Catalyst deactivation scheme

The purpose of this method for passivating the catalyst is to enable its reuse.

For catalyst reuse: the hourly temperature rise at the outlet of the synthesis tower must be less than 5°C; for simple passivation: controlling the hourly temperature rise at the outlet of the synthesis tower to less than 25°C is sufficient.

Another version of the catalyst passivation plan for the methanol synthesis tower is provided here for everyone’s reference and study. * I. Principle of passivation: The XXX series catalysts used in this methanol synthesis tower (with the main chemical components being CuO, ZnO, and Al2O3) have CuO in their composition reduced to Cu+ or atomic copper during use; this Cu+ forms a solid mixture with ZnO, thereby maintaining the catalyst’s activity. When this atomic-state copper is discharged from the catalyst basket and comes into full contact with oxygen in the air, oxygen can rapidly penetrate the inner surface of the catalyst in a short period of time, generating a large amount of reaction heat. This leads to excessive local temperature rises or sharp increases in temperature differences. The resulting expansion stresses may cause certain components inside the synthesis tower to deform or crack, as well as lead to catalyst sintering, thereby damaging those components; or the catalyst may adhere to the interior parts of the synthesis tower. The passivation of a catalyst refers to the slow oxidation of the catalyst using oxidizing agents before it is removed from the synthesis tower, thereby forming an oxide coating on its surface and preventing further oxidation reactions once the copper catalyst is taken out. II. Passivation Method: During passivation, the compressor must be turned on to drive the circulation of the entire synthesis loop; nitrogen is used as the carrier gas for this circulation, and oxygen is added gradually. The following reaction takes place on the catalyst: 2Cu + O2 → 2CuO – ΔH (31416 kJ/mol). Since the reaction between elemental copper and oxygen releases a large amount of heat, an increase of 1% in oxygen concentration can result in a temperature rise of about 100°C at the beginning of the passivation process. To prevent local overheating that could damage the components inside the tower, it is necessary to maintain a stable temperature in the catalyst bed. III. Deep passivation of the catalyst: If the catalyst is not fully passivated, when the manhole of the synthesis tower is opened for natural ventilation, a large amount of air enters the tower. Given sufficient time, the catalyst reacts vigorously with oxygen; as a result, the catalyst may stick to the tubes inside the synthesis tower, or local areas of the catalyst bed may reach high temperatures, leading to catalyst sintering. The catalyst that has not reacted internally will undergo intense oxidation reactions upon coming into contact with large amounts of air during removal, which can cause the catalyst to smoke or even catch fire, posing a threat to the safety of the construction operations. To prevent the occurrence of such conditions at the site, deep passivation is required, so that all the copper atoms on the surface of the catalyst react with oxygen to form a stable and complete oxide coating. This oxide layer prevents oxygen from further reacting with the copper atoms inside the catalyst, thereby avoiding catalyst sintering; hence, the high-temperature oxygen immersion passivation method is employed. The passivation curve is shown in the figure. wps2A4.tmp.jpg 1. Temperature increase: In the nitrogen displacement synthesis system, the volume fraction of flammable gases should be ≤0.2%. The compressor is started to establish a nitrogen circulation; the flow rate of nitrogen must be at least 7000 Nm3/h. The pressure from the nitrogen supply valve at the compressor inlet to the compressor outlet should not exceed 0.6 MPa, while the pressure within the synthesis system should be around 0.35 MPa. Online analyzers are installed at the inlet and outlet of the synthesis tower. The drum liquid level is within the normal range; continuous water feeding is in progress. Open the start-up injector of the synthesis tower to keep the inlet temperature of the synthesis tower stable at 100°C. When steam begins to emerge from the vent valve of the synthesis drum, reduce the opening of that vent valve in order to maintain the pressure in the synthesis drum and keep the inlet temperature of the synthesis tower stable. During the passivation process, the principle for increasing oxygen content is to ensure that the oxygen level at the exit of the synthesis tower is equal to that at the inlet, while keeping the temperature at the exit of the synthesis tower constant. And it follows the principle of \"increasing oxygen without increasing temperature, and increasing temperature without increasing oxygen.