Since asphaltenes are a class of molecules that can be separated from crude oil or distillation residue by the difference of solubility parameters in distinct solvents, it is common for asphaltenes to be defined as the fraction of crude oil that is insoluble in short-chain n- alkanes and soluble in monoaromatic solvents like benzene and toluene.
The stability of the asphaltene fractions in petroleum has been widely studied, 8 - 12 and one of the main factors that acts to destabilize asphaltenes in oil mixtures is the variation of composition.
Therefore, the precipitation onset is based on the volume of n- heptane necessary to reach the minimum absorption intensity. The precipitation onset is thus expressed in terms of the volume of n- heptane mL necessary to induce the start of asphaltene precipitation from 1 g of petroleum. The oil is considered more stable the greater the volume of n- heptane necessary to cause the asphaltenes to precipitate. Ascertaining the solubility parameter of a crude oil sample by experimental determination of the asphaltene precipitation onset is unreliable when this onset point is hard to detect.
This occurs when visualization of the particles by optical microscopy is difficult and the absorption intensity curve in function of n- heptane volume is poorly defined. This limitation can be overcome by estimating the precipitation onset by mixing the target petroleum with another crude oil that has well-defined precipitation onset, called the standard petroleum.
In a previous study, 30 a Brazilian crude oil, called APS, was used as the standard to determine the precipitation onset and hence the solubility parameter of two other oils. Since the solubility parameter values obtained are not exactly equal to those determined directly by using the precipitation onset of the target oil sample itself, it is necessary that the standard petroleum always to be the same.
This is only possible when the same type of petroleum is produced. An alternative to this method can be implemented by using synthetic molecules instead of the standard petroleum, tailored to have similar behavior when added to oil samples for which it is hard to detect the precipitation onset.
Some studies have been carried out to investigate model asphaltene molecules. The modeling of the chemical structure and reactivity of the asphaltenes enables, for instance, predicting the formation of coke and the properties of various refinery processes, allowing reduction of costs. Based on characterization results obtained, for example, from nuclear magnetic resonance NMR , Fourier-transform ion cyclotron resonance-mass spectrometry FTICR-MS and collision-induced dissociation CID mass spectrometry, researchers have suggested that asphaltene molecules contain a large polynuclear aromatic PNA nucleus with smaller aromatic rings linked to it.
The nucleus is basically composed of aromatics, but also contains naphthenic compounds. Several structures have been proposed to model asphaltene molecules, the main ones being the continental and archipelago models. Based on characterization results of asphaltene fractions obtained by solid-state NMR, X-ray photoelectron spectroscopy XPS and elemental analysis, molecular structures have been proposed for different types of crude oil, with the objective of simulating the cracking behavior of vacuum distillation residues.
Molecular simulation has also been employed to improve the relation between the physico-mechanical properties of model molecules and those of real asphaltenes, 34 , 37 , 50 - 52 as well as to shed light on the aggregation and separation of asphaltenes in binary solvent systems. As already mentioned, in a previous work, 30 we used a petroleum defined as standard APS to improve the profile of absorption curve by NIR of crude oils called APA and APB in function of the volume of n- heptane titrated. In this study, we synthesized and characterized model molecules to be added to these two crude oils, seeking to find systems whose asphaltenes precipitation onset values are close to those obtained by the addition of APS.
Figure 1a , formaldehyde P. Duque de Caxias, Brazil. Tetrahydrofuran THF P. Cardanol Figure 1c , presenting the approximate concentrations of the components of the mixture 41 was supplied by Satya Cashew Chemicals Tamil Nadu, India.
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The relevant characteristics of these samples Table 1 were published previously. Pyrene was chemically modified acylated utilizing 2. Dichloromethane was used as solvent. The reaction was stopped with ice and addition of 1 mL of hydrochloric acid HCl. The final product was removed from the aqueous phase by decantation.
Cardanol was nitrated utilizing 1 mol of cardanol, 1 mol of nitric acid and methanol as solvent, at room temperature for 30 min under magnetic stirring. At the end of the reaction, 10 mL of ethyl acetate was added and the solvent was evaporated at room temperature. The cardanol epoxidation reaction was performed using 0. The reaction was kept under magnetic stirring for 18 h at room temperature.
Then 50 mL of dichloromethane and ethanol mixture was added under magnetic stirring to form two phases. The final product was separated in a separation funnel, washed with distilled water and dried with anhydrous sodium sulfate. Nonylphenol was nitrated using 1 mol of nonylphenol, 1 mol of nitric acid and methanol as solvent, for 30 min at room temperature.
Then 10 mL of ethyl acetate was added and the solvent was evaporated at room temperature. Polycardanol, called PCC0, was synthesized by polycondensation employing a molar ratio of To obtain PCC1, 40 mL of cardanol, The same reaction conditions were employed to obtain PCC2 except that the oxalic acid was added directly in the reaction solution, while to synthesize PCC3, oxalic acid was added in two steps: at the start of the reaction and after 1 h.
