Crude Oil Refining - Coursework Example

Crude Oil Refining Need to Refine Crude Oil Crude oil is present in large underground vaults where millions of years of decomposition and pressure have turned biological matter into oil. However crude oil is present with other factions such as gases. Once crude oil is extracted it is itself a mixture of various lighter factions of organic liquids. The composition of crude oil varies from location to location though the change is not highly significant. The commonest factions that compose crude oil are generally alkanes, cycloalkanes, aromatic hydrocarbons and other organic compounds that are constituted from sulphur, oxygen and nitrogen along with trace amounts of transition metals such as vanadium, nickel, iron and copper. The specific molecular makeup of crude oil varies from location to location as mentioned above however the chemical elements required to form crude oil vary over a fairly narrow margin. (Speight, 1999) The major hydrocarbon components of crude oil include paraffins, naphthenes, aromatics and asphaltics which display consistency in their presence in crude oil. The variation in these hydrocarbon components determines the exact properties of the crude oil in question. (Hyne, 2001) Crude oil contains a number of lighter more usable factions such as HFO, LFO, diesel, gasoline and a number of components for isomers of various kinds, asphalt etc. These factions can only be separated if crude oil is subjected to various processes. The processes used to separate factions of crude oil are better known collectively as petroleum refining and fractional distillation is a critical process. Fractional distillation relies on the physical property of different liquids to boil out at specific temperatures only. Since crude oil is a mixture of various different lighter oils and solids, the constituents all tend to turn to vapour at different temperatures. Once these temperatures are achieved, the particular faction in question is collected downstream and is thus separated from crude oil’s other constituents. Processes carried out in Oil Refineries The crude oil that is brought in from an oil field is separated into various useful constituents through a variety of different techniques. Not only are the constituents of crude oil separated but they are often treated into different other useful products as well. The entire process begins in the crude oil distillation unit (CDU). All forms of oil refineries possess a CDU which intimates the separation. More separation may be carried out after the CDU too. The basic job of the CDU is to distil various larger constituents in crude oil into smaller more easily processed factions. The CDU for most oil refineries is operated at or around atmospheric pressure (760 mm Hg) so the CDU is also referred to as the atmospheric distillation unit. (Kister, 1992) The crude oil is at ambient temperature when it is introduced into the refining process so it needs to be heated before separation. This is done by heating the crude oil with the lighter fractions of oil that have already been distilled as they already contain a lot of heat. The heated crude oil is then taken through a process where any inorganic salts are removed from it. Typically the crude oil contains a sizable amount of sodium chloride and it is removed. After the salt has been removed from the crude oil, it is then heated again by exchanging heat with other distilled fractions of oil. This stage is better known as pre-heating because the crude oil does not reach a temperature where the fractions begin to separate. The heat required to begin the process of fractional distillation is gathered through a fuel fired furnace in which fuel is combusted to produce large amounts of heat. The heat produced in this manner is generally around 398oC in temperature and the stream is then fed to the bottom most section of the distillation unit or column. As the crude oil is heated and various fractions separate from it there is a need to cool those fractions to liquefy them. Generally this is achieved by coolers and condensers that are placed near the top section of the distillation tower. The heat removed is often used to heat the incoming crude oil as mentioned above. In some cases where there is additional heat available, it is removed by using air cooled and water cooled condensers. A complete pumping system must be installed and used in order to utilise the water condensers. (Leffler, 1985) The top most section of the distillation unit produces naphtha as the dominant fraction. Other fractions that are separated from the crude oil before naphtha are removed from the sides of the distillation unit and are referred to as side cuts. These side cuts are often in the form of vapours and generally carry large amounts of heat with themselves. In order to store these removed fractions safely and simply it is necessary to remove the heat that these fractions posses. However, if the heat removed in this fashion is not utilised elsewhere in the process then the efficiency of the process falls. In order to utilise the heat carried by these fractions the incoming crude oil is generally heated through the use of heat exchangers. The side cuts generally are composed of kerosene, light gas oils, heavy gas oils and similar oil products. These products are cooled as mentioned above and are then sent for storage before they are either transported for use or are processed further to produce even lighter fractions of oil. (Beychok, 1967) The separation of crude oil into lighter fractions often produces four distinct categories of oils. These are light distillates, middle distillate, heavy distillates and miscellaneous distillates. Light distillates are composed of LPG (liquefied petroleum gas), gasoline (or better known as petrol), kerosene, jet oil fuel and other kinds of aircraft fuels. The middle distillates are generally composed of diesel (both for use in automobiles and rail road engines), residential use