Biodiesel production from highly unsaturated feedstock via simultaneous transesterification and partial hydrogenation in supercritical methanol

Biodiesel production from highly unsaturated feedstock via simultaneous transesterification and partial hydrogenation in supercritical methanol

J. of Supercritical Fluids 82 (2013) 251–255 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.els...

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J. of Supercritical Fluids 82 (2013) 251–255

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu

Biodiesel production from highly unsaturated feedstock via simultaneous transesterification and partial hydrogenation in supercritical methanol Hee-Yong Shin a , Jae-Hun Ryu a , Seong-Youl Bae a,∗ , Young Chai Kim b a b

Department of Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Korea Department of Chemical Engineering, Hanyang University, Seoul 133-781, Republic of Korea

a r t i c l e

i n f o

Article history: Received 18 December 2012 Received in revised form 29 July 2013 Accepted 5 August 2013 Keywords: One-pot process Biodiesel Transesterification Partial hydrogenation Supercritical methanol

a b s t r a c t In this study, a supercritical one-pot process combining transesterification and partial hydrogenation was proposed to test its technical feasibility. Simultaneous transesterification of soybean oil and partial hydrogenation of polyunsaturated compounds over Cu catalyst in supercritical methanol was performed at 320 ◦ C and 20 MPa. Hydrogenation proceeded simultaneously during the transesterification of soybean oil in supercritical methanol, and hydrogenation occurred during the reaction despite the absence of hydrogen gas. The polyunsaturated methyl esters obtained in the biodiesel were mainly converted to monounsaturated methyl esters by partial hydrogenation. Key properties of the partially hydrogenated methyl esters were improved and complied with standard specifications for biodiesel. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel has recently attracted a great deal of attention due to its environmental benefits and molecular similarities to petroleum diesel. In general, biodiesel consists of fatty acid methyl esters (FAMEs) and is produced from renewable sources such as vegetable oils or animal fats. As the production of biodiesel has been increasing worldwide, the use of newly available feedstock different from established raw materials is essential. However, the use of alternative feedstock may not be compatible with typical transesterification technologies, such as highly acidic or unsaturated oils and fats [1]. The fatty acid composition of the feedstock has a significant impact on the properties of the biodiesel obtained because the fatty acid profile in the feedstock is identical to that of the resulting biodiesel [2]. According to previous research, the presence of polyunsaturated fatty esters leads to low oxidation stability and low cetane numbers [1–4], while high content of fully saturated fatty esters has a negative effect on cold flow properties [1,2,4]. It has been reported that monounsaturated methyl esters such as methyl oleate (18:1) and methyl palmitoleate (16:1) are the ideal components of biodiesel [1,2,5]. Therefore, partial hydrogenation

∗ Corresponding author. Tel.: +82 31 400 5272; fax: +82 31 406 2406. E-mail address: [email protected] (S.-Y. Bae). 0896-8446/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2013.08.004

of polyunsaturated FAMEs to monounsaturated compounds can substantially increase fuel quality. Partial hydrogenation reactions catalyzed by nickel [6] and noble metal-based catalysts [7–10] have recently been tested to improve biodiesel fuel quality, but showed rather low monounsaturated FAME selectivity. On the other hand, copper catalysts have good activity and selectivity in partial hydrogenation of highly unsaturated FAMEs, which reduces the degree of unsaturation without increasing the fully saturated methyl esters [1,11]. It should be pointed out, however, that previous studies employed two separate processes, i.e., transesterification for biodiesel production followed by partial hydrogenation of FAMEs, which significantly increases the cost of biodiesel production. Supercritical methanol techniques for the synthesis of biodiesel have been studied over the past decade due to their advantages over conventional homogeneous basic catalysis [12–23]. Supercritical methanol can react with triglycerides and free fatty acids without catalysts and can provide a high reaction rate to produce biodiesel. As a result, the purification of products after reaction is much simpler, no wastewater is generated, and high biodiesel yield is attained in a very short time. For these reasons, this supercritical methanol technique has been found to be more economically viable [24–26] and environmentally friendly than the use of conventional homogeneous basic catalysis. If partial hydrogenation of polyunsaturated compounds can occur simultaneously during supercritical transesterification, this one-pot process would be a cost effective and environmentally

