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Supercritical ethanol technology for the production of biodiesel: Process optimization studies
J. of Supercritical Fluids 49 (2009) 286–292
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Supercritical ethanol technology for the production of biodiesel: Process optimization studies Meei Mei Gui, Keat Teong Lee ∗ , Subhash Bhatia School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
a r t i c l e
i n f o
Article history: Received 11 September 2008 Received in revised form 7 November 2008 Accepted 30 December 2008 Keywords: Biodiesel Non-catalytic Transesterification Supercritical ethanol Optimization
a b s t r a c t Biodiesel is currently produced from transesterification reaction of various types of edible oil with methanol. However, the requirement of methanol makes the current biodiesel produce not totally 100% renewable as methanol is derived from fossil based products. Ethanol, on the other hand, can be produced from agricultural biomass via fermentation technology and is already easily available in the market at a high purity. Thus, in this work, possible 100% renewable biodiesel fuel was prepared from refined palm oil by using non-catalytic transesterification reaction in supercritical ethanol. The effect of various process parameters on the yield of biodiesel was studied using design of experiments (DOE). The process parameters studied are: reaction temperature (300–400 ◦ C), reaction period (2–30 min) and ethanol-to-oil ratio (5–50). The optimum process conditions were then obtained using response surface methodology (RSM) coupled with center composite design (CCD). The results revealed that at the following optimum process conditions; reaction temperature of 349 ◦ C, reaction period of 30 min and ethanol-to-oil ratio of 33, a biodiesel yield of 79.2 wt.% can be obtained. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is an alternative to petroleum-based fuels derived from vegetable oils, animal fats, and used cooking oil including triglycerides. Vegetable oils have become more attractive recently because of its environmental benefits and the fact that it is made from renewable resources. In 1970, scientists discovered that the viscosity of vegetable oils could be reduced by a simple chemical process and that it could perform as diesel fuel in modern engine. Table 1 shows the chemical and physical properties of biodiesel derived from various types of vegetable oil as compared to petroleum-derived diesel. Fundamentally, high viscosity appears to be a property at the root of many problems associated with direct use of vegetable oils as engine fuel. Different ways have been considered to reduce the high viscosity of vegetable oils: (a) dilution, (b) microemulsions, (c) pryolysis, (d) catalytic cracking, and (e) transesterification [1]. Among the four techniques, chemical conversion of oil through transesterification of the oil with short-chain alcohols, such as methanol or ethanol, to its corresponding fatty ester appears to be the most promising solution to the high-viscosity problem. The transesterification reaction is affected by alcohol type, molar ratio of glycerides to alcohol, type and amount of catalyst, reaction tem-
∗ Corresponding author. Tel.: +604 5996467; fax: +604 5941013. E-mail address: [email protected] (K.T. Lee). 0896-8446/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.12.014
perature, reaction time and free fatty acids and water content of vegetable oils or animal fats [6].
(1) Eq. (1) represents the reaction equation of transesterification reaction of triglycerides and alcohol into biodiesel. During the transesterification reaction, the alkoxy groups in triglyceride are exchange with the alkyl group from the alcohol, resulting in the formation of fatty acid alkyl esters as the product and glycerol as the by-product [3]. Transesterification reaction in normal room conditions is relatively slow (almost nil) due to the two-phase nature of alcohol-oil mixture that has contrast polarity. A catalyst is usually used to overcome this limitation and thus improve the reaction rate and product yield. There are various types of alkali and acidic catalysts, either in the form of homogenous or heterogeneous that are being used to improve the transesterification reaction. The most preferred alkali catalysts are sodium hydroxide, potassium hydroxide and sodium methoxide; meanwhile hydrochloric acid
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Table 1 Comparisons of physical and chemical properties of biodiesel from different sources with petroleum-derived diesel [2]. Properties
Non-edible oil
Viscosity at 40 ◦ C (cSt) Specific gravity Calorific value (MJ/kg) Flash point (◦ C) Pour point (◦ C) Ash content (wt.%) Acid value (mg KOH/g)
Edible oil
Petroleum-derived diesel
Jatropha
Castor
Rapeseed
Palm
4.80 – 39.23 135 2 0.012 0.40
0.960 39.50 260 −32 0.02 –
4.50 0.882 37.00 170 −12 – –
4.42 0.860–0.900 – 182 15 0.02 0.08
and sulfonic acid are the most commonly used acidic catalysts [6]. Nevertheless, the usage of catalyst complicates the transesterification reaction mainly in the need to separate the catalyst from the product mixture. In a more recent development, it was reported that biodiesel can be produced at a relatively fast rate without the presence of catalyst by heating up methanol to its supercritical stage and reacting it with rapeseed oil [7]. It was also reported that with methanol at supercritical stage, a single phase of methanol–oil mixture can be obtained instead of the two phase methanol–oil mixture observed at room conditions. This is because at supercritical stage, the dielectric constant of liquid methanol tends to decrease and thus increase the solubility of oil in methanol and enables the transesterification reaction to complete in a very short reaction time. Since catalyst is not being used, this process is much simpler and superior as compared to conventional catalytic transesterification process in terms of cost saving and purification of the product mixture [7]. On the other hand, currently all commercial biodiesel are produced by vegetable oil and methanol as the source of alcohol, producing fatty acid methyl esters. Methanol is selected because the fatty acid methyl esters obtained after the transesterification have very similar properties with petroleum-derived diesel [3]. Nevertheless, the utilization of methanol shows that the biodiesel produced is not entirely renewable as methanol is mainly derived from fossil sources namely petroleum and natural gas [8]. Furthermore, the depleting fossil sources have caused uncertainty to the supply and cost of products derived from fossil sources, including methanol. Therefore, one possible solution to overcome this limitation is to use other alcohol sources that are derived from renewable sources such as ethanol. Since the molecular structure of ethanol and methanol only differs by one methyl group, there is not much difference between the chemical and physical properties of both alcohols. However, when ethanol is used, fatty acid ethyl esters (FAEE) will be obtained as the products of the transesterification reaction. In recent studies, various types of alcohols such as ethanol, propanol and butanol have also been proven can be used as the source of alcohol for production of biodiesel in supercritical technology [8,9]. At the moment, ethanol can be easily produced from biomass that can be found abundantly via fermentation process and this type of ethanol is commonly known as bioethanol. Besides that, since there is already commercially production of bioethanol as a substitute fuel for gasoline, the supply of bioethanol for the production of biodiesel will be very promising. With the utilization of bioethanol as the source of alcohol, the biodiesel obtained can then be claimed
Soybean 4.08 0.885 39.76 69 −3 – –
2.60 0.850 42.00 68 −20 0.01 –
