Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process Sumit M. Dubey a, Vitthal L. Gole b, Parag R. Gogate a,⇑ a b
Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 40019, India Chemical Engineering Department, AISSMS College of Engineering, Pune 411 001, India
a r t i c l e
i n f o
Article history: Received 25 June 2014 Received in revised form 19 August 2014 Accepted 19 August 2014 Available online xxxx Keywords: Nagchampa oil FAME Cavitational effects High acid value Two step processing Intensification
a b s t r a c t The present work reports the intensification aspects for the synthesis of fatty acid methyl esters (FAME) from a non-edible high acid value Nagchampa oil (31 mg of KOH/g of oil) using two stage acid esterification (catalyzed by H2SO4) followed by transesterification in the presence of heterogeneous catalyst (CaO). Intensification aspects of both stages have been investigated using sonochemical reactors and the obtained degree of intensification has been established by comparison with the conventional approach based on mechanical agitation. It has been observed that reaction temperature for esterification reduced from 65 to 40 °C for the ultrasonic approach whereas there was a significant reduction in the optimum reaction time for transesterification from 4 h for the conventional approach to 2.5 h for the ultrasound assisted approach. Also the reaction temperature reduced marginally from 65 to 60 °C and yield increased from 76% to 79% for the ultrasound assisted approach. Energy requirement and activation energy for both esterification and transesterification was lower for the ultrasound based approach as compared to the conventional approach. The present work has clearly established the intensification obtained due to the use of ultrasound and also illustrated the two step approach for the synthesis of FAME from high acid value feedstock based on the use of heterogeneous catalyst for the transesterification step. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, synthesis of fatty acid methyl esters (FAME) based on esterification or transesterification reactions has gained worldwide attention due to number of applications in perfume and flavor industry and also due to the use of biodiesel as a source of alternative fuel [1–4]. For the specific use of FAME as biodiesel, it is important to note that the biodiesel demand in India is increasing with rapid industrialization, globalization and rapid changing lifestyle and considering the fact that India is planning to use biodiesel blend B20 by 2017, the expected increase is at a rate of 6.2% per annum. Biodiesel is a mixture of alkyl esters, which can be used in conventional compression ignition engines, without any significant modifications. Transesterification has been generally used for the synthesis of fatty acid esters from vegetable oils and animal fats using homogeneous catalyst (mainly sodium or potassium hydroxide dissolved in methanol). Traditional homogeneous catalysts (basic or acid) possess advantages including high activity and requirement of mild reaction conditions (from 40 to 65 °C and atmospheric pressure). However, the use of homogeneous ⇑ Corresponding author. Tel.: +91 22 3361 2024; fax: +91 22 3361 1020. E-mail address: [email protected] (P.R. Gogate).
catalysts can lead to soap formation especially in the case of feedstock with high free fatty acid content. The total cost of the production using homogeneous catalysis, is not yet sufficiently competitive as compared to the cost of diesel production from petroleum. An alternative approach for synthesis of fatty acid methyl esters can be based on the use of heterogeneous catalysts. Use of heterogeneous catalyst can be helpful in avoiding the soap formation (produced in presence of the homogenous alkali catalyst) and also reduce the load on the downstream processing of separations possibly leading to considerable saving in terms of energy requirements and avoiding wastewater generation. There have been many studies reported in the literature relating to the synthesis of biodiesel using heterogeneous catalyst [5–8]. Georgogianni et al. [5] studied the transesterification of rapeseed oil in the presence of heterogeneous catalysts (Mg MCM-41, Mg–Al Hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasound reactor (24 kHz). It has been reported that Mg–Al hydrotalcite gives the highest activity with 97% conversion in a reaction time of 5 h. Biodiesel synthesis from Jatropha curcas oil using solid catalyst Na/SiO2 in the presence of ultrasound gave yield of 98.53% at optimal molar ratio of oil to methanol as 1:9, catalyst concentration of 3 wt% of oil and 15 min of reaction time [6]. The ultrasound assisted biodiesel synthesis from palm oil [7] gave yield of 77.3%
http://dx.doi.org/10.1016/j.ultsonch.2014.08.019 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
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S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
(CaO), 94.2% (SrO) and 95.2% (BaO) for a reaction time of 60 min. The optimum parameters for the ultrasound assisted biodiesel synthesis from soybean oil were catalyst (CaO) loading of 6 wt%, temperature of 62 °C, and molar ratio of oil to methanol as 1:10. The approach of ultrasound assisted biodiesel synthesis from palm oil using calcium oxide was reported to give a yield of nearly 100% in 2 h of the operation [7]. The reported work in the literature on heterogeneous catalyzed biodiesel synthesis in the presence of ultrasound has concentrated mainly on the feed stock (virgin oil) with lower acid value requiring only one step processing. The current work presents a novel approach of intensification of two step processing for the sustainable feed stock containing higher acid value (which is expected to create problems of soap formation if used with alkaline catalyst) based on the use of heterogeneous catalyst. The feedstock used in the present work is Nagchampa oil containing higher amount of the oil (65–70%) which is almost twice as compared to the other non-edible feedstock like Karanja (25–30%), Jatropha (30–35%) [9]. Nagchampa is abundantly found in the costal part of the India. The major problem associated with Nagchampa as a feedstock is initial higher amount of free fatty acid (31 mg of KOH/g of oil), which requires more processing like two stage approach of esterification (to reduce the free fatty acid content) followed by transesterification [9]. It is important to note here that the two step synthesis is highly energy intensive operation and hence investigating the process intensification aspects with an objective of decreasing the material/temperature requirement is significant. In the past, the various approaches used for intensification of biodiesel synthesis include ultrasound, microwave, use of supercritical conditions, hydrodynamic cavitation, etc. [1–5]. Process intensification using cavitational reactors can be a promising approach due to the fact that operation is at ambient conditions and the physical effects of cavitation such as liquid circulation associated with turbulence can eliminate the mass transfer resistances and mixing issues associated with esterification/transesterification [9,10]. The work also presents comparison of the FAME synthesis from Nagchampa oil using conventional reflux method with ultrasound assisted approach to establish the degree of intensification. The effect of molar ratio, catalyst concentration and temperature has been investigated for both the steps of esterification and transesterification. 2. Materials and methods 2.1. Materials The raw Nagchampa oil was procured from M/s Amit Oil Mill, Vengurla, Dist: Sindhudurgah, Maharashtra, India. The chemical composition of Nagchampa oil is Palmitic acid as 12%, Stearic acid as 13%, Oleic acid as 34.1%, Linoleic acid as 38.3% and Linolenic acid as 0.3%. Methanol (HPLC grade), sulfuric acid (98%) (A.R.) and hexane (HPLC grade) were obtained from M/s Thomas Baker Chem. Pvt. Ltd., Mumbai, India. Calcium oxide (A.R.) was obtained from S.D. Fine-Chem. Ltd., Mumbai, India.
Condenser Stirrer Glass reactor Speed Regulator
Fig. 1a. Experimental setup for conventional approach.
The equipment used to study the effect of cavitation on synthesis was an ultrasonic horn, procured from M/s Dakshin Pvt. Ltd. Mumbai, India. Ultrasonic horn operates at a frequency of 20 kHz with a power rating of 120 W. The ultrasonic horn was fitted with a piezoelectric transducer with a tip diameter of 1 cm and immersed to a depth of 1 cm below the liquid level in a 150 ml capacity glass reactor. Reactor was kept in a water bath to maintain desired temperature within ±1 °C. The typical arrangement of the experimental setup used for the ultrasound assisted approach is shown in Fig. 1b. 2.3. Experimental methodology 100 ml of Nagchampa oil was taken in a glass reactor and preheated to the desired value. After 10 min, when the temperature of oil uniformly remained constant, mixture of methanol (at the desired molar ratio) and sulfuric acid was added in appropriate proportions for the first stage of esterification. The acid value was checked after every 1 h of reaction time. After reaching the constant acid value, esterified oil was kept for phase separation for 4 h in a separating funnel. The lower layer was separated and upper acid layer was used for second stage. The upper layer from first stage esterification was now taken in the reactor and kept for 10 min so as to reach constant temperature. Calculated amount of catalyst (H2SO4) and methanol was added. The acid value was checked after every 1 h of time. After reaching the desired acid value, esterified oil was kept for phase separation for 4 h in separating funnel. For preparing the catalyst to be used in the actual reaction, CaO was heated to remove moisture and volatile matter
2.2. Reactor configuration The reactions were performed in a 5.0 cm (ID), 150 cm3 capacity glass reactor equipped with reflux condenser. Reactor was kept in a water bath to maintain constant temperature. Water bath was procured from M/s Ganesh Scientific Industries, Mumbai, India and has provision for maintaining the temperature within ±1 °C. A pitched blade glass stirrer having 1 inch diameter was used to achieve uniform mixing of the reactants in the conventional approach. The schematic representation of experimental setup is shown in Fig. 1a.
Water bath
Condenser Ultrasound Horn Glass Reactor Generator
Water Bath
Fig. 1b. Experimental setup for ultrasound assisted approach.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
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acid value (mg of KOH/mg of oil
35 50°C 60°C 65°C 70°C
30 25 20 15 10 5 0 0
1
2
3
4
5
time(h) Fig. 3a. Effect of temperature on acid value at constant catalyst loading of 1% and oil to methanol molar ratio as 1:4 using conventional method. Fig. 2. HPLC chromatogram for FAME analysis.
