On the Nanofiltration of Asphaltene Solutions, Crude Oils, and Emulsions

It has long been established that crude oils contain a colloidal suspension of asphaltenes where the mass fraction can range from 0 to 15%. The size of the colloidal particles is less well understood. Crude oil properties depend critically on the asphaltene content, yet measurement of these properties can be hindered by emulsified water and inorganic particulates in heavier crude oils. Here we explore the ability to isolate pure and unaltered crude oil samples using Gore-Tex membranes with nominal pore sizes as small as 30 nm where a transmembrane differential pressure no greater than 60 psi is employed. We first demonstrate that conventional crude oils can pass through a Gore-Tex membrane without alteration of their asphaltene content or their transport properties. Second, we demonstrate that the same filtration process can be used to extract crude oil from a water-in-oil emulsion where the extracted crude's properties are not distinguishable from those of the original crude. Third, we show that these experiments put an upper bound on the asphaltene nanoaggregate size in crude oils. For all experiments, visible near infrared (VIS-NIR) spectroscopy and Karl Fischer titration analysis are used to determine the potential loss of asphaltenes upon filtration and the effectiveness of the removal of water from emulsions. In all cases, negligible asphaltene loss was detected, indicating that asphaltene nanoaggregates and nanoaggregate clusters are smaller than the 30 nm nominal pore size of the membranes used here. While only small sample volumes were used, membrane separation of pure crude oil is an excellent method for isolating pure, representative crude oil samples.