Catalytically stabilized thermal operating conditions

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

Catalytic Activity. The world-wide interest focused in the catalytic partial oxidation of methane to formaldehyde has led to a great variety of conflicting results (9), The main reason of such discrepancies lies in the lack of a generally valid rule for evaluating and comparing the proposed catalytic systems. In effect, this reaction involves a very complex pathway since the desired partial oxidation product, HCHO, exhibits a limited thermal stability at T>4(X)°C and can be oxidized to more easily than CH itself. Hence, a suitable reactor device and appropriate operating conditions result to be of fundamental importance in order to attain reliable data unaffected by experimental artefacts. 

The anode material must possess, imder the operating conditions of the fuel cell good physical and chemical stability, chemical and structural compatibility with the electrolyte and interconnect, high ionic and electronic conductivity and catalytic activity for fuel oxidation (Ralph et al., 2001). The thermal stability is an important aspect to maintain the structural integrity throughout the temperature variations at which this component is subjected. 

Stability limits are provided in Fig. 6.6 for two inlet velocities, p = 5 bar and Tjj,j = 700 K, in terms of the critical heat transfer coefficient for extinction. For low thermal conductivities k < 2 W/mK), the reduced upstream heat transfer hinders catalytic ignition (light-off) and causes blowout. The stability limits at low fcs are narrower at higher inlet velocities (Fig. 6.6). In comparison to pure gas-phase combustion studies [18], there is a marked difference at the low behavior, which is discussed qualitatively (since the aforementioned work refers to different geometry and operating conditions). In gas-phase combustion, the blowout limits extend over a narrower range of ( 0.4-0.8 W/mK) and are nearly independent of h (the blowout limit line is almost parallel to the /i-axis). This is because low... 

Another type of stability problem arises in reactors containing reactive solid or catalyst particles. During chemical reaction the particles themselves pass through various states of thermal equilibrium, and regions of instability will exist along the reactor bed. Consider, for example, a first-order catalytic reaction in an adiabatic tubular reactor and further suppose that the reactor operates in a region where there is no diffusion limitation within the particles. The steady state condition for reaction in the particle may then be expressed by equating the rate of chemical reaction to the rate of mass transfer. The rate of chemical reaction per unit reactor volume will be (1 - e)kCAi since the effectiveness factor rj is considered to be unity. From equation 3.66 the rate of mass transfer per unit volume is (1 - e) (Sx/Vp)hD(CAG CAl) so the steady state condition is ... 

As in all catalytic processes, catalyst stability is an essential feature. We have investigated the stability of acylase in conditions that are pertinent for large-scale processes. Instead of just determining thermal stability, which can be done by measuring storage stability of the enzyme in particular conditions of temperature and pH, we have also determined operational stability. The relevant parameter for operational stability studies of enzymes is the product of active enzyme concentration [E] and residence time t, [E] t. For a CSTR, the quantities [E] and t are linked by Eq. (19.37), where [S0] = initial substrate concentration, % = degree of conversion, and r(x) = reaction rate (Wandrey, 1977 Bommarius, 1992b). 

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