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Biodiesel production in a membrane reactor using MCM-41 supported solid acid catalyst
Bioresource Technology 159 (2014) 286–291
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Biodiesel production in a membrane reactor using MCM-41 supported solid acid catalyst Wei Xu, Lijing Gao, Songcheng Wang, Guomin Xiao ⇑ School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China
h i g h l i g h t s Solid acid catalysis and membrane separation was coupled for biodiesel production. PTSA/MCM-41 was used as a catalyst for biodiesel production in a membrane reactor. Box–Behnken design was employed to analyze and optimize the process.
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 26 February 2014 Accepted 1 March 2014 Available online 12 March 2014 Keywords: Biodiesel Membrane reactor Transesterification Solid acid catalyst
a b s t r a c t Production of biodiesel from the transesterification between soybean oil and methanol was conducted in this study by a membrane reactor, in which ceramic membrane was packed with MCM-41 supported p-toluenesulfonic acid (PTSA). Box–Behnken design and response surface methodology (RSM) were used to investigate the effects of reaction temperature, catalyst amount and circulation velocity on the yield of biodiesel. A reduced cubic model was developed to navigate the design space. Reaction temperature was found to have most significant effect on the biodiesel yield while the interaction of catalyst amount and circulation velocity have minor effect on it. 80 °C of reaction temperature, 0.27 g/cm3 of catalyst amount and 4.15 mL/min of circulation velocity were proved to be the optimum conditions to achieve the highest biodiesel yield. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The environmental and society crisis including air pollution and limitation of crude oil resources has encouraged the development of alternative energy (Omraei et al., 2013). Fatty acid methyl ester (FAME), commonly known as biodiesel, is a renewable, biodegradable and non-petroleum fuel that can alleviate the crisis caused by overuse of petroleum-based diesel. Biodiesel is conventionally produced through the catalytic or non-catalytic transesterification process between vegetable oil/animal fat and methanol (Ng et al., 2010; Olutoye and Hameed, 2013; Tuli et al., 2011). Since the transesterification downstream processes will obtain a mixture of biodiesel, glycerol, remaining reactant and catalyst. The purification of the biodiesel product is large expenses, which will further increase its cost and limit its utilization and popularization when the raw materials are fixed (Atadashi et al., 2011; Shi et al., 2013). More recently, process intensification device have been used to increase the yield of biodiesel and decrease the cost ⇑ Corresponding author. Tel./fax: +86 25 52090612. E-mail address: [email protected] (G. Xiao). http://dx.doi.org/10.1016/j.biortech.2014.03.004 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
of it. Especially, reactive separation technologies, such as reactive distillation (Noshadi et al., 2012), reactive absorption (Kiss and Bildea, 2011), reactive extraction (Shuit et al., 2010) and membrane reactors (Cao et al., 2008a), offer practical and exciting approach for industrial production of biodiesel. Among them, membrane reaction process for simultaneous transesterification and membrane separation has attracted attention for its ability of providing high quality biodiesel as well as remarkable biodiesel yield (Kiss and Bildea, 2012; Shuit et al., 2012). Some researchers believe that the membrane reactor can overcome the equilibrium limitation by continuous removal of the products (Cao et al., 2009). In addition, the porous structure of the ceramic membrane also obtains high mass-transfer between the immiscible phases (Kiss and Bildea, 2012). The principle of membrane reactor operation for biodiesel production can be explained that (Dube et al., 2007; Falahati and Tremblay, 2012): FAME is miscible in methanol at normal temperatures for transesterification, while methanol and vegetable oil phases are immiscible. As a consequence, the mixture in phase equilibrium will form a binary system, denoted as methanol rich phase and oil rich phase. Since the oil rich phase exists in the form
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of an emulsion due to the various surface forces when the amount of methanol and the mixing degree are enough. The oil droplets are retained by the membrane, while the biodiesel will pass though the membrane along with the methanol rich phase. Based on this principle, membrane reactor is particularly useful in getting biodiesel product without any unreacted oil and shifting the reaction equilibrium to the product side for both acid/base-catalysis processes (He et al., 2012). However, most of the catalysts used in relevant literatures are homogeneous ones, such as NaOH or H2SO4 (Cao et al., 2008b; Falahati and Tremblay, 2012; Tremblay et al., 2008), which cannot avoid the inherent disadvantages of homogenous catalysis. Moreover, the catalyst may dissolve in the permeate stream and directly affect the quality of biodiesel. A simple approach to this problem is the utilization of heterogeneous catalyst. To date, such solid alkaline as KOH/activated carbon (Baroutian et al., 2011) and KF/hydrotalcite (Xu et al., 2013) have been used in the membrane reaction process for biodiesel production. The utilization of heterogeneous catalyst is proved to be useful and efficient for obtaining high quality