Abstract: In a refining and chemical company in central China, the pressure drop in the vacuum tower of an 8.0 M/a atmospheric and vacuum distillation unit increased significantly in the middle and late stages of the second operation cycle after commissioning, which seriously affected the smooth operation of the unit and the total distillate yield. Through analysis, this was mainly caused by the coking in the washing section of the vacuum tower which resulted in the reduction of the gas-phase channel and the rise of the pressure drop, thereby leading to the reduction of the flash volume in the feed section and the decrease of the distillate yield of the vacuum tower. During the major overhaul in 2017, the bed height of the packing was modified and the corrosion and coking resistant surface treatment were carried out on the packing of the washing section. The pressure drop in the vacuum tower and the distillate yield of the unit were stable for 1 cycle after the modification. And coking of the packing was found to be significantly improved in the overhaul in 2021. The transformation results show that the application of surface modification technology, in combination with the adjustment of operation, can effectively slow down the coking trend of the packing surface in the washing section, avoid the reduction of the distillate yield due to the increase of the pressure drop of the vacuum tower, and achieve remarkable economic benefits.

Key words: vacuum tower, metal surface modification, coking, pressure drop, total distillate yield