Wire-gauze reactors

FIGURE 11.31 A typical catalytic wire-gauze reactor. 

In addition to the two most common reactors described above, other designs have also been used. The most important of these reactors are the radial-flow and catalytic wire-gauze reactors, followed by the rather infrequently used spherical reactor. 

Figure 8.12 A typical catalytic wire-gauze reactor. (Adapted from Joshi, J.B. and Doraiswamy, L.K. Chemical Reaction Engineering, in Albright s Chemical Engineering Handbook, CRC Press, Albright, Boca Raton, FL, 2009.)...

A platinum-rhodium ahoy is used as a catalyst at 1100°C. Approximately equal amounts of ammonia and methane with 75 vol % air are introduced to the preheated reactor. The catalyst has several layers of wire gauze with a special mesh size (approximately 100 mesh). 

In this process (Fig. 1), the reactor contains a rhodium-platinum catalyst (2 to 10% rhodium) as wire gauzes in layers of 10 to 30 sheets at 750 to 920°C, 100 psi, and a contact time of 3 X 10"4 second. After cooling, the product gas enters the absorption tower with water and more air to oxidize the nitric oxide and hydrate it to nitric acid in water. Waste gases contain nitric oxide or nitrogen dioxide, and these are reduced with hydrogen or methane to ammonia or nitrogen gas. Traces of nitrogen oxides can be... 

Wire Gauzes Wire screens are used for very fast catalytic reactions or reactions that require a bulk noble metal surface for reaction and must be quenched rapidly. The nature and morphology of the gauze or the finely divided catalyst are important in reactor design. Reaction temperatures are typically high, and the residence times are on the order of milliseconds. 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

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. 

A platinum-rhodium alloy is used as a catalyst at 1100°C. Approximately equal amounts of ammonia and methane with 75 vol% air are introduced to the preheated reactor. The catalyst has several layers of wire gauze with a special mesh size (approximately 100 mesh). The Degussa process, on the other hand, reacts ammonia with methane in the absence of air using a platinum aluminum-ruthenium alloy as a catalyst at approximately 1200°C. The reaction produces hydrogen cyanide and hydrogen, and the yield is over 90%. The reaction is endothermic and requires 251 kJ/mol. 

Wire-gauze envelopes filled with catalyst pellets can also be placed on trays, either across the tray [33] or in the downcomer section [34]. Catalyst beads can be immobilized, as in a fixed-bed reactor, between two nonreactive distillation trays [35, 36] as well as in an external sidestream reactor [37, 38]. 

The cobalt oxide catalyst for oxidation of ammonia, worked out in our laboratory, has the form of granules of high mechanical strength, owing to which it may be applied both in stationary and in fluidized beds. The yields of ammonia oxidation to NO measured during laboratory and large laboratory studies of that catalyst exceeded 95%. Optimum temperature of ammonia oxidation process carried out on our catalyst (760-780 ) is lower than that needed for platinum-rhodium wire gauze currently appli in industrial reactors. 

Industrial fertilizer synthesis starts from ammonia synthesis, and ammonia is then easily oxidized in a separate reactor to nitric oxide over PtRh wire gauze catalyst. Formation of nitric acid requires further oxidation of nitric oxide to nitrogen dioxide (NO2) and absorption of the nitrogen dioxide in water. Overall, three different chemical process plants are used for the synthesis of nitric acid. The ammonia synthesis reaction takes place in a high-tem-perature, high-pressure reactor that requires recycling of products due to the thermodynamic limitations of chanical conversion. The ammonia oxidation reaction is very fast and takes place over a very small reactor length. Finally, nitric acid synthesis takes place in absorption columns. 

Critical effects in CO oxidation over Pt catalysts were obtained [33, 34, 63-85] in various catalytic systems over wires, foils and gauzes, on single pellets and fixed beds, in isothermal and adiabatic reactors (differential and integral). The literature also reported the oscillating behaviour of the homogeneous oxidation of CO [86, 87]. 

These catalysts are manufactured as smooth wires with no internal pores and then woven into gauze pads. Mechanical rigidity is important since the reactors are usually large in diameter (i.e., 4-12 ft) and are used in the reactor with minimum physical support. Furthermore, the conditions of operation are quite severe with respect to temperature and corrosion, and thus metallurgical integrity must be maximized. The most important properties are the purity of composition, wire diameter, and mesh size as well as mechanical strength. 

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