2.4. Method of analysis The acid value of the samples in the first and second stage esterification was determined using the acid base titration technique. A standard solution of 0.05 N potassium hydroxide solution and ethanol for dissolving the withdrawn sample was used. A sample of 1 g of oil was dissolved in 20 ml of ethanol and heated for 5–10 min. Then 10 ml of sample was titrated against alcoholic potassium hydroxide using phenolphthalein as indicator. The acid value can be calculated as follows:
AV ¼
Burette reading Mol: Wt: KOH Normality of KOH Wt: of Oil
The samples from transesterification stage were analyzed by High Pressure Liquid Chromatography (HPLC). Fatty acid methyl ester (FAME) was analyzed by HPLC with a C18 Phenomenex column (25 cm 4.6 mm, 5 lm particle size) using UV/Visible detection (Hitachi) at 210 nm. The sample was analyzed using linear elution of methanol and hexane. The mobile phase consists of 90% methanol and 10% hexane. The mobile phase flow rate was maintained as 1 ml/min. Samples were prepared by using 10 ll of the reaction mixture diluted with 10 ml of methanol. A sample HPLC chromatogram has been given in Fig. 2. 3. Results and discussion 3.1. Conventional approach 3.1.1. Esterification 3.1.1.1. Effect of reaction temperature. The effect of temperature on the esterification using the conventional approach has been studied by changing the temperature of reaction at different values as 50, 60, 65 and 70 °C. The molar ratio of 1:4 and catalyst concentration of 1% (w/w) was kept constant for all the experimental runs related to the effect of temperature. The obtained results for effect of temperature have been shown in Fig. 3a. It has been observed that acid value decreases from 31 to 9.2 mg KOH/g of oil with an increase in reaction temperature from 50 to 65 °C in 2.5 h and further increase in temperature does not yield any additional
3.1.1.2. Effect of catalyst loading. Esterification was also performed using different catalyst concentrations as 0.5%, 1%, 1.5%, and 2% (w/w) of oil at optimized constant reaction temperature as 65 °C and molar ratio of 1:4. The obtained results for the effect of catalyst loading have been given in Fig. 3b. It was observed that acid value decreased from 31 to 9.2 mg KOH/g of oil at optimum catalyst loading of 1%. However, above 1% catalyst loading, there was not much decrease in the final acid value. Esterification reaction after some time approaches to equilibrium and hence further reduction in acid value is not observed. Similar optimum loading for reducing acid value of Jatropha oil from 15 to 1 mg of KOH/g of oil has been reported as 1% [16]. Thiruvengadaravi et al. [14] reported an optimum acid catalyst loading of 1% for esterification process for synthesis of biodiesel from waste cooking oil. 3.1.1.3. Effect of molar ratio. Esterification is a reversible reaction and hence in order to maintain the forward path of reaction, methanol is generally taken in excess. Esterification was performed 35 acid value (mg of KOH/mg of oil)
using a Muffle Furnace at 700 °C for 4 h. The upper layer from second stage esterification was taken in reactor and kept for 10 min for achieving desired temperature of oil. Calculated amount of calcined catalyst (CaO) and methanol at desired molar ratio was added. The samples were withdrawn at regular interval and analyzed by HPLC.
benefits. An increase in the operating temperature results in enhanced solubility of methanol in the oil phase thereby increasing the rate of reaction and decreasing the final acid value of oil. The reduction in acid value is restricted to the equilibrium conversion and hence further reduction in acid value requires additional esterification stage. The existence of optimum temperature has been also observed for various studies in the literature using different feed stocks such as Nagchampa oil [10], waste cooking oil [11,12], soybean oil [13], Karanja oil [14] and J. curcas oil [15].
0.5% 1% 1.5% 2%
30 25 20 15 10 5 0 0
1
2
3
4
5
time (h) Fig. 3b. Effect of catalyst loading on acid value at constant temperature of 65 °C and oil to methanol ratio as 1:4 using conventional method.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
3.1.1.4. Second stage esterification. In first stage esterification process, it has been observed that acid value of feedstock was reduced from 31 to 9.2 mg of KOH/g of oil at optimum molar ratio of 1:4, catalyst concentration of 1% w/w and temperature as 65 °C. In order to avoid the saponification reaction (in the presence of alkali catalyst) during transesterification process, the acid value should be less than 2 mg of KOH/g of oil (i.e., 1% of free fatty acid). In order to reduce the acid value below desired value, an additional esterification stage was used for re-esterification of first stage esterified oil. The results for the second stage esterification have been shown in Fig. 3d. It has been observed that conditions of molar ratio of 1:1, catalyst concentration of 1% w/w and temperature of 65 °C was sufficient to reduce the acid value from 9.2 to 2 mg of KOH/g of oil in 3 h of reaction. 3.1.2. Transesterification The two stage esterified oil has been used for the transesterification and similar to esterification process, the effect of temperature, catalyst concentration and molar ratio has been investigated. 3.1.2.1. Effect of reaction temperature. The effect of temperature on the transesterification stage was studied by using different temperatures of reaction as 40, 50, 65 and 70 °C. The molar ratio of 1:10 and catalyst (CaO) concentration of 2.5% (w/w) was kept constant for these studies and the obtained results have been shown in Fig. 4a. It was found that the yield of FAME increases with an increase in temperature to 65 °C. An increase in the operating
acid value (mg of KOH/mg of oil)
35 (1:2) (1:3) (1:4) (1:6)
30 25 20 15 10 5 0 0
1
2
3
4
10 acid Value (mg of KOH/mg of oil)
using varying oil to methanol molar ratio as 1:2, 1:3, 1:4, and 1:6 at optimized constant H2SO4 concentration of 1% weight of oil and reaction temperature as 65 °C. The obtained results for the effect of molar ratio have been given in Fig. 3c. It was observed that the acid value decreases from 31 to 9.2 mg KOH/g of oil with an increase in molar ratio to 1:4 and there was no significant reduction in the acid value for further increase in the ratio to 1:6. This can be attributed to the fact that water produced during esterification of free fatty acids prevents further reaction [14]. Water formed during esterification dissolves in the excess methanol and may disturb the equilibrium of the reaction. Selection of optimum molar ratio is significant for esterification as it decides the load on the downstream process for removal of excess methanol. Similar results have been reported in the literature for Jatropha oil where acid value reduction was achieved from 15 to 1 mg of KOH/g of oil for optimum molar ratio of 1:6 and 2 h reaction time [16]. Similar trend of existence of optimum conditions to reduce acid value has been reported for esterification of soybean oil [17].
8
6
4
2
0 0
1
2
3
4
5
time (h) Fig. 3d. Acid value for second stage esterification for 65 °C temperature, 1% catalyst concentration and 1:1 oil to methanol molar ratio using conventional method.
100 80
%, yield
4
60 40°C
40
50°C 60°C
20
65°C 0 0
2
4
6
time, hr Fig. 4a. Effect of temperature on yield of FAME at constant catalyst loading as 2.5% and oil to methanol ratio as 1:10 using conventional method.
temperature result in enhanced solubility of methanol in the oil resulting in increased extent of conversion. It was also observed that beyond a reaction time of 4 h, the FAME yield marginally decreased (the extent of decrease was more significant at higher temperatures), which may be due to the possible degradation of product under prolonged exposure at higher temperature. It has been observed that 76% yield was obtained in 4 h of operation at 65 °C. 3.1.2.2. Effect of catalyst loading. The effect of amount of heterogeneous catalyst (CaO) on transesterification was investigated by changing the catalyst concentration as 1, 2.5, 5 and 6 (w/w) of oil at optimized reaction temperature of 65 °C and molar ratio of 1:10. The obtained results for the effect of catalyst loading have been given in Fig. 4b. It was observed that fatty acid methyl ester yield increases with an increase in catalyst concentration from 1 to 2.5% (w/w) for 4 h operation. Further increase in catalyst concentration does not give any significant effect on the final yield and hence catalyst concentration of 2.5% has been considered as optimum for further processing. Deshamane et al. [18] reported an optimum catalyst loading as 1% for the heterogeneous calcium methoxide assisted synthesis of biodiesel from Soybean oil. Similar results were also reported for synthesis of biodiesel from Palm oil where 3% optimum heterogeneous catalyst concentration was observed [19].
5
time (h) Fig. 3c. Effect of molar ratio on acid value at constant temperature of 65 °C and catalyst concentration as 1%.
3.1.2.3. Effect of molar ratio. In transesterification process, three moles of methanol are required per mole of triglyceride to yield three moles of fatty acid methyl ester and one mole of glycerol.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
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100
%, yield
80 60 40 1% 2.5%
20
5% 6%
0 0
2
4
6
time (hr) Fig. 4b. Effect of catalyst loading on yield of FAME at constant temperature of 65 °C and oil to methanol molar ratio 1:10 using conventional method.
