1.2 the products of refining - treccani

1.2.1 Introduction

Crude oil refineries carry out two groups of successiveoperations to obtain finished products. As a first step,crude oil is treated to obtain a series of intermediateproducts, consisting in fractions having differentdistillation ranges, and a residue. These intermediateproducts are then subjected to specific processes foreach fraction, followed by blending and additivation,and so forth, to obtain the finished products that willbe put on the market.

The finished products obtained with these differenttreatments, listed in increasing order of molecularcomplexity and C/H ratio, are: Liquefied PetroleumGas (LPG), gasolines, solvents, kerosenes, gas oils,lubricating oils, paraffins, fuel oils and bitumens.

All of these products are characterized by a seriesof quality requirements, or ‘specifications’, whichdistinguish them both during production and on themarketplace.

A specification is the set of minimum requirementsthat a product must meet, and more specifically thedocument listing them. These requirements areexpressed in terms of a set of properties, testmethodologies and limits (i.e. minimum and/ormaximum values) that must be met. The propertiesincluded in the specifications for petroleum productshave different origins and aims.

Application or utilization properties ensure thatthe product performs adequately when it reaches theend user, and are required by the manufacturers ofengines, combustion plants or industrial machinery.Usually the specification limits are agreed upon bythe oil industries and the manufacturers of engines.In many instances, they are defined bystandardization bodies (such as ISO or CEN, seebelow), in which all interested parties arerepresented.

Handling properties aim to ensure that noproblems arise during the transport, storage, anddelivery of the product.

Environmental properties stem from regulationsaimed at protecting health and the environment,published in international and national legislations; forinstance, sulphur content of fuels.

Customs and fiscal properties allow one todifferentiate between various classes of commercialproducts, in order to determine the relevant fiscaltreatment.

Safety properties are determined by nationalauthorities or by international bodies responsible forthe safety of transport (IMO, IATA, etc.) and the safetyof storage regarding the choice of tanks and fittings(API, BSI, DIN, etc.); an example is the flash point.

Legal properties are those specified by the lawsand decrees in force in individual countries.

These properties are determined using analyticalmethodologies. The reference methodologies are valid

25VOLUME II / REFINING AND PETROCHEMICALS

1.2

The products of refining

Table 1. Main standards agencies

AFNOR Association Française de NORmalisation

API American Petroleum Institute

ASTM American Society for Testing and Materials

BSI British Standards Institute

CEN Comitato Europeo di Normazione

CUNA Commissione di UNificazione nell’Autoveicolo

DIN Deutsches Institut für Normung

EN European Norm

IATA International Air Transport Association

IMO International Maritime Organization

IP Institute of Petroleum

ISO International Organization for Standardization

internationally, and are defined by standardizationbodies (Table 1).

Since the results of analytical methods are subject,like those of all measurements, to a degree ofimprecision, they must always be accompanied both byinformation on how the tests were carried out, and byan indication of precision expressed in terms ofrepeatability and reproducibility.

Repeatability is the value below which the absolutedifference between two single results, obtained using thesame method on the same sample material under thesame conditions (same operator, same apparatus, samelaboratory within a short time interval), is expected tolie with a 95% probability.

Reproducibility is the value below which theabsolute difference between two single resultsobtained using the same method on the same samplematerial, under different conditions (differentoperators, different apparatus, different laboratoriesand/or at different times) is expected to lie within a95% probability.

In other words, repeatability and reproducibilitymeasure the statistical variance of the results, obtainedunder the conditions specified above, and represent thevalues within which the difference between twomeasurements, carried out under the conditionsspecified above, must fall in order to be consideredvalid.

One of the most important additional notes in anyspecification is the criterion to be used for theinterpretation of results, in the case of a disputebetween supplier and customer.

In most cases, disputes are resolved by applyingthe rules set down in standard ISO 4259, Petroleumproducts – Determination and application of precisiondata in relation to methods of test. This standardspecifies (as the criterion for the acceptability of avalue obtained experimentally) a limit of �0.59 Rwith respect to the specification value, where R is thevalue of the reproducibility of the method and 0.59 astatistical factor.

1.2.2 Liquefied petroleum gas

Liquefied Petroleum Gas (LPG) consists mainly of amixture of paraffinic hydrocarbons with three or fourcarbon atoms (propane, butane and isobutane) andsmaller amounts of unsaturated hydrocarbons(olefins). It may also contain small amounts ofhydrocarbons with two or five carbon atoms, and isobtained by extraction from so-called natural gas or, inrefineries, by treating crude oil.

LPG has a sufficiently low vapour pressure toallow it to be compressed and stored in the liquid state

at normal ambient temperature. When pressurized inmetal containers (pressure bottles) or in tanks, LPGcan be transported easily and can be used immediatelyfor numerous applications, for example as a fuel.

Classification and productionThe main applications of LPG are: automotive fuel

(as an alternative to gasoline) in vehicles withspecially modified fuel lines and tanks; boiler fuel forheating and the production of hot water, stored intanks outside the building; domestic and camping fuelfor cooking and heating, using pressure bottles.

In Europe, the set of requirements for LPGconstituting the reference specifications for thisproduct as an automotive fuel are contained in theEN 589 standard, whereas LPG for heat productionmust meet national specifications. In the UnitedStates, reference specifications for LPG, both as anautomotive fuel and as an industrial and domesticfuel, are set out in the ASTM D 1835 standards.

The LPG produced in refineries from thetreatment of crude oil is largely obtained from theinitial distillation, where it is collected at the head ofthe atmospheric fractionation (topping) column, andfrom the naphtha reforming process. LPG is alsoproduced by conversion processes (catalyticcracking, hydrocracking, visbreaking). Unlike theother streams, the LPGs from topping and reforminggenerally have a low content of unsaturatedhydrocarbons (olefins).

In many plants, the finished product is prepared byappropriately blending the LPG streams from thevarious processes, and by subjecting the final blend toa specific treatment for the removal of sulphurcompounds.

Since LPG may cause explosions if it accumulatesin closed areas, the product must be odorized forsafety reasons before it can be sold, in order to give itan unpleasant and perceptible odour that makes leakseasily detectable by smell.

PropertiesThe main properties of LPG are listed below,

divided according to their function; some of these onlyconcern specific uses of the product.

Visual propertiesThe presence of water (free, emulsified or

dissolved) in LPG may derive from the productionprocess or from handling. It is important to control thepresence of water since the transition from the liquidto the gas phase during use may entail strong cooling,below 0°C, causing any water present to solidify. Inthis case, the ice crystals may block the fuel pressurereducers.

26 ENCYCLOPAEDIA OF HYDROCARBONS

OIL REFINING INDUSTRY: GENERAL ASPECTS

Compositional propertiesC2 hydrocarbons (ethane and ethylene) have a high

vapour pressure; therefore their presence in LPG mustbe limited in order to avoid an overly volatile product.

The content of C3 and C4 hydrocarbons is checkedfor commercial and fiscal reasons; this also makes itpossible to characterize and distinguish betweenpropane, butane and LPG mix. The following limitsare usually applied: for propane, at least 85% mol ofC3; for butane, at least 85% mol of C4; for LPG mix atleast 20% mol of C3.

Hydrocarbons with five or more carbon atoms (C5�)are not particularly volatile and thus cannot be vaporizedin pressure reducers; therefore, they cannot be used forthe purposes for which LPG is destined. An excessivecontent of olefins (hydrocarbons with one or moredouble bonds) may cause malfunctions during use, dueto the formation of gummy sediments (polymers). Thedienes (olefinic hydrocarbons with two double bonds)content is controlled to avoid the formation of gums andresins that may clog pressure reducers.

Combustion propertiesThe Octane Number (ON) is a parameter that is

relevant only for the use of LPG as an automotive fuel,and represents a measurement of its antiknockcapacity, i.e. the capacity to withstand strongcompression without detonating, undergoing a seriesof anomalous combustion phenomena causing violentpressure transitions.

The Octane Number of LPG is measured using theMotor method. This provides a measurement(MON, Motor Octane Number) of the fuelantiknocking capacity in spark ignition engines.A sufficiently high value for MON ensures that theLPG provides an acceptable performance level inmotor vehicles. Since each hydrocarbon has its ownMON value, the specification leaves room for somecompositional flexibility.

The Net Calorific Value (NCV) is a parametercharacterizing the use of LPG as a fuel. It representsthe amount of energy released by the combustion of 1kg of product, after subtracting the heat of evaporationof any water present and the water formed duringcombustion. The NCV depends on the hydrogen-carbon ratio, and thus on the composition of the LPG.

Volatility propertiesMass per unit volume (density) is used for mass (or

weight) to volume conversions; it may also provide anapproximate indication of the composition of the LPG(or more generally of any petroleum product), and istherefore also considered a volatility property.

The values of Vapour Pressure (VP) at 40°C and10°C are extremely important for LPGs. The vapour

pressure at 40°C is correlated with operational safetyof pressurized appliances (tanks, piping, etc.); vapourpressure at 10°C is correlated with the possibility ofproviding an adequate feed pressure to engines andcombustion plants under all circumstances.

The evaporation residue is correlated with thecontrol of the LPG’s non-volatile heavy hydrocarboncontent, in order to guarantee a correct functioning ofthe vaporizer.

Corrosion propertiesA copper strip corrosion test is carried out because

the product may present a tendency to corrode thematerials used in fuel lines due to acidity andimpurities.

Hydrogen sulphide is a gas whose presence in LPGis not permitted, since it is highly corrosive to all themetal parts of storage systems and fuel lines. It is alsoextremely toxic.

1.2.3 Gasolines

Gasoline is a mixture of relatively light hydrocarbons,containing 4 to 12 carbon atoms and with a typicaldistillation range from 30°C to about 220°C. It is usedas a fuel for spark ignition internal combustionengines, sometimes blended with other products ofnon-petroleum origin (mainly oxygenate compounds,such as alcohols and ethers). More broadly, the termgasoline is also applied to other products with adistillation interval comprised lying within the samelimits, even if they are destined for other purposes.

Different qualities of gasoline are produced tosatisfy the various needs of consumers, depending onthe type of engine, climatic conditions and drivinghabits. Over the course of the year, the formulation ofgasoline may be modified to adapt it to seasonalvariations. Frequently, very small quantities of specificchemical products, so-called additives, may also beadded to gasolines with the aim of improving someaspects of their performance.

Classification and productionThe most widespread classification criterion for

gasolines is based on one of the main properties of thisfuel, that is, the antiknocking capacity. This property isexpressed by the Octane Number on a conventionalscale, defined using a number of pure hydrocarbons asreference points.

Gasoline is produced in the refinery by suitablyblending various hydrocarbon fractions obtainedwith the application of available technologies,together with other products of different origins(such as oxygenates and additives), taking care to


THE PRODUCTS OF REFINING

meet a predetermined set of minimum qualityrequirements. The set of requirements(specifications) consists of a list of properties,limits, and test methods. The European referencespecifications are contained in the EN 228 standard,whereas in the US reference is made to the ASTMstandards, to mention only the most widespread ones(see again Table 1).

The composition and properties of thehydrocarbon components of a gasoline may varydepending on the type and nature of the crude oiltreated, the transformation processes available inthe refinery, the thermodynamic conditions underwhich the process takes place, the overall balancebetween the market demand for gasoline and otherpetroleum products, and finally by the referencespecification.

The refinery hydrocarbon fractions suitable for thepreparation of gasoline are usually: a) butanes (C4)from various refinery plants; b) light virgin naphthafrom primary distillation (C5-C6); c) gasoline fromcatalytic reforming; d ) light catalytic naphtha; e) isomerate (fraction from the isomerization process);f ) alkylate (fraction from the alkylation process).

These fractions, often known as components,are present in gasoline in percentages that basicallydepend on the overall balance between the variousproductions inside the refinery and on the qualityrestrictions imposed by the referencespecifications.

The oxygenate components which may potentiallybe added are governed by law, in terms of both qualityand quantity. In Europe, the following products may beadded: methanol, ethanol, isopropyl alcohol, isobutylalcohol, tert-butyl alcohol, methyl tert-butyl ether(MTBE), ethyl tert-butyl ether (ETBE), tert-amylmethyl ether (TAME), and other ethers with more than5 carbon atoms (as long as their final distillation pointis no higher than that of gasoline).

As noted above, the commercial product may alsocontain specific additives, which serve to improvesome properties. The main additives have thefollowing functions: antioxidants, antiknocking,corrosion inhibitors and metal deactivators. Detergentsare also added to limit the formation of resins andgums, and to keep clean or clean up the fuel lines ofengines, including the intake valves.

Finally, gasolines may be dyed for tax purposes orto distinguish visually between the various grades. Thetotal percentage of all these additives, however, doesnot exceed 0.1% in weight.

PropertiesThe main properties of gasoline are listed below,

divided according to their various functions.

Visual propertiesGasoline must not contain suspended solid or

liquid impurities; in other words, it must appear clearand ‘sparkling’. The presence of these impurities mayfoul fuel lines, or hinder the correct flow of the fuelthrough filters or other devices regulating the amountof gasoline introduced to the combustion chamber,such as the jets of carburettors or injectors.

The natural colour of gasoline depends on the typeof hydrocarbons that are present in the product. Often,gasoline has an artificial colour deriving from theaddition of a dye prescribed by law, in order to allowthe product to be identified visually for tax inspection.

Compositional propertiesAromatic hydrocarbons are characterized by the

presence of at least one aromatic (benzene) ring intheir structure. In gasoline, the aromatic hydrocarbonspresent in the largest amounts are the so-called BTX:Benzene, Toluene and Xylene. This class ofcompounds presents a greater hazard to humans thanother naphthenic and/or paraffinic hydrocarbons. Ingasolines, the level of aromatics is controlled in orderto limit the harmfulness of emissions due toevaporation and vehicle exhaust fumes. Benzene is thefirst hydrocarbon of the aromatic series. It is foundnaturally in crude oil, and forms in some gasolineproduction processes; it is a carcinogenic substance tohumans. To safeguard health and protect theenvironment, in Europe the benzene content ofgasoline must not exceed 1% in volume.

Olefins are a family of hydrocarbons characterizedby one or more double C�C bonds. Their presence ingasoline derives mainly from the fraction produced bycatalytic cracking. The olefin content must becontrolled for reasons of performance andenvironmental protection: a high olefin content leadsboth to an increase in sensitivity (the parameter thatmeasures the variation in the antiknocking capacity asthe severity of the engine’s operating conditions varies;see below) and, given the high reactivity of this classof compounds, an increase in the product’s ability toproduce photochemical smog.

The total sulphur content is a compositionalproperty linked to the origin of the crude oil and thetype of treatment carried out in the refinery. Highersulphur contents are found in gasolines containingcracking fractions. Limiting the sulphur content ofgasoline allows to contain the efficiency losses ofcatalytic exhaust converters. This also prevents theemission of other types of pollutants. The sulphur ingasoline is oxidized and turned into sulphur oxides(corrosive and pollutant), whereas in catalyticvehicles, it may also be reduced to hydrogen sulphide,a foul-smelling and toxic compound.



