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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


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


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


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.


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


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


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


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.


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.


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


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


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


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


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


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


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


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.


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


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


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).


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


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.


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.


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%).


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.


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


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.


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.


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


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


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.


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.


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;


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.


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


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


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.

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