Controlling contaminants in vacuum gasoil
How revamps can increase VDU capacity and reliability while controlling contaminant levels to acceptable limits.
Shell Global Solutions International
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For many refiners, the amount of vacuum gasoil (VGO) that their vacuum distillation units (VDU) can generate is a critical constraint. Have you ever tried to lift more from the vacuum residue (VR) in order to fill the hydrocracker, fluid catalytic cracker (FCC) or other conversion unit, only for that downstream unit to suffer reliability issues as a consequence? When lifting additional VGO with standard hardware, elevated levels of metals and other harmful contaminants can be entrained in the VGO. However, with the latest technology it is possible to increase the VGO yield of a VDU substantially, at low cost, and without affecting the downstream units’ reliability.
Several refiners worldwide have reported cost-effectively increasing the capacity and reliability of their VDUs by revamping them using Shell deep-flash technology. Crucially, this also improves the quality of the VGO that the unit provides. The deep-flash technology keeps the levels of contaminants such as nickel, vanadium and Conradson Carbon Residue (CCR) to within the conversion units’ acceptable limits, thereby safeguarding their reliability.
Such a change can unlock substantial value. For example, compared with a typical conventional unit, an 8.9 million t/y or 150000 b/d unit that uses Shell deep-flash technology can improve a refinery’s margin by some $7.8 million per year. This is because it can generate 1-3% more distillates, cut energy costs by 25%, and improve mechanical availability by 1.5%.1
Common VDU issues that can seriously downgrade refinery performance
The VDU is a key processing unit whose capabilities affect the performance of other units and, therefore, the overall refinery margin. Suboptimal operation here can mean that substantial value could be lost.
Do you ever find that your VDU’s capacity is constraining your hydrocracker or FCC? With demand for diesel fuel continuing to increase in many markets, many refiners are keen to increase the throughput of their conversion units, but the capacity of the upstream VDU prevents them from doing so.
Do you have difficulty achieving the desired turnaround cycle in your furnace and VDU column? Cycle-limiting coking in the furnace and the wash bed is a common problem across the industry. However, applying best practices and latest generation technologies can help to avoid this; many refiners are achieving run lengths in excess of four years.
Are you experiencing suboptimal reliability in your downstream conversion units? The origin of hydrocracker or FCC under-performance is often traceable back to the VDU. When VR containing metals and other harmful contaminants is lifted into the VGO, it can seriously affect the performance or cycle length of the conversion unit.
Fortunately, the capacity of the VDU and the quality of the VGO that it provides can often be cost-effectively increased. By revamping the VDU using deep-flash technology, refiners have been able to increase their VGO yields without affecting the reliability of their downstream units.
Shell deep-flash technology has been developed through extensive research on mass transfer and separation equipment, and been supported by operating experience from numerous units over many years. Shell Global Solutions has licensed deep-flash technology in 26 revamped and 24 grassroots units since 1985.
As Figure 1 shows, the technology includes five key design features that can be tailored to a refiner’s specific revamp requirements. It is important to note, however, that, because each refiner’s circumstances and business objectives are unique, revamp projects are highly tailored, so some projects may not incorporate all of these design features.
The first of these design differentiators is Shell’s proprietary furnace coil design. Conventional technologies can be prone to coking, but Shell Global Solutions has developed technology involving suppressed vaporisation that controls the flow regime in a furnace. The liquid coats the walls of the heated coils continuously throughout the furnace and the vapour passes through the core, thereby completely avoiding a mist flow region where the droplets can hit the very hot walls and form coke. Consequently, units typically achieve pace setting cycle lengths (see Figure 2).
Shell direct-contact spray condensation sections also contribute to improved distillate recovery. Conventional VDUs often have packed pumparound condensation sections, but this packing can increase pressure drop and reduce yields. The Shell approach is to achieve vapour condensation by direct contact with liquid in an empty space. This removes the need for packing and reduces column height. Crucially, it also maximises distillate recovery, as the pressure drop is lower than for packed condensation sections, thus enabling lower flash zone pressures.
Entrainment of VR from the flash zone often limits the VGO yield of conventional VDUs owing to quality concerns. To address this, the Shell Schoepentoeter Plus vane-type feed inlet device is designed to achieve high de-entrainment efficiency at a lower pressure drop than conventional feed inlet devices. It also offers good vapour distribution. This efficient feed inlet device can help operators to process more difficult crudes and meet tighter product specifications.
Coking of the wash oil section, which washes down the remnants of entrained VR, is a common reason for premature VDU shutdowns. Shell-designed wash oil sections and wash rate management aim to avoid coking and, thus, extend cycle lengths, increase distillate yields and meet tighter product specifications. Maintaining a low pressure drop across the wash oil section is key to this: conventional designs may see a gradual increase in pressure drop over the wash oil packing due to coking, which can cause deterioration in distillate yield and quality.
Based on its operating experience, Shell Global Solutions has developed guidelines for the design and operation of wash oil beds that can enable continuous operation for over four years without coking of the wash oil packing. As Figure 3 shows, Shell-designed and advised units can typically operate for over four years without coking of the wash oil bed packing.
Shell low pressure drop, insulated draw-off trays help to maximise VGO yield by: having minimum liquid hold-up on the tray to avoid coking; insulating the bottom trays to minimise condensation of hot vapour (wild reflux) against the relatively cold tray surface by having an empty space between double walls; and leaving large open areas for vapour passage and thus providing a very low pressure drop, good vapour distribution and a lower flash zone pressure.
In addition, the draw-off trays’ shallow design helps to reduce the column height, thereby saving capital costs, and making them easy to install because they do not require seal welding.
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