as 100% renewable based. Nevertheless, there are still not sufficient research findings on the utilization of ethanol for biodiesel production in order to convince the commercial biodiesel producers to switch to bioethanol for its environmental advantages. On top of that, researches on production of biodiesel from ethanol via non-catalytic transesterification in supercritical conditions are still very limited and much more information is required in detail such as; interaction effects among the process parameters and process optimization for maximum yield of biodiesel. Thus, the aim of this study is to obtain more data on the production of biodiesel from ethanol and palm oil using supercritical ethanol technology. In this study, the effect of reaction time, reaction temperature and the ratio of ethanol-to-oil on the yield of biodiesel will be emphasized. The data obtained will then be utilized to optimize the conditions for maximum biodiesel production. 2. Experimental 2.1. Materials and apparatus The raw materials used in this study are refined palm oil (Yee Lee Edible Oils Ltd. Co., Malaysia) and absolute ethanol (Sigma–Aldrich, Malaysia). For the analysis of samples using gas chromatography, analytical grade hexane (Merck, Malaysia) is used as solvent and various analytical grade ethyl esters, 98–99% (Sigma–Aldrich, Malaysia) such as ethyl myristate, ethyl palmitate, ethyl stearate, ethyl oleate, and ethyl linoleate are used as standards. In addition, methyl heptadecanoate, 99% (Sigma–Aldrich, Malaysia) is selected as the internal standard. The transesterification reaction was carried out in a batch-type tubular reactor system consists of an 11 mL tube-reactor made from Stainless Steel 316, furnace, and a chilled-water bath with temperature and pressure controller. The furnace is used to heat up the reactor to the desired reaction temperature, while the chilled-water bath is used to quench the reaction immediately after the desired reaction duration. 2.2. Procedures Initially, refined palm oil and ethanol (pre-calculated amount and ratio) was charged into the tubular reactor. The reactor was then inserted into the furnace which has been preheated to desired reaction temperature. The experiments were conducted generally at conditions above the critical temperature and pressure of ethanol,
Table 2 Process parameters in central composite design: coded and natural values. Variables
Reaction time Reaction temperature Ethanol-to-oil ratio
Parameter code
A B C
Unit
min ◦ C N/A
Level −2 (−␣)
−1
0
+1
+2 (+␣)
2 300 5
9 325 16.25
16 350 27.50
23 375 38.75
30 400 50.00
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Table 3 Experiment matrix of 23 center composite design and the response obtained. Run
Parameters A, Reaction time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Repeated experiments 15 16 17 18 19 20
Yield (wt.%) ◦
B, Reaction temperature ( C)
C, Ethanol-to-oil ratio
9 23 9 23 9 23 9 23 2 30 16 16 16 16
325 325 375 375 325 325 375 375 350 350 300 400 350 350
16.25 16.25 16.25 16.25 38.75 38.75 38.75 38.75 27.50 27.50 27.50 27.50 5.00 50.00
15.2 19.3 45.4 56.4 22.1 72.7 25.2 56.7 16.3 79.1 22.9 43.7 22.7 41.9
350 350 350 350 350 350
16 16 16 16 16 16
27.50 27.50 27.50 27.50 27.50 27.50
33.3 31.6 33.1 34.1 35.9 33.1
which is above 243 ◦ C and 6.38 MPa, respectively. During the reaction, the reaction temperature was monitored by a temperature controller system using PID controller. The reaction was carried out for a specific duration ranging from 2 to 30 min. After the reaction duration was reached, the reaction was quenched immediately by immersing the reactor into the chilled-water bath. The products were then removed from the tubular reactor and excess ethanol was removed by evaporation. The remaining products will form a two layer solution, the top layer contains of biodiesel and unreacted oil, meanwhile the undesired by-product; glycerol remained at the bottom layer. Sample from the top layer was then taken for gas chromatography analysis. 2.3. Analysis The composition and yield of FAEE in the product samples were determined using gas chromatography (PerkinElmer, claurus 500) equipped with flamed ionized detector (FID) and NukolTM (15 m × 0.53 mm; 0.5 m film) capillary column. Hexane was used as the solvent while helium gas was used as the carrier gas. The identification of the peaks of the various ethyl esters was made by comparing the retention time of each compound in the sample with the standard compound. The yield of FAEE was then calculated using the ratio of the peak area of the sample to the internal standard.
2.4. Statistical analysis using design of experiments The effect of process parameters in the non-catalytic transesterification reaction and the optimum conditions for the yield of FAEE was studied by using design of experiments. In this study, the design of experiment selected was Response Surface Method (RSM) coupled with Central Composite Design (CCD) using the Design-Expert Version 6.0.6 (Stat-Ease, Inc.) software. The process parameters selected for this study are reaction temperature, reaction time and ratio of ethanol-to-palm oil. Table 2 shows the coded and actual values of the process parameters used in the design of experiments. The experiments were then conducted based on the design matrix shown in Table 3. Upon completion of all the experimental runs, the responses (yield of biodiesel) obtained were then fitted in a quadratic model using regression analysis. 3. Result and discussion 3.1. Development of regression model The CCD matrix and the response obtained from the experimental runs are shown in Table 3. The results obtained were then analyzed using analysis of variance (ANOVA) and is shown in Table 4. After eliminating the insignificant parameters, multiple regression analysis gives the following quadratic model expressed
Table 4 Analysis of variance (ANOVA) for response surface quadratic model for the yield of FAEE. Source
Sum of squares
DF
Mean square
F value
Prob > F
Model A B C A2 B2 C2 AB AC BC
5826.99 3100.26 575.52 385.14 355.29 0.71 0.19 18.85 561.46 803.20
9 1 1 1 1 1 1 1 1 1
647.44 3100.26 575.52 385.14 355.29 0.71 0.19 18.85 561.46 803.20
35.76 171.25 31.79 21.27 19.63 0.039 0.011 1.04 31.01 44.37
<0.0001a <0.0001a 0.0002a 0.0010a 0.0013a 0.8472b 0.9202b 0.3316b 0.0002a <0.0001a
R2 , 0.9699; Adjusted R2 , 0.9427; Predicted R2 , 0.7822. a Significant at 95% confidence interval. b Not significant at 95% confidence interval.
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Fig. 1. A comparative plot between experimental FAEE yield and predicted FAEE yield.
in coded factors (Eq. (2)): FAEE Yield (wt.%) = +33.97 + 13.92A + 6.00B + 4.91C + 3.76A2 + 8.38AC − 10.02BC
(2)
As shown in Table 4, F-test (Fisher) on the developed model gives a value of 35.76 indicating that the model is significant at 95% CI (confidence interval). Fig. 1 shows the experimental values versus predicted values using the model equation developed. A line of unit slope, the line of perfect fit with points corresponding to zero error between experimental and predicted values is also shown in Fig. 1. The results in Fig. 1 demonstrated that the regression model equation provided a very accurate description of the experimental data, indicating that it was successful in capturing the correlation between the three transesterification process parameters to the yield of palm oil FAEE. This is further supported by the value of the correlation coefficient, R2 which was found to be very close to unity (0.9699). 3.2. Effects of process parameters 3.2.1. Single parameter effect Based on the developed model, all three single parameters were found to have significant positive effect on the yield of FAEE as indicated by the positive values of all three regression coefficient estimates. This result is consistent with those reported in literature in which longer reaction time will eventually allow the transesterification reaction to proceed towards complete conversion while higher molar ratio of ethanol-to-oil will shift the reversible transesterification reaction forward, resulting in increasing the yield of FAEE [3–5]. For the effect of reaction temperature, Saka and Kusdiana [7] and Song et al. [3] also reported an increase in the yield of FAEE with reaction temperature, provided the reaction temperature is below 380 ◦ C. However, in their study, methanol was used instead of ethanol. Reaction temperature above 380 ◦ C was reported not suitable for transesterification reaction as oil and the alkyl esters tend to decompose at the highest rate [10,11]. Apart from that, the degree of significance of each parameter can be evaluated according to its F-test value obtained using ANOVA. By referring to Table 4, the parameter which has the highest significant effect (highest F-test value) on the yield of FAEE is reaction time (parameter A), while reaction temperature (parameter B) and molar ratio of ethanol-tooil (parameter C) have almost similar significance.
