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Refining Process. Chapter1 Composition of petroleum. Chapter1- Composition of petroleum.
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Chapter1- Composition of petroleum 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 produce salable products. 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 hydrocarbons present in crude petroleum are classified into three general types: paraffin Naphthenes aromatics
What is Crude Oil? • Mixture of organic carbon chain molecules • Impurities include sulfur and nitrogen compounds • Some metals and salts too
Straight-Chain Hydrocarbons Olefins Cyclic H/C Aromatics (Benzene, toluene, xylenes) Mercaptans Hydrogen Sulfide (H2S) Greases Propane LPG Components such as . . .
Paraffins • 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 CnH2n2. • For example, the motor octane number of n-octane is 17 and that of isooctane (2,2,4-trimethyl pentane) is 100. • The number of possible isomers increases in geometric progression as the number of carbon atoms increases. • Crude oil contains molecules with up to 70 carbon atoms, and the number of possible paraf-finic hydrocarbons is very high.
Olefins • 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 CnH2n. • 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. • In gasoline boiling-range fractions, some olefins are desirable because olefins have higher research octane numbers than paraffin compounds with the same number of carbon atoms.
Naphthenes (Cycloparaffins) • 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 compounds such as cyclopentane and cyclohexane, are generally not handled as individual compounds.
Aromatics • Aromatic hydrocarbons contain a benzene ring which is unsaturated but very stable and frequently behaves as a saturated compound. • 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. These mixed types have many of the chemical and physical characteristics of both of the parent compounds,
Chapter 2-Refinery Feed stocks The chemical compositions of crude oils are surprisingly uniform even though their physical characteristics vary widely.
Gravity The density of petroleum oils is expressed in terms of API gravity rather than specific gravity an increase in API gravity corresponds to a decrease in specific gravity most crudes fall in the 20 to 45°API range API gravities are not linear and, therefore, cannot be averaged.
Sulfur Content, wt% Sulfur content and API gravity are two properties which have had the greatest influence on the value of crude oil, although nitrogen and metals contents are increasing in importance. The sulfur content is expressed as percent sulfur by weight and varies from less than 0.1% to greater than 5%. Although the term ‘‘sour’’ crude initially had reference to those crudes containing dissolved hydrogen sulfide independent of total sulfur content, it has come to mean any crude oil with a sulfur content high enough to require special processing. There is no sharp dividing line between sour and sweet crudes, but 0.5% sulfur content is frequently used as the criterion. Crudes with greater than 0.5% sulfur generally require more extensive processing than those with lower sulfur content.
Pour Point, °F (°C) The pour point of the crude oil, in °F or °C, is a rough indicator of the relative paraffinicityand aromaticity of the crude. The lower the pour point, the lower the paraffin content and the greater the content of aromatics.
Carbon Residue, wt% Carbon residue is determined by distillation to a coke residue in the absence of air. The carbon residue is roughly related to the asphalt content of the crude and to the quantity of the lubricating oil fraction that can be recovered. In most cases the lower the carbon residue, the more valuable the crude.
Salt Content, lb/1000 bbl If the salt content of the crude is greater than 10 lb/1000 bbl, it is generally necessary to desalt the crude before processing. -corrosion problems -Catalyst inhibitor
Nitrogen Content, wt% Crudes containing nitrogen in amounts above 0.25% by weight require special processing to remove the nitrogen.
Metals Content, ppm Minute quantities of some of these metals (nickel, vanadium, and copper) can severely affect the activities of catalysts and result in a lower value product distribution. Vanadium concentrations above 2 ppm in fuel oils can lead to severe corrosion to turbine blades and deterioration of refractory furnace linings and stacks
Characterization Factors The correlation index is useful in evaluating individual fractions from crude oils. The correlation index or CI scale is based upon straight-chain paraffins having a CI value of 0 and benzene having a CI value of 100. UOP or Watson ‘‘characterization factor’’ (KW) ranges from less than 10 for highly aromatic materials to almost 15 for highly paraffinic compounds. Crude oils show a narrower range of KW and vary from 10.5 for a highly naphthenic crude to 12.9 for a paraffinic base crude.
Distillation Range The boiling range of the crude gives an indication of the quantities of the various products present. /???????????????????
Chapter 3- Refinery Products American Petroleum Institute (API) of the petroleum refineries and petrochemical plants revealed over 2,000 products made to individual specifications
Products Made by the U.S. Petroleum Industry In general, the products which dictate refinery design are relatively few in number, and the basic refinery processes are based on the large-quantity products such as gasoline, diesel, jet fuel, and home heating oils.
