283

15Diesel Fuel

15.1Introduction

Diesel fuel refers to the fuel that is used in compression-ignition engines (diesel engines). It isgenerally material boiling in the C11 –C22 hydrocarbon range. The requirements for diesel fuelare often diametrically opposed to that of motor-gasoline (Chapter 13). In motor-gasoline, it isimportant to suppress autoignition of the fuel to allow spark ignition to be correctly timed forgood engine performance. In diesel fuel, it is important that the fuel autoignites and that thedelay between fuel injection and the start of combustion is short.

Compression-ignition engines operate at high compression ratios, typically around 15–17 : 1.This improves the thermodynamic efficiency of the engine, and on average compression-ignitionengines are more efficient than spark-ignition engines. The engine itself is heavier, andtraditionally it has been applied mainly for heavier vehicles. This situation has changed and,despite the higher cost of compression-ignition engines as compared to spark-ignition engines,there has been a marked shift in preference for diesel-powered passenger vehicles in Europe.This change is partly due to an increase in environmental awareness. Better engine efficiencyleads to lower fuel consumption and can be translated into less CO2 emitted per distance traveled.The move to vehicles with lower rated emissions (in grams of CO2 per kilometer) is supportedby the policies of the European Union.

This shift in transportation fuel preference for the passenger vehicle market had a markedimpact on refineries. The hydrogen in a typical conventional crude oil refinery is obtained fromcatalytic naphtha reforming. As the demand for motor-gasoline decreases, the need for reformatealso decreases, which in turn reduces hydrogen production. Yet, refining crude oil to dieselfuel requires more hydrogen than refining crude oil to motor-gasoline. In each refinery thereis a minimum motor-gasoline to diesel fuel ratio beyond which refining becomes inefficientand costly. The shift toward diesel fuel has created an imbalance in refinery production and inpractice this imbalance in Europe is corrected by exporting motor-gasoline and importing dieselfuel. In the long run, it will be interesting to see how this might affect diesel fuel specifications,which over time have made diesel fuel production more difficult.

Diesel fuel, like motor-gasoline, is a major consumer product and exerts a tremendousinfluence on the economy and the environment. Diesel fuel specifications are consequentlysubject to technical, environmental, and political pressures (Section 13.1).

Fischer–Tropsch Refining, First Edition. Arno de Klerk. 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

284 15 Diesel Fuel

This chapter will employ a similar format as the preceding two chapters that dealt withmotor-gasoline and jet fuel (Chapters 13 and 14). More detailed descriptions of fuels and fuel speci-fications can be found in the literature [1, 2]. The following aspects will be discussed in this chapter:

1) Current diesel fuel specifications, regional differences, and how the specifications relate tothe molecular requirements of the fuel.

2) Direction of anticipated changes that will affect refinery designs in future.3) The considerable difference in distillate properties from high-temperature Fischer–Tropsch

(HTFT) synthesis and low-temperature Fischer–Tropsch (LTFT) synthesis.4) The different demands placed on HTFT and LTFT refineries by the diesel fuel property

requirements.

15.2Diesel Fuel Specifications

Compression-ignition engines are robust and can operate reliably with heavy fuels of quite avaried composition. There are divergent views about diesel fuel quality and specifications. Thiscan be seen when comparing the European EN590 diesel fuel specifications, the American ASTMD 975 specification, and the World Wide Fuel Charter (WWFC) guidelines (Tables 15.1 and 15.2).

Table 15.1 American and European diesel fuel specifications.

Specification D-1 D-2 Euro-2 Euro-3 Euro-4

ASTM D 975 EN590:1993 EN590:1999 EN590:2004

Density at 15 ◦C (kg·m−3) – a – a 820–860 820–845 820–845Cetane number, min. 40b 40b 49 51 51c

Viscosity at 40 ◦C (cSt) 1.3–2.4 1.9–4.1 2.0–4.5 2.0–4.5 2.0–4.5Flash point (◦C), min. 38 52 55 55 55Lubricity, HFRR at 60 ◦C (µm), max. 520 520 460 460 460Hydrocarbon content (mass%), max.

Total aromatics – a – a – a – a – a

Polycyclic aromatics – a – a 11 11 11Sulfur content (µg·g−1), max. 500/15 500/15 2000/500 350 50/10Water content (µg·g−1), max. 500 500 200 200 200Distillation (◦C)

T90, min. – a 282 – a – a – a

T90, max. 288 338 – a – a – a

T95, max. – a – a 370 360 360FAME addition (vol%), max.d – a – a – a – a 5

aNot regulated by the fuel specification.bEngine operation at higher altitude and low ambient temperature may require a higher cetane number;minimum cetane index requirement is also 40.cMinimum cetane index of 46 is required.dFatty acid methyl ester (FAME) inclusion allow the blending of renewable material.

