A general description from Googleing on catalyst pinning:
A phenomenon known as "pinning" inhibits catalyst transfer in many reactor arrangements. "Pinning" is the phenomenon wherein the flow of reactant gas at sufficient velocity can block the downward movement of catalyst. "Pinning" is a function of the gas velocity and the physical characteristics of the flow channel in which the flowing gaseous reactants contact the catalyst. As the gas flows through the channels that retain the catalyst, the gas impacts the catalyst particles and raises intergranular friction between the particles. When the vertical component of the frictional forces between the particles overcomes the force of gravity on the particles the particles become pinned. As the flow path length of gas through the catalyst particles becomes longer, the forces on the particles progressively increase from the outlet to the inlet of the flow channel. In addition, as the catalyst flow channel becomes more confined, the gravity flow of catalyst particles becomes more hindered. Accordingly, as the size of the flow channel becomes more confined, wall effects increasingly add to the vertical hold-up force on the catalyst particles. As a result narrow flow channels have a greater susceptibility to pinning and cannot normally provide continuous catalyst circulation.
In the case of reactors providing indirect heat exchange, the arrangement of the reactor exacerbates the problem of catalyst pinning. Increasing the number of channels by decreasing their size facilitates heat transfer by increasing the surface area between the heat exchange fluid and the catalyst.
UOP have one catalyst version with higher density to overcome the pinning. Think it is the 200 version vs. the older 100 version (i.e. R-264 instead of R-164 etc.)The higher density will counteract the pinning effect as descibed by 25362.
Usuaully the mayor pinning effect is seen in the regenerator according to what I was told.
Havent seen a pnning problem myself on a CCR but (according to UOP) this is a problem debottelnecking a CCR as well as just pushing the unit to its limits.
