Continuous Synthesis and in Situ Monitoring of Biodiesel Production

Article pubs.acs.org/IECR

Continuous Synthesis and in Situ Monitoring of Biodiesel Production in Different Microfluidic Devices Edgar L. Martínez Arias,*,† Patricia Fazzio Martins,†,‡ André L. Jardini Munhoz,† Luis Gutierrez-Rivera,§ and Rubens Maciel Filho† †

Laboratory of Optimization, Design, and Advanced Control (LOPCA), School of Chemical Engineering, University of Campinas (UNICAMP), Avenida Albert Einstein, 500, 13083-852, Campinas, São Paulo, Brazil ‡ Department of Earth and Exact Sciences, Federal University of São Paulo (UNIFESP), Rua Prof. Artur Riedel, 275, 09972-270, Diadema, São Paulo, Brazil § National Institute for Nanotechnology, National Research Council Canada, 11421 Saskatchewan Drive NW, Edmonton, Alberta, Canada ABSTRACT: Currently, there is great interest in developing new processes for continuous biodiesel synthesis in order to overcome problems imposed by biphasic reaction and thermodynamic equilibrium, as well as to reduce production costs related to the conventional batch reaction processes. The use of microreactors can significantly improve the mixing between reactants and phases, enhancing the transfer rates, and, consequently, increasing biodiesel yields. Therefore, in this work, microreactors with different internal geometries have been fabricated and used for continuous production of biodiesel from castor oil and ethanol. The influences of the microchannel geometry (Tesla-, omega-, and T-shaped) on the performance of the biodiesel synthesis were experimentally studied. Higher biodiesel yields were reached using the Tesla- and omega-shaped microchannels than during the T-shaped microchannels due to better mixture mechanism efficiency. Using a catalyst loading of 1.0 wt % NaOH and a reaction temperature of 50 °C, ethyl ester conversions of 96.7, 95.3, and 93.5% were achieved using Tesla-, omega-, and Tshaped microreactors, respectively. In addition, transesterification reaction in situ monitoring by near-infrared spectroscopy using a fiber-optic probe was evaluated, showing that more studies must be performed in order to allow its use in online monitoring of continuous processes.

1. INTRODUCTION Concerns about petroleum supplies due to increasing demand and decreasing reserves, added to environmental pollution, high energy prices, and energy and environmental security are stimulating the research for environmentally friendly and renewable biofuels as an alternative to petroleum-derived energy sources. Biodiesel, a mixture of fatty acid alkyl esters derived from animal fats or vegetable oils, is rapidly moving toward becoming one of the main fossil fuel substitutes. The most common way to produce biodiesel is through the transesterification reaction, which refers to a chemical reaction between a vegetable oil (constituted mainly by triacylglycerols) and an alcohol over a homogeneous catalyst to yield fatty acid alkyl esters (biodiesel) and glycerol (Figure 1). Triacylglycerols, also known as triglycerides, consist of three long chains of fatty acids esterified to the same glycerol backbone. When triacylglycerol reacts with alcohol, three fatty acid chains are released from the glycerol skeleton and combine with three alcohol molecules to yield fatty acid alkyl esters as shown in Figure 1. Glycerol is formed as a byproduct of the reaction, and can be used for different applications.1−3 Several aspects, including the type of catalyst, type of alcohol, alcohol/vegetable oil molar ratio, temperature, water content, and free fatty acid content, influence the course of transesterification. Methanol and ethanol are the most commonly used alcohols because of their low cost.4 The use of ethanol for ethyl ester production is of considerable interest because it allows biodiesel production from entirely renewable sources, © 2012 American Chemical Society

especially for those countries in which ethanol is produced from sugarcane, sugar beets, and corn. In addition, biodiesel can also be prepared by noncatalyzed reaction5−7 and by using lipase as catalysts,8−12 but to date, transesterification reactions are conventionally produced through base catalysts and acid catalysts depending on the vegetable oil acid content.13−16 Although the majority of commercial biodiesel production be made in stirred tank reactors, Qiu et al.17 have identified some challenges related to this process: (i) the reaction rate can be limited by mass transfer between the oils and alcohol because they are immiscible; (ii) transesterification reaction is a reversible reaction (Figure 2) and, therefore, there is an upper limit to conversion in the absence of product removal; (iii) most commercial processes run in a batch mode and, thus, do not present the advantages of continuous operation. Aiming to overcome these problems related to the batch reaction process, a relatively long reaction time, a high molar ratio of alcohol to oil, and a high catalyst concentration have been traditionally used. In addition, in order to achieve the final product specifications, regulated by ASTM 6751-11a18 and EN 1421419 standards, acid is used to neutralize the alkali catalyst and to remove it.20 The conventional process used to produce biodiesel under law specifications generates significant amounts Received: Revised: Accepted: Published: 10755

February 23, 2012 July 10, 2012 July 23, 2012 July 23, 2012 dx.doi.org/10.1021/ie300486v | Ind. Eng. Chem. Res. 2012, 51, 10755−10767

Industrial & Engineering Chemistry Research

Article

Figure 1. Transesterification of triacylglycerols. Ri represents a fatty acid chain.

