| This work investigated hindered diffusion of model compounds as well as coal and petroleum asphaltenes in fresh, spent, and regenerated NiMo/{dollar}rm Alsb2Osb3{dollar} catalysts. A general mathematical model was developed for the adsorptive diffusion of solute in a solvent onto catalyst pores, and the model equations were solved using various methods. Experimental measurements together with mathematical modeling were conducted to determine how the diffusion rates of asphaltenes and model compounds depended on various parameters such as solute molecule sizes and shapes, catalyst pore structures, catalyst aging and regeneration, solute surface adsorption, experimental temperature.; Experimental results showed that fresh bimodal catalyst had several advantages over fresh unimodal catalyst, e.g., larger pore volume, higher surface area, better adsorption capacity, and greater effective diffusivity. However, after use in direct coal liquefaction, most advantages for the bimodal catalyst were lost due to heavy depositions in the spent catalyst pores. Nonuniformly distributed carbonaceous deposits, rather than metals, were the dominant factor causing a decrease in the diffusivity, and this decrease was much greater than would be estimated from the decrease in average pore diameter. A model incorporating the nonuniformities in the spent catalysts yielded results in satisfactory agreement with experimental measurements and provided a reasonable rationalization for the very low diffusivities. Regeneration by controlled oxidation of the spent catalysts significantly restored the diffusivities and other physical parameters.; Solute adsorption on catalyst pore walls could, depending on parameters such as molecular size and surface adsorption isotherm, significantly restrict the pore diffusion process by decreasing the effective local pore size and increasing hindrance effects. A mathematical model, termed the shrinking pore model, was developed and incorporated the enhanced restrictive effect of an adsorbed solute layer of finite thickness on the pore walls. Simulation results showed that although both the shrinking and the uniform pore models fitted the experimental uptake data of quinoline well, the shrinking pore model was required to obtain a satisfactory representation of the adsorptive uptake for a larger polystyrene solute with a stronger surface adsorption capacity.; Hindered diffusion of coal and petroleum asphaltenes was investigated by the aid of SEC on which the asphaltenes were grouped into various fractions according to their molecular sizes. Simulation results showed that even though the properties of the coal and petroleum asphaltenes were quite different, the values of model parameters for both asphaltenes fractions had the same trend and could be estimated by same numerical expressions, providing the appropriate allowance for the differences in molecular weights, sizes, and diffusivities. The effect of temperature on the adsorptive diffusion of asphaltenes was complex. As expected, for smaller asphaltene fractions, the adsorption isotherms decreased with temperature; however, for larger fractions, the adsorption isotherms increased with the increase in temperature. |
