Internal and external transport

Figure 9. Effects of internal and external transport resistances on the computed step-response of CO adsorption. Curve A corresponds to our experimental conditions. Key A, km = 60 cm/s, Deff = 0.0246 cm2/s B, km —r oo, Detl = 0.246 cms/s and C, km — oo, Dell oo.

The differential equations involve the effective diffusivity and thermal conductivity of the porous pellet. Data and theories for these quantities are given in Secs. 11-1 to 11-5, and in Secs. 11-6 to 11-11 the results are used to establish the rate for the whole pellet. The method of combining the effects of internal and external transport resistance to give the global rate in terms of bulk fluid properties is discussed in Chap. 12. 

Unfortunately, to the best of our knowledge, there is no appropriate published experimental data on a 3D spray drying process for the pilot-scale spray chamber used here. However, the developed model of internal and external transport phenomena can be validated by experimental values available for a closely similar drying process, for example, pneumatic drying. For this purpose, the presented model, with slight modifications, has been utilized to perform steady-state numerical simulations of 3D pneumatic... 

Theron, J. N., Dry, M. E., Steen, E. V., and Fletcher, J. C. G. Internal and external transport effects during the oxidative reforming of methane on a commercial steam reforming catalyst. Studies Surface Sci. Catal. 107, 455-460, 1997. 

The importance of internal diffusion can also be appreciated from a different point of view the fact that the internal diffusion plays a pivotal role in internal and external transport processes. For negligible concentration gradient in the pellet, Eq. 4.57 still holds. However, the value of r Da will be larger than that for diffusion-limited case for the same intrinsic rate since 17 is larger and therefore the pellet will be more isothermal as Figure 4.7 reveals. Further, a relativdy large Biot number for mass under realistic conditions still ensures negligible external mass transfer resistance. It is seen then that in the absence of diffusional resistance, the pellet tends to be more isothermal and the only major resistance is likely to be external heat transfer. 

Carefully plaimed diagnostic experiments can be used to evaluate the effect of both internal and external transport on catalyst performance. 

Haase R (1991) Internal and external transport numbers in ionic melts. Z Phys Chem (Muenchen) 174 77-87... 

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