One question per week on synthetic ammonia knowledge
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This post was last edited by Yan Qiusheng on 2010-11-23 09:44. The standard answer: Taken from the book \"Fertilizer Production Operators.\" When synthetic catalysts are being reduced, maintaining a high space velocity is beneficial for the reduction process; it allows the water vapor generated by the reduction reaction to be removed promptly, thus keeping the water vapor concentration in the gas low, which facilitates the progress of the reduction reaction. A high space velocity also helps to ensure uniform temperature across the catalyst bed, reducing the temperature difference between the inlet and outlet. In actual production, the space velocity is primarily limited by the capacity of the heating furnaces used for startup. Why is a high space velocity used during the temperature-raising reduction of ammonia synthesis catalysts?
Increasing the space velocity can reduce the water vapor concentration in the reducing gas, decrease the axial temperature difference, and enhance the catalyst’s activity. However, it should not be too high, as this will affect the increase in the reduction temperature; it must be determined based on the capacity of the electric furnace, while also meeting the requirements regarding the reduction temperature. Generally, it is low at the initial stage of reduction, and a higher space velocity can be used during the peak effluent production period.
Increasing the space velocity can reduce the water vapor concentration in the reducing gas, decrease the axial temperature difference, and enhance the catalyst’s activity. However, too high an air velocity will affect the increase in reduction temperature; therefore, when increasing the air velocity, the capacity of the electric heater must also be taken into account. It must also meet the requirements regarding the reduction temperature. Generally, a lower space velocity is used in the early stages of reduction, while a higher space velocity can be employed during the peak growth period
The main purpose of using a high space velocity for the temperature-raising reduction in ammonia synthesis is to reduce water vapor concentration, minimize repeated oxidation and reduction of the catalyst, and enhance its activity. Later on, the high space velocity is utilized to carry the reaction heat to the lower catalyst layer, thereby accelerating the reduction process and ensuring thorough catalyst reduction.
This post was last edited by Chemical Gas Purification on 2010-11-28 at 11:42. Firstly, it is to avoid deviation in flow rate at low speeds; secondly, high flow rates enable the reaction heat to be carried away promptly, preventing overheating
The higher the high-velocity air velocity, the shorter the reaction time; the lower the conversion rate of ammonia synthesis, and the lower the ammonia content in the gas exiting the tower. At the same time, it is possible to transfer the ammonia synthesis reaction zone from the upper layer to the lower layer, thereby ensuring an appropriate temperature for heating and reduction in the lower layer.
Increasing the air velocity can reduce water vapor in the reducing gas and decrease the axial temperature difference. The magnitude of the air velocity is primarily determined by the power of the electric furnace; given that the furnace’s power permits it, it also depends on the water vapor concentration and an appropriate reduction temperature.
To remove the water generated during the reduction process in a timely manner, if the water is produced too quickly and the vapor concentration becomes too high, the reverse reaction, namely the oxidation of iron, increases, which leads to a decrease in CAT activity. Therefore, it is necessary to control the vapor concentration during the reduction process; the main method for doing this is to increase the space velocity. If the circulation volume remains unchanged, reducing the pressure is a better approach
This post was last edited by Chemical Gas Purification on 2010-11-22 at 17:11. A large amount of heat is generated during catalytic reduction, and a high space velocity helps to carry away this heat.
This is because the requirements for catalyst reduction conditions are \"three highs and three lows\", namely high hydrogen level, high circulation rate, and high electric furnace power (for catalyst reduction in the middle and lower sections), as well as low water vapor concentration, low reduction temperature, and low ammonia cooling temperature. When the catalyst is heated, to meet the aforementioned requirements—low water vapor concentration, low reduction temperature, and low ammonia temperature—it is necessary to increase the circulation rate. On the other hand, if the temperature cannot be effectively reduced, the amount of water produced by the reaction of H2 with the catalyst per unit of time increases, thereby reducing the efficiency of the reduction process.
The main purpose is to allow the water generated during the reduction process to be removed promptly; if the water is produced too quickly and its vapor concentration becomes too high, the reverse reaction, namely the oxidation of iron, increases, which reduces the catalyst’s activity. Therefore, it is necessary to control the vapor concentration during the reduction process, and the primary method for doing this is by increasing the air velocity. If the circulation rate remains unchanged, reducing the pressure is a better approach.