1992 chemical stability of diesel fuels and sediment formation therein - 1. evaluation of the...

8/13/2019 1992 Chemical Stability of Diesel Fuels and Sediment Formation Therein - 1. Evaluation of the Chemical Stability of Diesel Fuels by Following the Kinetics of S

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Chemical stability of diesel fuels andsediment formation therein1. Evaluation of the chemical stability of diesel fuels byfollowing the kinetics of sediment formationZh. D. Kalitchin, SI. K. lvanov, S. K. Tanielyan, M. I. Boneva,P. T. Georgiev, A. Ivanov* and K. Kanariev*Institute of Organic Chemistry, Bulgarian Academy of Sciences, Sofia 17 13, Bulgaria*Institute of Petrorefining and Petrochemistry, SF Neftochim, Bout-gas 8000, BulgariaReceived 73 May 1991; revised 20 September 1997)

The chemical stability and the kinetics of formation of insolubles in model diesel fuels and of the differentcomponents used in their formulation are studied at elevated temperatures. The parameters which changeduring the accelerated ageing are studied. A new method is developed to follow the kinetics of formationof total insolubles in diesel fuels using light scattering on the basis of nephelometry. Comparing theexperimental data and values of the initiation rate constant for total insolubles formation calculated bythe kinetic approach showed that the process is not just a polymerization reaction. Sulphur content hasa significant influence on the formation of total insolubles and should be considered in the formulation ofdiesel fuels.Keywords: diesel; kinetics; sediment formation)

The possibility of long term storage of diesel fuels isdetermined by their physical and chemical stability. Toevaluate chemical stability the following are studied : acidnumber, oxygen absorption, optical density, amount ofinsoluble gum, amount of total insolubles, etc. The lasttwo parameters are considered the most importantfactors in the stability of diesel fuels.

It has been shown by several authors that theauto-oxidation of hydrocarbons to hydroperoxides is themain reaction leading to the formation of soluble andinsoluble gums - .l3 The decrease in chemical stability offuels is connected to the presence of mono- and di-olefinsand alkyl aromatic hydrocarbons with short sidechains7v8. A synergistic action has been found to occurbetween mono- and di-olefinsO. The influence of sulphurcontent on the stability of diesel fuels is studied in Ref.14. Our data show that there are two basic processesleading to the formation of total insolubles in dieselfuels 5 : formation of gum as a result of thermal andchemical processes in the absence of atmospheric oxygen ;and oxidation of hydrocarbons by atmospheric oxygenwith the formation of gum and total insolubles.

We consider that the basic shortcoming for methodsdescribed in the literature for studying the chemicalstability of diesel fuels is the fact that the experimentsare not carried out in the kinetic region. We were unableto find any data in the literature concerning thedetermination of the kinetics of formation of totalinsolubles in diesel fuels at elevated temperatures. Suchstudies have been published recently for jet fuels16.Ritchie concluded that standard methods are not usefulfor predicting the storage stability of diesel fuels.

Our studies18 show that the logarithm of the rate offormation of total insolubles shows a linear dependencewith the inverse of temperature (Arrhenius dependence)over a wide temperature range (393-433 K) 5,18. There-fore, the processes in our method for studying the rateof total insolubles formation proceed in the kinetic region(with no significant diffusion or other factors involved).The measured activation energies of two samples (2 and8) are of the order of 70 kJ mol- which proves thatchemical processes but no diffusion occur18.

The purpose of this paper is to study the chemicalstability and the kinetics of formation of total insolublesboth in model diesel fuels and of the different componentsused for their formulation, at elevated temperatures. Theparameters which change during the accelerated ageingare studied in order to obtain a suitable parameter toevaluate and predict chemical stability.

EXPERIMENTALThe basic physico-chemical characteristics of the com-ponents used in the production of diesel fuels are givenin Table I. They include the following fractions obtainedby the atmospheric distillation of petroleum : light dieselfraction (LDF, cut 453-513 K), hydrogenate (HG, cut513-633 K) and denormalizate (DN, cut 473-573 K).The latter is obtained after hydropurification and isolationof the n-paraffin hydrocarbons (Parex process). We havealso used a source for the production of normal paraffins(NP) and a heavy diesel fraction.

