PETROLEUM REFINING AND PETROCHEMICALS

PE 411

THE UNIVERSITY OF DODOMA

• Pre-requisites:• CH 105 - Organic chemistry I• PE 303 - Petroleum Production Engineering I• PE 306 - Petroleum Production Engineering II• PE 305 - Reservoir Engineering I

• Mode of Delivery and Assessment:• Delivery: 3 hours Lecture + 1 hours tutorial per week

Assessment:• 40% continuous assessment• 60% University examination

• Recommended books1. Introduction to Petroleum Production-I & II, Dr. Skimmer.2. Surface Operation in Petroleum Production, G.V. Chillingarian, J.O. Robertsin and S. Kumar.3. Handbook of petroleum refining processes, by Robert A. Meyers, MaC-Graw Hill, 20034. Petroleum Refining: Technology and Economics by James H. Gary and Glenn E. Handwerk, CRC

Press, 4th Edition, 2001

Petroleum (L. petroleum, from Greek: Πέτρα(rock) + Latin: oleum (oil)) is a naturally occurringflammable liquid consisting of a complex mixture of hydrocarbons of various molecularweights and other Liquid organic compounds, that are found in geologic formations beneaththe Earth's surface.

Petroleum - Crude oil

• The name Petroleum covers both naturally occurring unprocessed crude oils andpetroleum products that are made up of refined crude oil.

• Petroleum contains different hydrocarbons; the most commonly found molecules arealkanes (linear or branched), cycloalkanes, aromatic hydrocarbons, or morecomplicated chemicals like asphaltenes.

• Petroleum is recovered mostly through oil drilling. This comes after the studies ofstructural geology (at the reservoir scale), sedimentary basin analysis, reservoircharacterization (mainly in terms of porosity and permeable structures).

• It is refined and separated, most easily by boiling point, into a large number ofconsumer products, from petrol (or gasoline) and kerosene to asphalt and chemicalreagents used to make plastics and pharmaceuticals.

• Petroleum is used in manufacturing a wide variety of materials, and it is estimatedthat the world consumes about 88 million barrels each day.

COMPOSITION OF CRUDE OILS

• Crude oil (petroleum) is a naturally occurring brown to black flammable liquid.

• Crude oils are principally found in oil reservoirs associated with sedimentary rocks beneath the earth’ssurface. Although exactly how crude oils originated is not established, it is generally agreed that crudeoils derived from marine animal and plant debris subjected to high temperatures and pressures.

• It is also suspected that the transformation may have been catalyzed by rock constituents.

• Regardless of their origins, all crude oils are mainly constituted of hydrocarbonsmixed with variable amounts of sulfur, nitrogen, and oxygen compounds.

• Metals in the forms of inorganic salts or organometallic compounds are present inthe crude mixture in trace amounts.

• The ratio of the different constituents in crude oils, however, vary appreciably fromone reservoir to another.

• Normally, crude oils are not used directly as fuels or as feedstocks for theproduction of chemicals. This is due to the complex nature of the crude oilmixture and the presence of some impurities that are corrosive or poisonous toprocessing catalysts.

• Crude oils are refined to separate the mixture into simpler fractions that can beused as fuels, lubricants, or as intermediate feedstock to the petrochemicalindustries.

• A general knowledge of this composite mixture is essential for establishing aprocessing strategy.

CRUDE OIL

• The hydrocarbons in crude oil are mostly alkanes (paraffins), cycloalkanes (naphthenes) and variousaromatic hydrocarbons

• The other organic compounds contain nitrogen, oxygen and sulphur, and trace amounts of metalssuch as iron, nickel, copper and vanadium.

• The exact molecular composition varies widely from formation to formation but the proportion of chemical elements vary over fairly narrow limits as follows:

• Elemental composition by weight

Composition

Element Percent range

Carbon 83 to 87%

Hydrogen 10 to 14%

Nitrogen 0.1 to 2%

Oxygen 0.05 to 1.5%

Sulfur 0.05 to 6.0%

Metals < 0.1%

CRUDE OIL CONITUNUES…..

