Biodiesel Microfiltration Dynamics During

Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Biobased Materials and Bioenergy Vol. 8, 1–6, 2014

Biodiesel Microfiltration Dynamics During Transesterification of Rapeseed Oil Robinson Muñoz1, Isaac Reyes1 , Gustavo Ciudad1 2 , David Jeison1 2 , and Rodrigo Navia1 2 ∗ 1

Scientific and Technological Bioresources Nucleus, Universidad de La Frontera, P. O. Box 54-D, Temuco, Chile 2 Departament of Chemical Engineering, Universidad de La Frontera, P. O. Box 54-D, Temuco, Chile

The aim of this work was to evaluate fatty acid methyl esters (FAME or biodiesel) microfiltration dynamics during transesterification of rapeseed oil using a ceramic membrane of 0.2 m size pore. Moreover, ternary phase diagrams were studied and applied for glycerol separation. At the beginning, membrane filtration causes TG (Triglycerides) retention, allowing methanol to cross through the membrane. However, a low methanol to oil molar ratio allows TG to cross the membrane. After this stage, the predominant blend is constituted by a one-phase FAME and TG mixture. Finally, glycerol can be completely retained in all the evaluated trials. Regarding FAME/TG/methanol system, immiscibility between TG and FAME is achieved at 73 wt% in FAME, 12 wt% in methanol and 15% wt in TG. This is the critical composition of the non-polar phase for obtaining an efficient FAME/TG separation during transesterification reaction. Regarding TG/glycerol/methanol system, TG retention is not a problem if TG concentration exceeds 10 wt%.

Keywords: Emulsion, Biodiesel, Fatty Acid Methyl Ester, Ceramic Membrane, Refining.

The worldwide production of biofuels rose from 34.4 Mt in 2007 to 51.8 Mt in 2009. The same trend is observed in biodiesel production, increasing from 2.2 Mt in 2002 to 11.1 Mt in 2008, being 80% produced and consumed in Europe.1 Biodiesel or fatty acid methyl ester (FAME) is a renewable and biodegradable biofuel with a neutral CO2 balance compared to fossil fuels. However, its production is associated with high costs mainly related to vegetable oils use which can represent up to 80% of the overall costs.2 In addition, the costs associated with FAME refining to achieve strict quality standards such as ASTMD-6751 and EN-14214 can be also significant. Therefore, important efforts to improve biodiesel separation and refining processes have been continuously performed.3–5 In fact, the presence of impurities may affect FAME quality, decreasing both the calorific value and FAME yield, simultaneously increasing engine corrosion when the biofuel is burned.6 7 To diminish these effects, the removal of water, residual alcohol, glycerol, intermediary products (such as mono- and di-glycerides) and the remaining catalyst is necessary.8 Several methods have been applied to improve FAME purity, being water washing the most used.9 Other methods ∗

Author to whom correspondence should be addressed. Email: [email protected]

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consider FAME distillation, the use of solvents and the neutralization with acid or alkaline compounds.10 The use of water generates a wastewater stream with high organic matter content and an extreme alkaline pH, causing an increase in the production cost due to wastewater treatment.11–14 Recently, the use of membranes has been successfully applied in FAME refining.15–25 This technology allows obtaining a high quality FAME, avoiding both the use of chemicals and a settler to separate glycerol. In FAME refining, both micro and ultrafiltration membranes have been used. In this process ceramic and organic (polymeric) membranes have been tested, and ceramic membranes have demonstrated a higher resistance to temperature and chemicals, being the most used.26–28 Ceramic membranes have a porous size in the range of 0.01 to 0.8 m, allowing an emulsion separation by means of drops retention and continuous phase permeation. This technology has been applied to diminish the use of water in biodiesel refining process, separating the hydrophobic phase (FAME) in the permeate and the hydrophilic phase (glycerol, water and alcohol) in the retentate.17 21 25 One important challenge in FAME production is the possibility of using membrane technology as a continuous process,18 19 29–31 but however, it is still necessary to understand the phenomena that are occurring during the transesterification reaction, where the dynamics of the formed

