Ammonia Synthesis—Simple Kinetics

Ammonia synthesis is one of the most important processes of chemical industry tens of millions of tons of this product are synthesized annually in various countries of the world. On a commercial scale the reaction is operated on promoted iron catalysts at temperatures near to 500°C and high pressures, mostly at 300 atm. At present K20, A1203, and CaO in amounts of several parts by weight per 100 parts of catalyst are usually employed as promoters. The application of high pressure is caused by the reversibility of the reaction molar fraction of ammonia corresponding to the equilibrium 

From the viewpoint of the experimenter, the ammonia synthesis reaction is an advantageous subject for kinetic studies since it proceeds only in one direction without any by-products the activity of catalysts is usually sufficiently stable, it being an important condition for the success of kinetic investigations. 

Many works were devoted to the mechanism and kinetics of ammonia synthesis. In accordance with the outline of this article, only the main results of studies of ammonia synthesis by the author with co-workers will be presented here other works will be cited, but in connection with our studies. The discussions of kinetics and the mechanism of ammonia synthesis in the reviews published in this series (90) and in the contribution by Nielsen et al. (91) can serve as supplementary sources of information. The contribution by Malina (92) contains interesting historical data and an extensive list of references. 

The kinetics of reaction (294) at the conditions of its commercial realization answers to the following mechanism  

At equilibrium r = 0, taking (303) into account, we obtain from (305) 

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Unfortunately, these requirements have not yet fully been met for any catalytic reaction, although for some simple catalytic reactions reasonable approaches are known. Such reactions are the oxidation of CO over a supported Rh catalyst [46,47], ammonia synthesis over iron [48, 49], and the HCN synthesis over a Pt gauze catalyst. More recently Wolf [50] carried out a micro-kinetic analysis of the primary reaction steps in the oxidative coupling of methane and also related the rate... 

Most of the theory of diffusion and chemical reaction in gas-solid catalytic systems has been developed for these simple, unimolecular and irreversible reactions (SUIR). Of course this is understandable due to the obvious simplicity associated with this simple network both conceptually and practically. However, most industrial reactions are more complex than this SUIR, and this complexity varies considerably from single irreversible but bimolecular reactions to multiple reversible multimolecular reactions. For single reactions which are bimolecular but still irreversible, one of the added complexities associated with this case is the non-monotonic kinetics which lead to bifurcation (multiplicity) behaviour even under isothermal conditions. When the diffusivities of the different components are close to each other that added complexity may be the only one. However, when the diffusiv-ities of the different components are appreciably different, then extra complexities may arise. For reversible reactions added phenomena are introduced one of them is discussed in connection with the ammonia synthesis reaction in chapter 6. 

Chemically, the ammonia synthesis reaction is very simple. There is only one reaction, with no possibility for side reactions. No selectivity problem exist. The situation in methanol synthesis is different. The reaction mechanism and the reaction kinetics for methanol synthesis on copper catalyst has been studied and some results were presented by B0gild Hansen et al [i. Since this... 

Ammonia synthesis is a relatively simple reaction without the complication of any secondary reaction product, and is especially suitable for a theoretical approach to its kinetics. In fact, the most used kinetic equation for ammonia synthesis was developed by Temkin on the basis of theoretical assumptions about 50 years ago and is still used successfully by chemists and engineers. 

In solving (134) Di is usually considered constant, although Dj depends on the gas composition. This approximation seems reasonable considering the uncertainty in t. For simple reaction kinetics, the solution to (134) is given in terms of the Thiele modulus [102]. The more complicated kinetics for ammonia synthesis does not give a simple analytical solution. Generally, a numerical integration has to be carried out. 

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