Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

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acetic anhydride

(ethanoic anhydride)

COCCH

−−−−

Carboxylic acid esters are formed by replacing the –COOH by –COOR

group. Examples are:

ethyl acetate

(ethyl ethanoate)

phenyl acetate

−−−−

CO −−

Ketones are compounds with two carbon atoms bounded to the carbon

of a carbonyl group

C =

. Furans are hetroaromatic compounds with five-

member oxygenated rings. Benzofuran is a furan condensed with an aromatic

ring. Examples are:

dimethyl ketone

(acetone)

furan Benzofuran

CCH

−−

2.2.7. Nitrogen Compounds

Crude oils contain very low amounts of nitrogen compounds. In general,

the more asphaltic the oil, the higher its nitrogen content. Nitrogen com-

pounds are more stable than sulphur compounds and therefore are harder to

remove. Even though they are present at very low concentrations, nitrogen

compounds have great significance in refinery operations. They can be

responsible for the poisoning of a cracking catalyst, and they also contribute

to gum formation in finished products.

The nitrogen compounds in crude oils may be classified as basic or

non-basic. Basic nitrogen compounds consist of pyridines. The greater

part of the nitrogen in crude oils is the non-basic nitrogen compounds,

which are generally of pyrrole types.

Pyridines are six-membered heteroaromatic compounds containing one

nitrogen atom. When fused with benzene rings, pyridines are converted to

the polycyclic heteroaromatic compounds quinolines and isoquinolines.

18 Chapter 2

pyridine

N )

quinoline

N )

isoquinoline

N )

N N

In non-basic nitrogen compounds, pyrroles are five-membered hetero-

aromatic compounds containing one nitrogen atom. When fused with

benzene ring, pyrrole is converted to the polycyclic heteroaromatic com-

pounds indole and carbazole.

pyrrole

N )

indole

N )

carbazole

N )

2.2.8. Metallic Compounds

Metallic compounds exist in all crude oil types in very small amounts. Their

concentration must be reduced to avoid operational problems and to

prevent them from contaminating the products. Metals affect many upgrad-

ing processes. They cause poisoning to the catalysts used for hydroprocessing

and cracking. Even minute amounts of metals (iron, nickel and vanadium)

in the feedstock to the catalytic cracker affect the activity of the catalyst

and result in increased gas and coke formation and reduced gasoline yields.

For high-temperature power generators, the presence of vanadium in the

fuel may lead to ash deposits on turbine blades and cause severe corrosion,

and the deterioration of refractory furnace linings.

Part of the metallic constituents of crude oils exist as inorganic water-soluble

salts, mainly as chlorides and sulphates of sodium, potassium, magnesium and

calcium. These are removed in desalting operations. More important are metals

which are present in form of oil-soluble organometallic compounds. Zinc,

titanium, calcium and magnesium appear in the form of organometallic soaps.

However, vanadium, nickel, copper and iron are present as oil-soluble com-

pounds, capable of complexing with pyrrole compounds.

2.2.9. Asphaltenes and Resins

The physical properties of crude oils, such as the specific gravity (or API),

are considerably influenced by high-boiling constituents, in which the

heteroatoms (sulphur, nitrogen and metals) concentrate. It is therefore

Refinery Feedstocks and Products 19

important to characterize the heaviest fractions of crude oils in order to

determine their properties and ease of processing. This calls for determining

the percentage of two generally defined classes of compounds, namely

asphaltenes and resins.

Asphaltenes are dark brown friable solids that have no definite melting

point and usually leave carbonaceous residue on heating. They are made up

of condensed polynuclear aromatic layers linked by saturated links. These

layers are folded, creating a solid structure known as a micelle. Their

molecular weights span a wide range, from a few hundred to several million.

Asphaltenes are separated from petroleum in the laboratory using non-polar

solvents such as pentane and n-heptane. Liquefied petroleum fractions

(propane and butane) are used commercially in deasphalting residues and

lube stock oils.

The presence of high amounts of asphaltenes in crude oil can create

tremendous problems in production because they tend to precipitate inside

the pores of rock formations, well heads and surface processing equipments.

They may also lead to transportation problems because they contribute to

gravity and viscosity increases of crude oils. In refinery operations, asphal-

tenes have markedly adverse effects on the processability of crude oils. They

lead to coke formation and metal deposition on the catalyst surface causing

catalyst deactivation.

