Originally By Tony Ackland
Reflux Ratio within a column Still
These HETPs change depending on how much liquid & vapour are
flowing around them. This ratio can be described by the Reflux Ratio -
the ratio of Liquid flowing down the column over the amount of distillate
drawn off :
This can be easily measured if the still design is like Stone & Nixon's where all the vapour is condensed separately, and you control the amount withdrawn vs returned (refluxed). It's a little harder with the Stillmaker design where the refluxing liquid is determined by the amount of cooling done by the first condensers, and you never get to single it out, but you should be able to estimate the amount of vapour from the amount of heat you apply.
As the reflux ratio increases, so the HETP improves. Generally though, you can see that choosing the right packing to start with does the greatest improvement; increasing the Reflux ratio only squeezes the last extra bit out of it (at the cost of having to wait longer too). Where you will notice it is when the design is poor to start with - increasing the reflux ratio will help out quite a bit.
Calculate the HETP for your still ...
See http://www.raschig-rings.com for more information on other column packing details. Note also that when real plates are used in a column, you also need to do a similar calculation - they are often far from ideal in operation, and you may need several to achieve one HETP.
Jan Willem experimented with this ...
At my latest distilling escapade I turned the reflux ratio up. Just as a test that would show me how pure it could get AND if there was a taste diffirence (after dilluting ofcourse) Collected the good stuff at 100mL/hour (a long wait) Then the score was initialy 95.?% and was going down a bit to 94.5% (Dunno if it was 94.3 or 94.8 so I say 94.5%) After the taste test I noticed NO difference, but I'm no expert at wodka tasting.
The improvement isn't linear either - you can halve the HETP for Stainless Steel Wool (SS below) by going from "bugger-all" reflux to "some" reflux, but there is little improvement winding it up too far past there.
SS = Stainless Steel Wool Scrubbers, RR6 = 6mm Ceramic Raschig Rings,
RR13 = 13mm Ceramic Raschig Rings, M = 10mm Marbles