Intensification of Synthesis of Biodiesel from Nonedible Oils Using

Article pubs.acs.org/IECR

Intensification of Synthesis of Biodiesel from Nonedible Oils Using Sonochemical Reactors Vitthal L. Gole and Parag R. Gogate* Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai-40019, India ABSTRACT: Biodiesel synthesis from nonedible oils, which offer excellent potential as sustainable feed stock, is highly energyintensive and slow operation, because it requires considerable processing due to higher initial acid values and due to the fact that the reaction is mass-transfer-controlled. The present work reports the intensification of synthesis of biodiesel from the high-acidvalue Nagchampa oil using sonochemical reactors. The synthesis process is a two-step method of esterification in the presence of homogeneous acid catalyst followed by transesterification using an alkaline (KOH) catalyst. The synthesis has also been attempted using conventional methods of reflux for analyzing the degree of intensification. With an objective of avoiding possible saponification reaction in the transesterification based on use of an alkaline catalyst, the acid value of oil was reduced from 18.4 mg KOH/g of oil to 1.4 mg KOH/g of oil, using the first-stage esterification method. The reduction in the acid value allows for an efficient second transesterification stage. The reaction temperature, molar ratio, and catalyst concentration were optimized for esterification and transesterification stages for the ultrasound and conventional techniques. It has been observed that the reaction temperature and reaction time required for esterification, as well as the transesterification stages, are substantially lower in the case of sonochemical reactors, compared to the conventional heating method. Also, the percentage excess of the reactants is significantly reduced, leading to energy savings in the subsequent separation processes for getting the purified product. Overall, the present work has clearly established the efficacy of sonochemical reactors for the intensification of biodiesel synthesis based on a sustainable raw material.

1. INTRODUCTION In recent years, energy production from the renewable sources has been looked upon as a viable and necessary option for sustainability and reducing the dependency on the conventional fossil fuels. The stable economy of any country depends on the energy sustainability of the country. In India, there is a severe problem in terms of increasing pollution due to emissions and a continuous shortage in the energy supply based on the conventional fuels, which makes it imperative to look for alternative to fossil fuels.1 Biodiesel is one of the promising sources to replace the petro-based fuel sources. In India, biodiesel is generally produced from nonedible sources, such as Karanja, Jatropha, Kusum, Nagchampa, etc., and also the waste oil and waste cooking oil have been considered to be sustainable feedstock.2 Conventional transesterification using alkaline catalysts, although much faster, cannot be used for nonedible oils, because of the possibility of soap formation from the reaction of free acids with alkali. In the case of waste oil or nonedible oils, where the initial free acid value is substantially high, a two-step approach of esterification followed by transesterification is generally used, which requires a greater amount of energy and time for processing.3 Thus, there is a need to develop sustainable process intensification technology for biodiesel processing from nonedible oil sources with an objective of reducing the cost of treatment. Process intensification is any chemical engineering development that leads to a substantially smaller, cleaner, safer, and more-energyefficient technology.4 There are various process intensification technologies based on the use of alternate energy sources such as ultrasound and microwave, which can be effectively applied to meet the demand of cutting edge technologies. Acoustic © 2012 American Chemical Society

cavitation based technologies eliminate the mass transfer barriers in the process while microwave-based technologies enhances the rate of heat transfer. 5 Microwave-based technologies have the limitations of penetration depth of the microwave, as well as uncontrolled and uneven distribution of the temperature (i.e., hot and cold spot generation in same system).6 Other process intensification technologies that can be applied for the production of biodiesel are based on the use of a static mixer, microchannel reactor, and oscillatory flow reactor, but these intensification processes have the limitations related to scaleup and continuous operational aspects.7 The current work reports the use of sonochemical reactors for the intensification of biodiesel synthesis from nonedible oils. The intensification using sonochemical reactors is based on the generation of cavitation events in the reactor due to the pressure fluctuations induced by the incident ultrasound waves. Cavitation is a phenomenon of nucleation, growth, and subsequent collapse (quasi-adiabatic) of micro bubbles in a liquid medium. Cavitation results in the generation of high temperature (in the range of 1000−15000 K) and pressure (in the range of 500−5000 bar) locally but at millions of locations in the reactor. In addition to the generation of hotspots, cavitation also results in strong acoustic streaming (liquid circulation), high shear stress near the bubble wall, formation of microjets near the solid surface (due to asymmetric collapse of Special Issue: Alternative Energy Systems Received: Revised: Accepted: Published: 11866

September 23, 2011 January 25, 2012 February 11, 2012 February 11, 2012 dx.doi.org/10.1021/ie2029442 | Ind. Eng. Chem. Res. 2012, 51, 11866−11874