Resins are polar molecules in the molecular weight range of 500–1000,

which are insoluble in liquid propane but soluble in n-heptane. It is believed

that the resins are responsible for dissolving and stabilizing the solid asphal-

tene molecules in petroleum. The resin molecules surround the asphaltene

clusters (micelles) and suspend them in liquid oil. Because each asphaltene is

surrounded by a number of resin molecules, the content of resins in crude

oils is higher than that of the asphaltenes.

The characterization of crude oils and petroleum fractions involves frac-

tionation techniques, measurement of physical and chemical properties and

analytical techniques for compositional measurements.

2.3. Products Composition

There are specifications for over 2000 individual refinery products.

Intermediate feed stocks can be routed to various units to produce different

blend products depending on market demand. Figure 2.1 shows typical

refinery products with their carbon atom contents and boiling ranges.

The specifications of each product are discussed in detail in the coming

subsections (Roussel and Boulet, 1995a).

20 Chapter 2

2.3.1. Liquefied Petroleum Gas (LPG)

Liquified petroleum gas is a group of hydrocarbon-based gases derived from

crude oil refining or natural gas fractionation. They include ethane, ethyl-

ene, propane, propylene, normal butane, butylene, isobutane and isobutyl-

ene. For convenience of transportation, these gases are liquefied through

pressurization.

2.3.2. Gasoline

Gasoline is classified by octane ratings (conventional, oxygenated and

reformulated) into three grades: Regular, Midgrade and Premium.

Asphalt

Petroleum

Coke

600

Boiling Range ⬚C

Number of Carbon Atoms

Wax

500

400

300

200

100

−100

Lube

Oil

Heavy

Fuel

Oil

Paraffins

Kerosene

Automotive

Gasoline

Jet

Fuel

Diesel

Fuel

Naphtha

LPG

Aviation

Gasoline

Figure 2.1 Principal petroleum products with carbon numbers and boiling ranges

Refinery Feedstocks and Products 21



Regular gasoline: Gasoline having an antiknock index, i.e. octane

rating, greater than or equal to 85 and less than 88.



Mid-grade gasoline: Gasoline having octane rating, greater than or

equal to 88 and less than or equal to 90.



Premium gasoline: Gasoline having octane rating greater than 90.

Premium and regular grade motor gasoline are used depending on the

octane rating. In addition, aviation gasoline, which is a complex mixture

of relatively volatile hydrocarbons, is blended with additives to form suitable

fuel for aviation engines.

2.3.3. Kerosene

Kerosene is a light petroleum distillate that is used in space heaters, cook

stoves and water heaters and which is suitable for use as a light source.

Kerosene has a maximum distillation temperature of 204



C (400



F) at the

10% recovery point, a final boiling point of 300



C (572



F), and a mini-

mum flash point of 37.8



C (100



F). The two grades are recognized by

ASTM Specification D3699. A kerosene-type jet fuel-based product is

having a maximum distillation temperature of 204



C (400



F) at the 10%

recovery point and a final maximum boiling point of 300



C (572



F) and

meeting ASTM Specification D1655.

2.3.4. Jet Fuel

This category comprises both gasoline and kerosene and meets specifica-

tions for use in aviation turbine power units.

2.3.5. Diesel Fuel

The quality of diesel fuels can be expressed as cetane number or cetane

index. The cetane number (CN) is expressed in terms of the volume percent

of cetane (C

) which has high ignition (CN = 100) in a mixture

with alpha-methyl-naphthalene (C

) which has low ignition quality

(CN = 0). Diesel fuel includes No.1 diesel (Super-diesel) which has cetane

number of 45 and it is used in high speed engines, trucks and buses. No. 2

diesel has 40 cetane number. Railroad diesel fuels are similar to the heavier

automotive diesel fuels, but have higher boiling ranges upto 400



(750



F) and lower cetane numbers (CN = 30).

2.3.6. Fuel Oil

The fuel oils are mainly used in space heating and thus the market is

quite high specially in cold climates. No. 1 fuel oil is similar to kerosene

and No. 2 fuel oil is very similar to No. 2 diesel fuel. Heavier grades of

No. 3 and 4 are also available.

22 Chapter 2

2.3.7. Residual Fuel Oil

It is mainly composed of vacuum residue. Critical specifications are viscosity

and sulphur content. Low sulphur residues are in more demand in the

market.

2.3.8. Lube Oil

Lubricants are based on the viscosity index. Paraffinic and naphthenic

lubricants have a finished viscosity index of more than 75.

2.3.9. Asphalt

Asphalt is an important product in the construction industry and comprises

upto 20% of products. It can be produced only from crude containing

asphaltenic material.

