Cubic EOS calculates heavy oil SARA fractions

Dec. 11, 2006
A new cubic equation of state (EOS) predicts densities of Western Canadian heavy oils and their SARA (saturates, aromatics, resins, and asphaltenes) fractions.

A new cubic equation of state (EOS) predicts densities of Western Canadian heavy oils and their SARA (saturates, aromatics, resins, and asphaltenes) fractions. The method considers the SARA fraction of heavy oil as a pseudocomponent.

A comparison to experimental density data shows that the new EOS is more accurate than the Peng-Robinson (PR) EOS and Soave-Redlich-Kwong (SRK) EOS.

The new EOS is a simple and accurate correlation for modeling and simulating oil and gas processes.

SARA correlations

The industry has several correlations for estimating phase behavior of asphaltene and their SARA fractions in heavy oils.1 2 Also, some have used cubic equations of state to predict molar volume of heavy oils and asphaltene.3 4

Cubic equations of state without volume translation cannot estimate molar volume of the SARA fractions.

The new method set forth in this article discusses the application of a new cubic equation of state for predicting the densities of western Canadian heavy oils and their SARA fractions.

The results are better than with the PR EOS5 and SRK EOS.6

New EOS

The new cubic equation of state is based on the authors’ previous work (OGJ, Nov. 28, 2005, p. 46).

The new EOS is expressed as Equations 1-7, in which P is pressure, T is temperature, and v is molar volume.

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The calculation of a and b parameters of the EOS requires critical properties and the acentric factor. These calculations use the correlation of Akbarzadeh for obtaining the critical properties and acentric factors of SARA fractions (Equations 8-10).7

In Equations 8-10, M is molar mass of the fractions and K is an empirically obtained adjustable parameter.

Calculations

Petroleum engineers need heavy oil density and SARA fractions for modeling the process of asphaltene precipitation. The first step, therefore, is to calculate the volumetric property of heavy oils with a cubic equations of state.

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Table 1 shows the analysis of the SARA fraction for four western Canadian heavy oils. The acentric factors of SARA fractions require parameter K as a correction factor. The correction factor is an adjustable parameter.

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Table 2 shows the values of K for each SARA fraction calculated with the new EOS, PR EOS, and SRK EOS. Reference 8 provided the correction factor values for the PR EOS and SRK EOS.

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Table 3 presents the experimental and calculated densities of SARA fractions of the Western Canadian heavy oils and also compares the results with those from the PR EOS and SRK EOS.

The results show that the new EOS better calculates the densities of SARA fractions than the PR and SRK equations of state.

Calculating the densities of heavy oils from their SARA fractions requires a mixing rule (Equation 11, in which wi is the mass fraction).

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Table 4 shows the experimental and calculated densities of the Western Canadian heavy oil and bitumen.

The average absolute deviation (ADD) was 16.523 for the new EOS, 17.287 for the PR EOS and 32.985 for the SRK EOS.

References

  1. Yarranton, H.W., and Masliyah, J.H., “Molar Mass Distribution and Solubility of Asphaltenes,” AIChE Journal, No. 42, 1996, p. 3,533.
  2. Rassmdana, H., et al., “Asphalt Flocculation and Deposition: I, The Onset of Precipitation,” AIChE Journal, No.42, 1996, p. 10.
  3. Akbarzadeh, K., et al., “ Prediction of the Densities of Western Canadian Heavy Oils and Their SARA Fractions from the Cubic Equations of State, ” Iran Journal of Science & Technology, No. 28, 2004, p. 695.
  4. Pazuki, G.R., and Nikookar, M., “A Modified Flory-Huggins Model for Prediction of Asphaltenes Precipitation in Crude Oil,” Fuel, No. 85, 2006, p. 1,083.
  5. Peng, D.Y., and Robinson, D.B., “A New Two-Constant Equation of State,” Ind. Eng. Chem. Fundam. No. 15, 1976, p. 59.
  6. Soave, G., “Equilibrium Constants from a Modified Redlich-Kwong Equation of State,” Chem. Eng. Sci., No. 72, 1972, p. 1,197.
  7. Akbarzadeh, K., et al., “ Equations Lead to Asphaltene Deposition Predictions, ” OGJ, Oct. 28, 2002, p. 51.

The authors

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Gholam Reza Pazuki ([email protected]) is a PhD student in thermodynamics at Sharif University of Technology, Iran and a member at the Malek Ashtar University of Technology, Tehran. Pazuki holds a BSc in petroleum process engineering from Sharif University of Technology, Iran, and an MSc in chemical engineering from Tarbiat Modarres University, Tehran..

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Shiva Mansouri (shmj4@ yahoo.com) is a senior process engineer at Pars Oil Co., Tehran. Mansouri holds a BSc in gas process engineering from Sharif University of Technology and an MSc in chemical engineering from the Science and Technology University, Tehran.