 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 product becomes lighter or have 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 catalytic surfaces so that the remaining product is much lighter or in 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 carbon ashes 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 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 you have higher yields of the desirable lighter products as 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 processes. So in a refinery both of these pathways carbon rejection and hydrogen addition are used to produce the products that are in demand for the market in the optimum fashion. That means that the cheapest way in essence.