October 10-11 2013, Bandung, Indonesia
BIODIESEL
BD.09 International Seminar on Biorenewable Resources Utilization for Energy and Chemicals 2013 In conjunction with Chemical Engineering Seminar of SoehadiReksowardojo 2013
A Kinetics Study of Fatty Acid Esterification over Sulfated Zirconia/Zeolite Catalyst for Biodiesel Production Ratna Dewi Kusumaningtyas 1,2, Masduki1, Arif Hidayat1,3, Prima Astuti Handayani2, Rochmadi1, Suryo Purwono1, Arief Budiman1* 1
Department of Chemical Engineering Gadjah Mada University, Yogyakarta 55281 Indonesia 2 Faculty of Engineering Semarang State University, Semarang 50229 Indonesia 3 Faculty of Engineering Islamic University of Indonesia, Yogyakarta 55581 Indonesia Email:
[email protected]
Abstract. Development of a sustainable alternative energy is important today to cope with the current world's energy crisis. Biodiesel is among the promising energy since it is an environmental friendly biofuel and has high cetane number. Biodiesel can be produced via transesterification of the triglyceride or esterification of the free fatty acid (FFA). FFA esterification is the best route when the cheap and low grade feedstocks which contains high FFA, are employed. FFA esterification can be performed in the presence of either homogeneous or heterogeneous bronsted acid catalyst. Homogeneous catalyst is very active, but it exhibits some problems associated with the environmental, corrosion, separation, and by products formation issues. To overcome this problem, heterogeneous catalyst is applied. In this work, esterification of the FFAs in PFAD was performed using sulfated zeolite-zirconium catalyst in a batch reactor. The kinetic of the esterification reaction was also studied. Different kinetic models based on the Eley-Rideal and Langmuir Hinshelwood assumption, which take into account both the ideal and non-ideal solution approach, were proposed. UNIFAC thermodynamic model was employed for the calculation of the non-ideal model. It was revealed that the Eley-Rideal model with non-ideal approach best fits the experimental data. Keywords: Eley-Rideal; esterification; fatty acid; Langmuir Hinshelwood; sulfated zeolite-zirconium.
1
Introduction
Biodiesel is among the attractive alternative energy resources that is currently becoming a fast-growing market product [1]. The rising interest in biodiesel is because of the great advantages that come with it. Biodiesel is environmentally
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friendly and has high cetane number that makes vehicle can perform better. Biodiesel fuel is also produced locally, thus it will be more cost efficient. Traditionally Biodiesel is produced by transesterifying vegetable oils using alkaline catalyst. However, this process is only suitable for feedstocks having free fatty acids (FFAs) less that 1%. When the FFAs amount exceeds this limit, they will react with alkali catalyst to form soap by-products which deactivates the catalyst and inhibits transesterification for biodiesel production. In addition, this unwated soap causes difficulties in the biodiesel purification [2]. Feedstocks containing a small amount of FFAs are usually refined, expensive, and edible vegetable oils. On the other hand, the low-cost biodiesel raw materials, such as non-edible oils and waste cooking oils, are usually high in FFAs. Thus, for the high FFAs feedstocks, biodiesel production should be performed by applying an integrated process which includes a pretreatment step of FFA esterification followed by alkaline transesterification. Furthermore, when the FFAs contain in certain vegetable oils are extremely high, the esterification reaction to convert FFAs into methyl ester becomes the main route of the biodiesel production. Hence, the esterification is a crucial process, both for the pretreatment process prior to the transesterification and as the main process of biodiesel production from high FFAs vegetable oils [3, 4]. Esterification of FFAs can be performed in the presence of either homogeneous or heterogeneous brΓΈnsted and lewis acid catalyst. Currently, the common-used catalysts in biodiesel production at industrial scale are the conventional homogeneous mineral acids such as sulfuric acid. This catalyst is very active, but it exhibits some problems associated with the environmental and corrosion issues, as well as the high quantity