• Four different types of hydrocarbon molecules appear in crude oil.

• The relative percentage of each varies from oil to oil, determining the properties of each oil.

• Petroleum - Composition by weight

Hydrocarbon Average Range

Paraffins 30% 15 to 60%

Naphthenes 49% 30 to 60%

Aromatics 15% 3 to 30%

Asphaltics 6% remainder

CRUDE OIL CONTINUES…..

• Petroleum is used mostly, by volume, for producing fuel oil and petrol, both important "primary energy" sources.

• 84 vol. % of the hydrocarbons present in petroleum is converted into energy-rich fuels (petroleum-based fuels), including petrol, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas.

• The lighter grades of crude oil produce the best yields of these products, but asthe world's reserves of light and medium oil are depleted, oil refineries areincreasingly having to process heavy oil and bitumen, and use more complex andexpensive methods to produce the products required.

• Because heavier crude oils have too much carbon and not enough hydrogen, theseprocesses generally involve removing carbon from or adding hydrogen to themolecules, and using fluid catalytic cracking to convert the longer, more complexmolecules in the oil to the shorter, simpler ones in the fuels.

• In its strictest sense, petroleum includes only crude oil, but in common usage it includes all liquid, gaseous, and solid hydrocarbons.

• Under surface pressure and temperature conditions, lighter hydrocarbons methane, ethane, propane and butane occur as gases, while pentane and heavier ones are in the form of liquids or solids.

• However, in an underground oil reservoir the proportions of gas, liquid, and solid depend on subsurface conditions and on the phase diagram of the petroleum mixture.

• 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 predominantly 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. At surface conditions these will condense out of the gas to form natural gas condensate, often shortened to condensate.

• Condensate resembles petrol in appearance and is similar in composition to some volatile light crude oils.

• The proportion of light hydrocarbons in the petroleum mixture varies greatly among different oil fields, ranging from as much as 97 percent by weight in the lighter oils to as little as 50 percent in the heavier oils and bitumens.

Crude oil mixture is composed of the following groups:

1. Hydrocarbon compounds(compounds made of carbon and hydrogen).

2. Non-hydrocarbon compounds.

3. Organometallic compounds and inorganic salts (metallic compounds).

• The principal constituents of most crude oils are hydrocarbon compounds.

• All hydrocarbon classes are present in the crude mixture, except alkenes and alkynes.

Alkanes (Paraffins)

• Alkanes are saturated hydrocarbons having the general formula CnH2n+2

• The simplest alkane, methane (CH4), is the principal constituent of natural gas. Methane, ethane, propane, and butane are gaseous hydrocarbons at ambient temperatures and atmospheric pressure.

• They are usually found associated with crude oils in a dissolved state.

• Normal alkanes (n-alkanes, n-paraffins) are straight-chain hydrocarbons having no branches.

• Branched alkanes are saturated hydrocarbons with an alkyl substituent or a side branch from the main chain.

• A branched alkane with the same number of carbons and hydrogens as an n-alkane is called an isomer.

For example, butane (C4H10) has two isomers, n-butane and 2-methyl propane (isobutane). As the molecular weight of the hydrocarbon increases, the number of isomers also increases. Pentane (C5 - C12) has three isomers; hexane (C6H14) has five. The following shows the isomers of hexane:

• Alkanes from pentane (C5H12) to octane (C8H18) are refined into petrol, the ones from nonane (C9H20) to hexadecane (C16H34) into diesel fuel, kerosene and jet fuel.

• Alkanes with more than 16 carbon atoms can be refined into fuel oil and lubricating oil.

• At the heavier end of the range, paraffin wax is an alkane with approximately 25 carbon atoms, while asphalt has 35 and up, although these are usually cracked by modern refineries into more valuable products.

• The shortest molecules, those with four or fewer carbon atoms, are in a gaseous state at room temperature. They are the petroleum gases.

• Depending on demand and the cost of recovery, these gases are either flared off, sold as liquified petroleum gas under pressure, or used to power the refinery's own burners.

