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Renewable fuels and lubricants from Lunaria annua L.
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Renewable fuels and lubricants from Lunaria annua L. George S. Dodos ∗ , Dimitrios Karonis, Fanourios Zannikos, Evripidis Lois School of Chemical Engineering, Laboratory of Fuel Technology and Lubricants, National Technical University of Athens, 15780 Athens, Greece
a r t i c l e
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Article history: Received 16 February 2015 Received in revised form 20 May 2015 Accepted 21 May 2015 Available online xxx Keywords: Lunaria annua L. Transesterification Biodiesel Lunaria oil methyl esters Biolubricants Lunaria oil trimethylolpropane esters
a b s t r a c t A non-edible fatty oil coming from Lunaria annua plant (honesty plant) was investigated as a starting material for the sequential production of biodiesel (fatty acid methyl esters—FAME) and biolubricants (trimethylolpropane esters). Lunaria oil constitutes an attractive feedstock because of its unusual fatty acid profile comprising mainly of erucic (22:1) and nervonic (24:1) acid. For the production of the desired biobased products, the following transesterification methodology was applied. At first, Lunaria oil was converted to the corresponding methyl esters via methanolysis reaction and the produced FAME was assessed as automotive fuel according to the specified requirements and test methods included in the European Standard EN14214. In order to synthesize the environmentally adapted lubricants, part of the previously prepared methylesters was converted to trimethylolpropane (TMP) esters via alkaline transesterification reaction. Lunaria TMP esters were evaluated as potential lubricant basestock regarding their physicochemical properties and their lubricating performance. Concerning fuel properties, Lunaria methyl esters appear to satisfy the majority of the EN14214 requirements with the exception of kinematic viscosity due to the high content of long chain fatty acids. However, the later could be counterbalanced though blending with other types of low quality feedstock. On the other hand, the high monounsaturated content is beneficial to other properties such as oxidation stability. The synthesized TMP esters demonstrate very good lubricating properties as well as oxidation stability, and thus, they could be utilized as lubricant basestock in the formulation of high added value biolubricants for special and environmentally sensitive applications. As a whole, Lunaria oil appears to be an interesting feedstock for the production of biodiesel and especially biobased lubricant basestock. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The environmental concern for petroleum products, the dictates of sustainable development and the geopolitical strategies regarding crude oil reserves are some of the driving forces toward the development of alternative renewable fuels and lubricants from oilseeds over the last decades. The European Comission has set a number of targets by year 2020. According to RED (Renewable Energy Directive – 2009/28/EC) and FQD (Fuel Quality Directive – 2009/30/EC), a 10% share in all forms of transport concerning the energy from renewable sources in the transport has to be met, whereas a 6% reduction in the greenhouse gases of fuels used in road transport has to be achieved by the same year. Moreover, a proposed sustainability criterion designates that the share of energy from biofuels produced from food crops shall be limited to 5% of the final consumption of energy in transport in order to meet the RED 10% target (COM, 2012, 595). Biodiesel
∗ Corresponding author. Tel.: +30 210 7723213; fax: +30 210 7723163. E-mail address: [email protected] (G.S. Dodos).
is one of the most widely used first generation biofuel. It is a renewable substitute of conventional petroleum diesel fuel consisting of fatty acid methyl esters (FAME) that is being added nowadays as a mixing component at a maximum concentration of 7% v/v in the EU. In Europe, a continuing shift from gasoline to diesel fuel is observed leading to increased demands of diesel and subsequently biodiesel fuel (Maniatis, 2012). Rapeseed, soybean, palm, and sunflower oil are the main source materials for biodiesel production; however, those constitute edible crops (Escobar et al., 2009; Singh and Singh, 2010). Since the 1st generation biofuels (FAME) are still the prominent widely available diesel substitutes, the exploitation of alternative non-food crops as feedstock is advised. On the other hand, biobased lubricants (or biolubricants) are high added value commodities, and their market is considered to be one of the most promising sectors universally. Nowadays, the consumption of biolubricants in the EU-27 is estimated to be about 100.000 t/a largerly in total loss and high risk application. However, according to various forecasts, the volume could be quadrupled in the 2020th in case of binding political framework for supporting biobased (Luther, 2014). A first significant step was made by CEN in 2011 by publishing the Technical Report 16227 that sets the
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Picture 1. Example of Lunaria annua (Honesty) plant.
minimum requirements for bio-lubricants including renewability, biodegradability, toxicity and technical performance (PD CEN/TR 16,227:2011). Moreover, the European and national ecolabelling shemes for commercial biobased lubricants can further promote the market share. Alternative renewable lubricants are usually formulated from fatty oil derivatives such as fatty acid mono-esters of ethylhexyl alcohol and fatty acid polyolesters of trimethylolparopane (TMP), pentaerythritol (PE) and neopentylglycol (NPG). Lubricants based on oleochemicals from vegetable oils possess certain advantageous characteristics not only regarding biodegradability and eco-toxicity but also high lubricity, high viscosity index (VI), high flash point, and low evaporative loss. Additionally, these esters exhibit good low temperature performance and improved oxidation-thermal stability due to the presence of a quaternary C-atom as well as the absence of a secondary hydrogen in the -position (Salimon et al., 2010; Honary and Richter, 2011; Mobarak et al., 2014). Based on the above, the aim of this study was to examine the utilization of the Lunaria annua deriving fatty oil as a starting material for the production of biodiesel (FAME) and biolubricants (TMP esters). L. annua L. – also known as Honesty – is generally a self-sown plant which is very common in south eastern Europe and south west Asia (Zanetti et al., 2013). It is usually grown as a biennial, although some annual species also exist. It can be found commonly along roads and in semi-shady places. It is also cultivated as an ornamental flower. Lunaria is a cruciferous plant growing to a maximum of 80–100 cm tall with large, coarsely toothed oval leaves (Picture 1). During spring and summer, it produces purple or white flowers, followed after a while by circular pods containing the seeds as shown in Picture 2 (Mastebroek and Marvin, 2000). Regarding seeding rate, it has been reported that field yields of 2.0–2.5 t/ha could be achieved with advanced crop management (Cromack, 1998). Lunaria seeds contain about 30–40% oil consisting mainly of long chain fatty acids such as erucic and nervonic acids (Martin et al.,
Picture 2. Lunaria annua – the circular pods containing the oilseeds.
