EXP-PR-RT030-EN-R0_1 - Intro HC.pdf

THEORY OUTLINE INTRODUCTION TO HYDROCARBONS TRAINING MANUAL Course EXP-PR-RT030 Revision 0.1

Exploration & Production Theory Outline Introduction to Hydrocarbons

THEORY OUTLINE INTRODUCTION TO HYDROCARBONS CONTENTS 1. OBJECTIVES ..................................................................................................................4 2. INTRODUCTION TO THE ORGANIC CHEMISTRY OF HYDROCARBONS ..................5 2.1. BASICS .....................................................................................................................5 2.2. ORIGIN OF HYDROCARBONS ................................................................................8 2.3. CARBON AND HYDROGEN.....................................................................................9 2.4. ISOMERS................................................................................................................11 2.4.1. Definition...........................................................................................................11 2.4.2. Constitutional isomers ......................................................................................12 2.4.3. Stereoisometry..................................................................................................12 2.4.4. Alkyl radicals.....................................................................................................13 3. THE SIX DIFFERENT HYDROCARBON GROUPS ......................................................15 3.1. THE ALKANE GROUP............................................................................................15 3.1.1. Linear or normal alkanes (n) .............................................................................15 3.1.2. Carbon chain alkanes or isomers (i) .................................................................17 3.2. THE ALKENE GROUP............................................................................................18 3.3. THE ALKYNE GROUP............................................................................................20 3.4. THE CYCLANE GROUP .........................................................................................21 3.5. THE AROMATIC GROUP .......................................................................................22 3.6. THE CYCLEN GROUP ...........................................................................................22 4. MAIN PHYSICAL CHARACTERISTICS OF HYDROCARBONS...................................23 4.1. GENERAL ASPECTS .............................................................................................23 4.2. STATE CHANGING TEMPERATURES ..................................................................25 4.3. DENSITY / °API ......................................................................................................25 4.4. VISCOSITY .............................................................................................................25 4.5. COMBUSTION HEAT .............................................................................................25 5. INTRODUCTION TO HYDROCARBON PROCESSING ...............................................26 5.1. CRUDE OIL PROCESSING ....................................................................................26 5.2. GAS PROCESSING................................................................................................28 5.2.1. Transport in gaseous form ................................................................................29 5.2.2. Transport in liquid form .....................................................................................29 5.2.2.1. Liquefied natural gas: LNG.........................................................................31 5.2.2.2. LPG – liquefied petroleum gas ...................................................................32 5.2.2.3. NGL - Natural Gas Liquids .........................................................................32 5.2.2.4. Condensates or natural gasoline................................................................33 6. HYDROCARBONS AS A SOURCE OF ENERGY.........................................................34 6.1. INTRODUCTION.....................................................................................................34 6.2. COMBUSTION OF ALKANES ................................................................................34 6.2.1. Example: the combustion of methane:..............................................................34 7. NAMES ASSOCIATED WITH HYDROCARBONS ........................................................36 7.1. GAS AND CRUDE ..................................................................................................36 Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

7.2. NATURAL GAS .......................................................................................................36 7.3. CRUDE OIL.............................................................................................................36 7.4. DENSITY OF OIL: THE API DEGREE ....................................................................37 7.5. ASSOCIATED GAS.................................................................................................37 7.6. ARABIAN LIGHT .....................................................................................................37 7.7. BRENT ....................................................................................................................38 7.8. LIGHT AND HEAVY OILS (HYDROCARBONS) .....................................................38 7.9. TOE: TONNE OIL EQUIVALENT ............................................................................38 7.10. GOR (Gas Oil Ratio) .............................................................................................38 7.11. BSW (Bottom Sediment and Water – also known as watercut).............................38 7.12. HYDRATES...........................................................................................................39 7.13. DEW POINT ..........................................................................................................39 7.14. WATER CONTENT ...............................................................................................39 7.15. WOBBE INDEX .....................................................................................................40 8. EXERCISES ..................................................................................................................41 9. GLOSSARY ...................................................................................................................44 10. LIST OF FIGURES ......................................................................................................45 11. LIST OF TABLES ........................................................................................................46 12. EXERCISE SOLUTIONS .............................................................................................47

Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

1. OBJECTIVES The objective of this course is to allow future operators to understand the bases of the hydrocarbons domain. Upon conclusion of this course, they should be able to: Explain the origin of hydrocarbons Interpret the basic chemical principles of Carbon - Hydrogen bonds Distinguish hydrocarbon components by their names and formulas Define the composition of isomers Name the different hydrocarbon groups Write the chemical formulas of the components in the groups above Define the main physical characteristics of hydrocarbons Interpret changes of state in hydrocarbons Integrate Pressure and Temperature values to define the characteristics of hydrocarbons Explain, interpret density terms, °API, viscosity, combustion heat…, and any other term relative to hydrocarbon production Name the different stages in the processing of crude oil Name the different states in the processing of a natural gas Differentiate the different solutions for transporting hydrocarbons in gas and / or liquid form Interpret the combustion of a liquid and / or gas hydrocarbon Distinguish the different names of crude oils and gases Define, compare and calculate with the different measurement units associated with hydrocarbons Interpret the terms and characteristics such as “GOR, dew point, BSW (or watercut), water content, Wobbe index and any other term employed in the “routine” activities of an operator.

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Exploration & Production Theory Outline Introduction to Hydrocarbons

2. INTRODUCTION TO THE ORGANIC CHEMISTRY OF HYDROCARBONS 2.1. BASICS Organic chemistry is the study of molecules which contain carbon and hydrogen atoms. This is the common point between all organic molecules. Some molecules contain only carbon and hydrogen atoms whilst others also contain other atoms (for example, oxygen, nitrogen atoms, etc.).

Figure 1: Propane

Figure 2: Glycol

Figure 3: Ethylamine

A wide range of organic molecules therefore exist and the thing that differentiates them is the number and different types of atoms which they contain. Another characteristic which may differentiate these molecules is their structure (a single bond or a double bond between two atoms for example), and the way in which these arrangements are made in space (flat or other molecule). From a physical point of view, each given molecule is associated with precise characteristics. Certain molecules with similar physical/chemical behaviour are grouped together in families which makes it possible to examine the characteristics of a given family as an initial approach. For the purposes of this course, we will be concentrating on the study of hydrocarbon molecules which are made up only of carbon and hydrogen. This family is divided into 6 groups but the 5 most important ones are: Alkanes or paraffins, Alkenes or olefins (former name), Alkynes or acetylenes, cyclanes or cyclo-alkanes ou les naphtenes aromatics. Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

Name of the function/ group

Structure of the group

Comments

Consisting only of atoms connected by single bonds

Alkane

Alkene

Contains at least one double bond

Alkyne

Presence of a triple bond between two carbons

cyclopropane cyvlobutane cyclohexane, etc

Cyclo-alkane

Presence of 2 hydrogen atoms per carbon atom, with a ring structure

Aromatics

Presence of a 6-atom cycle with alternating single and double bonds (benzene core)

Table 1: The most important hydrocarbons

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Exploration & Production Theory Outline Introduction to Hydrocarbons

A basic specificity of this group is the fact that the hydrocarbon molecules of each group have different boiling point temperatures. It is this characteristic which makes it possible to manufacture finished (marketable) products using treatment processes which allow for the selective separation of the different types of hydrocarbon molecules (e.g. distillation, etc.).

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Exploration & Production Theory Outline Introduction to Hydrocarbons

2.2. ORIGIN OF HYDROCARBONS Fossil hydrocarbons are the components of petroleum and natural gas and are produced by the decomposition of organic matter accumulated more than 500 million years ago. These compounds (organic molecules) are formed exclusively of carbon and hydrogen. However, the hydrocarbons are mixed with other, often undesirable, elements (nitrogen, oxygen, sulphur, etc.). Petroleum is always composed mainly of hydrocarbons, of which alkanes are the main compounds (which can reach more than forty carbon atoms). It also contains cyclanes, aromatics, and products containing sulphur, nitrogen and oxygen in variable quantities depending on the deposit. It is therefore necessary to separate these different products (distillation), transform them chemically (cracking and reforming) and purify them (actual refining) to obtain usable products. All these operations are carried out in refineries and the procedure is known as refining.

