Purity is improved during distillation by allowing the rising vapour to mingle with some liquid at a slightly cooler temperature. In doing so, some of the water rich vapour will condense, supplying a bit of energy to allow some alcohol rich vapour to form from the liquid , and join the existing vapour. Each time this "mingling" is sufficient to reach equilibrium, the purity takes a "step" on the graph below:

You can see that due to the shape of the curve, most of the gains are early on; to get to the really high % purity, you need to take lots of steps later on. There is no way around this. If you want high purity, you have to work hard for it. Also note (particularly for inefficient columns with the equivalent of only 1-2 plates) that the starting % can also affect the final % achieved - hence a good idea to use the better yeasts.

Each of these "steps" represents an "ideal plate" where enough mingling of liquid & vapour allows them to come to equilibrium. If you don’t allow enough mingling (equilibrium), then you won’t achieve a full step, but end up a little shy of the target. You get the first step free - its the boiler/pot.

One way of doing these steps is to do many single distillations, collect the vapour that comes off, condense it, clean out the still, and run it through the still again. This why pot stillers do double & triple distillations to get into the 80+ % range. But a Reflux column allows this to happen continuously; if given enough surface area to equilibrate on, the vapour can have gone through multiple distillations by the time it gets to the top of the column.

For each plate to work, it has to be at a particular temperature, slightly cooler than the one below, and warmer than the one above. Only then will it achieve its equilibrium and an increase in the alcohol purity. The differences are really fine too – its all happening only between 78.1 C and 82.2 C – quite a tight band to walk between.

Mike Nixon explains in a bit more detail ...

The process of separation depends on two facts:

1) when a vapor condenses then the resulting liquid has the same composition as the vapor, and the temperature at which this occurs is the same as the boiling point of that mixture. The boiling point lowers as the proportion of volatiles increases, so the temperature as you go up a column naturally decreases. One sticking point is that many think that a vapor only condenses when it encounters a surface that is cooler than the boiling point, but this is not so. Condensation occurs when there is a path for the latent heat of vaporization/condensation to be removed from the vapor, and the resulting liquid will remain at its boiling point if no further heat is removed.

2) when this liquid re-evaporates then the resulting vapor is richer in the most volatile components.

The packing is there simply to hold intermediate distillate in place so it can be bathed in hot, rising vapor and allow this second process to occur. As volatiles are further extracted from the intermediate distillate, the boiling point of what remains increases and the depleted liquid builds up, eventually dripping down the packing to a hotter level where it can again be stripped of more volatiles.

A cooling tube placed near the bottom of a column simply interrupts this natural progression and serves no useful purpose in the separation process. In contrast, the top cooling tube IS useful as it helps to return some of the vapor arriving at the top of the column to the packing, where it has a further chance of being stripped more thoroughly. This is what a condenser placed on top of a compound column does, but with more efficiency.

This is why you should also (see my interactive Heat & Mass Balance page to play with these and see it for yourself):

• Insulate the column well (don’t want breezes causing additional cooling out of sequence),
• Let the column run at total reflux for a while (to allow the packings to heat up to their equilibrium temperature). This is also important so that the methanol is given a chance to all work its way to the top of the column, so that it will all come off in the first off-take,
• Only have condensors for reflux above the packing, and
• Use a stable/continuous heat source (you don’t want it switching off & on all the time causing surges of vapour going up the column then periods of nothing; it has to be a steady continuous flow of vapour & liquid)

So you can easily work out what is required to get a particular % purity; just look up the number of ideal plates needed, eg 2 plates = 87%, 3=90%, 4=92% and so on. Remember you get the first one free - its the pot.

A pot still is the equivalent of a single plate; if it has "thumpers" attached to it, each of these can act as an extra plate.