2.3.10. Petroleum Coke

Carbon compounds formed from thermal conversion of petroleum con-

taining resins and asphaltenes are called petroleum cokes. Fuel grade coke

contains about 85% carbon and 4% hydrogen. The balance is made up of

sulphur, nitrogen, oxygen, vanadium and nickel.

2.4. Physical Property Characterization Data

2.4.1. Fractionation

Distillation of crude oils determines the yield of the products that can be

obtained from this crude oil when it is processed in a refinery. A light crude

oil will produce higher amounts of gasoline than a heavier crude oil.

Different standard distillation tests can be performed on crude oil or

petroleum fractions.

2.4.2. True Boiling Point Distillation

The boiling point distribution of crude oil (boiling point versus volume

or mass percent distilled) is obtained through a batch distillation test

ASTM 2892. The distillation apparatus has 15–18 theoretical plates with a

5:1 reflux ratio. For boiling points below 340



C (644



F) the distillation is

performed at atmospheric pressure. The residue is distilled under vacuum

(1–10 mm Hg). The boiling points under vacuum are converted to normal

boiling points. The distillation continues to a normal boiling point of

Refinery Feedstocks and Products 23

535



C (995



F). This test allows for the collection of sample cuts at

different boiling point ranges. These cuts can be subjected to physical and

chemical measurements.

2.4.3. ASTM Distillation

The distillation of petroleum cuts is done in a simple distillation apparatus

which does not have a fractionation column. For light cuts (gasoline, kerosene,

diesel and heating oil) the distillation is run at atmosphe ric pressure under

ASTM D86 test. For heavier fractions an ASTM D1160 test at reduced

pressure is employed.

2.4.4. Simulated Distillation by Gas Chromatography

The boiling point distribution of the whole crude oil can be determined by

an injection of the sample in a gas chromatograph which separates the

hydrocarbons in boiling point order. The retention time is related to the

boiling point through a calibration curve. The results of this test are

comparable to the true boiling point tests. In addition, the boiling point

distribution of light and heavy petroleum cuts can also be done by gas

chromatography. One of the standards methods of measurements is

ASTM D5307.

2.4.5. API Gravity

The gravity of crude oil determines its price commercially. It is generally

expressed as API gravity defined as:

API ¼

141:5

 131:5 ð2:1Þ

where SG is the specific gravity defined as the density of the crude oil

relative to the density of water both at 15.6



C (60



F). The API gravity can

range from 8.5 for very heavy crudes to 44 for light crudes. Crude oils can

generally be classified according to gravity as shown in Table 2.3.

Table 2.3 Classiﬁcation of crude oils

Crude Category Gravity

Light crudes API > 38

Medium crudes 38 > API > 29

Heavy crudes 29 > API > 8.5

Very heavy crudes API < 8.5

24 Chapter 2

The API gravity is also measured for various petroleum fractions. One of

the standard tests is ASTM D1298.

2.4.6. Pour Point

The pour point is defined as the lowest temperature at which the sample

will flow. It indicates how easy or difficult it is to pump the oil, especially in

cold weather. It also indicates the aromaticity or the paraffinity of the crude

oil or the fraction. A lower pour point means that the paraffin content is

low. Pour points for the whole crude and fractions boiling above 232



(450



F) are determined by standard tests like ASTM D97.

2.4.7. Viscosity

The resistance to flow or the pumpability of the crude oil or petroleum

fraction is indicated by the viscosity. More viscous oils create a greater

pressure drop when they flow in pipes. Viscosity measurement is expressed

in terms of kinematic viscosity in centiStocks (cSt) and can also be expressed

in saybolt seconds. The viscosity is measured at 37.8



C (100



F) by ASTM

D445 and by ASTM D446 at 99



C (210



F).

2.4.8. Refractive Index

The refractive index is the ratio of the velocity of light in a vacuum to the

velocity of light in the oil. This parameter is used as a characterization

parameter for petroleum fraction composition as will be discussed in Chap-

ter 3. It is measured according to ASTM D1218.

2.4.9. Freezing Point

Petroleum fractions are mostly liquids at ambient conditions. However,

heavy oils contain heavy compounds such as waxes or asphaltenes. These

compounds tend to solidify at low temperatures, thus restricting flow. The

freezing point is the temperature at which the hydrocarbon liquid solidifies

at atmospheric pressure. It is one of the important property specifications

for kerosene and jet fuels due to the very low temperatures encountered at

high altitudes in jet planes. One of the standard tests is ASTM D4790 and

ASTM D16.

