[jsmith613]

Fractional Distillation is only used to separate mixtures that have b.p with a difference smaller than 25. I would presume this is becasue normal distillation would be less efficient and may not actually seperate the mixtures properly. Is that correct.
Additionally, why do we use fractional distillation for crude oil, surly simple distillation would be cheaper and more economic (I make my assumptions from this picture http://en.wikipedia.org/wiki/File:Fractional_distillation_lab_apparatus.svg

[typhoon2028]

Fractional distillation columns are expensive.

[jsmith613]

Fractional distillation columns are expensive.

exactly, so why do we use them for crude oil

[DrCMS]

What are the materials present in crude oil and how close are their boiling points?

[jsmith613]

What are the materials present in crude oil and how close are their boiling points?

the b.p are quite far apart, as far as i know

components include
refinery gas
petrol
naphtha
kerosene
diesel oil
bitumen

[typhoon2028]

You do realize that none of the materials listed are pure substances?

The materials on your list will can be removed from a distillation column.  To remove them at the right boiling point, you need a fractional distillation column.

Differents cuts or fractions inside the column produce your listed components:  refinery gas at the top, then moving down the columne to gasoline (petrol), kerosene, diesel fuel, and bitumen at the bottom {from your list}.

I believe naphta is between petrol and kerosene.

[jsmith613]

You do realize that none of the materials listed are pure substances?

The materials on your list will can be removed from a distillation column.  To remove them at the right boiling point, you need a fractional distillation column.

Differents cuts or fractions inside the column produce your listed components:  refinery gas at the top, then moving down the columne to gasoline (petrol), kerosene, diesel fuel, and bitumen at the bottom {from your list}.

I believe naphta is between petrol and kerosene.

so are you saying that the reason we use a fractioning column is to obtain a pure substance, and that simple distillation would produce impure mixtures?
If not please explain

[DrCMS]

You do realize that none of the materials listed are pure substances?

The materials on your list will can be removed from a distillation column.  To remove them at the right boiling point, you need a fractional distillation column.

Differents cuts or fractions inside the column produce your listed components:  refinery gas at the top, then moving down the columne to gasoline (petrol), kerosene, diesel fuel, and bitumen at the bottom {from your list}.

I believe naphta is between petrol and kerosene.

so are you saying that the reason we use a fractioning column is to obtain a pure substance, and that simple distillation would produce impure mixtures?
If not please explain

Crude oil is a complex mixture of longer chain hydrocarbons.  Crude oil is cracked to give shorter chains which are separated by fractional distillations. 

refinery gas
petrol
naphtha
kerosene
diesel oil
bitumen

The materials you have listed are mixtures they are not pure substances.  They are boiling point fractions from the cracked crude oil.

[jsmith613]

You do realize that none of the materials listed are pure substances?

The materials on your list will can be removed from a distillation column.  To remove them at the right boiling point, you need a fractional distillation column.

Differents cuts or fractions inside the column produce your listed components:  refinery gas at the top, then moving down the columne to gasoline (petrol), kerosene, diesel fuel, and bitumen at the bottom {from your list}.

I believe naphta is between petrol and kerosene.

so are you saying that the reason we use a fractioning column is to obtain a pure substance, and that simple distillation would produce impure mixtures?
If not please explain

Crude oil is a complex mixture of longer chain hydrocarbons.  Crude oil is cracked to give shorter chains which are separated by fractional distillations. 

refinery gas
petrol
naphtha
kerosene
diesel oil
bitumen

The materials you have listed are mixtures they are not pure substances.  They are boiling point fractions from the cracked crude oil.

so getting back to the bid question, what is the actual purpose of fractional distilation as oppose to simple distillation if it does not even produce pure substances

[typhoon2028]

Simple distillation won't be able to separate the components and give you the products you want.

If you want the products: gasoline, diesel fuel, kerosene, jet fuel from crude oil you need a fractional distillation column.  Simple distillation is great for binary systems with large differences in BP.  Crude oil is too complex of a mixture to use simple distillation.  You would have to distill crude oil many times using simple distillation.