Finding ways to minimise the amount of bottoms sent to the bunker fuel pool has become a strategic priority for many refiners. Although many technical solutions are available, the optimum response for a specific refiner depends on individual circumstances. Refiners that already have a visbreaker unit (VBU) and are capital constrained may find integrating it with the vacuum distillation unit (VDU) solvent deasphalting (SDA) unit and hydrocracking unit (HCU) or fluidised catalytic cracker (FCC) to be particularly attractive. Here is why.
Visbreaking is a well-established process that has been around for more than 50 years. It has been a particularly popular bottom-of-the-barrel upgrading option in some regions, especially much of Europe and parts of Asia Pacific that have had a strong market for fuel oil. Although a VBU produces lower distillate yields than a delayed coker, it has clear strengths, in particular, a lower capital cost. It can also be revamped easily to achieve higher conversions if there is no need to produce stable fuel oil.
With outlets for fuel oil now diminishing, however, refiners with a VBU must evaluate new bottom-of-the-barrel solutions. Those with an abundance of capital could consider investing in the technologies that provide the highest conversion levels, such as residue gasification, slurry hydrocracking or ebullated bed residue hydrocracking, but these will be out of reach for most refiners.
However, as a VBU can integrate seamlessly with an SDA unit, which has a low investment cost, there is the opportunity to reduce fuel oil exposure significantly at a low cost.
Before installing any kind of residue upgrading technology, the first step when seeking to reduce refinery fuel oil production should always be to maximise the distillate yield from the VDU (either as a diesel pool component or as a feedstock for a secondary processing unit such as an HCU or an FCC). The production costs for straight-run distillates are lower than those for a thermal cracking or solvent extraction unit are.
This can be done at relatively low cost by revamping the VDU using Shell’s deep-flash, high-vacuum technology. Typically, this can generate 1–3% more distillates, with a corresponding reduction in short-residue (SR) yield.
Next, how to deal with the SR? Some refiners consider putting an SDA unit upstream of the VBU. Shell Global Solutions’ view is that cracking SR in a VBU before sending it to the SDA unit is preferable, as it leads to a smaller and, therefore, less expensive SDA unit.
The quality of the deasphalted oil (DAO) produced from the SDA unit will depend on the unit’s extraction depth. A higher extraction depth results in DAO with higher levels of resins, asphaltenes and contaminants. In contrast, the heavy distillates produced by the VBU will be of higher quality. Furthermore, the VBU can be upgraded, also at relatively low cost, by adding a vacuum flasher. This will increase conversion and distillate recovery.
The visbroken vacuum gas oil (VGO) can be routed to the HCU or the FCC, while the high-sulphur vacuum-flashed cracking residue that remains can go to an SDA process such as the residuum oil supercritical extraction (ROSE®) process (see below, KBR’s ROSE SDA technology) that can handle such difficult feedstocks. The extraction depth achievable is decided by the quality of the pitch (asphalt) that is produced and its intended use.
In turn, the SDA unit will produce paraffinic DAO with a high hydrogen content that is, therefore, suitable for further cracking. The pitch that remains, which contains most of the residue’s contaminants, can be routed to bitumen blending or for pelletising. A possible line-up is shown in Figure 1.