Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-017-0296-0
REVIEW ARTICLE
Microreactor technology for biodiesel production: a review Akansha Madhawan 1 & Arzoo Arora 1 & Jyoti Das 1 & Arindam Kuila 1 & Vinay Sharma 1 Received: 27 April 2017 / Revised: 11 September 2017 / Accepted: 16 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017
Abstract Due to a rise in global demand for energy and an increase in greenhouse gases, biodiesel has been accepted as an alternative fuel because of its biodegradability, low environment detrimental effect, better quality of exhaust gas emission, and renewability. Biodiesel is a mixture of monoalkyl esters of long-chain fatty acids also known as fatty acid methyl esters. Transesterification is the most commonly adopted technique for the production process. But the conventional biodiesel technology has its own disadvantages. Process intensification technologies can overcome these drawbacks. Some novel reactors such as microchannel reactor, static mixers, oscillatory flow reactors, and spinning tube reactors have been developed so as to improve mass transfer and mixing. These technologies can achieve rapid and high reaction rates due to the high surface area/volume ratio and short diffusion distance thus intensifying the transesterification process. Various factors such as alcohol to triglyceride molar ratio, microchannel size, residence time, reaction temperature, mixing mechanism, and catalyst affect the production process. Although biodiesel production has been commercialized in several countries, it still requires a clean, effective, and environment-friendly technology to make it cost-effective and increase its competency against conventional fossil fuels. Microreactor technology, however, has proved to be a benchmark to serve this purpose. The current review paper provides an overview about different types of microreactors used in biodiesel production and the parameters affecting biodiesel production in microreactors. The microreactor technology discussed in this paper aims to improve the production process by decreasing the reaction time from hours to minutes. Keywords Fatty acid methyl ester . Transesterification . Microreactor technology . Catalyst
1 Introduction An increase in human population has led to a rise in the global demand for energy, and most of the energy requirement is derived from fossil fuels which comprises about 88% of total energy consumption with an oil share of 35%, and natural gases accounting 24% and coal 29% as major fuels, while hydroelectricity and nuclear energy comprise 6 and 5% of total energy consumption, respectively [1]. But in recent years, due to environmental concern with increase in greenhouse gases and limitation of traditional fossil fuel resources, great attention has been paid towards an alternative source of energy to meet the energy requirement. For this reason,
Akansha Madhawan, Arzoo Arora, and Jyoti Das have equal contribution. * Vinay Sharma
[email protected] 1
Department of Bioscience & Biotechnology, Banasthali University, Rajasthan 304022, India
biodiesel has gained a lot of attention in recent years because of its biodegradability, low environment detrimental effect, better quality of exhaust gas emission, and renewability [2]. Biodiesel is a mixture of monoalkyl esters of long-chain fatty acids derived from lipid feedstocks such as animal fats and vegetable oils [3]. It is also known as fatty acid methyl ester (FAME). These lipid feedstocks can be categorized under the following: waste vegetable oil which is not suitable for human consumption [4]; non-edible oils including Jatropha, neem oil, and castor oil [5, 6]; animal fats like lard, yellow grease, and tallow [7]; and virgin vegetable oil feedstocks such as rapeseed, soybean, sunflower, and palm oil [8]. Patle et al. [9] analyzed the organic waste and heat duty in comparison to the process of biodiesel production and suggested that a study on the detailed composition of the substrate along with the reaction kinetics is necessary for the stimulation of the biodiesel production process. Different techniques are used to produce biodiesel with its own advantages and disadvantages such as microemulsion [10], transesterification [11], direct/blends [12, 13], and pyrolysis [14] Table 1.
Biomass Conv. Bioref. Table 1 Methods of production of biodiesel [15]
Sr. no.
Methods
Advantages
Disadvantages
1
Pyrolysis
Low processing cost and simple process
High temperature required
2
Microemulsion
Lower fuel viscosity
Lower cetane number and energy content
3
Direct use and blending Transesterification
Liquid nature and renewability
Low volatility and high viscosity
High conversion energy, renewability, higher cetane number
Glycerol disposal and multiple downstream processing.
