Ammonia synthesis reaction conditions

The production of ammonia is of historical interest because it represents the first important application of thermodynamics to an industrial process. Considering the synthesis reaction of ammonia from its elements, the calculated reaction heat (AH) and free energy change (AG) at room temperature are approximately -46 and -16.5 KJ/mol, respectively. Although the calculated equilibrium constant = 3.6 X 108 at room temperature is substantially high, no reaction occurs under these conditions, and the rate is practically zero. The ammonia synthesis reaction could be represented as follows ... 

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. 

The synthesis of the pyrazino[l,2-a]indole nucleus (64)/(65) was attained by intramolecular cyclization of several 2-carbonyl-1-propargylindoles (63) in the presence of ammonia. The reaction conditions were optimized using microwave heating and a... 

In 1905 Haber reported a successful experiment in which he succeeded in producing NH3 catalytically. However, under the conditions he used (1293 K) he only found minor amounts of NH3. He extrapolated his value to lower temperatures (at 1 bar) and concluded that a temperature of 520 K was the maximum temperature for a commercial process. This was the first application of chemical thermodynamics to catalysis, and precise thermodynamic data were not then known. At that time Haber regarded the development of a commercial process for ammonia synthesis as hopeless and he stopped his work. Meanwhile, Nernst had also investigated the ammonia synthesis reaction and concluded that the thermodynamic data Haber used were not correct. He arrived at different values and this led Haber to continue his work at higher pressures. Haber tried many catalysts and found that a particular sample of osmium was the most active one. This osmium was a very fine amorphous powder. He approached BASF and they decided to start a large program in which Bosch also became involved. 

All steam reforming catalysts in the activated form contain metallic nickel as active component, but the composition and structure of the support and the nickel content differ considerably in the various commercial brands. Thus the theoretical picture is less uniform than for the ammonia synthesis reaction, and the number of scientific publications is much smaller. The literature on steam reforming kinetics published before 1993 is summarized by Rostrup - Nielsen [362], and a more recent review is given by K. Kochloefl [422]. There is a general agreement that the steam reforming reaction is first order with respect to methane, but for the other kinetic parameters the results from experimental investigations differ considerably for various catalysts and reaction conditions studied by a number of researchers. 

A practical application of such a parametric study might be the determination of optimal conditions for a particular batch of catalyst. For example the Temkin equation, which is of the form (10), is used with some confidence for the ammonia synthesis reaction (Annable, 1952). However, the values of the constants k, k%, Ei, and E2 may vary with different catalysts and so the constants p, K, and A and the consequent optimal policy. As we shall now show the results of this section could be used to determine the maximum conversion from a fixed length of converter with the current brand of catalyst. 

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. 

The conventional ammonia production line consists of seven gas-solid catalytic reactors, namely desulfurization unit, primary reformer, secondary reformer, high temperature shift, low temperature shift, methanator and finally the ammonia converter. In addition the production line includes an absorption-stripping unit for the removal of CO2 from the gas stream leaving the low temperature shift converter. The ammonia converter is certainly the heart of the process with all the other units serving to prepare the gases for the ammonia synthesis reaction which takes place over an iron promoted catalyst under conditions of high temperature and pressure. 

In certain conditions the rate of the ammonia synthesis reaction in the reverse direction is very small and the overall reaction rate is determined by the ammonia synthesis rate in the forward direction. According to the conventional derivation based on the model of biographically nonuniform surfaces this rate in the forward direction is given by... 

What is the equilibrium of the ammonia synthesis reaction at the following conditions ... 

Note that since K 1, we expect this reaction to favor formation of product, leaving an equilibrium mixture that is predominantly ammonia. While this is a valid thermodynamic conclusion, it is incomplete because, in fact, at ambient conditions this ammonia-synthesis reaction proceeds slowly. To be industrially viable, the reaction must be carried out at elevated temperatures, where the equilibrium constant is actually smaller than it is at 25°C compensation is achieved by increasing the reaction pressure and using a catalyst. The controlling factor is a meager reaction rate, but thermodynamics cannot address rates in analyzing any reaction-equilibrium situation, thermodynamics can only bound what will be observed at the completion of a... 

