- Email: [email protected]
Pumpkin (Cucurbita pepo L.) seed oil as an alternative feedstock for the production of biodiesel in Greece
ARTICLE IN PRESS BIOMASS AND BIOENERGY
33 (2009) 44 – 49
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Pumpkin (Cucurbita pepo L.) seed oil as an alternative feedstock for the production of biodiesel in Greece P. Schinas, G. Karavalakis, C. Davaris, G. Anastopoulos, D. Karonis, F. Zannikos, S. Stournas, E. Lois Laboratory of Fuels and Lubricants Technology, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Street, Zografou Campus, 157 80 Athens, Greece
ar t ic l e i n f o
abs tra ct
Article history:
In recent years, the acceptance of fatty acid methyl esters (biodiesel) as a substitute to
Received 19 December 2006
petroleum diesel has rapidly grown in Greece. The raw materials for biodiesel production in
Received in revised form
this country mainly include traditional seed oils (cotton seed oil, sunflower oil, soybean oil
9 April 2008
and rapeseed oil) and used frying oils. In the search for new low-cost alternative feedstocks
Accepted 10 April 2008
for biodiesel production, this study emphasizes the evaluation of pumpkin seed oil. The
Available online 2 June 2008
experimental results showed that the oil content of pumpkin seeds was remarkably high
Keywords: Pumpkin seed oil Biodiesel Alkaline transesterification Fatty acid methyl esters Biodiesel properties
(45%). The fatty acid profile of the oil showed that is composed primarily of linoleic, oleic, palmitic and stearic acids. The oil was chemically converted via an alkaline transesterification reaction with methanol to methyl esters, with a yield nearly 97.5 wt%. All of the measured properties of the produced biodiesel met the current quality requirements according to EN 14214. Although this study showed that pumpkin oil could be a promising feedstock for biodiesel production within the EU, it is rather difficult for this production to be achieved on a large scale. & 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Biodiesel is made up of mono-alkyl esters of fatty acids derived from vegetable oils, used frying oils and animal fats. Most of the commercial biodiesel production is performed through a transestrification reaction of triglycerides in vegetable oils and animal fats with mono-alkyl alcohols in the presence of homogeneous base or acid catalysts [1]. The choice of the fat or oil to be used in biodiesel production is a process involving both chemistry and economics. With respect to process chemistry, the greatest difference among the choices of fats and oils is the amount of free fatty acids (FFAs) that is associated with the triglycerides. The iodine value is also a major parameter that associates with the choice of oil. According to the European Standard EN 14214, the iodine value is limited to 120. This parameter Corresponding author. Tel.: +30 210 7723213; fax: +30 210 7723163.
E-mail address: [email protected] (G. Karavalakis). 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.04.008
excludes the use of biodiesel from sunflower oil, which is one of the main oleaginous crops cultivated in the southern countries of the EU. Along with sunflower oil, tobacco seed oil, which is one of the most dynamic, low-cost oleaginous crops in Greece, is also excluded. New low-cost oil crops are needed to produce economical oils suitable for biodiesel production. One of the possible alternative oil crops for the Mediterranean area is pumpkin seed (Cucurbita pepo L.). To the authors’ best knowledge, studies on pumpkin seed oil as the feedstock for methyl ester production were never conducted. In the search for alternative oils for biodiesel production, pumpkin seed oil presents a promising choice; however, this cannot be regarded as a massive raw material for biodiesel production on a large scale. The seeds of pumpkin are a common snack food in several cultures, including Greece, and pumpkin seed
ARTICLE IN PRESS BIOMASS AND BIOENERGY
oil is a common salad oil with limited application for cooking. It is also used for medicinal purposes. Important considerations in selecting feedstocks for biodiesel production include (1) cost, (2) variability in quality and the chemical content feedstock, (3) regular availability of the feedstock, (4) flexibility to increase supply and (5) cost of transport and pretreatment. At present, it is not clear how pumpkin oil measures against most of these factors. The cost of raw pumpkin oil is rather difficult to be estimated since there is no commercial production in Greece and it has only limited use as cooking oil in some countries. It is a fact that the supply and cost of various oils largely depend on the level of demand in another market. In the case of pumpkin seed oil choice, the supply will be determined by the demand for the seeds used as a snack food. Although, increased use of pumpkin as a biodiesel feedstock could drive up the price, it is unlikely to exceed the cost of virgin vegetable oils, such as sunflower oil, corn oil and soybean oil. Even though there is no commercial pumpkin oil production in Greece, with an average seed production of 1.2 t ha1, [2,3] and total area in Greece used for gourd planting is about 4000 ha, an annual seed production of 4800 t can be estimated. In this study, the profile of pumpkin seed oil and its production to fatty acid methyl esters via a base-catalyzed transesterification were investigated. The advantage of pumpkin oil over sunflower oil, soy been oil and rapeseed oil would lie in the oil price. The reason of this price variation is possible due to the fact that pumpkin seed oil is not widely used as edible oil. Pumpkin seed contain 45% oil average (dry weight basis), which is a major advantage over a number of vegetable oils cultivated in Greece. The oil was extracted using a Soxhlet apparatus and was characterized by its physicochemical properties. Furthermore, the intent of this work was to investigate and to determine the influence of the chemical properties of the oil in the transesterification and to examine the quality parameters of the methyl ester according to the European standard EN 14214.
2.
Experimental
2.1.
Pumpkin seed oil extraction
Currently, there is no commercial production of pumpkin seed oil in Greece; hence it was not available in the market. The collected seeds were extracted in laboratory scale, and the procedure is as follows. The seeds were ground to a fine powder and then dried for 2 h at 100 1C. For the continuous extraction of the oil, the Soxhlet extraction apparatus was employed and hexane was used as solvent in the extraction process. The Soxhlet device temperature was kept at 65–70 1C and the overall process lasted 24 h. At the end of the process, the oil was separated from the organic solvent using a rotary vacuum evaporator, dried at 60 1C and weighed. Yield was calculated on dry weight basis.
