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Supercritical carbon dioxide-mediated esterification in a microfluidic reactor
Chemical Engineering & Processing: Process Intensification 123 (2018) 168–173
Contents lists available at ScienceDirect
Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Supercritical carbon dioxide-mediated esterification in a microfluidic reactor
T
⁎
Armando T. Quitaina,b, , Elaine G. Missionc, Yoshifumi Sumigawac, Mitsuru Sasakid a
Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan International Research Organization for Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan c Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan d Institute of Pulsed Power Science, Kumamoto University, Kumamoto 860-8555, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Microfluidic device Supercritical carbon dioxide Biodiesel Free fatty acid
The immiscibility of vegetable oil and alcohol during the initial step in biodiesel synthesis has slowed down the process resulting to increase production costs. Thus, process intensification techniques that would enhance mixing or molecular interactions of the reactants, and enable short reaction time should be carried out in order to increase yield and minimize costs. We proposed coupling of two process intensification schemes in enhancing molecular interactions, i.e. the use of supercritical carbon dioxide as a solvent and microfluidic device, as applied to esterification of oleic acid (OA) with methanol (MeOH). We found this superior over the commonly applied microwave. Hence, we further explored the influences of reaction temperature, OA-to-MeOH molar ratio, residence time, water contents, pressure and catalyst addition on the oleic acid methyl ester (OAME) yield. Results showed that at temperature range of 60 to 120 °C and pressure of 10 MPa, the esterification reaction proceeds even without any catalyst in less than 1 min. Addition of relatively small amount of catalyst (0.1 wt% H2SO4) dramatically increased the yield 4-fold to 90%. Recirculation of the product even without a catalyst significantly increased yield. The experimental analysis confirms the applicability potential of the microfluidic reactor in the non-catalytic biodiesel synthesis.
1. Introduction With the impending fossil fuel depletion, massive researches have focused in developing sustainable alternatives such as biodiesel (BDF) for internal combustion engines. BDF has attracted intense attention as it is a renewable, biodegradable and carbon neutral alternative for petroleum [1–3]. Recently, waste and non-edible oils were evaluated as suitable feedstock for BDF synthesis, which would enable BDF pricing market competitive [4–6]. Since waste oils contain significant amounts of free fatty acids, esterification process has been developed as another route for BDF synthesis. In this process, fatty acids react with alcohol to produce alkyl esters in the presence of acid catalyst. The fatty acids and alcohol are immiscible, hence there is a high mass transfer resistance (or low collision coefficient) between the two reacting molecules at the beginning of the esterification process. This is heightened as homogeneous acids, i.e. sulfuric acid, intended to catalyze the reaction preferentially co-locates in the alcohol phase [7,8]. As a result, the overall reaction rate is slowed down resulting to higher production costs [9]. With such a challenge, process technologies which are energy efficient, environment-friendly and with low residence time and higher ⁎
molecular collision should be carried out [10]. To address the immiscibility problems, we proposed the use of environmentally benign supercritical carbon dioxide (hereby abbreviated as SCCO2) as a solvent for reaction. Both reactants, methanol (hereby abbreviated as MeOH) and oleic acid (hereby abbreviated as OA), are known to be soluble in SCCO2 even at low pressure conditions [11,12]. Besides, unlike the commonly used organic solvent such as hexane, separation of the products can be easily carried out by simple depressurization steps. For enhancement of reaction rates, two of the promising process intensification techniques related to improving molecular collision and interactions are microwave irradiation (MW) and the use of microfluidic device. MW has attracted interest in accelerating reactions since heating is produced within the system by the molecular motion of absorbing polar molecules. This is known as the reversed heating phenomena, which increases the chance for reacting molecules to collide and interact [13,14]. The collision of molecules are also thought to contribute to the reaction, and is considered as non-thermal effect of MW irradiation [15]. On the other hand, a microfluidic reactor has an internal diameter of a few millimeters and which allow for continuous-flow processes. Processing in microscale enhances heat and
Corresponding author at: Faculty of Advanced Science and Technology ,Kumamoto University, Kumamoto 860-8555 Japan. E-mail address: [email protected] (A.T. Quitain).
https://doi.org/10.1016/j.cep.2017.11.002 Received 12 February 2017; Received in revised form 17 October 2017; Accepted 4 November 2017 Available online 09 November 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 123 (2018) 168–173
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molecular collision due to short diffusion paths and large surface areas, thus, making it easy to operate and control even with sudden temperature changes [16]. With these characteristics, the use of a microfluidic reactors could bring about development of more energy efficient and cost-effective processes. For industrial applications, instead of scaling up, numbering up of microfluidic reactors to meet the required throughput can simplify the rigorous conventional process design methodology. These devices have been applied in the direct synthesis of hydrogen peroxide from hydrogen and oxygen [17]; lignin oxidation [18]; and in the direct conversion of syngas to dimethyl ether [19] at a safer, cleaner and more efficient way. Furthermore, coupling these devices with supercritical fluid (SCF) further intensifies the process since SCF can easily penetrate the sample matrix and induce miscibility [20]. Thus, performing supercritical fluid-based processes in a microfluidic device has a great potential for process intensification. In this work, preliminary experiments were carried out to compare the application of two process intensification technologies, namely MW irradiation and microfluidic reactor for the synthesis of oleic acid methyl ester (OAME) from OA and MeOH. The microfluidic device was complemented with the use of environmentally benign SCCO2 to further intensify the process. Due to the superiority of the latter approach, various process variables were investigated to increase the OAME yield.