4
Among these, transesterification is the most commonly adopted technique, and the mechanism involves three reversible reactions: conversion of triglycerides to diglycerides and then to monoglycerides, and subsequently to glycerol. An alkyl ester is produced at each step of the reaction thus producing three alkyl esters [16]. But the conventional biodiesel technology has its own disadvantages. The production of biodiesel is dependent on the reactant and transesterification conditions. A large amount of alcohol is required to shift the equilibrium of the reaction to the product side thus increasing the biodiesel yield [17]. High alcohol consumption unfortunately results in high production cost. Use of a homogenous acid catalyst could be used to reduce alcohol consumption, but the process requires high reaction time and is corrosive in nature. Although homogenous alkaline catalysts could overcome these limitations, the presence of free fatty acids and water in raw feedstock can result in a saponification process. These dissolved soaps in the glycerol phase increase the solubility of methyl ester and complicate the separation process [18]. A heterogeneous catalyst has been suggested as an alternative, but the three-phase system consisting of triglycerides, alcohol, and solid catalyst in the reaction mixture reduces the availability of an active site for catalytic reaction thus decreasing the reaction rate [19]. However, supercritical transesterification without the aid of a catalyst can overcome mass transfer limitation, but the process requires a large amount of energy [20] and is not feasible for large-scale production. Moreover, multiple downstream processing decreases the yield of biodiesel. Process intensification technologies can overcome these drawbacks. Some novel reactors such as microchannel reactor, static mixers, oscillatory flow reactors, and spinning tube reactors have been developed so as to improve mass transfer and mixing. These technologies can achieve rapid and high reaction rates due to the high surface area/volume ratio and short diffusion distance [21] thus intensifying the transesterification process.
2 Types of microreactor for biodiesel production Microreactor technology has been proposed to carry out the transesterification process in a short time interval. This comes
in various shapes and structures and is designed in such a way that proper mixing and reaction can be carried out. Biodiesel production by transesterification using microreactors can result in a higher chemical reaction rate and is able to reduce the reaction time thus increasing the rate of product synthesis. Capillary microreactors were the first reported microreactors used for biodiesel synthesis. Later on, various advanced microreactors were fabricated using different materials and techniques.
2.1 Membrane microreactor The membrane system is known for its high selectivity, as well as its ability to control the mixing of components between two phases and provide high surface areas per unit volume. These reactors are generally classified on the basis of four concepts: the membrane used in the reaction (organic, inorganic, porous, or dense membrane), the reactor design (extractor, distributor, or contactor), whether it is an inert or catalytic membrane reactor, and the reactions that occur in the membrane reactor such as dehydrogenation [22] and esterification [22, 23]. Using different mass transfer rates, the membrane acts as a selective barrier thus regulating the transport of substances. During biodiesel production, the membrane separates glycerol from the product stream [24, 25] or retains the unreacted triglycerides within the membrane [26–28] (Fig. 1). To deal with acid and base concentrations during the reaction, determining the type of membrane to be used is of most significance. Dube et al. [28] suggested that the carbon membrane resisted high acid and base environments and can have several advantages over conventional reactors such as resistance to corrosion. They carried out acid- and base-catalyzed transesterification of canola oil in a membrane reactor of pore size 0.05 m with an internal diameter of 6 mm, outer diameter of 8 mm, and surface area of 0.022 m2. The reactor was operated at a pressure of 138 kPa and pump flow rate of 6.1 mL/min at a temperature of 60 °C for 30 min and 65 °C for 40 min thus separating the unreacted canola oil from the reaction product. Also, Barredo-Damas et al. [29] stated that ceramic carbon membranes offer many advantages over polymeric membranes such as high chemical, mechanical, and thermal resistance to degradation and higher permeability rates. However, the cost of ceramic membranes is high. Polyethersulfone used in
Biomass Conv. Bioref. Fig. 1 Basic layout of the integrated membrane reactor system
the biocatalytic membrane microreactor also showed good stability with no decay of its catalytic activity for at least 12 days of continuous operation [30]. Cao et al. [27] developed a membrane reactor for the transesterification of a number of vegetable oils such as soybean, yellow grease lipid, and palm. The reactor was able to carry out the process using high-quality biodiesel fuel which was confirmed by GC analysis. Similarly, Boroutian et al. [26] also developed a ceramic (TiO2/Al2O3) membrane reactor comprising a pore size of 0.05 m, inner and outer diameters of 1.60 and 2.54 cm, respectively, and length of 40 cm. A Chem-Durance chemical-resistant pump tubing with size of 16 was used. The membrane was able to block the triglycerides, and biodiesel was able to pass through the membrane, thus obtaining 94% conversion providing 157.04 g of catalyst per unit volume of reactor at 70 °C reaction temperature. Achmadin et al. [31] developed a membrane microreactor with a diameter of 63.5 mm for transesterification of triolein and methanol. The pores in the membrane act as a site of reaction and, using this, obtained 80% conversion with a reaction time of 19 min at 35 °C. In other reports, a Al2O3 ceramic membrane reactor was used for transesterification of canola oil with 0.05% water content. A membrane with a pore size of 0.05 mm, length of 30 cm, and inner and outer diameters of 6 and 10 mm, respectively, was used, and a high quality of biodiesel was produced without need of washing and purification step. High conversion (96.42%) was obtained by adding 1% KOH as catalyst and a methanol to oil ratio of 6:1 for a reaction time of 7 h. The study showed that the membrane reactor could enhance the reaction rate by excellent mixing in the membrane reactor loop and the continuous removal of the product and unreacted triglyceride from the reaction medium. Moreover, none of these novel reactors, except the membrane reactor, is able to overcome the limitations caused by chemical equilibrium in transesterification.
