PrintPaths for Upgrading Heavy Oil
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.
Video: FSC 432 Upgrading Heavy Oil (3:48)
PRESENTER: Upgrading heavy oil has become a major task in refineries. Heavy oils contain higher-density compounds that tend to be more aromatic in nature while the desirable products from refinery are light distillates, like gasoline, or middle distillates, like kerosene or diesel. Now, light distillates and middle distillates have higher hydrogen to carbon ratios because they tend to be paraffinic compared to the aromatic heavy oil. So upgrading heavy oil really means managing the hydrogen to carbon ratio.
There are two major pathways to manage or change the hydrogen to carbon ratio. The first pathway is called "carbon rejection." By rejecting carbon from the heavy oil, say through coking-- making a carbon-rich byproduct, coke. The remainder from that product becomes lighter or has a high hydrogen to carbon ratio.
Our solvent extraction, as in de-asphalting-- rejecting asphalt-- makes the de-asphalted oil lighter. Similarly, in visbreaking and catalytic cracking processes, carbon is rejected in the reactor tubes or on catalyst surfaces so that the remaining product is much lighter-- or a higher hydrogen to carbon ratio, which is essentially the upgrading process.
So through carbon rejection, we produce lighter products with high hydrogen to carbon ratio, as in light distillates or middle distillates. And carbonaceous-- or carbon-rich-- byproducts with low hydrogen to carbon ratio. One disadvantage of carbon rejection is actually losing carbon as a waste or a low-value product.
The second upgrading pathway requires hydrogen-- external hydrogen. It's a hydrogen addition process, as in hydrocracking or hydrogenation.
So you need to bring in hydrogen from an external resource to be able to add hydrogen to your heavy oil for upgrading purposes. Hydrogen addition would also lead to lighter products with higher hydrogen to carbon ratio, as we have seen with carbon rejection. But in this case, there is no rejection of carbon or losing part of your feedstock as a low-value or a waste product. So they have higher yields of the desirable lighter products. That's the advantage of hydrogen addition.
The disadvantage is that hydrogen is expensive to produce, and the catalysts used in hydrocracking and hydrogenation processes are also expensive. So higher yields of lighter products, but more costly process.
So in a refinery, both of these pathways-- carbon rejection and hydrogen addition-- are used to produce to get the products that are in demand for the market in the optimum fashion. That means the cheapest way, in essence.