Length grading separates longer catalyst particles from shorter ones. Conventional screening can only remove fines and particles that are shorter than the catalyst diameter. In many cases, the screened material does not meet the length specifications for a given unit. Length grading can be used to remove the short catalyst particles and produce material of acceptable length while minimizing the loss of desirable longer particles.


In the example shown below, 1.6 mm cobalt molybdenum catalyst was length graded to remove particles shorter than 2.5 mm. The figures show the particle length distributions for the feed and the short and long fractions. About 24% of the catalyst was rejected in the short fraction, while the average length of the recovered catalyst was increased from 3.2 to 3.7 mm.