Use of Near-to-Microporous Silica Gel for Selective Separation of Petroleum Vanadyl Porphyrins from Asphaltene Clusters

The possibility of separation of petroleum vanadyl porphyrins from asphaltene clusters on the silica gel adsorbent whose mean pore diameter (27 Å) exceeds the size of petroporphyrin molecules but not of asphaltene clusters has been studied. For this purpose, the time- and concentration-dependent adsorption of heavy oil asphaltenes and vanadyl porphyrins contained in them was studied by a UV–vis spectroscopic method in order to identify differences in the mechanism and rate of their adsorption. It has been established that at initial asphaltene concentrations above the critical nanoaggregate concentration (CNAC) and critical clustering concentration (CCC), the shape of the kinetic adsorption curves is significantly influenced by nonaggregated asphaltene molecules still remaining in solution. The rate of attainment of adsorption equilibrium is limited by the rate of adsorption of asphaltene nanoaggregates. The rate of adsorption of free vanadyl porphyrins is up to 2 orders of magnitude greater than the adsorption rate of nanoaggregates, which indicates independent diffusion of these two types of components inside the adsorbent pores. Asphaltene clusters do not make a significant contribution to the total adsorption due to the size-exclusion effect. Using experimental data on time-course and equilibrium adsorption, we revealed the impact of kinetic and concentration factors on the efficacy of vanadyl porphyrin isolation. Due to the size-exclusion effect toward asphaltene clusters, a 6-fold enrichment of the adsorbate with petroporphyrins is achieved. The results of the study can lay the basis for more effective approaches for petroporphyrin isolation.