of by-products formed. It is also difficult to recycle. The further drawbacks of the homogenous catalyst is the fact that the reaction dealing with the complicated and costly product separation [5, 6, 7]. In order to overcome the disadvantages of homogenous acid catalysis, several methods have been proposed. Many researchers has worked on utilization of enzyme catalysts. Meanwhile, the application of a catalyst-free production method using supercritical alcohols has also been tested [2]. However, these methods are limited due to the high cost and unstable activity of enzyme, and the unfavorable reaction conditions of high temperature and pressure required in the supercritical method [8]. As alternative, the use of heterogeneous (solid) acid catalysts might have a greater potential for a large-scale production. Solid acid catalysis is favorable as it offers significant advantages of eliminating corrosion, toxicity and environmental problems. The other benefit of using solid acid catalysts is the easy separation of the catalyst from the product stream since
A Kinetics Study of Fatty Acid Esterification over Sulfated Zeolite-Zirconium Catalyst for Biodiesel Production 3 they do not mix with the biodiesel product. They are also superior in terms of recyclability and reusability, therefore, have attracted considerable attention [9]. The appropriate solid acid catalyst for biodiesel preparation should have high stability, good catalytic activity, abundant acid sites, large pores to ensure the diffusion of the large fatty acid molecules, hydrophobic surface and low cost. Among the excellent solid acid catalyst for the FFAs esterification is sulfated zirconia (SO2-/ ZrO2). This catalyst is classified as heterogeneous super-acid catalyst with strong acid properties [10]. It exhibits both active Bronsted and Lewis acidic sites and shows a higher acid strength than the other heterogeneous acid catalysts. In addition, sulfated zirconia has been intensively examined and found potential for various esterification reactions, for instance esterification of oleic acid with methanol, myristic acid in rapeseed oil, oleic acid in soybean oil, and palmitic acid [11, 12, 13, 14]. However, it has a drawbacks associated with the expensive price since zirconium is a rare and costly metal. To overcome this problem, this precious metal catalyst is impregnated on an inexpensive support. Preparing the noble catalyst on the support is important to provide a highly active catalyst with sufficient thermal and mechanical stability at a lower cost. Material for the support should be a highly stable inorganic support and also has an appropriate interaction with zirconia, allowing a good dispersion of the catalyst species on the support. Among the proper material for the support is zeolite. Zeolites are microporous crystalline solids with well-defined structures, containing silicon, aluminum and oxygen in their framework and cations. As catalysts, zeolites exhibit appreciable acid activity with shape selective features. Zeolite has also been studied for several fatty acid esterifications [13, 15]. Hence, zeolite has a benefit as a catalytically active support. In this work, esterification of FFAs in Palm Fatty Acid Distillate (PFAD) has been performed over sulfated zirconia catalyst supported on natural zeolite. This type of catalyst is appropriate for the FFAs esterification in various reactor systems, from the conventional batch ones to the more sophisticated configurations. Thus, for process development, kinetics study of this reaction is essential. A reliable model is needed to accurately represent the kinetic behavior of such reacting systems [16]. In this study, kinetics of FFA esterification in PFAD in the presence of sulfated zirconia/natural zeolite catalyst has been investigated. Several kinetic models were proposed to explain the phenomenon of reaction and determine the corresponding kinetic parameters. The best model was determined by a computer simulation utilizing Matlab 7.
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Experimental
2. 1. Material Natural zeolite was obtained from Wonosari, Gunung Kidul, Indonesia. Zirconium (IV) oxychloride octahydrate (ZrOCl2.8H2O), Amonium sulfat (NH4)2SO4, and 99.99% of methanol were from Merck. Palm Fatty Acid Distillate with FFA content of 82.5% (w/w) obtained from PT. Sinar Mas Tbk.