289
These results can also be easily verified by visually inspecting Table 3. For instance, comparison between runs 1 and 2 (and also all comparable runs), an increase in reaction time (while other parameters remain constant) always resulted in higher yield of FAEE. This indicates the significant positive effect of reaction time on the yield of FAEE which is in agreement with the highest F-Test value obtained for reaction time in the ANOVA. Nevertheless, the same cannot be said for the effect of reaction temperature and molar ratio of ethanol-to-oil. Although visual inspection on the results in Table 3 shows that in most cases, an increase in reaction temperature (runs 1 and 3, runs 2 and 4, runs 5 and 7) and molar ratio of ethanol-to-oil (runs 1 and 5, runs 2 and 6) resulted in an increase in the yield of FAEE, but there are exceptional cases. For reaction temperature, comparison between runs 6 and 8 shows a drop in yield. On the other hand, for molar ratio of ethanol-to-oil, comparison between runs 3 and 7 shows a drop in yield while runs 4 and 8 show a relatively similar yield. This is again consistent with the ANOVA results, whereby the F-Test value for the term reaction temperature and molar ratio of ethanol-to-oil is not that high as compared to reaction time. These results show that the effect of reaction temperature and molar ratio of ethanol-to-oil on the yield of FAEE is not solely dependent on its individual effect, but its interaction effect with other parameters must also be considered. Therefore, the result up to this point stress the importance and need of using design of experiment approach in order to be able to study the interaction effect between parameters efficiently, which will be presented in the subsequent section. 3.2.2. Effect of interaction between parameters According to the ANOVA presented in Table 4, two interaction terms show significant effect on the yield of FAEE, which are: term AC (reaction time and molar ratio of ethanol-to-oil) and term BC (reaction temperature and molar ratio of ethanol-to-oil). The developed model was then used to construct a response surface plot to facilitate a straightforward investigation on the interaction between the parameters. Fig. 2 shows the interaction between reaction time and molar ratio of ethanol-to-oil at a fixed reaction temperature of 350 ◦ C and Fig. 3 shows the response surface plot of FAEE yield against reaction time and molar ratio of ethanol-to-oil. As expected and explained in the previous section, higher reaction time allows the transesterification to proceed to completion and resulting in higher yield of FAEE. Nevertheless, Fig. 2 shows that the effect of reaction time is more prominent at higher molar ratio of ethanol-to-oil. As seen in Fig. 2, at a molar ratio of 40, the yield of FAEE increase rapidly with longer reaction time; meanwhile, the
Fig. 2. Two-dimensional plot of FAEE yield against reaction time and ethanol-to-oil ratio.
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Fig. 3. Response surface plot of FAEE yield against reaction time and ethanol-to-oil ratio.
increase in the yield of FAEE with reaction time is much slower at a molar ratio of 16. The result trend reveals that reaction rate increases much faster with high molar ratio of ethanol-to-oil as excess ethanol will shift the reversible transesterification reaction forward, resulting in higher conversion of triglycerides to FAEE. Fig. 4 shows the interaction effect of reaction temperature and molar ratio of ethanol-to-oil on the yield of FAEE with the reaction time kept constant at 16 min. The response surface plot of FAEE yield against reaction temperature and ethanol-to-oil ratio is shown in Fig. 5. The graph shows two contrast trends for the yield of FAEE, indicating a significant interaction between both the parameters; reaction temperature and molar ratio of ethanol-to-oil. At a higher molar ratio of ethanol-to-oil (40), an increase in reaction temperature reduces the yield of FAEE. Contrary, the yield of FAEE increases with higher reaction temperature at a lower ratio of 16. The significant interaction between reaction temperature and molar ratio of ethanol-to-oil is also picked up by the ANOVA in which the term BC has a very high F-value. Therefore the design of experiments approach used in this study present advantage in detecting interaction effect between parameters that might not have been detected in the conventional approach of studying one parameter at one time while keeping the rest constant. In normal circumstances, it is expected that higher reaction temperature will increases any rate of reaction, leading to an increase
Fig. 4. Two-dimensional plot of FAEE yield against reaction temperature and ethanol-to-oil ratio.
Fig. 5. Response surface plot of FAEE yield against reaction temperature and ethanolto-oil ratio.
in yield, as in the case when the molar ratio of ethanol-to-oil is 16. However, this was found not to be true in the case for the ratio of 40. Possible reason for this phenomenon is as follow. When the molar ratio of ethanol-to-oil used is as high as 40, the critical temperature of the reactant/product mixture between ethanol–oil now becomes highly dependent on the concentration of ethanol, in which the critical temperature of the reactant/product mixture decreases with an increase in ethanol-to-oil ratio [11]. This may be rationalized as oil has a higher critical temperature compared to ethanol. Furthermore, Hegel et al. [11] reported that the critical temperature for a product mixture with high methanol concentration is at about 280 ◦ C, while for system with low methanol concentration is at about 377 ◦ C. Considering the fact that the critical temperature of methanol and ethanol is close, therefore, the lowest reaction temperature used in this study is already above the critical temperature of the reactant/product mixture for the case with ethanol-to-oil molar ratio of 40. When reactant/product mixture is heated above its critical temperature, it has a high tendency to decompose easily, leading to lower yield of FAEE. The decomposition of products at high reaction temperature is also supported by the recent finding by Imahara et al. [12]. In their study, it was reported that unsaturated fatty acid methyl esters are unstable and start to decompose at temperature above 300 ◦ C. At high temperature, unsaturated fatty acids such as oleic acid and linoleic acid tends to decompose via isomerization of the double bond functional group from cis-type carbon bonding (C C) into trans-type carbon bonding (C C), which is naturally unstable fatty acids. Therefore, in this study, it is highly possible that the drop in
Fig. 6. FT-IR analysis on the product sample at reaction temperature (a) 325 ◦ C, (b) 375 ◦ C.
