Porosity, catalyst pellet total

Now consider a catalyst pellet with a random network of zig-zag pores. The surface of the pellet is composed of both solid material and pores. The flux equation derived earlier must be modified to account for the fact that the flux, N, is based only on the area of a pore. A parameter called the porosity of the pellet, or Bp, is defined as the ratio of void volume within the pellet to the total pellet volume (void -f solid). The flux can be expressed in moles of A diffusing per unit pellet surface area (containing both solids and pores) by using 8 as... 

The Parallel-pore Model Wheeler proposed a model, based on the first three of these properties, to represent the monodisperse pore-size distribution in a catalyst pellet. From p and Vg the porosity e is obtained from Eq. (8-16). Then a mean pore radius d is evaluated by writing equations for the total pore volume and total pore surface in a pellet. The result, developed as Eq. (8-26), is... 

Qe is the energy transferred per imit total area of the particle normal to the direction of heat transfer. The effective thermal conductivities of catalyst pellets are remarkably low because of the pore structure. The contribution of the thermal conductivity of the solid skeleton is little, since the extremely small heat transfer areas existing at solid-solid contact points offer substantial resistance to heat transfer. The gas phase filling the void spaces in the pores also participates in hindering heat conduction experimental results indicate that decreases as Gp increases. At low pressures, when the mean free path of molecules is greater than or equal to pore size, increases with total pressure since free-molecule conduction starts to dominate. There are no general correlations for predicting Ae from the physical properties of the solid and fluid phases involved. An approximate correlation based on the thermal conductivities of the individual phases and the porosity of the particle has been proposed ... 

Liquid holdup in bed porosity, dimensionless = Bed void fraction, dimensionless = Catalyst pellet void fraction, dimensionless = Effectiveness factor of totally wetted catalyst, dimensionless = Catalyst wetting efficiency, dimensionless = Overall effectiveness factor, dimensionless = Overall efficiency of a trickle bed, dimensionless... 

Procedures outlined in previous sections were used to compute the curves in Fig. 1.8. Three variables arc shown. The abscissa is the chemical rate applicable if all the surface is available. It is the chemical rate built into the catalyst. The ordinate represents values of an index that combines all diffusional tendencies of the particle. Moving in the direction of more diffusion control decreases this index. For example, higher porosity increases (providing it is not accomplished by decreasing the pore radius), increasing total pressure decreases Dg (and thus and increasing the pellet diameter, Pp, lowers both the effectiveness factor and mass transfer coefficient. Parameters on the curves are pellet rates. The ratio between the pellet rate and the intrinsic chemical rate is the effectiveness factor. 

Catalyst Apparent density of particle or pellet (g/cm ) Equivalent diameter (m /g) (cmVg) Total porosity, P ... 

---