Chemical and physical properties of hydrocarbons

Chapter 9

Chemical and physical properties of hydrocarbons 1. Introduction Hydrocarbon derivatives (hydrocarbons) are the principal compounds of crude oil and natural gas and to some extent the major constituents of the gaseous and liquid products produced by the thermal decomposition of coal (Chapter 5), oil shale (Chapter 6), and biomass (Chapter 7). In many cases, the hydrocarbon derivatives produced from these sources have to be chemically altered to make useful products and materials. This conversion to saleable products is carried out by changes in the chemical and physical structure. Such differences in molecular structure, even though the empirical formula can remain the same, cause significant difference in the properties and behavior of hydrocarbon derivatives and fuels that are predominantly composed of hydrocarbon derivatives. A hydrocarbon is any chemical compound that consists only of the elements carbon (C) and hydrogen (H) (Chapter 1). With the exception of methane (CH4), all hydrocarbon derivatives contain a carbon-chain skeleton and have hydrogen atoms attached to the carbon skeleton. Most hydrocarbon derivatives are readily combustible (Chapter 10). Almost all usable supplies of hydrocarbon derivatives are currently obtained from crude oil and natural as. Hydrocarbon derivatives are the simplest organic compounds and contain only carbon and hydrogen but they can be straight-chain or branched-chain (Stoker, 2008; Klein, 2013) with the same empirical formula but showing difference in properties. The hydrocarbon derivatives can be divided into various homologous series (Chapter 1) and each member of such a series shows a definite relationship in its structural formula to the members preceding and following it, and there is generally some regularity in changes in physical properties of successive members of a series. The alkane derivatives are a homologous series of saturated aliphatic hydrocarbon derivatives. The first and simplest member of this series is methane, CH4; the series is sometimes called the methane series. Each successive member of a homologous series of hydrocarbon derivatives has one more carbon and two more hydrogen atoms in its molecule than the preceding member. The second alkane is ethane, C2H6, and the third is Handbook of Industrial Hydrocarbon Processes. https://doi.org/10.1016/B978-0-12-809923-0.00009-6 Copyright © 2020 Elsevier Inc. All rights reserved.

387

388 Handbook of Industrial Hydrocarbon Processes

propane, C3H8. Alkane derivatives have the general formula CnH2nþ2 (where n is an integer greater than or equal to 1). Other homologous series of hydrocarbon derivatives include the alkene derivatives and the alkyne derivatives (Chapter 1). Hydrocarbon mixtures are composed of hydrocarbon derivatives, naphtha, benzine, and petroleum ether. Naphtha is a flammable liquid hydrocarbon mixture that typically boils over the range 0e200 C (32e390 F). Benzine (which should not be confused with benzened an aromatic hydrocarbon) also known as petroleum ether is a hydrocarbon mixture of alkane derivatives, such as pentane, hexane, and heptane. Petroleum ether is obtained from crude oil refineries as the portion of the distillate which is intermediate between the low boiling naphtha and the higher boiling kerosene. It has a specific gravity of between 0.6 and 0.8 depending on its composition. Petroleum ether should not be confused with the class of organic compounds called ethers. The physical properties of the unsaturated hydrocarbons (alkane derivatives and cycloalkanes derivatives) are similar to (but not the same as) the properties of the corresponding saturated hydrocarbons (alkene derivatives and alkyne derivatives). The molecules are nonpolar and thus relatively insoluble in water and any intermolecular bonds are weak van der Waal’s bonds. Melting points and boiling points for the small molecules are fairly low. The higher the molecular weight of the hydrocarbon derivative, the higher the melting point and the boiling point. Finally, the chemical and physical properties of hydrocarbon derivatives are also dictated by stereochemistry (Olah and Molna´r, 2003). Both types of properties are related and the proportions of the stereoisomers serve to influence the chemical and/or physical properties.

2. Stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. Stereochemistry is an important facet of chemistry and the study of stereochemical effects spans the entire range of chemical and physical properties (Eliel and Wilen, 1994; Eliel et al., 2001). Stereochemistry (molecular geometry) refers to the threedimensional arrangement of a molecule. Organic molecules of the same chemical formula can have their atoms arranged differently in space, often leading to significantly different chemical properties. Isomers are those types of compounds which have the same chemical formula but different atomic arrangements in space. Isomers can be divided into stereoisomers and structural isomers. Stereoisomers change their atomic arrangement as a result of changes in pressure or temperature. All bonds and types of bonds (single, double, triple) are conserved in the same original fashion, however. Structural isomers have atoms which change their position

Chemical and physical properties of hydrocarbons Chapter | 9

389

in a molecule. One example is a linear compound (where all of the carbon atoms are lined up in linear fashion), compared to the same chemical formula compound with a shorter linear structure and branching (chain isomerism). Functional groups can change their position (functional isomerism), or can differ from another isomer in the position of a double or triple bond (bond isomerism). The number of carbon atoms in a hydrocarbon determines how many forms that compound can takedor the number of possible isomers. The number of possible isomers in a compound rises as the number of carbon atoms it contains rises. The molecular structure of the alkane derivatives directly affects their physical and chemical characteristics. It is derived from the electron configuration of carbon (Chapter 1), which has four valence electrons. The carbon atoms in alkane derivatives are always sp3 hybridized, that is to say that the valence electrons are said to be in four equivalent orbitals derived from the combination of the 2s orbital and the three 2p orbitals. These orbitals, which have identical energies, are arranged spatially in the form of a tetrahedron, the angle of cos1(1/3 ) z 109.47 degrees between them. An alkane molecule has only CeH and CeC single bonds. The former result from the overlap of a sp3-orbital of carbon with the 1s-orbital of a hydrogen; the latter by the overlap of two sp3-orbitals on different carbon atoms. The bond lengths amount to 1.09  1010 m for a carbon-hydrogen (CeH) bond and 1.54  1010 m for a carbon-carbon (CeC) bond.

The tetrahedral structure of methane.

The spatial arrangement of the bonds is similar to that of four sp3orbitalsdthey are tetrahedrally arranged, with an angle of 109.47 degrees between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not correspond with the reality. The structural formula and the bond angles are not usually sufficient to completely describe the geometry of a molecule. There is a further degree of freedom for each carbon-carbon bond: the torsion angle between the atoms or groups bound to the atoms at each end of the bond. The spatial arrangement described by the torsion angles of the molecule is known as its conformation.

390 Handbook of Industrial Hydrocarbon Processes

Newman projections of the two conformations of ethane: eclipsed on the left, staggered on the right.

Ball-and-stick models of the two rotamers of ethane.

Ethane forms the simplest case for studying the conformation of alkane derivatives, as there is only one carbon-carbon bond. If one looks down the axis of the carbon-carbon bond, one will see the so-called Newman projection. The hydrogen atoms on both the front and rear carbon atoms have an angle of 120 degrees between them, resulting from the projection of the base of the tetrahedron onto a flat plane. However, the torsion angle between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the rear carbon can vary freely between 0 and 360 degrees. This is a consequence of the free rotation about a carbon-carbon single bond. Despite this apparent freedom, only two limiting conformations are important: eclipsed conformation and staggered conformation. The two conformations, also known as rotamers, differ in energy: the staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation (the least stable). This difference in energy between the two conformations, known as the torsion energy, is low compared to the thermal energy of an ethane molecule at ambient temperature. There is constant rotation about the carbon-carbon single bond. The time taken for an ethane molecule to pass from one staggered conformation to the next, equivalent to the rotation of one methyl (CH3) group by 120 degrees relative to the other, is of the order of 10e11 s. The case of higher molecular weight alkane derivatives is more complex but based on similar principles, with the antiperiplanar conformation always being the most favored around each carbon-carbon bond. For this reason, alkane derivatives are usually shown in a zigzag arrangement in diagrams or in models. The actual structure will always differ somewhat from these idealized forms, as the differences in energy between the conformations are small compared to the thermal energy of the molecules: Alkane molecules have no fixed structural form, whatever the models may suggest. The geometry of acetylene is linear but the structure of ethylene and propylene are different because the two double bonded carbons are sp2 hybridized and therefore are trigonal planar. There is no free rotation of a double bond or triple. Therefore, many alkene derivatives and alkyne derivatives exhibit geometric isomerism. For example,

Chemical and physical properties of hydrocarbons Chapter | 9

391

cis-2-butene and trans-2-butene are geometric isomersd cis means on the same side, while trans means on opposite sidesd and refer to (in the case of the butylenes) the relative position of the methyl groups.

