First Example Ammonia Synthesis

Oxygen-containing molecules cannot be tolerated in the ammonia synthesis, primarily because they form iron oxide that blocks the active surface. First the CO2 is removed, through a scrubber, by reaction with a strong base. The remaining CO (and CO2) is then removed by the methanation reaction, converting the CO into methane and water. Finally the water is removed by, for example, molecular sieves. Methane does not present problems because it interacts weakly with the catalyst surface. The gas mixture (Tab. 8.6) is compressed to the roughly 200 bar needed for ammonia synthesis and admitted to the reactor. 

The first example of a dynamic flux analysis was a study performed in the 1960s [269]. In the yeast Candida utilis, the authors determined metabolic fluxes via the amino acid synthesis network by applying a pulse with 15N-labeled ammonia and chasing the label with unlabeled ammonia. Differential equations were then used to calculate the isotope abundance of intermediates in these pathways, with unknown rate values fitted to experimental data. In this way, the authors could show that only glutamic acid and glutamine-amide receive their nitrogen atoms directly from ammonia, to then pass it on to the other amino acids. 

Our first example involves an industrial process of immense importance the catalytic synthesis of ammonia (NH3). Ammonia is a central component of... 

The second major class of enamines contains the enamino ketones, made from ammonia or an amine and a 1,3-diketone. By this route the same 1,3-diketone can provide both halves of the resulting pyridine such an example is provided by the synthesis of 5-benzoyl-2-phenylpyridine (444) (54YZ259). As in the case of the crotonates, the first examples of the condensation of an enamino ketone with an a,/ -unsaturated ketone were provided by Knoevenagel and Ruschhaupt (1898CB1025), who prepared the range of 3-acetyldihy-dropyridines (445) to (447). Similarly, Mumm and Bohme obtained the 3-acetyl- and 3-benzoyl-2,6-dimethylpyridines (448) and (449) from ethyl acetylpyruvate (21CB726). 

Alkoxylation of MHal is performed by KOR, NaOR, or LiOR or by ammonia in solution in alcohol (formally ammonium alkoxide). Reaction with NaOR is accompanied by formation of NaCl, which is almost completely insoluble in organic solvents and is, therefore, used in the synthesis of soluble metal alkoxides. One of the first examples of application of such reactions has not lost its importance as a method of synthesis to this day ... 

A sphingosine derivative, symbioramide (308), was obtained from the laboratory-cultured dinofagellate Symbiodinium sp. as a Ca2+-ATPase activator in the sarcoplasmic reticulum (250). The complete stereostructure of 308 was established by total synthesis (251). The occurrence of eo>f/iro-docosasphinga-4,8-dienine, as an ester (309), was found in Ammonia sulcata (252). This is the first example of a sphinga-4,8-dienine ester. A... 

The first examples of these ring closure reactions were reported by Bard and Bunnett318 and Beugelmans and Roussi319, who studied the photostimulated reactions of oamino-halobenzenes (257, X = Br,I) with carbanions in liquid ammonia to perform the synthesis of indoles 259 (equation 167). 

Poly(ethylenimine) (PEI) has been examined extensively both in its classical, random branched topology [125] and in its linear form [126]. The various architectural and topological forms of PEI have been reviewed recently [127], Here we describe the first example of this polymer system as an ideal, hyper-branched molecular assembly. Synthesis of a tri-dendron poly(ethyleneimine) dendrimer derived from an ammonia core involved, first the selective alkylation of diethylenetriamine (DETA) with aziridine to produce a symmetrical core cell, namely tris-(-2-aminoethyl)amine. Subsequent exhaustive alkylations of the terminal amino moieties with activated aziridines [2, 127, 128], such as IV-tosyl- or N-mesylaziridine gave very good conversions to the first-generation protected... 

Around 1900 Fritz Haber began to investigate the ammonia equilibrium [11] at atmospheric pressure and found minimal ammonia concentrations at around 1000 °C (0.012 %). Apart from Haber, Ostwald and Nernst were also closely involved in the ammonia synthesis problem, but a series of mistakes and misunderstandings occurred during the research. For example, Ostwald withdrew a patent application for an iron ammonia synthesis catalyst because of an erroneous experiment, while Nernst concluded that commercial ammonia synthesis was not feasible in view of the low conversion he found when he first measured the equilibrium at 50 - 70 bar [12] - [14],... 

If the objective in design or operation were optimizing catalyst utilization, then Figure 82 shows that the converter temperature-composition profile should follow curve (a), which corresponds to maximum reaction rate at all points. It is also obvious that in reality this ideal temperature - concentration profile cannot be achieved. For example, a synthesis gas with about 3 % ammonia concentration entering the converter cannot be heated to the ideal temperature by heat exchange because the very high temperature required does not exist in the converter system. To reach the ideal temperature, the first portion of the catalyst must initially operate adiabatically. Consideration of the service life of the catalyst requires that this maximum initial temperature not exceed that recommended by the manufacturer, usually 530 °C (cf. Section... 

We will describe here the general operation of the algorithm for the construction of direct mechanisms for chemical systems. We will examine the algorithm only in its simplest form. Mavrovouniotis (1992) presents various enhancements and discusses computational implementation considerations. We will begin with two examples (ammonia and methanol synthesis) that illustrate the step-by-step operation of the algorithm these should be particularly useful for readers that are not accustomed to the abstract description of algorithms, who may wish to study the examples first, or study the abstract algorithm and the examples simultaneously. 

We first consider an example on ammonia synthesis that was used by Happel and Sellers (1982, 1983) and Happel et al. (1990) and contains only... 

In order to prolong the time between regenerations and shutdowns, the reactor tube may be made longer than required for the reaction itself. For example, suppose a length of 3 ft is necessary to approach the equilibrium conversion with fresh catalyst of high activity. The reactor may be built with tubes 10 ft long. Initially, the desired conversion will be obtained in the first 3 ft. As the catalyst activity falls off, the section of the bed in which the reaction is mainly accomplished will move up the bed, until finally all 10 ft are deactivated. This technique can be used only with certain types of reactions but it has been employed successfully in the ammonia synthesis. 

In addition to the skeptic s view of LH correlations, however, there must be additional reasons why this model has been used successfully for the correlation of kinetics in so many different types of catalytic reactions. Two factors seem to be most important. First is the fact that the expressions are fairly insensitive to the precise sort of kinetic scheme involved. Many widely differing types of reaction sequences can be shown to give approximate LH forms, as given in Tables 3.2 and 3.3 [see also a further good example based on the ammonia synthesis reaction by G. Buzzi Ferraris, G. Donati, F. Rejna and S. Carra, Chem. Eng. Sci., 29, 1621 (1974)]. Second is the fact that for reasonable assumptions concerning the nature of typical nonuniform surfaces it can be shown that they tend to look like uniform surfaces in overall behavior. This second point can be illustrated by an example scheme in which the heat of formation of surface complexes is a linear function of surface coverage. Consider the two-step sequence... 

---