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blogfast25
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Again, relatively minor gripe.
In modern fractionating theory, the process is considered just another counter-current separation method like liquid-liquid separations for instance.
Vapour gets sent up by boiling and liquid gets sent down by the refluxer. Mass transfer occurs between the two phases with the more volatile component
being enriched in the vapour phase according to the x,y operating diagram.
You want to call that an 'interaction' but that's unnecessarily vague.
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deltaH
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Quote: Originally posted by blogfast25  | | Quote: Originally posted by deltaH | Real life is typically twice that , as a very crude rule-of-thumb, because of inefficiencies. It's a really quick way to estimate if you want
to go from say 0.05 mole fraction ethanol to say 0.5, how many stages you're going to need.
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A minor gripe. That statement is fairly meaningless.
Fractionated columns in industry are designed using fairly accurate design criteria to make them fit-for-purpose.
Actual column performance is then determined mainly as the Number of Theoretical Plates (as well as other parameters) (McCabe Thiele, in essence).
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When I say 'real life', I mean real vs ideal, nothing about professional industrial column design. This amateur friendly rule of thumb for
distillation does in fact have a theoretical basis... the Gilliland correlation used by professional chemical engineers. As you probably know, but for
the benefit of others, Gilland undertook a study of actual commercial distillation over a wide range of types and purposes and plotted them on scales
so that an emperical correlation could be drawn between the number of real trays and the real reflux ratios in actual operation versus the theoretical
minimum stages and minimum reflux. What he found was this:
Now, notice that there is a much larger number of dots near the elbow region. If you take my abovementioned rule of thumb of N = 2N_min, substitute
that into his y axis formula and make the assumption that N >> 1, you arrive at an y-axis value of 0.5, which is the upper-bound of that main
grouping. That means this is a safe value to pick for most cases to get the job done within reason.
Professional column design, like you rightly say, are done to strict guidelines, but this rule of thumb is very amateur friendly if you're not trained
in chemical engineering.
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j_sum1
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Delta, you are not living up to your tagline very well.
| Quote: | | Dangerous source of unreferenced speculation |
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deltaH
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blogfast25
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Delta:
That is a very interesting study and new to me. Time allowing, I will look into that with more zeal.
I still prefer (old habits die hard!) the cumbersome calculations needed to design a suitable rectifying system from scratch because it's such a good
example of where rationalism, empiricism and historicism blend well and produce real world results.
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deltaH
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The Gilliland correlation is old, it's part of what's called the FUG[K] shortcut column design method (stands for Fenske, Underwood, Gilliland,
Kirkbride). First you calculate the minimum number of theoretical plates using the Fenske equation, then you calculate Rmin from the Underwood
equation, then use those values in the Gilland correlation with your rule of thumb of choice and finally the Kirkbride tells you the best tray to feed
on. Easy peasy... kinda 
It's covered over the net and in many chem. eng. separation texts.
***************************************
The free software Chemsep LITE, for those who are familiar with distillation column, is a very powerful and easy software to simulate equilibrium
columns, draw McCabe-Thiele diagrams, phase diagrams and much, much more. It comes with quite an extensive library of components and thermodynamic
models and data. For a modest fee, the author will sell you the full version that can do non-equilibrium column design with a very wide range of
industrial packings and trays. It's impressive to use. Chemsep LITE also automatically comes when you download COCO (which it integrates with), the
free process simulator for multiple unit operations.
Anyhow, I'm digressing from thumpers...
[Edited on 18-2-2015 by deltaH]
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blogfast25
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I'm only 18 so I wouldn't know about that... Nope, 81. Dammit, I'm so old I can't remember when I was born!
[Edited on 18-2-2015 by blogfast25]
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deltaH
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| Quote: Originally posted by blogfast25 |
I'm only 18 so I wouldn't know about that... Nope, 81. Dammit, I'm so old I can't remember when I was born!
[Edited on 18-2-2015 by blogfast25] |
You're as young as the woman you feel 
[Edited on 18-2-2015 by deltaH]
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blogfast25
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Question for Zombie:
What method do you use for alcohol determination?
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Magpie
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One method that may be of value would be "determination of boiling point." Toggle the diagram below. However, I would not try to use this particular
diagram as the axes are not scaled. Surely there are scaled ones available on the internet.
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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Magpie:
Accurate BP determinations (e.g. capillary method) aren't easy and are time consuming. We may have to analyse up to 50 samples.
