Few refiners will respond to the International Maritime Organization’s global fuel oil sulphur cap (IMO 2020) with a single, major investment that will eliminate the bottom of the barrel. Most will implement several low-capital expenditure solutions that partially reduce it. One relatively inexpensive option that may be particularly attractive is an innovation in fluidised catalytic cracking (FCC) feed nozzle technology , previously thought to be a mature technology. Recent developments here provide the opportunity to increase slurry oil conversion through better atomisation and to, therefore, reduce this heavy, high-sulphur stream, which traditionally goes to the marine fuel oil pool.
In recent years, FCC feed nozzles have seen only small incremental design improvements and conventional analysis techniques have provided few insights into improving their performance further.
However, after a major Shell Global Solutions research and development programme into feed nozzle technology using new analytical techniques, the organisation has developed upgraded feed nozzle technology that is pushing the boundaries for feed atomisation and enhancing unit reliability.
Consequently, refiners can use this technology to increase the conversion of slurry oil, the heavy aromatic by-product that is the lowest-value stream produced by an FCC unit, to higher value products.
FCC unit performance at two Shell refineries, Deer Park in the USA and Sarnia in Canada, has significantly improved after installing the new feed nozzle technology. Additional feed nozzle upgrades are planned at other Shell refineries, and the technology is available for licensing for non-Shell sites.
Development 1: Feed atomisation
When atomised sprays are discharged from feed nozzles they transition from liquid sheets to non-spherical ligaments of poorly atomised agglomerations of liquid and then, ultimately, to droplets. However, atomised spray distributions that include non-spherical droplets and ligaments present a challenge to most spray characterisation methods, which tend to rely on spherical and near-spherical liquid droplet distributions.
Looking to improve feed atomisation, Shell tested a range of droplet measurement techniques including phase Doppler and laser diffraction. Finding that non-spherical droplets and ligaments could be improperly characterised, Shell questioned the validity of these methods and sought a more-appropriate spray characterisation technique.
Key to Shell’s innovation was the decision to use the shadowgraph particle image velocimetry (PIV) technique. The shadowgraph PIV data the organisation collected for a wide range of commercially implemented feed nozzle designs indicated an opportunity to improve feed atomisation. This programme also provided considerable insights into feed nozzle performance, which Shell leveraged to develop its latest-generation technology.
Crucially, the upgraded technology reduces the quantity and size of globules in the spray. Globules are defined as large droplets plus poorly atomised ligaments; collectively, these can take substantial time to vaporise in the riser.
Development 2: Reliability and integrity
Globules in an atomised spray can lead to coke deposition along the riser wall, throughout the reactor internals , along the overhead vapour line wall and at the main fractionator inlet. Fouling of the slurry system can also result. This deposition and fouling negatively affect the pressure balance, the unit capacity, the catalyst circulation and the slurry heat removal. These effects become progressively worse as the unit gets further into its operating cycle.
As the latest-generation feed nozzle technology can reduce spray globules and improve slurry conversion and liquid yield, it also helps to mitigate these reliability threats.
Two mechanical upgrades supplement the process design improvements. The first is a tapered sleeve design that enables easier extraction of the feed nozzles during unit turnarounds, thereby reducing the time required for feed nozzle replacement. The second is a proprietary, shield-like sacrificial shroud that covers most of the nozzle tip surface yet allows the feed injection to pass through while minimising tip erosion from the flowing catalyst in the riser.
Case study: Deer park refinery
In 2015, Shell Global Solutions evaluated Deer Park’s FCC unit. Shadowgraph PIV identified opportunities to improve the droplet size distribution and to reduce the globule content, which would increase the number of droplets and reduce the feed vaporisation time.
So, during its 2016 FCC unit turnaround, Deer Park refinery installed the improved feed nozzle technology. Evidence of improved feedstock atomisation is a dramatically lower riser mix zone temperature at a constant riser temperature. By improving the feed atomisation, the new feed nozzle design has reduced the apparent existence of varying local catalyst-to-oil ratios and increased the effective riser residence time. The net effect is a boost in bottoms destruction (slurry) and additional gasoline and light cycle oil (LCO) production (see Table 1).
|Yield shifts, Vol% feed||Audited|
|Dry gas + coke||0.4|
|Gasoline + LCO||1.5|
|LPG + gasoline||0.9|
Table 1: Deer Park feed nozzle shift audit.
Case study: Sarnia refinery
Sarnia refinery also installed the improved feed nozzle technology during a 2016 turnaround.
Unit monitoring data is showing measureable yield improvements with a lower dry gas yield. A shift in the mix zone riser temperature profile (Figure 1) for the same riser top temperature demonstrates the improvement in feed atomisation and vaporisation consistent with observations from Deer Park. Furthermore, the success of the feed nozzle shroud in protecting the feed nozzle head has provided the site with confidence that it will achieve another five years of exceptional feed nozzle performance.