Crude Oil (5): Matching supply and demand for crude oil products

What is cracking and why is it important in the oil industry?

Here are some details of the problem of supply versus demand for crude oil fractions (the figures are percentages)

As you can probably see the excessive demand is for petrol//gasoline and there is an oversupply of fuel oil.

So smaller molecule hydrocarbons are in demand and larger molecule hydrocarbons are in excess.  

How can one molecule change into another because if that happened it might be possible to match supply with demand.

Breaking a large alkane molecule into smaller molecules is called cracking.

How is cracking achieved?

First here is a link to a school experiment you might have done on this. 

And here is a video about the industrial process of cracking:

 

Note these points about the laboratory process:

• usually liquid paraffin is cracked
• it needs a high temperature and a catalyst (usually aluminium oxide: Al2O3)
• it converts large hydrocarbon molecules into smaller ones that are more useful
• it makes more petrol

You can tell cracking converts large molecules into smaller ones because the molecules cracked are liquids but the smaller ones produced are gases at the same temperature.  

Here's another thing the product molecules are of two basic types of hydrocarbon: alkanes and alkenes.

Alkanes are generally CnH2n+2   alkenes are generally CnH2n

Examples:

butene C4H8

butane C4H10

Thing is alkanes are pretty unreactive and tend to be used as fuels or solvents.

Alkenes are much more reactive (its the double bond) and there's a much wider group of products they can be converted into including polymers (more about this in another blog) 

So here are three equations to show you what happens when different alkanes are cracked:

You ought to try and build your own equations. 

The key to getting this type of equation right is to remember that all the carbons and hydrogens on the left add up to the same number of carbons and hydrogens on the right hand side. 

Look at equation (b), there are 12 carbons on the left and 6+2+4=12 carbons on the right.  

You can check the hydrogens for yourself.

So here's a task for you: write three cracking equations (no pun intended!!) for the cracking of undecane C11H24.  

Name all the products you come up with.  Have fun!!!!

Pages on the "Mole" and "Using the Mole" in chemical calculations are here