The integrated picture to minimize bottoms sent to bunker fuel - Shell & KBR
Finding ways to minimize 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.
The rationale
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.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 pelletizing. A possible line-up is shown in Figure 1.
The DAO produced can be cracked in a Shell thermal distillate conversion (TC) unit. The conversion of DAO into distillates takes place in a heater. The advantages here are that DAO is cracked to distillates without using a catalyst or hydrogen, and the investment costs are significantly lower than those for catalytic conversion processes. When the distillate cracking unit is integrated with a VBU, the cracked feed is routed to a combined fractionator. Additional processing, usually hydrotreating, is required to make final products. Adjusting the heater operating conditions maximizes the overall yield of distillates (520 minus) to as high as 75 wt%. With its low capital cost, reasonably high conversion and wide flexibility in feed quality, such a line-up could be ideal for refineries with capital constraints in revamping secondary processing units.
Phased investment opportunity
Increasingly, refiners are keen to avoid regret investments; a line-up such as this would support such an objective. For example, if they later wanted to install one of the highest residue upgrading technologies, the SDA unit would continue to be relevant.
In terms of bottoms reduction, it would provide an incremental benefit from unit to unit: the VDU revamp lifts more distillate, so, fewer bottoms are produced; the vacuum flasher further increases distillate recovery; and the fuel oil that is produced by the VBU is reduced further by the SDA unit that follows.
Consequently, such a line-up could be attractive to refiners worldwide and, with the International Maritime Organization’s global fuel oil sulphur cap (IMO 2020) on the horizon, might provide a much-needed low-capital cost option for residue conversion.
KBR’s Rose SDA Technology
One of the leading SDA technologies on the market is ROSE technology, which is licensed by the technology and engineering firm KBR. The DAO it produces contains very low quantities of metals, asphaltenes, sulfur and Conradson carbon residue, and is an excellent feedstock for processing in conventional refinery units such as fixed-bed VGO hydrotreaters and FCCs. It can also be processed in high-pressure HCUs and TC units.
The increased hydrogen content and lower contaminants of the DAO relative to the residue, together with the low investment and operating costs, often makes ROSE technology an economical option for producing good-quality feed from residue for secondary processing units. According to KBR, the ROSE process is also highly efficient and requires up to 60% less energy than other technologies.
KBR is one of Shell Global Solutions’ alliance partners, so when customers receive a Shell-designed revamp that includes the addition of a ROSE SDA unit, all the units are optimized to work together. This includes, for example, the feed specifications and the battery limit conditions.
To learn more, go to Shell Global Solutions "Seeing the integrated picture"
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