Effect of winterization and plant phenolic-additives on the cold-flow properties and oxidative stability of Karanja biodiesel

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Effect of winterization and plant phenolic-additives on the cold-flow properties and oxidative stability of Karanja biodiesel Dipesh Kumar, Bhaskar Singh

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Department of Environmental Sciences, Central University of Jharkhand, Ranchi 835 205, India

ARTICLE INFO

ABSTRACT

Keywords: Biodiesel Oxidative stability Cloud point Pour point Winterization Natural antioxidant

Poor stability and low-temperature operability are among the major hurdles in the commercialization of biodiesel. The presence of polyunsaturated fatty acid esters renders the fuel susceptible to oxidative attack while the long-chain saturated components limit its utility under low-temperature conditions. In this study, an attempt was made to improve these properties of Karanja biodiesel. Karanja biodiesel synthesized via a two-step alkalicatalyzed process exhibited poor stability and cold-flow properties. Karanja biodiesel was winterized to limit the content of long-chain saturates, and it had a favorable effect on the cloud and pour point of the fuel. Removal of long-chain saturated components led to an enrichment of the fuel in unsaturated fractions, and as a result, the stability of the fuel further deteriorated. For improving, the stability of the fuel T. cordifolia stem extract rich in phenolic constituents was added to winterized biodiesel. The combined treatment of winterization and phenolicrich extract (1000 ppm) had a pronounced effect on fuel quality as it led to a reduction in the cloud (by 7 °C) and pour point (by 6 °C) and substantially improved the stability of the fuel under accelerated oxidative test conditions. The ASTM D6751, IS 15607, and EN 14214 specifications for the minimum induction period for blendstock biodiesel were satisfied. Thus, coupling the use of winterization and natural antioxidants offers novel opportunities in improving the fuel properties and acceptability of biodiesel in an efficient, economical, and environment-friendly manner.

1. Introduction Biodiesel, as defined by the ASTM D6751, IS 15607 and EN 14214 is “a mixture of mono-alkyl (methyl) esters of long-chain fatty acids derived from vegetable oil or animal fat.” However, several other oleaginous materials including single cell (microalgae and yeast) oil [1,2], municipal sludge [3], waste cooking oil [4], etc. have also been transesterified for the production of biodiesel [5]. The single most significant hindrance in the direct utility of vegetable oil (or other oleaginous matter) as an alternative fuel for compression ignition engine is its higher viscosity [6,7]. To circumvent the challenges arising from high viscosity of triglyceride-rich oleaginous matter, the utility of direct use and blending, microemulsion, pyrolysis, transesterification, and hydroprocessing have been reported [8,9]. Transesterification remains to be the most commonly adopted strategy for the purpose [10,11]. Hydroprocessing of oleaginous matter to renewable diesel (also termed as green diesel or hydroprocessed vegetable oil) is emerging as an attractive approach, but its overall feasibility is yet to be clearly understood [12,13]. Biodiesel is an attractive alternative fuel for climate change mitigation commitments and for meeting the ever-increasing

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demand for fuel [14,15]. The past few decades have witnessed a surge in demand for biodiesel as several countries have incorporated policy mandates for biodiesel in transport energy mix [16,17]. Renewability, cleaner emission profile, biodegradability, miscibility in diesel, and comparable fuel properties are among the significant advantages of biodiesel [18]. Some of the properties of biodiesel, including its viscosity, density, cold flow properties, oxidative stability, cetane number, etc. are intricately related to the fatty acid profile of the feedstock [19]. The widespread acceptability of the fuel is limited by its poor stability and inferior performance under low-temperature conditions [20,21]. The saturated fractions of biodiesel have a favorable effect on its stability, but they (particularly the long-chain saturates) can potentially compromise the low-temperature performance (cold-flow properties; CFPs) of the fuel. Long-chain saturated esters have a very high melting point, and they tend to crystallize under conditions of low temperature [22]. The highest temperature at which these crystals are first detected under a strictly controlled cooling test is referred as the cloud point (CP), and if the temperature continues to fall beyond the CP the crystals continue to agglomerate and grow in size until a point is reached when

Corresponding author. E-mail address: [email protected] (B. Singh).