Why call them plates ? In large distillation columns, they are exactly that; large metal plates or trays, which the liquid flows over, and the gas bubbles up through holes in them. However they are quite tricky to design & build, and not really suited for small column diameters (say less than 1 ft diameter) - they’re just too fiddly. Below this size, its easier to use a Packed Column; where the packing can be random (eg just dumped in there and given a shake), or carefully positioned & stacked . For any particular type of packing, we can estimate how much of it is required to make one of these "ideal plates". See http://www.5continentsusa.com/cer-pack.htm for examples of different commercially available types of packing. These commercial packings are quite difficult to source, then expensive to purchase. They're designed for an industrial operation, where they're expected to be run continuously 24/7 for weeks or months at a time without fouling up. For a hobby distiller it is far easier, and with higher performance (%purity), to use common pot scourers (non-rusting stainless steel or copper) instead for packing, as we'll be cleaning them frequently (like after every 20L run).

Jim adds:

While gathering materials for my (first) reflux still, I came across an interesting material used for making batteries. It's a fine-mesh expanded metal made from copper by the Exmet Corporation. They make expanded metal from a variety of metals besides copper in sheets varying in width from .099 in. to 60 in.
 Their spec sheet is found at: http://www.exmet-corp.com/chart.html
I don't know how you would calculate the void to surface ratio to get optimum results. I leave that to the "experts".One could roll a 30" wide sheet, for example, into a single piece that could be inserted into the column. It would have a very consistent internal structure. It would be easy to remove and clean.

Have you checked the aquarium shops for ceramic rings used for pre filtration. I recently bought 2 litres for change from £10 (£4.85/l). I suspect they would do the same job. They also come in hex or tube shapes. There are also similar rings for bio filtration that have an open surface area so would perhaps be more efficient, though could be a bugger to clean

Ken : The idea with a fractionating column is a temperature gradient (falling of course) as you go UP the column, but a constant temperature ACROSS the column at any height. As the diameter of the column increases, it gets harder to achieve the constant temperature at a given height if you continue to use a packing material -- hence the plates. Plates give you resistance to flow in an upwards direction, but very easy "spreading" horizontally.

George : From what I have read, a general rule of thumb is; up to 4" you would be better off with scrubbers, anything bigger than 4" and scrubber will tend to channel, from 4" up to 8" you would do well with packing like pall rings and the like, from 10" and up you would do well to use plates. However I have seen plans for a 4" still with bubble cap plates and they claim to be the most efficient still made. Their claim not mine. To make the 10" bubble cap plate type still work you would need somewhere around a 10 hp boiler or around 350,000 btu input. It would also produce up to or around 30 gallon of ethanol per hou using a 10% wash. I have gleaned this information from a lot of different sources and complyed it myself, none of it is to be considered absolute. Their has been some writings about using some sort of perforated plate with the packing on the 6 to 8 inch stills to help even out the vapor flow.

On spacing I read once, and do not remember where that the spacing should be double the diameter as a rule of thumb. But the heat input, the quanity output, and the wash percent all effect this so it is hard to say. Their is no set rule to follow. Perforated plates require a lot of drilling and the bubble caps are hard to construct. Anything over a 6 to 8 inch would require quite the effort to bring up to speed. Unless you have a cheap source of heat, a motor of some type that runs constancely , the expense of bringing one of the bigger one up to steam would be very high.

One other thing that effects the plate type stills is wheather or not you are going to filter out the solids in your wash. If you are not then your plates would have to be designed to be self cleaning. If you do then the solids need to be compressed to get as much alcohol recovered as possible.

Gaw : Using the photos ... of a bubble type plate still in Holland I built a four inch eight plate still which seems to work quite well on top of a six gallon electric water heater with benefit of a thermostat which I added. To further the experiment of continuous distilling I added a thirty gallon pot with a connection three plates above the smaller unit and after the complete system reaches operating temps the unit seems to function quite well at 94-95 per cent. I used ss plates which I found in a salvage yard and soldered the bubblers into separate units which I then bolted together, believe it or not, with ss bolts and neoprene gaskets on each end of the four inch pieces.

I think the best solution for an ethanol distillation would be a packing of copper rings. These should not be too difficult to manufacture. Winding a copperwire on a thin rod with an electric drill and cutting the created spring to rings shouldn't be too difficult.

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 :

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 :

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.

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

So, put these together to work out your still performance;

• Determine the HETP for the packing you are using, then
• Work out for the height of packing you have, how many Ideal Plates you have, then
• Look up the purity expected for that number of plates.