15.2 Diesel Fuel Specifications 285

Table 15.2 Selected diesel fuel quality guidelines proposed by the WWFC.

Specification Category 1 Category 2 Category 3 Category 4

Density at 15 ◦C (kg·m−3) 820–860 820–850 820–840 820–840Cetane number, min. 48 51 53 55Cetane index, min.a 48/45 51/48 53/50 55/52Viscosity at 40 ◦C (cSt) 2.0–4.5 2.0–4.0 2.0–4.0 2.0–4.0Flash point (◦C), min. 55 55 55 55Lubricity, HFRR at 60 ◦C (µm), max. 400 400 400 400Hydrocarbon content (mass%), max.

Total aromatics – b 25 20 15Polycyclic aromatics – b 5 3 2

Sulfur content (µg·g−1), max. 2000 300 50 10Water content (µg·g−1), max. 500 200 200 200Distillation (◦C), max.

T90 – b 340 320 320T95 370 355 340 340FBP – b 365 350 350

FAME addition (vol%), max 5 5 5 0

aThe higher value must be used if no cetane number improver is used; the lower value is valid only for the basefuel before addition of a cetane number improver.bNot specified by the guidelines.

Some trends can be seen from the changes in the European EN590 over time as well as thedifferent WWFC categories (Table 13.2) [3]. There is a steady increase in the cetane number(CN) requirement for diesel fuel. This is mainly related to reported beneficial effects, such aslower crank time and lower hydrocarbon and CO emissions. Other changes are also motivatedby emission reduction. In order to limit deactivation of tailpipe catalytic converters and reduceexhaust emissions in general, the maximum sulfur content in diesel fuel was dramaticallyreduced. The maximum polycyclic aromatic content was decreased in tandem with a lowering ofthe T95 distillation temperature to reduce particulate emissions.

The ASTM D 975 specifications [4] are much less restrictive, although provision was made forlow-sulfur diesel fuels.

The diesel fuel specifications listed in Tables 15.1 and 15.2 are not a complete set ofspecifications. Cold-flow properties, which have not been listed, are important specificationsand are dependent on the climate where the diesel fuel will be used. There are also otherspecifications, such as fuel stability requirements, that have not been listed.

As in the case of motor-gasoline and jet fuel, final on-specification diesel fuel includes additives[2]. Additives address some of the properties that are not regulated by the fuel specifications,such as detergency, antifoaming properties, and water de-emulsifying behavior. There are alsomajor diesel fuel specification requirements that can be addressed through the judicious useof additives. Since these additives make refining less onerous and consequently have a directimpact on refinery design, such additives will be discussed separately (Section 15.4).

286 15 Diesel Fuel

15.3Diesel Fuel Properties

The molecular properties of a diesel fuel have a significant impact on engine performance andemissions. Even for Fischer–Tropsch-derived diesel fuels that have almost identical ASTM D 975properties, observed NOx and particulate matter (PM) emissions can be quite different [5]. Thepoint made by Bisio and Atkinson is pertinent and it is well stated [5]: ‘‘Often it is not appreciatedthat the properties of (and the nature and distribution of molecular species) in Fischer–Tropschdiesel are established in the downstream refinery processes . . . and not in the Fischer–Tropschreactor.’’

Cookson, Lloyd, and Smith applied the same property correlation methodology as that employedfor jet fuel (Section 14.3) to diesel fuel [6]. Diesel fuel properties were linearly correlated to themass percentage of [n] the n-alkanes, [BC] the branched alkanes and cycloalkanes, and [Ar] thearomatics (Equation 14.1). It was noted that an expansion of the correlation to differentiatebetween mononuclear and dinuclear aromatics did not improve its predictive ability much. Amore complex, but analogous approach has successfully been followed to relate PM emissions tothe molecular properties of diesel fuels [7].

Using the approach of Cookson, Lloyd, and Smith [6], a specification domain can be con-structed for diesel fuel that shows the compositional space allowed by diesel fuel specifications(Figure 15.1).

As in the case of jet fuels, the inherent shortcoming of lumping branched alkanes andcycloalkanes into one parameter, [BC], is that it is not representative of Fischer–Tropsch-deriveddiesel fuel. The key difference between branched alkanes and cycloalkanes is density. It turnsout that density is a critical property for meeting EN 590 type and WWFC type diesel fuels [8].The diesel fuel density specification is constraining and, although Figure 15.1 gives a reasonablequalitative indication of the specification domain, it does not reflect the density deficiencyassociated with a low cycloalkane content.

15.3.1Cetane Number

The cetane number is a measure of the compression–ignition delay of a fuel. The ignition delay isthe time difference between injecting the fuel into the combustion chamber and the autoignitionof the fuel. If the ignition delay is too long, it leads to noisy combustion, very high pressuresin the combustion chamber, and increased NOx emissions. The ignition delay is one of the keyquality parameters for a diesel fuel.

