How is the filling and reduction of ammonia synthesis pre-reduction catalysts carried out?

If possible, nitrogen can be introduced for filling; otherwise, the time of exposure to air should be minimized as much as possible. Prevent the pre-reduced catalyst from being oxidized again. Reduction must be carried out strictly in accordance with the instructions provided by the catalyst manufacturer.

The catalysts used in filling have their surfaces passivated, so oxidation does not occur during normal filling; the reactor is sealed promptly, and nitrogen purging is employed for protection. Reduction is primarily carried out in accordance with the technical guidelines provided by the catalyst supplier. Generally, it is divided into four stages: Stage 1: Preheating and temperature rise; partial pre-reduction begins to occur, and the water vapor concentration increases. Phase 2: The catalysts begin to reduce the first catalyst bed in sequence, with the water vapor concentration kept below 1000 ppm. Stage 3: When the outlet temperature of the first catalyst bed reaches the required temperature and the water vapor concentration stabilizes, it indicates that the reduction in the first bed is complete, and the reduction of the remaining beds begins. Stage 4: Once all catalyst reduction is complete, the ammonia concentration at the outlet reaches a certain value, the water vapor concentration decreases; at this point the heater can be turned off and left on for a certain period of time. Increasing the hydrogen concentration in the reducing gas and reducing the water vapor concentration is beneficial for reduction.

After the temperature is lowered, ideally to below 50 degrees, nitrogen is used for protection; the old catalyst is removed using a vacuum pump, and then a pre-reduced catalyst is installed. The reactor is sealed, followed by reduction according to the reduction procedure provided by the catalyst supplier.

The amount of pre-reduction catalyst used and the loading scheme are usually of three types. 1. Fill entirely with pre-reduced catalyst. The internal structure of some ammonia synthesis towers consists of only two adiabatic sections, with cooling taking place between them. For towers of this structure, it is easier to implement safety measures when loading the second stage (lower) ammonia synthesis catalyst, such as the S-200 model tower manufactured by Topsoe in Denmark. 2. Only about 20% of the catalyst is pre-reduced, while the remaining 80% remains in its oxidized state. When the ammonia synthesis tower has 3 to 4 adiabatic reaction sections inside, it is difficult to install pre-reduction catalysts in the lower sections, such as the third and fourth adiabatic sections. It is necessary to thoroughly consider measures for the rapid evacuation of personnel inside, as well as to have an ample supply of inert gas (nitrogen) on hand for emergency use. If there is no complete certainty regarding safety, a pre-reduction catalyst is usually installed only in the first adiabatic section. Since it requires only 10% of the oxygen that is present in the catalytic material in the oxidized state to be reduced, this allows the first adiabatic section to be reduced completely and ammonia to be produced rapidly. The heat generated by this reaction can be used to supplement the heating provided by external furnaces, thereby enabling the catalytic material in the lower sections to be reduced more quickly. 3. Between the two above, that is, 40% to 60% loaded with pre-reduced catalyst, with the remainder being oxidized catalyst. For a tower with four adiabatic sections, the pre-reduction catalyst is usually installed in the first and second sections, or in the first, second, and third sections.