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Exploration & Production Theory Outline Introduction to Hydrocarbons

2.3. CARBON AND HYDROGEN Carbon and hydrogen are the 2 components of hydrocarbons. The molecules consisting of C and H atoms represent 90% (in weight) of the natural hydrocarbons and the remainder consists mainly of nitrogen, oxygen and sulphur. If we refer to what has been said about the composition of the molecules: •

Carbon possesses 6 electrons (see periodic table) and the layer k is saturated with 2 electrons and it possesses 4 electrons on layer L, there are therefore 4 “available spaces” remaining for electrons of other atoms and it is said that carbon has a valency of 4.

•

Hydrogen possesses 1 electron (see periodic table), therefore only layer K is occupied by an electron and there is 1 “available space” for one electron from another atom.

Therefore, a wide range of structures is possible (and therefore molecule types) containing only C and H atoms, not only depending on the number of atoms involved but also depending on the type of bond between these atoms. For example, molecules with the maximum amount of hydrogen atoms are said to be saturated and are Alkanes. These organic molecules can be represented in a variety of ways, the 3 main ones being: representation by an empirical formula: CH4 (methane), C2H6 (ethane) semi-developed formula CH4 and CH3-CH3 developed representation.

Figure 4: Methane CH4

Figure 5: Ethane C2H6

Whilst it is not possible to use a single formula to define hydrocarbons, we can nevertheless use the different possible bonds between the carbon and hydrogen to group Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

hydrocarbons together in six different families whose components possess similar properties. Alkanes (saturated aliphatic carbonaceous chains) Alkenes (unsaturated aliphatic carbonaceous chains) Alkynes (unsaturated carbonaceous chains comprising a triple carbon/carbon bond) Cyclanes (saturated alicyclic carbonaceous chains ) Aromatics (unsaturated cyclic carbonaceous chains comprising a hexagonal carbon cycle) Cyclens (unsaturated cyclic carbonaceous chains )

Comments: A molecule is said to be saturated when each carbon atom which composes it contains the maximum possible number of hydrogen atoms (all the carbon bonds are single). It is said to be saturated because hydrogen cannot be added to this molecule. A molecule (saturated or otherwise) is said to be aliphatic when its carbon chain is open and it does not possess a closed carbon cycle (these are alkanes, alkenes and alkynes) A molecule is said to be cyclic when its carbon chain closes on itself without there being any aromatic cycle (these are cyclanes and cyclens). A molecule is said to be aromatic when it possesses an unsaturated cycle of 6 carbon atoms. Although we shall be examining the main hydrocarbons, the emphasis will be placed on paraffinic hydrocarbons and alkanes since alkanes are found throughout the entire oil exploitation chain.

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Exploration & Production Theory Outline Introduction to Hydrocarbons

2.4. ISOMERS We have seen that it is useful to group hydrocarbons together in “families” in order to obtain molecules with similar properties with these groups corresponding to molecules of a similar constitution (example: Alkanes are saturated aliphatic carbonaceous chains). Conversely, identical molecules also exist (by their empirical formula) which have different properties. These molecules are known as isomers.

