Adsorption of asphaltenes on multiscale porous alumina

Abstract

Alumina catalysts are frequently used in refineries for the hydrotreatment of heavy petroleum fraction that are enriched in asphaltenes. The transport of real and model asphaltenes molecules through powder and alumina’s extrudates treated or not at 150 °C to remove or not surface-adsorbed water was studied. The kinetics and isotherms of adsorption at 298 K were obtained by the solution depletion method. Calorimetric experiments were also investigated. The kinetic is faster on the powder than on the alumina extrudates where the equilibrium is reached after 24 h (against 1 h for the powder) due to mass transfer limitation. The capacity of adsorption of model asphaltenes on untreated powder and extrudates is comparable around 1.1 and 1.2 mg.m− 2 and increases with the heat treatment due to water removal. Both adsorption strength and capacity of real asphaltenes on alumina is lower compared to the model asphaltene molecule which could be explained by the strong interaction between the acidic function of the model molecule and the alumina surface. The calorimetric study in absence of alumina shows the dimerization of model asphaltene molecules. In presence of alumina, the enthalpy of adsorption of model and real asphaltenes on alumina is determined. The enthalpy of adsorption of model asphaltenes on treated powder is higher than on untreated powder meaning that more energetic sites are available (probably due to the release of water-occupied sites) and the curve obtained for treated powder suggests different adsorption sites. The enthalpy of adsorption of model asphaltene is higher for the treated extrudates but these results must be taken carefully because the kinetic of adsorption is very slow (24 h) for extrudates. The effect of flow rate was studied by saturating an extrudate column with model asphaltene molecules. The adsorption increases as the flow rate decreases which could be explained by higher friction in the macropores leading to the release of weakly retained asphaltenes as the flow rate increases or by less intermediate pore blocking by asphaltenes as the flow rate and thus the pressure increases. This study shows that the transport of asphaltenes through porous alumina supports is a complex process depending on many parameters.