Lesson 6 Overview

Now, having done all the physical separation processes on the crude oil, such as distillation or deasphalting, dewaxing, we have pretty much squeezed out what we could from the crude oil. But the yield of these products, particularly the light distillates like gasoline or kerosene, does not really meet the demand. The larger demand for distillate fuels, particularly gasoline, started with the introduction of the mass-produced automobile Model T. That's in the 1920s, in essence.

So we need to now change the composition of the crude oil. Separating it into desirable fractions does not cut it anymore. We need to use chemistry. So what does, really, chemistry mean? We need to break bonds and make new bonds.

The first chemical process introduced right around that time, thermal cracking process, meant to convert these heavier hydrocarbons, longer chains of paraffins, to the light distillates or the gasoline boiling range. By breaking these bonds by brutal heat, heating these temperatures until they cracked, pretty much, until the compounds crack or break the chemical bonds. So the thermal cracking process delivers the gasoline that was needed for the automobiles that were introduced at the time.

Currently, thermal cracking is not a significant process in a refinery in the US because the gasoline produced from thermal cracking would not really work in the current automobiles that require higher octane number fuel. And that is the reason why, pretty quickly, right around the Second World War, the catalytic cracking processes were introduced, making higher octane gasoline.

Now, there's a chemical difference between thermal cracking and catalytic cracking that is beyond just using catalysts in cat cracking processes. That is the chemistry of reactive species. Now, thermal cracking proceeds through neutral reactive species that we call free radicals. The chain reaction of free radicals govern the thermal cracking process, whereas, as we'll see in the next lesson, ions, charged species, carbocations govern the catalytic cracking process.

Another use for thermal cracking was to really convert the bottom of the barrel, the vacuum distillation residue, into usable products such as fuel oils. And in this breaking process, where we break some bonds to reduce the viscosity of VDR, so we can have a fuel that would flow, that's really a residual fuel oil, or heat the vacuum distillation residue to such temperatures that we pretty much destroy the molecular makeup to make a byproduct, coke, reject carbon through this byproduct to make lighter compounds, lighter distillates, like gasoline, diesel, or kerosene from this very heavy end. So this is essentially a thermal upgrading process.