• During the winter, butane (C4H10), is blended into the petrol pool at high rates, because its high vapor pressure assists with cold starts

CYCLOPARAFFINS (NAPHTHENES)

• Saturated cyclic hydrocarbons, normally known as naphthenes, are also part of thehydrocarbon constituents of crude oils. Their ratio, however, depends on the crudetype.

• The lower members of naphthenes are cyclopentane, cyclohexane, and their mono-substituted compounds.

• They are normally present in the light and the heavy naphtha fractions.• Cyclohexanes, substituted cyclopentanes, and substituted cyclohexanes are important

precursors for aromatic hydrocarbons.

CYCLOALKANES / NAPHTHENES

• The cycloalkanes(naphthenes), are saturated hydrocarbonswhich have one or more carbon rings to which hydrogenatoms are attached according to the formula CnH2n.

• Cycloalkanes have similar properties to alkanes but havehigher boiling points.

• The smallest cycloalkane is cyclopropane• Cycloalkanes are very similar to the alkanes in reactivity,

except for the very small ones - especially cyclopropane.Cyclopropane is much more reactive than expected.

• The reason has to do with the bond angles in the ring.Normally, when carbon forms four single bonds, the bondangles are about 109.5°. In cyclopropane, they are 60°.

• With the electron pairs this close together, there is a lotof repulsion between the bonding pairs joining the carbonatoms. That makes the bonds easier to break.

AROMATIC COMPOUNDS

• Lower members of aromatic compounds are present in small amounts in crude oils and lightpetroleum fractions.

• The simplest mononuclear aromatic compound is benzene (C6H6). Toluene (C7H8) and xylene (C8H10) arealso mononuclear aromatic compounds found in variable amounts in crude oils.

• Benzene, toluene, and xylenes (BTX) are important petrochemical intermediates as well as valuablegasoline components.

• Separating BTX aromatics from crude oil distillates is not feasible because they are present in lowconcentrations. Enriching a naphtha fraction with these aromatics is possible through a catalyticreforming process.

• Binuclear aromatic hydrocarbons are found in heavier fractions than naphtha.• Trinuclear and polynuclear aromatic hydrocarbons, in combination with heterocyclic compounds, are

major constituents of heavy crudes and crude residues.• Asphaltenes are a complex mixture of aromatic and heterocyclic compounds.

The following are representative examples of some aromatic compounds found in crude oils:

SULPHUR COMPOUNDS

• Sulfur in crude oils is mainly present in the form of organosulphur compounds.• Hydrogen sulfide is the only important inorganic sulfur compound found in crude oil. • Its presence, however, is harmful because of its corrosive nature. • Organosulphur compounds may generally be classified as acidic and non-acidic. • Acidic sulfur compounds are the thiols (mercaptans).• Thiophene, sulfides, and disulfides are examples of non-acidic sulfur compounds found

in crude fractions. • Extensive research has been carried out to identify some sulfur compounds in a

narrow light petroleum fraction. Examples of some sulfur compounds from the two types are:

NITROGEN COMPOUNDS

• Organic nitrogen compounds occur in crude oils either in a simple heterocyclic form as in pyridine (C5H5N) and pyrrole (C4H5N), or in a complex structure as in porphyrin.

• The nitrogen content in most crudes is very low and does not exceed 0.1 wt%. • In some heavy crudes, however, the nitrogen content may reach up to 0.9 wt %. • Nitrogen compounds are more thermally stable than sulfur compounds and accordingly are

concentrated in heavier petroleum fractions and residues. • Light petroleum streams may contain trace amounts of nitrogen compounds, which should

be removed because they poison many processing catalysts. • During hydro treatment of petroleum fractions, nitrogen compounds are hydro

denitrogenated to ammonia and the corresponding hydrocarbon.• Nitrogen compounds in crudes may generally be classified into basic and non-basic

categories. • Basic nitrogen compounds are mainly those having a pyridine ring, and the non-basic

compounds have a pyrrole structure. • Both pyridine and pyrrole are stable compounds due to their aromatic nature

• Porphyrins are non-basic nitrogen compounds. • The porphyrin ring system is composed of four pyrrole rings joined by =CH- groups. • The entire ring system is aromatic. Many metal ions can replace the pyrrole hydrogens and

form chelates. The chelate is planar around the metal ion and resonance results in four equivalent bonds from the nitrogen atoms to the metal.