In recent years, continuous synthesis of biodiesel fuel using microreactor systems has been considered (Table 1). These reports show the microreactor technology as a new promising way for the high-efficiency industrial production of biodiesel; however, none of them have explored the use of different microchannel geometries. Scale-up of this technology to industrial scale could be easily implemented by numbering up, which consists in extrapolating a process by putting devices in parallel. The advantage in the implementation of the microreactor to obtain biodiesel is based on the small volumes of microreaction systems that greatly intensify heat and mass transfer due to a significant decrease in diffusion path length between reacting molecules and to a large increase of the surface-area-to-volume ratio.38 In microreaction systems, the time that a reactant molecule takes to diffuse through the interface and react with another species is drastically decreased,

Figure 2. Steps of transesterification reaction. TG, DG, MG, GL, Ri, and ki represent triacylglycerides, diacylglycerides, monoacylglycerides, glycerol, fatty acid chains, and rate constants, respectively.

of toxic wastewater that requires a treatment step, increasing the operating cost and energy consumption required. Therefore, the development of continuous processes that will reduce production costs and increase the product uniformity for largescale production has been investigated.21−24 Table 1. Continuous Biodiesel Production Using Microreactors reference

catal concna [wt %]

reactants

Canter25,26

soybean oil + methanol

NaOH

Sun and co-workers27

cottonseed oil + methanol

1.0% KOH

Guan and co-workers28

sunflower oil + methanol

1.0% KOH

Jachuck and coworkers29

canola oil + methanol

1.0% NaOH



1.0% KOH



4.5% KOH

Wen and co-workers32


1.2% KOH

Silva and co-workers33

soybean oil + ethanol

−



1.0% KOH



3.0% H2SO4

Trentin and coworkers36

soybean oil + ethanol

−

Kalu and co-workers37


0.0263 g of NaOH/mL of CH3OH

channel dimensionsb [μm]

residence time [min]

w = 100 d = ni l = ni Ø = 2.5 × 102 l = 3.0 ×107 Ø = 1.0 × 103 l = 1.6 × 105 Ø = 1.5 × 103 l = 1.5 × 107 c Ø = 9.6 × 102 l = 3.6 × 105 Ø = 8.0 × 102 l = 3.0 × 105 w = 3.0 × 102 d = 2.0 × 102 l = 1.1 × 106 Ø = 7.6 × 102 l = 8.0 × 107 c Ø = 6.0 × 102 l = 5.0 × 106 Ø = 6.0 × 102 l = 1.4 × 107 c Ø = 571 l = 9.7 × 107 c w = 2.0 × 103 d = 1.0 × 103 l = 1.5 × 105

10.0

temp [°C]

conv [%]

25

96.0

5.89

60

99.4

1.87

60

100.0

3.0

60

99.8

1.55

25

92.8

1.67

60

100.0

0.47

56

99.5

325

70.0

70

94.8

20.0

120

97.5

50.0

325

84.0

3.0

55−65

100.0

45.0 0.73

a

Weight percent based on the vegetable oil weight. bw = width; d = depth; l = length; Ø = diameter; ni = no information. cCalculated value from reported data. 10756

dx.doi.org/10.1021/ie300486v | Ind. Eng. Chem. Res. 2012, 51, 10755−10767


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Figure 3. Fabrication of microchannels by soft lithography: (a) spin-coating of a glass wafer with SU-8, UV exposure using a clear field mask, and development of SU-8 master; (b) pouring PDMS on the mold and peeling off the PDMS part; (c) surface treatment of two PDMS layers with oxygen plasma and bonding.

developed to determine the biodiesel content in blends with diesel and to evaluate contamination by raw vegetable oil.54,71−76 Additionally, NIR77,78 and MIR67 spectroscopies have been employed to monitor batch transesterification reactions. NIR spectroscopy associated with different multivariate data analysis techniques has been used to predict important biodiesel properties: iodine value,61,79 kinematic viscosity,60,61 density,60,61,80 cold filter plugging point,61,79 the end boiling point (50% v/v),79,80 the end boiling point (85% v/ v),80 the end boiling point (90% v/v),79 methanol content,60,81 sulfur content,80 and water content.60,81 Nevertheless, methods to monitor in situ biodiesel continuous synthesis have not been developed yet. The majority of biodiesel production studies in microreactors have been limited to simple microtube reactors, and only some researchers have investigated the effect of zigzag geometry on biodiesel production.32 However, internal geometries inside them have a huge impact in mass and heat transfer.82 Therefore, in this work microreactors with different channel geometries such as omega shape, Tesla shape, and T-shape have been developed in order to improve the efficiency of the homogeneously alkali-catalyzed biodiesel synthesis. The effect of geometric parameters such as the fluid path inside the microreactor for biodiesel synthesis was experimentally investigated. The effects of catalyst amount, reaction temperature, molar ratio of ethanol to oil, and residence time have also been studied. In addition, the potential use of NIR in a portable spectrometer coupled fiber-optic device for in situ monitoring of the transesterification process in microreactors was evaluated.

