Kinetics high-temperature ammonia oxidation

Case Study Kinetics of High-Temperature Ammonia Oxidation in an Annular Reactor... 

At high temperature, ammonia is oxidized by oxygen, which decreases the selectivity of the standard SCR reaction. In a kinetic model that should describe a broad temperature interval, it is therefore crucial to have a good description of the ammonia oxidation. Accordingly, we investigated ammonia oxidation separately in order to decrease the correlation between the parameters. In the global kinetic model, the following reaction was added in which ammonia stored on the surface is oxidized to produce Na and H2O ... 

Ammonia Oxidation Kinetics in a High Temperature Flow Reactor , Univ California, Berkeley UCB-TS-71-6, AFOSR (1971)... 

One of the main goals of chemical kinetics is to understand the steps by which a reaction takes place. This series of steps is called the reaction mechanism. Understanding the mechanism allows us to find ways to facilitate the reaction. For example, the Haber process for the production of ammonia requires high temperatures to achieve commercially feasible reaction rates. However, even higher temperatures (and more cost) would be required without the use of iron oxide, which speeds up the reaction. 

Obviously, the kinetics and the thermodynamics of this reaction are in opposition. A compromise must be reached, involving high pressure to force the equilibrium to the right and high temperature to produce a reasonable rate. The Haber process for manufacturing ammonia represents such a compromise (see Fig. 19.6). The process is carried out at a pressure of about 250 atm and a temperature of approximately 400°C. Even higher temperatures would be required if a catalyst consisting of a solid iron oxide mixed with small amounts of potassium oxide and aluminum oxide were not used to facilitate the reaction. 

At high pressures the heat evolved is sufficient for adiabatic operation of reactors. Since the normal operating temperatures of industrial converters lie between 800 and 1100°C and pressures between 1 and 10 atm, homogeneous gas-phase reactions may contribute to the observed product distribution. The initiation temperatures for homogeneous oxidation are of course a function of pressure, so that in the laboratory the catalytic reaction may be isolated by working at low pressures under molecular beam conditions. In this way the reaction kinetics can be observed over very wide temperature ranges (usually up to 1200 °C). Studies of ammonia oxidation carried out in this way range from 0.1 to 10 Torr total pressure. ... 

The most recent work on ammonia oxidation by Gland and Korchak is different in that they have used a Pt(lll) stepped single-crystal face (as used in adsorption studies) at much lower pressures (10 —10 Torr) and related kinetic data to quantitative surface composition data obtained by AES their maximum temperature was 700 °C. Under these reaction conditions LEED proved that the Pt(S)-[12(l 11) x (111)] surface was stable throughout the work. The results are of high quality and extremely revealing. 

A good description of the ammonia storage and desorption is critical in order to describe transient features of the SCR system, and usually a Temkin type of kinetics is used that considers the adsorbate-adsorbate interactions. The parameters for these reactions are usually fitted to TPD experiments, but also microcalorimetry studies are presented. The most common approach is to consider one ammonia adsorption site, but more detailed kinetic models use several adsorption sites. Ammonia oxidation is a reaction occurring at high temperatures, which unfortunately decreases the selectivity of the NOx reduction in SCR. It is therefore crucial to include this reaction in kinetic models for this system. 

The reaction NH2 + O2 was studied at high temperatures in shock waves [66 to 68]. The induction period for the appearance of OH absorption and the consumption of NH3 was measured in shock-heated NH3-O2 mixtures in the temperature range 1500 to 2800 K an activation energy of = 178 kJ/mol was obtained for the reaction NH2 + 02 NH + H02 [67]. The kinetics of ammonia oxidation in reflected shock waves was followed by observing the induction period. Computer simulation of the NH3-O2 system with the reaction scheme... 

Motekaitis determined the kinetics of hydrolysis and ammonolysis of EDTA at temperatures of interest for chemical cleaning activities. The rate increases with decreasing pH in the presence of ammonia. The rates are high enough at temperatures above 160°C (320°F) to affect the commercial use of ammonium EDTA. The molecule is cleaved at the C—N and C—C bonds. EDTA is rapidly oxidized by ferric ions. Motekaitis also determined that ferric EDTA would oxidize other EDTA molecules rapidly at temperatures as low as 125 C (257°F). The major reaction cuts off carboxy-late groups in a sequential fashion. NTA is oxidized more slowly than EDTA. 

---