Adsorption of asphaltenes on kaolinite as an irreversible process

• Experiments with different initial conditions demonstrate that equilibrium isotherms do not apply to asphaltene adsorption.
• First examination of irreversible adsorption as a kinetically limited deposition analogous to other colloids such as proteins and polymers.
• Define the surface coverage of the minerals and the desorption of the asphaltenes in terms of irreversible adsorption processes.

The adsorption of asphaltenes on mineral surfaces is important in petroleum production and in oilsands extraction. Adsorption of Athabasca asphaltenes from toluene solutions onto kaolinite was investigated over a wide range of asphaltene and kaolinite concentrations. Although the adsorption appeared to follow the functional form of the Langmuir isotherm, the amount adsorbed depended on the kaolinite concentration so that the result depended on the experimental path, not the final solution concentration of asphaltenes. The results are consistent with kinetically limited adsorption, analogous to proteins, with a functional dependence on the asphaltene/kaolinite ratio. Multiple contacts between dried asphaltene-coated kaolinite and fresh asphaltene solutions, and between “spent” asphaltene solutions and fresh samples of kaolinite further confirmed that conventional isotherms were inadequate to describe the adsorption behavior. Repeated contact of asphaltene solution with kaolinite adsorbed up to 98% of the asphaltenes, therefore, the adsorption was not selective for specific species in the asphaltene mixture. Kaolinite samples before and after multiple contacts with asphaltene were analyzed by elemental analysis and X-ray photoelectron spectroscopy (XPS). The adsorption density of asphaltene on kaolinite increased dramatically after each contact with a fresh asphaltene solution, but the surface composition detected by XPS on the kaolinite surface was unchanged. The presence of Al and Si, detectable even at high asphaltene adsorption density, proved that asphaltene adsorption layer is not continuous on the kaolinite surface, and that subsequent asphaltene adsorption occurred preferentially on the asphaltenes already adsorbed on the kaolinite surface.

Adsorption of asphaltenes with increasing solution concentration (adsorption), then decreasing concentration (desorption) with and without irreversible interaction with the surface.Figure optionsDownload as PowerPoint slide