External transport processes

This is a different kind of heterogeneous reaction—a gas-solid noncatalytic one. Let us examine the process at initial conditions (t 0), so that there has been no opportunity for a layer of UF4.(5) to be formed around the UO pellet The process is much like that for gas-solid catalytic reactions. Hydrogen fluoride gas is transferred from the bulk gas to the surface of the UO2 pellets and reacts at the pellet-gas interface, and H2O diffuses out into the bulk gas. If the pellet is nonporous, all the reaction occurs at the outer surface of the UO2 pellet, and only an external transport process is possible. Costa studied this system by suspending spherical pellets 2 cm in diameter in a stirred-tank reactor. In one run, at a bulk-gas temperature of 377°C, the surface temperature was 462°C and the observed rate was — Tuo = 6.9 x 10 mole U02/(sec) (cm reaction surface). At these conditions the concentrations of... 

CHAPTER 10 EXTERNAL TRANSPORT PROCESSES IN HETEROGENEOUS REACTIONS... 

In biosensors the biocatalyst and the signal transducer are spatially combined, i.e., the enzyme reaction proceeds in a layer separated from the measuring solution. The substrates reach the membrane system of the biosensor by convective diffusion from the solution. The rate of this external transport process depends essentially on the degree of mixing. In the multilayer system in front of the sensor the substrates and products are... 

Notwithstanding the wealth of theory about such external transport processes, the most accurate equations available for estimating transport coefficients are empirical. 

The intrinsic catalytic cycle contains only the chemical steps 3-4-5 of the 7-step sequence listed in the so-called continuous reaction model. It is necessary to make the assumption of zero gradients with respect to heat and mass transport both outside and within the catalyst particle. Therefore, experimental conditions in the laboratory have to be adjusted to ensure that (i) external transport processes (steps 1 and 7 of the sequence) are very rapid compared to chemical steps and (ii) internal transport processes (steps 2 and 6 of the sequence) are negligible, that is, particle sizes are small enough to ignore pore structure. In Figure 2.3, the reactant concentration profile labeled as IV represents the case for intrinsic kinetics. 

Furthermore, the external transport processes are also assumed to be linear ... 

When the catalyst is not supported, as in the case of metal wire or foil, only the external transport processes affect the reaction rate. In such a case, Eqs. 4.35 and 4.36 with rj replaced by rjx can be used in the definition of rjx given in Eq. 4.46 ... 

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. 

---