Stirring was provided using magnetic bar. After setting up the apparatus, the mixture was stirred for a few minutes, then MW was irradiated at 200 W for the specified duration. The reaction temperature was maintained between 60 and 70 °C. After each reaction, the mixture was cooled down to room temperature and collected for GC-FID analysis. Similarly, a mixture of the OA and MeOH prepared at specified molar ratios were used for microfluidic reactions. The mixture was pumped into the reactor at a constant flowrate (0.02–0.1 mL/min) and with the same flowrate as CO2. Experiments on the synthesis of OAME was carried out at a pressure of 10 MPa and temperatures of 60 to 120 °C. Samples were immediately collected at the outlet and also subjected to GC-FID analysis. 2.3. GC-FID analysis of reaction products Gas chromatographic method with flame ionization detection (GCFID) was used to analyze the reaction products, especially for the presence of OAME. Components separation was carried out in a capillary column CP-SIL 8CB-MS (GL Sciences, Tokyo, Japan). Hexane (Wako, Osaka, Japan) was used as a solvent, and 2, 6-dimethylnaphthalene (Wako, Osaka, Japan) was used as an internal standard. Analysis was operated under programmed temperature conditions: 40 °C for 10 min, increased by 30 °C/min until 250 °C, maintained for 1 min then increased by 10 °C/min until 320 °C was reached then maintained at 10 min (detector and injector temperatures of 300 and 270 °C, respectively), injection volume of 0.2 μL and helium as carrier gas (150 kPa). The yield was calculated following Eq. (1):
2. Experimental methodology 2.1. Experimental set-up Batch MW irradiation experiments for the synthesis of OAME were carried out using a μReactor EX (Shikoku Instrumentation Co., Ltd.) equipped with a fiber optic thermometer for temperature measurement and a condenser for MeOH reflux were attached as shown in the schematic diagram (Fig. 1). It is capable of generating up to 1000 W of MW power. Flow reactions were performed using the microfluidic device shown in Fig. 2. This apparatus consisted of a carbon dioxide (CO2) pump (Teledyne, ISCO, Japan), UFLC pump (LC-20AD, Shimadzu, Japan), micro device (SUS316L, Kobe Steel Co., Japan) and an oven. Fig. 3 shows the schematic diagram of the microfluidic reactor used in the experiments. The internal diameter of the microfluidic device is 0.9 mm while the length is 500 mm. The cross-section has a semi-circle shape. It is housed inside a stainless steel casing with dimensions of 200 × 100 mm, and can be operated at a maximum pressure of 20 MPa.
Yield (%) =
OAME concentration (
mol ) L
× 100
OAfeed concentration (mol/ L)
(1)
3. Results and discussion 3.1. Preliminary studies on the non-catalytic biodiesel synthesis using MW and SCCO2-microfluidic reactor We initially explored the intensification of non-catalytic biodiesel synthesis employing batch reaction through MW heating. We hypothesized that inherent internal and localized heating facilitate esterification reaction in a shorter reaction time. MeOH, being a polar solvent, acts as the microwave absorber system. First, a 5-min noncatalytic esterification reaction was performed, which resulted to a very poor yield of 0.62 wt% (Fig. 4–1). In order to improve the yield, we added a small amount of sulfuric acid (0.1 wt%). Yields were 22.1% at 60 °C and 30 s (Fig. 4–2) and 36% at 70 °C and 60 s (Fig. 4–3) at an OAto-MeOH ratio of 1:4. Indeed, both the addition of the acid catalyst and the increase in temperature has increased the yield but still remains relatively low. Due to the limitations in our reactor for high-temperature experiments, we benchmarked our results with that of Melo-Junior et al. [21]. They carried out the reactions at elevated temperatures since esterification is an endothermic process and increasing temperatures could drive the equilibrium towards product formation [22]. At 150 °C, their uncatalyzed yield amounted to about 3% (Fig. 4–4). Further increasing to 200 °C yielded 25% and 60% at reaction times of 5 and 30 min (Fig. 4–5 and 4–6), respectively. Because of the relatively low yield obtained using MW-assisted batch reaction and we cannot elevate reaction temperatures, we decided to explore the use of a microfluidic device which would enable us to carry out a continuous process at elevated temperatures. Using an OA: MeOH ratio of 1:4 and 60 s at 120 °C, the yield already reached 19% (Fig. 4–7). As in the majority of researches which compared continuous-flow microfluidic device with batch processes, the continuous process outperforms the batch process under specified
2.2. Experiments for non-catalytic biodiesel synthesis For MW experiments, oleic acid (Wako, Osaka, Japan) and methanol (Wako, Osaka, Japan) at 1:4 ratio were placed in a 100-mLthree-necked flask. The flask was placed inside the cavity of the MW apparatus, attached to the fiber optic thermometer and condenser.
