Separation Technology & Materials
Catalyst Density & Length Grading
Length grading is used to increase the average particle length of a batch of catalyst (extrudates). Short particles can reduce the void fraction in a catalyst bed and thereby increase pressure drop over the reactor. In order to recover particles of good length, Catalyst Recovery has developed a sophisticated screening technology that allows the separation of catalysts particles with a length up to twice the diameter to be efficiently removed without excessive catalyst losses. Length grading can be performed on any batch of catalyst or extrudates; fresh, used, or regenerated. Length grading as part of a regeneration job is best performed before regeneration so that short particles are not unnecessarily regenerated.
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%.
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 low-carbon catalyst contaminated with heel catalyst. Because the heel catalyst has a 40-80% higher density than the low-carbon catalyst, a precise separation can be made.