Biodiesel Production via Transesterification of Soybean Oil Using Acid

Sep 4, 2012 - The parameters affecting the fatty acid methyl esters. (FAME) yield, such as pressure, temperature, and the molar ratio of methanol to s...
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Biodiesel Production via Transesterification of Soybean Oil Using Acid Catalyst in CO2 Expanded Methanol Liquids Zhen Ma, Zi-Yang Shang, En-Jun Wang, Jun-Chen Xu, Qin-Qin Xu, and Jian-Zhong Yin* State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China ABSTRACT: The transesterification of soybean oil and methanol to prepare biodiesel using H2SO4 and solid acid (NaHSO4) as catalysts was conducted in CO2-expanded liquids (CXLs). The aim of adding CO2 in the system is to intensify the reaction, and then to shorten the reaction time and decrease the methanol consumption in the traditional acid catalysis method or decrease the high temperature and pressure in the supercritical methanol method. The parameters affecting the fatty acid methyl esters (FAME) yield, such as pressure, temperature, and the molar ratio of methanol to soybean oil (M/O ratio), as well as catalyst amount were investigated. The results indicated that a complete oil conversion happened at 10 MPa, 70 °C (reaction time 6 h) and 80 °C (reaction time 4 h) with M/O ratio of 12:1, and H2SO4 amount of 4%, while for NaHSO4 catalytic reactions, the FAME yield was 80.94% after 6 h at 10 MPa, 90 °C, and the M/O ratio 9:1, NaHSO4 amount of 5% (based on oil weight). The underlying reasons why CO2 enhances the transesterification of oil and methanol were highlighted.

1. INTRODUCTION Conventional biodiesel processes still suffer from some economic and environmental problems associated with the use of acid or base catalysts.1−5 A newly developed supercritical methanol (scMeOH) method has attracted great attention due to its specific merits such as no catalysts, high reaction rate (5− 10 min), high conversion, and easy separation and purification.6,7 Although this method requires high temperature (350 °C) and pressure (10−50 MPa), which give rise to great challenges for apparatus, the comprehensive economic evaluation is still attractive.8 To make the scMeOH method beneficial to the industry and maintain the advantages of the method while reducing its operating temperatures and pressures, many studies have focused on the intensification of this process.9,10 Modifications include such changes as adding organic solvents or CO2 as cosolvents,11−13 or adding a small amount of alkali catalyst.9,14−16 It was found that the reaction temperature was reduced from 350 °C to 160−220 °C and the pressure was reduced from 10 to 50 MPa to less than 2−4 MPa in these reports. Some papers17,18 have reported a method using CXLs to prepare biodiesel with catalyst recently. It can decrease the experimental temperatures of the modified scMeOH method further. What’s more, with the addition of CO2, a CXLs region (methanol-rich phase) is formed; thus the transesterification of soybean oil and methanol could possibly be a homogeneous reaction in this region. Higher FAME yield could be accomplished with less methanol and shorter reaction time compared to the traditional acid catalysis method. Wyatt et al.18 made use of a supercritical extraction technique and the solubilization ability of CXLs to synthesize FAME from soy flakes and methanol using H2SO4 as a catalyst. The results indicated that reaction rate increased significantly and the amount of methanol was obviously reduced compared to a traditional acid catalysis method. However, the reaction temperature 121 °C is higher than the data in other literatures. Moreover there were no details about the role of CXLs or discussions about the influence of temperature in the © 2012 American Chemical Society

experiment. To study the universality of biodiesel production by transesterification using catalyst in CXLs, this paper investigated the feasibility of using CO2 expanded methanol with H2SO4 and solid acid (NaHSO4) as catalyst to produce biodiesel and discussed the effect of the operation parameters on the FAME yield. Furthermore, the underlying reasons such as enhancement of miscibility, reduction in viscosity, and increase of diffusion coefficients and reaction rate enhancement in the presence of CO2 in the system were discussed.

