crude oil

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Petroleum or crude oil is a naturally occurring, toxic, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights, and other organic compounds, that are found in geologic formations beneath the Earth's surface. Introduction

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In its strictest sense, petroleum includes only crude oil, but in common usage it includes both crude oil and natural gas. Both crude oil and natural gas are predominantly a mixture of hydrocarbons. Under surface pressure and temperature conditions, the lighter hydrocarbons methane, ethane, propane and butane occur as gases, while the heavier ones from pentane and up are in the form of liquids or solids. However, in the underground oil reservoir the proportion which is gas or liquid varies depending on the subsurface conditions, and on the phase diagram of the petroleum mixture.

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An oil well produces predominantly crude oil, with some natural gas dissolved in it. Because the pressure is lower at the surface than underground, some of the gas will come out of solution and be recovered (or burned) as associated gas or solution gas. A gas well produces predominately natural gas. However, because the underground temperature and pressure are higher than at the surface, the gas may contain heavier hydrocarbons such as pentane, hexane, and heptane in the gaseous state. Under surface conditions these will condense out of the gas and form natural gas condensate, often shortened to condensate. Condensate resembles gasoline in appearance and is similar in composition to some volatile light crude oils.

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Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs.

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Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.

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Refining of Crude Oil Fractional distillation of crude oil is the first step in the production of many of the materials we have come to rely on in modern life. All our fossil fuels, virtually all our plastics, detergents and commercial alcohols are made from products of this process.

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Oil Field to the Refinery The first step is the transport of the crude oil from its natural location to the refinery. Oil drilling occurs both at sea and on land, depending on the size and profitability of the oil deposits located. Fossil fuel formation is the result of environmental conditions in the distant past and the geological processes that have occurred since the laying down of the original organic matter from which the oil is formed. Once obtained from the ground, the oil is transported by ship, truck or pipeline to the refinery.

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At the Refinery Once the oil reaches the refinery the work to separate it into useful products begins. Oil refineries are enormously complex and each part of the distilled oil goes through several stages of processing. However, the very first step is to break up the crude oil. The crude oil is a mixture of many different chemicals. The majority of these are hydrocarbons, which are molecules made only from the element Carbon and the element Hydrogen. The mixture of hydrocarbons contains both alkane and alkene molecules and the length of the chains vary wildly, from five Carbon atoms long to 60 Carbons or more. Since fuels need to be very specific in terms of the length of the Carbon chain, the different lengths need to be separated. These different length chains are called FRACTIONS.

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The boiling point of a Hydrocarbon fraction, which is the temperature at which it evaporates, is dependent on the length of the Carbon chain. Those fractions with shorter chains evaporate more easily than those with longer chains. This explains why petrol, which is mainly made of the 8-Carbon molecule octane, evaporates more easily than engine oil which has carbon chains in the range of 20 or more.

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Fractional distillation of Crude Oil In order to separate the different length chains in the crude mix, it is heated to a very high temperature. The temperature is set so that all those fractions with a Carbon chain length of 20 and below are evaporated from the crude mix. The temperature cannot be set higher than this as there is a risk that the lighter fractions will ignite. The remaining liquid, which is composed of only the heavier fractions, passes to a second location where it is heated to a similar temperature, but at lower pressure. This has the effect of making the heavy Hydrocarbon fractions more likely to evaporate.

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The way the Distillation Tower works is by becoming progressively cooler from the base to the top. All the Hydrocarbon fractions start off in gas form, as they have been heated to that point. The gases then rise up the tower. The gas mixture then encounters a barrier through which there are only openings into the bubble caps. The gas mixture is then forced to go through a liquid before continuing upwards. The liquid in the first tray is at a cool enough temperature to get the heaviest gas fractions to condense into liquid form, while the lighter fractions stay gaseous. Working of the Distillation Tower

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In this way the heaviest hydrocarbon fractions are separated out from the mixed gas. The remaining gas continues its journey up the tower until it reaches another barrier. Here the bubble cap process is repeated but at a lower temperature than before, which then filters out the next lightest set of fractions. This process continues until only the very lightest fractions, those of 1 to 4 Carbon atoms, are left. These stay in gas form and are collected at the top of the tower. The separation of the heavier elements in the second tower follows exactly the same process but at lower pressure

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Introduction which has carbon chains in the range of 20 or more.

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Introduction which has carbon chains in the range of 20 or more.

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Introduction which has carbon chains in the range of 20 or more.

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