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

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Crude

Coker

FCC Gasoline

Vac Gas Oil

Hydrotreater

(HDT)

Hydrotreater

(HDT)

Hydrotreater & Reformer

Vac Resid

Jet Fuel/Stove Oil

Light Ends

Fuel Gas

LPG

Gasoline

Alkylate

Butane

Slurry

Gas Oil

Lt Naphtha

Kerosene

Hvy Naphtha

Diesel/Heating Oil

Light Crude Oil

Atmospheric Resid

VGO HDT

FCC/Hydrocracker

Alkylation

Coke

Atmospheric Distillation

Vac Dist.

Figure 1.3 A high co nversion refiner y

Introduction 7

LPG

Acetone

LPG

Bis-Phenol A

Phenol

Kerosene

Diesel

Polypropylene

Ethylene

Glycol

Ethylene

Oxide

Gasoline

Methanol

Hydrogen

Carbon

Oxide

Crude

HSRN

Kerosene

Diesel

AGO

VGO

LSRN

C5/C6

Isom

HDS

FCC

HDS

Cat Ref

Cumene Phenol

Air

Olefins

Cracking

Gasoline

Blending

Fuel

Gas

Purer

MTBE

Hydrogen

Methanol

MTBE

Off -Gas

Bis-

Phenol A

Acetone

Syn Gas

Electric Power

Stream

Synthesis Gas to

Derivatives Plants

Isomerate

’s

Splitter

Debutanizer

Splitter

Vacuum Dist.

Crude Dist.

Splitter

Stabilizer

Resid

Oxygen

Petrochemical

Units

Gasification

Cooling &

Shift RX

Methanol

CO2

Recovery

Combined

Cycle

Cogen

Alkylation

Ethylene

Oxide

Polypropylene

Ethylene

Glycol

Figure 1.4 Refinery-petrochemical integration

8 Chapter 1

1.3.4. Refinery-petrochemical Integration

The growth of the petrochemical industry has put pressure on refineries to

either change their configuration or operating conditions to produce more

aromatics and gases. FCC has been developed to petro-FCC which pro-

duces high yield of gases. The phasing out of the idea of increasing the

octane number by increasing aromatic content has changed the role of the

catalytic reformer to produce a high yield of aromatics as BTX feedstock.

The addition of gasification units to process vacuum residue has opened

the way for the addition of a variety of petrochemicals. Such a refinery-

petrochemical configuration is shown in Fig. 1.4 (Crawford et al., 2002).

1.3.5. Development of New Technology

If a new technology is developed to give better yields, save energy, meet

environmental regulations and product specifications, then this technology

might replace old technology in existing and new refineries, depending on

the economics. Other factors, which might influence the refinery configu-

ration, are feedstock availability, product markets and a company’s strategic

objectives.

REFERENCES

Crawford, C. D., Bharvani, P. R., and Chapel, D. G. (2002). Integrating petrochemicals

into the refinery. Hydrocarbon Process 7, 35–38.

Gary, J. H., and Handwerk, G. E. (2001). ‘‘Petroleum Refining Technology and Econom-

ics.’’ 4th ed. Marcel Dekker, New York.

Jones, D. S. J, and Pufado, P. R. (2005). ‘‘Handbook Of Petroleum Processing.’’ Springer,

Berlin.

Speight, J. G. (1999). ‘‘The Chemistry and Technology of Petroleum,’’ 3rd ed. Marcel

Dekker, New York.

Introduction 9

CHAPTER TWO

Refinery Feedstocks and Products

2.1. Introduction

A petroleum refining study starts with describing its feedstock, the

crude oil and the range of products that are produced by the various

processes. Crude oil comes from different parts of the world and has

different physical and chemical characteristics. On the other hand, the

products that are produced have to meet market requirements and as

such, should comply with certain specifications.

2.2. Composition of Crude Oils

Crude oil is a complex liquid mixture made up of a vast number of

hydrocarbon compounds that consist mainly of carbon and hydrogen in

differing proportions. In addition, small amounts of organic compounds

containing sulphur, oxygen, nitrogen and metals such as vanadium, nickel,

iron and copper are also present. Hydrogen to carbon ratios affect the

physical properties of crude oil. As the hydrogen to carbon ratio decreases,

the gravity and boiling point of the hydrocarbon compounds increases.

Moreover, the higher the hydrogen to carbon ratio of the feedstock, the

higher its value is to a refinery because less hydrogen is required.

The composition of crude oil, on an elemental basis, falls within certain

ranges regardless of its origin. Table 2.1 shows that carbon and hydrogen

contents vary within narrow ranges. For this reason, crude oil is not

classified on the basis of carbon content. Despite their low concentrations,

impurities such as sulphur, nitrogen, oxygen and metals are undesirable

because they cause concerns in the processability of crude feedstock

and because they affect the quality of the produced products. Catalyst

poisoning and corrosion are the most noticeable effects during refining.