Industrial & Engineering Chemistry Research

Article

conventional two-stage approach can be energy-intensive, because of the much-larger excess of methanol or higher operating temperatures, giving much higher processing costs and, hence, intensification using sonochemical reactors would be very important. The present work is based on using sonochemical reactors for the intensification of biodiesel synthesis using Nagchampa oil as a starting raw material (Calophyllum Innophyllum Linn), which can be promising option for biodiesel production, because of its high oil content over the range of ∼65%−75%.22 Biodiesel production from this source has limitations, in terms of high acid value (18−20 mg of KOH/g of oil). The novelty of the present work is that intensification of synthesis of biodiesel based on nonedible oils with high-acid-value content, using a two-stage process, has been investigated; this has not been done in any of the earlier works to the best of our knowledge. Nagchampa is found extensively along the seashores in the tropical Asia, and its geographical distribution area includes Philippines, Melanesia, and coastal regions of India and Polynesia. Its seeds sprout easily in muddy and saline soils. The tree can flower and bear fruit all year round under Indian conditions. Kernels have very high oil content (75%). Once grown, a tree produces up to 100 kg of fruits and ∼ 18 kg of oil.23 Although the oil content is very high, not much work has been concentrated on the production of biodiesel from this oil mostly due to the fact that the initial acid value of oil is quite high posing significant problems for the transesterification reaction, which is generally recommended. The present work reports the synthesis of biodiesel from high-acid-value Nagchampa oil using the two-stage approach of esterification (H2SO4 as a catalyst) followed by transesterification (KOH as a catalyst). The first stage esterification is mainly with an objective of reducing the acid value of the feed stock so that alkaline transesterification can be effectively applied. The effect of different operating parameters such as reaction temperature, molar ratio (oil to methanol), and catalyst concentration has been investigated. Biodiesel properties synthesized from these methods have also been evaluated in order to match with ASTM standards.

bubbles), generation of highly reactive free radicals, and turbulence resulting in enhanced transport properties of the species.8 There have been some reports for application of sonochemical reactors for biodiesel production from edible sources such as beef tallow,9 sunflower oil,10 canola oil,11 and soybean12 in the presence of alkali catalyst. These oils typically contain lower initial free fatty acid content and, hence, singlestep processing is sufficient without any problems for the alkalicatalyzed transesterification. However, if the free fatty acid content of starting raw material is higher as is the case with nagchampa oil, the two-step processing must be employed. In the conventional approach, the processing rate is low, because of the mass-transfer limitations between heterogeneous liquid phases and this would significantly affect the processing rates in the case of the two-step approach. For both reactionsthe esterification of free fatty acid (FFA in Nagchampa oil) with methanol and the transesterification of triglycerides with methanolblending the reagents together is crucially important. Because of the fact that oil and alcohols are not totally miscible, the synthesis of biodiesel is a relatively slow process under the conventional method. Ultrasonic power is a useful tool for intensifying the mass transfer of a liquid−liquid heterogeneous system. Ultrasound can be employed to overcome the mass-transfer limitations, and there are very few works reporting such studies for the intensification of biodiesel synthesis from feedstock containing a higher amount free fatty acids, such as rapeseed oil deodorizer distillate,13 jatropha oil,14 and vegetable oils.15 These works have been carried out at the constant operating parameters and mainly highlighted the reduction in the processing time without any detailed effect of the operating parameters. The work on biodiesel synthesis from high-acid-value Jatropha oil reported that ultrasonication reduced the time required to reduce the acid value from 10.4 mg KOH/g of oil to 1.2 mg KOH/g of oil from 4 h to 1 h, and, similarly, the time required for transesterification was reduced from 1.5 to 0.5 h, compared to the conventional method.14 Also, there have been some other reports indicating that the synthesis of biodiesel from palm,16 coconut,17 and soybean oils18 in the presence of heterogeneous catalyst can be intensified using ultrasound; however, all these studies have been single-stage operations either resulting in lower rates for homogeneous catalyst or based on the lower acid values of feedstocks, where single stage transesterification can be used. It should be noted here that use of edible oils as a starting raw material is not considered to be a sustainable approach and efforts have been made to use some of the sustainable raw materials such as waste cooking oil,19 fatty acid distillate,20 and Jatropha.21 The studies reported earlier with the use of ultrasonic irradiations for sustainable raw materials19−21 have been based on the use of the single-stage transesterification route, because of the much lower initial acid content associated with these raw materials. The nonedible oils offer a huge potential as a starting raw material for biodiesel synthesis, but the synthesis process is hampered by the presence of high initial acid content, which prevents the use of a transesterification approach using alkaline catalysts, because of the problems associated with soap formation. Adequate pretreatment is required for removal of the initial high acid content or a two-stage approach based on esterification− transesterification should be used; however, both of these approaches add considerable processing costs and the overall economics may not be feasible, even after considering the government subsidies that are available in some countries. The

