CHAPTER33Hydrocarbons from PetroleumContents
1. Introduction 862. Gaseous products 893. Naphtha 92
3.1. Composition 923.2. Manufacture 933.3. Properties and uses 99
4. Gasoline 1004.1. Composition 1014.2. Manufacture 1024.3. Properties and uses 1054.4. Octane numbers 106
5. Kerosene and related fuels 1075.1. Composition 1095.2. Manufacture 1095.3. Properties and uses 110
6. Diesel fuel 1107. Gas oil and fuel oil 1118. Lubricating oil 113
8.1. Composition 1148.2. Manufacture 115
8.2.1. Chemical refining processes 1168.2.2. Hydroprocessing 1168.2.3. Solvent refining processes 1178.2.4. Catalytic dewaxing 1178.2.5. Solvent dewaxing 1178.2.6. Finishing processes 1188.2.7. Older processes 119
8.3. Properties and uses 1219. Wax 122
9.1. Composition 1229.2. Manufacture 1239.3. Properties and uses 124
References 125
Handbook of Industrial Hydrocarbon Processes � 2011 Elsevier Inc.ISBN 978-0-7506-8632-7, doi:10.1016/B978-0-7506-8632-7.10003-9 All rights reserved. 85 j
86 Hydrocarbons from Petroleum
1. INTRODUCTION
The constant demand for hydrocarbon products such as liquid fuels is
a major driving force behind the petroleum industry.
Petroleum products (in contrast to petrochemicals) are those hydrocarbon
fractions that are derived from petroleum and have commercial value as
a bulk product (Table 3.1). A major group of hydrocarbon products from
petroleum (petrochemicals) are the basis of a major industry. They are, in
the strictest sense, different to petroleum products insofar as the petro-
chemicals are the basic building blocks of the chemical industry.
There is a myriad of other products that have evolved through the short
life of the petroleum industry, either as single hydrocarbons or as hydro-
carbon fractions (Table 3.2). And the complexities of product composition
have matched the evolution of the products. In fact, it is the complexity of
product composition that has served the industry well and, at the same time,
had an adverse effect on product use. Product complexity has made the
industry unique among industries. Indeed, current analytical techniques
that are accepted as standard methods for, as an example, the aromatics
content of fuels (ASTM D-1319, ASTM D-2425, ASTM D-2549, ASTM
Table 3.1 Hydrocarbon number range for petroleum products
Product
Lowercarbonlimit
Uppercarbonlimit
Lowerboilingpoint°C
Upperboilingpoint°C
Lowerboilingpoint°F
Upperboilingpoint°F
Refinery gas C1 C4 e161 e1 e259 31
Liquefied
petroleum gas
C3 C4 e42 e1 e44 31
Naphtha C5 C17 36 302 97 575
Gasoline C4 C12 e1 216 31 421
Kerosene/diesel
fuel
C8 C18 126 258 302 575
Aviation
turbine fuel
C8 C16 126 287 302 548
Fuel oil C12 >C20 216 421 >343 >649
Lubricating oil >C20 >343 >649
Wax C17 >C20 302 >343 575 >649
Asphalt >C20 >343 >649
Coke >C50* >1000* >1832*
*Carbon number and boiling point difficult to assess; inserted for illustrative purpose only.
Table 3.2 Properties of hydrocarbon products from petroleum
Molecularweight
Specificgravity
Boilingpoint°F
Ignitiontemperature°F
Flashpoint°F
Flammabilitylimits in air% v/v
Benzene 78.1 0.879 176.2 1040 12 1.35e6.65
n-Butane 58.1 0.601 31.1 761 e76 1.86e8.41
iso-Butane 58.1 10.9 864 e117 1.80e8.44
n-Butene 56.1 0.595 21.2 829 Gas 1.98e9.65
iso-Butene 56.1 19.6 869 Gas 1.8e9.0Diesel fuel 170e198 0.875 100e130Ethane 30.1 0.572 e127.5 959 Gas 3.0e12.5Ethylene 28.0 e154.7 914 Gas 2.8e28.6Fuel oil No. 1 0.875 304e574 410 100e162 0.7e5.0Fuel oil No. 2 0.920 494 126e204Fuel oil No. 4 198.0 0.959 505 142e240Fuel oil No. 5 0.960 156e336Fuel oil No. 6 0.960 150
Gasoline 113.0 0.720 100e400 536 e45 1.4e7.6n-Hexane 86.2 0.659 155.7 437 e7 1.25e7.0
n-Heptane 100.2 0.668 419.0 419 25 1.00e6.00
Kerosene 154.0 0.800 304e574 410 100e162 0.7e5.0Methane 16.0 0.553 e258.7 900e1170 Gas 5.0e15.0Naphthalene 128.2 424.4 959 174 0.90e5.90
Neohexane 86.2 0.649 121.5 797 e54 1.19e7.58
Neopentane 72.1 49.1 841 Gas 1.38e7.11
n-Octane 114.2 0.707 258.3 428 56 0.95e3.2
iso-Octane 114.2 0.702 243.9 837 10 0.79e5.94
n-Pentane 72.1 0.626 97.0 500 e40 1.40e7.80
iso-Pentane 72.1 0.621 82.2 788 e60 1.31e9.16
n-Pentene 70.1 0.641 86.0 569 e 1.65e7.70
Propane 44.1 e43.8 842 Gas 2.1e10.1Propylene 42.1 e53.9 856 Gas 2.00e11.1
Toluene 92.1 0.867 321.1 992 40 1.27e6.75
Xylene 106.2 0.861 281.1 867 63 1.00e6.00
Hydrocarbons from Petroleum 87
D-2786, ASTM D-2789), as well as proton and carbon nuclear magnetic
resonance methods, yield different information. Each method will yield the
“% aromatics” in the sample but the data must be evaluated within the
context of the method.
The customary processing of petroleum does not usually involve the
separation and handling of pure hydrocarbons (Figure 3.1). Indeed, petroleum-
derived products are always mixtures: occasionally simple but more often very
complex. Thus, for the purposes of this chapter, such materials as the gross
fractions of petroleum (e.g., gasoline, naphtha, kerosene, and the like) which
are usually obtained by distillation and/or refining are classed as petroleum
Figure 3.1 Schematic of a modern refinery
88Hydrocarbons
fromPetroleum
Hydrocarbons from Petroleum 89
products; asphalt and other solid products (e.g., wax) are also included in this
division.
This type of classification separates this group of products from those
obtained as petroleum chemicals (petrochemicals), for which the emphasis is
on separation and purification of single chemical compounds, which are in
fact starting materials for a host of other chemical products.
2. GASEOUS PRODUCTS
Natural gas, which is predominantly methane, occurs in underground
reservoirs separately or in association with crude oil (Chapter 3). The
principal types of gaseous fuels are oil (distillation) gas, reformed natural gas,
and reformed propane or liquefied petroleum gas (LPG).
Liquefied petroleum gas (LPG) is the term applied to certain specific
hydrocarbons and their mixtures, which exist in the gaseous state under
atmospheric ambient conditions but can be converted to the liquid state under
conditions of moderate pressure at ambient temperature. These are the light
hydrocarbons fraction of the paraffin series, derived from refinery processes,
crude oil stabilization plants and natural gas processing plants comprising
propane (CH3CH2CH3), butane (CH3CH2CH2CH3), iso-butane [CH3CH
(CH3)CH3] and to a lesser extent propylene (CH3CH¼CH2), or butylene
(CH3CH2CH¼CH2). Themost common commercial products are propane,
butane, or some mixture of the two (Table 3.3) and are generally extracted
from natural gas or crude petroleum. Propylene and butylenes result from
cracking other hydrocarbons in a petroleum refinery and are two important
chemical feedstocks.
Mixed gas is a gas prepared by adding natural gas or liquefied petroleum
gas to a manufactured gas, giving a product of better utility and higher heat
content or Btu value.
The principal constituent of natural gas is methane (CH4). Other
constituents are paraffinic hydrocarbons such as ethane (CH3CH3), propane
(CH3CH2CH3), and the butanes [CH3CH2CH2CH3 and/or (CH3)3CH].
Many natural gases contain nitrogen (N2) as well as carbon dioxide (CO2) and
hydrogen sulfide (H2S). Trace quantities of argon, hydrogen, and heliummay
also be present. Generally, the hydrocarbons having a higher molecular weight
thanmethane, carbon dioxide, and hydrogen sulfide are removed fromnatural
gas prior to its use as a fuel. Gases produced in a refinery contain methane,
ethane, ethylene, propylene, hydrogen, carbon monoxide, carbon dioxide,
andnitrogen,with lowconcentrationsofwater vapor, oxygen, andother gases.
Table 3.3 Properties of propane and butanePropane Butane
Formula C3H8 C4H10
Boiling point, �F e44� 32�
Specific gravity e gas (air ¼ 1.00) 1.53 2.00
Specific gravity e liquid (water ¼ 1.00) 0.51 0.58
lb/gallon e liquid at 60�F 4.24 4.81
Btu/gallon e gas at 60�F 91,690 102,032
Btu/lb e gas 21,591 21,221
Btu/ft3 e gas at 60�F 2,516 3,280
Flash point, �F e156 e96
Ignition temperature in air, �F 920e1,020 900e1,000
Maximum flame temperature in air, �F 3,595 3,615
Octane number (iso-octane ¼ 100) 100þ 92
90 Hydrocarbons from Petroleum
Unless produced specifically as a product (e.g., liquefied petroleum gas),
the gaseous products of refinery operations are mixtures of various gases.
Each gas is a by-product of a refining process. Thus, the compositions of
natural, manufactured, and mixed gases can vary so widely, no single set of
specifications could cover all situations.
As already noted, the compositions of natural, manufactured, and mixed
gases can vary so widely, no single set of specifications could cover all
situations. The requirements are usually based on performances in burners
and equipment, on minimum heat content, and on maximum sulfur
content. Gas utilities in most states come under the supervision of state
commissions or regulatory bodies and the utilities must provide a gas that is
acceptable to all types of consumers and that will give satisfactory perfor-
mance in all kinds of consuming equipment. However, there are specifi-
cations for liquefied petroleum gas (ASTMD1835) which depend upon the
required volatility.
Since natural gas as delivered to pipelines has practically no odor, the
addition of an odorant is required by most regulations in order that the
presence of the gas can be detected readily in case of accidents and leaks.
This odorization is provided by the addition of trace amounts of some
organic sulfur compounds to the gas before it reaches the consumer. The
standard requirement is that a user will be able to detect the presence of the
gas by odor when the concentration reaches 1% of gas in air. Since the lower
limit of flammability of natural gas is approximately 5%, this 1% requirement
is essentially equivalent to one-fifth the lower limit of flammability. The
Hydrocarbons from Petroleum 91
combustion of these trace amounts of odorant does not create any serious
problems of sulfur content or toxicity.
