Kinetic study on microwave-assisted esterification of free fatty acids derived from Ceiba pentandra Seed Oil

Accepted Manuscript Kinetic Study on Microwave-Assisted Esterification of Free Fatty Acids derived from Ceiba Pentandra Seed Oil Thanh Lieu, Suzana Yusup, Muhammad Moniruzzaman PII: DOI: Reference:

S0960-8524(16)30408-4 http://dx.doi.org/10.1016/j.biortech.2016.03.105 BITE 16295

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

19 January 2016 18 March 2016 19 March 2016

Please cite this article as: Lieu, T., Yusup, S., Moniruzzaman, M., Kinetic Study on Microwave-Assisted Esterification of Free Fatty Acids derived from Ceiba Pentandra Seed Oil, Bioresource Technology (2016), doi: http://dx.doi.org/10.1016/j.biortech.2016.03.105

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Kinetic Study on Microwave-Assisted Esterification of Free Fatty Acids derived from Ceiba Pentandra Seed Oil Thanh Lieu, Suzana Yusup*, Muhammad Moniruzzaman Biomass Processing Lab, Center for Biofuel and Biochemical Research, Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia E-mail:1) [email protected] (Thanh Lieu) 2) [email protected] (AP Dr Suzana Yusup)* 3) [email protected] (Dr. Muhammad Moniruzzaman) * Corresponding author HIGHLIGHTS • Time energy-saving of microwave technique in biodiesel pretreatment process. • Higher conversion obtained using sulfuric acid catalyst under microwave conditions. • Development of pseudo-homogeneous second-order kinetics under microwave method. ABSTRACT Recently, a great attention has been paid to advanced microwave technology that can be used to markedly enhance the biodiesel production process. Ceiba Pentandra Seed Oil containing high free fatty acids (FFA) was utilized as a non-edible feedstock for biodiesel production. Microwave-assisted esterification pretreatment was conducted to reduce the FFA content for promoting a high-quality product in the next step. At optimum condition, the conversion was achieved 94.43% using 2 wt% of sulfuric acid as catalyst where as 20.83% conversion was attained without catalyst. The kinetics of this esterification reaction was also studied to determine the influence of factors on the rate of reaction and reaction mechanisms. The results indicated that microwave-assisted esterification was of endothermic second-order reaction with the activation energy of 53.717 kJ/mol.

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Keywords Microwave, biodiesel pretreatment, free fatty acids, kinetic, second-order reaction 1. Introduction Energy plays a vital and fundamental role to the quality of human life. Considering the dramatic growth of world population and the socioeconomic uplift of countries around the globe, it is a key challenge to satisfy the increasing energy demand in a safe and environmentally responsible manner. Owing to the depletion of non-renewable energy and environmental concerns, biodiesel has received worldwide attention as one of the most sustainable, attractive renewable and green energy resources that can help to fulfill energy demand in future (Lam et al., 2010). Biodiesel has the potential to mitigate pollution from the transport industry which is a significant contributor to global warming through emission of carbon dioxide (Acevedo et al., 2015). In recent years, biodiesel is considered as a sustainable choice due to its physical properties as substitute for diesel fuel. The production of biodiesel has increased in the world, specifically in Europe (Zhu et al., 2016; Acevedo et al., 2015). However, the most significant drawback for biodiesel production is feedstocks cost, which is found to be 70 – 95% of total operating cost (Bankovic-Ilic et al., 2012). Using virgin vegetable oils has a negative impact on the global imbalance between food supply and biodiesel production, hence abrupt increase in the cost. This results in demand for alternative cheap materials, such as non-edible feedstocks. In this regard, the oil from tropical plant Ceiba Pentandra is an attractive potential feedstock. Nonetheless, low-cost oil generally contains a high level of free fatty acids (FFA) which lead to severe deterioration of product quality. Therefore, Ceiba Pentandra needs to undergo a pretreatment process called esterification reaction to reduce the concentration of free fatty acids. Generally, esterification is conducted with acid catalysts in lieu of base catalysts to prevent soap formation, thus improving the final yield of biodiesel. However, this reaction