which is highly improbable to reach even in the most vigorously agitated usual batch reactor. Consequently, the residence time needed to achieve high conversion is reduced to the order of a few minutes since the diffusion becomes a less significant resistance to the reaction. In addition, decreasing microchannel linear dimensions allows converting a batch reaction into a continuous flow system and implementing a more precise temperature control. Furthermore, considerable efforts have also been made in order to monitor the transesterification reaction and to analyze the biodiesel quality. Currently, several methods have been developed for analyzing samples obtained by the transesterification reaction of vegetable oils,39 including techniques such as thin layer chromatography (TLC),40−42 gas chromatography (GC),43−45 high performance liquid chromatography (HPLC),46,47 gel permeation chromatography (GPC),48−50 and nuclear magnetic resonance (NMR).51−54 However, all of these techniques rely on extensive sample preparation, which is timeconsuming and expensive, preventing in situ monitoring of continuous processes. An alternative analytical technique which is noninvasive, is cheap, requires minimal sample preparation, and can yield a response in real time is near-infrared (NIR) spectroscopy. 55−60 NIR spectroscopy is based on the absorption of electromagnetic radiation in the region from 12 820 to 4000 cm−1 (from 780 to 2500 nm).61 It generates a spectrum based on the relatively weak and broad overtone and combination bands of fundamental vibrational transitions associated mainly with C−H, N−H, and O−H functional groups.62 Analyses of fatty materials by NIR spectroscopy have become widespread,63,64 generating results similar to those obtained by time-consuming analytical techniques such as GC and HPLC.65 The NIR spectroscopy method for determining the biodiesel content in conventional diesel fuel and lubricating oil was reported by Knothe54 and Sadeghi-Jorabchi et al.,66 respectively. However, limitations have been found due to the dependence of reflectance on the scattering properties of the sample and the existence of overlapping absorption bands, which may confound peaks of interest.64,67 In order to overcome these problems, NIR spectroscopy is associated with multivariate data analysis, becoming a powerful tool in the analysis of a variety of fuels. Multivariate calibration methods provide empirical models that relate the multiple spectral intensities from many calibration samples to the known analytic concentrations or properties of biodiesel samples.58,68−70 For instance, Balabin and Safieva59 correlated the near-infrared spectra of biodiesel samples and their feedstock. Multivariable calibration models based on mid-infrared (MIR) and near-infrared (NIR) spectroscopies have been

2. EXPERIMENTAL SECTION 2.1. Chemicals. Castor oil was purchased from Campestre Indústria e Comércio de Ó leos Vegetais Ltda (São Bernardo do Campo, Brazil). It presented an acid value of 1.3 mg of KOH/g measured according to the AOCS Ca 5a-40 method.83 Ethanol and sodium hydroxide were purchased from Merck (São Paulo, Brazil). Tetrahydrofuran (THF), HPLC grade (B&J Brand), was used as solvent for transesterification product analysis. Polydimethylsiloxane (PDMS) used to build microreactors was purchased from Dow Corning (Midland, MI, USA). 2.2. Fabrication of Microreactors. The microreactors were made of polydimethylsiloxane (PDMS) by a soft lithography process as shown in Figure 3. First, PDMS was obtained from the mixture of two commercially prepolymers available: one of them consists of a long chain polymer known as Sylgard 184 (base), and the other one consists of a short chain polymer with an initiator (curing agent). The weight ratio of the base and the curing agent was 10:1. The solid master was 10757



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Figure 4. PDMS microreactor internal geometries: (a) omega shaped, (b) Tesla shaped, and (c) T-shaped.

Figure 5. Experimental setup for transesterification reaction in the microreactors.

surface-activated PDMS (Figure 3c); they were pressed against each other manually and allowed to stand for 2 h. This process forms a watertight and irreversible seal. Figure 4 illustrates different microreactor internal geometries used in this work and their main dimensions. Depending on the channel geometry, the microreactors were named “omega shaped”, “Tesla shaped”, and “T-shaped”. The Tesla and omega microreactor designs were based on internal geometries proposed by Hong et al.86 and Yu et al.,87 respectively. The microchannels were of quadratic cross section with width and height of 500 μm and length of 1 m (referred to the longitudinal direction). The reaction fluid was fed to and removed from microreactor headers via 1/16 in. tubing and fittings. The microreactor manufacturing was carried out in the

fabricated using an SU-8 negative photoresist (Figure 3a). The PDMS mixture was poured onto the replication master and degassed in a desiccator at 5−6 Pa for 1 h to eliminate air bubbles. The whole set was then cured at relatively low temperature (100 °C) for 1 h. Afterward, the PDMS layer was peeled off and the external access to the microfluidic array was obtained by drilling holes in the PDMS layer84 (Figure 3b). The sealing process was carried out by oxidizing the PDMS surface through a radio frequency (rf) oxygen plasma using a PLAB SE80 plasma cleaner (Plasma Technology, Wrington, England). The plasma working parameters were obtained from Jo et al.85 and were 16 Pa of O2, 70 W rf power, and 20 s exposition. After plasma oxidation, the PDMS layer was brought into contact with another piece of 10758



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shown in Table 2. Additionally, spectra of the pure oils and ethyl esters samples were obtained.

