Gasoline

So if you've seen the new documentary, Mad Max: Fury Road or the originals, you'll know that even if the world ended today, gasoline (or guzzoline) would still be very important to society. While the world still hasn't ended, if you'll just look around you can probably spot a car or two without trying that hard. Gas and oil-products are everywhere and this post will specifically focus on the production of gasoline from raw crude oil, a process known as cracking. Cracking is based in the chemical engineering process of chemical separation, specifically the process of cracking is a modified form of distillation. Now I'm going to have to go into some of the basics of organic chemistry for this to make sense but basically crude oil is a mixture of various lengths of chain made of carbon atoms surrounded by hydogren as seen below.
 
Basically crude oil starts with mostly longer chains of carbon and then through cracking (which can be heat based or catalytic), the chains break into smaller chains. Generally crude oil is comprised of very long chains (50-100 carbons long) and the cracking process is useful for breaking these into smaller, more sought-after chain lengths. Steam heating is generally the most common where super-heated steam is used to provide heat to trays inside of a distillation column. By setting the temperature of the rising trays to be slightly lower than each one below them, the cracked components can filter out at each stage. Basically, the shorter carbon chains are lighter and will float higher in the column, Think about how liquid water will sit in a pot while steam from it will rise up. Something very similar is happening here except instead of just water molecules, the molecules have different lengths here. As the feed is cracked in the column, various outlets are used to collect varying components. You'll see things like heating oil or diesel on lower levels of the tower and near the top, you'll get some of the lighter components like gasoline (octane) or natural gas (a mixture of methane, ethane, and propane).


You may have noticed that when you buy gas at the station, there's usually three buttons with different numbers for the octane rating. Octane is a chain of eight carbon molecules and additives such as branched or cyclic alkanes are added to the processed fuel to prevent the octane chains from bonding up with other chains again. The reason that the higher octane fuel costs more is because it is more expensive to separate fuel into purer concentrations of octane. Simply cracking and distilling may not be enough to create pure enough feeds for high octane fuels. To get purer fuels, further separation may required where desiccants may be used to absorb undesirables or a membrane filter may be used to further remove any larger molecules than the octane. As you approach 100% octane, it gets harder and harder to remove the other components and thus it makes sense that 95% octane fuel would costs more than 85% octane fuel. Though the actual octane number you see on a gas pump isn't actually a percentage, it is a metric used to define the performance of the fuel for gasoline engines, but the higher number still generally refers to higher concentrations of octane.

And that's basically how you get gasoline. You start with thick crude oil, heat it, or break it up with a catalyst, and you get smaller and smaller chains of carbon. If you want carbon chains that are eight carbons long, then you want to extract the octane. If you want lighter fuel still like propane or methane, then you will need more separations trays to get lighter and lighter outlet components. The more trays you have, the more separated the higher components will be from the lower ones. It's arguably one of the most important chemical processes in the world today and many chemical engineers find themselves working on process design and management in the oil industry where they'll oversee the cracking process and the distillation columns it occurs in.



Image sources:
http://www.bbc.co.uk/schools/gcsebitesize/science/21c/materials_choices/crude_oil_usesrev2.shtml

http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/rocks/fuelsrev3.shtml