Biodiesel production in micro-reactors: A review

Energy for Sustainable Development 43 (2018) 143–161

Contents lists available at ScienceDirect

Energy for Sustainable Development

Review

Biodiesel production in micro-reactors: A review Appurva Tiwari, V.M. Rajesh ⁎, Sanjeev Yadav Department of Chemical Engineering, Shiv Nadar University, Tehsil Dadri, Gautam Buddha Nagar, Greater Noida, Uttar Pradesh 201314, India

a r t i c l e

i n f o

Article history: Received 14 September 2017 Revised 9 January 2018 Accepted 9 January 2018 Available online xxxx Keywords: Renewable energy Micro-reactors Biodiesel Transesterification Bio-fuel

a b s t r a c t In recent years, biodiesel, as a renewable and environment-friendly fuel has emerged as one of the most investigated bio-fuel with a goal to decrease our dependence on petroleum fuels and reduce environmental pollution. The current commercial technique of biodiesel production via transesterification is constrained by high operating cost and energy requirements, long residence time and limited conversion due to equilibrium limitations. On the other hand, the process intensification using the micro-reactor technology demonstrated an excellent performance ascribed to short diffusion distance and high surface area-to-volume ratio that can lead to high heat and mass transfer rates and improved mixing compared to the conventional reactors. This review provides an overview of the current status of biodiesel production in micro-reactors. It includes various types of microreactors used in the production of biodiesel, factors affecting the biodiesel production (i.e., temperature, residence time, alcohol to oil molar ratio, micro-channel size, inlet mixer type, internal geometries, co-solvent, catalyst type and concentration). This review also includes the factors affecting the liquid-liquid flow patterns and application of micro-reactor technology in the purification of biodiesel. Some of the critical observations from this review are, 1) inlet mixer type, channel size, and internal channel geometry (zig-zag, omega, and tesla shaped channels) have shown a significant effect on mixing in micro-channels. 2) In case of base-catalyzed transesterification, the biodiesel yield was found to increase up to the reaction temperature of 60–65 °C. 3) Homogeneous alkaline catalyst (NaOH, KOH, CH3ONa) was preferred for the feedstock with low free fatty acid content (b1 wt%). However, an acid catalyst with high concentration, a significant amount of methanol and long reaction time were required for high free fatty acid feedstock (N1 wt%). Therefore, the current research is more focused on the investigation of heterogeneous catalysts in micro-reactors to develop an ecologically friendly process for the production of biodiesel. 4) Also, the reaction temperature and inlet mixer type had shown a significant effect on liquid-liquid flow patterns. This review also addressed the following literature gaps; a numberingup technique to increase the throughput; catalyst development for high free fatty acid feedstock; continuous production of biodiesel in micro-reactors with in-line purification step to meet the energy demand and quality standards. © 2018 International Energy Initiative. Published by Elsevier Inc. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . Present state of the art: micro-reactors used in biodiesel synthesis Factors affecting biodiesel production . . . . . . . . . . . . . Effect of reaction temperature . . . . . . . . . . . . . . Residence time . . . . . . . . . . . . . . . . . . . . Alcohol to oil molar ratio . . . . . . . . . . . . . . . . Effects of geometrical configurations . . . . . . . . . . . Effect of channel size . . . . . . . . . . . . . . Effect of inlet mixer type . . . . . . . . . . . . . Effect of internal geometries . . . . . . . . . . . Effect of co-solvent . . . . . . . . . . . . . . . . . . . Effects of catalyst type and concentration . . . . . . . . . Homogeneous catalysts . . . . . . . . . . . . . Heterogeneous catalysts . . . . . . . . . . . . .

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⁎ Corresponding author. E-mail address: [email protected] (V.M. Rajesh).

https://doi.org/10.1016/j.esd.2018.01.002 0973-0826/© 2018 International Energy Initiative. Published by Elsevier Inc. All rights reserved.

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Catalyst-free process . . . . . . . . Liquid-liquid two-phase flows in micro-reactors for biodiesel production . . . . . . . . . . . . . . . Factors affecting flow patterns . . . . . . . Effect of temperature on flow patterns Effect of alcohol on oil molar ratio . . Effect of inlet mixer type . . . . . . Purification of biodiesel in micro-reactors . . . . Scope for future work . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

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Introduction With the rapid depletion of fossil fuels and increasing environmental concerns, the need for an alternative fuel that is cheaper, renewable and environment-friendly is growing consistently. Biodiesel has emerged as a potential alternative fuel to fossil fuels, which has similar properties to fossil diesel with some advantages (Freedman, Pryde, & Mounts, 1984; Van Gerpen, 2005b). Biodiesel is a renewable fuel, less-toxic than conventional diesel and also biodegradable (Al-Zuhair, 2007; Bozbas, 2008; Knothe, Sharp, & Ryan, 2006; Koonin, 2006; Meher, Vidyasagar, & Naik, 2006). Biodiesel blends with conventional diesel up to B20 (20% biodiesel and 80% diesel) can be used in a diesel engine without engine modification. However, biodiesel N20% in the blend required few engine modifications (Canakci, Erdil, & Arcaklioğlu, 2006; Sharma, Singh, & Upadhyay, 2008; Van Gerpen, 2005a; Xue, Grift, & Hansen, 2011). Biodiesel is produced via transesterification of vegetable oils or fats with alcohol, typically methanol, in the presence of a catalyst (Demirbaş, 2003; Van Gerpen, 2005b). The transesterification reaction takes place in a liquid-liquid two-phase system and the reaction rate is limited by mass transfer due to immiscibility of oil and alcohol (Crawford et al., 2008; Guan, Kusakabe, Moriyama, & Sakurai, 2009a). Therefore, an efficient contact between the feed oil and the alcoholcatalyst mixture becomes crucial for achieving the high rate of reaction. Several studies on biodiesel production were carried out using batch reactors (Dorado, Ballesteros, López, & Mittelbach, 2004; Lotero et al., 2005; Shah, Sharma, & Gupta, 2004; Vicente, Martınez, & Aracil, 2004; Xie, Peng, & Chen, 2006; Zhang, Dube, McLean, & Kates, 2003). These reactors were also commercially used for the production of biodiesel at industrial scale (Haas, McAloon, Yee, & Foglia, 2006). However, unfavourable economics and other design and operational issues at large-scale batch reactors pose challenges to the commercialization of biodiesel (Maddikeri, Pandit, & Gogate, 2012; Qiu, Zhao, & Weatherley, 2010; Santacesaria, Vicente, Di Serio, & Tesser, 2012). The critical challenges are, the long residence time of several hours, considerable investment in equipment and manufacturing floor space, high energy consumption and low production efficiency (Jachuck, Pherwani, & Gorton, 2009; Qiu et al., 2010; Slinn & Kendall, 2009; Warabi, Kusdiana, & Saka, 2004). On the other hand, with these limitations on a conventional batch process, several alternative highly energy efficient technologies to produce biodiesel in a continuous mode of operation were investigated (Maddikeri et al., 2012; Qiu et al., 2010). In recent years, micro-reactor technology has emerged as a promising technique for high-efficiency continuous biodiesel production (Xie, Zhang, & Xu, 2012). Micro-reactors are miniaturized reaction systems with reduced internal characteristic dimensions of 1000 μm–10 μm. Micro-reactors offer small diffusion distances and large surface area to volume ratio (10,000–50,000 m2 m−3) and thus lead to high heat and mass transfer rates. As a result higher conversion is achieved in shorter residence time (Kashid & Kiwi-Minsker, 2009). Mixing time in micro-reactors is also considerably reduced due to their reduced internal dimensions. Moreover, small volume favours micro-reactors to perform highly

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exothermic reactions safely with precise control over the operating conditions (Benke, 2014; Burns & Ramshaw, 1999; Kobayashi, Mori, & Kobayashi, 2006). However, the production capacity of micro-reactors can be increased by the scale-out approach (Aran & Lammertink, 2013; Geyer & Seeberger, 2008). Some researchers have successfully applied micro-reactors for biodiesel synthesis via transesterification (Aghel, Rahimi, Sepahvand, Alitabar, & Ghasempour, 2014; Buddoo, Siyakatshana, & Pongoma, 2008; Guan et al., 2009a; Martínez Arias, Fazzio Martins, Jardini Munhoz, Gutierrez-Rivera, & Maciel, 2012; Sun, Wang, Yao, Zhang, & Xu, 2009; Wen, Yu, Tu, Yan, & Dahlquist, 2009). It was demonstrated that as compared to conventional batch reactors, higher conversion in shorter residence time could be achieved in micro-reactors (Dai, Li, Zhao, & Xiu, 2014; Elkady, Zaatout, & Balbaa, 2015; Guan et al., 2009a; Martínez Arias et al., 2012; Rahimi, Aghel, Alitabar, Sepahvand, & Ghasempour, 2014; Santacesaria et al., 2012; Santana, Tortola, Reis, Silva, & Taranto, 2016; Schürer, Thiele, Wiborg, Ziogas, & Kolb, 2014; Sun, Ju, Ji, Zhang, & Xu, 2008). It should also be noted that as compared to conventional reactors, biodiesel could be produced 10 to 100 times faster in a micro-reactor. Moreover, the requirements of high energy in the mixing of reactants, ample floor space and standing time for the separation of products are eliminated using micro-reactors (Buddoo et al., 2008; Jovanovic, 2006). Although the amount of biodiesel produced using a single micro-reactor is a trickle, each unit can be numbered up to increase the throughput from lab to commercial scale (Billo et al., 2015; Jovanovic, 2006; Schürer et al., 2016). The process of transesterification in micro-reactors is observed to be affected by the operating conditions as well as the type and geometrical parameters of micro-reactor (Xie et al., 2012). With this background, the present review has been organized as following sections: 1) the present state of the art: the types of micro-reactors used in biodiesel synthesis with different feedstock have been discussed. 2) The factors affecting biodiesel production in micro-reactors, in which, the effect of operating parameters such as temperature, alcohol to oil molar ratio, the effect of cosolvent addition, catalyst type and concentration as well as the effect of the geometrical configuration of micro-reactors have been discussed. 3) Liquid-liquid two-phase flows in micro-reactors for biodiesel production: flow patterns observed during production of biodiesel and the factors affecting the flow patterns have been discussed. 4) Purification of biodiesel in micro-reactors: the current research techniques applied for biodiesel separation from glycerol at micro-scale are mentioned. Finally, the current research gaps that need further investigations into full-scale commercialization of biodiesel production using microreactors are also addressed. Present state of the art: micro-reactors used in biodiesel synthesis Micro-reactors can be categorized by geometries, fabrication material, and techniques. For the production of biodiesel, mainly microtubular and micro-channel reactors have been applied. In general, the micro-reactor system is composed of a mixer for intensifying mixing


and a micro-channel or micro-tube for completion of the reaction. Various combinations of mixers and reaction loop (micro-channel or micro-tube), fabricated with different materials have been used in biodiesel synthesis as shown in Table 1. To demonstrate that the production of biodiesel is feasible using micro-reactor technology Al-Dhubabian (2005) developed a micro-reactor with channel dimensions; 23.3 mm length, 10.5 mm width, and 0.1 mm height as shown in (Fig. 1). Under operating conditions of 7.2:1 methanol/soya bean oil molar ratio, 1 wt% NaOH catalyst, at atmospheric pressure and 25 °C, a conversion of 91% was obtained in 10 min. Similarly, Canter (2006) reported a microreactor with parallel micro-channels cut into a thin plastic plate could be used for making biodiesel under mild operating conditions. Biodiesel yield above 90% was obtained at a residence time of about 4 min. However, the details of the reactor and operating parameters were not described completely. These works proved that micro-reactors could be potentially applied for the production of biodiesel commercially. The findings of Al Dhubabian (2005) and Canter (2006) were supported by Sun et al. (2008), where they performed transesterification of

