How to protect the catalyst in the ammonia synthesis tower during major repairs

To protect the catalyst in the ammonia synthesis tower, it is first necessary to isolate the synthesis tower system, fill it with nitrogen for protection, and closely monitor the bed temperature. Nitrogen is periodically introduced to maintain a slight positive pressure in the synthesis tower. Prevent air from entering.

If the system is not maintained, it becomes isolated from other units, with the pressure being released to near atmospheric levels. If the time interval is not too long, the ammonia vapor and the liquid ammonia inside the cold exchanger will continue to evaporate, and generally no air will enter the system. For maintenance of the system, it is necessary to isolate the synthesis tower from the rest of the system; if required, blind flanges should be installed, and pressure filling points should be provided so that nitrogen can be used to maintain a slight positive pressure to protect the catalyst. The nitrogen must be of high purity, as otherwise it may cause poisoning or oxidation.

When shutting down the synthesis tower, cooling by circulation is carried out first, with the cooling rate controlled within the range of 30–50 degrees Celsius, until the temperature of the catalyst layer drops below 100 degrees Celsius, at which point cooling is stopped. Drain the liquid level from the condenser separator, release the system pressure, and purge it with nitrogen of 99.99% purity. The rate of pressure change during this process must be strictly controlled in accordance with the specified guidelines; a combustible gas content of less than 0.5% (H2 + NH3) is considered acceptable. Release the pressure, install blind flanges on all entrances and exits of the synthesis tower to isolate it from other areas, thereby preventing air from entering the catalyst layer during maintenance and causing the catalyst to oxidize. When driving, after removing the blind flange, nitrogen purging should also be carried out, and the 02 content must be less than 0.5% to be considered acceptable.

If the system is to be overhauled, blind flanges must be installed at both the inlet and outlet of the synthesis tower. Nitrogen can generally be introduced through the sampling points; if, due to the overhaul, the sampling points are not within the range covered by the blind flanges, a bypass connection must be prepared in advance. The nitrogen gas needs to have its CO2 and O2 content analyzed, and an experienced technician should be assigned to oversee this process. This time, before removing the blind flanges, we replaced the gas in the synthesis circuit outside the synthesis tower by releasing air from the liquid ammonia storage tank. This information is provided for the reference of the original poster.

Isolate all pipes leading to and from the synthesis tower, and purge them with nitrogen