Kinetics for Low-Temperature Catalytic Oxidation of Ammonia with Ferric-Oxide System

Abstract

In chemical reactions occurring within porous solid catalyst, the rates are affected not only by intrinsic catalytic activities, but also by the resistance of internal diffusion due to pore structures. Therefore, in the analysis of true kinetics, the decision of effective particle diameter or the estimation of catalytic activity, effectiveness factor for each catalyst should be predicted.From this point of view, kinetics for low-temperature oxidation of ammonia with air in an iso-thermal flow system (inside diameter of reactor: 1.0cm) involving fixed bed of several coprecipitated porous Fe2O3-Bi2O3-MnO2 catalysts (calcined at 550°C) were investigated at atmospheric pressure in steady state under the following experimental conditions: reaction temperature 180 to 340°, mass of catalyst 0.2 to 0.6g, average particle diameter 3.56×10-2 to 14.10×10-2cm, partial pressure of ammnia 0.25atm. and feed velocity 1.6 to 12.4cm/sec.The experimental reaction rates under the resistance of negligible external diffusion were determined, and on the basis of Wheeler's theory the calculation methods of effectiveness factor were examined.On the other hand, the physical properties of internal surface area, pore volume, average pore diameter, etc. as shown in Table 1 were measured by means of the B. E. T. and mercury-water displacement methods, and then the effects of pore structures on reaction rates were examined.In this way, the effectiveness factor for each catalyst was estimated with considerable precision as mentioned below.Since, the experimental results generally gave a first-order irreversible reaction with respect to ammonia (it was confirmed that under conditions described above, principal reaction was a conversion to nitrous oxide), effectiveness factor can be expressed by a function of a dimensionless modulus φas given by Eq.(1) for a single spherical particle or by Eq.(3) for a single cylindrical pore.When the catalysts with different particle diameters were employed, the ratio of moduli φ became equal to the ratio of particle diameters as given by Eq.(5), for in Eq.(2) or (4), rate constant, pore radius and effective diffusion coefficient could be assumed to be approximately constant in the same catalysts, which was experimentally confirmed from the physical properties as shown in Table 1. The ratio of experimental rates (N) uninfluenced by external diffusion was equal to the ratio of effectiveness factor as given by Eq.(6).Accordingly, by introducing η3/η1=N3/N1=α and φ3/φ1=Dp3/Dp1=β, the correlations given by Eqs.(8) and (9) were derived from Eqs.(1) and (3), respectively.Now that the relationships between different particle diameters of catalysts employed were ap-proximately Dp3/Dpi=β=0.25, 0.35, 0.50 and 0.70, effectiveness factor could be easily evaluated from either the curves for Eqs.(1) and (8) or those for Eqs.(3) and (9), as given in Fig. 2.Both effectiveness factors obtained from Eqs.(1) and (3) lay very close to each other, though the one obtained from the former equation was a little lower than that given by the latter, as shown in Table 3.Comparison of effectiveness factors obtained from η3/ηi=N3/Ni=α and φ3/φi=Dpi/Dpi=β proved that they held good for each particle. For example, each effectiveness factor η1 showed good agreement within ±0.05 as shown in Table 3.