2005; Gunstone et al., 2007). It is not considered an edible oil and due to its composition it has been studied as a promising crop for medicinal or cosmetic uses. Besides, it has also been characterized as suitable for other high added value commodities such as lubricants (Gunstone et al., 2007; Zanetti et al., 2013). The utilization of L. annua as an industrial oil crop is quite recent. The biennial character of Lunaria is usually regarded as a main constraint for an economical viable cultivation. Also, the commercially available varieties are still few (Zanetti et al., 2013). Efforts are currently being made in order to overcome the limited knowledge of crop culture potential and to optimize the production conditions especially of the annual species. As a result, Lunaria oil was explored as a renewable source for biodiesel and biolubricants based on the following principles. First and foremost, it contains an unconventional fatty acid profile that requires examination and could be advantageous for several quality parameters of renewable fuels and lubricants compared to other commonly utilized fatty oils. Secondly, L. annua is a non edible oilcrop so it does not directly compete with the food sector. Last but not least, the Mediterranean climate appears to be optimum for this plant and so the latter’s exploitation would be beneficial for the local economy. 2. Experimental 2.1. Materials and reagents Crude Lunaria oil was extracted from lunaria seeds with hexane in laboratory scale by using a Soxhlet apparatus. Table 1 lists the properties of the extracted fatty oil while a sample of the extracted fatty oil is depicted in Picture 3. Methanol (CAS: 67-56-1, EINECS: 200-659-6), 99.99% purity was obtained from Fisher Scien-
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Table 1 Physicochemical properties of crude Lunaria oil. Property
Units
Lunaria oil
Method
Density at 15 ◦ C KV at 40 ◦ C Water content Acid value Saponification value
kg/m3 mm2 /s mg/kg mg KOH/g ◦ C
897.6 37.18 200 2.70 167
EN ISO 12185 EN ISO 3104 EN ISO 12937 EN 14104 AOCS Cd3-25 Equation 1. Synthesis of Lunaria Methyl Esters (LUNME).
tific. Sodium methoxide (CAS: 124-41-4, EINECS: 204-699-5), pure, anhydrous powder, was obtained from Acros Organics. Trimethylolpropane – TMP (CAS: 77-99-6, EINECS: 201-074-9), >99% purity was obtained from Merck. 2.2. Methodology A two-stages transesterification methodology was applied so as to synthesize the Lunaria based alternative diesel and lubricant, as illustrated in Fig. 1. Initially Lunaria oil was converted to the corresponding methylester. Subsequently, the LUNME were transesterified with trimethylolpropane (TMP) producing the desired polyol esters. 2.3. Preparation of Lunaria oil methyl ester (LUNME) LUNME was produced via methanolysis of Lunaria oil (Equation 1). Sodium methoxide (CH3 ONa) was used as catalyst at a concentration of 1.0 wt% and a 6:1 methanol/oil molar ratio was employed. The transesterification reaction was carried out at 65 ◦ C for 2 h and after the completion, the upper methyl esters phase was separated
from the glycerol phase and a dry purification procedure was followed. The excess of methanol was removed by rotary evaporator. The purified methyl esters were dried over anhydrous sodium sulphate (Na2 SO4 ) and after vacuum filtration the final LUNME were obtained. 2.4. Synthesis of Lunaria oil TMP esters (LUNTMPE) In order to synthesize, the Lunaria-based lubricant basestock a second sequential transeterification reaction was conducted using the previously mentioned methyl esters as starting material (Equation 2). A flask equipped with a Dean-Stark cap and a reflux condenser was filled with LUNME and TMP at stoichiometric ratio followed by the addition of isooctane – as azeotropic agent – and 2 wt% of the catalyst. The latter was a Ca alkoxide alkaline catalyst prepared in the laboratory as described in a previous publication of the authors (Dodos et al., 2012). Methanol that was formed over the reaction was constantly removed, to favor the forward reaction and was collected in the cap as a means for monitoring the conversion. When the reaction was complete, iso-ocatne was removed by evaporation while the catalyst was separated from the synthesized ester by centrifuge. 2.5. Quality assessment The analysis of the LUNME as alternative diesel fuel was performed according to the specified requirements and test methods indicated in the European Standard EN14214. The fatty acid composition of LUNME was determined by gas chromatography using a GC apparatus in accordance with EN14103. Measurements of cetane number, determined as derived cetane number (DCN) was performed by EN16144 standard method. The synthesized TMP esters were evaluated regarding their physicochemical properties as potential lubricant basestocks. The kinematic viscosities (KV) at 40 ◦ C and 100 ◦ C and the viscosity index (VI) were determined according to methods EN3104 and ISO 2909, respectively. The pour point (PP) measurements were carried out in accordance to ISO 3016 standard method. For the determination of acid value (AV), the European Standard EN14104 for oil and fat derivatives were followed. A Karl–Fischer Coulometer was used for the determination of the water content following the EN ISO 12937 procedure. Oxidation stability was measured in the Rancimat unit (EN14112). The lubricating properties of the esters were investigated by employing both a Four Ball test machine and a High Frequency Reciprocating Rig (HFRR) apparatus. The wear preventive characteristics were examined in the Four Ball tester in
Picture 3. Extracted Lunaria oil.
Equation 2. Synthesis of Lunaria TMP Esters (LUNTMPE).
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Fig. 1. Simplified flow diagram for the sequential production of Lunaria based biodiesel and biolubricants.
accordance to IP 239 standard method. The HFRR unit was used to assess the antiwear and tribological properties of the produced biolubricants under boundary lubricating conditions. Samples were subjected to oscillating motion of 1.0 mm amplitude and 50 Hz frequency between a moving test ball and a fixed specimen under loading of 1 kg. The sample temperature was 100 ◦ C and the test duration was 60 min. The air relative humidity was kept at 45% while the ambient temperature in the laboratory was approximately 25 ◦ C. The coefficient of friction (CoF) and the film were continuously recorded, and at the end of the test the average values was reported. The antiwear characteristics of the biolubricants were estimated by measuring the mean wear scar diameter (MWSD) on the test ball, using the stereoscope at 120× magnification. The MWSD was corrected to the standardized water vapour pressure of 1.4 kPa and the WS 1.4 value, in micrometers (m), was reported. A conventional additive-free mineral base oil (Group I) was used as a reference lubricating fluid and was subjected to a similar series of analyses. In all cases, measurements were carried out at least in duplicate, and their mean value is reported. Unless otherwise stated both determinations were found to be within the repeatability of each method employed. 3. Results and discussion 3.1. Fatty acid profile of Lunaria oil Table 2 presents the fatty acid composition of Lunaria oil measured on methylesters basis. It is clear that the distribution of fatty acids is rather unsual and substantially varies from the majority of other commonly used fatty oils (Gunstone et al., 2007; Hoekman et al., 2012). Mono-unsaturated fatty acids are predominant in Lunaria oil (∼90 wt%). It consists mainly of nervonic (C24:1), erucic (C22:1) and oleic (C18:1) acid at a concentration of 43.25 wt%,
