une revue de méthodes de modélisation cinétique pour des procédés complexesDans cet article, les techniques de modélisation cinétique des processus chimiques complexes sont examinées. Après un bref aperçu historique de la cinétique chimique, un aperçu des bases théoriques de la modélisation cinétique d'étapes élémentaires et de réactions globales est présenté. Les techniques classiques de regroupement (lumping) sont ensuite présentées et analysées. Deux exemples de modèles cinétiques regroupés (pour l'hydrotraitement de gazole atmosphérique et pour l'hydrotraitement de résidus) développés à IFP Energies nouvelles (IFPEN) sont présentés. La plus grande partie de cette revue décrit des stratégies avancées de modélisation cinétique, dans lesquelles le détail moléculaire est retenu : les réactions entre les molécules sont représentées ou même subdivisées en étapes élémentaires. Pour être en mesure de conserver ce niveau moléculaire à la fois dans le modèle cinétique et dans les simulations de réacteurs, plusieurs obstacles doivent d'abord être éliminés : (i) la charge doit être décrite en termes de molécules, (ii) les grands réseaux réactionnels doivent être générés automatiquement et (iii) un grand nombre d'équations de vitesse avec leurs paramètres de vitesse doit être dérivé. Pour ces trois obstacles, des techniques de reconstruction moléculaire, des programmes de génération de réseaux déterministes ou stochastiques, et des modèles microcinétiques basés sur des événements constitutifs (single events) et/ou des

The mean molecular weight and particle size of asphaltene aggregates extracted from Buzurgan feedstock have been evaluated by measuring the diffusivity by means of 1H diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR). This technique, recently applied in the petroleum industry, appeared as a key tool to investigate the behavior of asphaltenes diluted in toluene over a wide range of concentrations (from 0.01 to 15 wt %). The results show that the diffusivity is highly dependent upon solute concentration. Indeed, interactions occurring in dilute systems differ from interactions occurring in the semi-dilute regime. In the dilute regime (below 0.25 wt %), physical characterization of the nanoaggregates detected by 1H DOSY NMR could be achieved. An average molecular weight of roughly 6900 g mol−1 (taken at the highest diffusion peak and given as a polystyrene equivalent) with a range of 1500−85000 g mol−1 was obtained, while a mean radius of 15.6 Å was determined from the solute diffusivity at infinite dilution [D ∞asp ∼ (2.4 ± 0.1) × 10−10 m2 s−1 for Buzurgan asphaltenes]. Average masses (M n and M w) were also calculated from DOSY NMR data and compared to results obtained from size exclusion chromatography (SEC) analyses. We also used DOSY NMR techniques to investigate the molecular dynamics of asphaltenes. A concentration of 0.25 wt % was found to represent the onset of the aggregation process. We believe DOSY NMR allowed us to observe the beginning of aggregation in phase transition. The asphaltene system is very polydisperse. At low concentrations, it is a polydisperse diluted system, but increasing the solute concentration induces a distinction of different aggregates, presenting various sizes in a macroscopic homogeneous phase. When the distinction occurs, there is a zone that is richer in high molecular weight aggregates of asphaltenes, and as a result, a second zone is impoverished in high molecular weight aggregates. In a microscopic point of view, there is a difference in nano or macroaggregate concentration, whereas in a macroscopic point of view, an average density is observed. For the first time, a clear separation between two families of aggregates of asphaltenes is presented in the diffusion dimension for concentrations higher than 3 wt %. 1H DOSY spectra and diffusion profiles confirm these results. The key point of this study resides in the detection and presentation of two classes of aggregates of asphaltenes achieved for concentrated solutions, without any assumption concerning the composition of the mixture.

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Heavy crude oils are more and more of interest for the oil industry to meet the growing worldwide energy demand. Asphaltenes, which are the heaviest and least reactive molecules in crude oils, have received great attention over the last few decades, because they are responsible for many problems occurring during hydrotreatment and hydroconversion. 1H diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) based on pulsed field gradient (PFG) sequences is a powerful tool to analyze polydisperse samples. The key advantage of such a technique compared to traditional NMR diffusion sequences, such as pulsed field gradient spin−echo (PFGSE), is to obtain both physical and chemical information in a single experiment. 1H DOSY NMR has been carried out on different types of samples to obtain a deeper insight into the physicochemical properties of petroleum samples, because diffusion coefficients are both sensitive to molecular weights and sizes. The application of DOSY NMR to analyze hydrocarbon mixtures and asphaltenes is assessed. It is shown that both solute and solvent diffusion coefficients decrease with an increasing concentration of solute. Different types of intermolecular interactions were observed on petroleum samples depending upon the sample concentration. Dilute and semi-dilute regimes have also been detected. 1H DOSY spectra applied to diesel and asphaltene samples in toluene are presented and interpreted to demonstrate the potential of DOSY techniques to analyze heterogeneous petroleum samples. The data obtained for the diesel sample enable us to establish that monoaromatics were connected to long alkyl chains, whereas di- or triaromatic molecules were linked to shorter hydrocarbon chains. However, a clear conclusion could not be reached for the asphaltene sample because there were only a few aromatic protons that were not detected. This observation is most consistent with a continental model of these asphaltenes.

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