The CN is a derived property value, which is a measure of the ignition delay of a fuel. Alow value signifies a long compression–ignition delay, while a high value signifies a shortdelay. The CN is measured on a test engine in accordance with the ASTM D 613 standardtest method [9]. It is defined on an arbitrary scale where n-hexadecane (n-cetane) has avalue of 100 and 1-methylnaphthalene has a value of 0. Since 1962, 1-methylnaphthalenehas been replaced by 2,2,4,4,6,8,8-heptamethylnonane as a primary reference fuel. The2,2,4,4,6,8,8-heptamethylnonane has a cetane value of 15 as measured relative to that of theoriginal definition of CN.

15.3 Diesel Fuel Properties 287

[BC] [Ar]

[n]

Maximumcold-flowproperties

Minimumdensity

Minimumcetane number

On-specificationEN590-typediesel fuel

Figure 15.1 Relationship between diesel fuel compositionand the EN590 diesel fuel specification domain. The com-position is defined by grouping the n-alkanes as [n], thebranched alkanes and cycloalkanes as [BC], and the aromat-ics as [Ar].

The ASTM D 613 is an expensive and material-intensive test method. A derived CN can also beobtained from the more recently developed Ignition Quality Tester (IQT) [10], which measuresthe time from the start of fuel injection into a constant-volume combustion chamber to the startof combustion. The derived CN from the IQT is one-to-one correlated with the CN.

The inverse relationship between CN and the ignition delay is the opposite of octane number(Section 13.3.1), which is a measure of autoignition resistance. The autoignition resistance isdirectly related to ignition delay. CN is correlated to the blending value of the motor octanenumber by (Equation 15.1) [11]:

Cetane number = 58.9 − 0.47 · MON (15.1)

As anticipated by this correlation, the compound classes that display poor octane numbers(Section 13.3.1) are those that have high CNs (Table 15.3) [12]. The CNs for isostructural aliphatichydrocarbons increase with increasing carbon number [13], as one would expect from the changein octane number.

Various derived methods have been developed to calculate the ignition delay properties ofdiesel fuels without having to resort to the difficult, time-consuming, and costly ASTM D 613method for CN determination. Crude oil refiners often make use of the cetane index as analternative to the CN. The cetane index is a calculated value based on the density and distillationproperties of a diesel fuel. It correlates with the CN before the addition of cetane improvers

288 15 Diesel Fuel

Table 15.3 Cetane numbers of various hydrocarbon compounds.

Compound class Compound Cetane number

n-Alkane n-Decane 77n-Dodecane 87n-Tetradecane 95n-Hexadecane 100n-Octadecane 106

Branched alkane 2,2,4,4,6,8,8-Heptamethylnonane 157,8-Dimethyltetradecane 405-Butyldodecane 45

n-1-Alkene 1-Decene 561-Dodecene 711-Tetradecene 801-Hexadecene 861-Octadecene 90

Branched alkene 5-Butyl-4-dodecene 45Cycloalkane n-Butyldecalin 31

n-Octyldecalin 313-Cyclohexylhexane 36trans-Decalin 46

Aromatic 2,6-Dimethylnaphthalene −131-n-Butylnaphthalene 6Tetralin 13n-Butyltetralin 16n-Hexylbenzene 26n-Octylbenzene 32

(Section 15.4) and has a limited range of applicability (32.5 < CN < 56.5). It is generally notvalid for Fischer–Tropsch-derived diesel fuels, since the correlation is implicit in assuming anaverage composition based on the refining of crude oil. Many similar correlations can be foundin the literature, employing the physical and chemical characteristics of the diesel fuel [13–17].Of these, the methods based on nuclear magnetic resonance (NMR) spectrometry has beenfound to be especially useful for very paraffinic distillates, typical of LTFT wax hydrocracking,and hydrogenated oligomers from alkene oligomerization.

For good compression-ignition engine performance, a diesel fuel should have a CN thatexceeds a minimum CN, which places an upper limit on the ignition delay. From the differentspecification values (Tables 15.1 and 15.2), it is clear that there is no consensus about theminimum value. Since the distillates refined from Fischer–Tropsch syncrude tend to have highCNs, the minimum CN requirement is not critical to Fischer–Tropsch refining. An aspectthat is more interesting is the maximum CN, where LTFT distillates have an advantage overpetroleum-derived diesel fuel.