2.2.
Physicochemical parameters of pumpkin seed oil
The pumpkin seed oil was examined in order to evaluate its use either as a blendstock in automotive or heating diesel fuel
45
33 (2009) 44 – 49
Table 1 – Physicochemical properties of pumpkin seed oil Property
Pumpkin seed oil
Test method
35.6
EN ISO 3104 EN ISO 3675 EN 22719 EN ISO 6245 EN ISO 20846 EN ISO 12937 EN 14111 EN 14104 EN ISO 10370 ASTM D 97 IP 12
Kinematic viscosity (40 1C), mm2 s1 Density (15 1C), kg m3
921.6
Flash point, 1C Sulphated ash content, wt%
4230 o0.01
Sulphur content, mg g1 Water content, mg g1 Iodine number Acid value, KOH, mg g1 Carbon residue, wt% Pour point, 1C Gross calorific value, MJ kg1
2 584 115 0.55 0.1754 12 39.0
or as a suitable raw material for biodiesel production. Its major quality properties using standard test methods are shown in Table 1.
2.3.
Transesterification
The transesterification reaction of pumpkin seed oil was carried out in a 500 mL spherical flask, with anhydrous methanol in molar ratio methanol to oil 6:1, using sodium hydroxide (NaOH) as catalyst. The reaction was carried out at 65 1C for 1 h [4–6]. By the end of the experiment the reaction mixture was transferred to a decanter for glycerol and methyl ester separation, allowing glycerol to separate by gravity for 24 h. Once the two phases were separated, the excess alcohol in each phase was removed by flash evaporation at 90 1C. The methanol was recovered and re-used. The purity level of the biodiesel has strong effects on its fuel properties [7]. Therefore, the methyl ester was purified by washing gently with three volumes of warm deionized water to remove residual catalyst, glycerol, methanol and soap using a centrifuge. A small amount of sulphuric acid (H2SO4) was used in the second washing to neutralize remaining soaps and other catalyst impurities. The washed methyl ester is then dried over the heated anhydrous sodium sulphate (Na2SO4). Solid traces from the methyl ester were removed with a filtration process. The schematic of the complete oil extraction and oil transesterification process flow diagram is shown in Fig. 1.
2.4.
Pumpkin seed oil methyl ester analysis
The fuel properties of pumpkin seed oil methyl ester, along with the test methods adopted for the fuel property analysis, are given in Table 2. Most of the fuel properties of the pumpkin methyl ester are quite comparable to those of other methyl esters reported in the literature, and fulfil the
ARTICLE IN PRESS 46
BIOMASS AND BIOENERGY
Pumpkin Seeds
Oil Extraction
Grinding
Oil (T 50 °C)
33 (2009) 44 – 49
MeOH+NaOH
Transesterification (T 65 °C)
Alcohol Removal
Methyl Ester Washing
Glycerol Removal
Drying Process
EN 14214 FAME
Catalyst Removal
Fig. 1 – Pumpkin oil extraction and transesterification process flow diagram. Table 2 – Properties of pumpkin oil methyl ester Property Kinematic viscosity (40 1C), mm2 s1 Density (15 1C), kg m3 Flash point, 1C Sulphur content, mg g1 Water content, mg g1 Iodine number Acid value, KOH, mg g1 Cold filter plugging point (CFPP), 1C Gross calorific value, MJ kg1 Group I metals Na+K, mg g1
Phosphorus content, mg g1 Monoglyceride content, %(m/m) Diglyceride content, %(m/m) Triglyceride content, %(m/m) Total glycerol, %(m/m) Free glycerol content, %(m/m)
Pumpkin oil methyl ester
EN 14214 limits
Test methods
4.41 883.7 4120 2 490 115 0.48 9 38.08 1.2, o0.5
3.50–5.00 0.860–0.900 120 min 10 max 500 max 120 max 0.5 max +5 max 5 max
EN ISO 3104 EN ISO 3675 EN 22719 EN ISO 20846 EN ISO 12937 EN 14111 EN 14104 EN 116 IP 12 EN 14108 EN 14109
3 0.47 0.15 0.14 0.16 0.00
10 max 0.80 max 0.20 max 0.20 max 0.25 max 0.02 max
European Standard EN 14214. Glyceride analysis was performed with a Carlo Erba Instrument—GC 3000 Series (Fig. 2).
2.5.
Fatty acid composition of pumpkin oil methyl ester
Fatty acid composition of pumpkin seed oil methyl ester was also determined by gas chromatography analysis. The fatty acid profile of the methyl ester was identified and quantified as shown in Table 3.
3.
Results and discussion
3.1.
Pumpkin oil characterization
The oil content of pumpkin seed ranged from 42% to 45% on dry weight basis. This may result in lower operation costs
EN EN EN EN EN EN EN
14107 14105 14105 14105 14105 14105 14106
compared to some other oilseeds, such as soybeans and cottonseeds, which have average oil contents of only 20% and 14%, respectively. Lower operation costs result from higher oil percentage mainly due to less capacity needed for the extruder and oilseed press.
3.2. Effect of pumpkin oil properties on the transesterification reaction The two major quality parameters that influence the production process of biodiesel are the FFAs and water content. Several studies showed that the raw oil acid value should be less than KOH 1.0 mg g1 and that all raw materials should be anhydrous (water content o0.3%) [8,9]. If the above requirements are not met it is still possible to produce biodiesel, but the overall yield of the reaction is significantly reduced due to the deactivation of the catalyst and the formation of soaps.
ARTICLE IN PRESS BIOMASS AND BIOENERGY
47
33 (2009) 44 – 49
Fig. 2 – Glyceride analysis of the pumpkin oil methyl ester.