Fig. 1. Schematic diagram of microwave-experimental apparatus.
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Fig. 2. Schematic diagram of the experimental apparatus for supercritical carbon dioxide-mediated microfluidic reactor.
conditions [8]. This indicates that microfluidic reactor-SCCO2 performance is much better than MW. Thus, we next explored several conditions to further increase product yield.
addition of water. This has been attributed to the promotion of backward reaction as the formation of OAME by esterification is a reversible reaction. The presence of water shifted equilibrium concentrations resulting to lower OAME yields [25,26]. Next, the effects of OA-to-MeOH molar ratio were investigated. Since esterification is a reversible reaction, the required OA-to-MeOH molar ratio should be less than the theoretical value of 1 to drive the reaction forward [22]. Yet, an optimum molar ratio is desired in order to minimize raw material costs and avoid the excess MeOH separation and recycling. As illustrated by the results in Fig. 6, at an OA-to-MeOH ratio of 1:2, OAME was hardly produced. This trend was also reported in previous studies despite the differences in synthesis route [23]. A significant increase in yield was achieved at 1:4 ratio, also in agreement with previous reports [24]. Mohod et al. has emphasized that the exact ratio may vary based on the actual feedstock used although the trend is similar [27]. Residence time effects were also investigated. Experiments were conducted continuously, for several minutes and samples were obtained at certain periods of time. The possible effect of residence time distribution inside the reactor was not considered in this current study. Besides, high diffusivities are inherent characteristics of supercritical CO2, thus, we assumed the residence time to be uniform all throughout the reactor. In this current work, the reaction time inside the microreactor was considered similar to the residence time. The results, as shown in Fig. 7 indicate that in general, yield increased with increasing residence time. A longer residence time ensures complete mixing and sufficient contact between the OA and MeOH. Also, sufficient residence time could potentially give rise to the occurrence of micro-segmentation as pointed out by Budden et al. [28]. In such phenomenon, segment-internal convection occur inside the fluid segments, enabling fast
3.2. Factors affecting yields in microfluidic reactor The reaction temperature is typically regarded as an important parameter for chemical reactions with faster rates taking place when elevated temperatures are utilized. Being an endothermic process, a higher temperature is expected to increase conversion as the reaction equilibrium is shifted towards product formation during esterification [22,23]. Esterification reactions are typically carried out between 60 and 120 °C, where product yields are typically insignificant beyond 120 °C presumably due to the evaporation of alcohol despite efforts to contain it [22,24]. The same temperature range was investigated for OAME synthesis without catalyst using the microfluidic reactor-SCCO2 system. The results are shown in Fig. 5. The graph indicates that OAME could be synthesized at 60 °C without any catalysts in less than 30 s. As expected, the yield increased with temperature rise. However, the yield was relatively low reaching only about 5% at 120 °C. The data suggests that esterification reaction is possible in the absence of a catalyst at a very short period of time and that restricting the reaction volume in a microreactor indeed improves contact between OA and MeOH thereby producing OAME. However, the very low yield was unacceptable at this point, so other process variables were investigated in order to increase the yield. For example, the addition of slight amount of water was initially thought to have positive effect on reaction due to possible formation of carbonic acid (H2CO3) that could serve as catalyst for the reaction. On the contrary, the yield decreased from 5% to 4% at 120 °C with the
Fig. 3. Schematic diagram of the microfluidic reactor.
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Fig. 4. Oleic acid methyl ester yields using microwave and microfluidic device. ([4–6] *C. A. R. Melo-Junior, C. E. R. Albuquerque, M. Fortuny, C. Dariva, S. Egues, A. F. Santos, A. L. D. Ramos, Use of Microwave Irradiation in the Noncatalytic Esterification of C18 Fatty Acids, Energy & Fuel 23 (2009) 580–585.)
Fig. 5. Dependence of oleic acid methyl ester yields on reaction temperatures without catalyst.
Fig. 7. Influence of residence time on the oleic acid methyl ester.