1556-6560/2014/8/001/006

doi:10.1166/jbmb.2014.1469

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RESEARCH ARTICLE

1. INTRODUCTION

Biodiesel Microfiltration Dynamics During Transesterification of Rapeseed Oil

Muñoz et al.

emulsion observed during the reaction is directly related with FAME separation efficiency.15 Therefore, the aim of this work was to determine the effect of synthetic bicomponent blends and emulsion formation in a biodiesel microfiltration process by using ceramic membranes. In addition, ternary phase diagrams of FAME/triglyceride (TG)/methanol and TG/glycerol/methanol were studied and applied to glycerol separation and FAME refining.

2. MATERIAL AND METHODS

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2.1. Raw Materials For this study rapeseed oil was facilitated by Molino Gorbea Ltda., an oil producing company from Southern Chile. The main fatty acids present in rapeseed oil were C16:0, C:18:0, C:18:1, C18:2 and C18:3. Water content was ≤0.01%, the acid value was 0.8 mg KOH/g oil with a 1.6% FFA. Methanol (99.9 wt%), glycerol (99.0 wt%) and NaOH pellets (used as catalyst) were supplied by Winkler Co. 2.2. FAME Production FAME was produced from rapeseed oil in 2 L Erlenmeyer flasks. As catalyst, a 1% solution of NaOH was used, while methanol was used in a methanol to oil molar ratio of 6:1. Transesterification reaction was performed at 60 C during 1 h and 200 rpm in the reactor shown in Figure 2. After reaction, the blend was maintained in a settler for 12 h to separate FAME from glycerol. The FAME enriched phase (non-polar phase) was washed with water to remove the remaining catalyst, residual glycerol, soaps and methanol. To neutralize FAME, a second washing step with a sulfuric acid solution of 0.002 M was used, until achieving pH 7.0. Water was then removed by heating the non-polar phase at 110 C for 1 h. Finally, FAME purity was determined. 2.3. Effect of Blend Composition in FAME Microfiltration To determine the effect of the different compounds present during transesterification in FAME separation, synthetic bi-component blends were tested. These bicomponent blends were determined and used according to the emulsion dynamics observed during transesterification reaction,15 considering a estequiometric methanol to oil molar ratio of 3:1. During transesterification reaction three periods can be observed (Fig. 1): during the first period, the blend is rich in reactants, during the second period, the blend is rich in reactants and products and during the third period the blend is rich in products. As shown in Table I, different synthetic bi-component blends of FAME-methanol, FAME-TG and FAMEglycerol were investigated regarding these three periods. In addition, two bi-component blends with high glycerol accumulation and methanol excess were also evaluated (TG-glycerol and TG-methanol in Table II). These two 2

Fig. 1. Compounds reaction dynamics during the conversion of oil into FAME. TG: Triglyceride, FAME: Fatty acid methyl ester.

types of extreme conditions have been already reported in the literature. In fact, the first blend represents the high glycerol accumulation as reported by Reyes et al.15 and the methanol excess case has been already observed by Saka and Kusdiana.32 2.4. Experimental Set Up The microfiltration system was specially designed and implemented for this work, where the refining process was carried out using a hydrophilic tubular ceramic membrane made of -Al2 O3 /TiO2 , with an internal diameter of 6 mm, a length of 0.2 m, a filtration area of 0.004 m2 and 0.2 m pore size. The membrane was purchased in Atech Innovations, Gladbeck, Germany. The system was operated by a non-pulsating positive displacement pump (Moyno 500) and the transmembrane pressure was regulated by using a manual valve. Both permeate and retentate were continuously recycled to the reactor to maintain constant the proportion of the bi-component blends. The membrane was operated in a tangential flux mode (Fig. 2). 2.5. Bi-Component Blends Separation Assays were performed varying the concentration of each of the bi-component blends. These experiments were carried out at 55 C, 1 bar of trans-membrane pressure (TMP) and stirring rate of 200 rpm in the reactor (see Fig. 2). To form an emulsion, the bi-component blends were recirculated (container-membrane-container) during 20 min without applying any trans-membrane pressure. Table I. Synthetic bi-component blends evaluated.

