Ammonia synthesis catalyst development

The thermodynamic equilibrium is most favourable at high pressure and low temperature. The methanol synthesis process was developed at the same time as NH3 synthesis. In the development of a commercial process for NH3 synthesis it was observed that, depending on the catalyst and reaction conditions, oxygenated products were formed as well. Compared with ammonia synthesis, catalyst development for methanol synthesis was more difficult because selectivity is crucial besides activity. In the CO hydrogenation other products can be formed, such as higher alcohols and hydrocarbons that are thermodynamically favoured. Figure 2.19 illustrates this. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Explain why it is important to develop an ammonia synthesis catalyst with high activity at low temperatures. 

N2 and CO, respectively [31,32], Empirical knowledge about the promoting effect of many elements has been available since the development of the iron ammonia synthesis catalyst, for which some 8000 different catalyst formulations were tested. Recent research in surface science and theoretical chemistry has led to a fairly complete understanding of how a promoter works [33,34],... 

The ammonia synthesis catalyst problem could be considered solved when the catalytic effectiveness of iron in conversion and its onstream life were successfully and substantially improved by adding reduction-resistant metal oxides [232] (Table 15). The iron catalysts promoted with aluminum and potassium oxides proved to be most serviceable [238]. Later, calcium was added as the third activator. Development work in the United States from 1922 can be found in [239]. 

Efforts to develop more active ammonia synthesis catalysts have been ongoing. As an example, a Ru-based ammonia synthesis catalyst has been introduced to the market. This catalyst is more active than the iron-based catalyst, but the cost is also much higher due to the high Ru price, and the use of the catalyst is very limited. 

In 1920s, the studies on the catalysts for ammonia sjmthesis were performed sporadically in BASF, instead, the company mainly focused on the organic synthesis under high pressm-es and the new fields in heterogeneous catalysis. Dm-ing the development of ammonia synthesis catalysts, researchers provided valuable information about the dm-ability, thermal stability, sensitivity to poisons, and in particular to the concept of promoter. Mittasch smnmarized the roles of various additives as shown in Fig. 1.9. The hypothesis of successful catalyst is multi-component system proposed by Mittasch was confirmed to be very successful. Iron-chromium catalysts for water gas shift reaction, zinc hromium catalystfor methanol synthesis, bismuth iron catalysts for ammonia oxidation and iron/zinc/alkali catalysts for coal hydrogenation were successively developed in BASF laboratories. 

In 1951, A102 catalyst, the first ammonia synthesis catalyst in China, was developed and manufactured by Nanjing Chemical Industry Corporation. A106 and A109 catalysts were developed in 1956 and 1967, respectively, and were widely used in industry, but their activity were low and running temperatures were high. 

In 1979, Zhejiang University of Technology developed successfully, AllO-2 low temperature ammonia synthesis catalysts by adjusting interaction between electronic promoters and structural promoters. This catalyst was the pioneer of Chinese Alio series catalysts. Later, Nanjing Research Institute of Chemical Industry, Fuzhou University, Linqu catalyst plant, Zhengzhou University and Hubei Institute of Chemistry developed AllO-1, AllO-3, AllO-4, A110-5Q (spherical) and AllO-6 ammonia synthesis catalysts, which forms the AllO series catalysts and have been most widely used in China since 1980s. 

Since 1980s, ruthenium based catalysts discovered by British Petroleum of and Fei xO based catalyst developed by China had made new progresses on ammonia synthesis catalysts. Three technical routes were developed including the magnetite based (Fe3C4) route, Fei xC catalysts and Ru catalysts, and have achieved significant progresses, respectively. 

Table 1.6 History and development of ammonia synthesis catalysts... 

Ammonia synthesis catalyst with Fes04 as precursor has been studied widely and deeply in the past one century. " These results have greatly promoted the development of heterogeneous catalysis and surface science. Ammonia synthesis reaction is a green chemical reaction without side reaction and with molecular efficiency and selectivity of 100%. It is used as the ideal model reaction in heterogeneous catalysis, and all general concepts of catalysis were developed and formulated in relation to ammonia synthesis. So ammonia synthesis catalyst is also called textbook catalyst. ... 

Since ruthenium catalyst is expensive, highly active and readily inhibited by H2, the process for the ammonia synthesis must be modified to fit these features. In 1980, BP and Kellogg Corporation cooperated to develop a novel ammonia synthesis system, in which BP was to develop a new ammonia synthesis catalyst with high activity at low temperatures and low pressures, while Kellogg was responsible for the development of the matching technology for the process of ammonia synthesis. After a joint effort for 10 years, a process called Kellogg Advanced Ammonia Process (KAAP) was developed successfully (Fig. 1.26). 

Based on the method mentioned above, the authors carried out the kinetic measurement for 110-2 and ZA-5 ammonia synthesis catalysts that were developed lately, at medium and low temperature, and the results obtained by treating these data are given in Table 2.15. 

Dm-ing the early stages in the development of ammonia synthesis catalysts, German chemists investigated over 20,000 catalyst formula proportions and spent amazing efforts into it. Over the century, the compositions, properties, reaction mechanism and kinetics of the catalysts have been extensively studied in the world and we have accmnulated large munber of data and information. Now, it is possible to design the ammonia synthesis catalysts on those bases. 

Ruhler et al. considered that Ba-Ru/MgO catalysts have high activities and stabilities. The researchers in Tops Company, Denmark developed series of ruthenium based ammonia synthesis catalysts support on different supports, with electronic and structural promoters. In these studies, the ruthenium catalysts supported on the Mg Al spinel and high surface area graphite show promising activity (see Table 6.6). However, the stability has some problems under industrial conditions. ... 

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