3. Molecular weight The molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass units (Drews, 1998; Speight, 2015). This is distinct from the relative molecular mass of a molecule, frequently referred to by the term molecular weight, which is the ratio of the mass of that molecule to 1/12 of the mass of carbon 12 and is a dimensionless number. Thus, it is incorrect to express relative molecular mass (molecular weight) in Daltons (Da) or kilo-Daltons (kDa) (unfortunately, the terms molecular weight and molecular mass have been confused on numerous websites, which often state that molecular weight was used in the past as another term for molecular mass). For a well-defined molecular structure, the molecular weight is calculated from the atomic masses of the constituents. In the case of mixtures, the average molecular weight can be measured by the following methods: vapor pressure osmometry, freezing point depression, boiling point elevation, gel permeation chromatography, and nonfragmenting mass spectrometry (Speight, 2014, 2015). The different methods have advantages and drawbacks, which make them suitable for different molecular weight ranges. Vapor pressure osmometry, freezing point depression, and boiling point elevation are all based on the assumption that the change in the corresponding properties (vapor pressure, freezing point, and boiling point) in a pure solvent caused by introduction of a solute at low concentration is directly proportional to the concentration of the solute. Gel permeation chromatography, also known as size exclusion chromatography, takes advantage of the difference in elution time between molecules with different sizes. Nonfragmenting mass spectrometry principally provides detailed information of the hydrocarbon types, the formulae, and the concentration of all the components in a fraction. Generally, hydrocarbon derivatives of low molecular weight, e.g., methane, ethane, and propane, are gases; those of intermediate molecular weight, e.g., hexane, heptane, and octane, are liquids; and those of high molecular weight, e.g., eicosane (C20H42) and polyethylene, are solids. Paraffin is a mixture of high molecular weight alkane derivatives.

4. Chemical properties By way of recall (Chapter 1), the hydrocarbon family falls into five subgroups: (i) paraffin derivatives, (ii) cycloparaffin derivatives, also called naphthene derivatives, (iii) olefin derivatives, also called alkene derivatives, (iv) acetylene derivatives, also called alkyne derivatives, and (v) aromatic derivatives, also

392 Handbook of Industrial Hydrocarbon Processes

called arene derivatives. The chemical properties of hydrocarbon derivatives describe the potential of hydrocarbon derivatives to undergo chemical change or reaction by virtue of the hydrocarbon structure (Howard and Meylan, 1997; Yaws, 1999). Chemical change results in the hydrocarbon yielding a product that may be entirely different in composition to the starting hydrocarbondthe exception is the isomerization reaction where a straight-chain hydrocarbon is converted to a branched-chain hydrocarbon. In such a case, the composition of the product is not changed over the composition of the starting material but the structure has been changed: CH3CH2CH2CH2CH3 / CH3CH2CH(CH3)CH3 n-pentane, C5H12 isopentane, C5H12 Thus, since a chemical change alters the composition of the original matter, the expected outcome is usually the presence of different elements or compounds at the end of the chemical change. The atoms in compounds are rearranged to make new and different compounds. In the absence of a spark or a high-intensity light source, alkane derivatives are generally inert to chemical reactions. However, it must be recognized that alkane derivatives burst into flame in the presence of a spark. It doesn’t matter whether the starting material is (i) the methane found in natural gas, (ii) the mixture of butane and isobutane used in disposable cigarette lighters, (iii) the mixture of pentane (C5) to hexane (C6) hydrocarbon derivatives in charcoal lighter fluid, or (iv) the complex mixture of hexane (C6) to octane (C8) hydrocarbon derivatives in naphtha or gasoline; once the reaction is ignited by a spark, these hydrocarbon derivatives burn to form carbon dioxide and water and give energy: CH4(g) þ 2O2(g) / CO2(g) þ 2H2O(g) 2C4H10(g) þ 13O2(g) / 8CO2(g) þ 10H2O(g) C5H12(g) þ 8O2(g) / 5 CO2(g) þ 6 H2O(g) 2C8H18(l) þ 25 O2(g) / 16CO2(g) þ 18H2O(g) In the presence of light, or at high temperatures, alkane derivatives react with halogens to form alkyl halides in which the alkyl group is a derivative of the related hydrocarbon (Table 9.1). Reaction with chlorine gives an alkyl chloride and the ease with which chlorine can be introduced into the molecules of all the hydrocarbon types present in petroleum has resulted in the commercial production of a number of widely used compounds (Speight, 2019). With saturated hydrocarbon derivatives the reactions are predominantly substitution of hydrogen by chloride and are strongly exothermic, difficult to control, and inclined to become explosively violent: RH þ Cl2 / RCl þ HCl

Chemical and physical properties of hydrocarbons Chapter | 9

393

TABLE 9.1 General comparison of selected properties of alkane, alkene, and alkyne derivatives. Alkanes

Alkenes

Alkynes

General formula

CHnH2nþ2

CHnH2n

CHnH2n-2

Naming

All the members end with ane

All the members end with ene

All the members end with yne

Physical state

Members having 1 e4 carbon atoms per molecule are gases Members having 5 e17 carbon atoms are liquids Higher members are solids at room temperature

Members having 2e4 carbon atoms per molecule are gases Members having 5e15 carbon atoms per molecule are liquids Higher members are solids

Members having 2 e4 carbon atoms per molecule are gases Members having 5 e13 carbon atoms per molecule are liquids Higher members are solids

Boiling points and melting points

Melting and boiling points increase with increase in molecular mass

Melting point and boiling point increase with the increase in molecular mass

Melting point and boiling point increase with the increase in molecular mass

Combustion

Undergo complete combustion Produce CO2, H2O, and heat

Burn with a sooty flame because of the higher percentage of carbon Produce CO2, H2O, and heat.

Burn with a sooty flame because of the higher percentage of carbon Produce CO2, H2O, and heat.

Moderately high temperatures are used, about 250e300 C (480e570 F), for the thermal chlorination of methane, but as the molecular weight of the paraffin increases the temperature may generally be lowered. A mixture of chlorinated derivatives is always obtained, and many variables, such as choice of catalyst, dilution of inert gases, and presence of other chlorinating agents (antimony pentachloride, sulfuryl chloride, and phosgene), have been tried in an effort to direct the path of the reaction. Methane yields four compounds upon chlorination in the presence of heat or light: CH4 þ Cl2 / CH3Cl, CH2Cl2, CHCl3, CCl4 These compounds, known as chloromethane or methyl chloride, dichloromethane or methylene chloride, trichloromethane or chloroform, and

394 Handbook of Industrial Hydrocarbon Processes

tetrachloromethane or carbon tetrachloride, are used as solvents or in the production of chlorinated materials. The successive substitution of methane hydrogens with chlorine produces a mixture of four chloromethanes: (i) methyl chloride, CH3Cl, also known as monochloromethane, (ii) methylene dichloride, CH2Cl2, also known as dichloromethane, (iii) chloroform, CHC13, also known as trichloromethane, and (iv) carbon tetrachloride, CCl4, also known as tetrachloromethane. Each of these four compounds has many industrial applications. Methane is the most difficult alkane to chlorinate. The reaction is initiated by chlorine free radicals obtained via the application of heat (thermal) or light (hv). Thermal chlorination (more widely used industrially) occurs at approximately 350e370 C (660e700 F) and atmospheric pressure. A typical product distribution for a feedstock (methane-chlorine) ratio of 1.7 v/v is: methyl chloride: 58.7% v/v, methylene dichloride: 29.3% v/v, chloroform: 9.7% v/v, and carbon tetrachloride: 2.3% v/v. The first step in the process involves the breaking of the chlorine-chlorine bond which forms two chlorine (Cll) free radicals after which the chlorine free radical reacts with methane to form a methyl free radical (CH3l) and hydrogen chloride. The methyl radical then reacts in a subsequent step with a chlorine molecule, forming methyl chloride and a chlorine radical: Cll þ CH4 / CH3 þ HCl CH3l þ Cl2 / CH3Cl þ Cll The freshly generated chlorine radical Cl either attacks another methane molecule and repeats the above reaction, or it reacts with a methyl chloride molecule to form a chloromethyl free radical (lCH2Cl, in which the free electron resides on the carbon atom) and hydrogen chloride: Cll þ CH3Cl / lCH2Cl The chloromethyl free radical then attacks another chlorine molecule and produces dichloromethane along with a Cl atom: l

CH2Cl þ Cl2 / CH2Cl2 þ Cll

This formation of chlorine free radicals continues until all chlorine is consumed. Chloroform and carbon tetrachloride are formed in a similar way by reaction of dichloromethyl (lCHC12) and trichloromethyl (lCC13) free radicals with chlorine. Product distribution among the products depends primarily on the mole ratio of the reactants in the feedstock. For example, the yield of methyl chloride (CH3Cl) chloromethane could be increased to 80% v/v by increasing the methane-chlorine mole ratio to 10:1 at 450 C (840 F). If dichloromethane is the desired product, the methane-chlorine mole ratio is lowered and the monochloromethane recycled. Decreasing the methane-chlorine mole ratio