Microfabrication Laboratory of the Brazilian Synchrotron Light National Laboratory (LNLS). 2.3. Transesterification Reaction. Two ST670 syringe pumps (Samtronic Infusion Systems, Brazil) were used to inject the castor oil and ethanol at different flow rates inside the microreactor. The flow rates used ranged from 0.5 to 22.6 mL/ h with a standard deviation of 1% of the set value. Preliminary tests showed that there is a flow rate upper limit to work with the developed devices. Using feed flow rates higher than 25 mL/h caused fluid leaking at microreactor connections and problems in dosing the liquids due to pressure increase. The microreactors were placed into the HPLC furnace to ensure the accuracy of the reaction temperature. The experiments were carried out at temperatures of 30, 50, and 70 °C with accuracy control of 0.1 °C. The outlet of the microreactor was connected to a collector flask placed in an ice−water vessel in order to finish the transesterification reaction quickly. The experimental setup is shown in Figure 5. Catalyst amounts of 0.5, 1.0, and 1.5 wt % referred to the oil mass were prepared. The molar ratios of ethanol to castor oil were adjusted to 9:1, 12:1, and 17:1 by changing the flow rates. The molecular weight of the castor oil (926 g/mol) was estimated based on its fatty acid composition according to the AOCS Ce 1f-96 method.88,89 Samples (0.1 mL) were withdrawn during the experiments, immediately diluted in 10 mL of tetrahydrofuran (HPLC grade), and cooled. Samples prepared this way were ready for chemical analysis. Runs performed in triplicate using a temperature of 50 °C, an ethanol/oil molar ratio of 9:1, a catalyst amount of 1 wt %, and feed flow rates of 1 mL/h of castor oil and 0.5 m/h of ethanol were considered to calculate the standard deviation (σ = 1.04%). It was assumed that all runs presented the same standard deviation. 2.4. Quantitative Analysis. Size-exclusion chromatography (SEC) was used for the triacylglycerol, diacylglycerol, monoacylglycerol, ethyl ester, and glycerol analysis, according to the Shoenfelder method.48,50 The system consisted of a VISCOTECK GPC/SEC TDA max chromatograph with a triple detector array with refractive index (RI), viscometer, and light scattering detectors. Data collection and analysis were performed with GPC software. The mobile phase was tetrahydrofuran (THF), HPLC grade (JT Baker, USA), at a flow rate of 0.8 mL/min. Three GPC/SEC Phenogel analytical columns connected in series were used (Phenomenex, Torrance, CA), of 300 mm × 7.8 mm, packed with spherical styrene−divinylbenzene copolymer beads with an average particle size of 5 μm. First, a column with a pore size of 100 Å, corresponding to a molecular weight (MW) range of 100− 6000, was placed. Then, two columns with a pore size of 50 Å, corresponding to a MW range of 50−3000, were connected. The sample injection volume was 20 μL, and all analyses were carried out at 40 °C. Reference standard substances (1,2,4butanetriol, diolein, glycerol, monoolein, tricaprin, triolein, tripalmitin, trilinolein, ethyl palmitate, ethyl stearate, ethyl oleate, ethyl ricinoleate, and glycerol) were used to identify their retention times. Identification of GPC/SEC peaks was performed comparing their retention times with the ones of the above-mentioned standards. 2.5. In Situ Monitoring Reaction. Samples were prepared in a beaker by mixing ethyl esters of castor oil (biodiesel), ethanol, glycerol, and castor oil in different proportions. After each addition of reaction subproduct, the mixture was stirred for 5 min and the spectrum was recorded. The ethyl ester content in these blends ranged from 81.25 to 66.67 wt % as

Table 2. Samples Prepared with Different Amounts of Transesterification Reaction Subproducts sample component

composition (wt %)

sample

component

composition (wt %)

ethyl esters ethanol ethyl esters glycerol ethyl esters castor oil ethanol ethyl esters castor oil glycerol ethyl esters castor oil ethanol glycerol

77.78 22.22 81.25 18.75 75.00 12.50 12.50 75.00 12.50 12.50 66.67 11.11 11.11 11.11

1

ethyl esters

100.0

6

2

castor oil

100.0

7

3

glycerol

100.0

8

4

ethanol

100.0

9

5

ethyl esters castor oil

81.25 18.75

10

The mixtures were checked first by a conventional nearinfrared (NIR) spectrometer. The NIR spectra (900−2100 nm) were obtained with a Perkin-Elmer Spectrum GX spectrometer. Spectra were acquired, in the NIR region, using a quartz flow cell with a 1.0 mm path length positioned directly in front of the near-infrared radiation beam. In all cases, the spectra were recorded at room temperature with a spectral resolution of 8 cm−1 and 16 coaveraged scans. After recording a spectrum, the cell was cleaned by successive treatments with ethanol and acetone. The obtained spectra were digitalized and exported for subsequent analysis. The in situ reaction monitoring process in three microreactors was made by portable near-infrared spectroscopy using a fiber-optic probe as shown in Figure 6. The NIR spectra were obtained on an Ocean Optics Spectrum 2000 spectrometer equipped with a Galileo transmission-type fiber-optic probe. The transesterification reactions were conducted at room temperature with an ethanol/castor oil molar ratio of 6:1 and 1.0 wt % NaOH based on castor oil weight. The reactions were analyzed in situ through a fiber-optic probe coupled to the spectrometer positioned directly in front of the microchannels and the near-infrared radiation beam (Figure 6). After recording a spectrum, the fiber-optic probe was cleaned by successive treatments with ethanol and acetone by immersion into each stirred solvent for several minutes. The obtained spectra were digitalized and exported for subsequent analysis on a personal computer (Hewlett-Packard, HP).

