DETERMINAÇÃO DO INÍCIO DE PRECIPITAÇÃO DOS ASFALTENOS EM PETRÓLEOS BRASILEIROS E IMPLICAÇÕES NA ESTABILIDADE DE MISTURAS DE PETRÓLEOS

Abstract

The crude oil can be understood as a mixture sufficiently polydisperse where its diverse components form a dispersion and/or solution whose original state can be modified in function of variations of temperature, pressure and composition causing, for example, the formation of organic deposits. The asphaltene constitutes heavy fractions of oils with great capacity for precipitation and consequent deposition. Better understanding of the asphaltenes behavior is essential to reduce the negative impacts caused by these fractions in petroleum production and processing. An important subject is the relation between the asphaltenes stability in the oils and the measurements of asphaltenes onset precipitation, which represent the minimum amount of a flocculent (n-alkane of low molar mass) necessary to initiate the formation of the precipitated ones in the oils. These measurements, in general, are executed in different conditions against the found ones during the production, and are found also significant errors for oils with low amounts of asphaltenes and with particles in suspension. Distinct interpretations in relation to the precipitation phenomenon are still verified when comparing the results between diverse techniques. In this work, the results of different techniques in the determination of the asphaltenes onset precipitation have been evaluated by optical microscopy, viscosimetry and impedance spectroscopy in ten samples of Brazilian crude oils with different characteristics. The results of precipitation onset also had been related with the asphaltenes stability in the pure oils and mixtures of oils. The oil samples had been assigned P1-P10, A and B. The precipitation onset was induced by the addition of heptane in the oil and the toluene oil (2:1) mixture. The results showed good agreement with the values of precipitation onset for the three evaluated techniques, indicating that the flocculation and precipitation represent the same phenomenon. The addition of the toluene provoked a displacement of the asphaltene onset precipitation point strengthening the influence of the aromaticity in the solubilization of the asphaltenes. It was still observed that the stability of the asphaltene in the oil depends on a relation between all the physico-chemical parameters, however parameters such as resins and aromatics exert a bigger influence on the stability. The study in oil mixtures had indicated that the precipitation onset is not a parameter that can be used exclusively as indicative of asphaltenes stability.