The different methods for gas analysis include absorption, distillation,
combustion, mass spectroscopy, infrared spectroscopy, and gas chroma-
tography (ASTM D2163, ASTM D2650, and ASTM D4424). Absorption
methods involve absorbing individual constituents one at a time in suitable
solvents and recording of contraction in volume measured. Distillation
methods depend on the separation of constituents by fractional distillation
and measurement of the volumes distilled. In combustion methods,
certain combustible elements are caused to burn to carbon dioxide and
water, and the volume changes are used to calculate composition. Infrared
spectroscopy is useful in particular applications. For the most accurate
analyses, mass spectroscopy and gas chromatography are the preferred
methods.
The specific gravity of product gases, including liquefied petroleum gas,
may be determined conveniently by a number of methods and a variety of
instruments (ASTM D1070, ASTM D4891).
The heat value of gases is generally determined at constant pressure in
a flow calorimeter in which the heat released by the combustion of a defi-
nite quantity of gas is absorbed by a measured quantity of water or air. A
continuous recording calorimeter is available for measuring heat values of
natural gases (ASTM D1826).
The lower and upper limits of flammability of organic compounds
indicate the percentage of combustible gas in air below which and above
which flame will not propagate. When flame is initiated in mixtures having
compositions within these limits, it will propagate and therefore the
mixtures are flammable. Knowledge of flammable limits and their use in
establishing safe practices in handling gaseous fuels is important, e.g., when
purging equipment used in gas service, in controlling factory or mine
atmospheres, or in handling liquefied gases.
Many factors enter into the experimental determination of flammable
limits of gas mixtures, including the diameter and length of the tube or
vessel used for the test, the temperature and pressure of the gases, and the
direction of flame propagation – upward or downward. For these and other
reasons, great care must be used in the application of the data. In monitoring
closed spaces where small amounts of gases enter the atmosphere, often the
maximum concentration of the combustible gas is limited to one-fifth of the
concentration of the gas at the lower limit of flammability of the gas–air
mixture.
92 Hydrocarbons from Petroleum
3. NAPHTHA
The term petroleum solvent describes the liquid hydrocarbon fractions
obtained from petroleum and used in industrial processes and formulations.
These fractions are also referred to as naphtha or industrial naphtha. By
definition the solvents obtained from the petrochemical industry such as
alcohols, ethers, and the like are not included in this chapter. A refinery is
capable of producing hydrocarbons of a high degree of purity and at the
present time petroleum solvents are available covering a wide range of
solvent properties including both volatile and high boiling qualities.
Naphtha (often referred to as naft in the older literature) is actually
a general term applied to refined, partly refined, or unrefined petroleum
products. In the strictest sense of the term, not less than 10% of the material
should distill below 175�C (345�F); not less than 95% of the material should
distill below 240�C (465�F) under standardized distillation conditions
(ASTM D-86).
Naphtha has been available since the early days of the petroleum
industry. Indeed, the infamous Greek fire documented as being used in
warfare during the last three millennia is a petroleum derivative. It was
produced either by distillation of crude oil isolated from a surface seepage or
(more likely) by destructive distillation of the bituminous material obtained
from bitumen seepages (of which there are/were many known during the
heyday of the civilizations of the Fertile Crescent). The bitumen obtained
from the area of Hit (Tuttul) in Iraq (Mesopotamia) is an example of such an
occurrence (Abraham, 1945; Forbes, 1958a).
Other petroleum products boiling within the naphtha boiling range
include industrial spirit and white spirit.
Industrial spirit comprises liquids distilling between 30 and 200�C (–1 to
390�F), with a temperature difference between 5% volume and 90% volume
distillation points, including losses, of not more than 60�C (140�F). Thereare several (up to eight) grades of industrial spirit, depending on the position
of the cut in the distillation range defined above. On the other hand, white
spirit is an industrial spirit with a flash point above 30�C (99�F) and has
a distillation range from 135 to 200�C (275–390�F).
3.1. CompositionNaphtha is divided into two main types, aliphatic and aromatic. The two
types differ in two ways: first, in the kind of hydrocarbons making up the
solvent, and second, in the methods used for their manufacture. Aliphatic
Hydrocarbons from Petroleum 93
solvents are composed of paraffinic hydrocarbons and cycloparaffins
(naphthenes), and may be obtained directly from crude petroleum by
distillation. The second type of naphtha contains aromatics, usually alkyl-
substituted benzene, and is very rarely, if at all, obtained from petroleum as
straight-run materials.
Stoddard solvent is a petroleum distillate widely used as a dry cleaning
solvent and as a general cleaner and degreaser. It may also be used as paint
thinner, as a solvent in some types of photocopier toners, in some types of
printing inks, and in some adhesives. Stoddard solvent is considered to be
a form of mineral spirits, white spirits, and naphtha but not all forms of
mineral spirits, white spirits, and naphtha are considered to be Stoddard
solvent. Stoddard solvent consists of linear alkanes (30–50%), branched
alkanes (20–40%), cycloalkanes (30–40%), and aromatic hydrocarbons (10–
20%). The typical hydrocarbon chain ranges from C7 through C12 in length.
3.2. ManufactureIn general, naphtha may be prepared by any one of several methods, which
include: (1) fractionation of straight-run, cracked, and reforming distillates,
or even fractionation of crude petroleum; (2) solvent extraction; (3)
hydrogenation of cracked distillates; (4) polymerization of unsaturated
compounds (olefins); and (5) alkylation processes. In fact, the naphtha may
be a combination of product streams from more than one of these processes.
The more common method of naphtha preparation is distillation.
Depending on the design of the distillation unit, either one or two naphtha
steams may be produced: (1) a single naphtha with an end point of about
205�C (400�F) and similar to straight-run gasoline or (2) this same fraction
divided into a light naphtha and a heavy naphtha. The end point of the light
naphtha is varied to suit the subsequent subdivision of the naphtha into
narrower boiling fractions and may be of the order of 120�C (250�F).Before the naphtha is redistilled into a number of fractions with boiling
ranges suitable for aliphatic solvents, the naphtha is usually treated to remove
sulfur compounds, as well as aromatic hydrocarbons, which are present in
sufficient quantity to cause an odor. Aliphatic solvents that are specially
treated to remove aromatic hydrocarbons are known as deodorized solvents.
Odorless solvent is the name given to heavy alkylate used as an aliphatic solvent,
which is a by-product in the manufacture of aviation alkylate.
Sulfur compounds are most commonly removed or converted to
a harmless form by chemical treatment with lye, doctor solution, copper
chloride, or similar treating agents. Hydrorefining processes are also often
94 Hydrocarbons from Petroleum
used in place of chemical treatment. Solvent naphtha is solvents selected for
low sulfur content, and the usual treatment processes, if required, remove
only sulfur compounds. Naphtha with a small aromatic content has a slight
odor, but the aromatic constituents increase the solvent power of the
naphtha and there is no need to remove aromatics unless an odor-free
solvent is specified.
Naphtha that is either naturally sweet (no odor), or has been treated until
sweet, is subdivided into several fractions in efficient fractional distillation
towers frequently called pipe stills, columns, and column steam stills. A
typical arrangement consists of primary and secondary fractional distillation
towers and a stripper. Heavy naphtha, for example, is heated by a steam
heater and passed into the primary tower, which is usually operated under
vacuum. The vacuum permits vaporization of the naphtha at the temper-
atures obtainable from the steam heater.
The primary tower separates the naphtha into three parts:
1. A high boiling hydrocarbon fraction that is removed as a bottom product
and sent to a cracking unit.
2. A side stream hydrocarbon product of narrow boiling range that, after
passing through the stripper, may be suitable for the aliphatic solvent
Varsol.
3. An overhead hydrocarbon product that is sent to the secondary
(vacuum) tower where the overhead product from the primary tower is
divided into an overhead and a bottom product in the secondary tower,
which operates under a partial vacuum with steam injected into the
bottom of the tower to assist in the fractionation. The overhead and
bottom products are finished aliphatic solvents, or if the feed to the
primary tower is light naphtha instead of heavy naphtha, other aliphatic
solvents of different boiling ranges are produced.
Superfractionation (Speight, 2007) is a highly efficient fractionating tower
used to separate ordinary petroleum products and isolate narrow-boiling
hydrocarbon fractions. For example, to increase the yield of furnace fuel oil,
heavy naphtha may be redistilled in a tower that is capable of making a better
separation of the naphtha and the fuel oil components. The latter, obtained
as a bottom product, is diverted to furnace fuel oil.
Fractional distillation as normally carried out in a refinery does not
completely separate one petroleum fraction from another. One product
overlaps another, depending on the efficiency of the fractionation, which in
turn depends on the number of trays in the tower, the amount of reflux
used, and the rate of distillation. Kerosene, for example, normally contains
Hydrocarbons from Petroleum 95
a small percentage of hydrocarbons that (according to their boiling points)
belong in the naphtha fraction and a small percentage that should be in the
gas oil fraction. Complete separation is not required for the ordinary uses of
these materials, but certain materials, such as solvents for particular purposes
(hexane, heptane, and aromatics), are required as essentially pure compo-
unds. Since they occur in mixtures of hydrocarbons they must be separated
by distillation and with no overlap of one hydrocarbon with another. This
requires highly efficient fractional distillation towers specially designed for
the purpose and referred to as superfractionators. Several towers with
50–100 trays operated with a high reflux ratio may be required to separate a
single compound with the necessary purity.
Azeotropic distillation (Speight, 2007) is the use of a third component to
separate two close-boiling components by means of the formation of an
azeotropic mixture between one of the original components and the third
component to increase the difference in the boiling points and facilitates
separation by distillation.
All compounds have definite boiling temperatures, but a mixture of
chemically dissimilar compounds sometimes causes one or both of the
components to boil at a temperature other than that expected. For example,
benzene boils at 80�C (176�F), but if it is mixed with hexane, it distills at
69�C (156�F). A mixture that boils at a temperature lower than the boiling
point of either of the components is called an azeotropic mixture.
Twomain types of azeotropes exist, i.e., the homogeneous azeotrope, where
a single liquid phase is in the equilibrium with a vapor phase; and the
heterogeneous azeotropes, where the overall liquid composition, which forms,
two liquid phases, is identical to the vapor composition. Most methods of
distilling azeotropes and low relative volatility mixtures rely on the addition
of specially chosen chemicals to facilitate the separation.
The five methods for separating azeotropic mixtures are:
1. Extractive distillation and homogeneous azeotropic distillation where the
liquid-separating agent is completely miscible.
