An overview on the process intensification of microchannel reactors for biodiesel production

Accepted Manuscript Title: An overview on the process intensification of microchannel reactors for biodiesel production Authors: N. Yasvanthrajan, Abhinandan Nabera, Saman Salike, D.T. Valan, P. Sivakumar, K. Muthukumar, A. Arunagiri PII: DOI: Reference:

S0255-2701(18)31065-1 https://doi.org/10.1016/j.cep.2018.12.008 CEP 7446

To appear in:

Chemical Engineering and Processing

Received date: Revised date: Accepted date:

6 September 2018 9 November 2018 19 December 2018

Please cite this article as: Yasvanthrajan N, Nabera A, Salike S, Valan DT, Sivakumar P, Muthukumar K, Arunagiri A, An overview on the process intensification of microchannel reactors for biodiesel production, Chemical Engineering and Processing - Process Intensification (2018), https://doi.org/10.1016/j.cep.2018.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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An overview on the process intensification of microchannel reactors for biodiesel production N. Yasvanthrajan1, Abhinandan Nabera1, Saman Salike1, D.T. Valan1, P. Sivakumar2, K. Muthukumar1, A. Arunagiri1* 1

Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli,

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Tamil Nadu, India, 620015. School of Petroleum Technology, Pandit Deendayal Petroleum University, Gandhinagar,

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Gujarat, India, 382421. *

Corresponding author Email id: [email protected]

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Graphical abstract

Highlights  Reviews the recent developments in biodiesel production using microchannel reactors  Intensification of microreactors by varying mixers and influencing parameters is elaborated  Future challenges and area to be focused are identified

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Abstract Biodiesel produced from esterification or transesterification of oils from renewable energy sources is the only alternate to liquid fuels and can be directly used in currently existent diesel engines. The cost of biodiesel is an important issue and the commercial production of

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biodiesel should require low energy and minimum downstream steps. The intensification of biodiesel production using microchannel reactors is an exquisite option that improves

conversion, lessens the reaction time and enriches product purity. The increased mass transfer

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rates and high surface to volume ratio are the striking features of this reactor. This review

focusses on the feasibility of utilizing microchannel reactors for biodiesel production. The emphasis is given to mixers used, channel configurations, reactor dimensions, flow behaviour

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and diffusion pathways. The influence of methanol to oil ratio, reaction temperature and

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catalyst concentration on biodiesel yield is discussed at length. Finally, the problems and the

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challenges pertaining to these reactors to obtain improved performance are elaborated.

1. Introduction

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Keywords: Microchannel reactor, Micromixer, Biodiesel, intensification,

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At present, major body of research is focussed on weaning the world from fossil fuels.

Despite fossil fuels domination in the energy sector, renewable energy sources had shared about 12% in 2016 and this would double in 2025 and may increase to 30-35% in 2040. Most

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of the developed countries have invested significantly for the development of renewable energy sector [1]. Importantly, the utilization of fossil fuels greatly affects sustainable development and climatic conditions which may lead to the extinction of millions of species present on the earth. These problems accelerate the need for alternative and renewable energy resources [2,3]. Biodiesel, Fatty Acid Methyl Ester (FAME), is produced majorly from lipids such as edible oil, non-edible oil, microalgae oil, animal fats containing triglycerides by esterification and transesterification using a catalyst [4]. Biodiesel has an immense potential to be an inspiring

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sustainable energy owing to its non-toxic, renewable, recyclable, environmental friendly and engine friendly characteristics [5, 6]. Biodiesel has no sulphur and its inbuilt 10-12 wt% oxygen helps in reducing the release of toxic gases [7]. Moreover, it is similar to petro-diesel due to its lubricity characteristics, cetane number and flash point [8]. The sequence of operations involved in the production of biodiesel include mixing of feedstock with catalyst, chemical transformation, separation of biodiesel from glycerol and

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removal of catalyst. The conventional reactors are typically expensive due to their large size. Moreover, it requires 50-300% excess alcohol to favour the forward reaction, high residence time and additional energy to attain chemical equilibrium. Therefore, mixing of these excess

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reactants, which are at different phases, requires excess energy [9]. Additionally, the time required to separate ester from glycerol is approximately 1.5-2 days. In general, the time required to separate glycerol will be sixteen times more than that of the reaction time required to achieve 95% conversion in the conventional batch reactor systems [10]. But in microchannel

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reactor, the separation of two phases starts when the products are formed inside the channel,

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which significantly reduces the time required for separation besides offering 99% conversion

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[11]. The efficiency of conventional reacting systems was reported to increase due to ultrasound and microwave intensification. This offers higher yield and reduces the residence

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time requirement besides offering better mass transfer [12–15]. Though the process intensification using microwaves is feasible on an industrial scale [16], it is greatly affected by

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its high complexity, reactor cost and the difficulty in achieving a homogeneous intensity [17]. To overcome these limitations, static mixers were used to enhance mass transfer and interaction

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between the phases [18,19]. They are typically used in continuous processing but can also be employed in closed loop systems or for premixing. The important features include lesser power

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requirement and higher reaction conversions in a short span of time. Moreover, this can be intensified by employing small scale reactors such as microchannel reactors, slit channel reactors, fixed bed microreactors and membrane microreactors [20]. The increased interest in microscale technology, in particular for continuous processing, is due to its advantages such as

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higher energy efficiency, lower residence time, no secondary reactions, better heat management, easy to scale-up, increased safety and effective control [21]. The overall outline of the micromixers and microchannel reactors used for biodiesel production is given below in Fig.1. Fig.1. Outline of micromixers and microchannel reactors for biodiesel production

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This review mainly focuses on process intensification features of microchannel reactors for continuous biodiesel production. Firstly, the parameters influencing the biodiesel yield such as catalyst concentration, methanol to oil molar ratio and reaction temperature are discussed at length. Secondly, the importance of micromixers, mixer types, methods used to improve mixing inside the channels, channel dimensions, geometry of the microchannel, impact of fluid flows, co-solvent behaviour, power consumption, catalysts used and challenges in devising the microchannel reactors are discussed elaborately. Thirdly, the future scope of microchannel

appropriate places in the discussion of above mentioned parameters.

