Gauze Mass Transfer Limited

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. 

Mayer et al. [43] compared their results generated at a micro structured monolith (see Section 2.4.3) with literature data [137]. The degree of conversion and the hydrogen selectivity of the rhodium monolith outperformed both metal-coated foam monoliths, Pt-Rh gauzes and extruded monoliths. This was partially attributed to the higher activity of the rhodium monolith, but also to the lower cross-sectional channel area of the metallic monolith, which reduced mass transfer limitations, and to the improved heat conductivity. 

The MTL (Mass Transfer Limited) gauzes use Pd/Ni (95/5) or Pd/Cu (95/5) alloys. The base metals in the alloys are not stable at the nitric acid plant operating conditions. They are rapidly oxidized and vaporized in the gas stream. This rapid oxidation makes the wire swell by a factor of 2 after ten days of operation. The larger surface area results in a large improvement in the capture of platinum oxide vapor237. 

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... 

It has long been generally assumed that the reaction is film diffusion limited in ammonia (the usual feed is oxygen-rich). Mass transfer coefficients on gauzes have been examined by several groups over a range of conditions. Loflfier and... 

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