Explaining the Paradox of Asphaltene Precipitation with n-Alkanes

Explaining the paradox of asphaltene precipitation with n-alkanes

Md Rashedul Islam and Chau-Chyun Chen

Department of Chemical Engineering, Texas Tech University

Abstract

Paraffins, specifically alkanes, promotes asphaltene precipitation when they are blended with

asphaltene-containing petroleum fluids. The alkane/oil volume ratio required for onset of

asphaltene precipitation increases with the carbon number and yields a maxima. Experimental

study indicates that the maxima lays at C7[1] or between C8 -- C10 [2]. However, precipitation

yield decreases asymptotically with increase of the carbon number even if excessively high

alkane/oil volume ratio is used [3]. For example, n-pentane be a better solvent for asphaltene

than n-hexane in regard of both onset of precipitation and precipitation yield. However, high

carbon number alkane beyond the maxima, n-hexadecane for instance, can be a better solvent at

onset of precipitation point and a poorer solvent in regard of precipitation yield than n-

pentadecane. A thermodynamic explanation for this paradox is necessarily important to find the

compatibility of bitumen or crude oil with paraffinic hydrocarbons. In this work, this paradox is

explained by the aggregation thermodynamics of asphaltene precipitation [4]. Both the enthalpy

and entropy of mixing are investigated to explain the effect of paraffin carbon number on

asphaltene precipitation. Athabasca and cold lake bitumen samples are studied.

References

[1] G. Hotier and M. Robin, "Action de divers diluants sur les produits pétroliers lourds:

mesure, interprétation et prévision de la floculation des asphalténes," Oil & Gas Science and

Technology, vol. 38, pp. 101-120, 1983.

[2] I. A. Wiehe, H. W. Yarranton, K. Akbarzadeh, P. M. Rahimi, and A. Teclemariam, "The

paradox of asphaltene precipitation with normal paraffins," Energy & fuels, vol. 19, pp.

1261-1267, 2005.

[3] D. L. Mitchell and J. G. Speight, "The solubility of asphaltenes in hydrocarbon solvents,"

Fuel, vol. 52, pp. 149-152, 1973.

[4] M. Wang, Y. Hao, M. R. Islam, and C. C. Chen, "Aggregation Thermodynamics for

Asphaltene Precipitation," AIChE Journal, Submitted.