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
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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 , °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 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 s 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 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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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 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 compri 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.
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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 ()
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
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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 (<30Km) the gas may be transported without treatment ensuring that there is no hydrate formation and no corrosion phenomena by injecting chemicals (for example glycol) and by providing liquid recuperation facilities at the discharge point. For longer distances, it is necessary to “dry the gas” to a level of dryness that prevents the formation of hydrates (solid plugs which form by a reaction between the hydrocarbons and the water they contain, under specific conditions. These plugs may block the pipes).
5.2.2. Transport in liquid form The liquefaction of gas is a necessary stage in its marketing if transport via a pipeline is impossible. In this case, it has to be transported by boat and therefore in small transport-unit volumes and liquefaction which reduces the volume 600 times is therefore the only possibility for this type of transport. The treatment of gas before marketing may be more or less complex, depending on the intended aim and in particular depending on the products required from liquefaction: (LNG, LPG, Propane, Butane, etc. see “Liquefaction”).
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Figure 25: Gas processing
A number of processes exist for the liquefaction of natural gas and the chosen process depends above all on the composition of the gas to be liquefied. This composition will define the temperature which needs to be attained for liquefaction. In all cases, the gas is purified by the elimination of undesirable components, particularly CO2, H2S and H2O, before liquefaction. The principle of the liquefaction of natural gas is based on a series of "refrigerating cycles", using one or more cooling gases, alternating compressions, high-pressure condensation and low-pressure evaporation and expansion. The succession of these cycles using heat exchangers brings the gas to a sufficiently low temperature to transform it to its liquid state at atmospheric pressure. On average a liquefaction plant consumes 12 % of the incoming natural gas for its own operations.
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The general principle may be represented as follows:
Figure 26: Gas treatment 5.2.2.1. Liquefied natural gas: LNG Liquefied Natural Gas (LNG): methane (CH4) and ethane (C2H6) Natural gas consists mainly of methane. Therefore LNG essentially contains methane with ethane, propane and butane.
LNG Properties & mole%
GL 4Z
KENAI NIKISI
MARSA EL BREGA
GL 1K
(ALGERIA)
(LYBIA)
(LYBIA)
ALGERIA)
Actual LNG to ship
Actual LNG to ship
Actual LNG to ship Actual LNG to ship
N2 (mole %) C1 C2 C3 iC4 nC4 C5+ Molecular weight
0.71 86.98 9.35 2.33 0.27 0.36
0.40 99.60
67.70 19.80 8.70 3.10 0.60
1.25 92.55 5.37 0.59 0.10 0.14
18.36
16.39
22.89
17.18
Boiling point at 760mm Hg
deg. C
-163
-162
-160
-164
Density at storage conditions
kg/m3
474
428
537
456
Gas to liquid ratio Gross calorific value
Sm3/m3
618
636
555
634
Kcal/Sm3 Kcal/m3 Liq
9920 6 140 000
8879 5 651 000
12 224 6 800 000
9290 5 889 000
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Natural gas is liquefied (at around -163°C at atmospheric pressure), transported in this form by liquid natural gas tankers and restored to is gaseous state on arrival in the country. (-161°C=boiling point temperature of methane). 5.2.2.2. LPG – liquefied petroleum gas This is a mixture in variable proportions of propane (C3H8) and Butane (C4H10); propane is generally the preponderant element. They are gases under normal temperature and pressure conditions, liquefied and maintained in their liquid state when the pressure is increased or when the temperature is reduced. Liquefied gas is obtained from certain natural gases or from the refining of oil. 60 % of worldwide production is obtained from natural gas and 40 % from the refining of crude oil (1 t of oil produces between 20 and 30 kg of LPG in the refining process). LPG cannot be obtained directly from natural gas (which contains other hydrocarbons: CH4, C2H6 for example). In order to obtain LPG, it is necessary to go through an intermediate stage, namely the manufacturing of NGL, which is a mixture of gaseous hydrocarbons which are liquefied by cooling. The C3 and C4 mixture is isolated by fractionating NGL in order to obtain LPG.
5.2.2.3. NGL - Natural Gas Liquids This is a general term which is applied to all liquids obtained from the associated gas. They are heavier mixtures of ethane and hydrocarbons; propane, butanes, condensates, etc. They are gaseous in the reservoir (like condensates). They are liquefied by cryogenics before use and are sold (C3 and C4) under the name of LPG. NGL are obtained fairly easily since it is not necessary to cool to very low temperatures. Boiling point temperature of C2 approximately -88°C compared with that of C1 -161°C).
