Introduction to refinery

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  Introduction: Composition of petroleum, laboratory tests, refinery feedstocks and products   Fig:1.1   Fig.1.2 Distribution of world energy resources. (From World Energy Outlook 2005, International Energy Agency.)    Introduction to petroleum refinery Petroleum refineries have goal to convert as much of the barrel of crude oil into transportation fuels which is economically practical. Refineries produce many profitable products however, the high-volume profitable products are the transportation fuel gasoline, diesel and turbine (jet) fuels, and the light heating oils. These transportation fuels have boiling points between 25 and 350 o C. Although products such as lubricating oils, refrigeration and transformer oils, and  petrochemical feedstocks are profitable.They amount to less than 5 percent of the total crude oil charged to refineries. The processing equipment indicated is for processing crude oils of average gravities and sulfur contents. Crude oils with low API gravities (high specific gravities) and high sulfur contents require additional hydrotreating equipment. The quality of crude oils  processed by worldwide refineries is expected to worsen slowly in the future with the sulfur contents and densities to increase. Therefore refineries will require processing the entire barrel of crude rather than just the material boiling below (550 o C).Sulfur restrictions on fuels, coke and heavy fuel oils affects the bottom-of-the-barrel processing as well. These factors requires extensive refinery additions modernization the shift in market requirements among gasolines and reformulated fuels for transportation challenges. Refinery Overview The crude oil is heated in a furnace and charged to an atmospheric distillation tower, where it is separated into light gas (C 1 -C 4 ), light naphtha, heavy naphtha, kerosine, atmospheric gas oil, and reduced (topped) crude . The reduced crude is sent to the vacuum distillation tower and separated into vacuum gas oil stream and vacuum reduced crude bottoms (residua, resid). The reduced crude bottoms from the vacuum distillation tower is thermally cracked in a delayed coker to produce gas, coker gasoline, coker gas oil, and coke. The atmospheric and vacuum crude unit gas oils and coker gas oil are used as feedstocks for the catalytic cracking or hydrocracking units where heavy molecules get converted into lower molecular weight compounds boiling in the gasoline and distillate fuel ranges. The hydrocracked products are saturated whereas catalytic cracker products are unsaturated and further need improvement in quality by either hydrotreating or by reforming. The light naphtha streams from the crude tower, coker and cracking units are sent to an isomerization unit to convert straight-chain  paraffins into isomers which have higher octane numbers. The heavy naphtha streams from the crude tower, coker, and cracking units are fed to the catalytic reformer to improve octane numbers. The products from the catalytic reformer can be blended into regular and premium gasolines for marketing. The wet gas streams from the crude unit, coker, and cracking units are separated in the vapor recovery section (gas plant) into fuel gas, liquefied petroleum gas (LPG), unsaturated hydrocarbons (propylene, butylenes, and pentenes), normal butane, and isobutane. The fuel gas is burned as a fuel in refinery furnaces and the normal butane is blended into gasoline or LPG. The unsaturated hydrocarbons and isobutane are sent to the alkylation unit to react olefins with isobutane to yield isoparaffins. The alkylation is done at high pressure and low temperature in the presence of sulfuric or hydrofluoric acid as catalyst. The product is called alkylated gasoline, which is a high-octane product blended into premium motor gasoline and aviation gasoline. The middle distillates from the crude unit, coker, and cracking units are  blended into diesel and jet fuels and furnace oils. In some refineries, the heavy vacuum gas oil and reduced crude from paraffinic or naphthenic base crude oils are processed into lubricating oils. The asphaltenes are removed in a propane deasphalting unit, and the reduced crude from  bottoms are processed with the vacuum gas oils to produce lubeoil base stocks (LOBS). The vacuum gas oils and deasphalted stocks are solvent-extracted to remove the aromatic compounds followed by dewaxing to improve the pour point. These LOBS are further treated  with acid clays to improve their color and stability before being blended into lubricating oils. Each refinery has its own unique processing scheme which is determined by the process equipment available, crude oil characteristics, operating costs, and product demand. Refinery Feed Stocks The basic raw material for refineries is petroleum or crude oil. The chemical compositions of crude oils obtained from various sources are almost uniform although their physical characteristics vary widely. The elementary composition of crude oil usually falls within the following ranges: C 84-88; H 11-15; S up to 5%, N up to 0.5 %. Crude oils are classified as  paraffin base, naphthene base, asphalt base, or mixed base depending upon the composition of the residue left after distillation. Crude oils which have up