June 11, 2008 > TechKnow Talk: From fossils to fuel: how is gasoline produced?
TechKnow Talk: From fossils to fuel: how is gasoline produced?
Based on the price at the pump, one might suspect gasoline is made from gold or diamonds, or extracted from the rare giant squid. But in fact it is a petroleum product, meaning it is derived from oil. Oil is also the source of motor oil, kerosene, jet fuel, diesel fuel, heating oil, liquefied petroleum gas (LPG), and a variety of materials used to make products such as plastics, tires, and crayons.
How is oil created, located, extracted, and processed? The oil we are using today originated primarily from the decayed remains of ocean animals and plants that lived more than 60 million years ago. For example, the rich North Sea oil fields began forming during the Jurassic period, some 160 million years ago, from a shallow sea that was teeming with life at that time. Some oil fields are hundreds of millions of years old. In fact, oil is considered a non-renewable resource, but new oil is being formed now - we will just have to wait 100 million years or so for the process to complete.
As marine organisms such as plankton and algae died, they sank to the bottom of these ancient seas, where they were covered with mud. There, they slowly decomposed with the help of anaerobic bacteria, or bacteria that live in the absence of oxygen. This process resulted in carbon-rich organic material called kerogen, which combined with the sediments in the sea bed, hardened into shale, and continued to sink deeper into the earth.
As layers of rock were deposited above, these materials were pushed deeper, and the temperature and pressure increased. This "cooking" under pressure caused the kerogen to liquefy into the substance we recognize as oil, comprised of hydrogen and carbon (hydrocarbons). The oil seeped through holes and fissures in the soft, porous rock and was contained by "traps" or "caps" of denser rock such as marble or granite. Because the original organisms and conditions creating the oil varied from place to place, no two oil fields produce exactly the same type of oil.
A competing theory of oil formation has been advanced, proposing that oil is not a fossil fuel at all, but was created from inorganic materials such as magma very deep in the Earth's crust or even in the mantle, and then seeped upward to pool in the reservoirs we find today. However, this "abiotic" (non-biological) theory is currently dismissed by the vast majority of scientists in favor of the process of organic decomposition described above.
Geologists use a variety of techniques to locate oil, including satellite imagery, fine measurement of the earth's gravitational and magnetic fields, and seismology, which sends shock waves into the earth and measures their echoes, similar to radar or sonar. This latter technique can detect the types and densities of rocks deep under the surface.
Even with all these sophisticated technologies, the only way to determine with certainty if oil is present is to drill. Drilling for oil is an expensive and complicated process. Since some oil fields are in remote areas, access roads must be constructed and heavy equipment brought in. Drilling requires a lot of water, so a well must be dug or a surface source of water located. Oil exploration is also governed by very strict environmental protection laws; steps must be taken throughout the process to minimize the impact to the natural environment.
Large liquid oil reservoirs are preferred, but as fewer of these are available, oil companies are turning to oil that is combined with rocks, as well as oil that is younger and older (and shallower and deeper) than is ideal. These deposits are more difficult to extract and may be of different composition as well, containing a lower percentage of the types of hydrocarbons that can be refined into gasoline. More exploration is also being conducted offshore (in the ocean) and over inland lakes, presenting additional drilling challenges.
A derrick is constructed over the drill site, and the complex process of drilling a hole thousands of feet through rock is begun. This requires a great deal of specialized equipment and consumes considerable energy. A typical oil well may be a mile deep. Some extend to a depth of several miles. As the drilling proceeds, a concrete casing is used to line the hole and prevent it from collapsing.
Once it is determined that a reservoir of usable oil has been penetrated, the drilling equipment is removed and the shaft is used to bring oil to the surface. Unless there is sufficient natural pressure to push the oil out, a pump is installed. The familiar "seesaw" rocking arm often seen in oil fields is powered by an electric motor and is driving a pump in the shaft below. Other techniques are also used to drive the oil to the surface, including digging a hole nearby and injecting water or steam to increase the pressure in the reservoir.
Once the oil has been recovered, it is transported to a refinery. Before exploring the refining process, let's take a closer look at the composition of the oil. Crude oil is composed of hydrocarbons, molecules of hydrogen and carbon containing very small amounts of a few other elements such as oxygen, nitrogen, and sulfur. These molecules vary from a few atoms to hundreds of atoms in length.
Each type of molecule has different properties, and ultimately results in a different petroleum product. For example, small molecules break apart easily, boil at relatively low temperatures, and produce gasoline, kerosene, and LPG (primarily propane). Larger molecules require more energy to break apart, boil at higher temperatures, and produce motor oil, fuel (heating) oil, and materials used to produce plastics.
Though oil refineries are very complex factories, the basic concept is simple. The oil is heated until all its components boil. The vapor is routed into a "fractional distillation column." As the vapor rises up the column it gets cooler and cooler. Each type of molecule returns to its liquid state as it cools through its boiling point. The liquid is collected at various heights within the column and directed to separate tanks, thus separating each of the components. In this way, a typical barrel of crude oil may produce about 20 gallons of gasoline, ten gallons of diesel fuel or heating oil, four gallons of jet fuel, two gallons of LPG, and about eight gallons of other products.
However, modern refineries are able to use chemical processes to alter the length of hydrocarbon molecules and convert them to other products. For example, by breaking longer molecules into shorter ones, diesel oil may be converted into gasoline, increasing the yield of gasoline from each barrel of crude oil. Different conversion processes are used to create and separate the various gasoline octane levels; these are determined by the composition of the molecule rather than its length.
Once the gasoline and other products have completed the distillation and conversion processes, they may undergo some additional chemical processing to remove impurities before being pumped into storage tanks to await transportation to gas stations, airports, and other destinations.
The U.S. consumes roughly 400 million gallons of gasoline each day or 150 billion gallons annually. This requires about 22 million barrels of crude oil per day. Approximately 40% of this is produced domestically, with Texas and the Gulf of Mexico the top oil producing regions, followed by Alaska and California. Worldwide, 86 million barrels of oil are produced daily. The leading countries, in order of oil production, are Saudi Arabia, Russia, the USA, and Iran.
Oil accounts for about 40% of total energy needs in the U.S. and about 37% worldwide. Demand for oil is increasing due to increasing population and the energy needs of emerging nations. How much oil is left? Some experts have estimated that the world's supply of oil will last another 50-80 years. However, much of the "easy" oil has already been extracted, and oil companies are turning to more difficult and expensive oil fields. While better technology to extract and process these deposits may possibly stretch the supply a little further into the future, it is clear that our grandchildren need to find other sources of energy.