Molar masses of both n-pentane-extracted and n-heptane-extracted Athabasca asphaltenes were
measured in toluene or 1,2-dichlorobenzene with a vapor pressure osmometer (VPO). The initial
asphaltene molar mass, at concentrations below 0.5 kg/m3, is ≈1800 g/mol. The asphaltene molar
mass is found to increase with asphaltene concentration until a limiting value is reached at a
concentration between 10 and 20 kg/m3. The limiting value ranges from 4000 to 10 000 g/mol
and depends on the solvent, temperature, and asphaltene fraction. The results suggest that
asphaltenes form aggregates of 2−6 molecules in aromatic solvents. Interfacial tensions of
asphaltenes in toluene or 1,2-dichlorobenzene versus water were measured for asphaltene
concentrations from 0.3 to 100 kg/m3 using a drop volume tensiometer. The interfacial tension
decreases linearly with concentration, indicating that no micelles are formed. Hence, the
aggregation observed with VPO does not appear to be micellization. Similar results are obtained
for Cold Lake asphaltenes.
Asphaltene properties vary with separation method and sometimes with individual technique. Factors such as contact time, solvent-to-crude oil ratio, and temperature influence asphaltene precipitation and are somewhat standardized. However, the final step in most separations, washing the asphaltene filter cake with solvent, is not standardized. Asphaltene properties can be very sensitive to small amounts of resins and therefore may be sensitive to the amount of washing. Asphaltenes were extracted with three different levels of washing from four source oils (Athabasca, Cold Lake, Lloydminster, and Peace River). In all cases, increased washing decreased asphaltene yield and slightly increased asphaltene density. Increased washing significantly increased molar mass and decreased the solubility of the extracted asphaltenes. A new washing method using a Soxhlet apparatus removed the largest amount of resinous material and yielded asphaltenes with significantly different properties from conventionally washed asphaltenes. Since more resinous material was removed, the Soxhlet method allows a more direct comparison between asphaltenes from different sources. Asphaltenes were also extracted using three standard separation methods, IP 143, ASTM D4124, and a method proposed by Speight. Some property variations between the methods were observed and a set of criteria to obtain consistent samples is proposed.
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