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Uncomfortable Reality?

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.

Uncomfortable Reality?

Postby Manderson » Thu Oct 08, 2015 1:27 pm

This post is aimed only at those interested in the Science behind component separation via fractionation with the objective of deeper dissemination of understanding the key factors involved.
Especially by Continuous fractionation.
Note that I didn't say simply "distillation".
Pot distillers with a scientific bent and using fractionating devices (plate, packed columns etc) may also find the expected discussion of strong interest. Their stills are just more complex to quantify because they don't operate at a Steady State condition. Everything varies for them, all of the time!


The following link presents the Science behind a routine phenomenon called Steam Distillation:-
http://www.chemguide.co.uk/physical/phaseeqia/immiscible.html

All of the following presumes a fractionator operating at ambient pressure and containing no hidden different pressure zones within it.

Wherever you read "Phenylamine" in the link, replace it in your mind's eye, with "Congener X", and imagine innumerable different congeners y,z,a,b etc etc.

Wherever you read "Steam" replace it with "MANY relatively volatile compounds". And think of Alcohol as one such compound, too!

What's going on, as it applies to us 'stillers, in particular those who distill continuously; because we can see cause and effect more easily and clearly in our stable, Steady State stills?

It's not that obvious that once even some of the wash is vapourised and moving up a fractionator, and reflux is passing down that fractionator, the action between the gas and liquid phases enriches the vapour with alcohol, and the falling liquid with water. To different degrees and at different temperatures in different places in the fractionator. Almost like a plate fractionator with an infinite number of plates, perhaps.
Not rocket science, and nothing new about that, 'eh?
Well, not yet anyway.
Now we throw a Big Spoke into the "understanding" wheel.....

Our wash doesn't just contain water and alcohol; there are small amounts of hundreds of other things in there. Literally HUNDREDS.
And they all are able to "Steam Distil" up the fractionator, AT DIFFERENT mobility rates depending on their individual vapour pressures, at the temperature of their environment at ANY particular point as they travel upwards. Each of these myriad Steam Distillations is running separately, and in parallel with the better understood simple alcohol/water molecular exchanges already mentioned: a multi-variable, multi-condition system.
Because Vapour Pressures vary with temperature, they aren't usually linear either!

If we aim for Azeotrope output, and the fractionator efficiency is high, only low-boiling liquid (alcohol + volatile "heads") will get to the top.
But aiming at somewhere less than azeotrope, some of the heavy "congeners" will always be present.

Ah, but Master Distillers know, from experience, that careful management of the throughput of the fractionator allows them to "steer" the output so that desirable congeners make it upwards, and undesirables ("tailsies") tend to fall (mostly) downwards.

We knew that!!!!
What's the beef?

Few have twigged that if the fractionator's resolving power (the ease with which it can separate all mixture components) combined with its throughput capacity are the sole regulators of how easy or difficult that is to achieve. Which is why an SPP-filled column - IF NOT DRIVEN TOO HARD - can produce VERY clean "cuts" on a pot still, and SUPER clean cuts on a continuous still. At low throughputs it indeed has very high component resolving power.
Overdrive it with throughput though (to the point of choking maybe?) and its resolving power breaks down and output quality is destroyed.
Somewhere in-between, stillers refer to "smearing": in other words,lousy resolving performance
It really is, scientifically, THAT simple!

1. Get the bestest fractionator you can build in terms of resolving power AND throughput.
2. Don't worry too much about the pot (or reboiler) or the condenser. If they are poorly designed, the effects are obvious and easily rectified.
Building a QUALITY fractionator though, is something else entirely!
Even being able to accurately measure its Resolving Power vs Throughput Capability is almost unheard of outside of scientific circles.
You CANNOT separate alcohol from water AND steer the desired congeners into your output without building a fractionator of substantially better quality than usual today, in terms of resolving power and throughput performance together.
HETP alone is worthless if taken alone (it doesn't encompass throughput).

But this isn't rocket science either, if you apply yourself to the problem step-by-step.....

We can discuss further these things?

Teatime!
Manderson.
:8)
Manderson
 
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