long stability

7/27/2019 Long Stability

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Long storage stability of biodiesel from vegetable and used frying oils

Abderrahim Bouaid, Mercedes Martinez, Jose Aracil *

Department of Chemical Engineering, Faculty of Chemistry, Complutense University, 28040 Madrid, Spain

Received 31 August 2006; received in revised form 1 February 2007; accepted 6 February 2007Available online 13 March 2007

Abstract

Biodiesel is defined as the mono-alkyl esters of vegetable oils. Production of biodiesel has grown tremendously in European Union inthe last years. Though the commercial prospects for biodiesel have also grown, there remains some concern with respect to its resistanceto oxidative degradation during storage. Due to the chemical structure of biodiesel the presence of the double bond in the molecule pro-duce a high level of reactivity with the oxygen, especially when it placed in contact with air. Consequently, storage of biodiesel overextended periods may lead to degradation of fuel properties that can compromise fuel quality.

This study used samples of biodiesel prepared by the process of transesterification from different vegetable oils: high oleic sunfloweroil (HOSO), high and low erucic Brassica carinata oil (HEBO and LEBO) respectively and used frying oil (UFO). These biodiesels, pro-duced from different sources, were used to determine the effects of long storage under different conditions on oxidation stability. Sampleswere stored in white (exposed) and amber (not exposed) glass containers at room temperature.

The study was conducted for a period of 30-months. At regular intervals, samples were taken to measure the following physicochem-ical quality parameters: acid value (AV), peroxide value (PV), viscosity (m), iodine value (IV) and insoluble impurities (II). Results showedthat AV, PV, m and II increased, while IV decreased with increasing storage time of biodiesel samples. However, slight differences werefound between biodiesel samples exposed and not exposed to daylight before a storage time of 12 months. But after this period the dif-

ferences were significant. 2007 Elsevier Ltd. All rights reserved.

Keywords: Biodiesel; Vegetable oils; Used frying oil; Storage stability

1. Introduction

Biodiesel is an alternative diesel fuel consisting of alkylmonoesters of fatty acids prepared from vegetable oils. Ithas been the focus of a considerable amount of recentresearch because it is renewable, reduces the emission of

some pollutants, and is also readily biodegradable in theenvironment [1].Biodiesel has become a fast growing renewable liquid

biofuel within the European Union. In order to ensure cus-tomers acceptance, standardization and quality assuranceare key factors for the market introduction of biodieselas fuel for transport and heating. One of the main criteria

for the quality of a biofuel is its storage stability. The prob-lems arising from the deterioration of the fuel properties ofbiodiesel during storage are expected to be more severethan for commercial diesel fuel. Resistance to oxidativedegradation during storage is an increasingly importantissue for the successful development and viability of alter-

native fuels.Oxidation of unsaturated esters in biodiesel occurs bycontact with air and other pro-oxidizing conditions duringlong term storage. Thus, oxidative stability is an importantissue that biodiesel research must address since oxidationproduct may impair fuel quality and, subsequently, engineperformance.

Bondioli et al. and Thompson et al. [2,3], studied thedeterioration of rapeseed oil methyl esters (RME) underdifferent storage conditions, including changes in acidity,peroxide value, and viscosity, and found that acid value

0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2007.02.014

* Corresponding author. Tel.: +34 91 394 4167.E-mail address: [email protected] (J. Aracil).

www.fuelfirst.com

Fuel 86 (2007) 25962602
mailto:[email protected]:[email protected]


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(AV), peroxide value (PV) and viscosity (m) increased withtime. Other authors [4] found a correlation between theoxidation stability and the content of tocopherol (naturalantioxidant). Additives, such as antioxidants and stabiliz-ers, are frequently added to diesel fuels to inhibit fuel deg-radation and prolong storage life. In the BIOSTAB [57]project several expert teams from Research Institutes, Uni-

versities and biodiesel producers were pooled in order tofind answers to the several still open questions concerningbiodiesel stability. They found that several typical dieselfuel additives and the antioxidant, tert-butylhydroquinone(TBHQ), in combination with a fuel additive, were investi-gated for their effectiveness in inhibiting sediment forma-tion during aging in the biodiesel, one petrodiesel, andbiodiesel/petrodiesel blends. The results showed a large dif-ference in effectiveness between the additives, and also alarge difference in additive response between petrodieseland biodiesel. According to [8,9], the oxidation of fattyderivatives such as biodiesel is affected by other factors,

such as elevated temperature, light, the presence of metals,and other parameters that may accelerate oxidation.

