Conductivity of porous catalysts, thermal

Effective Thermal Conductivities of Porous Catalysts. The effective thermal conductivity of a porous catalyst plays a key role in determining whether or not appreciable temperature gradients will exist within a given catalyst pellet. By the term effective thermal conductivity , we imply that it is a parameter characteristic of the porous solid structure that is based on the gross geometric area of the pellet perpendicular to the direction of heat transfer. For example, if one considers the radial heat flux in a spherical pellet one can say that... 

THERMAL CONDUCTIVITY OF POROUS CATALYST PELLETS. MASAMUNE S SMITH J M J CHEM ENGR DATA... 

U. Hoffmann, G. Emig and H. Hofmann Comparison of different determination methods for effective thermal conductivity of porous catalysts, ACS Symp.Ser. 65(1978)189-200 /23/ R. Broucek, G. Emig and H. Hofmann Rechnergesteuerter Kreislaufreaktor fiir kinetische Untersuchungen,Chem.Ing.Techn. (1984) 236-237... 

Oamparison of Different Determination Methods for Effective Thermal Conductivity of Porous Catalysts... 

Dynamic methods for the determination of thermal conductivity of porous catalyst pellets normally consist of experiments in which a spherically or cylindri-... 

Table 3 Effective Thermal Conductivities of Porous Catalysts, Determined bv Different Methods...

An extensive discussion of the thermal conductivity of porous catalyst particles is presented by Satterfield [94], who suggested that the effective thermal conductivity of most catalyst pellets falls in the range 8-4 x lO"" cal/cm sec °C. 

Thermal conductivities of porous oxide catalysts, which are of major practical interest, (at atmospheric pressure in air) are within the range 0.2 - 0.5 W nr K 1. The spread of values of the thermal conductivity is remarkably small for the wide range of catalysts studied by several investigators. It does not vary greatly with major differences in void fraction and pore size distribution. 

Method lie requires that (i) The thermal conductivity of the catalyst pellets be within the range of the thermal conductivities of the pure liquid components (ii) the thermal conductivity of the liquid mixture be known as a function of its composition. This means that catalysts with a thermal conductivity greater than water can not be tested in this way, e.g. catalysts A and E in this study. Naturally this method can also be used to determine the thermal conductivity of solid catalyst material if the pellets are so finely ground that the liquid can penetrate into all pores. This value is then the basis for determining the effective thermal conductivity of the porous pellet using known models [, 9 1. 

Here Iq is the thermal conductivity of the system, consisting of the porous solid and the reacting fluid inside the pores. This is the most uncertain value, while everything else is measurable. Two things must be remembered. First, data on thermal conductivity of catalysts are approximate. The solid fraction of the catalyst (1-0) always reduces the possibility for diflhision, while the solid can contribute to the thermal conductivity. Second, the outside temperature difference normal to the surface or Daiv, will become too high, much before the inside gradient can cause a problem. See Hutching and Carberry (19), Carberry (20). 

Temperature gradients within the porous catalyst could not be very large, due to the low concentration of combustibles in the exhaust gas. Assuming a concentration of 5% CO, a diffusion coefficient in the porous structure of 0.01 cms/sec, and a thermal conductivity of 4 X 10-4 caI/sec°C cm, one can calculate a Prater temperature of 1.0°C—the maximum possible temperature gradient in the porous structure (107). The simultaneous heat and mass diffusion is not likely to lead to multiple steady states and instability, since the value of the 0 parameter in the Weisz and Hicks theory would be much less than 0.02 (108). 

The effective thermal conductivities of common commercial porous catalysts are quite low and fall within a surprisingly narrow range. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Measurements of A, are scarce. The available data are reviewed by Satterfield [2], The effective thermal conductivity of a porous catalyst can be estimated from the correlation of Russell [30] ... 

Strongly exothermic or endothermic reactions may cause a temperature profile within the catalyst layer, and reaction rates and thus selectivity may be altered. The importance of the temperature profile depends on the reaction rate, the layer thickness, the reaction enthalpy, AHR, the activation energy, E, and the thermal conductivity of the porous catalyst, le. To achieve what is considered quasi-isothermal behavior, the observed rate, reff, must not differ from the rate at uniform temperature by more than about 5%. The following criterion, designed for a catalyst layer in a microchannel, can be formulated ... 

In the general case where the active material is dispersed through the pellet and the catalyst is porous, internal diffusion of the species within the pores of the pellet must be included. In fact, for many cases diffusion through catalyst pores represents the main resistance to mass transfer. Therefore, the concentration and temperature profiles inside the catalyst particles are usually not flat and the reaction rates in the solid phase are not constant. As there is a continuous variation in concentration and temperature inside the pellet, differential conservation equations are required to describe the concentration and temperature profiles. These profiles are used with intrinsic rate equations to integrate through the pellet and to obtain the overall rate of reaction for the pellet. The differential equations for the catalyst pellet are two point boundary value differential equations and besides the intrinsic kinetics they require the effective diffusivity and thermal conductivity of the porous pellet. 

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 ... 

The effective thermal conductivities of common commercial porous catalysts are quite low and fall within a surprisingly narrow range. The heat transfer path through the solid phase offers considerable thermal resistance for many porous materials, particularly if the pellet is formed by tableting of microporous particles. Such pellets may be regarded as an assembly of particles that contact one another at only a relatively small number of points that act as regions of high thermal resistance. 

In addition to temperature differences between fluid and solid catalysts, temperature profiles within the porous catalyst may occur. However, because of the higher thermal conductivity of the solid, temperature differences are normally small and exceed rarely several degrees. 

For the design and analysis of fixed-bed catalytic reactors as well as the determination of catalyst efficiency under nonisothermal conditions, the effective thermal conductivity of the porous pellet must be known. A collection of thermal conductivity data of solids published by the Thermophysical Properties Research Centre at Purdue University [ ] shows "a disparity in data probably greater than that of any other physical property ". Some of these differences naturally can be explained, as no two samples of solids, especially porous catalysts, can be made completely identical. However, the main reason is that the assumed boundary conditions for the Fourier heat conduction equation... 

Air cathodes are typically constructed in layers. The uppermost layer is exposed to the air and must allow oxygen gas to enter, so this material is a porous structure often made of PTFE (polytetrafluoroethylene). The catalysts are then found in the next layer. This layer is a porous carbon layer that contains the catalysts. This carbon layer can function on its own for reaction with oxygen gas, but the presence of catalysts can improve the performance of the cell. Catalysts are added to this layer as the pure catalyst itself or as a catalyst coated on carbon powder. This can be done with a wet or dry process. Depending on the wet process to manufacture the air cathode, the catalyst must be able to not decompose in the solvent such as water or alcohol. For the dry process the catalyst must readily mix with the carbon power and binders. Many air cathode manufacturing processes involve heat, some up to 300 °C, which requires thermal stability of the catalyst. The next layer down is a metal mesh current collector that is used to conduct the electrons and provide mechanical support. This air cathode represented here is typical of an air cathode manufactured by Yardney Technical Products Inc. and it has a second layer for catalysts. This second layer improves performance of the air cathode. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

As with thermal conductivity, we see in this section that disorder can greatly affect the mechanism of diffusion and the magnitude of diffusivities, so that crystalline ceramics and oxide glasses will be treated separately. Finally, we will briefly describe an important topic relevant to all material classes, but especially appropriate for ceramics such as catalyst supports—namely, diffusion in porous solids. 

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