biodiesel with high yield. And an efficient and stable heterogeneous catalyst will make the membrane process cleaner and more environmentally friendly. P-toluenesulfonic (PTSA) is found to be an effective acid catalyst for esterification or transesterification reaction (de Jong et al., 2009; Guan et al., 2009), and MCM-41 presents significant advantages as a catalyst support due to its high specific surface areas, large pore size and high thermal stability. In addition, the integrating of solid acid catalyzed transesterification and membrane separation has not been reported by other researchers. So, in this study, the main aim was to integrate the solid acid catalyzed transesterification and membrane separation in a membrane reactor and then optimize the methanolysis of soybean oil. For this purpose, microfiltration membrane was used as separation media and PTSA/ MCM-41 was used as a catalyst. Response surface methodology (RSM) coupled with Box–Behnken design was utilized to analyze and optimize the process. 2. Methods
The PTSA/MCM-41 was prepared by the following steps: immersing MCM-41 into 0.2 mol/L PTSA solution with a ratio of 15 mL/g for 24 h. Then the mixture was stirred at 80 °C to evaporate the water, and then dried at 140 °C overnight to obtain the powder PTSA/MCM-41. The powder PTSA/MCM-41 was preformed using tablet compressing machine and then grounded into 20–40 mesh. 2.2. Membrane reaction process for biodiesel production The schematic diagram of laboratory scale membrane reactor for biodiesel production was shown in Fig. 1. The length, inner diameter and outer diameter of the ceramic membrane tube were 200, 6 and 10 mm, respectively. Certain amount of catalyst was packed into single channel ceramic membrane tube and fixed in the reactor, and then the reactor was heated to the fixed temperature. The trans-membrane pressure was kept at 80 kPa. Typically, 30 g of soybean oil were initially mixed with methanol (molar ratio of oil to methanol was 1:24) in the mixing vessel. The reactant mixture was preheated and then continuously charged into the reactor by the circulating pump. The retentate stream, which consists of oil, FAME, methanol and glycerol, was returned to the feed flask. While the permeate stream containing FAME, methanol and glycerol, was collected in a vessel equipped with a methanol recycle unit, in which methanol was distilled and then returned to the feed flask. After a period of time (typically 3 h), the products in the product vessel were transferred into a separating funnel to separate biodiesel from glycerol. The mass of the product was measured to calculate the yield of biodiesel. After each run, the system was fully drained and then flushed for 10 min with methanol. The contents of biodiesel in the samples were analyzed by the gas chromatograph (Ouhua GC 9160) equipped with a DB-5Ht capillary column (15 m 0.25 mm 0.25 mm) and a flame ionization detector (FID). Nitrogen was used as the carrier gas and the injector and detector temperature were both 360 °C. In this work, the yield of biodiesel was calculated based on the biodiesel collected in the product vessel as follows:
Biodiesel yield ð%Þ ¼
2.1. Chemicals and catalyst preparation
mass of biodiesel in the product vessel initial mass of oil in the mixing vessel ð1Þ
PTSA (AR) and methanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial food grade soybean oil was bought from local market. MCM-41 was purchased from Catalyst Plant of Nankai University, Tianjin. Ceramic membrane was manufactured by Nanjing Aiyuqi Membrane Technology Co., Ltd. Tablet compressing machine (FY-40) was purchased from Tianjin Sichuang Science and Technology Development Co., Ltd.
2.3. Experimental design In this study, Design Expert software Version 8.0 (Stat-Ease Inc., USA) was used to design the experiments. Box–Behnken method was used for the statistical design of experiments and data analysis. The independent parameters were selected as reaction temper-
Fig. 1. Schematic diagram of membrane reactor for biodiesel production.
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ature, catalyst amount and circulation velocity and they were studied at three levels: low (1), medium (0) and high (+1). The response was biodiesel yield within the reaction time. The general form of equation fitted to the quadratic response surface model was: 3 3 X 3 3 X X X Y ¼ b0 þ bi xi þ bij xi xj þ bii x2i i¼1
i¼1 j¼iþ1
transformation is not required to improve the model. The quadratic models to predict the biodiesel yield in the fixed-bed membrane reactor in terms of coded and actual factors are as below: Equation in the terms of coded factors
Biodiesel yield ð%Þ ¼ þ69:5 þ 10:92 A þ 5:53 B þ 3:39 C þ 1:65 A B þ 3:8 A C
ð2Þ
3:96 A2 2:94 B2 10:39 C2
i¼1
where Y was the predicted response, xi and xj were independent variables, b0 was the offset term, while bi, bij and bii was the linear, quadratic and interaction coefficient, respectively. The number of replicates was chosen as three to provide a relatively stable standard error of prediction. The ranges and designate levels of the variables in this study were listed in Table 1. Selection of levels of reaction temperature was based on the conventional reaction temperature of transesterifications using PTSA/MCM-41 catalyst. The levels of circulation velocity were selected according to the capacity of the circulation pump and the catalytic efficient of the catalyst. A relatively higher circulation velocity (lower resistance time) was selected in order to keep the system within two-phase area (Falahati and Tremblay, 2012).