https://doi.org/10.1016/j.fuel.2019.116631 Received 25 June 2019; Received in revised form 7 November 2019; Accepted 9 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Dipesh Kumar and Bhaskar Singh, Fuel, https://doi.org/10.1016/j.fuel.2019.116631

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the fuel can no longer be pumped (pour point; PP) [23]. Thus, biodiesel containing high levels of long-chain saturated components may not perform ideally under certain climatic conditions. Blending (in diesel or biodiesel containing high levels of unsaturates), use of cold-flow improvers, use of long/branched-chain allylic group donors (alcohols), and selective crystallization-fractionation (winterization) have received considerable attention as strategies for improving the low-temperature performance of fuel [22]. The points of unsaturation in biodiesel, on the other hand, are readily attacked, leading to a chain of radical-mediated oxidative reactions in which the parent molecule is broken down to a range of small chain compounds. The condition is aggravated under certain conditions of storage, such as exposure to air, sunlight, moisture, metals, and other extraneous materials [24]. Thus, there are advantages and trade-offs associated with either type (saturated and unsaturated) of fatty acids [19]. Biodiesel is commonly used in its blended form (usually as B5-B20) with diesel. The ASTM D975 (specifications for diesel) allows a blend of up to 5 vol% of biodiesel (meeting ASTM D6751 specifications), while for regulating higher blend proportions (> B5 to ≤B20) the ASTM 7467 specifications are in place. Similarly, the Indian specification for diesel (IS 1460-16) allows up to 7 vol% of biodiesel, and the specifications for higher blends are under consideration. Since the low-temperature operability and stability requirements for diesel are more stringent, any blend should only be prepared with biodiesel compliant with the prescribed specifications (ASTM D6751/IS15607/EN 14214). For instance, the minimum stability requirement for B5 and > B5 to ≤B20 blends in the American specification is 24 and 6 h, respectively, while the Indian specification for blends up to 7 vol% (IS 1460-17) is a minimum of 20 h (Bharat Stage IV and VI). The same (Indian) specification suggests that the PP for blends up to 7% should be a maximum of 3 and 15 °C in winter and summer, respectively. The American, European and Indian specifications for blendstock biodiesel (B100) include a minimum induction time of 3, 6 and 8 h, respectively. These standards do not specify any limits for CP and PP of biodiesel (B100), but the same is to be reported to the consumer. Diesel generally performs better than biodiesel under low-temperature conditions (has lower CP and PP), and hence, biodiesel blends prepared from samples enriched in long-chain saturates and high-melting components are likely to pose challenges. Studies on the effect of blending biodiesel in diesel invariably suggest that for lower blends (up to B5), the CP largely remains unchanged, and as the concentration of biodiesel in the blend increases, there is a proportionate increase in the CP of the blend. Palm oil biodiesel (CP = 16 °C) blends of B5 had the same CP as that of diesel (-5°C), while B20 exhibited a CP of 0 °C [25]. Likewise, a strong inverse correlation between the concentration of biodiesel in the blend and stability of the blend is reported, and higher blends invariably lead to poor stability characteristics [26]. Blends prepared with biodiesel containing high levels of unsaturates (and those not compliant with minimum stability requirement) can drastically lower the stability of the blend even for low blend proportions (> B10) [27,28]. Winterization involves selective crystallization of long-chain saturated components under a strictly controlled condition of cooling, and the crystallized fractions are then removed [28]. Selective removal of saturated fractions enriches the fuel in unsaturated mono-alkyl esters [19]. Unsaturated fractions have a favorable effect on CFPs and viscosity, but it renders the fuel susceptible to oxidative damage [19]. Of particular concern are the fatty acid esters having multiple double bonds (polyunsaturates) [29]. Poor stability hampers the long-term storage potential of the fuel. In order to circumvent such problems, several synthetic radical scavengers (antioxidants) are being added to biodiesel, and their effect on the stability of the fuel is assessed under accelerated oxidative conditions (EN 15751). These additives are very effective in low dosage [30], but many of them are very expensive, nonrenewable, toxic, and have shown carcinogenic effects [31,32]. More recently, numerous research investigations have established the

