Estudo da estabilidade de asfaltenos em função da temperatura e contribuição das interações entre asfaltenos via modelagem coloidal

Abstract

In this thesis, a study of the stability of asphaltenes in crude oil was carried out, as a function of temperature, comparing different techniques for detecting the onset of precipitation (onset), through the identification and quantification of asphaltene particles and their size distribution, imposing temperature variation cycles. In the microscopy analysis, it was possible to obtain photos of the slides, which were later treated with the help of ImageJ software. Next, an experimental study of the evolution of the distribution of asphaltenes as a function of temperature was carried out, a temperature cycle was carried out, heating followed by cooling with a return to the initial temperature. The comparison of different techniques for detecting the beginning of precipitation allowed us to conclude that optical microscopy presents more reliable results for the beginning of this detection but requires analyst training to identify the beginning of flocculation, especially when the sample is oil, due to the presence of components other than asphaltenes. Refractive index analysis can be used in conjunction with density to help quickly infer the region where asphaltene precipitation begins, but this analysis must be done with caution as the onset may not be identified. Using only the refractive index did not lead to the identification of the onset. The analysis by UV-Vis spectroscopy presented some limitations, therefore the suggestion for future work is the use of diffuse reflectance spectroscopy or using smaller optical paths, or even a possible use of DLS (Dynamic light scattering) to identify small clusters that were not detected by optical microscopy due to limitation of detection size. The data obtained through optical microscopy were processed allowing the particle size distribution to be quantified, depending on the system conditions. Statistical analysis of optical microscopy results allowed a detailed analysis of the phenomenon. When comparing the volumes of precipitates before and after cooling, it is possible to observe that the volume of smaller particles does not vary significantly in each class, but the volume of larger particles continues to increase even with cooling, showing that the phenomenon is irreversible. Another contribution is the modeling of interaction forces, which was carried out through theoretical analysis based on Hamaker theory and De Gennes repulsive theory. When the parameters of the attractive term and repulsive term of the model were kept constant, there was greater stability with increasing temperature, different from what was found experimentally. On the other hand, it was observed that the reduction of the parameter related to the length of the alkyl chains can explain the experimental macroscopic behavior of the Shale oil studied and some different behaviors reported in the literature, such as an increase in the flocculate volume with increasing temperature. This size reduction in the length of the alkyl chains can be attributed to a desolvation effect. Thus, adequate parameterization of the model can mimic the behavior of the temperature effect. Furthermore, proper use of this theory appears to be a promising way to represent microscopic interactions in asphaltene systems, including previous results from the literature.