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pre-mixed rapeseed oil and methanol using KOH as a catalyst in quartz capillary of 0.25 mm inner diameter and length of 30 m. At a catalyst concentration of 1% by weight and methanol to oil molar ratio of 6:1, a fatty acid methyl ester (FAME) yield of 98.80% was obtained in 6 min at 60 °C. Instead of using a microfluidic mixer the reactants were a premixed in a tank reactor with constant stirring where the yield reached 81% before injecting the mixture into the capillary. Hence, the merits of using a micro-tube capillary as a reactor were not clearly observed. Guan et al. (2008) carried out continuous production of biodiesel from sunflower oil using a micro-reactor system consisting of Fluorinated ethylene propylene (FEP) tube with an inner diameter of 1 mm and 160 mm in length and a mixer (split and recombine mixer or a Ttype) to create dispersion. Oil conversion reached 100% in 112 s, using methanol to oil molar ratio of 23.9:1 and 1 wt% KOH concentration and split and recombine mixer. It was noted that for complete conversion of oil, the residence time decreased drastically from 600 s to b240 s when lab scale batch reactor was replaced by a micro-reactor, under same operating conditions. In their progressive work, Guan

Table 1 Micro-reactors used in the production of biodiesel. Mixer type

Reaction loop

Reactants

Catalyst amount (wt%)

T (°C)

Alcohol/oil molar ratio

Residence time

Biodiesel yield (%)

Ref.

Stirred tank

i.d. = 0.25 mm l = 30 m Quartz capillary i.d. = 0.25 mm l = 30 m Stainless-steel capillary i.d. = 1 mm l = 160 mm FEP tube i.d. = 0.8 mm l = 300 mm FEP tube i.d. = 1.5 mm l = n.i. PTFE tube dh = 0.24 mm l = 1.07 m Stainless-steel zig-zag channel i.d. = 3 mm l = n.i. PTFE tube packed with Dixon rings dh = 0.5 mm l=1m Tesla-shaped PDMS micro-channel dh = 0.5 mm l=1m Omega-shaped PDMS micro-channel dh = 0.5 mm l=1m T-shaped PDMS micro-channel dh = 1.5 mm l = n.i. Stainless-steel zig-zag reaction channel i.d. = 0.90 mm l = n.i. Stainless micro-tube embedded with wire coil (30 cm length, 0.5 mm pitch) i.d. = 1.58 mm l = n.i. Stainless-steel tube

Rapeseed oil + methanol

1% KOH

60 °C

6:1

6 min

98.80%

Sun et al. (2008)

Cottonseed oil + methanol

1% KOH

60 °C

6:1

5.89 min

99.40%

Sun et al. (2008)

Sunflower oil + methanol

1% KOH

60 °C

23.9:1

112 s

100%

Sunflower oil + methanol

4.5% KOH

60 °C

23.9:1

100 s

100%

Guan, Kusakabe, Moriyama, and Sakurai (2008) Guan et al. (2009a)

Canola oil + methanol

1% NaOH

60 °C

6:1

3 min

99.80%

Jachuck et al. (2009)

Soya bean oil + methanol

1.2% NaOH

56 °C

17:1

28 s

99.5%

Wen et al. (2009)

Cottonseed oil + methanol

1% KOH

70 °C

8:1

17 s

99.5%

Sun et al. (2009)

Castor oil + ethanol

1% NaOH

50 °C

12:1

10 min

96.70%

Martínez Arias et al. (2012)


1% NaOH

50 °C

12:1

10 min

95.30%



1% NaOH

50 °C

12:1

10 min

93.50%



1.20% KOH

59 °C

8.5:1

14.9 s

99.50%

Dai et al. (2014)


1.20% KOH

60 °C

9:1

180 s

99%

Aghel et al. (2014)

Soya bean oil + methanol + hexane (co-solvent)

1% KOH

57.2 °C

6:1

9.05 s

98.80%

Waste vegetable oil + methanol + THF (co-solvent)

1% NaOH

70 °C

12:1

n.i.

97%

Rahimi, Mohammadi, Basiri, Parsamoghadam, and Masahi (2016) Elkady et al. (2015)

Stirred tank

Split and recombine micromixer T-type

T-mixer

T-shaped junction

Split interdigital micromixer (SIMM-V2) T-Type

T-Type

T-Type

Zig-zag micro-channel mixer

T-type

Four-way micromixer with 45° confluence angle KM micromixer

n.i. = no information, i.d. = inner diameter, l = length, dh = hydraulic diameter.

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Fig. 1. Schematic diagram of a micro-reactor system with a digital image of the side view of the micro-reactor. Reprinted with permission from Al-Dhubabian (2005). Copyright 2005 Oregon State University.

et al. (2009a) performed transesterification of sunflower oil using a FEP tube of 0.8 mm in diameter with T-shaped junction. A complete conversion of oil was obtained at 300 mm tube length, 4.5 wt% KOH concentration, in a residence time of 100 s using the same alcohol to oil molar ratio and operating conditions as their previous work (Guan et al., 2008). Jachuck et al. (2009) used a PTFE (Polytetrafluoroethylene) narrow channel reactor with an internal diameter of 1.5 mm, coupled with a T-mixer to intensify the biodiesel production process. Conversions higher than 98% were obtained in a residence time of 3 min with NaOH loading of 1 wt% and methanol to canola oil molar ratio of 6:1 at 60 °C. In the previous works, simple T-flow structures were applied to generate dispersion in micro-reactors (Canter, 2006; Guan et al., 2009a; Sun et al., 2008). To investigate another high efficiency passive microstructured mixers for biodiesel production, Wen et al. (2009) developed nine different zig-zag micro-channel reactors, zig-zag-1 to

zig-zag-9. The micro-channel reactors were fabricated on stainless-steel sheet by electric spark processing and assembled with a T-shaped three-way mixer. The micro-channels of all reactors were of rectangular cross-section with the same length of 1.07 m but a different number of periodic turns (10, 50, 100, 200 & 350) and hydraulic diameter (240–900 μm) as shown in Table 3. Using the micro-channel reactor with a smallest hydraulic diameter (240 μm) and most number of turns (350), a maximum yield of 99.5% was reached with methanol to soya bean oil molar ratio of 9:1, NaOH concentration of 1.2 wt% in a very short residence time of 28 s. Based on the same idea, Dai et al. (2014) developed a micro-reactor in which the conventional T-type mixer used by Wen et al. (2009) was replaced by zig-zag micro-channel mixers patterned on stainless-steel sheet along with zig-zag reaction channels. The mixing and reacting channels were linked through a connecting channel as shown in Fig. 2. Using a micro-channel of hydraulic diameter

Fig. 2. The configuration of zig-zag micro-channel reactor with zig-zag mixers. Reprinted with permission from Dai et al. (2014). Copyright 2014 American Chemical Society.


(A)

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(B)

Fig. 3. (A) Mixing forms and channel dimensions (width × height) of various micromixers: (a) T-mixer, (b) J-mixer, (c) rectangular interdigital micromixer (RIMM), and (d) split interdigital micromixer (SIMM-V2). M, an inlet of the methanol phase; O, an inlet of oil. “Reprinted with permission from Sun et al. (2009). Copyright 2010 American Chemical Society.” (B) PDMS micro-reactor internal geometries: (a) Omega-shaped, (b) Tesla shaped, and (c) T-shaped. Reprinted with permission from Martínez Arias et al. (2012). Copyright 2012 American Chemical Society.

1.5 mm, a biodiesel yield of 99.5% was obtained at a residence time of 14.9 s, methanol to soya bean oil molar ratio of 8.5:1 and KOH concentration of 1.2 wt% at 59 °C. Sun et al. (2009) performed experiments on the fast synthesis of biodiesel from cottonseed oil using high throughput micro-structured reactor with various mixer types as shown in Fig. 3(A). The FAME yields obtained using the micromixers were almost twice of the yields obtained using T- and J-mixers due to more intense mixing in micromixers. Using rectangular interdigital micromixer (RIMM) a maximum yield of 99.5% was achieved under the conditions of methanol to oil molar ratio of 8:1, a reaction temperature of 70 °C, the residence time of 17 s and a flow rate of 10 mL/min. Similarly, Bhoi et al. (2014) investigated KOH catalyzed biodiesel production using three different reaction systems having different microfluidic junctions, namely, T-type, cross-type and split and recombine mixer followed by serpentine mixing and reaction channels etched on a glass chip. It was observed that using methanol to sunflower oil molar ratio of 10.3:1, conversion above 90% could be obtained in a residence time of 0.5 min, 1 min, and 5 min with, T-type, cross-type and split and recombine mixer respectively. Avellaneda and Salvadó (2011) demonstrated a helicoidal system comprising a T-mixer for aggregation of reactants and a series of spirals (i.d. 6 mm) connected consecutively by a fixture, for the continuous production of biodiesel using pre-treated recycled oil. It was determined that 89% yield of fatty acid methyl ester could be attained in 13 min at 60 °C, methanol to oil molar ratio of 6:1 and using 0.6 g of NaOH as catalyst per 100 g of oil. For the same yield in the batch reactor, a residence time of 75 min was required. Santacesaria et al. (2011) proposed the use of a tubular stainless-steel reactor of 10 mm inner diameter and 20 cm in length with three different packing configurations, that were, (a) spheres of 2.5 mm diameter, (b) spheres of 2.5 and 1 mm diameters and (c) spheres of 2.5 and 0.39 mm diameters, forming micro-channels of about 1000 μm, 500 μm and 300 μm respectively, for studying KOH catalyzed methanolysis of soya bean oil. Using a methanol to oil ratio of 6:1 at 60 °C, very high oil conversions were observed in residence time of b1 min. In their subsequent work, Santacesaria, Di Serio, et al. (2012) used the same cylindrical tube filled with stainless-steel wool to obtain a higher void fraction which resulted in increased productivity. Martínez Arias et al. (2012) employed different channel geometries (Omega, Tesla, and T-shaped), as shown in Fig. 3(B) made of polydimethylsiloxane (PDMS) using soft lithography process for producing biodiesel from canola oil and ethanol in presence of NaOH as a catalyst.