2.4.1. Definition There is only one methane, one ethane and one propane, but using butane there are several possibilities for bonding carbon and hydrogen atoms. The isomer is a molecule with the same empirical formula as the “normal” molecule, the difference being that the carbon atoms of the “normal” molecule form a chain whereas the isomer possesses a different semi-developed formula (therefore a different atom arrangement and structure) and different properties or a different atom arrangement in space. Isomers are molecules with the same empirical formula but different structures. The name is often preceded by the letters n or i to differentiate between the “normal” molecule and the corresponding isomer, Thus there are two alkanes with four carbon atoms: normal butane and isobutane. They have the same formula C4H10, but are differentiated notably by their boiling point temperature. The number of isomers increases rapidly in excess of six carbon atoms. There are 2 types of isomer: Constitutional isomers for which there is only on spatial geometry possible for the molecule concerned are known as isomers. Isomers which can be differentiated by their spatial geometry are known as stereoisomers

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Exploration & Production Theory Outline Introduction to Hydrocarbons

No of C. atoms 4 5 6 7 8

No of isomers 2 3 5 9 18

No of C. atoms 9 10 12 15 20

No of isomers 35 75 355 4347 366319

Table 2: Number of alkane isomers

2.4.2. Constitutional isomers A molecule which possesses n carbon atoms and 2n+2 hydrogen atoms if n > 3, may present branching (non-linear carbon backbone). This is the case of butane and 2-methylpropane which have the same empirical formula C4H10 In order to determine the name of a branched alkane, we consider that it consists of a principal chain to which more or less complex groups are attached. The main groups are known as alkyls: The name given to the molecule is the name of the longest linear chain in which alkyl groups are added (see below) to numbered carbons.

2.4.3. Stereoisometry When we consider a molecule to be fully developed in space, new cases of isometry in addition to constitutional isometry may exist. They are known as spatial isometries or stereoisometries (the molecules have the same formula as the normal molecule or the constitutional isomer but do not have the same properties). Two stereoisomers have the same semi-developed formula but different forms in threedimensional space. We will consider only stereoisometry with the configuration Z / E (the prefixes come from German Z: zusammen = together, E: entgegen: opposing)

Comments: Z and E isomerism can be applied to all molecules of the type CHA = CHB. Groups A and B are not hydrogen atoms. Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

The 2 groups A and B are “together”

The 2 groups A and B are “diametrically opposed”

Figure 6: Stereoisomers NB: isomers have relatively little importance for the oil industry

2.4.4. Alkyl radicals Alkyl radicals are derivatives of an alkane which lacks hydrogen and which can therefore be bonded to a carbon chain by forming a branch known as an alkyl radical. The name linear alkyl radicals derives from the linear alkane by replacing the ending “ane” by “yl”; the first 5 Alkyl radicals are:

Radical

Empirical formula

Semi-developed formula

Methyl

CH3 -

CH3 -

Ethyl

C2H5 -

CH3 - CH2-

Propyl

C3H7 -

CH3 - CH2 - CH2 -

Isopropyl

C3H7 -

CH3 - CH (CH3) -

Butyl

C4H9 -

CH3 - CH2 - CH2 - CH2 -

Table 3: The first 5 alkyl radicals Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

Explanations concerning the definition method for Alkyl radical isomers: The longest carbon chain is identified. This is the one which gives the alkane its name. The name (without the final e) of the group attached to the principal chain is added as a prefix. Its position is identified by numbering the principal chain to give the smallest number to the carbon carrying the group. This name is placed in front of the group name. In the case of several identical groups, the prefix di-, tri-, tétra- is placed in front of the group name. In the case of different groups, they are named in alphabetical order. The smallest number is allocated to the group in first position in alphabetical order.

3-ethyl-5-methylhexane

2,4-dimethyl hexane

3-ethyl-5-methyl-4-propylheptane

Figure 7: Iso-alkanes

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Exploration & Production Theory Outline Introduction to Hydrocarbons

3. THE SIX DIFFERENT HYDROCARBON GROUPS 3.1. THE ALKANE GROUP The alkanes: CnH2n+2 (or saturated aliphatic hydrocarbons): These are molecules whose carbon atom chain consists only of single bonds (each carbon atom possesses 4 bonds either with H atoms or with a C atom to form a chain). All the possible carbon bonds are used, which is why these molecules are known as saturated aliphatic hydrocarbons. General alkane formula: CnH2n+2 irrespective of whether their carbon backbone is linear or branched. A distinction is made between normal molecules (n) with a linear chain structure and

isomolecules (i) with a branched structure (iso as in isomer) As from five carbon atoms, alkanes are liquids or solids in their natural state (it is commonly said that the liquids are C5+).

