
It should be clear from this quick tour of a refinery, that the most valuable products from a refinery include light distillates (gasoline) and middle distillates (jet fuel and diesel). These products are mostly paraffinic and contain relatively short paraffin chains, or small molecules, or, in other words, high H/C ratios. In this context, one could summarize the overall goal of petroleum refining as managing the H/C ratio of the products for the optimum distribution of hydrogen into products to maximize profits. Controlling the H/C ratio of the products would require either lowering the C content of the products (i.e., carbon rejection), or increasing the H content (i.e., hydrogen addition). The animation below depicts these two major paths for upgrading heavy oil (or crude oil) with some examples for each path. The processes, coking, solvent extraction (e.g., deasphalting), visbreaking, and catalytic cracking reject carbon in the coke (carbonaceous) product so that lighter products (with high H/C) ratios can be obtained in these processes. Carbon in the coke, or in the heavier product is considered to have been rejected (and potentially lost) since the carbonaceous byproducts have much lower value in comparison to those of the lighter products. In contrast, hydrogen addition, as in the processes of hydrogenation and hydrocracking, enables the conversion of all the carbon present in heavy oil (or crude oil) to high value products without rejecting, or sacrificing, any. One might ask, then, why would any refinery carry out any carbon rejection process instead of hydrogen addition? A short answer to this question involves basic refinery economics; the hydrogen addition processes cost much more than carbon rejection processes, because producing hydrogen and the catalysts used in hydrogen addition processes are very expensive.