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Biodiesel production by two-stage transesterification with ethanol
Bioresource Technology 102 (2011) 10407–10413
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Biodiesel production by two-stage transesterification with ethanol G. Mendow, N.S. Veizaga, B.S. Sánchez, C.A. Querini ⇑ Instituto de Investigaciones en Catálisis y Petroquímica-INCAPE-(FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 5 August 2011 Accepted 11 August 2011 Available online 22 August 2011 Keywords: Biodiesel Ethanol Ethyl ester Transesterification
a b s t r a c t A two-stage process consisting of two reactions steps with glycerin separation and ethanol/catalyst addition in each of them was optimized for ethyl esters production. The optimal reaction temperature was 55 °C. At an ethanol/oil molar ratio of 4.25:1 (25% v/v alcohol with respect to oil), a 99% conversion value was obtained with low ethanol consumption. In contrast to methoxide catalysts, sodium and potassium hydroxide catalysts severely complicate the purification since no phase separation took place under most conditions. With a total sodium methoxide concentration of 1.06 g catalyst/100 g oil, and adding 50% of the catalyst in each reaction step, biodiesel with a total glycerin content of 0.172% was obtained. The optimal conditions found in this study make it possible to use the same industrial facility to produce either methyl or ethyl esters. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is defined as a mixture of alkyl esters obtained by transesterification of vegetable oils or animal fats with a shortchain alcohol, typically methanol or ethanol. Ethyl esters-based biodiesel presents numerous advantages over more commonly used methyl esters. Ethyl esters exhibit lower particulate matter and green-house gases emissions, such as carbon dioxide and nitrogen oxides (NOx), and are more biodegradable in water than methyl esters (Boehman, 2005; Makareviciene and Janulis, 2003). They present a higher cetane index and heating power (Clark et al., 1984) and lower cloud, cold filter plugging, and pour points (Encinar et al., 2007). Ethanol is generally obtained from agricultural sources and thus ethyl esters are a renewable biofuel. From an economical point of view, there is another important advantage of ethyl ester, which is related to the reaction stoichiometry (Scheme 1). When the process yield is defined as the ethyl ester/triglyceride mass ratio, the maximum yield is 105.2% (ton of biodiesel/ton of oil) for ethanol, as compared to 100.5% for methanol. In the case of a production plant with a 300,000 ton/year capacity, for instance, this increase in yield represents and extra production of 12,000 ton/year, as compared to the production with methanol. The technological and environmental advantages of ethyl esters, as well as the potential economical benefits, make their production process an interesting area of research. Ethyl esters can be obtained from vegetable oils and animal fats by alkaline (Alamu et al., 2008), acid (Morin et al., 2007), enzymatic (Moreira et al., 2007), or heterogeneously (Kim et al., 2010; Li et al., 2009) catalyzed reactions, and ⇑ Corresponding author. Tel.: +54 342 4533858; fax: +54 342 4531068. E-mail address: querini@fiq.unl.edu.ar (C.A. Querini). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.08.052
by non-catalyzed processes under sub or supercritical conditions (Warabi et al., 2004). However, alkaline catalysis is the most effective and widely used method (Meneghetti et al., 2006). Although transesterification with ethanol has been studied repeatedly (Bouˇ ernoch et al., 2010; Encinar et al., 2007; Marjanovic aid et al., 2009; C et al., 2010; Zhou et al., 2003), procedures that allow ethanol-based biodiesel production with high conversion levels under conditions suitable for industrial application have not been developed. Ethanol-based transesterification is extremely sensitive to minor changes in water content, reaction temperature, oil/ethanol ratio, and catalyst concentration; that can lead to a system with one or two phases at the end of the reaction. Determining accurate yields of ethanol-based biodiesel production is complicated by the fact that GC methods as described in the UNE-EN 14105 (2003) can be errorprone due to peak overlapping when monoglycerides are present. Therefore, in the current study, total glycerin content was analyzed by a volumetric procedure (Pisarello et al., 2010) which has no limitations regarding the raw material or the alcohol used in the reaction. For total ester content determination carried out according to the EN 14103 standard using the C17 methyl ester as internal standard, the response factor of ethyl esters must be corrected. These issues could account for the differences in ethyl ester yields reported by Soares et al. (2010), Bouiard et al. (2009), Encinar et al. (2007), and Issariyakul et al. (2008). Obtaining economically acceptable transesterification is more difficult to achieve with ethanol than with methanol since glycerin and ethyl esters in the presence of ethanol are mutually soluble, which severely complicates phase separation after the reaction. Depending upon the ethanol/oil volume ratio fed to the reactor, phase separation may not occur spontaneously and necessitates the addition of glycerin (Encinar et al., 2007; Issariyakul et al., 2008) or evaporation of the ethanol (Bouaid et al., 2009). More
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Triglyceride
G. Mendow et al. / Bioresource Technology 102 (2011) 10407–10413
+
3 Ethanol
881 g/mol
138 g/mol
3 Ethyl ester 927 g/mol
+
Glycerin 92 g/mol
Scheme 1. Ethanolysis reaction stoichiometry.
soap is formed in the presence of ethanol than methanol (Mendow et al., 2011) and the resulting stable emulsions complicate phase separation and necessitates washing procedure with large volumes of water (Encinar et al., 2007). Mendow et al. (2011) studied single-step sunflower oil transesterification with ethanol in the presence of different catalysts and under various conditions, but were unable to attain biodiesel conforming to the international standards of 0.23 wt% bound glycerin. This outcome was mainly attributed to a very fast saponification, which consumes the catalyst. In the current study a two-stage process, consisting of two reactions steps with glycerin separation and ethanol/catalyst addition in each of them was optimized for the production of ethyl esters, was implemented to increase the reaction conversion and thus achieve values required by the EN 14214 standard. Several catalysts and reaction conditions were used with the aim of optimizing the production process.
2. Experimental 2.1. Biodiesel production process 2.1.1. 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 a stirring speed of 1200 rpm. Refined sunflower oil (acidity <0.1 g oleic acid/100 g oil) was used as raw material to study the effect of different operation variables on the conversion. The effect of the temperature was studied using the refined sunflower oil and refined beef tallow (acidity <0.1 g oleic acid/100 g oil). Table 1 shows the fatty acid composition of the sunflower oil. The reaction was carried out in two stages. The reaction temperature was varied between 45 and 70 °C, and reaction times from 30 to 120 minutes were tested. For the first stage, the oil was loaded into the reactor, and the temperature adjusted to the desired value. Once the oil reached this value, the alcohol containing the catalyst was added within a time range of 3 to 5 seconds. Time zero for the soap and catalyst measurements was the moment at which the ethanol/catalyst mixture was transferred to the reaction vessel. Ethanol 99.5% purity (water content 1150 ppm) from Cicarelli was used as transesterification alcohol. Typically, the ethanol/oil molar ratios employed were 2.55:1 for the first stage and 1.7:1 in the second stage. On a volumetric basis, these ratios correspond to 15% and 10% v/v (volume of ethanol/volume of oil 100),
Table 1 Fatty acid composition of refined sunflower oil. Fatty acid
Composition wt.%
14:0 16:0 16:1 18:0 18:1 18:2 18:3 20:0 20:1 22:0
0.09 5.98 0.17 3.15 28.49 58.13 0.19 0.26 0.30 1.24