Generally, the rate of oxidation of fatty compoundsdepends on the number of double bonds and their position[10]. The oxidation chain reaction is usually initiated at thepositions allylic to double bounds. Thus fatty acids (FA)with methylene-interrupted double bonds, for example, lin-oleic acid [(9Z,12Z)-octadecadienoic acid], are more sus-ceptible to oxidation because they contain methylenegroups that are allylic to two double bonds. Fatty acidswith conjugated double bonds, for example, linolenic acid[(9Z,12Z,15Z)-octadecatrienoic acid], are even more sus-ceptible to oxidation.

Modern analytical methods are often being used for thepurpose of determining structure indices such as iodinevalue (IV). The IV, which measures total unsaturation,has even been included in some standards for industrialproducts such as biodiesel. However, the IV index is toogeneral to allow the correlation of physical and chemicalproperties with FA compositions. The IV is treated in atheoretical fashion regarding biodiesel- and oxidative sta-bility-related issues. That the concept of iodine value as astructure index is unsatisfactory and need to be investi-gated in more detail, has led to the development of alterna-tive indices that take into account more accurately

compound structure and properties [11]. Possible alterna-

tives are the allylic position equivalent (APE) and the bis-allylic position equivalent (BAPE), which better relatestructure and amount of common component of fatty acidsin vegetable oils to observed properties. The APE andBAPE indices are based on the number of reactive posi-tions in oxidation. The use of the APE and BAPE indicesinstead of IV also implies that oxidative stability may be

more strongly influenced by the presence of small amountsof more highly unsaturated fatty compounds than byincreasing amounts thereof.

In this work the storage stability of biodiesel made fromthree different vegetable oils and used frying oil was inves-tigated over a storage time of 30-months under differentstorage conditions. Analysis of fuel properties such as per-oxide value (PV), acid value (AV), iodine value (IV), vis-cosity (m), and insolubles impurities (II) were made fromsamples taken from each storage container for a 30-monthperiod. It is expected that the results will lead to a betterunderstanding of the influence of storage conditions on

the quality of pure biodiesel.

2. Materials and methods

High oleic sunflower oil, high erucic and low erucicBrassica carinata oil were purchased from Koipe (Sevilla,Spain) and used frying oil obtained from the UniversityComplutense restaurant. Certified methanol of 99.8% pur-ity was obtained from Aroca (Madrid, Spain). The catalyst,potassium hydroxide, was pure grade from Merck (Barce-lona, Spain).

Three liters of biodiesel were made in our laboratoryaccording to a methodology previously described [12,13],for different vegetable oils, high oleic sunflower oil(HOSO), high erucic Brassica oil (HEBO), low erucic Bras-sica oil (LEBO) and used frying oil (UFO) at the beginningof the study. Table 1 shows the characteristics of the oilsused in this study.

2.1. The storage conditions and analysis

The biodiesel samples (3000 mL each) were stored for30-months at room temperature at different storage condi-tions. Samples were stored in glass bottles that were closedto the air and exposed (B1B5) or not exposed (B1 0B5 0) to

daylight. During storage, samples were taken out periodi-

Nomenclature

SSL samples stored in the daylightSSD samples stored in the darkHOSO high oleic sunflower oil

HEBO high erucic Brassica carinata oilLEBO low erucic Brassica carinata oilUFO used frying oilME methyl ester

APE allylic position equivalentBAPE bis-allylic position equivalentAV acid value

PV peroxide valuem viscosityIV iodine valueII insoluble impurities

A. Bouaid et al. / Fuel 86 (2007) 25962602 2597


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cally (each month) and different quality parameters (PV,AV, IV, m and II) were monitored. Samples were analyzedaccording to the following procedures, PV (AOCS Cd 8-53), AV (AOCS Ca 5a-40), m (ISO 3104), IV (AOCS Cd1-25), insoluble impurities (AOCS Ca 3a-46), the method

determines dirt, meal and other foreign substances insolu-ble in kerosene and petroleum ether is applicable to all nor-mal fats and oils and allows the calculations of insolubleimpurities, % = gain in mass of crucible/mass of sampletaken for moisture * 100 [14]. In order to obtain a quicklycomparison between the two units (%) and mg/100 mL, weinclude the conversion factor (*88.4) between the abovementioned units.

B1: biodiesel from high oleic sunflower oil (moisturecontent 0.69%), B2: biodiesel from high oleic sunfloweroil (moisture content 0.10%), B3: biodiesel from high erucicBrassica carinata oil, B4: biodiesel from low erucic Brassicacarinata oil, B5: biodiesel from used frying oil.

3. Results and discussion

The information about biodiesel samples from differentraw materials and storage conditions are shown in Table 2.

