Hindered diffusion of asphaltenes through microporous membranes

The relationship between the diffusion coefficient of asphaltenes and pore size was studied experimentally using track-etched mica membranes. The asphaltenes were extracted from Kuwait atmospheric bottoms using criteria of n-pentane insolubility and tetrahydrofuran solubility at 25°C. Data were obtained for asphaltene fractions of narrow size range by using gel permeation chromatography to analyze the diffusion samples. The results show a very strong pore size effect on diffusion coefficient in the range 70–500 Å pore radius for asphaltenes whose GPC-equivalent polystyrene molecular weight is 3000 or greater. By properly accounting for the molecular size of the asphaltene fractions, we are able to obtain good agreement between the measured diffusion coefficients and those predicted from a hydrodynamic theory of hindered diffusion.