SS = Stainless Steel Wool Scrubbers, RR6 = 6mm Ceramic Raschig Rings,
RR13 = 13mm Ceramic Raschig Rings, M = 10mm Marbles

But what diameter should the column be ? This needs to be worked out from the amount of heat you are putting in. The more heat, the more vapour you generate. If the vapour rate is too great, then instead of having your refluxing liquid flowing down the column, it will be blown out the top. You also need to consider how much space the packing is taking up too. The following diagram is based on the calculations - unfortunately the sizes are about 50% smaller than what appears the actual limit, so scale up the calculated result if you plan on following them.

SS = Stainless Steel Wool Scrubbers, RR6 = 6mm Ceramic Raschig Rings,
RR13 = 13mm Ceramic Raschig Rings, M = 10mm Marbles

All these figures above equate to a vapor speed up the column of 119 cm/sec (47"/sec) Assuming for ease a column length of 119cm, that means that vapor will traverse the column in one second, and during that time it is busy condensing and being reboiled many times.

Tony reported that a 25mm diameter column at 1350W gave very poor results, but a 36mm diameter column handled that power well. Vapor speed through the 25mm column would have been 211 cm/sec, so it's not surprising that it didn't work well as the vapor had only 0.56 seconds to traverse the column.  Increasing the column diameter to 36mm brought the speed down to 101 cm/sec, so the transit time increased to 1.2 seconds.  This is on the 'safe side' of the recommended figures in the table.  

I personally favor lower power settings, and the reason lies in measurements I did with column stability, measuring the temperature gradient at steady state for different power settings, and first using a simple reflux column that relied solely in internal condensation for reflux.  (A not-to-scale schematic has been posted in the Photos section of Distillers. It's called 'Temperature gradients')

  I used a 2" diameter column and found that at 750W the temperature gradient settled out 2/3 of the way up the column, at around 80cm.  This told me that vapor moving at 28 cm/sec had fully separated after spending 2.5 seconds in the packing.  Increasing the heat input raised the point at which the straight line met the curve, but when the curve reached the top of the packing, the curve switched to the straight line sharply, not asymptotically, and the temp in the top rose to match the increasing temp at the top of the packing. So using 1350W with a 25mm column would result in full separation occurring at a point right at the top of the column, with little or no leeway.  

Adding imposed reflux with a compound column did little to change the temperature gradient up the column, but what it did do was add that touch more separation in the region above the top of the packing. I could detect no change in the 'asymptotic' nature of the temp gradient when the straight section still lay inside the packing, but when power was increased to raise that point to the tip of the packing then the more I increased the power a sharp 'step' began to appear.  The straight line went down to the top of the packing, then quickly jumped to meet the temp in the top section of packing.  This indicated to me that the composition of the cycled vapour in the void between the top of the packing and the top condenser was the result of further separation imposed by the imposed reflux operation in that region.  In effect, I had two stills one on top of the other, the bottom being a simple reflux still relying on internal reflux, and a recycling still that took what the reflux still gave it and used that as its starting point.  This held true until either the power was increased to a point when the curve would have settled down itself in a longer column (about a quarter extra length) or the take-off ratio was increased to a stage when the sharp step suddenly broke down and the old asymptotic curve re-asserted itself, and quality instantly dropped.  

OK ... so what has all this got to do with those figures in the table? Essentially, it is that the figures in the table are good for indicating the maximum you can push a simple reflux column to and attain full separation ... just!   

If consistent results are wanted, then the aim should surely be to allow some leeway and try to get that curve settling down before the top of the packing is reached.  That way, the reflux column has a chance to do its job as fully as it can before either taking off product, as in a simple reflux still, or passing on the results to a secondary top section that operates with imposed reflux for that final touch of separation.  My personal 'cautious old fuddy-duddy' approach would be to reduce all those wattage figures in the table to 1/4 of what they are now and regard that as a good guide for reliable operation.  This sounds drastic but, when you think about it, gives much greater assurance of high quality with simple reflux stills, and greater flexibility in take-off rates with a compound still.  Maybe I'm just an aging Sunday Driver, but I find that I get to where I'm going with less hassle than a Boy Racer, and both my passengers and booze samplers enjoy the ride better!