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2. Catalysts for Microchannel Reactors

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rectors in biodiesel production is discussed. The intensification aspects are elaborated at

Catalysts play a major role in transesterification to achieve better conversion and determine the feedstock composition limits, reaction conditions as well as post separation steps

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[22]. The introduction of catalyst reduces the activation energy required to initiate the reaction and increases the reaction rate. Understanding the advantages of using a particular type of

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catalyst for biodiesel production in the context of microscale technology is important to

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determine its potential. Commonly, homogeneous alkali catalyst offers higher reaction rates

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compared to heterogeneous alkali catalysts. However solid catalysts eliminate complex separation issues and exhibit longer stability. Despite the fact that homogeneous and

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heterogeneous acid catalysts require more rigorous reaction conditions and they are mainly used for treating high free fatty acid (FFA) [23–26]. Therefore, the choice and use of catalysts are very crucial in biodiesel production using microscale technology. In microchannel reactors,

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homogenous base catalysts (NaOH and KOH) are employed mainly due to the ease of operation. While, homogeneous acid catalyst (H2SO4) are used for the pretreatment of

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feedstock to carryout esterification. Moreover, enzyme catalysts (Candida antartica, Thermomyces lanuginosus) and heterogeneous catalysts (CaO) are studied rarely due to incompetence of these catalysts with microchannel reactors [27–30]. However, these reactions demand refined feedstock since the presence of water or FFA interferes with the reaction [31].

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In some cases, catalyst reacts with FFA and induces soap formation via saponification that affects the yield. The existence of water retards transesterification reaction by hydrolysing the triglycerides [32,33]. Therefore, feedstock having higher FFA generally requires a two-step process. The first step converts FFA into alkyl esters by using an acid catalyst and in the second step, alkali catalysed transesterification takes place [23]. In the recent past, enzymatic transesterification has drawn tremendous interest due to its less response to FFA, requirement

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of mild operating conditions, easy downstream processing, reduction in by-products generation and reusability. However, catalyst deactivation, enzyme cost and leisurely reaction rates restrict its application on industrial scale [34]. Supercritical methanol formed at temperature and pressure beyond critical point offers high solubility, less mass transfer limitations, greater reaction rates, easier separation and refinement of products [19,35]. However, high methanol to oil ratio requirement and

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application of high temperature and pressure to achieve satisfactory conversion increase the processing costs and often degrade fatty acid esters [36–39]. But, the operational costs and product degradation can be reduced using co-solvents such as carbon dioxide or propane which

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decreases the mixture critical point and permits the reaction to be supported under reduced process conditions with higher yields [14,40,41]. 3. Types of Micromixers

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Mixing two immiscible phases such as alcohol and oil is a challenging step in

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microchannel reactors. The most common types of micromixers used for injecting reactants are T and J-type. The difference between these mixers are represented in Figs.2A and B. In T-

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mixers, oil and alcohol meet at an impact angle of 180° and produces tiny droplets due to high

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pressure. On the other hand, J-mixers have an impact angle of 90°. The head on collision at low pressure creates larger droplets. Studies were conducted to elucidate the effect of geometric

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patterns such as hydraulic diameter and droplet size distribution on FAME production. T-type mixers showed superior mixing performance than J-type mixers [42]. Rarely employed co-

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axial fluid mixing has an impact angle of 0° and it induces circular motion of the droplets inside the microchannel which also results in efficient mixing. The pictorial representation of co-axial fluidic mixing is shown in Fig.3. Importantly, it works with lesser energy than T and J-mixers

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[30].

Fig.2A. T-mixer, B. J-mixer [42] Fig.3. Co-axial fluidic system [30]

The microchannel reactor with a T-mixer showed a higher oil conversion with the

increase in methanol to oil ratio and reaction temperature [43]. Furthermore, the segmented flow of oil droplets and methanol at the entry of microtube reactor exhibited a quasihomogeneous behaviour because of intense aggregation of fine droplets at the exit of the reactor. Similar study conducted with T-mixers having three different internal diameters such

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as 1.27, 2.286 and 0.33 mm indicated that fatty acid ethyl ester yield was strongly dependent on mass transfer rates and was influenced by internal diameter of the micromixer [44]. To improve the efficiency of mixing, three different four way micromixers with a confluence angle of 45°, 90° and 135° and hexane as a co-solvent were used. The different confluence angle is shown in Fig.4. Enhanced performance was observed at 45° followed by 135° and 90° which is due to the hydrophobic nature of hexane affecting the interaction

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between the molecules to maintain homogeneity of the fluids [45] Fig. 4. Four way micromixers with different confluence angle 45°, 90°, 135° [45]

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In addition to T and J-mixers, rectangular and slit interdigital micromixers were also

used. The flow pathways of the above mixers are illustrated in Fig.5 [46]. The rectangular mixer consists of a large number of interconnected micropores (60 µm) through which fluid is passed and converged into thin tubes using a rectangular sheet connected to the reactor. The

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slit interdigital micromixers consist of two similar channel arrangements converging through a

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slit which flows outward and through which the fluid passes to the reactor [47]. These mixers enhance homogeneity and reduce gel formation inside the reactor. Rectangular and slit

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interdigital micromixers give better FAME yield than T and J-mixers. Furthermore, rectangular

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interdigital micromixer provides higher yield compared to slit. [46] Fig.5. Flow pathways in different mixers: A.T- type mixers, B. J- type mixers, C.

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Rectangular interdigital micromixer, D. Slit interdigital micromixer [46] The effects of micromixing induced by three different micromixers such as T-mixer,

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lamellar mixer and split & recombine mixer in seven different continuous reactor systems are reported. The flow passage is shown in Fig. 6A and B. In lamellar mixer, the inner diameter of

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the channel changes with the mixing distance whereas in split and recombine type, the flow splits into two streams and then combine again through the mixing distance. The highest yield was observed with split & recombine type mixer and the internal diameter was found to significantly influence the yield. The increase in internal diameter above the optimum value

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decreases the interfacial area, which affect the biodiesel conversion [48]. In a similar vein, another study reported that the performance of split & recombine micromixer was poor when compared to T and †- mixing due to the complexity of two liquid phase flow [49]. The pictorial representation of †- mixing is given in Fig.7. Fig. 6.A. Lamellar mixer, B. Split and recombine type mixer [48]

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Fig. 7. †- mixing pattern [49] Kaewchada et al (2016) studied transesterification in a microtube reactor with T/J-type mixer [42]. The shorter residence time or higher flow rate enhanced FAME yield and T-mixer showed superior performance due to the head on collisions of the two streams resulting in reduced droplet size. Guan et al (2008) carried out transesterification in a microtube having an internal diameter of 1 mm equipped with a traditional T-type mixer and split & recombine type

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micromixer [43]. The oil conversion improved when a T-type mixer at the inlet of the microtube was replaced with split & recombine type micromixer due to its larger surface area

and an evenly distributed flow of small methanol droplets. The microtube provided higher

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surface area of contact between the immiscible phases due to the segmented flow along the length of the tube which enhanced mass transfer rate and phase internal flow. These findings

were reinforced by Sun et al (2010a) who compared the performance of four micromixers such

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as T, J, rectangular and slit interdigital micromixer. FAME yields obtained with the latter micromixers were almost two times higher than T or J-type mixer. The yield increased with

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the increase in flowrate and temperature because of better mass transfer [46].

