Heterogeneous Multiphase Reactions

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 production of species i (number of moles per unit volume and time) is the velocity of reaction,. In the same sense, one understands the molar flux, jh of particles / per unit cross section and unit time. In a linear theory, the rate and the deviation from equilibrium are proportional to each other. The factors of proportionality are called reaction rate constants and transport coefficients respectively. They are state properties and thus depend only on the (local) thermodynamic state variables and not on their derivatives. They can be rationalized by crystal dynamics and atomic kinetics with the help of statistical theories. Irreversible thermodynamics is the theory of the rates of chemical processes in both spatially homogeneous systems (homogeneous reactions) and inhomogeneous systems (transport processes). If transport processes occur in multiphase systems, one is dealing with heterogeneous reactions. Heterogeneous systems stop reacting once one or more of the reactants are consumed and the systems became nonvariant. 

Figure 6-14. Course of component k chemical potential during a heterogeneous multiphase reaction which is partly interface controlled.

In contrast to equilibrium thermodynamics, the thermodynamics of irreversible processes portray the application of thermodynamic methods as dynamic and therefore time-dependent procedures. The name Prigo-gine must be mentioned in relationship to this—he received for his work in this area the Nobel Prize in the year 1977. A new, very complex thermodynamics originated from his examination method for chemical reactions, and was developed by us, to come to a successful description of heterogenous multiphase polymer systems. This theory interprets crazing fracture energy dissipation and fracture mechanism in a totally new way on the basis of dissipative structures in polymer blends and their dynamics, For a list of abbreviations used in this section sec page 610,... 

To adapt the above-mentioned hierarchy properly to the desired performance, interactions between the phenomena involved must be taken into account Indeed, the fundamental phenomena presented in Table 2.1 can exhibit strong couplings that radically change the system efficiency a heterogeneous reaction is coupled to a mass transfer of reactants from the bulk fluid to the catalyst, multiphase reactions combine multiphase transfer and reactions and so forth. 

Advanced learners should then also study Section 4.11.4 (on transport limitations in experimental catalytic reactors) and Sections 4.11.5.2-4.11.5.4, where some more complex examples are given (heterogeneously catalyzed gas-phase reaction, catalytic multiphase reaction, and non-isothermal oxidation of carbon). 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Laboratory experiments by our group showed that reaction 13 occurs very slowly in the gas phase (10). However, in the presence of ice surfaces the reaction proceeds very efficiently the product CI2 is immediately released to the gas phase, whereas HNO3 remains frozen in the ice (11). Other groups also found that this heterogeneous (i.e., multiphase) process occurs efficiently (12,13), and that a similar reaction also occurs with N2O5 as a reactant ... 

The role of mixing in heterogeneous reactions is obvious. In multiphase processes mixing imposed by a stirrer or an external pump is necessary to increase the interface through which reactants pass to meet their partner in the other phase and/or to intensify mass transfer between phases. Mixing can also play a significant role in the case of homogeneous reactions. Chemical reactions occur at the molecular level. Reactant molecules introduced into a reactor encounter the environment in the vicinity of the inlet. The composition of the mixture there is obviously... 

---