to 80% aromatic content are known as aromatic-base oils. The U.S. Bureau of Mines has developed a system which classifies the crude according to two key fractions obtained in distillation: No. 1 from 250 to 275 o C at atmospheric pressure and No. 2 from 275 to 300 o C at 40 mmHg pressure. The API gravity of these fractions varies depending upon paraffinic and naphthenic grade of the crude(Paraffin : API 40 for No. 1 and 30 for No. 2, Naphthene : API < 30 for No. 1 oil and <=20 for No. 2 oil). The paraffinic and asphailtic classifications in common use are based on the properties of the residuum left from nondestructive distillation and are more descriptive to the refiner because they convey the nature of the products to be expected and the processing necessary. Composition Of Petroleum Crude oils are composed of many members of homologous series of hydrocarbons. Petroleum is essentially a mixture of hydrocarbons, and even the non-hydrocarbon elements are generally  present as components of complex molecules predominantly hydrocarbon in character, but containing small quantities of oxygen, sulfur, nitrogen, vanadium nickel, and chromium. The composition of the total mixture, in terms of elementary composition, does not vary a great deal, but small differences in composition can greatly affect the physical properties and the  processing required to production marketable products. The hydrocarbons present in crude  petroleum are classified into three general types: paraffins, naphthenes, and aromatics. Olefins are generally not present in crude oil, however these are formed during processing by the dehydrogenation of paraffins and naphthenes. The paraffin series of hydrocarbons is characterized by the rule that the carbon atoms are connected by a single bond and the other  bonds are saturated with hydrogen atoms.The general formula for paraffins is C n H 2n+2 . Crude oil contains molecules with up to 70 carbon atoms, and the number of possible paraffinic hydrocarbons is very high .Olefins do not naturally occur in crude oils but are formed during the processing. They are very similar in structure to paraffins but at least two of the carbon atoms are joined by double bonds. The general formula is C n H 2n . Olefins are generally undesirable in finished products because the double bonds are reactive and the compounds are more easily oxidized and polymerized to form gums and varnishes. Olefins containing five carbon atoms have high reaction rates with compounds in the atmosphere that form pollutants and, even though they have high research octane numbers, are considered generally undesirable. Some diolefins (containing two double bonds) are also formed during processing,  but they react very rapidly with olefins to form high-molecular-weight polymers consisting of many simple unsaturated molecules joined together. Diolefins are very undesirable in products  because they are so reactive they polymerize and form filter and equipment plugging compounds. Cycloparaffin hydrocarbons in which all of the available bonds of the carbon atoms are saturated with hydrogen are called naphthenes. There are many types of naphthenes  present in crude oil, but except for the lower-molecular-weight such as cyclopentane and  cyclohexane, are generally not handled as individual compounds. They are classified according to boiling range and their properties determined with the help of correlation factors such as the characterization (K  w ) factor or correlation index ( CI) . The aromatic series of hydrocarbons is chemically and physically very different from the paraffins and cycloparaffins (naphthenes). The cyclic hydrocarbons, both naphthenic and aromatic, can add paraffin side chains in place of some of the hydrogen attached to the ring carbons and form a mixed structure. Petroleum /crude oil    Petroleum (also called crude oil) is a mixture of gaseous, liquid , and solid hydrocarbon compounds.    Petroleum occurs in sedimentary rock deposits throughout the world and also contains small quantities of nitrogen oxygen and sulfur-containing compounds as well as trace amounts of metallic constituents. Petroleum    The fuels derived from Petroleum contribute approximately one-third to one-half of the total world energy supply and are used for transportation fuels (gasoline,diesel fuel,and aviation fuel,among others) and heating buildings.    Petroleum products have a wide variety of uses that vary from gaseous and liquid fuels to near-solid machinery lubricants. Residue of many refinery processes asphalt  —  is now a premium value product for highway surfaces, roofing materials, and miscellaneous waterproofing uses.    Crude petroleum is a mixture of compounds boiling at different temperatures that can  be separated into a variety of different generic fractions by distillation and the terminology of these fractions has been bound by utility and often bears little relationship to composition. Major Processes    Desalting    Sweetening    Hydrogen Generation Unit    DHDS/DHDT    Reformer    Isomerisation    Amine Treating    Sulphur Recovery Unit    Bitumen Blowing Unit    Lube and wax    Solvent Extraction    Solvent dewaxing    Solvent Deoiling    Solvent deasphalting    Lube isomerisation    Hydrodesulfurisation
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