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Furthermore, the performance of micromixers like T, cross and double-T-micromixer

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were compared and reported. In a T-mixer, oil and alcohol pass through two inlets, whereas in a cross mixer oil passes through the middle inlet and alcohol passes through the other two

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inlets. In a double-T-micromixer, oil passes through two alternate inlets whereas alcohol passes through the remaining two inlets. The differences in the above mixers are shown in Fig.8. The

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cross micromixer showed highest mixing index and conversion, which were found to increase with residence time [50].

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Fig. 8A. T-mixer, B. Cross T- mixer, C. Double cross T-mixer [50] Alkali catalysed transesterification was carried out on a microchannel reactor consisting

of zig-zag mixers. In this mixer, pumped oil and methanol meet at a right angled point which acts like a multi-laminated mixer as shown in Fig. 9. The continued mixing in the reaction

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channels improves the performance much better than a single mixer. The resistance in the reaction sheet is higher than in mixing sheet and the process can be scaled up by increasing the number of parallel couples of mixing and reaction sheets. The flow maldistribution can be mitigated by splitting the flow in the mixing units. Zig-zag microchannel reactor greatly reduces the flow maldistribution due to the enhanced mixing rate inside the zig-zag channels and shows excellent performance for transesterification reaction [28].

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Fig. 9. Mixing configuration in the Zig-zag microchannel reactor [28] 4. Mixing in the Channels Mixing inside the channel is another important parameter that significantly affects the yield. Essentially, the flow behaviour inside the channel should be laminar for efficient mixing. The increase in Reynolds number changes the flow from laminar to turbulent creating vortices that resist the intermolecular diffusion of methanol-oil mixture [50]. In fact, the residence time

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would be shorter for turbulent flow which significantly impacts the mixing and ends in lower

conversion. The improper mixing will lead to uneven distribution of catalyst or shortage of any

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one of the reactants.

Aghel et al (2014) reported that placing a wire coil inside the microchannel intensified mixing and helped in attaining equilibrium within a shorter time. The pictorial representation is given in Fig. 10. Two reactors with and without wire coil were compared on the basis of

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FAME yield [51]. The reactor with wire coil gave higher yield and was directly proportional

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to the length of the wire coil and inversely proportional to residence time. The spirally wound microchannel generates centrifugal movement of fluids inside the channel due to its curved

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trajectory arrangements. The raise in flowrate causes advection resulting in a maximum mixing

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index of 0.99. The pictorial representation is shown in Fig. 11. Its performance was compared using straight channel microreactor. It was observed that the increase in FAME yield was more

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for spiral microchannel reactor than for straight channel reactor [52]. Fig. 10. Schematic representation of wire coil reactor

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Fig. 11. Spiral coil reactor

To improve the mixing inside the channels, Martinez et al (2012) proposed tesla, omega

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and T-shaped channels having different geometries in microchannel reactors as shown in Fig. 12. They are sequentially arranged to gain efficient mixing. Tesla shaped channels have angled surfaces where effective collision occurs due to the split and redirection. Omega shaped

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channels create vortices in the fluid due to non-uniform velocities and split the flow, which then converges in the adjacent channels. In T-shaped channel, intermolecular diffusion occurs in relation to the collision between fluids having the same mixing time that corresponds to mixing length. Moreover, the lack of curves inside this channel makes the flow laminar [53]. Fig. 12. Omega and Tesla shaped mixing channels [53]

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In co-axial fluidic system, oil collides with methanol at the inlet of the channel to form methanol droplets that are surrounded by oil. With this feature, reactions can be carried out with stoichiometric proportion and this arrangement significantly decreases the methanol requirement. The induced circular motion of the droplets along the channels enhances the mixing rate and conversion and reduces the energy requirement [30]. Wen et al (2009) studied zig-zag microchannel reactors with many turns of 90° impact

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angle to improve passive mixing as shown in Fig. 13. The mean droplet size decreased with the number of turns due to the periodic reorientation of the flow. Moreover, the mean droplet size decreased with hydraulic diameter due to the dominating laminar flow at the microscale.

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The raise in shear strain with a reduction in channel size resulted in smaller droplets. Zig-zag microchannels significantly improved the mixing rate inside the channel as compared to conventional reactors [54].

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Fig. 13. Zig-zag microchannel with turns [54].

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The mixing patterns inside the T channels with circular and alternate circular obstructions were compared at different Reynolds numbers. The pictorial representation is

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improved the mixing index [55].

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given in Fig. 14. A, B, C. This showed maximum conversion and the existence of obstacles

Fig. 14. A. T-channel, B. T-channel with circular obstructions and C. T-channel

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with alternate circular obstructions [55]

Mohammadi et al (2017) conducted studies with ring shaped and pitted mixing channel

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with and without T-junction. Static and revolving magnetic fields created by permanent magnets were used to induce mixing in the presence of synthesized Fe3O4 nanoparticles.

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Efficient mixing was observed with rotating magnetic field due to the aggregation of magnetic chains inside the fluid that act as micro and nano stirrers which in turn cause stretching and folding of the fluid. Importantly, a concentration of 0.01% (w/v) nanoparticles significantly

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enhanced mixing. [56]. The effect of ultrasound in microchannels was investigated by Avellaneda and Salvado

(2011) and ultrasound was supplied externally to the helicoidal microchannel reactor. Ultrasound irradiation produces cavitational bubbling which enhances mass transfer across the interface. Though the low residence time negatively affected the impact of ultrasound, it enhanced the formation of emulsion causing back flow of fluids due to its vibration. The yield of this process was found to be equivalent to that of batch reactor under similar operating

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conditions. Ultrasound also favours the formation of emulsions such as soaps or gels even with low catalyst concentration (0.6 & 1 wt%) [57]. 5. Microchannel Reactors Configuration Microchannel reactors have gained noteworthy attention because of their ease of operation in continuous mode and in a constrained environment. The main advantages of microchannel reactor are its large surface to volume ratio and shorter reaction time. The

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specific surface area of these reactors lies between 10000 and 50000 m2 m-3 that allows active molecular diffusion. This eliminates adverse side reactions and the creation of hot spots inside the channels besides inducing rapid mixing rates. Moreover, these reactor systems can be

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scaled up by parallelization [58]. The technical specifications and other relevant details of different microchannel reactors discussed in this study are listed in Table 1.