3. RESULTS AND DISCUSSION 3.1. Development of Internal Geometries That Act As Passive Micromixers. Process intensification (PI) technologies have been developed and applied to drastically improve equipment and process efficiency. According to Górak and Stankiewicz,90 PI follows four main goals: to maximize the effectiveness of intra- and intermolecular events, to give each molecule the same processing experience, to optimize the driving forces/maximize specific interfacial areas, and to maximize the synergistic effects of partial processes. In addition, Gerven and Stankiewicz91 defined fundamental approaches of PI in four domains: spatial, thermodynamic, functional, and 10759



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Figure 6. Experimental setup for transesterification reaction in the microreactors.

velocities and nonuniform flow velocities, and that generate vortices in the flow. Each omega channel has a curve that hampers the oncoming flow and separates it into two streams, which converge with other flows in adjacent omega channels (Figure 4a). The obstacles imposed by the omega channel force the flow to commingle back and forth between the center and the channel wall. The velocity of flow in the omega channel varies significantly from point to point and its variance is large, which improves the interfacial mass transfer.87 On the other hand, in the Tesla-shaped micromixer, the fluids are first bilaminated in a T-type configuration and then pass a so-called Tesla structure which comprises angled surfaces86 (Figure 4b). By flowing along the latter, splitting and redirection of the flow are achieved, which leads to a kind of collision. While one stream passes the major angled passage, the neighboring stream approaches both this major passage and a smaller secondary passage, set in a Y-type flow configuration. This stream splits into two substreams according to the different pressure losses of both passages. The flow of both passages is so oriented in such way that leads to an effective collision. Thus, a larger stream with predominantly one fluid collides with a smaller stream of the pure other liquid. The Tesla structure is repeated many times in a row so that a sequential mixing is achieved. 3.2. Effect of Microreactor Geometric Parameters. To study the effect of the microchannel geometry, the ethyl ester yields in the microreactors for a catalyst amount of 1.0 wt % (based on the castor oil weight) and temperature of 50 °C were investigated. The molar ratio of ethanol to castor oil was kept at 9:1, and the residence time was varied by adjusting flow rates. All the microchannels used in this work had a 500 μm quadratic cross section, corresponding to a hydraulic diameter of 500 μm and length of 1 m (referred to the longitudinal direction). As depicted in Table 3, the biodiesel synthesis was greatly dependent on the geometry of the microreactor. For each experimental condition, the Tesla-shaped microreactor resulted in higher biodiesel yields than the omega- and T-shaped microreactors. After a residence time of 15 min, the yield of ethyl ester for omega- and Tesla-shaped microreactors was

temporal. This shows the potential of PI technologies as one promising development path for the process industry and modern chemical engineering. Among the various engineered components used in PI, micromixers play an important role. As discussed by Hessel and Löwe,92 micromixers have a large potential of application in tasks, such as mixing, blending, emulsification, and suspension, as well as their use in reactors. If the mixing process is poor, the reaction process may be slowed down by a local shortage of one of the reactants, uneven catalyst distribution, thermal nonuniformities, or ignition delays. The mixing efficiency in microprocesses is, therefore, very important to improve process performance, and it will affect various parameters including heat and mass transfer rates, process operating time, costs, and ultimately the product quality.93 Several different types of mixers for microfluidic applications have been proposed.94−96 Mixing at the microscale can be classified into passive mixing and active mixing. In active mixing, external energy is used for the mixing process. In passive mixing, external energy is not required but diffusion and chaotic advection induce mixing. In this work, microreactors with different internal geometries were developed, taking into account the passive mixing concept. First, a T-shaped microreactor was used. It has two inlet channels that merge into a common mixing channel (Figure 4c). In this case, the dominant mixture effect is molecular diffusion, in which the mixing length is proportional to the mixing time and the diffusion coefficient. Owing to the small size of the channels and the use of a simple microchannel, the absence of internal structures inside the microreactors makes the flow predominantly laminar and excludes the possibility of turbulent mixing. In order to effectively mix at this scale in a reasonable time, internal structures can be used to increase the interfacial surface area between the fluids and to decrease the diffusional path, thereby enhancing molecular diffusion to complete the mixing process.38 Thus, omega- and Tesla-shaped microreactors were also built. In these cases, chaotic flow caused by internal structures speeded up mixing. An omega-shaped microreactor consists of a series of omega structures that lead to high 10760



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reaction yield in all of the microfluidic devices would reach the equilibrium value. 3.4. Effect of Catalyst Concentration. Experiments were conducted using three different NaOH concentrations (0.5, 1.0, and 1.5 wt % based on castor oil weight) for each microreactor. The residence time of 10 min, molar ratio of ethanol to castor oil of 9:1, and temperature of 50 °C were held constant. Figure 8 summarizes the experimental results of the effect of catalyst

Table 3. Ethyl Ester Yields for Different Volumetric Rates Using Ethanol/Oil Molar Ratio of 9:1, Catalyst Amount of 1.0 wt % (Based on Oil Weight), and Temperature of 50 °C volumetric rate (mL/h)

geometry trans section (μm × μm), 500 × 500

hydraulic diam, 500 μm

castor oil 15.0 7.5 5.0 3.8 3.0 2.1 1.5 1.0

ethyl ester conversion (%)

ethanol

residence time (min)

