Consequently, it is attractive to recover as much hydrocarbon feedstock, such as heavy vacuum gas oil (HVGO), as possible from the atmospheric residue (AR) that is routed to conversion units.

Shell Global Solutions’ deep-flash, high-vacuum technology has been developed through extensive research on mass transfer and separation equipment, and has also been supported by operating experience from numerous Shell and Shell-advised units over many years. This technology includes five key design features that can be tailored to meet a refiner’s specific revamp requirements. These are the:

  1. proprietary furnace coil design;
  2. Schoepentoeter* inlet device;
  3. wash-oil section design;
  4. insulated, low pressure drop draw-off trays; and
  5. direct contact condensation sections.

In this article, we profile two HVU revamp projects at European refineries. When scoping these projects, it became clear that both refiners had limited capital and a strong desire to minimise unit downtime, so Shell Global Solutions’ consultants, working with the refinery teams, elected to install only a selection of the key design features. This demonstrates the flexibility of the technology.

Case study 1: Increasing HVGO yield

This refiner’s objective was to increase recovery of HVGO from the AR to feed the hydrocracker and, at the same time, improve the reliability of the HVU so that a longer run length, a target four to five years, was achievable between maintenance turnarounds. For this project, the allowable changes were restricted to the HVU column internals. The revamp was successfully executed using Shell Global Solutions’ deep-flash technology.

To increase the feed rate to a hydrocracker, more hydrocarbons must be recovered from the vacuum residue (VR). This means that the volume of the HVGO stream must increase, up to acceptable limits of heavy metals (vanadium and nickel) and Conradson carbon residue, for reliable hydrocracker operation.

The modifications were carried out during a routine refinery maintenance turnaround and are described in Figure 1.

New spray distributor and wash-oil bed

A very common reliability problem for HVUs is coke formation within the wash-oil bed’s structured packing. With elevated flash zone temperatures and a deep vacuum, entrainment of VR in the rising feed vapour can cause coke formation within “dry” sections of the bed. This can increase the pressure drop over the wash-oil bed. With a higher pressure drop in the wash-oil bed, the flash zone pressure increases, thus leading to loss of HVGO yield to the VR.

The coke build-up and resultant high pressure drop can only be resolved by stopping the HVU’s operation to replace the wash-oil bed’s structured packing. A new spray distributor was designed to ensure that there is adequate wetting of the wash-oil bed at all times. The free-passage area of the nozzles is adequate to prevent plugging and fouling. Filters were provided upstream of the spray distributor to remove any large particles that might block the spray nozzles. The spray distributor was also moved closer to the top of the wash-oil bed section to reduce re-entrainment of liquid from the nozzles.

The wash-oil bed itself was replaced with six layers of a higher-capacity structured packing to help avoid VR re‑entrainment and reduce the pressure drop over the wash-oil bed. 

New, insulated, low pressure drop draw-off tray

Hot vapours from the HVU flash zone rise and contact the underside of the “dirty” wash-oil draw-off tray first. The temperature difference between the draw-off tray and the rising hot vapours can cause condensation of HVGO, which will be downgraded to VR. An insulated, low pressure drop draw-off tray was installed to help minimise this effect, which is known as wild reflux.

New stripping trays

To maximise the recovery of HVGO from the VR, the amount of stripping steam applied at the bottom of the HVU was doubled. The existing ejector system was adequate to handle this higher vapour loading. High-capacity Shell calming section sieve trays were installed within the existing column insert below the feed location. These trays were sized to handle the increased vapour loading. 

Case study 2: Improving HVGO quality

In this case, the refiner was experiencing poor quality HVGO from the HVU: high metals content and almost daily quality fluctuations. This indicates relatively poor performance from the lower wash-oil zone, which was permitting entrainment of residue to the HVGO, so the revamp focused on this area.

The performance of the HVU lower wash-oil zone system is critical to achieving high vacuum gas oil (VGO) yields with no residue entrainment. HVGO quality is key because contaminants such as metals, Conradson carbon residue and heptane insolubles are all detrimental to the performance of a VGO hydrocracker or hydrotreater. The wash-oil zone system performance relies on the hardware (especially the spray distributor, the draw-off tray and the wash-oil bed), but it is also important to understand how to operate the system for optimum HVGO recovery.

During the planned turnaround, the lower wash-oil zone hardware was changed and post start-up the operation was optimised. Optimising the wash-oil system ensures removal of any VR entrainment in the wash-oil bed so that the HVGO yield can be maximised to ensure ongoing unit reliability. The revamp helped to improve the HVGO quality, as demonstrated by the nickel content of the HVGO (see Figure 2).

Key takeaways

By applying Shell deep-flash, high-vacuum technology, existing HVUs can be revamped at relatively low capital costs. The simple payback times for such revamps are very attractive, and they can be undertaken within an existing turnaround window.

More in Industry Focus