2. Heterogeneous azeotropic distillation, or more commonly, azeotropic distil-
lationwhere the liquid-separating agent (the entrainer) forms one or more
azeotropes with the other components in the mixture and causes two
liquid phases to exist over a wide range of compositions. This immis-
cibility is the key to making the distillation sequence work.
3. Distillation using ionic salts. The salts dissociate in the liquid mixture and
alter the relative volatilities sufficiently that the separation becomes
possible.
96 Hydrocarbons from Petroleum
4. Pressure-swing distillation where a series of columns operating at different
pressures are used to separate binary azeotropes which change appre-
ciably in composition over a moderate pressure range or where a sepa-
rating agent which forms a pressure-sensitive azeotrope is added to
separate a pressure-insensitive azeotrope.
5. Reactive distillation where the separating agent reacts preferentially and
reversibly with one of the azeotropic constituents. The reaction product
is then distilled from the non-reacting components and the reaction is
reversed to recover the initial component.
In simple distillation (Speight, 2007) a multi-component liquid mixture is
slowly boiled in a heated zone and the vapors are continuously removed as
they form and, at any instant in time, the vapor is in equilibrium with the
liquid remaining on the still. Because the vapor is always richer in the more
volatile components than the liquid, the liquid composition changes
continuously with time, becoming more and more concentrated in the least
volatile species. A simple distillation residue curve (Speight, 2007) is a means by
which the changes in the composition of the liquid residue curves on the
pot change over time. A residue curve map is a collection of the liquid residue
curves originating from different initial compositions. Residue curve maps
contain the same information as phase diagrams, but represent this infor-
mation in a way that is more useful for understanding how to synthesize
a distillation sequence to separate a mixture.
All of the residue curves originate at the light (lowest boiling) pure
component in a region, move towards the intermediate boiling component,
and end at the heavy (highest boiling) pure component in the same region.
The lowest temperature nodes are termed as unstable nodes, as all trajectories
leave from them, while the highest temperature points in the region are
termed stable nodes, as all trajectories ultimately reach them. The point that
the trajectories approach from one direction and end in a different direction
(as always is the point of intermediate boiling component) is termed saddle
point. Residue curves that divide the composition space into different
distillation regions are called distillation boundaries.
Many different residue curve maps are possible when azeotropes are
present. Ternary mixtures containing only one azeotrope may exhibit six
possible residue curve maps that differ by the binary pair forming the
azeotrope and by whether the azeotrope is minimum or maximum boiling.
By identifying the limiting separation achievable by distillation, residue
curve maps are also useful in synthesizing separation sequences combining
distillation with other methods.
Hydrocarbons from Petroleum 97
However, the separation of components of similar volatility may become
economical if an entrainer can be found that effectively changes the relative
volatility. It is also desirable that the entrainer be reasonably cheap, stable,
non-toxic, and readily recoverable from the components. In practice it is
probably this last criterion that severely limits the application of extractive
and azeotropic distillation. The majority of successful processes, in fact, are
those in which the entrainer and one of the components separate into two
liquid phases on cooling if direct recovery by distillation is not feasible.
A further restriction in the selection of an azeotropic entrainer is that the
boiling point of the entrainer be in the range 10–40�C (18–72�F) belowthat of the components. Thus, although the entrainer is more volatile than
the components and distills off in the overhead product, it is present in
a sufficiently high concentration in the rectification section of the column.
Extractive distillation (Speight, 2007) is the use of a third component
to separate two close-boiling components in which one of the original
components in the mixture is extracted by the third component and
retained in the liquid phase to facilitate separation by distillation.
Using acetone–water as an extractive solvent for butanes and butenes,
butane is removed as overhead from the extractive distillation column with
acetone–water charged at a point close to the top of the column. The
bottom product of butenes and the extractive solvent are fed to a second
column where the butenes are removed as overhead. The acetone–water
solvent from the base of this column is recycled to the first column.
Extractive distillation may also be used for the continuous recovery of
individual aromatics, such as benzene, toluene, or xylene(s), from the
appropriate petroleum fractions. Prefractionation concentrates a single
aromatic cut into a close-boiling cut, after which the aromatic concentrate is
distilled with a solvent (usually phenol) for benzene or toluene recovery.
Mixed cresylic acids (cresols and methylphenols) are used as the solvent for
xylene recovery.
Extractive distillation is successful because the solvent is specially chosen
to interact differently with the components of the original mixture, thereby
altering their relative volatilities. Because these interactions occur
predominantly in the liquid phase, the solvent is continuously added near
the top of the extractive distillation column so that an appreciable amount is
present in the liquid phase on all of the trays below. The mixture to be
separated is added through a second feed point further down the column. In
the extractive column, the component having the greater volatility, not
necessarily the component having the lowest boiling point, is taken
98 Hydrocarbons from Petroleum
overhead as a relatively pure distillate. The other component leaves with the
solvent via the column bottoms. The solvent is separated from the
remaining components in a second distillation column and then recycled
back to the first column.
Several methods, involving solvent extraction (Speight, 2007) or destructive
hydrogenation (hydrocracking) (Speight, 2007), can accomplish the removal of
aromatic hydrocarbons from naphtha. By this latter method, aromatic
hydrocarbon constituents are converted into odorless, straight-chain
paraffin hydrocarbons that are required in aliphatic solvents.
The Edeleanu process (Speight, 2007) was originally developed to
improve the burning characteristics of kerosene by extraction of the smoke-
forming aromatic compounds. Thus it is not surprising that its use has been
extended to the improvement of other products as well as to the segregation
of aromatic hydrocarbons for use as solvents. Naphtha fractions rich in
aromatics may be treated by the Edeleanu process for the purpose of
recovering the aromatics, or the product stream from a catalytic reformer
unit – particularly when the unit is operated to produce maximum
aromatics – may be Edeleanu treated to recover the aromatics. The other
most widely used processes for this purpose are the extractive distillation
process and the Udex processes. Processes such as the Arosorb process and
cyclic adsorption processes are used to a lesser extent.
The Udex process (Speight, 2007) is also employed to recover aromatic
streams from reformate fractions. This process uses a mixture of water and
diethylene glycol to extract aromatics. Unlike extractive distillation, an
aromatic concentrate is not required and the solvent removes all the
aromatics, which are separated from one another by subsequent fractional
distillation.
The reformate is pumped into the base of an extractor tower. The feed
rises in the tower countercurrent to the descending diethylene glycol–water
solution, which extracts the aromatics from the feed. The non-aromatic
portion of the feed leaves the top of the tower, and the aromatic-rich solvent
leaves the bottom of the tower. Distillation in a solvent stripper separates the
solvent from the aromatics, which are sulfuric acid and clay treated and then
separated into individual aromatics by fractional distillation.
Silica gel (SiO2) is an adsorbent for aromatics and has found use in
extracting aromatics from refinery streams (Arosorb and cyclic adsorption
processes) (Speight, 2007). Silica gel is manufactured amorphous silica that is
extremely porous and has the property of selectively removing and holding
certain chemical compounds from mixtures. For example, silica gel
Hydrocarbons from Petroleum 99
selectively removes aromatics from a petroleum fraction, and after the non-
aromatic portion of the fraction is drained from the silica gel, the adsorbed
aromatics are washed from the silica gel by a stripper solvent (or desorbent).
Depending on the kind of feedstock, xylene, kerosene, or pentane may be
used as the desorbent.
However, silica gel can be poisoned by contaminants, and the feedstock
must be treated to remove water as well as nitrogen, oxygen, and sulfur-
containing compounds by passing the feedstock through beds of alumina
and/or other materials that remove impurities. The treated feedstock then
enters one of several silica gel cases (columns) where the aromatics are
adsorbed. The time period required for adsorption depends on the nature of
the feedstock; for example, reformate product streams have been known to
require substantially less treatment time than kerosene fractions.
3.3. Properties and usesGenerally, naphtha is valuable as a solvent because of good dissolving power.
The wide range of naphtha available, from the ordinary paraffin straight-run
to the highly aromatic types, and the varying degree of volatility possible
offer products suitable for many uses (Boenheim and Pearson, 1973; Hadley
and Turner, 1973).
The main uses of naphtha fall into the general areas of: (1) solvents
(diluents) for paints, for example; (2) dry-cleaning solvents; (3) solvents for
cutback asphalt; (4) solvents in the rubber industry; and (5) solvents for
industrial extraction processes.
Turpentine, the older, more conventional solvent for paints, has now been
almost completely replaced with the discovery that the cheaper and more
abundant petroleum naphtha is equally satisfactory. The differences in
application are slight: naphtha causes a slightly greater decrease in viscosity
when added to some paints than does turpentine, and depending on the
boiling range, may also show difference in evaporation rate.
The boiling ranges of fractions that evaporate at rates permitting the
deposition of good films have been fairly well established. Depending on
conditions, products are employed as light as those boiling from 38 to 150�C(100–300�F) and as heavy as those boiling between 150 and 230�C (300 and
450�F). The latter are used mainly in the manufacture of backed and forced-
drying products.
The solvent power required for conventional paint diluents is low and
can be reached by distillates from paraffinic crude oils, which are usually
recognized as the poorest solvents in the petroleum naphtha group. In
100 Hydrocarbons from Petroleum
addition to solvent power and correct evaporation rate, a paint thinner
should also be resistant to oxidation, i.e., the thinner should not develop bad
color and odor during use. The thinner should be free of corrosive impu-
rities and reactive materials, such as certain types of sulfur compounds,
when employed with paints containing lead and similar metals. The
requirements are best met by straight-run distillates from paraffinic crude
oils that boil from 120 to 205�C (250–400�F). The components of enamels,
varnishes, nitrocellulose lacquers, and synthetic resin finishes are not as
soluble in paraffinic naphtha as the materials in conventional paints, and
hence naphthenic and aromatic naphtha are favored for such uses.
Naphtha is used in the rubber industry for dampening the play and tread
stocks of automobile tires during manufacture to obtain better adhesion
between the units of the tire. They are also consumed extensively in making
rubber cements (adhesives) or are employed in the fabrication of rubberized
cloth, hot-water bottles, bathing caps, gloves, overshoes, and toys. These
cements are solutions of rubber and were formerly made with benzene, but
petroleum naphtha is now preferred because of the less toxic character.
Petroleum hydrocarbon distillates are also added in amounts up to 25%
and higher at various stages in the polymerization of butadiene-styrene
to synthetic rubber. Those employed in oil-extended rubber are of the
aromatic type. These distillates are generally high boiling fractions and
preferably contain no wax, boil from 425 to 510�C (800–950�F), havecharacterization factors of 10.5–11.6, a viscosity index lower than 0,
bromine numbers of 6–30, and API gravity of 3–24.