2. 2. Synthesis of The Catalyst Prior to the main process of the catalyst synthesis, zeolite was primarily activated by heating it in the oven at the temperature of 100oC. Then, it was weighed for 50 gr and mixed with 0.5 M of H2SO4. Subsequently, it was stirred for 12 hour at room temperature, filtered and washed by using aquadest. In another place, 5 gr of ZrOCl2.8H2O was dissolved in aquadest and stirred for 15 minute. Afterwards, the prepared zeolite was added into the solution and stirred for 24 hour. It was then evaporated in the oven with the temperature of 105oC to attain it completely dry. Finally, the sample was mixed with (NH4)2SO4 with the mass ratio of 2:5, and left for 18 hours and now the sulfated zirconia on natural zeolite support was obtained. To end the process, sulfated zirconia/zeolite was calcinated at the temperature of 400oC for 4 hours.
2. 3. The Esterification Reaction The reaction was carried out in a 500 ml three-neck flask batch reactors equipped with magnetic stirrer and reflux condenser as illustrated in Figure 1. Initially, 50 g of PFAD was melted and introduced into the flask along with methanol in a certain ratio. The solution was then heated and the sample was immediately taken to measure its acidity, which was considered as the initial acidity. When the desired temperature was reached, the catalyst was afterwards added into the mixture and the sample was taken right away to measure the acidity that was considered as time of zero. Subsequently, samples were withdrawn periodically every 10 minutes. Reaction conversions were estimated from the FFA content of the medium by NaOH titration [17 ]. The acidity was estimated by the following equation: π=
π π‘ππ‘π .πππ΄ .πΆπ‘ππ‘π π π πππππ .1000
And the conversion was calculated by the equation as follows:
(1)
A Kinetics Study of Fatty Acid Esterification over Sulfated Zeolite-Zirconium Catalyst for Biodiesel Production 5 π₯π΄ =
π π βπ π‘ ππ
π₯ 100%
(2)
Notes: 1. Three neck flask 2. Magnetic stirrer 3. Oil bath
5
4. Magnet 5. Condenser 6. Thermometer
6 1 3 4 4 2 4 4
Figure 1. Batch reactor apparatus set
2. 4. Kinetic Study Kinetics study of the FFA esterification was conducted through a simulation taking into account adsorption-based heterogeneous kinetics models, with ideal and non-ideal solution approach, utilizing Matlab 7. The simulation results were compared with the experimental for validation. The nonideality behavior of the system was represented by the activity coefficients. The values of these activity coefficient were determined using a predictive model UNIFAC.
3.
Result and Discussion
3. 1. Kinetics Models The rate of a heterogeneous reaction is related to external/internal diffusion, adsorption/desorption, and surface reaction. Various models have been
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proposed to express the kinetic behaviors of the heterogeneous esterification. The simplest model is quasi-homogeneous (Q-H) model, which is in the same form as the power-law model [16]. In this model, the adsorption and desorption of reactants and products are neglected [7]. However, when the adsorption of reacting species is significant, the adsorption effects need to be considered in the derivation of the kinetic model. The popular adsorption-based kinetics approach are the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) models. L-H Model can describe the kinetic data rooted in the assumption that both reactants are addsorbed. However, if a model derived on the basis of the assumption that one of the adsorbed reactants is reacting with another in the bulk fluid, then it can be described by the E-R models. For this condition, there are possibly three basic different reaction mechanisms. First basic model, adsorbed fatty acid is reacting with adsorbed and the rate-limiting step is the surface reaction between the adsorbed molecules. Second basic model, the rate-determining step is the surface reaction between the adsorbed fatty acid with the alcohol in the bulk fluid. The third basic model states that the overall reaction rate is determined by the surface reaction between fatty acid in the bulk fluid, and third with adsorbed alcohol [16]. Because of the strong water affinity of zeolite, the activity of water in the solid catalyst phase, where the reaction occurs, may markedly differ from that in the liquid phase. It is introduced in the form of an empirical exponent to the activity of water in the rate expression. By taking both the ideal and the non-ideal solution assumption into