Our investigations on the formation of total insolublesin diesel fuels were based on ASTM D 2274-8019.

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Chemical stabi l i ty o f diesel fuels: Zh 0 Kali tchin et alTable 1 Components and physico-chemical characteristics of the samplesComponents

DNHGLDFNPCharacteristicsDensity at 20CFr(agctZaY )contentboiling point (C)

10%50%90%

End of boiling (C)Kinematic viscosity at20C (mm2 s-i)Sulphur (mass % )Mercaptan sulphur(%)Ash (%)Corrosion on copperplateWater soluble acidsAcidity (mg KOH per100 ml)Resins (mgper lOOmI)

Content (vol %)100 100 80 8020

90108020 8020 20

70 6030 40 90 70 6010 30 40

0.832 0.820 0.832 0.832 0.821 0.823 0.832 0.832 0.828 0.821 0.821 0.822

196 185 178 188 185 188 180 176 183 185 185 179218 215 212 216 215 214 216 213 209 214 211 215248 248 246 246 247 251 247 251 247 247 249 247280 281 281 280 283 283 280 283 286 283 284 284300 300 298 304 300 301 301 301 301 302 305 306

3.4 3.3 3.3 3.4 3.2 3.3 3.4 3.3 3.1 3.3 3.2 3.10.04 0.03 0.15 0.23 0.14 0.22 0.09 0.18 0.26 0.09 0.19 0.26

0.0048 0.0052 0.0052 0.0050 0.0028 0.0080 0.0101 0.0032 0.0074 0.0100_ - _ -

+ +- - + + + + + + + + + +- - _ _ - - _ - - _0.17 0.17 0.17 0.23 0.11 0.23 0.35 0.11 0.23 0.35_ - - _ _ - - - _ _

+, Stable

Table 2 Results of the studies according to ASTM D 2274-80 modified for the experiment in the kinetic region at 368 KColour(ASTM-1500)

Non-oxid. Oxidized1 21 21 21 21 2.51 2.51 1.51 21 21 21 21 21 21 21 21 23 4

[ROOH], (Acid no.), (Acid no.), Total(mol I-) (mg KOH per (mg KOH per insolubles(x 103) 100 ml) 100 ml) (mg per 100 ml)o Sample12345678910

11121314151617

10% LDF + 90% HG20% LDF + 80% HG30% LDF + 70% HG40% LDF + 60% HGHGDN10% LDF + 90% DN20% LDF + 80% DN30% LDF + 70% DN40% LDF + 60% DN20% LDF + 80% DN + additive I20% LDF + 80% DN + additive II20% LDF + 80% DN + additive III20% LDF + 80% DN + ADM 15-120% SNP + 80% DN20% SNP + 80% HGHeavy diesel fraction

0 0 0 0.600 0 0 0.300 0 0 0.500 0 0 1.24

26 0 0 6.9046 0 0 4.330 0 0 0.620 0 0 0.580 0 0 1.220 0 0 1.270 0 0 0.220 0 0 0.540 0 0 0.540 0 0 0.340 0 0 0.930 0 0 0.670 1.3 7.4 1.62

*[ROOH], and (acid no.), are determined after 16 h of accelerated ageing

According to the standard the oxidation is carried outat 368 K with an oxygen flow rate of 5 1 h-i. Our resultsobtained at 368 and 413 K with additional stirring of thereactors (60 rev min- ), and an oxygen flow rate of14 1 h-i, showed that more information about thestability of the samples is given at the higher temperatureTables 2 and 3). We used a modified apparatus whereaccelerated ageing of six samples was carried outsimultaneously. The temperature change was kept in the

range of + 1 K. In order to have conditions for theprocess to occur in the kinetic region and to preventsedimentation of the solid phase, the thermostaticallycontrolled part of the apparatus was attached to a stirringmechanism. The construction of the stirring mechanismis given in Ref. 20.For a more precise and uninterrupted study of theformation of total insolubles we used a nephelometricmethod in parallel to the gravimetric measurements.

