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Asphaltene Precipitation using VMGSim's PIONA Characterization

Introduction

Asphaltene precipitation is, by design, a part of several processes including solvent deasphalting and paraffinic oil sands froth treatment; and yet, one of the historical challenges in phase behavior simulation is the prediction of the conditions at which asphaltene precipitation occurs, the amount of precipitate, and the properties of the asphaltene-rich phase [1]. VMGSim recently added a regular solution based asphaltene precipitation model, the SARA Regular Solution property package, to its property package list specifically designed to address these issues.

This package allows for more accurate modeling of asphaltene precipitation processes through the estimation of asphaltene’s yield, precipitation onset and physical properties. The regular solution based model has a proven track record for accurate simulation of asphaltene precipitation for heavy oils diluted with solvents, blended oils, and even depressurized live oils [2].

In order to make use of this package in VMGSim a SARA (Saturates, Aromatics, Resins and Asphaltenes) characterization of the oil is needed. When the oil is characterized, the appropriate chemical families are assigned to the pseudo-components and the appropriate physical properties for pure components and mixtures are automatically calculated. Once the oil has been characterized it can be used in a number of ways. A typical study is to understand the effect of solvent flow rate on the formation of a precipitated asphaltene phase and precipitation onset conditions. Studies can also be done to see the effects of different solvents, as well as the behaviour of mixtures of different solvents. The solvents that the model was developed for include n-alkanes from propane to eicosane.

The oil’s SARA characterization can now be performed in VMGSim 9.5 using VMG’s structure oriented characterization based on PIONA (Paraffins, Iso-Paraffins, Olefins, Naphthenes and Aromatics) components. The key advantage of using the PIONA approach is the capture of the essential chemistry of the feedstock. This method is flexible enough to encode in the characterized compounds known chemical characteristics of the feed ranging from simple properties such as molecular weight and density to PIONA characterization data. Also, the use of this method allows the characterization of several oils using the same slate of components meaning that asphaltene precipitation studies for different oils can be performed within the same flowsheet.

Asphaltene Precipitation Modelling using PIONA based Characterization – An Example

This example will go through the necessary steps for creating an Oil Source unit operation capable of modeling the asphaltene precipitation of bitumen using different solvents at different operating conditions.

The objective of this example is to fit experimental asphaltene precipitation from Athabasca bitumen diluted with n-heptane and n-hexane. These experimental data were obtained from Akbarzadeh et al. [2]. The following table shows the SARA Analysis that represents Athabasca Bitumen.

PIONA_1.png

To start, open a new case in VMGSim 9.5 and select SARA Regular Solution as the Property Package and SI as the Unit System, then click on the PIONA Slate button to access its environment and define the characterization.

PIONA_2.png

In the PIONA Slate form enter the following values:

PIONA_3.png

Note that the Asphaltene Precipitation box is checked in order to create an extended list of dehydrated aromatic components that will represent the Resin and Asphaltene components. The Asph. Precip. Settings box inside the Slate Range Settings is also checked to allow a fair number of resin and asphaltene components to be created. Click on the Create Slate button to create the PIONA components.

For this example, two pure components will be used as the solvents: n-Hexane and n-Heptane. The solvents must be also added to the list of components inside the property package:

PIONA_4.png

Once the components list has been defined, go to the flowsheeting environment and add an Oil Source unit operation (drag the Oil Source icon from the Oil Data Regression shape window to the main flowsheet), open the unit operation and select Deasphalting as the Application type. Note that the Asphaltene Precipitation tab will be available once the selection is done:

PIONA_5.png

Go to the Asphaltene Precipitation tab and add the SARA Analysis of the Athabasca bitumen from the table above; be sure that the SARA Distribution basis is Mass.

PIONA_6.png

The following step is to add the asphaltene yield data. Akbarzadeh et al. [2] published the following data for the asphaltene precipitation from Athabasca bitumen diluted with n-hexane and n-heptane:

PIONA_7.png

PIONA_8.png

To add this data, check the Asphaltene Precipitation Data box and type 2 as the number of available experiments. Add the temperature, pressure, solvent composition, and asphaltene yield data for the Athabasca bitumen diluted with n-heptane in the Experiment_1 frame:

PIONA_9.png

Do the same for the Athabasca bitumen diluted with n-hexane in the Experiment_2 frame:

PIONA_10.png

Take a look to the calculated results and Asphaltene Yield Curve tab. This tab shows a plot comparing the calculated an experimental data, it can be noted that the calculated data is far from the expected experimental results; therefore, it is necessary to regress the parameters of the Oil Source unit operation to match the results.

Before doing the regression calculations and in order to guarantee a good agreement in the results let's change the Regression Tolerance in the Settings tab to 1.00E-04:

PIONA_11.png

Now click on the Regress Parameters button (at the left bottom corner of the unit operation) to start the regression process and wait until the calculations are done. Once the process is finished take a look again to the calculated results and the Asphaltene Yield Curve tab. It can be seen now the calculated and experimental results are very close as it can be seen in the following figures:

PIONA_12.png

PIONA_13.png

The results can be validated if we take, for example, the first experimental point for the asphaltene yield of Athabasca bitumen with n-heptane. If we mix 1 kg/h of a Material Stream attached to the Oil Source with another material stream containing only n-heptane with a flow of 1.368 kg/h, we will see that the mixed stream contains the same amount of precipitated asphaltene as calculated in the Oil Source, the amount of asphaltene precipitated can be seen in the Equilibrium Results tab of the mixed stream, the mass flow of the Liq1 phase (heavy liquid / asphaltene phase) will contain this value.

PIONA_14.png

References

[1] Landra, C., Loria, H. and Yarranton, H.W. SARA Regular Solution Method, Inside VMG, November 2013

[2] Akbarzadeh, K., Alboudwarej, H., Svrcek, W.Y., Yarranton, H.W., A Generalized Regular Solution Model for the Prediction of Asphaltene Precipitation from n-Alkane Diluted Heavy Oils and Bitumens, Fluid Phase Equilibria, 232, 2005, 159-170

Herbert Loria, Ph.D, P.Eng, VMG Calgary

Please contact your local VMG office for more information.

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