consideration, these there basic models can be derived into numerous different kinetics model. In this work, the five different adsorption-based models were applied to evaluate the kinetic behaviors of the heterogeneous esterification reaction of fatty acid in PFAD with methanol over sulfated zirconia catalyst supported on natural zeolite. The Quasi-Homogeneous models were also evaluated as benchmarking kinetics models. The rate expression for those seven kinetics models are given as follow: Model 1: E-RNIDS Model This Model is based on the Eley Rideal (E-R) Model with Non-Ideal-Solution Assumption [16]. It is assumed that that the methanol acid has strong interaction with the sulfate group of the sulfated zirconia catalyst. Hence, it is regarded as an adsorbed species at the catalyst surface. Furthermore, it is also considered that the zeolite support interacts strongly with water. Therefore, water can be regarded as an adsorbed species too. On the other hand, the possibilities of ester and fatty acid with the sulfate group are much weaker, therefore can be neglected. The reaction occurred between the adsorbed methanol with fatty acid
A Kinetics Study of Fatty Acid Esterification over Sulfated Zeolite-Zirconium Catalyst for Biodiesel Production 7 in bulk. The Rate Expression: βππ¨ =
ππ β ππ¨ ππ© β ππ β ππ ππ« π+π²π,π« ππ«
(3)
The temperature dependence of the kinetic constant is fitted with the Arrhenius equation: ππ = π¨π ππ±π© π¨
ππ = π¨π πππ π
βπ¬π,π
(4)
πΉπ» βπ πΉπ»
(5)
On the other hand, the temperature dependent of the adsorption equilibrium constant are fitted with: π²π,π =
π¨π π¨π
πππ
βπ―π πΉπ»
(6)
Model 2: E-RIDS Model This Model is developed based on the E-R model with Ideal-Solution assumption [18]. The Rate Expression is then written as: βππ¨ =
πͺ πͺ ππ (πͺπ¨ πͺπ©β πͺ π« ) π²
π+π²π,π¨ ππ¨ +π²π,π« ππ«
(7)
Model 3: L-HNIDS Model This Model is based on L-H with Non-Ideal-Solution Assumption [16]. The Rate Expression: βππ¨ =
ππ β ππ¨ ππ© β ππ β ππ ππ« π+π²π,π« ππ«
π
(8)
Model 4: L-H-H-WNIDS Model This Model is a modification of Langmuir-Hinshelwood model called Langmuir-Hinshelwood-Hougen-Watson (LHHW) with Non β Ideal - Solution assumption. This model assumes that surface reaction is the controlling steps with the other steps remaining in equilibrium. The adsorption behavior of the compounds of the reaction mixture in the surface of the catalyst. To develop the adsoption-based kinetic model, only the components having the strongest adsorption are considered. In this case, the most polar molecules (water and methanol) are considered to have the strongest adsorption strengths on the
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catalyst surface, so the adsorption of fatty acid and methyl ester can be disregarded. Thus the simplified rate expression can be written as follows [19]: βππ¨ = ππ
π π ππ¨ ππ© β πͺ π« π²
π+π²π,π© ππ© +π²π,π« ππ«
π
(9)
MODEL 5: Modified L-H-H-WNIDS Model This Model is quite similar to the Model 5 (LHHW with Non β Ideal - Solution Assumption) but it is modified to be: βππ¨ = ππ
π π ππ¨ ππ© β πͺ π« π²
π+π²π,π© ππ© +π²π,π« ππ«
(10)
Model 6: Q-HIDS Model This is the Quasi-Homogeneous Reaction Model with Ideal β Solution Assumption [16]. The Rate Expression: βππ¨ = ππ β ππ¨ ππ© β ππ β ππͺ ππ«
(11)
Model 7: Q-HNIDS Model This is the Quasi-Homogeneous Reaction Model with Non-IdealβSolution Assumption [16]. The Rate Expression: βππ¨ = ππ β ππ¨ ππ© β ππ β ππͺ ππ«
(12)
3. 2. Estimation of Activity Coefficient The activity coefficients estimation in non-ideal system mixtures can be calculated by the method that is based on the interaction between the molecules in the mixture or the prediction using the contributions of interactions between functional groups. In this paper, the activity coefficients were determined using UNIFAC method. This method can be done by assembling the pure component from individual groups and assessing the contributions of their interactions. UNIFAC uses the functional groups present on the molecules that make up the liquid mixture to calculate activity coefficients. By utilizing interactions for each of the functional groups present on the molecules, as well as some binary interaction coefficients, the activity of each of the solutions can be calculated. UNIFAC was reported to be accurate to predict the system non-linearity [7]. This thermodynamic method has been found precise to predict both the LiquidLiquid Equilibrium (LLE) and Vapor-Liquid Equilibrium (VLE) of the biodiesel as well as fatty acid systems [20, 21, 22].
