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Ind. Eng. Chem. Res. 2005, 44, 7978-7982
Kinetics of Oleic Acid Esterification with Methanol in the Presence of Triglycerides R. Tesser,† M. Di Serio,† M. Guida,‡ M. Nastasi,‡ and E. Santacesaria*,† Dipartimento di Chimica, Universita` di Napoli Federico II, via Cintia 80126, Napoli, Italy, and ASER srl, S.S. n.11 Padana Superiore 2/B, 20063 Cernusco sul Naviglio, Milano, Italy
In the present work, the kinetics of oleic acid esterification with methanol has been studied by using an acid ion-exchange polymeric resin (Relite CFS) as the heterogeneous catalyst. The kinetics of the reaction has been studied by performing batch runs at different temperatures and in the presence of a certain amount of triglyceride for simulating an oil with a high content of free fatty acids, which is of great potential interest as a raw material in the biodiesel production process. The experimental data have been interpreted with a second-order, pseudo-homogeneous kinetic model, and a good agreement between the experimental data and the model has been obtained. Introduction One among the main topics in the framework of a “sustainable development” is represented by the production and usage of biofuels as a competitive renewable energy source. Actually, the most attractive biofuel is represented by biodiesel, which is constituted by a mixture of fatty acids methylesters (FAME) or ethylesters (FAEE), produced by a transesterification reaction performed on high-quality vegetable oils with methanol or ethanol. This transesterification process is affected by several factors such as catalyst concentration, reactant molar ratio, and water and free fatty acids content in the raw materials.1-3 The catalysts that are more widely used, and which are the most effective in this step of the process, are sodium and potassium hydroxide;1,2 acid catalysts, such as sulfuric or hydrochloric acid, have also been proven as effective in this reaction.1 Despite the fact that alkaline catalysis is characterized by a higher reaction rate with respect to an acid-catalyzed reaction, some severe drawbacks must be accounted for in this case: the presence of moisture and free acidity that strongly influences the process performance and economics. In fact, both water and free fatty acids (FFAs) rapidly react with the catalyst, consuming it and giving way to longchain soaps for which the tenside properties do not allow an efficient separation of the pure glycerol in the final step of the process. In the case of an oil with a high content of free fatty acids (FFAs), like usually happens in waste materials, a pretreatment esterification step must be considered as mandatory in order to eliminate the free acidity that must be reduced below 1 wt %. On the other hand, the opportunity to employ low-quality or waste raw materials, as a starting point in the biodiesel production cycle, is an essential aspect for decreasing the production costs and making this alternative fuel competitive with traditional fossils fuels. On the basis of these considerations, the improvement of the esterification step could represent one of the key * To whom correspondence should be addressed. E-mail:
[email protected]. † Universita` di Napoli Federico II. ‡ ASER srl.
points through which the whole process can result in being economically convenient. In the present work, the kinetics of the esterification reaction of oleic acid with methanol, over a sulfonic ionexchange resin catalyst, has been investigated. The experimental runs have been performed in the presence of acidity-free soybean oil in mixture with oleic acid with the aim to simulate the real mixture that must be submitted to the preliminary esterification step in the biodiesel production process. While a lot of papers have been reported in the literature concerning the esterification reaction of a short-chain carboxylic acid with an alcohol,4-8 heterogeneously catalyzed by acid resin with sulfonic groups, works devoted to the same reaction performed on longchain acids are rather scarce. Moreover, very few data have been published regarding the esterification of high free-acidity oils with methanol that, on the contrary, are of great interest in biodiesel production. As an example, very recently, Sendzikiene et al.9 have investigated the effect of catalyst concentration and of the reactants’ molar ratio on the esterification rate of an artificially acidified rapeseed oil with methanol. The reaction has been studied in homogeneous catalysis conditions, using sulfuric acid as the catalyst, and the authors have found that the esterification rate, in the temperature range 20-60 °C, depends both on the amount of used catalyst and on the initial oleic acid concentration. A similar reaction system has also been investigated by Steinigeweg and Gmehling:10 pure decanoic acid has been converted to the corresponding methyl ester using methanol and an ion-exchange resin as the catalyst in a reactive distillation device. In this reactor configuration, a simultaneous reactants conversion and products separation were achieved, although a reactive column section of about 10 m-long was required for obtaining an almost complete carboxylic acid conversion. A more sophisticated and innovative approach has been proposed by Chemseddine and Audinos11 which has adopted a membrane reactor in which the ionexchange is performed by the membrane itself. This device allows an almost complete esterification of oleic acid with methanol, at room temperature, by shifting the equilibrium reaction toward ester formation caused by water osmosis migration at one side of the mem-
10.1021/ie050588o CCC: $30.25 © 2005 American Chemical Society Published on Web 09/17/2005
Ind. Eng. Chem. Res., Vol. 44, No. 21, 2005 7979 Table 1. Properties of Catalyst commercial name and producer matrix functional groups acidity particles mean diameter particles size range total exchange capacity maximum operating temperature bulk density
Figure 1. Scheme of the reactor: 1, reactor; 2, circulation thermostat; 3, stirrer; 4, sampling valve; 5, indicator for pressure transducer; 6, indicator for liquid-phase thermocouple; 7, reactor jacket.
brane. However, the described operation is rather complex, and no information is given, in the corresponding paper, about the possibility of utilizing the presence of waste fried oils or fats of animal derivation as representative feedstocks with high FFA content. In all the mentioned papers, some kinetic data related to the esterification of long-chain fatty acids have been reported. In our work, we have deepened, in particular, the kinetic approach by studying the esterification performed in the presence of triglycerides with high FFA content.
Relite CFS by Resindion porous copolymer styrene-DVB sulfonics 3.6 meq/g 0.7 mm 0.3-1.18 mm 2.0 equiv/L 140 °C 0.840 g/cm3
An acid sulfonic resin (Relite CFS furnished by Resindion) has been used as the catalyst, and its main properties are summarized in Table 1. The reactor was heated after the acid oil and catalyst were charged, and when the chosen reaction temperature was reached, methanol was added in the desired amount. This instant of time is assumed as the initial time for the reaction. The total pressure of the system is the methanol vapor pressure at the temperature of the experiment. In Table 2 are summarized the experimental conditions adopted in each run and the used reactants and catalyst amounts. At the temperatures of 50, 65, and 85 °C, the runs have been repeated by increasing the reaction time by a factor of 5 and 10 in order to also investigate the equilibrium behavior. This aspect is of crucial importance because, for a high efficiency in the successive transesterification step of the process, the free acidity must be lowered below the value of 1 wt %.
Experimental Section
Results and Discussion
The experimental runs have been performed in a batchwise-operated stainless steel reactor (total volume 1 L) equipped with a magnetically driven stirrer and a jacket. The thermal control is implemented by a circulation thermostat that continuously feeds a thermal fluid into the reactor jacket, allowing temperature control within (0.5 °C. Moreover, the reactor is equipped with a pressure transducer fitted in the vapor space and with a thermocouple in the liquid phase for the measurement of the reaction temperature. Liquid-phase samples were withdrawn at different times during a run, by means of a sampling line equipped with a stopping valve. A scheme of the reactor is reported in Figure 1. The reactants employed are methanol (Carlo Erba, purity 99.9% w/w), oleic acid (Carlo Erba, purity 99.9% w/w), and a commercially available acidity-free soybean oil (acidity
