Common assumption is that a VM still works with the principle of ethanol will “sink into the produccondenser”.
The correct term should be “ alcohol enriched vapour is heavier than atmospheric air”. That’s the reason why some had more or less dramatic experiences when vapor hits their burner.
Following statement is my point of view, others may have a different opinion.
Water has a covalent bond of oxygen and hydrogen. The simple construction of the watermolecule (H2O), results in watermolecules can be tighter packed together than a typical ethanolmolecule (CH3CH2OH or C2H5OH) as ethanol also contains carbon atoms.
Therefore more watermolecules can be placed at a given space than ethanolmolecules, thus the lower density of ethanol.
We still agree on one ethanolmolecule is heavier than one watermolecule! The space that each molecule will fill is the key to all this, remember the SG 0,78 to 1,0 for 1 cm3.
As we heat our VM column, a separation of the different molecules that the vapor contains take place. A well known fact is that the higher boilingpoint , the lower in the column molecules will be. So some of the unwanted stuff will start to rise early in this process, that’s the reason we have foreshots. Then ethanol starts to boil off. Unfortunatly it will take some of the water with it on it’s way up the column. The explanation for this are the boilingpoints of water and ethanol are close 78,3C to 100C and..ethanol is hydroscopic (it loves water). Which is also the reason why we can’t distill higher than azeotropic conditions (95,6 ethanol by mass, NOT alcohol by volume ,ABV) under normal atmospheric pressure. Furthermore ethanol is a solvent, this means it reduces the surface tension on water. Now we can see why alcohols and water are fully miscible. We can also understand why we can’t just heat until 78,3C and all the ethanol magical draws off the still. Do you see were we get ?
So a mixture of water and ethanol is on it’s way up the column. We are still not violating any laws of nature here, and thermodynamics does it’s work. We know for a fact that warmer gasses will ascent (any hotair ballon proves that). Before that mix meets the refluxcondenser anyway, it has to displace the gasses that can’t be condensed in our still, the air. The constructor of this apparatus have seen that coming and placed a ventilation hole or tube at the top of that still. Just one more reason for that hole. There will be temperature fluctuations, that indicates things are about to happen in there.
As the vapormix rises it meets a cold surface (our refluxcondenser) now comes the point: Water will sustain more heatenergy than ethanol, thus the ethanol condenses faster than water and falls back down. Because ethanol by now has made a phase change and turned into liquid again (liquid drops, vapor rises) So here we have it…it’s not because ethanol is heavier, it’s because it gives off it’s energy faster than water. In other words,the phase change happens quicker for ethanol than water and so it enriches ascending vapor.Of course water also condenses and falls back the column. Now comes the point were we want to establish equilibrium in order to stack the vapors according to their boilingpoints. In this whole process of condensing and reboiling (as the NOW liquid hits the packing), the column establishes a temperature gradient dividing vapor according to their specific gravity. BUT…as watervapor contains more energy than all other vapor in the column it stays longer at the top, creating a “bufferzone” in the uppermost part. Still not violating any nature given laws we can now see, why we don’t get any output in a “water only” run on a VM still. It takes more energy to vaporize water than any other alcohol. The vapor MUST contain alcohol for this system to work. We also see why there is a typical auto shut off at the end of a run, as the alcohol is depleted and composition of the mix gets inferior. We can also understand why steamdistillation works…ethanol being heated by the heatenergy of water (steam).
There are other things to influence the vapor and Bernoullis law is one of the important factors. The simple law of energy preservation and pressure, makes a plane fly and a VM still work. Reynolds numbers also play a role. As we want a laminar flow through the column and a turbular flow (to cause whirl and eddys) before the takeoff point. Going much further into those well proven and empirical tested subjects, would perhaps blow the frame of theorys for hobbydistillation. Everyone however could take a look at some videos on youtube, that explains very good what goes on when we talk laminar vs. turbular flow and the transition that happens in between. Several threads in here are scratching those subjects too, but here is a link to underline my statement and FYI :
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A little resume on it would be that vapor till a certain degree can be compared with liquid. We just have to remember vapor (gasses) can be compressed, liquid can’t. That said we can compare a VM branch to a Y-piece used by the firedepartment, it divides the water coming from the main tube into two other connections. The water will now divide according to the path given for it and the speed will rise or lower with the diameter of the tube decreasing or increasing. Another fact we should be aware about is nature is lazy, it won’t use more energy than necessary. This means liquid and vapor will always take the easy way. That fact shows us how important correct dimensions on the takeoff branch are. We can’t expect a reasonable output with a 15 mm takeoff on a 2” column. A respectable distiller claims to have a good output with only 10 mm the vapor can pass through. I have no reason to question the mans credibility or competence ( and I certainly won’t). However he can’t change laws of nature and this could not be done without modifications above the takeoff, forcing vapor to take that path. Common consensus seems to be a equal dimensioned tee, is the way to go. This provides possible 1:1 refluxratio, as the amount of reflux depends on the ratio of “smallest crosssectional area before the productcondenser (usually the valve) and the path to refluxcondenser. Take a 2”x2”x2” tee and you got a 50/50 vaporsplit (that’s what a VM is..a vaporsplitter). Now narrow the outlet to 1”, it will change the whole setup as you now have a cross sectional areal 4 times smaller, throtteling output.
I think there is also a need for debunking an old myth about “heads compression” in a VM still. This is pure nonsense, as we can not compress anything in a system that is supposed to be open to atmosphere two different places in the construction! What we CAN talk about, is “heads concentration” as we concentrate those heads to the uppermost point in the still on smallest possible space. Not to dig into semantics, but a compression can only take place when there is pressure above atmospheric. Just that we don’t want anywhere in our still, thus the vent at top. Another common misconception might as well be killed here. There is no such thing as “sucktion in the productcondenser” or “chimney effect downwards the productcondenser”. Why is it that people want to see a vacuum or underpressure in a system open to atmosphere? Any “drag” downwards, would immediately be equalized by vapor produced in the boiler. And that boiler will give us just short of 46 L vapor pr minute for each KW that is converted into heat. The massive amount of vapor makes one understand that cross sectional areal of the column is very important. If you heat a tiny column of 1” with 3 KW, you will soon now what flooding means.
Those were some thoughts about a VM and it’s design, if I missed anything or any statement is not valid, please comment. After all we are pursuing the same goal…to understand distillation and stilldesigns better.
You still awake?
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