A Kinetics Study of Fatty Acid Esterification over Sulfated Zeolite-Zirconium Catalyst for Biodiesel Production 9
3. 3. Simulation and Experimental Results The kinetics of fatty acid esterification in PFAD with methanol over sulfated zirconia was studied at the temperature of 323, 328, 333, and 338 K. In the modeling and simulation, fatty acid was represented by oleic acid as the dominant compound in the feedstock. The activity coefficients of the reactants and products used in this equations were determined using UNIFAC model. To calculated the UNIFAC activity coefficient in this system, the four component present in the system were divided into the following sub-groups: oleic acid (1 CH3, 14 CH2, 2 CH, and 1 COOH); methanol (1CH3 and 1 OH); ethyl oleate (2 CH3, 14 CH2, 2 CH, and 1 COO -); and water (1 H2O). Experimental data fitted to the 7 rate expressions were tested using Matlab application software. The aims of this work was to arrive at the reaction mechanism which best described the kinetics of this heterogeneous system. It could be evaluated by minimizing the Mean Squared Errors (MSE) between calculated values of rate (xcalc) obtained using Matlab and the experimental data (xexp) as shown below: π΄πΊπ¬ =
π π΅
π΅
( πΏπ¨,ππππ β πΏπ¨,πππ )π
(13)
The best kinetic model would provide a good agreement between the simulation result and experimental data. It was observed based on the MSE values given by each model. This comparison were also graphically shown in Figure 2-8.
Figure 2. Comparison of simulation and experimental data (Model 1 - ERNIDS)
Figure 2 and 3 are the comparison between the experimental data and the simulation results of E-R based model. Meanwhile, the simulation results based
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on the L-H concepts are demonstrated in Figure 4, 5 and 6. For benchmarking, the results of the Quasi-Homogeneous models are disclosed in Figure 7 and 8.
Figure 3. Comparison of simulation and experimental data (Model 2 - ERIDS)
Figure 4. Comparison of simulation and experimental data (Model 3 - LHNIDS)
A Kinetics Study of Fatty Acid Esterification over Sulfated Zeolite-Zirconium Catalyst for Biodiesel Production 11
Figure 5. Comparison of simulation and experimental data (Model 4 -LHHWNIDS)
Figure 6. Comparison of simulation and experimental data (Model 5 β modified LHHWNIDS)
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Figure 7. Comparison of simulation and experimental data (Model 6 β QHIDS)
Figure 8. Comparison of simulation and experimental data (Model 7 β QHNIDS)
The data-fitting was also performed to obtain the model parameters. The parameters obtained for each model are exhibited in Table 1. The most suitable model for describing the reaction kinetics provided reasonable values of the parameters with the least MSE. Table 1. Parameters Obtained for Each Model Model Model 1 Model 2 Model 3 Model 4
Af Mol/min/kg 8.20 x 109 3.98 x 104 3.20 x 1012 2.16 x 1011
Eo,f kJ/ mol 7.90 x 104 3.68 x 104 9.51 x 104 9.07 x 104
Af/ Ar 6.62 x 10-2 2.17 x 10-4 4.28 x 104 -
βh kJ/ mol -3.34 x 103 -1.75 x 104 3.95 x 103 -
MSE 3.98 x 10-2 0.108 5.07 x 10-2 0.424
A Kinetics Study of Fatty Acid Esterification over Sulfated Zeolite-Zirconium Catalyst for Biodiesel Production 13 Model 5 Model 6 Model 7
1.85 x 1018 1.11 x 1012 2.55 x 1015
1.36 x 104 8.69 x 104 1.14 x 105
2.33 x 107 8.29 x 109
