Regarding the issue of recycled hydrogen
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Dear experts, I have a question: Recently, the pressure drop in the reactor of our hydrogenation unit has been high. In order to maintain the situation until the next major maintenance session, management ordered that the speed of the recycle hydrogen compressor be reduced, thereby decreasing the amount of recycle hydrogen and lowering the pressure drop. This measure proved effective – the pressure drop was reduced by half a kilogram. What I want to ask is, if the original amount of circulating hydrogen was 10,000, and it drops to 8,000 after the speed is reduced, then where does that 2,000 units of hydrogen go? From a reaction principle perspective, as the light oil ratio increases, the reaction depth definitely increases, and so does the hydrogen consumption. But with a decrease in the amount of circulating hydrogen while the feed oil remains unchanged, will the reaction space velocity decrease? In that case, the degree of desulfurization should increase to some extent, or at least it shows that a higher hydrogen-to-oil ratio does not necessarily lead to a higher degree of reaction. This is especially true if the 2,000 units of hydrogen mentioned earlier are present in the reactor, which supports this idea. However, I myself think this theory is not valid; I hope experts can help me figure this out. Also, we suspect that the increase in bed pressure drop is due to a problem with the raw material filter, which fails to remove impurities. Even if the compressor speed is reduced to lower the amount of circulating hydrogen and thereby reduce the bed pressure drop, what’s the point of that? Is it to prevent high pressure differences from damaging the catalyst? Otherwise, why not increase the rotation speed to boost the cyclic power? I hope the experts can give some advice! ! Thank you first
I’m not quite sure about this. An increase in reactor pressure drop indicates carbon deposition and coking in the bed layer. Reducing the amount of recycle hydrogen will only exacerbate this trend. If the high pressure drop causes the recycle hydrogen compressor to operate under overload, then there’s no other choice but to reduce the hydrogen flow rate
I don’t quite understand either – why does the bed pressure drop decrease when the amount of circulating hydrogen decreases. Could it be that the gas used to measure the bed pressure drop comes from the circulating hydrogen, and as the amount of circulating hydrogen changes, the pressure drop also changes? Seeking advice
The outlet pressure of the reactor remains unchanged, while the inlet pressure decreases; as a result, the pressure difference naturally decreases
It’s possible that this is the case. Assume that the original air intake volume is 20,000 cubic units, of which 10,000 are recycled air and 10,000 are fresh air. The pressure difference is large. The one-time reaction rate is 75%. Now that the circulation volume has been reduced to 8 thousand, the one-time reaction rate has increased to 80%; the volume of fresh gas has been lowered to 8,500 cubic units, while the total inlet gas volume is 16,500 cubic units. This results in a lower pressure difference compared to the original value of 20,000, and the production volume also decreases
An increase in reactor pressure drop is usually caused by impurity blockages, catalyst pulverization, carbon deposition on the catalyst surface, coking, and a decrease in porosity. With the same gas flow rate, the pressure drop will certainly increase. Reducing the hydrogen circulation rate can help lower this pressure drop. If the hydrogen-to-oil ratio is still acceptable, then there is no major problem; otherwise, it may be necessary to reduce the load as well.
What was said upstairs is correct: when the amount of circulating hydrogen decreases, the pressure at the reactor inlet drops, while the outlet pressure remains unchanged; thus, the pressure drop naturally decreases. A decrease in the flow rate of recycled hydrogen leads to a lower hydrogen-to-oil ratio, which is unfavorable for the progress of the forward reaction.
Besides, where can one find hydrogen at a concentration of 2,000? If the system pressure remains constant, the amount of hydrogen in the system stays fixed; whatever amount of hydrogen is added must be equal to the amount that is removed. When the compressor slows down, it’s just the flow rate that decreases, meaning the circulation of hydrogen becomes slower.
I agree with what was said above: the amount of hydrogen hasn’t decreased; reducing the compressor’s speed only lowers its power, and the air flow rate also decreases
When the inlet pressure and inlet flow rate of the compressor drop to a certain level, surge occurs, which can in severe cases lead to an emergency shutdown of the compressor. Therefore, the reason your supervisor reduced the compressor’s speed is to decrease the pressure difference at the compressor’s inlet and outlet, thereby preventing surge. However, this will reduce the hydrogen-to-oil ratio and decrease the depth of the reaction, which is not conducive to the progress of the forward reaction. That is, it only lasts for a short time. The only option left is to reduce the feed rate. Additionally, the overall system pressure drop increases; aside from the rise in the reactor’s pressure drop, it is recommended to check the pressure drop in the heat exchangers. In many cases, ammonium salt crystallization in the heat exchangers leads to blockages in the tube bundles, thereby causing an increase in system pressure.