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No. Sample1 10% LDF + 90% HG2 20% LDF + 80% HG3 30% LDF + 70% HG4 40% LDF + 60% HF5 HG6 DN7 10% LDF + 90% DN8 20% LDF + 80% DN9 30% LDF + 70% DN

10 40% LDF + 60% DN11 20% LDF + 80% DN + additive I12 20% LDF + 80% DN + additive II13 20% LDF + 80% DN + additive III14 20% LDF + 80% DN + ADM 15-I15 20% SNP + 80% DN16 20% SNP + 80% HG17 Heavy diesel fraction

Chemical stability of diesel fuels: Zh. D. Kal it chin et al.Table 3 Results of the studies for samples of diesel fuel at 413 K

Total (Acid no.), Maximum lightinsolubles (mg KOH scattering (%)(mg per per lOOmI) -____-100 ml) 575 nm 600 nm

9.60 0 160 404.30 0 30 II5.21 0 38 6

12.68 0 155 712321.00 836 5000 2900

833.03 306 2400 145039.79 0 155 59

6.79 0 50 815.20 0 170 7016.06 0 175 657.18 6.1 84 8

33.91 0 134 822.84 0 19 6

15.64 0 59 2231.50 0 280 12510.60 0 163 12536.47 48.2 285 126~-

Total InductionMaximum light insolubles period of totalscattering (%) under Ar insolubles--_____~ (mg per formation575 nm 600 nm 100 ml) (h)0 0 _ 1.50 0 0.70 0

11 3 0.54 4 0.37 00 0 2.50 0 _. 3.0

12 4 _ 1.54 0 0.38 00 0 _ 06 4 1.20 0 _ 12.03 0 ._ 00 0 _ 5.50 0 00 0 00 0 _ 00 0 0

The concentration of hydroperoxides in the samples is zero within the limits of experimental error

350 400 450 500 550 600A nm)

igure I Spectral distribution of the intensity of the scattered lightfor sample containing 20% LDF and 80% HG and its change withtime: 0, at the beginning of the experiment; 0, after 3.5 h; A, after6h;A,after9h;n,afterl2h;m,after16h

This enabled us to make a quantitative assessment of thepresence of a heterogeneous phase in the oxidized dieselfuels. In order to exclude the possibility of mixing in thespectra from light scattering and fluorescence, due to thepresence of compounds with condensed nuclei2, westudied the fluorescence spectra of the initial samples.These spectra showed that no fluorescence is observedin the region from 350 to 800 nm on irradiation in theU.V. region. The fluorescence spectra of the oxidizedsamples (after the total insolubles were filtered) showedtwo weak maxima at 385 and 468 nm, respectively. Thewavelength of the irradiation light in this case was350 nm. Hence the observed increase in the region500-800 nm (see Figure 1) is due only to light scattering.The nephelometric measurements were carried out onapparatus which enabled us to measure the intensity ofthe light flux perpendicular to that coming from the light

sources, in the visible region, at the same wavelength.Figure 1 gives the spectral distribution of the intensityof the scattered light for a sample containing 20% LDFand 80% HG and its dependence on time. Similarrelationships are observed for the other samples. Theexperiments were carried out in conditions of acceleratedageing at 413 K in an oxygen atmosphere. As can be seenfrom Figure 1 the intensity of the scattered light in theregion from 500 to 700 nm increases with time. Thereason for the decrease in intensity of the light at350-500 nm is the formation of compounds as a resultof oxidation, which absorb in this spectral region.Gravimetric measurements were carried out after thesamples subjected to accelerated ageing were filteredthrough Schott-filter 4G, under vacuum, according toRef. 19.In order to use the results obtained by the nephelometricmethod for evaluating the formation of solid phase inthe sample we studied the correlation between theamount of total insolubles determined gravimetricallyand the intensity of the scattered light at 575 nm. Figure

2 shows that good correlation is obtained between thosetwo parameters. For samples HG and DN (5 and 6 inTable 3) a plateau is observed in the nephelometrickinetic curve after 16 h. This is most probably due to thefact that the total insolubles particles formed are too bigand sedimentation takes place due to cohesion ofparticles. In our studies we used nephelometric kineticcurves in the time interval 0- 16 h, where there is a goodcorrelation with the gravimetric results. The decrease inthe measured intensity of the scattered light after thefiltration by more than 20-fold Table 3) is additionalproof that the intensity of the scattered light is due tothe heterogeneous phase formed. The amount of totalinsolubles determined at the end of the accelerated ageingfor 16 h at 413 K corresponds to the measured maximumof the light scattering reflection determined for the samesamples Table 3).