The general structure of the PCC molecule is represented by Figure 2a. Polycardanol, called PCA, was synthesized by polyaddition via cationic initiation. For this purpose, 30 mL of cardanol in 0. O C 2 H 5 2 reacted under an inert atmosphere in a glove bag. After this, the reaction was stopped by placing the system in an ice bath.
The general structure of PCA molecule is represented by Figure 2b.
PCA was nitrated utilizing 0. The samples were analyzed in KBr pellets and the spectra were obtained with a scan from to cm -1 with resolution of 4 cm -1 at room temperature.
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The sample was prepared at a concentration of 10 mg mL -1 in tetrahydrofuran THF. Although they have been subjected to modern analytical methods, including the well known SARA analysis, mass spectrometry, electron paramagnetic resonance and nuclear magnetic resonance , the exact molecular structures are difficult to determine. Given this limitation, asphaltenes are composed mainly of polyaromatic carbon ring units with oxygen , nitrogen , and sulfur heteroatoms , combined with trace amounts of heavy metals, particularly chelated vanadium and nickel, and aliphatic side chains of various lengths.
Speight  reports a simplified representation of the separation of petroleum into the following six major fractions: volatile saturates, volatile aromatics, nonvolatile saturates, nonvolatile aromatics, resins and asphaltenes. He also reports arbitrarily defined physical boundaries for petroleum using carbon-number and boiling point. Asphaltenes are today widely recognised as dispersed, chemically altered fragments of kerogen , which migrated out of the source rock for the oil, during oil catagenesis. Asphaltenes had been thought to be held in solution in oil by resins similar structure and chemistry, but smaller , but recent data shows that this is incorrect.
Indeed, it has recently been suggested that asphaltenes are nanocolloidally suspended in crude oil and in toluene solutions of sufficient concentrations. In any event, for low surface tension liquids, such as alkanes and toluene, surfactants are not necessary to maintain nanocolloidal suspensions of asphaltenes. The nickel to vanadium contents of asphaltenes reflect the pH and Eh conditions of the paleo-depositional environment of the source rock for oil Lewan, ; , and this ratio is, therefore, in use in the petroleum industry for oil-oil correlation and for identification of potential source rocks for oil oil exploration.
Heavy oils, oil sands , bitumen and biodegraded oils as bacteria cannot assimilate asphaltenes, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers — enzymatically controlled contain much higher proportions of asphaltenes than do medium- API oils or light oils. Condensates are virtually devoid of asphaltenes.
Asphaltene aggregation, precipitation or deposition can be predicted by modeling or artificial intelligent methods. Asphaltenes impart high viscosity to crude oils, negatively impacting production, also the variable asphaltene concentration in crude oils within individual reservoirs creates a myriad of production problems.
Asphaltenes, Heavy Oils, and Petroleomics
Asphaltenes are known to be one of the largest causes of fouling in the heat exchangers of the crude oil distillation preheat train. Paper, ACS Div. Preprint, 35, Mullins 1. Introduction 1. Overview The most important attribute of any chemical compound is its elemental constituents. There is, fortunately, no uncertainty about the elemental composition of asphaltenes.
The second most important attribute of any chemical compound is its molecular structure and, as a prerequisite to that information, molecular weight. Although the set of structures of individual chemical units constituting asphaltene, such as the number of fused aromatic rings, length of aliphatic chains, and com- mon functional groups is mostly agreed upon, the asphaltene molecular weight has been the subject of a large and long-standing controversy. For the most part, literature reports differ by a factor of 10, but some reports differ by many orders of magnitude.
The question is essentially if and how the chemical units are linked. These uncertainties are exacerbated by the corresponding possibilities that differ- ent asphaltenes are variable, thus prohibiting facile comparison of results across different laboratories on different asphaltenes.
Consequently, a phenomenological approach has been routine in asphaltene science. We employ time-resolved fluorescence depolarization TRFD to measure the molecular rotational correlation time of a large variety of asphaltenes. TRFD methods naturally allow interrogation of different chromophore classes in the as- phaltenes enabling stringent predictions to be tested regarding molecular weight and molecular structure. There is little variation of molecular weight among virgin crude Henning Groenzin and Oliver C.
Model Molecules for Evaluating Asphaltene Precipitation Onset of Crude Oils
Mullins oil petroleum asphaltenes. A variety of other asphaltene samples are investigated as well. Furthermore, all TRFD results are consistent with a molec- ular structure that has a single fused ring system of 4 to 10 rings per petroleum asphaltene molecule including a small number of aliphatic chains.
These results are exploited to develop structure-function relations for asphaltenes; implications are discussed in terms of asphaltene nanoaggregate formation. Finally, we note that asphaltenes are polydisperse, other molecular structures and likely present but only in small mass fraction. Chemical Bonding of Functional Groups in Asphaltenes Molecular weight is one of the most fundamental attributes of any chem- ical compound.