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Table 5 Comparisons of FAEE yield obtained with different reaction conditions and types of oil. Reference
Type of oil
Optimum reaction conditions ◦
This study Warabi et al. [9] Demirbas [13] Madras et al. [14] Varma and Madras [15] Varma and Madras [15] Vieitez et. al. [16]
Palm Rapeseed Linseed Sunflower Linseed Castor Soybean
Yield/conversion (%)
Reaction temperature ( C)
Reaction time (min)
Ethanol-to-oil ratio
349 350 250 400 300 300 350
30 45 8 30 40 30 Flow rate: 1.0 mL/min
33 42 41 40 40 40 40
the yield of FAEE at increasing temperature with a high ethanol-tooil molar ratio (such as in the case of 40) is due to the decomposition of FAEE. This is especially true when the reactions are carried out above the critical temperature of the reactant/product mixture. In order to confirm the decomposition of FAEE at high temperature, selected product samples were subjected to FT-IR (Fourier Transform Infrared Spectrometry) analysis. Fig. 6 shows a comparison of FT-IR spectrum for two samples collected from reaction conducted at different reaction temperatures with the reaction time and ethanol–oil molar ratio fixed at 23 min and 40, respectively. Graph (a) is the FT-IR spectrum for the product obtained from reaction at lower temperature (325 ◦ C) and graph (b) shows the FT-IR spectrum for the product obtained from reaction at higher temperature (375 ◦ C). As shown in Fig. 6, the FT-IR spectrum for (b) shows a trans-type carbon bonding (C C) group at the wavelength of 967.27 cm−1 . On the other hand, there is no peak observed at this wavelength for (a). This observation therefore confirmed that thermal decomposition of FAEE did occur at higher reaction temperature for reactants with high ethanol-to-oil molar ratio, due to the isomerization of cis-type (C = C) into trans-type (C C). The presence of trans-type (C C) which is unstable ultimately reduces the yield of FAEE. Another significant interaction between the molar ratio of ethanol-to-oil and reaction temperature is that, at lower reaction temperature (<363 ◦ C), higher molar ratio of ethanol resulted in higher yield of FAEE as compared to lower molar ratio of ethanol. However, the inverse trend was observed at higher reaction temperature (>363 ◦ C), higher molar ratio of ethanol gives a lower yield of FAEE as compared to lower molar ratio of ethanol. As for the case involving lower reaction temperature, it can be easily explained as follow. The higher molar ratio of ethanol-to-oil used will shift the reaction forward leading to higher yield of FAEE as compared to lower amount of ethanol used. Although, the reactant/product mixture at higher molar ratio of ethanol might have started to decompose (due to lower reactant/product mixture critical temperature), even at lower reaction temperature, however, the positive effect of having higher quantity of ethanol to push the reaction forward is more prominent as compared to lowering the critical temperature of the reactant/product that leads to the decomposition of FAEE. However, at higher reaction temperature, the decomposition rate of the reaction products has overcome the positive effect of having higher ethanol ratio that shifts the reaction forward. Thus, when the reaction temperature is higher, the thermal decomposition of FAEE in the reaction with higher ethanol molar ratio was so significant that it caused a reduction in the yield of FAEE to a point that it is even lower than the yield of FAEE obtained using lower molar ratio of ethanol. This result therefore illustrates the trade off when higher molar ratio of ethanol-to-oil is used, between shifting the reaction forward and reducing the critical temperature of the reactant/product mixture that could eventual lead to decomposition. In the case shown in Fig. 5, when the reaction time was fixed at 16 min, 363 ◦ C seems to be the trade off temperature between the positive and negative effect of using higher molar ratio of ethanol-to-oil.
79.2 95.0 90.0 95.0 90.0 95.0 77.5
3.3. Optimization Up to this point, apart from the three individual parameters, the interactions between parameters were also found to have significant effect on the yield of FAEE. Higher molar ratio of ethanol-to-oil was found to able to shift the reversible reaction forward, resulting in higher yield of FAEE. However, its interaction with reaction temperature can on the other hand cause a reduction in the yield of FAEE due to the decomposition of FAEE. Taking this factor into consideration, therefore, the optimization process must be able to take into account all the effects of the individual parameters and also the interaction between parameters, in order to maximize the conversion of palm oil and ethanol into FAEE, while at the same time, minimizing the thermal decomposition of FAEE which will eventually caused a reduction in the yield of FAEE. In order to perform this task, the mathematical model developed was used to obtain the process parameters that can give the optimum yield of FAEE. This was carried out with the aid of the optimization function embedded in the Design-Expert software. It was predicted that an optimum FAEE yield of 83.1 wt.% can be achieved by using the following process condition; reaction temperature of 349 ◦ C, molar ratio of ethanol-to-oil of 33 and reaction time of 29 min. The predicted optimum yield was verified by carrying out experiment runs using the optimum process condition suggested. The experimental run gives an actual optimum yield of 79.2 wt.% which is close to the predicted value (with less than 5% error) indicating that the predicted optimum yield is valid for this study. The result obtained in this optimization study was compared with the results reported by other researchers for non-catalytic transesterification in supercritical ethanol. Table 5 summarizes the comparison. The optimum yield obtained in this study is relatively slightly lower than those reported in the literature. This may be due to the type of oil used as palm oil has a high-saturated fatty acid content compared to other types of oils reported. Despite the relatively lower yield, however, it should be noted that apart from the reaction conditions reported by Demirbas [13], the optimum reactions conditions obtained in this study is all towards the lower end. This factor is very crucial in terms of economic viability to commercialize this process. On top of that, comparison on the data shown in Table 5 also showed that the process conditions for optimum yield of biodiesel is not only dependent on the types of oil used, but also on the experimental reaction system set-up. Although in the study reported by Demirbas [13] and Varma and Madras [15] both used linseed oil as the feedstock, the process conditions for the obtaining the optimum yield is very much different, indicating that process conditions for optimum yield is not only dependent on types of oil used but also reactor system set-up. 4. Conclusion Based on the design of experiment approach over conventional strategy, it was found that interaction between the process parameters significantly affect the yield of biodiesel obtained via non-catalytic supercritical ethanol transesterification. Once the
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interaction effects were identified, it can be used to optimize the transesterification process. However, in this study, only selected critical process parameters were studied, leaving behind other parameters such as content of free fatty acid that may have significant effect on the yield. Furthermore, this study revealed that owing to the high content of saturated fatty acid in palm oil, the optimum yield is achieved at different process parameters as compared to oil with high-unsaturated fatty acid content. Therefore, this warrant for more studies on supercritical ethanol transesterification using oil with high-saturated fatty acid content. Acknowledgements The authors would like to acknowledge Prof. Shiro Saka from Kyoto University, Japan for his guidance given in this research. Apart from that, the authors would also like to acknowledge Ministry of Science, Technology and Innovation (Science Fund No: 03-01-05SF0138) and Universiti Sains Malaysia (Research University Grant) for the financial support given. References [1] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. 70 (1999) 1–15. [2] M.M. Gui, K.T. Lee, S. Bhatia, Feasibility of edible oil vs. non-edible oil vs. waste edible oil as biodiesel feedstock, Energy 33 (2008) 1646–1653.
