Intensified Biodiesel Production Using a Rotating Tube Reactor

Article pubs.acs.org/EF

Intensified Biodiesel Production Using a Rotating Tube Reactor Himanshu Lodha,* Roshan Jachuck, and Saravanan Suppiah Singaram Process Intensification and Clean Technology (PICT) Group, Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, New York 13699, United States ABSTRACT: Biodiesel is an alternative fuel, produced by the transesterification of oils or fats with alcohols. The present work investigates the use of a thin-film system, a rotating tube reactor (RTR) for continuous production and separation of biodiesel. A thin liquid film is created in RTR by strong centrifugal forces generated in the RTR. These thin, intensely mixed films are capable of delivering high heat- and mass-transport rates. In the current study, biodiesel is synthesized from canola oil, methanol, and sodium hydroxide as a catalyst. Parameters such as rotational speed, flow rate, temperature, and catalyst concentration were investigated in terms of bound glycerol. Atmospheric pressure and low operating temperature (40−65 °C) have been used to achieve conversions in excess of 98% in residence time of less than 1 min.

1. INTRODUCTION Extensive research has been performed for biodiesel synthesis using various sources of triglyceride and alcohol.1−12 The most commonly used alcohols for the transesterification reaction are methanol, ethanol, n-propanol, and n-butanol. Methanol and ethanol are preferred because they give stable products and higher conversions compared to longer chain alcohols. Methanol is cheaper compared to ethanol2,13 and can be easily separated during downstream separation, owing to its higher polarity. It dissolves sodium hydroxide more easily compared to other alcohols.14 It has been observed that the viscosities of ethyl esters are higher than methyl esters,15 have higher glycerol content, and give more injector coking than methyl esters.15 Thus, methanol is used as the alcohol feedstock for the present work. Presently, research is being carried out to intensify the biodiesel reaction using alternative energy sources, such as microwaves10,16 and ultrasound.9,17 Also, a lot of effort is being put into developing continuous processes with new reactor designs.6−8 The present work uses a continuous, novel rotating tube reactor (RTR) that uses the centrifugal force to intensify both the reaction and downstream separation steps. In RTR, because of its rotation, centrifugal forces are generated on the rotating tube surface, producing sheared thin films. The film thickness varies from 700 to 1400 μm. This small thickness results in a very high surface area/volume ratio (∼1000), which improves the interactions of film with its surroundings.18 Thin films also correspond to a small conduction path length, thus improving heat and mass transfer. The presence of wavy films or ripples corresponds to improve mixing. Thus, RTR, owing to its better mixing, small conduction path lengths, and better control over the heattransfer characteristics, has the ability of enhancing reaction rates. Figure 1 shows a few different hydrodynamic regimes observed in the RTR. Experiments were conducted in annular flow and pool flow regimes.

Figure 1. Different hydrodynamic regimes inside RTR. methanol phases are weighed to maintain a molar ratio of 1:6, respectively. Oil and methanol phases were mixed, and the mixture was pumped into the RTR, which is rotating at a fixed rotational speed. RTR is maintained at a specific wall temperature by a heat-transfer jacket. At the rear end of the RTR (product collection), a lip (weir) was attached. An in situ condenser was used for all experiments to condense methanol vapors formed during the reaction. The product is collected, quenched in an ice bath, centrifuged, and analyzed using a gas chromatograph (GC). The chemicals used for carrying out transesterification are shown in Table 1. 2.1. Analytical Procedure. The product collected was centrifuged for 9 min at 4000 rpm using a Hermle centrifuge. Then, 100 mg of product from the top layer (biodiesel layer) of the centrifuged product was taken. The sample was prepared and analyzed for free and bound glycerol content following the ASTM D6584 specifications.19 For the analysis, a GC fitted was used. The conversion was calculated using the formula written below conversion (%) =

where BG is bound glycerol.

3. RESULT AND DISCUSSION Experiments were conducted to study the influence of RTR on key parameters affecting conversion of the transesterification

2. EXPERIMENTAL PROCEDURE

Received: July 24, 2012 Revised: October 3, 2012 Published: October 4, 2012

The experimental setup is shown in Figure 2. Methanol and sodium methoxide were premixed and dried using molecular sieves. Oil and © 2012 American Chemical Society

10.44 − BG × 100% 10.44

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Figure 2. Schematic of the experimental setup.