þ 3:57 A2 B
ð3Þ
Equation in the terms of actual factors
Biodiesel yieldð%Þ ¼ 612:9755 þ 14:876 Reaction temperature þ 2010 Catalyst amount þ 55:7325 Circulation velocity 53:77778 Reaction temperature Catalyst amount þ 0:38 Reaction temperature Circulation velocity 0:11112 Reaction temperature2 362:65432 Catalyst amount2 10:3875 2
3. Results and discussion
Circulation velocity þ 0:39722 Reaction temperature2 Catalyst amount ð4Þ
3.1. Experimental design and analysis of variance The results of the experimental design matrix and biodiesel yield are shown in Table 1. A quadratic model fitted to the experimental results is presented assisted by the Design Expert software. Eqs. (3) and (4) show the quadratic models to predict the biodiesel yield, where A, B and C represent the reaction temperature (°C), catalyst amount (g/cm3), and circulation velocity (mL/min), respectively. Suggested by the software, reaction temperature, catalyst amount, circulation velocity, the quadratic of the three independent parameters and the interaction of reaction temperature-circulation velocity are significant items. However, the lack of fitness for the model is undesired and cannot be improved by removing the insignificant terms. Therefore, the quadratic model is modified by adding the interactive term of square reaction temperature and catalyst amount (A2B), which makes a reduced cubic model for the model. In addition, the insignificant item BC (interaction between catalyst amount and circulation velocity) is eliminated while AB (interaction between reaction temperature and catalyst amount) cannot be eliminated otherwise the hierarchy of the model will be disarrayed. Diagnostics of the residuals indicate that
The actual values for the biodiesel yield versus the predicted ones calculated from the model equation are shown in Fig. 2. The model used to evaluate the biodiesel yield fits the experimental data well with a correlation coefficient R2 = 0.9933, which indicates the interpretation is correct. The statistical analysis of variance (ANOVA) is performed by the software and shown in Table 2. Term of ‘‘lack of fit’’ reflects the consistence between predict model and experimental data, while ‘‘adequate precision’’ reflects the signal to noise ratio. Values of p-value (probability of error value) less than 0.05 indicate model terms are significant, while values greater than 0.1 indicate the model terms are not significant. As is shown in the table, the p-value of the model is less than 0.05, which demonstrates that the model is highly significant and suitable in predicting the biodiesel yield. P-value of ‘‘lack of fit’’ is 0.3592, implying the model is fitted to all data. An ‘‘adequate precision’’ of 30.273 indicates an adequate signal, which means this model can be used to navigate the design space. The coefficient of determination (R2) calculated to be 0.99 for the response, which also implies that 99% of the experimental data confirm compatibility with the data predicted by the model.
Table 1 Experimental design and experimental results. Reaction temperature (°C)
Catalyst amount (g/mL)
Circulation velocity (mL/min)
Run
Actual
Coded
Actual
Coded
Actual
Coded
9 1 14 2 7 10 3 8 5 4 12 15 6 11 13
70 60 70 80 60 70 60 80 60 80 70 70 80 70 70
0 1 0 1 1 0 1 1 1 1 0 0 1 0 0
0.09 0.09 0.18 0.09 0.18 0.27 0.27 0.18 0.18 0.27 0.27 0.18 0.18 0.09 0.18
1 1 0 1 0 1 1 0 0 1 1 0 0 1 0
2.8 3.8 3.8 3.8 4.8 2.8 3.8 4.8 2.8 3.8 4.8 3.8 2.8 4.8 3.8
1 0 0 0 1 1 0 1 1 0 1 0 1 1 0
Yield (%)
47.5 45.3 70.9 61.7 42.6 57.8 60.2 74.2 43.7 83.2 65.6 68.3 60.1 53.8 69.3
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Fig. 2. Comparison of biodiesel yield between predicted and measured values.
Table 2 Analysis of variance (ANOVA) for the model. Source
P-value, probability > F
Model A B C AB AC A2 B2 C2 A2B Lack of fit R2 Adjusted R2 Predicted R2 Adequate precision
<0.0001, significant <0.0001 0.0011 0.0021 0.0999 0.0056 0.0056 0.0183 <0.0001 0.0272 0.3592, not significant 0.99 0.98 0.81 30.273
3.2. Influence of the parameters on the biodiesel yield Three-dimension surface plots of the predicted biodiesel yield are illustrated in Fig. 3. Based on the model analysis, it can be observed that reaction temperature and catalyst amount have positive effect on the biodiesel yield. This is also represented in the Fig. 3(a): the yield of biodiesel presents an increase trend with the increase of reaction temperature and catalyst amount. However, at higher circulation velocities, a reduction of biodiesel yield can be observed because the quadratic term of circulation velocity is more significant with a negative effect (Eq. (3)). This trend can be observed in Fig. 3(b): biodiesel yield increases initially and then decreases with the increase of circulation velocity. The reason for this phenomenon may be that the mixing intensity causes the increase of oil conversion when the circulation velocity increases initially, while the reduction in residence time and the decrease in permeate stream when the circulation velocity increases continuously lead to the reduction of the biodiesel yield. Fig. 4 shows the contour plots for the predicted biodiesel yield in the studied range of the parameters. Each contour indicates the combination of two parameters when the third parameter is fixed at its center point, where the same biodiesel yield is obtained. 3.3. Optimization of the operating conditions The optimization of biodiesel yield is conducted both graphically and numerically. The optimum conditions in this study mean
Fig. 3. (a) 3-D surface plot of biodiesel yield versus catalyst amount and reaction temperature at circulation velocity of 4.15 mL/min. (b) 3-D surface plot of biodiesel yield versus circulation velocity and reaction temperature at catalyst amount of 0.27 g/cm3.
parameters with respectively high and low limits to achieve the highest biodiesel yield. Assisted by the software, the optimal values of the parameters are a reaction temperature of 80 °C, a catalyst amount of 0.27 g/mL and a circulation velocity of 4.15 mL/ min. A biodiesel yield of 85.5% is achieved under the optimum conditions. The biodiesel yield is tested under these predicted optimum conditions, an error of 1.72% between predicted and measured values of the optimum biodiesel yield shows a reasonable accuracy. Thus, it can be concluded that the generated model has sufficient accuracy to predict the biodiesel yield. 3.4. Catalyst reusability and comparison with other catalysts The stability of the catalysts is investigated by repeatedly using the catalyst. After each reaction cycle, the catalyst is taken out of the reactor and dried for the repeated use. All experiments are performed under the optimum conditions calculated from the model. PTSA/MCM-41 catalyst in this study is compared with other catalysts for biodiesel production. The results are shown in Table 3. It can be seen that PTSA obtains a relatively lower biodiesel yield, although the reaction is pseudo-homogeneous system and conducted in a stirred vessel reactor (Guan et al., 2009). The catalytic efficiency of PTSA/MCM-41 in the membrane reactor is lower than
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Fig. 5. Biodiesel yield under different circulation time.
