Investigation on the reversibility of asphaltene precipitation

The precipitation and redissolution of asphaltenes upon the addition and re moval of solvent were investigated for Athabasca and Cold Lake bitumens usi ng a flow-loop apparatus. The presence of precipitate was detected through an increase in pressure drop across an in-line filter. These solvent-revers ibility experiments were conducted at 40 and 60 degreesC with n-heptane sol vent. A significant hysteresis was observed for Athabasca bitumen while lit tle 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 t o 160 degreesC. A hysteresis was observed for both bitumens and only partia l reversibility was achieved. Benchtop solvent-reversibility experiments we re also conducted on the two bitumens at room temperature. In this case, th e precipitate was recovered by centrifugation. The benchtop results were in good agreement with the flow-loop experimental results. Two heavy oils wer e also tested and both exhibited hysteresis and complete reversibility. Pre cipitation and redissolution in n-heptane were measured over time for the A thabasca bitumen. Precipitation increased over time reaching an apparent eq uilibrium after 8 days. Redissolution experiments reached virtually the sam e equilibrium position in less than 1 day. The slow kinetics of precipitati on suggests rate-limiting nucleation, growth, or flocculation of the asphal tenes. Redissolution experiments with their more rapid kinetics are better suited for obtaining equilibrium solubility data.