Table 2 Fatty acid composition of Lunaria oil methyl ester (LUNME). Fatty acids
Chemical structure
Weight%
Myristic Palmitic Palmitoleic Stearic Oleic Linoleic Linolenic Gadoleic Erucic Nervonic
C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:1 C22:1 C24:1
0.24 1.17 0.28 0.20 23.00 6.31 0.95 0.50 43.25 23.01
CH3 (CH2 ) 12 COOH CH3 (CH2 ) 14 COOH CH3 (CH2 ) 5 CH CH(CH2 ) 7 COOH CH3 (CH2 ) 16 COOH CH3 (CH2 ) 7 CH CH(CH2 ) 7 COOH CH3 (CH2 ) 3 (CH2 CH CH) 2 (CH2 ) 7 COOH CH3 (CH2 CH CH) 3 (CH2 ) 7 COOH CH3 (CH2 ) 8 CH CH(CH2 ) 8 COOH CH3 (CH2 ) 7 CH CH(CH2 ) 11 COOH CH3 (CH2 ) 7 CH CH(CH2 ) 13 COOH
23.01 wt% and 23 wt%, respectively. The rest of the fatty acids is linoleic (6.31 wt%) and palmitic (1.17 wt%). Very low levels of linolenic acid have been detected (0.95 wt%). This unique composition of Lunaria oil is the main motive behind its expoitation as industrial crop. 3.2. LUNME as automotive fuel for diesel engines Lunaria oil biodiesel can satisfy the majority of the measured applicable requirements as automotive diesel fuel outlined in the European Standard EN14214 (Table 3). Under the methanolysis conditions employed in this study the ester content was 96.6% which is just above the lower limiting value but still is a quite satisfactory conversion implying that during the transesterification process the undesirable saponification and neutralization side reactions were significantly reduced. Density, water content and acid value parameters were measured within the acceptable limits. A high water concentration may cause either engine corrosion or a reversion of fatty acid methyl esters to fatty acids, while increased acid value imply high content of free fatty acids. Iodine value was determined around 80, well below the EN specification limits, in spite of the abundance in unsaturated fatty acids. The GC glycerides analysis of the methylesters showed that the remaining levels of mono-, di- and tri-glycerides are below the maximum permitted values. In general, high contents of glycerides are undesirable as they may cause formation of deposits on injections and valves. Free glycerol content was also found to be relatively low indicating effectual glycerol separation and methylester purification. The high conversion of the oil to its methylesters and the final good quality of the resulting biodiesel was demonstrated by the fairly low levels of total glycerol detected. Gross calorific value or higher heating value is not specified in either EN14214 or ASTM D6751; however, it is important since it impacts the fuel efficiency and consumption. Usually the mass energy content of biodiesel is lower than that of conventional diesel fuel by approximately 10–12% due to the structural oxygen content (Hoekman et al., 2012; Knothe et al., 2005). In the case of LUNME, the gross heat of combustion was determined to be 40.64 MJ/kg, a quite satisfactory number and within the range for a biodiesel fuel which is typically between 39 and 41 MJ/kg (Atabani et al., 2012; Demirbas, 2008). However, the main quality parameters that LUNME appears to excel are the cold flow properties, the oxidative behaviour and the cetane number. CFPP was measured as low as −12 ◦ C and with this value the low temperature characteristics of LUNME are far better than the average value demonstrated by other FAMEs, as shown in Table 4. Furthermore, the oxidation stability of LUNME seems extraordinarily high either in Rancimat or RSSOT (Rapid Small Scale
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Table 3 Properties of Lunaria oil methyl ester (LUNME). Property
Units
LUNME
EN14214 limits
Method
Ester content Density at 15 ◦ C K. viscosity at 40 ◦ C Water content CFPP Linolenic acid methyl ester Oxidation stability Rancimat Acid value Iodine value Cetane number (DCN) Methanol content Monoglyceride cont. Diglyceride content Triglyceride content Free glycerol Total glycerol Sulfur content Group I metals (Na + K) Group II metals (Ca + Mg) Phosphorous content Gross calorific value
%m/m kg/m3 mm2 /s mg/kg ◦ C %m/m
96.6 877 6.815 220 −12 0.95
± ± ± ± ± ±
1.6 2 0.034 20 2 0.10
min 96.5 860–900 3.50–5.00 max 500 (climate related) max 12
EN 14103 EN ISO 12185 EN ISO 3104 EN ISO 12937 EN 116 EN 14103
hours mg KOH/g g I2 /100g – %(m/m) %(m/m) %(m/m) %(m/m) %(m/m) %(m/m) mg/kg mg/kg mg/kg mg/kg MJ/kg
30.5 ± 0.42 ± 80 ± 78.9 ± 0.12 ± 0.361 ± 0.056 ± 0.037 ± 0.007 ± 0.10 ± 14.0 ± <1.0 <1.0 <1.0 40.64 ±
2.9 0.02 2 0.8 0.03 0.047 0.007 0.010 0.002 0.01 1.3
min 8 max 0.50 max 120 min 51 max 0.2 max 0.7 max 0.2 max 0.2 max 0.02 max 0.25 max 10.0 max 5.0 max 5.0 max 4.0 –
EN 14112 EN 14104 EN 14111 EN 16144 EN 14110 EN 14105
Oxidation Test) methods. Especially, in the Rancimat apparatus, the lower limiting value of 8 h is very easily surpassed. The oxidation rate is very low, resulting in an unprecedented induction period of 30.5 h, which is far beyond of what the rest of FAMEs can achieve (see Table 4). The abundance of high molecular wieght mono-unsaturated fatty acids – and the subsequent low levels of poly-unsaturated fatty acids – might be one reason for this unforeseen oxidation stability. It is known that FAMEs with increased content of poly unsatureated fatty acids are more vulnerable to oxidative deterioration due to the existence of a number of bisallylic moieties (Pullen and Saeed, 2012). The DCN measurements revealed that LUNME possess a rather high cetane number which is just below 80. CN is a non-linear parameter indicative of the ignition delay time of a diesel fuel upon injection into the combustion chamber. A high CN implies short ignition delay and in general is a desired characteristic for a diesel fuel since it can contribute to reduced engine knock as well as reduced NOx emissions (Knothe, 2014; Wadumesthrige et al., 2008). Although, it is known that CN of biodiesel is generally higher than conventional diesel (Demirbas, 2008), the obtained value is much higher than the reported range of 45–67 for biodiesel fuels from various common feedstocks (Atabani et al., 2012; Wadumesthrige et al., 2008). The unconventional fatty acid composition of Lunaria oil is the main parameter that gives rise to this CN. The long-chain fatty acids erucic and nervonic sum up to more that 66 wt%, while, the mono-unsaturated content is over 90 wt%. The positive effect of increasing carbon chain length and decreasing unsaturation on CN is thoroughly studied. The measured DCN for LUNME also agrees with the high CN of methyl erucate (74.2) as reported in a recent study (Knothe, 2014).
Table 4 Comparison of low temperature and oxidation properties between Lunaria methyl esters (ME) and other types of FAMEs. FAME type
CFPP (◦ C)
Rancimat oxidation stability (h)
Lunaria ME Cotton ME Sunflower ME Palm ME Pomace olive ME Rapeseed oil ME
−12 1 −2 9 −4 −12
30.5 2.5 2.5 14.0 10.5 5
Source: Dodos (2013), Hoekman et al. (2012), and Serrano et al. (2013).
