Elsevier

Bioresource Technology

Volume 102, Issue 11, June 2011, Pages 6385-6391
Bioresource Technology

Case Study
Ethyl ester production by homogeneous alkaline transesterification: Influence of the catalyst

https://doi.org/10.1016/j.biortech.2011.01.072Get rights and content

Abstract

In this work, the process for ethyl ester production is studied using refined sunflower oil, and NaOH, KOH, CH3ONa, and CH3OK, as catalysts. In all cases, the reaction is carried out in a single reaction step. The best conversion is obtained when the catalyst is either sodium methoxide or potassium methoxide. We found that during the transesterification with ethanol, soap formation is more important than in the case of methanol. The saponification reaction consumes an important fraction of the catalyst. The amount of catalyst consumed by this reaction is 100% in the case of using hydroxides as catalyst (KOH or NaOH), and 25%, and 28% when using CH3ONa and CH3OK as catalysts, respectively. Ethanol increases the catalyst solubility in the oil–ethyl ester phase, thus accelerating the saponification reaction.

It is possible to obtain high conversions in a one-step reaction, with a total glycerine concentration close to 0.25%.

Introduction

Biodiesel is a renewable fuel that can be used pure (B100) or mixed with mineral gasoil in diesel engines. It can be obtained by transesterification of vegetable oils and animal fats (triglycerides) with an alcohol in presence of a catalyst. Typically, the alcohol is methanol, and the catalyst is sodium methoxide. As a result, methyl esters and glycerine are obtained as products. If the alcohol used in the reaction is ethanol, ethyl esters and glycerine are the products. As indicated by Boehman (2005), these fuels have numerous advantages compared to those obtained from petroleum. For example, lower emissions of particulate matter (smoke), carbon monoxide, sulfur dioxide, and hydrocarbons. On the other hand, the biodiesel has better cetane index and lubricity, lower toxicity and higher biodegradability. For these reasons, this fuel has been considered as an alternative in order to partially substitute the mineral gasoil (Freedman et al., 1984, Freedman et al., 1986, Schwad et al., 1987, Shay, 1993), gaining increasing acceptance throughout the world.

The main component of the biodiesel final cost is the raw material (oil or fat, and alcohol). The largest facilities throughout the world, used soybean oil (USA, Argentina and Brazil), rapeseed oil (Europe), and palm oil (Indonesia). In medium size industries, tallow is also used, such as those found in Paraguay and Brazil. However, the alcohol that has been used almost exclusively is the methanol. Due to the high methanol consumption, the strong dependency of the biodiesel industry on this alcohol implies certain risks, such as lack of supply and high prices. The methanol price follows the crude oil price. For example, in Brazil the cost of methanol in January 2008 was 1 u$s/L, while anhydrous ethanol was below 0.6 u$s/L. At that time the crude oil had a very high price. In April 2010, both alcohols cost approximately 0.52 u$s/L (http://www.unica.com.br, 2010; http://www.methanex.com, 2010). Nevertheless, the price of methanol is currently lower than that of ethanol, except in countries like Brazil, where the ethanol production is very important. Since the use of ethanol mixed with gasoline is also increasing in different countries, the production capacity is also increasing worldwide and, therefore, it can be expected a decrease in its cost.

Ethanol is a good candidate to replace methanol, and has several advantages. One of the most important is the fact that it is a renewable fuel and, consequently, the sustainability of the biodiesel obtained by transesterification of vegetable oils (or animal fats) with ethanol, is enhanced. In addition, the ethyl esters have higher cetane number and heating power (Clark et al., 1984). Another very important advantage of ethyl esters is that they have better cold properties, such as cloud point, cold filter plugging point, or pour point (Encinar et al., 2007). From the environmental point of view, the ethyl esters lead to lower emissions of nitrogen oxides (NOx), carbon monoxide, particulate matter, and has better biodegradability, compared to the methyl esters (Boehman, 2005, Makareviciene and Janulis, 2003). However, there is another important advantage of ethyl ester, which is related to the reaction stoichiometry:Triglyceride+3Ethanol3Ethyl ester+Glycerine881g/mol138g/mol927g/mol92g/mol

If the process yield is defined as the ethyl ester/triglyceride mass ratio, the yield in this case is 1.052% (ton of biodiesel/ton of oil). In the case of a production plant with a capacity of 300.000 ton/year, this increase in yield represent and extra production of 12.000 ton/year, as compared to the production with methanol.

