FSC 432
Petroleum Processing

Lesson 6 Overview


Video: FSC 432 Lesson 6 (3:47)

Click here for transcript of 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.

Credit: Dutton Institute © Penn State is licensed under CC BY-NC-SA 4.0


Thermal cracking is the first commercial conversion process developed in the early 1900s principally to produce more motor gasoline from crude oils and produce high-octane gasoline for aircraft use, initiating an attempt to change the composition of crude oil in petroleum refinery. The purpose of thermal cracking is to make light middle distillates from heavier ends by pyrolysis, or thermolysis. With the advent of catalytic cracking in the 1930s and 1940s and its capability to produce higher yields of gasoline with higher octane number, thermal cracking of gas oils has ceased to be an important process for gasoline production in modern refineries. In countries where the principal petroleum fuel with a high demand is diesel fuel, thermal cracking is still important in fuel refineries. A principal application of thermal cracking of distillate fractions in current refineries is limited to naphtha cracking for the purpose of producing ethylene (C2H4) for the petrochemical industry. However, thermal cracking of residual fractions, particularly VDR, is still practiced in association with visbreaking and coking processes in the refineries. The chemistry of thermal cracking and thermal cracking processes is discussed in this section.

Learning Outcomes

By the end of this lesson, you should be able to:

  • summarize the chemistry of thermal cracking and free radical chain reactions;
  • apply thermal cracking of gas oil to produce lighter distillates and examine how thermal reactivity affects process configuration;
  • appraise thermal cracking for upgrading of residual fractions (visbreaking and coking) and interpret thermal severity to compare visbreaking processes;
  • analyze and compare different coking processes: Delayed Coking, Fluid Coking and Flexicoking.

What is due for Lesson 6?

This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignments below can be found on the Assignments page within this lesson.

Lesson 6 Tasks
Readings J. H. Gary, G. E. Handwerk, Mark J. Kaiser, Chapter 5
Assignments Exercise 5: A coil visbreaker operates at 500°C for 1 min. How long will it take to achieve the same thermal severity at 450°C in a soaker visbreaking process? An apparent Arrhenius activation energy for thermal cracking is given as 50 kcal/mol.


If you have any questions, please post them to our Help Discussion (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.