Energy & Fuels 2008, 22, 1493–1501
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A Pilot-Scale Study of Alkali-Catalyzed Sunflower Oil Transesterification with Static Mixing and with Mechanical Agitation Dario Frascari,* Michele Zuccaro, Davide Pinelli, and Alessandro Paglianti Department of Chemical, Mining and EnVironmental Engineering, UniVersity of Bologna, Via Terracini 34, 40131 Bologna, Italy ReceiVed October 3, 2007. ReVised Manuscript ReceiVed January 8, 2008
The utilization of Sulzer 15 mm SMV static mixers (SM) for the KOH-catalyzed transesterification of sunflower oil was studied by means of batch tests conducted in an experimental ring equipped with a 22 L reactor with a 6:1 methanol:oil molar ratio. The effect of SM number and superficial velocity was investigated. The results were compared to those of analogous tests performed with only mechanical agitation, at different rotational speeds and catalyst concentrations. The tests conducted with one single SM at a 1.3 m/s superficial velocity (Re ) 1490) resulted in a profile of sunflower oil conversion versus time equivalent to that obtained in the best-performing test with mechanical agitation, indicating the attainment of a reaction run not affected by mass transfer limitations. In an evaluation of the energy requirement for the attainment of the alcohol/oil dispersion, the SM tests performed better than those with mechanical agitation (17 vs 35 J/kgbiodiesel, in the reaction conditions without mass transfer constraints). This analysis was scaled-up to industrial-scale biodiesel production plants, confirming the favorable performance of static mixers. On the whole, the results suggest that static mixing can be effectively applied to oil transesterification processes for biodiesel production.
Introduction Biodiesel is an alternative fuel that can be produced from fresh and used vegetable oils and animal fats by means of a transesterification reaction. It represents a promising alternative to fossil diesel, thanks to its renewable origin, high biodegradability, and low pollutant emissions in engines.1–5 The reaction of oil or fat conversion into biodiesel can be operated in both homogeneous and heterogeneous catalysis with acids and bases. Basic homogeneous catalysis, which generally allows the attainment of higher reaction rates, remains the most common biodiesel production method.6,7 A large number of bench-scale literature studies on homogeneous-catalyzed processes examined the effect of the different operational parameters on the rate of biodiesel production and on the equilibrium conversion of the * Corresponding author. E-mail address:
[email protected]. Fax: +39 051 6347788. (1) Ma, F.; Hanna, M. A. Biodiesel production: a review. Biores. Technol. 1999, 70, 1–15. (2) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 2001, 92, 405–416. (3) Van Gerpen, J. Biodiesel processing and production. Fuel Process. Technol. 2005, 86, 1097–1107. (4) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Possible methods for biodiesel production. Renewable Sustainable Energy ReV. 2007, 11, 1300– 1311. (5) Srivastava, A.; Prasad, R. Triglycerides-based diesel fuels. Renewable Sustainable Energy ReV. 2000, 4, 111–133. (6) Lopez Granados, M.; Zafra Poves, M. D.; Martin Alonso, D.; Mariscal, R.; Cabello Galisteo, F.; Moreno-Tost, R.; Santamaria, J.; Fierro, J. L. G. Biodiesel from sunflower oil by using activated calcium oxide. Appl. Catal. B: EnViron. 2007, 73, 317–326. (7) Bambase, M. E. Jr.; Nakamura, N.; Tanaka, J.; Matsumura, M. Kinetics of hydroxide-catalyzed methanolysis of crude sunflower oil for the production of fuel-grade methyl esters. J. Chem. Technol. Biotechnol. 2007, 82, 273–280.