Table 4 Comparison of the soybean oil FAME and EN14214. Properties
Units
Measured value
EN14214
Density at 15 °C Easter content Flash point Cloud point Water Viscosity at 40 °C Cetane number Acid number Sulfur Sulfated ash content
kg/m3 % °C °C mg/kg mm2/s – mg KOH/g mg/kg % (mass/mass)
867 99 154 11 200 4.3 53 0.3 0.36 0.012
860 900 >96.5 >101 – <500 3.5 5 >51 <0.5 <10 0.02
the increase of reaction time. And this increasing tendency slows down with the increase of reaction time, since the oil concentration decreases and the reaction rate slows down gradually. In addition, the reaction obtains a biodiesel yield of 79.1% with a reaction time of 3 h, which is in reasonable agreement with the value calculated from the model (80%). When ceramic membranes of different pore sizes are used in the membrane reactor, it is found that all the membranes (pore sizes: 0.05 0.8 lm) used in this study are suitable for membrane reaction system for biodiesel production. This is in agreement with the lower drop size limit and the research of Cao et al. (Cao et al., 2007). In addition, the biodiesel yields are almost the same with the same reaction time (3 h), which means the membrane pore size does not have significant effect on biodiesel yield.
Fig. 4. (a) Contour plot of biodiesel yield versus catalyst amount and reaction temperature at circulation velocity of 4.15 mL/min. (b) Contour plot of biodiesel yield versus circulation velocity and reaction temperature at catalyst amount of 0.27 g/cm3.
that of solid alkaline due to the difference of catalytic activity for transesterification between acid and alkali catalyst. It also can be seen that the biodiesel yield remains 91.7% of the fresh catalyst when the PTSA/MCM-41 used for a third time, and this value is within the range of reported by other researches (Baroutian et al., 2011; Xie and Yang, 2012; Xu et al., 2013). 3.5. Influence of reaction time and membrane pore size Fig. 5 shows the biodiesel yield with different reaction times under the condition of 75 °C of reaction temperature, 0.27 g/mL of catalyst amount and 4.1 mL/min of circulation velocity. It can be seen from the figure that the biodiesel yield increases with
3.6. Biodiesel characterization The properties of the biodiesel produced under optimal conditions is characterized and compared with EN14214 standard. As
Table 3 Catalytic efficacy and reusability comparison between the catalysts.
a
Catalyst
Reactor
Biodiesel yield (fresh) (%)
Repeated run number
Remained catalytic activity (%)a
References
PTSA/MCM-41 PTSA KOH/activated carbon KF/hydrotalcite WO3/AlPO4
Membrane reactor Autoclave reactor Membrane reactor Membrane reactor Autoclave reactor
84.1 66.3 93.2 91 72.5
3 – 3 3 5
91.7 – 89.3 93.4 90.2
This work Guan et al. (2009) Baroutian et al. (2011) Xu et al. (2013) Xie and Yang (2012)
Based on the activity of fresh catalyst.
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presented in Table 4, the biodiesel produced in this study meets the standard limits.
4. Conclusion A fixed-bed membrane reactor packed with PTSA/MCM-41 catalyst was successfully employed for biodiesel production, in which solid acid catalyzed transesterification and membrane separation were integrated to obtain high quality biodiesel. Reaction temperature, catalyst amount and circulation velocity were found to be significant to the process according to the analysis of RSM based on Box–Behnken design. The highest biodiesel yield of 84.1% was obtained at 80 °C of reaction temperature, 0.27 g/mL of catalyst amount and 4.15 mL/min of circulation velocity with a trans-membrane pressure of 80 kPa. Acknowledgements The paper was financially supported by National Natural Science Foundation of China (Nos. 21076044 and 21276050), National Development Program for Basic Research of China (973 Program, No. 2010CB732206). References Atadashi, I.M., Aroua, M.K., Aziz, A.R.A., Sulaiman, N.M.N., 2011. Membrane biodiesel production and refining technology: a critical review. Renew. Sust. Energ. Rev. 15, 5051–5062. Baroutian, S., Aroua, M.K., Raman, A.A.A., Sulaiman, N.M.N., 2011. A packed bed membrane reactor for production of biodiesel using activated carbon supported catalyst. Bioresour. Technol. 102, 1095–1102. Cao, P.G., Tremblay, A.Y., Dube, M.A., Morse, K., 2007. Effect of membrane pore size on the performance of a membrane reactor for biodiesel production. Ind. Eng. Chem. Res. 46, 52–58. Cao, P.G., Dube, M.A., Tremblay, A.Y., 2008a. High-purity fatty acid methyl ester production from canola, soybean, palm and yellow grease lipids by means of a membrane reactor. Biomass Bioenergy 32, 1028–1036. Cao, P.G., Dube, M.A., Tremblay, A.Y., 2008b. Methanol recycling in the production of biodiesel in a membrane reactor. Fuel 87, 825–833. Cao, P.G., Tremblay, A.Y., Dube, M.A., 2009. Kinetics of canola oil transesterification in a membrane reactor. Ind. Eng. Chem. Res. 48, 2533–2541.