Similar to esterification, higher molar ratio is required in order to drive the forward reaction. At significantly higher molar ratio, glycerine formed during reaction dissolves into the excess methanol which may lower the yield of FAME. Thus, the optimization of molar ratio for transesterification is important. The effect of molar ratio was studied at different ratios as 1:6, 1:8, 1:10 and 1:12 at optimum catalyst concentration as 2.5 w/w and reaction temperature of 65 °C. The obtained results for the effect of molar ratio have been given in Fig. 4c. It has been observed that the yield increases with an increase in molar ratio from 1:8 to 1:10 but a further increase in molar ratio does not result in significant increase in the yield of fatty acid methyl ester. Yield of process also decreased marginally after the 4 h of reaction. Since transesterification is an equilibrium reaction, a larger amount of methanol initially shifts the methanolysis towards forward direction, resulting in favorable conditions for the FAME formation. Deshamane et al. [18] reported similar requirement of higher molar ratio of 1:12 for synthesis of biodiesel from soybean oil using heterogeneous calcium methoxide catalyst. 3.2. Ultrasound assisted approach 3.2.1. Esterification 3.2.1.1. Effect of reaction temperature. The effect of temperature on the esterification was studied using different values of temperature as 30, 40, 50 and 65 °C. The molar ratio of 1:4 and catalyst concentration of 1% (w/w) was kept constant. It has been observed that acid value decreases from 31 to 10.1 mg KOH/g of oil at reaction
100
%, yield
80
5
temperature of 40 °C in 3 h and further increase in temperature does not yield any additional benefits. The observed trend for the effect of temperature was similar to that observed for the conventional approach. An increase in reaction temperature causes the decrease in oil viscosity and gives enhanced cavitational effects improving the contact between reactants and hence the acid value decreases with an increase in temperature [20]. The physical effects of cavitation like acoustic streaming and microemulsification are useful in enhancing the contact between the immiscible phases of methanol, oil and catalyst. Use of temperature as 40 °C also assists in reducing the attenuation effect of ultrasound in the viscous medium of oil. The existence of optimum temperature has been also observed for Tung oil [21] and Zanthoxylum bungeanum seed oil [22]. 3.2.1.2. Effect of catalyst loading. Optimization of catalyst concentration is important in order to avoid the problems related to the removal of catalyst from the product stream. Esterification was studied by changing the catalyst concentration at different value of 0.5%, 1%, 1.5% and 2% (w/w) of oil at optimum temperature of 40 °C and molar ratio of 1:4. The observed trend for the effect of catalyst loading was similar to that observed for the conventional approach. It has been observed that acid value decreases from around 31 to 10.1 mg KOH/gm for 1% w/w catalyst loading in 3 h. Similar effect has been reported for esterification of fatty acid cuts with optimum concentration as 2% [23], while esterification of palm fatty acid distillate required 5% catalyst concentration as optimum for the ultrasound assisted synthesis [18]. 3.2.1.3. Effect of molar ratio. The effect of molar ratio on esterification using ultrasound was studied at varying oil to methanol molar ratio as 1:2, 1:3, 1:4 and 1:6 at optimum catalyst concentration of 1% (w/w) and temperature of 40 °C. It was observed that acid value decreases from around 31–10.1 mg KOH/g of oil at 1:4 molar ratio and any further increase in molar ratio does not give significant reduction in acid value. Since this reaction is reversible, excess amount of methanol is required to drive the reaction in the forward direction. Thus, the final acid value would be lower where a higher excess of methanol is available in the reaction mixture as explained in details for the conventional approach. 3.2.1.4. Second stage esterification. Similar to the conventional approach, it has been observed that the single stage esterification does not reduce the acid value below 2 mg of KOH/g of oil required for avoiding the saponification reaction during the transesterification stage. Re-esterification of the processed oil was performed and molar ratio, catalyst concentration and reaction temperature has been again optimized for the second stage esterification. The effect of temperature on the esterification stage was studied using different temperatures as 30, 40, 50 and 65 °C, catalyst concentration as 0.5%, 1%, 1.5% and 2% and molar ratio of 1:0.5, 1:1, 1:2 and 1:3. It has been observed that acid value decreases from around 10 to 1.69 mg KOH/g of oil in 3 h operation at optimum reaction temperature as 40 °C, catalyst concentration of 1% and molar ratio of 1:1.
60 40 1:6 1:8
20
1:10 1:12
0 0
2
4
6
time (hr) Fig. 4c. Effect of molar ratio on FAME at constant temperature of 65 °C and catalyst concentration as 2.5% using conventional method.
3.2.2. Transesterification CaO catalyst being hydrophilic in nature preferably stays in the methanol phase. At the catalyst surface, methoxy ions are generated as a result of adsorption of methanol molecule and subsequently transferred to the organic phase at the interface between the two phases and reaction proceeds. Convection generated by ultrasound helps in efficient dispersion of the aqueous and organic phases forming a fine emulsion with significantly higher interfacial area. Due to high viscosity of oil, the micro-streaming and microconvection generated in methanol mostly contributes to the convection generated in the reaction system.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
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3.2.2.2. Effect of catalyst loading. The effect of heterogeneous catalyst (CaO) loading on transesterification was investigated by changing the catalyst concentration as 1%, 2.5%, and 5% (w/w) of oil at 60 °C temperature and molar ratio of 1:10. It was observed that FAME yield increases with an increase in catalyst concentration from 1% to 2.5% (w/w), which can be attributed to the increase in active sites in the reaction system. Further increase in catalyst concentration does not give significant effect on the yield. The change in the yield with catalyst concentration has been shown in Fig. 5b. The catalytic activities were mainly related to their basic strength [24–26], which should increase with an increase in the catalyst loading till an optimum. Xie et al. [26] reported that when the Mg/Al catalyst quantum exceeds beyond an optimum value, the catalytic activity decreases due to the formation of new weaker basic sites. At higher catalyst concentrations, the extent of increase in the FAME is observed to be lower which may also be attributed to possible formation of agglomerates of catalyst particles and also it is important to note that any excess use of catalyst will lead to more energy consumption in the separation process. Hence the selection of the optimum catalyst concentration is important [24–26]. 3.2.2.3. Effect of molar ratio. The effect of molar ratio was studied at different molar ratios as 1:6, 1:8, 1:10 and 1:12. The change in the 100
100 2.5% 1.00%
80
%, yield
3.2.2.1. Effect of reaction temperature. The effect of temperature on the transesterification rate was studied using different temperatures of reaction as 40, 50 and 60 °C at constant molar ratio of 1:10 and catalyst (CaO) concentration of 2.5% (w/w). The effect of temperature on FAME yield has been shown in Fig. 5a. It can be seen from the figure that the biodiesel yield increases with an increase in reaction time till an optimum value beyond which yield decreases possibly due to the degradation of the FAME. It was also found the optimum reaction time in the case of ultrasound assisted approach is 2.5 h as compared to 4 h in the case of conventional synthesis, which is expected as the possible degradation will be faster in the presence of ultrasound. Ultrasound assisted reaction gave yield of 79% in 2.5 h at 60 °C as compared to 76% yield at 65 °C obtained in 4 h using the conventional approach. Similar results of intensified synthesis have also been reported in the literature for single step processing. Arzamendi et al. [24] reported that CaO catalyst could give 30% of biodiesel yield in 5 h using conventional approach and higher yield was achieved in a much shorter reaction time when ultrasound was used. At longer reaction time, the promotion of reverse reaction and/or possible degradation rendered the decrease in the biodiesel yield with time. The catalysts could also last longer as new catalytic active sites could be exposed to the reactants due to the cleaning effects caused by the ultrasound [22,23].
5%
60
40
20
0 0
1
2
time (h)
3
4
5
Fig. 5b. Effect of catalyst loading on yield of FAME at constant temperature of 60 °C and oil to methanol molar ratio as 1:10 using ultrasound method.
100 80
%, yield
6
60 40 1:6 1:8
20
1:10 1:12
0 0
1
2
3
4
5
time (hr) Fig. 5c. Effect of molar ratio on yield of FAME at constant temperature of 60 °C and catalyst concentration as 2.5% using ultrasound method.
yield with molar ratio has been shown in Fig. 5c. It was observed that the yield increases with an increase in molar ratio till an optimum molar ratio as 1:10. The initial increase in the yield with an increase in molar ratio can be attributed to the fact that higher molar ratio works favorably to maintain the forward path of the reaction. As the reaction proceeds, glycerol produced during the reaction may dissolve in the excess methanol and disturb the reaction equilibrium resulting in lower yield. Hence the selection of the optimum molar ratio is very crucial [24–27]. Also any unutilized methanol needs to be separated which can increase the energy requirements in the downstream processing and hence using too much excess of methanol is not recommended.
60°C 50°C
80
4. Comparative study between conventional and ultrasound assisted synthesis
%, yield
40°C 60 40 20 0 0
1
2
time (h)
3
4
5
Fig. 5a. Effect of temperature on yield of FAME at constant catalyst concentration 2.5% and oil to methanol ratio as 1:10 using ultrasound method.
