For many catalytic reactions, significant temperature gradients may develop within the porous catalysts. Especially for systems with high heat of reaction and low thermal conductivity of pellets, considerable variation of the observed rate from the rate evaluated a t the surface temperature is to be expected. Excellent reviews of heat effects and heat conduction in porous catalysts were reported in the well known books of Satterfield (1970), Petersen (1965), and Carberry (1976.Heat conduction through porous catalysts takes place during the solid and gas phases in parallel with interchange of heat between the two phases. A model was proposed by Butt (1965) for the prediction of effective thermal conductivities. The thermal conductivity of porous solids depends strongly upon geometrical factors and porosity. Most of the effective thermal conductivity values reported in the literature are in the range of to J/scm a "C (Dogu, 1986). Effective thermal conductivities of some catalysts were reported by Sehr (1958), and by Mischke and Smith (1962).In this study, a new dynamic technique was introduced for the measurement of effective thermal conductivity and the Biot number for heat transfer, for porous solids. For this purpose, pulse-response experiments were conducted in a single-pellet reactor. Application of this technique is not limited to porous catalysts. The technique allows fast and precise determination of thermal conductivity of any porous solid.
Method and Theoretical DevelopmentThe diffusion cell used for the measurement of effective diffusion, adsorption and reaction rate parameters (Dogu and Smith, 1975; Dogu 1984; Dogu and Ercan, 1983;Dogu et al., 1986) was Correspondence concerning this paper should be addressed to T. Dogu modified and used for the measurement of effective thermal conductivity of a porous solid (Miirtezaoglu, 1988). As shown in Figure 1, a two-zone cylindrical pellet was prepared in a teflon mold and placed into the single-pellet cell. The lower pellet's effective thermal conductivity was to be measured. In this work, the lower zone was made from -Alumina. The upper zone of the pellet was made from (Pt-AI2O3 containing 0.5% Pt-active catalyst), and it acts as the heat source. Hydrogen gas streams passed over both end faces of the pellet in the single-pellet reactor. A pulse of oxygen gas (6% 0, in H,) was injected into the hydrogen stream flowing over the upper face. Oxygen tracer reacted with hydrogen within the upper active zone, and the heat liberated due to reaction in this zone was conducted through the porous solid placed to the lower zone of the pellet. Bell-shaped temperature-time curves were measured at the interface of upper and lower zones, and at the lower end face of the pellet by using carefully placed thin thermocouples. It was shown that the ratio of zeroth moments and the difference of first absolute moments of the temperature-time curves, were functions of Biot number and the effective thermal diffusivity of the lower pellet.The controlling differential equation for t...