2.0. MATERIALS AND METHODS 2.1. Materials. The raw Nagchampa oil was procured from M/s Amit oil Mill (Vengurla, Dist: Sindhugrah, Maharashtra, India). Initially, the oil was filtered to remove traces of particles and mud. The initial moisture content was calculated using Karl Fisher apparatus. Table 1 gives the typical composition of oil, Table 1. Fatty Acid Composition of Nagchampa Oil fatty acid

composition (%)

palmitic, C16:0 stearic, C18:0 oleic, C18:1 linoleic, C18:2 linolenic, C18:3

12 13 34.1 38.3 0.3

consisting of 25% of saturated acid (stearic and palmitic) and 72.7% of unsaturated acid (oleic, linoleic, and linolenic). The initial acid value for the oil was observed to be 18.4 mg of KOH/g of oil. Sulfuric acid (98% concentrated), methanol, ethanol, and potassium hydroxide (of GR grade, from Merck), as well as methanol and hexane (of HPLC grade), were procured from the supplier M/s Phenonix Marketing, Pune. 11867

dx.doi.org/10.1021/ie2029442 | Ind. Eng. Chem. Res. 2012, 51, 11866−11874


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times with hot distilled water in order to remove traces of sulfuric acid. The washed oil is heated to 75 °C under vacuum of 25 mmHg for 20 min to remove traces of water. Moisture content in oil was recorded and it was reduced to 0.05% which is just sufficient for further processing.24 The acid value of the lower layer of dried oil is measured to monitor the progress of the reaction. In the second stage of synthesis, alkaline transesterification was carried out using esterified oil in the presence of potassium hydroxide as a catalyst. The final transesterified product was separated as two phases: a heavier glycerine phase and a lighter fatty acid methyl ester phase. The excess amount of methanol was removed from the mixture by heating the mixture in a rotary evaporator at 80 °C. The product was then washed with a mixture of 5% (by weight) ortho-phosphoric acid and water, in order to remove the catalyst from the mixture. The washed product is heated to 75 °C under a vacuum of 25 mmHg for 20 min to remove the traces of water. The fatty acid methyl ester is measured using high-performance liquid chromatography (HPLC), in order to monitor the progress of the reaction. The process parameters such as reaction temperature, molar ratio (oil to methanol), and catalyst concentration (percentage weight of catalyst/weight of oil) were optimized for both esterification and transesterification reactions. Density, kinematic viscosity, and acid value of final transesterified product were evaluated as per the ASTM method 2.4. Method of Analysis. In the case of esterification, since the free acid value was much higher and the main objective was to reduce the free acid value, titration was performed to monitor the progress of the reaction. The acid value of the reaction mixture in the first stage was determined by the acid− base titration technique. A standard solution of 0.1 N potassium hydroxide solution and ethanol for dissolving the sample was used. A sample of 1 g of oil is dissolved in ethanol and heated for 5−10 min. The known quantity of sample is then titrated against potassium hydroxide using phenolphthalein as an indicator. In the case of transesterification reaction, the formation of esters have been monitored using HPLC analysis, because it was simpler, compared to monitoring the oil conversion directly. A sample HPLC chromatogram has been given in Figure 2. The concentration was calculated based on the area under the peak for the retention time of standard samples of methyl oleate and methyl linoleate procured from Sigma− Aldrich. Based on the reaction stoichiometry, the oil conversion was calculated from the product formation using the extent of completion of reaction under equilibrium conditions. For the actual analysis, fatty acid methyl ester (FAME) was analyzed by using a C18 Phenomenex column (25 cm × 4.6 mm, 5 μm particle size) using UV/visible detection (Hitachi) at 210 nm. The sample was analyzed using linear elution of methanol and hexane; during the first 5 min, methanol concentration was changed from 100% to 90%, flow rate gradient was maintained as 1 mL/min for first 2 min and then 1.2 mL/min was maintained. Samples were prepared by using 10 μL of the reaction mixture diluted with 10 mL of methanol.

HPLC standards of methyl oleate and methyl linoate were procured from M/s Sigma−Aldrich. All the chemicals were used as received from the supplier. 2.2. Reactor Details. The sonochemical reactor used in the present work is an ultrasonic bath with three transducers arranged in triangular pitch at the bottom of tank, operating at a frequency of 20 kHz with a power dissipation of 120 W, which was procured from M/s Oscar Ultrasonics Pvt. Ltd., Mumbai. The ultrasonic bath has the dimensions of 25 cm × 17.5 cm × 10 cm and has maximum capacity of 3 L. The calorimetric measurement revealed that the exact power dissipation was 45 W, which indicates ∼37.5% energy efficiency. The three-neck, flat-bottom flask was placed at the center of the bath, where the maximum irradiation effect is observed, based on the study of aluminum foil irradiation.8 A schematic of the experimental setup has been shown in Figure 1. Stirring at 350 rpm was also

Figure 1. Schematic representation of a sonochemical reactor.

introduced, in order to uniformly distribute reactant as well as the cavitation effects in the reaction mixture. The temperature in the bath was maintained constant at the desired value. The conventional reflux experiments were carried out in 250mL-capacity, three-neck, flat-bottom flask. The openings are used for the condenser (to condense the methanol evaporated during reaction), a stirrer (for uniform mixing of the reaction mixture (stirring rate was kept constant as 350 rpm)), and for the introduction of the thermometer for recording the temperature change during the reaction. The flask was immersed in a constant-temperature oil bath (Equiptron). The temperature of the bath was kept constant and adjusted as per the required reaction temperature. 2.3. Experimental Procedure. Initially, 50 g of the oil is placed in the flask and heated to 120 °C for 2 h in order the remove the moisture content in the oil. The moisture content in the oil was monitored using Karl Fisher titration. The initial moisture content in the oil was reduced from 0.12% to 0.05%. The oil is then cooled to 60 °C, and a mixture of methanol and sulfuric acid is added in appropriate proportions for the first stage of esterification. The reaction mixture was heated for a specific interval of reaction time. Excess amount of methanol is removed from the mixture by heating the mixture in rotary evaporator at 80 °C. The reaction mixture is then cooled to room temperature and separation is carried out using separating funnel. The lower layer mixture is removed and washed 3−4

