 

HETP properties in Reflux Still's (Theoretical Plates)The height of packing needed in order to do the same job as an ideal plate is called the HETP  Height Equivalent to a Theoretical Plate. Smaller HETP's are better than large ones, as it means that for a given column height (say 1m) you end up with more ideal plates, eg only 2 plates (87% purity) if the HETP= 0.5m, but 4 plates (92% purity) if HETP = 0.25 m. If you don't have an exact number of plates, that's still OK; you'll end up somewhere proportionally between the two. So an empty column, with no packing, ain't going to do a lot. Sure, you might get a little liquid running down the sides of it, but this has got nowhere near the same surface area as using packing. The HETP for a packing depends on its:
Typical HETPs for common packings are :
 