Naphtha is used for extraction on a fairly wide scale, such as the
extraction of residual oil from castor beans, soybeans, cottonseed, and wheat
germ and in the recovery of grease from mixed garbage and refuse. The
solvent employed in these cases is a hexane cut, boiling from about 65 to
120�C (150–250�F). When the oils recovered are of edible grade or
intended for refined purposes, stable solvents completely free of residual
odor and taste are necessary, and straight-run streams from low-sulfur,
paraffinic crude oils are generally satisfactory.
4. GASOLINE
Gasoline, also called gas (United States and Canada), petrol (Great Britain), or
benzine (Europe), is a mixture of volatile, flammable liquid hydrocarbons
derived from petroleum and used as fuel for internal-combustion engines. It
is also used as a solvent for oils and fats. Originally a by-product of the
Hydrocarbons from Petroleum 101
petroleum industry (kerosene being the principal product), gasoline became
the preferred automobile fuel because of its high energy of combustion and
capacity to mix readily with air in a carburetor.
Gasoline is a mixture of hydrocarbons that usually boil below 180�C(355�F) or, at most, below 200�C (390�F). The hydrocarbon constituents
in this boiling range are those that have four to 12 carbon atoms in their
molecular structure and fall into three general types: paraffins (including the
cycloparaffins and branched materials), olefins, and aromatics.
Gasoline is still in great demand as a major product from petroleum. The
network of interstate highways that links towns and cities in the United
States is dotted with frequent service centers where motorists can obtain
refreshment not only for themselves but also for their vehicles.
4.1. CompositionGasoline is manufactured to meet specifications and regulations and not to
achieve a specific distribution of hydrocarbons by class and size. However,
chemical composition often defines properties. For example, volatility is
defined by the individual hydrocarbon constituents and the lowest boiling
constituent(s) defines the volatility as determined by specific test methods.
Automotive gasoline typically contains almost two hundred (if not several
hundred) hydrocarbon compounds. The relative concentrations of the
compounds vary considerably depending on the source of crude oil,
refinery process, and product specifications. Typical hydrocarbon chain
lengths range from C4 through Cl2 with a general hydrocarbon distribution
consisting of alkanes (4–8%), alkenes (2–5%), iso-alkanes (25–40%),
cycloalkanes (3–7%), cycloalkenes (l–4%), and aromatics (20–50%).
However, these proportions vary greatly.
The majority of the members of the paraffin, olefin, and aromatic series
(of which there are about 500) boiling below 200�C (390�F) have been
found in the gasoline fraction of petroleum. However, it appears that the
distribution of the individual members of straight-run gasoline (i.e., distilled
from petroleum without thermal alteration) is not even.
Highly branched paraffins, which are particularly valuable constituents
of gasoline(s), are not usually the principal paraffinic constituents of straight-
run gasoline. The more predominant paraffinic constituents are usually the
normal (straight-chain) isomers, which may dominate the branched isomer(s)
by a factor of 2 or more. This is presumed to indicate the tendency to
produce long uninterrupted carbon chains during petroleum maturation
rather than those in which branching occurs. However, this trend is
102 Hydrocarbons from Petroleum
somewhat different for the cyclic constituents of gasoline, i.e., cycloparaffins
(naphthenes) and aromatics. In these cases, the preference appears to be for
several short side chains rather than one long substituent.
Gasoline can vary widely in composition: even those with the same
octane number may be quite different, not only in the physical makeup but
also in the molecular structure of the constituents. For example, Pennsyl-
vania petroleum is high in paraffins (normal and branched), but California
and Gulf Coast crude oils are high in cycloparaffins. Low-boiling distillates
with high content of aromatic constituents (above 20%) can be obtained
from some Gulf Coast and West Texas crude oils, as well as from crude oils
from the Far East. The variation in aromatics content as well as the variation
in the content of normal paraffins, branched paraffins, cyclopentanes, and
cyclohexanes involve characteristics of any one individual crude oil and may
in some instances be used for crude oil identification. Furthermore, straight-
run gasoline generally shows a decrease in paraffin content with an increase
in molecular weight, but the cycloparaffins (naphthenes) and aromatics
increase with increasing molecular weight. Indeed, the hydrocarbon type
variation may also vary markedly from process to process.
The reduction in the lead content of gasoline and the introduction of
reformulated gasoline has been very successful in reducing automobile
emissions (Wittcoff, 1987; Absi-Halabi et al., 1997). Further improvements
in fuel quality have been proposed for the years 2000 and beyond. These
projections are accompanied by a noticeable and measurable decrease in
crude oil quality and the reformulated gasoline will help meet environ-
mental regulations for emissions for liquid fuels.
4.2. ManufactureGasoline was at first produced by distillation, simply separating the volatile,
more valuable fractions of crude petroleum. Later processes, designed to
raise the yield of gasoline from crude oil, decomposed higher-molecular-
weight constituents into lower-molecular-weight products by processes
known as cracking. And like typical gasoline, several processes produce the
blending stocks for gasoline (Figure 3.2).
Up to and during the first decade of the present century, the gasoline
produced was that originally present in crude oil or that could be condensed
from natural gas. However, it was soon discovered that if the heavier
portions of petroleum (such as the fraction that boiled higher than kerosene,
e.g., gas oil) were heated to more severe temperatures, thermal degradation
(or cracking) occurred to produce smaller molecules within the range
Figure 3.2 Refinery streams that are blended to roduce gasoline
Hydrocarbons
fromPetroleum
103
p
104 Hydrocarbons from Petroleum
suitable for gasoline. Therefore, gasoline that was not originally in the crude
petroleum could be manufactured.
Thermal cracking, employing heat and high pressures, was introduced in
1913 but was replaced after 1937 by catalytic cracking, the application of
catalysts that facilitate chemical reactions producing more gasoline. Other
methods used to improve the quality of gasoline and increase its supply
include polymerization, alkylation, isomerization, and reforming.
Polymerization is the conversion of gaseous olefins, such as propylene and
butylene, into larger molecules in the gasoline range. Alkylation is a process
combining an olefin and paraffin (such as iso-butane). Isomerization is the
conversion of straight-chain hydrocarbons to branched-chain hydrocarbons.
Reforming is the use of either heat or a catalyst to rearrange the molecular
structure.
Aviation gasoline is a form of motor gasoline that has been especially
prepared for use for aviation piston engines. It has an octane number suited
to the engine, a freezing point of –60�C (–76�F), and a distillation range
usually within the limits of 30–180�C (86–356�F) compared to –1 to 200�C(30–390�F) for automobile gasoline. The narrower boiling range ensures
better distribution of the vaporized fuel through the more complicated
induction systems of aircraft engines. Aircraft operate at altitudes at which
the prevailing pressure is less than the pressure at the surface of the earth
(pressure at 17,500 feet is 7.5 psi compared to 14.7 psi at the surface of the
earth). Thus, the vapor pressure of aviation gasoline must be limited to
reduce boiling in the tanks, fuel lines, and carburetors. Thus, the aviation
gasoline does not usually contain the gaseous hydrocarbons (butanes) that
give automobile gasoline the higher vapor pressures.
Aviation gasoline is strictly limited regarding hydrocarbon composition.
The important properties of the hydrocarbons are the highest octane
numbers economically possible, boiling points in the limited temperature
range of aviation gasoline, maximum heat contents per pound (high
proportion of combined hydrogen), and high chemical stability to withstand
storage. Aviation gasoline is composed of paraffins and iso-paraffins
(50–60%), moderate amounts of naphthenes (20–30%), small amounts of
aromatics (10%), and usually no olefins, whereas motor gasoline may
contain up to 30% olefins and up to 40% aromatics.
Under conditions of use in aircraft, olefins have a tendency to form gum,
cause pre-ignition, and have relatively poor antiknock characteristics under
lean mixture (cruising) conditions; for these reasons olefins are detrimental
to aviation gasoline. Aromatics have excellent antiknock characteristics
Hydrocarbons from Petroleum 105
under rich mixture (takeoff) conditions, but are much like the olefins under
lean mixture conditions; hence the proportion of aromatics in aviation
gasoline is limited. Some naphthenes with suitable boiling temperatures are
excellent aviation gasoline components but are not segregated as such in
refinery operations. They are usually natural components of the straight-run
naphtha (aviation base stocks) used in blending aviation gasoline. The lower
boiling paraffins (pentane and hexane), and both the high-boiling and low-
boiling iso-paraffins (iso-pentane to iso-octane) are excellent aviation
gasoline components. These hydrocarbons have high heat contents per
pound and are chemically stable, and the iso-paraffins have high octane
numbers under both lean and rich mixture conditions.
The manufacture of aviation gasoline is thus dependent on the avail-
ability and selection of fractions containing suitable hydrocarbons. The
lower boiling hydrocarbons are usually found in straight-run naphtha from
certain types of crude petroleum. These fractions have high contents of
iso-pentanes and iso-hexane and provide needed volatility, as well as high
octane number components. Higher boiling iso-paraffins are provided by
aviation alkylate, which consists mostly of branched octanes. Aromatics,
such as benzene, toluene, and xylene, are obtained from catalytic reforming
or a similar source.
To increase the proportion of higher boiling octane components, such as
aviation alkylate and xylenes, the proportion of lower boiling components
must also be increased to maintain the proper volatility. Iso-pentane and,
to some extent, iso-hexane are the lower boiling components used.
Iso-pentane and iso-hexane may be separated from selected naphtha by
superfractionators or synthesized from the normal hydrocarbons by iso-
merization. In general, most aviation gasolines are made by blending
a selected straight-run naphtha fraction (aviation base stock) with iso-
pentane and aviation alkylate.
4.3. Properties and usesDespite the diversity of the processes within a modern petroleum refinery,
no single hydrocarbon stream meets all the requirements of gasoline. Thus,
the final step in gasoline manufacture is blending the various streams into
a finished product (Figure 3.2). It is not uncommon for the finished gasoline
to be made up of six or more streams and several factors make this flexibility
critical: (1) the requirements of the gasoline specification (ASTM D-4814)
and the regulatory requirements, and (2) performance specifications that are
subject to local climatic conditions and regulations.
106 Hydrocarbons from Petroleum
The early criterion for gasoline quality was Baume (or API) gravity.
For example, a 70� API gravity gasoline contained fewer, if any, of the
heavier gasoline constituents than a 60�API gasoline. Therefore, the 70�APIgasoline was a higher quality and, hence, economically more valuable
gasoline. However, apart from being used as a rough estimation of quality
(not only for petroleum products but also for crude petroleum), specific
gravity is no longer of any significance as a true indicator of gasoline quality.