LOW-BOILING PRODUCTS The classification low-boiling products encompasses the compounds which are in the gas phase at ambient temperatures and pressures: methane, ethane, propane, butane, and the corresponding olefins. Methane (C1) is usually used as a refinery fuel, but can be used as a feedstock for hydrogen production by pyrolytic cracking and reaction with steam. Ethane (C2) can be used as refinery fuel or as a feedstock to produce hydrogen or ethylene, which are used in petrochemical processes. Propane (C3) is frequently used as a refinery fuel but is also sold as a liquefied petroleum gas (LPG) The butanes present in crude oils and produced by refinery processes are used as components of gasoline and in refinery processing as well as in LPG. Normal butane (nC4) has a lower vapor pressure than isobutane (iC4), and is usually preferred for blending into gasoline to regulate its vapor pressure and promote better starting in cold weather.
LOW-BOILING PRODUCTS • The butanes present in crude oils and produced by refinery processes are used as components of gasoline and in refinery processing as well as in LPG. • Normal butane (nC4) has a lower vapor pressure than isobutane (iC4), and is usually preferred for blending into gasoline to regulate its vapor pressure and promote better starting in cold weather. • Isobutane has its greatest value when used as a feedstock to alkylation units, where it is reacted with unsaturated materials (propenes, butenes, and pentenes) to form high-octane isoparaffin compounds in the gasoline boiling range. • Although, isobutaneis present in crude oils, its principal sources of supply are from fluid catalytic cracking (FCC) and hydrocracking (HC) units in the refinery and from natural gas processing plants. • Isobutane not used for alkylation unit feed can be sold as LPG or used as a feedstock for propylene (propene) manufacture. • A significant amount of isobutane is converted to isobutylene which is reacted with methanol to produce methyl tertiary butyl ether (MTBE). • Butane–propane mixtures are also sold as LPG.
GASOLINE Most refiners produce gasoline in two or three grades, unleaded regular, premium super-premium The principal difference between the regular and premium fuels is the antiknock performance. ضد کوبه Gasolines are complex mixtures of hydrocarbons having typical boilingranges from 100 to 400°F (38 to 205°C) as determined by the ASTM method.Components are blended to promote high antiknock quality, ease of starting,quick warm-up, low tendency to vapor lock, and low engine deposits.
GASOLINE Light straight-run (LSR) gasoline consists of the C5-190°F (C5-88°C) fractionof the naphtha cuts from the atmospheric crude still. The reformer increases the octane by converting low-octane paraffinstohigh-octane aromatics. Some aromatics have high rates of reaction with ozoneto form visual pollutants in the air and some are claimed to be potentially carcinogenic. Polymer gasoline is manufactured by polymerizing olefinichydrocarbonsto produce higher molecular weight olefins in the gasoline boiling range. Alkylate gasoline is the product of the reaction of isobutane with propylene,butylene, or pentylene to produce branched-chain hydrocarbons in the gasolineboiling range.
GASOLINE- SPECIFICATIONS The Reid vapor pressure (RVP) and boiling range of gasoline governs ease of starting, engine warm-up rate of acceleration loss by crankcase dilution mileage economy vapor lock.
GASOLINE-types of octane numbers motor method(MON) : is a guide to engine performance on the highway or under heavy load Conditions. Researchmethod (RON): represents the performance during city driving when acceleration is relatively frequent, Effects of Variables on Octane Requirements
What Goes on at a Refinery. . .? • Separation of components by distillation, e.g.: • Atmospheric • Vacuum • Hydrotreating (uses excess hydrogen) • Breaking apart molecules to make smaller ones, e.g.: • catalytic cracking • hydrocracking • Joining molecules to make bigger ones, e.g.: • Reforming - alkylation that lengthens the hydrocarbon chain • Reforming - cyclic that generates hydrogen
4. Crude Distillation • The crude stills are the first major processing units in the refinery. • They are used to separate the crude oils by distillation into fractions according to boiling point. • crude oil separation is accomplished in two steps: • first by fractionating the total crude oil at essentially atmospheric pressure • feeding the high-boiling bottoms from the atmospheric still to a second fractionator operated at a high vacuum.
DESALTING CRUDE OILS • If the salt content of the crude oil is greater than 10 lb/1000 bbl, the crude requires desalting to minimize: • fouling • corrosion caused by salt deposition on heat transfer surfaces • acids formed by decomposition of the chloride salts • metals in inorganic compounds dissolved in water emulsified with the crude oil, which can cause catalyst deactivation in catalytic processing units • removal of suspended solids from the crude oil (fine sand, clay, and soil particles; iron oxide and iron sulfide particles from pipelines, tanks, or tankers; and other contaminants picked up in transit or production.
DESALTING CRUDE OILS • The basic principle is to wash the salt from the crude oil with water. • Desalting is carried out by mixing the crude oil with from 3 to 10 vol% water at temperatures from 90 to 150°C. • the ratio of the water to oil and the temperature of operation are functions of the density of the oil.