transesterification reaction.7–19 In particular, the investigations focused on the role of alcohol type, alcohol:oil molar ratio, type and concentration of catalyst, temperature, and intensity of agitation. On the other hand, recent studies led to major advancements toward the developments of suitable heterogeneous catalysts and evidenced some relevant advantages of heterogeneous-catalyzed biodiesel production, such as the possibility to reutilize the catalyst, the simplification of the (8) Vicente, G.; Martinez, M.; Aracil, J.; Esteban, A. Kinetics of sunflower oil methanolysis. Ind. Eng. Chem. Res. 2005, 44, 5447–5454. (9) Komers, K.; Skopal, F.; Stloukal, R.; Machek, J. Kinetics and mechanism of the KOH-catalyzed methanolysis of rapseed oil for biodiesel production. Eur. J. Lipid Sci. Technol. 2002, 104, 728–737. (10) Goff, M. J.; Bauer, N. S.; Lopes, S.; Sutterlin, W. R.; Suppes, G. J. Acid-catalyzed alcoholysis of soybean oil. J. Am. Oil Chem. Soc. 2004, 81, 415–420. (11) Ataya, F.; Dubè, M. A.; Ternan, M. Acid-catalyzed transesterification of canola oil to biodiesel under single- and two-phase reaction conditions. Energy Fuels 2007, 21, 2450–2459. (12) Dorado, M. P.; Ballestros, E.; Lopez, F. J.; Mittlebach, M. Optimization of alkali-catalyzed transesterification of Brassica carinata oil for biodiesel production. Energy Fuels 2004, 18, 77–83. (13) Ma, F.; Clements, L. D.; Hanna, M. A. The effect of mixing on the transesterification of beef tallow. Biores. Technol. 1999, 69, 289–293. (14) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification kinetics of soybean oil. J. Am. Oil Chem. Soc. 1986, 63, 1375–1380. (15) Vicente, G.; Coteron, A.; Martinez, M.; Aracil, J. Application of the factorial design of experiments and response surface methodology to optimize biodiesel production. Ind. Crop. Prod. 1998, 8, 29–35. (16) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterification in a batch reactor. J. Am. Oil Chem. Soc. 2000, 77, 1263–1267. (17) Noureddini, H.; Zhu, D. Kinetics of transesterification soybean oil. J. Am. Oil Chem. Soc. 1997, 74, 1457–1463. (18) Encinar, J. M.; Gonzales, J. F.; Rodriguez, J. J.; Tejedor, A. Biodiesel fuels from vegetable oils: transesterification of Cynara cardunculus oils with ethanol. Energy Fuels 2002, 16, 443–450. (19) Freedman, B.; Pryde, E. H.; Mounts, T. L. Variables affecting the yields of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc. 1984, 61, 1638–1643.
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product separation process, and the higher purity of the ester and glycerol phases produced.6,20–24 Because vegetable oil and alcohol form two almost immiscible phases, no matter what type of catalysis is selected, an intense mixing of the two reagents is required in order to break the alcohol phase into small drops, thus providing sufficient interfacial area for the reaction. The vast majority of the studies on transesterification focused on mechanically agitated systems, either with impellers or with magnetic stirrers. Conversely, a limited number of preliminary works investigated the utilization of static mixers (SM) in biodiesel production processes.25–27 SM consist of motionless mixing elements placed within a tube or column. The utilization of SM for dispersing two immiscible liquids finds applications in liquid–liquid extraction, metal extraction, emulsification processes, and some liquid–liquid reactions.28,29 SM generally require less power input than mechanical agitation at equivalent performance, and thanks to the absence of moving parts they are characterized by minimum maintenance requirements. They also present the advantage of narrow residence time distributions.28,30–32 Noureddini et al.25 examined a combination of static mixing and mechanical agitation with an online mixer in a pilot plant in which continuous transesterification occurred in a 1 L tubular reactor. With a 10 min residence time, they showed that even without mechanical agitation static mixing allows the attainment of a 99% oil conversion. Utilizing KOH as the catalyst, Peterson et al.26 studied canola oil transesterification with ethanol in a 27 L continuous tubular reactor preceded by 12 Cole-Parmer static mixers and followed by a centrifuge for biodiesel/glycerol separation. At a 55 min residence time, they obtained a biodiesel respecting the ASTM PS121-99 specification (free glycerol 0.05. On the basis of this analysis, no significant differences were found (at 0.8 wt % KOH) between the reference condition not limited by mass-transfer (700 rpm), the assay at 400 rpm and the 1-SM test with VSM ) 1.3 m/s. This important result indicates that the utilization of a single SM at 1.3 m/s leads to the formation