i need help with the fractional distillation method of a couple of substances at least more than two substances , just like air actually what am i looking for is that why do substances in air with a low boiling point like nitrogen reach higher the fractionating column even if it looks like that substances with higher boiling point will absorb more energy and thus eventually result in greater kinetic energy (at least to me). please help . heres how i came to know that nitrogen rises higher the fractionating column

distillation process


Distillation relies on the fact that, at the same stage (tray), all the substances are approximately at the same temperature. Then, at the same temperature, the vapour will be composed preferably by substances with a lower boiling point and the liquid composition will be dominated by higher boiling point compounds.

Very quickly, distillation works as follows. The vapour flow from one stage (i) is then richer in the light compound (with lower boiling point). That vapour is enters the immediately higher stage (i-1) making the composition richer in the light compound. The vapour from stage i-1 is even richer in the light compound. This process is repeated until you reach the top stage where the light compound can be almost pure. Likewise, the liquid of stage i is rich in the heavy compound and goes down to stage i+1. The liquid leaving stage i+1 is even richer in the heavy compound. At the bottom, you can reach and almost pure heavy compound.

Formally, the fractionation of a mixture is dictated by the relative volatility of the compounds (which for ideal solutions does not vary along the column). At low/moderate pressures and for an ideal solution, the relative volatility is simply
$$ \alpha =\dfrac{\dfrac{y_1}{x_1}}{\dfrac{y_2}{x_2}} = \dfrac{P^s_1}{P^s_2} $$ We usually take the heaviest compound as the bottom line for comparison. In your example, at the pressure and temperature conditions of the column, the relative volatility of nitrogen/oxygen is higher than argon/oxygen. Therefore, nitrogen leaves the column at the top, argon at an intermediate stage and oxygen at the bottom.

  • $\begingroup$ that hopefully explains why at the same stage(tray) the substance with lesser boiling point dominates . but why do the higher melting points condense at a particular tray and the lower melting points rise higher ? thanks in advance $\endgroup$ – user24065 Dec 23 '15 at 13:12
  • $\begingroup$ I have edited my answer to respond your query $\endgroup$ – Toulousain Dec 23 '15 at 13:38
  • $\begingroup$ thanks , it explains everything now is this your own opinion or have you taken it from a source please mention if there is any i am asking you this because i had a similar post and someone posted some diffrent explanation. i would had provided th link but the post is gone now $\endgroup$ – user24065 Dec 23 '15 at 14:14
  • $\begingroup$ No, this is not my own opinion. You can take any textbook on distillation and it will say the same. I would recomend "Separation Process Principles" by Seader and Henley; "Separation Process Engineering" by Wankat $\endgroup$ – Toulousain Dec 23 '15 at 15:24
  • $\begingroup$ one second i pondered over your explanation and it suggests that if we achieve the boiling points of any two of three liquids in diffrent containers , then in a specific interval of time all should have achieved the same height ? but we know that this is not the case ! please explain $\endgroup$ – user24065 Dec 23 '15 at 15:43

The gases in the distillation column boil off according to their boiling point.

   Gas          Boiling point
   Nitrogen     77.355 K ​(−195.795 °C)
   Argon        87.302 K (−185.848 °C)
   Oxygen       90.188 K ​(−182.962 °C)

The distillation column shown is setup for continuous distillation. But imagine that a pot distillation was being done. Nitrogen would boil off first, then argon, then oxygen. So the various trays in the distillation column represent different temperatures. Nitrogen boils off at the top since it has the lowest boiling point, and oxygen collects in the "pot" since it has the highest boiling point.

  • $\begingroup$ that answered my whole question , thanks ! actually i did not knew that the trays in distillation flask are not of the same trempreature throughout . But what about a fractionatig column with marbles in it ? it has no trempreature diffrences $\endgroup$ – user24065 Dec 24 '15 at 0:50
  • $\begingroup$ The marbles would act as the trays. There has to be a temperature difference from the bottom to the top of the column or it isn't a "fractioning column." $\endgroup$ – MaxW Dec 24 '15 at 0:55
  • $\begingroup$ but the laboratory setup for the marble distillation column does not show any trempreature difference ! $\endgroup$ – user24065 Dec 24 '15 at 1:25
  • $\begingroup$ In a typical pot chemistry experiment you heat the liquid in the pot and the vapor carries the heat up the column. $\endgroup$ – MaxW Dec 24 '15 at 1:28
  • $\begingroup$ thanks ! for a second i thought that in the marble fractionating column you need to collect the pure gas from below the container! hopefully i was wrong , right ? $\endgroup$ – user24065 Dec 24 '15 at 1:33

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