3. RESULTS AND DISCUSSION 3.1. Esterification. Acid esterification of free fatty acid (FFA) is a slow reaction, compared to alkaline esterification. The water formed during esterification reacts with triglycerides to form the diglycerides, monoglycerides, and free fatty acid. In the case of oils with high initial acid content, the presence of 11868



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and ultrasound. In the conventional method, it has been observed that acid value decreases from 13.5 to 1.4 mg KOH/g of oil with an increase in reaction temperature from 40 to 60 °C in 20 min, further increase in temperature does not yield any additional benefits. In the case of ultrasonic irradiation, a decrease in the acid value to 1.4 mg of KOH/g of oil was obtained in 15 min at an operating temperature of 40 °C and further increase in temperature again did not yield any benefits. The extent of acid value reduction obtained at higher temperatures of 60 °C was not significantly different, compared to that obtained at 40 °C; hence, 40 °C was selected as the optimum operating temperature to measure the effect of other operating parameters. It is also important to note here that, at an operating temperature of 40 °C, the desired acid value is not achieved in the case of the conventional method and, hence, there is a necessity to use higher operating temperatures. In the case of ultrasonic irradiations though the overall temperature is lower, the reactants are exposed to higher temperatures locally, although for few microseconds, due to the continuous process of generation and collapse of cavities. It is well-known that, due to acoustic cavitation events, high temperatures (in the range of 1000−15 000 K) and pressures (in the range of 500−5000 bar) are developed at millions of locations in the reactor. The existence of optimum temperature has been also observed for various studies in the literature such as Jatropha oil,25 waste restaurant oil,26 and edible vegetable oil,27 using the conventional method. Similar results have also been observed for the esterification of fatty acid cuts28 and palm fatty acid distillate,29 using the sonochemical reactors. Thus, it can be concluded that the temperature required to decrease the acid value desirable for the transesterification reaction is reduced, because of the use of ultrasound (40 °C), compared to the conventional technique (60 °C). 3.1.2. Effect of Molar Ratio. Since esterification is a reversible reaction, usually methanol is taken in excess, to drive the reaction in the forward direction. To investigate the effect of excess methanol on the progress of the reaction, esterification was carried out using various oil-to-methanol molar ratios (1:6, 1:4, 1:3, and 1:2) at a constant H2SO4 concentration of 1 wt % of oil, and reaction temperatures of 60 and 40 °C, for the conventional approach and the ultrasound method, respectively. In the case of the conventional reflux method, a higher temperature was selected, based on the earlier investigation-related requirement of higher temperatures for reducing the free acid content. Since the ultrasound method creates the higher temperature, a pressure effect, and emulsification locally in the reactor, lower temperatures are sufficient for the synthesis approach,8 as also discussed in the earlier section. The changes in acid value are shown in Figure 4 for conventional and ultrasound techniques. It has been observed that the final acid value of oil increases with a reduction in the molar ratio, i.e., when a lower excess of methanol is used. From the reaction stoichiometry, it is observed that one mole of methanol is required to complete the reaction. Since this reaction is reversible, the 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. It has been observed that a molar ratio of 1:3 and reaction time of 20 min is sufficient to reduce the acid value for carrying out the transesterification reaction using the conventional approach, whereas a molar ratio of 1:2 and 15 min time is sufficient for the ultrasound-based

Figure 2. High-performance liquid chromatography (HPLC) chromatogram for identification of FAME.

more free fatty acid in the oil leads to a saponification reaction in the presence of an alkali catalyst during the transesterification reaction. This necessitates the first stage of acid-catalyzed esterification. The acid value must be reduced below 2 mg KOH/g of oil for effective processing of the transesterification reaction. In order to effectively reduce the FFA content, optimization of process parameters of esterification reaction are important. 3.1.1. Effect of Reaction Temperature. The effect of temperature on the esterification rate has been studied by changing the reaction temperature to 40, 50, 60, and 65 °C for the conventional method and over the range of 30−60 °C for the sonochemical reactor operation. The objective of using lower temperatures in the case of operation with sonochemical reactors was to check whether the use of sonochemical reactors results in reducing the temperature requirement, which can lead to cost savings. The molar ratios of 1:3 and 1:2 were kept constant for the conventional and ultrasound methods, respectively (again, a lower molar ratio has been selected with similar objective of process intensification) and catalyst concentration of 1% was kept constant for both methods. The change in acid value at different operating temperatures has been shown in Figure 3 for the case of conventional method

Figure 3. Effect of reaction temperature on the acid value for a 1:3 molar ratio of methanol to oil and catalyst concentration of 1 wt % for the conventional method and for 1:2 molar ratio of methanol to oil and a catalyst concentration of 1 wt % for the ultrasound method. 11869



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Figure 4. Effect of molar ratio of methanol to oil on the acid value for 1 wt % of H2SO4 catalyst and a temperature of 65 °C for the conventional method, and 1 wt % of H2SO4 catalyst and a temperature 40 °C for the ultrasound method.