respectively. Experiments employing 30%, 40% and 50% v/v of ethanol were also carried out to study the effect of the ethanol content on conversion. The catalyst was either sodium or potassium hydroxide dissolved in methanol or ethanol, or sodium or potassium methoxide solutions (30 and 32 wt.% in methanol respectively) provided by Evonik-Degussa. The amount of catalyst was calculated such that it could neutralize the free fatty acids and catalyze the reaction. The catalyst concentration was varied between 1 and 1.44 wt.% (g catalyst/100 g oil). At the end of the first reaction step, phase separation was carried out in a separatory funnel at room temperature, the biodiesel-rich phase, which also contained the remaining ethanol, dissolved glycerin, and catalyst, was transferred to a 0.5 L flask, and the second reaction step was performed using the same procedure as the one employed during the first reaction step. The glycerin-rich phase formed during the second stage was separated using a separatory funnel. The experiments were repeated once and the results for the two experiments differed by less than 11%. 2.1.2. Biodiesel purification The biodiesel-rich phase was extracted with an aqueous solution of HCl 5 wt.% followed by extraction with water. Both extraction stages (washings) were carried out with gentle agitation at 60 °C, during 15 min. The volume of aqueous phase was 30% v/v relative to the biodiesel phase. The biodiesel was dried by stripping with nitrogen at 80 °C. 2.1.3. Effect of reaction temperature A set of experiments was carried out to determine the effect of the reaction temperature. The total glycerin analyses were performed by gaseous chromatography, and also using the volumetric technique above mentioned. The total glycerin content is the sum of the bound glycerin (as mono-, di- and tri-glycerides) and the free glycerin. In this work, the biodiesel purification techniques employed were optimized in order to assure negligible free glycerin content in the final product. For this reason, the total glycerin content measured is the same as the bound glycerin, representing the percentage of non-converted glycerides. The total glycerin (T.G.) value is calculated with the following equation, from EN 14105:
%T:G: ¼ %GF þ 0:255 ð%M:G:Þ þ 0:146 ð%D:G:Þ þ 0:103 ð%T:G:Þ
Where %T.G. is the total glycerin percentage, %GF is the free glycerin percentage, and %M.G., %D.G. and %T.G. are the monoglycerides, diglycerides and triglycerides percentages, respectively, obtained by gaseous chromatography. The reaction temperature was varied between 45 and 65 °C. The raw materials evaluated were refined sunflower oil and refined beef tallow. The stirring speed was 1200 rpm, while the ethanol/ oil molar ratio was 4.25:1 in all the experiments. The total catalyst concentration (sodium methoxide) used was 1.13 wt.% for the sunflower oil and 1.07 wt.% for the beef tallow. 2.1.4. Effect of the ethanol/oil ratio The experiments were performed in two stages, using refined sunflower oil, 70 °C temperature, total catalyst concentration 1.06 wt.%., reaction time 30 and 60 minutes in the first and second stage, respectively, and stirring speed 1200 rpm. The ethanol used in the first and in the second reactions stages were (in a volumetric basis): 15%/10%; 15%/15%, 20%/20%, and 25%/25%, in the experiments carried out using a total amount of ethanol of 25%, 30%, 40%, and 50% v/v, respectively. These values correspond to a volumetric basis (volume of ethanol/volume of oil 100). The ethanol/
G. Mendow et al. / Bioresource Technology 102 (2011) 10407–10413
oil molar ratios that correspond to these experiments are 4.25:1, 5.1:1, 6.8:1, and 8.5:1, respectively. 2.1.5. Effect of catalyst distribution in each reaction stage A set of experiments was designed in order to optimize the concentration of catalyst to be used in each reaction step. These experiments were performed varying the proportions of catalyst added at each step, but using the same total amount of catalyst. In all cases, the raw material used was refined beef tallow (acid value = 0.56 mg KOH/100 g), the stirring speed was 1200 rpm and the reaction time 30 and 60 minutes for the first and second stage, respectively. The total catalyst concentration was 1.06 wt.%. Two series of experiments were carried out, being the alcohol volume 25 and 31% v/v (expressed in a volumetric basis), and the temperature 55 and 60 °C, respectively. The catalyst dosages evaluated were (1° Stage/2° Stage): 0.24/0.72; 0.36/0.6; 0.48/0.48; 0.6/0.36 and 0.72/0.24 g NaCH3O/100 mL oil, which correspond to the following distribution: 25/75%; 37.5/62.5%; 50/50%; 62.5/37.5% and 75/25%, respectively. 2.1.6. Glycerin recirculation The glycerin obtained in the second reaction stage was loaded in the first reaction stage in the following cycle. The objective is to feed the catalyst and the alcohol contained in the glycerin-rich phase. The reaction conditions were: stirring speed 1200 rpm; temperature 55 °C in both stages; 15% v/v ethanol in the first stage and 10% v/v ethanol in the second stage (expressed in a volumetric basis); 0.6 wt.% and 0.58 wt.% NaCH3O in the first and second stage, respectively; reaction time 30 and 60 minutes in the first and second stage, respectively. 2.2. Catalyst analysis Small samples (approx. 5 g) of the reacting mixture were taken at different times to determine the catalyst content by an acid–base titration method (AOCS Cc17-79 or IRAM 5599). The biodiesel sample was dissolved in a toluene-ethanol mixture (50:50 v/v), and titrated with 0.1 N HCl solution, using phenolphthalein as indicator. 2.3. Total glycerin analysis by a volumetric procedure The triglycerides conversion was followed by measuring the bound-glycerin by GC (see Section 2.4), as specified in the EN 14105 and ASTM D-6584 standards. Since these standards are validated only for the transesterification of soybean, rapeseed, or sunflower oils, with methanol, an alternative method was also used. In this volumetric procedure (Pisarello et al., 2010), the total glycerin content was obtained after the mono-, di- and tri-glycerides present in the sample were quantitatively transformed into methyl esters and glycerin by transesterification. The glycerin was extracted first with acidified water, and then with water. Finally, glycerin was titrated according to standard procedures, such as ASTM D1615, IRAM 41089, BS-5711, or AOCS 14-56. Unless otherwise stated, the progress of the reaction is represented by the total glycerin value (GT), determined by the volumetric procedure. 2.4. GC analysis Mono-, di-, and tri-glycerides contents (MG, DG, and TG, respectively) were determined according to UNE-EN 14105. 2.5. Glycerin-rich phase analyses The soap and catalyst content in the glycerin-rich phase were determined by an acid–base titration method (AOCS Cc17-79 or IRAM 5599). The glycerin sample was dissolved in distilled water,
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and titrated with 0.1 N HCl solution, using phenolphthalein as indicator to determine the catalyst concentration, and bromophenol blue to quantify the soaps concentration. The glycerin content was determined by titration as indicated in the ASTM D-1615, IRAM 41089, BS-5711, or AOCS 14-56. The ethanol content in the glycerol phase was measured using the head space method in accordance with UNE-EN 14110, using isopropanol as the internal standard and an adequate calibration curve. 3. Results and discussion 3.1. Transesterification process The two-step transesterification process is based on the Henkel industrial process, which is carried out in a two-stage reaction with glycerin separation after each stage (Assmann et al., 1991). Since it was previously shown (Mendow et al., 2011) that approximately 91% of the catalyst that was not consumed in the soap formation reaction accumulated in the glycerin-rich phase, removal of this phase and addition of new catalyst should promote further transesterification. It is very important to limit the progress of the transesterification in the first stage so that sufficient glycerol can be produced in the second stage to allow phase separation. In order to quantify this limit, systematic experiments were performed, in which the amount of catalyst dosed in the first stage was progressively increased, in order to achieve higher conversion values. Then, the second stage was performed, observing whether or not phase separation occurred. It could be established that above a glycerides conversion of 90–92% in the first stage, a spontaneous phase separation in the second stage can not be accomplished and the catalyst and the soap remained in the biodiesel phase causing emulsion formation during the washing steps. 3.2. Effect of reaction temperature The optimal temperature for transesterification of beef tallow and sunflower oil was 55 °C (Fig. 1); a temperature that minimizes total glycerin. At temperatures below this value, the reaction rate is lower in agreement with the Arrhenius law. At temperatures higher than 55 °C, the reaction rate decreases due to an increase of the ethanol partial pressure that diminishes its concentration in the reacting phase. These results agree with those of Mendow et al. (2011), who showed that in a single reaction step, the best conversion was obtained at 55 °C. At temperatures higher than 55 °C fast saponification also occurs (see Section 3.3) causing a decrease in catalyst concentration and lower final conversion. 3.3. Effect of catalyst Fig. 2 shows the mono, di and triglycerides content of the samples after the first reaction step. When potassium hydroxide was used as catalyst, 51.34% of triglycerides remain unconverted and with sodium hydroxide, this amount was only slightly lower. The amount of diglycerides obtained with these two catalysts was also practically the same, but the monoglycerides content was higher when sodium hydroxide was the catalyst, as compared with the use of potassium hydroxide. Thus, sodium hydroxide was slightly more active than potassium hydroxide, but the conversion values were lower than those obtained with methoxides as catalysts. Both potassium and sodium hydroxides were completely consumed after the first minutes (Fig. 3) due to saponification (Mendow et al, 2011). In the case of the NaOH, 93.5% of the initial
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100
Sunflower Oil Beef Tallow
0.21 0.18 0.15 0.12 0.09 40
45
50 55 60 Temperature (ºC)
65
70
% MG, DG, TG.