Combustion propertiesThe best antiknocking properties are guaranteed by

branched chain paraffinic hydrocarbons, followed bylinear-chain aromatic, naphthenic and paraffinichydrocarbons. Good antiknocking properties are alsoguaranteed by oxygenate compounds, such as MTBE,widely used for this reason. The antiknocking capacityof gasoline is expressed by the Octane Number, whichis measured on special laboratory engines bycomparison with reference fuels. There are various testprocedures, and the indication of the Octane Numbermust therefore be accompanied by a reference to themethod used. The two most important methods are: theMotor method, which indicates the resistance ofgasoline to detonation at high engine speed, forexample, in motorway use (the corresponding OctaneNumber is indicated with the acronym MON, MotorOctane Number); the Research method, whichindicates the resistance to detonation at start-upconditions and acceleration from low speed in highgear; this is the number most frequently referred to,when distinguishing between the various grades ofgasoline; the corresponding Octane Number isindicated with the acronym RON (Research OctaneNumber).

A further parameter linked to the Octane Numberis sensitivity, which represents the difference between

the Research and Motor octane numbers. It defines thebalance between RON and MON in relation tocomposition, production processes and the functioningof the engine. Acceptable values for this parameter arethose lower than 10.

Volatility propertiesThe distillation curve is of fundamental

importance in the characterization of gasoline. It isadjusted during production by acting on the propertiesof the components and the formulation of the finishedproduct, with the aim of obtaining the mostappropriate progressive evaporation properties, inorder for the engine to function correctly and ensurethe adequate driveability of vehicles. The parametersfor the distillation curve generally established byquality specifications are shown in Table 2.

The vapour pressure provides an empiricalmeasurement of the tendency of gasoline to evaporate.It is adjusted during production by the addition of thelightest fraction (C4). The vapour pressure must beabove a minimum value to ensure that the vehiclestarts when cold, but must be kept below a maximumvalue in order to contain the evaporation losses ofgasoline from storage facilities, tanks, service stationsand fuel lines. Evaporation is responsible for productlosses and environmental problems, with the emission



Table 2. Values for distillation curve parameters prescribed by quality specifications for gasolines

Evaporated at 70°C

This is the percentage in volume of evaporated liquid at the temperature of 70°C.The value prescribed by the specification serves to ensure the presence of a balanced light fraction to ease starting (minimum value), and the driveability of the vehicle, as well as to avoid the formation of vapour bubbles in the fuel lines (maximum value). The limits must fall within a predeterminedinterval, which depends on climatic conditions. Values are lower in summer and higher in winter

Evaporated at 100°C

This is the percentage in volume of evaporated liquid at the temperature of 100°C.The value prescribed by the specification serves to ensure the presence of a balanced ‘body’in the distillation curve to facilitate the vehicle’s ‘driveability’. The limits must fall within a predetermined interval, which depends on the season. Values are lower in summer and higher in winter

Evaporated at 180°C

This is the percentage in volume of evaporated liquid at the temperature of 180°C.A minimum of 85% is required

Difference between 90% distilled and 5% distilled

This is the difference between the temperatures at which 5% and 90% of the volume distil.This measurement has fiscal purposes, because it serves to distinguish between automotive gasoline andthat for industrial use. The minimum margin of 60°C allows a distinction to be made between the taxclass of gasolines with a broad distillation interval, and the class of ‘special’ gasolines with a narrowdistillation interval (such as turpentine)

End Point

This is the temperature at which the vaporization of the product during the distillation test is complete.Limiting this value serves to control the presence of heavy hydrocarbons, which do not vaporize easilyand therefore compromise correct combustion.The limits generally fall between 180 and 210°C. Higher values are found only in the case ofcontamination with heavier products (such as gas oil)

ResidueThis is the percentage of liquid that does not vaporize during the distillation test.This value serves to limit the presence of very heavy fractions which may compromise the correctfunctioning of fuel lines and seep into the lubricant, reducing its viscosity and increasing consumption

of Volatile Organic Compounds (VOCs) and theresulting formation of atmospheric ozone in cities,especially during the summer. The limits varydepending on the country and the season, vapourpressure being higher in winter and cold climates, andlower in summer and hot climates.

The Vapour Lock Index (VLI) is a number thatexpresses the tendency of a gasoline to form vapourbubbles, and combines the value of evaporate at 70°Cwith the vapour pressure, using an empirical equation.It is important to keep the value of vapour pressureand/or evaporate at 70°C within limits that guaranteethe correct working of the engine. The limit for VLI isset according to the climate, the country and theseason; higher values are needed in winter and coldclimates, and lower values in summer and hotclimates.

The mass per unit volume (density) is afundamentally important parameter for enginedesigners, because it allows them to define the mostsuitable air/fuel ratios for the various operatingregimes of the engine. At the same time, it is equallyimportant in commercial transactions because it allowsmass (or weight) to be calculated from volume, andvice versa.

The flash point expresses the minimum fueltemperature at which its vapours ignite in contact witha spark. This is a property of gasoline, which providesuseful indications for the safe handling of the product,especially when managing large quantities.

Cleanliness propertiesGums are polymerized hydrocarbons which

hinder the functioning of fuel lines. For this reason, itis necessary to limit the formation of deposits, gumsand resins on the carburettor, injectors and intakevalves. The presence of gums indicates the presenceof heavy olefinic or non-volatile compounds. The useof special detergents to remove the deposits that mayform on carburettors, injectors and intake valves(clean up), or to hinder their formation (keep clean)is increasingly frequent.

Corrosion propertiesSubstances with acid functional groups may be

present in the product. These may in turn causecorrosion and act as precursors to the formation ofproducts of degradation and oxidation. The absence ofacidity indicates that the product is of high quality, andincreases the operational reliability of the logistic chain.

A test (i.e. the copper strip corrosion test) isusually carried out to evaluate the product’s tendencyto attack the materials in a vehicle’s fuel lines.

Since hydrogen sulphide and mercaptans may bepresent in gasolines that have not been adequately

desulphurized, it is important to check the content ofthese products, which have a corrosive effect on tanks,using suitable tests (e.g. the Doctor Test).

Stability propertiesThe induction period is an empirical test, which

evaluates the tendency of a gasoline to decompose byself-oxidation, giving rise, over time, to the formationof gums. Determining this property helps to ensuregood stability during storage.

1.2.4 Hydrocarbon solvents

Hydrocarbon solvents are a class of compoundsderiving from crude oil. Within the solvents class, adistinction can be made for commercial purposesbetween various types of products: aliphatics,isoparaffins (which chemically are still aliphaticcompounds), cyclics, distillates with a narrowdistillation range, and aromatics. Some solvents arespecific chemical compounds; others are mixtures ofhydrocarbons. The mixtures are usually distinguishedon the basis of their distillation range.

SBS (Special Boiling Solvents) are mixturescomposed of aliphatic hydrocarbons C5-C9 (linear andbranched paraffins, cycloparaffins) with a boilingrange of 30-160°C.

Other aliphatic solvents have higher distillationranges, 150-220°C, and longer chains, C7-C12. Thesesolvents also contain aromatic components.

On the market there are also solvents with aboiling interval of 60-300°C, and more than ninecarbon atoms. Aromatic solvents, with theexception of toluene, are mixtures of isomers.

Classification and productionUnlike other petroleum products, solvents are often

classified on the basis of their industrial applications(solvents for protective coatings, inks, paints andvarnishes, adhesives, aerosol, extractions, cleaning, asreaction media).

For this reason, there are no referencespecifications for solvents; each industrial sector setsspecific quality requirements depending on theapplication for which the products are intended.

Aromatic products are obtained by extraction fromcracking and reforming fractions.

Non-aromatic products are obtained by distillationof light virgin naphtha fractions or desulphurizedkerosenes.

PropertiesThe essential properties for all applications are

described below.

30


ENCYCLOPAEDIA OF HYDROCARBONS

The solvent power is the ability to dissolve anothercomponent, known as the solute. This is an importantindication for all those purposes (protective coatings,varnishes, paints, adhesives, inks and aerosols) inwhich the solvent acts as a carrier for an activecomponent and evaporates after application. The valueof the solvent power, determined by comparison withother solvents, is specified using the kauri-butanolnumber for light aliphatic solvents or the aniline pointfor other products. The aniline point is also used toobtain an indication of the aromatic content of amixture, since aromatic hydrocarbons have low values,while paraffinic hydrocarbons have higher ones.

Volatility is one of the parameters that determinethe behaviour of a solvent. This property is stronglyinfluenced by the boiling point (range), vapourpressure and evaporation velocity.

The boiling point of a pure liquid is thetemperature at which the vapour pressure of the liquidis identical to the atmospheric pressure. When a ‘pure’liquid shows a narrow boiling interval (a few °C), thismeans either that impurities are present, as is the casefor industrial solvents, or that it is a mixture ofisomers. When dealing with a mixture, there is not aboiling point, but rather a wider boiling interval. Inthis case, for practical purposes, it is necessary toknow the initial boiling point and the final boilingpoint. In the technology of petroleum products,the initial boiling point refers to the temperature atwhich the first condensed drop of evaporated solventforms; the final boiling point refers to the temperatureat which the final drop of distillate forms.

The vapour pressure at a given temperature is anindirect measurement of the ease with which a productevaporates. For users, it is a necessary indication toevaluate the ease with which the product passes from theliquid to the vapour state at the application temperature.

The evaporation velocity is a relative measurementof the velocity with which the solvent passes from theliquid to the vapour state. It is determined bycomparison with one or more reference solvents. Thisis an extremely important application parameter; forpractical purposes its value must not be too low, sincein most applications the solvent must evaporateleaving the active part, nor must it be too high, sincethis would encourage the condensation by cooling ofatmospheric humidity on the layer of active product,and, in spray application, would cause the formation ofpointed or bubbled surfaces. The evaporation ofsolvents contributes significantly to the amount ofVolatile Organic Compounds (VOCs) in the air.

Viscosity, which expresses the internal resistanceof a fluid to flow, depends on the hydrocarboncomposition of the solvent, and in particular on thedistillation curve (it increases passing from light to

heavy fractions). Furthermore, it varies significantlywith temperature (decreasing as temperatureincreases). The knowledge of its value makes itpossible to select the most suitable solvent forstabilizing the active part and facilitating theapplication of the final solution.

In addition to the properties mentioned above, andthose peculiar to each specific use, a good industrialsolvent must also possess the following additionalproperties: it must be clear, colourless and sufficientlyvolatile to be removed without leaving residues; itmust not react chemically with the substance insolution; it must have an acceptable smell, constantphysical properties, low toxicity and ecotoxicity, andgood biodegradability.

1.2.5 Kerosenes

Kerosene is a mixture of hydrocarbons, characterizedby 9-16 carbon atoms, with a typical distillation rangebetween 180 and 280°C. It is obtained by treatingcrude oil, essentially by atmospheric distillationprocesses, and is an intermediate distillate betweengasoline and gas oil.

Kerosene is mainly used as a fuel for turbineengines, which are widely adopted for aircraft,helicopters and other vehicles. On-board aircraft,kerosene is also used as a hydraulic and cooling fluid.

In the past, it was used as a fuel for heating andlighting; this use is now marginal.

Classification and productionThe quality classification depends on the sector in

which the product is to be used. The kerosene used foraviation is generally known as Jet Fuel: the mostwidespread products are those used in aircraft forcivilian transport (Jet A-1) and military uses (JP-5 andJP-8). Generally speaking, these fuels contain a highproportion of additives alongside the hydrocarbonfraction. The quality requirements are set down inreference specifications, which are generally validworldwide.

The major reference specifications for civilaviation kerosene (Jet A-1) are the following: a) ASTM D 1655 Jet A-1 (issued by ASTM in theUnited States); b) DEF STAN 91 (issued by the BritishMinistry of Defence); c) IATA Guidance List (issuedby IATA, the association of airline companies); d) Joint Fuelling System Check List (JFSCL) forJet A-1: this is the specification used by the major oilcompanies to produce and commercialize Jet Fuel forcivilian use. The quality requirements included in thislast specification meet those of all three previousspecifications.



The reference specifications for JP-5 and JP-8 aredrawn up by the United States Department of Defence.

JP-8 is the aviation fuel used by the Air Forces of NATO countries, and is also known by the relevantNATO code (F 34).

The Jet Fuel sector was among the first tointroduce detailed quality control procedures (JFSCLGuidelines) due to the sensitive nature of this product’suse. The quality specification is extremely complexand detailed, and the relevant certification must meetwell-defined and precise criteria. Furthermore, thehandling of the product from the refinery to itsdelivery to the aircraft is subject to predeterminedrules and procedures, with intermediate qualitycontrols.

Generally speaking, all of the properties includedin the specifications mentioned above result both frompractical requirements and the need to ensuremaximum safety.

A large proportion of kerosene (especially thatdestined for aviation) is produced by the atmosphericdistillation of crude oil. Cuts from other refiningprocesses, such as hydrocracking, may be added to thisfraction. Generally, cuts from catalytic cracking orthermal cracking units are not used.

The final blend is subjected to desulphurizationprocesses in which sulphur compounds are removedbefore it is used.

The product for civil aviation, as prescribed by thereference specifications, must contain antioxidant andantistatic additives; that used for military aviationmust also contain icing inhibitor, anti-corrosionadditives and metal deactivators. Sometimes, additivesthat improve the lubricating power are also added.

PropertiesThe main properties of the kerosene used as a fuel

for turbine engines are listed below, subdividedaccording to their functions.

Visual propertiesThe product must appear clear, ‘sparkling’ and

must not show the presence of contaminants, in orderto guarantee the absence of obvious quality problems(the visible presence of water, impurities and solidsediments).

Compositional propertiesThe product may contain substances with acid

groups, capable of corroding tanks and piping.The absence of acidity is an indication that the

product is of high quality, and increases theoperational reliability of the fuel lines in aircraft.

The presence of aromatic hydrocarbons leads toincreased smoke production and a higher degree of

heat radiation as compared to other families ofhydrocarbons. A minimal aromatic content isnonetheless necessary to avoid the breakage of theelastomeric sleeves and gaskets used on aircraft.

Mercaptans are substances characterized by thepresence of one or more functional �C�SH groupsin their molecular structure. The presence ofsignificant amounts of mercaptan sulphur may resultfrom an ineffective desulphurization process. Inaddition to causing unpleasant smells, mercaptansulphur has a detrimental effect on elastomers (sleevesand gaskets) and may corrode some parts of the fuellines in aircraft. Limiting mercaptan sulphur lengthensmaintenance intervals and increases the operationalreliability of the engine.

The presence of sulphur is a compositionalcharacteristic linked to the origin of the crude oil andto refinery treatments. Sulphur causes sulphur oxideemissions, which are responsible for air pollution inproximity of airports; the global environmental effectsof sulphur oxides released at altitude are still beingstudied.

Naphthalenes are hydrocarbons with two aromaticrings, which produce a smoky flame and a high degreeof heat radiation when burned. Their level must becontrolled to ensure the effective combustion of thefuel, and maximize the working life of combustors andother hot parts of turbines.

Olefins may be present in significant quantities inJet Fuel if components from cracking processes areused for blending. Their presence must be limited toavoid problems such as instability and the formation ofgums, thus allowing for adequate storage times intanks and making it possible to use Jet Fuel as acooling fluid on-board of aircraft.

Combustion propertiesThe Net Calorific Value (NCV) is the amount of

energy released by the combustion of 1 kg of product,after subtracting the latent heat of evaporation of thewater formed during combustion. The NCV dependson the hydrogen-carbon ratio, and therefore on theprevalent type of hydrocarbon (paraffinic, naphthenicand aromatic hydrocarbons have decreasing NCVs).This property guarantees the energy content of theproduct.