Fig. 6. Oleic acid methyl ester yields under various oleic acid-methanol molar ratio.
mixing and heat transfer to take place [29] too. The influence of SCCO2 pressure was studied just above the critical pressure of CO2 (7.38 MPa) at 8, 10 and 12 MPa under catalyzed conditions for a temperature of 120 °C. Working at high pressures is necessary especially if the reaction temperature is relatively high to avoid evaporation of the reactants [18]. Besides, it is speculated that high
Fig. 8. Influence of supercritical carbon dioxide pressure on oleic acid methyl ester yield.
pressures could increase solubility of the reactants thus enhancing molecular interactions, resulting into high yields. However, on the contrary, results in Fig. 8 shows that yields decreased as pressure was 171
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reactions at a residence time of less than 60 s. The OAME yield for catalyzed and uncatalyzed microfluidic reactions follow first order kinetics as shown in Fig. 10. The calculated rate constants, k, for the uncatalyzed and catalyzed reactions were 0.0036 and 0.0634 s−1, respectively. This result is very promising considering that near complete conversion could be obtained in a very short reaction time. 3.3. Enhancement of yield in the non-catalytic approach Since, the non-catalytic approach remains a challenge, increasing the residence time by incorporating a recirculation loop was investigated. This approach is unique for continuous flow reactors and cannot be performed in batch reactors with ease. As shown in Fig. 11, recirculating the products obtained from the first pass significantly increased the yield to 78% (Fig. 9). It is supposed that the OAME formed from the first pass acts like a surfactant during the second pass; the mutual solubility of MeOH and OA improves and coupled with efficient mixing of devices has greatly increased yield [22]. From this result, it is considered possible to improve the yield even under non-catalytic conditions by supplying recovered samples into the system again.
Fig. 9. Effect of 0.1 wt% sulfuric acid addition on oleic acid methyl ester yield.
4. Summary Both MW and microfluidic device has been utilized to demonstrate oleic acid methyl ester (OAME) synthesis representing batch and continuous reactions, respectively. Microfluidic device has outperformed microwave (MW) irradiation reactions owing to the efficient molecular contact between OA (oleic acid) and methanol (MeOH). OAME yields were obtained in as short as 20 s between 60 and 120 °C. Reaction temperature, molar ratio and residence time were found to be significant in driving the reaction forward. The introduction of 0.1% wt H2SO4 catalyst allowed for almost complete conversion, which can potentially be achieved in an uncatalyzed reaction by recirculating products from the first pass back into the reactor. Future studies that will focus on further optimization of the process to get high yield will also be carried out.
Fig. 10. Correlation between the rate of reaction and residence time in a microfluidic reactor with and without catalysts.
Acknowledgements The microfluidic device used in this study was kindly provided by Dr. Ryuichi Fukuzato of SCF Techno-Link (Japan). We also acknowledge Prof. Motonobu Goto of Nagoya University for valuable technical advices and insightful thoughts regarding the use of the device under supercritical fluid conditions. References [1] J. Armendáriz, M. Lapuerta, F. Zavala, E. García-Zambrano, M. Del Carmen Ojeda, Evaluation of eleven genotypes of castor oil plant (Ricinus communis L.) for the production of biodiesel, Ind. Crops Prod. 77 (2015) 484–490, http://dx.doi.org/10. 1016/j.indcrop.2015.09.023. [2] B. Hajra, N. Sultana, A.K. Pathak, C. Guria, Response surface method and genetic algorithm assisted optimal synthesis of biodiesel from high free fatty acid sal oil (Shorea robusta) using ion-exchange resin at high temperature, J. Environ. Chem. Eng. 3 (4) (2015) 2378–2392, http://dx.doi.org/10.1016/j.jece.2015.08.015. [3] H. Zhang, Q. Zhou, F. Chang, H. Pan, X. Liu, H. Li, S. Yang, Production and fuel properties of biodiesel from Firmiana platanifolia L. f. as a potential non-food oil source, Ind. Crops Prod. 76 (2015) 768–771, http://dx.doi.org/10.1016/j.indcrop. 2015.08.002. [4] G. Chiatti, O. Chiavola, F. Palmieri, Impact of waste cooking oil in biodiesel blends on particle size distributions from a city-car engine, J. Energy Inst. (2016), http:// dx.doi.org/10.1016/j.joei.2016.11.009. [5] M.D. D́ Agosto, M.A. Da Silva, L.S. Franca, C.M. De Oliveira, M.O. Alexandre, L.G. Da Costa Marques, M.A. De Freitas, Comparative study of emissions from stationary engines using biodiesel made from soybean oil, palm oil and waste frying oil, Renew. Sustain. Energy Rev. (2016), http://dx.doi.org/10.1016/j.rser.2016.12. 040. [6] T. Maneerung, S. Kawi, Y. Dai, C. Wang, Sustainable biodiesel production via transesterification of waste cooking oil by using CaO catalysts prepared from chicken manure, Energy Convers. Manage. 123 (2016) 487–497, http://dx.doi.org/ 10.1016/j.enconman.2016.06.071.