antiradical properties of plant phenolics, and therefore such compounds have found increasing applications in several fields, including their use as an antioxidant additive for biodiesel [33–35]. In our previous work, we had optimized the extraction conditions for phenolic compounds from the stem of T. cordifolia [36]. Under the optimized set of conditions, the stem extract contained high levels of phenolic compounds (64 mg GAE g−1 biomass) and exhibited excellent antiradical activity when tested against a model radical (DPPH). The stem extract could delay the onset of Karanja biodiesel oxidation by several hours, and at a loading of 600 ppm, the American (ASTM D6751; 3 h min.) and European (EN 14214; 6 h min.) biodiesel specifications were met. The Indian specification for the minimum induction time (IS 15607-16; 8 h) is more stringent than the ASTM and EN limits for blendstock biodiesel, and the same was met at an extract loading of 1000 ppm (induction time of 8.56 h) [36]. Since the removal of long-chain saturated components through winterization is expected to improve the CFPs of the fuel at the expense of its stability, the utility of plant phenolics as an antioxidant additive for winterized biodiesel is worth investigation. In this analysis, we, for the very first time, investigate the combined effect of winterization and plant phenolics on the CFPs and stability of Karanja biodiesel. 2. Materials and method 2.1. Materials Methanol (≥99.5%), Sulfuric Acid (≥98%), and anhydrous Sodium Sulphate (≥99%) were sourced from Merck, India, while Sodium Methoxide (≥98%) was procured from Loba Chemicals, Mumbai. Karanja oil was sourced from a soap manufacturing industry in Ranchi. 2.2. Synthesis of biodiesel The Karanja biodiesel was synthesized as per the conventional twostep process for feedstocks rich in the content of free fatty acids (FFA) [36]. The Karanja oil was heated at 105 °C overnight for the removal of moisture. The transesterification of the Karanja oil was preceded by its esterification. The esterification experiment was aimed at limiting the content of FFA in Karanja oil to levels acceptable for alkali-catalyzed transesterification (< 4.0 mg KOH g−1 Oil) [37,38]. The acid value of the Karanja oil was estimated as per the ASTM protocol (D 664) through acid-base titrimetry, and the content of FFA was estimated to be 17 mg KOH g−1 oil. The esterification setup consisted of a single neck flat bottom flask (50 mL) fitted with a reflux condenser. The flask was dipped in an oil bath maintained at a constant temperature of 60 °C ( ± 1.5 °C) with the help of a magnetic stirrer cum hot plate (Tarson 6090). The pre-heated Karanja oil was added to the glass reactor to which a mixture of methanol (oil to methanol molar ratio of 1:6) and sulphuric acid (1 wt% of oil) was later added [39]. The reactor contents were vigorously stirred at 800 rpm with the help of a Teflon rotor, and the reaction was allowed to continue under the reflux conditions (60 °C) of methanol for 1 h. It was followed by the density-driven separation of water (bottom layer), esterified oil (middle layer), and methanol (topmost layer) in a separatory funnel overnight. The bottom layer of water was drained out from the bottom, followed by the heating of the esterified oil and methanol mixture at 105 °C for 2 h for the removal of residual moisture and un-reacted methanol. The esterified oil was allowed to cool and was subsequently subjected to the analysis of FFA content. The esterification led to an FFA content of 3.5 mg KOH g−1 esterified oil. The transesterification of Karanja oil involved the same reaction setup as its esterification. The pre-heated (65 °C) Karanja oil was added to the glass reactor to which a pre-treated (heated at 65 °C for 0.25 h) mixture of methanol (oil to alcohol molar ratio of 1:10) and sodium methoxide (1.5 wt% of oil) was added. The reaction was allowed to 2

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continue for 1.5 h, after which the contents were transferred to a separatory funnel for the separation of biodiesel and glycerol layers overnight. The bottom layer of glycerol was removed, and the crude supernatant was subjected to repeated cycles of gentle hot water wash until the effluent was visibly clear to eliminate the traces of un-used catalyst, unreacted methanol, and residual glycerol. It was followed by the heating (at 105 °C for 5 h) of the washed biodiesel for the removal of moisture. Traces of moisture present were removed with the help of anhydrous sodium sulfate. 2.3. Characterization of biodiesel and assessment of fuel properties The estimation of feedstock conversion to biodiesel was based on Nuclear Magnetic Resonance (NMR) Spectroscopy. Briefly, 3.0 mg of the sample was dissolved in deuterated chloroform to which tetramethylsilane (TMS) was added as the internal standard. The 1H NMR spectra of transesterified Karanja oil was recorded on the JEOL 400 MHz spectrophotometer. The conversion was estimated as per the relation provided below (Eq. (1)) [40,41].

