Gasoline Refinement

Gasoline

Gasoline as we know it today comes from crude oil. Crude oil, which is a fossil fuel, is a nonrenewable (once its gone, its gone for good) fuel source. Fossil fuel is comprised of naturally decaying plants and animals which once lived in oceans and seas millions of years ago. Where we find crude oil deposits we also find ancient oceans and seabeds. When extracted from the earth, crude oil may have colors ranging from clear to jet black and resistance to flow or viscosity from water to a molasses type substance.

Crude oil that is low in metals and sulfur content, light in color and flow easy are said to be "light" and cost more where those which are high in metal and sulfur content dark in color and must be heated to be fluid are called "heavy" and are less expensive. Light crude is less expensive to refine and heavy crude is more expensive to refine. The term "sweet" is used to define crude oil that is low in sulfurous compounds such as hydrogen sulfide and mercaptans, which is an organic compound, produced by decaying animals and plant matter and has bonded to a carbon atom. "Sour" crude oil is high in sulfurous compounds.

Once extracted from the earth, crude oil must be refined to become useful. Crude oil contains over 500 hundred hydrocarbons plus many other elements and additives that are all combined into one product. The refining process separates and groups these hydrocarbons together to make things that are of value to us such as gasoline and diesel fuel, waxes, asphalts, household fuel oil, industrial lubricating oils, greases and other petrochemicals.

Upon extraction, the crude oil is transferred to a refinery. This is normally done either by pipeline or supertanker ships that are up 1500 feet in length and carry as much as 2 million barrels of crude oil or about 84 million gallons. When the raw material arrives at the refinery it is transferred into holding tanks until the raw crude oil is ready to be refined. Holding tanks can hold up 16 millions gallons of crude per tank. Holding tanks have containment areas surrounding them to hold the total contents of the tank in a rare case that the tank was ruptured.

When the refinery is ready to turn the crude oil into useful substances such as gasoline, diesel fuel and other products. It goes through several basic steps, which are distilling, cracking, treating, reforming and blending. The crude oil is pumped through pipes into a hot furnace; this first step is the distilling process (involves several columns or towers which, most of us believe to be smoke stacks) that separates light hydrocarbons from heavy ones. This process must be monitored very closely. If the crude oil is over heated it will cause thermal decomposition and create unwanted waste. As the crude oil is heated the hundreds of different hydrocarbons vaporize at different temperatures. Light materials like butane, propane, natural gas and petroleum ether will vaporize between 68 and 140 degrees Fahrenheit. It will then condense at the top of the column. Medium weight materials such as gasoline and diesel fuel will vaporize between 104 and 390 degrees Fahrenheit, then condenses in the middle of the column. Kerosene vaporizes at 150-500 degrees Fahrenheit, fuel oils between 500-750 degrees Fahrenheit and lubricants above 750 degrees Fahrenheit. Heavy material also called residuum will remain at the bottom since it does not vaporize.

Some refineries operate their distillers in a vacuum, also called vacuum distillation. This allows the furnace to operate at lower temperatures and in theory reducing the chance of thermal decomposition, decreasing waste and residuum, and increasing efficiency.

As the different products are distilled, they travel through piping in the top, middle and bottom of the columns to different parts of the refinery for additional refining. This starts the second step "cracking" in which there are two major methods. The first one being fluid catalytic cracking (FCC), which produces high amounts of gasoline and liquefied petroleum gas (LPG). The second method hydrocracking, produces jet fuel and LPG.

During the fluid catalytic cracking process catalyst's, which accelerate the chemical reaction, are added to the product in order to expedite the process. In the hydrocracking process catalysts in addition to hydrogen are used to elevate the pressure.

After the cracking process the product must be treated using a process called hydrotreating. This is a mild form of hydrocracking and is designed to remove impurities and reduced air pollution when the fuel is used. Hydrotreating also removes sulfur and nitrogen. The sulfur is converted into elemental sulfur and the nitrogen is transformed into ammonia for further use in the production of fertilizers.

The next step of the process is the reforming, when we are this far in the process the product does not have enough octane to work well in automobiles. Gasoline coming from being hydrotreated is sent to be reformed. This process uses metal catalysts such as platinum and rhenium. Through this process hydrocarbons are transformed into high-octane gasoline components. The reforming process also removes hydrogen from the low octane gasoline. The hydrogen once siphoned off is diverted and used in the hydrocracking and hydrotreating process.

In the refined state gasoline has between 5 and 12 carbon atoms (C5-C12) however it is most common to see between 6 and 8 carbon atoms (C6-C8). Most automobile internal combustion engines have a compression ratio of 8.5:1 or 9:1, meaning that the gasoline and air mixture is compressed in the cylinder of the engine by a factor of 8.5 or 9 before it is ignited by the sparkplug.

In its current state the gasoline has a very low octane level, which would cause the gasoline to burn unevenly causing the engine to knock, ping and run rough. To prevent unwanted knocks, pings and rough running the gasoline goes through the last step of the refining process, which is blending.

During the blending step operators add hydrocarbons to the gasoline to increase the octane. These hydrocarbons include Benzene (C6H6), Toluene (C7H8), Naphthalene (C10H8) and Methyl Tert-Butyl Ether (MTBE.) The gasoline is also tested