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Ind. Eng. Chem. Res. 2009, 48, 1068–1071
Simulation of Biodiesel Production through Transesterification of Vegetable Oils Scott Stiefel and Gustavo Dassori* Archer Daniels Midland, ADM Sustainable BioEnergy Modeling Center, 2021 South First Street, Suite 112, Champaign, Illinois 61821
Biodiesel can be produced from a number of natural, renewable sources, but vegetable oils are the main feedstocks. Most existing biodiesel plants currently rely upon the use of a homogeneous catalyst in a continuous reactor system, using the transesterification of soybean or rapeseed oil with methanol into alkyl esters. A key differentiation characteristic among the existing processes is that of the mixing pattern employed in the system. The present study compares reactor performances in terms of biodiesel yields for plug flow and complete mixing behaviors, along with the use of interstage phase separation. It is concluded that plug flow pattern shows a distinctive benefit in terms of yields and reactor volume reduction compared to complete mixing. Interstage separation improves the reacting system yields when proper phase separation can be achieved, maximizing glycerine removal from the downstream system. Staged mechanically stirred tank reactors can reach similar performances as plug flow behavior systems without resorting to interstage separation under the conditions here studied. Methanol/oil ratio can improve the reacting system performance and reduce differences, even if plug flow reactors are not employed. Introduction Biodiesel is a bioenergy alternative to petroleum fuels. It can be produced from a number of natural, renewable sources, but vegetable oils are the main feedstocks. Outstanding benefits of biodiesel are the high cetane numbers that it naturally achieves and the lack of polluting heteroatoms like sulfur or nitrogen. Cetane number increase and sulfur/nitrogen content reduction are the most capital intensive hydrotreatment processes that a conventional refinery has to handle, a fact that lays the ground for the biodiesel niche in the near-future refining business. Most existing biodiesel plants currently rely upon the use of a homogeneous catalyst in a continuous reactor system, using the transesterification of soybean or rapeseed oil with methanol into alkyl esters. Diglycerides and monoglycerides are the intermediates in this process, and glycerol is a major byproduct.1 These reactions are affected by alcohol/oil feed ratio, free fatty acids, moisture, catalyst concentration, space velocity, temperature, and mixing. Although the relevance of biodiesel production is self-evident, a better understanding of process operation and optimization based on kinetic models that account for all of the above parameters is still needed. Several studies have been carried out for soybean oil transesterification.2-4 Optimization of the amount of alcohol and base catalyst was studied, aiming for reduced alcohol usage, varying all other reactor and process parameters. There is a clear incentive for minimizing alcohol handling because of downstream separation costs. Different degrees of mixing have also been evaluated for their impact upon biodiesel yields and selectivity. There are two distinctive mixing patterns that are currently employed: complete mixing when using stirred tank reactors and plug flow when using pipe reactors. Also, continuous multistage mechanically agitated reactors have been recently studied.5 Biodiesel production economics is constrained by the virgin vegetable oil cost. For instance, in the case of soybean oil, raw materials constitute 88% of the overall production cost.6 This substantial impact calls for reducing production costs and biodiesel process optimization. * To whom correspondence should be addressed. Fax: (217) 6934035. E-mail:
[email protected].
