Homogeneous and Wall-catalyzed Reactions

We now consider the case of coupled homogeneous and wall-catalyzed reactions involving M species occurring in a tubular reactor. The homogeneous reaction is of the form 

Non-isothermal Reactor Models 1. Wall-catalyzed Reactions 

The low-dimensional model for non-isothermal wall-catalyzed reaction in a tubular reactor is given by Eqs. (283)-(285) and 

---

Mills PL, Nicole JF Multiple automated reactor systems (MARS). 2. Effect of microreactor configurations on homogeneous gas-phase and wall-catalyzed reactions for 1,3-butadiene oxidation, Ind Eng Chem Rjes 44 6453—6465, 2005b. 

For the case of the wall-catalyzed reaction A — B, the global equation is closed for any type of kinetic expression rw(c). [Unlike the homogeneous reaction case, here the reaction rate r is evaluated at the surface concentration cs, which contains all the fluctuation modes (c ) about the mean .] Thus, convergence of the model depends only on the local equation. For the special case of linear kinetics, flat velocity, and Per -> oo, the local equation was determined to be... 

Evidence for the participation of homogeneous or wall-catalyzed reduction of NO2 to NO by Reaction 3 at temperatures above about 300-350°C was observed by Andersson et al. (8, 9) and by workers in this laboratory (75). Its contribution is typically on the order of 5-12% in the range of 350-500°C. Any NO produced by Reaction 3 would probably react readily via Reaction 5 to N2 since Reaction 4 is negligible on H-mordenite (16. Since the overall stoichiometries of Reaction 3 and 5 predict higher NH3 consumption relative to NO2, this route would not account for the lower NH3 conversion relative to that of NOx. 

Fio. 19. Influence of micromixing and mass transfer limitations on the yield of competitive-consecutive reactions (of which one reaction is homogeneous and the other is wall-catalyzed) in a tubular reactor. 

We may first divide tubular reactors into those designed for homogeneous reactions, and therefore basically just an empty tube, and those designed for a heterogeneously catalyzed reaction, and hence to be packed with a catalyst. Both types can of course be operated adiabatically, and it was the simplest model of these that we discussed in the last chapter. If the temperature of the reactor is to be controlled this is through the wall, and the associated problems of heat transfer now arise. These include transfer at the wall and subsequent radial diffusion across the flowing reactants. In the empty tubular reactor there may be considerable variations in flow rate across the tube. For example, in the slow laminar flow the fluid... 

Reactions and Kinetic Description. The physical system exhibits homogeneous catalysis, but simultaneous stoichiometric reactions (unassociated with the homogeneous catalysis) and even heterogeneous catalytic reactions can be assumed to occur. The latter case is mentioned since (1) vessel walls can sometimes catalyze reactions, particularly when they are contaminated with a previously... 

Results When we assume that the catalyzed wall reaction takes place homogeneously and can be described as being first order in the hydrocarbon concentration, while the gas flow through the tubes is supposed to be ideal plug flow, an impression of the magnitude of the rate constant can be obtained by using the relation c... 

Most of the gas-liquid applications of monoliths have used a heterogeneous catalyst (be it supported noble metals or immobihzed enzymes) on the channel walls. Here, we also consider the use of monohths without a catalyst on the walls in gas-liquid applications, i.e. homogeneously catalyzed liquid-phase reactions. The fluid mechanics of the system do not change appreciably by lethng the reaction take place in the liquid bulk instead of in a washcoat layer, and it is interesting to consider such reactions in a discussion of mass transfer and power-input requirement. Of course, the mass-transfer behavior does change by changing the locahon where the reaction takes place, and we will discuss gas-hquid reactors and gas-liquid-solid reactors separately. 

---