NaOH KOH
60 40 20 0 5
10
15 20 Time (min.)
25
30
Fig. 3. Changes in catalyst concentration over time for the different catalysts used in the first transesterification step. Reaction conditions: ethanol/oil molar ratio 2.55:1 (15% v/v); temperature 55 °C; stirring speed 1200 rpm; raw material, sunflower oil; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil).
Table 2 Mono-, di- and tri-glycerides content at the end of the second reaction stage with different catalysts. Reaction conditions: ethanol/oil molar ratio 1.7:1; temperature 55 °C; stirring speed 1200 rpm; raw material, sunflower oil; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil); reaction time 60 min.
% MG % DG
40
KCOH3
0
Fig. 1. Total glycerin content vs. temperature, using refined sunflower oil and refined beef tallow. Reaction conditions: stirring speed 1200 rpm; ethanol/oil molar ratio 4.25:1; total sodium methoxide concentration 1.13 wt.% for sunflower oil and 1.06 wt.%. for beef tallow; reaction time: 30 and 60 minutes for the first and second stages, respectively.
50
NaCOH3
80 Catalyst (mmol/Kg)
Total Glycerin (%)
0.24
% TG
30
Catalyst
% MG
% DG
% TG
NaCH3O KCH3O EN 14214 Standard
0.52 0.73 0.8
0.14 0.16 0.2
0.05 0.01 0.2
obtained meets the quality standards, but based on its lower cost and higher availability, sodium methoxide would be preferable.
20
3.4. Reaction time
10 0
NaCH3O
KCH3O
NaOH
KOH
Catalyst Fig. 2. Mono-, di- and tri-glycerides (MG, DG, TG, respectively) content of biodiesel obtained at the end of the first reaction step. Reaction conditions: ethanol/oil molar ratio 2.55:1 (15% v/v); temperature 55 °C; stirring speed 1200 rpm; raw material, sunflower oil; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil); reaction time 30 minutes.
amount was consumed after 5 minutes of reaction, while in the case of the KOH, 99.2% was consumed in the same time. No phase separation was observed when the hydroxides were used, as was observed by others (Cˇernoch et al., 2010; Encinar et al., 2007; Issariyakul et al., 2008). In contrast, when sodium or potassium methoxides were used, a fast phase separation was achieved. When the methoxides were used, the triglycerides conversion was much higher than that obtained with hydroxides (Fig. 2). The amount of non-converted triglycerides was 8.98% and 5.17% when sodium metoxide and potassium methoxide were used, respectively. Therefore, potassium methoxide was more active than sodium methoxide for the conversion of tri- to di-glycerides. Table 2 shows the non-reacted mono-, di- and tri-glycerides after the second reaction stage, as well as the values established by the European standard EN 14214. It can be concluded that both catalysts present excellent performance, since the biodiesel
Fig. 4A shows the evolution of mono-, di- and tri-glycerides with time in the first reaction step. After approximately 15 minutes, the concentrations of mono- and tri-glycerides reached a constant value, whereas the diglycerides slightly decreased between 15 and 30 minutes. These data indicate that the first reaction can be completed in 30 minutes. Fig. 4B shows the glycerides concentration as a function of time during the second reaction step. After 30 minutes of reaction, the mono-, di- and tri-glycerides concentrations reached constant values and therefore, the reaction time for the second stage should be 30 minutes, same as the first stage. Thus, the chemical reactors can have approximately the same volume. This finding is important since industrial production of methyl esters is carried out with this configuration and consequently, conversion to ethanol-based transesterification would not require new equipment. As it is shown in Fig. 4, in the first reaction step (Fig. 4A) the triglycerides concentration was reduced from the initial 100 wt.% to 17 wt.% in the first 5 minutes. In the following 25 minutes the triglycerides concentration reached 9 wt.%. The second reaction stage presented a similar behavior (Fig. 4B). The triglycerides concentration decreased from 9 to 3 wt.% within the first 5 minutes, and then, they disappeared very slowly, reaching a final value concentration of 0.08 wt.%. 3.5. Effect of ethanol/oil molar ratio Fig. 5 shows the effect of the total ethanol/oil molar ratio on the glycerides conversion, in experiments carried out as described in
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A
1st Stage
100
B
2nd Stage
10
% MG
% MG
90
% DG
% DG
8
% TG Glycerides (%)
Glycerides (%)
% TG
25 20 15 10
6 4 2
5 0
0
5
10
15
20
25
30
Time (min.)
0
0
10
20
30 40 Time (min)
50
60
Fig. 4. Mono-, di- and tri-glycerides (MG, DG, TG, respectively) concentrations measured during the transesterification reaction of refined sunflower oil using sodium methoxide as catalyst. Stirring speed 1200 rpm; temperature 55 °C; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil). (A) First reaction stage. Ethanol:oil molar ratio 2.55:1 (15% v/v). (B) Second reaction stage. Ethanol/oil molar ratio 1.7:1.
Section 2.1.4. This Figure shows the total glycerin content (wt.%) in the final product vs the total amount of alcohol fed to the reacting system, taking into account the two reaction stages (% v/v). Increasing the alcohol concentration up to 36% v/v led to a decrease in the total glycerin concentration, i.e., the conversion improved. These were expected results and are in agreement with previous reports (Bouaid et al. 2007; Encinar et al., 2007; Fillières et al., 1995; Zhou et al., 2003). However, when the alcohol concentration was increased above 36% v/v, the conversion dropped. This concentration corresponds to an ethanol/oil molar ratio of 6:1. Above this value, the catalyst dilution becomes more important than the increase in alcohol concentration. In addition, if the ethanol/oil molar ratios are higher than 6:1, the separation of glycerol is difficult, since the excess ethanol hindered the decantation by gravity because both phases have similar densities. Nevertheless, in all cases the conversion values were high, and the total glycerin percentages were below the maximum value establishes by the international standards (Total Glycerin = 0.25 wt.%). For example,
EN14214 Limit (0.25 %)
Total Glycerin (%)
0.25 0.20 0.15 0.10 0.05 0.00 15
20
25
30 35 40 % Ethanol
45
50
Fig. 5. Total glycerin content vs. total ethanol percentage using refined sunflower oil as raw material and sodium methoxide as catalyst. Reaction conditions: temperature 70 °C; total catalyst concentration 1.06 wt.%; reaction time, 30 and 60 minutes for the first and second stage, respectively; stirring speed 1200 rpm.
using an ethanol/oil molar ratio of 4.25:1, which equals 25% v/v ethanol with respect to oil, the total glycerin content was lower than 0.15 wt.%, which is very good, and corresponds to a triglyceride conversion of 99%. Therefore, the ethanol/oil molar ratio of 4.25:1 was selected in order to obtain an appropriate conversion value using an ethanol:oil ratio lower than 6:1.
3.6. Effect of catalyst distribution in each reaction stage The effect of catalyst dosing in each reaction stage was studied as described in Section 2.1.5. With any of the temperatures and ethanol concentrations used, there was an optimum catalyst dosage of 50% of the total amount in reaction stage. This value maximized the conversion, and made it possible to obtain the biodiesel within specifications. Once the reaction ended, most of the catalyst remained in the glycerin phase. During the transesterification reaction, the volume of the glycerol-rich phase increased, and due to its capacity to extract the catalyst, the sodium methoxide concentration in the reacting phase (ethanol, biodiesel, glycerides) dropped. Consequently, the reaction rate decreased. When the amount of catalyst added in the first stage was low, and high in the second stage, the final conversion did not reach the required value (Total Glycerin% <0.25 wt.%), because a low catalyst concentration in the first step, led to a low amount of glycerin formed during this stage. Then, in the second stage, when the catalyst concentration was high, a large amount of glycerin was quickly generated. This glycerin extracted the sodium methoxide and, consequently, the amount of catalyst available in the oil-rich phase was not enough to complete the reaction. With the 37.5/ 62.5 dosage, the conversion significantly improved. When adding more catalyst in the first stage, the amount of glycerin generated was higher, and thus, the production of glycerin in the second stage decreased. Therefore, less catalyst migrated to the glycerin phase during the second stage, making it more available for the reaction and improving the global conversion. As shown in Fig. 6, with the 62.5/37.5% dosage, the conversion was lower than with the 50/50%, because both a percentage of the catalyst added in the first stage was eliminated from the system when the glycerin was removed, and the catalyst concentration in the second step was not enough to complete the reaction.