of an oil/methanol dispersion sufficiently fine and stable to prevent mass-transfer limitations in the first critical minutes of the transesterification reaction at 60 °C and 0.8 wt % KOH. Besides, no significant differences were found among the tests operated with varying number of SM. According to the literature on static mixing, higher values of the L/D ratio of the SM section determine a higher homogeneity of the exiting solution or dispersion.28,30,40 The insignificant effect of the number of SM seems therefore to indicate that, in our experimental setup, the homogeneity of the alcohol/oil dispersion does not have any detectable effect on the kinetic of the transesterification reaction. The results of these SM tests are in agreement with the outcome of the test with mechanical agitation at 400 rpm (1 min) + 100 rpm. Both type of assays, in fact, show that once a suitable methanol/oil dispersion has been created by an initial “high-energy” event, the transesterification reaction, at 60 °C and 0.8 wt % KOH, advances without any appreciable mass-transfer limitation even in the case of a very low agitation speed (100 rpm). In order to distinguish between the contribution of the SM and that of the two-phase flow from the tee of the methanol and oil loading system to the reactor inlet, a further test was operated with the same loading system but in the absence of SM. This test was conducted at the flow rate corresponding to a 1.3 m/s superficial velocity in the SM tests. The results, illustrated in Figure 4a, show that the two-phase flow through the loading system (followed by a mechanical agitation at 100 rpm in the reactor) led to the formation of some dispersion of methanol in oil, and consequently to a higher advancement of the reaction than that obtained with the mere mechanical agitation at 100 rpm (Figure 3). On the other hand, the “twophase flow” test was characterized by significantly lower values of reaction rate and 2 h conversion than the test with SM at 1.3 m/s. These data therefore indicate that, while the flow through the loading system gives a relevant contribution to the dispersion obtained in the SM tests, the mean drop diameter obtained in the absence of the SM is too high to allow the transesterification reaction to proceed without significant mass transfer limitations. A second group of tests, operated with 1 SM, 0.8 wt % KOH and 100 rpm in the reactor, focused on the effect of the superficial velocity in the SM section (Figure 4b). On the basis (40) Middleman, S. Drop size distributions produced by turbulent pipe flow of immiscible fluids through a static mixer. Ind. Eng. Chem. Process. Des. DeV. 1974, 13, 78–83.
of the statistical analysis, increasing the SM velocity from 1.3 to 2.5 m/s did not lead to any significant improvement of the observed biodiesel production rate. This result confirms the observation that, with the dispersion obtained at 1.3 m/s, the reaction is not significantly mass-transfer limited. Conversely, the decrease in SM velocity from 1.3 to 0.4 m/s led to a significantly different profile of oil conversion (p < 0.05). In particular, the reaction rate was slower during the first 10–15 min (for example, the average biodiesel rate during the first 2 min was 0.32–0.36 kg/L/min at 1.3 or 2.5 m/s, versus 0.24 at 0.4 m/s). Analysis of the Experimental Results: Comparison of Mean Drop Diameters. The results illustrated in the previous subsections indicate that the drop size distribution that prevents the reaction from being mass-transfer limited corresponds to a rotational speed between 200 and 400 rpm in the case of mechanical agitation, and to a superficial velocity in the 0.4–1.3 m/s range in the case of a single static mixer. This observation suggested the idea to compare the estimates of the mean drop diameter obtained in the two systems. The d32 were estimated using eqs 2 and 4, for mechanical agitation and static mixing respectively. The results, summarized in Table 5, show a good agreement between the evaluations of d32 and the experimental profiles of biodiesel mass fraction versus time. In fact, the lowest d32 (22 µm) was obtained at 700 rpm, the condition that was the least mass-transfer limited on the basis of the data of oil conversion versus time (70% conversion after 1 min). In the tests at 400 rpm, at VSM ) 2.5 m/s and at VSM ) 1.3 m/s s three experimental conditions that on the bases of the initial reaction rates appeared to be slightly mass-transfer limited during the initial minutes of reaction s the estimates of d32 varied between 36 and 63 µm. Finally, in the tests at 200 rpm and at VSM ) 0.4 m/s, which were mass-transfer limited up to a reaction time of 10–15 min, we obtained estimates of d32 g 99 