4.4. Octane numbersGasoline performance and hence quality of an automobile gasoline is
determined by its resistance to knock, for example detonation or ping during
service. The antiknock quality of the fuel limits the power and economy
that an engine using that fuel can produce: the higher the antiknock quality
of the fuel, the more the power and efficiency of the engine.
Octane numbers are obtained by the two test procedures. Those obtained
by the first method are called motor octane numbers (indicative of high-speed
performance) (ASTM D-2700 and ASTM D-2723). Those obtained by
the second method are called research octane numbers (indicative of normal
road performance) (ASTM D-2699 and ASTM D-2722). Octane numbers
quoted are usually, unless stated otherwise, research octane numbers.
In the test methods used to determine the antiknock properties of
gasoline, comparisons are made with blends of two pure hydrocarbons,
n-heptane and iso-octane (2,2,4-trimethylpentane). Iso-octane has an
octane number of 100 and is high in its resistance to knocking; n-heptane is
quite low (with an octane number of 0) in its resistance to knocking.
Extensive studies of the octane numbers of individual hydrocarbons have
brought to light some general rules. For example, normal paraffins have the
least desirable knocking characteristics, and these become progressively
worse as the molecular weight increases. Iso-paraffins have higher octane
numbers than the corresponding normal isomers, and the octane number
increases as the degree of branching of the chain is increased. Olefins have
markedly higher octane numbers than the related paraffins; naphthenes are
usually better than the corresponding normal paraffins but rarely have very
high octane numbers; aromatics usually have quite high octane numbers.
Blends of n-heptane and iso-octane thus serve as a reference system for
gasoline and provide a wide range of quality used as an antiknock scale. The
exact blend, which matches identically the antiknock resistance of the fuel
under test, is found, and the percentage of iso-octane in that blend is termed
the octane number of the gasoline. For example, gasoline with a knocking
Hydrocarbons from Petroleum 107
ability which matches that of a blend of 90% iso-octane and 10% n-heptane
has an octane number of 90. However, many pure hydrocarbons and even
commercial gasoline have antiknock quality above an octane number of
100. In this range it is common practice to extend the reference values by
the use of varying amounts of tetraethyl lead in pure iso-octane.
With an accurate and reliable means of measuring octane numbers, it
was possible to determine the cracking conditions – temperature, cracking
time, and pressure – that caused increases in the antiknock characteristics of
cracked gasoline. In general it was found that higher cracking temperatures
and lower pressures produced higher octane gasoline, but unfortunately
more gas, cracked residua, and coke were formed at the expense of the
volume of cracked gasoline.
To produce higher-octane gasoline, cracking coil temperatures were
pushed up to 510�C (950�F), and pressures dropped from 1000 to 350 psi.
This was the limit of thermal cracking units, for at temperatures over 510�C(950�F) coke formed so rapidly in the cracking coil that the unit became
inoperative after only a short time on-stream. Hence it was at this stage that
the nature of the gasoline-producing process was re-examined, leading to
the development of other processes, such as reforming, polymerization, and
alkylation for the production of gasoline components having suitably high
octane numbers.
It is worthy of note here that the continued decline in petroleum reserves
and the issue of environmental protection has emerged as of extreme
importance in the search for alternatives to petroleum. In this light,
oxygenates, either neat or as additives to fuels, appear to be the principal
alternative fuel candidates beyond the petroleum refinery.
5. KEROSENE AND RELATED FUELS
Kerosene (kerosine), also called paraffin or paraffin oil, is a flammable pale-
yellow or colorless oily liquid with a characteristic odor. It is obtained from
petroleum and used for burning in lamps and domestic heaters or furnaces,
as a fuel or fuel component for jet engines, and as a solvent for greases and
insecticides.
Kerosene is intermediate in volatility between gasoline and gas/diesel
oil. It is a medium oil distilling between 150 and 300�C (300–570�F).Kerosene has a flash point about 25�C (77�F) and is suitable for use as an
illuminant when burned in a wide lamp. The term kerosene is also too often
incorrectly applied to various fuel oils, but a fuel oil is actually any liquid or
108 Hydrocarbons from Petroleum
liquid petroleum product that produces heat when burned in a suitable
container or that produces power when burned in an engine.
Kerosene was the major refinery product before the onset of the auto-
mobile age, but now kerosene can be termed one of several secondary
petroleum products after the primary refinery product – gasoline. Kerosene
originated as a straight-run petroleum fraction that boiled between
approximately 205 and 260�C (400–500�F) (Walmsley, 1973). Some crude
oils, for example those from the Pennsylvania oil fields, contain kerosene
fractions of very high quality, but other crude oils, such as those having an
asphalt base, must be thoroughly refined to remove aromatics and sulfur
compounds before a satisfactory kerosene fraction can be obtained.
Jet fuel comprises both gasoline- and kerosene-type jet fuels meeting
specifications for use in aviation turbine power units and is often referred to
as gasoline-type jet fuel or kerosene-type jet fuel.
Jet fuel is a light petroleum distillate that is available in several forms
suitable for use in various types of jet engines. The major jet fuels used by
the military are JP-4, JP-5, JP-6, JP-7, and JP-8.
Briefly, JP-4 is a wide-cut fuel developed for broad availability. JP-6 is
a higher cut than JP-4 and is characterized by fewer impurities. JP-5 is
specially blended kerosene, and JP-7 is high-flash-point special kerosene
used in advanced supersonic aircraft. JP-8 is kerosene modeled on Jet A-l
fuel (used in civilian aircraft). From what data are available, typical hydro-
carbon chain lengths characterizing JP-4 range from C4 to C16.
Aviation fuels consist primarily of straight and branched alkanes and
cycloalkanes. Aromatic hydrocarbons are limited to 20–25% of the total
mixture because they produce smoke when burned. A maximum of 5%
alkenes is specified for JP-4. The approximate distribution by chemical class
is: straight-chain alkanes (32%), branched alkanes (31%), cycloalkanes
(16%), and aromatic hydrocarbons (21%).
Gasoline-type jet fuel includes all light hydrocarbon oils for use in aviation
turbine power units that distill between 100 and 250�C (212–480�F). It isobtained by blending kerosene and gasoline or naphtha in such a way that
the aromatic content does not exceed 25% in volume. Additives can be
included to improve fuel stability and combustibility.
Kerosene-type jet fuel is a medium distillate product that is used for aviation
turbine power units. It has the same distillation characteristics and flash point
as kerosene (150–300�C, 300–570�F, but not generally above 250�C,480�F). In addition, it has particular specifications (such as freezing point)
which are established by the International Air Transport Association (IATA).
Hydrocarbons from Petroleum 109
5.1. CompositionChemically, kerosene is a mixture of hydrocarbons; the chemical compo-
sition depends on its source, but it usually consists of about ten different
hydrocarbons, each containing from 10 to 16 carbon atoms per molecule;
the constituents include n-dodecane (n-C12H26), alkyl benzenes, and
naphthalene and its derivatives. Kerosene is less volatile than gasoline; it
boils between about 140�C (285�F) and 320�C (610�F).Kerosene, because of its use as a burning oil, must be free of aromatic and
unsaturated hydrocarbons, as well as free of the more obnoxious sulfur
compounds. The desirable constituents of kerosene are saturated hydro-
carbons, and it is for this reason that kerosene is manufactured as a straight-
run fraction, not by a cracking process.
Although the kerosene constituents are predominantly saturated mate-
rials, there is evidence for the presence of substituted tetrahydronaph-
thalene. Dicycloparaffins also occur in substantial amounts in kerosene.
Other hydrocarbons with both aromatic and cycloparaffin rings in the same
molecule, such as substituted indan, also occur in kerosene. The predom-
inant structure of the dinuclear aromatics appears to be that in which the
aromatic rings are condensed, such as naphthalene, whereas the isolated two-
ring compounds, such as biphenyl, are only present in traces, if at all.
5.2. ManufactureKerosene was first manufactured in the 1850s from coal tar, hence the name
coal oil was often applied to kerosene, but petroleum became the major
source after 1859. From that time, the kerosene fraction is, and has
remained, a distillation fraction of petroleum. However, the quantity and
quality vary with the type of crude oil, and although some crude oils yield
excellent kerosene quite simply, others produce kerosene that requires
substantial refining.
Kerosene is now largely produced by cracking the less volatile portion of
crude oil at atmospheric pressure and elevated temperatures.
In the early days, the poorer quality kerosene was treated with large
quantities of sulfuric acid to convert them to marketable products. However,
this treatment resulted in high acid and kerosene losses, but the later devel-
opment of the Edeleanu process overcame these problems (Speight, 2007).
Kerosene is a very stable product, and additives are not required to
improve the quality. Apart from the removal of excessive quantities of
aromatics by the Edeleanu process, kerosene fractions may need only a lye
110 Hydrocarbons from Petroleum
wash or a doctor treatment if hydrogen sulfide is present to remove
mercaptans.
5.3. Properties and usesKerosene is by nature a fraction distilled from petroleum that has been used
as a fuel oil from the beginning of the petroleum-refining industry. As such,
low proportions of aromatic and unsaturated hydrocarbons are desirable to
maintain the lowest possible level of smoke during burning. Although some
aromatics may occur within the boiling range assigned to kerosene,
excessive amounts can be removed by extraction; that kerosene is not usually
prepared from cracked products almost certainly excludes the presence of
unsaturated hydrocarbons.
The essential properties of kerosene are flash point, fire point, distillation
range, burning, sulfur content, color, and cloud point. In the case of the
flash point (ASTM D-56), the minimum flash temperature is generally
placed above the prevailing ambient temperature; the fire point (ASTM
D-92) determines the fire hazard associated with its handling and use.
The boiling range (ASTM D-86) is of less importance for kerosene than
for gasoline, but it can be taken as an indication of the viscosity of the
product, for which there is no requirement for kerosene. The ability of
kerosene to burn steadily and cleanly over an extended period (ASTM
D-187) is an important property and gives some indication of the purity or
composition of the product.
The significance of the total sulfur content of a fuel oil varies greatly
with the type of oil and the use to which it is put. Sulfur content is of great
importance when the oil to be burned produces sulfur oxides that
contaminate the surroundings. The color of kerosene is of little significance,
but a product darker than usual may have resulted from contamination or
aging, and in fact a color darker than specified (ASTM D-156) may be
considered by some users as unsatisfactory. Finally, the cloud point of
kerosene (ASTM D-2500) gives an indication of the temperature at which
the wick may become coated with wax particles, thus lowering the burning
qualities of the oil.
6. DIESEL FUEL
Diesel fuel oil is essentially the same as furnace fuel oil, but the proportion of
cracked gas oil is usually less since the high aromatic content of the cracked
gas oil reduces the cetane value of the diesel fuel.
