[NaDoume]

Why are spherical catalyst pellets used if they give the lowest surface area to volume ratio and highest pressure drop?

[Arkcon]

You're asking why, as an example, heterogeneous catalysts are typically spherical?  Here's a good source on Wikipedia,better than other Wikipedia topics, but feel free to browse around to see if you can glean a better answer. https://en.wikipedia.org/wiki/Catalysis#Heterogeneous_catalysts

Briefly, they're not often spherical , they are often cylindrical.  They're also usually porous, so surface area means more than the mathematical surface of sphere or cylinder.  In all likelihood,it has to do with fabrication, or perhaps space filling in a reaction chamber or catalyst holder.

[Woopy]

I agree, the packing density of a sphere is better than a cylinder. By the way for the OP, how does the size of the pellet affect the effectiveness of the catalyst?