How does coking convert heavy petroleum residue into lighter products?

Coking is a severe form of thermal cracking used in petroleum refining to convert heavy residual feedstocks into more valuable, lighter products.1

The process functions through the following steps:

• Heating: The heavy residue is heated in a furnace to temperatures between 475 and 520 °C (890 to 970 °F) with a very low residence time to initiate the cracking process.1
• Reaction: The heated feed is discharged into a large vessel known as a coke drum. Within this drum, extensive and controlled cracking occurs.1
• Separation: As the heavy molecules break apart, lighter cracked products rise to the top of the drum and are drawn off. These are subsequently sent to a fractionator to be separated into naphtha, diesel oils, and heavy gas oils, which serve as feedstock for further processing.1
• Coke Formation: The remaining heavy material continues to crack due to retained heat until it is converted into coke, a solid carbonaceous substance.1

Once a drum is filled with solid coke, it is taken out of service for decoking, while another drum continues the process.1