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0.35
Table 3 Amount and composition of the glycerin-rich phases generated in the first and second reaction stages. Catalyst: sodium methoxide.
% Total Glycerin
0.30 0.25 0.20
a
0.15 0.10 0.05
Phase
Glycerin (wt.%)
Soaps (wt.%)
Glycerol 1st Stage Glycerol 2nd Stage
89.7
1.3
80.9
2.9
37.25 % EtOH - 60 ºC
0
6.9
2.1
5.3
10.5
5.7
4.2
Mass ratio of glycerin and oil phases.
Exp.
25/75 37.5/62.5 50/50 62.5/37.5 75/25 st
Glycerin/oila (%)
Table 4 Conversion during transesterification: (A, C) with glycerin recirculation; (B, D) without glycerin recirculation. Reaction conditions: stirring speed 1200 rpm; temperature 70 °C; reaction time 30 and 60 minutes for the first and second stage; ethanol: first stage 15% v/v, second stage 10% v/v. Catalyst: sodium metoxide.
25 % EtOH - 55 ºC
% Catalyst (1 Stage/2
Ethanol (wt.%)
Catalyst (wt.%)
nd
Stage)
Fig. 6. Total glycerin content vs. sodium methoxide concentration used in the first/ second reaction stage. Reaction conditions: Alcohol percentage 25% and 31% and temperature 55 and 60 °C, respectively. Stirring speed 1200 rpm; reaction time, 30 and 60 minutes for the first and second stage, respectively; total catalyst concentration 1.06 g NaCH3O/100 g oil. Raw material: refined beef tallow.
A similar and even more pronounced phenomenon was observed in the case of the 75/25% dosage. Fig. 6 also shows the effect of the temperature. It can be observed that at 60 °C, and in spite of using a higher ethanol concentration, the conversion was lower than at 55 °C. This result agrees with the data shown in Fig. 1.
3.7. Glycerin recirculation The effect of glycerin recirculation from the second decanter into the first reactor was investigated, using the conditions described in Section 2.1.6. The composition of the glycerin phases at the exit of first and second decanters are shown in Table 3. In addition, the last column shows the mass percentage of the glycerin phase, relative to the oil fed. The total glycerin value measured in the biodiesel obtained using the conditions described in Section 2.1.6, was 0.12 wt.%. It can be observed in Table 3 that the catalyst content of the glycerin phases was very high. The amount of sodium metoxide contained in the glycerol generated in the second reaction stage represented approximately 0.44 wt.% of catalyst, which is equivalent to 73.5% of the total amount of catalyst needed for the first reaction step. However, regarding the ethanol provided by this recirculation process, and taking into account that the ethanol content in the glycerol phase was 10.5 wt.%, the amount of alcohol entering to first reactor with the glycerin stream is below 0.5% v/ v (volume of ethanol/volume of oil 100). The results of recirculating experiments are summarized in Table 4. It can be observed that although the amount of catalyst added in experiment A was substantially higher than in experiment B, there was only a slight improvement in the conversion. Experiments C and D were carried out to address the effect of the presence of a glycerin-rich phase on the conversion. In these two experiments, the same total amount of catalyst (0.88 wt.%) was loaded to the first reactor. In experiment C, this amount corresponded to the contribution of both fresh catalyst (0.33 wt.%), and the catalyst contained in the recycled glycerin (0.55 wt.%). In experiment D, no glycerin recirculation was carried out and, therefore, only fresh catalyst was added (0.89 wt.%). The results shown
A B C D
1st Stage
2nd Stage
Catalyst fed (wt.%)
Catalyst in glycerol (wt.%)
Total catalyst (wt.%)
Catalyst fed (wt.%)
Total glycerin (wt.%)
0.38 0.38 0.33 0.89
0.29 – 0.55 –
0.67 0.38 0.88 0.89
0.38 0.38 0.58 0.58
0.268 0.307 0.347 0.196
in Table 4 indicate that the presence of the glycerin phase led to lower conversion because the equilibrium was shifted towards the reagents side, and the glycerin acted as an extracting medium, diminishing the amount of catalyst and alcohol available in the reacting phase. This observation shows the importance of glycerin withdrawal to avoid the preferential concentration of catalyst and alcohol in this phase. The results shown in Table 4 provide additional evidences of the effect of the reaction temperature. The data shown in this table and Fig. 1 show that the reaction rate at 70 °C is lower than at 65 °C. 4. Conclusions An efficient two-stage ethyl esters production process was developed, using sodium methoxide as catalyst and 25% v/v of ethanol, which in a molar basis is lower than the ratio used with methanol. The conversion is very sensitive to the way that the catalyst is distributed between the two reaction stages, because the glycerin extracts it from the reaction media. This two-stage strategy made it possible to obtain biodiesel with a conversion that meets the requirements of the international standards, using 30 minutes of residence time in each stage, and a volume of alcohol equal to that used in the case of methanol. Thus, it is possible to use the same industrial facilities either with methanol or with ethanol. References Alamu, O.J., Waheed, M.A., Jekayinfa, S.O., 2008. Effect of ethanol–palm kernel oil ratio on alkali-catalyzed biodiesel yield. Fuel 87, 1529–1533. Assmann, G., Blassé, G., Gutsche, B., Jeromin, L., Rigal, J., Armengaud, R., Cormary, B., 1991. World patent WO 91/05034. Boehman, L., 2005. Biodiesel production and processing. Fuel Process. Technol. 86, 1057–1058. Bouaid, A., Martínez, M., Aracil, J., 2007. A comparative study of the production of ethyl esters from vegetable oils as a biodiesel fuel optimization by factorial design. Chem. Eng. J. 134, 93–99. Bouaid, A., Martínez, M., Aracil, J., 2009. Production of biodiesel from bioethanol and Brassica carinata oil: oxidation stability study. Bioresour. Technol. 100, 2234– 2239. ˇ ernoch, M., Hájek, M., Skopal, F., 2010. Ethanolysis of rapeseed oil – Distribution of C ethyl esters, glycerides and glycerol between ester and glycerol phases. Bioresour. Technol. 101, 2071–2075.
G. Mendow et al. / Bioresource Technology 102 (2011) 10407–10413 Clark, S.J., Wagner, L., Schrock, M.D., Pienaar, P.G., 1984. Methyl and ethyl soybean esters as renewable fuels for diesel engines. J. Am. Oil Chem. Soc. 61, 1632– 1638. Encinar, J.M., González, J.F., Rodríguez-Reinares, A., 2007. Ethanolysis of used frying oil. Biodiesel preparation and characterization. Fuel Process. Technol. 88, 513– 522. Fillières, R., Benjelloun-Mlayah, B., Delmas, M., 1995. Ethanolysis of rapeseed oil: quantification of ethyl esters, mono-, di- and tri-glycerides and glycerol by high-performance size-exclusion chromatography. J. Am. Oil Chem. Soc. 72, 427–432. Issariyakul, T., Kulkarni, M.G., Meher, L.C., Dalai, A.K., Bakhshi, N.N., 2008. Biodiesel production from mixtures of canola oil and used cooking oil. Chem. Eng. J. 140, 77–85. Kim, M., Yan, S., Salley, S.O., Ng, K.Y.S., 2010. Competitive transesterification of soybean oil with mixed methanol/ethanol over heterogeneous catalysts. Bioresour. Technol. 101, 4409–4414. Li, E., Xu, Z.P., Rudolph, V., 2009. MgCoAl–LDH derived heterogeneous catalysts for the ethanol transesterification of canola oil to biodiesel. Appl. Catal. B: Environ. 88, 42–49. Makareviciene, V., Janulis, P., 2003. Environmental effect of rapeseed oil ethyl ester. Renewable Energy 28, 2395–2403. Marjanovic´, A.V., Stamenkovic´, O.S., Todorovic´, Z.B., Lazic´, M.L., Veljkovic´, V.B., 2010. Kinetics of the base-catalyzed sunflower oil ethanolysis. Fuel 89, 665–671. Mendow, G., Veizaga, N.S., Querini, C.A., 2011. Ethyl ester production by homogeneous alkaline transesterification: influence of the catalyst. Bioresour. Technol. 102, 6385–6391.
