Development trend of ammonia synthesis catalysts

It is well known that the conversion of hydrogen and nitrogen per pass is only 20%-30% for the present catalytic ammonia synthesis technology (Table 1.4). Most synthesis gases need to be returned to the reaction system, which increases power consumption. In order to increase conversion per pass, it must increase the outlet ammonia concentration of reactor. Accordingly, it can be seen from Table 1.4 that it is necessary to increase reaction pressure for small and medimn scale aimnonia plants and Topspe process, or to reduce the content of inert gas in sjmthesis gas for Topspe and Braun processes, or to reduce ammonia concentration in the inlet of converter for small and medium scale ammonia plants and Kellogg process. But all of these operations will add the power consumption or unit gas consumption. 

Processes Small and medium scale ammonia plants Kellogg Topsoe Braun ICl AMV 

For the improvement of synthetic quotient per pass, it is also an effective approach to increase volume of catalyst in order to reduce space velocity such as ICI-AMV and Braun processes. 

Nevertheless, either increasing the outlet ammonia concentration or reducing space velocity is confined by the equilibrium ammonia concentration. Where, the catalyst efficiency ( ex/ e) is defined as the ratio of outlet ammonia concentration ( ex) to equilibrium ammonia concentration (y e) under same conditions, which indicates the degree of the reaction close to the equilibrium. The efficiency of the present catalysts used in industry is about 90% at higher temperatures, as shown in Table 1.5. For example, the efficiency of ZA-5 catalyst is about 95% at about 475°C at the space velocity of 3 x 10 h and pressure of 15 MPa, which is very close to the equilibrium concentration of ammonia. With low space velocities, the operating temperature can be decreased as the outlet ammonia concentration is close to equilibrium concentration. As shown above, the efficiency of ZA-5 is about 98% at above 450°C and at the space velocity of 1 x 10 h Thus, it is impossible to further increase the outlet ammonia concentration under these conditions. As a result, the activity at low temperatures must be increased, since equilibrium ammonia concentration is higher at low temperatures. The outlet ammonia concentration is 16.68% on ZA-5 catalyst at 400°C and with the space velocity of 3 x 10 h and pressure of 15 MPa, the equilibrium concentration is 32.83%, and therefore the catalyst efficiency is only 50.8%. If the catalytic efficiency can be increased to more than 95% at 400°C by increasing the catalyst s activity at this temperature, the synthetic quotient per pass could be about 50%. 

Obviously, the most effective approach for increasing the synthetic quotient per pass is increasing the catalyst s activity at low temperatures. It is necessary for ammonia synthesis industry to develop the catalysts with higher activities at lower temperatures and pressures. Correspondingly new process and reactors should be developed based on these novel catalysts.  

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