The micro-channels were of the quadratic cross-section with width and height of 500 μm and length of 1 m. For T-, Tesla and Omega shaped micro-reactors, ethyl ester conversions of 93.5%, 95.3%, and 96.7% were attained respectively, at alcohol to oil molar ratio of 25:1, catalyst loading of 1 wt% and a reaction temperature of 50 °C in 10 min. Aghel et al. (2014) studied biodiesel synthesis using a modified microreactor consisting of a silver wire coil inserted in a stainless-steel tube of 0.9 mm inner diameter and a micromixer. At T- shaped junction of micromixer, soya bean oil and methanol were mixed in a molar ratio of 1:9 with 1.2 wt% of KOH as a catalyst. Using a wire coil with 0.8 mm average diameter, 30 cm length and 0.5 mm average pitch length, a FAME conversion of 99% was obtained at 60 °C in residence time of 180 s. Recently, Santana et al. (2016) performed experiments in a PDMS based T-junction micro-channel (l × w × h; 41.1 cm × 1.5 cm × 0.2 cm) reactor. In their experiments, sunflower oil was used as feedstock with ethanol to produce biodiesel through transesterification with sodium hydroxide as a catalyst. It was observed that minimum residence time of 1 min was needed at room temperature for a conversion of 95.8% as compared to the batch process in which 94.1% conversion was achieved in 180 min. In order to increase the conversion and reduce the residence time, the same group (Santana, Tortola, Silva, & Taranto, 2017) performed experiments in a PDMS micro-channel (l × w × h; 35.1 cm × 1.5 cm × 0.2 cm) reactor with internal static elements (1000 μm × 100 μm) for the improved mixing. It was found that maximum yield (99.53%) was obtained with 1 wt% catalyst concentration, residence time of ~12 s and ethanol to oil molar ratio of 9:1 at 50 °C. Guan, Sakurai, and Kusakabe (2009b) and Rahimi et al. (2016) studied the production of biodiesel in the presence of co-solvent at room temperature using micro-tube reactors. Guan, Sakurai, and Kusakabe (2009b) reported biodiesel production at room temperature in the presence of cosolvent in a FEP tube (0.96 mm i.d) micro-reactor with the T-shaped junction. Using diethyl ether (DEE) as co-solvent in a molar ratio of 0.73 to methanol, 1 wt% KOH as catalyst and methanol to sunflower oil molar ratio of 8:1, a conversion of 93.2% at a tube length of 36 cm was observed in 93 s. Later, Rahimi et al. (2016) performed transesterification of soya bean oil in the presence of hexane as a co-solvent using three different four-way micromixers with 0.8 mm inner diameter followed by a stainless-steel tube with inner diameter of 1.58 mm. The micromixers were designed with various confluence angles of 45o, 90o and 135o namely E1, E2 and E3 respectively on a polymethylmethacrylate (PMMA) flat plate by milling as shown in Fig. 4. The highest FAME

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Fig. 4. The experimental setup used by Rahimi et al. (2016). Reprinted from Rahimi et al. (2016), with permission from Elsevier.

yield of 98.8% was observed using E1 mixer under optimal conditions of methanol to oil volumetric ratio of 3:1, hexane to methanol volumetric ratio of 0.45, reaction temperature of 57.2 °C and 1 wt% KOH concentration. Several investigations were performed on the continuous catalystfree production of biodiesel using micro-reactors in the presence of co-solvent and heterogeneous catalyst under supercritical conditions (Bertoldi et al., 2009; Silva, Castilhos, Oliveira, & Filho, 2010; Schürer et al., 2014, 2016; Trentin et al., 2011). Bertoldi et al. (2009) carried out transesterification of soya bean oil using supercritical ethanol with carbon dioxide as a co-solvent in a tubular micro-reactor which was made of stainless-steel with an internal diameter of 3.2 mm or 0.76 mm. The reactions were performed in the absence of a catalyst at the temperature and pressure ranges from 300 to 350 °C and 7.5 to 20 MPa respectively. In addition, the ranges of oil to methanol molar ratio and co-solvent to substrate mass ratio were also maintained as 1:10 to 1:40 and 0:1 to 0.5:1 respectively. It was found that the FAEE (fatty acid ethyl ester) yield was improved at the optimum design and operating conditions such as 0.76 mm diameter tube, CO2 to substrate mass ratio of 0.05:1, oil to ethanol molar ratio of 1:40 and at 350 °C and 10 MPa. Sootchiewcharn, Attanatho, and Reubroycharoen (2015) produced biodiesel from palm oil with supercritical ethylacetate in a stainlesssteel micro-reactor with 0.762 mm i.d. and 15.14 m in length. At a temperature of 350 °C, oil to the ethyl acetate molar ratio of 1:50, and ethyl acetate pressure of 20 MPa, biodiesel yield of 78.3% was obtained in 20 min. Recently, Schürer et al. (2014) reported a stainless-steel micro-channel reactor coated with alumina as a catalyst to synthesize biodiesel under supercritical conditions. A complete conversion of tricaprin in residence time of b30 s was observed at 375 °C and 10 MPa pressure. Unlike the conventional micro-tubular and micro-channel reactors used for biodiesel production, Machsun, Gozan, Nasikin, Setyahadi,

and Yoo (2011) developed a continuous bio catalytic membrane micro-reactor using an asymmetric polyether sulfone membrane with a nominal molecular weight limit of 300 kDa (PES 300) as an enzyme carrier. Each pore of membrane acted as a particular micro-system in which biocatalytic transesterification of triolein with methanol occurred. With a membrane diameter of 63.5 mm and 0.28 mm thickness, 80% conversion of triolein to methyl oleate (biodiesel) was acquired in a reaction time of 19 min. Kurayama et al. (2010) reported that microcapsules could be used as a micro-reactor for biodiesel production. The transesterification of rapeseed oil was carried out by adding methanol to Calcium oxide (CaO) loaded micro-capsules and oil mixture in the batch-type reactor. A maximum FAME yield was reached under optimum conditions of methanol to oil molar ratio of 8:1, 20 wt% of CaO content in micro-capsule and the temperature of 65 °C. From the above literature, it is evident that biodiesel production process is highly intensified applying micro-reactor technology. The Table 2 Comparison of batch versus micro-reactor plant for production of biodiesel. Reprinted with permission from Buddoo et al. (2008). Production of biodiesel

Batch plant

Micro-reactor plant

Plant output (tonne/yr) Reactor volume (m3) Plant footprint (m2) Surface area to volume ratio (m2/m3) Productivity (kg/h/m3) Energy input (kJ per kg) Mass transfer coefficient kla (s−1) Heat transfer coefficient (kJ/m3) Mixing efficiency (Re) Capital cost (Rm) Manufacturing costs (R/L)

20,000 10 149 14.9 250 7.1 10−2–10 628 7 × 105 8.6 6.60

20,000 2.4 × 10−3 60 2.5 × 104 10.4 × 105 0.4 10–100 2.86 × 106 10 6.5 5.87

Comments

4167× smaller 60% smaller 1678× higher 4167× higher 18× lower 104 higher 4554× higher 7 × 104 higher 24.4% saving 11.1% saving


residence time of several hours in a conventional stirred tank reactor can be minimized up to several seconds using a micro-reactor. A comparison between batch and micro-reactor plant for biodiesel production (Table 2) exhibited a superior performance of the micro-reactors over the batch plant regarding productivity, energy input and capital cost for same plant capacity (Buddoo et al., 2008). However, in most of the studies application of this technology is confined to lab scale, producing biodiesel only in small quantities. Billo et al. (2015) developed a cellular manufacturing process for fabrication and assembly of a full-scale micro-reactor to produce biodiesel at the rate of 2.47 L/min and a capacity of 1.2 million L per year. The scale-up of micro-reactor was done through numberingup of individual micro-channel laminae into subassemblies and assemblies. Schürer et al. (2016) designed a micro-reactor plant with annual capacity of 40 t for production of biodiesel under super critical conditions in the presence of a heterogeneous catalyst. The micro-reactor consisted of three stainless-steel plates on which the micro-channels were etched by wet chemical etching. 30 wt% La2O3 supported by γ-Al2O3 was coated on to the micro-channels as a catalyst. The plates were stacked and sealed by laser welding. At a reaction conditions of 375 °C and 175 bar and methanol to oil molar ratio of 40:1, a complete conversion of rapeseed oil was obtained within a few seconds. Factors affecting biodiesel production The process of biodiesel production is affected by various factors depending on the reaction conditions and the geometrical parameters of micro-reactor used. The effects of these factors are discussed below. Effect of reaction temperature The effect of temperature on the transesterification of vegetable oils has been studied over a wide range of reaction temperatures in microreactors. It was noticed that the temperature of reaction clearly influences the yield of the biodiesel product (Aghel et al., 2014; Jachuck et al., 2009; Martínez Arias et al., 2012). Using a micro-tube reactor, it was found that the conversion of sunflower oil increased with increase in reaction temperature (Guan et al., 2009a). Martínez Arias et al., 2012 studied the effect of temperature on biodiesel yield using three different micro-reactors. As the temperature was increased from 30 to 70 °C the biodiesel yield increased from 67.3 to 89.0%, from 73.9 to 92.2% and from 75.9 to 92.6% for T-, Omega-, and Tesla-shaped micro-reactors,

149

respectively as shown in Fig. 5. It can be ascribed to enhanced mass transfer caused by an improvement in alcohol–oil miscibility and increased rate of reaction at elevated temperature (Čerče, Peter, & Weidner, 2005; Zhou, Lu, & Liang, 2006). Moreover, it was observed that on increasing the temperature from 46 to 60 °C the percentage increase in conversion was higher at lower catalyst loading in a microchannel reactor. At a catalyst loading of 0.25 wt%, the conversion increased by 14.80% while only 2.5% increase in conversion was obtained for 1 wt% catalyst loading (Jachuck et al., 2009). Most of the works were carried out at temperatures below the normal boiling point of methanol. Sun et al. (2009) examined the effect of reaction temperature on transesterification of cottonseed oil both below and above the boiling point of methanol using a micro-reactor with rectangular interdigital micromixer (RIMM). The fatty acid methyl ester yield increased as the reaction temperature was increased from 60 to 80 °C at methanol to oil molar ratio b9:1. The two-phase slug flow at 60 °C was transformed to gas-liquid two-phase slug-annular flow when the temperature was raised to 80 °C. However, Aghel et al. (2014) and Sun et al. (2008) found that when the reaction temperature increases beyond the optimum level, the biodiesel yield decreases. The fatty acid methyl ester yield increased from 96.2% to 99.4% as the temperature increased from 30 to 60 °C, but further increase in temperature to 70 °C resulted in a decrease of FAME yield to 99.1% (Sun et al., 2008). Aghel et al. (2014) observed an increase in the percentage of FAME yield with increase in temperature from 55 °C to 60 °C but the yield decreased rapidly when the temperature was increased from 60 °C to 65 °C. It may be because of the change in flow pattern from slug to bubble flow and the accelerated saponification reaction of triglycerides in the presence of an alkaline catalyst at higher temperatures (Aghel et al., 2014; Leung & Guo, 2006; Sun et al., 2008). To perform continuous biodiesel production by two-step acid catalyzed process using acid oil, temperatures N100 °C were needed. Both esterification and transesterification steps were observed to be positively affected by the reaction temperature (Sun, Sun, Yao, Zhang, & Xu, 2010). Even higher reaction temperatures were required for production of biodiesel under supercritical conditions in micro-reactors (Gonzalez et al., 2013; Silva et al., 2014; Silva & Oliveira, 2014; Sootchiewcharn et al., 2015). An increase in the FAEE yield was reported with increase in the reaction temperature. However, at prolonged residence times, too high temperatures led to thermal decomposition of the product (Gonzalez et al., 2013; Sootchiewcharn et al., 2015). Compared to higher reaction temperatures, when the reactions were carried out at room temperature, a longer residence time was required to obtain a FAME yield above 90% (Canter, 2006). Residence time

Fig. 5. Influence of reaction temperature on yield of ethyl ester using an ethanol/oil molar ratio of 9:1, NaOH concentration of 1 wt%, and residence time of 10 min in T-, omega-, and Tesla-shaped micro-reactors. Reprinted with permission from Martínez Arias et al. (2012). Copyright 2012 American Chemical Society.