Conversion (%) =

2A1 × 100 3A2

(1)

where A1 is the integration of the methoxy protons in biodiesel (a mixture of Fatty Acid Methyl Esters; FAME) resonating at 3.6 ppm, and A2 is the integration value of the Methylene protons in α carbon in the fatty acid chain resonating at 2.3 ppm. Some of the essential properties of Karanja biodiesel, including its viscosity, density, acid value, and moisture content were estimated as per the standard ASTM protocols. The fatty acid profiling of Karanja biodiesel was performed as per EN 14103 on Thermo, Trace-Ultra gas chromatograph. To a 100 mg sample of biodiesel, 100 mg of methyl heptadecanoate (C17:0) was added, and the mixture was then dissolved in 10 mL of toluene. 1 µL of the sample was split-injected into an HP-INNOWax (polyethylene glycol) column through an injection port maintained at 250 °C. Helium (at 1 mL min−1) was used as the carrier gas. The column temperature was initially programmed to hold at 60 °C for 2 min and was then ramped at a rate of 10 °C min−1 until 200 °C. Finally, the temperature was raised up to 240 °C at 5 °C min−1 and was held constant for 7 min. The eluting components were detected with the help of a flame ionization detector maintained at 250 °C. The oxidative stability of Karanja biodiesel was estimated by Rancimat analysis in line with the standard EN 15715 protocol on Metrohm 873 Biodiesel Rancimat. Briefly, 3.0 g of biodiesel sample was maintained at a temperature of 110 °C to which air was pumped continuously at 10 L h−1, and the analysis was continued until an inflection point on the conductivity vs. time plot was recorded. The time until the inflection point is recorded (induction period) represents the stability of the fuel. The CFPs (CP and PP) of the fuel were measured on a manual cold flow apparatus (Swastik Scientific; T 8025). Any moisture present in the test specimen was removed by heating followed by subsequent adsorption on anhydrous sodium sulfate, and the specimen was allowed to cool until room temperature. The test jar loaded with biodiesel specimen was capped with a rubber cork, which in turn was centrally bored for the test thermometer. The temperature of the test bath was maintained at 0 ± 1.5 °C using crushed ice and water as a coolant. The test jar was placed in the test bath. The specimen in the test jar was allowed to cool, and the specimen was periodically examined at an interval of 1 °C fall in temperature to detect the appearance of a cloud. The PP of biodiesel specimen was estimated as per the ASTM protocol (D 97) using the same apparatus that was used for the analysis of CP. The test bath was maintained at a temperature of 0 ± 1.5 °C using ice and water as a coolant. The test jar containing the specimen was lowered in the test bath and was periodically analyzed. The examination of the test jar was started at 15 °C with periodical visualization for the

Fig.1. Cloud point apparatus used in the present investigation.

cessation of any movement when the test jar was kept horizontally for 5 s. The visualization was carried out at every 3 °C decrease in the temperature of the test specimen. The cold flow apparatus used in the present investigation is shown in Fig. 1. 2.4. Winterization of Karanja biodiesel The winterization experiments were performed in the same test jar that was used for the estimation of CFPs. The test specimen was maintained at a temperature of 5 °C lower than its CP for 15 min. It allowed the cloud to grow in size at a fixed rate, and then the specimen was quickly poured into another test jar maintained at the same temperature. The entire liquid fraction was transferred, while the crystallized fraction remained in the 1st jar. The supernatant layer was maintained in the 2nd test jar for another 15 min, and the formation of any crystal was carefully monitored. The test jars were recovered and subjected to gravimetric analysis. The effect of winterization on the fuel properties and the fatty acid profile of Karanja biodiesel were estimated as per the methodology described in Section 2.3. 2.5. Extraction of phenolic compounds from the stem of T. cordifolia The extraction of phenolic compounds from the stem of T. cordifolia was based on our previously published work [36]. The extraction was performed under the optimized conditions using aqueous methanol (85:15 ratio of methanol and de-ionized water) as the extraction solvent at a solvent to solute ratio of 5:1 (vol/wt), extraction temperature of 40 °C and extraction time of 5 h under vigorous stirring at 1000 rpm. Extraction under these sets of conditions led to an extract rich in phenolic compounds (64 mg GAE g−1 biomass) and excellent radical scavenging activity [36]. The extract was equally efficient in delaying the onset of biodiesel oxidation, and at a loading of 1000 ppm, all the major national/international requirements for the stability of 3