µm. These results therefore indicate that, at 60 °C and with KOH equal to 0.8% of oil mass, the transesterification of sunflower oil with methanol is not significantly mass-transfer limited (with the exception of the initial few minutes) when d32 is smaller than about 65 µm. This condition corresponds, at a 0.2 volume fraction of dispersed phase, to an interfacial area per unit volume >18 500 m2/m3. The good agreement between the estimates of d32 and the experimental profiles of oil conversion versus time represents a qualitative validation of the correlations utilized to evaluate d32. However, in future work, the above-reported estimates of d32 should be confirmed by means of direct measurements of drop size. Analysis of the Experimental Results: Evaluation of Energy Consumption. As a further step toward the comparison of the different experimental conditions tested in this study, for each assay we built a plot of cumulative biodiesel mass produced versus cumulative energy consumption. The results of this elaboration are shown in Figure 5. In each SM test (empty symbols), the first point (positioned on the x-axis) indicates the energy dissipated in the SM section, whereas the subsequent increases in energy consumption are due to the mechanical agitation at 100 rpm. From the energetic point of view, the most interesting conditions are the SM tests at VSM ) 0.4 or 1.3 m/s and the test with only mechanical agitation at 200 rpm. Conversely, the tests at higher rotational speed or higher SM velocity are characterized by elevated energy consumptions. In order to allow an easier comparison of the different experimental conditions tested, we determined for each test the values of reaction time (tr) and energy consumed per kg of biodiesel produced (e) corresponding to a 75% and a 90% oil
Alkali-Catalyzed Sunflower Oil Transesterification
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Figure 7. Specific energy consumption at 90% oil conversion.
Figure 5. Profiles of cumulative biodiesel mass produced versus cumulative energy consumed in the tests with static mixing and with mechanical agitation.
Figure 6. Specific energy consumption and total operational time at 75% oil conversion.
conversion. These two values were chosen on the basis of the following considerations: 75% is far enough from the equilibrium conversion (equal to about 93%) to evaluate tr with an acceptable accuracy; thus, it allows to discriminate the different tests on the basis of the reaction rate. 90%, on the contrary, allows to discriminate the tests on the basis of the energetic requirement at a conversion closer to those typically aimed at in industrial applications. At 90% conversion, the different tests were therefore compared only in terms of specific energy consumption. At 75% conversion, the different operational conditions were compared in terms of total operational time, obtained by summing tr to half of the loading time (the other half-being already included in the reaction time, as explained in the Experimental Section). The results are presented in Figures 6 (75% conversion) and 7 (90%). As can be observed in Figure 6, in the tests at 200 rpm and at VSM ) 0.4 m/s we obtained the lowest e75 (12–13
J/kgbiodiesel) but the resulting operational times were significantly higher than in the other cases (4–5.5 min). Conversely, the tests at 400 rpm, 700 rpm and at VSM g 1.3 m/s were characterized by very short operational times (about 2 min), and by energy requirements that increased with increasing number of SM, SM velocity, and impeller rotational speed. In particular, the test with VSM ) 1.3 m/s was the most interesting one: in fact it had an operational time for 75% conversion analogous to that of the reference condition not limited by mass transfer (no SM, 700 rpm), and an estimate of e75 3–17 times lower than those obtained at 400 and 700 rpm. On the other hand, also the tests at 200 rpm and at VSM ) 0.4 m/s represent interesting conditions: despite their higher t75, at 90% oil conversion they are characterized by a reaction time about equal (200 rpm) or slightly higher (0.4 m/s) than that obtained at 700 rpm (Figures 3 and 4). As can be observed in Figure 7, if the energy requirement is evaluated at 90% oil conversion, the advantage of the SM tests over the tests with mechanical agitation is more evident: for example, in the SM assay at 1.3 m/s e90 is about half that at 200 rpm and 23 times lower than that at 400 rpm. In fact, in the SM tests the prevailing fraction of the energy requirement is due to the passage through the SM section, the subsequent consumption due to the 100 rpm agitation being almost negligible. Therefore, the energy utilized per unit mass of reagents is