0.13
EN ISO 20846 EN 14538 EN 14107 ASTM D 240
The considerable amounts of high-molecular weight fatty acids (erucic and nervonic), however, have a negative effect on the viscosity (Knothe and Steidley, 2011). Viscosity is an important parameter especially when fuel atomization is concerned. LUNME shows a viscosity of 6.8 mm2 /s, that does not meet the allowable EN14214 range i.e., between 3.5 and 5.0 mm2 /s. The same applies to sulfur content, as well, which was found to be just above the limiting value of 10 mg/kg, although this value cannot be considered as characteristic of Lunaria oil in general. Nevertheless, the above mentioned drawback might not be deemed as a limitative factor for utilizing Lunaria oil in the biodiesel section. Taking into consideration its advantageous characteristics, such as oxidative behavior, CFPP, and cetane number Lunaria oil could still be considered as an interesting mixing component of a FAME’s feedstock, especially if an upgrade of other poor quality starting materials is targeted. 3.3. LUNTMPE as biobased lubricant basestock 3.3.1. Identifiers After a transesterification reaction time of 8 h, a quite satisfactory conversion of LUNME to LUNTMPE was achieved, equal to 97%. An attempt to verify the identity of the produced substance was made by measuring the refractive index (nD 20 ) and by performing an FTIR–ATR analysis. The refractive index of LUNTMPE is given in Table 5 and is similar to published nD values regarding fatty acid TMP tri-esters (Happe et al., 2012). Fig. 2 shows the FTIR spectrum of LUNTMPE. The noticeable absorbance band in the range of 2970–2850 cm−1 is due to CH stretching vibrations of methyl and methylene moieties. The ester group is clearly imprinted by the characteristic strong peak appearing in the band of 1730–1750 cm−1 . Finally, there is an obvious evolution of a peak within the wavenumbers 1060–1050 cm−1 , relating to C O vibrations. According to the previous studies, the appearance of this peak is attributed to the formation of TMP tri-esters (Arbain and Salimon, 2011; Eychenne et al., 1998). 3.3.2. Quality parameters The measured physicochemical properties of LUNTMPE along with the standard methods employed for the analyses are listed in Table 5. For comparison, a second column has been inserted showing some the corresponding parameters of a typical Group
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6 Table 5 Properties of Lunaria oil TMP ester (LUNTMPE). Property
Units
LUNTMPE
Density @ 15 ◦ C KV @ 40 ◦ C KV @ 100 ◦ C Viscosity index (VI) Water content Oxidation stability (Rancimat) Pour point (PP) Acid value Refractive index (nD 20 )
g/cm3 cSt cSt – mg/kg hours ◦ C mg KOH/g –
0.9077 ± 0.0015 29.97 ± 0.033 6.532 ± 0.007 181 <100 >150 −18 ± 3 <0.02 1.4724 ± 0.0002
(Gp I B.O.) (0.8620) (28.10) (5.07) (108) (<50) (n/a) (−6) (<0.05) –
Method EN ISO 12185 EN 3104 EN 3104 ISO 2909 EN ISO 12937 EN 14112 ISO 3016 EN 14104 ASTM D 1218
Fig. 3. Wear preventive characteristics (Four Ball test) of LUNTMPE compared to conventional Gp I base oil. Fig. 2. LUNTMPE FTIR spectrum.
I conventional base oil. The synthesized tri-ester has a kinematic viscosity of 29.97 mm2 /s at 40 ◦ C, whereas at 100 ◦ C the k. viscosity is reduced at 6.53 mm2 /s which is considerably higher than the k. viscosity of the conventional base oil at the same temperature. As a result, LUNTME ends up with a much higher Viscosity Index (181), as well, which is a desirable characteristic because in the majority of applications a resistance a lubricant will be asked to perform with not so significant changes in its viscosity throughout a temperature range. Acid value was minimized in the case of LUNTMPE. It was measured below the marginal value of 0.02 mgKOH/g and this is beneficial not only towards oxidation stability but also to lower corrosive tendency. Regarding cold flow characteristics, LUNTMPE possess inherently a pretty low pour point value equal to −18 ◦ C,
that can ensure pumpability in cold starting conditions. This good low temperature behavior can be attributed to the abundance of mono-unsaturated fatty acids. Besides, the increased levels of high molecular weight mono-unsaturated fatty acids along with the presence of a quaternary C-atom in the triester structure may contribute, up to an extend to the very high resistance to oxidative deterioration that LUNTMPE demonstrate. The induction time determined in the Rancimat apparatus was over 150 h which is actually five times higher than the stability of the previously prepared Lunaria methylesters. 3.3.3. Lubricating properties The results obtained from the tribological assesement in the Four Ball and HFRR units are depicted graphically in Figs. 3 and 4,
Fig. 4. Antiwear properties (HFRR Test) of LUNTMPE compared to conventional Gp I base oil.
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respectively. Under the conditions of both methods the superior boundary lubricating properties of LUNTMPE compared to the conventional Group 1 base oil are demonstrated. In particular, in the four ball results, the system lubricated with the biobased ester produced a MWSD of 0.53 which is considerably lower than the 0.9 wear scar of the conventional basestock. The results indicate the advanced wear preventive characteristics that a final formulation based on LUTMPE would be able to possess. Similarly, the antiwear properties and tribological characteristics measured in the HFRR show the same trend, i.e., the biobased TMP ester exhibit lower WS1.4 value compared to the mineral oil. Additionally, the CoF was higher when the system was lubricated with the mineral oil (0.147) and the percent of film formation ability was very poor (20%). On the contrary, the prepared biolubricant exhibited much better friction characteristics, and CoF was reduced to a value of less than 0.08 while the contact resistance between the sliding surfaces ascended to 85%. The ester functionality of LUNTMPE and the ability of the contained polar compounds – such as fatty acid – to reduce surfaceto-surface energy and subsequently friction, is the main reason for the lower WSD values and generally for the elevated lubrication characteristics of LUNTME versus mineral oils (Stachowiak and Batchelor, 2013; Quinchia et al., 2014). 4. Conclusions In this study, Lunaria (L. annua L.) oil was examined as a starting material for the production of two types of biobased oleochemical esters that could be utilized as renewable substitutes of diesel fuels and lubricants. Lunaria methylesters and Lunaria TMP esters were synthesized by applying a two-stages sequential transesterification process. The former were assessed according to FAME European standard EN14214 whereas the latter were evaluated as as potential lubricant basestock regarding their physicochemical properties and their lubricating performance. The results can be interpreted as follows: • Lunaria oil is abundant in monounsaturated fatty acids, in particular nervonic, erucic acid. The unusual fatty acid composition of this fatty oil along with its non-edible nature can make its utilization as industrial crop very attractive. • A quite satisfactory conversion of Lunaria oil to Lunaria methyl esters was achieved with sodium methoxide as catalyst at a concentration of 1 wt%. As a result, LUNME can meet the majority of EN14214 requirements. The remarkable oxidation stability and the excellent cold flow properties are the key advantages of LUNME, whereas the main weakness lies in the high kinematic viscosity value. However, this could be counterbalanced though blending in the feedstock with other low quality starting materials (e.g., UFO). The use of Lunaria oil as a mixing component in biodiesel’s feedstock is currently under investigation. • The Lunaria based biolubricant demonstrate very good low temperature behavior as well as elevated resistance to oxidation. LUNTMPE provide superior lubricating properties compared to conventional mineral base oil and so it could be utilized as lubricant basestock in the formulation of high added value biobased lubricants for special and environmentally sensitive applications. • From a technical point of view, Lunaria oil appears to be an interesting feedstock for the production of biodiesel and especially biobased lubricant basestock. Currently, the limited knowledge on systematic culture potential cannot help assessing the viability of the crop from an economic point of view. Optimization of cultivation techniques of L. annua – especially the annual species – in the near future, could contribute to the successful industrialization of Lunaria oil in the non-food sector.
7
Acknowledgments The authors would like to express their appreciation to ELDON’s S.A. – Lubricants company, Greece, for their assistance in measuring some of the biobased lubricant properties.