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Meneghetti, S.M.P., Meneghetti, M.R., Wolf, C.R., Silva, E.C., Lima, G.E.S., Coimbra, M.A., Soletti, J.I., Carvalho, S.H.V., 2006. Ethanolysis of castor oil and cottonseed oil: a systematic study using classical catalysts. J. Am. Chem. Soc. 83, 819–822. Moreira, A.B.R., Perez, V., Zanin, G.M., de Castro, H.F., 2007. Biodiesel synthesis by enzymatic transesterification of palm oil with ethanol using lipases from several sources immobilized on silica–PVA composite. Energy Fuels 21, 3689– 3694. Morin, P., Hamad, B., Sapaly, G., Carneiro Rocha, M.G., Pries de Oliveira, P.G., Gonzalez, W.A., Andrade Sales, E., Essayem, N., 2007. Transesterification of rapeseed oil with ethanol I. Catalysis with homogeneous Keggin heteropolyacids. Appl. Catal. A: General 330, 69–76. Pisarello, M.L., Dalla Costa, B.O., Veizaga, N.S., Querini, C.A., 2010. Volumetric method for free and total glycerin determination in biodiesel. Ind. Eng. Chem. Res. 49, 8935–8941. Soares, C.M., Itavo, L.C.V., Dias, A.M., Arruda, E.J., Delben, A.A.S.T., Oliveira, S.L., de Oliveira, L.C.S., 2010. Forage turnip, sunflower, and soybean biodiesel obtained by ethanol synthesis: Production protocols and thermal behavior. Fuel 89, 3725–3729. Warabi, Y., Kusdiana, D., Saka, S., 2004. Reactivity of triglycerides and fatty acids of rapeseed oil in supercritical alcohols. Bioresour. Technol. 91, 283–287. Zhou, W., Konar, S.K., Boocock, D.G.B., 2003. Ethyl esters from the single-phase basecatalyzed ethanolysis of vegetable oils. J. Am. Oil Chem. Soc. 80, 367–371.
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Biodiesel production by two-stage transesterification with ethanol G. Mendow, N.S. Veizaga, B.S. Sánchez, C.A. Querini ⇑ Instituto de Investigaciones en Catálisis y Petroquímica-INCAPE-(FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 5 August 2011 Accepted 11 August 2011 Available online 22 August 2011 Keywords: Biodiesel Ethanol Ethyl ester Transesterification
a b s t r a c t A two-stage process consisting of two reactions steps with glycerin separation and ethanol/catalyst addition in each of them was optimized for ethyl esters production. The optimal reaction temperature was 55 °C. At an ethanol/oil molar ratio of 4.25:1 (25% v/v alcohol with respect to oil), a 99% conversion value was obtained with low ethanol consumption. In contrast to methoxide catalysts, sodium and potassium hydroxide catalysts severely complicate the purification since no phase separation took place under most conditions. With a total sodium methoxide concentration of 1.06 g catalyst/100 g oil, and adding 50% of the catalyst in each reaction step, biodiesel with a total glycerin content of 0.172% was obtained. The optimal conditions found in this study make it possible to use the same industrial facility to produce either methyl or ethyl esters. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is defined as a mixture of alkyl esters obtained by transesterification of vegetable oils or animal fats with a shortchain alcohol, typically methanol or ethanol. Ethyl esters-based biodiesel presents numerous advantages over more commonly used methyl esters. Ethyl esters exhibit lower particulate matter and green-house gases emissions, such as carbon dioxide and nitrogen oxides (NOx), and are more biodegradable in water than methyl esters (Boehman, 2005; Makareviciene and Janulis, 2003). They present a higher cetane index and heating power (Clark et al., 1984) and lower cloud, cold filter plugging, and pour points (Encinar et al., 2007). Ethanol is generally obtained from agricultural sources and thus ethyl esters are a renewable biofuel. From an economical point of view, there is another important advantage of ethyl ester, which is related to the reaction stoichiometry (Scheme 1). When the process yield is defined as the ethyl ester/triglyceride mass ratio, the maximum yield is 105.2% (ton of biodiesel/ton of oil) for ethanol, as compared to 100.5% for methanol. In the case of a production plant with a 300,000 ton/year capacity, for instance, this increase in yield represents and extra production of 12,000 ton/year, as compared to the production with methanol. The technological and environmental advantages of ethyl esters, as well as the potential economical benefits, make their production process an interesting area of research. Ethyl esters can be obtained from vegetable oils and animal fats by alkaline (Alamu et al., 2008), acid (Morin et al., 2007), enzymatic (Moreira et al., 2007), or heterogeneously (Kim et al., 2010; Li et al., 2009) catalyzed reactions, and ⇑ Corresponding author. Tel.: +54 342 4533858; fax: +54 342 4531068. E-mail address: querini@fiq.unl.edu.ar (C.A. Querini). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.08.052
by non-catalyzed processes under sub or supercritical conditions (Warabi et al., 2004). However, alkaline catalysis is the most effective and widely used method (Meneghetti et al., 2006). Although transesterification with ethanol has been studied repeatedly (Bouˇ ernoch et al., 2010; Encinar et al., 2007; Marjanovic aid et al., 2009; C et al., 2010; Zhou et al., 2003), procedures that allow ethanol-based biodiesel production with high conversion levels under conditions suitable for industrial application have not been developed. Ethanol-based transesterification is extremely sensitive to minor changes in water content, reaction temperature, oil/ethanol ratio, and catalyst concentration; that can lead to a system with one or two phases at the end of the reaction. Determining accurate yields of ethanol-based biodiesel production is complicated by the fact that GC methods as described in the UNE-EN 14105 (2003) can be errorprone due to peak overlapping when monoglycerides are present. Therefore, in the current study, total glycerin content was analyzed by a volumetric procedure (Pisarello et al., 2010) which has no limitations regarding the raw material or the alcohol used in the reaction. For total ester content determination carried out according to the EN 14103 standard using the C17 methyl ester as internal standard, the response factor of ethyl esters must be corrected. These issues could account for the differences in ethyl ester yields reported by Soares et al. (2010), Bouiard et al. (2009), Encinar et al. (2007), and Issariyakul et al. (2008). Obtaining economically acceptable transesterification is more difficult to achieve with ethanol than with methanol since glycerin and ethyl esters in the presence of ethanol are mutually soluble, which severely complicates phase separation after the reaction. Depending upon the ethanol/oil volume ratio fed to the reactor, phase separation may not occur spontaneously and necessitates the addition of glycerin (Encinar et al., 2007; Issariyakul et al., 2008) or evaporation of the ethanol (Bouaid et al., 2009). More
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Triglyceride
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+
3 Ethanol
881 g/mol
138 g/mol
3 Ethyl ester 927 g/mol
+
Glycerin 92 g/mol
Scheme 1. Ethanolysis reaction stoichiometry.
soap is formed in the presence of ethanol than methanol (Mendow et al., 2011) and the resulting stable emulsions complicate phase separation and necessitates washing procedure with large volumes of water (Encinar et al., 2007). Mendow et al. (2011) studied single-step sunflower oil transesterification with ethanol in the presence of different catalysts and under various conditions, but were unable to attain biodiesel conforming to the international standards of 0.23 wt% bound glycerin. This outcome was mainly attributed to a very fast saponification, which consumes the catalyst. In the current study a two-stage process, consisting of two reactions steps with glycerin separation and ethanol/catalyst addition in each of them was optimized for the production of ethyl esters, was implemented to increase the reaction conversion and thus achieve values required by the EN 14214 standard. Several catalysts and reaction conditions were used with the aim of optimizing the production process.
