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The vacuum distillation (VD) unit in the petroleum refinery separates the heavier fraction of crude oil from the atmospheric distillation unit (ADU) into light vacuum gasoil (LVGO) and heavy VGO (HVGO) under a vacuum to avoid thermal cracking of the heavier hydrocarbons. The two-phase mixture of the reduced crude oil (RCO) liquid and vapor from the ADU enters the VD column flash zone and flashes further under low pressure.
While the lighter materials, mainly VGO as vapor, move up the column, the heavier liquid materials, vacuum residue (VR), move to the column bottom. However, some VR liquid droplets are entrained within the vapor and must be washed off to maintain HVGO product quality. A slop wax or wash bed is used to separate out the entrained liquids from the vapor.
Background
The vapors rising from the flash zone are contacted with the hot internal reflux of HVGO from the VDU inside the slop-wax bed, as shown in Fig. 1. Normally, structured packing/grid is used for contacting the wash oil with the vapor carrying the entrained droplets due to the low liquid retention property and, consequently, low residence time of the liquid. The slop wax is drawn from the bottom of the slop-wax bed, and it is considered as HVGO from the slop-wax bed in liquid form, i.e., overflash and entrained VR from the flash zone.
Fig. 1. Wash-zone section of vacuum column.
Operation trends
At present, a growing trend among refiners is to maximize VGO recovery from the crude unit. However, the 95% cutpoint of VGO exceeds the conventional operational temperature limits of 565 [degrees]C to 580 [degrees]C. Increasing the VGO cutpoint comes with the potential to increase coke formation inside the slop-wax or washing bed unless proper engineering design and operational measures are applied.
Engineering design plays an important role in determining the coking rate inside the slop-wax bed as defined by the liquid retention time on the packing, vapor distribution within the packing, entrainment from the flash zone, etc. From the operations point of view, controlling the HVGO internal reflux at optimal values is most critical. With an increase in the internal reflux, the HVGO product becomes a purer product but at the expense of lower HVGO yields. Conversely, providing very low...