On the control of hot spot temperature in reactor beds

I fully understand the difficult problem you are facing at the moment. A hot spot at 425°C occurred in the refining reactor, and the cold hydrogen control failed; this is a very dangerous and urgent signal during the operation of the hydrogenation unit, indicating that local coking and catalyst deactivation may be accelerating, with a risk of temperature out of control and reactor damage. Under the premise of \"maintaining production,\" ensuring the safety of the equipment must be the top priority; no operations shall be carried out at the expense of safety. The following are the response strategies that can be considered and implemented, ranked by order of priority: 1. Immediate emergency adjustment measures (top priority: reducing the intensity of hotspots). The purpose of these actions is to cool down the hotspots as much as possible without causing a significant reduction in performance, thereby buying time for further action. 1. Try to adjust the cold hydrogen distribution: · Although the opening degree of the cold hydrogen valve in this bed layer has reached 65%, please check whether there are any blockages or flow deviation issues with the cold hydrogen injector. If possible, try to slightly reduce the cold hydrogen valve value for this bed layer (for example, to 60%), and then quickly increase it again (to over 70%), using the changes in flow rate and pressure to impact any potentially clogged distributors in an attempt to improve the mixing of cold hydrogen with the reactants. This operation requires extreme caution, with close monitoring of heat spot temperature changes. · Check whether the outlet temperature of the upper reactor is too high. If the reaction depth in the upper layer is too great, resulting in a large temperature difference of the material entering this bed layer, it will also exacerbate flow deviation. Consider appropriately reducing the setpoint for the outlet temperature of the upper reactor. 2. Slowly reduce the reaction depth: · Slightly lower the reactor inlet temperature: Slowly decrease the inlet temperature of the refining reactor at a rate of 1–2°C per hour. Lowering the temperature can slow down the reaction rate and reduce heat release, thereby cooling the reaction at its source. This is one of the most effective and safe methods. · Consider reducing the raw material feed rate slightly: if conditions permit, and without affecting the balance in the upstream and downstream processes, reducing the feed rate by 2-5% can lower the thermal load on the entire reactor, thereby providing more room for temperature control. 3. Adjust the quench hydrogen injection strategy: · You mentioned that the cold hydrogen valve at the bottom of the bed has been closed. You can try opening the cold hydrogen valve of the lower bed layer slightly (for example, 5%-10%). Although this will result in a lower temperature in the lower bed layer, it allows for a reduction in the overall temperature of the material entering that bed layer, thereby providing a cooler \"cold source\" for the hot spots in the upper layers. It is a strategy of sacrificing the minor to save the major. II. Medium-term operation optimization strategies (stabilizing production and delaying coking) If, through the aforementioned emergency measures, the hotspot temperature can be maintained at 425°C or slightly reduced, with no further rapid increase, then the following strategies can be implemented to sustain operations. 1. Optimize hydrogen distribution and hydrogen-to-oil ratio: · Within the limits permitted by the compressor’s capacity, appropriately increase the amount of circulating hydrogen or the amount of fresh hydrogen supplied. Increasing the hydrogen partial pressure helps to suppress coking reactions and enhances the heat-carrying capacity of hydrogen, thereby facilitating the removal of heat from the bed. · Ensure that the hydrogen-to-oil ratio operates at the upper limit of the design value; a higher hydrogen-to-oil ratio means that more heat can be removed per unit of feedstock. 2. Monitor the properties of raw materials: · Analyze the properties of the current crude oil immediately, with a focus on whether levels of asphaltenes, residue, and metal content are above acceptable limits. Low-quality raw materials are the main cause of localized coking. · If the quality of the raw materials does deteriorate, strengthen communication with the scheduling team to arrange for high-quality materials to be used instead, or increase the proportion of diluents (such as light oil) used. 3. Increase the frequency of key monitoring: · Raise the monitoring frequency of hotspot temperatures to every 15 minutes or even less. · Closely monitor the trend of changes in reactor pressure drop. An increase in pressure drop is the most direct sign of worsening coking. If the pressure drop begins to rise more rapidly, more stringent measures must be taken without hesitation. III. Parking contingency plans required (safety baseline) It must be clearly understood that hot issues are likely to cannot be resolved completely, only delayed. Once the following situations occur, the shutdown procedure must be carried out without hesitation; otherwise, it may lead to damage to the reactor’s internal components, catalyst sintering, or even more serious safety accidents. 1. Temperature out of control: The temperature at hot spots continues to rise rapidly, exceeding 450°C and remaining uncontrolled. 2. Sudden increase in pressure drop: The reactor’s pressure drop increases by more than 30%-50% compared to normal levels, or there is a continuous upward trend on an hourly basis. 3. Deterioration of product quality: Due to localized deactivation of the catalyst, parameters such as the color and nitrogen content of the refined products (such as diesel) continue to worsen, and this cannot be corrected by adjusting the operating conditions. The shutdown plan should include: · Formulating detailed plans for reducing flow rate, temperature, pressure, and carrying out replacement during shutdown. · Prepare a testing plan after shutting down the operation: use infrared thermal imaging to determine the exact location of hot spots, and employ catalyst diameter meters to detect voids and coked areas in the catalyst bed, thereby providing a basis for deciding whether it is necessary to adjust the setup or replace the catalyst. Summary and Recommendations Under the current circumstances, the key to maintaining production is \"reducing temperature, suppressing coking, and ensuring safety.\" Immediate action checklist: 1. Slightly reduce the inlet temperature (highly recommended). 2. Try fine-tuning the cold hydrogen valve to target areas that may be blocked. 3. Consider slightly opening the lower layer of cold hydrogen to reduce the feed temperature. 4. Increase the amount of circulating hydrogen to raise the hydrogen partial pressure and the hydrogen-to-oil ratio. 5. Strengthen monitoring, especially of temperature and pressure drop trends. Please remember that all these measures are aimed at buying time for the final decision. It is essential to maintain close communication with your technical team, equipment manufacturers, and catalyst suppliers to jointly assess the risks. Work safety must always come first; once it can no longer be controlled, shutting down operations promptly is the most economical and safest option. I hope these suggestions will be helpful to you!