3.1. Peroxide value (PV)

Although PV is not specified in current biodiesel fuelstandards, this parameter influences cetane number (CN),

a parameter that is specified in the fuel standard. IncreasingPV increases CN, this may reduce ignition delay time [15],but also have negative effects and in particular there areconcerns about compatibility with certain plastics andelastomers.

Peroxide values, measured in milliequivalents of perox-

ide per kilogram of sample for each sampling period, areshown in Fig. 1a (SSL) and Fig. 1b (SSD). Samples exposedto daylight showed an increase in peroxides with time,which increased acceleration after the 6th month. Thiswas especially noted for samples (B1 and B1 0), where perox-ide numbers reached a value of over 200 meq per/kg sam-ple; this result may be due to the presence of traces ofwater (0.69% moisture content) which causes oxidation ofthe sample. The same tendency was observed for the sam-ples stored in the dark; these samples showed a differencewith an increase at a slower rate compared to other samplesexposed to daylight, except for B4 0, the sample from LEBOstored in the dark; this result may be due to the storage con-

dition of this sample with a large air surface contact.

3.2. Acid value (AV)

As expected, the acid number increased with an increasein peroxides because the esters were first oxidized to form

Table 1Characteristics of oils used in this study

Characteristic High oleicsunflower oil

High erucicBrassica oil

Low erucicBrassica oil

Usedfryingoil

Acid number(mg kOH/g)

0.05 0.833 1.161 2.73

Iodine number(I2/100 g)

90.43 114.6 132.5 119.6

Peroxidenumber(meq per/kg)

10.1 27.1 43.8 37.7

Viscosity (40 C)(mm2/s)

46.61 55.07 37.88 80.81

Table 2

Biodiesel samples and storage conditionsSample Oil Source Volume

(mL)Storageconditions

Temperaturerange (C)

B1 High oleic sunflowermoisture content (0.69%)

3000 Daylight RoomB1 0 Dark Room

B2 High oleic sunflowermoisture content (0.1%)

3000 Daylight RoomB2 0 Dark Room

B3 High erucic Brassica 3000 Daylight RoomB3 0 Dark Room

B4 Low erucic Brassica 3000 Daylight RoomB4 0 Dark Room

B5 Used frying oil 3000 Daylight Room

B5

0

Dark Room

Daylight

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Months of storage

Peroxidnumber(meqPer/kg

sample)

B1

B2

B3

B4

B5

a

b Dark

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Months of storage

Peroxidnumber(meqPer/Kgsample)

B1

B2B3

B4B5

Fig. 1. Evolution of the peroxide value of biodiesel samples over a

30-months storage period: (a) exposed, (b) not exposed to light.

2598 A. Bouaid et al. / Fuel 86 (2007) 25962602


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peroxides, which then undergo complex reactions includinga split into more reactive aldehydes which further oxidize

into acids. Acids can also be formed when traces of watercauses hydrolysis of the esters into alcohol and acids Fig. 2.The acid value (AV) of biodiesel samples also increased

with increasing storage time as a result of hydrolysis offatty acids methyl esters (FAME) to fatty acids (FA).The specification limit of 0.5 mg KOH/g was exceeded withthe methyl esters (ME) samples exposed to daylight (B3, B4and B5) after a storage time of 12 months, whereas forsamples without light exposure only (B 04) led to a valueexceeding 0.5 mg KOH/g. This may be due to the storageconditions with a large air surface contact of the sample(B 04) and probably due to the fatty acids composition ofthe vegetable oils used as raw materials. The acid values

remained constant until the 4th month and then took a sig-nificant upward trend except for used frying oil methylester (UFOME), which showed an increase over time. Dur-ing the frying of vegetable oils, most of the antioxidants areconsumed [8], and it is possible to assume poor oxidativestability of biodiesel made from used frying oil. The fuelstored in the daylight increased its AV at a faster rate thanthe fuel stored in the dark after 4 months of storage.

3.3. Viscosity (m)

Oxidation of methyl ester begins with build-up of perox-

ides; viscosity starts to increase only after the peroxides

have reached a certain level. During storage, the viscosityof the methyl esters increases by the formation of morepolar, oxygen containing molecules and also by the forma-tion of oxidized polymeric compounds that can lead to theformation of gums and sediments that clog filters. In thiswork all biodiesel samples under these storage conditions

showed a slight increase in viscosity after being stored for30-months (Fig. 3), except for samples with high moisturecontents (B1and B1 0) and samples with a relatively highiodine value (IV = 130) (B4 and B4 0) that showed a signif-icant increase in viscosity. There may be a slight depen-dence between IV and an increase of viscosity.