Fig. 11. Recirculation of products for a second pass.
increased. Looking at the over-all process, this decrease in the yield could be due most likely to formation of two phases and difficulty in recovering the products during depressurization steps at higher pressures. Thus, a pressure of about 8 MPa is suggested at the conditions indicated in Fig. 8. This is expected to vary depending on temperature, flowrate and OA-to-MeOH ratio. Finally, in order to increase the yield, a relatively small amount of catalyst (0.1 wt% H2SO4) was introduced into the system then similar experiments were performed. The results illustrated in Fig. 9 indicated a significant increase in yield of about 4-fold compared to uncatalyzed
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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Supercritical carbon dioxide-mediated esterification in a microfluidic reactor
T
⁎
Armando T. Quitaina,b, , Elaine G. Missionc, Yoshifumi Sumigawac, Mitsuru Sasakid a
Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan International Research Organization for Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan c Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan d Institute of Pulsed Power Science, Kumamoto University, Kumamoto 860-8555, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Microfluidic device Supercritical carbon dioxide Biodiesel Free fatty acid
The immiscibility of vegetable oil and alcohol during the initial step in biodiesel synthesis has slowed down the process resulting to increase production costs. Thus, process intensification techniques that would enhance mixing or molecular interactions of the reactants, and enable short reaction time should be carried out in order to increase yield and minimize costs. We proposed coupling of two process intensification schemes in enhancing molecular interactions, i.e. the use of supercritical carbon dioxide as a solvent and microfluidic device, as applied to esterification of oleic acid (OA) with methanol (MeOH). We found this superior over the commonly applied microwave. Hence, we further explored the influences of reaction temperature, OA-to-MeOH molar ratio, residence time, water contents, pressure and catalyst addition on the oleic acid methyl ester (OAME) yield. Results showed that at temperature range of 60 to 120 °C and pressure of 10 MPa, the esterification reaction proceeds even without any catalyst in less than 1 min. Addition of relatively small amount of catalyst (0.1 wt% H2SO4) dramatically increased the yield 4-fold to 90%. Recirculation of the product even without a catalyst significantly increased yield. The experimental analysis confirms the applicability potential of the microfluidic reactor in the non-catalytic biodiesel synthesis.
1. Introduction With the impending fossil fuel depletion, massive researches have focused in developing sustainable alternatives such as biodiesel (BDF) for internal combustion engines. BDF has attracted intense attention as it is a renewable, biodegradable and carbon neutral alternative for petroleum [1–3]. Recently, waste and non-edible oils were evaluated as suitable feedstock for BDF synthesis, which would enable BDF pricing market competitive [4–6]. Since waste oils contain significant amounts of free fatty acids, esterification process has been developed as another route for BDF synthesis. In this process, fatty acids react with alcohol to produce alkyl esters in the presence of acid catalyst. The fatty acids and alcohol are immiscible, hence there is a high mass transfer resistance (or low collision coefficient) between the two reacting molecules at the beginning of the esterification process. This is heightened as homogeneous acids, i.e. sulfuric acid, intended to catalyze the reaction preferentially co-locates in the alcohol phase [7,8]. As a result, the overall reaction rate is slowed down resulting to higher production costs [9]. With such a challenge, process technologies which are energy efficient, environment-friendly and with low residence time and higher ⁎
molecular collision should be carried out [10]. To address the immiscibility problems, we proposed the use of environmentally benign supercritical carbon dioxide (hereby abbreviated as SCCO2) as a solvent for reaction. Both reactants, methanol (hereby abbreviated as MeOH) and oleic acid (hereby abbreviated as OA), are known to be soluble in SCCO2 even at low pressure conditions [11,12]. Besides, unlike the commonly used organic solvent such as hexane, separation of the products can be easily carried out by simple depressurization steps. For enhancement of reaction rates, two of the promising process intensification techniques related to improving molecular collision and interactions are microwave irradiation (MW) and the use of microfluidic device. MW has attracted interest in accelerating reactions since heating is produced within the system by the molecular motion of absorbing polar molecules. This is known as the reversed heating phenomena, which increases the chance for reacting molecules to collide and interact [13,14]. The collision of molecules are also thought to contribute to the reaction, and is considered as non-thermal effect of MW irradiation [15]. On the other hand, a microfluidic reactor has an internal diameter of a few millimeters and which allow for continuous-flow processes. Processing in microscale enhances heat and
Corresponding author at: Faculty of Advanced Science and Technology ,Kumamoto University, Kumamoto 860-8555 Japan. E-mail address: [email protected] (A.T. Quitain).
