Effect of Reactant Gas Composition

The voltage of MCFCs varies with the composition of the reactant gases. The effect of reactant gas partial pressure, however, is somewhat difficult to analyze. One reason involves the water gas shift reaction at the anode due to the presence of CO. The other reason is related to the consumption of both CO2 and O2 at the cathode. Data (55,64,65,66) show that increasing the reactant gas utilization generally decreases cell performance. 

As reactant gases are consumed in an operating cell, the cell voltage decreases in response to the polarization (i.e., activation, concentration) and to the changing gas composition (see discussion in Section 2). These effects are related to the partial pressures of the reactant gases. 

These reactions can be used to estimate the effect of changes in operating parameters on gas composition. As temperature increases, endothermic reactions are favored over exothermic reactions. Methane production will decrease, and CO production will be favored as reactions are shifted in the direction in which heat absorption takes place. An increase in pressure favors reactions in which the number of moles of products is less than the number of moles of reactants. At higher pressure, production of CO2 and CH4 will be favored. 

The SNCR process is characterized by a selectivity in the reaction pathways, as the injected agent (NH3) may react with NO to form N2 (the desired reaction) or be oxidized to NO by reaction with O2 (undesired). The selectivity toward NO or N2 depends mainly on the temperature and gas composition, but also the mixing of reactants is conceivably important because changes in the local conditions may favor different reaction pathways. As a continuation of the previous exercise we will use the Zwietering approach to assess qualitatively the effect of mixing on the SNCR process. 

The gas composition may vary considerably across a catalyst pellet. If the concentration of the reactants is high, this may result in two effects ... 

Chemical Composition Concentration of Reactants. The first important studies of reaction systems were begun with a study of the effect of the concentrations of the reacting species on the rate of a reaction. For gas reactions, concentrations are directly related through the equation of state to pressure, volume, and temperature. For liquid reactions the pressure assumes secondary importance as a variable, the volume of the system being very insensitive to either pressure or temperature changes. 

The general conclusion from this result is that a reaction will go fastest in the medium that favors the association of reactants. The magnitude to be expected for such an effect can be estimated for nonionic reactions by using a simple model of a solution. If we assume that A, B,. . . and X form ideal solutions with the solvent S which follow Raoult s law over the entire range of compositions, then we can write for the relation between mole fraction X of the fth component and its e(iuilibrium vapor pressure above the solution = x p, where pt is the vapor pressure of the pure i species at temperature T. On converting these to concentrations, we have for ideal gases Pi = C gRT, while for dilute solutions Xi = CtJCs = C Fs where Fs is the molar volume of the solvent and C = 1/Fs its concentration Cts refers to solution and C g to gas). 

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