Table 3 – Fatty acid composition of pumpkin seed oil methyl ester Number of carbons 8 10 12 14 16 16 17 17 18 18 18 18 18 18 20 20 22 22 24
Fatty acid
Chemical structure
Weight %
Caprylic Capric Lauric Myristic Palmitic Palmitoleic Heptadecanoic Heptadecenoic Stearic Oleic Linoleic Linolenic Elaidic C18:2+C18:3 trans Eicosanoic Eicosenoic Behenic Erucic Lignoceric
CH3(CH2)6COOH CH3(CH2)8COOH CH3(CH2)10COOH CH3(CH2)12COOH CH3(CH2)14COOH CH3(CH2)5CH¼CH(CH2)7COOH CH3(CH2)15COOH CH3(CH2)6CH¼CH(CH2)7COOH CH3(CH2)16COOH CH3(CH2)7CH¼CH(CH2)7COOH CH3(CH2)3(CH2CH¼CH)2(CH2)7COOH CH3(CH2CH¼CH)3(CH2)7COOH CH3(CH2)7CH¼CH(CH2)7COOH CH3(CH2)3(CH2CH¼CH)2(CH2)7COOH+CH3(CH2CH¼CH)3(CH2)7COOH CH3(CH2)18COOH CH3(CH2)8CH¼CH(CH2)8COOH CH3(CH2)20COOH CH3(CH2)7CH¼CH(CH2)11COOH CH3(CH2)22COOH
o0.01 o0.01 0.01 0.11 12.51 0.15 0.08 0.04 5.43 37.07 43.72 0.18 0.03 0.04 0.39 0.11 0.12 0.02 0.06
High acid values may be corrected by the addition of sodium hydroxide or by heterogeneous acid catalysts [10]. In this study, the amount of FFAs measured in the oil was 0.27% while the water content of the oil was rather low (500 mg g1). Based on these properties, the selected catalyst for the reaction was NaOH and the molar ratio of methanol to oil was 6:1. From the GC analysis, it was observed that no soaps
or other impurities were produced during the reaction process. Although the iodine number is not included in the properties that influence the transesterification reaction, it should be considered in determining the oil of choice. The specified limit for this parameter is 120 according to the FAME standard EN 14214. In this sense, methyl esters obtained
ARTICLE IN PRESS 48
BIOMASS AND BIOENERGY
from sunflower oil, soy oil or tobacco seed oil cannot meet this specification because of their high proportion of unsaturated chains. However, if commercial production of pumpkin oil is desired, the iodine number will not be a constraint.
3.3.
Biodiesel yield
In order to estimate the biodiesel yield after the reaction and separation stages, the methyl ester weight yield, relative to the initial amount of vegetable oil, was worked out. The calculated methyl ester concentration was nearly 97.5 wt%, and according to this result, the alkaline transesterification reaction was completed. However, if there are no side reactions, the biodiesel weight yields, relative to the initial amount of pumpkin oil, should be nearly 99–100 wt%. In this sense, two possible side reactions could occur, the saponification of triglycerides or neutralization of the FFAs of the vegetable oil. Both of them produce soaps and, therefore, decrease the biodiesel yield. FFA neutralization could not be substantial in the present investigation, since the acid number of the oil was only KOH 0.55 mg g1.
3.4.
Characterization of biodiesel
The standard for biodiesel states that the fuel should have a density between 860 and 900 kg m3. Density is an important parameter for diesel fuel injection systems. The results obtained showed that the produced methyl ester was within the specification limits. Viscosity represents flow characteristics and the tendency of fluids to deform with stress. One of the main reasons for processing vegetable oils for use in internal combustion engines is to reduce the viscosity, thereby improving fuel flow characteristics. Even more than density, kinematic viscosity is an important parameter regarding fuel atomization as well as fuel distribution [11]. For biodiesel to be used in diesel engines, the viscosity must be between 3.5 and 5.0 mm2 s1. Pumpkin seed oil methyl ester had a viscosity of 4.41 mm2 s1. Viscosities above the specification limit can be attributed to the incomplete reaction or to the inefficient purification steps of the process, leaving glycerine in the ester phase. The iodine number is an index of the number of double bonds in biodiesel, and therefore is a parameter that quantifies the degree of unsaturation of biodiesel. This property greatly influences oxidation stability and the polymerization of glycerides. This can lead to the formation of deposits formed in diesel engines injectors. The iodine number is directly correlated to biodiesel viscosity, cetane number and cold flow characteristics (cold filter plugging point). At high saturation, the cetane number is improved while poor low-temperature qualities may be observed. In this work, the iodine number of the produced methyl ester was 115, below the specification maximum limit [12]. Fuel contaminated with water can cause engine corrosion or may cause a reversion of fatty acid methyl esters to fatty acids, which can lead to filter plugging [13]. Therefore, EN 14214 imposed a maximum content of 500 mg g1 of water in biodiesel. In this study, the produced methyl ester was routinely dried over anhydrous sodium sulphate. This tech-
33 (2009) 44 – 49
nique was proved efficient enough and the water content of pumpkin methyl ester was below the specification limits. Alkaline metals (Na+K) result from the metal catalyst used in the transesterification. Because the presence of high amounts of alkaline metals in biodiesel may be linked with ash formation in the combustion engine, EN 14214 specifies a maximum value of 5 mg g1. High content of metals in the ester phase indicates insufficient washing and purification during the production process. The sodium content of the produced methyl ester was 1.2–5 mg g1, a result that indicates that the washing procedure used was efficient in catalyst removal. Sulphur and phosphorus content in biodiesel is sourced mainly from the crops and animal fats used as feedstock. Most virgin or used vegetable oil and animal fat-based biodiesel have very low levels of sulphur content. However, methyl esters obtained from used frying oils have detected high levels of sulphur, which is suspected to originate from food cooked in the oil. Combustion of fuel containing sulphur causes emissions of sulphur oxides, particulate matter and can also lead to poisoning of post-treatment devices. On the other hand, the phosphorus content in biodiesel is generally low. However, specifying this parameter is important for engine operability. The present results agree well with the specification limits of sulphur and phosphorus content [13,14]. Mono- and diglycerides as well as triglycerides are referred to as bound glycerol. They are present in the feedstock oil and can remain in the final product in small quantities. A high excess of alcohol in the transesterification reaction should ensure that all triglycerides (the major component of vegetable oil) are reacted. A higher content of glycerides in the ester, especially triglycerides, may cause formation of deposits at the injection nozzles and at the valves [15–17]. The GC analysis of the produced methyl ester showed that the triglycerides of the parent oil reacted at a satisfactory yield to mono- and diglycerides. Their values were found to agree with the specified EN 14214 limits. Free glycerol is a by-product of the transesterification process and is separated from the ester. The present results showed that free glycerol content was below the detection range of the test method. This result indicates efficient separation and sufficient methyl ester washing. Total glycerol content (the sum of free and bound glycerol) is one of the main parameters indicating the final quality of biodiesel and is specified by EN 14214 to a maximum limit of 0.25% m/m. The analysis of total glycerol content in pumpkin oil methyl ester was relatively low (0.16% m/m), which was attributed to the high conversion of the oil into its mono-alkyl esters. The fatty acid profile of the methyl ester showed that it is composed primarily of linoleic and oleic acids with 43.72 and 37.07 wt%, respectively. The other main fatty acids were palmitic (12.51 wt%) and stearic (5.43 wt%) acids. Lower proportions of eicosanoic, linolenic, behenic and palmitoleic acids were also observed.
