Talk:Fractional distillation

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Contents

[edit] Flawed example

Before I go bold and remove the ethanol-water example, anyone interested in choosing something else and with a more accurate explanation? Here are two reasons:

  • In fractional distillation, the vapors and liquid have varying equilibrium concentrations along the vertical fractionator, with the most volatile component being most abundant at the top. However, since the batch setup in the example will slowly deplete in the more volatile component during operation, its concentration at the top of the column will continuously decrease in time and there will be no sharp "switch" to the less volatile component. Only the first few drops condensed out from this batch fractional distillation setup will have a relatively high purity in the most volatile component. The explanation in the text is better fit for the simple batch distillation (on which there is no article yet).
  • The particular ethanol-water mixture will form an azeotrope containing about 95% ethanol, which is actually more volatile than pure ethanol. See for example http://www.ces.purdue.edu/extmedia/AE/AE-117.html. So this will be the component that will separate first at the top of the column. --Unconcerned 23:52, 17 Aug 2004 (UTC)
I had the same thought while reading the example. It seems sufficient for its scope (lab benchtop distillation), however the explanation and results described seem to confuse the concept of batch distillation with continuous process distillation. Perhaps a separation of components with less affintity for one another such as a C3 splitter (propane-propylene splitter) would be more appropriate . . . C3 splitters are the biggest distillation towers I´ve ever seen (with respect to the number of trays 200+).;--StateOfTheUnion

[edit] trays vs. random packing

So can someone explain trays in the process (and how that would fit in with different faction outlets), and what they're good for, and why you might want raschig rings or ordered packing instead of them (in certain situations)? And link it up to reflux as well? I'd like to learn more about fusel alcohols and how they're taken out of ordinary distillation. I was pleased to learn about azeotropes, but there's more that I could learn...
~ender 2005-10-04 22:12:MST

[edit] Logical Inconsistency

The following sentence is illogical, and is factually flawed:

" In this example, a mixture of 95% ethanol and 5% water boils at 78.2°C, being more volatile than pure ethanol, so the ethanol cannot be completely purified by distillation."

The two concepts may be true from some point fo view (completely purified? Fine precious metals are never better than 99.98% pure, so how pure is "completely purified" ethanol? Can any method produce it? I think 99% or better is called "ethanol absolute", perhaps that's "complete purification".), but they do not follow from one another as written.

The reason that complete puriication by distillation is impossible has little to do with the boiling point, it's about both water and alcohol evaporationg into a less than saturated gaseous medium, and how it's impossible to saturate the gas with water and keep the alcohol at less than saturation. There is no temperature and pressure that will allow the distillation to perfectly separate the liquids. The purity of the distillate can be improved by distillation at a temperature significantly below the boiling point of either water or alcohol or of the mixture by taking advantage of evaporation and partial pressures in the surrounding gas.

Thinking about this, it would be interesting to see what happens in a mixture at around -20c, where the water has turned solid and the alcohol is still a liquid, to use vacuum distillation to take off the liquid alcohol, leaving the solid water ice behind. Clearly this would be expensive and impractical in a commercial operation, but it may be a method by which ethanol is completely purified (whatever the boundary of that could be) by distillation, in contrast with the statement in the article. Furthermore, absolute ethanol is obtained by distillation with benzine.

I have also identified that this is actually a misquoting/misunderstanding of the entry from the Columbia Encyclopedia - http://www.bartleby.com/65/et/ethanol.html - where they refer to simple distillation. Presumably the writer did not understand that the meaterial lost integrity when the word simple was omitted to avoid misappropriation of the material. Clearly this should be rewritten. Octothorn 02:51, 10 January 2006 (UTC)

See freeze distillation, where alcohol and water cannot be seperated by freezing. (Maybe vacuum freeze distillation might work?) :~ender 2006-01-31 19:29:PM MST

[edit] Bunsen Burner

Who uses a bunsen burner as the heat source for a fractional distillation? Most of the time, you risk setting your lab bench on fire! —Preceding unsigned comment added by 64.53.218.27 (talk • contribs) 7 February 2006 (UTC)

...I use one. Kilo-Lima|(talk) 14:32, 23 April 2006 (UTC)
Depends what you're fractionating really. Then again, strictly speaking it's probably safer to move away from the open flames, but if you need fast powerful heat...GreatMizuti 13:47, 29 July 2006 (UTC)

[edit] Deleted expansion stub

I deleted the expansion stub because:

  • I expanded the article by adding a schematic diagram and a discussion of reflux used in large-scale industrial towers.
  • I expanded the "See also" section considerably by adding links to the articles that deal with the various other types of distillation such as azeotropic distillation, freeze distillation, extractive distillation, etc.

In my personal opinion, what this article needs is to downplay its focus on laboratory-scale glassware distillation and upgrade its focus on the use of large-scale fractionators in industry. Perhaps that could be done by simply reversing the location of the glassware section and the industrial section?? What do others think? - mbeychok 00:08, 5 June 2006 (UTC)

[edit] Categories

Fractional distillation is indeed a vital part of the oil industry, if you don't think it fits under Industrial processes, then, quite frankly, you are quite mistaken. GreatMizuti 00:47, 18 September 2006 (UTC)

GreatMizuti, good for you! I agree thoroughly. Here is what I posted on Wsloand's Talk page:
I am curious as to why you are removing the "parent categories" from a number of articles. Regardless of the Wikipedia Manual of Style, I thought one of the prime purposes of Wikipedia was to help unkowledgeable readers and/or newcomers to Wikipedia find what they are looking for as easily as possible. I would point out that the Wikipedia Manual of Style is not written in stone and is not meant to be completely inflexible. It is meant to be guidance. Please explain your thinking on this subject. Thanks, - mbeychok 00:30, 18 September 2006 (UTC)
As an addendum to my above posting here is a quote from the Manual of style:
"This Manual of Style has the simple purpose of making the encyclopedia easy to read by following a consistent format — it is a style guide. The following rules do not claim to be the last word on Wikipedia style. One way is often as good as any other, but if everyone does it the same way, Wikipedia will be easier to read and use, not to mention easier to write and edit. In this regard, the following quotation from The Chicago Manual of Style deserves notice:
Rules and regulations such as these, in the nature of the case, cannot be endowed with the fixity of rock-ribbed law. They are meant for the average case, and must be applied with a certain degree of elasticity." - mbeychok 00:30, 18 September 2006 (UTC)

[edit] Trays

Is it possible to use a spiral for the column?Arnero 19:01, 22 October 2006 (UTC)

Arnero, If you are asking about industrial fractionators, I don't understand what you mean by a spiral. Industrial fractionators (or distillation columns) usually use vapor-liquid contacting devices known as trays (or plates) or some type of packing. There are various types of trays ... the most prevalent are those using either bubble caps or valve caps. As for packing (as noted in this article), it may be random dumped packing or structured sheet metal. - mbeychok 19:25, 22 October 2006 (UTC)
I suspect he's thinking of a graham condenser (for example, this Ace Glass item [1]). That's very good for condensing a vapor, due to lots of thermal contact with the cooling jacket, but I don't know why it would be a good for the fractionating column. Alternatively, there are spiral packing techniques for straight Liebig condensers. DMacks 08:00, 23 October 2006 (UTC)