3.1.1. Linear or normal alkanes (n) These are alkanes with a linear carbon backbone (with no branching). The beginning of the name of a linear alkane depends on the number of carbon atoms in the molecule and the names ends in "ane"

Number of C

1

2

3

4

5

6

7

8

9

10

prefix

meth

eth

prop

but

pent

hex

hept

oct

non

dec

Table 4: The linear alkanes This accounts for the names: methane, ethane, propane, butane,pentane...with the respective empirical formulae C1H4, C2H6, C3H8, C4H10, C5H12,

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Exploration & Production Theory Outline Introduction to Hydrocarbons

A few representations:

Figure 8: Methane – CH4 (gas)

Figure 9: Ethane – C2H6 (gas)

Figure 11: Butane – C4H10 (gas)

Figure 10: Propane – C3H8 (gas)

Figure 12: Pentane – C5H12 (liquid)

Melting point Boiling point State at 25oC (oC) (oC)

Name

Molecule

methane

CH4

-182

-162

gas

ethane

C2H6

-183

-88.7

gas

propane

C3H8

-188

-42.

gas

butane

C4H10

-138

-0.5

gas

pentane

C5H12

-130

36

liquid

hexane

C6H14

-95

68.9

liquid

heptane

C7H16

-90.6

98.4

liquid

octane

C8H18

-56.8

125.6

liquid

nonane

C9H20

-51

150.8

liquid

decane

C10H22

-29.7

174.1

liquid

undecane

C11H24

-24.6

195.9

liquid

dodecane

C12H26

-9.6

216.3

liquid

eicosane

C20H42

36.8

343

solid

triacontane

C30H62

65.8

449.7

solid

Table 5: Linear saturated hydrocarbons or Alkanes Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

The table above shows that the compounds C1 to C4 (methane to butane) are gaseous at room temperature, in excess of five carbon atoms the compounds are liquid and in excess of fifteen carbon atoms they are solids.

3.1.2. Carbon chain alkanes or isomers (i) By applying what has been outlined in the section on Alkyl radicals, it is possible to describe the isomers of the first 3 Alkanes

Isomers of butane C4H10 Butane, whose empirical formula is C4H10, has two common isomers, normal butane (nbutane) and 2-methyl-propane (formerly isobutane), whose empirical formulae are as follows:

CH3-CH2-CH2-CH3

CH3-CH(CH3)-CH3

Figure 13: n-butane – C4H10

Figure 14: 2-methyl-propane(isobutane) – C4H10

A methyl group (CH3-) has attached itself to the second carbon of a propane to form the methyl-propane isomer, which explains the origin of the name 2-methylpropane.

Pentane isomers C5H12: Using the same principle as for butane isomers, 3 isomers exist for pentane CH3-CH2-CH2-CH2-CH3

n-pentane

C5H12

CH3-CH(CH3)-CH2-CH3

2-methylbutane

C5H12

CH3-C(CH3)2-CH3

2,2-dimethylpropane

C5H12

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Exploration & Production Theory Outline Introduction to Hydrocarbons

Hexane isomers C6H14: Still using the same principle, the 5 hexane isomers are: CH3- CH2-CH2-CH2-CH2-CH3

n-hexane

C6H14

CH3-CH(CH3)-CH2-CH2-CH3

2-methylpentane

C6H14

CH3- CH2-CH(CH3)-CH2-CH3

3-methylpentane

C6H14

CH3-C(CH3)2-CH2-CH3

2,2-dimethylbutane

C6H14

CH3-C(CH3)-CH(CH3)-CH3

2,3-dimethylbutane

C6H14

3.2. THE ALKENE GROUP The Cn H2n alkenes (or unsaturated hydrocarbons with only one double bond): Alkenes are unsaturated hydrocarbons which possess only one double bond. Their general formula is therefore derived from the alkane formula after removing 2 H atoms. Alkenes are also known as oleofines, literally “to make oil” because they have a tendency to remain in the liquid state when they cool. The name of an alkene is derived from the name of the corresponding alkane by replacing the ending –ane with the ending –ene.