Table 1. Chemicals Used during the Transesterification Reaction and Analysis chemical methanol canola oil sodium hydroxide molecular sieves

grade 98% histological grade commercially available pellets anhydrous 3 Å, 8−12 mesh

use alcohol source triglyceride source catalyst absorb water from methanol phase

reaction, such as catalyst concentration, residence time, mixing intensity, and operating temperature. Conditions derived are based on the conversion of oil into methyl esters. All of the experiments were performed using a molar ratio of 1:6 for oil/ methanol, which has been reported as the optimum ratio by earlier studies.20 Also, an in situ condenser is used for all experiments. The hydrodynamic condition inside a RTR helps in achieving high heat- and mass-transfer rates, which, in turn, help in achieving faster conversions. The liquid mixture (oil and methanol phases) pumped inside the RTR forms a thin film on the inner surface of the RTR, owing to the high rotational speed. This liquid mixture nearly attains the wall temperature. Reaction progresses along the length of the reactor tube. As the reaction proceeds, glycerol begins to separate out from the reacting mixture. Because of its polarity, glycerol molecules are bound to the methanol molecules. Because of the difference in the density of glycerol (1261 kg/m3) and biodiesel (880 kg/ m3), this glycerol is thrown by the centrifugal forces toward the heated wall of the rotating tube. This glycerol layer along with methanol is heated through the heat-transfer liquid flowing in the heat-transfer jacket. This heating activates the methanol and pushes the reaction further. Also, the separation of glycerol from the reaction mixture helps in pushing the reaction forward. Thus, an in situ separation of glycerol can be seen in RTR. RTR was equipped with an in situ condenser to condense methanol vapors formed when the reaction mixture is above the boiling point of methanol, thus maintaining the mole ratio of oil/methanol. 3.1. Experiments with the Lip and in Situ Condenser. 3.1.1. Effect of the Rotational Speed. Figure 3 summarizes the effect of the rotational speed on the conversion of canola oil. For these experiments, the flow rate (15 mL/s), film thickness

Figure 3. Effect of the variation in rotational speed on the conversion.

(1.1 cm), and catalyst concentration (1.5 wt %) were kept constant. As seen in Figure 3, at a fixed flow rate and film thickness, with an increase in the rotational speed, the hydrodynamic regime changes from pool flow at 270 rpm to annular flow at 470 rpm, and with a further increase in the rotational speed, the reacting fluid remains in the annular flow regime. In the annular flow regime, an increase in rotational speed corresponds to a decrease in residence time and an increase in centrifugal force. The results shown in Figure 3 can be attributed to interplay between the higher centrifugal force acting to separate out the glycerol molecules from the reaction mixture, thus leading to higher conversion and a decreasing residence time with an increasing rotational speed. 3.1.2. Effect of the Flow Rate. Figure 4 summarizes the experimental results of effect of variations in the flow rate. For this study, the rotational speed (670 rpm), film thickness (1.1 cm), and catalyst concentration (2 wt %) have been kept constant. At a fixed rotational speed and film thickness, with an increase in the flow rate, the residence time decreases and the mixing improves because of the creation of ripples and waves. Mixing because of ripples and waves is useful at higher temperatures when the condensed methanol falls back into the reacting mixture, thus helping to maintain the molar ratio of methanol/oil. It can also be observed that, at a wall temperature of 26 °C, the conversions for increasing the flow rate tend to lie on a straight line, because at these conditions, the residence time, 7038

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Figure 4. Effect of the flow rate on the conversion.

Figure 6. Effect of the variation in lip thickness on the conversion.

which changes with the flow rate, is the main conversioninfluencing parameter. At conversions close to 97%, all of the data points tend to converge. This is due to the fact that, close to this conversion value, the reactions are trying to reach completion. 3.1.3. Effect of the Catalyst Concentration. Experiments have been conducted to study the influence of the catalyst concentration on the conversion of canola oil. The flow rate (15 mL/s), rotational speed (870 rpm), and film thickness (1.1 cm) have been held constant for these experiments. Figure 5

tube. As seen in Figure 6, the conversion increased with an increase in the lip thickness, i.e., with an increase in the residence time. 3.2. Experiment with the in Situ Condenser Only. 3.2.1. Effect of the Condenser. The RTR was equipped with an in situ condenser to condense any methanol vapors formed during the course of the reaction. Experiments were performed to test the efficiency of this condenser. As seen in Figure 7, for

Figure 7. Effect of the condenser on the conversion. Figure 5. Effect of the catalyst concentration on the conversion.

low temperatures, i.e., below the boiling point of methanol, the conversions achieved with and without the use of the condenser are very similar. However, when the temperature was raised to 77 °C, a significant increase in the conversion was observed when the condenser was used. This increase in conversion may be attributed to the fact that, at higher temperatures, the condenser helps to maintain the methanol/oil ratio in the system by condensing methanol evaporated during the reaction.

summarizes the experimental results of the effect of variations in the catalyst concentration. It shows that, as the catalyst concentration increases, the triglyceride conversion also increases. This is in agreement with the observations by Darnoko and Cheryan.21 These observations also indicate that conversion increases with an increase in the temperature. However, with an increase in the catalyst concentration and temperature, it was observed that side reactions (saponification) also increase considerably. The highest drop in conversion because of soap formation was observed in conditions of the highest catalyst concentration (2 wt % of oil) and highest temperature. It can be concluded that, with an increase in catalyst concentrations, higher conversions were obtained but the product yield decreased. 3.1.4. Effect of the Lip. Figure 6 summarizes the effect of the lip thickness on the conversion of canola oil to biodiesel. To study this, lips of different thickness (5000 and 11 000 μm) were attached at the rear end (product end) of the rotating