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reaches equilibrium state slowly, leading to a low conversion and high energy consumption (Leung et al., 2010; Shahid and Jamal, 2011). To overcome this limitation, microwave irradiation is an innovative energy source which has been extensively utilized for higher yield in shorter reaction time under mild reaction conditions as well as greater product purity (Patil et al., 2012). During the past two decades, the use of microwave irradiation has gained popularity instead of conventional heating system in various chemical reactions. Microwave heating is extensively accepted, non-conventional energy source which is very attractive for chemical applications, including organic synthesis. Compared with the wall heat transfer, microwave heating remains overwhelmingly dominant. Microwave energy is a non-ionizing radiation incapable of breaking bonds. It is very low compared to ionization energies of biological compounds, of covalent bonds and hydrogen bonds, van der Waals intermolecular interactions. Moreover, it is much lower than the energy of Brownian motion, thus it is not strong enough to break chemical bonds (Gude et al., 2013). This results in the incapacity of microwaves for chemical reactions inducement. Several researches have been carried out work on the optimization of esterification under microwave irradiation. According to Shi et al., 2010, the yield of esterification of salicylic acid could reach 93.6% using SO3Hfunctionalized ionic liquids with HSO4– with the help of microwave technique in 20 min. As mentioned by Kim et al., 2011, more than 90% conversion of microwave-accelerated esterification was obtained in 20 min using 5 wt% sulfated zirconia and 1:20 M ration of oil to methanol at 60 oC. From the work of Kamath et al., 2011, the FFA of crude karanja oil was decreased to 1.11% using 33.83 wt% methanol to oil ratio and 3.73 wt% sulfuric acid concentration in 190 seconds through the Box-Behnken experimental design. Chemical kinetics is of primary concern in the exploitation of chemical reactions in industrial production (Levenspiel, 1972). The understanding of chemical kinetics is of vital importance

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in order that the selected reaction system can operate in the safest and most efficient manner at a commercial scale (Fogler, 2006). As aforementioned, non-edible oils used in alkaline transesterification reaction contain high free fatty acids in which saponification formation impedes the separation of the esters from glycerine, leading to the decrease in biodiesel yield (Berrios et al., 2007). To address this problem, esterification reaction is carried out as a pretreatment process for biodiesel production. Following that, a kinetic study which investigates the rate of esterification reaction is conducted. Under conventional heating method, a first-order kinetic law is found to be fit for the forward reaction in the work of Berrios et al. (2007) and Chai et al. (2014). According to Berrios et al. (2007), the activation energy for the forward reaction reduced with the rise of sulfuric acid catalyst concentration from 5 wt% to 10 wt% (50.745 kJ/mol and 44.559 kJ/mol, respectively). Conversely, the pre-exponential factor for the forward reaction rose from 2.869 x 106 min-1 to 3.913 x 10 6 min-1 when the catalyst concentration was increased from 5 wt% to 10 wt%. This fact may be attributed to the beneficial effect of catalyst on the conversion of free fatty acids (FFA). However, the kinetic constant for the reverse reaction was negligible irrespective of the catalyst concentration, which implied that it hardly occurred the hydrolysis reaction. Similarly, it is mentioned in the work of Chai et al. (2014) that the activation energy reduced from 42.007 kJ/mol to 20.747 kJ/mol with increase in sulfuric acid catalyst concentration from 5 wt% to 12.5 wt%. Moreover, it was also reported in their work that the reverse reaction could be neglected when significantly excess alcohol such as methanol was used. Under microwave heating method, limited researches are found on kinetic study of acid catalyzed esterification reaction. As it is referenced by Mazubert et al. (2014), the conversion of FFA from waste cooking oil using sulfuric acid catalyst increased at higher temperature and the kinetic model was a pseudo-homogeneous second-order model with the activation energy value of 45.4 kJ/mol and the pre-exponential factor value of 7.0 x 10 7 L/mol/min.

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Biodiesel production from Ceiba Pentandra using microwave-assisted technique has not been reported in previous literature. The objectives of this work are to study the feasibility of using microwave-assisted technique for FFA reduction in acid catalyzed esterification process and investigate the effect of parameters on the response using three-dimensional graphs. Limited publications were reported on the kinetics of biodiesel pretreatment. The present work also studies the type of the reaction and the influence of factors on the rate of esterification reaction from Ceiba Pentandra Seed Oil assisted by microwave system. 2. Materials and Methods 2.1 Materials Ceiba Pentandra Seed Oil was purchased from BUNGAKEMBANG ENTERPRISE CV in Indonesia. Solvent methanol (99.9% purity); acid catalyst H2SO4 (95 – 97 %); titrant KOH pellet; drying agent Na2SO4 anhydrous (purity 99 %) and qualitative filter papers were purchased from Merck Chemical Company (Darmstadt, Germany) and Sigma Aldrich Chemical Company (United States). All chemicals were analytical grade. 2.2 Microwave-assisted biodiesel pretreatment The esterification reaction of free fatty acids was performed in a batch mode, using a microwave synthesis reactor (MARS 6 SYNTHESIS, CEM Corporation PO Box 200, Matthews, NC 28106, United States) with two magnetrons (1800W) that operates at the frequency of 2450 MHz to deliver more power. The microwave worked at atmospheric pressure and was equipped with a 500 ml three-necked round-bottomed glass reactor. Initially, a fixed amount of the sample i.e., crude Ceiba Pentandra Seed Oil was transferred to the glass. A known amount of sulfuric acid was mixed with specific quantity of methanol and the mixture was stirred thoroughly until the acid was completely dissolved. Then, the solution was added to the oil sample and esterification reaction occurred for a certain period of reaction time.

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During the reaction, a reflux condenser was set on the main neck of the flask to prevent the methanol loss. A magnetic stirring bar was put inside the reactor to attain a completely homogeneous mixture among the reactants at constant rate. The maximum power for this type of glass was 500 W. The power was automatically adjusted to the set temperature via a fiber optic probe which provided accurate measurement every time. The agitation speed remained constant at 900 rpm. Once the desired reaction was completed, the mixture was poured into a separating funnel and allowed to cool down to separate the excess methanol, the sulfuric acid catalyst and any impurities existed in the upper layer. Following that, the esterified oil at the lower layer was separated and washed with distilled water at 50 oC to remove impurities, including sulfuric acid and methanol. Thereafter, anhydrous Na2SO4 was added to eliminate water and filter paper was used to filter any traces of Na2SO4. Ultimately, the product was poured into a rotary evaporator set at 60 oC under vacuum conditions for 1 hour to remove extra methanol and water. In the present study, Response Surface Methodology (RSM) was applied to determine the optimized operating condition using microwave technique for biodiesel pretreatment of Ceiba Pentandra Seed Oil. A total of 30 experimental trials were designed by Central Composite Design (CCD) using the Design-Expert software 9.0 (Stat-Ease, Inc., USA) to develop a response surface quadratic model for describing the effect of four parameters on conversion of FFA (Lieu et al., 2015). This response was assumed to be affected by four main parameters i.e., acid catalyst concentration (A), methanol to oil molar ratio (B), reaction temperature (C) and reaction time (D), which was also studied in previous work for Ceiba Pentandra Seed Oil using conventional method (Sivakumar et al., 2013).

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2.3 Mechanism for esterification reaction In the esterification of FFA with methanol, concentrated sulfuric acid acts as a proton donor, increasing the rate of reaction between two reactants. This reaction mechanism has six steps as described in Fig. 1. In the initial step, a proton from sulfuric acid is transferred to carbonyl oxygen, increasing the electrophilicity of the carbonyl carbon atom. The following step is the nucleophilic oxygen atom attack of the alcohol on that carbonyl carbon atom. Then proton transfer from the oxonium ion to a second molecule of the alcohol forms a complex conglomerate. This arrangement of atoms and charge is not stable, so it undergoes a protonation of one of the hydroxyl groups to give a new oxonium ion. Leaving of water molecule from this oxonium ion gives increased stabilization. Finally, deprotonation of sulfuric acid completes the process, producing the ester. Since concentrated sulfuric acid is regenerated but not degenerated by the reaction, it is considered as a mineral acid catalyst, not a reactant (Loudon, 2009). 2.4 Acid value test The initial acid value of Ceiba Pentandra seed oil was 13.922 mg KOH/g oil. The acid value test for Ceiba Pentandra seed oil was performed to determine the amount of acid reduction through titration based on AOCS official method (Cd, 3d-63), as shown in Eq. (1) (Firestone, 2009). Acid value =

( A − B) × M × 56.1 , mg KOH/g oil m

(1)

Where, A is the volume of titrant solution used in the titration of the sample, mL B is the volume of titrant solution used in the titration of the blank, mL M is the molarity of the titrant solution, mol/L m is the mass of the sample, g

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The conversion of free fatty acids, X was calculated using the following Eq. (2) (Man et al., 2013).

§ AV − AVt X = ¨¨ i AVi ©

· ¸¸ × 100,% ¹

(2)

Where AVi is initial acid value of the mixture and AVt is the acid value at any time, t. 2.5 Kinetic of biodiesel pretreatment from Ceiba Pentandra Seed Oil The methanolysis of free fatty acids is a reversible reaction catalyzed by catalyst. It produces methyl ester and water as by-product (Khan et al., 2010). ுమ ௌைర

ܴ‫ ܪܱܱܥ‬൅ ‫ܪܥ‬ଷ ܱ‫ ܪ‬ሯልልሰ ܴ‫ܪܥܱܱܥ‬ଷ ൅ ‫ܪ‬ଶ ܱ

In general, the reaction rate equation is defined as − rA = k1C Aa C Bb − k 2 C Rr C Ss

(3)

Where CA, CB, CR, and CS represent the concentrations of free fatty acids, methanol, methyl ester and water, respectively. k1 and k2 are the forward and reverse reaction rate constants, respectively. a, b, c and d are the reaction orders with respect to A, B, C and D, respectively. Based on Le Chatelier’s principle on the concentration changes, any change to the concentration of a chemical will shift the equilibrium to the side that minimizes the change in concentration. As aforementioned, with the excessive presence of methanol, the reaction was considered irreversible and also the concentration of methanol was considered constant. The reaction rate equation was simplified to pseudo-homogeneous equation with Į as the reaction order.

− rA = −

dC A = kCAa dt

(4)

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For a constant density system, volume does not change, a given conversion XA of free fatty acids is a convenient variable often used in place of FFA concentration. Following expression relates concentration and fractional conversion CA =

and

N A N Ao (1 − X A ) = = C Ao (1 − X A ) V V

− dC A = C A0 dX A

(5)

(6)

Hence Eq. (4) becomes

C A0

dX A = kC Aa0 (1 − X A ) a dt

rearranging gives

(7)

dX A = kC Aa0−1 (1 − X A ) a dt

(8)

After esterification reaction completion, the optimum result with respect to catalyst concentration and methanol to oil molar ratio was used to carry out further experiments at three different temperatures (50, 60 and 70 oC) and different reaction time (2, 4, 8, 12, 16, 20, 24, 28, 32, 36 min) for study of esterification kinetics. The oil was preheated in the reactor before adding the methanol and sulfuric acid. The time required for each run was recorded when the set temperature was reached. All the experiments were repeated two times. After every experiment, the acid value of treated oil was analyzed to find out the conversion of FFA. 3. Results and discussions

3.1 Optimization study of acid catalyzed esterification process As a non-edible feedstock, Ceiba Pentandra seed oil possesses high free fatty acids which dramatically affect the transesterification reaction due to soap formation. Therefore, the aim of pretreatment process is to minimize the amount of FFA in the oil up to 1%. In this study, microwave-assisted esterification reaction was carried out to investigate the effects of four important process parameters on the reduction of FFA. As aforementioned, the Response Surface Methodology (RSM) approach in conjunction with Central Composite Design (CCD) 9

was applied to determine the interactions between inputs and output. Moreover, the optimum conditions for lowest acid value were also investigated. The design of experiments with response and predicted results are displayed in Table 1. It can be observed that the acid value reduced from 13.922 mg KOH/g oil to 0.776 mg KOH/g oil with respect to the range of process parameters studied, in which reaction time was significantly diminished from three hours (Ong et al., 2013) to twelve minutes in comparison with conventional heating system. In the conventional heating, heat is transferred to the sample volume from the surface towards the center of the material by conduction, convection and radiation, which increases the temperature of the surface of the vessel and then, diffuses inward (Gude et al., 2013; Refaat and Sheltawy, 2008). As a result, vast amounts of energy supplied are lost to the environment, leading to the increase of reaction time. In contrast, microwaves are a form of energy conversion rather than a type of heat that is manifested as heat through the interaction of electromagnetic waves with the materials at molecular level. As a volumetric heating process, such interaction in microwave heating can affect molecular actions at a very fast rate, contributing to localized and rapid superheating of the sample (Gude et al., 2013; Sajjadi et al., 2014). This leads to heat the sample in the entire volume at similar rate due to internal thermal dissipation of the vibrational energy of the particles. Therefore, esterification under microwave irradiation is marked by even heating, short reaction time and being environmentally friendly (Shi et al., 2010). As discussed in the previous paper (Lieu et al., 2015), the analysis of variance (ANOVA) illustrated that the observed model for acid value gave good prediction of the average outcomes at the confidence level of 95%. The value of adjusted determination coefficient (R2a = 0.9070) was high and in reasonable agreement with the predicted determination coefficient (R2p = 0.7240). The second order polynomial model selected for response of acid value in terms of coded factors is attained in Eq. (9)

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Acid value (mg KOH/ g oil) = 1.700 – 0.683 A – 0.347 B – 0.375 C – 0.286 D – 0.068 AB + 0.068 AC + 0.061 AD – 0.006 BC – 0.076 BD + 0.001 CD + 0.260 A2 + 0.195 B2 + 0.064 C2 + 0.028 D2

(9)

3.1.1. Optimization and Reproducibility of experiment results By the aid of RSM and Design Expert Software 9.0 ®, the optimum conditions for biodiesel pretreatment has been determined to be 60 oC, 12 min, 2 wt% H2SO4 and 10:1 methanol to oil molar ratio. The reproducible results were comparable with the value of the model prediction. The optimum acid value of predicted result versus the experimental result was found to be 0.537 ± 0.274 mg KOH/g oil and 0.778 ± 0.004 mg KOH/g oil, respectively. It can be concluded that the experimental results are in harmony with the model prediction. In terms of reactor performance, along with conversion, product selectivity is a crucial concept in investigating the effect of process parameters, i.e. alcohol to oil molar ratio, catalyst concentration and reaction temperature to maximize the production of desired product (Nasir et al., 2014). The term selectivity can be described as a ratio between moles of desired product formed and moles of substrate consumed (Patel and Singh, 2014). At optimum conditions, the selectivity was determined to be 83.30%, which is relatively high in a batch reactor. Further experiment has been done at optimum conditions without catalyst. FFA conversion of 20.83% was obtained in 12 min without catalyst under microwave condition implies the inefficiency of esterification reaction in converting free fatty acids into esters. However, the result also showed a significant reduction in FFA (20.83% in 12 min) for microwave heating method compared to that of conventional heating (about 70% in 120 min) (Ong et al., 2013). Again, it is ascertained that microwave-assisted acid catalyzed esterification reaction merits special attention for acquiring higher quality biodiesel in terms of time and energy.

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3.1.2. Three-dimensional response surface plots Besides being designed to optimize processes, RSM is also used to investigate the interactions among various factors. The three-dimensional response surface plots provide a vivid description of the group of two process parameters interactions on the response. In this part, 3-D plots are depicted for the two combined independent variables of factors A, B, C and D interactions on the reduction of acid value. The three-dimensional response surface plot with two design parameters interaction on acid value, i.e. catalyst concentration and methanol to oil molar ratio is shown in Fig. 2a. Generally, the acid value reduced with the increase of each factor. It can be seen that catalyst concentration has higher significance level than methanol to oil molar ratio in minimizing the response. At lower catalyst concentration of 1 wt%, the acid value relatively reduced from 3.11 mg KOH/ g oil to 2.55 mg KOH/ g oil by 18.0% when methanol to oil molar ratio increased from 6:1 to 10:10. At lower methanol to oil molar ratio of 6:1, the acid value significantly reduced from 3.11 mg KOH/ g oil to 1.89 mg KOH/ g oil by 39.2% when catalyst concentration increased from 1 wt% to 2 wt%. Another observation is that the acid value tended to sharply reduce in the area shaped by quadratic curve when catalyst concentration and methanol to oil molar ratio increased. This may be attributed to the increase of the reaction rate between two reactants as a result of miscibility enhancement of methanol in oil phase via proton donation and ionic interaction under microwave condition (Gude et al., 2013). Fig. 2b demonstrates the three-dimensional response surface plot for acid value on the effect of catalyst concentration and reaction temperature. It can be seen that the acid value reduced drastically with the increase in catalyst concentration while it tended to decrease slightly with the rise in temperature. Increasing temperature from 50 oC to 60 oC reduced the acid value from 3.14 mg KOH/ g oil to 2.25 mg KOH/ g oil by 28.3% with the use of 1 wt% H2SO4 and

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from 1.65 mg KOH/ g oil to 1.04 mg KOH/ g oil by 37.0% with the use of 2 wt% H2SO4. Conversely, the acid value can be diminished from 3.14 mg KOH/ g oil to 1.65 mg KOH/ g oil by 47.5% when catalyst concentration rose from 1 wt% to 2 wt% at 50 oC and from 2.25 mg KOH/ g oil to 1.04 mg KOH/ g oil by 53.8% at 60 oC. It is clear that the influence of catalyst concentration on reduction of acid value outweighs that of reaction temperature. Fig. 2c displays the three-dimensional response surface plot on the effect of catalyst concentration and reaction time on acid value. Similarly, the plot reveals the marked influence of catalyst concentration on acid value compared to that of reaction time. It is interesting to observe that when reaction time increased from 5 min to 12 min, the acid value reduced from 3.01 mg KOH/ g oil to 2.32 mg KOH/ g oil by 22.9% and from 1.54 mg KOH/ g oil to 1.08 mg KOH/ g oil by 29.9% with the use of 1 wt% and 2 wt% H2SO4, respectively. This indicates that the rise of reaction time does not show much effect on the response but the higher catalyst concentration gives the better decrement of acid value. The three-dimensional response surface plot on the effect of methanol to oil molar ratio and reaction temperature on acid value is presented in Fig. 2d. It is easy to recognize that both methanol to oil molar ratio and reaction temperature have a positive effect on the reduction of acid value. At lower temperature of 50 oC, the acid value reduced from 2.67 mg KOH/ g oil to 1.99 mg KOH/ g oil by 25.5% with the increase of methanol to oil molar ratio from 6:1 to 10:1. At higher temperature of 60 oC, the acid value reduced from 1.93 mg KOH/ g oil to 1.23 mg KOH/ g oil by 36.3% with the increase of methanol to oil molar ratio from 6:1 to 10:1. Similarly, when temperature increased from 50 oC to 60 oC, the acid value reduced by 27.7% using 6:1 methanol to oil molar ratio while the acid value reduced by 37.9% using 10:1 methanol to oil molar ratio. These data shows the good agreement in the increase of two factors with the decrease of response and methanol to oil molar ratio reflects a major influence on the reduction of acid value.

