What exactly is fractional distillation?

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Could someone please explain fractional distillation? I find it really confusing to understand.
Thanks!
 

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  • #2
berkeman
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Could someone please explain fractional distillation? I find it really confusing to understand.
Thanks!
What specific questions do you have when reading introductory articles like the one at Wikipedia? Is it the function of the column that is confusing?

https://en.wikipedia.org/wiki/Fractional_distillation

1581773941260.png
 
  • #3
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What specific questions do you have when reading introductory articles like the one at Wikipedia? Is it the function of the column that is confusing?

https://en.wikipedia.org/wiki/Fractional_distillation

View attachment 257118
It says that there is a temperature gradient in the fractional column. So the component with higher boiling point condenses before the one with a lower boiling point as it rises up the column. This condensation and vapourization occur and gradually they are separated.
But how does it reach the top i.e, the condenser?
 
  • #4
hutchphd
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This works like an old "single pipe" steam radiator in a house. The condensation pulls vapor that is pushed by the boiler. The vapor necessarily comes from the cooler top of the distillation column (which will gradually get hotter as the batch is fractionally evaporated)
 
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DaveE
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But how does it reach the top i.e, the condenser?
The basic source of movement is the boiling in the flask. When liquids are vaporized they expand greatly, so there is a net flow away from the flask. The distillation process is a selection of which molecules travel farthest in a decreasing temperature environment to get out to the condenser. Some will condense back to liquid in the column and fall back into the flask. Note that if the temperature at the top of the column is cold enough that everything re-condenses, then there will be no net flow out to the condenser.
 
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chemisttree
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This works like an old "single pipe" steam radiator in a house. The condensation pulls vapor that is pushed by the boiler. The vapor necessarily comes from the cooler top of the distillation column (which will gradually get hotter as the batch is fractionally evaporated)
Nah!
 
  • #7
chemisttree
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The mixture of compounds in the pot boils. The vapor is enriched in the lowest boiling component. It rises into the column and condenses. This droplet of condensate does not have the same composition as the pot it just left; it is enriched in the lowest boiling component.

The boiling point of this new drop of condensate is a bit lower, closer to the BP of the lowest boiling component. It starts on its way back to the pot but it is blasted by the hot vapor rising from the pot and itself begins to boil. The vapor from that first drop of condensate is further enriched in the lowest boiling component and moves even further up the column until it condenses.

This second drop of condensate has a different composition than both the pot and the first drop of condensate. Its boiling point is even lower and closer to the BP of the lowest boiling component. It too starts back down toward the pot but it is blasted by hot vapor rising from below and then it boils too. Its BP is lower than both the pot and the first drop of condensate just below it in the column.

A series of these boiling, movement upwards and condensations occurs until we run out of length of fractionating column and the next drop falls over into cooled condensation column and runs downward, this time into the receiver. Hopefully, the fractionating column was long enough to nearly completely isolate the lowest boiling component.

Its BP will register on the column’s thermometer and is lower than the temperature of the pot and all of the fractions condensing below it in the column.
Each of these boiling/condensation events is referred to as a “theoretical plate”. Fractionating columns should have many theoretical plates and putting high surface area stuff into it tends to increase the theoretical plates.
 
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