A step change in feed nozzle design

How Shell’s Max Atomisation Feed Nozzle technology can improve FCC unit margins with minimal capital investment, including audited results from three refinery scenarios.

Todd Foshee
Shell Catalysts & Technologies

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Article Summary

In a climate in which conservative approaches to capital investment are the norm, fluidised catalytic cracking (FCC) unit operators may be seeking low-cost opportunities to increase their margins. After all, investing in margin-improvement projects is usually key to maintaining an operator’s competitive position.

What would a creative project that could deliver the desired marginal gains look like on the ground? It may be characterised by one or more of these outcomes:
- The ability to process cheaper, lower-quality feeds, such as residues and difficult-to-crack materials
- A yield increase from reducing the dry gas yield and increasing conversion, which would shift the yield structure to more valuable product slates
- Maximisation of the operating severity. If the operator is seeking to realise this scenario, it will be necessary to relieve any constraints imposed by the capacity of the wet gas compressor or the air blower and heat-balance apparatus.

Is it possible to invest in a low-cost technology that could deliver these desirable outcomes and increase margins? For an FCC unit, there is a compelling solution.

Following a major research and development programme using new analytical techniques, Shell Catalysts & Technologies has recently developed new feed nozzles that push the boundaries for feed atomisation, thereby enabling refiners around the world to capture substantial margin increases. Upgrading feed nozzles is a low-capital opportunity with a quick payback that boosts margins and adds flexibility to the FCC and catalytic feed hydrotreating units.

Rerun or replace? Dispelling the myth
Refiners can occasionally be reluctant to replace or upgrade their feed nozzles, instead preferring to get another run out of their existing ones. Indeed, Shell feed nozzles, including legacy generations, are robust enough to operate for more than one run cycle. This operating strategy can offer a relatively modest reduction in capital cost, but it comes with risk.

Although inspecting feed nozzles during a turnaround can determine the amount of erosion or plugging that has occurred during the previous cycle, it cannot predict the nozzles’ ability to complete a second cycle in such a harsh environment.

Therefore, it is necessary to consider whether the capital saving is outweighed by the risk of loss in performance during the second cycle, given that feed nozzle replacement is low capital cost and small shifts in yield performance can pay for feed nozzles in a short time. In the pursuit of margin improvements, FCC operators must consider the big question: is there more value in a rerun or in replacing the existing feed nozzles?

In most cases, replacing the feed nozzles provides a shift in yields and feed flexibility that unlocks profitability over the duration of the entire run cycle. The payback time for the latest-generation Shell feed nozzles is typically less than one year. In fact, feed nozzle replacement compares favourably against other technology-based solutions that FCC unit operators are likely to consider. In many cases, the capital cost of replacing the feed nozzles is equivalent to the operating expense of trialling a new catalyst but without the associated risk.

Improving feed atomisation: the benefits
When atomised sprays are discharged from feed nozzles, they transition from liquid sheets to non-spherical ligaments and agglomerations of liquid (globules) and then, ultimately, to droplets (see Figure 1). 

At Shell Technology Center Houston, shadowgraph particle image velocimetry studies have been used to characterise the droplets in an FCC facility and have revealed the previously unidentified presence of liquid globules and ligaments. These are not apparent or quantified when referring to spray Sauter mean diameter, or D32, data.

Technology providers frequently quote low D32 values that may be associated with their feed nozzles. Although important, when taken in isolation D32 values do not reveal the whole story about feed atomisation. In fact, two atomised feeds can have very similar D32 values but very different characteristics. Imaging (see Figure 2) shows how the troublesome ligament content for one 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 reactions and lowers conversion.

Using these state-of-the-art spray characterisation techniques, 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 atomisation, 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 cracking 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.

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 constraints 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.

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