Ammonia synthesis pressure selection

Ammonia synthesis process selective hydrogen adsorption process by pressure swing adsorption with both co-current and countercurrent depressurization steps. A. Fuderer (Union Carbide Corp.). US 4475929 (1984). 

Figure 3 illustrates the shift and methanation conversion. The resulting methane is inert and the water is condensed. Thus purified, the hydrogen-nitrogen mixture with the ratio of 3H2 pressed to the pressure selected for ammonia synthesis. 

Iron catalysts have found only limited use in usual hydrogenations, although they play industrially important roles in the ammonia synthesis and Fischer-Tropsch process. Iron catalysts have been reported to be selective for the hydrogenation of alkynes to alkenes at elevated temperatures and pressures. Examples of the use of Raney Fe, Fe from Fe(CO)5, and Urushibara Fe are seen in eqs. 4.27,4.28, and 4.29, respectively. 

The thermodynamic equilibrium is most favourable at high pressure and low temperature. The methanol synthesis process was developed at the same time as NH3 synthesis. In the development of a commercial process for NH3 synthesis it was observed that, depending on the catalyst and reaction conditions, oxygenated products were formed as well. Compared with ammonia synthesis, catalyst development for methanol synthesis was more difficult because selectivity is crucial besides activity. In the CO hydrogenation other products can be formed, such as higher alcohols and hydrocarbons that are thermodynamically favoured. Figure 2.19 illustrates this. 

Selection of the laboratory reactor requires considerable attention. There is no such thing as a universal laboratory reactor. Nor should the laboratory reactor necessarily be a reduced replica of the envisioned industrial reactor. Figure 1 illustrates this point for ammonia synthesis. The industrial reactor (5) makes effective use of the heat of reaction, considering the non-isothermal behavior of the reaction. The reactor internals allow heat to exchange between reactants and products. The radial flow of reactants and products through the various catalyst beds minimizes the pressure drop. In the laboratory, intrinsic catalyst characterization is done with an isothermally operated plug flow microreactor (6). 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

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... 

Marnellos G, Stoukides M (1998) Ammonia synthesis at atmospheric pressure. Science 282 98-100 Marques FMB, Wirtz GP (1991) Electrical properties of ceria-doped yttria. J Am Ceram Soc 74 598-605 Marques FMB, Kharton W, Naumovich EN, Shaula AL, Kovalevsky AV, Yaremchenko AA (2006) Oxygen ion conductors for fuel cells and membranes selected developments. Sohd State Ionics 177 1697-1703 Marsal A, Comet A, Morante JR (2003a) Study of the CO and humidity interference in La doped tin oxide CO gas sensor. Sens Actuators B 94 324-329... 

The same type of fused iron catalyst may exhibit different structures and activities after reduction under different conditions (e.g., temperature, pressure, space velocity and gas composition etc.). Reduction condition is the external factor which affects the physical-chemical properties of catalysts. Thus, different reduction conditions are required for catalysts with different t3rpes, particle sizes or different types and content of promoters. The selection of the optimized reduction condition is very important to obtain a high performance for ammonia synthesis catalysts. It is the main reason to study the reductive performance and related kinetics of catalysts. 

Catal ic reactor is the heart of catal3h ic activity evaluation device, which is considered as the center to organize the experimental process. The reactor is controlled by external conditions, such as the supply of raw materials, analysis and the measurement, preheating or (and) the pressurized device and so on. After reaction, it needs the means of separation, measurement and analysis to provide necessary flow and concentration data of reaction mixture to determine and to calculate the activity, and selectivity of catalysts. At present, the most commonly used device in the evaluation of catalysts is tubular reactor. Here, the high-pressure experimental device of ammonia synthesis catalyst is taken as an example. 

The direct comparison of the catalytic activity and selectivity of surfaces with different orientations provides information abont the influence of the atomic structure. This has been well described (for example, see [15]). It is well established that catalytic reactions may depend on the atomic structure of the surface (i.e., they are structure sensitive). A classic example of a catalytic reaction that is sensitive to the atomic sUucture of the catalyst s surface is ammonia synthesis on iron surfaces [15], The (111) and (211) surfaces of iron exhibit a significantly higher reaction rate than the (100), (210), and (110) faces. This structural effect has been ascribed to C7 sites (i.e., Fe atoms with a coordination number of 7, or number of nearest neighbors), which exist only on the (111) and (211) surfaces. Now, what if the structure of the catalytic surface during the reaction differs substantially from the initially pure, well-defined crystalline metal surface For example, depending on the gas pressure (i.e., the chemical potential) new structures may become stable (see Sect. 8.2.2). Or what if only a small percentage of uncontrolled or varying defects and steps completely dominate the activity In the remainder of this chapter, these questions will be addressed. 

Quartulli et al. " published a detailed study of the factors to be considered in selecting the proper synthesis pressure. The reaction equilibrium, reaction rate, and condensation of ammonia all are favored by high pressure. Mechanical problems and cost considerations also are considered. In general, the study concluded that operation at 2100 psi is more economical than at 3300, 4700, or 6100 psi. The following were some of the factors considered catalyst volume, recycle flow, horsepower for makeup-gas compression, recycle-gas flow, refrigeration, equipment cost, and equipment reliability. 

The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

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