Sun et al (2008) reported higher methyl ester yield in shorter residence time in a

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capillary microchannel reactor by intensification of mass transfer using larger capillary specific

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surface area [59]. An optimum FAME yield was obtained with 1 wt% KOH but an further increase in the concentration of catalyst decreased the yield due to saponification reaction.

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Similar studies conducted by Ab Rashid et al (2014) using a capillary millichannel reactor

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under the same operating conditions resulted in a FAME yield lesser than that of capillary microchannel reactor [60]. Therefore, these studies prove that channels having smaller

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dimensions are more effective and efficient. Likewise, a study by Azam et al (2016) discussed the eff ects of catalyst concentration and residence time at diff erent internal diameters of a

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microchannel reactor. The FAME yield was higher in a channel with smaller internal diameter due to higher fluid velocities and mass transfer [61].

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The flow patterns in a microchannel were investigated using transparent Teflon tubes of diameters of 0.46, 0.68, 0.86, and 0.96 mm [62]. The increase in diameter significantly affected FAME yield. The soap formation due to the presence of FFA in the feedstock affected the formation of fine droplets. On the other hand, higher FAME yield was obtained with

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feedstock having lower FFA. Richard et al (2013) studied ethanolysis in a continuous microscale and batch process.

These reactors were connected with NMR spectroscopy for kinetic data acquisition. Further, the reaction rate was accelerated by varying the molar ratios of ethanol to oil from 6.0 to 45.4:1. Unlike batch process, higher alcohol to oil molar ratio requirement was observed for microchannel reactors to avoid soap formation [63]. Rahimi et al (2014) studied the

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transesterification of soybean oil in circular tubes and reported a maximum FAME yield at optimum KOH concentration. The increase in temperature above the optimum level decreased the yield due to glyceride saponification by the alkaline catalyst. Moreover, lower flow rates resulted in inefficient mixing, which lowered FAME yields though the residence time was sufficient [64]. Yeh et al (2016) devised a millimetre channel device that employs a co-axial mixing

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system for alkali catalysed transesterification. Circulation of droplets and higher surface to volume ratio are the two factors responsible for improved reaction rate. Higher amount of FFA or water may decrease the conversion by blocking the device channel. The interfacial collision

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between the reactants was the main feature influencing the reaction that was inversely proportional to the size of droplet. Hence, an intensified post treatment is required for the separation of residual alkali catalyst [30].

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Tanawannapong et al (2013) carried out waste cooking oil esterification using a microtube reactor. The reduction in acid value from 3.96 mg KOH/g to 1 mg KOH/g was

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observed. The rate of esterification reaction in the microchannel reactor was lower than that of

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batch process as the first step occurred within 5 sec of the reaction. But, higher FAME yield

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attained through transesterification was 91.76 wt% [65]. Mohammadi et al (2017) compared four different micromixers with an inlet and outlet

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channel depth and width of 0.4 and 0.8 mm respectively. The diameter and depth of the cylindrical pit for pitted type with and without T-junction was 50 and 0.4 mm, respectively

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while the diameter of the circular ring and the width of the channel for ring shaped with and without T-junction was 50 and 0.8 mm, respectively. The maximum FAME yield of 98.1 % was obtained using pitted type with T-junction where the mixing was performed by magnetized

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nanoparticles [56]. Schwarz et al (2013) studied seven diff erent continuous reactor systems assembled using three different mixers namely T, lamellar and split & recombine type [48]. The mixers made of Teflon capillaries had the same inner diameter of 1 mm and a mixing

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volume of 0.2 ml. They are connected to channels of 1.8 mm diameter having diff erent volumes ranging from 1.3 to 16.6 ml. The modified Villermaux–Dushman method was employed to rank these devices based on their effectiveness of mixing [66]. Highest yields were observed with split & recombine type mixer for the entire temperature range studied which was due to the intensity of micro mixing that increases mass transfer rate.

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Conversely, Bhoi et al (2014) investigated the performance of three different types of microreactors such as T-type, †-type and split & recombine type having a glass chip with a serpentine microchannel etched on it. Neither T-type nor †-type outperformed each other in terms of conversion. The unexpected trend observed with a methanol to oil ratio of 6:1 was the conversion did not decrease with the increase in flow rate but showed more chaotic behaviour. However, this was not true when the molar ratio was raised from 10.3:1 to 16:1. Though the conversion improved with feed molar ratio, no clear tendencies were reported yet. The

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negligible eff ect of temperature on conversion indicated that the mass transfer resistance at the liquid–liquid interface was significant. The results showed that very high conversion could be

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achieved with low residence time [49].

The performance of zig-zag microchannel reactors was analysed based on channel size/hydraulic diameter and number of turns. Mixing behaviour was analysed using laser scattering which indicated that methyl ester yield was strongly dependent on the droplet size.

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Smaller droplets in the reactor showed higher activity owing to increase in the specific

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interfacial area [54]. A helicoidal microchannel tube incorporated with a T-micromixer and

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energy from ultrasound under controlled conditions was used. The comparison with batch scale indicated that the yield obtained was similar to batch and continuous processes [57]. Aghel et

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al (2014) analysed the influence of mixing in three different cases - (i) coil with a length of 30 cm for the first case and (ii) 10 cm in the next two cases. The average pitch length of 0.5 mm

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was used in first two cases and 1 mm was used in the third. The influence of wire coil and its absence were compared for the above cases. Higher reaction yield was obtained with a

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difference of 6% in a reactor with a wire coil of 30 cm length and 0.5 mm of pitch length than in a reactor without wire coil. The addition of wire coil is favourable for cases having a coil

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with 30 cm length and 0.5 mm average pitch length. The wire coil was used to induce mixing in the reactor and the major factor to be considered is that the insertion of wire coil resists the flow passage necessitating additional energy to obtain higher yield [51]. Although most of the studies are conducted at atmospheric pressure, a study on the

