[kenny1999]

i am not kidding. Although i am not a chemist or professional in this area, i learnt about

fractional distillation of liquid air more than 10 years ago, but I still do not understand

why nitrogen comes at the top of the fractionating tower while oxygen comes at the bottom,

I only memorize the facts over years. Now, I really want a reason. I have checked numerous

textbooks in my local areas and researched on web before I ask the question. It seems that

no people try to explain the concept very clearly.



I just want to know why oxygen comes out at the bottom and nitrogen comes out at the top.

I know that nitrogen has a lower b.p.than oxygen has, so I can understand nitrogen boils first,

but why doesn't it come out at the bottom? Why it will "fly" to the top of the bottom before it

comes out? Can't the nitrogen gas "fly" out at the bottom?

Same, as the temp increases, liquid oxygen would boil, so why doesn't it "fly" up to the top of the tower?

but it stays at the bottom?

When the oxygen or nitrogen comes out of the fractionating tower ,what are their states? (Liquid or Gas)?


I am really confused.

[fledarmus]

A fractionating tower works by creating a temperature gradient from the bottom to the top of the column, with the highest temperature at the bottom. The vapors rise up the column until they reach the part of the column that is at their condensation temperature, where they condense to form a liquid. The lower the boiling point of the liquid, the higher up the column they will have to go before the temperature is low enough for them to condense. Oxygen, which has a higher boiling point, will condense lower on the column, while nitrogen, with a lower boiling point, rises higher in the column.

This is a different system from what you typically see in a lab distillation system. In a lab system, you slowly raise the temperature of the pot, and as the boiling point of each component is reached, that component will boil off and the vapors can be collected at the top of the column. With a fractionating tower, the temperature of the pot is high enough to boil all of the components, and the temperature gradient along the tower determines where the vapors of each component will recondense.

This is a very simplified explanation and ignores the entire theoretical plate concept of fractional distillation, but it should at least explain why the lowest boiling materials are removed from the highest parts of the column.

[kenny1999]

A fractionating tower works by creating a temperature gradient from the bottom to the top of the column, with the highest temperature at the bottom. The vapors rise up the column until they reach the part of the column that is at their condensation temperature, where they condense to form a liquid. The lower the boiling point of the liquid, the higher up the column they will have to go before the temperature is low enough for them to condense. Oxygen, which has a higher boiling point, will condense lower on the column, while nitrogen, with a lower boiling point, rises higher in the column.

This is a different system from what you typically see in a lab distillation system. In a lab system, you slowly raise the temperature of the pot, and as the boiling point of each component is reached, that component will boil off and the vapors can be collected at the top of the column. With a fractionating tower, the temperature of the pot is high enough to boil all of the components, and the temperature gradient along the tower determines where the vapors of each component will recondense.

This is a very simplified explanation and ignores the entire theoretical plate concept of fractional distillation, but it should at least explain why the lowest boiling materials are removed from the highest parts of the column.

I understand that it has ignored the detailed stuff because I learnt about the detailed stuff many years ago but all has been given back to the teachers. I hope I understand what you are talking about. THanks again