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Author Topic: Steam Distillation - a question  (Read 11331 times)

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Steam Distillation - a question
« on: October 28, 2005, 07:30:44 PM »

Assume a mixture of toluene (bp 110.8C) and water that is steam disilled.
After the process is over, if you look at the distillate, you see that there is more toluene than water. But if the bp of water is 100 C - so how can it be ?

Thanks in advance !


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Re:Steam Distillation - a question
« Reply #1 on: October 28, 2005, 09:26:54 PM »


do you know about azeotropic mixtures?.

An azeotropic mixture has a boiling opoint that does not change when vapour is removed by evaporaion. The boiling point may be higher or lower than any of the components of the mixture.

toluene / water mixture has bp 85.0oC, but bp toluene is 110.6oC and bp water is 100.0oC.  For the toluene-water system, below 85oC the two liquid phases coexist. At 85oC the sum of the vapour pressures of water and toluene = one atmosphere, and boiling starts.

In a mixture of toluene and water, boiling occurs at 85oC and both water and toluene are in the vapour that condenses to form two layers. The bottom layer of water has 0.06% toluene dissolved in it. The top layer of toluene has 0.05% water dissolved in it. The relative volumes are 18% water and 82% toluene. The excess toluene flows back into the flask and distils back over with more water. At 85oC, vapour presure of water = 57.7 kPA and vapour pressure of toluene = 50.6 kPa. Weight of water = (molar mass water /molar mass toluene) X (vapour pressure water / vapour pressure toluene) = (18 / 92) X (57.7 / 50.6) = 0.223 = 20%.

I hope this will clarify your doubt.


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Re:Steam Distillation - a question
« Reply #2 on: October 29, 2005, 07:17:37 AM »

hi, first - thanks a bounch !
No, I don't know about such solution.
I did understnat what you wrote - but - I did not understand the molecular reason it happend.


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Re:Steam Distillation - a question
« Reply #3 on: November 01, 2005, 02:50:45 PM »

Toluene and water differs by their intermolecular bonding forces. The water is held together by hydrogen bonding and the toluene by mostly van der Waals. When the two are mixed or steam distilled the intermolecular bonding forces lower because the hydrogen bonding of water is disrupted by the presence of toluene and the van der Waals of toluene is disrupted by the presence water. While neither benefit by bonding with the other. The results is a lowering boiling point.

Steam distillation is quite slick. One development project I worked on was steam distilling chloronated solvents out of machine oils. The direct boiling of the oil /solvent was high in temp and left too much solvent residue, but add a steam lance into the oil/solvent tank, and chlorocarbons stripped off at the boiling point of water leaving nice clean oil. In this case, the chloro-solvents would H-bond to the water and become stripped from the oil. They would then phase separate from water upon cooling for clean recycle.  
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