2. Experimental 2.1. Biodiesel production process 2.1.1. 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 a stirring speed of 1200 rpm. Refined sunflower oil (acidity <0.1 g oleic acid/100 g oil) was used as raw material to study the effect of different operation variables on the conversion. The effect of the temperature was studied using the refined sunflower oil and refined beef tallow (acidity <0.1 g oleic acid/100 g oil). Table 1 shows the fatty acid composition of the sunflower oil. The reaction was carried out in two stages. The reaction temperature was varied between 45 and 70 °C, and reaction times from 30 to 120 minutes were tested. For the first stage, the oil was loaded into the reactor, and the temperature adjusted to the desired value. Once the oil reached this value, the alcohol containing the catalyst was added within a time range of 3 to 5 seconds. Time zero for the soap and catalyst measurements was the moment at which the ethanol/catalyst mixture was transferred to the reaction vessel. Ethanol 99.5% purity (water content 1150 ppm) from Cicarelli was used as transesterification alcohol. Typically, the ethanol/oil molar ratios employed were 2.55:1 for the first stage and 1.7:1 in the second stage. On a volumetric basis, these ratios correspond to 15% and 10% v/v (volume of ethanol/volume of oil 100),
Table 1 Fatty acid composition of refined sunflower oil. Fatty acid
Composition wt.%
14:0 16:0 16:1 18:0 18:1 18:2 18:3 20:0 20:1 22:0
0.09 5.98 0.17 3.15 28.49 58.13 0.19 0.26 0.30 1.24
respectively. Experiments employing 30%, 40% and 50% v/v of ethanol were also carried out to study the effect of the ethanol content on conversion. The catalyst was either sodium or potassium hydroxide dissolved in methanol or ethanol, or sodium or potassium methoxide solutions (30 and 32 wt.% in methanol respectively) provided by Evonik-Degussa. The amount of catalyst was calculated such that it could neutralize the free fatty acids and catalyze the reaction. The catalyst concentration was varied between 1 and 1.44 wt.% (g catalyst/100 g oil). At the end of the first reaction step, phase separation was carried out in a separatory funnel at room temperature, the biodiesel-rich phase, which also contained the remaining ethanol, dissolved glycerin, and catalyst, was transferred to a 0.5 L flask, and the second reaction step was performed using the same procedure as the one employed during the first reaction step. The glycerin-rich phase formed during the second stage was separated using a separatory funnel. The experiments were repeated once and the results for the two experiments differed by less than 11%. 2.1.2. Biodiesel purification The biodiesel-rich phase was extracted with an aqueous solution of HCl 5 wt.% followed by extraction with water. Both extraction stages (washings) were carried out with gentle agitation at 60 °C, during 15 min. The volume of aqueous phase was 30% v/v relative to the biodiesel phase. The biodiesel was dried by stripping with nitrogen at 80 °C. 2.1.3. Effect of reaction temperature A set of experiments was carried out to determine the effect of the reaction temperature. The total glycerin analyses were performed by gaseous chromatography, and also using the volumetric technique above mentioned. The total glycerin content is the sum of the bound glycerin (as mono-, di- and tri-glycerides) and the free glycerin. In this work, the biodiesel purification techniques employed were optimized in order to assure negligible free glycerin content in the final product. For this reason, the total glycerin content measured is the same as the bound glycerin, representing the percentage of non-converted glycerides. The total glycerin (T.G.) value is calculated with the following equation, from EN 14105:
%T:G: ¼ %GF þ 0:255 ð%M:G:Þ þ 0:146 ð%D:G:Þ þ 0:103 ð%T:G:Þ
Where %T.G. is the total glycerin percentage, %GF is the free glycerin percentage, and %M.G., %D.G. and %T.G. are the monoglycerides, diglycerides and triglycerides percentages, respectively, obtained by gaseous chromatography. The reaction temperature was varied between 45 and 65 °C. The raw materials evaluated were refined sunflower oil and refined beef tallow. The stirring speed was 1200 rpm, while the ethanol/ oil molar ratio was 4.25:1 in all the experiments. The total catalyst concentration (sodium methoxide) used was 1.13 wt.% for the sunflower oil and 1.07 wt.% for the beef tallow. 2.1.4. Effect of the ethanol/oil ratio The experiments were performed in two stages, using refined sunflower oil, 70 °C temperature, total catalyst concentration 1.06 wt.%., reaction time 30 and 60 minutes in the first and second stage, respectively, and stirring speed 1200 rpm. The ethanol used in the first and in the second reactions stages were (in a volumetric basis): 15%/10%; 15%/15%, 20%/20%, and 25%/25%, in the experiments carried out using a total amount of ethanol of 25%, 30%, 40%, and 50% v/v, respectively. These values correspond to a volumetric basis (volume of ethanol/volume of oil 100). The ethanol/
G. Mendow et al. / Bioresource Technology 102 (2011) 10407–10413
oil molar ratios that correspond to these experiments are 4.25:1, 5.1:1, 6.8:1, and 8.5:1, respectively. 2.1.5. Effect of catalyst distribution in each reaction stage A set of experiments was designed in order to optimize the concentration of catalyst to be used in each reaction step. These experiments were performed varying the proportions of catalyst added at each step, but using the same total amount of catalyst. In all cases, the raw material used was refined beef tallow (acid value = 0.56 mg KOH/100 g), the stirring speed was 1200 rpm and the reaction time 30 and 60 minutes for the first and second stage, respectively. The total catalyst concentration was 1.06 wt.%. Two series of experiments were carried out, being the alcohol volume 25 and 31% v/v (expressed in a volumetric basis), and the temperature 55 and 60 °C, respectively. The catalyst dosages evaluated were (1° Stage/2° Stage): 0.24/0.72; 0.36/0.6; 0.48/0.48; 0.6/0.36 and 0.72/0.24 g NaCH3O/100 mL oil, which correspond to the following distribution: 25/75%; 37.5/62.5%; 50/50%; 62.5/37.5% and 75/25%, respectively. 2.1.6. Glycerin recirculation The glycerin obtained in the second reaction stage was loaded in the first reaction stage in the following cycle. The objective is to feed the catalyst and the alcohol contained in the glycerin-rich phase. The reaction conditions were: stirring speed 1200 rpm; temperature 55 °C in both stages; 15% v/v ethanol in the first stage and 10% v/v ethanol in the second stage (expressed in a volumetric basis); 0.6 wt.% and 0.58 wt.% NaCH3O in the first and second stage, respectively; reaction time 30 and 60 minutes in the first and second stage, respectively. 2.2. Catalyst analysis Small samples (approx. 5 g) of the reacting mixture were taken at different times to determine the catalyst content by an acid–base titration method (AOCS Cc17-79 or IRAM 5599). The biodiesel sample was dissolved in a toluene-ethanol mixture (50:50 v/v), and titrated with 0.1 N HCl solution, using phenolphthalein as indicator. 2.3. Total glycerin analysis by a volumetric procedure The triglycerides conversion was followed by measuring the bound-glycerin by GC (see Section 2.4), as specified in the EN 14105 and ASTM D-6584 standards. Since these standards are validated only for the transesterification of soybean, rapeseed, or sunflower oils, with methanol, an alternative method was also used. In this volumetric procedure (Pisarello et al., 2010), the total glycerin content was obtained after the mono-, di- and tri-glycerides present in the sample were quantitatively transformed into methyl esters and glycerin by transesterification. The glycerin was extracted first with acidified water, and then with water. Finally, glycerin was titrated according to standard procedures, such as ASTM D1615, IRAM 41089, BS-5711, or AOCS 14-56. Unless otherwise stated, the progress of the reaction is represented by the total glycerin value (GT), determined by the volumetric procedure. 2.4. GC analysis Mono-, di-, and tri-glycerides contents (MG, DG, and TG, respectively) were determined according to UNE-EN 14105. 2.5. Glycerin-rich phase analyses The soap and catalyst content in the glycerin-rich phase were determined by an acid–base titration method (AOCS Cc17-79 or IRAM 5599). The glycerin sample was dissolved in distilled water,
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and titrated with 0.1 N HCl solution, using phenolphthalein as indicator to determine the catalyst concentration, and bromophenol blue to quantify the soaps concentration. The glycerin content was determined by titration as indicated in the ASTM D-1615, IRAM 41089, BS-5711, or AOCS 14-56. The ethanol content in the glycerol phase was measured using the head space method in accordance with UNE-EN 14110, using isopropanol as the internal standard and an adequate calibration curve. 3. Results and discussion 3.1. Transesterification process The two-step transesterification process is based on the Henkel industrial process, which is carried out in a two-stage reaction with glycerin separation after each stage (Assmann et al., 1991). Since it was previously shown (Mendow et al., 2011) that approximately 91% of the catalyst that was not consumed in the soap formation reaction accumulated in the glycerin-rich phase, removal of this phase and addition of new catalyst should promote further transesterification. It is very important to limit the progress of the transesterification in the first stage so that sufficient glycerol can be produced in the second stage to allow phase separation. In order to quantify this limit, systematic experiments were performed, in which the amount of catalyst dosed in the first stage was progressively increased, in order to achieve higher conversion values. Then, the second stage was performed, observing whether or not phase separation occurred. It could be established that above a glycerides conversion of 90–92% in the first stage, a spontaneous phase separation in the second stage can not be accomplished and the catalyst and the soap remained in the biodiesel phase causing emulsion formation during the washing steps. 3.2. Effect of reaction temperature The optimal temperature for transesterification of beef tallow and sunflower oil was 55 °C (Fig. 1); a temperature that minimizes total glycerin. At temperatures below this value, the reaction rate is lower in agreement with the Arrhenius law. At temperatures higher than 55 °C, the reaction rate decreases due to an increase of the ethanol partial pressure that diminishes its concentration in the reacting phase. These results agree with those of Mendow et al. (2011), who showed that in a single reaction step, the best conversion was obtained at 55 °C. At temperatures higher than 55 °C fast saponification also occurs (see Section 3.3) causing a decrease in catalyst concentration and lower final conversion. 3.3. Effect of catalyst Fig. 2 shows the mono, di and triglycerides content of the samples after the first reaction step. When potassium hydroxide was used as catalyst, 51.34% of triglycerides remain unconverted and with sodium hydroxide, this amount was only slightly lower. The amount of diglycerides obtained with these two catalysts was also practically the same, but the monoglycerides content was higher when sodium hydroxide was the catalyst, as compared with the use of potassium hydroxide. Thus, sodium hydroxide was slightly more active than potassium hydroxide, but the conversion values were lower than those obtained with methoxides as catalysts. Both potassium and sodium hydroxides were completely consumed after the first minutes (Fig. 3) due to saponification (Mendow et al, 2011). In the case of the NaOH, 93.5% of the initial
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100
Sunflower Oil Beef Tallow
0.21 0.18 0.15 0.12 0.09 40
45
50 55 60 Temperature (ºC)
65
70
% MG, DG, TG.