The view has been expressed that an ideal diesel fuel should have the highest possible CN[18]. This view was motivated by the better combustion performance and lower emissionsthat can, in principle, be obtained by high CN diesel fuels. An almost diametrically opposedview is expressed in the ASTM D 975 specifications in order to motivate the recommended

15.3 Diesel Fuel Properties 289

minimum CN requirement [4]: ‘‘Increase in cetane number over values actually required doesnot materially improve engine performance. Accordingly, the cetane number specified should beas low as possible to assure maximum fuel availability.’’ One may argue that synthetic diesel fuelavailability is not necessarily constrained by a high CN. It would therefore be best to maximize CNwhenever possible to take maximum advantage of the reduced emissions from high CN dieselfuels. However, in practice there is an upper limit to CN beyond which emission performancemay be degraded. Tests with a high-speed direct-injection diesel engine indicated poorer PMemission performance by an 81 CN diesel fuel than a well-formulated 53 CN diesel fuel [7]. Thiswas ascribed to the very short ignition delay that resulted in combustion commencing beforesufficient fuel–air mixing occurred.

The definition of CN is such that it is mainly a measure of the time required for free radicalinitiation leading to ignition. The CN of a diesel fuel can therefore be improved by blendingthe fuel with a thermally labile compound. Typical cetane improvers are compounds such asperoxides and alkyl nitrates. However, if CN was only a measure of initiation, the CN gainobtained by thermally labile compounds should have been independent of the fuel matrix, whichis not the case. Ignition requires radical propagation, and a low CN base stock cannot be improvedmuch by the addition of a cetane improver.

If we translate these properties to the molecular requirements for CN, molecules will have ahigh CN if they are thermally labile or readily autoxidized. At this point, we can also anticipate thatthere are potential trade-offs involved in having a high CN and good fuel stability (Section 15.3.8).For example, the CN can be improved by autoxidation [19], and one may expect a slight increasein CN after prolonged storage.

Resasco and coworkers made an exhaustive study of the relationship between CN and molecularproperties in order to suggest refining pathways to improve diesel fuel quality [12]. It was foundthat the most effective description resulted from dividing the hydrocarbons into two groups: thefirst group consisting of the alkanes and cycloalkanes and the second group consisting of thealkenes and aromatics. Some common features that correlated well with CN emerged fromthe analysis, despite differences in the detail level description:

1) The highest positive charge on a hydrogen atom in the molecule.2) Ovality of the molecule, expressed as the actual surface area relative to that of an equivalent

sphere.3) The number of –CH2 – groups in the molecule.4) Some description of the connectivity.

The study yielded valuable insights relevant to crude oil refining. The study also indirectlyexplained why straight-run HTFT and LTFT syncrudes have high CNs. In the case of HTFTsyncrude, the alkyl aromatics and mixtures of linear and branched hydrocarbons (alkanes andalkenes) all have reasonable CNs, which can be further improved by mild hydrotreating. In thecase of LTFT syncrude, which contains mainly n-alkanes, the CN is of course extremely high.

The alcohols in Fischer–Tropsch syncrude can also be employed as diesel fuel components.The alcohols can be directly used as diesel fuel extenders [20]. However, of the oxygenate classes,the linear ethers have been found to provide the best compromise between CN and cold-flowproperties [21]. Methoxymethane (dimethyl ether, DME) is the simplest of the linear ethers, witha CN of 55–60 and it has been extensively investigated as conventional diesel fuel substitute [22].Longer chain linear ethers have been successfully tested as blend components with HTFT diesel

290 15 Diesel Fuel

fuel [23]. Etherification (Chapter 17) is a potentially useful refining technology for diesel fuelproduction in a Fischer–Tropsch refinery.

15.3.2Density and Viscosity

The injection of diesel fuel into a compression-ignition engine is controlled either by a solenoidvalve or by positive volumetric displacement. The density and viscosity of the fuel determine theperformance of the injection system. It affects not only the energy value of the material that isinjected, but also the droplet size distribution (Section 14.2.2). If the diesel density and viscosityare not within the narrow range for which the engine was calibrated, it will degrade the engineperformance and increase emissions [3]. The progressive narrowing of both the density andviscosity range of diesel fuel specifications is therefore intentional.

Fischer–Tropsch syncrude inherently has low density and viscosity. The diesel fuel obtainedfrom HTFT syncrude is at the lower end of the acceptable range, but that from LTFT syncrudeis usually well below the density of crude-oil-derived diesel fuels. When LTFT syncrude ishydroprocessed, the acyclic aliphatic nature of the syncrude is retained and the distillate has ahigh CN but low density. The low density is advantageous for the Fischer–Tropsch refiner, butnot for the consumer. It was found that LTFT distillate had a 2.7% better gravimetric energyefficiency than crude-oil-derived diesel fuel, but a 7.2% lower density [24]. On a volumetric basis,it implies that LTFT distillate results in 5% higher volumetric fuel consumption for the sameenergy delivery.

Refining strategies have been suggested to improve the density of Fischer–Tropsch-deriveddistillates without degrading the CN to an unacceptable level [8]. Such interventions are necessaryonly for stand-alone refineries that have to produce on-specification diesel fuel as the final product.In most situations, the density deficiency can be overcome by blending with petroleum productsor pyrolysis products such as coal liquids.