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P

2 1

3

2

T1

TC Electric furnace

T2 Methanol

Magnetic bar

Cu Oil 4

as the carrier gas. The treated liquid product was filtered to separate the catalyst, and excess methanol was removed with a rotary evaporator. Layer separation was then performed to separate the biodiesel and glycerin phases. The top ester layer was collected, and the FAME content (%) was analyzed using the European Standard EN 14103 [27]. FAME content was analyzed by a gas chromatograph (Agilent, HP-6890) with FID and a capillary column (Agilent, HP-INNOWAX). The identification of FAMEs in biodiesel was performed by comparing the retention times with those of authentic standards. A solution of methyl heptadecanoate in heptane was used as an internal standard for quantification. The degree of hydrogenation was determined by the degree of unsaturation (DU) of biodiesel, which was calculated from the following equation: DU = methyl oleate (wt.%) + 2 × methyl linoleate (wt.%)

1. Autoclave reactor 2. Shut -off valve 3. Pressure gauge

4 . Hot plate with magnetic stirrer T1,T2 : Thermocouple TC: Temperature controller

Fig. 1. Experimental apparatus used for the supercritical one-pot process combining transesterification and partial hydrogenation.

benign process. We report here the discovery of a supercritical one-pot process combining transesterification and partial hydrogenation for biodiesel production from highly unsaturated feedstock.

+ 3 × methyl linolenate (wt.%) Iodine values of biodiesel samples were evaluated according to EN 14111 [28]. The oxidation stability and cold filter plugging point (CFPP) of the samples were predicted by using the correlations between these properties and fatty acid content of biodiesel samples [4]. Cetane numbers of biodiesel samples were determined by using a quadratic correlation with the number of carbon atoms in the original fatty acid and the number of double bonds [29]. All of the experiments were replicated three times and the mean average values were used as experimental data.

2. Experimental 3. Results and discussion Refined soybean oil and copper particles (99.7% pure, Sigma–Aldrich 357456, Korea) were used as a model compound of highly unsaturated feedstock and catalyst, respectively. The catalyst was used in powder form, and its particle size was 3 ␮m. Methanol was used as the supercritical reaction medium. The critical temperature and pressure of methanol are 239.4 ◦ C and 8.09 MPa, respectively. A mixture of FAMEs (Sigma–Aldrich, Korea) was used as a biodiesel standard, and methyl heptadecanoate (Sigma–Aldrich, Korea) was used as an internal standard for the analysis of FAMEs. Fig. 1 shows the experimental apparatus used for the supercritical one-pot process combining transesterification and partial hydrogenation. The reaction system consists of a 25 ml autoclave made of 316 grade stainless steel, K-type thermocouples (T1, T2) for sensing temperature, a pressure gauge, a magnetic stirrer, an electric furnace, and a PID temperature controller. Soybean oil (4.61 g) was loaded into an autoclave with a prescribed amount of methanol and Cu catalyst. The molar ratio of methanol to oil and the amount of catalyst were fixed at 45 and 10% (wt. catalyst/wt. oil), respectively. Argon was used to purge the autoclave, which was subsequently filled with hydrogen until the initial pressure of the reactor reached 1 MPa. The reactor was heated to a desired reaction temperature with a heating rate of 15 ◦ C/min. The final temperature was controlled within ±2 ◦ C for a set time. Zero reaction time was defined as the point when the reaction temperature reached the set-point, and the reaction pressure was controlled by varying the volumes of methanol and oil fed into the reactor. Pressure inside the vessel was monitored using a pressure gauge. The agitation speed was adjusted at 1000 rpm. After reaching the prescribed reaction conditions, the reaction was continued for a certain period of time that had been set in advance. When the reaction finished, the reactor was moved to a water bath. After the vessel was rapidly cooled to room temperature, gaseous product was collected using a Tedlar bag and its volume was measured by water displacement to calculate the weight of collected gas. The gaseous product was also analyzed by gas chromatograph (Agilent, HP-6890) equipped with TCD and a packed column (Carbosphere 80/100). Argon was used