Blend Blend Blend Blend

1 2 3 4

FAME [wt%]

TG [wt%]

FAME [wt%]

Methanol [wt%]

Glycerol [wt%]

TG [wt%]

20 40 60 80

80 60 40 20

20 40 60 80

80 60 40 20

20 40 60 80

80 60 40 20

Notes: TG: Triglyceride; FAME: Fatty acid methyl ester.

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Table II. Bi-component blends evaluated for high glycerol accumulation and methanol excess. TG/glycerol blends TG [wt%]

TG/methanol blends

Glycerol [wt%]

TG [wt%]

Methanol [wt%]

80 60 40 20

20 40 60 80

80 60 40 20

20 40 60 80 Note: TG: Triglyceride.

Subsequently, during the separation of the blend by membrane, 1 bar of TMP was used. One mL samples were taken after 5 min (25 minutes in total) of operation from the feed, permeate and retentate. 2.6. Membrane Cleaning After each assay, the membrane system was cleaned using 250 mL of ethanol, recirculating it for 5 min at 40 C. This process allows removing residues, avoiding membrane clogging and so, reproducible results during the different assays were assured.

Fig. 2. Microfiltration membranes.

system for liquid blends using ceramic


2.8. Ternary Phase Equilibrium Diagrams Titration experiments were carried out in vessel flasks (50 mL) using a hot plate and a magnetic stirrer. The total volume was 50 mL, which was distributed in different ratios at 20 C and atmospheric pressure. These ratios were 1:1, 1:2, 1:5, 1:6 for all blends studied. The titrant used for FAME/TG blends was methanol while for glycerol/methanol blend was TG. The titration data obtained was used to build the ternary phase equilibrium diagram using the ProSim Software (ProSim S.A. Labege-France).

3. RESULTS AND DISCUSSION 3.1. Bi-Component Blends Emulsion Behavior During Transesterification and Its Effect on FAME Refining by Ceramic Membranes To determine the effect of emulsion formation during transesterification and its effect on FAME refining by using ceramic membranes, different synthetic bi-component blends were studied. At the beginning of transesterification reaction (period 1 in Fig. 1) in a batch configuration process, reactants presence is predominant (high content of both methanol and TG). The application of membrane filtration causes TG retention, allowing methanol to cross through the membrane (Fig. 3(A)). However, when a low methanol to oil molar ratio is used, this decrease in methanol content allows TG to cross through the membrane (Fig. 3(A)). During the transition stage of transesterification reaction (period 2, Fig. 1), FAME content starts to increase, and therefore, the predominant blend is constituted by FAME and TG. This bi-component blend forms only one phase, not allowing its separation by the application of membrane filtration, causing that permeate and retentate streams show the same composition. The miscibility between FAME and lipids can be explained as TG has a positive charge that interacts with the negative charge of FAME, forming a hydrogen bond.33 TG-glycerol is another important bi-component blend present during transesterification reaction, as glycerol is produced while TG is being consumed. In this case, glycerol was completely retained by the membrane in all the evaluated trials for the bi-component blend TG-glycerol (Fig. 3(B)), showing the same behavior of other previous works.17 21 30 34 35 In the case of the bi-component blend formed by FAME-methanol, as methanol is present in excess in the 3

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2.7. Analytical Procedures FAME and glycerol quantification was carried out using a Clarus 600 chromatograph coupled with a Clarus 500T mass spectrometer from Perkin Elmer (GC-MS). An Elite5 ms capillary column with a length of 30 m, thickness of 0.1 m and internal diameter of 0.25 mm was used. For FAME quantification, the vials were prepared by adding 3 g of the sample to 100 L methyl heptadecanoate used as internal standard (initial concentration of 1,300 mg/L). The following temperature program was used: 50 C for 1 min and then increasing temperature at a rate of 1.1 C/min up to 187 C. Both the injector and detector temperatures were 250 C and He was used as the carrier gas. Glycerol was quantified by using the normative UNEEN 14105. The vials were prepared with 10 mg of sample adding 8 L of 1,2,4-butanotriol dissolved in pyridine as

internal standard. Then, this preparation was dissolved in 8 mL of heptane. The following temperature program was used: 50 C for 1 min, 15 C/min to 180 C, hold 0 min, 7 C/min to 230 C, hold 0 min, 30 C/min to 380 C, hold 8 min. As the bi-component blends were weighed and heated at 70 C during 2 h, the evaporated methanol was measured gravimetrically.