Chemical and physical properties of hydrocarbons Chapter | 9

395

generally increases polysubstitution and, hence, the yield of chloroform and carbon tetrachloride. Similarly, the reaction of methane with bromine gives methyl bromide. CH4(g) þ Br2(l) / CH3Br(g) þ HBr(g) However, the ease of halogenation is influenced by the halogensdfluorine (fluorination) and chlorinedwhich are more electrophilic and are more aggressive halogenating agents. Bromine (bromination) is a weaker halogenating agent than both fluorine and chlorine and iodine (iodination) is the least reactive halogen. The facility of dehydrohalogenation follows the reverse trend: iodine is most easily removed from organic compounds, and organofluorine compounds are highly stable. Alkene derivatives (olefin derivatives, CnH2n) are unsaturated compounds of carbon with hydrogen which contain one or two double bonds between atoms of carbon. They burn to form carbon soot and carbon dioxide and water. They are more reactive than alkane derivatives because of the fact that they contain double bonds. Multiple bonds (double, triple bonds) are energetically less advantageous for atoms than corresponding single bonds. For this reason, the atoms in a compound will attempt to break multiple bonds to form single bonds, which are more advantageous energetically. This explains why compounds which contain double and triple bonds are so much more reactive than those which contain single bonds. The alkene derivatives include ethylene (C2H4), propylene (C3H6), butylene (C4H8), and pentylene (pentene, C5H10). Up to butylene, the alkene derivatives occur as gases. Up to hexadecene (C16H32) they are liquids, with higher alkene derivatives found in the solid state of matter. Hydrocarbon derivatives with double bonds make up the alkene family while hydrocarbon derivatives with triple bonds make up the alkyne family and there are similarities in physical properties (Table 9.1). Open chain alkene derivatives with one double bond have the general formula, CnH2n, where n equals the number of carbon atoms. Open chain alkyne derivatives with one triple bond have the general formula CnH2n-2. Like the alkane derivatives and other hydrocarbon derivatives, they are insoluble in water and are flammable. The most familiar alkene derivatives are ethylene and propylene. Ethyne (acetylene) is an important alkyne. Unsaturated hydrocarbon derivatives such as alkene derivatives and alkyne derivatives are much more reactive than the parent alkane derivatives. They react rapidly with bromine, for example, to add a Br2 molecule across the C] C double bond.

396 Handbook of Industrial Hydrocarbon Processes

This reaction provides a way to test for alkene derivatives or alkyne derivatives. Solutions of bromine in carbon tetrachloride have an intense redorange color. When bromine in carbon tetrachloride is mixed with a sample of an alkane, no change is initially observed. When it is mixed with an alkene or alkyne, the color of bromine rapidly disappears. The reaction between 2-butene and bromine to form 2,3-dibromobutane is just one example of the addition reactions of alkene derivatives and alkyne derivatives. Hydrogen bromide (HBr) adds across a C]C double bond to form the corresponding alkyl bromide, in which the hydrogen ends up on the carbon atom that had more hydrogen atoms to begin with. Addition of HBr to 2-butene, for example, gives 2-bromobutane.

H2 adds across double (or triple bonds) in the presence of a suitable catalyst to convert an alkene (or alkyne) to the corresponding alkane.

In the presence of an acid catalyst, it is even possible to add a molecule of water across a C]C double bond.

Addition reactions provide a way to add new substituents to a hydrocarbon chain and thereby produce new derivatives of the parent alkane derivatives. In theory, two products can form when an unsymmetrical reagent such as HBr is added to an unsymmetrical C]C double bond. In practice, only one product is obtained. When HBr is added to 2-methylpropene, for example, the product is 2-bromo-2-methylpropane, not 1-bromo-2-methylpropane.

In 1870, after careful study of many examples of addition reactions, the Russian chemist Vladimir Markovnikov formulated a rule for predicting the product of these reactions. The Markovnikov rule states that the hydrogen atom adds to the carbon atom that already has the larger number of hydrogen

Chemical and physical properties of hydrocarbons Chapter | 9

397

atoms when HX adds to an alkene. Thus, water (HeOH) adds to propene to form the product in which the OH group is on the middle carbon atom.

Alkyne derivatives (acetylene derivatives, CnH2n-2) are unsaturated hydrocarbon derivatives which contain one or more triple bonds between atoms of carbon. When they burn, they tend to form carbon soot. When oxygen is present during burning, high temperatures can be reached. The simplest (lowest molecular weight) alkyne derivatives are acetylene (C2H2, HC^CH), propyne (C3H4, CH3C^CH) and butyne (C4H6, CH3C^CCH3 or CH3CH2C^CH). Cycloalkane derivatives (cyclic alkane derivatives) are differentiated from aliphatic hydrocarbon derivatives insofar as they contain a ring structure and form a homologous group of compounds. The first member of the series is cyclopentane followed by cyclohexane. Cycloalkane derivatives are saturated compounds and, like linear alkane derivatives, are not very reactive. Aromatic hydrocarbon derivatives are derived from benzene. Group members have six free valence electrons which are distributed in a circle in the form of a charged cloud. Because of the presence of these valence electrons, we can predict that the reactivity of these aromatic compounds will be similar to other unsaturated hydrocarbon derivatives. However, benzene is much less reactive than other unsaturated hydrocarbon derivatives. Only at high temperatures and in the presence of a catalyst can benzene take on another hydrogen atom. When it does, cyclohexane is the resultant product.

5. Physical properties Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter (Howard and Meylan, 1997; Yaws, 1999). The three states of matter are: solid, liquid, and gas. The melting point and boiling point are related to changes of the state of matter. All matter may exist in any of three physical states of matter. A physical change takes place without any changes in molecular composition. The same element or compound is present before and after the change. The same molecule is present throughout the changes. Physical changes are related to physical properties since some changes require a change in the three-dimensional structure of the molecule.

398 Handbook of Industrial Hydrocarbon Processes

Physical properties that are of interest in the current context include: (i) boiling point, (ii) density and specific gravity, (iii) dew point, (iv) flash point and ignition temperature, (v) melting point, and (vi) vapor density. These properties are listed in alphabetical order rather than attempt to assign importance to any individual property. The properties present indications the behavior of hydrocarbon derives as determined by application of standard test method (Speight, 2015). The physical properties of alkene derivatives are similar to those of the alkane derivatives. The boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. For molecules with the same number of carbon atoms and the same general shape, the boiling points usually differ only slightly, just as would be expected for chemicals in which whose molar mass differs by only one to two hydrogen atoms (i.e., RCH2CH]CH2 compared to RCH2CH2CH3). Like other hydrocarbons, the alkene derivatives are insoluble in water but soluble in organic solvents.

5.1 Boiling point The boiling point of an organic compound is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid. Thus, the normal boiling point of a pure hydrocarbon is the temperature

TABLE 9.2 Selected properties of lower molecular weight alkane derivatives.

Name

Molecular formula

Structural formula

Boiling point ( C)

Melting point ( C)

Density (g/ml, 20 C)

Methane

CH4

CH4

161.5

182.5

Ethane

C2H6

CH3CH3

88.6

183.3

Propane

C3H8

CH3CH2CH3

42.1

189.7

Butane

C4H10

CH3(CH2)2CH3

0.5

138.4

Pentane

C5H12

CH3(CH2)3CH3

36.1

129.7

0.626

Hexane

C6H14

CH3(CH2)4CH3

68.7

95.3

0.659

Heptane

C7H16

CH3(CH2)5CH3

98.4

90.6

0.684

Octane

C8H18

CH3(CH2)6CH3

125.7

56.8

0.703

Nonane

C9H20

CH3(CH2)7CH3

150.8

53.5

0.718

Decane

C10H22

CH3(CH2)8CH3

174.1

29.7

0.730

Chemical and physical properties of hydrocarbons Chapter | 9

399

TABLE 9.3 Common alkyl groupsa. Group

Carbon atoms

Formula

Methyl

1

CH 3

Ethyl

2

CH3CH 2

Propyl

3

CH3CH2CH 2

Butyl

4

CH3CH2CH2CH 2

Pentyl

5

CH3CH2CH2CH2CH 2

Hexyl

6

CH3CH2CH2CH2CH2CH 2

Heptyl

7

CH3CH2CH2CH2CH2CH2CH 2

Octyl

8

CH3CH2CH2 CH2CH2CH2CH2CH 2

Nonyl

9

CH3CH2CH2CHCH2CH2CH2CH2CH 2

Decyl

10

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH 2

a

For convenience, only the n-isomers are shown.

at which the substance changes state from liquid to vapor at a pressure of 1 atm (14.7 psi) (Table 9.2) from which the common alkyl groups are derived (Table 9.3). This property is known and tabulated for all the pure components normally considered in petroleum engineering except for the high boiling substances that do not have a boiling point at atmospheric pressure. For the distillation fractions of a hydrocarbon fuel, the normal boiling point is usually chosen to be a certain average value of the temperature interval of the distillation fraction. The boiling points of organic compounds can give important clues to other physical properties and structural characteristics. A liquid boils when its vapor pressure is equal to the atmospheric pressure. Vapor pressure is determined by the kinetic energy of molecules. Kinetic energy is related to temperature and the mass and velocity of the molecules (K.E. ¼ 1/2 mv2). When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state. Vapor pressure is caused by equilibrium between molecules in the gaseous state and molecules in the liquid state. When molecules in the liquid state have sufficient kinetic energy, they may escape from the surface and turn into a gas. Molecules with the most independence in individual motions achieve sufficient kinetic energy (velocities) to escape as gases at lower temperatures. The vapor pressure will be higher (more gas molecules are present) and therefore the compound will boil at a lower temperature.