2.2 Microtube microreactor Providing efficient heat dissipation, high mass transfer, and short diffusion distance, microtube technology has gained a
lot of attention in recent years [32] (Fig. 2). Bertoldi et al. [33] and Trentin et al. [34] respectively employed a tubular reactor for the production of biodiesel from soybean oil using carbon dioxide and supercritical ethanol as a co-solvent. The capacity of the tubular reactor was 88 mL with an outer diameter of 1.59. Guan et al. [35] used a transparent fluorinated ethylene propylene tube as microtube reactor with 8-mm internal diameter for sunflower oil transesterification. With this, 100% FAME yield was obtained using KOH concentration of 4.5 wt%, methanol to oil molar ratio of 23.9:1, and flow rate of 8.2 cm3/h. They showed that the microtube reactor shows better performance in oil conversion than the lab-scale batch reactor. Silva et al. [36] worked on fatty acid ethyl ester production from transesterification of soybean oil. Reaction was carried out in the microtube reactor with an internal diameter of 0.76 mm having a capacity of 36.5 mL. Appreciable yields were obtained at a temperature of 398 K, pressure of 20 MPa, and oil to ethanol molar ratio of 1:20. Similarly, Tanawannapong et al. [37] reported biodiesel production from waste cooking oil using a microtube reactor with 1.2 cm length and internal diameter of 0.508 mm along with a Tmixer at the inlet of the reactor. Their study suggested that transesterification reaction in the microtube reactor takes a shorter time as compared to the batch reactor, and 91.7% methyl ester content was obtained using an oil to methanol ratio of 1:9 and KOH concentration of 1 wt% at 65 °C for 5 s reaction time. Similarly, Kaewchada et al. [38] studied transesterification of palm oil and methanol with KOH as catalyst in a microtube reactor of 1.2 m length and internal diameter of 0.508 mm for biodiesel synthesis. They reported that an increase in catalyst amount (5 to 10 mg/g) and short residence time cause a sharp increase in %FAME. FAME (97.1%) was obtained for a residence time of 5 s and 10 mg/ g catalyst with a methanol to oil ratio of 6:1. One of the subgroups of microtube reactors are capillary microreactors as the principle of operation is identical. Capillaries are the simplest and the first microreactors to be reported for biodiesel production. Sun et al. [39] used a
Biomass Conv. Bioref. Fig. 2 Experimental setup (diagrammatic) of a microreactor for biodiesel production. The microreactor depicted in this figure represents a microtube reactor (made up of polytetrafluoroethylene, generally 1.2 m long with internal diameter of 0.508 mm) with a T-mixer as a mixing unit
capillary microreactor with an inner diameter of 0.25 mm for transesterification of unrefined cottonseed oil and rapeseed oil. KOH-catalyzed reaction was carried out at a concentration of 1 wt% KOH and methanol to oil molar ratio of 6:1. Providing a residence time of 5.89 min at 60 °C, they obtained 99.4% methyl ester. Similarly, Wen et al. [40] observed that the high reaction efficiency of the microchannel reactor is attributed to the intensification of overall volumetric mass transfer by passive mixing at the micro scale. Using a capillary microreactor of 250 μm diameter, KOH concentration 1.0%, and a methanol to oil molar ratio of 6:1 at a reaction temperature of 60 °C, residence time was decreased. Recently, Rashid et al. [41] carried out biodiesel synthesis in a capillary millichannel reactor with an inner diameter of 1.59 mm. Using methanol and potassium hydroxide (KOH) as base catalyst with palm oil as a feedstock, the highest FAME yield that has been achieved in this study was 91%. Billo et al. [42] worked to scale up microreactor technology such that the capacity of the microreactor for biodiesel production increases. They developed a microreactor system that improved the production capacity per reactor volume by decreasing the required reaction time from hours to minutes. The microreactor unit contains thousands of microchannels that organize the bulk flow into parallel streams of alternating reactant slugs that are laminar. This flow pattern intensifies the mass transfer due to the high surface area to volume ratio of the slugs and essentially accelerates the reaction [43]. The product stream exiting the microreactor is an emulsion of biodiesel and glycerol. To develop a full-scale microreactor, individual microreactor laminae of 0.76 mm thickness and 5.7 cm × 5.7 cm planar dimensions were fabricated; stacks of 50 laminae were then assembled into modules, modules were assembled into manifolds, and manifolds were then assembled into the final full-scale microreactor. For laminae, the material substrate that was selected was required to be
chemically inert and low-cost so high-density polyethylene (HDPE), being highly chemically resistant to the biodiesel process reactants and products, was used. To obtain the production rate of 1.9 L/min of biodiesel fuel, the final full-scale microreactor consisted of 35 manifolds connected in parallel, containing over 14,000 individual laminae operated at 65 °C, methanol to oil ratio 1:3, catalyst concentration 0.9 M, and overall residence time of 2.6 min including the time spent for static mixing.
