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Mechanism of the Int r ins icReaction. Classical Iron CatalystsComposition. Particle Size and Shape. Other Catalysts. Metals with Catalytic Potential. Synthesis Gas Produc t ionSteam Reforming.

Mechanisms and Kinetics of SteamReforming Catalysts. Primary Reformer. Secondary ReformerReduced Primary Reforming. Heat-Exchange Reforming.

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Fully Autothermal Reforming. Synthetic Gas. Shift Conversion in Steam4. High-Temperature Shift Conversion4. Low-Tem perature Shift Conversion4. Intermediate-Temperature Shift4. Shift Conversion in Partial. Process Configurati4. Chemical Absorption Systems4. Physical Absorption Solvents. Sour Gas Removal in Partial. Oxidation Processes.

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Final Purification. Selectoxo Proc4. Cyrogenic Methods. Reciprocating C4. Synthesis Loop Configurations.

Preparation of Ammonia in Industry

Formation of Ammonia in the. Converter Commercial Ammonia Converters Principal Converter Configurations Tube-Cooled Converters.

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Ammonia Plants based on HeavyHydrocarbons. Revamping Objectives Revamping Options. Material Considerations forEquipment Fabrication. Transport by Pipelines. Quality S pecifications andAnalysis. Health AspectsEnvironmental. The name ammonia for the nitrogen hydrogen compound NH, is derived from theoasis Amm on today Siwa in Egypt, where Amm onia salts were already known inancient times and also the Arabs were aware of ammonium carbonate.

The distance between the triply bonded nitrogen atoms of 1. Because of the high dissociation energy the direct formation of oxides is "endergo- nisch [ They are thermodynamically unstable compared to N, and O,, which explains why the oxidation route for nitrogen fixation was not competitive. The only reactions of molecular nitrogen at ambient temperature are the formation of lithium nitride Li3N, reactions with certain transition metal complexes, and nitrogen fixation with nitrogenase in the bacteria of the root nodules of legumes and in blue algae Sections Above "C nitrogen reacts with some elements, especially with metals nitride formation.

For purely thermal energy supply with a favorable collision yield, this activation barrier requires temperatures well above - K to achieve measurable reaction rates.

Ammonia : principles and industrial practice / Max Appl.

However, at such high temperatures and industrially reasonable pressures, the theore- tically achievable ammonia yield is extremely small because of the unfavorable position of the thermodynamic equilibrium. In fact, all older attempts to combine molecular nitrogen purely thermally with atomic or molecular hydrogen failed.

On the other. Heat of reaction AH in hJ for the reaction 0. U I, "C pah,. At pressures above MPa bar , the synthesis of ammonia proceeds even in the absence of specific catalysts. At such extreme pressures the vessel walls appear to catalyze the formation of ammonia. In the homogeneous phase under thermodynamically favorable temperature condi- tions, the formation of ammonia may be forced by employing other forms of energy, such as electrical energy or ionizing radiation.

The principal difficulty with these so- called plasma processes, which also impedes their economic use, is that the energy supplied is useful only in part for ammonia formation. A greater part is transformed in primary collision and exothermic secondary processes into undesirable heat or unu- sable incidental radiation. In the catalytic combination of nitrogen and hydrogen, the molecules lose their translational degrees of freedom by fixation on the catalyst surface.

The reaction may then proceed in the temperature range "C. In , it was discovered that electron donor - acceptor EDA complexes permit making ammonia with measurable reaction rate at room temperature. Iron Y catalysts which are generally used until today in commercial production units are composed in unreduced form of iron oxides mainly magnetite and a few percent of Al, Ca, and K; other elements such as Mg and Si may also be present in small amounts. Activation is usually accomplished in situ by reduction with synthesis gas. Prereduced catalysts are also commercially available.

Numerous investigations have been performed to elucidate the mechanism of cat- alytic reaction of nitrogen and hydrogen to form ammonia. References [ l o l l - give reviews of the older and some of the newer literature. During the past two decades a large variety of surface science techniques involving Auger electron spectroscopy, X- ray photoelectron spectroscopy, work-function measurements, temperature-pro- grammed adsorption and desorption, scanning tunnelling microscopy, and others have been developed [], [].

Many of these methods are based on interaction of slow electrons, ions, or neutral particles and exhibit high sensitivity to surface structures. With these powerful tools the kinetics of nitrogen and hydrogen adsorption and desorption could be investigated, and it was also possible to identify adsorbed inter- mediates.

The results of these experiments allow the mechanism of ammonia synthesis in the pressure range of industrial interest to be elucidated [ As with every catalytic gas-phase reaction, the course of ammonia synthesis by the Haber - Bosch process can be divided into the following steps : 1 Transport of the reactants by difision and convection out of the bulk gas stream, through a laminar boundary layer, to the outer surface of the catalyst particles, and further through the pore system to the inner surface pore walls 2 Adsorption of the reactants and catalyst poisons on the inner surface 3 Reaction of the adsorbed species, if need be with participation by hydrogen from the gas phase, to form activated intermediate compounds 4 Desorption of the ammonia formed into the gas phase 5 Transport of the ammonia through the pore system and the laminar boundary layer into the bulk gas stream Only the portion of the sequence that occurs on the catalyst surface is significant for the intrinsic catalytic reaction.

Of special importance is the adsorption of nitrogen. This assumption is decisive in representing the synthesis reaction kinetics. The transport processes occurring in the pores of the catalyst in accordance with the classical laws of difision are of importance in industrial synthesis see also Sections 3. This has now been fully confirmed by microkinetic simulations 5 based on the results of surface science studies [], [ l a l ] , [l In single-crystal experiments it was found that the activation energy for dissociative nitrogen adsorption 5 Q.

From experimental results it was concluded that for hydrogen a direct J U transition from the gaseous H, molecule into the chemisorbed H,, is most likely, and evidence for the stepwise hydrogenation of surface nitrogen atomic species was found. It was further shown that at lower temperatures nitrogen becomes adsorbed only in the molecular state, but subsequently dissociates when the tempera- ture is raised.

Isotopic experiments with 30N, and 29N2showed that the surface species resulting from low-temperature adsorption was molecular, whereas that from high- temperature adsorption was atomic [ Ammonia synthesis is highly sensitive to the orientation of the different crystal planes of iron in the catalyst [] - []. Measure- ments on defined single crystal surfaces of pure iron performed under ultrahigh vacuum [] - [] clearly showed that Fe ll1 is the most active surface. This was also demonstrated by the rate of ammonia formation on five different crystallographic planes for unpromoted iron at 20 bar, as shown in Figure 7.