Abstract: There exists many organic solid substance with high molecular weight in crude oil. Therefore, in order to describe phase equilibrium appearing in oil/gas system accurately, it is essential to study vapor/liquid/solid three phase equilibrium. On the basis of understanding to asphaltene precipitation mechanism, the paper comes up with the view that the incompatibility between asphaltene components and light hydrocarbons can be described
with larger interaction coefficients. The present paper also puts forward vapor/liquid/solid material balance equations which has their own characterization in line with the generic opinion and the method for characterizing asphaltene components.With three phase algorithm which takes asphaltene precipitation into account, the technique can model asphaltene precipitation quantitatively. Moreover, by examples described in the paper, calculation method of reference fugacity and matching method of
saturation pressure and amount of precipitated asphaltene have been presented.

Key words: equation of state, asphaltene precipitation, flash calculation, fugacity, saturation pressure