4.
Conclusions
The aim of this study was to evaluate pumpkin seed oil as a potential raw material for biodiesel production. Pumpkin oil
ARTICLE IN PRESS BIOMASS AND BIOENERGY
was extracted and chemically converted via an alkaline transesterification reaction to fatty acid methyl ester. The experimental results are described as follows:
Pumpkin seeds were found to be rich in oil, with an average yield of approximately 45%.
The emphasis from the European Union that is given to
biodiesel can be a strong incentive for pumpkin seedgrowing countries, to further study pumpkin oil as a possible venue for biodiesel production. The major physicochemical properties of pumpkin seed oil makes it an attractive alternative application of the existing feedstocks for biodiesel production in Greece. Based on the content of FFAs and water in the oil, the optimal reaction conditions were a 6:1 molar ratio of methanol to oil; temperature, 65 1C; NaOH amount, 1% (by the weight of the oil); pressure, atmospheric; and reaction time, 60 min. Biodiesel production in laboratory scale provided a yield of nearly 97.5 wt%. All of the determined parameters of pumpkin seed oil methyl ester were found to comply with the European FAME standard EN 14214. The refining process of methyl esters (washing with warm distilled water) was proved effective in regard to reducing free glycerol content. In this study, the sufficient number of washing steps was determined to be three. Further investigation of biodiesel originated from pumpkin seed oil can be widened by the examination of different catalyst types and the impact of pumpkin seed oil methyl ester on the exhaust emissions in respect to other biodiesels.
Acknowledgements This research was conducted under the project ‘HERAKLEITOS’, which is co-funded by the European Social Fund (75%) and National Resources (25%). R E F E R E N C E S
[1] Berrios M, Siles J, Martin MA, Martin A. A kinetic study of the esterification of free fatty acids (FFA) in sunflower oil. Fuel 2007;86:2383–8.
33 (2009) 44 – 49
49
[2] Nerson H. Effects of fruit shape and plant density on seed yield and quality of squash. Scientia Horticulturae 2005;105:293–304. [3] Bavec F, Gril L, Grobelnik-Mlakar S, Bavec M. Production of pumpkin for oil. In: Janick J, Whipkey A, editors. Trends in new crops and new uses. Alexandria, VA: ASHS Press; 2002. p. 187–93. [4] Tomasevic AV, Siler-Marinkovic SS. Methanolysis of used frying oil. Fuel Processing Technology 2003;81:1–6. [5] Wang Y, Ou S, Liu P, Xue L, Tang S. Comparison of two different processes to synthesize biodiesel by waste cooking oil. Journal of Molecular Catalysis A: Chemical 2006;252:107–12. [6] Demirbas A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Progress in Energy and Combustion Science 2005;31:466–87. [7] Centinkaya M, Karaosmanoglu F. Optimization of basecatalyzed transesterification reaction of used cooking oil. Energy and Fuels 2004;18:1888–95. [8] Haas MJ. Improving the economics of biodiesel production through the use of low value lipids as feedstocks: vegetable oil soapstock. Fuel Processing Technology 2005;86(10):1087–96. [9] Leung DYC, Guo Y. Transesterification of neat and used frying oil: optimization for biodiesel production. Fuel Processing Technology 2006;87:883–90. [10] Ramadhas AS, Jayaraj S, Muraleedharan C. Biodiesel production from high FFA rubber seed oil. Fuel 2005;84:335–40. [11] Mittelbach M. Diesel fuels derived from vegetable oils VI: specifications and quality control of biodiesel. Bioresource Technology 1996;56:7–11. [12] Azam MM, Waris A, Nahar NM. Prospects and potential of fatty acid methyl esters of some non-traditional seed oils for use as biodiesel in India. Biomass and Bioenergy 2005;29(4):293–302. [13] Setting National Fuel Quality Standards, Paper 6, National Standard for Biodiesel—discussion paper, 2003 /www.ea. gov.au/atmosphere/transport/biodieselS. [14] Dmytryshyn SL, Dalai AK, Chaudhari ST, Mishra HK, Reaney MJ. Synthesis and characterization of vegetable oil derived esters: evaluation of their diesel additive properties. Bioresource Technology 2004;92:55–64. [15] Karavalakis G, Stournas S, Karonis D, Zannikos F, Anastopoulos G. Tobacco seed as an alternative oil crop for the production of biodiesel in Greece. In: Proceedings of the 14th European Biomass Conference & Exhibition; 2005. p. 1411–4. [16] Felizardo P, Correia MJN, Raposo I, Mendes FJ, Berkemeier R, Bordado JM. Production of biodiesel from waste frying oils. Waste Management 2006;26:487–94. [17] Vicente G, Martinez M, Aracil J. A comparative study of vegetable oils for biodiesel production in Spain. Energy and Fuels 2006;20:394–8.