Fundamentals of Petroleum Refining

2010 Elsevier B.V.

There are three main classes of hydrocarbons. These are based on the

type of carbon–carbon bonds present (Roussel and Boulet, 1995b). These

classes are:



Saturated hydrocarbons contain only carbon–carbon single bonds. They

are known as paraffins (or alkanes) if they are acyclic, or naphthenes (or

cycloalkanes) if they are cyclic.



Unsaturated hydrocarbons contain carbon–carbon multiple bonds (dou-

ble, triple or both). These are unsaturated because they contain fewer

hydrogens per carbon than paraffins. Unsaturated hydrocarbons are

known as olefins. Those that contain a carbon–carbon double bond are

called alkenes, while those with carbon–carbon triple bond are alkyenes.



Aromatic hydrocarbons are special class of cyclic compounds related in

structure to benzene.

2.2.1. Paraffins

Paraffins, also known as alkanes, are saturated compounds that have the

general formula C

2nþ2

, where n is the number of carbon atoms. The

simplest alkane is methane (CH

), which is also represented as C

Normal paraffins (n-paraffins or n-alkanes) are unbranched straight-

chain molecules. Each member of these paraffins differs from the next

higher and the next lower member by a –CH

– group called a methylene

group (Table 2.2). They have similar chemical and physical properties,

which change gradually as carbon atoms are added to the chain.

Isoparaffins (or isoalkanes) are branched-type hydrocarbons that exhibit

structural isomerization. Structural isomerization occurs when two molecules

have the same atoms but different bonds. In other words, the molecules

have the same formulas but different arrangements of atoms, known as

isomers. Butane and all succeeding alkanes can exist as straight-chain mole-

cules (n-paraffins) or with a branched-chain structure (isoparaffins). For

example, butane and pentane have the following structural isomers:

Table 2.1 Elemental composition of crude oils (Roussel and

Boulet, 1995c)

Element Composition (wt%)

Carbon 83.0–87.0

Hydrogen 10.0–14.0

Sulphur 0.05–6.0

Nitrogen 0.1–0.2

Oxygen 0.05–2.0

Ni <120 ppm

V <1200 ppm

12 Chapter 2

−−−

n-Butane

−−−−

n-Pentane

Isobutane

(2-methylpropane)

isopentane

(2-methylbutane)

neopentane

(2,2-dimethylpropane)

CHCH

The number of isomers increases geometrically with the carbon number.

While there are two isomers for butane and three for pentane, there are

75 isomers for decane (C

). For paraffins in the range of C

–C

, there

are more than 600 isomers with only 200–400 that are identified in petro-

leum fractions. Because of their different structures, these isomers have dif-

ferent properties. For instance, the presence of isoparaffins in gasoline is

essential for increasing the octane number of gasoline fuels.

2.2.2. Olefins

Olefins, also known as alkenes, are unsaturated hydrocarbons containing

carbon–carbon double bonds. Compounds containing carbon–carbon triple

bonds are known as acetylenes, and are also known as biolefins or alkynes. The

general formulas of olefins and acetylenes are C

(R–CH ¼CH–R

) and

2n2

(R–CH C–R

), respectively. Unsaturated compounds may

have more than one double or triple bond. If two double bonds are present,

the compounds are called alkadienes or, more commonly, dienes (R–CH ¼

CH–CH ¼R

). There are also trienes, tetraenes and even polyenes.

Table 2.2 Names and formulas of the ﬁrst ten parraﬁns (alkanes)

Name

Number of

carbon atoms

Molecular

formula Structural formula

Number

of isomers

Methane 1 CH

Ethane 2 C

Propane 3 C

Butane 4 C

Pentane 5 C

(CH

)

Hexane 6 C

(CH

)

Heptane 7 C

(CH

)

Octane 8 C

(CH

)

Nonane 9 C

(CH

)

Decane 10 C

(CH

)

Refinery Feedstocks and Products 13

Olefins are not naturally present in crude oils but they are formed during

the conversion processes. They are more reactive than paraffins. The light-

est alkenes are ethylene (C

) and propylene (C

), which are important

feedstocks for the petrochemical industry. The lightest alkyne is acetylene.

CHCH

CHHC

Ethylene

(ethene)

Propylene

(propene)

Acetylene

(ethyne)

2.2.3. Naphthenes (cycloalkanes)

Naphthenes, also known as cycloalkanes, are saturated hydrocarbons that have

at least one ring of carbon atoms. They have the general formula C

A common example is cyclohexane (C

Cyclohexane

The boiling point and densities of naphthenes are higher than those of

alkanes having the same number of carbon atoms. Naphthenes commonly

present in crude oil are rings with five or six carbon atoms. These rings

usually have alkyl substituents attached to them. Mutli-ring naphthenes are

present in the heavier parts of the crude oil. Examples of naphthenes

are shown below.