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Zanetti, F., Monti, A., Berti, M.T., 2013. Challenges and opportunities for new industrial oilseed crops in EU-27: a review. Ind. Crops. Prod. 50, 580–595.
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Renewable fuels and lubricants from Lunaria annua L. George S. Dodos ∗ , Dimitrios Karonis, Fanourios Zannikos, Evripidis Lois School of Chemical Engineering, Laboratory of Fuel Technology and Lubricants, National Technical University of Athens, 15780 Athens, Greece
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 20 May 2015 Accepted 21 May 2015 Available online xxx Keywords: Lunaria annua L. Transesterification Biodiesel Lunaria oil methyl esters Biolubricants Lunaria oil trimethylolpropane esters
a b s t r a c t A non-edible fatty oil coming from Lunaria annua plant (honesty plant) was investigated as a starting material for the sequential production of biodiesel (fatty acid methyl esters—FAME) and biolubricants (trimethylolpropane esters). Lunaria oil constitutes an attractive feedstock because of its unusual fatty acid profile comprising mainly of erucic (22:1) and nervonic (24:1) acid. For the production of the desired biobased products, the following transesterification methodology was applied. At first, Lunaria oil was converted to the corresponding methyl esters via methanolysis reaction and the produced FAME was assessed as automotive fuel according to the specified requirements and test methods included in the European Standard EN14214. In order to synthesize the environmentally adapted lubricants, part of the previously prepared methylesters was converted to trimethylolpropane (TMP) esters via alkaline transesterification reaction. Lunaria TMP esters were evaluated as potential lubricant basestock regarding their physicochemical properties and their lubricating performance. Concerning fuel properties, Lunaria methyl esters appear to satisfy the majority of the EN14214 requirements with the exception of kinematic viscosity due to the high content of long chain fatty acids. However, the later could be counterbalanced though blending with other types of low quality feedstock. On the other hand, the high monounsaturated content is beneficial to other properties such as oxidation stability. The synthesized TMP esters demonstrate very good lubricating properties as well as oxidation stability, and thus, they could be utilized as lubricant basestock in the formulation of high added value biolubricants for special and environmentally sensitive applications. As a whole, Lunaria oil appears to be an interesting feedstock for the production of biodiesel and especially biobased lubricant basestock. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The environmental concern for petroleum products, the dictates of sustainable development and the geopolitical strategies regarding crude oil reserves are some of the driving forces toward the development of alternative renewable fuels and lubricants from oilseeds over the last decades. The European Comission has set a number of targets by year 2020. According to RED (Renewable Energy Directive – 2009/28/EC) and FQD (Fuel Quality Directive – 2009/30/EC), a 10% share in all forms of transport concerning the energy from renewable sources in the transport has to be met, whereas a 6% reduction in the greenhouse gases of fuels used in road transport has to be achieved by the same year. Moreover, a proposed sustainability criterion designates that the share of energy from biofuels produced from food crops shall be limited to 5% of the final consumption of energy in transport in order to meet the RED 10% target (COM, 2012, 595). Biodiesel
∗ Corresponding author. Tel.: +30 210 7723213; fax: +30 210 7723163. E-mail address: [email protected] (G.S. Dodos).
is one of the most widely used first generation biofuel. It is a renewable substitute of conventional petroleum diesel fuel consisting of fatty acid methyl esters (FAME) that is being added nowadays as a mixing component at a maximum concentration of 7% v/v in the EU. In Europe, a continuing shift from gasoline to diesel fuel is observed leading to increased demands of diesel and subsequently biodiesel fuel (Maniatis, 2012). Rapeseed, soybean, palm, and sunflower oil are the main source materials for biodiesel production; however, those constitute edible crops (Escobar et al., 2009; Singh and Singh, 2010). Since the 1st generation biofuels (FAME) are still the prominent widely available diesel substitutes, the exploitation of alternative non-food crops as feedstock is advised. On the other hand, biobased lubricants (or biolubricants) are high added value commodities, and their market is considered to be one of the most promising sectors universally. Nowadays, the consumption of biolubricants in the EU-27 is estimated to be about 100.000 t/a largerly in total loss and high risk application. However, according to various forecasts, the volume could be quadrupled in the 2020th in case of binding political framework for supporting biobased (Luther, 2014). A first significant step was made by CEN in 2011 by publishing the Technical Report 16227 that sets the
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Picture 1. Example of Lunaria annua (Honesty) plant.
minimum requirements for bio-lubricants including renewability, biodegradability, toxicity and technical performance (PD CEN/TR 16,227:2011). Moreover, the European and national ecolabelling shemes for commercial biobased lubricants can further promote the market share. Alternative renewable lubricants are usually formulated from fatty oil derivatives such as fatty acid mono-esters of ethylhexyl alcohol and fatty acid polyolesters of trimethylolparopane (TMP), pentaerythritol (PE) and neopentylglycol (NPG). Lubricants based on oleochemicals from vegetable oils possess certain advantageous characteristics not only regarding biodegradability and eco-toxicity but also high lubricity, high viscosity index (VI), high flash point, and low evaporative loss. Additionally, these esters exhibit good low temperature performance and improved oxidation-thermal stability due to the presence of a quaternary C-atom as well as the absence of a secondary hydrogen in the -position (Salimon et al., 2010; Honary and Richter, 2011; Mobarak et al., 2014). Based on the above, the aim of this study was to examine the utilization of the Lunaria annua deriving fatty oil as a starting material for the production of biodiesel (FAME) and biolubricants (TMP esters). L. annua L. – also known as Honesty – is generally a self-sown plant which is very common in south eastern Europe and south west Asia (Zanetti et al., 2013). It is usually grown as a biennial, although some annual species also exist. It can be found commonly along roads and in semi-shady places. It is also cultivated as an ornamental flower. Lunaria is a cruciferous plant growing to a maximum of 80–100 cm tall with large, coarsely toothed oval leaves (Picture 1). During spring and summer, it produces purple or white flowers, followed after a while by circular pods containing the seeds as shown in Picture 2 (Mastebroek and Marvin, 2000). Regarding seeding rate, it has been reported that field yields of 2.0–2.5 t/ha could be achieved with advanced crop management (Cromack, 1998). Lunaria seeds contain about 30–40% oil consisting mainly of long chain fatty acids such as erucic and nervonic acids (Martin et al.,
Picture 2. Lunaria annua – the circular pods containing the oilseeds.