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Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Biodiesel production in a membrane reactor using MCM-41 supported solid acid catalyst Wei Xu, Lijing Gao, Songcheng Wang, Guomin Xiao ⇑ School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China
h i g h l i g h t s Solid acid catalysis and membrane separation was coupled for biodiesel production. PTSA/MCM-41 was used as a catalyst for biodiesel production in a membrane reactor. Box–Behnken design was employed to analyze and optimize the process.
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 26 February 2014 Accepted 1 March 2014 Available online 12 March 2014 Keywords: Biodiesel Membrane reactor Transesterification Solid acid catalyst
a b s t r a c t Production of biodiesel from the transesterification between soybean oil and methanol was conducted in this study by a membrane reactor, in which ceramic membrane was packed with MCM-41 supported p-toluenesulfonic acid (PTSA). Box–Behnken design and response surface methodology (RSM) were used to investigate the effects of reaction temperature, catalyst amount and circulation velocity on the yield of biodiesel. A reduced cubic model was developed to navigate the design space. Reaction temperature was found to have most significant effect on the biodiesel yield while the interaction of catalyst amount and circulation velocity have minor effect on it. 80 °C of reaction temperature, 0.27 g/cm3 of catalyst amount and 4.15 mL/min of circulation velocity were proved to be the optimum conditions to achieve the highest biodiesel yield. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The environmental and society crisis including air pollution and limitation of crude oil resources has encouraged the development of alternative energy (Omraei et al., 2013). Fatty acid methyl ester (FAME), commonly known as biodiesel, is a renewable, biodegradable and non-petroleum fuel that can alleviate the crisis caused by overuse of petroleum-based diesel. Biodiesel is conventionally produced through the catalytic or non-catalytic transesterification process between vegetable oil/animal fat and methanol (Ng et al., 2010; Olutoye and Hameed, 2013; Tuli et al., 2011). Since the transesterification downstream processes will obtain a mixture of biodiesel, glycerol, remaining reactant and catalyst. The purification of the biodiesel product is large expenses, which will further increase its cost and limit its utilization and popularization when the raw materials are fixed (Atadashi et al., 2011; Shi et al., 2013). More recently, process intensification device have been used to increase the yield of biodiesel and decrease the cost ⇑ Corresponding author. Tel./fax: +86 25 52090612. E-mail address: [email protected] (G. Xiao). http://dx.doi.org/10.1016/j.biortech.2014.03.004 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
of it. Especially, reactive separation technologies, such as reactive distillation (Noshadi et al., 2012), reactive absorption (Kiss and Bildea, 2011), reactive extraction (Shuit et al., 2010) and membrane reactors (Cao et al., 2008a), offer practical and exciting approach for industrial production of biodiesel. Among them, membrane reaction process for simultaneous transesterification and membrane separation has attracted attention for its ability of providing high quality biodiesel as well as remarkable biodiesel yield (Kiss and Bildea, 2012; Shuit et al., 2012). Some researchers believe that the membrane reactor can overcome the equilibrium limitation by continuous removal of the products (Cao et al., 2009). In addition, the porous structure of the ceramic membrane also obtains high mass-transfer between the immiscible phases (Kiss and Bildea, 2012). The principle of membrane reactor operation for biodiesel production can be explained that (Dube et al., 2007; Falahati and Tremblay, 2012): FAME is miscible in methanol at normal temperatures for transesterification, while methanol and vegetable oil phases are immiscible. As a consequence, the mixture in phase equilibrium will form a binary system, denoted as methanol rich phase and oil rich phase. Since the oil rich phase exists in the form
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of an emulsion due to the various surface forces when the amount of methanol and the mixing degree are enough. The oil droplets are retained by the membrane, while the biodiesel will pass though the membrane along with the methanol rich phase. Based on this principle, membrane reactor is particularly useful in getting biodiesel product without any unreacted oil and shifting the reaction equilibrium to the product side for both acid/base-catalysis processes (He et al., 2012). However, most of the catalysts used in relevant literatures are homogeneous ones, such as NaOH or H2SO4 (Cao et al., 2008b; Falahati and Tremblay, 2012; Tremblay et al., 2008), which cannot avoid the inherent disadvantages of homogenous catalysis. Moreover, the catalyst may dissolve in the permeate stream and directly affect the quality of biodiesel. A simple approach to this problem is the utilization of heterogeneous catalyst. To date, such solid alkaline as KOH/activated carbon (Baroutian et al., 2011) and KF/hydrotalcite (Xu et al., 2013) have been used in the membrane reaction process for biodiesel production. The utilization of heterogeneous catalyst is proved to be useful and efficient for obtaining high quality biodiesel with high yield. And an efficient and stable heterogeneous