400 Handbook of Industrial Hydrocarbon Processes

In each homologous series of hydrocarbon derivatives, the boiling points increase with molecular weight and structure also has a marked influence since it is a general rule that branched paraffin isomers have lower boiling points than the corresponding n-alkane. In any given series, steric effects notwithstanding, there is an increase in boiling point with an increase in carbon number of the alkyl side chain. This particularly applies to alkyl aromatic compounds where alkyl-substituted aromatic compounds can have higher boiling points than polycondensed aromatic systems. The boiling points of hydrocarbon fuels are rarely, if ever, distinct temperatures; it is, in fact, more correct to refer to the boiling ranges of the various fuels. To determine these ranges, the crude oil is tested in various methods of distillation, either at atmospheric pressure or at reduced pressure. In general, the limiting molecular weight range for distillation at atmospheric pressure without thermal degradation is 200e250, whereas the limiting molecular weight range for conventional vacuum distillation is 500e600. Alkane derivatives experience intermolecular van der Waals forces. Stronger intermolecular van der Waals forces give rise to greater boiling points of alkane derivatives. There are two determinants for the strength of the van der Waals forces: (i) the number of electrons surrounding the molecule, which increases with the molecular weight of the alkane, and (ii) the surface area of the molecule. Under standard conditions (STP), alkane derivatives from methane (CH4) to butane (C4H10) are gaseous; from pentane (C5H12) to C17H36 they are liquids; and after C18H38 they are solids. As the boiling point of alkane derivatives is primarily determined by weight, it should not be a surprise that the boiling point has almost a linear relationship with the size (molecular weight) of the molecule. As a general rule, the boiling point rises 20e30 C for each carbon added to the chain; this rule applies to other homologous series. A straight-chain alkane will have a boiling point higher than a branchedchain alkane due to the greater surface area in contact, which results in greater van der Waals forces between adjacent molecules. For example, compare iso-butane (2-methylpropane) and n-butane (butane), which boil at 12 and 0 C, respectively, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 C (122 F) and 58 C (136 F), respectively. For the latter case, two molecules of 2,3-dimethylbutane can associate with each other better than the cross-shaped 2,2-dimethylbutane, hence the greater van der Waals forces. On the other hand, cycloalkane derivatives tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact. The boiling points of alkene derivatives, like the boiling points of the alkane derivatives, increase with molecular weight (Table 9.4). Branchedchain alkene derivatives have lower boiling points than the corresponding straight-chain alkene derivatives. However, the boiling point of each alkene is

Chemical and physical properties of hydrocarbons Chapter | 9

401

TABLE 9.4 Boiling points of alkene derivatives. Alkene

Boiling point ( C)

Ethylene

104

Propylene

47

Trans-2-butene

0.9

Cis-2-butene

3.7

1-Pentene

30

Trans-2-pentene

36

Cis-2-pentene

37

1-Heptene

115

3-Octene

122

3-Nonene

147

5-Decene

170

very similar to that of the alkane with the same number of carbon atoms (Table 9.2, Table 9.4). Ethylene, propylene, and the various butenes are gases at room temperature. The higher boiling alkene derivatives are liquids. In each case, the alkene has a boiling point which is slightly lower than the boiling point of the corresponding alkane. The only attractions involved are van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has two fewer electrons than the alkane with the same number of carbons. The melting points of alkene derivatives also increase with molecular weight (Table 9.5). Generally, alkene derivatives have similar melting points to that of corresponding alkane derivatives. However, melting points of alkene derivatives depend on the packaging of the moleculesdcis isomers are packaged in a U-bending shape and, therefore, display lower melting points compared to their respective trans isomers. In keeping with the general trend of alkane derivatives and alkene derivatives (and to no surprise), the boiling points and melting points of alkyne derivatives increase as the number of carbon atoms (i.e., molecular weight) increases (Table 9.6). However, alkyne derivatives have higher boiling points than alkane derivatives or alkene derivatives, with its increased number of weakly held P electrons because of the triple bond. Because of these weakly held electrons, the electric field is more easily distorted, producing stronger attractive forces between molecules. This holds the molecules together at higher temps preventing vaporization.

402 Handbook of Industrial Hydrocarbon Processes

TABLE 9.5 Melting points of alkene derivatives. Alkene

Melting point ( C)

Ethene

169

Propene

185

Butene

138

1-Pentene

165

Trans-2-pentene

135

Cis-2-pentene

180

1-Heptene

119

3-Octene

101.9

3-Nonene

81.4

5-Decene

66.3

TABLE 9.6 Physical properties of alkyne derivatives.

Name

Formula

Acetylene

HC^CH

Propyne

Melting point ( C)

Boiling point ( C)

Density (20 C)

82

75

HC^CCH3

101.5

23

1-Butyne

HC^CCH2CH3

122

91

2-Butyne

CH3C^CCH3

24

27

0.694

1-Pentyne

HC^C(CH2)2CH3

98

40

0.695

2-Pentyne

CH3C^CCH2CH3

101

55

0.714

1-Hexyne

HC^C(CH2)3CH3

124

72

0.719

1-Heptyne

HC^C(CH2)4CH3

80

100

0.733

1-Octyne

HC^C(CH2)5CH3

70

126

0.747

1-Nonyne

HC^C(CH2)6CH3

65

151

0.763

1-Decyne

HC^C(CH2)7CH3

35

182

0.770

Chemical and physical properties of hydrocarbons Chapter | 9

403

Cycloalkane derivatives are similar to alkane derivatives in their general physical properties, but they have higher boiling points, melting points, and densities than alkane derivatives. This is due to stronger London forces because the ring shape allows for a larger area of contact. Containing only carbon-carbon (CeC) and carbon-hydrogen (CeH) bonds, unreactivity of cycloalkane derivatives with little or no ring strain (see below) are comparable to noncyclic alkane derivatives. The London dispersion force is the weakest intermolecular force. The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. This force is sometimes called an induced dipole-induced dipole attraction. London forces are the attractive forces that cause nonpolar substances to condense to liquids and freeze into solids when the temperature is lowered sufficiently. Because of the constant motion of the electrons, an atom or molecule can develop a temporary (instantaneous) dipole when its electrons are distributed unsymmetrically about the nucleus. A second atom or molecule, in turn, can be distorted by the appearance of the dipole in the first atom or molecule (because electrons repel one another) which leads to an electrostatic attraction between the two atoms or the two molecules.

5.2 Cloud point and pour point The relationship of cloud point, pour point, melting point, and freezing point to one another varies widely from one hydrocarbon fuel to another, depending upon the composition of the fuel in terms of the hydrocarbon derivative present in the fuel. Hence, the significance of the data also varies for the different types of product. In general, cloud, melting, and freezing points are of more limited value and each is of narrower range of application than the pour point. The cloud point of hydrocarbon fuel is the temperature at which paraffin wax or other solidifiable compounds present in the oil appear as a haze when the sample is chilled under definitely prescribed conditions (ASTM D2500; ASTM D6751). To determine the cloud point and the pour point (ASTM D97, ASTM D512, ASTM D1835, ASTM D524, ASTM D5501, ASTM D975) the oil is contained in a glass test tube fitted with a thermometer and immersed in one of three baths containing coolants. The sample is dehydrated and filtered at a temperature higher than the above anticipated cloud point. It is then placed in a test tube and cooled progressively in coolants held at 1 to þ2 C (30e35 F), 18 to 20 C (4 to 0 F) and 32 to 35 C (26 to 31 F), respectively. The sample is inspected for cloudiness at temperature intervals of 1 C (2 F). If conditions or oil properties are such that reduced temperatures are required to determine the pour point, alternate tests are available that accommodate the various types of samples. Related to the cloud point, the wax appearance temperature or wax appearance point is also determined (ASTM D6751).