2005; Gunstone et al., 2007). It is not considered an edible oil and due to its composition it has been studied as a promising crop for medicinal or cosmetic uses. Besides, it has also been characterized as suitable for other high added value commodities such as lubricants (Gunstone et al., 2007; Zanetti et al., 2013). The utilization of L. annua as an industrial oil crop is quite recent. The biennial character of Lunaria is usually regarded as a main constraint for an economical viable cultivation. Also, the commercially available varieties are still few (Zanetti et al., 2013). Efforts are currently being made in order to overcome the limited knowledge of crop culture potential and to optimize the production conditions especially of the annual species. As a result, Lunaria oil was explored as a renewable source for biodiesel and biolubricants based on the following principles. First and foremost, it contains an unconventional fatty acid profile that requires examination and could be advantageous for several quality parameters of renewable fuels and lubricants compared to other commonly utilized fatty oils. Secondly, L. annua is a non edible oilcrop so it does not directly compete with the food sector. Last but not least, the Mediterranean climate appears to be optimum for this plant and so the latter’s exploitation would be beneficial for the local economy. 2. Experimental 2.1. Materials and reagents Crude Lunaria oil was extracted from lunaria seeds with hexane in laboratory scale by using a Soxhlet apparatus. Table 1 lists the properties of the extracted fatty oil while a sample of the extracted fatty oil is depicted in Picture 3. Methanol (CAS: 67-56-1, EINECS: 200-659-6), 99.99% purity was obtained from Fisher Scien-
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Table 1 Physicochemical properties of crude Lunaria oil. Property
Units
Lunaria oil
Method
Density at 15 ◦ C KV at 40 ◦ C Water content Acid value Saponification value
kg/m3 mm2 /s mg/kg mg KOH/g ◦ C
897.6 37.18 200 2.70 167
EN ISO 12185 EN ISO 3104 EN ISO 12937 EN 14104 AOCS Cd3-25 Equation 1. Synthesis of Lunaria Methyl Esters (LUNME).
tific. Sodium methoxide (CAS: 124-41-4, EINECS: 204-699-5), pure, anhydrous powder, was obtained from Acros Organics. Trimethylolpropane – TMP (CAS: 77-99-6, EINECS: 201-074-9), >99% purity was obtained from Merck. 2.2. Methodology A two-stages transesterification methodology was applied so as to synthesize the Lunaria based alternative diesel and lubricant, as illustrated in Fig. 1. Initially Lunaria oil was converted to the corresponding methylester. Subsequently, the LUNME were transesterified with trimethylolpropane (TMP) producing the desired polyol esters. 2.3. Preparation of Lunaria oil methyl ester (LUNME) LUNME was produced via methanolysis of Lunaria oil (Equation 1). Sodium methoxide (CH3 ONa) was used as catalyst at a concentration of 1.0 wt% and a 6:1 methanol/oil molar ratio was employed. The transesterification reaction was carried out at 65 ◦ C for 2 h and after the completion, the upper methyl esters phase was separated
from the glycerol phase and a dry purification procedure was followed. The excess of methanol was removed by rotary evaporator. The purified methyl esters were dried over anhydrous sodium sulphate (Na2 SO4 ) and after vacuum filtration the final LUNME were obtained. 2.4. Synthesis of Lunaria oil TMP esters (LUNTMPE) In order to synthesize, the Lunaria-based lubricant basestock a second sequential transeterification reaction was conducted using the previously mentioned methyl esters as starting material (Equation 2). A flask equipped with a Dean-Stark cap and a reflux condenser was filled with LUNME and TMP at stoichiometric ratio followed by the addition of isooctane – as azeotropic agent – and 2 wt% of the catalyst. The latter was a Ca alkoxide alkaline catalyst prepared in the laboratory as described in a previous publication of the authors (Dodos et al., 2012). Methanol that was formed over the reaction was constantly removed, to favor the forward reaction and was collected in the cap as a means for monitoring the conversion. When the reaction was complete, iso-ocatne was removed by evaporation while the catalyst was separated from the synthesized ester by centrifuge. 2.5. Quality assessment The analysis of the LUNME as alternative diesel fuel was performed according to the specified requirements and test methods indicated in the European Standard EN14214. The fatty acid composition of LUNME was determined by gas chromatography using a GC apparatus in accordance with EN14103. Measurements of cetane number, determined as derived cetane number (DCN) was performed by EN16144 standard method. The synthesized TMP esters were evaluated regarding their physicochemical properties as potential lubricant basestocks. The kinematic viscosities (KV) at 40 ◦ C and 100 ◦ C and the viscosity index (VI) were determined according to methods EN3104 and ISO 2909, respectively. The pour point (PP) measurements were carried out in accordance to ISO 3016 standard method. For the determination of acid value (AV), the European Standard EN14104 for oil and fat derivatives were followed. A Karl–Fischer Coulometer was used for the determination of the water content following the EN ISO 12937 procedure. Oxidation stability was measured in the Rancimat unit (EN14112). The lubricating properties of the esters were investigated by employing both a Four Ball test machine and a High Frequency Reciprocating Rig (HFRR) apparatus. The wear preventive characteristics were examined in the Four Ball tester in
Picture 3. Extracted Lunaria oil.
Equation 2. Synthesis of Lunaria TMP Esters (LUNTMPE).
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Fig. 1. Simplified flow diagram for the sequential production of Lunaria based biodiesel and biolubricants.
accordance to IP 239 standard method. The HFRR unit was used to assess the antiwear and tribological properties of the produced biolubricants under boundary lubricating conditions. Samples were subjected to oscillating motion of 1.0 mm amplitude and 50 Hz frequency between a moving test ball and a fixed specimen under loading of 1 kg. The sample temperature was 100 ◦ C and the test duration was 60 min. The air relative humidity was kept at 45% while the ambient temperature in the laboratory was approximately 25 ◦ C. The coefficient of friction (CoF) and the film were continuously recorded, and at the end of the test the average values was reported. The antiwear characteristics of the biolubricants were estimated by measuring the mean wear scar diameter (MWSD) on the test ball, using the stereoscope at 120× magnification. The MWSD was corrected to the standardized water vapour pressure of 1.4 kPa and the WS 1.4 value, in micrometers (m), was reported. A conventional additive-free mineral base oil (Group I) was used as a reference lubricating fluid and was subjected to a similar series of analyses. In all cases, measurements were carried out at least in duplicate, and their mean value is reported. Unless otherwise stated both determinations were found to be within the repeatability of each method employed. 3. Results and discussion 3.1. Fatty acid profile of Lunaria oil Table 2 presents the fatty acid composition of Lunaria oil measured on methylesters basis. It is clear that the distribution of fatty acids is rather unsual and substantially varies from the majority of other commonly used fatty oils (Gunstone et al., 2007; Hoekman et al., 2012). Mono-unsaturated fatty acids are predominant in Lunaria oil (∼90 wt%). It consists mainly of nervonic (C24:1), erucic (C22:1) and oleic (C18:1) acid at a concentration of 43.25 wt%,