basically independent of the degree of advancement of the reaction. Conversely, in the assays with mechanical agitation, each increase in reaction time implies a marked increase in energy consumption. Unfortunately, the configuration of the stirred tank reactor did not allow to validate the abovereported estimations of energy consumption in the tests with mechanical agitation by means of direct torque measurements in the stirrer. Therefore, the estimates of e75 and e90 relative to the tests with mechanical agitation might be affected by a higher uncertainty than those relative to the SM tests. However, given the marked differences in the evaluations of specific energy consumption obtained, in particular at 90% conversion, in the two types of reaction tests, the higher uncertainties relative to the tests with mechanical agitation should not significantly affect the comparison of the two systems. The comparison of the estimates of d32 (Table 5) and e (Figures 6, 7) relative to the tests with 1 SM (0.4 and 1.3 m/s) and with mechanical agitation (200 and 400 rpm) is generally
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Table 6. Estimated Energy Requirement of Industrial-Scale Biodiesel Production in Plants of Different Sizes with Static Mixing condition a: reaction with mass-transfer limitation in the initial 10–15 min (average rate at 90% conversion ) 0.055 kgbiodiesel/L/min)a 50 0.9 3.0 × 104 2.6
DSM (mm) VSM (m/s) plant size (tbiodiesel/y) e90,SM (J/kgbiodiesel)c
100 1.5 2.0 × 105 6.3
condition b: reaction without mass-transfer limitation (average rate at 90% conversion ) 0.10 kgbiodiesel/L/min)b 50 3.0 9.8 × 104 25
100 5.0 6.4 × 105 67
a Condition a corresponds to the results obtained in the laboratory-scale pilot at V b Condition b SM ) 0.4 m/s and without SM at N ) 200 rpm. corresponds to the results obtained in the laboratory-scale pilot at VSM ) 1.3 m/s and without SM at N ) 400 rpm. c Specific energy consumption at 90% oil conversion, in a continuous reactor with static mixing.
in agreement with the observations of Davies,41 who showed that, in the formation of liquid–liquid dispersions, static mixing is generally characterized by lower specific energy consumptions and higher drop diameters than mechanical agitation. Evaluation of the Energy Requirement for Biodiesel Production in Industrial-Scale Plants with Static Mixing. The good match between the estimates of the average drop diameter and the transesterification rates suggests that the scale-up of the biodiesel production process with SM should be operated at constant d32, in order to attain time profiles of oil conversion similar to those obtained in the laboratory-scale pilot. By rearranging eq 4, the d32 at the exit of an SMV static mixing section can be expressed as d32 )
(
)
0.65 0.21σ0.5 dh Fc0.35µc0.15 VSM0.85
(7)
The scale-up condition that, at equal values of the chemical– physical properties, ensures an equal drop size distribution is therefore
(
dh,ind. plant VSM,ind. plant ) VSM,lab pilot dh,lab pilot
)
0.65/0.85
(8)
Because the SM hydraulic diameter increases with increasing inner diameter DSM, eq 8 implies that, to operate at constant d32, bigger static mixers must be operated at higher superficial velocities. To take into account the variation of energy dissipation in the SM section resulting from this increase in SM velocity, the SM pressure drop ∆pSM can be evaluated as a function of VSM using the expression of Thakur et al.:28 ∆pSM ) nSMNeFmixVSM2(LSM/DSM)
(9)
where Ne indicates the Newton number and Fmix the density of the reagent mixture. The SM manufacturer provided the value of Ne (constant and equal to 2) only for ReSM > 5000, where ReSM, the Reynolds number, referred to the entire pipe holding the SM, is given by FmixVSMDSM/µmix. This evaluation of Ne is in agreement with the indication of Etchels III and Meyer,42 who report values of Ne ranging between 1 and 2 for SMV SM in the turbulent regime. For ReSM < 5000, we interpolated on a Ne–ReSM plot the points corresponding to the three experimental conditions of this study (each value of Ne being obtained from eq 9, utilizing the experimental values of ∆pSM). The resulting correlation, valid in the transition regime, shows a marked decrease of Ne as ReSM increases. Therefore, in the evaluation of ∆pSM, the increase of VSM resulting from the scaleup condition (eq 8) might be offset by the decrease of Newton number resulting from the increase of DSM and VSM, and consequently of ReSM. The ultimate result is that the specific energy consumption of biodiesel production with SM is affected by a scale factor. Consequently, the values of e75 and e90 reported in Figures 6 and 7 are size-specific and cannot be generalized.