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Supercritical ethanol technology for the production of biodiesel: Process optimization studies Meei Mei Gui, Keat Teong Lee ∗ , Subhash Bhatia School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
a r t i c l e
i n f o
Article history: Received 11 September 2008 Received in revised form 7 November 2008 Accepted 30 December 2008 Keywords: Biodiesel Non-catalytic Transesterification Supercritical ethanol Optimization
a b s t r a c t Biodiesel is currently produced from transesterification reaction of various types of edible oil with methanol. However, the requirement of methanol makes the current biodiesel produce not totally 100% renewable as methanol is derived from fossil based products. Ethanol, on the other hand, can be produced from agricultural biomass via fermentation technology and is already easily available in the market at a high purity. Thus, in this work, possible 100% renewable biodiesel fuel was prepared from refined palm oil by using non-catalytic transesterification reaction in supercritical ethanol. The effect of various process parameters on the yield of biodiesel was studied using design of experiments (DOE). The process parameters studied are: reaction temperature (300–400 ◦ C), reaction period (2–30 min) and ethanol-to-oil ratio (5–50). The optimum process conditions were then obtained using response surface methodology (RSM) coupled with center composite design (CCD). The results revealed that at the following optimum process conditions; reaction temperature of 349 ◦ C, reaction period of 30 min and ethanol-to-oil ratio of 33, a biodiesel yield of 79.2 wt.% can be obtained. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is an alternative to petroleum-based fuels derived from vegetable oils, animal fats, and used cooking oil including triglycerides. Vegetable oils have become more attractive recently because of its environmental benefits and the fact that it is made from renewable resources. In 1970, scientists discovered that the viscosity of vegetable oils could be reduced by a simple chemical process and that it could perform as diesel fuel in modern engine. Table 1 shows the chemical and physical properties of biodiesel derived from various types of vegetable oil as compared to petroleum-derived diesel. Fundamentally, high viscosity appears to be a property at the root of many problems associated with direct use of vegetable oils as engine fuel. Different ways have been considered to reduce the high viscosity of vegetable oils: (a) dilution, (b) microemulsions, (c) pryolysis, (d) catalytic cracking, and (e) transesterification [1]. Among the four techniques, chemical conversion of oil through transesterification of the oil with short-chain alcohols, such as methanol or ethanol, to its corresponding fatty ester appears to be the most promising solution to the high-viscosity problem. The transesterification reaction is affected by alcohol type, molar ratio of glycerides to alcohol, type and amount of catalyst, reaction tem-
∗ Corresponding author. Tel.: +604 5996467; fax: +604 5941013. E-mail address: [email protected] (K.T. Lee). 0896-8446/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.12.014
perature, reaction time and free fatty acids and water content of vegetable oils or animal fats [6].
(1) Eq. (1) represents the reaction equation of transesterification reaction of triglycerides and alcohol into biodiesel. During the transesterification reaction, the alkoxy groups in triglyceride are exchange with the alkyl group from the alcohol, resulting in the formation of fatty acid alkyl esters as the product and glycerol as the by-product [3]. Transesterification reaction in normal room conditions is relatively slow (almost nil) due to the two-phase nature of alcohol-oil mixture that has contrast polarity. A catalyst is usually used to overcome this limitation and thus improve the reaction rate and product yield. There are various types of alkali and acidic catalysts, either in the form of homogenous or heterogeneous that are being used to improve the transesterification reaction. The most preferred alkali catalysts are sodium hydroxide, potassium hydroxide and sodium methoxide; meanwhile hydrochloric acid
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Table 1 Comparisons of physical and chemical properties of biodiesel from different sources with petroleum-derived diesel [2]. Properties
Non-edible oil
Viscosity at 40 ◦ C (cSt) Specific gravity Calorific value (MJ/kg) Flash point (◦ C) Pour point (◦ C) Ash content (wt.%) Acid value (mg KOH/g)
Edible oil
Petroleum-derived diesel
Jatropha
Castor
Rapeseed
Palm
4.80 – 39.23 135 2 0.012 0.40
0.960 39.50 260 −32 0.02 –
4.50 0.882 37.00 170 −12 – –
4.42 0.860–0.900 – 182 15 0.02 0.08
and sulfonic acid are the most commonly used acidic catalysts [6]. Nevertheless, the usage of catalyst complicates the transesterification reaction mainly in the need to separate the catalyst from the product mixture. In a more recent development, it was reported that biodiesel can be produced at a relatively fast rate without the presence of catalyst by heating up methanol to its supercritical stage and reacting it with rapeseed oil [7]. It was also reported that with methanol at supercritical stage, a single phase of methanol–oil mixture can be obtained instead of the two phase methanol–oil mixture observed at room conditions. This is because at supercritical stage, the dielectric constant of liquid methanol tends to decrease and thus increase the solubility of oil in methanol and enables the transesterification reaction to complete in a very short reaction time. Since catalyst is not being used, this process is much simpler and superior as compared to conventional catalytic transesterification process in terms of cost saving and purification of the product mixture [7]. On the other hand, currently all commercial biodiesel are produced by vegetable oil and methanol as the source of alcohol, producing fatty acid methyl esters. Methanol is selected because the fatty acid methyl esters obtained after the transesterification have very similar properties with petroleum-derived diesel [3]. Nevertheless, the utilization of methanol shows that the biodiesel produced is not entirely renewable as methanol is mainly derived from fossil sources namely petroleum and natural gas [8]. Furthermore, the depleting fossil sources have caused uncertainty to the supply and cost of products derived from fossil sources, including methanol. Therefore, one possible solution to overcome this limitation is to use other alcohol sources that are derived from renewable sources such as ethanol. Since the molecular structure of ethanol and methanol only differs by one methyl group, there is not much difference between the chemical and physical properties of both alcohols. However, when ethanol is used, fatty acid ethyl esters (FAEE) will be obtained as the products of the transesterification reaction. In recent studies, various types of alcohols such as ethanol, propanol and butanol have also been proven can be used as the source of alcohol for production of biodiesel in supercritical technology [8,9]. At the moment, ethanol can be easily produced from biomass that can be found abundantly via fermentation process and this type of ethanol is commonly known as bioethanol. Besides that, since there is already commercially production of bioethanol as a substitute fuel for gasoline, the supply of bioethanol for the production of biodiesel will be very promising. With the utilization of bioethanol as the source of alcohol, the biodiesel obtained can then be claimed
Soybean 4.08 0.885 39.76 69 −3 – –
2.60 0.850 42.00 68 −20 0.01 –