There are numerous papers related to the kinetics of biodiesel production and the process economics, but no attempt has been made so far to link the kinetics to the reactor embodiment and its respective performance. Therefore, the analysis of the reactor mixing pattern that maximizes biodiesel yield can provide fruitful insights into process improvements that can be realized for the homogeneous transesterification of refined vegetable oils. The present study is focused on the mixing pattern effect upon process performance, coupled with an analysis of interstage separation impact upon overall production. Open literature data3 is used for the kinetic parameters, and the model is built within a process simulator environment.7 Interstage separation efficiencies are fixed according to literature information.6 Model Formulation. Kinetic parameters for soybean oil transesterification have been obtained from Noureddini and Zhu,3 who used a methanolic solution of NaOH as catalyst. In particular, the following reaction system, reaction rate equations and kinetic parameters were employed: k1
TG + CH3OHS DG + R1COOCH3 k2
(I)
k3
DG + CH3OHS MG + R2COOCH3 k4
(II)
k5
MG + CH3OHS GL + R3COOCH3
(III)
-rI ) k1CTGCCH3OH - k2CDGCR1COOCH3
(1)
-rII ) k3CDGCCH3OH - k4CMGCR2COOCH3
(2)
-rIII ) k5CMGCCH3OH - k6CGLCR3COOCH3
(3)
k6
with:
where TG represents a triglyceride, DG a dygliceride, and MG a monoglyceride, and GL represents glycerol. RiCOOCH3 are the respective methyl esters (biodiesel). Also ki represents the respective kinetic constant that follows the Arrhenius equation: ki ) k∞i e-E⁄RT
10.1021/ie8005512 CCC: $40.75 2009 American Chemical Society Published on Web 09/03/2008
(4)
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Table 1. Kinetic Parameters reaction TG f DG DG f TG DG f MG MG f DG MG f GL GL f MG
activation energy (kcal/mol) k values at 50 °C (1/mol · min) 13.145 9.932 19.860 14.639 6.421 9.588
0.050 0.110 0.215 1.228 0.242 0.007
where E stands for the activation energy, R is the ideal gas constant, T is the absolute temperature, and ki∞ is the preexponential factor. The kinetic parameters values used in this study are as shown in Table 1 above. Glycerides were represented by pure components within the process simulator. These species are the same ones as used by Haas et al.6 Those are triolein (C57H104O6) for triglycerides, diolein (C39H72O5) for diglycerides, monoolein (C21H40O4) for monoglycerides, and methyl oleate (C19H36O2) for all methyl esters. The consistency of the model built in Aspen Plus7 was verified by comparing the predicted results using a batch mode model with the literature experimental data,3 as can be seen in Figure 1. The BATCH reactor option (a default option provided by the process simulator) was employed in order to check that for the reacting conditions used by the quoted authors we could verify a good prediction of the main product concentrations they obtained in their laboratory. It was done this way because the original data by Noureddini and Zhu3 was generated in a batch reactor. The oil and products were characterized by the same method as in the case of the continuous reactors. Oil, methanol, and catalyst addition were used as referred in the paper by Noureddini and Zhu,3 as well as reaction conditions. All cases considered that sodium hydroxide is used as catalyst with a concentration of 0.2 wt % based on the mass of the soybean oil, here represented by triolein. Free fatty acids and water content in the feed are considered to be small enough in refined oil feedstocks to neglect any saponification effect downstream in the process as it is considered in Noureddini and Zhu3 and Vicente et al.8 However, if significant water and free fatty acids content is initially present, as in crude or used oil, the catalyst consumption through the saponification reaction will require a different analysis. In all liquid-liquid separators, the weight fractions in the heavy phase referred to the incoming component mass flow rate in the feed to said unit were taken as in Haas et al.6 original computer model: oil 0.001 as in Haas et al,6 diglycerides 0.001 same as oil in Haas et al.;6 monoglycerides 0.001 same as oil in Haas et al.;6 glycerol 0.94 as in Haas et al.;6 methanol 0.4 as in Haas et al.;6 sodium hydroxide 0.4 as in Haas et al.;6 biodiesel 0.001, Haas et al.6 considered it to be 0.0. Results and Discussion Continuous processes for vegetable oil transesterification are based on continuous mixed tank reactors6 with interstage separation or pipe reactors9 with interstage separation. This separation is intended for increasing methyl esters production by removing the glycerol produced at each stage, thus shifting the equilibrium in the set of reactions toward the biodiesel final product. The efficiency in the interstage separation is a key issue in these processes, because the reverse reactions can prevail under certain scenarios, particularly in the downstream stages, where di- and monoglycerides are present in the inlet stream. Some research has been focused on phase distribution data collection for this kind of system;10,11 however, more work in this area is still needed.
Figure 1. Model validation. Comparison with Noureddini and Zhu3 data (Figure 5 in referenced source). Transesterification of soybean oil at 50 °C, 1 atm in a batch reactor.
Figure 2. Two-stage reactor schemes.