15.3.3Flash Point

The flash point of diesel fuels is mainly controlled from a safety point of view, since it gives anindication of the tendency of the fuel to form a flammable mixture with air. It is extensively usedto assess the fire risk associated with the storage of potentially flammable products. The flashpoint is controlled by distillation, and, by increasing the temperature of the initial boiling pointof the diesel, the flash point can also be increased.

15.3.4Lubricity

Diesel fuel pumps that do not have an external lubricating system rely on the lubricity of the fuelto reduce wear. Inadequate lubricity can result in excessive pump wear and even pump failure.Historically, lubricity of diesel fuel was not an issue. The importance of specifying lubricitybecame apparent only after ultralow sulfur diesel (sulfur content of <50 µg·g−1) was introduced.Shortly after the introduction of the ultralow sulfur diesel into the Swedish market in 1992,

15.3 Diesel Fuel Properties 291

diesel-powered passenger cars started experiencing problems with their fuel pumps. Cars usingthe Bosch rotary pump reported failures within 3000 to 10 000 km, while other manufacturersreported reduced pump performance at short service life [25]. It turned out that the naturallubricating properties of the diesel were destroyed during the severe hydroprocessing to reducethe sulfur content of the diesel fuel.

Diesel lubricity is determined by the ASTM D 6079 standard test method [26], which makesuse of a high-frequency reciprocating rig (HFRR). Although other test methods can be used, thistest has been shown to have a good correlation with actual automotive diesel fuel pump wear.

The loss of lubricity during the production of low-sulfur diesel from crude oil seemed toindicate that lubricity is linked to the sulfur-containing species. Superficially, this contentionwas supported by the observation that hydroprocessed sulfur-free Fischer–Tropsch distillatesalso had poor lubricity [27]. However, it was shown not to be the case. The heteroatom-derivedlubricity-improving properties of compounds in diesel fuel followed the trend O > N >> S [28].Straight-run Fischer–Tropsch syncrude should consequently have good boundary layer lubricity,which it indeed has. Boundary layer lubricity improvement is not the only lubricity requirement;hydrodynamic lubricity that is related to viscosity is also important.

Boundary layer lubricity is imparted by surface-active species. A critical surface concentrationis necessary to provide adequate lubricity. The nonlinear response of diesel fuel lubricity inresponse to the addition of a lubricity improver is to be expected (Figure 15.2) [29].

The lubricity-enhancing properties of various oxygenates were investigated [25, 28–33]. Not allclasses of oxygenates are equally effective as lubricity providers, and it has been shown that thelubricity of biodiesel, which is rich in methyl esters, can itself be improved by mild oxidation[34]. When different oxygenate classes of the same chain length were added to diesel fuel, thelubricity improved in relation to the polarity of the oxygenate used: ethers < ketones < methylesters < 1-alcohols < aldehydes < carboxylic acids [28].

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1

Concentration (%)

Lubr

icity

, HF

RR

wea

r sc

ar (

µm) Sunflower oil

Olive oilCorn oilUsed frying oil

Figure 15.2 Lubricity response of diesel fuel to the additionof different amounts of methyl esters obtained from differentoils.

292 15 Diesel Fuel

During the refining of Fischer–Tropsch syncrude, it is important not to over-hydrogenate thedistillate. With proper catalyst selection and operation, it is possible to hydrotreat syncrude insuch a way that enough natural lubricity is retained to meet the diesel fuel specifications [35].Bypassing hydroprocessing steps with some of the straight-run Fischer–Tropsch distillate canalso markedly improve lubricity. Because of the nonlinear response of lubricity to surface-activematerial (Figure 15.2), this does not have to be a large fraction of the syncrude.

15.3.5Aromatic Content

According to the diesel fuel specifications, a molecule is considered an aromatic if it contains atleast one benzene ring in its structure. Consequently, there are a wide variety of molecules, withvery different fuel properties, that can be classified as aromatics. This is why the fuel specificationsdifferentiate between total aromatics and polynuclear aromatics, which are aromatics with morethan one benzene ring. The total aromatic content determines the flame temperature duringcombustion. A higher aromatic content leads to higher PM and NOx emissions, but this does notimply that aromatics are the sole source of these emissions. In a study by ExxonMobil and Toyotausing a modern high-speed direct-injection engine, it was found that PM showed a statisticallysignificant correlation with aromatics, cycloalkanes, CN, and density [7].

The aromatics in straight-run Fischer–Tropsch syncrude and in the hydroprocessed materialfrom HTFT and LTFT synthesis are mainly mononuclear (Table 15.4) [24, 36, 37].

15.3.6Sulfur Content

The sulfur in diesel fuel has a negative influence on engine performance and exhaust emissions.It contributes significantly to the formation of fine PM through the formation of sulfates inthe exhaust stream and atmosphere. It can also lead to corrosion and wear of engine systemsand reduce the efficiency of the exhaust emission after-treatment systems. The maximum sulfurcontent in diesel fuel has therefore, over time, been reduced to very low levels.