catalyst will make the membrane process cleaner and more environmentally friendly. P-toluenesulfonic (PTSA) is found to be an effective acid catalyst for esterification or transesterification reaction (de Jong et al., 2009; Guan et al., 2009), and MCM-41 presents significant advantages as a catalyst support due to its high specific surface areas, large pore size and high thermal stability. In addition, the integrating of solid acid catalyzed transesterification and membrane separation has not been reported by other researchers. So, in this study, the main aim was to integrate the solid acid catalyzed transesterification and membrane separation in a membrane reactor and then optimize the methanolysis of soybean oil. For this purpose, microfiltration membrane was used as separation media and PTSA/ MCM-41 was used as a catalyst. Response surface methodology (RSM) coupled with Box–Behnken design was utilized to analyze and optimize the process. 2. Methods
The PTSA/MCM-41 was prepared by the following steps: immersing MCM-41 into 0.2 mol/L PTSA solution with a ratio of 15 mL/g for 24 h. Then the mixture was stirred at 80 °C to evaporate the water, and then dried at 140 °C overnight to obtain the powder PTSA/MCM-41. The powder PTSA/MCM-41 was preformed using tablet compressing machine and then grounded into 20–40 mesh. 2.2. Membrane reaction process for biodiesel production The schematic diagram of laboratory scale membrane reactor for biodiesel production was shown in Fig. 1. The length, inner diameter and outer diameter of the ceramic membrane tube were 200, 6 and 10 mm, respectively. Certain amount of catalyst was packed into single channel ceramic membrane tube and fixed in the reactor, and then the reactor was heated to the fixed temperature. The trans-membrane pressure was kept at 80 kPa. Typically, 30 g of soybean oil were initially mixed with methanol (molar ratio of oil to methanol was 1:24) in the mixing vessel. The reactant mixture was preheated and then continuously charged into the reactor by the circulating pump. The retentate stream, which consists of oil, FAME, methanol and glycerol, was returned to the feed flask. While the permeate stream containing FAME, methanol and glycerol, was collected in a vessel equipped with a methanol recycle unit, in which methanol was distilled and then returned to the feed flask. After a period of time (typically 3 h), the products in the product vessel were transferred into a separating funnel to separate biodiesel from glycerol. The mass of the product was measured to calculate the yield of biodiesel. After each run, the system was fully drained and then flushed for 10 min with methanol. The contents of biodiesel in the samples were analyzed by the gas chromatograph (Ouhua GC 9160) equipped with a DB-5Ht capillary column (15 m 0.25 mm 0.25 mm) and a flame ionization detector (FID). Nitrogen was used as the carrier gas and the injector and detector temperature were both 360 °C. In this work, the yield of biodiesel was calculated based on the biodiesel collected in the product vessel as follows:
Biodiesel yield ð%Þ ¼
2.1. Chemicals and catalyst preparation
mass of biodiesel in the product vessel initial mass of oil in the mixing vessel ð1Þ
PTSA (AR) and methanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial food grade soybean oil was bought from local market. MCM-41 was purchased from Catalyst Plant of Nankai University, Tianjin. Ceramic membrane was manufactured by Nanjing Aiyuqi Membrane Technology Co., Ltd. Tablet compressing machine (FY-40) was purchased from Tianjin Sichuang Science and Technology Development Co., Ltd.
2.3. Experimental design In this study, Design Expert software Version 8.0 (Stat-Ease Inc., USA) was used to design the experiments. Box–Behnken method was used for the statistical design of experiments and data analysis. The independent parameters were selected as reaction temper-
Fig. 1. Schematic diagram of membrane reactor for biodiesel production.
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ature, catalyst amount and circulation velocity and they were studied at three levels: low (1), medium (0) and high (+1). The response was biodiesel yield within the reaction time. The general form of equation fitted to the quadratic response surface model was: 3 3 X 3 3 X X X Y ¼ b0 þ bi xi þ bij xi xj þ bii x2i i¼1
i¼1 j¼iþ1
transformation is not required to improve the model. The quadratic models to predict the biodiesel yield in the fixed-bed membrane reactor in terms of coded and actual factors are as below: Equation in the terms of coded factors
Biodiesel yield ð%Þ ¼ þ69:5 þ 10:92 A þ 5:53 B þ 3:39 C þ 1:65 A B þ 3:8 A C
ð2Þ
3:96 A2 2:94 B2 10:39 C2
i¼1
where Y was the predicted response, xi and xj were independent variables, b0 was the offset term, while bi, bij and bii was the linear, quadratic and interaction coefficient, respectively. The number of replicates was chosen as three to provide a relatively stable standard error of prediction. The ranges and designate levels of the variables in this study were listed in Table 1. Selection of levels of reaction temperature was based on the conventional reaction temperature of transesterifications using PTSA/MCM-41 catalyst. The levels of circulation velocity were selected according to the capacity of the circulation pump and the catalytic efficient of the catalyst. A relatively higher circulation velocity (lower resistance time) was selected in order to keep the system within two-phase area (Falahati and Tremblay, 2012).
