How are ammonia synthesis catalysts deactivated

First, lower the furnace temperature below 50 degrees, then introduce nitrogen for circulation. Add air slowly, depending on the rate of change in furnace temperature. The oxygen content must be strictly controlled. It can’t be added too quickly; it needs to be added gradually. It is best if the furnace temperature does not rise. Keep it around 50 degrees. In the final analysis, the oxygen content in the outlet reaches 20%, marking the end of the passivation process. The oxygen content in the recycle gas needs to be analyzed frequently. If the temperature rises quickly, reduce the amount of air added; if it drops, increase it slightly.

If passivation is carried out using process gas, the oxygen content is 0.1–0.2%, with the temperature kept below 95 degrees. For pure nitrogen, the oxygen content is 0.2-0.5%, with the temperature kept below 140 degrees. If the temperature exceeds the limit, reduce the oxygen content or stop supplying oxygen. When the catalyst temperature shows a downward trend or the oxygen contents at the inlet and outlet are similar, the oxygen content is gradually increased to 20%; once the temperature stops rising, passivation is complete. Content

The process parameters for catalyst deactivation vary, but the operation procedures are essentially the same.

First, heat with nitrogen, then passivate with air.

In my company, we first cool the nitrogen to 100°C, then loosen the inlet flange of the hydrogen-nitrogen mixer, and replace the pressure gauges for stage 1 and stage 3 with those rated for 0.5–1.0 MPa. Air is gradually introduced into the synthesis tower; the amount of air added is determined based on the increase in catalyst temperature, while also taking into account the oxygen content. When the temperature starts to drop, more air is added. Throughout this process, the catalyst temperature is kept below 550°C. In fact, I have already carried out passivation procedures on the catalysts in both towers, and in those cases the temperature rise never exceeded 400°C. The final criterion for success is that the catalyst temperature drops to 80°C as a whole after 4 hours. However, when the catalyst is actually removed, especially from the lowest layer, it is dark red in color when poured out.

We passivate it with water. But that is a method used only when replacing the catalyst

In the actual production process, if production cannot continue due to catalyst degradation or damage to the inner cylinder, the catalyst must be removed. Since iron-based catalysts undergo intense oxidation upon contact with air and sinter into lumps, it is necessary to passivate the catalysts. If the catalyst still has good activity, the passivated catalyst can be reused after reduction. The passivation procedure is as follows: 1. Cool down and reduce pressure ； Cool the catalyst to room temperature and relieve the system pressure. 2. Exhaust displacement ； After deoxygenation using an inert gas through copper, the system is purged until the ammonia content in the gas leaving the tower is below 1%. 3. Preparations before blunting ； Replace the system pressure gauge with a low-pressure gauge ranging from 0 to 1.6 MPa. Connect the air compressor (which can also be powered by an air compressor used for supplying air to instruments) to the drain line of the pressure gauge at the outlet of the circulation machine. Using air as an oxidizing agent, store qualified inert gas in the preceding section to replenish the system when necessary. 4. Key points for passivation operation 1: Temperature. Temperature reflects the oxidation state of the catalyst, and it has a significant impact on the activity of the catalyst after passivation. The higher the temperature, the greater the degree of oxidation. Therefore, strict control is necessary. To maintain a consistent degree of oxidation and facilitate operational control, it is best to raise the temperature to the operating level 3–4 hours after passivation, and then keep it at that level. From the perspective of passivation, when the temperature is maintained at 80–90°C, the oxidation reaction becomes intense. Therefore, the passivation temperature should generally not exceed 100°C, and it is adjusted by varying the cycle volume and controlling the oxygen content entering the tower. In practical operation, due to low pressure, low space velocity, and low oxygen content, the hotspot temperature of the catalyst gradually decreases as passivation progresses. When the hotspot temperature moves to the lower layer, stops rising and shows a slight downward trend, the oxygen contents at the inlet and outlet are essentially equal. This marks the end of passivation. 2 Pressure: The level of pressure affects the partial pressure of oxygen, the flow rate, as well as the temperature difference between the upper and lower parts of the catalyst; it also influences the reaction rate. For ease of control, low-pressure passivation is generally used, that is, passivation is carried out by controlling the pressure within the range of 0.3–0.6 MPa. Moreover, the control is low in the early stage and high in the middle and later stages. 3 Oxygen content: The level of oxygen content in the gases entering and leaving the tower is one of the key parameters to be controlled during the passivation process, and it must be strictly regulated. In practice, only the oxygen content in the gas entering the tower is controlled, while the oxygen content in the gas exiting the tower serves merely as an indicator of the degree of passivation. The control of oxygen content in the gas entering the tower is adjusted based on the degree of passivation. In the initial stage, due to the intense oxidation reaction of the catalyst, the oxygen content in the gas entering the tower should be kept low, generally between 0.2–0.5%. It can be increased slightly later on, with the control range being around 0.5–2.0%. Later on, as the oxidation reaction rate of the catalyst gradually slows down, it is mainly controlled by adjusting the amount of air added. 4. Control of circulation volume: Due to the low pressure and low space velocity during the passivation process, temperature regulation relies mainly on the circulation volume. The level of circulation volume directly affects the passivation temperature, as well as the reaction rate of passivation and the temperature difference between the upper and lower layers of the catalyst. Therefore, during the passivation process, the circulation rate should not be too low. 5 When the temperature of the catalyst layer stops rising and shows a downward trend, and the oxygen content in the gas entering and leaving the tower is equal, it indicates that the passivation process has come to an end. If the catalyst needs to be replaced, merely to avoid accidents caused by severe oxidation during its removal, passivation can be carried out using the table below. It is also possible to first poison the catalyst and then pass air through it using a high-pressure pump for passivation. Passivation stage: Pressure of gas entering the tower, in MPa; Hotspot temperature, in °C; Temperature fluctuation, in °C/h. Notes: Cyclical cooling using hydrogen and nitrogen – up to 12; Operating temperature < 60; Pressure is reduced to 200°C; H2 concentration < 4%, O2 concentration < 0.1%; 0.2–0.3150; Pressure < 60; Replacement with inert gas during passivation; O2 concentration < 3%, temperature < 2, < 300, pressure < 50. End of passivation: O2 concentration ≥ 20%, temperature < 100℃

