Reduction of fused iron catalysts

It is crucial for ammonia plant to choose ammonia synthesis catalyst with excellent capability to increase productivity and decrease energy consumption. 

Fused iron catalysts are composed of iron oxides (Fes04, FeO) and a small amount of promoters which are usually metal oxides with high melting points such as AI2O3, K2O, CaO, MgO and Si02 etc. The iron oxides must be reduced to metal state to have catalytic activity, but other oxides which act as promoters carmot be reduced. The activity and durability of catalysts are dependent upon the chemical composition, preparation method, reduction procedm-e and conditions. Thus the reduction is a crucial step in the manufacture and application of catalysts, and the performance and kinetics of reduction are important aspects in the study of catalysts. 

Macroscopically, the fused iron catalyst is a non-porous solid before reduction. Due to the existence of structural promoters (AI2O3 etc.), the contraction of the catalyst pellet does not take place during reduction. After oxygen anions are removed by hydrogen during catalyst reduction, it forms a porous structure. The surface area increases greatly and the catalyst exhibits high activity. Nevertheless, the microstructure has big differences such as the surface area, pore size and their distribution for different types of catalysts. 

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. 

The reduction process of the fused iron catalyst in mixture of H2 and N2 is a complex physical-chemical process as displayed in Fig. 5.1. Reduction includes several processes as follows  

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D. Iron catalysts for medium-pressure require pretreatment with reducing gases. A very cautious reduction of fused iron catalysts with pure dry hydrogen was necessary while active catalysts could be pretreated with synthesis gas only. 

Solid state reactions discussed here refer to the reactions which have at least one solid as reactants or products. Both the preparation and reduction of fused iron catalyst are solid state reaction processes, but the role of solid reaction has never been studied thoroughly yet, although there are few reports in literatme and textbooks on the preparation and reduction of fused iron catalyst. There is no doubt that the basic reactions during preparation and reduction of fused iron catalysts belong to orderliness of solid state reactions. The reduction and oxidation of solid oxide, the decomposition of carbonates and hydrates, and the oxidation of sulfides etc belongs to solid state reaction. The solid state reaction follows its imique law, and it must be considered in the analysis and interpretation of preparation and reduction of fused iron catalyst. Therefore, it should be understood on the basic law of solid state chemical reactions. 

According to the studies on reduction of fused iron catalysts following three premises can be established primarily. ... 

Kinetic model for reduction of fused iron catalyst... 

The intrinsic kinetics of reduction. The reduction rate was measured by thermogravimetric analysis (TGA) under conditions of particle sizes of 0.034 mm-0.054mm, H2 flow of 175ml-min temperatures at 350°C, 375°C, 400°C, 475°C, and 500° C, respectively. The effect of diffusion has been eliminated at such particle size range. If the shape of catalyst is taken as spheroid, reduction of fused iron catalysts can be described by SCM. The intrinsic kinetics of reduction is expressed as in Eq. (5.45). 

For the reduction of fused iron catalysts, the reduction temperature is lower than 570°C, regardless of iron oxide in catalysts is Fes04, FeO or their mixtures (of the Fe +/Fe + >0.5, Fe20s does not exist in this system), therefore they can be directly reduced to Fe. ... 

Reduction of fused iron catalyst is a reversible and endothermic reaction. It can be known from thermodynamics that increasing temperature benefits for the generation of o-Fe and complete reduction of the catalyst. At the same time, the reduction time can be shortened. The reduction of fused iron catalyst is a gas-solid non-catalytic chemical reaction. As a single-phase reaction, the Arrhenius equation can be used to describe the relationship between reaction rate constant and temperature. 

The reduction of fused iron catalyst commences from the external surface of particles, and then expands inward. The reduction rate can be increased obviously by increasing the space velocity of reducing gas. The higher gas space velocity, the more favorable the reduction is, i.e., the lower the concentration of water vapor in gas, the faster the diffusion rate, the easier for the water molecules in the pore of catalyst to escape. As a result, the poisoning effect of water vapor is decreased to minimum. In addition, it is also conducive for the reduction reaction to move to the right and to raise the rate of reduction. However, when the space velocity continues to increase, the extent of increasing reduction rate will be minor. When it reaches the critical value, the space velocity of reductant gas on reduction rate has almost no impact. At the same time, in industrial production, increasing the space velocity is limited by the furnace heat supply and the temperature. 

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