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2.6. Combined effect of winterization and stem extract on fuel properties

Table 1 Gradient elution design used for RP-HPLC analysis of stem extract. Time (in min)

0–10 10–30 30–50 50–60 60–70 70–105 105–110 110–120

The biodiesel sample obtained after its fractional crystallization (winterization) was loaded with different concentrations of the stem extract (100–1000 ppm) and was once again subjected to fuel quality tests as per the methodology described in Section 2.3. All the experiments were performed as triplicates, and the results have been presented as their mean value.

Concentration of solvents (in %) A

B

C

100.0 0.0 10.0 20.0 30.0 0.0 100.0 100.0

0.0 100.0 90.0 80.0 70.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 100.0 0.0 0.0

3. Results and discussion 3.1. Characterization of biodiesel and its fuel properties The conversion of Karanja oil to its biodiesel was estimated to be 98.76%, and thus the EN 14214 specification for minimum ester (monoalkyl ester) content in biodiesel was met. The 1H NMR spectra of transesterified Karanja oil (Karanja biodiesel) is shown in the form of Fig. 2. The conversion of Karanja oil to its biodiesel is marked with a characteristic peak between 3.60 and 3.70 ppm for methoxy protons in biodiesel. The presence of all the saturated, oleic, and linoleic acyl groups is characterized by a peak between 0.83 and 0.93 ppm while that for the linolenic acyl group is apparent between 0.93 and 1.03 ppm [43,44]. The characteristic chemical shifts of protons present in different chemical environments in a biodiesel sample are shown in the form of Table 2. The synthesized biodiesel complied with most of the ASTM and EN

blendstock biodiesel were met. In the present investigation, the extract was further characterized by reverse phase-high performance liquid chromatography (RP-HPLC). The analysis was performed on Shimadzu Prominence (LC 20-AD) liquid chromatograph coupled to a UV–VIS SPD-PDA detector as per the conditions described by Proestos et al. [42]. The isothermal analysis involved an RP-C18 column (length of 25 cm, internal diameter of 4.6 mm and a particle size of 5 µm) and a gradient elution design. Three different solvents namely A (aq. glacial acetic acid; 1%), B (aq. glacial acetic acid; 6%) and C (65:30:5 ratio of water, acetonitrile, and glacial acetic acid) were used to aid in effective separation and elution of the components. The gradient elution design is shown in the form of Table 1.

Fig. 2. 1H NMR spectra of transesterified Karanja oil (Karanja Biodiesel). 4

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Table 2 Assignment of chemical shifts in 1H NMR spectra of Biodiesel. S. No

Chemical Shift

1

1 2 3 4 5 6 7 8 9

0.83–0.93 0.93–1.03 1.22–1.42 1.52–1.70 1.94–2.14 2.23–2.36 2.30–2.34 2.70–2.84 3.60–3.70