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Chemical stabi l i ty of diesel fuel s: Z h. D. Kal i t chin et al.

2

1

Figure

5.0 10.0 15.0Time [h)

Dependence of the scattered light 0 ) and the total insolubles( on time for a sample containing 20% LDF and 80% DN at 413 K

RESULTS AND DISCUSSIONThe most important parameter in determining thechemical stability of diesel fuels is their stability towardsformation of total insolubles. Stringent demands involvingthis parameter under the conditions of accelerated ageingat 368 K are stipulated in USA and USSR standards,19.Comparison of the conditions given in ASTM D2274-8019 and the considerations given in Ref. 23 showsthat the oxidation in the former is not in the kineticregion, because of the low gas flow rate and thesedimentation of the total insolubles because of lack ofstirring. In order to distinguish the chemical stability ofthe diesel fuels the accelerated ageing experiments shouldbe carried out in the kinetic region. The results of thecomparative studies of different model diesel fuels andfractions at 368 K are given in Tabl e 2. The initial samplesdo not contain hydroperoxides according to the methoddescribed in Ref. 24. The same applies to the samplessubjected to 16 h of accelerated ageing in an oxygenatmosphere. It has been reported in the literaturez5 thatsamples HG and DN contain a small amount of sulphurTable I . Sulphur compounds in the samples are in theform of organic compounds (sulphides, thiophenes,thiophanes, etc.) which decompose hydroperoxides26.The latter are found in large amounts in samples 5 and 6(HG and DN), where the sulphur content is insignificant.This result shows that the samples are not an exceptionto the other light fuels with the oxidation proceedingby a radical chain mechanism with degenerate chainbranching.Both the initial samples and those after oxidationunder the same experimental conditions do not containacids*. Taking into account the above concerning theoxidation mechanism there should be acids formed.Obviously, the acids are rapidly consumed during theformation of total insolubles. The data in Tabl e 2 showthat a temperature of 368 K does not enable anydistinction to be made between the samples with respectto their ability to form total insolubles. The same appliesto the influence of additives. This is confirmed by thedata in Tabl e 2 for samples where the HG and DNcontents are changing (samples l-4 and 7-10). Thedifferences between the experimental data in these casesare in the region of experimental error. In order to achievean improved differentiation in the thermo-oxidative andthermal formation stages of total insolubles and to takeinto account the factors influencing the chemical and

oxidative stability of diesel fuels, the samples listed inTabl e 2 were studied in the same apparatus, but atelevated temperature (4 13 K ).The amount of total insolubles was determinedgravimetrically at the end of the experiment and atmaximum light scattering. The results show that thereexists a dependence between the amount of totalinsolubles and the sulphur content ; a sharp minimum isshown in Figure 3. Sharp differences are observedbetween HG and DN and the other samples with respectto the total insolubles formed at the end of the acceleratedageing. Taking into account these results we carried outour studies at 413 K and we recommend this temperatureas being the more appropriate for studying total insolublesformation in accelerated ageing.It can be seen that hydroperoxides are not formed atelevated temperatures. The kinetic studies of hydro-peroxides formation showed that maximum productionis reached 6 h after the beginning of the oxidation, afterwhich their decomposition rate is greater than the overallrate of formation Figure 4).As can be seen from the data in Tabl e 3 for most ofthe samples at 413 K the formation of organic acids isnot observed. The exceptions are samples 5, 6 and 17,where a dramatic rise in the acid number is observed.Comparing these results for the first two samples (DNand HG) with the results of total insolubles formation,then these acids can be regarded as precursors of thetotal insolubles. Obviously, the presence of a moderateamount of sulphur (0.09-0.36 mass %) suppresses theformation of carboxylic acids and of total insolubles.We studied whether total insolubles formation is dueto oxidation reactions or whether they can form in ananaerobic atmosphere. For this purpose some of thesamples were subjected to accelerated ageing at 413 Kfor 30 h under an argon atmosphere Table 3). Theamount of the total insolubles formed is less than in an