To obtain a high content of FAME through supercritical transesterification, the effects of operating parameters such as temperature, pressure, reaction time, and molar ratio of methanol to oils are crucial issues. In recent years, operating conditions for high conversion efficiency of lipid to biodiesel with supercritical methanol have been reviewed [19,22], and this supercritical methanol process has been statistically optimized [30,31]. Also, preventing thermal degradation of FAMEs is one of the most important concerns in supercritical high-yield biodiesel production. Polyunsaturated FAMEs are much less stable than saturated and monounsaturated FAMEs in supercritical methanol [32]. In our previous study [33], the thermal stability of FAMEs in supercritical methanol was evaluated, and the results demonstrated that transesterification in supercritical methanol should be performed below 325 ◦ C (at 23 MPa) to avoid thermal degradation of polyunsaturated FAMEs. In the present study, 320 ◦ C, molar ratio of 45 (methanol/oil) and 20 MPa were selected as appropriate reaction conditions to avoid the thermal decomposition and to achieve the high content of FAMEs. The following reactions were performed to validate the potential of supercritical one-pot synthesis at the fixed reaction conditions: (1) non-catalytic transesterification of soybean oil in supercritical methanol with H2 (reaction A) or without H2 (reaction B), and (2) simultaneous transesterification of soybean oil and partial hydrogenation of polyunsaturated compounds over Cu catalyst in supercritical methanol with H2 (reaction C) or without H2 (reaction D). Fig. 2 shows changes of FAME content and DU of biodiesel obtained by the reactions used in this study. In Fig. 2(A) and (B), FAME contents in biodiesel obtained by non-catalytic processes rapidly increased for 15 min and then increased slightly up to approximately 98% before remaining almost constant. As expected, the decomposition of FAMEs was not detected during the reaction. The values of DU were unchanged as the reaction proceeded, which means that no hydrogenation occurred without Cu catalyst. It was observed from Fig. 2(C) that hydrogenation proceeded

H.-Y. Shin et al. / J. of Supercritical Fluids 82 (2013) 251–255 100

253

160 (A)

90

The degree of unsaturation

FAME content (%)

140 80 70

(B) (A)

60 50 40

(B) (D)

120

(C)

100

80

(D)

60

(C)

30 0

10

20

30

40

50

0

10

20

Time (min)

30

40

50

Time (min)

Fig. 2. Changes of FAME content and DU of biodiesel obtained by the reactions used in this study: reaction A (A), reaction B (B), reaction C (C), and reaction D (D) (all reactions were performed at 320 ◦ C, 20 MPa).

simultaneously during the transesterification of soybean oil in supercritical methanol. Both reaction rates increased considerably for 30 min and then increased slowly thereafter. The FAME content had reached around 90% and 92.3% after 30 min and 40 min, respectively. The transesterification reaction rates of supercritical one-pot processes (reactions C and D) were considerably slower than those of the non-catalytic supercritical processes (reactions A and B). Interestingly, in the case of reaction D, hydrogenation occurred during the reaction despite the absence of hydrogen gas, and both the hydrogenation and transesterification rates of reactions C and D were almost the same. From this result, it is inferred that, during reaction D, hydrogen gas may be generated by methanol decomposition: CH3 OH → CO + 2H2 Thermal decomposition of methanol over Cu catalyst in the supercritical state was carried out to confirm the generation of hydrogen. Methanol and copper particles were loaded into an autoclave and were treated under the same reaction conditions as were used in the supercritical one-pot process, and then the gaseous product was analyzed and compared with that of reaction D. Chromatograms of gaseous product for the decomposition of methanol and reaction D are shown in Fig. 3. From Fig. 3(A) and (B), the products were mainly composed of H2 , CO, and CO2 in both cases, which are main products of methanol decomposition over Cubased catalysts at higher temperatures above 300 ◦ C [34,35]. In most cases of methanol decomposition using Cu-based catalysts, methyl formate, dimethyl ether, methane, and CO2 can be produced as by-products [34–41]. In general the largest by-product is methyl formate and it decomposes further into H2 , CO or CO2 with increasing reaction temperature [34,35]. It can be assumed that hydrogen generated by methanol decomposition was used for the simultaneous hydrogenation during transesterification in supercritical methanol, indicating that the supercritical one-pot process does not require additional H2 supply for hydrogenation. The molar ratio of methanol to oil generally has a significant effect on the reaction rate in supercritical transesterification [13–15,21,30]. Thus, it is expected that the FAME content will be reduced due to the decrease in molar ratio of methanol to oil when methanol decomposition occurs during supercritical onepot process. In order to estimate the changes in the amount of methanol, we collected the gaseous product after reaction D (at 30 min) and measured the volume of the gas mixture (molar ratio of H2 :CO:CO2 = 2:0.3:0.15). The weight of collected gas was calculated using the ideal gas law and its value was 0.116 g, which indicates

that the amount of remained methanol is 7.584 g (initial amount of methanol: 7.7 g) after methanol decomposition. This shows that the molar ratio of methanol to oil was almost unchanged (from 45 to 44.37), indicating that methanol decomposition scarcely affects the FAME yield in this supercritical one-pot process. Table 1 shows fatty acid compositions of refined soybean oil and biodiesel samples obtained by the reactions used in this study. The fatty acid compositions of biodiesel obtained by reactions A and B were similar to that of refined soybean oil. This also indicates that no hydrogenation proceeded during the supercritical transesterification without Cu catalyst. As shown, fatty acid compositions of biodiesel obtained by reactions C and D were almost identical regardless of the presence of hydrogen. The compositions of polyunsaturated FAMEs decreased considerably, whereas the contents of monounsaturated and saturated FAMEs increased. The