5.13 x 104 7.30 x 104
0.363 9.07 x 10-2 9.32 x 10-2
Compared with the other adsorptive-based kinetics models, the E-R and L-H models with non ideal assumption revealed much better results, which were closed to the experimental data. It indicated that methanol was adsorbed much more strongly on the active site of the catalyst than the other compounds in the system. It was due to the strong affinity of catalyst with sulfate group in sulfated zirconia catalyst and zeolite support. Besides, the small size of the methanol molecule was easier to be adsorbed in the pore of the catalyst than the FFAs. Based on the calculation of MSE values, it was found that the model giving the least error was the E-R model with Non-Ideal-Solution Assumption (Model 1), with MSE of 3.98 x 10-2. Therefore, among the models proposed, Model 1 was selected as the best one. The rate equation was expressed as: βππ¨ =
π.ππ π± πππ ππ±π©
βπ.ππ π± πππ πΉπ»
ππ¨ ππ© β π.ππ π± ππβπ πππ
π+π.ππ π± πππ πππ
βπ.ππ π± πππ πΉπ»
ππ«
βπ.ππ π± πππ πΉπ»
ππ ππ«
(14)
For the wide range of the temperatures, the value of πΎπ ,π· may significantly change with the change of the reaction temperature. Hence, the rate equation was stated in a formula in which πΎπ ,π· was considered as a temperature dependent constant. The selected model in this work (ER model with non-ideal approach) was based on a controlling step of surface reaction between adsorbed methanol with fatty acid in bulk liquid, forming methyl ester and water. UNIFAC was successfully employed to account for the non-ideal thermodynamic behavior of the system.
4.
Conclusion
The kinetic behavior for the esterification of fatty acid in PFAD with methanol over sulfated zirconia/ zeolite at temperatures from 323 to 338K and at molar feed ratios of 10 has been investigated experimentally in a batch reactor. The E-R model with Ideal and Non- Ideal-Solution Assumption, L-H model with Non-Ideal-Solution Assumption, LHHW and modified LHHW model with Non β Ideal - Solution Assumption, as well as Q-H Model with Ideal and Non Ideal β Solution Assumption, have been applied to correlate the kinetics data. E-R model with non-ideal solution assumption appears to represent the kinetics behavior of this FFA esterification over wide range of reaction temperature.
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Acknowledgement
The authors gratefully acknowledge the financial support from the Indonesian Directorate of Higher Education (DIKTI)
6.
Nomenclature
π = Activity A, B, C, D = Fatty acid, methanol, methyl ester, and water, respectively π¨π = Arrhenius preexponential factor of the forward reaction, (mol/ min/kg) π¨π = Arrhenius preexponential factor of the reverse reaction, (mol/ min/ kg) π¬π,π = Activation energy of the forward reaction (kJ/ mol) βπ = Molar heat of the reaction, kJ/ mol βπ―π = Enthalpy of Adsorption, (kJ/ mol) ππ = Kinetic costant for forward reaction, mol/kg/min ππ = Kinetic costant for reverse reaction π²π,π = Constant for previous equation π²π = Adsorption constant of a certain component N = Total number of experiments performed πΉ = Gas constant, kJ/mol/ K π» = Temperature, K π = Mole fraction of the component πΏπ¨ = Conversion of fatty acid Subscript A, B, C, D = Fatty acid, methanol, methyl ester, and water, respectively exp = experiment calc = calculation
7. [1] [2]
[3]
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