The residence time for biodiesel production in micro-reactors varies with the type of micro-reactor applied. In recent years, several studies on biodiesel production in micro-reactors have reported that an increase in residence time is favourable for higher biodiesel yield. Martínez Arias et al. (2012) transesterified castor oil under the conditions of ethanol to oil molar ratio of 9:1, catalyst amount of 1 wt% at 60 °C in three different micro-reactors. It was observed that for first 4 min of the reaction, with increase in the residence time the biodiesel yield increased, and after that, the yield attained almost a constant value as shown in Fig. 6. After a residence time of 15 min, biodiesel yield of 75.9%, 94.1%, and 93.7% was acquired for T-, Omega and Tesla shaped micro-reactors respectively. Aghel et al. (2014) observed a similar effect of residence time on transesterification of soya bean oil with methanol using micro-reactor with a wire coil. The percentage of methyl ester conversion increased from 87.9% to 99% with an increase in residence time from 20 to 180 s. The conversion increased rapidly for first 20 to 60 s and then very slightly as the residence time was prolonged from 60 to 180 s. However, in the study of Sun et al. (2008) methyl ester yield first increased with increase in residence time from

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Fig. 6. Ethyl ester yield of castor oil transesterification carried out in T-, omega-, and Teslashaped micro-reactors at different residence times and using an ethanol/oil molar ratio of 9:1, catalyst amount of 1 wt% and temperature of 50 °C. Reprinted with permission from Martínez Arias et al. (2012). Copyright 2012 American Chemical Society.

3.68 to 5.89 min then on the further prolongation of residence time the yield decreased to about 92%. It may have resulted due to weakened mass transfer (decrease in internal circulations) at prolonged residence time resulted by a decrease in average velocity for a fixed length of capillary (Dummann et al., 2003; Sun et al., 2008). Under supercritical conditions, the residence time for transesterification of oils in micro-reactors can be shortened up to a few seconds. Using supercritical methanol in absence of catalyst, a complete conversion of triglyceride was achieved in a micro-channel reactor at a residence time of 5 min (Schürer et al., 2014). However, when the supercritical transesterification was performed in presence of a heterogeneous catalyst coated on the micro-channels, a complete conversion was obtained in a very short residence time of b15 s (Schürer et al., 2014; Schürer et al., 2016).

Alcohol to oil molar ratio The molar ratio of alcohol to oil is one of the most important parameters affecting the yield of alkyl ester. Stoichiometrically, 3 mol of alcohol is required for transesterification of 1 mol of oil to yield 3 mol of alkyl ester and 1 mol of glycerol. However, an excess of alcohol to oil molar ratio results in greater oil conversions (Freedman et al., 1984). A linear increase in fatty acid methyl ester yield was observed as the methanol to cottonseed oil molar ratio was raised from 3:1 to 9:1 between 60 and 80 °C with 1 wt% KOH catalyst, in a micro-reactor assembly consisting of rectangular interdigital micromixer (RIMM) connected to a stainless-steel delay loop (Sun et al., 2009). Aghel et al. (2014) examined the effect of alcohol to oil molar ratio on ester yield with alkali catalyst using methanol to oil molar ratios of 6:1, 9:1 and 12:1 in two different micro-reactors, one with an inserted wire coil and other without the coil. The results indicated that increase in a molar ratio from 6:1 to 9:1 led to 4.6% and 7.2% improvement in the percentage of methyl ester for micro-reactor with and without coil respectively as shown in Fig. 7. However, as the molar ratio was increased above 9:1, a slight decrease in the percentage of methyl ester was observed. This behavior between methanol to oil molar ratio and ester yield agreed with Sun et al. (2008) observations. Sun et al. (2008) concluded that in a capillary micro-reactor when the methanol to soya bean oil molar ratio was increased from 3 to 6, the methyl ester yield increased from 82.1% to 99.3% at 60 °C and 1 wt% KOH concentration. However, a further increase in the molar ratio resulted in a decrease in yield to 95.3%. This unexpected decrease in fatty acid methyl ester yield was probably because at too high methanol to oil ratio; methanol acts as an emulsifier that

Fig. 7. Effect of molar ratio (methanol/oil) on FAME%. Reprinted from Aghel et al. (2014), with permission from Elsevier.

could cause a part of glycerol to remain in biodiesel phase (Leung & Guo, 2006). Compared to alkali-catalyzed transesterification higher alcohol to oil molar ratios were required under acid catalyzed transesterification of oils with a significant amount of free fatty acid content in oil. A molar ratio of 30:1 and 20:1 was used for esterification and transesterification respectively, with sulfuric acid as a catalyst to obtain a yield of 99.5% in a stainless-steel capillary micro-reactor. In the esterification step, the conversion of FFA (free fatty acid) increased with increase in methanol to FFA molar ratio from 10:1 to 30:1. Whereas, in the transesterification step the yield first increased from 96.1 to 99.9% as the methanol to triglyceride molar ratio was increased from 6 to 20 and then decreased on further increase in the molar ratio (Sun et al., 2010). Under supercritical conditions, much higher alcohol to oil molar ratios were required in comparison to subcritical transesterification (Gonzalez et al., 2013; Silva & Oliveira, 2014; Trentin et al., 2011). Hence, transesterification in microreactors is positively affected by an increase in alcohol to oil molar ratio, but too high alcohol to oil molar ratios may lead to a decrease in biodiesel yield. Moreover, the alcohol to oil molar ratio depends on the type of micro-reactor applied, reaction conditions (subcritical or supercritical) and catalyst used for biodiesel production. Effects of geometrical configurations Effect of channel size The micro-channel size in a micro-reactor has a strong influence on the yield of methyl ester. It could be attributed to different mass transfer distance and rate between the oil and alcohol phases in various microchannel sizes. Using 0.25 mm inner diameter capillary micro-reactor the methyl ester yield of 98.8% was obtained in a residence time of 6 min at methanol to oil ratio of 6:1 and 1 wt% KOH concentration while the yield reached only 96.7% in a residence time of 8.2 min when 0.53 mm inner diameter capillary micro-reactor was used under same operating conditions (Sun et al., 2008). Guan et al. (2009a) observed a linear increase in sunflower oil conversion as the micro-tube diameter was decreased from 1 mm to 0.8 mm, 0.6 mm and 0.4 mm. A similar relation between micro-channel size and biodiesel yield was noticed when zigzag micro-channel reactors with a hydraulic diameter between 240 and 900 μm were used for transesterification of soya bean oil. The microchannel with smallest hydraulic diameter gave the maximum biodiesel yield as shown in Table 3 (Wen et al., 2009). Kalu, Chen, and Gedris (2011) studied the effect of channel depth on the performance of the reactor using slit channel reactor for biodiesel synthesis. The result indicated that with a decrease in channel depth from 10, 5 and 2 to 1 mm the

A. Tiwari et al. / Energy for Sustainable Development 43 (2018) 143–161 Table 3 Geometric parameters and performances of micro-channel reactors. Reprinted from Wen et al. (2009), with permission from Elsevier. Name

Section (μm × μm)

Hydraulic diameter (μm)

Turns

Yield (%)a

Zig-zag-1 Zig-zag-2 Zig-zag-3 Zig-zag-4 Zig-zag-5 Zig-zag-6 Zig-zag-7 Zig-zag-8 Zig-zag-9

200 × 300 300 × 500 500 × 500 500 × 900 900 × 900 500 × 500 500 × 500 500 × 500 500 × 500

240 375 500 643 900 500 500 500 500

350 350 350 350 350 10 50 100 200

97.3 91.3 81.1 80.9 77.8 60.0 63.2 70.2 79.8

a Reaction conditions: molar ratio of methanol/oil 6, catalyst amount 1% (based on oil weight), temperature 60 °C, and residence time 28 s.

conversion of soya bean oil increased. With increasing depth, the channels approach the batch system and hence are less efficient than shallow channels. Effect of inlet mixer type The mixing mechanism has a strong influence on the reaction when it is carried out between two immiscible liquids. Therefore, the biodiesel production in micro-reactor is affected by the configuration of mixer applied to enhance the mixing of alcohol and oil phase. The microfluidic systems for biodiesel synthesis consist of an integrated mixer to generate dispersion, along with the reaction channel. A FAME yield of 98.80% was obtained in a residence time of 6 min when instead of a microfluidic mixer, the reactants were mixed in a batch reactor then injected into 0.25 mm capillary micro-reactor. The yield already reached 81% before the mixture was pumped into the micro-reactor (Sun et al., 2008). Guan et al. (2008) carried out transesterification of sunflower oil using a T-type and a split and recombine micromixer. The oil conversion was higher when the micromixer was used instead of a T-type mixer. Applying the split and recombine micromixer it was noticed that sunflower oil could be completely converted to biodiesel in a residence time of as short as 112 s. Even shorter residence time of 14.9 s was observed to reach a yield of 99.5% when zig-zag mixer channels with a zig-zag reaction channel (Fig. 2) were used for the production of biodiesel using soya bean oil (Dai et al., 2014). Sun et al. (2009) studied the effect of mixer type on the biodiesel yields using four different mixers as shown in Fig. 3(A), namely, a Tmixer, a J-mixer, a rectangular interdigital micromixer (RIMM) and a

(a)