40.0 -=E8i 30.0 -PBI)3 20.0 -sl.tm;;+

10.0 -

t

\ .i

I I I I I0.1 0.2 0.3 0.4

Sulphur content mass 8)Figure 3 Dependence of the total insolubles after 16 h on the sulphurcontent for samples containing different amounts of DN (0) and HG(0 ) at 413 K. The values on the abscissa are taken from Tab l e I fordifferent blends where the amount of DN and HG varies from 60 to100 vol%

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7000 -

00

3.0 6.0 9.0 12. 0 16. 0Ti me h)

Figure 4 Dependence of the concentration of hydroperoxides and acidon time for: DN - 0, hydroperoxides, 0, acids; HG - 0,hydroperoxides, acids

oxygen atmosphere after 16 h. We tested three samples(2, 4 and 8) which are the most stable towards totalinsolubles formation in an oxygen atmosphere. Thesedata confirm the existence of two processes: thermalreactions and reactions influenced by oxygen. Theamount of total insolubles formed under an argonatmosphere is only 16% of that formed in an oxygenatmosphere for sample 2.In the literature 1o-13,17 it is stated that the formationof total insolubles can be considered as a polymer chainreaction. We can express the oxidation rate in an oxygenatmosphere by the following equation :Wp, = k; CWg~~~W/J k, 1)

where c is the maximum rate of total insolublesformation in an oxygen atmosphere, k: is the rateconstant of chain propagation, [M] is the concentrationof monomer in the diesel fuel, [0,] is the oxygenconcentration in the sample, ko is the rate constant ofchain initiation and k, is the rate constant of chaintermination. The equation for the rate of total insolublesformation in an argon atmosphere is :

2)where Wi is the maximum rate of total insolublesformation in an argon atmosphere, ki is the rate constantof chain propagation and kT is the rate constant of chaininitiation. If it is taken into account that the rateconstants k, and k, in Equations (1) and (2) have thesame value, for the ratio of the rates of total insolublesformation the following equation is obtained :g = Jmco,lIJm (3)s

Our experiments showed that the left-hand side of theequation has a value of 6.25 for sample 2 Tabl e 3) seeabove). If we substitute the oxygen concentrationi inthe sample (7.8 x 10m3 mol 1-l) for the concentrationof the monomer we obtain the following expression:

[M] = a/k (4)where a has a value of 1.4 x 10m4 s.Obviously, according to Equation (4) the concentrationof the monomer is inversely proportional to the rate

Chemical stability of diesel fuels: Zh. D. Kalitchin et al.constant of initiation. However, according to Equation2), [M] - l/,,@, if the total insolubles formation isdue only to a polymerization process. Therefore, theformation of total insolubles is not just a polymerizationreaction in diesel fuels.The influence of sulphur content on the stability of

diesel fuels was also studied. The dependence of theisolated total insolubles on the sulphur content is givenin Figure 3. A sharp minimum is observed at a sulphurcontent of 0.1 mass%. With LDF, with a sulphurcontent of 0.36 mass% the formation of total insolublesis suppressed. Our experimental data show that theoptimal concentration of LDF in a model diesel fuel is20 ~01% with respect to the formation of total insolubles(samples 2 and 8 in Tabl e 3). Up to a concentration of 0.3mass% the sulphur compounds inhibit the formation oftotal insolubles. Their increase leads to the formation ofinsoluble compounds due to reactions with the organicbases in the model diesel fuels. It should be noted thatno additional sulphur compounds are doped in thesample, and the plot in Figure 3 is made by expressingthe amount of total insolubles versus the sulphur contentin the samples Table 1).The temperature of 413 K enabled us to study theability of several additives to inhibit the formation oftotal insolubles Tabl e 3). We have studied the com-mercially available additives I, II, III and ADM 15-I.(ADM 15-I represents a mixture of metal deactivator(Schiff base) and free radical acceptor inhibitor.) Theexperimental data in Tables 2 and 3 show that due tothe presence of a natural sulphur containing inhibitor inthe diesel fuels the additives do not have a significantinhibiting effect on total insolubles formation at 368 KTable 2) and 413 K Table 3). By choosing componentswith different sulphur contents and by analysis of thekinetic data one can obtain diesel fuels stable with respectto the formation of total insolubles. This is illustrated inFigure 3.The nephelometric method developed by us to followthe kinetics of formation of total insolubles gave us theopportunity to determine the induction period of thisprocess Table 3). No induction periods are observed forsamples 2, 3, 8, 9, 11, 14, 15, 16 and 17 Table 3).Relatively high values are observed for samples HG andDN, although the rate of formation of total insolubles isan order of magnitude greater for the rest of the samples.Additives I and III cause relatively high induction periodsand lead to small amounts of total insolubles after 16 hof accelerated ageing at 413 K. The induction period isnot a reliable parameter for predicting the stability ofdiesel fuels with respect to the formation of totalinsolubles.