3.4. Iodine value (IV)

Oxidation consists of a complex series of chemical reac-tions characterized by a decrease in the total unsaturatedcontent of biodiesel due to elimination of hydrogen adja-cent to a double bond and the formation of free radicals[16]. Iodine values of all the samples exposed and notexposed to day light showed a slight decrease with a lowerIV for samples stored in daylight (SSL) compared with thesamples kept in the dark after storage time (Fig. 4). Exceptfor B1, B1 0 B4, and B4 0 samples, may be due to the reasonscited before the presence of traces of water (0.69% moisture

Daylight

0

0,5

1

1,5

2

2,5

3

0 5 10 15 20 25 30 35

Months of storage

Acidnumber(mgKOH/g

sample)

B1

B2

B3

B4

B5

a

b Dark

0

0,5

1

1,5

2

2,5

3

0 5 10 15 20 25 30 35

Months of storage

Acidnumber(mgkOH/g

sam

ple)

B1

B2

B3

B4

B5

Fig. 2. Evolution of the acid number of biodiesel samples over a30-months storage period: (a) exposed, (b) not exposed to light.

Daylight

0,000

2,000

4,000

6,000

8,000

10,000

12,000

14,000

0 10 20 30 40

Months of storage

Viscosity

R1

R2

R3

R4

R5

a

b Dark

0,000

2,000

4,000

6,000

8,000

10,000

12,000

14,000

0 10 20 30 40

Months of storage

Viscosity

B1

B2

B3

B4

B5

Fig. 3. Evolution of the viscosity of biodiesel samples over the 30-months

storage: (a) exposed, (b) not exposed to light.



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content) B1, B1 0 and samples with a relatively high iodinevalue (IV = 130) (B4 and B4 0), which could accelerate theoxidation in the biodiesel at a faster rate.

The IV is a structure-related index in FA chemistry thatrelates to the total number of double bonds in a fat or oil(or derivatives thereof). A major drawback of the IV is thatit does not distinguish structural differences present in dif-ferent fatty compounds such as nature, position in thechain, etc., and amount olefinic carbons. Iodine value con-siders these as being equally reactive.

The lack of correlation between oxidative stability, aswell as other properties of biodiesel and iodine value, hasbeen noted previously by other researchers [17,18]. How-ever, the alternative concept of the allylic position equiva-lent (APE) and bis-allylic equivalent (BAPE) is based onthe relative rates of oxidation of these positions in unsatu-rated FA as well as their amounts [19] and gives more use-ful results. The BAPE value is the more significant foroxidation of unsaturated fatty compounds due to the sig-nificantly higher relative rate of oxidation of bis-allylicCH2 positions [20]. The iodine value of the vegetable oilsused in this study was below the recent specified limit,120, in the draft of European Union Standards, except

for low erucic Brassica carinata oil (IV = 130).

Biodiesel sample from HEBO, B3 (IV = 114.9) and bio-diesel sample from LEBO, B4 (IV = 130.4) had differentIV, different BAPE and showed considerable differencesin oxidation stability referring to other parameters (PV,AV, m). This result may be due to it is difference in chemicalstructure and storage conditions. B3, with a relatively high

IV compared to other biodiesel samples (HOSO), has highoxidative stability and is much less susceptible to oxida-tion. This sample contains a high level of monosaturated(C18:1 and C22:1) FAME (Table 3) and low levels of poly-unsaturated (C18:2, C18:3) FAME, and gives a low BAPEvalue.

Table 3 shows BAPE values for all biodiesel samples,low BAPE show a high oxidative stability. As an example,B2, B3 and B5 biodiesel samples with a BAPE value (24.74,19.89, 26.06) lower than B4 (36.51) are less susceptible tooxidation.

Generally, the present results confirmed the observationthat small amounts of more highly unsaturated fatty

Daylight

0,000

20,000

40,000

60,000

80,000

100,000

120,000

140,000

0 10 20 30 40

Months of storage

Iodine

value

B1

B2

B3B4

B5

a

b Dark

0,000

20,000

40,000

60,000

80,000

100,000

120,000

140,000

0 10 20 30 40

Months of storage

Iodinevalue(IV

)B1B2

B3

B4

B5

Fig. 4. Evolution of the iodine number of biodiesel samples over a 30-months storage: (a) exposed, (b) not exposed to light.