1

2

3

(A)

3

(B)

1

2

0

5 10 15 Retention time (min)

20

Fig. 3. Chromatograms of gaseous product for the decomposition of methanol (methanol decomposition over Cu catalyst at 320 ◦ C, 20 MPa for 30 min) (A) and reaction D (supercritical one-pot process without H2 at 320 ◦ C, 20 MPa for 30 min) (B): H2 (1), CO (2), CO2 (3).

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Table 1 Fatty acid compositions of refined soybean oil and biodiesel samples obtained by the reactions used in this study (all reactions were performed at 320 ◦ C, 20 MPa for 30 min). Samples

Refined soybean oil Reaction A Reaction B Reaction C Reaction D a b

Fatty acid compositions (wt.%) 16:0

18:0

18:1

18:2

18:3

Others

Total SFAa

Total PUFAb

10.4 10.8 10.7 10.2 10.3

4.4 4.1 3.9 8.9 7.1

24.6 22.1 21.2 57.1 54.3

52.5 51.6 50.8 18.0 20.6

6.8 6.5 6.6 1.9 1.6

1.3 4.9 6.8 3.9 6.1

14.8 14.9 14.6 19.1 17.4

59.3 58.1 57.4 19.9 22.2

SFA, saturated fatty acids. PUFA, polyunsaturated fatty acids.

Table 2 Key properties of biodiesel samples obtained by the reactions used in this study and biodiesel standard specifications (all reactions were performed at 320 ◦ C, 20 MPa for 30 min). Property (units)

Oxidation stability (h) Iodine value (g/100 g) Cetane number CFPP (◦ C)

Test method

EN 14112 EN 14111 EN 15195 KS M 2411

Limit

6 min 120 max 51 min 0 max

increase in monounsaturated methyl ester was significant, while the saturated esters increased only slightly. These results are similar to those of previous research [1,11] in which the selectivity of monounsaturated FAMEs drastically increased during the partial hydrogenation process. It is apparent that polyunsaturated compounds were mainly converted to monounsaturated compounds by partial hydrogenation during the supercritical one-pot process. Oxidative stability, iodine value, cetane number, and cold-flow properties are the major factors limiting the utilization and market acceptance of biodiesel. As mentioned above, these properties of biodiesel fuel are directly determined by the degree of unsaturation of its component fatty esters [2] and can be improved by partial hydrogenation. In order to confirm the improvement of fuel quality, the fuel properties of methyl esters obtained by the reactions used in this study were determined and were compared to each other and biodiesel standard specifications (Table 2). The biodiesel samples obtained by both reactions A and B did not fulfill the given limits of oxidation stability, iodine value, and cetane number. However, these properties of biodiesels produced by reactions C and D were considerably improved and met the requirements of biodiesel standards. The CFPP of all biodiesel samples ranged from −3.1 ◦ C to −1.3 ◦ C. The values of partially hydrogenated samples were somewhat higher than those of non-hydrogenated samples, but they were all within the specified limit of Korean biodiesel standards. Through the supercritical one-pot process, key properties of the partially hydrogenated methyl esters complied with the standard specifications for biodiesel. Based on these results, it can be concluded that simultaneous transesterification and partial hydrogenation in supercritical methanol is a feasible and effective method for the production of high-quality biodiesel from highly unsaturated feedstock. Also, the biodiesel obtained through the supercritical one-pot process could be a competitive alternative to conventional diesel fuel. 4. Conclusions In this study, a supercritical one-pot processing combining transesterification and partial hydrogenation was proposed, and its technical feasibility was successfully validated. The merit of this new process is that, in addition to the advantages of the supercritical methanol technique for biodiesel production, it requires a simple single step to produce biodiesel and improve its fuel quality. In addition, there is no need to supply hydrogen. In the near future,

Reactions used in this study Reaction A

Reaction B

Reaction C

Reaction D

4.6 121.3 47.5 −3.1

4.6 122.9 46.9 −2.3

8.5 78.0 59.4 −1.5

7.9 79.7 57.5 −1.3

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