Gibbs Free Energy of the Precipitated Phase

Gibbs free energy of the precipitated phase. In order to evaluate the Gibbs free energy of the precipitated phase, first the state of this phase should be ascertained. Most of the asphaltene precipitation calculations are based on the assumption that the precipitate is a solid phase. This assumption is most probably true at room temperature but may not be valid at reservoir temperatures which are often higher than 330 K. Experimental studies have shown that the precipitated phase is in the form of dark solid particles when a crude oil is diluted with propane at room temperature (Kokal et al., 1992), When the same crude oil is diluted with n-pentane or a heavier normal alkane at room temperature, the precipitated phase has a crystalline state (Kokal et al., 1992). Chung et al. (1991) also observed the precipitate to be in a solid state when a crude oil was mixed with n-pentane at room temperature. 

At high temperatures, Kokal et al. (1992), Hirschberg et al. (1984) and Godbole et al. (1995) observed the precipitation of a black-liquid mixture. A transition from solid to liquid state was observed by Storm et al. (1996) at above 330 K. Here we will formulate the Gibbs free energy of the precipitated phase as a liquid. The expression for the Gibbs free energy of the precipitated solid phase will be presented in Example 5.7. 

In the following, we will assume that the precipitated phase only consists of asphaltenes and resins. We will also assume that the precipitated phase does not form a micellar solution there will be no association either between asphaltene molecules or between asphaltene and resin molecules in the precipitate. The Gibbs free energy expression for the precipitated liquid phase L2 will be simply 

Gibbs free energy of the liquid and the precipitate system and equilibrium 

The total Gibbs free energy of the light-liquid phase and the precipitated phase L2 is given by G = G -f G. Using Eqs. (5.81) and (5.83), 

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