Mass transfer gauze reactors

For processes controlled by external mass transfer, a reactor length of only about ten particle diameters is needed for almost complete conversion, as shown by Example 4.5.3. Technically relevant examples are catalytic ammonia oxidation on a platin gauze (Section 6.4) and the combustion of solid fuels like coal where we have the same interplay of mass transfer and chemical reaction as in heterogeneously catalyzed reactions. 

The difference in reactor performance is due to the difference in hydraulic diameters of the reaction channels, i.e. related to varying mass-transfer limitations. The micro channels of the p-gauze platinum catalyst amount to 70 pm, whereas the monoliths have channel/pore diameters of 500-1200 pm. 

The reaction is carried out over a silver gauze or low surface supported catalyst at 600—700°C, indicating a very fast chemical reaction. This implies that determination of the intrinsic reaction rate in laboratory reactors is complicated by the interference of heat and mass transfer limitations. To avoid this problem, studies have been made at much lower temperatures, which in turn run the risk of being non-representative. 

Modeling considerations for monoliths are similar to those of gauze catalysts however, since the flow and temperature in each channel may be assumed to be identical to those in the next channel the solution for a single channel may reflect the performance of the reactor. For an application in which the reaction rate is mass-transfer-limited, the reactant concentration at the wall of the catalyst is much lower than in the bulk and may be neglected. In such a case, the fractional conversion % is... 

Hickman and Schmidt (158,159) thoroughly studied Eq. (33) over Pt-and Pt-Rh-coated monoliths and gauzes. Best results are obtained at about 1100°C with residence times of lO " to 10 s. High flow rates minimize mass transfer effects. Since the gas at the exit of the reactor is almost in... 

In this section we develop the design equations and give the mass transfer correlations for two common types of catalytic reactors the wire screen or catalyst gauze reactor and the monolith reactor. 

The feed temperature for the NHs/air mixture is 250-350°C, but onee ignited the gauzes are at a mueh higher temperature that is elose to the feed temperature plus the adiabatic temperature rise. Intermediate temperatures are unstable, and the reactor operates at an upper stable point where the reaction is mass transfer controlled. The temperature and conversion profiles are sketched in Figure 10.8. 

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