Ammonia radial

The conduction electrons are scattered by the alkali atoms, the coherence implicit in the radial distribution function. Unlike the case of the scattering of a single electron in a plane wave state by a liquid, discussed previously, in this case the structure factor S(k) must be known up to the Fermi energy (which is 0.5 e.v. — 1 e.v. in saturated metal ammonia solutions). 

Figure 8. Designs of ammonia synthesis converters (a) Principle of the autothermal ammonia synthesis reactor (b) Radial flow converter with capacities of 1,800 tpd (c) Horizontal three-bed converter and detail of the catalyst cartridge. (Source Walas, M. S., Chemical Process Equipment, Selection and Design, Butterworth Series in Chemical Engineering, 1988.)...

The first application in 1992 used a two-bed, hot-wall KAAP reactor that featured a low pressure drop and radial flow. Because of the KAAP catalyst s high activity, thin beds are necessary to keep operating temperatures within the desired range203. In 2002 the KAAP reactor had evolved to a four-bed design. A magnetite catalyst is used in the first bed of the synthesis loop when the ammonia concentration is below 2% of the feed. Then the ruthenium catalyst is used in the next three beds to bring the ammonia level up to 18% or more215. 

In most radial-flow converters, the upper portion of the bed is sealed with excess, unused catalyst. This design prevents feed gas from by-passing the reaction section when the catalyst settles. The KAAP reactor uses a proprietary sealing system to overcome this problem. This sealing system avoids the catalyst maldistribution that can lead to formation of hot spots in the catalyst bed. The system also allows 100% of the loaded catalyst volume to be utilized for the ammonia conversion reaction203. 

Haldor Topspe s ammonia synthesis technology is based on the S-200 ammonia converter. This is a two-bed radial flow converter with indirect cooling between the beds. Features of the S-200 include efficient use of converter volume and low pressure drop (factors related to the use of small catalyst particles 1.5 to 3.0 mm), and high conversion per pass due to indirect cooling85. 

The ammonia loop is based on the Ammonia Casale axial-radial three-bed converter with internal heat exchangers. Heat from the ammonia synthesis is used to 1) generate high-pressure steam and 2) preheat feed gas. The gas is then cooled and refrigerated to separate ammonia product. Unconverted gas is recycled to the syngas compressor208 214... 

Figure 6.11. Ammonia Casale Axial-Radial Reactor Design. (Reproduced by permission of Casale Group)...

Analysis of the radial pair distribution function for the electron centroid and solvent center-of-mass computed at different densities reveals some very interesting features. At high densities, the essentially localized electron is surrounded by the solvent resembling the solvation of a classical anion such as Cr or Br. At low densities, however, the electron is sufficiently extended (delocalized) such that its wavefunction tunnels through several neighboring water or ammonia molecules (Figure 16-9). 

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). 

Synthesis gas is compressed to the synthesis pressure, typically ranging from 140 to 220 kg/cm2g and converted into ammonia in a synthesis loop using radial flow synthesis converters, either the two-bed S-200, the three-bed S-300, or the S-250 concept using an S-200 converter followed by a boiler or steam superheater, and a one-bed S-50 converter. Ammonia product is condensed and separated by refrigeration. This process layout is flexible, and each ammonia plant will be optimized for the local conditions by adjustment of various process parameters. Topsoe supplies all catalysts used in the catalytic process steps for ammonia production. 

The ammonia synthesis loop uses two ammonia converters with three catalyst beds. Waste heat is used for steam generation downstream the second and third bed. Waste-heat steam generators with integrated boiler feedwater preheater are supplied with a special cooled tubesheet to minimize skin temperatures and material stresses. The converters themselves have radial catalyst beds with standard small grain iron catalyst. The radial flow concept minimizes pressure drop in the... 

Megammonia A process for making ammonia. It uses oxygen instead of steam, and novel axial-radial reactors that reduce pressure-drop and catalyst quantities. Developed by Lurgi Oel-Gas-Chemie and Ammonia Casale, for which they received the AstraZeneca Award for Green Chemistry in 2003. 

---