DIFFUSION-CONTROLLED ADSORPTIVE UPTAKE OF COAL AND PETROLEUM ASPHALTENES IN A NiMo/Al₂O₃ HYDROTREATING CATALYST

In this work, hindered diffusion of one coal and two petroleum asphaltenes was studied by adsorptive uptake in THF from a bath surrounding a NiMo/Al2O3 catalyst. A mathematical model for the adsorption-diffusion of asphaltenes was developed. The model parameters were obtained by simulating the experimental data with the model solution. Several asphaltene fractions were defined via SEC (Size Exclusion Chromatography), with the molecular weight of each fraction being determined by its elution characteristics. It was found that both the coal and petroleum asphaltenes have very broad molecular weight distributions; however, the molecular weights of the coal asphaltenes (50-2000) were much smaller than those of the petroleum asphaltenes (200-30000). The uptake rates for asphaltene fractions with different molecular weights varied, depending on their diffusion rates and adsorption constants. Simulation results showed that even though the properties of coal and petroleum asphaltenes were quite different, the values of model parameters for the fractions of the three asphaltenes had the same trend and could be estimated by same numerical expressions; with increasing molecular weight of the fraction, the adsorption constant monotonically increased, and the effective diffusivity decreased. The experimental uptake data for the three asphaltenes as a function of molecular weight were well represented by same mathematical model.