The change in the sulfur specification of diesel fuel has no impact on Fischer–Tropsch refining,because the syncrude is sulfur-free. However, it influenced the marketing of Fischer–Tropsch

Table 15.4 Aromatic content of different Fischer–Tropsch-derived distillates.

Origin of distillate Aromatic content (mass%)

Total Mononuclear Dinuclear Trinuclear

SMDS distillate 1.4 1 0.4a –Sasol SPD distillate 0.47 0.44 0.03 <0.01Straight-run HTFT distillate 27 26 0.9 0.1Hydrotreated HTFT distillate 22.45 22.2 0.24 <0.01

aNo distinction made between di- and trinuclear aromatics.

15.3 Diesel Fuel Properties 293

products. The sulfur specification has become so stringent that it is no longer possible to useFischer–Tropsch distillate as a low-sulfur blend stock with crude oil to help crude oil refinersmeet the sulfur specification.

15.3.7Cold-Flow Properties

The specifications for cold-flow properties are region-specific. It is a very important fuelspecification. As the temperature of a diesel fuel is lowered, the highest cetane numbercomponents, that is the n-alkanes, tend to crystallize out of solution as a wax. The wax may blockthe fuel filter and fuel lines, rendering engine operation difficult or impossible.

The cold-flow properties of a diesel are therefore defined by wax-related tests, such as the cloudpoint (CP) and cold filter plugging point (CFPP). CP is the temperature at which the diesel fuelbecomes cloudy due to the formation of small wax crystals in the fuel. The observed turbidity isdue to light scattering by the small crystals. This is not an accurate measure of the operabilityof the fuel, which may remain pumpable and filterable at temperatures below the CP. The CPis determined by the ASTM D 2500 standard test method [38]. The advantage of the CP isthat it gives a conservative estimate of the temperature limit where solid formation becomessignificant. The CFPP is a filterability test that gives a better reflection of the temperature atwhich operational problems will occur during actual engine operation. The CFPP will in generalbe lower than the CP, since it will be possible for small wax crystals to pass through the fuel filterwithout blinding or blocking it.

The cold-flow behavior can be modified by additives (Section 15.4), but ultimately the dieselfuel must be refined to produce molecules with acceptable cold-flow properties.

The cold-flow properties are related to the freezing points of the compounds in the mixture andto the mutual solubility of the compounds in each other. Freezing point generally increases withmolecular mass, and the heavier fraction of the diesel fuel is the more likely fraction to determinecold flow. The compound classes that have the worst cold-flow properties are the n-alkanesand unsubstituted aromatics. Much of the discussion on jet fuel freezing point (Section 14.3.3)applies. Although the cold-flow criterion is less strict, the material is heavier.

HTFT syncrude has a small heavy-end fraction and, despite being called a waxy oil, it is rich inmononuclear alkyl aromatic compounds (Table 15.4). Ideally, one would like to hydroisomerizethe alkyl groups of such molecules without causing dealkylation. The aliphatic compounds includeboth n-alkanes and branched alkanes. The cold-flow properties of the aliphatic compounds canalso be improved by hydroisomerization (Chapter 18).

LTFT syncrude contains a large fraction of n-alkane waxes that are solids at ambient conditions.The straight-run distillate fraction is also rich in n-alkanes and, except for the light distillatefraction, it requires hydroisomerization to be fluid. It has been reported that poor cold flow is thelargest development obstacle to direct blending of straight-run distillate [39]. Direct application ofstraight-run material avoids yield losses associated with further hydroprocessing, and it retainsthe beneficial syncrude properties, such as lubricity.

When most of the distillate is produced by LTFT wax hydrocracking (Chapter 21), the cold-flowproperties are determined by the isomerizing nature of the hydrocracking catalyst.

294 15 Diesel Fuel

15.3.8Stability

The stability of diesel fuel is measured in terms of its oxidation, storage, and thermal stabilities.Oxidation stability is expressed in terms of the insoluble material formed during oxidation asdescribed by the ASTM D 2274 standard test method [40]. It is stated that this method is notapplicable to fuels containing a significant amount of material derived from nonpetroleumsources. Storage stability requires a longer test and is performed in accordance with the ASTMD 4625 standard test method [41]. Thermal stability can be determined using the ASTM D 6468standard test method [42].

Diesel fuel with a high CN inherently has a high autoxidation propensity, which underminesits storage stability. The situation is more precarious when thermal stability is considered.CN improvers are thermally labile compounds and will, by definition, decompose at elevatedtemperatures. Hydroperoxides which are formed during autoxidation are such compounds andare indeed employed as cetane improvers. Stability is therefore an issue for very high CNFischer–Tropsch-derived fuels that are readily autoxidized. In fact, stability has been pointed outas one of the greatest concerns associated with diesel fuel blends containing Fischer–Tropschmaterial [43].