4. CONCLUSION We summarize that we have developed a novel and continuous process for the transesterification of biodiesel fuel, which operates at atmospheric pressure and moderate temperatures. The influence of the catalyst concentration, flow rate, rotational speed, temperature, lip thickness, and condenser were studied for RTR equipped with a lip and condenser. The increase in catalyst loading (1−2 wt %) showed an increase in the conversion of triglycerides to methyl ester (biodiesel fuel) for all cases, except at the conditions of the 7039

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Table 2. Rotating Tube versus Other Methodologies literature

reactors

Dubé et al. Harvey et al. Leevijit et al. Noureddini et al. Lodha et al.

membrane reactor OFR CSTR motionless, shear mixer RTR

reactants oil oil oil oil oil

+ + + + +

MeOH MeOH MeOH MeOH MeOH

catalyst concentration

residence time

flow rate/volume reactor

conversion

1 wt % 32.4 g/L MeOH 1 wt % 0.4 wt % 1.5 wt %

6h 30 min 6 min 6.67 min 0.75 min

6.1 mL/min 1.56 L 378 mL/min 300 mL/min 900 mL/min

96% ASTM passed 97.5% 98% 97.65%

(9) Stavarachea, C.; Vinatoru, M.; Nishimurab, R.; Maeda, Y. Fatty acids methyl esters from vegetable oil by means of ultrasonic energy. Ultrason. Sonochem. 2005, 12 (5), 367−372. (10) Saifuddin, N.; Chua, K. H. Production of ethyl ester (biodiesel) from used frying oil: optimization of transesterification process using microwave irradiation. Malays. J. Chem. 2004, 6 (1), 077−082. (11) Lotero, E.; Goodwin, J. G., Jr.; Bruce, D. A.; Suwannakarn, K.; Liu, Y.; Lopez, D. E. The catalysis of biodiesel synthesis. Catalysis 2006, 19, 41−83. (12) Noureddini, H.; Harkey, D.; Medikonduru, V. A continuous process for the conversion of vegetable oils into methyl esters of fatty acids. J. Am. Oil Chem. Soc. 1998, 75 (12), 1775−1783. (13) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 2001, 92 (5), 405−416. (14) Ma, F.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1−5. (15) Encinar, J. M.; González, J. F. G.; Rodríguez-Reinares, A. Biodiesel from used frying oil. Variables affecting the yields and characteristics of the biodiesel. Ind. Eng. Chem. Res. 2005, 44 (15), 5491−5499. (16) Lertsathapornsuk, V.; Ruangying, P.; Pairintra, R.; Krisnangkura, K. Continuous transethylation of vegetable oils by microwave irradiation. pp 11−13; http://e-nett.sut.ac.th/download/RE/RE11. pdf. (17) Hanh, H. D.; Dong, N. T.; Okitsu, K.; Nishimura, R.; Yasuaki, M. Effects of molar ratio, catalyst concentration and temperature on transesterification of triolein with ethanol under ultrasonic irradiation. J. Jpn. Pet. Inst. 2007, 50 (4), 195−199. (18) Cowen, G.; Norton-Berry, P.; Steel, M. L. Chemical process on the surface of rotating body. U.S. Patent 4,311,570, 1982. (19) American Society for Testing and Materials (ASTM). ASTM D6584, Standard Test Method for Determination of Free and Total Glycerin in B-100 Biodiesel Methyl Esters by Gas Chromatography; ASTM: West Conshohocken, PA, 2005. (20) Noureddini, H.; Zhu, D. Kinetics of transesterification of soybean oil. J. Am. Oil Chem. Soc. 1997, 74 (11), 1457−1463. (21) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterification in a batch reactor. J. Am. Oil Chem. Soc. 2000, 77 (12), 1263−1267.

highest temperature, catalyst concentration, and rotational speed, where the saponification (side reaction) increased considerably. A decrease in the flow rate (21−5 mL/s) showed an increase in the conversion of triglyceride to methyl esters. The study for the influence of the rotational speed showed that 670 rpm showed the best results among the four rotational speeds studied. In a residence time of 45 s, 98% conversion was achieved. The RTR also made the downstream separation process easier and helped to separate the biodiesel and glycerol layers. A current investigation would be helpful in developing an intensified biodiesel production plant in terms of reduction in energy consumption, reaction time, and separation needed for biodiesel production. Table 2 compares the present study to those of other researchers in terms of residence time, catalyst concentration, and flow rate.6,8,12

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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 This work was performed while Himanshu Lodha and Saravanan Suppiah Singaram were students and Roshan Jachuck was a professor at Clarkson University. The authors thank Clarkson University for providing the infrastructure and other necessities for supporting the research.

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REFERENCES

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