Alumina effective thermal conductivity

We assume that the adsorbent mass used in the kinetic test consists of a sphere of radius R. It may be composed of several microsize particles (such as zeolite crystals) bonded together as in a commercial zeolite bead or simply an assemblage of the microparticles. It may also be composed of a noncrystalline material such as gels or aluminas or activated carbons. The resistance to mass transfer may occur at the surface of the sphere or at the surface of each microparticle. The heat transfer inside the adsorbent mass is controlled by its effective thermal conductivity. Each microparticle is at a uniform temperature dependent on time and its position in the sphere. 

Fig. 11-3 Effective thermal conductivity of alumina [boehmite) catalyst pellets at 10 to 25 microns Hg pressure...

Fig. 11-5 Effective thermal conductivity of alumina (hoehmite) pellets I j void fraction at 120 F...

The boundary conditions at the external surface of the catalyst are T = Tsurface and Ca = Ca surface, and A effeciive is the effective thermal conductivity of the composite catalyst structure (i.e., 1.6 x 10 J/cm s K for alumina). Initially, the surface temperature and concentration of reactant A in Uie vicinity of a single isolated catalytic peUet are chosen to match the inlet values to the packed reactor. If external mass and heat transfer resistances are minimal, then bulk gas-phase temperature and reactant concentration at each axial position in the reactor represent the characteristic quantities that should be used to calculate the intrapellet Damkohler number for nth-order chemical kinetics ... 

The enthalpy change for reaction is exothermic and varies from 50 to 80 kJ/mol. The activation energy for the forward reaction varies from 25 to 27 kJ/mol. The temperature at the external surface of the pellet is constant at 350 K. The effective thermal conductivity of alumina catalysts is 1.6 x 10 J/cm s K. The chemical reaction is first-order and irreversible and the catalysts exhibit rectangular symmetry. When a(0) 1 in the mass transfer equation, simulations in... 

The results obtained on samples containg A alumina are in good agreement with those of previous studies given in the earlier references. The thermal conductivity of these composites is less than that of the pure resin below about 9 K, with a saturation effect at concentrations near 56 wt.%. The results are very different for A2 alumina. The thermal conductivity becomes lower than that of the pure resin below about 20 K, and the saturation is not yet observed at 65 wt.%. This difference is attributed to some porosity, which is suggested by the appreciable differences between the calculated and measured densities (see Table III). Near 4 K, thermal conductivity is reduced with respect to the resin value by a factor of 2 for A alumina and more than 10 for A2 alumina. 

A. J. Slavin, F. A. Londry, and J. Harrison. "A new model for the effective thermal conductivity of packed beds of solid spheroids Alumina in helium between 100 and 500°C," Int J. Heat Mass Transfer, 43, 2059-2073, 2000. 

The interelectrode insulators, an integral part of the electrode wall stmcture, are required to stand off interelectrode voltages and resist attack by slag. Well cooled, by contact with neighboring copper electrodes, thin insulators have proven to be very effective, particularly those made of alumina or boron nitride. Alumina is cheaper and also provides good anchoring points for the slag layer. Boron nitride has superior thermal conductivity and thermal shock resistance. 

Fig. 3. Effect of density on thermal conductivity. A, 48-mg/cm siUca fiber B, 96-mg/cm siUca fiber C, 128-mg/cm alumina—siUca fiber D, 192-mg/cm ...

For certain products, skill is required to estimate a product s performance under steady-state heat-flow conditions, especially those made of RPs (Fig. 7-19). The method and repeatability of the processing technique can have a significant effect. In general, thermal conductivity is low for plastics and the plastic s structure does not alter its value significantly. To increase it the usual approach is to add metallic fillers, glass fibers, or electrically insulating fillers such as alumina. Foaming can be used to decrease thermal conductivity. 

Catalyst supports such as silica and alumina have low thermal conductivities so that temperature gradients within catalyst particles are likely in all but the finely ground powders used for infrinsic kinetic studies. There may also be a film resisfance fo heaf fransfer af fhe exfemal surface of the catalyst. Thus the internal temperatures in a catalyst pellet may be substantially different than the bulk gas temperature. The definition of the effectiveness factor, Equation 10.23, is unchanged, but an exothermic reaction can have reaction rates inside the pellet that are higher than would be predicted using the bulk gas temperature. In the absence of a diffusion limitation, rj > 1 would be expected for an exothermic reaction. (The case > 1 is also possible for some isothermal reactions with weird kinetics.) Mass transfer limitations may have a larger... 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

The effective bed conductivity has a static or zero-flow term, which is usually about 5k when the particles are a porous inorganic material such as alumina, silica gel, or an impregnated catalyst, and kg is the thermal conductivity of the gas. The turbulent flow contribution to the conductivity is proportional to the mass flow rate and particle diameter, and the factor 0.1 in the following equation agrees with the theory for turbulent diffusion in packed beds ... 

This difference can be easily explained. For formation of solid product on the reactant surface, heating of the sample in a high vacuum by radiation (for example, from the walls of an alumina container) occurs through an intermediate product layer, for example, CaO for the CaCOs decomposition. This means that the effective value of the emittance for heat transfer from the container walls to the calcite crystal covered by a CaO layer, is the product of the corresponding coefficients for the four surfaces AI2O3, CaO (outward side), CaO (inward side), and CaCOs. (The residual thermal conductivity through point contacts between CaCOs crystal and CaO nanoparticles is neglected.) If... 

In a typical case where a silicon IC is attached to an alumina ceramic substrate that, in turn, is attached to the inside of a metal or ceramic package, the two epoxy interfaces can easily contribute 2.5 °C/watt to the total resistance. However, some silver-filled epoxies are reported to have high thermal conductivities, thus contributing 0.6 °C to 1 °C/watt. Actual measurements may differ considerably from calculated values because of reported thermal conductivities that differ from the actual, differences in the thicknesses of bond lines, voids in the adhesive, and incomplete mating of surfaces. Further in the analysis, the effects of lateral flow of heat and interactions of heat flow among adjacent components are often neglected. 

Since the present day TBCs are used in conjunction with alumina-forming alloys, this imposes an additional constraint on the selection of prospective coatings. They must be thermodynamically stable with alumina at high temperature. Although a great effort has been made in searching out low thermal conductivity materials for high temperature applications, there are three difficulties in selection of candidate materials for TBCs. First, an effective model is needed to understand and... 

The package base is 0.040 in. (1.02 x 10 m) thick 92% alumina, which has a thermal conductivity of 17 W/mK [22]. The thermal resistance of the package base 0g is calculated as follows. Assuming 45° spreading, the effective area in the alumina base can be approximated by the geometric mean... 

---