Ethanolysis of used frying oil. Biodiesel preparation and characterization
Introduction
The worldwide worry about the protection of environment and the conservation of non-renewable natural resources, has given rise to the alternative development of sources of energy as substitute of traditional fossil fuels. Among the most used fuels, the diesel oils of automotive industry, known like no. 2 diesel, are the more important. Diesel fuels are used in city buses, locomotives, electric generators, etc., and they have an essential function in the industrial economy of a country. The alternatives to diesel fuel must be technically feasible, economically competitive, environmentally acceptable, and readily available [1]. Many of these requisites are satisfied by vegetable oils or, in general, by triglycerides. Indeed, vegetable oils are widely available from a variety of sources, and they are renewable. As indicated by Boehman [2], vegetable oil and animal fat based biodiesel fuels, as methyl or ethyl esters, have the following advantages over diesel fuel: as a neat fuel or in blends with diesel fuel they produce less smoke and particulates, have higher cetane numbers, produce lower carbon monoxide and hydrocarbon emissions, are biodegradable and non-toxic, and provide engine lubricity to low sulfur diesel fuels. Conversely, they present other technical challenges such as: low volatility; high pour points, cloud points and cold filter plugging temperatures; elevated NOx emissions; and incomplete combustion. Their advantages as cetane improvers and lubricity additives for diesel fuels have not been fully appreciated, but the advent of ultra low sulfur diesel fuels and their reduced lubricity may motivate use of low concentration biodiesel blends to ensure adequate lubricity. Consequently, these products can be considered viable alternatives for diesel fuel [3], [4], [5], [6]. Their main drawback is price, which is higher than for oil-derived diesels. In consequence, their use must be accompanied by a policy oriented towards their total tax exemption.
The high cost of biodiesel is mainly due to the cost of virgin vegetable oil. Therefore, it is not surprising that the biodiesel produced from vegetable oil (for example, pure soybean oil) costs much more than petroleum-based diesel [7], [8]. Therefore, it is necessary to explore ways to reduce production costs of biodiesel. In this sense, methods that permit to minimize the costs of the raw material are of special interest. The use of waste frying oil, instead of virgin oil, to produce biodiesel is an effective way to reduce the raw material cost because waste frying oil is estimated to be about half the price of virgin oil [8], [9]. In addition, the utilization of waste frying oils diminishes the problems of contamination, because the reusing of these waste greases can reduce the burden of the government in disposing of the waste, maintaining public sewers, and treating the oil wastewater. The fact is, so far, that only a very small percentage of these oils has been collected and used for soap production.
Nevertheless, it is necessary to consider that the used frying oils have properties different from those of refined and crude oils. The high temperatures of typical cooking processes and the water from the foods accelerate the hydrolysis of triglycerides and increase the free fatty acid content in the oil. Also, problems with the stability of the mixtures and increases in the peroxide value have been observed. Likewise, the viscosity, iodine value, saponification value, and density are different when refined and crude oils are used [10]. Many questions about the optimization of methanolysis or ethanolysis of used frying oil have not been reported. The resolution of these questions is very important for biodiesel manufacture.
Generally, biodiesel is produced by means of transesterification. Transesterification is the reaction of a lipid with an alcohol to form esters and a byproduct, glycerol. It is, in principle, the action of one alcohol displacing another from an ester, referred to as alcoholysis (cleavage by an alcohol). The reaction, as shown in Eq. (1), is reversible, and thus an excess of alcohol is usually used to force the equilibrium to the product side. The stoichiometry for the reaction is 3:1 alcohol to lipids. However, in practice this is usually increased to 6:1 to raise the product yield.
Transesterification consists of a sequence of three consecutive reversible reactions. The first step is the conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides, and finally monoglycerides into glycerol, yielding one ester molecule from each glyceride at each step. The reactions are reversible, although the equilibrium lies towards the production of fatty acid esters and glycerol [11], [12]. The mechanism of transesterification is described in Eqs. (2), (3), (4).Triglyceride+ROH⇄Diglyceride+R′COORDiglyceride+ROH⇄Monoglyceride+R′COORMonoglyceride+ROH⇄Glycerol+R′COOR
The catalyst used has a determinant effect on the reaction, raising the rate notably. It is known that basic catalysts require short times (30 min) to complete the reaction even at room temperature, while acid catalysts, such as sulfuric acid, require higher temperatures (100 °C) and longer reaction times (3–4 h) [4], [11], [12], [13]. The alkalis that are used generally include sodium and potassium hydroxides, carbonates, and alkoxides such as methoxide, ethoxide, propoxide, and butoxide. Eqs. (5), (6), (7), (8) summarize the mechanism of alkali-catalyzed transesterification. The first step is an attack on the carbonyl carbon atom of the triglyceride molecule by the anion of the alcohol (alkoside ion) to form a tetrahedral intermediate. In the second step, the intermediate reacts with an alcohol to regenerate the anion of the alcohol (alkoside ion). In the last step, rearrangement of the tetrahedral intermediate results in the formation of a fatty acid ester and a diglyceride. In the pre-step, when NaOH, KOH or other similar catalysts are mixed with alcohol, the actual catalyst, alkoxide group is formed [11], [12].
