Ammonia synthesis influence

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

These effects profoundly influence the ammonia, as shown in Fig. 8.28 where the ammonia synthesis rate is plotted for two basal planes of iron and the same iron surfaces modified with 0.1 Ml of potassium. 

Poisoning of iron catalysts during ammonia synthesis by sulfur compounds has received relatively little attention (154, 240-244). Nevertheless, the previous work provides information on the poisoning mechanism and interesting examples of how oxide promoters may influence the sulfur poisoning behavior of a catalytic metal. 

Equations for describing ammonia synthesis under industrial operating conditions must represent the influence of the temperature, the pressure, the gas composition, and the equilibrium composition. Moreover, they must also take into consideration the dependence of the ammonia formation rate on the concentration of catalyst poisons and the influence of mass-transfer resistances, which are significant in industrial ammonia synthesis. 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

The previous sections mainly considered the individual process steps involved in the production of ammonia and the progress made in recent years. The way in which these process components are combined with respect to mass and energy flow has a major influence on efficiency and reliability. Apart from the feedstock, many of the differences between various commercial ammonia processes lie in the way in which the process elements are integrated. Formerly the term ammonia technology referred mostly to ammonia synthesis technology (catalyst, converters, and synthesis loop), whereas today it is interpreted as the complete series of industrial operations leading from the primary feedstock to the final product ammonia. 

It all started in 1938 when Temkin first applied transition state theory to heterogeneous catalysis. Soon after, he published with V. Pyzhev one of the most frequently cited papers in catalytic ammonia synthesis. Since both Mikhail Temkin and Paul Emmett had a profound influence on the theory and practice of this famous reaction, it seems proper to quote here Emmett s assessment of the 1939-1940 paper of Temkin and Pyazhev. ... 

For completeness, we note that Nerskov and co-workers have published extensive calculations concerning the adsorption of N2 on Fe lll [66], Fe 110 [47,67] and Fe 001 [59]. Detailed comparison of their calculations against experimental work undwlines the ability of modem DFT calculations to provide valuable insight into real catalytic problems (in this case the energetics and kinetics of the rate-determining-step for ammonia synthesis), but no details of the influence of adsorption on siuface magnetism are reported. 

All catalysts, operated either in laboratory or conmiercially, are deactivated during their use. Deactivation is very important in commercial operation because it influences the choice of the operational conditions and fixes the cycle length between regenerations and the total life of the catalyst. Some catalysts remain active for a decade (catalysts for oxidation of SO2 and for ammonia synthesis) whereas others must be regenerated after a few minutes of operation (catalysts for fluidized bed hydrocarbon cracking). 

The Influence of Oxygen Poisoning on a Multiply Promoted Iron Catalyst Used for Ammonia Synthesis A Temperature-Programmed Desorption and Reaction Study... 

Alkali metals are often used as additives during catalytic reactions. They are bonding modifiers that is, they influence the bonding and thus the reactivity of the coadsorbed molecules. Potassium is a promoter in CO hydrogenation reactions where CO dissociation is desired and is one of the elementary reaction steps. The alkali metal also reduces the hydrogen chemisorption capacity of the transition metal. Potassium is a promoter in ammonia synthesis for the opposite reason, because it weakens the NH3 product molecule bonding to the metal, thereby reducing its sur-... 

The loop pressure has an important influence on the performance of the ammonia synthesis loop because of its influence on the reaction equilibrium, reaction kinetics, and gas/liquid equilibrium in the product separation. Actual selection of loop pressure is in many cases a compromise between selecting a high pressure to favour the ammonia synthesis reaction, and on the other hand selecting a reasonable pressure to minimise the compression power of the synthesis gas compressor, which compresses the synthesis gas to the desired loop pressure. The loop pressure also has a significant impact on the ammonia refrigeration system, since a high loop pressure favours condensation of the ammonia product in the loop water cooler and saves compression power on the refrigeration compressor. On the other hand, a low loop pressure saves compression power on the synthesis gas compressor, but increases the... 

Another example of the influence of the metal/metal oxide interface on the chemistry of the metal refers to the promotional effects that take place in the presence of alkali metal oxides on transition-metal catalyzed reactions. Alkah and alkahne earth metal oxides are known to promote the catalytic activation of different molecules such as CO (in Fischer-Tropsch) and N2 (in ammonia synthesis) over supported metal particles. Coadsorbed alkali... 

In the synthesis of ammonia, under industrial conditions, the reaction normally comes sufficiently close to equilibrium for the applications of thermodjaiamics to prove of immense value, f Thus it will predict the influence of changes of pressure, temperature and composition on the maximum attainable yield. By contrast in the catalytic oxidation of ammonia the yield of nitric oxide is determined, not by the opposition of forward and backward reactions, as in ammonia synthesis, but by the relative speeds of two independent processes which compete with each other for the available ammonia. These are the reactions producing nitric oxide and nitrogen respectively, the latter being an undesired and wasteful product. The useful yield of nitric oxide is thus determined by the relative speeds of these two reactions on the surface of the catalyst. It is therefore a problem of rates and not of equilibria. 

---