NaOH KOH
60 40 20 0 5
10
15 20 Time (min.)
25
30
Fig. 3. Changes in catalyst concentration over time for the different catalysts used in the first transesterification step. Reaction conditions: ethanol/oil molar ratio 2.55:1 (15% v/v); temperature 55 °C; stirring speed 1200 rpm; raw material, sunflower oil; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil).
Table 2 Mono-, di- and tri-glycerides content at the end of the second reaction stage with different catalysts. Reaction conditions: ethanol/oil molar ratio 1.7:1; temperature 55 °C; stirring speed 1200 rpm; raw material, sunflower oil; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil); reaction time 60 min.
% MG % DG
40
KCOH3
0
Fig. 1. Total glycerin content vs. temperature, using refined sunflower oil and refined beef tallow. Reaction conditions: stirring speed 1200 rpm; ethanol/oil molar ratio 4.25:1; total sodium methoxide concentration 1.13 wt.% for sunflower oil and 1.06 wt.%. for beef tallow; reaction time: 30 and 60 minutes for the first and second stages, respectively.
50
NaCOH3
80 Catalyst (mmol/Kg)
Total Glycerin (%)
0.24
% TG
30
Catalyst
% MG
% DG
% TG
NaCH3O KCH3O EN 14214 Standard
0.52 0.73 0.8
0.14 0.16 0.2
0.05 0.01 0.2
obtained meets the quality standards, but based on its lower cost and higher availability, sodium methoxide would be preferable.
20
3.4. Reaction time
10 0
NaCH3O
KCH3O
NaOH
KOH
Catalyst Fig. 2. Mono-, di- and tri-glycerides (MG, DG, TG, respectively) content of biodiesel obtained at the end of the first reaction step. Reaction conditions: ethanol/oil molar ratio 2.55:1 (15% v/v); temperature 55 °C; stirring speed 1200 rpm; raw material, sunflower oil; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil); reaction time 30 minutes.
amount was consumed after 5 minutes of reaction, while in the case of the KOH, 99.2% was consumed in the same time. No phase separation was observed when the hydroxides were used, as was observed by others (Cˇernoch et al., 2010; Encinar et al., 2007; Issariyakul et al., 2008). In contrast, when sodium or potassium methoxides were used, a fast phase separation was achieved. When the methoxides were used, the triglycerides conversion was much higher than that obtained with hydroxides (Fig. 2). The amount of non-converted triglycerides was 8.98% and 5.17% when sodium metoxide and potassium methoxide were used, respectively. Therefore, potassium methoxide was more active than sodium methoxide for the conversion of tri- to di-glycerides. Table 2 shows the non-reacted mono-, di- and tri-glycerides after the second reaction stage, as well as the values established by the European standard EN 14214. It can be concluded that both catalysts present excellent performance, since the biodiesel
Fig. 4A shows the evolution of mono-, di- and tri-glycerides with time in the first reaction step. After approximately 15 minutes, the concentrations of mono- and tri-glycerides reached a constant value, whereas the diglycerides slightly decreased between 15 and 30 minutes. These data indicate that the first reaction can be completed in 30 minutes. Fig. 4B shows the glycerides concentration as a function of time during the second reaction step. After 30 minutes of reaction, the mono-, di- and tri-glycerides concentrations reached constant values and therefore, the reaction time for the second stage should be 30 minutes, same as the first stage. Thus, the chemical reactors can have approximately the same volume. This finding is important since industrial production of methyl esters is carried out with this configuration and consequently, conversion to ethanol-based transesterification would not require new equipment. As it is shown in Fig. 4, in the first reaction step (Fig. 4A) the triglycerides concentration was reduced from the initial 100 wt.% to 17 wt.% in the first 5 minutes. In the following 25 minutes the triglycerides concentration reached 9 wt.%. The second reaction stage presented a similar behavior (Fig. 4B). The triglycerides concentration decreased from 9 to 3 wt.% within the first 5 minutes, and then, they disappeared very slowly, reaching a final value concentration of 0.08 wt.%. 3.5. Effect of ethanol/oil molar ratio Fig. 5 shows the effect of the total ethanol/oil molar ratio on the glycerides conversion, in experiments carried out as described in
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A
1st Stage
100
B
2nd Stage
10
% MG
% MG
90
% DG
% DG
8
% TG Glycerides (%)
Glycerides (%)
% TG
25 20 15 10
6 4 2
5 0
0
5
10
15
20
25
30
Time (min.)
0
0
10
20
30 40 Time (min)
50
60
Fig. 4. Mono-, di- and tri-glycerides (MG, DG, TG, respectively) concentrations measured during the transesterification reaction of refined sunflower oil using sodium methoxide as catalyst. Stirring speed 1200 rpm; temperature 55 °C; catalyst concentration 0.55 wt.% (0.094 mol catalyst/L oil). (A) First reaction stage. Ethanol:oil molar ratio 2.55:1 (15% v/v). (B) Second reaction stage. Ethanol/oil molar ratio 1.7:1.
Section 2.1.4. This Figure shows the total glycerin content (wt.%) in the final product vs the total amount of alcohol fed to the reacting system, taking into account the two reaction stages (% v/v). Increasing the alcohol concentration up to 36% v/v led to a decrease in the total glycerin concentration, i.e., the conversion improved. These were expected results and are in agreement with previous reports (Bouaid et al. 2007; Encinar et al., 2007; Fillières et al., 1995; Zhou et al., 2003). However, when the alcohol concentration was increased above 36% v/v, the conversion dropped. This concentration corresponds to an ethanol/oil molar ratio of 6:1. Above this value, the catalyst dilution becomes more important than the increase in alcohol concentration. In addition, if the ethanol/oil molar ratios are higher than 6:1, the separation of glycerol is difficult, since the excess ethanol hindered the decantation by gravity because both phases have similar densities. Nevertheless, in all cases the conversion values were high, and the total glycerin percentages were below the maximum value establishes by the international standards (Total Glycerin = 0.25 wt.%). For example,
EN14214 Limit (0.25 %)
Total Glycerin (%)
0.25 0.20 0.15 0.10 0.05 0.00 15
20
25
30 35 40 % Ethanol
45
50
Fig. 5. Total glycerin content vs. total ethanol percentage using refined sunflower oil as raw material and sodium methoxide as catalyst. Reaction conditions: temperature 70 °C; total catalyst concentration 1.06 wt.%; reaction time, 30 and 60 minutes for the first and second stage, respectively; stirring speed 1200 rpm.
using an ethanol/oil molar ratio of 4.25:1, which equals 25% v/v ethanol with respect to oil, the total glycerin content was lower than 0.15 wt.%, which is very good, and corresponds to a triglyceride conversion of 99%. Therefore, the ethanol/oil molar ratio of 4.25:1 was selected in order to obtain an appropriate conversion value using an ethanol:oil ratio lower than 6:1.