Figure 5. Effect of catalyst concentration on the acid value for a 1:3 molar ratio of methanol to oil, and a temperature of 65 °C for the conventional method and a 1:2 molar ratio of methanol to oil and a temperature of 40 °C for the ultrasound method.

operation. In ultrasonication, the microemulsion effect produces more interfacial area, which enhances the masstransfer rate between the heterogeneous liquid phases of methanol and oil, compared to the conventional stirring approach, and, hence, gives higher reduction in the acid value at comparatively lower excess of the methanol. Similar effects have been reported in the literature for different systems, such as Jatropha oil, where acid value reduction was achieved from 15 mg of KOH/g of oil to 1 mg of KOH/g of oil for a molar ratio of 1:6 and 2 h of reaction time;25 waste restaurant oil, where the acid value can be reduced from a 90.8 mg of acid value to a 5.06 mg of KOH/g oil for a molar ratio of 1:9 and a reaction time of 1 h26 and for edible vegetable oil, wherea reduction of acid value is possible from the initial 141 mg of KOH/g of oil to 1 mg of KOH/g of oil for a molar ratio of 1:3 and reaction time of 2 h,27 using the conventional method. For the ultrasound-induced biodiesel synthesis, it has also been shown that the optimum molar ratio for esterification of fatty acid cuts is 1:9, requiring a reaction time of 90 min,28 while for the esterification of palm fatty acid distillate, the optimum ratio is 1:3 and 6 h of reaction time.29 A quick comparison of the present work with the literature illustrations reveals that using the Nagchampa oil as feed stock offers a good alternative, compared to other sustainable feed stocks, such as offspecification fatty acid cuts or waste cooking oils, since the esterification is much faster, and requires a lower excess of methanol. 3.1.2. Effect of Catalyst Concentration. Esterification was also studied by changing the catalyst concentration from 0.5 wt % to 1, 1.25, and 1.5 wt % of oil at reaction temperatures of 60 and 40 °C, and molar ratios of 1:3 and 1:2 for the conventional and ultrasound techniques (optimum molar ratios and temperatures, as observed in the earlier experimental runs), respectively. The changes in the acid value with catalyst concentrations have been given in Figure 5. It was been observed that for the 0.5% catalyst concentration the acid value reduces to 5.5 and 2.3 for the conventional and ultrasound procedures, respectively, whereas at 1% catalyst concentration, it decreases to a required value for transesterification processing for both the conventional and ultrasound approaches (the final value is similar at 1% concentration) and with further increase in concentration, it does not show any effect on the final acid value. Thus, based on experiments with varying catalyst concentrations, it can be said that the optimum catalyst loading of 1% is the same for both the approaches and, as such, the use

of the ultrasound method does not alter this catalyst requirement. Similar optimum concentrations for reducing acid value of Jatropha oil from 15 mg of KOH/g to 1 mg of KOH/g has been reported as 1% catalyst concentration,25 whereas for waste restaurant oil, the optimum catalyst loading was 1.5%. For esterification of fatty acid cuts, the required optimum concentration was 2%,28 while esterification of palm fatty acid distillate required 5% catalyst concentration,29 using the ultrasound method. It is also worthwhile to mention here that the catalyst requirements are also dependent on the feed stocks and, in this aspect, again, Nagchampa oil, as a starting feedstock, offers benefits as relatively lower concentrations of catalyst are sufficient for desired progress of the esterification reaction. 3.2. Transesterification. Similar to the esterification reaction, the operating temperature, the molar ratio, and the catalyst concentration are the important parameters for the transesterification step that must be optimized for a costeffective operation and economic feasibility of the biodiesel synthesis process. 3.2.1. Effect of Reaction Temperature. In order to study the effect of reaction temperature on methyl ester formation, the transesterification reaction was carried out under the fixed conditions of reactant molar ratio and catalyst loading (i.e., a 1:6 oil-to-methanol molar ratio and 1 wt % KOH concentration) and at different operating temperatures ranging from 30 °C to 50 °C for the ultrasound method and from 45 °C to 65 °C for the conventional method. The effect of reaction temperature on the extent of conversion has been presented in Figure 6 for the conventional approach and in the presence of ultrasonic irradiations. Experimental results indicate that, at lower temperature, the extent of conversion is lower and when the temperature is increased, the conversion also increased, showing a significant positive impact, up to a certain limit. To give a quantitative idea, an increase in temperature from 30 °C to 40 °C resulted in an increase in the conversion from 65.5% to 92.6% for the operation using ultrasound, while in the case of the conventional method, the increase in conversion was from 65.2% to 91.5% for an increase in temperature from 40 °C to 60 °C. A further increase in the operating temperature (beyond 40 °C for the case of ultrasound and beyond 60 °C for the conventional method) did not yield any further increase in the extent of conversion. An increase in the operating temperature results in enhanced solubility of methanol in the other phase, thereby increasing the extent of conversion initially; however, 11870