Hydrocarbons from Petroleum 111
Diesel fuels originally were straight-run products obtained from the
distillation of crude oil. However, with the use of various cracking processes
to produce diesel constituents, diesel fuels also may contain varying amounts
of selected cracked distillates to increase the volume available for meeting
the growing demand. Care is taken to select the cracked stocks in such
a manner that specifications are met as simply as possible.
Under the broad definition of diesel fuel, many possible combinations of
characteristics (such as volatility, ignition quality, viscosity, gravity, stability,
and other properties) exist. To characterize diesel fuels and thereby establish
a framework of definition and reference, various classifications are used in
different countries. An example is ASTM D-975 in the United States in
which grades No. l-D and 2-D are distillate fuels, the types most commonly
used in high-speed engines of the mobile type, in medium-speed stationary
engines, and in railroad engines. Grade 4-D covers the class of more viscous
distillates and, at times, blends of these distillates with residual fuel oils. No.
4-D fuels are applicable for use in low- andmedium-speed engines employed
in services involving sustained load and predominantly constant speed.
Cetane number is a measure of the tendency of a diesel fuel to knock in
a diesel engine. The scale is based upon the ignition characteristics of two
hydrocarbons, n-hexadecane (cetane) and 2,3,4,5,6,7,8-heptamethylno-
nane. Cetane has a short delay period during ignition and is assigned
a cetane number of 100; heptamethylnonane has a long delay period and has
been assigned a cetane number of 15. Just as the octane number is mean-
ingful for automobile fuels, the cetane number is a means of determining
the ignition quality of diesel fuels and is equivalent to the percentage by
volume of cetane in the blend with heptamethylnonane, which matches the
ignition quality of the test fuel (ASTM D-613).
7. GAS OIL AND FUEL OIL
Fuel oil is classified in several ways but generally may be divided into two
main types: distillate fuel oil and residual fuel oil. Distillate fuel oil is vaporized
and condensed during a distillation process and thus has a definite boiling
range and does not contain high-boiling constituents. A fuel oil that contains
any amount of the residue from crude distillation of thermal cracking is
a residual fuel oil. The terms distillate fuel oil and residual fuel oil are losing their
significance, since fuel oil is now made for specific uses and may be either
distillates or residuals or mixtures of the two. The terms domestic fuel oil, diesel
fuel oil, and heavy fuel oil are more indicative of the uses of fuel oils.
112 Hydrocarbons from Petroleum
Domestic fuel oil is fuel oil that is used primarily in the home. This
category of fuel oil includes kerosene, stove oil, and furnace fuel oil; they are
distillate fuel oils.
Diesel fuel oil is also a distillate fuel oil that distills between 180 and
380�C (356–716�F). Several grades are available depending on uses: diesel
oil for diesel compression ignition (cars, trucks, and marine engines) and
light heating oil for industrial and commercial uses.
Heavy fuel oil comprises all residual fuel oils (including those obtained
by blending). Heavy fuel oil constituents range from distillable constitu-
ents to residual (non-distillable) constituents that must be heated to 260�C(500�F) or more before they can be used. The kinematic viscosity is
above 10 centistokes at 80�C (176�F). The flash point is always above
50�C (122�F) and the density is always higher than 0.900. In general,
heavy fuel oil usually contains cracked residua, reduced crude, or cracking
coil heavy product which is mixed (cut back) to a specified viscosity with
cracked gas oils and fractionator bottoms. For some industrial purposes in
which flames or flue gases contact the product (ceramics, glass, heat
treating, and open hearth furnaces) fuel oils must be blended to contain
minimum sulfur contents, and hence low-sulfur residues are preferable for
these fuels.
No. 1 fuel oil is a petroleum distillate that is one of the most widely used
of the fuel oil types. It is used in atomizing burners that spray fuel into
a combustion chamber where the tiny droplets burn while in suspension. It
is also used as a carrier for pesticides, as a weed killer, as a mold release agent
in the ceramic and pottery industry, and in the cleaning industry. It is found
in asphalt coatings, enamels, paints, thinners, and varnishes. No. 1 fuel oil is
a light petroleum distillate (straight-run kerosene) consisting primarily of
hydrocarbons in the range C9–C16. Fuel oil No. l is very similar in
composition to diesel fuel; the primary difference is in the additives.
No. 2 fuel oil is a petroleum distillate that may be referred to as domestic
or industrial. The domestic fuel oil is usually lower boiling and a straight-
run product. It is used primarily for home heating. Industrial distillate is
a cracked product or a blend of both. It is used in smelting furnaces, ceramic
kilns, and packaged boilers. No. 2 fuel oil is characterized by hydrocarbon
chain lengths in the C11–C20 range. The composition consists of aliphatic
hydrocarbons (straight-chain alkanes and cycloalkanes) (64%), l–2% unsat-
urated hydrocarbons (alkenes), and aromatic hydrocarbons (including alkyl
benzenes and 2-ring, 3-ring aromatics) (35%) but contains only low
amounts of the polycyclic aromatic hydrocarbons (<5%).
Hydrocarbons from Petroleum 113
No. 6 fuel oil (also called Bunker C oil or residual fuel oil) is the residuum
from crude oil after naphtha-gasoline, No. 1 fuel oil, and No. 2 fuel oil have
been removed. No. 6 fuel oil can be blended directly to heavy fuel oil or
made into asphalt. Residual fuel oil is more complex in composition and
impurities than distillate fuels. Limited data are available on the composition
of No. 6 fuel oil. Polycyclic aromatic hydrocarbons (including the alkylated
derivatives) and metal-containing constituents are components of No. 6
fuel oil.
Stove oil, like kerosene, is always a straight-run fraction from suitable
crude oils, whereas other fuel oils are usually blends of two or more frac-
tions, one of which is usually cracked gas oil. The straight-run fractions
available for blending into fuel oils are heavy naphtha, light and heavy gas
oils, reduced crude, and pitch. Cracked fractions such as light and heavy gas
oils from catalytic cracking, cracking coil tar, and fractionator bottoms from
catalytic cracking may also be used as blends to meet the specifications of the
different fuel oils.
Since the boiling ranges, sulfur contents, and other properties of even
the same fraction vary from crude oil to crude oil and with the way the
crude oil is processed, it is difficult to specify which fractions are blended to
produce specific fuel oils. In general, however, furnace fuel oil is a blend of
straight-run gas oil and cracked gas oil to produce a product boiling in the
175–345�C (350–650�F) range.The manufacture of fuel oils at one time largely involved using what was
left after removing desired products from crude petroleum. Now fuel oil
manufacture is a complexmatter of selecting and blending various petroleum
fractions to meet definite specifications, and the production of a homo-
geneous, stable fuel oil requires experience backed by laboratory control.
8. LUBRICATING OIL
After kerosene the early petroleum refiners wanted paraffin wax for the
manufacture of candles, and lubricating oil was, at first, a by-product of wax
manufacture. The preferred lubricants in the 1860s were lard oil, sperm oil,
and tallow. The demand that existed for kerosene did not develop for
petroleum-derived lubricating oils. In fact, oils were used to supplement the
animal and vegetable oils used as lubricants. However, as the trend to heavier
industry increased, the demand for mineral lubricating oils increased, and
after the 1890s petroleum displaced animal and vegetable oils as the source
of lubricants for most purposes.
114 Hydrocarbons from Petroleum
Mineral oils are often used as lubricating oils but also have medicinal and
food uses. A major type of hydraulic fluid is the mineral oil class of hydraulic
fluids. The mineral-based oils are produced from heavy-end crude oil
distillates. Hydrocarbon numbers ranging from C15 to C50 occur in the
various types of mineral oils, with the heavier distillates having higher
percentages of the higher carbon number compounds.
Crankcase oil (motor oil) may be either mineral-based or synthetic. The
mineral-based oils are more widely used than the synthetic oils and may be
used in automotive engines, railroad and truck diesel engines, marine
equipment, jet and other aircraft engines, and most small 2- and 4-stroke
engines. The mineral-based oils contain hundreds to thousands of hydro-
carbon compounds, including a substantial fraction of nitrogen- and sulfur-
containing compounds. The hydrocarbons are mainly mixtures of straight
and branched chain hydrocarbons (alkanes), cycloalkanes, and aromatic
hydrocarbons. Polynuclear aromatic hydrocarbons (and the alkyl deriva-
tives) and metal-containing constituents are components of motor oils and
crankcase oils, with the used oils typically having higher concentrations
than the new unused oils. Typical carbon number chain lengths range from
C15 to C50.
8.1. CompositionLubricating oils are distinguished from other fractions of crude oil by their
usually high (>400�C,>750�F) boiling point, as well as their high viscosity.Materials suitable for the production of lubricating oils are comprised
principally of hydrocarbons containing from 25 to 35 or even 40 carbon
atoms per molecule, whereas residual stocks may contain hydrocarbons with
50 or more (up to 80 or so) carbon atoms per molecule. The composition of
lubricating oil may be substantially different from the lubricant fraction from
which it was derived, since wax (normal paraffins) is removed by distillation
or refining by solvent extraction and adsorption preferentially removes non-
hydrocarbon constituents as well as polynuclear aromatic compounds and
the multi-ring cycloparaffins.
Normal paraffins up to C36 have been isolated from petroleum, but it is
difficult to isolate any hydrocarbon from the lubricant fraction of petro-
leum. Various methods have been used in the analysis of products in the
lubricating oil range, but the most successful procedure involves a technique
based on the correlation of simple physical properties, such as refractive
index, density, and molecular weight or viscosity. Results are obtained in the
form of carbon distribution and the methods may also be applied to oils that
Hydrocarbons from Petroleum 115
have not been subjected to extensive fractionation. Although they are
relatively rapid methods of analysis, the lack of information concerning the
arrangement of the structural groups within the component molecules is
a major disadvantage.
Nevertheless, there are general indications that the lubricant fraction
contains a greater proportion of normal and branched paraffins than the
lower boiling portions of petroleum. For the polycycloparaffin derivatives,
a good proportion of the rings appear to be in condensed structures, and
both cyclopentyl and cyclohexyl nuclei are present. The methylene groups
appear principally in unsubstituted chains at least four carbon atoms in
length, but the cycloparaffin rings are highly substituted with relatively short
side chains.
Mono-, di-, and trinuclear aromatic compounds appear to be the main
constituents of the aromatic portion, but material with more aromatic
nuclei per molecule may also be present. For the dinuclear aromatics, most
of the material consists of naphthalene types. For the trinuclear aromatics,
the phenanthrene type of structure predominates over the anthracene type.
There are also indications that the greater part of the aromatic compounds
occurs as mixed aromatic–cycloparaffin compounds.