as 100% renewable based. Nevertheless, there are still not sufficient research findings on the utilization of ethanol for biodiesel production in order to convince the commercial biodiesel producers to switch to bioethanol for its environmental advantages. On top of that, researches on production of biodiesel from ethanol via non-catalytic transesterification in supercritical conditions are still very limited and much more information is required in detail such as; interaction effects among the process parameters and process optimization for maximum yield of biodiesel. Thus, the aim of this study is to obtain more data on the production of biodiesel from ethanol and palm oil using supercritical ethanol technology. In this study, the effect of reaction time, reaction temperature and the ratio of ethanol-to-oil on the yield of biodiesel will be emphasized. The data obtained will then be utilized to optimize the conditions for maximum biodiesel production. 2. Experimental 2.1. Materials and apparatus The raw materials used in this study are refined palm oil (Yee Lee Edible Oils Ltd. Co., Malaysia) and absolute ethanol (Sigma–Aldrich, Malaysia). For the analysis of samples using gas chromatography, analytical grade hexane (Merck, Malaysia) is used as solvent and various analytical grade ethyl esters, 98–99% (Sigma–Aldrich, Malaysia) such as ethyl myristate, ethyl palmitate, ethyl stearate, ethyl oleate, and ethyl linoleate are used as standards. In addition, methyl heptadecanoate, 99% (Sigma–Aldrich, Malaysia) is selected as the internal standard. The transesterification reaction was carried out in a batch-type tubular reactor system consists of an 11 mL tube-reactor made from Stainless Steel 316, furnace, and a chilled-water bath with temperature and pressure controller. The furnace is used to heat up the reactor to the desired reaction temperature, while the chilled-water bath is used to quench the reaction immediately after the desired reaction duration. 2.2. Procedures Initially, refined palm oil and ethanol (pre-calculated amount and ratio) was charged into the tubular reactor. The reactor was then inserted into the furnace which has been preheated to desired reaction temperature. The experiments were conducted generally at conditions above the critical temperature and pressure of ethanol,
Table 2 Process parameters in central composite design: coded and natural values. Variables
Reaction time Reaction temperature Ethanol-to-oil ratio
Parameter code
A B C
Unit
min ◦ C N/A
Level −2 (−␣)
−1
0
+1
+2 (+␣)
2 300 5
9 325 16.25
16 350 27.50
23 375 38.75
30 400 50.00
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Table 3 Experiment matrix of 23 center composite design and the response obtained. Run
Parameters A, Reaction time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Repeated experiments 15 16 17 18 19 20
Yield (wt.%) ◦
B, Reaction temperature ( C)
C, Ethanol-to-oil ratio
9 23 9 23 9 23 9 23 2 30 16 16 16 16
325 325 375 375 325 325 375 375 350 350 300 400 350 350
16.25 16.25 16.25 16.25 38.75 38.75 38.75 38.75 27.50 27.50 27.50 27.50 5.00 50.00
15.2 19.3 45.4 56.4 22.1 72.7 25.2 56.7 16.3 79.1 22.9 43.7 22.7 41.9
350 350 350 350 350 350
16 16 16 16 16 16
27.50 27.50 27.50 27.50 27.50 27.50
33.3 31.6 33.1 34.1 35.9 33.1
which is above 243 ◦ C and 6.38 MPa, respectively. During the reaction, the reaction temperature was monitored by a temperature controller system using PID controller. The reaction was carried out for a specific duration ranging from 2 to 30 min. After the reaction duration was reached, the reaction was quenched immediately by immersing the reactor into the chilled-water bath. The products were then removed from the tubular reactor and excess ethanol was removed by evaporation. The remaining products will form a two layer solution, the top layer contains of biodiesel and unreacted oil, meanwhile the undesired by-product; glycerol remained at the bottom layer. Sample from the top layer was then taken for gas chromatography analysis. 2.3. Analysis The composition and yield of FAEE in the product samples were determined using gas chromatography (PerkinElmer, claurus 500) equipped with flamed ionized detector (FID) and NukolTM (15 m × 0.53 mm; 0.5 m film) capillary column. Hexane was used as the solvent while helium gas was used as the carrier gas. The identification of the peaks of the various ethyl esters was made by comparing the retention time of each compound in the sample with the standard compound. The yield of FAEE was then calculated using the ratio of the peak area of the sample to the internal standard.
2.4. Statistical analysis using design of experiments The effect of process parameters in the non-catalytic transesterification reaction and the optimum conditions for the yield of FAEE was studied by using design of experiments. In this study, the design of experiment selected was Response Surface Method (RSM) coupled with Central Composite Design (CCD) using the Design-Expert Version 6.0.6 (Stat-Ease, Inc.) software. The process parameters selected for this study are reaction temperature, reaction time and ratio of ethanol-to-palm oil. Table 2 shows the coded and actual values of the process parameters used in the design of experiments. The experiments were then conducted based on the design matrix shown in Table 3. Upon completion of all the experimental runs, the responses (yield of biodiesel) obtained were then fitted in a quadratic model using regression analysis. 3. Result and discussion 3.1. Development of regression model The CCD matrix and the response obtained from the experimental runs are shown in Table 3. The results obtained were then analyzed using analysis of variance (ANOVA) and is shown in Table 4. After eliminating the insignificant parameters, multiple regression analysis gives the following quadratic model expressed
Table 4 Analysis of variance (ANOVA) for response surface quadratic model for the yield of FAEE. Source
Sum of squares
DF
Mean square
F value
Prob > F
Model A B C A2 B2 C2 AB AC BC
5826.99 3100.26 575.52 385.14 355.29 0.71 0.19 18.85 561.46 803.20
9 1 1 1 1 1 1 1 1 1
647.44 3100.26 575.52 385.14 355.29 0.71 0.19 18.85 561.46 803.20
35.76 171.25 31.79 21.27 19.63 0.039 0.011 1.04 31.01 44.37
<0.0001a <0.0001a 0.0002a 0.0010a 0.0013a 0.8472b 0.9202b 0.3316b 0.0002a <0.0001a
R2 , 0.9699; Adjusted R2 , 0.9427; Predicted R2 , 0.7822. a Significant at 95% confidence interval. b Not significant at 95% confidence interval.
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Fig. 1. A comparative plot between experimental FAEE yield and predicted FAEE yield.
in coded factors (Eq. (2)): FAEE Yield (wt.%) = +33.97 + 13.92A + 6.00B + 4.91C + 3.76A2 + 8.38AC − 10.02BC
(2)
As shown in Table 4, F-test (Fisher) on the developed model gives a value of 35.76 indicating that the model is significant at 95% CI (confidence interval). Fig. 1 shows the experimental values versus predicted values using the model equation developed. A line of unit slope, the line of perfect fit with points corresponding to zero error between experimental and predicted values is also shown in Fig. 1. The results in Fig. 1 demonstrated that the regression model equation provided a very accurate description of the experimental data, indicating that it was successful in capturing the correlation between the three transesterification process parameters to the yield of palm oil FAEE. This is further supported by the value of the correlation coefficient, R2 which was found to be very close to unity (0.9699). 3.2. Effects of process parameters 3.2.1. Single parameter effect Based on the developed model, all three single parameters were found to have significant positive effect on the yield of FAEE as indicated by the positive values of all three regression coefficient estimates. This result is consistent with those reported in literature in which longer reaction time will eventually allow the transesterification reaction to proceed towards complete conversion while higher molar ratio of ethanol-to-oil will shift the reversible transesterification reaction forward, resulting in increasing the yield of FAEE [3–5]. For the effect of reaction temperature, Saka and Kusdiana [7] and Song et al. [3] also reported an increase in the yield of FAEE with reaction temperature, provided the reaction temperature is below 380 ◦ C. However, in their study, methanol was used instead of ethanol. Reaction temperature above 380 ◦ C was reported not suitable for transesterification reaction as oil and the alkyl esters tend to decompose at the highest rate [10,11]. Apart from that, the degree of significance of each parameter can be evaluated according to its F-test value obtained using ANOVA. By referring to Table 4, the parameter which has the highest significant effect (highest F-test value) on the yield of FAEE is reaction time (parameter A), while reaction temperature (parameter B) and molar ratio of ethanol-tooil (parameter C) have almost similar significance.