2. MATERIALS AND METHODS Refined soybean oil (free fatty acid (FFA) content 0.19%, which was measured according to ref 16, Shanghai Fulinmen Food Co. Ltd.) and methanol (Tianjin Lianbang Co. Ltd.) were used as reactants. H2SO4 (98%, Dalian Haiyun Co. Ltd.) and NaHSO4 (Tianjin Kermel Co. Ltd.) were used as catalysts. CaO was obtained from Shenyang Xinxing reagent factory and used to neutralize H2SO4 in the product. CO2 was supplied by Dalian Guangming Gas, Inc. All chemicals were obtained commercially and of analytical grade. Figure 1 is the schematic diagram of experimental apparatus. For a typical reaction, 20 g of soybean oil and a given amount of methanol were charged into an 80 mL stainless steel autoclave, with catalyst amounts of H2SO4 and NaHSO4 ranging from 3% to 7% (based on oil weight), respectively. The autoclave was then kept at a certain temperature by a water bath. After 30 min, CO2 was pumped into the autoclave until the pressure reached a certain value. At the same time, the liquid solution was stirred at a constant speed by a magnetic stirrer. After the prescribed reaction period, the reactor was quickly immersed in an ice-water bath to quench the reaction, and then the CO2 was vented slowly. The product was then neutralized by CaO (just for H2SO4 Received: Revised: Accepted: Published: 12199

May 8, 2012 August 26, 2012 September 4, 2012 September 4, 2012 dx.doi.org/10.1021/ie3011929 | Ind. Eng. Chem. Res. 2012, 51, 12199−12204

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Figure 1. Schematic diagram of experimental apparatus: (1) CO2 tank, (2) regulator, (3) filter, (4) coil, (5) pump, (6) surge tank, (7,11,12) needle valve, (8) reactor, (9) magnetic stirrer, (10) water bath, (13) collecting bottle, (14) gas flow meter.

Figure 3. Schematic diagram of phase behavior of transesterification in CO2-expanded methanol.

immiscible with each other. When CO2 was charged into the reactant mixtures, a CXLs region (methanol-rich phase) is formed; thus a continuum of liquid media ranging from pure methanol phase to compressed CO2 phase is generated by tuning CO2 pressure and a possibly homogeneous transesterification reaction could occur in this region. Then the reaction is controlled by a chemical reaction instead of the interphase mass transfer between methanol and oil and the transesterification reaction is greatly promoted. To understand the influence of the CO2 pressure on the FAME yield, the Peng−Robinson equation of state was used to calculate CO2 mole fraction in methanol, and results are shown in Table 1. It

catalyst) and separated by centrifuge and vacuum distillation. The products were allowed to stand 30 min to delaminate, and the samples from the upper layer were diluted by n-hexane for analysis. The details of the gas chromatography analysis have been described elsewhere.15

3. RESULTS AND DISCUSSION 3.1. Production of Biodiesel Catalyzed by Sulfuric Acid. 3.1.1. Effect of Reaction Pressure on the FAME Yield. Figure 2 presents the variations of the FAME yield with the

Table 1. Solubility of CO2 (xCO2) in Methanol at Various Temperatures and Pressures pressure (MPa)

60 °C

70 °C

80 °C

90 °C

6 8 10

0.27 0.35 0.58

0.24 0.34 0.46

0.22 0.30 0.40

0.20 0.27 0.36

demonstrates that there would be more CO2 dissolution in methanol with increasing pressure at a fixed temperature. Therefore the reaction region shown in Figure 3 will be expanded more at higher pressure, which can explain why the FAME yield at 10 MPa was higher than that at 6 MPa in Figure 2. Song et al.17 have also studied the phase behavior of the glycerol monostearate+methanol+CO2 ternary system. Results show that CO2 can enhance the miscibility of the methanol and glycerol monostearate at suitable pressures. Besides improving the miscibility of the reactants, addition of CO2 could also influence the physical properties of the methanol and oil binary system, such as reducing the viscosity and increasing the diffusion coefficients of the reactants. The diffusion coefficients of oil in CO2-expanded methanol and the viscosity of the mixtures were calculated. The viscosity of the mixtures with or without CO2 was estimated by the Chung− Lee−Starling equation19 which can be used to calculate the viscosity of gas and liquid at both high and low pressures, and the diffusion coefficient was obtained by the Wilke−Chang equation.20,21 It was found that the viscosity of the mixture was decreased from 1.33 × 10−3 Pa·s to 2.62 × 10−5 Pa·s by adding CO2, and the diffusion coefficient of soybean oil and methanol was promoted from 1.26 × 10−10 m2·s−1 and 5.78 × 10−9 m2·s−1 (without CO2) to 3.23 × 10−9 m2·s−1 and 6.30 × 10−8 m2·s−1 (with CO2, 60 °C, 10 MPa), respectively. Lin et al.22 measured the diffusion coefficient of benzonitrile in the CO2-expanded ethanol and employed a modified Wilke−Chang equation to

Figure 2. Effect of reaction pressure on the FAME yield (70 °C, methanol/oil = 9:1, 4% H2SO4).