2.3 Microstructured reactors In comparison to the microtube reactors, microstructured reactors manufactured with different principles of mixing are accepted to further enhance the mixing of methanol and oil. Microstructured reactors have brought attention towards energy technology due to its potential in process intensification in which compact and decentralized solutions are needed [44]. For biodiesel production, Canter [45] used a vegetable oil and methanol/NaOH, fed by two syringe pumps into the microstructured reactor with a parallel microchannel cut in a thin plate of plastic. Yields of FAME of > 90 and 96% could be obtained at temperatures of 40 and 45 °C and residence times of 4 and 10 min, respectively. In a base-catalyzed transesterification reaction, using soybean and sunflower as the oil source with methanol, microstructured reactors including HiPress Slit Interdigital Micro Mixer, Slit Interdigital Micro Mixer V2, and Caterpillar Split-Recombine Micro Mixer have been utilized for production of biodiesel. Sun et al. [46] operated microstructured reactors constructed with two multilamination micromixers and devised a PTFE tube of 3 mm i.d. packed with Dixon rings as the delay loop for transesterification of methanol and cottonseed oil with KOH catalyst. The two micromixers, a slit interdigital micromixer (SIMM-V2) and a rectangular interdigital micromixer
Biomass Conv. Bioref.
(RIMM), express more enhanced mixing than the J-mixer and T-mixer, hence resulting in a greater FAME yield. The biodiesel yield reaches 99.5% at 8:1 molar ratio of methanol to oil, a flow rate of 10 mL/min, a reaction temperature of 70 °C, and a residence time of 17 s. For continuous alkali-catalyzed synthesis of biodiesel, Wen et al. [40] used channels of zigzag shape (Fig. 3). The smaller the channels generated, the smaller the droplets. Ester yield of 99.5% was obtained by the conversion of soybean oil (21.4% oleic acid, 10.2% palmitic acid, 57.2% linoleic acid, 4.0% stearic acid, and 7.2% linolenic acid) within 28 s. As compared to conventional technology, the specific energy input needed for formation of droplet by zigzag-shaped channels was extremely smaller. Fig. 4 Flow pattern inside the column of an oscillatory flow reactor
2.4 Oscillatory flow reactors Oscillatory flow reactors (OFRs) are proving to be novel reactors with tubes containing equally spaced orifice plate baffles. With the net flow of process fluid, an oscillatory motion is superimposed creating a flow pattern which helps in efficient mixing and mass transfer (Fig. 4). It has proved to be advantageous over conventional plug flow reactors, where a minimum Reynolds number must be maintained. The degree of mixing is independent of the net flow thus allowing long residence times to be achieved in a reactor of greatly reduced length to diameter ratio [47]. The controlled oscillatory motion enhances radial mixing. Harvey et al. [47] developed a reactor with two vertically positioned jacketed QVF tubes of 25 mm internal diameter and 1.5 m length. Rapeseed oil was transesterified with a methanol to oil ratio maintained at 1.5 in a 1.56-dm3 reactor. Conversion of biodiesel up to 98% was achieved in the presence of NaOH catalyst after 30 min at 50 °C. Phan et al. [48] continuously screened the base-catalyzed biodiesel production using a newly designed mesoscale oscillatory baffled reactor which consists a series of 340-mm-long, 5-mm-diameter tubes. The study was done using two different types of baffles (helical baffle and central sharp-edged baffle). Rapeseed oil with methanol and solution of methanol and potassium hydroxide was added continuously.
