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 :

R = L / D = (V-D) / D

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if the still design is like Stone & Nixon's where all the vapour is condensed separately, and you control the amount withdrawn vs returned (refluxed).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 ...

Select Packing Type
(note: the values these selections generate are only approximate guesses)
MaterialDiameterSurface AreaSurface Tension
Marbles 0.010 m 50 m2/m3 0.061 N/m
6 mm Raschig rings 0.006 m 140 m2/m3 0.061 N/m
13 mm Raschig rings 0.013 m 40 m2/m3 0.061 N/m
Stainless Steel Wool 0.001 m 800 m2/m3 0.075 N/m
or your own values ... m m2/m3 N/m
Still Characteristics
Column Diameter
Packing Height
Reflux Ratio
Power Input

Estimate of Packed Column Performance
Height Equivalent of Packing (HETP): m
Number of Theoretical Plates :   = (height/HETP) + 1 for pot
Approx Vapour Purity : %

See 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 ...
    I get 94% at a rate of ~ 500mL/hour. My column is 115cm long and 42mm wide Filled with potscrubbers from the undersite to just under the precooling coil. (Tony - ie its of a good design already - heaps of HETP)

    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     This page last modified Mon, 05 Mar 2012 08:07:08 -0800