Take a look at this link to see if it’s helpful to you – our plant’s catalyst passivation solution for ammonia synthesis

The passivation of ammonia catalysts generally involves the following steps: 1. Displacement – The system is cooled to below 100°C, and pure nitrogen (of 99.9% purity or higher) is used for displacement to ensure that the H2 content in the system is ≤3%. The temperature should be below 50°C at this time. 2. Oxygen passivation: After circulating nitrogen until the temperatures in all sections of the catalyst bed have stabilized, pure air is introduced. The initial oxygen content should be less than 0.3% in order to cause a temperature rise in the bed; this temperature increase will first occur at the topmost part of the catalyst layer, with the highest temperature gradually moving downward. Control the temperature at the highest point; if the catalyst is still to be used, the stability at that highest point must not exceed 60℃ ； If the catalyst is no longer in use, the temperature can be adjusted to a higher level, but it must not exceed 100°C. Since the temperature we measure is the temperature of the gas phase, and there are also several layers of thermocouple sheaths providing insulation, the actual temperature of the catalyst particles will be much higher than the measured temperature. As the hotspot moves, it indicates that the oxidation rate of the catalyst under this oxygen content drive is close to dynamic equilibrium; therefore, the oxygen content is increased to boost the drive, or in other words to accelerate the oxidation reaction, with the oxygen content being raised gradually to 0.5%, 1.0%, 1.5%, 0.0%, …… 20.5%. Keep the catalyst completely in an air-filled environment, with the temperature not exceeding 50°C at this time. (Because the catalyst needs to be unloaded, the ambient temperature will not exceed 50°C.) If it is difficult to reach a bed temperature of 50°C, heating the inlet gas must be used to maintain the bed temperature. 3. Indicators of passivation completion: a) Oxygen content in the circulating gas ≥ 20.5% b) Catalyst bed temperature is around 50°C, with no significant temperature rise points in the bed c) Oxygen content at the inlet and outlet is the same. In this way, the passivation depth is controlled at around 10% (except when the passivation process exceeds the optimal temperature). 4. After reaching the state described in 3, unload after several hours of cycling. If it is still to be used, it needs to be packaged in a dry container and stored tightly, in a cool and well-ventilated place, away from direct sunlight. If it is no longer in use, it can be thoroughly rinsed with water right after being unloaded. 5. The passivated catalyst is reused, with heating carried out in the same manner as for the pre-reduced catalyst