• Almost all crude oils and bitumens contain detectable amounts of vanadyl and nickel porphyrins.

OXYGEN COMPOUNDS

• Oxygen compounds in crude oils are more complexthan the sulfur types. However, their presence inpetroleum streams is not poisonous to processingcatalysts.

• Many of the oxygen compounds found in crude oilsare weakly acidic. They are carboxylic acids, cresylicacid, phenol, and naphthenic acid. Naphthenic acidsare mainly cyclopentane and cyclohexane derivativeshaving a carboxyalkyl side chain.

• Naphthenic acids in the naphtha fraction have aspecial commercial importance and can be extractedby using dilute caustic solutions. The total acidcontent of most crudes is generally low, but mayreach as much as 3%, as in some California crudes.

• Non-acidic oxygen compounds such as esters, ketones, and amides are less abundant than acidic compounds.

• They are of no commercial value.

METALLIC COMPOUNDS

Many metals occur in crude oils. • Some of the more abundant aresodium (Na), calcium (Ca), magnesium (Mg), aluminium

(Al), iron (Fe), vanadium (V), and nickel (Ni).• They are present either as inorganic salts, such as sodium and magnesium chlorides, or

in the form of organometallic compounds, such as those of Ni and V (as in porphyrins). • Calcium and magnesium can form salts or soaps with carboxylic acids. These compounds

act as emulsifiers, and their presence is undesirable.• Although metals in crudes are found in trace amounts, their presence is harmful and

should be removed. When crude oil is processed, sodium and magnesium chloridesproduce hydrochloric acid, which is very corrosive.

• Desalting crude oils is a necessary step to reduce these salts.• Vanadium and nickel are poisons to many catalysts and should be reduced to very low

levels. • Most of the vanadium and nickel compounds are concentrated in the heavy residues. • Solvent extraction processes are used to reduce the concentration of heavy metals in

petroleum residues.

CRUDE OIL CLASSIFICATION

• Appreciable property differences appear between crude oils as a result of the variable ratios of the crude oil components.

• Crude oils can be arbitrarily classified into three or four groups depending on the relative ratio of the hydrocarbon classes that predominates in the mixture.

The following describes three types of crudes:1. Paraffinic- the ratio of paraffinic hydrocarbons is high compared to aromatics and naphthenes.2. Naphthenic- the ratios of naphthenic and aromatic hydrocarbons are relatively higher than in paraffinic

crudes.3. Asphaltic- contain relatively a large amount of polynuclear aromatics, a high asphaltene content, and

relatively less paraffins than paraffinic crudes.

General characteristics- Content of impurities: water, sediment, salts, metals, nitrogen- Sulfur: 0.1 – 0.5 wt.% sweet / 0.5 – 0.8 wt.% semi-sweet / 0.8 – 5 wt.% sour- Density: 38 – 45 oAPI light / 28 – 38 oAPI medium / 12 – 28 oAPI heavy

NOTE: API gravity, is a measure of how heavy or light a petroleum liquid is

compared to water. API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks

EVALUATION OF CRUDE OIL

• Crude oils differ appreciably in their properties according to origin and the ratio of the different components in the mixture.

• Lighter crudes generally yield more valuable light and middle distillates and are sold at higher prices.

• Evaluation of crude oil is important for refiner because it gives the following types of information:I. Base and general properties of the crude oil. II. Presence of impurities such as sulfur, salt, and emulsions which cause general

difficulties in processing. III. Operating or design data. Primarily this necessitates curves of temperature

and gravity vs. per cent distilled. a) Fractionating or true boiling point distillation curve. b) Equilibrium or flash-vaporization curve. c) API or specific gravity curve of each fraction distilled.