The present analysis on mixing patterns is based on evaluating five different arrangements for biodiesel production, keeping catalyst concentration and pressure fixed. Under these conditions, there were considered as reactor systems a continuous stirred tank reactor (CSTR), a pipe flow reactor (PFR), a 6-CSTR staged mechanically agitated reactor, two CSTRs with intermediate liquid/liquid separation, and two PFRs with intermediate liquid-liquid separation. This set covers the major process options used today. For the two-stage cases with intermediate separation, in order to have the same initial sodium hydroxide concentration in the second stage as in the first one (0.2 wt % based on fed oil) we introduced, as can be seen in Figure 2, an additional feed of NaOH after the interstage separation to compensate for the base loss in said operation as it is usually done in practice. Temperature Effect. Soybean oil transesterification is usually carried out at temperatures just below the methanol boiling point, when methanolysis is the selected route. Figure 3 shows the different performances for the systems under study when temperature is varied within normal operating ranges, keeping space velocity and the molar methanol/oil ratio fixed, for the NaOH 0.2 wt % case. The PFR with interstage separation has a better response to temperature increments than the single PFR or the staged CSTR. In fact, it shows a better performance when temperature gets closer to the methanol normal boiling point than at lower temperatures. However, there is no significant difference for the CSTR with interstage separation in comparison with the single CSTR for temperature increments. It is clear
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Figure 3. Biodiesel yield vs temperature. LHSV ) 1 h ; P ) 1 atm; MeOH/oil molar ratio, 6. Maximum represents the maximum attainable biodiesel fraction yield.
Figure 5. Biodiesel yield vs MeOH/oil molar ratio. Temperature, 60 °C; P ) 1 atm; space velocity, 1 h-1. Maximum represents the maximum attainable biodiesel fraction yield.
Figure 6. Minor species mass fraction vs temperature. LHSV ) 1 h-1; P ) 1 atm; MeOH/oil molar ratio, 6. Figure 4. Biodiesel yield vs space velocity. Temperature, 60 °C; P ) 1 atm; MeOH/oil molar ratio, 6. Maximum represents the maximum attainable biodiesel fraction yield.
that the systems with plug flow behavior present a better performance than the ones with complete mixing. Space Velocity Effect. Usual space velocity ranges are within 0.5-2 h-1, depending on the catalyst concentration and the effectiveness of phase separation. Figure 4 shows the different performances for the systems under study when space velocity is varied within 0.5-2 h-1, keeping temperature and the molar methanol/oil ratio fixed, for the NaOH 0.2 wt % case. The PFR with interstage separation is more sensitive to LHSV increments than the single PRF or the staged CSTR, showing some decay at higher space velocity. However, there is a slight improvement for the CSTR with interstage separation in comparison with the single CSTR for LHSV increments. This overall reduction in yield can be compensated by increasing catalyst concentration. Methanol/Oil Molar Ratio Effect. The reversible nature of the transesterification reacting system calls for an excess methanol concentration in order to shift the reactions toward maximum methyl esters production. However, from a practical viewpoint, there are incentives to keep methanol levels low because of the increasing costs of downstream methanol separation. Figure 5 shows the different performances for the systems under study when the methanol/oil molar ratio is varied between 6 and 20, keeping temperature and space velocity fixed,
for the NaOH 0.2 wt % case. With the exception of 1 CSTR for the methanol range here studied, all other configurations can minimize differences when a very high methanol feed is used. However, the preferred methanol/oil range used in commercial units is around 5-6, because of associated separation costs. It can be seen again that those systems showing a plug flow behavior have a distinctive advantage over those with complete mixing for the catalyst concentration considered here. These differences can be amplified when interstage separation is not good enough to remove most of the glycerol produced, because downstream reactors could run under conditions where the reverse reactions prevail. Intermediate Glycerides Production. The main target for soybean transesterification is the production of methyl esters, that is, optimizing its yield. Some reacting systems can achieve high soybean conversions, but at the expense of producing more intermediate di- and monoglycerides, rather than going all the way to the final methyl esters desired product. In Figure 6 is examined the final outlet stream composition in terms of unreacted oil and the intermediate glycerides for the cases of one PFR and two PFRs with interstage separation. The PFR with interstage separation is more effective in converting oil into biodiesel compared with the single PFR, because the latter tends to accumulate more di- and monoglycerides, even when reaching higher oil conversions. Systems with Interstage Separation. A sharper distinction among the two systems with interstage separation can be
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shows a 15% increase in biodiesel yield for a two-stage plug flow reactor versus the two-stage mixed tank case, when using an interstage separator. The case is for a temperature of 60 °C, methanol/oil ratio of 6, and LHSV equal to 1 h-1. This increase in yield directly translates into a proportional increase in benefits when using a plug flow reactor instead of a mixed tank reactor, assuming a similar capital investment for both cases. Consequently, one will obtain a better net present value and internal rate of return for the prevailing case. Conclusions
Figure 7. Volume and temperature effects for 2 CSTRs vs 2 PFRs with interstage separation. LHSV ) 1 h-1; P ) 1 atm; MeOH/oil molar ratio, 6.