operation pressure. The reaction pressure was tuned by adding a different amount of CO2 into the reactants. The transesterification reaction was performed at a fixed M/O ratio of 9:1 and 4% H2SO4 at 70 °C. It was observed that the FAME yield was only 46.6% in the absence of CO2 even after 6 h. However, the FAME yield reached 85.3% and 92.4% when the reaction pressure increased to 6 and 10 MPa, respectively, by adding CO2 into the reaction system. Song et al.17 also studied the effect of CO2 on the reaction rate of transesterification of palm oil and methanol with H2SO4 as catalyst, it was found that complete conversion of glycerol monostearate was accomplished at 70 °C, 10.5 MPa within 5 h, while about 35 h was required to obtain similar conversion rate in the absence of CO2. The higher FAME yield after the addition of CO2 into the system is mainly due to the increase of reactants miscibility and reducing of viscosity. Figure 3 shows that there are three phases including the vapor methanol phase, the liquid methanol phase, and the oil phase for the mixture of oil and methanol, from which we could clearly see that methanol and oil is almost 12200

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estimate the viscosity of CO2-expanded ethanol. The results indicate that the benzonitrile diffusion coefficients in the CO2expanded ethanol increase with increasing CO2 mole fraction while the viscosity of CXLs decreases. Houndonougbo et al.23 have conducted MD simulations to calculate the diffusion coefficients and shear viscosities in CO2 expanded acetonitrile, and the same trend was obtained. On the basis of the discussed results, CO2 can enhance the miscibility of the reactants, reduce the viscosity of the reaction mixture, and increase the diffusion coefficients of the reactants; the chemical reaction process could be possibly intensified. 3.1.2. Effect of M/O Ratio on the FAME Yield. The M/O ratio is one of the most important variables affecting the FAME yield. Theoretically, increasing the concentration of the reactant will be beneficial for biodiesel production. To investigate the effect of the M/O ratio on the FAME yield, transesterification was performed at 70 °C, 10 MPa, and 4% H2SO4. As shown in Figure 4, the FAME yield increased evidently with the increase

Figure 5. Effect of reaction temperature on the FAME yield (methanol/oil = 12:1, H2SO4 4%).

methanol and oil in the reaction phase at a higher reaction temperature (i.e., 80 °C in this study) which makes a homogeneous reaction occur and leads to a higher reaction rate. The transesterification reaction is almost conducted completely after 6 h at 70 °C and 4 h at 80 °C, respectively. Taking the comprehensive economic evaluation into consideration, 70 °C is suggested to be the appropriate reaction temperature. 3.1.4. Effect of the H2SO4 Amount on the FAME Yield. To discuss the effect of the H2SO4 amount on the FAME yield, the reaction conditions were set at 70 °C, 10 MPa, and a M/O ratio of 12:1. The results of the experiment are shown in Figure 6.

Figure 4. Effect of molar ratio of methanol to soybean oil on the FAME yield (70 °C, 10 MPa, 4% H2SO4).

of M/O ratio from 6:1 to 12:1. First, the M/O ratio should make the reactants match the stoichiometric ratio (3:1), and methanol should be in excess so as to drive the equilibrium toward products because the reaction is reversible. Second, besides the molar fraction of CO2 (depending on the temperature and pressure of the system) a certain dosage of methanol is essential to obtain the CXLs and a suitable reaction region. 3.1.3. Effect of Reaction Temperature on the FAME Yield. Reaction temperature is another important factor affecting the FAME yield. To understand how the temperature influences the FAME yield, the transesterification reaction was carried out at the M/O ratio 12:1, H2SO4 amount of 4%, with CO2 (10 MPa) and without CO2 at 60, 70, and 80 °C, respectively. Figure 5 shows that the FAME yield increased as the itemperature increased with or without CO2. The FAME yield was greatly increased after adding CO2 into the system, and the increment of the FAME yield with the addition of CO2 at 60 and 70 °C is more significant than that at 80 °C. However, the FAME yield at 80 °C is remarkably higher than that at 60 and 70 °C. Therefore, higher temperatures are still beneficial to the transesterification reaction, particularly for the transesterification reaction in CXLs. The composition of the reaction system of methanol, CO2, and oil is varied when the temperature changed at a fixed pressure. The composition of the reaction system (molar fraction of per component) should 70°C 60°C 80°C °C 80°C 70°C 60°C be X80 MeOH > XMeOH > XMeOH, XCO2 < XCO2 < XCO2 < and Xoil

Figure 6. Effect of the H2SO4 content on the FAME yield. (70 °C; 10 MPa; methanol/oil = 12:1).