151

slit interdigital micromixer (SIMM-V2) followed by stainless-steel capillary with an i.d. of 0.6 mm. The ester yields obtained using T and J mixer were almost equal and half of those achieved with the RIMM and SIMM-V2 as represented in Fig. 8(a). It may be due to smaller mixer dimensions and more intensified mixing in RIMM and SIMM-V2 micromixers. Similarly, three different mixers such as T-type, cross(†) type and split and recombine were used for methanolysis of sunflower oil. Micro-reactor with †-type mixer appeared to be better than the micro-reactor with a T-type mixer and the micro-reactor with split and recombine micromixer (Bhoi et al., 2014). Rahimi et al. (2016) designed three unlike micromixers with different confluence angles of 45°, 90° and 135° namely E1, E2 and E3 respectively (Fig. 4) for biodiesel production from soya bean oil. The results demonstrated that the reaction was most enhanced in E1 mixer followed by E3 and E2 respectively. Hence from the above review, it can be concluded that efficiency the mixer is an effective parameter on the transesterification of oil to fatty acid alkyl ester. It can be attributed to varying dimensions of different mixers and the flow behavior at the outlet of the mixers. Apart from the mixer type, Shaaban et al. (2015) investigated the effect of mixer inner diameter and geometry on the yield of fatty acid ethyl ester at same reactor volume, flow rate and residence time using three different inlet mixers T1, T2 and T3 as shown in Fig. 8(b). The T1 mixer recorded highest conversion of 97% in 80 s using 9:1 methanol to oil molar ratio. This was ascribed to its smaller internal dimensions which led to formation of smaller slugs and increase in interfacial area. Effect of internal geometries Apart from diffusion, advection is another important form of mass transfer in a microfluidic system. The so-called chaotic advection can significantly improve mixing of oil and alcohol. For passive mixing in micro-channels, the chaotic advection can either be introduced by modifications in channel geometry or by inserting an obstacle in the channel (Nguyen & Wu, 2005). Several studies have been conducted for the production of biodiesel in micro-reactors with simple Y and T structures (Canter, 2006; Guan et al., 2009a; Jachuck et al., 2009). Moreover, there are few efforts on modified channel configurations for biodiesel synthesis. Using a zig-zag micro-channel reactor, the efficiency of biodiesel production process improved significantly. The FAME yield reached 99.5% in few seconds under mild operating conditions (Dai et al., 2014; Wen et al., 2009). Martínez Arias et al. (2012) investigated the effect of channel geometries on alkali-catalyzed transesterification of castor oil using a T-shaped, Tesla shaped and Omega shaped micro-reactor. Under

(b)

Fig. 8. (a) FAME yields of different micromixers at 60 °C for a methanol to oil molar ratio of 8:1, a KOH concentration of 1 wt%, a total flow rate of 1 mL/min, and a residence time of 44 s: (T) T-mixer, (J) J-mixer, (RIMM) rectangular interdigital micromixer, (V2) (SIMM-V2) split interdigital micromixer. Reprinted with permission from Sun et al. (2009). Copyright 2010 American Chemical Society. (b) Specifications of the micromixers used by Shaaban, El-Shazly, Elkady, and Ohshima (2015). Reprinted with permission from Shaaban et al. (2015).

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Table 4 Ethyl ester yields for different volumetric rates using ethanol/oil molar ratio of 9:1, catalyst amount of 1 wt% and temperature of 50 °C. Reprinted with permission from Martínez Arias et al. (2012). Copyright 2012 American Chemical Society. Geometry

Transsection (μm × μm), 500 × 500 hydraulic dia, 500 μm

Volumetric rate (mL/h) Castor oil

Ethanol

15.0 7.5 5.0 3.8 3.0 2.1 1.5 1.0

8.1 4.1 2.7 2.1 1.6 1.1 0.8 0.5

Residence time (min)

Ethyl ester conversion (%) T-shaped Omega shaped

Tesla shaped

1 2 3 4 5 7 10 15

58.9 69.4 73.1 74.7 71.1 75.1 73.4 75.9

69.2 83.8 88.8 91.5 91.1 91.7 92.2 93.7

67.1 79. 3 87.5 88.3 89.7 88.5 87.4 91.4

same operating conditions, the Omega and Tesla shaped micro-reactors resulted in higher ethyl ester conversions, followed by T-shaped microreactor as shown in Table 4. It can be ascribed to chaotic flow in Tesla and Omega micro-reactor which favoured better contact between the oil and alcohol phase. Recently, Aghel et al. (2014) developed a micro-reactor with an inserted wire coil to improve the mixing efficiency. Compared to a simple T-shaped micro-reactor used in their previous work (Rahimi et al., 2014), the micro-reactor equipped with wire coil produced a higher percentage of FAME at the same reaction conditions. Similarly, the efficiency of micro-channel reactor with static elements (MSE) in terms of the mixing and reaction was also compared with plain T-shaped microchannel reactor without static elements. It was found that the microchannel with static elements showed superior performance of mixing index and oil conversion due to induced changes in the direction of flow, disturbances in boundary layer and formation of vortex (Santana et al., 2017). From the above review, it is noticeable that better conversion in micro-reactors with modified micro-channel structure can be obtained as a result of chaotic mixing in comparison to laminar mixing in simple T and Y micro-channels. Effect of co-solvent The biggest obstacle in the transesterification of vegetable oil is immiscibility of alcohol and oil phase that slows down the reaction significantly. In order to overcome slow reaction rates caused by the extremely low solubility of the alcohol in the triglyceride phase, addition of an organic co-solvent has been recommended by Meher et al. (2006). Several works on transesterification of triglycerides to fatty acid alkyl ester using an organic co-solvent have been performed at micro-scale to take the advantages of micro-reactors simultaneously (Elkady et al., 2015; Guan, Sakurai, & Kusakabe, 2009b; Rahimi et al., 2016). An improved process was investigated for methanolysis of sunflower oil with co-solvent in a micro-tube reactor. At a methanol to oil molar ratio of 8:1 and 25 °C, oil conversion was considerably high in the presence of co-solvent (Diethyl ether, DEE) as compared to conversion obtained without co-solvent. It was observed that the flow was homogenous at the entrance of the micro-tube and with the formation of immiscible glycerol the homogenous flow was disturbed (Guan, Sakurai, & Kusakabe, 2009b). Using Tetrahydrofuran (THF) as co-solvent, transesterification of waste vegetable oil was observed at methanol volumetric ratio ranges of 0.2 to 1 in a KM micromixer. As shown in Fig. 9, an increase in the biodiesel yield was seen with an increase in the THF to methanol volumetric ratio. A maximum yield of 97.30% was obtained at THF to methanol volumetric ratio of 0.3:1. However, while increasing the ratio above 0.3:1, no noticeable increase in the yield was found. In particular, THF was chosen because its boiling point of 67 °C is only two degrees higher

Fig. 9. Effect of co-solvent volumetric ratio (THF/methanol) on percentage biodiesel yield. Reprinted with permission from Elkady et al. (2015).

than that of methanol. Therefore, at the end of the reaction, the unreacted methanol and THF can be co-distilled and recycled (Elkady et al., 2015). Rahimi et al. (2016) studied the influence of co-solvent (hexane) to methanol volumetric ratio on the FAME content. Using an E1 mixer (see Fig. 4), an increase in the percentage of FAME from 81% to 88% was reported as the co-solvent to methanol volume ratio was increased from 0.1 to 0.4 at 45 °C, 2:1 oil to methanol volumetric ratio and 9 s residence time. However, on further increasing the ratio, the FAME content decreased. It may be due to dilution of reactants at large amounts of co-solvent (Mohammed-Dabo, Ahmad, Hamza, Muazu, & Aliyu, 2012; Pardal, Encinar, González, & Martínez, 2010; Zhang, Jin, Zhang, Huang, & Wang, 2012).The addition of co-solvent to alcohol to overcome the immiscibility improves the contact between the alcohol and oil phase but increases the number of downstream purification steps.

Effects of catalyst type and concentration Depending on the characteristics of feedstock, the nature of catalyst employed plays an important role for transesterification of triglycerides to biodiesel. The production of biodiesel can be carried out by acid, alkali or enzyme-catalyzed processes or in the absence of catalyst using supercritical methanol. The alkali and acid catalysts include both homogeneous and heterogeneous catalysts. Homogenous alkaline catalysts are most commonly used to catalyze transesterification due to their low cost, shorter reaction times and high conversion under mild operation conditions (Casas, Fernández, Ramos, Pérez, & Rodríguez, 2010; Demirbas, 2007; Freedman, Butterfield, & Pryde, 1986; Wang, Ou, Liu, & Zhang, 2007). However, the process involving homogeneous alkaline catalysts needs feedstock with free fatty acid content b1 wt%. In the case of high FFA levels, a considerable amount of alkaline catalyst reacts with FFAs to form soaps which hinder biodiesel purification process and disturbs the flow patterns in micro-reactors (Guan et al., 2010). Although alkali catalyzed, reactions are much faster compared to acid catalysts, to overcome the difficulties due to soap formation acid catalyzed transesterification is preferable when high content of FFA (N1 wt%) is present in the oil. The acidic catalysts yield high biodiesel content for high FFA feedstock, but it is greatly ignored due to long reaction times, high catalyst concentration and a large amount of methanol requirement (Sun et al., 2010). The heterogeneous catalysts are still under development, and only a limited number of investigations have been done on heterogeneous catalysts for the production of biodiesel in micro-reactors (Kurayama et al., 2013; Kurayama et al., 2010; Machsun et al., 2011; Schürer et al., 2014; Schürer et al., 2016). The heterogeneous catalyzed process exhibits some advantages, such as good tolerance to free fatty acids, simple product purification, no waste water treatment and mild operating conditions but because of high production cost and short lifetime, their application for biodiesel production is limited (Gorji, 2015; Sun et al., 2010).


Apart from catalyzed process in micro-reactors, research has been done using supercritical alcohols for biodiesel synthesis (Silva et al., 2012; Farobie, Sasanami, & Matsumura, 2015; Gonzalez et al., 2013; Schürer et al., 2014; Schürer et al., 2016; Silva et al., 2014; Sootchiewcharn et al., 2015). However, the high cost of the production process in the supercritical process due to strict demand of equipment and high temperature and pressure requirements is a major limitation (Chen et al., 2008). Besides the type of catalyst, the concentration of catalyst used in the reaction has an optimum value below which the reaction remains incomplete, and an excess catalyst amount may lead to reinforcing the lateral saponification reaction (Dunn, 2001). The optimum concentration of catalyst depends on the operating parameters as well as catalyst type (Gorji, 2015). Several investigations on the effect of catalyst concentration on biodiesel yield in micro-reactors have been performed (Guan et al., 2009a; Kaewchada, Pungchaicharn, & Jaree, 2016; Rashid, Uemura, Kusakabe, Osman, & Abdullah, 2014). A similar influence as observed for the conventional batch process was observed in microreactors. Homogeneous catalysts The majority of research for biodiesel production using micro-reactor technology has been done using low FFA feedstock or pre-treated feedstock in the presence of homogeneous alkaline catalysts. On the contrary, transesterification of high acid value oils catalyzed by homogeneous acid catalyzed has not been explored much. It may be due to simplicity, and strong catalytic activity of an alkali-catalyzed process in which the reaction proceeded in a single step while using acid catalyst required removal of water as it may hinder the transesterification process to some extent (Guan, Teshima, et al., 2009; Sun et al., 2010). However, use of alkali catalyst is limited to oils with low acid content because of the formation of soaps when a great amount of free fatty acid is present which disturbs the flow behavior in micro-channels. Alkaline catalysts. In micro-reactors, potassium hydroxide and sodium hydroxide are the most investigated homogeneous base catalysts for transesterification of vegetable oils. Sun et al. (2008) used KOH as a catalyst, with methyl alcohol to convert the cottonseed oil into biodiesel. By varying the concentration of catalyst at a constant methanol to oil molar ratio of 6, residence time of 6 min and 60 °C reaction temperature, they concluded that an increase in KOH concentration from 0.40 wt% to 1 wt% led to an increase in methyl ester yield from 86% to a maximum of 99.3% but with further increase in KOH concentration the FAME yield decreased to 94.80%. This reduction in FAME yield was ascribed to saponification of oil with KOH since an increase in soap concentration was observed from 0.2 to 0.34 wt%, when catalyst concentration was increased from 1.0 to 1.2 wt%. Similarly, the influence of catalyst amount was studied on transesterification of soya bean oil with three different KOH concentrations of 0.6, 1.2 and 1.8 wt%. The results indicated that the percentage of methyl ester increased when the catalyst concentration was raised from 0.6 to 1.2 wt% while a slight decrease in the proportion of methyl ester was observed when a KOH concentration of 1.8 wt% was used (Aghel et al., 2014). Dai et al. (2014) performed series of experiments using KOH as a catalyst in the range of 0.8–1.5 wt% for converting soya bean oil to methanol in a zig-zag micro-channel reactor. A maximum yield of 99.5% was obtained at 59 °C using 1.2 wt% KOH and 8.5:1 methanol to oil molar ratio in 14.9 s. Kaewchada et al. (2016) and Azam, Uemura, Kusakabe, and Bustam (2016) carried out KOH catalyzed transesterification of palm oil. It was noted that an increase in the concentration of catalyst positively affected the biodiesel yield. When NaOH is used as a catalyst, the reaction temperature applied is usually low to avoid emulsification and saponification (Xie et al., 2012). Experiments were conducted using four different NaOH concentrations from 0.25 to 1 wt%, for transesterification of canola oil in a small channel reactor. The residence time, methanol to oil molar ratio (6:1), temperature and pressure of reaction were kept constant for each catalyst loading. The results indicated that the use of higher catalyst

