Influence of fatty acid composition of raw materials on biodiesel properties
Introduction
The major part of all energy consumed worldwide comes from fossil sources (petroleum, coal and natural gas). However, these sources are limited, and will be exhausted by the near future (Dermibas, 2005). Biodiesel, an alternative diesel fuel, is made from renewable biological sources such as vegetable oils and animal fats. This fuel is biodegradable and non-toxic and has low emission profiles as compared to petroleum diesel. Usage of biodiesel will allow a balance to be sought between agriculture, economic development and the environment (Meher et al., 2006). Recently, the demand of biodiesel has increased due to petroleum price rises in the last few months and the development of government measures like the EU Directive 2003/30/EC on the promotion of the use of biofuels or other renewable fuels for transport (Vicente et al., 2007).
Biodiesel, is produced through a reaction known as transesterification. In a transesterification or alcoholysis reaction, one mole of triglyceride reacts with three moles of alcohol (molar ratio of methanol to vegetable oil of 3:1) to form one mole of glycerol and three moles of the respective fatty acid alkyl esters. The process is a sequence of three reversible reactions, in which the triglyceride molecule is converted step by step into diglyceride, monoglyceride and glycerol (Mittelbach and Remschmidt, 2004).
Several reviews dealing with the production of biodiesel by transesterification have been published (Dermibas, 2005, Fukuda et al., 2001, Ma and Hanna, 1999, Schuchardt et al., 1998). Generally, transesterification can proceed by base or acid catalysis. However, in homogeneous catalysis, alkali catalysis (sodium or potassium hydroxide; or the corresponding alkoxides) is a much more rapid process than acid catalysis (Freedman et al., 1984).
Several types of vegetable oils, with a varied composition in fatty acids, can be used for the preparation of biodiesel. Four oil crops clearly dominate the feedstock sources used for world-wide biodiesel production. Soybean (Sensöz and Kaynar, 2006, Xie et al., 2006), rapeseed (Cvengros and Povazanec, 1996, Peterson et al., 1996), palm (Kalam and Masjuki, 2002) and sunflower (Antolín et al., 2002, Vicente et al., 2005) oils are the most studied. However, there are no technical restrictions to the use of other types of vegetable oils. Often the vegetable oils investigated for their suitability as biodiesel are those which occur abundantly in a specific area. Thus, Spain, along with France and Italy, is one of the major world producers of grape seed oil, obtained from the seeds left following pressing of the juice from grapes for wine making. Biodiesel production from grape seed oil constitutes an economic alternative for valuation of by-product obtained from the wine manufacture in the region of Castilla-La Mancha (Spain).
Vicente et al. (2006) examined the methanolysis of 21 different vegetable oils to produce biodiesel in Spain, including traditional vegetable seed oils (sunflower and rapeseed), alternative vegetable oils (Brassica carinata), genetically modified vegetable oils (high oleic sunflower), and used frying oils.
The properties of the triglyceride and the biodiesel fuel are determined by the amounts of each fatty acid that are present in the molecules. Chain length and number of double bonds determine the physical characteristics of both fatty acids and triglycerides (Mittelbach and Remschmidt, 2004). Transesterification does not alter the fatty acid composition of the feedstocks and this composition plays an important role in some critical parameters of the biodiesel, as cetane number and cold flow properties.
Studies about the influence of the triglycerides composition in the biodiesel quality are scarce. Muniyappa et al. (1996) reported density, viscosity and cloud point of two biodiesels synthesized by soybean and beef tallow oil. The high cloud point of methyl esters from beef tallow oil was indicative of a high concentration of saturated fatty esters. Lang et al. (2001) tested several oils (rapeseed, sunflower, canola and linseed oil) on the biodiesel production and compared some physical and fuel properties of biodiesels with those of conventional diesel fuels. Cardone et al. (2003) reported a comparison of the performance of B. carinata oil-derived biodiesel with a commercial biodiesel and petroleum diesel fuel. This study was centred in the oxidation stability on the basis of linolenic acid content. Dmytryshyn et al. (2004) performed the transesterification of four vegetable oils, comparing properties like density, viscosity, cloud point and pour point, and establishing differences between them. Sarin et al. (2007) reported some blends of biodiesel from Jatropha and Palm oils to study their physico-chemical properties in order to improve the oxidation stability.
The main purpose of this work is based on the study of the influence of vegetable oils properties and composition on the quality of biodiesel synthesized. It has been reported the transesterification of common oils (rapeseed, soybean, sunflower and palm oil) and others less investigated as olive, almond, corn, grape seed and high oleic sunflower oil. The fatty acid profile of vegetable oils used was measured following the International and European Standards ISO 5509 and EN 14103. The quality of the biodiesels synthesized was tested according to the European Standard EN 14214. Relations between the composition of vegetable oils and critical parameters of biodiesel were obtained allowing us to predict some critical parameters in the biodiesel knowing the composition of the raw material.
Section snippets
Materials
Refined palm, olive, peanut, rape, soybean, sunflower and almond oil were provided by FLUKA. Refined grape seed oil was supplied by BORGES. Refined high oleic sunflower (HOS) was obtained from a local store and, finally, refined corn oil from SIGMA. Anhydrous methanol (99.8%) and acetic acid were obtained from PANREAC. The catalyst (potassium methoxide) was 97% purity from BASF. The following chemicals were supplied by FLUKA: heptane (puriss. p.a., ⩾99.5% GC); methyl heptadecanoate (puriss.
Results and discussion
The fatty acid profile of the vegetable oils used in this work is summarized in Table 1. There are three main types of fatty acids that can be present in a triglyceride: saturated (Cn:0), monounsaturated (Cn:1) and polyunsaturated with two or three double bonds (Cn:2,3). The percentage of these compounds for each vegetable oil is given in Table 2. According to this composition, two parameters based on the type of fatty acids were defined: degree of unsaturation (DU) and long chain saturated
Conclusions
The main purpose of this work was based on the study of the influence of fatty acid composition of vegetable oils on the quality of biodiesel synthesized. Low cetane numbers have been associated with more highly unsaturated components (C18:2 and C18:3). Polyunsaturated fuels that contain high levels of these components include soybean, sunflower and grape seed oils. In addition, these biodiesels showed high iodine values. Furthermore, the oxidation stability decreased with the increase of the
Acknowledgement
Financial support by the Junta de Comunidades de Castilla-La Mancha (Project PIB07-0031-5471) is gratefully acknowledged.
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