Reaction Rates in the Synthesis of Ammonia. I. Dependence upon Reaction Pressure and Hydrogen-Nitrogen Ratio

Abstract

Abstract This series of experiments presents the well known fact about the different concentration increases as well as the different decreases of the conversion efficiency with the increasing pressure depending upon the kinds of catalysts in greater detail. At extremely high space velocities, the ammonia concentration remains constant independent of the change of the total pressure when the mixture of a constant gas ratio (H2: N2=3: 1) is used; however, it increases with increasing nitrogen pressure when the mixtures of different initial compositions are used at a constant pressure. As predicted by Temkin’s equation, the nitrogen percentage or hydrogen percentage, at which a maximum ammonia concentration or a minimum conversion efficiency is observed, shifts towards the higher percentage with the increasing space velocity; however, the nitrogen percentage corresponding to the maximum shifts further through the critical value predicted by Temkin. The shifting is-more or less rapid depending on the kind of catalyst. To the extent that the space velocity is very high and the total pressure remains constant the equation by S. L. Kiperman can represent the shifting in a better agreement together with the observed proportionality between the ammonia concentration and the reciprocal space velocity. The rate equations of M. I. Temkin and M. Shindo, which are based on different concepts, have been compared with the present results, and neither M. I. Temkin’s nor M-Shindo’s (unimproved) is in satisfactory agreement with the results. An equation based on the concept by M. I. Temkin and provided with another factor characteristic of the kind of catalyst has been proposed. The equation is in better agreement with the experimental results concerning the ammonia concentration expressed as functions of the space velocity and the pressure. The maximum of the ammonia concentration or the minimum of the conversion efficiency related to the ratio of hydrogen to nitrogen is represented in a somewhat better agreement with the singly promoted catalyst, while yet unsatisfactorily with the triply promoted.