[umunch]

Please help I'm stuck on this question.

Hydrocarbons such as gasoline are purified by fractional distillation of crude oil. Imagine that the height of the fractionating tower in the figure were tripled. Based on the diagram below, what consequence might be experienced? You may assume the packing material in the column remains constant.

here's the image that came with the question (scroll down): http://www.aapgsuez.net/Scientific%20Articles/Distillation.php

possible answers -  choose only one (2 and 4 both seem correct to me)
1. Less energy is required to distill aviation gasoline.
2. Gas Oil is now able to be separated from Heavy Furnace Oil.
3. The boiling point of lubricating oil goes down.
4. The alpha value for a 50:50 mixture of kerosene and petrochemicals increases.

[Arkcon]

Greetings, Ndunguru:, I'd like to welcome you and all the new students to the Chemical Forums.  But I'd like to ask you to trouble yourself to read our Forum Rules{click}. You already accepted them when you signed up for our forum, and they apply to you, whether you agree with them or not, or even if you're unaware of them. 

Our rules specify that we want to see your work, and then, we want to give you hints, so that you learn for yourself.  It doesn't matter if you're a new student, or a dedicated amateur, we need to know what you know, and what you think, so we know what level you're at, so we can give useful hints, so your knowledge can grow.   

Let's get started.

You say you're stuck, but you say 2 and 4 seem correct.  Why do think that they're correct?  What's wrong with 1 and 3?

[umunch]

Thanks for the reply.

I know that increasing height of the fractionating column increases the number of theoretical plates. This is why I believe answer 2 to be correct, because more theoretical plates makes separation more efficient and therefore you can separate compounds with more similar boiling points.

However, isn't alpha also a measure of efficiency? So increasing height should also increase alpha? Or is alpha simply a baseline value independent of any fractionating apparatus?

Option 3 is obviously not correct because boiling point is physical constant. And I don't see why it would take less energy to distill aviation gasoline (option 1).

[umunch]

I still need help on this.

[Arkcon]

Thanks for the reply.

I know that increasing height of the fractionating column increases the number of theoretical plates. This is why I believe answer 2 to be correct, because more theoretical plates makes separation more efficient and therefore you can separate compounds with more similar boiling points.

Good work, that's the definition.   But without being quantitative, we can't say one fraction will or won't separate from another.  Why not plug in 3x into the relevant equation, and see what happens?

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

However, isn't alpha also a measure of efficiency? So increasing height should also increase alpha? Or is alpha simply a baseline value independent of any fractionating apparatus?

Your second conclusion is correct.  The references I have suggest that alpha isn't changeable for distillation.  (HPLC also uses alpha, but we change it by changing media or eluent)

Option 3 is obviously not correct because boiling point is physical constant. And I don't see why it would take less energy to distill aviation gasoline (option 1).

Likewise, I can't see how there could be true.