When the H2SO4 amount was 1%, the FAME yield was about 75%. While the FAME yield was almost 99% after increasing the amount of H2SO4 to 4%. Freedman24 reported the same reaction conducted at M/O of 30:1, H2SO4 amount of 1% at 65 °C, and the FAME yield was only 60% after 30 h. Wang25 also conducted transesterification of waste cooking oil and methanol at M/O of 16:1, H2SO4 amount of 4% at 95 °C, and the FAME yield was 90% after 10 h. In summary, in contrast with the traditional acid catalyst method, a higher FAME yield by transesterification of soybean oil and methanol in CXLs could be accomplished with less methanol and shorter reaction time. It can be concluded that this technique has potential commercial value. 3.2. Production of Biodiesel Catalyzed by NaHSO4. NaHSO4·H2O is a stable inorganic crystal, the solution of which is strongly acidic, belonging to the Brφnsted acids, so it can catalyze the esterification and transesterification reactions. Furthermore, NaHSO4·H2O is insoluble in organic reaction systems. Compared to homogeneous catalysts (e.g., H2SO4), it

°C °C > X70 > X60 >. It indicates that there would be more oil oil

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3.2.2. Effect of Temperature and Catalyst Amount on the FAME Yield. To investigate the effect of temperature and the catalyst amount on the FAME yield, the reactions were carried out at a fixed M/O ratio 9:1 and a fixed reaction pressure at 10 MPa for 6 h. Figure 8 shows that the FAME yield was

has the advantages such as no corrosion to the apparatus, facile separation from the product, and easy recovery, etc. However, there were no reports about transesterification catalyzed by NaHSO4·H2O to prepare biodiesel as we know, it is meaningful to prove the possibility of using this simple solid catalyst to catalyze the transesterification of oil and methanol in CXLs. 3.2.1. Effect of M/O ratio and CO2 Pressure on the FAME Yield. As described above, the addition of CO2 in the reaction system will enhance the mass transfer of the transesterification reaction by reducing the viscosity and the diffusion coefficients of the reactants. As the amount of CO2 increased in CXLs, the expansion of its volume increased while the polarity as well as the hydrogen bond were weakened.26 A large amount of polar methanol molecules will aggregate and gather around the oil drops; consequently, the reaction rate is accelerated. As seen in Figure 7, there were no positive effects for the FAME yield by

Figure 8. Effects of temperature and catalyst amount on the FAME (10 MPa; methanol/oil = 9:1; 6 h).

promoted by higher temperature. In the presence of 5% catalyst amount, the FAME yield was about 19% at 60 °C while it was 80.94% at 90 °C, and the FAME yield increased 3 times from 60 to 90 °C. The increase of the catalyst amount also had a positive effect on the FAME yield when the catalyst amount was less than 5%. However, the FAME yield was decreased when the catalyst amount was up to 7%. It seems like 5% of NaHSO4 was the optimum catalyst amount at temperatures from 70 to 90 °C, but the FAME yield did not show maxima with the catalyst amount when H2SO4 was used as catalyst. It was assumed that CH3ONa is introduced when NaHSO4 is added as catalyst in the system of methanol and soybean oil. Therefore, Na+ becomes one of the active centers in the transesterification of triglycerides with alcohols. In addition, NaHSO4 shows the Brφnsted acid catalysis characteristic with the crystal water which is similar to H2SO4. When the amount of catalyst is increased redundantly, the active centers mentioned above may be capped with the excess catalyst making the catalyst poisoning lead to a lower yield of FAME. The highest yield was obtained at 90 °C with 5% catalyst amount in our experiment. 3.2.3. Effect of Reaction Time on the FAME Yield. To investigate the effect of reaction time on the FAME yield, the transesterification reactions were carried out at a fixed M/O ratio of 9:1, NaHSO4 amount of 5% at 10 MPa, and 80 and 90 °C, respectively. As seen in Figure 9, at a reaction temperature

Figure 7. Effects of molar ratio of alcohol to oil and CO2 pressure on the FAME (90 °C; 5% NaHSO4; 6 h).

