Asphaltene Diffusion and Adsorption in Modified NiMo Alumina Catalysts Followed by Ultraviolet (UV) Spectroscopy

Hindered diffusion and adsorption phenomena of asphaltene molecules inside the porous network of hydrotreatement catalysts are discussed in this paper. The kinetics of the global process was followed by visible absorption spectroscopy measurements. After n-heptane extraction, asphaltenes were dissolved in toluene at low concentration and put in contact with a given amount of catalyst. The dynamic of the diffusion adsorption process was investigated by measuring the absorbance of monochromatic visible radiation (750 nm) versus contact time. The effects of the composition of the catalysts (alumina-supported NiMo with or without dopants, such as sodium and tin) have been investigated. Coke formation and asphaltene conversion have also been studied during the hydroconversion catalytic test. A modeling approach based on the Stefan-Maxwell formalism was applied, which takes into account the volume constraints by Fornasiero's formulation and assumes that the molecules collide only by an equivalent volume. The different parameter estimations have been performed and discussed to identify the controlling step of the process: mass transfer or asphaltene adsorption on the surface. The results show that dopants have only a moderate effect, with sodium addition decreasing the asphaltene adsorption constant, whereas it is slightly increased by tin. On the other hand, the coke deposit has a dramatic impact on the overall process, decreasing the effective diffusion coefficient as well as the adsorption constant of asphaltene. Moreover, after deep hydroconversion, the size of the asphaltene molecules is strongly decreased, the adsorption coefficient is enhanced, and diffusional coefficients increased by 1 order of magnitude.