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At the end of the year, due to a sudden loss of flow at the bottom of the vacuum distillation tower in the slurry-bed residue hydrogenation process, emergency repairs were carried out using the \"vacuum distillation tower bypass for emergency shutdown\" and \"vacuum distillation tower bypass isolation mode\". Lacking experience and means to make predictions, significant difficulties were encountered and the work had to be repeated; looking back on that period after operations were resumed, one truly realizes how tough such situations can be. The slurry in the vacuum tower is fed at 360°C to enable saturated flashing and rapid separation; the bottom stream is quenched at 300°C. Why does coking occur both at the top and bottom of the feed? The analysis is as follows: The feed composition of the slurry-bed vacuum tower includes converted aromatics, unconverted wax oil (the dispersed phase), washing oil, converted diesel, and solid insolutes (metals, MoS2 coke). The feed distributor rapidly separates the mixture into a rising gas phase and a descending liquid phase; the rising gas phase carries solid insolutes, aromatics, and distillates. During this upward movement, heat and mass transfer occur within the packing material, during which aromatics and insolutes deposit in the gaps between the packing elements and condense into semi-coke, which in turn blocks those gaps. Over time, this leads to an increase in pressure drop, a decrease in evaporation rate, and a reduction in the thermal load ； Depressurized dehydrogenation of the descending liquid phase, concentration and aggregation of aromatic hydrocarbons; condensation and deposition due to low-speed residence on the chevron baffles. At locations with relatively low flow rates and long residence times, a lumped form of coke is formed (attached, suspended, hung, or stuck). During operation, this coke can break off unstablely and block the conical extraction outlet. Therefore, in addition to suppressing coke formation through reaction kinetics, another prerequisite for the long-term operation of slurry bed reactors is the application of engineering expertise in the design of the depressurization tower. It is recommended that to achieve solid-liquid separation through vacuum flashing in a slurry-bed vacuum tower, two key factors must be properly controlled: first, the simpler the internal structure, the better ； Secondly, there is no need to pursue a high evaporation rate (in fact, due to the strong intermolecular forces of aromatic hydrocarbons, the ideal liquid yield at the bottom of the tower is limited to the target value); by addressing these two aspects, the dilemma between the economic efficiency of separation and the periodic nature of operation can be resolved.