On the basis of the above-described procedure, we performed a preliminary evaluation of the values of e90 that might be obtained in industrial-scale biodiesel production plants operated with SMV-type SM. This evaluation was done for the case of a continuous tubular reactor. The hypothesis of a continuous reactor is justified by the fact that continuous biodiesel production is becoming the industrial standard, in particular for large production facilities.27,43,44 The scale-up was conducted with regard to both the SM tests that performed interestingly from the energetic standpoint: that at VSM ) 1.3 m/s (transesterification reaction not significantly limited by mass transfer) and that at VSM ) 0.4 m/s (mass-transfer limitation during the initial 10–15 min of reaction). These two experimental conditions, which will be referred to in the following as a and b, gave significantly different results in terms of average biodiesel production rate at 90% conversion. The analysis was extended to two sizes of SMV-type SM: DSM ) 50 mm and DSM ) 100 mm. These values were chosen among the commercially available diameters (15, 25, 50, 100, 200 mm) on the basis of the corresponding biodiesel yearly productions obtainable in the two abovedescribed experimental conditions. In fact, as reported in Table 6, in the 50 mm SM, on the basis of eq 8 the low-velocity case (a) corresponds to 0.9 m/s and the high-velocity case (b) to 3.0 m/s; the corresponding biodiesel yearly productions (estimated at 90% conversion and for a 300 d/y continuous process) are typical of a plant of medium-large size ((3.0–9.8) × 104 t/y). Conversely in the 100 mm SM the low-velocity condition (1.5 m/s) corresponds to a very large plant (2.0 × 105 t/y), whereas the high velocity (5.0 m/s) yields a yearly production higher than those typical of the largest biodiesel production facilities. For each case examined, e90 was estimated as e90 )
∆pSM 0.90Freagent mixtureωoil
(10)
where ωoil represents the oil mass fraction in the reagent mixture, and the denominator therefore represents the biodiesel mass produced (at 90% conversion) per unit volume of reagent mixture. As shown in Table 6, the low-velocity condition resulted, for both DSM ) 50 and 100 mm, in an estimate of e90 lower than that obtained in the laboratory-scale pilot (3–6 versus 13 J/kg). This indicates that, in the expression of ∆pSM, the decrease of Ne (resulting from the increase of ReSM) prevailed (41) Davies, J. T. A physical interpretation of drop sizes in homogenizers and agitated tanks, including the dispersion of viscous oils. Chem. Eng. Sci. 1987, 42, 1671–1676. (42) Etchel III, A. W.; Meyer, C. F. Mixing in Pipelines; In Handbook of Industrial Mixing. Science and Practice; Paul, E. L., Atiemo-Obeng, V. A., Kresta, S. M., Eds.; John Wiley & Sons: Hoboken, NJ, 2004. (43) Bournay, L.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. New heterogeneous process for biodiesel production: a way to improve the quality and value of the crude glycerin produced by biodiesel plants. Catal. Today 2007, 106, 190–192. (44) Darnoko, D.; Cheryan, M. Continuous production of palm methyl esters. J. Am. Oil Chem. Soc. 2000, 77, 1269–1272.