IV. Finished products. Having established the general properties and yield by means of distillation and property curves and exploring the economy of the various break-ups of the crude oil

V. Curves of the properties of the fractions vs. percent distilled (mid percent curves) or the average properties of a series of fractions vs. Percentage yield (yield curve) by which common realization of yields can by prepared. Among property curves are

a) Viscosity of lubricating-oil fractions b) Octane number of gasoline fractions. c) Aniline point of solvents, kerosene, or diesel fractions. d) Percentage of asphaltic residues. e) Viscosity of distillation residues.

• A more useful unit used by the petroleum industry is specific gravity, which is the ratio of the weight of a given volume of a material to the weight of the same volume of water measured at the same temperature.

• Specific gravity is used to calculate the mass of crude oils and its products.• Usually, crude oils and their liquid products are first measured on a volume basis, then changed to

the corresponding masses using the specific gravity.• The API (American Petroleum Institute) gravity is another way to express the relative masses of crude

oils. The API gravity could be calculated mathematically using the following equation:

• Specific gravities of crude oils roughly range from 0.82 for lighter crudes to over 1.0 for heavier crudes (41 - 10 °API scale).

• Higher °API , more paraffinic crude, higher yields of gasoline.• Lower °API , more aromatic crude, lower yields of gasoline.

Density, Specific Gravity and API Gravity

Sulfur Content

• 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%.

• Crudes with greater than 0.5% sulfur generally require more extensive processing than those with lower sulfur content.

Pour Point, (°C)

• The pour point of the crude oil, in °For °C, is a rough indicator of the relative paraffinicity and 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. This is expressed in terms of the weight percent carbon residue by either the Ramsbottom (RCR) or Conradson (CCR) ASTM test procedures (D-524 and D-189).

• If the salt content of the crude, when expressed as NaCl, is greater than 10lb/1000 bbl, it is generally necessary to desalt the crude before processing.

• If the salt is not removed, severe corrosion problems may be encountered. • If residua are processed catalytically, desalting is desirable at even lower salt

contents of the crude. • Although it is not possible to have an accurate conversion unit between lb/1000

bbl and ppm by weight because of the different densities of crude oils, 1lb/1000 bbl is approximately 3 ppm

Salt Content

Characterization Factor (Kw) and Correlation Index (CI)

There are several correlations between yield and the aromaticity and paraffinicityof crude oils, but the two most widely used are the UOP or Watson ‘‘characterization factor’’ (KW) and the U.S. Bureau of Mines ‘‘correlation index’’ (CI).

• 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.

• The correlation index (CI) is useful in evaluating individual fractions from crude oils.

• The CI scale is based upon straight-chain paraffins having a CI value of 0 and benzene having a CI value of 100.

• The CI values are not quantitative, but the lower the CI value, the greater the concentrations of paraffin hydrocarbons in the fraction; and the higher the CI value, the greater the concentrations of naphthenes and aromatics.

• For petroleum fractions 10 < Kw < 15 (Watson characterization factor ranges)

↑ ↑contain containhighly highly

aromatic paraffiniccompounds compounds

• For crude oil 10.5 < Kw < 12.9

↑ ↑

highly highly

naphthenic paraffinic

crude crude

• For pure hydrocarbons• Kw = 13 for paraffins.

• Kw = 12 for HC with equivalent chain and ring weights.

• Kw = 11 for pure naphthenes.

• Kw = 10 for pure aromatics.

Nitrogen Content, wt%

• A high nitrogen content is undesirable in crude oils because organic nitrogen compounds cause severe poisoning of catalysts used in processing and cause corrosion problems such as hydrogen blistering.

• Crudes containing nitrogen in amounts above 0.25% by weight require special processing to remove the nitrogen.

Viscosity Index• This is a series of numbers ranging from 0-100 which indicate the rate of

change of viscosity with temperature.Paraffinic base C.O V.I =100Naphthenic base C.O V.I = 40Some Naphthenic base C.O V.I =0

Metals Content, ppm

• The metals content of crude oils can vary from a few parts per million to more than 1000 ppm and, in spite of their relatively low concentrations, are of considerable importance.