–CH3 –CH3 –C–CH2–C– –OCO–CH2–CH2– –CH2–CH]CH– ROCO–CH2– HOCO–CH2– ]HC–CH2–CH] –O–CH3

H Type

Position Terminal methyl group in all the saturates, oleic and linoleic acyl groups Terminal methyl group in linolenic acyl group Methylene proton in all the acyl group Methylene protons in β carbon attached to carboxyl group Allylic (methylene) proton Methylene protons in α carbon attached to ester Methylene protons in α carbon attached to acid Methylene protons in Polyunsaturated acids (bis-allylic protons) Methoxy protons in FAME

specifications for blendstock biodiesel (Table 3). However, it failed to comply with the ASTM D6751, IS 15607 and EN 14214 specifications for the stability of the fuel. Moreover, the CP and PP were unacceptably high at 16 and 9 °C, respectively. Thus, the fuel is likely to be troublesome in cold-climatic conditions even for tropical countries like India when the overnight ambient temperature approaches 16–17 °C. Although the fuel can be pumped at temperatures slightly lower than its CP, it can lead to plugging of fuel filters and can cause overnight startup issues. Therefore, it is highly pertinent that these properties of the fuel are improved for its widespread acceptability and commercialization. The fatty acid profile of Karanja biodiesel indicates the presence of high levels of unsaturated components (Table 4 and Fig. 3). Since the 1 H NMR spectroscopy and GC results did not detect any signals of unreacted/partially converted triglyceride/free glycerol (no resonating signals of glyceridic protons/free glycerol in NMR spectra and no overlapping/un-allocated peak in Gas chromatogram), there were sufficient reasons to believe that their content was very low and within acceptable limits. The unsaturated fatty acid esters render biodiesel susceptible to oxidative attack, and the effect is particularly aggravated in the presence of their polyunsaturated counterparts. The poor stability of the fuel can be attributed to the high content of polyunsaturated components (linoleic acid 9.78% and linolenic acid 4.24%). The overall

content of unsaturated fractions stood at 87.69%. In contrast to the stability, the acceptability of the fuel in terms of its low-temperature operability is hampered in the presence of long-chain saturates. A powerful positive correlation between the content of long-chain saturates, and the cold-flow behavior of the fuel has been reported [45,46]. The fatty acid profiling revealed the presence of high melting behenic (C22:0; 1.85%, mp of 57 °C) stearic (18:0; 4.58%, mp of 37.66 °C) and palmitic (C16:0; 5.89%, mp of 28.48 °C) acid methyl esters. Thus the poor low-temperature operability of the fuel could be attributed to the presence of such high melting point fractions. 3.2. Combined effect of winterization and stem extracts The utility of winterization, also known as fractional crystallization in the improvement of the fuel operability under cold climatic conditions, has been previously established [6–9]. During winterization, a significant fraction (91.33%) of the fuel was recovered as the liquid fraction, while the crystallized fraction only constituted 8.67% of the initial sample weight. The crystallized fraction contained the entire mass of behenic and stearic acid methyl esters originally present in Karanja Biodiesel, and the rest (25.95%) was constituted by Palmitic acid methyl ester (Table 4). [48]. As expected, the effect on

Table 3 Fuel properties of Karanja Biodiesel. S. No

Property

Test Method

Karanja Biodiesel

ASTM D6751-15 Specifications

EN 14214-14 Specifications

IS 15607-16 Specifications

1. 2. 3. 4. 5. 6. 7. 8.

Viscosity (at 40 °C; in mm s−1) Density (at 15 °C; in g cm−3) Oxidative Stability (in h) Cloud Point (in °C) Pour Point (in °C) Ester Content (in %) Content of Linolenic Acid (in %) Polyunsaturated Fatty Acids (≥4C]C; in %) Moisture Content (in %) Acid Value (in mg KOH g−1)

D 445 D 1298 EN 15751 D 2500 D 97 1 H NMR EN 14103 EN 14103

4.7 0.868 2.65 16 9 98.76 0

1.9–6.0 0.86–0.9 3 (min.) Report to customer Report to customer not specified not specified not specified

3.5–5.0 0.86–0.9 6 (min.) Report to customer Report to customer 96.5 (min.) 12 (max.) 1 (max.)

3.5–5.0 0.86–0.9 8 (min.) Report to customer Report to customer 96.5 (min.) 12 (max.) 1 (max.)

D 2709 D 664

0.03 0.20

0.05 (max.) 0.50 (max.)

0.05 (max.) 0.50 (max.)

0.05 (max.) 0.50 (max.)

9. 10.

