| In coal liquefaction hindered diffusion is a detrimental factor to catalyst activity, selectivity and deactivation. The influence of hindered diffusion can be alleviated by modifying the catalyst pore structure. The concept of using a bimodal catalyst consisting of a macro and micropore system is studied with the objective of increasing the accessibility of large reactant molecules.; To elucidate the effect of pore geometry on the performance of catalysts in reactions related to coal liquefaction, several catalysts with different pore structures were prepared and their activities were evaluated in terms of pentane soluble oils yield and sulfur removal. Several alumina extrudates were prepared to investigate effects of catalyst preparation variables on pore structure, including sintering time and temperature, acid type, and fiber type and loading. Bimodal NiMo catalysts were effectively prepared without losing surface area by using a combined method of coextrusion and fiber incorporation.; Mass transfer effects were observed to be important in both model compound and coal liquefaction/coprocessing reactions performed using tubing bombing microreactors. The transfer of molecular hydrogen to the solid and liquid phases primarily caused the mass transfer limitation. By orienting the reactor in a horizontal mode, mass transfer effects were reduced to the level obtained by introducing mixing balls as an agitation aid into a vertical tubing bomb microreactor.; The conversion of naphthalene at 380 {dollar}spcirc{dollar}C was a highly transitory phenomenon associated with the initial state of the catalyst, catalyst sulfiding, and hydrogen sulfide (H{dollar}sb2{dollar}S) partial pressure. Because of the inhibiting effect of H{dollar}sb2{dollar}S on hydrogenation rates, the regulation of H{dollar}sb2{dollar}S concentration provided an additional process variable for maximizing the concentration of intermediate hydroaromatic species.; A maximal point in catalyst activity, observed using unimodal catalysts in both coal-tetralin reactions and in petroleum desulfurization experiments, resulted from the trade-off between the hindered diffusivity and surface area. Compared to unimodal catalysts, bimodal catalysts showed better catalyst activity. A simple model describing the diffusion and reaction processes was found to be highly applicable to the prediction of catalyst activity and took into account most important aspects of catalyst properties and reaction. The effective diffusivity of the catalyst was experimentally measured in both octane-decane and polystyrene-chloroform systems for comparison with effects identified in the model work. |