The smoke point is an empirical parametercorresponding to the maximum flame height obtainablewithout the formation of smoke, using a wick immersedin kerosene. The smoke point correlates with the type ofhydrocarbon composition: the higher the aromaticcontent, the lower the smoke point; by contrast,paraffinic hydrocarbons have a high smoke point. A‘high’ smoke point is an indication of low smokeproduction during use. The smoke point is qualitatively



linked to the transmission of heat by radiation from theflame to the walls of the combustion chamber and theturbine blades. Radiation strongly influences theworking life-span of these parts of the turbine: thegreater the radiation, the more frequently maintenancework must be carried out and parts replaced. The smokepoint thus provides an empirical basis for a correlationbetween the properties of the fuel and the power plant’soperational life; based on this, the design of the turbinecan be improved to optimize combustion efficiency. Alow smoke point is also linked to the formation ofcarbonaceous particles, which damage the turbineblades, and carbonaceous deposits in the combustionchamber.

The Luminometer Number (LN) provides ameasurement of the flame’s radiation intensity at agiven temperature and within a predetermined visiblewavelength range, in relation to two reference flames.The Luminometer Number can be correlated with thecombustion properties of a kerosene in aviationturbine engines. High LN numbers correspond to alow transmission of heat by radiation from theproducts of combustion to the walls of the combustionchamber. Since the transmission of heat by radiationstrongly influences the temperature of the metal of theexposed surfaces of the turbine, the LN represents abasis for a correlation between the properties of thefuel and the life expectancy of these components.

Volatility propertiesThe distillation curve is of fundamental

importance for the characterization of Jet Fuel. It iscontrolled during production by acting on thefractionation in the distillation column, and on theformulation of the product.

In contrast to what happens in internal combustionengines, the performance of turbine engines isrelatively insensitive to the distillation curve; however,other restrictions do exist, resulting from the followingneeds: to limit the presence of light fractions in orderto contain evaporation losses at altitude; to contain theformation of vapour locks before the fuel pumps andthe danger of fire; to limit the presence of heavy‘tails’, which raise the freezing point and which mayalso lead to the formation of deposits in thecombustion chamber. The distillation curveparameters, which are important to control, are shownin Table 3.

The mass per unit volume (density) is linked to thetype of hydrocarbons prevalent in the product and tothe distillation range. Density increases passing fromparaffinic to naphthenic and aromatic hydrocarbonsand is used for weight/volume conversions. Inaddition, an adequate density interval ensures a correctenergy supply to the turbine, an effective atomizationof the fuel in the engine’s combustors and finally thesafety and good performance of the fuel as a lubricantand hydraulic fluid.

The flash point is the temperature at which theproduct’s vapours, under specific conditions,ignite in the presence of a flame, and depends onthe presence of light volatile fractions. As such,the flash point, together with the freezing point(see below), is one of the two binding parameterswhich delimits the distillation interval of aviationkerosene.

Cleanliness propertiesThe formation of gums in tanks and fuel lines must

be controlled to avoid the danger of fuel filters



Table 3. Values for distillation curve parameters prescribed by quality specifications for kerosenes

Initial Point The temperature at which distillation begins. The flash point depends on the initial point

10% distillateThe temperature at which 10% of the liquid has distilled.This serves to ensure the presence of a light fraction sufficient to guarantee the vaporization of the jet in the turbine engine’s combustion chamber

Distillate at 210°C

Percentage in volume of liquid distilled at 210°C.This parameter is of concern only in the fiscal classification of the product: the maximum value of 90% distilled liquid distinguishes the tax class of medium oils (kerosene) from light oils (gasoline)


Percentage in volume of liquid distilled at 250°C.This parameter is also of concern only in the fiscal classification of the product: the minimum value of 65% distilled liquid distinguishes the tax class of medium oils from the tax class of heavy oils (gas oil)

End PointThis is the temperature at which the vaporization of the product during the distillation test is complete.Limiting this value serves to control the presence of heavy fractions which do not allow the freezingpoint to be exceeded

ResidueThis is the percentage of liquid which does not vaporize during the distillation test.It serves to limit the presence of heavy ‘tail ends’ due to contamination with heavier products

becoming clogged. To this end, the olefin content islimited by hydrotreating the product and then usingantioxidant additives.

The reaction with water assesses the product’stendency to give rise to stable emulsions of water.Emulsified water may rapidly cause a breakdown ofthe coalescing filters installed in airports, allowingwater and particles to directly reach the fuel tanks ofaircraft. Water is undesirable on-board aircraftbecause it freezes at temperatures below 0°C andmay therefore form ice crystals, which eventuallyblock fuel piping and filters.

The WSEP (Water SEParometer) propertyprovides a measure of the presence of detergents,additives, residues or other soluble contaminantswhich may decrease the capacity of filters toseparate free water from the fuel. As already noted,such water freezes at temperatures below 0°C andmay therefore form ice crystals which block the fuelpiping and filters.

The presence of particulate matter (i.e. rust, sandand particles) in the product may clog filters andcause other operational problems. Sometimes thecolour of the sediments deposited on the filter isalso examined.

Any contaminants present in the product maycause filters on the ground to become blocked andbreak, decreasing the in-flight reliability of theaircraft. The value of this parameter is determinedusing the test known as the Filtration Time, andmust ensure that the product is sufficientlyfilterable.

Fluidity propertiesThe temperature of the fuel inside an aircraft’s

tanks decreases gradually during the flight, dependingon the external air temperature, the speed and altitudereached, and the flight’s duration. The freezing pointis an indication of the tendency of Jet Fuels to solidifyat low temperatures. It is controlled in the refineryby acting on the final distillation point, sinceheavy hydrocarbons solidify at lower temperaturesthan light hydrocarbons.

At low temperatures, the viscosity of the productmust not be too high, to avoid problems with thehandling of the fuel in product storage facilities and inengine fuel lines. A suitable viscosity ensures aneffective atomization in the combustor nozzles, safety,good performance as a lubricant and hydraulic fluid,and that the fuel can be pumped.

Corrosion propertiesThe copper strip corrosion test allows an

evaluation of the product’s tendency to attack thematerials in the aircraft’s injection system.

Stability propertiesAt high temperatures, hydrocarbons decompose

more or less rapidly, giving rise to the formation ofdeposits. In the JFTOT test (thermal stability at260°C, or Jet Fuel Thermal Oxidation Tester), asample is subjected to thermal stress; the type ofdeposits formed and the pressure differentialthrough a filter are then evaluated. The presence ofchemically instable hydrocarbons and/or copper,even in traces, has a significantly negative effect onthe stability test.

Together with limiting the formation of deposits oncombustors and heat exchangers, stability also allowsthe fuel to be used as a cooling medium and hydraulicfluid.

Electrostatic propertiesThis property is expressed by the capacity to

dissipate the electrostatic charge generated in theproduct during pumping and filtering operations,and depends on the fuel’s electrical conductivity.Conductivity in turn depends on the content in ioniccompounds and may be improved by the use ofadditives. It is important to guarantee a sufficientlyhigh conductivity to dissipate the electrostaticcharge accumulated by the fuel, thus avoiding theformation of dangerous differences of voltage(which may cause sparks, and thus a risk of fires andexplosions).

AdditivesFuel System Icing Inhibitor (FSII). Given the

high flying altitudes of military aircraft, the fuel mayreach extremely low temperatures, leading to adanger that paraffin crystals may form and block thefilters. Icing inhibitor suppresses the formation ofparaffin crystals, and thus lowers the freezing point.In recent years, diethylene glycol monomethyl ether(DiEGME) has replaced ethylene glycolmonomethyl ether (EGME). Its use in JP-8 isobligatory.

Corrosion improver/lubricity improver. Usuallypaired with FSII, this improves both the product’santi-corrosion properties and the properties ofJP-8 as a lubricant in feed pumps. Severelyhydrotreated kerosenes have insufficientlubricating power to ensure that the pumpsinstalled in some aircraft engines work adequately.Its use in JP-8 is obligatory.

Metal deactivator. Some metals (e.g. copper)present in traces act as catalysts for the formation ofperoxides and gums in Jet Fuel; in some cases, theproduct may degrade during storage or on-board theaircraft. Metal deactivators inhibit this action. Its use isoptional.



1.2.6 Gas oils

Gas oil is a complex blend of hydrocarbonscharacterized by 13-20 carbon atoms and with atypical distillation range between 160°C and 380°C. Itis used as a fuel for internal combustion engines(diesel cycle). Combustion in thermal plants for heatproduction is another very widespread use for gas oil.

The specifications for gas oil greatly vary,depending on the product use. Specifications forautomotive fuel (diesel) are far more restrictive thanthose for the gas oil used in heating or powerproduction plants, and must therefore be discussedseparately.

Classification and productionGas oil is generally classified on the basis of its

end use. The main distinction, as already noted,concerns its use as a fuel for diesel engines or for heatproduction, but there are also gas oils used as fuels formarine engines or for stationary turbines for powergeneration.

Gas oil is also classified in accordance withenvironmental needs, almost always on the basis of itssulphur content and climatic conditions. This isbecause the formulation of gas oil is modified toconform to seasonal variations or to the geographicalarea where it is to be used.

Gas oil is produced in the refinery by suitablyblending various hydrocarbon fractions, obtained byapplying available technologies (processes), togetherwith other components of different origin, such asbiodiesel. Furthermore, tiny amounts of specificchemical products (additives) are almost always addedto gas oil with the aim of improving some aspects ofits performance.

Like all other petroleum products, gas oil iscarefully produced to meet the series of predeterminedminimum quality requirements or specifications – thatis to say, as already stated, a list of properties, limits,and test methodologies.

For automotive gas oil, the European referencespecifications are contained in the EN 590 standard,whereas in the United States reference is made to theASTM standards, which are the most widespread (seeagain Table 1).

For the gas oil used in marine engines, used for theso-called ‘bunkerage’ of ships, reference is generallymade to the international standards represented by theISO 8217 specifications (DMA and DMB grades).

For the gas oil used for heating, there are nointernationally valid reference specifications, but onlylocal specifications.

The composition and properties of the hydrocarboncomponent of a gas oil may vary depending on the

type and nature of the crude oil, the transformationprocesses available in the refinery, the thermodynamicprocess conditions, the overall balance between themarket demand for gas oil and the demand for otherpetroleum products, and the reference specifications.

The refinery hydrocarbon fractions suitable for thepreparation of gas oil are generally the following: a) gas oil from atmospheric distillation; b) light gas oilfrom vacuum distillation; c) gas oil from dewaxing; d) gas oil from hydrocracking; e) gas oil from catalyticcracking; f ) gas oil from visbreaking and thermalcracking; and g) kerosene.

In the majority of cases, the cuts listed above aresubjected to a desulphurization process to lower theirsulphur content.

As mentioned above, the commercial product mayalso contain specific additives which help improvesome properties. The main additives have thefollowing functions: a) to improve behaviour at lowworking temperatures; b) to improve the lubricatingpower; c) to improve the cetane number; d ) to avoidthe formation of foam; e) to increase electricalconductivity. Furthermore, detergents are added inorder to keep clean or clean up the fuel lines ofengines.

The total percentage of all these additives,however, does not exceed 0.1% in weight.

PropertiesThe main properties of gas oil are listed below,

subdivided according to their various functions, withreference to the most widespread typology, which isautomotive fuel.

Visual propertiesGas oil must not contain suspended solid or liquid

impurities; in other words, it must appear clear and‘sparkling’. The presence of these impurities couldblock fuel lines or prevent the correct flow of the fuelthrough filters, injectors, etc.

The natural colour of gas oil depends on thetypology of the petroleum components used toformulate the finished product. A dark colour mayindicate the presence of unstable cracked componentsor contamination with fuel oil. Often the colouring ofgas oil results from the addition of a dye prescribed bylaw, in order to make the product visually recognizableor for tax purposes.

Compositional propertiesThe ash content indicates the quantity of metallic

material present in the gas oil, which may formdeposits in engines and boilers.

The total sulphur content is a compositionalproperty linked to the type of crude and to refinery



treatments. Limiting the sulphur in gas oil prevents theemission of some types of pollutants. This is becausethe sulphur present in the fuel is oxidized duringcombustion to sulphur oxides, which are emitted in theatmosphere with the exhaust gases. The sulphurcontent of automotive gas oil is also controlled toavoid the formation of sulphates, which increaseparticulate emissions in the exhaust gases of dieselengines.

The presence of polyaromatics has a negativeinfluence on the quality of emissions and, especially,on the particulate content of exhaust gases from dieselvehicles.

The term ‘biodiesel’ refers to a blend of estersobtained by subjecting vegetable fatty acids (rapeseed,sunflower, soybean oil) to a transesterification processwith methanol. The chemical name of this product isFatty Acid Methyl Esters (FAME); it is usuallyreferred to with this acronym in the standards. It canbe used pure (100%) as a fuel, but its use is generallylimited to an amount of 5%. Biodiesel has excellentlubricating properties, does not contain sulphur andallows for an overall lowering of emissions ofgreenhouse gases (carbon dioxide) since, being ofplant origin, it recycles this substance.

Combustion propertiesThe cetane number expresses the delay with which

gas oil ignites and starts to burn from the moment atwhich it begins to be injected into the engine’scombustion chamber. A gas oil with a high cetanenumber shows a gentler and more gradual combustion,has low combustion noise, allows the engine to starteasily at low temperatures and reduces the emissionsof white smoke at start-up and smoke emission at theexhaust. An insufficient cetane number may causeproblems at start-up and high emissions. The bestignition properties are guaranteed by linear chainparaffinic hydrocarbons, followed by branchedchain paraffinic hydrocarbons, naphthenic and finallyaromatic hydrocarbons. The cetane number ismeasured using special one-cylinder laboratoryengines in comparison with reference gas oils.

The cetane index is an indicator that is used toevaluate the cetane number of gas oil. It is obtained bycalculation from other properties of the product(density and distillation parameters). It depends on thecrude oil, the distillation curve, and the hydrocarboncomposition.

The carbonaceous residue provides an indicationof gas oil’s tendency to form carbonaceous deposits oninjectors, piston rings and combustion chambers indiesel engines, and, in the case of gas oil used forheating, the emissions of particles and carbonaceousdeposits in the burners of boilers.

The Net Calorific Value (NCV), an importantproperty of the gas oil used for heating, is the amountof heat released by the combustion of 1 kg of productafter subtracting the latent heat of evaporation of anywater present. It depends on the hydrogen-carbon ratioand thus on the prevalent type of hydrocarbon(paraffinic, olefinic and aromatic hydrocarbons have adecreasing NCV given an equal number of carbonatoms). Its value varies according to the product’scomposition.

Volatility propertiesThe mass per unit volume (density) is a

fundamentally important parameter for enginedesigners, since it makes it possible to define themost suitable air/fuel ratios for the engine’s variousoperating regimes. This is because a density lowerthan that for which the injection pump is calibratedleads to a loss of power, whereas at a higher densitythe air/fuel ratio is noticeably lowered, the engineruns irregularly and the smoke emission at theexhaust increases. At the same time, the densityvalue is equally important in commercial transactionsbecause it allows one to perform weight to volumeconversion, and vice versa.

The distillation curve is also of fundamentalimportance for the characterization of gas oil. Thevalues for distillation curve parameters prescribed byquality specifications are shown in Table 4.

The flash point is the temperature at which theproduct vapours, under specified conditions, ignite inthe presence of a flame, and depends on the presenceof light volatile fractions. A sufficiently high flashpoint value is required to ensure safety duringtransport, handling, storage and use.

Cleanliness propertiesWater is an unwanted component since it does not

burn, may block filters and electrovalves, and causethe premature wear and tear of the vehicle’s injectionpump. Specifications prescribe the absence of freewater, and restrict the presence of suspended water towithin low and controlled limits.