Biodiesel Microfiltration Dynamics During Transesterification of Rapeseed Oil (A)

(C)

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(B)

(D)

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Fig. 3. Separation performance of bi-component blends by microfiltration at 55 C and 1 bar transmembrane pressure (TMP). TG: Triglyceride, FAME: Fatty acid methyl ester.

reaction while FAME is being produced, although an emulsion is formed no clear separation by membrane application was observed (Fig. 3(C)). The repulsive forces between polar components (methanol and glycerol) and non-polar components (TG and FAME) promote the emulsion formation36 allowing an easy separation by membrane application.26 37 However, as FAME has a slight polar behavior, the emulsion is weak and FAME behaves as partially miscible with methanol.21 This blend can be completely miscible at both high temperature and pressure.38 Therefore, inside the membrane module the blend can behave as a one phase blend, explaining the obtained result. So, in this particular case the refining process should be performed at low pressure and low temperature. Finally, at the end of the transesterification reaction (period 3 in Fig. 1) glycerol and FAME predominate, and the best separation efficiency was reached when a 60% of glycerol content was used. By a higher glycerol content, drops tend to coalesce forming a continuous phase, allowing glycerol to cross through the membrane (see Fig. 3(D)). As demonstrated, the different component fractions present in the bi-component blends affect FAME separation efficiency. This fact can be related with blend components that promote a disperse phase formation, decreasing drops coalescence probability for an efficient microfiltration.39 In this sense, Saleh et al.21 improved 4

glycerol retention by water addition, causing an increase in glycerol drops size. According to these results, it can be inferred that avoiding the use of this membrane refining process during the beginning of the transesterification reaction would be the appropriate strategy, due to a low glycerol content, which can cross through the membrane.40–42 In addition, the bi-component blends composed by TG-methanol, glycerol-TG, FAME-methanol and glycerol-FAME may form emulsions and therefore the application of the proposed membrane refining process could be suitable depending on the transesterification reaction conditions, as small amounts of additional components can have large impacts on the solubility of the phases and emulsion stability. 3.2. Ternary Phase Equilibrium Studies 3.2.1. Ternary Phase Diagram of FAME/TG/Methanol System To study the behavior between the different components involved in the transesterification reaction, ternary phase diagrams were studied. Thus, the FAAE/TG/methanol mixture would be representative of the composition expected in the non-polar phase (or permeate), while the TG/glycerol/methanol mixture would be representative of the retentate composition after microfiltration. Ternary phase diagrams of FAME/TG/methanol and TG/glycerol/methanol systems were built and are shown J. Biobased Mater. Bioenergy 8, 1–6, 2014

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3.3. Ternary Phase Diagram of TG/Glycerol/Methanol System The ternary phase diagram of TG/glycerol/methanol system is shown in Figure 5. Considering the differences in

B’

A’ Fig. 4. Ternary phase diagram of the FAME/TG/Me–OH system at 20 C. FAME: Fatty acid methyl ester, TG: Triglyceride, Me–OH: methanol. Shaded area represents the formation of a single homogeneous phase.


