Problems with the shutdown of hydrogen production facilities

The inlet temperature of the converter should be reduced to below 450°C, and the inlet temperature of the hydrogenation reactor should be reduced to below 280°C. A large-scale circulation within the plant should be established gradually, with feedstock supply and hydrogen addition stopped. If the temperature in the hydrogenation reactor rises rapidly, the medium-pressure gas should be removed and the system filled with nitrogen for displacement; at this point, methanation reactions have already begun in the hydrogenation reactor.

What is your hydrogen production medium?

Currently, hydrogenation cracking off-gases and natural gas are mainly used

I believe that cooling the reactor inlet is intended to prevent methanation, while cooling the conversion inlet is done because a decrease in flow rate corresponds to a reduction in space velocity, which increases the reaction depth; high temperatures can easily lead to coking. That’s my opinion. But I’m skeptical about the way you have stopped operations; with medium-pressure shift gas undergoing large-scale circulation at this temperature, aren’t you worried that the catalysts in the hydrogenation reactor might get reduced?

Ah, I’m a beginner too; I don’t understand it

After modifying the cycle, the hydrogenation reactor is removed; reduction will not occur

The hydrogenation reactor is disconnected directly and routed via a bypass line; then the system pressure is reduced, the circulation rate is increased, nitrogen is used for purging, steam supply is stopped, the purification is completed, cooling is finished, and the compressor is shut down.

How to cool down after the reactor is removed?

