Hydrocracking emergency shutdown!
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The hydrocracking unit of our plant is equipped with one recycle hydrogen compressor and two fresh hydrogen compressors (one in operation and one as backup). A nitrogen line with a nominal diameter of 25 is provided at the inlet of the cyclic hydrogen compressor (K102), while a nitrogen line with a nominal diameter of 100 is provided at the inlet of the fresh hydrogen compressor (K101/AB). 1. Our operating procedures state that when the hydrocracking unit is shut down urgently, it may be necessary to supply nitrogen (with qualified nitrogen). A recycle hydrogen compressor is used for replenishment instead of a new hydrogen compressor. 1. I don’t quite understand why a new hydrogen compressor isn’t used (the nitrogen inlet pipeline of the new hydrogen compressor is thicker than that of the recycled hydrogen compressor, allowing for more replenishment) 2. Is nitrogen displacement required for the compressor when adding nitrogen? Second, another question is: under what circumstances is it generally done to replace the compressor with nitrogen?
Regarding the emergency shutdown of the hydrogenation system and the introduction of N2, I believe it is done first to protect the catalysts, and second to purge the system to bring it into a safe state; in other words, it is a process of reducing temperature and pressure. After shutting down the feed pump and heating equipment, the remaining material will continue to react in the system for some time. To bring the reaction to an end as quickly as possible and to facilitate control, it is necessary to use the recycle hydrogen compressor to perform a large-scale circulation throughout the system. The new hydrogen compressor is not able to do this well, as it is installed in the middle of the main reactor to supply fresh H2 to the system. If I encountered this situation, I would also use a circulating hydrogen compressor. We use N2 purging when servicing compressors.
Since the new hydrogen compressor is used for pressure increase rather than circulation, compressors designed for circulation are generally used instead of new hydrogen compressors! ~
Nitrogen is added at the outlet of the circulating hydrogen compressor primarily to protect the catalyst in case of a leak in the high-pressure system. Hydrogen is not added at the inlet of the new hydrogen compressor because the pressure of nitrogen could be higher than the design pressure of the new hydrogen separator tank, posing a safety risk; moreover, there is a certain pressure drop after nitrogen passes through the compressor valves.
The nitrogen line at the outlet of the cycle compressor is used in case of accidents and for purging during startup and shutdown. The nitrogen line at the inlet of the new hydrogen compressor is used to pressurize the reaction system during the plant’s first start-up.
Our nitrogen setup is the same as yours, but after an emergency shutdown, we usually use a new hydrogen generator to produce nitrogen, applying a slight pressure increase to it. Of course, the circulation pump is also in operation to remove heat from the system as quickly as possible and cool it down to a safe level; once that is achieved, the nitrogen is removed, and the new hydrogen generator is prepared to supply hydrogen so that the system can be restarted, and so on. . . .
It is recommended that you check the pressure ratings of these two nitrogen pipelines. As an inert gas, the nitrogen at the inlet of the cyclohydrogen compressor is used to prevent further expansion of accidents, as well as to dilute and displace hydrogen.
Should the nitrogen valve be replaced when using nitrogen?
The new hydrogen compressor is used for pressure increase; it has a low displacement, of around 20,000 to 30,000 cubic meters per hour. The circulating hydrogen compressor, on the other hand, is used for circulation and has a high displacement, generally exceeding 200,000 cubic meters per hour. If the new hydrogen compressor is used to inject nitrogen for cooling, it operates in a one-way manner: the compressor supplies nitrogen while the pressure is released through a vent line. The displacement of the new hydrogen compressor is far insufficient to remove enough reaction heat, and it cannot prevent the reactor’s temperature from rising too high. The circulating hydrogen compressor is different – with a displacement of over 200,000 cubic meters per hour, it is capable of removing sufficient heat. In emergency situations, to avoid affecting the overall hydrogen pipeline system, the new hydrogen compressor is stopped immediately. The nitrogen line of the new hydrogen compressor is used, on one hand, for nitrogen purging of the system and for ensuring airtightness, and on the other hand, for nitrogen purging before machine maintenance.
Would you like to change it? It’s just that nitrogen molecules have a high molecular weight, resulting in a high outlet temperature