Problem analysis and countermeasures of the vacuum deep extraction in distillation unit

In 2017, cavitation occurred in the washing oil pump of the atmospheric and vacuum distillation unit. In order to prevent pump cavitation, the washing oil tank was operated with full liquid level. During the overhaul in 2019, it was found that the steel net of third line reflux filter was broken. There was more oil sludge in the third line reflux main and distribution pipe of the vacuum column. The closer it was to the end of the main pipe, the more oil sludge there was. There were 6 layers of packing in the washing section of vacuum column, and coking gradually increased from top to bottom. There were more coke powder and broken coke blocks in the washing oil collection tank and extraction outlet. Combined with the situation of overhaul, the reasons of washing oil pump cavitation and coking of packing layer in washing section were analyzed, and the measures to reduce coking of packing layer were formulated: (1) Replace all the packings in the washing section and clean up the third line reflux pipe during the overhaul. (2) During the production process, the third line reflux filter was checked and cleaned regularly, the wetting amount of the washing section packing was strictly controlled, and the atmospheric extraction rate was improved. During the implementation of vacuum deep extraction, the feed temperature of vacuum column was high and the residual oil cracking reaction was intensified, which led to the increase of vacuum top gas flow. The high content of organic sulfur in vacuum top gas led to the SO₂ mass concentration in the flue gas of vacuum furnace exceeding standard. Therefore, diesel absorption measures were added to desulfurization system of vacuum top gas, which effectively reduced the content of organic sulfur and total sulfur in vacuum top gas, and successfully achieved the standard emission of the flue gas of vacuum furnace.