INFLUENCE OF THE LINEAR VELOCITY OF THE REACTANT FLOW ON A HETEROGENEOUS PROCESS IN THE EXTERNAL TRANSITION REGION

V. M. MIRONOV, S. I. SMOL’YANINOV

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

Using the method of Ya. B. Zel’dovich [1], it is possible to qualitatively estimate the nature of the influence of the linear velocity of the reactant flow on heterogeneous reactions occurring in the external transition region.

Figure 1 shows part of the theoretical temperature dependence of the rate of a heterogeneous process in the interval of the regions of external and internal diffusion, and, at the same time, the distribution of reactant concentrations on the active surface and inside the pores of a lump material corresponding to each region.

Under unchanged conditions of external mass transfer, the dependence of the rate of the heterogeneous process on temperature will be determined by the broken line \(OABC\). An increase in temperature leads to an increase in the rate of the surface reaction and, correspondingly, to an increase in the difference between the surface and bulk concentrations of the reactants, while an increase in the flow velocity leads to a decrease in this difference. Since a change in the conditions of external mass transfer has practically no effect on the rate of the process in the region of internal diffusion, the equilibrium line of the resultant influence of temperature and linear velocity on the temperature dependence of the reaction rate may be considered to be the segment \(OA\). Therefore, an increase in the linear velocity, leading to equality of the bulk and surface concentrations (position II, Fig. 1), transfers the process (at constant temperature) from the external-transition region into the internal-diffusion region.

Fig. 1. To explain the influence of external diffusion conditions on the temperature dependence of the reaction rate.

Then an increase in temperature (at a constant increased linear flow velocity) will lead the process along the broken line \(OA_1B_1C_1\), etc.

Thus, the influence of the lin-

...of the linear velocity of the reactant flow on the temperature dependence of the rate of a heterogeneous reaction proceeding in the external transition region is determined by a parallel displacement of the straight lines (curves) in Arrhenius coordinates that characterize this region (the lines \(AB\), \(A_1B_1\), \(A_2B_2\), etc.). The indicated character of the dependence of the effective rate constant on temperature was obtained by us experimentally in studying the macrokinetics of synthesis from carbon monoxide and water vapor at atmospheric pressure [2].

At very high velocities of flow past a piece of granular material, the external transition region may practically disappear, and the process will pass completely to the external surface (broken line 125, position II, Fig. 1), i.e., to the external kinetic region, which is described by the line \(OA_2B_3C_3\).

Conclusion

The character of the temperature dependence of the rate of a heterogeneous process under changing conditions of external diffusion, in the case where it proceeds in the external transition region, has been qualitatively assessed. The dependence considered is confirmed experimentally.

References

1. Ya. B. Zel’dovich, ZhFKh, 13, no. 2, 162—165, 1939.
2. S. I. Smol’yaninov, V. M. Mironov (this collection).