404 Handbook of Industrial Hydrocarbon Processes

The pour point of crude oil hydrocarbon derivatives or a crude oil product is determined using this same technique (ASTM D97), and it is the lowest temperature at which the oil pours or flows. It is actually 2 C (3 F) above the temperature at which the oil ceases to flow under these definitely prescribed conditions when it is chilled without disturbance at a standard rate. To determine the pour point, the sample is first heated to 46 C (115 F) and cooled in air to 32 C (90 F) before the tube is immersed in the same series of coolants as used for the determination of the cloud point. The sample is inspected at temperature intervals of 2 C (3 F) by withdrawal and holding horizontal for 5 s until no flow is observed during this time interval. Cloud and pour points are useful for predicting the temperature at which the observed viscosity of oil deviates from the true (Newtonian) viscosity in the low-temperature range. They are also useful for identification of oils or when planning the storage of oil supplies, as low temperatures may cause handling difficulties with some oils. The pour point of a crude oil was originally applied to crude oil that had high wax content. More recently, the pour point, like the viscosity, is determined principally for use in pumping arid pipeline design calculations. Difficulty occurs in these determinations with waxy crude oils that begin to exhibit irregular flow behavior when wax begins to separate. These crude oils possess viscosity relationships that are difficult to predict in pipeline operation. In addition, some waxy crude oils are sensitive to heat treatment that can also affect their viscosity characteristics. This complex behavior limits the value of viscosity and pour point tests on waxy crude oils. At the present time, multimile crude oil pipelines and the increasing production of waxy crude oils make it necessary to make an assessment of the pumpability of a waxcontaining crude oil through a given system because it may be a matter of some difficulty that can often only be resolved after field trials. Alkene derivatives contain a carbon-carbon double bond, which affects the physical properties of alkene derivatives relative to the physical properties of alkane derivatives. At room temperature, alkene derivatives exist in all three phases, solid, liquids, and gases. Melting and boiling points of alkene derivatives are similar to that of alkane derivatives but isomers of the cis-alkene derivatives have lower melting points than that of trans-isomers. Alkene derivatives display a weak dipole-dipole interaction due to the electron-attracting sp2 carbon. The physical properties of alkene derivatives are comparable with those of alkane derivatives but the physical state does depend on the molecular weight. The lower molecular weight alkene derivatives (ethylene, propylene, and butylene) are gases while linear alkene derivatives of approximately 5 to 16 carbons are liquids, and higher alkene derivatives are waxy solids. Cis-isomers and trans-isomers often have different physical properties. Differences between isomers, in general, arise from the differences in the shape of the molecule or the overall dipole moment of the molecule. This difference can be small as in the case of the boiling point of straight-chain

Chemical and physical properties of hydrocarbons Chapter | 9

405

alkene derivatives, such as 2-pentene which is 37 C (98 F), in the cis-isomer and 36 C (96 F) in the trans-isomer.

5.3 Density and specific gravity Density is the mass of a unit volume of material at a specified temperature and has the dimensions of grams per cubic centimeter (a close approximation to grams per milliliter). Specific gravity (also called the relative density) is the ratio of the mass of a volume of the substance to the mass of the same volume of water and is dependent on two temperatures, those at which the masses of the sample and the water are measured. When the water temperature is 4 C (39 F), the specific gravity is equal to the density in the centimeter-gramsecond (cgs) system, since the volume of 1 g of water at that temperature is, by definition, 1 mL. Thus the density of water, for example, varies with temperature, and its specific gravity at equal temperatures is always unity. The standard temperatures for a specific gravity in the crude oil industry in North America are 60/60 F (15.6/15.6 C). Specific gravity usually meant to imply the relative density of the substance with respect to the density of water. If the specific gravity of a hydrocarbon is less than one then it is less dense than water (Table 9.2), or the reference chemical. Conversely, if the specific density if greater than one, it is denser than the reference. If the relative density is exactly one then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water then a substance with a relative density (or specific gravity) less than one will float in water. Temperature and pressure must be specified for both the sample and the reference pressure is typically 14.7 psi (1 atm) but, for correctness, the pressure should be stated. Where it is not it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice the specific gravity as specified above is multiplied by 1000. Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brine, sugar solutions, and acids. The density and specific gravity of crude oil and hydrocarbon fuels (ASTM D70; ASTM D71; ASTM D287; ASTM D1217; ASTM D1298; ASTM D1480; ASTM D1481; ASTM D1555; ASTM D1657; ASTM D4052) are two properties that have found wide use in the industry for preliminary assessment of the character and quality of crude oil. In the early years of the crude oil industry, and hydrocarbon properties, density was the principal specification for crude oil and refinery products; it was used to give an estimation of the gasoline and, more particularly, the kerosene present in the crude oil. However, the derived relationships between the density of crude oil and its fractional composition were valid only if they were applied to a certain type of crude oil and lost some of their significance

406 Handbook of Industrial Hydrocarbon Processes

when applied to different types of crude oil. Nevertheless, density is still used to give a rough estimation of the nature of crude oil and crude oil products. Although density and specific gravity are used extensively, the API (American Petroleum Institute) gravity is the preferred property. This property was derived from the Baume´ scale: Degrees Baume´ ¼ (140/sp gr @ 60/60oF) - 130 However, a considerable number of hydrometers calibrated according to the Baum scale were found at an early period to be in error by a consistent amount, and this led to the adoption of the equation: Degrees API ¼ (141.5/sp gr @ 60/60oF) - 131.5 The specific gravity of crude oil usually ranges from approximately 0.8 (45.3 API) for the lighter crude oils to over 1.0 (less than 10 API) for heavy crude oil and bitumen. Specific gravity is influenced by the chemical composition of crude oil, but quantitative correlation is difficult to establish. Nevertheless, it is generally recognized that increased amounts of aromatic compounds result in an increase in density, whereas an increase in saturated compounds results in a decrease in density. Indeed, it is also possible to recognize certain preferred trends between the density of crude oil and one or other of the physical properties. For example, an approximate correlation exists between the density (API gravity) and sulfur content, Conradson carbon residue, viscosity, and nitrogen content (Speight, 2017). Density, specific gravity, or API gravity may be measured by means of a hydrometer (ASTM D287; ASTM D1298; ASTM D1657, IP 160), a pycnometer (ASTM D70, ASTM D1217, ASTM D1480, and ASTM D1481), or by means of a digital density meter (ASTM D4052) and a digital density analyzer (ASTM D5002). The pycnometer method (ASTM D70; ASTM D1217; ASTM D1480; ASTM D1481) for determining density is reliable, precise, and requires relatively small test samples. However, because of the time required, other methods such as using the hydrometer (ASTM D1298, ASTM D1298), the density meter (ASTM D4052), and the digital density analyzer (ASTM D5002) are often preferred. However, surface tension effects can affect the displacement method and the density meter method loses some of its advantage when measuring the density of heavy oil and bitumen. The pycnometer method (ASTM D70, ASTM D1217, ASTM D1480, and ASTM D1481) is routinely used to measure the density of samples being charged to a distillation flask, where volume charge is needed, but the volume is not conveniently measured. The volume may be found weighing the sample, and determining the sample density. It is also used in routine measurements of material properties. It is worthy of note that even a small amount of solids in the sample will influence its measured density. For example, 1% by weight solids in the sample can raise the density by 0.007 g/cm3.