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These results can also be easily verified by visually inspecting Table 3. For instance, comparison between runs 1 and 2 (and also all comparable runs), an increase in reaction time (while other parameters remain constant) always resulted in higher yield of FAEE. This indicates the significant positive effect of reaction time on the yield of FAEE which is in agreement with the highest F-Test value obtained for reaction time in the ANOVA. Nevertheless, the same cannot be said for the effect of reaction temperature and molar ratio of ethanol-to-oil. Although visual inspection on the results in Table 3 shows that in most cases, an increase in reaction temperature (runs 1 and 3, runs 2 and 4, runs 5 and 7) and molar ratio of ethanol-to-oil (runs 1 and 5, runs 2 and 6) resulted in an increase in the yield of FAEE, but there are exceptional cases. For reaction temperature, comparison between runs 6 and 8 shows a drop in yield. On the other hand, for molar ratio of ethanol-to-oil, comparison between runs 3 and 7 shows a drop in yield while runs 4 and 8 show a relatively similar yield. This is again consistent with the ANOVA results, whereby the F-Test value for the term reaction temperature and molar ratio of ethanol-to-oil is not that high as compared to reaction time. These results show that the effect of reaction temperature and molar ratio of ethanol-to-oil on the yield of FAEE is not solely dependent on its individual effect, but its interaction effect with other parameters must also be considered. Therefore, the result up to this point stress the importance and need of using design of experiment approach in order to be able to study the interaction effect between parameters efficiently, which will be presented in the subsequent section. 3.2.2. Effect of interaction between parameters According to the ANOVA presented in Table 4, two interaction terms show significant effect on the yield of FAEE, which are: term AC (reaction time and molar ratio of ethanol-to-oil) and term BC (reaction temperature and molar ratio of ethanol-to-oil). The developed model was then used to construct a response surface plot to facilitate a straightforward investigation on the interaction between the parameters. Fig. 2 shows the interaction between reaction time and molar ratio of ethanol-to-oil at a fixed reaction temperature of 350 ◦ C and Fig. 3 shows the response surface plot of FAEE yield against reaction time and molar ratio of ethanol-to-oil. As expected and explained in the previous section, higher reaction time allows the transesterification to proceed to completion and resulting in higher yield of FAEE. Nevertheless, Fig. 2 shows that the effect of reaction time is more prominent at higher molar ratio of ethanol-to-oil. As seen in Fig. 2, at a molar ratio of 40, the yield of FAEE increase rapidly with longer reaction time; meanwhile, the
Fig. 2. Two-dimensional plot of FAEE yield against reaction time and ethanol-to-oil ratio.
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Fig. 3. Response surface plot of FAEE yield against reaction time and ethanol-to-oil ratio.
increase in the yield of FAEE with reaction time is much slower at a molar ratio of 16. The result trend reveals that reaction rate increases much faster with high molar ratio of ethanol-to-oil as excess ethanol will shift the reversible transesterification reaction forward, resulting in higher conversion of triglycerides to FAEE. Fig. 4 shows the interaction effect of reaction temperature and molar ratio of ethanol-to-oil on the yield of FAEE with the reaction time kept constant at 16 min. The response surface plot of FAEE yield against reaction temperature and ethanol-to-oil ratio is shown in Fig. 5. The graph shows two contrast trends for the yield of FAEE, indicating a significant interaction between both the parameters; reaction temperature and molar ratio of ethanol-to-oil. At a higher molar ratio of ethanol-to-oil (40), an increase in reaction temperature reduces the yield of FAEE. Contrary, the yield of FAEE increases with higher reaction temperature at a lower ratio of 16. The significant interaction between reaction temperature and molar ratio of ethanol-to-oil is also picked up by the ANOVA in which the term BC has a very high F-value. Therefore the design of experiments approach used in this study present advantage in detecting interaction effect between parameters that might not have been detected in the conventional approach of studying one parameter at one time while keeping the rest constant. In normal circumstances, it is expected that higher reaction temperature will increases any rate of reaction, leading to an increase
Fig. 4. Two-dimensional plot of FAEE yield against reaction temperature and ethanol-to-oil ratio.
Fig. 5. Response surface plot of FAEE yield against reaction temperature and ethanolto-oil ratio.
in yield, as in the case when the molar ratio of ethanol-to-oil is 16. However, this was found not to be true in the case for the ratio of 40. Possible reason for this phenomenon is as follow. When the molar ratio of ethanol-to-oil used is as high as 40, the critical temperature of the reactant/product mixture between ethanol–oil now becomes highly dependent on the concentration of ethanol, in which the critical temperature of the reactant/product mixture decreases with an increase in ethanol-to-oil ratio [11]. This may be rationalized as oil has a higher critical temperature compared to ethanol. Furthermore, Hegel et al. [11] reported that the critical temperature for a product mixture with high methanol concentration is at about 280 ◦ C, while for system with low methanol concentration is at about 377 ◦ C. Considering the fact that the critical temperature of methanol and ethanol is close, therefore, the lowest reaction temperature used in this study is already above the critical temperature of the reactant/product mixture for the case with ethanol-to-oil molar ratio of 40. When reactant/product mixture is heated above its critical temperature, it has a high tendency to decompose easily, leading to lower yield of FAEE. The decomposition of products at high reaction temperature is also supported by the recent finding by Imahara et al. [12]. In their study, it was reported that unsaturated fatty acid methyl esters are unstable and start to decompose at temperature above 300 ◦ C. At high temperature, unsaturated fatty acids such as oleic acid and linoleic acid tends to decompose via isomerization of the double bond functional group from cis-type carbon bonding (C C) into trans-type carbon bonding (C C), which is naturally unstable fatty acids. Therefore, in this study, it is highly possible that the drop in
Fig. 6. FT-IR analysis on the product sample at reaction temperature (a) 325 ◦ C, (b) 375 ◦ C.