Table 4 Abundance of different methyl esters in unmodified and winterized Karanja Biodiesel and their melting points. S. No

Retention Time (Min)

Common Name

Lipid Number

Abundance in unmodified biodiesel (%)

Melting Point of methyl esters (°C ± s.d) [48]

Abundance in winterized biodiesel (%)

1 2 3 4 5 6 7 8

19.497 21.056 21.877 22.259 23.602 24.873 25.229 28.980

Palmitic Acid Linoleic Acid Oleic Acid Stearic Acid Gondoic Acid Linolenic Acid Erucic Acid Behenic Acid

16:0 18:2 18:1 18:0 20:1 18:3 22:1 22:0

5.890 9.780 46.08 4.580 6.030 4.240 21.550 1.850

28.48 ± 0.44 −43.09 ± 0.71 −20.21 ± 0.51 37.66 ± 0.25 −7.79 ± 0.34 −57.1 ± 0.42 8.47 ± 0.15 53.22 ± 0.26

5.095 11.225 47.525 nd 7.475 5.685 22.995 nd

nd = not detected. 5

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Fig. 3. Gas chromatogram of Karanja Biodiesel.

winterization on the CFPs was well pronounced. The CP and the PP of the biodiesel came down from the initial levels of 16 and 9 °C to 10 and 3 °C, respectively. The relative abundance of FAMEs in unmodified and winterized Karanja biodiesel and their melting temperature are listed in Table 4. Perez et al. tested several winterization strategies to improve the cold-filter plugging point (CFPP) of peanut biodiesel and with a reduction in CFPP by 25 °C and biodiesel loss of 8.93%, the crystallization fractionation with methanol was identified to be the best strategy [28]. Lee et al. attempted the winterization of soybean biodiesel (diluted in hexane) having a crystallization onset temperature (COT) of 3.7 °C and achieved a maximum reduction in COT by 9.5 °C with a yield of 77% [49]. Beef tallow-based biodiesel having a saturated ester content of 86.91% was subjected to winterization, and with a reduction in saturated fractions by 13.35% a minimum COT of 16.3 °C was attained in six sequential steps [47]. In contrast to these studies, our work did not involve any solvent, and with a biodiesel loss of only 8.6%, a considerable reduction in the CP (by 7 °C) and PP (by 6 °C) was attained. Several potential applications for the high melting components recovered after winterization can be envisaged that may include an application as a solvent, lubricant, phase-change material, blendstock for biodiesel highly enriched in polyunsaturated fractions, etc. [28]. Winterization improved the low-temperature operability range of the fuel; however, the removal of long-chain saturated components further enriched the fuel in unsaturated fractions, and hence its effect on the stability of the fuel deserved attention. The stability of the winterized biodiesel (2.49 h) was slightly lower than the unmodified fuel (2.65 h). We have previously shown the utility of T. cordifolia stem extracts as an antioxidant additive for Karanja biodiesel (unmodified), where it could augment the stability of the fuel to comply with the American, European and the Indian specifications at a loading of 1000 ppm [36]. Similar results were obtained for ginger [33], moringa [35] potato peel [50], and coriander [51] extracts, but the radical scavenging activity of T. cordifolia is slightly superior. Moreover, the utilization of edible resources for such purposes raises several sustainability-related questions. RP-HPLC analysis of the extract helped identify six major phenolic compounds in the stem of T. cordifolia, including three phenolic acids (tannic, caffeic and ferulic) and three flavonoids (naringenin, apigenin, and luteolin). The antioxidant activity of these constituents has already been established (Table 5). Incremental loadings of stem extracts led to a proportionate increase in induction time. Although the effect of stem extracts on winterized biodiesel was slightly less pronounced (in comparison to unmodified biodiesel), it could still satisfy the specifications of a minimum induction time requirement of 8 h at a loading of 1000 ppm. The conductivity vs. time plot for the winterized fuel loaded with 1000 ppm of extract indicated an inflection point after 8.04 h. For unmodified biodiesel containing an identical concentration of extracts, the induction time was attained at 8.56 h [36]. After the recovery of a fraction of saturated

Table 5 Phenolic compounds identified in the stem extract of T. cordifolia. S.No

Retention Time

Phenolic Compound

Nature of the compound

Antioxidant Activity [Reference]