Table 2 Fatty acid composition of Lunaria oil methyl ester (LUNME). Fatty acids
Chemical structure
Weight%
Myristic Palmitic Palmitoleic Stearic Oleic Linoleic Linolenic Gadoleic Erucic Nervonic
C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:1 C22:1 C24:1
0.24 1.17 0.28 0.20 23.00 6.31 0.95 0.50 43.25 23.01
CH3 (CH2 ) 12 COOH CH3 (CH2 ) 14 COOH CH3 (CH2 ) 5 CH CH(CH2 ) 7 COOH CH3 (CH2 ) 16 COOH CH3 (CH2 ) 7 CH CH(CH2 ) 7 COOH CH3 (CH2 ) 3 (CH2 CH CH) 2 (CH2 ) 7 COOH CH3 (CH2 CH CH) 3 (CH2 ) 7 COOH CH3 (CH2 ) 8 CH CH(CH2 ) 8 COOH CH3 (CH2 ) 7 CH CH(CH2 ) 11 COOH CH3 (CH2 ) 7 CH CH(CH2 ) 13 COOH
23.01 wt% and 23 wt%, respectively. The rest of the fatty acids is linoleic (6.31 wt%) and palmitic (1.17 wt%). Very low levels of linolenic acid have been detected (0.95 wt%). This unique composition of Lunaria oil is the main motive behind its expoitation as industrial crop. 3.2. LUNME as automotive fuel for diesel engines Lunaria oil biodiesel can satisfy the majority of the measured applicable requirements as automotive diesel fuel outlined in the European Standard EN14214 (Table 3). Under the methanolysis conditions employed in this study the ester content was 96.6% which is just above the lower limiting value but still is a quite satisfactory conversion implying that during the transesterification process the undesirable saponification and neutralization side reactions were significantly reduced. Density, water content and acid value parameters were measured within the acceptable limits. A high water concentration may cause either engine corrosion or a reversion of fatty acid methyl esters to fatty acids, while increased acid value imply high content of free fatty acids. Iodine value was determined around 80, well below the EN specification limits, in spite of the abundance in unsaturated fatty acids. The GC glycerides analysis of the methylesters showed that the remaining levels of mono-, di- and tri-glycerides are below the maximum permitted values. In general, high contents of glycerides are undesirable as they may cause formation of deposits on injections and valves. Free glycerol content was also found to be relatively low indicating effectual glycerol separation and methylester purification. The high conversion of the oil to its methylesters and the final good quality of the resulting biodiesel was demonstrated by the fairly low levels of total glycerol detected. Gross calorific value or higher heating value is not specified in either EN14214 or ASTM D6751; however, it is important since it impacts the fuel efficiency and consumption. Usually the mass energy content of biodiesel is lower than that of conventional diesel fuel by approximately 10–12% due to the structural oxygen content (Hoekman et al., 2012; Knothe et al., 2005). In the case of LUNME, the gross heat of combustion was determined to be 40.64 MJ/kg, a quite satisfactory number and within the range for a biodiesel fuel which is typically between 39 and 41 MJ/kg (Atabani et al., 2012; Demirbas, 2008). However, the main quality parameters that LUNME appears to excel are the cold flow properties, the oxidative behaviour and the cetane number. CFPP was measured as low as −12 ◦ C and with this value the low temperature characteristics of LUNME are far better than the average value demonstrated by other FAMEs, as shown in Table 4. Furthermore, the oxidation stability of LUNME seems extraordinarily high either in Rancimat or RSSOT (Rapid Small Scale
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Table 3 Properties of Lunaria oil methyl ester (LUNME). Property
Units
LUNME
EN14214 limits
Method
Ester content Density at 15 ◦ C K. viscosity at 40 ◦ C Water content CFPP Linolenic acid methyl ester Oxidation stability Rancimat Acid value Iodine value Cetane number (DCN) Methanol content Monoglyceride cont. Diglyceride content Triglyceride content Free glycerol Total glycerol Sulfur content Group I metals (Na + K) Group II metals (Ca + Mg) Phosphorous content Gross calorific value
%m/m kg/m3 mm2 /s mg/kg ◦ C %m/m
96.6 877 6.815 220 −12 0.95
± ± ± ± ± ±
1.6 2 0.034 20 2 0.10
min 96.5 860–900 3.50–5.00 max 500 (climate related) max 12
EN 14103 EN ISO 12185 EN ISO 3104 EN ISO 12937 EN 116 EN 14103
hours mg KOH/g g I2 /100g – %(m/m) %(m/m) %(m/m) %(m/m) %(m/m) %(m/m) mg/kg mg/kg mg/kg mg/kg MJ/kg
30.5 ± 0.42 ± 80 ± 78.9 ± 0.12 ± 0.361 ± 0.056 ± 0.037 ± 0.007 ± 0.10 ± 14.0 ± <1.0 <1.0 <1.0 40.64 ±
2.9 0.02 2 0.8 0.03 0.047 0.007 0.010 0.002 0.01 1.3
min 8 max 0.50 max 120 min 51 max 0.2 max 0.7 max 0.2 max 0.2 max 0.02 max 0.25 max 10.0 max 5.0 max 5.0 max 4.0 –
EN 14112 EN 14104 EN 14111 EN 16144 EN 14110 EN 14105
Oxidation Test) methods. Especially, in the Rancimat apparatus, the lower limiting value of 8 h is very easily surpassed. The oxidation rate is very low, resulting in an unprecedented induction period of 30.5 h, which is far beyond of what the rest of FAMEs can achieve (see Table 4). The abundance of high molecular wieght mono-unsaturated fatty acids – and the subsequent low levels of poly-unsaturated fatty acids – might be one reason for this unforeseen oxidation stability. It is known that FAMEs with increased content of poly unsatureated fatty acids are more vulnerable to oxidative deterioration due to the existence of a number of bisallylic moieties (Pullen and Saeed, 2012). The DCN measurements revealed that LUNME possess a rather high cetane number which is just below 80. CN is a non-linear parameter indicative of the ignition delay time of a diesel fuel upon injection into the combustion chamber. A high CN implies short ignition delay and in general is a desired characteristic for a diesel fuel since it can contribute to reduced engine knock as well as reduced NOx emissions (Knothe, 2014; Wadumesthrige et al., 2008). Although, it is known that CN of biodiesel is generally higher than conventional diesel (Demirbas, 2008), the obtained value is much higher than the reported range of 45–67 for biodiesel fuels from various common feedstocks (Atabani et al., 2012; Wadumesthrige et al., 2008). The unconventional fatty acid composition of Lunaria oil is the main parameter that gives rise to this CN. The long-chain fatty acids erucic and nervonic sum up to more that 66 wt%, while, the mono-unsaturated content is over 90 wt%. The positive effect of increasing carbon chain length and decreasing unsaturation on CN is thoroughly studied. The measured DCN for LUNME also agrees with the high CN of methyl erucate (74.2) as reported in a recent study (Knothe, 2014).
Table 4 Comparison of low temperature and oxidation properties between Lunaria methyl esters (ME) and other types of FAMEs. FAME type
CFPP (◦ C)
Rancimat oxidation stability (h)
Lunaria ME Cotton ME Sunflower ME Palm ME Pomace olive ME Rapeseed oil ME
−12 1 −2 9 −4 −12
30.5 2.5 2.5 14.0 10.5 5
Source: Dodos (2013), Hoekman et al. (2012), and Serrano et al. (2013).