Thus, it was concluded that the reactor provides sufficient mixing and produces a homogenous mixture into the two-phase liquid reaction giving a yield of methyl ester at mixing intensity of (ReoN130). Similarly, Phan et al. [49] reported continuous alkali-catalyzed transesterification of rapeseed oil with methanol in a ratio of 1:6 in a 29-mL reactor. Three oscillatory flow mesoreactors (integral baffles, wire wool, and sharp-edged helical baffles with a central rod) were assessed. Reaction was carried out 60 °C with residence time of 5 min and flow rate below 3 mL per minute. FAME content obtained was about 97% with KOH as catalyst while that with sodium methoxide FAME content was 94–96% at 5–10 min residence time. Highina et al. [50] reported transesterification of Jatropha oil to biodiesel in a reactor consisting of two vertically positioned jacketed stainless steel tubes with internal diameter 0.1 m and length 1.4 m, and steel baffles were welded in a tube wall. Using this oscillatory baffled reactor system, 94% of Jatropha oil was converted with a methanol to oil ratio of 6:1 in a reaction time of 5 min adding 1 wt% NaOH as catalyst. Suryanto et al. [51] worked on optimizing the oscillatory flow reactor for continuous biodiesel production. A continuous reactor tube of 3.6 m was used along with 76 baffles installed within the cylindrical tube. Palm oil and methanol in 10:1.7 were used with NaOH as a catalyst, and the production rate reaching 12 times faster was observed using this new innovation. Because the oscillatory flow reactor can achieve long residence times with lower methanol to oil ratio, it can be designed with a smaller length to diameter ratio, which eventually helps to improve the economy of biodiesel production because of the smaller Bfootprint,^ lower capital, reduced pumping cost, and ease of control.
3 Parameters affecting biodiesel production in microreactor Fig. 3 Schematic diagram of a microchannel microreactor
Different parameters affecting the transesterification process are alcohol to triglyceride molar ratio, microchannel size,
Biomass Conv. Bioref.
residence time, reaction temperature, mixing mechanism, and catalyst.
3.1 Molar ratio of alcohol to triglyceride During the transesterification reaction, one of the important variables affecting the yield of methyl ester production is the molar ratio of alcohol to triglyceride. In order to drive the equilibrium reaction, an excess amount of alcohol is needed to move the reaction towards right for the formation of methyl ester [52]. An ideal ratio of alcohol to oil has to be empirically established because a large amount of alcohol makes the glycerol recovery difficult [53]. The reactants’ molar ratios of methanol to oil for biodiesel production have been studied from 6:1 up to 48:1. It was noticed that the reactants’ molar ratio of 12:1 recorded the maximum conversion of biodiesel as compared with a 6:1 molar ratio that attained the lowest conversion of biodiesel. Biodiesel conversion declines if there is an increase in molar ratio of alcohol to oil above 12:1 due to reversible transesterification reaction. Yuan et al. [54] carried out a transesterification of waste-pretreated rapeseed oil and found maximum conversion at 6.5:1 of methanol to oil ratio, while an earlier report by Leung and Guo [55] showed a maximum conversion at a molar ratio of 7:1. The reactants’ molar ratio 6:1 of alcohol to triglyceride is more acceptable for the maximum conversion to methyl ester [56–61].
3.2 Microchannel size In a capillary microreactor, for biodiesel production, yield of methyl ester > 95% could be achieved using a φ 0.25-mm capillary microreactor, which is greater than the value achieved when a φ 0.53-mm capillary microreactor was utilized at a molar ratio of 6:1 of methanol to oil, 1% concentration of KOH catalyst, and 6 min residence time [39]. An identical trend was also examined when zigzag microchannel reactors with hydraulic diameters between 240 and 900 μm were used for NaOH-catalyzed biodiesel production [40]. When the size of the channel is over 1 mm, there is a dramatic increase in the conversion of soybean oil, with a decrease of the depth of channel from 10, 5, and 2 to 1 mm in a slitchannel reactor with a channel length of 15.24 cm and channel width of 2 mm [62]. The higher yield of methyl ester could be achieved when the size of the microchannel is smaller. However, by decreasing the size of the microchannel, the pressure drop will rise significantly, hence resulting in enlarged difficulty in operating and cost of production. An adequate solution is packing of Dixon rings into a large tube [46], which can definitely decrease the pressure drop along with obtainment of a high yield of methyl ester in a less time.
