Thermal field-flow fractionation of asphaltenes

Abstract The thermal field-flow fractionation technique has been applied to the analysis of three samples of asphalt and asphaltenes. These samples were effectively retained in all three of the solvents used as eluants, with toluene giving the best retention level and solubility. The influence of eluant flow rate on peak retention and peak broadening was studied. It has been established, by means of the thermogravitational effect, that asphaltenes, like polymers, migrate toward decreasing temperatures under the influence of a thermal gradient. Asphaltene sample fractions were collected at the separator outlet and then reinjected into the channel, demonstrating an important fractionation effect. U.v.-visible spectrometric analysis of the fractions revealed a continuous modification of the spectra with the fraction collection order. The retention of asphaltenes was compared with that of polystyrenes. The sample molecular weight distribution and the average molecular weights were determined by assuming that the relation between thermal diffusion factor and molecular weight is the same for these two kinds of substances. The average diffusion coefficient of asphaltenes was independently determined by the chromatographic peak dispersion method; in terms of molecular weight, this provided a result coherent with the retention measurements. It is therefore proposed that polystyrene be used as a convenient reference material for the determination of molecular weights of heavy oil fractions and for the comparison of samples from different origins.