https://doi.org/10.1016/j.cep.2017.11.002 Received 12 February 2017; Received in revised form 17 October 2017; Accepted 4 November 2017 Available online 09 November 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 123 (2018) 168–173
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molecular collision due to short diffusion paths and large surface areas, thus, making it easy to operate and control even with sudden temperature changes [16]. With these characteristics, the use of a microfluidic reactors could bring about development of more energy efficient and cost-effective processes. For industrial applications, instead of scaling up, numbering up of microfluidic reactors to meet the required throughput can simplify the rigorous conventional process design methodology. These devices have been applied in the direct synthesis of hydrogen peroxide from hydrogen and oxygen [17]; lignin oxidation [18]; and in the direct conversion of syngas to dimethyl ether [19] at a safer, cleaner and more efficient way. Furthermore, coupling these devices with supercritical fluid (SCF) further intensifies the process since SCF can easily penetrate the sample matrix and induce miscibility [20]. Thus, performing supercritical fluid-based processes in a microfluidic device has a great potential for process intensification. In this work, preliminary experiments were carried out to compare the application of two process intensification technologies, namely MW irradiation and microfluidic reactor for the synthesis of oleic acid methyl ester (OAME) from OA and MeOH. The microfluidic device was complemented with the use of environmentally benign SCCO2 to further intensify the process. Due to the superiority of the latter approach, various process variables were investigated to increase the OAME yield.
Stirring was provided using magnetic bar. After setting up the apparatus, the mixture was stirred for a few minutes, then MW was irradiated at 200 W for the specified duration. The reaction temperature was maintained between 60 and 70 °C. After each reaction, the mixture was cooled down to room temperature and collected for GC-FID analysis. Similarly, a mixture of the OA and MeOH prepared at specified molar ratios were used for microfluidic reactions. The mixture was pumped into the reactor at a constant flowrate (0.02–0.1 mL/min) and with the same flowrate as CO2. Experiments on the synthesis of OAME was carried out at a pressure of 10 MPa and temperatures of 60 to 120 °C. Samples were immediately collected at the outlet and also subjected to GC-FID analysis. 2.3. GC-FID analysis of reaction products Gas chromatographic method with flame ionization detection (GCFID) was used to analyze the reaction products, especially for the presence of OAME. Components separation was carried out in a capillary column CP-SIL 8CB-MS (GL Sciences, Tokyo, Japan). Hexane (Wako, Osaka, Japan) was used as a solvent, and 2, 6-dimethylnaphthalene (Wako, Osaka, Japan) was used as an internal standard. Analysis was operated under programmed temperature conditions: 40 °C for 10 min, increased by 30 °C/min until 250 °C, maintained for 1 min then increased by 10 °C/min until 320 °C was reached then maintained at 10 min (detector and injector temperatures of 300 and 270 °C, respectively), injection volume of 0.2 μL and helium as carrier gas (150 kPa). The yield was calculated following Eq. (1):
2. Experimental methodology 2.1. Experimental set-up Batch MW irradiation experiments for the synthesis of OAME were carried out using a μReactor EX (Shikoku Instrumentation Co., Ltd.) equipped with a fiber optic thermometer for temperature measurement and a condenser for MeOH reflux were attached as shown in the schematic diagram (Fig. 1). It is capable of generating up to 1000 W of MW power. Flow reactions were performed using the microfluidic device shown in Fig. 2. This apparatus consisted of a carbon dioxide (CO2) pump (Teledyne, ISCO, Japan), UFLC pump (LC-20AD, Shimadzu, Japan), micro device (SUS316L, Kobe Steel Co., Japan) and an oven. Fig. 3 shows the schematic diagram of the microfluidic reactor used in the experiments. The internal diameter of the microfluidic device is 0.9 mm while the length is 500 mm. The cross-section has a semi-circle shape. It is housed inside a stainless steel casing with dimensions of 200 × 100 mm, and can be operated at a maximum pressure of 20 MPa.
Yield (%) =
OAME concentration (
mol ) L
× 100
OAfeed concentration (mol/ L)
(1)
3. Results and discussion 3.1. Preliminary studies on the non-catalytic biodiesel synthesis using MW and SCCO2-microfluidic reactor We initially explored the intensification of non-catalytic biodiesel synthesis employing batch reaction through MW heating. We hypothesized that inherent internal and localized heating facilitate esterification reaction in a shorter reaction time. MeOH, being a polar solvent, acts as the microwave absorber system. First, a 5-min noncatalytic esterification reaction was performed, which resulted to a very poor yield of 0.62 wt% (Fig. 4–1). In order to improve the yield, we added a small amount of sulfuric acid (0.1 wt%). Yields were 22.1% at 60 °C and 30 s (Fig. 4–2) and 36% at 70 °C and 60 s (Fig. 4–3) at an OAto-MeOH ratio of 1:4. Indeed, both the addition of the acid catalyst and the increase in temperature has increased the yield but still remains relatively low. Due to the limitations in our reactor for high-temperature experiments, we benchmarked our results with that of Melo-Junior et al. [21]. They carried out the reactions at elevated temperatures since esterification is an endothermic process and increasing temperatures could drive the equilibrium towards product formation [22]. At 150 °C, their uncatalyzed yield amounted to about 3% (Fig. 4–4). Further increasing to 200 °C yielded 25% and 60% at reaction times of 5 and 30 min (Fig. 4–5 and 4–6), respectively. Because of the relatively low yield obtained using MW-assisted batch reaction and we cannot elevate reaction temperatures, we decided to explore the use of a microfluidic device which would enable us to carry out a continuous process at elevated temperatures. Using an OA: MeOH ratio of 1:4 and 60 s at 120 °C, the yield already reached 19% (Fig. 4–7). As in the majority of researches which compared continuous-flow microfluidic device with batch processes, the continuous process outperforms the batch process under specified
2.2. Experiments for non-catalytic biodiesel synthesis For MW experiments, oleic acid (Wako, Osaka, Japan) and methanol (Wako, Osaka, Japan) at 1:4 ratio were placed in a 100-mLthree-necked flask. The flask was placed inside the cavity of the MW apparatus, attached to the fiber optic thermometer and condenser.