Solid substances in suspension and othercontaminants (rust, sand, organic matter) are known asactual sediments. These contaminants causeoperational problems such as the blockage of the gasoil filters in fuel lines and the wear and tear byabrasion of injection pumps.

Fluidity propertiesThe cold properties define the behaviour of gas

oil at low temperatures. Cold properties dependmainly on the type of hydrocarbons present in theproduct and the distillation curve, and therefore on



the type of crude oil treated, and the proportions ofthe various refinery components. Aromatichydrocarbons and light products have the best coldproperties; linear paraffins and heavier products, onthe other hand, have the worst. Cold properties arean important factor in the quality and value ofautomotive gas oils. Good cold properties can beobtained with the use of additives and/or theaddition, during blending, of a certain amount ofkerosene.

As temperature decreases, the paraffins present ingas oil begin to form crystals which, in an initialphase, cause the product to become cloudy (cloudpoint); at lower temperatures the crystals increase innumber and become larger, and may clog the filters invehicles (see below); finally, at even lowertemperatures, a solid gel forms that prevents theproduct from flowing (pour point).

The cloud point (i.e. the temperature at whichthe first paraffin crystals form in gas oil) is aparameter that indicates the natural cold propertiesof a gas oil.

The CFPP (Cold Filter Plugging Point), whichmeasures the limit temperature of filterability,provides an indication of the ease with which a gas oilpasses through a filter at low temperature.It can be improved by additivation with a flowimprover in the refinery. The CFPP simulates theworking conditions of the gas oil filters installed incars, trucks and burners. CFPP values above theworking temperature cause operational problems dueto the blockage of the filters.

The pour point is the lowest temperature at whichthe gas oil is still able to flow. Below the pour point,gas oil has a semisolid appearance, cannot be pumpedand does not flow through piping. The pour point canbe lowered in the refinery with the use of additives(pour point depressants).

The viscosity, which expresses the internalresistance of a fluid to flow, depends onthe hydrocarbon composition of the gas oil, and inparticular on the distillation curve (it increasesfrom light to heavy fractions). It variessignificantly with temperature (decreasing astemperature increases).

For the gas oil used as automotive fuel, theviscosity value must fall within an interval whichguarantees the lubrication of the injection pumpswhere necessary, and the correct atomization of thefuel by the injectors.

Corrosion propertiesThe copper strip corrosion test makes it possible to

evaluate gas oil’s tendency to attack the metallicmaterials in the fuel lines of an engine or boiler plant.

Gas oil may contain substances with acidfunctional groups, which may be subject todegradation and oxidation. Mineral acidityevaluates the presence of inorganic acids. Totalacidity refers to the sum of both organic andinorganic acids. The absence of acidity representsa guarantee against problems during refineryoperations, and is an indicator of the good qualityof the product.



Table 4. Values for distillation curve parameters prescribed by quality specifications for gas oils

Initial PointThis is the temperature at which the distillation of the product begins.The flash point depends on the initial point. A low initial point value indicates a significant presence of light fractions

Distillate at 150°CThis is the volume of liquid distilled at the temperature of 150°C.Limiting this parameter serves to control the presence of light fractions


This is the volume of liquid distilled at the temperature of 250°C.It is aimed at commercial and fiscal classification: the minimum value of 65% in volume of distillatemakes it possible to distinguish between the tax classes of heavy oils (gas oils) and medium oils(kerosenes)

Distillate at 350°CThis is the volume of liquid distilled at the temperature of 350°C.This has the same aims as the 250°C point: the maximum value of 85% in volume of distillate makes it possible to distinguish between the tax classes of gas oils and fuel oils

95% distillatetemperature

This is the temperature at which 95% of the product has distilled.Controlling this parameter serves to limit the presence of heavier fractions in gas oil. These may giverise to difficulties in vaporization and combustion, and to the build-up of deposits on injectors, with negative effects on particulate emissions from motor vehicles

End PointThis is the temperature at which the vaporization of the product during the distillation test is complete.Controlling this serves to limit the presence of heavy tail ends which may lead to the engine functioningpoorly, dilute the quantity of lubricant and cause other problems during use

Distillation Residue This is a measure of the presence of heavy tail ends which cause the engine to function poorly

Stability propertiesThe stability to oxidation (potential sediments) is a

measure of gas oil’s tendency towards chemicalinstability during storage (formation of solidpolymeric substances). It is evaluated from thepresence of non-filterable sediments after thermalstress, to which a sample of the gas oil is subjected inthe laboratory to accelerate the natural process.

Lubrication propertiesThese properties represent the ability of a gas oil to

lubricate the feed pump of light diesel engines, in theabsence of a separate lubricating device. It is usuallyindicated with the term lubricity.

Electrostatic propertiesThe conductivity expresses the capacity of a gas oil

to dissipate the electrostatic charges that may build upas an effect of the handling of the product, especiallywhen done rapidly. These build-ups are extremelydangerous, as they may generate sparks andconsequently explosions. In general, gas oils with a lowimpurity content, such as those with a low or very lowsulphur content, have lower conductivity, and are themost critical from this point of view.

AdditivesGas oil, especially when used as automotive fuel,

may contain additives, sometimes in the form of‘packages’ with multiple functions. The main additivesare listed below.

Flow improvers are added in the refinery toimprove the cold properties of gas oil, and particularlyto lower the CFPP.

Lubricity improvers serve to increase the ability ofa gas oil to lubricate fuel injection pumps.

Antifoaming agents decrease the tendency of a gasoil to form foam during the loading of tanker trucksand the filling of motor vehicles.

Cetane improvers allow gas oil’s natural cetanenumber to be increased.

Detergents, often used in multi-purpose packages,have the aim of ensuring the optimal cleanliness ofinjectors and the entire fuel system of diesel engines.

Antioxidants and metal deactivators improve gasoil’s stability during storage.

Antistatic additives increase the electricalconductivity of the product, thus avoiding the build-upof electrostatic charges.

1.2.7 Fuel oils

Fuel oil is a mixture of heavy hydrocarbons, which isobtained starting from a residual high viscosity

product, and diluting this with a less viscous distillate(known as the diluent), in order to obtain productswith a viscosity suitable for use as a fuel in heatproduction plants producing heat or electricity, or forpowering large marine diesel engines.

Classification and productionFuel oil is generally classified according to its use

and properties, with particular reference to viscosity andsulphur content. The various types of commercialproducts are generally known as ‘grades’.

On the basis of their viscosity, the various gradesare usually described as dense, semifluid, fluid, veryfluid, whereas on the basis of their sulphur contentthey are known as LSC (Low Sulphur Content) orHSC (High Sulphur Content).

Fuel oils are prepared in the refinery by blendingthe heavy residues from various processes, taking careto meet the reference specifications. As in other cases,these are lists of properties, limits and testmethodologies aimed at defining minimum qualityrequirements.

For purposes such as fuel for marine dieselengines, reference is made to the international ISO8217 standard, which establishes a series of gradesas a function of the viscosity value, whereas for theproduction of heat for civil and industrial purposes,each country adopts its own regulations. In Italy, forthis purpose, reference is made to the UNI-CTI 6579standard, which prescribes the specifications for thevarious grades of fuel oil as a function of viscosityand sulphur content. For the production of electricalpower, on the other hand, reference is made tospecifications that are generally formulated by thepurchaser of the product, in other words, the powercompanies.

The components used in refineries to prepare thevarious types of fuel oil are: residue from atmosphericdistillation, residue from vacuum distillation,residue from visbreaking or thermal cracking units,residue from hydrocracking units, heavy residue fromthe production of lubricants, heavy cycle oil fromcatalytic cracking and, as diluents, kerosene, gas oiland light cycle oil from catalytic cracking.

Properties

Compositional propertiesAsphaltenes are natural components of crude oil,

consisting of very complex and heavy molecules,which are only partially cracked by conversion units.Due to their high molecular weight, asphaltenes arepresent in large quantities in residues, and remain insuspension in the liquid phase in fuel oil, thanks to thepresence of complex aromatic molecules (resins).



Disturbing the phase equilibrium may cause theasphaltenes to precipitate.

Asphaltenes do not burn easily and contribute tothe emission of carbonaceous particulate. If theyprecipitate, this may give rise to the instability andincompatibility of the product, unwantedphenomena especially in bunker fuel oils, since theytend to generate sludges in the bottoms of tanks orcause serious problems during the use of theproduct.

Ashes represent the material of metallic typepresent in the product (metallic and metallic-organiccompounds, dirt, rust). These may form deposits inships’ engines and boilers, hindering the transmissionof heat and lowering energy levels.

The mass per unit volume (density) depends onthe hydrocarbon composition of the fuel oil, andthus on the type of crude oil, the relativeproportions of the refinery components used toproduce the bases for fuel oil, their distillationcurve, and the quantity of diluent used. In general,the density is lower for LSC (paraffinic) fuel oilsthan for HSC (more aromatic/asphaltenic) fueloils. The density is determined in order to carryout weight to volume conversions and to controlthe heat generated by combustion. Furthermore,the water separation centrifuges on-board shipsrequire a product with a density which iscontrolled and sufficiently lower than that ofwater, in order to work properly.

Combustion propertiesThe Net Calorific Value (NCV) depends on the

hydrogen-carbon ratio and therefore on the prevalenttype of hydrocarbon (paraffinic, olefinic and aromatichydrocarbons have decreasing NCVs).

The flash point depends on the presence ofvolatile light fractions. A high value increases safetyduring transport and storage operations, and duringuse.

The carbonaceous residue provides anindication of the tendency of a fuel oil to generatehigh percentages of particulate in fumes, and toform deposits in the burners of boilers and in thecombustion chambers of marine diesel engines.

Volatility propertiesThe distillation test is generally used for

commercial and fiscal purposes. Obviously, this onlyallows the lighter fractions of fuel oil to becharacterized. The distillation points generally adoptedare described in Table 5.

Cleanliness propertiesThe presence of water reduces the true quantity of

combustible product and is also the cause of problemssuch as the instability and interruption of combustion,and the erosion of burner nozzles and othermechanical parts.

Aluminium and silicon may be present ascontaminants in fuel oil, especially if the product isprepared using heavy cycle oils from catalyticcracking containing spent catalyst powder. Theirpresence in bunker fuel oils may cause serious damage(premature wear and tear and scratching) to the linersof cylinders in marine diesel engines, especially if theparticles have a high granulometry, whereas in the fueloil used for heat generation, this is much less critical.

Sodium is present in fuel oil since it is contained inthe saltwater present in the original crude oil. In therefinery, the sodium content is reduced and controlledby desalting the crude oil. In combination withvanadium, sodium may lead to the formation ofcompounds that cause encrustations and damages tothe combustion chambers and burners in thermal andelectric power stations.

Vanadium is found in fuel oil since it is present inthe original crude oil. In association with sodium,vanadium may cause the formation of compoundswhich encrust and corrode the metallic materials in fluestacks. These compounds are also damaging to health.

The sulphur present in fuel oil derives from theoriginal crude oil; it is considered an indicator of theoverall quality of the crude and the fuel oils madefrom it, to the extent that LSC fuel oils are consideredmore valuable than HSC oils. During combustion,sulphur forms corrosive sulphur oxides, which are oneof the most serious air pollutants.

The nickel present in fuel oils also derives from theoriginal crude oil. It is generally controlled because itposes a severe danger to human health and the



Table 5. Values for distillation curve parameters prescribed by quality specifications for fuel oils

Distillate at 250°CThis is the volume of liquid distilled at the temperature of 250°C.It serves fiscal purposes. The value of 65% distillate out of the total volume makes it possible to distinguish between the tax class of heavy oils and that of medium oils

Distillate at 350°CThis is the volume of liquid distilled at the temperature of 350°C.It serves fiscal purposes. The value of 85% distillate out of the total volume makes it possibleto distinguish between the tax class of fuel oils and that of heavy oils deriving from gas oil

environment when it is released to the air throughexhaust gases or the fumes from flue stacks.

The actual sediments in a fuel oil consist of thefraction which is insoluble in a paraffinic solvent; itcan be removed by filtration. The presence ofsignificant quantities of actual sediments may causefouling of fuel pipelines and burners. Furthermore,these sediments may accumulate in storage tanks, andon filters and burners, obstructing the flow of fuelfrom the tank to the boilers.

The potential sediments of a fuel oil are thefraction present after suffering thermal or chemicalstress, which can be removed by filtration. Theformation of significant quantities of sediments afterthermal stress is an indication of the tendency of a fueloil to form deposits during storage and handling.Chemical stress, by contrast, indicates the potentialincapacity of the product to maintain the asphaltenesin solution. In both cases, the precipitation of a fuel oilfraction may cause serious operational problems, andmay even make the product totally unsuitable andirrecuperable for use in boilers and marine engines.Potential sediments may be critical when the fuel oil ismade using base products with a high asphaltenecontent; in these cases, the oil phase does not containenough aromatic hydrocarbons to keep the asphaltenesin solution. The problem worsens if these baseproducts are blended with paraffinic fuel oil ordiluents; in this case, one speaks of instability of thefuel oil, while the term incompatibility is used if thesame phenomenon occurs as a result of blending twofinished fuel oils.

Sediments by extraction are the fraction insolublein aromatic solvents, and essentially consist of metallicparticles, sand, earth and foreign bodies which depositon the bottom of the tanks. These sediments areresponsible for problems such as instability of theflame, erosion of burner nozzles and other mechanicalparts, and clogging of filters.

Fluidity propertiesThe pour point can be lowered in the refinery by

diluting with gas oil or using suitable additives.

In fuel oils, viscosity is strictly dependent on theamount of diluent added to the base, and decreases astemperature increases. This is a parameter used for thefiscal classification of fuel oils. From a practical pointof view, it is also a design parameter for fuel pumpsand burner nozzles. For these purposes, values forviscosity measured at different predeterminedtemperatures are used.

Bibliography

ASTM (American Society for Testing and Materials) (2002)Standard terminology relating to petroleum, petroleumproducts and lubricants, ASTM D 4175-02a.

ASTM (American Society for Testing and Materials) (2004)Standard specification for aviation turbine fuels, ASTMD 1655-04a.

British Ministry of Defence (2005) Defence standard 91-91. Turbine fuel, aviation kerosene type, jet A-1, NATO codeF-35, Joint Service Designation AVTUR, issue 5, 8 February.

CEN (European Committee for Standardization) (2004)Automotive fuels. Diesel. Requirements and test methods,EN 590.

CEN (European Committee for Standardization) (2004)Automotive fuels. LPG. Requirements and test methods,EN 589.

CEN (European Committee for Standardization) (2004)Automotive fuels. Unleaded petrol. Requirements and testmethods, EN 228.

IATA (International Air Transport Association) (2004) JIGguidelines for aviation quality control & operatingprocedures into jointly operated systems, issue 9, January.

ISO (International Organization for Standardization) (1996)Petroleum products. Fuels (class F). Specifications of marinefuels, ISO 8217.

JIG (Joint Inspection Group) (2005) Aviation fuel qualityrequirements for jointly operated systems, bulletin 4, issue20, March.

UNI (Ente Nazionale Italiano di Unificazione) (2004)Combustibili liquidi per usi termici industriali e civili.Classificazione e caratteristiche, UNI 6579.

