Introduction :Composition of petroleum,laboratory tests,refineryfeedstocks and products

Fig:1.1[15]

Fig.1.2 Distribution of world energy resources. (From World Energy Outlook 2005, International Energy Agency.)[16,17]

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, thehigh­volume 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 350oC.Although products such as lubricating oils, refrigeration and transformer oils, and petrochemicalfeedstocks are profitable.They amount to less than 5 percent of the total crude oil charged torefineries. The processing equipment indicated is for processing crude oils of average gravities andsulfur contents. Crude oils with low API gravities (high specific gravities) and high sulfur contentsrequire additional hydrotreating equipment. The quality of crude oils processed by worldwiderefineries is expected to worsen slowly in the future with the sulfur contents and densities toincrease. Therefore refineries will require processing the entire barrel of crude rather than just thematerial boiling below (550 oC).Sulfur restrictions on fuels, coke and heavy fuel oils affects thebottom­of­the­barrel processing as well. These factors requires extensive refinery additionsmodernization the shift in market requirements among gasolines and reformulated fuels fortransportation challenges.[16] Refinery Overview[15­20]

The crude oil is heated in a furnace and charged to an atmospheric distillation tower, where it isseparated into light gas (C1­C4), light naphtha, heavy naphtha, kerosine, atmospheric gas oil, andreduced (topped) crude . The reduced crude is sent to the vacuum distillation tower and separatedinto vacuum gas oil stream and vacuum reduced crude bottoms (residua, resid). The reducedcrude bottoms from the vacuum distillation tower is thermally cracked in a delayed coker toproduce gas, coker gasoline, coker gas oil, and coke. The atmospheric and vacuum crude unit gasoils and coker gas oil are used as feedstocks for the catalytic cracking or hydrocracking unitswhere heavy molecules get converted into lower molecular weight compounds boiling in thegasoline and distillate fuel ranges. The hydrocracked products are saturated whereas catalyticcracker products are unsaturated and further need improvement in quality by either hydrotreatingor by reforming. The light naphtha streams from the crude tower, coker and cracking units aresent to an isomerization unit to convert straight­chain paraffins into isomers which have higheroctane numbers. The heavy naphtha streams from the crude tower, coker, and cracking units arefed to the catalytic reformer to improve octane numbers. The products from the catalytic reformercan be blended into regular and premium gasolines for marketing. The wet gas streams from thecrude unit, coker, and cracking units are separated in the vapor recovery section (gas plant) intofuel gas, liquefied petroleum gas (LPG), unsaturated hydrocarbons (propylene, butylenes, andpentenes), normal butane, and isobutane. The fuel gas is burned as a fuel in refinery furnaces andthe normal butane is blended into gasoline or LPG. The unsaturated hydrocarbons and isobutaneare sent to the alkylation unit to react olefins with isobutane to yield isoparaffins. The alkylation isdone at high pressure and low temperature in the presence of sulfuric or hydrofluoric acid ascatalyst. The product is called alkylated gasoline, which is a high­octane product blended intopremium 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, theheavy vacuum gas oil and reduced crude from paraffinic or naphthenic base crude oils areprocessed into lubricating oils. The asphaltenes are removed in a propane deasphalting unit, andthe reduced crude from bottoms are processed with the vacuum gas oils to produce lubeoil basestocks (LOBS). The vacuum gas oils and deasphalted stocks are solvent­extracted to remove thearomatic compounds followed by dewaxing to improve the pour point. These LOBS are furthertreated with acid clays to improve their color and stability before being blended into lubricatingoils. Each refinery has its own unique processing scheme which is determined by the processequipment available, crude oil characteristics, operating costs, and product demand.

Refinery Feed Stocks[19­20] 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 varywidely. The elementary composition of crude oil usually falls within the following ranges: C 84­88;H 11­15; 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 aromatic­base oils. The U.S.Bureau of Mines has developed a system which classifies the crude according to two key fractionsobtained in distillation: No. 1 from 250 to 275 oC at atmospheric pressure and No. 2 from 275 to300 oC at 40 mmHg pressure. The API gravity of these fractions varies depending upon paraffinicand 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 useare based on the properties of the residuum left from nondestructive distillation and are moredescriptive to the refiner because they convey the nature of the products to be expected and theprocessing necessary. Composition Of Petroleum[19­20]