The optimized parameters for conventional and ultrasound assisted FAME synthesis from Nagchampa oil have been established and a summary of obtained results is shown in Table 1. The optimum reaction time in the esterification stage was not affected meaning that the rate of reaction is more dominant on the temperature and the molar ratio of the reactants. Though reaction time for ultrasound and conventional approach is same, reaction temperature was reduced from 60 °C (conventional approach) to 40 °C(ultrasound approach). Another advantage of ultrasound approach for first stage esterification is lower energy requirement for conversion of free fatty acid to biodiesel as compared to the
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
7
S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 1 Optimized parameters for conventional and ultrasound method. ES-I
Reaction time (h) Molar ratio (Oil:Methanol) Catalyst concentration (%w/w) Reaction temperature (°C) Conversion/yield (%) Energy consumption (g/J 105)
ES-II
y = 19.30x R² = 0.990
1.4
T
1.2
C
US
C
US
C
US
3 1:4 1
3 1:4 1
3 1:1 1
3 1:1 1
4 1:10 2.5
2.5 1:10 2.5
65 71.2 8.9
40 67.8 11.27
65 80.2 10.04
40 84.4 14.02
65 76.4 8.28
60 78.7 15.84
1/ [A] - 1/[A]0
Parameters
1.6
1 0.8 0.6 0.4 0.2 0 0
1
2
ES-I, esterification stage-I; ES-II, esterification stage-II; T, transesterification; US, ultrasound; C, conventional.
conventional approach. Biodiesel production per unit energy consumption for first stage esterification was 8.9 105 and 11.27 105 g/J for the conventional approach and ultrasound based approach respectively and for the second stage esterification, it was 10.04 105 and 14.02 105 g/J. The details of the calculation of energy requirement and biodiesel production per unit energy consumed have been given in Appendix I. Similar trend has been observed for transesterification stage with a decrease in the reaction temperature from 65 °C for the conventional approach to 60 °C for the ultrasound assisted approach. In addition to that, there was reduction in processing time from 4 h required for the conventional approach to 2.5 h using ultrasound approach. The yield was also found to be increased to 79% using the ultrasound approach as compared to 76% in conventional approach. The yield of biodiesel per unit energy consumed for transesterification stage was 15.84 105 g/J in the case of ultrasound based approach whereas for the conventional approach the extent of productivity was 8.28 105 g/J. The physical effects of ultrasound induced cavitation such as acoustic streaming [28] and micro-emulsification enhance the rate of esterification and transesterification process. These effects are useful for increasing the yield of reaction, for reducing the temperature of reaction and also the reaction times. Deshmane et al. [29] have reported similar intensification with the use of cavitation generated using
3
4
time, hrs
Fig. 6. Fitting curve for transesterification (conventional method) reaction for second order reaction kinetics ([A]: concentration of any time, [A]0: initial concentration).
ultrasonic irradiation for synthesis of biodiesel from palm fatty acid distillate, which is a comparatively much cheaper raw material, as compared to vegetable oils. Kinetic rate constants have been obtained, considering secondorder kinetics for each step of synthesis and the values of the rate constants for both stages of esterification, as well as transesterification, have been given in Table 2. The kinetic rate constant was evaluated assuming second-order reaction (equilibrium was assumed to proceed in a forward path in the presence of excess methanol) for esterification [20] and transesterification reactions [30]. It has been observed that the rate constant (sample graph has been shown in Fig. 6) for both esterification and transesterification reactions increased with temperature, molar ratio and catalyst concentration (till optimum value, significant increase is observed) for conventional method as well as ultrasound-assisted synthesis. The rate constant was observed to be higher using the ultrasound approach for both reactions as compared to the conventional method. The activation energy was also determined using the Arrhenius equation which gives a relationship between the specific reaction
Table 2 Second order rate constant for esterification and transesterification for conventional and ultrasound approach. Esterification (Conventional approach) (k 102, L mol1 min1) Temperature (°C)
Molar ratio
Catalyst concentration (% w/w)
Stage I k
50 0.5
60 1.7
65 2.7
70 3.1
1:2 1.1
1:3 1.9
1:4 2.7
1:6 2.9
0.5 1.7
1 2.7
1.5 2.9
2 2.9
Stage II k
50 0.67
60 7.7
65 13.3
70 14.8
1:05 5.6
1:1 13.3
1:2 16.3
1:3 16.2
0.5 6.8
1 13.3
1.5 14.3
2 14.1
Esterification (Ultrasound approach) (k 102, L mol1 min1) Temperature, (°C)
Molar ratio
Stage I k
30 0.8
40 2.4
50 2.3
65 2.8
1:2 0.6
1:3 1.3
1:4 2.4
1:6 2.6
0.5 1.4
1 2.4
1.5 2.6
2 2.9
Stage II k
30 4.1
40 16.5
50 17.1
65 17.2
1:05 2.5
1:1 16.5
1:2 15.2
1:3 15.1
0.5 6.5
1 16.5
1.5 17.2
2 16.8
1:6 14.2
1:8 19.3
1:10 20.8
1:12 25.5
1 17.0
2.5 20.8
5 26.8
6 25.4
Transesterification (Ultrasound approach) (k 102, L mol1 min1) 30 40 50 60 1:6 k 11.2 23.6 25.5 32.8 21.3
1:8 30.3
1:10 32.8
1:12 35.0
1 27.6
2.5 32.8
5 35.9
6 34.7
Catalyst concentration (% w/w)
Transesterification (Conventional approach) (k 102, L mol1 min1) Temperature (°C) Molar ratio k
40 14.3
50 16.6
65 20.8
70 19.4
Catalyst concentration (% w/w)
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Table 3 Activation energy.
Appendix I
Activation energy (kJ/mol)
Esterification stage I Esterification stage II Transesterification
Conventional method
Ultrasound approach
86.66 61.86 18.03
28.18 59.22 14.71
Sample calculation of energy requirement for transesterification using conventional as well as ultrasound approach is given below: A.1. Conventional approach Total energy requirement ¼ Energy for stirring þ Energy for heating Energy for stirring ¼ Power required for stirring time in sec
8.2 8
¼ 1728 103 J
Ultrasound
Energy for heating ¼ m CpðT2 T1 Þ
ln k
7.8
where
7.6 7.4
y = -1768.x + 12.88 R² = 0.992
7.2 7 6.8 0.0029
¼ 120 4 60 60
Conventional
y = -2169.x + 14.44 R² = 0.927
0.003
0.0031
0.0032
0.0033
0.0034
1/T Fig. 7. Change in logarithm of specific rate constant with reciprocal of temperature for transesterification.
rate constant (k), absolute temperature (T) and the energy of activation (E) as:
m = mass of water = 5000 g Cp = specific heat of water = 4.18 J/kg, K T1 = initial temperature of water = 298.15 K t2 = final temperature of water = 338.15 K
¼ 836 103 J Total energy ¼ 2564 103 J Mass of biodiesel formed ¼ yield of biodiesel Average molecular weight of biodiesel ¼ 0:76 279:4 ¼ 212:34 g Energy required ¼
k ¼ A eðE=RTÞ where A is the frequency factor and R is universal gas constant. Activation energy obtained for the case of conventional and ultrasound approach is given in Table 3. The activation energy (sample graph is shown in Fig. 7) for the first esterification stage in conventional synthesis is 86.66 kJ/mol which is much higher than the value observed for the ultrasound assisted approach as 28.18 kJ/mol. For the second esterification stage the activation energy is marginally low for the ultrasound assisted approach. In the transesterification stage, the activation energy for ultrasound approach was also lower as compared to conventional synthesis approach.
Mass of biodiesel formed; g total energy required; J
¼ 8:28 105 g=J
A.2. Ultrasound based approach
Total energy requirement ¼ Energy for equipment þ Energy for heating Energy for stirring ¼ Power required for equipment time in sec ¼ 120 2:5 60 60 ¼ 1080 103 J Energy for heating ¼ m CpðT2 T1 Þ where
5. Conclusions The present work has established a novel process for the FAME synthesis from higher acid value feedstock using two stage esterification followed by transesterification approach. The physical effects of cavitation were found to be beneficial in enhancing the rate of esterification and transesterification reactions. The required reaction temperature for both the processes was lower in the case of ultrasound assisted approach. Also the yield of reaction increased for transesterification with a simultaneous reduction in the reaction time. Rate constant for both esterification and transesterification reactions increased with temperature, molar ratio and catalyst concentration (till an optimum value) for conventional method and ultrasound assisted synthesis. The rate constant was observed to be higher for the ultrasound approach for both reactions compared to the conventional method. Use of heterogeneous catalyst eliminates the separation problems that might be associated with homogeneous catalyst in transesterification reaction. Overall, it can be said that the ultrasound assisted process enhanced the rate of reaction, reduced the temperature requirement which can possibly lead to considerable saving in terms of energy.
m = mass of water = 5000 g Cp = Specific heat of water = 4.18 J/kg, K T1 = Initial temperature of water = 298.15 K T2 = Final temperature of water = 313.15 K
¼ 313:5 103 J Total energy ¼ 1393:5 103 J Mass of biodiesel formed ¼ yield of biodiesel molecular weight of biodiesel ¼ 0:79 279:4 ¼ 220:7 g Energy required ¼
Mass of biodiesel formed; g total energy required; J
¼ 15:84 105 g=J Similarly productivity of biodiesel per unit power consumption is calculated for esterification stages for ultrasound and conventional approach.