3.3 Residence time Residence time of biodiesel synthesis in microchannel reactors is 10–100 times lesser as compared to that in conventional batch reactors [45]. Usually, in both microstructured and capillary microreactors, yield of FAME increases with prolonged residence time. However, there is change in residence time in different kinds of microreactor. To achieve a high yield of FAME, the residence time required in capillary reactors is usually longer as compared to microstructured reactors (Table 2). Very short time of residence at the range of 17– 28 s under optimized conditions of reaction can result in yield of FAME > 99% [33]. Sun et al. [39] observed that by increasing the residence time from 3.68 to 5.89 min, there was an increase first in the methyl ester yield from 92.5 to 99.4%, and then methyl ester yield decreased to about 92%, by further increasing the time of residence when biodiesel production took place in an inner diameter of 0.25 mm capillary from solutions premixed with methanol, rapeseed oil, and KOH, with 1% concentration of KOH and the molar ratio of methanol to oil of 6 at 60 °C reaction temperature. This may be due to the reverse reaction in this type of microreactor at long time of residence.
3.4 Reaction temperature Sun et al. [39] investigated the effect of reaction temperature on the cottonseed oil methyl ester yield with a molar ratio of 6 of methanol to oil and 1% KOH concentration at 6 min residence time using a microchannel reactor joined by two parallel stainless steel capillaries having an inner diameter of 0.25 mm with length of 30 m. It was observed that methyl ester yield increased from 96.2 to 99.4% as the temperature increases from 30 to 60 °C. There was a slight decrease in the yield of methyl ester to 99.1% when temperature further increased to 70 °C. It seems that the optimal temperature for biodiesel production under these conditions of reaction is 60 °C. This would conclude from the factor that the 70 °C temperature is higher than the methanol boiling point (64.7 °C). Thus, in the microreactor at 70 °C, methanol existed as gas, as we could identify the popping out of some bubbles from the capillary. Therefore, transformation of a flow pattern in the microreactor was observed from slug flow to bubble flow. Also, temperature increase to 70 °C led to a speed-up of the saponification of glycerides before alcoholysis completion by the alkaline catalyst [55].
3.5 Mixing mechanism The mixing mechanism has a powerful control on the mass transfer of the microreactor, hence exerting an impact on the results of the reaction. The methanol phase and the oil phase are separated from each other because of the high interfacial
Biomass Conv. Bioref. Table 2
Comparison of reactors for production of biodiesel
Type of reactor Unit of mixing
Residence time loop
Molar ratio of methanol to oil
Amount of catalyst (wt%)
Time
T Yield of (°C) FAME (%)
References
Microstructured SIMM-V2
3 mm ID PTFE tube packed with Dixon rings 0.8 mm ID FEP tube
8:1
1% KOH
17 s
70
[46]
23.9:1
4.5% KOH
100 s
60
Microtube
Oscillatory flow reactor
T-shaped joint Stirred tank T-shaped junction Orifice plate baffles
99.5 100
[35, 43]
0.25 mm ID quartz capillary
6:1
1% KOH
5.89 min 60
99.4
[39]
Zigzag microchannels
9:1
1.2% NaOH
28 s
56
99.5
[40]
25 mm ID QVF tube
3:2
1.2% NaOH
30 min
50
forces between the two phases in capillary reactors and as a result in a slug flow. Along with the proceeding of the reaction, the pattern of flow does not change at the capillary outer part with glycerol and methyl ester formation [39]. During the reaction, a quasi-homogenous one-phase flow [35] is developed, resulting from the droplet aggregation, the reduction of volume of the methanol phase in the course of the reaction, and the FAME formation. An identical slug flow is also examined at the outlets of the J-mixer and T-mixer; however, a few slugs and many droplets of methanol with 50–500-μm diameters well dispersed into the oil phase are developed at the outlets of the SIMM-V2 and RIMM micromixers, after mixing methanol with cottonseed oil [46]. The patterns of flow in SIMM-V2 change at various molar ratios of methanol to oil, residence time of 0–44 s, and temperature of 60–80 °C.
3.6 Catalysts Although biodiesel production can be carried out in the absence of catalysts, but then the reaction would require higher temperature and pressure with increased time of residence. Hence, to obtain optimum ester yields under moderate experimental conditions, various catalysts are used (Table 3). Alkaline catalysts favor higher conversions under moderate conditions within a much less reaction time in comparison to acid catalysts, but these are not suitable for feedstocks with higher free fatty acid content. Generally, homogeneous catalysts (both acid and alkaline) are used commercially while heterogeneous ones are susceptible to further research and development.
