Thermal cracking

In the progression through the oil window, thermally driven reactions break kerogen and larger hydrocarbon molecules into smaller ones. In a refinery, analogous processes take some of the heavier products and break them into smaller molecules. This shifts the molecular weight downward, increasing the amounts of relatively small molecules boiling in the gasoline range. To operate on a human rather than a geologic time scale requires running at much higher temperatures than are encountered in the oil window. Processes that rely entirely on heat for breaking down large petroleum molecules into smaller ones are called thermal cracking.

With the steady increase in use of automobiles and trucks in the early decades of the past century, market demand for gasoline exceeded what could be supplied by straight-run gasoline, even augmented with natural gasoline. Cracking processes can increase the relative proportion of molecules in the C5–C10 range, at the expense of larger molecules in products having a lower value than gasoline.

Refinery cracking processes are of two types, thermal (that rely entirely on temperature to drive the cracking reactions) or catalytic. Chapter 14 included a discussion of catalytic cracking reactions and processes. Thermal processes were developed starting around 1913. Numerous thermal cracking processes were developed in the early decades of the twentieth century. They helped meet the increasing demand for gasoline in the 1920s and 30s. The process developed by C.P. Dubbs [A] provides an example (see Figure 16.1).