2.4.10. Aniline Point

The lowest temperature at which an equal volume mixture of the petro-

leum oil and aniline are miscible is the aniline point. Since aniline is an

aromatic compound, a petroleum fractions with high aromatic content will

Refinery Feedstocks and Products 25

be miscible in aniline at ambient conditions. However, if the oil has more

paraffins, it will require a higher temperature and thus higher aniline point

in order to be miscible in aniline. This property is important for the

specifications of diesel fuels. It is measured by ASTM D611.

2.4.11. Flash Point

The flash point of a liquid hydrocarbon is the lowest temperature at which

sufficient vapours are produced above the liquid such that spontaneous

ignition will occur if a spark is present. It is an important specification for

gasoline and naphtha related to safety in storage and transport in high-

temperature environments. Flash point indicates the fire and explosion

potential of a fuel. A low flash point fuel is a higher fire hazard. One of

the standard tests is ASTM D1711, D09 and D1695.

2.4.12. Octane Number

An octane number is a measure of the knocking tendency of gasoline fuels

in spark ignition engines. The ability of a fuel to resist auto-ignition during

compression and prior to the spark ignition gives it a high octane number.

The octane number of a fuel is determined by measuring its knocking value

compared to the knocking of a mixture of n-heptane and isooctane (2,2,4-

trimethyl pentane). Pure n-heptane is assigned a value of zero octane while

isooctane is assigned 100 octane. Hence, an 80 vol% isooctane mixture has

an octane number of 80. Two octane tests can be performed for gasoline.

The motor octane number (MON) indicates engine performance at high-

way conditions with high speeds (900 rpm). On the other hand, the

research octane number is indicative of low-speed city driving (600 rpm).

The posted octane number (PON) is the arithmetic average of MON and

RON. One of the standard tests is ASTM D2700.

2.4.13. Cetane Number

The cetane number measures the ability for auto ignition and is essentially

the opposite of the octane number. The cetane number is the percentage of

pure cetane (n-hexadecane) in a blend of cetane and alpha methyl naphtha-

lene which matches the ignition quality of a diesel fuel sample. This quality is

specified for middle distillate fuels. One of the standard tests is ASTM D976.

2.4.14. Smoke Point

The smoke point is a test measures the burning qualities of kerosene and jet

fuel. It is defined as the maximum height in mm, of a smokeless flame of

fuel. One of the standard tests is ASTM D1322.

26 Chapter 2

2.4.15. Reid Vapour Pressure

The reid vapour pressure (RVP) of a product is the vapour pressure

determined in a volume of air four times the liquid volume at 37.8



(100



F). This property measures the vapour-lock tendency of a motor

gasoline in which excessive vapours are produced in the fuel line causing

interruption of the supply of liquid fuel to the engine. It also indicates the

explosion and evaporation hazards of the fuel. One of the standard tests is

ASTM D323.

2.4.16. Water, Salt and Sediment

Crude oil contains small amounts of water, mineral salts and sediments.

Most of the salts are dissolved in the water, and the remainder is present in

the oil as fine crystals. Chlorides of magnesium, calcium and sodium are the

most common salts. The presence of salts causes problems in processing,

such as corrosion, erosion and plugging of equipment, and catalyst deacti-

vation. Sediments are solid material that are not soluble in the hydrocarbon

or water and can be comprised of sand, drilling mud, rock or minerals

coming from erosion of metal pipes, tanks and equipment. One of the

standard tests is ASTM D6470.

2.4.17. Molecular Weight

Most crude oils and petroleum fractions have average molecular weights

from 100 to 500. Although, there are several methods for measuring the

molecular weight, the most suitable method is that based on freezing point

depression.

2.5. Chemical Analysis Data

2.5.1. Elemental Analysis

Carbon, hydrogen and nitrogen content of a petroleum fraction or crude oil

can be determined by elemental analysis. The sample is combusted to

carbon dioxide, sulphur dioxide, water and nitrogen oxides. The gases are

separated and their quantities determined by different methods.

Of utmost importance is the determination of the sulphur content as it

determines the processing scheme of the crude oil and thus the market value

of the products. The analytical methods for sulphur determination are

numerous and depend on the level of sulphur in the sample. The most

widely used methods are based on combustion and X-ray fluorescence.

Elemental analysis can be estimated by ASTM D5251.

Refinery Feedstocks and Products 27