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5.2.2.4. Condensates or natural gasoline (Condensate or natural gasoline): liquid mixture C5 to C8. During extraction, expansion at the wellhead triggers the condensation of hydrocarbons C5 to C8 (which were gases in the reservoir conditions). The recovered liquids, known as “natural gas condensates” correspond to extremely light oil with a very high value (giving petroleum and naphtha). The remainder (hydrocarbons C1 to C4, CO2, H2S and He) is gaseous at room temperature and is dispatched through a pipeline to a gas treatment factors. Two collection networks are therefore required, one for the gas and one for the condensates. The market value of the condensates and the LPG is such that certain deposits are produced only for this purpose, with the “poor gas” (methane) being gradually reinjected in the absence of local outlets. Even when most of the poor gas is sold, a proportion is often reinjected into the deposit to compensate for the pressure drop and eventually to recover a larger proportion of condensates and LPG.
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6. HYDROCARBONS AS A SOURCE OF ENERGY 6.1. INTRODUCTION NB: see also course entitled “From atoms to hydrocarbons”. Combustion is a chemical reaction which generates heat. During this reaction, a fuel (in this case a hydrocarbon) reacts with a combustive fuel (mainly oxygen O2). The combustion of an organic component produces carbon dioxide (CO2) and water (H2O). The heat energy produced is related to the transformation of the hydrocarbon molecules, the principle being that the breaking of the C-H bonds produces much more energy than required to create bonds for the CO2 and H2O molecules produced by combustion. NB: the heating value of a fuel is the quantity of heat produced by its combustion at a constant pressure and under normal temperature and pressure conditions (in other words at 0°C and with a pressure of 1013 mbar).
6.2. COMBUSTION OF ALKANES The combustion of alkanes in the air is a highly exothermal reaction which is widely used for the production of heat and mechanical energy.
6.2.1. Example: the combustion of methane: The chemical reaction corresponding to the combustion of methane ( g: the elements are gaseous) is written as follows: CH4 (g) + 2O2 (g)
(the reagents)
→
CO2(g) + 2H2O(g)
(the products)
The balance equation (see course entitled “From atoms to hydrocarbons”) shows that this reaction produces more energy than it consumes and is said to be exothermal. The energy released (3460 kJ. mol-1) by the creation of product bonds is 820 kJ. mol-1 more than the quantity consumed ( 2640 kJ. mol-1) to break the reagent bonds (therefore the methane molecules). Comments: The combustion heat of methane is - 820 kJ. mol-1 if water is obtained in the gaseous state, but is - 890 kJ. mol-1 if water is obtained in the liquid state: the reaction is more exothermal if the water is in the liquid state (the energy required for the vaporisation of the water is not expended). Training course: EXP-PR-RT030-EN Last revised: 26/04/2007
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This explains why the combustion of hydrocarbons produces energy.
Figure 27: Combustion of hydrocarbons If we express this energy in kJ.kg-1, we simply need to that the mass (quantity of matter) of one mole of an element is equal to the value of its molar mass expressed in grams. Therefore the molar mass of methane =16g/mol i.e. 1kg of methane=1000/16 moles, releasing 890 x 1000/16 = 55625 (kJ.kg-1). In general, the combustion heat of alkanes with N carbon atoms is approximately 658 N+ 243 kJ/mole. The first are relatively richer in hydrogen and their combustion heat is slightly more intense. If we apply this estimation to methane the energy produced = 658+243=901 kJ. mol-1. The important thing to is that the heat energy released by one mole of alkane is greater when more carbon atoms are present (one mole of a methane molecule is made up of 6.023 1023 molecules of methane). By comparison, the combustion of butane (C4H10) produces 11062-8388=2674 kJ/mol (using the same approach with the corresponding combustion chemical reaction).
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7. NAMES ASSOCIATED WITH HYDROCARBONS 7.1. GAS AND CRUDE Natural gas and crude oil are often associated and extracted at the same time from the same deposits or the same production zones. An average proportion of liquid hydrocarbons, corresponding to around 80%, are obtained from crude oil; the remaining 20%, among the lightest fractions, contain propane and butane and are nearly always liquefied to facilitate transport.
7.2. NATURAL GAS This is a mixture of hydrocarbons which are in a gaseous state under reservoir conditions. This gas has a high methane content (between 70% and 100%) and consists manly of gaseous hydrocarbon compounds (C1 to C5). It also contains a smaller quantity of heavier molecules in gaseous form (nitrogen, carbon dioxide, hydrogen sulphide.) The production of this gas generally generates liquid hydrocarbons. This liquefaction occurs during expansion in the production facilities. This is why a natural gas is often characterised by the associated quantity of condensates.