þ 3:57 A2 B
ð3Þ
Equation in the terms of actual factors
Biodiesel yieldð%Þ ¼ 612:9755 þ 14:876 Reaction temperature þ 2010 Catalyst amount þ 55:7325 Circulation velocity 53:77778 Reaction temperature Catalyst amount þ 0:38 Reaction temperature Circulation velocity 0:11112 Reaction temperature2 362:65432 Catalyst amount2 10:3875 2
3. Results and discussion
Circulation velocity þ 0:39722 Reaction temperature2 Catalyst amount ð4Þ
3.1. Experimental design and analysis of variance The results of the experimental design matrix and biodiesel yield are shown in Table 1. A quadratic model fitted to the experimental results is presented assisted by the Design Expert software. Eqs. (3) and (4) show the quadratic models to predict the biodiesel yield, where A, B and C represent the reaction temperature (°C), catalyst amount (g/cm3), and circulation velocity (mL/min), respectively. Suggested by the software, reaction temperature, catalyst amount, circulation velocity, the quadratic of the three independent parameters and the interaction of reaction temperature-circulation velocity are significant items. However, the lack of fitness for the model is undesired and cannot be improved by removing the insignificant terms. Therefore, the quadratic model is modified by adding the interactive term of square reaction temperature and catalyst amount (A2B), which makes a reduced cubic model for the model. In addition, the insignificant item BC (interaction between catalyst amount and circulation velocity) is eliminated while AB (interaction between reaction temperature and catalyst amount) cannot be eliminated otherwise the hierarchy of the model will be disarrayed. Diagnostics of the residuals indicate that
The actual values for the biodiesel yield versus the predicted ones calculated from the model equation are shown in Fig. 2. The model used to evaluate the biodiesel yield fits the experimental data well with a correlation coefficient R2 = 0.9933, which indicates the interpretation is correct. The statistical analysis of variance (ANOVA) is performed by the software and shown in Table 2. Term of ‘‘lack of fit’’ reflects the consistence between predict model and experimental data, while ‘‘adequate precision’’ reflects the signal to noise ratio. Values of p-value (probability of error value) less than 0.05 indicate model terms are significant, while values greater than 0.1 indicate the model terms are not significant. As is shown in the table, the p-value of the model is less than 0.05, which demonstrates that the model is highly significant and suitable in predicting the biodiesel yield. P-value of ‘‘lack of fit’’ is 0.3592, implying the model is fitted to all data. An ‘‘adequate precision’’ of 30.273 indicates an adequate signal, which means this model can be used to navigate the design space. The coefficient of determination (R2) calculated to be 0.99 for the response, which also implies that 99% of the experimental data confirm compatibility with the data predicted by the model.
Table 1 Experimental design and experimental results. Reaction temperature (°C)
Catalyst amount (g/mL)
Circulation velocity (mL/min)
Run
Actual
Coded
Actual
Coded
Actual
Coded
9 1 14 2 7 10 3 8 5 4 12 15 6 11 13
70 60 70 80 60 70 60 80 60 80 70 70 80 70 70
0 1 0 1 1 0 1 1 1 1 0 0 1 0 0
0.09 0.09 0.18 0.09 0.18 0.27 0.27 0.18 0.18 0.27 0.27 0.18 0.18 0.09 0.18
1 1 0 1 0 1 1 0 0 1 1 0 0 1 0
2.8 3.8 3.8 3.8 4.8 2.8 3.8 4.8 2.8 3.8 4.8 3.8 2.8 4.8 3.8
1 0 0 0 1 1 0 1 1 0 1 0 1 1 0
Yield (%)
47.5 45.3 70.9 61.7 42.6 57.8 60.2 74.2 43.7 83.2 65.6 68.3 60.1 53.8 69.3
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Fig. 2. Comparison of biodiesel yield between predicted and measured values.
Table 2 Analysis of variance (ANOVA) for the model. Source
P-value, probability > F
Model A B C AB AC A2 B2 C2 A2B Lack of fit R2 Adjusted R2 Predicted R2 Adequate precision
<0.0001, significant <0.0001 0.0011 0.0021 0.0999 0.0056 0.0056 0.0183 <0.0001 0.0272 0.3592, not significant 0.99 0.98 0.81 30.273
3.2. Influence of the parameters on the biodiesel yield Three-dimension surface plots of the predicted biodiesel yield are illustrated in Fig. 3. Based on the model analysis, it can be observed that reaction temperature and catalyst amount have positive effect on the biodiesel yield. This is also represented in the Fig. 3(a): the yield of biodiesel presents an increase trend with the increase of reaction temperature and catalyst amount. However, at higher circulation velocities, a reduction of biodiesel yield can be observed because the quadratic term of circulation velocity is more significant with a negative effect (Eq. (3)). This trend can be observed in Fig. 3(b): biodiesel yield increases initially and then decreases with the increase of circulation velocity. The reason for this phenomenon may be that the mixing intensity causes the increase of oil conversion when the circulation velocity increases initially, while the reduction in residence time and the decrease in permeate stream when the circulation velocity increases continuously lead to the reduction of the biodiesel yield. Fig. 4 shows the contour plots for the predicted biodiesel yield in the studied range of the parameters. Each contour indicates the combination of two parameters when the third parameter is fixed at its center point, where the same biodiesel yield is obtained. 3.3. Optimization of the operating conditions The optimization of biodiesel yield is conducted both graphically and numerically. The optimum conditions in this study mean
Fig. 3. (a) 3-D surface plot of biodiesel yield versus catalyst amount and reaction temperature at circulation velocity of 4.15 mL/min. (b) 3-D surface plot of biodiesel yield versus circulation velocity and reaction temperature at catalyst amount of 0.27 g/cm3.
parameters with respectively high and low limits to achieve the highest biodiesel yield. Assisted by the software, the optimal values of the parameters are a reaction temperature of 80 °C, a catalyst amount of 0.27 g/mL and a circulation velocity of 4.15 mL/ min. A biodiesel yield of 85.5% is achieved under the optimum conditions. The biodiesel yield is tested under these predicted optimum conditions, an error of 1.72% between predicted and measured values of the optimum biodiesel yield shows a reasonable accuracy. Thus, it can be concluded that the generated model has sufficient accuracy to predict the biodiesel yield. 3.4. Catalyst reusability and comparison with other catalysts The stability of the catalysts is investigated by repeatedly using the catalyst. After each reaction cycle, the catalyst is taken out of the reactor and dried for the repeated use. All experiments are performed under the optimum conditions calculated from the model. PTSA/MCM-41 catalyst in this study is compared with other catalysts for biodiesel production. The results are shown in Table 3. It can be seen that PTSA obtains a relatively lower biodiesel yield, although the reaction is pseudo-homogeneous system and conducted in a stirred vessel reactor (Guan et al., 2009). The catalytic efficiency of PTSA/MCM-41 in the membrane reactor is lower than
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Fig. 5. Biodiesel yield under different circulation time.
