Modelling of diffusion of molecules in biporous structure supports
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media - I (T2-7a)
Keywords: biporous structure, diffusion, up-scaling, size effect, PFG-NMR
Modelling of diffusion of molecules in biporous structure supports
Raphaël Mesnier1, Loïc Sorbier1, Michel Quintard2
1 IFP LYON, BP3, 69390 Vernaison, FRANCE
2 IMF Toulouse, Allée du Professeur Camille Soula, 31400 Toulouse, FRANCE
Residual oils treatments are a big issue for refiners. The control of hydrotreating processes requires an overall understanding of the various stages of the catalytic reactions taking place, especially the ones concerning the transport of the molecules of the load to the active sites. Vacuum residues mainly contain asphaltenes which lead to problems associated to diffusion in the porosity of hydrotreating catalysts supports, because of their size. In the industry, the used hydrodemetalation catalysts are porous support having two scales of porosity : a macroporosity, in order to have a great porous volume and a mesoporosity to have a great active area.
In our study, we measured the diffusion coefficients of three molecules of different sizes in this kind of support. These molecules are supposed representative of asphaltenes. Coefficients have been measured at a microscopic level by pulsed field gradient NMR (PFG-NMR). Based on these results, we built a model describing the kinetics of the molecules diffusion at a macroscopic level, i.e., for a catalysts pellet. Using upscaling theories, the mesoporous domain can be homogenized and this leads to an effective diffusion coefficient. The macro-scale model will follow by a subsequent upscaling using this mesopore effective diffusion coefficient and the molecular diffusion coefficient in the macropores. The resulting macro-scale model takes into account the non linear adsorption of the molecules (Langmuir type). In fact, the accurate description of mesoporosity is not easy with the actual experimental techniques. Therefore, the idea is to determine experimentally the diffusion properties in the mesoporous domain, and the macropore structure as a starting point for the upscaling theory. This is the objective of this paper.
Indeed, PFG-NMR measurements show there are two diffusion coefficients in biporous structures. It was verified that the value of the diffusion coefficient measured in the macropores is the same as the value in a free liquid. For the mesoporosity, it was found that the diffusion coefficient depends on the mesopore size and molecules diffusing. In addition, we can see an evolution of the effective diffusion coefficient with the mesoporous pore structure. Biporous structures are good candidates for a double-porosity behaviour, i.e., rapid diffusion in the macropores followed by a relaxation of the concentration in the mesoporous domain. Based on our experimental data, a numerical solution over a structure close to the real structure shows that relaxation time in the mesoporous domain is fast with regard to transport in macroporous network, at least for molecules of intermediate sizes. Therefore, this suggests that a single macro-scale effective diffusion coefficient ay be used for the description at the pellet-scale.
Keywords : biporous structure, diffusion, up-scaling, size effect, PFG-NMR
Presented Tuesday 18, 12:00 to 12:20, in session Transport Phenomena in Porous/Granular Media - I (T2-7a).
