① For extended shutdown periods, the
ammonia synthesis catalyst undergoes passivation. This process involves reducing pressure and lowering the temperature appropriately.
② After purging with nitrogen to displace remaining gases, the synthesis tower is sealed, allowing it to naturally cool to room temperature.
③ During passivation, the catalyst must be cooled to below 50°C. Nitrogen is then introduced, and under a pressure of 0.3~0.5 MPa, nitrogen circulation begins. This continues until the hydrogen content is less than 4%.
④ Dry compressed air is then introduced, ensuring that the airflow maintains the catalyst bed's maximum temperature below 100°C.
⑤ The passivation is considered complete when there is no change in the catalyst bed temperature after increasing the airflow, provided that the oxygen content at the inlet and outlet of the tower is the same.
⑥ In the absence of nitrogen, hydrogen-nitrogen gas circulation, pressure reduction, and modifications in gas synthesis operations are necessary to reduce hydrogen content to below 40%. Extreme caution is needed to ensure the bed temperature does not exceed 100°C, preferably staying below 80°C.