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effect of pressure is very essential. A narrow channel Teflon tube reactor with 80 psi pressure was investigated. Higher methanol solubility and conversion was detected at low Reynolds numbers. A distinct change from slug flow at the inlet to stratified flow at the outlet was observed and this indicated in-situ separation of biodiesel and glycerol. Furthermore, higher conversions were observed at lower catalyst concentrations and residence times [67]. Most of the studies reported that laminar flow in T-shaped reactors offer better yield. The investigations

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on tesla, omega and T-shaped microchannels with a quadratic cross-section of 500 µm showed that tesla and omega shaped microreactors were good to provide higher yields than T-shaped reactor owing to chaotic flow patterns offering higher inter phase contact. [53]. Experimental investigations using ring and pitted mixing channels with and without Tjunction were reported and a higher yield was observed with ring and pitted micromixers with T-junction because of the strong collision of methanol and oil. The mixing by nano-magnetic

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material under revolving magnetic field showed better performance than static magnetic field at a varied range of residence time. The increase in nanoparticles concentration from 0.0025 to 0.01% (w/v) had positive eff ect on biodiesel production. However, the magnetic field increases

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the pressure drop, energy consumption and performance ratio [56].

Chueluecha et al (2017a) explored the use of packed bed microchannel reactor (configuration of 60 × 1 × 0.5 mm) loaded with activated CaO catalyst [27]. The reactants were

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fed using T-mixer and the channel was packed with 30 mg of catalyst powder. The activated CaO gave higher FAME yield compared to non-activated CaO for varying residence time (90.5

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% of FAME for activated CaO whereas only 82.7 % for non-activated CaO). The catalyst gave

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consistently higher yield during 24 h of continuous operation. A conversion of 99 % was

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obtained at the optimum reaction time of 8.9 min at 65°C and methanol to oil molar ratio of 24:1. The same microreactor setup was used to study the influence of different co-solvents such

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as tetrahydrofuran (THF), ethyl acetate, iso-propanol and the best co-solvent was selected based on the purity of biodiesel obtained. The highest FAME yield was achieved at a residence time of 7.1 min when 40 wt% THF was fed to the reactor whereas, iso-propanol achieved the

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highest FAME yield with 20 wt% under the same residence time. On the contrary, a maximum FAME yield of 20 wt% was attained with ethyl acetate at a residence time of 8.9 min. This

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indicated that iso-propanol was the eff icient co-solvent offering higher homogeneity in the reaction mixture than other co-solvents. Moreover, negative eff ect on FAME yield was observed with the use of co-solvents such as THF and ethyl acetate when the reaction was carried out at shorter residence times. The premixing of oil and co-solvent improved the

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conversion while the reaction efficiency improvement was due to lesser residence time and methanol to oil ratio. The optimum process conditions of 6.5 min residence time, 20:1 methanol to oil molar ratio and 14.5 wt% co-solvent gave 99% pure methyl esters [68]. Continuous biodiesel production from high acid value oils in a two-step process was studied using a micro-structured reactor assembled with a slit interdigital micromixer. The reduction in the acid value of oil from 160 to 1.1 mg KOH g-1 was achieved in the first step at

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the optimum conditions of methanol to oleic acid molar ratio of 30:1, 3 wt% H2SO4 and 7 min residence time at 100 °C. In the second step, a higher FAME yield of 99.5 % was achieved in 5 min with a methanol to oil molar ratio of 20:1 at 120 °C. Further increase in methanol to oil molar ratio decreased FAME yield since the first step contained about 75% FAME that enriched the solubility of the reactants [69]. A novel catalytic fixed bed reactor was developed to carry out both esterification and

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transesterification simultaneously using porous zirconia, titania and alumina particles [70]. The catalyst performance was analysed with surface modified and unmodified particles. These

catalysts are capable of operating under supercritical alcohol conditions at high pressure (2500

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psi) and temperature (300-450 °C). The feedstock was passed through a reactor of 15 cm long

and with 10 mm inner diameter and 2 µm stainless steel frits were used to combine the feedstock and methanol using T-mixer. Biodiesel was obtained from different feeds such as soybean oil, acidulated soap stock, tall oil, algae oil and corn oil with diff erent alcohols. The

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experiment results revealed that surface modification of the catalyst is not essential for Mcgyan

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process. Moreover, this process was scaled up to a continuous operation of 115 h without any

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significant decrease in the biodiesel conversion. The main highlight of this developed Mcgyan process is that it does not require continual addition of catalyst which significantly reduces

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soap formation.

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6. Impact of Co-solvents

The biodiesel production in a microtube reactor using various co-solvents such as tert-

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butyl methyl ether, dimethyl ether, diethyl ether and THF with KOH was investigated by Guan et al (2009a) [71]. Optimum co-solvent to methanol ratio is essential to achieve better performance. On excessive addition, co-solvent dilutes the reactant and the collision between methanol and oil is affected. The analysis of flow pattern in the presence of co-solvent showed

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homogeneous flow in the microtube at the entry which gradually transformed into a dispersed flow with the separation of glycerol at the outlet. The influence of co-solvent THF on the production of biodiesel in a Kinetic energy and Molecular diffusion (KM) micromixer consisting of 3 stainless steel plates was studied by Elkady et al (2015) [72]. The KM mixer exhibited superior performance than other mixers. The maximum yield was obtained with a THF to methanol volume ratio of 0.3 at 60 mL h-1. KM

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micromixer connected with fourteen microchannels was found to be eff ective in completing the transesterification reaction. Another study used hexane as a co-solvent in three diff erent configurations with confluence angles of 45°, 90° and 135° in a four way micromixer. The optimum co-solvent to methanol volume ratio of 0.4 gave higher FAME yield and a maximum yield was obtained for the microreactor operated with a hexane to methanol volume ratio of 0.45. At varying

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temperatures, highest FAME yield was obtained at 45° followed by 135° and 90° which was either due to the hydrophobic nature of hexane that allows to weaken oil-oil unified forces or due to the slight mixing to influence a substantial homogeneity [45].

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7. Microchannel reactors with enzyme catalyst

The process of immobilizing enzymes in a microchannel reactor was explored in three ways such as packing enzyme inside the reactor, immobilizing lipase on a medium like porous

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monolith or depositing on a reactor surface. The disadvantages associated with enzyme include

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the requirement of longer reaction time, difficulty in the maintenance of its activity and sensitivity to higher temperatures. However, enzyme is the best alternate to chemical catalysts

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and offers higher yield and consumes less energy [73]. The technical details of microchannel

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reactors using enzyme catalysts are listed in Table 2.