A’ B’ Fig. 5. Ternary phase diagram of the TG/Gly/Me–OH system at 20 C. TG: Triglyceride, Gly: Glycerol, Me–OH: methanol. Shaded area represents the formation of a single homogeneous phase.

polarity of the mixture components (methanol and glycerol are polar while TG is non-polar), the system will behave preferably as a heterogeneous one. The non-shaded area shows different ratios where the system forms two phases. A high area indicates the surfactant effect of methanol.17 The alcohol reduces the surface tension between the nonpolar phase and glycerol, maximizing their superficial contact area and causing the formation of small droplets of the disperse phase.17 43 Thus, a decrease in the interfacial tension makes easier to break down the glycerol droplets into smaller ones. However, TG retention by the membrane is not a problem if TG concentration exceeds 10 wt% (nonshaded area in Fig. 5) in comparison to the other components (methanol and glycerol). When TG concentration is less than 10 wt%, the methanol/glycerol mixture forms a homogenous phase in all cases analyzed (shaded area in Fig. 5). Thus, a blend with a TG concentration of less than 10%, glycerol 80% and methanol 10%, under favorable operating conditions may be perfectly permeable through the membrane. This could even increase as methanol is distributed in the glycerol-rich phase and the FAME-rich phase.44 The complete separation of the remaining methanol and not-reacted TG from the products is especially important at the end of the transesterification reaction. In Figure 6 the points A (90/0/10) and B (10/90/0) represent a theoretical condition at the beginning and end of the transesterification, respectively. As shown in the diagram (Fig. 6) during the transition from point A to B or, while TG/methanol react and glycerol is formed, the TG/glycerol/methanol system forms strictly a biphasic emulsion that can be retained by a microfiltration membrane process ensuring free-FAME permeation. In fact, the system studied (Fig. 5) could be useful to explain the permeation possibilities of retentate after a transesterification/membrane refining process, where a low concentration of non-reactive TG, 5

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in Figures 4 and 5, respectively. Each diagram represents the behavior of three components at different proportion in the defined blends. The points on the curve, as well as in the shaded area represent the different ratios where the system begins to form a single homogeneous phase. For mixtures under the curve, the blend behaves as a biphasic system. Looking at Figure 5, when FAME content is 73 wt%, TG is 15 wt% and methanol is 12 wt% (top point of the curve), the blend forms a homogeneous phase. However, at a lower FAME percentage and higher methanol and TG percentage (under this curve) the system forms two phases (Fig. 4). Therefore immiscibility between TG and FAME at the highest FFAE concentration is achieved at 73 wt% in FAME, 12 wt% in methanol and 15% wt in TG. This can be seen as the critical composition in the non-polar phase for obtaining an efficient FAME/TG separation during transesterification reaction at 20 C. The distribution of each component in the different phases formed can be estimated tracing a tie line. In Figure 4, the points A (0/90/10) and B (90/10/0) represent a theoretical condition of the beginning and end of the transesterification reaction, respectively. Thus, for point A no FAME production is assumed while for point B is assumed that methanol is completely reacted. The diagram shows that transesterification begins as a biphasic phase (non-shaded area) and finishes as a single-phase blend (shaded area) when the concentration of FAME in the FAME/TG/methanol system is sufficiently high. This increased miscibility between TG/methanol due to increased FAME has been previously reported by Cheng et al.24


low concentration of remaining methanol, and high glycerol concentration will be present.

4. CONCLUSIONS

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FAME-TG and FAME-methanol blends did not show any significant difference between permeate and retentate, indicating that separation is only feasible when glycerol is present. Regarding FAME/TG/methanol system, immiscibility between TG and FAME is achieved at a concentration of less than 73 wt% FAME and 12 wt% methanol. This is the critical composition for obtaining an efficient FAME/TG separation. Regarding TG/glycerol/methanol system, TG retention by the membrane is not a problem if TG concentration exceeds 10 wt%. When TG concentration is less than 10 wt%, the methanol/glycerol mixture forms a homogenous phase in all cases analyzed. A better understanding of the separation process of FAAE blends can be clarified by means of phase equilibrium diagrams involving the different components of the transesterification reaction. Thus, microfiltration assays of synthetic blends and phase diagrams can be a useful tool to develop a continuous operation strategy of biodiesel refining by microfiltration. Acknowledgments: This work was partially supported by Chilean CONICYT Project 78110106 and FONDECYT Project 1090382.

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Received: xx Xxxx xxxx. Accepted: xx Xxxx xxxx.

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