Ammonia synthesis reaction rate

The optimum ammonia synthesis reaction rate depends on several factors including pressure, temperature, H2-to-N2 molar ratio, concentration of impurities and catalyst activity. Therefore the H2-to-N2 molar ratio is adjusted to suit the requirements in the ammonia synthesis. This adjustment occurs before the compression step. 

Ammonia synthesis reaction rate about three kinds of catalysts with different loading amount of ruthenium and promoters were studied. Based on O2 chemisorption data, the relationship between the turnover of frequency (TOF) and the ruthenium particle size is shown in Fig. 6.51. A monotonic increase in TOF02 vs do2 is characteristic for each system. Extrapolation of the results to small crystallite diameters suggests (Fig. 6.51) that extra fine particles smaller than 0.7nm 0.8 nm (critical size) might be totally inactive. Analogous trends in the surface activities were found (not shown) when the amount of adsorbed CO were used instead of O2 uptake for the particle diameter and TOF calculation. 

Fig. 6.69 Relation between TOP of ammonia synthesis reaction rate and mole fraction of N2(d) and H2(o) (623 K) (mole fraction of H2 is 0.25 when change N2 and mole fraction of N2 is 0.25 when change H2)...

Because the ammonia synthesis reaction is an equiUbrium, the quantity of ammonia depends on temperature, pressure, and the H2 to-N2 ratio. At 500°C and 20.3 MPa (200 atm), the equiUbrium mixture contains 17.6% ammonia. The ammonia formed is removed from the exit gases by condensation at about —20° C, and the gases are recirculated with fresh synthesis gas into the reactor. The ammonia must be removed continually as its presence decreases both the equiUbrium yield and the reaction rate by reducing the partial pressure of the N2—H2 mixture. 

Synthesis Pressure. The ammonia reaction proceeds with a decrease in volume therefore, according to Le Chateher s principle, an increase in pressure increases the equiUbrium percentage of ammonia. The reaction rate is also accelerated by increasing the pressure. 

Synthesis Temperature. Because of the exothermic nature of the ammonia synthesis reaction, higher temperatures increase reaction rates, but the equihbrium amount of ammonia decreases. Thermal degradation of the catalyst also increases with temperature. 

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 ... 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

What is the rate-determining step of the ammonia synthesis reaction Describe the function of promoters in the ammonia synthesis catalyst. 

Kinetics of Ammonia Synthesis on Re Crystal Surfaces. The kinetics of the ammonia synthesis reaction on Re were studied on the Re(ll50) surface which is composed of atoms having seven-fold coordination. The rate of production at 70 K and 20 atm. ... 

Figure 6. Steady-state (curve 1) and cycle-average (curve 2) reaction rate versus feed composition. Experimental data for ammonia synthesis reaction by Jain, Silveston and Hudgins adapted from Chemical Reaction Engineering, ACS Symposium Ser, 1982, 196, 97-107.

In the ammonia synthesis reaction, four molecules of reactant, H2 and N2, produce two molecules of product, NH3. According to Le Ghate-lier s principle, if pressure on the reaction or the system is increased, the forward reaction will speed up to lessen the stress because two molecules exert less pressure than four molecules. Increased pressure wiU also cause the reactants to collide more often, thus increasing the reaction rate. Haber s apparatus used a total pressure of 2X10 kPa, which was the highest pressure he could achieve in his laboratory. 

Like the ammonia synthesis reaction (discussed in section 3.2.4), the shift reaction although known to take place according to a CSD mechanism, power law kinetics are adequate for accurate design and simulation of industrial shift converters. The most successful rate equation is that of Rase (1977) obtained from industrial data (and therefore includes diffusional limitations). 

Buzzi Ferraris et al. (1974) use the mathematical analysis of kinetic data to build rate equations for the ammonia synthesis reaction. They offer more than 20 models which give good fitting of experimental data. 

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... 

Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV The rate-determining ammonia synthesis reaction is widely accepted to be the dissociation of nitrogen [32, 55-57]. Consequently the direct interaction between nitrogen and iron has been studied [43, 44] together with the addition of submonolayer amounts of potassium [56, 58]. All the... 

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. 

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... 

Table 1.19 Rate of ammonia synthesis reaction on rare earth metal inter-metallic compounds ...

The role of a catalyst is, first, to form a complex molecule with reactant molecule coordination through its appropriate frontal orbital and thus fully weaken the H H bond and triple N=N bond. For example, H-H changes to two coordinated hydrogens and then may easily form new bond with the activated reaction intermediates. In other words, catalyst can participate in the reaction and form unstable intermediate complexes with reactants and form products at last. The activation energies required for every step are much lower than that for the reaction without catalyst, and therefore the reaction rate is accelerated. Consequently, the reaction pathway is changed by catalysts. For the case of ammonia synthesis reaction on Fe (111), Ertl, a winner of the Nobel chemistry prize in 2007, proposed a thermochemical kinetic profile, as shown in Fig. 2.1. 

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