Tshaped

omega shaped

Tesla shaped

8.1 4.1 2.7 2.1 1.6 1.1 0.8 0.5

1 2 3 4 5 7 10 15

58.9 69.4 73.1 74.7 71.1 75.1 73.4 75.9

67.1 79.3 87.5 88.3 89.7 88.5 87.4 91.4

69.2 83.8 88.8 91.5 91.1 91.7 92.2 93.7

about 1.2-fold greater than that for the T-shaped microreactor. Therefore, the higher efficiency of reaction for biodiesel production through the Tesla-shaped microreactor is attributed to the higher intensification of overall volumetric mass transfer between the reactants. 3.3. Effect of Residence Time. Figure 7 shows the effect of the residence time on the biodiesel synthesis in the Figure 8. Influence of NaOH concentration on yield of ethyl ester using an ethanol/oil molar ratio of 9:1, temperature of 50 °C, and residence time of 10 min in T-, omega-, and Tesla-shaped microreactors.

amount on the performance of the T-, omega-, and Teslashaped microreactors. As shown in Figure 8, the use of a higher catalyst concentration resulted in higher ethyl ester yields. The yield of ethyl ester increased from 50.6 to 79.1%, from 54.3 to 96.2%, and from 56.3 to 98.9% with increase of the catalyst concentration from 0.5 to 1.5% for T-, omega-, and Teslashaped microreactors, respectively. In addition, Tesla- and omega-shaped microreactors resulted in higher biodiesel yields than the T-shaped microreactor for the same catalyst concentration. This can be attributed to the fact that the chaotic flow patterns promoted by Tesla and omega geometries favor the contact between phases and, consequently, increase yields. Since the transesterification reaction is heterogeneous, the channel geometry influences the available interfacial area in which the reaction takes place. During the reaction course, the mixture passes from a biphasic system (alcohol and oil) to a biphasic system (ester rich and glycerol rich). The formed glycerol is insoluble in the ester-rich and oil phases.31 On the other hand, the catalyst is very soluble in the glycerol-rich phase,98 with decreasing catalyst amount in the reacting medium. 3.5. Effect of Temperature. The effect of temperature on the biodiesel yield for three different microreactors is presented in Figure 9. The biodiesel yield increased, with enhancing the reaction temperature from 30 to 70 °C. Ethyl ester yields rose from 67.3 to 89.0%, from 73.9 to 92.2% and from 75.9 to 92.6% for T-, omega-, and Tesla-shaped microreactors, respectively. Transesterification is an endothermic reaction in which the temperature increasing moves the equilibrium composition toward ethyl ester synthesis. In addition, the miscibility of ethanol and triacylglycerols is enhanced at high temperature,99,100 favoring the contact between different phases. Figure 9 also shows that the conversion increases rapidly when

Figure 7. Ethyl ester yield of castor oil transesterification carried out in T-, omega-, and Tesla-shaped microreactors at different residence times and using an ethanol/oil molar ratio of 9:1, catalyst amount of 1.0 wt % (based on oil weight), and temperature of 50 °C.

microreactors at an ethanol/oil molar ratio of 9:1, a catalyst amount of 1.0 wt % (based on oil weight), and temperature of 50 °C. As depicted in Figure 7, the yield of ethyl ester reached values of 75.9, 91.4, and 93.7% for T-, omega-, and Teslashaped microreactors, respectively, after a residence time of 15 min. It can be observed that, for the first 4 min of reaction, with increasing residence time the ethyl ester yields also increase. After this time, the ethyl ester yield reaches, practically, a constant value; however, for the T-shaped microreactor it is significantly inferior because of the microchannel geometry that promotes a lower interfacial contact area. As a consequence, there is a decrease in the mass transfer rates between the immiscible reactants, even though longer reaction time leads to a smaller average velocity for a fixed-length microchannel.97 Nevertheless, considering an infinite residence time, the 10761



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1.0 wt %, and temperature of 50 °C for T-, omega-, and Teslashaped microreactors, respectively. It is the shortest time for the nearly complete ethyl ester yield at such a mild reaction condition. Currently, the supercritical transesterification has been shown as an alternative to the intensified transesterification reaction of vegetable oils. For the supercritical transesterification without catalyst, the corresponding time is in the range 5− 12 min.101 McNeff et al.102 reported that a biodiesel yield of 92.6% was obtained at the residence time of 5.4 s using a metal oxide based catalyst with the temperature of 455 °C in a continuous fixed bed reactor. However, these new processes results in more power consumption and technical difficulties than the use of microreactors. In the case of microreactor technology, the improvement of the throughput of biodiesel can be easily implemented by just increasing the number of these microreactors and the intensification of overall volumetric mass transfer can be increased by passive mixing application. Therefore, the microscale appears as a promising technology to create a compact and mini fuel processing plant in the future. 3.7. NIR Spectra of the Samples. NIR spectra of castor oil, glycerol, and ethanol were compared with those of the ethyl esters, as shown in Figure 11. There are no absorption peaks in

Figure 9. Influence of reaction temperature on yield of ethyl ester using an ethanol/oil molar ratio of 9:1, NaOH concentration of 1.0 wt %, and residence time of 10 min in T-, omega-, and Tesla-shaped microreactors.

