In this paper, the Soave-Redlich- Kwong equation of state is modified based on perturbation theory. The parameter of the new cubic equation of state considered reduced temperature and acentric factor dependent. The average of absolute deviations of predicted saturated vapor pressure, vapor volume and saturated liquid density of 40 pure hydrocarbon and non-hydrocarbon compounds are 1.116, 3.083 and 5.696 percent, respectively. Also comparison with the new equation of vaporization of pure compounds is given. The equilibrium parameters such as ΔH, ΔU, and ν are calculated using modified equation of state and willapplied in Flory-Huggins (FH) model to predict of asphaltene precipitation during solvent titration in heavy oil.

At the second stage the Flory-Huggins (FH) model is modified by three adjustable parameters (a, b, c) topredict asphaltene precipitation more accurately. These parameters are defined based on molecular weight of asphaltene to molecular weight of heavy oil ratio in form of second polynomial function. The phase behavior of asphaltene was extended by these modifications and the precipitation of asphaltene is calculated while adding three n-alkanes solvents. The new model is relatively accurate in predicting phase behavior of asphaltene in heavy oil.


Asphaltene Precipitation in oil reservoir is Prejudicial to the economy of oil production because its deposition impairs oil flow to the wellbore. In site asphaltene deposition causes permeability reduction and wettability changes. It is of great interest to be able to predict the condition that lead to such precipitation and to quantify the amount of precipitated asphaltenes.

Gradual progress has been made in the modeling of asphaltene precipitation from Petroleum fluids in the last two decades. The modeling approaches can be classified into five different categories:

  1. polymer solubility representation,

  2. equation of state,

  3. colloidal,

  4. thermodynamic micellization, and

  5. molecular thermodynamics.

The solubility model has been developed by Hirshberg et al(1). 4) and further applied by several authors(2),(3). The reversible liquid-liquid equilibrium (LLE) is governed by activity coefficients that are calculated from the Flory-Huggins polymer solution theory in order to account for the non- ideality of asphaltene and resin molecules. Another group of authors(4),(5) used a similar approach but considered asphaltenes as a solid component. The general equation that relates the solid to the liquid fugacity of pure sphaltenes depends on temperature and fusion properties(6). Recently, G.R. Pazuki and M. Nikookar et. al. (7) presented better results by modification of Flory- Huggins model. The equation of state model has been established by Gupta(8), and later modified by Nghiem et al. (9) It is also referred to as the solid model. The precipitated asphaltene phase is represented by a pure solid component while the liquid and vapor phases are modeled with an equation of state (EOS). The Peng- Robinson EOS was used to calculate the asphaltene precipitation by assigning different values of binary interaction coefficients to the precipitating and nonprecipitating pseudo components with light components. The model is easy to implement, however, it requires tuning of many parameters to match experimental data.

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