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process Sumit M. Dubey a, Vitthal L. Gole b, Parag R. Gogate a,⇑ a b
Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 40019, India Chemical Engineering Department, AISSMS College of Engineering, Pune 411 001, India
a r t i c l e
i n f o
Article history: Received 25 June 2014 Received in revised form 19 August 2014 Accepted 19 August 2014 Available online xxxx Keywords: Nagchampa oil FAME Cavitational effects High acid value Two step processing Intensification
a b s t r a c t The present work reports the intensification aspects for the synthesis of fatty acid methyl esters (FAME) from a non-edible high acid value Nagchampa oil (31 mg of KOH/g of oil) using two stage acid esterification (catalyzed by H2SO4) followed by transesterification in the presence of heterogeneous catalyst (CaO). Intensification aspects of both stages have been investigated using sonochemical reactors and the obtained degree of intensification has been established by comparison with the conventional approach based on mechanical agitation. It has been observed that reaction temperature for esterification reduced from 65 to 40 °C for the ultrasonic approach whereas there was a significant reduction in the optimum reaction time for transesterification from 4 h for the conventional approach to 2.5 h for the ultrasound assisted approach. Also the reaction temperature reduced marginally from 65 to 60 °C and yield increased from 76% to 79% for the ultrasound assisted approach. Energy requirement and activation energy for both esterification and transesterification was lower for the ultrasound based approach as compared to the conventional approach. The present work has clearly established the intensification obtained due to the use of ultrasound and also illustrated the two step approach for the synthesis of FAME from high acid value feedstock based on the use of heterogeneous catalyst for the transesterification step. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, synthesis of fatty acid methyl esters (FAME) based on esterification or transesterification reactions has gained worldwide attention due to number of applications in perfume and flavor industry and also due to the use of biodiesel as a source of alternative fuel [1–4]. For the specific use of FAME as biodiesel, it is important to note that the biodiesel demand in India is increasing with rapid industrialization, globalization and rapid changing lifestyle and considering the fact that India is planning to use biodiesel blend B20 by 2017, the expected increase is at a rate of 6.2% per annum. Biodiesel is a mixture of alkyl esters, which can be used in conventional compression ignition engines, without any significant modifications. Transesterification has been generally used for the synthesis of fatty acid esters from vegetable oils and animal fats using homogeneous catalyst (mainly sodium or potassium hydroxide dissolved in methanol). Traditional homogeneous catalysts (basic or acid) possess advantages including high activity and requirement of mild reaction conditions (from 40 to 65 °C and atmospheric pressure). However, the use of homogeneous ⇑ Corresponding author. Tel.: +91 22 3361 2024; fax: +91 22 3361 1020. E-mail address: [email protected] (P.R. Gogate).
catalysts can lead to soap formation especially in the case of feedstock with high free fatty acid content. The total cost of the production using homogeneous catalysis, is not yet sufficiently competitive as compared to the cost of diesel production from petroleum. An alternative approach for synthesis of fatty acid methyl esters can be based on the use of heterogeneous catalysts. Use of heterogeneous catalyst can be helpful in avoiding the soap formation (produced in presence of the homogenous alkali catalyst) and also reduce the load on the downstream processing of separations possibly leading to considerable saving in terms of energy requirements and avoiding wastewater generation. There have been many studies reported in the literature relating to the synthesis of biodiesel using heterogeneous catalyst [5–8]. Georgogianni et al. [5] studied the transesterification of rapeseed oil in the presence of heterogeneous catalysts (Mg MCM-41, Mg–Al Hydrotalcite, and K+ impregnated zirconia), using low frequency ultrasound reactor (24 kHz). It has been reported that Mg–Al hydrotalcite gives the highest activity with 97% conversion in a reaction time of 5 h. Biodiesel synthesis from Jatropha curcas oil using solid catalyst Na/SiO2 in the presence of ultrasound gave yield of 98.53% at optimal molar ratio of oil to methanol as 1:9, catalyst concentration of 3 wt% of oil and 15 min of reaction time [6]. The ultrasound assisted biodiesel synthesis from palm oil [7] gave yield of 77.3%
http://dx.doi.org/10.1016/j.ultsonch.2014.08.019 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
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(CaO), 94.2% (SrO) and 95.2% (BaO) for a reaction time of 60 min. The optimum parameters for the ultrasound assisted biodiesel synthesis from soybean oil were catalyst (CaO) loading of 6 wt%, temperature of 62 °C, and molar ratio of oil to methanol as 1:10. The approach of ultrasound assisted biodiesel synthesis from palm oil using calcium oxide was reported to give a yield of nearly 100% in 2 h of the operation [7]. The reported work in the literature on heterogeneous catalyzed biodiesel synthesis in the presence of ultrasound has concentrated mainly on the feed stock (virgin oil) with lower acid value requiring only one step processing. The current work presents a novel approach of intensification of two step processing for the sustainable feed stock containing higher acid value (which is expected to create problems of soap formation if used with alkaline catalyst) based on the use of heterogeneous catalyst. The feedstock used in the present work is Nagchampa oil containing higher amount of the oil (65–70%) which is almost twice as compared to the other non-edible feedstock like Karanja (25–30%), Jatropha (30–35%) [9]. Nagchampa is abundantly found in the costal part of the India. The major problem associated with Nagchampa as a feedstock is initial higher amount of free fatty acid (31 mg of KOH/g of oil), which requires more processing like two stage approach of esterification (to reduce the free fatty acid content) followed by transesterification [9]. It is important to note here that the two step synthesis is highly energy intensive operation and hence investigating the process intensification aspects with an objective of decreasing the material/temperature requirement is significant. In the past, the various approaches used for intensification of biodiesel synthesis include ultrasound, microwave, use of supercritical conditions, hydrodynamic cavitation, etc. [1–5]. Process intensification using cavitational reactors can be a promising approach due to the fact that operation is at ambient conditions and the physical effects of cavitation such as liquid circulation associated with turbulence can eliminate the mass transfer resistances and mixing issues associated with esterification/transesterification [9,10]. The work also presents comparison of the FAME synthesis from Nagchampa oil using conventional reflux method with ultrasound assisted approach to establish the degree of intensification. The effect of molar ratio, catalyst concentration and temperature has been investigated for both the steps of esterification and transesterification. 2. Materials and methods 2.1. Materials The raw Nagchampa oil was procured from M/s Amit Oil Mill, Vengurla, Dist: Sindhudurgah, Maharashtra, India. The chemical composition of Nagchampa oil is Palmitic acid as 12%, Stearic acid as 13%, Oleic acid as 34.1%, Linoleic acid as 38.3% and Linolenic acid as 0.3%. Methanol (HPLC grade), sulfuric acid (98%) (A.R.) and hexane (HPLC grade) were obtained from M/s Thomas Baker Chem. Pvt. Ltd., Mumbai, India. Calcium oxide (A.R.) was obtained from S.D. Fine-Chem. Ltd., Mumbai, India.
Condenser Stirrer Glass reactor Speed Regulator
Fig. 1a. Experimental setup for conventional approach.
The equipment used to study the effect of cavitation on synthesis was an ultrasonic horn, procured from M/s Dakshin Pvt. Ltd. Mumbai, India. Ultrasonic horn operates at a frequency of 20 kHz with a power rating of 120 W. The ultrasonic horn was fitted with a piezoelectric transducer with a tip diameter of 1 cm and immersed to a depth of 1 cm below the liquid level in a 150 ml capacity glass reactor. Reactor was kept in a water bath to maintain desired temperature within ±1 °C. The typical arrangement of the experimental setup used for the ultrasound assisted approach is shown in Fig. 1b. 2.3. Experimental methodology 100 ml of Nagchampa oil was taken in a glass reactor and preheated to the desired value. After 10 min, when the temperature of oil uniformly remained constant, mixture of methanol (at the desired molar ratio) and sulfuric acid was added in appropriate proportions for the first stage of esterification. The acid value was checked after every 1 h of reaction time. After reaching the constant acid value, esterified oil was kept for phase separation for 4 h in a separating funnel. The lower layer was separated and upper acid layer was used for second stage. The upper layer from first stage esterification was now taken in the reactor and kept for 10 min so as to reach constant temperature. Calculated amount of catalyst (H2SO4) and methanol was added. The acid value was checked after every 1 h of time. After reaching the desired acid value, esterified oil was kept for phase separation for 4 h in separating funnel. For preparing the catalyst to be used in the actual reaction, CaO was heated to remove moisture and volatile matter
2.2. Reactor configuration The reactions were performed in a 5.0 cm (ID), 150 cm3 capacity glass reactor equipped with reflux condenser. Reactor was kept in a water bath to maintain constant temperature. Water bath was procured from M/s Ganesh Scientific Industries, Mumbai, India and has provision for maintaining the temperature within ±1 °C. A pitched blade glass stirrer having 1 inch diameter was used to achieve uniform mixing of the reactants in the conventional approach. The schematic representation of experimental setup is shown in Fig. 1a.
Water bath
Condenser Ultrasound Horn Glass Reactor Generator
Water Bath
Fig. 1b. Experimental setup for ultrasound assisted approach.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
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acid value (mg of KOH/mg of oil
35 50°C 60°C 65°C 70°C
30 25 20 15 10 5 0 0
1
2
3
4
5
time(h) Fig. 3a. Effect of temperature on acid value at constant catalyst loading of 1% and oil to methanol molar ratio as 1:4 using conventional method. Fig. 2. HPLC chromatogram for FAME analysis.
