Asphaltenes Transport into Catalysts under Hydroprocessing Conditions

Abstract

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.

Quantitative imaging of carbon in heterogeneous refining catalysts

“…To simulate process conditions and interactions with crude impurities, alumina supports were immersed with varying impregnation times in a 2 wt% asphaltene solution prepared by dissolution in toluene at a target temperature of 250 C and a high pressure in a hermetic cell. 30 Asphaltenes are a heavy cut from crude that contain polyaromatic hydrocarbons that form aggregates over several size ranges and are considered under strong diffusion limitations in heterogeneous hydrodemetallization rening catalysts. 31,32 The asphaltenes were previously extracted by n-heptane precipitation from a Safaniya vacuum residue, as described in the French standard NF T60-115 method.…”

“…For each sample, the global carbon concentration in the bulk was estimated by considering the concentration of carbon in Safaniya asphaltenes measured by combustion analysis (equal to 82.3 wt% AE 0.4) and the asphaltene concentrations in supports, estimated by mass balance calculations achieved for each batch. 30 The fresh catalyst support (not impregnated with asphaltenes) is known to hold a residual homogeneous carbon content (equal to 0.12 wt%, as measured by combustion analysis). This fresh support will be considered as the reference sample in the following sections.…”

Asphaltenes diffusion/adsorption through catalyst alumina supports – Influence on catalytic activity

“…After a contact time support-asphaltene solution, varying from few hours to several days, the catalytic supports were recovered from the asphaltene solution by filtration and then the supports, consecutively dried, and the asphaltene were submitted to various characterizations. A precise mass balance on asphaltenes was calculated and is closed to 90 % +/-5%, allowing the determination of the asphaltenes weight repartition between the solution and the support [39].…”

“…Other contributions dealt with catalytic solids [31][32][33][34][35][36][37][38]. A more recent strategy previously detailed was proposed in order to cope with the temperature effect on diffusion process [39]. From the experimental data giving asphaltene concentration versus time and using diffusion and adsorption models, it was possible to deduce some key parameters as the average size of the molecules, the diffusion coefficient and some parameters characteristic of the adsorption [32][33][34].…”

“…This paper gathers experimental results on the asphaltenes transport into various catalytic supports used for refining catalytic hydrotreatments. The methodology is based on the use of a new high temperature and pressure unit which was previously detailed in a recent contribution aforementioned [39]. Alumina supports are put in contact with asphaltenes dispersed in toluene and their penetration through the porosity is followed versus time by ex-situ analysis.…”