3.6. Effect of catalyst distribution in each reaction stage The effect of catalyst dosing in each reaction stage was studied as described in Section 2.1.5. With any of the temperatures and ethanol concentrations used, there was an optimum catalyst dosage of 50% of the total amount in reaction stage. This value maximized the conversion, and made it possible to obtain the biodiesel within specifications. Once the reaction ended, most of the catalyst remained in the glycerin phase. During the transesterification reaction, the volume of the glycerol-rich phase increased, and due to its capacity to extract the catalyst, the sodium methoxide concentration in the reacting phase (ethanol, biodiesel, glycerides) dropped. Consequently, the reaction rate decreased. When the amount of catalyst added in the first stage was low, and high in the second stage, the final conversion did not reach the required value (Total Glycerin% <0.25 wt.%), because a low catalyst concentration in the first step, led to a low amount of glycerin formed during this stage. Then, in the second stage, when the catalyst concentration was high, a large amount of glycerin was quickly generated. This glycerin extracted the sodium methoxide and, consequently, the amount of catalyst available in the oil-rich phase was not enough to complete the reaction. With the 37.5/ 62.5 dosage, the conversion significantly improved. When adding more catalyst in the first stage, the amount of glycerin generated was higher, and thus, the production of glycerin in the second stage decreased. Therefore, less catalyst migrated to the glycerin phase during the second stage, making it more available for the reaction and improving the global conversion. As shown in Fig. 6, with the 62.5/37.5% dosage, the conversion was lower than with the 50/50%, because both a percentage of the catalyst added in the first stage was eliminated from the system when the glycerin was removed, and the catalyst concentration in the second step was not enough to complete the reaction.
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0.35
Table 3 Amount and composition of the glycerin-rich phases generated in the first and second reaction stages. Catalyst: sodium methoxide.
% Total Glycerin
0.30 0.25 0.20
a
0.15 0.10 0.05
Phase
Glycerin (wt.%)
Soaps (wt.%)
Glycerol 1st Stage Glycerol 2nd Stage
89.7
1.3
80.9
2.9
37.25 % EtOH - 60 ºC
0
6.9
2.1
5.3
10.5
5.7
4.2
Mass ratio of glycerin and oil phases.
Exp.
25/75 37.5/62.5 50/50 62.5/37.5 75/25 st
Glycerin/oila (%)
Table 4 Conversion during transesterification: (A, C) with glycerin recirculation; (B, D) without glycerin recirculation. Reaction conditions: stirring speed 1200 rpm; temperature 70 °C; reaction time 30 and 60 minutes for the first and second stage; ethanol: first stage 15% v/v, second stage 10% v/v. Catalyst: sodium metoxide.
25 % EtOH - 55 ºC
% Catalyst (1 Stage/2
Ethanol (wt.%)
Catalyst (wt.%)
nd
Stage)
Fig. 6. Total glycerin content vs. sodium methoxide concentration used in the first/ second reaction stage. Reaction conditions: Alcohol percentage 25% and 31% and temperature 55 and 60 °C, respectively. Stirring speed 1200 rpm; reaction time, 30 and 60 minutes for the first and second stage, respectively; total catalyst concentration 1.06 g NaCH3O/100 g oil. Raw material: refined beef tallow.
A similar and even more pronounced phenomenon was observed in the case of the 75/25% dosage. Fig. 6 also shows the effect of the temperature. It can be observed that at 60 °C, and in spite of using a higher ethanol concentration, the conversion was lower than at 55 °C. This result agrees with the data shown in Fig. 1.
3.7. Glycerin recirculation The effect of glycerin recirculation from the second decanter into the first reactor was investigated, using the conditions described in Section 2.1.6. The composition of the glycerin phases at the exit of first and second decanters are shown in Table 3. In addition, the last column shows the mass percentage of the glycerin phase, relative to the oil fed. The total glycerin value measured in the biodiesel obtained using the conditions described in Section 2.1.6, was 0.12 wt.%. It can be observed in Table 3 that the catalyst content of the glycerin phases was very high. The amount of sodium metoxide contained in the glycerol generated in the second reaction stage represented approximately 0.44 wt.% of catalyst, which is equivalent to 73.5% of the total amount of catalyst needed for the first reaction step. However, regarding the ethanol provided by this recirculation process, and taking into account that the ethanol content in the glycerol phase was 10.5 wt.%, the amount of alcohol entering to first reactor with the glycerin stream is below 0.5% v/ v (volume of ethanol/volume of oil 100). The results of recirculating experiments are summarized in Table 4. It can be observed that although the amount of catalyst added in experiment A was substantially higher than in experiment B, there was only a slight improvement in the conversion. Experiments C and D were carried out to address the effect of the presence of a glycerin-rich phase on the conversion. In these two experiments, the same total amount of catalyst (0.88 wt.%) was loaded to the first reactor. In experiment C, this amount corresponded to the contribution of both fresh catalyst (0.33 wt.%), and the catalyst contained in the recycled glycerin (0.55 wt.%). In experiment D, no glycerin recirculation was carried out and, therefore, only fresh catalyst was added (0.89 wt.%). The results shown
A B C D
1st Stage
2nd Stage
Catalyst fed (wt.%)
Catalyst in glycerol (wt.%)
Total catalyst (wt.%)
Catalyst fed (wt.%)
Total glycerin (wt.%)
0.38 0.38 0.33 0.89
0.29 – 0.55 –
0.67 0.38 0.88 0.89
0.38 0.38 0.58 0.58
0.268 0.307 0.347 0.196
in Table 4 indicate that the presence of the glycerin phase led to lower conversion because the equilibrium was shifted towards the reagents side, and the glycerin acted as an extracting medium, diminishing the amount of catalyst and alcohol available in the reacting phase. This observation shows the importance of glycerin withdrawal to avoid the preferential concentration of catalyst and alcohol in this phase. The results shown in Table 4 provide additional evidences of the effect of the reaction temperature. The data shown in this table and Fig. 1 show that the reaction rate at 70 °C is lower than at 65 °C. 4. Conclusions An efficient two-stage ethyl esters production process was developed, using sodium methoxide as catalyst and 25% v/v of ethanol, which in a molar basis is lower than the ratio used with methanol. The conversion is very sensitive to the way that the catalyst is distributed between the two reaction stages, because the glycerin extracts it from the reaction media. This two-stage strategy made it possible to obtain biodiesel with a conversion that meets the requirements of the international standards, using 30 minutes of residence time in each stage, and a volume of alcohol equal to that used in the case of methanol. Thus, it is possible to use the same industrial facilities either with methanol or with ethanol. References Alamu, O.J., Waheed, M.A., Jekayinfa, S.O., 2008. Effect of ethanol–palm kernel oil ratio on alkali-catalyzed biodiesel yield. Fuel 87, 1529–1533. Assmann, G., Blassé, G., Gutsche, B., Jeromin, L., Rigal, J., Armengaud, R., Cormary, B., 1991. World patent WO 91/05034. Boehman, L., 2005. Biodiesel production and processing. Fuel Process. Technol. 86, 1057–1058. Bouaid, A., Martínez, M., Aracil, J., 2007. A comparative study of the production of ethyl esters from vegetable oils as a biodiesel fuel optimization by factorial design. Chem. Eng. J. 134, 93–99. Bouaid, A., Martínez, M., Aracil, J., 2009. Production of biodiesel from bioethanol and Brassica carinata oil: oxidation stability study. Bioresour. Technol. 100, 2234– 2239. ˇ ernoch, M., Hájek, M., Skopal, F., 2010. Ethanolysis of rapeseed oil – Distribution of C ethyl esters, glycerides and glycerol between ester and glycerol phases. Bioresour. Technol. 101, 2071–2075.
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