Fiorella IobbiEni - Divisione Refining & Marketing

Roma, Italy



1.2.8 The petroleum waxes

When petroleum oils consist of paraffins in therange from about 50% to nearly 100%, they result insemi-solid or solid gels at room temperature andpressure. These gels are called petroleum waxes onaccount of their appearance and because they areused in the same industrial applications as vegetable,animal and synthetic waxes (Botteri, 1954; Fieserand Fieser, 1962).

Petroleum waxes are generally produced duringthe refining of the paraffinic base oils (see Chapter8.3), however heavy grades of these waxes are alsoobtained by refining the bottoms from the storagetanks of paraffinic crude oils and, in smallquantities, by treating the wax mined in ozokeritedeposits (from Greek åzein “to emit odour” andkhrãj “wax”) mainly located in the Ukraine andAsia. Such deposits of paraffinic hydrocarbons,which are present on the surface of the Earth or atsuperficial depths, are also located in a number ofother countries, including the United States (Utah),Austria, Romania and Italy (Botteri, 1954), however,at present, they are generally not exploited due totheir small sizes and the high costs involved.

In addition, the Fischer-Tropsch processproduces synthetic wax exhibiting higher normalparaffin content and average molecular weight thanpetroleum wax.

CharacteristicsA brief outline of the main properties follows

below (Botteri, 1954; Tuttle, 1960; Costantinides,1969).

The appearance is defined by standard analyticmethods such as the colour arising from theobservation of the light transmitted through themelted wax. This colour varies from colourless(water-white), to whitish, to yellow, to pale/darkorange and up until dark brown.

Depending on the type of wax, the liquid/solid orliquid/semi-solid transition at atmospheric pressureis characterized in a different manner. In hard wax(see below) the high quantity of normal paraffinsand either a low or very low oil content cause therelease of the latent heat of solidification at aconstant temperature during the cooling of the

liquid. This results in a temporary step (i.e. themelting point) in the time-temperature cooling curveat atmospheric pressure.

In Fig. 1 the melting and boiling points of the normalparaffins present in waxes, measured in vacuum andextrapolated to atmospheric pressure, are related to thenumber of carbon atoms. The normal paraffins haveboiling points which fall in the ranges typical ofvacuum distillates and residua (corresponding toapprox. 18-41 and 40-70 carbon atoms, respectively).Their boiling points result in higher values as comparedwith alkyl benzenes and alkyl naphthenes with the samenumber of carbon atoms and long paraffinic chains(Costantinides, 1969).

The lowest melting points are a property ofisoparaffins with a long linear branching chain inthe middle of the molecule. For example, themelting point of n-hexacosane (i.e. a normalparaffin with 26 carbon atoms) is 330 K (57°C) andshifts from 294 K (21°C) to 273 K (0°C) for then-butyldocosanes (i.e. isoparaffins with the samenumber of carbon atoms) by moving the n-butylradical from position 5 to position 11, in the centreof the chain. Given the same number of carbonatoms, the density, refractive index and viscosity ofthe isoparaffins with side chains in the middle of themolecule have higher values as compared with thenormal paraffins. This comparison is reversed if thebranches occur near the ends of the isoparaffin(Sachanen, 1950).


850

750

650

550

450

350

25010 20 30 40 50 60 70

tem

pera

ture

(K

)

carbon atoms

boiling pointmelting point

Fig. 1. Melting and boiling points of normal paraffins at atmospheric pressure (Fieser and Fieser, 1962; American Petroleum Institute, 1976).

When the quantity of isoparaffins is large and/orthe oil content is high, the transition to the solidstate and the release of latent heat occur gradually.In these cases the analysis of the melting point cannot be carried out and the congealing point (i.e. thetemperature at which the melted wax ceases to flowwhen cooled on the bulb of a rotating thermometer)is generally utilized.

The oil content is the quantity of oil weighed afterseparation at 241 K (�32°C, 0°F) and filtration of thewax diluted in methyl ethyl ketone. The consistency ofthe waxes is generally measured at 298 K (25°C), orsometimes at higher temperatures, by the measurementof needle penetration on the solid waxes or the conepenetration on the semi-solid waxes. Its valuedecreases as the oil content increases, and increases asthe quantity of normal paraffins increases. In waxeswith very low oil content the quantitative analysis ofthe paraffinic hydrocarbons can be carried out bymeans of gas chromatography.

The use of the waxes in contact with food, aswell as in the pharmaceutical and cosmeticindustries, is allowed when they comply with thespecifications reported by law and pharmacopoeiawith regard to the possible presence of aromatichydrocarbons which is detected by means ofultraviolet (UV) absorbance.

For the sake of brevity, odour, acidity, vacuumdistillation curve, inflammability, etc. will not covered.

Products and their denominationsDepending on the dimensions of the crystals, the

waxes can be classified as macro- andmicrocrystalline. These are characterized by

different properties: microcrystalline wax hasgreater flexibility and plasticity, mechanicalstrength, resistance to passage of water vapour andretains more oil (Costantinides, 1969).Macrocrystalline wax is produced by dewaxing (seeChapter 8.4) vacuum distillates, whilemicrocrystalline wax is obtained from heaviervacuum distillates and vacuum residua.

The extent of the wax refining determines thevalues of the oil content, melting point, penetration,colour and UV absorbance.

On the basis of their oil content, the waxes can besubdivided in three groups:• Oily wax, in semi-solid state (slack wax, called

petrolatum when microcrystalline, and known aspetroleum jelly when microcrystalline with thehighest oil content). Oily wax with high isoparaffincontent (soft wax) is also know as slack wax.

• Deoiled wax, in solid state (scale wax, also knownas slack wax).

• Wax with a very low oil content and a highquantity of normal paraffin (hard wax). Thesewaxes, which are the hardest and exhibit the lowestpenetration, are characterized by gradesdetermined by the range of temperature where themelting point occurs (in degrees Fahrenheit), e.g.131-135°F (328-330 K or 55-57°C).Further distinctions can be made within each of

these three groups on the basis of colour. Wax is saidto be crude when its colour is that of the productstraight from the production plant. After sometreatment the colour of the resulting semi-refined waxranges between white and dark yellow (or even darkorange), while after full treatment the colour of the



Oil Content

Presence of iso- andnormal paraffins Products

Main denominations(physical state at room

temperature and pressure)

iso- normal- crude waxsemi-refined

waxfully refined

waxmacrocrystalline

waxmacrocrystalline

wax

Oily wax yes yes • • •slack wax*(semi-solid)

petrolatum(semi-solid)

Deoiled wax yes yes • • •scale wax

(solid)petrolatum

(solid)

Almost freefrom oil(hard wax)

yesprevalent over iso-

– • •paraffin wax

(solid)ceresin (solid)

(*) The denomination slack wax also refers to petrolatum and to scale wax, and can include soft wax with high oil and isoparaffin content.

Table 1. Composition, denomination and physical state of the petroleum waxes at room temperature and pressure

resulting fully refined wax is water-white, or whitish,and the wax complies with the UV absorption test (seeChapter 8.4). The products in highest demand are thefully refined waxes; crude hard wax is generally notmarketable.

Composition, denomination and physical state atroom temperature and pressure are summarized inTable 1.

UsesThe uses of the waxes are numerous, very diverse

and generally established over time. It is oftennecessary to blend different grades of waxes while inthe liquid state. It is also common to blend inadditives, such as natural waxes and partially oxidizedand saponified or esterified waxes if greatadhesiveness is required. Petroleum waxes are alsoblended with synthetic waxes of high molecularweights (such as polyethylene and its co-polymerswith a-olefins) if more adhesiveness and increasedmechanical strength, as well as resistance to humidity,are needed. Some of the most important uses ofpetroleum wax are summarized below (Botteri,1954;Tuttle, 1960; Costantinides, 1969).

The main use is in the paper, cardboard andpacking industries as a binding agent of cellulosefibres (sizing), as a waterproofing agent, etc. In orderto evaluate the products obtained, a number oftechnological tests are performed in the laboratory,such as the determination of the surface gloss, theblocking point (i.e. the temperature at which twosheets significantly adhere to each other), and thepicking point (i.e. the temperature at which a layer ofthe wax begins to break).

On account of the high dielectric constant, waxedpaper can be utilized as an insulator (e.g. incondensers) and wax can be used for impregnation andfilling, e.g. in dry batteries, and as insulator in cablesfor the transmission of electrical energy.

When wax is mixed with stearic acid in order toincrease the melting point, it can be used for theproduction of various types of candles and night lights.The ‘lost-wax’ technique is used for casting infoundries, for works of art and in the production ofjewellery. Slack wax and petrolatum are utilized in the

production of protective materials for machinery andin floor wax.

Wax is added in the production of rubber,particularly in the case of tyres, since it facilitates themanufacturing process; surplus wax rises to thesurface of the rubber and protects it from the oxidationand the effects of light. In the textile industry, waxemulsions are used in order to return to natural fibresthe substances lost during the manufacturing process,as well as for lubrication.

Waxes are also used in the manufacture ofexplosives, such as trinitrotoluene (TNT), since theyreduce their sensitivity, and in the manufacture of shoepolishes, matches, printing inks, pencils, crayons andlip pencils. Preparations with a wax base are used indentistry to make dental imprints.

The surface of the cheeses is coated with wax,while oranges, lemons, mandarins and apples are oftentreated with wax emulsions in water, in order toconserve them longer and to improve their appearance.Petroleum jelly is also used as hair gel and as the basefor active ingredients in ointments and creamsproduced by the cosmetics and pharmaceuticalindustries.

References

API (American Petroleum Institute) (1976) Technical data book.Petroleum refining, Washington (D.C.), API, 2v., 1-94.

Botteri M. (1954) Cere industriali naturali e sintetiche, Milano,Hoepli, 1-4, 11-31, 152, 224-278.

Costantinides G. (1969) Paraffina, in: Girelli A. (a cura di)Petrolio. Grezzo, raffinazione, prodotti, Milano, Tamburini,552-568.

Fieser L.F., Fieser M. (1962) Trattato di chimica organica,Milano, Manfredi, 36, 448, 459.

Sachanen A.N. (1950) Hydrocarbons in petroleum, in: BrooksB.T., Dunstan A.E. (editors) The science of petroleum. Acomprehensive treatise of the principles and practice of theproduction, refining, transport and distribution of mineraloil, London, Oxford University Press, 1938 - ; v. V/1, 68-73.

Tuttle J.B. (1960) The petroleum waxes, in: Guthrie V.B.(editor) Petroleum products handbook, New York, McGraw-Hill, 10-1/10-30.

Alessandro BelliScientific Consultant



1.2.9 Lubricants

Finished lubricants are obtained from mixtures ofbase oils and additives; each formulation is studiedand defined so as to guarantee when used aperformance level that is suitable for the applicationfor which the lubricant is intended.

Base oils are the chief component in the vastmajority of lubricants; therefore, it is clear that thequality of the finished oil depends on that of thebase oils. Besides mineral base oils, obtained fromthe processing of crude oil, non-conventional andsynthetic bases are becoming increasinglyimportant.

Components of base oilsIn base oils there are various types of

hydrocarbons, as indicated below.Alkanes. Known also as paraffins, they are

saturated compounds of a linear (normal paraffin) orbranched (isoparaffin) structure.

Alkenes. Known also as olefins, they areunsaturated compounds which are relatively rare incrude oil. They are formed in certain refiningprocesses, such as for example cracking ordehydrogenation. The lack of saturation causesinstability in most applications, which is alsopromoted by the temperature and the presence of airand other agents, with the formation of deposits andundesired components.

Alicyclics. Known also as naphthenes, they aresaturated compounds containing at least one ring(naphthenes present in base oils normally have ringswith 5 or 6 carbon atoms).

Aromatics. These are cyclic compounds withconjugated double bonds, based on the benzene ring;they are among the least useful hydrocarbonsbecause of their negative impact on the viscosityindex of the whole base oil. In addition they worsenthe characteristics of the base oil under variousaspects, in particular by increasing the propensity toform deposits since they have little resistance tooxidation. Aromatics have very low melting points,while they have excellent solvent power.

Mixed hydrocarbons. Where in the samemolecule, groups that are characteristic ofparaffinic, naphthenic and aromatic hydrocarbons

are present. They represent the majority ofhydrocarbons in crude oils.

Besides the hydrocarbons, base oils contain non-hydrocarbon compounds, which were alreadypresent in the original crude oil and are of differentchemical structures. Processing tends to reduce theircontent in the base oil; a further reduction is madethrough finishing treatments at the end of theproduction process (hydrofinishing).

The main non-hydrocarbon components areheterocyclic containing sulphur (the most abundant)or nitrogen or oxygen.

The hydrocarbon components influence thecharacteristics linked to viscosity and the viscosityindex. The other components influence, positively ornegatively, characteristics such as oxidation stability,foaming, demulsibility, and anti-corrosive power.

The components of base oils (type and relatedquantity) depend closely on the original crude andcan only be partially changed through the productionprocess.

Mineral base oilsMineral base oils are obtained from the

processing of crude oil, through a refining processthat may be integrated with a medium or lowintensity hydrofinishing treatment.

The various fractions, or grades, of base oilsproduced are classified internationally on the basisof the SUS (Saybolt Universal Seconds) viscosity,measured at 40 or 100°C (100 or 210°F).

In addition, the number which indicates the SUSviscosity is preceded by a code, such as for exampleSN (Solvent Neutral) or HVI (High ViscosityIndex), which indicates the production process used.The code BS (Bright Stock) is used for the heaviestgrade which may be obtained from the processing ofthe bottom of the vacuum distillation column. Forexample: SN 150 indicates a viscosity grade of 150SUS at 40°C (approximately 30 cSt) obtainedthrough solvent extraction; HVI 56 indicates aviscosity grade of 56 SUS at 100°C (approximately10 cSt) obtained with an extraction process whichproduces a high viscosity index; BS 150 indicates aBright Stock with viscosity of 150 SUS at 100°C.

The number of grades and the relatedviscosimetric characteristics depend on the producer



and the type of process. Normally productionincludes: a) a very fluid grade (SN 80-100 orspindle); b) a fluid grade (SN 125-170, typically150); c) a medium grade (SN 350-600); d) a BrightStock (BS 150 or 200).

Paraffinic base oilsIn such base oils the paraffinic hydrocarbons are

preponderant. They are the most common base oils,since they derive from crude oil which is largelyparaffinic and which is the most widely used for theproduction of base oils.

The overall characteristics of these base oilsdepend on the hydrocarbon distribution of the crude,as well as the severity of the extraction anddeparaffination process.

In this regard it should be noted that normalparaffins and isoparaffins with fairly short side-chainsare hydrocarbons characterized by a very high level ofviscosity (200), but are penalised by the fact that theysolidify at relatively high temperatures. This makesthem unsuitable for most applications, and, therefore,are removed from the finished base oil through aprocess of deparaffination.

The other isoparaffins, on the other hand, arecharacterized by a viscosity index below that of normalparaffins, but which is still very high (approximately140), and have low pour points. In fact they are the mosthighly valued fractions of the base oils.

The viscosity index of paraffinic base oils isgenerally over 95 and the pour point is relatively high.It is possible to obtain paraffinic base oils with bettercharacteristics for the viscosity index, by making theirextraction conditions even more severe; this is to thedetriment of the yield in the lubricant fraction. It isalso possible to increase the index by reducing thedeparaffination (with a further advantage in terms ofyield), but to the detriment of the cold properties ofthe base oil.

Naphthenic base oilsIn these base oils there is a prevalence of naphthenic

hydrocarbons. They may be obtained from just a fewcrude oils (Venezuela, United States, and Russia), andare currently used only in particular applications and,for toxicological reasons, only if subjected to processeswith solvents or through hydrogenation.