Choi et al (2016) investigated the fatty acid ethyl ester synthesis via lipase catalysed

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transesterification of acid oil with ethanol in a microchannel packed bed reactor [29]. The stainless steel reactor channel of 5.1 cm length and 4.8 mm inner diameter was packed with 0.5

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g catalyst (Lipozyme TL). The optimum water content, temperature and molar ratio of acid oil to ethanol were 4 wt%, 20 °C and 1:4 respectively. The increase in water content enhanced the

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enzyme catalysed hydrolysis reaction rather than the alcoholysis reaction. Highest yield of 92 wt% was attained under optimal conditions. Ethanol washing enhanced the relative activity of the catalyst and removed the adsorbed glycerol from the reactor. Enzymatic transesterification of tert-butanol solvent system was analysed in a packed

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bed microreactor using Novozym 435 with methanol and soybean oil as reactants [74]. The reaction was carried out in a stainless steel tube of 25 cm length and 4.6 mm inner diameter packed with 1.7 g lipase. All experiments were performed with 32.5 wt% tert-butanol. The flow rate and temperature showed significant eff ects on the molar conversion. The optimum flow rate of 0.1 mL min-1, temperature of 52.1 °C and substrate molar ratio of 1:4 gave 82.81

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% yield. Moreover, the conversion was found to remain same even up to 30 days without significant decrease. Anuar et al (2013) used lipase immobilized onto silica monolith in a microreactor to perform lipid transformation. The reactor is fused with a monolithic network having a silica capillary of 320 µm internal diameter and this provided a high surface area for immobilization. Candida antarctica lipase was covalently bonded and cross linked using glutaraldehyde. The operated under room temperature with a flow rate less than 1 L min-1 [75].

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activity of the immobilized enzyme was found to be reusable without any loss for 15 runs

Mugo and Ayton (2013) studied the lipid transformation using lipase immobilized poly

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glycidyl methacrylate-co-ethylene dimethacrylate [poly (GMA-co-EDMA)] in a monolith microreactor [76]. Candida antartica lipase B was covalently immobilized on poly (GMA-coEDMA) monolith arranged in a silica capillary of 0.7 mm internal diameter. The enzyme was

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reused up to 15 times without any noticeable loss in its activity over a period of one month operated at room temperature. Though the cost of the enzyme is very high, the reusability of

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the lipase loaded microreactor overcomes this limitation and makes this as a better option.

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The performance of microfluidic reaction system during the hydrolysis of soybean oil

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catalysed using Thermomyces lanuginosus lipase (Lipolase 100 L) was testified by Cech et al [77]. The microfluidic slug flow generator made up of plexiglass and consisting of T-slug flow

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generator, reaction tube, primary and secondary separators for collection and separation of oil and water was used. An estimated cross section of 400 × 500 µm was made on 75 mm diameter

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plexiglass microchannels whose ends had holes for inserting the inlet and outlet tube with a diameter of 1.6 mm. The results showed 25–30% and 50% conversions in 10 min and 1 h respectively due to the constraints of triglyceride hydrolysis on the interfacial area. The energy

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consumption was significantly lower in classical agitated arrangements and the mass transfer coefficient was also smaller due to lower characteristic velocities used in slug flow experiments. Bi et al (2017) designed a simple device made up of polytetrafluoroethylene for

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improving the enzymatic transesterification in microchannel reactors [73]. Inside the microchannel surface, polyethyleneimine and lipase were deposited one over the other as multilayers. The existence of the layers significantly increased the enzyme loading. Candida antartica enzyme was immobilized by adsorption and to avoid enzyme inhibition, tert-butanol was used instead of methanol. About 95.2% conversion was observed in 53 min at 40 °C. Further increase in temperature led to lipase inactivity and affected the conversion negatively.

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The increase in the length of the reactor enhanced the reaction rate up to 5 m and further increase to 7.5 from 5 m had no impact on the conversion except for the increase in residence time. Habibi et al (2016) studied the enzymatic transesterification of canola oil and methanol in a shake flask and capillary channel reactors using Candida rugosa lipase without any solvent [78]. The reactor had a microsize T-type junction with an internal diameter of 0.8 mm and

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connected to a channel with a length and dimension of 100 cm and 3 × 1 mm respectively. In order to improve the mixing, nine stainless steel coils with a length of 10 cm and a stride length of 0.5 mm were inserted in the beginning of the channel after T conjunction and also at all bend

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sections of the capillary channel. This capillary reactor enhanced the methanolysis yield up to

four fold as compared to shake flask experiments. A proper mixing at a narrow size channel could sufficiently reduce the reaction time and raw materials consumption.

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8. Kinetic and Simulation Studies in Microchannel Reactors

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Pontes et al (2016) developed coupled nonlinear mathematical model by Generalised Integral Transform Technique (GITT) approach and using the simulation software COMSOL

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Multiphysics to interpret the transesterification studies. Eff ects of volumetric flow rate,

M

microreactor dimensions, residence time and flow temperature on microreactor performance were evaluated [79]. The simulation results reported that higher conversion of triglycerides can

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be attained at higher residence times and smaller microreactor heights due to the superior surface to volume ratios. Higher temperatures improved the chemical process which is the

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result of the influence of temperature observed early in the reaction that corresponds to the inlet region of the microreactor. A three-dimensional non-linear mathematical model was proposed by Pontes et al (2017) to describe the transesterification reaction in microreactors by

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considering the diff usion, reaction and convection phenomena. The results indicated that enhanced conversion of triglyceride occurs at higher residence time, larger reaction

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temperature and lower microreactor height [80]. The numerical simulations based on computational model was developed using

ANSYS ICEM and CFX for geometries and fluid dynamics analysis of reactive studies [52]. Micromixer types like T, Cross and Double-T-micromixer were analysed based on mixing index and Reynolds number. The cross micromixer showed highest mixing index whereas Reynolds number varied greatly with all the micromixers. The mixing pattern using T with circular obstructions and T with alternate circular obstructions was also reported. Numerical

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and experimental studies showed similar results and it proved that higher obstructions in the channel are responsible for effective mixing due to the higher interaction between the fluids. T with alternate circular obstructions showed maximum conversion and the presence of obstacles in micromixers improved the mixing index which did not happen in T with circular obstructions [55]. The experiments as well as numerical simulations for T-micromixer and spiral micromixer with different heights and widths were also reported. It was also proved that spiral micromixer had maximum mixing index whereas T-mixer showed maximum conversion [52].