• Minute quantities of some of these metals (nickel, vanadium, and copper) can severely affect the activities of catalysts and result in a lowervalue 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 .

• Distillation concentrates the metallic constituents of crude in the residues, but some of the organometallic compounds are actually volatilized at refinery distillation temperatures and appear in the higher-boiling distillates.

• The metallic content may be reduced by solvent extraction with propaneor similar solvents as the organometallic compounds are precipitated with the asphaltenes and resins.

ANALYSIS OF CRUDE PETROLEUM

1. Distillation Range (Curve)

• The boiling point range is an alternate method to represent the composition of petroleum and its products.

• It gives an indication of the quantities and qualities of the various products present in crude oil (i.e. naphtha, kerosene, diesel, gas oil, residue, etc).

• Can be used to determine the most desirable processing sequence to obtain the required products.

• Can be used to determine whether the crude is suitable for asphalt or lube oil manufacture.

• Types of distillation curves1. TBP (True Boiling Point) distillation curve.2. EFV (Equilibrium Flash Vaporization)3. ASTM (D86/D1160) distillation curve.

1. TBP (True Boiling Point)TBP is the most useful.

• However, no standard test exists for measuring the TBP.• Most common TBP test is Hempel & D-285. (Neither specifies # of stages or reflex ratio

used).• Trend is toward 15/5 distillation (D-2892) to stand for TBP (assumed to be the same as

TBP).• An estimate of the composition of the butane and lighter components is frequently added

to the lowboiling end or the IBP of the TBP curve to compensate for the loss duringdistillation.

• The feed material is heated as it flows continuously through a heating coil.• As vapor is formed it kept cohesively with liquid at some temperature and a

sudden release of pressure quickly flashes or separates the vapor from themixture without any rectification.

• By successive flash evaporation like this the stock can be progressively distilledat different increasing temperatures.

• a curve of percentage vaporized vs. temperature may be plotted. travels along inthe tube with remaining liquid until separation is permitted in a vaporseparator or vaporizer.

• By conducting the operation at a series of outlet temperature, a curve ofpercentage vaporized vs. temperature may be plotted.

2 Equilibrium or Flash Vaporization (EFV).

3 ASTM or no fractionating distillation: (no fractional , run on fractions) .

• It is supposed to be like EFV, a non fractionating distillation system, distinguishing • itself as differential distillation. It is a simple distillation carried out with standard • ASTM flasks 100,200,500 ml flasks. The data obtained is similar to TBP data

Mid Percent Curves

• The physical properties of an oil found to vary gradually throughout therange of compounds that constitute the oil.

• Distillation is a means of arranging of compounds these chemical compoundsin order of their boiling points.

• The properties such as color, specific gravity, and viscosity are found to bedifferent for each drop or fraction of the material distilled.

• The rate at which these properties change from drop to drop may plot asmid per cent curves.

• In reality, the specific gravity or viscosity of a fraction is an average of the propertiesof the many drops that constitute the fraction.

• If each drop is equally different from the last drop and from the succeeding one, thenthe drop that distills at exactly half of the fraction has the same property as theaverage of all the drops.

• This would be the condition for a mid per cent that is a straight line, but they aresubstantially straight through any short range of percentage. For a short range ofpercentage the average property is equal to the property at the mid-point of thefraction.

• The arithmetical average of the properties of these small fractions is the property ofthe total or large fraction, or even the entire sample.

• Integral- averaging by adding together the properties of a series of short fractions anddividing by number of fractions can not be used on properties that are not additive.

• Specific gravity (not API gravity) is an example of an additive property, e. g. 10volumes of an oil of specific gravity 0.8 when mixed with an equal volume of 0.9specific gravity oil yields a mixture that has a specific gravity 0.85

Additive Properties Non additive Properties

Boiling Point (T.B.P) API Gravity

Vapour Pressure Viscosity

Specific Gravity Colour

Aniline Point Flash Point

Sulphur Content

Hydrogen/Carbon ratio