Figure 15: Alkane: Ethane – C2H6

Figure 16: Alkene: Ethene – C2H4

The double bond may be located at different points in the chain. To specify this position we indicate (in the name) the position of the double bond by numbering the carbon atoms in the carbon chain giving the carbon atoms with a double bond the smallest numbers (this numbering is not applicable for the first two alkenes, namely ethane, commonly known as ethylene, and propene). Only then do we consider the numbers of the carbon atoms in which the methyl, ethyl groups etc. are found. Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

Ethene (ethylene)

Propene

But-1-ene

But-2-ene (2 stereoisomers)

Pent-1-ene

Methylpropene

3-ethyl-2-methylpent-2-ene

2,3-diethylhex-1-ene Figure 17: Alkenes Certain alkenes possess several doubles bonds. Alkenes with 2 double bonds are known as dienes and those with 3 are known as trienes.

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Exploration & Production Theory Outline Introduction to Hydrocarbons

3.3. THE ALKYNE GROUP The Alkynes: Cn H2n-2 (or unsaturated hydrocarbons with a single triple bond): An alkyne is a hydrocarbon whose molecule contains a triple carbon-carbon bond. Alkynes are therefore unsaturated hydrocarbons as a given carbon atom is not surrounded by 4 distinct elements due to the triple bond between the carbons. Particular reference should be made to the structure of the ethyne molecule, commonly known as acetylene (deceptive ending as it is similar to the alkene family):

Compound

Ethyne (acetylene)

External electrons

Lewis model

C: 4 external electrons H: 1 external electron (4)2 + (1)2 = 10 electrons 5 bonding doubles

Figure 18: The alkynes

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Exploration & Production Theory Outline Introduction to Hydrocarbons

3.4. THE CYCLANE GROUP The Cyclanes: CnH2n (or saturated alicyclic hydrocarbons): Examples:

C6H12 cyclohexane

C9H18 1-ethyl-3-methylcycohexane

Figure 19: Cyclanes

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Exploration & Production Theory Outline Introduction to Hydrocarbons

3.5. THE AROMATIC GROUP (or unsaturated hydrocarbons with a benzenic cycle): The aromatics are unsaturated hydrocarbons which possess one or more “phenyl” radicals (see representation below) bonded to carbon chains. There are at least 6 carbons. Empirical formula: C6H5-Y: C6H5 is the phenyl radical and Y a molecule attached to this radical..

Figure 20:Phenyl Radical C6H5

Figure 21:Benzene C6H6

Benzene C6H6 is the simplest aromatic hydrocarbon

3.6. THE CYCLEN GROUP (or unsaturated alicyclic hydrocarbons): Cyclens are unsaturated alicyclic hydrocarbons which possess one or more double bonds (C=C).

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Exploration & Production Theory Outline Introduction to Hydrocarbons

4. MAIN PHYSICAL CHARACTERISTICS OF HYDROCARBONS 4.1. GENERAL ASPECTS In general terms we have seen that the characteristics of a hydrocarbon are directly linked to the family (group) to which it belongs. Crude oil and natural gas contain different components. Depending on the characteristics of the alkanes we have seen that the compounds C1 to C4 are gaseous in their natural state whilst from pentane onwards, hydrocarbons are liquid. In fact, an oil or a gas in its natural state will have more or less the same components; it is the proportion of these components in the mixture which determines whether the it is a liquid or a gas, as shown in the table below.

Figure 22: Composition of a crude oil and raw natural gas Within the alkane family, these characteristics are directly linked to the number of carbon atoms making up the molecule as shown in the table below.

