I could make up a very plausible reason. Handwaving is a key job skill for physicists and con men alike. Example: It's about the surface area of the catalyst.

I could make up a very plausible reason. Handwaving is a key job skill for physicists and con men alike. Example: It's about the surface area of the catalyst. You can't just use one big cylinder of catalyst because that doesn't have enough surface. You need 40 or 50 little cylinders. And then you can't put all of those in one tank because then the flow of reactant past the surfaces of the cylinders isn't properly laminar, so you put each one in its individual cell. Then you build each cell with a separate housing and so forth, to contain the damage from the occasional explosions. As for why the magic number "43": We built 52, but 9 of them have, um, failed, so now we have 43. Soon there will only be 42. By the way, I hope you are wearing your safety goggles? ;) [REITERATION OF DISCLAIMER: I am not describing a real experiment. I am describing a figment of my imagination. Please do not send money.]