8.2. ManufactureLubricating oil manufacture was well established by 1880, and the method
depended on whether the crude petroleum was processed primarily for
kerosene or for lubricating oils. Usually the crude oil was processed for
kerosene, and primary distillation separated the crude into three fractions,
naphtha, kerosene, and a residuum. To increase the production of kerosene
the cracking distillation technique was used, and this converted a large part
of the gas oils and lubricating oils into kerosene. The cracking reactions
also produced coke products and asphalt-like materials, which gave the
residuum a black color, and hence it was often referred to as tar (Speight,
2007).
The production of lubricating oils is well established (Sequeira, 1992)
and consists of four basic processes: (1) distillation to remove the lower
boiling and lower-molecular-weight constituents of the feedstock; (2)
solvent refining, such as deasphalting, and/or hydrogen treatment to remove
the non-hydrocarbon constituents and to improve the feedstock quality; (3)
dewaxing to remove the wax constituents and improve the low-temperature
properties; and (4) clay treatment or hydrogen treatment to prevent insta-
bility of the product.
116 Hydrocarbons from Petroleum
Chemical, solvent, and hydrogen refining processes have been devel-
oped and are used to remove aromatics and other undesirable constituents,
and to improve the viscosity index and quality of lube base stocks. Tradi-
tional chemical processes that use sulfuric acid and clay refining have been
replaced by solvent extraction/refining and hydrotreating which are more
effective, cost efficient, and generally more environmentally acceptable.
Chemical refining is used most often for the reclamation of used lubricating
oils or in combination with solvent or hydrogen refining processes for the
manufacture of specialty lubricating oils and by-products.
8.2.1. Chemical refining processesAcid–alkali refining, also called wet refining, is a process where lubricating
oils are contacted with sulfuric acid followed by neutralization with alkali.
Oil and acid are mixed and an acid sludge is allowed to coagulate. The
sludge is removed or the oil is decanted after settling, and more acid is added
and the process repeated.
Acid–clay refining, also called dry refining, is similar to acid–alkali
refining with the exception that clay and a neutralizing agent are used for
neutralization. This process is used for oils that form emulsions during
neutralization. Neutralization with aqueous and alcoholic caustic, soda ash
lime, and other neutralizing agents is used to remove organic acids from
some feedstocks. This process is conducted to reduce organic acid corrosion
in downstream units or to improve the refining response and color stability
of lube feedstocks.
8.2.2. HydroprocessingHydroprocessing, which has been generally replaced with solvent refining,
consists of lube hydrocracking as an alternative to solvent extraction, and
hydrorefining to prepare specialty products or to stabilize hydrocracked base
stocks. Hydrocracking catalysts consist of mixtures of cobalt, nickel,
molybdenum, and tungsten on an alumina or silica–alumina-based carrier.
Hydrotreating catalysts are proprietary but usually consist of nickel–
molybdenum on alumina. The hydrocracking catalysts are used to remove
nitrogen, oxygen, and sulfur, and convert polynuclear aromatics and
polynuclear naphthenes to mononuclear naphthenes, aromatics, and iso-
paraffins, which are typically desired in lube base stocks. Feedstocks consist
of unrefined distillates and deasphalted oils, solvent-extracted distillates and
deasphalted oils, cycle oils, hydrogen refined oils, and mixtures of these
hydrocarbon fractions.
Hydrocarbons from Petroleum 117
Lube hydrorefining processes are used to stabilize or improve the quality
of lube base stocks from lube hydrocracking processes and for manufacture
of specialty oils. Feedstocks are dependent on the nature of the crude source
but generally consist of waxy or dewaxed-solvent extracted or hydrogen-
refined paraffinic oils and refined or unrefined naphthenic and paraffinic oils
from some selected crude oils.
8.2.3. Solvent refining processesFeedstocks from solvent refining processes consist of paraffinic and naph-
thenic distillates, deasphalted oils, hydrogen refined distillates and deas-
phalted oils, cycle oils, and dewaxed oils. The products are refined oils
destined for further processing or finished lube base stocks. The by-products
are aromatic extracts which are used in the manufacture of rubber, carbon
black, petrochemicals, catalytic cracking feedstock, fuel oil, or asphalt. The
major solvents in use areN-methyl-2-pyrrolidone (NMP) and furfural, with
phenol and liquid sulfur dioxide used to a lesser extent.
The solvents are typically recovered in a series of flash towers. Steam or
inert gas strippers are used to remove traces of solvent, and a solvent
purification system is used to remove water and other impurities from the
recovered solvent.
Lube feedstocks typically contain increased wax content resulting from
deasphalting and refining processes. These waxes are normally solid at
ambient temperatures and must be removed to manufacture lube oil
products with the necessary low-temperature properties.
Catalytic dewaxing and solvent dewaxing (the most prevalent) are
processes currently in use. Older technologies include cold settling, pressure
filtration, and centrifuge dewaxing.
8.2.4. Catalytic dewaxingBecause solvent dewaxing is relatively expensive for the production of low
pour point oils, various catalytic dewaxing (selective hydrocracking)
processes have been developed for the manufacture of lube oil base stocks.
The basic process consists of a reactor containing a proprietary dewaxing
catalyst followed by a second reactor containing a hydrogen finishing catalyst
to saturate olefins created by the dewaxing reaction and to improve stability,
color, and demulsibility of the finished lube oil.
8.2.5. Solvent dewaxingSolvent dewaxing consists of the following steps: crystallization, filtration,
and solvent recovery. In the crystallization step, the feedstock is diluted with
118 Hydrocarbons from Petroleum
the solvent and chilled, solidifying the wax components. The filtration step
removes the wax from the solution of dewaxed oil and solvent. Solvent
recovery removes the solvent from the wax cake and filtrate for recycling by
flash distillation and stripping. The major processes in use today are the
ketone dewaxing processes. Other processes that are used to a lesser degree
include the Di/Me process and the propane dewaxing process. The most
widely used ketone processes are the Texaco solvent dewaxing process and
the Exxon Dilchill process. Both processes consist of diluting the waxy
feedstock with solvent while chilling at a controlled rate to produce a slurry.
The slurry is filtered using rotary vacuum filters and the wax cake is washed
with cold solvent. The filtrate is used to chill the feedstock and solvent
mixture. The primary wax cake is diluted with additional solvent and
filtered again to reduce the oil content in the wax. The solvent is recovered
from the dewaxed oil and wax cake by flash vaporization and recycled back
into the process.
The Texaco solvent dewaxing process (also called the MEK process) uses
a mixture of MEK and toluene as the dewaxing solvent, and sometimes uses
mixtures of other ketones and aromatic solvents. The Exxon Dilchill
dewaxing process uses a direct cold solvent dilution-chilling process in
a special crystallizer in place of the scraped surface exchangers used in the
Texaco process. The Di/Me dewaxing process uses a mixture of dichloro-
ethane and methylene dichloride as the dewaxing solvent. The propane
dewaxing process is essentially the same as the ketone process except for the
following: propane is used as the dewaxing solvent and higher-pressure
equipment is required, and chilling is done in evaporative chillers by
vaporizing a portion of the dewaxing solvent. Although this process
generates a better product and does not require crystallizers, the temperature
differential between the dewaxed oil and the filtration temperature is higher
than for the ketone processes (higher energy costs), and dewaxing aids are
required to get good filtration rates.
8.2.6. Finishing processesHydrogen finishing processes have largely replaced acid and clay finishing
processes. The hydrogen finishing processes are mild hydrogenation
processes used to improve the color, odor, thermal, and oxidative stability,
and demulsibility of lube base stocks.
The process consists of fixed bed catalytic reactors that typically use
a nickel–molybdenum catalyst to neutralize, desulfurize, and denitrify lube
base stocks. These processes do not saturate aromatics or break carbon–carbon
Hydrocarbons from Petroleum 119
bonds as in other hydrogen finishing processes. Sulfuric acid treating is still
used by some refiners for themanufacture of specialty oils and the reclamation
of used oils. This process is typically conducted in batch or continuous
processes similar to the chemical refining processes with the exception that
the amount of acid used is much lower than that used in acid refining.
Clay contacting involves mixing the oil with fine bleaching clay at
elevated temperature followed by separation of the oil and clay. This process
improves color and chemical, thermal, and color stability of the lube base
stock, and is often combined with acid finishing. Clay percolation is a static
bed absorption process used to purify, decolorize, and finish lube stocks and
waxes. It is still used in the manufacture of refrigeration oils, transformer
oils, turbine oils, white oils, and waxes.
8.2.7. Older processesBecause of cracking distillation in the primary distillation and the high
temperatures used in the still, the paraffin distillate contained dark-colored,
sludge-forming asphaltic materials. These undesirable materials were
removed by treatment with sulfuric acid followed by lye washing. Then, to
separate the wax from the acid-treated paraffin distillate, the latter was
chilled and filtered. The chilled, semisolid paraffin distillate was then
squeezed in canvas bags in a knuckle or rack press (similar to a cider press) so
that the oil would filter through the canvas, leaving the wax crystals in the
bag. Later developments saw chilled paraffin distillate filtered in hydrauli-
cally operated plate and frame presses, and the use of these continued almost
to the present time.
The oil from the press was known as pressed distillate, which was sub-
divided into three fractions by redistillation. Two overhead fractions of
increasing viscosity, the heavier with a Society of Automotive Engineers
(SAE) viscosity of about 10, were called paraffin oils. The residue in the still
(viscosity equivalent to a light SAE 30) was known as red oil. All three
fractions were again acid and lye treated and then washed with water. The
treated oils were pumped into shallow pans in the bleacher house, where air
blown through the oil and exposure to the sun through the glass roof of the
bleacher house or pan removed cloudiness or made the oils bright.
Further treatment of the paraffin oil produced pale oil; thus if the
paraffin oil was filtered through bone charcoal, fuller’s earth, clay, or similar
absorptive material, the color was changed from a deep yellow to a pale
yellow. The filtered paraffin oil was called pale oil to differentiate it from the
non-filtered paraffin oil, which was considered of lower quality.
120 Hydrocarbons from Petroleum
The wax separated from paraffin distillate by cold pressing contained
about 50% oil and was known as slack wax. The slack wax was melted and
cast into cakes, which were again pressed in a hot or hard press. This
squeezed more oil from the wax, which was known as scale wax. By a process
known as sweating, the scale wax was subdivided into several paraffin waxes
with different melting points.
In contrast, crude petroleum processed primarily as a source of lubri-
cating oil was handled differently from crude oils processed primarily for
kerosene. The primary distillation removed naphtha and kerosene fractions,
but without using temperatures high enough to cause cracking. The yield of
kerosene was thus much lower, but the absence of cracking reactions
increased the yield of lubricating oil fractions. Furthermore, the residuum
was distilled using steam, which eliminated the need for high distillation
temperatures, and cracking reactions were thus prevented. Thus, various
overhead fractions suitable for lubricating oils and known as neutral oils
were obtained; many of these were so light that they did not contain wax
and did not need dewaxing; the more viscous oils could be dewaxed by cold
pressing.
