Hoppers at a refinery are usually catalysts or filtering agents like bentonite clay. I don't know if Google Earth has photos of the Holley (ex-Sun Oil refinery) in Tulsa, OK, or the Texaco refinery here. They are both very small refineries that are practical to look at for modeling. An N-scale model of some big refineries would fill a 150,000 sq. ft. building with some selective compression!