Chemical and physical properties of hydrocarbons Chapter | 9

407

The densimeter method (ASTM D4052) uses an instrument that measures the total mass of a tube by determining its natural frequency of vibration. This frequency is a function of the dimensions and the elastic properties of the tube, and the weight of the tube and contents. Calibration with water and air provides data for the determination of the instrument constraints which allow conversion of the natural frequency of vibration to sample density. The variation of density with temperature, effectively the coefficient of expansion, is a property of great technical importance, since most crude oil products are sold by volume and specific gravity is usually determined at the prevailing temperature (21 C, 70 F) rather than at the standard temperature (15.6 C, 60 F). The tables of gravity corrections (ASTM D1555) are based on an assumption that the coefficient of expansion of all crude oil products is a function (at fixed temperatures) of density only. However, not all of these methods are suitable for measuring the density (or specific gravity) of heavy oil and bitumen although some methods lend themselves to adaptation. The API gravity of a feedstock (ASTM D287) is calculated directly from the specific gravity. The specific gravity of bitumen shows a fairly wide range of variation. The largest degree of variation is usually due to local conditions that affect material close to the faces, or exposures, occurring in surface oil sand beds. There are also variations in the specific gravity of the bitumen found in beds that have not been exposed to weathering or other external factors. The range of specific gravity usually varies over the range of the order of 0.995e1.04. A very important property of the Athabasca bitumen (which also accounts for the success of the hot water separation process) is the variation in density (specific gravity) of the bitumen with temperature. Over the temperature range 30e130 C (85e265 F) the bitumen is lighter than water. Flotation of the bitumen (with aeration) on the water is facilitated, hence the logic of the hot water separation process (Speight, 2014). The density of the alkane derivatives usually increases with increasing number of carbon atoms, but remains less than that of water (Table 9.6). Hence, alkanes form the upper layer in an alkane-water mixture. The density of the alkene derivatives is higher than the density of the corresponding alkane derivatives. Again, density usually increases with increasing number of carbon atoms, but all alkene derivatives have the density smaller than 1. The density of the cycloalkane derivatives is higher than the density of the corresponding alkane derivatives (Table 9.7).

5.4 Dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). The measurement of the dew point is related to humiditydfor example, a higher dew point means there will be

408 Handbook of Industrial Hydrocarbon Processes

TABLE 9.7 Physical properties of cycloalkane derivatives compared to the properties of the corresponding (matching number of carbon atoms) alkane derivatives. Hydrocarbon

Melting point,  C

Boiling point,  C

Cyclopropane

127

33

Propane

167

42

80

13

138

0

94

49

0.746

130

36

0.626

81

0.778

Cyclobutane Butane Cyclopentane Pentane Cyclohexane

6.5

Density, @20 C

Hexane

95

69

0.659

Cycloheptane

12

118

0.810

Heptane

90

98

0.684

14

149

0.830

57

126

0.703

Cyclooctane Octane

more moisture in the air. In addtion, when air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. When the temperature is below the freezing point of water, the dew point is called the frost point because frost is formed rather than dew. Briefly, the frost point is similar to the dew point in that it is the temperature to which a given amount of humid air must be cooled, at constant atmospheric pressure (14.7 psi) for water vapor to be deposited on a surface as ice crystals without passing through an intermediate liquid phase. The frost point for a given amount of air is always higher than the dew point, as the stronger bonding between water molecules on the surface of ice requires higher temperature to break. On the other hand, as a reversal of the frost point, sublimation is the transition of a substance directly from the solid phase to the gas phase without passing through the intermediate liquid phase. Sublimation is an endothermic process that occurs at temperatures and pressures below the triple point of a substance in the phase diagram which corresponds to the lowest pressure at which the substance can exist as a liquid. Sublimation has also been used as a generic term to describe a solid-to-gas transition (sublimation) followed by a gas-to-solid transition (deposition). While a transition from liquid to gas is described as evaporation if it occurs below the boiling point of the liquid, and as boiling if it occurs at the boiling point, there is no

Chemical and physical properties of hydrocarbons Chapter | 9

409

such distinction within the solid-to-gas transition, which is always described as sublimation. The hydrocarbon dew point (HDP) is a function of the composition of the gas mixture and is strongly influenced by the concentration of the higher molecular weight hydrocarbon derivatives, especially C6þ. The presence of higher molecular weight hydrocarbon derivatives will increase the HDP and failure to include them in a hydrocarbon dew point calculation will underpredict the hydrocarbon dew point. For most pipeline conditions, the HDP temperature at a given pressure increases as the concentration of heavier hydrocarbon derivatives increases. Thus, the potential to form liquids at certain pipeline conditions exists for gases rich in C6þ. Processing of the gas stream primarily removes or extracts higher molecular weight hydrocarbon derivatives and thus reduces the HDP of a given mixture. The level of hydrocarbon removal directly impacts the HDP. The HDP curve is plotted as a function of gas pressure (P) and temperature (T) (Fig. 9.2). The left-hand side of the curve is the bubble point line and divides the single-phase liquid region from the two-phase gas-liquid region. The right-hand side of the curve is the dew point line and divides the twophase gas-liquid region and the single-phase gas region. The bubble point and dew point lines intersect at the critical point, where the distinction between gas and liquid properties disappears. Note that two dew point temperatures are possible at a given pressure (P3) and two dew point pressures are possible at a given temperature (T3). This phase envelope phenomenon provides for behavior known as retrograde condensation. The retrograde phenomenon occurs when liquids form at a given temperature when the pressure is lowered (see red arrow).

FIGURE 9.1 Melting points (lower line) and boiling points (upper line) of the C1eC14 alkane derivatives.

410 Handbook of Industrial Hydrocarbon Processes

FIGURE 9.2 Hydrocarbon dew point curve for natural gas.

The word retrograde indicates a phenomenon that is contradictory to the phase behavior of pure components, which condense with increasing pressure and or decreasing temperature. The maximum pressure at which phase change occurs (Pmax) is called the cricondenbar, and the maximum temperature (Tmax) at which phase change l occurs from is called the cricondentherm. For unprocessed and processed gas mixtures there are variations in the HDP (Fig. 9.3). The unprocessed HDP curve has a higher cricondentherm temperature than the processed HDP curve and illustrates the impact of

FIGURE 9.3 Contrast between unprocessed (black line) and processed (gray line) natural gas.

Chemical and physical properties of hydrocarbons Chapter | 9

411

processing on the HDP. The significance of the HDP curve for gas transmission and distribution operations lies in the potential transition from the single-phase gas region to the two-phase gas-liquid region.

5.5 Flash point and ignition temperature The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air (Table 9.8). At the flash point, the vapor may cease to burn when the source of ignition is removed. The flash point is often used as a descriptive characteristic of liquid fuel, and it is also used to describe liquids that are not normally used as fuels but are flammable liquids and/or combustible liquids. There are various international standards for defining each, but most agree that liquids with a flash point less than 43 C (109 F) are flammable, while those having a flash point above this temperature are combustible. The fire point is a slightly higher temperature and is the temperature at which the vapor continues to burn after being ignited. Neither the flash point

TABLE 9.8 Flash points, autoignition temperature, and flammability limits for various hydrocarbons. Flammable limits Upper

Lower

Hydrocarbon

Flash point  C

Autoignition temperature  C

Methane

188

630

5.0

15.0

Ethane

135

515

3.0

12.4

Propane

104

450

2.1

9.5

n-Butane

74

370

1.8

8.4

n-Pentane

49

260

1.4

7.8

n-Hexane

23

225

1.2

7.4

n-Heptane

3

225

1.1

6.7

n-Octane

14

220

0.95

6.5

n-Nonane

31

205

0.85

n-Decane

46

210

0.75

n-Dodecane

74

204

0.60

-

n-Tetradecane

99

200

0.50

-

(vol % at 25 C)

5.6

412 Handbook of Industrial Hydrocarbon Processes

nor the fire point is related to the temperature of the ignition source or of the burning liquid, which are much higher. The flash point is not to be confused with the autoignition temperature, which does not require an ignition source. The ignition temperature is the minimum temperature to which a substance must be heated before it will spontaneously burn independently of the source of heat. The autoignition temperature (kindling point) of a substance (Table 9.8) is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical will ignite decreases as the pressure increases or as oxygen concentration increases. It is usually applied to a combustible fuel mixture. Autoignition temperatures of liquid chemicals are typically measured using a 500 mL flask placed in a temperature controlled oven in accordance with a standard test procedure (ASTM E659). The flash point of a volatile liquid is the lowest temperature at which the liquid can vaporize to form an ignitable mixture in air. Measuring the flash point of a liquid requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed. The flash point is not to be confused with the autoignition temperature, which does not require an ignition source. The fire point, a slightly higher temperature than the flash point, is the temperature at which the vapor continues to burn after being ignited. Neither the flash point nor the fire point is related to the temperature of the ignition source or of the burning liquid, which are much higher. The flash point is often used as a descriptive characteristic of hydrocarbon derivatives (Table 9.8) and hydrocarbon fuels (Table 9.9) and it is also used to describe other liquids, including those that are not normally used as fuels. Flash point refers to both flammable liquids and combustible liquids. There are various international standards for defining each, but most agree that liquids with a flash point less than 43 C (109 F) are flammable, while those having a flash point above this temperature are combustible. There are two basic types of flash point measurement: open cup and closed cup. The best known example of the open cup method is the Cleveland Open

TABLE 9.9 Examples of the flash points of hydrocarbon fuels. Fuel

Flash point 

Autoignition temperature

Gasoline



<40 C (40 F)

246 C (475 F)

Diesel fuel

>62 C (143 F)

210 C (410 F)

Jet fuel

>60 C (140 F)