The way in which the stability is measured is important though. Deposit formation subsequentto oxidation requires three sequential steps: initiation by autoxidation, propagation of the freeradicals, and termination in such a way that the products will form deposits on filtration. Thereis also a time factor involved. The autoxidation sequence may be interrupted at any point toimprove stability, but by doing so the CN is undermined. However, the duration of the test setsa limit on the amount of inhibition that is required. The volume of oxidation inhibitor that isadded (or naturally present) in the fuel is therefore also of consequence. During the oxidativeprocess, the oxidation inhibitor is consumed and, once consumed, autoxidation can readily takeplace. Adequate oxidation stability of diesel fuel is therefore a trade-off between the duration ofstability required and the extent of CN reduction that can be endured.

Stability is not exclusively related to the oxygenate content of a fuel, but oxygenates andoxygenate chemistry play a dominant role. Straight-run Fischer–Trosch syncrude containsoxygenates and, even in conventional crude-oil-derived diesel fuels, oxygenates are employed aslubricity improvers and CN improvers. Balancing oxygenate content and stability is importantand necessary to produce a good diesel fuel. Very paraffinic Fischer–Tropsch distillates, as isobtained from LTFT syncrude (not HTFT syncrude), has the added disadvantage of decreaseddissolving power. The alkanes are poor solvents for heavier oxidation products, which increasesprecipitation of insoluble matter.

15.3.9Elastomer Compatibility

Elastomer compatibility issues have already been described for jet fuels (Section 14.3.8). Thedifference in seal swelling that is caused by changing from an aromatic diesel fuel to a moreparaffinic diesel fuel has been evaluated for Fischer–Tropsch distillates [44]. LTFT distillate has amuch lower aromatic content than HTFT distillate (Table 15.4) and blending of HTFT and LTFT

15.4 Diesel Fuel Additives That Affect Refinery Design 295

Table 15.5 Elastomer compatibility of standard nitrile buta-diene rubber when changing from a US D-2 diesel fuel thatwas used as reference standard to other diesel fuels.

Property Diesel fuel

US D-2 UK EN590 HTFTa LTFTb

Fuel propertiesDensity (kg·m−3) 858 833 824 765Cetane number 41 56 52 72Viscosity at 40 ◦C (cSt) 2.6 3.4 2.3 2.0Flash point (◦C) 39 91 71 59Cold filter plugging point (◦C) −14 −24 −3 −19Total aromatics (mass%) 34.5 18 30.8 0.14Polynuclear aromatics (mass%) 5.9 4.2 2.6 0Sulfur content (µg·g−1) 400 11 2 1

Elastomer property (% change)Mass 0c −9.4 −7.7 −12.3Thickness 0c −4.9 −3.3 −4.9Hardness 0c 6.1 5.2 8.5

aHTFT diesel fuel from Sasol Synfuels, which contains some hydrogenated coal liquids.bHydrocracked LTFT distillate prepared by Chevron at their Richmond pilot plant facilities.cReference fuel used to evaluate elastomer compatibility, by definition zero.

distillate is advantageous [27]. However, the aromatic content is not the only aspect influencingthe changes in elastomer mass, dimension, hardness, and tensile strength.

The polar material in the diesel fuel also influences elastomer properties. This was especiallyapparent from the elastomer compatibility comparison between South African HTFT-deriveddiesel fuel, United Kingdom EN590-compliant diesel fuel, and United States D-2 diesel fuel(Table 15.5) [44]. Although the HTFT and US D-2 diesel fuels were closer to each other in termsof aromatics content, the elastomer compatibility behavior of HTFT and the lower aromaticEN590 diesel fuel was better matched.

Elastomer compatibility is of concern only in countries where mixed fuel availability enablesfuel users to change from one fuel type to another. In this respect, LTFT-derived distillates benefitfrom blending with crude oil or HTFT distillates to reduce the risk of incompatibility.

15.4Diesel Fuel Additives That Affect Refinery Design

Additives are compounds that are added to the diesel fuel base stock during final blending inorder to produce on-specification diesel fuels [2]. When additives can be employed to meet fuelspecifications that would otherwise require more refining effort, it is usually cheaper to do sorather than to increase the refining intensity. In such cases, the refinery design is directly affectedby the ability to improve product quality by the additives. Such diesel fuel additives effectively

296 15 Diesel Fuel

reduce the specifications that must be met by the refinery design, and for design purposes it isequivalent to relaxing some of the fuel specifications.