0.13
EN ISO 20846 EN 14538 EN 14107 ASTM D 240
The considerable amounts of high-molecular weight fatty acids (erucic and nervonic), however, have a negative effect on the viscosity (Knothe and Steidley, 2011). Viscosity is an important parameter especially when fuel atomization is concerned. LUNME shows a viscosity of 6.8 mm2 /s, that does not meet the allowable EN14214 range i.e., between 3.5 and 5.0 mm2 /s. The same applies to sulfur content, as well, which was found to be just above the limiting value of 10 mg/kg, although this value cannot be considered as characteristic of Lunaria oil in general. Nevertheless, the above mentioned drawback might not be deemed as a limitative factor for utilizing Lunaria oil in the biodiesel section. Taking into consideration its advantageous characteristics, such as oxidative behavior, CFPP, and cetane number Lunaria oil could still be considered as an interesting mixing component of a FAME’s feedstock, especially if an upgrade of other poor quality starting materials is targeted. 3.3. LUNTMPE as biobased lubricant basestock 3.3.1. Identifiers After a transesterification reaction time of 8 h, a quite satisfactory conversion of LUNME to LUNTMPE was achieved, equal to 97%. An attempt to verify the identity of the produced substance was made by measuring the refractive index (nD 20 ) and by performing an FTIR–ATR analysis. The refractive index of LUNTMPE is given in Table 5 and is similar to published nD values regarding fatty acid TMP tri-esters (Happe et al., 2012). Fig. 2 shows the FTIR spectrum of LUNTMPE. The noticeable absorbance band in the range of 2970–2850 cm−1 is due to CH stretching vibrations of methyl and methylene moieties. The ester group is clearly imprinted by the characteristic strong peak appearing in the band of 1730–1750 cm−1 . Finally, there is an obvious evolution of a peak within the wavenumbers 1060–1050 cm−1 , relating to C O vibrations. According to the previous studies, the appearance of this peak is attributed to the formation of TMP tri-esters (Arbain and Salimon, 2011; Eychenne et al., 1998). 3.3.2. Quality parameters The measured physicochemical properties of LUNTMPE along with the standard methods employed for the analyses are listed in Table 5. For comparison, a second column has been inserted showing some the corresponding parameters of a typical Group
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6 Table 5 Properties of Lunaria oil TMP ester (LUNTMPE). Property
Units
LUNTMPE
Density @ 15 ◦ C KV @ 40 ◦ C KV @ 100 ◦ C Viscosity index (VI) Water content Oxidation stability (Rancimat) Pour point (PP) Acid value Refractive index (nD 20 )
g/cm3 cSt cSt – mg/kg hours ◦ C mg KOH/g –
0.9077 ± 0.0015 29.97 ± 0.033 6.532 ± 0.007 181 <100 >150 −18 ± 3 <0.02 1.4724 ± 0.0002
(Gp I B.O.) (0.8620) (28.10) (5.07) (108) (<50) (n/a) (−6) (<0.05) –
Method EN ISO 12185 EN 3104 EN 3104 ISO 2909 EN ISO 12937 EN 14112 ISO 3016 EN 14104 ASTM D 1218
Fig. 3. Wear preventive characteristics (Four Ball test) of LUNTMPE compared to conventional Gp I base oil. Fig. 2. LUNTMPE FTIR spectrum.
I conventional base oil. The synthesized tri-ester has a kinematic viscosity of 29.97 mm2 /s at 40 ◦ C, whereas at 100 ◦ C the k. viscosity is reduced at 6.53 mm2 /s which is considerably higher than the k. viscosity of the conventional base oil at the same temperature. As a result, LUNTME ends up with a much higher Viscosity Index (181), as well, which is a desirable characteristic because in the majority of applications a resistance a lubricant will be asked to perform with not so significant changes in its viscosity throughout a temperature range. Acid value was minimized in the case of LUNTMPE. It was measured below the marginal value of 0.02 mgKOH/g and this is beneficial not only towards oxidation stability but also to lower corrosive tendency. Regarding cold flow characteristics, LUNTMPE possess inherently a pretty low pour point value equal to −18 ◦ C,
that can ensure pumpability in cold starting conditions. This good low temperature behavior can be attributed to the abundance of mono-unsaturated fatty acids. Besides, the increased levels of high molecular weight mono-unsaturated fatty acids along with the presence of a quaternary C-atom in the triester structure may contribute, up to an extend to the very high resistance to oxidative deterioration that LUNTMPE demonstrate. The induction time determined in the Rancimat apparatus was over 150 h which is actually five times higher than the stability of the previously prepared Lunaria methylesters. 3.3.3. Lubricating properties The results obtained from the tribological assesement in the Four Ball and HFRR units are depicted graphically in Figs. 3 and 4,
Fig. 4. Antiwear properties (HFRR Test) of LUNTMPE compared to conventional Gp I base oil.
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respectively. Under the conditions of both methods the superior boundary lubricating properties of LUNTMPE compared to the conventional Group 1 base oil are demonstrated. In particular, in the four ball results, the system lubricated with the biobased ester produced a MWSD of 0.53 which is considerably lower than the 0.9 wear scar of the conventional basestock. The results indicate the advanced wear preventive characteristics that a final formulation based on LUTMPE would be able to possess. Similarly, the antiwear properties and tribological characteristics measured in the HFRR show the same trend, i.e., the biobased TMP ester exhibit lower WS1.4 value compared to the mineral oil. Additionally, the CoF was higher when the system was lubricated with the mineral oil (0.147) and the percent of film formation ability was very poor (20%). On the contrary, the prepared biolubricant exhibited much better friction characteristics, and CoF was reduced to a value of less than 0.08 while the contact resistance between the sliding surfaces ascended to 85%. The ester functionality of LUNTMPE and the ability of the contained polar compounds – such as fatty acid – to reduce surfaceto-surface energy and subsequently friction, is the main reason for the lower WSD values and generally for the elevated lubrication characteristics of LUNTME versus mineral oils (Stachowiak and Batchelor, 2013; Quinchia et al., 2014). 4. Conclusions In this study, Lunaria (L. annua L.) oil was examined as a starting material for the production of two types of biobased oleochemical esters that could be utilized as renewable substitutes of diesel fuels and lubricants. Lunaria methylesters and Lunaria TMP esters were synthesized by applying a two-stages sequential transesterification process. The former were assessed according to FAME European standard EN14214 whereas the latter were evaluated as as potential lubricant basestock regarding their physicochemical properties and their lubricating performance. The results can be interpreted as follows: • Lunaria oil is abundant in monounsaturated fatty acids, in particular nervonic, erucic acid. The unusual fatty acid composition of this fatty oil along with its non-edible nature can make its utilization as industrial crop very attractive. • A quite satisfactory conversion of Lunaria oil to Lunaria methyl esters was achieved with sodium methoxide as catalyst at a concentration of 1 wt%. As a result, LUNME can meet the majority of EN14214 requirements. The remarkable oxidation stability and the excellent cold flow properties are the key advantages of LUNME, whereas the main weakness lies in the high kinematic viscosity value. However, this could be counterbalanced though blending in the feedstock with other low quality starting materials (e.g., UFO). The use of Lunaria oil as a mixing component in biodiesel’s feedstock is currently under investigation. • The Lunaria based biolubricant demonstrate very good low temperature behavior as well as elevated resistance to oxidation. LUNTMPE provide superior lubricating properties compared to conventional mineral base oil and so it could be utilized as lubricant basestock in the formulation of high added value biobased lubricants for special and environmentally sensitive applications. • From a technical point of view, Lunaria oil appears to be an interesting feedstock for the production of biodiesel and especially biobased lubricant basestock. Currently, the limited knowledge on systematic culture potential cannot help assessing the viability of the crop from an economic point of view. Optimization of cultivation techniques of L. annua – especially the annual species – in the near future, could contribute to the successful industrialization of Lunaria oil in the non-food sector.
7
Acknowledgments The authors would like to express their appreciation to ELDON’s S.A. – Lubricants company, Greece, for their assistance in measuring some of the biobased lubricant properties.
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