DESALTING CRUDE OILS • The salts are dissolved in the wash water and the oil and water phases separated in a settling vessel either by adding chemicals to assist in breaking the emulsion or by developing a high-potential electrical field across the settling vessel to coalesce the droplets of salty water more rapidly. • Either AC or DC fields may be used and potentials from 12,000 to 35,000 volts are used to promote coalescence. • For single-stage desalting units 90 to 95% efficiencies are obtained and two-stage processes achieve 99% or better efficiency. • If the pH of the brine exceeds 7, emulsions can be formed because of the sodium naphthenate and sodium sulfide Present. • For most crude oils it is desirable to keep the pH below 8.0. Better dehydration is obtained in electrical desalters when they are operated in the pH range of 6 to 8 with the best dehydration obtained at a pH near 6. • The pH value is controlled by using another water source or by the addition of acid to the inlet or recycled water.
4.2 ATMOSPHERIC TOPPING UNIT • The temperature of crude oil is raised to about 288°C by heat exchange with product and reflux streams. • It is then further heated to about 399°C in a furnace and charged to the flash zone of the atmospheric fractionators. • The furnace discharge temperature is sufficiently high 343 to 399°C to cause vaporization of all products withdrawn above the flash zone plus about 10 to 20% of the bottoms product. • Reflux is provided by • condensing the tower overhead vapors and returning a portion of the liquid to the top of the tower • pump-around and pumpback streams lower in the tower. • Each of the sidestream products removed from the tower decreases the amount of reflux below the point of draw off. • Maximum reflux and fractionation is obtained by removing all heat at the top of the tower, but this results in an inverted cone-type liquid loading which requires a very large diameter at the top of the tower.
4.2 ATMOSPHERIC TOPPING UNIT • To reduce the top diameter of the tower and even the liquid loading over the length of the tower, intermediate heat-removal streams are used to generate reflux below the sidestream removal points. To accomplish this, liquid is removed from the tower, cooled by a heat exchanger, and returned to the tower or, alternatively, a portion of the cooled sidestream may be returned to the tower. This cold stream condenses more of the vapors coming up the lower and thereby increases the reflux below that point. • Although crude towers do not normally use reboilers, several trays are generally incorporated below the flash zone and steam is introduced below the bottom tray to strip any remaining gas oil from the liquid in the flash zone and to produce a high-flash-point bottoms. • The atmospheric fractionator normally contains 30 to 50 fractionation trays. • Separation of the complex mixtures in crude oils is relatively easy and generally five to eight trays are needed for each sidestream product plus the same number above and below the feed plate. Thus, a crude oil atmospheric fractionation tower with four liquid sidestreamdrawoffs will require from 30 to 42 trays.
ATMOSPHERIC TOPPING UNIT • The liquid sidestream withdrawn from the tower will contain low-boiling components which lower the flashpoint. These ‘‘light ends’’ are stripped from each sidestream in a separate small stripping tower containing four to ten trays with steam introduced under the bottom tray. The steam and stripped light ends are vented back into the vapor zone of the atmospheric fractionator above the corresponding side-draw tray. • The overhead condenser on the atmospheric tower condenses the pentaneand- heavier fraction of the vapors that passes out of the top of the tower. This is the light gasoline portion of the overhead, containing some propane and butanes and essentially all of the higher-boiling components in the tower overhead vapor. Some of this condensate is returned to the top of the tower as reflux, and the remainder is sent to the stabilization section of the refinery gas plant where the butanes and propane are separated from the C5-180°F (C5-82°C) LSR gasoline.
4.3 VACUUM DISTILLATION • The furnace outlet temperatures required for atmospheric pressure distillation of the heavier fractions of crude oil are so high that thermal cracking would occur, with the resultant loss of product and equipment fouling. • Distillation is carried out with absolute pressures in the tower flash zone area of 25 to 40 mmHg. • Addition of steam to the furnace inlet increases the furnace tube velocity and minimizes coke formation in the furnace as well as decreasing the total hydrocarbon partial pressure in the vacuum tower. • Furnace outlet temperatures are also a function of the boiling range of the feed and the fraction vaporized as well as of the feed coking characteristics. • furnace outlet temperatures in the range of 388 to 454°C are generally used The lower operating pressures cause significant increases in the volume of vapor per barrel vaporized and, as a result, the vacuum distillation columns are much larger in diameter than atmospheric towers. It is not unusual to have vacuum towers up to 40 feet in diameter.
4.4 CRUDE DISTILLATION UNIT PRODUCTS • Fuel gas. The fuel gas consists mainly of methane and ethane. In some refineries, propane in excess of LPG requirements is also included in the fuel gas stream. This stream is also referred to as ‘‘dry gas.’’ • Wet gas. The wet gas stream contains propane and butanes as well as methane and ethane. The propane and butanes are separated to be used for LPG and, in the case of butanes, for gasoline blending and alkylation unit feed. • LSR naphtha. The stabilized LSR naphtha (or LSR gasoline) stream is desulfurized and used in gasoline blending or processed in an isomerization unit to improve octane before blending into gasoline. • HSR naphtha or HSR gasoline. The naphtha cuts are generally used as catalytic reformer feed to produce high-octane reformate for gasoline blending and aromatics.