7.3. CRUDE OIL Crude oil is a mixture, in variable proportions, of hydrocarbons which are liquid in their natural state under varying pressure and temperature conditions in their reservoir. It may contain small quantities of matter other than hydrocarbons. Crude oils are classified according to their specific gravity or density API as: extra-heavy: in excess of 1 000 kg/m3 (less than 10 °API), heavy: 1 000-920 kg/m3 (10-22.3 °API), average: 920-870 kg/m3 (22.3-31.1 °API).
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7.4. DENSITY OF OIL: THE API DEGREE Crude oil is classified as light, average and heavy depending on its gravity or density, as measured on the scale of the American Petroleum Institute (API). The “API degree” is used in the Anglo Saxon system to measure the density of an oil, whereby the temperature is always specified. API degree (temp)= (141.5/ Density at temp) – 131.5. Density ( temp) = 141.5/( 131.5 + °API at temp) The API density is expressed in API degrees Examples: a liquid with a density of 1.00 at 15°C (H2O =1kg/litre) has an API density of 10°API at15°C. (API degree= 141.5/1 – 131.5 = 10°API) a liquid with 22°API at 15°C has a density at 15°C = 0.9218 (Density at 15°C = 141.5/(131.5+22) = 141.5/153.5 = 0.9218) a liquid with 35°API at 15°C has a density at 15°C = 0.8498 (Density at 15°C = 141.5/(131.5+35) = 141.5/166.5 = 0.8498) The conventional lower limit for oil is generally set at 15°API. Light crude has an API density in excess of 31.1; average crude has a density of between 22.3° and 31.1° and heavy oil has a density of less than 22.3°.
7.5. ASSOCIATED GAS During the production of crude oil, the products extracted from the reservoir are stabilised at the surface (to evacuate the gases dissolved in the liquid). The gas which is released as a result is known as the associated gas. The potential of a crude to produce associated has is known by definition as GOR (gas oil ratio). GOR = total gas produced (SCF or m3)/ oil produced (bbl or m3).
7.6. ARABIAN LIGHT Quality of crude oil produced in Saudi Arabia with a density of 34° API. Its price on leaving the Gulf has formed the reference for OPEC prices for many years. Training course: EXP-PR-RT030-EN Last revised: 26/04/2007
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7.7. BRENT Mixture of crude oils produced by seven neighbouring fields in the North Sea linked to the same pipeline.
7.8. LIGHT AND HEAVY OILS (HYDROCARBONS) Light oils are hydrocarbons which consist mainly of light molecules (which contain few carbon atoms) and are therefore essentially: Methane(CH4) Ethane(C2H6) Propane(C3H8) Butane (C4H10) Which are all below C5 (see API degree for further details)
7.9. TOE: TONNE OIL EQUIVALENT This approximate equivalence is used to express combustible reserves of TOE. 1 tonne of oil = 1000 m3 of gas = 1.5 tonnes of coal
7.10. GOR (Gas Oil Ratio) This is the total volume of gas expressed under standard conditions associated with the production of a unit volume expressed under reference conditions for storage oil. It is expressed as m3/m3 standards.
7.11. BSW (Bottom Sediment and Water – also known as watercut) This is the percentage of water and sediments in relation to the liquid phase (oil +water + sediment). During a delivery of crude, disregarding sediments, this corresponds to the percentage of water contained in this crude. BSW = WATER / OIL + WATER.
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7.12. HYDRATES Hydrates are akin to ice in which a molecule of gas is encaged by molecules of water. These cages group together and quickly form solid plugs. The hydrates form at fairly high pressures and low temperatures but free water must always be present to enable the hydrates to form. Hydrates may create solid plugs in pipes, preventing the circulation of gas, which is why before gas is transported though a pipe it is dehydrated to ensure that in the worst case any water which may remain dissolved in the gas is not transformed into free water. The precautions to be taken (level of dehydration, P, T) can be obtained by calculations or from curves. Hydrates also exist in nature and form pockets of gas in the form of ice which compose a wonderful source of energy, equivalent to twice the methane from identified reserves of coal, oil and gas combined.
7.13. DEW POINT (for H2O and HC). All gases are products formed from a group of components, particularly different types of hydrocarbons and water (in gaseous form). The dew point (for a given pressure) is the temperature above which the component es from the gaseous state to the liquid state. Therefore for water the dew point of a given gas at a given pressure is the temperature above which water in gaseous form liquefies. The same applies to gaseous hydrocarbons (HC dew point).
7.14. WATER CONTENT All gases contain water in their gaseous form. The water content is the quantity of water/value of gas generally expressed in Kg/Sm3. This value depends on the pressure of the gas concerned and its temperature. The water content which is obtained from curves can be used to determine the quantity of water which must be removed from the gas by the drying process.