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trends indicates that the extent of increase in the conversion is significant up to an oil-to-methanol ratio of 1:6, and beyond this, only a marginal increase in the extent of conversion is observed when the ratio is further increased to 1:10 for both the methods. Leung and Guo30 have also reported similar results for the variation oil to methanol molar ratio for both neat canola oil and used frying oil. Since the transesterification reaction is controlled by physical effects,31 an excess molar ratio is used to shift the equilibrium towards a favorable direction. It has been observed that the chemical and physical effects of cavitation phenomena intensify the reaction by ∼50%, but these have a marginal effect on the reduction in the molar ratio (1:6 molar ratio is the optimum in both cases). Similar trends have been observed for the transesterification of beef tallow, where a molar ratio of 1:6 has been reported to be optimum for both cases9 and the use of ultrasound results in significant process intensification (reaction time for achieving similar levels of conversion are 60 min and 70s using conventional and ultrasound methods, respectively). Another investigation involving the transesterification of sunflower oil reported an optimum molar ratio of 1:5 and reaction times of 60 and 20 min for the conventional and ultrasound methods, respectively.10 3.2.3. Effect of Catalyst Concentration. The effect of potassium hydroxide (KOH) concentration on the transesterification of esterified oil was investigated with its concentration varying from 0.5 wt % to 2.0 wt % (based on the weight of oil) at constant reaction temperatures of 65 and 40 °C, and a molar ratio of 1:6, for the conventional and ultrasound techniques (optimum molar ratios), respectively. The obtained results have been depicted in Figure 8 for

Figure 6. Effect of reaction temperature on conversion for a 1:6 molar ratio of methanol to oil and a catalyst concentration of 1 wt % for the conventional method and for a 1:6 molar ratio of methanol to oil and a catalyst concentration of 1 wt % for the ultrasound method.

the extent of cavitational effects is dampened at higher operating temperatures and, hence, an optimum temperature exists. 3.2.2. Effect of Molar Ratio. The methanol-to-oil molar ratio is also an equally important parameter in determining the extent of conversion in the case of transesterification, and usually a much larger quantity of methanol is required.26,30 The transesterification reaction requires three moles of methanol per mole of triglyceride to yield three moles of fatty acid methyl ester and one mole of glycerol. The theoretically minimum required molar ratio of methanol to oil should therefore be 1:3, hence accounting for equilibrium limitations, and suppression of the rate of reversible reactions, ratios ranging from 1:4 onward were selected in the present work. Experiments were conducted with oil-to-methanol molar ratios ranging from 1:4, 1:6, 1:8 and 1:10 at a constant KOH concentration of 1 wt % of oil, and reaction temperatures of 65 and 40 °C for the conventional approach and ultrasound method, respectively. The results obtained are given in Figure 7 for the conventional reflux method and sonochemical reactors. It has been observed that, with an increase in the molar ratio from 1:4 to 1:10, the extent of conversion increases from 75.8% to 90.6% for reaction times of 40 and 90 min for ultrasound and conventional methods, respectively. A careful observation of the obtained

Figure 8. Effect of catalyst concentration on conversion for a methanol-to-oil molar ratio of 1:6 and a temperature 65 °C for the conventional method, and for a methanol-to-oil 1:6 molar ratio and a temperature of 40 °C for the ultrasound method.

conventional and ultrasonic irradiations. It can be seen from the figure that an increase in the catalyst concentration from 0.5 wt % to 1 wt % results in an increase in the conversion from 77.8% to 90.2%. Initially, an insufficient amount of KOH results in incomplete conversion of triglycerides into the esters, as indicated from its lower ester content. However, above a catalyst concentration of 1%, further increase in the catalyst concentration to 2 wt % does not show a significant increase in the extent of conversion. The optimum amount of catalyst required for both methods is 1 wt %, similar to that observed for the esterification reactions earlier. The hydrolysis of product may start when more amount of water is present in the product

Figure 7. Effect of the methanol-to-oil molar ratio on conversion for 1 wt % of KOH catalyst and a temperature of 65 °C for the conventional method and for 1 wt % of KOH catalyst and a temperature of 40 °C for the ultrasound method. 11871



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approach. Kinetic rate constants have been obtained, considering second-order kinetics for each step of biodiesel synthesis and the values of the rate constants for esterification, as well as transesterfication, have been given in Table 3, whereas the optimized set of operating parameters have been given in Table 4. 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 esterification35 and transesterification36 reactions. It has been observed that the rate constant for both reactions increased with temperature from 40 °C to 60 °C for conventional method, and it was constant for ultrasoundassisted synthesis, which is consistent with the earlier discussion. In the case of conventional method, the rate constant initially increased with molar ratio from 1:2 to 1:3 and 1:4 to 1:8 for esterification and transesterification, respectively, while in the case of the ultrasound method, it was constant for esterification but for transesterification, the rate constant increased with molar ratio from 1:4 to1:8. As the catalyst concentration increased from 0.5 to 1% w/w, the rate constant for both reactions and approaches increased. The rate constant for both reactions and appraoches using the ultrasound approach was observed to be higher, compared to the conventional method. Based on the optimized set of parameters as given in the Table 4, it can be easily established that the reaction time and reaction temperature for the ultrasonication is reduced, compared to the conventional method. The intensification obtained because of the use of ultrasonic irradiations is attributed to the physical effects of the cavitation phenomena, mainly in terms of the intense levels of turbulence and mixing generated in the reactor. Because of the generation of microemulsions between the two immiscible phases taking part in the reaction, the available interfacial area between the reactants increases enormously, giving faster reaction rates and the requirement of less-severe conditions, in terms of the operating temperature.29 Another advantage offered by the use of the ultrasound approach is in terms of the requirements of a smaller amount of methanol (lower molar ratios) for achieving a similar degree of progress, especially in the case of first stage of the synthesis process. A lower requirement of excess methanol will certainly reduce the energy requirements for the overall process, because methanol separation using distillation is a significantly energy-intensive operation, controlling the overall economics of the biodiesel synthesis process. The ultrasound-based process also offers easy purification of the product and the final product properties are more suitable, compared to the conventional approach.

