 Now, having done all the physical separation processes on the crude oil, such as distillation or deasphalting, de-waxing, we have pretty much squeezed out what we could from the crude oil, but that 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 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, the thermal cracking process, are 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 crack pretty much, until the compounds crack or we break the chemical bonds. So this 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 refiner 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 is a difference, chemical difference between thermal cracking and catalytic cracking that is beyond just using catalyst in cut cracking processes. That is the chemistry of reactive species. Now thermal cracking proceeds through neutral reactive species that we call free radicals. The chain reactions of free radicals govern the thermal cracking process, whereas as we will see in the next lesson, ions, charged species, carbocations govern the catalytic cracking processes. Another use for thermal cracking was to really convert the bottom of the barrel, the vacuum distillation visit into usable products, such as fuel oils in the visbreaking 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 residue of fuel oil, or heat the vacuum distillation visit to such temperatures that we pretty much destroy the molecular makeup to make a byproduct conch, 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.