2.4. Method of analysis The acid value of the samples in the first and second stage esterification was determined using the acid base titration technique. A standard solution of 0.05 N potassium hydroxide solution and ethanol for dissolving the withdrawn sample was used. A sample of 1 g of oil was dissolved in 20 ml of ethanol and heated for 5–10 min. Then 10 ml of sample was titrated against alcoholic potassium hydroxide using phenolphthalein as indicator. The acid value can be calculated as follows:
AV ¼
Burette reading Mol: Wt: KOH Normality of KOH Wt: of Oil
The samples from transesterification stage were analyzed by High Pressure Liquid Chromatography (HPLC). Fatty acid methyl ester (FAME) was analyzed by HPLC with a C18 Phenomenex column (25 cm 4.6 mm, 5 lm particle size) using UV/Visible detection (Hitachi) at 210 nm. The sample was analyzed using linear elution of methanol and hexane. The mobile phase consists of 90% methanol and 10% hexane. The mobile phase flow rate was maintained as 1 ml/min. Samples were prepared by using 10 ll of the reaction mixture diluted with 10 ml of methanol. A sample HPLC chromatogram has been given in Fig. 2. 3. Results and discussion 3.1. Conventional approach 3.1.1. Esterification 3.1.1.1. Effect of reaction temperature. The effect of temperature on the esterification using the conventional approach has been studied by changing the temperature of reaction at different values as 50, 60, 65 and 70 °C. The molar ratio of 1:4 and catalyst concentration of 1% (w/w) was kept constant for all the experimental runs related to the effect of temperature. The obtained results for effect of temperature have been shown in Fig. 3a. It has been observed that acid value decreases from 31 to 9.2 mg KOH/g of oil with an increase in reaction temperature from 50 to 65 °C in 2.5 h and further increase in temperature does not yield any additional
3.1.1.2. Effect of catalyst loading. Esterification was also performed using different catalyst concentrations as 0.5%, 1%, 1.5%, and 2% (w/w) of oil at optimized constant reaction temperature as 65 °C and molar ratio of 1:4. The obtained results for the effect of catalyst loading have been given in Fig. 3b. It was observed that acid value decreased from 31 to 9.2 mg KOH/g of oil at optimum catalyst loading of 1%. However, above 1% catalyst loading, there was not much decrease in the final acid value. Esterification reaction after some time approaches to equilibrium and hence further reduction in acid value is not observed. Similar optimum loading for reducing acid value of Jatropha oil from 15 to 1 mg of KOH/g of oil has been reported as 1% [16]. Thiruvengadaravi et al. [14] reported an optimum acid catalyst loading of 1% for esterification process for synthesis of biodiesel from waste cooking oil. 3.1.1.3. Effect of molar ratio. Esterification is a reversible reaction and hence in order to maintain the forward path of reaction, methanol is generally taken in excess. Esterification was performed 35 acid value (mg of KOH/mg of oil)
using a Muffle Furnace at 700 °C for 4 h. The upper layer from second stage esterification was taken in reactor and kept for 10 min for achieving desired temperature of oil. Calculated amount of calcined catalyst (CaO) and methanol at desired molar ratio was added. The samples were withdrawn at regular interval and analyzed by HPLC.
benefits. An increase in the operating temperature results in enhanced solubility of methanol in the oil phase thereby increasing the rate of reaction and decreasing the final acid value of oil. The reduction in acid value is restricted to the equilibrium conversion and hence further reduction in acid value requires additional esterification stage. The existence of optimum temperature has been also observed for various studies in the literature using different feed stocks such as Nagchampa oil [10], waste cooking oil [11,12], soybean oil [13], Karanja oil [14] and J. curcas oil [15].
0.5% 1% 1.5% 2%
30 25 20 15 10 5 0 0
1
2
3
4
5
time (h) Fig. 3b. Effect of catalyst loading on acid value at constant temperature of 65 °C and oil to methanol ratio as 1:4 using conventional method.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
3.1.1.4. Second stage esterification. In first stage esterification process, it has been observed that acid value of feedstock was reduced from 31 to 9.2 mg of KOH/g of oil at optimum molar ratio of 1:4, catalyst concentration of 1% w/w and temperature as 65 °C. In order to avoid the saponification reaction (in the presence of alkali catalyst) during transesterification process, the acid value should be less than 2 mg of KOH/g of oil (i.e., 1% of free fatty acid). In order to reduce the acid value below desired value, an additional esterification stage was used for re-esterification of first stage esterified oil. The results for the second stage esterification have been shown in Fig. 3d. It has been observed that conditions of molar ratio of 1:1, catalyst concentration of 1% w/w and temperature of 65 °C was sufficient to reduce the acid value from 9.2 to 2 mg of KOH/g of oil in 3 h of reaction. 3.1.2. Transesterification The two stage esterified oil has been used for the transesterification and similar to esterification process, the effect of temperature, catalyst concentration and molar ratio has been investigated. 3.1.2.1. Effect of reaction temperature. The effect of temperature on the transesterification stage was studied by using different temperatures of reaction as 40, 50, 65 and 70 °C. The molar ratio of 1:10 and catalyst (CaO) concentration of 2.5% (w/w) was kept constant for these studies and the obtained results have been shown in Fig. 4a. It was found that the yield of FAME increases with an increase in temperature to 65 °C. An increase in the operating
acid value (mg of KOH/mg of oil)
35 (1:2) (1:3) (1:4) (1:6)
30 25 20 15 10 5 0 0
1
2
3
4
10 acid Value (mg of KOH/mg of oil)
using varying oil to methanol molar ratio as 1:2, 1:3, 1:4, and 1:6 at optimized constant H2SO4 concentration of 1% weight of oil and reaction temperature as 65 °C. The obtained results for the effect of molar ratio have been given in Fig. 3c. It was observed that the acid value decreases from 31 to 9.2 mg KOH/g of oil with an increase in molar ratio to 1:4 and there was no significant reduction in the acid value for further increase in the ratio to 1:6. This can be attributed to the fact that water produced during esterification of free fatty acids prevents further reaction [14]. Water formed during esterification dissolves in the excess methanol and may disturb the equilibrium of the reaction. Selection of optimum molar ratio is significant for esterification as it decides the load on the downstream process for removal of excess methanol. Similar results have been reported in the literature for Jatropha oil where acid value reduction was achieved from 15 to 1 mg of KOH/g of oil for optimum molar ratio of 1:6 and 2 h reaction time [16]. Similar trend of existence of optimum conditions to reduce acid value has been reported for esterification of soybean oil [17].
8
6
4
2
0 0
1
2
3
4
5
time (h) Fig. 3d. Acid value for second stage esterification for 65 °C temperature, 1% catalyst concentration and 1:1 oil to methanol molar ratio using conventional method.
100 80
%, yield
4
60 40°C
40
50°C 60°C
20
65°C 0 0
2
4
6
time, hr Fig. 4a. Effect of temperature on yield of FAME at constant catalyst loading as 2.5% and oil to methanol ratio as 1:10 using conventional method.
temperature result in enhanced solubility of methanol in the oil resulting in increased extent of conversion. It was also observed that beyond a reaction time of 4 h, the FAME yield marginally decreased (the extent of decrease was more significant at higher temperatures), which may be due to the possible degradation of product under prolonged exposure at higher temperature. It has been observed that 76% yield was obtained in 4 h of operation at 65 °C. 3.1.2.2. Effect of catalyst loading. The effect of amount of heterogeneous catalyst (CaO) on transesterification was investigated by changing the catalyst concentration as 1, 2.5, 5 and 6 (w/w) of oil at optimized reaction temperature of 65 °C and molar ratio of 1:10. The obtained results for the effect of catalyst loading have been given in Fig. 4b. It was observed that fatty acid methyl ester yield increases with an increase in catalyst concentration from 1 to 2.5% (w/w) for 4 h operation. Further increase in catalyst concentration does not give any significant effect on the final yield and hence catalyst concentration of 2.5% has been considered as optimum for further processing. Deshamane et al. [18] reported an optimum catalyst loading as 1% for the heterogeneous calcium methoxide assisted synthesis of biodiesel from Soybean oil. Similar results were also reported for synthesis of biodiesel from Palm oil where 3% optimum heterogeneous catalyst concentration was observed [19].
5
time (h) Fig. 3c. Effect of molar ratio on acid value at constant temperature of 65 °C and catalyst concentration as 1%.
3.1.2.3. Effect of molar ratio. In transesterification process, three moles of methanol are required per mole of triglyceride to yield three moles of fatty acid methyl ester and one mole of glycerol.
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
100
%, yield
80 60 40 1% 2.5%
20
5% 6%
0 0
2
4
6
time (hr) Fig. 4b. Effect of catalyst loading on yield of FAME at constant temperature of 65 °C and oil to methanol molar ratio 1:10 using conventional method.