Naphthenic base oils can behave differentlydepending on the presence of more or less widespreadparaffinic side-chains. Depending on the prevalence ofparaffinic chains on the naphthenic rings, thesefractions present a higher or lower viscosity index.These base oils have better solvent power with regardto additives, but worse resistance to oxidationcompared to paraffinic oils. They also have a relatively

low viscosity index (generally between 40 and 80) anda relatively low pour point, owing to the absence ofparaffin. Currently they are used in applications whichrequire cold properties and where the viscosity indexis of secondary importance.

The characteristics of oxidation resistance areintrinsically lower compared to those of paraffinicbase oils; it is possible to compensate partly for thisfeature through the use of additives.

Non-conventional base oilsBase oils which are commonly defined as non-

conventional (NCBO, Non-Conventional Base Oils)are obtained by using components that are directlyavailable from the refining process, through hydrogenbased processes. The two main types are derived fromhydrocracking or hydroisomerisation of wax. The waxcan also be obtained from Fischer-Tropsch synthetictechnologies; in this case high quality base oils areobtained (viscosity index >140).

The advantages offered by these base oils are oftwo kinds: firstly, the processes used, which cansubstitute totally or in part the treatment with solvent,allow to achieve a final composition of the grades thatis relatively independent from the characteristics of theoriginal crude; secondly, the quality of the base oilsobtained from such processes is superior to that ofbase oils derived from the traditional cycle withsolvent, compared to which they are less volatile at asimilar level of viscosity, have a higher viscosityindex, have better stability against changes intemperature and a low or insignificant sulphur content.

Main properties of base oilsBase oils are duly classified in order to check their

suitability for their intended uses. It is worth pointingout that there are no preset criteria for a qualitativeselection and classification, since the tendencycharacteristics of base oils can be changed, as necessary,via the use of additives. In any case, these characteristicsprovide useful indications for a preselection and for thesetting of preliminary formulations to be subjected,nonetheless, to performance tests. The maincharacteristics normally referred to are: a) viscosityindex; b) oxidation stability; c) pour point; d ) cloudpoint; e) flash point; f ) colour; g) colour stability; h) carbon residue. Other characteristics that aresometimes considered are volatility, density,demulsibility, foaming and air release properties.

ViscosityThe viscosity of an oil is the measure of resistance

(internal friction) to forces that tend to make it flow andwhich decreases as the temperature increases. In thepast, viscosity was expressed in Engler degrees (in



Europe), in Redwood degrees (in the United Kingdom)and in Saybolt degrees (in the United States). The SI(International System) units are the mm2/s for thekinematic viscosity and the mPa·s for the dynamicviscosity. The former units of measure are however stillcommonly used: these are the centistoke, cSt (1 cSt�1 mm2/s) and the centipoise, cP (1 cP�1mPa�s).

Viscosity indexSince viscosity depends on temperature, a measure

of sensitiveness to temperature is expressed by theViscosity Index (VI or IV). The viscosity index of a baseoil is determined by comparing the change in viscosity,as the temperature changes, with that for two referenceoils. Of these, one (paraffinic) has a low variation inviscosity compared to temperature (by convention, thisoil is rated VI�100), while the other has oppositecharacteristics (and is given a value of VI�0).

The viscosity index is given by the followingexpression: VI�100(L�U)/(L�H), where L is theviscosity at 100°F of the reference oil with VI�0 andwith viscosity at 210°F, equal to that of the oil to beassessed; H is the viscosity at 100°F of the referenceoil with VI�100 and with viscosity at 210°F, equal tothat of the oil to be assessed; U is the viscosity at100°F of the oil to be assessed.

An oil with VI�95 presents lower variations inviscosity against temperature compared to an oil withVI�90.

Oxidation stabilityMost oils, when exposed to the air, react over time

with oxygen. In the field of base oils oxidation testsare used, including IP (Institute of Petroleum) 48, DSC(Differential Scanning Calorimetry) and TGA(Thermogravimetric analysis). Base oils must behighly stable against oxidation. If base oils withinsufficient oxidation stability are used, hightemperatures and/or their prolonged use can lead to theformation of corrosive acids and insolublecompounds. This can, for example, affect theperformance of engines through the formation ofsignificant deposits in piston ring grooves.

Cloud point and pour pointThe cloud point of an oil is the temperature at

which the oil becomes cloudy; this is determined bythe first formation of paraffin crystals. If thetemperature is further reduced, increasing quantities ofparaffin will crystallize until the point where the oil nolonger pours. The temperature at which this happens isthe oil’s pour point.

The oils used for lubricants must have asufficiently low pour point so that, should they be used

in low temperature conditions, they maintain theirliquid state. A good cloud point ensures that theyremain bright and clear in such conditions.

Flash pointThe flash point of an oil is the temperature at

which the related vapour burns if exposed to a nakedflame. A minimum flash point is normally specifiedfor safety reasons. This is an important feature forbase oils.

ColourThe colour of a base oil does not influence its

properties in terms of performance. Nonetheless, sinceit is easy to measure, it is often used to provideindications of possible contamination or excessivepresence of heteroatoms. Colour is determinedthrough an ASTM (American Society for Testing andMaterials) test which converts the colour of an oil intoa single scale:• Pale, colour of 4.5 ASTM or paler.• Red, darker than 4.5 ASTM.• Dark, darker than 8.0 ASTM.

In general, the more viscous the base oil is, thedarker its colour. A dark colour can also be indicativeof the process of oxidation degradation. Non-conventional base oils have a very pale colour.

Carbon residueThe carbon residue of an oil is how much of it

remains once it has been burnt. Base oils used forlubricants must have a low carbon residue, so that theydo not leave deposits should they be exposed tocombustion during application.

Rerefined base oilsRerefined base oils are obtained through

suitable processing procedures for used oils. Oncethe lubricant has fulfilled its function oflubrication it should not be disposed of but takento authorized collection centres. The usedlubricant is then sent to suitable combustion plantsor for rerefining.

Rerefining plants use production processes thatallow the creation of base lubricants which have thesame characteristics as mineral lubricants.

Rerefining enables about 60 kg of rerefined baseoil to be obtained for every 100 kg of spent oil and,therefore, to reuse a product that is potentiallyhighly polluting by transforming it once again into araw material, with a saving on the importation ofcrude oil.

From the processing viewpoint, rerefiningrequires:• A treatment for the elimination of volatile



compounds (solvents, fuel residues, etc.).Generally a high temperature flash is used.

• A treatment to eliminate insoluble compounds andthe residues of additives.

• Traditional operations such as distillation andhydrofinishing or earth treatment complete thecycle; the final hydrogen treatment enables theelimination or reduction of the content ofpolynuclear aromatics (PNAs), which arecarcinogens.

Synthetic base oilsThe most widely used synthetic bases in the

lubricants sector are polyalphaolefins (PAOs),aromatic alkylates, esters, polyglycols, polybutenesand polyinternalolefins (PIOs).

Polyalphaolefins (PAOs)These derive from the oligomerisation of alpha

decene. They have very good low temperaturecharacteristics thanks to their high degree of branchingand their much lower volatility compared to mineraloils derived both from solvent extraction processes andfrom hydrocracking or hydroisomerisation.

On the other hand, since PAO bases are ‘pure’, insome oxidation tests (for example DSC) if they are notmixed with additives they seem less resistant thanmineral bases. This behaviour may be attributed to theabsence of natural antioxidants which, vice versa, arepresent in mineral oils.

PAOs are only slightly polar and consequently havelimited solvency and this is to the detriment of theirability to dissolve polar additives present in thelubricant oil and oxidation products (rubber) whichform during use.

For these reasons it is a good idea to use PAOsmixed with other more polar bases, for examplealiphatic esters (synthetic lubricants) and mineralbases refined with solvent (semi-synthetic lubricants).The broad temperature range in which PAOs can work,together with their excellent chemical, physical andthermo-oxidative characteristics, enable their use in awide range of applications.

Alkylated aromaticsAlkylbenzenes have poorer characteristics

compared to PAOs, thanks to their excellent solvencypower and low pour point, they are used inrefrigerating oils.

PolyglycolsGenerally they have a high viscosity index,

poor oxidation resistance but high lubricatingproperties, which make them particularly suitable toformulate oils for warm gear lubrication.

PolybutenesDerived from the polymerisation of isobutenes

and of polyisobutenes (PIBs), these are polymerswith good shear stability and are used as ViscosityIndex Improvers (VII). They have higher volatility,less oxidation resistance and a lower viscosity indexcompared to PAOs and esters. In syntheticlubricants, combined with esters and PAOs,polybutenes can have an influence on the control ofthe viscosity of the lubricant, giving them propertiesof low formation of deposits and thickening.

Synthetic estersThe most immediate effect of the ester group on

the physical properties of the lubricant is to lower itsvolatility and raise its flash point. Esters influenceother properties such as: a) the thermal and hydrolyticstability; b) solvency; c) lubricating property; d ) biodegradability.

In the case of lubricants for transport, mixes ofesters-PAOs have advantages in terms of cold starting,fuel economy, engine cleanliness, and protectionagainst wear.

Polyinternalolefins (PIOs)Just like PAOs they derive from the

oligomerisation and subsequent hydrogenation ofolefins. The substantial difference consists in thedifferent feed of the starting olefins (linear monoolefins C14-C18).

They feature a high viscosity index, excellentrheological behaviour at high and low temperatures,low volatility and good thermal-oxidativeperformance. They are used in lubricants for internalcombustion engines and for industrial machinery.

API and ATIEL classification of base oilsThe American API and the European ATIEL

systems include a classification criterion for base oilsin relation to their content of sulphur and saturates,their viscosity index and their typology for syntheticbases (Table 1).



Group Saturates (% by weight)

ViscosityIndex

I

II

III

IVV

VI*

(*) only ATIEL

Table 1. API and ATIEL classification of base oils

Sulphur (% by weight)

�90% �0.03 80-120

�90% �0.03 80-120

�90% �0.03 �120polyalphaolefinsall the other base oils

polyinternalolefins

Both API and ATIEL envisage regulationsregarding the interchangeability of base oils whichguarantee the quality of the finished lubricants. Thesubstitution of the bases in a finished product entailsthe repetition of some tests, in relation to well-definedinterchangeability grids.

Production processesBelow are given some essential processing

schemes for the production of bases for lubricants(for a more detailed analysis see Chapter 8.3 ). Inpractice even more complex process configurationsmay be applied, using different combinations of theoperative units.

Mineral basesFig. 1 sets out the scheme of a unit for the

production of mineral bases which use the solvexprocess, so far still the most commonly usedworldwide. The distillation allows the removal ofhigh and low boiling components and leaves asuitable residue. The extraction of aromatics permitsan improvement in the thermo-oxidative index andstability; it also eliminates the most dangerousproducts (PNAs). To this end various solvents areused from furfural to N-methylpyrrolidone.Dewaxing eliminates the components at a higher

melting point and improves the characteristics at lowtemperature. This process is carried out at lowtemperature by using a mix of toluene-methyl ethylketone (MEK). Hydrofinishing removes the polarcompounds by improving the colour and stabilitythrough a medium severity hydrogen treatment.

Non-conventional basesIn Figs. 2 and 3 there are two alternative productionschemes for non-conventional bases which usefeedstock other than the classic atmospheric residue.These schemes refer to processes for the productionof bases respectively by hydrocracking and byhydroisomerisation of wax. Hydrogenation generallyreduces the content of aromatics and creates baseoils that have a high viscosity index and low nitrogen and sulphur content; it can be ‘mild’ or‘severe’ and proportionally also the level ofeffectiveness varies. Catalytic dewaxing can breakthe long chains of normal paraffins (or of onlyslightly branched isoparaffins), so as to obtainhydrocarbons with a lower molecular weight whichare more fluid in the cold. Unlike traditionaldeparaffination, normal paraffins are thus notremoved from the base oil, with an advantage interms of yield in the lubricant fraction. Isodewaxing,unlike catalytic dewaxing, does not break normal



atmosphericresidue

aromaticextracts

waxes lightproducts

fraction Afraction B

fraction C

light SN

medium SN

heavy SN

brightstock

vacuum gas oil

deasphalted oil

asphaltenic residue

Fig. 1. Production process for mineral base oils (solvent process).

light productsvacuumgas oil

base oils

Fig. 2. Productionprocess for base oilsfrom hydrocracking.

paraffins but converts them into isoparaffins; theresult is better, in terms of both the viscosity indexand the general performance. Hydroisomerisationworks at very high hydrogen pressure; the process isaimed at treating normal paraffins of heavymolecular weight (waxes), where the long chains arebroken to be recombined in isoparaffins.

1.2.10 Bitumens

Introduction

General features and definitionBitumens are a solid or semisolid material at

ambient temperature, their colour ranges from black todark brown and they behave like thermoplasticmaterials. They are mainly used for road surfacingand, in a wide range of other applications, where thereis a need for adhesion and impermeability. They canbe found in natural deposits, but today are commonlyobtained by means of refining during the processing ofcrude oil.

The definition of bitumen given in the Europeanstandard (EN) 12597:2000 Bitumen and bituminousbinders – Terminology is as follows: “virtuallyinvolatile, adhesive and waterproofing materialderived from crude petroleum, or present in naturalasphalt, completely or nearly completely soluble intoluene and very viscous or nearly solid at ambienttemperature”. It should be noted that in the USbitumen is usually called “asphalt” (ASTM, 2005).

Historical notesBitumen was the first petroleum product used by

man thanks to its significant adhesive, protective andwaterproofing capabilities. Archaeologicaldiscoveries in Mesopotamia show for example that itwas used as a waterproof coating of temple baths

used for ritual bathing and in water cisterns.Evidence of the use of bitumen starting from 3000BC up to 500 BC has also been found in Iran and innorth-west India, where it was used as a binder inbuilding works and as waterproofing in water tanks.Over the same period bitumen was used in Egypt formummification and as a binding agent for the rockslaid on the banks of the Nile to protect it againsterosion.

The first applications of bitumen in the industrialera date from the end of the Ninenteenth century,especially in the United States, in works forwaterproofing, protection and construction of roads.Only with the development of refining methods forcrude oil, starting in 1920, and subsequently with thedevelopment of methods for transporting the productwhen it is hot, did bitumen spread rapidly to a largenumber of applications.

Natural bitumens and industrially manufacturedbitumens

In various parts of the world there are deposits ofbitumen in an almost pure natural state and ofbitumen mixed with minerals. The deposits of nativebitumens have largely variable geological structures.Their formation may be traced back to the upwellingand infiltration among the rocks of crude fromnearby oil deposits. During these processes, underparticular conditions, separation of the aromatichydrocarbons from the other hydrocarbons mayoccur, leading to the formation of asphaltenes andresins (Abraham, 1960-1963; Giavarini andScarsella, 1993).

In industrially manufactured bitumens theseparation is performed by means of crude oilrefining processes, mainly fractionated distillation,deasphalting or precipitation using solvents,oxidation or blowing and thermal conversion(thermal cracking or visbreaking). Distillation andprecipitation enable separation of the more volatilesubstances and the concentration of the asphalticsubstances to occur, without causing any chemicaltransformation and without cracking effects. On theother hand, the process of oxidation leads tochemical reactions and, therefore, like thermalcracking, can be considered a transformationprocess. Therefore, the characteristics of thebitumens vary depending on the production processused and on the nature of the original crude.

