Oil is pumped from deep underground and underseas. Once it has been collected, it is transported to a refinery where it is turned into useful products for customers. A barrel of crude oil is 42-gallons. From this, regulations force refineries to turn at least 50 percent into gasoline. Crude oil from each reservoir has certain characteristics. They are classified by the density and sulfur amounts in the oil. Reservoirs with less dense crude produce more valued products. A less dense or ‘lighter’ crude requires much less work and therefore has a better price margin.
Once crude oil is at the refinery there are a large number of various methods used to distill the crude oil. A simple description of a refinery is a large distillation plant where distillation, conversion, and treating take place. Crude oil is not useful in its raw state and must be refined into usable petroleum products. The oil contains hydrocarbons of varying lengths. These lengths allow refineries to break apart the crude to collect streams of products such as diesel, kerosene, gasoline, and petroleum gas.
Oil refining begins with distillation. The tall columns that are seen in a refinery complex have catch trays at various levels inside. Crude is pumped in and heated. The lightest materials such as propane and butane go to the highest point. Medium weight materials stay in the middle areas and the heaviest materials stay low never the bottom. The temperatures in the distillation chambers are controlled using computer systems to get the maximum amounts from the crude. Once the various products coalesce into liquid form, they are sent through different pipes for different refining techniques.
The next step most products go through is conversion. This is the where most components become useful and the refinery makes money. The liquids from the medium and heavy ranges are first treated with hydrogen to remove as many contaminants as possible. One of the most common conversion processes is cracking.
Cracking involves breaking or ‘cracking’ the hydrocarbon chains in the oil. This allows the medium and heavy fluids to be converted to high-end light densities where the most useful products are. There are two common methods of cracking, Fluid Catalytic Cracker (FCC) and Hydrocracking. Fluid catalytic cracking uses temperatures around 1,000 degrees Fahrenheit, low pressure and a powdered catalyst to break apart the heavy mixture into gasoline.
Hydrocracking uses a different catalyst, lower temperatures and higher pressure to make gasoline and jet fuel. Other conversion processes used are combining and rearranging. Since most gasoline from these processes does not have enough octane to burn in automobiles, it is rearranged to form high-octane gasoline.
All petroleum products contain impurities such as nitrogen and sulfur. The treating process removes these compounds to produce cleaner burning fuels, which reduces air pollution. Hydrotreating is a milder form of hydrocracking and is used to remove impurities.
The final process is blending. Most gasoline is a blend of various treated components. These blends vary from state to state depending on regulations.