coupled with higher loadings of the catalyst. In order to suppress these side reactions, optimum amount of catalyst is always preferred, which will be sufficient to render the transesterification process. Similar trends have been observed for the transesterification of beef tallow where an optimum loading as 0.5% catalyst concentration has been established.9

4. PROPERTIES OF BIODIESEL Density, flash point, kinematic viscosity, and acid value of the final dried product obtained using both methods are shown in Table 2. It is observed that the properties of biodiesel obtained Table 2. Properties of Biodiesel Value conventional

ultrasound

ASTM D 6751

density

property

0.877 g/cm3

kinematic viscosity, 40 °C

2.64 mm2/s

0.874 g/cm3 2.78 mm2/s

159 °C 0.41

151 °C 0.32

0.86−0.9 g/cm3 1.9−6.0 mm2/s 130 min °C 0.5 max

flash point acid value (mg of KOH/g of oil)

by both methods match with the ASTM D 6751 standards. However, more washing is required to remove the traces of catalyst in the case of the conventional method, compared with the ultrasound method. It was observed that, in the case of ultrasound-assisted synthesis approach, the fine KOH particles form a thin interface between the glycerine and fatty acid methyl ester layers, which can be easily removed, and it helps to reduce the washing period, and increased purity of the fatty acid methyl ester. The flash point is one of the key parameters for biodiesel, which indicate the working feasibility engine performance and depends on the quality of separation of the triglyceride/glycerine from the product, fatty acid methyl esters. The flash point of the biodiesel synthesized from ultrasound (151 °C) is lower than that obtained using the conventional method (159 °C) which can be attributed to higher purity of the final product. Flash point of biodiesel from this nonedible oil is less, compared with biodiesel produced from other sources (Karanja (230 °C),32 Madhuca indica (208 °C),33 and waste frying oil (160−170 °C)).34

5. COMPARATIVE PERFORMANCE OF ULTRASOUND AND CONVENTIONAL APPROACHES It is worthwhile to summarize the extent of intensification obtained using sonochemical reactors versus the conventional

Table 3. Second-Order Rate Constant for Esterification and Transesterification Rate Constant for Esterification Processing, k (× 10−2 L mol−1 min−1) Based on Temperature method

40 °C

50 °C

conventional ultrasound

0.1 2.7

0.1 2.8

60 °C

Based on Molar Ratio 1:2

1:3

1:4

Based on Catalyst Concentration 1:6

0.5 w/w

1.7 0.3 1.7 1.8 1.8 0.2 2.8 2.5 2.9 2.9 3.0 1.2 Rate Constant for Transesterification Processing, k (× 10−2 L mol−1 min−1)

Based on Temperature

Based on Molar Ratio

1 w/w

1.25 w/w

1.5 w/w

1.8 2.8

1.8 2.8

1.8 2.8

Based on Catalyst Concentration

method

40 °C

50 °C

60 °C

1:4

1:6

1:8

1:10

0.5 w/w

1 w/w

1.5 w/w

2 w/w

conventional ultrasound

2.8 25.6

7.2 27.7

9.6 27.6

4.1 9.7

8.8 21.2

9.0 22.1

9.1 22.4

4.3 11.0

8.9 21.8

9.0 22.1

9.0 22.1

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Table 4. Optimized Parameters of Transesterification and Esterification Processes Conventional

