Figure 2 FCC sprays characterised by comparing D32 data with ligament percentage
Unburdening the wet gas compressor and the air blower Because most refiners operate to the limits imposed by one or both the WGC and the air blower, lifting these con- straints can be the key to squeezing greater margins from the FCC unit. Improved atomisation and dispersion of the feed can reduce the loading on the WGC and air blower, which are common objectives for refiners. Coking of the regenerated catalyst in the vicinity of the injection zone, caused by unvaporised feed, is a consequence of poor and incomplete feed atomisation with inadequate feed and catalyst contacting. This creates a larger demand for combustion air at a lower conversion because more of the coke is coming from non-vaporised feed instead of a catalytic reaction. By installing Shell Max Atomisation Feed Nozzles, the feed contacts the catalyst with greater intimacy. Feed coking is reduced, which frees up combustion air for additional riser feed rate and/or conversion within the coke- burning capacity of the unit. Other ways to take advantage of the more complete feed atomisation include keeping the same conversion while realising lower regenerator bed temperatures or providing opportunities to process more difficult feedstocks, such as residue, at the same conversion. Better feed atomisation and catalyst contacting can allevi- ate the formation of large temperature gradients within the riser, which produces more dry gas from thermal cracking reactions. By reducing these, reaction selectivity is enhanced, and the lower dry gas yield results in lower loadings on the wet gas compressor and rectifying absorber column. The reactor severity and post-riser cracking time are key parameters in determining the impact of improved feed atomisation and dispersion on the dry gas yield. Riser out- let temperature is the key metric for reactor severity and generally aligns with the dry gas yield from the operation. Figure 3 presents the reduction in dry gas yield for sev- eral FCC units that have upgraded their feed nozzle tech- nologies. All units have modern riser termination separation systems. In each case, unit conversion increases were achieved. Feedstock qualities ranged widely, including the extent of feedstock hydrotreating and the Conradson car- bon residue (CCR) value.
atomised feed can be almost double that of another feed with a similar D32 value. Ligaments and globules contain a larger volume of liquid for the same surface area and, as a result, take longer to vaporise in the riser, which reduces the amount of riser time used for catalytic cracking reac- tions and lowers conversion. Using these state-of-the-art spray characterisation tech- niques, Shell has developed feed nozzle technology that reduces both droplet size and the non-atomised portion of the spray in the form of globules and ligaments, which take a substantial time to vaporise in the riser. By producing smaller droplets and more complete atomi- sation, with fewer ligaments and globules, there is better contacting of feed and catalyst in the feed mix zone of the riser, resulting in faster vaporisation. The benefits of improving atomisation in this way include: • An increase in the usable riser volume for catalytic crack- ing reactions • Increased conversion and improved product yield • Less slurry oil • Less thermal cracking, so lower dry gas production and improved product distribution for more high-value products • Lower loading of the wet gas compressor (WGC) and air blower as a result of lower dry gas and coke yields.
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Riser outlet temperature, ˚F
Figure 3 Dry-gas reductions following feed nozzle upgrades
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PTQ Q3 2022
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