Thermal chemical reaction characteristics of autothermal fixed
bed reactors with structured catalysts and different surface features
Conventional methods of producing a synthesis gas are expensive and complex installations. In order to overcome the complexity and expense of such installations, it is proposed to generate the synthesis gas within autothermal fixed bed reactors that utilize structured catalysts and different surface features to generate the heat necessary to support endothermic heating requirements of the steam reforming reactions. The present study is focused primarily upon the thermal chemical reaction characteristics of autothermal fixed-bed reactors with structured catalysts and different surface features. The heat capacity, thermal conductivity, and viscosity of the mixture are calculated as a mass-weighted average of the values for each constituent. In addition, the material properties of each individual species are functions of the local temperature. When solving the species mass transport equations, binary mass diffusion coefficients are used directly. Only isotropic catalyst structures are considered and the permeability and inertial resistance factor are held constant. The effective thermal conductivity of the catalyst structure system including the porous structure and intervening gas is calculated based upon the porosity of the porous medium, the fluid phase thermal conductivity, and the porous medium effective thermal conductivity as measured in air at ambient conditions. The present study aims to explore how to effectively generate the synthesis gas within autothermal fixed bed reactors that utilize structured catalysts and different surface features. Particular emphasis is placed upon the heat and mass transport phenomena involved in autothermal fixed bed steam reforming reactors. The results indicate that for catalytic thermal chemical reactions, both kinetic impediments are substantially reduced permitting realization of theoretical or near theoretical reaction kinetics. The heat transfer chamber is in thermal contact with the reaction chamber volume, the heat transfer chamber transferring heat at the enhanced heat transfer rate across the wall between the heat transfer chamber and the reaction chamber, thereby obtaining the enhanced production rate per reaction chamber volume for the thermal chemical reaction. Structured catalyst configurations result in high rates of heat transport from the oxidation side to the reforming side. Typically, combustion takes place at low or near-atmospheric pressure, although high pressure combustion is widely practiced. The heat that is generated on the combustion side is quickly transferred on the reforming side. The heat integrated reforming reactor offers several advantages over conventional flame-based reforming reactors. The incorporation of a simultaneous endothermic reaction to provide an improved heat sink may enable a typical heat flux of roughly an order of magnitude above the convective cooling heat flux. The wall can be constructed from any material, but materials that offer low resistance to heat transfer such as metals and metallic alloys are preferred. In this configuration, heat is generated by combustion in the catalytic chamber and is transported very easily and efficiently though the wall to the reforming chamber where the heat demanding reforming reactions take place. The presence of a catalyst and lower temperatures permit significantly higher space velocities to be used compared to flame-based reformers.
Keywords: Hydrocarbons; Reactors; Structures; Properties; Reforming; Combustion