heating fuel and other kinds of light fuel oils. In comparison heavy distillates are composed of heavy fuel oils (used in large power generation plants), bunker oils as well as other kinds of residual fuel oils. Miscellaneous distillates are specialised in the sense that they may not be produced in every oil refinery. Depending on the crude oil received by each refinery and depending on the design of the processes in use at the oil refinery different kinds of miscellaneous distillates may be produced. The various kinds of miscellaneous distillates include (but are not limited to) specialised petroleum naphthas, specialised solvents derived from oils, sulphur (in elemental forms) as well as sulphuric acid, various kinds of feed stocks for petrochemical industries, tar as well as asphalt (used for insulating buildings and making paved roads), coke derived from petroleum, lubricating grade mineral oils, greases and waxes (also used for lubrication), oils used in transformers and cables to keep them cool as well as black carbon. (Black, 2000) Cracking Chemically cracking refers to breaking down larger and more complex molecules into smaller and simpler molecules. This tends to modify the properties of the resulting molecules considerably in comparison to the parent material’s molecules. Often cracking is employed with fractions obtained from crude oil in order to make them more useful and simpler to use. Cracking is carried out by disintegrating the carbon to carbon bonds present within larger hydrocarbon molecules. Cracking rate depends primarily on the temperature at which cracking is carried out as well as the kinds of catalysts employed. Various kinds of cracking processes are employed in oil refineries to produce more usable forms of oils and other petroleum derived products. Probably the most important of all cracking processes employed in oil refineries is known as FCC (fluid catalytic cracking). FCC is employed in all petroleum refiners to produce lighter and more usable fractions from heavier, high molecular weight and high boiling point fractions present within crude oil. The smaller fractions produced in this manner are generally more valuable and include gasoline, olefinic gases and other similar products. (Gary & Handwerk, 2001) Previously such cracking of hydrocarbons was done through thermal cracking but it has been replaced nearly everywhere because FCC is far more efficient and tends to produce more gasoline with a higher octane rating which is more valuable. The by-products produced are also more valuable kinds of olefins. FCC is applied to the feedstock derived from crude oil that possesses a boiling point of greater than 340oC (at atmospheric pressure) and with an average molecular weight of between 200 and 600 and sometimes even higher. This particular fraction of crude oil is better known as heavy gas oil. During FCC the longer chain molecules are vaporised and then broken down as the feed stock comes in contact with high temperatures and medium pressure levels. A fluidized (powdered) catalyst is also used to speed up the entire process. (Sadeghbeigi, 2000) FCC is widely employed in order to produce lighter more valuable fractions from heavier fractions of oil to increase the productivity of petroleum refining. Currently FCC is used in over 400 refineries around the world and around one-third of all processed crude oil is routed through FCC to create more valuable derivatives. (Jones & Pujado, 2006) In 2007 alone various FCC units around the US produced some 5.3 million barrels of feedstock per day for cracking. During FCC the feedstock contacts the catalyst which is pulverised to improve the total surface area in contact. The straight chain alkane molecules (paraffins) which possess a large structure are further broken down into smaller molecules such as straight chained alkanes, branched chained alkanes, branched alkenes (olefins) as well as cycloalkanes (naphthenes). Chemically this process includes the “scission” of the carbon to carbon bonds in the larger molecules to create smaller molecules. The smaller alkanes derived in this manner are further decomposed into smaller alkenes as well as branched alkenes mostly in gaseous forms such as ethylene, propylene, butylenes and isobutylene. These gases are all olefins and are crucial to petrochemical feed stock purposes. These olefins can also be converted in certain processes to obtain gasoline blended constituents which are also valuable compounds. The cycloalkanes that were generated initially are also further broken down into smaller molecules. These molecules include compounds better known as aromatics (for their pleasant smell) and include benzene, toluene, xylene and other similar compounds. All of these compounds have the capability to boil near the boiling point of gasoline and posses far greater octane ratings than gasoline and hence more valuable too. Multiple other similar reactions occur during cracking in general and during FCC in particular. The base contention always remains the same that is to convert larger molecules to smaller more usable molecules. (Jones & Pujado, 2006) Bibliography Beychok, M. R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants. New York: John Wiley & Sons. Black, B. (2000). Petrolia: the landscape of Americas first oil boom. Johns Hopkins University Press. Gary, J. H., & Handwerk, G. E. (2001). Petroleum Refining: Technology and Economics. London: CRC Press. Hyne, N. J. (2001). Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production. New York: PennWell Corporation. Jones, D. S., & Pujado, P. P. (2006). Handbook of Petroleum Processing. New York: Springer. Kister, H. Z. (1992). Distillation Design (1st Edition ed.). New York: McGraw-Hill. Leffler, W. (1985). Petroleum refining for the nontechnical person. PennWell Books. Sadeghbeigi, R. (2000). Fluid Catalytic Cracking Handbook. Tehran: Gulf Publishing. Speight, J. G. (1999). The Chemistry and Technology of Petroleum. London: Marcel Dekker. Read More