1 2 3 4 5 6

3.361 46.602 79.953 88.264 98.941 100.117

Tannic Acid Caffeic Acid Ferulic Acid Naringenin Apigenin Luteolin

Phenolic Acid Phenolic Acid Phenolic Acid Flavonoid Flavonoid Flavonoid

[52,53] [54,55] [56,57] [58,59] [60,61] [62,63]

components, a marginal decrease in the induction time of Karanja biodiesel was not surprising. The effect of winterization and loading stem extract in winterized biodiesel (1000 ppm) on the fuel properties of Karanja biodiesel are shown in the form of Table 6. Winterization had a noticeable effect on several of the important fuel properties of biodiesel, including its viscosity, density, stability, and cold flow properties. It had a favorable effect on the viscosity and cold flow properties of the fuel while the stability and the density of the fuel were adversely affected. A small increment in the viscosity and CP of the fuel was observed after the addition of stem extract (Table 5). It could well be attributed to the incomplete solubilization of the extract. The increase in CP could be explained in the light of the fact that the solubility of extract decreases with a decrease in temperature, and it could serve as a condensation platform for the long-chain saturated constituents of the fuel. However, unlike the stability, the quantum of change for other properties was only marginal. Moreover, several of the studies have suggested a positive role of radical scavenging antioxidants in checking the emission of oxides of nitrogen (NOx) from biodiesel fuelled engines [64-66]. The property is attributed to the curtailment of the radical-mediated formation of NOX (Fenimore mechanism) by radical scavenging species. The current study clearly demonstrates the utility of plant extracts and winterization in the simultaneous improvement of stability and CFPs of biodiesel. Coupling the use of winterization and natural antioxidants offers novel opportunities in improving the fuel properties and acceptability of biodiesel in an efficient, economical, and environment-friendly manner. 4. Conclusion In this investigation, the utility of winterization and the combined effect of winterization and T. cordifolia stem extract on the oxidative stability and CFPs of Karanja biodiesel were assessed. The said properties of unmodified Karanja biodiesel did not meet the prescribed specifications. The winterization of Karanja biodiesel could improve the CFPs of the fuel, but it led to a further deterioration of its stability. T. cordifolia stem extract as an antioxidant additive could substantially augment the induction period of the winterized fuel. The combined treatment of Karanja biodiesel led to a fuel having an acceptable cloud 6

Fuel xxx (xxxx) xxxx

and pour point, and at an extract loading of 1000 ppm (induction time of 8.04 h), all major international specifications for the stability of the fuel were satisfied. Overall, with a product loss of only 8.67%, the combined treatment offers a novel prospect for the simultaneous improvement of fuel stability and low-temperature operability.

3.5–5.0 0.86–0.9 8 (min.) Report to customer Report to customer

IS 15607–16

D. Kumar and B. Singh

3.5–5.0 0.86–0.9 6 (min.) Report to customer Report to customer

Acknowledgments

1.9–6.0 0.86–0.9 3 (min.) Report to customer Report to customer

Mr. Dipesh Kumar gratefully acknowledges the University Grants Commission (UGC), New Delhi, for providing Senior Research Fellowship. We acknowledge BIT Mesra, Ranchi, and CSIR-CSMCRI Bhavnagar for extending analytical facilities. References

4.3 ± 0.22 0.871 ± 0.17 2.49 ± 0.02 9±1 3±1

4.4 ± 0.35 0.871 ± 0.21 8.04 ± 0.04 10 ± 1 3±1

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Viscosity (at 40 °C; in mm s ) Density (at 15 °C; in g cm−3) Oxidative Stability (in h) Cloud Point (in °C) Pour Point (in °C)

−1

1. 2. 3. 4. 5.

ASTM D6751-12

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

4.7 ± 0.32 0.868 ± 0.10 2.65 ± 0.04 16 ± 1 9±1

Winterized Biodiesel + stem extract (1000 ppm) Winterized Biodiesel Unmodified Biodiesel Property S. No

Table 6 Fuel properties of unmodified Karanja biodiesel, winterized Karanja biodiesel, and winterized biodiesel containing 1000 ppm of stem extract.

Specifications

EN 14114–12

Declaration of Competing Interest

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