Classification and types of bitumensA classification criterion based on the final

destination of the product divides bitumens intobitumens for use on paving roads and bitumens forindustrial use.



waxes

hydrogen

recycling of unconverted waxes

light products

base oils

Fig. 3. Production process for base oils from hydroisomerization of wax.

Industrially manufactured bitumens can also beclassified on the basis of their physical state at the endof the production cycle: they can be semi-solid, liquidor in form of bituminous emulsions; in their turn, thesemi-solid bitumens can be unprocessed, oxidized(blown) or modified using polymers.

The unprocessed bitumens represent the vastmajority of commercial bitumens. The gradations foruse on roads include penetration intervals from 20-30to 250-330 dmm (in particular, the classes are 20-30,30-45, 35-50, 40-60, 50-70, 70-100, 100-150, 160-220 and 250-330). It should be recalled that thepenetration of a bituminous binder is given by thedepth reached in the material by a standardizedneedle under precise test conditions (see below:Mechanical properties).

Oxidized bitumens are produced by blowing withan air stream of appropriate bituminous bases at hightemperature. Oxidized bitumens are harder and havereduced susceptibility to change with temperature.They are used for waterproofing, sealing, protectionand insulation coatings, although for some years nowthe use of bituminous membranes for theseapplications has been preferred.

Polymer modified bitumens are binders whoserheological properties have been modified duringproduction by means of the use of one or morepolymer compounds. The modification processes aimto give the final bitumen superior rheologicalcharacteristics and performance and at the same timeto preserve all of the binding properties of thebituminous base (Giavarini, 1994). Changing thestructure of the bitumen with a polymer signifies theinversion of the continuous phase of the mix from abituminous to a polymeric phase (Fig. 1). The successof modified bitumens allowed the development ofspecial asphalt mixes such as the porous variety usedfor drainage and sound absorption.

Liquid (or cut-back) bitumens are bituminoussolutions in a suitable liquid solvent. The bitumen baseis chosen according to the climatic conditions in whichthe product is used, while the diluent (solvent oil usedto soften bitumen which is too hard) must be quitevolatile in order to evaporate in a relatively short time.

Bituminous emulsions (SFERB, 1991) aresuspensions of bitumen (dispersed phase) in water(continuous phase), whose stability is allowed thanksto the addition of appropriate additives calledemulsifier agents. The preparation of bituminousemulsions takes place in special plants and involvesthe mixing of bitumen with the aqueous phase at 70-80°C and contains the previously dissolved stabilizingagent. Emulsions are used for road surface rougheningtreatments (necessary to restore the surfaceroughness), for tack coating applications (to avoid twolayers of blends slipping on each other), as sealantsand in cold mixes recycling treatments.

Composition and structureFrom the chemical viewpoint bitumen is a complex

mix consisting of hydrocarbons with high molecularweight and other organic compounds containingdifferent atoms besides carbon, such as sulphur,nitrogen, and oxygen. The average elementarycomposition of bitumen depends on the nature of thecrude oil and the production process; indicatively,percentages of the elements are the following: 80-88%carbon, 7-12% hydrogen, 0-6% sulphur, 1-4% oxygen,0-1% nitrogen, and some trace metals such asvanadium, nickel, iron, magnesium and calcium.

The hydrocarbons present include alkanes,cycloalkanes, aromatic and polycyclic hydrocarbons;they are mainly complex hydrocarbons formed bynaphthenic and aromatic systems, which are dispersedor condensed and interconnected by side chains.

Due to the complexity of the structures present andthe large number of compounds, it is not possible toidentify the chemical composition of bitumen; usingsolvent extraction processes it is, however, possible toidentify some relatively homogenous fractions. These are not fractions with a set composition and chemical formula, but of a kind of solubilityclassification that is determined by the solubility orlack of solubility in particular solvents; in fact there isno discontinuity in the composition of the bitumen(Giavarini and Scarsella, 1993).

Bitumen fractionsAmong bitumen fractions, asphaltenes are by

definition the components that are insoluble inaliphatic solvents such as n-pentane or n-heptane; inspecifying the content of asphaltenes it is, therefore,necessary to specify the solvent used to separate them,since the use of a different solvent leads to theseparation of different asphaltenes, in terms both ofquality and quantity. Asphaltenes are present inpercentages that range from approximately 6 to 25%and represent the ‘backbone’ of the bitumen and giveit its black colour. Once separated they are solid, at



Fig. 1. Electron microscope photograph: phases in theprogress of the bitumen-polymer modification (A and B)and achievement of phase inversion (C).

A B C

ambient temperature, with very complex structuresthat are rich in polynuclear (especially aromatic) andheterocyclic compounds. The molecular weight is veryhigh (from several thousands to hundreds ofthousands) and depends on their state of aggregation.The particular structure of the asphaltenes and theirmarked polarity are largely responsible for thebehaviour of bitumen as a very viscous material, withgreat plasticity and elasticity, that is resistant tomechanical stress, and is highly adhesive and cohesive(Yen and Chiligarian, 1994).

Traditionally, the fraction that is soluble in n-pentane (or in another n-alkane) is given the namemaltenes; maltenes can be further subdivided into resinsand oils. The oils act as dispersants of the asphaltenes,together with the resins. The resins are a class ofcompounds that are relatively polar, solid or semi-solidat ambient temperature, and brown or reddish in colour;they have a stabilizing effect on the asphaltenes, andhave a very important role in the balance of thecolloidal state. They also help to make the bitumenductile with binding power and good adhesion.

Two other fractions, which can be considered asfurther subdivisions of the oil fraction are thearomatics and the saturates. The aromatics fractionincludes aromatic and naphthenic compounds at alower molecular weight and is the fraction mostresponsible for the dispersion of asphaltenes; atambient temperature, aromatics are a viscous, darkbrown liquid. Saturates represent the fractioncomposed of straight or branched chain aliphatichydrocarbons, together with alkyl naphthenes andalkyl aromatics. They are straw-coloured, non-polarviscous oils; the average molecular weight is similar tothat of aromatics. This fraction represents from 5 to20% of the bitumen.

The analytical techniques used to separate thefractions described above are fractional solubilization,chemical precipitation and absorption followed byselective desorption; among these the most common ischromatography. Using thin layer chromatographytogether with a flame ionization detector it is possible,for example, to determine the content in terms ofasphaltenes, resins, aromatics and saturates.

Physical structure From the chemical and physical viewpoint bitumen

is described as a multi-phase system of a colloidalnature, consisting of micelles, formed by asphalteniccompounds of high molecular weight and by aromaticcompounds with high solvating power, dispersed in aviscous fluid consisting of maltenes (Yen andChiligarian, 1994). Given enough resins andaromatics, the asphaltenes are adequately solvated, thelevel of dispersion of the micelles is very high and inthis case the bitumen is classified as sol type: themicelles are small and the liquid character is prevalent.On the other hand, a gel-type structure is found whenthere is not enough of a solvating fraction for theasphaltenes, and consequently there is a partialaggregation of the micelles, which are large andpoorly dispersed (Fig. 2).

The colloidal behaviour of the asphaltenes inbitumen and their degree of aggregation, therefore,influence their viscosity and rheological properties.Bitumen with a sol type colloidal structure behaveselastically at high load speeds and has Newtonianviscous reactions at low speeds. If the colloidalstructure has a lower level of dispersion, the viscousreaction is usually of a slightly non-Newtonian type.Finally, for gel-type bitumens the viscosity isdecidedly non-Newtonian.

The equilibrium between the phases is markedlyinfluenced by the temperature and therefore, as the lattervaries, it is possible to observe the same bitumen in bothof the two extreme situations, respectively sol and gel,and a continuous and gradual series of intermediateconditions (Bonemazzi and Giavarini, 1999).

Fundamental properties of bitumensand their assessment

Rheological propertiesFrom the rheological viewpoint bitumen is a

viscoelastic material and in order to describe itsresponse to stress, it is useful to consider thefundamental rheological models in variouscombinations and measures. These models includeelastic response, creep phenomena, delayed elasticityand plastic deformation. The behaviour of bitumen canin fact vary from that of an elastic to a viscous body,



AA

B

Fig. 2. Representation of the structure of bitumen: A) sol-type structure; B) gel-type structure.

taking on all the intermediate elastoplastic aspects,depending on the temperature, duration and frequencyof the application of the stress.

Therefore, when characterizing bitumenrheological tests are conducted under such conditionsas to ensure the control of the temperature and perfectknowledge of the response to the loads at every instantand at every point of the specimen considered. Thesetests can be tests of viscosity or tests in which it ispossible to highlight the components of elasticresponse, such as deformation tests under constantloading, relaxation tests at a set deformation andoscillation tests (dynamic tests); these tests can becarried out by using various geometric configurationsand loads, with reference to shear or normaldeformation and stresses (Eurobitume, 1999).

Creep tests, normally carried out under uniformshear stress, allow the visualization of the variouscomponents of the mechanical response of thebitumen. Starting from a parallel plate geometry andapplying, at a given temperature, a constant tangentialstress t on a sample of bitumen, a shear deformation gis obtained which can be represented in relation to thelength of time the load is applied (Fig. 3); thus threeregions of behaviour of the bitumen may be identified:the elastic region, the delayed elasticity region and theviscous region, characterized respectively by a shear

deformation gE, gDE, gn. For very short loading timesand very low temperatures, the elastic response isdominant; for long loading times or high temperaturesthe viscous response prevails; for intermediate loadingtimes and temperature levels the delayed elasticresponse is important. From the parameters imposedand measured in a creep test, two viscoelastic featurescan be identified which effectively describe therheological properties of the bitumen: the sheardeformability J(t)�g(t)/t and its inverse, the shearstiffness modulus, G(t)�t/g(t).

The rheological behaviour of bitumen can bedescribed by tests in oscillation mode, in which asample of bitumen is subjected to a tangential orperpendicular deformation with a sinusoidal motionover time; the resulting stress or deformationmeasured will also be sinusoidal. The two viscoelasticparameters that can be calculated in this type of testare the complex modulus G*�t/g0 and the phaseangle d. The complex modulus G*, defined as therelationship between the maximum stress t and themaximum deformation g0, expresses the overallstiffness of the material, while the phase angle dexpresses the delay between the stress imposed and thedeformation of the material and depends on its degreeof elasticity. For elastic materials d�0° and forviscous material d�90°. On the basis of suchparameters it is possible to calculate the elasticmodulus G�G*cosd, which represents therelationship between the components of stress anddeformation that are perfectly in phase with eachanother, and the viscous modulus G”�G*sind, whichis given by the ration of the components out of phaseby 90°; these moduli are indicative respectively of theenergy used for the development of a completelyreversible elastic deformation and of the energydissipated within the material during deformation. Athigh frequencies or reduced loading times bitumenbecomes practically elastic and tends to a limitingvalue of the complex modulus; at low frequencies orfor long loading times the bitumen behaves in aviscous manner, which occurs for various times orfrequencies depending on the temperature at which thetest is performed. In intermediate situations, theperformance corresponds to viscoelastic behaviourwith values of d around 45°.

The viscosity of bitumens is a measure of theresistance (internal friction) of the material to theforces which tend to make it flow. There are varioustests to determine viscosity and each takes advantageof a particular physical phenomenon: the time requiredfor the material to flow through a nozzle; the time fora sphere to fall through a given column of bitumen;and the power absorbed during the movement of acylinder in contact with the product. For bitumens, as



uP

h

t0

t�t0

gE

gDEgn

gn

gE

t

g

section A

time

gDE

Fig. 3. Behaviour of bitumen in a creep test followingapplication of a tangential stress t0.

for all materials, it is possible to refer to dynamic andstatic viscosity. Dynamic viscosity is given by therelationship between the tangential force applied to asample of bitumen and the speed gradient of the flow:m�t/(du/dy); its unit of measurement in theInternational System is the Pa�s (1 Pa�s�10 poise).Dynamic viscosity is measured by a rotationalviscometer: measurements are made of the shear forceapplied to a film of bitumen placed between twoelements and the speed of the resulting flow. It is alsopossible to consider the kinematic viscosity recordedby means of a capillary viscometer; this viscosity isrelated to dynamic viscosity by means of therelationship: kinematic viscosity�dynamicviscosity/density.

Depending on the characteristic frequencies andtemperatures employed in the applications in whichbitumen is used, the information collected throughthe rheological tests allows the selection of the mostsuitable materials for a given application. It shouldalso be noted that some traditional tests (penetration,ring and ball softening point, viscosity) were alreadyrelated to the rheological properties of the bitumens.In the last decades of the Twentieth century specificequipment as well as measurement methodologieswere developed for the rheological characterizationof bitumens, principally due to the work of theSHRP (Strategic Highway Research Program, see below).

Mechanical properties Among the properties related to the behaviour of

bitumen subject to mechanical stresses it is necessaryto mention the consistency which expresses theresistance offered by the bitumen to the penetration ofa foreign body, determined using penetration tests andthe softening temperature (Fig. 4).

Penetration expresses the toughness of a sample,measuring in decimillimetres the vertical penetration

of a standardized needle at a certain temperature,under a fixed load (100 g�0.98 N) applied for a settime interval (5 seconds); penetration is greatlyinfluenced by temperature. The measure of penetrationin Europe is still used to make a technical andcommercial classification of bitumens: depending onthe environmental, climatic and constructionconditions it is necessary to use a product that canresist the thermal and mechanical stresses envisagedfor a given application.

The softening point traditionally represents thetemperature at which the bitumen changes from thesemi-solid to the liquid state, even though, asbitumen consists of numerous different chemicalcompounds that are interconnected in thousands ofdifferent structural configurations, it does not have amelting point, but rather a temperature range duringwhich the passage to the liquid state occurs. Thesoftening point is defined by the ring and ballmethod: the bitumen sample is placed in a specialbrass ring and loaded in the centre with a steelsphere of a fixed size and weight. The ring is placedin a heated bath; as the temperature increases, thebitumen deforms under the weight of the spherewhich gradually descends until it reaches a targetplaced underneath the starting level: thecorresponding temperature of the bath (Tpa)represents the softening point.

Once the values of penetration and softening pointsare known, the penetration index is defined. Thisnumber is chosen by convention to classify thebitumen on the basis of its susceptibility to thermalactivity. Experience has shown that asphaltenicbitumens of a gel type, with a non-Newtonianstructure, show little susceptibility to heat and resistthermal stress better than others. On the other hand,bitumens which are low in asphaltenes, or in any case,of a sol-type structure, have high thermalsusceptibility.



A B C

start after 5 s

penetration (dmm)

Tpa

TFraass

5°C/min

1°C/min

25.4 mm

100 g

100 g

25°C3.5 g

A B C

Fig. 4. Representation of the determination of penetration (A), of the softening-point ring and ball test (B),and of the Fraass breaking point (C).

Adhesion and cohesion The binding properties of bitumen enable it to

adhere and maintain adhesion to a range of bodies,such as inorganic aggregates, binding with them sothat they are held tight.

The phenomenon of adhesion, i.e. the property ofbinding and remaining soundly anchored to the surfaceof the material covered, is explained by variousmechanisms: neutralization chemical reactions of anacid-base type between the acid compounds of thebitumen and the basic compounds present on thesurface of aggregates; surface phenomena linked tothe formation of interfacial tension between thebitumen and the aggregates (the interface increases instability as the polarity of the substances underexamination rises, while it is markedly reduced by thepresence of water on the surface to be treated); andmechanical factors linked to the level of roughness andporosity of the surface of the body to be treated (theformation of a stable bituminous film is easier toachieve, the rougher and more porous the surface ofthe aggregate).

