Crude Oil (GCSE Chemistry)


Uploaded by freeeschool on 04.09.2012

Transcript:
GCSE Chemistry – Crude Oil
Hello and welcome to a video about Crude Oil and we are going to look at, in particular,
Crude oil and the fractional distillation in having it separated out to the different
parts or the different sections of the mixture. It starts off if you will imagine what crude
oil look like. It is tough, kind of, to give you an impression here. But is a very thick,
very sticky black liquid and the key thing about it is that it contains a mixture of
compounds. The actual compounds are hydrocarbons. These hydrocarbons are very useful for a whole
variety of things which we shall look at in a minute.
But it is a mixture of compounds and because it is a mixture and mixture is a science or
a chemistry keyword, it means that because it is a mixture, it can be reasonably easily
separated out into the different parts.
How would we actually do that? Well, you may have seen a laboratory experiment where you
take something like crude oil or a special mixture that is made to be like crude oil
and separated out to different parts. The key way you might do it in a lab is very simply
you take some in a boiling tube, and you apply a small amount of heat. Now the key thing
about the different parts of the crude oil is that each fraction or each compound in
the mixture has a different boiling point. The boiling point is the temperature at which
the chemical compound will evaporate.
If we heated it by a very small amount, say up to 30?; the compound that boils at 30?
will evaporate away. Now imagine we then carried on heating it and heated it to a higher amount,
let’s just say, for arguments sake, to 60?C, the compound that is in there with the boiling
point of 60?C will evaporate away. In this way, you can, one at a time, heat different
temperatures and each compound will evaporate at different temperatures. In that way, you
can separate it out.
Now your next question might be, how’s that useful? These compounds are all escaping up
into the air. Scientists have developed a way in which they can heat it and separate
out the compounds and actually recapture them. This process is called “fractional distillation”.
It’s probably a good idea to write that out.
The way this works is we take the crude oil, but in much larger amounts than we showed
in our laboratory demonstration there and we pump it into this fractionating tower.
As we pumped it in, we heat it to a high temperature and as we heat it up to a high temperature,
most of the compounds, most of the hydrocarbons evaporate. As they evaporate, they rise up
the tower.
Now, you might notice that the colors change as we go up and that is because as we rise
up the tower, the vapors cool down. What we got to remember is the boiling point of a
compound is the point where it changes from a liquid to a vapor, but it is also the point
at which it changes from vapor back into liquid again. You can imagine all of these different
compounds evaporate and as they cool down, they are going to all condense or turn back
into liquid at different temperatures.
So those compounds, for example, petrol that evaporates at 70? will also condense and turn
back into liquid at 70?. Let’s take petrol as our example. Right down here, it is very
hot and it will evaporate and rise up the tower. It will cool down and eventually when
it gets to 70?, it will turn back into liquid. At that point there, because it condensed
back into liquid, it can be collected.
In a similar way, it is the same for all the other different kinds of chemicals that we
have. The vapor rises, cools down and it is separated out into the different fractions.
Now, these are the common names of the different fractions. We have refinery gas which is a
gas at around 20? comes right out the top; we have petrol which is right about 70?; naptha;
kerosene; jet fuel; diesel fuels; fuel oil for ships and things like that; and right
at the bottom, we have bitumen.
As we go down, there are certain characteristics that you need to know about all these different
kinds of chemicals. You don’t necessarily need to know the temperature in which they
all come up the tower, but you certainly need to be aware which ones they are and their
particular uses.
Let’s have a look at some of the patterns and some of the characteristics of the different
compounds. Let’s start with the first one up here. All the compounds that come out of
the top have the least viscosity. The word viscosity, for want of a better way of explaining
it, means thickness. We mean thickness of a liquid. So something that is very viscous
or has a high viscosity is something like syrup, very thick. Something which has low
viscosity, you say water is not very thick and it’s quite runny; so the least viscosity
at the top.
The other features that you should be aware of are the fact that we have the highest flammability
and the smallest molecules. As we go down the tower, that will change to most viscosity,
lowest flammability and largest molecules. That pattern goes all the way through for
each one. So far, just show some arrows to show that’s happening.
We could say that the compounds with the highest flammability at the top and as you go down,
they become less and less flammable. That means that the compounds that come out of
the top are the most useful for fuels because they burn very easily. Compounds that come
out at the bottom have low flammability, don’t burn very well at all and so least useful
in that respect.
The other thing that you should be able to do is to link these three characteristics
together. We have the lowest flammability because we have the largest molecules. These
large hydrocarbon molecules make the characteristics so that its got low flammability and it make
use of the characteristics of most viscosity. The same with the top because we have small
molecules. That gives us a high flammability for these hydrocarbons and it makes them least
viscous out of all of them.
So the key thing to remember here really is how we separate out the fractions in the fractionating
tower; the names of some of these and their uses; the kinds of temperatures, but you really
don’t have to memorize those, you just have to remember that as they get higher as you
go down; and these three very key and important features of the hydrocarbons.
[end of audio – 6:41] GCSE Chemistry – Crude Oil Page…1