Fig. 1. Schematic diagram of microwave-experimental apparatus.
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Fig. 2. Schematic diagram of the experimental apparatus for supercritical carbon dioxide-mediated microfluidic reactor.
conditions [8]. This indicates that microfluidic reactor-SCCO2 performance is much better than MW. Thus, we next explored several conditions to further increase product yield.
addition of water. This has been attributed to the promotion of backward reaction as the formation of OAME by esterification is a reversible reaction. The presence of water shifted equilibrium concentrations resulting to lower OAME yields [25,26]. Next, the effects of OA-to-MeOH molar ratio were investigated. Since esterification is a reversible reaction, the required OA-to-MeOH molar ratio should be less than the theoretical value of 1 to drive the reaction forward [22]. Yet, an optimum molar ratio is desired in order to minimize raw material costs and avoid the excess MeOH separation and recycling. As illustrated by the results in Fig. 6, at an OA-to-MeOH ratio of 1:2, OAME was hardly produced. This trend was also reported in previous studies despite the differences in synthesis route [23]. A significant increase in yield was achieved at 1:4 ratio, also in agreement with previous reports [24]. Mohod et al. has emphasized that the exact ratio may vary based on the actual feedstock used although the trend is similar [27]. Residence time effects were also investigated. Experiments were conducted continuously, for several minutes and samples were obtained at certain periods of time. The possible effect of residence time distribution inside the reactor was not considered in this current study. Besides, high diffusivities are inherent characteristics of supercritical CO2, thus, we assumed the residence time to be uniform all throughout the reactor. In this current work, the reaction time inside the microreactor was considered similar to the residence time. The results, as shown in Fig. 7 indicate that in general, yield increased with increasing residence time. A longer residence time ensures complete mixing and sufficient contact between the OA and MeOH. Also, sufficient residence time could potentially give rise to the occurrence of micro-segmentation as pointed out by Budden et al. [28]. In such phenomenon, segment-internal convection occur inside the fluid segments, enabling fast
3.2. Factors affecting yields in microfluidic reactor The reaction temperature is typically regarded as an important parameter for chemical reactions with faster rates taking place when elevated temperatures are utilized. Being an endothermic process, a higher temperature is expected to increase conversion as the reaction equilibrium is shifted towards product formation during esterification [22,23]. Esterification reactions are typically carried out between 60 and 120 °C, where product yields are typically insignificant beyond 120 °C presumably due to the evaporation of alcohol despite efforts to contain it [22,24]. The same temperature range was investigated for OAME synthesis without catalyst using the microfluidic reactor-SCCO2 system. The results are shown in Fig. 5. The graph indicates that OAME could be synthesized at 60 °C without any catalysts in less than 30 s. As expected, the yield increased with temperature rise. However, the yield was relatively low reaching only about 5% at 120 °C. The data suggests that esterification reaction is possible in the absence of a catalyst at a very short period of time and that restricting the reaction volume in a microreactor indeed improves contact between OA and MeOH thereby producing OAME. However, the very low yield was unacceptable at this point, so other process variables were investigated in order to increase the yield. For example, the addition of slight amount of water was initially thought to have positive effect on reaction due to possible formation of carbonic acid (H2CO3) that could serve as catalyst for the reaction. On the contrary, the yield decreased from 5% to 4% at 120 °C with the
Fig. 3. Schematic diagram of the microfluidic reactor.
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Fig. 4. Oleic acid methyl ester yields using microwave and microfluidic device. ([4–6] *C. A. R. Melo-Junior, C. E. R. Albuquerque, M. Fortuny, C. Dariva, S. Egues, A. F. Santos, A. L. D. Ramos, Use of Microwave Irradiation in the Noncatalytic Esterification of C18 Fatty Acids, Energy & Fuel 23 (2009) 580–585.)
Fig. 5. Dependence of oleic acid methyl ester yields on reaction temperatures without catalyst.
Fig. 7. Influence of residence time on the oleic acid methyl ester.