Similar to esterification, higher molar ratio is required in order to drive the forward reaction. At significantly higher molar ratio, glycerine formed during reaction dissolves into the excess methanol which may lower the yield of FAME. Thus, the optimization of molar ratio for transesterification is important. The effect of molar ratio was studied at different ratios as 1:6, 1:8, 1:10 and 1:12 at optimum catalyst concentration as 2.5 w/w and reaction temperature of 65 °C. The obtained results for the effect of molar ratio have been given in Fig. 4c. It has been observed that the yield increases with an increase in molar ratio from 1:8 to 1:10 but a further increase in molar ratio does not result in significant increase in the yield of fatty acid methyl ester. Yield of process also decreased marginally after the 4 h of reaction. Since transesterification is an equilibrium reaction, a larger amount of methanol initially shifts the methanolysis towards forward direction, resulting in favorable conditions for the FAME formation. Deshamane et al. [18] reported similar requirement of higher molar ratio of 1:12 for synthesis of biodiesel from soybean oil using heterogeneous calcium methoxide catalyst. 3.2. Ultrasound assisted approach 3.2.1. Esterification 3.2.1.1. Effect of reaction temperature. The effect of temperature on the esterification was studied using different values of temperature as 30, 40, 50 and 65 °C. The molar ratio of 1:4 and catalyst concentration of 1% (w/w) was kept constant. It has been observed that acid value decreases from 31 to 10.1 mg KOH/g of oil at reaction
100
%, yield
80
5
temperature of 40 °C in 3 h and further increase in temperature does not yield any additional benefits. The observed trend for the effect of temperature was similar to that observed for the conventional approach. An increase in reaction temperature causes the decrease in oil viscosity and gives enhanced cavitational effects improving the contact between reactants and hence the acid value decreases with an increase in temperature [20]. The physical effects of cavitation like acoustic streaming and microemulsification are useful in enhancing the contact between the immiscible phases of methanol, oil and catalyst. Use of temperature as 40 °C also assists in reducing the attenuation effect of ultrasound in the viscous medium of oil. The existence of optimum temperature has been also observed for Tung oil [21] and Zanthoxylum bungeanum seed oil [22]. 3.2.1.2. Effect of catalyst loading. Optimization of catalyst concentration is important in order to avoid the problems related to the removal of catalyst from the product stream. Esterification was studied by changing the catalyst concentration at different value of 0.5%, 1%, 1.5% and 2% (w/w) of oil at optimum temperature of 40 °C and molar ratio of 1:4. The observed trend for the effect of catalyst loading was similar to that observed for the conventional approach. It has been observed that acid value decreases from around 31 to 10.1 mg KOH/gm for 1% w/w catalyst loading in 3 h. Similar effect has been reported for esterification of fatty acid cuts with optimum concentration as 2% [23], while esterification of palm fatty acid distillate required 5% catalyst concentration as optimum for the ultrasound assisted synthesis [18]. 3.2.1.3. Effect of molar ratio. The effect of molar ratio on esterification using ultrasound was studied at varying oil to methanol molar ratio as 1:2, 1:3, 1:4 and 1:6 at optimum catalyst concentration of 1% (w/w) and temperature of 40 °C. It was observed that acid value decreases from around 31–10.1 mg KOH/g of oil at 1:4 molar ratio and any further increase in molar ratio does not give significant reduction in acid value. Since this reaction is reversible, excess amount of methanol is required to drive the reaction in the forward direction. Thus, the final acid value would be lower where a higher excess of methanol is available in the reaction mixture as explained in details for the conventional approach. 3.2.1.4. Second stage esterification. Similar to the conventional approach, it has been observed that the single stage esterification does not reduce the acid value below 2 mg of KOH/g of oil required for avoiding the saponification reaction during the transesterification stage. Re-esterification of the processed oil was performed and molar ratio, catalyst concentration and reaction temperature has been again optimized for the second stage esterification. The effect of temperature on the esterification stage was studied using different temperatures as 30, 40, 50 and 65 °C, catalyst concentration as 0.5%, 1%, 1.5% and 2% and molar ratio of 1:0.5, 1:1, 1:2 and 1:3. It has been observed that acid value decreases from around 10 to 1.69 mg KOH/g of oil in 3 h operation at optimum reaction temperature as 40 °C, catalyst concentration of 1% and molar ratio of 1:1.
60 40 1:6 1:8
20
1:10 1:12
0 0
2
4
6
time (hr) Fig. 4c. Effect of molar ratio on FAME at constant temperature of 65 °C and catalyst concentration as 2.5% using conventional method.
3.2.2. Transesterification CaO catalyst being hydrophilic in nature preferably stays in the methanol phase. At the catalyst surface, methoxy ions are generated as a result of adsorption of methanol molecule and subsequently transferred to the organic phase at the interface between the two phases and reaction proceeds. Convection generated by ultrasound helps in efficient dispersion of the aqueous and organic phases forming a fine emulsion with significantly higher interfacial area. Due to high viscosity of oil, the micro-streaming and microconvection generated in methanol mostly contributes to the convection generated in the reaction system.
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S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
3.2.2.2. Effect of catalyst loading. The effect of heterogeneous catalyst (CaO) loading on transesterification was investigated by changing the catalyst concentration as 1%, 2.5%, and 5% (w/w) of oil at 60 °C temperature and molar ratio of 1:10. It was observed that FAME yield increases with an increase in catalyst concentration from 1% to 2.5% (w/w), which can be attributed to the increase in active sites in the reaction system. Further increase in catalyst concentration does not give significant effect on the yield. The change in the yield with catalyst concentration has been shown in Fig. 5b. The catalytic activities were mainly related to their basic strength [24–26], which should increase with an increase in the catalyst loading till an optimum. Xie et al. [26] reported that when the Mg/Al catalyst quantum exceeds beyond an optimum value, the catalytic activity decreases due to the formation of new weaker basic sites. At higher catalyst concentrations, the extent of increase in the FAME is observed to be lower which may also be attributed to possible formation of agglomerates of catalyst particles and also it is important to note that any excess use of catalyst will lead to more energy consumption in the separation process. Hence the selection of the optimum catalyst concentration is important [24–26]. 3.2.2.3. Effect of molar ratio. The effect of molar ratio was studied at different molar ratios as 1:6, 1:8, 1:10 and 1:12. The change in the 100
100 2.5% 1.00%
80
%, yield
3.2.2.1. Effect of reaction temperature. The effect of temperature on the transesterification rate was studied using different temperatures of reaction as 40, 50 and 60 °C at constant molar ratio of 1:10 and catalyst (CaO) concentration of 2.5% (w/w). The effect of temperature on FAME yield has been shown in Fig. 5a. It can be seen from the figure that the biodiesel yield increases with an increase in reaction time till an optimum value beyond which yield decreases possibly due to the degradation of the FAME. It was also found the optimum reaction time in the case of ultrasound assisted approach is 2.5 h as compared to 4 h in the case of conventional synthesis, which is expected as the possible degradation will be faster in the presence of ultrasound. Ultrasound assisted reaction gave yield of 79% in 2.5 h at 60 °C as compared to 76% yield at 65 °C obtained in 4 h using the conventional approach. Similar results of intensified synthesis have also been reported in the literature for single step processing. Arzamendi et al. [24] reported that CaO catalyst could give 30% of biodiesel yield in 5 h using conventional approach and higher yield was achieved in a much shorter reaction time when ultrasound was used. At longer reaction time, the promotion of reverse reaction and/or possible degradation rendered the decrease in the biodiesel yield with time. The catalysts could also last longer as new catalytic active sites could be exposed to the reactants due to the cleaning effects caused by the ultrasound [22,23].
5%
60
40
20
0 0
1
2
time (h)
3
4
5
Fig. 5b. Effect of catalyst loading on yield of FAME at constant temperature of 60 °C and oil to methanol molar ratio as 1:10 using ultrasound method.
100 80
%, yield
6
60 40 1:6 1:8
20
1:10 1:12
0 0
1
2
3
4
5
time (hr) Fig. 5c. Effect of molar ratio on yield of FAME at constant temperature of 60 °C and catalyst concentration as 2.5% using ultrasound method.
yield with molar ratio has been shown in Fig. 5c. It was observed that the yield increases with an increase in molar ratio till an optimum molar ratio as 1:10. The initial increase in the yield with an increase in molar ratio can be attributed to the fact that higher molar ratio works favorably to maintain the forward path of the reaction. As the reaction proceeds, glycerol produced during the reaction may dissolve in the excess methanol and disturb the reaction equilibrium resulting in lower yield. Hence the selection of the optimum molar ratio is very crucial [24–27]. Also any unutilized methanol needs to be separated which can increase the energy requirements in the downstream processing and hence using too much excess of methanol is not recommended.
60°C 50°C
80
4. Comparative study between conventional and ultrasound assisted synthesis
%, yield
40°C 60 40 20 0 0
1
2
time (h)
3
4
5
Fig. 5a. Effect of temperature on yield of FAME at constant catalyst concentration 2.5% and oil to methanol ratio as 1:10 using ultrasound method.