Short-chain alcohols such as methanol, ethanol, and butanol are the most frequently employed. Although the use of different alcohols presents some differences with regard to the reaction kinetics, the final yield of esters remains more or less inalterable. Therefore, selection of the alcohol is based on cost and performance consideration. Ethanol can be produced from agricultural renewable resources, thereby attaining total independence from petroleum-based alcohols. Also, ethanol, as extraction solvent, is preferable to methanol because of its much higher dissolving power for oils. For this cause, ethanol is often used as an appropriate alcohol for the transesterification of vegetables oils. Therefore, producing ethyl esters rather than methyl esters is of considerable interest, because, in addition to the entirely agricultural nature of the ethanol, the extra carbon atom brought by the ethanol molecule slightly increases the heat content and the cetane number. Finally, another important advantage in the use of ethanol is that the ethyl esters have cloud and pour points that are lower than the methyl esters. This fact improves the cold start.
Pre-stepOH-+ROH⇄RO−+H2O or NaOR⇄RO−+Na+
Step 1
Step 2
Step 3where:
From one environmental point of view, ethyl esters utilization is also more advantageous than the utilization of methyl esters. According to Makareviciene and Janulis [14], the results showed that when considering emissions of nitrogen oxides (NOx), carbon monoxide (CO) and smoke density, rapeseed oil ethyl ester had less negative effect on the environment in comparison with that of rapeseed oil methyl ester. When fuelled with pure rapeseed oil ethyl ester, HC emissions decreased by 53%, CO emissions by 7.2% and smoke density by 72.6% if compared with the emissions when fossil diesel fuel was used. Also carbon dioxide (CO2) emissions, which cause greenhouse effect, decreased by 782.87 g/kWh when rapeseed oil ethyl ester was used instead of fossil diesel fuel. Finally, these authors found than the rapeseed oil ethyl ester was more rapidly biodegradable in aqua environment than rapeseed oil methyl ester and especially with fossil diesel fuel.
However, the utilization of ethanol also presents inconveniences. Effectively, as it is indicated in the literature [15], the base-catalyzed formation of ethyl ester is difficult compared to the formation of methyl esters. Specifically the formation of stable emulsion during ethanolysis is a problem. Methanol and ethanol are not miscible with triglycerides at room temperature, and the reaction mixture is usually mechanically stirred to enhance mass transfer. During the curse of reaction, emulsions are usually formed. In the case of methanolysis, these emulsions break down quickly and easily to form a lower glycerol rich layer and upper methyl ester rich layer. In ethanolysis, these emulsions are more stable and severely complicate the separation and purification of esters.
With these considerations, and as a continuation of previous works [16], [17], [18], we carried out a study on the transesterification process of used frying oil utilizing ethanol, in order to characterize the ethyl esters obtained for their applications as biodiesel in internal combustion engines.
Section snippets
Experimental methods and materials
The used frying oil, originally a mixture of olive oil and sunflower oil, was supplied by Rograsa (Mérida-Spain), and was free of meat. Anhydrous ethanol, sodium methoxide, potassium methoxide, sodium hydroxide, and potassium hydroxide were supplied by Panreac. Ethyl esters of palmitic, stearic, oleic, linoleic, linolenic, and erucic acids (employed as standards in the chromatographic determination) were supplied by Merck and Sigma. All reagents were of analytical grade.
The reaction of
Results and discussion
The operation variables employed were ethanol/oil molar ratio (6:1–15:1), catalyst type (sodium hydroxide, potassium hydroxide, sodium methoxide, and potassium methoxide), catalyst concentration (0.1–1.5 wt.%), and temperature (35–78 °C). Oil mass (500 g), reaction time (120 min), and alcohol type (ethanol) were fixed as common parameters in all experiments.
In the process of transesterification in two stages, the initial concentration of ethyl esters was 74.2%. Only two variables were studied:
Conclusions
The transesterification of used frying oil yielded biodiesel with properties similar to those of no. 2 diesel. Consequently, the use of waste frying oil is an effective way to reduce the raw material cost. In addition, the pollution problems could be reduced.
The ethanol/oil molar ratio was one of the variables that had more influence on the process. Within the range of molar ratios employed, the best results were obtained for a 12:1 ratio.
Potassium hydroxide gave the highest ester yield,
Acknowledgments
The authors express their gratitude to the “Junta de Extremadura” for the financial support received to perform this study by means of Project No. 2PR01A034.
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