Fig. 6. Oleic acid methyl ester yields under various oleic acid-methanol molar ratio.
mixing and heat transfer to take place [29] too. The influence of SCCO2 pressure was studied just above the critical pressure of CO2 (7.38 MPa) at 8, 10 and 12 MPa under catalyzed conditions for a temperature of 120 °C. Working at high pressures is necessary especially if the reaction temperature is relatively high to avoid evaporation of the reactants [18]. Besides, it is speculated that high
Fig. 8. Influence of supercritical carbon dioxide pressure on oleic acid methyl ester yield.
pressures could increase solubility of the reactants thus enhancing molecular interactions, resulting into high yields. However, on the contrary, results in Fig. 8 shows that yields decreased as pressure was 171
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reactions at a residence time of less than 60 s. The OAME yield for catalyzed and uncatalyzed microfluidic reactions follow first order kinetics as shown in Fig. 10. The calculated rate constants, k, for the uncatalyzed and catalyzed reactions were 0.0036 and 0.0634 s−1, respectively. This result is very promising considering that near complete conversion could be obtained in a very short reaction time. 3.3. Enhancement of yield in the non-catalytic approach Since, the non-catalytic approach remains a challenge, increasing the residence time by incorporating a recirculation loop was investigated. This approach is unique for continuous flow reactors and cannot be performed in batch reactors with ease. As shown in Fig. 11, recirculating the products obtained from the first pass significantly increased the yield to 78% (Fig. 9). It is supposed that the OAME formed from the first pass acts like a surfactant during the second pass; the mutual solubility of MeOH and OA improves and coupled with efficient mixing of devices has greatly increased yield [22]. From this result, it is considered possible to improve the yield even under non-catalytic conditions by supplying recovered samples into the system again.
Fig. 9. Effect of 0.1 wt% sulfuric acid addition on oleic acid methyl ester yield.
4. Summary Both MW and microfluidic device has been utilized to demonstrate oleic acid methyl ester (OAME) synthesis representing batch and continuous reactions, respectively. Microfluidic device has outperformed microwave (MW) irradiation reactions owing to the efficient molecular contact between OA (oleic acid) and methanol (MeOH). OAME yields were obtained in as short as 20 s between 60 and 120 °C. Reaction temperature, molar ratio and residence time were found to be significant in driving the reaction forward. The introduction of 0.1% wt H2SO4 catalyst allowed for almost complete conversion, which can potentially be achieved in an uncatalyzed reaction by recirculating products from the first pass back into the reactor. Future studies that will focus on further optimization of the process to get high yield will also be carried out.
Fig. 10. Correlation between the rate of reaction and residence time in a microfluidic reactor with and without catalysts.
Acknowledgements The microfluidic device used in this study was kindly provided by Dr. Ryuichi Fukuzato of SCF Techno-Link (Japan). We also acknowledge Prof. Motonobu Goto of Nagoya University for valuable technical advices and insightful thoughts regarding the use of the device under supercritical fluid conditions. References [1] J. Armendáriz, M. Lapuerta, F. Zavala, E. García-Zambrano, M. Del Carmen Ojeda, Evaluation of eleven genotypes of castor oil plant (Ricinus communis L.) for the production of biodiesel, Ind. Crops Prod. 77 (2015) 484–490, http://dx.doi.org/10. 1016/j.indcrop.2015.09.023. [2] B. Hajra, N. Sultana, A.K. Pathak, C. Guria, Response surface method and genetic algorithm assisted optimal synthesis of biodiesel from high free fatty acid sal oil (Shorea robusta) using ion-exchange resin at high temperature, J. Environ. Chem. Eng. 3 (4) (2015) 2378–2392, http://dx.doi.org/10.1016/j.jece.2015.08.015. [3] H. Zhang, Q. Zhou, F. Chang, H. Pan, X. Liu, H. Li, S. Yang, Production and fuel properties of biodiesel from Firmiana platanifolia L. f. as a potential non-food oil source, Ind. Crops Prod. 76 (2015) 768–771, http://dx.doi.org/10.1016/j.indcrop. 2015.08.002. [4] G. Chiatti, O. Chiavola, F. Palmieri, Impact of waste cooking oil in biodiesel blends on particle size distributions from a city-car engine, J. Energy Inst. (2016), http:// dx.doi.org/10.1016/j.joei.2016.11.009. [5] M.D. D́ Agosto, M.A. Da Silva, L.S. Franca, C.M. De Oliveira, M.O. Alexandre, L.G. Da Costa Marques, M.A. De Freitas, Comparative study of emissions from stationary engines using biodiesel made from soybean oil, palm oil and waste frying oil, Renew. Sustain. Energy Rev. (2016), http://dx.doi.org/10.1016/j.rser.2016.12. 040. [6] T. Maneerung, S. Kawi, Y. Dai, C. Wang, Sustainable biodiesel production via transesterification of waste cooking oil by using CaO catalysts prepared from chicken manure, Energy Convers. Manage. 123 (2016) 487–497, http://dx.doi.org/ 10.1016/j.enconman.2016.06.071.
Fig. 11. Recirculation of products for a second pass.
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