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Biodiesel synthesis in a T- shaped microchannel reactor with static elements was evaluated

using both numerical and experimental methods and the results were similar in both the cases.

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The characteristic reaction kinetics of a batch reactor was also compared with a microreactor [81]. 9. Challenges in Microchannel Reactors

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The material selection for the construction of microchannel reactors is a major challenge and materials such as polycarbonate, Teflon, polysulfone, polymethylmethacrylate

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etc., on prolonged usage exhibit stress cracks at the joints due to the reaction with

A

reactants/products. Fouling is a common problem in microchannel reactors and is unavoidable

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irrespective of the materials used for the construction. It affects the durability and lifetime of the material. Since studies have not been conducted to overcome the risk of fouling, care has

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to be taken in selecting the right material for the construction of microreactors. Parallelization of biodiesel synthesis in microreactors made up of high density

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polyethylene and arresting the leakage in the joints between the modules of full scale microreactors are the challenges reported by Billo et al [11]. The determination of efficient mechanism for arresting the leakage between the modules using gaskets and with mechanical

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fasteners that are suitable for prolonged usage is not easy and hence further research is required to find reliable materials. The research works done on small scale production of biodiesel in microreactors have reported the advantages of different micromixers. The primary aim of the

A

mixers is to maximise the homogeneity of the immiscible phases before entering the channels to promote molecular interactions. The selection of the suitable micromixer is also tedious and is influenced by the nature of the feed and catalyst. The impact of co-solvent is another important challenge and the addition of co-solvents increases the molecular interaction and improves the flow behaviour. The scope of replicating the same in large scale production is uncertain because online monitoring of the flow behaviour

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of every individual module inside the microchannel is practically not possible. Hence, it requires an automated monitoring system to exactly control the process. The another difficulty is immobilizing the catalysts on the microchannel surface without the loss of its catalyst activity. Also, research has not yet completely explored the use of heterogeneous catalysts in microchannel reactors due to their practical constraints. The mass transfer limitations associated with immiscible phases and catalysts affect the reaction rate. Hence, further research

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in this area is essential. Though packed bed microreactor gives appreciable yield with immobilized enzymes, continuous use of packed bed at a larger scale is still a challenge. Also, preventing the enzymes

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from mechanical and other stresses is intimidating. Another problem that needs to be addressed

is the creation of high pressure drop when the enzymes are packed into the microreactor by immobilization [73]. Microchannel reactors used for biodiesel synthesis face the major problem of soap formation due to high FFA and water content because of which the

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physicochemical properties and the yield of biodiesel are affected. So, it necessitates proper

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pretreatment of feedstock to reduce the above limitations. Hence, selecting a proper, efficient

A

and economical pretreatment is a major concern. Furthermore, maintaining the even

M

distribution of temperature inside the channel for higher yield is necessary. Various mathematical models such as Minitab, GITT, ANSYS ICEM, CFD analysis,

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etc. are employed to analyze the kinetic studies involved in process intensification in microchannel reactors for biodiesel production. But the experimental validation has practical constraints because of challenges involved obtaining the data at different time intervals inside

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the microchannel reactor.

Microscale technology is not yet used extensively in an industrial scale. However,

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conventional reactors consume more power though they have high productivity, product feasibility and well controlled reaction environment. The research on biodiesel production still requires tools and strategies that can minimize the gap between industry and laboratory and to

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fulfil the industrial needs. 10. Conclusion The high production cost of biodiesel necessitates process intensification techniques using microchannel reactors that are the best substitutes to conventional techniques and efficient methods to achieve the target. In this study, an extensive review on the microchannel reactor design, fabrication, different types of mixers and mixing inside the channels was carried

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out. The advancements in microchannel and near microchannel dimensions are analysed for different systems. The reported works are feasible only on the laboratory scale and parallelization of reactors is difficult. Homogeneous base catalysts are most widely employed due to low catalyst consumption, high product purity and higher reaction rates. On the other hand, fewer attempts have been made with heterogeneous catalysts in microchannel reactors due to their practical constraints. The use of enzymes in microreactors has great potential for bringing down the cost of biodiesel production. Enzyme catalysts are used utilized in packed

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bed, monolith and surface coating and the reported works suggest that catalyst reusability

enhanced greatly without a noteworthy decrease in the yield along with desired product purity.

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But still, a research focussing on immobilizing the enzyme inside the channels to improve its stability and to prevent its denaturation is essential. Hence, a distinctive approach towards reactor design for utilizing different catalysts still needs to be explored for the betterment of

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the process.

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Acknowledgment

The authors would like to express their gratitude to National Institute of Technology, Tiruchirappalli, India under the Ministry of Human Resource Development for providing a

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platform to carry out research and fellowship for doctoral degree to Mr. N. Yasvanthrajan.

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29

List of figures Outline of Micromixers and microchannel reactors for biodiesel production

Fig. 2.

T-mixer, B. J-mixer

Fig. 3.

Co-axial fluidic system

Fig. 4.

Four way micromixers with different confluence angle 45°, 90°, 135°

Fig. 5.

Flow pathways in different mixers, A. T- type mixers, B. J- type mixers, and

IP T

Fig. 1.

SC R

C. Rectangular interdigital micromixer, D. Slit interdigital micromixer A. Lamellar mixer, B. Split and recombine type mixer

Fig. 7.

†- mixing pattern

Fig. 8.

A. T-mixer, B. Cross T- mixer, C. Double cross T-mixer

Fig. 9.

Mixing configuration in the Zig-zag microchannel reactor

Fig. 10.

Schematic representation of wire coil reactor

Fig. 11.

Spiral coil reactor

Fig. 12.

Omega and Tesla shaped mixing channels

Fig. 13.

Zig-zag microchannel with turns

N

A

M

ED

PT

CC E

Fig. 14.

U

Fig. 6.

A. T-channel, B. T-channel with circular obstructions and C. T-channel with

A

alternate circular obstructions

SC R

IP T

30

A

CC E

PT

ED

M

A

N

U

Fig.1. Outline of Micromixers and microchannel reactors for biodiesel production

Fig.2A. T-mixer, B. J-mixer [36]

PT

ED

M

A

N

U

SC R

Fig.3. Co-axial fluidic system [37]

IP T

31

A

CC E

Fig. 4. Four way micromixers with different confluence angle 45°, 90°, 135°.[40]

SC R

IP T

32

U

Fig.5. Flow pathways in different mixers. A.T- type mixers, B. J- type mixers, C. Rectangular

A

CC E

PT

ED

M

A

N

interdigital micromixer, D. Slit interdigital micromixer.[41]

Fig.6.A. Lamellar mixer, B. Split and recombine type mixer [43]

ED

M

A

N

U

SC R

Fig.7. †- mixing pattern

IP T

33

A

CC E

PT

Fig. 8 A. T-mixer, B. Cross T- mixer, C. Double cross T-mixer [45].

