Heat & Mass Balance calculations in Reflux Still's

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.
Phil suggests a cheap supply of ceramic packings though ...
    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
More about using plates (rather than packed columns ...
    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.
Hennie writes:
    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.


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