Investigation on the Reversibility of Asphaltene Precipitation

The precipitation and redissolution of asphaltenes upon the addition and removal of solvent were investigated for Athabasca and Cold Lake bitumens using a flow-loop apparatus. The presence of precipitate was detected through an increase in pressure drop across an in-line filter. These solvent-reversibility experiments were conducted at 40 and 60 °C with n-heptane solvent. A significant hysteresis was observed for Athabasca bitumen while little or no hysteresis was observed for Cold Lake bitumen. In both cases, the precipitation could be completely reversed. Temperature-reversibility was also investigated with n-dodecane solvent at temperatures ranging from 40 to 160 °C. A hysteresis was observed for both bitumens and only partial reversibility was achieved. Benchtop solvent-reversibility experiments were also conducted on the two bitumens at room temperature. In this case, the precipitate was recovered by centrifugation. The benchtop results were in good agreement with the flow-loop experimental results. Two heavy oils were also tested and both exhibited hysteresis and complete reversibility. Precipitation and redissolution in n-heptane were measured over time for the Athabasca bitumen. Precipitation increased over time reaching an apparent equilibrium after 8 days. Redissolution experiments reached virtually the same equilibrium position in less than 1 day. The slow kinetics of precipitation suggests rate-limiting nucleation, growth, or flocculation of the asphaltenes. Redissolution experiments with their more rapid kinetics are better suited for obtaining equilibrium solubility data.