Table 4 Comparison of the soybean oil FAME and EN14214. Properties
Units
Measured value
EN14214
Density at 15 °C Easter content Flash point Cloud point Water Viscosity at 40 °C Cetane number Acid number Sulfur Sulfated ash content
kg/m3 % °C °C mg/kg mm2/s – mg KOH/g mg/kg % (mass/mass)
867 99 154 11 200 4.3 53 0.3 0.36 0.012
860 900 >96.5 >101 – <500 3.5 5 >51 <0.5 <10 0.02
the increase of reaction time. And this increasing tendency slows down with the increase of reaction time, since the oil concentration decreases and the reaction rate slows down gradually. In addition, the reaction obtains a biodiesel yield of 79.1% with a reaction time of 3 h, which is in reasonable agreement with the value calculated from the model (80%). When ceramic membranes of different pore sizes are used in the membrane reactor, it is found that all the membranes (pore sizes: 0.05 0.8 lm) used in this study are suitable for membrane reaction system for biodiesel production. This is in agreement with the lower drop size limit and the research of Cao et al. (Cao et al., 2007). In addition, the biodiesel yields are almost the same with the same reaction time (3 h), which means the membrane pore size does not have significant effect on biodiesel yield.
Fig. 4. (a) Contour plot of biodiesel yield versus catalyst amount and reaction temperature at circulation velocity of 4.15 mL/min. (b) Contour plot of biodiesel yield versus circulation velocity and reaction temperature at catalyst amount of 0.27 g/cm3.
that of solid alkaline due to the difference of catalytic activity for transesterification between acid and alkali catalyst. It also can be seen that the biodiesel yield remains 91.7% of the fresh catalyst when the PTSA/MCM-41 used for a third time, and this value is within the range of reported by other researches (Baroutian et al., 2011; Xie and Yang, 2012; Xu et al., 2013). 3.5. Influence of reaction time and membrane pore size Fig. 5 shows the biodiesel yield with different reaction times under the condition of 75 °C of reaction temperature, 0.27 g/mL of catalyst amount and 4.1 mL/min of circulation velocity. It can be seen from the figure that the biodiesel yield increases with
3.6. Biodiesel characterization The properties of the biodiesel produced under optimal conditions is characterized and compared with EN14214 standard. As
Table 3 Catalytic efficacy and reusability comparison between the catalysts.
a
Catalyst
Reactor
Biodiesel yield (fresh) (%)
Repeated run number
Remained catalytic activity (%)a
References
PTSA/MCM-41 PTSA KOH/activated carbon KF/hydrotalcite WO3/AlPO4
Membrane reactor Autoclave reactor Membrane reactor Membrane reactor Autoclave reactor
84.1 66.3 93.2 91 72.5
3 – 3 3 5
91.7 – 89.3 93.4 90.2
This work Guan et al. (2009) Baroutian et al. (2011) Xu et al. (2013) Xie and Yang (2012)
Based on the activity of fresh catalyst.
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presented in Table 4, the biodiesel produced in this study meets the standard limits.
4. Conclusion A fixed-bed membrane reactor packed with PTSA/MCM-41 catalyst was successfully employed for biodiesel production, in which solid acid catalyzed transesterification and membrane separation were integrated to obtain high quality biodiesel. Reaction temperature, catalyst amount and circulation velocity were found to be significant to the process according to the analysis of RSM based on Box–Behnken design. The highest biodiesel yield of 84.1% was obtained at 80 °C of reaction temperature, 0.27 g/mL of catalyst amount and 4.15 mL/min of circulation velocity with a trans-membrane pressure of 80 kPa. Acknowledgements The paper was financially supported by National Natural Science Foundation of China (Nos. 21076044 and 21276050), National Development Program for Basic Research of China (973 Program, No. 2010CB732206). References Atadashi, I.M., Aroua, M.K., Aziz, A.R.A., Sulaiman, N.M.N., 2011. Membrane biodiesel production and refining technology: a critical review. Renew. Sust. Energ. Rev. 15, 5051–5062. Baroutian, S., Aroua, M.K., Raman, A.A.A., Sulaiman, N.M.N., 2011. A packed bed membrane reactor for production of biodiesel using activated carbon supported catalyst. Bioresour. Technol. 102, 1095–1102. Cao, P.G., Tremblay, A.Y., Dube, M.A., Morse, K., 2007. Effect of membrane pore size on the performance of a membrane reactor for biodiesel production. Ind. Eng. Chem. Res. 46, 52–58. Cao, P.G., Dube, M.A., Tremblay, A.Y., 2008a. High-purity fatty acid methyl ester production from canola, soybean, palm and yellow grease lipids by means of a membrane reactor. Biomass Bioenergy 32, 1028–1036. Cao, P.G., Dube, M.A., Tremblay, A.Y., 2008b. Methanol recycling in the production of biodiesel in a membrane reactor. Fuel 87, 825–833. Cao, P.G., Tremblay, A.Y., Dube, M.A., 2009. Kinetics of canola oil transesterification in a membrane reactor. Ind. Eng. Chem. Res. 48, 2533–2541.
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