(352f) Enthalpy of Solution Differences Between Chemically Separated Asphaltenes and Physically Separated Asphaltene-Rich Materials in Athabasca Bitumen

Asphaltenes, defined as a fraction of crude oils that can be filtered from admixtures with n-alkanes¹ are a surrogate for nanoaggregated materials present² with which they share many but not all properties. These nanoaggregated materials, however defined, comprise up to 20 wt % of heavy oils. Once destabilized, they may separate as a solid or viscous liquid phase and stick to surfaces in reservoirs, pipelines and refining process equipment. Deposition of asphaltene-rich materials can cause wellbore or pipeline plugging, degradation of process performance and process failure. During bitumen extraction, asphaltene-rich materials absorb at oil/water interfaces and stabilize oil in water (O/W) or water in oil (W/O) emulsions.³ Stabilization of water in oil emulsions by asphaltene-rich materials results in inefficient separation of water from the oil phase. Since water drops in these emulsions contain dissolved salts, inefficient separation can cause equipment corrosion in downstream processes. Incomplete removal of oil drops from the water phase reduces the recovery of bitumen and causes difficulties during wastewater treatment.

Many studies, both experimental and theoretical, purport to address the conditions under which asphaltenes or asphaltenes-rich materials deposit on substrates and the extent of deposition. Frequently, the formation of asphaltene phases, as a dispersion, and their deposition on surfaces is not discriminated and correlations identifying conditions where deposition occurs remain unreliable when applied in the field.^4–8 This uncertainty contributes to the technical and hence the economic risk associated with process design and process operation. Further, recent studies have shown that the properties, compositions and phase behaviors of asphaltenes separated from crude oils upon addition of n-pentane, namely C₅ asphaltenes, or n-heptane, C₇ asphaltenes, on which correlations are largely based, are not representative of the asphaltene-rich materials that aggregate naturally in crude oils.^5,9

In this study the enthalpies of solution of chemically separated C₅ asphaltenes, and physically separated asphaltene-rich nanoaggregated materials are compared in toluene, heptane, and de-ionized water using a precision solution calorimeter. The repeatability and reliability of data obtained with this equipment was validated in a recent prior study.¹⁰ Physical separation of asphaltene-rich nanoaggregated materials from Athabasca bitumen was conducted using nanofiltration with commercially available ceramic membranes at 473 K, with a moderate differential pressure and without solvent addition.² The solution behaviors of chemically separated C₅ asphaltenes, and physically separated asphaltene-rich materials are shown to be both quantitatively and qualitatively different. This additional difference between the behaviors of chemically separated C₅ asphaltenes and asphaltene-rich nanoaggregated materials in crude oil underscores the need to work directly with materials in oils that are aggregated or that form aggregates, and not with operationally defined fractions with poorly correlated properties. By doing so, there is potential to develop generalizable mitigation strategies and reliable predictive tools for “asphaltene deposition” in field applications.

(1)       Buenrostro-Gonzalez, E.; Lira-Galeana, C.; Gil-Villegas, A.; Wu, J. Asphaltene Precipitation in Crude Oils: Theory and Experiments. AIChE J. 2004, 50, 2552–2570.

(2)       Zhao, B.; Shaw, J. M. Composition and Size Distribution of Coherent Nanostructures in Athabasca Bitumen and Maya Crude Oil. Energy & Fuels 2007, 21, 2795–2804.

(3)       Yang, X.; Hamza, H.; Czarnecki, J. Investigation of Subfractions of Athabasca Asphaltenes and Their Role in Emulsion Stability. Energy & Fuels 2004, 18, 770–777.

(4)       Likhatsky, V. V.; Syunyaev, R. Z. New Colloidal Stability Index for Crude Oils Based on Polarity of Crude Oil Components. Energy and Fuels 2010, 24, 6483–6488.

(5)       Mohammadi, A. H.; Eslamimanesh, A.; Gharagheizi, F.; Richon, D. A Novel Method for Evaluation of Asphaltene Precipitation Titration Data. Chem. Eng. Sci. 2012, 78, 181–185.

(6)       Rogel, E.; Leon, O.; Contreras, E.; Carbognani, L.; Torres, G.; Espidel, J.; Zambrano, A. Assessment of Asphaltene Stability in Crude Oils Using Conventional Techniques. Energy & Fuels 2003, 17, 1583–1590.

(7)       Wang, J. X.; Creek, J. L.; Buckley, J. S. Screening for Potential Asphaltene Problems. In SPE Annual Technical Conference and Exhibition; Society of Petroleum Engineers: San Antonio, 2006.

(8)       Cheng Xing, Robert W. Hilts, and John M. Shaw, Sorption of Athabasca Vacuum Residue Constituents on Synthetic Mineral and Process Equipment Surfaces from Mixtures with Pentane, Energy & Fuels, 2010,24(4) 2500-2513. 

(9)       Amundaraín Hurtado, J. L.; Chodakowski, M.; Long, B.; Shaw, J. M. Characterization of Physically and Chemically Separated Athabasca Asphaltenes Using Small-Angle X-Ray Scattering. Energy & Fuels 2011, 25, 5100–5112.

(10)     Pourmohammadbagher, A.; Shaw, J. M. Excess Enthalpy and Excess Volume for Pyridine + Methyldiethanolamine and Pyridine + Ethanolamine Mixtures. J. Chem. Eng. Data 2013, 58, 2202–2209.