Higher and foundation tiers
The fractional distillation of crude oil turns a pretty useless substance
into lots of very valuable
hydrocarbons that are in huge demand. However there is a slight problem with the amounts of
each fraction produced in the distillation process. The small volatile hydrocarbons such as the petrol and
diesel fractions are in large demand but the distillation process does not produce enough of these
hydrocarbon molecules to meet worldwide demand. Whereas the fractions containing large
hydrocarbon molecules such as bitumen/tar and
heavy oils are in much lower demand but the fractional distillation process produces large amounts
of these heavy fractions. The solution to this problem is not to simply "chuck away" the large
unwanted hydrocarbon molecules but to break up or crack the large hydrocarbon molecules into the
smaller high demand high value hydrocarbon molecules. This process of breaking up the large unwanted hydrocarbon molecules into smaller one is called cracking.
In the science lab it is relatively easy to crack a large chain hydrocarbon using the apparatus below.
Here heat resistant mineral wool has been soaked in liquid paraffin. To crack the paraffin molecule you could simply heat it up to a high temperature. As you heat the paraffin molecules they vibrate faster and faster and eventually if they are heated to approximately 7000C the molecules would simply shake themselves to pieces. Industrially as well as using a high temperature a high pressure; around 70 atmospheres is also used to crack or break up large hydrocarbon molecules into smaller more useful molecules, this is called thermal cracking. However a catalyst is often used to speed up the cracking process and also allow it to occur at a lower temperature; around 450-5000C; a catalyst of aluminium oxide or pieces of broken crockery is often used. In industry zelolite catalyts are used, zeolite is a common mineral containing aluminium, silicon and oxygen.
To crack the paraffin molecules into smaller more useful molecules heat up the catalyst for a few minutes with a hot Bunsen flame. Once the catalyst is hot move the Bunsen flame back and forward between the paraffin and the catalyst. Once the paraffin vapour hits the hot catalyst it will be adsorbed onto the surface of the catalyst and it will be cracked into smaller hydrocarbon molecules. These smaller molecules are likely to be a gas and they can be collected underwater as shown in the diagram. The cracked gas is not soluble in water so collecting in this way is an efficient way to do it. It is likely there will be a high proportion of unsaturated hydrocarbons in the cracked gas. You could prove this by simply testing with red/orange bromine water.
Steam cracking is the most common method used
to obtain the unsaturated alkenes ethene and propene, which are used to make
polymers.
Steam cracking is a type of thermal cracking. The hydrocarbon to be
cracked, which is called the feed stock;
is mixed with high temperature steam. The steam helps reduce unwanted side reactions from taking place. The
mixture of hydrocarbons to be cracked and the steam are pumped very very quickly through a series of
tubes inside a furnace at a high temperature; typically the mixtures of gases are inside the hot tubes
for between 0.1-0.5 seconds. Cracking happens inside the tubes and the cracked gases leave the furnace and are
quickly cooled by water in a condenser. The mixture of cracked gases and the steam are separated later
in the industrial process. A basic outline of the steam cracking is shown opposite.
The products of cracking depend on the temperature and which type of cracking is used. However cracking generally produces a mixture of saturated alkanes and unsaturated alkenes. We can show this as: