CHEMICAL ENGINEERING TRANSACTIONS VOL. 78, 2020 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2020, AIDIC Servizi S.r.l. ISBN 978-88-95608-76-1; ISSN 2283-9216 Kinetic Study of Esterification of Palm Fatty Acid Distillate Using Sulfonated Glucose Prepared via Microwave-Assisted Heating Method Nur Nazina Saimon, Mazura Jusoh, Norzita Ngadi, Zaki Yamani Zakaria* School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai. zakiyamani@utm.my Kinetic models play an important role especially in designing reactor, simulation and optimization of a particular chemical reaction. In this study, kinetic model of the esterification of palm fatty acid distillate (PFAD) using sulfonated incomplete carbonized glucose (SO3H/ICG) prepared via microwave-assisted heating method was developed. D(+)-Glucose was carbonized in a microwave to produce incomplete carbonized glucose (ICG). Then the ICG was sulfonated with H2SO4, heated in a microwave and stirred. The catalyst prepared was then used for esterification of PFAD at reaction temperatures of 65 °C, 75 °C and 85 °C. The highest conversion exhibited was 95.76 % performed at 85 °C, 2.5 wt% catalyst, 10:1 molar ratio of methanol to PFAD, 120 min reaction time. A kinetic model of PFAD’s esterification was developed and the model was described as a pseudo first-order reaction rate law. The value of the activation energy was 28.8 kJ/mol with R2 of 0.985. 1. Introduction Biodiesel, scientifically identified as fatty acid methyl esters (FAME) is getting acknowledged due to its properties which effectively reduce the emission of greenhouse gases (GHG) to atmosphere. The usage of biodiesel especially in transportation provides the environment a possible solution to the alarming air pollution problems. However, synthesis of biodiesel consumes high cost and energy (Mohammadshirazi et al., 2014). Part of it was due to that fact that vegetable oil which was used as biodiesel feedstock competes with the food industry. Hence, second and third generation biodiesel that employs inedible sources emerged in the form of biomass and engineered crops. Palm fatty acid distillate (PFAD) (Saimon et al., 2018) and algae (Peralta-Ruiz et al., 2018) had been used as biodiesel feedstock which possess substantial potential in cost reduction. Esterification of the feedstock with high free fatty acid (FFA) requires the existence of heterogeneous acid catalyst to avoid saponification and separation problem (Gaurav et al., 2019). The biodiesel production is started with a lab-scale to a pilot-scale and demonstration scale and finally develops into an industrial-scale production. In order to operate an industrial-scale biodiesel production efficiently, kinetic study is the most important aspect to be comprehended. It is a powerful tool in designing reactor for synthesis of higher added- value products of chemical processes. The development of kinetic model will be a great help for the process simulation (Harsha Hebbar et al., 2018). In order to scale up a process, the concentration of reactants, temperature of the reaction as well as the pressure profile should be determined through reactor modeling (Chai et al., 2014). Through a kinetic study on the esterification reaction, the parameters to predict the extent of reaction under particular reaction conditions will be provided. Basically a kinetic study will be covering the determination of reaction rate equation, rate constant as well as activation energy (Marchetti et al., 2011). Years of study shown that description of the kinetics of esterification for biodiesel remains controversial. There are conflicting findings on the reaction order and the estimation of reaction rate constants (Turner, 2005). Various researchers are more focused on determining the best fit of empirical data to simple models of reaction order (Silva et al., 2009). Common acid or base catalyst esterification adopt kinetic model developed by (Bernard Freedman et al., 1986). This study promotes the development of acid heterogeneous catalyst prepared via DOI: 10.3303/CET2078068 Paper Received: 05/05/2019; Revised: 16/09/2019; Accepted: 20/11/2019 Please cite this article as: Saimon N.N., Jusoh M., Ngadi N., Zakaria Z.Y., 2020, Kinetic Study of Esterification of Palm Fatty Acid Distillate Using Sulfonated Glucose Prepared via Microwave-Assisted Heating Method, Chemical Engineering Transactions, 78, 403-408 DOI:10.3303/CET2078068 403 microwave-assisted heating technique to esterify the PFAD with methanol. The main objective of this study is to develop a kinetic model from esterification of PFAD with methanol using the prepared catalyst. It is imperative to investigate the performance of reaction kinetic resulted from the catalyst that is prepared from a non-conventional approach. Most of the previous study is mainly developed a kinetic model from transesterification. This kinetic model is also applicable to esterification of high free fatty acid using sulfonated glucose via microwave-heating method. 2. Methodology 2.1 Materials PFAD was supplied by Mewaholeo Industries Sdn. Bhd., Pasir Gudang, Johor and and methanol (95 %) was supplied by Merck chemical company. Materials to synthesis catalyst include D(+)-Glucose, potassium hydroxide (KOH) as well as sulphuric acid (H2SO4), which were bought from J.T. Baker. All chemicals were all analytical grade products, thus no further purification is required. 2.2 Catalyst preparation ICG was produced from heating 20 g of D(+)-glucose powder in microwave. The ICG produced was crushed into powder form. Sulfonated glucose is prepared by heating 4g of ICG with 158.11 mL concentrated H₂SO₄ at 7.5 min, using 400 W of microwave’s power level and stirred at 700 rpm. All parameters were obtained from the result of optimization of the sulfonation process. The mixture was then washed with hot distilled water and filtered until the filtrate turn to a clear solution. The purpose of the washing is to remove the excess sulphate ions and impurities from the precipitate. The moisture of the sulfonated glucose produced was removed by heating inside a microwave for 10 min. Characterizations of the sulfonated glucose catalyst have been described in our previous work (Saimon et al., 2019). 2.3 Esterification of PFAD PFAD was liquefied at 70 °C. The amount of the feedstock was prepared based on the 10:1 molar ratio of methanol and PFAD. The preheated PFAD was poured into a 100 mL three-necks-round-bottom-flask. The reflux condenser was installed onto the neck of the flask mainly to re-condense the evaporated methanol. As shown in Figure 1. The methanol and 2.5 wt% sulfonated glucose catalyst were added into the flask and heated at 65 °C, 75 °C and 85 °C for 2 h. 2.5 wt% of the prepared catalyst is selected based on the previous literature review (Lokman et al., 2015). Then the liquid was poured into a separating funnel and was allowed to settle for around 60 min. The bottom layer containing FAME was collected and the conversion of the PFAD to methyl ester was calculated using Eq(1) and (2): Acid value = N x Mw KOH x Volume of KOH titrate used (mL) / mass of biodiesel (1) where N is normality of KOH and Mw is molecular weight of KOH (56.11 g/mol). FFA conversion (%) = (Acid value of PFAD – Acid value of sample) x 100 / (Acid value of PFAD) (2) Figure 1: Experimental set-up for the esterification reaction. 404 2.4 Kinetic Modelling The activation energy (Ea) can be determined by using Arrhenius Eq(3) as shown below: k = Ae-Ea/RT (3) where A is the frequency factor, R is 8.31 Jmol-1K-1 and T is temperature in K. The Ea can be calculated via integration of the Eq(3) and expressed as Eq(4) and Eq(5) as follow: ln k = ln (Ae-Ea/RT) (4) ln k = -Ea/R (1/T) + ln A (5) Generally, Eq(5) can be defined as y=mx+c thus, Ea can be determined by the slope of the ln k against 1/T graph as shown in Eq(6) below: Slope = Ea/RT (6) Thus, the activation energy can be calculated by Eq(7): Ea = slope x RT (7) Esterification of PFAD with methanol catalyzed by SO3H/ICG to produce FAME and water can be described by the reaction in Eq(8): R1COOH + MeOH ⇄ R1COOCH3 + H2O (8) Eq(8) can be simplified as Eq(9) as follows: A + B ⇄ C + D (9) The rate of reaction for Eq(9) can be described mathematically as Eq(10): -rA =-d[A]/dt = k1 [A]a [B]b – k2[C]c [D]d (10) which -rA is the reaction rate of the PFAD; k1 and k2 are reaction rate constant and equilibrium constant; [A] is the concentration of PFAD; [B] is the concentration of methanol; [C] is concentration of FAME while [D] is concentration of water. The kinetic model is based on several assumptions: (i) concentration of methanol is constant due to the excess of methanol used (ii) the reverse reaction can be neglected due to the excessive methanol (Berrios M, 2007). Thus, [B]b is assumed fixed and k1[B]b can be symbolized as k. The reversible reaction from Eq(9) can be ignored. The rate of reaction after following the assumptions can be written as in Eq(11) and Eq(12) as follow: -rA = -d[A]/dt = k[A] (11) d[A]/[A] = -kdt (12) the integration of Eq(12) will obtain following Eq(13): ln [Ao] / [A] = -kt (13) which [Ao] is the initial concentration of the PFAD and [A] is the concentration of the PFAD at time t. k value is obtained using non-linear regression which basically provides the Sum of Square Quake Error (SSE) between the experimental data and the calculated data as shown in Eq(14) below. SSE (k) = ∑(xexp – xcalc)2 (14) 3. Results and discussion Table 1 tabulates the conversion of PFAD to biodiesel for both experimental and calculated esterification reaction taking place at 10:1 molar ratio of methanol to PFAD; 2.5 wt% catalyst; for 120 min of reaction time. Based on Table 1, highest conversion obtained from the esterification process of PFAD and methanol using the prepared sulfonated incomplete glucose acid catalyst was 95.76 % at 85 °C of reaction temperature. Figure 2 illustrates the experimental and calculated PFAD conversion (%) at 65 °C, 75 °C and 85 °C. Via best fit curve, it can be observed that the percentage FFA conversion increased from between 31-36 % to between 80-100% at the end of the experiment. Extending experiment time would result in 100 % FFA conversion but may not be economically viable due to various cost factor such as power consumption management and 405 others. Hence, it is best to optimize the reaction synergistically through the operating temperature, catalyst mass, catalyst acid strength and other factors. Table 1: Conversion of PFAD (X), experimental and calculated in esterification of PFAD at 10:1 molar ratio of Methanol to PFAD; 2.5 wt% catalyst; for 120 min of reaction time Temp °C 65 75 85 time, min XAexp XAcalc Error2 XAexp XAcalc Error2 XAexp XAcalc Error2 0 33.67 49.0742 0.2093 35.63 52.9575 0.2365 32.33 54.2949 0.4616 12 38.97 51.2552 0.0994 39.55 54.9018 0.1507 41.02 58.9414 0.1909 24 43.75 53.2221 0.0469 43.19 56.7072 0.0980 48.19 62.7752 0.0916 36 49.33 55.5183 0.0157 50.93 60.5463 0.0357 59.61 68.8815 0.0242 48 55.04 57.8680 0.0026 59.31 64.7028 0.0083 66.34 72.4800 0.0086 60 64.66 61.8266 0.0019 69.21 69.6132 0.0000 72.44 75.7417 0.0021 72 68.22 63.2915 0.0052 75.73 72.8471 0.0014 79.52 79.5273 0.0000 84 71.09 64.4725 0.0087 81.66 75.7884 0.0052 83.31 81.5539 0.0004 96 75.49 66.2831 0.0149 85.83 77.8567 0.0086 89.16 84.6819 0.0025 108 78.31 67.4436 0.0193 87.04 78.4568 0.0097 92.31 86.3662 0.0041 120 80.44 68.3201 0.0227 87.43 78.6503 0.0101 95.76 88.2109 0.0062 k= 0.4466 k= 0.5641 k= 0.7922 Figure 2: Experimental and calculated PFAD conversion (%) at 65 °C, 75 °C and 85 °C. Based on Figure 3, the slope obtained is 3,464, thus the activation energy calculated is 28.8 kJ/mol and the collision frequency factor, A is equivalent to 8.124 x 10-5 min-1. The data shows that the increment of temperature will increase the k value as well. Reaction occur when particles collide with each other. If temperature is increased, the particles are frequently move and collide thus speeding up the rate of reaction (Gaurav et al., 2019). This study fits to first order reaction. Talebian-Kiakalaieh (2013) also found the same order of reaction for the forward reaction in trans-esterification of free waste cooking oil (WCO) by heteropoly acid (HPA) catalyst by making similar assumption of high methanol concentration employed in the reaction (Talebian-Kiakalaieh et al., 2013). Figure 3: Graph of –ln k versus 1/T (K-1) for Arrhenius equation of the esterification reaction 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 F F A C o n v e rs io n ( % ) Time (min) 65 °C 75 °C 85 °C y = 3,464x - 9.4181 R² = 0.985 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.00275 0.0028 0.00285 0.0029 0.00295 0.003 -l n k 1/T (K-¹) 406 Table 2 shows the comparison of the kinetic study of esterification from various feedstocks using acid heterogeneous catalyst. Based on the Table 2 below, it can be seen that activation energy of this study is comparable and the lowest among the activation energy from other study. Wang et al. (2019) obtained the lowest activation energy which is 24.7 kJ/mol, however the catalyst prepared took quite a long time since the catalyst is conventionally prepared by sulfonation for 16 h. Shu et al. (2019) manages to obtain Ea value of 32.7 kJ/mol from esterification of oleic acid and F--SO2-4/ MWCNTs catalyst. The catalyst was also prepared in 30 to 60 min at higher temperature which was between 500 to 700 °C. Li et al. (2018) managed to obtain 47.9 kJ/mol of the activation energy, a little bit higher than the rest by using rubber seed oil as the feedstock and SO42−/ZrO2 as the catalyst which was prepared at 550 °C for 4 h. Lieu et al. (2016) however obtained the highest activation energy of 53.7 kJ/mol using Ceiba pentandra seed oil and H2SO4 as a catalyst. The esterification of the seed oil was done in a microwave and obtained 94.43 % of FFA conversion. Mazubert et al. (2014) also study the esterification of waste cooking oil in a microwave using H2SO4 as a catalyst and obtained 37.1 kJ/mol of activation energy. These two studies were proofs that the microwave irradiation is an effective tool for biodiesel pretreatment. However, most of kinetic study using microwave heating method offered within the range of esterification process and not covering the preparation of catalyst using microwave- irradiation thoroughly. Most of the activation energy values in Table 2 are almost analogous with this study, however, the preparation of the catalyst offers significant differences in term of the energy utilization and time consumption. Table 2: Comparison of kinetic study using acid heterogeneous esterification reaction Feedstock Catalyst Reaction order Activation Energy, Ea (kJ/mol) Pre-Exponential factor, A References Oleic Acid Zr-SO3H 1st 24.7 134.4 min-1 (Wang et al., 2019) Oleic Acid F--SO2-4/ MWCNTs 2nd 32.7 1.88 L/mol.h (Shu et al., 2019) Canola + 10 % Palmitic Acid HSiW/Al2O3 2nd 32.9 7.9 x 10-2 mol/g.min (Gaurav et al., 2019) Rubber Seed Oil SO42−/ZrO2 1st 47.9 7.71 x 106 mol/L.min (Li et al., 2018) PFAD Tulsion 42SM 1st 34.0 n/a (Augustia et al., 2018) Ceiba Pentandra Seed Oil H2SO4 2nd 53.7 3.98 x 109 min-1 (Lieu et al., 2016) Waste Cooking Oil H2SO4 - 37.1 1.3 x 106 L/mol/min (Mazubert et al., 2014) PFAD ICG/SO3H 1st 28.8 8.124 x 10-5 min-1 This study 4. Conclusions This study revealed that the sulfonated glucose catalyst prepared via microwave-assisted heating method produced better PFAD conversion to methyl ester as reaction temperature increase. The PFAD conversion obtained was 95.76 % using 2.5 wt% sulfonated glucose catalyst. The esterification of PFAD followed pseudo first-order reaction with R2 value 0.985 and 28.8 kJ/mol of activation energy. The study attained in this work is comparable with others recent investigations. For future work, it is highly recommended to use the pre-exponential factor (A) and activation energy (Ea) obtained from this study to expand the reaction to pilot and commercial scale. This is imperative to evaluate the viability of the reaction on a higher scale and to enable adjustment to be made if necessary. 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