Mass transfer, unusual effects

Reaction rates typically are strongly affected by temperature (76,77), usually according to the Arrhenius exponential relationship. However, side reactions, catalytic or equiHbrium effects, mass-transfer limitations in heterogeneous (multiphase) reactions, and formation of intermediates may produce unusual behavior (76,77). Proposed or existing reactions should be examined carefully for possible intermediate or side reactions, and the kinetics of these side reactions also should be observed and understood. 

The central difficulty in applying Equations (11.42) and (11.43) is the usual one of estimating parameters. Order-of-magnitude values for the liquid holdup and kiA are given for packed beds in Table 11.3. Empirical correlations are unusually difficult for trickle beds. Vaporization of the liquid phase is common. From a formal viewpoint, this effect can be accounted for through the mass transfer term in Equation (11.42) and (11.43). In practice, results are specific to a particular chemical system and operating mode. Most models are proprietary. 

Spacer chain catalysts 3, 4, and 19 have been investigated under carefully controlled conditions in which mass transfer is unimportant (Table 5)80). Activity increased as chain length increased. Fig. 7 shows that catalysts 3 and 4 were more active with 17-19% RS than with 7-9% RS for cyanide reaction with 1-bromooctane (Eq. (3)) but not for the slower cyanide reaction with 1-chlorooctane (Eq. (1)). The unusual behavior in the 1-bromooctane reactions must have been due to intraparticle diffusional effects, not to intrinsic reactivity effects. The aliphatic spacer chains made the catalyst more lipophilic, and caused ion transport to become a limiting factor in the case of the 7-9 % RS catalysts. At > 30 % RS organic reactant transport was a rate limiting factor in the 1-bromooctane reations80), In contrast, the rate constants for the 1 -chlorooctane reactions were so small that they were likely limited only by intrinsic reactivity. (The rate constants were even smaller than those for the analogous reactions of 1-bromooctane and of benzyl chloride catalyzed by polystyrene-bound benzyl-... 

In many of these experiments, interfacial turbulence was the obvious visible cause of the unusual features of the rate of mass transfer. There are, however, experimental results in which no interfacial activity was observed. Brian et al. [108] have drawn attention to the severe disagreement existing between the penetration theory and data for the absorption of carbon dioxide in monoethanolamine. They have performed experiments on the absorption of C02 with simultaneous desorption of propylene in a short, wetted wall column. The desorption of propylene without absorption of C02 agrees closely with the predictions of the penetration theory. If, however, both processes take place simultaneously, the rate of desorption is greatly increased. This enhancement must be linked to a hydrodynamic effect induced by the absorption of C02 and the only one which can occur appears to be the interfacial turbulence caused by the Marangoni effect. No interfacial activity was observed because of the small scale and small intensity of the induced turbulence. 

Thermal and mass-transfer effects are matters of reaction engineering and reactor design, not of kinetics as understood here. For standard situations—that is, reactions with positive activation energies and rate equations with reaction orders that are positive or zero for reactants and negative or zero for products—current texts on reaction engineering provide excellent treatments [1-10]. In some instances, however, the special nature of a multistep reaction may result in unusual and quite different behavior. Only such cases will be examined here. 

Chapter 12. Unusual thermal and mass-transfer effects... 

Mass transfer may be more important when processing certain unusual types of pure hydrocarbons (208). The effect of diffusion is more pronounced when employing large catalyst particles, especially at high temperatures (49). 

It is difficult to estimate all the constants in quation 31 in order to arrive at a quantitative evaluation. However, since the current increases with increasing mean velocity,it might be said the effects nearly cancel in the denominator of equation 31. This would mean that indeed the cathodic current becomes almost proportional to the mean velocity with the 1/6 power. Even though this argument is not strictly quantitative, it still suggests a reasonable explantion for the rather unusual mass transfer effect observed in Fig. 20. The above rationale applies only to the cathodic current, however, a similar derivation can be used to calculate the anodic current dependence on flow rate. 

In effect, the two phases and the interface can be in physicochemical equilibrium or not [Mior to emulsification. If the SOW system is preequilibrated there will be no mass transfer or chemical potential driving force, only adsorptimi on newly created surface. If not. mass transfer through interface could produce spontaneous emulsification and other unusual phenomena, both during the emulsification and during the decay. 

Polymer transport processes that are relevant to the FRRPP process are fluid flow, heat transport, and mass transfer. Fluid flow is evident during fluid conveying and mixing of reactor fluids. Heat transfer is always relevant not only for dissipation of the heat of polymerization but also in an unusual heat trapping effect found in FRRPP systems. Finally, mass transfer is relevant to the diffusion of reactants into polymerization reactive sites, in measurement of spinodal curves, and in study of the evolution of composition profiles during phase separation. 

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