Shutting-down plan: Normal shutdown. The hydrogen production unit can undergo a planned shutdown (normal shutdown) after the hydrogenation unit stops using hydrogen. I. Preparation work: 1. Check the nitrogen line for oil; nitrogen can be used only after ensuring there is no oil present. ⒉The standby dry gas compressor passed the trial operation. ⒊Ensure an adequate supply of nitrogen is available. II. Reducing the load: 1. After receiving the shutdown notice, gradually reduce the air intake volume. Up to 30% load, the load reduction rate is 5% per 20 minutes. Pay attention to controlling the inlet and outlet temperatures of the preheating furnace and the conversion furnace. ⒉As the intake air decreases, the process parameters are adjusted accordingly. ①Converter inlet temperature: 450–470°C ② Converter outlet temperature: 720°C ③ Water-to-carbon ratio: 3.5–4.5 (applicable during startup and shutdown) ⒊ Principle for reducing load: First reduce the amount of feedstock, then reduce the steam supply, and finally lower the furnace temperature. III. Cutting off the PSA system 1. As the intake air volume decreases, the PSA system adjusts the adsorption time. ⒉After the load is reduced to 30% and operation stabilizes, the fuel supply to the converter is gradually decreased; it is also possible to shut off several burners. At the same time, slowly close the desorption gas valve; the temperature of the conversion furnace is adjusted using fuel gas. Pay attention to the negative pressure in the conversion furnace during adjustment. The desorption gas is sent to the flare line via the vent line. ⒊PSA shutdown: Manually press the PSA stop button to shut down the PSA system; at this point, the system pressure is controlled through the intermediate pressure relief valve, and the manual valve for the PSA system is closed. After complete shutdown, the PSA system is purged with N2, and then filled with N2 to maintain a pressure of 0.2 MPa. IV. Removal of raw materials and establishment of a circulating cooling system in the large-scale system: 1. After the PSA unit is taken out of service, operations for a circulating system in the desulfurization, conversion, and medium-pressure transformation systems are established. 2. Slowly open the valve on the inlet line of the medium-pressure return compressor, while slowly closing the regulator valve for the feed gas entering the unit. Maintain the compressor inlet pressure between 0.1 MPa and 0.7 MPa, with the system pressure around 1.0 MPa. After closing the feed regulator valve, close the manual valves located before and after it. During the switch-over process, be sure to monitor closely that the compressor inlet pressure does not exceed the specified limits or become negative. Establish a large-scale system circulation process. Ensure that the hydrogen purity in the medium-pressure gas is above 60%; reduce the steam input to the converter by about 8 t/h, keep the H2O/H2 ratio below 7, and maintain a steam flow of 110 Kg per furnace tube. 3. After increasing the circulation rate of the unit, the hydrogenation reactor was immediately disconnected, and the system was adjusted to operate at the maximum circulation rate for cooling. 4. Adjust the preheating furnace to cool at a rate of 30°C/h, and the conversion furnace should also cool at a rate of 30°C/h. When the inlet temperature of the converter is 400–420°C and the outlet temperature is 600°C, and the bed temperature of the medium-shift reactor is 200–220°C, steam supply is stopped. The control valve for the steam supplied to the converter is gradually closed, followed by closing the manual valves before and after that control valve. A drain valve in the pipeline is opened to drain excess liquid. When steam supply is cut off, the pressure in the drum is controlled through its vent to prevent overpressure. After the steam is removed, the medium-temperature reactor is removed. When the steam distribution is cut off. After all the liquid level control valves of the respective water separation tanks have been closed, stop the acid water pump. Stop the stripping steam to the acidic water stripping tower. 5. After the conversion cooling, stop feeding steam at 3.5 MPa into the system. ①Steam is vented through a silencer to control the drum pressure. ②The steam produced is routed to a 3.5 MPa steam pipeline network; as the converter cools down, when the pressure in the drum falls below 3.2 MPa, the pressure is controlled by venting it through a silencer. The drum liquid level is maintained at 60%. 6. Desulfurization and conversion continue with a cooling rate of 30°C/h. When the temperature at the outlet of the conversion furnace falls below 400°C, nitrogen is introduced at the compressor inlet to completely displace the hydrogen from the system. After the hydrogenation and medium-shift reactors have also been purged with nitrogen, these reactors are put into operation to facilitate further cooling. 7. When the preheating furnace cools down to 150°C, turn off the heating furnace, fully open the flue dampers, and allow the heating furnace to cool down naturally. The converter should be cooled at a rate of 30°C/h; steam supply should be cut off to prevent the converter from overheating, and some burners can be turned off as required by the actual situation. When the exhaust gas temperature at the converter outlet reaches 200°C, the fuel gas to the converter can be shut off, causing the converter to go out. At the same time, close the main gas valve and allow any remaining pressure to be released into the flare system. When the temperature at the outlet of the converter flue gas is less than 100°C, stop the blower and exhaust fan in accordance with their respective operating procedures. Open the fire viewing port and use the inlet and outlet valves of the exhaust fan for natural cooling. 8. After shutting down the furnace, open the vent valve at the top of the steam drum to release all the steam, stop the boiler feed water pump, and cease feeding water to the steam drum. When the boiler water temperature drops to 70°C, the drum water can be discharged as appropriate. After releasing the drum water, nitrogen should be introduced into the drum to maintain a pressure of over 0.1 MPa. 9. During the systematic cyclic cooling process, gas samples from various locations are taken for analysis, with 2 analyses per shift. The parameters are considered acceptable when H2 < 0.3%, CH4 < 0.3%, CO2 < 0.3%, and C1 and C2 < 0.5%. After the replacement is successful, stop the compressor in accordance with the operating procedures, and fill the system with N2 to a pressure of over 0.1 MPa to maintain that pressure. 10. According to the specific maintenance requirements, install or remove blind flanges as necessary, purge all areas thoroughly, open the vent valves of all containers, and open the drain valves and drip valves at all low points. Once the conditions for performing hot work are met, hand over the area for maintenance.

IV. Removal of raw materials and establishment of a circulating cooling system in the large-scale system: 1. After the PSA unit is taken out of service, operations for the circulation within the desulfurization, conversion, and medium-pressure transformation systems are established. Is it not necessary to conduct analytical tests on the carbon monoxide and carbon dioxide levels in the medium-temperature converted gas before establishing a cycle for the desulfurization, conversion, and medium-temperature transformation systems? According to your plan, the reactor inlet isn’t cooled yet, right? Aren’t you worried about a methanation reaction occurring? 4. Adjust the preheating furnace to cool at a rate of 30°C/h; … after the steam supply is cut off, the medium-temperature reactor shall also be shut down. When the steam distribution is cut off. After all the liquid level control valves of the respective water separation tanks have been closed, stop the acid water pump. Stop the stripping steam to the acidic water stripping tower. After the steam supply is cut off, the temperature in your medium-temperature reactor should still be around 200°C. At this point, if the medium-temperature reactor is shut down, how can its temperature be reduced?