Mass heterogeneous reactions

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. 

Surface area effects in mass transfer or heterogeneous reactions... 

SURFACE AREA EFEECTS IN MASS TRANSFER OR HETEROGENEOUS REACTIONS 5 1... 

The reactor volume is calculated from Mj and the bulk density of the catalyst material, (-r ) depends not only on composition and temperature, but also on the nature and size of the catalyst pellets and the flow velocity of the mixture. In a heterogeneous reaction where a solid catalyst is used, the reactor load is often determined by the term space velocity, SV. This is defined as the volumetric flow at the inlet of the reactor divided by the reaction volume (or the total mass of catalyst), that is... 

Mass Transfer. The reaction rate of heterogeneous reactions may be controlled by the rates of diffusion of the reacting species, rather than the chemical kinetics. 

In a heterogeneous reaction two or more phases e.xist and the overriding problem in reactor design is to promote mass transfer between the phases. The possible combinations of phases are... 

Whitaker, S, Transport Processes with Heterogeneous Reaction. In Concepts and Design of Chemical Reactors Whitaker, S Cassano, AE, eds. Gordon and Breach Newark, NJ 1986 1. Whitaker, S, Mass Transport and Reaction in Catalyst Pellets, Transport in Porous Media 2, 269, 1987. 

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... 

Consideration thus far has been on only balanced reactions which occur in one phase, that is, homogeneous reactions. There are, of course, a great many reactions which occur between substances in different phases, and these are known as heterogeneous reactions. Numerous reversible, heterogeneous reactions are known, and it is pertinent now to bestow consideration on how far the law of mass action can be applied to such cases. The familiar reaction of the decomposition of calcium carbonate thermally - a well-known example of a reversible reaction represented by the equation... 

The effect of temperature on the rate of a typical heterogeneous reaction is shown in Figure 3.25. At low temperatures the reaction is chemically controlled and at high temperatures it is diffusion or mass transport controlled. 

The reaction plane model with heterogeneous reactions was discussed at length for acid-base reactions in the previous section. The same modeling technique, of confining the reactions to planes, can be applied to micelle-facilitated dissolution. As with the acid-base model, one starts with a one-dimensional steady-state equation for mass transfer that includes diffusion, convection, and reaction. This equation is then applied to the individual species i, i.e., the solute, s, the micelle, m, and the drug-loaded micelle, sm, to yield... 

In Section 9.3, we focus more on the intrinsic rates for reactions involving solids, since there are some modem processes in which mass transport rates play a relatively small role. Examples in materials engineering are chemical vapor deposition (CVD) and etching operations. We describe some mechanisms associated with such heterogeneous reactions and the intrinsic rate laws that arise. 

Then the fraction of the total irreversible process due to each of the reactions can be determined. Like ANg, av is a sum °f terms. For the same initial mass of solid NgjCD ANg i i are obtained from individual rate curves. Unless each ANg,i depends on its 5 in the same way, the relative importance of the various heterogeneous reactions changes with the total extent of reaction. In any event, it is necessary to express each ANs,i as a function of av. 

Just as with the metal surface reactions described above, the efficiency of heterogeneous reactions involving solids dispersed in liquids will depend upon the available reactive surface area and mass transfer. 

The rate of a generic reaction j is represented by its absolute velocity Vj, which may be obtained from experiments, provided that the mass of solvent is held constant throughout the experiment and no additional homogeneous or heterogeneous reactions concur to modify the molality of the ion in solution (Delany et al., 1986) ... 

In a heterogeneous reaction, a potentially high chemical reaction rate may be limited by the rate at which reactants can reach the reaction zone mass transfer is then of the utmost significance. 

The above analysis, which is exceedingly brief and simplified is designed to demonstrate how, even in a pre-mixed flame, the question arises as to what is the appropriate reaction volume (i.e. the flame thickness). In heterogeneous reactions, this is a question that will recur again and ain and the designer of reactors must not attempt to avoid it. It is interesting to note that, in the next but one example to be treated, the overall reaction rate (a flame speed cm s in the above) becomes a mass transfer coefficient (also cms" ) when considering the absorption of gas into a liquid with which it reacts quickly. Furthermore, exactly the same sort of analysis as the above leads to the dependence of the mass transfer coefficient fej on the reaction rate coefficient and the diffusivity, D, in the liquid phase, of ki o. (rD), cf. z a RKY above. 

In Chapter 17 we pointed out that the treatment of heterogeneous reaction required the consideration of two factors in addition to those normally encountered in homogeneous reactions the modification of the kinetic expressions resulting from the mass transfer between phases and the contacting patterns of the reacting phases. 

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