Therefore, due to the technological advantages of ethyl esters, and the potential economical benefits, it is of interest to study the production process of ethyl esters.

Most of the works already reported on the ethyl esters production, did not find a suitable process. In some cases, excess of anhydrous ethanol are used, such as ethanol/triglycerides molar ratio 6:1, which in volume it represents 36 v/v% (Issariyakul et al., 2008, Marjanovic et al., 2010, Zhou et al., 2003). Another alternative presented in the literature, is the use of a cosolvent such as tetrahydrofuran (THF) (Zhou et al., 2003), which has a negative effect in the economy of the process.

There are several differences in the physicochemistry of the reacting system based on ethanol compared to methanol. For example, the higher mutual miscibility between the glycerine and the esters in the presence of ethanol, severely complicate the phase separation operation after the reaction. Depending upon the ethanol/oil volume fraction loaded to the reactor, the phase separation may not occur, being necessary to add glycerine (Encinar et al., 2007, Issariyakul et al., 2008) or to evaporate the ethanol (Bouaid et al., 2009) in order to induce phase separation. Another problem is the intensive soap formation that occurs in this system, and leads to the formation of stable emulsions that also complicates the separation of phases. Therefore, the washing procedure requires large volumes of water being necessary to improve this part of the process (Encinar et al., 2007).

There are several publications in which the transesterification with ethanol has been addressed (Bouaid et al., 2009, Černoch et al., 2010, Encinar et al., 2007, Marjanovic et al., 2010, Zhou et al., 2003). However, the results are very different among them, not being possible to draw general conclusions regarding the best conditions in order to carry out the reaction with high conversion levels. There are several possible reasons for these discrepancies. On one hand, we have observed that the system is extremely sensitive to minor changes in the experimental conditions, such as water content, reaction temperature fluctuations and deviations, the oil/ethanol ratio, and catalyst concentration. These variables, even with minor changes, might lead to a system with one or two phases at the end of the reaction, and this has an important effect on the final conversion. On the other hand, in several cases the analytical technique used to follow the conversion may lead to important errors. For example, the GC method, as described in the UNE-EN 14105 (2003) can be applied to methyl esters obtained with soybean, rapeseed, or sunflower oils. Therefore, this can be a source of errors in the analysis due to peak overlapping, mainly in the case of the monoglycerides quantification. In each case, this should be verified.

In this work, we present the study that we carried out in order to determine the effect of different homogeneous catalysts and reaction conditions, on the transesterification reaction using ethanol in a one-stage reaction. The formation of soaps in each case is analyzed, and correlated with the catalyst activity. Results obtained with methanol are included as comparison. The total glycerine content is analyzed by a volumetric procedure (Pisarello et al., 2010), which has no limitations regarding the raw material or the alcohol used in the reaction.

Section snippets

Transesterification reaction and phase separation

The reaction was carried out in a 0.5 L flask, with magnetic stirring, using a 50 mm Teflon-coated magnetic bar, and 800 rpm. Reaction temperature was in the range 20–70 °C, and reaction time between 1 and 3 h. Refined sunflower oil with acidity less than 0.1 oleic acid/100 g sample was used as raw material. Preliminar experiments were carried out, in order to determine if the speed of agitation was enough in order to avoid mass transfer limitations. We found that in the above-described reacting

Results and discussion

We carried out experiments in similar conditions to those used by other authors in order to determine the reproducibility of the experiments of transesterification with ethanol. For example, Bouaid et al. (2007) reported an experiment carried out with sunflower oil, 1.5 wt.% KOH, ethanol/oil molar ratio = 5:1, at 32 °C. They obtained that the final product contained 0% MG, 0% DG, 0%TG and 0.003% GT. We obtained under the same conditions the following composition: 1.79% MG, 2.21% DG y 2.96% TG.

Conclusions

In this work, we showed that the saponification reaction is much faster in the case of using ethanol during the transesterification, and because of this, higher catalyst consumption takes place and consequently, a lower reaction rate with a lower final conversion is observed. On the other hand, in the presence of methanol, the amount of soaps formed during the reaction is considerably lower, and this is another reason for the much higher activity observed when using this alcohol.

Using

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