CONCLUSIONS1. A new method is developed for following the kineticsof formation of total insolubles in diesel fuels using

light scattering on the basis of nephelometry.2. Twenty-two samples of model diesel fuels are analysedwith respect to their chemical stability. They includedenormalizate, hydrogenate and four mixtures of bothcomponents with a light diesel fraction and a sourcefor normal paraffins.3. By comparing experimental data with values of theinitiation rate constant for total insolubles formation

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Chemical stability of diesel fuels: Zh. D. Kalitchin et al.calculated by the kinetic approach it is shown thatthis process is not just a polymerization reaction.4. The sulphur content in diesel fuels up to a concentra-tion of 0.3 mass% inhibits total insolubles formation.By choosing components with different sulphur con-tents and on the basis of kinetic data, diesel fuels stabletowards the formation of total insolubles can beformulated more successfully than by using someconventional additives.

REFERENCES1 Gureev, A. A., Seregin, E. P. and Azev, V. S. Qualification forPetrol Fuels, Khimia, Moscow, 1984, p. 104 (in Russian)2 Voorhees, V. and Eiseinger, J. J. Sot. Auto. Eng. 1929,24,5843 Mardles, T. and Ross, C. J. Inst. Pew. Technol. 1929, 15, 6574 Wagner, C. and Human, J. Oil Gas J. 1929, 125 Brooks, B. Ind. Eng. Chem. 1929, 18, 11986 Flood, D., Hladky, J. and Edgar, A. Ind. Eng. Chem. 1933,25,

12347 Martin, S., Grull, W. and Lowy, A. Ind. Eng. Chem 1933,25,3818 Morrell, J., Dryer, C., Lowry, G. et al. Ind. Eng. Chem. 1934,26,497, 885

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Roel, R. J. Inst. Petr. 1964, SO, 22Skepfer, M. W. Oil Gas J. 1984,52, 79Taylor, W. and Frankfield, J. Ind. Eng. Chem. Prod. Res. Dev.1978, 17,86Taylor, W. Ind. Eng. Chem. Prod. Res. Dev. 1976, 15, 64Schwartz, F. and Whisman, M. Mines Bull. 1972, 660, 58Schwartz, F. and Whisman, M. Mines Bull. 1964, 662, 44Kalitchin, Zh. D. PhD Thesis IOC BAS, Bulgaria, 1988Li. F. and Li. N. Fuel 1985. 64. 1041. 1047Ritchie, J. J. inst. Petr. 196j, gl, 298Kalitchin, Zh. D., Ivanov, Sl. K., Tanielyan, S. K. et al. FuelsubmittedASTM D 2274-80, Sofia, 1983Ivanov, Sl. K., Tanielyan, S. K. and Tzonkovski, I. M. Khim.Ind. 1985, 27, 13 (in Bulgarian)Kalitchin, Zh. D., Schmulovitch, V., Ivanov, S. K. et al. Dokl.AN SSSR 1986,290, 880Calvert, J. and Pitts, J. Photochemistry, Mir, 1968, p. 238Schmulovitch, V. G. and Goldenberg, V. I. Nefekhimia 1972,19,912Uvanov, S. K. and Kateva, J. Compt. Rend. Acad. Bulg. Sci.1968, 21, 681Kanariev, K., Ivanov, At. and Kutzarova, Z. Khim. Znd. 1986,58, 346Ivanov, S. K. in Developments in Polymer Stabilization - III,Applied Science, 1982, p. 55Bulgarian State Standard BDS 14369-77, Sofia, 1983

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1992 chemical stability of diesel fuels and sediment formation therein - 1. evaluation of the...

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