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Table 5 Comparisons of FAEE yield obtained with different reaction conditions and types of oil. Reference
Type of oil
Optimum reaction conditions ◦
This study Warabi et al. [9] Demirbas [13] Madras et al. [14] Varma and Madras [15] Varma and Madras [15] Vieitez et. al. [16]
Palm Rapeseed Linseed Sunflower Linseed Castor Soybean
Yield/conversion (%)
Reaction temperature ( C)
Reaction time (min)
Ethanol-to-oil ratio
349 350 250 400 300 300 350
30 45 8 30 40 30 Flow rate: 1.0 mL/min
33 42 41 40 40 40 40
the yield of FAEE at increasing temperature with a high ethanol-tooil molar ratio (such as in the case of 40) is due to the decomposition of FAEE. This is especially true when the reactions are carried out above the critical temperature of the reactant/product mixture. In order to confirm the decomposition of FAEE at high temperature, selected product samples were subjected to FT-IR (Fourier Transform Infrared Spectrometry) analysis. Fig. 6 shows a comparison of FT-IR spectrum for two samples collected from reaction conducted at different reaction temperatures with the reaction time and ethanol–oil molar ratio fixed at 23 min and 40, respectively. Graph (a) is the FT-IR spectrum for the product obtained from reaction at lower temperature (325 ◦ C) and graph (b) shows the FT-IR spectrum for the product obtained from reaction at higher temperature (375 ◦ C). As shown in Fig. 6, the FT-IR spectrum for (b) shows a trans-type carbon bonding (C C) group at the wavelength of 967.27 cm−1 . On the other hand, there is no peak observed at this wavelength for (a). This observation therefore confirmed that thermal decomposition of FAEE did occur at higher reaction temperature for reactants with high ethanol-to-oil molar ratio, due to the isomerization of cis-type (C = C) into trans-type (C C). The presence of trans-type (C C) which is unstable ultimately reduces the yield of FAEE. Another significant interaction between the molar ratio of ethanol-to-oil and reaction temperature is that, at lower reaction temperature (<363 ◦ C), higher molar ratio of ethanol resulted in higher yield of FAEE as compared to lower molar ratio of ethanol. However, the inverse trend was observed at higher reaction temperature (>363 ◦ C), higher molar ratio of ethanol gives a lower yield of FAEE as compared to lower molar ratio of ethanol. As for the case involving lower reaction temperature, it can be easily explained as follow. The higher molar ratio of ethanol-to-oil used will shift the reaction forward leading to higher yield of FAEE as compared to lower amount of ethanol used. Although, the reactant/product mixture at higher molar ratio of ethanol might have started to decompose (due to lower reactant/product mixture critical temperature), even at lower reaction temperature, however, the positive effect of having higher quantity of ethanol to push the reaction forward is more prominent as compared to lowering the critical temperature of the reactant/product that leads to the decomposition of FAEE. However, at higher reaction temperature, the decomposition rate of the reaction products has overcome the positive effect of having higher ethanol ratio that shifts the reaction forward. Thus, when the reaction temperature is higher, the thermal decomposition of FAEE in the reaction with higher ethanol molar ratio was so significant that it caused a reduction in the yield of FAEE to a point that it is even lower than the yield of FAEE obtained using lower molar ratio of ethanol. This result therefore illustrates the trade off when higher molar ratio of ethanol-to-oil is used, between shifting the reaction forward and reducing the critical temperature of the reactant/product mixture that could eventual lead to decomposition. In the case shown in Fig. 5, when the reaction time was fixed at 16 min, 363 ◦ C seems to be the trade off temperature between the positive and negative effect of using higher molar ratio of ethanol-to-oil.
79.2 95.0 90.0 95.0 90.0 95.0 77.5
3.3. Optimization Up to this point, apart from the three individual parameters, the interactions between parameters were also found to have significant effect on the yield of FAEE. Higher molar ratio of ethanol-to-oil was found to able to shift the reversible reaction forward, resulting in higher yield of FAEE. However, its interaction with reaction temperature can on the other hand cause a reduction in the yield of FAEE due to the decomposition of FAEE. Taking this factor into consideration, therefore, the optimization process must be able to take into account all the effects of the individual parameters and also the interaction between parameters, in order to maximize the conversion of palm oil and ethanol into FAEE, while at the same time, minimizing the thermal decomposition of FAEE which will eventually caused a reduction in the yield of FAEE. In order to perform this task, the mathematical model developed was used to obtain the process parameters that can give the optimum yield of FAEE. This was carried out with the aid of the optimization function embedded in the Design-Expert software. It was predicted that an optimum FAEE yield of 83.1 wt.% can be achieved by using the following process condition; reaction temperature of 349 ◦ C, molar ratio of ethanol-to-oil of 33 and reaction time of 29 min. The predicted optimum yield was verified by carrying out experiment runs using the optimum process condition suggested. The experimental run gives an actual optimum yield of 79.2 wt.% which is close to the predicted value (with less than 5% error) indicating that the predicted optimum yield is valid for this study. The result obtained in this optimization study was compared with the results reported by other researchers for non-catalytic transesterification in supercritical ethanol. Table 5 summarizes the comparison. The optimum yield obtained in this study is relatively slightly lower than those reported in the literature. This may be due to the type of oil used as palm oil has a high-saturated fatty acid content compared to other types of oils reported. Despite the relatively lower yield, however, it should be noted that apart from the reaction conditions reported by Demirbas [13], the optimum reactions conditions obtained in this study is all towards the lower end. This factor is very crucial in terms of economic viability to commercialize this process. On top of that, comparison on the data shown in Table 5 also showed that the process conditions for optimum yield of biodiesel is not only dependent on the types of oil used, but also on the experimental reaction system set-up. Although in the study reported by Demirbas [13] and Varma and Madras [15] both used linseed oil as the feedstock, the process conditions for the obtaining the optimum yield is very much different, indicating that process conditions for optimum yield is not only dependent on types of oil used but also reactor system set-up. 4. Conclusion Based on the design of experiment approach over conventional strategy, it was found that interaction between the process parameters significantly affect the yield of biodiesel obtained via non-catalytic supercritical ethanol transesterification. Once the
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interaction effects were identified, it can be used to optimize the transesterification process. However, in this study, only selected critical process parameters were studied, leaving behind other parameters such as content of free fatty acid that may have significant effect on the yield. Furthermore, this study revealed that owing to the high content of saturated fatty acid in palm oil, the optimum yield is achieved at different process parameters as compared to oil with high-unsaturated fatty acid content. Therefore, this warrant for more studies on supercritical ethanol transesterification using oil with high-saturated fatty acid content. Acknowledgements The authors would like to acknowledge Prof. Shiro Saka from Kyoto University, Japan for his guidance given in this research. Apart from that, the authors would also like to acknowledge Ministry of Science, Technology and Innovation (Science Fund No: 03-01-05SF0138) and Universiti Sains Malaysia (Research University Grant) for the financial support given. References [1] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. 70 (1999) 1–15. [2] M.M. Gui, K.T. Lee, S. Bhatia, Feasibility of edible oil vs. non-edible oil vs. waste edible oil as biodiesel feedstock, Energy 33 (2008) 1646–1653.
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