210 C (410 F)

Kerosene

>38 C e72 C (100 F e162 F)

220 C (428 F)

Chemical and physical properties of hydrocarbons Chapter | 9

413

Cup (COC). In the open-cup method, the sample is contained in an open cup which is heated, and at intervals a flame is brought over the surface. The measured flash point will actually vary with the height of the flame above the liquid surface, and at sufficient height the measured flash point temperature will coincide with the fire point. There are two types of closed cup testers: nonequilibrium, such as PenskyMartens where the vapors above the liquid are not in temperature equilibrium with the liquid, and equilibrium where the vapors are deemed to be in temperature equilibrium with the liquid. In both these types the cups are sealed with a lid through which the ignition source can be introduced. Closed cup testers normally give lower values for the flash point than open cup (typically 5e10 C) and are a better approximation to the temperature at which the vapor pressure reaches the lower flammability limit (LFL). The flash point is an empirical measurement rather than a fundamental physical parameter. The measured value will vary with equipment and test protocol variations, including temperature ramp rate (in automated testers), time allowed for the sample to equilibrate, sample volume, and whether the sample is stirred. Methods for determining the flash point of a liquid are specified in many standards. For example, testing by the Pensky-Martens closed cup method is detailed in ASTM D93. Determination of flash point by an alternate closed cup method is detailed in ASTM D3828, ASTM D3278. Gasoline is designed for use in an engine which is driven by a spark and the fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature (Table 9.8). Diesel fuel flash points vary between 52 and 96 C (126e204 F). Diesel is designed for use in a high compression engine in which air is compressed until it has been heated above the autoignition temperature of the fuel. The diesel fuel is then injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source and, therefore, diesel is required to have a high flash point and a low autoignition temperature (Table 9.9). Gasoline is designed for use in a spark ignition engine and the hydrocarbon fuel is premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The hydrocarbon fuel should not preignite in the hot engine and, therefore, gasoline is required to have a low flash point and a high autoignition temperature. The flash point of diesel fuel varies between 52 and 96 C (126e204 F). Diesel is designed for use in a high compression engine in which air is compressed until it has been heated above the autoignition temperature of the diesel fuel. Then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source and, therefore, diesel is required to have a high flash point and a low autoignition temperature.

414 Handbook of Industrial Hydrocarbon Processes

The flash point of jet fuel also varies considerably. Both Jet A and Jet A-1 have flash points between 38 and 66 C (100e150 F).

5.6 Melting point The melting point of a solid is the temperature at which the vapor pressure of the solid and the liquid are equal. At the melting point, the solid and liquid phases exist in equilibrium. The melting point (sometimes referred to as the liquefaction point) is the point where a chemical changes from the solid state to the liquid state and at the melting point the solid and liquid phases exist in equilibrium. The melting point of a substance depends on the pressure and is usually specified at a standard pressure such as 1 atm (14.7 psi). When the change of phase is reversed (that is from the liquid phase to the solid phase), the temperature at which this phase change occurs is referred to as the freezing point or the crystallization point. Because of the ability of some substances to supercool, the freezing point is not always considered to be a characteristic property of a substance. The melting points of the alkane derivatives follow a similar trend to boiling points of alkane derivatives (Table 9.2, Fig. 9.1) for the same reason as outlined above. That is (all other things being equal), the larger the molecule the higher the melting point. There is one significant difference between boiling points and melting points. Solids have more rigid and fixed structure than liquids. This rigid structure requires energy to break down. Thus the stronger, better put together solid structures will require more energy to break apart. For alkane derivatives, the odd-numbered alkane derivatives have a lower trend in melting points than even-numbered alkane derivatives (Fig. 9.1). Even-numbered alkane derivatives pack well in the solid phase, forming a well-organized structure, which requires more energy to break apart. The odd-number alkane derivatives pack less well and so the looser organized solid packing structure requires less energy to break apart. The melting points of branched-chain alkane derivatives can be either higher or lower than those of the corresponding straight-chain alkane derivatives, again depending on the ability of the alkane in question to packing well in the solid phase. This is particularly true for iso-alkane derivatives (2methyl isomers), which often have melting points higher than those of the linear analogs. Hydrocarbon derivatives that are liquids at ambient temperature, and problems that may arise from solidification during normal use are not common. Nevertheless, the melting point is a test (ASTM D87 and ASTM D127) that is widely used by suppliers of wax and wax consumers; it is particularly applied to the highly paraffinic or crystalline waxes. Quantitative prediction of the melting point of pure hydrocarbon derivatives is difficult, but the melting point tends to increase qualitatively with the molecular weight and symmetry of the molecule.

Chemical and physical properties of hydrocarbons Chapter | 9

415

Unsubstituted and symmetrically substituted compounds (e.g., benzene, cyclohexane, p-xylene, and naphthalene) melt at higher temperatures relative to the paraffin compounds of similar molecular weight: the unsymmetrical isomers generally melt at lower temperatures than the aliphatic hydrocarbon derivatives of the same molecular weight. Unsaturation affects the melting point principally by its alteration of symmetry; thus ethane (172 C, 278 F) and ethylene (169.5 C, 273 F) differ only slightly, but the melting points of cyclohexane (6.2 C, 21 F) and cyclohexene (104 C, 155 F) contrast strongly. All types of highly unsymmetrical hydrocarbon derivatives are difficult to crystallize; asymmetrically branched aliphatic hydrocarbon derivatives as low as octane and most substituted cyclic hydrocarbon derivatives comprise the greater part of the lubricating fractions of crude oil, crystallize slowly, if at all, and on cooling merely take the form of glasslike solids. Although the melting points of crude oil and crude oil products are of limited usefulness, except to estimate the purity or perhaps the composition of waxes, the reverse process, solidification, has received attention in crude oil chemistry. In fact, solidification of crude oil and crude oil products has been differentiated into four categories, namely, freezing point, congealing point, cloud point, and pour point. Crude oil becomes more or less a plastic solid when cooled to sufficiently low temperatures. This is due to the congealing of the various hydrocarbon derivatives that constitute the oil. The cloud point of a crude oil or a crude oil product is the temperature at which paraffin wax or other solidifiable compounds present in the oil appear as a haze when the oil is chilled under definitely prescribed conditions (ASTM D2500; ASTM D6751). As cooling is continued, all crude oils and crude oil products become more and more viscous and flow becomes slower and slower. The pour point of crude oil or crude oil product is the lowest temperature at which the oil pours or flows under definitely prescribed conditions when it is chilled without disturbance at a standard rate (ASTM D97). The solidification characteristics of a hydrocarbon or hydrocarbon fuel depend on its grade or kind. For grease, the temperature of interest is that at which fluidity occurs, commonly known as the dropping point. The dropping point of grease is the temperature at which the grease passes from a plastic solid to a liquid state and begins to flow under the conditions of the test (ASTM D56 and ASTM D2650). For another type of plastic solid, including petrolatum and microcrystalline wax, both melting point and congealing point are of interest. The melting point of wax is the temperature at which the wax becomes sufficiently fluid to drop from the thermometer; the congealing point is the temperature at which melted petrolatum ceases to flow when allowed to cool under definitely prescribed conditions (ASTM D93). For paraffin wax, the solidification temperature is of interest. For such purposes, the melting point is

416 Handbook of Industrial Hydrocarbon Processes

the temperature at which the melted paraffin wax begins to solidify, as shown by the minimum rate of temperature change, when cooled under prescribed conditions. For pure or essentially pure hydrocarbon derivatives, the solidification temperature is the freezing point, the temperature at which a hydrocarbon passes from a liquid to a solid state.

5.7 Vapor density Vapor density is the density of a vapor in relation to that of airdhydrogen may also be used as the standard of comparison. In the case of air (which is commonly used in relation to hydrocarbon derivatives and hydrocarbon fuels), the vapor density is the mass of a specified volume of the substance divided by mass of same volume of air and air is given an arbitrary vapor density of one. With this definition, the vapor density would indicate whether a gas is denser (greater than one) or less dense (less than one) than air. The vapor density has implications for container storage and personnel safetydif a container can release a dense gas, its vapor could sink and, if flammable, collect until it is at a concentration sufficient for ignition. Even if not flammable, it could collect in the lower floor or level of a confined space and displace air, possibly presenting a smothering hazard to individuals entering the lower part of that space.