the reaction temperature increases from 30 to 50 °C; whereas this increase is not as significant for a temperature near ethanol’s boiling point (76 °C) for omega- and Tesla-shaped microreactors, it is for the T-shaped microreactor. Besides the kinetic effect, this is due to the fact that the mass transfer resistance decreases at higher temperatures, increasing phase miscibility. 3.6. Effect of Ethanol/Castor Oil Molar Ratio. It is wellknown that, according to the stoichiometry of the transesterification reaction, 3 mol of ethanol for each mole of oil is required. However, in practice, the ethanol/oil molar ratio should be higher than the stoichiometric ratio in order to drive the reaction toward completion and produce more ethyl esters as products. The reaction in the microreactors also exhibited the same trend; i.e., with increasing the alcohol amount the equilibrium is moved toward ethyl ester production, as shown in Figure 10. This trend was more obvious when the ethanol to oil molar ratio was raised from 9:1 to 12:1, increasing ethyl ester yields about 1.1 times. Higher ethyl esters yields of 93.5, 95.3, and 96.7% were obtained with a residence time of 10 min at the molar ratio of ethanol to oil of 25:1, catalyst amount of

Figure 11. NIR spectra from 4800 to 11 000 cm−1 for castor oil, ethanol, glycerol, and ethyl esters.

the region 9000−11 000 cm−1. However, NIR spectra of castor oil and its corresponding ethyl esters showed significant differences. Differences in the NIR spectra of castor oil, glycerol, ethanol, and ethyl esters are shown in detail in the upper right corner of Figure 11 for the region 5000−5500 cm−1, which can be assigned to CO and C−O stretching combinations.103 While the midrange infrared spectra of triacylglycerols (vegetable oils) and their corresponding ethyl esters are similar,71 their NIR spectra reveal the possibility of distinguishing them. The ethyl ester (biodiesel) presents an absorbance at wavenumbers between 5000 and 5500 cm−1 (Figure 11). In this region ethyl esters display a characteristic peak that is not presented by castor oil, ethanol, and glycerol. In order to evaluate the NIR spectra for a mixture of ethyl esters, glycerol, ethanol, and castor oil, which are compounds found during the transesterification reaction, blends of ethyl esters were prepared with the compositions shown in Table 2. The differences in NIR spectra of the subproduct blend of the transesterification reaction are shown in Figure 12. It was

Figure 10. Influence of ethanol/castor oil molar ratio on yield of ethyl ester using NaOH concentration of 1.0 wt %, temperature of 50 °C, and residence time of 10 min in T-, omega-, and Tesla-shaped microreactors. 10762



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Figure 12. NIR spectra from 4800 to 11 000 cm−1 for mixtures of (A) 81.3% ethyl ester and 18.7% castor oil; (B) 77.8% ethyl ester and 22.2% ethanol; (C) 81.3% ethyl ester and 18.7% glycerol; (D) 75.0% ethyl ester, 12.5% castor oil, and 12.5% ethanol; (E) 75.0% ethyl ester, 12.5% castor oil, and 12.5% glycerol; and (F) 66.7% ethyl ester, 11.1% castor oil, 11.1% ethanol, and 11.1% glycerol. Concentrations are based on mass percentage (% w/w).

Figure 14. NIR spectra from 5000 to 11 000 cm−1 of different in situ monitoring times using volumetric rates of 4.5 and 3.0 mL/h for ethanol and castor oil, respectively, catalyst amount of 1.0 wt % based on castor oil weight, and room temperature.

observed that all blends showed the ethyl ester characteristic absorbance at 5000−5500 cm−1 as shown in detail in the upper central box of Figure 12. Therefore, the absorbance in this region could be used to carry out ethyl ester quantification through multivariate calibration. 3.8. In Situ Monitoring in Microreactors. To study the in situ monitoring of the transesterification reaction performed in microreactors, a reaction was conducted at room temperature, using 1.0 wt % NaOH and a volumetric ratio (ethanol:castor oil) of 1.5. Ethanol flow rates ranged from 1.5 to 11.3 mL/h and castor oil flow rates ranged from 1.0 to 7.5 mL/h. NIR spectra were taken at different times, and the results are presented at Figures 13−15. It was observed that it is possible to monitor the transesterification reaction in situ through fiber-optic probes in continuous processes. However, the results did not show a constant baseline of the NIR spectra due to the inherent characteristic of a continuous process. The baseline is very sensitive to any disturbance of flow inside the microchannel.

Figure 15. NIR spectra from 5000 to 11 000 cm−1 of different in situ monitoring times using volumetric rates of 11.3 and 7.5 mL/h for ethanol and castor oil, respectively, catalyst amount of 1.0 wt % based on castor oil weight, and room temperature.

