Density grading separates catalyst particles of similar size based on individual particle density.

CCR reforming units continuously circulate catalyst through the reactor. Catalyst exiting the reactor typically contains about 5% carbon. It is regenerated online and fed back into the reactor.

In many units, a small fraction of the catalyst does not circulate through the reactors. This non-flowing “heel” catalyst is held up in the bottom and along the walls of the reactors. Over time, the heel catalyst builds up carbon levels as high as 50%.

When the catalyst is unloaded for unit maintenance, the heel catalyst along the reactor walls is released, contaminating the low-carbon circulating catalyst. Typically, the last 10-20% of circulating catalyst that is unloaded is contaminated with heel catalyst. The figure below shows carbon analyses of catalyst removed from one refiner’s circulating catalyst reactors. Near the end of the unloading, the carbon levels increase as the catalyst is contaminated with high-carbon heel catalyst.

Heel-contaminated catalyst cannot be reused because its high carbon content causes temperature excursions in the regenerator tower. When a plant plans to reuse the catalyst removed in a turnaround, there is a strong economic incentive to recover the low-carbon circulating catalyst. If the low-carbon catalyst is not recovered, it must be sent to platinum recovery and replaced with fresh, makeup catalyst at considerable cost.

Density grading can be used to recover the low-carbon catalyst. Because the heel catalyst has a 40-80% higher density than the low-carbon catalyst, a precise separation can be made.




One refiner had shut down its reformer for a maintenance turnaround. The refiner planned to reuse the catalyst, so the heel-contaminated catalyst was sent for density grading.



The table above demonstrates that the results of the separation were excellent. The purity and recovery of the light fraction were very good, and the amount of the low-carbon catalyst in the heavy portion was minimal.



The figure above shows carbon analyses on the light material produced. The first 10 containers held high-quality material with peak carbon levels less than 8% and average carbon levels 2-3%. Peak carbon levels were determined by examining the 5 darkest pellets from a population of 300-400 pellets. This catalyst could easily be reused with satisfactory performance.

The last two containers were contaminated with a small amount (10-15%) of medium-carbon catalyst. This material is considered marginal for reuse. However, the refiner had the early reactor design, which allowed placement of this material behind the catalyst withdrawal scoops. This minimized the impact of the higher-carbon pellets. Recovery of reusable catalyst was high. By weight, 86.4% of good, light catalyst was recovered from the mixture. Recoveries of 60-90% are typical in the density grading of heel-contaminated catalyst.

The recovered catalyst was reloaded into the reformer and the unit successfully started up. By using density grading, the refiner recovered 30,714 pounds of catalyst that otherwise would have been sent out for disposal. Considering the savings in new catalyst purchases and platinum recovery costs, the refiner saved more than $300,000.