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:

  • Size (smaller objects pack together better). The size also needs to be in proportion to the column diameter too
  • Voidage (need to allow room for the gas & liquid to flow around them, don't want to block the column off)
  • Surface area (eg how many square meters of surface you have per cubic meter of packing — the more surface area, the more places for the liquid & vapour to mingle)
  • The amount of liquid & vapour flowing around it

Typical HETPs for common packings are :

Stainless Steel Wool Scrubbers0.13 m
Marbles (10mm diameter)0.33 m
6mm Ceramic Raschig Rings0.24 m
13mm Ceramic Raschig Rings0.38 m
Zoran suggests that in some cases marbles may be as effective as a 0.2m HETP.

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