Ammonia synthesis process description

The kinetics of the ammonia synthesis have been discussed as an example of micro-kinetic modeling in Chapter 7. Here we present a brief description of the process, concentrating on how process variables are related to the microscopic details and the optimization of the synthesis. 

The application of the fusion process can lead to a control over structure-sensitive reactions for unsupported catalysts. The prototype example for such a catalyst is the multiply-promoted iron oxide precursor used for ammonia synthesis. In Section B.2.1.1 a detailed description is given of the necessity for oxide fusion and the consequences of the metastable oxide mixture for the catalytic action of the final metal catalyst. 

Description Natural gas or another hydrocarbon feedstock is compressed (if required), desulfurized, mixed with steam and then converted into synthesis gas. The reforming section comprises a prereformer (optional, but gives particular benefits when the feedstock is higher hydrocarbons or naphtha), a fired tubular reformer and a secondary reformer, where process air is added. The amount of air is adjusted to obtain an H2/N2 ratio of 3.0 as required by the ammonia synthesis reaction. The tubular steam reformer is Topsoe s proprietary side-wall-fired design. After the reforming section, the synthesis gas undergoes high- and low-temperature shift conversion, carbon dioxide removal and methanation. 

Description The key steps in the KAAP plus process are reforming using the KBR reforming exchanger system (KRES), cryogenic purification of the synthesis gas and low-pressure ammonia synthesis using KAAP catalyst. 

In the kinetic modelling of catalytic reactions, one typically takes into account the presence of many different surface species and many reaction steps. Their relative importance will depend on reaction conditions (conversion, temperature, pressure, etc.) and as a result, it is generally desirable to introduce complete kinetic fundamental descriptions using, for example, the microkinetic treatment [1]. In many cases, such models can be based on detailed molecular information about the elementary steps obtained from, for example, surface science or in situ studies. Such kinetic models may be used as an important tool in catalyst and process development. In recent years, this field has attracted much attention and, for example, we have in our laboratories found the microkinetic treatment very useful for modelling such reactions as ammonia synthesis [2-4], water gas shift and methanol synthesis [5,6,7,8], methane decomposition [9], CO methanation [10,11], and SCR deNO [12,13]. 

Just these two statements are sufficient to emphasize the complexity of the process of activation of the technical ammonia synthesis catalyst. This process is essentially the reduction of iron oxides to iron metal using synthesis gas mixtures. Each catalyst is delivered to the customer with detailed descriptions of the reduction parameters. These describe a temperature program ranging from room temperature to ca 790 K which extends typically over 50-120 hours and they further contain a set of specified boundary conditions such as space velocity, pressure, and exit water content. In order to simplify the start-up of a fresh catalyst the reduction process may now be split into a prereduction at the catalyst factory, followed by... 

These sets of parameters appear to be rather arbitrary and may have been established by empirical methods. It is the purpose of this chapter to describe some of the underlying solid state chemical principles which will allow us to systematize the complex phenomena of activation. A description of the resulting micromorphology will be followed by an analysis of the activated surfaces. The properties of the resulting gas-solid interface, as described by elemental and structural compositions and their changes with time, determine the usefulness of the activated catalyst. Finally, an empirical model of the active catalyst surface is presented which provides the basis for the discussion of the process of ammonia synthesis in terms of a comparison between the technical catalyst and model surfaces based on single crystals of iron. 

Only few descriptions of the surface constitution of technical ammonia synthesis catalysts can be found in the literature.An extensive investigation of such a catalyst, similar to one of the samples used in this study, was carried out by Ertl and Thiele who employed a similar activation process to the dry method used in the present investigation. Although there is good general agreement between the two studies, some differences will be discussed. 

The oldest process for final purification of ammonia synthesis gas is the copper liquor wash, which was used in the world s first ammonia plant in Oppau, Germany in 1913 [321]. Descriptions of the process are given in [321-323]. The... 

It has also been considered to use metal hydrides for hydrogen recovery from ammonia synthesis purge gas. Hydrogen may react with certain metals with the formation of hydrides, and the reaction can be reversed by increasing the temperature and/or reducing the pressure. The process has not yet found acceptance in the ammonia industry. Description of the technology may be found in [674, 675]. 

After wrestling with the ammonia synthesis problem for about a decade, Ertl and coworkers were able to meet Emmett s challenge They showed that a combination of the kinetic parameters associated with the individual reaction steps shown in Fig. 5.22 furnishes a steady-state yield of ammonia from the elements which, for a range of conditions, accurately reproduces the real-life yields measured at industrial plants. This agreement has demonstrated that, in the case of the ammonia synthesis, the surface science approach to catalysis is capable of providing no less than a quantitative description of an industrial process. 

Description The key features of the KBR Purifier Process are mild primary reforming, secondary reforming with excess air, cryogenic purification of syngas, and synthesis of ammonia over magnetite catalyst in a horizontal converter. 

Description Ammonia and C02 react at synthesis pressure of 140 bar to urea and carbamate (Fig. 1). The conversion of ammonia as well as C02 in the synthesis section is 80% resulting in an extreme low recycle flow of carbamate. Because of the high-ammonia efficiency, no pure ammonia is recycled in this process. The synthesis temperature of 185°C is low, and, consequently, corrosion in the plant is negligible. 

It is unknown when and how cooperation with amino acids, peptides, and proteins started to evolve into an RNA-protein world. However, there is an upper size limit of RNAs, which is due to a threshold error of RNA replication. The heart of the core necessary to launch the process of chemical evolution towards the RNA world must have consisted of a number of pathways for the synthesis of organic molecules from CO2, N2, and H2. Additional pathways for the synthesis of amino acids, ribose, purines, pyrimidines, coenzymes, and lipids likely combined into this core. Overall, the number of pathways required to generate nucleotides is relatively small. Pyruvate, ammonia, carbon dioxide, ATP, and glyoxalate suffice to synthesize virtually the compounds required for metabolic cycles. It seems likely that once the RNA world existed that thereafter an RNA-Peptide world developed. Details are on the following website http //www.sciencedirect.com - Cell, Volumel36, Issue 4, page 599, and a description follow below. 

Because nitrogen catabolic pathways are similar in many organisms and most research efforts in nitrogen catabolism have concentrated on mammals, the mammalian pathways are the focus of this chapter. Chapter 15 begins with a discussion of the pathways that degrade amino acids to form ammonia and the carbon skeletons used in anabolic and catabolic processes. This is followed by a discussion of urea synthesis. Chapter 15 ends with descriptions of the degradation of several amine neurotransmitters, the nucleotides, and the porphyrin heme. 

See in particular Roy MacLeod s contribution to this volume. For French attempts to imitate German processes in the immediate post-war period see Lothar Meinzer s paper. Compare also Rolf Petri s description of the situation in Italy, where notable progress was made in the high pressure synthesis of ammonia. Unlike most other contributors to this volume, I place the greatest emphasis on products and... 

The absorption of carbon dioxide in water at elevated pressure was formerly an important industrial process, particularly for the purification of synthesis gas for ammonia production. The process has now generally been replaced by more efficient systems which employ chemical or physical solvents with much higher capacities for carbon dioxide than water. Such systems are described in Chapters 2, 3, 3, and 14. A description of the water wash process for carbon dioxide removal is included in this chapter because of its historical interest, its technical value as a classical liquid film-controlled operation, and the hope that the extensive work done on the process will prove usefril in the development of new processes or applications. 

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