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Ray Hill, in his Jan 1981 article in Popular Science, prudently emphasizes that producing ethanol for use in vehicles is not an economical process, unless you have at your disposal a great deal of cheap energy or have a large installation that makes good use of heat recycling. If using propane to heat the boiler, then you would do well to weigh up the costs of building and operating a still compared with converting your car to using propane direct. He emphasizes this by comparing two installations: a small one built by Bob Brautigam and Lynn Doxon at the Tallgrass Research Center in Formoso, Kansas, and a large installation operated by Gene Schroder on his farm in Campo, Colo. Bob and Lynn manage to produce ethanol up to 180 proof, which is about as far as you can go with a simple still, but Gene clearly uses further chemical processing to remove all the water to get 200 proof pure ethanol. Such further processing is rather beyond what most amateur enthusiasts would care to undertake, but they may well be attracted by the design of the Tallgrass still. However, before anyone is tempted to start work making a still like that, it would be prudent to make one or two modifications to improve both safety and efficiency.
Ray describes the processes involved in this still, and it is clear from the diagram that those who designed it believed that distillation worked in this way. Unfortunately, it doesn't. The major error lies in believing that components in a mixture of liquids boil and condense at the same temperatures as they do when on their own. We therefore get the notion that all you have to do is provide a coil at the top of the tower which is maintained at precisely the boiling point of ethanol for separation of ethanol and water to take place - ethanol gets through, water doesn't. I'm afraid it just doesn't work that way. If it did, then why bother with all the business of a tower etc? All you'd have to do is heat the boiler to somewhere above the BP of ethanol, but below that of water, and voila! Instant ethanol vapor, as only the ethanol is boiling. Wouldn't that be wonderful?
These facts need emphasizing, as they have a direct bearing on how you go about designing an efficient still.
a) The condensation temperature of PURE ethanol vapor - with nothing else
with it - is the same as its BP - 78.5 deg C at standard pressure.
b) The condensation temperature of PURE water vapor - with nothing else with it - is the same as its BP - 100 deg C at standard pressure.
c) The condensation temperature of a MIXTURE of ethanol and water vapor - a mixture that is as intermingled as much as it is when liquid - is the same as the BP of that MIXTURE at standard pressure.
Molecules when mixed together do not, repeat NOT, behave the same as when they are separated!
So, with this in mind, what modifications would you make to the still shown in the article? For a start, no purpose is achieved by bubbling the vapor from the boiler through water at the bottom of the column. All this would do is cause vapor to be condensed until the water (now a mixture of ethanol and water from the condensate) reaches the same temperature as the vapor. You can bubble all you like for the good it will do you - it doesn't serve to strip any water from the vapor. An overflow, as shown in the diagram is needed - but it's needed to collect and then bleed off the ethanol depleted water that eventually comes down the column. Secondly, the 'stripping coil' at the top of the column serves no purpose save that of providing a little reflux (true definition of reflux is 'flow back' - not re-evaporation. It's the deliberate flow back into the column of ethanol rich condensate in order to shorten the length of column required). The main condenser at the top of the column is all that's required. Thirdly, and the MOST IMPORTANT, there should be NO CAP on top of the column. This is dangerous as it can lead to a pressure build-up resulting in an explosion should the boiler be delivering vapor at a rate too fast for the condenser to handle. When I first saw the diagram I thought it was a pipe bomb! Properly designed, the condenser should be able to extract all the heat thrown at it by the rising vapor, and nothing will come out the top. There SHOULD be a pressure relief valve in the line from the boiler however, as shown in the diagram, in case the packing in the column gets choked with too much fluid.
So not much needs to be done to improve on the design that's in Ray's article. I would throw in a just few suggestions:
a) Let the vapor get to the column directly instead of bubbling through the stripped
liquid in the bottom.
b) Do away with the 'stripping coil' half way up and use the space profitably with more packing material. The more surface area you have the better the column will work, so think of something better than marbles which have the least surface area of all shapes.
c) Increase the length and diameter of the column - a good working ratio is length/diameter = 20 to 25 for a middling sized assembly.
d) Throw away the cap and leave the top of the column open (think what raising the pressure does to the BPs all the way up the column - quite apart from the very real danger)
e) Use lagging all the way up the column to insulate as well as you can. You save energy and you get better stability.
f) Put a pressure gauge in the feed line from the boiler - you should be operating as close to ambient pressure as possible, and it would also serve as a timely reminder to turn the propane down if you're boiling too fast for the column to cope.
g) Lag the feed line from the boiler well, and place the column as far away from the propane flame as possible - no point in asking for trouble!
h) Keep the excellent idea of an inverted funnel to ensure that you have a good reflux ratio (you should try to aim to draw off only 10% of all that arrives at the top of the column at any time. The 90% goes back down as reflux to get vaporized again and be further enriched).
i) Be very,very happy if you manage to get 180 proof alcohol. If you don't, try slowing down and let the molecules work it out for themselves. Patience is the greatest design feature you can put into any still.
j) Don't do this in the house!!!
Drink it, or drive on it - don't do both!