The baseline variation can lead to problems in considering ethyl ester quantification, once it is carried out considering the area under the peak, mainly when the concentration of esters is low. Similar results were found by Knothe,77,78 who used NIR spectroscopy to monitor a batch biodiesel synthesis and to determine the content of soybean biodiesel in conventional diesel fuel in the mass fraction range between 0 and 100%. However, the method was not precise for detecting low amounts of ester in petrodiesel in the mass fraction range of 0− 2%. As observed in Figure 13, for 1 min of reaction it is not possible to distinguish an ethyl ester peak. In addition, it was observed that using higher volumetric flow rates produced higher ethyl ester yields. These observations are based on peak areas under the NIR spectra in the range 5000− 5500 cm−1. Higher ethyl ester yields are due to the use of higher volumetric flow rates which favor chaotic flow inside the microchannels, decreasing the mass transfer resistance and contributing to increased biodiesel production. More studies need to be conducted to improve the baseline stability. Probably the baseline instability is caused by the small optical path traveled by the infrared beam, due to small

Figure 13. NIR spectra from 5000 to 11 000 cm−1 of different in situ monitoring times using volumetric rates of 1.5 and 1.0 mL/h for ethanol and castor oil, respectively, catalyst amount of 1.0 wt % based on castor oil weight, and room temperature. 10763



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reaction temperature. Ethyl ester yields of 89.0, 92.2, and 92.6% were reached for T-, omega-, and Tesla-shaped microreactors, respectively. This is due to the mass transfer enhancement caused by the miscibility improvement of ethanol and triglyceride at high temperature. When the ethanol/oil molar ratio was increased from 9:1 to 12:1 at 50 °C, the ethyl ester yields increased about 1.1 times. The ethyl ester yields of 93.5, 95.3 and 96.7% were obtained for T-, omega-, and Tesla-shaped microreactors, respectively, with a residence time of 10 min, molar ratio of ethanol to oil of 25:1, catalyst amount of 1.0 wt %, and temperature of 50 °C. Ethyl esters showed a characteristic absorbance at 5000− 5500 cm−1. This characteristic can be use to perform the quantification analysis of the transesterification reaction using castor oil and ethanol. Higher ethyl ester yields were observed with the use of higher volumetric flow rates, which favor chaotic flow, decreasing the mass transfer resistance and contributing to increased biodiesel production. However, the in situ monitoring of microreactors showed baseline instability and more studies should be performed to try to improve the baseline stability in order to quantify the ester concentration. Possibly baseline instability is caused by the small optical path traveled by the infrared beam, due to small microreactor dimensions. Increasing the infrared beam optical path can be made by increasing the diameter of the channel where the infrared beam focuses.

microreactor dimensions. One possibility that can be explored is to measure the concentration of esters out of the microchannels. This can be done by increasing the diameter of the channel where the infrared beam focuses and, consequently, increasing the optical path of the infrared beam. To do this, new microreactors have to be built. Despite the challenges involving the continuous online monitoring of biodiesel production, this methodology stills appears to be an attractive analytical technique to monitor the reactions in microreactors and to be used in control of portable plants because of its low cost and simplicity and because it is less timeconsuming.

4. CONCLUSIONS Experimental investigations on the improvement of the biodiesel production process using microreactors with different geometries were carried out. The influences of process parameters such as geometry, catalyst amount, temperature, ethanol/castor oil molar ratio, and residence time were studied. The biodiesel synthesis was greatly dependent on the geometry of the microreactor. The Tesla-shaped microreactor resulted in higher biodiesel yields than the omega- and Tshaped microreactors. The high efficiency of reaction for biodiesel production by the Tesla-shaped microreactor is attributed to the higher intensification of overall volumetric mass transfer between the reactants. It could be seen that the yield of ethyl esters after a residence time of 15 min for omegaand Tesla-shaped microreactors was about 1.2-fold greater than that for the T-shaped microreactor considering a catalyst amount of 1.0 wt % based on the castor oil weight, a temperature of 50 °C, and a molar ratio of ethanol/castor oil of 9:1. The enhanced rate of reaction may be attributed to specific characteristics of the intensified module employed in the experiments. It is important to note that the transesterification reaction is diffusion controlled. Therefore, it can be concluded that the reason for obtaining better conversion in the Teslaand omega-shaped microreactors used in this work is due to convective mixing. In the T-shaped microreactor, conversion is based on diffusive and laminar mixing. Additionally, the use of an intensified process provides good control of the process parameters due to smaller volumes and shorter path lengths which contribute to higher transport rates. Thus, microreactors showed to be a promising technology for biodiesel synthesis in order to design a compact and mini fuel processing plant. The yields of ethyl ester reached values of 75.9, 91.4, and 93.7% after residence times of 15 min for T-, omega-, and Tesla-shaped microreactors, respectively. It could be seen that, for the first 4 min of reaction, with increasing residence time, the ethyl ester yield also increases. After this time, the ethyl ester yield reaches a practically constant value; however, for the T-shaped microreactor it was significantly inferior because of the microchannel geometry that promotes a lower interfacial contact area and consequently decreases the mass transfer rates between the immiscible reactants. Castor oil conversion in microreactors increased with the ethanol/castor oil molar ratio, catalyst amount, and reaction temperature. The highest yields of ethyl ester in this study were 79.1, 96.2, and 98.9%, using a catalyst concentration of 1.5%, residence time of 10 min, molar ratio of ethanol to castor oil of 9:1, and temperature of 50 °C, for T-, omega-, and Teslashaped microreactors, respectively. The study of the temperature effect on the biodiesel yield resulted that the biodiesel yield increased with increasing

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by The Scientific Research Foundation for the State of São Paulo (FAPESP) and research support by The Brazilian Synchrotron Light Laboratory (LNLS).

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REFERENCES

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Continuous Synthesis and in Situ Monitoring of Biodiesel Production

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