Monolith Fecralloy

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.

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. 

The rhodium monolith showed a lower hydrogen selectivity of 50% at the same temperature and residence time. For the FeCrAlloy monolith impregnated with nickel, full conversion was achieved at 900 °C, but at a lower hydrogen yield [55],... 

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. 

Recent research in packed beds [38], ceramic monoliths [20] and microreactors [30] also revealed an excellent performance of a Ce-Zr support mixture, which was explained by an increase in surface area by the addition of zirconium oxide [38]. A zirconium oxide-supported rhodium catalyst also revealed similar ignition performance to a mixed Ce-Zr oxide supported rhodium catalyst on the surface of Fecralloy monoliths with trapezoidal channels [9]. A novel route to a support, which might be useful for CPO and OSR, is the synthesis of silicon carbide foam, recently used for steam reforming [44] (Figure 25.2). This support would also be less acidic and is suitable for building a compact foam catalyst... 

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... 

Figure 25.12 Conversion and selectivity to main products (a and b) and by-products (c and d) as a function of residence time during CPO (a and c) and OSR (b and d). The left-hand series in each diagram represents experiments overthe 1.0 mg Rh/AhOs/Fecralloy microchannel monoliths for a total reactant flowof400-2000mLmin STP...

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. 

An alternative to Fecralloy is aluminium when the operating temperature of the monolith is limited to below about 450 °C. An alumina layer may be formed on the aluminium, for example, by anodic oxidation. 

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. 

It is difficult to clearly establish the role of additives since they can produce competitive or synergic effects affecting the rheological properties of the slurry. The most commonly used additives are binders and surfactants. Usual binders are colloidal silica or alumina. Alumina is more thermostable but can increase the acidity of the catalyst. Whatever the chosen binder, the minimum amount should be added [75]. Almeida et al. [9] studied the influence of the slurry composition on viscosity, load, and adherence of washcoated Fecralloy monoliths. They verified how colloidal alumina increased viscosity... 

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)... 

Table 2. Effect of slurry composition on viscosity, specific load and adherence of washcoated Fecralloy monoliths.

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