Alkali-Catalyzed Sunflower Oil Transesterification
on the increase of VSM2. Conversely, the high-velocity condition resulted in a 1.5–5 fold increase of e90 in comparison with the estimate obtained in the laboratory-scale pilot, as a consequence of the strong increase of VSM2. In order to assess the values of e90 reported in Table 6 for SM continuous plants, it is possible to compare them with tentative evaluations of the corresponding values that may be obtained in industrial-scale facilities operating with mechanically agitated batch reactors. According to Leng and Calabrese,36 in the case of a liquid–liquid dispersion in a tank reactor, operating a scale-up at constant power per unit volume of reagent mixture (P/V) leads to analogous liquid/liquid dispersions. Therefore, the power consumptions per unit mass of biodiesel produced (e) evaluated for our tests with mechanical agitation represent reasonable estimates of the corresponding values that might be obtained in industrial-scale facilities, provided the two systems utilize the same impeller configuration. On the basis of the data reported in Figures 3 and 4 and of the significance tests, the high-velocity condition of our SM tests is comparable with the test with mechanical agitation at 400 rpm (no significant masstransfer limitations), whereas the low-velocity condition with SM is analogous to the test at 200 rpm (mass-transfer constraints during the initial 10–15 min of reaction). On the basis of the batch reaction times typically utilized in the industrial-scale biodiesel production plants, the specific energy at 200 rpm was evaluated at a 30 min reaction time (corresponding in our test to an oil conversion of about 92%), whereas the specific energy at 400 rpm was evaluated at 15 min (about 90% conversion in our test). The resulting values of e are 105 J/kgbiodiesel in the first condition (versus 3–6 J/kgbiodiesel for the SM continuous reactor), and 470 J/kgbiodiesel in the second one (versus 25–67 J/kgbiodiesel for the SM plant). Despite the high uncertainties affecting these evaluations, these data seem to suggest that, from an energetic point of view, the utilization of static mixing represents an interesting option in biodiesel production plants. Conclusions The utilization of one single 15 mm SMV static mixer at a 1.3 m/s superficial velocity yielded a profile of sunflower oil conversion versus time equivalent to that obtained in the bestperforming tests with mechanical agitation, indicating the attainment of a transesterification reaction not significantly limited by mass transfer. Even at 0.4 m/s, the SM test resulted in an acceptable biodiesel production rate. The analysis of the energetic consumptions showed that SM perform favorably in comparison with mechanically agitated reactors. This indication was confirmed by the extension of the energetic assessment to biodiesel production facilities of medium and large size. The overall results indicate that static mixing represents an interesting option for the biodiesel production process. The energetic optimization of the transesterification reaction, which can lead to a marked improvement of the biodiesel energy balance, might be further investigated by testing different types of SM, different combinations of impellers, other temporal
Energy & Fuels, Vol. 22, No. 3, 2008 1501
schedules of impeller rotational speed and alternative ways for creating the oil/alcohol dispersion. Acknowledgment. This work was financed by the EU research project “BIOCARD”, contract no. 019829. The authors acknowledge the BIOCARD partner Instituto de Catalisis y Petroleoquimica— CSIC for certifying the biodiesel standards, and Sulzer Chemtech Ltd. for supplying the static mixers.
Nomenclature aV ) interfacial area per unit volume (m2/m3) d32 ) Sauter mean diameter of dispersed phase droplets (m) dh ) hydraulic diameter of the static mixer (m) di ) nominal drop diameter D ) impeller diameter (m) DSM ) static mixer diameter (m) e ) specific energy consumption evaluated at a given oil conversion (J/kgbiodiesel) HL ) liquid height in the reactor (m) Hr ) height of the reactor (m) LSM ) length of a single static mixer (m) N ) impeller rotational speed (min-1) n ) number of samples in a specific reaction test nd,i ) number of drops in size class i ni ) number of impellers nSM ) number of static mixers Ne ) static mixer Newton number (–) Np,j ) impeller power number at time tj (–) p ) probability of the F test Pj ) impeller power consumption at time tj (W) Rei,j ) impeller Reynolds number at time tj (–) ∆pSM ) static mixer pressure drop (Pa) ReSM ) static mixer Reynolds number, defined with respect to the entire pipe holding the SM (–) ReSM,ch ) static mixer Reynolds number, defined with respect to the mixing channels (–) t ) overall time (residence time + ½ loading time) evaluated at a given oil conversion (min) T ) reactor diameter (m) tr ) reaction time (min) VSM ) static mixer superficial velocity (m/s) W ) impeller height (m) Wei ) impeller Weber number (–) WeSM,ch ) static mixer Weber number, defined with respect to the mixing channels (–) Φ ) volume fraction of the dispersed phase (–) µc ) viscosity of the continuous phase (Pa s) µmix,j ) viscosity of the reagent–product mixture at time tj (Pa s) Fc ) density of the continuous (oil) phase (kg/m3) Fmix,j ) density of the reagent–product mixture at time tj (kg/m3) σ ) superficial or interfacial tension (N/m) σrt2 ) variance of the reference test (only mechanical agitation, N ) 700 rpm) (–) σk2 ) variance of test k (–) ωi ) biodiesel mass fraction in sample i Note Added after ASAP Publication. There was an error in equation 2 in the version published ASAP February 28, 2008; the corrected version was published ASAP March 28, 2008. EF700584H
