Product Lift in Vacuum Columns
Background
A common process configuration in Refinery crude distillation units is an atmospheric distillation unit (ADU) with a downstream vacuum distillation unit (VDU). In the VDU, vacuum distillates are recovered from the long residue (LR) for use as feed to a conversion unit, such as a hydrocracker (HCU). Due to the good separation in a VDU column, it’s not unusual that the vacuum gas oil (VGO) properties are similar to the atmospheric gas oil (AGO) produced in the ADU. The short residue (SR) stream from the bottom of the vacuum column can be routed to coking or visbreaking process units.
A key operating parameter for an ADU is the cutpoint between LR and the AGO. The front-end (i.e. the lightest) components of the LR stream have a significant lifting effect in the VDU flash zone and, therefore, on the total VGO recovery.
Figure 1: Simplified Refinery Crude Unit (ADU and VDU)
Lift Effect
With the shift of gas oil from the ADU to the VDU, the partial pressure of the heavy-boiling components in the LR stream is reduced due to the higher amount of gas oil and lighter components. This reduction in partial pressure facilitates the vaporization of the heavy components (i.e. “lifts” the heavy components), particularly the heavy vacuum distillate components. The same effect is achieved when stripping steam is injected at the bottom of an ADU or VDU column.
“The reduction in partial pressure of heavy-boiling components due to a higher quantity of lighter-boiling components.”
For a constant flash zone temperature and pressure, a 1.0% weight shift on crude (woc) from the ADU to the VDU will result in an approximate 0.2% woc higher vacuum distillate yield at the expense of SR. Therefore, a gas oil shift from the ADU to the VDU improves the vacuum distillate recovery.
It should be noted that shifting ADU gas oil has several drawbacks that must be considered when optimizing the ADU-VDU cutpoint. These include,
- The increase in VDU column load can be problematic, especially if any column sections are already operating near their loading factor limits. This can result in poor separation and increased entrainment of SR in the HVGO and wash oil (WO).
- The latent heat of the gas oil shifted from the ADU to the VDU is recovered as lower-level heat (60-100°C/140-212°F in the VDU-TCR instead of 200-290°C/392-554°F in the ADU-LCR). As a result, the ADU furnace inlet temperature will decrease. This can be problematic if the furnace is limited, although the resulting lower furnace outlet temperature will compensate.
Recommendation
As a general guideline, a VGO yield of 4-8% weight on LR feed typically gives a good balance between the lifting effect and the drawbacks listed above. This VGO yield normally corresponds to an ADU furnace outlet temperature (FOT) of approximately 360°C/680°F.
To continuously optimize the cutpoint between LR and AGO (in addition to other crude unit operating parameters) given changing economics, equipment constraints, and product quality requirements, the benefits of a real-time optimization (RTO) system should be evaluated.