The design decision to rely on additives to address specific limitations must be consciouslymade. Once it is decided to employ additives to avoid the associated refining cost to upgradethe fuel to the required specification limit, blending becomes dependent on the additive tomeet specification. It will then no longer be possible for the refinery design to meet the fuelspecifications without the additive. There are mainly three diesel fuel additive classes that canmake refining less onerous:

1) CN improvers, also known as ignition improvers, are able to increase the CN of the dieselfuel. These additives include compounds such as alkyl nitrates (e.g., 2-ethylhexyl nitrate) andperoxides. The CN that can be achieved with a fixed volume addition of the CN improverironically increases as the quality of the base diesel fuel improves. A diesel fuel with a CNin the range 40–50 will typically register a CN increase of 3–4 when dosing 0.05 vol% ofthe CN improver as additive. Because the CN improver reduces the actual ignition delay, theimprovement will be reflected by the ASTM D 613 standard test method [9], as well as by theASTM D 6890 (IQT-based) standard test method [10]. However, it will not affect the calculatedcetane index measurement, because the additive volume is too small to substantially affectthe density and distillation profile of the diesel fuel.

2) Lubricity additives are surface-active compounds and are required only in small volumesto provide sufficient boundary layer lubrication (Section 15.3.4). It compensates for theeffects of severe hydroprocessing, which destroys the surface-active compounds. In a crudeoil refinery where the sulfur specification necessitates severe hydrotreating, reliance on alubricity additive is inevitable. The same is not true in a Fischer–Tropsch refinery where itis not necessary to ensure hydrodeoxygenation to the same level.

3) Cold-flow improvers affect the way in which the n-alkanes in the diesel fuel crystallize at lowtemperatures. The CP can be lowered only by increasing the solubility of the n-alkanes inthe fuel. CP depressants can be added and a lowering of around 3 ◦C can be achieved at adosage level of 500 µg·g−1 of an appropriate depressant [2]. Since the CP is less importantthan the CFPP as a measure of engine operability at low temperatures, flow improvers thatlower the CFPP are more important additives for diesel fuel. Flow improvers do not affectthe CP, but modify the crystallization process and the nature of the wax crystals that areformed. By forming more, but smaller wax crystals that do not agglomerate, the crystals canstill pass through the fuel filter and do not affect fuel flow to the engine. The use of cold-flowimprovers as additives allow the n-alkane content of the diesel fuel to be higher and thusreduce the level of refining that is required in order for the cold-flow requirements to be met.This also results in a CN benefit, because less n-alkanes have to be isomerized to branchedalkanes. Cold-flow improvers are therefore indirectly also CN improvers.

15.5Future Diesel Fuel Specification Changes

The most significant change in diesel specifications by far was the drastic reduction in sulfurcontent. At a maximum sulfur content of 10 µg·g−1, refineries are approaching the limit of sulfurremoval. Beyond that it becomes difficult to reduce sulfur when considering practical limitations,such as hydrodynamic effects in fixed bed reactors.

References 297

Many of the other diesel specification changes have been incremental. The CN is nudgingup, the density and viscosity ranges are narrowing, and the heavy end of the distillation profileis becoming lighter. These changes are significant, but not disruptive. Indirectly, these changesreduced the polynuclear aromatic content of diesel fuel. Whether more stringent limitations ofthe polynuclear aromatic content of diesel fuel will serve a purpose is doubtful, unless a verylow limit is set. Deeper hydrogenation may not be economical even at high crude oil pricesand in practice refiners may opt to reduce the cut point of diesel fuel rather than to increasehydrogenation severity.

The only specification that is of concern to Fischer–Tropsch refiners is the lower specificationlimit on diesel fuel density. There is a density–cetane–yield triangle that limits on-specificationdiesel fuel production in high yield from Fischer–Tropsch syncrude [8]. This is discussed in detailin Chapter 27, which deals with diesel fuel refining. The density deficiency can be overcome byblending with heavier distillates, but it is an issue for stand-alone refineries where such blendingis limited or logistically challenging.

There is a definite trend, spurred by environmental and political reasoning, to includerenewable material in diesel fuel. Much work has been done on esterification of plant and animalfats and oils as blending components in diesel. Diesel fuel specifications already make provisionfor limited inclusion of fatty acid methyl esters (FAMEs). However, it has been reported thatblends of LTFT distillate, FAME, and petroleum have storage stability issues [45].

Future legislation concerning the inclusion of bio-derived oxygenates in diesel fuel is likely tobe a subject of political expediency.

Fischer–Tropsch refiners may have the option of including bio-derived material in the rawmaterial used as feed for synthesis gas production. Legislation to ensure the inclusion of aminimum amount of renewable material in diesel fuel may therefore be met either by using thebiomass in syncrude production or by co-refining the bio-derived products in the refinery. Assuch, co-refining of biomass can more easily be accommodated in a Fischer–Tropsch refinerythan a crude oil refinery, since a Fischer–Tropsch refinery has to process oxygenates in any case.This may resolve some of the blending issues related to oxygenated products in diesel fuel.

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