The optimized parameters for conventional and ultrasound assisted FAME synthesis from Nagchampa oil have been established and a summary of obtained results is shown in Table 1. The optimum reaction time in the esterification stage was not affected meaning that the rate of reaction is more dominant on the temperature and the molar ratio of the reactants. Though reaction time for ultrasound and conventional approach is same, reaction temperature was reduced from 60 °C (conventional approach) to 40 °C(ultrasound approach). Another advantage of ultrasound approach for first stage esterification is lower energy requirement for conversion of free fatty acid to biodiesel as compared to the
Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
7
S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 1 Optimized parameters for conventional and ultrasound method. ES-I
Reaction time (h) Molar ratio (Oil:Methanol) Catalyst concentration (%w/w) Reaction temperature (°C) Conversion/yield (%) Energy consumption (g/J 105)
ES-II
y = 19.30x R² = 0.990
1.4
T
1.2
C
US
C
US
C
US
3 1:4 1
3 1:4 1
3 1:1 1
3 1:1 1
4 1:10 2.5
2.5 1:10 2.5
65 71.2 8.9
40 67.8 11.27
65 80.2 10.04
40 84.4 14.02
65 76.4 8.28
60 78.7 15.84
1/ [A] - 1/[A]0
Parameters
1.6
1 0.8 0.6 0.4 0.2 0 0
1
2
ES-I, esterification stage-I; ES-II, esterification stage-II; T, transesterification; US, ultrasound; C, conventional.
conventional approach. Biodiesel production per unit energy consumption for first stage esterification was 8.9 105 and 11.27 105 g/J for the conventional approach and ultrasound based approach respectively and for the second stage esterification, it was 10.04 105 and 14.02 105 g/J. The details of the calculation of energy requirement and biodiesel production per unit energy consumed have been given in Appendix I. Similar trend has been observed for transesterification stage with a decrease in the reaction temperature from 65 °C for the conventional approach to 60 °C for the ultrasound assisted approach. In addition to that, there was reduction in processing time from 4 h required for the conventional approach to 2.5 h using ultrasound approach. The yield was also found to be increased to 79% using the ultrasound approach as compared to 76% in conventional approach. The yield of biodiesel per unit energy consumed for transesterification stage was 15.84 105 g/J in the case of ultrasound based approach whereas for the conventional approach the extent of productivity was 8.28 105 g/J. The physical effects of ultrasound induced cavitation such as acoustic streaming [28] and micro-emulsification enhance the rate of esterification and transesterification process. These effects are useful for increasing the yield of reaction, for reducing the temperature of reaction and also the reaction times. Deshmane et al. [29] have reported similar intensification with the use of cavitation generated using
3
4
time, hrs
Fig. 6. Fitting curve for transesterification (conventional method) reaction for second order reaction kinetics ([A]: concentration of any time, [A]0: initial concentration).
ultrasonic irradiation for synthesis of biodiesel from palm fatty acid distillate, which is a comparatively much cheaper raw material, as compared to vegetable oils. Kinetic rate constants have been obtained, considering secondorder kinetics for each step of synthesis and the values of the rate constants for both stages of esterification, as well as transesterification, have been given in Table 2. The kinetic rate constant was evaluated assuming second-order reaction (equilibrium was assumed to proceed in a forward path in the presence of excess methanol) for esterification [20] and transesterification reactions [30]. It has been observed that the rate constant (sample graph has been shown in Fig. 6) for both esterification and transesterification reactions increased with temperature, molar ratio and catalyst concentration (till optimum value, significant increase is observed) for conventional method as well as ultrasound-assisted synthesis. The rate constant was observed to be higher using the ultrasound approach for both reactions as compared to the conventional method. The activation energy was also determined using the Arrhenius equation which gives a relationship between the specific reaction
Table 2 Second order rate constant for esterification and transesterification for conventional and ultrasound approach. Esterification (Conventional approach) (k 102, L mol1 min1) Temperature (°C)
Molar ratio
Catalyst concentration (% w/w)
Stage I k
50 0.5
60 1.7
65 2.7
70 3.1
1:2 1.1
1:3 1.9
1:4 2.7
1:6 2.9
0.5 1.7
1 2.7
1.5 2.9
2 2.9
Stage II k
50 0.67
60 7.7
65 13.3
70 14.8
1:05 5.6
1:1 13.3
1:2 16.3
1:3 16.2
0.5 6.8
1 13.3
1.5 14.3
2 14.1
Esterification (Ultrasound approach) (k 102, L mol1 min1) Temperature, (°C)
Molar ratio
Stage I k
30 0.8
40 2.4
50 2.3
65 2.8
1:2 0.6
1:3 1.3
1:4 2.4
1:6 2.6
0.5 1.4
1 2.4
1.5 2.6
2 2.9
Stage II k
30 4.1
40 16.5
50 17.1
65 17.2
1:05 2.5
1:1 16.5
1:2 15.2
1:3 15.1
0.5 6.5
1 16.5
1.5 17.2
2 16.8
1:6 14.2
1:8 19.3
1:10 20.8
1:12 25.5
1 17.0
2.5 20.8
5 26.8
6 25.4
Transesterification (Ultrasound approach) (k 102, L mol1 min1) 30 40 50 60 1:6 k 11.2 23.6 25.5 32.8 21.3
1:8 30.3
1:10 32.8
1:12 35.0
1 27.6
2.5 32.8
5 35.9
6 34.7
Catalyst concentration (% w/w)
Transesterification (Conventional approach) (k 102, L mol1 min1) Temperature (°C) Molar ratio k
40 14.3
50 16.6
65 20.8
70 19.4
Catalyst concentration (% w/w)
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S.M. Dubey et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
Table 3 Activation energy.
Appendix I
Activation energy (kJ/mol)
Esterification stage I Esterification stage II Transesterification
Conventional method
Ultrasound approach
86.66 61.86 18.03
28.18 59.22 14.71
Sample calculation of energy requirement for transesterification using conventional as well as ultrasound approach is given below: A.1. Conventional approach Total energy requirement ¼ Energy for stirring þ Energy for heating Energy for stirring ¼ Power required for stirring time in sec
8.2 8
¼ 1728 103 J
Ultrasound
Energy for heating ¼ m CpðT2 T1 Þ
ln k
7.8
where
7.6 7.4
y = -1768.x + 12.88 R² = 0.992
7.2 7 6.8 0.0029
¼ 120 4 60 60
Conventional
y = -2169.x + 14.44 R² = 0.927
0.003
0.0031
0.0032
0.0033
0.0034
1/T Fig. 7. Change in logarithm of specific rate constant with reciprocal of temperature for transesterification.
rate constant (k), absolute temperature (T) and the energy of activation (E) as:
m = mass of water = 5000 g Cp = specific heat of water = 4.18 J/kg, K T1 = initial temperature of water = 298.15 K t2 = final temperature of water = 338.15 K
¼ 836 103 J Total energy ¼ 2564 103 J Mass of biodiesel formed ¼ yield of biodiesel Average molecular weight of biodiesel ¼ 0:76 279:4 ¼ 212:34 g Energy required ¼
k ¼ A eðE=RTÞ where A is the frequency factor and R is universal gas constant. Activation energy obtained for the case of conventional and ultrasound approach is given in Table 3. The activation energy (sample graph is shown in Fig. 7) for the first esterification stage in conventional synthesis is 86.66 kJ/mol which is much higher than the value observed for the ultrasound assisted approach as 28.18 kJ/mol. For the second esterification stage the activation energy is marginally low for the ultrasound assisted approach. In the transesterification stage, the activation energy for ultrasound approach was also lower as compared to conventional synthesis approach.
Mass of biodiesel formed; g total energy required; J
¼ 8:28 105 g=J
A.2. Ultrasound based approach
Total energy requirement ¼ Energy for equipment þ Energy for heating Energy for stirring ¼ Power required for equipment time in sec ¼ 120 2:5 60 60 ¼ 1080 103 J Energy for heating ¼ m CpðT2 T1 Þ where
5. Conclusions The present work has established a novel process for the FAME synthesis from higher acid value feedstock using two stage esterification followed by transesterification approach. The physical effects of cavitation were found to be beneficial in enhancing the rate of esterification and transesterification reactions. The required reaction temperature for both the processes was lower in the case of ultrasound assisted approach. Also the yield of reaction increased for transesterification with a simultaneous reduction in the reaction time. Rate constant for both esterification and transesterification reactions increased with temperature, molar ratio and catalyst concentration (till an optimum value) for conventional method and ultrasound assisted synthesis. The rate constant was observed to be higher for the ultrasound approach for both reactions compared to the conventional method. Use of heterogeneous catalyst eliminates the separation problems that might be associated with homogeneous catalyst in transesterification reaction. Overall, it can be said that the ultrasound assisted process enhanced the rate of reaction, reduced the temperature requirement which can possibly lead to considerable saving in terms of energy.
m = mass of water = 5000 g Cp = Specific heat of water = 4.18 J/kg, K T1 = Initial temperature of water = 298.15 K T2 = Final temperature of water = 313.15 K
¼ 313:5 103 J Total energy ¼ 1393:5 103 J Mass of biodiesel formed ¼ yield of biodiesel molecular weight of biodiesel ¼ 0:79 279:4 ¼ 220:7 g Energy required ¼
Mass of biodiesel formed; g total energy required; J
¼ 15:84 105 g=J Similarly productivity of biodiesel per unit power consumption is calculated for esterification stages for ultrasound and conventional approach.
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Please cite this article in press as: S.M. Dubey et al., Cavitation assisted synthesis of fatty acid methyl esters from sustainable feedstock in presence of heterogeneous catalyst using two step process, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.08.019