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concentration led to higher triglyceride conversion. However, the rate of change of conversion on catalyst loading was negligible at catalyst concentrations above 0.75% (Jachuck et al., 2009). Martínez Arias et al. (2012) carried out biodiesel synthesis in three different micro-reactors i.e. a T-, Tesla, and Omega shaped micro-reactors using different NaOH concentrations (0.5, 1.0, and 1.5 wt%) for each micro-reactor. The residence time, temperature and ethanol to castor oil molar ratio were held constant at 10 min, 50 °C and 9:1 respectively. It was observed that with an increase in catalyst concentration from 0.5 to 1.5 wt% the yield of ethyl ester increased from 50.6 to 79.1%, from 54.3 to 96.2% and from 56.3 to 98.9% for T-, Omega and Tesla shaped micro-reactors, respectively. Likewise, NaOH was tested as a catalyst in a range of 0.5 to 2 wt% for biodiesel production from waste vegetable oil in a KM micromixer. A maximum yield of 95% was achieved with 1 wt% catalyst concentration. Using 0.5 wt% catalyst concentration the oil conversion was observed to be incomplete while biodiesel yield decreased significantly as the NaOH concentration was increased above 1 wt% (Elkady et al., 2015). Santana et al. (2016) studied the relation between fatty acid ethyl ester percentage and catalyst concentration. The results indicated that the percentage of FAEE (fatty acid ethyl ester) yield increased slightly from 89.13% to 89.89% when the NaOH concentration was increased from 0.2 to 0.85 wt%, but it diminished when NaOH concentration was higher than 0.85%. Crawford et al. (2008) performed transesterification of triolein catalyzed by sodium methoxide in a commercial Syrris 250 μL micro-reactor. Using a 0.1 M catalyst concentration, a complete conversion of triolein to methyl oleate was obtained at room temperature in 2.5 min. It has been reported that compared to sodium hydroxide (NaOH), sodium methoxide (NaOCH3) is more efficient because it is disintegrated into CH3O− and Na+ and does not form water as in the case of NaOH/KOH. Additionally, in contrast to sodium hydroxide the required amount of sodium methoxide is about 50% lower, but due to its high cost it is not used widely (Shahid & Jamal, 2011). Acid catalysts. Despite the fact that alkaline catalysts are very efficient and common for production of biodiesel, these catalysts do not exhibit good results when the acid value and water content in oil are high (N1 wt%). The water contents and acid values of most of the nonedible and cheap feedstock are higher than performance range of base catalysts. In such cases, acid catalysts are preferred (Fukuda, Kondo, & Noda, 2001). Sulfuric acid is the most widely used acid catalyst, which can be used to conduct both esterification and transesterification simultaneously. Acid catalyzed transesterification of high FFA oils in micro-reactors has not been examined much. Sun et al. (2010) carried out production of biodiesel with acid oil as a feedstock by a two-step sulfuric acid catalyzed process in a micro-reactor assembled with a split interdigital micromixer (SIMM-V2) and a 0.6 mm i.d. stainless-steel capillary delay loop. Esterification of oleic acid and transesterification of cottonseed oil with methanol were investigated as model reactions to explore optimum condition for the two-step process. Based on these reactions, an acid catalyzed two-step process was developed with the first and second steps conducted separately under the optimized esterification and transesterification conditions. The results indicated that the acid value of the acid oil was reduced from 160 to 1.1 mg KOH/g, with a methanol to the acid molar ratio of 30, the H2SO4 concentration of 3 wt%, and a residence time of 7 min at 100 °C in the first step. The final FAME yield reached 99.5% with a methanol to triglyceride molar ratio of 20, the H2SO4 concentration of 3 wt%, and a residence time of 5 min at 120 °C in the second step. Thus, biodiesel can be produced in a residence time of b15 min by an acid catalyzed process in a microreactor. However, higher concentration of catalyst is required for acid catalyzed transesterification compared to base catalyzed transesterification. Heterogeneous catalysts Homogeneous catalysts are very efficient and are most frequently used in biodiesel production. However, these catalysts require excessive

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washing for their removal from the final product and subsequent treatment of waste water. As a result, a lot of water, energy and time are consumed. Besides that, these catalysts cannot be reused (Fukuda et al., 2001). The use of heterogeneous catalysts offers several advantages over homogeneous catalysts, such as; simple product separation, simple catalyst recovery, catalyst reusability, less energy requirements and less added cost of purification (Atadashi, Aroua, Aziz, & Sulaiman, 2013). Biodiesel synthesis in micro-reactors applying heterogeneous catalysts is still under investigations. To the best of our knowledge only heterogeneous alkaline and enzyme catalysts have been examined to catalyze transesterification of oils in micro-reactors. Kurayama et al. (2013) used CaO loaded micro-capsules as a micro-reactor for biodiesel production from rapeseed oil. The micro-capsules were prepared by encapsulation of CaO particles with butanol modified alginate as a shell material using co-extrusion technique. It was suggested that CaO-loaded micro-capsules could be successfully reused for three times without the loss of the catalytic activity. Under supercritical conditions, the application of heterogeneous catalyst coatings in microchannels for biodiesel production was first investigated by Schürer et al. (2014). They performed experiments on transesterification of tricaprin under supercritical conditions using alumina as a catalyst, coated onto stainless-steel micro-channels by applying wash coating process. A complete conversion was obtained in a very short residence time of the 30 s. In their successive work, Schürer et al. (2016) designed a small-scale heterogeneous alkali catalyzed (30 wt% La2O3 supported by γ-Al2O3) plant with annual capacity of 40 t for biodiesel synthesis under supercritical conditions. The implementation of heterogeneous catalyst to micro-reactors minimized the residence time significantly. Machsun et al. (2011) used a bio catalytic membrane micro-reactor with immobilized lipase in the pores of an asymmetric polyether sulfone (PES) membrane for transesterification of triolein with methanol. Each membrane pore acted as a micro-reactor in which the reaction occurred. A maximum conversion of 80% was observed in a residence time of 19 min. No activity decay of immobilized lipase was observed over a period of 12 days of continuous operation. Catalyst-free process In the catalyst-free process, transesterification of oils is performed in the absence of catalyst using supercritical alcohol at extremely high pressure and temperature (Demirbaş, 2002; Kusdiana & Saka, 2001; Saka & Kusdiana, 2001; Schürer et al., 2014; Warabi et al., 2004). Biodiesel production by the catalyst-free supercritical process has several advantages over catalytic processes, including higher reaction rates, easier separation, and purification of products, environment friendliness and more tolerance to the presence of water and free fatty acids (Kusdiana & Saka, 2004; Rathore & Madras, 2007). Bertoldi et al. (2009) and Trentin et al. (2011) carried out continuous biodiesel production in a stainless-steel micro-tube reactor through catalyst-free transesterification of soya bean oil in supercritical ethanol in presence of CO2 as a co-solvent. Bertoldi et al. (2009) performed experiments in temperatures between 300 and 350 °C, pressure from 7.5 to 20 MPa, oil to ethanol molar ratio of 1:6 to 1:40, co-solvent to substrate mass ratio from 0:1 to 0.5:1. An appreciable yield was obtained at 350 °C and 10 MPa pressure, using oil to ethanol molar ratio of 1:40 and CO2 to substrate mass ratio of 0.05:1. Trentin et al. (2011) used the similar experimental reaction system previously used by Bertoldi et al. (2009) but a different range of operating parameters. In the investigated experimental range it was observed that temperature, pressure, and co-solvent to substrate mass ratio had a positive effect on FAEE (fatty acid ethyl ester) yield and significant yields were achieved at 325 °C, 20 MPa, oil to ethanol molar ratio of 1:20, 0.8 mL/min substrates flow rate and CO2 to substrate mass ratio of 0.2:1. Sootchiewcharn et al. (2015) investigated the production of FAEE from refined palm oil with supercritical ethyl acetate in a micro-reactor. A biodiesel yield of 78.3% was obtained at oil to ethyl acetate molar ratio of 1:50, the temperature of 350 °C and 20 min of residence time. Using supercritical methanol,