increasing CO2 pressure when the M/O ratio was equal to the stoichiometric ratio. But when the M/O ratio was higher than the stoichiometric ratio, the FAME yield increased with the increasing CO2 partial pressure. When the M/O ratio was 9:1, the FAME yield was 66.84% at 6 MPa and 80.93% at 10 MPa (the relative increment of FAME yield was 21.1%). However, the FAME yield increased slightly or even decreased when the CO2 pressure was promoted to a higher value. Experiments showed that the FAME yields were 81.36% at 12 MPa and 73.24% at 14 MPa, respectively. The M/O ratio is an important factor for biodiesel production. Because a large M/O ratio could shift the reaction equilibrium to the biodiesel production side and accelerate the reaction rate, the M/O ratio in the experiment is usually much greater than the stoichiometric ratio especially for the supercritical method (42:1).13 While taking commercial feasibility and environmental problems into consideration, it is worth discussing how to lower the M/O ratio to the economic level. As shown in Figure 7, when the M/O ratio was less than 9:1, the FAME yield increased as the increasing M/O ratio, while the FAME yield decreased slightly and then was maintained at some level even after a long reaction period. However, if the M/O ratio was larger than 9:1, the mixture of reactants would be diluted by a large amount of methanol, which led to lower reaction rate. A study on biodiesel preparation from Chlorella protothecoides also showed that excess methanol in large quantities reduced the amount of products and complicated the separation process of glycerol and FAME.27 The suggested pressure and M/O ratio are 10 MPa and 9:1, respectively.

Figure 9. Effect of reaction time on the FAME yield (10 MPa; methanol/oil = 9:1; 5% NaHSO4; 6 h). 12202

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of 90 °C, the reaction can be divided into three stages according to the reaction time. Initially, the yield increased rapidly within 6 h, which was a quick reaction stage, while the yield changed slowly after 6 h. The yield increased only by 8% when the reaction time increased from 6 to 10 h. After 10 h, the reaction was very slow and the yield stayed at an almost constant value from 10 to 12 h. A similar trend is observed when the temperature is 80 °C. It can be seen from Table 2 that the FAME yield is lower with the higher M/O ratio using NaHSO4 as catalyst without

temperature (°C)

catalyst amount (wt %)

3:1

90

5

6:1

90

5

9:1

90

5

time (h)

yield (%)

6 9 12 6 9 12 6 9 12

51.5 58.8 63.1 39.6 55.2 61.8 2.4 39.2 45.2

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ABBREVIATIONS CXLs = CO2 expanded liquids FAME = fatty acid methyl esters FFA = free fatty acid M/O ratio = molar ratio of methanol to soybean oil scMeOH = supercritical methanol REFERENCES

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CO2, and the highest FAME yield is only 63.1% at a reaction time of 12 h. The investigation of the effect of catalyst amount on the FAME yield shows that when the reaction temperature is 90 °C, the M/O ratio is 3:1, the FAME yield was less than 65% with the catalyst of 9 wt %. Compared with the transesterification of oil and methanol in CXLs, the reaction without CO2 needs more catalyst and longer reaction time.

4. CONCLUSION The transesterification of methanol and soybean oil, catalyzed by H2SO4 or NaHSO4, could be greatly enhanced in CXLs medium compared to the traditional acid catalysis method, and the severe operating conditions were also remarkably improved compared to the scMeOH method. The transesterification reactions can be conducted completely with an M/O ratio of 12:1, 4% H2SO4, at 10 MPa, 70 °C, 6 h and 80 °C, 4 h, respectively. The FAME yield was 80.94% at 10 MPa, 90 °C, 6 h with an M/O ratio of 9:1, as well as 5% NaHSO4. The effects of CXLs is to tune the phase behavior of the reactant system, allowing the reaction to occur under a homogeneous or quasihomogeneous phase state. Then the transesterification in the reaction region is controlled by the chemical reaction instead of controlled by mass transfer like the reaction conducted using only methanol and oil. The present results are possibly helpful to lower the biodiesel production cost and enhance the process of biodiesel production for the industrial biodiesel applications; nevertheless, how to recover the catalyst easily should be the subject of future studies.

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ACKNOWLEDGMENTS

This work was financially supported by National Natural Science Foundation of China (20976026, 20976028) and the Natural Science Foundation of Liaoning Province (20102030).

Table 2. Effect of M/O Ratio and Reaction Time on FAME Yield without CO2 Using NaHSO4 as Catalyst M/O ratio

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Address: Jianzhong Yin, School of Chemical Machinery, Dalian University of Technology, Linggong Road 2, Dalian, 116024, Liaoning Province, China. Tel: +86-411-84986274. Fax: +86-411-84986274. Notes

The authors declare no competing financial interest. 12203

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