If the wax in the residual oil could not be removed by cold pressing it was
removed by cold settling. This involved admixture of the residual oil with
a large volume of naphtha, which was then allowed to stand for as long as
necessary in a tank exposed to low temperature, usually climatic cold
(winter). This caused the waxy components to congeal and settle to the
bottom of the tank. In the spring the supernatant naphtha–oil mixture was
pumped to a steam still, where the naphtha was removed as an overhead
stream; the bottom product was known as steam-refined stock. If the steam-
refined stock (bright stock) was filtered through charcoal or a similar filter
material the improvement in color caused the oil to be known as bright
stock. Mixtures of steam-refined stock with the much lighter paraffin, pale,
red, and neutral oils produced oils of any desired viscosity.
The wax material that settled to the bottom of the cold settling tank was
crude petrolatum. This was removed from the tank, heated, and filtered
through a vessel containing clay, which changed its red color to brown or
yellow. Further treatment with sulfuric acid produced white grades of
petrolatum.
If the crude oil used for the manufacture of lubricating oils contained
asphalt, it was necessary to acid treat the steam-refined oil before cold
settling. Acid-treated, settled steam-refined stock was widely used as steam
cylinder oils.
Hydrocarbons from Petroleum 121
The crude oils available in North America until about 1900 were either
paraffin base or mixed base; hence paraffin wax was always a component of
the raw lubricating oil fraction. The mixed-base crude oils also contained
asphalt, and this made acid treatment necessary in the manufacture of
lubricating oils. However, the asphalt-base crude oils (also referred to as
naphthene-base crude oils) that contained little or nowax yielded a different
kind of lubricating oil. Since wax was not present, the oils would flow at
much lower temperatures than the oils from paraffin- and mixed-base crude
oils even when the latter had been dewaxed. Hence lubricating oils from
asphalt-base crude oils became known as low cold-test oils; furthermore,
these lubricating oils boiled at a lower temperature than oils of similar
viscosity from paraffin-base crude oils. Thus higher-viscosity oils could be
distilled from asphalt-base crude oils at relatively low temperatures, and the
low cold-test oils were preferred because they left less carbon residue in
gasoline engines.
The development of vacuum distillation led to a major improvement in
both paraffinic and naphthenic (low cold-test) oils. By vacuum distillation
the more viscous paraffinic oils (even oils suitable for bright stocks) could be
distilled overhead and could be separated completely from residual asphaltic
components. Vacuum distillation provided the means of separating more
suitable lubricating oil fractions with predetermined viscosity ranges and
removed the limit on the maximum viscosity that might be obtained in
a distillate oil.
However, although vacuum distillation effectively prevented residual
asphaltic material from contaminating lubricating oils, it did not remove
other undesirable components. The naphthenic oils, for example, contained
components (naphthenic acids) that caused the oil to form emulsions with
water. In particular, naphthenic oils contained components that caused oil
to thicken excessively when cold and become very thin when hot. The
degree to which the viscosity of an oil is affected by temperature is measured
on a scale that originally ranged from 0 to 100 and is called the viscosity
index. An oil that changes the least in viscosity when the temperature is
changed has a high viscosity index. Naphthenic oils have viscosity indices of
35 or less, compared to 70 or more for paraffinic oils.
8.3. Properties and usesLubricating oil may be divided into many categories according to the types
of service they are intended to perform. However, there are two main
groups: (1) oils used in intermittent service, such as motor and aviation oils;
122 Hydrocarbons from Petroleum
and (2) oils designed for continuous service, such as turbine oils. Lubricating
oil is distinguished from other fractions of crude oil by a high (>400�C,>750�F) boiling point, as well as a high viscosity and, in fact, lubricating oil
is identified by viscosity.
This classification is based on the SAE (Society of Automotive Engi-
neers) J 300 specification. The single grade oils (e.g., SAE 20, etc.) corre-
spond to a single class and have to be selected according to engine
manufacturer specifications, operating conditions, and climatic conditions.
At –20�C (–68�F), multi-grade lubricating oil such as SAE 10W-30
possesses the viscosity of a 10Woil and at 100�C (212�F) the multi-grade oil
possesses the viscosity of an SAE 30 oil.
Oils used in intermittent service must show the least possible change in
viscosity with temperature; that is, their viscosity indices must be high.
These oils must be changed at frequent intervals to remove the foreign
matter collected during service. The stability of such oils is therefore of less
importance than the stability of oils used in continuous service for pro-
longed periods without renewal.
Oils used in continuous service must be extremely stable, but their
viscosity indices may be low because the engines operate at fairly constant
temperature without frequent shutdown.
9. WAX
Petroleum wax is of two general types: (1) paraffin wax in petroleum distillates
and (2) microcrystalline wax in petroleum residua. The melting point of wax is
not directly related to its boiling point, because waxes contain hydrocarbons
of different chemical nature. Nevertheless, waxes are graded according to
their melting point and oil content.
9.1. CompositionParaffin wax is a solid crystalline mixture of straight-chain (normal)
hydrocarbons ranging from C20 to C30 and possibly higher, that is,
CH3(CH2)nCH3 where n � 18.
It is distinguished by its solid state at ordinary temperatures (25�C,77�F) and low viscosity (35–45 SUS at 99�C, 210�F) when melted.
However, in contrast to petroleum wax, petrolatum (petroleum jelly),
although solid at ordinary temperatures, does in fact contain both solid
and liquid hydrocarbons. It is essentially a low-melting, ductile, micro-
crystalline wax.
Hydrocarbons from Petroleum 123
9.2. ManufactureParaffin wax from a solvent dewaxing operation is commonly known as slack
wax, and the processes employed for the production of waxes are aimed at
de-oiling the slack wax (petroleum wax concentrate).
Wax sweating was originally used in Scotland to separate wax fractions
with various melting points from the wax obtained from shale oils. Wax
sweating is still used to some extent but is being replaced by the more
convenient wax recrystallization process. In wax sweating, a cake of slack
wax is slowly warmed to a temperature at which the oil in the wax and the
lower melting waxes become fluid and drip (or sweat) from the bottom of
the cake, leaving a residue of higher melting wax. However, wax sweating
can be carried out only when the residual wax consists of large crystals that
have spaces between them, through which the oil and lower melting waxes
can percolate; it is therefore limited to wax obtained from light paraffin
distillate.
The amount of oil separated by sweating is now much smaller than it
used to be owing to the development of highly efficient solvent dewaxing
techniques. In fact, wax sweating is now more concerned with the sepa-
ration of slack wax into fractions with different melting points. A wax
sweater consists of a series of about nine shallow pans arranged one above
the other in a sweater house or oven, and each pan is divided horizontally by
a wire screen. The pan is filled to the level of the screen with cold water.
Molten wax is then introduced and allowed to solidify, and the water is then
drained from the pan leaving the wax cake supported on the screen.
A single sweater oven may contain more than 600 barrels of wax, and
steam coils arranged on the walls of the oven slowly heat the wax cakes,
allowing oil and the lower melting waxes to sweat from the cakes and drip
into the pans. The first liquid removed from the pans is called foots oil, which
melts at 38�C (100�F) or lower, followed by interfoots oil, which melts in the
range 38–44�C (100–112�F). Crude scale wax next drips from the wax cake
and consists of wax fractions with melting points over 44�C (112�F).When oil removal was an important function of sweating, the sweating
operation was continued until the residual wax cake on the screen was free
of oil. When the melting point of the wax on the screen has increased to the
required level, allowing the oven to cool terminates sweating. The wax on
the screen is a sweated wax with the melting point of a commercial grade of
paraffin wax, which after a finished treatment becomes refined paraffinic
wax. The crude scale wax obtained in the sweating operation may be
124 Hydrocarbons from Petroleum
recovered as such or treated to improve the color, in which case it is white
crude scale wax. The crude scale wax and interfoots, however, are the
sources of more waxes with lower melting points. The crude scale wax and
interfoots are re-sweated several times to yield sweated waxes, which are
treated to produce a series of refined paraffin waxes with melting points
ranging from about 50 to 65�C (125–150�F).Sweated waxes generally contain small amounts of unsaturated aromatic
and sulfur compounds, which are the source of unwanted color, odor,
and taste that reduce the ability of the wax to resist oxidation; the com-
monly used method of removing these impurities is clay treatment of the
molten wax.
Wax recrystallization, like wax sweating, separates slack wax into fractions,
but instead of using the differences in melting points, it makes use of the
different solubility of the wax fractions in a solvent, such as the ketone used
in the dewaxing process. When a mixture of ketone and slack wax is heated,
the slack wax usually dissolves completely, and if the solution is cooled
slowly, a temperature is reached at which a crop of wax crystals is formed.
These crystals will all be of the same melting point, and if they are removed
by filtration, a wax fraction with a specific melting point is obtained. If the
clear filtrate is further cooled, a second crop of wax crystals with a lower
melting point is obtained. Thus by alternate cooling and filtration the slack
wax can be subdivided into a large number of wax fractions, each with
different melting points.
This method of producing wax fractions is much faster and more
convenient than sweating and results in a much more complete separation of
the various fractions. Furthermore, recrystallization can also be applied to
the microcrystalline waxes obtained from intermediate and heavy paraffin
distillates, which cannot be sweated. Indeed, the microcrystalline waxes
have higher melting points and differ in their properties from the paraffin
waxes obtained from light paraffin distillates; thus wax recrystallization
makes new kinds of waxes available.
9.3. Properties and usesThe melting point of paraffin wax (ASTM D-87) has both direct and
indirect significance in most wax utilization. All wax grades are commer-
cially indicated in a range of melting temperatures rather than at a single
value, and a range of 1�C (2�F) usually indicates a good degree of refine-
ment. Other common physical properties that help to illustrate the degree of
refinement of the wax are color (ASTM D-156), oil content (ASTM
Hydrocarbons from Petroleum 125
D-721), API gravity (ASTM D-287), flash point (ASTM D-92), and
viscosity (ASTM D-88 and ASTM D-445), although the last three prop-
erties are not usually given by the producer unless specifically requested.
Petroleum waxes (and petrolatum) find many uses in pharmaceuticals,
cosmetics, paper manufacturing, candle making, electrical goods, rubber
compounding, textiles, and many more too numerous to mention here. For
additional information, more specific texts on petroleum waxes should be
consulted.
REFERENCESAbraham, H., 1945. Asphalt and Allied Substances, fifth ed. Van Nostrand Inc., New York,
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