Structured packing is chosen on two competing numbers: efficiency and capacity. A higher surface area — 500 over 125 — packs more mass-transfer area into the column and gives more theoretical stages per metre, but leaves less open space, so it handles a lower load before flooding. The corrugation angle trades the same way: the forty-five-degree Y-type is more efficient, the thirty-degree X-type takes more load at a lower pressure drop. Stainless steel gives the corrosion resistance and the smooth, anti-fouling surface for a long, clean-running life, and copper or other alloys can be supplied where the process needs them. The size and type are matched to the separation, the loads and the column.
