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Heavy oil fractions can be upgraded through various processes, such as catalytic residue hydrotreatments. Mass transfer of macromolecules present in the heavy oil fraction, so-called asphaltenes, from feedstock to catalytic active sites is limited during hydroprocesses. Mechanisms of the diffusion of asphaltenes through pore network, adsorption, and pore plugging are no well-known under process conditions. A new method has been developed to characterize and investigate asphaltene diffusion phenomenon in catalysts under a high temperature and pressure. Alumina supports immersed in asphaltene solution are left to evolve at 250 °C and 5.0 MPa. Solutions and supports are analyzed to quantify the mass transfer, penetration depth, and change in support porosity of asphaltenes. This procedure was evaluated in terms of reproducibility and sensitivity. The impact of several parameters, such as pressure, was appraised. With this powerful procedure, for the first time, asphaltene diffusion without conversion into the pore network of a catalyst at a high temperature and pressure has been monitored over time. In accordance with analytical results, we proposed a primary model for the asphaltene adsorption and pore network cluttering mechanism under hydroprocessing conditions.

The upgrading of heavy petroleum fractions needs the development of more and more efficient heterogeneous catalysts. One of the major issues of these processes is the diffusion of asphaltenes to the active site through the porous network of the alumina support. The catalytic efficiency is deeply impacted by the transport phenomena and the interfacial interactions. The aim of this work is to capture the extent to which low-field two-dimensional (2D) 1H NMR relaxation time correlations can contribute to a better understanding of the dynamics of asphaltene in solution and within the pores of catalyst supports. Two-dimensional T 1–T 2 maps for asphaltenes in solution in toluene exhibit several T 1–T 2 contributions, varying with the asphaltene concentration and the size of the asphaltenic fractions obtained by ultrafiltration separation. According to the nanoaggregate structure proposed by the Yen–Mullins aggregation model of asphaltenes, it was possible to unravel the different asphaltenic proton relaxation behaviors. By the use of NMR relaxometry, we have confirmed the stronger interaction of water with alumina than the one of toluene. The presence of macropores in catalyst clearly boosts the toluene mobility through the porous network. Two-dimensional T 1–T 2 maps for asphaltenes inside the pores show various types of protons, all of them with a severe constrained dynamics. Asphaltene nanoaggregates and clusters can be seen as large entities jammed into the pores, slowly mobile and affecting the solvent (toluene) mobility. When macroporosity exists in the support, the asphaltene overcrowding is less sensitive, enabling a faster dynamics of asphaltenes and toluene.

This study deals with the role of the alumina nanoporous texture in the accessibility of the asphaltene molecules to the active sites of resid hydroconversion catalysts. We have proposed in this contribution to investigate the diffusion and adsorption process into the porosity of various monomodal and bimodal alumina supports. It consists of immersing catalytic supports, under the form of extrudates, into an asphaltene solution in toluene. Optical microscopy observations of a cut of the cylindrical extrudates were carried out to follow the asphaltene penetration as a function of the contact time. We aimed at reaching a good asphaltene mass balance, taking into account (i) the evolution of the asphaltene concentration in solution and (ii) the precise determination of the amount of asphaltenes deposited into the support. The influence, on the mass transfer and the penetration depth, of various parameters such as temperature, asphaltene concentration of the solution, asphaltene size polydispersity, and alumina porous texture was appraised. The novelty of this Article resides in the evaluation of diffusion and adsorption of asphaltenes at high temperature (250 °C). Finally, an endeavor for the process modeling is presented. The adsorption equilibrium was modeled according to the Langmuir-type isotherm. A model considering the radial diffusion in a cylinder (the extrudate) was used providing an average diffusion coefficient.