The plug flow pattern in transesterification reactions for biodiesel production shows a distinctive benefit in terms of yields and reactor volume reduction. Interstage separation improves the reacting system yields when proper phase separation can be achieved, maximizing glycerine removal from the downstream system. Staged mechanically stirred tank reactors can reach performances similar to that of plug flow behavior systems without resorting to interstage separation under the conditions here studied. Reverse reactions can prevail when interstage separation is not efficient enough at removing the glycerine coproduct. Methanol/oil ratio can improve any reacting system performance and reduce differences, even if plug flow reactors are not employed. Literature Cited
Figure 8. Volume and space velocity effects for 2 CSTRs vs 2 PFRs with interstage separation. Temperature, 60 °C; P ) 1 atm; MeOH/oil molar ratio, 6.
obtained when comparing them in terms of total reactor volume for fixed conditions. This is that both cases are run for the same total reactor volume and biodiesel yields are computed for similar scenarios. Figure 7 shows that for fixed pressure, catalyst concentration, and space velocity, there is a significant difference in terms of volume and temperature for achieving the same biodiesel yield. In Figure 7 it is shown that 2 CSTRs with interstage separation at 60 °C need twice as much volume as 2 PFRs with interstage separation running 10 °C below for obtaining the same biodiesel yield. If the comparison is made by changing the space velocity, as is shown in Figure 8, then it can be observed that at half the reactor volume for 2 PFRs with interstage separation (2 h-1) one needs to reduce by half the space velocity (1 h-1) for the same methyl esters yield when using 2 CSTRs with interstage separation. The difference in reactor performance because of the mixing pattern, for the same space velocity, allows us to drive some economic conclusions by comparing obtained results. Figure 7
(1) Ma, F.; Hanna, M. A. Biodiesel production: a review. Bioresour. Technol. 1999, 70, 1. (2) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification Kinetics of Soybean Oil. J. Am. Oil Chem. Soc. 1986, 63, 1375. (3) Noureddini, H.; Zhu, D. Kinetics of Transesterification of Soybean Oil. J. Am. Oil Chem. Soc. 1997, 74, 1457. (4) Morgenstern, M.; Cline, J.; Meyer, S.; Cataldo, S. Determination of the Kinetics of Biodiesel Production Using Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR). Energy Fuels 2006, 20, 1350. (5) Leevijit, T.; Tongurai, C.; Prateepchaikul, G.; Wisutmethangoon, W. Performance test of a 6-stage continuous reactor for palm methyl ester production. Bioresour. Technol. 2008, 99, 214. (6) Haas, M. J.; McAloon, A. J.; Yee, W. C.; Foglia, T. A. A process model to estimate biodiesel production costs. Bioresour. Technol. 2006, 97, 671. (7) Aspen Plus, Version 12.1, 2007. Aspen Technology, Inc., Cambridge, MA, 02141. (8) Vicente, G.; Martinez, M.; Aracil, J.; Esteban, A. Kinetics of Sunflower Oil Methanolysis. Ind. Eng. Chem. Res. 2005, 44, 5447. (9) Connemann, J.; Krallmann, A.; Fischer, E. Process for the continuous production of lower alkyl esters of higher fatty acids. U.S. Patent 5,354,878, 1994. (10) Komers, K.; Tichy, J.; Skopal, F. Ternares Phasendiagramm Biodiesel-Methanol-Glyzerin. J. Prakt. Chemie Chemiker 1995, 337, 328. (11) Chiu, C.-W.; Goff, M. J.; Suppes, G. J. Distribution of Methanol and Catalysts between Biodiesel and Glycerin Phases. AIChE J. 2005, 51, 1274.
ReceiVed for reView April 6, 2008 ReVised manuscript receiVed July 15, 2008 Accepted July 21, 2008 IE8005512
