Coking and activity of a reforming catalyst in near-critical and dense supercritical reaction mixtures

Along near-critical isotherms (1.01-1.2 T{dollar}sb{lcub}c{rcub}{dollar}), fluids display properties ranging from gas-like to liquid-like with small variations in pressure (0.9-1.2 P{dollar}sb{lcub}c{rcub}{dollar}). in this region an optimum combination of solvent and transport properties intermediate to those of a liquid and of a gas can be obtained. Employing the Pt/{dollar}gamma{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} catalyzed isomerization of 1-hexene (P{dollar}sb{lcub}c{rcub}=31.7{dollar} bar; T{dollar}sb{lcub}c{rcub} =231.1 spcirc{dollar}C) as a model reaction, it is shown that such an optimum combination of fluid properties can be exploited for mitigating coke buildup in porous catalysts.; Experiments were performed at 281{dollar}spcirc{dollar}C (1.1 T{dollar}sb{lcub}c{rcub}{dollar}) at pressures from 0.7-11 P{dollar}sb{lcub}rm C{rcub}{dollar} in a tubular reactor at a space velocity of 135 g hexene/g cat./hr. Eight hour isomerization rates are nearly twofold higher and deactivation rates nearly threefold lower at near-critical conditions (1.1 T{dollar}sb{lcub}c{rcub}{dollar}; 0.85 {dollar}rhosb{lcub}c{rcub}{dollar}, relative to subcritical conditions, (1.1 T{dollar}sb{lcub}c{rcub}{dollar}; 0.42 {dollar}rhosb{lcub}c{rcub}{dollar}). Coke laydown is 2.5 times lower and remaining surface area 2.5 times higher in near-critical relative to subcritical conditions. The in situ extraction of the coke compounds alleviates pore-choking and prevents pore-plugging that otherwise occurs at subcritical conditions during the eight hour runs. Although coke laydown decreases at supercritical conditions ({dollar}>{dollar} 1.5 {dollar}rhosb{lcub}c{rcub}{dollar}), isomerization rates are lower and deactivation rates are higher due to pore-diffusion limitations in liquid-like reaction mixtures. We therefore conclude that near-critical reaction mixtures provide an optimum combination of solvent and transport properties for maximizing reaction rates and minimizing deactivation rates.; A mathematical model incorporating transport of the reactants into the pore, formation of products, coke deposition, coke extraction and transport of the products, was developed. Numerical simulation results are consistent with the experimental trends noted above. Data on intrinsic solubilities of coke compounds and transport properties in near-critical mixtures are needed to improve quantitative model predictive.; Reactions requiring liquid-like reaction mixtures for coke removal and gas-like diffusivities can benefit from the use of near-critical reaction media that provide and optimum combination of these properties. The Fischer-Tropsch synthesis is an example of such a reaction.