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Exploration & Production Theory Outline Introduction to Hydrocarbons

Methane

Ethane

Propane

Butane

CH4

C2H6

C3H8

C4H10

Molar mass (g.mol-1)

16

30

44

58

Density in Kg/l (liquid)

0.3

0.37

0.51

0.58

Densityof gas at15°c and 760mmHg in Kg/m3

0.667

1.27

1.86

2.45

Melting temperature (°C)

-182

-183

-188

-138

Boiling point temperature (°C) - P = 101325 Pa

-162

-88,7

-42

-0,5

Vapour pressure10°C (Kg/cm2)

370

32

6.2

1.5

Litres of gas obtained from a litre of liquid

443

294

273

238

Net calorific value (kcal/m3 of gas)

9490

16630

23660

30665

Gross calorific value (Kcal/Kg)

13288

12417

11980

11586

Energy released by the combustion of one mole (kJ.mol-1)

890

1 560

2 220

2 880

Energy released by the combustion of one kilogramme (kJ.kg-1)

55 600

52 000

50 400

49 700

Name

Empirical formula

Table 6: Properties of alkanes Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

We have therefore seen that production of crude oil is always associated with the production of gas (associated gas). This gas is obtained by degassing the crude at different separation levels (the recovery of light hydrocarbons C1 to C4) In addition, the production of gas from a gas field is generally associated with the production of a certain quantity of liquid hydrocarbons (from pentane, known as condensates) which require a specific installation before they can be commercialised.

4.2. STATE CHANGING TEMPERATURES The length of the carbon chain affects the melting and boiling point temperatures. These state changing temperatures increase with the length of the carbon chain. We have seen that at a normal temperature with five carbon atoms, alkanes with 5 to 16 C atoms are liquid and these are the ones which are stored.

4.3. DENSITY / °API The density compared with air for gaseous alkanes or compared with water for liquid or solid alkanes also increases with the length of the carbon chain (see details on the API degree in the final chapter). API degree (temp)= (141.5/ Density at temp) – 131.5. Density ( temp) = 141.5/( 131.5 + °API at temp)

4.4. VISCOSITY The viscosity of alkanes varies rapidly depending on the number of carbon atoms which they contain; when the number of carbon atoms in an alkane is greater than 16 it solidifies at a temperature exceeding 18°C.

4.5. COMBUSTION HEAT The combustion heat of alkanes with N carbon atoms is approximately 658 N+ 243 kJ/mole. Since the first ones are relatively richer in hydrogen, their combustion heat is slightly higher. It has also been shown that the calorific value, or the quantity of heat emitted by the combustion of 1 kg of alkane, tends towards a limit of 46 000 kJ/kg when N increases indefinitely. Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

5. INTRODUCTION TO HYDROCARBON PROCESSING 5.1. CRUDE OIL PROCESSING Crude oil is a mixture of a wide range of hydrocarbon molecules in which alkanes, notably linear ones, are predominant. The proportions of the mixture vary from one reservoir to the next and may vary over time for a given reservoir. Commercial products must correspond to specific characteristics. It is therefore necessary to obtain precise specific products from a crude oil, which involves transforming the crude into a series of products with specific hydrocarbon groups: this is achieved by distillation. The lightest oils are the most highly sought after by refiners as they directly give large quantities of light cuts with a high value (see introduction to the treatment of crude oil). Conversely, heavy oils give lager quantities of products such as asphalts and residual fuel which either have to be sold as they are at a low price or converted into lighter cuts, notably by hydrocracking (addition of hydrogen).

Figure 23: The distillation cuts

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Exploration & Production Theory Outline Introduction to Hydrocarbons