\" 2. Early stage of passivation: Oxygen is initially introduced to achieve a φ(O2) level of 0.25%. In the compression and synthesis section, compressed air is gradually added to increase the oxygen content at the inlet of the synthesis tower, until φ(O2) in the gas entering the tower stabilizes at 0.25%. The time required to raise the φ(O2) level to 0.25% must be no less than 2 hours, with this level being maintained for 1 hour ； Once the oxygen contents at the inlet and outlet of the synthesis tower become equal, adding air will cause the temperature at the outlet of the synthesis tower to rise gradually; the inlet temperature can be maintained at 100°C by reducing the opening degree of the start-up injector or by adjusting the pressure in the synthesis drum. 3. Intermediate passivation stage: ① Increase φ(O2) to a stable value of 2.5%: While maintaining the inlet temperature of the synthesis tower at 100°C, gradually increase φ(O2) at the inlet of the synthesis tower (by 0.25% each time), depending on changes in the oxygen content at the outlet of the synthesis tower and its outlet temperature, up to a maximum of 2.5%. ②100°C passivation zone: The inlet temperature of the synthesis tower is maintained at 100°C by reducing or increasing the opening degree of the start-up injector, or by adjusting the pressure in the synthesis drum. Allow oxygen to react layer by layer from the upper part of the catalyst downward. When the temperature reading at the outlet of the synthesis tower begins to drop, it indicates that the reaction front of the catalyst is reaching the bottom of the tower. Once the reaction front reaches the bottom, the oxygen consumption rate will decrease, as well as the amount of heat released during the reaction. To keep the φ(O2) at the inlet of the synthesis tower below the maximum allowable value of 2.5%, it is necessary to gradually reduce the amount of air supplied in advance, adjust the opening degree of the inlet valve of the start-up injector, or increase the pressure in the synthesis drum in order to maintain stability in the outlet temperature of the synthesis tower ； It ends when the oxygen content at the inlet and outlet of the synthesis tower is equal. ③150°C passivation zone: When oxygen consumption ceases, the supply of air should be stopped (at this point, the oxygen content at the inlet and outlet of the synthesis tower is equal). The inlet valve of the start-up injector should be opened wider or the pressure in the synthesis drum increased, in order to raise the temperature at the inlet of the synthesis tower to 150°C and maintain it there. At this point, the passivation reaction will accelerate again, and oxygen will be consumed once more. Depending on the changes in the oxygen content at the outlet of the synthesis tower as well as the temperature there, the value of φ(O2) at the inlet of the synthesis tower will be increased gradually (by 0.25% each time), up to a maximum of 2.5%. This allows oxygen to react layer by layer from the upper part of the catalyst downward, until the oxygen content at the inlet and outlet becomes equal. ④200°C passivation zone: Raise the inlet temperature of the synthesis tower to 200°C again and repeat step ③ above. Under a φ(O2) of 2.5%, oxygen was allowed to react layer by layer from the upper part of the catalyst downward until the oxygen content at the inlet and outlet became equal. 4. Oxygen immersion period: Raise the inlet temperature of the synthesis tower to above 210°C, with the outlet temperature not exceeding 225°C. Over a period of more than 1 hour, the φ(O2) at the inlet of the synthesis tower was gradually increased to 10%. By controlling the pressure in the synthesis drum or the amount of steam supplied by the start-up injector, the temperatures at the inlet and outlet of the tower were kept within the specified ranges, and this condition was maintained for several hours, until no more heat release was detected in the entire bed layer and the outlet temperature stopped rising. 5. Completion of passivation: Over a period of not less than 3 hours, the gas in the system should be gradually replaced with air (with φ(O2) increasing by 4% per hour), and this process should continue for 4 hours. It should be ensured that φ(O2) at the outlet of the synthesis tower remains above 20%, after which the compressor should be shut down. At this stage, the water on the shell side of the synthesis drum and synthesis tower is not discharged yet; it will be emptied completely after the catalyst has been removed. The system is gradually reduced to atmospheric pressure through the compressor’s vent, allowing for natural cooling. Thus, the catalyst deactivation process is completely complete.

That initial 0.2% is almost impossible to achieve.

The plan is good and worth considering