Determination of the mass fraction of crystals in partly frozen hydrocarbon fuels

The per cent by mass of crystalline solid in a partly frozen hydrocarbon fuel cannot be determined by a filtration experiment alone because the precipitate retained by the filter entraps large amounts of the liquid phase. A technique has been developed which augments the filtration experiment with an independent determination of the amount of liquid in the precipitate and hence allows calculation of the true value of the mass percent of crystalline solid, % S. This method involves the addition to the initial fuel of a small amount of a non-crystallizing dye as a tracer and the determination of the amount of entrapped liquid in the precipitate fraction by a spectrophotometric comparison of the dye concentrations in the precipitate and filtrate. The technique has been tested at several temperatures on both a diesel and a jet fuel. % S increased with decreasing temperature below the melting point. The % S values for the jet fuel were in good agreement with results obtained in another laboratory using a filtration/g.c. analysis method. Precipitates obtained by filtration of the partly frozen fuels were found to contain 50–80% entrapped liquid.