Regarding the shutdown phase of the hydrogenation unit
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In hydrogenation units, it is usual to first reduce the temperature and flow rate, then carry out a full cycle of circulating hydrogen, before raising the temperature to enable hot hydrogen to carry the oil. I have an unrefined idea: is it possible to avoid reducing the temperature, and instead keep the temperature at that level suitable for hot hydrogen to carry the oil from the moment the unit starts to be prepared for shutdown and reduced operation, while only reducing the feed rate?
For safety reasons, it’s better to follow the operating procedures. It’s not worth the extra time; keeping the temperature high all the time carries significant risks
The operating principle for hydrogenation is to first reduce the temperature and then reduce the flow rate. If only the flow rate is reduced without lowering the temperature, the reaction depth and speed will definitely increase under unchanged other conditions, and the reaction temperature is likely to get out of control, leading to temperature runaway accidents. Moreover, to ensure that there are no residual unreacted feed materials in the reactor catalyst bed during the period of hot hydrogen carrying oil, circulating oil is generally used in the reactor for about 4 hours after stopping the fresh feed, in order to prevent a temperature rebound during this period.
I’m not sure if my understanding is correct: how can it lead to overheating? It’s because the heat of reaction is too high, and this heat arises from the intense reaction of the raw materials. So when I reduce the amount used, there are fewer raw materials available; wouldn’t that mean the heat of reaction would also be less? Moreover, in this process, the recycled hydrogen is fully circulated, and the hydrogen-to-oil ratio increases as well; a large amount of hydrogen should quickly remove the heat generated by the reaction, or in other words, increase the heat capacity, thereby preventing the bed temperature from rising too high Moreover, high temperatures and a high hydrogen-to-oil ratio are favorable for the reaction; therefore, there should be no unreacted materials. I hope experts will not hesitate to offer their guidance!
If the flow rate is reduced, the space velocity decreases, and the reaction becomes more intense. If the temperature is not lowered and the flow rate is increased further, the intensity of the reaction will increase, posing a risk of excessive temperature rise!
Hydrogenation is exothermic due to the saturation of unsaturated bonds; with less feedstock, shouldn’t this heat amount decrease as well?
Deepens the reaction depth, promoting cracking and condensation coking reactions; there is a risk of excessive temperature rise
Isn’t there a large amount of recycled hydrogen?