For a given packed column, at the high end of liquid and vapour rates we encounter flooding as liquid backs up the column and fills all the void space in the packing bed. Poor disengagement between vapour and liquid (back mixing) reduces the separation efficiency, and the high liquid hold-up in the bed increases the pressure drop.

The traditional approach to analysing flooding in packed columns relies on measuring pressure drop. At low liquid rates, the open area of the packing is practically the same as for dry packing. In this regime the pressure drop is proportional to the square of the vapour flowrate. As the vapour rate continues to increase, eventually a point is reached when the vapour begins to interfere with the downward liquid flow, holding up liquid in the packing. The increase in the pressure drop is proportional to a power greater than 2.

At this point, the pressure drop starts to increase rapidly because the accumulation of liquid in the packing reduces the void area available for the vapour flow. This area is called the "loading region". As the liquid accumulation increases, a condition is reached where the liquid phase becomes continuous .....

The problem with this traditional approach is the difficulty in differentiating between the transition points of the loading or flooding in the pressure drop curve. Some suggestions for the definition of when a packed column become fully "flooded" are :
* the slope of the pressure drop curve goes to infinity
* the gas velocity is so great that efficiency goes to zero
* pressure drop reaches 2 in.H2O per foot of packing
* pressure drop rapidly increases in a region, with simultaneous loss of mass-transfer efficiency ......

There are two forms of liquid hold-up in packed columns. One is referred to as static hold-up. Static hold-up is the amount of liquid that is held onto the packing after it has been wetted, then drained - the film of liquid or droplets of liquid that adhere to the packing. This amount jointly depends upon the physical properties of the liquid and the type and material of the packing.

The second aspect is the operating or dynamic hold-up. Dynamic hold-up is the amount of liquid held in the packing by the interaction of the vapour and liquid flows. Dynamic hold-up must be measured experimentally. To measure this amount, instantaneously stop the liquid and vapour flows, then collect and measure the volume of liquid that drains from the packing. The total liquid hold-up in packing is the sum of these two forms of hold-up.....since the static hold-up is constant, the operating or dynamic hold-up changes in proportion to changes in liquid and vapour rates. The void fractions in a packed bed may change across the bed due to fouling or damage, and vapour-liquid loads may be different along the bed for different operating conditions. The peak loading could occur anywhere in a packed bed, or a liquid distributor could initiate the flooding......

An interesting phenomenon for random packing and most corrugated sheet packing is that the separation efficiency of an "initial flooding" bed could be better than a "normal" bed, because of high liquid hold-up and intimate vapour-liquid contact in the "frothing" regime..... But at the high-efficiency state it is difficult to keep the column stable, and the column could go out of control as a result of any slight process turbulence. For this reason it is always recommended to avoid designing a packed column close to the initial flooding point. In operation we would not then be overly concerned with some liquid accumulation or hold-up, as long as the column could be kept stable and under control......

I use 3M ones myself as I have found them the best quality. Use a good quality one preferably. On a 1.5 or 2" column each should fill 55mm to 75mm (max) of column (less on 2"). Even less if you prefer. I tend to work in the vicinity of each filling somewhere between 55 and 63mm. At 55mm on a 36" column this equates to almost 17 from which I deduct 1 to allow for space at the top ie. =16. Allow at least 2" to 2.5" of clear space between the top of the scrubbers and the takeoff point for the vapour to expand into and so the reflux falls back into the scrubbers.

Do not unravel but tease them out by hand a bit so they fill the whole column diameter rather than just a part of it. Most of the ones I have seen in NZ do not have rubber bands around them. Place them into the column from the bottom one at a time using some sort of restriction at the top and bottom to prevent them going further or dropping out back into the boiler.. I use a 2" pall ring which works well.

On a slightly longer column (1m = 39.37" ) I use 19 off memory so 17 is probably around the right number. You dont want them too loose or too tight. If too tight they will compact more. The main thing is to have an even constant heat so you dont get surging. Surging causes compaction.

• Physical data for Ethanol & Water
• Pot Still calcs
• Reflux Still calcs.
• Detail of the Equations used
• Interactive Heat & Mass Balance model