3.6.1 Homogeneous catalysts In conventional production processes, the most common alkaline catalysts used are alkoxides and carbonates, alkali metal hydroxides, guanidine compounds, and organic amines while their acid counterparts include dihydrochloride, benzenesulfonic acid, phosphoric acid, sulfuric acid, and so
98%
[47]
on [4]. These are preferred in microreactors because of their higher catalytic efficiency and operational simplicity. Alkaline catalysts The most favored alkaline catalysts used in microreactors for biodiesel production are potassium hydroxide, sodium methoxide, and sodium hydroxide. The base catalyst has approximately 4000 times higher activity than the homogenous acid catalyst, and it causes less corrosion problems [71]. Low temperatures are favorable for sodium hydroxide to prevent saponification and emulsification. Higher FAME yields are obtained by potassium hydroxide and sodium methoxide. Acid catalyst Sulfuric acid is the most preferred one because of its simultaneous esterification and transesterification action [72, 73]. Sun et al. [46] worked upon a two-step process for fast catalysis from substrates with high oil content within a microstructured reactor integrated with a SIMM-V2 micromixer. The overall reaction time was reduced by using a microreactor. Corrosion is the main problem caused by acid catalysts like sulfonic acid. However, the use of a homogenous catalyst during the transesterification process for biodiesel production requires additional downstream processing to remove residual inorganic components, which leads to higher production cost. Therefore, many researchers have tried to develop heterogeneous acid and base catalysts, which have the advantage of simple separation from the product and reuse in the biodiesel production process [71]. 3.6.2 Heterogeneous catalysts The use of heterogeneous catalysts offers several advantages such as operational simplicity, cost-effective separation, less wastewater generation, and minimal energy and capital investment [74]. However, the reaction rate of transesterification with heterogeneous catalysts is lower than that using homogeneous catalysts due to the catalytic activity and mass transfer resistance. Lui et al. [68] used microcapsules loaded with calcium oxide in the form of microreactors for the production of biodiesel. Lipase was immobilized within an asymmetric
Biomass Conv. Bioref. Table 3 Various catalysts used for biodiesel production
Catalyst
Methanol to oil ratio
FAME Yield%
References
(Alkaline) NaOH
KOH
7.2:1
91
[34]
9:1 6:1
99.5 94
[45] [50]
23.9:1
(Acid) H2SO4
100
[63]
8:1
99.5
[46]
6:1
99.1
[21]
6:1
90
[64]
(Heterogeneous catalysts) Mesoporous silica loaded with MgO
8
96
[65]
NaOH/alumina CaO/SBA-14
6 12
99 95
[66] [67]
CaO, SrO
12 40 30
95 90 86.6
[68] [69] [70]
WO3/ZrO2, zirconia–alumina and sulfated tin oxide Mg–Al–CO3 (hydrotalcite)
polyethersulfone membrane so that transesterification could take place in micropores [31]. Guan and Kusakabe [71] used tri-potassium phosphate (K3PO4) as a solid catalyst for transesterification of waste cooking oil with methanol. Tripotassium phosphate has high catalytic activity for the transesterification reaction, compared to CaO and tri-sodium phosphate. The FAME yield reached 97.3% when transesterification was performed with a catalyst concentration of 4 wt% at 60 °C for 120 min. After regeneration of the used tri-potassium phosphate catalyst with aqueous KOH solution, the FAME yield was 88%.
alternative co-solvent for biodiesel production. The reaction rate was greatly improved by addition of DME as a co-solvent because of the disappearance of mass transfer resistance between the two phases. Various ether-related co-solvents, i.e., dimethyl ether (DME), diethyl ether (DEE), t-butyl methyl ether (TBME), and tetrahydrofuran (THF), have also been investigated. However, even if the cosolvent_ methanol_oil system did not become homogeneous, the transesterification rate was improved compared to the system without cosolvents.