33 (2009) 44 – 49
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Pumpkin (Cucurbita pepo L.) seed oil as an alternative feedstock for the production of biodiesel in Greece P. Schinas, G. Karavalakis, C. Davaris, G. Anastopoulos, D. Karonis, F. Zannikos, S. Stournas, E. Lois Laboratory of Fuels and Lubricants Technology, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Street, Zografou Campus, 157 80 Athens, Greece
ar t ic l e i n f o
abs tra ct
Article history:
In recent years, the acceptance of fatty acid methyl esters (biodiesel) as a substitute to
Received 19 December 2006
petroleum diesel has rapidly grown in Greece. The raw materials for biodiesel production in
Received in revised form
this country mainly include traditional seed oils (cotton seed oil, sunflower oil, soybean oil
9 April 2008
and rapeseed oil) and used frying oils. In the search for new low-cost alternative feedstocks
Accepted 10 April 2008
for biodiesel production, this study emphasizes the evaluation of pumpkin seed oil. The
Available online 2 June 2008
experimental results showed that the oil content of pumpkin seeds was remarkably high
Keywords: Pumpkin seed oil Biodiesel Alkaline transesterification Fatty acid methyl esters Biodiesel properties
(45%). The fatty acid profile of the oil showed that is composed primarily of linoleic, oleic, palmitic and stearic acids. The oil was chemically converted via an alkaline transesterification reaction with methanol to methyl esters, with a yield nearly 97.5 wt%. All of the measured properties of the produced biodiesel met the current quality requirements according to EN 14214. Although this study showed that pumpkin oil could be a promising feedstock for biodiesel production within the EU, it is rather difficult for this production to be achieved on a large scale. & 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Biodiesel is made up of mono-alkyl esters of fatty acids derived from vegetable oils, used frying oils and animal fats. Most of the commercial biodiesel production is performed through a transestrification reaction of triglycerides in vegetable oils and animal fats with mono-alkyl alcohols in the presence of homogeneous base or acid catalysts [1]. The choice of the fat or oil to be used in biodiesel production is a process involving both chemistry and economics. With respect to process chemistry, the greatest difference among the choices of fats and oils is the amount of free fatty acids (FFAs) that is associated with the triglycerides. The iodine value is also a major parameter that associates with the choice of oil. According to the European Standard EN 14214, the iodine value is limited to 120. This parameter Corresponding author. Tel.: +30 210 7723213; fax: +30 210 7723163.
E-mail address: [email protected] (G. Karavalakis). 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.04.008
excludes the use of biodiesel from sunflower oil, which is one of the main oleaginous crops cultivated in the southern countries of the EU. Along with sunflower oil, tobacco seed oil, which is one of the most dynamic, low-cost oleaginous crops in Greece, is also excluded. New low-cost oil crops are needed to produce economical oils suitable for biodiesel production. One of the possible alternative oil crops for the Mediterranean area is pumpkin seed (Cucurbita pepo L.). To the authors’ best knowledge, studies on pumpkin seed oil as the feedstock for methyl ester production were never conducted. In the search for alternative oils for biodiesel production, pumpkin seed oil presents a promising choice; however, this cannot be regarded as a massive raw material for biodiesel production on a large scale. The seeds of pumpkin are a common snack food in several cultures, including Greece, and pumpkin seed
ARTICLE IN PRESS BIOMASS AND BIOENERGY
oil is a common salad oil with limited application for cooking. It is also used for medicinal purposes. Important considerations in selecting feedstocks for biodiesel production include (1) cost, (2) variability in quality and the chemical content feedstock, (3) regular availability of the feedstock, (4) flexibility to increase supply and (5) cost of transport and pretreatment. At present, it is not clear how pumpkin oil measures against most of these factors. The cost of raw pumpkin oil is rather difficult to be estimated since there is no commercial production in Greece and it has only limited use as cooking oil in some countries. It is a fact that the supply and cost of various oils largely depend on the level of demand in another market. In the case of pumpkin seed oil choice, the supply will be determined by the demand for the seeds used as a snack food. Although, increased use of pumpkin as a biodiesel feedstock could drive up the price, it is unlikely to exceed the cost of virgin vegetable oils, such as sunflower oil, corn oil and soybean oil. Even though there is no commercial pumpkin oil production in Greece, with an average seed production of 1.2 t ha1, [2,3] and total area in Greece used for gourd planting is about 4000 ha, an annual seed production of 4800 t can be estimated. In this study, the profile of pumpkin seed oil and its production to fatty acid methyl esters via a base-catalyzed transesterification were investigated. The advantage of pumpkin oil over sunflower oil, soy been oil and rapeseed oil would lie in the oil price. The reason of this price variation is possible due to the fact that pumpkin seed oil is not widely used as edible oil. Pumpkin seed contain 45% oil average (dry weight basis), which is a major advantage over a number of vegetable oils cultivated in Greece. The oil was extracted using a Soxhlet apparatus and was characterized by its physicochemical properties. Furthermore, the intent of this work was to investigate and to determine the influence of the chemical properties of the oil in the transesterification and to examine the quality parameters of the methyl ester according to the European standard EN 14214.
2.
Experimental
2.1.
Pumpkin seed oil extraction
Currently, there is no commercial production of pumpkin seed oil in Greece; hence it was not available in the market. The collected seeds were extracted in laboratory scale, and the procedure is as follows. The seeds were ground to a fine powder and then dried for 2 h at 100 1C. For the continuous extraction of the oil, the Soxhlet extraction apparatus was employed and hexane was used as solvent in the extraction process. The Soxhlet device temperature was kept at 65–70 1C and the overall process lasted 24 h. At the end of the process, the oil was separated from the organic solvent using a rotary vacuum evaporator, dried at 60 1C and weighed. Yield was calculated on dry weight basis.
