Development and application of wüstite-based ammonia synthesis catalysts

Abstract

In the 1980s, Zhejiang University of Technology (ZJUT) started developing a new catalyst for ammonia synthesis which in final form comprises multi-promoters wüstite-based catalysts with Fe1-xO as precursor. This became the first new iron catalyst for ammonia synthesis commercialized since Fritz Haber’s promoted iron catalyst. The catalyst is commercialized in the existing or new low pressure process. In this paper, the basic theory, method, technology and practice of the wüstite-based catalyst in industrial production and application are reviewed. The wüstite-based catalysts are still iron-based, however, they are completely different from the magnetite-based catalyst in the preparation principle, chemical composition, crystal structure, and chemical-physical properties. Wüstite does not exist in nature, and is a thermodynamically meta-stable phase. Therefore, the Fe1-xO must be firstly synthesized by a chemical method and avoid occurrence reactions of oxidation and disproportionation to obtain a commercial wüstite-based catalyst with single phase and stability during the preparation of catalyst. We described the development of the catalyst, starting from the 1980s with studies to elucidate the inhibition mechanism of disproportionation reaction and the effects of temperature and promoters, and to ensure that a single wüstite phase is obtained during preparation. This is a prerequisite for ensuring high activity of the catalyst. Scale up of catalyst production and testing were described briefly, as are some process considerations. Secondly, we described the industrial application of the catalyst with studies to elucidate including the industrial bypass testing, the foundation research on the industrial application technology such as industrial reduction technology due to too fast reduction rate, development of industrial technology taking out the excess reaction heat, optimization of reaction conditions using wüstite-based catalyst, low-pressure ammonia process, and optimization of synthesis loop process and convertor for existing synthesis ammonia plant, etc. The wüstite-based catalysts exhibit about 70 % higher catalytic activity, and 65.9 % higher the TOF. It is the most advanced commercial iron catalyst with the highest activity and low production cost in the world, and it is competitive with ruthenium-based ammonia synthesis catalysts. In industrial application, compared to the classic magnetite-based catalyst for ammonia synthesis, the higher activity allows lower operating temperatures and loop pressures. Existing loops can utilize it by running at minimum possible loop pressure for extended period of time. This allows to increase ammonia plant performance, efficiency and economy without the need of any revamping. New plants could be designed for lower pressure operation and with a smaller synthesis converter. This all together leads to lower capital expend tureen lower energy costs. The activation of this catalyst is fast and easy, which saves time and cost. Wüstite-based catalyst is clearly a perfect low cost alternative to Ru/C catalyst.