Once good adherence of the bitumen to theaggregates has been achieved, the bituminous bindermust also demonstrate effective cohesion, i.e. it mustkeep the disaggregated particles tightly bound to eachother and coated in bitumen.

As for tests to measure the above properties,generally the adhesion of bitumen to an aggregate isestimated by assessing the percentage of surface areaof an aggregate sample (originally covered with a setamount of bitumen) which is uncovered followingimmersion in distilled water of increasing values oftemperature. The cohesion of the bituminous binders isassessed by the measurement, at a low temperature, ofthe tensile properties of a specimen which is stretchedby means of a tensile test (in such a way as to acquirea filiform structure under the forces of traction).

Other physical properties of bitumenOther typical properties of bitumen are: a) the

specific weight, which is used to carry out weight-volume conversion (transformation calculations); b) the flash point, the value of which determines thelevel of safety of manipulation; c) the solubility inorganic solvents, which is useful in order to determinethe level of purity; and d) the paraffin content, used toassess its tendency to brittleness.

Mechanical properties at high temperaturesThe properties required of bitumen at high

temperatures are consistency in order to resistpermanent deformations and elasticity to recoverdeformations at high temperatures of service. In orderto assess these properties reference is made to the

determination of the viscosity, of the softening pointby the ring and ball test method, and of the complexmodulus by means of the Dynamic Shear Rheometer(DSR): using this device it is possible to measure thestiffness G*, the phase angle d and the viscosity athigh temperatures. These measurements have beendeveloped as part of the SHRP program in relation tothe problem of cutting (left by vehicles on the roadsurface): the studies carried out have revealed acorrelation between the cut depth of asphalt at a hightemperature and the stiffness performance of thebinder.

Mechanical properties at low temperaturesPerformance at low temperatures is associated with

the loss of the elastic and mechanical properties of thebitumen. In order to define the fragility of a bitumen,generally, the lowest temperature at which the bitumenresists a given mechanical stress is determined. To dothis reference is made to the Fraass breaking point,which conventionally measures the temperature(TFraass) at which a film of bitumen subject to flexingshows the first signs of breaking.

In order to determine the performance of abituminous binder at low temperatures there are twodevices, both developed as part of the SHRP program:the Bending Beam Rheometer (BBR; Fig. 5), whichallows the measurement, against time, of the creepdeflection of a sample loaded at its centre, and theDirect Tension Tester (DTT) which allows themeasurement of the fracture behaviour of a samplesubject to traction.

AgeingBinder ageing indicates a gradual reduction over

time in the physical and mechanical properties of thebitumen and is a consequence of chemical changesinduced by external factors. Bitumen in fact suffers



thermostatic bathbitumen bar

thermometer

data and control

load cell

pneumatic system

LVDT

Fig. 5. Diagram of the bending beam rheometer. LVDT, Linear Variable Differential Transformer.

from exposure to oxygen, atmospheric agents,ultraviolet radiation and the heat to which it is subjectduring storage operations, preparation and laying ofthe asphalt mixes and in the working conditions of theroad surface of which it is part. From the rheologicalviewpoint ageing brings a change in the flow and anincrease in the stiffness of the bitumen. This isperceived as a reduction in the viscosity and ductilityand an increase in the brittleness at low temperatures,with the consequence that the ageing asphalt mixes areless resistant to stress.

Two related phenomena contribute to ageing, i.e.evaporation of volatile substances and oxidization,which both increase the tendency of resin-asphaltstructures to associate to give rise to more complexstructural configurations with a reduction in theproperties of the binder, such as cohesion andadhesion.

Validation tests in the field have highlighted thefact that ageing of bitumens occurs above all duringthe preparation and laying of the mixes (short-termageing), while during the life of the road ageingfollows much slower kinetics (long-term ageing); themanipulation of bitumen during the preparation andlaying of the mix, in fact, occurs at high temperaturesand in an oxidizing atmosphere, while in service theageing conditions of the bitumen are gentler anddepend above all on climatic and environmentalfactors.

The resistance of bitumen to short-term ageing isassessed using various tests conducted to measure thetendency of the product to harden in variousoperational stages carried out at high temperatures.The best known and most widely used methods are:the Rolling Thin Film Oven Test (RTFOT), a dynamictest that simulates the conditions of the mixing of thebinder with the aggregates; the Thin Film Oven Test(TFOT), a test which simulates storage and serves todetermine ageing under static conditions; and theRotating Flask Test (RFT), a test to determine ageingunder dynamic conditions.

To assess long-term ageing the Pressure AgeingVessel (PAV) method is used, based on a pressurechamber that can work with pressures of 21 bar andtemperatures between 80 and 115°C.

The ageing tests have the twofold aim of assessingthe resistance to ageing and pretreatment of the binderin relation to further determinations in order to assessits ability to resist oxidation.

Normative referencesIn Europe as from 1999 some standards have been

approved regarding the classification and assessmentof bituminous binders, developed by the technicalcommittee CEN to specify the properties of bitumens.

The standard (EN) 12591:1999 Bitumen andbituminous binders – Specifications for paving gradebitumens relates to traditional bitumens for road useand classifies them on the basis of the classes ofpenetration. It divides the bitumens into three groups:penetration of between 20 and 330 dmm at 25°C, intotal 9 grades; penetration of between 250 and 900dmm at 25°C, in total 4 grades; and soft bitumensdefined by a kinematic viscosity interval at 60°C, intotal 4 grades.

The European standard on modified bitumens,(EN) 14023:2004 Bitumen and bituminous binders –Framework specification for polymer modifiedbitumens, is based on a classification of adescriptive type similar to that for traditionalbitumens and proposes a model for the variousclasses of product. The main properties to which itrefers are: elastic recovery, Fraass breaking point,plasticity range, storage stability, cohesion, forceductility, flash point, penetration and softening pointafter RTFOT ageing test.

Also for bituminous emulsions, the Europeanstandard (EN) 13808:2003 Framework specificationfor bituminous emulsion proposes a framework forvarious classes of product and sets out the features ofthe emulsion, of the recovered bitumen (evaporationmethod) and of the bitumen after ageing.

The standards described above represent aharmonization of the various national specifications,but are based on traditional and empirical tests. At thesame time as they came into force, therefore, therebegan a process to define a second generation ofspecifications based on the performance of the binder.The performance characteristics identified are: theability of the bituminous binder to respond elasticallywhen subject to stress at high temperatures (reducedpermanent deformations), the ability of the binder torespond elastically to stress at low temperatures(reduced loss of elastic properties), the durability ofthe bitumen, in other words, its resistance to ageing.For each of these properties the reference will be thetest methods developed as part of the SHRP program,which have already been widely adopted in Europe.

Strategic Highway Research Program (SHRP)In the United States, the SHRP program, promoted by

the Federal Highway Administration at the end of the1980s, defined new methodologies to assess theproperties of bituminous binders and to check theirresponse under real working conditions. The SHRPregulations allow classification on the basis of criteria thattake account of the performance required from the binderonce it has been laid (Giavarini and Speight, 1991).

The studies carried out in this field have shownthat the most common forms of deterioration of road



surfaces are mainly due to three phenomena: theformation of cuts on the road surface, fatiguecracking and thermal cracking, linked to therheological characteristics of bitumen, which in theirturn depend on the temperature and the frequency ofloading.

According to the SHRP studies, in order toprevent the formation of cuts it is necessary to checkthe factor G*/sind�1.00 kPa, since this ratio hasbeen recognized as a valid parameter that caninterpret the viscous behaviour of the material: thelower the dephasing between the load imposed andthe resulting deformation, the lower the phase angle.In the complex modulus of the binder the elasticcomponent is greater and the viscous componentlower, which results in the formation of the cuts. Byincreasing the test temperature the value of thecomplex modulus reduces; in practice it is necessaryto identify an upper temperature limit (maximumproject temperature) for a bituminous binder, beyondwhich the previous relationship is no longerrespected. The viscoelastic parameters of thebitumen are established in correspondence to a loadapplication rate of 10 rad/s (1.59Hz), on the originalbinder and after ageing on the RTFOT (with a valueof G*/sind�1.00 kPa).

To prevent fatigue cracking it must be verified thatG*sind�5,000 kPa (50 bar). The parameter G*sindcorresponds to the viscous component of the complexmodulus and represents the energy dissipated by thematerial under loading. It is this energy which leads tothe formation of cracks. By reducing the testtemperature the value of the complex modulusincreases, and therefore in practice it is a matter ofidentifying a lower temperature limit (minimumproject temperature) below which the previousrelationship breaks down. The test is carried out onaged bitumen. In particularly harsh climates, thebitumen fractures both in conditions of repeated trafficloads (fatigue cracking), and in conditions ofpersistent low temperatures (low temperaturecracking). To measure the fragility, the followingequipment has been devised: the direct tension tester,to assess the ductility of the bitumen, and the bendingbeam rheometer to test the creep stiffness at lowtemperature. The SHRP specifications requireverification that the elastic modulus is low and the rateof its fall over time is high. The temperature at whichthese conditions are verified is defined as the limitingstiffness temperature.

In this way it is possible to identify the maximumand minimum temperatures at which the bituminousbinder is intended to work once laid (within thisinterval there are no road deterioration phenomenathat can be ascribed to the binder).

For their commercial classification, binders areidentified by a Performance Grade (PG) whichrepresents the interval between the highest and lowesttemperatures recorded on the road (as an average overseven days). The territory of the United States hasbeen subdivided into isothermal zones and, therefore,the task of whoever has to prescribe the type of binderto adopt has been made easier. A binder which, forexample, may be classified as PG 64-28 can be usedwithout producing deterioration phenomena due toperformance problems in the range between 64°Cand �28°C.

Applications of bitumenThe applications of bitumen can be broadly

subdivided into road and industrial use.The use of bitumen in the construction of road

surfaces covers approximately 80% of the marketand is mainly done with the production and layingof bituminous blends, mixes of stone aggregatesand appropriate loads mixed in special plantsgenerally at a high temperature with a bituminousbinder. A road made of bituminous blend must beable to withstand, without any significantpermanent deformation, the vertical and tangentialstresses imposed by the circulating traffic. Therole of the road surface is, therefore, to uniformlydistribute the mechanical stresses in such a way asto make them compatible with the load bearingcapacity of the foundation. Depending on thetraffic conditions, the structure can vary from asimple surface covering placed on a road-bed to amultilayer surface up to 50 cm thick. Generally aroad structure consists of a base course, a binderand wearing course (Fig. 6). The base course isimmediately above foundation ground and has theaim of absorbing the loads and mechanical stressesdue to the traffic coming from the higher layersand to transmit them reduced and uniformlydistributed to the foundation. The binder musttransmit to the base course the stresses coming



wearing course

binder course

base course

sub-base

Fig. 6. Layers in a road structure.

from the surface, and contribute to sharing theloads gradually towards the foundation. Thewearing course is the part directly exposed to thetraffic movements and climate and over time mustresist static or dynamic loads, generated byvehicles, withstand the pressure and shear loadsand stresses and resist deterioration; given theflexibility of this layer, due to the elasticproperties of the bitumen, most of the stresses aretransmitted to the underlying layers. The top layermust be waterproof and be very stable chemicallyand physically, have good surface regularity toguarantee a smooth ride and at the same time havethe ideal level of roughness to allow vehicles tomaintain correct adherence in all climaticconditions. The material used is an asphalt mixwhich is generally richer in bitumen and fineaggregates than that of the other layers.

Due to its waterproofing properties, thermalinsulation and chemical resistance, bitumen is usedfor making bitumen-polymer waterproofingmembranes, destined for the construction sector toprotect and waterproof foundations, roofs, terracesand covers from the action of water and atmosphericagents. Bitumen-polymer membranes consist of asublayer or support, for example in polyester fibre,which gives the product resistance to the mechanicalactions to which it is subject; this sublayer is coveredon one or both sides by a film of polymer modifiedbitumen and is filled with an inorganic fill.Waterproofing membranes made in this way arehighly flexible, which allows preservation of theimpermeability of the cover even in the presence ofsmall deformations in the sublayer and makes itpossible to apply it in situ. These bituminousmembranes are also used to waterproof river banks,irrigation canals, drains, tanks and swimming pools.Bitumen is a suitable material since it is resistant tochemical agents such as acids, bases and salts; andgiven its plasticity it is easily adaptable to the surfaceirregularities of the structures.

Bitumen is also used for its isolating properties ininsulation applications, in covering and insulatingelectrical equipment due to its water-repellent natureand dielectric properties, in protective coverings forits ability to provide protection against corrosion andfinally in noise-insulating coatings in the carindustry. Bitumen is also used in the preparation ofmastics, paints or dyes, lubrificants for chains andgears, adhesives and plastificants for the rubberindustry.

Bibliography

ASTM (American Society for Testing and Materials) (1990)Annual book of ASTM standard. Section 5: Petroleumproducts, lubricants and fossil fuels, Philadelphia (PA),ASTM.

Del Ross S. (1977) I lubrificanti. Caratteristiche fisiche,chimiche e tecnologiche, Milano, ETAS.

Giavarini C., Scarsella M. (1995) Caratteristiche del bitumein rapporto a salute e sicurezza, «Rassegna del bitume», 25.

Klamann D. (1984) Lubricants and related products,Weinheim, Chemie.

Mortier R.M., Orszulik S.T. (edited by) (1997) Chemistryand technology of lubricants, London, Blackie Academic& Professional.

PIARC (World Road Association)/SITEB (Associazione ItalianaAsfalto Strade) (1998) Modified binders, special bitumensand additives in road contruction. Proceedings of theInternational workshop on modified bitumens, Rome(Italy), 17-19 June.

Read J., Whiteoak D. (2003) The Shell bitumen handbook,London, Telford.

Shubkin R.L. (editor) (1993) Synthetic lubricants and high-performance functional fluids, New York, Marcel Dekker.

References

Abraham H. (1960-1963) Asphalts and allied substances. Theiroccurrence, modes of production, uses in the arts, and methodsof testing, New York, Van Nostrand Reinhold, 5v.; v.I.

ASTM (American Society for Testing and Materials) (2005)Annual book of ASTM standards. Section 4: Construction;4.03: Road and paving materials. Vehicle-pavement systems,Philadelphia (PA), ASTM.

Bonemazzi F., Giavarini C. (1999) Shifting the bitumenstructure from sol to gel, «Journal of Petroleum Scienceand Engineering», 22, 17.

Eurobitume (1999) Eurobitume workshop 99. Performancerelated properties for bitominous binders. Proceedings ofthe Eurobitume workshop, Luxembourg, 3-6 May.

Giavarini C. (1994) Polymer modified bitumens, in: Yen T.F.,Chiligarian G.V. (edited by), Asphaltenes and asphalts,Amsterdam, Elsevier.

Giavarini C., Scarsella M. (1993) Struttura e composizionedel bitume, «La chimica e l’industria», 75, 754.

Giavarini C., Speight J. (editors) (1991) Chemistry ofbitumens. Proceedings of the International symposium onchemistry of bitumens, Rome (Italy), 5-8 June.

SFERB (Syndacat des Frabricants d’Émulsions Routières deBitume) (1991) Bitumen emissions, Paris, SFERB.

Yen T.F., Chiligarian G.V. (edited by) (1994) Asphaltenesand asphalts, Amsterdam, Elsevier.

Riccardo MaioneEni - Divisione Refining & Marketing

Roma, Italy



1.2 the products of refining - treccani

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