Several issues regarding bed temperature spikes in hydrogenation units

What is the normal operating temperature? What is the interlock value? To deal with excessive temperature in hydrocracking, pressure relief is used for emergency cooling – and pressure relief is the most effective method for cooling! When the pressure drops to a certain level and the bed temperature has not yet decreased, nitrogen may be used to continue cooling down. However, your facility has an adequate supply of N2, it’s just not very convenient!

"The inlet of the refining reactor is equipped with a high temperature alarm and interlock, and the same type of interlock is installed at both the inlet and outlet of the cracking reactor. It’s not clear what actions will be triggered by such interlocks; moreover, the reaction temperature is determined based on the properties of the catalyst, so it’s not certain that it must be exactly this or that temperature. You have quite an adequate supply of nitrogen for emergency use – 20 MPA. In an emergency, this nitrogen is sent through the outlet pipeline of the recycle hydrogen compressor and injected into the reactor to cool the temperature of the entire catalyst bed.

The operating temperature and interlock values have not been determined yet, as we still don’t know what kind of catalyst it is; the relevant parameters have not been provided yet. Our emergency nitrogen supply is directly connected to the inlet pipeline of the reactor

I believe the interlock at the inlet of the refining reactor is connected to that of the heating furnace. When the temperature at the inlet of the refining reactor is high, which means the temperature at the outlet of the furnace is also high, it causes the temperature of the reactor bed to rise, thereby intensifying the cracking reaction and leading to excessive temperature rises. When the temperature at the inlet of the refining reactor exceeds the set interlock value, the system automatically controls the amount of gas supplied to the heating furnace to prevent the temperature from rising too high. As for nitrogen, there is no need to add nitrogen if the bed temperature does not rise excessively; nitrogen is used for displacement in cases of excessive temperature. Under normal circumstances, emergency pressure relief will reduce the bed temperature by 30 degrees below the reaction temperature. If the temperature remains high even after the pressure is reduced to 0.04 MPa, then nitrogen must be used for displacement.

There is temperature control at the outlet of the heating furnace; my description was incorrect. The alarm interlocks for the two reactors are located at the upper part inside the reactors, near the inlet, and also at a location inside the reactors near the inlet and outlet, with a three-way temperature comparison system in place to trigger high-temperature alarms

We are a hydrogenation refining unit with a pressure of 6.0 MPa; the catalyst used is an FHJ nickel-based catalyst. Excessive levels of CO and CO2 in the hydrogen caused the catalyst to overheat. The most effective solution was to shut off all feed streams, turn off the heater, and rapidly vent the pressure down to below 0.5 MPa at a rate of 0.7 MPa/min, after which high-purity nitrogen was introduced to cool the catalyst bed.

When the bed temperature exceeds the limit, the quench hydrogen should first be increased, or even turned up to full capacity. If this still isn’t sufficient, nitrogen must be injected to lower the temperature (provided that the purity of the nitrogen is maintained; low purity may cause the catalyst to burn or even lead to an explosion due to the mixture of hydrogen and oxygen). If there’s really no other option, then we have to activate the 7-bar air release system. However, our setup also includes a 21-bar release system as a last resort. Activating the 21-bar system means that the air supply will have to be stopped, haha

I’m sorry; the answer I gave regarding this issue yesterday was incorrect. Hehe. Injecting nitrogen into the reactor to lower the temperature is a method used when the bed temperature remains high even after venting to relieve pressure. This procedure is rather complicated, as it requires changing the feed to the compressor to nitrogen. Hehe. This applies to our facility, and I welcome any corrections.

For hydrocracking, we only have an interlock for the temperature at the reactor outlet; the main burner is turned off, but the pilot light remains active. There is no interlock at the inlet side, and I don’t think it’s necessary. If the instruments fail, it could lead to unplanned shutdowns. It’s still necessary to accurately judge situations involving excessive temperature rise. Nitrogen protection is also used, as long as its purity remains above 99%. Otherwise, the presence of oxygen might cause further temperature spikes. In theory, as long as the pressure is reduced to the lowest level possible, excessive temperature rise will not occur

I think the high temperature alarm at the inlet of the refined reactor is used to interlock with the feed heater, while the interlocks at the inlet and outlet of the cracking reactor are connected to the emergency pressure relief system and the heater!

In the case of an excessive bed temperature, the cold hydrogen valve is first opened to carry the hot reaction zone out of the reactor. Generally, the opening degree of the cold hydrogen valve is set at 60%, mainly to enable a rapid increase in the amount of cold hydrogen in the event of an accident! If the temperature still cannot be controlled after introducing cold hydrogen, emergency pressure relief is required. The cold hydrogen valve should still be kept at full open during pressure relief. When using nitrogen protection, it is essential to ensure the purity of the nitrogen; otherwise, it is very likely that the temperature will rise excessively again!

Generally, thermal cracking reactions occur when the temperature exceeds around 480°C. If no measures are taken, as these reactions progress to a certain extent, the heat released during hydrogenation becomes significant, and the reaction temperature can reach around 815°C within a few minutes. During operation, it is essential to strictly prevent overheating or sudden temperature spikes. Reactor overheating phenomenon: The reactor temperature rises normally. Reason for reactor over-temperature: The outlet temperature of the reaction feed heater is excessively high ； Sudden changes in the amount and properties of raw materials ； For some reason, the amount of circulating hydrogen has decreased sharply ； Quench hydrogen regulation failed. Handling of reactor over-temperature: 1. If the temperature at any point in the bed exceeds the normal operating temperature by 8°C, the following measures should be taken: reduce the outlet temperature of the reaction feed heater ； And increase the amount of cold hydrogen supplied at the inlet of the overheated bed layer to reduce the temperature of that layer ； Increase the amount of circulating hydrogen and reduce the temperature in each bed layer ； Continue feeding at the normal feed rate ； If necessary, measures such as reducing the temperature of the feed buffer tank and bypassing the feed heat exchanger can be taken. 2. If it is determined that the cause of excessive bed temperature is a change in the properties of the raw material, operations should be adjusted promptly, or a decision should be made based on the actual situation as to whether to stop feeding from the tank area; if feeding is stopped, the circulation rate can be increased. Avoid excessive system pressure drop, which can lead to a reduction in the amount of circulating hydrogen. If the system pressure drop increases, the most likely cause is blockage of the reactor internals; in such cases, the reactor inlet temperature should be reduced promptly, and the plant should be shut down urgently. If rapid cooling hydrogen regulation fails, contact the instrumentation team promptly for handling. 3. If the over-temperature condition cannot be controlled even after 8–10 minutes and the bed temperature continues to rise, reaching emergency conditions, the emergency pressure relief system shall be activated ； Activate the emergency pressure relief; at this point the heating furnace will automatically enter interlock mode, and the gas valve of the heating furnace should be closed on-site ； Stop adding new hydrogen ； The system has been changed to a cold large-cycle mode, with a circulation volume equal to the normal feed rate ； Close attention should be paid to the high liquid level during pressure relief ； Stop the water injection. 4. If the temperature continues to rise, the pressure is reduced to 1/4 of the operating pressure. When the reaction temperature drops to 260°C, feeding to the reaction system is stopped; the catalyst is subjected to thermal hydrogen stripping, and the distillation system is switched to a short cycle. It is decided whether to shut down the system based on the circumstances. Once the temperature is under control and the cause of the issue has been identified, it can be decided whether to stop pressure relief and resume normal operations based on the actual situation. When feeding again, the reactor inlet temperature must be at least 55°C lower than the normal operating temperature. This post was last edited by hgkklzp on 2009-2-28 17:13]