Ultrasound

parameter

esterification

transesterification

esterification

transesterification

reaction time molar ratio catalyst concentration reaction temperature

20 min 1:3 1 wt % 60 °C

90 min 1:6 1 wt % 65 °C

15 min 1:2 1 wt % 40 °C

40 min 1:6 1 wt % 40 °C

(6) Stankiewicz, A. Alternative Sources and Forms of Energy for Intensification of Chemical and Biochemical Processes. Chem. Eng. Res. Des. 2006, 84 (A7), 511. (7) Zheyan, Q.; Lina, Z.; Laurence, W. Process intensification technologies in continuous biodiesel production. Chem. Eng. Process. 2010, 49, 323. (8) Gogate, P. R. Cavitational reactors for process intensification of chemical process applications: A critical review. Chem. Eng. Process. 2008, 47, 515. (9) Leonardo, S. G. T.; Julio, C. R. A.; Daniel, R. M.; Iran, T.V. S.; Paulo, R. B. G.; Luiz, A. M. P.; Josanaide, S. R. T. Comparison between conventional and ultrasonic preparation of beef tallow biodiesel. Fuel Process. Technol. 2009, 90, 1164. (10) Georgogianni, K. G.; Kontominasa, M. G.; Pomonisa, P. J.; Avlonitis, D.; Gergisc, V. Conventional and in situ transesterification of sunflower seed oil for the production of biodiesel. Fuel Process. Technol. 2008, 89, 503. (11) Le, T. T.; Kenji, O.; Yasuhiro, S.; Norimichi, T.; Yasuaki, M.; Hiroshi, B. Ultrasound-assisted production of biodiesel fuel from vegetable oils in a small scale circulation process. Bioresour. Technol. 2010, 101, 639. (12) Jianbing, J.; Jianli, W.; Yongchao, L.; Yunliang, Y.; Zhichao, X. Preparation of biodiesel with the help of ultrasonic and hydrodynamic cavitation. Ultrasonics 2006, 44, e411. (13) Liu, Y.; Wang, L.; Yan, Y. Biodiesel synthesis combining preesterification with alkali catalyzed process from rapeseed oil deodorizer distillate. Fuel Process. Technol. 2009, 90, 857. (14) Deng, X.; Fang, Z.; Liu, Y. Ultrasonic transesterification of Jatropha curcas L. oil to biodiesel by a two-step process. Energ. Convers. Manage. 2010, 51, 2807. (15) Lee, S. B.; Lee, J. D.; Hong, I. K. Ultrasonic energy effect on vegetable oil based biodiesel synthetic process. J. Ind. Eng. Chem. 2011, 17, 138. (16) Hamed, M.; Babak, S.; Subhash, B.; Ahmad, Z. A. Ultrasonicassisted biodiesel production process from palm oil using alkaline earth metal oxides as the heterogeneous catalysts. Fuel 2010, 89, 1818. (17) Dharmendra, K.; Gajendra, K. P.; Singh, C. P. Fast, easy ethanolysis of coconut oil for biodiesel production assisted by ultrasonication. Ultrason. Sonochem. 2010, 17, 555. (18) Yi, Z.; Maria, S.; Michio, I. Rapid transesterification of soybean oil with phase transfer catalysts. Appl. Catal., A, 2009, 366, 176. (19) Hingu, S. M.; Gogate, P. R.; Rathod, V. K. Synthesis of biodiesel from waste cooking oil using sonochemical reactors. Ultrason. Sonochem. 2010, 17, 827. (20) Deshmane, V. G.; Gogate, P. R.; Pandit, A. B. Ultrasound assisted synthesis of biodiesel from palm fatty acid distillate. Ind. Eng. Chem. Res. 2009, 48, 7923. (21) Kumar, D.; Kumar, G.; Poonam; Singh, C. P. Ultrasonic-assisted transesterification of Jatropha curcus oil using solid catalyst, Na/SiO2. Ultrason. Sonochem. 2010, 17, 839. (22) Sahoo, P. K.; Das, L. M.; Babu., M. K. G.; Naik, S. N. Biodiesel development from high acid value polanga seed oil and performance evaluation in a CI engine. Fuel 2007, 86, 448. (23) Venkanna, B. K.; Venkataramana, C. R. Biodiesel production and optimization from Calophyllum inophyllum linn oil (honne oil)-A three stage method. Bioresour. Technol. 2009, 100, 5122. (24) Haas, M. J.; Scott, K. M. Moisture Removal Substantially Improves the Efficiency of in Situ Biodiesel Production from Soybeans. J. Am. Oil Chem. Soc. 2007, 84, 197.

6. CONCLUSIONS The present work has clearly established the utility of high-acidvalue nonedible oils for the synthesis of biodiesel using a novel two-stage approach of esterification, followed by transesterification, processes. Sonochemical reactors based on the principle of generation of cavitation events using ultrasonic irradiations have also been confirmed to give significant process intensification effects for the synthesis of biodiesel. The advantages offered due to the use of sonochemical reactors, as revealed by optimization studies related to the various operating parameters such as molar ratio, catalyst concentration, and reaction temperature, are a reduced reaction temperature (savings in the energy required for heating of the process streams), a lower reaction time (energy savings in reactor processing), and the requirement of a smaller amount of excess methanol for equivalent levels of equilibrium conversion (considerable energy savings in the methanol separation units). Also, in the case of ultrasonication, the ease of separation of glycerine and catalyst is higher, which reduces the purification time and energy. The properties of biodiesel produced from both methods matches the ASTM standards. Overall, it can be said that use of sonochemical reactors considerably enhances the rates of biodiesel synthesis and would also lead to substantial energy savings, because of various process improvements, as observed in the present work.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-22-33612222. Fax: +91-22-3361 1020. E-mail: pr. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS One of the authors, V.L.G., would like to acknowledge the funding of The Institution of Engineers, Kolkata, India while both the authors (V.L.G. and P.R.G.) would like to acknowledge the support of UGC Networking Resource Centre of the Chemical Engineering Department.

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REFERENCES

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Intensification of Synthesis of Biodiesel from Nonedible Oils Using

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