SC R

IP T

34

A

CC E

PT

ED

M

A

N

U

Fig.9. Mixing configuration in the Zig-zag microchannel reactor.

Fig.10. Schematic representation of wire coil reactor

A

CC E

PT

ED

M

A

N

U

Fig.11. Spiral coil reactor

SC R

IP T

35

Fig.12. Omega and Tesla shaped mixing channels

SC R

IP T

36

A

CC E

PT

ED

M

A

N

U

Fig. 13. Zig-zag microchannel with turns [50].

Fig. 14.

A. T-channel, B. T-channel with circular obstructions and C. T-channel with alternate circular obstructions

37

List of Tables 1. Technical details of microchannel reactors used for biodiesel production

A

CC E

PT

ED

M

A

N

U

SC R

IP T

2. Technical details of enzymatic microchannel reactors.

I N U SC R

38

Table.1. Technical details of microchannel reactors used for biodiesel production Type of reactor

Material of construction

Reactor diameter (mm)

Feedstock

Catalyst used

Catalyst Quantity (wt%)

1.

T-type tubular with ultrasound KM micromixer

Stainless steel Stainless steel

2.15900

NaOH

Narrow channel with T-mixer Tesla shaped with T-mixer Omega shaped with T-mixer T-shaped shaped with T-mixer Microtube with wire coil with Tmixer Microtube without wire coil with Tmixer Microtube with T-mixer Microtube with 4-way Micromixer

Teflon

1.5

Recycled oil Waste vegetable oil Canola oil

Polydimethyl siloxane

0.5

Castor oil

3.

M

0.22

Temperature (°C)

Yield %

Reference

0.6

Methanol – oil molar ratio 6:1

60

87

[53]

NaOH

1

12:1

70

97

[68]

NaOH

1

6:1

60

98

[63]

NaOH

1

9:1

50

96.7

[49]

95.3

CC E

PT

4.

ED

2.

A

S. No.

A

5.

6. 7.

Stainless steel

93.5 0.9

Soybean oil

KOH

1.2

9:1

60

94.5

[47]

88.7

Teflon

0.96

Stainless steel

1.58

Sunflower oil Soybean oil

KOH

1

6:1

25

92.8

[38]

KOH

1

3:1

57.2

98.8

[40]

I Teflon

1.59

9.

Capillary Microchannel with external mixer Circular microtube withT-mixer Mircotube (Milli & micro) with T-mixer Circular Microtube with T-micromixer Co-axial fluidic system Microtube with T-micromixer

Stainless Steel Quartz

2

12.

CC E

13. 14.

KOH

5

23:1

60

91

[56]

Rapeseed oil, cotton seed oil

KOH

1

6:1

60

96.7

[55]

Teflon

M

98.8

0.5

Sunflower oil

Sodium ethoxide

1

45.4:1

65

98

[59]

Teflon

Refined palm oil

KOH

4.5

21:1

60

100

[57]

1.6

Stainless steel

0.8

Soybean oil

KOH

1.2

9:1

60

89

[60]

Teflon

0.55

NaOH

1

1:3

55

98.6

[37]

Teflon

0.508

H2SO4, KOH

1

9:1

65

91.76

[61]

Teflon

0.8

KOH

4.5

23.9:1

60

100

[38]

Stainless steel

0.6

Soybean oil Waste cooking oil Sunflower oil Cotton seed oil

KOH

1

8:1

70

99.5

[41]

Glass chip

10

Sunflower oil

KOH

2

10.3:1

60

99.2

[44]

15. Microtube with T-micromixer 16. Microstructure with T and J mixers 17. Serpentine microchannel with three different mixers

A

Palm oil

ED

11.

0.53

PT

10.

N U SC R

Capillary Millichannel with T-mixer

A

8.

39

0.58

3.5

I Stainless steel

0.9

CC E A

23. Microtube with T and J- mixer

N U SC R

0.24

Soybean oil

NaOH

1.2

6:1

56

99.5

[50]

Soybean oil

KOH

1.17

8.5:1

59

99.5

[46]

Soybean oil

KOH

1

12:1

45

98.1

[52]

0.6

Cotton seed oil

H2SO4

3

20:1

120

99.9

[66]

Teflon

0.46 0.68 0.86 0.96

KOH

4.5

-

59.85

89 92.8 91.4 89.2

[58]

Teflon

0.508

Waste cooking oil, Sunflower oil Palm oil

KOH

1 mass%

6:1

60

97.14

[36]

ED

Stainless steel

1.58

M

Stainless steel

A

Stainless steel

PT

18. Zig-zag microchannel with T-mixer 19. Zig-zag microchannel with zig-zag mixing mixer 20. Microchannel with different micromixers 21. Microstructure with slit interdigital micromixer 22. Microtube with T-mixer

40

I N U SC R

41

Table.2. Technical details of enzymatic microchannel reactors.

3.

4.

5.

A

6.

Catalyst Quantity (g) 0.5

Alcohol – oil molar ratio 4:1

20

92

[71]

Soybean oil

4.6

1.7

4:1

52.1

82.81

[72]

Silica gel

Triolein

0.32

-

-

22

72

[73]

Poly (GMA-coEDMA)

Lauric acid

0.25

0.7

1:1

50

97

[74]

Rugose Lipase

-

0.8

0.05

3:1

37

91.8

[76]

Candida Antarctica

Polyethylenimine

Canola Oil Soybean oil

0.38

-

7:1

40

95.2

[70]

A

Immobilisation Carrier Silica gel

Novozym 435

Acrylic resin

M

Lipozyme TL

ED

2.

Reactor diameter (mm) 4.8

Enzyme

Candida Antarctica

PT

1.

Type of micro reactor and material used Packed bed microreactor, Stainless steel Packed bed microreactor, Stainless steel Polymer monolith microreactor Capillary Polymer microreactor Capillary microreactor Packed bed microreactor, Polytetrafluoroet hylene

Candida Antarctica

CC E

S. No.

Feedstock

Acid oil

Temperature (°C)

Yield %

Reference