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Fig. 2e depicts the three-dimensional response surface plot on the effect of methanol to oil molar ratio and reaction time on acid value. Within 5 min of reaction time, the decrement of acid value was found to be 22.1% when methanol to oil molar ratio rose from 6:1 to 10:1 while the acid value reduced from 2.06 mg KOH/ g oil to 1.22 mg KOH/ g oil by 40.8% in 12 min of reaction time in the same manner. However, when the reaction time increased from 5 min to 12 min, the acid value merely reduced from 2.48 mg KOH/ g oil to 2.06 mg KOH/ g oil by 16.9% using 6:1 methanol to oil molar ratio while it moderately decreased from 1.93 mg KOH/ g oil to 1.21 mg KOH/ g oil by 37.3% using 10:1 methanol to oil molar ratio. It can be concluded that methanol to oil molar ratio and reaction time exhibit positive influence on the acid value and the effect of methanol to oil molar ratio is more significant than that of reaction time on the output. Fig. 2f depicts the three-dimensional response surface plot for acid value on the effect of reaction temperature and reaction time. It is observed that the acid value slightly reduced in linear curve area when reaction temperature and time increased. Compared to the previous plot in Fig. 1a, the effect of temperature and time on the reduction of acid value is less significant than the influence of catalyst concentration and methanol to oil molar ratio. At lower temperature of 50 oC, the acid value moderately decreased from 2.45 mg KOH/ g oil to 1.88 mg KOH/ g oil by 23.3% when the reaction time increased from 5 min to 12 min. Similarly, the acid value comparatively reduced from 2.45 mg KOH/ g oil to 1.70 mg KOH/ g oil by 30.6% when reaction temperature increased from 50 oC to 60 oC in the reaction time of 5 min. In summary, the three-dimensional response surface plots reveal the significance level of four factors towards the response in which catalyst concentration gains a dominant influence, followed by methanol to oil molar ratio, temperature and time which present a modest effect on the reduction of acid value.

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3.2 Kinetic analysis of acid catalyzed esterification reaction As can be seen in Fig. 3, when using 2 wt% of H2SO4 in oil and 10:1 methanol to oil molar ratio, the conversion of FFA is found to increase with the reaction time. It is obvious that in the first two minutes, the FFA conversion rate was remarkably fast, and then slowed down gradually. The conversion for two minutes at 50 oC, 60 oC and 70 oC was 77.31%, 84.26% and 86.41%, respectively. In the field of electromagnetic waves, the interaction of microwave irradiation with materials can be described in terms of the absorbability of absorbers, which decreases the available microwave energy and rapidly heats the sample (Motasemi and Afzal, 2013; Varma, 2001). In other words, microwave heating or dielectric heating is based on the ability of a particular substance. Polar solvents or substrates with high dielectric constant and low molecular weight (such as methanol) can absorb microwave energy and successfully convert the electromagnetic energy to heat (kinetic energy) (Sajjadi et al., 2014; Leadbeater, 2011). The selective heating of certain compositions may result in the formation of high temperature microzones called “hot spots”, leading to an increase in the escalation of chemical reaction rate (Manco, 2012). After twelve minutes, more than 90% of FFA was converted into methyl ester, which has been proven in previous papers that microwave irradiation is a fast method which rapidly reduces the FFA content within the first 15 min (Suppalakpanya et al., 2010). This is also evidenced that the free fatty acids of crude Karanjja oil decreased to 87.39% in 190 sec under microwave irradiation using 3.73 wt% of sulfuric acid and 33.83 wt% methanol-oil ratio (Kamath et al., 2011). On the other hand, it is observed that the FFA conversion rate increased considerably with an increase in reaction temperature from 50 oC to 70 oC and it reached maximum conversion at 70 oC in this study. It may be due to the fact that when the methanol was vaporized, alcohol and oil come into intimate interfacial contact throughout the reactor, giving high conversion in relatively short reaction time. Even though from 60 oC to 70 oC, there was a slight decrease

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in acid value of Ceiba Pentandra Seed Oil from 24 min to 28 min of reaction time, it can still be seen that the trend of esterification reaction progressing forward. This can be explained due to the fact that increasing the temperature apparently favours the acceleration of the forward reaction as the reaction is endothermic under a kinetically controlled regime, which has been also demonstrated in the publications of Su (2013) and Liu et al. (2013). From the theory of Le Châtelier’s principle, for endothermic reactions, the equilibrium shifts to the right as the temperature increases (Fogler, 2006). Therefore, reaction temperature shows a positive effect on the conversion of FFA. It is also indicated that FFA conversion stopped increasing from 12 min to 28 min at 70 oC. Methanol evaporation may be the reason for slow reduction of FFA content during the extended period of reaction time. However, the conversion continued rising slightly after 28 min at 70 oC and remained constant because of the fast reaction rate and the equilibrium reached. 3.2.1. Determination of kinetic parameters Unlike other publications on kinetic study which assumed the order for esterification reaction, in this research, the reaction order is figured out by plotting the graph of dX/dt versus (1-X) at three different temperatures based on differential Eq. (8). From the plots in Fig. 4a, it is indicated that the order of the reaction increases with higher temperature and is nearly two. This may be due to the fact that the rate of reaction is dramatically enhanced in microwave heating system and reaction rate has dimensions of concentration per unit time, thus it depends on the concentration as a direct proportionality, in which the concentrations may be raised to be the first or second power. Assuming the model used is a pseudo-homogeneous second-order model, hence Eq. (8) becomes

dX A = kC A0 (1 − X A ) 2 = k ' (1 − X A ) 2 dt

(10) 16

' With k = kCA0

Fig. 4b depicts the plots of dXA/ dt versus (1 – XA)2 where the slopes are equal to the reaction rates. The kinetic data are well fitted with the pseudo-homogeneous second-order equation with high regression coefficients (R2 = 0.981, 0.982, 0.956 for 50 oC, 60 oC and 70 oC, respectively). Fig. 5 describes the experimental and prediction plot for FFA conversion of esterification reaction at three different temperatures. It can be observed that the data points narrowly scattered around the diagonals at three different temperature with the regression coefficients close to unity (R2 = 0.984, 0.972 and 0.947 for 50 oC, 60 oC and 70 oC, respectively). This indicates a particularly good agreement corresponding pairs of data in terms of kinetic study of acid catalyzed esterification reaction. 3.2.2. Arrhenius’ plot and activation energy The reaction rate constant, k is almost always strongly dependent on temperature. Therefore, at a given temperature level, it is merely independent of the FFA concentration of the esterification reaction. It depends on whether or not a catalyst is present. The quantity k is referred to as either the specific reaction rate or the rate constant. Arrhenius’ equation shows the temperature dependence of the specific reaction rate kA as follows (Fogler, 2006)

k A (T ) = Ae− E / RT

(11)

Where A = pre-exponential factor or frequency factor E = activation energy, J/mol or cal/mol R = gas constant = 8.314 J/mol.K = 1.987 cal/mol.K T = absolute temperature, K After taking the natural logarithm of Eq. (11)

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ln k A = ln A −

E§1· ¨ ¸ (12) R ©T ¹

The plot of ln kA vs 1/T in Fig. 6 is found to be linear with high regression coefficient (R2 = 0.974). The calculated activation energy EA is determined to be 53.717 kJ/mol from the slope of this plot, which is similar to the result from the work of Mazubert et al. (2014) with the maximum activation energy of 58.9 kJ/mol. Then relation between equation and reaction rate constant to temperature is

dX A − rA = C A0 = C A0 (1 − X A ) 2 × 3.98 × 109 e dt

−53.717 RT

(13)

As can be seen in Table 2, the pre-exponential factor of the forward reaction is significantly higher than that of the reverse reaction. It proves that the reverse reaction can be neglected when the significantly excess alcohol (methanol or ethanol) is used. Moreover, it can also be observed that the pre-exponential factor for microwave system is relatively higher than that for conventional system, particularly in the present work with the substantially high preexponential factor value of 3.98x10 9 min-1. It is due to the fact that in microwave field, the increase of the molecular mobility leads to the rise of value of the Arrhenius pre-exponential factor A (Gude et al., 2013). 4. Conclusion

This study revealed that microwave irradiation is an effective tool for biodiesel pretreatment. With higher reaction temperature, the conversion attained was higher and reached equilibrium faster as esterification reaction is endothermic. The FFA conversion was obtained 94.43% using 2 wt% of sulfuric acid catalyst and 20.83% without catalyst. In terms of chemical kinetics, the results obtained showed that the microwave-accelerated esterification of free fatty acids with a mineral acid catalyst was pseudo-homogeneous second-order reaction. The

18

R2 value 0.974 for Arrhenius’ plot with high pre-exponential factor A (3.98x109 min-1) proved energy-efficiency of microwave for esterification reaction. Acknowledgements

The authors would like to express gratitude to University Teknologi PETRONAS and Exploring Research Grant Scheme (ERGS) for the supports given to conduct the research work. References