2.2.
Physicochemical parameters of pumpkin seed oil
The pumpkin seed oil was examined in order to evaluate its use either as a blendstock in automotive or heating diesel fuel
45
33 (2009) 44 – 49
Table 1 – Physicochemical properties of pumpkin seed oil Property
Pumpkin seed oil
Test method
35.6
EN ISO 3104 EN ISO 3675 EN 22719 EN ISO 6245 EN ISO 20846 EN ISO 12937 EN 14111 EN 14104 EN ISO 10370 ASTM D 97 IP 12
Kinematic viscosity (40 1C), mm2 s1 Density (15 1C), kg m3
921.6
Flash point, 1C Sulphated ash content, wt%
4230 o0.01
Sulphur content, mg g1 Water content, mg g1 Iodine number Acid value, KOH, mg g1 Carbon residue, wt% Pour point, 1C Gross calorific value, MJ kg1
2 584 115 0.55 0.1754 12 39.0
or as a suitable raw material for biodiesel production. Its major quality properties using standard test methods are shown in Table 1.
2.3.
Transesterification
The transesterification reaction of pumpkin seed oil was carried out in a 500 mL spherical flask, with anhydrous methanol in molar ratio methanol to oil 6:1, using sodium hydroxide (NaOH) as catalyst. The reaction was carried out at 65 1C for 1 h [4–6]. By the end of the experiment the reaction mixture was transferred to a decanter for glycerol and methyl ester separation, allowing glycerol to separate by gravity for 24 h. Once the two phases were separated, the excess alcohol in each phase was removed by flash evaporation at 90 1C. The methanol was recovered and re-used. The purity level of the biodiesel has strong effects on its fuel properties [7]. Therefore, the methyl ester was purified by washing gently with three volumes of warm deionized water to remove residual catalyst, glycerol, methanol and soap using a centrifuge. A small amount of sulphuric acid (H2SO4) was used in the second washing to neutralize remaining soaps and other catalyst impurities. The washed methyl ester is then dried over the heated anhydrous sodium sulphate (Na2SO4). Solid traces from the methyl ester were removed with a filtration process. The schematic of the complete oil extraction and oil transesterification process flow diagram is shown in Fig. 1.
2.4.
Pumpkin seed oil methyl ester analysis
The fuel properties of pumpkin seed oil methyl ester, along with the test methods adopted for the fuel property analysis, are given in Table 2. Most of the fuel properties of the pumpkin methyl ester are quite comparable to those of other methyl esters reported in the literature, and fulfil the
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BIOMASS AND BIOENERGY
Pumpkin Seeds
Oil Extraction
Grinding
Oil (T 50 °C)
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MeOH+NaOH
Transesterification (T 65 °C)
Alcohol Removal
Methyl Ester Washing
Glycerol Removal
Drying Process
EN 14214 FAME
Catalyst Removal
Fig. 1 – Pumpkin oil extraction and transesterification process flow diagram. Table 2 – Properties of pumpkin oil methyl ester Property Kinematic viscosity (40 1C), mm2 s1 Density (15 1C), kg m3 Flash point, 1C Sulphur content, mg g1 Water content, mg g1 Iodine number Acid value, KOH, mg g1 Cold filter plugging point (CFPP), 1C Gross calorific value, MJ kg1 Group I metals Na+K, mg g1
Phosphorus content, mg g1 Monoglyceride content, %(m/m) Diglyceride content, %(m/m) Triglyceride content, %(m/m) Total glycerol, %(m/m) Free glycerol content, %(m/m)
Pumpkin oil methyl ester
EN 14214 limits
Test methods
4.41 883.7 4120 2 490 115 0.48 9 38.08 1.2, o0.5
3.50–5.00 0.860–0.900 120 min 10 max 500 max 120 max 0.5 max +5 max 5 max
EN ISO 3104 EN ISO 3675 EN 22719 EN ISO 20846 EN ISO 12937 EN 14111 EN 14104 EN 116 IP 12 EN 14108 EN 14109
3 0.47 0.15 0.14 0.16 0.00
10 max 0.80 max 0.20 max 0.20 max 0.25 max 0.02 max
European Standard EN 14214. Glyceride analysis was performed with a Carlo Erba Instrument—GC 3000 Series (Fig. 2).
2.5.
Fatty acid composition of pumpkin oil methyl ester
Fatty acid composition of pumpkin seed oil methyl ester was also determined by gas chromatography analysis. The fatty acid profile of the methyl ester was identified and quantified as shown in Table 3.
3.
Results and discussion
3.1.
Pumpkin oil characterization
The oil content of pumpkin seed ranged from 42% to 45% on dry weight basis. This may result in lower operation costs
EN EN EN EN EN EN EN
14107 14105 14105 14105 14105 14105 14106
compared to some other oilseeds, such as soybeans and cottonseeds, which have average oil contents of only 20% and 14%, respectively. Lower operation costs result from higher oil percentage mainly due to less capacity needed for the extruder and oilseed press.
3.2. Effect of pumpkin oil properties on the transesterification reaction The two major quality parameters that influence the production process of biodiesel are the FFAs and water content. Several studies showed that the raw oil acid value should be less than KOH 1.0 mg g1 and that all raw materials should be anhydrous (water content o0.3%) [8,9]. If the above requirements are not met it is still possible to produce biodiesel, but the overall yield of the reaction is significantly reduced due to the deactivation of the catalyst and the formation of soaps.
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47
33 (2009) 44 – 49
Fig. 2 – Glyceride analysis of the pumpkin oil methyl ester.