[Edited on 18-2-2015 by blogfast25]
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deltaH
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Ethanol and water have strongly differing dielectric constants, so measuring the capacitance of the solution with a multimeter using a small vial and
two fixed electrodes should work well. You would need to calibrate of course from a set of alcohol solutions of known strength.
NB: the electrodes would need to be physically well fixed in the vial to not move at all so as to maintain consistency and insulate the electrodes
with a thin layer of... whatever 
Physics is NOT my strong point, so not sure if this can work.
If too small a capacitance is a problem, one could use an open multiplate variable capacitor and immerse it (not sure if immersion would be a
problem). The only danger there is that you would need to make sure that no air bubbles trap between the plates as this would introduce an error. Tap
it?
[Edited on 18-2-2015 by deltaH]
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aga
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Whatever method of determining the EtOH concentration won't matter muchly, so long as it's granularity is reasonably fine (e.g. 0.5%), is done with
the same equipment, and by the same operator.
It's a Comparison, so the absolute accuracy will not be vital.
Your suggested refractometry thing would be fine for this purpose.
Some calibration would make your data more generally useful of course.
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blogfast25
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| Quote: Originally posted by aga |
Your suggested refractometry thing would be fine for this purpose.
Some calibration would make your data more generally useful of course. |
In all likelihood, yes. A rough idea of both accuracy and repeatability would be useful. A simple statistical analysis on the obtained data would give
an idea of confidence levels.
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deltaH
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| Quote: Originally posted by aga | Whatever method of determining the EtOH concentration won't matter muchly, so long as it's granularity is reasonably fine (e.g. 0.5%), is done with
the same equipment, and by the same operator.
It's a Comparison, so the absolute accuracy will not be vital.
Your suggested refractometry thing would be fine for this purpose.
Some calibration would make your data more generally useful of course. |
The refractive indices of water and ethanol vary by only 0.027. So increments of 0.5% would mean increments in refraction index measurements down to
0.000135 Is that typical of these instruments (never worked with one, so
wouldn't know)?
[Edited on 18-2-2015 by deltaH]
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blogfast25
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| Quote: Originally posted by deltaH |
The refractive indices of water and ethanol vary by only 0.027. So increments of 0.5% would mean increments in refraction index measurements down to
0.000135 Is that typical of these instruments (never worked with one, so
wouldn't know)?
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Yes. There are refractometers that are specifically designed for EtOH/water, in that range of index.
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deltaH
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Nice! Cost?
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blogfast25
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About £25.
I think 1.0 % will be about the accuracy to be expected but if the overall variation S is good then that will be good enough.
Better than 'billy bubbles'!
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deltaH
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Yeah, PERFECT!
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chemrox
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The thumper serves as a partial takeoff head to drain off water and some of the "fusel oil". It's not as efficient as some of the fractionators built
into modern so-called moonshine stills.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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aga
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There's a Digital refractometer :-
http://www.ebay.co.uk/itm/Refractometro-digital-de-Alcohol-p...
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Zombie
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| Quote: Originally posted by blogfast25 |
Again, relatively minor gripe.
In modern fractionating theory, the process is considered just another counter-current separation method like liquid-liquid separations for instance.
Vapour gets sent up by boiling and liquid gets sent down by the refluxer. Mass transfer occurs between the two phases with the more volatile component
being enriched in the vapour phase according to the x,y operating diagram.
You want to call that an 'interaction' but that's unnecessarily vague. |
Mass Transfer. Correct. Even when I know the lingo I tend to oversimplify.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Zombie
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Breathalyzer... 
No, I have a Chase Inst. hydrometer calibrated at 60*f, and use a Kessler -20*C / +150* C thermometer.
My runs begin @ 80% abv, and I stop @ 20% abv.
Average or final proof @ 100.
[Edited on 18-2-2015 by Zombie]
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Zombie
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| Quote: Originally posted by Magpie | One method that may be of value would be "determination of boiling point." Toggle the diagram below. However, I would not try to use this particular
diagram as the axes are not scaled. Surely there are scaled ones available on the internet.
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I posted one up in the first few pages here.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Zombie
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| Quote: Originally posted by chemrox | | The thumper serves as a partial takeoff head to drain off water and some of the "fusel oil". It's not as efficient as some of the fractionators built
into modern so-called moonshine stills. |
Pretty much on the money there.
A large part of what they do is compress the fractions allowing the lightest to come of cleaner. Same with the desired middle fraction (EtOH), and the
less volatile keytones, fuesils last.
[Edited on 18-2-2015 by Zombie]
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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