In principle all reactions are reversible just as reactants have a tendency to combine and form products, products have the tendency to recombine and form the initial reactants. At equilibrium, the forward rate is balanced by the reverse rate, all net conversion ceases, and the composition of the system becomes constant in time. Suppose we load a closed reactor with a mixture that contains arbitrary amounts of the reactant and product species, and initiate the reaction while maintaining constant temperature and pressure. If we monitor the progress to equilibrium through the extent of reaction, we will observe it to increase in the positive or negative direction, indicating that the reaction progresses in the forward or reverse direction, until equilibrium is reached. Since temperature and pressure are held constant, the equilibrium state corresponds to conditions that minimize the Gibbs free ener. This condition allows us to obtain precise mathematical relationships for the equilibrium constant of the reaction. As an example, consider the ammonia synthesis reaction. 

For the ammonia synthesis reaction in Sample Exercise 19.5, is the enthalpy change for the reaction under standard conditions, AH°, so the changes in entropy will be standard entropy changes, AS°. Therefore, using the procedures described in Section 5.7, we have... 

Synthesis pressure, synthesis temperature, space velocity, inlet gas composition, and catalyst particle size all affect ammonia synthesis. LeChatelier s principle helps explain how synthesis pressure affects the synthesis of ammonia. As the ammonia reaction takes place, there is a decrease in volume. Thus, raising the pressure increases the equilibrium percentage of ammonia and accelerates the reaction rate. The ammonia synthesis reaction is exothermic therefore, higher temperatures increase reaction rates and thermal degradation of the catalyst. But the equilibrium amount of ammonia decreases with an increase in temperature. Space velocity, the ratio of the volumetric rate of gas at standard conditions to the volume of the catalyst, decreases the... 

Apart from the Fe catalyst, osmium and mthenium catalysts have been applied in ammonia synthesis reaction. The mthenium catalyst was found to be more active than the Fe catalyst, so it can perform at milder conditions than the Fe catalyst (Dahl et al., 2001 Logadottir et al., 2001 Rossetti, Pemicone, Ferrero, Fomi, 2006). However, Ru is more expensive than Fe, and the lifetime of the Ru catalyst is shorter than that of the Fe catalyst. 

The microkinetics analysis, as a useful and powerful tool to interpret, harmonize and consolidate the study of catal3dic phenomena, can describe various results obtained at wide experimental conditions. For ammonia synthesis reaction discussed in this section, the microkinetic models are evaluated from the experimental data such as the sticking coefficient of dissociated nitrogen adsorption, the spectrum of programmed-temperature desorption of adsorbed nitrogen as well as the kinetics of ammonia synthesis at industrial conditions and at laboratory conditions are far from equilibrium. 

There is no consistent understanding on the role of mechanism of alkali metals and alkaline earth metals on ruthenium catalysts and their state under the operating conditions. The current studies show that the dynamics of the ammonia synthesis reaction are different using the alkali metals and alkaline earth metals as the promoters, respectively. Therefore, using combination of promoters is more favorable to increase the catalytic activity. 

Due to the equilibrium ammonia concentration changes with H2/N2 according to thermodynamics of ammonia synthesis reaction. Therefore, outlet ammonia concentration cannot directly reflect the influence of H2/N2 on catal3dic activity. Hence, the catalytic efficiency is defined as the ratio of outlet ammonia concentration to equilibrium ammonia concentration at the same conditions. The effect of H2/N2 on catalytic efficiency for Ba-Ru-K/AC catalyst is shown in Table 6.45. It can be... 

The ammonia synthesis reaction is affected by many factors. The fundamental basis of choosing process conditions is the thermodynamics and kinetics of the ammonia synthesis reaction, and the activity of the catalyst. In selecting the optimum conditions, it is necessary to consider many factors including thermodynamics, reaction kinetics, yields, and energy consumption. Both the process and the equipment need to be considered, all of which must be subjected to the constraints of the catalyst. ... 

The hot spot temperature of the catalyst used in different period(s) is shown in Table 8.41. Under the condition of steady production capacity and stable catalytic activity, the hot spot temperature should be maintained as low as possible. This will not only benefit in maintaining the low-temperature activity of the catalyst, but also enhances the equilibrium of the ammonia synthesis reaction. Especially for the A301, ZA-5 and other low-temperature and low-pressure catalysts, because the temperature for high activity is low, the hot spot temperature should be strictly controlled and should not exceed the designed limit. When operators who were used to using the medium-temperatme catalyst change to the new low-temperature catalyst, they are not accustomed to operate the process at low temperatures. They might raise the hot spot temperatme intentionally or unintentionally, which would reduce the catalyst utilization efficiency and shorten the catalyst life. This issue has been discussed in Section 8.1.2.4. 

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