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It should be noted that: For a given gas pressure, the lower the temperature of the gas, the lower the water content For a given temperature, the higher the gas pressure, the higher the water content.
7.15. WOBBE INDEX This is the highest heating value of a gas divided by the square root of its density; it is a parameter used to compare the combustion energy of a gas. Two gases with the same index are interchangeable (from the point of view of the energy supplied) without altering the combustion system. It is expressed, for example, in J/m3 .
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8. EXERCISES 1. The 3 molecules below have the same empirical formula C6H14, which ones have the same carbon chain?
2. Determine the number of constitutional isomers of hexane C6H14
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3. Name the following molecule:
4. Name the following molecule:
5. Name the following molecule:
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9. GLOSSARY
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10. LIST OF FIGURES Figure 1: Propane ................................................................................................................5 Figure 2: Glycol....................................................................................................................5 Figure 3: Ethylamine ............................................................................................................5 Figure 4: Methane CH4 ........................................................................................................9 Figure 5: Ethane C2H6 .........................................................................................................9 Figure 6: Stereoisomers.....................................................................................................13 Figure 7: Iso-alkanes .........................................................................................................14 Figure 8: Methane – CH4 (gas) ..........................................................................................16 Figure 9: Ethane – C2H6 (gas) ...........................................................................................16 Figure 10: Propane – C3H8 (gas) .......................................................................................16 Figure 11: Butane – C4H10 (gas) ........................................................................................16 Figure 12: Pentane – C5H12 (liquid) ...................................................................................16 Figure 13: n-butane – C4H10 ..............................................................................................17 Figure 14: 2-methyl-propane(isobutane) – C4H10 ...............................................................17 Figure 15: Alkane: Ethane – C2H6......................................................................................18 Figure 16: Alkene: Ethene – C2H4......................................................................................18 Figure 17: Alkenes .............................................................................................................19 Figure 18: The alkynes ......................................................................................................20 Figure 19: Cyclanes ...........................................................................................................21 Figure 20:Phenyl Radical C6H5 ..........................................................................................22 Figure 21:Benzene C6H6 ....................................................................................................22 Figure 22: Composition of a crude oil and raw natural gas ...............................................23 Figure 23: The distillation cuts ...........................................................................................26 Figure 24: Characteristics of a number of natural gas deposits .........................................28 Figure 25: Gas processing.................................................................................................30 Figure 26: Gas treatment ...................................................................................................31 Figure 27: Combustion of hydrocarbons ............................................................................35
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11. LIST OF TABLES Table 1: The most important hydrocarbons .........................................................................6 Table 2: Number of alkane isomers ...................................................................................12 Table 3: The first 5 alkyl radicals .......................................................................................13 Table 4: The linear alkanes................................................................................................15 Table 5: Linear saturated hydrocarbons or Alkanes ..........................................................16 Table 6: Properties of alkanes ...........................................................................................24
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12. EXERCISE SOLUTIONS 1. The 3 molecules below have the same empirical formula C6H14, which ones have the same carbon chain?
There is no difference between molecules A and B; they both contain a (linear) chain of 5 carbon atoms with a CH3 branch on the 2nd carbon atom. However, molecule C only contains one (linear) chain of 4 carbon atoms and 2 CH3 branches, therefore C is different from A and B.
2. Determine the number of constitutional isomers of hexane C6H14 There are 5 constitutional isomers: the simple chain which defines the “normal” isomer:
There are 2 isomers with a single carbon branch:
and there are 2 isomers with 2 carbon branches:
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3. Name the following molecule:
This molecule is derived from pentane as its longest chain contains 5 carbon atoms – there are 3 identical alkyl radicals (CH3 – methyl)- 2 of these radicals are in position 2 and one radical is on the 4th carbon. This molecule is therefore 2,2,4-trimethylpentane. – but since it contains 8 carbon atoms and its formula is C8H18 it is isooctane.
4. Name the following molecule:
This molecule is derived from heptane as its longest chain contains 7 carbon atoms.- there are 2 alkyl radicals, one CH3 (methyl) on the carbon 2 and one CH2CH3 (ethyl) on the carbon 4 This molecule is therefore 4-ethyl-2-methylheptane. – but since it contains 10 carbon atoms and its formula is C10H22 it is isodecane.
5. Name the following molecule:
This molecule is derived from hexane as its longest carbon chain contains six carbon atoms. It also contains a double carbon bond C=C, this molecule therefore belongs to the hexene family. It possesses 2 alkyl radicals(CH3 methyl) which are bonded to the carbons in positions 3 and 5 – this molecule is dimethyl-2-hexene. It contains 8 carbon atoms and its formula is C8H16. It is not the octane isomer as the empirical formula of octane is C8H18.
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