Chemistry of inorganic compounds – Nitrogen or compound thereof – Ammonia or ammonium hydroxide
Patent
1996-07-02
1998-12-08
Langel, Wayne
Chemistry of inorganic compounds
Nitrogen or compound thereof
Ammonia or ammonium hydroxide
423363, 423632, 502241, 502242, 502243, 502247, 502248, 502250, 502254, 502255, 502257, 502258, 502302, 502305, 502306, 502308, 502312, 502314, 502316, 502319, 502320, 502321, 502322, 502323, 502324, 502328, 502330, 502332, 502338, C01C 104, B01J 23745
Patent
active
058465073
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a solid catalyst, a process for its preparation and its use in the ammonia synthesis industry.
At present, the prior art ammonia synthesis catalysts are commonly prepared by melting magnetite and minor amount of promoters in an arc furnace (or resistance furnace, or inductor furnace). During the melting process, small amount of pure iron (less than 5% by weight ) is added in most case so as to adjust the ratio Fe.sup.2+ /Fe.sup.3+ to a specified value. The iron oxide formed in the catalyst, generally referred to as the "main phase" or the "main component" is mainly Fe.sub.3 O.sub.4, its content ranging from 78 to 95% by weight, and its chemical composition is 24-40% by weight of FeO, 52-70% by weight of Fe.sub.2 O.sub.3 and 4-7% by weight of promoters (based on the total weight of the catalyst, and the same hereafter). The ratio of divalent iron to trivalent iron (Fe.sup.2+ /Fe.sup.3+) ranges from 0.4 to 0.8. Ammonia synthesis catalysts available from different countries over the world such as KM catalyst made in Denmark, C73 catalyst in U.S.A., IC135-4 and 74-1 catalysts in U.K., BASF catalyst in Germany, CA catalyst in former U.S.S.R., and A series catalysts in China, all have Fe.sub.3 O.sub.4 as their main component without any exception (Timm.B., 8th International Congress on Catalysis, Berlin, 2-6, Jul., 1984), whose ratios of Fe.sup.2+ /Fe.sup.3+ all range between 0.4 and 0.8. For example, BASF-E1 ammonia synthesis catalyst made by BASF Co. Germany has a chemical composition as follows: 31.3% by weight of FeO and 62.53% by weight of Fe.sub.2 O.sub.3 with the Fe.sup.2+ /Fe.sup.3+ ratio of 0.56, as well as small amount of promoters composed of 2.9% by weight of Al.sub.2 O.sub.3, 0.97% by weight of K.sub.2 O, 1.65% by weight of CaO, 0.30% by weight of MgO and 0.35% by weight of SiO.sub.2 (Slack AV and Russell James G, Ammonia (in four parts) part III, Marcel Dekler Inc, New York, 1977: 84-87).
The conventional ammonia synthesis catalysts are based on Fe.sub.3 O.sub.4 whose crystal phase is in magnetite form having spinet structure with lattice parameter of 0.838. 0.841 nm (XRD) and having strong magnetism, this is the esseutial feature decisive to the performance of the pior art catalyst products. The bulk density before reduction is 2.7-2.9g/cm.sup.3 and the specific surface area after reduction is 13-16m.sup.2 /g (BET). Under the experimental conditions of pressure at 30MPa, temperature at 425.degree. C., space velocity of 30,000m.sup.3 /m.sup.3 h and a stoichiometric mixture of nitrogen and hydrogen, the ammonia concentration at the reactor exit is 22-24% by volume, and the ammonia concentration in equilibrium is 41.78% by volume under the same condition. Therefore, as far as the prior art ammonia synthesis catalysts are concerned, not only high pressure and temperature are needed, but also the reaction for ammonia synthesis is far from ammonia equilibrium concentration, especially at low temperature. Furthermore, 5-7 days of reduction time is needed when the conventional ammonia catalysts are put to use in industry, which indicates their slower reduction rate. As for the conventional catalysts containing cobalt (Co) (e.g., IC174-1 catalysts), cost for preparing them is even higher. All these defects are due to the disadvantages of the magnetite which constitutes the main phase of the conventional catalyst.
For more than eighty years, ammonia synthesis catalysts have been researched extensively and intensively by different countries over the world, but all these efforts are limited to changing types and quantities of promoters under the condition of remaining Fe.sub.3 O.sub.4 as the main phase unchanged, and they neglect the crucial value of improving the composition and structure of the main phase to the performances of the catalysts, so, the progress in developing new system of ammonia synthesis catalyst having better performance is little. The research work of ammonia synthesis catalysts carried out in different countries over the world is almost a repetition on the same
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Hu Zhangneng
Jiang Zurong
Li Xiaonian
Li Yanying
Liu Huazhang
Langel Wayne
Zhejiang University of Technology
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