Collapse of the hydrogenation catalyst bed and excessive pressure difference

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Poor uniformity in the radial packing of the catalyst within the reactor can lead to phenomena such as \"channeling,\" \"wall flow,\" and \"collapse\" of the reaction mixture within the catalyst bed. Most reactors in hydrogenation processes are trickle-bed reactors, and hydrogenation operations typically employ a high hydrogen-to-oil ratio; as a result, the flow velocity of the gaseous phase within the reactor is much higher than that of the liquid phase. This difference in flow velocities between the gas and liquid phases leads to phase separation. Once the catalyst bed becomes radially uneven, with different channel resistances within it, the gas stream, which is primarily composed of recycled hydrogen, tends to flow through the channels with lower resistance and easier passage, while the liquid stream, which is mainly made up of feed oil, is forced to flow through the areas of the catalyst bed where the particles are more densely packed. This leads to a separation between the gas and liquid phases, reducing the mass transfer rate between them and thereby diminishing the efficiency of the reaction. In this state, the heat transfer effect of the circulating hydrogen is poor, which easily leads to the formation of high-temperature \"hot spots\" in the bed. This accelerates catalyst coking in those hot spot areas, increasing the pressure drop in that region of the bed. As a result, less gaseous material flows through that section of the bed, and the reaction heat cannot be removed in time, causing the temperature at that point to rise further – creating a vicious cycle.