Schürer et al. (2014) performed transesterification of tricaprin in stainless-steel micro-channels. A complete conversion was achieved in residence time of 5 min at 40-fold excess of methanol, 200 bar reaction pressure and 375 °C reaction temperature. However, by application of heterogeneous catalyst under supercritical conditions the required pressure was reduced to 100 bar and a residence time of b30 s was required for 100% conversion. Liquid-liquid two-phase flows in micro-reactors for biodiesel production It is crucial to study hydrodynamics in the multiphase reactor because different flow patterns influence mass transfer and axial dispersion, which affect the conversion and selectivity of the reactions. Depending on the volumetric flow ratio of the two phases and total flow rate, different flow patterns can be attained in micro-channels. In two phase liquid-liquid system the conventional flow regimes include as droplet (Fig. 10A), parallel (Fig. 10B), or slug flow (Fig. 10C). For liquid-liquid two-phase mass transfer limited reactions, application of slug flow has been suggested (Burns & Ramshaw, 2001; Dummann et al., 2003; Kashid et al., 2005). In slug flow, the mass transfer between two immiscible phases is significantly intensified due to the presence of internal circulations (Burns & Ramshaw, 1999, 2001; Kashid et al., 2005; Khan, Günther, Schmidt, & Jensen, 2004). These internal circulations are induced as a result of shear force between the two liquid phases as well as by liquid/wall friction (Malsch et al., 2008). It has been reported that enhanced mixing and rapid reaction rates can be achieved using segmented flow with internal circulations (Burns & Ramshaw, 1999; Tice, Song, Lyon, & Ismagilov, 2003). Using a quartz capillary micro-reactor, Sun et al. (2008) observed that the methanol and oil phases were separated from each other forming a slug flow due to high interfacial forces between the two phases. An alternating long oil slug and a short methanol slug was formed at the capillary inlet. They also reported a presence of a thin methanol wall film surrounding the oil slug due to superior wettability of methanol phase. The dimensions of oil slugs decreased with a decrease in capillary diameter suggesting an increased mass transfer at smaller channel diameters due to an increase in interfacial area. As the reaction between the two phases progressed, a similar alternative slug flow of methyl ester and glycerol was seen at the outlet part of the capillary. However, the flow patterns in the middle of capillary were not clearly identified and the effect of reaction conditions and geometrical configurations of micro-reactor were not investigated. Assuming droplet flow could occur in tubular micro-reactor, de Mas et al. (2005) suggested application of the droplet flow to achieve high interfacial area and intense local mixing for higher conversions. At a constant methanol volume of 3.97 × 10−7 m3, the highest interfacial area was reported for droplet flow as shown in Table 5. Yeh, Huang, Cheng, Cheng, and Yang (2016) devised a millimetrically scaled device that employed a droplet-based co-axial fluidic system to complete alkali-catalyzed transesterification for biodiesel production. The large surface area-to-volume ratio of the droplet-based system, and the internal circulation induced inside the moving droplets, significantly enhanced the reaction rate of immiscible liquids i.e. soya bean oil and methanol. The droplet-based co-axial fluidic system performed better than other methods of continuous-flow production. This study demonstrated the high potential of droplet-based fluidic chips for energy production. Factors affecting flow patterns Effect of temperature on flow patterns Temperature clearly influences the methanol-oil two-phase flow behavior in micro-channels as the miscibility of oil and methanol increases with increase in temperature (Čerče et al., 2005; Zhou et al., 2006). Guan et al. (2009a) used a transparent fluorinated ethylene propylene (FEP) tube to observe the flow pattern during biodiesel production under


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Fig. 10. Scheme of flow patterns of the reactants during the synthesis of biodiesel in the tubular micro-reactor: (A) droplet flow; (B) parallel flow; (C) slug flow. Reprinted with permission from de Mas, Günther, Kraus, Schmidt, and Jensen (2005). Copyright 2017 Taylor & Francis.

different reaction conditions. In order to obtain a clear view of flow, methanol was dyed with inert red phloxine B which is also soluble in glycerol but insoluble in oil. The flow patterns were observed at different temperatures for two different alcohol to sunflower oil molar ratios of 4.6 and 23.9. A gradual change in flow patterns was observed with increase in temperature from 20 to 60 °C. When the methanol to oil molar flow ratio was 4.6, at 20 °C a clear segmented flow was formed 400 mm apart from the tube inlet and then fine red droplets composed of methanol and glycerol were observed in oil slugs which agglomerated and merged into the methanol segment to form larger red segments near the exit region of tube (Fig. 11a). On the increase in temperature, the agglomeration of segments began closer to the tube inlet (Fig. 11b). At 60 °C the agglomeration started at 100 mm from the inlet of the tube, and a quasi-homogenous phase was formed at the outlet region of the microtube (Fig. 11c). When the tube outlet was cooled to room temperature, the quasi-homogenous segments were separated into a segmented flow of red segments containing methanol and glycerol, and oil segments containing unreacted oil and FAMEs. For methanol to oil molar ratio of 23.9, agglomeration of the red droplets appeared at 100 mm and 50 mm from reaction inlet at 20 °C and 60 °C respectively. Hence, the time required to reach the quasi-homogenous phase became shorter as the temperature is increased. Previous research on biodiesel synthesis studied flow patterns up to temperatures below the boiling point of methanol. Sun et al. (2009) used a rectangular interdigital micromixer (RIMM) coupled with polyvinyl chloride (PVC) tube to examine the flow behavior both below and above the boiling point of methanol. At 60–65 °C a fully developed droplet flow of methanol dispersed in oil phase was seen at the outlet of the mixer. As the temperature was increased, the flow transformed into a mixture of many droplets and a small number of bubbles at 70 °C and to a mixture of many bubbles and a smaller number of droplets at 75 °C. Finally, at 80 °C a gas-liquid two-phase slug-annular flow (Yue, Chen, Yuan, Luo, & Gonthier, 2007) was formed. At the outlet part of the tube, a two-phase slug flow was observed between 60 and 70 °C and however, some methanol bubbles were seen in the slugs at 70 °C. At 75 °C, a gasTable 5 Estimated total interfacial areas for different flow patterns with a constant methanol volume of 3.97 × 10−7 m3. Reprinted with permission from de Mas et al. (2005). Copyright 2017 Taylor & Francis. Flow pattern

Volume of single droplet-slug (m3)

N° droplets-lugs in TMR

Total interfacial area (m2/m3)

Drop flow Slug flow Parallel flow

4.2 × 10−15 4.82 × 10−10 N.A.a

9.45 × 107 822 N.A.a

6.00 × 104 1.46 × 103 6.31 × 102

a

N.A.: not available.

liquid two-phase slug-annular flow, similar to that at 80 °C during all reaction times, was formed. Thus, the reaction temperature has a significant influence on the flow behavior. Effect of alcohol on oil molar ratio It has been reported that alcohol to oil molar ratio strongly affects the flow patterns (Guan et al., 2009a; Sun et al., 2009). At a methanol to oil molar ratio of 4.6, short methanol segments dispersed in long oil segments were observed. When the methanol to oil molar ratio was increased to 23.9, the number of methanol segments per unit length of the tube slightly decreased compared to the number of segments at 4.6 methanol to oil molar ratio as shown in Fig. 11. Moreover, the length of methanol segments increased, while the length of oil segments decreased. The formation of red droplets containing glycerol and methanol and their aggregation was more intense for methanol to oil molar ratio of 23.6, and a complete conversion was obtained at 300 mm from tube inlet (Guan et al., 2009a). Sun et al. (2009) reported that on increasing the methanol to oil molar ratio from 3:1 to 15:1 at 60 °C, the droplets of glycerol at the outlet part of the tube are enlarged gradually. Thus, depicting higher conversion at higher methanol to oil molar ratio. Effect of inlet mixer type The mixer type also has a strong influence on the flow pattern in micro-reactors. Sun et al. (2009) observed the flow patterns at the outlet of four different types of micromixers, namely, a T-mixer, a J-mixer, a rectangular interdigital micromixer (RIMM) and a slit interdigital micromixer (SIMM-V2). As shown in Fig. 12, a uniform slug flow with methanol slug dispersed in oil phase was formed at the outlet T-mixer and J mixer, whereas a mixture of some slugs and many methanol droplets appeared at the outlet of RIMM and SIMM-V2. Hence, the contact area between the methanol and oil phase was significantly increased using RIMM and SIMM-V2 resulting in higher FAME yields. Similarly, Bhoi et al. (2014) investigated the flow in micro-reactors with three different mixers, a T-type, a cross (†) type, and a split and recombine mixer at various flow rates and constant methanol to oil molar ratio of 10.3:1 during methanolysis of sunflower oil. The flow pattern for micro-reactor with T-mixer was mainly a parallel flow with intermittent droplets which were formed due to interface instabilities. This parallel flow existed for a complete range of flow rates depicting no change in the interfacial area with the flow rate. For micro-reactor with the †-type junction, a mixture of slug and droplet flow was observed at a flow rate of 0.5 mL/min. When the flow rate was decreased gradually, short slugs were seen at 0.25 mL/min which continued up to a flow rate of 0.10 mL/min. As the flow rate was decreased further to 0.07 and 0.05 mL/min annular flow was observed. Hence, in the flow rate range of 0.05 to 0.5 mL/min, three different flow patterns were observed for a

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Fig. 11. Total flow patterns in a transparent FEP micro-tube reactor (inner diameter = 0.8 mm, length = 1000 mm). Total flow rate =8.2 cm3/h. Arrows stand for the flow direction. Reaction temperature: (a, d) 20, (b, e) 40, and (c, f) 60 °C. Methanol/oil molar ratio: (a–c) 4.6 and (d–f) 23.9. Reprinted with permission from Guan et al. (2009a). Copyright 2009 American Chemical Society.

cross-type junction. For micro-reactor with split and recombine mixer, at flow rates equal to 0.05, 0.07 and 0.20 mL/min a slug flow was seen while parallel flow with intermittent droplets was observed at a flow rate equal to 0.10, 0.15, 0.25 and 0.5 mL/min. Purification of biodiesel in micro-reactors Transesterification of oils in the presence of methanol in microreactors involves the formation of FAME and glycerol as the reaction progresses. Separation of glycerol phase from biodiesel is a major step to meet the quality standards. As both FAME and glycerol are immiscible, they have to be separated from each other by density difference in batch process. In the conventional batch process, an emulsion of biodiesel and glycerol is formed which is hard to break and consequently the separation of the two phases is considerably slow. However, traces of glycerol remain in the biodiesel phase after phase separation. Conventionally, this small amount of glycerol is removed from the biodiesel phase through washing followed by drying to remove the moisture (Perez et al., 2014; Shahid & Jamal, 2011). For microfluidics devices with small dimensions the separation of two phases on the basis of density difference is inappropriate due to negligible gravitational forces. Separation and purification in microfluidic devices have gained much importance in recent years (Hartman & Jensen, 2009; Jahnisch,

Hessel, Lowe, & Baerns, 2004; Naleini, Rahimi, & Heydari, 2015; Sahoo, Kralj, & Jensen, 2007; Singh, Renjith, & Shenoy, 2015) but only limited work has been performed on purification of biodiesel product in micro-reactors (Xie et al., 2012). Crawford et al. (2008) used a Syrris Flow Liquid-Liquid Extraction (FLLEX) module for in-line continuous separation of fatty acid methyl esters and glycerol. The FLLEX module consisted of a microfluidic channel fitted with a polytetrafluoroethylene (PTFE) membrane designed by Kralj, Sahoo, and Jensen (2007). The separation was achieved by the addition of water to the mixture leaving the micro-reactor chip at a rate of 100 μL/m in the FLLEX module. The hydrophobic biodiesel phase was able to traverse the membrane pores, while the aqueous phase containing glycerol and methanol did not. According to a review article by Xie et al. (2012), Sun (2010) performed the separation of biodiesel from glycerol by washing in a micro-separator with channel dimensions of 500 × 500 × 500,000 μm. A sandwich flow pattern between the water and raw biodiesel was observed in the separator. The glycerol content in the final biodiesel product was decreased to 0.02%. Purification of biodiesel in microreactors is still under development and new technologies are needed for continuous separation of biodiesel from glycerol. Several phase separation techniques have been developed and implemented for separation of two liquid phases (Fig. 13) which may be investigated for separation of biodiesel from glycerol at micro-scale (Tsaoulidis, 2015). Scope for future work

Fig. 12. Flow patterns in the transparent PVC tubes (inner diameter 1.2 mm) at the outlets of different micromixers: (a) T-mixer, (b) J-mixer, (c) rectangular interdigital micromixer (RIMM), (d) split interdigital micromixer (SIMM-V2). Reprinted with permission from Sun et al. (2009). Copyright 2010 American Chemical Society.

