Calculation of a Reflux Still Heat & Mass Balance

This here lets you model the heat and mass balances around a typical 4 plate column.

Picture a portion of a distillation column ... hot rising vapour gets to meet some cooler liquid dripping down past it (over your marbles or stainless steel scrubber packing). As it cools a little, and some of the vapour that is high in water content condenses out, it releases enough heat to warm up the liquid and also vapourise a ethanol rich vapour from the liquid (and thus improves the % alcohol in the vapour as it goes up the column). Each gram of water that condenses out of the vapour gives up enough energy to vapourise 2.6 g of ethanol out of the liquid.

If you put a cooling source midway through the column you disrupt this; the liquid dripping down will get quite a bit colder there, and the rising vapour will no longer have enough available heat content to get the alcohol out of it. So this portion of the column doesn't do anything. No gain in alcohol % . No point having it there. The liquid is going to have to drip down quite a bit further, and pass quite a bit more vapour before it gets back up to working temperature. The rising vapour is going to be cooled too - so it won't be as effective on its upward path either. Same reason why you should insulate the column - avoid unnessesary heat losses sapping the energy from the vapour. What you are after is a nice steady transition in temperature up the column, from its base (96C ish) to the top (around 78-79C).

To run this model, just select the heat input into the pot, the cooling rate at the condensor (these values should roughly be the same), and the reflux ratio (how much liquid you return down the column vs the amount you keep). You can also elect to do some additional cooling by any of the plates (eg put cooling pipes through the column).

How do you read it ? The Purity & Temperature are already determined by the vapour-liquid equilibrium. Each plate here is the end-point on one of the "steps" as shown in the Theory section earlier. The "Vapour" column shows you the flowrate of vapour entering the plate, both the total flowrate, and the flowrate of just Ethanol vapour. The "Liquid" column is the amount of liquid entering the plate, total & ethanol too.
Inputs
Heat Input to Pot W Wash Purity % alcohol
Heat Out of Condenser W Reflux Ratio mL refluxed/mL kept
Cooling Water Flowrate mL/s (Do Iterations)

StageTemperature
(oC)
Vapour Flowrate
Total & EtOH (g/s)
Liquid Flowrate
Total & EtOH (g/s)
Extra Heat
Loss (W)
Stage 4    
Stage 3    
Stage 2    
Stage 1    
Distillate Flowrate: (g/s) at oC (subcooling of oC )
Heat to be Removed at Condensor : Min. = W : Max. = W
Change in Condenser Cooling Water Temperature = oC

Try a couple of exercises/examples with this ...

  • Cooling Rate - Minimum & Maximum The heat removed from the column has to be the same as that put in to it. This means that for any given heat input, there is a minimum rate at which you should cool it (total of condensor heat loss and extra losses). If you go below the minimum, not all the vapour will get condensed (the "subcooling" value will be negative (eg hotter than its dew point)) and will be lost as a vapour (good for setting fire to the place). If the cooling rate is too great (eg 1400 W vs 1200 W) the distillate gets too cold, and ends up affecting the top couple of plates. This is because there isn't enough heat in the rising vapour to both heat the down-coming liquid (because its now so cold) and extract the most ethanol from it. So in practice don't run your cooling water too fast - keep the distillate fairly warm.

  • Pass-through Cooling Pipes - see what happens if you do additional cooling halfway down the column (eg set the heat loss at plate 3 to 200 W). The flowrate of ethanol vapour stops improving, and that section of the column basically ends up doing nothing. This is because the only heat available (in the rising vapour) is now going into trying to heat the cold liquid there back up to its equilibrium temperature, and so doesn't get to vapourise any ethanol out of the liquid. So in practice, do all your cooling/condensing at the very TOP of the column, and not part-way up it. This lets the liquid come to the right temperature at the top, so that it can then progress down the column smoothly, with all the heat in the vapour being used to vapourise ethanol out of the liquid. Also insulate your column, so as to avoid unnessessary heat losses.

Re-reading the explanation in "Moonshine made simple and still makers manual", by Byron Ford, of how a reflux column works, its apparent that there's still a bit of confusion about, of exactly how refluxing works.

When the vapour comes off the wash, it is a MIXTURE of both ethanol and water vapour.If you put something cold in its path, the MIXTURE will condense.

You do not get just the "heavier fractions" condensing, letting the "lighter fractions" go past. If it were that simple, all we'd need to do is put something in-between the pot and the condensor at say 85-90 C, and it would cause all the water to drop out, and let pure ethanol go past, and deliver 100% purity in a single step. But life ain't so.

What does happen instead is that the cooler liquid mixture starts dripping back down the column, going past the hot vapour on its way up. This causes the vapour to cool a little (just a weeny bit). As it does so, it finds that it can now hold more alcohol & less water. At the same time, the liquid as it warms a little, realises that it can hold less ethanol and more water. To get back to "equilibrium", they swap a little between the two of them - the vapour will take some of the ethanol from the liquid, and the liquid will take a little of the water from the vapour. The exact content of each depends on the temperature. But the overall effect is that the vapour gets richer in ethanol % the higher up the column it goes (and into the cooler sections at the top of the column), provided it is passing by some liquid which it can "equilbriate" with.

Similar result you say to just calling it as "just having the water condense out" ? Why is he so bloody pedantic about it ? Because its not quite the same. If it were "just the water condensing out", then you wouldn't need any packing below the "through tubes". You could just have a really really short column, with heaps of through tubes to knock out all the water, and get great results. But this doesn't work. What happens is that you'll just end up recycling heaps of your mixture, and very little would sneak past to get to the condensor. Go on, try it sometime (sarcasm).

You need the "equilbrium" to occur. This needs surface area to spread the liquid out over (in a thin film), so that the vapour can mingle together with it. This is why we need taller columns, with packing inside.

Aha you say - but the "Still Spirits" reflux stills with no packing, still work somewhat, getting say 75% purity, improved over a regular pot still at 60% purity. Yes, this is because they still do have a little surface area for the reflux liquid to spread out over - the walls of the column on the way back to the pot. But this is really ineffective, because theres not a lot of it. Note the great results that Mos&Liz, and Kev have got, simply by putting some packing beneath, for the same amount of reflux to spread out over. Into the mid 90's %. Makes a difference huh ? The purity hasn't increased because they suddenly started refluxing more liquid, but because they've given it a surface to spread out over.

Sorry if I'm upsetting anyone by riding my "hobby horse" again, but it upsets me when a book showing still design gets it wrong. It would lead you to the wrong decisions when trying to improve your still. Yes, lower through tubes will help if you've over-sized your heating element, and you need to limit the amount of vapour going up the column, but they're not doing anything for the purity. Its only the liquid dribbling down over the packing which will do that for you.

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