5.8 Use of the data Thus, this chapter has been devoted to presentation of the various methods of hydrocarbon analysis and there is no doubt that the analysis of petroleum products is used to ensure that the products match the specifications that are required for their sale and use. In the modern era, as a result of the expansion of computer simulators and advanced analytical tools accompanied by the availability of more accurate analytical data, the fields of petroleum analysis and petroleum product analysis have expanded even further. The data derived from any one, or more, of the evaluation techniques described in this chapter give an indication of the characteristics of the hydrocarbon as well as options for use or other form of processing as well as for the prediction of product properties. Other properties may also be required for more detailed evaluation of the hydrocarbon and for comparison between hydrocarbons. However, proceeding from the raw evaluation data to use may not always be the preferred step. Further evaluation of the hydrocarbon is usually necessary in order to develop accurate and realistic relationships using the data obtained from the test methods. After that, there may be other test methods that can also play an important role to assist in the various tweaks that are needed to produce a saleable product with the necessary properties. It is, of course, a matter of choosing the relevant and meaningful properties to meet the nature of the task.

Chemical and physical properties of hydrocarbons Chapter | 9

417

However, as analytical procedures evolved and data become available, it became obvious that each piece of data not only reflected the test method applied to the sample and the exact steps followed in the analysis of a sample depend, among other factors, on the interpretation by the analyst of the written test method and the unique characteristics of the test samplednot all samples of petroleum or of a specific petroleum product are the same or even equivalent. Thus, the procedure and the resulting data will (i) reflect the background of the investigator, (ii) reflect the ability of the investigator to interpret the results in relation to the application, (iii) be dependent upon the analytical facilities available, and (iv) be shown the required levels of accuracy and precision. As a result, it has been necessary to support the objectives of standard organizations (such as ASTM International), which provide a forum for the exchange of information related to test methods, discussion of any issues, and cooperative development of modified and/or standard test methods. Indeed, whatever the source, hydrocarbon fuels are complex mixtures of hydrocarbon compounds, ranging from low molecular weight, low-boiling volatile organic compounds to high molecular weight, high-boiling constituents (even nonvolatile under achievable temperatures). Furthermore, the composition of petroleum products varies depending upon (i) the source of the crude oildcrude oil is derived from a variety of source materials which vary greatly in chemical composition, and (ii) the refining processes used to produce the hydrocarbon product. When the data from the various analytical tests and investigations have been collected, the obvious question that comes to mind relates to use of the data. For the most part, the data are used to determine whether or not a crude oil may produce a certain hydrocarbon product and whether or not the product can meet the specifications for use. However since the early days of analysis, there has been a growing tendency to use the analytical data as a means of more detailed and accurate projections of (i) the refinery process by which the hydrocarbon is produced, (ii) the yield of the hydrocarbon product, (iii) properties of the hydrocarbon product, and (iv) predictability of the behavior of the product. For example, predictive methods to determine the composition of the hydrocarbon product is vital to determine if the product meets specifications set by the federal government or by local authorities. Thus, the data derived from any one or more of the analytical techniques applied to a hydrocarbon product not only give an indication of the product characteristics but also whether or not the product is suitable (i.e., meets specification) for the proposed use. Indeed, the use of physical properties for hydrocarbon evaluation and yields and, in some cases, product properties has continued in process research laboratories to the present time and will continue for some time. Thus, using the data derived from the application of a test method, it is possible to assess hydrocarbon quality and acquire a degree of predictability of performance during use. However, knowledge of the basic concepts of

418 Handbook of Industrial Hydrocarbon Processes

hydrocarbon production will help the analyst understand the production and, to a large extent, the anticipated properties of the product, which in turn is related to storage, sampling, and handling the products. In addition, there are many instances in which interrelationships of the specification data enable properties to be predicted from the measured properties with as good precision as can be obtained by a single test. It would be possible to examine in this way the relationships between all the specified properties of a hydrocarbon product and to establish certain key properties from which the remainder could be predicted, but this would be a tedious task. An alternative approach to that of picking out the essential tests in a specification (especially a hydrocarbon mixture such as a fuel) using regression analysis is to examine the specifications as a whole, and to use the necessary component. This is termed principal components analysis. And in this method a set of data as points in multidimensional space (n-dimensional, corresponding to n original tests) is examined to determine the direction that accounts for the biggest variability in the data (first principal component). The process is repeated until n principal components are evaluated, but it must be determined which are of practical importance since some principal components may be due to experimental error. The number of significant principal components shows the number of independent properties being measured by the tests considered. Having established the number of independent properties for such a product, it is also necessary to examine the basis for making the specification more efficient. In the long-term, it might be possible to obtain new tests of a fundamental nature to replace existing tests. In the short-term, selecting the best of the existing tests to define product quality will be beneficial. In summary, hydrocarbon product analysis is a complex discipline involving a variety of standard test methods, some of which have been mentioned above, and which needs a multidimensional approach. No single technique should supersede the other without adequate testing, along with an explanation of the data that are obtained with adequate interpretation.

References ASTM D56, 2019. Standard Test Method for Flash Point by Tag Closed Cup Tester. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D70, 2019. Standard Test Method for Density of Semi-solid Bituminous Materials (Pycnometer Method). Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D71, 2019. Standard Test Method for Relative Density of Solid Pitch and Asphalt (Displacement Method). Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D87, 2019. Standard Test Method for Melting Point of Petroleum Wax (Cooling Curve). Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania.

Chemical and physical properties of hydrocarbons Chapter | 9

419

ASTM D93, 2019. Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D97, 2019. Standard Test Method for Pour Point of Petroleum Products. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D127, 2019. Standard Test Method for Drop Melting Point of Petroleum Wax, Including Petrolatum. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D287, 2019. Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method). Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D512, 2019. Standard Test Methods for Chloride Ion in Water. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D524, 2019. Standard Test Method for Ramsbottom Carbon Residue of Petroleum Products. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D975, 2019. Standard Specification for Diesel Fuel Oils. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1217, 2019. Standard Test Method for Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1298, 2019. Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1480, 2019. Standard Test Method for Density and Relative Density (Specific Gravity) of Viscous Materials by Bingham Pycnometer. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1481, 2019. Standard Test Method for Density and Relative Density (Specific Gravity) of Viscous Materials by Lipkin Bicapillary Pycnometer. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1555, 2019. Standard Test Method for Calculation of Volume and Weight of Industrial Aromatic Hydrocarbons and Cyclohexane [Metric]. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1657, 2019. Standard Test Method for Density or Relative Density of Light Hydrocarbons by Pressure Hydrometer. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D1835, 2019. Standard Specification for Liquefied Petroleum (LP) Gases. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D2500, 2019. Standard Test Method for Cloud Point of Petroleum Products. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D2650, 2019. Standard Test Method for Chemical Composition of Gases by Mass Spectrometry. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D3828, 2019. Standard Test Method for pH of Activated Carbon. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D4052, 2019. Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania.

420 Handbook of Industrial Hydrocarbon Processes ASTM D5002, 2019. Standard Test Method for Density, Relative Density, and API Gravity of Crude Oils by Digital Density Analyzer. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM D6751, 2019. Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. ASTM E659, 2019. Standard Test Method for Autoignition Temperature of Liquid Chemicals. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania. Drews, A.W. (Ed.), 1998. Manual on Hydrocarbon Analysis. American Society for Testing and Materials, West Conshohocken, Pennsylvania. Eliel, E.L., Wilen, S.H., 1994. Stereochemistry of Organic Compounds. John Wiley & Sons Inc., New York. Eliel, E.L., Wilen, S.H., Doyle, M.P., 2001. Basic Organic Stereochemistry. John Wiley & Sons Inc., New York. Howard, P.H., Meylan, 1997. W.M. Handbook of Physical Properties of Organic Chemicals. CRC Press, Taylor & Francis Group, Boca Raton, Florida. Klein, D., 2013. Organic Chemistry, second ed. John Wiley & Sons Inc., Hoboken, New Jersey. Olah, G.A., Molna´r, A., 2003. Hydrocarbon Chemistry, second ed. John Wiley & Sons Inc., Hoboken, New Jersey. Speight, J.G., 2014. The Chemistry and Technology of Petroleum, fifth ed. CRC Press, Taylor & Francis Group, Boca Raton, Florida. Speight, 2015. Handbook of Petroleum Product Analysis, second ed. John Wiley & Sons Inc., Hoboken, New Jersey. Speight, J.G., 2017. Lange’s Handbook of Chemistry, seventeenth ed. McGraw Hill, New York. Speight, J.G., 2019. Handbook of Petrochemical Processes. CRC Press, Taylor & Francis Group, Boca Raton, Florida. Stoker, H.S., 2008. General, Organic, and Biological Chemistry. Florence Kentucky. Yaws, C.L., 1999. Chemical Properties Handbook. McGraw-Hill, New York.

Further reading ASTM D5502, 2019. Standard Test Method for Density and Relative Density of Crude Oils by Digital Density Analyzer. Annual Book of Standards. ASTM International, West Conshohocken, Pennsylvania.