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List of figure captions

Fig.1: Mechanism for esterification reaction Fig. 2: 3-D response surface plots on the effect of four parameters on acid value (a) the effect of catalyst concentration and methanol to oil molar ratio; (b) the effect of catalyst concentration and reaction temperature; (c) the effect of catalyst concentration and reaction time; (d) the effect of methanol to oil molar ratio and reaction temperature; (e) the effect of methanol to oil molar ratio and reaction time; (f) the effect of reaction temperature and reaction time. Fig. 3: FFA conversion as a function of time at three different temperatures Fig. 4: (a) the reaction rate equations for FFA conversion at three different temperatures; (b) typical kinetic plots for the effect of temperature on acid catalyzed esterification reaction Fig. 5: Experimental vs. prediction plot for FFA conversion of esterification reaction at (a) 50 o

C; (b) 60 oC; (c) 70 oC

Fig. 6: Arrhenius’ plot of ln kA versus 1/T

23

List of figures

Fig.1

24

(a)

(b)

25

(c)

(d)

26

(e)

(f)

Fig. 2

27

Fig. 3

28

(a)

(b)

Fig. 4

29

Fig. 5

Fig. 6

30

List of table captions

Table 1: Experimental runs for acid catalyzed esterification with response and predicted results Table 2: Kinetic study on acid catalyzed esterification reaction

31

List of tables

Table 1: Experimental runs for acid catalyzed esterification with response and predicted results Experimental

Catalyst concentration

Methanol to oil

Temperature

Time

Response Acid value

Predicted Acid value

Run

(wt%)

(molar ratio)

(oC)

(min)

(mg KOH/g oil)

(mg KOH/g oil)

1

1

6

50

5

3.541

3.919

2

1

6

50

12

3.111

3.376

3

1

6

60

5

2.849

3.043

4

1

6

60

12

2.449

2.504

5

1

10

50

5

3.505

3.524

6

1

10

50

12

2.812

2.677

7

1

10

60

5

2.505

2.624

8

1

10

60

12

1.607

1.782

9

2

6

50

5

2.522

2.432

10

2

6

50

12

2.112

2.132

11

2

6

60

5

1.552

1.826

12

2

6

60

12

1.466

1.532

13

2

10

50

5

1.682

1.766

14

2

10

50

12

1.271

1.162

15

2

10

60

5

1.316

1.136

32

16

2

10

60

12

0.776

0.537

17

0.5

8

55

8.5

4.529

4.107

18

1.5

4

55

8.5

3.645

3.176

19

1.5

8

45

8.5

2.812

2.708

20

1.5

8

55

1.5

2.671

2.384

21

1.5

8

55

8.5

1.663

1.700

22

1.5

8

55

8.5

1.710

1.700

23

1.5

8

55

8.5

1.653

1.700

24

1.5

8

55

8.5

1.738

1.700

25

1.5

8

55

8.5

1.738

1.699

26

1.5

8

55

8.5

1.700

1.700

27

1.5

12

55

8.5

1.542

1.787

28

1.5

8

65

8.5

1.327

1.207

29

1.5

8

55

15.5

1.178

1.241

30

2.5

8

55

8.5

1.177

1.376

33

Table 2: Kinetic study on acid catalyzed esterification reaction Feedstock

Method

Reaction order

Activation energy

Pre-exponential factor

E1 = 44.559 kJ/mol

A1 = 3.913 x 106 min-1

Pseudo-homogeneous first-order in forward reaction Sunflower oil

Reference

Berrios et al.

Conventional Pseudo-homogeneous second-

(2007) E2 = 42.761 kJ/mol

A2 = 707.166 L/mol/min

E1 = 46.69 kJ/mol

A1 = 240.635 min-1

order in reverse reaction Pseudo first-order irreversible Oleic acid

Conventional reaction

(2008)

Pseudo-homogeneous first-order Used cooking oil

Conventional

E1 = 45.937 kJ/mol

A1 = 1.795 x 105 min-1

irreversible reaction E1 = 31.6 kJ/mola

A1 = 1.1 x 105 L/mol/min

Pseudo-homogeneous second-

E2 = 9.7 kJ/mola

A2 = 6.7 x 10-1 L/mol/min

Mazubert et al.

order reversible reaction

E1 = 37.1 kJ/mola

A1 = 1.3 x 106 L/mol/min

(2014)

E2 = 17.9 kJ/mola

A2 = 2.1 x 101 L/mol/min

E1 = 53.717 kJ/mol

A1 = 3.98 x 109 min-1b

Waste cooking oil

Microwave Pseudo-homogeneous secondMicrowave Seed Oil

Chai et al. (2014)

Conventional

Ceiba Pentandra

Cardoso et al.

order irreversible reaction

E1 and E2 are activation energies of forward and reverse reactions A1 and A2 are pre-exponential factors of forward and reverse reactions

34

a

average value

b

using conversion

This work

GRAPHICAL ABSTRACT Increasing the reaction temperature accelerates the forward of reaction as esterification reaction is endothermic under a kinetically controlled regime.