Why Do High-Matrix FCC Catalysts Often Test Poorly?
High-matrix FCC catalysts often appear to perform worse in laboratory testing than they do in commercial FCC units because laboratory conditions do not fully replicate what happens inside an operating FCC. Understanding these differences can help refiners interpret test results more effectively when evaluating catalyst performance.
Becht’s Fun with FCC Catalyst series takes a conversational look at how catalyst choices shape FCC performance. From tweaking frequency to test strategy and cost trade-offs, each post aims to spark debate and share real-world insights – because great ideas often start with great discussions.
Key takeaways
- Laboratory testing can overstate the coke penalty associated with higher matrix catalysts.
- Riser residence time and diffusion limitations differ significantly between pilot plants and commercial FCC units.
- Stripper performance can influence how higher-matrix activity affects coke yield.
- Laboratory results should always be considered alongside unit operating experience and economics.
FCC catalyst alumina matrix surface area is one of the key parameters that can be adjusted to increase or decrease main column bottoms yield. Increasing matrix activity promotes additional heavy oil cracking. The big question to consider is: will it also increase coke yield?
Laboratory testing consistently indicates coke yield will increase with additional matrix activity, yet many units have switched from high to low Z/M (and vice versa) without seeing the predicted shift in coke yield. Several theories have been proposed to explain this discrepancy. Here, I offer my perspective and invite others to share their views while keeping the discussion constructive and collaborative.
Why FCC units and laboratory testing produce different results
To understand why laboratory results and commercial performance sometimes disagree, it helps to review two important differences between FCC operation and pilot plant testing.
Riser residence time
In an FCC unit, feed is injected into the riser where it must be vaporized by hot regenerated catalyst before cracking can occur in the zeolite pore. In most FCC risers, all the magic occurs in only two to four seconds. Diffusion limitations likely have a much greater influence in the riser than they do in a pilot plant, where residence time can exceed 10 seconds.
Zeolite cracking is the primary route for maximizing LPG and gasoline yields. Molecules larger than the zeolite pore can’t initially reach the active sites. Instead, they undergo thermal cracking or matrix cracking. Once these larger molecules are “pre-cracked,” portions may become small enough to enter the zeolite and undergo catalytic cracking. Increased matrix cracking may allow more heavy oil molecules to become small enough to reach the zeolite before leaving the riser.
Stripper residence time
The second major difference between a pilot plant and a commercial FCC unit is stripper residence time. Most pilot plants use extended stripping times to remove hydrocarbons from coked catalyst. Typical pilot plant stripping time ranges from one to three minutes, compared to roughly 30 seconds in an operating FCC unit.
Higher matrix catalysts may exhibit improved stripping and/or additional cracking of heavier hydrocarbons in the stripper. If stripper performance influences regenerator temperature, the additional coke associated with bottoms upgrading may be partially offset by improved stripping and continued heavy oil cracking. The overall impact depends on stripper design and operating conditions, which is why results often vary from one FCC unit to another.
Practical considerations when increasing bottoms cracking
- Define your objective. If your goal is to reduce slurry yield, you may need to generate a little more coke. The last heavy molecules that you crack typically produce proportionally more coke. Stripper performance may or may not offset part of that increase.
- Interpret pilot plant coke yields with a grain of salt. Pilot plants often exaggerate coke and yield differences. A good rule of thumb is to discount coke yield changes by 75 to 80% in your modeling.
- Keep the economics in mind. Even though slurry is of lower value than LCO, it’s still worth more than if you were to burn it in the regenerator.
- Consider bottoms operating limits. If the unit is already operating at its minimum bottoms rundown rate, additional bottoms cracking may require dropping LCO into the bottoms pool to maintain rundown rate.
- Optimize rather than maximize conversion. A truly optimized bottoms yield will continue cracking until the only incremental products are coke and dry gas!
Final thoughts
Laboratory testing remains an important tool for catalyst evaluation, but it doesn’t perfectly replicate commercial FCC operation. Differences in how cracking occurs inside the riser and how hydrocarbons are removed in the stripper can significantly influence how higher-matrix catalysts perform in practice. That’s why catalyst selection should combine laboratory data with operating experience and unit-specific economics. Looking at all three together provides a more realistic basis for evaluating catalyst performance.
If you’d like to learn more, reach out to connect with one of our FCC subject matter experts. We’re always glad to discuss catalyst addition strategies and help optimize unit performance. Have an FCC topic you’d like us to cover in a future blog? Let us know – your ideas help shape our content and ensure we’re addressing the challenges that matter most to you.
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