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Ultra-Simplistic View of Fractionation

Any sort that runs continuous. Examples are beer stripping stills that have a continuously, vodka stills fed from a beer stripper or any sort of spirit doubler, meaning continuously fed pot still.

Ultra-Simplistic View of Fractionation

Postby Abussive » Sun Aug 05, 2012 1:01 pm

With a sting in the tail.......

Imagine a thin film of liquid falling downwards over a small, regular area (Pale Blue)
Immediately above its surface lies a layer of gas (Purple), separated from it by an infinitesmally narrow gap.


The liquid falls downwards, and the gas flows upwards.
The two phases are separated only by the infinitesmally thin gap.
The gap is so thin that any molecules moving in either layer (gas or liquid) which "hit" the gap, cross the gap.
With reference to Mike Nixon's excellent summary of these circumstances (where there is no heat input capable of actually BOILING the liquid) molecules of both gas and alcohol will "exchange" between the layers until a dynamic equilibrium of alcohol/water gas and alcohol/water liquid is established... over the entire available surface area where the gas meets the liquid. It all happens completely by standard Evaporation (NO boiling).

Of interest to us is that the thinner the gas and liquid layers (in other words the LOWER the volume needed to create the gas/liquid "tablet") and the bigger the area of available interchange, the FASTER the attainment of that equilibrium.
(That's classically put into scientific form by Langmuir, but be warned, it's complex stuff!)

All we need do is imagine a vertical stack of these "tablets", where the falling liquid starts as recycled distillate (i.e. "reflux"). As it falls downwards from tablet to tablet, it gradually depletes its alcohol. Conversely, as the gas phase rises, it gradually picks up alcohol from the falling liquid.

The rising gas is classically delivered from a boiling Pot, and may attain concentrations as high as 95% ABV by the time it (the vapour) reaches the top of the stack.

The sting in the tail:

Consider the bottom tablet in the imaginary stack.
It's liquid has reached equilibrium (for the associated temperature and pressure) with the newly-admitted vapour passing by, upwards.
So it cannot be much different in composition to the liquid currently boiling in the pot!
It drops straight back into the pot, to be reboiled.

In a continuous fractionator, the gas admitted to the bottom tablet may be PURE water (steam at 100C), so that it is entirely possible for there to be ALMOST NO ALCOHOL AT ALL in the effluent falling out of it as "bottoms". Just water and relatively non-volatile, high-BP compounds.

A complicating factor for continuously-operated fractionators is that ALL fractions must be removed continuously too, otherwise they accrue, and will gradually pervade (smear) into other collected fractions.
If "nasties" are close to "flavours" we can expect difficulty!

Important limiting factors:
If the thickness of the liquid layer is made too small, it will not fall in the expected manner due to capillary effects and/or entrainment (as droplets or mist) by the rising gas.
Under those circumstances, the fractionator will be "choking" and totally fail to resolve alcohol. The temperature differential between its top and bottom will disappear too.
If the gas speed is too high, it might encourage entrainment of liquid as droplets or mist however thick the liquid layer.
Again, choking begins.

Note though, that the amount (volume) of liquid retained (aka "hold-up") on the fractionator support surface is pretty much irrelevant as long as the choking point is avoided.
What really matters is maximising and maintaining the available liquid/gas contact surface area, the speed at which they pass each other, the temperature and pressure (which establish the point at which equilibrium occurs).
The astute reader might notice that via the Bernoulli Effect, high gas speeds may create a pressure drop, and thus alter the equilibrium point(s) from that expected.........but whatever is done, in an un-choking contiinuous still, it will stay "the way it is set" due to the Steady State characteristics. Anything and everything discovered/unveilled should be repeatable and transferable!


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