Table 3 – Fatty acid composition of pumpkin seed oil methyl ester Number of carbons 8 10 12 14 16 16 17 17 18 18 18 18 18 18 20 20 22 22 24
Fatty acid
Chemical structure
Weight %
Caprylic Capric Lauric Myristic Palmitic Palmitoleic Heptadecanoic Heptadecenoic Stearic Oleic Linoleic Linolenic Elaidic C18:2+C18:3 trans Eicosanoic Eicosenoic Behenic Erucic Lignoceric
CH3(CH2)6COOH CH3(CH2)8COOH CH3(CH2)10COOH CH3(CH2)12COOH CH3(CH2)14COOH CH3(CH2)5CH¼CH(CH2)7COOH CH3(CH2)15COOH CH3(CH2)6CH¼CH(CH2)7COOH CH3(CH2)16COOH CH3(CH2)7CH¼CH(CH2)7COOH CH3(CH2)3(CH2CH¼CH)2(CH2)7COOH CH3(CH2CH¼CH)3(CH2)7COOH CH3(CH2)7CH¼CH(CH2)7COOH CH3(CH2)3(CH2CH¼CH)2(CH2)7COOH+CH3(CH2CH¼CH)3(CH2)7COOH CH3(CH2)18COOH CH3(CH2)8CH¼CH(CH2)8COOH CH3(CH2)20COOH CH3(CH2)7CH¼CH(CH2)11COOH CH3(CH2)22COOH
o0.01 o0.01 0.01 0.11 12.51 0.15 0.08 0.04 5.43 37.07 43.72 0.18 0.03 0.04 0.39 0.11 0.12 0.02 0.06
High acid values may be corrected by the addition of sodium hydroxide or by heterogeneous acid catalysts [10]. In this study, the amount of FFAs measured in the oil was 0.27% while the water content of the oil was rather low (500 mg g1). Based on these properties, the selected catalyst for the reaction was NaOH and the molar ratio of methanol to oil was 6:1. From the GC analysis, it was observed that no soaps
or other impurities were produced during the reaction process. Although the iodine number is not included in the properties that influence the transesterification reaction, it should be considered in determining the oil of choice. The specified limit for this parameter is 120 according to the FAME standard EN 14214. In this sense, methyl esters obtained
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BIOMASS AND BIOENERGY
from sunflower oil, soy oil or tobacco seed oil cannot meet this specification because of their high proportion of unsaturated chains. However, if commercial production of pumpkin oil is desired, the iodine number will not be a constraint.
3.3.
Biodiesel yield
In order to estimate the biodiesel yield after the reaction and separation stages, the methyl ester weight yield, relative to the initial amount of vegetable oil, was worked out. The calculated methyl ester concentration was nearly 97.5 wt%, and according to this result, the alkaline transesterification reaction was completed. However, if there are no side reactions, the biodiesel weight yields, relative to the initial amount of pumpkin oil, should be nearly 99–100 wt%. In this sense, two possible side reactions could occur, the saponification of triglycerides or neutralization of the FFAs of the vegetable oil. Both of them produce soaps and, therefore, decrease the biodiesel yield. FFA neutralization could not be substantial in the present investigation, since the acid number of the oil was only KOH 0.55 mg g1.
3.4.
Characterization of biodiesel
The standard for biodiesel states that the fuel should have a density between 860 and 900 kg m3. Density is an important parameter for diesel fuel injection systems. The results obtained showed that the produced methyl ester was within the specification limits. Viscosity represents flow characteristics and the tendency of fluids to deform with stress. One of the main reasons for processing vegetable oils for use in internal combustion engines is to reduce the viscosity, thereby improving fuel flow characteristics. Even more than density, kinematic viscosity is an important parameter regarding fuel atomization as well as fuel distribution [11]. For biodiesel to be used in diesel engines, the viscosity must be between 3.5 and 5.0 mm2 s1. Pumpkin seed oil methyl ester had a viscosity of 4.41 mm2 s1. Viscosities above the specification limit can be attributed to the incomplete reaction or to the inefficient purification steps of the process, leaving glycerine in the ester phase. The iodine number is an index of the number of double bonds in biodiesel, and therefore is a parameter that quantifies the degree of unsaturation of biodiesel. This property greatly influences oxidation stability and the polymerization of glycerides. This can lead to the formation of deposits formed in diesel engines injectors. The iodine number is directly correlated to biodiesel viscosity, cetane number and cold flow characteristics (cold filter plugging point). At high saturation, the cetane number is improved while poor low-temperature qualities may be observed. In this work, the iodine number of the produced methyl ester was 115, below the specification maximum limit [12]. Fuel contaminated with water can cause engine corrosion or may cause a reversion of fatty acid methyl esters to fatty acids, which can lead to filter plugging [13]. Therefore, EN 14214 imposed a maximum content of 500 mg g1 of water in biodiesel. In this study, the produced methyl ester was routinely dried over anhydrous sodium sulphate. This tech-
33 (2009) 44 – 49
nique was proved efficient enough and the water content of pumpkin methyl ester was below the specification limits. Alkaline metals (Na+K) result from the metal catalyst used in the transesterification. Because the presence of high amounts of alkaline metals in biodiesel may be linked with ash formation in the combustion engine, EN 14214 specifies a maximum value of 5 mg g1. High content of metals in the ester phase indicates insufficient washing and purification during the production process. The sodium content of the produced methyl ester was 1.2–5 mg g1, a result that indicates that the washing procedure used was efficient in catalyst removal. Sulphur and phosphorus content in biodiesel is sourced mainly from the crops and animal fats used as feedstock. Most virgin or used vegetable oil and animal fat-based biodiesel have very low levels of sulphur content. However, methyl esters obtained from used frying oils have detected high levels of sulphur, which is suspected to originate from food cooked in the oil. Combustion of fuel containing sulphur causes emissions of sulphur oxides, particulate matter and can also lead to poisoning of post-treatment devices. On the other hand, the phosphorus content in biodiesel is generally low. However, specifying this parameter is important for engine operability. The present results agree well with the specification limits of sulphur and phosphorus content [13,14]. Mono- and diglycerides as well as triglycerides are referred to as bound glycerol. They are present in the feedstock oil and can remain in the final product in small quantities. A high excess of alcohol in the transesterification reaction should ensure that all triglycerides (the major component of vegetable oil) are reacted. A higher content of glycerides in the ester, especially triglycerides, may cause formation of deposits at the injection nozzles and at the valves [15–17]. The GC analysis of the produced methyl ester showed that the triglycerides of the parent oil reacted at a satisfactory yield to mono- and diglycerides. Their values were found to agree with the specified EN 14214 limits. Free glycerol is a by-product of the transesterification process and is separated from the ester. The present results showed that free glycerol content was below the detection range of the test method. This result indicates efficient separation and sufficient methyl ester washing. Total glycerol content (the sum of free and bound glycerol) is one of the main parameters indicating the final quality of biodiesel and is specified by EN 14214 to a maximum limit of 0.25% m/m. The analysis of total glycerol content in pumpkin oil methyl ester was relatively low (0.16% m/m), which was attributed to the high conversion of the oil into its mono-alkyl esters. The fatty acid profile of the methyl ester showed that it is composed primarily of linoleic and oleic acids with 43.72 and 37.07 wt%, respectively. The other main fatty acids were palmitic (12.51 wt%) and stearic (5.43 wt%) acids. Lower proportions of eicosanoic, linolenic, behenic and palmitoleic acids were also observed.
