In a refinery, components are primarily separated using “fractional distillation”. After being sent through a furnace, the crude petroleum enters a fractionating column, where the products condense at different temperatures within the column, so that the lighter components separate out at the top of the column (they have lower boiling points than heavier ones) and the heavier ones fall towards the bottom. Because this process occurs at atmospheric pressure, it may be called atmospheric distillation. Some of the heavier components that are difficult to separate may then undergo vacuum distillation (fractional distillation in a vacuum) for further separation. The heaviest components are then commonly “cracked” (undergoing catagenesis) to form lighter hydrocarbons, which may be more useful. In the same manner that natural mineral catalysts help to transform kerogen to crude oil through the process of catagenesis, metal catalysts can help transform large hydrocarbons into smaller ones. The modern form of “catalytic cracking” utilizes hydrogen as catalyst, and is thus termed “hydrocracking”. This is a primary process used in modern petroleum refining to form more valuable lighter fuels from heavier ones. All of the products then undergo further refinement in different units that produce the desired products.