Alkylcyclopentanes Alkylcyclohexanes

methylcyclopentane 1,2-dimethylcyclopentane 1-ethyl-2-methylcyclopentane

Bicycloalkanes

2.2.4. Aromatics

Aromatics are unsaturated cyclic compounds composed of one or more

benzene rings. The benzene ring has three double bonds with unique

electron arrangements that make it quite stable.

14 Chapter 2

Benzene

or or

Crude oils from various origins contain different types of aromatic

compounds in different concentrations. Light petroleum fractions contain

mono-aromatics, which have one benzene ring with one or more of the

hydrogen atoms substituted by another atom or alkyl groups. Examples of

these compounds are toluene and xylene. Together with benzene, such

compounds are important petrochemical feedstocks, and their presence in

gasoline increases the octane number.

Toluene

(methylbenzene)

Styrene o-xylene

(1,2-Dimethylbenzene)

CH = CH

More complex aromatic compounds consist of a number of ‘‘fused’’

benzene rings. These are known as polynuclear aromatic compounds. They are

found in the heavy petroleum cuts, and their presence is undesirable because

they cause catalyst deactivation and coke deposition during processing,

besides causing environmental problems when they are present in diesel

and fuel oils. The heaviest portion of the crude oil contains asphaltenes,

which are condensed polynuclear aromatic compounds of complex struc-

ture. Examples of polynuclear aromatic compounds are shown below.

Aromatic-cycloalkanes Fluorenes Binuclear aromatics

Phenanthrene Pyrene Chrysene

Refinery Feedstocks and Products 15

2.2.5. Sulphur Compounds

The Sulphur content of crude oils varies from less than 0.05 to more than

10 wt% but generally falls in the range 1–4 wt%. Crude oil with less than 1 wt

% sulphur is referred to as low sulphur or sweet, and that with more than

1 wt% sulphur is referred to as high sulphur or sour.

Crude oils contain sulphur heteroatoms in the form of elemental sulphur

S, dissolved hydrogen sulphide H

S, carbonyl sulphide COS, inorganic

forms and most importantly organic forms, in which sulphur atoms are

positioned within the organic hydrocarbon molecules.

Sulphur containing constituents of crude oils vary from simple mercap-

tans, also known as thiols, to sulphides and polycyclic sulphides. Mercaptans

are made of an alkyl chain with –SH group at the end (R–SH). Examples of

mercaptans and sulphides are as follows:

SH CH

methyl mercaptan

(methanethiol)

n-butyl mercaptan

(1-butanethiol)

phenyl mercaptan

(thiophenol)

In sulphides and disulphides, the sulphur atom replaces one or two

carbon atoms in the chain (R–S–R

or R–S–S–R

). These compounds are

often present in light fractions. Sulphides and disulphides may also be cyclic

or aromatic.

cyclic sulfides Aromatic disulfide

RCH

−−

S-S-R

Thiophenes are polynuclear aromatic compounds (Chatila, 1995)in

which the sulphur atom replaces one or more carbon atoms in the aromatic

ring. They are normally present in heavier fractions. Thiophenes present in

crude oils may have the following formulas:

Thiophene Benzothiophene Dibenzothiophene

Naphthobenzothiophene

16 Chapter 2

2.2.6. Oxygen Compounds

The oxygen content of crude oil is usually less than 2 wt%. A phenomenally

high oxygen content indicates that the oil has suffered prolonged exposure to

the atmosphere. Oxygen in crude oil can occur in a variety of forms. These

include alcohols, ethers, carboxylic acids, phenolic compounds, ketones,

esters and anhydrides. The presence of such compounds causes the crude

to be acidic with consequent processing problems such as corrosion.

Alcohols have the general formula R–OH and are structurally similar to

water but with one of the hydrogen atoms replaced by an alkyl group.

In phenols, one of the hydrogen atoms in the aromatic ring is replaced with

a hydroxyl group (–OH). Ethers have two organic groups connected to

a single oxygen atom (R–O–R

). Examples of alcohols, phenols and

ethers are:

− OH

methyl alcohol

(methanol)

isopropyl alcohol

(2-propanol)

phenyl alcohol

(phenol)

− O − CH

ethyl methyl ether diphenyl ether

CHCH

−−

tetrahydropyran

(cyclic ether)

pentamethylene oxide

Carboxylic acids have a carboxyl group as their functional group

(–COOH), and their general formula can be written as:

carboxylic acidcarboxyl group

−C

R −COO

R − C

Examples of aliphatic and aromatic carboxylic acids are:

−COOH

acetic acid

(ethanoic acid)

benzoic acid

(benzenecarboxylic acid)

COOH

Carboxylic acid anhydrides are formed by removing water from two

carboxyl groups and connecting the fragments. The most important ali-

phatic anhydride is acetic anhydride.

Refinery Feedstocks and Products 17