3.6.3 Catalyst-free process
4 Role of microfluidics
Supercritical conditions in the absence of catalysts offer several advantages such as higher efficiency of production, ecofriendly nature, and usage of a wide variety of feedstocks [75]. Microtube reactors were used in a catalyst-free process under continuous operation for the transesterification of soya bean oil using supercritical ethanol, and high yields of FAME were obtained under specified conditions [33, 34]. 3.6.4 Presence of co-solvents Addition of co-solvent enhances the reaction rate. Tetrahydrofuran (THF) is a widely used co-solvent, but when stored it forms peroxides. Another most commonly used cosolvent is dimethyl ether (DME). Guan and Kusakabe [71] reported 100% conversion of corn oil using a KOH concentration of 0.5 wt% with added DME. Moreover, DME can be easily recovered from the product by a depressurization procedure. Therefore, DME can prove to be a promising
Microfluidics, generally referred to as lab-on-a-chip technology, relies upon the micron-ranged interconnected channels and provides a base to study and analyze much smaller volumes of liquids on nanoliter (and further smaller) ranges. These devices (~ $5–10 per device) are made up of a microchannel network that helps in driving and storing small aliquots of liquids and droplets (droplets-in-channel microfluidics) [76–79]. Although these devices offer minimal usage of reagents, their serial nature may hamper simultaneous handling of different reagents. An alternative to serve the purpose is DMF (digital microfluidics), which is an arraybased technology and offers integrated fluid operations amidst multiplexing purposes [80–82]. Several devices based on this technology have been developed for the transesterification resulting in biodiesel production taking into account microreactors [40, 83–85] and capillary reactors [39, 86]. A droplet-based microfluidic system increases the material interface between the reagents as they possess a large surface-
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to-volume ratio and hence is more reliable for transesterification purposes [87, 88]. It has been observed that the mass transfer between the boundaries of immiscible liquids within a microchannel is significantly enhanced via circulation within the segmented liquid. The efficient intermixing of immiscible liquids within a microchannel has advanced chemical engineering application. Fatty acid methyl esters (FAME) and glycerol are the two major products in the process of biodiesel production and can be separated spontaneously via phase separation provided the quantity of glycerol is optimum. But the process is slow and leftovers still remain. Thus, the traditional washing method is applied to obtain the phase-separated product which adds on to the wastewater generation and subsequent drying. Recently, mixture separation and purification through microfluidic systems have gained much recognition [89]. Liquid-liquid microseparators increase the efficiency of separation between aqueous and organic phases [90] as applied for the separation of glycerol and FAME. The separation of biodiesel from glycerol was achieved in a microseparator having channel dimensions of 500 × 500 × 500000 μm, following a sandwiched flow pattern to ensure efficient separation [46]. The glycerol content was reported to be lowered to 0.02% depicting reliable separation. Hence, microfluidic technology on the one hand seems to revolutionize the area of liquid-based reactions and separation procedures but at the same time more research and development is required to enhance the surface design, channel features, inlet and outlet methods, and ambient environment control within the microreactor to further increase its commercial accessibility.
5 Conclusion Biodiesel, being an eco-friendly and non-perishable fuel, has gained importance since past decades. Although its production has been commercialized in several countries of the world, it still requires a clean, effective, and environmentfriendly technology to make it cost-effective and increase its competency against conventional fossil fuels. Microreactor technology, however, has proved to be a benchmark to serve this purpose. In this study, a detailed analysis of different types of microreactors, currently used for the production of biodiesel, is carried out with an emphasis on the factors affecting the production process. It provides higher FAME yields resulting within a micronranged reactor with minimal energy requirement, moderate reaction conditions, and reduced time of residence. Thus, it seems to be reliable both from commercial as well as scientific points of view.
However, the technology still requires advancements in certain areas (Table 4). Further research is required to develop operating procedures that include usage of feedstock possessing increased levels of free fatty acids and exploit heterogeneous catalysts within these chip-based reactors. Limited use of solvents and high-temperature incompatibility for lowcost polydimethylsiloxane microchannels needs to be managed. Although satisfactory initial results have been obtained regarding purification procedures via microfluidic technology, still the design and surface structure requires more analysis and enhancement. High costs and complexities in the fabrication of glass and silicon microdevices need to be resolved. Moreover, the development of further miniature version of these microreactors will enable continued operations of reaction and separation cycles with much smaller inoculum size, Table 4 Future prospects and challenges associated with different microreactors Microreactor
Future Prospects
Challenges
References
Can combine both Membrane systems [26, 91] are limited by reaction and pore size and separation shape of simultaneously, materials to be and can filtered. Ceramic effectively block membranes are unreacted very expensive triglycerides. for the targeted applications. [92, 93] Further Oscillatory flow Very low molar intensification reactors ratio of methanol has to be to oil required. developed when Mixing is no a heterogeneous longer directly catalyst is used, dependent upon as the OFR has the Reynolds been shown to be number of the net ideal for use of flow through the homogenous reactor, but is solid catalyst or dependent upon polymer the oscillation supported conditions. catalyst. Hence, the OFR allows many Blong^ batch processes to be converted to more efficient continuous processes. Decreasing the flow [94] Microtube Efficient heat rate while microreactors dissipation, high keeping the tube mass transfer, length constant and short results in a diffusion decrease in distance. pressure drop and a significant drop in %FAME.
Membrane microreactor
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reduced work space requirement, higher yield, and better control of reaction parameters which would hence offer a more promising way to fulfill the ever-increasing energy demands along with a sustainable and eco-friendly fuel production technology. Acknowledgements Authors sincerely acknowledge Prof. Aditya Shastri, Vice Chancellor, Banasthali University, for necessary infrastructure and facilities. Compliance with ethical standards
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Conflict of interest The authors declare that they have no conflict of interest. 18.
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