Table 3Fatty acids composition of biodiesel samples

Veg. oilsamples

Methyl ester FA Amount(%)

Iodinevalue

APEa BAPEb

HOSO C16:0 4.23 90.3 185.6 24.74C18:1 80C18:2 10C18:3 2.83C20:1 0.29C22:1 0.11

Other minorcomponents

Rest to100

HEBO C16:0 3 14 03.68 19.89C18:1 32.75C18:2 9.29C18:3 5.3C20:1 9.8C22:1 40.84Other minorcomponents

Rest to100

LEBO C16:0 4.75 130.43 177.24 36.51C18:1 59.39C18:2 21.95C18:3 7.28

C20:1 2.43C22:1 1.46Other minorcomponents

Rest to100

UFO C16:0 8.85 118.8 165.34 26.06C18:1 61.52C18:2 16.24C18:3 4.91C20:1 C22:1 0.91Other minorcomponents

Rest to100

a APE: allylic position equivalent. Calculated from Eq. (7) in Ref. [22].b BAPE: bis-allylic position equivalent. Calculated from Eq. (7) in Ref.

[22].



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compounds containing bis-allylic carbons have a dispro-portional strong effect on oxidative stability.

3.5. Insolubles impurities (II)

With diesel fuel, most tests focus on the development of

insoluble products that can block fuel filters or fuelling sys-tems, which are important factors for a consumer of dieselfuel. In addition, the information contained in the litera-ture [21], indicates that many of the mechanisms for thechanges in diesel fuel are most likely not present in biodie-sel and vice-versa.

The analysis of biodiesel has demonstrated the tendencyto form deleterious particles in the different biodieselsamples.

The total insoluble content of biodiesel samplesincreased with increasing storage time. Fig. 5 shows anincrease in insolubles impurities for all biodiesel samples,

except for B5 that show a faster increase. Samples storedin the daylight showed the same tendency as the dark oneswith an increase at a faster rate for (B1, B4 and B5) andlower rate for B2 and B3.

In this work the storage stability, especially (oxidativestability) for the biodiesel made from used frying oil,showed a fast increase in insoluble impurities (II) comparedwith that of other biodiesel samples may be due to the

characteristic of the raw material, which has been heatedover 300 C during frying.

4. Conclusions

According to results from this 30-month study of

HOSME (high oleic sunflower oil methyl ester), HEBO(high -erucic Brassica oil) ME, LEBO (low erucic Brassicaoil) ME and UFO (used frying oil) ME, all biodiesel sam-ples are very stable because they did not demonstrate rapidincrease in peroxide value (PV), acid value (AV), viscosity(m), and insoluble impurities (II). However, there was adeterioration of the fuel after 12 months of storage. Signif-icant differences were, however, found in the value of themeasured parameters for all fuels type and storage condi-tions with the passage of time. For all biodiesel samples,the peroxide, acid values, viscosity, and insoluble impuri-ties, tended to increase and iodine value (IV) to decrease

over time. Fuels exposed to daylight tended to degrade atfaster rate than did the others fuels, particularly as indi-cated by their peroxide and acid values. The specificationlimit of the parameters studied was exceeded with biodieselsamples after a storage time of 12 months.

The use of the APE and BAPE indices instead of IV alsoimplies that oxidative stability may be more strongly influ-enced by the presence of small amounts of more highlyunsaturated fatty compounds than by increasing amountsthereof. The oxidation stability depends not only on howthe oil is pressed and refined but also on the raw materialand on the production process of biodiesel.

Biodiesel sample from low erucic Brassica carinata oilshowed worse oxidation stability than the other biodieselsamples. This may be due to its quite elevated level of lin-oleic and linolenic acid (characterized by two and threeunsaturated bonds). The results obtained in this work haveconfirmed the literature information that biodiesel storagepromotes a rise in the peroxide number, acid number, vis-cosity, and insolubles of the fuel. Long-term storage studygives a better understanding of the effect of the differentconditions and chemical composition on the stability ofbiodiesel. In conclusion, it can be observed that water con-tent and air exposure are two important factors affectingthe degradation of biodiesel.

Results of this study suggest that to obtain a highly sta-ble biodiesel and to avoid oxidation, it is necessary to takeespecial precaution during the storage such as limitingaccess to oxygen and exposure to light and moisture.

These promising results need to be completed by a moredetailed study of the effect of parameters such as elevatedtemperature, light, air contact and other parameters thataffect the oxidation stability of biodiesel.

Acknowledgement

Financial support from the (CICYT), Spanish project

CTQ-2006-10-467-PPQ is gratefully acknowledged.

Daylight

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10 20 30 40

Months of storage

Impurties(%) B1

B2

B3

B4

B5

a

b Dark

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,80,9

1,0

0 10 20 30 40

Months of storage

Impurties(%)

B1

B2

B3

B4

B5

Fig. 5. Insoluble impurities for a 30-months storage period for methyl

esters.



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