4.
Conclusions
The aim of this study was to evaluate pumpkin seed oil as a potential raw material for biodiesel production. Pumpkin oil
ARTICLE IN PRESS BIOMASS AND BIOENERGY
was extracted and chemically converted via an alkaline transesterification reaction to fatty acid methyl ester. The experimental results are described as follows:
Pumpkin seeds were found to be rich in oil, with an average yield of approximately 45%.
The emphasis from the European Union that is given to
biodiesel can be a strong incentive for pumpkin seedgrowing countries, to further study pumpkin oil as a possible venue for biodiesel production. The major physicochemical properties of pumpkin seed oil makes it an attractive alternative application of the existing feedstocks for biodiesel production in Greece. Based on the content of FFAs and water in the oil, the optimal reaction conditions were a 6:1 molar ratio of methanol to oil; temperature, 65 1C; NaOH amount, 1% (by the weight of the oil); pressure, atmospheric; and reaction time, 60 min. Biodiesel production in laboratory scale provided a yield of nearly 97.5 wt%. All of the determined parameters of pumpkin seed oil methyl ester were found to comply with the European FAME standard EN 14214. The refining process of methyl esters (washing with warm distilled water) was proved effective in regard to reducing free glycerol content. In this study, the sufficient number of washing steps was determined to be three. Further investigation of biodiesel originated from pumpkin seed oil can be widened by the examination of different catalyst types and the impact of pumpkin seed oil methyl ester on the exhaust emissions in respect to other biodiesels.
Acknowledgements This research was conducted under the project ‘HERAKLEITOS’, which is co-funded by the European Social Fund (75%) and National Resources (25%). R E F E R E N C E S
[1] Berrios M, Siles J, Martin MA, Martin A. A kinetic study of the esterification of free fatty acids (FFA) in sunflower oil. Fuel 2007;86:2383–8.
33 (2009) 44 – 49
49
[2] Nerson H. Effects of fruit shape and plant density on seed yield and quality of squash. Scientia Horticulturae 2005;105:293–304. [3] Bavec F, Gril L, Grobelnik-Mlakar S, Bavec M. Production of pumpkin for oil. In: Janick J, Whipkey A, editors. Trends in new crops and new uses. Alexandria, VA: ASHS Press; 2002. p. 187–93. [4] Tomasevic AV, Siler-Marinkovic SS. Methanolysis of used frying oil. Fuel Processing Technology 2003;81:1–6. [5] Wang Y, Ou S, Liu P, Xue L, Tang S. Comparison of two different processes to synthesize biodiesel by waste cooking oil. Journal of Molecular Catalysis A: Chemical 2006;252:107–12. [6] Demirbas A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Progress in Energy and Combustion Science 2005;31:466–87. [7] Centinkaya M, Karaosmanoglu F. Optimization of basecatalyzed transesterification reaction of used cooking oil. Energy and Fuels 2004;18:1888–95. [8] Haas MJ. Improving the economics of biodiesel production through the use of low value lipids as feedstocks: vegetable oil soapstock. Fuel Processing Technology 2005;86(10):1087–96. [9] Leung DYC, Guo Y. Transesterification of neat and used frying oil: optimization for biodiesel production. Fuel Processing Technology 2006;87:883–90. [10] Ramadhas AS, Jayaraj S, Muraleedharan C. Biodiesel production from high FFA rubber seed oil. Fuel 2005;84:335–40. [11] Mittelbach M. Diesel fuels derived from vegetable oils VI: specifications and quality control of biodiesel. Bioresource Technology 1996;56:7–11. [12] Azam MM, Waris A, Nahar NM. Prospects and potential of fatty acid methyl esters of some non-traditional seed oils for use as biodiesel in India. Biomass and Bioenergy 2005;29(4):293–302. [13] Setting National Fuel Quality Standards, Paper 6, National Standard for Biodiesel—discussion paper, 2003 /www.ea. gov.au/atmosphere/transport/biodieselS. [14] Dmytryshyn SL, Dalai AK, Chaudhari ST, Mishra HK, Reaney MJ. Synthesis and characterization of vegetable oil derived esters: evaluation of their diesel additive properties. Bioresource Technology 2004;92:55–64. [15] Karavalakis G, Stournas S, Karonis D, Zannikos F, Anastopoulos G. Tobacco seed as an alternative oil crop for the production of biodiesel in Greece. In: Proceedings of the 14th European Biomass Conference & Exhibition; 2005. p. 1411–4. [16] Felizardo P, Correia MJN, Raposo I, Mendes FJ, Berkemeier R, Bordado JM. Production of biodiesel from waste frying oils. Waste Management 2006;26:487–94. [17] Vicente G, Martinez M, Aracil J. A comparative study of vegetable oils for biodiesel production in Spain. Energy and Fuels 2006;20:394–8.