Chemical plant separations are defined less by scale than by service. Hydrogen chloride, chlorine, caustic and organic acids all attack the internals, so material selection and lining are frequently the whole engineering question rather than a detail at the end.

Many of these columns also run in campaigns, handling different products through the year. That makes turndown and cleanability more valuable than peak efficiency at a single point — a column optimised for one case can be useless for the next.