Metal Fecralloy monolith

Metallic (Fecralloy) monoliths based on Mg-Al HT MM Mg Al HT Co-ppt, calcined at 500°C Free-alkali co-ppt, template with polystryne Sunflower oil in methanol Triglycerides (C4-C18)... 

In another brief examination [ 15] of the impact of monolith supports for mcthanation catalysts, a comparison between nickel and ruthenium catalysts was made utilizing a metal (Fecralloy) support. The conversion tests were run at 673 K, 5400 kPa, 3.47 sec", and with a gas composition of 62% hydrogen, 18% carbon monoxide, and 20% water vapor. A ruthenium pellet catalyst that was run in comparison was approximately twice as active as ruthenium on the monolith. However, the difference in product (methane) selectivity was 97% for the metal monolith catalyst and 83% for the pellet bed. In the comparison between nickel and ruthenium, shown in Fig. 14, the ruthenium was more active and selective. The lack of impact on activity or selectivity as a result of steam addition to the reactant mixture provided useful practical data as well. No further details regarding the catalyst characteristics were provided. 

One approach to reactors with additional supports is the use of metallic monoliths with trapezoidal channels of 1.2 mm hydraulic diameter for methane CPO for catalytically rich combustion in a gas turbine ]9]. Fuel is turned partly into hydrogen before combustion. The catalyst was Rh/Zr02 coated on a Fecralloy monolith. 

A comparison between ceramic foams and metallic microstructured monoliths of Fecralloy was performed for the OSR and CPO of propane under similar conditions of total catalyst amount, catalyst composition and modified catalyst residence time [40], A major difference in this investigation was, however, that uncoated foam was located before and after the catalytically coated foam to prevent gas-phase ignition. Irrespective of this difference, two main conclusions can be drawn first, the catalyst supported on the microchaimel structure did not deactivate like the foam catalyst under OSR conditions. This could be due to the fact that peak temperature... 

Metal monoliths show good thermal characteristics. A typical support with herringbone channels made from Fecralloy performed satisfactory in automotive applications [27]. Modeling showed that overall heat transfer was about 2 times higher than for conventional pellets [28,29]. Hence, there is potential for structured catalysts for gas-phase catalytic processes in multitubular reactors. 

While conventional monoliths contain parallel channels, in practice, systems are often made from alternate layers that allow lighter structures with better mass transfer characteristics in gas-phase applications, see Figure 9.6 showing interconnected flow paths. They are usually made from metal, mostly Fecralloy , Kanthal , or stainless steel, and widely used in autocatalysts and in environmental... 

Metallic monoliths made of both rhodium ([HCR 1]) and FeCrAlloy (72.6% Fe, 22% Cr and 4.8% Al ([HCR 3]) carrying micro channels of 120 pm x 130 pm cross-section at various length (5 and 20 mm) were applied. The monoliths were prepared of micro structured foils by electron beam welding. After bonding, the FeCrAlloy was oxidized in air at 1 000 °C for 4 h to form an a-alumina layer, which was verified by XRD. Its thickness was determined as < 10 pm by SEM/EDX. The alumina layer was impregnated with rhodium chloride and alternatively with a nickel salt solution. The catalyst loading with nickel (30 mg) was much higher than that with rhodium (1 mg) (see Table 2.4). The amount of rhodium on the catalyst surface was determined as 3% by XPS. 

Keywords Methane combustion, metallic monolith, FeCrAlloy, hexaaluminate... 

A parametric study on the effects of axial heat conduction in the solid matrix has shown that i) such effects are negligible in ceramic monoliths (cordierite, kj = 1.4 w/m/K) but expectedly significant in metallic monoliths (Fecralloy, k i = 35 W/m/K) when a constant heat flux is imposed at the external matrix wall ii) however, the influence of axial conduction in metallic monoliths is much less apparent if a constant wall temperature condition is applied, since the monolith tends to an isothermal behavior. Metallic matrices exhibit very flat axial and radial temperature profiles, which seems promising for their use as catalyst supports in non-adiabatic chemical reactors. 

Figure 4 - Effect of cell density and of channel size on heat transfer in a monolith with constant external temperature = 500 K. Case of metallic monolith (Fecralloy). Gas = air.

Materials that are routinely coated with catalyst wash-coats are ceramics such as cordierite, which is the construction material of ceramic monoliths, metals such as Fecralloy, the construction material of metallic monoliths (see Section 6.2) and stainless steel [57]. The amount of catalyst material that can be coated onto a monolith ranges between 20 and 40 g m , while plate heat-exchangers may even take up more catalyst when coated prior to the sealing procedure, because the access to the channels is better. 

When metallic surfaces are coated with catalyst, a pre-treatment to improve the adherence is usually applied [122]. Besides mechanical roughening, chemical and thermal pre-treatments are methods that are frequently used. Fecralloys, which are the construction material for metallic monoliths, are usually pre-treated at about 900-1000 °C, because they form an alumina layer of about 1-pm thickness on their surface. This is an ideal basis for catalyst coatings. However, metal oxide layers are formed on stainless steel and may also serve as an adhesion layer. 

The constmction material of metallic monoliths are alloys of iron, about 15-20 wL % chromium and 5 wt.% aluminium (Fecralloy). The unique feature of these alloys is the formation of a thin alumina layer (0.5 pm) on their outer surface when treated at temperatures above 850 °C [130]. Long alumina whiskers are formed at temperatures of around 900 °C and an oxidation time of greater than 12h (see Figure 10.6). This alumina layer then acts as an adhesion layer for the catalyst coating. On top of this, the alumina layer protects the alloy from corrosion, which allows the operation of ultra-thin Fecralloy foils at high temperatures. 

Another option are Fecralloys, which are also used for fabrication of metallic monoliths and are mechanically stable up to very high temperatures exceeding 1200 °C. However, the material is britde, which generates practical problems with respect to mechanical stability. 

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