Crude oils are composed of many members of homologous series of hydrocarbons. Petroleum isessentially a mixture of hydrocarbons, and even the non­hydrocarbon elements are generallypresent as components of complex molecules predominantly hydrocarbon in character, butcontaining small quantities of oxygen, sulfur, nitrogen, vanadium nickel, and chromium. Thecomposition 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 processingrequired to production marketable products. The hydrocarbons present in crude petroleum areclassified into three general types: paraffins, naphthenes, and aromatics. Olefins are generally notpresent in crude oil, however these are formed during processing by the dehydrogenation ofparaffins and naphthenes. The paraffin series of hydrocarbons is characterized by the rule that thecarbon atoms are connected by a single bond and the other bonds are saturated with hydrogenatoms.The general formula for paraffins is CnH2n+2. Crude oil contains molecules with up to 70carbon atoms, and the number of possible paraffinic hydrocarbons is very high .Olefins do notnaturally occur in crude oils but are formed during the processing. They are very similar instructure to paraffins but at least two of the carbon atoms are joined by double bonds. Thegeneral formula is CnH2n. Olefins are generally undesirable in finished products because the doublebonds are reactive and the compounds are more easily oxidized and polymerized to form gumsand varnishes. Olefins containing five carbon atoms have high reaction rates with compounds inthe atmosphere that form pollutants and, even though they have high research octane numbers,are considered generally undesirable. Some diolefins (containing two double bonds) are alsoformed during processing, but they react very rapidly with olefins to form high­molecular­weightpolymers consisting of many simple unsaturated molecules joined together. Diolefins are veryundesirable in products because they are so reactive they polymerize and form filter andequipment plugging compounds. Cycloparaffin hydrocarbons in which all of the available bonds ofthe carbon atoms are saturated with hydrogen are called naphthenes. There are many types ofnaphthenes present in crude oil, but except for the lower­molecular­weight such as cyclopentaneand cyclohexane, are generally not handled as individual compounds. They are classified accordingto boiling range and their properties determined with the help of correlation factors such as thecharacterization (Kw) factor or correlation index ( CI) . The aromatic series of hydrocarbons ischemically and physically very different from the paraffins and cycloparaffins (naphthenes). Thecyclic hydrocarbons, both naphthenic and aromatic, can add paraffin side chains in place of someof the hydrogen attached to the ring carbons and form a mixed structure. Petroleum /crude oil[19]

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 smallquantities of nitrogen oxygen and sulfur­containing compounds as well as trace amounts ofmetallic constituents.

Petroleum[19]

The fuels derived from Petroleum contribute approximately one­third to one­half of the totalworld energy supply and are used for transportation fuels (gasoline,diesel fuel,and aviationfuel,among others) and heating buildings.Petroleum products have a wide variety of uses that vary from gaseous and liquid fuels tonear­solid machinery lubricants. Residue of many refinery processes asphalt—is now apremium value product for highway surfaces, roofing materials, and miscellaneouswaterproofing uses.Crude petroleum is a mixture of compounds boiling at different temperatures that can beseparated into a variety of different generic fractions by distillation and the terminology ofthese fractions has been bound by utility and often bears little relationship to composition.

Major Processes:[19­20]

DesaltingSweeteningHydrogen Generation UnitDHDS/DHDTReformerIsomerisationAmine TreatingSulphur Recovery UnitBitumen Blowing UnitLube and waxSolvent ExtractionSolvent dewaxingSolvent DeoilingSolvent deasphaltingLube isomerisationHydrodesulfurisation

Fig:1.3[15]A typical REFINERY PROCESS CHART

Typical Refinery Products

LPG 2.1%NAPHTHA 5.0%MS 11.2%OTHERS 18.3%LIGHT DISTILLATE 36.6%ATF 9.0

Chemistry in Fuel refining Key properties of Fuels

MS: Octane number, Sulphur, distillation, Oxidation stability, Bz/ Aromatics/ Olefin content.HSD: Cetane number,Sulphur,distillation,lubricity.Kerosene: Smoke Point.ATF: Freezing point.Furnace oil: Stability.Bitumen: Penetration, viscosity, Softening point.LOBS: Viscosity, Viscosity Index.

Definations and Terminology[19,20]

Petroleum is a mixture of gaseous, liquid, and solid hydrocarbon compounds that occur insedimentary rock deposits throughout the world and also contains small quantities of nitrogen,oxygen, and sulfur­containing compounds as well as trace amounts of metallicconstituents.Petroleum is a naturally occurring mixture of hydrocarbons, generally in a liquidstate, which may also include compounds of sulfur, nitrogen, oxygen, metals, other elements .Anynaturally occurring hydrocarbon, whether in a liquid, gaseous, or solid state may foundexplorations. Complexity of a refinery[19,20] The combination of refining processes and operations employed (complexity) varies from onerefinery to another.Modern refinery is highly complex, energy and capital intensive. Role ofcatalytic processes, secondary processing and hydroprocessing is increasing .Factors deciding thecomplexity of a refinery.

Nature/source of crude oils­ flexibility to process variety of crudes.Demand pattern in the markets.Product quality ­ current/ future.Production of feedstocks for downstream units.Inter­fuel substitution.

Separation : Heavy on the bottom, light on the top

The separation of crude oil by atmospheric and vacuum distillation into groups ofhydrocarbon compounds of different boiling point ranges called “fractions” or “cuts”.The first step in crude oil processing, where the first separation takes place is called CrudeDistillation Unit (CDU) Atmospheric & Vacuum Unit (AVU).This step is performed in all refineries :These units are called “Mother Units”Typical products from CDU are : Gas, LPG, naphtha, SKO/ATF, HSD and RCO.Vacuum Distillation of RCO produces VGO (or LOBS cuts) and VRAll products need further treatment/processing.