The principle of distillation (or fractionating) is simply based on the fact that the boiling point temperatures of different alkanes are clearly separated between one alkane group and the next, which is known as fractionating by cut. The fact that not all hydrocarbons pass through the gaseous state at the same temperature is related to the molar mass of the molecule concerned. The greater the volatility of a hydrocarbon, the smaller its molar mass (small number of carbon atoms). In an industrial context, this distillation is conducted in a 40 to 60 meter high tower comprising up to 50 plates. The crude is heated in the furnace to 370°C and transmitted to the tower where the pressure is equal to atmospheric pressure (atmospheric distillation). The products collected on the different levels (the least volatile at the bottom and the most volatile at the top) are mixtures with similar properties and are known as cuts: The different distillation cuts obtained along the column are: At the top of the column, the most volatile products are collected in their gaseous state (C1-C4). The boiling point temperature of the mixture decreases with its height in the column and gas oils (C13-C20), kerosene (C10-C13), jet fuel and naphtha (C5-C10) are separated allowing for the production of normal and super petrol. At the bottom of the column are the heaviest hydrocarbons (C>20, with more than 20 carbon atoms per molecule), known as “atmospheric residue”. These are the ones with the highest boiling point. In order to undergo a more complex separation they must be distilled in a vacuum. The most precise cuts can often be obtained with different temperature ranges which correspond to specific recovery heights in the distillation column. Cuts C1-C4: the combinations of alkanes with molecules of CH4 to C4H10).. (these are the lightest), are gaseous at normal pressure and temperature and are most commonly used as combustible gases (domestic and industrial use) and as raw materials in the petrochemical industry. Cuts C5-C6: boiling point 20-60°C Give solvents and petroleum ether Cuts C6-C7: (boiling point 60-100°C) Give light naphtha or white spirit and are mainly used as solvents. Cut C6-C11 (boiling point 60-200°C) Gives gasoline, the basis for the manufacturing of fuels, and also, for the naphtha part (C6-C10), the raw material which is subjected to steam cracking for the petrochemical industry.

Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

The fraction C11-C16 (boiling point 180-280°C), Gives kerosene, mainly used as a fuel in jet engines and diesel engines and as a fuel (light fuel) for domestic heating. The fraction exceeding C18 (boiling point > 350°C) Composes atmospheric residue and is used as a fuel (heavy fuel) for industrial heating (thermal power plants). It is subject to reduced pressure distillation and produces light (C18-C25, boiling point 300-400°C) and heavy lubricating oils (C26C36, boiling point 400-500°C). The residues of vacuum distillation are asphalts. Additional processing exist to upgrade the crude and to produce the largest possible number of high-value molecules, but the principle is the same and consists of using the difference between the characteristics of the molecules to obtain the desired product.

5.2. GAS PROCESSING In sufficient quantities, gas is upgraded by specific gas treatment facilities and is either commercialised in its gaseous form or as a liquid (after liquefaction) depending on the distances between the production point and the consumption point.

Frigg (North Sea)

Lacq (France)

Urengoï (Russia)

Hassi R'Mel (Algeria)

Groningue (Netherlands)

Methane (%)

95.7

69.2

98

83.5

81.3

Ethane (%)

3.6

3.3-3.6

7.9

2.9

Propane (%)

0.04

1.0-1.2

2.1

0.4

Butane (%)

0.01

0.6-0.9

1.0

0.2

Nitrogen gas (%)

0.4

0.6

1.2

5.3

14.3

Carbon dioxide (%)

0.3

9.3

0.3

0.2

0.9

-

15.3

-

-

-

Hydrogen sulphide (%)

The compositions are given as a volume %. Figure 24: Characteristics of a number of natural gas deposits

Training course: EXP-PR-RT030-EN Last revised: 26/04/2007

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Exploration & Production Theory Outline Introduction to Hydrocarbons

5.2.1. Transport in gaseous form In addition to hydrocarbons (which can be upgraded), natural gas contains impurities which have to be removed in order to prevent problems in the facilities, particularly since the specifications imposed by the purchasers include identical clauses to preserve their own facilities. As far as transport in gaseous form is concerned, it is important to ensure that there is no formation of liquid in the pipe (condensates and / or water) and no risk of corrosion (CO2 s and / or H2S) The pressure and temperature conditions which develop in the pipe may give rise to the formation of liquids, water or HC. The formation of liquid must be avoided for three main reasons: Accumulation of liquids at the low points of the pipe creating liquid slugs and instabilities in the flow (two phase), Internal pipe corrosion phenomena if the gas contains H2S or CO2 and water, Formation of hydrates (water which reacts with HC under particular T/P conditions). Over short distances (