It is evident from the reported literature that production of biodiesel through transesterification of oils with alcohol in the presence of a catalyst have been investigated experimentally using various types of micro-reactors, micro-tubes and micro-channels. Moreover, the advantages of micro-reactors were also demonstrated and the potential of micro-reactors for small-scale production of biodiesel fuel, rather than large-scale centralized production using batch reactors. Through the reduced footprint of the entire production system which can be made possible through the application of micro-reactor, the portable biodiesel


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Fig. 13. A schematic of separation devices during liquid-liquid plug or parallel flow. (a) Phase separation using a Y-splitter Xue et al. (2011), (b) phase separation by capillary forces (Angelescu, Mercier, Siess, & Schroeder, 2010), (c) phase separation during parallel flow (Aota, Mawatari, & Kitamori, 2009) and (d) rhase separation by wettability combined with pressure balance (Scheiff, Holbach, & Agar, 2013). Reprinted with permission from Tsaoulidis (2015). Copyright 2015 Springer.

system could be easily installed in the fields where oil-producing feedstock exists rather than transporting feedstock. Thus, avoiding the transportation cost and loss of oil due to inevitable drying and spilling of the feedstock oil while in transit. However, in reviewing the published research on biodiesel production in micro-reactors, most of the production scale was relatively small and limited effort has been done towards the development and scale-up of micro-reactors for production of biodiesel at pilot scale (Billo et al., 2015; Schürer et al., 2016). The throughput of a single unit of micro-reactor is too low to meet the production rates at industrial scale. Hence, the scale-up of micro-reactors units is

highly desirable and this can be achieved using the concept of numbering-up. Numbering-up is the multiple, parallel repetitions of micro-reactor units to increase the throughput without altering the microfluidic flow properties (Schenk, Hessel, Hofmann, Löwe, & Schönfeld, 2003). The concept of numbering-up becomes important to prove the applicability of micro-reactor at industrial scale. Kashid, Gupta, Renken, and KiwiMinsker (2010) demonstrated the different numbering-up techniques such as external numbering-up and internal numbering-up as shown in Fig. 14(a & b). In internal numbering-up the two liquid phases are

(c) Fig. 14. Numbering-up concept for two-phase micro-reactor. (a) Internal numbering-up and (b) external numbering-up. Reprinted with permission from Kashid et al. (2010). Copyright 2010 Elsevier Ltd. (c) Splitting distributor developed by Adamson, Mustafi, Zhang, Zheng, and Ismagilov (2006). Reprinted with permission from Adamson et al. (2006). Copyright 2006 Royal Society of Chemistry.

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mixed at the junction and the mixture is distributed in several parallel channels. The advantage of internal numbering-up is that only a single pumping system and one mixing element is required. However, equal distribution of two phase mixture in parallel channels is a challenging task. In external numbering-up several mixing elements are arranged in parallel and the two phases are mixed in each mixing element independently. Hence, in external numbering-up performance of single micro-reactor is ensured along with increased throughput but requirement of multiple independent reaction systems limits the use of this strategy. In recent years, a new numbering-up strategy of designing a splitting distributor has emerged in which a single micro-channel is fabricated on a plate through etching and branched into multiple microchannels to form a tree like structure as shown in Fig. 14(c). Splitting distributors ensure better flow uniformity as compared to external and internal numbering-up solutions (Adamson et al., 2006; AlRawashdeh et al., 2012). To overcome the disadvantages of internal and external numberingup, Schenk et al. (2004) developed a first liquid-flow splitting unit which ensured the flow equidistribution. The flow equipartition was achieved by building up a pressure barrier. It was equipped with six docking stations and three dampening elements for six micro-devices. Separation-layer mixers were chosen as micro-devices, since the carrying out of fouling-sensitive reactions was considered as one major application of the flow splitting unit. Kashid et al. (2010) developed a capillary micro-structured reactor, numbered-up for six capillaries to investigate the effect of interfacial tension on the mass transfer performance for various operating conditions. To reduce the equipment cost and size, they integrated internal numbering-up approach for distributing single phase fluids and external numbering-up for two-phase contacting. Yue et al. (2010) developed an external numbering-up geometry of a parallel micro-channel contactor with two constructal distributors to study the flow distribution and mass transfer characteristics during CO2-water flow. Each distributor comprised of a dichotomic tree structure that fed 16 micro-channels. It was found that constructal distributors could ensure a nearly uniform distribution of the two phases when the ideal flow pattern was slug-annular flow. Hoang et al. (2014) designed a splitting distributor for uniform distribution of bubbles over the exit channel. They observed that all bubbles at the junctions will be broken if the value of capillary number is greater than a critical value i.e. Ca N Cacritical. Kriel, Woollam, Gordon, Grant, and Priest (2016) performed experimental numbering-up of microfluidic chips from one to five and then ten in an extraction module to extract Platinum (IV) Chloride using a secondary amine. The extraction performance was observed to remain unchanged with increasing throughput. Al-Rawashdeh et al. (2012) developed barrier-based distributor which assured flow uniformity and no channelling between the two phases. With large constrictions in the upstream a flow uniformity of N90% was observed. Hence, it is evident from the reported literature that several attempts were made to numbering-up the channels using different techniques. However, application of these techniques for production of biodiesel were not explored. Most of the current researches on biodiesel production in microreactors were performed through transesterification of low free fatty acid (FFA) edible oils using homogenous base catalysts to avoid the formation of soaps. A confined work has been done on biodiesel production using low-cost non-edible feedstock with high FFA content. The conventional process of two-step acid-base transesterification for biodiesel production using high FFA feedstock is yet to be applied at the micro-scale. Alternative design and modifications of the microfluidic devices for continuous removal of soap have to be carried out for biodiesel production using oil with high FFA, which may minimize the cost of production. Moreover, to overcome the difficulties due to soap formation development of alternative catalysts with good tolerance to FFA and adaptability at micro-scale needs to be investigated. From the above review, it is evident that the current research is focused on increasing the production efficiency through application of micro-reactors. However,

additional studies are required for in-line continuous purification of biodiesel to meet the industrial quality standards. Conclusions • In recent years, biodiesel has gained significant importance with an aim to replace fossil diesel fuel. It is evident from the reported literature that many nations of the world are commercially producing biodiesel using conventional reactors. However, conventional technique for biodiesel production faces several challenges of long residence time, high investment in equipment and manufacturing floor space, high cost of operation and energy requirements and low production efficiency. Micro-reactor technology can be successfully applied to overcome these challenges. Attributed to small diffusion distances and large surface area to volume ratio in micro-reactors, heat, and mass transfer are significantly intensified, thus resulting in higher conversions in shorter residence times. Moreover, the energy requirements in the mixing of reactants, ample floor space and standing time for the separation of products are also eliminated. • Production of biodiesel through transesterification in micro-reactors is observed to be affected by operating parameters such as temperature, alcohol to oil molar ratio, the effect of co-solvent addition, catalyst type and concentration. Additionally, the geometrical configuration of micro-reactors as well as the flow regime have significant effect on the biodiesel yield. • Production of biodiesel in micro-reactors has been investigated for a wide range of temperatures. Most of the reported work has been performed below the boiling temperature of methanol (60–65 °C). For alkali catalyzed transesterification the biodiesel yield increases with increase in temperature up to an optimum temperature beyond which the yield decreases. This can be ascribed to accelerated saponification reaction of triglycerides in the presence of an alkaline catalyst at higher temperatures. However, to perform acid catalyzed transesterification and transesterification under supercritical condition, temperature above 100 °C and 300 °C were required respectively. • Apart from the temperature it is important to optimize the residence time for the reaction. The residence time for biodiesel production varies with the type of micro-reactor applied. However, at prolonged residence time resulted by a decrease in average velocity for fixed length micro-channel may cause a decrease in internal circulations leading to weakened mass transfer. Hence, a decrease in the yield may be noticed at decreased flow velocities. Similarly, under supercritical conditions the biodiesel yield decreases due to decomposition of the reactants and products at too long residence time. • Alcohol to oil molar ratio is another important factor that affects the yield of biodiesel during transesterification. Methanol and ethanol are most investigated for biodiesel production in micro-reactors. Under base catalyzed transesterification, most of the researchers obtained the maximum yield at alcohol to oil molar ratio of 6:1. However, the yield varied with the type of micro-reactor used. When the alcohol to oil molar ratio was increased above the optimum value a decrease in the biodiesel yield decreases. This unexpected decrease in yield is probably because at too high alcohol to oil molar ratio; the alcohol acts as an emulsifying agent which causes a part of glycerol to remain in biodiesel phase. Compared to base catalyzed transesterification a high alcohol to oil molar ratio of up to 20:1 may be required for acid catalyzed transesterification. Even higher excess alcohol to oil molar ratio (up to 40:1) is required for transesterification under supercritical conditions. • The reaction temperature, residence time and alcohol to oil molar ratio depend upon type of catalyst, and catalyst concentration. For biodiesel production in micro-reactors generally homogenous base catalysts are used. Most of the researchers have used Potassium hydroxide (KOH) or Sodium hydroxide (NaOH). For feedstock with high free fatty content acid catalyst (H2SO4) is preferred over base catalyst. However,


•

•

•

•

high reaction temperature, residence time and alcohol to oil molar ratio are required for acid catalyst. The application of heterogeneous catalyst makes the catalyst separation obsolete but requires high durability of the catalyst. Heterogeneous catalyst micro-reactors are still under development. Catalyst free process using supercritical alcohol can also be used to accelerate the transesterification but the high cost of the production process in the supercritical process due to strict demand of equipment and high temperature and pressure requirements is a major limitation for this method. The conversion efficiency is further improved when supercritical transesterification is performed in presence of a cosolvent or a heterogeneous catalyst. The geometrical configurations of micro-reactor i.e. channel size, inlet mixer type and internal geometry of the channels also affect the ester yield significantly. Shorter dimensions of channel lead to shorter diffusion distances between the reactant molecules which enhances the mass transfer. Hence, with decrease in the channel size (hydraulic diameter) the yield of product increases. However, small dimension of channels may lead to low throughput therefore apart from diffusion, advection can be used to intensify the transesterification reaction through improved mixing of oil and alcohol. Micro-reactors in which chaotic advection is induced by modifications in channel geometry like application of zig-zag, omega, tesla shaped channels or by inserting an obstacle in the channel tend to provide higher yields. Moreover, the yield of biodiesel also depends on the inlet mixer type. It can be attributed to varying dimensions of different mixers and the flow behavior at the outlet of the mixers. The nature of flow between the two phases determines the surface area to volume ratio. The droplet-based flow performs better than other methods of continuous-flow production. Optimization of operating parameters and design of micro-reactor is necessary to gain higher productivity. However, for biodiesel production in micro-reactors, there are some issues still needed to be tackled with, including the numbering-up of micro-reactor from lab to pilot scale, design, and development of micro-reactors for handling high free fatty acids feedstock and heterogeneous catalyst micro-reactors. Furthermore, continuous biodiesel purification using an integrated microfluidic device is still under development. More design and surface modification of the microfluidic devices have to be carried out to meet the quality standards.

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